Brown fat biology and thermogenesis

Brown fat (brown adipose tissue, BAT) primary function is to produce heat. There is now compelling evidence to indicate that brown fat cells in some BAT depots share their predecessor cells with myocytes. Brown adipocyte (trans) differentiation depends on various receptors / transcription factors that include peroxisome proliferator-activated receptor gamma (PPAR gamma), PPAR gamma-coactivator-1 alpha (PGC1 alpha), PRD1-BF1-RIZ1 homologous domain-containing 16 (PRDM16), CCAAT/enhancer-binding protein beta (C/EBP-beta) and bone morphogenetic protein 7 (BMP7). Such mediators also help BAT to acquire its thermogenic phenotype, which is essentially conferred by uncoupling protein 1 (UCP1). UCP1 uncouples adenosine-5'-triphosphate (ATP) synthesis from substrate oxidation in brown adipocytes. Its activity depends on the availability of fatty acids delivered upon BAT's beta-adrenergic activation, which, physiologically, ensues from the sympathetic nervous system (SNS) activation of the tissue. SNS-mediated thermogenesis is largely controlled by the hypothalamus and brainstem. Recently, positron emission tomography / computed tomography (PET/CT) scanning investigations have revealed the presence in adult humans of important neck and shoulder BAT depots. That finding has contributed to reinstate a strong interest for brown adipocyte biology and thermogenesis. This review aims at the unique biology of BAT with the emphasis put on the recent discoveries regarding the brown adipocyte development and function.