Dual effect of the novel peptide antagonist K-14585 on proteinase-activated receptor-2-mediated signalling

Protease-activated receptors in health and disease

Proteases are signaling molecules that specifically control cellular functions by cleaving protease-activated receptors (PARs). The four known PARs are members of the large family of G protein-coupled receptors. These transmembrane receptors control most physiological and pathological processes and are the target of a large proportion of therapeutic drugs. Signaling proteases include enzymes from the circulation; from immune, inflammatory epithelial, and cancer cells; as well as from commensal and pathogenic bacteria. Advances in our understanding of the structure and function of PARs provide insights into how diverse proteases activate these receptors to regulate physiological and pathological processes in most tissues and organ systems. The realization that proteases and PARs are key mediators of disease, coupled with advances in understanding the atomic level structure of PARs and their mechanisms of signaling in subcellular microdomains, has spurred the development of antagonists, some of which have advanced to the clinic. Herein we review the discovery, structure, and function of this receptor system, highlight the contribution of PARs to homeostatic control, and discuss the potential of PAR antagonists for the treatment of major diseases.

GB83, an Agonist of PAR2 with a Unique Mechanism of Action Distinct from Trypsin and PAR2-AP

Protease-activated receptor 2 (PAR2) is a G-protein-coupled receptor (GPCR) activated by proteolytic cleavage of its N-terminal domain. Once activated, PAR2 is rapidly desensitized and internalized by phosphorylation and β-arrestin recruitment. Due to its irreversible activation mechanism, some agonists that rapidly desensitized PAR2 have been misconceived as antagonists, and this has impeded a better understanding of the pathophysiological role of PAR2. In the present study, we found that GB83, initially identified as a PAR2 antagonist, is a bona fide agonist of PAR2 that induces unique cellular signaling, distinct from trypsin and PAR2-activating peptide (AP). Activation of PAR2 by GB83 markedly elicited an increase in intracellular calcium levels and phosphorylation of MAPKs, but in a delayed and sustained manner compared to the rapid and transient signals induced by trypsin and PAR2-AP. Interestingly, unlike PAR2-AP, GB83 and trypsin induced sustained receptor endocytosis and PAR2 colocalization with β-arrestin. Moreover, the recovery of the localization and function of PAR2 was significantly delayed after stimulation by GB83, which may be the reason why GB83 is recognized as an antagonist of PAR2. Our results revealed that GB83 is a bona fide agonist of PAR2 that uniquely modulates PAR2-mediated cellular signaling and is a useful pharmacological tool for studying the pathophysiological role of PAR2.

Proteinase-activated receptor-2 antagonist C391 inhibits Alternaria-induced airway epithelial signalling and asthma indicators in acute exposure mouse models

BACKGROUND AND PURPOSE

Despite the availability of a variety of treatment options, many asthma patients have poorly controlled disease with frequent exacerbations. Proteinase-activated receptor-2 (PAR2) has been identified in preclinical animal models as important to asthma initiation and progression following allergen exposure. Proteinase activation of PAR2 raises intracellular Ca²⁺ , inducing MAPK and β-arrestin signalling in the airway, leading to inflammatory and protective effects. We have developed C391, a potent PAR2 antagonist effective in blocking peptidomimetic- and trypsin-induced PAR2 signalling in vitro as well as reducing inflammatory PAR2-associated pain in vivo. We hypothesized that PAR2 antagonism by C391 would attenuate allergen-induced acutely expressed asthma indicators in murine models.

EXPERIMENTAL APPROACH

We evaluated the ability of C391 to alter Alternaria alternata-induced PAR2 signalling pathways in vitro using a human airway epithelial cell line that naturally expresses PAR2 (16HBE14o-) and a transfected embryonic cell line (HEK 293). We next evaluated the ability for C391 to reduce A. alternata-induced acutely expressed asthma indicators in vivo in two murine strains.

KEY RESULTS

C391 blocked A. alternata-induced, PAR2-dependent Ca²⁺ and MAPK signalling in 16HBE14o- cells, as well as β-arrestin recruitment in HEK 293 cells. C391 effectively attenuated A. alternata-induced inflammation, mucus production, mucus cell hyperplasia and airway hyperresponsiveness in acute allergen-challenged murine models.

CONCLUSIONS AND IMPLICATIONS

To our best knowledge, this is the first demonstration of pharmacological intervention of PAR2 to reduce allergen-induced asthma indicators in vivo. These data support further development of PAR2 antagonists as potential first-in-class allergic asthma drugs.

The development of proteinase-activated receptor-2 modulators and the challenges involved

Protease-activated receptor-2 (PAR2) has been extensively studied since its discovery in the mid-1990. Despite the advances in understanding PAR2 pharmacology, it has taken almost 25 years for the first inhibitor to reach clinical trials, and so far, no PAR2 antagonist has been approved for human use. Research has employed classical approaches to develop a wide array of PAR2 agonists and antagonists, consisting of peptides, peptoids and antibodies to name a few, with a surge in patent applications over this period. Recent breakthroughs in PAR2 structure determination has provided a unique insight into proposed PAR2 ligand binding sites. Publication of the first crystal structures of PAR2 resolved in complex with two novel non-peptide small molecule antagonists (AZ8838 and AZ3451) revealed two distinct binding pockets, originally presumed to be allosteric sites, with a PAR2 antibody (Fab3949) used to block tethered ligand engagement with the peptide-binding domain of the receptor. Further studies have proposed orthosteric site occupancy for AZ8838 as a competitive antagonist. One company has taken the first PAR2 antibody (MEDI0618) into phase I clinical trial (NCT04198558). While this first-in-human trial is at the early stages of the assessment of safety, other research into the structural characterisation of PAR2 is still ongoing in an attempt to identify new ways to target receptor activity. This review will focus on the development of novel PAR2 modulators developed to date, with an emphasis placed upon the advances made in the pharmacological targeting of PAR2 activity as a strategy to limit chronic inflammatory disease.

Legumain Induces Oral Cancer Pain by Biased Agonism of Protease-Activated Receptor-2

Oral squamous cell carcinoma (OSCC) is one of the most painful cancers, which interferes with orofacial function including talking and eating. We report that legumain (Lgmn) cleaves protease-activated receptor-2 (PAR₂) in the acidic OSCC microenvironment to cause pain. Lgmn is a cysteine protease of late endosomes and lysosomes that can be secreted; it exhibits maximal activity in acidic environments. The role of Lgmn in PAR₂-dependent cancer pain is unknown. We studied Lgmn activation in human oral cancers and oral cancer mouse models. Lgmn was activated in OSCC patient tumors, compared with matched normal oral tissue. After intraplantar, facial or lingual injection, Lgmn evoked nociception in wild-type (WT) female mice but not in female mice lacking PAR₂ in Na_V1.8-positive neurons (Par₂Na_v1.8), nor in female mice treated with a Lgmn inhibitor, LI-1. Inoculation of an OSCC cell line caused mechanical and thermal hyperalgesia that was reversed by LI-1. Par₂Na_v1.8 and Lgmn deletion attenuated mechanical allodynia in female mice with carcinogen-induced OSCC. Lgmn caused PAR₂-dependent hyperexcitability of trigeminal neurons from WT female mice. Par₂ deletion, LI-1, and inhibitors of adenylyl cyclase or protein kinase A (PKA) prevented the effects of Lgmn. Under acidified conditions, Lgmn cleaved within the extracellular N terminus of PAR₂ at Asn³⁰↓Arg³¹, proximal to the canonical trypsin activation site. Lgmn activated PAR₂ by biased mechanisms in HEK293 cells to induce Ca²⁺ mobilization, cAMP formation, and PKA/protein kinase D (PKD) activation, but not β-arrestin recruitment or PAR₂ endocytosis. Thus, in the acidified OSCC microenvironment, Lgmn activates PAR₂ by biased mechanisms that evoke cancer pain.SIGNIFICANCE STATEMENT Oral squamous cell carcinoma (OSCC) is one of the most painful cancers. We report that legumain (Lgmn), which exhibits maximal activity in acidic environments, cleaves protease-activated receptor-2 (PAR₂) on neurons to produce OSCC pain. Active Lgmn was elevated in OSCC patient tumors, compared with matched normal oral tissue. Lgmn evokes pain-like behavior through PAR₂ Exposure of pain-sensing neurons to Lgmn decreased the current required to generate an action potential through PAR₂ Inhibitors of adenylyl cyclase and protein kinase A (PKA) prevented the effects of Lgmn. Lgmn activated PAR₂ to induce calcium mobilization, cAMP formation, and activation of protein kinase D (PKD) and PKA, but not β-arrestin recruitment or PAR₂ endocytosis. Thus, Lgmn is a biased agonist of PAR₂ that evokes cancer pain.

Protease-activated receptors (PARs): mechanisms of action and potential therapeutic modulators in PAR-driven inflammatory diseases

Inflammatory diseases have become increasingly prevalent with industrialization. To address this, numerous anti-inflammatory agents and molecular targets have been considered in clinical trials. Among molecular targets, protease-activated receptors (PARs) are abundantly recognized for their roles in the development of chronic inflammatory diseases. In particular, several inflammatory effects are directly mediated by the sensing of proteolytic activity by PARs.

PARs belong to the seven transmembrane domain G protein-coupled receptor family, but are unique in their lack of physiologically soluble ligands. In contrast with classical receptors, PARs are activated by N-terminal proteolytic cleavage. Upon removal of specific N-terminal peptides, the resulting N-termini serve as tethered activation ligands that interact with the extracellular loop 2 domain and initiate receptor signaling. In the classical pathway, activated receptors mediate signaling by recruiting G proteins. However, activation of PARs alternatively lead to the transactivation of and signaling through receptors such as co-localized PARs, ion channels, and toll-like receptors.

In this review we consider PARs and their modulators as potential therapeutic agents, and summarize the current understanding of PAR functions from clinical and in vitro studies of PAR-related inflammation.

Rational design of a heterotrimeric G protein α subunit with artificial inhibitor sensitivity

Transmembrane signals initiated by a range of extracellular stimuli converge on members of the Gq family of heterotrimeric G proteins, which relay these signals in target cells. Gq family G proteins comprise Gq, G11, G14, and G16, which upon activation mediate their cellular effects via inositol lipid–dependent and –independent signaling to control fundamental processes in mammalian physiology. To date, highly specific inhibition of Gq/11/14 signaling can be achieved only with FR900359 (FR) and YM-254890 (YM), two naturally occurring cyclic depsipeptides. To further development of FR or YM mimics for other Gα subunits, we here set out to rationally design Gα16 proteins with artificial FR/YM sensitivity by introducing an engineered depsipeptide-binding site. Thereby we permit control of G16 function through ligands that are inactive on the WT protein. Using CRISPR/Cas9-generated Gαq/Gα11-null cells and loss- and gain-of-function mutagenesis along with label-free whole-cell biosensing, we determined the molecular coordinates for FR/YM inhibition of Gq and transplanted these to FR/YM-insensitive G16. Intriguingly, despite having close structural similarity, FR and YM yielded biologically distinct activities: it was more difficult to perturb Gq inhibition by FR and easier to install FR inhibition onto G16 than perturb or install inhibition with YM. A unique hydrophobic network utilized by FR accounted for these unexpected discrepancies. Our results suggest that non-Gq/11/14 proteins should be amenable to inhibition by FR scaffold–based inhibitors, provided that these inhibitors mimic the interaction of FR with Gα proteins harboring engineered FR-binding sites.

Protease-activated receptor-2 signaling through β-arrestin-2 mediates Alternaria alkaline serine protease-induced airway inflammation

Alternaria alternata is a fungal allergen associated with severe asthma and asthma exacerbations. Similarly to other asthma-associated allergens, Alternaria secretes a serine-like trypsin protease(s) that is thought to act through the G protein-coupled receptor protease-activated receptor-2 (PAR2) to induce asthma symptoms. However, specific mechanisms underlying Alternaria-induced PAR2 activation and signaling remain ill-defined. We sought to determine whether Alternaria-induced PAR2 signaling contributed to asthma symptoms via a PAR2/β-arrestin signaling axis, identify the protease activity responsible for PAR2 signaling, and determine whether protease activity was sufficient for Alternaria-induced asthma symptoms in animal models. We initially used in vitro models to demonstrate Alternaria-induced PAR2/β-arrestin-2 signaling. Alternaria filtrates were then used to sensitize and challenge wild-type, PAR2-/- and β-arrestin-2-/- mice in vivo. Intranasal administration of Alternaria filtrate resulted in a protease-dependent increase of airway inflammation and mucin production in wild-type but not PAR2-/- or β-arrestin-2-/- mice. Protease was isolated from Alternaria preparations, and select in vitro and in vivo experiments were repeated to evaluate sufficiency of the isolated Alternaria protease to induce asthma phenotype. Administration of a single isolated serine protease from Alternaria, Alternaria alkaline serine protease (AASP), was sufficient to fully activate PAR2 signaling and induce β-arrestin-2-/--dependent eosinophil and lymphocyte recruitment in vivo. In conclusion, Alternaria filtrates induce airway inflammation and mucus hyperplasia largely via AASP using the PAR2/β-arrestin signaling axis. Thus, β-arrestin-biased PAR2 antagonists represent novel therapeutic targets for treating aeroallergen-induced asthma.

Signaling Crosstalk of TGF-β/ALK5 and PAR2/PAR1: A Complex Regulatory Network Controlling Fibrosis and Cancer

Both signaling by transforming growth factor-β (TGF-β) and agonists of the G Protein-coupled receptors proteinase-activated receptor-1 (PAR1) and -2 (PAR2) have been linked to tissue fibrosis and cancer. Intriguingly, TGF-β and PAR signaling either converge on the regulation of certain matrix genes overexpressed in these pathologies or display mutual regulation of their signaling components, which is mediated in part through sphingosine kinases and sphingosine-1-phosphate and indicative of an intimate signaling crosstalk between the two pathways. In the first part of this review, we summarize the various regulatory interactions that have been discovered so far according to the organ/tissue in which they were described. In the second part, we highlight the types of signaling crosstalk between TGF-β on the one hand and PAR2/PAR1 on the other hand. Both ligand–receptor systems interact at various levels and by several mechanisms including mutual regulation of ligand–ligand, ligand–receptor, and receptor–receptor at the transcriptional, post-transcriptional, and receptor transactivation levels. These mutual interactions between PAR2/PAR1 and TGF-β signaling components eventually result in feed-forward loops/vicious cycles of matrix deposition and malignant traits that exacerbate fibrosis and oncogenesis, respectively. Given the crucial role of PAR2 and PAR1 in controlling TGF-β receptor activation, signaling, TGF-β synthesis and bioactivation, combining PAR inhibitors with TGF-β blocking agents may turn out to be more efficient than targeting TGF-β alone in alleviating unwanted TGF-β-dependent responses but retaining the beneficial ones.

Neutrophilia, gelatinase release and microvascular leakage induced by human mast cell tryptase in a mouse model: Lack of a role of protease-activated receptor 2 (PAR2)

BACKGROUND

Tryptase, the most abundant protease of the human mast cell, has been implicated as a key mediator of allergic inflammation that acts through activation of PAR2.

OBJECTIVES

To investigate the contribution of PAR2 in the pro-inflammatory actions mediated by tryptase in a mice model.

METHODS

We have injected recombinant human βII-tryptase into the peritoneum of PAR2-deficient and wild-type C57BL/6 mice. After 6, 12 and 24 hours, mice were killed, peritoneal lavage performed and inflammatory changes investigated.

RESULTS

Tryptase stimulated an increase in neutrophil numbers in the peritoneum, but responses did not differ between PAR2-deficient and wild-type mice. Heat inactivation of tryptase or pre-incubation with a selective tryptase inhibitor reduced neutrophilia, but neutrophil accumulation was not elicited with a peptide agonist of PAR2 (SLIGRL-NH2 ). Zymography indicated that tryptase stimulated the release of matrix metalloproteinases (MMP) 2 and 9 in the peritoneum of both mouse strains. Studies involving immunomagnetic isolation of neutrophils suggested that neutrophils represent the major cellular source of tryptase-induced MMP2 and MMP9. At 24 hours after tryptase injection, there was increased microvascular leakage as indicated by high levels of albumin in peritoneal lavage fluid, and this appeared to be partially abolished by heat-inactivating tryptase or addition of a protease inhibitor. There was no corresponding increase in levels of histamine or total protein. The extent of tryptase-induced microvascular leakage or gelatinase release into the peritoneum did not differ between PAR2-deficient and wild-type mice.

CONCLUSIONS

Our findings indicate that tryptase is a potent stimulus for neutrophil accumulation, MMP release and microvascular leakage. Although these actions required an intact catalytic site, the primary mechanism of tryptase in vivo would appear to involve processes independent of PAR2.

A Potent Antagonist of Protease-Activated Receptor 2 That Inhibits Multiple Signaling Functions in Human Cancer Cells

Protease-activated receptor 2 (PAR2) is a cell surface protein linked to G-protein dependent and independent intracellular signaling pathways that produce a wide range of physiological responses, including those related to metabolism, inflammation, pain, and cancer. Certain proteases, peptides, and nonpeptides are known to potently activate PAR2. However, no effective potent PAR2 antagonists have been reported yet despite their anticipated therapeutic potential. This study investigates antagonism of key PAR2-dependent signaling properties and functions by the imidazopyridazine compound I-191 (4-(8-(tert-butyl)-6-(4-fluorophenyl)imidazo[1,2-b]pyridazine-2-carbonyl)-3,3-dimethylpiperazin-2-one) in cancer cells. At nanomolar concentrations, I-191 inhibited PAR2 binding of and activation by structurally distinct PAR2 agonists (trypsin, peptide, nonpeptide) in a concentration-dependent manner in cells of the human colon adenocarcinoma grade II cell line (HT29). I-191 potently attenuated multiple PAR2-mediated intracellular signaling pathways leading to Ca²⁺ release, extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation, Ras homologue gene family, member A (RhoA) activation, and inhibition of forskolin-induced cAMP accumulation. The mechanism of action of I-191 was investigated using binding and calcium mobilization studies in HT29 cells where I-191 was shown to be noncompetitive and a negative allosteric modulator of the agonist 2f-LIGRL-NH₂ The compound alone did not activate these PAR2-mediated pathways, even at high micromolar concentrations, indicating no bias in these signaling properties. I-191 also potently inhibited PAR2-mediated downstream functional responses, including expression and secretion of inflammatory cytokines and cell apoptosis and migration, in human colon adenocarcinoma grade II cell line (HT29) and human breast adenocarcinoma cells (MDA-MB-231). These findings indicate that I-191 is a potent PAR2 antagonist that inhibits multiple PAR2-induced signaling pathways and functional responses. I-191 may be a valuable tool for characterizing PAR2 functions in cancer and in other cellular, physiological, and disease settings.

PAR2 Modulators Derived from GB88

PAR2 antagonists have potential for treating inflammatory, respiratory, gastrointestinal, neurological, and metabolic disorders, but few antagonists are known. Derivatives of GB88 (3) suggest that all four of its components bind at distinct PAR2 sites with the isoxazole, cyclohexylalanine, and isoleucine determining affinity and selectivity, while the C-terminal substituent determines agonist/antagonist function. Here we report structurally similar PAR2 ligands with opposing functions (agonist vs antagonist) upon binding to PAR2. A biased ligand AY117 (65) was found to antagonize calcium release induced by PAR2 agonists trypsin and hexapeptide 2f-LIGRLO-NH₂ (IC₅₀ 2.2 and 0.7 μM, HT29 cells), but it was a selective PAR2 agonist in inhibiting cAMP stimulation and activating ERK1/2 phosphorylation. It showed anti-inflammatory properties both in vitro and in vivo.

Antagonism of the proinflammatory and pronociceptive actions of canonical and biased agonists of protease-activated receptor-2

BACKGROUND AND PURPOSE

Diverse proteases cleave protease-activated receptor-2 (PAR2) on primary sensory neurons and epithelial cells to evoke pain and inflammation. Trypsin and tryptase activate PAR2 by a canonical mechanism that entails cleavage within the extracellular N-terminus revealing a tethered ligand that activates the cleaved receptor. Cathepsin-S and elastase are biased agonists that cleave PAR2 at different sites to activate distinct signalling pathways. Although PAR2 is a therapeutic target for inflammatory and painful diseases, the divergent mechanisms of proteolytic activation complicate the development of therapeutically useful antagonists.

EXPERIMENTAL APPROACH

We investigated whether the PAR2 antagonist GB88 inhibits protease-evoked activation of nociceptors and protease-stimulated oedema and hyperalgesia in rodents.

KEY RESULTS

Intraplantar injection of trypsin, cathespsin-S or elastase stimulated mechanical and thermal hyperalgesia and oedema in mice. Oral GB88 or par2 deletion inhibited the algesic and proinflammatory actions of all three proteases, but did not affect basal responses. GB88 also prevented pronociceptive and proinflammatory effects of the PAR2-selective agonists 2-furoyl-LIGRLO-NH2 and AC264613. GB88 did not affect capsaicin-evoked hyperalgesia or inflammation. Trypsin, cathepsin-S and elastase increased [Ca(2+) ]i in rat nociceptors, which expressed PAR2. GB88 inhibited this activation of nociceptors by all three proteases, but did not affect capsaicin-evoked activation of nociceptors or inhibit the catalytic activity of the three proteases.

CONCLUSIONS AND IMPLICATIONS

GB88 inhibits the capacity of canonical and biased protease agonists of PAR2 to cause nociception and inflammation.

Teleocidin A2 inhibits human proteinase-activated receptor 2 signaling in tumor cells

Enhanced expression of the proteinase‐activated receptor 2 (PAR2) is linked to cell proliferation and migration in many cancer cell types. The role of PAR2 in cancer progression strongly illustrates the need for PAR2‐inhibiting compounds. However, to date, potent and selective PAR2 antagonists have not been reported. The natural product teleocidin A2 was characterized against PAR2‐activating peptide SLIGKV‐NH₂, and trypsin‐induced PAR2‐dependent intracellular Ca²⁺ mobilization in tumor and in primary endothelial or epithelial cells. Further biochemical and cell‐based studies were conducted to evaluate teleocidin specificity. The antagonizing effect of teleocidin A2 was confirmed in PAR2‐dependent cell migration and rearrangement of actin cytoskeleton of human breast adenocarcinoma cell line (MDA‐MB 231) breast cancer cells. Teleocidin A2 antagonizes PAR2‐dependent intracellular Ca²⁺ mobilization induced by either SLIGKV‐NH₂ or trypsin with IC₅₀ values from 15 to 25 nmol/L in MDA‐MB 231, lung carcinoma cell line, and human umbilical vein endothelial cell. Half maximal inhibition of either PAR1 or P2Y receptor‐dependent Ca²⁺ release is only achieved with 10‐ to 20‐fold higher concentrations of teleocidin A2. In low nanomolar concentrations, teleocidin A2 reverses both SLIGKV‐NH₂ and trypsin‐mediated PAR2‐dependent migration of MDA‐MB 231 cells, and has no effect itself on cell migration and no effect on cell viability. Teleocidin A2 further controls PAR2‐induced actin cytoskeleton rearrangement of MDA‐MB 231 cells. Thus, for the first time, the small molecule natural product teleocidin A2 exhibiting PAR2 antagonism in the low nanomolar range with potent antimigratory activity is described.

Protease activated receptor 2 (PAR2) modulators: a patent review (2010-2015)

INTRODUCTION

Protease activated receptor 2 (PAR2) is a self-activated G protein-coupled receptor that has been implicated in several diseases, including inflammatory, gastrointestinal, respiratory, metabolic diseases, cancers and others, making it an important prospective drug target. No known endogenous ligands are available for PAR2, so having potent exogenous agonists and antagonists can be helpful for studying physiological functions of PAR2.

AREAS COVERED

This review covers agonist-, antagonist-, antibody- and pepducin-based modulators of PAR2 reported in patent applications between 2010-2015, along with their available structure-activity relationships, biological activities and potential uses for studying PAR2.

EXPERT OPINION

In the last six years, substantial efforts were made towards developing PAR2 modulators, but most lack potency or selectivity or have poor pharmacokinetic profiles. Many PAR2 modulators were assessed by measuring Gαq protein-mediated calcium release in cells. This may be insufficient to fully characterize ligand function, since different ligands signal through PAR2 via multiple signaling pathways. It may be feasible to develop biased ligands as drugs that can selectively modulate one or more specific signaling pathways linking PAR2 to a specific diseased state. Accordingly, potent, orally bioavailable, pathway- and receptor-selective PAR2 modulators may be an achievable goal to realizing effective drugs that can treat PAR2-mediated diseases.

The novel PAR2 ligand C391 blocks multiple PAR2 signalling pathways in vitro and in vivo

BACKGROUND AND PURPOSE

Proteinase-activated receptor-2 (PAR2) is a GPCR linked to diverse pathologies, including acute and chronic pain. PAR2 is one of the four PARs that are activated by proteolytic cleavage of the extracellular amino terminus, resulting in an exposed, tethered peptide agonist. Several peptide and peptidomimetic agonists, with high potency and efficacy, have been developed to probe the functions of PAR2, in vitro and in vivo. However, few similarly potent and effective antagonists have been described.

EXPERIMENTAL APPROACH

We modified the peptidomimetic PAR2 agonist, 2-furoyl-LIGRLO-NH₂ , to create a novel PAR2 peptidomimetic ligand, C391. C391 was evaluated for PAR2 agonist/antagonist activity to PAR2 across G_q signalling pathways using the naturally expressing PAR2 cell line 16HBE14o-. For antagonist studies, a highly potent and specific peptidomimetic agonist (2-aminothiazo-4-yl-LIGRL-NH₂ ) and proteinase agonist (trypsin) were used to activate PAR2. C391 was also evaluated in vivo for reduction of thermal hyperalgesia, mediated by mast cell degranulation, in mice.

KEY RESULTS

C391 is a potent and specific peptidomimetic antagonist, blocking multiple signalling pathways (G_q -dependent Ca²⁺ , MAPK) induced following peptidomimetic or proteinase activation of human PAR2. In a PAR2-dependent behavioural assay in mice, C391 dose-dependently (75 μg maximum effect) blocked the thermal hyperalgesia, mediated by mast cell degranulation.

CONCLUSIONS AND IMPLICATIONS

C391 is the first low MW antagonist to block both PAR2 Ca²⁺ and MAPK signalling pathways activated by peptidomimetics and/or proteinase activation. C391 represents a new molecular structure for PAR2 antagonism and can serve as a basis for further development for this important therapeutic target.

Biased signaling of protease-activated receptors

In addition to their role in protein degradation and digestion, proteases can also function as hormone-like signaling molecules that regulate vital patho-physiological processes, including inflammation, hemostasis, pain, and repair mechanisms. Certain proteases can signal to cells by cleaving protease-activated receptors (PARs), a family of four G protein-coupled receptors. PARs are expressed by almost all cell types, control important physiological and disease-relevant processes, and are an emerging therapeutic target for major diseases. Most information about PAR activation and function derives from studies of a few proteases, for example thrombin in the case of PAR1, PAR3, and PAR4, and trypsin in the case of PAR2 and PAR4. These proteases cleave PARs at established sites with the extracellular N-terminal domains, and expose tethered ligands that stabilize conformations of the cleaved receptors that activate the canonical pathways of G protein- and/or β-arrestin-dependent signaling. However, a growing number of proteases have been identified that cleave PARs at divergent sites to activate distinct patterns of receptor signaling and trafficking. The capacity of these proteases to trigger distinct signaling pathways is referred to as biased signaling, and can lead to unique patho-physiological outcomes. Given that a different repertoire of proteases are activated in various patho-physiological conditions that may activate PARs by different mechanisms, signaling bias may account for the divergent actions of proteases and PARs. Moreover, therapies that target disease-relevant biased signaling pathways may be more effective and selective approaches for the treatment of protease- and PAR-driven diseases. Thus, rather than mediating the actions of a few proteases, PARs may integrate the biological actions of a wide spectrum of proteases in different patho-physiological conditions.

Toward drugs for protease-activated receptor 2 (PAR2)

PAR2 has a distinctive functional phenotype among an unusual group of GPCRs called protease activated receptors, which self-activate after cleavage of their N-termini by mainly serine proteases. PAR2 is the most highly expressed PAR on certain immune cells, and it is activated by multiple proteases (but not thrombin) in inflammation. PAR2 is expressed on many types of primary human cells and cancer cells. PAR2 knockout mice and PAR2 agonists and antagonists have implicated PAR2 as a promising target in inflammatory conditions; respiratory, gastrointestinal, metabolic, cardiovascular, and neurological dysfunction; and cancers. This article summarizes salient features of PAR2 structure, activation, and function; opportunities for disease intervention via PAR2; pharmacological properties of published or patented PAR2 modulators (small molecule agonists and antagonists, pepducins, antibodies); and some personal perspectives on limitations of assessing their properties and on promising new directions for PAR2 modulation.

Roles of proteolysis in regulation of GPCR function

The enzymatic activity of peptidases must be tightly regulated to prevent uncontrolled hydrolysis of peptide bonds, which could have devastating effects on biological systems. Peptidases are often generated as inactive propeptidases, secreted with endogenous inhibitors, or they are compartmentalized. Propeptidases become active after proteolytic removal of N-terminal activation peptides by other peptidases. Some peptidases only become active towards substrates only at certain pHs, thus confining activity to specific compartments or conditions. This review discusses the different roles proteolysis plays in regulating GPCRs. At the cell-surface, certain GPCRs are regulated by the hydrolytic inactivation of bioactive peptides by membrane-anchored peptidases, which prevent signalling. Conversely, cell-surface peptidases can also generate bioactive peptides, which directly activate GPCRs. Alternatively, cell-surface peptidases activated by GPCRs, can generate bioactive peptides to cause transactivation of receptor tyrosine kinases, thereby promoting signalling. Certain peptidases can signal directly to cells, by cleaving GPCR to initiate intracellular signalling cascades. Intracellular peptidases also regulate GPCRs; lysosomal peptidases destroy GPCRs in lysosomes to permanently terminate signalling and mediate down-regulation; endosomal peptidases cleave internalized peptide agonists to regulate GPCR recycling, resensitization and signalling; and soluble intracellular peptidases also participate in GPCR function by regulating the ubiquitination state of GPCRs, thereby altering GPCR signalling and fate. Although the use of peptidase inhibitors has already brought success in the treatment of diseases such as hypertension, the discovery of new regulatory mechanisms involving proteolysis that control GPCRs may provide additional targets to modulate dysregulated GPCR signalling in disease.

PAR(2) expression in peripheral blood monocytes of patients with rheumatoid arthritis

OBJECTIVES

Proteinase-activated receptor 2 (PAR(2)) is a G protein-coupled receptor activated by serine proteinases with proinflammatory activity. A study was undertaken to investigate the presence and functional significance of PAR(2) expression on rheumatoid arthritis (RA)-derived leucocyte subsets.

METHODS

Venous blood was obtained from patients with RA and osteoarthritis (OA) as well as healthy control subjects. Surface expression of PAR(2) on peripheral blood mononuclear cells (PBMCs) was analysed by flow cytometry and interleukin 6 (IL-6) generation by ELISA.

RESULTS

Patients with RA had elevated but variable surface expression of PAR(2) on CD14+ monocytes compared with control subjects (median (1st to 3rd quartiles) 1.76% (0.86-4.10%) vs 0.06% (0.03-0.81%), p<0.0001). CD3+ T cells showed a similar pattern with significantly higher PAR(2) expression in patients with RA compared with controls (3.05% (0.36-11.82%) vs 0.08% (0.02-0.28%), p<0.0001). For both subsets, PAR(2) expression was significantly higher (p<0.00001) in patients with high levels of disease activity: PAR(2) expression for both CD14+ and CD3+ cells correlated to C reactive protein and erythrocyte sedimentation rate. Furthermore, in a cohort of patients with newly diagnosed RA, elevated PAR(2) expression in both CD14+ and CD3+ cells was significantly reduced 3 months after methotrexate or sulfasalazine treatment and this reduction correlated significantly with the reduction in the 28-joint Disease Activity Scale score (p<0.05). PAR(2) expression on cells from patients with OA was low, similar to levels seen in control subjects. Generation of IL-6 by monocytes in response to a selective PAR(2) agonist was significantly greater in patients with RA than in patients with OA and control subjects (p<0.05).

CONCLUSIONS

These findings are consistent with a pathogenic role for PAR(2) in RA.

Targeting proteinase-activated receptors: therapeutic potential and challenges

Proteinase-activated receptors (PARs), a family of four seven-transmembrane G protein-coupled receptors, act as targets for signalling by various proteolytic enzymes. PARs are characterized by a unique activation mechanism involving the proteolytic unmasking of a tethered ligand that stimulates the receptor. Given the emerging roles of these receptors in cancer as well as in disorders of the cardiovascular, musculoskeletal, gastrointestinal, respiratory and central nervous system, PARs have become attractive targets for the development of novel therapeutics. In this Review we summarize the mechanisms by which PARs modulate cell function and the roles they can have in physiology and diseases. Furthermore, we provide an overview of possible strategies for developing PAR antagonists.

Protease-activated receptor 2 signaling in inflammation

Protease-activated receptors (PARs) are G protein-coupled receptors that are activated by proteolytical cleavage of the amino-terminus and thereby act as sensors for extracellular proteases. While coagulation proteases activate PARs to regulate hemostasis, thrombosis, and cardiovascular function, PAR2 is also activated in extravascular locations by a broad array of serine proteases, including trypsin, tissue kallikreins, coagulation factors VIIa and Xa, mast cell tryptase, and transmembrane serine proteases. Administration of PAR2-specific agonistic and antagonistic peptides, as well as studies in PAR2 knockout mice, identified critical functions of PAR2 in development, inflammation, immunity, and angiogenesis. Here, we review these roles of PAR2 with an emphasis on the role of coagulation and other extracellular protease pathways that cleave PAR2 in epithelial, immune, and neuronal cells to regulate physiological and pathophysiological processes.

Neutrophil elastase acts as a biased agonist for proteinase-activated receptor-2 (PAR2)

Human neutrophil proteinases (elastase, proteinase-3, and cathepsin-G) are released at sites of acute inflammation. We hypothesized that these inflammation-associated proteinases can affect cell signaling by targeting proteinase-activated receptor-2 (PAR(2)). The PAR family of G protein-coupled receptors is triggered by a unique mechanism involving the proteolytic unmasking of an N-terminal self-activating tethered ligand (TL). Proteinases can either activate PAR signaling by unmasking the TL sequence or disarm the receptor for subsequent enzyme activation by cleaving downstream from the TL sequence. We found that none of neutrophil elastase, cathepsin-G, and proteinase-3 can activate G(q)-coupled PAR(2) calcium signaling; but all of these proteinases can disarm PAR(2), releasing the N-terminal TL sequence, thereby preventing G(q)-coupled PAR(2) signaling by trypsin. Interestingly, elastase (but neither cathepsin-G nor proteinase-3) causes a TL-independent PAR(2)-mediated activation of MAPK that, unlike the canonical trypsin activation, does not involve either receptor internalization or recruitment of β-arrestin. Cleavage of synthetic peptides derived from the extracellular N terminus of PAR(2), downstream of the TL sequence, demonstrated distinct proteolytic sites for all three neutrophil-derived enzymes. We conclude that in inflammation, neutrophil proteinases can modulate PAR(2) signaling by preventing/disarming the G(q)/calcium signal pathway and, via elastase, can selectively activate the p44/42 MAPK pathway. Our data illustrate a new mode of PAR regulation that involves biased PAR(2) signaling by neutrophil elastase and a disarming/silencing effect of cathepsin-G and proteinase-3.

Interdicting protease-activated receptor-2-driven inflammation with cell-penetrating pepducins

Protease-activated receptor-2 (PAR2), a cell surface receptor for trypsin-like proteases, plays a key role in a number of acute and chronic inflammatory diseases of the joints, lungs, brain, gastrointestinal tract, and vascular systems. Despite considerable effort by the pharmaceutical industry, PAR2 has proven recalcitrant to targeting by small molecule inhibitors, which have been unable to effectively prevent the interaction of the protease-generated tethered ligand with the body of the receptor. Here, we report the development of first-in-class cell-penetrating lipopeptide "pepducin" antagonists of PAR2. The design of the third intracellular (i3) loop pepducins were based on a structural model of a PAR2 dimer and by mutating key pharmacophores in the receptor intracellular loops and analogous pepducins. Individual pharmacophores were identified, which controlled constitutive, agonist, and antagonist activities. This approach culminated in the identification of the P2pal-18S pepducin which completely suppressed trypsin and mast cell tryptase signaling through PAR2 in neutrophils and colon cancer cells. The PAR2 pepducin was highly efficacious in blocking PAR2-dependent inflammatory responses in mouse models. These effects were lost in PAR2-deficient and mast-cell-deficient mice, thereby validating the specificity of the pepducin in vivo. These data provide proof of concept that PAR2 pepducin antagonists may afford effective treatments of potentially debilitating inflammatory diseases and serve as a blueprint for developing highly potent and specific i3-loop-based pepducins for other G protein-coupled receptors (GPCRs).

Structure, function and pathophysiology of protease activated receptors

Discovered in the 1990s, protease activated receptors(1) (PARs) are membrane-spanning cell surface proteins that belong to the G protein coupled receptor (GPCR) family. A defining feature of these receptors is their irreversible activation by proteases; mainly serine. Proteolytic agonists remove the PAR extracellular amino terminal pro-domain to expose a new amino terminus, or tethered ligand, that binds intramolecularly to induce intracellular signal transduction via a number of molecular pathways that regulate a variety of cellular responses. By these mechanisms PARs function as cell surface sensors of extracellular and cell surface associated proteases, contributing extensively to regulation of homeostasis, as well as to dysfunctional responses required for progression of a number of diseases. This review examines common and distinguishing structural features of PARs, mechanisms of receptor activation, trafficking and signal termination, and discusses the physiological and pathological roles of these receptors and emerging approaches for modulating PAR-mediated signaling in disease.

Protease and protease-activated receptor-2 signaling in the pathogenesis of atopic dermatitis

Proteases in the skin are essential to epidermal permeability barrier homeostasis. In addition to their direct proteolytic effects, certain proteases signal to cells by activating protease-activated receptors (PARs), the G-protein-coupled receptors. The expression of functional PAR-2 on human skin and its role in inflammation, pruritus, and skin barrier homeostasis have been demonstrated. Atopic dermatitis (AD) is a multifactorial inflammatory skin disease characterized by genetic barrier defects and allergic inflammation, which is sustained by gene-environmental interactions. Recent studies have revealed aberrant expression and activation of serine proteases and PAR-2 in the lesional skin of AD patients. The imbalance between proteases and protease inhibitors associated with genetic defects in the protease/protease inhibitor encoding genes, increase in skin surface pH, and exposure to proteolytically active allergens contribute to this aberrant protease/ PAR-2 signaling in AD. The increased protease activity in AD leads to abnormal desquamation, degradation of lipid-processing enzymes and antimicrobial peptides, and activation of primary cytokines, thereby leading to permeability barrier dysfunction, inflammation, and defects in the antimicrobial barrier. Moreover, up-regulated proteases stimulate PAR-2 in lesional skin of AD and lead to the production of cytokines and chemokines involved in inflammation and immune responses, itching sensation, and sustained epidermal barrier perturbation with easier allergen penetration. In addition, PAR-2 is an important sensor for exogenous danger molecules, such as exogenous proteases from various allergens, and plays an important role in AD pathogenesis. Together, these findings suggest that protease activity or PAR-2 may be a future target for therapeutic intervention for the treatment of AD.