Cytoprotective Effect of Glucosylceramide Synthase Inhibition against Daunorubicin-induced Apoptosis in Human Leukemic

Several studies have shown that ceramide (CER) glucosylation contributes to drug resistance in multidrugresistant cells and that inhibition of glucosylceramide synthase sensitizes cells to various drug treatments. However, the role of glucosylceramide synthase has not been studied in drug-sensitive cancer cells. We have demonstrated previously that the anthracycline daunorubicin (DNR) rapidly induces interphasic apoptosis through neutral sphingomyelinase-mediated CER generation in human leukemic cell lines. We now report that inhibition of glucosylceramide synthase using D,Lthreo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) or 1-phenyl-2-palmitoylamino-3-morpholino-1-propanol (PPMP) protected U937 and HL-60 cells from DNR-induced apoptosis. Moreover, blocking CER glucosylation did not lead to increased CER levels but to increased CER galactosylation. We also observed that pretreating cells with galactosylceramide (GalCER) significantly inhibited DNR-induced apoptosis. Finally, we show that GalCER-enriched lymphoblast cells (Krabbe’s disease) were significantly more resistant to DNRand cytosine arabinoside-induced apoptosis as compared with normal lymphoblasts, whereas glucosylceramideenriched cells (Gaucher’s disease) were more sensitive. In conclusion, this study suggests that sphingomyelinderived CER in itself is not a second messenger but rather a precursor of both an apoptosis second messenger (GD3) and an apoptosis “protector” (GalCER).