Elevation of Cytoplasmic Cytochrome c in Radiation‐induced Apoptosis

The signal transduction pathway involved in radiation‐induced apoptosis remains unclear, especially as regards the site at which the primary effect of radiation occurs. In this study, we demonstrate that cytochrome c may be released from mitochondria into the cytosol after irradiation, but that direct irradiation of isolated mitochondria induces no elevation of cytochrome c release. These observations suggest that cytochrome c and mitochondria may be involved in the radiation‐induced apoptotic pathway, but mitochondria might not be a target of radiation, and that a signal transduction pathway from an unknown target might exist upstream of mitochondria during radiation‐induced apoptosis.