Direct thrombin inhibition reduces lung collagen, accumulation, and connective tissue growth factor mRNA levels in bleomycin-induced pulmonary fibrosis

Dramatic activation of the coagulation cascade has been extensively documen ted for pulmonary fibrosis associated with acute and chronic lung injury. I n addition to its role in hemostasis, thrombin exerts profibrotic effects v ia activation of the major thrombin receptor, protease-activated receptor-1 . In this study, we examined the effect of the direct thrombin inhibitor, U K-156406 on fibroblast responses in vitro and on bleomycin-induced pulmonar y fibrosis in rats. UK-156406 significantly inhibited thrombin-induced fibr oblast proliferation, procollagen production, and connective tissue growth factor (CTGF) mRNA levels when used at equimolar concentration to the prote ase. Thrombin levels in bronchoalveolar lavage fluid and expression of thro mbin and protease-activated receptor-1 in lung tissue were increased after intratracheal. instillation of bleomycin. The characteristic doubling in lu ng collagen in bleomycin-treated animals (38.4 +/- 2.0 mg versus 17.1 +/- 1 .4 mg, P < 0.01) was preceded by significant elevations in alpha (1)(I) pro collagen and CTGF mRNA levels (3.0 +/- 0.4-fold and 6.3 +/- 0.4-fold respec tively, (P < 0.01), and total inflammatory cell. number. UK-156406, adminis tered at an anticoagulant dose, attenuated lung collagen accumulation in re sponse to bleomycin by 35 +/- 12% (P < 0.05), inhibited alpha (1)(I)procoll agen and CTGF mRNA levels by 50% and 35%, respectively (P < 0.05), but had no effect on inflammatory cell recruitment. This is the first report showin g that direct thrombin inhibition abrogates lung collagen accumulation in b leomycin-induced pulmonary fibrosis.