The Outer Membrane-Translocator Protein Mediates Activation of the Mitochondrial Permeability Transition by Porphyrin-Based Photooxidative Stress

A prominent feature of successful photodynamic therapy (PDT) is the targeting of mitochondria, because these organelles are critical sites for initiating both necrotic and apoptotic cell death. Among the variety of structures identified as targets for PDT, the outer membrane (OMM)- bound translocator protein, TSPO (formerly known as the peripheral benzodiazepine receptor), is of particular interest because it binds photosensitizers such as dicarboxylic porphyrins with nanomolar affinity and it is present at elevated levels in cancer cells. TSPO has also been postulated to be a component of the mitochondrial permeability transition (PT) pore (PTP), a protein channel that opens in the inner membrane (IMM) under the action of various stimuli, mainly matrix Ca2+ overload and oxidative stress, leading the cell toward death. In this study, photooxidation experiments with porphyrins indicated a strict correlation between porphyrin affinity for TSPO and the extent of mitochondrial photosensitization; moreover, they revealed a dual role of TSPO: (1) as a transport protein that facilitates diffusion of porphyrins into IMM, and (2) as a PTP regulatory protein when it binds porphyrins at its selective sites. Only photoactivation of TSPO-bound porphyrins leads to the opening of the PTP and mitochondrial dysfunction, whereas photoactivation of IMM-bound porphyrins maintains the PTP in the closed state