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 Ca²⁺ homeostasis and cell death. Protection of SERCA2 protein rescues ER-Ca²⁺ levels and prevents cell death, suggesting that SERCA2 photodestruction with consequent incapability of the ER to maintain intracellular Ca²⁺ homeostasis is causal to cell killing. Apoptosis is rapidly initiated after ER-Ca²⁺ depletion and strictly requires the BAX/BAK gateway at the mitochondria. Bax^−/−Bak^−/− double-knockout (DKO) cells are protected from apoptosis but undergo autophagy-associated 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 Ca²⁺ 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