The mitochondrial uncoupling protein-2: current status

In eukaryotic cells ATP is generated by oxidative phosphorylation, an energ etic coupling at the mitochondrial level. The oxidative reactions occurring in the respiratory chain generate an electrochemical proton gradient on bo th sides of the inner membrane. This gradient is used by the ATPsynthase to phosphorylate ADP into ATP. The coupling between respiration and ADP phosp horylation is only partial in brown adipose tissue (BAT) mitochondria, wher e the uncoupling protein UCP1 causes a reentry of protons into the matrix a nd abolishes the electrochemical proton gradient. The liberated energy is t hen dissipated as heat and ATP synthesis is reduced. This property was for a long time considered as an exception and specific to the non-shivering th ermogenesis found in BAT. The recent cloning of new UCPs expressed in other tissues revealed the importance of this kind of regulation of respiratory control in metabolism and energy expenditure. The newly characterised UCPs are potential targets for obesity treatment drugs which could favour energy expenditure and diminish the metabolic efficiency. In 1997, we cloned UCP2 and proposed a role for this new uncoupling protein in diet-induced thermogenesis, obesity, hyperinsulinemia, fever and restin g metabolic rate. Currently, an abundant literature deals with UCP2, but it s biochemical and physiological functions and regulation remain unclear. Th e present review reports the status of our knowledge of this mitochondrial carrier in terms of sequence, activity, tissue distribution and regulation of expression. The putative physiological roles of UCP2 will be introduced and discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.