APOPTOSIS IN ERYTHROID PROGENITORS DEPRIVED OF ERYTHROPOIETIN OCCURS DURING G(1)-PHASE AND S-PHASE OF THE CELL-CYCLE WITHOUT GROWTH ARREST OR STABILIZATION OF WILD-TYPE P53

Erythropoietin (Epo) inhibits apoptosis in murine proerythroblasts infected with the anemia-inducing strain of Friend virus (FVA cells). We have shown that the apoptotic process in FVA cell populations deprived of Epo is asynchronous as a result of a heterogeneity in Epo dependence among individual cells. Here we investigated whether apoptosis in FVA cells correlated with cell cycle phase or stabilization of p53 tumor suppressor protein. DNA analysis in nonapoptotic FVA. cell subpopulations cultured without Epo demonstrated little change in the percentages of cells in G(1), S, and G(2)/M phases over time. Analysis of the apoptotic subpopulation revealed high percentages of cells in G(1) and S, with few cells in G(2)/M at any time. When cells were sorted from G(1) and S phases prior to culture without Epo, apoptotic cells appeared at the same rate in both populations, indicating that no prior commitment step had occurred in either G(1) or S phase. Steady-state wild-type p53 protein levels were very low in FVA cells compared with control cell lines and did not accumulate in Epo-deprived cultures; however, p53 protein did accumulate when FVA cells were treated with the, DNA-damaging agent actinomycin D. These data indicate that erythroblast apoptosis caused by Epo deprivation (i) occurs throughout G(1) and S phases and does not require cell cycle arrest, (ii) does not have a commitment event related to cell cycle phase, and (iii) is not associated with conformational changes or stabilization of wild-type p53 protein.