Overexpression of extracellular superoxide dismutase protects mice from radiation-induced lung injury

Purpose: The purpose of this study was to determine if radiation-induced lung injury is associated with prolonged oxidative stress, and whether chronic overexpression of extracellular superoxide dismutase (EC-SOD) in the lung of transgenic mice protects against radiation-induced lung injury. Methods and Materials: Whole-lung radiation was delivered to EC-SOD overexpressing B6C3 transgenic (XRT-TG) mice and wild-type littermates (XRT-WT). Pulmonary function was assessed by breathing frequency. Right lung wet weight was used as a gross indicator of lung damage. Histopathology was used to assess collagen deposition and tissue fibrosis according to an established grading system. Immunohistochemistry was used to stain and quantify the number of macrophages. ELISA was used to measure activated TGF-β1. Oxidative stress was assessed by measuring lipid oxidation products (malondialic acid) by HPLC. Results: Four of six XRT-WT mice required euthanasia at 15-19 weeks postradiation because of respiratory distress, whereas no XRT-TG mouse developed distress. All assessments of lung damage at 15-20 weeks postradiation were higher for XRT-WT mice compared with the XRT-TG mice, including breathing frequency (380 vs. 286 bpm, p ≤ 0.0004), right lung weight (228 vs. 113 mg, p ≤ 0.06), macrophage count (48 vs. 5 per 40× field, p ≤ 0.06), and percent activated TGF-β1 (37 vs. 11%, p ≤ 0.06). Semiquantitative measures, including fibrosis and collagen deposition, were also higher for XRT-WT mice, with an exact Fisher p value of ≤0.03 for both variables. In addition, malondialic acid was elevated in XRT-WT mice 15-20 weeks after radiation delivery, and levels were lower in the XRT-TG mice (624 vs. 323 pmol/mg protein, p ≤ 0.06). Conclusions: After radiation therapy, oxidative stress is present at 15-20 weeks after initial exposure, which correlates with the delayed clinical onset of radiation-induced lung damage. Overexpression of EC-SOD in transgenic mice appears to confer protection against this radiation-induced lung injury, with a corresponding decrease in oxidative stress. EC-SOD may be a potential therapeutic agent for radioprotection in the treatment of thoracic malignancies. Further investigation is needed to confirm and expand on the current results.