Hypoxia induces apoptosis in human neuroblastoma SK‐N‐MC cells by caspase activation accompanying cytochrome c release from mitochondria

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Abstract
We have attempted to elucidate the mechanism of apoptotic cell death induced by hypoxia (very low oxygen conditions) in neuronal cells. Human neuroblastoma SK‐N‐MC cells under hypoxic conditions resulted in apoptosis in a time‐dependent manner estimated by DNA fragmentation assay and nuclear morphology stained with fluorescent chromatin dye. Pretreatment with Z‐Asp‐CH2‐DCB, a caspase inhibitor, suppressed the DNA ladder in response to hypoxia in a concentration‐dependent manner. An increase in caspase‐3‐like protease (DEVDase) activity was observed during apoptosis, but no caspase‐1 activity (YVADase) was detected. To confirm the involvement of caspase‐3 during apoptosis, Western blot analysis was performed using anti‐caspase‐3 antibody. The 20‐ and 17‐kDa proteins, corresponding to the active products of caspase‐3, were generated in hypoxia‐challenged lysates in which processing of the full length form of caspase‐3 was evident. With a time course similar to this caspase‐3 activation, hypoxic stress caused the cleavage of PARP, yielding an 85‐kDa fragment typical of caspase activity. In addition, caspase‐2 was also activated by hypoxia, and the stress elicited the release of cytochrome c into the cytosol during apoptosis. These results suggest that caspase activation and cytochrome c release play roles in hypoxia‐induced neuronal apoptosis.