Reactive oxidizing species produced near the plasma membrane induce apoptosis in bovine aorta endothelial cells

Many cytotoxic agents initiate apoptosis by generating reactive oxidizing species (ROS). The goal of this study was to determine whether apoptosis could be induced by initial reactions of ROS near the plasma membrane. Bovine aorta endothelial cells (BAEC) were illuminated with evanescent wave visible radiation, which has limited penetration into the basal surface of cells, or by trans-radiation. Imaging of fluorescent dyes localizing in the plasma membrane, mitochondria, or nucleus confirmed that evanescent wave radiation excited only dyes in and near the plasma membrane. Singlet oxygen, an ROS generated by photosensitization, has a very short lifetime, ensuring that it oxidizes molecules residing in or very close to the plasma membrane when evanescent wave radiation is used. Cells with condensed nuclei were considered apoptotic and were quantified after treatment with varying doses of light. Annexin V staining without propidium iodide staining confirmed that these cells were apoptotic. The doses required to induce apoptosis using evanescent wave radiation were 10-fold greater than those needed for trans-irradiation. Quantitative analysis of the evanescent wave penetration into cells supports a mechanism in which the singlet oxygen created near the plasma membrane, rather than at intracellular sites, was responsible for initiation of apoptosis.