Brown adipose tissue: Updates in cellular and molecular biology

Brown adipose tissue (BAT) is mainly composed of adipocytes, it is highly vascularized and innervated, and can be activated in adult humans. Brown adipocytes are responsible for performing non-shivering thermogenesis, which is exclusively mediated by uncoupling protein (UCP) -1 (a protein found in the inner mitochondrial membrane), the hallmark of BAT, responsible for the uncoupling of the proton leakage from the ATP production, therefore, generating heat (i.e. thermogenesis). Besides UCP1, other compounds are essential not only to thermogenesis, but also to the proliferation and differentiation of BAT, including peroxisome proliferator-activated receptor (PPAR) family, PPARgamma coactivator 1 (PGC1)-alpha, and PRD1-BF-1-RIZ1 homologous domain protein containing protein (PRDM) -16. The sympathetic nervous system centrally regulates thermogenesis through norepinephrine, which acts on the adrenergic receptors of BAT. This bound leads to the initialization of the many pathways that may activate thermogenesis in acute and/or chronic ways. In summary, this mini-review aims to demonstrate the latest advances in the knowledge of BAT.     

UCP1 mRNA does not produce heat

Because of the possible role of brown adipose tissue and UCP1 in metabolic regulation, even in adult humans, there is presently considerable interest in quantifying, from in-vitro data, the thermogenic capacities of brown and brite/beige adipose tissues. An important issue is therefore to establish which parameters are the most adequate for this. A particularly important issue is the relevance of UCP1 mRNA levels as estimates of the degree of recruitment and of the thermogenic capacity resulting from differences in physiological conditions and from experimental manipulations. By solely following UCP1 mRNA levels in brown adipose tissue, the conclusion would be made that the tissue's highest activation occurs after only 6 h in the cold and then successively decreases to being only some 50% elevated after 1 month in the cold. However, measurement of total UCP1 protein levels per depot ("mouse") reveals that the maximal thermogenic capacity estimated in this way is reached first after 1 month but represents an approx. 10-fold increase in thermogenic capacity. Since this in-vitro measure correlates quantitatively and temporally with the acquisition of nonshivering thermogenesis, this must be considered the most physiologically relevant parameter. Similarly, observations that cold acclimation barely increases UCP1 mRNA levels in classical brown adipose tissue but leads to a 200-fold increase in UCP1 mRNA levels in brite/beige adipose tissue depots may overemphasise the physiological significance of these depots, as the high fold-increases are due to very low initial levels, and the UCP1 mRNA levels reached are at least an order of magnitude lower than in brown adipose tissue; furthermore, based on total UCP1 protein amounts, the brite/beige depots attain only about 10% of the thermogenic capacity of the classical brown adipose tissue depots. Consequently, inadequate conclusions may be reached if UCP1 mRNA levels are used as a proxy for the metabolic significance of recruited versus non-recruited brown adipose tissue and for estimating the metabolic significance of brown versus brite/beige adipose tissues. This article is part of a Special Issue entitled Brown and White Fat: From Signaling to Disease.     

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.     

Effect of unilateral surgical denervation of brown adipose tissue on uncoupling protein mRNA level and cytochrom-c-oxidase activity in the Djungarian hamster

The bilateral lobe of interscapular brown adipose tissue of the Djungarian hamster was unilaterally denervated in order to study the role of the sympathetic innervation for maintenance and cold-induced increase of non-shivering thermogenesis. Denervation decreased the noradrenaline content of brown adipose tissue to less than 9% of the intact contralateral pad. This low noradrenaline level was maintained for 1–14 days after denervation. First, to study the role of the sympathetic innervation of brown adipose tissue in the maintenance of the high thermogenic capacity characteristic of the cold acclimated state, brown adipose tissue was denervated in hamsters either kept at thermoneutrality or acclimated to 5°C ambient temperature for 4 weeks. Cold-acclimated hamsters had elevated levels of uncoupling protein messenger ribonucleic acid (8.1-fold) and cytochrom-c oxidase-activity (3-fold). Denervation of brown adipose tissue decreased uncoupling protein-messenger ribonucleic acid level and cytochrom-c-oxidase-activity as compared to the intact pad in thermoneutral and in cold-acclimated hamsters. However, in cold-acclimated hamsters uncoupling protein-messenger ribonucleic acid level and cytochrom-c-oxidase-activity in denervated brown adipose tissue both were maintained on an elevated 6-fold higher levels as compared to thermoneutral controls. Second, to study the role of the sympathetic innervation of brown adipose tissue in the cold-induced increase in thermogenic capacity, hamsters were denervated prior to cold acclimation and responses were measured after 3 and 14 days of cold exposure. Uncoupling protein-messenger ribonucleic acid level and cytochrom-c-oxidase-activity of intact brown adipose tissue increased after 14 days cold acclimation. Denervation did not completely prevent a cold-induced 1.5-fold increase of cytochrom-c-oxidase-activity and a 3.2-fold increase of the uncoupling protein-messenger ribonucleic acid level in denervated brown adipose tissue after 14 days of cold acclimation. In conclusion, high levels of uncoupling protein-messenger ribonucleic acid and cytochrom-c-oxidase activity in brown adipose tissue of cold-acclimated hamsters can partially be maintained without intact sympathetic innervation, suggesting a considerable contribution of trophic factors not requiring sympathetic innervation for maintenance. The cold-induced increase of cytochrom-c-oxidase activity and expression of uncoupling protein-messenger ribonucleic acid largely depends upon sympathetic innervation of brown adipose tissue.Abbreviations ANOVA analysis of variance - BAT brown adipose tissue - COX cytochrom-c-oxidase - HPLC high performance liquid chromatography - mRNA messenger ribonucleie acid - NA noradrenaline - T _a ambient temperature - UCP uncoupling protein     

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.     

青藏高原小型哺乳动物的生理生态学研究：从个体到生态系统

本文回顾了青藏高原小型哺乳动物生理生态学研究取得的主要成就,包括能量代谢特征与环境适应性、适应性产热与体温调节、能量平衡与体重调节、生理极限值和种群能流估算等,总结了近年一些新兴领域的最新进展,包括双标记水方法测定能量消耗、肠道菌群的功能、地理生理学、种群生理学、植物次生代谢产物及其生理功能等。有些工作引领了中国动物生理生态学的发展,如生态能量学、适应性产热和生理适应等。本文对未来需要发展的领域和深入方面提出了建议,希望能建立青藏高原小型哺乳动物生理生态学学科体系。     

刺猬冬眠过程中褐色脂肪和非颤抖性产热研究

本文对刺猬在冬眠过程中褐色脂肪（ＢＡＴ）的相对重量、超微结构和非颤抖性产热（ＮＳＴ）的变化进行了测定,探讨了ＢＡＴ产热在热在冬眠刺猬醒眠过程中的作用。结果表明,夏季（７月份）刺猬的ＢＡＴ相对重量最低,为０．２３％。秋季迅速增加,至入眠前达到最高,为０、４６％。冬眠期中缓慢减少,冬眠后显著下降（０．３２％）,这与醒眠时需要快速产热密切相关。冬眠前和冬眠中刺猬的ＢＴＡ脂滴和线粒体嵴数量明显高于冬眠后,     

Intrinsic differences in BRITE adipogenesis of primary adipocytes from two different mouse strains

BRITE (brown-in-white) cells are brown adipocyte-like cells found in white adipose tissue (WAT) of rodents and/or humans. The recruitment of BRITE adipocytes, referred to as the browning of WAT, is hallmarked by the expression of UCP1 and exerts beneficial metabolic effects. Here we address whether beyond systemic cues depot- and strain-specific variation in BRITE recruitment is determined by a cellular program intrinsic to progenitors. Therefore we compared the browning capacity of serum and investigated brown and BRITE adipogenesis in primary cultures of stromal-vascular cells isolated from interscapular brown adipose tissue (iBAT), inguinal white adipose tissue (iWAT) and epididymal white adipose tissue (eWAT) in two inbred mouse strains C57BL/6J (B6, a strain with low browning propensity) and 129/S6SvEv (129, a strain with high browning propensity). Paradoxically, serum collected from B6 mice was more potent in the promotion of browning than serum collected from 129 mice. Nevertheless, we demonstrate that depot- and strain-specific differences observed in vivo are pheno-copied in primary cultures in vitro, as judged by UCP1 expression and by functional analysis. Notably, primary adipocytes from 129 mice had a higher capacity for isoproterenol-induced uncoupled respiration than B6. We conclude that cues intrinsic to the progenitor cells contribute to differential BRITE adipogenesis. Further analyses demonstrate that these cues are independent of autocrine/paracrine mechanisms, BRITE progenitor abundance and genetic variation in the gene regulatory region of Ucp1 but rather depend on trans-acting factors. These results provide new insights on the molecular basis of strain and depot-specific differences in BRITE adipogenesis.     

Cold adaptive thermogenesis in small mammals from different geographical zones of China

The mechanisms of thermogenesis and thermoregulation were studied in the tree shrew (Tupaia belangeri) and greater vole (Eothenomys miletus) of the subtropical region, and Brandt's vole (Microtus brandti), Mongolian gerbil (Meriones unguiculatus), Daurian ground squirrel (Spermophilus dauricus) and plateau pika (Ochotona curzoniae) of the northern temperate zone. Resting metabolic rate (RMR) and non-shivering thermogenesis (NST) increased significantly in T. belangeri, E. miletus, M. brandti and M. unguiculatus after cold acclimation (4 degrees C) for 4 weeks. In T. belangeri, the increase in RMR and thermogenesis at liver cellular level were responsible for enhancing the capacity of enduring cold stress, and homeothermia was simultaneously extended. Stable body temperature in M. brandti, E. miletus, M. unguiculatus and O. curzoniae was maintained mainly through increase in NST, brown adipose tissue (BAT) mass and its mitochondrial protein content, and the upregulation of uncoupling protein (UCP1) mRNA, as well as enhancement of the activity of cytochrome C oxidase, alpha-glycerophosphate oxidase and T(4) 5'-deiodinase in BAT mitochondria. The RMR in O. curzoniae and euthermic S. dauricus was not changed, while NST significantly increased during cold exposure; the former maintained their stable body temperature and mass, while body temperature in the latter declined by 4.8 degrees C. The serum T(3) concentration or ratio of T(3)/T(4) in all the species was enhanced after cold acclimation. Results indicated that: (1) the adaptive mechanisms of T. belangeri residing in the subtropical region to cold are primarily by increasing RMR and secondly by increasing NST, and the mechanisms of thermogenesis are similar to those in tropical mammals; (2) in small mammals residing in northern regions, the adaptation to cold is chiefly to increase NST; (3) the mechanism of cold-induced thermogenesis in E. miletus residing in subtropical and high mountain regions is similar to that in the north; (4) a low RMR in warm environments and peak RMR and NST in cold environments enabled M. unguiculatus to tolerate a semi-desert climate; (5) O. curzoniae has unusually high RMR and high NST, acting mainly via increasing NST to adapt to extreme cold of the Qinghai-Tibet Plateau; (6) the adaptation of euthermic S. dauricus to cold is due to an increase in NST and a relaxed homeothermia; and lastly (7) the thyroid hormone is involved in the regulation of cold adaptive thermogenesis in all the species studied.     

中缅树鼩的非颤抖性产热及细胞呼吸特征

中缅树鼩(Tupaia belangeri chinensis)是东南亚树鼩中分布最北的一个种。在热中性区内的非颤抖性产热(nonshivering thermogenesis NST)分别为2.57±0.21(冬)和2.21±0.12(夏)mlO_2/(g·h);分别为体重预期值的75.9％和61.2％,两者不仅冬季显著高于夏季,而且亦高于典型的热带种类,但低于温带类群。褐色脂肪组织(brown adipose tissue,BAT)的重量冬季为0.622±0.015 S,夏季0.532±0.80 g,冬季也显著高于夏季;同时,BAT总蛋白含量、线粒体蛋白含量以及细胞α-磷酸甘油氧化酶和细胞色素C氧化酶活性,冬季也显著高于夏季,但增加的比例较温带种类低;而肝脏细胞的上述指标及线粒体状态Ⅲ、状态Ⅳ呼吸等,冬夏两季均无显著差异。因此,中缅树鼩的NST和细胞产热能力介于热带与温带类群之间,显示出向温带类型过渡的趋势。     

长爪沙鼠褐色脂肪组织的适应性产热

低温（５±２℃）环境暴露１天、１周、４周长爪沙鼠的静止代谢率和非颤抖性产热、褐色脂肪组织蛋白质含量、线粒体细胞色素－Ｃ氧化酶和α－磷酸甘油氧化酶活力、钠钾泵活力以及环腺苷酸含量、血清Ｔ３含量等项产热指标逐渐提高;而低温驯化８周动物各项指标基本维持在４周水平上,表明长爪沙鼠在冷适应过程中产热能力逐渐提高,而后维持一稳定水平,其中褐色脂肪组织起重要调节作用。     

Brown adipose tissue specific lack of uncoupling protein 3 is associated with impaired cold tolerance and reduced transcript levels of metabolic genes

Uncoupling protein 3 (Ucp3) is located within the mitochondrial inner membrane of brown adipose tissue and skeletal muscle. It is thought to be implicated in lipid metabolism and defense against reactive oxygen species. We previously reported on a mutation in our breeding colony of Djungarian hamsters (Phodopus sungorus) that leads to brown adipose tissue specific lack of Ucp3 expression. In this study we compared wildtype with mutant hamsters on a broad genetic background. Hamsters lacking Ucp3 in brown adipose tissue displayed a reduced cold tolerance due to impaired nonshivering thermogenesis. This phenotype is associated with a global decrease in expression of metabolic genes but not of uncoupling protein 1. These data implicate that Ucp3 is necessary to sustain high metabolic rates in brown adipose tissue.     

The effect of pinealectomy on non-shivering thermogenesis and hibernation of the wyoming ground squirrel, Spermophilus elegans

1. 1.|Pineal gland removal (PX) performed on Wyoming ground squirrels (Spermophilus elegans) during the summer significantly lowered their non-shivering thermogenic (NST) response and impaired their ability to hibernate the subsequent winter.

2. 2.|This impairment was not observed in previous studies on squirrels PX'd later in the season. A period of responsiveness for involvement of the pineal gland in NST and hibernation is, therefore, presented.

3. 3.|Environmental signals translated by the pineal gland which influence the NST response do not appear to be thermal cues but are more than likely photic information.

4. 4.|Reduced impairment of the NST response was observed in squirrels PX'd for 3 yr compared to the 1st PX'd group. However, this may simply be a result of the 3-yr PX'd group being tested later in the hibernation cycle.

达乌尔黄鼠产热的季节性变化

达乌尔黄鼠（Ｃｉｔｅｌｌｕｓｄａｕｒｉｃｕｓ）的产热表现出明显的季节性变化。在非冬眠期,静止代谢率（ＲＭＲ）和非颤抖性产热（ＮＳＴ）于春季最高,秋季次之,夏季最低。冬眠期,ＲＭＲ降到极低水平,只为春季的３．０％。肝脏的线粒体蛋白含量、线粒体呼吸和细胞色素Ｃ氧化酶活力在秋季显著高于其它各季。褐色脂肪组织（ＢＡＴ）的重量、线粒体蛋白含量、细胞色素Ｃ氧化酶活力和α－磷酸甘油氧化酶活力,在夏季处于一年中的最低水平,到了冬季这些指标达到一年中的最高水平。在非冬眠季节ＢＡＴ产热能力升高时,ＮＳＴ能力也相应升高,这表明ＢＡＴ产热能力的增强是ＮＳＴ能力提高的部分机制。达乌尔黄鼠血清Ｔ＿４含量在年周期中没有明显改变,冬眠时血清Ｔ＿３含量显著高于其它各季。     

Specific binding of chartreusin, an antitumor antibiotic, to DNA

Chartreusin, an antitumor and antibacterial antibiotic, was found to inhibit negatively superhelical DNA-relaxation catalyzed by prokaryotic topoisomerase I and conversion of the superhelical DNA into unit length linear form catalyzed by single-strand-specific S1 nuclease. The inhibitory effect of the agent was due to the binding to DNA causing the alteration of tertiary structure. To characterize the binding specificity, we investigated the protection of DNA against cleavages by various restriction endonucleases. It was evidenced that the binding of the agent is not at random and correlates to the sequence 5'CGC 3' 3'GCG 5' on DNA stretch.     

Modulation of adipose tissue thermogenesis as a method for increasing energy expenditure

There is a renewed interest in the role of adipose tissue in energy utilization and thermogenesis and its potential application in the treatment of metabolic disorders such as obesity and diabetes. The last few years have seen the identification of brown adipose tissue capable of metabolic activation in adult humans, the possibility of recruiting ‘beige’ adipocytes to increase energy expenditure, and the implication of molecules such as FGF21 and irisin in inducing increases in energy expenditure in adipose tissue. The translation of these findings into human trials to deliver safe, efficacious medicines remains a challenge.     

Decreased brown adipose tissue thermogenic activity following a reduction in brain serotonin by intraventricular p-chlorophenylalanine

The effects of reducing brain serotonin (5-HT) levels by means of intracerebral-ventricular injections of the tryptophan antagonist p-chlorophenylalanine (PCPA) were investigated in male rats. Six days after the operation, PCPA-treated rats, either fedad libitum or pair-fed to the food intake of control rats, showed decreased thermogenic activity and capacity in their interscapular brown adipose tissue (BAT) and also increased fat storage in their white adipose tissue (WAT). These results indicate that serotonergic synapses might play a regulatory role in the sympathetic control of BAT thermogenesis and in the rate of WAT deposition (by an as yet unidentified mechanism), in addition to their well established role in controlling food intake.     

Interaction between heat acclimation and exogenous insulin in brown adipose tissue of rats

Seventy-one male Wistar strain rats (7 weeks old) were kept at 5, 25, or 34° C, respectively, for 2 weeks with or without insulin administration. Insulin (Novo Lente MC) was given subcutaneously in a dose of 3.62 nmol/125 µl saline per 100 g body weight. An apparent effect of insulin treatment was noted only in heat-exposed rats, resulting in a remarkable gain in inter-scapular brown adipose tissue (BAT) mass of heat-acclimated, insulin-treated rats in terms of weight or weight per unit body weight. The BAT from heat-acclimated, insulin-treated rats had significantly higher levels of protein, DNA, RNA, and triglyceride than BAT from heat-acclimated, saline-treated rats. Therefore, it seems likely that the growth of BAT in heat-acclimated, insulin-treated rats was mostly due to the anabolic effects of insulin. The uncoupling protein mRNA was, however, present in BAT of heat-acclimated, insulin-treated rats at rather a depressed level, explaining a corresponding decrease in cold tolerance. On the other hand, the expression of insulin receptor mRNA was attenuated in BAT of rats from all the insulin-treated groups, possibly due to the down-regulation of insulin. Thus, there appeared to be some linkage among BAT, heat acclimation, and insulin.