Photosensitizer Activation Drives Apoptosis by Interorganellar Ca(2+) Transfer and Superoxide Production in Bystander Cancer Cells

In cells, photosensitizer (PS) activation by visible light irradiation triggers reactive oxygen species (ROS) formation, followed by a cascade of cellular responses involving calcium (Ca(2+)) and other second messengers, resulting in cell demise. Cytotoxic effects spread to nearby cells not exposed to light by poorly characterized so-called “bystander effects”. To elucidate the mechanisms involved in bystander cell death, we used both genetically encoded biosensors and fluorescent dyes. In particular, we monitored the kinetics of interorganellar Ca(2+) transfer and the production of mitochondrial superoxide anion (O(2)(−)∙) and hydrogen peroxide (H(2)O(2)) in irradiated and bystander B16-F10 mouse melanoma cancer cells. We determined that focal PS photoactivation in a single cell triggers Ca(2+) release from the endoplasmic reticulum (ER) also in the surrounding nonexposed cells, paralleled by mitochondrial Ca(2+) uptake. Efficient Ca(2+) efflux from the ER was required to promote mitochondrial O(2)(−)∙ production in these bystander cells. Our results support a key role for ER–mitochondria communication in the induction of ROS-mediated apoptosis in both direct and indirect photodynamical cancer cell killing.