Downregulation of Bax mRNA expression and protein stability by the E6 protein of human papillomavirus 16

Previous studies have shown that human papillomavirus (HPV) 16 E6 inhibits apoptosis induced during terminal differentiation of primary human keratinocytes (PHKs) triggered by serum and calcium. E6 inhibition of apoptosis was accompanied with prolonged expression of Bcl-2 and reduced elevation of Bax levels. In the present study, the effect of E6 on Bax mRNA expression and protein stability was investigated. These studies indicate that stable E6 expression in differentiating keratinocytes reduced the steady-state levels of Bax mRNA and shortened the half-life of Bax protein. These results were confirmed in transiently transfected 293T cells where E6 degraded Bax in a dose-dependent manner. Bax degradation was also exhibited in Saos-2 cells that lack p53, indicating its p53 independence. E6 did not form complexes with Bax and did not induce Bax degradation in vitro under experimental conditions where p53 was degraded. Finally, E6 aa 120-132 were shown to be necessary for Bax destabilization and, more importantly, for abrogating the ability of Bax to induce cellular apoptosis, highlighting the functional consequences of the E6-induced alterations in Bax expression.