Immunocytochemical localization of the sites of superoxide dismutase induction by hyperoxia in rat lungs.

BACKGROUND: Rats exposed to 85% O2 can survive subsequent exposures to 100% O2. The development of this tolerance to oxygen toxicity is associated with an increase in total pulmonary content of both CuZn and Mn superoxide dismutases (SOD). It is not known, however, in which cells increases in these antioxidant enzymes occur. We carried out an electron microscopic immunocytochemical study to define the distribution of CuZnSOD and MnSOD and their sites of induction in rat lungs. EXPERIMENTAL
DESIGN: The lungs of rats exposed to either air or 85% O2 were fixed by instillation of fixative 7 and 14 days after the exposures were started. Cryoultrathin sections were prepared from randomly selected, frozen tissue blocks and were immunolabeled for either CuZnSOD or MnSOD. The labeling densities of the two enzymes in the major alveolar parenchymal cells (type I and type II epithelial cells, interstitial fibroblasts, and endothelial cells), were evaluated.
RESULTS: All of the alveolar septal cells studied displayed similar concentrations of CuZnSOD. After 7 or 14 days of exposure to 85% O2, the total lung content of CuZnSOD was increased because of cell hyperplasia and hypertrophy, but the intracellular concentrations of CuZnSOD did not increase. MnSOD was present in the mitochondria of all lung cells in control rat lungs but was highest in alveolar type II epithelial cells. Exposure to 85% O2 increased the concentration of MnSOD in the mitochondria of interstitial fibroblasts by 197 and 139% after O2 exposure of 7 days and 14 days, respectively. A significant induction of MnSOD also occurred in the mitochondria of alveolar type II cells after 7 days of hyperoxia exposure. Biochemical analyses showed that a significant fraction of the hyperoxia-induced MnSOD was in the form of an enzymatically inactive protein.
CONCLUSIONS: Hyperoxia induced total lung CuZnSOD in proportion to lung cell hypertrophy and hyperplasia. Total lung MnSOD was increased in association with specific inductions of MnSOD protein expression in the mitochondria of type II alveolar epithelial cells and interstitial fibroblasts. Hyperoxia appeared to result in inactivation and accumulation of the inactive MnSOD as well as induction of enzymatically active MnSOD. The ability of fibroblasts and alveolar type II cells to replace inactive MnSOD or to augment active MnSOD in the mitochondria appears to play important roles in the development of tolerance to hyperoxia by rats.