Absence of proteinase-activated receptor-1 signaling in mice confers protection from fMLP-induced goblet cell metaplasia

The morphological features of chronic obstructive pulmonary disease in man include emphysema and chronic bronchitis associated with mucus hypersecretion. These alterations can be induced in mice by a single intratracheal instillation of N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP), a chemoattractant and degranulating agent for neutrophils. The mechanisms underlying excessive mucus production and, in particular, goblet cell hyperplasia/metaplasia in chronic obstructive pulmonary disease remain poorly understood. The proteinase-activated receptors (PARs) are widely recognized for their modulatory properties during inflammation. In this study, we examined whether PAR-1 contributes to inflammation and lung damage induced by fMLP by comparing the response of PAR-1-deficient (PAR-1(-/-)) mice with that of wild-type (WT) mice. Mice were killed at various time points after fMLP instillation (200 microg/50 microl). WT mice developed emphysema and goblet cell metaplasia. The onset of pulmonary lesions was preceded by an increase in thrombin immunoreactivity in bronchial airways and alveolar tissue. This was followed by a decrease in PAR-1 immunoreactivity, and by an increase in IL-13 immunostaining on the luminal surface of airway epithelial cells. In PAR-1(-/-) mice, fMLP administration induced similar responses in terms of inflammation and emphysema, but these mice were protected from the development of goblet cell metaplasia. The involvement of PAR-1 in airway epithelial cell transdifferentiation was confirmed by demonstrating that intratracheal instillation of the selective PAR-1 agonist (TFLLR) induced goblet cell metaplasia in the airways of WT mice only. These data suggest that emphysema and goblet cell metaplasia occur independently, and that PAR-1 signaling through IL-13 stimulation may play an important role in inducing goblet cell metaplasia.

Procoagulant signalling mechanisms in lung inflammation and fibrosis: novel opportunities for pharmacological intervention?

There is compelling evidence that uncontrolled activation of the coagulation cascade following lung injury contributes to the development of lung inflammation and fibrosis in acute lung injury/acute respiratory distress syndrome (ALI/ARDS) and fibrotic lung disease. This article reviews our current understanding of the mechanisms leading to the activation of the coagulation cascade in response to lung injury and the evidence that excessive procoagulant activity is of pathophysiological significance in these disease settings. Current evidence suggests that the tissue factor-dependent extrinsic pathway is the predominant mechanism by which the coagulation cascade is locally activated in the lungs of patients with ALI/ARDS and pulmonary fibrosis. Whilst, fibrin deposition might contribute to the pathophysiology of ALI/ARDS following systemic insult; current evidence suggests that the cellular effects mediated via activation of proteinase-activated receptors (PARs) may be of particular importance in influencing inflammatory and fibroproliferative responses in experimental models involving direct injury to the lung. In this regard, studies in PAR(1) knockout mice have shown that this receptor plays a major role in orchestrating the interplay between coagulation, inflammation and lung fibrosis. This review will focus on our current understanding of excessive procoagulant signalling in acute and chronic lung injury and will highlight the novel opportunities that this may present for therapeutic intervention.

A novel therapeutic target in various lung diseases: Airway proteases and protease-activated receptors

Protease-activated receptors (PAR), which are G protein-coupled receptors, have 4 members, PAR-1 to PAR-4. PARs are activated by proteolysis of a peptide bond at the N-terminal domain of the receptor. PARs are widely distributed throughout the airways. Their activity is modulated by airway proteases of endogenous and exogenous origin, which can either activate or disable the receptors. The regulation of PAR activity by proteases is important under pathological conditions when the activity of proteases is increased. Moreover, various inflammatory mediators, such as cytokines, growth factors, or prostanoids, alter the PAR expression level. Elevated PAR levels are observed in various lung disorders, and their significance in the development of pathological situations in the lung is currently intensively investigated. Consequences of PAR activation can be either beneficial or deleterious, depending on the PAR subtype. PAR-1 has been shown to be an important player in the development of pulmonary fibrosis. Thus, PAR-1 represents an exciting target for clinical intervention in fibrotic diseases. PAR-2 contributes to allergic airway inflammation. However, the question whether the impact of PAR-2 is beneficial or deleterious is still under intensive discussion. Therefore, precise information concerning the participation of PAR-2 in various lesions is required. Moreover, it is necessary to generate selective PAR- and organ-targeted approaches for treating the diseases. A thorough understanding of PAR-induced cellular events and the consequences of receptor blockade may help in the development of novel therapeutic strategies targeted to prevent lung destruction and to avoid deterioration of conditions of patients with inflammatory or fibrotic lung diseases.

Signaling pathways involved in proteinase-activated receptor1-induced proinflammatory and profibrotic mediator release following lung injury

The proteinase-activated receptors (PAR1 to 4) represent important novel therapeutic targets in fibroproliferative lung disorders, linking signals from coagulation proteinases to the production of proinflammatory and profibrotic mediators, in response to lung injury. This review focuses on the current understanding of cytokine and chemokine release downstream of PAR1 activation in the context of fibroproliferative lung disease, with an emphasis on the induction of the chemokine CCL2/MCP-1.

Absence of proteinase-activated receptor-1 signaling affords protection from bleomycin-induced lung inflammation and fibrosis

Activation of the coagulation cascade is commonly observed in the lungs of patients with both acute and chronic inflammatory and fibrotic lung disorders, as well as in animal models of these disorders. The aim of this study was to examine the contribution of the major thrombin receptor, proteinase-activated receptor-1 (PAR-1), during the acute inflammatory and chronic fibrotic phases of lung injury induced by intratracheal instillation of bleomycin in mice. Inflammatory cell recruitment and increases in bronchoalveolar lavage fluid (BALF) protein were attenuated by 56 +/- 10% (P < 0.05) and 53 +/- 12% (P < 0.05), respectively, in PAR-1-deficient (PAR-1-/-) mice compared with wild-type (WT) mice. PAR-1-/- mice were also protected from bleomycin-induced pulmonary fibrosis with total lung collagen accumulation reduced by 59 +/- 5% (P < 0.05). The protection afforded by PAR-1 deficiency was accompanied by significant reductions in pulmonary levels of the potent PAR-1-inducible proinflammatory and profibrotic mediators, monocyte chemoattractant protein-1 (MCP-1), transforming growth factor-beta-1 (TGF-beta1), and connective tissue growth factor/fibroblast-inducible secreted protein-12 (CTGF/FISP12). In addition, PAR-1 was highly expressed in inflammatory and fibroproliferative lesions in lung sections obtained from patients with fibrotic lung disease. These data show for the first time that PAR-1 signaling plays a key role in experimentally induced lung injury, and they further identify PAR-1 as one of the critical receptors involved in orchestrating the interplay between coagulation, inflammation, and remodeling in response to tissue injury.