Background & Aims:The mechanisms of liver injury in chronic hepatitis C virus (HCV) infection are poorly understood. Indirect evidence suggests that oxidative stress and mitochondrial injury play a role. The aim of this study was to determine if the HCV core protein itself alters mitochondrial function and contributes to oxidative stress. Methods:HCV core protein was expressed in 3 different cell lines, and reactive oxygen species (ROS) and lipid peroxidation products were measured. Results:Core expression uniformly increased ROS. In 2 inducible expression systems, core protein also increased lipid peroxidation products and induced antioxidant gene expression as well. A mitochondrial electron transport inhibitor prevented the core-induced increase in ROS. A fraction of the expressed core protein localized to the mitochondria and was associated with redistribution of cytochrome c from mitochondrial to cytosolic fractions. Sensitivity to oxidative stress was also seen in HCV transgenic mice in which increased intrahepatic lipid peroxidation products occurred in response to carbon tetrachloride. Conclusions:Oxidative injury occurs as a direct result of HCV core protein expression both in vitro and in vivo and may involve a direct effect of core protein on mitochondria. These results provide new in sight into the pathogenesis of hepatitis C and provide an experimental rationale for investigation of antioxidant therapy.