HPV-16 E6 oncoprotein induces mutations via p53-dependent and -independentpathways

The E6 oncoprotein of 'high risk' human papillomaviruses (HPV) promotes onc ogenesis by inactivating tumor suppressor protein p53 i.e. it binds to and enhances the degradation of p53. To study whether inactivation of p53 is so lely responsible for E6-induced oncogenesis, we constructed several plasmid vectors expressing wild-type (wt) or mutant (mt) E6 proteins. RKO cells th at express wt p53 were stably transfected with these plasmids and challenge d with DNA damaging agents. The level of p53 was significantly increased by DNA damaging agents in control cells and cells transfected with plasmids e xpressing mt EG that do not bind to p53. As expected, p53 did not increase in cells transfected with plasmids expressing mt E6 that do bind to p53. To investigate the oncogenic effect of these various E6 proteins, we determin ed the mutation frequency of the hprt locus in control cells and cells expr essing different E6 proteins. We found that cells expressing wt E6 and mt E 6 (capable or incapable of binding to p53) showed notable increases in the mutation frequency at hprt locus compared with that of control cells. The e levation of mutation frequency in cells expressing mt E6 was similar to tha t in cells expressing wt E6. These data indicate that EG-induced mutagenici ty is induced not only via p53 inactivation, but also via p53-independent p athways.