Role of endoplasmic reticulum depletion and multidomain proapoptotic BAX and BAK proteins in shaping cell death after hypericin‐mediated photodynamic therapy

Both the commitment event and the modality of cell death in photodynamic therapy (PDT) remain poorly defined.We report that PDT with endoplasmic reticulum (ER)-associating hypericin leads to an immediate loss of SERCA2 protein levels, causing disruption of Ca 2+ homeostasis and cell death.Protection of SERCA2 protein rescues ER-Ca 2+ levels and prevents cell death, suggesting that SERCA2 photodestruction with consequent incapability of the ER to maintain intracellular Ca 2+ homeostasis is causal to cell killing.Apoptosis is rapidly initiated after ER-Ca 2+ depletion and strictly requires the BAX/BAK gateway at the mitochondria.Bax -/-Bak -/-double-knockout (DKO) cells are protected from apoptosis but undergo autophagyassociated cell death as revealed by electron microscopy and biochemical analysis.Autophagy inhibitors, but not caspase antagonists, significantly reduce death of DKO cells, suggesting that sustained autophagy is lethal.Thus, following ER photodamage and consequent disruption of Ca 2+ homeostasis, BAX and BAK proteins model PDT-mediated cell killing, which is executed through apoptosis in their presence or via an autophagic pathway in their absence.Key words: mitochondria • reactive oxygen species • cancer therapy hotodynamic therapy (PDT) is an established and useful modality for the treatment of cancer.This therapy requires a photosensitizing agent (photosensitizer) selectively taken up by tumor cells, visible light, and molecular oxygen to generate reactive oxygen species (ROS), which ultimately cause tumor destruction (1).Hypericin, a powerful naturally occurring photosensitizer, is endowed with remarkable tumor localizing properties and in vivo antineoplastic activity upon irradiation, suggesting its clinical use as a promising PDT agent (2).Since apoptosis is a major in vivo response to hypericin-mediated PDT (3), modulation of the apoptotic vulnerability of the cancer cells is critical for the therapeutic efficacy of this drug.