Proteinase activated receptor 2 a protean effector comes of ag

June 2004 EDITORIALS 1907 Proteinase Activated Receptor 2: A Protean Effector Comes of Age See article on page 1844 P roteinase-activated receptors (PARs) are G proteincoupled receptors with seven transmembrane domains that are activated following proteolysis rather than through binding to an extracellular ligand.1 Proteolytic cleavage within the receptor extracellular N-terminal domain uncovers a new N-terminus that serves as an internal “tethered ligand” to activate the remaining receptor Four PARs have been identified, PAR-1 (the original thrombin receptor) and PAR-3, activated by thrombin, PAR-4, activated by both thrombin and trypsin, and PAR-2, activated by trypsin, but not thrombin.2 Further studies demonstrated that additional activators of PAR-2 include trypsin II and IV, different mast cell tryptases, and factor VIIa in the presence of tissue factor and factor X.3–5 These serine endoproteases cleave the N-terminal SKGR2SLIG site of human PAR-2, allowing the new N-terminus, beginning at SLIG, to activate the receptor Accordingly, synthetic peptides corresponding to this tethered ligand, such as the mouse PAR-2 activating peptide (sequence, SLIGRL), selectively activate PAR-2 without proteolysis The gene encoding PAR-2 has been identified in humans, mouse, and rats Immunoreactive PAR-2 has been localized, in the GI tract, to the epithelial cells and smooth muscles of the pancreas, stomach, and intestine, as well as smooth muscle, epithelial, and endothelial cells of the kidney, lung, ovary, uterus, and blood vessels.6 In the digestive system, functional PAR-2 are expressed by pancreatic ducts,7 intestinal cells,1 colonic myocytes,1 and neurons.1 Since its original discovery 10 years ago,2 PAR-2 has come of age, with an increased recognition of its role in diverse physiologic functions and pathologic conditions Biologic Effects of PAR-2 PAR-2 agonists stimulate IP3 production to increase [Ca2ϩ]i in numerous cell types They also activate MAP kinases ERK-1/-2, induce arachidonic acid release through activation of phospholipase A2, and stimulate regeneration of prostaglandins via activation of COX-1.1 Through these intracellular signaling pathways, PAR-2 mediate the following functions: Secretion: PAR-2 agonists regulate the activity of different ion transport pathways (e.g., Ca2ϩ-acti- vated Kϩ and ClϪ channels) and of secretion (e.g., mucin) in many epithelial cells, including those in the pancreas, bronchi, colon, and kidney.7–9 Smooth muscle contractility: PAR-2 agonists modulate smooth muscle function either directly or through release of NO or PGE2 from neighboring cells.1,10 In the respiratory system, PAR-2 inhibits airway bronchoconstriction, possibly serving a protective function.10 Neuromodulation: PAR-2 are expressed on myenteric and submucosal neurons of the enteric nervous system and on the spinal afferent neurons whose cell bodies are located within the dorsal root ganglia.1,11 PAR-2 activation in these neurons affects inflammation and nociception Indeed, activation of PAR-2 on spinal afferent neurons stimulate the release of substance P (SP) and calcitonin gene-related peptide (CGRP) from peripheral endings; these neuropeptides may, in turn, induce neurogenic edema and inflammation through vasodilation, plasma extravasation, and immune cell adhesion and infiltration,12 or stimulates secretion of cytoprotective mucus in the stomach.13 In addition, activation of PAR-2 on afferent fibers may sensitize these fibers, resulting in the amplification and persistence of pain.14,15 Inflammation: PAR-2 activation induces production of IL-1␤, IL-8, and ICAM-1 by lung epithelial cells and umbilical vein endothelial cells.16 However, in a model of colitis, it prevented induction of IL-2, IL-12, TNF-␣, and INF-␥, inhibited the up-regulation of COX-2 and iNOS, and downregulated CD44 expression from lamina propria T lymphocytes.17 Fibrosis: Activated mast cells are present in many fibrotic conditions, suggesting their key role in fibrogenesis This role appears to be mediated by the tryptase secreted from these cells which activates PAR-2, resulting in induction of COX-2, synthesis of 15d-PGJ2, activation of PPAR␥, and proliferation of fibroblasts.18 PAR-2 and Pancreatitis In this issue of GASTROENTEROLOGY, Namkung et al extensively document how the different effects transduced by PAR-2 combine to modulate the severity of acute pancreatitis.19 Because PAR-2 mediates a wide 1908 EDITORIALS variety of functions, its activation produces dual and divergent effects in acute pancreatitis: it provides local pancreatic protection, but partially mediates the distant systemic complications Indeed, in both in vitro and in vivo models of pancreatitis (induced by bile acid and cerulein), PAR-2 activation protected against pancreatic cellular damage and death While unexpected, these local protective effects are also consistent with the preliminary observations made by other investigators using PAR-2 knock-out mice.20 The most intriguing question raised by these findings is the mechanism underlying this protective effect The authors suggest that this protection results from PAR-2 stimulation of HCO3Ϫ secretion However, in the pertinent in vitro experiments, acinar cells were used to establish PAR-2 protection while ducts cells were used to verify PAR-2 stimulation of HCO3Ϫ secretion Defining the mechanism for PAR-2 mediated protection will likely remain an active field of investigation To examine the basis for the extrapancreatic effects of PAR-2, the authors established that PAR-2 activation in THP-1 monocyte-derived cells (a model for immune cells) induced NF-␬B activation and IL-8 production, while PAR-2 activation in HUVEC cells (a model for endothelial cells) stimulated NO production They next demonstrated that the decreases in both systolic and diastolic blood pressures and the widening of the pulse pressure observed with pancreatitis were partially reproduced with an infusion of either trypsin, at a circulating concentration determined experimentally, or PAR-2 activating peptide PAR-2 activation by trypsin may therefore partially mediate the hemodynamic response to pancreatitis, possibly through systemic vasodilation This vasodilation may result from vascular smooth muscle relaxation, caused by NO released from endothelial cells that have been stimulated by PAR-2; alternatively, vasodilation may also result from direct activation of PAR-2 on smooth muscle or activation of PAR-2 on afferent nerves, causing secretion of the vasodilating agent, CGRP Again, clarifying whether and how PAR-2 may mediate or modulate the dysfunctions of distant organs in severe pancreatitis will be a fertile field of research However, we should recognize that while Namkung et al aptly raise our awareness about the role of PAR-2 in acute pancreatitis, PAR-2 is probably only one amongst many factors that modulates the severity of this ailment In addition to a role in acute pancreatitis, PAR-2 activation may also contribute to the pathology of chronic pancreatitis As mentioned previously, PAR-2 activation sensitizes afferent nerve fibers to amplify and prolong pain sensation Activation of PAR-2 in the pancreas activates and sensitizes pancreas-specific afferent GASTROENTEROLOGY Vol 126, No neurons in the corresponding dorsal root ganglions.15 A role for PAR-2 in modulating pain, one of the most vexing manifestations of chronic pancreatitis, is therefore plausible Along with pain and pancreatic insufficiency, pancreatic fibrosis is another hallmark of chronic pancreatitis Activation of PAR-2 by tryptase released from stimulated mast cells may lead to active deposition of collagen.21 Indeed mast cell tryptase promotes repair following inflammation, including fibrogenesis, as it induces angiogenesis and stimulates proliferation of epithelial cells, fibroblasts, myocytes, and endothelial cells In the respiratory system, tryptase, acting through PAR-2 induces pulmonary fibrosis through multiple mechanisms: stimulation of the recruitment and growth of airway fibroblasts and enhanced collagen production by these cells.21 In the digestive system, the presence and/or activation of mast cells correlate with fibrotic disorders, such as collagenous colitis,22 intestinal strictures in Crohn’s disease,23 radiation-induced fibrosis, and hepatic cirrhosis.24 It is therefore plausible that PAR-2 activation may also contribute to the fibrosis observed in chronic pancreatitis As pointed out by the authors, this report may also have clinical implications Gabexate, a protease inhibitor, can be effective in preventing the generation of pancreatitis, such as the one observed following ERCP However, this prophylactic use is not widely adopted, as it is difficult to predict accurately individual bouts of pancreatitis As this article shows that trypsin is not only important in initiating pancreatitis, but also in modulating its severity, the window for administering gabexate may be extended to the early phase of this disorder In Greek mythology, Proteus, son of Poseidon, was a seer who knew everything about the past, present and future, but did not foretell the future to anyone unless he was seized and held on tightly, even as he took on different shapes The word protean is derived from his name Like Proteus, PAR-2 may mediate multiple effects resulting in different manifestations; however, with a firm grasp of its function, PAR-2 will help us clarify many of the challenging enigmas in digestive physiology and pathology TOAN D NGUYEN Division of Gastroenterology University of Washington & Puget Sound VA Health Care System Seattle, Washington References De´ry O, Corvera CU, Steinhoff M, Bunnett NW Proteinase-activated receptors: novel mechanisms of signaling by serine proteases Am J Physiol 1998;274:C1429 –C1452 June 2004 Nystedt S, Emilsson K, Wahlestedt C, Sundelin J Molecular cloning of a potential proteinase activated receptor Proc Natl Acad Sci U S A 1994;91:9208 –9212 Alm A-K, Gagnemo-Persson R, Sorsa T, Sundelin J Extrapancreatic trypsin-2 cleaves proteinase-activated receptor-2 Biochem Biophys Res Comm 2000;275:77– 83 Cottrell GS, Amadesi S, Grady EF, Bunnett NW Trypsin IV: a novel agonist of protease-activated receptors and J Biol Chem 2004;In Press Camerer E, Huang W, Coughlin SR Tissue factor- and factor X-dependent activation of protease-activated receptor by factor VIIa Proc Natl Acad Sci U S A 2000;97:5255–5260 D’Andrea M, Derian C, Leturcq D, Baker S, Brunmark A, Ling P, Darrow A, Santulli R, Brass L, Andrade-Gordon P Characterization of protease activated receptor-2 immunoreactivity in normal human tissues J Histochem Cytochem 1998;46:157–164 Nguyen TD, Moody MW, Steinhoff M, Okolo C, Koh D-S, Bunnett NW Trypsin activates pancreatic duct epithelial cell ion channels through proteinase-activated receptor-2 J Clin Invest 1999;103: 261–269 Danahay H, Withey L, Poll CT, Van De Graaf SFJ, Bridges RJ Protease-activated receptor-2-mediated inhibition of ion transport in human bronchial epithelial cells Am J Physiol Cell Physiol 2001;280:C1455–C1464 Bertog M, Letz B, Kong W, Steinhoff M, Higgins MA, BielfeldAckermann A, Froămter E, Bunnett NW, Korbmacher C Basolateral proteinase-activated receptor (PAR-2) induces chloride secretion in M-1 mouse renal cortical collecting duct cells J Physiol 1999; 521:3–17 10 Cocks TM, Fong B, Chow JM, Anderson GP, Frauman AG, Goldie RG, Henry PJ, Carr MJ, Hamilton JR, Moffatt JD A protective role for protease-activated receptors in the airways Nature (London) 1999;398:156 –160 11 Steinhoff M, Vergnolle N, Young SH, Tognetto M, Amadesi S, Ennes HS, Trevisani M, Hollenberg MD, Wallace JL, Caughey GH, Mitchell SE, Williams LM, Geppetti P, Mayer EA, Bunnett NW Agonists of proteinase-activated receptor induce inflammation by a neurogenic mechanism Nat Med 2000;6:151–158 12 Figini M, Emanueli C, Grady EF, Kirkwood K, Payan DG, Ansel J, Gerard C, Geppetti P, Bunnett N Substance P and bradykinin stimulate plasma extravasation in the mouse gastrointestinal tract and pancreas Am J Physiol 1997;272:G785–G793 13 Kawabata A, Kinoshita M, Nishikawa H, Kuroda R, Nishida M, Araki H, Arizono N, Oda Y, Kakehi K The protease-activated receptor-2 agonist induces gastric mucus secretion and mucosal cytoprotection J Clin Invest 2001;107:1443–1450 14 Vergnolle N, Bunnett NW, Sharkey KA, Brussee V, Compton SJ, Grady EF, Cirino G, Gerard N, Basbaum AI, Andrade-Gordon P, EDITORIALS 15 16 17 18 19 20 21 22 23 24 1909 Hollenberg MD, Wallace JL Proteinase-activated receptor-2 and hyperalgesia: a novel pain pathway Nat Med 2001;7:821– 826 Hoogerwerf WA, Zou L, Shenoy M, Sun D, Micci MA, Lee-Hellmich H, Xiao SY, Winston JH, Pasricha PJ The proteinase-activated receptor is involved in nociception J Neurosci 2001;21:9036 – 9042 Compton SJ, Cairns JA, Holgate ST, Walls AF The role of mast cell tryptase in regulating endothelial cell proliferation, cytokine release, and adhesion molecule expression J Immunol 1998; 161:1939 –1946 Fiorucci S, Mencarelli A, Palazzetti B, Distrutti E, Vergnolle N, Hollenberg MD, Wallace JL, Morelli A, Cirino G Proteinase-activated receptor is an anti-inflammatory signal for colonic lamina propria lymphocytes in a mouse model of colitis Proc Natl Acad Sci U S A 2001;98:13936 –13941 Frungieri MB, Weidinger S, Meineke V, Kohn FM, Mayerhofer A Proliferative action of mast-cell tryptase is mediated by PAR-2, COX-2, prostaglandins, and PPAR␥: possible relevance to human fibrotic disorders Proc Natl Acad Sci U S A 2002;99:15072– 15077 Namkung W, Han W, Luo X, Muallem S, Cho KH, Kim KH, Lee MG Protease-activated receptor exerts local protection and mediates part of systemic complications in acute pancreatitis Gastroenterology 2004;126:1844 –1859 Singh VP, Saluja AK, Navina S, Bhagat L, Andrade-Gordon P, Steer ML PAR-2 plays a protective role in pancreatitis Gastroenterology 2003;124(Suppl 1):A-94 Akers IA, Parsons M, Hill MR, Hollenberg MD, Sanjar S, Laurent GJ, McAnulty RJ Mast cell tryptase stimulates human lung fibroblast proliferation via protease-activated receptor-2 Am J Physiol 2000;278:L193–L201 Schwab D, Raithel M, Hahn EG Evidence for mast cell activation in collagenous colitis Inflamm Res 1998;47(Suppl 1):S64 – S65 Gelbmann CM, Mestermann S, Gross V, Koăllinger M, Schoălmerich J, Falk W Strictures in Crohn’s disease are characterized by an accumulation of mast cells colocalised with laminin but not with fibronectin or vitronectin Gut 1999;45:210 –217 Matsunaga Y, Kawasaki H, Terada T Stromal mast cells and nerve fibers in various chronic liver diseases: relevance to hepatic fibrosis Am J Gastroenterol 1999;94:1923–1932 Address requests for reprints to: Toan D Nguyen, M.D., GI Section (S-111-Gastro) Puget Sound VA Health Care System, Seattle, 1660 S Columbian Way, Washington 98108 e-mail: T1Nguyen@ u.washington.edu; fax: (206) 764-2232 © 2004 by the American Gastroenterological Association 0016-5085/04/$30.00 doi:10.1053/j.gastro.2004.04.026 ... Molecular cloning of a potential proteinase activated receptor Proc Natl Acad Sci U S A 1994;91: 920 8 – 921 2 Alm A- K, Gagnemo-Persson R, Sorsa T, Sundelin J Extrapancreatic trypsin -2 cleaves proteinase- activated. .. Substance P and bradykinin stimulate plasma extravasation in the mouse gastrointestinal tract and pancreas Am J Physiol 1997 ;27 2:G785–G793 13 Kawabata A, Kinoshita M, Nishikawa H, Kuroda R, Nishida... Press Camerer E, Huang W, Coughlin SR Tissue factor- and factor X-dependent activation of protease -activated receptor by factor VIIa Proc Natl Acad Sci U S A 20 00;97: 525 5– 526 0 D’Andrea M, Derian