C2-ceramide signaling in glioma cells: synergistic enhancement of CD95-mediated, caspase-dependent apoptosis

Most human malignant glioma cell lines are susceptible to CD95 ligand (CD95 L)-induced apoptosis, Here, we report that glioma cells are also susceptibl e to the cytotoxic effects of exogenous C2-ceramide. This form of cell deat h exhibits some morphological features of apoptosis as assessed by electron microscopy, but is unaffected by the broad spectrum caspase inhibitor, zVA D-fmk. Further, CD95L-induced apoptosis is synergistically enhanced by coex posure of the glioma cells to CD95L and CP-ceramide. CD95L-induced caspase 3-like activity, cytochrome c release and cleavage of caspases 3, 8, 9 and poly(ADP-ribose)polymerase (PARP) increase substantially after cotreatment with CD95L and CS-ceramide compared with CD95L treatment alone. None of the se events occur in response to cytotoxic concentrations of CS-ceramide alon e. C2-ceramide does not alter CD95 expression. Gene transfer-mediated enhan cement of CD95 expression results not only in increased susceptibility to C D95L, but also in increased sensitivity to CS-ceramide. We conclude that (i ) synergistic induction of apoptos is by CP-ceramide and CD95 L depend on a cross-talk between the two signal transduction pathways and that (ii) CP c eramide, independently of its sensitizing effects on CD95-dependent caspase activation, is also capable of triggering an apoptotic signaling cascade t hat is unaffected by zVAD-fmk-mediated caspase inhibition, but promoted by high levels of CD95 expression.