Proteases and Protease‐activated receptors signalling: at the crossroads of acquired and innate immunity

Protease-activated receptors (PARs) are seven-transmembrane G-protein coupled receptors activated by serine proteases (Fig. 1). Proteases cleave the extracellular Nterminus of the molecule to expose a new tethered ligand, which in turn binds and activates the cleaved receptor. Four PARs have been cloned and each has a unique cleavage site amino acid sequence. Tryspin activates PAR2 and PAR4. Thrombin activates PAR1 and PAR3. Of particular interest for allergic disease, mast cell tryptase activates PAR2. Much that is known about PARs and their function in the respiratory system comes from studies in the lower airways. All four PARs are expressed in airway epithelium and smooth muscle. PAR2 expression is increased in the asthmatic epithelium [1]. In isolated mouse airways, activation of epithelial PAR1, PAR2 and PAR4 by the proteases trypsin and thrombin, as well as by specific activating peptides corresponding to each receptor’s tethered ligand amino acid sequence, cause airway relaxation via release of a cyclooxygenase product, probably prostaglandin E2 (PGE2) [2, 3]. PAR2 activation also causes relaxation of isolated rat, guinea pig and human bronchi, and induces bronchodilation in mice in vivo [2]. On the other hand, thrombin stimulates airway smooth muscle contraction, likely by activation of airway smooth muscle PAR1 [4]. Activation of PAR1, PAR2 and PAR4 stimulates IL-6, IL-8/CXCL8 and PGE2 release from airway epithelial cells [5, 6]. Activation of PAR2 also induces airway epithelial cell release of granulocyte-macrophage-colony stimulating factor (GM-CSF), eotaxin/CCL11 and matrix metalloproteinase (MMP)-9 [7–9]. Basolateral stimulation of PAR2 receptors in mouse and human airways results in phospholipase C and Ca-dependent inhibition of amiloride-sensitive Na conductance and stimulation of both luminal Cl channels and basolateral K channels, leading to a secretory response [10]. PAR2 activation interrupts E-cadherin adhesion and compromises the airway epithelial barrier [11]. PARs are also present on mast cells, eosinophils, neutrophils, alveolar macrophages, monocytes and lymphocytes [12]. Thrombin and the PAR1 activating peptide induce b-hexosaminidase, IL-6 and MMP-9 release from mouse bone marrow mast cells, as well as mast cell adhesion to fibronectin [13]. Trypsin induces activation and superoxide release from human eosinophils through PAR2 [14]. PAR2 stimulation of peripheral blood monocytes induces Ca flux and production of IL-1b, IL-6, and IL-8/CXCL8 [15]. Stimulation of human peripheral monocytes and monocyte-derived macrophages with thrombin or PAR1 activating peptide triggers expression of monocyte chemoattractant protein (MCP)-1/CCL2 [16]. Thrombin, trypsin and the PAR2 activating peptide induce calcium flux in human T cell lines [17]. The abundant effects of PAR2 activation on airway cell and leucocyte function are consistent with the notion that PAR2 plays a critical role in the pathogenesis of allergic airways disease. To test this, Schmidlin et al. [18] examined the response to ovalbumin (OVA) sensitization and challenge in PAR2 knockout mice, as well as mice undergoing intranasal administration of the PAR2 activating peptide SLIGRL-NH2. Compared with wild-type animals, eosinophil infiltration was inhibited by 73% in mice lacking PAR2 and increased by 88% in mice overexpressing PAR2. Similarly, compared with wild-type animals, airway cholinergic responsiveness was diminished 38% in mice lacking PAR2 and increased by 52% in mice overexpressing PAR2. PAR2 deletion also reduced IgE levels to OVA sensitization by fourfold compared with those of wild-type animals. Thus, PAR2 significantly contributes to the development of acquired immunity and allergic inflammation in the airways. Correspondence: Marc B. Hershenson, Department of Pediatrics and Communicable Diseases, Department of Molecular and Integrative Physiology, University of Michigan Medical School, Ann Arbor, MI, USA 48109. E-mail: mhershen@umich.edu doi: 10.1111/j.1365-2222.2007.02738.x Clinical and Experimental Allergy, 37, 963–966