Early Molecular and Cellular Events of Oxidant-Induced Pulmonary Fibrosis in Rats

Abstract To evaluate the early molecular events of oxidant-induced pulmonary fibrosis, rats were continuously exposed to 0.4 ppm ozone and 7 ppm nitrogen dioxide. The early responses to the combined gases could be divided into three phases. Acute pulmonary inflammation indicated by an increase in pulmonary edema as well as an influx of neutrophils into the airspaces first occurred on days 1 to 3 of the exposure. The pulmonary inflammation was reversed by day 8, and no biochemical or morphologic aspects of tissue responses were detected from days 15 to 45, suggesting that rats adapted to the stimuli during that period. Pulmonary fibrosis could be detected by an increase in the biomarker of lung collagen content at day 60 and by histopathologic evaluation by day 90. Enhanced expression of macrophage inflammatory protein-2 was observed only at day 1, whereas the pulmonary expression of transforming growth factor-β was upregulated on days 60 and 90 of the exposure. Macrophage expressions of interleukin-1β and interleukin-6 were enhanced during acute pulmonary inflammation; however, macrophage expression of tumor necrosis factor-α was elevated at both day 1 and days 60–90. Activation of nuclear factor-kappa B and increased expression of thioredoxin in the lungs was also observed at day 1 and days 60–90. The expression of antioxidant enzymes, such as manganeous superoxide dismutase and glutathione peroxidase, was not altered during exposure. These results indicate that macrophage activation and the expression of macrophage-derived cytokines may play an important role in the early pulmonary responses against the combined gases.