Photodamage to multiple Bcl-xL isoforms by photodynamic therapy with the phthalocyanine photosensitizer Pc 4

The antiapoptotic oncoprotein Bcl-2 is now a recognized phototarget of photodynamic therapy (PDT) with the phthalocyanine Pc 4 and with other mitochondrion-targeting photosensitizers. Photodamage, observed on Western blots as the loss of the native 26-kDa Bcl-2 protein, is PDT dose dependent and occurs in multiple cell lines, in the cold, and immediately upon photoirradiation. In our initial study, no photochemical damage was observed to Bcl-xL, in spite of its similarity in size, sequence, location and function to Bcl-2. The original study used a commercial anti-Bcl-xS/L antibody. We have revisited this issue by examining Western blots developed using one of three epitope-specific anti-Bcl-xL antibodies from commercial sources, a polyclonal antibody generated to the entire protein, as well as the antibody used previously. All five Bcl-xL antibodies recognized bacterially expressed Bcl-xL, but not Bcl-2, whereas an anti-Bcl-2 antibody recognized Bcl-2 and not Bcl-xL. All five Bcl-xL antibodies recognized at least one protein migrating at ∼30 kDa; two of the antibodies recognized an additional band, migrating at ∼33 or ∼24 kDa. We now observe Pc 4-PDT-induced photodamage to all Bcl-xL-related proteins, except the 33-kDa species, in several human cancer cell lines. The results indicate that, in addition to the expected quantitative differences that may reflect exposure of individual epitopes, the antibodies also detect proteins of different apparent molecular weights that may be distinct isoforms or post-translationally modified forms of Bcl-xL. No evidence for PDT-induced phosphorylation or degradation was observed. Bcl-xL localized to mitochondria was considerably more sensitive to photodamage than was Bcl-xL in the cytosol, indicating that as previously found for Bcl-2, Bcl-xL must be membrane localized to be photosensitive.