Bcl2 inhibits apoptosis associated with terminal differentiation of HL-60 myeloid leukemia cells

The Bcl2 protein inhibits apoptosis (programmed cell death) induced by a variety of noxious stimuli. However, relatively little is known about its effect on apoptosis that occurs after terminal differentiation. Bcl2 protein levels decrease during differentiation of myeloid cells into granulocytes that subsequently undergo apoptosis, but the potential role of Bcl2 in coupling survival and differentiation remains undefined. To ascertain the relationship between decreasing Bcl2 levels and the onset of apoptosis in differentiating myeloid cells, Bcl2 was hyperexpressed in the HL-60 cell line after retroviral gene transfer. After treatment of HL-60/BCL2 cells with all-trans retinoic acid or phorbol myristic acid, Bcl2 levels did not decrease as in normal HL-60 cells but, rather, increased because of activation of the viral promoter. Differentiation of the Bcl2-overexpressing cells was similar to that of normal HL-60 cells, but they showed little evidence for apoptosis and had a prolonged survival. These studies show that the survival-enhancing properties of Bcl2 counteract programmed cell death that accompanies terminal differentiation; however, Bcl2 has no significant effect on differentiation itself, suggesting that apoptosis and differentiation are regulated independently in myeloid cells.