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  • Published: 06 September 2022

Browning of the white adipose tissue regulation: new insights into nutritional and metabolic relevance in health and diseases

  • Sabrina Azevedo Machado 1   na1 ,
  • Gabriel Pasquarelli-do-Nascimento 1   na1 ,
  • Debora Santos da Silva 1 ,
  • Gabriel Ribeiro Farias 1 ,
  • Igor de Oliveira Santos 1 ,
  • Luana Borges Baptista 1 &
  • Kelly Grace Magalhães   ORCID: orcid.org/0000-0002-7435-5272 1  

Nutrition & Metabolism volume  19 , Article number:  61 ( 2022 ) Cite this article

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Adipose tissues are dynamic tissues that play crucial physiological roles in maintaining health and homeostasis. Although white adipose tissue and brown adipose tissue are currently considered key endocrine organs, they differ functionally and morphologically. The existence of the beige or brite adipocytes, cells displaying intermediary characteristics between white and brown adipocytes, illustrates the plastic nature of the adipose tissue. These cells are generated through white adipose tissue browning, a process associated with augmented non-shivering thermogenesis and metabolic capacity. This process involves the upregulation of the uncoupling protein 1, a molecule that uncouples the respiratory chain from Adenosine triphosphate synthesis, producing heat. β-3 adrenergic receptor system is one important mediator of white adipose tissue browning, during cold exposure. Surprisingly, hyperthermia may also induce beige activation and white adipose tissue beiging. Physical exercising copes with increased levels of specific molecules, including Beta-Aminoisobutyric acid, irisin, and Fibroblast growth factor 21 (FGF21), which induce adipose tissue browning. FGF21 is a stress-responsive hormone that interacts with beta-klotho. The central roles played by hormones in the browning process highlight the relevance of the individual lifestyle, including circadian rhythm and diet. Circadian rhythm involves the sleep–wake cycle and is regulated by melatonin, a hormone associated with UCP1 level upregulation. In contrast to the pro-inflammatory and adipose tissue disrupting effects of the western diet, specific food items, including capsaicin and n-3 polyunsaturated fatty acids, and dietary interventions such as calorie restriction and intermittent fasting, favor white adipose tissue browning and metabolic efficiency. The intestinal microbiome has also been pictured as a key factor in regulating white tissue browning, as it modulates bile acid levels, important molecules for the thermogenic program activation. During embryogenesis, in which adipose tissue formation is affected by Bone morphogenetic proteins that regulate gene expression, the stimuli herein discussed influence an orchestra of gene expression regulators, including a plethora of transcription factors, and chromatin remodeling enzymes, and non-coding RNAs. Considering the detrimental effects of adipose tissue browning and the disparities between adipose tissue characteristics in mice and humans, further efforts will benefit a better understanding of adipose tissue plasticity biology and its applicability to managing the overwhelming burden of several chronic diseases.

The adipose tissues (ATs) are endocrine and dynamic organs that display high morphological and functional plasticity. White AT (WAT) was named that way because it presents white adipocytes in its composition. In contrast, brown AT (BAT) has as its main integrant the brown adipocytes [ 1 ]. Both ATs play various physiological roles, including energy storage, endocrine regulation, and thermogenesis. As a means of adapting, mammalians developed a mechanism to maintain their body temperatures under unfavorable climates [ 2 ]. This process, called adaptative thermogenesis, occurs due to the elevated plasticity of ATs, which allows reversible changes in their morphology and functions [ 3 ]. Studies have shown the capacity of progenitor cells as well as mature adipocytes to differentiate into a model that presents similarities with the brown profile, called brite or beige AT [ 4 , 5 , 6 ]. When this phenomenon occurs in mature adipocytes it is called browning of WAT [ 4 ].

During the browning process, WAT show increased mitochondrial number, augmented energy expenditure, fat multilocularization, and thermogenic genes expressions, such as Peroxisome proliferator-activated receptor-a (PPARα) and PPARγ, PR domain containing 16 (PRDM16), peroxisome proliferator-activated receptor-gamma coactivator 1 a (PGC1-α), cell death-inducing DNA fragmentation factor-like effector A (CIDEA), and Uncoupling Protein 1 (UCP1) [ 7 , 8 , 9 ]. WAT browning occurs in specific conditions through exposure to certain stimuli such as cold, thyroid hormones, diet, natriuretic peptides, medication, and exercise [ 10 , 11 , 12 , 13 , 14 ] (Fig.  1 ).

figure 1

Brown adipocyte regulation by exogenous agents. Thyroid hormones act on thermogenesis through interaction with their Thyroid receptors (TR) and the G-protein-coupled receptor (GPCR). TRα promotes an increase in adrenergic signaling while TRβ acts to stimulate uncoupling protein 1 (UCP1). β3 receptor (β3-AR) is expressed constitutively on the surface of the adipocyte and acts to regulate the transcription and activation of genes related to mitochondrial biogenesis, brown adipocyte differentiation, and lipid storage. This receptor can be activated through cold, which is the main mechanism for activating browning, agonist drugs, or diet. Chronic exposure to cold and food intake, such as curcumin and fish oil, promotes thermogenesis by releasing catecholamines from the central nervous system (CNS) that bind to β-AR, thus initiating a signaling cascade. An increase in the concentration of cAMP is elicited which consequently leads to the enhanced activity of protein kinase A (PKA) which promotes the cAMP-response element-binding protein (CREB). This pathway is related to the transcription of thermogenic genes such as peroxisome proliferator-activated receptor γ (PPARγ), Type II iodothyronine deiodinase (DIO2), PR domain containing 16 (PRDM16), Peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) and Cell death-inducing DNA fragmentation factor-like effector A (CIDEA). The other dietary components can participate in the induction of browning through the modulation of the gut microbiota promoting the increase of the expression of thermogenic genes. The release of growth factor 21 (FGF21) is mediated by the practice of physical exercises and physiological changes. FGF21 will interact with its FGFR receptor and that activation induces a self-phosphorylation of the FGFR that mediates the activation of pathways related to increased expression of UCP1

The increase in energy expenditure can act as a therapeutic approach to metabolic syndrome, and also can be associated with poor prognosis of diseases associated with hypermetabolism [ 15 , 16 , 17 , 18 ]. For this reason, efforts have been employed to identify the key participants in the regulation of browning. This review aims to present and describe the current studies related to both endogenous and exogenous most relevant agents and their biological mechanisms at biochemical and molecular levels.

Introduction

Originally cited as energy storage organs, exclusively, ATs are currently known to express and secrete a variety of bioactive peptides, the adipokines, including leptin, resistin, vaspin, visfatin, hepcidin, adiponectin, and inflammatory cytokines. These bioactive secreted factors act both locally and systematically, modulating different biological processes and consequently influencing the metabolism of various organs, such as the liver, muscle, pancreas, and brain via endocrine mechanisms [ 19 , 20 ]. Besides adipocytes, AT contains an extracellular matrix, nerve tissue, stromovascular cells, and immune cells, which together act as an integrated unit [ 21 ].

Presently, two main subtypes of ATs have been described: WAT and BAT. Brown and white adipocytes have widely different morphologies, not only in terms of composition but also in the form of lipid storage (number and size of lipid droplets) and the disposition and number of mitochondria. These differences correspond to distinct functional roles, diverging in energy metabolism, storage, and distribution [ 19 , 22 ].

WAT, the most abundant AT in the body, contains the white adipocytes, which present unilocular lipid droplets, scarce mitochondria, and lipid storage capacity [ 23 ]. Since the discovery of the adipokines, WAT is also recognized as an important endocrine organ, actively participating in the regulation of physiologic and pathologic processes, including immunity and inflammation [ 24 , 25 ]. Widely distributed throughout the body, there are two main representative types of WATs, the visceral WAT (vWAT) and the subcutaneous WAT (scWAT). While one is distributed around organs and provides protective padding, the other is located under the skin and provides insulation against heat or cold, respectively [ 26 ].

In contrast, brown adipocytes display multilocular lipid droplets, a large number of mitochondria, and thermogenic capacity due to elevated uncoupling protein 1 (UCP1) amounts anchored in its mitochondrial inner membrane [ 27 ]. The BAT utilizes this high mitochondrial content and elevated UCP1 amounts to uncouple oxidative phosphorylation from adenosine triphosphate (ATP) synthesis to dissipate chemical energy as heat [ 28 ]. Thus, BAT affects the metabolism of the entire body, being able to alter insulin sensitivity and modify the susceptibility to increase weight. For a long time, BAT was only considered an energy-producing organ in rodents and newborns, undergoing involution with age. However, BAT has also been identified in human adults near the aorta and within the supraclavicular region of the neck. Nevertheless, the origin of BAT is still under debate [ 26 , 29 ].

Recently, a type of AT showing intermediary characteristics between that of white and brown adipocytes, which has mixed structural features of both, was identified as beige AT [ 29 ]. This type of AT was reported as a set of adipocytes in WAT that might acquire a thermogenic phenotype with higher UCP1 expression, similar to brown adipocytes after enough stimulus [ 29 , 30 ].

There are two major mechanisms described related to beige cells arising: d e novo differentiation which occurs from a progenitor resident cell and transdifferentiation which consist of differentiation of a mature white adipocyte through a molecular mechanism. The first theory is based on that beige adipocytes come from progenitor cells differentiation induced by adipogenic stimulation such as cold exposure, adrenergic signaling, exercise, natriuretic peptides, thyroid hormones, diets, and food components [ 31 , 32 ]. Currently, several specific cell markers were identified in various types of progenitor cells such as in smooth muscle-like cells (Myh11 + ), preadipocytes (Pdgfrb + , SMA + ), adipocytes progenitor cells (Sca-1 + Pdgfra + CD81 + ) [ 33 , 34 , 35 ]. These adipogenic stimulation actives transcriptional machinery of browning that is characterized by the expression of Ucp1, Prdm16, Zfp516, and Pgc1a genes that will promote a beige differentiation [ 36 ].

On the other hand, the transdifferentiation hypothesis proposes beige cells arise from mature white adipocytes, in a reversible process, after adipogenic stimulus without the participation of a progenitor-like state of cells [ 37 ]. The underlying molecular mechanisms for transdifferentiation are under intensive research, but some studies already show that this plasticity process occurs mainly in scWAT depots [ 29 ]. Known as browning, this process has gained increasing attention in the research area as an alternative method of energy stimulation. UCP1 expression can be stimulated when white adipocytes are exposed to stimuli, previously referred as to adipogenic stimulus, [ 20 , 27 , 29 ], driven by a set of molecules known as browning markers.

The uncoupling protein 1 (UCP-1)

The non-shivering thermogenesis is a phenomenon that occurs in brown and beige ATs due mostly to the action of UCP1 [ 38 ]. UCPs are transmembrane proteins that belong to the mitochondrial anion carrier family (MACF), i.e., mediate specific metabolite exchanges between the cell cytoplasm and the mitochondrial matrix and thus enable the activation of essential biochemical pathways [ 39 , 40 ]. The UCPs exhibit 5 isoforms, ranging from UCP1 to UCP5 are present in several tissues [ 41 , 42 ]. UCP1 is the main isoform associated with thermogenesis, it is widely and selectively expressed in the inner mitochondrial membrane of the adipocyte, representing about 10% of the total mitochondrial protein in human epicardial AT [ 43 , 44 , 45 , 46 ].

UCP1 protein is described as participating in thermogenesis by interfering in proton leakage within the chemiosmotic gradient during the mitochondrial oxidative phosphorylation by the translocating fatty acids (FAs). This gradient is obtained from the oxidation of substrates and provides the required force to induce the respiratory machinery to produce ATP. Once UCP1 promotes proton leakage, the energy obtained cannot be stored in the form of ATP and is alternatively dissipated as heat [ 47 , 48 ]. Thus, it is evident that direct regulation of UCP1 protein activity is one of the means of regulating thermogenesis, and that occurs in opposite ways by cytosolic purine nucleotides and long-chain fatty acids (LCFA), promoting inhibition or activation of UCP1, respectively [ 49 ].

The other form of regulating UCP1 is at the transcriptional level. UCP1 gene is transcribed only in brown and beige adipocytes, associates with the differentiation state of these cells, and is quantitatively regulated in response to many physiological signals [ 9 ]. These characteristics are consequences of the transcriptional control mediated by trans-acting factors on regulatory regions found in the 5’ non-coding region of the UCP1 gene. The proximal regulatory region, which is found immediately upstream of the transcription start site, contains cAMP response element-binding protein (CREB) [ 50 , 51 ] and CCAAT-enhancer-binding protein (C/EBP) [ 52 ] binding sites. Also in the proximity of the site of transcription start, activating transcription factor-2 (ATF2)-binding site interacts with transcriptional coregulators, such as PGC-1α, impacting UCP1 gene transcription [ 9 ]. In opposition to these proximal regulatory sites, a strong enhancer region is placed more than 2 kb upstream of the transcription initiation site [ 50 , 53 ] and contains a cluster of response elements for nuclear hormone receptors [ 7 , 54 ].

UCP1 gene activation and repression depend on which trans-acting factors bind to the regulatory region. For example, CREB binding sites mediate a positive transcriptional response to cAMP [ 50 ] and a negative response to AP2 (c-Jun/c- Fos) complexes [ 51 ]. Another example is the PPARγ binding site found in the distal enhancer region, which associates with gene activation after binding to its main ligand but represses UCP1 transcription when interacting with liver X receptor (LXR) and its corepressor receptor-interacting protein 140 (RIP140) [ 55 ]. RIP140 inhibits UCP1 gene transcription by enabling the assembly of DNA and histone methyltransferases on the UCP1 gene, altering the methylation status of CpG islands in the promoter region and histones, impacting gene expression through transcription machinery accessibility [ 56 ].

Although some epigenetic modifications are associated with repressed UCP1 gene expression, as in H3K9 demethylation marks, chromatin modifications indicative of activation of this gene also occur, such as in the case of H3K4 trimethylated marks, which are enriched in BAT [ 57 ]. Also participating in fine-tuning of gene expression, microRNAs (miRs) are characterized to be a group of short non-coding RNAs (ncRNAs) generated by the sequential processing of longer ribonucleic acid molecules [ 58 ]. While miR-328 [ 59 ] and miR-455 [ 60 ] are described to be activators of UCP1 gene expression, miR-27 [ 61 , 62 ], and miR-133 [ 63 ] display UCP1 gene transcription inhibitory activity .

The roles of WAT and BAT in metabolic syndrome is well characterized, but the physiological and biochemical modulations of BAT remain unclear [ 64 , 65 , 66 ]. Several studies showed that UCP1-dependent BAT activity was mostly found to be beneficial in decreasing inflammation, and improving cardiometabolic homeostasis [ 67 , 68 , 69 ]. However, this tissue has a lower activity in obese in comparison to healthy individuals [ 70 ].

It is well established that the deficiency of the UCP1 gene is not enough to protect against diet-induced obesity (DIO), but can modulate important physiological and metabolic parameters in mice [ 64 , 65 ]. The food intake-induced browning is inhibited in the absence of UCP1, demonstrating the intimate relationship between this differentiation process and UCP1 [ 71 , 72 ]. More than that, UCP1 -/- female mice fed with a western diet displayed increased whitening of BAT, as well as metabolic disruption indicated by glucose intolerance, upregulation of genes related to inflammation, liver steatosis, immune cell infiltration, and endoplasmic reticulum (ER)/oxidative stress [ 73 ]. Accordingly, even during standard diet under cold exposure, UCP1−/−, male mice showed BAT immune cell infiltration and ER stress profile [ 74 ]. The lack of UCP1 promotes de novo lipogenesis and hyperplasia of inguinal WAT, leading to an increase in FA trafficking to the liver [ 75 ].

In contrast, the upregulation of UCP1 or even only its activation can perform a paradoxical role in hypermetabolic scenarios and associate with a worse prognosis [ 76 , 77 ]. It is proven that diet-induced whitening is related to the upregulation of this gene. A greater expression of browning markers (e.g., UCP1, PGC-1α, TBX1) was found in obese human patients mainly in vWAT [ 78 ] Besides that, mice affected by cancer-associated cachexia tend to show increased thermogenesis gene expression and BAT activation [ 79 ].

The browning process is spontaneously induced by tumor-secreted factors and IL-6 during cachexia development, which can lead to full depletion of AT [ 80 , 81 ]. Interestingly, the elicitation of browning after burn injury is associated with the hypermetabolic response, as well as an increase in lipolysis and free fatty acid efflux that can outcome in liver steatosis [ 82 , 83 ]. In addition to the therapeutic impact of the browning process in obesity and metabolic diseases, recent discoveries regarding the impact of UCP1-dependent BAT activity in hypermetabolism conditions should be further investigated in the context of UCP1 to appropriately regulate browning for application in different situations.

Beta 3 adrenergic receptor activation

Increasing energy expenditure through activation of BAT shows potential for treating metabolic diseases, and that is the reason this approach has been deeply investigated [ 84 ]. The white/brown plasticity of ATs and tissue thermogenesis appear to be activated by a β-3 adrenergic receptor (β-3AR) system [ 4 , 30 , 85 ]. β3-ARs are expressed predominantly on white and brown adipocytes [ 86 ]. Murine WAT expresses β3-AR transcripts in a greater proportion compared to other β-ARs, similar to BAT [ 87 ]. Although β3-AR mRNA levels are lower in humans than in rodent AT, its roles seem to be fundamental in the regulation of energy balance and glucose homeostasis [ 88 ].

Browning of WAT occurs mainly by noradrenaline and adrenaline stimulation, which influence lipolysis after binding to different adrenoceptor subtypes on the cell-surface membrane of fat cells. The interaction with β3-AR initiates a cascade of signal transduction that ends with the overexpression of thermogenic proteins, such as UCP-1 [ 88 , 89 ]. The adaptive thermogenic response is initiated by the central (CNS) and sympathetic (SNS) nervous systems with the release of norepinephrine (NE) and stimulation of β3-AR, through the G protein-coupled receptor Gs, which in turn activates the adenylyl cyclase (AC), stimulating the production of cyclic adenosine monophosphate (cAMP), and activating the protein kinase A (PKA) pathway. Then, these signals from the cAMP pathway, finally, upregulate UCP-1 and lipolysis [ 88 , 89 , 90 , 91 , 92 ].

A distinguishing feature of the β3-AR, already seen in past studies, is that it appears to be relatively resistant to desensitization and down-regulation, leading to the hypothesis that one of its functions might be to maintain signaling during periods of sustained sympathetic stimulation, as in diet-associated β3-AR activation or cold exposure [ 87 ]. Cold temperature exposure elicits a coordinated physiological response aimed at maintaining their body temperature. This response activates the mentioned cascade and generates heat in beige adipocytes within scWAT and BAT [ 85 ]. Thus, it was seen that mice with a combined target disruption of the three β1, β2, and β3 adrenergic receptors (TKO mice) have increased susceptibility to cold-induced hypothermia as well as diet-induced obesity [ 91 ]. Thereby, mice β3-AR activation started to be studied, effectively mimicking cold exposure effects [ 84 , 91 ].

Initial studies demonstrated that WAT UCP1 mRNA and protein levels are strongly decreased in β3-AR knockout (KO) mice [ 30 , 93 ]. In addition, β3-AR agonists are well-known for inducing ectopic UCP1 expression in WAT coupled with a significant mitochondrial enhancement in rodents, and for augmenting glucose homeostatic activity of their BAT [ 84 , 94 ]. On the other hand, in humans, early efforts to increase browning activation with the use of β3-adrenoreceptor agonists have failed in clinical trials because of their β1- and β2-AR-mediated cardiovascular effects [ 13 , 84 , 94 ]. However, a recent study showed that mirabegron, a selective β3-agonist previously developed for the treatment of overactive bladder, was shown to increase BAT activity as compared to placebo. This study used an oral dose of 200 mg in healthy male subjects, and despite not having severe cardiovascular side effects, they have been shown to increase heart rate and systolic blood pressure [ 84 ]. That is the reason long-term studies are warranted to investigate the effectiveness and cardiovascular safety of this type of treatment to induce weight loss and metabolic health improvements.

Genetic factors must be considered in influencing adipocyte lipolysis regulation. Genetic variance in β3-AR and its specific G-coupling protein has functional effects on lipolysis. Polymorphism in the G-β3 gene, for example, influences catecholamine-induced lipolysis in human fat cells by altering the coupling of β3-AR to G-proteins [ 88 ]. This proves once again the importance of the β3-AR presence for the thermogenic process.

Temperature-induced browning

It is well established that temperature can modulate biochemical, inflammatory, and immunological processes systemically, displaying relevant physiological impact [ 95 , 96 ]. Despite this, the influence of warmer temperatures is better described compared to cold conditions due to their immediate danger. Fever, triggered by infectious and inflammatory processes, was associated with a worse prognosis in the past centuries, demanding greater medical attention for a long time [ 97 ]. However, currently, it is recognized that both hyper and hypothermia, in properly regulated circumstances, are beneficial response mechanisms to infection in mild and severe profiles, respectively [ 98 ].

Hypothermia is also associated with an advantageous mechanism against severe systemic inflammation. In experimental studies, the infectious or aseptic systemic inflammation process is elicited by the intravenous administration of bacterial lipopolysaccharide (LPS) in mice [ 99 , 100 , 101 ]. The variances of body temperature are modulated by the environmental temperature and concentration of LPS introduced [ 102 ]. Animals housed in hyperthermal conditions or exposed to lower LPS concentrations displayed polyphasic fever. In thermoneutral conditions, the fever was also usually elicited to induce the immunological response. However, if the mice were housed in cooler acclimation or administered with higher LPS concentrations, the effect elicited was hypothermia, which associates with arterial hypotension aimed to avoid infection spread, followed by polyphasic fever.

The ideal body temperature is obtained by the modulation of blood vessel tension degree, and heat production by thermogenesis. Cutaneous vasoconstriction and thermogenesis processes occur to increase body temperature and avoid heat loss. Conversely, the opposite effect, skin vasodilatation and thermogenesis inhibition, is stimulated to induce hypothermia [ 102 , 103 , 104 , 105 ].

Temperature is a paradoxical agent with important roles not only in biological events but also in the development of several diseases [ 106 , 107 , 108 , 109 , 110 ]. Hypothermia, specifically, displays a typical profile, in which the energy is preserved. The decrease in body temperature also favors the development of an anti-inflammatory profile and immunosuppression, which can act as a double-edged sword depending on the condition [ 108 , 111 , 112 , 113 ]. In the tumoral context, hypothermia provides an immunosuppressive, hence, pro-tumoral microenvironmental due to impaired CD8+ T cell function, an increase of the regulatory and Th2 cells, and higher levels of the cytokines IL-4 and IL-10, which increase cancer progression and metastasis [ 114 , 115 , 116 ].On the other hand, hyperthermia is recognized to elicit a more robust immune response against infection, injury, and cancer [ 117 ]. Several studies reported an increase in IFN-γ and IL-2 secretion from peripheric T cells, enhancement of cytotoxicity, DC maturation, and increase of tumor-specific CD8+ T cells [ 118 , 119 , 120 , 121 ].

In inflammatory conditions, such as neurological damage, atherosclerosis, systemic inflammation, and hypothermia (cryotherapy) can be beneficial [ 122 , 123 , 124 , 125 , 126 ]. Cryotherapy benefits are illustrated by an experimental approach that submitted healthy and physical activity practitioners men to intense exercises aimed to induce muscle injury [ 127 ]. It was observed that cryotherapy mediated the increase of IL-10, reduction of pro-inflammatory cytokine IL-1, reduction of muscle damage and blood cholesterol, decrease oxidative stress and improve the lipid profile not only in healthy patients but also in patients with active-phase ankylosing spondylitis [ 124 , 128 , 129 ]. Cryotherapy also shows to be neuroprotective capacity, alleviating sequelae from ischemic or hemorrhage stroke, cardiac arrest, intracranial pressure elevation, and traumatic brain injury [ 130 ].

In the same line, recent research evaluated the impact of spontaneous body hyperthermia after brain injury. Metabolic modulations were observed as the diminishment of both cerebral and arterial glucose levels and increase of lactate-pyruvate ratio. However, these changes were not associated with a worse prognostic [ 131 ]. Additionally, induced hyperthermia in healthy men promoted an increase in cerebral metabolic rate of oxygen (CMRO 2 ), also increase IL-6 and myeloperoxidase (MPO) systemically, but did not promote the same inflammatory and oxidative phenomenon in the brain [ 132 ]. In peripheral organs such as the liver, hyperthermia is associated with an increase in oxidative metabolism, vasodilation, and an increase of heat shock proteins (HSP) expression. HSP displays an important role in metabolism such as modulation of both glucose and lipid metabolism in the liver and improving the mitochondrial skeletal muscle functionality [ 133 ].

In addition, temperature modulates directly the shivering and non-shivering thermogenesis processes. The occurrence of these events maintains proper body temperature under adverse thermal acclimation. Once shivering thermogenesis is decreased in cold acclimation (around 4 °C), non-shivering thermogenesis is the major way to produce heat in this context [ 2 , 4 ]. The detection of the thermal changes begins with the capture of sensory stimuli by cutaneous thermoreceptors, which promote the sensitization of afferent nerves. The stimuli are directed to the CNS, which then induces thermoregulatory responses, including vasoconstriction and catecholamines secretion. These catecholamines, mainly NE, increase BAT activation, hence heat production through a UCP1-dependent manner [ 134 , 135 ].

BAT is a highly innervated and vascularized organ that displays considerable amounts of β3-ARs, which is also expressed in WAT, though at a lower level. NE binding to β3-ARs promotes systemic adrenergic activation, which induces a signal cascade culminating in the accumulation of adipokines, such as Zinc-α2-glycoprotein (ZAG), increase in thermogenesis-related gene expression, as UCP1, thus enabling mobilization and oxidation of free fatty acids (FFAs) in both tissues, increasing BAT activity and promoting browning in WAT. [ 84 , 136 , 137 ].

It is known that the results observed in humans do not always represent the same effects previously described in mice or even contradictory results can be obtained under similar conditions for the same species [ 138 ]. Unfortunately, this premise can also be applied to cold-induced browning. Leitner and colleagues showed that in human fewer than half of the BAT deposits is stimulated by cold exposure, hence, the thermogenic function was lower than expected [ 44 ].

Brychta and others demonstrated that the profile of men with obesity was associated with a reduced tolerance limit to chill temperatures, suggesting that thermogenesis was diminished in these individuals, as well as energy expenditure [ 139 ]. Blauw et al. [ 140 ] demonstrated a surprising association between the impairment of glycemic homeostasis, diabetes, and obesity in humans housed in the United States of America (USA) at warmer temperatures and associated these results with a decrease in BAT activity. Taking the assessment on a global scale, Kanazawa evaluated the parallel between higher temperatures, weight gain, and obesity. The data analyzed allowed to predict that global warming could be responsible for an increase of more than 10% in the obesity rates in 120 years, counting from 1961 [ 141 ].

Notably, the use of cold as a browning inducer has been carefully applied not only because of the side effects that can be displayed at the whole-body level but also due to the contradictory effect observed in humans. If on one hand, the anti-inflammatory and immunosuppressive role mediated by cold and cold-induced browning is beneficial in healthy individuals, for ill people these same effects may become harmful. In contrast, intriguing research has brought a new perspective on temperature-based browning. Li and colleagues discovered that a hydrogel-based photothermal therapy leads to a successful increase of beige activation in both mice and humans. The therapy consists of increasing the local temperature, around 41˚C, without evident stress on skin or adjacent tissues [ 142 ]. This promising study succeeds previous findings that pointed to the occurrence of WAT browning after burn injury [ 143 , 144 ]. The characterization of possible inducers of browning is a strongly growing field since the applicability of these inducers as therapy in humans has proven to be a major clinical hurdle. However, even under promising advances is clear that further investigation regarding the mechanisms triggered by this stimulus pathway should be conducted.

Exercise-induced browning

Physical exercising is already associated with improvements in several processes related to the cardiovascular system, skeletal muscle, and ATs [ 193 ]. Following this, several studies show that physical activity provides better quality of life [ 194 ] and helps in the treatment of several metabolic diseases and obesity [ 195 ] through increasing AT lipolysis, vascularization, blood flow, and promoting the secretion of hormones and adipokines [ 196 ]. Among the main adipokines, FGF21 (Fibroblast growth factor 21) and leptin stand out, which act in an autocrine/paracrine manner, regulating WAT browning process [ 197 ]. After physical activities, the adipokine leptin stimulates activity in the sympathetic nerve and together with insulin act synergistically in different neuronal subsets of proopiomelanocortin (POMC) inducing browning of WAT through decreased hypothalamic inflammation caused by exercise [ 198 ]. During exercise, the increase in glucagon, which already has thermogenic potential [ 199 ], and the decrease in insulin in the liver lead to FGF21 secretion [ 200 ].

The exercise induces pleiotropic effects in the liver, AT, immune system, and skeletal muscle by enabling myokine secretion upon contraction [ 201 ]. After activities, muscle cells increase the expression of PGC-1α, inducing BAT thermogenesis and mitochondrial biogenesis. Among the myokines involved in the browning process, interleukin-6 (IL-6) is a modulatory cytokine secreted by several tissues, including skeletal muscle and AT. A study showed that mouse AT when treated with IL-6 for 6 h induces the expression of PGC-1α and mitochondrial enzymes [ 202 ]. In addition, analyses showed that IL-6 is involved in the increase of UCP1 mRNA in inguinal WAT (igWAT) stimulated by physical activity [ 203 ]. Another relevant myokine is Irisin; Once exercising increases the expression of PGC-1α, it induces increased levels of the fibronectin domain-containing protein 5 (FNDC5) protein, which, after being cleaved, is released as the hormone irisin [ 204 ]. Irisin was shown to stimulate UCP1 expression and thermogenic differentiation of white fat precursor cells in vitro and in vivo [ 145 ].

The myokine myostatin (Mstn), a growth factor that limits muscle growth and development, is negatively involved in the WAT browning process, as Mstn-deficient mice showed high expression of genes associated with FA oxidation, mitochondrial biogenesis, lipid transport together with the positive regulation of PGC-1α and UCP1, this mechanism occurs through the phosphorylation of AMPK, necessary for the activation of PGC1α and FNDC5 [ 199 ]. Metrnl, the gene encoding for Meteorin-like protein, is a myokine known to be induced by resistance exercise dependently on PGC-1α4. Metrnl regulates genes involved in thermogenesis, as it is capable of promoting the activation of M2 macrophages by inciting the expression of IL-4 and thus triggering the production of catecholamines [ 206 ], responsible for favoring thermogenesis in AT [ 207 ]. Beta-Aminoisobutyric acid is another myokine that has increased levels during exercise and can induce the brown adipocyte phenotype in human-induced pluripotent stem cells during differentiation to mature white adipocytes [ 146 ].

Intense physical activity causes increased heart rate and stretching of cardiomyocytes, which cause the secretion of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP), molecules that stimulate lipolysis, UCP1 expression, and mitochondrial biogenesis [ 208 ]. It also induces an increase in lactate, which binds to receptor GRP81 on adipocytes, leads to an increase in P38 phosphorylation, and thus mediates the browning of WAT by activating the PGC-1a, PPAR, γ, and Ucp1 genes [ 147 ]. During lipolysis, FAs are not only used as an energy source but also undergo the re-esterification process where they are converted into triglycerides in AT. This re-esterification consumes ATP generating AMP. AMP in turn can activate AMPK, which then induces greater expression of PGC-1α and mitochondrial biogenesis [ 210 ].Another the important effect induced by exercise that plays an important role in the browning of WAT is oxidative stress in skeletal muscle, whish it responsible for the increase in H 2 O 2 through the reduction of glutathione levels, a molecule capable of supplying electrons to glutathione peroxidase, thus increasing H 2 O 2 levels. And also by increasing the activity of superoxide dismutase 2 (SOD2), which reduces ROS to H 2 O 2 . When H 2 O 2 enters the circulation, it is directed to WAT and subsequently induces the expression of thermogenic genes [ 148 ]. Exercise also increases the level of succinate, resulting in augmented levels of mitochondrial reactive oxygen species, which in turn promotes the sulphenylation of Cys253 to increase UCP1 activity [ 149 ].

Although the studies conducted in mice seem promising, the effect of exercise on WAT browning in humans has proven to be controversial. A survey conducted with sedentary subjects participating in a 12-week bicycle-training program showed scWAT increased expression of UCP1, carnitine palmitoyltransferase 1B (CPT1B), TBX1 [ 15 ]. However, other studies have not achieved similar effects. Tsiloulis and colleagues collected scWAT of obese men after 6 weeks of physical training and the mRNA levels of UCP1, CD137, CITED, TBX1, LHX8, and TCF21 were not altered [ 211 ]. Many factors may be involved in this diversity of results since the duration, frequency, and degree of intensity are associated with these effects. Thus, more human studies need to be conducted as many questions still need to be clarified.

Fibroblast growth factor 21

The fibroblast growth factor family (FGF) performs a range of cellular metabolic and physiological responses to maintain overall homeostasis. FGF 21 was first identified in mice and humans in 2,000 by Nishimura and colleagues through cDNA identification in different organs [ 150 ]. While the gene in mice is located in chromosome 7 and encodes a preprotein of 210 amino acids (aa), in humans it is found in chromosome 19 and encodes a preprotein of 209 aa. Proteins belonging to the FGF family exert a wide range of functions, from promoting cell proliferation and differentiation to systemic effects [ 151 , 152 ], acting as autocrine/paracrine/endocrine factors [ 153 ]. Most FGF family members have a high affinity to heparin sulfate, except for the endocrine FGF (FGF) subgroup, which consists of FGF 19 (FGF 15 in rodents), FGF 21 e FGF 23 in humans [ 151 ]. FGF molecules lack an extracellular heparin-binding domain and thus can enter the blood system [ 154 ].

FGF 21 binds to a fibroblast growth factor tyrosine kinase receptor (FGFR), which can be found in seven isoforms: 1b, 1c, 2b, 2c, 3b, 3c, and 4. The FGF 21 requires its dimerization with a klotho protein, called beta-klotho (KLB). Thus, the FGFR-KLB receptors lead to the intracellular cascade that goes through the phosphorylation of FGFR substrate 2α (FRS2α) and the activation of Ras-MAPKs and PI3K-Akt kinases [ 154 , 155 , 156 ]. Once FGF21 signaling requires KLB to activate FGFRs, the co-expression of these two receptors determines the sensitivity of a tissue or organ to FGF21 [ 157 ]. FGF 21 is defined as a stress-responsive hormone [ 152 ], which effect is subtle in physiological conditions but significantly exacerbated under nutritional, metabolic, oxidative, hormonal, or environmental challenges. Consequently, starvation and overfeeding, ketogenic and high-carbohydrate diets, physical exercises, protein restriction [ 158 ], type 2 diabetes (T2D), obesity [ 153 ], and nonalcoholic fatty liver disease (NAFLD) [ 159 ] can induce the expression or/and signaling of FGF 21 [ 158 ].

FGF 21 is synthesized mainly in the liver and thymus but is also detected in skeletal muscle, pancreas, intestine, heart, β cells, and WAT and BAT [ 160 ]. As an important metabolic regulator, acting mostly in glucose and lipid homeostasis, FGF 21 triggers lipolysis and FFAs released in circulation from WAT during prolonged fasting or starvation [ 29 , 160 ]. PPAR-α is activated in the presence of FFA and improves FFA oxidation and ketone bodies formation for acting as energy sources during prolonged fasting. The interaction between FFA/PPAR-α/retinoid X receptor (RXR) has a PPAR-α response element, which activates FGF 21 promoter [ 161 , 162 ]. Thus, when PPAR-α activity increases, the production of FGF 21 in the liver also augments, leading to energy production, increased ketogenesis, gluconeogenesis, appetite, and systemic glucose uptake as adaptive responses to starvation [ 163 ]. The activity of FGF 21 is not limited to starvation conditions, but it is also increased in adaptation to high-fat (HF) intake [ 164 ].

Human studies inform that FGF21 production is stimulated in situations of decreased thermogenesis, reduction in adiponectin levels, and tissue breakdown markers, such as transaminases elevation mare than changes in levels of FFAs [ 165 ]. Another means of increasing FGF21 levels, through PPAR-α activity, is through intense physical activity, growth hormone therapy, lactation, and milk ingestion in neonates [ 163 , 166 ]. Macronutrients such as proteins also regulate FGF 21 production through amino acid restriction [ 167 ]. This process starts when the general control non-derepressible 2 (GCN2)-eukaryotic initiation factor 2 (eIF2) α pathway is activated inducing the binding of activating transcription factor 4 (ATF4) to PGC-1 α [ 168 , 169 ].

After being secreted, its most important target is WAT, where FGF21 improves insulin sensitivity [ 153 , 160 ] and increments GLUT1 expression and consequently glucose uptake, as shown by in vitro 3T3-L1 adipocyte analyses [ 153 , 170 ]. The response element-binding protein (ChREBP) is sensitive to carbohydrates in the liver and ChREBP interaction with PPAR-γ in adipocytes modulates the expression of FGF21. In other words, the upregulation of ChREBP may induce the expression of this FGF [ 160 ]. Another example of FGF 21 influence on carbohydrate metabolism is through the suppression of hepatic pyruvate dehydrogenase (PD) complex through PD kinase 4 activity [ 171 ]. Additional transcription factors, such as retinoic acid (RA) receptor β (RARβ), TRβ, cyclic AMP response element-binding protein H (CREBH), RA receptor-related orphan receptor α (RORα), respond to determinants in the liver and regulates FGF 21 production [ 151 ].

WAT is not only a target of FGF21, but it is the major mediator of its effects. The processes of glucose- and insulin-sensitive responses depend on adiponectin production and secretion by this tissue [ 172 ]. Adiponectin also reduces the levels of sphingolipid ceramides in obese animals, which have been associated with lipotoxicity [ 173 ]. The action of FGF21 in WAT includes paracrine and autocrine actions and is mediated through the induction of PGC-1α protein in cold and through the enhanced levels of the thermogenic protein UCP1, which is a key protein for heat production [ 174 ]. BAT requires the FGFR1/KLB complex to respond to FGF21, which induces glucose uptake and thermogenesis through the induction of UCP1, in response to its autocrine and paracrine production. FGF 21 impact derives from increased PGC-1α levels and, consequently, expression of UCP1 [ 174 ]. In conclusion, FGF 21 is involved in glucose uptake, lipogenesis, and lipolysis, depending on the metabolic state of the adipocytes. This dual phenomenon may depend on nutritional condition, FGF21 concentrations reached between pharmacological administration and physiological secretion [ 175 ].

Thyroid-hormone-induced browning

Thyroid Hormone (TH) is essential for metabolism in mammals and associates with many processes, including organism development, metabolic regulation, neural differentiation, and growth [ 176 ]. Many genes are regulated after its conversion from the prohormone thyroxine (T4) to the activated form triiodothyronine (T3) [ 177 ] by 5′-deiodinase type 2 (D2), enzyme known to be expressed in the hypothalamus, WAT, BAT, and skeletal muscle, and to be required for adaptive thermogenesis [ 178 ]. TH is produced in the follicles of the thyroid gland and is synthesized through iodination of tyrosine residues in the glycoprotein thyroglobulin [ 179 , 180 ]. The main means of regulator its production is through thyroid-stimulating hormone (TSH), which binds to the TSH receptor (TSH-R) expressed in the thyroid follicular cell basolateral membrane and is released by the anterior pituitary in response to a circulating TH [ 181 ].

The biological response of TH is complex and highly regulated. It is mediated by thyroid hormone nuclear receptors (TRs). The TR genes produce two main types of receptors, α and β, and their isoforms α1, α2, α3, β1, β2, and β3, but only α1, β1, β2, and β3 are T3-binding receptors, which are differentially expressed in tissues and have distinct roles in TH signaling [ 178 , 182 ]. TH enters the cell through membrane proteins monocarboxylate transporter 8 (MCT8) and solute carrier organic anion transporter family member 1C1 9 (OATP1C1), then interacts with TR in the nucleus, which binds to the genomic thyroid-hormone responsive elements (TREs) and other nuclear proteins, including corepressors, coactivators, and cointegrators, leading to chromatin remodeling and the regulation of the UCP1 gene transcription [ 183 , 184 ].

This hormone is correlated with weight and energy expenditure. Thus, hypothyroidism, characterized by diminished TH levels, leads to hypometabolism, a condition associated with reduced resting energy expenditure, weight gain, high cholesterol levels, reduced lipolysis, and gluconeogenesis. On the other hand, hyperthyroidism, and elevated TH levels, induce a hypermetabolic state, characterized by increased resting energy expenditure, lower cholesterol levels, increased lipolysis and gluconeogenesis, and weight loss. Consequently, TH controls energy balance by regulating energy storage and expenditure regulating key metabolic pathways [ 178 ].

TH regulates basal metabolic rate (BMR) through ATP production, used for metabolic processes, and by generating and maintaining ion gradient [ 185 , 186 , 187 ]. BMR is induced by the stimulation of two main gradients, the Na+/K+ gradient across the cell membrane and the Ca 2+  gradient between the cytoplasm and sarcoplasmic reticulum and produces heat during ATP hydrolysis [ 188 ]. TH maintains the BMR levels through the uncoupling oxidative phosphorylation in the mitochondria. When ATP production is compromised in skeletal muscle, TH increases the leak of protons through the mitochondrial inner membrane, stimulating more oxidation to maintain ATP synthesis [ 189 ].

TH regulates metabolism primarily through actions in the brain, WAT, BAT, skeletal muscle, liver, and pancreas [ 178 ]. This action, as already said, is through TH receptors (TR) isoforms, WAT has the adrenergic signaling increased by TRα [ 190 ], otherwise BAT expresses TR α and β, as it needs TRα for adrenergic stimulation and TRβ for stimulating of UCP1, both for thermogenesis [ 176 ]. In humans, T3 administration induced UCP1 expression, dependent on the presence of TRβ, which induces “browning” [ 191 ]. TH regulates several aspects of lipid metabolism and human BAT from lipogenesis to lipoprotein signaling [ 192 ]. Rats administrated with T3 showed how the central nervous system is important to the activation of BAT by TH through inhibition of hypothalamic AMP-activated protein kinase (AMPK). Stimulation of sympathetic nervous system (SNS) activity leads to thermogenic gene expression in BAT [ 193 ].

As discussed previously, β-AR is stimulated by NE in response to SNS [ 1 ]. The expression of UCP1, required for BAT thermogenesis, is regulated by NE and T3 synergistically, once the induction in separate is twofold, while combined is 20 -fold [ 194 ]. Another way that UCP1 expression and thermogenesis are induced is through bile acid stimulation. G protein-coupled membrane bile acid receptor (TGR5) is stimulated in BAT and results in D2 stimulation and local T3 production [ 178 ].

In conclusion, several mechanisms have been proposed for the TH influence in the browning process, including cold exposure, adrenergic activation [ 167 ], and bile acid signal [ 178 ]. Thus, the stimulation of BAT activation and WAT browning increase the energy expenditure, loss of weight [ 195 ], D2 activation, UCP1 level increase, and consequent thermogenesis [ 192 ].

Circadian rhythm and browning

As previously discussed here, several exogenous factors are able to elicit browning of WAT and BAT activation. However, endogenous factors also play an important role in regulating the phenotype and physiology of these tissues. One of the most important endogenous factors that are related to the regulation of AT is the circadian rhythm, which is a refined system that acts as a master biological clock synchronizing daily and seasonal variations with the behavioral, cellular and tissue-autonomous clock, as well as several biological processes that include sleep–wake cycle, hormone secretion, lipid and glucose homeostasis, energy balance and body temperature [ 196 ]. The circadian rhythm is controlled by melatonin synthesis, which can occur both in the CNS, more specifically in the pineal gland being regulated by light/dark stimulus via the suprachiasmatic nucleus of the hypothalamus (SCN), and in peripheral tissues where its regulation remains unclear [ 197 ].

Disruption of circadian rhythm caused by aging, shift-work, irregular sleep, insomnia, or long exposure to light during the night is associated with sleep and metabolic disorders such as cardiovascular diseases, diabetes type 2 and obesity. Regarding metabolic diseases, AT plays a central role in metabolic and whole-body energy homeostasis, once its secretes several adipokines that regulate diverse processes in CNS and peripheral tissues. Leptin, a hormone mainly produced by adipocytes, is released into the circulation where it crosses the blood–brain barrier (BBB), through a saturable system, and interact with its receptor in the hypothalamus LepRb [ 198 , 199 ]. Hsuchou and colleagues demonstrated that leptin signaling disruption through a pan-leptin receptor knockout (POKO) in mice was able to dysregulate feeding behavior, metabolic and circadian rhythm profile and thus promote an accentuating of obesity [ 200 ]. Beyond control of feeding and metabolic processes, leptin also displays a role in energy balance through the increase of AT thermogenesis in BAT by sympathetic activation [ 201 , 202 ].

Recent studies have proposed that diurnal rhythm promotes differential modulation in activity, thermogenesis and fat oxidation in BAT. It was observed that plasmatic lipid metabolism was improved during daytime with a higher expression of lipoprotein lipase, FA uptake, and modulates lipid plasmatic concentration in BAT [ 203 ]. In the same line, Matsushita and colleagues, assessed forty-four healthy men who received diet-induced thermogenesis (DIT) under room temperature (27 °C) and cold (19 °C) in the morning and in the evening by using 18 F-fluoro-2-deoxy-D-glucose positron emission tomography. It was observed that thermogenic parameters presented better performance during the morning [ 204 ].

Moreover, several studies have established that melatonin directly impacts BAT morphology and function, also, in a mechanism dependent on adrenergic activation mediated by NE release. Melatonin is related to an increase of BAT volume, and thermogenic capacity, associated with the increase of UCP1 mRNA expression and mitochondrial mass and functionality, as well as seric lipid concentration. These profiles are significatively impaired under melatonin deficiency but reverse with oral melatonin replacement [ 205 , 206 , 207 ]. Growing evidence confirms the intimate relationship between circadian rhythm and AT, with emphasis on metabolic homeostasis and modulation of BAT activity. The characterization of how this process happens emerges as a strong diagnostic tool as well as a therapeutic approach concerning sleep disorders and metabolic diseases.

Food-intake and browning

Several studies suggest that food items can affect AT function. Among them curcumin, present in saffron, proved to be involved in the browning process, as mice treated with 50 or 100 mg/kg/day of this compound increased inguinal WAT expression of several browning-associated genes, such as Ucp1, Pgc1a, Prdm16, Dio2, PPARα, and CIDEA, and displayed mitochondrial biogenesis in this tissue. Curcumin stimulation was unable to induce the same effects in the epididymal WAT, though. This process was mediated by the NE-β3-AR pathway since the levels of NE and β3-AR were elevated in the inguinal WAT [ 208 ]. Although studies are scarce regarding the impact of thyme in the WAT browning process, it was observed that 20 µM of thymol, a substance present in the essential oils of thyme, in the complete medium when placed in contact with 3T3-LI preadipocytes for 6–8 days was able to induce an increased gene and protein expression of the PGC-1α, PPARγ, and UCP1. Such increases were related to the activation of β3-AR, AMPK, PKA, and Mitogen-activated protein kinase (p38 MAPK) being accompanied by an increase in mitochondrial biogenesis [ 209 ].

Cinnamon oil contains trans-cinnamic acid, which exposure to 3T3-L1 white adipocytes at 100 µM high gene expression of Lhx8, Ppargc1, Prdm16, Ucp1, and Zic1 and markers of UCP1, PRDM16, and PGC-1α, indicating WAT browning [ 210 ]. Quercetin, a flavonoid present in the onion, also proved to be efficient in the browning process since mice fed for 8 days with 0.5% onion peel extract (OPE) during a high-fat diet (HFD) exhibited increased expression levels OF UCP-1, PRMD16, AND PGC1-α in retroperitoneal white adipose tissue (rWAT) [ 211 ]. Just as the combination of quercetin and resveratrol also induces the WAT browning phenotype [ 212 ]. The resveratrol, present in the bark of grapes and other plants, also increases the expression of UCP-1, PRDM16, and PPARγ, suggesting that resveratrol induces the formation of beige adipocytes through the phosphorylation of AMPK, once treatment coupled with inhibition or the deletion of AMPK did not produce the same effects [ 208 ]. The same was observed in the substances found in the mushroom and honey, which induced increased expression of brown fat markers via AMPK and PGC-1α [ 213 ].

The peppers have capsaicin, an active compound responsible for the burning sensation that is also involved in the browning of WAT. One study used wild-type (WT) and TRPV1 −/− mice fed with HFD containing 0.01% capsaicin. The WT animals showed an increase in the expression of Ucp-1, Pgc-1α, Sirt-1, Prdm16, and exhibited browning of WAT via activation of the transient receptor potential vanilloid 1(TRPV1), which is related to the synthesis of catecholamine or sirtuin 1 (SIRT1)-mediated deacetylation of PPARγ, facilitating PPARγ-PRDM-16 interaction. The same did not happen in animals TRPV1 −/− , demonstrating that capsaicin depends on the role of TRPV1 in the browning process [ 214 ]. Menthol, an organic compound extracted from Mentha piperite oil, was shown to activate Transient Receptor Potential Cation Channel Subfamily M Member 8 (TRPM8), and this matched the increase in the thermogenic Ucp1 gene and the expression of Pgc-1α through PKA phosphorylation induced by free intracellular Ca 2+ in adipocytes treated with menthol for 8 h [ 215 ]. Other substances, such as carotenoids, are involved in the WAT browning process. Fucoxanthin, β-carotene, and citrus fruits are efficient in modulating the Ucp1 expression ( 216 , 217 , 218 ).

Another food component that is involved in the browning process of WAT is berberine, a molecule derived from the plants Coptis chinensis and Hydrastis canadensis . Obese male C57BLKS/J-Leprdb/Leprdb mice (db/db) were injected for 4 weeks with berberine (5 mg/kg/day). The group discovered berberine promotes BAT thermogenesis and WAT browning, since the igWAT, but not the epididymal, showed high levels of mRNA and UCP1 protein expression and increased mitochondrial biogenesis after injections. The brown adipocyte markers PGC-1α, CIDEA, Cox8b, and lsdp5 were also elevated and AMPK and PGC-1α are involved [ 219 ]. In another study, the polyphenols from tea extracts (0.5%) present in the high-fat diet for 8 weeks reduced the size of adipocytes and induced browning markers in WAT, and the size of lipid droplets and whitening markers were reduced in the BAT [ 220 ]. Another analysis with the extract induced 77.5 or 155 mg/kg/day for 8 days, in which there was an increase in UCP-1 and PPARγ [ 221 ].

Genistein, present in soy, indirectly induces browning since it is capable of increasing irisin levels, through PGC-1α / FNDC5, which increases Ucp1 and Tmem26 expression ( 222 ). In Magnolia Officinalis, two magnolol compounds (20 µM) and Honokiol (1–20 µM) when used to stimulate 3T3-L1 adipocytes increased protein levels of PGC-1α, PRDM16, and UCP-1 [ 223 ]. Honokiol also increased protein expression levels of CIDEA, COX8, FGF21, PGC-1α, and UCP1 [ 224 ]. The herb panax ginseng contains ginsenoside Rg1 (10 μM of ginsenoside Rb1), which is capable of considerably increasing the mRNA expression of UCP1, PGC-1α, and PRDM16 in mature 3T3-L1 adipocytes via PPARγ [ 225 ], as well as activating the AMP-activated protein kinase pathway [ 226 ].

The fish oil is rich in n-3 polyunsaturated fatty acids (PUFAs), components that are associated with the formation of beige adipocytes, among them is eicosapentaenoic acid (EPA). Mice fed different diets, including with EPA, for 8 weeks showed increased expression of β3-AR, PGC-1α, and UCP1 and exhibited high expression of PPAR [ 227 ], though this effect is controversial since another animal study investigating a diet containing pure EPA (3.6% as EPA ethyl ester) did not show the expression of beige adipocyte marker genes of inguinal and visceral WAT, but only in BAT [ 228 ]. Docosahexaenoic acid (DHA) (1.2%) together with EPA (2.4%) increased oxygen consumption and rectal temperature, as well as UCP1 and β3AR levels via the central nervous system. However, knockout mice for TRPV1 did not achieve the same effect, showing that such events were mediated by SNS, TRPV1, and catecholamines [ 229 ]. Conjugated linoleic acids (CLAs) also showed potential to induce browning process in the WAT [ 230 ].

Once the overwhelming impact of infectious diseases has been alleviated by the development of efficient therapeutics, life expectancy has been continuously increasing (World Health Organization, 2019). Age-associated diseases, including type 2 diabetes (T2D), cardiovascular diseases (CVDs), neurodegenerative pathologies, and obesity statistics are alarming and correlates with changes in the lifestyle of individuals throughout the world, including the diet, and impair the health spam rise. Western diets (WDs) are composed by food items enriched in processed sugar, white flour and salt and poor in fibers, vitamins and minerals [ 231 ]. At the same time, the diet may be the remedy against the burden caused by these chronic diseases. While overnutrition often correlates with inflammatory and metabolic detrimental effects at molecular level, undernutrition without starvation presents many benefits. Calorie restriction (CR) and intermittent fasting (IF) are promising interventions against the overweight and obesity numbers, climbing specially in Western countries [ 232 ].

CR, defined as reduced calorie consumption without malnutrition, is the best studied dietary intervention that increase health spam in experimental models. A plethora of human studies place CR as beneficial for expanding the health spam [ 232 ]. These studies proceeded Weindruch and Sohal positive correlations between CR and health spam [ 233 ] Click or tap here to enter text.. AT plasticity is one of the connections between CR and health benefits. Fabbiano and colleagues analyzed mice under CR and described that this regimen induces functional beige fat development in WAT, phenomenon that occur via enhanced type 2 immune response and SIRT1 expression in AT macrophages [ 234 ].

The stress resistance provided by the IF practice places this regimen as a feasible dietary intervention against various devastating complex pathologies. Differently from CR, intermittent fasting (IF) does not influence the meal size, but decrease the number of meals in a given period [ 232 ]. The fasting state leads to a metabolic switch, which increases the usage of free fatty acid (FFA) as energy source in comparison to glucose. In addition, IF favors the synthesis of ketone bodies (KBs) by the liver, molecules that act as an energy source during nutrient deprivation and induce a plethora of beneficial effects on the organism by acting upon the muscle, liver, heart, brain, intestine and AT [ 235 , 236 , 237 ]. Moreover, during prolonged fasting periods, the levels of the bioenergetic sensors NADH, ATP, and acetyl-CoA decrease and the amounts of NAD+, AMP, CoA rise, molecules that act as epigenetic cofactors and lead to the activation of stress resistance mediators, as sirtuins (SIRTs), NRF2 (nuclear factor erythroid 2–related factor 2), and AMPK (AMP-activated protein kinase). IF also impacts positively on AT remodeling. A DIO animal model submitted to repetitive fasting cycles displayed increased glucose tolerance, and diminished adipocyte hypertrophy and tissue inflammation [ 238 ]. Mouse studies show that IF induces WAT mass decrease, elevation of AT UCP1 expression and thermogenic capacity [ 239 , 240 ], and augmented beige pre-adipocytes recruitment to WAT [ 241 , 242 , 243 ] (Fig.  2 ).

figure 2

The impact of circadian rhythm and different diets on the WAT browning modulation. The secretion of melatonin, a circadian rhythm regulating neurohormone, is mediated by the release of Norepinephrine (NE), which binds to β-adrenergic receptors. Adrenergic activation is one of the main mechanisms of WAT browning induction and BAT activation. Intermittent fasting (IF) associates with weight reduction, improved metabolic status due to increased glycemic tolerance, decreased white adipocyte hypertrophy and AT inflammation, and augmented expression of thermogenic genes (such as UCP1) and recruitment of beige adipocytes. IF is also modulates the intestinal microbiome composition and diversity, a shift closely related to the induction of browning in the WAT. Caloric restriction (CR) is also associated with weight loss, promotes greater recruitment of beige adipocytes through the participation of M2 macrophage and eosinophil infiltration and in WAT. Finally, obesity-inducing diets correlate with increased lipid accumulation, WAT unhealthy expansion and dysregulation. Abnormal expansion of WAT promotes ER stress, greater induction of adipose cell apoptosis and inflammation through NF-κB transcription factor activation and increased pro-inflammatory cytokines secretion

An elegant study conducted by Li and colleagues informed that mice under IF cycles display an intestinal microbiome composition shift associated with increased levels of the fermentation products lactate and acetate. They also show that the modulation of the gut microbiota by IF is crucial for its browning effect, as microbiota-depleted mice present impaired IF-induced AT beiging and fecal microbiota transfer from these mice to antibiotics treated animals display increased browning of WAT [ 244 ] (Fig.  2 ). Unexpectedly, a human study conducted by von Schwartzenberg and colleagues showed that CR may diminish bacterial abundance, deeply change gut microbiome composition and diversity, impair nutrient absorption, and favor the outgrowth of the pathobiont ( Clostridioides difficile ). This diet also led to a decrease in bile acid (BA) levels [ 245 ].

BA, nonesterified fatty acids, are synthesized during the browning of WAT, a phenomena associated with the potentiation of the lipolytic machinery [ 246 ]. These fatty acids can not only activate UCP-1 allosterically, but also serve as fuel for oxidative phosphorylation and consequently heat generation in BAT [ 1 ]. Furthermore, in the liver they are used for the generation of acylcarnitines and VLDL which is used as source for thermogenesis [ 247 ]. Moreover, studies show that the increase in brite and brown adipocytes in WAT leads to an elevation in lipoprotein lipase (LPL) activity and subsequently an increase in circulating lipids available for BAT through intravascular hydrolysis of chylomicron triglycerides [ 248 ]. Consequently, these mechanisms result in the generation of cholesterol-enriched lipoprotein remnants, which upon activation of BAT accelerates the flow of cholesterol to the liver [ 249 ].

BA are steroid acids derived from dietary cholesterol catabolism. These acids are synthesized in the liver and act to aid digestion and absorption of fat in the intestine, in addition to playing an essential role in lipid metabolism. BA act in other tissues, such as AT, as signaling molecules through interaction with the nuclear Farnesoid X receptor (FXR) and the G protein-coupled membrane receptor (TGR5) [ 250 ]. Recent studies have shown that BA play a relevant role in BAT activation and increased thermogenesis in adipocytes. In rodents, the activation of BAT by BA is dependent on its interaction with the TGR5 receptor and expression of the enzyme type 2 iodothyronine deiodinase (DIO2). Additionally, experiments with oral supplementation of BA in humans indicated increased BAT activity in humans [ 251 , 252 ]. Another experiment performed under thermoneutrality, demonstrated an improvement in glycemic metabolism and lipogenesis in the liver and fat accumulation in the TA and also induced an improvement in thermogenic parameters and mitigation of the impact of diet-induced obesity after feeding mice with HFD associated with BA [ 253 ].

Moreover, BAT activation also promotes liver protection. In a study performed with animals under alcohol-induced hepatic steatosis or liver injury, activation of the TGR5 receptor induced improvement of clinical condition. The increase in thermogenesis in BAT promotes an increase in lipid metabolism with lower availability of circulating FFA and, consequently, lower absorption of these molecules by the liver [ 254 ]. However, if on the one hand BAs have been shown to be effective in inducing browning, on the other hand the excess of these acids is capable of promoting an antagonistic effect, such as mitochondrial dysfunction and expression of genes associated with cellular senescence in adipose cells [ 255 ].

Transcriptional regulation of WAT browning

ATs are embryologically distinct from other tissues and are formed according to specific stimuli during embryo development, including Bone morphogenetic proteins (BMPs), pleiotropic molecules that interact with type I and type II BMP receptors and influence embryogenesis [ 256 , 257 ]. BMPs interact with type I and type II BMP receptors serine/threonine kinase activity and influence lineage determination [ 258 ]. Tang and colleagues showed that transfection of C3H10T1/2 cells with BMPs coped with phenotypes: while BMP2 associated with the osteogenic lineage, BMP4 led to adipogenic differentiation [ 259 ].Noteworthy, BMP4 overexpression was found to increase UCP1 and other beiging markers, as Hoxc9, Tbx1, and Tbx15 [ 260 ]. These different phenotypes induced by the BMPs, including the induced beige adipocytes, highlight how relevant transcriptional regulation is for determining the cells functions and characteristics. The main proteins that regulate gene expression are the transcriptional factors (TFs), DNA binding proteins that modulate gene transcription by interacting with the gene promoter or cis-regulatory elements, such as enhancers and silencers, and include PPAR proteins, PGC-1α, and PRDM16 [ 261 , 262 ].

In addition to its roles in ATs development [ 263 ], PPARγ is a central TF for adipogenesis and lipid storage regulation, influences cell thermogenic capacity, and impacts lipid metabolism and insulin sensitivity [ 264 ]. This TF is expressed in elevated levels in ATs [ 265 ], and upon ligand binding PPARγ recruits different cofactor sets for controlling the expression of specific genes. PPARγ cooperates with the basic leucine-zipper factor C/EBPα and interacts with the majority of adipocyte-selective genes [ 266 , 267 ]. The use of PPARγ full agonists is associated with improved insulin sensitivity and induces WAT browning, but can cause detrimental effects, such as undesirable weight gain and augmented visceral adiposity [ 264 ].

Although PPARγ is sufficient for converting white adipocytes into cells that display a brown-like phenotype in vitro [ 268 ], PPAR-α and PPAR-β/δ also influence the browning process. PPAR-β/δ agonist enhances beta-oxidation and improves glucose tolerance, key characteristics of the white-to-beige plasticity [ 269 ]. PPARα acts synergistically with PPARγ in inducing robust WAT browning in vivo [ 268 ], and is currently considered a prominent target for treating metabolic disorders [ 269 ].

A way that PPAR agonists provoke WAT browning is by stabilizing PRDM16 [ 270 ], a protein that activates a complete set of thermogenic genes in WAT [ 271 ]. PRDM16 is essential for browning particularly in scWAT, once its induction in visceral depots does not correlate with thermogenesis [ 272 ]. Mice lacking Prdm16 in scWAT are unable to induce browning within subcutaneous depots after stimuli [ 273 ]. Ectopic PRDM16 expression induce thermogenic genes in several cell types [ 274 ]. PRDM16 AT overexpression in rodents copes with augmented energy expenditure and DIO resistance [ 272 ] PRDM16 acts by binding to specific regulatory sequences in DNA and by interacting with other proteins [ 271 ], such as PGC-1α [ 275 ].

PGC-1α plays a key role in the adapting thermogenesis. First described in cold-induced adaptive thermogenesis analyses [ 276 ], this transcriptional coactivator participates in the regulation of a plethora of cellular functions, including mitochondrial biogenesis, oxidative phosphorylation, and gluconeogenesis [ 277 , 278 ]. Once PGC-1α influences genes related to energy metabolism, it is expressed mostly in tissues that require an elevated amount of energy, like AT, liver, skeletal muscle, and brain [ 279 ]. Pgc1α is activated by the action of the cAMP-PKA-p38/MAPK signaling pathway and physically interacts with Nuclear Respiratory Factors 1 and 2 (NRF1 and NRF2) and co-activates PPARγ, PPARα, and ERRα/β/γ [ 280 ]. When overexpressed, Pgc1α induces mitochondrial biogenesis [ 281 ].

Another key regulator of the browning process is CIDEA. Initially described as a mitochondrial protein, CIDEA was further discovered to be associated with cell lipid droplets (LD) [ 282 , 283 , 284 ]. This molecule leads to the occurrence of browning by inhibiting the suppression of UCP1 gene expression mediated by liver-X receptors (LXRs) and increasing PPARγ binding strength to the UCP1 enhancer [ 285 ]. As detailed in this review, UCP1 is found in the inner mitochondrial membrane and acts by uncoupling the electron transport chain and oxidative phosphorylation, releasing energy as heat [ 1 ]. The existence of molecular markers for the browning process can be useful for investigating AT plasticity status and correlate with health and disease.

Zfp516 is a TF that directly binds to the UCP1 and PGC1α promoters and induce WAT browning upon cold exposure. Zfp516 overexpression copes with augmented multilocular lipid droplets (LDs) biogenesis and increased oxygen consumption and UCP1 levels [ 286 ]. HSF1 was described to cooperate with PGC1α in igWAT, favoring the induction of the thermogenic and mitochondrial gene programs, which leads to augmented energy consumption. HSF1-deficient mice are cold intolerant due to decreased β oxidation and UCP1 expression. HSF1 activation associates with scWAT browning, non-shivering thermogenesis and energy consumption [ 287 , 288 ]. IRF4 also cooperates with PGC1α and was described to inhibit lipogenesis in adipocytes [ 289 ]. While IRF4 overexpression favors beiging in epididymal WAT, the absence of this factor is linked to diminished energy expenditure and increased risk to hypothermia [ 290 ]. The TF NFIA presents a crucial role in the initial steps of thermogenic gene regulation, as its increased levels in thermogenic adipocytes precedes PPARγ upregulation in these cells [ 291 ].

Members of “Early B-Cell Factor” (EBF) protein family play key roles in the regulation of thermogenesis. The TF EBF2 uncouples adipocyte mitochondrial respiration and is sufficient for WAT browning. Increased EBF2 levels in WAT leads to activation of the thermogenic program, favoring increased oxygen consumption and resistance for weight gain. EBF2-KO mice show impaired WAT browning and ablates the brown fat-specific characteristics of BAT [ 292 , 293 , 294 ]. EBF2 activity is regulated by the action of ZFP423, which binds to EBF2 and recruits NuRD (nucleosome remodeling deacetylase) corepressor complex to suppress EBF2 activity in thermogenic genes regulatory sequences [ 295 ]. Absence of ZFP423 associates with PPARγ binding to thermogenic gene enhancers, WAT browning and non-shivering thermogenesis [ 296 ]. The protein transducin-like enhancer of split 3 (TLE3) was first described by Villanueva and colleagues to increase PPARγ adipogenic activity [ 297 ]. Deletion of TLE3 copes with increased energy expenditure and mitochondrial oxidative metabolism in adipocytes, characteristics associated with the browning process [ 298 ] KLF11, TAF7L, ZBTB16, EWS, PLAC8, ERRα, ΕRRγ and other TFs that were described to promote WAT browning. In contrast, FOXO1, TWIST1, p107, LXRα, pRB, RIP140, REVERBα acting repressing AT beiging by impacting on the activity of EBF2, PRDM16, PGC1α and other activating TF [ 299 , 300 , 301 ].

In order the transcription to occur, chromatin needs to be accessible to polymerases and TFs. Remodeling enzymes, which are classified in covalent histone modifiers and ATP-dependent chromatin remodeling complexes, actively modify chromatin status in response to environmental cues [ 302 ]. Histone covalent modifiers are enzymes that chemically modify positively-charged amino acids (mainly lysine) present in these proteins. Histone acetyltransferases (HATs) and histone deacetylases (HDACs) catalyze the removal or insertion of acetyl group, respectively, influencing histone acetylation pattern [ 303 ]. Acetylation decreases histones’ positive charge, leading to histone-DNA looser interaction, chromatin decompaction and gene activation. These epigenetic elements classified into three major families based on the acetyl group transfer mechanisms: the CREB binding protein (CBP)/P300 family (CBP, P300), the GCN5-related N-acetyltransferases (GNAT) family (GCN5, PCAF, and Hat1), and the MYST family (MYST1, MYST2, TIP60) [ 304 ]. The HAT CBP was shown to inhibit the browning process, once it is key in white adipocyte differentiation [ 305 ]. Histone acetylation pattern has been described to present a critical role in adipocyte identity, as cold exposure leads beige adipocytes to show histone acetylation pattern associated with brown phenotype [ 306 ]. The HATs GCN5/PCAF were also shown to participate in the browning process [ 307 ].

HDACs catalyze the deacetylation of amino acid residues in histones, reactions favor gene repression. These enzymes are categorized into four groups, namely class I (HDAC1–3 and 8), class IIa (HDAC4, 5, 7, and 9), class IIb (HDAC6 and 10), class III (SIRT1–7), and class IV (HDAC11) [ 304 ]. HDAC1 expression is augmented in WAT and copes with decreased levels of proteins associated with non-shivering thermogenesis, as UCP1 and PPARγ [ 308 ]. HDAC3 levels also suppress WAT browning, as absence in WAT correlates with H3K27ac on enhancers of Ucp1 and Ppar g , signature associated with tissue improved oxidative capacity, mitochondrial biogenesis, and thermogenesis [ 309 ]. Experiments involving the deletion of HDAC9 shows that this enzyme also contributes for the metabolic dysfunction characteristic of HFD-fed rodents [ 310 ]. HDAC11 is another element that impairs WAT beiging, once its removal favors at thermogenesis in diet-induced obese (DIO) mice [ 311 ]. Other key HDAC is the SIRT family (SIRT1, SIRT2, SIRT3, SIRT5, and SIRT6). SIRT1 deletion in HFD-treated mice is associated with diminished amounts of PGC-1α, FGF21, and UCP1 in epididymal WAT (eWAT) [ 312 ].

Histones can also be post-translationally modified by histone methyltransferases (HMTs) and histone demethylases, which influence gene activation or suppression depending on the modified residue position and valency [ 313 ]. There are two categories of HMTs: lysine methyltransferases (KMTs) and arginine methyltransferases (RMTs). Rodent studies show that inactivation of the KMT MLL3 favors improved insulin sensitivity and augmented energy expenditure [ 314 ]. Absence of another KMT, the EHMT1, was shown to impair the thermogenic program in WAT [ 315 ]. Histone demethylases (HDM) catalyze histone demethylation processes and are also classified according to the characteristics of the modified amino acid. The KDM LSD1 expression, which was found to be induced by chronic cold exposure and β3-adrenergic stimulation, leads to increased mitochondrial activity in WAT by cooperating with NRF1 [ 316 , 317 , 318 ]. In addition, rodents presenting increased LSD1 levels are associated with igWAT browning in lean animals and with lower weight gain in the context of DIO [ 317 ] Also induced by β3-adrenergic stimulation, the KDM3A JMJD1A directly regulates ppar a and ucp1 genes and was found to be crucial for WAT browning [ 319 ]. Specific DNA sequences called CpG islands can also be methylated for gene transcription regulation by DNA methyltransferases (DNMTs), which switches off gene expression. Ten-eleven translocation (TET) family enzymes switch on the gene expression by demethylation [ 320 ].

Gene transcription can also be impacted by the action of ATP-dependent chromatin remodeling complexes, which, differently from the covalent histone modifiers, alter the interaction between the DNA and the nucleosome non-covalently using ATP hydrolysis as an energy source. The chromatin remodelers are categorized in INO80, CHD, ISWI, and SWI/SNF [ 321 ]. The mammalian SWI/SNF (mSWI/SNF or BAF) complex can act with either Brahma homolog (BRM) or BRM-related gene 1 (BRG1) ATPases [ 322 ]. Abe and colleagues suggested that BRG1 is necessary for thermogenesis induction [ 319 ].

Other key actors in gene transcription regulation are the microRNAs (MiR), small non-coding regulatory ribonucleic acids (maximum 200 nucleotides) that influence gene expression post transcriptionally. These elements regulate gene expression by several mechanisms, including binding to mRNA strands to repress protein translation and to favor decapping, deadenylation and ultimately degradation of target mRNA in P-bodies, and direct translation inhibition. However, many studies suggest that miRNAs are also capable of activating transcription protein translation, fact that highlights miRNAs as central participants in the fine regulation of gene expression in response to specific stimuli [ 323 ]. Raymond and others described that miR-32 inhibition copes with impaired WAT browning [ 324 ]. MiR-181, which is induced by tryptophan‐derived metabolites produced by the intestinal microbiota, was also connected with WAT physiology, once it promotes energy expenditure and insulin sensitivity in rodent models and its removal may cope with the development of obesity in these animals [ 325 ]. Another miRNA that influences WAT homeostasis is miR-26, which deletion leads to WAT enlargement and overexpression inhibits the progression of obesity in the DIO in mice [ 326 ]. miR-196a, miR-455, miR-30b/c are other examples of miRNAs that are induced by browning inducers and promote this WAT plasticity process [ 327 , 328 ].

In contrast, miR-27, miR-133, and miR-150 inhibit TFs associated with the browning process [ 61 , 63 , 329 ]. Fu and colleagues showed that miR-34a inhibits WAT beiging via FGF21 [ 330 ], miRNA-155, miRNA Let-7i-5p and miR-125b- 5p are molecules that impair WAT browning process, which inhibition promotes beiging [ 331 , 332 , 333 ] (Fig.  3 ). Other small non-coding RNAs (sncRNAs) impact on gene expression and are extensively reviewed elsewhere.

figure 3

WAT browning transcriptional regulation. The transcriptional regulation of the WAT beiging process involves the action of a plethora of specific proteins and nucleic acids. This orchestra of trans-acting factors modulates genes associated with oxidative capacity, mitochondrial biogenesis and non-shivering thermogenesis. While the action of some factors, such as PGC-1α, IRF4, PRDM16, ZFP516, EHMT1 and RNAs, copes with WAT browning, TLE3, ZFP423, and the NuRD complex, another set of RNAs e others inhibit this process, favoring adipogenesis and white adipocyte differentiation

Non-coding RNAs that present average length longer than 200 nucleotides are called LncRNAs [ 302 , 334 ] These nucleic acids have been described to influence gene transcription through several mechanisms, including the induction of more efficient protein translation by binding to an internal ribosome entry site (IRES), increase of mRNAs stability, polyubiquitination process inhibition thus increasing protein stability, binding to specific proteins in the cytoplasm, and acting as a sponge sequestering miRNAs [ 335 , 336 , 337 , 338 , 339 ] . Zhao and collaborators identified lncRNA 1 (Blnc1) as a driver for beige adipocyte differentiation and WAT browning [ 293 ].

All the regulatory elements discussed here cooperate for generating a phenotype in response to environmental stimuli, such as physical exercising, intermittent fasting, calorie restriction, thyroid hormones, microbiota-associated metabolites, thyroid hormones and others [ 299 ] The lncRNA Blnc1 forms a Blnc1/hnRNP-U/EBF2 ribonucleoprotein complex and is required for EBF2 transcriptional activity [ 292 , 340 ]. The TF ZBTB7B also interacts with this lncRNA through the hnRNP-U/Blnc1 for beige adipocyte differentiation and thermogenesis [ 341 ]. LncRNAs also interact with other TFs, such as PGC-1α, ZFP516, and PRDM16 for full transcription [ 274 , 280 , 286 ]. PRDM16, a coregulator of PPAR, cooperates with EHMT1 for WAT browning induction. As EHMT1, other covalent histone modifiers form complexes with ATP-dependent chromatin-remodelers [ 342 ]. The chromatin-remodelers BRG1 and the BAF also interact with the TF EBF2 for gene transcription [ 292 , 293 ]. PGC1α favors increased transcription by recruiting HATs, as CBP/p300 and GCN5 [ 343 ].

Noteworthy, AT functional and morphological plasticity is represented by the extreme dynamic beige adipocyte chromatin state. Beige adipocytes show a chromatin signature similar to the pattern presented by brown adipocytes after cold exposure and display a chromatin signature associated with white adipocytes after re-warming to 30 °C. However, if the beige adipocytes were cold-induced, these cells displayed epigenetic marks that favored rapid thermogenic genes expression upon β-adrenergic stimulation, even after temperature rise, suggesting the occurrence of epigenomic memory [ 306 , 344 ].

WAT browning, health impact and applications

For the past decades, the browning process has been deeply explored for managing both metabolic syndromes, and hypermetabolic diseases. Proposing treatment, the cold was the pioneer mechanism associated with a great elicitation of this event in mice. However, the applicability and outcomes of this approach in humans are far from exciting. Technological advances provide efficient mechanisms that can be employed to improve a biological system. Recent studies applied bioengineers to achieve the improvement of browning in adipose tissue. The injection of M2 AT macrophages (ATMs) from transgenic mwRIP140KD mice, a specific knockdown of RIP140 that is related to activation of M1 polarization, in HFD-fed obese wild type mice could recover the disruption induced by obesity through browning induction [ 345 ]. WAT-derived stem cells (ASCs) from humans and rats could be differentiated into BAT under browning conditions in three-dimensional (3D) polyethylene glycol (PEG) hydrogel [ 346 ].

In addition to bioengineering, a field that has been explored is the transplantation or re-implantation of BAT, also termed ex-BAT, to increase this endogenous tissue. The harvested scWAT is differentiated into brite and then is re-implanted in the same area promoting the local increase of BAT, displaying great outcomes [ 347 ]. Aiming for a less invasive procedure, the use of a microneedle patch to address the browning agents directly to the scWAT for inducing browning of this tissue is another strategy investigated presently [ 348 ]. These strategies aim to promote the increase of endogenous BAT, as well as its activity with an applicable method, reducing the degree of invasiveness and risks of rejection by the body.

Several studies have targeted increasing BAT mass and activity, but when the aim is inhibiting the browning process to avoid a worsening prognosis, what is proposed? Expanding knowledge of the pathways involved is the main way of inhibiting that event. It is well established the role of parathyroid hormone (PTH) in the elicitation of browning. In the tumor context, it was identified as a peptide-derived tumor, parathyroid hormone-related protein (PTHrP) that favors browning and cachexia [ 16 ]. Blocking PTHR specifically in ATs inhibits browning and wasting of this tissue, as well preserves muscle integrity, and ameliorates muscle-related strength, protecting mice from cachexia [ 349 ].

Currently, some pharmacological approaches have displayed alternative and prominent ways to restrain browning. Metformin demonstrated to be efficient to prevent the browning of the scWAT, as well as the inhibition of mevalonate pathways by the use of Statin or Fluvastatin, which implicates in the disruption of browning [ 350 , 351 ]. However, further studies should be conducted to promote a broader and more detailed debate on the topic.

Given the data presented, it is evident that the benefits and dangers associated with activation or inhibition of the browning process are closely linked to the type of disorders displayed in the individual's body at the metabolic, cellular, or physiological level. Taking as criteria patients with metabolic syndromes and obesity, browning process emerges as a promising therapeutic approach, mainly due to its several benefits, such as improvement of clinical conditions associated with lower side effects, and the multi-stimulatory character, which allows numerous safe and feasible ways of induction. In contrast, the ways and means of inhibiting browning to prevent the development of comorbidities in cases of chronic or acute hypermetabolism are still poorly explored. In this way, browning process needs to be deeply explored and can be used as a key factor in different therapeutical approaches in health and diseases. Emerging insights into the metabolic and immunological role of browning of the white adipose tissue are also discussed, along with the developments that can be expected from these promising targets for therapy of metabolic and chronic disease in the forthcoming future.

Mouse and human AT research

A major discussion pointed by the scientific community about the investigation of AT in mice is how much the results obtained in the animal model would reproduce the anatomorphophysiologies of these tissues in humans, since, unlike humans, mice have a significant amount of BAT during embryonic and adult, as well as they differ in several other factors, such as expression of molecular markers, activation profile and location. Human BAT was discovered to be more similar to beige compared to classical BAT markers [ 352 , 353 ]. Another relevant point concerns the fact that the BAT deposits most used for research purposes are the BAT located in the interscapular region (iBAT), while in humans there is a greater abundance of this tissue in the clavicular and neck regions, presenting compositional differences that represent obstacles in the overlapping of scientific findings. In this sense, Mo and colleagues identified an analogous deposit of BAT in mouse embryos, which is maintained during adulthood, and in humans called supraclavicular BAT (scBAT). The scBAT shows similarities to scBAT in humans in terms of location, morphology and thermogenic capacity [ 354 ].

Another alternative developed in order to attenuate the anatomical, physiological and molecular differences between the organisms was to submit mice under conditions called “physiologically humanized” in which middle-aged animals under thermoneutrality (30˚C) and diets aligned with the dietary pattern of human beings are used. humans. In which the authors observed a greater similarity between several cellular and molecular parameters between the BAT of humans and mice [ 355 ]. The technique generated controversies that were discussed by Kajimura and Spiegelman and replicated by the authors of the original article ( 356 , 357 ).

Conclusions and perspectives

Current research place WAT browning as an extremely dynamic process that is influenced by several factors , including temperature, physical exercising, thyroid hormones, circardian rhythm, food components and dietary regimens. The participation of AT plasticity on the organism metabolic health and inflammatory status spot this process as a promising therapeutic target for decreasing the risk associated with many chronic diseases. Further efforts in investigating AT plasticity must alleviate the burden of these devastating life-style associated pathologies.

Availability of data and materials

Not applicable.

Abbreviations

5′-Deiodinase type 2

Amino acids

Adenylyl cyclase

AMP-activated protein kinase

Atrial natriuretic peptide

Adipose stromal cells

Adipose tissue

Activating transcription factor

Adipose tissue macrophages

Adenosine triphosphate

Brown adipose tissue

Blood–brain barrier

Bone morphogenetic proteins

Basal metabolic rate

Brain natriuretic peptide

BRM-related gene 1

Brahma homolog

CCAAT-enhancer-binding protein

Calcium ion

Cyclic adenosine monophosphate

CREB binding protein

Response element-binding protein

Cell death-inducing DNA fragmentation factor-like effector A

Conjugated linoleic acids

Central nervous systems

Carnitine palmitoyltransferase 1B

Calorie restriction

CAMP response element-binding protein

Cyclic AMP response element-binding protein H

Docosahexaenoic acid

Diet-induced obese

2 Iodothyronine deiodinase

Diet-induced thermogenesis

DNA methyltransferases

Epicardial AT

Early B-cell factor

Endocrine FGF

Eukaryotic initiation factor

Eicosapentaenoic acid

Epididymal WAT

Fatty acids

Free fatty acids

Fibroblast growth factor family

Fibroblast growth factor tyrosine kinase receptor

Fibronectin domain-containing protein 5

FGFR substrate 2α

Farnesoid X receptor

Non-derepressible 2

Glucose transporter

GCN5-related N-acetyltransferases

Histone acetyltransferase

Histone deacetylase

Histone demethylase

High-fat diet

Histone methytransferases

Interscapular BAT

Intermittent fasting

Interleukin

Internal ribosome entry site

Beta-Klotho

Lysine methyltransferases

Long-chain fatty acids

Lipid droplet

Long non-coding RNA

Lipopolysaccharide

Liver X receptor

Mitochondrial anion carrier family

Membrane proteins monocarboxylate transporter 8

Messenger RNA

Myogenic factor 5

Nonalcoholic fatty liver disease

Non-coding RNAs

Norepinephrine

Nuclear factor erythroid-related factor 2

Nuclear respiratory factor 1

Nuclear respiratory factor 2

Nucleosome remodeling deacetylase

Organic anion transporter family member 1C1 9

Onion peel extract

Mitogen-activated protein kinase

Pyruvate dehydrogenase

Polyethylene glycol

Peroxisome proliferator-activated receptor-gamma coactivator 1 α

Protein kinase A

Proopiomelanocortin

Peroxisome proliferator-activated receptor

PR domain containing 16

Parathyroid hormone

Parathyroid hormone receptor

Parathyroid hormone-related protein

Polyunsaturated fatty acids

Retinoic acid

Retinoic acid receptor

Receptor-interacting protein 140

Arginine methyltransferases

Retinoic acid receptor-related orphan receptor α

Retroperitoneal white adipose tissue

Retinoid X receptor

Supraclavicular BAT

Suprachiasmatic nucleus

Subcutaneous WAT

Small non-coding RNA

Sympathetic nervous system

Type 2 diabetes

Triiodothyronine

T-box transcription factor TBX1

Ten-eleven translocation

Transcription factor

Transforming growth fator beta

G protein-coupled membrane bile acid receptor

Thyroid Hormone

T help 2 cells

Transducin-like enhancer of split 3

Thyroid-hormone responsive elements

Transient receptor potential cation channel subfamily M member 8

Transient receptor potential vanilloid 1

Thyroid hormone nuclear receptors

Thyroid stimulating hormone

Thyroid stimulating hormone receptor

Uncoupling protein 1

Visceral WAT

  • White adipose tissue

Zinc-α2-glycoprotein

β-3 Adrenergic receptor

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Sabrina Azevedo Machado, Gabriel Pasquarelli-do-Nascimento, Debora Santos da Silva, Gabriel Ribeiro Farias, Igor de Oliveira Santos, Luana Borges Baptista & Kelly Grace Magalhães

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Machado, S.A., Pasquarelli-do-Nascimento, G., da Silva, D. et al. Browning of the white adipose tissue regulation: new insights into nutritional and metabolic relevance in health and diseases. Nutr Metab (Lond) 19 , 61 (2022). https://doi.org/10.1186/s12986-022-00694-0

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Adipose Tissue: Physiology to Metabolic Dysfunction

Allison J. Richard , PhD, Ursula White , PhD, Carrie M. Elks , PhD, RD, and Jacqueline M. Stephens , PhD.

Last Update: April 4, 2020 .

Like the obesity epidemic, our understanding of adipocytes and adipose tissue is expanding. Just in the past decade, substantial advances have led to new insights into the contributions of adipose tissue to normal physiology and obesity-related complications, which places adipocyte biology at the epicenter of a global pandemic of metabolic diseases. In addition to detailing the types, locations, and functions of different adipose tissue depots, this chapter will review the secretory capacities of adipose tissue. Arguably one of the most significant discoveries in the last two decades of adipocyte research is that not only do adipocytes release endocrine hormones, but fat cells and adipose tissue secrete a variety of effectors, including exosomes, miRNA, lipids, inflammatory cytokines, and peptide hormones that act in both paracrine and endocrine capacities to impact local and systemic metabolic responses. The origins of adipocytes via progenitor cells and the process of adipocyte development are discussed. Inflammation, metabolically healthy fat, and adipose tissue expansion are also considered. Finally, several emerging research areas in fat cell biology with therapeutic potential in the management patients who are overweight and have obesity are summarized. For complete coverage of all related areas of Endocrinology, please visit our on-line FREE web-text, WWW.ENDOTEXT.ORG .

  • introduction: AN Historical Perspective on Adipose Tissue biology

The first published citation referencing adipose tissue (AT) dates to 1837. Subsequent sporadic single AT citations appeared in in the literature until the 1940’s, including a 1933 publication in Biochemical Journal examining the degree of fatty acid unsaturation in human AT in relation to its depth from the skin surface ( 1 ). The first year in which two AT-related citations were recorded was in 1942. In 1947, nearly ten AT citations appeared. Adipose tissue remained understudied for decades due to the misconception that it was simply an inert energy storage depot, but recent discoveries of AT’s wider role in cell and whole-body signaling have created a scientific renaissance in this field. As of early 2019, over 139,000 citations involving adipocytes or AT are now discoverable.

The earliest recognized function of adipocytes was the storage of energy in the form of triacylglycerols (TAGs). It was not until the mid-1980s that the secretory functions of AT and the production of adipocyte-specific proteins were revealed. At that time, a serine protease named adipsin was shown to be secreted from cultured adipocytes and reported to be reduced in mouse models of obesity compared to lean littermates ( 2 ). Acylation stimulating protein, a member of the alternative complement family, was also revealed to be produced by AT ( 3 ) and implicated in lipid storage ( 4 ). Although the functions of these AT secretory products remain poorly understood, their discovery revealed adipocytes and AT to be significant sources of a variety of protein products, including many endocrine hormones. Arguably one of the most important of these discoveries was leptin ( 5 ), a bona-fide adipocyte-derived hormone that clearly acts not only as an afferent “adipostat” signal of fat mass to central brain centers in the regulation of body weight ( 5 ) but also has peripheral actions that impact glucose metabolism ( 6 ) and immune function ( 7 ).

In addition, adipocytes are also highly sensitive to insulin and involved in the regulation of blood glucose levels. Insulin action on fat cells stimulates glucose uptake and modulates lipid metabolism by increasing the accumulation and decreasing the breakdown of TAGs (and subsequent release of free fatty acids into the circulation) within the adipocyte. The importance of each of these 3 fat cell functions ( Figure 1 ) – lipid storage, secretory function, and insulin sensitivity – is underscored by the demonstration that disruption of any one role has profound systemic ramifications in mice and man that can contribute to a variety of obesity-related metabolic disease states ( 8 ).

The first CDC statistics reporting obesity rates over 20% in many US states also appeared in the late 1990’s, as did literature from a variety of disciplines showing that obesity, or excess adipose tissue, enhanced the risk of metabolic diseases, particularly type 2 diabetes (T2D). This was a substantial shift in thinking from the previous two decades when AT was not considered to have much importance or relevance to T2D. In addition to metabolic diseases, obesity is associated with increased risk of 13 types of cancer that account for ~40% of all cancers diagnosed in the United States ( 9 ).

Today, obesity and accompanying epidemics of co-morbidities have become global problems. While in 2015–2016 the prevalence of obesity was 39.8% in adults and 18.5% in youth in the USA ( 10 ), the World Health Organization (WHO) reports that obesity has nearly tripled across the world since 1975, and in 2016 more than 1.9 billion adults were overweight and over 650 million were obese. Today, with most of the world's population living in countries where overweight and obesity account for more deaths than malnutrition (underweight), excess AT presents a major challenge to chronic disease prevention and health across the planet. This global epidemic can be attributed to advancing economies and the adoption of mechanized transport, urbanization, commercial growth, industrialization, a progressively more sedentary lifestyle, and a nutritional transition to processed foods and high calorie diets over the last 30 years ( 11 ). Besides preventing obesity by promoting a healthy lifestyle through diet and exercise, one of the best ways for modern-day physicians and scientists to combat the global menace of obesity is to better understand AT.

Figure 1. . Physiological characteristics of adipocytes.

Physiological characteristics of adipocytes. Disruption of any one of these fat cell functions may lead to the development of systemic metabolic dysfunction.

  • adipocyte physiology

Adipocyte Hues – White, Brown, Beige and Pink

Adipose tissue has historically been classified into two types, white adipose tissue (WAT) and brown adipose tissue (BAT), which are visibly distinguishable based on tissue color. The white and brown adipocytes comprising these depots exhibit physiological differences, which give rise to specialized tissue functions. White adipose tissue, which is critical for energy storage, endocrine communication, and insulin sensitivity, comprises the largest AT volume in most mammals including humans. In contrast, BAT is largely present in mammals postnatally and during hibernation. Brown adipose tissue uses energy for non-shivering heat production, which is critical for body temperature maintenance. While BAT was originally thought to only be present in infant humans, imaging studies have revealed metabolically active BAT in the supraclavicular and thoracic regions of adults ( 12 – 14 ). Although women have increased BAT mass and activity over men ( 14 , 15 ), the chance of detecting BAT activity in either sex has been shown to be inversely correlated with age and body mass index (BMI) ( 14 ). Seasonal correlations have also been observed with BAT activity being higher in the winter and lower in the summer, possibly due to either the temperature or, more likely, the photoperiod ( 14 , 15 ). In healthy humans, BAT activity contributes to whole-body fat oxidation and diet-induced thermogenesis ( 16 ), supporting a physiological role for this AT depot in adults.

Brown and white adipocytes differ in shape, size, and the intracellular structure of their organelles ( Figure 2 ). White adipocytes are generally spherical in shape and each contains a large, single lipid droplet that pushes all other organelles, including the nucleus, to the cell’s periphery. Brown adipocytes contain multiple lipid droplets dispersed throughout a more ellipsoidal-shaped cell that is enriched with iron-containing mitochondria, giving the cell (and the BAT as a whole) a brownish hue. The thermogenic activity of brown adipocytes is conferred by the presence of its numerous mitochondria containing uncoupling protein 1 (UCP-1), a proton transporter that short-circuits the ATP (energy)-generating proton gradient and allows for concurrent heat production as protons flow back into the mitochondrial matrix ( 17 ). Brown fat cells typically grow to 15 to 50 µm, while white fat cells have a larger capacity for lipid storage and can expand to nearly 100 µm in diameter ( 18 ). The capacity of white adipocytes to expand in number and size is depot-dependent and is discussed in more detail in the Adipose Tissue Expandability and Metabolic Health section.

Figure 2. . Adipocyte types are described by color hues.

Adipocyte types are described by color hues. The primary characteristic of an adipocyte is its ability to store lipid; white, brown, beige, and pink adipocytes all share this property. However, each type of fat cell is somewhat specialized and has a distinct intracellular distribution of organelles and gene expression profile. All fat cells have Golgi and endoplasmic reticulum, but these organelles make up a more significant portion of pink adipocytes than other adipocyte types.

Recently, two additional adipocyte hues – beige and pink – have been described. Beige adipocytes display characteristics of both brown and white fat cells ( Figure 2 ) and typically develop within subcutaneous WAT from a distinct subset of preadipocytes ( 19 ) or via the transdifferentiation of existing white adipocytes ( 20 , 21 ). However, gene expression analyses indicate that beige fat cells represent a distinct type of thermogenic fat cell ( 19 ). Beige adipocytes were originally observed to arise in response to cold exposure in rodents ( 22 , 23 ); however, many studies have since identified that diet ( 24 ), exercise ( 25 ), pre- and post-biotics ( 26 ), pharmaceutical agents, numerous plant-based bioactives, and even adipokines ( 27 ) can also induce “beiging” or “browning” of WAT, which may protect against obesity and associated metabolic dysfunction. The “beiging” of WAT is inducible in both mice and humans ( 28 ), but this process is more highly observed in mice.

Pink adipocytes were first described in 2014, arising in the subcutaneous WAT of female mice during days 17-18 of pregnancy and persisting throughout lactation. These fat cells appear to derive from white adipocytes that take on epithelial-like features to form milk-secreting alveoli, giving the tissue a pink hue ( 29 ). Pink adipocytes are characterized by compartmentalized lipid droplets, cytoplasmic projections, and abundant organelles including mitochondria, peroxisomes, and rough endoplasmic reticulum, that show a structure more typical of epithelial cells. While reversible transdifferentiation appears to be responsible for the development and disappearance of pink adipocytes during pregnancy, lactation, and post-lactation in rodents ( 30 ), it remains uncertain whether or not pink adipocytes form in humans. Notably, loss of a key adipogenic transcription factor within the mammary secretory epithelium creates a pro-breast tumorigenic environment and indicates that the reversible white-to-pink transition might reveal insights into breast cancer biology ( 29 , 31 ). Further investigations into adipocyte plasticity might therefore identify novel therapeutic targets to combat obesity and its pathological consequences, as well as cancer. However, since WAT makes up the largest AT volume in the human body and undergoes the most expansion during obesity, in this chapter we will focus on the roles that white adipocytes and WAT play in normal physiology and metabolic dysfunction.

Adipose Tissue in the Regulation of Lipid Metabolism

Adipose tissue stores body fat as neutral TAGs and represents the chief energy reservoir within mammals. Although many diverse cell types are found in whole AT, adipocytes constitute the largest cell volumes and are the defining AT cell type. White adipocytes are characterized by their large unilocular central lipid droplets (cLDs). However, the biogenesis of unilocular LDs in adipocytes is poorly understood due to the fragile nature of WAT.

Using live-cell imaging combined with fluorescent labeling techniques, the cytoarchitecture of unilocular adipocytes ( Figure 3 ) and spatiotemporal dynamics of lipid droplet formation have been investigated ( 32 ). As shown in Figure 3 , cytoplasmic nodules containing micro LDs (mLDs; small green fluorescent protein (GFP)-negative spheres within the cytoplasm) appear on the surface of fat cells, pushed to the edges by the large cLD. Surprisingly, the cytoplasm and organelles do not distribute uniformly around the edge of the cell, but instead form numerous, discrete cytoplasmic nodules connected via a thin layer of GFP-positive cytoplasm. The largest nodule also contains the nucleus, which is surrounded by a thicker layer of cytoplasm. The electron micrograph ( Figure 3F ) shows the close contacts between mLDs and mitochondria. Furthermore, additional nascent lipid droplets can be visualized budding off from the smooth ER (sER). Studies using a fluorescent-labeled free fatty acid (FFA) analog revealed that exogenously added lipids were rapidly taken up by the fat cell and concurrently esterified to TAG and absorbed by mLDs prior to packaging within the cLD. The lipid transfer followed a unidirectional path from mLD to cLD and provides insight into adipose tissue growth via fat cell hypertrophy ( 32 ).

Figure 3. . Architecture of primary unilocular adipocytes.

Architecture of primary unilocular adipocytes. Figure adapted from ( 32 ). The cytoplasm and nuclei of adipocytes and stromovascular cells were labeled by infecting visceral WAT explants from nonhuman primates with an adenoviral vector encoding enhanced green fluorescent protein (eGFP). Two days after infection, live explants were examined by for GFP expression using confocal microscopy. Cellular and subcellular features are labeled: cLD, central lipid droplet; Cyt, cytoplasm; LDM, lipid droplet membrane; mLD, micro-LD; N, nucleus; PM, plasma membrane; sER, smooth ER. (A) GFP-positive unilocular adipocytes (spheres) and stromovascular cells (asterisks) residing in WAT. The image represents the sum of all confocal slices. Bar, 10 um. (B) Single confocal section of the image in A. Enhanced magnification of adipocytes containing cytoplasmic nodules (C) and perinuclear cytoplasm (D). (E) Schematic representation a unilocular adipocyte demonstrates that the cLD is a sphere tightly fitted within the cell, whereas the cytoplasm collects in multiple organelle- and mLD-containing nodules. (F) Electron micrograph of a unilocular adipocyte from a visceral WAT explant that was fixed and processed for electron microscopy. Asterisks mark contact sites between mitochondria and mLDs, whereas arrowheads point towards vesicles budding off the ER tubules. Bar, 500 nm.

Adipocytes store TAG under conditions of energy surplus and release fatty acids to supply to other tissues during fasting or times of high energy demand. As such, AT is central to the regulation of systemic lipid metabolism, and nutritional and hormonal cues serve to balance lipid storage and breakdown within the fat cell ( Figure 4 ).

Figure 4. . A critical balance between lipogenesis and lipolysis within adipocytes must be established to maintain whole body insulin sensitivity and energy homeostasis.

A critical balance between lipogenesis and lipolysis within adipocytes must be established to maintain whole body insulin sensitivity and energy homeostasis. Lipogenesis is shown on the left (gray arrows mark the pathway), whereas lipolysis is shown on the right and is marked by black arrows. Nutritional and hormonal cues regulate both processes. Lipid droplet associated proteins, such as perilipin and comparative gene identification-58 (CGI-58) are not shown but play important roles in lipolysis. CD36 (cluster of differentiation 36) is a fatty acid transporter that facilitates entry of free fatty acids (FFAs) into the cell. Insulin stimulates glucose uptake into fat cells by increasing the localization of the insulin responsive glucose transporter, GLUT4, within the plasma membrane. Other abbreviations: VLDL-TG – triglyceride-containing very low density lipoprotein; LPL – lipoprotein lipase; ACC - acetyl-CoA carboxylase 1; FAS – fatty acid synthase; G3P – glycerol 3 phosphate; DGAT - diacylglycerol acyltransferase; β-AR – β-adrenergic receptor; NA – noradrenaline; AC – adenylyl cyclase; PKA – protein kinase A; ATGL - adipocyte triglyceride lipase; HSL - hormone sensitive lipase; MGL - monoacylglycerol lipase; TAG – triacylglycerol; DAG – diacylglycerol; MAG – monoacylglycerol.

LIPOGENESIS

Adipocytes accumulate lipid via one of two processes ( Figure 4 ). In the first process, under normal daily feeding conditions adipocytes take up dietary lipids from the circulation in the form of FFA’s liberated from circulating TAGs via the action of lipoprotein lipase (LPL) ( 33 ). Adipocytes secrete LPL, which is transported to the adjacent capillary lumen to catalyze the hydrolysis of FFA’s from circulating triglyceride-containing lipoproteins ( 34 , 35 ), such as chylomicrons produced in the small intestine and very low density lipoproteins (VLDLs) synthesized by the liver ( 36 ). Adipocytes also take up glucose, which is converted to glycerol and serves as the backbone for the sequential esterification of fatty acids for form TAG. The final step in TAG synthesis, re-esterification of circulating free fatty acids, mediated by diacylglycerol acyltransferase (DGAT) ( 37 , 38 ). The second process is by de novo lipogenesis (DNL) within the adipocytes themselves. Lipogenesis comprises both de novo synthesis of fatty acids from acetyl-coenzyme A (acetyl-CoA) and the esterification of these fatty acids to a glycerol backbone producing TAGs ( Figure 4 ) . De novo lipogenesis can occur in the fasting and fed states ( 36 ). Following a meal, especially one high in carbohydrates, excess glucose oxidation yields elevated levels of acetyl-CoA that become substrate to generate fatty acids. This occurs through actions of the DNL enzymes acetyl-CoA carboxylase 1 (ACC1) and fatty acid synthase (FAS) to convert acetyl-CoA to palmitate, which can then be elongated and desaturated to form other fatty acid species ( 39 ).

Surprisingly, in rodents DNL is relatively low in WAT compared to BAT and liver, and it plays an even lesser role in WAT lipid storage in humans under physiological conditions ( 40 , 41 ). Typically, hepatic DNL activity exceeds that of AT and is a more substantial contributor DNL-generated circulating lipids. However, in humans fed high-carbohydrate diets, liver DNL contributes only a small portion of total de novo fat biosynthesis, suggesting that AT contributes significantly to whole body DNL when there is a carbohydrate surplus ( 39 , 42 ). Under this condition, adipocyte DNL is usually quite low but has been shown to be important for whole body substrate metabolism ( 43 , 44 ) as inhibition of WAT DNL is associated with insulin resistance ( 45 ).

A primary transcriptional regulator of adipocyte DNL is carbohydrate response element-binding protein (ChREBP) ( 39 ). Mice lacking AT ChREBP have decreased DNL and insulin resistance ( 46 ). The other major DNL regulator in AT is sterol regulatory element-binding protein 1 (SREBP1). Mice with whole body knockout of SREBP1 do not display decreased lipogenic gene expression in AT ( 45 , 47 ), thus supporting ChREBP as the primary lipogenic transcription factor driving AT DNL. However, a new mouse model of inducible, overexpression of insulin-induced gene 1 (Insig1), an inhibitor of SREBP1 activation and transcriptional activity, demonstrated that several acute and chronic white adipocyte-specific compensatory mechanisms are activated to restore adipocyte DNL in the absence of SREBP1 activity ( 44 ). Decreased SREBP1 activity prior to this compensation and during conditions where compensation was inactivated result in decreased lipogenic gene expression, impaired whole body glucose tolerance, and elevated lipid clearance ( 44 ) suggesting that both SREBP1 and ChREBP play important roles in adipocyte DNL.

Enhanced AT DNL can produce favorable lipid species that may be therapeutically advantageous in the context of obesity and insulin resistance ( 48 ). Adipocytes synthesize and secrete a novel family of bioactive lipids, known as the branched fatty acid esters of hydroxyl fatty acids (FAHFAs). Although FAHFAs are found in many tissues, the highest levels are in white and brown AT, and their production is likely dependent on AT lipogenesis as disruption of adipocyte DNL impairs their synthesis ( 39 , 49 ). Over 1000 structurally distinct FAHFAs have been predicted based on in silico analyses and at least 20 FAHFA families have already been identified in mammalian tissues ( 50 ). The serum and subcutaneous AT levels of one FAHFA family, palmitic acid esters of hydroxyl steric acids (PAHSAs) have been shown to be higher in insulin-sensitive compared to insulin-resistant individuals ( 51 ). In animal models, PAHSAs have been shown to decrease inflammation and enhance whole body insulin sensitivity ( 39 , 49 ). Recent evidence from a mouse model of high-fat diet (HFD)-induced insulin resistance demonstrates that PAHSAs act via both direct and indirect mechanisms to improve insulin sensitivity in multiple metabolic tissues, such glycolytic skeletal muscle, heart, liver, and AT. In WAT explants, PAHSAs directly inhibit lipolysis and enhance insulin’s ability to suppress lipolysis. While PASHAs can also directly inhibit endogenous glucose production (EGP) in isolated hepatocytes, the decreased AT lipolysis indirectly attenuates EGP because of reduced glycerol (gluconeogenic substrate) delivery to the liver ( 50 ).

Additional evidence from humans support a role for increased DNL and ChREBP activity in maintaining metabolic health. These include restoration of DNL in WAT following as bariatric surgery-induced weight loss ( 52 ) and reported observations of elevated WAT DNL in other metabolically favorable states including caloric restriction and adaptive thermogenesis ( 53 , 54 ). Collectively, these studies in mice and man support a potential role of WAT DNL in metabolic health.

Under physiological conditions when metabolic fuels are low and/or energy demand is high, such as fasting, exercise, and cold exposure, adipocytes mobilize their TAG stores via the catabolic process of lipolysis to supply fuel to peripheral tissues ( 55 ). Lipolysis is a highly regulated biochemical process that generates glycerol and FFAs from the enzymatic cleavage of TAGs by lipases ( 36 ) and can occur in all tissues, although it is most prevalent in AT where the bulk of TAG is stored. As shown in Figure 4 , TAGs are broken down into diacylglycerols (DAGs) and monoacylglycerols (MAGs) by the sequential action of adipocyte triglyceride lipase (ATGL), hormone sensitive lipase (HSL), and monoacylglycerol lipase (MGL). At each step a single FFA is released, and in the final step MGL releases the glycerol backbone from the last FFA. These breakdown products can be re-esterified within the adipocyte or released into circulation to be used by other tissues ( 36 , 55 ), including by the liver for gluconeogenesis (glycerol) and for oxidative phosphorylation by muscle or other oxidative tissues ( 56 ).

Lipolysis is controlled by sympathetic nervous system (SNS) input as well as a variety of hormones ( 55 ). The best understood of these regulators is the catecholamine, noradrenaline (NA), also known as norepinephrine. Noradrenaline stimulates β-adrenergic receptors ( Figure 4 ), which, in turn, stimulate protein kinase A (PKA) via adenylyl cylase (AC)-mediated production of cyclic AMP (cAMP). PKA activates the lipolytic action of ATGL and HSL by different mechanisms. Several lipid droplet-associated proteins, such as perilipin 1 (PLIN1) and comparative gene identification-58 (CGI-58) are also important in regulating lipolysis ( 57 , 58 ). Whereas PKA can directly phosphorylate and activate HSL ( 59 – 62 ), it primarily stimulates ATGL activity indirectly by phosphorylating PLIN1. This phosphorylation releases CGI-58 to potently activate ATGL ( 58 , 63 , 64 ). Non-adrenergic lipolytic stimuli include glucocorticoids, natriuretic peptides, growth hormone, and tumor necrosis factor alpha (TNFα) ( 58 ). These hormones are typically less potent lipolytic inducers than β-adrenergic stimulation and the molecular mechanisms responsible for their lipolytic abilities have not been clearly elucidated. However, some of these hormones clearly utilize different pathways than β-adrenergic signaling with additive or synergistic affects to increase lipolysis ( 55 , 58 ).

After a meal, the post-prandial increase in circulating insulin readily suppresses lipolysis ( 65 ) by increasing the activity of phosphodiesterase 3 (PDE3B) and decreasing cAMP levels ( 58 ). In the fasting state, insulin levels drop and NA is released, thus promoting lipolysis ( 66 ). Physiologically, exercise is another major pro-lipolytic stimulus in humans ( 58 ). Growth hormone, along with NA, adrenaline, and cortisol increase with exercise intensity, while insulin levels decrease. These changes culminate in an overall lipolytic response, the magnitude of which depends on exercise intensity and duration ( 58 , 67 ).

When AT becomes insulin resistant, as occurs in patients with diabetes and may also be present in patients with obesity, insulin’s ability to inhibit adipocyte lipolysis and reduce serum levels of FFA and glycerol are impaired. As a result, excessive lipolysis leads to increased FFA levels in both the fasted and fed state. Constant exposure of the liver and muscle to these high FFA levels is thought to promote the uptake and ectopic storage of lipids in these tissues ( 68 ). Ectopic lipids have been shown to impair insulin signaling, and thus insulin resistance at the level of adipocyte via increased lipolysis may be a major contributor to whole body insulin resistance ( 69 ). In addition to impaired insulin responsiveness in fat cells, elevated lipolysis in obesity may be mediated by decreased expression of adipocyte lipid droplet proteins such as PLIN1 and Fsp27/Cidec (fat-specific protein 27/cell death-inducing DFFA-like effector c) ( 70 ). These proteins coat the lipid droplet and promote TAG retention via the inhibition of lipolysis, and mice or humans deficient for PLIN1 ( 71 ) or Fsp27 ( 72 , 73 ) exhibit lipodystrophy and insulin resistance ( 70 ). Interestingly, adipocyte-selective gene deletions or transgenic overexpression mouse models of proteins involved in insulin signaling, glucose and lipid metabolism demonstrate parallel modulation of adipocyte insulin action and systemic insulin sensitivity or glucose tolerance.

In addition to potent anti-lipolytic action ( 58 ), insulin also stimulates lipogenesis ( 74 ) by activating LPL (activation) and increasing the transcription of lipogenic enzymes ( 74 ). Growth hormone antagonizes insulin by promoting lipolysis and inhibiting lipogenesis ( 36 , 58 ). The insulin-sensitizing, anti-inflammatory lipids (PAHSAs) generated during AT DNL and the excess basal lipolysis associated with ectopic lipid deposition and insulin resistance make both AT lipogenesis and lipolysis attractive targets for pharmaceutical intervention. On the other hand, the balance between lipid storage, mobilization, and utilization is homeostatically regulated through a complex interaction of often redundant hormonal signaling, neurological input, and nutrient flow. These intricacies complicate attempts to develop therapies targeting one aspect of lipid metabolism since disrupting the balance between lipolysis and lipogenesis may, in turn, have unanticipated effects on insulin sensitivity and whole-body energy homeostasis.

Figure 5. . The Adipocyte Secretome.

The Adipocyte Secretome. Fat cells express and release numerous protein, lipid, and nucleic acid factors that can act on other nearby or distant tissues within the body in a paracrine or endocrine manner. Leptin, adiponectin, and resistin are highlighted here because they are exclusively secreted from mouse adipocytes, while the other factors can also be secreted from other cell types. The arrow-headed line representing secretion of resistin is dashed since in humans, macrophages, and not adipocytes, primarily produce this adipokine. Abbreviations are RBP4 – retinol binding protein 4, BMPs – bone morphogenetic proteins, PAI-1 – plasminogen activator inhibitor 1, miRNA – microRNA, FFA – free fatty acid, FAHFA - fatty acid esters of hydroxyl fatty acids, PAHSA – palmitic-acid-hydroxy-steric-acid, FGF21 – fibroblast growth factor 21.

Endocrine Properties of Adipose Tissue

Adipocytes and other AT cells secrete a variety of mediators, including exosomes, miRNA, lipids, inflammatory cytokines, and peptide hormones that act in both paracrine and endocrine modes ( Figure 5 ) ( 75 ). Although adipocytes secrete a large variety of bioactive molecules with widespread systemic effects contributing to numerous physiological and pathological processes, the autocrine and paracrine actions of these molecules are highly complex, and our understanding of these processes is likely rudimentary. However, substantial progress has been made studying three endocrine hormones that are almost exclusively produced in adipocytes and function to regulate food intake, the reproductive axis, insulin sensitivity, and immune responses. These hormones are leptin, adiponectin, and resistin, and we review their expression in obesity, their receptors, and effects in target tissues including metabolic actions ( Figure 6 ). While not produced in human adipocytes directly but secreted instead by AT macrophages, resistin has similar functions in mouse and man. The dysregulation of any one of these hormones can contribute to systemic metabolic dysfunction, as well as to the pathogenesis of chronic metabolic diseases and some types of cancer.

Figure 6. . Summary of adipocyte-specific adipokines, and their actions on other tissues.

Summary of adipocyte-specific adipokines, and their actions on other tissues. Abbreviations: TLR4 - Toll-like receptor 4; CAP1 - adenylyl cyclase-associated protein 1; AdipoR1 & R2 - Adiponectin receptors 1 and 2; CNS - Central nervous system; FAO – fatty acid oxidation; EE – energy expenditure.

The first discovered endocrine hormone of adipocyte origin was leptin ( 5 ). In 1949 spontaneously occurring obese offspring in a Jackson Laboratories’ non-obese mouse colony were determined to be homozygous for a recessive mutation, termed “obese” ( ob ) ( 76 ). These ob/ob mice appear normal at birth, but soon begin gaining excess fat mass and displaying hyperglycemia and hyperinsulinemia ( 77 ). In the 1950s, ob/ob mice and their non-obese littermates underwent parabiosis experiments, where two animals are surgically joined (usually by peritoneum or a long bone of the leg) to allow for the exchange of whole blood between them ( 78 ). Weight gain was inhibited in ob/ob mice parabiosed with non-obese littermates, providing evidence that the ob/ob gene product was a circulating factor transferred from the blood of the lean littermate. In 1972, a similar study demonstrated that parabiosis with lean animals not only reduced weight gain in ob/ob mice, but also improved hyperglycemia, hyperinsulinemia, and insulin sensitivity ( 6 ). Finally, in the 1990s, positional cloning studies identified the product of the ob gene, dubbed leptin, which was derived from the Greek word “leptos” meaning to be thin. Further characterization of leptin revealed that adipocytes were its predominant source ( 5 ). Following this discovery, the first directly observed function of leptin was its effect on food intake ( 79 ) followed shortly thereafter by demonstration that leptin levels in mice and men strongly correlate with fat mass and play a key role in body-weight (energy) homeostasis as described below.

Leptin Receptor and Signaling

Another spontaneously arising mutant mouse that developed obesity and type 2 diabetes is the diabetes or db/db mouse ( 80 ). In contrast to ob/ob mice, parabiosis of db/db mice to wild type littermates did not improve body weight or diabetes, but instead resulted in unhealthy weight loss in the lean littermates, leading investigators to deduce that mice with the db mutation lacked a functioning receptor for the ob gene but still manufactured a circulating protein that crossed over to the lean littermates to induce anorexia ( 81 ). Further confirmation of these hypotheses came when parabiosis of ob/ob with db/db mice induced weight loss in the ob/ob mouse while the obese state was preserved in the db/db mouse ( 81 – 83 ). Although the db gene was cloned in 1990 ( 84 ), it was not until almost 5 years later ( 85 ) following the identification of leptin that the db gene was identified to encode the leptin receptor (LR).

The LR is a class 1 cytokine receptor with substantial homology to glycoprotein 130, a plasma membrane receptor that mediates the actions of many cytokines. Unlike other plasma membrane receptors, such as the insulin receptor, the LR lacks intrinsic kinase activity and signals via Janus kinases (JAKs). Six LR isoforms exist, designated LRa-LRf, with LRb being the best characterized. It is the longest LR isoform that is capable of full signaling via the JAK/STAT pathway ( 86 ).

Leptin-regulated circuits involved in energy homeostasis have been mapped to distinct yet diverse brain regions ( 87 ) expressing the long form of the LR ( 88 ). Increased central leptin signaling inhibits food intake and elevates energy expenditure, while leptin deficiency (such as during fasting or starvation) has opposite effects. Expression of the LR has also been detected in peripheral tissues, but the exclusivity of the central leptin circuits to modulate energy intake and expenditure is supported by studies showing that deletion of the long form LR in peripheral tissues had no effects on these processes ( 89 ). Leptin levels strongly correlate with fat mass in mice and men ( 90 , 91 ), and as such leptin acts as a sensor of energy stores signaling the availability of body fat to the brain and regulating adipose reserves. However, during obesity the negative feedback loop between increasing leptin levels that signal high energy availability and inhibit food intake becomes disrupted due to the development of leptin resistance ( 92 ) — the inability to respond to leptin despite having sufficient or excess levels in circulation during accumulation of excess adipose stores. Although the physiological causes of leptin resistance are not well understood, it has been shown that hyperleptinemia is required for the development of leptin resistance during obesity. When leptin levels of mice are clamped to low levels (similar to lean mice), these clamped mice still develop obesity on HFD, but they do not become leptin resistant ( 93 ). The inability to overcome leptin resistance by giving supplemental doses has precluded leptin’s use as an anti-obesity therapeutic. Interestingly, leptin resistance that accompanies obesity appears to result from selective impairment of leptin’s ability to reduce food intake, while preserving its other capacity to raise energy expenditure ( 94 ). The molecular basis for this phenomenon has not yet been elucidated and remains under active investigation.

Leptin also has central nervous system effects not directly related to energy balance, including modulation of reproduction and thermoregulation. Additionally, research into the role of leptin to mediate anxiety and depression is currently ongoing ( 95 ). Leptin can also act peripherally on hepatocytes and pancreatic β-cells to regulate glucose and lipid metabolism independently of its central effects ( 96 ). Leptin has also been shown to affect innate and adaptive immunity ( 7 ), bone formation ( 97 ), bone metabolism ( 98 , 99 ), angiogenesis, and wound healing ( 88 ). Skeletal muscle, liver, and intestines have been described as targets for leptin action ( 100 ), and some evidence suggests that leptin may also act in an autocrine manner on AT ( 101 ). How leptin mediates responses in peripheral tissues is poorly understood and complicated by the existence of its six receptor isoforms, their differential expression across tissues, the pleiotropic nature of leptin’s effects, the demonstration of “selective” or tissue-specific leptin resistance, and the complexity of the signaling pathways involved.

Leptin and Cancer

Given the elevated risk for many cancers in patients with obesity in whom leptin levels are also high, it is not surprising that leptin has been implicated in tumorigenesis. Indeed, leptin levels or leptin signaling has been found to be dysregulated in breast, thyroid, endometrial, and gastrointestinal malignancies ( 102 ). Ectopic leptin expression in colorectal adenomas increases during the progression to colorectal cancer ( 103 , 104 ) yet associates with a favorable prognosis of the cancer ( 103 ). In papillary thyroid cancer, increased circulating leptin levels occur independently of body mass index (BMI), coincide with elevated LR expression on the tumor cells, and associate with aggressive carcinogenesis and poor prognosis ( 105 , 106 ). In contrast, reported associations between leptin levels and endometrial cancer are not maintained when adjusted for BMI, suggesting that leptin is not likely a causative factor in the development of this cancer ( 107 – 109 ).

Studies show that postmenopausal women with obesity have a 20-40% greater risk of developing breast cancer compared to normal weight women ( 110 ). In breast cancer, particularly in high-grade tumors, overexpression of both leptin and LR is associated with cancer progression and poor patient survival. Leptin’s ability to stimulate angiogenesis, regulate endothelial cell proliferation, and crosstalk with insulin and human epidermal growth factor receptor 2 (HER2) signaling pathways represent a few of the possible mechanisms by which leptin plays a role in breast cancer ( 111 ). As obesity rates continue to rise, it is likely that studies examining the relationship between leptin and cancer will become even more relevant.

Adiponectin

Adiponectin is a unique and extensively studied adipocyte-derived hormone with complex biology. Efforts to identify genes regulated during adipogenesis led to the discovery of adiponectin in 1995 ( 112 ) and 1996 by three separate research groups employing different approaches ( 113 – 115 ). Secreted by adipocytes, adiponectin is characterized by its remarkably high circulating levels reaching plasma concentrations in humans of 2-20 ug/ml ( 116 ), values that are more than 1000-fold higher than most other secreted factors ( Figure 7 ). Unlike leptin, adiponectin levels decease as a function of increasing fat mass in both rodents and humans with obesity ( 115 ); thus, they are lower in patients with obesity than those who are lean. Adiponectin’s widely reported anti-hyperglycemic, anti-atherogenic, and anti-inflammatory effects have made it an attractive therapeutic target for the treatment of obesity and insulin resistance. However, efforts to develop therapies targeting adiponectin function have been impeded by its complex structure and regulation ( 117 ).

Figure 7. . Typical circulating concentrations of select adipokines and insulin for normal weight, healthy humans.

Typical circulating concentrations of select adipokines and insulin for normal weight, healthy humans. Adipokine levels in the blood are several orders of magnitude higher than that of insulin.

Adipocytes secrete different forms of adiponectin: low-molecular weight (LMW) trimers (the most basic form), medium-molecular weight (MMW) hexamers, and high-molecular weight (HMW) oligomers ( 118 ), as well as globular adiponectin, a proteolytic fragment of the protein ( 119 , 120 ). In humans, the MMW and HMW oligomers make up most of the circulating adiponectin while the LMW trimer constitutes less than 30% of serum adiponectin. The HMW oligomer is most closely associated with enhanced insulin sensitivity and reduced glucose levels ( 121 ).

Adiponectin signaling is complex and incompletely understood. Three adiponectin receptors have been identified. Adiponectin receptor-1 and -2, referred to as AdipoR1 and AdipoR2, bind the LMW and globular forms ( 122 ). T-cadherin binds HMW adiponectin ( 123 ). Both AdipoR1 and AdipoR2 can modulate insulin sensitivity and metabolic gene expression in insulin-responsive tissues, and both receptors have demonstrated roles in the pathophysiology of insulin resistance and T2D ( 124 , 125 ). T-cadherin, which is expressed in a variety of tissues including the liver ( 126 ), belongs to a family of cell surface proteins involved in cell-cell interactions ( 127 ). Mice lacking T-cadherin accumulate adiponectin in circulation and have a similar cardiovascular phenotype to adiponectin knockout mice ( 128 ), suggesting that T-cadherin is the primary effector of cardioprotection by adiponectin ( 129 – 131 ).

Adiponectin enhances fatty acid oxidation through activation of AMP-activated kinase ( AMPK ) ( 132 , 133 ), a cellular energy sensor, which then inhibits acetyl CoA carboxylase, a rate limiting enzyme in DNL ( 134 ). This, in turn, reduces malonyl-CoA production and enhances fatty acid oxidation. Adiponectin can activate AMPK through two independent pathways, and can also modulate lipid metabolism by increasing mitochondrial density and mitochondrial DNA content ( 135 , 136 ). Adiponectin has diverse effects in many tissues, including bone and cartilage ( 137 ), and can act in an autocrine or paracrine manner in AT and other tissues ( 138 ). Adiponectin also appears to modulate a wide range of biological processes, including reproduction and embryonic development ( 139 , 140 ). The heart, liver, and skeletal muscle are considered the primary targets for adiponectin action, and adiponectin’s prominent insulin-sensitizing effects have been most fully characterized at the mechanistic level in liver and muscle ( 117 ) ( Figure 6 ).

The liver performs a critical function in maintaining normal blood glucose levels by releasing glucose (i.e. hepatic glucose output) into circulation in conditions such as fasting, exercise, and pregnancy. Conversely, the ability of the liver to reduce its glucose output when demand is low, as in the fed state, is also crucial to preventing hyperglycemia, and this process is often impaired with obesity and insulin resistance. Adiponectin can robustly reduce plasma glucose levels predominantly by inhibition of hepatic glucose production as opposed to effects on whole-body glucose uptake into cells and glycolysis ( 141 ). The importance of adiponectin in regulating glucose output in the liver is underscored by studies showing that mouse models with genetic deletion ( 142 ) or overexpression ( 143 ) of adiponectin have impaired or enhanced hepatic insulin sensitivity, respectively. Adiponectin levels are increased by thiazolidinediones (TZDs), which is thought to be the predominant mechanism of action that improves insulin sensitivity and glucose tolerance with this class of medications ( 142 – 144 ). Adiponectin also promotes hepatocyte survival, inhibits hepatic fibrosis and inflammation, stimulates fatty acid oxidation ( 133 , 145 ) and modulates fatty acid uptake and metabolism ( 146 ). In patients with nonalcoholic fatty liver disease (NAFLD) who are insulin resistant, low plasma adiponectin levels are associated with the progression of NAFLD and non-alcoholic steatohepatitis ( 147 , 148 ). In summary, adiponectin has beneficial effects in the liver, where it protects against metabolic dysfunction and hepatic diseases ( Figure 6 ).

Skeletal muscle is responsible for up to 80% of insulin-mediated glucose uptake in healthy individuals ( 149 , 150 ). Adiponectin can promote glucose uptake ( 151 , 152 ), enhance fatty acid oxidation ( 152 , 153 ), and enhance insulin sensitivity ( 154 ) in cultured muscle cell lines and mouse skeletal muscle. Adiponectin administration to obese, insulin-resistant adiponectin-knockout mice improves skeletal muscle insulin sensitivity ( 146 , 155 , 156 ). In human myotubes, adiponectin promotes fat oxidation via AMPK activation; this response is impaired in myotubes from patients with T2D and obesity ( 157 ). Thus, adiponectin has an important role in skeletal muscle metabolism in humans as well as rodents, and defective adiponectin signaling in skeletal muscle may contribute to insulin resistance.

Finally, in addition to its insulin-sensitizing and glucose-lowering effects in liver and skeletal muscle, adiponectin is also cardioprotective. Low circulating adiponectin levels correlate significantly and independently with coronary artery disease ( 158 ), and are considered a risk factor for cardiovascular diseases (CVD) such as hypertension, coronary artery disease, and restenosis ( 159 ). The vascular endothelium is believed to mediate some of the cardioprotective effects of adiponectin via AMPK activation and subsequent activation of eNOS (endothelial nitric oxide synthase) ( 160 ).

In light of these beneficial functions, adiponectin has significant therapeutic potential in the treatment of T2D, CVD, and NAFLD. Several years ago, small molecule screening efforts produced the first small molecule AdipoR agonist. “AdipoRon”, as it was named, not only recapitulated adiponectin’s effects on AdipoR signaling pathways but also had profound anti-hyperglycemia effects in both diet-induced obese mice and a genetic mouse model for diabetes ( 161 ) . A more recent study has now shown that AdipoRon can also decrease ceramides and lipotoxicity, and mitigate diabetic nephropathy ( 162 ). Hence, small molecule activators of adiponectin signaling show promise in the management of obesity-associated metabolic diseases like insulin resistance, NAFLD, and T2D.

Resistin, the most recently discovered of the major adipocyte-derived hormones, was independently identified by two laboratories. In one case, the gene coding for this novel endocrine factor was identified in a screen for genes inhibited by TZD drugs and was named “resistin” because it induced insulin resistance ( 163 ). Another group identified the same gene in a screen for genes expressed exclusively in adipocytes and induced during adipogenesis; they named it ADSF, for adipose tissue-specific secretory factor. In this study, the product of the gene was shown to inhibit differentiation of adipocytes in vitro ( 164 ), later confirmed in a separate study ( 165 ).

Elucidating resistin’s role in physiology has been challenging. While resistin is expressed in both white and brown fat in mice, the various WAT depots (inguinal, gonadal, retroperitoneal, and mesenteric) and BAT exhibit distinct patterns of resistin expression ( 166 ). In addition, circulating levels of resistin are directly proportional to its gene expression in some conditions, but inversely proportional in others ( 167 , 168 ). Remarkably, while resistin produces similar metabolic and inflammatory effects in humans and mice, human resistin is predominantly secreted from macrophages, not adipocytes ( 169 – 171 ). The complex regulation of resistin expression and the fundamental differences in resistin biology between species are significant obstacles to fully understanding this hormone’s functions and mechanisms of action in humans.

Resistin interacts with two known receptors: the toll-like receptor 4 (TLR4) and adenylyl cyclase-associated protein 1 (CAP1) ( 172 , 173 ). Resistin signaling through TLR4 contributes to monocyte recruitment and chemokine expression, and is involved in inflammatory responses in atherosclerosis and acute lung injury ( 135 , 174 ). Both knockout and overexpression studies of CAP1 indicate that this receptor can also mediate proinflammatory effects of resistin ( 173 ). Overall, the similarities, differences, and tissue specificity of resistin signaling through TLR4 versus CAP1 remains poorly understood.

Resistin has also been shown to regulate fasting blood glucose levels in mice ( 175 ). Elevated levels of circulating resistin are reported in genetic and diet-induced mouse models of obesity ( 163 ). Anti-resistin antibody administration improves insulin sensitivity in diet-induced obese mice, and conversely, resistin injection impairs glucose tolerance in normal mice; supporting a causative role of resistin in mediating insulin resistance in mouse models ( 176 ). Moreover, both human and mouse resistin have been shown to impair insulin-stimulated glucose uptake in cultured murine myocytes in vitro ( 177 ). Other studies have shown similar insulin desensitizing effects of resistin in liver and brain ( 178 , 179 ).

The finding that human resistin originates not in adipocytes but in mononuclear lymphocytes raised the possibility that the hormone may have distinct roles in the two species. An elegant mouse model was generated to address this issue, the so-called humanized resistin mouse. In these mice, the endogenous resistin gene (normally expressed in adipocytes) was deleted, and the macrophage-expressed human resistin gene was inserted ( 180 ). Data from this study revealed that like murine adipocyte-derived resistin, the humanized resistin induced systemic insulin resistance, adipose tissue inflammation, and elevated circulating free fatty acids in high-fat diet (HFD)-fed mice.

In humans, epidemiological, genetic, and clinical data support a role for resistin in dysfunctional metabolism and related pathologies ( 181 ). As in mouse models, serum resistin levels are elevated during human obesity ( 182 , 183 ). Furthermore, high circulating resistin concentrations in humans have been associated with atherosclerosis, coronary heart disease, congestive heart failure, as well as inflammatory conditions including systemic lupus erythematosus, inflammatory bowel disease, and rheumatoid arthritis ( 184 – 188 ). Whether the relationship between resistin and insulin resistant states is merely correlative and whether interventions to antagonize resistin action will be of therapeutic value in the treatment of metabolic or cardiovascular disease in humans remains undetermined.

Cell Types in Adipose Tissue

Besides adipocytes, AT is comprised of endothelial cells, blood cells, fibroblasts, pericytes, preadipocytes, macrophages, and several types of immune cells ( 189 ). These non-adipocyte cell types are commonly referred to as the AT stromal vascular fraction (SVF) ( Figure 8 ). Our understanding of the complexity of the cell types present in the SVF and how this milieu is altered by metabolic disease states is an area of active investigation. Cells in the SVF produce hormones and cytokines that can act in a paracrine manner on adjacent adipocytes. In the early 1990s, it was shown that TNF alpha production was increased in AT during metabolic disease states, in particular, T2D ( 190 ). Yet, it wasn’t for another ten years that adipose tissue macrophages (ATMs) were identified as the primary cellular source of AT TNF alpha ( 191 ). It is now largely accepted that in conditions of obesity and T2D, TNF alpha is produced in ATMs and acts on adjacent adipocytes within AT to promote insulin resistance. Hence, it is important to consider the presence and dynamic interactions of the SVF cells, especially when determining the cellular sources of AT-derived paracrine and endocrine hormones.

Figure 8. . Constituents of adipose tissue (AT).

Constituents of adipose tissue (AT). Left: Along with mature, functional adipocytes and precursor cells, many cell types related to vasculature and immune function reside within AT. They perform both physiological and pathophysiological functions by communicating with the adipocytes via secreted factors and scavenging lipid from dying fat cells. The number and diversity of these cell types increases with developing obesity and metabolic dysfunction. Right: The non-adipocyte cells are collectively referred to as the stromal vascular fraction (SVF), and the SVF can be separated from lipid-containing adipocytes by digesting the extracellular matrix (ECM) and centrifuging the cellular mixture. The SVF will form a pellet at the bottom of the tube, while the adipocytes will float and form a visible lipid layer at the top of the aqueous medium. This separation technique is critical to studying the cellular composition of adipose tissue and gaining insight regarding the individual functions of these diverse and distinct cell types under physiological and pathophysiological conditions.

Adipogenesis

To understand how adipocytes contribute to systemic metabolic regulation, it is important to understand their development. Adipogenesis refers to the process by which precursor cells differentiate and become committed to storing lipid and maintaining energy homeostasis as adipocytes. Adipogenesis is regulated by hundreds of factors, including nutrients, cellular signaling pathways, miRNAs, cytoskeletal proteins, and endocrine hormones such as growth hormone, insulin-like growth factor 1, insulin, and several steroid hormones, as well as cytokines. Generally, pro-inflammatory cytokines inhibit adipogenesis ( 192 ), although some cytokines within the same family exert opposing effects ( 192 ). Cytoskeletal proteins ( 193 ), ECM proteins and their regulators ( 194 ), microRNAs (miRNAs) ( 195 ), and long noncoding RNAs (lncRNAs) ( 196 ) differentially modulate adipogenesis. Dozens of different transcription factors, briefly described below, also regulate adipogenesis ( Figure 9 ).

Figure 9. . Transcriptional regulation of adipogenesis as determined in vitro in a fibroblast-like preadipocyte clonal cell line.

Transcriptional regulation of adipogenesis as determined in vitro in a fibroblast-like preadipocyte clonal cell line. Preadipocytes are grown to confluence and become growth arrested. Following induction of differentiation, they re-enter the cell cycle and undergo several rounds of proliferation, a process known as mitotic clonal expansion. At the end of this short proliferative phase, preadipocytes terminally differentiate into adipocytes as they begin synthesizing lipid and assume characteristics of mature fat cells. Numerous transcription factors have been determined to promote (green arrows) or inhibit (orange horizontal ended line) adipogenesis either during clonal expansion or at later stages of terminal differentiation. The timing of activation (i.e. when each transcription factor is turned on and off) is critical to the progression of adipocyte differentiation.

Promotors of Adipogenesis

The transcription factor peroxisome proliferator activated receptor gamma (PPARg) is considered the principal adipogenesis regulator ( 197 ). Its discovery substantially enhanced our understanding of the adipocyte and its role in metabolic disease. For example, mice with adipocyte-specific PPARg deletion have decreased AT mass and are insulin resistant ( 198 ). In humans, PPARg gene mutations can also cause lipodystrophy (partial or generalized loss of fat in the body) and severe insulin resistance ( 199 – 201 ). The discovery of PPARg as the functional receptor for the insulin-sensitizing TZDs resulted in a significant effort to understand PPARg action and identify additional agonists. Synthetic TZDs induce weight gain in humans and rodents by increasing fat mass, more so in the subcutaneous adipose depot, which is associated with improved metabolic outcomes. However, this weight gain is also considered as a negative side effect of TZD treatment, especially in the typical patient who has pre-existing obesity. Other adverse side effects of TZDs, such as bone fractures and heart failure, have spawned the search for structurally distinct PPARg ligands capable of inducing unique receptor-ligand conformations with signature affinities for diverse co-regulators ( 202 ). Several selective PPARg modulators (SPPARMs) with fewer side effects have been identified. These act as partial PPARg agonists, alter specific post-translational modifications of PPARg, and preserve anti-hyperglycemia effects while minimizing or eliminating the adipogenic effect that leads to increased fat mass via activation of distinct gene profiles that may be cell and tissue specific ( 203 , 204 ). Interestingly, the TZD, rosiglitazone, is capable of improving glucose homeostasis even in the absence of PPARg in mature adipocytes ( 205 ), suggesting that its adipogenic effects (in addition to its non-adipogenic ones) may also be important for its anti-hyperglycemic action.

In addition to TZDs, PPARg binds endogenous lipophilic molecules, including: long chain fatty acids (LCFAs), oxidized or nitrated FAs, prostaglandins, and arachidonic acid derivatives ( 206 ). Interestingly, serotonin (5-hydroxytryptamine, 5-HT) has also been shown to be a high affinity agonist for PPARg ( 207 ). Many of the endogenous PPARg ligands enhance adipocyte differentiation and regulate fat cell functions such as lipolysis, glucose uptake, and lipogenesis through PPARg-dependent and independent methods ( 208 – 214 ). Overall, these endogenous ligands have low affinity and limited subtype selectivity for PPARg relative to other PPARs, suggesting that much remains to be understood regarding this critical adipogenesis regulator. While there is no question that PPARg is essential for adipogenesis and lipid accumulation within fat cells, a better mapping of its gene expression profiles in discrete cell and tissue types and with endogenous and synthetic ligands will improve our understanding of AT development and function under both physiological and pathophysiological conditions.

The CCAAT/enhancer-binding proteins (C/EBPs) are widely expressed transcription factors that regulate proliferation and differentiation of various cell types in mammals. Studies in vivo and in vitro have identified C/EBP isoforms α, β and δ as important regulators of adipogenesis ( 215 ). C/EBPs β and δ work together in early adipogenesis to promote fat cell differentiation by inducing expression of C/EBPα and PPARg ( 216 ). Additionally, the transcription factors Krox20 and ZNF638 can modulate adipogenesis by affecting C/EBPβ function ( 217 , 218 ).

The Signal Transducer and Activator of Transcription (STAT) family of transcription factors was first identified over 20 years ago ( 219 ). Both the protein expression of STATs and their ability to regulate gene expression are tissue-specific ( 220 ). In AT, STATs regulate gene expression during adipogenesis, and the expression of STATs 1, 3, 5A, and 5B is induced during differentiation of murine and human preadipocytes ( 221 , 222 ). Notably, the ability of STAT5 proteins to promote adipogenesis has been documented by over a dozen independent laboratories using both in vitro and in vivo approaches ( 17 ).

Of the three isoforms of Sterol Response Element Binding Proteins (SREBP-1a, SREBP-1c, and SREBP-2), SREBP-1c is the predominant form expressed in white AT ( 223 , 224 ) and is an important regulator of lipogenesis genes, while SREBP-2 regulates the expression of cholesterol biosynthesis genes ( 225 ). Intriguingly, two miRNAs (miR-33a and miR-33b) located within the SREBP genes are highly induced during adipogenesis ( 226 ). Although SREBP-1 clearly plays a promoting role in adipogenesis in vitro , in vivo studies suggest that SREBP-1 is not critical for AT development and/or expansion, perhaps due to compensatory SREBP-2 overexpression ( 47 , 227 ).

Members of the early B-cell factor (EBF) family of transcription factors are characterized for their ability to modulate islet beta-cell maturation and neural development. Three primary members of this family (EBFs 1, 2, and 3) are expressed in fat cells. EBFs 1 and 2 can promote adipogenesis ( 228 , 229 ), and EBF2 can also play roles in determining brown versus white adipocyte identity in vivo ( 230 ) and the beiging process of adipose tissue in mice ( 231 ).

Inhibitors of Adipogenesis

The interferon-regulatory factor (IRF) family of transcription factors has functionally diverse roles in the immune system, but also plays a role in adipocyte development. All nine IRF family members are regulated to different degrees during adipogenesis in vitro , and some members can repress adipogenesis ( 232 ) and contribute to insulin resistance ( 233 ). For example, knockdown of IRF3, whose is expression is elevated in visceral and subcutaneous AT of obese mice as well as in subcutaneous AT from humans with obesity and diabetes decreases fat mass and prevents insulin resistance in high fat diet-fed mice ( 233 ).

Wingless-related integration site (Wnt) proteins regulate development and cell fate through both autocrine and paracrine signaling ( 234 ) by using three well-characterized pathways: the canonical Wnt signaling and the planar cell polarity and Wnt/calcium pathways, which are non-canonical. The canonical pathway is dependent upon the transcription factor, β-catenin ( 235 ). Wnt10b is the best studied member of the Wnt signaling family in terms of adipocyte development. In the presence of Wnt10b, β-catenin translocates to the nucleus where it inhibits PPARγ and C/EBPα activity, thereby impeding adipogenesis ( 236 , 237 ). On the other hand, extracellular antagonists of Wnt/β-catenin signaling have been reported to promote adipocyte differentiation ( 238 , 239 ).

The GATA family of transcription factors were named based on their ability to bind the DNA sequence GATA ( 240 ). Only GATAs 2 and 3 are expressed in preadipocytes residing in white AT ( 241 ), and both are repressed during adipogenesis. In fact, GATA2 can directly bind to the PPARγ promoter to suppress its activity ( 241 ). In addition to inhibition of PPARγ expression, GATAs 2 and 3 can also associate with C/EBPs to disrupt their transcriptional activity ( 242 ). GATA3 expression is driven by the canonical Wnt signaling pathway ( 243 , 244 ). Collectively, these studies demonstrate that two GATA proteins can attenuate adipocyte development via multiple transcriptional and signaling pathways.

Transcription Factor Families that can either Promote or Inhibit Adipogenesis

The Krüppel-like transcription factors (KLFs) include 17 members that can either activate or repress transcription. In relation to adipocyte development, KLFs 4, 5, 6, 9 and 15 can promote adipogenesis, while KLFs 2, 3 and 7 repress adipocyte development. Most studies on the roles of KLFs in adipogenesis have been performed in vitro using a variety of cell culture models, and have demonstrated that KLFs act in concert with other transcription factors modulate adipogenesis ( 245 ).

The transcription factor activator protein 1 (AP-1) consists of Jun proteins (c-Jun, JunB, and JunD), Fos proteins (c-Fos, FosB, Fra1, and Fra2), ATF and JDP family members, several of which are induced during adipogenesis ( 222 ). In humans, a mutation in the c-fos gene that is associated with lipodystrophy has been shown to reduce c-fos activity and adipocyte development ( 246 ). Many in vitro and in vivo studies demonstrate that, like KLFs, AP-1 transcription factors can positively and negatively regulate adipogenesis.

Many of the zinc finger proteins (ZFPs) function as transcription factors with several contributing to adipocyte determination and/or adipogenesis. Zfp423 and Zfp467 can promote adipocyte differentiation by enhancing PPARγ expression and activity ( 247 , 248 ). In addition to stimulating adipogenesis, Zfp423 can suppress ‘beige-like’ properties in white adipocytes that are typically associated with improved metabolic health ( 249 ). Zfp521 can inhibit Zfp423 to reduce adipocyte development and is also considered a critical regulator of the commitment to either osteogenic or adipogenic lineages ( 250 , 251 ).

The transforming growth factor beta (TGF-β) superfamily encompasses a large number of proteins, including bone morphogenetic proteins (BMPs) ( 252 ). BMPs and TGF-β have been reported to be involved in both adipocyte commitment and differentiation ( 253 – 255 ). Specifically, BMPs 2 and BMP4 can promote adipogenesis via the Smad signaling pathway ( 256 ) to regulate transcription of target genes such as PPARγ ( 257 , 258 ). While BMPs are known to promote adipogenesis, in vitro and in vivo studies demonstrated that TGF-β primarily inhibits fat cell differentiation.

Hormonal Regulation of Adipogenesis

Steroids are prominent regulators of AT development and distribution, and adipocytes express high levels of many steroid hormone receptors. These lipophilic hormones diffuse through plasma membranes, dimerize, and bind to their specific receptors to impart both genomic and non-genomic responses ( 259 , 260 ). Since steroid-bound receptors act as transcription factors, their capabilities should be fully considered in the transcriptional regulation of adipogenesis.

Two types of estrogen receptors, ERα and ERβ, are expressed in rat and human preadipocytes, mature adipocytes, and in other AT cells ( 261 – 263 ). Although many studies describing the role of estrogens in AT are contradictory, most investigations indicate that estrogen inhibits adipocyte differentiation ( 245 ) and the adipogenic action of PPARγ ( 264 ). Aromatase is an enzyme found in several tissues, including AT, that aromatizes androgens into estrogens . Both ERα- and aromatase-knockout mice have increased adiposity, suggesting that both estrogen and its receptor can reduce adipocyte development ( 265 , 266 ). Mice lacking ERα have enhanced visceral AT deposition and increased weight gain compared with wild-type mice ( 267 ).

Androgen receptors (AR) are also expressed in rodent ( 268 , 269 ) and human AT ( 270 ). Similar to estrogen, many studies report contradictory actions of androgens on the differentiation and function of adipocytes. These inconsistent results highlight the importance of accounting for sex-, depot- and organism-specific effects. In studies of human AT, testosterone and the non-aromatizable androgen, dihydrotestosterone, inhibit differentiation of preadipocytes obtained from subcutaneous and omental depots of both men and women, although the magnitude of the inhibitory effect may differ between the sexes ( 271 , 272 ). Overall, most studies indicate that androgens exert inhibitory effects on adipogenesis.

Glucocorticoids (GCs) are well-known promoters of adipocyte development. GCs also promote adipocyte hypertrophy and differentiation of central fat depots that can lead to abdominal obesity and insulin resistance ( 273 ). In vitro adipogenesis studies include the wide use of the synthetic GC, dexamethasone. Although the mechanisms of action and target genes of GCs involved in adipocyte differentiation are not completely clear, it is known that GCs induce expression of C/EBPs beta and δ and that GC-induced C/EBPδ coordinates with C/EBPβ to induce PPARg expression and adipogenesis ( 274 ).

To understand the actions of GCs via the glucocorticoid receptor (GR), it is important consider the enzyme that affects circulating levels of cortisol, the active form of GR’s endogenous ligand. 11β-hydroxysteroid dehydrogenase type 1 (11 beta HSD1) is an enzyme highly expressed in AT and liver that in AT converts inactive cortisone to the active hormone cortisol . Hence, it is not surprising that 11 beta HSD1 mRNA expression and activity is essential for the induction of human adipogenesis and that adipocyte development can be blocked with a 11 beta HSD1 specific inhibitor ( 275 ). In addition to inducing the expression of early adipogenic transcription factors, GCs promote adipocyte development by mechanisms that include suppression of anti-adipogenic factors (Pref-1 and Runx2); anti-proliferative effects on preadipocytes; and sensitizing or ‘priming’ of human preadipocytes to insulin action ( 276 ). Recent attention has focused on the potential contributions of environmental pollutants known as endocrine disrupting chemicals (EDCs) in the development of metabolic diseases. Studies reveal that EDCs can promote adipogenesis through GR activation ( 277 ), thereby implicating these compounds in the rising rates of obesity and diabetes.

In addition to regulating water and salt homeostasis, the mineralocorticoid aldosterone and its receptor (MR) have also been shown to play a role in the regulation of adipocyte development. This is important since MR is a high-affinity receptor for both mineralocorticoids and GCs. Aldosterone promotes adipogenesis in an MR-dependent manner ( 278 ) and a MR antagonist can inhibit adipogenesis ( 279 ). Although GRs and MRs are expressed in AT and thought to mediate cortisol’s actions on AT, the levels of GR are several hundred-fold higher than MR in both human preadipocytes and adipocytes ( 280 ). Loss of GR, but not MR, blocks the adipogenic capabilities of cortisol in human preadipocytes ( 280 ). However, MR expression is higher in omental than in subcutaneous AT, so there could potentially be depot differences in the relative importance of MR and GR in cortisol-induced adipogenesis ( 280 ). There could also be differences in the contribution of MR to adipogenesis during obesity when MR and 11 beta HSD1 expression levels are increased, while the GR and 11 beta HSD2 (the enzyme that deactivates cortisol) levels do not increase accordingly ( 280 ). Most of the current evidence suggests that the ability of aldosterone to modulate adipogenesis in vitro is largely dependent on MR. Additional studies are needed to determine if MR plays a role in adipocyte development in vivo .

Vitamin D is another steroid hormone with strong experimental evidence that it can regulate adipogenesis. Unlike most of the water-soluble vitamins that are excreted via urine when in excess, Vitamin D, along with the other fat-soluble vitamins (A, E, and K), can be stored within fat-laden adipose tissue. The vitamin D receptor (VDR) and 1α-hydroxylase (CYP27B1), the enzyme that activates vitamin D, are expressed in human AT, primary preadipocytes, and newly-differentiated adipocytes ( 281 ). The most active form of Vitamin D, 1, 25-Dihydroxyvitamin D, represses adipocyte differentiation ( 282 , 283 ) and the VDR can block adipogenesis by inhibiting C/EBPβ expression ( 284 ). Vitamin D-induced inhibition of adipogenesis also involves direct suppression of C/EBPα and PPARg ( 285 ). Vitamin D and VDR also play a role in the inhibition of adipogenesis of bone marrow stromal cells ( 286 ), in part by suppressing the expression of inhibitors of the canonical Wnt/β-catenin signaling pathway ( 287 ). Although vitamin D inhibits adipogenesis in the widely used murine and bone marrow-derived cells, both 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D 3 can promote the differentiation of human subcutaneous preadipocytes ( 281 ). Overall, a case could be made that concentrations of vitamin D as well as the type of adipocyte precursor determine whether this hormone exerts pro- or anti-adipogenic actions via the VDR.

On the other hand, evidence regarding Vitamin D’s role in adipocyte development in humans is controversial and contradictory. According to a systematic review and meta-analysis of 23 studies between 2002 and 2014, overweight or obese subjects exhibit a higher prevalence of Vitamin D deficiency ( 288 ). In two double-blind, placebo-controlled randomized clinical trials, Vitamin D-supplemented individuals with healthy overweight or obesity lost significantly more fat mass than the placebo group when fed either a calorie-restriction ( 289 ) or weight-maintenance ( 290 ) diet for 12 weeks. While decreased fat mass may result from Vitamin-D induced inhibition of adipogenesis, this hypothesis was not directly tested in the studies, and two other longer term studies demonstrated no change in fat mass with Vitamin D supplementation between 14,000 and 20,000 IU per week ( 291 , 292 ).

The relationship between adipocyte development and thyroid hormones has been recognized since 1888 when a report on myxedema proposed that obesity was a requirement for a diagnosis of hypothyroidism ( 293 ). The most biologically active form of thyroid hormone, T3, can induce brown adipocyte differentiation ( 294 ). Hyperthyroidism in rodents induces adipocyte hyperplasia, whereas hypothyroidism impedes AT development ( 295 ). Overall, studies on the involvement of thyroid hormones in AT development are controversial. While the induction of adipogenesis is differentially regulated by various thyroid hormone receptor (TR) isoforms, studies largely indicate that TRs promote adipogenesis in the majority of model systems ( 245 ).

Adipocyte Progenitors

In AT, pools of adipose stem/progenitor cells (APs) exist that can differentiate into mature adipocytes ( 296 , 297 ). At least two distinct progenitor populations give rise to adipocytes: developmental APs and adult APs ( 296 , 298 ). Our understanding of the molecular characteristics of APs has dramatically increased in recent years as discussed below.

APs in Adipose Development

AT organogenesis in mice and humans begins during embryogenesis, and ends in the postnatal period for mice and just before birth in humans ( 296 , 297 , 299 ). AT is widely accepted to be of mesodermal origin ( 297 ). However, some of the spatiotemporal and molecular differences observed in formation of different AT depots suggest diverse developmental origins ( 297 , 300 ). Further, white and brown adipocytes, once considered to have common APs are now known to have different origins ( 297 , 301 , 302 ).

In the generation of white adipocytes, developmental APs express the master adipogenesis regulator PPARg but have distinct functional and molecular properties compared to adult APs ( 298 , 302 ). Developmental APs do not contain lipid but express the mature adipocyte markers perilipin and adiponectin, are able to replicate, and are located along the vasculature in developing white adipose tissue ( 298 , 302 , 303 ). Brown adipocytes can arise from myogenic Myf5-expressing precursors that also give rise to skeletal myocytes ( 297 , 302 , 304 ). Interestingly, brown-like adipocytes, known as beige adipocytes, emerge in white adipose from Myf5-negative precursors in response to cold or adrenergic stimuli, which suggests that the developmental origins of brown adipocytes and beige adipocytes are different ( 297 , 304 ). Collectively, these findings highlight the complex developmental heterogeneity of APs observed among adipose tissue depots in animals and humans.

APs in Adult Adipose Tissue

The notion that we are born with all the fat cells we will ever have is now considered archaic and inaccurate. Adipose tissue continues to generate new adipocytes throughout the lifespan, with a median adipocyte turnover rate of 8.3 years ( 302 , 305 ). Adult APs have been found in the SVF of AT depots in both rodents and humans ( 302 , 306 – 308 ) and are thought to represent an AP pool that contributes to this adipocyte renewal. Flow cytometry techniques that use a variety of cell surface and stem cell markers, have helped identify stromal cells that can undergo adipogenesis ( 302 , 306 , 307 , 309 ). These adult APs arise from tissue-resident mesenchymal stem cells, and are a major source of new adipocytes in AT ( 297 , 310 ). Bone marrow-derived APs from the myeloid lineage can also be recruited to AT where they become adipocytes ( Figure 10 ). Bone marrow-derived adipocytes (BMDAs) are more abundant in female mice and are more frequent in visceral depots ( 297 , 311 , 312 ). Though BMDAs have been observed in human AT and are increased in patients with obesity, the processes and factors involved in BMDA recruitment to AT remain unclear. Compared to normal adipocytes, BMDAs have reduced expression of lipid metabolism genes and increased pro-inflammatory gene expression, suggesting that they may have negative metabolic effects ( 297 , 311 , 313 ).

Figure 10. . Adipocytes are derived from both resident mesenchymal cells in the stromal-vascular fraction of adipose tissue and hematopoietic progenitors that reside in the bone marrow.

Adipocytes are derived from both resident mesenchymal cells in the stromal-vascular fraction of adipose tissue and hematopoietic progenitors that reside in the bone marrow. In addition to adipocytes, mesenchymal progenitors can form other connective tissue cells, such as myocytes and osteocytes. Myeloid progenitors derived from hematopoietic progenitors in bone marrow give rise to adipocytes as well as neutrophils, macrophages, dendritic cells, and granulocytes.

Most of the information regarding AP proliferation in obesity comes from rodent models. In mice fed high-fat diets to induce obesity, APs form new adipocytes primarily in the visceral depot ( 299 , 302 ). Although limited data report decreased AP proliferation and differentiation capacity from humans with obesity compared to lean individuals ( 314 ), convincing evidence for depot-specific AP populations in humans has emerged. Subcutaneous APs were shown to have a higher growth rate and adipogenic potential than visceral APs, giving rise to more functional adipocytes ( 315 , 316 ). Increasingly sophisticated methods for assessing APs in mice will help facilitate the identification of the origins of all APs for each adipose depot as well as the niches in which they reside.

Adipose Extracellular Matrix: From Normal Development to Fibrosis

An underappreciated influence on AT physiology is the adipose extracellular matrix (ECM). The dynamics and composition of the ECM are critical for proper adipocyte development and function ( 317 ). During adipogenesis, there is increased synthesis of laminar ECM constituents and maintenance of peri-adipocyte fibrillar collagens that ultimately allows the adipocyte to embed itself in the basal lamina ( 317 ). In the growth phase of adipogenesis, adipocytes require ECM-mediated traction forces to properly accumulate lipids and increase in size. A number of inhibitors, enzymes, and modifiers contribute to adipocyte ECM maintenance and renewal; these reactions consume a large amount of energy in the mature fat cell ( 317 ). In obesity, the ECM expands to accommodate the adipocyte hypertrophy and hyperplasia, and subsequent tissue growth, induced by the increased demand for lipid storage ( 317 – 321 ). This process appears to occur in a similar fashion in both animal models and humans.

Figure 11. . Differences in AT between lean and obese mammals.

Differences in AT between lean and obese mammals. The AT extracellular matrix (ECM) is important for normal tissue function but can also contribute to its dysfunction. In obesity, accumulation of ECM components can restrict AT expansion, promote inflammation by recruiting immune cells, and impair adipogenesis. These combined effects can worsen insulin resistance.

Adipose tissue expansion during obesity, coupled with immune cell accumulation and hypoxia, can lead to AT fibrosis ( Figure 11 ) ( 317 , 319 , 322 ). Fibrosis is the excessive accumulation of ECM components, such as collagens, that typically results from an imbalance of the synthesis and degradation of ECM components ( 319 , 323 ). Ultimately, adipocyte dysfunction will result from the decreased ECM flexibility conferred by the accumulation of fibrillar ECM components ( 317 , 320 , 323 ). Abnormal ECM collagen deposition is associated with immune cell infiltration, which can worsen fibrosis and contribute to AT dysfunction that often occurs in obesity ( 319 , 323 ). The removal of collagen VI, a major AT ECM component, improves adipocyte function and metabolism in obese mice by both decreasing AT immune cell infiltration and “weakening” the ECM, which allows uninhibited adipocyte hypertrophy ( 317 , 318 , 323 ). In humans, AT collagen VI expression is increased in obesity, and subjects with higher collagen VI have increased macrophage content and AT inflammation ( 324 ). Endotrophin, an adipocyte-derived cleavage product of collagen VI, directly stimulates AT fibrosis and macrophage accumulation, and can lead to systemic insulin resistance ( 325 ). Endotrophin can also cause fibrosis and endothelial cell migration in mammary tumors, leading to tumor expansion and the enhancement of metastatic growth ( 325 , 326 ).

Accumulation of ECM components and increased ECM-receptor signaling are associated with insulin resistance in obesity thought to be mediated by several possible mechanisms. In addition to physically restricting AT expansion, excess ECM components can also increase AT inflammation by interactions with their cell surface receptors (CD44, CD36, and integrins) ( 320 ). These ECM-receptor interactions can induce adipocyte death, inhibit angiogenesis, and promote macrophage infiltration and inflammation in adipose tissue, thereby driving insulin resistance ( 320 ). Interestingly, these downstream effector pathways of ECM-receptor signaling are similar to those involved in tumor growth and pulmonary fibrosis development.

The ECM has clear roles in the normal development and function of adipocytes, but in excess can also play roles in obesity development and metabolic dysfunction. Our understanding of the adipose ECM has deepened in recent years, but more research is necessary to better delineate how ECM components and their interactions can directly influence AT physiology and pathophysiology. Since many AT cell types produce ECM components, studies to determine the specific contributions of adipocyte-derived ECM components to normal AT function as well as dysfunction will be required.

Rodent versus Human Adipose Depots

Much of the knowledge about the depot-specific characteristics and metabolic profiles of AT has been obtained from rodents. However, the validity of translating studies conducted in rodent fat to humans remains controversial. Relative to humans, rodents have substantially more BAT and rely heavily on this highly-inducible depot to stimulate thermogenesis ( 327 ). While BAT activation in rodents has been shown to elicit beneficial effects, including improvements in glucose and lipid metabolism ( 328 , 329 ), BAT function in humans is more controversial. Overall, the majority of studies have reported that the amount of active BAT in humans appears insufficient to induce meaningful changes in energy metabolism and, thus, is not thought to impact whole-body physiology and metabolic control in humans ( 330 ) as described in rodents.

With regard to white AT, notable differences exist with respect to fat depot structure and function between species ( 18 ). Humans have subcutaneous depots primarily in the abdominal and gluteal-femoral regions; whereas rodents have subcutaneous fat pads located anteriorly and posteriorly ( Figure 12 ). With regards to location, the inguinal (posterior) fat pad in rodents is considered comparable to the gluteal and femoral depots in humans. Human subcutaneous abdominal AT can be categorized as superficial SAT or deep SAT ( 331 ), which are morphologically and metabolically different. Deep SAT has been reported to be closely related to the pathophysiology of obesity-related metabolic complications, while superficial SAT is more closely related to the protective lower-body SAT ( 332 – 334 ). However, these subcutaneous layers are not present in rodents. In humans, intra-abdominal fat refers to visceral AT, which surrounds the inner organs, and includes omental, mesenteric, retroperitoneal, gonadal, and pericardial depots ( 335 ). For most purposes, however, when used in reference to human studies, visceral AT refers to omental and mesenteric depots that are quantified by abdominal computed tomography or MRI scans. On the other hand, visceral fat pads in rodents are classified as perigonadal (epididymal in males and periovarian in females), retroperitoneal, and mesenteric. While the mesenteric fat pad is most analogous to abdominal (visceral) AT in humans, it is not often studied in rodents due to surgical limitations. The perigonadal fat pads are the largest and most the readily assessable fat in rodents; hence, they are most frequently used in mouse studies and cited the most often in the literature as surrogates for human visceral AT. However, humans do not have an AT depot analogous to the rodent perigonadal fat pads. In addition, the omental depot is clearly defined in humans, but in mice it is difficult to detect. Overall, striking anatomical differences in AT distribution exist between rodents and humans, and these differences should be considered when interpreting rodent studies and potentially translating these observations to humans.

Figure 12. . Rodent versus Human AT depots.

Rodent versus Human AT depots. Several differences exist between rodent and human subcutaneous (SubQ) and visceral AT depots. In the figure SubQ depots are colored as beige, while visceral depots are white, and BAT or BAT-like depots are brown.

It is well-established that the various adipose depots display metabolic heterogeneity and are intrinsically different within each species. In humans, fat deposition in the upper body, mainly the visceral but also the subcutaneous abdominal depot, is linked to a higher risk of metabolic dysfunction; while lower body adiposity in the subcutaneous gluteal and femoral regions is associated with lower risk and may even be protective ( 336 ). Rodent studies reveal that surgical removal of visceral fat pads improves insulin action, glucose tolerance, and longevity ( 337 , 338 ), while the removal of subcutaneous fat pads can cause metabolic syndrome ( 339 ). In addition, subcutaneous, but not visceral, donor AT transplanted into the visceral region of recipient mice improves glucose metabolism ( 340 ). In contrast, human studies have shown that the removal of small amounts of omental AT in individuals with obesity provided no metabolic health benefits ( 341 ). Likewise, liposuction (~10 kg) of subcutaneous AT in humans neither harmed nor improved the cardio-metabolic profile ( 342 , 343 ). Nevertheless, fat is redistributed from the subcutaneous to visceral depots during aging ( 344 ) in conjunction with increasing prevalence of chronic diseases such as hypertension, T2DM, and cardiovascular disease, suggesting that subcutaneous AT may be metabolically beneficial in humans as has been extensively reported in rodents.

Studies of depot-specific expression patterns have enhanced our understanding of the mechanisms underlying abdominal versus gluteal and femoral adiposity ( 345 – 347 ). Unique expression patterns in different adipose tissue depots in mice indicate substantial difference in the expression of homeobox (HOX) developmental genes ( 348 ). Not surprisingly, HOX genes exhibit differential expression patterns in human compared to mouse fat depots ( 346 , 347 ). In contrast, structural and hormonal regulators, including collagen VI ( 349 , 350 ) and glucocorticoids ( 351 , 352 ), respectively, that influence fat distribution are similarly associated with AT expansion in both rodents and humans.

Similar to humans ( 353 ), female rodents have a higher percentage fat mass relative to males, yet remain more insulin sensitive ( 354 ). However, there are many notable sex differences in rodent versus human depots. The inguinal depots of female mice contain mammary glands and the gonadal fat pad is near reproductive tissue, which is not the case in humans. In addition, high-fat diet-induced obesity affect men and women alike, but in many strains of mice females are resistant HFD obesity, unlike male mice ( 355 , 356 ). Furthermore, the periovarian (visceral) fat pad in female mice has been shown to be more insulin sensitive than the inguinal fat pad ( 354 ), which is contrary to human data that indicates in women the gluteal and femoral depots are more insulin sensitive relative to the visceral AT ( 357 ).

Current literature suggests that the secretion patterns of adipokines (including leptin, interleukin-6, and tumor necrosis factor α) in the visceral versus subcutaneous depots of humans are relatively similar to that of rodents. Interestingly, lower body AT has been shown to secrete more metabolically favorable adipokines such as adiponectin ( 358 ). These observations are similar in rodents studies ( 340 ).

While lipolysis can be stimulated in rodents and humans under similar physiological conditions, important biological differences in AT lipolysis among these species have been suggested. The β1 and β2 adrenergic receptors (AR) are ubiquitously expressed in rodents and humans, while β3-AR expression is confined to white AT in rodents and only marginally expressed in human adipocytes ( 359 ). The α2-ARs are highly expressed in the subcutaneous AT of humans and act to inhibit lipolysis ( 360 ), but are absent in rodent adipocytes. Though common factors, including catecholamines, growth hormone, and cortisol, are similar among species in regulating lipolysis, differences in the response to other lipolytic agents have also been reported. Natriuretic peptide induces lipolysis in humans, but not in rodents ( 361 ), while adrenocorticotropic hormone and alpha-melanocyte-stimulating hormone modulate lipolysis in rodent but not human adipocytes ( 362 , 363 ). Therefore, it is important to account for these differences and commonalities in AT lipolysis among species.

Rodent studies are essential to expand our understanding of pathways underlying the associations between fat distribution and metabolic health and disease. Fortunately, there are many shared traits among rodent fat pads and human fat depots. However, given the clear differences in adipose depot location and physiology between the species, interpretation of experimental data and the extrapolation of conclusions drawn from rodent data to humans should be conducted with appropriate caution and caveats.

Dermal Adipose Tissue

A thick layer of adipocytes, historically referred to as subcutaneous AT, underlies the reticular dermis in both rodents and humans ( 364 ). Recent studies have revealed major differences between the adipocytes from this dermal layer and more typical subcutaneous adipocytes found in other locations ( 364 – 366 ). Today, dermal adipose tissue (dWAT) is considered a separate adipose depot that is distinguishable from subcutaneous fat ( 364 ). Two unique features of dermal adipocytes in this regard are that they can alter their cellular characteristics and have high turnover rates ( 366 ). An additional distinguishing factor for dWAT is its organization. In rodents, dWAT forms several adipocyte layers between the dermis and muscle layer (panniculus carnosus) ( 367 ). Human dWAT is present as individual units referred to as dermal cones. These cones are concentrated around pilosebaceous units that functionally interact with each other to form the dWAT structure ( 366 , 368 ). Interestingly, only body regions prone to scarring contain dermal cones ( 368 ), indicating a potential role for dWAT in scarring and wound healing. Also, dWAT can regenerate after injury. Following injury, adipogenesis is activated in the proliferative phase of wound healing and dermal adipocytes repopulate the wound ( 366 , 367 ). This is a critical event, as mouse models lacking mature adipocytes cannot recruit the fibroblasts required for wound healing ( 369 – 371 ).

Other identified roles for dWAT include insulation ( 372 ), barrier protection from skin infection ( 373 ), and hair follicle cycling ( 374 ). It is well known that brown adipose tissue (BAT) rapidly responds to cold temperature challenges by mobilizing lipids for heat generation (adaptive thermogenesis), yet dWAT slowly responds to these challenges by thickening/expanding over days to provide an effective layer of insulation ( 367 , 372 ). Mouse models lacking adequate dWAT undergo chronic activation of BAT since the dWAT cannot provide adequate mitigation of body temperature ( 367 ). Conversely, obese mice with excess dWAT undergo minimal adaptive thermogenesis ( 367 ). The dWAT thickening observed with cold exposure also occurs with bacterial exposure. Adipocytes in dWAT differentiate and become hypertrophic and result in a thicker dWAT layer in response to epidermal Staphylococcus aureus. This dWAT adipocyte reaction is also critical for immune response to bacterial invasion ( 373 ). Hair follicles go through repeated rounds of death and regrowth, referred to as the hair follicle cycle ( 367 ). Robust dWAT expansion is characterized by increased adipogenesis and dermal adipocyte hypertrophy that accompanies the regrowth of hair follicles ( 374 ). Conversely, inhibiting adipogenesis impedes hair follicle regeneration. In several species of mammals, a thickening of the hair coat accompanies dWAT expansion in response to cold exposure ( 367 ). In summary, dWAT has distinct roles from subcutaneous AT. Thus far, unlike other AT depots, the contribution of dWAT to metabolic health has not been investigated. Nonetheless, there is clear evidence that dWAT has distinct structures and functions and plays a role in variety of physiological processes.

Epicardial AT

Epicardial AT (EAT) has recently emerged as an important player in the development of cardiovascular disorders ( 375 , 376 ). Notably, EAT is distinct from pericardial fat. While pericardial AT surrounds the pericardium, EAT lies between the visceral pericardium and the myocardium and shares a blood supply with the coronary arteries ( 375 – 378 ). The adipocytes in EAT are smaller than those in other visceral or subcutaneous depots and are outnumbered by preadipocytes; this is thought to be related to the high energy requirement of the heart, which normally favors oxidation of fatty acids over other substrates ( 376 , 379 ). Furthermore, the gene expression and adipokine secretion profiles of EAT are unique from those of other depots ( 376 , 380 , 381 ).

In normal physiological conditions, EAT behaves like BAT and serves to protect the coronary vessels and myocardium against hypothermia ( 376 , 382 ). In pathologies such as coronary artery disease and type 2 diabetes, EAT can display an extensive pro-inflammatory signature ( 383 – 385 ). Macrophages and mast cells have been shown to infiltrate EAT, undergo activation, and through a cascade of signaling events facilitate lipid accumulation in atherosclerotic plaques ( 376 , 384 ). Pro-inflammatory adipokine secretion from EAT has also been shown to induce atrial fibrosis ( 381 ). Further, insulin sensitivity and EAT thickness are inversely correlated, whereas fasting glucose and EAT size are positively correlated, with enlarged EAT depots often found in individuals with type 2 diabetes ( 376 , 386 , 387 ). These data suggest that EAT functions as a distinct fat depot with important physiological and pathological roles.

  • Metabolic dysfunction associated with Adipose Tissue

Adipose Tissue Expandability and Metabolic Health

White AT retains the ability to expand during adult life to accommodate chronic excess caloric intake. AT expansion is characterized by adipocytes accumulating lipid and growing in size (hypertrophy) or number (hyperplasia or adipogenesis) or increasing in both size and number. Evidence suggests that the capacity of subcutaneous AT to expand as well as the manner of expansion (hypertrophy vs. hyperplasia) can influence cardiometabolic health. This mechanism is thought to underlie the benefits of thiazolidinedione (TZD) medications, which are approved for the treatment of type 2 diabetes ( 388 , 389 ). These PPARγ agonists stimulate preadipocyte differentiation and the proliferation of adipocytes ( 390 , 391 ), especially in subcutaneous depots as compared to visceral adipose tissue ( 392 ), which leads to increased adiponectin levels and improved insulin sensitivity ( 393 , 394 ). Hence, there is a clear rationale to further characterize the mechanisms of AT expansion through adipocyte proliferation in humans that may inform future effective drug therapies.

On the other hand, the presence of enlarged, hypertrophic adipocytes, a lack of hyperplasia, and development of AT inflammation and fibrosis reflect impaired AT expansion and is associated with metabolic derangements ( 395 – 398 ). These observations support the “AT expandability hypothesis”, which postulates that a lack of adipogenesis (or hyperplasia) results in the limited capacity of AT to expand and store lipid, causing ectopic fat accretion and “lipotoxicity” in non-adipose tissues such as skeletal muscle and liver ( 399 – 401 ). The degree of ectopic lipid deposition in the liver and skeletal muscle is a significant determinant of metabolic syndrome (MetS) and the development of T2D and CVD ( 402 ).

Other findings do not support the AT expandability hypothesis and indicate that higher adipogenesis does not necessarily denote improvements in metabolic health. These studies report a higher population of small adipocytes (a measure of hyperplasia) in the AT of individuals with insulin resistance and T2D ( 403 – 406 ) and in those with more visceral AT and liver fat ( 406 , 407 ). Experimental overfeeding intervention studies have shown that individuals with smaller adipocytes at baseline have poorer metabolic health outcomes (i.e. impaired insulin sensitivity) in response to substantial weight gain than those with larger adipocytes ( 408 , 409 ). In addition, one in vivo analysis in humans demonstrated that increased hyperplastic expansion correlated with an increased number of metabolic syndrome components ( 410 ). Collectively, these data imply an alternative model of impaired AT expansion, as compared to the mechanisms proposed by the AT expandability hypothesis, and suggest that there is not a deficiency in hyperplasia but an abundance of adipocytes with a limited capacity to adequately expand and accommodate lipid, whether large or small. This inability to store excess lipid in AT is thought to be a key feature that leads to metabolic dysfunction.

Although the mechanisms of adipose expansion and its precise role in promoting glucolipid dysregulation remain a matter of debate, all of the aforementioned studies support the view that AT’s capacity to expand is intimately related to metabolic homeostasis, as the failure to store excess lipid appropriately in AT can contribute to many obesity-related complications.

AT Inflammation

A variety of cell types from both the innate and adaptive immune systems have been found in AT ( 411 – 413 ). Though resident AT immune cells are critical to normal adipocyte function in healthy individuals, AT inflammation, as mentioned in several preceding sections, is considered a major contributor to the metabolic dysfunction associated with obesity ( 413 , 414 ).

During nutrient excess as AT expandability reaches its limit, a strong association exists between adipocyte size and adipocyte death ( 415 ). In response to adipocyte death, pro-inflammatory macrophages surround dead and dying cells and remove debris from the damaged area. During this process, macrophages acutely produce inflammatory cytokines ( 413 , 416 ). In obesity, this cytokine production often fails to resolve, becomes chronic, and leads to impaired adipocyte insulin signaling, further inflammation, and a continued worsening of AT dysfunction ( 413 , 416 , 417 ). In a field that is rapidly changing, it is worth mentioning that some degree of inflammatory signaling might be required for normal AT function. The pro-inflammatory cytokines TNF alpha and oncostatin M have been shown to be required for proper AT expansion and maintenance of insulin sensitivity in mice ( 414 , 418 – 420 ). Although AT inflammation clearly has detrimental effects in obesity, evidence also indicates adaptive and homeostatic roles for pro-inflammatory signaling in AT expansion and function.

Metabolically Healthy (MHO) versus Metabolically Unhealthy (MUO) Obesity

An estimated 10-30% of individuals with obesity are considered to have “metabolically healthy obesity” (MHO) with favorable metabolic profiles ( 421 ). Although there is currently no consensus for parameters used to classify MHO, these individuals are characterized by normal insulin sensitivity, normal fasting glucose levels, low incidence of hypertension, and blood lipid profiles in the healthy range ( 422 , 423 ) ( Figure 13 ). In contrast, individuals with “metabolically unhealthy obesity” (MUO) have comparable body mass indices (BMI) but develop metabolic aberrations. Factors that distinguish individuals with MHO from MUO ( Figure 13 ) highlight the premise that metabolic health risk is not solely dependent on body weight and are described in more detail below. Understanding these characteristics and potential mechanisms underlying the MHO and the perceived healthy metabolic state of these individuals is an important area of ongoing research.

Figure 13. . Clinical and biological factors thought to distinguish metabolically healthy obesity (MHO) from metabolically unhealthy obesity (MUO).

Clinical and biological factors thought to distinguish metabolically healthy obesity (MHO) from metabolically unhealthy obesity (MUO). Abbreviations: VAT – Visceral AT, SubQ AT – Subcutaneous AT, EMCL - extramyocellular lipid; IMCL – intramyocellular lipid; HDL – high density lipoprotein.

Evidence suggests that WAT plays a critical role in the development of MHO vs MUO, as its properties, location, and function are closely linked with cardiometabolic risk. Fat distribution ( 422 ), as well as changes associated with AT expansion, including the capacity for adipocyte differentiation ( 403 ) and parameters related to ECM remodeling ( 424 ), may also contribute to the MHO phenotype. In addition, adipose-derived circulating factors that impact whole-body metabolism have been implicated in MHO vs MUO differences ( 425 ). However, studies have shown that the location of AT, rather than overall obesity, may be a stronger predictor of metabolic health risks ( 336 ). The accumulation of upper-body fat, namely visceral AT (VAT) but also subcutaneous abdominal (scABD) adipose tissue, confers a higher risk of obesity-related disorders ( 426 ), while lower-body fat (subcutaneous gluteal and femoral) may be metabolically protective ( 427 ). The preserved metabolic function of individuals with MHO may be attributed to significantly lower accumulation of VAT relative to MUO ( 422 , 428 , 429 ). As described in the previous section, enlarged adipocyte size, independent of adiposity, is positively correlated with the development of insulin resistance and impaired metabolic health ( 396 ). MUO individuals have been shown to have larger adipocytes than their MHO counterparts ( 430 , 431 ). Hypertrophic adipocytes may represent the failure of subcutaneous AT to expand and store excess fat, which can ultimately lead to ectopic lipid deposition in non-adipose tissues such as the liver and skeletal muscle ( 402 ).

Ectopic lipid accumulation in both the liver and skeletal muscle is of pathophysiological significance as part of the “lipotoxicity” hypothesis and may also impact the varying health risk of MHO vs MUO. Extramyocellular lipid (EMCL) and intramyocellular lipid (IMCL) are postulated to cause defects in insulin signaling and reduce insulin-stimulated skeletal muscle glucose uptake ( 432 ). These lipid stores are strong correlates of insulin resistance and are increased in individuals with T2D ( 402 ). Paradoxically, increased IMCL is also observed in ‘insulin sensitive’ athletes, which may be attributed to the oxidative capacity of skeletal muscle ( 433 ) and increased glucose transport in trained muscle ( 434 ). Intrahepatic lipid accumulation strongly associates with impaired insulin-induced suppression of hepatic glucose production, even independently of visceral AT amount, and the development of T2D ( 435 ). Ectopic fat in both the liver and skeletal muscle has been shown to be lower in MHO than MUO individuals ( 422 , 436 , 437 ) (refer to Figure 13 ).

The differential secretion of pro-inflammatory adipokines has also been proposed as a mechanism underlying the MHO phenotype ( 438 – 440 ) by some investigators, although others have reported conflicting results ( 441 ). Nevertheless, studies show reduced macrophage infiltration in MHO ( 442 , 443 ), supporting a reduced inflammatory state in these individuals. Intriguing data implicating potential genetic differences among MHO vs. MUO indicate that specific polymorphisms in genes, including the adiponectin receptor 1 and hepatic lipase, may be associated with the MHO phenotype ( 436 ). In addition, genes encoding some proinflammatory cytokines can be more highly expressed in the adipose tissue of MUO compared with MHO individuals ( 444 , 445 ).

A lingering question that remains unanswered is whether MHO subjects will sustain a healthy metabolic state throughout their lifespan or if they will eventually become MUO. An additional question is if a healthy lifestyle can help to maintain a favorable profile and prevent the transition to MUO. Indeed, longitudinal data clearly show that not all MHO individuals remain metabolically healthy, as up to 30% progress to MUO over a 5-10 year time frame ( 446 – 448 ). Of note, the length of time for follow-up assessments of MHO individuals is an important factor that may have considerable effects on the observed outcomes, because the total number of years as obese and aging can independently increase mortality risk. A major obstacle in advancing the understanding of the MHO phenotype is the manner by which metabolic health is described, including the parameters used to define insulin sensitivity and metabolic syndrome ( 449 ). Defining metabolic outcomes based on differing criteria can result in a broad range of reported prevalence, discrepancies regarding the observed characteristics, varied interpretations of health and mortality risks, and disagreement concerning the implications of therapeutic interventions. In addition, the use of the “healthy” descriptor may be misrepresentative of the true medical risks to these individuals, as long-term adverse health outcomes have been observed in individuals with MHO during follow-up years, thus no longer characterizing them as “metabolically healthy” ( 450 , 451 ). Additional long-term prospective studies are necessary to assess features of the MHO phenotype and to observe how the factors discussed above are altered over time. In addition, these studies may reveal if WAT function is a cause or consequence of the MHO and MUO phenotypes.

Lipodystrophy

While excessive adiposity, or obesity, can have adverse health consequences, deficiency of AT mass, as seen in lipodystrophy, can also lead to derangements in glucolipid metabolism. Lipodystrophy encompasses a group of rare, heterogeneous, genetic or acquired disorders characterized by varying degrees of severe reduction or absence of body fat ( 452 ) . Anatomically, this disease can present as a partial (i.e. localized to certain body areas) or generalized lack of AT. The combined overall prevalence of lipodystrophy is estimated to be 1 in 1,000,000 individuals, with ~1000 patients reported with genetic forms ( 453 ). Lipodystrophy associated with highly active antiretroviral therapy for HIV is one of the most common acquired forms worldwide ( 454 ). The diagnosis of this disorder mostly relies on clinical criteria. In most cases of generalized lipodystrophy, standard physical examination is sufficient to establish this diagnosis. In contrast, partial lipodystrophy may be represented by mild physical abnormalities and can sometimes be misdiagnosed as common forms of central (abdominal) obesity, suggesting that this form of lipodystrophy may be an underestimated condition ( 455 ). Although the pathological basis of most lipodystrophies remains unclear, it is well-accepted that AT dysfunction is a primary determinant of the resulting health consequences in these patients. Limited development and non-expandability of AT and failure of AT to accommodate excess lipid leads to the redistribution and storage of fat ectopically in the liver and skeletal muscle and the development of non-alcoholic fatty liver disease, often severe insulin resistance and type 2 diabetes, hypertriglyceridemia, and associated diseases ( 456 – 458 ).

Markedly reduced levels of leptin and adiponectin may also contribute to the pathology of lipodystrophy. As described above, leptin plays an important role in the regulation of body weight and energy metabolism, and leptin deficiency is common in lipodystrophic patients, due to the lack of AT ( 452 ). Low leptin levels can not only impact glucose metabolism ( 459 ) but also contribute to increased appetite and excessive caloric intake in these patients ( 460 ). Transgenic animal models shed light on the pathology of lipodystrophy and confirm the importance of AT in normal physiology. Fatless rodents, created via AT ablation, display hypertriglyceridemia and ectopic lipid accumulation, along with severe insulin resistance ( 457 , 461 ). In addition, several groups have successfully treated the metabolic derangements in these fatless mice by transplanting AT from wild-type animals ( 461 – 464 ). However, transplantation of AT from leptin-deficient mice did not improve the metabolic abnormalities in fatless mice ( 465 ), while leptin administration in the fatless mice ameliorated insulin resistance and hepatic steatosis ( 466 ). In humans with total lipodystrophy, leptin treatment also markedly improves the severe hypertriglyceridemia and insulin resistance that accompanies this disorder ( 467 ). These studies confirm that both AT and leptin deficiency play a central role in lipodystrophy-associated pathologies.

Adiponectin has insulin-sensitizing and anti-inflammatory effects, and low levels of this adipokine have also been observed in patients with lipodystrophies ( 468 ). Recombinant adiponectin, adiponectin analogues (i.e. osmotin), and compounds that upregulate endogenous adiponectin (i.e. TZDs) have all been proposed as treatment approaches for lipodystrophy ( 469 ). In a fatless mouse model, treatment with the globular domain of adiponectin significantly improved the hyperglycemia and hyperinsulinemia characteristic of these lipoatrophic diabetic mice ( 146 ). Interestingly, the insulin resistance observed in these mice was completely ameliorated by treatment with both adiponectin and leptin, but only partially by either adiponectin or leptin alone ( 146 ), suggesting that both adiponectin and leptin deficiency may contribute to the insulin resistance in humans with lipodystrophy.

Studies to date support the premise that too little AT, as seen in lipodystrophy, appears to be just as deleterious as too much AT. Emerging data reveal that patients with lipodystrophy may have reduced survival and high mortality at an early age, predominantly due to cardiometabolic complications ( 470 – 472 ). Lipodystrophy has no cure; therefore, the primary treatment option is to improve metabolic outcomes via physical activity and dietary and pharmacological interventions. Conventional insulin-sensitizing agents, such as metformin and TZDs, are often used ( 453 , 473 ), and leptin replacement is also an approved therapy for total congenital lipodystrophy ( 467 ). Future investigations to better understand the pathogenesis and the clinical manifestation of lipodystrophy syndromes are essential for the development of improved therapeutics.

Adipose Tissue and Reproduction

While many studies have primarily examined the influence of white AT on the metabolic consequences associated with obesity, less frequently mentioned is the interplay between AT and reproductive health. Nevertheless, it is well established that AT is important for the normal function of the reproductive system, including the production and regulation of sex and reproductive hormones, pubertal development, and the maintenance of pregnancy and lactation ( 474 ).

Leptin and adiponectin are the most investigated adipokines as mediators of reproductive health and pathology. Receptors for both leptin and adiponectin have been identified in all major reproductive tissues, including the testes, placenta, ovaries, oviducts and endometrium ( 475 ). Obese mice that are deficient in leptin or the leptin receptor are unable to reproduce ( 476 ). Although rare, humans with leptin mutations have been identified, and studies in these individuals have validated the infertility findings in rodents ( 477 ).

Leptin administration in rodents was shown to increase luteinizing hormone (LH), follicle stimulating hormone (FSH), and ovarian and uterine weights in females, and testosterone, testicular weights and sperm counts in males ( 478 , 479 ). During human pubertal development, there is a steady increase in leptin, stimulating a rise in testosterone levels and fat-free mass in boys and in estradiol and fat mass in girls ( 480 , 481 ). Adiponectin administration was shown to inhibit gonadotrophin releasing hormone (GnRH), LH, and FSH ( 482 , 483 ) in pigs and increase estrogen and progesterone ( 484 ) in rats, while circulating levels in humans have been shown to be associated with serum levels of sex hormones (primarily estrogens), though this correlation was largely mediated by body weight ( 485 ).

In humans, leptin levels increase during pregnancy and rapidly fall in the post-partum period ( 486 ), and other reports have suggested that adiponectin may influence the amount of gestational weight gain and weight maintenance post-partum, even after adjusting for the sum of skinfold thickness and BMI ( 487 , 488 ). The effects of leptin on fetal development continue to be investigated, and have been suggested to correlate with fetal growth, birth weight, and organogenesis ( 489 ). Adiponectin plasma levels were shown to be significantly lower in overweight patients than normal weight women during pregnancy and negatively correlated with progressive gestational age and weight gain ( 490 ). In addition, women with low adiponectin concentrations experienced a significantly increased risk of gestational diabetes mellitus ( 491 , 492 ), and large reductions in adiponectin levels during pregnancy may also predict large-for-gestational-age offspring and increased birth weight ( 493 ). Interestingly, several studies have shown that higher adiponectin levels are associated with increased conception success in women undergoing assisted reproduction approaches ( 494 ). Overall, these studies are consistent with adipose-derived leptin and adiponectin having critical roles in reproductive function.

Many lines of evidence also demonstrate that either insufficient or excessive AT can have detrimental effects on reproductive health. Women with lipodystrophy disorders (see above), are characterized by AT and leptin deficiency and are frequently infertile ( 495 , 496 ). Anorexia nervosa is an eating disorder characterized by very low AT mass that is often accompanied by amenorrhea (absence of at least three menstrual periods in a row) ( 497 ). It is estimated that ~38% of women affected by anorexia experience infertility ( 498 ). Leptin deficiency is common in these patients ( 499 ) and may lead to disruptions in downstream neuroendocrine signaling ( 500 ). This was tested when leptin replacement to women with hypogonadotrophic hypogonadism due to anorexia nervosa or excessive exercise was found to restore normal periods ( 501 ). Estrogen deficiency in these women results in major implications for bone health, ultimately contributing to increased osteopenia or osteoporosis ( 502 ).

Body weight has been shown to predict testosterone levels in men ( 503 ); and obesity, specifically central adiposity, is associated with low testosterone levels ( 504 ). Increased AT also leads to elevated estradiol, resulting in reduced circulating testosterone through feed-back inhibition of gonadotrophs ( 504 , 505 ). A common medical condition in women at the crossroads of dysfunctional AT and reproduction is polycystic ovary syndrome (PCOS), which in roughly 50% of affected women is associated with increased central obesity and metabolic health risk. PCOS is commonly defined using the consensus of Rotterdam, which requires two of three criteria: polycystic ovaries on ultrasonography, hyperandrogenism, or amenorrhea. Studies of PCOS generally show that adiponectin levels are lower in these patients ( 506 ). Another burgeoning area of research is the study of excessive AT and reproductive malignancies, as obesity is known to increase the risk of breast, uterine, cervical, and prostate cancers ( 507 ). Studies have reported inverse relationships between leptin and adiponectin levels with breast, endometrial, ovarian, and prostate cancers ( 508 , 509 ).

  • Emerging Areas in Adipocyte Biology

Critical considerations in the study of fat tissue are its cellular complexity and heterogeneity. AT depots can exist in close association with other organs and act physiologically as metabolic “sinks” that store excess energy as lipid in a protective manner, or they can promote systemic metabolic dysfunction by secreting excess lipid or inflammatory adipokines. As the recognition of distinct AT depots increases, so does our understanding of their diversity. A recent review considers the locations and functions of several depots, ranging from facial AT to cardiovascular AT as well as the presence of adipocytes in bone marrow, within and between muscle beds, and joints ( 510 ). Currently, we are experiencing a new and exciting period in AT research with the focus shifting toward recognizing neglected AT depots, the expanding types of adipocytes, and the complex developmental and sex-regulated origins of adipocytes. Adipocytes are critical secretory cells that contribute a variety of circulating proteins, including endocrine hormones. Of course, adipocytes also produce lipids and can release genetic material that can have profound systemic functions.

Much remains to be discovered about the types of nerves present in fat tissue and how they vary according to AT type and location. How these AT nerves act to regulate metabolic homeostasis is a current focus of fat cell biology. Recent advances in whole tissue AT imaging and studies on brain-adipose communication suggest we are just beginning to uncover the capabilities and function of AT nerves, and there are many unanswered questions in this field ( 511 ). Research on the molecular pathways that connect AT innervation to insulin action in obesity and diabetes may provide insight into our understanding of the pathogenesis of metabolic disease states.

Another developing area of fat cell biology is the effects of exercise on adipocyte function. Recent studies have shown that transplantation of subcutaneous AT from exercise-trained mice improves glucose tolerance and insulin sensitivity in recipient, non-exercised mice ( 512 ), and strongly suggest that exercise favorably remodels AT to improve systemic metabolic health. Recently, an AT-derived lipid was shown to increase fatty acid uptake in skeletal muscle ( 513 ). The importance of AT to whole-body energy metabolism is well established; yet, the impacts of different types of endurance or resistance exercise on adipose tissue dynamics remains largely understudied, particularly in the context of obesity and other metabolic disease states.

A newly discovered pathway shows that lipids can be released by adipocytes in the form of exosome-sized, lipid-filled vesicles ( 514 ). This process occurs independently of canonical lipolytic pathways, and adipocyte exosomes deliver excess lipid to local macrophages in obesity ( 514 ). Other novel pathways of paracrine regulation have also been demonstrated in AT. These paradigm-shifting observations demonstrated that extracellular vesicles (EV) from endothelial cells in adipose tissue can provide lipids and proteins to adjacent adipocytes. This EV communication between endothelial cells and adipocytes within AT is bi-directional and is regulated by fasting/refeeding and in conditions of obesity ( 515 ). These very recent observations reveal the highly complex signaling mechanisms that exist in AT.

Though it was once considered a mere energy storage site, AT is now considered an important endocrine organ and site of inflammatory cell signaling that governs not only survival but also plays critical roles in reproduction and in glucometabolic homeostasis. As scientific methods for the study of AT continue to rapidly evolve, so does our understanding regarding the metabolic, biomechanical, immune, and secretory functions of AT in normal physiology and metabolic disease.

  • Acknowledgements

The authors are grateful to Anik Boudreau and Christina Zunica for their assistance in editing and referencing the chapter. This work was supported by National Institutes of Health Grant R01 DK052968.

This electronic version has been made freely available under a Creative Commons (CC-BY-NC-ND) license. A copy of the license can be viewed at http://creativecommons.org/licenses/by-nc-nd/2.0/ .

  • Cite this Page Richard AJ, White U, Elks CM, et al. Adipose Tissue: Physiology to Metabolic Dysfunction. [Updated 2020 Apr 4]. In: Feingold KR, Anawalt B, Blackman MR, et al., editors. Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc.; 2000-.

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Remodeling of white adipose tissue microenvironment against obesity by phytochemicals

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  • 1 Institute of Pharmacology, Department of Pharmaceutical Sciences, Zhejiang University of Technology, Hangzhou, People's Republic of China.
  • 2 Research and Development Department, Zhejiang Starry Pharmaceutical Co., Ltd., Taizhou, People's Republic of China.
  • PMID: 36786412
  • DOI: 10.1002/ptr.7758

Obesity is a kind of chronic disease due to a long-term imbalance between energy intake and expenditure. In recent years, the number of obese people around the world has soared, and obesity problem should not be underestimated. Obesity is characterized by changes in the adipose microenvironment, mainly manifested as hypertrophy, chronic inflammatory status, hypoxia, and fibrosis, thus contributing to the pathological changes of other tissues. A plethora of phytochemicals have been found to improve adipose microenvironment, thus prevent and resist obesity, providing a new research direction for the treatment of obesity and related diseases. This paper discusses remodeling of the adipose tissue microenvironment as a therapeutic avenue and reviews the progress of phytochemicals in fighting obesity by improving the adipose microenvironment.

Keywords: adipose microenvironment; fibrosis; hypoxia; obesity; phytochemicals.

© 2023 John Wiley & Sons Ltd.

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Characteristics of Abdominal Visceral Adipose Tissue, Metabolic Health and the Gut Microbiome in Adults

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Mueez U-Din, Basma A Ahmed, Saad A Syed, Frank J Ong, Stephan M Oreskovich, Elizabeth Gunn, Michael G Surette, Zubin Punthakee, Gregory R Steinberg, Katherine M Morrison, Characteristics of Abdominal Visceral Adipose Tissue, Metabolic Health and the Gut Microbiome in Adults, The Journal of Clinical Endocrinology & Metabolism , Volume 109, Issue 3, March 2024, Pages 680–690, https://doi.org/10.1210/clinem/dgad604

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Compared with the relatively benign effects of increased subcutaneous adipose tissue (SAT), increased visceral adipose tissue (VAT) volume is a causal risk factor for hypertension, hyperlipidemia, type 2 diabetes, and cardiovascular disease. In rodents, increased VAT volume and triglyceride density and ectopic lipid accumulation in kidneys and liver have been induced by alterations in the gut microbiome. However, few studies have characterized these relationships in humans.

To evaluate the tissue triglyceride content of VAT and SAT, liver, kidneys, and pancreas in male and female adults and assess associations with markers of glucose tolerance, serum insulin, and lipids and characteristics of the gut microbiome.

Cross-sectional observational study of healthy human adults (n = 60) at a clinical research center. Body mass index (BMI), body composition, and oral glucose tolerance were assessed. Microbiome analysis was conducted on stool samples using 16S rRNA v3 amplicon sequencing. The triglyceride content of VAT, SAT, liver, kidney and pancreas were determined by assessing proton density fat fraction (PDFF) with magnetic resonance imaging (MRI).

Higher VAT PDFF and the ratio of VAT to SAT PDFF were related to higher BMI, HbA1c, HOMA-IR, non-high–density lipoprotein cholesterol, plasma triglycerides, low-density lipoprotein (LDL) cholesterol, and lower high-density lipoprotein (HDL) cholesterol. A higher VAT PDFF and VAT to SAT PDFF ratio were associated with lower alpha diversity and altered beta diversity of the gut microbiome. Differences in VAT were associated with higher relative abundance of the phylum Firmicutes, lower relative abundance of the phylum Bacteroidetes, and enrichment of the bacterial genera Dorea , Streptococcus , and Solobacterium .

VAT PDFF measured with MRI is related to impaired glucose homeostasis, dyslipidemia, and differences in the gut microbiome, independently of the total body fat percentage.

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  • Published: 04 January 2021

Brown adipose tissue is associated with cardiometabolic health

  • Tobias Becher   ORCID: orcid.org/0000-0001-7670-757X 1 , 2 , 3 ,
  • Srikanth Palanisamy 1 , 4 ,
  • Daniel J. Kramer 1 , 4 , 5 ,
  • Mahmoud Eljalby   ORCID: orcid.org/0000-0003-4951-7257 1 , 4 ,
  • Sarah J. Marx 1 ,
  • Andreas G. Wibmer 6 ,
  • Scott D. Butler 7 ,
  • Caroline S. Jiang 8 ,
  • Roger Vaughan 4 , 8 ,
  • Heiko Schöder 6 ,
  • Allyn Mark 9 &
  • Paul Cohen   ORCID: orcid.org/0000-0002-2786-8585 1  

Nature Medicine volume  27 ,  pages 58–65 ( 2021 ) Cite this article

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White fat stores excess energy, whereas brown and beige fat are thermogenic and dissipate energy as heat. Thermogenic adipose tissues markedly improve glucose and lipid homeostasis in mouse models, although the extent to which brown adipose tissue (BAT) influences metabolic and cardiovascular disease in humans is unclear 1 , 2 . Here we retrospectively categorized 134,529 18 F-fluorodeoxyglucose positron emission tomography–computed tomography scans from 52,487 patients, by presence or absence of BAT, and used propensity score matching to assemble a study cohort. Scans in the study population were initially conducted for indications related to cancer diagnosis, treatment or surveillance, without previous stimulation. We report that individuals with BAT had lower prevalences of cardiometabolic diseases, and the presence of BAT was independently correlated with lower odds of type 2 diabetes, dyslipidemia, coronary artery disease, cerebrovascular disease, congestive heart failure and hypertension. These findings were supported by improved blood glucose, triglyceride and high-density lipoprotein values. The beneficial effects of BAT were more pronounced in individuals with overweight or obesity, indicating that BAT might play a role in mitigating the deleterious effects of obesity. Taken together, our findings highlight a potential role for BAT in promoting cardiometabolic health.

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research paper on white adipose tissue

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Average monthly temperatures in Central Park were obtained from the National Weather Service at https://www.weather.gov/media/okx/Climate/CentralPark/monthlyannualtemp.pdf . Anonymized clinical data are available upon request, subject to an internal review by T.B., P.C., A.G.W. and H.S. to ensure that the participants’ anonymity and confidentiality are protected and completion of a data sharing agreement, and in accordance with The Rockefeller University and Memorial Sloan Kettering Cancer Center Institutional Review Board and Institutional Guidelines. Source data are provided with this paper.

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Acknowledgements

We thank M.N. Singer and C. Poon for expert IT support and data extraction; R. Teng for assistance in accessing data; and B.S. Coller, A.J. Dannenberg, J.J. Moslehi and M.D. Curtis for valuable discussions during the preparation of this manuscript. T.B. was supported in part by the National Center for Advancing Translational Sciences, NIH, through The Rockefeller University (grant UL1TR001866). P.C. was supported by the Sinsheimer Foundation and by the American Diabetes Association Pathway Program Accelerator Award (grant 1-17-ACE-17).

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Laboratory of Molecular Metabolism, The Rockefeller University, New York, NY, USA

Tobias Becher, Srikanth Palanisamy, Daniel J. Kramer, Mahmoud Eljalby, Sarah J. Marx & Paul Cohen

DZHK (German Centre for Cardiovascular Research), Partner Site Heidelberg/Mannheim, Mannheim, Germany

Tobias Becher

Division of Cardiology, First Department of Medicine, University Medical Center Mannheim, Mannheim, Germany

Weill Cornell Medicine, New York, NY, USA

Srikanth Palanisamy, Daniel J. Kramer, Mahmoud Eljalby & Roger Vaughan

Weill Cornell/Rockefeller/Sloan Kettering Tri-Institutional MD-PhD Program, New York, NY, USA

Daniel J. Kramer

Department of Radiology, Memorial Sloan Kettering Cancer Center, New York, NY, USA

Andreas G. Wibmer & Heiko Schöder

Department of Biomedical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY, USA

Scott D. Butler

Center for Clinical and Translational Science, The Rockefeller University, New York, NY, USA

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Contributions

P.C. and T.B. designed and conceived the study. T.B., S.J.M., A.G.W., S.D.B. and H.S. acquired data. P.C., T.B., S.P., M.E., A.G.W., R.V., H.S., C.S.J. and A.M. analyzed and interpreted the data. T.B, D.J.K., M.E. and P.C. wrote the manuscript with input from all authors.

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Correspondence to Paul Cohen .

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Extended data

Extended data fig. 1 number of 18 f-fdg pet/ct scans and brown fat prevalence between 06/2009 - 03/2018..

a , The total number of 18 F-FDG PET/CT scans performed at MSKCC between 2009 and 2018 and prevalence of reported brown fat. Dots depict total number of 18 F-FDG PET/CT scan in each year, half filled dots illustrate years in which only part of the year was analyzed. Bars represent prevalence of 18 F-FDG PET/CT scans with reported brown fat.

Source data

Extended data fig. 2 correlation between brown fat prevalence and body mass index and outside temperature..

a , Correlation between body mass index and prevalence of brown fat reported on 18 F-FDG PET/CT. b , Correlation between outside temperature in the month of the scan and prevalence of brown fat reported on 18 F-FDG PET/CT.

Extended Data Fig. 3 Index scan position and cancer treatment.

a , Percentage of patients with BAT (BAT+) and without BAT (BAT-) receiving cancer therapy within 90 days of the index scan. Number in parenthesis indicates position of index scan. b , Percentage of patients with BAT (BAT+) and without 18 F-FDG uptake on PET in regions corresponding to fat on CT (BAT-) and the position of the index scan depicted in parenthesis.

Extended Data Fig. 4 Standardized mean differences before and after propensity score matching.

Propensity score matching was assessed by comparing standardized mean differences before and after the matching process. The blue shaded area indicates standardized mean differences between 0.1 and −0.1.

Extended Data Fig. 5 Assocation of brown fat and cardiometabolic disease and additional adjustment for type II diabetes or index scan position.

a, Forest plots illustrate the association between brown fat status and cardiometabolic disease in the propensity score matched cohort, with additional adjustment for T2DM. Circles and bars represent odds ratios (ORs) and 95% confidence intervals (CIs) respectively. b , Forest plots illustrate the association between brown fat status and cardiometabolic disease in the propensity score matched cohort, with additional adjustment for index scan position. Circles and bars represent odds ratios (ORs) and 95% confidence intervals (CIs), respectively.

Extended Data Fig. 6 Association of brown fat and cardiometabolic disease in the entire cohort.

a, Association between brown fat and cardiometabolic disease in the entire study cohort. b , Forest plots illustrate the association between brown fat status and cardiometabolic disease in the entire study cohort. Circles and bars represent odds ratios (ORs) and 95% confidence intervals (CIs), respectively.

Extended Data Fig. 7 Examples of 18 F-FDG uptake on PET in regions corresponding to adipose tissue on CT in patients with cancers of the head and neck.

a , 52-year old, female patient with keratinizing squamous cell carcinoma of the gums (green arrow). 18 F-FDG PET/CT indicated for cancer staging. Symmetrical, bilateral 18 F-FDG uptake on PET in cervical regions corresponding to adipose tissue on CT (blue arrow). b , 34-year old, female patient with marginal zone B-cell lymphoma of the mouth floor, initial staging scan (green arrow). 18 F-FDG PET/CT indicated for cancer staging. Symmetrical, bilateral 18 F-FDG uptake on PET in cervical regions corresponding to adipose tissue on CT (blue arrow).

Supplementary information

Reporting summary, supplementary tables 1–11, source data fig. 1.

Statistical source data.

Source Data Fig. 2

Source data fig. 3, source data extended data fig. 1, source data extended data fig. 2, source data extended data fig. 3, rights and permissions.

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Becher, T., Palanisamy, S., Kramer, D.J. et al. Brown adipose tissue is associated with cardiometabolic health. Nat Med 27 , 58–65 (2021). https://doi.org/10.1038/s41591-020-1126-7

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Received : 14 February 2020

Accepted : 09 October 2020

Published : 04 January 2021

Issue Date : January 2021

DOI : https://doi.org/10.1038/s41591-020-1126-7

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