Hepatitis B virus X protein stimulates cell growth by downregulating p16 levels via PA28γ-mediated proteasomal degradation

Proteasomal activator 28 gamma (PA28γ), an essential constituent of the 20S proteasome responsible for ubiquitin-independent degradation of target proteins, is frequently overexpressed in hepatocellular carcinoma. Recently, we have reported that hepatitis B virus (HBV) X protein (HBx) activates PA28γ expression in human hepatocytes via upregulation of p53 levels; however, its role in HBV tumorigenesis remains unknown. Here, we found that HBx-activated PA28γ downregulates p16 levels via ubiquitin-independent proteasomal degradation. As a result, HBx activated the Rb-E2F pathway and stimulated G1/S cell cycle progression, resulting in an increase in cell proliferation. The potential of HBx to induce these effects was reproduced in a 1.2-mer HBV replicon and in in vitro HBV infection systems and was almost completely abolished by either PA28γ knockdown or p16 overexpression, demonstrating the critical role of the PA28γ-mediated p16 degradation in HBV tumorigenesis.