UCP1 in brite/beige adipose tissue mitochondria is functionally thermogenic

Brown adipose tissue: physiological function and evolutionary significance

In modern eutherian (placental) mammals, brown adipose tissue (BAT) evolved as a specialized thermogenic organ that is responsible for adaptive non-shivering thermogenesis (NST). For NST, energy metabolism of BAT mitochondria is increased by activation of uncoupling protein 1 (UCP1), which dissipates the proton motive force as heat. Despite the presence of UCP1 orthologues prior to the divergence of teleost fish and mammalian lineages, UCP1's significance for thermogenic adipose tissue emerged at later evolutionary stages. Recent studies on the presence of BAT in metatherians (marsupials) and eutherians of the afrotherian clade provide novel insights into the evolution of adaptive NST in mammals. In particular studies on the 'protoendothermic' lesser hedgehog tenrec (Afrotheria) suggest an evolutionary scenario linking BAT to the onset of eutherian endothermy. Here, we review the physiological function and distribution of BAT in an evolutionary context by focusing on the latest research on phylogenetically distinct species.

The Ontogeny of Brown Adipose Tissue

There are three different types of adipose tissue (AT)-brown, white, and beige-that differ with stage of development, species, and anatomical location. Of these, brown AT (BAT) is the least abundant but has the greatest potential impact on energy balance. BAT is capable of rapidly producing large amounts of heat through activation of the unique uncoupling protein 1 (UCP1) located within the inner mitochondrial membrane. White AT is an endocrine organ and site of lipid storage, whereas beige AT is primarily white but contains some cells that possess UCP1. BAT first appears in the fetus around mid-gestation and is then gradually lost through childhood, adolescence, and adulthood. We focus on the interrelationships between adipocyte classification, anatomical location, and impact of diet in early life together with the extent to which fat development differs between the major species examined. Ultimately, novel dietary interventions designed to reactivate BAT could be possible.

Pharmacologic Effects of FGF21 Are Independent of the "Browning" of White Adipose Tissue

"Browning," the appearance and activation of brown-in-white (brite) adipose cells within inguinal white adipose tissue (iWAT), and induction of uncoupling protein 1 (UCP1) correlate with fibroblast growth factor-21 (FGF21)-induced weight loss and glucose homeostasis improvements. Therefore, antiobesity therapies targeting browning and brite adipocyte activation are currently being sought. To test the dependence of weight loss on browning, we examined whether this event was responsible for FGF21-Fc's beneficial effects. Lean and diet-induced obese mice housed at 21°C or 30°C that received FGF21-Fc exhibited similar degrees of body weight reduction and glucose homeostasis improvement. Substantial browning of iWAT occurred only in FGF21-Fc-treated lean mice housed at 21°C. Further, FGF21-Fc-treated Ucp1(-/-) mice showed robust improvements in body weight, glucose homeostasis, and plasma lipids, associated with increased energy expenditure and FGF21-Fc-induced Ppargc1 expression in iWAT. We conclude that FGF21 requires neither UCP1 nor brite adipocytes to elicit weight loss and improve glucose homeostasis.

Central serotonergic neurons activate and recruit thermogenic brown and beige fat and regulate glucose and lipid homeostasis

Thermogenic brown and beige adipocytes convert chemical energy to heat by metabolizing glucose and lipids. Serotonin (5-HT) neurons in the CNS are essential for thermoregulation and accordingly may control metabolic activity of thermogenic fat. To test this, we generated mice in which the human diphtheria toxin receptor (DTR) was selectively expressed in central 5-HT neurons. Treatment with diphtheria toxin (DT) eliminated 5-HT neurons and caused loss of thermoregulation, brown adipose tissue (BAT) steatosis, and a >50% decrease in uncoupling protein 1 (Ucp1) expression in BAT and inguinal white adipose tissue (WAT). In parallel, blood glucose increased 3.5-fold, free fatty acids 13.4-fold, and triglycerides 6.5-fold. Similar BAT and beige fat defects occurred in Lmx1b(f/f)ePet1(Cre) mice in which 5-HT neurons fail to develop in utero. We conclude 5-HT neurons play a major role in regulating glucose and lipid homeostasis, in part through recruitment and metabolic activation of brown and beige adipocytes.

Adrenergically stimulated blood flow in brown adipose tissue is not dependent on thermogenesis

Brown adipose tissue (BAT) thermogenesis relies on blood flow to be supplied with nutrients and oxygen and for the distribution of the generated heat to the rest of the body. Therefore, it is fundamental to understand the mechanisms by which blood flow is regulated and its relation to thermogenesis. Here, we present high-resolution laser-Doppler imaging (HR-LDR) as a novel method for noninvasive in vivo measurement of BAT blood flow in mice. Using HR-LDR, we found that norepinephrine stimulation increases BAT blood flow in a dose-dependent manner and that this response is profoundly modulated by environmental temperature acclimation. Surprisingly, we found that mice lacking uncoupling protein 1 (UCP1) have fully preserved BAT blood flow response to norepinephrine despite failing to perform thermogenesis. BAT blood flow was not directly correlated to systemic glycemia, but glucose injections could transiently increase tissue perfusion. Inguinal white adipose tissue, also known as a brite/beige adipose tissue, was also sensitive to cold acclimation and similarly increased blood flow in response to norepinephrine. In conclusion, using a novel noninvasive method to detect BAT perfusion, we demonstrate that adrenergically stimulated BAT blood flow is qualitatively and quantitatively fully independent of thermogenesis, and therefore, it is not a reliable parameter for the estimation of BAT activation and heat generation.

Leydig cell steroidogenesis unexpectedly escapes mitochondrial dysfunction in prematurely aging mice

Point mutations and deletions of mitochondrial DNA (mtDNA) accumulate in tissues during aging in animals and humans and are the basis for mitochondrial diseases. Testosterone synthesis occurs in the mitochondria of Leydig cells. Mitochondrial dysfunction (as induced here experimentally in mtDNA mutator mice that carry a proofreading-deficient form of mtDNA polymerase γ, leading to mitochondrial dysfunction in all cells types so far studied) would therefore be expected to lead to low testosterone levels. Although mtDNA mutator mice showed a dramatic reduction in testicle weight (only 15% remaining) and similar decreases in number of spermatozoa, testosterone levels in mtDNA mutator mice were unexpectedly fully unchanged. Leydig cell did not escape mitochondrial damage (only 20% of complex I and complex IV remaining) and did show high levels of reactive oxygen species (ROS) production (>5-fold increased), and permeabilized cells demonstrated absence of normal mitochondrial function. Nevertheless, within intact cells, mitochondrial membrane potential remained high, and testosterone production was maintained. This implies development of a compensatory mechanism. A rescuing mechanism involving electrons from the pentose phosphate pathway transferred via a 3-fold up-regulated cytochrome b5 to cytochrome c, allowing for mitochondrial energization, is suggested. Thus, the Leydig cells escape mitochondrial dysfunction via a unique rescue pathway. Such a pathway, bypassing respiratory chain dysfunction, may be of relevance with regard to mitochondrial disease therapy and to managing ageing in general.

Leptin and insulin act on POMC neurons to promote the browning of white fat

The primary task of white adipose tissue (WAT) is the storage of lipids. However, "beige" adipocytes also exist in WAT. Beige adipocytes burn fat and dissipate the energy as heat, but their abundance is diminished in obesity. Stimulating beige adipocyte development, or WAT browning, increases energy expenditure and holds potential for combating metabolic disease and obesity. Here, we report that insulin and leptin act together on hypothalamic neurons to promote WAT browning and weight loss. Deletion of the phosphatases PTP1B and TCPTP enhanced insulin and leptin signaling in proopiomelanocortin neurons and prevented diet-induced obesity by increasing WAT browning and energy expenditure. The coinfusion of insulin plus leptin into the CNS or the activation of proopiomelanocortin neurons also increased WAT browning and decreased adiposity. Our findings identify a homeostatic mechanism for coordinating the status of energy stores, as relayed by insulin and leptin, with the central control of WAT browning.

Overexpression of SIRT1 in rat skeletal muscle does not alter glucose induced insulin resistance

SIRT1 is a NAD+-dependent deacetylase thought to regulate cellular metabolic pathways in response to alterations in nutrient flux. In the current study we investigated whether acute changes in SIRT1 expression affect markers of muscle mitochondrial content and also determined whether SIRT1 influenced muscle insulin resistance induced by acute glucose oversupply. In male Wistar rats either SIRT1 or a deacetylase inactive mutant form (H363Y) was electroprated into the tibialis cranialis (TC) muscle. The other leg was electroporated with an empty control vector. One week later, glucose was infused and hyperglycaemia was maintained at ~11mM. After 5 hours, 11mM glucose induced significant insulin resistance in skeletal muscle. Interestingly, overexpression of either SIRT1 or SIRT1 (H363Y) for 1 week did not change markers of mitochondrial content or function. SIRT1 or SIRT1 (H363Y) overexpression had no effect on the reduction in glucose uptake and glycogen synthesis in muscle in response to hyperglycemia. Therefore we conclude that acute increases in SIRT1 protein have little impact on mitochondrial content and that overexpressing SIRT1 does not prevent the development of insulin resistance during hyperglycaemia.

Brown fat activation reduces hypercholesterolaemia and protects from atherosclerosis development

Brown adipose tissue (BAT) combusts high amounts of fatty acids, thereby lowering plasma triglyceride levels and reducing obesity. However, the precise role of BAT in plasma cholesterol metabolism and atherosclerosis development remains unclear. Here we show that BAT activation by β3-adrenergic receptor stimulation protects from atherosclerosis in hyperlipidemic APOE*3-Leiden.CETP mice, a well-established model for human-like lipoprotein metabolism that unlike hyperlipidemic Apoe^−/− and Ldlr^−/− mice expresses functional apoE and LDLR. BAT activation increases energy expenditure and decreases plasma triglyceride and cholesterol levels. Mechanistically, we demonstrate that BAT activation enhances the selective uptake of fatty acids from triglyceride-rich lipoproteins into BAT, subsequently accelerating the hepatic clearance of the cholesterol-enriched remnants. These effects depend on a functional hepatic apoE-LDLR clearance pathway as BAT activation in Apoe^−/− and Ldlr^−/− mice does not attenuate hypercholesterolaemia and atherosclerosis. We conclude that activation of BAT is a powerful therapeutic avenue to ameliorate hyperlipidaemia and protect from atherosclerosis.

Brown adipose tissue (BAT) produces heat by burning lipid triglycerides. Here, Berbée et al. show that pharmacological BAT activation protects hyperlipidemic mice from atherosclerosis, provided mice retain the metabolic capacity to clear cholesterol-enriched lipoprotein remnants by the liver.

The brain and brown fat

Brown adipose tissue (BAT) is a specialized organ responsible for thermogenesis, a process required for maintaining body temperature. BAT is regulated by the sympathetic nervous system (SNS), which activates lipolysis and mitochondrial uncoupling in brown adipocytes. For many years, BAT was considered to be important only in small mammals and newborn humans, but recent data have shown that BAT is also functional in adult humans. On the basis of this evidence, extensive research has been focused on BAT function, where new molecules, such as irisin and bone morphogenetic proteins, particularly BMP7 and BMP8B, as well as novel central factors and new regulatory mechanisms, such as orexins and the canonical ventomedial nucleus of the hypothalamus (VMH) AMP- activated protein kinase (AMPK)-SNS-BAT axis, have been discovered and emerged as potential drug targets to combat obesity. In this review we provide an overview of the complex central regulation of BAT and how different neuronal cell populations co-ordinately work to maintain energy homeostasis.

Potential novel therapeutic strategies from understanding adipocyte transdifferentiation mechanisms

Brown adipocytes are located in discrete anatomical locations in both small mammals and in humans. 'Brown-like' adipocytes, also known as brite (brown in white) or beige adipocytes are found interspersed among white adipocytes in several fat depots. From a functional point of view, the activity of brown and brite cells is similar, that is, heat production mediated by uncoupling protein 1. The morphology and expression of 'thermogenic' genes is also very similar in these two cell types. The origin of brite adipocytes is under intense investigation because enhancing their presence and activity has the potential to promote a healthy metabolic profile. Transdifferentiation mechanisms as well as de novo recruitment have been investigated. The characterization of the mechanisms involved in the recruitment and activation of brown/brite adipocytes in adult humans, could open the avenue for promising therapeutic strategies to curb metabolic diseases.

Brown adipose tissue during puberty and with aging

It was previously assumed that brown adipose tissue (BAT) is present in humans only for a short period following birth, the time in which mechanisms of generating heat by way of shivering are not yet developed. Although BAT is maximally recruited in early infancy, findings in recent years have led to a new consensus that metabolically active BAT remains present in most children and many adult humans. Evidence to date supports a slow and steady decline in BAT activity throughout life, with the exception of an intriguing spike in the prevalence and volume of BAT around the time of puberty that remains poorly understood. Because BAT activity is more commonly observed in individuals with a lower body mass index, an association seen in both adult and pediatric populations, there is the exciting possibility that BAT is protective against childhood and adult obesity. Indeed, the function and metabolic relevance of human BAT is currently an area of vigorous research. The goal of this review is to summarize what is currently known about changes that occur in BAT during various stages of life, with a particular emphasis on puberty and aging.

Thermogenic brown and beige/brite adipogenesis in humans

Evidence from rodents established an important role of brown adipose tissue (BAT) in energy expenditure. Moreover, to sustain thermogenesis, BAT has been shown to be a powerful sink for draining and oxidation of glucose and triglycerides from blood. The potential of BAT activity in protection against obesity and metabolic syndrome is recognized. Recently, an unexpected presence and activity of BAT has been found in adult humans. Here we review the most recent research in this field and, specifically, how new findings apply to humans. Moreover, we seek to clarify the underlying biological processes occurring beyond the burst of new nomenclature in the field. The cell type responsible for thermogenesis, the brown adipocyte, arises from complex developmental processes. In addition to 'classical' brown adipocytes, present in developmentally programmed BAT depots, there are brown adipocytes, named 'brite' (from 'brown-in-white') or 'beige', which appear in response to thermogenic stimuli in white fat due to the so-called 'browning' process. Beige/brite cells appear to be important components of BAT depots in adult humans. In addition to the known control of BAT activity by the sympathetic nervous system, metabolic and hormonal signals originating in muscle or liver (e.g. irisin, FGF21) are recognized as activators of BAT and beige/brite adipocytes.

When pigs fly, UCP1 makes heat

Brown and beige adipose tissue may represent important therapeutic targets for the treatment of diabetes and obesity as these organs dissipate nutrient energy as heat through the thermogenic uncoupling protein 1 (UCP1). While mice are commonly used to mimic the potential effects of brown/beige adipose tissue that may act in human metabolism, new animal models are edging into the market for translational medicine. Pigs reflect human metabolism better than mice in multiple parameters such as obesity-induced hyperglycemia, cholesterol profiles and energy metabolism. Recently, it was reported that energy expenditure and body temperature in pigs is induced by the hormone leptin, and that leptin's action is mediated by UCP1 in adipose tissue. Given the tremendous importance of identifying molecular mechanisms for targeting therapeutics, we critically examine the evidence supporting the presence of UCP1 in pigs and conclude that methodological shortcomings prevent an unequivocal claim for the presence of UCP1 in pigs. Despite this, we believe that leptin's effects on energy expenditure in pigs are potentially more transformative to human medicine in the absence of UCP1, as adult and obese humans possess only minor amounts of UCP1. In general, we propose that the biology of new animal models requires attention to comparative studies with humans given the increasing amount of genomic information for various animal species.

In vivo measurement of energy substrate contribution to cold-induced brown adipose tissue thermogenesis

The present study was designed to investigate the effects of cold on brown adipose tissue (BAT) energy substrate utilization in vivo using the positron emission tomography tracers [(18)F]fluorodeoxyglucose (glucose uptake), 14(R,S)-[(18)F]fluoro-6-thia-heptadecanoic acid [nonesterified fatty acid (NEFA) uptake], and [(11)C]acetate (oxidative activity). The measurements were performed in rats adapted to 27°C, which were acutely subjected to cold (10°C) for 2 and 6 hours, and in rats chronically adapted to 10°C for 21 days, which were returned to 27°C for 2 and 6 hours. Cold exposure (acutely and chronically) led to increases in BAT oxidative activity, which was accompanied by concomitant increases in glucose and NEFA uptake. The increases were particularly high in cold-adapted rats and largely readily reduced by the return to a warm environment. The cold-induced increase in oxidative activity was meaningfully blunted by nicotinic acid, a lipolysis inhibitor, which emphasizes in vivo the key role of intracellular lipid in BAT thermogenesis. The changes in BAT oxidative activity and glucose and NEFA uptakes were paralleled by inductions of genes involved in not only oxidative metabolism but also in energy substrate replenishment (triglyceride and glycogen synthesis). The capacity of BAT for energy substrate replenishment is remarkable.

Mitochondrial uncoupling proteins and energy metabolism

Understanding the metabolic factors that contribute to energy metabolism (EM) is critical for the development of new treatments for obesity and related diseases. Mitochondrial oxidative phosphorylation is not perfectly coupled to ATP synthesis, and the process of proton-leak plays a crucial role. Proton-leak accounts for a significant part of the resting metabolic rate (RMR) and therefore enhancement of this process represents a potential target for obesity treatment. Since their discovery, uncoupling proteins have stimulated great interest due to their involvement in mitochondrial-inducible proton-leak. Despite the widely accepted uncoupling/thermogenic effect of uncoupling protein one (UCP₁), which was the first in this family to be discovered, the reactions catalyzed by its homolog UCP₃ and the physiological role remain under debate. This review provides an overview of the role played by UCP₁ and UCP₃ in mitochondrial uncoupling/functionality as well as EM and suggests that they are a potential therapeutic target for treating obesity and its related diseases such as type II diabetes mellitus.

Brown and beige fat in humans: thermogenic adipocytes that control energy and glucose homeostasis

Brown adipose tissue (BAT), a specialized fat that dissipates energy to produce heat, plays an important role in the regulation of energy balance. Two types of thermogenic adipocytes with distinct developmental and anatomical features exist in rodents and humans: classical brown adipocytes and beige (also referred to as brite) adipocytes. While classical brown adipocytes are located mainly in dedicated BAT depots of rodents and infants, beige adipocytes sporadically reside with white adipocytes and emerge in response to certain environmental cues, such as chronic cold exposure, a process often referred to as "browning" of white adipose tissue. Recent studies indicate the existence of beige adipocytes in adult humans, making this cell type an attractive therapeutic target for obesity and obesity-related diseases, including type 2 diabetes. This Review aims to cover recent progress in our understanding of the anatomical, developmental, and functional characteristics of brown and beige adipocytes and discuss emerging questions, with a special emphasis on adult human BAT.

Prospective heterotopic ossification progenitors in adult human skeletal muscle

Skeletal muscle has strong regenerative capabilities. However, failed regeneration can lead to complications where aberrant tissue forms as is the case with heterotopic ossification (HO), in which chondrocytes, osteoblasts and white and brown adipocytes can arise following severe trauma. In humans, the various HO cell types likely originate from multipotent mesenchymal stromal cells (MSCs) in skeletal muscle, which have not been identified in humans until now. In the present study, adherent cells from freshly digested skeletal muscle tissue were expanded in defined culture medium and were FACS-enriched for the CD73(+)CD105(+)CD90(-) population, which displayed robust multilineage potential. Clonal differentiation assays confirmed that all three lineages originated from a single multipotent progenitor. In addition to differentiating into typical HO lineages, human muscle resident MSCs (hmrMSCs) also differentiated into brown adipocytes expressing uncoupling protein 1 (UCP1). Characterizing this novel multipotent hmrMSC population with a brown adipocyte differentiation capacity has enhanced our understanding of the contribution of non-myogenic progenitor cells to regeneration and aberrant tissue formation in human skeletal muscle.

Mitochondrial turnover: a phenotype distinguishing brown adipocytes from interscapular brown adipose tissue and white adipose tissue

To determine the differences between brown adipocytes from interscapular brown tissue (iBAT) and those induced in white adipose tissue (WAT) with respect to their thermogenic capacity, we examined two essential characteristics: the dynamics of mitochondrial turnover during reversible transitions from 29 °C to 4 °C and the quantitative relationship between UCP1 and selected subunits of mitochondrial respiratory complex in the fully recruited state. To follow the kinetics of induction and involution of mitochondria, we determined the expression pattern of UCP1 and other mitochondrial proteins as well as analyzed mtDNA content after cold stimulation and reacclimation to thermoneutrality. We showed that UCP1 turnover is very different in iBAT and inguinal WAT (ingWAT); the former showed minimal changes in protein content, whereas the latter showed major changes. Similarly, in iBAT both mtDNA content and the expression of mitochondrial proteins were stable and expressed at similar levels during reversible transitions from 29 °C to 4 °C, whereas ingWAT revealed dynamic changes. Further analysis showed that in iBAT, the expression patterns for UCP1 and other mitochondrial proteins resembled each other, whereas in ingWAT, UCP1 varied ∼100-fold during the transition from cold to warmth, and no other mitochondrial proteins matched UCP1. In turn, quantitative analysis of thermogenic capacity determined by estimating the proportion of UCP1 to respiratory complex components showed no significant differences between brown and brite adipocytes, suggesting similar thermogenic potentiality. Our results indicate that dynamics of brown adipocytes turnover during reversible transition from warm to cold may determine the thermogenic capacity of an individual in a changing temperature environment.

Myocardin-related transcription factor A regulates conversion of progenitors to beige adipocytes

Adipose tissue is an essential regulator of metabolic homeostasis. In contrast with white adipose tissue, which stores excess energy in the form of triglycerides, brown adipose tissue is thermogenic, dissipating energy as heat via the unique expression of the mitochondrial uncoupling protein UCP1. A subset of UCP1+ adipocytes develops within white adipose tissue in response to physiological stimuli; however, the developmental origin of these "brite" or "beige" adipocytes is unclear. Here, we report the identification of a BMP7-ROCK signaling axis regulating beige adipocyte formation via control of the G-actin-regulated transcriptional coactivator myocardin-related transcription factor A, MRTFA. White adipose tissue from MRTFA(-/-) mice contains more multilocular adipocytes and expresses enhanced levels of brown-selective proteins, including UCP1. MRTFA(-/-) mice also show improved metabolic profiles and protection from diet-induced obesity and insulin resistance. Our study hence unravels a central pathway driving the development of physiologically functional beige adipocytes.

The Therapeutic Potential of Brown Adipocytes in Humans

Obesity and its metabolic consequences represent a significant clinical problem. From a thermodynamic standpoint, obesity results from a discord in energy intake and expenditure. To date, lifestyle interventions based on reducing energy intake and/or increasing energy expenditure have proved ineffective in the prevention and/or treatment of obesity, owing to poor long-term adherence to such interventions. Thus, an effective strategy to prevent or correct obesity is currently lacking. As the combustion engines of our cells, mitochondria play a critical role in energy expenditure. At a whole-body level, approximately 80% of mitochondrial membrane potential generated by fuel oxidation is used to produce ATP, and the remaining 20% is lost through heat-producing uncoupling reactions. The coupling of mitochondrial respiration to ATP production represents an important component in whole-body energy expenditure. Brown adipose tissue (BAT) is densely populated with mitochondria containing the inner mitochondrial proton carrier uncoupling protein 1 (UCP1). UCP1 uncouples oxidative phosphorylation, meaning that mitochondrial membrane potential is dissipated as heat. The recent rediscovery of BAT depots in adult humans has rekindled scientific interest in the manipulation of mitochondrial uncoupling reactions as a means to increase metabolic rate, thereby counteracting obesity and its associated metabolic phenotype. In this article, we discuss the evidence for the role BAT plays in metabolic rate and glucose and lipid metabolism in humans and the potential for UCP1 recruitment in the white adipose tissue of humans. While the future holds much promise for a therapeutic role of UCP1 expressing adipocytes in human energy metabolism, particularly in the context of obesity, tissue-specific strategies that activate or recruit UCP1 in human adipocytes represent an obligatory translational step for this early promise to be realized.

Unlock the Thermogenic Potential of Adipose Tissue: Pharmacological Modulation and Implications for Treatment of Diabetes and Obesity

Brown adipose tissue (BAT) is considered an interesting target organ for the treatment of metabolic disease due to its high metabolic capacity. Non-shivering thermogenesis, once activated, can lead to enhanced partitioning and oxidation of fuels in adipose tissues, and reduce the burden of glucose and lipids on other metabolic organs such as liver, pancreas, and skeletal muscle. Sustained long-term activation of BAT may also lead to meaningful bodyweight loss. In this review, we discuss three different drug classes [the thiazolidinedione (TZD) class of PPARγ agonists, β3-adrenergic receptor agonists, and fibroblast growth factor 21 (FGF21) analogs] that have been proposed to regulate BAT and beige recruitment or activation, or both, and which have been tested in both rodent and human. The learnings from these classes suggest that restoration of functional BAT and beige mass as well as improved activation might be required to fully realize the metabolic potential of these tissues. Whether this can be achieved without the undesired cardiovascular side effects exhibited by the TZD PPARγ agonists and β3-adrenergic receptor agonists remains to be resolved.

Group 2 innate lymphoid cells promote beiging of white adipose tissue and limit obesity

Obesity is an increasingly prevalent disease regulated by genetic and environmental factors. Emerging studies indicate that immune cells, including monocytes, granulocytes and lymphocytes, regulate metabolic homeostasis and are dysregulated in obesity. Group 2 innate lymphoid cells (ILC2s) can regulate adaptive immunity and eosinophil and alternatively activated macrophage responses, and were recently identified in murine white adipose tissue (WAT) where they may act to limit the development of obesity. However, ILC2s have not been identified in human adipose tissue, and the mechanisms by which ILC2s regulate metabolic homeostasis remain unknown. Here we identify ILC2s in human WAT and demonstrate that decreased ILC2 responses in WAT are a conserved characteristic of obesity in humans and mice. Interleukin (IL)-33 was found to be critical for the maintenance of ILC2s in WAT and in limiting adiposity in mice by increasing caloric expenditure. This was associated with recruitment of uncoupling protein 1 (UCP1)(+) beige adipocytes in WAT, a process known as beiging or browning that regulates caloric expenditure. IL-33-induced beiging was dependent on ILC2s, and IL-33 treatment or transfer of IL-33-elicited ILC2s was sufficient to drive beiging independently of the adaptive immune system, eosinophils or IL-4 receptor signalling. We found that ILC2s produce methionine-enkephalin peptides that can act directly on adipocytes to upregulate Ucp1 expression in vitro and that promote beiging in vivo. Collectively, these studies indicate that, in addition to responding to infection or tissue damage, ILC2s can regulate adipose function and metabolic homeostasis in part via production of enkephalin peptides that elicit beiging.

Inhibiting peripheral serotonin synthesis reduces obesity and metabolic dysfunction by promoting brown adipose tissue thermogenesis

Mitochondrial uncoupling protein 1 (UCP1) is enriched within interscapular brown adipose tissue (iBAT) and beige (also known as brite) adipose tissue, but its thermogenic potential is reduced with obesity and type 2 diabetes for reasons that are not understood. Serotonin (5-hydroxytryptamine, 5-HT) is a highly conserved biogenic amine that resides in non-neuronal and neuronal tissues that are specifically regulated via tryptophan hydroxylase 1 (Tph1) and Tph2, respectively. Recent findings suggest that increased peripheral serotonin and polymorphisms in TPH1 are associated with obesity; however, whether this is directly related to reduced BAT thermogenesis and obesity is not known. We find that Tph1-deficient mice fed a high-fat diet (HFD) are protected from obesity, insulin resistance and nonalcoholic fatty liver disease (NAFLD) while exhibiting greater energy expenditure by BAT. Small-molecule chemical inhibition of Tph1 in HFD-fed mice mimics the benefits ascribed to Tph1 genetic deletion, effects that depend on UCP1-mediated thermogenesis. The inhibitory effects of serotonin on energy expenditure are cell autonomous, as serotonin blunts β-adrenergic induction of the thermogenic program in brown and beige adipocytes in vitro. As obesity increases peripheral serotonin, the inhibition of serotonin signaling or its synthesis in adipose tissue may be an effective treatment for obesity and its comorbidities.

MicroRNAs are required for the feature maintenance and differentiation of brown adipocytes

Brown adipose tissue (BAT) is specialized to burn lipids for heat generation as a natural defense against cold and obesity. Previous studies established microRNAs (miRNAs) as essential regulators of brown adipocyte differentiation, but whether miRNAs are required for the feature maintenance of mature brown adipocytes remains unknown. To address this question, we ablated Dgcr8, a key regulator of the miRNA biogenesis pathway, in mature brown as well as in white adipocytes. Adipose tissue-specific Dgcr8 knockout mice displayed enlarged but pale interscapular brown fat with decreased expression of genes characteristic of brown fat and were intolerant to cold exposure. Primary brown adipocyte cultures in vitro confirmed that miRNAs are required for marker gene expression in mature brown adipocytes. We also demonstrated that miRNAs are essential for the browning of subcutaneous white adipocytes in vitro and in vivo. Using this animal model, we performed miRNA expression profiling analysis and identified a set of BAT-specific miRNAs that are upregulated during brown adipocyte differentiation and enriched in brown fat compared with other organs. We identified miR-182 and miR-203 as new regulators of brown adipocyte development. Taken together, our study demonstrates an essential role of miRNAs in the maintenance as well as in the differentiation of brown adipocytes.

Myeloperoxidase deletion prevents high-fat diet-induced obesity and insulin resistance

Activation of myeloperoxidase (MPO), a heme protein primarily expressed in granules of neutrophils, is associated with the development of obesity. However, whether MPO mediates high-fat diet (HFD)-induced obesity and obesity-associated insulin resistance remains to be determined. Here, we found that consumption of an HFD resulted in neutrophil infiltration and enhanced MPO expression and activity in epididymal white adipose tissue, with an increase in body weight gain and impaired insulin signaling. MPO knockout (MPO(-/-)) mice were protected from HFD-enhanced body weight gain and insulin resistance. The MPO inhibitor 4-aminobenzoic acid hydrazide reduced peroxidase activity of neutrophils and prevented HFD-enhanced insulin resistance. MPO deficiency caused high body temperature via upregulation of uncoupling protein-1 and mitochondrial oxygen consumption in brown adipose tissue. Lack of MPO also attenuated HFD-induced macrophage infiltration and expression of proinflammatory cytokines. We conclude that activation of MPO in adipose tissue contributes to the development of obesity and obesity-associated insulin resistance. Inhibition of MPO may be a potential strategy for prevention and treatment of obesity and insulin resistance.

Is the heat surrounding adipose tissue mitochondria warranted?

PURPOSE OF REVIEW

Mitochondrial uncoupling proteins uncouple oxidative phosphorylation. The physiological role ascribed to this process is thermoregulation. The metabolic consequence of mitochondrial respiration uncoupled from ATP production is increased substrate oxidation and metabolic rate. The recent discovery of uncoupling protein 1 (UCP1) positive mitochondria in human adipose tissue has rekindled interest in the role of UCP1 in energy balance and metabolic health.

RECENT FINDINGS

Recently, there have been numerous reports of functional brown adipose tissue in humans. Further, data from cell and murine studies suggest that beige adipocytes can be induced within white adipose tissue. The presence of brown/beige adipocytes with mitochondria expressing UCP1 negatively correlates with adiposity. Further, activation of these adipocytes alters energy balance and substrate metabolism. However, in humans, brown fat content varies significantly. Further, although beige adipocytes can be induced in white adipose tissue of rodents, whether this is also true in humans remains unclear.

SUMMARY

The presence of UCP1-positive mitochondria in human adipose tissue represents an exciting therapeutic target for treating obesity and its metabolic complications. Understanding the mechanisms governing brown fat activation will be crucial if the therapeutic potential of UCP1 is to be realized.

Cold-activated brown adipose tissue in human adults: methodological issues

The relevance of functional brown adipose tissue (BAT) depots in human adults was undisputedly proven approximately seven years ago. Here we give an overview of all dedicated studies that were published on cold-induced BAT activity in adult humans that appeared since then. Different cooling protocols and imaging techniques to determine BAT activity are reviewed. BAT activation can be achieved by means of air- or water-cooling protocols. The most promising approach is individualized cooling, during which subjects are studied at the lowest temperature for nonshivering condition, probably revealing maximal nonshivering thermogenesis. The highest BAT prevalence (i.e., close to 100%) is observed using the individualized cooling protocol. Currently, the most widely used technique to study the metabolic activity of BAT is deoxy-2-[18F]fluoro-d-glucose ([18F]FDG)-positron emission tomography/computed tomography (PET/CT) imaging. Dynamic imaging provides quantitative information about glucose uptake rates, whereas static imaging reflects overall BAT glucose uptake, localization, and distribution. In general, standardized uptake values (SUV) are used to quantify BAT activity. An accurate determination of total BAT volume is hampered by the limited spatial resolution of the PET image, leading to spillover. Different research groups use different SUV threshold values, which make it difficult to directly compare BAT activity levels between studies. Another issue is the comparison of [18F]FDG uptake in BAT with respect to other tissues or upon with baseline values. This comparison can be performed by using the “fixed volume” methodology. Finally, the potential use of other relatively noninvasive methods to quantify BAT, like magnetic resonance imaging or thermography, is discussed.

Fighting obesity: When muscle meets fat

The prevalence of obesity has risen to an unprecedented level. According to World Health Organization, over 500 million adults, equivalent to 10%-14% of the world population, were obese with a body mass index (BMI) of 30 kg/m(2) or greater in 2008.(1) This rising prevalence and earlier onset of obesity is believed to be resulted from an interplay of genetic factors, over-nutrition and physical inactivity in modern lifestyles. Obesity also increases the susceptibility to metabolic syndromes, hypertension, cardiovascular diseases, Type 2 diabetes mellitus (T2DM) and cancer.(2-4) The global obesity epidemic has sparked substantial interests in the biology of adipose tissue (fat). In addition, the skeletal muscle and its secretive factors (myokines) have also been shown to play a critical role in controlling body energy balance, adipose homeostasis and inflammation status.(5) Interestingly, skeletal muscle cells share a common developmental origin with brown adipocytes,(6,7) which breaks down lipids to generate heat - thus reducing obesity. Here, we provide a brief overview of the basics and recent progress in muscle-fat crosstalk in the context of body energy metabolism, obesity, and diabetes. We summarize the different types of adipocytes, their developmental origins and implications in body composition. We highlight the role of several novel myokines in regulating fat mass and systemic energy balance, and evaluate the potential of skeletal muscles as a therapeutic target to treat obesity.

Thermogenic adipocytes: from cells to physiology and medicine

The identification of active brown fat in humans has evoked widespread interest in the biology of non-shivering thermogenesis among basic and clinical researchers. As a consequence we have experienced a plethora of contributions related to cellular and molecular processes in thermogenic adipocytes as well as their function in the organismal context and their relevance to human physiology. In this review we focus on the cellular basis of non-shivering thermogenesis, particularly in relation to human health and metabolic disease. We provide an overview of the cellular function and distribution of thermogenic adipocytes in mouse and humans, and how this can be affected by environmental factors, such as prolonged cold exposure. We elaborate on recent evidence and open questions on the distinction of classical brown versus beige/brite adipocytes. Further, the origin of thermogenic adipocytes as well as current models for the recruitment of beige/brite adipocytes is discussed with an emphasis on the role of progenitor cells. Focusing on humans, we describe the expanding evidence for the activity, function and physiological relevance of thermogenic adipocytes. Finally, as the potential of thermogenic adipocyte activation as a therapeutic approach for the treatment of obesity and associated metabolic diseases becomes evident, we highlight goals and challenges for current research on the road to clinical translation.

Unraveling biochemical pathways affected by mitochondrial dysfunctions using metabolomic approaches

Mitochondrial dysfunction(s) (MDs) can be defined as alterations in the mitochondria, including mitochondrial uncoupling, mitochondrial depolarization, inhibition of the mitochondrial respiratory chain, mitochondrial network fragmentation, mitochondrial or nuclear DNA mutations and the mitochondrial accumulation of protein aggregates. All these MDs are known to alter the capacity of ATP production and are observed in several pathological states/diseases, including cancer, obesity, muscle and neurological disorders. The induction of MDs can also alter the secretion of several metabolites, reactive oxygen species production and modify several cell-signalling pathways to resolve the mitochondrial dysfunction or ultimately trigger cell death. Many metabolites, such as fatty acids and derived compounds, could be secreted into the blood stream by cells suffering from mitochondrial alterations. In this review, we summarize how a mitochondrial uncoupling can modify metabolites, the signalling pathways and transcription factors involved in this process. We describe how to identify the causes or consequences of mitochondrial dysfunction using metabolomics (liquid and gas chromatography associated with mass spectrometry analysis, NMR spectroscopy) in the obesity and insulin resistance thematic.

Role of forkhead box protein A3 in age-associated metabolic decline

Aging is associated with increased adiposity and diminished thermogenesis, but the critical transcription factors influencing these metabolic changes late in life are poorly understood. We recently demonstrated that the winged helix factor forkhead box protein A3 (Foxa3) regulates the expansion of visceral adipose tissue in high-fat diet regimens; however, whether Foxa3 also contributes to the increase in adiposity and the decrease in brown fat activity observed during the normal aging process is currently unknown. Here we report that during aging, levels of Foxa3 are significantly and selectively up-regulated in brown and inguinal white fat depots, and that midage Foxa3-null mice have increased white fat browning and thermogenic capacity, decreased adipose tissue expansion, improved insulin sensitivity, and increased longevity. Foxa3 gain-of-function and loss-of-function studies in inguinal adipose depots demonstrated a cell-autonomous function for Foxa3 in white fat tissue browning. Furthermore, our analysis revealed that the mechanisms of Foxa3 modulation of brown fat gene programs involve the suppression of peroxisome proliferator activated receptor γ coactivtor 1 α (PGC1α) levels through interference with cAMP responsive element binding protein 1-mediated transcriptional regulation of the PGC1α promoter. Overall, our data demonstrate a role for Foxa3 in energy expenditure and in age-associated metabolic disorders.

Coupling of lipolysis and de novo lipogenesis in brown, beige, and white adipose tissues during chronic β3-adrenergic receptor activation

Chronic activation of β3-adrenergic receptors (β3-ARs) expands the catabolic activity of both brown and white adipose tissue by engaging uncoupling protein 1 (UCP1)-dependent and UCP1-independent processes. The present work examined de novo lipogenesis (DNL) and TG/glycerol dynamics in classic brown, subcutaneous "beige," and classic white adipose tissues during sustained β3-AR activation by CL 316,243 (CL) and also addressed the contribution of TG hydrolysis to these dynamics. CL treatment for 7 days dramatically increased DNL and TG turnover similarly in all adipose depots, despite great differences in UCP1 abundance. Increased lipid turnover was accompanied by the simultaneous upregulation of genes involved in FAS, glycerol metabolism, and FA oxidation. Inducible, adipocyte-specific deletion of adipose TG lipase (ATGL), the rate-limiting enzyme for lipolysis, demonstrates that TG hydrolysis is required for CL-induced increases in DNL, TG turnover, and mitochondrial electron transport in all depots. Interestingly, the effect of ATGL deletion on induction of specific genes involved in FA oxidation and synthesis varied among fat depots. Overall, these studies indicate that FAS and FA oxidation are tightly coupled in adipose tissues during chronic adrenergic activation, and this effect critically depends on the activity of adipocyte ATGL.

Ebf2 is a selective marker of brown and beige adipogenic precursor cells

Brown adipocytes and muscle and dorsal dermis descend from precursor cells in the dermomyotome, but the factors that regulate commitment to the brown adipose lineage are unknown. Here, we prospectively isolated and determined the molecular profile of embryonic brown preadipose cells. Brown adipogenic precursor activity in embryos was confined to platelet-derived growth factor α(+), myogenic factor 5(Cre)-lineage-marked cells. RNA-sequence analysis identified early B-cell factor 2 (Ebf2) as one of the most selectively expressed genes in this cell fraction. Importantly, Ebf2-expressing cells purified from Ebf2(GFP) embryos or brown fat tissue did not express myoblast or dermal cell markers and uniformly differentiated into brown adipocytes. Interestingly, Ebf2-expressing cells from white fat tissue in adult animals differentiated into brown-like (or beige) adipocytes. Loss of Ebf2 in brown preadipose cells reduced the expression levels of brown preadipose-signature genes, whereas ectopic Ebf2 expression in myoblasts activated brown preadipose-specific genes. Altogether, these results indicate that Ebf2 specifically marks and regulates the molecular profile of brown preadipose cells.

A switch from white to brown fat increases energy expenditure in cancer-associated cachexia

Cancer-associated cachexia (CAC) is a wasting syndrome characterized by systemic inflammation, body weight loss, atrophy of white adipose tissue (WAT) and skeletal muscle. Limited therapeutic options are available and the underlying mechanisms are poorly defined. Here we show that a phenotypic switch from WAT to brown fat, a phenomenon termed WAT browning, takes place in the initial stages of CAC, before skeletal muscle atrophy. WAT browning is associated with increased expression of uncoupling protein 1 (UCP1), which uncouples mitochondrial respiration toward thermogenesis instead of ATP synthesis, leading to increased lipid mobilization and energy expenditure in cachectic mice. Chronic inflammation and the cytokine interleukin-6 increase UCP1 expression in WAT, and treatments that reduce inflammation or β-adrenergic blockade reduce WAT browning and ameliorate the severity of cachexia. Importantly, UCP1 staining is observed in WAT from CAC patients. Thus, inhibition of WAT browning represents a promising approach to ameliorate cachexia in cancer patients.

Taking control over intracellular fatty acid levels is essential for the analysis of thermogenic function in cultured primary brown and brite/beige adipocytes

Thermogenesis in brown adipocytes, conferred by mitochondrial uncoupling protein 1 (UCP1), is receiving great attention because metabolically active brown adipose tissue may protect humans from metabolic diseases. In particular, the thermogenic function of brown-like adipocytes in white adipose tissue, known as brite (or beige) adipocytes, is currently of prime interest. A valid procedure to quantify the specific contribution of UCP1 to thermogenesis is thus of vital importance. Adrenergic stimulation of lipolysis is a common way to activate UCP1. We here report, however, that in this frequently applied setup, taking control over intracellular fatty acid levels is essential for the analysis of thermogenic function in cultured brown and brite adipocytes. By the application of these findings, we demonstrate that UCP1 is functionally thermogenic in intact brite adipocytes and adrenergic UCP1 activation is largely dependent on adipose triglyceride lipase (ATGL) rather than hormone sensitive lipase (HSL).

Eosinophils and type 2 cytokine signaling in macrophages orchestrate development of functional beige fat

Beige fat, which expresses the thermogenic protein UCP1, provides a defense against cold and obesity. Although a cold environment is the physiologic stimulus for inducing beige fat in mice and humans, the events that lead from the sensing of cold to the development of beige fat remain poorly understood. Here, we identify the efferent beige fat thermogenic circuit, consisting of eosinophils, type 2 cytokines interleukin (IL)-4/13, and alternatively activated macrophages. Genetic loss of eosinophils or IL-4/13 signaling impairs cold-induced biogenesis of beige fat. Mechanistically, macrophages recruited to cold-stressed subcutaneous white adipose tissue (scWAT) undergo alternative activation to induce tyrosine hydroxylase expression and catecholamine production, factors required for browning of scWAT. Conversely, administration of IL-4 to thermoneutral mice increases beige fat mass and thermogenic capacity to ameliorate pre-established obesity. Together, our findings have uncovered the efferent circuit controlling biogenesis of beige fat and provide support for its targeting to treat obesity.

Inhibition of Notch signaling promotes browning of white adipose tissue and ameliorates obesity

Beige adipocytes in white adipose tissue (WAT) are similar to classical brown adipocytes in that they can burn lipids to produce heat. Thus, an increase in beige adipocyte content in WAT browning would raise energy expenditure and reduce adiposity. Here we report that adipose-specific inactivation of Notch1 or its signaling mediator Rbpj in mice results in browning of WAT and elevated expression of uncoupling protein 1 (Ucp1), a key regulator of thermogenesis. Consequently, as compared to wild-type mice, Notch mutants exhibit elevated energy expenditure, better glucose tolerance and improved insulin sensitivity and are more resistant to high fat diet-induced obesity. By contrast, adipose-specific activation of Notch1 leads to the opposite phenotypes. At the molecular level, constitutive activation of Notch signaling inhibits, whereas Notch inhibition induces, Ppargc1a and Prdm16 transcription in white adipocytes. Notably, pharmacological inhibition of Notch signaling in obese mice ameliorates obesity, reduces blood glucose and increases Ucp1 expression in white fat. Therefore, Notch signaling may be therapeutically targeted to treat obesity and type 2 diabetes.

The different shades of fat

Our understanding of adipose tissue biology has progressed rapidly since the turn of the century. White adipose tissue has emerged as a key determinant of healthy metabolism and metabolic dysfunction. This realization is paralleled only by the confirmation that adult humans have heat-dissipating brown adipose tissue, an important contributor to energy balance and a possible therapeutic target for the treatment of metabolic disease. We propose that the development of successful strategies to target brown and white adipose tissues will depend on investigations that elucidate their developmental origins and cell-type-specific functional regulators.

Muscle uncoupling protein 3 expression is unchanged by chronic ephedrine/caffeine treatment: results of a double blind, randomised clinical trial in morbidly obese females

Ephedrine/caffeine combination (EC) has been shown to induce a small-to-moderate weight loss in obese patients. Several mechanisms have been proposed, among which an increased thermogenic capacity of skeletal muscle consequent to the EC-induced up-regulation of uncoupling protein 3 (UCP3) gene expression. We did a parallel group double-blind, placebo-controlled, 4-week trial to investigate this hypothesis. Thirteen morbidly obese women (25-52 years of age, body-mass index 48.0±4.0 kg/m2, range 41.1-57.6) were randomly assigned to EC (200/20 mg, n = 6) or to placebo (n = 7) administered three times a day orally, before undergoing bariatric surgery. All individuals had an energy-deficit diet equal to about 70% of resting metabolic rate (RMR) diet (mean 5769±1105 kJ/day). The RMR analysed by intention to treat and the UCP3 (long and short isoform) mRNA levels in rectus abdominis were the primary outcomes. Body weight, plasma levels of adrenaline, noradrenaline, triglycerides, free fatty acids, glycerol, TSH, fT4, and fT3 were assessed, as well as fasting glucose, insulin and HOMA index, at baseline and at the end of treatments. Body weight loss was evident in both groups when compared to baseline values (overall -5.2±3.2%, p<0.0001) without significant differences between the treated groups. EC treatment increased the RMR (+9.2±6.8%, p = 0.020), differently from placebo which was linked to a reduction of RMR (-7.6±6.5%, p = 0.029). No significant differences were seen in other metabolic parameters. Notably, no changes of either UCP3 short or UCP3 long isoform mRNA levels were evident between EC and placebo group. Our study provides evidence that 4-week EC administration resulted in a pronounced thermogenic effect not related to muscle UCP3 gene expression and weight loss in morbidly obese females under controlled conditions.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02048215.

Sestrin2 inhibits uncoupling protein 1 expression through suppressing reactive oxygen species

Uncoupling protein 1 (Ucp1), which is localized in the mitochondrial inner membrane of mammalian brown adipose tissue (BAT), generates heat by uncoupling oxidative phosphorylation. Upon cold exposure or nutritional abundance, sympathetic neurons stimulate BAT to express Ucp1 to induce energy dissipation and thermogenesis. Accordingly, increased Ucp1 expression reduces obesity in mice and is correlated with leanness in humans. Despite this significance, there is currently a limited understanding of how Ucp1 expression is physiologically regulated at the molecular level. Here, we describe the involvement of Sestrin2 and reactive oxygen species (ROS) in regulation of Ucp1 expression. Transgenic overexpression of Sestrin2 in adipose tissues inhibited both basal and cold-induced Ucp1 expression in interscapular BAT, culminating in decreased thermogenesis and increased fat accumulation. Endogenous Sestrin2 is also important for suppressing Ucp1 expression because BAT from Sestrin2(-/-) mice exhibited a highly elevated level of Ucp1 expression. The redox-inactive mutant of Sestrin2 was incapable of regulating Ucp1 expression, suggesting that Sestrin2 inhibits Ucp1 expression primarily through reducing ROS accumulation. Consistently, ROS-suppressing antioxidant chemicals, such as butylated hydroxyanisole and N-acetylcysteine, inhibited cold- or cAMP-induced Ucp1 expression as well. p38 MAPK, a signaling mediator required for cAMP-induced Ucp1 expression, was inhibited by either Sestrin2 overexpression or antioxidant treatments. Taken together, these results suggest that Sestrin2 and antioxidants inhibit Ucp1 expression through suppressing ROS-mediated p38 MAPK activation, implying a critical role of ROS in proper BAT metabolism.

Expression of adipocyte biomarkers in a primary cell culture models reflects preweaning adipobiology

A cohort of genes was selected to characterize the adipogenic phenotype in primary cell cultures from three tissue sources. We compared the quantitative expression of biomarkers in culture relative to their expression in vivo because the mere presence or absence of expression is minimally informative. Although all biomarkers analyzed have biochemical functions in adipocytes, the expression of some of the biomarkers varied enormously in culture relative to their expression in the adult fat tissues in vivo, i.e. inguinal fat for white adipocytes and brite cells, interscapular brown adipose tissue for brown adipocytes, and ear mesenchymal stem cells for white adipocytes from adult mice. We propose that the pattern of expression in vitro does not reflect gene expression in the adult mouse; rather it is predominantly the expression pattern of adipose tissue of the developing mouse between birth and weaning. The variation in gene expression among fat depots in both human and rodent has been an extensively studied phenomenon, and as recently reviewed, it is related to subphenotypes associated with immune function, the inflammatory response, fat depot blood flow, and insulin sensitivity. We suggest that adipose tissue biology in the period from birth to weaning is not just a staging platform for the emergence of adult white fat but that it has properties to serve the unique needs of energy metabolism in the newborn. A case in point is the differentiation of brite cells that occurs during this period followed by their involution immediately following weaning.

On the evolutionary origins of obesity: a new hypothesis

Obesity is an escalating threat of pandemic proportions, currently affecting billions of people worldwide and exerting a devastating socioeconomic influence in industrialized countries. Despite intensive efforts to curtail obesity, results have proved disappointing. Although it is well recognized that obesity is a result of gene-environment interactions and that predisposition to obesity lies predominantly in our evolutionary past, there is much debate as to the precise nature of how our evolutionary past contributed to obesity. The "thrifty genotype" hypothesis suggests that obesity in industrialized countries is a throwback to our ancestors having undergone positive selection for genes that favored energy storage as a consequence of the cyclical episodes of famine and surplus after the advent of farming 10 000 years ago. Conversely, the "drifty genotype" hypothesis contends that the prevalence of thrifty genes is not a result of positive selection for energy-storage genes but attributable to genetic drift resulting from the removal of predative selection pressures. Both theories, however, assume that selection pressures the ancestors of modern humans living in western societies faced were the same. Moreover, neither theory adequately explains the impact of globalization and changing population demographics on the genetic basis for obesity in developed countries, despite clear evidence for ethnic variation in obesity susceptibility and related metabolic disorders. In this article, we propose that the modern obesity pandemic in industrialized countries is a result of the differential exposure of the ancestors of modern humans to environmental factors that began when modern humans left Africa around 70 000 years ago and migrated through the globe, reaching the Americas around 20 000 years ago. This article serves to elucidate how an understanding of ethnic differences in genetic susceptibility to obesity and the metabolic syndrome, in the context of historic human population redistribution, could be used in the treatment of obesity in industrialized countries.

ROS production in brown adipose tissue mitochondria: the question of UCP1-dependence

Whether active UCP1 can reduce ROS production in brown-fat mitochondria is presently not settled. The issue is of principal significance, as it can be seen as a proof- or disproof-of-principle concerning the ability of any protein to diminish ROS production through membrane depolarization. We therefore undertook a comprehensive investigation of the significance of UCP1 for ROS production, by comparing the ROS production in brown-fat mitochondria isolated from wildtype mice (that display membrane depolarization) or from UCP1(-/-) mice (with a high membrane potential). We tested the significance of UCP1 for glycerol-3-phosphate-supported ROS production by three methods (fluorescent dihydroethidium and the ESR probe PHH for superoxide, and fluorescent Amplex Red for hydrogen peroxide), and followed ROS production also with succinate, acyl-CoA or pyruvate as substrate. We studied the effects of the reverse electron flow inhibitor rotenone, the UCP1 activity inhibitor GDP, and the uncoupler FCCP. We also examined the effect of a physiologically induced increase in UCP1 amount. We noted GDP effects that were not UCP1-related. We conclude that only ROS production supported by exogenously added succinate was affected by the presence of active UCP1; ROS production supported by any other tested substrate (including endogenously generated succinate) was unaffected. This conclusion indicates that UCP1 is not involved in control of ROS production in brown-fat mitochondria. Extrapolation of these data to other tissues would imply that membrane depolarization may not necessarily decrease physiologically relevant ROS production. This article is a part of a Special Issue entitled: 18th European Bioenergetics Conference (Biochim. Biophys. Acta, Volume 1837, Issue 7, July 2014).

Brite/beige fat and UCP1 - is it thermogenesis?

The presence of two distinct types of adipose tissue, which have opposing functions, has been known for decades. White adipose tissue (WAT) is the main tissue of energy storage, while brown adipose tissue (BAT) dissipates energy as heat and is required for non-shivering thermoregulation. In the last few years, a third type of adipocyte was identified, termed the brite ("brown and white") or beige adipocyte. Their physiological control and role, however, are not fully clarified. Brite/beige adipocytes have a positive impact on systemic metabolism that is generally explained by the thermogenesis of brite/beige adipocytes; although thermogenesis has not been directly measured but is mostly inferred by gene expression data of typical thermogenic genes such as uncoupling protein 1 (UCP1). Here we critically review functional evidence for the thermogenic potential of brite/beige adipocytes, leading to the conclusion that direct measurements of brite/beige adipocyte bioenergetics, beyond gene regulation, are pivotal to quantify their thermogenic potential. In particular, we exemplified that the massive induction of UCP1 mRNA during the browning of isolated subcutaneous adipocytes in vitro is not reflected in significant alterations of cellular bioenergetics. Herein, we demonstrate that increases in mitochondrial respiration in response to beta-adrenergic stimulus can be independent of UCP1. Using HEK293 cells expressing UCP1, we show how to directly assess UCP1 function by adequate activation in intact cells. Finally, we provide a guide on the interpretation of UCP1 activity and the pitfalls by solely using respiration measurements. The functional analysis of beige adipocyte bioenergetics will assist to delineate the impact of browning on thermogenesis, possibly elucidating additional physiological roles and its contribution to systemic metabolism, highlighting possible avenues for future research. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference.