5-Aminolevulinic acid enhances mitochondrial stress upon ionizing irradiation exposure and increases delayed production of reactive oxygen species and cell death in glioma cells.

5-Aminolevulinic acid (5-ALA) can accumulate protoporphyrin IX (PpIX) in tumour cell mitochondria and is well known for its utility in fluorescence-guided resection of malignant gliomas as a live molecular marker. Previously, we and other authors demonstrated that 5-ALA has a radiosensitizing effect for tumours. In the present study, we aimed to investigate the mechanism underlying the radiosensitizing effect of 5-ALA by focusing on glioma cell mitochondria. Using an enhancer (ciprofloxacin) of 5-ALA-induced PpIX accumulation, we evaluated the influence of ionizing irradiation (IR) and delayed reactive oxygen species (ROS) production 12 h after IR by colony-forming assay and flow cytometry (FCM) with different amounts of PpIX accumulation. The mitochondrial mass and mitochondrial electron transport chain (mtETC) activity were evaluated by FCM and western blot analysis. Cell death and delayed ROS production after IR in glioma cells were increased in proportion to 5-ALA-induced PpIX accumulation. Delayed ROS production enhanced by 5-ALA localized to the glioma cell mitochondria. Mitochondrial mass and mitochondrial complex III activity, among mtETC factors, were also increased 12 h after IR in glioma cells in proportion to 5-ALA-induced PpIX accumulation with some variation. These results suggest that 5-ALA enhances IR-induced mitochondrial oxidative stress and leads to increased cell death with mitochondrial changes, thereby acting as a targeting mitochondrial drug, and so‑called radiosensitizer in glioma cells.