| At present, studies on relationship between the human diseases and cell signal transduction are very popular. The intracellular signal transduction may play an important role in disclosing the nature of disease. The pathogenic effects of a number of viruses result from the disturbance of intracellular signal cascades caused by viral proteins. Protein-protein interaction of viral antigens result in modulation of the biological activities of cellular proteins.Hepatitis C virus (HCV) is a main causative agent of human severe liver disorders worldwide. It was reported that chronic HCV infection occured in about 3% of the world's population. Moreover, HCV infection leads to more than 50% of liver diseases such as chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma. Treatment with interferon is the only available specific therapy for HCV infection, but most HCV isolates are resistant. Since the absence of efficient cell culture systems and appropriate animal model for HCV replication, the molecular mechanisms of HCV persistence and the pathogenesis of hepatitis C remain unclear. HCV infection is also associated with lymphoproliferative disorders which include autoantibody production, immune-mediated diseases such as non-Hodgkin's lymphoma and cryoglobulinemia. We assumed that the pathogenic effects of HCV result from the disturbance of intracellular signal cascades caused by HCV proteins. Although the mechanism by which HCV enters host cells is unknown, the second envelope protein (E2) is thought to be responsible for initiating virus attachment to a receptor on host cells. That is to say, HCV E2 protein possibly mediates viral entry into target cells. Human CD81, a putative cellular receptor for HCV, is a widely expressed cell-surface protein involved in variety of biologic responses such as adhesion, morphology, activation, proliferation and differentiation of cells. At the same time, HCV E2-CD81 interaction also may be important for the processing of viral proteins and intracellular virion formation. Mitogen-activated protein kinase (MAPK) is a serine-threonine kinase that performs important functions as mediators of cellular responses to a variety of extracellular stimuli. It was known that the interaction ofenvelope protein of a virus with a cellular receptor is a major event in determining the subsequent events in replication and the outcome of infections. The response of cells to extracellular signals is mediated by surface receptors and subsequent second messengers. Therefore, it is conceivable that relationship of HCV E2-CD81 and MAPK may be a mechanism which is involved in HCV pathogenesis. To test this hypothesis, effects and biological significance of HCV E2 protein on MAPK/ERK and p38MAPK pathways via human CD81 were investigated in this study.Studies on effects of intracellular MAPK/ERK pathway initiated by HCV E2 proteinTo compare the expression levels of CD81 on U937, Daudi, Molt-4, HepG2 and Huh-7 cells of human origin, cells were assessed for CD81 expression by FACS analysis using anti-CD81 MAb JS81. Human CD81 expression, independent of cell cycle, was significantly high in Huh-7, HepG2 and Molt-4 cells, but not detected in U937 cells. So CD81 was expressed to different extents among of different cell types as follows: Huh-7＞HepG2＞Molt-4＞Daudi cells. Human U937, Molt-4 and HepG2 cells were treated with purified recombinant E2 protein from HCV subtype 1a expressed in CHO cells in different concentrations and time lengths, and the activation of extracellular signal-regulated kinase (MAPK/ERK, also called p44/42 MAPK) in the cells were assessed by Western blotting analysis. Phospho- MAPK/ERK antibody(Thr 202/Tyr 204)detects p42 and p44 MAP kinase only when catalytically activated by phosphorylation at Thr 202 and Tyr 204 threonine. The results showed that the phosphorylation of MAPK/ERK was detectable in the above cells. Stimulation of the cells with 1 mg/L HCV E2 protein for 15 minutes resulted in significant phosphorylation of MAPK/ERK. The dynamic change...
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