Protease-mediated signalling: new paradigms for cell regulation and drug development

Thrombin stories in the gut

Many studies have demonstrated the involvement of proteases in gut physiology and pathophysiology over the recent years. Among them, thrombin has appeared for a long time as an old player only involved in blood clotting upon tissue injury. The fact that thrombin receptors (Protease-Activated Receptors-1 and -4) are expressed and functional in almost all cell types of the gut, contributing to barrier, immune or motility functions, suggested that thrombin could actually be at the crossroad of intestinal physiology. Recent work has unraveled the constitutive release of active thrombin by intestinal epithelial cells, opening new research avenues on the role of thrombin in the gut. These roles are considered in the present review, as well as the regulation of thrombin in the gut. The potential of thrombin as a target for treatments of intestinal pathologies is also discussed here.

Protease Activated Receptors and Arthritis

The catabolic and destructive activity of serine proteases in arthritic joints is well known; however, these enzymes can also signal pain and inflammation in joints. For example, thrombin, trypsin, tryptase, and neutrophil elastase cleave the extracellular N-terminus of a family of G protein-coupled receptors and the remaining tethered ligand sequence then binds to the same receptor to initiate a series of molecular signalling processes. These protease activated receptors (PARs) pervade multiple tissues and cells throughout joints where they have the potential to regulate joint homeostasis. Overall, joint PARs contribute to pain, inflammation, and structural integrity by altering vascular reactivity, nociceptor sensitivity, and tissue remodelling. This review highlights the therapeutic potential of targeting PARs to alleviate the pain and destructive nature of elevated proteases in various arthritic conditions.

Pruritogenic mechanisms and gut sensation: putting the "irritant" into irritable bowel syndrome

Chronic abdominal pain is a common clinical condition experienced by patients with irritable bowel syndrome (IBS). A general lack of suitable treatment options for the management of visceral pain is the major contributing factor to the debilitating nature of the disease. Understanding the underlying causes of chronic visceral pain is pivotal to identifying new effective therapies for IBS. This review provides the current evidence, demonstrating that mediators and receptors that induce itch in the skin also act as "gut irritants" in the gastrointestinal tract. Activation of these receptors triggers specific changes in the neuronal excitability of sensory pathways responsible for the transmission of nociceptive information from the periphery to the central nervous system leading to visceral hypersensitivity and visceral pain. Accumulating evidence points to significant roles of irritant mediators and their receptors in visceral hypersensitivity and thus constitutes potential targets for the development of more effective therapeutic options for IBS.

Effect of Protease-Activated Receptor-2-Activating Peptide on Guinea Pig Airway Resistance and Isolated Tracheal Strips

Purpose:

Protease-activated receptors (PARs) are a family of G-protein-coupled receptors distributed in a number of tissues. PAR-2 is expressed on airway epithelium and smooth muscles and overexpressed under pathological conditions, such as asthma and chronic obstructive pulmonary disease. However, the role of PAR-2 in airways has not yet been defined. In this study, we investigated the role of PAR-2-activating peptide (SLIGRL) on histamine-induced bronchoconstriction and the mechanisms underlying the bronchoprotective effect both in vivo and in vitro.

Materials and Methods:

The effect of SLIGRL was tested in vivo using histamine-induced bronchoconstriction in the guinea pig and in vitro using isolated tracheal spiral strips.

Results:

In vivo pretreatment with SLIGRL significantly reduced the histamine-induced increased bronchoconstriction. Neither propranolol nor vagotomy abolished the inhibitory effect of SLIGRL. Furthermore, indomethacin or glibenclamide did not antagonize the inhibitory response to SLIGRL. In isolated tracheal spiral strips in vitro, SLIGRL did not affect the contractile response to acetylcholine or potassium chloride; however, histamine-induced contraction was inhibited in a dose-dependent manner.

Conclusion:

Our data demonstrate the protective effect of SLIGRL in airways; however, this effect appears to be mediated independently of prostanoids, nitric oxide, circulating adrenaline, ATP-sensitive K + channels, and vagal stimulation.

Protease inhibition as new therapeutic strategy for GI diseases

The GI tract is the most exposed organ to proteases, both in physiological and pathophysiological conditions. For digestive purposes, the lumen of the upper GI tract contains large amounts of pancreatic proteases, but studies have also demonstrated increased proteolytic activity into mucosal tissues (both in the upper and lower GI tract), associated with pathological conditions. This review aims at outlining the evidences for dysregulated proteolytic homeostasis in GI diseases and the pathogenic mechanisms of increased proteolytic activity. The therapeutic potential of protease inhibition in GI diseases is discussed, with a particular focus on IBDs, functional GI disorders and colorectal cancer.

The catalytic triad of testes-specific protease 50 (TSP50) is essential for its function in cell proliferation

Testes-specific protease 50 (TSP50) is a novelly identified pro-oncogene and it shares a similar enzymatic structure with many serine proteases. Our previous results suggested that TSP50 could promote tumorigenesis through degradation of IκBα protein and activating NF-κB signaling, and the threonine mutation in its catalytic triad could depress TSP50-mediated cell proliferation. However, whether the two other residues in the catalytic triad of TSP50 play a role in maintaining protease activity and tumorigenesis, and the mechanisms involved in this process remain unclear. Here, we constructed and characterized three catalytic triad mutants of TSP50 and found that all the mutants could significantly depress TSP50-induced cell proliferation and colony formation in vitro and tumor formation in vivo, and the aspartic acid at position 206 in the catalytic triad played a more crucial role than threonine and histidine in this process. Mechanistic studies revealed that the mutants in the catalytic triad abolished the enzyme activity of TSP50, but did not change the cellular localization. Furthermore, our data indicated that all the three mutants suppressed activation of NF-κB signal by preventing the interaction between TSP50 and the NF-κB:IκBα complex. Most importantly, we demonstrated that TSP50 could interact with IκBα protein and cleave it directly as a new protease in vitro.

Protease-activated receptors as therapeutic targets in visceral pain

The protease-activated receptors (PARs) play a pivotal role in inflammatory and nociceptive processes. PARs have raised considerable interest because of their capacity to regulate numerous aspects of viscera physiology and pathophysiology. The present article summarizes research on PARs and proteases as signalling molecules in visceral pain. In particular, experiments in animal models suggest that PAR2 is important for visceral hypersensitivity. Moreover, endogenous PAR2 agonists seem to be released by colonic tissue of patients suffering from irritable bowel syndrome, suggesting a role for this receptor in visceral pain perception. Thus, PARs, together with proteases that activate them, represent exciting targets for therapeutic intervention on visceral pain.

Role of cockroach proteases in allergic disease

Allergic asthma is on the rise in developed countries, and cockroach exposure is a major risk factor for the development of asthma. In recent years, a number of studies have investigated the importance of allergen-associated proteases in modulating allergic airway inflammation. Many of the studies have suggested the importance of allergen-associated proteases as having a direct role on airway epithelial cells and dendritic cells. In most cases, activation of the protease activated receptor (PAR)-2 has been implicated as a mechanism behind the potent allergenicity associated with cockroaches. In this review, we focus on recent evidence linking cockroach proteases to activation of a variety of cells important in allergic airway inflammation and the role of PAR-2 in this process. We will highlight recent data exploring the potential mechanisms involved in the biological effects of the allergen.

RNA interference-based silencing reveals the regulatory role of fatty acid-binding protein 4 in the production of IL-6 and vascular endothelial growth factor in 3T3-L1 adipocytes

The fatty acid-binding protein 4 (FABP4) is believed to play an important role in maintaining glucose and lipid homeostasis. However, the physiological functions of FABP4 in adipocytes have not been fully elucidated because of difficulties associated with the effective transfection of small interfering RNA (siRNA) to differentiated adipocytes. The aim of this study was to clarify the physiological roles of FABP4 in adipocytes by establishing an efficient, universal technique for endogenous gene silencing in fully differentiated 3T3-L1 cells. Confocal-based three-dimensional observations demonstrated that, in traditionally cultured 3T3-L1 cells, multilayers of undifferentiated cells were formed. As a result, small interfering RNA failed to reach many of the differentiated cells. To solve this problem, we developed a reliable method, denoted as density-based separation followed by replating of enriched adipocytes in a monolayer (DREAM) and, using the developed method, succeeded in a significant knockdown of FABP4. Loss-of-function analyses revealed that FABP4 regulates the expression of IL-6 and vascular endothelial growth factor (VEGF) mediated by the protease-activated receptor 1 (PAR1), a thrombin receptor, in adipocytes. In addition, the basal IL-6 production was partially suppressed by PAR1 knockdown. Moreover, we also demonstrated that IL-6 stimulates the proliferation of primary endothelial cells isolated from murine adipose tissue. These findings indicate that FABP4 may have a crucial role in modulating IL-6 and vascular endothelial growth factor as angiogenesis inducers stimulated by the cellular action of thrombin on adipocytes via PAR1. These findings promise to be helpful for developing an understanding of physiological counterparts with respect to the inflammatory and angiogenic properties of adipose tissue.

Protease-activated receptor-2 regulates cyclooxygenase-2 expression in human bile duct cancer via the pathways of mitogen-activated protein kinases and nuclear factor kappa B

BACKGROUND/PURPOSE

Recent studies have suggested that protease-activated receptor-2 (PAR-2) activity correlates with cell proliferation and tumor growth, and its activation induces expression of cyclooxygenase-2 (COX-2). However, no previous reports have investigated PAR-2 signaling pathways in bile duct cancer. The aim of this study was to determine whether PAR-2 activation can regulate COX-2 expression via mitogen-activated protein kinases (MAPKs) and nuclear factor kappa B (NF-κB) in human bile duct cancer cells.

METHODS

We immunohistochemically examined PAR-2 and COX-2 expression in 104 resected human specimens of extrahepatic bile duct cancer. We then determined how inhibitors of MAPKs and NF-κB signaling pathways influence COX-2 expression under PAR-2 activation in HuCCT1 and TKKK, human bile duct cancer cell lines.

RESULTS

PAR-2 and COX-2 proteins were immunohistochemically recognized in 63 and 57% of specimens and were significantly correlated. PAR-2 agonist peptide activated mRNA and protein expression of COX-2 in HuCCT1 and TKKK. Pharmacologic blockade of p44/42 or p38 MAPK significantly inhibited PAR-2-activated mRNA and protein expression of COX-2 in both cells. COX-2 protein expression was also inhibited by the blocker of NF-κB pathway in both cells.

CONCLUSIONS

PAR-2 may regulate COX-2 expression in human bile duct cancer via the MAPKs and NF-κB pathways.

German cockroach frass proteases modulate the innate immune response via activation of protease-activated receptor-2

Allergen exposure can induce an early innate immune response; however, the mechanism by which this occurs has not been addressed. In this report, we demonstrate a role for the active serine proteases in German cockroach (GC) feces (frass) and protease-activated receptor (PAR)-2 in modulating the innate immune response. A single exposure of GC frass induced inflammatory cytokine production and cellular infiltration in the airways of mice. In comparison, exposure to protease-depleted GC frass resulted in diminution of inflammatory cytokine production and airway neutrophilia, but had no effect on macrophage infiltration. Selective activation of PAR-2 confirmed that PAR-2 was sufficient to induce airway inflammation. Exposure of GC frass to PAR-2-deficient mice led to decreased immune responses to GC frass compared to wild-type mice. Using the macrophage as an early marker of the innate immune response, we found that GC frass induced significant release of tumor necrosis factor-alpha from primary alveolar macrophages. This effect was dependent on the intrinsic proteases in GC frass. We confirmed GC frass-induced cytokine expression was mediated by activation of NF-kappaB and ERK in a macrophage cell line. Collectively, these data suggest a central role for GC frass protease-PAR-2 activation in regulating the innate immune response through the activation of alveolar macrophages. Understanding the potential role of protease-PAR-2 activation as a danger signal or adjuvant could yield attractive therapeutic targets.

Thrombin increases inflammatory cytokine and angiogenic growth factor secretion in human adipose cells in vitro

Background

Abdominal obesity is associated with pro-thrombotic and inflammatory states. Therefore, the purpose of this study was to examine the expression of thrombin receptors (PAR1 and PAR4) human adipose tissue and whether thrombin stimulates an inflammatory cytokine and growth factor profile in human adipose tissue.

Methods

Human adipose tissue, isolated preadipocytes and differentiated adipocytes were used in this study. PAR1 and PAR4 mRNA and protein were detected by RT-PCR and immunoblot analysis in both adipose tissue and adipose microvessels. In separate studies, IL-1β, IL-6, MCP-1, TNF-α, IL-10, FGF-2, VEGF, and PDGF production were measured from adipose tissue (n = 5), adipocytes (n = 5), and preadipocytes (n = 3) supernatants with and without thrombin (1 or 10 U/ml; 24 hrs) treatment.

Results

Thrombin increased cytokine secretion of IL-1β, IL-6, MCP-1 and TNF-α and growth factor secretion of VEGF from adipocytes along with MCP-1 and VEGF from preadipocytes. The direct thrombin inhibitor lepirudin given in conjunction with thrombin prevented the thrombin-mediated increase in cytokine and growth factor secretion.

Conclusion

Here we show that thrombin PAR1 and PAR4 receptors are present and that thrombin stimulates inflammatory cytokine generation and growth factor release in human adipose tissue and cells in vitro. These data suggest that thrombin may represent a molecular link between obesity and associated inflammation.

Differential effects of serine proteases on the migration of normal and tumor cells: implications for tumor microenvironment

The supporting role of proteases in tumor progression and invasion is well known; however, the use of proteases as therapeutic agents has also been demonstrated. In this article, the authors report on the differential effects of exogenous serine proteases on the motility of tumor and normal cells. The treatment of normal and tumor cells with a single dose of pancreatic serine proteases, trypsin (TR) and chymotrypsin (CH), leads to a concentration-dependent response by cells, first accelerating and then slowing mobility. Tumor cells are 10 to 20 times more sensitive to exogenous TR/CH, suggesting that a single dose of proteases may cause discordant movements of normal and tumor cells within the tumor environment. The inhibitory effects of TR on cell motility are contradicted by thrombin (TH), particularly in the regulation of normal cells' migration. The purpose of this investigation was to ascertain the role of protease-activated receptors (PARs) in terms of normal and tumor cell motility. Duplicate treatments with proteases resulted in diminished mobility of both normal and tumor cells. Repeated application of TR and TH in 1-hour treatment intervals initially desensitizes cell surface PARs. However, cell surface PARs reappear regardless of subsequent protease treatments in both normal and tumor cells. The resensitization process is retarded in tumor cells when compared with normal cells. This is evidenced by lower expression of PARs as well as by their relocalization at the tumor cell surfaces. Under these conditions, normal cells remain responsive to exogenous proteases in terms of cell motility. Exogenous proteases do not modulate motility of repeatedly stimulated tumor cells, and consequently, the migration of tumor cells appears disconnected from the PAR signaling pathways. The use of activating peptides in lieu of the cognate proteases for a given PAR system indicated that proteases may act through additional targets not regulated by PAR signaling. We hypothesize that the divergent migration patterns of normal and tumor cells due to exposure to proteases is in part mediated by PARs. Thus, treatment with exogenous proteases may cause rearrangement of the tumor and stromal cells within the tumor microenvironment. Such topographical effects may lead to the inhibition of tumor progression and metastasis development.

SCH 79797, a selective PAR1 antagonist, limits myocardial ischemia/reperfusion injury in rat hearts

Myocardial ischemia/reperfusion (I/R) injury is partly mediated by thrombin. In support, the functional inhibition of thrombin has been shown to decrease infarct size after I/R. Several cellular responses to thrombin are mediated by a G-protein coupled protease-activated receptor 1 (PAR1).However, the role of PAR1 in myocardial I/R injury has not been well characterized. Therefore, we hypothesized that PAR1 inhibition will reduce the amount of myocardial I/R injury. After we detected the presence of PAR1 mRNA and protein in the rat heart by RT-PCR and immunoblot analysis,we assessed the potential protective role of SCH 79797, a selective PAR1 antagonist, in two rat models of myocardial I/R injury. SCH 79797 treatment immediately before or during ischemia reduced myocardial necrosis following I/R in the intact rat heart. This response was dose-dependent with the optimal dose being 25 microg/kg IV. Likewise, SCH 79797 treatment before ischemia in the isolated heart model reduced infarct size and increased ventricular recovery following I/R in the isolated heart model with an optimal concentration of 1 microM. This reduction was abolished by a PAR1 selective agonist. SCH 79797-induced resistance to myocardial ischemia was abolished by wortmannin, an inhibitor of PI3 kinase; L-NMA, a NOS inhibitor; and glibenclamide, a nonselective K(ATP) channel blocker. PAR1 activating peptide,wortmannin, L-NMA and glibenclamide alone had no effect on functional recovery or infarct size. A single treatment of SCH 79797 administered prior to or during ischemia confers immediate cardioprotection suggesting a potential therapeutic role of PAR1 antagonist in the treatment of injury resulting from myocardial ischemia and reperfusion.

The membrane-anchored serine protease, TMPRSS2, activates PAR-2 in prostate cancer cells

TMPRSS2 is a type II transmembrane-bound serine protease that has gained interest owing to its highly localized expression in the prostate and its overexpression in neoplastic prostate epithelium. Once activated, the serine protease domain of TMPRSS2 is released from the cell surface into the extracellular space. PAR (protease-activated receptor)-2 belongs to a family of G-protein-coupled receptors (PAR-1-4) that are activated by specific serine proteases, which are expressed in many normal and malignant cell types. Previous in vitro studies on prostate cancer cells suggest a role for PAR-2 in prostate cancer metastasis. A polyclonal anti-human TMPRSS2 antibody was generated against the TMPRSS2 serine protease domain. The antibody showed specific reactivity with recombinant expressed TMPRSS2, and so was used to extract and purify the cleaved active TMPRSS2 protease from prostate cancer cells. Reverse transcriptase PCR and Western blot analysis were used to show the expression of both TMPRSS2 and PAR-2 in the androgen-dependent LNCaP prostate cancer cell line. Treatment of LNCaP cells with the cellular immunopurified TMPRSS2 protease induced a transient increase in intracellular calcium, which is indicative of G-protein-coupled-receptor activation. This calcium mobilization was inhibited by cellular pre-treatment with a specific PAR-2 antagonist, but not with a PAR-1 antagonist; inhibition of the protease activity also failed to mobilize calcium, suggesting that TMPRSS2 is capable of cleaving and thereby activating the PAR-2 receptor. The calcium mobilization was also inhibited by cellular pre-treatment with suramin or 2-APB (2-aminoethoxydiphenyl borate), indicating that a G-protein pathway is involved and that subsequent calcium release is mainly from intracellular stores. The present study describes how TMPRSS2 may contribute to prostate tumour metastasis via the activation of PAR-2.

Nuclear distribution and chromatin association of DNA polymerase alpha-primase is affected by TEV protease cleavage of Cdc23 (Mcm10) in fission yeast

BACKGROUND

Cdc23/Mcm10 is required for the initiation and elongation steps of DNA replication but its biochemical function is unclear. Here, we probe its function using a novel approach in fission yeast, involving Cdc23 cleavage by the TEV protease.

RESULTS

Insertion of a TEV protease cleavage site into Cdc23 allows in vivo removal of the C-terminal 170 aa of the protein by TEV protease induction, resulting in an S phase arrest. This C-terminal fragment of Cdc23 is not retained in the nucleus after cleavage, showing that it lacks a nuclear localization signal and ability to bind to chromatin. Using an in situ chromatin binding procedure we have determined how the S phase chromatin association of DNA polymerase alpha-primase and the GINS (Sld5-Psf1-Psf2-Psf3) complex is affected by Cdc23 inactivation. The chromatin binding and sub-nuclear distribution of DNA primase catalytic subunit (Spp1) is affected by Cdc23 cleavage and also by inactivation of Cdc23 using a degron allele, implying that DNA polymerase alpha-primase function is dependent on Cdc23. In contrast to the effect on Spp1, the chromatin association of the Psf2 subunit of the GINS complex is not affected by Cdc23 inactivation.

CONCLUSION

An important function of Cdc23 in the elongation step of DNA replication may be to assist in the docking of DNA polymerase alpha-primase to chromatin.

Effects of thrombin, PAR-1 activating peptide and a PAR-1 antagonist on umbilical artery resistance in vitro

Background

The non-thrombotic effects of thrombin in cardiovascular tissues, as mediated via the protease activated receptors (PARs), and particularly PAR-1, have been the focus of much recent research. The aims of this study were to evaluate the effects of thrombin, a specific PAR-1 activating peptide (PAR1-AP), and a PAR-1 antagonist on human umbilical artery tone in vitro.

Methods

Human umbilical artery samples were obtained from 17 women at term. Arterial rings were suspended under physiologic conditions for isometric recording. The in vitro effects of thrombin (0.5 units/mL to 3 units/mL), PAR1-AP TFLLR-NH2 [10(-9) to 10(-6) M], and PAR-1 antagonist (N-trans cinnamoyl- p-fluoroPhe-p-guanidinoPhe-Leu-Arg-Orn-NH2) [10(-9) M to 10(-5) M] on umbilical artery tone were measured.

Results

Both thrombin and TFLLR-NH2 exerted a potent cumulative vasodilatory effect on human umbilical artery resistance (P < 0.001). The mean net maximal inhibition (MMI) for thrombin was 53.05% (n = 6; SEM = 1.43) at tissue bath concentration of 3 units/mL. The MMI with TFLLR-NH2 was 61.50 % (n = 6; SEM = 1.43) at bath concentration of 10(-6) M. In comparison to vehicle control, the PAR-1 antagonist did not show a significant relaxant or contractile effect (P > 0.05).

Conclusion

These findings highlight a potential role for thrombin and PAR-1 receptors in vascular regulation of feto-placental blood flow in normal pregnancy, and in association with the vascular lesions associated with IUGR and pre-eclampsia.

Distinct PAR/IQGAP expression patterns during murine development: implications for thrombin-associated cytoskeletal reorganization

Thrombin has a critical role in many adult and embryologic cellular processes, exerting its effects through two high-affinity thrombin receptor systems: protease-activated receptor 1 (PAR1) and the PAR3/PAR4 system. Both hPAR1 and hPAR3 are coclustered in the human genome, with hPAR3 encompassed within hIQGAP2, a putative GTPase activating protein with actin polymerizing functions linked to cytoskeletal reorganization. Since hPARs colocalize with hIQGAP2 in the human genome and function coordinately with this protein in platelet thrombin signaling pathways, we have further characterized these genes in developing embryonic and adult tissues. We confirmed the presence of a mIQGAP2/ mPAR gene cluster on murine Chromosome 13 and showed it to be organized similarly to that in humans, except that murine PAR3 is translated off the forward (sense) strand. Northern analysis demonstrated limited mPAR3 expression in adult tissues, although its expression during embryogenesis was evident at E15 in cartilage, brain, and keratinocytes. mIQGAPs 1 and 2 had congruent expression patterns in 11 of 15 adult tissues studied. In contrast, whole embryos demonstrated predominant mIQGAP1 expression starting at E7 and evident to E17. In situ hybridization of whole embryos (E9-E16) demonstrated distinct patterns of tissue-dependent mIQGAP1/ mIQGAP2 expression. Concordant expression (absence or presence) of mPAR1 with either mIQGAP1 or mIQGAP2 was seen in the majority (12 of 15) of adult tissues studied. Similarly, there was no evidence for mPAR3 expression during embryogenesis in the absence of either mIQGAP1 or mIQGAP2. These data provide a panoramic survey of PAR/ IQGAP expression as an initial approach to dissect thrombin signaling pathways linked to cytoskeletal reorganization.

Neurogenic inflammation and pancreatitis

Stimulation of primary sensory neurons produces local vasodilation, plasma extravasation, and pain and is due largely to the release of the tachykinins substance P and calcitonin-gene-related peptide. Pathological activation of sensory neurons and the inflammatory sequelae are known as neurogenic inflammation and appear to be important in many organ systems, including the pancreas. Factors that stimulate primary sensory neurons include hydrogen ions, heat, leukotrienes, arachidonic acid metabolites, bradykinin, and proteases such as trypsin, all of which may participate in the generation of acute pancreatitis. The current review examines the cellular and molecular mechanisms involved in sensory nerve activation within the pancreas and the potential contribution of neurogenic inflammation to the pathogenesis of pancreatitis.

Thrombin and PAR1-activating peptide: effects on human uterine contractility in vitro

OBJECTIVE

Thrombin enhances uterine contractions in animal models, which is an effect that is mediated through protease-activated receptors. The aims of this study were to investigate the effects of thrombin on spontaneous human uterine contractility in vitro, in tissue obtained in the presence and absence of pregnancy, and to investigate the effects of a specific protease-activated receptor 1-activating peptide on human pregnant myometrial contractility.

STUDY DESIGN

Isometric recordings were performed under physiologic conditions on myometrial strips that were obtained from elective cesarean delivery and premenopausal hysterectomy specimens. The effects of thrombin (0.5 to 5.0 U/mL) and a specific protease-activated receptor 1-activating peptide (1 nmol/L to 10 micromol/L) on integrals of contractile activity were measured and compared with control values.

RESULTS

Thrombin exerted a potent stimulatory effect on human myometrial contractility. For pregnant and nonpregnant myometrium, this effect was significant at concentrations of > or =3.0 U/mL and 1.0 U/mL, respectively, with net maximal stimulatory effects of 44.5% (5.0 U/mL, P<.001) and 40.42% (3.0 U/mL, P<.001), respectively. The protease-activated receptor 1-activating peptide also mediated a significant uterotonic effect (55.1% increase) on human pregnant myometrial contractility at 10 micromol/L concentration (P<.001).

CONCLUSION

This uterotonic effect of thrombin suggests that it may play a role in the pathophysiologic condition of human uterine contractions in association with intrauterine bleeding. The similar uterotonic effect that is elicited by protease-activated receptor 1-activating peptide suggests a major role for protease-activated receptor-1 in the thrombin-mediated contractile effect.

Proteinase-mediated signaling: proteinase-activated receptors (PARs) and much more

Quite apart from their ability to generate active polypeptides from hormone precursors and to function as digestive enzymes, proteinases are now known to play a hormone-like role by triggering signal transduction pathways in target cells. The best understood example of proteinase-mediated signaling can be seen in the action of thrombin, which in addition to triggering the coagulation cascade, regulates platelet and endothelial cell function via its serine proteinase activity. The discovery of the G-protein-coupled 'receptor' responsible for these cellular actions of thrombin (Proteinase-activated Receptor-1, or PAR(1)) represents one of the more intriguing signal transduction stories elucidated over past decade or so. It is the objective of this brief review to provide an overview of the discovery and molecular pharmacology of the PAR family and to indicate the widespread roles these receptor systems can play in a variety of tissues. Further, the article (1) illustrates the utility of employing receptor-selective PAR-activating peptides to determine the potential physiological roles these receptors play in vivo and (2) describes how these agonists have identified receptors other than the PARs. Finally, the mechanisms other than via the PARs by which proteinases can generate cellular signals are summarized.

Proteinase-activated receptor-1 (PAR-1) activation contracts the isolated human renal artery in vitro

1. The in vitro motor function of protease-activated recepter-1 (PAR-1), PAR-2 and PAR-4 and the presence by immunohistochemistry of PAR-1 in the human renal artery have been investigated. 2. Thrombin and the human PAR-1 (SFLLRN-NH(2)) activating peptide, but not the PAR-1 reverse peptide (NRLLFS-NH(2)), contracted both endothelial-intact and endothelial-denuded human renal artery strips, whereas no relaxation was observed either in strips non-precontracted or precontracted with phenylephrine. Maximum contraction by thrombin or SFLLRN-NH(2) was about 60% of phenylephrine. However, thrombin was approximately 1000-fold more potent than SFLLRN-NH(2). 3. PAR-1 desensitisation, using repeated applications of SFLLRN-NH(2), almost completely blocked the response to thrombin. The contractile effect produced by thrombin and SFLLRN-NH(2) was not affected by nitric oxide synthase inhibition, but was significantly reduced by cyclooxygenase blockade. 4. Trypsin, the PAR-2 (SLIGKV-NH(2) and SLIGRL-NH(2)) and PAR-4 (GYPGQV-NH(2) and AYPGKF-NH(2)) activating peptides did not produce any significant contraction or relaxation. 5. In agreement with the motor function data immunohistochemistry showed specific staining patterns for PAR-1 in the human renal artery. 6. Combined, these studies would suggest a possible role for PAR-1 in renal vascular homeostasis.

Expression of protease-activated receptor-1, -2, -3, and -4 in control and experimentally inflamed mouse bladder

Inflammation underlines all major bladder pathologies and represents a defense reaction to injury involving a mandatory participation of mast cells and sensory nerves. Mast cells are particularly frequent in close proximity to epithelial surfaces where they are strategically located in the bladder and release their mediators in response to inflammation. Tryptase is specifically produced by mast cells and modulates inflammation by activating protease-activated receptors (PARs). We recently found that PAR-4 mRNA is up-regulated in experimental bladder inflammation regardless of the initiating stimulus. Because it has been reported that PAR-1, PAR-2, and PAR-3 may also be involved in the processes of inflammation, we used immunohistochemistry to characterize the expression of all known PARs in normal, acute, and chronic inflamed mouse bladder. We found that all four PARs are present in the control mouse bladder, and follow a unique distribution. All four PARs are co-expressed in the urothelium, whereas PAR-1 and PAR-2 are predominant in the detrusor muscle, and PAR-4 is expressed in peripheral nerves and plexus cell bodies. The strong expression of PARs in the detrusor muscle indicates the need for studies on the role of these receptors in motility whereas the presence of PAR-4 in nerves may indicate its participation in neurogenic inflammation. In addition, PARs are differentially modulated during inflammation. PAR-1 and PAR-2 are down-regulated in acute inflammation whereas PAR-3 and PAR-4 are up-regulated. Bladder fibroblasts were found to present a clear demarcation in PAR expression secondary to acute and chronic inflammation. Our findings provide evidence of participation of PARs in the urinary system, provide a working model for mast cell tryptase signaling in the mouse bladder, and evoke testable hypotheses regarding the roles of PARs in bladder inflammation. It is timely to understand the role of tryptase signaling and PARs in the context of bladder biology.

A yeast-based growth assay for the analysis of site-specific proteases

Many cellular and viral processes depend on site-specific proteolysis. Here, a genetic system for the identification of such proteases and inhibitors is described. The system utilizes the temperature- sensitive Saccharomyces cerevisiae CDC25-2 mutant strain and exploits the strict requirement of membrane localization of a constitutively active Ras mutant for the complementation of the yeast growth defect at the non-permissive temperature. Expression of a fusion protein in which a substrate peptide of the TEV protease separates a myristoylation signal from a constitutively active human Ras protein confers temperature insensitivity. Co-expression of the protease results in release of the Ras mutant from the membrane and growth arrest at the non-permissive temperature. This non-transcriptional assay represents a new approach to the in vivo analysis of site-specific proteases and may be a valuable alternative to existing methods. It has significant potential for the selection of inhibitors of cytoplasmic and membrane-associated proteases of biotechnical and clinical relevance.

Minimal structural requirements for agonist activity of PAR-2 activating peptides

Protease-activated receptor 2 (PAR-2) is involved in inflammatory, gastrointestinal, and vascular diseases. The aim of the present work was to elucidate the minimal structural features for PAR-2 agonist activity in short peptides. Our study resulted in the discovery of dipeptide derivatives of N(alpha)-benzoyl-Arg(NO(2))-Leu-NH(2) displaying a potency comparable to that of the full-length rat PAR-2 activating peptide (Ser-Leu-Ile-Gly-Arg-Leu-NH(2)).

Potent, low molecular weight thrombin receptor antagonists

Several benzimidazole derivatives have been identified as potent thrombin receptor (PAR-1) antagonists as represented by compound 1h, which showed an IC(50) of 33 nM.

Initiation of human colon cancer cell proliferation by trypsin acting at protease-activated receptor-2

The protease-activated receptor-2 (PAR-2) is a G protein-coupled receptor that is cleaved and activated by trypsin. We investigated the expression of PAR-2 and the role of trypsin in cell proliferation in human colon cancer cell lines. A total of 10 cell lines were tested for expression of PAR-2 mRNA by Northern blot and RT-PCR. PAR-2 protein was detected by immunofluorescence. Trypsin and the peptide agonist SLIGKV (AP2) were tested for their ability to induce calcium mobilization and to promote cell proliferation on serum-deprived cells. PAR-2 mRNA was detected by Northern blot analysis in 6 out of 10 cell lines [HT-29, Cl.19A, Caco-2, SW480, HCT-8 and T84]. Other cell lines expressed low levels of transcripts, which were detected only by RT-PCR. Further results were obtained with HT-29 cells: (1) PAR-2 protein is expressed at the cell surface; (2) an increase in intracellular calcium concentration was observed upon trypsin (1-100 nM) or AP2 (10-100 microM) challenges; (3) cells grown in serum-deprived media supplemented with trypsin (0.1-1 nM) or AP2 (1-300 microM) exhibited important mitogenic responses (3-fold increase of cell number). Proliferative effects of trypsin or AP2 were also observed in other cell lines expressing PAR-2. These data show that subnanomolar concentrations of trypsin, acting at PAR-2, promoted the proliferation of human colon cancer cells. The results of this study indicate that trypsin could be considered as a growth factor and unravel a new mechanism whereby serine proteases control colon tumours.

Selected contribution: tryptase-induced PAR-2-mediated Ca(2+) signaling in human airway smooth muscle cells

Tryptase, the major mast cell product, is considered to play an important role in airway inflammation and hyperresponsiveness. Tryptase produces different, sometimes opposite, effects on airway responsiveness (bronchoprotection and/or airway contraction). This study was designed to examine the effect of human lung tryptase and activation of protease-activated receptor (PAR)-2 by synthetic activated peptide (AP) SLIGKV-NH(2) on Ca(2+) signaling in human airway smooth muscle (HASM) cells. Immunocytochemistry revealed that PAR-2 was expressed by HASM cells. Tryptase (7.5--30 mU/ml) induced a concentration-dependent transient relative rise in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) that reached 207 +/- 32 nM (n = 10) measured by indo 1 spectrofluorometry. The protease inhibitors leupeptin or benzamidine (100 microM) abolished tryptase-induced [Ca(2+)](i) increase. Activation of PAR-2 by AP (1-100 microM) also induced a concentration-dependent transient rise in [Ca(2+)](i), whereas the reverse peptide produced no effect. There was a homologous desensitization of the [Ca(2+)](i) response on repeated stimulation with tryptase or AP. U-73122, a specific phospholipase C (PLC) antagonist, xestospongin, an inositol trisphosphate (IP(3))-receptor antagonist, or thapsigargin, a sarcoplamic Ca(2+)-ATPase inhibitor, abolished tryptase-induced [Ca(2+)](i) response, whereas Ca(2+) removal, in the additional presence of EGTA, had no effect. Calphostin C, a protein kinase C inhibitor, increased PAR-2 [Ca(2+)](i) response. Our results indicate that tryptase activates a [Ca(2+)](i) response, which appears as PAR-2 mediated in HASM cells. Signal transduction implicates the intracellular Ca(2+) store via PLC activation and thus via the IP(3) pathway. This study provides evidence that tryptase, which is increasingly recognized as an important mediator in airway inflammation and hyperresponsiveness, is also a potent direct agonist at the site of airway smooth muscle.

Differential expression of protease-activated receptors-1 and -2 in stromal fibroblasts of normal, benign, and malignant human tissues

The serine proteases thrombin and trypsin are among many factors that malignant cells secrete into the extracellular space to mediate metastatic processes such as cellular invasion, extracellular matrix degradation, angiogenesis, and tissue remodeling. The degree of protease secretion from malignant cells has been correlated to their metastatic potential. Protease activated receptors (PAR)-1 and -2, which are activated by thrombin and trypsin respectively, have not been extensively characterized in human tumors in situ. We investigated the presence of PAR-1 and PAR-2 in human normal, benign and malignant tissues using immunohistochemistry and in situ hybridization. Our results demonstrate PAR-1 and PAR-2 expression in the tumor cells, mast cells, macrophages, endothelial cells, and vascular smooth muscle cells of the metastatic tumor microenvironment. Most notably, an up-regulation of PAR-1 and PAR-2 observed in proliferating, smooth muscle actin (SMA)-positive stromal fibroblasts surrounding the carcinoma cells was not observed in normal or benign conditions. Furthermore, in vitro studies using proliferating, SMA-positive, human dermal fibroblasts, and scrape-wounded human dermal fibroblasts demonstrated the presence of PAR-1 and PAR-2 not detected in quiescent, SMA-negative cultures. PAR-1 and PAR-2 in the cells forming the tumor microenvironment suggest that these receptors mediate the signaling of secreted thrombin and trypsin in the processes of cellular metastasis.

Receptor nomenclature guidelines

Receptors are typically characterized via two distinct approaches: (1) the identification and pharmacological characterization of a receptor-mediated response using classical pharmacological and/or radioligand approaches in tissues and animal models using selective agonist and antagonist ligands, and; (2) the cloning and expression of proteins with structural homology to known receptors, the function of which is subsequently established by studying the structure activity relationship (SAR) of receptor-mediated responses. An additional means to characterize receptors proceeded, and evolved, with the structural approach, namely classification in terms of signal transduction mechanisms. The International Union of Pharmacology (IUPHAR) created guidelines and selected working groups for each receptor family to establish a common nomenclature system. Reports from those groups that have reached some degree of consensus have been summarized in this appendix.

Protease-activated receptors in inflammation, neuronal signaling and pain

The ability of proteases to regulate cell function via protease-activated receptors (PARs) has led to new insights about the potential physiological functions of these enzymes. Several studies suggest that PARs play roles in both inflammation and tissue repair, depending on the cellular environment in which they act. The recent detection of PARs on peripheral and central neurons suggests that neuronal PARs might be involved not only in neurogenic inflammation and neurodegenerative processes, but also in nociception. Thus, the list of potential roles for PARs has lengthened considerably and their physiological course of action might be much broader than initially anticipated.

Evidence that PAR-1 and PAR-2 mediate prostanoid-dependent contraction in isolated guinea-pig gallbladder

We have investigated the ability of protease-activated receptor-1 (PAR-1), PAR-2, PAR-3 and PAR-4 agonists to induce contractile responses in isolated guinea-pig gallbladder. Thrombin, trypsin, mouse PAR-1 activating (SFLLRN-NH(2)) peptide, and mouse PAR-2 activating (SLIGRL-NH(2)) and human PAR-2 activating (SLIGKV-NH(2)) peptides produced a concentration-dependent contractile response. Mouse PAR-4 activating (GYPGKF-NH(2)) peptide, the mouse PAR-1 reverse (NRLLFS-NH(2)) peptide, the mouse PAR-2 reverse (LRGILS-NH(2)) and human PAR-2 reverse (VKGILS-NH(2)) peptides caused negligible contractile responses at the highest concentrations tested. An additive effect was observed following the contractile response induced by either trypsin or thrombin, with the addition of a different PAR agonist (SFLLRN-NH(2) and SLIGRL-NH(2), respectively). Desensitization to PAR-2 activating peptide attenuated the response to trypsin but failed to attenuate the response to PAR-1 agonists, and conversely desensitization to PAR-1 attenuated the response to thrombin but failed to alter contractile responses to PAR-2 agonists. The contractile responses produced by thrombin, trypsin, SFLLRN-NH(2) and SLIGRL-NH(2) were markedly reduced in the presence of the cyclo-oxygenase inhibitor, indomethacin, whilst the small contractile response produced by NRLLFS-NH(2) and LRGILS-NH(2) were insensitive to indomethacin. The contractile responses to thrombin, trypsin, SFLLRN-NH(2) and SLIGRL-NH(2) were unaffected by the presence of: the non-selective muscarinic antagonist, atropine; the nitric oxide synthase inhibitor, L-NAME; the sodium channel blocker, tetrodotoxin; the combination of selective tachykinin NK(1) and NK(2) receptor antagonists, (S)-1-[2-[3-(3,4-dichlorphenyl)-1 (3-isopropoxyphenylacetyl) piperidin-3-yl] ethyl]-4-phenyl-1 azaniabicyclo [2.2.2] octane chloride (SR140333) and (S)-N-methyl-N-[4-acetylamino-4-phenylpiperidino-2-(3, 4-dichlorophenyl)-butyl] benzamide (SR48968), respectively. The results indicate that PAR-1 and PAR-2 activation causes contractile responses in the guinea-pig gallbladder, an effect that is mediated principally by prostanoid release, and is independent of neural mechanisms.

Normal endothelial cell function

Endothelial cell functions, primarily involving regulated mediator secretion or altered surface protein expression, are vital for normal homeostasis. Endothelial cells secrete the potent vasodilator and anti-platelet agent prostacyclin and nitric oxide, and also the potent vasoconstrictor peptide endothelin-1; they control the selective adhesion and emigration of leukocytes from the bloodstream; and they are the source of circulating von Willebrand factor, tissue plasminogen activator and type 1 plasminogen activator inhibitor. The properties of healthy endothelium ensure that an antithrombotic and anticoagulant balance is maintained in the bloodstream, and provide a tonic vasodilator action that controls blood flow and pressure on a minute-to-minute basis. Disturbances of normal endothelial function are strongly implicated in the pathogenesis of atherosclerosis and autoimmune vasculitic diseases including lupus.

Presence and bronchomotor activity of protease-activated receptor-2 in guinea pig airways

The protease activated receptor-2 (PAR-2) belongs to a family of G-protein-coupled receptors that are activated by proteolysis. Trypsin cleaves PAR-2, exposing an N-terminal tethered ligand (SLIGRL) that activates the receptor. Messenger RNA (mRNA) for PAR-2 was found in guinea pig airway tissue by reverse transcription-polymerase chain reaction, and PAR-2 was found by immunohistochemistry in airway epithelial and smooth-muscle cells. In anesthetized guinea pigs, trypsin and SLIGRL-NH(2) (given intratracheally or intravenously) caused a bronchoconstriction that was inhibited by the combination of tachykinin-NK(1) and -NK(2) receptor antagonists and was potentiated by inhibition of nitric oxide synthase (NOS). Trypsin and SLIGRL-NH(2) relaxed isolated trachea and main bronchi, and contracted intrapulmonary bronchi. Relaxation of main bronchi was abolished or reversed to contraction by removal of epithelium, administration of indomethacin, and NOS inhibition. PAR-1, PAR-3, and PAR-4 were not involved in the bronchomotor action of either trypsin or SLIGRL-NH(2), because ligands of these receptors were inactive either in vitro or in vivo, and because thrombin (a PAR-1 and PAR-3 agonist) did not show cross-desensitization with PAR-2 agonists in vivo. Thus, we have localized PAR-2 to the guinea-pig airways, and have shown that activation of PAR-2 causes multiple motor effects in these airways, including in vivo bronchoconstriction, which is in part mediated by a neural mechanism.

Proteinase-activated receptors (PARs): activation of PAR1 and PAR2 by a proteolytic fragment of the neuronal growth associated protein B-50/GAP-43

The neuronal growth associated protein B-50/GAP-43 has been localized in synaptosomes both as an intact protein and as a partial proteolysis product (termed B-60) that has an N-terminal sequence SFRGHITR.... Because of the relationship of this amino acid sequence to those of the tethered ligand for the human proteinase activated receptors PAR1 (SFLLRN...) and PAR2 (SLIGKV...), we wished to determine whether the B-50/GAP-43-derived proteolytic fragment SFRGHITR (SFRB60) might function as a PAR-activating peptide (PAR-AP) to stimulate either PAR1 or PAR2. With the use of a newly developed PAR1/PAR2 receptor activation-desensitization assay, employing PAR1/PAR2-bearing cultured human embryonic kidney (HEK293) cells, we found that SFRB60 could activate both PAR1 and PAR2 so as to elevate intracellular calcium with EC50 values of approximately 200 and 50 µM, respectively. We also showed that trypsin can rapidly degrade B-50 to smaller fragments that would include the sequence SFRB60. Because PAR1 and PAR2 are present on neurones, our data raise the possibility that in certain circumstances in vivo, B-50/GAP-43 may play a signalling role by serving as a precursor for proteolytically generated PAR-activating peptides.Key words: proteinase-activated receptors, neurones, B-50/GAP-43, calcium signalling.

Endothelial cell function and thrombosis

The endothelium is pivotal in the control of haemostasis and thrombosis because it is the primary source of many of the major haemostatic regulatory molecules. Healthy endothelial cells, unlike extravascular cells, are anticoagulant and antithrombotic. This is due to the regulated secretion of antiplatelet agents, including prostacyclin and nitric oxide. Following vessel injury, platelet adhesion to exposed matrix requires von Willebrand Factor, another endothelial cell product. Local generation of thrombin causes a series of receptor-mediated endothelial cell functional responses, while the surface of the endothelium is additionally the site for inactivation of thrombin by antithrombin, and its conversion to a coagulation inhibitor by interaction with thrombomodulin. Endothelial cells are also the source of circulating tissue-type plasminogen activator and its inhibitor, and Tissue Factor pathway inhibitor. In disease states, many of these endothelial cell properties are perturbed towards a more procoagulant and prothrombotic phenotype.

Enhancement of vascular permeability by specific activation of protease-activated receptor-1 in rat hindpaw: a protective role of endogenous and exogenous nitric oxide

1. To clarify the role of the first thrombin receptor/protease-activated receptor (PAR)-1 in an inflammatory process, we tested and characterized the effect of intraplantar (i.pl.) administration of the highly specific PAR-1 agonist TFLLR-NH2 in rat hindpaw. 2. TFLLR-NH2 administered i.pl. at 0.01-0.03 micromol per paw enhanced vascular permeability in the hindpaw and produced paw oedema in a dose-dependent manner. This effect was almost completely abolished by repeated pretreatment with compound 48/80 to deplete inflammatory mediators in mast cells. 3. The NO synthase inhibitor N(G)-nitro-L-arginine methyl ester or N-iminoethyl-L-ornithine, preadministered i.pl., stereospecifically potentiated the i.pl. TFLLR-NH2-induced permeability increase, while the NO donor sodium nitroprusside or NOC-18, given i.pl., suppressed the effect of TFLLR-NH2. 4. These findings demonstrate that specific activation of PAR-1 produces increased vascular permeability accompanied by oedema formation in the rat hindpaw, predominantly via mast cell degranulation, and that endogenous and exogenous NO plays a protective role in the PAR-1-mediated inflammatory event.

Pro- and anti-inflammatory actions of thrombin: a distinct role for proteinase-activated receptor-1 (PAR1)

1. Thrombin has well characterized pro-inflammatory actions that have recently been suggested to occur via activation of its receptor, proteinase-activated receptor-1 (PAR1). 2. In the present study, we have compared the effects of thrombin to those of two peptides that selectively activate the PAR1 receptor, in a rat hindpaw oedema model. We have also examined whether or not thrombin can exert anti-inflammatory activity in this model. 3. Both thrombin and the two PAR1 activating peptides induced significant oedema in the rat hindpaw following subplantar injection. 4. The oedema induced by thrombin was abolished by pre-incubation with hirudin, and was markedly reduced in rats in which mast cells were depleted through treatment with compound 48/80 and in rats pretreated with indomethacin. In contrast, administration of the PAR1 activating peptides produced an oedema response that was not reduced by indomethacin and was only slightly reduced in rats pretreated with compound 48/80. 5. Co-administration of thrombin together with a PAR1 activating receptor resulted in a significantly smaller oedema response than that seen with the PAR1 activating peptide alone. This anti-inflammatory effect of thrombin was abolished by pre-incubation with hirudin. 6. These results demonstrate that the pro-inflammatory effects of thrombin occur through a mast-cell dependent mechanism that is, at least in part, independent of activation of the PAR1 receptor. Moreover, thrombin is able to exert anti-inflammatory effects that are also unrelated to the activation of PAR1.

Increased vascular permeability by a specific agonist of protease-activated receptor-2 in rat hindpaw

The present study examined the effect of intraplantar (i.pl.) administration of a selective agonist of protease-activated receptor (PAR)-2, SLIGRL-NH2(PP6-NH2), on vascular permeability in rat hindpaw. PP6-NH2, administered i.pl. at 10-100 nmol per paw, enhanced vascular permeability and caused oedema formation in rat hindpaw. SLIGRL (PP6-OH) and trypsin, by i.pl. administration, also elicited an increase in vascular permeability, although i.pl. administration of the mixture of constituent amino acids of PP6-OH at an equivalent dose did not. The PP6-NH2-induced increase in vascular permeability was abolished by repeated pretreatment with compound 48/80 to deplete bioactive amines in mast cells. These findings suggest that the activation of PAR-2 induces acute inflammation, at least partially, via mast cell degranulation in rat hindpaw.

The human proteinase-activated receptor-3 (PAR-3) gene. Identification within a Par gene cluster and characterization in vascular endothelial cells and platelets

Proteolytically activated receptors (PARs) represent an emerging subset of seven transmembrane G protein-coupled receptors that mediate cell activation events by receptor cleavage at distinct scissile bonds located within receptor amino termini. Differential genomic blotting using a yeast artificial chromosome known to contain the PAR-1 and PAR-2 genes identified the PAR-3 gene within a PAR gene cluster spanning approximately 100 kilobases at 5q13. The PAR-3 gene is relatively small (approximately 12 kilobases); and, like the PAR-1 and PAR-2 genes, it displays a two-exon structure, with the majority of the coding sequence and the proteolytic cleavage site contained within the larger second exon. Sequence analysis of the 5'-flanking region demonstrates that the promoter is TATA-less, similar to that seen with PAR-1, with the identification of nucleic acid motifs potentially involved in transcriptional gene regulation, including AP-1, GATA, and octameric sequences. PAR-3 transcripts were apparent in human vascular endothelial cells, although at considerably lower levels than those of PAR-1 and not significantly modulated by the endothelial cell stimulus tumor necrosis factor-alpha. Likewise, although PAR-3 mRNA was evident in human platelets, receptor cell surface expression was modest (approximately 10%) compared with that of PAR-1. Thus, although PAR-3 is postulated to represent a second thrombin receptor, its modest endothelial cell and platelet expression suggest that PAR-3 activation by alpha-thrombin is less relevant for physiological responses in these mature cells. Rather, given its disparately greater expression in megakaryocytes (and megakaryocyte-like human erythroleukemia cells), a regulatory role in cellular development (by protease activation) could be postulated.

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

Although serine proteases are usually considered to act principally as degradative enzymes, certain proteases are signaling molecules that specifically regulate cells by cleaving and triggering members of a new family of proteinase-activated receptors (PARs). There are three members of this family, PAR-1 and PAR-3, which are receptors for thrombin, and PAR-2, a receptor for trypsin and mast cell tryptase. Proteases cleave within the extracellular NH2-terminus of their receptors to expose a new NH2-terminus. Specific residues within this tethered ligand domain interact with extracellular domains of the cleaved receptor, resulting in activation. In common with many G protein-coupled receptors, PARs couple to multiple G proteins and thereby activate many parallel mechanisms of signal transduction. PARs are expressed in multiple tissues by a wide variety of cells, where they are involved in several pathophysiological processes, including growth and development, mitogenesis, and inflammation. Because the cleaved receptor is physically coupled to its agonist, efficient mechanisms exist to terminate signaling and prevent uncontrolled stimulation. These include cleavage of the tethered ligand, receptor phosphorylation and uncoupling from G proteins, and endocytosis and lysosomal degradation of activated receptors.

Characterization of a 34-kDa soybean binding protein for the syringolide elicitors

Syringolides are water-soluble, low-molecular-weight elicitors that trigger defense responses in soybean cultivars carrying the Rpg4 disease-resistance gene but not in rpg4 cultivars. 125I-syringolide 1 previously was shown to bind to a soluble protein(s) in extracts from soybean leaves. A 34-kDa protein that accounted for 125I-syringolide 1 binding activity was isolated with a syringolide affinity-gel column. Partial sequences of internal peptides of the 34-kDa protein were identical to P34, a previously described soybean seed allergen. In soybean seeds, P34 is processed from a 46-kDa precursor protein and was shown to have homology with thiol proteases. P34 is a moderately abundant protein in soybean seeds and cotyledons but its level in leaves is low. cDNAs encoding 46-, 34-, and 32-kDa forms of the soybean protein were cloned into the baculovirus vector, pVL1392, and expressed in insect cells. The resulting 32- and 34-kDa proteins, but not the 46-kDa protein, exhibited ligand-specific 125I-syringolide binding activity. These results suggest that P34 may be the receptor that mediates syringolide signaling.

A genetic screen for the isolation and characterization of site-specific proteases

Site-specific proteolysis is an important regulatory mechanism in basic cellular and viral processes. Using the protease of the HIV as a model, a genetic system has been developed for the isolation and characterization of site-specific proteases. The system utilizes the well defined bacteriophage lambda regulatory circuit where the viral repressor, cI, is specifically cleaved to initiate the lysogenic-to-lytic switch. The model system is rapid, highly specific, and demonstrates the ability to isolate and characterize enzymes of limited expression or activity. In addition, the system has a significant potential for the selection of clinically relevant mutant enzymes and in the development of anti-protease therapeutics.

The thrombin receptor in adrenal medullary microvascular endothelial cells is negatively coupled to adenylyl cyclase through a Gi protein

The effects of thrombin on adenylyl cyclase activity were examined in rat adrenal medullary microvascular endothelial cells (RAMEC). Confluent RAMEC monolayers were stimulated for 5 min with cAMP-generating agents in the absence and presence of thrombin, and intracellular cAMP was measured with a radioligand binding assay. Thrombin (0.001-0.25 U/ml) dose-dependently inhibited IBMX-, isoproterenol- and forskolin-stimulated cAMP accumulation. A peptide agonist of the thrombin receptor, gamma-thrombin, and the serine proteases trypsin and plasmin, also inhibited agonist-stimulated cAMP levels, while proteolytically inactive PPACK- or DIP-alpha-thrombins were without effect. Moreover, the thrombin inhibitor hirudin abolished the inhibitory effect of thrombin but not of the peptide agonist. These results suggest that the inhibitory action of thrombin on cAMP accumulation is mediated by a proteolytically-activated thrombin receptor. The inhibitor of G(i)-proteins pertussis toxin abolished the inhibitory effect of thrombin on isoproterenol- or IBMX-stimulated cAMP production, while the phorbol ester PMA partly impaired it. The protein kinase C inhibitors staurosporine or H7 and the intracellular Ca2+ chelator BAPTA-AM were without effect. Collectively, our data suggest that the thrombin receptor in RAMEC is negatively coupled to adenylyl cyclase through a pertussis toxin-sensitive G(i)-protein.