Hepatitis C virus (HCV) is the major causative agent of hepatocellular carcinoma. However, the precise mechanism underlying the carcinogenesis is yet to be elucidated. It has recently been reported that Syk, a non-receptor protein tyrosine kinase, functions as a potent tumour suppressor in human breast carcinoma. This study first examined the possible effect of HCV infection on expression of Syk in vivo. Immunohistochemical analysis revealed that endogenous Syk, which otherwise was expressed diffusely in the cytoplasm of normal hepatocytes, was localized near the cell membrane with a patchy pattern in HCV-infected hepatocytes. The possible interaction between HCV proteins and Syk in human hepatoma-derived Huh-7 cells was then examined. Immunoprecipitation analysis revealed that NS5A interacted strongly with Syk. Deletion-mutation analysis revealed that an N-terminal portion of NS5A (aa 1-175) was involved in the physical interaction with Syk. An in vitro kinase assay demonstrated that NS5A inhibited the enzymic activity of Syk and that, in addition to the N-terminal 175 residues, a central portion of NS5A (aa 237-302) was required for inhibition of Syk. Moreover, Syk-mediated phosphorylation of phospholipase C-c1 was downregulated by NS5A. An interaction of NS5A with Syk was also detected in Huh-7.5 cells harbouring an HCV RNA replicon or infected with HCV. In conclusion, these results demonstrated that NS5A interacts with Syk resulting in negative regulation of its kinase activity. The results indicate that NS5A may be involved in the carcinogenesis of hepatocytes through the suppression of Syk kinase activities.
INTRODUCTIONHepatitis C virus (HCV) is the major aetiological agent of viral hepatitis worldwide after hepatitis A and B viruses (Choo et al., 1989), with about 170 million people being infected. The majority of HCV-infected individuals develop chronic infection, which may progress to liver cirrhosis and hepatocellular carcinoma (HCC). HCV is a member of the family Flaviviridae and its genome consists of a single-stranded, positive-sense RNA of approximately 9600 nt, which encodes a polyprotein precursor of about 3010 aa. Currently, clinical HCV isolates are classified into six genotypes and more than 60 subtypes (Doi et al., 1996;Mellor et al., 1995;Robertson et al., 1998). The polyprotein is cleaved by signal peptidase, signal peptide peptidase and two virally encoded proteases to generate at least ten mature proteins: core, envelope glycoprotein 1 (E1), E2, p7, non-structural protein 2 (NS2), NS3, NS4A, NS4B, NS5A and NS5B Reed & Rice, 2000).HCV NS5A is part of the replication complex that catalyses replication of the viral genome. NS5A takes two forms, p56 and p58, with different degrees of phosphorylation, which may play distinct roles in the virus replication cycle (Evans 3These authors contributed equally to this work. YXX(L/I)X 6-8 YXX(L/I)] in the cytoplasmic tail of the Fc receptor c-chain or B-cell receptor subunit Iga to be activated after the engagement of immune receptors (Kurosaki et al...