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Cell cycle attenuation by p120E4F is accompanied by increased mitotic dysfunction
1Department of Genetics, Duke University Medical Center, Durham, North Carolina 27710, USA. robert.rooney@duke.edu
Abstract:
In addition to their well-established roles at the G1-S checkpoint, recent reports support a role for universal cyclin-dependent kinase (CDK) inhibitors in the control of G2-M and suggest that their induction may stimulate the occurrence of endomitosis or polyploidy in a number of physiological settings. In this report, the stable expression of the p120E4F transcription factor, which attenuates G1-S progression by elevating p21WAF1 and p27KIP1 protein levels, was shown to also interfere with the regulation of G2-M and cytokinesis. Exponentially growing cultures of p120E4F-expressing fibroblast cell lines had reduced levels of CDC2 kinase activity, elevated levels of Cyclin B1 protein, and continuously generated a subpopulation of tetraploid cells and elevated numbers of multinucleated cells. Coexpression of activated Ras, which stimulates Cyclin D1 expression and G1-S-specific cyclin-CDK kinase activities, alleviated these effects without reducing p21WAF1 or p27KIP1 protein levels; p120E4F/ras-expressing cell lines contained reduced levels of Cyclin B1 protein, a restoration of Cyclin B-CDC2 kinase activity to control levels, and exhibited no increase of tetraploid or multinucleated cells. Interestingly, changes in the expression of Cyclin B1 and, to a lesser extent, CDC2 were primarily regulated by post-transcriptional mechanisms. The results indicate that mechanisms which moderately elevate CDK inhibitor levels can reduce CDC2 kinase activity to the point of impeding normal G2-M function and suggest that two molecular determinants commonly associated with the induction of polyploidy in a number of tissues, i.e., elevated levels of universal CDK inhibitors and sustained CDK2 kinase activity, may be solely sufficient to initiate endomitosis.
Insights
Elevated cyclin-dependent kinase (CDK) inhibitors can disrupt G2-M cell cycle progression and cytokinesis, leading to polyploidy. This study shows moderate CDK inhibitor increases impede G2-M function and may initiate endomitosis.
Area of Science:
- Cell Biology
- Molecular Biology
- Genetics
Background:
- Cyclin-dependent kinase (CDK) inhibitors are known regulators of the G1-S checkpoint.
- Emerging evidence suggests universal CDK inhibitors also play a role in G2-M checkpoint control.
- Their induction may contribute to endomitosis and polyploidy in various physiological contexts.
Purpose of the Study:
- To investigate the role of the p120E4F transcription factor in cell cycle regulation beyond G1-S.
- To determine the impact of p120E4F expression on G2-M progression and cytokinesis.
- To explore the relationship between CDK inhibitors, cell cycle progression, and polyploidy.
Main Methods:
- Stable expression of the p120E4F transcription factor in fibroblast cell lines.
- Analysis of CDC2 kinase activity and Cyclin B1 protein levels.
- Assessment of cell ploidy and multinucleation.
- Coexpression of activated Ras to modulate G1-S specific cyclin-CDK activities.
Main Results:
- p120E4F expression reduced CDC2 kinase activity and elevated Cyclin B1 protein levels.
- Fibroblast cell lines expressing p120E4F showed increased tetraploid and multinucleated cells.
- Coexpression with Ras alleviated these effects, restoring CDC2-Cyclin B activity and reducing polyploidy.
- Changes in Cyclin B1 and CDC2 levels were mainly post-transcriptional.
Conclusions:
- Moderate elevation of CDK inhibitors can impair G2-M function by reducing CDC2 kinase activity.
- Sustained CDK2 kinase activity and elevated CDK inhibitors may be sufficient to induce endomitosis.
- The findings highlight a potential mechanism for polyploidy development in physiological settings.