A new, powerful player in lipoprotein metabolism: brown adipose tissue

Adipo-oncology: adipocyte-derived factors govern engraftment, survival, and progression of metastatic cancers

Conventional therapies for metastatic cancers have limited efficacy. Recently, cancer therapies targeting noncancerous cells in tumor microenvironments have shown improved clinical outcomes in patients. However, further advances in our understanding of the metastatic tumor microenvironment are required to improve treatment outcomes. Adipocytes are distributed throughout the body, and as a part of the metastatic tumor microenvironment, they interact with cancer cells in almost all organs. Adipocytes secrete various factors that are reported to exert clinical effects on cancer progression, including engraftment, survival, and expansion at the metastatic sites. However, only a few studies have comprehensively examined their impact on cancer cells. In this review, we examined the impact of adipocytes on cancer by describing the adipocyte-secreted factors that are involved in controlling metastatic cancer, focusing on adipokines, such as adiponectin, leptin, visfatin, chemerin, resistin, apelin, and omentin. Adipocyte-secreted factors promote cancer metastasis and contribute to various biological functions of cancer cells, including migration, invasion, proliferation, immune evasion, and drug resistance at the metastatic sites. We propose the establishment and expansion of "adipo-oncology" as a research field to enhance the comprehensive understanding of the role of adipocytes in metastatic cancers and the development of more robust metastatic cancer treatments.

In-vivo detection of white adipose tissue browning: a multimodality imaging approach

Detection and differentiation of brown fat in humans poses several challenges, as this tissue is sparse and often mixed with white adipose tissue. Non-invasive detection of beige fat represents an even greater challenge as this tissue is structurally and functionally more like white fat than brown fat. Here we used positron emission tomography with 18F-fluorodeoxyglucose, computed tomography, xenon-enhanced computed tomography, and dynamic contrast-enhanced ultrasound, to non-invasively detect functional and structural changes associated with the browning process of inguinal white fat, induced in mice by chronic stimulation with the β3-adrenergic receptor agonist CL-316243. These studies reveal a very heterogeneous increase in baseline tissue radiodensity and xenon-enhanced radiodensity, indicative of both an increase in adipocytes water and protein content as well as tissue perfusion, mostly in regions that showed enhanced norepinephrine-stimulated perfusion before CL-316243 treatment. No statistically significant increase in 18F-fluorodeoxyglucose uptake or norepinephrine-stimulated tissue perfusion were observed in the mice after the CL-316243 treatment. The increase in tissue-water content and perfusion, along with the negligible increase in the tissue glucose uptake and norepinephrine-stimulated perfusion deserve more attention, especially considering the potential metabolic role that this tissue may play in whole body metabolism.

"Humanizing" mouse environments: Humidity, diurnal cycles and thermoneutrality

Thermoneutral housing has been shown to promote more accurate and robust development of several pathologies in mice. Raising animal housing temperatures a few degrees may create a relatively straightforward opportunity to improve translatability of mouse models. In this commentary, we discuss the changes of physiology induced in mice housed at thermoneutrality, and review techniques for measuring systemic thermogenesis, specifically those affecting storage and mobilization of lipids in adipose depots. Environmental cues are a component of the information integrated by the brain to calculate food consumption and calorie deposition. We show that relative humidity is one of those cues, inducing a rapid sensory response that is converted to a more chronic susceptibility to obesity. Given high inter-institutional variability in the regulation of relative humidity, study reproducibility may be improved by consideration of this factor. We evaluate a "humanized" environmental cycling protocol, where mice sleep in warm temperature housing, and are cool during the wake cycle. We show that this protocol suppresses adaptation to cool exposure, with consequence for adipose-associated lipid storage. To evaluate systemic cues in mice housed at thermoneutral temperatures, we characterized the circulating lipidome, and show that sera are highly depleted in some HDL-associated phospholipids, specifically phospholipids containing the essential fatty acid, 18:2 linoleic acid, and its derivative, arachidonic acid (20:4) and related ether-phospholipids. Given the role of these fatty acids in inflammatory responses, we propose they may underlie the differences in disease progression observed at thermoneutrality.

Adipose Tissue Remodeling in Obesity: An Overview of the Actions of Thyroid Hormones and Their Derivatives

Metabolic syndrome and obesity have become important health issues of epidemic proportions and are often the cause of related pathologies such as type 2 diabetes (T2DM), hypertension, and cardiovascular disease. Adipose tissues (ATs) are dynamic tissues that play crucial physiological roles in maintaining health and homeostasis. An ample body of evidence indicates that in some pathophysiological conditions, the aberrant remodeling of adipose tissue may provoke dysregulation in the production of various adipocytokines and metabolites, thus leading to disorders in metabolic organs. Thyroid hormones (THs) and some of their derivatives, such as 3,5-diiodo-l-thyronine (T2), exert numerous functions in a variety of tissues, including adipose tissues. It is known that they can improve serum lipid profiles and reduce fat accumulation. The thyroid hormone acts on the brown and/or white adipose tissues to induce uncoupled respiration through the induction of the uncoupling protein 1 (UCP1) to generate heat. Multitudinous investigations suggest that 3,3',5-triiodothyronine (T3) induces the recruitment of brown adipocytes in white adipose depots, causing the activation of a process known as "browning". Moreover, in vivo studies on adipose tissues show that T2, in addition to activating brown adipose tissue (BAT) thermogenesis, may further promote the browning of white adipose tissue (WAT), and affect adipocyte morphology, tissue vascularization, and the adipose inflammatory state in rats receiving a high-fat diet (HFD). In this review, we summarize the mechanism by which THs and thyroid hormone derivatives mediate adipose tissue activity and remodeling, thus providing noteworthy perspectives on their efficacy as therapeutic agents to counteract such morbidities as obesity, hypercholesterolemia, hypertriglyceridemia, and insulin resistance.

Brown adipose TRX2 deficiency activates mtDNA-NLRP3 to impair thermogenesis and protect against diet-induced insulin resistance

Brown adipose tissue (BAT), a crucial heat-generating organ, regulates whole-body energy metabolism by mediating thermogenesis. BAT inflammation is implicated in the pathogenesis of mitochondrial dysfunction and impaired thermogenesis. However, the link between BAT inflammation and systematic metabolism remains unclear. Herein, we use mice with BAT deficiency of thioredoxin-2 (TRX2), a protein that scavenges mitochondrial reactive oxygen species (ROS), to evaluate the impact of BAT inflammation on metabolism and thermogenesis and its underlying mechanism. Our results show that BAT-specific TRX2 ablation improves systematic metabolic performance via enhancing lipid uptake, which protects mice from diet-induced obesity, hypertriglyceridemia, and insulin resistance. TRX2 deficiency impairs adaptive thermogenesis by suppressing fatty acid oxidation. Mechanistically, loss of TRX2 induces excessive mitochondrial ROS, mitochondrial integrity disruption, and cytosolic release of mitochondrial DNA, which in turn activate aberrant innate immune responses in BAT, including the cGAS/STING and the NLRP3 inflammasome pathways. We identify NLRP3 as a key converging point, as its inhibition reverses both the thermogenesis defect and the metabolic benefits seen under nutrient overload in BAT-specific Trx2-deficient mice. In conclusion, we identify TRX2 as a critical hub integrating oxidative stress, inflammation, and lipid metabolism in BAT, uncovering an adaptive mechanism underlying the link between BAT inflammation and systematic metabolism.

Effects of Gouda cheese and Allium hookeri on thermogenesis in mice

Cheese contains various beneficial nutrients, including calcium and whey protein, as well as large amounts of saturated fatty acids. Thus, intake of cheese increases the production of low-density lipoprotein-cholesterol (LDL-C), a well-defined risk factor for cardiovascular disease. Therefore, identification of natural products that inhibit LDL-C production following cheese intake and verification of the efficacy of such products in animal models are essential. Here, we evaluated the effects of Allium hookeri, a well-known traditional herbal remedy, on metabolism and thermogenesis in mice consuming a cheese-containing diet. Intake of A. hookeri extracts significantly blocked increases in body weight and fat mass caused by intake of Gouda cheese in mice. Additionally, increases in blood triglyceride levels following intake of Gouda cheese were alleviated by A. hookeri. Moreover, intake of Gouda cheese enhanced thermogenesis efficiency. Thus, A. hookeri may have applications as an important additive for reducing the risk of metabolic disease resulting from cheese consumption.

Obese Adipose Tissue Secretion Induces Inflammation in Preadipocytes: Role of Toll-Like Receptor-4

In obesity, the dysfunctional adipose tissue (AT) releases increased levels of proinflammatory adipokines such as TNFα, IL-6, and IL-1β and free fatty acids (FFAs), characterizing a chronic, low-grade inflammation. Whilst FFAs and proinflammatory adipokines are known to elicit an inflammatory response within AT, their relative influence upon preadipocytes, the precursors of mature adipocytes, is yet to be determined. Our results demonstrated that the conditioned medium (CM) derived from obese AT was rich in FFAs, which guided us to evaluate the role of TLR4 in the induction of inflammation in preadipocytes. We observed that CM derived from obese AT increased reactive oxygen species (ROS) levels and NF-ĸB nuclear translocation together with IL-6, TNFα, and IL-1β in 3T3-L1 cells in a TLR4-dependent manner. Furthermore, TLR4 signaling was involved in the increased expression of C/EBPα together with the release of leptin, adiponectin, and proinflammatory mediators, in response to the CM derived from obese AT. Our results suggest that obese AT milieu secretes lipokines, which act in a combined paracrine/autocrine manner, inducing inflammation in preadipocytes via TLR4 and ROS, thus creating a paracrine loop that facilitates the differentiation of adipocytes with a proinflammatory profile.

Characterization of BAT activity in rats using invasive and non-invasive techniques

Introduction

Brown adipose tissue (BAT) is considered as a potential target for combating obesity in humans where active BAT metabolizes glucose and fatty acids as fuel resulting in heat production. Prospective studies in humans have been set up to further study the presence and metabolic activity of BAT mostly using Positron Emission Tomography (PET) imaging in cold-stimulated conditions with the radiolabeled glucose derivative [¹⁸F]FDG. However, radiotracers beyond [¹⁸F]FDG have been proposed to investigate BAT activity, targeting various aspects of BAT metabolism. It remains questionable which tracer is best suited to detect metabolic BAT activity and to what extent those results correlate with ex vivo metabolic BAT activity.

Methods

PET and Single Photon Emission Computed Tomography (SPECT) imaging, targeting different aspects of BAT activation such as glucose metabolism, fatty acid metabolism, noradrenergic stimulation, blood perfusion and amino acid transport system, was performed immediately after injection of the tracer in rats under different temperatures: room temperature, acute cold (4 ⁰C for 4 h) or acclimated to cold (4 ⁰C for 6 h per day during 28 days). Furthermore, Magnetic Resonance Spectroscopy (MRS)-derived BAT temperature was measured in control and cold-acclimated rats.

Results

At room temperature, only [¹⁸F]FDG visualized BAT. Glucose metabolism, fatty acid metabolism, noradrenergic stimulation and blood perfusion showed a clear tracer-dependent twofold increase in BAT uptake upon cold exposure. Only the tracer for the amino acid transport system did not show BAT specific uptake under any of the experimental conditions. MRS demonstrated that cold-acclimated animals had BAT with a stronger heat-production compared to control animals.

Conclusion

BAT activity following cold exposure in rats was visualized by several tracers, while only [¹⁸F]FDG was also able to show BAT activity under non-stimulated conditions (room temperature). The variances in uptake of the different tracers should be taken into account when developing future clinical applications in humans.

Effect of Iodothyronines on Thermogenesis: Focus on Brown Adipose Tissue

Thyroid hormones significantly influence energy expenditure by affecting the activity of metabolic active tissues, among which, mammalian brown adipose tissue (BAT) plays a significant role. For a long time, the modulation of BAT activity by 3,3',5-triiodo-l-thyronine (T3) has been ascribed to its direct actions on this tissue; however, recent evidence indicates that T3, by stimulating specific brain centers, activates the metabolism of BAT via the sympathetic nervous system. These distinct mechanisms of action are not mutually exclusive. New evidence indicates that 3,5-diiodo-l-thyronine (3,5-T2), a thyroid hormone derivative, exerts thermogenic effects, by influencing mitochondrial activity in metabolically active tissues, such as liver, skeletal muscle, and BAT. At the moment, due to the absence of experiments finalized to render a clear cut discrimination between peripheral and central effects induced by 3,5-T2, it is not possible to exclude that some of the metabolic effects exerted by 3,5-T2 may be mediated centrally. Despite this, some evidence suggests that 3,5-T2 plays a role in adrenergic stimulation of thermogenesis in BAT. This mini-review provides an overview of the effects induced by T3 and 3,5-T2 on BAT thermogenesis, with a focus on data suggesting the involvement of central adrenergic stimulation. These aspects may reveal new perspectives in thyroid physiology and in the control of energy metabolism.

Brown adipose tissue activation in a rat model of Parkinson's disease

Loss of body weight and fat mass is one of the nonmotor symptoms of Parkinson's disease (PD). Weight loss is due primarily to reduced energy intake and increased energy expenditure. Whereas inadequate energy intake in PD patients is caused mainly by appetite loss and impaired gastrointestinal absorption, the underlying mechanisms for increased energy expenditure remain largely unknown. Brown adipose tissue (BAT), a key thermogenic tissue in humans and other mammals, plays an important role in thermoregulation and energy metabolism; however, it has not been tested whether BAT is involved in the negative energy balance in PD. Here, using the 6-hydroxydopamine (6-OHDA) rat model of PD, we found that the activity of sympathetic nerve (SN), the expression of Ucp1 in BAT, and thermogenesis were increased in PD rats. BAT sympathetic denervation blocked sympathetic activity and decreased UCP1 expression in BAT and attenuated the loss of body weight in PD rats. Interestingly, sympathetic denervation of BAT was associated with decreased sympathetic tone and lipolysis in retroperitoneal and epididymal white adipose tissue. Our data suggeste that BAT-mediated thermogenesis may contribute to weight loss in PD.

Quantification of Bone Fatty Acid Metabolism and Its Regulation by Adipocyte Lipoprotein Lipase

Adipocytes are master regulators of energy homeostasis. Although the contributions of classical brown and white adipose tissue (BAT and WAT, respectively) to glucose and fatty acid metabolism are well characterized, the metabolic role of adipocytes in bone marrow remains largely unclear. Here, we quantify bone fatty acid metabolism and its contribution to systemic nutrient handling in mice. Whereas in parts of the skeleton the specific amount of nutrients taken-up from the circulation was lower than in other metabolically active tissues such as BAT or liver, the overall contribution of the skeleton as a whole organ was remarkable, placing it among the top organs involved in systemic glucose as well as fatty acid clearance. We show that there are considerable site-specific variations in bone marrow fatty acid composition throughout the skeleton and that, especially in the tibia, marrow fatty acid profiles resemble classical BAT and WAT. Using a mouse model lacking lipoprotein lipase (LPL), a master regulator of plasma lipid turnover specifically in adipocytes, we show that impaired fatty acid flux leads to reduced amounts of dietary essential fatty acids while there was a profound increase in de novo produced fatty acids in both bone marrow and cortical bone. Notably, these changes in fatty acid profiles were not associated with any gross skeletal phenotype. These results identify LPL as an important regulator of fatty acid transport to skeletal compartments and demonstrate an intricate functional link between systemic and skeletal fatty acid and glucose metabolism.

Thermogenic adipocytes promote HDL turnover and reverse cholesterol transport

Brown and beige adipocytes combust nutrients for thermogenesis and through their metabolic activity decrease pro-atherogenic remnant lipoproteins in hyperlipidemic mice. However, whether the activation of thermogenic adipocytes affects the metabolism and anti-atherogenic properties of high-density lipoproteins (HDL) is unknown. Here, we report a reduction in atherosclerosis in response to pharmacological stimulation of thermogenesis linked to increased HDL levels in APOE*3-Leiden.CETP mice. Both cold-induced and pharmacological thermogenic activation enhances HDL remodelling, which is associated with specific lipidomic changes in mouse and human HDL. Furthermore, thermogenic stimulation promotes HDL-cholesterol clearance and increases macrophage-to-faeces reverse cholesterol transport in mice. Mechanistically, we show that intravascular lipolysis by adipocyte lipoprotein lipase and hepatic uptake of HDL by scavenger receptor B-I are the driving forces of HDL-cholesterol disposal in liver. Our findings corroborate the notion that high metabolic activity of thermogenic adipocytes confers atheroprotective properties via increased systemic cholesterol flux through the HDL compartment.

Activation of brown adipose tissue (BAT) reduces the development of atherosclerosis in animal models. Here the authors show that BAT activation also increases reverse cholesterol transport and turnover of high-density lipoprotein, which likely contributes to the anti-atherosclerotic effect of BAT activation.

Differential Role of Adipose Tissues in Obesity and Related Metabolic and Vascular Complications

This review focuses on the contribution of white, brown, and perivascular adipose tissues to the pathophysiology of obesity and its associated metabolic and vascular complications. Weight gain in obesity generates excess of fat, usually visceral fat, and activates the inflammatory response in the adipocytes and then in other tissues such as liver. Therefore, low systemic inflammation responsible for insulin resistance contributes to atherosclerotic process. Furthermore, an inverse relationship between body mass index and brown adipose tissue activity has been described. For these reasons, in recent years, in order to combat obesity and its related complications, as a complement to conventional treatments, a new insight is focusing on the role of the thermogenic function of brown and perivascular adipose tissues as a promising therapy in humans. These lines of knowledge are focused on the design of new drugs, or other approaches, in order to increase the mass and/or activity of brown adipose tissue or the browning process of beige cells from white adipose tissue. These new treatments may contribute not only to reduce obesity but also to prevent highly prevalent complications such as type 2 diabetes and other vascular alterations, such as hypertension or atherosclerosis.

Fatty acid metabolism and the basis of brown adipose tissue function

Obesity has reached epidemic proportions, leading to severe associated pathologies such as insulin resistance, cardiovascular disease, cancer and type 2 diabetes. Adipose tissue has become crucial due to its involvement in the pathogenesis of obesity-induced insulin resistance, and traditionally white adipose tissue has captured the most attention. However in the last decade the presence and activity of heat-generating brown adipose tissue (BAT) in adult humans has been rediscovered. BAT decreases with age and in obese and diabetic patients. It has thus attracted strong scientific interest, and any strategy to increase its mass or activity might lead to new therapeutic approaches to obesity and associated metabolic diseases. In this review we highlight the mechanisms of fatty acid uptake, trafficking and oxidation in brown fat thermogenesis. We focus on BAT's morphological and functional characteristics and fatty acid synthesis, storage, oxidation and use as a source of energy.

Genetic Dissection of Tissue-Specific Apolipoprotein E Function for Hypercholesterolemia and Diet-Induced Obesity

ApoE deficiency in mice (Apoe^−/−) results in severe hypercholesterolemia and atherosclerosis. In diet-induced obesity, Apoe^−/− display steatohepatitis but reduced accumulation of triacylglycerides and enhanced insulin sensitivity in white adipose tissue (WAT). Although the vast majority of apoE is expressed by hepatocytes apoE is also abundantly expressed in WAT. As liver and adipose tissue play important roles for metabolism, this study aims to outline functions of both hepatocyte- and adipocyte-derived apoE separately by investigating a novel mouse model of tissue-specific apoE deficiency. Therefore we generated transgenic mice carrying homozygous floxed Apoe alleles. Mice lacking apoE either in hepatocytes (Apoe^ΔHep) or in adipose tissue (Apoe^ΔAT) were fed experimental diets. Apoe^ΔHep exhibited slightly higher body weights, adiposity and liver weights on diabetogenic high fat diet (HFD). Accordingly, hepatic steatosis and markers of inflammation were more pronounced compared to controls. Hypercholesterolemia evoked by lipoprotein remnant accumulation was present in Apoe^ΔHep mice fed a Western type diet (WTD). Lipidation of VLDL particles and tissue uptake of VLDL were disturbed in Apoe^ΔHep while the plasma clearance rate remained unaltered. Apoe^ΔAT did not display any detectable phenotype, neither on HFD nor on WTD. In conclusion, our novel conditional apoE deletion model has proven here the role of hepatocyte apoE for VLDL production and diet-induced dyslipidemia. Specific deletion of apoE in adipocytes cannot reproduce the adipose phenotype of global Apoe^−/− mice, suggesting that apoE produced in other cell types than hepatocytes or adipocytes explains the lean and insulin-sensitive phenotype described for Apoe^−/− mice.

Of mice and men: novel insights regarding constitutive and recruitable brown adipocytes

Recently, there has been great attention given to the possibility of combating obesity by targeting brown fat activity or increasing differentiation of brown adipocytes in white fat depots through a process termed 'browning'. Sympathetic innervation of brown and white adipose tissues provides adrenergic input that drives thermogenesis and regulates fatty acid metabolism, as well as stimulating adipogenesis of recruitable brown adipocyte tissue (rBAT, also known as beige or brite) in white fat. Other factors acting in an endocrine or autocrine/paracrine manner in adipose tissue may also stimulate browning. There have been significant recent advances in understanding the mechanisms of increasing adipose tissue energy expenditure, as well as how brown adipocytes appear in white fat depots, including via de novo adipogenesis from tissue precursor cells. In this article, we integrate this new knowledge with a historical perspective on the discovery of 'browning'. We also provide an overview of constitutive BAT vs rBAT in mouse and human.

Brown Adipose Tissue Response Dynamics: In Vivo Insights with the Voltage Sensor 18F-Fluorobenzyl Triphenyl Phosphonium

Brown adipose tissue (BAT) thermogenesis is an emerging target for prevention and treatment of obesity. Mitochondria are the heat generators of BAT. Yet, there is no noninvasive means to image the temporal dynamics of the mitochondrial activity in BAT in vivo. Here, we report a technology for quantitative monitoring of principal kinetic components of BAT adaptive thermogenesis in the living animal, using the PET imaging voltage sensor ¹⁸F-fluorobenzyltriphenylphosphonium (¹⁸F-FBnTP). ¹⁸F-FBnTP targets the mitochondrial membrane potential (ΔΨm)—the voltage analog of heat produced by mitochondria. Dynamic ¹⁸F-FBnTP PET imaging of rat’s BAT was acquired just before and during localized skin cooling or systemic pharmacologic stimulation, with and without administration of propranolol. At ambient temperature, ¹⁸F-FBnTP demonstrated rapid uptake and prolonged steady-state retention in BAT. Conversely, cold-induced mitochondrial uncoupling resulted in an immediate washout of ¹⁸F-FBnTP from BAT, which was blocked by propranolol. Specific variables of BAT evoked activity were identified and quantified, including response latency, magnitude and kinetics. Cold stimulation resulted in partial washout of ¹⁸F-FBnTP (39.1%±14.4% of basal activity). The bulk of ¹⁸F-FBnTP washout response occurred within the first minutes of the cold stimulation, while colonic temperature remained nearly intact. Drop of colonic temperature to shivering zone did not have an additive effect. The ß3-adrenergic agonist CL-316,243 elicited ¹⁸F-FBnTP washout from BAT of kinetics similar to those caused by cold stimulation. Thus, monitoring ΔΨm in vivo using ¹⁸F-FBnTP PET provides insights into the kinetic physiology of BAT. ¹⁸F-FBnTP PET depicts BAT as a highly sensitive and rapidly responsive organ, emitting heat in short burst during the first minutes of stimulation, and preceding change in core temperature. ¹⁸F-FBnTP PET provides a novel set of quantitative metrics highly important for identifying novel therapeutic targets at the mitochondrial level, for developing means to maximize BAT mass and activity, and assessing intervention efficacy.

Metabolic differences between white and brown fat from fasting rabbits at physiological temperature

It has been suggested that activated brown adipose tissue (BAT) shows increased glucose metabolic activity. However, less is known about metabolic activity of BAT under conditions of fasting and normal temperature. The aim of this study was to compare the possible differences in energetic metabolism between BAT and white adipose tissue (WAT) obtained from rabbits under the conditions of physiological temperature and 24 h after fasting conditions. The study was carried out on New Zealand rabbits (n=10) maintained for a period of 8 weeks at 23±2 °C. Food was removed 24 h before BAT and WAT were obtained. Protein expression levels of the glycolytic-related protein, glyceraldehyde-3-phosphate dehydrogenase, and pyruvate dehydrogenase were higher in WAT than that in BAT. The expression level of carnitine palmitoyltransferase 1 (CPT1) and CPT2, two fatty acid mitochondrial transporters, and the fatty acid β-oxidation-related enzyme, acyl CoA dehydrogenase, was higher in BAT than in WAT. Cytosolic malate dehydrogenase expression and malate dehydrogenase activity were higher in WAT than in BAT. However, lactate dehydrogenase expression and lactate content were significantly higher in BAT than in WAT. In summary, this study for the first time, to our knowledge, has described how under fasting and normal temperature conditions rabbit BAT seems to use anaerobic metabolism to provide energetic fuel, as opposed to WAT, where the malate-aspartate shuttle and, therefore, the gluconeogenic pathway seem to be potentiated.

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.

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.

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.

3,5-Diiodo-L-thyronine activates brown adipose tissue thermogenesis in hypothyroid rats

3,5-Diiodo-l-thyronine (T2), a thyroid hormone derivative, is capable of increasing energy expenditure, as well as preventing high fat diet-induced overweight and related metabolic dysfunction. Most studies to date on T2 have been carried out on liver and skeletal muscle. Considering the role of brown adipose tissue (BAT) in energy and metabolic homeostasis, we explored whether T2 could activate BAT thermogenesis. Using euthyroid, hypothyroid, and T2-treated hypothyroid rats (all maintained at thermoneutrality) in morphological and functional studies, we found that hypothyroidism suppresses the maximal oxidative capacity of BAT and thermogenesis, as revealed by reduced mitochondrial content and respiration, enlarged cells and lipid droplets, and increased number of unilocular cells within the tissue. In vivo administration of T2 to hypothyroid rats activated BAT thermogenesis and increased the sympathetic innervation and vascularization of tissue. Likewise, T2 increased BAT oxidative capacity in vitro when added to BAT homogenates from hypothyroid rats. In vivo administration of T2 to hypothyroid rats enhanced mitochondrial respiration. Moreover, UCP1 seems to be a molecular determinant underlying the effect of T2 on mitochondrial thermogenesis. In fact, inhibition of mitochondrial respiration by GDP and its reactivation by fatty acids were greater in mitochondria from T2-treated hypothyroid rats than untreated hypothyroid rats. In vivo administration of T2 led to an increase in PGC-1α protein levels in nuclei (transient) and mitochondria (longer lasting), suggesting a coordinate effect of T2 in these organelles that ultimately promotes net activation of mitochondrial biogenesis and BAT thermogenesis. The effect of T2 on PGC-1α is similar to that elicited by triiodothyronine. As a whole, the data reported here indicate T2 is a thyroid hormone derivative able to activate BAT thermogenesis.

CD36 is indispensable for thermogenesis under conditions of fasting and cold stress

Hypothermia can occur during fasting when thermoregulatory mechanisms, involving fatty acid (FA) utilization, are disturbed. CD36/FA translocase is a membrane protein which facilitates membrane transport of long-chain FA in the FA consuming heart, skeletal muscle (SkM) and adipose tissues. It also accelerates uptake of triglyceride-rich lipoprotein by brown adipose tissue (BAT) in a cold environment. In mice deficient for CD36 (CD36(-/-) mice), FA uptake is markedly reduced with a compensatory increase in glucose uptake in the heart and SkM, resulting in lower levels of blood glucose especially during fasting. However, the role of CD36 in thermogenic activity during fasting remains to be determined. In fasted CD36(-/-) mice, body temperature drastically decreased shortly after cold exposure. The hypothermia was accompanied by a marked reduction in blood glucose and in stores of triacylglycerols in BAT and of glycogen in glycolytic SkM. Biodistribution analysis using the FA analogue (125)I-BMIPP and the glucose analogue (18)F-FDG revealed that uptake of FA and glucose was severely impaired in BAT and glycolytic SkM in cold-exposed CD36(-/-) mice. Further, induction of the genes of thermogenesis in BAT was blunted in fasted CD36(-/-) mice after cold exposure. These findings strongly suggest that CD36(-/-) mice exhibit pronounced hypothermia after fasting due to depletion of energy storage in BAT and glycolytic SkM and to reduced supply of energy substrates to these tissues. Our study underscores the importance of CD36 for nutrient homeostasis to survive potentially life-threatening challenges, such as cold and starvation.

Hypothalamic control of adipose tissue

A detailed appreciation of the control of adipose tissue whether it be white, brown or brite/beige has never been more important to the development of a framework on which to build therapeutic strategies to combat obesity. This is because 1) the rate of fatty acid release into the circulation from lipolysis in white adipose tissue (WAT) is integrally important to the development of obesity, 2) brown adipose tissue (BAT) has now moved back to center stage with the realization that it is present in adult humans and, in its activated form, is inversely proportional to levels of obesity and 3) the identification and characterization of "brown-like" or brite/beige fat is likely to be one of the most exciting developments in adipose tissue biology in the last decade. Central to all of these developments is the role of the CNS in the control of different fat cell functions and central to CNS control is the integrative capacity of the hypothalamus. In this chapter we will attempt to detail key issues relevant to the structure and function of hypothalamic and downstream control of WAT and BAT and highlight the importance of developing an understanding of the neural input to brite/beige fat cells as a precursor to its recruitment as therapeutic target.

The cell-type specific uptake of polymer-coated or micelle-embedded QDs and SPIOs does not provoke an acute pro-inflammatory response in the liver

Semiconductor quantum dots (QD) and superparamagnetic iron oxide nanocrystals (SPIO) have exceptional physical properties that are well suited for biomedical applications in vitro and in vivo. For future applications, the direct injection of nanocrystals for imaging and therapy represents an important entry route into the human body. Therefore, it is crucial to investigate biological responses of the body to nanocrystals to avoid harmful side effects. In recent years, we established a system to embed nanocrystals with a hydrophobic oleic acid shell either by lipid micelles or by the amphiphilic polymer poly(maleic anhydride-alt-1-octadecene) (PMAOD). The goal of the current study is to investigate the uptake processes as well as pro-inflammatory responses in the liver after the injection of these encapsulated nanocrystals. By immunofluorescence and electron microscopy studies using wild type mice, we show that 30 min after injection polymer-coated nanocrystals are primarily taken up by liver sinusoidal endothelial cells. In contrast, by using wild type, Ldlr (-/-) as well as Apoe (-/-) mice we show that nanocrystals embedded within lipid micelles are internalized by Kupffer cells and, in a process that is dependent on the LDL receptor and apolipoprotein E, by hepatocytes. Gene expression analysis of pro-inflammatory markers such as tumor necrosis factor alpha (TNFα) or chemokine (C-X-C motif) ligand 10 (Cxcl10) indicated that 48 h after injection internalized nanocrystals did not provoke pro-inflammatory pathways. In conclusion, internalized nanocrystals at least in mouse liver cells, namely endothelial cells, Kupffer cells and hepatocytes are at least not acutely associated with potential adverse side effects, underlining their potential for biomedical applications.

Syndecan-1 is required to maintain intradermal fat and prevent cold stress

Homeostatic temperature regulation is fundamental to mammalian physiology and is controlled by acute and chronic responses of local, endocrine and nervous regulators. Here, we report that loss of the heparan sulfate proteoglycan, syndecan-1, causes a profoundly depleted intradermal fat layer, which provides crucial thermogenic insulation for mammals. Mice without syndecan-1 enter torpor upon fasting and show multiple indicators of cold stress, including activation of the stress checkpoint p38α in brown adipose tissue, liver and lung. The metabolic phenotype in mutant mice, including reduced liver glycogen, is rescued by housing at thermoneutrality, suggesting that reduced insulation in cool temperatures underlies the observed phenotypes. We find that syndecan-1, which functions as a facultative lipoprotein uptake receptor, is required for adipocyte differentiation in vitro. Intradermal fat shows highly dynamic differentiation, continuously expanding and involuting in response to hair cycle and ambient temperature. This physiology probably confers a unique role for Sdc1 in this adipocyte sub-type. The PPARγ agonist rosiglitazone rescues Sdc1−/− intradermal adipose tissue, placing PPARγ downstream of Sdc1 in triggering adipocyte differentiation. Our study indicates that disruption of intradermal adipose tissue development results in cold stress and complex metabolic pathology.

All mammals strive to maintain a fixed body temperature, and do so using a remarkable array of different strategies, which vary depending upon the degree of cold challenge. Physiologists many decades ago observed that a fat layer right underneath the epidermis (and above the dermal muscle layer) thickens in response to colder ambient temperatures. This “intradermal fat” provided insulation within days of climate changes. We have found that syndecan-1, which functions as a facultative lipoprotein uptake receptor, is required for intradermal fat expansion in response to cold exposure. This is a highly specific phenotype not shared by other adipocytes. When intradermal fat is absent, mice do not adapt normally to cold stress, and show altered systemic physiologies, including increased brown adipose tissue thermogenesis and hyper-activation of a stress checkpoint (p38α), designed to protect the body against mutagenic and oxidative stressors. The phenotypes associated with loss of Sdc1 function are reversed when mice are housed in warm temperatures, where defense of body temperature is not required. This study is the first to show that intradermal fat can be genetically regulated, with systemic effects on physiology.

Hepatic lipase is expressed by osteoblasts and modulates bone remodeling in obesity

A number of unexpected molecules were recently identified as products of osteoblasts, linking bone homeostasis to systemic energy metabolism. Here we identify the lipolytic enzyme hepatic lipase (HL, encoded by Lipc) as a novel cell-autonomous regulator of osteoblast function. In an unbiased genome-wide expression analysis, we find Lipc to be highly induced upon osteoblast differentiation, verified by quantitative Taqman analyses of primary osteoblasts in vitro and of bone samples in vivo. Functionally, loss of HL in vitro leads to increased expression and secretion of osteoprotegerin (OPG), while expression of some osteoblast differentiation makers is impaired. When challenging energy metabolism in a diet-induced obesity (DIO) study, lack of HL leads to a significant increase in bone formation markers and a decrease in bone resorption markers. Accordingly, in the DIO setting, we observe in Lipc(-/-) animals but not in wild-type controls a significant increase in lumbar vertebral trabecular bone mass and formation rate as well as in femoral trabecular bone mass and cortical thickness. Taken together, we demonstrate that HL expressed by osteoblasts has an impact on osteoblast OPG expression and that lack of HL leads to increased bone mass in DIO. These data provide a novel and completely unexpected molecular link in the complex interplay of osteoblasts and systemic energy metabolism.

Brown fat fuel utilization and thermogenesis

Brown adipose tissue (BAT) dissipates energy as heat to maintain optimal thermogenesis and to contribute to energy expenditure in rodents and possibly humans. The energetic processes executed by BAT require a readily-available fuel supply, which includes glucose and fatty acids (FAs). FAs become available by cellular uptake, de novo lipogenesis, and multilocular lipid droplets in brown adipocytes. BAT also possesses a great capacity for glucose uptake and metabolism, and an ability to regulate insulin sensitivity. These properties make BAT an appealing target for the treatment of obesity, diabetes, and other metabolic disorders. Recent research has provided a better understanding of the processes of fuel utilization carried out by brown adipocytes, which is the focus of the current review.

Brown adipose tissue dynamics in wild-type and UCP1-knockout mice: in vivo insights with magnetic resonance

We used noninvasive magnetic resonance imaging (MRI) and magnetic resonance spectroscopy to compare interscapular brown adipose tissue (iBAT) of wild-type (WT) and uncoupling protein 1 (UCP1)-knockout mice lacking UCP1-mediated nonshivering thermogenesis (NST). Mice were sequentially acclimated to an ambient temperature of 30°C, 18°C, and 5°C. We detected a remodeling of iBAT and a decrease in its lipid content in all mice during cold exposure. Ratios of energy-rich phosphates (ATP/ADP, phosphocreatine/ATP) in iBAT were maintained stable during noradrenergic stimulation of thermogenesis in cold- and warm-adapted mice and no difference between the genotypes was observed. As free fatty acids (FFAs) serve as fuel for thermogenesis and activate UCP1 for uncoupling of oxidative phosphorylation, brown adipose tissue is considered to be a main acceptor and consumer of FFAs. We measured a major loss of FFAs from iBAT during noradrenergic stimulation of thermogenesis. This mobilization of FFAs was observed in iBAT of WT mice as well as in mice lacking UCP1. The high turnover and the release of FFAs from iBAT suggests an enhancement of lipid metabolism, which in itself contributes to the sympathetically activated NST and which is independent from uncoupled respiration mediated by UCP1. Our study demonstrates that MRI, besides its potential for visualizing and quantification of fat tissue, is a valuable tool for monitoring functional in vivo processes like lipid and phosphate metabolism during NST.

Adipose tissue browning and metabolic health

Accumulation of excess white adipose tissue (WAT) has deleterious consequences for metabolic health. The activation of brown adipose tissue (BAT), the primary organ for heat production, confers beneficial effects on adiposity, insulin resistance and hyperlipidaemia, at least in mice. As the amount of metabolically active BAT seems to be particularly low in patients with obesity or diabetes mellitus who require immediate therapy, new avenues are needed to increase the capacity for adaptive thermogenesis. In this light, we review the findings that BAT in human adults might consist of not only classic brown adipocytes but also inducible brown adipocytes (also called beige, brown-in-white, or brite adipocytes), which are phenotypically distinct from both white and brown adipocytes. Stimulating the development of beige adipocytes in WAT (so called 'browning') might reduce adverse effects of WAT and could help to improve metabolic health. This article focuses on the development and regulatory control of beige adipocytes at the transcriptional and hormonal levels. Emerging insights into the metabolic role of beige adipocytes are also discussed, along with the developments that can be expected from these promising targets for therapy of metabolic disease in the future.

Translocator protein 18 kDa (TSPO) is regulated in white and brown adipose tissue by obesity

Translocator protein 18 kDa (TSPO) is an outer-mitochondrial membrane transporter which has many functions including participation in the mitochondrial permeability transition pore, regulation of reactive oxygen species (ROS), production of cellular energy, and is the rate-limiting step in the uptake of cholesterol. TSPO expression is dysregulated during disease pathologies involving changes in tissue energy demands such as cancer, and is up-regulated in activated macrophages during the inflammatory response. Obesity is associated with decreased energy expenditure, mitochondrial dysfunction, and chronic low-grade inflammation which collectively contribute to the development of the Metabolic Syndrome. Therefore, we hypothesized that dysregulation of TSPO in adipose tissue may be a feature of disease pathology in obesity. Radioligand binding studies revealed a significant reduction in TSPO ligand binding sites in mitochondrial extracts from both white (WAT) and brown adipose tissue (BAT) in mouse models of obesity (diet-induced and genetic) compared to control animals. We also confirmed a reduction in TSPO gene expression in whole tissue extracts from WAT and BAT. Immunohistochemistry in WAT confirmed TSPO expression in adipocytes but also revealed high-levels of TSPO expression in WAT macrophages in obese animals. No changes in TSPO expression were observed in WAT or BAT after a 17 hour fast or 4 hour cold exposure. Treatment of mice with the TSPO ligand PK11195 resulted in regulation of metabolic genes in WAT. Together, these results suggest a potential role for TSPO in mediating adipose tissue homeostasis.

Probiotics determine hypolipidemic and antioxidant effects in hyperlipidemic hamsters

SCOPE

Hyperlipidemia, hyperglycemia, and the oxidative stress are among the known risk factors of atherosclerosis. Our aim was to assess the hypolipidemic and antioxidant effects of a probiotic mix (Lactobacillus acidophilus and Bifidobacterium animalis) in hyperlipidemic hamsters (HL).

METHODS AND RESULTS

Male Golden Syrian hamsters developed hyperlipidemia after 21 weeks of fat diet. For the last 5 weeks of experiment, ten HL were treated with the probiotic mix (HLP), ten received water (HL). Ten animals received standard chow (N). Increase of plasma total cholesterol (TC), triglycerides (TG), phospholipids (PL), oxidized LDL, glucose, of 4-hydroxynonenal (4-HNE) in plasma, liver, and myocardium, and of intestinal Niemann Pick C1 like 1 (NPC1L1) and microsomal TG transfer protein (MTTP) expression was observed in HL versus N. The probiotic mix decreased plasma TC, TG, PL, oxidized LDL, 4-HNE, and glucose levels and increased paraoxonase-1 activity, decreased NPC1L1 and MTTP protein expression compared to HL. In HLP liver, a significant reduction of TC, TG, and fatty acids was observed. PL increased and 4-HNE levels decreased in the liver and myocardium of HLP versus HL.

CONCLUSION

Our data support the administration of probiotics to humans because of their hypolipidemic (through decreasing intestinal NPC1L1 and MTTP) and antioxidant effects (stimulating HDL-associated paraoxonase-1).

BMP7 activates brown adipose tissue and reduces diet-induced obesity only at subthermoneutrality

Background/Aims

Brown adipose tissue (BAT) dissipates energy stored in triglycerides as heat via the uncoupling protein UCP-1 and is a promising target to combat hyperlipidemia and obesity. BAT is densely innervated by the sympathetic nervous system, which increases BAT differentiation and activity upon cold exposure. Recently, Bone Morphogenetic Protein 7 (BMP7) was identified as an inducer of BAT differentiation. We aimed to elucidate the role of sympathetic activation in the effect of BMP7 on BAT by treating mice with BMP7 at varying ambient temperature, and assessed the therapeutic potential of BMP7 in combating obesity.

Methods and Results

High-fat diet fed lean C57Bl6/J mice were treated with BMP7 via subcutaneous osmotic minipumps for 4 weeks at 21°C or 28°C, the latter being a thermoneutral temperature in which sympathetic activation of BAT is largely diminished. At 21°C, BMP7 increased BAT weight, increased the expression of Ucp1, Cd36 and hormone-sensitive lipase in BAT, and increased total energy expenditure. BMP7 treatment markedly increased food intake without affecting physical activity. Despite that, BMP7 diminished white adipose tissue (WAT) mass, accompanied by increased expression of genes related to intracellular lipolysis in WAT. All these effects were blunted at 28°C. Additionally, BMP7 resulted in extensive ‘browning’ of WAT, as evidenced by increased expression of BAT markers and the appearance of whole clusters of brown adipocytes via immunohistochemistry, independent of environmental temperature. Treatment of diet-induced obese C57Bl6/J mice with BMP7 led to an improved metabolic phenotype, consisting of a decreased fat mass and liver lipids as well as attenuated dyslipidemia and hyperglycemia.

Conclusion

Together, these data show that BMP7-mediated recruitment and activation of BAT only occurs at subthermoneutral temperature, and is thus likely dependent on sympathetic activation of BAT, and that BMP7 may be a promising tool to combat obesity and associated disorders.

Increased mitochondrial activity in BMP7-treated brown adipocytes, due to increased CPT1- and CD36-mediated fatty acid uptake

AIMS

Brown adipose tissue dissipates chemical energy in the form of heat and regulates triglyceride and glucose metabolism in the body. Factors that regulate fatty acid uptake and oxidation in brown adipocytes have not yet been fully elucidated. Bone morphogenetic protein 7 (BMP7) is a growth factor capable of inducing brown fat mitochondrial biogenesis during differentiation from adipocyte progenitors. Administration of BMP7 to mice also results in increased energy expenditure. To determine if BMP7 is able to affect the mitochondrial activity of mature brown adipocytes, independent of the differentiation process, we delivered BMP7 to mature brown adipocytes and measured mitochondrial activity.

RESULTS

We found that BMP7 increased mitochondrial activity, including fatty acid oxidation and citrate synthase activity, without increasing the mitochondrial number. This was accompanied by an increase in fatty acid uptake and increased protein expression of CPT1 and CD36, which import fatty acids into the mitochondria and the cell, respectively. Importantly, inhibition of either CPT1 or CD36 resulted in a blunting of the mitochondrial activity of BMP7-treated cells.

INNOVATION

These findings uncover a novel pathway regulating mitochondrial activities in mature brown adipocytes by BMP7-mediated fatty acid uptake and oxidation.

CONCLUSION

In conclusion, BMP7 increases mitochondrial activity in mature brown adipocytes via increased fatty acid uptake and oxidation, a process that requires the fatty acid transporters CPT1 and CD36.

Human mediastinal adipose tissue displays certain characteristics of brown fat

Background:

The amount of intra-thoracic fat, of which mediastinal adipose tissue comprises the major depot, is related to various cardiometabolic risk factors. Autopsy and imaging studies indicate that the mediastinal depot in adult humans could contain brown adipose tissue (BAT). To gain a better understanding of this intra-thoracic fat depot, we examined possible BAT characteristics of human mediastinal in comparison with subcutaneous adipose tissue.

Materials and methods:

Adipose tissue biopsies from thoracic subcutaneous and mediastinal depots were obtained during open-heart surgery from 33 subjects (26 male, 63.7±13.8 years, body mass index 29.3±5.1 kg m⁻²). Microarray analysis was performed on 10 patients and genes of interest confirmed by quantitative PCR (qPCR) in samples from another group of 23 patients. Adipocyte size was determined and uncoupling protein 1 (UCP1) protein expression investigated with immunohistochemistry.

Results:

The microarray data showed that a number of BAT-specific genes had significantly higher expression in the mediastinal depot than in the subcutaneous depot. Higher expression of UCP1 (24-fold, P<0.001) and PPARGC1A (1.7-fold, P=0.0047), and lower expression of SHOX2 (0.12-fold, P<0.001) and HOXC8 (0.14-fold, P<0.001) in the mediastinal depot was confirmed by qPCR. Gene set enrichment analysis identified two gene sets related to mitochondria, which were significantly more highly expressed in the mediastinal than in the subcutaneous depot (P<0.01). No significant changes in UCP1 gene expression were observed in the subcutaneous or mediastinal depots following lowering of body temperature during surgery. UCP1 messenger RNA levels in the mediastinal depot were lower than those in murine BAT and white adipose tissue. In some mediastinal adipose tissue biopsies, a small number of multilocular adipocytes that stained positively for UCP1 were observed. Adipocytes were significantly smaller in the mediastinal than the subcutaneous depot (cross-sectional area 2400±810 versus 3260±980 μm², P<0.001).

Conclusions:

Human mediastinal adipose tissue displays some characteristics of BAT when compared with the subcutaneous depot at microscopic and molecular levels.

Novel aspects of brown adipose tissue biology

During the last decades, obesity research has focused on food intake regulation, whereas energy expenditure has been mainly measured based on whole-body oxygen consumption. With the renaissance of brown adipose tissue (BAT) thermogenesis as a potential drug target in humans, more thought is put into alternative heat-producing mechanisms. Also, the interaction of peripheral and central components to regulate thermogenesis requires further studies. Certainly, several of the novel molecular genetic tools available now, compared with 40 years ago, will be helpful to gain new insights in BAT-controlled energy homeostasis and promises new approaches to pharmacologically control body weight.