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 divisions followed by irreversible cell cycle withdrawal of hemoglobanized cells. The mechanisms leadlng to cell cycle withdrawal were assessed in stable transfectants of mutine erythroleukemia cells, in which the activiries of cyclin-dependent kinases (CDKs) and CDK inhibitors (CDKIs) could be tightly regulated during differentiation. Cell cycle withdrawal of differentiating cells is medlated by induction of several CDKIs, thereby leading to inhibition of CDK2 and CDK4. Manipulation of CDK activity in differentiating cells demonstrates that the onset of cell cycle withdrawal can be either greatly accelerated or greatly delayed without affecting hemoglobln levels. Extending the proliferation of differentiating cells requires the synergisfic action of CDK2 and CDK4. Importantly, CDK6 cannot substitute for CDK4 in this role, which demonstrates that the 2 cyclin D-dependent kinases are funcfionally different. The results show that differentiating hemoglobinized cells can be made to proliferate far beyond their normal capacity to divide. (C) 2000 by The Amerlcan Society of Hematology.