Cytopathic effects are currently believed to contribute to hepatitis C virus (HCV)-induced liver injury and are readily observed in Huh7.5 cells infected with the JFH-1 HCV strain, manifesting as apoptosis highly correlated with growth arrest. Reactive oxygen species, which are induced by HCV infection, have recently emerged as activators of AMP-activated protein kinase. The net effect is ATP conservation via on/off switching of metabolic pathways that produce/consume ATP. Depending on the scenario, this can have either pro-survival or pro-apoptotic effects. We demonstrate reactive oxygen species-mediated activation of AMP-activated kinase in Huh7.5 cells during HCV (JFH-1)-induced growth arrest. Metabolic labeling experiments provided direct evidence that lipid synthesis is attenuated, and -oxidation is enhanced in these cells. A striking increase in nuclear peroxisome proliferator-activated receptor ␣, which plays a dominant role in the expression of -oxidation genes after ligand-induced activation, was also observed, and we provide evidence that peroxisome proliferator-activated receptor ␣ is constitutively activated in these cells. The combination of attenuated lipid synthesis and enhanced -oxidation is not conducive to lipid accumulation, yet cellular lipids still accumulated during this stage of infection. Notably, the serum in the culture media was the only available source for polyunsaturated fatty acids, which were elevated (2-fold) in the infected cells, implicating altered lipid import/export pathways in these cells. This study also provided the first in vivo evidence for enhanced -oxidation during HCV infection because HCV-infected SCID/Alb-uPA mice accumulated higher plasma ketones while fasting than did control mice. Overall, this study highlights the reprogramming of hepatocellular lipid metabolism and bioenergetics during HCV infection, which are predicted to impact both the HCV life cycle and pathogenesis.With ϳ200 million people infected worldwide, hepatitis C virus (HCV) 2 is a global health problem and a major cause of viral hepatitis. Persistent infection occurs in ϳ70% of infected patients leading to inflammation, insulin resistance, steatosis, fibrosis, and hepatocellular carcinoma (1). Current direct-acting antivirals are predicted to be a cure for most patients, but the high cost of this treatment means that the pathological consequences of persistent HCV infection will remain a concern.Although the pathology associated with chronic HCV infection was initially thought to be due to HCV-specific immune responses (2), the current opinion is that direct cytopathic effects in virally infected cells also contribute to HCV-associated liver injury (3, 4). The cellular mechanisms by which HCV replication might mediate liver injury are unclear, but there is no doubt that oxidative/nitrosative stress in HCV-infected cells plays an important role in the initiation and progression of liver damage (3, 5, 6). Oxidative/nitrosative stress essentially arises when the production of reactive oxygen (...