Hepatitis C virus (HCV) is an etiologic agent of severe liver diseases in humans. HCV infection has been associated with severe alterations of the host redox status, and oxidative stress has been identified as a key mechanism of HCV-induced pathogenesis. Nevertheless, the source of hydrogen peroxide (H₂O₂) and superoxide (O₂ ^⋅-) during HCV infection is still incompletely characterized. Recently, hepatocytes have been found to express Nox family enzymes, but it is unknown if these proteins participate in HCV-induced oxidative stress. We found that Nox4 and Nox1 mRNA's were persistently elevated in Huh7 cells that produced infectious HCV particles, compared to mock-transfected controls. Duox2 mRNA was only transiently elevated with HCV. The same Nox/Duox mRNAs were also increased in telomerase-reconstituted primary human hepatocytes transfected with HCV RNA. In contrast, subgenomic HCV RNA did not increase Nox/Duox mRNA's, suggesting that the structural genes or the production of infectious virions is necessary. In addition, Nox4 and Nox1 protein contents were elevated in HCV RNA-transfected and -infected cells as well as human liver. HCV increased the oxidation of H₂-dihydrodichlorofluorescein diacetate generated from incubation of cells with (DCF-DA) that was sensitive to diphenyl iodonium, an inhibitor of flavoenzymes including Nox enzymes. Also, the levels of H₂O₂ and 2-OH-E+ were elevated in HCV-JFH1 RNA transfected hepatocytes, and siRNAs to Nox enzymes decreased H₂O₂ level. Finally, confocal laser scanning microscopy and subcellular fractionation studies showed that Nox4 was prominent in the nuclear compartment of these cells. Therefore, hepatocyte Nox proteins may act as an endogenous source of oxygen- centered reactive species during chronic HCV infection and are likely to contribute to the pathogenesis.