Mitochondrial Cytochrome C- and Smac-Dependent Apoptosis in Cerebral Ischemia: Role of Oxidative Signaling

Mitochondria are known to be involved in the early stage of apoptosis by releasing cytochrome c, caspase-9 and second mitochondria-derived activator of caspases (Smac). We have reported that overexpression of copper/zinc superoxide dismutase (SOD1) reduced superoxide production and ameliorated neuronal injury in the hippocampal CA1 subregion after global ischemia. However, the role of oxygen-free radicals produced after ischemia/reperfusion in the mitochondrial signaling pathway has not been clarified. Five minutes of global ischemia was induced in male SOD1 transgenic (Tg) and wild-type (Wt) littermate rats. In the Wt animals, early superoxide production, mitochondrial release of cytochrome c, Smac and cleaved caspase-9 were observed after ischemia. Active caspase-3 was subsequently increased and 85% of the hippocampal CA1 neurons showed apoptotic DNA damage 3 days after ischemia. Tg animals showed less superoxide production and cytochrome c and Smac release. These results suggest that overexpression of SOD1 reduced oxidative stress, thereby attenuating the mitochondrial release of cytochrome c and Smac, resulting in less caspase activation and apoptotic cell death. Oxygen-free radicals may play a pivotal role in the mitochondrial signaling pathway of apoptotic cell death in the hippocampal CA1 neurons after global ischemia.