Uncoupling protein 1 controls reactive oxygen species in brown adipose tissue.

Brown adipose tissue (BAT) is an organ specialized to fuel nonshivering thermogenesis for the defense of high body temperature of many eutherian mammals in the cold. Cold-induced sympathetic stimulation of brown adipocytes activates lipolysis, glucose uptake, and mitochondrial biogenesis, with the mitochondrial biogenesis providing a powerful cellular engine for heat generation. At the mitochondrial inner membrane, the energy of nutrients such as glucose and lipids is converted into a proton gradient, but instead of storing the potential energy in the generation of ATP, uncoupling protein 1 (UCP1) catalyzes an inducible proton leak to release the energy of the proton gradient directly as heat. The central role of UCP1 for nonshivering thermogenesis in rodents has been confirmed in knockout mice (1, 2), and the majority of research efforts on BAT and UCP1 centers on the physiological consequences of thermogenesis, including the regulation of body weight for treatment of obesity and other metabolic diseases. Although it appears that UCP1’s function to release the proton motive force as the final energy conversion step may only affect thermogenesis, reduction of the proton motive force will inevitably impact other mitochondrial and cellular processes. Reducing proton motive force by mitochondrial proton leak would alter the redox state of the respiratory chain and potentially reduce the production of reactive oxygen species (ROS) (3). The absence of UCP1 in BAT could be problematic, given the high concentration of respiratory chain complexes in cold-acclimated brown fat mitochondria and the minor concentrations of ATP synthase, with the latter not being able to release the enhanced, cold-induced proton motive force. Although the physiological consequences in UCP1 knockout mice have been almost exclusively attributed to thermogenesis, there may be further consequences on mitochondrial ROS biology and downstream effects that have been widely ignored. In PNAS, Kazak et al. (4) explore cold-induced molecular differences of BAT lacking UCP1 and show that mitochondrial calcium buffering is compromised through ROS production in an UCP1-dependent manner, demonstrating that UCP1 ablation is about more than thermogenesis. Mitochondrial ROS production has a major functional impact in all cells that has led to the emergence of new theories and biological disciplines such as the “free radical theory of aging” addressing the damage by ROS in aging (5) and the idea of ROS being an important signaling molecule. The sites of ROS production have been intensively explored in recent years, demonstrating that there are 11 molecular sites that are capable of producing ROS, at least in isolated mitochondria (6). The magnitude of ROS depends on the concentration of electron transport chain (ETC) complexes and their redox state, with the latter being affected by the concentration of electrons that are donated by substrates to the ETC and by their ability to be passed on from complex to complex so as to reduce oxygen finally to water. How effectively these electrons can be passed along the ETC also depends on the proton gradient generated by the same ETC. The higher the proton gradient, the more this gradient would stall the ETC, with the electrons possibly “overreducing” their carriers and prematurely escaping the ETC complexes toward oxygen, generating either superoxide or hydrogen peroxide (3). Thus, interspersed literature claiming that ROS production is positively associated with the rate of electron flux and oxygen consumption cannot be substantiated with the current understanding of bioenergetics. More than 20 y ago, it was already suggested that reduction of the proton motive force by uncoupling agents impacts mitochondrial production of ROS (7). Endogenous proton leak was proposed as the mechanism to reduce ROS (8), and a similar impact on ROS by UCP1 has been verified in isolated brown fat mitochondria by two laboratories (9, 10). However, the involvement of UCP1 in ROS biology has been disputed based mainly on conflicting studies, claiming that ROS directly activates UCPs, thus providing an elegant feedback mechanism to prevent excessive mitochondrial ROS levels (reviewed in ref. 11). Other studies have reported that this feedback mechanism does not exist (12–14) or is