Phorbol ester-induced monocytic differentiation is associated with G2 delay and down-regulation of cdc25 expression.

Treatment of human U-937 myeloid leukemia cells with 12-O-tetradecanoylphorbol-13-acetate (TPA) is associated with growth arrest and induction of monocytic differentiation. The present results demonstrate that exposure of G1- or S phase-enriched U-937 cells to TPA was associated with delayed transition through G2-M and then exit from the cell cycle in the subsequent G0-G1 phase. In contrast, G2-M cells treated with TPA progressed through G1-S, were delayed in the next G2-M phase, and then entered a nonproliferative G0-G1 state. The effects of TPA on expression of cell cycle control genes demonstrated down-regulation of the cdc25 mitotic inducer during S-G2 progression. In order to investigate the role of cdc25 in the control of cell cycle progression, U-937 cells were stably transfected with an antisense cdc25 construct under control of a metallothionein-inducible promoter. Expression of this construct in G1 phase cells was associated with a significant delay in transition through S and G2-M phase. Similar results were obtained when G1 phase cells were incubated with a 14-base pair antisense cdc25 oligomer. Taken together, these results indicate that the cdc25 gene product is functionally associated with S-G2 transition of proliferating myeloid leukemia cells and that down-regulation of this gene by TPA is associated with G2-M delay.