title: Ubiquitin-dependent degradation of cyclin B is accelerated in polyploid megakaryocytes.
abstract: During the endomitotic cell cycle of megakaryocytic cell lines, the levels of cyclin B1 and the activity of cyclin B1-dependent Cdc2 kinase, although detectable, are reduced as compared with megakaryocytes undergoing a mitotic cell cycle. The levels of cyclin A, however, are comparable during both cell cycles. The expression of cyclin B1 mRNA is also equivalent in proliferating and polyploidizing cells. In the current study, we found that the rate of cyclin B1 protein degradation is enhanced in polyploidizing megakaryocytes. This finding has led us to further investigate whether the ubiquitin-proteosome pathway responsible for cyclin B degradation is accelerated in these cells. Our data indicate that polyploidizing megakaryocytic cell lines nad primary bone marrow cells treated with the megakaryocyte proliferation- and ploidy-promoting factor, the c-Mpl ligand, display increased activities of the ubiquitin-proteosome pathway, which degrades cyclin B, as compared with proliferating megakaryocytic cell lines or diploid bone marrow cells, respectively. This degradation has all the hallmarks of a ubiquitin pathway, including the dependence on ATP, the appearance of high molecular weight conjugated forms of cyclin B, and inhibition of the proteolytic process by a mutated form of the ubiquitin-conjugating enzyme Ubc4. Our studies also indicate that the ability to degrade cyclin A is equivalent in both the mitotic and endomitotic cell cycles. The increased potential of polyploid megakaryocytes to degrade cyclin B may be part of the cellular programming that leads to aborted mitosis.
authors.affiliation: Department of Biochemistry, Whitaker Cardiovascular Institute, Boston University School of Medicine, Massachusetts 02118, USA.
authors.forename: Y
authors.initials: Y
authors.lastname: Zhang
authors.forename: Z
authors.initials: Z
authors.lastname: Wang
authors.forename: D X
authors.initials: DX
authors.lastname: Liu
authors.forename: M
authors.initials: M
authors.lastname: Pagano
authors.forename: K
authors.initials: K
authors.lastname: Ravid
mesh_terms: D001854:Bone Marrow Cells / Q000187:drug effects; D016203:CDC2 Protein Kinase / Q000378:metabolism; D002455:Cell Division; D002460:Cell Line; D019925:Cyclin A / Q000378:metabolism; D019926:Cyclin B / Q000378:metabolism*; D056744:Cyclin B1; D003546:Cysteine Endopeptidases / Q000378:metabolism; D006207:Half-Life; D006801:Humans; D008533:Megakaryocytes / Q000378:metabolism*; D008858:Microscopy, Phase-Contrast; D009097:Multienzyme Complexes / Q000378:metabolism; D011092:Polyethylene Glycols / Q000494:pharmacology; D011123:Polyploidy; D046988:Proteasome Endopeptidase Complex; D011994:Recombinant Proteins / Q000378:metabolism / Q000494:pharmacology; D013926:Thrombopoietin / Q000494:pharmacology; D014452:Ubiquitins / Q000378:metabolism*
chemical_list: C534699:CCNB1 protein, human; D019925:Cyclin A; D019926:Cyclin B; D056744:Cyclin B1; D009097:Multienzyme Complexes; D011994:Recombinant Proteins; D014452:Ubiquitins; C103117:polyethylene glycol-recombinant human megakaryocyte growth and development factor; D011092:Polyethylene Glycols; D013926:Thrombopoietin; D016203:CDC2 Protein Kinase; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex