Differential Expression of EC‐SOD, Mn‐SOD and CuZn‐SOD in Rat Lung Exposed to Crystalline Silica: Heungnam Kim, et al. Cell and Cancer Biology Branch, Center for Cancer Research, National Cancer Institute, National Institutes of Health, USA—Superoxide dismutases (SODs) are antioxidant enzymes that catalyze the dismutation of superoxide into hydrogen peroxide. There are 3 kinds of isozymes: extracellular superoxide dismutase (EC‐SOD), manganese‐containing superoxide dismutase (Mn‐SOD) and copper‐ and zinc‐containing superoxide dismutase (CuZn‐SOD). To examine the expression of SOD isozymes in lungs injured by crystalline silica, we intratracheally instilled male Wistar rats with 2 mg (8 mg/kg) of crystalline silica and investigated the mRNA, protein level and distribution of SOD isozymes in the rat lungs using RT‐PCR, western blot analysis and immunostaining, respectively at from 3 d to 180 d of recovery following the exposure. EC‐SOD mRNA levels significantly increased from 3 d to 90 d and the EC‐SOD protein level was significantly higher after 90 and 180 d recovery in the crystalline silica exposed groups than in the control groups. Mn‐SOD increased in silica treated rat lungs at both mRNA and protein levels, peaking at 30 d post‐exposure. CuZn‐SOD mRNA levels were decreased at 3, 7 and 30 d, and CuZn‐SOD protein levels were also significantly lower than the control group at 90 and 180 d recovery. There was prominent EC‐SOD immunostaining mainly in the plasma and alveolar macrophages and strong Mn‐SOD staining in alveolar macrophages and interstitial cells of the proximal and distal portions of the alveolar duct following crystalline silica exposure. There was less CuZn‐SOD staining in epithelial cells at terminal bronchioles in the crystalline silica‐exposed group. These findings suggest that these SOD isozymes may be related to lung injury induced by crystalline silica.