UCP2 transports C4 metabolites out of mitochondria, regulating glucose and glutamine oxidation

Abstract

Uncoupling protein 2 (UCP2) is involved in various physiological and pathological processes such as insulin secretion, stem cell differentiation, cancer, and aging. However, its biochemical and physiological function is still under debate. Here we show that UCP2 is a metabolite transporter that regulates substrate oxidation in mitochondria. To shed light on its biochemical role, we first studied the effects of its silencing on the mitochondrial oxidation of glucose and glutamine. Compared with wild-type, UCP2-silenced human hepatocellular carcinoma (HepG2) cells, grown in the presence of glucose, showed a higher inner mitochondrial membrane potential and ATP:ADP ratio associated with a lower lactate release. Opposite results were obtained in the presence of glutamine instead of glucose. UCP2 reconstituted in lipid vesicles catalyzed the exchange of malate, oxaloacetate, and aspartate for phosphate plus a proton from opposite sides of the membrane. The higher levels of citric acid cycle intermediates found in the mitochondria of siUCP2-HepG2 cells compared with those found in wild-type cells in addition to the transport data indicate that, by exporting C4 compounds out of mitochondria, UCP2 limits the oxidation of acetyl-CoA-producing substrates such as glucose and enhances glutaminolysis, preventing the mitochondrial accumulation of C4 metabolites derived from glutamine. Our work reveals a unique regulatory mechanism in cell bioenergetics and provokes a substantial reconsideration of the physiological and pathological functions ascribed to UCP2 based on its purported uncoupling properties. mitochondrial carrier | glucose and glutamine metabolism | Warburg effect | metabolic reprogramming | diabetes M itochondria couple respiratory oxidation of nutrients to ATP synthesis through an electrochemical proton gradient. Proton leak allows partial uncoupling of oxidative phosphorylation, producing heat. Through this mechanism, Uncoupling protein (UCP)1, a member of the mitochondrial carrier family (MCF), regulates adaptive thermogenesis in mammals. In 1997 a protein similar to UCP1 was cloned and named UCP2 (1) based on the assumption that the sequence homology implied a similar function. Whereas UCP1 has a clear-cut uncoupling activity relevant to nonshivering thermogenesis, this is not the case for UCP2. UCP2 has been involved in numerous physiopathological conditions including metabolic disorders, inflammation, ischemic shock, cancer, and aging. Furthermore, changes in UCP2 expression affect metabolic functions (2, 3). It has been suggested that these metabolic actions of UCP2 are due to a mild UCP1-like uncoupling activity (4, 5) that, combined with the generally low levels of UCP2 expression, would regulate the release of reactive oxygen species (ROS) (6) without significantly affecting energy conservation. Although fatty acid-dependent proton transport mediated by UCP2 was reported in reconstituted liposomes (7), a mounting body of evidence argues against UCP2 having an uncoupling activity in vivo (8, 9) and sugge...

Uncoupling protein 1 binds one nucleotide per monomer and is stabilized by tightly bound cardiolipin

“…Based on symmetry analysis across carriers, uncoupling proteins were predicted to transport small carboxylic or keto acids via a strict exchange process (49,50). In line with this notion, recombinant UCP2 was shown to exchange C4 metabolites, such as aspartate, malate, malonate, and oxaloacetate (52). However, recombinant UCP1 did not transport these substrates in parallel tests.…”

The Human SLC25A33 and SLC25A36 Genes of Solute Carrier Family 25 Encode Two Mitochondrial Pyrimidine Nucleotide Transporters

“…In recent years many of these transporters have been investigated at the molecular and biochemical levels. They transport inorganic anions, amino acids, carboxylic acids, nucleotides, and coenzymes across the inner mitochondrial membrane and in a few cases across other membranes (1,(3)(4)(5). The importance of mitochondrial carriers is demonstrated by their wide distribution in all eukaryotes, their role in numerous metabolic pathways and cell functions, and the identification of several diseases caused by alterations of their genes (1, 2, 6 -8).…”

Reactive oxygen species signaling influences feeding behaviour

“…UCP 2 therefore can reduce substrates of Krebs cycle, decrease the activity of the electron transport chain and this can reduce the capacity to produce ROS. 23 UCP 2 expression on mitochondrial membrane also increase the response to intraperitoneal injection of ghrelin, a hormone that is secreted in fasted state. Increased UCP 2 expression have been reported to subsequently scavenge ROS in NPY neurons without affecting ROS production in POMC neuron.…”