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Inhibition of cyclin D-CDK4/CDK6 activity is associated with an E2F-mediated induction of cyclin kinase inhibitor
S N Khleif1, J DeGregori, C L Yee
1National Cancer Institute-Navy Medical Oncology Branch, Bethesda, MD 20889, USA.
Abstract:
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 p16INK4a that control G1 cyclin-dependent kinase activity, and the loss of function of the retinoblastoma protein (RB), which is normally regulated by the G1 cyclin-dependent kinases. Various studies have revealed an inverse relationship in the expression of p16INK4a protein and the presence of functional RB in many cell lines. In this study we show that p16INK4a 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 p16INK4a 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 p16INK4a 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.
Insights
Cell cycle regulators like p16INK4a and retinoblastoma protein (RB) are crucial in cancer development. This study reveals how RB inactivation by viruses or mutations increases p16INK4a expression, impacting cell growth.
Area of Science:
- Cellular biology
- Cancer research
- Molecular oncology
Background:
- Cell cycle deregulation, including G1 cyclins and inhibitors like p16INK4a, is a hallmark of cancer.
- The retinoblastoma protein (RB) is a key regulator of the G1 phase, controlled by G1 cyclin-dependent kinases.
- Previous studies suggest an inverse relationship between p16INK4a expression and functional RB.
Purpose of the Study:
- To investigate the expression of p16INK4a in cervical cancer cell lines with non-functional RB.
- To examine the effect of DNA tumor virus proteins on RB inactivation and p16INK4a levels.
- To elucidate the role of the E2F transcription factor in regulating p16INK4a expression.
Main Methods:
- Analysis of p16INK4a expression in cervical cancer cell lines with Rb mutations or HPV E7 protein.
- Assessment of p16INK4a levels in primary cells with RB inactivated by viral proteins.
- Investigating E2F1's role in controlling cyclin D1-dependent kinase activity and p16-related transcript expression.
Main Results:
- p16INK4a is expressed in cervical cancer cell lines lacking functional RB due to mutation or HPV E7.
- RB inactivation by viral proteins leads to increased p16INK4a levels in primary cells.
- E2F1 overexpression inhibits cyclin D1-dependent kinase activity and induces p16-related transcripts.
Conclusions:
- Cell cycle progression involves G1 cyclin-dependent kinase activity accumulation, leading to RB inactivation and E2F accumulation.
- This E2F accumulation subsequently induces cyclin kinase inhibitor activity, shutting down G1 kinase activity.
- Understanding this regulatory loop provides insights into cancer development and potential therapeutic targets.