Oxidant-Dependent Sensitizing, Protective, and Mitigative Effects in X-Ray-Irradiated Pulmonary Endothelial Cells

The primary response of proliferating bovine pulmonary artery endothelial cells (BPAECs) after X-ray irradiation [≤10 gray (Gy)] is shown to be transient cell-cycle arrest. Accompanying oxidant-linked functional changes within the mitochondria are readily measured, but increased autophagy is not. Radiation-induced apoptosis is negligible in this line-important because cells undergoing apoptosis release oxygen-derived species that can overwhelm/mask the radiation-associated species and their effects that we wish to investigate. Cells irradiated and cultured at 3% oxygen exhibited delayed cell-cycle arrest (6-8 hours after 10 Gy irradiation) compared with those maintained at 20% oxygen (2-4 hours after 10 Gy irradiation). At 3% oxygen, either only during or only after irradiation, results intermediate between 20% and 3% oxygen throughout were obtained. No variability in cell-cycle distribution was observed for unirradiated cells cultured under different prevailing oxygen levels. Mitochondrially localized manganese superoxide dismutase delayed the X-ray-induced cell-cycle changes when over-expressed in BPAEC, indicating superoxide to be one of the key oxygen-derived cytotoxic species involved in the radiobiological response. Also, the peroxynitrite biomarker 3-nitrotyrosine was elevated, whereas hydrogen peroxide levels were not. Lastly, the utility of the BPAEC for screening potential countermeasures to ionizing radiation is demonstrated with some quinoline derivatives. Three of the five compounds appeared mitigative, and all were protective. It is suggested that the oxidation-reduction chemistry of these compounds probably offers a reasonable explanation for their observed ameliorative properties. Furthermore, the results suggest a promising new direction in the search for lead compounds as countermeasures to the effects of ionizing radiation. SIGNIFICANCE STATEMENT: The primary radiological response of proliferating bovine pulmonary artery endothelial cells is cell-cycle arrest, starting soon after X-ray irradiation (1-10 Gy) at 20% O₂ but delayed by 4 hours at systemic (3%) O₂. Oxygen/superoxide is found to be radio-sensitizing in at least two distinct time windows, during and after the irradiation, with both responses antagonized by various hydroxyquinoline derivatives. Similar responses in many other cell lines are likely to be masked by elevated oxidants associated with apoptosis.