Cellular Mechanisms of Photobiomodulation in Relation to HeLa Kyoto Tumor Cells Exposed to Ionizing Radiation

During the clinical use of photobiomodulation (PBM) to manage radiotherapy-induced side effects, tumor tissue may be exposed to low-intensity laser light. Therefore, it is necessary to evaluate potential unintended PBM stimulation of tumor cells when PBM combines with ionizing radiation (IR). We investigated the effects of PBM (0.3 J/cm²) on the cell cycle, mitochondrial potential, and cell death of HeLa Kyoto cells, comparing pre-irradiation (PBM→IR) and post-irradiation (IR→PBM) exposure to 2 Gy, 4 Gy, and 6 Gy of ionizing radiation. PBM prior to IR induced the radiation-induced arrest in the G₂/M phase of the cell cycle. PBM after IR resulted in a partial release of cells from the radiation-induced arrest in the G₀/G₁ phase, along with a decrease in the number of apoptotic cells and cells with depolarized mitochondrial membranes compared to samples treated with IR only. These findings provide a basis for further research into PBM timing to improve radiotherapy outcomes.