Phthalocyanine 4 photodynamic therapy-induced apoptosis of mouse L5178Y-R cells results from a delayed but extensive release of cytochrome c from mitochondria

Photodynamic therapy (PDT) activates the mitochondrial pathway of apoptosis, for which the release of cytochrome c into the cytosol is considered critical. To further elucidate the role of cytochrome c release in PDT-induced apoptosis, we monitored cytochrome c localization immunocytochemically and related it to nuclear apoptosis of the same cells. When mouse L5178Y-R cells were treated with 300 nM phthalocyanine (Pc) 4 and 0–75 mJ/cm2 red light, cytochrome c release had a dose response similar to that of clonogenic cell killing, with nearly identical threshold doses. Within individual cells, the release of cytochrome c appeared to be an all-or-none phenomenon. Moreover, it was tightly associated with activation of a caspase-3-like protease and changes in nuclear morphology. Thus, in response to Pc 4-PDT, the release of cytochrome c from mitochondria is a key determinant of apoptotic cell death.