To clarify the effects of hepatitis C virus (HCV) infection on hepatocytes, we analyzed and compared the induction of intracellular signals by HCV and hepatitis B virus (HBV) proteins. We examined the influence of 7 HCV (core, NS2, NS3, NS4A, NS4B, NS5A, and NS5B) and 4 HBV (precore, core, polymerase, and X) proteins on 5 well-defined intracellular signaling pathways associated with cell proliferation, differentiation, and apoptosis by use of a reporter assay. Viral protein-expression vectors were cotransfected into mammalian cells with reporter vectors having a luciferase gene driven by the following inducible cis-enhancer elements: the cyclic adenosine monophosphate response element, the serum response element (SRE), and the binding sites for nuclear factor kappaB (NF-kappaB), activator protein 1 (AP-1), and serum response factor (SRF). In addition, the activation of signals by HCV proteins was examined in a reporter plasmid having a natural interleukin-8 (IL-8) promoter upstream of a luciferase gene. Of 11 HCV and HBV proteins, HCV core had the strongest influence on intracellular signals, especially NF-kappaB-, AP-1-, and SRE-associated pathways. HCV core's activation level exceeded that of HBV X protein, a well-characterized transactivator of these signals. Moreover, HCV core activated the IL-8 promoter through NF-kappaB and AP-1. For the other proteins, HCV NS4B showed signal activation, but signals were activated at a lesser extent. The luciferase reporter assay, a recently introduced technique, helped in the elucidation of molecular events underlying the inflammatory and proliferation process in the liver induced by HCV.