Effect of Reoxygenation on Radioresistance of Chronically Hypoxic A549 Non-Small Cell Lung Cancer (NSCLC) Cells Following X-Ray and Carbon Ion Exposure

Hypoxia-induced radioresistance in non-small cell lung cancer (NSCLC) hinders radiotherapy efficacy. Fractionated schedules exploit reoxygenation between fractions to reverse this resistance, but the effects of <i>post-irradiation</i> reoxygenation remain unclear and may depend on radiation quality. We investigated survival, cell cycle progression, cytokine secretion, and gene expression in hypoxic (1 % O₂) and reoxygenated A549 cells irradiated with X-rays or carbon ions. Colony-forming assays revealed an Oxygen Enhancement Ratio (OER) > 1 for both hypoxic and reoxygenated cells after X-rays, indicating persistent radioresistance; carbon ion OER ≈ 1 reflected oxygen-independent cytotoxicity. Hypoxia weakened radiation-induced G₂ arrest, and this was unaffected by reoxygenation. IL-6 secretion increased after X-rays and IL-8 after carbon ions exposure; both were enhanced under hypoxia and reoxygenation. RNA sequencing revealed that hypoxia induced a pro-survival, epithelial-to-mesenchymal transition (EMT)-promoting, and immune-evasive transcriptional program, which was largely reversed by reoxygenation but without increased clonogenic killing. These findings indicate that short-term reoxygenation after irradiation can normalize hypoxia-driven transcriptional changes yet does not restore radiosensitivity, supporting the advantage of high-linear energy transfer (LET) carbon ions for targeting resistant hypoxic NSCLC cells.