Biosynthesis of interstitial and basement membrane collagens in pulmonary fibrosis

We have investigated the synthesis and matrix deposition of interstitial and basement membrane collagens in a rat model of bleomycin-induced pulmonary fibrosis. Rats weighing 175 to 200 g were injected intratracheally with bleomycin sulfate (1.5 units) or saline. At 10 days, lungs from bleomycin-treated rats showed multifocal interstitial and intraalveolar fibrosis, a nearly 2-fold increase in total lung hydroxyproline, and a 3-fold increase in the incorporation of [³H]proline into bacterial collagenase-sensitive protein. For characterization of the newly synthesized collagens, lung slices were labeled for 4 h with [³H]proline, homogenized in 4.5 M NaCl, and extracted at 4° C with 2 M guanidine-HCl and/or by reduction and alkylation. Extracts contained type IV procollagen, types I, III, and V collagen, and >95% of the nondialyzable [³H]hydroxyproline. At 10 days, bleomycin-treated rats showed a nearly 2-fold decrease in the proportion of type IV in extracts of the 4.5 M salt residue, a >3-fold increase in incorporation of proline into interstitial collagens, and an increase in the ratio of type I to type III collagen. Total incorporation of proline into type IV procollagen was not significantly increased. These results suggest that the period of maximal collagen synthetic activity observed at 1 to 2 wk after bleomycin administration is associated with a selective increase in the synthesis and accumulation of interstitial collagens.