Sensitization to CD95 ligand-induced apoptosis in human glioma cells by hyperthermia involves enhanced cytochrome c release

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Abstract
CD95L-induced apoptosis involves caspase activation and is facilitated when RNA and protein synthesis are inhibited. Here, we report that hyperthermia sensitizes malignant glioma cells to CD95L- and APO2L-induced apoptosis in the absence, but not in the presence, of inhibitors of RNA and protein synthesis. Hyperthermia does not alter CD95 expression at the cell surface and does not modulate the morphology of CD95-mediated cell death on electron microscopy. Bcl-2 gene transfer inhibits apoptosis and abrogates the sensitization mediated by hyperthermia. Hyperthermia does not overcome resistance to apoptosis conferred by the viral caspase inhibitor, crm-A, indicating the absolute requirement for the activation of crm-A-sensitive caspases, probably caspase 8, for apoptosis. CD95L-evoked DEVD-amc-cleaving caspase activity is enhanced by hyperthermia, suggesting that hyperthermia operates upstream of caspase processing to promote apoptosis. There is no uniformly enhanced processing of three caspase 3 substrates, poly-ADP ribose polymerase (PARP), protein kinase C (PKC) delta and DNA fragmentation factor (DFF) 45. Yet, hyperthermia promotes CD95L-evoked DNA fragmentation. Interestingly, hyperthermia enhances the CD95L-evoked release of cytochrome c in the absence, but not in the presence, of CHX. In contrast, the reduction of the mitochondrial membrane potential is enhanced by hyperthermia both in the absence and presence of CHX, and enhanced cytochrome c release is not associated with significantly enhanced caspase 9 processing. The potentiation of cytochrome c release at hyperthermic conditions in the absence of CHX is abrogated by Bcl-2. Thus, either hyperthermia or inhibition of protein synthesis by CHX potentiate cytotoxic cytokine-induced apoptosis. These pathways show no synergy, but rather redundance, indicating that CHX may function to promote apoptosis in response to cytotoxic cytokines by inhibiting the synthesis of specific proteins whose synthesis, function or degradation is temperature-sensitive.