Cell cycle attenuation by p120E4F is accompanied by increased mitotic dysfunction

In addition to their well-established roles at the G(1)-S checkpoint, recen t reports support a role for universal cyclin-dependent kinase (CDK) inhibi tors in the control of G(2)-M and suggest that their induction may stimulat e the occurrence of endomitosis or polyploidy in a number of physiological settings. In this report, the stable expression of the p120E4F transcriptio n factor, which attenuates G(1)-S progression by elevating p21(WAF1) and p2 7(KIP1) protein levels, was shown to also interfere with the regulation of G(2)-M and cytokinesis. Exponentially growing cultures of p120E4F-expressin g fibroblast cell lines had reduced levels of CDC2 kinase activity, elevate d levels of Cyclin B1 protein, and continuously generated a subpopulation o f tetraploid cells and elevated numbers of multinucleated cells. Coexpressi on of activated Ras, which stimulates Cyclin D1 expression and G(1)-S-speci fic cyclin-CDK kinase activities, alleviated these effects without reducing p21(WAF1) or p27(KIP1) protein levels; p120E4F/ras-expressing cell lines c ontained reduced levels of Cyclin B1 protein, a restoration of Cyclin B-CDC 2 kinase activity to control levels, and exhibited no increase of tetraploi d or multinucleated cells. Interestingly, changes in the expression of Cycl in B1 and, to a lesser extent, CDC2 were primarily regulated by posttranscr iptional mechanisms. The results indicate that mechanisms which moderately elevate CDK inhibitor levels can reduce CDC2 kinase activity to the point o f Impeding normal G(2)-M function and suggest that two molecular determinan ts commonly associated with the induction of polyploidy in a number of tiss ues, i.e., elevated levels of universal CDK inhibitors and sustained CDK2 k inase activity, may be solely sufficient to initiate endomitosis.