29th Annual Meeting of the American Society of Photobiology

Downtown Marriot

Chicago, Il.

Bcl-2: the target of 'mitochondrial' photodamage

Abstract-
We initially proposed that the sub-cellular target for a major group of photosensitizing agents was the mitochondrion. Sensitizers in this class include Photofrin, tin etiopurpurin (SnET2), a porphycene (CPO) and Foscan (mTHPC). This classification is based on several effects that occur within minutes of irradiation of photosensitized cells: a rapid loss of the mitochondrial membrane potential (_m), release of cytochrome c into the cytosol and activation of caspase-3. These events all occur within minutes after irradiation of photosensitized cells, and lead to the appearance of an apoptotic morphology within 30-90 min. More recent studies indicate a different sequence of events. A more sensitive fluorescence localization study revealed that these sensitizers do not localize in mitochondria, but are widely distributed among cytosolic membranes. Moreover, photodamage to the anti-apoptotic protein bcl-2 was detected directly after irradiation of photosensitized cells at 10^oC, while the pro-apoptotic protein bax was unaffected. Subsequent warming of the cell culture to 37^o resulted in loss of _m, release of cytochrome c, and activation of caspase-3. The latter appears to amplify the other two effects. Based on results reported here, we conclude that the apoptotic response to CPO, mTHPC and SnET2 is derived from bcl-2 photodamage, and that the initial target is not the mitochondrial membrane.

Keywords: photodynamic therapy, apoptosis, bcl-2, mitochondria