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

Direct neuronal protection by the protease-activated receptor PAR4 antagonist ML354 after experimental stroke in mice

BACKGROUND AND PURPOSE

Thrombo-inflammation is a key feature of stroke pathophysiology and provides multiple candidate drug targets. Thrombin exerts coagulation-independent actions via protease-activated receptors (PAR), of which PAR1 has been implicated in stroke-associated neuroinflammation. The role of PAR4 in this context is less clear. This study examined if the selective PAR4 antagonist ML354 provides neuroprotection in experimental stroke and explored the underlying mechanisms.

EXPERIMENTAL APPROACH

Mouse primary cortical neurons were exposed to oxygen-glucose deprivation (OGD) and simulated reperfusion ± ML354. For comparison, functional Ca2+-imaging was performed upon acute stimulation with a PAR4 activating peptide or glutamate. Male mice underwent sham operation or transient middle cerebral artery occlusion (tMCAO), with ML354 or vehicle treatment beginning at recanalization. A subset of mice received a platelet-depleting antibody. Stroke size and functional outcomes were assessed. Abundance of target genes, proteins, and cell markers was determined in cultured cells and tissues by qPCR, immunoblotting, and immunofluorescence.

KEY RESULTS

Stroke up-regulated PAR4 expression in cortical neurons in vitro and in vivo. OGD augments spontaneous and PAR4-mediated neuronal activity; ML354 suppresses OGD-induced neuronal excitotoxicity and apoptosis. ML354 applied in vivo after tMCAO reduced infarct size, apoptotic markers, macrophage accumulation, and interleukin-1β expression. Platelet depletion did not affect infarct size in mice with tMCAO ± ML354.

CONCLUSIONS AND IMPLICATIONS

Selective PAR4 inhibition during reperfusion improves infarct size and neurological function after experimental stroke by blunting neuronal excitability, apoptosis, and local inflammation. PAR4 antagonists may provide additional neuroprotective benefits in patients with acute stroke beyond their canonical antiplatelet action.

Discovery of Potent and Selective Quinoxaline-Based Protease-Activated Receptor 4 (PAR4) Antagonists for the Prevention of Arterial Thrombosis

PAR4 is a promising antithrombotic target with potential for separation of efficacy from bleeding risk relative to current antiplatelet therapies. In an effort to discover a novel PAR4 antagonist chemotype, a quinoxaline-based HTS hit 3 with low μM potency was identified. Optimization of the HTS hit through the use of positional SAR scanning and the design of conformationally constrained cores led to the discovery of a quinoxaline-benzothiazole series as potent and selective PAR4 antagonists. The lead compound 48, possessing a 2 nM IC50 against PAR4 activation by γ-thrombin in platelet-rich plasma (PRP) and greater than 2500-fold selectivity versus PAR1, demonstrated robust antithrombotic efficacy and minimal bleeding in the cynomolgus monkey models.

Protease-Activated Receptors (PARs): Biology and Therapeutic Potential in Perioperative Stroke

Perioperative stroke is a devastating complication that occurs during surgery or within 30 days following the surgical procedure. Its prevalence ranges from 0.08 to 10% although it is most likely an underestimation, as sedatives and narcotics can substantially mask symptomatology and clinical presentation. Understanding the underlying pathophysiology and identifying potential therapeutic targets are of paramount importance. Protease-activated receptors (PARs), a unique family of G-protein-coupled receptors, are widely expressed throughout the human body and play essential roles in various physiological and pathological processes. This review elucidates the biology and significance of PARs, outlining their diverse functions in health and disease, and their intricate involvement in cerebrovascular (patho)physiology and neuroprotection. PARs exhibit a dual role in cerebral ischemia, which underscores their potential as therapeutic targets to mitigate the devastating effects of stroke in surgical patients.

Platelets in Renal Disease

Kidney disease is a major global health concern, affecting millions of people. Nephrologists have shown interest in platelets because of coagulation disorders caused by renal diseases. With a better understanding of platelets, it has been found that these anucleate and abundant blood cells not only play a role in hemostasis, but also have important functions in inflammation and immunity. Platelets are not only affected by kidney disease, but may also contribute to kidney disease progression by mediating inflammation and immune effects. This review summarizes the current evidence regarding platelet abnormalities in renal disease, and the multiple effects of platelets on kidney disease progression. The relationship between platelets and kidney disease is still being explored, and further research can provide mechanistic insights into the relationship between thrombosis, bleeding, and inflammation related to kidney disease, and elucidate targeted therapies for patients with kidney disease.

Therapeutic charisma of imidazo [2,1-b] [1,3,4]-thiadiazole analogues: a patent review

Imidazothiadiazole was discovered around the 1950s era, containing an imidazole ring fused to a thiadiazole ring. Imidazothiadiazole exhibit versatile pharmacological properties including anticonvulsant, cardiotonic, anti-inflammatory, diuretic, antifungal, antibacterial and anticancer. Despite of the being discovered in 1950s, the imidazothiadiazole derivatives are unable to being processed to clinical trials because of lack of bioavailability, efficacy and cytotoxicity. The recent patent literature focused on structural modification of imidazothiadiazole core to overcome these problems. This review limelight a disease-centric perspective on patented imidazothiadiazole from 2015-2023 and to understand their mechanism of action in related diseases. The relevant granted patent applications were located using patent databases, Google Patents, USPTO, EPO, WIPO, Espacenet and Lens.

Synergistic inhibitory effects of clopidogrel and rivaroxaban on platelet function and platelet-dependent thrombin generation in cats

BACKGROUND

Dual antithrombotic treatment (DAT) with clopidogrel and rivaroxaban sometimes is prescribed to cats with hypertrophic cardiomyopathy at risk of thromboembolism. To date, no studies have evaluated their combined effects on platelet function.

OBJECTIVES/HYPOTHESIS

Evaluate the safety of DAT in healthy cats and compare, ex vivo, platelet-dependent thrombin generation and agonist-induced platelet activation and aggregation in cats treated with clopidogrel, rivaroxaban, or DAT. We hypothesized that DAT would safely modulate agonist-induced platelet activation and aggregation more effectively than single agent treatment.

ANIMALS

Nine apparently healthy 1-year-old cats selected from a research colony.

METHODS

Unblinded, nonrandomized ex vivo cross-over study. All cats received 7 days of rivaroxaban (0.6 ± 0.1 mg/kg PO), clopidogrel (4.7 ± 0.8 mg/kg PO), or DAT with defined washout periods between treatments. Before and after each treatment, adenosine diphosphate (ADP)- and thrombin-induced platelet P-selectin expression was evaluated using flow cytometry to assess platelet activation. Platelet-dependent thrombin generation was measured by fluorescence assay. Platelet aggregation was assessed using whole blood impedance platelet aggregometry.

RESULTS

No cats exhibited adverse effects. Of the 3 treatments, only DAT significantly decreased the number of activated platelets (P = .002), modulated platelet activation in response to thrombin (P = .01), dampened thrombin generation potential (P = .01), and delayed maximum reaction velocity (P = .004) in thrombin generation. Like clopidogrel, DAT inhibited ADP-mediated platelet aggregation. However, rivaroxaban alone resulted in increased aggregation and activation in response to ADP.

CONCLUSION AND CLINICAL IMPORTANCE

Treatment combining clopidogrel and rivaroxaban (DAT) safely decreases platelet activation, platelet response to agonists, and thrombin generation in feline platelets more effectively than monotherapy with either clopidogrel or rivaroxaban.

The mechanistic basis linking cytokine storm to thrombosis in COVID-19

It is now well established that infection with SARS-CoV-2 resulting in COVID-19 disease includes a severely symptomatic subset of patients in whom an aggressive and/or dysregulated host immune response leads to cytokine storm syndrome (CSS) that may be further complicated by thrombotic events, contributing to the severe morbidity and mortality observed in COVID-19. This review provides a brief overview of cytokine storm in COVID-19, and then presents a mechanistic discussion of how cytokine storm affects integrated pathways in thrombosis involving the endothelium, platelets, the coagulation cascade, eicosanoids, auto-antibody mediated thrombosis, and the fibrinolytic system.

The Role of Platelets in Diabetic Kidney Disease

Diabetic kidney disease (DKD) is among the most common microvascular complications in patients with diabetes, and it currently accounts for the majority of end-stage kidney disease cases worldwide. The pathogenesis of DKD is complex and multifactorial, including systemic and intra-renal inflammatory and coagulation processes. Activated platelets play a pivotal role in inflammation, coagulation, and fibrosis. Mounting evidence shows that platelets play a role in the pathogenesis and progression of DKD. The potentially beneficial effects of antiplatelet agents in preventing progression of DKD has been studied in animal models and clinical trials. This review summarizes the current knowledge on the role of platelets in DKD, including the potential therapeutic effects of antiplatelet therapies.

Epigenetic Regulation of F2RL3 Associates With Myocardial Infarction and Platelet Function

BACKGROUND

DNA hypomethylation at the F2RL3 (F2R like thrombin or trypsin receptor 3) locus has been associated with both smoking and atherosclerotic cardiovascular disease; whether these smoking-related associations form a pathway to disease is unknown. F2RL3 encodes protease-activated receptor 4, a potent thrombin receptor expressed on platelets. Given the role of thrombin in platelet activation and the role of thrombus formation in myocardial infarction, alterations to this biological pathway could be important for ischemic cardiovascular disease.

METHODS

We conducted multiple independent experiments to assess whether DNA hypomethylation at F2RL3 in response to smoking is associated with risk of myocardial infarction via changes to platelet reactivity. Using cohort data (N=3205), we explored the relationship between smoking, DNA hypomethylation at F2RL3, and myocardial infarction. We compared platelet reactivity in individuals with low versus high DNA methylation at F2RL3 (N=41). We used an in vitro model to explore the biological response of F2RL3 to cigarette smoke extract. Finally, a series of reporter constructs were used to investigate how differential methylation could impact F2RL3 gene expression.

RESULTS

Observationally, DNA methylation at F2RL3 mediated an estimated 34% of the smoking effect on increased risk of myocardial infarction. An association between methylation group (low/high) and platelet reactivity was observed in response to PAR4 (protease-activated receptor 4) stimulation. In cells, cigarette smoke extract exposure was associated with a 4.9% to 9.3% reduction in DNA methylation at F2RL3 and a corresponding 1.7-(95% CI, 1.2-2.4, P=0.04) fold increase in F2RL3 mRNA. Results from reporter assays suggest the exon 2 region of F2RL3 may help control gene expression.

CONCLUSIONS

Smoking-induced epigenetic DNA hypomethylation at F2RL3 appears to increase PAR4 expression with potential downstream consequences for platelet reactivity. Combined evidence here not only identifies F2RL3 DNA methylation as a possible contributory pathway from smoking to cardiovascular disease risk but from any feature potentially influencing F2RL3 regulation in a similar manner.

PAR4-Mediated PI3K/Akt and RhoA/ROCK Signaling Pathways Are Essential for Thrombin-Induced Morphological Changes in MEG-01 Cells

Thrombin stimulates platelets via a dual receptor system of protease-activated receptors (PARs): PAR1 and PAR4. PAR1 activation induces a rapid and transient signal associated with the initiation of platelet aggregation, whereas PAR4 activation results in a prolonged signal, required for later phases, that regulates the stable formation of thrombus. In this study, we observed differential signaling pathways for thrombin-induced PAR1 and PAR4 activation in a human megakaryoblastic leukemia cell line, MEG-01. Interestingly, thrombin induced both calcium signaling and morphological changes in MEG-01 cells via the activation of PAR1 and PAR4, and these intracellular events were very similar to those observed in platelets shown in previous studies. We developed a novel image-based assay to quantitatively measure the morphological changes in living cells, and observed the underlying mechanism for PAR1- and PAR4-mediated morphological changes in MEG-01 cells. Selective inhibition of PAR1 and PAR4 by vorapaxar and BMS-986120, respectively, showed that thrombin-induced morphological changes were primarily mediated by PAR4 activation. Treatment of a set of kinase inhibitors and 2-aminoethoxydiphenyl borate (2-APB) revealed that thrombin-mediated morphological changes were primarily regulated by calcium-independent pathways and PAR4 activation-induced PI3K/Akt and RhoA/ROCK signaling pathways in MEG-01 cells. These results indicate the importance of PAR4-mediated signaling pathways in thrombin-induced morphological changes in MEG-01 cells and provide a useful in vitro cellular model for platelet research.

The Emerging Role of Neutrophil Extracellular Traps in Arterial, Venous and Cancer-Associated Thrombosis

Neutrophils play a vital role in the formation of arterial, venous and cancer-related thrombosis. Recent studies have shown that in a process known as NETosis, neutrophils release proteins and enzymes complexed to DNA fibers, collectively called neutrophil extracellular traps (NETs). Although NETs were originally described as a way for the host to capture and kill bacteria, current knowledge indicates that NETs also play an important role in thrombosis. According to recent studies, the destruction of vascular microenvironmental homeostasis and excessive NET formation lead to pathological thrombosis. In vitro experiments have found that NETs provide skeletal support for platelets, red blood cells and procoagulant molecules to promote thrombosis. The protein components contained in NETs activate the endogenous coagulation pathway to promote thrombosis. Therefore, NETs play an important role in the formation of arterial thrombosis, venous thrombosis and cancer-related thrombosis. This review will systematically summarize and explain the study of NETs in thrombosis in animal models and in vivo experiments to provide new targets for thrombosis prevention and treatment.

Platelets Contribution to Thrombin Generation in Philadelphia-Negative Myeloproliferative Neoplasms: The "Circulating Wound" Model

Current cytoreductive and antithrombotic strategies in MPNs are mostly based on cell counts and on patient's demographic and clinical history. Despite the numerous studies conducted on platelet function and on the role of plasma factors, an accurate and reliable method to dynamically quantify the hypercoagulability states of these conditions is not yet part of clinical practice. Starting from our experience, and after having sifted through the literature, we propose an in-depth narrative report on the contribution of the clonal platelets of MPNs-rich in tissue factor (TF)-in promoting a perpetual procoagulant mechanism. The whole process results in an unbalanced generation of thrombin and is self-maintained by Protease Activated Receptors (PARs). We chose to define this model as a "circulating wound", as it indisputably links the coagulation, inflammation, and fibrotic progression of the disease, in analogy with what happens in some solid tumours. The platelet contribution to thrombin generation results in triggering a vicious circle supported by the PARs/TGF-beta axis. PAR antagonists could therefore be a good option for target therapy, both to contain the risk of vascular events and to slow the progression of the disease towards end-stage forms. Both the new and old strategies, however, will require tools capable of measuring procoagulant or prohaemorrhagic states in a more extensive and dynamic way to favour a less empirical management of MPNs and their potential clinical complications.

Protease Activated Receptors and Arthritis

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

Targeted nanotherapeutics in the prophylaxis and treatment of thrombosis

Currently available anti-thrombotic therapy for the prophylaxis and treatment of arterial and venous thrombosis includes intravenous administration of anti-thrombotic drugs which lead to severe bleeding risks such as cerebral hemorrhage and stroke. Targeting approaches that utilize nanosystems to reach the thrombus sites are emerging to increase the local effect of anti-thrombotic drugs, as well as to decrease these severe bleeding complications by diminishing the systemic availability of these drugs. This review emphasizes the emerging targeted nanomedicines (liposomes, micelles, polymeric nanoparticles, material bases nanoparticles and other biological vectors) for the prophylaxis and treatment of thrombotic events as well as multifunctional nanomedicines for theranostic applications. Nanomedicine offers a promising platform for a smart, safe, and effective approach for the management of thrombosis.

Protease-activated receptor 4 plays a role in lipopolysaccharide-induced inflammatory mechanisms in murine macrophages

The role of protease-activated receptor (PAR)4 in thrombin-induced platelet aggregation has been studied, and PAR4 blockade is thought to be useful as a new and promising approach in antiplatelet therapy in humans. In recent years, studies have been conducted to clarify the role of PAR4 in the host defense against invading microorganisms and pathogen-induced inflammation; however, to date, the role of PAR4 in mediating the LPS-induced inflammatory repertoire in macrophages remains to be elucidated. Here, we investigated the effects of the synthetic PAR4 agonist peptide (PAR4-AP) AYPGKF-NH₂ on the phagocytosis of zymosan-FITC particles; NO, ROS, and iNOS expression; and cytokine production in C57/BL6 macrophages cocultured with PAR4-AP/LPS. The PAR4-AP impaired LPS-induced and basal phagocytosis, which was restored by pharmacological PAR4 blockade. Coincubation with the PAR4-AP/LPS enhanced NO and ROS production and iNOS expression; decreased IL-10, but not TNF-α, in the culture supernatant; and increased translocation of the p65 subunit of the proinflammatory gene transcription factor NF-κ-B. Our results provide evidence for a complex mechanism and new approach by which PAR4 mediates the macrophage response triggered by LPS through counter-regulating the phagocytic activity of macrophages and innate response mechanisms implicated in the killing of invading pathogens.

Antithrombotic Effects of Combined PAR (Protease-Activated Receptor)-4 Antagonism and Factor Xa Inhibition

OBJECTIVE

PAR (protease-activated receptor)-4 antagonism has antiplatelet effects under conditions of high shear stress. We aimed to establish whether PAR4 antagonism had additive antithrombotic activity in the presence of factor Xa inhibition in an ex vivo model of acute arterial injury. Approach and Results: Fifteen healthy volunteers (29±6 years, 7 women) completed a phase zero double-blind randomized controlled crossover trial. Ex vivo platelet activation, platelet aggregation, and thrombus formation were measured following blood perfusion of low shear and high shear stress chambers. Upstream of the chambers, extracorporeal blood was admixed with (1) vehicle, (2) low-dose apixaban (20 ng/mL), (3) high-dose apixaban (80 ng/mL), (4) BMS-986141 (400 ng/mL), (5) BMS-968141 and low-dose apixaban, or (6) BMS-968141 and high-dose apixaban in 6 sequential studies performed in random order. Compared with vehicle, BMS-986141 demonstrated selective inhibition of PAR4-AP (agonist peptide)-stimulated platelet aggregation, platelet-monocyte aggregates, and P-selectin expression (P≤0.01 for all). Total thrombus area was reduced under both low shear and high shear stress conditions for all drug infusions (P<0.0001 for all versus vehicle). BMS-968141 reduced total (≤44.4%) and platelet-rich (≤39.3%) thrombus area, whereas apixaban reduced total (≤42.9%) and fibrin-rich (≤31.6%) thrombus area. Combination of BMS-986141 with apixaban caused a further modest reduction in total thrombus area (9.6%-12.4%), especially under conditions of high shear stress (P≤0.027).

CONCLUSIONS

In the presence of factor Xa inhibition, PAR4 antagonism with BMS-986141 further reduces thrombus formation, especially under conditions of high shear stress. This suggests the potential for additive efficacy of combination PAR4 antagonism and factor Xa inhibition in the prevention of atherothrombotic events.

Inflammation and thrombosis in COVID-19 pathophysiology: proteinase-activated and purinergic receptors as drivers and candidate therapeutic targets

Evolving information has identified disease mechanisms and dysregulation of host biology that might be targeted therapeutically in coronavirus disease 2019 (COVID-19). Thrombosis and coagulopathy, associated with pulmonary injury and inflammation, are emerging clinical features of COVID-19. We present a framework for mechanisms of thrombosis in COVID-19 that initially derive from interaction of SARS-CoV-2 with ACE2, resulting in dysregulation of angiotensin signaling and subsequent inflammation and tissue injury. These responses result in increased signaling by thrombin (proteinase-activated) and purinergic receptors, which promote platelet activation and exert pathological effects on other cell types (e.g., endothelial cells, epithelial cells, and fibroblasts), further enhancing inflammation and injury. Inhibitors of thrombin and purinergic receptors may, thus, have therapeutic effects by blunting platelet-mediated thromboinflammation and dysfunction in other cell types. Such inhibitors include agents (e.g., anti-platelet drugs) approved for other indications, and that could be repurposed to treat, and potentially improve the outcome of, COVID-19 patients. COVID-19, caused by the SARS-CoV-2 virus, drives dysregulation of angiotensin signaling, which, in turn, increases thrombin-mediated and purinergic-mediated activation of platelets and increase in inflammation. This thromboinflammation impacts the lungs and can also have systemic effects. Inhibitors of receptors that drive platelet activation or inhibitors of the coagulation cascade provide opportunities to treat COVID-19 thromboinflammation.

Protease activated receptor 4 (PAR4) antagonists: Research progress on small molecules in the field of antiplatelet agents

Protease activated receptor 4 (PAR4) is a key target in antiplatelet medication to reduce the risk of heart attack and thrombotic complications in stroke. PAR4 antagonists can prevent harmful and stable thrombus growth while retaining initial thrombus formation by acting on the late diffusion stage of platelet activation, which may provide a safer alternative than other antiplatelet agents. Currently, research on PAR4 antagonists is of increasing interest in the field of antiplatelet agents. This article provides an overview of the discovery and development of small-molecule antagonists of PAR4 as novel antiplatelet agents, including structure-activity relationship (SAR) analysis, progress of structure and bioassay optimization, and the latest structural and/or clinical information of representative small-molecule antagonists of PAR4.

Biased signaling in platelet G-protein coupled receptors

Platelets are small megakaryocyte-derived, anucleate, disk-like structures that play an outsized role in human health and disease. Both a decrease in the number of platelets and a variety of platelet function disorders result in petechiae or bleeding that can be life threatening. Conversely, the inappropriate activation of platelets, within diseased blood vessels, remains the leading cause of death and morbidity by affecting heart attacks and stroke. The fine balance of the platelet state in healthy individuals is controlled by a number of receptor-mediated signaling pathways that allow the platelet to rapidly respond and maintain haemostasis. G-protein coupled receptors (GPCRs) are particularly important regulators of platelet function. Here we focus on the major platelet-expressed GPCRs and discuss the roles of downstream signaling pathways (e.g., different G-protein subtypes or β-arrestin) in regulating the different phases of the platelet activation. Further, we consider the potential for selectively targeting signaling pathways that may contribute to platelet responses in disease through development of biased agonists. Such selective targeting of GPCR-mediated signaling pathways by drugs, often referred to as biased signaling, holds promise in delivering therapeutic interventions that do not present significant side effects, especially in finely balanced physiological systems such as platelet activation in haemostasis.

Regulators of G-protein signaling, RGS2 and RGS4, inhibit protease-activated receptor 4-mediated signaling by forming a complex with the receptor and Gα in live cells

BACKGROUND

Protease-activated receptor 4 (PAR4) is a seven transmembrane G-protein coupled receptor (GPCR) activated by endogenous proteases, such as thrombin. PAR4 is involved in various pathophysiologies including cancer, inflammation, pain, and thrombosis. Although regulators of G-protein signaling (RGS) are known to modulate GPCR/Gα-mediated pathways, their specific effects on PAR4 are not fully understood at present. We previously reported that RGS proteins attenuate PAR1- and PAR2-mediated signaling through interactions with these receptors in conjunction with distinct Gα subunits.

METHODS

We employed a bioluminescence resonance energy transfer technique and confocal microscopy to examine potential interactions among PAR4, RGS, and Gα subunits. The inhibitory effects of RGS proteins on PAR4-mediated downstream signaling and cancer progression were additionally investigated by using several assays including ERK phosphorylation, calcium mobilization, RhoA activity, cancer cell proliferation, and related gene expression.

RESULTS

In live cells, RGS2 interacts with PAR4 in the presence of Gα_q while RGS4 binding to PAR4 occurs in the presence of Gα_q and Gα_12/13. Co-expression of PAR4 and Gα_q induced a shift in the subcellular localization of RGS2 and RGS4 from the cytoplasm to plasma membrane. Combined PAR4 and Gα_12/13 expression additionally promoted translocation of RGS4 from the cytoplasm to the membrane. Both RGS2 and RGS4 abolished PAR4-activated ERK phosphorylation, calcium mobilization and RhoA activity, as well as PAR4-mediated colon cancer cell proliferation and related gene expression.

CONCLUSIONS

RGS2 and RGS4 forms ternary complex with PAR4 in Gα-dependent manner and inhibits its downstream signaling. Our findings support a novel physiological function of RGS2 and RGS4 as inhibitors of PAR4-mediated signaling through selective PAR4/RGS/Gα coupling. Video Abstract.

Molecular basis for activation and biased signaling at the thrombin-activated GPCR proteinase activated receptor-4 (PAR4)

Proteinase-activated receptor (PAR)-4 is a member of the proteolytically-activated PAR family of G-protein-coupled receptors (GPCR) that represents an important target in the development of anti-platelet therapeutics. PARs are activated by proteolytic cleavage of their receptor N terminus by enzymes such as thrombin, trypsin, and cathepsin-G. This reveals the receptor-activating motif, termed the tethered ligand that binds intramolecularly to the receptor and triggers signaling. However, PARs are also activated by exogenous application of synthetic peptides derived from the tethered-ligand sequence. To better understand the molecular basis for PAR4-dependent signaling, we examined PAR4-signaling responses to a peptide library derived from the canonical PAR4-agonist peptide, AYPGKF-NH2, and we monitored activation of the Gαq/11-coupled calcium-signaling pathway, β-arrestin recruitment, and mitogen-activated protein kinase (MAPK) pathway activation. We identified peptides that are poor activators of PAR4-dependent calcium signaling but were fully competent in recruiting β-arrestin-1 and -2. Peptides that were unable to stimulate PAR4-dependent calcium signaling could not trigger MAPK activation. Using in silico docking and site-directed mutagenesis, we identified Asp230 in the extracellular loop-2 as being critical for PAR4 activation by both agonist peptide and the tethered ligand. Probing the consequence of biased signaling on platelet activation, we found that a peptide that cannot activate calcium signaling fails to cause platelet aggregation, whereas a peptide that is able to stimulate calcium signaling and is more potent for β-arrestin recruitment triggered greater levels of platelet aggregation compared with the canonical PAR4 agonist peptide. These findings uncover molecular determinants critical for agonist binding and biased signaling through PAR4.

Selective Inhibition of PAR4 (Protease-Activated Receptor 4)-Mediated Platelet Activation by a Synthetic Nonanticoagulant Heparin Analog

Objective- PAR4 (protease-activated receptor 4), one of the thrombin receptors in human platelets, has emerged as a promising target for the treatment of arterial thrombotic disease. Previous studies implied that thrombin exosite II, known as a binding site for heparin, may be involved in thrombin-induced PAR4 activation. In the present study, a heparin octasaccharide analog containing the thrombin exosite II-binding domain of heparin was chemically synthesized and investigated for anti-PAR4 effect. Approach and Results- PAR4-mediated platelet aggregation was examined using either thrombin in the presence of a PAR1 antagonist or γ-thrombin, which selectively activates PAR4. SCH-28 specifically inhibits PAR4-mediated platelet aggregation, as well as the signaling events downstream of PAR4 in response to thrombin. Moreover, SCH-28 prevents thrombin-induced β-arrestin recruitment to PAR4 but not PAR1 in Chinese Hamster Ovary-K1 cells using a commercial enzymatic complementation assay. Compared with heparin, SCH-28 is more potent in inhibiting PAR4-mediated platelet aggregation but has no significant anticoagulant activity. In an in vitro thrombosis model, SCH-28 reduces thrombus formation under whole blood arterial flow conditions. Conclusions- SCH-28, a synthetic small-molecular and nonanticoagulant heparin analog, inhibits thrombin-induced PAR4 activation by interfering with thrombin exosite II, a mechanism of action distinct from other PAR4 inhibitors that target the receptor. The characteristics of SCH-28 provide a new strategy for targeting PAR4 with the potential for the treatment of arterial thrombosis.

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.

In-vitro Pre-Treatment of Cancer Cells with TGF-β1: A Novel Approach of Tail Vein Lung Cancer Metastasis Mouse Model for Anti-Metastatic Studies

Background:

Aggressive behavior of tumor metastasis comes from certain mutations, changes in cellular metabolic and signaling pathways that are majorly altered by tumor microenvironment (TME), its other components and growth factors like transforming growth factor-β1 (TGF-β1) which is chiefly known for its epithelial to mesenchymal transformation (EMT). EMT is a critical step of metastasis cascade in actual human lung cancer scenario.

Objective:

Our present study is focused on unveiling the in-vivo metastatic behavior of TGF-β1 treated lung cancer cells that undergo EMT.

Methods:

The lung cancer epithelial A549 cells were treated in-vitro with TGF-β1 (3-5ng/ml for 72 h) for EMT. After confirming the transformation of cells by phenotype modifications, wound healing and cell migration assay and qRT-PCR analyses of EMT biomarkers including E. Cadherin, Vimentin, Snail, Slug, MMP2 and MMP9; those TGF-β1 modified cells were probed with fluorescent trackers and were injected into the tail vein of BALB/c nude mice for metastatic dissemination studies.

Results:

Our findings indicate that the distribution of TGF-β1 treated A549 cells as compared to W.T A549 towards lungs is less in terms of total relative fluorescent cluster count, however, the difference is insignificant (52±4, 60±5 respectively). Additionally, we show that TGF-β1 treated cells tend to metastasize almost 2, 3, 1.5, 2 and 1.7 times more than W.T towards liver, brain, ovaries, bones and adrenal gland, respectively, which is very much like human lung cancer metastasis.

Conclusion:

Conclusively, it is the first study ever reporting that a pre-treatment of cells with TGF-β1 for experimental lung cancer metastasis mouse model may portray a more precise approach for the development of potential therapeutic treatments. Additional pre-treatment studies with the application of other TME conditions like hypoxia and factors like NFκB, VEGF etc. may be a future prospect to develop a better understanding.

Developments in inhibiting platelet aggregation based on different design strategies

Platelet aggregation is the central event in hemostasis and thrombosis. Up to now, many agents inhibiting platelet aggregation have been approved for the treatment of thrombotic disorders. In this review, we mainly summarized the progress in the research of platelet aggregation inhibitors based on different design strategies. The advantage and challenge of corresponding targets are also discussed in this article. We hope more platelet aggregation inhibitors with efficacy and safety will be discovered in the future.

Protease-activated receptor 4 protects mice from Coxsackievirus B3 and H1N1 influenza A virus infection

PAR4 is expressed by a variety of cells, including platelets, cardiac, lung and immune cells. We investigated the contribution of PAR4 to viral infections of the heart and lung. Toll-like receptor (TLR) 3-dependent immune responses were analyzed after co-stimulation of PAR4 in murine bone-marrow derived macrophages, embryonic fibroblasts and embryonic cardiomyocytes. In addition, we analyzed Coxsackievirus B3 (CVB3) or H1N1 influenza A virus (H1N1 IAV) infection of PAR4^-/- (ΔPAR4) and wild-type (WT) mice. Lastly, we investigated the effect of platelet inhibition on H1N1 IAV infection. In vitro experiments revealed that PAR4 stimulation enhances the expression of TLR3-dependent CXCL10 expression and decreases TLR3-dependent NFκB-mediated proinflammatory gene expression. Furthermore, CVB3-infected ΔPAR4 mice exhibited a decreased anti-viral response and increased viral genomes in the heart leading to more pronounced CVB3 myocarditis compared to WT mice. Similarly, H1N1 IAV-infected ΔPAR4 mice had increased immune cell numbers and inflammatory mediators in the lung, and increased mortality compared with infected WT controls. The study showed that PAR4 protects mice from viral infections of the heart and lung.

Clinical evaluation of carcinoembryonic and carbohydrate antigens as cancer biomarkers to monitor palliative chemotherapy in advanced stage gastric cancer

BACKGROUND

Carcinoembryonic antigen (CEA), carbohydrate antigen (CA)-125, CA19-9, and CA72-4 are often found modulated parameters in gastric cancer.

OBJECTIVE

Our present study is focused to evaluate the synchronization of these biomarkers in response to palliative chemotherapy.

METHOD

A retrospective study was conducted on 216 gastric cancer patients undergoing first-line cisplatin chemotherapy along with antiangiogenic regimen. Blood samples were taken and analyzed biochemically and statistically.

RESULTS

Progression occurred in 78 of 216 patients and the median progression-free survival (PFS) was 5 months. For serum CEA, the median PFS was 4 versus 7 months for elevated and normal groups respectively (P = 0.01). The median PFS for normal and elevated CA19-9 and CA72-4 was 6 vs 4 months respectively (P = 0.001). In the multivariate Cox regression model, elevated pretreatment level of CEA, CA19-9, and distant metastases were independent factors associated with increased risk of progression (P = 0.021, P = 0.000, P = 0.006, respectively).

CONCLUSIONS

Conclusively, elevated pretreatment level of CEA and CA19-9 is correlated with high risk of progression and worse prognosis. Moreover, an additional antiangiogenic therapy is more effective in decreasing cancer biomarker level after palliative chemotherapy that may be correlated with therapeutic triumph.