Impact of brown adipose tissue on body fatness and glucose metabolism in healthy humans

Macrophage infiltration into obese adipose tissues suppresses the induction of UCP1 level in mice

Emergence of thermogenic adipocytes such as brown and beige adipocytes is critical for whole body energy metabolism. Promoting the emergence of these adipocytes, which increase energy expenditure, could be a viable strategy in treating obesity and its related diseases. However, little is known regarding the mechanisms that regulate the emergence of these adipocytes in obese adipose tissue. Here, we demonstrated that classically activated macrophages (M1 Mϕ) suppress the induction of thermogenic adipocytes in obese adipose tissues of mice. Cold exposure significantly induced the expression levels of uncoupling protein-1 (UCP1), which is a mitochondrial protein unique in thermogenic adipocytes, in C57BL/6 mice fed a normal diet. However, UCP1 induction was significantly suppressed in adipose tissues of C57BL/6 mice fed a high-fat diet, into which M1 Mϕ infiltrated. Depletion of M1 Mϕ using clodronate liposomes eliminated the suppressive effect and markedly reduced the mRNA level of tumor necrosis factor-α (TNFα) in the adipose tissues. Importantly, consistent with the observed changes in the expression levels of marker genes for thermogenic adipocytes, combination treatment of clodronate liposome and cold exposure resulted in metabolic benefits such as lowered body weight and blood glucose level in obese mice. Moreover, intraperitoneal injection of recombinant TNFα protein suppressed UCP1 induction in lean adipose tissues of mice. Collectively, our data indicate that infiltrated M1 Mϕ suppress the induction of thermogenic adipocytes in obese adipose tissues via TNFα. This report suggests that inflammation induced by infiltrated Mϕ could cause not only insulin resistance but also reduction of energy expenditure in adipose tissues.

Brown Adipose Tissue Exhibits a Glucose-Responsive Thermogenic Biorhythm in Humans

High abundance of brown adipose tissue (BAT) is linked to lower glycaemia in humans, leading to the belief that BAT may protect against diabetes. The relationship between BAT glucose utilization and systemic glucose homeostasis has not been defined. In this paper we have characterized glycaemic excursions and BAT thermogenic responses in human brown adipocytes, BAT explants, and healthy adults through supraclavicular temperature profiling, revealing their circadian coupling in vivo and in vitro, orchestrated by UCP1, GLUT4, and Rev-erbα biorhythms. Extent of glycated haemoglobin also correlated positively with environmental temperature among community-dwelling patients. These data uncover potential crosstalk between BAT and glucose regulatory pathways, evident on cellular, tissue, individual, and population levels, and provide impetus to search for BAT harnessing strategies for therapeutic purposes.

Brown adipose tissue is involved in the seasonal variation of cold-induced thermogenesis in humans

Brown adipose tissue (BAT) contributes to whole-body energy expenditure (EE), especially cold-induced thermogenesis (CIT), in humans. Although it is known that EE and CIT vary seasonally, their relationship with BAT has not been investigated. In the present study, we examined the impact of BAT on seasonal variations of EE/CIT and thermal responses to cold exposure in a randomized crossover design. Forty-five healthy male volunteers participated, and their BAT was assessed by positron emission tomography and computed tomography. CIT, the difference of EE at 27ºC and after 2-h cold exposure at 19ºC, significantly increased in winter compared to summer, being greater in subjects with metabolically active BAT (High BAT, 185.6 kcal/d, 18.3 kcal/d, P<0.001) than those without (Low BAT, 90.6 kcal/d, -46.5 kcal/d, P<0.05). Multivariate regression analysis revealed a significant interaction effect between season and BAT on CIT (P<0.001). The cold-induced drop of tympanic temperature (T_ty) and skin temperature (T_skin) in the forehead region and in the supraclavicular region close to BAT deposits were smaller in the High BAT group than in the Low BAT group in winter but not in summer. In contrast, the drop of T_skinin the subclavicular and peripheral regions distant from BAT was similar in the two groups in both seasons. In conclusion, CIT increased from summer to winter in a BAT-dependent manner, paralleling cold-induced changes in T_ty/T_skin, indicating a role of BAT in seasonal changes in the thermogenic and thermal responses to cold exposure in humans.

An Emerging Role of Natriuretic Peptides: Igniting the Fat Furnace to Fuel and Warm the Heart

Natriuretic peptides are produced in the heart and have been well characterized for their actions in the cardiovascular system to promote diuresis and natriuresis, thereby contributing to maintenance of extracellular fluid volume and vascular tone. For this review, we scanned the literature using PubMed and MEDLINE using the following search terms: beiging, adipose tissue, natriuretic peptides, obesity, and metabolic syndrome. Articles were selected for inclusion if they represented primary data or review articles published from 1980 to 2015 from high-impact journals. With the advent of the newly approved class of drugs that inhibit the breakdown of natriuretic peptides, thereby increasing their circulation, we highlight additional functions for natriuretic peptides that have recently become appreciated, including their ability to drive lipolysis, facilitate beiging of adipose tissues, and promote lipid oxidation and mitochondrial respiration in skeletal muscle. We provide evidence for new roles for natriuretic peptides, emphasizing their ability to participate in body weight regulation and energy homeostasis and discuss how they may lead to novel strategies to treat obesity and the metabolic syndrome.

Type 2 diabetes in migrant south Asians: mechanisms, mitigation, and management

South Asians, particularly when living in high-income countries, are at a substantially elevated risk of type 2 diabetes compared with white Europeans, and typically develop the disease 5-10 years earlier and at a lower BMI. Migrant south Asians seem to be more insulin resistant than white Europeans across the life course and potentially experience β-cell exhaustion at a younger age. Differences in adiposity (high percentage of body fat and high proportion of deep subcutaneous and visceral fat) and skeletal muscle (low percentage of lean mass and low cardiorespiratory fitness) are likely to contribute these factors. No clear evidence is available suggesting genetic factors make a major contribution to the increased risk of diabetes in south Asians, but epigenetic factors might have a role. Irrespective of future mechanistic discoveries, south Asians need to be encouraged and helped-by various culturally appropriate methods--to maintain a high physical activity level and low bodyweight across the life course to prevent diabetes. In clinical terms, cardiovascular risks have attenuated over time in migrant south Asians with diabetes but retinopathy and renal complication risks remain high because of the high levels of glycaemia and rapid glycaemic deterioration noted in this population. We review these aspects and suggest areas for future research.

EBF2 promotes the recruitment of beige adipocytes in white adipose tissue

Objective

The induction of beige/brite adipose cells in white adipose tissue (WAT) is associated with protection against high fat diet-induced obesity and insulin resistance in animals. The helix-loop-helix transcription factor Early B-Cell Factor-2 (EBF2) regulates brown adipose tissue development. Here, we asked if EBF2 regulates beige fat cell biogenesis and protects animals against obesity.

Methods

In addition to primary cell culture studies, we used ​Ebf2 knockout mice and mice overexpressing EBF2 in the adipose tissue to study the necessity and sufficiency of EBF2 to induce beiging in vivo.

Results

We found that EBF2 is required for beige adipocyte development in mice. Subcutaneous WAT or primary adipose cell cultures from Ebf2 knockout mice did not induce Uncoupling Protein 1 (UCP1) or a thermogenic program following adrenergic stimulation. Conversely, over-expression of EBF2 in adipocyte cultures induced UCP1 expression and a brown-like/beige fat-selective differentiation program. Transgenic expression of Ebf2 in adipose tissues robustly stimulated beige adipocyte development in the WAT of mice, even while housed at thermoneutrality. EBF2 overexpression was sufficient to increase mitochondrial function in WAT and protect animals against high fat diet-induced weight gain.

Conclusions

Taken together, our results demonstrate that EBF2 controls the beiging process and suggest that activation of EBF2 in WAT could be used to reduce obesity.

•

Loss of Ebf2 prevents induction of beige adipocytes in inguinal WAT.

•

Ectopic expression of Ebf2 promotes beige fat induction in inguinal WAT.

•

Ectopic Ebf2 expression protects against high fat diet-induced obesity.

Capsaicin and Related Food Ingredients Reducing Body Fat Through the Activation of TRP and Brown Fat Thermogenesis

Brown adipose tissue (BAT) is a site of sympathetically activated adaptive nonshivering thermogenesis, thereby being involved in the regulation of energy balance and body fatness. Recent radionuclide imaging studies have revealed the existence of metabolically active BAT in adult humans. Human BAT is activated by acute cold exposure and contributes to cold-induced increase in whole-body energy expenditure. The metabolic activity of BAT is lower in older and obese individuals. The inverse relationship between the BAT activity and body fatness suggests that BAT, because of its energy dissipating activity, is protective against body fat accumulation. In fact, repeated cold exposure recruits BAT in association with increased energy expenditure and decreased body fatness. The stimulatory effects of cold are mediated through the activation of transient receptor potential (TRP) channels, most of which are also chemesthetic receptors for various naturally occurring substances including herbal plants and food ingredients. Capsaicin and its analog capsinoids, representative agonists of TRPV1, mimic the effects of cold to decrease body fatness through the activation and recruitment of BAT. The well-known antiobesity effect of green tea catechins is also attributable to the activation of the sympathetic nerve and BAT system. Thus, BAT is a promising target for combating obesity and related metabolic disorders in humans.

Paracrine, endocrine and neurocrine controls of the adipocyte color phenotype: view from the chair

After a long drought caused by misjudged irrelevance to human biology, the research field of brown adipose tissue has seen a period of resurgence since 2009 when discoveries of brown fat in adults were reported. However, the molecular and physiological regulators of the different types of adipose tissues-white, beige or brown-are still far from being fully determined. Speakers of the morning session of the 16th Annual Symposium of the Université Laval's Chair in Obesity, a series interestingly launched in 1998 precisely on the topic of uncoupling proteins, presented past and recent findings on non-adrenergic signaling pathways-both upstream and downstream-regulating the metabolic and thermogenic activities of adipose tissue. They went on to show that these pathways are altered in the contexts of obesity and aging, the latter being a very important factor involved in the decline of non-shivering thermogenesis. Whereas opinions diverged on readily applicable solutions for development of candidate therapeutics, the panelists agreed that the new factors involved in the control of the adipose thermogenic program hold great promise for innovation. This will likely depend on how this novel knowledge is integrated into the complex regulation of thermogenesis, which will be achieved through better-defined experimental protocols, both in humans and non-human models.

Brown adipose tissue and bone

Brown adipose tissue (BAT) is capable of transforming chemically stored energy, in the form of triglycerides, into heat. Recent studies have shown that metabolically active BAT is present in a large proportion of adult humans, where its activity correlates with a favorable metabolic status. Hence, the tissue is now regarded as an interesting target for therapies against obesity and associated diseases such as type 2 diabetes, the hypothesis being that an induction of BAT would be beneficial for these disease states. Apart from the association between BAT activity and a healthier metabolic status, later studies have also shown a positive correlation between BAT volume and both bone cross-sectional area and bone mineral density, suggesting that BAT might stimulate bone anabolism. The aim of this review is to give the reader a brief overview of the BAT research field and to summarize and discuss recent findings regarding BAT being a potential player in bone metabolism.

Opportunities and challenges in three-dimensional brown adipogenesis of stem cells

The formation of brown adipose tissue (BAT) via brown adipogenesis has become a notable process due to its ability to expend energy as heat with implications in the treatment of metabolic disorders and obesity. With the advent of complexity within white adipose tissue (WAT) along with inducible brown adipocytes (also known as brite and beige), there has been a surge in deciphering adipocyte biology as well as in vivo adipogenic microenvironments. A therapeutic outcome would benefit from understanding early events in brown adipogenesis, which can be accomplished by studying cellular differentiation. Pluripotent stem cells are an efficient model for differentiation and have been directed towards both white adipogenic and brown adipogenic lineages. The stem cell microenvironment greatly contributes to terminal cell fate and as such, has been mimicked extensively by various polymers including those that can form 3D hydrogel constructs capable of biochemical and/or mechanical modifications and modulations. Using bioengineering approaches towards the creation of 3D cell culture arrangements is more beneficial than traditional 2D culture in that it better recapitulates the native tissue biochemically and biomechanically. In addition, such an approach could potentially protect the tissue formed from necrosis and allow for more efficient implantation. In this review, we highlight the promise of brown adipocytes with a focus on brown adipogenic differentiation of stem cells using bioengineering approaches, along with potential challenges and opportunities that arise when considering the energy expenditure of BAT for prospective therapeutics.

Insufficient glucose supply is linked to hypothermia upon cold exposure in high-fat diet-fed mice lacking PEMT

Mice that lack phosphatidylethanolamine N-methyltransferase (Pemt(-/-) mice) are protected from high-fat (HF) diet-induced obesity. HF-fed Pemt(-/-) mice show higher oxygen consumption and heat production, indicating that more energy might be utilized for thermogenesis and might account for the resistance to diet-induced weight gain. To test this hypothesis, HF-fed Pemt(-/-) and Pemt(+/+) mice were challenged with acute cold exposure at 4°C. Unexpectedly, HF-fed Pemt(-/-) mice developed hypothermia within 3 h of cold exposure. In contrast, chow-fed Pemt(-/-) mice, possessing similar body mass, maintained body temperature. Lack of PEMT did not impair the capacity for thermogenesis in skeletal muscle or brown adipose tissue. Plasma catecholamines were not altered by Pemt genotype, and stimulation of lipolysis was intact in brown and white adipose tissue of Pemt(-/-) mice. HF-fed Pemt(-/-) mice also developed higher systolic blood pressure, accompanied by reduced cardiac output. Choline supplementation reversed the cold-induced hypothermia in HF-fed Pemt(-/-) mice with no effect on blood pressure. Plasma glucose levels were ∼50% lower in HF-fed Pemt(-/-) mice compared with Pemt(+/+) mice. Choline supplementation normalized plasma hypoglycemia and the expression of proteins involved in gluconeogenesis. We propose that cold-induced hypothermia in HF-fed Pemt(-/-) mice is linked to plasma hypoglycemia due to compromised hepatic glucose production.

Kaempferia parviflora extract increases whole-body energy expenditure in humans: roles of brown adipose tissue

Kaempferia parviflora extract (KP) has been reported to have a preventive effect on obesity in mice, probably by increasing energy expenditure (EE). The aims of the current study were to examine the acute effects of KP ingestion on whole-body EE in humans and to analyze its relation to the activity of brown adipose tissue (BAT), a site of non-shivering thermogenesis. After an oral ingestion of an ethanol extract of KP, EE increased significantly, showing a maximal increase of 229±69 kJ/d at 60 min, while it did not change after placebo ingestion. To evaluate BAT activity, the subjects underwent fluorodeoxyglucose-positron emission tomography, and divided into two groups with high- and low-BAT activities. A similar and greater response of EE to KP ingestion was observed in the high-BAT group (351±50 kJ/d at 60 min), but not in the low activity group. Placebo ingestion did not cause any significant EE change in either group. These results indicate that a single oral ingestion of the KP extract can potentially increase whole-body EE probably through the activation of BAT in healthy men, and may be useful as an anti-obesity regimen.

Genetic disruption of uncoupling protein 1 in mice renders brown adipose tissue a significant source of FGF21 secretion

OBJECTIVE

Circulating fibroblast growth factor 21 (FGF21) is an important auto- and endocrine player with beneficial metabolic effects on obesity and diabetes. In humans, thermogenic brown adipose tissue (BAT) was recently suggested as a source of FGF21 secretion during cold exposure. Here, we aim to clarify the role of UCP1 and ambient temperature in the regulation of FGF21 in mice.

METHODS

Wildtype (WT) and UCP1-knockout (UCP1 KO) mice, the latter being devoid of BAT-derived non-shivering thermogenesis, were exposed to different housing temperatures. Plasma metabolites and FGF21 levels were determined, gene expression was analyzed by qPCR, and tissue histology was performed with adipose tissue.

RESULTS

At thermoneutrality, FGF21 gene expression and serum levels were not different between WT and UCP1 KO mice. Cold exposure led to highly increased FGF21 serum levels in UCP1 KO mice, which were reflected in increased FGF21 gene expression in adipose tissues but not in liver and skeletal muscle. Ex vivo secretion assays revealed FGF21 release only from BAT, progressively increasing with decreasing ambient temperatures. In association with increased FGF21 serum levels in the UCP1 KO mouse, typical FGF21-related serum metabolites and inguinal white adipose tissue morphology and thermogenic gene expression were altered.

CONCLUSIONS

Here we show that the genetic ablation of UCP1 increases FGF21 gene expression in adipose tissue. The removal of adaptive nonshivering thermogenesis renders BAT a significant source of endogenous FGF21 under thermal stress. Thus, the thermogenic competence of BAT is not a requirement for FGF21 secretion. Notably, high endogenous FGF21 levels in UCP1-deficient models and subjects may confound pharmacological FGF21 treatments.

Brown adipose tissue triglyceride content is associated with decreased insulin sensitivity, independently of age and obesity

The aim of the present study was to determine whether single-voxel proton magnetic resonance spectroscopy ((1)H-MRS) can non-invasively assess triglyceride content in both supraclavicular fat depots and subcutaneous white adipose tissue (WAT) to determine whether these measurements correlate to metabolic variables. A total of 25 healthy volunteers were studied using (18)F-fluorodeoxyglucose positron emission tomography (PET) and (15)O-H2O PET perfusion during cold exposure, and (1)H-MRS at ambient temperature. Image-guided biopsies were collected from nine volunteers. The supraclavicular triglyceride content determined by (1)H-MRS varied between 60 and 91% [mean ± standard deviation (s.d.) 77 ± 10%]. It correlated positively with body mass index, waist circumference, subcutaneous and visceral fat masses and 8-year diabetes risk based on the Framingham risk score and inversely with HDL cholesterol and insulin sensitivity (M-value; euglycaemic-hyperinsulinaemic clamp). Subcutaneous WAT had a significantly higher triglyceride content, 76-95% (mean ± s.d. 87 ± 5%; p = 0.0002). In conclusion, the triglyceride content in supraclavicular fat deposits measured by (1)H-MRS may be an independent marker of whole-body insulin sensitivity, independent of brown adipose tissue metabolic activation.

Human brown adipose tissue assessed by simple, noninvasive near-infrared time-resolved spectroscopy

OBJECTIVE

Human brown adipose tissue (BAT) activity has been typically evaluated by (18) F-fluorodeoxyglucose (FDG)-positron emission tomography (PET) combined with computed tomography (CT). However, FDG-PET/CT has serious limitations (e.g., radiation and cold exposure). This study evaluated BAT density using near-infrared time-resolved spectroscopy (NIRTRS ), a simple and noninvasive method of measuring the indices of tissue hemoglobin concentration [total-Hb] and mitochondrial density (µs ').

METHODS

The NIRTRS parameters at 760, 800, and 830 nm in the supraclavicular region potentially containing BAT were evaluated. First, the NIRTRS parameters were compared at 27 °C and during a 2-h cold exposure (19 °C) in 18 men. Then, NIRTRS parameters at 27 °C were compared with mean standardized uptake values (SUVmean ) assessed by FDG-PET/CT after the 2-h cold exposure (19 °C) in 29 men.

RESULTS

There was no significant difference between the NIRTRS parameters at 27 °C and 19°C. The [total-Hb] and µs ' were significantly correlated to SUVmean (r = 0.73 and r = 0.64, respectively). A receiver operating characteristic analysis revealed that the sensitivity (75.0-82.4%), specificity (91.7-100%), and accuracy (82.8-86.2%) of the NIRTRS parameters were all good to determine the NIRTRS reliability.

CONCLUSIONS

Our novel NIRTRS method is noninvasive and simple and can reliably assess human BAT density in the supraclavicular region.

Transcriptional control and hormonal response of thermogenic fat

Obesity and its associated metabolic diseases present a major public health problem around the world. The discovery that thermogenic fat is active in adult humans has sparked a renewal of interest in the study of its development and function and in the feasibility of using modulators of thermogenesis to work against obesity. In recent years, it has been shown that there are at least two distinct types of thermogenic fat cells: brown and beige fat. In this review, we discuss the transcriptional mediators of thermogenesis and the signaling molecules that regulate thermogenic cells. We also review the effects of thermogenic fat activation on whole-body metabolic parameters and evaluate the increasing evidence that activating thermogenesis in humans can be a viable method of ameliorating obesity. In these discussions, we highlight targets that can potentially be stimulated or modified in anti-obesity treatments.

Brown adipose tissue activation is inversely related to central obesity and metabolic parameters in adult human

Background

Recent studies have shown that adult human possess active brown adipose tissue (BAT), which might be important in affecting obesity. However, the supporting evidence on the relationship between BAT and central obesity and metabolic profile in large population based studies is sparse.

Methodology/Principal Findings

We studied 4011 (2688 males and 1323 females) tumor-free Chinese adults aged 18-89 for BAT activities, visceral/subcutaneous fat areas (VFA/SFA), waist circumferences (WC) and metabolic parameters. We found that the prevalence of BAT was around 2.7% in our study participants, with a significant sexual difference (5.5% in the females vs. 1.3% in the males; p<0.0001). BAT detection was increased in low temperature and declined in elderly subjects. The BAT positive subjects had lower BMI (P<0.0001), less SFA (P<0.01), VFA (P<0.0001), WC (P<0.0001), lower fasting glucose and triglyceride levels (both P<0.01) and increased HDL cholesterol concentrations (P<0.0001), compared with the BAT negative subjects. Robust logistic regression revealed that after adjustment for covariates (including age, sex, BMI, VFA, SFA and WC), age and BMI in the males (0.92 [95%CI, 0.88-0.96] and 0.84 [95% CI, 0.75-0.96], both P ≤0.008) while age and VFA in the females (0.87 [95%CI, 0.83-0.91] and 0.98 [95%CI, 0.97-0.99], respectively, P<0.05) were independently associated with detectable BAT.

Conclusions/Significance

Our data suggest that decreased amount of active BAT might be associated with accumulation of visceral fat content and unfavorable metabolic outcomes.

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.

Brown adipose tissue activity as a target for the treatment of obesity/insulin resistance

Presence of brown adipose tissue (BAT), characterized by the expression of the thermogenic uncoupling protein 1 (UCP1), has recently been described in adult humans. UCP1 is expressed in classical brown adipocytes, as well as in “beige cells” in white adipose tissue (WAT). The thermogenic activity of BAT is mainly controlled by the sympathetic nervous system. Endocrine factors, such as fibroblast growth factor 21 (FGF21) and bone morphogenic protein factor-9 (BMP-9), predominantly produced in the liver, were shown to lead to activation of BAT thermogenesis, as well as to “browning” of WAT. This was also observed in response to irisin, a hormone secreted by skeletal muscles. Different approaches were used to delineate the impact of UCP1 on insulin sensitivity. When studied under thermoneutral conditions, UCP1 knockout mice exhibited markedly increased metabolic efficiency due to impaired thermogenesis. The impact of UCP1 deletion on insulin sensitivity in these mice was not reported. Conversely, several studies in both rodents and humans have shown that BAT activation (by cold exposure, β3-agonist treatment, transplantation and others) improves glucose tolerance and insulin sensitivity. Interestingly, similar results were obtained by adipose tissue-specific overexpression of PR-domain-containing 16 (PRDM16) or BMP4 in mice. The mediators of such beneficial effects seem to include FGF21, interleukin-6, BMP8B and prostaglandin D2 synthase. Interestingly, some of these molecules can be secreted by BAT itself, indicating the occurrence of autocrine effects. Stimulation of BAT activity and/or recruitment of UCP1-positive cells are therefore relevant targets for the treatment of obesity/type 2 diabetes in humans.

Insights into Brown Adipose Tissue Physiology as Revealed by Imaging Studies

There has been resurgence in interest in brown adipose tissue (BAT) following radiological and histological identification of metabolically active BAT in adult humans. Imaging enables BAT to be studied non-invasively and therefore imaging studies have contributed a significant amount to what is known about BAT function in humans. In this review the current knowledge (derived from imaging studies) about the prevalence, function, activity and regulation of BAT in humans (as well as relevant rodent studies), will be summarized.

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.

Brown Adipose Tissue: A Human Perspective

Since 2009, the presence of brown adipose tissue (BAT) in adult humans has been irrefutably proven. It is estimated that active BAT can contribute up to 2.5-5% of resting metabolic rate in humans, suggesting that sustained activation of BAT may alleviate obesity and associated disorders. In the current chapter, the discovery of BAT in adult humans will be discussed. Furthermore, the characteristics of human BAT, methods to visualize the tissue as well as physiological and pharmacological methods to enhance its activity will be stressed.

BMP7 drives human adipogenic stem cells into metabolically active beige adipocytes

Adult humans have a substantial amount of inducible-brown (or beige) fat, which is associated with increased energy expenditure and reduced weight gain via thermogenesis. Despite the identification of key regulators of beige adipogenesis, impacts of dietary factors on adaptive thermogenesis are largely unknown, partly due to a lack of validated human cell models. Bone morphogenetic protein 7 (BMP7) is known to promote brown adipogenesis in rodent and human progenitor cells. However, controversy still surrounds the cellular identity in BMP7-mediated transition of white to brown adipocytes. The aim of this study was to confirm BMP7-derived human adipocytes as a relevant in vitro model of human beige adipocyte by verifying the cellular lineage and metabolic activity. In this study, we hypothesized that pre-exposure of the stromal vascular (SV) fraction of primary human adipogenic precursor cells (hASC) to BMP7 would convert metabolically active brown adipocytes. Our results showed that exposure of hASC to human BMP7 was associated with significant escalation of (1) UCP1 gene expression, a signature gene of brown adipocytes, (2) beige specific marker gene expression (i.e., CD137 and TMEM26), (3) glucose and fatty acid uptake, and (4) basal and cAMP-stimulated oxygen consumption rate compared to white adipocyte control. Taken together, we demonstrated that BMP7 mediates conversion of hASC into metabolically active beige adipocytes. By confirming the cellular identity and metabolic activity, this BMP7-induced human beige adipocytes from hASC should aid in the discovery and assessment of bioactive molecules to promote adaptive thermogenesis.

Brown adipose tissue improves whole-body glucose homeostasis and insulin sensitivity in humans

Brown adipose tissue (BAT) has attracted scientific interest as an antidiabetic tissue owing to its ability to dissipate energy as heat. Despite a plethora of data concerning the role of BAT in glucose metabolism in rodents, the role of BAT (if any) in glucose metabolism in humans remains unclear. To investigate whether BAT activation alters whole-body glucose homeostasis and insulin sensitivity in humans, we studied seven BAT-positive (BAT(+)) men and five BAT-negative (BAT(-)) men under thermoneutral conditions and after prolonged (5-8 h) cold exposure (CE). The two groups were similar in age, BMI, and adiposity. CE significantly increased resting energy expenditure, whole-body glucose disposal, plasma glucose oxidation, and insulin sensitivity in the BAT(+) group only. These results demonstrate a physiologically significant role of BAT in whole-body energy expenditure, glucose homeostasis, and insulin sensitivity in humans, and support the notion that BAT may function as an antidiabetic tissue in humans.

Temperature-acclimated brown adipose tissue modulates insulin sensitivity in humans

In rodents, brown adipose tissue (BAT) regulates cold- and diet-induced thermogenesis (CIT; DIT). Whether BAT recruitment is reversible and how it impacts on energy metabolism have not been investigated in humans. We examined the effects of temperature acclimation on BAT, energy balance, and substrate metabolism in a prospective crossover study of 4-month duration, consisting of four consecutive blocks of 1-month overnight temperature acclimation (24 °C [month 1] → 19 °C [month 2] → 24 °C [month 3] → 27 °C [month 4]) of five healthy men in a temperature-controlled research facility. Sequential monthly acclimation modulated BAT reversibly, boosting and suppressing its abundance and activity in mild cold and warm conditions (P < 0.05), respectively, independent of seasonal fluctuations (P < 0.01). BAT acclimation did not alter CIT but was accompanied by DIT (P < 0.05) and postprandial insulin sensitivity enhancement (P < 0.05), evident only after cold acclimation. Circulating and adipose tissue, but not skeletal muscle, expression levels of leptin and adiponectin displayed reciprocal changes concordant with cold-acclimated insulin sensitization. These results suggest regulatory links between BAT thermal plasticity and glucose metabolism in humans, opening avenues to harnessing BAT for metabolic benefits.

Estrogen Deficiency and the Origin of Obesity during Menopause

Sex hormones strongly influence body fat distribution and adipocyte differentiation. Estrogens and testosterone differentially affect adipocyte physiology, but the importance of estrogens in the development of metabolic diseases during menopause is disputed. Estrogens and estrogen receptors regulate various aspects of glucose and lipid metabolism. Disturbances of this metabolic signal lead to the development of metabolic syndrome and a higher cardiovascular risk in women. The absence of estrogens is a clue factor in the onset of cardiovascular disease during the menopausal period, which is characterized by lipid profile variations and predominant abdominal fat accumulation. However, influence of the absence of these hormones and its relationship to higher obesity in women during menopause are not clear. This systematic review discusses of the role of estrogens and estrogen receptors in adipocyte differentiation, and its control by the central nervous systemn and the possible role of estrogen-like compounds and endocrine disruptors chemicals are discussed. Finally, the interaction between the decrease in estrogen secretion and the prevalence of obesity in menopausal women is examined. We will consider if the absence of estrogens have a significant effect of obesity in menopausal women.

Human brown adipose tissue: regulation and anti-obesity potential

Brown adipose tissue (BAT) is the site of sympathetically activated adaptive thermognenesis during cold exposure and after hyperphagia, thereby controlling whole-body energy expenditure (EE) and body fat. Radionuclide imaging studies have demonstrated that adult humans have metabolically active BAT composed of mainly beige/brite adipocytes, recently identified brown-like adipocytes. The inverse relationship between the BAT activity and body fatness suggests that BAT is, because of its energy dissipating activity, protective against body fat accumulation in humans as it is in small rodents. In fact, either repeated cold exposure or daily ingestion of some food ingredients acting on transient receptor potential channels recruits BAT in parallel with increased EE and decreased body fat. In addition to the sympathetic nervous system, several endocrine factors are also shown to recruit BAT. Thus, BAT is a promising therapeutic target for combating human obesity and related metabolic disorders.