Differential cytotoxic responses to low- and high-dose photodynamic therapy in human gastric and bladder cancer cells.

Here, we present differential cytotoxic responses to two different doses of photodynamic therapies (PDTs; low-dose PDT [LDP] and high-dose PDT [HDP]) using a chlorin-based photosensitizer, DH-II-24, in human gastric and bladder cancer cells. Fluorescence-activated cell sorting analysis using Annexin V and propidium iodide (PI) showed that LDP induced apoptotic cell death, whereas HDP predominantly caused necrotic cell death. The differential cytotoxic responses to the two PDTs were further confirmed by a DiOC(6) and PI double-staining assay via confocal microscopy. LDP, but not HDP, activated caspase-3, which was inhibited by Z-VAD, Trolox, and BAPTA-AM. LDP and HDP demonstrated opposite effects on intracellular reactive oxygen species (ROS)/Ca(2+) signals; LDP stimulated intracellular ROS production, contributing to a transient increase of intracellular Ca(2+) , whereas HDP induced a massive and prolonged elevation of intracellular Ca(2+) responsible for the transient production of intracellular ROS. In addition, the two PDTs also increased in situ transglutaminase 2 (TG2) activity, with a higher stimulation by HDP, and this increase in activity was prevented by Trolox, BAPTA-AM, and TG2-siRNA. LDP-induced apoptotic cell death was strongly inhibited by Trolox and TG2-siRNA and moderately suppressed by BAPTA-AM. However, HDP-mediated necrotic cell death was partially inhibited by BAPTA-AM but not by TG2-siRNA. Thus, these results demonstrate that LDP and HDP induced apoptotic and necrotic cell death by differential signaling mechanisms involving intracellular Ca(2+) , ROS, and TG2.