Regulation of programmed death in erythroid progenitor cells by erythropoietin: Effects of calcium and of protein and RNA syntheses

Abstract

Erythropoietin (EPO) retards DNA breakdown characteristic of programmed cell death (apoptosis) and promotes survival in erythroid progenitor cells. The mechanism by which EPO inhibits programmed death is unknown. In the well-characterized model of glucocorticoid-treated thymocytes, activation of a Ca²⁺/Mg²⁺-sensitive endonuclease and new protein and RNA syntheses have been found necessary for apoptosis. We examined the effects of EPO on the free intracellular calcium ion concentration ([Ca²⁺]_i), and the roles of Ca²⁺ and RNA and protein syntheses on DNA cleavage in erythroid progenitor cells. The murine model of erythroid differentiation using Friend leukemia virus-infected proerythroblasts (FVA cells) was used. EPO did not affect the [Ca²⁺]_i in FVA cells. Decreasing [Ca²⁺]_i by extracellular Ca²⁺ chelation with EGTA facilitated DNA breakdown. Increasing [Ca²⁺]_i with the calcium ionophore 4-bromo-A23187 increased DNA cleavage; however, DNA fragments generated by high [Ca²⁺]_i were much larger than those seen in the absence of EPO or presence of EGTA. Increased [Ca²⁺]_i also inhibited DNA breakdown to small oligonucleosomal fragments characteristic of cells cultured without EPO. However no concentration of ionophore protected the high molecular weight DNA as did EPO. Cycloheximide inhibited DNA breakdown in a dose dependent manner in cultures lacking EPO, but two other protein synthesis inhibitors, pactamycin and puromycin, did not prevent DNA breakdown. Inhibition of RNA synthesis with actinomycin D did not prevent DNA breakdown. Cells with morphological characteristics similar to those reported in other cells undergoing programmed death accumulated in EPO-deprived cultures. These studies demonstrate that although DNA cleavage and morphological changes are common to apoptotic cells, the roles for Ca²⁺ and protein and RNA syntheses are not universal and suggest that apoptosis can be regulated by different biochemical mechanisms in different cell types.