Autophagic Cell Death in Leukemia Cells Regulated by Ceramide and Sphingosine-1-Phosphate through Mammalian Target of Rapamycin.

Unlike conventional apoptosis, the mutually exclusive role of ceramide and sphingosine-1-phosphate (S1P) in autophagic cell death remains unclear in leukemia cells. Amino acids deprivation (AA(−))-induced autophagy and subsequent cell death of human leukemia HL-60 cells are preceded by the generation of intracellular ceramide with inhibition of mammalian target of rapamycin (mTOR) in a caspase-3-independent manner. S1P inhibits AA(−)- or N-acetylsphingosine (C2-ceramide)-induced autophagy through activation of an amino acids sensor mTOR, whereas C2-ceramide inhibits S1P-activated mTOR and overcomes inhibition of autophagy by S1P. Genetically activated mTOR inhibits AA(−)- or C2-ceramide-induced increases of autophagy and a mammalian homologue of yeast Apg8/Aut7, MAP-LC3. In contrast, overexpression of kinase-dead mTOR blocks S1P-induced inhibition of autophagy and LC3-II activation. We here suggest that in leukemia cells ceramide an S1P counteract as a novel mediator for autophagic cell death through regulation of mTOR-dependent MAP-LC3.