Bovine papillomavirus E2 protein activates a complex growth-inhibitory program in p53-negative HT-3 cervical carcinoma cells that includes repressionof cyclin A and cdc25A phosphatase genes and accumulation of hypophosphorylated retinoblastoma protein

hydroxyurea and mimosine, Based on these results and the known properties o f cell cycle components, we propose a model to account for EP-induced growt h inhibition of cervical carcinoma cell lines. The bovine papillomavirus E2 protein can inhibit the proliferation of HT-3 cells, a p53-negative cervic al carcinoma cell line containing integrated human papillomavirus type 30 D NA, Here, we analyzed HT-3 cells to explore the mechanism of p53-independen t E2-mediated growth inhibition. Expression of the E2 protein repressed exp ression of the endogenous human papillomavirus type 30 E6/E7 genes. This wa s accompanied by hypophosphorylation and increased accumulation of p105(Rb) and repression of E2F1 expression. The E2 protein also caused reduced cycl in-dependent kinase (cdk) 2 activity, but this did not appear to be due to increased expression of cdk inhibitors, Rather, expression of cyclin A, whi ch regulates cdk2 activity, and the cdc25A and cdc25B phosphatases, which a re thought to activate cdk2, was significantly reduced at both the RNA and protein levels in response to E2 expression, The E2 protein reduced express ion of cdc25A and cdc25B in both HT-3 and HeLa cells, but not in cells that were not growth-inhibited by the E2 protein. E2 point mutants unable to in hibit cell growth did not repress cdc25A and cdc25B expression, nor did the cell cycle inhibitors hydroxyurea and mimosine, Based on these results and the known properties of cell cycle components, we propose a model to accou nt for EP-induced growth inhibition of cervical carcinoma cell lines.