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

Abstract

Transforming growth factor-b (TGF-b) is a pleiotropic cytokine implicated as a pathogenic mediator in various liver diseases. Enhanced TGF-b production and lack of TGF-b responses are often observed during hepatitis C virus (HCV) infection. In this study, we demonstrate that TGF-b-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-b/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 TGFb, 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-b/Smad3-mediated apoptosis, only HCV core represses TGF-b-induced G1 phase arrest through downregulation of the TGF-b-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-b. In conclusion, HCV viral proteins interact with the TGF-b signaling mediator Smad3 and differentially impair TGF-b/Smad3-mediated transactivation and growth inhibition. This functional counteraction of TGF-b responses provides insights into possible mechanisms, whereby the HCV oncogenic proteins antagonize the host defenses during hepatocarcinogenesis.

Kaposi's Sarcoma-Associated Herpesvirus-Encoded MicroRNA miR-K12-11 Attenuates Transforming Growth Factor Beta Signaling through Suppression of SMAD5

“…The effect of oncogenic viral infection on TGF-␤ signaling has been described for several other viruses, including human T-cell lymphotropic virus type 1 (HTLV-1), adenovirus, human papillomavirus (HPV), hepatitis C virus (HCV), Marek's disease virus type 1 (MDV1), and Epstein-Barr virus (EBV) (1,9,18,19,29,30,33,41,46). In HTLV-1-infected adult T-cell leukemia (ATL) cells, the oncoprotein Tax induced the production of TGF-␤1, which is an inhibitor of T-cell proliferation and cytotoxicity and confers resistance to TGF-␤1-induced growth inhibition through JNK/cJun constitutive activation (1,18,29).…”

HTLV-1 bZIP factor enhances TGF-β signaling through p300 coactivator

“…Examples include hepatitis B virus pX; hepatitis C virus core protein, NS3 and NS5; Kaposi sarcoma-associated herpesvirus K-bZIP; Epstein-Barr virus LMP1; and severe acute respiratory syndrome-associated coronavirus N protein. [39][40][41][42][43][44] Like HBZ, the HBV pX and severe acute respiratory syndrome N protein enhance the transcriptional responses of TGF-␤. The common strategy used by viruses to modulate TGF-␤ signaling is the direct binding of viral proteins to Smad proteins.…”

Dysregulation of growth factor signaling in human hepatocellular carcinoma

“…Smad2 mutations (o5%) and loss of Smad4 expression (10%) are infrequent molecular events (Yakicier et al, 1999;Longerich et al, 2004). Smad3 mutations have not been described in human hepatocarcinogenesis yet; however, recently published data clearly show a physical interaction between Smad3 and the HCV core protein, consequently antagonizing DNA-binding capacity of Smad3 and therefore reducing TGFb-dependent signaling (Cheng et al, 2004;Pavio et al, 2005). Furthermore, the inhibitory Smad7 has been found to be upregulated in about 60% of advanced HCCs, but not in dysplastic nodules and early HCCs (Park et al, 2004).…”