IN-SITU HYBRIDIZATION ANALYSIS OF RAT LUNG ALPHA(1)(I) AND ALPHA(2)(I) COLLAGEN GENE-EXPRESSION IN PULMONARY FIBROSIS INDUCED BY ENDOTRACHEAL BLEOMYCIN INJECTION

BACKGROUND: Endotracheal bleomycin administration in rats and other an imal species causes rapid development of pulmonary fibrosis, character ized by a transiently increased number of contractile, filament-laden parenchymal cells and increased lung collagen synthesis and deposition . However, the identity and source of the cells that actively synthesi ze collagen and other extracellular matrix and their relationship to t he altered lung structure and function remain uncertain. EXPERIMENTAL DESIGN: In this study, the cells expressing alpha(1)(I) and alpha(2)(I ) procollagen genes were identified and their localization analyzed in control and bleomycin-treated rat lungs at different time points, by in situ and Northern hybridization analyses. RESULTS: In control lungs , only a few scattered fibroblasts with weak expression of the alpha(1 )(I) procollagen gene were localized exclusively in the adventitia of the primary and tertiary bronchi, as well as major blood vessels. At d ay 3 after bleomycin treatment, scattered interstitial cells with sign ificantly increased alpha(1)(I) and alpha(2)(I) procollagen gene expre ssion were observed in the adventitia of bronchioles, terminal brochio les, and adjacent small blood vessels. At days 7 and 14, there was a d ramatic increase in the number of interstitial cells expressing large amounts of alpha(1)(I) procollagen messenger RNA in these areas and ex tending to the lung parenchyma. This was followed on days 21 and 28 by significant decreases in procollagen gene expression and the number o f cells with increased collagen gene expression. Most of the cells wit h enhanced collagen gene expression were arrayed in a disorganized fas hion and were localized mainly around bronchioles, terminal bronchiole s, and adjacent small blood vessels as well as in the irregularly dist ributed fibrotic foci, some submesothelial areas, and injured parenchy ma. Northern blot analysis was consistent with the above in situ hybri dization observation of the kinetics of collagen gene expression. CONC LUSIONS: The results indicate that in this rat fibrosis model, increas ed numbers of the interstitial cells with high expression of type I pr ocollagen genes are derived primarily from the fibroblasts in the adve ntitia of bronchioles, terminal bronchioles, and adjacent blood vessel s, as well as the submesothelial region. This then can result in furth er expansion to adjacent parenchyma and alveolar areas.