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 inf ected 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 dependen ce among individual cells. Here we investigated whether apoptosis in F VA cells correlated with cell cycle phase or stabilization of p53 tumo r suppressor protein. DNA analysis in nonapoptotic FVA. cell subpopula tions cultured without Epo demonstrated little change in the percentag es of cells in G(1), S, and G(2)/M phases over time. Analysis of the a poptotic 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 appeare d at the same rate in both populations, indicating that no prior commi tment step had occurred in either G(1) or S phase. Steady-state wild-t ype p53 protein levels were very low in FVA cells compared with contro l 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 ap optosis caused by Epo deprivation (i) occurs throughout G(1) and S pha ses and does not require cell cycle arrest, (ii) does not have a commi tment event related to cell cycle phase, and (iii) is not associated w ith conformational changes or stabilization of wild-type p53 protein.