Evidence that bcl-2 is the target of three photosensitizers that induce a rapid apoptotic response

We originally proposed that the subcellular target for one class of photose nsitizing agents was the mitochondrion. This classification was based on ef fects that occur within minutes of irradiation of photosensitized cells: ra pid loss of the mitochondrial membrane potential (Delta Psi (m)), release o f cytochrome c into the cytosol and activation of caspase-3. These effects were followed by the appearance of an apoptotic morphology within 30-90 min . Fluorescence localization studies on three sensitizers initially classifi ed as 'mitochondrial' revealed that these agents bind to a variety of intra cellular membranes. The earliest detectable effect of photodamage is the se lective loss of the antiapoptotic protein bcl-2 leaving the proapoptotic pr otein bax undamaged. Bcl-2 photodamage can be detected directly after irrad iation of cells at 10 degreesC. Subsequent warming of cultures to 37 degree sC results in loss of Delta Psi (m), release of cytochrome c and activation of caspase-3. The latter appears to amplify the other two effects. Based o n results reported here we propose that the apoptotic response to these pho tosensitizers is derived from selective photodamage to the antiapoptotic pr otein bcl-2 while leaving the proapoptotic protein bax unaffected.