Manipulating the onset of cell cycle withdrawal in differentiated erythroid cells with cyclin-dependent kinases and inhibitors

Terminal differentiation of erythroid cells results in terminal cell divisi ons followed by irreversible cell cycle withdrawal of hemoglobinized cells. The mechanisms leading to cell cycle withdrawal were assessed in stable tr ansfectants of murine erythroleukemia cells, in which the activities of cyc lin-dependent kinases (CDKs) and CDK inhibitors (CDKIs) could be tightly re gulated during differentiation. Cell cycle withdrawal of differentiating ce lls is mediated by induction of several CDKIs, thereby leading to inhibitio n of CDK2 and CDK4, Manipulation of CDK activity in differentiating cells d emonstrates that the onset of cell cycle withdrawal can be either greatly a ccelerated or greatly delayed without affecting hemoglobin levels. Extendin g the proliferation of differentiating cells requires the synergistic actio n of CDK2 and CDK4, Importantly, CDK6 cannot substitute for CDK4 in this ro le, which demonstrates that the 2 cyclin D-dependent kinases are functional ly different. The results show that differentiating hemoglobinized cells ca n be made to proliferate far beyond their normal capacity to divide. (C) 20 00 by The American Society of Hematology.