Hepatitis C viral proteins interact with Smad3 and differentially regulate TGF-β/Smad3-mediated transcriptional activation

Transforming growth factor- β (TGF- β ) is a pleiotropic cytokine implicated as a pathogenic mediator in various liver diseases. Enhanced TGF- β production and lack of TGF- β responses are often observed during hepatitis C virus (HCV) infection. In this study, we demonstrate that TGF- β -mediated transactivation is decreased in cells exogenously expressing the intact HCV polyprotein. Among 10 viral products of HCV, only core and nonstructural protein 3 (NS3) physically interact with the MH1 (Mad homology 1) region of the Smad3 and block TGF- β /Smad3-mediated transcriptional activation through interference with the DNA-binding ability of Smad3, not the nuclear translocation. However, the interactive domain of NS3 extends to the MH2 (Mad homology 2) region of Smad3 and a distinction is found between effects mediated, respectively, by these two viral proteins. HCV core, in the presence or absence of TGF- β , has a stronger suppressive effect on the DNA-binding and transactivation ability of Smad3 than NS3. Although HCV core, NS3, and the HCV subgenomic replicon all attenuate TGF- β /Smad3-mediated apoptosis, only HCV core represses TGF- β -induced G1 phase arrest through downregulation of the TGF- β -induced p21 promoter activation. Along with this, HCV core, rather than NS3, exhibits a significant inhibitory effect on the binding of Smad3/Sp1 complex to the proximal p21 promoter in response to TGF- β . In conclusion, HCV viral proteins interact with the TGF- β signaling mediator Smad3 and differentially impair TGF- β /Smad3-mediated transactivation and growth inhibition. This functional counteraction of TGF- β responses provides insights into possible mechanisms, whereby the HCV oncogenic proteins antagonize the host defenses during hepatocarcinogenesis.