Brown adipose tissue: from thermal physiology to bioenergetics

Brown adipose tissue is an organ in mammals specialized for the generation of heat. The tissue plays an important role in thermoregulatory heat production (nonshivering thermogenesis), and in nutritional energetics (through the process of diet-induced thermogenesis). Much of the current interest in brown adipose tissue has been catalysed by the postulate (1970’s) that a reduced capacity for thermogenesis underlies the development of obesity. Heat is generated in brown fat by a controlled uncoupling of oxidative phosphorylation, a process regulated by a tissue-specific mitochondrial uncoupling protein,M _r 32–33,000. The immunological identification of uncoupling protein is now used as a biochemical criterion for distinguishing brown fat from white adipose tissue. The gene coding for uncoupling protein has been cloned in several species, and a number of factors regulating the expression of the gene, as well as the amount and activity of the protein itself, have been documented. In addition to its direct role in heat production, brown adipose tissue has some notable general metabolic properties, such as in the conversion of thyroxine to triiodothyronine. An overview of the biology of brown adipose tissue is presented in this article, with an emphasis on some recent developments.     

Uncoupling Protein 1 and Sarcolipin Are Required to Maintain Optimal Thermogenesis,and Loss of Both Systems Compromises Survival of Mice under Cold Stress

The importance of brown adipose tissue as a site of nonshivering thermogenesis has been well documented. Emerging studies suggest that skeletal muscle is also an important site of thermogenesis especially when brown adipose tissue function is lacking. We recently showed that sarcolipin (SLN), an uncoupler of the sarco(endo)plasmic reticulum Ca²⁺ ATPase (SERCA) pump, could contribute to heat production in skeletal muscle. In this study, we sought to understand how loss of UCP1 or SLN is compensated during cold exposure and whether they are both necessary for thermogenesis. Toward this goal, we generated a UCP1;SLN double knock-out (DKO) mouse model and challenged the single and DKO mice to acute and long-term cold exposures. Results from this study show that there is up-regulation of SLN expression in UCP1-KO mice, and loss of SLN is compensated by increased expression of UCP1 and browning of white adipose tissue. We found that the DKO mice were viable when reared at thermoneutrality. When challenged to acute cold, the DKO were extremely cold-sensitive and became hypothermic. Paradoxically, the DKO mice were able to survive gradual cold challenge, but these mice lost significant weight and depleted their fat stores, despite having higher caloric intake. These studies suggest that UCP1 and SLN are required to maintain optimal thermogenesis and that loss of both systems compromises survival of mice under cold stress.     

Adipose tissue development--impact of the early life environment

Increasing experimental and observational evidence in both animals and humans suggests that early life events are important in setting later fat mass. This includes both the number of adipocytes and the relative distribution of both brown and white adipose tissue. Brown adipose tissue is characterised as possessing a unique uncoupling protein (UCP)1 which enables the rapid generation of large amounts of heat and is most abundant in the newborn. In large mammals such as sheep and humans, brown fat that is located around the major internal organs, is largely lost during the postnatal period. However, it is retained in small and discrete areas into adulthood when it is sensitive to environmental cues such as changes in ambient temperature or day length. The extent to which brown adipose tissue is lost or replaced by white adipose tissue and/or undergoes a process of transdifferentiation remains controversial. Small amounts of UCP1 can also be present in skeletal muscle which now appears to share the same common precursor cell as brown adipose tissue. The functional consequences of UCP1 in muscle remain to be confirmed but it could contribute to dietary induced thermogenesis. Challenges in elucidating the primary mechanisms regulating adipose tissue development include changes in methylation status of key genes during development in different species, strains and adipose depots. A greater understanding of the mechanisms by which early life events regulate adipose tissue distribution in young offspring are likely to provide important insights for novel interventions that may prevent excess adiposity in later life.     

Uncoupling protein 1 expression and high-fat diets

Uncoupling protein 1 (Ucp1) is the key component of β-adrenergically controlled nonshivering thermogenesis in brown adipocytes. This process combusts stored and nutrient energy as heat. Cold exposure not only activates Ucp1-mediated thermogenesis to maintain normothermia but also results in adaptive thermogenesis, i.e., the recruitment of thermogenic capacity in brown adipose tissue. As a hallmark of adaptive thermogenesis, Ucp1 synthesis is increased proportionally to temperature and duration of exposure. Beyond this classical thermoregulatory function, it has been suggested that Ucp1-mediated thermogenesis can also be employed for metabolic thermogenesis to prevent the development of obesity. Accordingly, in times of excess caloric intake, one may expect a positive regulation of Ucp1. The general impression from an overview of the present literature is, indeed, an increased brown adipose tissue Ucp1 mRNA and protein content after feeding a high-fat diet (HFD) to mice and rats. The reported increases are very variable in magnitude, and the effect size seems to be independent of dietary fat content and duration of the feeding trial. In white adipose tissue depots Ucp1 mRNA is generally downregulated by HFD, indicating a decline in the number of interspersed brown adipocytes.     

Unexpected evidence for active brown adipose tissue in adult humans

The contention that brown adipose tissue is absent in adult man has meant that processes attributed to active brown adipose tissue in experimental animals (mainly rodents), i.e., classical nonshivering thermogenesis, adaptive adrenergic thermogenesis, diet-induced thermogenesis, and antiobesity, should be either absent or attributed to alternative (unknown) mechanisms in man. However, serendipidously, as a consequence of the use of fluorodeoxyglucose positron emission tomography (FDG PET) to trace tumor metastasis, observations that may change that notion have recently been made. These tomography scans have visualized symmetrical areas of increased tracer uptake in the upper parts of the human body; these areas of uptake correspond to brown adipose tissue. We examine here the published observations from a viewpoint of human physiology. The human depots are somewhat differently located from those in rodents, the main depots being found in the supraclavicular and the neck regions with some additional paravertebral, mediastinal, para-aortic, and suprarenal localizations (but no interscapular). Brown adipose tissue activity in man is acutely cold induced and is stimulated via the sympathetic nervous system. The prevalence of active brown adipose tissue in normal adult man can be only indirectly estimated, but it would seem that the prevalence of active brown adipose tissue in the population may be at least in the range of some tens of percent. We conclude that a substantial fraction of adult humans possess active brown adipose tissue that thus has the potential to be of metabolic significance for normal human physiology as well as to become pharmaceutically activated in efforts to combat obesity.     

Brown adipose tissue function in short-chain acyl-CoA dehydrogenase deficient mice

Brown adipose tissue is a highly specialized organ that uses mitochondrial fatty acid oxidation to fuel non-shivering thermogenesis. In mice, mutations in the acyl-CoA dehydrogenase family of fatty acid oxidation genes are associated with sensitivity to cold. Brown adipose tissue function has not previously been characterized in these knockout strains. Short-chain acyl-CoA dehydrogenase (SCAD) deficient mice were found to have increased brown adipose tissue mass as well as modest cardiac hypertrophy. Uncoupling protein-1 was reduced by 70% in brown adipose tissue and this was not due to a change in mitochondrial number, nor was it due to decreased signal transduction through protein kinase A which is known to be a major regulator of uncoupling protein-1 expression. PKA activity and in vitro lipolysis were normal in brown adipose tissue, although in white adipose tissue a modest increase in basal lipolysis was seen in SCAD−/− mice. Finally, an in vivo norepinephrine challenge of brown adipose tissue thermogenesis revealed normal heat production in SCAD−/− mice. These results suggest that reduced brown adipose tissue function is not the major factor causing cold sensitivity in acyl-CoA dehydrogenase knockout strains. We speculate that other mechanisms such as shivering capacity, cardiac function, and reduced hepatic glycogen stores are involved.     

Long photophase is not a sufficient stimulus to reduce thermogenic capacity in winter-acclimatized short-tailed field voles (Microtus agrestis) during long-term cold acclimation

The thermogenic capacity of brown adipose tissue in winter- and summer-acclimatized short-tailed field voles (Microtus agrestis) was investigated by examining changes in mass of brown adipose tissue, the ratio of white adipose tissue to brown adipose tissue, the concentration of the uncoupling protein (thermogenin) in whole depots (μg) and in mitochondrial mass (μg·mg^-1) and the activity of cytochrome c oxidase in the depots (mmol·min^-1). The concentration of thermogenin in winter-acclimatized voles (n=8), per brown adipose tissue depot and per mitochondrial mass, was significantly higher than in summer-acclimatized voles (n=6). There was no significant difference in the level of cytochrome c oxidase activity between these two groups. Four groups of winter-acclimatized voles (n=6 in each group) were exposed to 5°C for 10, 20, 50 and 100 days in a 14L:10D photoperiod. Body mass, brown adipose tissue mass, white adipose tissue mass and basal metabolic rate were significantly positively related to the length of time cold exposed up to 100 days. There was a significant inverse relationship between the ratio of white to brown adipose tissue mass and the duration of cold exposure. There was no significant relationship between thermogenin concentration, either per depot or in mitochondrial mass of brown adipose tissue, with the length of time cold exposed. The level of cytochrome c oxidase activity increased significantly from control levels to a maximum after 10 days in the cold but decreased from 10 days onwards. In winter-acclimatized M. agrestis, a 14L:10D photoperiod is not a sufficient stimulus to reduce thermogenic capacity during cold acclimation. Indeed, some changes in the indirect parameters reflecting thermogenesis, notably the increase in basal metabolic rate and the decrease in the ratio of white to brown adipose tissue mass, indicated that despite the long photophase the thermogenic capacity was slightly further enhanced during the cold acclimation.     

Blockade of the activin receptor IIb activates functional brown adipogenesis and thermogenesis by inducing mitochondrial oxidative metabolism

Brown adipose tissue (BAT) is a key tissue for energy expenditure via fat and glucose oxidation for thermogenesis. In this study, we demonstrate that the myostatin/activin receptor IIB (ActRIIB) pathway, which serves as an important negative regulator of muscle growth, is also a negative regulator of brown adipocyte differentiation. In parallel to the anticipated hypertrophy of skeletal muscle, the pharmacological inhibition of ActRIIB in mice, using a neutralizing antibody, increases the amount of BAT without directly affecting white adipose tissue. Mechanistically, inhibition of ActRIIB inhibits Smad3 signaling and activates the expression of myoglobin and PGC-1 coregulators in brown adipocytes. Consequently, ActRIIB blockade in brown adipose tissue enhances mitochondrial function and uncoupled respiration, translating into beneficial functional consequences, including enhanced cold tolerance and increased energy expenditure. Importantly, ActRIIB inhibition enhanced energy expenditure only at ambient temperature or in the cold and not at thermoneutrality, where nonshivering thermogenesis is minimal, strongly suggesting that brown fat activation plays a prominent role in the metabolic actions of ActRIIB inhibition.     

Activities of enzymes of amino acid metabolism in rat brown adipose tissue

There was a nil arginase and serine dehydratase activities in interscapular brown adipose tissue, but the activity of adenylate deaminase, glutamine synthetase, glutamate dehydrogenase and the aspartate, alanine and branched chain amino acid transaminases was higher than those of white adipose tissue; the differences were diminished when expressed per unit of protein weight. Brown adipose tissue enzyme activities were in a range between those of liver and muscle. The high amino acid handling capabilities, together with its physiological role, suggest that brown adipose tissue can metabolize significant amounts of amino acids, its enzyme pattern being different both from white adipose tissue, as well as of liver and muscle.     

The thermogenic activity of rat brown adipose tissue and rabbit white muscle Ca2+-ATPase

The Ca2+-ATPase (SERCA) found in vesicles derived from the sarco/endoplasmic reticulum vesicles of rats brown adipose tissue and rabbit white muscle were identified by gel electrophoresis, Western blot, electron microscopy and immunolabeling with gold particles. In both tissues, the isoform found was SERCA 1. The Ca2+ affinity of the fat SERCA 1 was different from the muscle isoform. The degree of uncoupling is estimated measuring the ratio between Ca2+ transport and ATP cleaved. In brown fat vesicles the degree of uncoupling varied depending on the Ca2+ concentration of the medium. This was not observed in vesicles derived from muscle. At all Ca2+ concentrations tested, the uncoupling was not related to Ca2+ leakage from the membrane and was far more pronounced in fat than in muscle vesicle. When a Ca2+ gradient was formed across the vesicles membrane the heat released during ATP hydrolysis varied between 22 and 26 Kcal/mol in both fat and muscle vesicles but in the absence of a gradient the heat released was 17 Kcal/mol in fat and 12 Kcal/mol in muscle. The data reported indicate that the SERCA 1 of brown adipocytes is far more thermogenic than the white muscle SERCA 1, and suggest that, in addition to storing Ca2+ inside the endoplasmic reticulum, the SERCA 1 may represent a source of heat production contributing to the thermogenic function of brown adipose tissue.     

An ancient look at UCP1

Brown adipose tissue serves as a thermogenic organ in placental mammals to defend body temperature in the cold by nonshivering thermogenesis. The thermogenic function of brown adipose tissue is enabled by several specialised features on the organ as well as on the cellular level, including dense sympathetic innervation and vascularisation, high lipolytic capacity and mitochondrial density and the unique expression of uncoupling protein 1 (UCP1). This mitochondrial carrier protein is inserted into the inner mitochondrial membrane and stimulates maximum mitochondrial respiration by dissipating proton-motive force as heat. Studies in knockout mice have clearly demonstrated that UCP1 is essential for nonshivering thermogenesis in brown adipose tissue. For a long time it had been presumed that brown adipose tissue and UCP1 emerged in placental mammals providing them with a unique advantage to survive in the cold. Our subsequent discoveries of UCP1 orthologues in ectotherm vertebrates and marsupials clearly refute this presumption. We can now initiate comparative studies on the structure-function relationships in UCP1 orthologues from different vertebrates to elucidate when during vertebrate evolution UCP1 gained the biochemical properties required for nonshivering thermogenesis.     

An ancient look at UCP1

Brown adipose tissue serves as a thermogenic organ in placental mammals to defend body temperature in the cold by nonshivering thermogenesis. The thermogenic function of brown adipose tissue is enabled by several specialised features on the organ as well as on the cellular level, including dense sympathetic innervation and vascularisation, high lipolytic capacity and mitochondrial density and the unique expression of uncoupling protein 1 (UCP1). This mitochondrial carrier protein is inserted into the inner mitochondrial membrane and stimulates maximum mitochondrial respiration by dissipating proton-motive force as heat. Studies in knockout mice have clearly demonstrated that UCP1 is essential for nonshivering thermogenesis in brown adipose tissue. For a long time it had been presumed that brown adipose tissue and UCP1 emerged in placental mammals providing them with a unique advantage to survive in the cold. Our subsequent discoveries of UCP1 orthologues in ectotherm vertebrates and marsupials clearly refute this presumption. We can now initiate comparative studies on the structure–function relationships in UCP1 orthologues from different vertebrates to elucidate when during vertebrate evolution UCP1 gained the biochemical properties required for nonshivering thermogenesis.     

Adipocyte lineages: Tracing back the origins of fat

The obesity epidemic has intensified efforts to understand the mechanisms controlling adipose tissue development. Adipose tissue is generally classified as white adipose tissue (WAT), the major energy storing tissue, or brown adipose tissue (BAT), which mediates non-shivering thermogenesis. It is hypothesized that brite adipocytes (brown in white) may represent a third adipocyte class. The recent realization that brown fat exist in adult humans suggests increasing brown fat energy expenditure could be a therapeutic strategy to combat obesity. To understand adipose tissue development, several groups are tracing the origins of mature adipocytes back to their adult precursor and embryonic ancestors. From these studies emerged a model that brown adipocytes originate from a precursor shared with skeletal muscle that expresses Myf5-Cre, while all white adipocytes originate from a Myf5-negative precursors. While this provided a rational explanation to why BAT is more metabolically favorable than WAT, recent work indicates the situation is more complex because subsets of white adipocytes also arise from Myf5-Cre expressing precursors. Lineage tracing studies further suggest that the vasculature may provide a niche supporting both brown and white adipocyte progenitors; however, the identity of the adipocyte progenitor cell is under debate. Differences in origin between adipocytes could explain metabolic heterogeneity between depots and/or influence body fat patterning particularly in lipodystrophy disorders. Here, we discuss recent insights into adipose tissue origins highlighting lineage-tracing studies in mice, how variations in metabolism or signaling between lineages could affect body fat distribution, and the questions that remain unresolved. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.     

Lack of TRPV2 impairs thermogenesis in mouse brown adipose tissue

Brown adipose tissue (BAT), a major site for mammalian non‐shivering thermogenesis, could be a target for prevention and treatment of human obesity. Transient receptor potential vanilloid 2 (TRPV2), a Ca²⁺‐permeable non‐selective cation channel, plays vital roles in the regulation of various cellular functions. Here, we show that TRPV2 is expressed in brown adipocytes and that mRNA levels of thermogenic genes are reduced in both cultured brown adipocytes and BAT from TRPV2 knockout (TRPV2KO) mice. The induction of thermogenic genes in response to β‐adrenergic receptor stimulation is also decreased in TRPV2KO brown adipocytes and suppressed by reduced intracellular Ca²⁺ concentrations in wild‐type brown adipocytes. In addition, TRPV2KO mice have more white adipose tissue and larger brown adipocytes and show cold intolerance, and lower BAT temperature increases in response to β‐adrenergic receptor stimulation. Furthermore, TRPV2KO mice have increased body weight and fat upon high‐fat‐diet treatment. Based on these findings, we conclude that TRPV2 has a role in BAT thermogenesis and could be a target for human obesity therapy.     

Hormone-sensitive lipase deficiency in mice causes diglyceride accumulation in adipose tissue, muscle, and testis.

Hormone-sensitive lipase (HSL) is expressed predominantly in white and brown adipose tissue where it is believed to play a crucial role in the lipolysis of stored triglycerides (TG), thereby providing the body with energy substrate in the form of free fatty acids (FFA). From in vitro assays, HSL is known to hydrolyze TG, diglycerides (DG), cholesteryl esters, and retinyl esters. In the current study we have generated HSL knock-out mice and demonstrate three lines of evidence that HSL is instrumental in the catabolism of DG in vivo. First, HSL deficiency in mice causes the accumulation of DG in white adipose tissue, brown adipose tissue, skeletal muscle, cardiac muscle, and testis. Second, when tissue extracts were used in an in vitro lipase assay, a reduced FFA release and the accumulation of DG was observed in HSL knock-out mice which did not occur when tissue extracts from control mice were used. Third, in vitro lipolysis experiments with HSL-deficient fat pads demonstrated that the isoproterenol-stimulated release of FFA was decreased and DG accumulated intracellularly resulting in the essential absence of the isoproterenol-stimulated glycerol formation typically observed in control fat pads. Additionally, the absence of HSL in white adipose tissue caused a shift of the fatty acid composition of the TG moiety toward increased long chain fatty acids implying a substrate specificity of the enzyme in vivo. From these in vivo results we conclude that HSL is the rate-limiting enzyme for the cellular catabolism of DG in adipose tissue and muscle.     

Distinction of white,beige and brown adipocytes derived from mesenchymal stem cells简

Adipose tissue is a major metabolic organ, and it has been traditionally classified as either white adipose tissue (WAT) or brown adipose tissue (BAT). WAT and BAT are characterized by different anatomical locations, morphological structures, functions, and regulations. WAT and BAT are both involved in energy balance. WAT is mainly involved in the storage and mobilization of energy in the form of triglycerides, whereas BAT specializes in dissipating energy as heat during cold- or diet-induced thermogenesis. Recently, brown-like adipocytes were discovered in WAT. These brown-like adipocytes that appear in WAT are called beige or brite adipocytes. Interestingly, these beige/brite cells resemble white fat cells in the basal state, but they respond to thermogenic stimuli with increased levels of thermogenic genes and increased respiration rates. In addition, beige/brite cells have a gene expression pattern distinct from that of either white or brown fat cells. The current epidemic of obesity has increased the interest in studying adipocyte formation (adipogenesis), especially in beige/brite cells. This review summarizes the developmental process of adipose tissues that originate from the mesenchymal stem cells and the features of these three different types of adipocytes.     

The Effects of Calcium Channel Blocker Benidipine and Calmodulin Antagonist W7 on GDP-Binding Capacity of Brown Adipose Tissue in Mice

It has been suggested that increased dietary calcium intake can attenuate obesity. Calcium antagonists, such as benidipine, also have been shown to have an anti-obesity effect. However, the mechanism for calcium-related anti-obesity effect has not yet been established. A defective brown adipose tissue thermogenesis has been shown in obese rodents. This study was designed to examine the direct effects of calcium channel blocker benidipine and calmodulin antagonist W7 administration on the adaptive thermogenesis in brown adipose tissue taken from the genetically obese mice and their lean controls. The GDP binding to brown-fat cell mitochondria was used as a brown adipose tissue thermogenic index. The results show that benidipine treatment had no marked effect on brown-fat cell GDP-binding capacities in both obese and lean mice. However, GDP-binding capacities were significantly reduced in both obese and lean mice after the W7 administration. The results of this study support the previous finding that benidipine did not have direct thermogenic effect on brown adipose tissue and suggest that the change in intracellular calmodulin availability might contribute to the adaptive thermogenesis in brown adipose tissue.