Ribavirin up-regulates the activity of double-stranded RNA-activated protein kinase and enhances the action of interferon-α against hepatitis C virus

Background. Ribavirin's mechanism of action in the treatment of chronic hepatitis C remains to be clarified. Double-stranded RNA-activated protein kinase (PKR) plays a role in cell defense against virus infection. This study investigated whether PKR is a mediator of the effectiveness of ribavirin, used either alone or in combination with interferon (IFN)-alpha, against hepatitis C virus (HCV) infection. Methods. Primary human hepatocytes and HCV-replicon cells were treated with ribavirin and/or IFN-alpha. PKR activity was assayed by immunoblotting. A pulse-chase assay of the half- life of PKR protein was performed to study whether ribavirin decreases PKR degradation. We used small-interference RNA (siRNA) to knock down PKR to assess its importance in the suppression of HCV-RNA replication in the replicon system. Results. Ribavirin was able to up-regulate the levels of phosphorylated PKR and phosphorylated eIF2 alpha, leading to suppression of HCV-RNA replication. The effects that treatment with ribavirin plus IFN-alpha had on PKR activity were greater than those observed for treatment with either ribavirin alone or IFN-alpha alone. Knockdown of PKR increased HCV-RNA replication, supporting the importance of PKR in the control of HCV-RNA replication. The pulse-chase experiment showed that ribavirin can reduce the degradation rate of PKR protein. Conclusion. These results suggest that the anti-HCV action of ribavirin is partly attributable to its ability to up-regulate PKR activity.