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 machinery that controls the progression of cells from a quiescent to a growing state contribute to the development of many human cancers. Such alterations include the deregulated expression of G₁ cyclins, the loss of function of activities such as those of protein p16^INK4a that control G₁ cyclin-dependent kinase activity, and the loss of function of the retinoblastoma protein (RB), which is normally regulated by the GI 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^INK4a is expressed in cervical cancer cell lines in which the RB gene, Rb, is not functional, either as a consequence of Rb mutation or expression 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 controlling E2F transcription factor activity, we investigated the role of E2F in controlling p16^INK4a expression. We found that E2F1 overexpression leads to an inhibition of cyclin D1-dependent kinase activity and induces the expression of a p16-related transcript. We conclude that the accumulation of G₁ cyclin-dependent kinase activity during normal G₁ progression leads to E2F accumulation through the inactivation of RB, and that this then leads to the induction of cyclin kinase inhibitor activity and a shutdown of G₁ kinase activity.