Abstract
BackgroundHepatitis C virus (HCV) encodes several proteins that interfere with the host cell antiviral response. Previously, the serine protease NS3/4A was shown to inhibit IFN-β gene expression by blocking dsRNA-activated retinoic acid-inducible gene I (RIG-I) and Toll-like receptor 3 (TLR3)-mediated signaling pathways.ResultsIn the present work, we systematically studied the effect of all HCV proteins on IFN gene expression. NS2 and NS3/4A inhibited IFN gene activation. NS3/4A inhibited the Sendai virus-induced expression of multiple IFN (IFN-α, IFN-β and IFN-λ1/IL-29) and chemokine (CCL5, CXCL8 and CXCL10) gene promoters. NS2 and NS3/4A, but not its proteolytically inactive form NS3/4A-S139A, were found to inhibit promoter activity induced by RIG-I or its adaptor protein Cardif (or IPS-1/MAVS/VISA). Both endogenous and transfected Cardif were proteolytically cleaved by NS3/4A but not by NS2 indicating different mechanisms of inhibition of host cell cytokine production by these HCV encoded proteases. Cardif also strongly colocalized with NS3/4A at the mitochondrial membrane, implicating the mitochondrial membrane as the site for proteolytic cleavage. In many experimental systems, IFN priming dramatically enhances RNA virus-induced IFN gene expression; pretreatment of HEK293 cells with IFN-α strongly enhanced RIG-I expression, but failed to protect Cardif from NS3/4A-mediated cleavage and failed to restore Sendai virus-induced IFN-β gene expression.ConclusionHCV NS2 and NS3/4A proteins were identified as potent inhibitors of cytokine gene expression suggesting an important role for HCV proteases in counteracting host cell antiviral response.
Highlights
Hepatitis C virus (HCV) encodes several proteins that interfere with the host cell antiviral response
Since many other HCV proteins are capable of interfering with host cell signalling pathways, we carried out a systematic analysis of all HCV proteins to determine their capacity to interfere with host cell signalling pathways regulating IFN gene expression
Expression plasmids encoding 11 HCV polypeptides were transfected into HEK293 cells together with IFN-β-Luc reporter plasmid; at 18 h after transfection, cells were infected with Sendai virus for 24 h, followed by preparation of cell lysates and measurement of luciferase activities (Fig. 1)
Summary
Hepatitis C virus (HCV) encodes several proteins that interfere with the host cell antiviral response. Virology Journal 2006, 3:66 http://www.virologyj.com/content/3/1/66 a polyprotein of about 3000 amino acids, which is cotranslationally and posttranslationally processed to mature proteins in the ER membrane. The core and envelope glycoproteins E1 and E2 form the structural proteins of the virion. Non-structural (NS) proteins NS2, NS3, NS4A, NS4B, NS5A and NS5B have important roles in the polyprotein processing and HCV replication [see for review [1]]. An alternative reading frame of the core region encodes for F protein, whose function is presently not known [2]. NS3 and NS4A proteins associate to form an active enzyme possessing RNA helicase and serine protease activities. NS3/4A has an ability to interfere with type I interferon (IFN) gene expression [3]
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