Differential modulation of G1-S-phase cyclin-dependent kinase 2/cyclin complexes occurs during the acquisition of a polyploid DNA content.

Despite a growing understanding of the biochemical mechanisms controlling the cell cycle, information regarding the temporal ordering of S phase and M phase remains scarce. Polyploid cells represent a useful model for examining S- and M-phase control, because their cell cycle machinery must be modulated to retain high levels of DNA content (ploidy) within a single nucleus. To evaluate the mechanisms of S-phase control during the process of polyploidization, we investigated the modulations that occur in cyclin-dependent kinase (CDK) complexes during the induction of megakaryocyte differentiation in human erythroleukemia cells. We report that during polyploidization, megakaryocytic human erythroleukemia cells undergo a dramatic modulation in the subunit composition of G1-associated and S phase-associated CDK complexes and a marked increase in their specific activities. This, in turn, is facilitated by a differential loss of the p21 or p27 CDK-inhibitory protein/kinase-inhibitory proteins (CIP/KIP) bound to specific cyclin/CDK complexes. The data show that the loss of S- and M-phase control in polyploid cells occurs within the context of an up-regulated function in those CDK complexes associated with both G1-S-phase transit and S-phase progression. Additional studies regarding the regulation of these complex CDK interactions will be important to understand cell cycle control in such diverse processes as megakaryocyte differentiation or the types of genomic instability that occur in cancer cells.