Nitric‐Oxide‐Induced Necrosis and Apoptosis in PC12 Cells Mediated by Mitochondria

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Abstract
Nitric oxide (NO) can trigger either necrotic or apoptotic cell death. We have used PC12 cells to investigate the extent to which NO‐induced cell death is mediated by mitochondria. Addition of NO donors, 1 mMS‐nitroso‐N‐acetyl‐DL‐penicillamine (SNAP) or 1 mMdiethylenetriamine‐NO adduct (NOC‐18), to PC12 cells resulted in a steady‐state level of 1‐3 μMNO, rapid and almost complete inhibition of cellular respiration (within 1 min), and a rapid decrease in mitochondrial membrane potential within the cells. A 24‐h incubation of PC12 cells with NO donors (SNAP or NOC‐18) or specific inhibitors of mitochondrial respiration (myxothiazol, rotenone, or azide), in the absence of glucose, caused total ATP depletion and resulted in 80‐100% necrosis. The presence of glucose almost completely prevented the decrease in ATP level and the increase in necrosis induced by the NO donors or mitochondrial inhibitors, suggesting that the NO‐induced necrosis in the absence of glucose was due to the inhibition of mitochondrial respiration and subsequent ATP depletion. However, in the presence of glucose, NO donors and mitochondrial inhibitors induced apoptosis of PC12 cells as determined by nuclear morphology. The presence of apoptotic cells was prevented completely by benzyloxycarbonyl‐Val‐Ala‐fluoromethyl ketone (a nonspecific caspase inhibitor), indicating that apoptosis was mediated by caspase activation. Indeed, both NO donors and mitochondrial inhibitors in PC12 cells caused the activation of caspase‐3‐ and caspase‐3‐processing‐like proteases. Caspase‐1 activity was not activated. Cyclosporin A (an inhibitor of the mitochondrial permeability transition pore) decreased the activity of caspase‐3‐ and caspase‐3‐processing‐like proteases after treatment with NO donors, but was not effective in the case of the mitochondrial inhibitors. The activation of caspases was accompanied by the release of cytochromecfrom mitochondria into the cytosol, which was partially prevented by cyclosporin A in the case of NO donors. These results indicate that NO donors (SNAP or NOC‐18) may trigger apoptosis in PC12 cells partially mediated by opening the mitochondrial permeability transition pores, release of cytochromec, and subsequent caspase activation. NO‐induced apoptosis is blocked completely in the absence of glucose, probably due to the lack of ATP. Our findings suggest that mitochondria may be involved in both types of cell death induced by NO donors: necrosis by respiratory inhibition and apoptosis by opening the permeability transition pore. Further, our results indicate that the mode of cell death (necrosis versus apoptosis) induced by either NO or mitochondrial inhibitors depends critically on the glycolytic capacity of the cell.