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J. Virol., May 1997, 3710-3718, Vol 71, No. 5
ES Hickman, S Bates and KH Vousden
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.
Copyright © 1997, American Society for Microbiology
Perturbation of the p53 response by human papillomavirus type 16 E7
Frederick Cancer Research and Development Center, National Cancer Institute, Maryland 21702, USA.
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