Comparative effects of temperature exposure on mass and oxidative enzyme activity of brown fat in insectivores, tupaiads and primates

The role of BAT in cardiometabolic disorders and aging

The demonstration of the presence of metabolically active brown adipose tissue (BAT) in adult humans using positron emission tomography (PET) over the past decade has lead to the rapid development of our knowledge regarding the role of BAT in energy metabolism in animal models and in humans. Although animal models continue to provide highly valuable information regarding the mechanisms regulating BAT development, mass and metabolic functions, these studies led to many assumptions that have been at best only partially verified in humans so far. Combined to some limitations of the current investigation approaches used in humans, this has lead to speculation on the potential role of BAT dysfunction in the development of cardiometabolic disorders and on the potential of BAT metabolic activation to treat these conditions. Here we propose a critical review of the evidence for the implication of BAT in cardiometabolic health.

Maintaining thermogenesis in cold exposed humans: relying on multiple metabolic pathways

In cold exposed humans, increasing thermogenic rate is essential to prevent decreases in core temperature. This review describes the metabolic requirements of thermogenic pathways, mainly shivering thermogenesis, the largest contributor of heat. Research has shown that thermogenesis is sustained from a combination of carbohydrates, lipids, and proteins. The mixture of fuels is influenced by shivering intensity and pattern as well as by modifications in energy reserves and nutritional status. To date, there are no indications that differences in the types of fuel being used can alter shivering and overall heat production. We also bring forth the potential contribution of nonshivering thermogenesis in adult humans via the activation of brown adipose tissue (BAT) and explore some means to stimulate the activity of this highly thermogenic tissue. Clearly, the potential role of BAT, especially in young lean adults, can no longer be ignored. However, much work remains to clearly identify the quantitative nature of this tissue's contribution to total thermogenic rate and influence on shivering thermogenesis. Identifying ways to potentiate the effects of BAT via cold acclimation and/or the ingestion of compounds that stimulate the thermogenic process may have important implications in cold endurance and survival.

Selective Impairment of Glucose but Not Fatty Acid or Oxidative Metabolism in Brown Adipose Tissue of Subjects With Type 2 Diabetes

Spontaneous glucose uptake by brown adipose tissue (BAT) is lower in overweight or obese individuals and in diabetes. However, BAT metabolism has not been previously investigated in patients with type 2 diabetes during controlled cold exposure. Using positron emission tomography with (11)C-acetate, (18)F-fluoro-deoxyglucose ((18)FDG), and (18)F-fluoro-thiaheptadecanoic acid ((18)FTHA), a fatty acid tracer, BAT oxidative metabolism and perfusion and glucose and nonesterified fatty acid (NEFA) turnover were determined in men with well-controlled type 2 diabetes and age-matched control subjects under experimental cold exposure designed to minimize shivering. Despite smaller volumes of (18)FDG-positive BAT and lower glucose uptake per volume of BAT compared with young healthy control subjects, cold-induced oxidative metabolism and NEFA uptake per BAT volume and an increase in total body energy expenditure did not differ in patients with type 2 diabetes or their age-matched control subjects. The reduction in (18)FDG-positive BAT volume and BAT glucose clearance were associated with a reduction in BAT radiodensity and perfusion. (18)FDG-positive BAT volume and the cold-induced increase in BAT radiodensity were associated with an increase in systemic NEFA turnover. These results show that cold-induced NEFA uptake and oxidative metabolism are not defective in type 2 diabetes despite reduced glucose uptake per BAT volume and BAT "whitening."

Sequential changes in the expression of mitochondrial protein mRNA during the development of brown adipose tissue in bovine and ovine species. Sudden occurrence of uncoupling protein mRNA during embryogenesis and its disappearance after birth

Samples of adipose tissue were obtained from different sites in bovine and ovine foetuses and newborns. RNA was isolated and analysed using bovine cDNA and ovine genomic probe for uncoupling protein (UCP), cDNA for subunits III and IV of cytochrome c oxidase and cDNA for ADP/ATP carrier. UCP mRNA was characterized for the first time in foetal bovine and ovine adipose tissue. It appeared later than mRNA of cytochrome c oxidase subunit III, and increased dramatically at birth (10-fold). ADP/ATP carrier mRNA was expressed at a lower level but also increased 10-fold at birth. It was demonstrated that UCP mRNA reached its highest level at birth in all bovine adipose tissues studied, except subcutaneous tissue. It disappeared quickly afterwards, being no longer detectable two days after birth. Similar variations were observed in newborn lambs. ADP/ATP carrier mRNA showed the same pattern of expression as UCP mRNA; although it was still lightly expressed two days after birth, it disappeared soon afterwards. Only mRNAs for cytochrome c oxidase subunits III and IV remained at the same level during the first postnatal week. On the basis of these data and of observations reported in the literature a sequence of events for the development of brown adipose cells in vivo is proposed. Soon after birth the perirenal adipose tissue of ruminants, which still contains mitochondria of typical brown adipose tissue morphology and high levels of cytochrome c oxidase mRNA, lacks UCP mRNA. Can it still be considered as brown fat? Ruminant species appear to be attractive models to study both the differentiation of brown adipose tissue and its possible conversion to white fat in large animals.

Characterization of mitochondrial-uncoupling protein in bovine fetus and newborn calf

Development changes in the content of the mitochondrial-uncoupling protein (UCP) have been studied in adipose depots of bovine fetuses and a newborn calf as well as in adipose depots of newborn and aging lambs. The occurrence of UCP unique to brown adipose tissue (BAT) was investigated by GDP binding, photoaffinity labeling with 8-azidoadenosine 5'-triphosphate, and immunoblots using specific antibodies directed against rat UCP. A protein of 32,000 relative molecular weight was characterized in both species with properties similar to those of rodent UCP. In bovine, UCP became detectable in the perirenal adipose tissue at day -80 and its content increased until birth. Both in bovine (perirenal, subscapular, and retroperitoneal sites) and in ovine (perirenal, subscapular, retroperitoneal, and pericardiac sites), all adipose tissues except the subcutaneous adipose tissue contained at birth UCP and thus can be considered as BAT. The data indicate that the perirenal adipose depot should play in bovine and ovine a major thermogenic role at birth, whereas perirenal and pericardiac adipose tissues of lambs held under cold conditions for 45 days after birth did not show any immunoreactive UCP.

Brown adipose tissue from fetal rhesus monkey (Macaca mulatta): morphological and biochemical aspects

Brown adipose tissue (BAT) from fetal rhesus monkeys microscopically resembled adult rodent BAT containing multiocular fat cells with numerous mitochondria. Mitochondrial carnitine palmitoyl transferase activity was lower than that in adult rodents and adenine nucleotide translocase activity was similar to that reported for rats. Rhesus monkey BAT mitochondria (BATM) possess an uncoupling protein that is characteristic of BAT as evidenced by the binding of [3H]GDP, the inhibition by GDP of the high Cl- permeability or rapid alpha-glycerol-3-phosphate oxidation. Electrophoretic analysis of BATM showed the presence of a 32,000 mol.wt protein which was enriched by procedures established for the isolation of BATM uncoupling protein.