Cytotoxic Effect of Protoporphyrin IX to Human Leukemia U937 Cells under Ultrasonic Irradiation

Background: Sonodynamic therapy (SDT) is an alternative strategy that manages malignancies via the generation of cytotoxic factors during ultrasound-activated sono-sensitive agents. However, the detailed mechanisms are not clear. This study was to identify the cytotoxic effects of ultrasound-activated protoporphyrin IX (PpIX) on U937 cells. Methods: Flow cytometry was performed to detect the time course for PpIX uptake in U937 cells. Sub-cellular localization of PpIX in U937 cells was visualized by inverted confocal laser scanning microscope. Following PpIX-mediated SDT treatment, cell viability was evaluated by the 3-(4, 5- dimethylthiazol-2-yl)-2, 5-diphenyltetrazoliumbromide (MTT) assay; nuclear damage was observed under fluorescent microscope; DNA fragmentation and mitochondrial membrane potential disruption were measured by flow cytometry. The role of reactive oxygen species (ROS) in SDT-induced cell death was also evaluated. Results: We observed that PpIX is mainly localized in the mitochondria, with a maximal uptake within 2 h. Compared with PpIX or ultrasound alone, PpIX plus ultrasound treatment significantly declined cell viability, caused more serious damage of cell morphology, DNA and mitochondria. In the combined treatment group, the intracellular ROS was greatly higher than in other groups; ROS scavenger N-acetylcysteine could effectively rescue the loss of mitochondria membrane potential and cell viability induced by SDT. Conclusion: Taken together, these findings primarily indicated that fatal damage could be induced by PpIX-mediated SDT in U937 cells, and the intracellular ROS was involved during this process.