Nonshivering thermogenesis in adult and aged C57BL/6J mice housed at 22°C and at 29°C

Twelve- and 28-month-old C57BL/6J male mice were housed either at room temperature of 22°C or at thermoneutrality (29°C) during the two months prior to experiments. Acute experiments were conducted under anesthesia, myorelaxation, and artificial ventilation. We recorded efferent electrical impulse activity in one of the sympathetic nerves innervating the interscapular brown adipose tissue in response to acute cold stimulation, when body temperature was lowered 7.5°C below control level. In separate experiments we measured O₂ consumption and CO₂ production and calculated the nonshivering thermogenesis. We also measured the concentration of uncoupling protein in interscapular brown adipose tissue before and after three-hour cold stress. In aged mice, both sympathetic nervous activity and nonshivering thermogenesis were lower in animals housed at thermoneutrality (29°C) than in mice housed at 22°. Among mice maintained at 22°C, but not at thermoneutrality, aged animals had greater nonshivering thermogenesis and greater cold induced concentration of uncoupling protein in the brown adipose tissue than adults. Sympathetic nervous outflow to brown adipose tissue was always greater in aged mice, regardless of the temperature of acclimation. We concluded that aged mice, housed at 22°C, showed the changes in nonshivering thermogenesis associated with cold acclimation. However, an increased sympathetic outflow to brown adipose tissue in aged animals reflects an age-related elevation of the tone and responsiveness of the sympathetic nervous system.     

The effect of exercise training in cold on shivering and nonshivering thermogenesis in adult and aged C57BL/6J mice

To understand the mechanisms of improvement of cold-induced heat production in aged mice following exercise training, the relative contributions of shivering and nonshivering thermogenesis to cold-induced metabolic responses were assessed in adult and aged C57BL/6J male mice, which inhabited sedentarily at room temperature, or were subjected either to a regimen of moderate intensity exercise training at 6°C, or to sedentary repeated exposures to the same temperature. The main findings were that (1) aged mice had greater cold-induced nonshivering thermogenesis, but lower shivering than adult mice; (2) exercise training in a cold environment enhanced cold-induced nonshivering thermogenesis in adult mice, but suppressed it in aged animals; (3) exercise training in a cold environment increased shivering thermogenesis in both age groups, but this increase was much greater in aged mice; (4) the increase of cold-induced shivering thermogenesis was mainly responsible for increased cold tolerance in aged mice after exercise training in a cold environment.     

Effects of a warm environment on energy balance in obese (ob/ob) mice

Low rates of thermoregulatory heat production associated with low metabolic activity of brown adipose tissue, the primary site of thermoregulatory heat production, contribute substantially to the high efficiency of energy retention in obese (ob/ob) mice housed at 20 degrees C to 28 degrees C. To eliminate the need for thermoregulatory heat production lean and ob/ob mice were housed at 34.5 degrees C. At this temperature ob/ob mice still retained energy with a greater efficiency than lean littermates. Next, we investigated the possibility that the high efficiency of energy retention in ob/ob mice housed at 34.5 degrees C was related to depressed dietary-induced thermogenesis associated with low metabolic activity of brown adipose tissue. The sympathetic nervous system is a primary regulator of brown adipose tissue metabolism. Therefore, rates of norepinephrine (NE) turnover in brown adipose tissue, as an indicator of sympathetic nervous system stimulation of the tissue, were measured. Lean and ob/ob mice housed at 34.5 degrees C had equally low rates of NE turnover in their brown adipose tissue. Thus, the high efficiency of energy retention in ob/ob mice maintained at 34.5 degrees C is caused by factors other than low sympathetic stimulation of brown adipose tissue.     

Metabolic consequences of the presence or absence of the thermogenic capacity of brown adipose tissue in mice (and probably in humans)

Only with the development of the uncoupling protein 1 (UCP1)-ablated mouse has it become possible to strictly delineate the physiological significance of the thermogenic capacity of brown adipose tissue. Considering the presence of active brown adipose tissue in adult humans, these insights may have direct human implications. In addition to classical nonshivering thermogenesis, all adaptive adrenergic thermogeneses, including diet-induced thermogenesis, is fully dependent on brown adipocyte activity. Any weight-reducing effect of β(3)-adrenergic agonists is fully dependent on UCP1 activity, as is any weight-reducing effect of leptin (in excess of its effect on reduction of food intake). Consequently, in the absence of the thermogenic activity of brown adipose tissue, obesity develops spontaneously. The ability of brown adipose tissue to contribute to glucose disposal is also mainly related to thermogenic activity. However, basal metabolic rate, cold-induced thermogenesis, acute cold tolerance, fevers, nonadaptive adrenergic thermogenesis and processes such as angiogenesis in brown adipose tissue itself are not dependent on UCP1 activity. Whereas it is likely that these conclusions are also qualitatively valid for adult humans, the quantitative significance of brown adipose tissue for human metabolism--and the metabolic consequences for a single individual possessing more or less brown adipose tissue--awaits clarification.     

Temperature-sensitive phenotype in mice lacking pituitary adenylate cyclase-activating polypeptide

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a highly conserved hormone. Targeted disruption of the PACAP gene has revealed a role for this peptide in lipid metabolism, carbohydrate metabolism, and the sympathetic response to insulin stress. We report here that PACAP null mice are temperature sensitive. When raised at 21 C, only 11% of the PACAP null mice survived past the first 2 wk after birth, but when raised at 24 C, most (76%) of the PACAP null mice survived. The question is the mechanism by which the absence of PACAP affects thermoregulation. Brown adipose tissue is the major site of adaptive thermogenesis in neonates and rodents. We show that PACAP null mice have brown adipocytes that differentiate normally and express two enzymes involved in thermogenesis, hormone-sensitive lipase and uncoupling protein 1. Likewise, levels of catecholamines in the adrenal medulla and plasma are normal in PACAP null mice raised at a lower temperature. In contrast, norepinephrine and its precursor dopamine extracted from brown adipose tissue are present at significantly lower levels in the PACAP null mice compared with controls. Also, PACAP null mice showed a greater loss of core body temperature compared with wild-type controls at 21 C. We conclude that under prolonged but mild cold stress, lack of PACAP results in inadequate heat production due to insufficient norepinephrine stimulation of brown adipose tissue.     

棕色脂肪组织与糖代谢的关系

研究证实成人体内存在有活性的棕色脂肪组织(BAT).BAT是非颤栗产热和饮食诱导产热的主要器官,其产热作用依赖线粒体内膜的解耦联蛋白1(UCP1).UCP1可使物质氧化与ATP生成解耦联(解耦联呼吸),减少ATP的生成,使能量以热量的形式释放,维持体温与能量的平衡.寒冷暴露、胰岛素、去甲肾上腺素、甲状腺激素等均可诱导UCP1表达使BAT活化,进而促进BAT摄取循环中的葡萄糖,加速循环中葡萄糖的清除.饮食因素以及可诱导BAT活化的因素均可影响BAT对葡萄糖的摄取.     

Fat cells directly sense temperature to activate thermogenesis

Classic brown fat and inducible beige fat both dissipate chemical energy in the form of heat through the actions of mitochondrial uncoupling protein 1. This nonshivering thermogenesis is crucial for mammals as a defense against cold and obesity/diabetes. Cold is known to act indirectly through the sympathetic nervous systems and β-adrenergic signaling, but here we report that cool temperature (27–33 °C) can directly activate a thermogenic gene program in adipocytes in a cell-autonomous manner. White and beige fat cells respond to cool temperatures, but classic brown fat cells do not. Importantly, this activation in isolated cells is independent of the canonical cAMP/Protein Kinase A/cAMP response element-binding protein pathway downstream of the β-adrenergic receptors. These findings provide an unusual insight into the role of adipose tissues in thermoregulation, as well as an alternative way to target nonshivering thermogenesis for treatment of obesity and metabolic diseases.     

The effect of cold acclimation and exercise training on cold tolerance in aged C57BL/6J mice

We investigated whether intermittent cold acclimation and low intensity exercise training, two interventions known to improve thermoregulatory function in young rodents, enhanced cold tolerance in aged C57BL/6J mice. Mice, aged 18 to 20 months, were randomly assigned to four treatment conditions: (a) intermittent cold acclimation (CA) (50 min per day, 5 times per week for 3 weeks at 5 degrees C), (b) submaximal treadmill exercise (EX) (15 m min-1 for 30 min, 5 times per week for 8 weeks, (c) sequential treatment of CA + EX, and (d) control group. Mice were exposed to a 3-hr cold stress test (15 degrees C) prior to and following treatment. CA aged mice maintained normothermia and demonstrated significant increases in oxygen consumption and brown adipose tissue protein concentration compared with controls; enhanced cold tolerance was probably due to increased utilization of nonshivering thermogenesis. EX aged mice had elevated O2 consumption and increased skeletal muscle enzyme activity compared with controls; however, cold tolerance was not enhanced compared with CA mice. The data suggest that intermittent CA effectively reduces hypothermia during cold challenge in aged mice.     

Decreased brown adipocyte recruitment and thermogenic capacity in mice with impaired peroxisome proliferator-activated receptor (P465L PPARgamma) function

Mice with a dominant-negative peroxisome proliferator-activated receptor gamma (PPARgamma) mutation (P465L) unexpectedly had normal amounts of adipose tissue. Here, we investigate the adipose tissue of the PPARgamma P465L mouse in detail. Microscopic analysis of interscapular adipose tissue of P465L PPARgamma mice revealed brown adipocytes with larger unilocular lipid droplets, indicative of reduced thermogenic capacity. Under conditions of cold exposure, the brown adipose tissue of the PPARgamma P465L mice was less active, a fact reflected in decreased uncoupling protein 1 levels. Analysis of the white adipocytes confirmed their normal cytoarchitecture and development, yet classical white adipose depots of the P465L PPARgamma mice had a striking reduction in brown adipocyte recruitment, a finding supported by reduced expression of UCP1 in the perigonadal adipose depot. Taken together, these data suggest that whole animal impairment of PPARgamma alters the cellular composition of the adipose organ to a more "white" adipose phenotype. Physiologically, this impairment in brown adipocyte recruitment is associated with decreased nonshivering thermogenic capacity after cold acclimation as revealed by norepinephrine responsiveness. Our results indicate that maintenance of oxidative brown-like adipose tissue is more dependent on PPARgamma function for development than white adipose tissue, an observation that may be relevant when considering PPARgamma-dependent strategies for the treatment of obesity.     

Brown adipose tissue thermogenesis in testosterone-treated rats.

The participation of sexual hormones in body weight regulation is partly accomplished by altering food intake. Nonetheless, female sexual hormones also alter brown adipose tissue thermogenesis in females. This study was aimed to find out if male hormones could alter brown adipose tissue thermogenesis in male rats. Testosterone was administered by means of Silastic capsules in adult male rats acclimated either at 28 degrees C (thermoneutrality) or at 6 degrees C (cold), treatment lasting 15 days. Food intake and body weight gain were reduced by hormonal treatment. However, brown adipose tissue mass, protein content, mitochondrial mass and GDP-binding were unchanged at both environmental temperatures. Accordingly, testosterone participation in body weight regulation is thought to be carried out without altering brown adipose tissue thermogenesis. A reduction in the weight of the sex accessory glands was also observed after cold acclimation.     

Hypothalamic melanin-concentrating hormone is induced by cold exposure and participates in the control of energy expenditure in rats

Short-term cold exposure of homeothermic animals leads to higher thermogenesis and food consumption accompanied by weight loss. An analysis of cDNA-macroarray was employed to identify candidate mRNA species that encode proteins involved in thermogenic adaptation to cold. A cDNA-macroarray analysis, confirmed by RT-PCR, immunoblot, and RIA, revealed that the hypothalamic expression of melanin-concentrating hormone (MCH) is enhanced by exposure of rats to cold environment. The blockade of hypothalamic MCH expression by antisense MCH oligonucleotide in cold-exposed rats promoted no changes in feeding behavior and body temperature. However, MCH blockade led to a significant drop in body weight, which was accompanied by decreased liver glycogen, increased relative body fat, increased absolute and relative interscapular brown adipose tissue mass, increased uncoupling protein 1 expression in brown adipose tissue, and increased consumption of lean body mass. Thus, increased hypothalamic MCH expression in rats exposed to cold may participate in the process that allows for efficient use of energy for heat production during thermogenic adaptation to cold.     

Defective thermoregulatory thermogenesis in monosodium glutamate-induced obesity in mice

We have studied thermoregulatory thermogenesis in mice rendered obese by neonatal administration of monosodium glutamate (MSG) and in saline treated controls. At 12 weeks of age MSG-treated mice maintained on a chow diet and housed at 24 degrees C, exhibited hypertrophy of brown adipose tissue (BAT) compared to controls (65% increase in wet weight and lipid content, no difference in DNA content). Acute cold exposure (4 degrees C for two hours) resulted in a significantly greater fall in core temperature in MSG-treated than control mice. After cold exposure to 4 degrees C for six hours, control animals mobilized BAT lipid whereas MSG-treated animals did not. Both groups showed comparable increments in oxygen consumption in response to exogenous norepinephrine. The above changes were qualitatively the same for both male and female animals. The following conclusions were reached: (1) MSG-treated mice have defective cold induced thermogenesis, indirect evidence suggests this results from impaired activation of thermogenic mechanisms in BAT; (2) the defect responsible for this lies extrinsic to BAT; and (3) the quantitative significance of defective thermoregulatory thermogenesis for the development of obesity in these mice is uncertain.     

Metabolic heat production and cold tolerance during cold stress of different intensity of adult and aged male C57BL/6J mice

Adult and aged male C57BL/6J mice were subjected to a 3-h cold stress test at either 24°C, 18°C, 12°C, or 6°C. Body mass was measured before the test, and colonic temperature, O₂ consumption, and CO₂ production were measured during the test. The slopes of colonic temperature over time of test and the mean metabolic heat production were calculated for each animal. While adult mice had a relatively small reduction in colonic temperature during the test at all four ambient temperatures, in the aged mice ambient temperatures resulted in steeper reductions of colonic temperature. In adult mice, an increase in metabolic heat production was proportional to ambient cold. The thermogenic response of aged mice at 24°C and at 18°C was similar to adult mice, suggesting that the ability of aged mice to respond to cold by increasing heat production does not diminish with age. However, in aged mice metabolic heat production at 12°C and 6°C was significantly below that of adult mice, which indicated a reduced capacity for thermogenesis.     

Lipoprotein lipase activity and cellularity in brown and white adipose tissue in Zucker obese rats

The genetically obese adult Zucker rat (fafa) exhibits reduced thermogenesis when stimulated by physiological agents (cold, catecholamines). Recent evidence suggests that this thermogenic defect may be important in the manifestation of the animal's obesity and that it reflects a reduced thermogenic contribution from brown adipose tissue, the major nonshivering thermogenic site in many mammals. The present study describes the effects of the obese genotype on brown (and white) adipocyte size, number, and lipid content and tissue lipoprotein lipase (LPL) activity. In the obese rats, brown fat depots were increased in mass. This increase could be accounted for by brown fat hypertrophy (due primarily to an increase in the amount of triglyceride present in each cell) rather than hyperplasia (there being no increase in the number of brown fat cells). In addition, unlike the situation in white fat, the brown fat from the obese rats did not exhibit higher LPL activity than did the brown fat from their lean littermates. This absence of an increased capacity for triglyceride uptake, coupled with the greater amount of triglyceride per brown adipocyte, is consistent with a reduction of triglyceride oxidation (and, thus, heat production) in the cells from the obese (v the lean) rats.     

Remodelling the inguinal adipose sensory system to switch on the furnace: Electroacupuncture stimulation induces brown adipose thermogenesis

Brown and white adipose tissue mediate thermogenesis through the thermogenetic centre of the brain, but safe methods for activating thermogensis and knowledge of the associated molecular mechanisms are lacking. We investigated body surface electroacupuncture stimulation (ES) at ST25 (targeted at the abdomen) induction of brown adipose thermogenesis and the neural mechanism of this process. Inguinal white adipose tissue (iWAT) and interscapular brown adipose tissue (iBAT) were collected and the thermogenic protein expression levels were measured to evaluate iBAT thermogenesis capacity. The thermogenic centre activating region and sympathetic outflow were evaluated based on neural electrical activity and c-fos expression levels. iWAT sensory axon plasticity was analysed with whole-mount adipose tissue imaging. ES activated the sympathetic nerves in iBAT and the c-fos-positive cells induced sympathetic outflow activation to the iBAT from the medial preoptic area (MPA), the dorsomedial hypothalamus (DM) and the raphe pallidus nucleus (RPA). iWAT denervation mice exhibited decreased c-fos-positive cells in the DM and RPA, and lower recombinant uncoupling orotein 1 peroxisome proliferator-activated receptor, β3-adrenergic receptor, and tyrosine hydroxylase expression. Remodelling the iWAT sensory axons recovered the signal from the MPA to the RPA and induced iBAT thermogenesis. The sympathetic denervation attenuated sensory nerve density. ES induced sympathetic outflow from the thermogenetic centres to iBAT, which mediated thermogenesis. iWAT sensory axon remodelling induced the MPA–DM–RPA–iBAT thermogenesis pathway.     

Endocrine regulation of uncoupling proteins and energy expenditure

Regulatory thermogenesis occurs upon exposure to the cold or during food intake. Among a variety of mechanisms leading to heat production, uncoupling of respiration in brown adipocyte mitochondria appears to be a major contributor to resistance to the cold in rodents. This uncoupling mechanism is due to the activity of uncoupling protein-1 (UCP-1), a specific carrier present in the inner membrane of mitochondria. The recent identification of UCP-2 and UCP-3, two homologues of the brown fat UCP, suggested that respiration uncoupling could contribute to thermogenesis in most tissues. Activity and expression of the three UCP's are stimulated by several neuromediators and hormones such as noradrenaline, tri-iodothyronine and leptin.     

Recent advances in adaptive thermogenesis: potential implications for the treatment of obesity

Large inter-individual differences in cold-induced (non-shivering) and diet-induced adaptive thermogenesis exist in animals and humans. These differences in energy expenditure can have a large impact on long-term energy balance and thus body weight (when other factors remain stable). Therefore, the level of adaptive thermogenesis might relate to the susceptibility to obesity; efforts to increase adaptive thermogenesis might be used to treat obesity. In small mammals, the main process involved is mitochondrial uncoupling in brown adipose tissue (BAT), which is regulated by the sympathetic nervous system. For a long time, it was assumed that mitochondrial uncoupling is not a major physiological contributor to adaptive thermogenesis in adult humans. However, several studies conducted in recent years suggest that mitochondrial uncoupling in BAT and skeletal muscle tissue in adult humans can be physiologically significant. Other mechanisms besides mitochondrial uncoupling that might be involved are futile calcium cycling, protein turnover and substrate cycling. In conjunction with recent advances on signal transduction studies, this knowledge makes manipulation of adaptive thermogenesis a more realistic option and thus a pharmacologically interesting target to treat obesity.     

Evidence for a compensated thermogenic defect in transgenic mice lacking the mitochondrial glycerol-3-phosphate dehydrogenase gene

A role for mitochondrial glycerol-3-phosphate dehydrogenase (mGPD) in thermogenesis was investigated in transgenic mice lacking the mGPD gene (mGPD-/-). Reared and studied at 22 C, these mice have a small, but significant, reduction (7-10%) in energy expenditure, as evidenced by oxygen consumption (QO2) and food intake, and show signs of increased brown adipose tissue (BAT) stimulation, higher plasma T4 and T3 concentrations, as well as increased uncoupling protein 3 (UCP3) expression in muscle. When acclimated at thermoneutrality temperature (32 C), QO2 decreased in both genotypes, but the difference between them widened to 16%, whereas BAT underwent atrophy, and plasma T4 and T3 levels and UCP3 mRNA decreased, yet T3 and UCP3 persisted at significantly higher levels in mGPD-/- mice. Such differences disappeared when the mice were rendered hypothyroid. A compensatory role for the observed changes in BAT, thyroid hormone levels, and UCP3 was investigated with a 2-h cold challenge of 12 C in euthyroid and hypothyroid mice. No hypothermia ensued if the mice had been acclimated at 22 C, but when acclimated at 32 C, euthyroid mGPD-/- mice became significantly more hypothermic than the wild-type controls. When rendered hypothyroid, this difference was accentuated, and the mGPD-/- mice developed profound hypothermia ( approximately 28 vs. 34 C in wild-type mice; P < 0.001). Thus, mGPD-deficient mice have, despite increased plasma T4 and T3, a small, but distinct, reduction in obligatory thermogenesis, which is compensated by increased BAT facultative thermogenesis and by thyroid hormone-dependent mechanisms using other proteins, possibly UCP3. The results support a role for mGPD in thyroid hormone thermogenesis.     

Unchanged nonshivering thermogenic capacity of dystrophic mice

The rate of oxygen consumption measured at 32.5 degrees C of lightly anesthetized 129/ReJ dy/dy mice was greater than that of dy/+ or +/+ control mice. However, the norepinephrine-stimulated rates of oxygen consumption of dystrophic and normal mice were similar. Brown adipose tissue cellularity (DNA content) of dystrophic mice was unchanged, and the tissue protein and succinate dehydrogenase contents were slightly reduced. The mitochondrial concentration of the uncoupling protein, thermogenin, and purine nucleotide binding to mitochondria isolated from brown fat of normal or dystrophic mice, were similar. These results indicate that the nonshivering thermogenic capacity of dystrophic mice is not significantly altered.