Challenges and Opportunities in Protease-Activated Receptor Drug Development

The Tissue Factor Pathway in Cancer: Overview and Role of Heparan Sulfate Proteoglycans

Historically, the only focus on tissue factor (TF) in clinical pathophysiology has been on its function as the initiation of the extrinsic coagulation cascade. This obsolete vessel-wall TF dogma is now being challenged by the findings that TF circulates throughout the body as a soluble form, a cell-associated protein, and a binding microparticle. Furthermore, it has been observed that TF is expressed by various cell types, including T-lymphocytes and platelets, and that certain pathological situations, such as chronic and acute inflammatory states, and cancer, may increase its expression and activity. Transmembrane G protein-coupled protease-activated receptors can be proteolytically cleaved by the TF:FVIIa complex that develops when TF binds to Factor VII (PARs). The TF:FVIIa complex can activate integrins, receptor tyrosine kinases (RTKs), and PARs in addition to PARs. Cancer cells use these signaling pathways to promote cell division, angiogenesis, metastasis, and the maintenance of cancer stem-like cells. Proteoglycans play a crucial role in the biochemical and mechanical properties of the cellular extracellular matrix, where they control cellular behavior via interacting with transmembrane receptors. For TFPI.fXa complexes, heparan sulfate proteoglycans (HSPGs) may serve as the primary receptor for uptake and degradation. The regulation of TF expression, TF signaling mechanisms, their pathogenic effects, and their therapeutic targeting in cancer are all covered in detail here.

A Pilot Proteomic Study of Vestibular Fluid From Patients With Vulvodynia

OBJECTIVE

Many women are affected by vulvodynia, but medical therapies to date have proven ineffective. We performed a pilot study using gel-based proteomics to develop a map of proteins present in vaginal/vestibular secretions and identify proteins that could be considered for future evaluation as potential therapeutic targets.

MATERIALS AND METHODS

We collected vestibular fluid from 4 controls and 4 patients with vulvodynia by placing a cotton swab in the vestibule and extracting the absorbed proteins. The proteins underwent 2-dimensional difference gel electrophoresis and mass spectrometry to develop a protein map. Immunohistochemistry was used to validate proteomic findings.

RESULTS

A map was constructed of 32 of the more abundant proteins in vestibular fluid and their levels compared in control subjects and vulvodynia patients. Among these were annexin A1, interleukin 1 receptor antagonist, protein S100 A9, and a number of antiproteases and proteases. Many of these proteins differed by at least 50% between groups, but only annexin A1, one of the protease inhibitors, and immunoglobulin G κ chain were significantly different. The results with annexin A1 were validated by similar findings with immunohistochemistry.

CONCLUSIONS

The findings of this pilot study demonstrate a set of vestibule mucosa proteins that differ significantly-either increasing or decreasing-in vulvodynia patients compared with controls, and several others that exhibited greater than 1.5-fold change but did not reach statistical significance. This study constitutes a proof-of-principle that an open, unbiased proteomic approach can identify molecular participants in vulvodynia, some of which had not been identified to date by hypothesis-driven studies.

Thromboinflammatory Processes at the Nexus of Metabolic Dysfunction and Prostate Cancer: The Emerging Role of Periprostatic Adipose Tissue

Simple Summary

As overweight and obesity increase among the population worldwide, a parallel increase in the number of individuals diagnosed with prostate cancer was observed. There appears to be a relationship between both diseases where the increase in the mass of fat tissue can lead to inflammation. Such a state of inflammation could produce many factors that increase the aggressiveness of prostate cancer, especially if this inflammation occurred in the fat stores adjacent to the prostate. Another important observation that links obesity, fat tissue inflammation, and prostate cancer is the increased production of blood clotting factors. In this article, we attempt to explain the role of these latter factors in the effect of increased body weight on the progression of prostate cancer and propose new ways of treatment that act by affecting how these clotting factors work.

Abstract

The increased global prevalence of metabolic disorders including obesity, insulin resistance, metabolic syndrome and diabetes is mirrored by an increased incidence of prostate cancer (PCa). Ample evidence suggests that these metabolic disorders, being characterized by adipose tissue (AT) expansion and inflammation, not only present as risk factors for the development of PCa, but also drive its increased aggressiveness, enhanced progression, and metastasis. Despite the emerging molecular mechanisms linking AT dysfunction to the various hallmarks of PCa, thromboinflammatory processes implicated in the crosstalk between these diseases have not been thoroughly investigated. This is of particular importance as both diseases present states of hypercoagulability. Accumulating evidence implicates tissue factor, thrombin, and active factor X as well as other players of the coagulation cascade in the pathophysiological processes driving cancer development and progression. In this regard, it becomes pivotal to elucidate the thromboinflammatory processes occurring in the periprostatic adipose tissue (PPAT), a fundamental microenvironmental niche of the prostate. Here, we highlight key findings linking thromboinflammation and the pleiotropic effects of coagulation factors and their inhibitors in metabolic diseases, PCa, and their crosstalk. We also propose several novel therapeutic targets and therapeutic interventions possibly modulating the interaction between these pathological states.

Histone 2B Facilitates Plasminogen-Enhanced Endothelial Migration through Protease-Activated Receptor 1 (PAR1) and Protease-Activated Receptor 2 (PAR2)

Plasminogen and its multiple receptors have been implicated in the responses of many different cell types. Among these receptors, histone 2B (H2B) has been shown to play a prominent role in macrophage responses. The contribution of H2B to plasminogen-induced endothelial migration, an event relevant to wound healing and angiogenesis, is unknown. Plasminogen enhanced the migration of endothelial cells, which was inhibited by both Protease-Activated Receptor-1 (PAR1) and 2 (PAR2) antagonists. H2B was detected on viable endothelial cells of venous and arterial origin, and an antibody to H2B that blocks plasminogen binding also inhibited the plasminogen-dependent migration by these cells. The antibody blockade was as effective as PAR1 or PAR2 antagonists in inhibiting endothelial cell migration. In pull-down experiments, H2B formed a complex with both PAR1 and PAR2 but not β3 integrin, another receptor implicated in endothelial migration in the presence of plasminogen. H2B was found to be associated with clathrin adapator protein, AP2µ (clathrin AP2µ) and β-arrestin2, which are central to the internationalization/signaling machinery of the PARs. These associations with PAR1-clathrin adaptor AP2µ- and PAR2-β-arrestin2-dependent internalization/signaling pathways provide a mechanism to link plasminogen to responses such as wound healing and angiogenesis.

Protease Activated Receptors: A Pathway to Boosting Mesenchymal Stromal Cell Therapeutic Efficacy in Acute Respiratory Distress Syndrome?

Acute Respiratory Distress Syndrome is the most common cause of respiratory failure among critically ill patients, and its importance has been heightened during the COVID-19 pandemic. Even with the best supportive care, the mortality rate in the most severe cases is 40–50%, and the only pharmacological agent shown to be of possible benefit has been steroids. Mesenchymal stromal cells (MSCs) have been tested in several pre-clinical models of lung injury and been found to have significant therapeutic benefit related to: (a) potent immunomodulation; (b) secretion of epithelial and endothelial growth factors; and (c) augmentation of host defense to infection. Initial translational efforts have shown signs of promise, but the results have not yielded the anticipated outcomes. One potential reason is the relatively low survival of MSCs in inflammatory conditions as shown in several studies. Therefore, strategies to boost the survival of MSCs are needed to enhance their therapeutic effect. Protease-activated receptors (PARs) may represent one such possibility as they are G-protein coupled receptors expressed by MSCs and control several facets of cell behavior. This review summarizes some of the existing literature about PARs and MSCs and presents possible future areas of investigation in order to develop potential, PAR-modified MSCs with enhanced therapeutic efficiency.

KLK8 promotes the proliferation and metastasis of colorectal cancer via the activation of EMT associated with PAR1

Kallikrein-related peptidase 8 (KLK8) acts as an oncogene or anti-oncogene in various tumours, and the abnormal expression of KLK8 is involved in the carcinogenesis of several tumours. However, the role of KLK8 in colorectal cancer (CRC) and the underlying mechanism remain largely unclear. In this study, the carcinogenic effect of KLK8 was determined via CCK-8 and colony formation assays in vitro and a xenograft model in nude mice in vivo. The metastasis-promoting effect of KLK8 was investigated with transwell migration and invasion assays and wound-healing assay in vitro and a metastasis model in nude mice in vivo. Bioinformatics analyses and mechanistic experiments were conducted to elucidate the molecular mechanism. Herein, we reported that KLK8 had a promotive effect on the proliferation, migration and invasion of RKO and SW480 cells. Epithelial-mesenchymal transition (EMT) played an important role in the promotive effects of KLK8 on CRC. In addition, protease-activated receptor-1 (PAR-1) antagonist SCH79797 but not protease-activated receptor-2 (PAR-2) antagonist FSLLRY-NH2 attenuated the proliferation, migration and invasion of KLK8-upregulated RKO and SW480 cells. PAR-1 antagonist SCH79797 reduced the tumour volume of xenograft model and decreased the metastatic nodules in the livers of metastasis model. Furthermore, SCH79797 could reverse the positive impact of KLK8 on the EMT process in CRC both in vitro and in vivo. Taken together, these findings demonstrated for the first time that KLK8 promoted EMT and CRC progression, and this effect might be, at least partly mediated by PAR1-dependent pathway.

A Role for Protease Activated Receptor Type 3 (PAR3) in Nociception Demonstrated Through Development of a Novel Peptide Agonist

The protease activated receptor (PAR) family is a group of G-protein coupled receptors (GPCRs) activated by proteolytic cleavage of the extracellular domain. PARs are expressed in a variety of cell types with crucial roles in homeostasis, immune responses, inflammation, and pain. PAR3 is the least researched of the four PARs, with little known about its expression and function. We sought to better understand its potential function in the peripheral sensory nervous system. Mouse single-cell RNA sequencing data demonstrates that PAR3 is widely expressed in dorsal root ganglion (DRG) neurons. Co-expression of PAR3 mRNA with other PARs was identified in various DRG neuron subpopulations, consistent with its proposed role as a coreceptor of other PARs. We developed a lipid tethered PAR3 agonist, C660, that selectively activates PAR3 by eliciting a Ca2+ response in DRG and trigeminal neurons. In vivo, C660 induces mechanical hypersensitivity and facial grimacing in WT but not PAR3-/- mice. We characterized other nociceptive phenotypes in PAR3-/- mice and found a loss of hyperalgesic priming in response to IL-6, carrageenan, and a PAR2 agonist, suggesting that PAR3 contributes to long-lasting nociceptor plasticity in some contexts. To examine the potential role of PAR3 in regulating the activity of other PARs in sensory neurons, we administered PAR1, PAR2, and PAR4 agonists and assessed mechanical and affective pain behaviors in WT and PAR3-/- mice. We observed that the nociceptive effects of PAR1 agonists were potentiated in the absence of PAR3. Our findings suggest a complex role of PAR3 in the physiology and plasticity of nociceptors. PERSPECTIVE: We evaluated the role of PAR3, a G-protein coupled receptor, in nociception by developing a selective peptide agonist. Our findings suggest that PAR3 contributes to nociception in various contexts and plays a role in modulating the activity of other PARs.

α-Arrestin ARRDC3 tumor suppressor function is linked to GPCR-induced TAZ activation and breast cancer metastasis

The α-arrestin domain containing protein 3 (ARRDC3) is a tumor suppressor in triple-negative breast carcinoma (TNBC), a highly metastatic subtype of breast cancer that lacks targeted therapies. Thus, understanding the mechanisms and targets of ARRDC3 in TNBC is important. ARRDC3 regulates trafficking of protease-activated receptor 1 (PAR1, also known as F2R), a G-protein-coupled receptor (GPCR) implicated in breast cancer metastasis. Loss of ARRDC3 causes overexpression of PAR1 and aberrant signaling. Moreover, dysregulation of GPCR-induced Hippo signaling is associated with breast cancer progression. However, the mechanisms responsible for Hippo dysregulation remain unknown. Here, we report that the Hippo pathway transcriptional co-activator TAZ (also known as WWTR1) is the major effector of GPCR signaling and is required for TNBC migration and invasion. Additionally, ARRDC3 suppresses PAR1-induced Hippo signaling via sequestration of TAZ, which occurs independently of ARRDC3-regulated PAR1 trafficking. The ARRDC3 C-terminal PPXY motifs and TAZ WW domain are crucial for this interaction and are required for suppression of TNBC migration and lung metastasis in vivo. These studies are the first to demonstrate a role for ARRDC3 in regulating GPCR-induced TAZ activity in TNBC and reveal multi-faceted tumor suppressor functions of ARRDC3. This article has an associated First Person interview with the first author of the paper.

Protease-activated receptors: An illustrated review

Proteases are important regulators of cell behavior, survival, and apoptosis. They communicate to cells directly through a special class of G‐protein–coupled receptors known as protease‐activated receptors (PARs). N‐terminal PAR proteolysis unmasks a neo‐N‐terminus, which serves as a tethered ligand to activate PARs. Using this unique irreversible activation mechanism, PARs relay information across cell membranes. The year 2020 is the 30th year since discovery of the first member of this family, PAR1. In this illustrated review, we highlight achievements in the PAR field over the past 3 decades. Additionally, the known expression profiles of PARs in human tissues and across species are portrayed. We also illustrate the tethered ligand activation mechanism, which is unique to PARs, and PAR regulatory mechanisms. PAR1 was originally named “thrombin receptor” because thrombin was the first protease identified to activate PAR1. However, over the past 30 years, a growing number of proteases have been found to cleave PARs and trigger differential downstream signaling depending on cleavage site, cell type, and species. We exemplify the diversity of PAR1‐mediated signaling outcomes in platelets and endothelial cells as pertinent examples to the hemostasis, thrombosis, and vascular biology fields. Further, the termination and regulation of PAR signaling via endocytosis and currently available pharmacologic approaches are depicted. We conclude with portrayal of clinically translational aspects of PAR biology including pharmacologic manipulation and single‐nucleotide polymorphisms.

Post-Translational Modifications of G Protein-Coupled Receptors Control Cellular Signaling Dynamics in Space and Time

G protein-coupled receptors (GPCRs) are a large family comprising >800 signaling receptors that regulate numerous cellular and physiologic responses. GPCRs have been implicated in numerous diseases and represent the largest class of drug targets. Although advances in GPCR structure and pharmacology have improved drug discovery, the regulation of GPCR function by diverse post-translational modifications (PTMs) has received minimal attention. Over 200 PTMs are known to exist in mammalian cells, yet only a few have been reported for GPCRs. Early studies revealed phosphorylation as a major regulator of GPCR signaling, whereas later reports implicated a function for ubiquitination, glycosylation, and palmitoylation in GPCR biology. Although our knowledge of GPCR phosphorylation is extensive, our knowledge of the modifying enzymes, regulation, and function of other GPCR PTMs is limited. In this review we provide a comprehensive overview of GPCR post-translational modifications with a greater focus on new discoveries. We discuss the subcellular location and regulatory mechanisms that control post-translational modifications of GPCRs. The functional implications of newly discovered GPCR PTMs on receptor folding, biosynthesis, endocytic trafficking, dimerization, compartmentalized signaling, and biased signaling are also provided. Methods to detect and study GPCR PTMs as well as PTM crosstalk are further highlighted. Finally, we conclude with a discussion of the implications of GPCR PTMs in human disease and their importance for drug discovery. SIGNIFICANCE STATEMENT: Post-translational modification of G protein-coupled receptors (GPCRs) controls all aspects of receptor function; however, the detection and study of diverse types of GPCR modifications are limited. A thorough understanding of the role and mechanisms by which diverse post-translational modifications regulate GPCR signaling and trafficking is essential for understanding dysregulated mechanisms in disease and for improving and refining drug development for GPCRs.

GPR50-Ctail cleavage and nuclear translocation: a new signal transduction mode for G protein-coupled receptors

Transmission of extracellular signals by G protein-coupled receptors typically relies on a cascade of intracellular events initiated by the activation of heterotrimeric G proteins or β-arrestins followed by effector activation/inhibition. Here, we report an alternative signal transduction mode used by the orphan GPR50 that relies on the nuclear translocation of its carboxyl-terminal domain (CTD). Activation of the calcium-dependent calpain protease cleaves off the CTD from the transmembrane-bound GPR50 core domain between Phe-408 and Ser-409 as determined by MALDI-TOF-mass spectrometry. The cytosolic CTD then translocates into the nucleus assisted by its 'DPD' motif, where it interacts with the general transcription factor TFII-I to regulate c-fos gene transcription. RNA-Seq analysis indicates a broad role of the CTD in modulating gene transcription with ~ 8000 differentially expressed genes. Our study describes a non-canonical, direct signaling mode of GPCRs to the nucleus with similarities to other receptor families such as the NOTCH receptor.

PAR4 activation involves extracellular loop 3 and transmembrane residue Thr153

Protease-activated receptor 4 (PAR4) mediates sustained thrombin signaling in platelets and is required for a stable thrombus. PAR4 is activated by proteolysis of the N terminus to expose a tethered ligand. The structural basis for PAR4 activation and the location of its ligand binding site (LBS) are unknown. Using hydrogen/deuterium exchange (H/D exchange), computational modeling, and signaling studies, we determined the molecular mechanism for tethered ligand-mediated PAR4 activation. H/D exchange identified that the LBS is composed of transmembrane 3 (TM3) domain and TM7. Unbiased computational modeling further predicted an interaction between Gly48 from the tethered ligand and Thr153 from the LBS. Mutating Thr153 significantly decreased PAR4 signaling. H/D exchange and modeling also showed that extracellular loop 3 (ECL3) serves as a gatekeeper for the interaction between the tethered ligand and LBS. A naturally occurring sequence variant (P310L, rs2227376) and 2 experimental mutations (S311A and P312L) determined that the rigidity conferred by prolines in ECL3 are essential for PAR4 activation. Finally, we examined the role of the polymorphism at position 310 in venous thromboembolism (VTE) using the International Network Against Venous Thrombosis (INVENT) consortium multi-ancestry genome-wide association study (GWAS) meta-analysis. Individuals with the PAR4 Leu310 allele had a 15% reduction in relative risk for VTE (odds ratio, 0.85; 95% confidence interval, 0.77-0.94) compared with the Pro310 allele. These data are consistent with our H/D exchange, molecular modeling, and signaling studies. In conclusion, we have uncovered the structural basis for PAR4 activation and identified a previously unrecognized role for PAR4 in VTE.

The domino effect triggered by the tethered ligand of the protease activated receptors

Protease activated receptors (PARs) are G-protein coupled receptors (GPCRs) that have a unique activation mechanism. Unlike other GPCRs that can be activated by free ligands, under physiological conditions, PARs are activated by the tethered ligand, which is a part of their N-terminus that is unmasked by proteolysis. It has been 30 years since the first member of the family, PAR1, was identified. In this review, we will discuss this unique tethered ligand mediate receptor activation of PARs in detail: how they interact with the proteases, the complex structural rearrangement of the receptors upon activation, and the termination of the signaling. We also summarize the structural studies of the PARs and how single nucleotide polymorphisms impact the receptor reactivity. Finally, we review the current strategies for inhibiting PAR function with therapeutic targets for anti-thrombosis. The focus of this review is PAR1 and PAR4 as they are the thrombin signal mediators on human platelets and therapeutics targets. We also include the structural studies of PAR2 as it informs the mechanism of action for PARs in general.

Beyond thrombosis: the impact of tissue factor signaling in cancer

Tissue factor (TF) is the primary initiator of the coagulation cascade, though its effects extend well beyond hemostasis. When TF binds to Factor VII, the resulting TF:FVIIa complex can proteolytically cleave transmembrane G protein-coupled protease-activated receptors (PARs). In addition to activating PARs, TF:FVIIa complex can also activate receptor tyrosine kinases (RTKs) and integrins. These signaling pathways are utilized by tumors to increase cell proliferation, angiogenesis, metastasis, and cancer stem-like cell maintenance. Herein, we review in detail the regulation of TF expression, mechanisms of TF signaling, their pathological consequences, and how it is being targeted in experimental cancer therapeutics.

Antiviral anticoagulation

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a novel envelope virus that causes coronavirus disease 2019 (COVID-19). Hallmarks of COVID-19 are a puzzling form of thrombophilia that has elevated D-dimer but only modest effects on other parameters of coagulopathy. This is combined with severe inflammation, often leading to acute respiratory distress and possible lethality. Coagulopathy and inflammation are interconnected by the transmembrane receptor, tissue factor (TF), which initiates blood clotting as a cofactor for factor VIIa (FVIIa)-mediated factor Xa (FXa) generation. TF also functions from within the nascent TF/FVIIa/FXa complex to trigger profound changes via protease-activated receptors (PARs) in many cell types, including SARS-CoV-2-trophic cells. Therefore, aberrant expression of TF may be the underlying basis of COVID-19 symptoms. Evidence suggests a correlation between infection with many virus types and development of clotting-related symptoms, ranging from heart disease to bleeding, depending on the virus. Since numerous cell types express TF and can act as sites for virus replication, a model envelope virus, herpes simplex virus type 1 (HSV1), has been used to investigate the uptake of TF into the envelope. Indeed, HSV1 and other viruses harbor surface TF antigen, which retains clotting and PAR signaling function. Strikingly, envelope TF is essential for HSV1 infection in mice, and the FXa-directed oral anticoagulant apixaban had remarkable antiviral efficacy. SARS-CoV-2 replicates in TF-bearing epithelial and endothelial cells and may stimulate and integrate host cell TF, like HSV1 and other known coagulopathic viruses. Combined with this possibility, the features of COVID-19 suggest that it is a TFopathy, and the TF/FVIIa/FXa complex is a feasible therapeutic target.

Recent advances in the development of legumain-selective chemical probes and peptide prodrugs

Legumain, which is also known as vacuolar processing enzyme (VPE) or asparaginyl endopeptidase (AEP), is a cysteine protease that was first discovered and characterized in the leguminous seeds of the moth bean in the early 1990s. Later, this enzyme was also detected in higher organisms, including eukaryotes. This pH-dependent protease displays the highest activity in acidic endolysosomal compartments; however, legumain also displays nuclear, cytosolic and extracellular activity when stabilized by other proteins or intramolecular complexes. Based on the results from over 25 years of research, this protease is involved in multiple cellular events, including protein degradation and antigen presentation. Moreover, when dysregulated, this protease contributes to the progression of several diseases, with cancer being the well-studied example. Research on legumain biology was undoubtedly facilitated by the use of small molecule chemical tools. Therefore, in this review, I present the historical perspectives and most current strategies for the development of small molecule substrates, inhibitors and activity-based probes for legumain. These tools are of paramount importance in elucidating the roles of legumain in multiple biological processes. Finally, as this enzyme appears to be a promising molecular target for anticancer therapies, the development of legumain-activated prodrugs is also described.

Molecular mechanism whereby paraoxonase-2 regulates coagulation activation through endothelial tissue factor in rat haemorrhagic shock model

We investigated the molecular mechanism of paraoxonase-2 (PON-2) in regulating blood coagulation activation in rats with haemorrhagic shock through endothelial tissue factor (TF). Thirty adult Sprague Dawley rats were randomly divided into three groups: healthy control group (group A), the haemorrhagic shock PON-2 treatment group (group B), and the haemorrhagic shock group (group C). After the model was established, blood was withdrawn from the inferior vena cava of all rats. The difference in plasma thrombomodulin (TM) levels of the three groups was determined by Western blotting. The expression of transcription factors Egr-1 and Sp1 was detected by Western blotting assays. reverse transcription-polymerase chain Reaction (RT-PCR) was used to determine the mRNA expression of t-PA, PAI-1, TM, and PON-2 in the serum of three groups of rats. Endothelial TF was measured by enzyme linked immunosorbent assay (ELISA), and coagulation assay was used to detect the activity of coagulation factor VIII. Histopathological examination of the arteries of the rats was performed. The molecular mechanism of PON-2 in regulating blood coagulation activation in haemorrhagic shock model rats by endothelial tissue factor was analysed. The expression of thrombin was determined by electrophoresis. Compared with the healthy control group, the expression of TM in groups B and C decreased, both 188.64 ± 12.47 and 137.48 ± 9.72, respectively, with a significant difference. The mRNA expression of TM and PON was determined by RT-PCR. The mRNA expression of TM and PON in group B was 0.97 ± 0.07 and 1.14 ± 0.09, compared with the control group, and the mRNA expression of TM and PON in group C was 0.86 ± 0.38 and 1.12 ± 0.41, both of which increased, and there were significant differences. By measuring the expression of endothelial TF, the expression of TF in groups B and C was elevated to 12.69 ± 1.07 and 11.59 ± 0.87, with significant differences. The enzyme activities of PON-2 in groups B and C, which were 110.34 ± 14.37 and 52.37 ± 8.06, respectively, were increased compared with the healthy control group and there were significant differences. PON-2 regulates the activation of coagulation in rats with haemorrhagic shock by regulating the expression of endothelial tissue-related genes such as plasma TM and endothelial TF under hypoxic and ischaemic conditions.

Cellular effects of factor VII activating protease (FSAP)

Factor VII activating protease (FSAP) is a circulating serine protease of broad specificity that is likely to be involved in many pathophysiological processes. The activation of the circulating zymogen form of FSAP by histones, released from damaged cells, underlines its roles in regulating host responses to tissue damage and inflammation. Some of the direct cellular effects of FSAP are mediated through protease-activated receptors (PARs). Knock-down of each one of the four PARs in endothelial cells indicated that PAR-1 and -3 are involved in regulating endothelial permeability in response to FSAP. Overexpression of PARs in cell lines led to the conclusion that PAR-2 and -1 were the main receptors for FSAP. Studies with synthetic peptides and receptor mutants demonstrate that FSAP cleaves PAR-1 and -2 at their canonical cleavage site. However, PAR-1 is not activated by FSAP in all cells, which may be related to other, as yet, undefined factors. Inhibition of apoptosis by FSAP is mediated through PAR-1 and was observed in neurons, astrocytes and A549 cells. FSAP also mediates cellular effects by modulating the activity of growth factors, generation of bradykinin, C5a and C3a generation or histone inactivation. These cellular effects need to be further investigated at the in vivo level.

Expression and Purification of Protease-Activated Receptor 4 (PAR4) and Analysis with Histidine Hydrogen-Deuterium Exchange

Protease-activated receptors (PARs) are G-protein-coupled receptors that are activated by proteolysis of the N-terminus, which exposes a tethered ligand that interacts with the receptor. Numerous studies have focused on the signaling pathways mediated by PARs. However, the structural basis for initiation of these pathways is unknown. Here, we describe a strategy for the expression and purification of PAR4. This is the first PAR family member to be isolated without stabilizing modifications for biophysical studies. We monitored PAR4 activation with histidine hydrogen-deuterium exchange. PAR4 has nine histidines that are spaced throughout the protein, allowing a global view of solvent accessible and nonaccessible regions. Peptides containing each of the nine His residues were used to determine the t_1/2 for each His residue in apo or thrombin-activated PAR4. The thrombin-cleaved PAR4 exhibited a 2-fold increase (p > 0.01) in t_1/2 values observed for four histidine residues (His¹⁸⁰, His²²⁹, His²⁴⁰, and His³⁸⁰), demonstrating that these regions have decreased solvent accessibility upon thrombin treatment. In agreement, thrombin-cleaved PAR4 also was resistant to thermolysin digestion. In contrast, the rate of thermolysin proteolysis following stimulation with the PAR4 activation peptide was the same as that of unstimulated PAR4. Further analysis showed the C-terminus is protected in thrombin-activated PAR4 compared to uncleaved or agonist peptide-treated PAR4. The studies described here are the first to examine the tethered ligand activation mechanism for a PAR family member biophysically and shed light on the overall conformational changes that follow activation of PARs by a protease.

G-protein-coupled Receptors in Fungi

G-protein-coupled receptors (GPCRs) are the largest family of transmembrane receptors in fungi. These receptors have an important role in the transduction of extracellular signals into intracellular sites in response to diverse stimuli. They enable fungi to coordinate cell function and metabolism, thereby promoting their survival and propagation, and sense certain fundamentally conserved elements, such as nutrients, pheromones, and stress, for adaptation to their niches, environmental stresses, and host environment, causing disease and pathogen virulence. This chapter highlights the role of GPCRs in fungi in coordinating cell function and metabolism. Fungal cells sense the molecular interactions between extracellular signals. Their respective sensory systems are described here in detail.

Protease activated receptors (PAR)-1 and -2 mediate cellular effects of factor VII activating protease (FSAP)

Factor VII activating protease (FSAP) is a circulating serine protease implicated in thrombosis, atherosclerosis, stroke, and cancer. Using an overexpression strategy, we have systematically investigated the role of protease activated receptors (PAR)-1, -2, -3, and -4 on FSAP-mediated signaling in HEK293T and A549 cells. Cleavage of PAR-reporter constructs and MAPK phosphorylation was used to monitor receptor activation. FSAP cleaved PAR-2 and to a lesser degree PAR-1, but not PAR-3 or PAR-4 in both cell types. Robust MAPK activation in response to FSAP was observed after PAR-2, but not PAR-1 overexpression in HEK293T. Recombinant serine protease domain of wild type FSAP, but not the Marburg I isoform of FSAP, could reproduce the effects of plasma purified FSAP. Canonical cleavage of both PARs was suggested by mass spectrometric analysis of synthetic peptide substrates from the N-terminus of PARs and site directed mutagenesis studies. Surprisingly, knockdown of endogenous PAR-1, but not PAR-2, prevented the apoptosis-inhibitory effect of FSAP, suggesting that PAR1 is nevertheless a direct or indirect target in some cell types. This molecular characterization of PAR-1 and -2 as cellular receptors of FSAP will help to define the actions of FSAP in the context of cancer and vascular biology.

Using PAR4 Inhibition as an Anti-Thrombotic Approach: Why, How, and When?

Protease-activated receptors (PARs) are a family of four GPCRs with a variety of cellular functions, yet the only advanced clinical endeavours to target these receptors for therapeutic gain to date relates to the impairment of platelet function for anti-thrombotic therapy. The only approved PAR antagonist is the PAR1 inhibitor, vorapaxar—the sole anti-platelet drug against a new target approved in the past 20 years. However, there are two PARs on human platelets, PAR1 and PAR4, and more recent efforts have focused on the development of the first PAR4 antagonists, with first-in-class agents recently beginning clinical trial. Here, we review the rationale for this approach, outline the various modes of PAR4 inhibition, and speculate on the specific therapeutic potential of targeting PAR4 for the prevention of thrombotic conditions.

Adhesion G protein-coupled receptors: opportunities for drug discovery

Adhesion G protein-coupled receptors (aGPCRs) - one of the five main families in the GPCR superfamily - have several atypical characteristics, including large, multi-domain N termini and a highly conserved region that can be autoproteolytically cleaved. Although GPCRs overall have well-established pharmacological tractability, currently no therapies that target any of the 33 members of the aGPCR family are either approved or in clinical trials. However, human genetics and preclinical research have strengthened the links between aGPCRs and disease in recent years. This, together with a greater understanding of their functional complexity, has led to growing interest in aGPCRs as drug targets. A framework for prioritizing aGPCR targets and supporting approaches to develop aGPCR modulators could therefore be valuable in harnessing the untapped therapeutic potential of this family. With this in mind, here we discuss the unique opportunities and challenges for drug discovery in modulating aGPCR functions, including target identification, target validation, assay development and safety considerations, using ADGRG1 as an illustrative example.

Conformational Plasticity of Human Protease-Activated Receptor 1 upon Antagonist- and Agonist-Binding

G protein-coupled receptors (GPCRs) show complex relationships between functional states and conformational plasticity that can be qualitatively and quantitatively described by contouring their free energy landscape. However, how ligands modulate the free energy landscape to direct conformation and function of GPCRs is not entirely understood. Here, we employ single-molecule force spectroscopy to parametrize the free energy landscape of the human protease-activated receptor 1 (PAR1), and delineate the mechanical, kinetic, and energetic properties of PAR1 being set into different functional states. Whereas in the inactive unliganded state PAR1 adopts mechanically rigid and stiff conformations, upon agonist or antagonist binding the receptor mechanically softens, while increasing its conformational flexibility, and kinetic and energetic stability. By mapping the free energy landscape to the PAR1 structure, we observe key structural regions putting this conformational plasticity into effect. Our insight, complemented with previously acquired knowledge on other GPCRs, outlines a more general framework to understand how GPCRs stabilize certain functional states.

PRSS21/testisin inhibits ovarian tumor metastasis and antagonizes proangiogenic angiopoietins ANG2 and ANGPTL4

Ovarian cancer is the leading cause of death among all the gynecological cancers in the USA. Ovarian cancer employs a unique mode of metastasis, as exfoliated tumor cells disseminate within the peritoneal cavity, colonizing in several sites as well as accumulating ascites. Tumor recurrence and widespread metastasis are significant factors contributing to poor prognosis. PRSS21 is a metastasis-associated ovarian cancer gene that encodes the glycosyl-phosphatidylinositol-linked serine protease, testisin. Testisin expression is increased in multiple ovarian tumor types, with relatively little expression in normal tissues, but is differentially decreased in metastatic ovarian serous carcinomas compared to primary tumors. Here we explored the function of testisin in late-stage ovarian cancer progression using a murine xenograft model of ovarian intraperitoneal tumor metastasis. Increased tumor testisin expression inhibited intra-peritoneal tumor seeding and colonization, ascites accumulation, and metastatic tumor burden that was dependent on catalytically active testisin. The known testisin substrate, protease-activated receptor-2 (PAR-2), is a target of testisin activity. Gene profiling and mechanistic studies demonstrate that testisin activity suppresses the synthesis and secretion of pro-angiogenic angiopoietins, ANG2 and ANGPTL4, which normally promote vascular leak and edema. These observations support a model wherein testisin activates PAR-2 to antagonize proangiogenic angiopoietins that modulate vascular permeability and ascites accumulation associated with ovarian tumor metastasis. KEY MESSAGES: Testisin inhibits metastatic ovarian tumor burden and ascites production. Testisin activity antagonizes ANG2 and ANGPTL4 synthesis and secretion. PAR-2 is a proteolytic target of testisin on the surface of ovarian cancer cells.

A function-blocking PAR4 antibody is markedly antithrombotic in the face of a hyperreactive PAR4 variant

Thrombin activates human platelets via 2 protease-activated receptors (PARs), PAR1 and PAR4, both of which are antithrombotic drug targets: a PAR1 inhibitor is approved for clinical use, and a PAR4 inhibitor is in trial. However, a common sequence variant in human PAR4 (rs773902, encoding Thr120 in place of Ala120) renders the receptor more sensitive to agonists and less sensitive to antagonists. Here, we develop the first human monoclonal function-blocking antibody to human PAR4 and show it provides equivalent efficacy against the Ala120 and Thr120 PAR4 variants. This candidate was generated from a panel of anti-PAR4 antibodies, was found to bind PAR4 with affinity (K_D ≈ 0.4 nM) and selectivity (no detectable binding to any of PAR1, PAR2, or PAR3), and is capable of near-complete inhibition of thrombin cleavage of either the Ala120 or Thr120 PAR4 variant. Platelets from individuals expressing the Thr120 PAR4 variant exhibit increased thrombin-induced aggregation and phosphatidylserine exposure vs those with the Ala120 PAR4 variant, yet the PAR4 antibody inhibited these responses equivalently (50% inhibitory concentration, 4.3 vs 3.2 µg/mL against Ala120 and Thr120, respectively). Further, the antibody significantly impairs platelet procoagulant activity in an ex vivo thrombosis assay, with equivalent inhibition of fibrin formation and overall thrombus size in blood from individuals expressing the Ala120 or Thr120 PAR4 variant. These findings reveal antibody-mediated inhibition of PAR4 cleavage and activation provides robust antithrombotic activity independent of the rs773902 PAR4 sequence variant and provides rationale for such an approach for antithrombotic therapy targeting this receptor.

Design and Evaluation of Heterobivalent PAR1-PAR2 Ligands as Antagonists of Calcium Mobilization

A novel class of bivalent ligands targeting putative protease-activated receptor (PAR) heteromers has been prepared based upon reported antagonists for the subtypes PAR1 and PAR2. Modified versions of the PAR1 antagonist RWJ-58259 containing alkyne adapters were connected via cycloaddition reactions to azide-capped polyethylene glycol (PEG) spacers attached to imidazopyridazine-based PAR2 antagonists. Initial studies of the PAR1-PAR2 antagonists indicated that they inhibited G alpha q-mediated calcium mobilization in endothelial and cancer cells driven by both PAR1 and PAR2 agonists. Compounds of this novel class hold promise for the prevention of restenosis, cancer cell metastasis, and other proliferative disorders.

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.

Tissue factor: newer concepts in thrombosis and its role beyond thrombosis and hemostasis

For many years, the attention on tissue factor (TF) in human pathophysiology has been limited to its role as initiator of extrinsic coagulation pathway. Moreover, it was described as a glycoprotein located in several tissue including vascular wall and atherosclerotic plaque. However, in the last two decades, the discovery that TF circulates in the blood as cell-associated protein, microparticles (MPs) bound and as soluble form, is changing this old vessel-wall TF dogma. Moreover, it has been reported that TF is expressed by different cell types, even T lymphocytes and platelets, and different pathological conditions, such as acute and chronic inflammatory status, and cancer, may enhance its expression and activity. Thus, recent advances in the biology of TF have clearly indicated that beyond its known effects on blood coagulation, it is a "true surface receptor" involved in many intracellular signaling, cell-survival, gene and protein expression, proliferation, angiogenesis and tumor metastasis. Finally, therapeutic modulation of TF expression and/or activity has been tested with controversial results. This report, starting from the old point of view about TF as initiator of extrinsic coagulation pathway, briefly illustrates the more recent concepts about TF and thrombosis and finally gives an overview about its role beyond thrombosis and haemostasis focusing on the different intracellular mechanisms triggered by its activation and potentially involved in atherosclerosis.

Edema Induced by a Crotalus durissus terrificus Venom Serine Protease (Cdtsp 2) Involves the PAR Pathway and PKC and PLC Activation

Snake venom serine proteases (SVSPs) represent an essential group of enzymatic toxins involved in several pathophysiological effects on blood homeostasis. Some findings suggest the involvement of this class of enzymatic toxins in inflammation. In this paper, we purified and isolated a new gyroxin isoform from the Crotalus durissus terrificus (Cdt) venom, designated as Cdtsp 2, which showed significant proinflammatory effects in a murine model. In addition, we performed several studies to elucidate the main pathway underlying the edematogenic effect induced by Cdtsp 2. Enzymatic assays and structural analysis (primary structure analysis and three-dimensional modeling) were closely performed with pharmacological assays. The determination of edematogenic activity was performed using Cdtsp 2 isolated from snake venom, and was applied to mice treated with protein kinase C (PKC) inhibitor, phospholipase C (PLC) inhibitor, dexamethasone (Dexa), antagonists for protease-activated receptors (PARs), or saline (negative control). Additionally, we measured the levels of cyclooxygenase 2 (COX-2), malondialdehyde (MDA), and prostaglandin E2 (PGE2). Cdtsp 2 is characterized by an approximate molecular mass of 27 kDa, an isoelectric point (pI) of 4.5, and significant fibrinolytic activity, as well as the ability to hydrolyze Nα-benzoyl-l-arginine 4-nitroanilide (BAPNA). Its primary and three-dimensional structures revealed Cdtsp 2 as a typical snake venom serine protease that induces significant edema via the metabolism of arachidonic acid (AA), involving PARs, PKC, PLC, and COX-2 receptors, as well as inducing a significant increase in MDA levels. Our results showed that Cdtsp 2 is a serine protease with significant enzymatic activity, and it may be involved in the degradation of PAR1 and PAR2, which activate PLC and PKC to mobilize AA, while increasing oxidative stress. In this article, we provide a new perspective for the role of SVSPs beyond their effects on blood homeostasis.

Role of the blood coagulation cascade in hepatic fibrosis

Liver is the primary source of numerous proteins that are critical for normal function of the blood coagulation cascade. Because of this, diseases of the liver, particularly when affiliated with severe complications like cirrhosis, are associated with abnormalities of blood clotting. Although conventional interpretation has inferred cirrhosis as a disorder of uniform bleeding risk, it is now increasingly appreciated as a disease wherein the coagulation cascade is precariously rebalanced. Moreover, prothrombotic risk factors are also associated with a more rapid progression of fibrosis in humans, suggesting that coagulation proteases participate in disease pathogenesis. Indeed, strong evidence drawn from experimental animal studies indicates that components of the coagulation cascade, particularly coagulation factor Xa and thrombin, drive profibrogenic events, leading to hepatic fibrosis. Here, we concisely review the evidence supporting a pathologic role for coagulation in the development of liver fibrosis and the potential mechanisms involved. Further, we highlight how studies in experimental animals may shed light on emerging clinical evidence, suggesting that beneficial effects of anticoagulation could extend beyond preventing thrombotic complications to include reducing pathologies like fibrosis.

Activated protein C, protease activated receptor 1, and neuroprotection

Protein C is a plasma serine protease zymogen whose active form, activated protein C (APC), exerts potent anticoagulant activity. In addition to its antithrombotic role as a plasma protease, pharmacologic APC is a pleiotropic protease that activates diverse homeostatic cell signaling pathways via multiple receptors on many cells. Engineering of APC by site-directed mutagenesis provided a signaling selective APC mutant with 3 Lys residues replaced by 3 Ala residues, 3K3A-APC, that lacks >90% anticoagulant activity but retains normal cell signaling activities. This 3K3A-APC mutant exerts multiple potent neuroprotective activities, which require the G-protein-coupled receptor, protease activated receptor 1. Potent neuroprotection in murine ischemic stroke models is linked to 3K3A-APC-induced signaling that arises due to APC's cleavage in protease activated receptor 1 at a noncanonical Arg46 site. This cleavage causes biased signaling that provides a major explanation for APC's in vivo mechanism of action for neuroprotective activities. 3K3A-APC appeared to be safe in ischemic stroke patients and reduced bleeding in the brain after tissue plasminogen activator therapy in a recent phase 2 clinical trial. Hence, it merits further clinical testing for its efficacy in ischemic stroke patients. Recent studies using human fetal neural stem and progenitor cells show that 3K3A-APC promotes neurogenesis in vitro as well as in vivo in the murine middle cerebral artery occlusion stroke model. These recent advances should encourage translational research centered on signaling selective APC's for both single-agent therapies and multiagent combination therapies for ischemic stroke and other neuropathologies.

Characterization of Protease-Activated Receptor (PAR) ligands: Parmodulins are reversible allosteric inhibitors of PAR1-driven calcium mobilization in endothelial cells

Several classes of ligands for Protease-Activated Receptors (PARs) have shown impressive anti-inflammatory and cytoprotective activities, including PAR2 antagonists and the PAR1-targeting parmodulins. In order to support medicinal chemistry studies with hundreds of compounds and to perform detailed mode-of-action studies, it became important to develop a reliable PAR assay that is operational with endothelial cells, which mediate the cytoprotective effects of interest. We report a detailed protocol for an intracellular calcium mobilization assay with adherent endothelial cells in multiwell plates that was used to study a number of known and new PAR1 and PAR2 ligands, including an alkynylated version of the PAR1 antagonist RWJ-58259 that is suitable for the preparation of tagged or conjugate compounds. Using the cell line EA.hy926, it was necessary to perform media exchanges with automated liquid handling equipment in order to obtain optimal and reproducible antagonist concentration-response curves. The assay is also suitable for study of PAR2 ligands; a peptide antagonist reported by Fairlie was synthesized and found to inhibit PAR2 in a manner consistent with reports using epithelial cells. The assay was used to confirm that vorapaxar acts as an irreversible antagonist of PAR1 in endothelium, and parmodulin 2 (ML161) and the related parmodulin RR-90 were found to inhibit PAR1 reversibly, in a manner consistent with negative allosteric modulation.

Enrichment-triggered prodrug activation demonstrated through mitochondria-targeted delivery of doxorubicin and carbon monoxide

Controlled activation is a critical component in prodrug development. Herein we report a concentration-sensitive platform approach for bioorthogonal prodrug activation by taking advantage of reaction kinetics. Using two “click and release” systems, we demonstrate enrichment and prodrug activation specifically in mitochondria to demonstrate the principle of this approach. In both cases, the payload (doxorubicin or carbon monoxide) was released inside the mitochondrial matrix upon the enrichment-initiated click reaction. Furthermore, mitochondria-targeted delivery yielded substantial augmentation of functional biological and therapeutic effects in vitro and in vivo, as compared to controls that did not result in enrichment. This method is thus a platform for targeted drug delivery amenable to conjugation with a variety of molecules and not limited to cell-surface delivery. Taken together, these two click and release pairs clearly demonstrate the concept of enrichment-triggered drug release and critical feasibility of treating clinically relevant diseases such as acute liver injury and cancer.

Genetic Variant in Human PAR (Protease-Activated Receptor) 4 Enhances Thrombus Formation Resulting in Resistance to Antiplatelet Therapeutics

OBJECTIVE

Platelet activation after stimulation of PAR (protease-activated receptor) 4 is heightened in platelets from blacks compared with those from whites. The difference in PAR4 signaling by race is partially explained by a single-nucleotide variant in PAR4 encoding for either an alanine or threonine at amino acid 120 in the second transmembrane domain. The current study sought to determine whether the difference in PAR4 signaling by this PAR4 variant is because of biased G_q signaling and whether the difference in PAR4 activity results in resistance to traditional antiplatelet intervention.

APPROACH AND RESULTS

Membranes expressing human PAR4-120 variants were reconstituted with either G_q or G₁₃ to determine the kinetics of G protein activation. The kinetics of G_q and G₁₃ activation were both increased in membranes expressing PAR4-Thr120 compared with those expressing PAR4-Ala120. Further, inhibiting PAR4-mediated platelet activation by targeting COX (cyclooxygenase) and P2Y₁₂ receptor was less effective in platelets from subjects expressing PAR4-Thr120 compared with PAR4-Ala120. Additionally, ex vivo thrombus formation in whole blood was evaluated at high shear to determine the relationship between PAR4 variant expression and response to antiplatelet drugs. Ex vivo thrombus formation was enhanced in blood from subjects expressing PAR4-Thr120 in the presence or absence of antiplatelet therapy.

CONCLUSIONS

Together, these data support that the signaling difference by the PAR4-120 variant results in the enhancement of both G_q and G₁₃ activation and an increase in thrombus formation resulting in a potential resistance to traditional antiplatelet therapies targeting COX-1 and the P2Y₁₂ receptor.

PAR1 biased signaling is required for activated protein C in vivo benefits in sepsis and stroke

Activated protein C (APC) cleaves protease-activated receptor 1 (PAR1) in vitro at R46 to initiate beneficial cell signaling; however, thrombin and APC can cleave at R41. To elucidate PAR1-dependent aspects of the pharmacologic in vivo mechanisms of APC, we generated C57BL/6 mouse strains carrying QQ41 or QQ46 point mutations in PAR1 (F2r gene). Using these strains, we determined whether or not recombinant murine signaling-selective APC mutants would reduce septic death or provide neuroprotection against ischemic stroke when mice carried PAR1-homozygous mutations that prevent cleavage at either R41 or R46. Intercrossing PAR1+/R46Q mice generated expected numbers of PAR1+/+, PAR1+/R46Q, and R46Q/R46Q offspring whereas intercrossing PAR1+/R41Q mice gave decreased R41Q/R41Q homozygotes (resembling intercrossing PAR1+/PAR1-knockout mice). QQ41-PAR1 and QQ46-PAR1 brain endothelial cells showed the predicted retention or loss of cellular responses to thrombin receptor-activating peptide, thrombin, or APC for each PAR1 mutation. In sepsis studies, exogenous APC reduced mortality from 50% to 10% in Escherichia coli-induced pneumonia for wild-type (Wt) PAR1 and QQ41-PAR1 mice (P < .01) but had no benefit for QQ46-PAR1 mice. In transient distal middle cerebral artery occlusion stroke studies, exogenous APC significantly reduced infarct size, edema, and neuronal apoptosis for Wt mice and QQ41-PAR1 mice but had no detectable benefits for mice carrying QQ46-PAR1. In functional studies of forelimb-asymmetry and foot-fault tests at 24 hours after stroke induction, signaling-selective APC was beneficial for Wt and QQ41-PAR1 mice but not QQ46-PAR1 mice. These results support the concept that APC-induced, PAR1-dependent biased signaling following R46 cleavage is central to the in vivo benefits of APC.

Protease-Activated Receptor 4 (PAR4): A Promising Target for Antiplatelet Therapy

Cardiovascular diseases (CVDs) are currently among the leading causes of death worldwide. Platelet aggregation is a key cellular component of arterial thrombi and major cause of CVDs. Protease-activated receptors (PARs), including PAR1, PAR2, PAR3 and PAR4, fall within a subfamily of seven-transmembrane G-protein-coupled receptors (GPCR). Human platelets express PAR1 and PAR4, which contribute to the signaling transduction processes. In association with CVDs, PAR4 not only contributes to platelet activation but also is a modulator of cellular responses that serve as hallmarks of inflammation. Although several antiplatelet drugs are available on the market, they have many side effects that limit their use. Emerging evidence shows that PAR4 targeting is a safer strategy for preventing thrombosis and consequently may improve the overall cardiac safety profile. Our present review summarizes the PAR4 structural characteristics, activation mechanism, role in the pathophysiology of diseases and understanding the association of PAR4 targeting for improved cardiac protection. Conclusively, this review highlights the importance of PAR4 antagonists and its potential utility in different CVDs.

The α-arrestin ARRDC3 suppresses breast carcinoma invasion by regulating G protein-coupled receptor lysosomal sorting and signaling

Aberrant G protein-coupled receptor (GPCR) expression and activation has been linked to tumor initiation, progression, invasion, and metastasis. However, compared with other cancer drivers, the exploitation of GPCRs as potential therapeutic targets has been largely ignored, despite the fact that GPCRs are highly druggable. Therefore, to advance the potential status of GPCRs as therapeutic targets, it is important to understand how GPCRs function together with other cancer drivers during tumor progression. We now report that the α-arrestin domain-containing protein-3 (ARRDC3) acts as a tumor suppressor in part by controlling signaling and trafficking of the GPCR, protease-activated receptor-1 (PAR1). In a series of highly invasive basal-like breast carcinomas, we found that expression of ARRDC3 is suppressed whereas PAR1 is aberrantly overexpressed because of defective lysosomal sorting that results in persistent signaling. Using a lentiviral doxycycline-inducible system, we demonstrate that re-expression of ARRDC3 in invasive breast carcinoma is sufficient to restore normal PAR1 trafficking through the ALG-interacting protein X (ALIX)-dependent lysosomal degradative pathway. We also show that ARRDC3 re-expression attenuates PAR1-stimulated persistent signaling of c-Jun N-terminal kinase (JNK) in invasive breast cancer. Remarkably, restoration of ARRDC3 expression significantly reduced activated PAR1-induced breast carcinoma invasion, which was also dependent on JNK signaling. These findings are the first to identify a critical link between the tumor suppressor ARRDC3 and regulation of GPCR trafficking and signaling in breast cancer.

Protease-activated receptor-4 and purinergic receptor P2Y12 dimerize, co-internalize, and activate Akt signaling via endosomal recruitment of β-arrestin

Vascular inflammation and thrombosis require the concerted actions of several different agonists, many of which act on G protein-coupled receptors (GPCRs). GPCR dimerization is a well-established phenomenon that can alter protomer function. In platelets and other cell types, protease-activated receptor-4 (PAR4) has been shown to dimerize with the purinergic receptor P2Y12 to coordinate β-arrestin-mediated Akt signaling, an important mediator of integrin activation. However, the mechanism by which the PAR4-P2Y12 dimer controls β-arrestin-dependent Akt signaling is not known. We now report that PAR4 and P2Y12 heterodimer internalization is required for β-arrestin recruitment to endosomes and Akt signaling. Using bioluminescence resonance energy transfer, immunofluorescence microscopy, and co-immunoprecipitation in cells expressing receptors exogenously and endogenously, we demonstrate that PAR4 and P2Y12 specifically interact and form dimers expressed at the cell surface. We also found that activation of PAR4 but not of P2Y12 drives internalization of the PAR4-P2Y12 heterodimer. Remarkably, activated PAR4 internalization was required for recruitment of β-arrestin to endocytic vesicles, which was dependent on co-expression of P2Y12. Interestingly, stimulation of the PAR4-P2Y12 heterodimer promotes β-arrestin and Akt co-localization to intracellular vesicles. Moreover, activated PAR4-P2Y12 internalization is required for sustained Akt activation. Thus, internalization of the PAR4-P2Y12 heterodimer is necessary for β-arrestin recruitment to endosomes and Akt signaling and lays the foundation for examining whether blockade of PAR4 internalization reduces integrin and platelet activation.