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.

We thank CM Rice and Apath (St Louis, MO, USA) for generously providing the Ava.5 cells and p90/HCVFLlongpU plasmid. We also thank L-H Hwang, R-H Chen, C-K Chou, K-H Lan, K Tokushige and K

Miyazono for providing plasmids used in this study. We are grateful to R Kirby for critical reading and comments on this manuscript. This work was supported by the following grants to Y-HW

Lee: NSC 89-2315-B-010-006-MH, NSC 89-2320-B-010-121, NSC 90-2320-B-010-077, NSC 91-2320-B-010-040, and NSC 92-2320-B-010-064 from National Science Council; and in part by grants

NHRI-EX91-9002BL, NHRI-EX92-9002BL, and NHRI-EX93-9002BL from the National Health Research Institute.

Institute of Biochemistry, National Yang-Ming University, Taipei, 112, Taiwan, Republic of China

Pei-Lin Cheng, Meng-Hsiung Chang, Chi-Hong Chao & Yan-Hwa Wu Lee

Anyone you share the following link with will be able to read this content: