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Journal of Virology, September 2005, p. 11392-11402, Vol. 79, No. 17
0022-538X/05/$08.00+0     doi:10.1128/JVI.79.17.11392-11402.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Examination of the pRb-Dependent and pRb-Independent Functions of E7 In Vivo

Scott Balsitis,1 Fred Dick,2,3 Denis Lee,1 Linda Farrell,1 R. Katherine Hyde,4 Anne E. Griep,4 Nicholas Dyson,3 and Paul F. Lambert1*

McArdle Laboratory for Cancer Research, University of Wisconsin Medical School, Madison, Wisconsin,1 Department of Biochemistry, University of Western Ontario, London, Ontario, Canada,2 MGH Cancer Center, Charlestown, Massachusetts,3 Department of Anatomy, University of Wisconsin Medical School, Madison, Wisconsin4

Received 3 April 2005/ Accepted 10 June 2005

High-risk human papillomaviruses encode two oncogenes, E6 and E7, expressed in nearly all cervical cancers. Although E7 protein is best known for its ability to inactivate the retinoblastoma tumor suppressor protein, pRb, many other activities for E7 have been proposed in in vitro studies. Herein, we describe studies that allowed us to define unambiguously the pRb-dependent and -independent activities of E7 for the first time in vivo. In these studies, we crossed mice transgenic for human papillomavirus 16 E7 to knock-in mice genetically engineered to express a mutant form of pRb (pRb{Delta}LXCXE) that is selectively defective for binding E7. pRb inactivation was necessary for E7 to induce DNA synthesis and to overcome differentiation-dependent cell cycle withdrawal and DNA damage-induced cell cycle arrest. While most of E7's effects on epidermal differentiation were found to require pRb inactivation, a modest delay in terminal differentiation with resulting hyperplasia was observed in E7 mice on the Rb{Delta}LXCXE mutant background. E7-induced p21 upregulation was also pRb dependent, and genetic Rb inactivation was sufficient to reproduce this effect. While E7-mediated p21 induction was partially p53 dependent, neither p53 nor p21 induction by E7 required p19ARF. These data show that E7 upregulates the expression of p53 and p21 via pRb-dependent mechanisms distinct from the proposed p19-Mdm2 pathway. These results extend our appreciation of the importance of pRb as a relevant target for high-risk E7 oncoproteins.

* Corresponding author. Mailing address: McArdle Laboratory for Cancer Research, 1400 University Ave., Madison, WI 53706. Phone: (608) 262-8533. Fax: (608) 262-2824. E-mail: lambert{at}oncology.wisc.edu.

Journal of Virology, September 2005, p. 11392-11402, Vol. 79, No. 17
0022-538X/05/$08.00+0     doi:10.1128/JVI.79.17.11392-11402.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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