The brown adipocyte: update on its metabolic role.

Brown adipocytes are multilocular lipid storage cells that play a crucial role in non-shivering thermogenesis. These cells are located in brown adipose tissue (BAT) depots which are found in abundance in small mammals as well as in newborns of larger mammals, including humans. Brown adipocytes comprise a very large number of mitochondria packed with cristae and are densely innervated by the sympathetic nervous system (SNS). Sympathetic nerve endings release noradrenaline (NA) in the proximity of brown fat cells, where noradrenaline activates G-protein-coupled beta-adrenergic receptors (AR) and by doing so initiates a cascade of metabolic events culminating in the activation of uncoupling protein 1 (UCP1). Uncoupling protein 1 is a unique feature of brown adipocytes that allows for the generation of heat upon sympathetic nervous system stimulation. It is found in the inner membrane of the mitochondrion, where uncoupling protein 1 uncouples the oxidation of fuel from adenosine triphosphate (ATP) production. The expression of uncoupling protein 1 is strongly induced by cold exposure, revealing the importance of this uncoupling protein in thermoregulation. The thermoregulatory role of uncoupling protein 1 has been emphasized in uncoupling protein 1-deficient mice, whose resistance to cold is impaired. Uncoupling protein 1 expression is modulated by diet and metabolic hormones such as leptin and glucocorticoids, which suggests that the protein is a player in energy balance regulation.