Perturbation of the p53 response by human papillomavirus type 16 E7.

The p53 tumor suppressor protein can induce both cell cycle arrest and apoptosis in DNA-damaged cells. In human carcinoma cell lines expressing wild-type p53, expression of E7 allowed the continuation of full cell cycle progression following DNA damage, indicating that E7 can overcome both G1 and G2 blocks imposed by p53. E7 does not interfere with the initial steps of the p53 response, however, and E7 expressing cells showed enhanced expression of p21(waf1/cip1) and reductions in cyclin E- and A-associated kinase activities following DNA damage. One function of cyclin-dependent kinases is to phosphorylate pRB and activate E2F, thus allowing entry into DNA synthesis. Although E7 may substitute for this activity during cell division by directly targeting pRB, continued cell cycle progression in E7-expressing cells was associated with phosphorylation of pRB, suggesting that E7 permits the retention of some cyclin-dependent kinase activity. One source of this activity may be the E7-associated kinase, which was not inhibited following DNA damage. Despite allowing cell cycle progression, E7 was unable to protect cells from p53-induced apoptosis, and the elevated apoptotic response seen in these cells correlated with the reduction of cyclin A-associated kinase activity. It is possible that inefficient cyclin A-dependent inactivation of E2F at the end of DNA synthesis contributes to the enhanced apoptosis displayed by E7-expressing cells.