Apoptosis induced by doxorubicin in neurotumor cells is divorced from drug effects on ceramide accumulation and may involve cell cycle-dependent caspase activation

Doxorubicin (0.5 microgram/ml) induced caspase-dependent apoptosis in SH-SY5Y neuroblastoma and CHP-100 neuroepithelioma cells. The apoptotic response started to be evident approximately 15 h after drug administration and, as monitored over a 48-h period, was more pronounced in CHP-100 than in SH-SY5Y cells. In both systems, apoptosis was accompanied by elevation of intracellular ceramide levels. Ceramide accumulation was blocked by the ceramide synthase inhibitor fumonisin B(1) (25 microM); this compound, however, did not prevent drug-induced apoptosis. Untreated cells from both lines expressed negligible p53 levels; on the other hand, whereas p53 and p21(Cip1/Waf1) were rapidly up-regulated in doxorubicin-treated SH-SY5Y cells, such a response was not observed in CHP-100 cells. Doxorubicin induced a G(2)/M phase block in both cell lines, but whereas the G(1) phase was markedly depleted in CHP-100 cells, it was substantially retained in SH-SY5Y cells. In the latter system, double G(1) and G(2)/M block largely preceded cell death; however, as apoptosis underwent completion, it selectively targeted late S and G(2)/M cells. Moreover, apoptosis suppression by caspase inhibition did not result in a recovery of the G(1) cell population. These results support the notion that doxorubicin-induced apoptosis and ceramide elevation are divorced events in neuroectodermal tumors and that p53 function is at least dispensable for apoptosis completion. Indeed, as G(1) cells appear to be refractory to doxorubicin-induced apoptosis, p53 up-regulation and p21(Cip1/Waf1) expression may provide an unfavorable setting for the apoptotic action of the drug.