Phosphatidylserine externalization during differentiation-triggered apoptosis of erythroleukemic cells

K562 erythroleukemia cells undergo apoptosis when induced to differentiate along the erythroid lineage with hemin. This event, characterized by DNA fragmentation, correlated with downregulation of the survival protein, BCL-x(L), and decrease in mitochondrial transmembrane potential (ΔΨ(m)) that ultimately resulted in cell death. Reorientation of phosphatidyl-serine (PS) from the cells inner-to-outer plasma membrane leaflet and inhibition of the aminophospholipid translocase was observed upon hemin-treatment. Constitutive expression of BCL-2 did not inhibit hemin-induced alterations in lipid asymmetry or decrease in ΔΨ(m), and only moderately prevented DNA fragmentation. BCL-2, on the other hand, effectively inhibited actinomycin D induced DNA fragmentation, the appearance of PS at the cells outer leaflet and the decrease in ΔΨ(m). The caspase inhibitor, z.VAD.fmk, blocked DNA fragmentation by both hemin and actinomycin D, but inhibited PS externalization only in the actinomycin D-treated cells. These results suggest that, unlike pharmacologically-induced apoptosis, PS externalization triggered by differentiation-induced apoptosis occurs by a mechanism that is associated with a decrease in ΔΨ(m), but independent of BCL-2 and caspases.