The cell cycle in polyploid megakaryocytes is associated with reduced activity of cyclin B1-dependent cdc2 kinase.
The platelet precursor, the megakaryocyte, matures to a polyploid cell as a result of DNA replication in the absence of mitosis (endomitosis). The factors controlling endomitosis are accessible to analysis in our megakaryocytic cell line, MegT, generated by targeted expression of temperature-sensitive simian virus 40 large T antigen to megakaryocytes of transgenic mice. We aimed to define whether endomitosis consists of a continuous phase of DNA synthesis (S) or of S phases interrupted by gaps. Analysis of the cell cycle in MegT cells revealed that, upon inactivation of large T antigen, the cells shifted from a mitotic cell cycle to an endomitotic cell cycle consisting of S/Gap phases. The level of the G1/S cyclin, cyclin A, as well as of the G1 phase cyclin, cyclin D3, were elevated at the onset of DNA synthesis, either in MegT cells undergoing a mitotic cell cycle or during endomitosis. In contrast, the level of the mitotic cyclin, cyclin B1, cycled in cells displaying a mitotic cell cycle while not detectable during endomitosis. Comparable levels of the mitotic kinase protein, Cdc2, were detected during the mitotic cell cycle or during endomitosis; however, cyclin B1-dependent Cdc2 kinase activity was largely abolished in the polyploid cells. Fibroblasts immortalized with the same heat-labile oncogene do not display reduced levels of cyclin B1 upon shifting to high temperature nor do they become polyploid, indicating that reduced levels of cyclin B1 is a property of megakaryocytes and not of the T-antigen mutant. We conclude that cellular programming during endoreduplication in megakaryocytes is associated with reduced levels of cyclin B1.
Zhang
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Y
Department of Biochemistry, Boston University School of Medicine, Boston, Massachusetts 02118, USA.
Wang
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Z
Ravid
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K
D000818:Animals; D000952:Antigens, Polyomavirus Transforming / Q000096:biosynthesis; D015153:Blotting, Western; D016203:CDC2 Protein Kinase / Q000378:metabolism*; D002448:Cell Adhesion; D002453:Cell Cycle; D002999:Clone Cells; D019926:Cyclin B; D056744:Cyclin B1; D016213:Cyclins / Q000096:biosynthesis*; D004247:DNA / Q000096:biosynthesis; D004261:DNA Replication; D016193:G1 Phase; D015870:Gene Expression; D008533:Megakaryocytes / Q000166:cytology* / Q000201:enzymology*; D051379:Mice; D008822:Mice, Transgenic; D011123:Polyploidy; D012333:RNA, Messenger / Q000096:biosynthesis; D011994:Recombinant Proteins / Q000096:biosynthesis; D016196:S Phase; D013539:Simian virus 40 / Q000235:genetics; D014162:Transfection
D000952:Antigens, Polyomavirus Transforming; C534700:Ccnb1 protein, mouse; D019926:Cyclin B; D056744:Cyclin B1; D016213:Cyclins; D012333:RNA, Messenger; D011994:Recombinant Proteins; D004247:DNA; D016203:CDC2 Protein Kinase