Interdicting protease-activated receptor-2-driven inflammation with cell-penetrating pepducins. Proc Natl Acad Sci U S A. 2011;108:8491-8496. [PMID: 21536878 DOI: 10.1073/pnas.1017091108]

γ-Secretase Cleaves Bifunctional Fatty Acid-Conjugated Small Molecules with Amide Bonds in Mammalian Cells

Connecting two small molecules, such as ligands, fluorophores, or lipids, together via a linker with amide bonds is a widely used strategy to generate synthetic bifunctional molecules for various biological and biomedical applications. Such bifunctional molecules have been used in live-cell experiments under the assumption that they should be stable in cells. However, we recently found that a membrane-targeting bifunctional molecule, composed of a lipopeptide and the small-molecule ligand trimethoprim, referred to as mgcTMP, underwent amide-bond cleavage in mammalian cells. In this work, we first identified γ-secretase as the major protease degrading mgcTMP in cells. We next investigated the intracellular degradation of several different types of amide-linked bifunctional compounds and found that N-terminally fatty acid-conjugated small molecules are susceptible to γ-secretase-mediated amide-bond cleavage. In contrast, amide-linked bifunctional molecules composed of two small molecules, such as ligands and hydrophobic groups, which lack lipid modification, did not undergo intracellular degradation. These findings highlight a previously overlooked consideration for the development and application of lipid-based bifunctional molecules in chemical biology research.

Protease-Activated Receptor 2 in inflammatory skin disease: current evidence and future perspectives

Protease-activated receptor-2 (PAR2) is a class-A G protein-coupled receptor (GPCR) activated by serine proteases and is expressed by multiple tissues, including the skin. PAR2 is involved in the skin inflammatory response, promoting Th2 inflammation, delaying skin barrier repair, and affecting the differentiation of keratinocytes. It also participates in the transmission of itch and pain sensations in the skin. Increasing evidence indicates that PAR2 plays an important role in the pathogenesis of inflammatory skin diseases such as acne vulgaris, rosacea, psoriasis, and atopic dermatitis. Additional focus will be placed on potential targeted therapies based on PAR2. The Goal of this review is to outline the emerging effects of PAR2 activation in inflammatory skin disease and highlight the promise of PAR2 modulators.

New Target(s) for RNF43 Regulation: Implications for Therapeutic Strategies

Cancer cells depend on specific oncogenic pathways or present a genetic alteration that leads to a particular disturbance. Still, personalized and targeted biological therapy remains challenging, with current efforts generally yielding disappointing results. Carefully assessing onco-target molecular pathways can, however, potently assist with such efforts for the selection of patient populations that would best respond to a given drug treatment. RNF43, an E3 ubiquitin ligase that negatively regulates Wnt/frizzled (FZD) receptors by their ubiquitination, internalization, and degradation, controls a key pathway in cancer. Recently, additional target proteins of RNF43 were described, including p85 of the PI3K/AKT/mTOR signaling pathway and protease-activated receptor 2 (PAR2), a G-protein-coupled receptor that potently induces β-catenin stabilization, independent of Wnts. RNF43 mutations with impaired E3 ligase activity were found in several types of cancers (e.g., gastrointestinal system tumors and endometrial and ovarian cancer), pointing to a high dependency on FZD receptors and possibly PAR2 and the PI3K/AKT/mTOR signaling pathway. The development of drugs toward these targets is essential for improved treatment of cancer patients.

Genome editing of PAR2 through targeted delivery of CRISPR-Cas9 system for alleviating acute lung inflammation via ERK/NLRP3/IL-1β and NO/iNOS signalling

Excessive and uncontrollable inflammatory responses in alveoli can dramatically exacerbate pulmonary disease progressions through vigorous cytokine releases, immune cell infiltration and protease-driven tissue damages. It is an urgent need to explore potential drug strategies for mitigating lung inflammation. Protease-activated receptor 2 (PAR2) as a vital molecular target principally participates in various inflammatory diseases via intracellular signal transduction. However, it has been rarely reported about the role of PAR2 in lung inflammation. This study applied CRISPR-Cas9 system encoding Cas9 and sgRNA (pCas9-PAR2) for PAR2 knockout and fabricated an anionic human serum albumin-based nanoparticles to deliver pCas9-PAR2 with superior inflammation-targeting efficiency and stability (TAP/pCas9-PAR2). TAP/pCas9-PAR2 robustly facilitated pCas9-PAR2 to enter and transfect inflammatory cells, eliciting precise gene editing of PAR2 in vitro and in vivo. Importantly, PAR2 deficiency by TAP/pCas9-PAR2 effectively and safely promoted macrophage polarization, suppressed pro-inflammatory cytokine releases and alleviated acute lung inflammation, uncovering a novel value of PAR2. It also revealed that PAR2-mediated pulmonary inflammation prevented by TAP/pCas9-PAR2 was mainly dependent on ERK-mediated NLRP3/IL-1β and NO/iNOS signalling. Therefore, this work indicated PAR2 as a novel target for lung inflammation and provided a potential nanodrug strategy for PAR2 deficiency in treating inflammatory diseases.

Low-Dose Trypsin Accelerates Wound Healing via Protease-Activated Receptor 2

The management of wounds remains a significant healthcare challenge, highlighting the need for effective wound healing strategies. To address this, it is crucial to explore the molecular mechanisms underlying tissue repair as well as explore potential therapeutic approaches. Trypsin, as a serine protease, has been clinically utilized for wound healing for decades; however, it still lacks systemic investigation on its role and related mechanism. This study aimed to investigate the effects of low-dose trypsin on wound healing both in vitro and in vivo. While trypsin is an endogenous stimulus for protease-activated receptor 2 (PAR2), we discovered that both low-dose trypsin and synthesized PAR2 agonists significantly enhanced the migration, adhesion, and proliferation of fibroblasts and macrophages, similar to the natural repair mechanism mediated by mast cell tryptase. Moreover, such cell functions induced by trypsin were largely inhibited by PAR2 blockade, indicating the participation of trypsin via PAR2 activation. Additionally, low-dose trypsin notably expedited healing and regeneration while enhancing collagen deposition in skin wounds in vivo. Importantly, upon stimulation of trypsin or PAR2 agonists, there were significant upregulations of genes including claudin-7 (Cldn7), occludin (Ocln), and interleukin-17A (IL-17A) associated with proliferation and migration, extracellular matrix (ECM), tight junction, and focal adhesion, which contributed to wound healing. In summary, our study suggested that a low-dose trypsin could be a promising strategy for wound healing, and its function was highly dependent on PAR2 activation.

Blockade of Proteinase-Activated Receptor 2 (PAR2) Attenuates Neuroinflammation in Experimental Autoimmune Encephalomyelitis

Proteinase-activated receptor-2 (PAR2), which modulates inflammatory responses, is elevated in the central nervous system in multiple sclerosis (MS) and in its murine model, experimental autoimmune encephalomyelitis (EAE). In PAR2-null mice, disease severity of EAE is markedly diminished. We therefore tested whether inhibiting PAR2 activation in vivo might be a viable strategy for the treatment of MS. Using the EAE model, we show that a PAR2 antagonist, the pepducin palmitoyl-RSSAMDENSEKKRKSAIK-amide (P2pal-18S), attenuates EAE progression by affecting immune cell function. P2pal-18S treatment markedly diminishes disease severity and reduces demyelination, as well as the infiltration of T-cells and macrophages into the central nervous system. Moreover, P2pal-18S decreases granulocyte-macrophage colony-stimulating factor (GM-CSF) production and T-cell activation in cultured splenocytes and prevents macrophage polarization in vitro. We conclude that PAR2 plays a key role in regulating neuroinflammation in EAE and that PAR2 antagonists represent promising therapeutic agents for treating MS and other neuroinflammatory diseases. SIGNIFICANCE STATEMENT: Proteinase-activated receptor-2 modulates inflammatory responses and is increased in multiple sclerosis lesions. We show that the proteinase-activated receptor-2 antagonist palmitoyl-RSSAMDENSEKKRKSAIK-amide reduces disease in the murine experimental autoimmune encephalomyelitis model of multiple sclerosis by inhibiting T-cell and macrophage activation and infiltration into the central nervous system, making it a potential treatment for multiple sclerosis.

C781, a β-Arrestin Biased Antagonist at Protease-Activated Receptor-2 (PAR2), Displays in vivo Efficacy Against Protease-Induced Pain in Mice

Given the limited options and often harmful side effects of current analgesics and the suffering caused by the opioid crisis, new classes of pain therapeutics are needed. Protease-activated receptors (PARs), particularly PAR2, are implicated in a variety of pathologies, including pain. Since the discovery of the role of PAR2 in pain, development of potent and specific antagonists has been slow. In this study, we describe the in vivo characterization of a novel small molecule/peptidomimetic hybrid compound, C781, as a β-arrestin-biased PAR2 antagonist. In vivo behavioral studies were done in mice using von Frey filaments and the Mouse Grimace Scale. Pharmacokinetic studies were done to assess pharmacokinetic/pharmacodynamic relationship in vivo. We used both prevention and reversal paradigms with protease treatment to determine whether C781 could attenuate protease-evoked pain. C781 effectively prevented and reversed mechanical and spontaneous nociceptive behaviors in response to small molecule PAR2 agonists, mast cell activators, and neutrophil elastase. The ED50 of C781 (intraperitoneal dosing) for inhibition of PAR2 agonist (20.9 ng 2-AT)-evoked nociception was 6.3 mg/kg. C781 was not efficacious in the carrageenan inflammation model. Pharmacokinetic studies indicated limited long-term systemic bioavailability for C781 suggesting that optimizing pharmacokinetic properties could improve in vivo efficacy. Our work demonstrates in vivo efficacy of a biased PAR2 antagonist that selectively inhibits β-arrestin/MAPK signaling downstream of PAR2. Given the importance of this signaling pathway in PAR2-evoked nociception, C781 exemplifies a key pharmacophore for PAR2 that can be optimized for clinical development. PERSPECTIVE: Our work provides evidence that PAR2 antagonists that only block certain aspects of signaling by the receptor can be effective for blocking protease-evoked pain in mice. This is important because it creates a rationale for developing safer PAR2-targeting approaches for pain treatment.

Allosteric Regulation of G-Protein-Coupled Receptors: From Diversity of Molecular Mechanisms to Multiple Allosteric Sites and Their Ligands

Allosteric regulation is critical for the functioning of G protein-coupled receptors (GPCRs) and their signaling pathways. Endogenous allosteric regulators of GPCRs are simple ions, various biomolecules, and protein components of GPCR signaling (G proteins and β-arrestins). The stability and functional activity of GPCR complexes is also due to multicenter allosteric interactions between protomers. The complexity of allosteric effects caused by numerous regulators differing in structure, availability, and mechanisms of action predetermines the multiplicity and different topology of allosteric sites in GPCRs. These sites can be localized in extracellular loops; inside the transmembrane tunnel and in its upper and lower vestibules; in cytoplasmic loops; and on the outer, membrane-contacting surface of the transmembrane domain. They are involved in the regulation of basal and orthosteric agonist-stimulated receptor activity, biased agonism, GPCR-complex formation, and endocytosis. They are targets for a large number of synthetic allosteric regulators and modulators, including those constructed using molecular docking. The review is devoted to the principles and mechanisms of GPCRs allosteric regulation, the multiplicity of allosteric sites and their topology, and the endogenous and synthetic allosteric regulators, including autoantibodies and pepducins. The allosteric regulation of chemokine receptors, proteinase-activated receptors, thyroid-stimulating and luteinizing hormone receptors, and beta-adrenergic receptors are described in more detail.

Tryptase and Exogenous Trypsin: Mechanisms and Ophthalmic Applications

Ocular injuries caused by inflammation, surgery or accidents are subject to a physiological healing process that ultimately restores the structure and function of the damaged tissue. Tryptase and trypsin are essential component of this process and they play a role in promoting and reducing the inflammatory response of tissues, respectively. Following injury, tryptase is endogenously produced by mast cells and can exacerbate the inflammatory response both by stimulating neutrophil secretion, and through its agonist action on proteinase-activated receptor 2 (PAR2). In contrast, exogenously introduced trypsin promotes wound healing by attenuating inflammatory responses, reducing oedema and protecting against infection. Thus, trypsin may help resolve ocular inflammatory symptoms and promote faster recovery from acute tissue injury associated with ophthalmic diseases. This article describes the roles of tryptase and exogenous trypsin in affected tissues after onset of ocular injury, and the clinical applications of trypsin injection.

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.

PAR2 promotes impaired glucose uptake and insulin resistance in NAFLD through GLUT2 and Akt interference

BACKGROUND AND AIMS

Insulin resistance and poor glycemic control are key drivers of the development of NAFLD and have recently been shown to be associated with fibrosis progression in NASH. However, the underlying mechanisms involving dysfunctional glucose metabolism and relationship with NAFLD/NASH progression remain poorly understood. We set out to determine whether protease-activated receptor 2 (PAR2), a sensor of extracellular inflammatory and coagulation proteases, links NAFLD and NASH with liver glucose metabolism.

APPROACH AND RESULTS

Here, we demonstrate that hepatic expression of PAR2 increases in patients and mice with diabetes and NAFLD/NASH. Mechanistic studies using whole-body and liver-specific PAR2-knockout mice reveal that hepatic PAR2 plays an unexpected role in suppressing glucose internalization, glycogen storage, and insulin signaling through a bifurcating Gq -dependent mechanism. PAR2 activation downregulates the major glucose transporter of liver, GLUT2, through Gq -MAPK-FoxA3 and inhibits insulin-Akt signaling through Gq -calcium-CaMKK2 pathways. Therapeutic dosing with a liver-homing pepducin, PZ-235, blocked PAR2-Gq signaling and afforded significant improvements in glycemic indices and HbA1c levels in severely diabetic mice.

CONCLUSIONS

This work provides evidence that PAR2 is a major regulator of liver glucose homeostasis and a potential target for the treatment of diabetes and NASH.

Diversification of PAR signaling through receptor crosstalk

Protease activated receptors (PARs) are among the first receptors shown to transactivate other receptors: noticeably, these interactions are not limited to members of the same family, but involve receptors as diverse as receptor kinases, prostanoid receptors, purinergic receptors and ionic channels among others. In this review, we will focus on the evidence for PAR interactions with members of their own family, as well as with other types of receptors. We will discuss recent evidence as well as what we consider as emerging areas to explore; from the signalling pathways triggered, to the physiological and pathological relevance of these interactions, since this additional level of molecular cross-talk between receptors and signaling pathways is only beginning to be explored and represents a novel mechanism providing diversity to receptor function and play important roles in physiology and disease.

Pepducin-mediated G Protein-Coupled Receptor Signaling in the Cardiovascular System

ABSTRACT

Pepducins are small-lipidated peptides designed from the intracellular loops of G protein-coupled receptors (GPCRs) that act in an allosteric manner to modulate the activity of GPCRs. Over the past 2 decades, pepducins have progressed initially from pharmacologic tools used to manipulate GPCR activity in an orthosteric site-independent manner to compounds with therapeutic potential that have even been used safely in phase 1 and 2 clinical trials in human subjects. The effect of pepducins at their cognate receptors has been shown to vary between antagonist, partial agonist, and biased agonist outcomes in various primary and clonal cell systems, with even small changes in amino acid sequence altering these properties and their receptor selectivity. To date, pepducins designed from numerous GPCRs have been studied for their impact on pathologic conditions, including cardiovascular diseases such as thrombosis, myocardial infarction, and atherosclerosis. This review will focus in particular on pepducins designed from protease-activated receptors, C-X-C motif chemokine receptors, formyl peptide receptors, and the β2-adrenergic receptor. We will discuss the historic context of pepducin development for each receptor, as well as the structural, signaling, pathophysiologic consequences, and therapeutic potential for each pepducin class.

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.

Revisiting the potential of South American rattlesnake Crotalus durissus terrificus toxins as therapeutic, theranostic and/or biotechnological agents

The potential biotechnological and biomedical applications of the animal venom components are widely recognized. Indeed, many components have been used either as drugs or as templates/prototypes for the development of innovative pharmaceutical drugs, among which many are still used for the treatment of human diseases. A specific South American rattlesnake, named Crotalus durissus terrificus, shows a venom composition relatively simpler compared to any viper or other snake species belonging to the Crotalus genus, although presenting a set of toxins with high potential for the treatment of several still unmet human therapeutic needs, as reviewed in this work. In addition to the main toxin named crotoxin, which is under clinical trials studies for antitumoral therapy and which has also anti-inflammatory and immunosuppressive activities, other toxins from the C. d. terrificus venom are also being studied, aiming for a wide variety of therapeutic applications, including as antinociceptive, anti-inflammatory, antimicrobial, antifungal, antitumoral or antiparasitic agent, or as modulator of animal metabolism, fibrin sealant (fibrin glue), gene carrier or theranostic agent. Among these rattlesnake toxins, the most relevant, considering the potential clinical applications, are crotamine, crotalphine and gyroxin. In this narrative revision, we propose to organize and present briefly the updates in the accumulated knowledge on potential therapeutic applications of toxins collectively found exclusively in the venom of this specific South American rattlesnake, with the objective of contributing to increase the chances of success in the discovery of drugs based on toxins.

Lipopeptide Pepducins as Therapeutic Agents

Pepducins are lipidated peptides that target the intracellular loops of G protein-coupled receptors (GPCRs) in order to modulate transmembrane signaling to internally located effectors. With a wide array of potential activities ranging from partial, biased, or full agonism to antagonism, pepducins represent a versatile class of compounds that can be used to potentially treat diverse human diseases or be employed as novel tools to probe complex mechanisms of receptor activation and signaling in cells and in animals. Here, we describe a number of different pepducins including an advanced compound, PZ-128, that has successfully progressed through phase 2 clinical trials in cardiac patients demonstrating safety and efficacy in suppressing myonecrosis and arterial thrombosis.

PAR2 blockade reverses osimertinib resistance in non-small-cell lung cancer cells via attenuating ERK-mediated EMT and PD-L1 expression

Osimertinib, as the third-generation EGFR tyrosine kinase inhibitors (EGFR-TKIs), is a first-line molecularly targeted drug for non-small cell lung cancer (NSCLC). However, the emergence of therapeutic resistance to osimertinib markedly impairs its efficiency and efficacy, leading to the failure of clinical applications. Novel molecular targets and drugs are urgently needed for reversing osimertinib resistance in NSCLC. Protease-activated receptor 2 (PAR2) that belongs to a subfamily of G protein-coupled receptors can stimulate the transactivation of EGFR to regulate multiple cellular signalling, actively participating in tumour progression. This study firstly discovered that PAR2 expression was notably enhanced when NSCLC cells became resistant to osimertinib. A PAR2 inhibitor facilitated osimertinib to attenuate EGFR transactivation, ERK phosphorylation, EMT and PD-L1 expression which were associated to osimertinib resistance. The combination of the PAR2 inhibitor and osimertinib also notably blocked cell viability, migration, 3D sphere formation and in vivo tumour growth whereas osimertinib itself lost such inhibitory effects in osimertinib-resistant NSCLC cells. Importantly, this reversal effect of PAR2 blockade was uncovered to depend on ERK-mediated EMT and PD-L1, since inhibition of β-arrestin or ERK, which could be modulated by PAR2, sensitized osimertinib to prevent EMT, PD-L1 expression and consequently overcame osimertinib resistance. Thus, this study demonstrated that PAR2 antagonism could limit ERK-mediated EMT and immune checkpoints, consequently attenuating EGFR transactivation and reactivate osimertinib. It suggested that PAR2 may be a novel drug target for osimertinib resistance, and PAR2 inhibition may be a promising strategy candidate for reversing EGFR-TKI resistance in NSCLC.

Ageing mechanisms that contribute to tissue remodeling in lung disease

Age is a major risk factor for chronic respiratory diseases such as idiopathic pulmonary fibrosis (IPF), chronic obstructive pulmonary disease (COPD) and certain phenotypes of asthma. The recent COVID-19 pandemic also highlights the increased susceptibility of the elderly to acute respiratory distress syndrome (ARDS), a diffuse inflammatory lung injury with often long-term effects (ie parenchymal fibrosis). Collectively, these lung conditions are characterized by a pathogenic reparative process that, rather than restoring organ function, contributes to structural and functional tissue decline. In the ageing lung, the homeostatic control of wound healing following challenge or injury has an increased likelihood of being perturbed, increasing susceptibility to disease. This loss of fidelity is a consequence of a diverse range of underlying ageing mechanisms including senescence, mitochondrial dysfunction, proteostatic stress and diminished autophagy that occur within the lung, as well as in other tissues, organs and systems of the body. These ageing pathways are highly interconnected, involving localized and systemic increases in inflammatory mediators and damage associated molecular patterns (DAMPs); along with corresponding changes in immune cell function, metabolism and composition of the pulmonary and gut microbiomes. Here we comprehensively review the roles of ageing mechanisms in the tissue remodeling of lung disease.

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.

Scabies itch: an update on neuroimmune interactions and novel targets

Frequently described as 'the worst itch' one can ever experience scabies itch is the hallmark of Sarcoptes scabiei mite infestation. Notably, the itchiness often persists for weeks despite scabicides therapy. The mechanism of scabies itch is not yet fully understood, and effective treatment modalities are still missing which can severely affect the quality of life. The aim of this review is to provide an overview of the scope of itch in scabies and highlight candidate mechanisms underlying this itch. We herein discuss scabies itch, with a focus on the nature, candidate underlying mechanisms and treatment options. We also synthesize this information with current understanding of the mechanisms contributing to non-histaminergic itch in other conditions. Itch is a major problem in scabies and can lead to grave consequences. We provide the latest insights on host-mite interaction, secondary microbial infection and neural sensitization with special emphasis on keratinocytes and mast cells to better understand the mechanism of itch in scabies. Also, the most relevant current modalities remaining under investigation that possess promising perspectives for scabies itch (i.e. protease-activated receptor-2 (PAR-2) inhibitor, Mas-related G protein-coupled receptor X2 (MRGPRX2) antagonist) are discussed. Greater understanding of these diverse mechanisms may provide a rational basis for the development of improved and targeted approaches to control itch in individuals with scabies.

Mimicry of Dopamine 1 Receptor Signaling with Cell-Penetrating Peptides

In this study, through the use of protein mimicry, a peptide was developed to activate the dopamine 1 receptor signaling pathway from the inside of the cell and in absence of the natural extracellular ligand. The sequence was initially derived from the intracellular interaction site between the activated receptor and the alpha domain of its associated G-protein and subsequently modified to increase its cell-penetrating properties. The peptide was then synthesized via solid phase peptide synthesis, purified and tested on cell models. This novel lipopeptide proved to be capable of efficiently ubiquitously penetrating the cell without the need for transfection agents or chiral recognition by specific pathways. Furthermore, the peptide induced the cellular response normally achieved through the activation of the receptor in cells that had not been treated with the natural ligand. The peptide could work as a candidate substitute to l-DOPA, leading the way for a peptides-based treatment for Parkinson’s disease.

The PAR2 inhibitor I-287 selectively targets Gαq and Gα12/13 signaling and has anti-inflammatory effects

Protease-activated receptor-2 (PAR2) is involved in inflammatory responses and pain, therefore representing a promising therapeutic target for the treatment of immune-mediated inflammatory diseases. However, as for other GPCRs, PAR2 can activate multiple signaling pathways and those involved in inflammatory responses remain poorly defined. Here, we describe a new selective and potent PAR2 inhibitor (I-287) that shows functional selectivity by acting as a negative allosteric regulator on Gα_q and Gα_12/13 activity and their downstream effectors, while having no effect on G_i/o signaling and βarrestin2 engagement. Such selective inhibition of only a subset of the pathways engaged by PAR2 was found to be sufficient to block inflammation in vivo. In addition to unraveling the PAR2 signaling pathways involved in the pro-inflammatory response, our study opens the path toward the development of new functionally selective drugs with reduced liabilities that could arise from blocking all the signaling activities controlled by the receptor.

Avet et al. characterize I-287, an inhibitor to protease-activated receptor 2 using BRET-assays. They find that I-287 selectively inhibits Gα_q and Gα_12/13 without affecting the activation of G_i/o or the recruitment of βarrestin2 and that it blocks inflammation in vitro and in vivo.

Targeting of G-protein coupled receptors in sepsis

The Third International Consensus Definitions (Sepsis-3) define sepsis as life-threatening multi-organ dysfunction caused by a dysregulated host response to infection. Sepsis can progress to septic shock-an even more lethal condition associated with profound circulatory, cellular and metabolic abnormalities. Septic shock remains a leading cause of death in intensive care units and carries a mortality of almost 25%. Despite significant advances in our understanding of the pathobiology of sepsis, therapeutic interventions have not translated into tangible differences in the overall outcome for patients. Clinical trials of antagonists of various pro-inflammatory mediators in sepsis have been largely unsuccessful in the past. Given the diverse physiologic roles played by G-protein coupled receptors (GPCR), modulation of GPCR signaling for the treatment of sepsis has also been explored. Traditional pharmacologic approaches have mainly focused on ligands targeting the extracellular domains of GPCR. However, novel techniques aimed at modulating GPCR intracellularly through aptamers, pepducins and intrabodies have opened a fresh avenue of therapeutic possibilities. In this review, we summarize the diverse roles played by various subfamilies of GPCR in the pathogenesis of sepsis and identify potential targets for pharmacotherapy through these novel approaches.

PAR2 controls cholesterol homeostasis and lipid metabolism in nonalcoholic fatty liver disease

OBJECTIVE

Increases in hepatic and plasma cholesterol occur in patients with nonalcoholic fatty liver disease (NAFLD), although the reason for this is not well understood. We investigated whether Protease-Activated Receptor 2 (PAR2) plays a role in cholesterol and lipid homeostasis in NAFLD.

METHODS

Human liver biopsies (n = 108) were quantified for PAR2 expression from NAFLD cases randomly selected and stratified by liver fibrosis stage, the primary predictor for clinical outcomes, while controlling for age, gender, and BMI between fibrosis groups. Demographic data and laboratory studies on plasma samples were obtained within 6 months of liver biopsy. Wild-type and PAR2-KO (C57BL/6 F2rl1-/-) mice were fed either normal or high fat diet for 16 weeks and plasma and liver assayed for lipids and soluble metabolites.

RESULTS

Severity of NAFLD and plasma cholesterol levels significantly correlated with hepatocyte PAR2 expression in NAFLD patients. Conversely, PAR2 deficiency in mice resulted in reduced expression of key hepatic genes involved in cholesterol synthesis, a 50% drop in plasma and total liver cholesterol, and induced a reverse cholesterol transport system that culminated in 25% higher fecal bile acid output. PAR2-deficient mice exhibited enhanced fatty acid β-oxidation with a ketogenic shift and an unexpected increase in liver glycogenesis. Mechanistic studies identified Gi-Jnk1/2 as key downstream effectors of protease-activated PAR2 in the regulation of lipid and cholesterol homeostasis in liver.

CONCLUSIONS

These data indicate that PAR2 may be a new target for the suppression of plasma cholesterol and hepatic fat accumulation in NAFLD and related metabolic conditions.

The parmodulin NRD-21 is an allosteric inhibitor of PAR1 Gq signaling with improved anti-inflammatory activity and stability

Novel analogs of the allosteric, biased PAR1 ligand ML161 (parmodulin 2, PM2) were prepared in order to identify potential anti-thrombotic and anti-inflammatory compounds of the parmodulin class with improved properties. Investigations of structure-activity relationships of the western portion of the 1,3-diaminobenzene scaffold were performed using an intracellular calcium mobilization assay with endothelial cells, and several heterocycles were identified that inhibited PAR1 at sub-micromolar concentrations. The oxazole NRD-21 was profiled in additional detail, and it was confirmed to act as a selective, reversible, negative allosteric modulator of PAR1. In addition to inhibiting human platelet aggregation, it showed superior anti-inflammatory activity to ML161 in a qPCR assay measuring the expression of tissue factor in response to the cytokine TNF-alpha in endothelial cells. Additionally, NRD-21 is much more plasma stable than ML161, and is a promising lead compound for the parmodulin class for anti-thrombotic and anti-inflammatory indications.

IgE-mediated mast cell activation promotes inflammation and cartilage destruction in osteoarthritis

Osteoarthritis is characterized by articular cartilage breakdown, and emerging evidence suggests that dysregulated innate immunity is likely involved. Here, we performed proteomic, transcriptomic, and electron microscopic analyses to demonstrate that mast cells are aberrantly activated in human and murine osteoarthritic joint tissues. Using genetic models of mast cell deficiency, we demonstrate that lack of mast cells attenuates osteoarthritis in mice. Using genetic and pharmacologic approaches, we show that the IgE/FcεRI/Syk signaling axis is critical for the development of osteoarthritis. We find that mast cell-derived tryptase induces inflammation, chondrocyte apoptosis, and cartilage breakdown. Our findings demonstrate a central role for IgE-dependent mast cell activation in the pathogenesis of osteoarthritis, suggesting that targeting mast cells could provide therapeutic benefit in human osteoarthritis.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Protease-activated receptor-2 regulates glial scar formation via JNK signaling

The study aimed to determine the effects of protease-activated receptor-2 (PAR-2) on glial scar formation after spinal cord injury (SCI) in Sprague-Dawley (SD) rats and the underlying mechanisms. Rivlin and Tator's acute extradural clip compression injury (CCI) model of severe SCI was established in this study. Animals were divided into four groups: 1) sham group (laminectomy only); 2) model group, treated with normal saline; 3) PAR-2 inhibitor group; 4) PAR-2 activator group. Enhanced GFAP and vimentin expression were the markers of glial scar formation. To determine whether JNK was involved in the effects of PAR-2 on GFAP and vimentin expression, we administered anisomycin (a JNK activator) in the presence of PAR-2 inhibitor and SP600125 (a JNK inhibitor) in the presence of PAR-2 activator. At 1, 7, 14 and 28 day after SCI, Basso, Beattie, and Bresnahan (BBB) locomotor score test was used to assess the locomotor functional recovery; immunofluorescence and western blot analysis were used to assess the expression level of GFAP, vimentin and p-JNK. Double immunofluorescence staining with GFAP and tubulin beta was used to assess the glial scar formation and the remaining neurons. Results suggested that PAR-2 is involved in glial scar formation and reduces neurons residues which can cause a further worsening in the functional outcomes after SCI via JNK signaling. Therefore, it may be effective to target PAR-2 in the treatment of SCI.

The functional role of protease-activated receptors on contractile responses by activation of Ca2+ sensitization pathways in simian colonic muscles

It has been known that activation of protease-activated receptors (PARs) affects gastrointestinal motility. In this study, we tested the effects of PAR agonists on electrical and contractile responses and Ca²⁺ sensitization pathways in simian colonic muscles. The Simian colonic muscle was initially hyperpolarized by PAR agonists. After the transient hyperpolarization, simian colonic muscle repolarized to the control resting membrane potential (RMP) without a delayed depolarization. Apamin significantly reduced the initial hyperpolarization, suggesting that activation of small conductance Ca²⁺-activated K⁺ (SK) channels is involved in the initial hyperpolarization. In contractile experiments, PAR agonists caused an initial relaxation followed by an increase in contractions. These delayed contractile responses were not matched with the electrical responses that showed no after depolarization of the RMP. To investigate the possible involvement of Rho-associated protein kinase 2 (ROCK) pathways in the PAR effects, muscle strips were treated with ROCK inhibitors, which significantly reduced the PAR agonist-induced contractions. Furthermore, PAR agonists increased MYPT1 phosphorylation, and ROCK inhibitors completely blocked MYPT1 phosphorylation. PAR agonists alone had no effect on CPI-17 phosphorylation. In the presence of apamin, PAR agonists significantly increased CPI-17 phosphorylation, which was blocked by protein kinase C (PKC) inhibitors suggesting that Ca²⁺ influx is increased by apamin and is activating PKC. In conclusion, these studies show that PAR activators induce biphasic responses in simian colonic muscles. The initial inhibitory responses by PAR agonists are mainly mediated by activation of SK channels and delayed contractile responses are mainly mediated by the CPI-17 and ROCK Ca²⁺ sensitization pathways in simian colonic muscles. NEW & NOTEWORTHY In the present study, we found that the contractile responses of simian colonic muscles to protease-activated receptor (PAR) agonists are different from the previously reported contractile responses of murine colonic muscles. Ca²⁺ sensitization pathways mediate the contractile responses of simian colonic muscles to PAR agonists without affecting the membrane potential. These findings emphasize novel mechanisms of PAR agonist-induced contractions possibly related to colonic dysmotility in inflammatory bowel disease.

Tissue factor at the crossroad of coagulation and cell signaling

The tissue factor (TF) pathway plays a central role in hemostasis and thrombo-inflammatory diseases. Although structure-function relationships of the TF initiation complex are elucidated, new facets of the dynamic regulation of TF's activities in cells continue to emerge. Cellular pathways that render TF non-coagulant participate in signaling of distinct TF complexes with associated proteases through the protease-activated receptor (PAR) family of G protein-coupled receptors. Additional co-receptors, including the endothelial protein C receptor (EPCR) and integrins, confer signaling specificity by directing subcellular localization and trafficking. We here review how TF is switched between its role in coagulation and cell signaling through thiol-disulfide exchange reactions in the context of physiologically relevant lipid microdomains. Inflammatory mediators, including reactive oxygen species, activators of the inflammasome, and the complement cascade play pivotal roles in TF procoagulant activation on monocytes, macrophages and endothelial cells. We furthermore discuss how TF, intracellular ligands, co-receptors and associated proteases are integrated in PAR-dependent cell signaling pathways controlling innate immunity, cancer and metabolic inflammation. Knowledge of the precise interactions of TF in coagulation and cell signaling is important for understanding effects of new anticoagulants beyond thrombosis and identification of new applications of these drugs for potential additional therapeutic benefits.

PAR2 Pepducin-Based Suppression of Inflammation and Itch in Atopic Dermatitis Models

PAR2 has been proposed to contribute to lesion formation and intense itch in atopic dermatitis. Here, we tested the ability of a cell-penetrating pepducin, PZ-235, to mitigate the potentially deleterious effects of PAR2 in models of atopic dermatitis. PZ-235 significantly inhibited PAR2-mediated expression of inflammatory factors NF-κB, TSLP, TNF-α, and differentiation marker K10 by 94%-98% (P < 0.001) in human keratinocytes and suppressed IL-4 and IL-13 by 68%-83% (P < 0.05) in mast cells. In delayed pepducin treatment models of oxazolone- and DNFB-induced dermatitis, PZ-235 significantly attenuated skin thickening by 43%-100% (P < 0.01) and leukocyte crusting by 57% (P < 0.05), and it inhibited ex vivo chemotaxis of leukocytes toward PAR2 agonists. Daily PZ-235 treatment of filaggrin-deficient mice exposed to dust mite allergens for 8 weeks significantly suppressed total leukocyte and T-cell infiltration by 50%-68%; epidermal thickness by 60%-77%; and skin thickening, scaling, excoriation, and total lesion severity score by 46%-56%. PZ-235 significantly reduced itching caused by wasp venom peptide degranulation of mast cells in mice by 51% (P < 0.05), which was comparable to the protective effects conferred by PAR2 deficiency. Taken together, these results suggest that a PAR2 pepducin may confer broad therapeutic benefits as a disease-modifying treatment for atopic dermatitis and itch.

Mast cells and mast cell tryptase enhance migration of human lung fibroblasts through protease-activated receptor 2

BACKGROUND

Mast cells may activate fibroblasts and contribute to remodeling processes in the lung. However, the mechanism behind these actions needs to be further investigated. Fibroblasts are major regulators of on-going remodeling processes. Protease activated receptor 2 (PAR2) expressed by fibroblasts may be activated by serine proteases, such as the mast cell mediator tryptase. The objective in this study was to investigate the effects of mast cells and specifically mast cell tryptase on fibroblast migration and the role of PAR2 activation.

METHODS

Human lung fibroblasts (HFL-1) were cultured together with human peripheral blood-derived mast cells or LAD2 mast cells and stimulated with either conditioned medium from LAD2 cells or tryptase. Analyses of immunological stimulation of mast cells by IgE/anti IgE in the co-culture system were also performed. The importance of PAR2 activation by mast cells and mast cell tryptase for the migratory effects of fibroblasts was investigated by pre-treatment with the PAR2 antagonist P2pal-18S. The expression of PAR2 was analyzed on fibroblasts and mast cells.

RESULTS

The migratory capacity of HFL-1 cells was enhanced by blood-derived mast cells (p < 0.02), LAD2 cells (p < 0.001), conditioned medium (p < 0.05) and tryptase (p < 0.006). P2pal-18S decreased the induced migration caused by mast cells (p < 0.001) and tryptase (p < 0.001) and the expression of PAR2 was verified in HFL-1 cells. Mast cells immunologically stimulated with IgE/Anti IgE had no further effects on fibroblast migration.

CONCLUSIONS

Mast cells and the mast cell mediator tryptase may have crucial roles in inducing lung fibroblast migration via PAR-2 activation, which may contribute to remodeling processes in chronic lung diseases.

Protease-Activated Receptor 1 as Therapeutic Target in Breast, Lung, and Ovarian Cancer: Pepducin Approach

The G-protein coupled receptors (GPCRs) belong to a large family of diverse receptors that are well recognized as pharmacological targets. However, very few of these receptors have been pursued as oncology drug targets. The Protease-activated receptor 1 (PAR1), which is a G-protein coupled receptor, has been shown to act as an oncogene and is an emerging anti-cancer drug target. In this paper, we provide an overview of PAR1’s biased signaling role in metastatic cancers of the breast, lungs, and ovaries and describe the development of PAR1 inhibitors that are currently in clinical use to treat acute coronary syndromes. PAR1 inhibitor PZ-128 is in a Phase II clinical trial and is being developed to prevent ischemic and thrombotic complication of patients undergoing cardiac catheterization. PZ-128 belongs to a new class of cell-penetrating, membrane-tethered peptides named pepducins that are based on the intracellular loops of receptors targeting the receptor G-protein interface. Application of PZ-128 as an anti-metastatic and anti-angiogenic therapeutic agent in breast, lung, and ovarian cancer is being reviewed.

Matrix metalloproteinase-2: A key regulator in coagulation proteases mediated human breast cancer progression through autocrine signaling

AIMS

Cell invasion is attributed to the synthesis and secretion of proteolytically active matrix-metalloproteinases (MMPs) by tumor cells to degrade extracellular matrix (ECM) and promote metastasis. The role of protease-activated receptor 2 (PAR2) in human breast cancer migration/invasion via MMP-2 up-regulation remains ill-defined; hence we investigated whether TF-FVIIa/trypsin-mediated PAR2 activation induces MMP-2 expression in human breast cancer.

MAIN METHODS

MMP-2 expression and the signaling mechanisms were analyzed by western blotting and RT-PCR. MMP-2 activity was measured by gelatin zymography. Cell invasion was analyzed by transwell invasion assay whereas; wound healing assay was performed to understand the cell migratory potential.

KEY FINDINGS

Here, we highlight that TF-FVIIa/trypsin-mediated PAR2 activation leads to enhanced MMP-2 expression in human breast cancer cells contributing to tumor progression. Knock-down of PAR2 abrogated TF-FVIIa/trypsin-induced up-regulation of MMP-2. Again, genetic manipulation of AKT or inhibition of NF-ĸB suggested that PAR2-mediated enhanced MMP-2 expression is dependent on the PI3K-AKT-NF-ĸB pathway. We also reveal that TF, PAR2, and MMP-2 are over-expressed in invasive breast carcinoma tissues as compared to normal. Knock-down of MMP-2 significantly impeded TF-FVIIa/trypsin-induced cell invasion. Further, we report that MMP-2 activates p38 MAPK-MK2-HSP27 signaling axis that leads to actin polymerization and induces cell migration. Pharmacological inhibition of p38 MAPK or MK2 attenuates MMP-2-induced cell migration.

SIGNIFICANCE

The study delineates a novel signaling pathway by which PAR2-induced MMP-2 expression regulates human breast cancer cell migration/invasion. Understanding these mechanistic details will certainly help to identify crucial targets for therapeutic interventions in breast cancer metastasis.

Pleiotropic effects of the hemostatic system

Atherothrombosis is characterized by the inflammatory process of atherosclerosis combined with a hypercoagulable state leading to superimposed thrombus formation. In atherosclerotic plaques, cell signaling can occur via protease-activated receptors (PARs), four of which have been identified so far (PAR1-PAR4). Proteases that are able to activate PARs can be produced systemically, but also at the sites of lesions, and they include thrombin and activated factor X. After PAR activation, downstream signaling can lead to both proinflammatory effects and a hypercoagulable state. Which specific effect occurs depends on the type of protease and activated PAR, and the site of activation. Hypercoagulable effects are mainly exerted through PAR1 and PAR4, whereas proinflammatory responses are mostly seen after PAR1 and PAR2 activation. PAR signaling pathways contribute to atherothrombosis, suggesting that inhibition of these pathways possibly prevents cardiovascular events based on this pathophysiological mechanism. In this review, we highlight the pathways by which PAR activation leads to proinflammatory responses and a hypercoagulable state. Furthermore, we give an overview of potential pharmacological treatment targets that promote vascular protection.

The Protease Activated Receptor2 Promotes Rab5a Mediated Generation of Pro-metastatic Microvesicles

Metastasis, the hallmark of cancer propagation is attributed by the modification of phenotypic/functional behavior of cells to break attachment and migrate to distant body parts. Cancer cell-secreted microvesicles (MVs) contribute immensely in disease propagation. These nano-vesicles, generated from plasma membrane outward budding are taken up by nearby healthy cells thereby inducing phenotypic alterations in those recipient cells. Protease activated receptor 2 (PAR2), activated by trypsin, also contributes to cancer progression by increasing metastasis, angiogenesis etc. Here, we report that PAR2 activation promotes pro-metastatic MVs generation from human breast cancer cell line, MDA-MB-231. Rab5a, located at the plasma membrane plays vital roles in MVs biogenesis. We show that PAR2 stimulation promotes AKT phosphorylation which activates Rab5a by converting inactive Rab5a-GDP to active Rab5a-GTP. Active Rab5a polymerizes actin which critically regulates MVs shedding. Not only MVs generation, has this Rab5a activation also promoted cell migration and invasion. We reveal that Rab5a is over-expressed in human breast tumor specimen and contributes MVs generation in those patients. The involvement of p38 MAPK in MVs-induced cell metastasis has also been highlighted in the present study. Blockade of Rab5a activation can be a potential therapeutic approach to restrict MVs shedding and associated breast cancer metastasis.

Intracellular Receptor Modulation: Novel Approach to Target GPCRs

Recent crystal structures of multiple G protein-coupled receptors (GPCRs) have revealed a highly conserved intracellular pocket that can be used to modulate these receptors from the inside. This novel intracellular site partially overlaps with the G protein and β-arrestin binding site, providing a new manner of pharmacological intervention. Here we provide an update of the architecture and function of the intracellular region of GPCRs, until now portrayed as the signaling domain. We review the available evidence on the presence of intracellular binding sites among chemokine receptors and other class A GPCRs, as well as different strategies to target it, including small molecules, pepducins, and nanobodies. Finally, the potential advantages of intracellular (allosteric) ligands over orthosteric ligands are also discussed.

The fibrogenic actions of the coagulant and plasminogen activation systems in pulmonary fibrosis

Fibrosis causes irreversible damage to lung structure and function in restrictive lung diseases such as idiopathic pulmonary fibrosis (IPF). Extravascular coagulation involving fibrin formation in the intra-alveolar compartment is postulated to have a pivotal role in the development of pulmonary fibrosis, serving as a provisional matrix for migrating fibroblasts. Furthermore, proteases of the coagulation and plasminogen activation (plasminergic) systems that form and breakdown fibrin respectively directly contribute to pulmonary fibrosis. The coagulants, thrombin and factor Xa (FXa) evoke fibrogenic effects via cleavage of the N-terminus of protease-activated receptors (PARs). Whilst the formation and activity of plasmin, the principle plasminergic mediator is suppressed in the airspaces of patients with IPF, localized increases are likely to occur in the lung interstitium. Plasmin-evoked proteolytic activation of factor XII (FXII), matrix metalloproteases (MMPs) and latent, matrix-bound growth factors such as epidermal growth factor (EGF) indirectly implicate plasmin in pulmonary fibrosis. Another plasminergic protease, urokinase plasminogen activator (uPA) is associated with regions of fibrosis in the remodelled lung of IPF patients and elicits fibrogenic activity via binding its receptor (uPAR). Plasminogen activator inhibitor-1 (PAI-1) formed in the injured alveolar epithelium also contributes to pulmonary fibrosis in a manner that involves vitronectin binding. This review describes the mechanisms by which components of the two systems primarily involved in fibrin homeostasis contribute to interstitial fibrosis, with a particular focus on IPF. Selectively targeting the receptor-mediated mechanisms of coagulant and plasminergic proteases may limit pulmonary fibrosis, without the bleeding complications associated with conventional anti-coagulant and thrombolytic therapies.

Proteinases and their receptors in inflammatory arthritis: an overview

Proteinases are enzymes with established roles in physiological and pathological processes such as digestion and the homeostasis, destruction and repair of tissues. Over the past few years, the hormone-like properties of circulating proteinases have become increasingly appreciated. Some proteolytic enzymes trigger cell signalling via proteinase-activated receptors, a family of G protein-coupled receptors that have been implicated in inflammation and pain in inflammatory arthritis. Proteinases can also regulate ion flux owing to the cross-sensitization of transient receptor potential cation channel subfamily V members 1 and 4, which are associated with mechanosensing and pain. In this Review, the idea that proteinases have the potential to orchestrate inflammatory signals by interacting with receptors on cells within the synovial microenvironment of an inflamed joint is revisited in three arthritic diseases: osteoarthritis, spondyloarthritis and rheumatoid arthritis. Unanswered questions are highlighted and the therapeutic potential of modulating this proteinase-receptor axis for the management of disease in patients with these types of arthritis is also discussed.

Homeostatic effects of coagulation protease-dependent signaling and protease activated receptors

A homeostatic function of the coagulation system in regard to hemostasis is well established. Homeostasis of blood coagulation depends partially on protease activated receptor (PAR)-signaling. Beyond coagulation proteases, numerous other soluble and cell-bound proteases convey cellular effects via PAR signaling. As we learn more about the mechanisms underlying cell-, tissue-, and context-specific PAR signaling, we concurrently gain new insights into physiological and pathophysiological functions of PARs. In this regard, regulation of cell and tissue homeostasis by PAR signaling is an evolving scheme. Akin to the control of blood clotting per se (the fibrin-platelet interaction) coagulation proteases coordinately regulate cell- and tissue-specific functions. This review summarizes recent insights into homeostatic regulation through PAR signaling, focusing on blood coagulation proteases. Considering the common use of drugs altering coagulation protease activity through either broad or targeted inhibitory activities, and the advent of PAR modulating drugs, an in-depth understanding of the mechanisms through which coagulation proteases and PAR signaling regulate not only hemostasis, but also cell and tissue homeostasis is required.

PAR-2信号通路与功能性胃肠病

功能性胃肠病(functional gastrointestinal disorders, FGIDs)是一组排除器质性病变的胃肠道疾病, 其症状复杂且无特异性. 该类疾病在人群中患病率不断升高, 虽不致死, 但伴随精神症状大大降低了患者生活质量, 病情反复且周期长, 给患者家庭和社会造成了一定经济压力. 探索其发病机制以制定更佳治疗策略成为当前重任. 近年研究证实蛋白酶激活受体2(protease-activated receptor 2, PAR-2)在FGIDs发病机制中的作用确切, 相关研究亦越来越深入. 但众多研究各持一角, 不免混杂, 故本文就近几年PAR-2的相关研究作了梳理, 以便后续研究能有所借鉴, 看到不足, 并能做进一步的深入研究.

Integration of endothelial protease-activated receptor-1 inflammatory signaling by ubiquitin

PURPOSE OF REVIEW

The maintenance and integrity of the endothelial barrier is essential for vascular homeostasis. Endothelial barrier dysfunction is mediated by various inflammatory factors, many of which act through G protein-coupled receptors including protease-activated receptors (PARs). PARs are expressed in multiple cell types in the vasculature and mediate cellular responses to thrombin, the key effector protease of the coagulation cascade. Thrombin activation of PAR1 induces endothelial barrier permeability through multiple pathways. Here, we discuss the mechanism by which thrombin activation of PAR1 promotes endothelial barrier breakdown and highlight recent advances that have provided new insight into molecular mechanisms that control endothelial barrier integrity.

RECENT FINDINGS

Although the signal transduction pathways induced by thrombin activation of PAR1 in endothelial cells have been extensively studied, the key regulatory mechanisms remain poorly understood. Posttranslational modifications are integral to the regulation of PAR1 signaling and recent studies suggest a novel function for ubiquitination of PAR1 in regulation of endothelial barrier permeability.

SUMMARY

An understanding of how endothelial barrier permeability is regulated by thrombin activation of PAR1 is important for the discovery of new drug targets that can be manipulated to control endothelial barrier permeability and prevent progression of vascular inflammation.

Targeting Liver Fibrosis with a Cell-penetrating Protease-activated Receptor-2 (PAR2) Pepducin

Chronic liver inflammation and fibrosis in nonalcoholic steatohepatitis can lead to cirrhosis and liver failure for which there are currently no approved treatments. Protease-activated receptor-2 (PAR2) is an emerging new target expressed on liver stellate cells and hepatocytes that regulates the response to liver injury and inflammation. Here, we identified a pepducin to block the deleterious actions of PAR2 in promoting liver fibrosis. Non-alcoholic fatty liver disease and early fibrosis were induced by the methionine-choline-deficient diet in mice. Fibrotic liver disease was induced by administering carbon tetrachloride for 8 weeks. Mice were treated with the pepducin PZ-235 either from onset of the experiment or after fibrosis was established. Hepatic fibrosis, collagen content, inflammatory cytokines, steatosis, triglycerides, and NAFLD activity score were assessed as primary outcome parameters depending on the model. The activity of the PAR2 pepducin on cultured stellate cell activation and hepatocyte reactive oxygen species production was evaluated. PZ-235 significantly suppressed liver fibrosis, collagen deposition, inflammatory cytokines, NAFLD activity score, steatosis, triglycerides, aspartate transaminase, alanine transaminase, and stellate cell proliferation by up to 50-100%. The PAR2 inhibitor afforded significant protective effects against hepatocellular necrosis and attenuated PAR2-mediated reactive oxygen species production in hepatocytes. PZ-235 was distributed to liver and other mouse tissues and was found to form a well structured α-helix that closely resembles the juxtamembrane helical region of the analogous TM6 and third intracellular region of the intact receptor that is critical for coupling to internal G proteins. The ability of PZ-235 to effectively suppress fibrosis, hepatocellular necrosis, reactive oxygen species production, steatosis, and inflammation indicates the potential for PAR2 pepducin inhibitors to be broadly efficacious in the treatment of liver fibrosis.

Proteinases, Their Extracellular Targets, and Inflammatory Signaling

Given that over 2% of the human genome codes for proteolytic enzymes and their inhibitors, it is not surprising that proteinases serve many physiologic-pathophysiological roles. In this context, we provide an overview of proteolytic mechanisms regulating inflammation, with a focus on cell signaling stimulated by the generation of inflammatory peptides; activation of the proteinase-activated receptor (PAR) family of G protein-coupled receptors (GPCR), with a mechanism in common with adhesion-triggered GPCRs (ADGRs); and by proteolytic ion channel regulation. These mechanisms are considered in the much wider context that proteolytic mechanisms serve, including the processing of growth factors and their receptors, the regulation of matrix-integrin signaling, and the generation and release of membrane-tethered receptor ligands. These signaling mechanisms are relevant for inflammatory, neurodegenerative, and cardiovascular diseases as well as for cancer. We propose that the inflammation-triggering proteinases and their proteolytically generated substrates represent attractive therapeutic targets and we discuss appropriate targeting strategies.

Protease-activated receptors (PARs)--biology and role in cancer invasion and metastasis

Although many studies have demonstrated that components of the hemostatic system may be involved in signaling leading to cancer progression, the potential mechanisms by which they contribute to cancer dissemination are not yet precisely understood. Among known coagulant factors, tissue factor (TF) and thrombin play a pivotal role in cancer invasion. They may be generated in the tumor microenvironment independently of blood coagulation and can induce cell signaling through activation of protease-activated receptors (PARs). PARs are transmembrane G-protein-coupled receptors (GPCRs) that are activated by a unique proteolytic mechanism. They play important roles in vascular physiology, neural tube closure, hemostasis, and inflammation. All of these agents (TF, thrombin, PARs—mainly PAR-1 and PAR-2) are thought to promote cancer invasion and metastasis at least in part by facilitating tumor cell migration, angiogenesis, and interactions with host vascular cells, including platelets, fibroblasts, and endothelial cells lining blood vessels. Here, we discuss the role of PARs and their activators in cancer progression, focusing on TF- and thrombin-mediated actions. Therapeutic options tailored specifically to inhibit PAR-induced signaling in cancer patients are presented as well.