Hepatitis C virus (HCV), the global leading cause of chronic
liver disease, has a positive-sense, ssRNA genome that encodes a large
polyprotein. HCV
polyprotein translation is initiated by an
internal ribosome-entry site (IRES) located at the 5' end of the viral genome, in a cap-independent manner, but the regulatory mechanism of this process remains poorly understood. In this study, we characterized the effect of HCV nonstructural
proteins on HCV IRES-directed translation in both HCV replicon cells and transiently transfected human liver cells expressing HCV nonstructural
proteins. Using bicistronic reporter gene constructs carrying either HCV or other viral IRES sequences, we found that the HCV IRES-mediated translation was specifically upregulated in HCV replicon cells. This enhancement of HCV IRES-mediated translation by the replicon cells was inhibited by treatment with either
type I interferon or
ribavirin, drugs that perturb HCV genome replication, suggesting that the enhancement is probably due to HCV-encoded
protein function(s). Reduced phosphorylation levels of both eIF2alpha and
eIF4E were observed in the replicon cells, which is consistent with our previous findings and indicates that the NS5A nonstructural
protein may be involved in the regulatory mechanism(s). Indeed, transient expression of NS5A or NS4B in human liver cells stimulated HCV IRES activity. Interestingly, mutation in the ISDR of NS5A perturbed this stimulation of HCV IRES activity. All these results suggest, for the first time, that HCV nonstructural
proteins preferentially stimulate the viral cap-independent, IRES-mediated translation.