HBx protein modulates PI3K/Akt pathway to overcome genotoxic stress-induced destabilization of cyclin D1 and arrest of cell cycle

Growth arrest represents an innate barrier to carcinogenesis. DNA damage and replicational stress are known to induce growth arrest and apoptotic death to avert genomic instability and consequently carcinogenesis. In this study, working on the genotoxic stress induced by hydroxyurea and methylmethanesulfone, we observed a growth arrest at G1/S-phase that was mediated by destabilization of cyclin D1. The growth arrest was independent of the stability of cdc25A and preceded transcriptional up-regulation of p21(waf1). Cyclin D1 destabilization involved its phosphorylation by GSK-3 beta at threonine-286, since overexpression of the kinase-dead mutant of GSK-3 beta or cyclin D1(T286A) mutant conferred stability to cyclin D1. Further, overexpression of cyclin D1(T216A) also helped in bypassing G1/S phase growth arrest. We also observed a rapid inactivation of Akt/PKB kinase in the presence of hydroxyurea. Enforced expression of the constitutively active Akt or viral oncoprotein HBx (Hepatitis B virus X protein) was sufficient to overcome growth arrest, independent of ATR signaling and stabilized cyclin D1. Thus, the present work not only establishes cyclin D1 to be a novel mediator of genotoxic stress signaling, but also explains how a deregulated mitogenic signaling or a viral oncoprotein can help bypass growth arrest.