Uncoupling protein-2 (UCP2) induces mitochondrial proton leak and increases susceptibility of non-alcoholic steatohepatitis (NASH) liver to ischaemia-reperfusion injury

Background: The mechanisms of progression from fatty liver to steatohepatitis and cirrhosis are not well elucidated. Mitochondrial dysfunction represents a key factor in the progression of non-alcoholic steatohepatitis (NASH) as mitochondria are the main cellular site of fatty acid oxidation, ATP synthesis and reactive oxygen species (ROS) production. Aims: (1) To evaluate the role of the uncoupling protein 2 in controlling mitochondrial proton leak and ROS production in NASH rats and humans; and (2) to assess the acute liver damage induced by ischaemia-reperfusion in rats with NASH. Methods: Mitochondria were extracted from the livers of NASH humans and rats fed a methionine and choline deficient diet. Proton leak, H2O2 synthesis, reduced glutathione/oxidised glutathione, 4-hydroxy-2-nonenal (HNE)-protein adducts, uncoupling protein-2 (UCP2) expression and ATP homeostasis were evaluated before and after ischaemia-reperfusion injury. Results: NASH mitochondria exhibited an increased rate of proton leak due to upregulation of UCP2. These results correlated with increased production of mitochondrial hydrogen peroxide and HNE-protein adducts, and decreased hepatic ATP content that was not dependent on mitochondrial ATPase dysfunction. The application of an ischaemia-reperfusion protocol to these livers strongly depleted hepatic ATP stores, significantly increased mitochondrial ROS production and impaired ATPase activity. Livers from patients with NASH exhibited UCP2 over-expression and mitochondrial oxidative stress. Conclusions: Upregulation of UCP2 in human and rat NASH liver induces mitochondrial uncoupling, lowers the redox pressure on the mitochondrial respiratory chain and acts as a protective mechanism against damage progression but compromises the liver capacity to respond to additional acute energy demands, such as ischaemia-reperfusion. These findings suggest that UCP2-dependent mitochondria uncoupling is an important factor underlying events leading to NASH and cirrhosis.