Hepatitis C virus core protein modulates the interferon-induced transacting factors of Jak/Stat signaling pathway but does not affect the activation of downstream IRF-1 or 561 gene

Hepatitis C virus (HCV) has a propensity to cause chronic infection, with a low proportion of patients exhibiting a sustained response to interferon-alpha (IFN alpha) therapy. An earlier report suggested that HCV inhibits IFN alpha -induced signal transduction through the Jak/Stat pathway by preventing the formation of the transacting factor ISGF3 complex, although the effect on downstream pathway and the specific viral protein responsible for inhibition of IFN alpha -mediated signal transduction were not elucidated. HCV core protein displays a number of intriguing functional properties and has been implicated in virus-mediated pathogenesis. In this study, we have analyzed the effect of core protein upon IFN alpha- or IFN gamma -induced regulation of the Jak/Stat signaling pathway. HCV core protein expression exhibited a reduced Stat1 expression in I FN-treated mammalian cells. A gel retardation assay suggested a reduced level of formation of the transacting factors, GAF and ISGF3, in IFN-treated cells. Further studies from protein expression and RNase protection assay revealed that the reduced level of GAF or ISGF3 formation could be attributed to modulation of Stat1 protein expression, an important player for innate immunity in host defense mechanism. However, these modulatory effects did not interfere with the activation of the downstream effector genes, IRF-1 and 561, in IFN-treated cells. Stable transfectants of cells after introduction of a plasmid DNA encoding both the structural and the nonstructural proteins of HCV also exhibited a similar effect. Taken together, these results suggest that although expression of the core protein alone or with other HCV proteins modulate transacting factors of Jak/Stat signaling pathway, expression of the downstream effector genes IRF-1 and 561 remains unaffected upon IFN treatment and may contribute to host defense mechanism. (C) 2001 Academic Press.