Cell cycle attenuation by p120E4F is accompanied by increased mitotic dysfunction.
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.
Rooney
R J
RJ
Department of Genetics, Duke University Medical Center, Durham, North Carolina 27710, USA. robert.rooney@duke.edu
D016475:3T3 Cells; D017504:Adenovirus E4 Proteins / Q000378:metabolism*; D000818:Animals; D015152:Blotting, Northern; D015153:Blotting, Western; D016203:CDC2 Protein Kinase / Q000378:metabolism; D002453:Cell Cycle; D002455:Cell Division; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism; D002469:Cell Separation; D019926:Cyclin B / Q000096:biosynthesis; D056744:Cyclin B1; D019938:Cyclin D1 / Q000378:metabolism; D004247:DNA / Q000378:metabolism; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005347:Fibroblasts / Q000378:metabolism; D005434:Flow Cytometry; D016195:G2 Phase; D051379:Mice; D008938:Mitosis; D010641:Phenotype; D011003:Ploidies; D011233:Precipitin Tests; D012333:RNA, Messenger / Q000378:metabolism; D012097:Repressor Proteins / Q000378:metabolism*; D013997:Time Factors; D018631:ras Proteins / Q000378:metabolism
D017504:Adenovirus E4 Proteins; C534700:Ccnb1 protein, mouse; D019926:Cyclin B; D056744:Cyclin B1; D004791:Enzyme Inhibitors; D012333:RNA, Messenger; D012097:Repressor Proteins; C411663:transcription factor p120(E4F); D019938:Cyclin D1; D004247:DNA; D016203:CDC2 Protein Kinase; D018631:ras Proteins