INHIBITION OF CYCLIN D-CDK4 CDK6 ACTIVITY IS ASSOCIATED WITH AN E2F-MEDIATED INDUCTION OF CYCLIN KINASE INHIBITOR ACTIVITY/

Alterations of various components of the cell cycle regulatory machine ry that controls the progression of cells from a quiescent to a growin g state contribute to the development of many human cancers. Such alte rations include the deregulated expression of G(1) cyclins, the loss o f function of activities such as those of protein p16(INK4a) that cont rol G(1) cyclin-dependent kinase activity, and the loss of function of the retinoblastoma protein (RB),which is normally regulated by the G( 1) cyclin-dependent kinases. Various studies have revealed an inverse relationship in the expression of p16(INK4a) protein and the presence of functional RB in many cell lines. In this study we show that p16(IN K4a) is expressed in cervical cancer cell lines in which the RB gene, Rb, is not functional, either as a consequence of Rb mutation or expre ssion of the human papillomavirus E7 protein. We also demonstrate that p16(INK4a) levels are increased in primary cells in which RB has been inactivated by DNA tumor virus proteins. Given the role of RB in cont rolling E2F transcription factor activity, we investigated the role of E2F in controlling p16(INK4a) expression. We found that E2F1 overexpr ession leads to an inhibition of cyclin D1-dependent kinase activity a nd induces the expression of a p16-related transcript. We conclude tha t the accumulation of G(1) cyclin-dependent kinase activity during nor mal G(1) progression leads to E2F accumulation through the inactivatio n of RB, and that this then leads to the induction of cyclin kinase in hibitor activity and a shutdown of G(1) kinase activity.