Down-regulation of ceramide production abrogates ionizing radiation-induced cytochrome c release and apoptosis

Previous work has demonstrated that down-regulation of ceramide production after selection of cells with N-oleoylethanolamine (OE), an inhibitor of ce ramidase, results in resistance to DNA damage-induced apoptosis. We report here that acute exposure of WEHI-231 cells (murine B-cell lymphoma) to OE a ctivates neutral sphingomyelinase, induces ceramide production and increase s intracellular reactive oxygen species. OE exposure also induces mitochond rial permeability, cytochrome c release, and apoptosis. Cells selected for resistance to OE exhibit little if any change in reactive oxygen species an d cytochrome c release when exposed either to OE or to toxic doses of ceram ide. Importantly, the OE resistant cells are also resistant to ionizing rad iation-induced cytochrome c release and apoptosis. These findings demonstra te that down-regulation of neutral sphingomyelinase activity is associated with decreased DNA-damage-induced apoptosis. In addition, the data suggests that agents that modify extranuclear targets responsible for ceramide prod uction select for cells resistant to ionizing radiation-induced apoptosis t hrough alterations in mitochondrial function.