Suppression of Bcl‐2 gene expression by sphingosine in the apoptosis of human leukemic HL‐60 cells during phorbol ester‐induced terminal differentiation

Abstract

Our recent studies have shown that intracellular levels of sphingosine, an endogenous PKC inhibitor, increase during apoptosis resulting from phorbol ester (PMA)-induced terminal differentiation of human myeloid leukemic HL-60 cells, and have suggested that sphingosine may function as an endogenous mediator of apoptosis in these cells [Ohta, et al. (1995) Cancer Res. 55, 691–697]. We report here that apoptosis induced by PMA, sphingosine, and N,N-dimethylsphingosine (DMS) was accompanied by a concomitant decrease of bcl-2 expression in both RNA and protein levels in HL-60 cells, while expression of bcl-X_L and bax mRNA did not change, and neither sphingosine nor DMS induced differentiation of HL-60 cells. In contrast, in apoptotic cells induced by pharmaceutical PKC inhibitors H7 or staurosporine, expression of bcl-2 did not change nor did the intracellular sphingosine concentration. These results suggest that sphingosine may function as an endogenous mediator of apoptotic signaling in PMA-induced terminal differentiation of HL-60 cells through bcl-2 down-regulation, probably independent from PKC inhibition.