Prevalence of Necrosis in C2-Ceramide–Induced Cytotoxicity in NB16 Neuroblastoma Cells

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Abstract
The mechanism of cell death triggered by C2-ceramide was investigated using the NB16 neuroblastoma cell line. Treatment of NB16 cells with 20 μM C2-ceramide for 20 h resulted in approximately 75% loss of cell viability, but only 25% of cells were scored as apoptotic based on terminal deoxynucleotidyl transferase nick-end labeling. Ultrastructural analysis revealed early development of necrotic cytoplasmic vacuolization. After 20 h of treatment with C2-ceramide, the majority of cells possessed necrotic morphology with pronounced cytoplasmic vacuolization and without any nuclear changes, although a quarter of the cell population also exhibited clear perinuclear chromatin condensation characteristic of apoptosis. Flow cytometric analysis of cells labeled with both annexin V and propidium iodide showed the rapid accumulation of C2-ceramide–treated cells in the necrotic/late apoptotic fraction. In contrast, cells treated with tumor necrosis factor α plus cycloheximide (TNFα + CHX) first appeared in the early apoptotic fraction and then accumulated in the necrotic/late apoptotic fraction. Both C2-ceramide and TNFα + CHX increased caspase 8- and 3-like activities in cytosolic extracts; however, treatment of cells with the broad-spectrum caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone protected NB16 cells from TNFα + CHX-induced cell death but did not prevent C2-ceramide cytotoxicity. Although C2-ceramide triggered apoptosis in a fraction of the cells, cell death in the population was primarily caused by necrosis. Thus, C2-ceramide does not faithfully mimic the effects of apoptotic ligands such as TNFα, which are thought to be mediated by an accumulation of endogenous ceramide. The inhibition of phosphatidylcholine synthesis is a target for C2-ceramide–mediated cytotoxicity, and this work suggests that other agents that kill cells by inhibiting this pathway may also use a mixture of mechanisms, including necrosis as well as apoptosis.