PDT-induced apoptosis: what are the critical molecular targets

Early molecular damages have been studied in a series ofhuman tumor and rodent cell lines following photodynamic therapy (PDT) sensitized by the silicon phthalocyanine Pc 4. Pc 4 preferentially localizes in mitochondria, and upon photoirradiation, immediate photodamage to the anti-apoptotic oncoprotein Bcl-2 is observed. The loss ofthe native 26-kDa protein, as evidenced by western blot analysis, is accompanied by the generation of a 23 -kDa fragment from a small portion of the molecules as well as a variety ofhigher molecular weight protein bands indicative ofphotochemical crosslinking ofBcl-2 to itself, to (pro-apoptotic) homologs, or to other nearby proteins. The changes in Bcl-2 are apparent immediately upon exposure ofPc 4-loaded cells to activating red light, occur in the cold, and are not dependent upon caspase-3 or other proteases. Crosslinking is also observed for the intermediate filament protein vimentin. The results implicate Bcl-2 (and perhaps vimentin) as important molecular targets that lead to apoptosis in Pc 4-PDT-treated cells.