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.