Critical role of a common transcription factor, IRF-1, in the regulation of IFN-beta and IFN-inducible genes.

Interferon regulatory factor 1 (IRF-1) is a protein that binds to cis-elements within the promoter of interferon (IFN)-beta and some IFN-inducible genes. We used a human fibroblast line, GM-637, to generate stable transfectants constitutively expressing IRF-1 mRNA in either the sense or antisense orientation. Upon induction with poly-(I).poly(C) or Newcastle disease virus, cells expressing sense IRF-1 mRNA produced significantly higher levels of IFN-beta mRNA and protein than control cells, whereas cells expressing antisense IRF-1 mRNA produced little or no IFN-beta mRNA and protein. Furthermore, clear differences were seen among the transfectants in the level of expression of two IFN-induced genes (2'-5'-oligoadenylate synthetase and class I HLA). Our data show that IRF-1 is essential for the induced expression of the IFN-beta gene. The results also indicate an important role of IRF-1 in the expression of IFN-inducible genes and suggest a role for IRF-1 in many other cytokine actions.     

Central role of interferon regulatory factor-1 (IRF-1) in controlling retinoic acid inducible gene-I (RIG-I) expression

Retinoic acid inducible gene-I (RIG-I) functions as the first line of defense against viral infection by sensing dsRNA and inducing type I interferon (IFN) production. The expression of RIG-I itself is induced by IFN-alpha/beta and dsRNA. To comprehend the molecular mechanism of expression regulation, we cloned the RIG-I promoter and analyzed its activity upon IFN-beta and dsRNA treatment. Under basal condition, RIG-I mRNA level and promoter activity were significantly higher in normal cells versus their tumor counterparts. In both normal and cancer cells, RIG-I expression was induced by IFN-beta and dsRNA. A single IRF-1 binding site in the proximal promoter functioned as a crucial regulator of basal, IFN-beta- and dsRNA-mediated induction of the RIG-I promoter. IFN-beta and dsRNA treatment increased IRF-1 binding to the RIG-I promoter. IRF-1 expression was also higher in normal cells than in cancer cells and it was induced by IFN-beta with similar kinetics as RIG-I. These results confirm that by controlling RIG-I expression, IRF-1 plays an essential role in anti-viral immunity. IRF-1 is a tumor suppressor and the expression profile of RIG-I together with its regulation by IRF-1 and the presence of a caspase-recruitment domain in RIG-I suggest that RIG-I might also possess tumor suppressor properties.     

Hepatitis C virus infection induces the beta interferon signaling pathway in immortalized human hepatocytes

Beta interferon (IFN-beta) expression is triggered by double-stranded RNA, a common intermediate in the replication of many viruses including hepatitis C virus (HCV). The recent development of cell culture-grown HCV allowed us to analyze the IFN signaling pathway following virus infection. In this study, we have examined the IFN-beta signaling pathway following infection of immortalized human hepatocytes (IHH) with HCV genotype 1a (clone H77) or 2a (clone JFH1). We observed that IHH possesses a functional Toll-like receptor 3 pathway. HCV infection in IHH enhanced IFN-beta and IFN-stimulated gene 56 (ISG56) promoter activities; however, poly(I-C)-induced IFN-beta and ISG56 expression levels were modestly inhibited upon HCV infection. IHH infected with HCV (genotype 1a or 2a) exhibited various levels of translocation of IRF-3 into the nucleus. The upregulation of endogenous IFN-beta and 2',5'-oligoadenylate synthetase 1 mRNA expression was also observed in HCV-infected IHH. Subsequent studies suggested that HCV infection in IHH enhanced STAT1 and ISG56 protein expression. A functional antiviral response of HCV-infected IHH was observed by the growth-inhibitory role in vesicular stomatitis virus. Together, our results suggested that HCV infection in IHH induces the IFN signaling pathway, which corroborates observations from natural HCV infection in humans.