Stabilization and functional impairment of the tumor suppressor p53 by the human papillomavirus type 16 E7 oncoprotein.

The p53 tumor suppressor is stabilized in cells expressing the human papillomavirus type 16 (HPV-16) E7 oncoprotein. In contrast, expression of the HPV-16 E6 protein inactivates p53 by targeting it for proteasomal degradation. Since p53 activation is associated with protein accumulation we investigated the biochemical mechanisms and biological consequences of p53 stabilization in HPV-16 E7-expressing cells. Transcriptional reporter assays, expression profiling studies using cDNA arrays, and immunoblot analyses of known p53 target genes suggest that p53 remains transcriptionally inert in E7-expressing cells. The stabilized p53 in E7-expressing cells is in a wild-type conformation and the same number of phospho-forms is present. Furthermore, E7 expression does not alter p53 localization or generally block nuclear export or proteasomal degradation of p53. Moreover, the stabilized p53 remains susceptible to mdm2-induced proteasome-mediated degradation, and exogenous transfected p53 is transcriptionally active in E7-expressing cells. Taken together, these results suggest that E7 can interfere with the normal turnover of p53 but that the resulting increase of p53 has no detectable transcriptional consequences on the p53 targets that we investigated.