The COX-2/PGE2 Response Pathway Upregulates Radioresistance in A549 Human Lung Cancer Cells through Radiation-Induced Bystander Signaling

SIMPLE SUMMARY: The radiation-induced bystander effect (RIBE) is a phenomenon in which unirradiated cells respond to the effects of irradiation due to signals received from nearby irradiated cells. Experiments have shown that the RIBE can enhance cell radioresistance, which reduces the effectiveness of radiation cancer therapy. However, the RIBE mechanisms in vivo are still poorly understood. The methods employed in many in vitro studies on RIBE lacked direct contact between irradiated and non-irradiated cells; thus, they were insufficient to capture the full effects of RIBE in radiation cancer therapy. In vivo, cells are in contact with each other and perform intercellular responses through the gap junctions. In this study, we mimicked the RIBE in radiation cancer therapy in vitro by irradiating the subcellular region while maintaining cell-to-cell contact using a single-particle irradiation system to cell (SPICE-QST microbeam) facility at Chiba, Japan. Then, we investigated the contribution of the RIBE in the radioresistance of cancer cells and its mechanism focusing on cyclooxygenase-2 and its metabolite prostaglandin E2. ABSTRACT: This study aimed to determine the mechanism underlying the modulation of radiosensitivity in cancer cells by the radiation-induced bystander effect (RIBE). We hypothesized that the RIBE mediates cyclooxygenase-2 (COX-2) and its metabolite prostaglandin E2 (PGE2) in elevating radioresistance in unirradiated cells. In this study, we used the SPICE-QST microbeam irradiation system to target 0.07–0.7% cells by 3.4-MeV proton microbeam in the cell culture sample, such that most cells in the dish became bystander cells. Twenty-four hours after irradiation, we observed COX-2 protein upregulation in microbeam-irradiated cells compared to that of controls. Additionally, 0.29% of the microbeam-irradiated cells exhibited increased cell survival and a reduced micronucleus rate against X-ray irradiation compared to that of non-microbeam irradiated cells. The radioresistance response was diminished in both cell groups with the hemichannel inhibitor and in COX-2-knockout cells under cell-to-cell contact and sparsely distributed conditions. The results indicate that the RIBE upregulates the cell radioresistance through COX-2/PGE2 intercellular responses, thereby contributing to issues, such as the risk of cancer recurrence.