Implications of nonshivering thermogenesis for energy balance regulation in humans

Brown adipose tissue: endocrine determinants of function and therapeutic manipulation as a novel treatment strategy for obesity

Introduction

Recent observation of brown adipose tissue (BAT) being functional in adult humans provides a rationale for its stimulation to increase energy expenditure through ‘adaptive thermogenesis’ for an anti-obesity strategy. Many endocrine dysfunctions are associated with changes in metabolic rate that over time may result in changes in body weight. It is likely that human BAT plays a role in such processes.

Review

In this brief review article, we explore the endocrine determinants of BAT activity, and discuss how these insights may provide a basis for future developments of novel therapeutic strategies for obesity management.

A review of electronic and print data comprising original and review articles retrieved from PubMed search up to December 2013 was conducted (Search terms: brown adipose tissue, brown fat, obesity, hormone). In addition, relevant references from the articles were screened for papers containing original data.

Conclusion

There is promising data to suggest that targeting endocrine hormones for BAT modulation can yield a cellular bioenergetics answer for successful prevention and management of human obesity. Further understanding of the physiological link between various endocrine hormones and BAT is necessary for the development of new therapeutic options.

Electronic supplementary material

The online version of this article (doi:10.1186/s40608-014-0013-5) contains supplementary material, which is available to authorized users.

Frequent extreme cold exposure and brown fat and cold-induced thermogenesis: a study in a monozygotic twin

INTRODUCTION

Mild cold acclimation is known to increase brown adipose tissue (BAT) activity and cold-induced thermogenesis (CIT) in humans. We here tested the effect of a lifestyle with frequent exposure to extreme cold on BAT and CIT in a Dutch man known as 'the Iceman', who has multiple world records in withstanding extreme cold challenges. Furthermore, his monozygotic twin brother who has a 'normal' sedentary lifestyle without extreme cold exposures was measured.

METHODS

The Iceman (subject A) and his brother (subject B) were studied during mild cold (13°C) and thermoneutral conditions (31°C). Measurements included BAT activity and respiratory muscle activity by [18F]FDG-PET/CT imaging and energy expenditure through indirect calorimetry. In addition, body temperatures, cardiovascular parameters, skin perfusion, and thermal sensation and comfort were measured. Finally, we determined polymorphisms for uncoupling protein-1 and β3-adrenergic receptor.

RESULTS

Subjects had comparable BAT activity (A: 1144 SUVtotal and B: 1325 SUVtotal), within the range previously observed in young adult men. They were genotyped with the polymorphism for uncoupling protein-1 (G/G). CIT was relatively high (A: 40.1% and B: 41.9%), but unlike during our previous cold exposure tests in young adult men, here both subjects practiced a g-Tummo like breathing technique, which involves vigorous respiratory muscle activity. This was confirmed by high [18F]FDG-uptake in respiratory muscle.

CONCLUSION

No significant differences were found between the two subjects, indicating that a lifestyle with frequent exposures to extreme cold does not seem to affect BAT activity and CIT. In both subjects, BAT was not higher compared to earlier observations, whereas CIT was very high, suggesting that g-Tummo like breathing during cold exposure may cause additional heat production by vigorous isometric respiratory muscle contraction. The results must be interpreted with caution given the low subject number and the fact that both participants practised the g-Tummo like breathing technique.

Human ortholog of a plant salicylic acid receptor found in SK-N-SH cell line

Our previous studies have described the purification and characterization of a novel plant NAD(P)-reductase like protein (RL) from the thermogenic appendix of the Sauromatum guttatum inflorescence. RL is mainly located in cytoplasm of thermogenic plants and it can act like a bistable switch. It adopts a compact conformation during heat-production and a more expanded conformation when heat is not generated. Addition of salicylic acid, a natural thermogenic inducer, at picomolar concentration to a solution of purified RL induced a discontinuous volume phase transition in which the volume of RL in the oligomeric form expanded and shrunk repeatedly every 4-5 min. In the present study using ESI-MS analysis we have demonstrated the existence of RL in the human SK-N-SH cell line and in mouse brain tissue. The molecular mass of human RL is in the same range as of its plant counterpart, 34,140 ± 34 Da. The charge state distribution of the human RL is identical to its plant counterpart from the Sauromatum appendix during heat-production. Human RL was present in the compact state when it was purified from the SK-N-SH cell line When these cells were treated with salicylic acid (10 μM) a shift to a much more compact conformation was observed. It seems that the potential of RL to respond to salicylic acid was conserved. These results may reveal the existence of a thermoregulation system that is evolutionarily conserved and is operating by conformational changes. This discovery may also represent an opportunity for a better understanding of some of the diverse functions of salicylic acid and aspirin in plants and humans.

Fetal programming of adipose tissue function: an evolutionary perspective

Obesity is an escalating threat of pandemic proportions and has risen to such unrivaled prominence in such a short period of time that it has come to define a whole generation in many countries around the globe. The burden of obesity, however, is not equally shared among the population, with certain ethnicities being more prone to obesity than others, while some appear to be resistant to obesity altogether. The reasons behind this ethnic basis for obesity resistance and susceptibility, however, have remained largely elusive. In recent years, much evidence has shown that the level of brown adipose tissue thermogenesis, which augments energy expenditure and is negatively associated with obesity in both rodents and humans, varies greatly between ethnicities. Interestingly, the incidence of low birth weight, which is associated with an increased propensity for obesity and cardiovascular disease in later life, has also been shown to vary by ethnic background. This review serves to reconcile ethnic variations in BAT development and function with ethnic differences in birth weight outcomes to argue that the variation in obesity susceptibility between ethnic groups may have its origins in the in utero programming of BAT development and function as a result of evolutionary adaptation to cold environments.

Supraclavicular skin temperature as a measure of 18F-FDG uptake by BAT in human subjects

BACKGROUND

Brown adipose tissue (BAT) has emerged as a novel player in energy homeostasis in humans and is considered a potential new target for combating obesity and related diseases. The current 'gold standard' for quantification of BAT volume and activity is cold-induced 18F-FDG uptake in BAT. However, use of this technique is limited by cost and radiation exposure. Given the fact that BAT is a thermogenic tissue, mainly located in the supraclavicular region, the aim of the current study was to investigate whether cold-induced supraclavicular skin temperature and core body temperature may be alternative markers of BAT activation in humans.

SUBJECTS/METHODS

BAT volume and activity were measured in 24 healthy lean adolescent males (mean age 24.1±0.8 years), using cold-induced 18F-FDG uptake with PET-CT. Core body temperature was measured continuously in the small intestine with use of an ingestible telemetric capsule and skin temperature was measured by eighteen wireless iButtons attached to the skin following ISO-defined locations.

RESULTS

Proximal and distal (hand/feet) skin temperatures markedly decreased upon cold exposure, while supraclavicular skin temperature significantly increased (35.2±0.1 vs. 35.5±0.1°C, p = 0.001). Furthermore, cold-induced supraclavicular skin temperature positively correlated with both total (R2 = 0.28, P = 0.010) and clavicular BAT volume (R2 = 0.20, P = 0.030) and clavicular SUVmax (R2 = 0.27, P = 0.010), while core body temperature did not.

CONCLUSIONS

Supraclavicular skin temperature as measured by iButtons may have predictive value for BAT detection in adult humans. This is highly desirable considering the increasing interest in pharmacological interventions to stimulate BAT in human subjects.

TRIAL REGISTRATION

NTR 2473.

What we talk about when we talk about fat

There has been an upsurge of interest in the adipocyte coincident with the onset of the obesity epidemic and the realization that adipose tissue plays a major role in the regulation of metabolic function. The past few years, in particular, have seen significant changes in the way that we classify adipocytes and how we view adipose development and differentiation. We have new perspective on the roles played by adipocytes in a variety of homeostatic processes and on the mechanisms used by adipocytes to communicate with other tissues. Finally, there has been significant progress in understanding how these relationships are altered during metabolic disease and how they might be manipulated to restore metabolic health.

Regulators of human white adipose browning: evidence for sympathetic control and sexual dimorphic responses to sprint interval training

The conversion of white adipose to the highly thermogenic beige adipose tissue has been proposed as a potential strategy to counter the unfavorable consequences of obesity. Three regulators of this conversion have recently emerged but information regarding their control is limited, and contradictory. We present two studies examining the control of these regulators. Study 1: In 10 young men, the plasma concentrations of irisin and fibroblast growth factor 21 (FGF21) were determined prior to and during activation of the sympathetic nervous system via hypoxic gas breathing (FIO2 = 0.11). The measurements were performed twice, once with and once without prior/concurrent sympathetic inhibition via transdermal clonidine administration. FGF21 was unaffected by basal sympathetic inhibition (338±113 vs. 295±80 pg/mL; P = 0.43; mean±SE), but was increased during hypoxia mediated sympathetic activation (368±135); this response was abrogated (P = 0.035) with clonidine (269±93). Irisin was unaffected by sympathetic inhibition and/or hypoxia (P>0.21). Study 2: The plasma concentration of irisin and FGF21, and the skeletal muscle protein content of fibronectin type III domain containing 5 (FNDC5) was determined in 19 young adults prior to and following three weeks of sprint interval training (SIT). SIT decreased FGF21 (338±78 vs. 251±36; P = 0.046) but did not affect FNDC5 (P = 0.79). Irisin was decreased in males (127±18 vs. 90±23 ng/mL; P = 0.045) and increased in females (139±14 vs. 170±18). Collectively, these data suggest a potential regulatory role of acute sympathetic activation pertaining to the browning of white adipose; further, there appears to be a sexual dimorphic response of irisin to SIT.

Brown adipose tissue volume in healthy lean south Asian adults compared with white Caucasians: a prospective, case-controlled observational study

BACKGROUND

Individuals of south Asian origin have a very high risk of developing type 2 diabetes compared with white Caucasians. We aimed to assess volume and activity of brown adipose tissue (BAT), which is thought to have a role in energy metabolism by combusting fatty acids and glucose to produce heat and might contribute to the difference in incidence of type 2 diabetes between ethnic groups.

METHODS

We enrolled Dutch nationals with south Asian ancestry and matched Caucasian participants at The Rijnland Hospital (Leiderdorp, Netherlands). Eligible participants were healthy lean men aged 18-28 years, and we matched groups for BMI. We measured BAT volume and activity with cold-induced (18)F-fluorodeoxyglucose ((18)F-FDG) PET CT scans, and assessed resting energy expenditure, non-shivering thermogenesis, and serum parameters. This study is registered with the Netherlands Trial Register, number 2473.

FINDINGS

Between March 1, 2013, and June 1, 2013, we enrolled 12 participants in each group; one Caucasian participant developed hyperventilation after (18)F-FDG administration, and was excluded from all cold-induced and BAT measurements. Compared with Caucasian participants, south Asian participants did not differ in age (mean 23.6 years [SD 2.8] for south Asians vs 24.6 years [2.8] for Caucasians) or BMI (21.5 kg/m(2) [2.0] vs 22.0 kg/m(2) [1.6]), but were shorter (1.74 m [0.06] vs 1.85 m [0.04]) and lighter (65.0 kg [8.5] vs 75.1 kg [7.2]). Thermoneutral resting energy expenditure was 1297 kcal per day (SD 123) in south Asian participants compared with 1689 kcal per day (193) in white Caucasian participants (difference -32%, p=0.0008). On cold exposure, shiver temperature of south Asians was 2.0°C higher than Caucasians (p=0.0067) and non-shivering thermogenesis was increased by 20% in white Caucasians (p<0.0001) but was not increased in south Asians. Although the maximum and mean standardised uptake values of (18)F-FDG in BAT did not differ between groups, total BAT volume was lower in south Asians (188 mL [SD 81]) than it was in Caucasians (287 mL [169]; difference -34%, p=0.04). Overall, BAT volume correlated positively with basal resting energy expenditure in all assessable individuals (β=0.44, p=0.04).

INTERPRETATION

Lower resting energy expenditure, non-shivering thermogenesis, and BAT volumes in south Asian populations might underlie their high susceptibility to metabolic disturbances, such as obesity and type 2 diabetes. Development of strategies to increase BAT volume and activity might help prevent and treat such disorders, particularly in south Asian individuals.

FUNDING

Dutch Heart Foundation (2009T038) and Dutch Diabetes Research Foundation (2012.11.1500).

Association of ambient indoor temperature with body mass index in England

OBJECTIVE

Raised ambient temperatures may result in a negative energy balance characterized by decreased food intake and raised energy expenditure. This study tested whether indoor temperatures above the thermoneutral zone for clothed humans (∼23 °C) were associated with a reduced body mass index (BMI).

METHODS

Participants were 100,152 adults (≥16 years) drawn from 13 consecutive annual waves of the nationally representative Health Survey for England (1995-2007).

RESULTS

BMI levels of those residing in air temperatures above 23 °C were lower than those living in an ambient temperature of under 19 °C (b = -0.233, SE = 0.053, P < 0.001), in analyses that adjusted for participant age, gender, social class, health and the month/year of assessment. Robustness tests showed that high indoor temperatures were associated with reduced BMI levels in winter and non-winter months and early (1995-2000) and later (2001-2007) survey waves. Including additional demographic, environmental, and health behavior variables did not diminish the link between high indoor temperatures and reduced BMI.

CONCLUSIONS

Elevated ambient indoor temperatures are associated with low BMI levels. Further research is needed to establish the potential causal nature of this relationship.

Body mass index as a determinant of brown adipose tissue function in healthy children

OBJECTIVE

To determine whether body mass index (BMI) percentile and ethnicity influence skin temperature overlying brown adipose tissue (BAT) depots in the supraclavicular region in healthy children.

STUDY DESIGN

Infrared thermography measured supraclavicular region temperature (T(SCR)) at baseline and after exposure to a mild cool stimulus (single hand immersion in water at 20.1 °C) for 5 minutes in children aged 6-11 years (n = 55). The studies were undertaken in a normal school environment.

RESULTS

BMI percentile and ethnicity were significant predictors of baseline T(SCR), with an inverse relationship between BMI percentile persisting after adjustment for ethnicity. Twenty-four children demonstrated a significant rise in T(SCR) after exposure to the cool stimulus. BMI percentile was a significant predictor of T(SCR) response, although there was no effect of ethnicity on T(SCR) change after exposure to the cool stimulus.

CONCLUSION

We have demonstrated a negative relationship between BMI percentile and both baseline T(SCR), colocating with the primary region of BAT, and the change in T(SCR) in response to the cool stimulus. Future studies aimed at determining the primary factors regulating BAT function in healthy children should be targeted at the goal of maintaining a healthy BMI trajectory during childhood.

Translational issues in targeting brown adipose tissue thermogenesis for human obesity management

The recent advancements in unraveling novel mechanisms that control the induction, (trans)differentiation, proliferation, and thermogenic activity and capacity of brown adipose tissue (BAT), together with the application of imaging techniques for human BAT visualization, have generated optimism that these advances will provide novel strategies for targeting BAT thermogenesis, leading to efficacious and safe obesity therapies. This paper first provides an overview of landmark events of the past few decades that have been driving the search for pharmaceutical and nutraceutical compounds that would increase BAT thermogenesis for obesity management. It then addresses issues about what could be expected from an ideal thermogenic antiobesity approach, in particular to what extent daily energy expenditure will need to increase in order to achieve long-term weight loss currently achievable only through bariatric surgery, and whether the human body will have enough thermogenic capacity to reach this target weight loss by future therapies focused on BAT.

Pathogenesis of type 2 diabetes in South Asians

The risk of developing type 2 diabetes mellitus (T2DM) is exceptionally high among both native and migrant South Asians. T2DM occurs more often and at a younger age and lower BMI, and the risk of coronary artery and cerebrovascular disease, and renal complications is higher for South Asians compared with people of White Caucasian descent. The high prevalence of T2DM and its related complications in South Asians, which comprise one-fifth of the total world's population, poses a major health and socioeconomic burden. The underlying cause of this excess risk, however, is still not completely understood. Therefore, gaining insight into the pathogenesis of T2DM in South Asians is of great importance. The predominant mechanism, in this ethnicity seems to be insulin resistance (IR) rather than an impaired β-cell function. In this systematic review, we describe several possible mechanisms that may underlie or contribute to the increased IR observed in South Asians.

Energy dissipation in brown adipose tissue: from mice to men

In rodents, brown adipose tissue (BAT) is a metabolic organ that produces heat in response to cold and dietary intake through mitochondrial uncoupling. For long time, BAT was considered to be solely important in small mammals and infants, however recent studies have shown that BAT is also functional in adult humans. Interestingly, the presence and/or functionality of this thermogenic tissue is diminished in obese people, suggesting a link between human BAT and body weight regulation. In the last years, evidence has also emerged for the existence of adipocytes that may have an intermediate thermogenic phenotype between white and brown adipocytes, so called brite or beige adipocytes. Together, these findings have resulted in a renewed interested in (human) brown adipose tissue and pathways to increase the activity and recruitment of these thermogenic cells. Stimulating BAT hypertrophy and hyperplasia in humans could be a potential strategy to target obesity. Here we will review suggested pathways leading to BAT activation in humans, and discuss novel putative BAT activators in rodents into human perspective.

Exercise training does not increase muscle FNDC5 protein or mRNA expression in pigs

BACKGROUND

Exercise training elevates circulating irisin and induces the expression of the FNDC5 gene in skeletal muscles of mice. Our objective was to determine whether exercise training also increases FNDC5 protein or mRNA expression in the skeletal muscles of pigs as well as plasma irisin.

METHODS

Castrated male pigs of the Rapacz familial hypercholesterolemic (FHM) strain and normal (Yucatan miniature) pigs were sacrificed after 16-20 weeks of exercise training. Samples of cardiac muscle, deltoid and triceps brachii muscle, subcutaneous and epicardial fat were obtained and FNDC5 mRNA, along with that of 6 other genes, was measured in all tissues of FHM pigs by reverse transcription polymerase chain reaction. FNDC protein in deltoid and triceps brachii was determined by Western blotting in both FHM and normal pigs. Citrate synthase activity was measured in the muscle samples of all pigs as an index of exercise training. Irisin was measured by an ELISA assay.

RESULTS

There was no statistically significant effect of exercise training on FNDC5 gene expression in epicardial or subcutaneous fat, deltoid muscle, triceps brachii muscle or heart muscle. Exercise-training elevated circulating levels of irisin in the FHM pigs and citrate synthase activity in deltoid and triceps brachii muscle. A similar increase in citrate synthase activity was seen in muscle extracts of exercise-trained normal pigs but there was no alteration in circulating irisin.

CONCLUSION

Exercise training in pigs does not increase FNDC5 mRNA or protein in the deltoid or triceps brachii of FHM or normal pigs while increasing circulating irisin only in the FHM pigs. These data indicate that the response to exercise training in normal pigs is not comparable to that seen in mice.

Mitochondrial uncoupling in skeletal muscle by UCP1 augments energy expenditure and glutathione content while mitigating ROS production

Enhancement of proton leaks in muscle tissue represents a potential target for obesity treatment. In this study, we examined the bioenergetic and physiological implications of increased proton leak in skeletal muscle. To induce muscle-specific increases in proton leak, we used mice that selectively express uncoupling protein-1 (UCP1) in skeletal muscle tissue. UCP1 expression in muscle mitochondria was ∼13% of levels in brown adipose tissue (BAT) mitochondria and caused increased GDP-sensitive proton leak. This was associated with an increase in whole body energy expenditure and a decrease in white adipose tissue content. Muscle UCP1 activity had divergent effects on mitochondrial ROS emission and glutathione levels compared with BAT. UCP1 in muscle increased total mitochondrial glutathione levels ∼7.6 fold. Intriguingly, unlike in BAT mitochondria, leak through UCP1 in muscle controlled mitochondrial ROS emission. Inhibition of UCP1 with GDP in muscle mitochondria increased ROS emission ∼2.8-fold relative to WT muscle mitochondria. GDP had no impact on ROS emission from BAT mitochondria from either genotype. Collectively, these findings indicate that selective induction of UCP1-mediated proton leak in muscle can increase whole body energy expenditure and decrease adiposity. Moreover, ectopic UCP1 expression in skeletal muscle can control mitochondrial ROS emission, while it apparently plays no such role in its endogenous tissue, brown fat.

Understanding the brown adipocyte as a contributor to energy homeostasis

Brown adipocytes are specialized cells capable of undergoing thermogenesis, a phenomenon regulated by the sympathetic nervous system, due to the presence of uncoupling protein 1 (UCP1). The recent demonstrations of their presence in adult humans, and the discovery that brown adipocytes can be derived from distinct precursors and express specific genes depending on their anatomic location, have sparked intense interest in enhancing the current understanding of their biology and relevance to human energy homeostasis. We provide an overview of the latest advances related to the developmental origins of brown adipocytes, discuss their regulation and function in both rodents and humans, and offer a critical perspective on the relevance of brown adipocyte-mediated thermogenesis in human physiology.

Brown adipose tissue activity after a high-calorie meal in humans

BACKGROUND

Studies in rodents have shown that brown adipose tissue (BAT) is activated on food intake, thereby reducing metabolic efficiency.

OBJECTIVE

The current study investigated whether a single high-calorie, carbohydrate-rich meal activates BAT in lean human adults.

DESIGN

BAT activity was studied in 11 lean adult men [age: 23.6 ± 2.1 y; body mass index (BMI; in kg/m(2)): 22.4 ± 2.1] after consumption of a high-calorie, carbohydrate-rich meal (1622 ± 222 kcal; 78% carbohydrate, 12% protein, 10% fat). BAT activity during 2 h of mild cold exposure served as a positive control experiment. BAT activity was assessed by [(18)F]fluorodeoxyglucose (FDG)-positron emission tomography-computed tomography. Energy expenditure was measured by indirect calorimetry.

RESULTS

Postprandial [(18)F]FDG uptake was significantly higher in BAT [1.65 ± 0.99 mean standard uptake value (SUVmean)] than in subcutaneous (0.35 ± 0.15 SUVmean; P < 0.05) and visceral (0.49 ± 0.24 SUVmean; P < 0.05) white adipose tissue and liver (0.95 ± 0.28 SUVmean; P < 0.05). Postprandial BAT activity was lower than cold-induced BAT activity (7.19 ± 2.09 SUVmean). However, postprandial BAT activity may have been underestimated because of high postprandial [(18)F]FDG uptake in skeletal muscle compared with cold (1.36 ± 0.31 compared with 0.59 ± 0.07 SUVmean, P < 0.05), which reduces [(18)F]FDG bioavailability for BAT and other tissues. No direct relation was found between BAT and diet-induced thermogenesis (DIT).

CONCLUSIONS

Glucose uptake in BAT increases after a meal in humans, which indicates a role for BAT in reducing metabolic efficiency. However, the quantitative contribution of BAT to DIT relative to other tissues, such as skeletal muscle, remains to be investigated. This trial was registered at www.controlled-trials.com as ISRCTN21413505.

Anatomical locations of human brown adipose tissue: functional relevance and implications in obesity and type 2 diabetes

We will review information about and present hypotheses as to the anatomy of brown adipose tissue (BAT). Why is it located where it is in humans? Its anatomical distribution is likely to confer survival value by protecting critical organs from hypothermia by adaptive thermogenesis. Ultimately, the location and function will be important when considering therapeutic strategies for preventing and treating obesity and type 2 diabetes, in which case successful interventions will need to have a significant effect on BAT function in subjects living in a thermoneutral environment. In view of the diverse locations and potential differences in responsiveness between BAT depots, it is likely that BAT will be shown to have much more subtle and thus previously overlooked functions and regulatory control mechanisms.

Brown adipose tissue in humans: therapeutic potential to combat obesity

Harnessing the considerable capacity of brown adipose tissue (BAT) to consume energy was first proposed as a potential target to control obesity nearly 40years ago. The plausibility of this approach was, however, questioned due to the prevailing view that BAT was either not present or not functional in adult humans. Recent definitive identification of functional BAT in adult humans as well as a number of important advances in the understanding of BAT biology has reignited interest in BAT as an anti-obesity target. Proof-of-concept evidence demonstrating drug-induced BAT activation provides an important foundation for development of targeted pharmacological approaches with clinical application. This review considers evidence from both human and relevant animal studies to determine whether harnessing BAT for the treatment of obesity via pharmacological intervention is a realistic goal.

Brown adipose tissue growth and development

Brown adipose tissue is uniquely able to rapidly produce large amounts of heat through activation of uncoupling protein (UCP) 1. Maximally stimulated brown fat can produce 300 watts/kg of heat compared to 1 watt/kg in all other tissues. UCP1 is only present in small amounts in the fetus and in precocious mammals, such as sheep and humans; it is rapidly activated around the time of birth following the substantial rise in endocrine stimulatory factors. Brown adipose tissue is then lost and/or replaced with white adipose tissue with age but may still contain small depots of beige adipocytes that have the potential to be reactivated. In humans brown adipose tissue is retained into adulthood, retains the capacity to have a significant role in energy balance, and is currently a primary target organ in obesity prevention strategies. Thermogenesis in brown fat humans is environmentally regulated and can be stimulated by cold exposure and diet, responses that may be further modulated by photoperiod. Increased understanding of the primary factors that regulate both the appearance and the disappearance of UCP1 in early life may therefore enable sustainable strategies in order to prevent excess white adipose tissue deposition through the life cycle.

Adaptive thermogenesis in adipocytes: is beige the new brown?

One of the most promising areas in the therapeutics for metabolic diseases centers around activation of the pathways of energy expenditure. Brown adipose tissue is a particularly appealing target for increasing energy expenditure, given its amazing capacity to transform chemical energy into heat. In addition to classical brown adipose tissue, the last few years have seen great advances in our understanding of inducible thermogenic adipose tissue, also referred to as beige fat. A deeper understanding of the molecular processes involved in the development and function of these cell types may lead to new therapeutics for obesity, diabetes, and other metabolic diseases.

Brown adipose tissue: a new human organ?

Metabolism & Endocrinology Themed Meeting of the Physiological Society. Brown adipose tissue: a new human organ? The Royal Society, London, UK, 11-13 December 2012 The prevalence of obesity and overweight is increasing rapidly and functional brown adipose tissue (BAT), with its role in energy expenditure, may provide one solution. However, several key questions remain: what is the role of BAT in body metabolism, does substantial diet-induced thermogenesis exist in BAT and can it have a significant impact on total energy expenditure? Brown adipocytes are present within white adipose depots (BRITE cells) and the transcriptional control of these and classical brown adipocytes, remains an area of immense research interest. In addition, BAT has a role in lipoprotein and glucose metabolism and may play a part in aging. These, and several other burning issues around BAT, were discussed at a meeting of the Physiological Society in London, UK (11-13 December 2012).

Ephedrine activates brown adipose tissue in lean but not obese humans

AIMS/HYPOTHESIS

Brown adipose tissue (BAT) activation increases energy consumption and may help in the treatment of obesity. Cold exposure is the main physiological stimulus for BAT thermogenesis and the sympathetic nervous system, which innervates BAT, is essential in this process. However, cold-induced BAT activation is impaired in obese humans. To explore the therapeutic potential of BAT, it is essential to determine whether pharmacological agents can activate BAT.

METHODS

We aimed to determine whether BAT can be activated in lean and obese humans after acute administration of an orally bioavailable sympathomimetic. In a randomised, double-blinded, crossover trial, we administered 2.5 mg/kg of oral ephedrine to nine lean (BMI 22 ± 1 kg/m²) and nine obese (BMI 36 ± 1 kg/m²) young men. On a separate day, a placebo was administered to the same participants. BAT activity was assessed by measuring glucose uptake with [¹⁸F]fluorodeoxyglucose and positron emission tomography-computed tomography imaging.

RESULTS

BAT activity was increased by ephedrine compared with placebo in the lean, but unchanged in the obese, participants. The change in BAT activity after ephedrine compared with placebo was negatively correlated with various indices of body fatness.

CONCLUSIONS/INTERPRETATION

BAT can be activated via acute, oral administration of the sympathomimetic ephedrine in lean, but not in obese humans.

Metabolic inflexibility of white and brown adipose tissues in abnormal fatty acid partitioning of type 2 diabetes

Type 2 diabetes (T2D) is characterized by a general dysregulation of postprandial energy substrate partitioning. Although classically described in regard to glucose metabolism, it is now evident that metabolic inflexibility of plasma lipid fluxes is also present in T2D. The organ that is most importantly involved in the latter metabolic defect is the white adipose tissue (WAT). Both catecholamine-induced nonesterified fatty acid mobilization and insulin-stimulated storage of meal fatty acids are impaired in many WAT depots of insulin-resistant individuals. Novel molecular imaging techniques now demonstrate that these defects are linked to increased dietary fatty acid fluxes toward lean organs and myocardial dysfunction in humans. Recent findings also demonstrate functional abnormalities of brown adipose tissues in T2D, thus suggesting that a generalized adipose tissue dysregulation of energy storage and dissipation may be at play in the development of lean tissue energy overload and lipotoxicity.

Brown adipose tissue and the regulation of nonshivering thermogenesis

PURPOSE OF REVIEW

The recent rediscovery of functional cold activated brown adipose tissue (BAT) in adult humans fuelled an uprise in studies on this tissue. This review focuses on the contribution of human BAT to nonshivering thermogenesis and on factors other than cold that activate BAT.

RECENT FINDINGS

Earlier studies revealed BAT activity using a glucose tracer for positron emission tomography/computed tomography (PET/CT) scanning. Several recent studies, using a mix of tracers and PET/CT dynamic scanning showed that human brown fat is metabolically active and related to the perfusion of the tissue. The actual contribution of BAT to nonshivering thermogenesis still needs to be explored.The last few years, several new factors that activate human BAT have been described. These studies also highlight the plasticity of brown and white adipose tissue. Some of these factors may have pharmacological significance.

SUMMARY

New PET/CT studies provide information on oxidative human BAT metabolism in vivo. This new information in combination with the study on factors activating BAT are promising with respect to management of obesity and related disorders.

Relationship between seasonal cold acclimatization and mtDNA haplogroup in Japanese

BACKGROUND

The purpose of this study was to elucidate the interaction between mtDNA haplogroup and seasonal variation that contributes to cold adaptation.

METHODS

There were 15 subjects (seven haplotype D subjects and eight haplotype non-D subjects). In summer and winter, the subjects were placed in an environment where the ambient temperature dropped from 27 °C to 10 °C in 30 minutes. After that, they were exposed to cold for 60 minutes.

RESULTS

In summer, the decrease in rectal temperature and increase in oxygen consumption was smaller and cold tolerance was higher in the haplotype non-D group than in the haplotype D group. In winter, no significant differences were seen in rectal temperature or oxygen consumption, but the respiratory exchange ratio decreased in the haplotype D group.

CONCLUSIONS

The results of the present study suggest that haplogroup D subjects are a group that changes energy metabolism more, and there appears to be a relationship between differences in cold adaptability and mtDNA polymorphism within the population. Moreover, group differences in cold adaptability seen in summer may decrease in winter due to supplementation by seasonal cold acclimatization.

Hibernoma with metabolic impact?

A hibernoma is a benign tumour of brown adipose tissue (BAT). BAT is also present in physiological depots, previously considered residual. However, in 2009 functional BAT in adults was reported. Metabolically active BAT is suggested to significantly influence the energy balance and increasing BAT activity could possibly treat obesity. This strongly increased the interest in BAT in man and currently more research is conducted. We describe the case of an intramuscular hibernoma and illustrate the pathophysiology of the observed hibernoma in relation to recent insights on physiological BAT. Immunofluorescence staining of the hibernoma showed the presence of uncoupling-protein-1, a protein specific for active BAT. This suggests that this hibernoma contains BAT that could possibly contribute to the human energy balance.

A role of active brown adipose tissue in cancer cachexia?

Until a few years ago, adult humans were not thought to have brown adipose tissue (BAT). Now, this is a rapidly evolving field of research with perspectives in metabolic syndromes such as obesity and new therapies targeting its bio-energetic pathways. White, brown and so-called brite adipose fat seem to be able to trans-differentiate into each other, emphasizing the dynamic nature of fat tissue for metabolism. Human and animal data in cancer cachexia to date provide some evidence for BAT activation, but its quantitative impact on energy expenditure and weight loss is controversial. Prospective clinical studies can address the potential role of BAT in cancer cachexia using ¹⁸F-fluoro- deoxyglucose positron emission tomography-computed tomography scanning, with careful consideration of co-factors such as diet, exposure to the cold, physical activity and body mass index, that all seem to act on BAT recruitment and activity.

Young women with cold-activated brown adipose tissue have higher bone mineral density and lower Pref-1 than women without brown adipose tissue: a study in women with anorexia nervosa, women recovered from anorexia nervosa, and normal-weight women

CONTEXT

Anorexia nervosa (AN) is associated with depletion of body fat, loss of bone mineral density (BMD), and impaired thermogenesis. Brown adipose tissue (BAT) is lower in obese individuals and decreases during aging. Recent studies have suggested a link between BAT and bone metabolism.

OBJECTIVE

Our objective was to investigate the presence and quantity of BAT in patients with AN, recovered AN (AN-R), and normal-weight controls and to study the relationship between BAT and BMD and body composition and investigate hormonal predictors of BAT.

DESIGN AND SETTING

This was a cross-sectional study at a clinical research center.

PATIENTS

Patients included 15 women: five with AN (mean age 30 ± 6.3 yr), five AN-R, and five healthy nonobese controls of comparable age.

MAIN OUTCOME MEASURES

Cold-activated BAT was determined by fluorodeoxyglucose-positron emission tomography/computed tomography. BMD of total-body, spine, and hip, fat and lean mass was determined by dual-energy x-ray absorptiometry. Single-slice magnetic resonance imaging at L4 was done for abdominal fat compartments, and preadipocyte factor-1 (Pref-1), T₃, and T₄ were measured.

RESULTS

Within the AN group, one of five; in the AN-R group, two of five; and in the healthy nonobese control group, four of five subjects were BAT positive. Subjects were divided into groups based on the presence (n = 7) or absence (n = 8) of BAT. Both groups were of comparable age and body mass index. Women with BAT had higher total-body BMD, higher T₃, and lower Pref-1 compared with women without BAT. There was a positive correlation between BAT and BMD that remained significant after controlling for disease status and body mass index.

CONCLUSION

Young women with AN have low cold-activated BAT, which may be due to impaired BAT thermogenesis. Young women with BAT have higher BMD and lower Pref-1 compared with women without BAT, suggesting that BAT may be involved in the regulation of stem cell differentiation into the bone lineage at the expense of adipogenesis.

Cardiac natriuretic peptides act via p38 MAPK to induce the brown fat thermogenic program in mouse and human adipocytes

The ability of mammals to resist body fat accumulation is linked to their ability to expand the number and activity of "brown adipocytes" within white fat depots. Activation of β-adrenergic receptors (β-ARs) can induce a functional "brown-like" adipocyte phenotype. As cardiac natriuretic peptides (NPs) and β-AR agonists are similarly potent at stimulating lipolysis in human adipocytes, we investigated whether NPs could induce human and mouse adipocytes to acquire brown adipocyte features, including a capacity for thermogenic energy expenditure mediated by uncoupling protein 1 (UCP1). In human adipocytes, atrial NP (ANP) and ventricular NP (BNP) activated PPARγ coactivator-1α (PGC-1α) and UCP1 expression, induced mitochondriogenesis, and increased uncoupled and total respiration. At low concentrations, ANP and β-AR agonists additively enhanced expression of brown fat and mitochondrial markers in a p38 MAPK-dependent manner. Mice exposed to cold temperatures had increased levels of circulating NPs as well as higher expression of NP signaling receptor and lower expression of the NP clearance receptor (Nprc) in brown adipose tissue (BAT) and white adipose tissue (WAT). NPR-C(-/-) mice had markedly smaller WAT and BAT depots but higher expression of thermogenic genes such as Ucp1. Infusion of BNP into mice robustly increased Ucp1 and Pgc-1α expression in WAT and BAT, with corresponding elevation of respiration and energy expenditure. These results suggest that NPs promote "browning" of white adipocytes to increase energy expenditure, defining the heart as a central regulator of adipose tissue biology.

Thermogenic characterization of ghrelin receptor null mice

Ghrelin is the only known circulating orexigenic hormone that increases food intake and promotes adiposity, and these physiological functions of ghrelin are mediated through its receptor growth hormone secretagogue receptor (GHS-R). Ghrelin/GHS-R signaling plays a crucial role in energy homeostasis. Old GHS-R null mice exhibit a healthy phenotype-lean and insulin sensitive. Interestingly, the GHS-R null mice have increased energy expenditure, yet exhibit no difference in food intake or locomotor activity compared to wild-type mice. We have found that GHS-R is expressed in brown adipose tissue (BAT) of old mice. Ablation of GHS-R attenuates age-associated decline in thermogenesis, exhibiting a higher core body temperature. Indeed, the BAT of old GHS-R null mice reveals enhanced thermogenic capacity, which is consistent with the gene expression profile of increases in glucose/lipid uptake, lipogenesis, and lipolysis in BAT. The data collectively suggest that ghrelin/GHS-R signaling has important roles in thermogenesis. The recent discovery that BAT also regulates energy homeostasis in adult humans makes the BAT a new antiobesity target. Understanding the roles and molecular mechanisms of ghrelin/GHS-R in thermogenesis is of great significance. GHS-R antagonists might be a novel means of combating obesity by shifting adiposity balance from obesogenesis to thermogenesis.

Therapeutic prospects of metabolically active brown adipose tissue in humans

The world-wide obesity epidemic constitutes a severe threat to human health and wellbeing and poses a major challenge to health-care systems. Current therapeutic approaches, relying mainly on reduced energy intake and/or increased exercise energy expenditure, are generally of limited effectiveness. Previously believed to be present only in children, the existence of metabolically active brown adipose tissue (BAT) was recently demonstrated also in healthy human adults. The physiological role of BAT is to dissipate chemical energy, mainly from fatty acids, as heat to maintain body temperature in cold environments. Recent studies indicate that the activity of BAT is negatively correlated with overweight and obesity, findings that raise the exciting possibility of new and effective weight reduction therapies based on increased BAT energy expenditure, a process likely to be amenable to pharmacological intervention.

Human brown fat and obesity: methodological aspects

Much is known about brown adipose tissue (BAT) in rodents. Its function is to generate heat in response to low environmental temperatures and to diet or overfeeding. The knowledge about BAT in humans is still rather limited despite the recent rediscovery of its functionality in adults. This review highlights the information available on the contribution of BAT in increasing human energy expenditure in relation to obesity. Besides that methodological aspects will be discussed that need special attention in order to unravel the heat producing capacity of human BAT, the recruitment of the tissue, and its functionality.