Intrahepatic levels of the inducible nitric oxide synthase (iNOS) are increased in chronic hepatitis C patients. As iNOS gene promoter contains Nuclear Factor (NF)-kappaB binding sites and hepatitis C virus (HCV) core protein activates NF-kappaB, the aim of this work was to study if HCV core protein transactivates iNOS promoter through NF-kappaB activation. iNOS mRNA and protein were determined by RT-PCR and western blot in HepG2 cells. The effect of HCV core protein on iNOS promoter was assayed by cotransfecting HepG2 cells with the core protein expression plasmid pHCV-Co and p1iNOS-CAT or p2iNOS-CAT plasmids. Formation of NF-kappaB-DNA complexes was determined by electrophoretic mobility shift assay. Transfection of HepG2 cells with pHCV-Co plasmid results in an increase in iNOS mRNA and protein levels. Cotransfection with pHCV-Co and p1iNOS-CAT or p2iNOS-CAT plasmids results in a transactivation of iNOS promoter, the presence of the proximal NF-kappaB binding site in the promoter being sufficient for the transactivation. Furthermore, the HCV core protein increases the formation of complexes between NF-kappaB and its binding sequence in the iNOS promoter. The expression of the NF-kappaB inhibitor IKB reverts the effect of the HCV core protein on the iNOS promoter. In conclusion, HCV core protein transactivates iNOS gene promoter through NF-kappaB activation.