Uncontrolled activation of the coagulation cascade contributes to the pathophysiology of several conditions, including acute and chronic lung diseases. Coagulation zymogens are considered to be largely derived from the circulation and locally activated in response to tissue injury and microvascular leak. Here we report that expression of coagulation factor X (FX) is locally increased in human and murine fibrotic lung tissue, with marked immunostaining associated with bronchial and alveolar epithelia. FXa was a potent inducer of the myofibroblast differentiation program in cultured primary human adult lung fibroblasts via TGF-β activation that was mediated by proteinase-activated receptor-1 (PAR1) and integrin α v β 5 . PAR1, α v β 5 , and α-SMA colocalized to fibrotic foci in lung biopsy specimens from individuals with idiopathic pulmonary fibrosis. Moreover, we demonstrated a causal link between FXa and fibrosis development by showing that a direct FXa inhibitor attenuated bleomycin-induced pulmonary fibrosis in mice. These data support what we believe to be a novel pathogenetic mechanism by which FXa, a central proteinase of the coagulation cascade, is locally expressed and drives the fibrotic response to lung injury. These findings herald a shift in our understanding of the origins of excessive procoagulant activity and place PAR1 central to the cross-talk between local procoagulant signaling and tissue remodeling.

Rationale: Studies in patients and experimental animals provide compelling evidence of the involvement of the major thrombin receptor, proteinase-activated receptor-1 (PAR 1 ), and the potent chemokine, chemokine (CC motif) ligand-2 (CCL2)/monocyte chemotactic protein-1, in the pathogenesis of idiopathic pulmonary fibrosis (IPF). PAR 1 knockout mice are protected from bleomycininduced lung inflammation and fibrosis and this protection is associated with marked attenuation in CCL2 induction. Objectives: The aim of this study was to determine which cell types represent the major source of PAR 1 -inducible CCL2 in the fibrotic lung. Methods: Using immunohistochemistry and dual immunofluorescence, we examined PAR 1 and CCL2 expression in the bleomycin model and human IPF lung. PAR 1 and CCL2 gene expression was also assessed in laser-captured alveolar septae from patients with IPF. The ability of PAR 1 to induce CCL2 production by lung epithelial cells was also examined in vitro. Measurements and Main Results: We report for the first time that PAR 1 and CCL2 are coexpressed and co-up-regulated on the activated epithelium in fibrotic areas in IPF. Similar observations were found in bleomycin-induced lung injury. Furthermore, we show that thrombin is a potent inducer of CCL2 gene expression and protein release by cultured lung epithelial cells via a PAR 1 -dependent mechanism. Conclusions: These data support the notion that PAR 1 activation on lung epithelial cells may represent an important mechanism leading to increased local CCL2 release in pulmonary fibrosis. Targeting PAR 1 on the pulmonary epithelium may offer a unique opportunity for therapeutic intervention in pulmonary fibrosis and other inflammatory and fibroproliferative conditions associated with excessive local generation of thrombin and CCL2 release.

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