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 crit ical. 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 cell s were treated with 300 nM phthalocyanine (Pc) 4 and 0-75 mJ/cm(2) red ligh t, cytochrome c release had a dose response similar to that of clonogenic c ell killing, with nearly identical threshold doses. Within individual cells , the release of cytochrome c appeared to be an all-or-none phenomenon. Mor eover, it was tightly associated with activation of a caspase-3-like protea se and changes in nuclear morphology. Thus, in response to Pc 4-PDT, the re lease of cytochrome c from mitochondria is a key determinant of apoptotic c ell death. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.