HCV core protein interferes with Smad1 and Smad3 dependent TGF-β signaling in HCC cells

TGF-β plays a pivotal role in cell signaling in liver. In healthy tissue and early stages of liver diesease, TGF-β signaling controls apoptosis, cell proliferation and differentiation, thereby acting as a tumor suppressor. In HCV-infected patients, TGF-β levels are upregulated. We investigated TGF-β signaling in Huh–7 cells stably overexpressing HCV core protein, or carrying the empty vector as a control. Transcriptional activity of members of the Smad signaling pathway was measured. In addition, we analyzed cell death dependent LDH release and cell proliferation by MTT assay. Expression of pro-apoptotic proteins was examined by RT-PCR. Expression of HCV core protein inhibits TGF-β-induced apoptosis, which is in concert with downregulation of pro-apoptotic markers p15, p21, and GADD45β. Reduced levels of Smad3 phosphorylation upon TGF-β treatment, leading to reduced transcriptional activity could explain these findings. Additionally, HCV inhibits Smad1 phosphorylation and Smad1 dependent transcriptional activity. Cell proliferation and expression of the fibrotic marker CTGF is highly induced by HCV core protein, but this effect is TGF-β signaling independent. These data suggest that HCV core protein inhibits canonical TGF-β signaling pathways, resulting in reduced TGF-β induced cell death and provides neoplastic hepatocytes a way to escape the TGF-β cytostatic response, thereby facilitating cancer progression.