Changes in p34cdc2 Kinase Activity and Cyclin A during Induced Differentiation of

Hexamethylene bisacetamide (HMBA)-induced murine erythroleukemia (MELC) differentiation is characterized by a prolongation of the initial C1 which follows passage through S phase in the presence of inducer. Commitment to terminal cell division is first detected in a portion of the cell population during this prolonged C1. HMBA-induced commitment is stochastic. This study has examined changes in two known cell cycle regulators, p34cdc2 and cyclin A, in cycle-synchronized MELC in the absence and presence of HMBA. Histone H 1 kinase activity of p34CdC2, and the levels of CDC2Mm mRNA, 1.8-kilobase mRNA of cyclin A, and cyclin A protein changed during cell cycle progression in MELC, and all of them were suppressed during C1. The suppression of the Hi kinase activity and cyclin A expression continued through the prolonged C1 in MEIC cultured with HMBA, whereas p342 protein level did not vary through the cell cycle in MELC cultured without or with inducer. Phosphorylation of p34CdC2 in uninduced MELC gradually increased as cells progressed from C1 to S. In induced MELC, an increase in phosphorylation of p342 occurred during the prolonged C1, and prior to the exit of the bulk of the cells from C1 to S. These results suggest that in HMBA-induced MELC, p34CdC2 phosphorylation per se is not a limiting factor in determining C1 to S progression. The persistent suppression of cyclin A expression and histone Hi kinase activity may play a role in HMBA-induced commitment to terminal differentiation