Using positron-emission tomography (PET), we found that cold-induced glucose uptake was increased by a factor of 15 in paracervical and supraclavicular adipose tissue in five healthy subjects. We obtained biopsy specimens of this tissue from the first three consecutive subjects and documented messenger RNA (mRNA) and protein levels of the brown-adipocyte marker, uncoupling protein 1 (UCP1). Together with morphologic assessment, which showed numerous multilocular, intracellular lipid droplets, and with the results of biochemical analysis, these findings document the presence of substantial amounts of metabolically active brown adipose tissue in healthy adult humans.
We investigated the metabolism of human brown adipose tissue (BAT) in healthy subjects by determining its cold-induced and insulin-stimulated glucose uptake and blood flow (perfusion) using positron emission tomography (PET) combined with computed tomography (CT). Second, we assessed gene expression in human BAT and white adipose tissue (WAT). Glucose uptake was induced 12-fold in BAT by cold, accompanied by doubling of perfusion. We found a positive association between whole-body energy expenditure and BAT perfusion. Insulin enhanced glucose uptake 5-fold in BAT independently of its perfusion, while the effect on WAT was weaker. The gene expression level of insulin-sensitive glucose transporter GLUT4 was also higher in BAT as compared to WAT. In conclusion, BAT appears to be differently activated by insulin and cold; in response to insulin, BAT displays high glucose uptake without increased perfusion, but when activated by cold, it dissipates energy in a perfusion-dependent manner.
Brown adipose tissue (BAT) has attracted scientific interest as an antidiabetic tissue owing to its ability to dissipate energy as heat. Despite a plethora of data concerning the role of BAT in glucose metabolism in rodents, the role of BAT (if any) in glucose metabolism in humans remains unclear. To investigate whether BAT activation alters whole-body glucose homeostasis and insulin sensitivity in humans, we studied seven BAT-positive (BAT+) men and five BAT-negative (BAT−) men under thermoneutral conditions and after prolonged (5–8 h) cold exposure (CE). The two groups were similar in age, BMI, and adiposity. CE significantly increased resting energy expenditure, whole-body glucose disposal, plasma glucose oxidation, and insulin sensitivity in the BAT+ group only. These results demonstrate a physiologically significant role of BAT in whole-body energy expenditure, glucose homeostasis, and insulin sensitivity in humans, and support the notion that BAT may function as an antidiabetic tissue in humans.
Based on the seminal observation by Cannon and Nedergaard 1 that human PET scans sometimes depicted a symmetric cold induced uptake of FDG-glucose, three independent studies, published in April 2009, demonstrated metabolically highly active brown adipose tissue (BAT) in adult humans [2][3][4] . Subsequent investigations demonstrated an inverse association of obesity and type 2 diabetes mellitus and the presence of active BAT [5][6][7] . A unique characteristic of BAT is the expression of uncoupling protein 1 (UCP1, also known as thermogenin). Activation of this transmembrane protein by fatty acids in response to adrenergic signaling short-circuits the inner mitochondrial membrane's proton gradient thereby uncoupling oxidative phosphorylation from ATP synthesis. Hence, chemical energy stored in the gradient is dissipated as heat allowing for efficient direct thermogenesis without shivering 8 . This adaptive defense against cold has been examined extensively in rodents and many aspects of BAT development and function have been elucidated. In rodents it is evident 3 that not only the distinct thermogenic BAT organ located in the interscapular region (iBAT) consists of brown adipocytes, but that a second type of brown adipocytes, so-called beige or brite cells can appear in white adipose tissue (WAT) depots in response to cold or 3-adrenergic stimuli 9,10 . Recently, lineage tracing experiments revealed that the two cell types have a different developmental origin 11 . While classical brown adipocytes and skeletal muscle cells arise from precursors in the dermomyotome 12 , beige/brite cells seem to originate from endothelial and perivascular cells within WAT depots [13][14][15] . A recent study by Wu et al suggests that the previously described depots of human BAT are of the beige/brite type and raises the question whether humans altogether lack classical brown adipocytes 16 , this has also been the topic of a recent review 17 . Histomorphological studies performed in the 1970s indicated the existence of brown adipocytes within the interscapular region in human infants and that these disappeared with age 18 . Using a combination of high resolution imaging techniques and morphological and biochemical analyses, we tested the hypothesis that human infants, like small mammals, possess an anatomically distinguishable iBAT depot consisting of classical brown adipocytes, a cell type so far not proven to exist in humans.In an attempt to visualize potential iBAT in humans we performed post mortem MR imaging of eight human infants. Using the fat fraction method 19 we did not only identify BAT depots in the supraclavicular region, but importantly also a fat depot in the interscapular region presenting with an intermediate fat fraction as opposed to the high fat fraction of the surrounding subcutaneous WAT (Supplementary Fig. 1). Using a three dimensional reconstruction we were able to compute the volume of the tissue depot with an average (±SD) volume of 3.6±2.4 ml. Figure 1 displays a representative reconstruction of the iBAT...
Brown adipose tissue (BAT) is specialized in energy expenditure, making it a potential target for anti-obesity therapies. Following exposure to cold, BAT is activated by the sympathetic nervous system with concomitant release of catecholamines and activation of β-adrenergic receptors. Because BAT therapies based on cold exposure or β-adrenergic agonists are clinically not feasible, alternative strategies must be explored. Purinergic co-transmission might be involved in sympathetic control of BAT and previous studies reported inhibitory effects of the purinergic transmitter adenosine in BAT from hamster or rat. However, the role of adenosine in human BAT is unknown. Here we show that adenosine activates human and murine brown adipocytes at low nanomolar concentrations. Adenosine is released in BAT during stimulation of sympathetic nerves as well as from brown adipocytes. The adenosine A2A receptor is the most abundant adenosine receptor in human and murine BAT. Pharmacological blockade or genetic loss of A2A receptors in mice causes a decrease in BAT-dependent thermogenesis, whereas treatment with A2A agonists significantly increases energy expenditure. Moreover, pharmacological stimulation of A2A receptors or injection of lentiviral vectors expressing the A2A receptor into white fat induces brown-like cells-so-called beige adipocytes. Importantly, mice fed a high-fat diet and treated with an A2A agonist are leaner with improved glucose tolerance. Taken together, our results demonstrate that adenosine-A2A signalling plays an unexpected physiological role in sympathetic BAT activation and protects mice from diet-induced obesity. Those findings reveal new possibilities for developing novel obesity therapies.
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