Changes in intracellular Ca2+ concentrations related to PDT-induced apoptosis in photosensitized human cancer cells

For photodynamic therapy (PDT), human squamous cell carcinoma cells (HSC-2) were treated with 3 microg/ml of photofrin 24 h prior to irradiating the cultures with the excimer dye laser at a dose of 2 J/cm2. Extensive DNA fragmentation was recognized within 2 h of PDT. The proportion of cells with DNA fragmentation on flow cytometric analysis was significantly increased to 44 and 78% at 1 and 2 h after PDT, respectively, compared to control groups. Confocal laser scanning microscopy revealed a slight change in the intracellular Ca2+ concentration occurring at 30 min after PDT and a subsequent marked increase in the Ca2+ concentration 1-2 h after PDT, but no change was observed in the cells exposed to laser irradiation alone and to photofrin alone and in the cells immediately after PDT. These findings suggest that an increase in the intracellular Ca2+ concentration may play an important role in the induction of PDT-induced apoptosis.