Hepatitis B virus X protein enhances Myc stability by inhibiting SCF Skp2 ubiquitin E3 ligase-mediated Myc ubiquitination and contributes to oncogenesis

The stability of Myc oncoprotein is regulated by Skp/cullin/F-box (SCF) ubiquitin E3 ligase, in particular, via either SCF Skp2 or SCF Fbw7 ubiquitin E3 ligases. An earlier study has shown that hepatitis B virus X protein (HBx) augments Myc stability via a mechanism involving the inhibition of Myc ubiquitination. However, the underlying mechanism by which HBx inhibits Myc ubiquitination remained to be elucidated. Moreover, to what extent HBx-mediated Myc stabilization contributes to viral oncogenesis is unknown. First, we corroborated the physiological significance of HBx-mediated Myc stabilization in HBV-replicating cells by demonstrating that (1) the elevation of Myc level in a HBV-replicating HepG2.2.15 cell compared to parental cells; (2) HBx-mediated Myc stabilization in a HBV replicon transfected cells; and (3) the inhibition of Myc ubiquitination by HBx in a HBV replicon transfected cells. Then, the molecular interaction between HBx and Myc protein was revealed via coimmunoprecipitation and immunofluorescence experiments. Subsequent analysis indicated that HBx stabilizes Myc oncoprotein by blocking Skp2, as opposed to Fbw7, -mediated Myc ubiquitination. Next, we defined the Myc-binding region to four residues (VFVL) near the C-terminus of HBx polypeptide. An HBx variant with mutated VFVL consistently failed to not only interact with Myc but also suppress Myc ubiquitination. Importantly, the VFVL mutant lost the ability to transform NIH3T3 cells in concert with Ras, implying that the HBx-Myc interaction is critical for viral oncogenesis. Consistently, immunohistochemistry of liver biopsies revealed that Myc protein is elevated in HBV-infected tissues. We concluded that HBx stabilizes Myc oncoprotein by inhibiting SCF Skp2 ubiquitin E3 ligase-mediated Myc ubiquitination, and the HBx-mediated Myc stabilization greatly contributes to viral oncogenesis.