Localization of Protease-Activated Receptors -1 and -2 in Human Mast Cells: Indications for an Amplified Mast Cell Degranulation Cascade

Protease-activated receptors (PARs) belong to a family of G-coupled seven transmembrane receptors that are activated by a proteolytic cleavage of their N-termini. Recent studies suggest the involvement of protease-activated receptors-1 and -2 (PAR-1, PAR-2) activators in mast cell de-granulation in various physiological and pathophysiological processes in inflammatory responses. Although PAR-1 and PAR-2 activating proteases, thrombin and tryptase, have been associated with mast cell activation, PAR-1 and PAR-2 have not been localized within these cells. We describe here the localization of PAR-1 and PAR-2 in mast cells from various normal human tissues using im-munohistochemical and double immunofluorescence techniques. The presence of these receptors on the membrane may explain the actions of accessible extracellular thrombin and tryptase for mast cell activation. In addition to the membrane labeling, these receptors are also localized on the membrane of the intracellular tryptase-positive granules, which may function to sustain further mast cell degranulation upon exocytosis. The localization of these two receptors in mast cells suggests a novel mechanism for controlling mast cell activation through regulation of PARI and PAR-2.     

Involvement of proteinase-activated receptor-2 in mast cell tryptase-induced barrier dysfunction in bovine aortic endothelial cells

We report here a direct modulation by mast cell tryptase of endothelial barrier function through activation of proteinase-activated receptor-2 (PAR-2). In cultured bovine aortic endothelial cells (BAECs), tryptase, trypsin and PAR-2 activating peptide impaired the barrier function as determined by the permeability of protein-conjugated Evans blue. The tryptase-induced barrier dysfunction was completely blocked by U73122, and partially reversed by xestospongin C, calphostin C or Y27632. The intracellular Ca(2+) was elevated by tryptase. It was notable that ioxaglate, a contrast material that degranulates mast cells, markedly increased the permeability when applied to BAECs in combination with mast cells, an action that was blocked by nafamostat, a potent tryptase inhibitor. Immunofluorescence analysis showed that actin stress fibre formation and disruption of VE-cadherin were observed after exposure to tryptase or ioxaglate in combination with mast cells. Therefore, it is suggested that mast cell tryptase impairs endothelial barrier function through activation of endothelial PAR-2 in a manner dependent on the phospholipase C activity.     

Proteinase-activated receptors: novel mechanisms of signaling by serine proteases

Although serineproteases are usually considered to act principally as degradativeenzymes, certain proteases are signaling molecules that specificallyregulate cells by cleaving and triggering members of a new family ofproteinase-activated receptors (PARs). There are three members of thisfamily, PAR-1 and PAR-3, which are receptors for thrombin, and PAR-2, areceptor for trypsin and mast cell tryptase. Proteases cleave withinthe extracellular NH₂-terminus oftheir receptors to expose a newNH₂-terminus. Specific residueswithin this tethered ligand domain interact with extracellular domainsof the cleaved receptor, resulting in activation. In common with many Gprotein-coupled receptors, PARs couple to multiple G proteins andthereby activate many parallel mechanisms of signal transduction. PARsare expressed in multiple tissues by a wide variety of cells, wherethey are involved in several pathophysiological processes, includinggrowth and development, mitogenesis, and inflammation. Because thecleaved receptor is physically coupled to its agonist, efficientmechanisms exist to terminate signaling and prevent uncontrolledstimulation. These include cleavage of the tethered ligand, receptorphosphorylation and uncoupling from G proteins, and endocytosis andlysosomal degradation of activated receptors.

     

Reaction of mast cell proteases tryptase and chymase with protease activated receptors (PARs) on keratinocytes and fibroblasts

Protease activated receptors (PARs) compose a family of G protein signal transduction receptors activated by proteolysis. In this study, the susceptibility of PARs expressed on human keratinocytes and dermal fibroblasts to the human mast cell proteases tryptase and chymase was evaluated. PAR activation was measured by monitoring cytosolic [Ca²⁺] in cells loaded with the fluorescent Ca²⁺ probe Fura-2. Tryptase produced transient cytosolic Ca²⁺ mobilization in keratinocytes, but not in fibroblasts. Ca²⁺ mobilization in keratinocytes required enzymatically active tryptase, demonstrated desensitization, and was blocked by pretreatment of cells with the PAR-2 peptide agonist SLIGKV, trypsin, or the phospholipase inhibitor U73122. Heparin, a GAG that binds to tryptase, stabilizing its functional form, also inhibited tryptase-induced Ca²⁺ mobilization. The maximal response elicited by tryptase was smaller than that observed upon treatment of keratinocytes with trypsin, a known activator of PAR-2, and keratinocytes made refractory to tryptase by pretreatment with the protease remained responsive to trypsin. Pretreatment of keratinocytes with thrombin, an activator of PAR-1 and -3 (thrombin receptors), had no detectable effect on the tryptase or trypsin responses. These data suggest that in keratinocytes tryptase may be activating a subpopulation of PAR-2 receptors. Treatment of keratinocytes or fibroblasts with human chymase did not produce Ca²⁺ mobilization, nor did it affect Ca²⁺ mobilization produced by trypsin. However, chymase pretreatment of fibroblasts rapidly inhibited the ability of these cells to respond to thrombin. Inhibition was dependent on chymase enzymatic activity and was not significantly affected by the presence of heparin. This finding is consistent with studies indicating that PAR-1 may be susceptible to proteases with chymotrypsin-like specificity. These results suggest that the proteases tryptase and chymase secreted from mast cells in skin may affect the behavior of surrounding cells by the hydrolysis of PARs expressed by these cells. J. Cell. Physiol. 176:365–373, 1998. © 1998 Wiley-Liss, Inc.     

Protease-activated receptors in the musculoskeletal system

Protease-activated receptors (PARs) mediate cellular responses to a subset of extracellular proteases, including blood coagulation factors and proteases produced by inflammatory cells. Cells in bone, cartilage and muscle exhibit cell type-specific expression patterns and functional responses for the different PARs. Activators of PAR-1 include thrombin, and activators of PAR-2 include trypsin and tryptase; PARs-3 and -4 are also receptors for thrombin. Thrombin stimulates PAR-1-mediated proliferative responses in osteoblasts, chondrocytes and myoblasts, and in developing muscle, PAR-1 activation by thrombin appears to mediate activity-dependent polyneuronal synapse reduction. In bone, activation of PAR-2 leads to inhibition of osteoblast-mediated osteoclast differentiation induced by hormones or cytokines, and in muscle, PAR-2 activation leads to stimulation of myoblast proliferation. Although there is some evidence for a role for PARs expressed by cells of the musculoskeletal system at specific stages of development, their major role appears to be in protecting the tissues from the destructive effects of inflammation and promoting regeneration. This review discusses the regulation of cell function in the musculoskeletal system by receptor-mediated responses to proteases. Expression patterns of PARs, the circumstances in which PAR activators are likely to be present, functional responses of PAR activation, and responses to thrombin for which receptors have not yet been identified are considered.     

蛋白酶激活受体2(PAR-2)激动剂对肥大细胞释放类胰蛋白酶的影响

研究反肉桂酰-亮-异亮-甘-精-亮-鸟-[酰胺]（tc-LIGRLO),一种PAR-2激动剂,对肥大细胞类胰蛋白酶释放的影响。结果显示,经过15min的培养,tc-LIGRLO可引起比基础分泌量增加1倍以上的类胰蛋白酶释放,作用强度超过抗IgE抗体和钙离子导入剂（calcium ionophore A23187,CI),而反PAR-2激动剂-反肉桂酰-鸟-亮-精-甘-异亮-亮-[酰胺]（tc-OLRGIL)无此作用,培养时间延长到30min时对tc-LIGRLO的作用无明显影响,其时间关系曲线表明,tc-LIGRLO的作用从1min开始,3min后达高峰,结果表明,PAR-2激动剂tc-LIGRLO是一种高效类胰蛋白酶释放刺激剂,在肥大细胞上可能有PAR-2存在。     

Induction of IL-4 release and upregulated expression of protease activated receptors by GM-CSF in P815 cells

GM-CSF has been showed to be able to induce up-regulated receptor and cytokine expression in mast cells in inflammatory conditions. However, little is known of its effects on protease activated receptor (PAR) expression and Th2 cytokine secretion from mast cells. In the present study, we examined potential influence of GM-CSF on mast cell PAR expression and IL-4 and IL-10 release by using flow cytometry analysis, quantitative real time PCR, ELISA and cellular activation of signaling ELISA (CASE) techniques. The results showed that GM-CSF induced up to 3.0-fold increase in IL-4 release from P815 cells, and FSLLRY-NH₂ and trans-cinnamoyl (tc)-YPGKF-NH₂ did not affect GM-CSF induced IL-4 release. GM-CSF reduced tryptase and trypsin induced IL-4 release by up to approximately 55.8% and 70.3%, respectively. GM-CSF elicited the upregulated expression of PAR-1, PAR-2, PAR-3 and PAR-4 mRNAs, but enhanced only PAR-4 protein expression in P815 cells. U0126, PD98059 and LY204002 almost completely abolished GM-CSF induced IL-4 release when they were preincubated with P815 cells for 30 min, indicating ERK and Akt cell signaling pathways may be involved in the event. In conclusion, GM-CSF can stimulate IL-4 release from mast cells through an ERK and Akt cell signaling pathway dependent, but PAR independent mechanism. GM-CSF may serve as a regulator for IL-4 production in mast cells and through which participates in the mast cell related inflammation.     

Human airway trypsin-like protease stimulates human bronchial fibroblast proliferation in a protease-activated receptor-2-dependent pathway

Human airway trypsin-like protease (HAT) was isolated from airway secretions and localized to bronchial epithelial cells by immunohistochemistry. In the present study, we examined whether HAT could stimulate DNA synthesis and proliferation of primary human bronchial fibroblasts (HBF). HAT significantly stimulated the proliferation of HBF by 20-55%, a level similar to that of the mitogenic activity of lung mast cell tryptase (MCT). HAT also stimulated the incorporation of [3H]thymidine in HBF, and this HAT-induced DNA synthesis was abolished by leupeptin. Protease-activated receptor-2 (PAR-2) mRNA was expressed and localized to the cell surface in HBF. PAR-2 activating peptide (AP) also enhanced DNA synthesis, and both HAT and PAR-2 AP induced receptor internalization, similar to the response to trypsin. Pretreatment of HBF with anti-PAR-2 antibody significantly suppressed both HAT and PAR-2 AP-induced DNA synthesis. In addition, HAT and PAR-2 AP induced intracellular Ca2+ mobilization in HBF. The HAT-induced increase in Ca2+ was desensitized by pretreatment with trypsin or PAR-2 AP. U0126, a specific MAPK inhibitor, completely inhibited HAT-induced DNA synthesis as well as HAT-induced phosphorylation of MAPK. The effect of HAT and MCT together was additive, whereas the effect of HAT and insulin together on HBF DNA synthesis was synergistic. These results indicate that HAT stimulates fibroblast proliferation in bronchial airways through a PAR-2-dependent MEK-MAPK mediated pathway and that HAT is linked to airway processes involving fibroblasts.     

Agonists of proteinase-activated receptor 2 induce TNF-alpha secretion from astrocytoma cells

Proteinase-activated receptor 2 (PAR2) is cleaved and activated by trypsin or mast cell tryptase, and may play an important role in inflammation. We have investigated the potential of PAR2 agonists to modulate TNF-alpha secretion from human astrocytoma cell line CCF-STTG1. We found that CCF-STTG1 expresses PAR2 by RT-PCR and Western blot analysis. Agonists such as trypsin, the peptide SLIGKV-NH(2) (corresponding to the PAR2 tethered ligand), or mast cell tryptase directly signal to CCF-STTG1 to stimulate secretion of TNF-alpha but do not stimulate in the presence of soybean trypsin inhibitor (SBTI) or VKGILS-NH(2) (reverse peptide). The secretion of TNF-alpha by trypsin was significantly blocked by pretreatment with either 50 microM PD98059 or 1 microM SB203580. Furthermore, trypsin stimulated the activation of extracellular signal-regulated kinase (ERK) and p38 MAP kinase homologue in CCF-STTG1 without any detectable activation of c-Jun N-terminal kinase (JNK). These results show that trypsin may induce TNF-alpha secretion following activation of ERK and p38 via PAR2 in CCF-STTG1.     

Tryptase promotes human monocyte-derived macrophage foam cell formation by suppressing LXRα activation

Accumulated mast cells in atherosclerotic plaques secrete a high level of tryptase that may participate in the pathogenesis of atherosclerotic disease by diverse pathways. However, the role of tryptase in the lipid metabolism of macrophages remains to be defined. In the present study, we found that the addition of tryptase into THP-1-derived macrophages increased both intracellular lipid accumulation and total cholesterol level. Tryptase promoting foam cell formation was also observed by transmission electron microscope. These effects were resisted by APC366, a selective inhibitor of mast cell tryptase. Tryptase dramatically resisted 22RHC induced activation of LXRα protein expression, which can be reversed by SAM-11 (a PAR-2-specific neutralizing antibody) and reduced LXRα, ABCG1, ABCA1 and SREBP-1c mRNA levels and ABCG1 protein level, which were all blocked by APC366. PAR-2 agonist also redeemed 22RHC stimulation to activate LXRα, ABCG1 protein expression, and mRNA levels of LXRα and its target genes in both THP-1-derived macrophages and primary human monocyte-derived macrophages. In primary macrophages that were first transfected with PAR-2 siRNA and then treated with tryptase, both the ABCG1 protein level and mRNA levels of LXRα and ABCG1 were higher than those in the control siRNA-treated cells. Taken together, our data clarified the PAR-2 expression of human macrophages and suggested that tryptase might promote lipid accumulation in macrophages and foam cell formation by suppressing LXRα activation via PAR-2/LXRα/LXRα target genes signaling pathway. This investigation sheds a new light on the role of tryptase in foam cell formation and pathogenesis of atherosclerosis.     

Mast cell tryptase stimulates human lung fibroblast proliferation via protease-activated receptor-2

Mast cells play a potentially important role in fibroproliferative diseases, releasing mediators including tryptase that are capable of stimulating fibroblast proliferation and procollagen synthesis. The mechanism by which tryptase stimulates fibroblast proliferation is unclear, although recent studies suggest it can activate protease-activated receptor (PAR)-2. We therefore investigated the role of PAR-2 in tryptase-induced proliferation of human fetal lung and adult lung parenchymal and airway fibroblasts and, for comparative purposes, adult dermal fibroblasts. Tryptase (0.7-70 mU/ml) induced concentration-dependent increases in proliferation of all fibroblasts studied. Antipain, bis(5-amidino-2-benzimidazolyl)methane, and benzamidine inhibited tryptase-induced fibroblast proliferation, demonstrating that proteolytic activity is required for the proliferative effects of tryptase. RT-PCR demonstrated the presence of PAR-2 mRNA, and immunohistochemical staining localized PAR-2 to the cell surface of lung fibroblasts. In addition, specific PAR-2 activating peptides, SLIGKV and SLIGRL, mimicked the proliferative effects of tryptase. In contrast, human dermal fibroblasts only weakly stained with the PAR-2 antibody, PAR-2 mRNA was almost undetectable, and fibroblasts did not respond to PAR-2 activating peptides. These results suggest that tryptase induces lung, but not dermal, fibroblast proliferation via activation of PAR-2 and are consistent with the hypothesis that the release of tryptase from activated mast cells may play an important role in the fibroproliferative response observed in asthma, chronic obstructive pulmonary disease, and patients with pulmonary fibrosis.     

PAR-2 activation,PGE2, and COX-2 in human asthmatic and nonasthmatic airway smooth muscle cells

The protease-activated receptor-2 (PAR-2) is present on human airway smooth muscle (ASM) cells and can be activated by mast cell tryptase, trypsin, or an activating peptide (AP). Trypsin induced significant increases in PGE2 release from human ASM cells after 6 and 24 h and also induced cyclooxygenase (COX)-2 mRNA expression and COX-2 protein. Tryptase and the PAR-2 AP did not alter PGE2 release or COX-2 protein levels, suggesting a lack of PAR-2 involvement. When we compared results in asthmatic and nonasthmatic muscle cells, both trypsin and bradykinin induced less PGE2 from asthmatic ASM cells, and bradykinin induced significantly less COX-2 mRNA in asthmatic cells. Significantly less PGE2 was released from proliferating ASM cells from asthmatic patients. In conclusion, trypsin induces PGE2 release and COX-2 in human ASM cells, which is unlikely to be via PAR-2 activation. In addition, ASM cells from asthmatic patients produce significantly less PGE2 and COX-2 compared with nonasthmatic cells. These findings may contribute to the increase in muscle mass evident in asthmatic airways.     

p24A, a type I transmembrane protein, controls ARF1-dependent resensitization of protease-activated receptor-2 by influence on receptor trafficking

Protease-activated receptor-2 (PAR-2), the second member of the G protein-coupled PAR family, is irreversibly activated by trypsin or tryptase and then targeted to lysosomes for degradation. Intracellular presynthesized receptors stored at the Golgi apparatus repopulate the cell surface after trypsin stimulation, thereby leading to rapid resensitization to trypsin signaling. However, the molecular mechanisms of the exocytic trafficking of PAR-2 from the Golgi apparatus to the plasma membrane remain largely unclear. Here we show that p24A, a type I transmembrane protein, which is a crucial constituent of the Golgi apparatus, associates with PAR-2 at the Golgi apparatus. The protein interaction occurs between the N-terminal region of p24A (residues 1-105; p24A-GL (GOLD domain with a small linker)) and the second extracellular loop of PAR-2. After receptor activation, PAR-2 dissociates from p24A. Importantly, we found that ADP-ribosylation factor 1 regulated the dissociation process and initiated PAR-2 trafficking to the plasma membrane. Conversely, overexpression of the fragment p24A-GL, but not other mutants containing the functional coiled-coil domain of p24A, arrested PAR-2 at the Golgi apparatus and inhibited receptor trafficking to the plasma membrane, which consequently prevented resensitization of PAR-2. These findings identify a new function of p24A as a regulator of signal-dependent trafficking that regulates the life cycle of PAR-2, Thus, we reveal a new molecular mechanism underlying resensitization of PAR-2.     

Protease-activated receptor-2 (PAR-2) is a weak enhancer of mucin secretion by human bronchial epithelial cells in vitro

PAR-2, a member of a family of G-protein-coupled receptors, can be activated by serine proteases via proteolytic cleavage. PAR-2 expression is known to be upregulated in respiratory epithelium subsequent to inflammation in asthma and chronic obstructive pulmonary disease (COPD). Since these diseases also are characterized by excessive mucus production and secretion, we investigated whether PAR-2 could be linked to mucin hypersecretion by airway epithelium. Normal human bronchial epithelial (NHBE) cells in primary culture or the human bronchial epithelial cell lines, NCI-H292 and HBE-1, were used. NHBE, NCI-H292, and HBE-1 cells expressed prominent levels of PAR-2 protein. Short-term (30min) exposure of cells to the synthetic PAR-2 agonist peptide (SLIGKV-NH2) elicited a small but statistically significant increase in mucin secretion at high concentrations (100microM and 1000microM), compared to a control peptide with reversed amino acid sequence (VKGILS-NH2). Neither human lung tryptase nor bovine pancreatic trypsin, both PAR-2 agonists, affected NHBE cell mucin secretion when added over a range of concentrations. Knockdown of PAR-2 expression by siRNA blocked the stimulatory effect of the AP. The results suggest that, since PAR-2 activation only weakly increases mucin secretion by human airway epithelial cells in vitro, PAR-2 probably is not a significant contributor to mucin hypersecretion in inflamed airways.     

Mast cell tryptase controls paracellular permeability of the intestine. Role of protease-activated receptor 2 and beta-arrestins

Tight junctions between intestinal epithelial cells prevent ingress of luminal macromolecules and bacteria and protect against inflammation and infection. During stress and inflammation, mast cells mediate increased mucosal permeability by unknown mechanisms. We hypothesized that mast cell tryptase cleaves protease-activated receptor 2 (PAR2) on colonocytes to increase paracellular permeability. Colonocytes expressed PAR2 mRNA and responded to PAR2 agonists with increased [Ca2+]i. Supernatant from degranulated mast cells increased [Ca2+]i in colonocytes, which was prevented by a tryptase inhibitor, and desensitized responses to PAR2 agonist, suggesting PAR2 cleavage. When applied to the basolateral surface of colonocytes, PAR2 agonists and mast cell supernatant decreased transepithelial resistance, increased transepithelial flux of macromolecules, and induced redistribution of tight junction ZO-1 and occludin and perijunctional F-actin. When mast cells were co-cultured with colonocytes, mast cell degranulation increased paracellular permeability of colonocytes. This was prevented by a tryptase inhibitor. We determined the role of ERK1/2 and of beta-arrestins, which recruit ERK1/2 to PAR2 in endosomes and retain ERK1/2 in the cytosol, on PAR2-mediated alterations in permeability. An ERK1/2 inhibitor abolished the effects of PAR2 agonist on permeability and redistribution of F-actin. Down-regulation of beta-arrestins with small interfering RNA inhibited PAR2-induced activation of ERK1/2 and suppressed PAR2-induced changes in permeability. Thus, mast cells signal to colonocytes in a paracrine manner by release of tryptase and activation of PAR2. PAR2 couples to beta-arrestin-dependent activation of ERK1/2, which regulates reorganization of perijunctional F-actin to increase epithelial permeability. These mechanisms may explain the increased epithelial permeability of the intestine during stress and inflammation.     

Autocrine Extra-Pancreatic Trypsin 3 Secretion Promotes Cell Proliferation and Survival in Esophageal Adenocarcinoma

Trypsin or Tumor associated trypsin (TAT) activation of Protease-activated receptor 2 (PAR-2) promotes tumor cell proliferation in gastrointestinal cancers. The role of the trypsin/PAR-2 network in esophageal adenocarcinoma (EA) development has not yet been investigated. The aim of this study is to investigate the role of trypsin/PAR-2 activation in EA tumorogenesis and therapy. We found that esophageal adenocarcinoma cells (EACs) and Barrett’s Metaplasia (BART) expressed high levels of type 3 extra-pancreatic trypsinogen (PRSS3), a novel type of TAT. Activity of secreted trypsin was detected in cultured media from EA OE19 and OE33 cultures but not from BART culture. Surface PAR-2 expression in BART and EACs was confirmed by both flow cytometry and immunofluorescence. Trypsin induced cell proliferation (∼ 2 fold; P<0.01) in all tested cell lines at a concentration of 10 nM. Inhibition of PAR-2 activity in EACs via the PAR-2 antagonist ENMD (500 µM), anti-PAR2 antibody SAM-11 (2 µg/ml), or siRNA PAR-2 knockdown, reduced cell proliferation and increased apoptosis by up to 4 fold (P<0.01). Trypsin stimulation led to phosphorylation of ERK1/2, suggesting involvement of MAPK pathway in PAR-2 signal transduction. Inhibition of PAR-2 activation or siRNA PAR-2 knockdown in EACs prior to treatment with 5 FU reduced cell viability of EACs by an additional 30% (P<0.01) compared to chemotherapy alone. Our data suggest that extra-pancreatic trypsinogen 3 is produced by EACs and activates PAR-2 in an autocrine manner. PAR-2 activation increases cancer cell proliferation, and promotes cancer cell survival. Targeting the trypsin activated PAR-2 pathway in conjunction with current chemotherapeutic agents may be a viable therapeutic strategy in EA.     

Human Tryptase Cleaves Pro-Nerve Growth Factor (Pro-NGF): HINTS OF LOCAL,MAST CELL-DEPENDENT REGULATION OF NGF/PRO-NGF ACTION*

Several factors regulate nerve growth factor (NGF), which is formed from pro-NGF by intracellular and extracellular enzymatic cleavage. The close proximity between mast cells expressing the protease tryptase and NGF-producing smooth muscle-like peritubular cells in the testes of infertile patients led us to examine whether tryptase is among those factors. Human peritubular cells express functional tryptase receptors (PAR-2). Recombinant enzymatically active β-tryptase increased NGF levels in the culture medium of primary human peritubular cells, but the peptide agonist for PAR-2 (SLIGKV) did not. Neither tryptase nor the peptide increased NGF mRNA levels. To test whether the increase in NGF is due to enzymatic activity of tryptase acting on pro-NGF, supernatants of peritubular cells and synthetic pro-NGF were treated with tryptase. Results of Western blot studies indicate enzymatic cleavage of pro-NGF by active tryptase. Heat-inactivated tryptase or SLIGKV was not effective. Mass spectrometry analysis of in vitro cleavage products from recombinant tryptase and synthetic pro-NGF revealed multiple cleavage sites within the pro-NGF sequence. The results also indicate the generation of mature NGF and smaller NGF fragments as a result of tryptase action. Thus, tryptase-secreting mast cells in the vicinity of pro-NGF/NGF-secreting cells in any human tissue are likely able to alter the ratios of pro-NGF/NGF. As NGF and pro-NGF have different affinities for their receptors, this indicates a novel way by which mast cells, via tryptase, can modify the microenvironment in human tissues with regard to neurotrophin actions.     

Jab1, a novel protease-activated receptor-2 (PAR-2)-interacting protein, is involved in PAR-2-induced activation of activator protein-1

Protease-activated receptor-2 (PAR-2), a G protein-coupled receptor for trypsin and tryptase, exerts important physiological and pathological functions in multiple systems. However, unlike PAR-1, the PAR-2-mediated intracellular signal transductions are hardly known. Here, using yeast two-hybrid screening with a human brain cDNA library, we identified an interacting partner of human PAR-2, the Jun activation domain-binding protein 1 (Jab1). The interaction was confirmed by glutathione S-transferase pull-down assays in vitro, and by co-immunoprecipitation assays in vivo. Jab1 was also shown to be colocalized with PAR-2 in both transfected HEK293 cells and in normal primary human astrocytes by double immunofluorescence staining. Further experiments demonstrated that multiple intracellular domains of PAR-2 are required for the interaction with Jab1. We then showed that agonist stimulation of PAR-2 disrupted the interaction, which could be prevented by the inhibitor of receptor endocytosis phenylarsine oxide, but not by the lysosomal protease inhibitor ZPAD. Importantly, we found that activation of PAR-2 induced the redistribution of Jab1 from the plasma membrane to the cytosol, but did not influence expression of Jab1. Furthermore, Jab1 mediated PAR-2-induced c-Jun activation, which was followed by increased activation of activator protein-1. Loss-of-function studies, using Jab1 small interfering RNA, demonstrated that Jab1 knockdown blocked PAR-2-induced activator protein-1 activation. Taken together, our data demonstrate that Jab1 is an important effector that mediates a novel signal transduction pathway for PAR-2-dependent gene expression.     

Kinetic analysis of the cleavage of human protease-activated receptor-1 / 2 / 3 and 4 using quenched-fluorescent peptide substrates

Protease-activated receptors [PARs] are a family of G-protein-coupled seven-transmembrane domain receptors that are activated by proteolytic cleavage of their amino-terminal exodomain. To characterize the cleavage rate of human PAR-1 / 2 / 3 and 4 by trypsin and thrombin, four synthetic quenched-fluorescent peptide substrates have been synthesized. Each substrate consisted of a ten-residue peptide spanning the receptor activation cleavage site and using progress-curve kinetics, k(cat) / K(m) values were determined.