Inhibition of RIG-I-dependent signaling to the interferon pathway during hepatitis C virus expression and restoration of signaling by IKKε

Interferon (IFN) is one important effector of the innate immune response, induced by different viral or bacterial components through Toll-like receptor (TLR)-dependent and -independent mechanisms. As part of its pathogenic strategy, hepatitis C virus (HCV) interferes with the innate immune response and induction of IFN-beta via the HCV NS3/4A protease activity which inhibits phosphorylation of IRF-3, a key transcriptional regulator of the IFN response. In the present study, we demonstrate that inhibition by the protease occurs upstream of the noncanonical IKK-related kinases IKK epsilon and TBK-1, which phosphorylate IRF-3, through partial inhibition of the TLR adapter protein TRIF/TICAM1 -dependent pathway. Use of TRIF-/- mouse embryo fibroblasts however revealed the presence of a TRIF-independent pathway involved in IFN induction that was also inhibited by NS3/4A. Importantly, we show that NS3/4A can strongly inhibit the ability of the recently described RIG-I protein to activate IFN, suggesting that RIG-I is a key factor in the TRIF-independent, NS3/4A-sensitive pathway. Expression of IFN signaling components including IKK epsilon, TBK-1, TRIF, and wild type or constitutively active forms of RIG-I in the HCV replicon cells resulted in IFN-beta promoter transactivation, with IKK epsilon displaying the highest efficiency. Subsequently, overexpression of IKK epsilon resulted in 80% inhibition of both the positive and negative replicative strands of the HCV replicon. The partial restoration of the capacity of the host cell to transcribe IFN-beta indicates that IKK epsilon expression is able to bypass the HCV-mediated inhibition and restore the innate antiviral response.