Different pathways mediate cytochrome c release after photodynamic therapywith hypericin

In this study we show that overexpression of Bcl-2 in PC60R1R2 cells reveal s a caspase-dependent mechanism of cytochrome c release following photodyna mic therapy (PDT) with hypericin. Bcl-2 overexpression remarkably delayed c ytochrome c release, procaspase-3 activation and poly(adenosine diphosphate -ribose)polymerase cleavage during PDT-induced apoptosis while it did not p rotect against PDT-induced necrosis. PDT-treated cells showed a reduction i n the mitochondrial membrane potential which occurred with similar kinetics in PC60R1R2 and PC60R1R2/Bcl-2 cells, and was affected neither by the perm eability transition pore inhibitor cyclosporin A nor by the caspase inhibit or N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (zVAD-fmk). Hypericin -induced mitochondrial depolarization coincided with cytochrome c release i n PC60R1R2 cells while it precedes massive cytochrome c efflux in PC60R1R2/ Bcl-2 cells. Preincubation of PC60R1R2 cells with zVAD-fmk or cyclosporin A did not prevent the mitochondrial efflux of cytochrome c, and caspase inhi bition only partially protected the cells from PDT-induced apoptosis. In co ntrast, in PC60R1R2/Bcl-2 cells cytochrome c release and apoptosis were sup pressed by addition of zVAD-fmk or cyclosporin A. These observations sugges t that the progression of the PDT-induced apoptotic process in Bcl-2-overex pressing cells involves a caspase-dependent feed-forward amplification loop for the release of cytochrome c.