Photochemical destruction of the Bcl-2 oncoprotein during photodynamic therapy with the phthalocyanine photosensitizer Pc 4

Photodynamic therapy (PDT), utilizing a photosensitizer and visible light, causes localized oxidative damage. With the mitochondrial photosensitizer P c 4, PDT induces apoptosis, yet its molecular targets are not known, Here, the anti-apoptotic protein Bcl-2 is shown to be highly sensitive to PDT, as judged on Western blots by the disappearance of anti-Bcl-2-reactive materi al from the position of the native 26 kDa protein. The loss of Bcl-2 was PD T dose dependent and was observed for both endogenous and overexpressed Bcl -2 in several cell lines, immediately after PDT, and with chilled cells. It was accompanied by a trace of a 23-kDa cleavage product as well as high-mo lecular weight products that may result from photochemical crosslinking. PD T-induced Bcl-2 loss occurred in MCF-7 cells that do not express caspase-3 or in the presence of protease inhibitors, but was prevented, along with th e induction of apoptosis, by the singlet oxygen scavenger L-histidine. Loss of FLAG-Bcl-2 was observed with both anti-FLAG and anti-Bcl-2 antibodies, indicating loss of native protein rather than simple BCL-2-epitope destruct ion. Photochemical damage was not observed in Bcl-x(L), Bax, Bad, the volta ge-dependent anion channel, or the adenine nucleotide translocator. Therefo re, Bcl-2 is one target of PDT with Pc 4, and PDT damage to Bcl-2 contribut es to its efficient induction of apoptosis.