Assessing autophagy in the context of photodynamic therapy

AbstractPhotodynamic therapy (PDT) is a procedure that has applications in the selective eradication of neoplasia where sites of malignant lesions are clearly delineated. It is a two-step process whereby cells are first sensitized to light and then photoirradiated. This results in the formation of singlet molecular oxygen and other reactive oxygen species that can cause photodamage at sites where the photosensitizing agent has localized. Photosensitizers found to be clinically useful show affinity for the endoplasmic reticulum (ER), mitochondria, lysosomes, or combinations of these sites. The induction of apoptosis and/or autophagy in photosensitized cells is a common outcome of PDT. This report explores the following issues: (1) Does the induction of autophagy in PDT protocols occur independent of, or in association with apoptosis? (2) Does the resulting autophagy play a pro-survival or pro-death role? (3) Do photosensitizers damage/inactivate specific proteins that are components of, or that modulate the autophagic process? Lastly, (4) can an autophagic response be mounted in cells in which lysosomes are specifically photodamaged? In brief, autophagy can occur independent of apoptosis in PDT protocols, and appears to play a pro-survival role in apoptosis competent cells, and a pro-death role in apoptosis incompetent cells. Mitochondrial and ER-localized sensitizers cause selective photodamage to some (i.e., Bcl-2, Bcl-xL, mTOR) proteins involved in the apoptotic/autophagic process. Finally, an aborted autophagic response occurs in cells with photodamaged lysosomes. Whereas autophagosomes form, digestion of their cargo is compromised because of the absence of functional lysosomes.