DRUG DEVELOPMENT RESEARCH 60:29–35 (2003) DDR Research Overview Proteinase-Activated Receptors and the Pathophysiology of Pulmonary Fibrosis Rachel C. Chambersn Centre for Respiratory Research, University College London, London, England Strategy, Management and Health Policy Preclinical Development Clinical Development Venture Capital Preclinical Toxicology, Formulation Phases I-III Postmarketing Enabling Research Drug Delivery, Regulatory, Quality, Phase IV Technology Pharmacokinetics Manufacturing ABSTRACT Interest in the role of proteinase-activated receptors (PARs) in the pathogenesis of fibrotic lung disease was fuelled by two central observations: first, the recognition that the coagulation cascade is activated in a number of these disorders; and second, the long-standing observation that thrombin, a principal regulator of coagulation, is a potent mitogen for fibroblasts, the key cell type responsible for the production of interstitial collagens in the fibrotic lung. This article will briefly review this evidence and focus on recent studies suggesting an important role for PAR1-mediated cellular responses in the pathophysiology of lung fibrosis. Drug Dev. Res. 60:29–35, 2003. c 2003 Wiley-Liss, Inc. Key words: PARs; pulmonary fibrosis; bleomycin ACTIVATION OF THE COAGULATION CASCADE IN recruited inflammatory cells into the injured lung FIBROTIC LUNG DISEASE injury, are thought to play a major role [reviewed in Pulmonary fibrosis is not a single disease but Chambers and Laurent, 1997]. In addition to classic represents the end stage of a heterogeneous group of fibrogenic mediators, it is increasingly recognized that disorders, of known and unknown cause, in which the coagulation cascade may also play an important role excessive deposition of collagen and other extracellular in fuelling the fibrotic response. Indeed, activation of matrix proteins within the pulmonary interstitium leads the coagulation cascade, which is manifest as either to progressive loss of lung function. There are currently increased thrombin levels, tissue factor–factor VIIa no adequate therapies, and it has been estimated that complexes or tissue factor expression and the resultant there are more than 3,000 deaths a year in the United extravascular deposition of fibrin, is a characteristic Kingdom from the most common form, cryptogenic feature of a number of respiratory diseases associated fibrosing alveolitis (CFA; also known as idiopathic with excessive deposition of connective tissue proteins. pulmonary fibrosis, IPF) alone [Hubbard et al., 1996]. These diseases include CFA/IPF [Imokawa et al., The pathogenesis of lung fibrosis remains incompletely 1997], pulmonary fibrosis associated with systemic understood, but current hypotheses propose that sclerosis [Ohba et al., 1994; Hernandez Rodriguez epithelial and/or endothelial injury leads to an ex- et al., 1995], chronic lung disease of prematurity [Dik aggerated fibroproliferative response during which fibroblasts transdifferentiate, proliferate, and increase Contract grant sponsors: the Wellcome Trust and the Medical Research Council, United Kingdom. extracellular matrix protein production. n In terms of the mediators involved in driving the Correspondence to: Rachel C. Chambers, Centre for Respiratory Research, University College London, 5 University fibrotic response, several potent fibrogenic cytokines Street, London WC1E 6JJ UK, England. and growth factors, including transforming growth E-mail: [email protected] factor-b (TGF-b) and platelet-derived growth factor Published online in Wiley InterScience (www.interscience. (PDGF), released from resident lung cells and wiley.com) DOI: 10.1002/ddr.10317 c 2003 Wiley-Liss, Inc. 30 CHAMBERS et al., 2003], and cryptogenic organising pneumonia role for this receptor in the pathogenesis of lung [Peyrol et al., 1990]. Extensive interstitial and intraal- fibrosis. veolar deposition of fibrin is also a characteristic feature in acute lung injury/adult respiratory distress ACTIVATION OF THE COAGULATION CASCADE IN syndrome (ARDS), in which rapid fibroproliferation EXPERIMENTALLY INDUCED LUNG FIBROSIS and matrix synthesis can lead to the development of The notion that inappropriate activation of the extensive fibrotic lesions. In the latter condition, coagulation cascade contributes to lung fibrosis is excessive procoagulant activity is mainly attributable supported by studies of animal models of lung injury to tissue factor–factor VII/VIIa complexes associated and fibrosis. The most widely used is that induced after with alveolar macrophages, in the face of reduced intratracheal instillation of the antineoplastic agent, levels of endogenous anticoagulants, including antith- bleomycin, to mice or rats. Instillation of this agent into rombin III, protein C, and protein S [recently reviewed the lung mimics some of the features of human fibrotic in Idell, 2003]. In patients with IPF, reduced protein C lung disease in that it leads to acute lung injury, plasma activity is further associated with abnormal collagen exudation, recruitment of inflammatory cells (predo- turnover in the intraalveolar space [Yasui et al., 2000]. minantly neutrophils, lymphocytes and macrophages), and eventually activation and proliferation of fibro- blasts and fibrosis. Intraalveolar and parenchymal fibrin ACTIVATORS OF PARs IN LUNG FIBROSIS formation is initiated by tissue factor [Olman et al., All four currently known PAR receptors are 1995] and active coagulation proteinases associated expressed in the lung, and collectively the proteinases with alveolar macrophages and epithelial cells, as well of the coagulation cascade can target all of them [see as fibroblasts within fibroproliferative foci [Howell et Riewald and Ruf, this Special Issue of Drug Develop- al., 2001]. Immunohistochemical studies performed in ment Research 59(4):400]. Thrombin is considered to our laboratory further revealed that PAR1 and active be one of the major physiologic activators of PAR1 [Vu thrombin colocalise to alveolar macrophages in inflam- et al., 1991]. Factor Xa, which is immediately upstream matory and fibroproliferative foci, as well as to of thrombin in the coagulation cascade, can activate interstitial spindle-shaped fibroblasts (Fig. 1). A direct PAR1 and PAR2, depending on the cell type, whereas causal role for the coagulation cascade in the tissue factor–factor VIIa complexes activate PAR2 pathogenesis of lung fibrosis in this model was [Bono et al., 2000; Riewald et al., 2001]. Transient provided by research performed in our laboratory ternary tissue factor–factor VIIa-Xa complexes activate and other laboratories that demonstrated that heparin, PAR1 and PAR2 with greater efficiency than the activated protein C, or a direct inhibitor of thrombin individual proteinases, indicating that tissue factor– proteolytic activity attenuated the fibrotic response factor VIIa initiated coagulation in the vasculature is obtained [Piguet et al., 1996; Yasui et al., 2000; Howell inseparably linked to PAR1 and PAR2 activation and et al., 2001]. cell signalling [Riewald and Ruf, 2001]. More recently, CONTRIBUTION OF THE PROCOAGULANT VERSUS PAR1 has also been shown to be activated by protein C bound to its receptor EPCR on endothelial cells PAR-MEDIATED CELLULAR EFFECTS IN LUNG [Riewald et al., 2002] and by plasmin, a proteinase of FIBROSIS the fibrinolytic pathway [Pendurthi et al., 2000]. Other One of the most interesting challenges for major physiologic activators of PAR2 that may be researchers working in this area centers on establishing relevant to lung fibrosis include trypsin [Belham et al., the relative contribution of the procoagulant (e.g., 1996], tryptase [Akers et al., 2000], membrane-type fibrin generation) versus the PAR-mediated cellular serine proteinase-1 (MT-SP-1) [Takeuchi et al., 2000], effects of coagulation proteinases in these models and and neutrophil proteinase 3 [Uehara et al., 2002]. The in human fibrotic lung disease. Early mechanistic physiologic functions of PAR3, which acts as a cofactor studies focussing on the contribution of fibrin to both for thrombin signalling [Nakanishi-Matsui et al., 2000], acute lung injury and progression to fibrosis provided and of PAR4, a more general serine proteinase evidence that fibrin and fibrinogen degradation pro- receptor, are also coming to light. Recent evidence ducts can influence tissue inflammation and repair by obtained in vitro suggests that PAR4, which is also acting as a provisional structural matrix and a source of activated by cathepsin G [Sambrano et al., 2000], may fibrogenic mediators for infiltrating fibroblasts and play a role in lung inflammatory responses [Asoka- inflammatory cells [recently reviewed in Idell, 2003]. nanthan et al., 2002]. However, the primary focus of Studies performed in genetically modified mice this article will be on discussing the potential role of in which the fibrinolytic capacity of the lung was PAR1, because there is growing in vivo evidence for a either up- or downregulated by knocking-out or PARs AND PULMONARY FIBROSIS 31 Fig. 1. Thrombin and proteinase-activated receptor 1 (PAR1) are localised to the same cell types in bleomycin-induced lung fibrosis. a and b: Normal rat lung stained for active thrombin at low ( Â 400) and high power ( Â 1,000). There is only very weak staining associated with alveolar macrophages. c ( Â 400) and d ( Â 1,000): Corresponding section for thrombin from rats with bleomycin-induced lung fibrosis. Thrombin is localised to macrophages in inflammatory and fibroproliferative foci, as well as to spindle-shaped fibroblasts.