Deficiency in apoptotic effectors bax and bak reveals an autophagic cell death pathway initiated by photodamage to the endoplasmic reticulum

Efficient exploitation of cell death mechanisms for therapeutic purpose requires the identification of the molecular events committing cancer cells to death and the intracellular elements of the pro-death and pro-survival machinery activated in response to the anticancer therapy. Photodynamic therapy (PDT) is a paradigm of anticancer therapy utilizing the generation of reactive oxygen species to kill the cancer cells. In this study we have identified the photodamage to the sarco(endo)plasmic-reticulum Cat(2+)-ATPase (SERCA) pump and consequent loss in the ER-Ca2+ homeostasis as the most apical molecular events leading to cell death in hypericin-photosensitized cells. Downstream of the ER-Ca2+ emptying, both caspase-dependent and -independent pathways are activated to ensure cell demise. The induction of apoptosis as a cell death modality is dependent on the availability of proapopototic Box and Bak proteins, which are essential effectors of the mitochondrial outer membrane permeabilization (MOMP) and subsequent caspase activation. In Bax(-/-)/Bak(-/-) cells a nonapoptotic pathway dependent on sustained autophagy commits the oxidatively damaged cells to death. These results argue that the decision to die in this paradigm of oxidative stress is taken upstream of Bax-dependent MOMP and that the irreversible photodamage to the ER induced by hypericin-PDT acts as a trigger for an autophagic cell death pathway in apoptosis-deficient cells.