Consequences of prolonged inhalation of ozone on F344/N rats: collaborative studies. Part IV: Effects on expression of extracellular matrix genes.

Increased deposition of lung extracellular matrix in terminal airways is associated with chronic ozone exposure. In situ hybridization was used to assess whether long-term ozone exposure causes elevated and continued expression of genes coding for connective tissue proteins. Accessory lobes were removed from the animals exposed to 0, 0.12, 0.5, or 1.0 parts per million (ppm)* ozone for 20 months as part of the National Toxicology Program (NTP)/HEI Collaborative Ozone Project. The lungs were perfused fixed under physiologic pressure and processed for in situ hybridization. Sections were hybridized with 35S-labeled probes for messenger RNA (mRNA) coding for various matrix proteins, including collagen types I and III, elastin, and fibronectin, and for interstitial collagenase, a matrix metalloproteinase. Fetal rat lung was used as a positive control for hybridization. No signal for any mRNA was detected in terminal airway stromal cells of lungs from animals exposed to ozone for 20 months or control animals breathing clean air. In all samples from animals exposed to ozone for 20 months and control animals, only a very weak signal was seen in occasional cells within the interstitial spaces around large airways and blood vessels. In contrast, a strong signal for matrix-related mRNA was detected in fetal lung tissue. These findings indicate that active or enhanced matrix production is turned off in the adult animals used in the ozone studies, suggesting that the increase in matrix deposition results from a transient and early fibrotic response. Indeed, signal for type I procollagen and tropoelastin mRNAs was seen in alveolar septal cells in lungs of rats exposed to ozone for two months. No signal was seen in alveolar cells of age-matched control animals. (These animals, exposed for two months, and age-matched controls were from earlier studies supported by the HEI.) These findings indicate that ozone mediates a transient fibrotic response that results in a sustained increase in lung extracellular matrix. Confirmation of this hypothesis would require additional studies using animals exposed to ozone for shorter times.