Ligand cross-reactivity within the protease-activated receptor family

Structural basis for the activation of proteinase-activated receptors PAR1 and PAR2

Members of the proteinase-activated receptor (PAR) subfamily of G protein-coupled receptors (GPCRs) play critical roles in processes like hemostasis, thrombosis, development, wound healing, inflammation, and cancer progression. Comprising PAR1-PAR4, these receptors are specifically activated by protease cleavage at their extracellular amino terminus, revealing a 'tethered ligand' that self-activates the receptor. This triggers complex intracellular signaling via G proteins and beta-arrestins, linking external protease signals to cellular functions. To date, direct structural visualization of these ligand-receptor complexes has been limited. Here, we present structural snapshots of activated PAR1 and PAR2 bound to their endogenous tethered ligands, revealing a shallow and constricted orthosteric binding pocket. Comparisons with antagonist-bound structures show minimal conformational changes in the TM6 helix and larger movements of TM7 upon activation. These findings reveal a common activation mechanism for PAR1 and PAR2, highlighting critical residues involved in ligand recognition. Additionally, the structure of PAR2 bound to a pathway selective antagonist, GB88, demonstrates how potent orthosteric engagement can be achieved by a small molecule mimicking the endogenous tethered ligand's interactions.

PAR2 Serves an Indispensable Role in Controlling PAR4 Oncogenicity: The β-Catenin-p53 Axis

Although the role of G-protein-coupled receptors (GPCRs) in cancer is acknowledged, GPCR-based cancer therapy is rare. Mammalian protease-activated receptors (PARs), a sub-group of GPCRs, comprise four family members, termed PAR1-4. Here, we demonstrate that PAR2 is dominant over PAR4 oncogene in cancer. We performed a knockdown of Par2/f2rl1 and expressed C-terminally truncated PAR2 (TrPAR2), incapable of inducing signaling, to assess the impact of PAR2 on PAR4 oncogenic function by β-catenin stabilization assessment, immunoprecipitation, and xenograft tumor generation in Nude/Nude mice. PAR2 and PAR4 act together to promote tumor generation. Knockdown Par2 and TrPAR2 inhibited the PAR2 and PAR4-induced β-catenin levels, nuclear dishevelled 1(DVL1), and TOPflash reporter activity. Likewise, PAR2 and PAR4-induced invasion and migration were inhibited when Par2 was knocked down or in the presence of TrPAR2. PAR cyclic (4-4) [Pc(4-4)], a PAR-based compound directed toward the PAR pleckstrin homology (PH)-binding site, effectively inhibited PAR2 oncogenic activity. Pc(4-4) inhibition is mediated via the increase in p53 level and the up-regulation of p21 as caspase-3 as well. Overall, we showed that in the absence of PAR2 signaling, the PAR4 pro-tumor functions are significantly inhibited. Pc(4-4) inhibits PAR2 acting via the modification of wt p53, thus offering a powerful drug measure for fighting cancer.

Integrative phosphoproteomic analyses reveal hemostatic-endothelial signaling interplay

BACKGROUND

The vascular endothelial cell (EC) monolayer plays a crucial part in maintaining hemostasis. An extensive array of G protein-coupled receptors allows ECs to dynamically act on key hemostatic stimuli such as thrombin and histamine. The impact of these individual stimuli on EC signal transduction has been the subject of various studies, but insight into discordant and concordant EC signaling between different G protein-coupled receptors remains limited.

OBJECTIVES

To elucidate histamine and protease-activated receptor (PAR1-4) signaling cascades in ECs, discern overlapping and diverging regulation between these stimuli and their effect on the EC monolayer.

METHODS

We employed stable isotope labeling by amino acids in cell culture mass spectrometry-based phosphoproteomics on in vitro cultured blood outgrowth ECs stimulated with histamine and different PAR1 to 4 peptides. We investigated key phosphosites through immuno(fluorescence) staining and determined effects on barrier function through transendothelial resistance assays.

RESULTS

EC histamine activation initiated an extensive (kinase) signaling network (including MAPK3, STAT3, and CTNND1). PAR1 and PAR2 receptors induced highly similar signaling cascades, whereas PAR3 and PAR4 induced minimal phospho-regulation. Integration of all applied stimuli indicated uniquely activated proteins between both stimuli, as well as a general overlapping activation of cell junction and actin cytoskeletal proteins.

CONCLUSION

We provide an integrative phosphoproteomic analysis of histamine and PAR agonists in the endothelium that highlights the endothelial response programs that are at the basis of regulating hemostasis.

Protease-activated receptors in health and disease

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

Diversification of PAR signaling through receptor crosstalk

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

Disengaging the COVID-19 Clutch as a Discerning Eye Over the Inflammatory Circuit During SARS-CoV-2 Infection

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes the cytokine release syndrome (CRS) and leads to multiorgan dysfunction. Mitochondrial dynamics are fundamental to protect against environmental insults, but they are highly susceptible to viral infections. Defective mitochondria are potential sources of reactive oxygen species (ROS). Infection with SARS-CoV-2 damages mitochondria, alters autophagy, reduces nitric oxide (NO), and increases both nicotinamide adenine dinucleotide phosphate oxidases (NOX) and ROS. Patients with coronavirus disease 2019 (COVID-19) exhibited activated toll-like receptors (TLRs) and the Nucleotide-binding and oligomerization domain (NOD-), leucine-rich repeat (LRR-), pyrin domain-containing protein 3 (NLRP3) inflammasome. The activation of TLRs and NLRP3 by SARS-CoV-2 induces interleukin 6 (IL-6), IL-1β, IL-18, and lactate dehydrogenase (LDH). Herein, we outline the inflammatory circuit of COVID-19 and what occurs behind the scene, the interplay of NOX/ROS and their role in hypoxia and thrombosis, and the important role of ROS scavengers to reduce COVID-19-related inflammation.

Protease-Activated Receptors - Key Regulators of Inflammatory Bowel Diseases Progression

The pathogenesis and course of inflammatory bowel diseases are related to both immune system disorders and dysfunction of colon permeability. Moreover, co-existing diseases in patients with Crohn's disease and ulcerative colitis are identified. Currently, there are some therapeutic strategies that affect the function of cytokine/s causing inflammation in the intestinal wall. However, additional approaches which target other components of inflammatory bowel diseases pathogenesis are still needed. Accumulating evidence suggests that proteases and protease-activated receptors seem to be responsible for colitis progression. Experimental and observational studies showed alteration of protease-activated receptors expression in the colon of patients with Crohn's disease and ulcerative colitis. Furthermore, it was suggested that the expression of protease-activated receptors correlated with inflammatory bowel diseases activity. Moreover, regulation of protease-activated receptors seems to be responsible for the modulation of colitis and clinical manifestation of inflammatory bowel diseases. In this review, we present the current state of knowledge about the contribution of protease-activated receptors to Crohn's disease and ulcerative colitis and its implications for diagnosis and treatment.

PAR2 Activation on Human Kidney Tubular Epithelial Cells Induces Tissue Factor Synthesis, That Enhances Blood Clotting

Coagulation abnormalities and increased risk of atherothrombosis are common in patients with chronic kidney diseases (CKD). Mechanisms that alter renal hemostasis and lead to thrombotic events are not fully understood. Here we show that activation of protease activated receptor-2 (PAR2) on human kidney tubular epithelial cells (HTECs), induces tissue factor (TF) synthesis and secretion that enhances blood clotting. PAR-activating coagulation-associated protease (thrombin), as well as specific PAR2 activators (matriptase, trypsin, or synthetic agonist 2f-LIGRLO-NH2 (2F), induced TF synthesis and secretion that were potently inhibited by PAR2 antagonist, I-191. Thrombin-induced TF was also inhibited by a PAR1 antagonist, Vorapaxar. Peptide activators of PAR1, PAR3, and PAR4 failed to induce TF synthesis. Differential centrifugation of the 2F-conditoned medium sedimented the secreted TF, together with the exosome marker ALG-2 interacting protein X (ALIX), indicating that secreted TF was associated with extracellular vesicles. 2F-treated HTEC conditioned medium significantly enhanced blood clotting, which was prevented by pre-incubating this medium with an antibody for TF. In summary, activation of PAR2 on HTEC stimulates synthesis and secretion of TF that induces blood clotting, and this is attenuated by PAR2 antagonism. Thrombin-induced TF synthesis is at least partly mediated by PAR1 transactivation of PAR2. These findings reveal how underlying hemostatic imbalances might increase thrombosis risk and subsequent chronic fibrin deposition in the kidneys of patients with CKD and suggest PAR2 antagonism as a potential therapeutic strategy for intervening in CKD progression.

Emerging Roles of Protease-Activated Receptors (PARs) in the Modulation of Synaptic Transmission and Plasticity

Protease-activated receptors (PARs) are a class of G protein-coupled receptors (GPCRs) with a unique mechanism of activation, prompted by a proteolytic cleavage in their N-terminal domain that uncovers a tethered ligand, which binds and stimulates the same receptor. PARs subtypes (PAR1-4) have well-documented roles in coagulation, hemostasis, and inflammation, and have been deeply investigated for their function in cellular survival/degeneration, while their roles in the brain in physiological conditions remain less appreciated. Here, we describe PARs' effects in the modulation of neurotransmission and synaptic plasticity. Available evidence, mainly concerning PAR1-mediated and PAR2-mediated regulation of glutamatergic and GABAergic transmission, supports that PARs are important modulators of synaptic efficacy and plasticity in normal conditions.

Liver fibrosis is driven by protease-activated receptor-1 expressed by hepatic stellate cells in experimental chronic liver injury

BACKGROUND

Blood coagulation protease activity is proposed to drive hepatic fibrosis through activation of protease-activated receptors (PARs). Whole-body PAR-1 deficiency reduces experimental hepatic fibrosis, and in vitro studies suggest a potential contribution by PAR-1 expressed by hepatic stellate cells. However, owing to a lack of specific tools, the cell-specific role of PAR-1 in experimental hepatic fibrosis has never been formally investigated. Using a novel mouse expressing a conditional PAR-1 allele, we tested the hypothesis that PAR-1 expressed by hepatic stellate cells contributes to hepatic fibrosis.

METHODS

PAR-1flox/flox mice were crossed with mice expressing Cre recombinase controlled by the lecithin retinol acyltransferase (LRAT) promoter, which induces recombination in hepatic stellate cells. Male PAR-1flox/flox/LRATCre and PAR-1flox/flox mice were challenged twice weekly with carbon tetrachloride (CCl4, 1 mL/kg i.p.) for 6 weeks to induce liver fibrosis.

RESULTS

PAR-1 mRNA levels were reduced (>95%) in hepatic stellate cells isolated from PAR-1flox/flox/LRATCre mice. Hepatic stellate cell activation was evident in CCl4-challenged PAR-1flox/flox mice, indicated by increased α-smooth muscle actin labeling and induction of several profibrogenic genes. CCl4-challenged PAR-1flox/flox mice displayed robust hepatic collagen deposition, indicated by picrosirius red staining and type I collagen immunolabeling. Notably, stellate cell activation and collagen deposition were significantly reduced (>30%) in PAR-1flox/flox/LRATCre mice. Importantly, the reduction in liver fibrosis was not a consequence of reduced acute CCl4 hepatotoxicity in PAR-1flox/flox/LRATCre mice.

CONCLUSIONS

The results constitute the first direct experimental evidence that PAR-1 expressed by stellate cells directly promotes their profibrogenic phenotype and hepatic fibrosis in vivo.

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.

Protease-activated receptor signalling initiates α5β1-integrin-mediated adhesion in non-haematopoietic cells

Haematopoietic cells and platelets employ G-protein-coupled receptors (GPCRs) to sense extracellular information and respond by initiating integrin-mediated adhesion. So far, such processes have not been demonstrated in non-haematopoietic cells. Here, we report that the activation of protease-activated receptors PAR1 and PAR2 induce multiple signalling pathways to establish α₅β₁-integrin-mediated adhesion. First, PARs signal via Gβγ and PI3K to α₅β₁-integrins to adopt a talin- and kindlin-dependent high-affinity conformation, which triggers fibronectin binding and initiates cell adhesion. Then, within 60 s, PARs signal via Gα₁₃, Gα_i, ROCK and Src to strengthen the α₅β₁-integrin-mediated adhesion. Furthermore, PAR signalling changes the abundance of numerous proteins in the adhesome assembled by α₅β₁-integrins, including Gα₁₃, vacuolar protein-sorting-associated protein 36, and band 4.1-like protein 4B or 5, and accelerates cell adhesion maturation, spreading and migration. The mechanistic insights describe how agonist binding to PAR employs GPCR and integrin-signalling pathways to initiate and regulate adhesion and to guide physiological responses of non-haematopoietic cells.

Cancer driver G-protein coupled receptor (GPCR) induced β-catenin nuclear localization: the transcriptional junction

G protein-coupled receptors (GPCRs) comprise the main signal-transmitting components in the cell membrane. Over the past several years, biochemical and structural analyses have immensely enhanced our knowledge of GPCR involvement in health and disease states. The present review focuses on GPCRs that are cancer drivers, involved in tumor growth and development. Our aim is to highlight the involvement of stabilized β-catenin molecular machinery with a specific array of GPCRs. We discuss recent advances in understanding the molecular path leading to β-catenin nuclear localization and transcriptional activity and their implications for future cancer therapy research.

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

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

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

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

Protease-Activated Receptors in the Intestine: Focus on Inflammation and Cancer

Protease-activated receptors (PARs) belong to the G protein-coupled receptor (GPCR) family. Compared to other GPCRs, the specificity of the four PARs is the lack of physiologically soluble ligands able to induce their activation. Indeed, PARs are physiologically activated after proteolytic cleavage of their N-terminal domain by proteases. The resulting N-terminal end becomes a tethered activation ligand that interact with the extracellular loop 2 domain and thus induce PAR signal. PARs expression is ubiquitous and these receptors have been largely described in chronic inflammatory diseases and cancer. In this review, after describing their discovery, structure, mechanisms of activation, we then focus on the roles of PARs in the intestine and the two main diseases affecting the organ, namely inflammatory bowel diseases and cancer.

Factor Xa inhibition by rivaroxaban regulates fibrogenesis in human atrial fibroblasts with modulation of nitric oxide synthesis and calcium homeostasis

BACKGROUND

Rivaroxaban, a widely used factor Xa inhibitor in reducing stroke in atrial fibrillation (AF) patients has multiple biological effects with activation of protease-activated receptor (PAR) signaling. Atrial fibrosis plays a critical role in the pathophysiology of AF. In this study, we evaluated whether rivaroxaban regulates atrial fibroblast activity and its underlying mechanisms.

METHODS AND RESULTS

Migration, proliferation analyses, nitric oxide (NO) production assay, calcium fluorescence imaging, and western blots were conducted in human atrial fibroblasts with or without rivaroxaban (100 nmol/L or 300 nmol/L) and co-administration of L-NAME (L-NG-nitro arginine methyl ester, 100 μmol/L), EGTA (Ethylene glycol tetra-acetic acid, 1 mmol/L), thrombin (0.5 U/mL), PAR1 agonist peptide (TFLLR-NH2, 100 μmol/L), PAR1 inhibitor (SCH79797, 0.5 μmol/L) and PAR2 inhibitor (GB83, 10 μmol/L). Atrial fibrosis was examined in isoproterenol (100 mg/kg, subcutaneous injection)-treated rats with or without rivaroxaban (10 mg/kg/day orally for 14 consecutive days). Rivaroxaban reduced the migration, pro-collagen type I production, and proliferation of atrial fibroblasts. Rivaroxaban decreased phosphorylated endothelial NO synthase (eNOS) (Thr 495, an inhibitory phosphorylated site of eNOS), and calcium (Ca²⁺) entry, and increased NO production. Moreover, L-NAME blocked the effects of rivaroxaban on fibroblast collagen and NO production. In the presence of EGTA, the migratory capability was similarly decreased in atrial fibroblasts with and without treatment with rivaroxaban (100 nmol/L), which suggests that rivaroxaban decreases migratory capability of atrial fibroblasts by inhibiting Ca²⁺ entry. Additionally, rivaroxaban significantly attenuated the effects of thrombin, and TFLLR-NH2 on migratory, proliferative, and pro-collagen type I production capability in atrial fibroblasts. SCH79797 or GB83 decreased pro-collagen type I production, migration, and proliferation capability in fibroblasts, but combined SCH79797 or GB83 with and without rivaroxaban had similar fibroblast activity. Moreover, rivaroxaban significantly decreased atrial fibrosis in isoproterenol-treated rats.

CONCLUSIONS

Rivaroxaban (100-300 nmol/L) regulates atrial fibroblast activity and atrial fibrosis by increasing NO production and decreasing Ca²⁺ entry through inhibition of PAR signaling.

Low-density lipoprotein receptor-related protein 6 is a novel coreceptor of protease-activated receptor-2 in the dynamics of cancer-associated β-catenin stabilization

Protease-activated receptor-2 (PAR₂) plays a central role in cancer; however, the molecular machinery of PAR₂-instigated tumors remains to be elucidated. We show that PAR₂ is a potent inducer of β-catenin stabilization, a core process in cancer biology, leading to its transcriptional activity. Novel association of low-density lipoprotein-related protein 6 (LRP6), a known coreceptor of Frizzleds (Fz), with PAR₂ takes place following PAR₂ activation. The association between PAR₂ and LRP6 was demonstrated employing co-immunoprecipitation, bioluminescence resonance energy transfer (BRET), and confocal microscopy analysis. The association was further supported by ZDOCK protein-protein server. PAR₂-LRP6 interaction promotes rapid phosphorylation of LRP6, which results in the recruitment of Axin. Confocal microscopy of PAR₂-driven mammary gland tumors in vivo, as well as in vitro confirms the association between PAR₂ and LRP6. Indeed, shRNA silencing of LRP6 potently inhibits PAR₂-induced β-catenin stabilization, demonstrating its critical role in the induced path. We have previously shown a novel link between protease-activated receptor-1 (PAR₁) and β-catenin stabilization, both in a transgenic (tg) mouse model with overexpression of human PAR₁ (hPar1) in the mammary glands, and in cancer epithelial cell lines. Unlike in PAR₁-G_α13 axis, both G_α12 and G_α13 are equally involved in PAR₂-induced β-catenin stabilization. Disheveled (DVL) is translocated to the cell nucleus through the DVL-PDZ domain. Collectively, our data demonstrate a novel PAR₂-LRP6-Axin interaction as a key axis of PAR₂-induced β-catenin stabilization in cancer. This newly described axis enhances our understanding of cancer biology, and opens new avenues for future development of anti-cancer therapies.

A rapid fluorogenic GPCR-β-arrestin interaction assay

Detection of protein-protein interactions involved in signal transduction in live cells and organisms has a variety of important applications. We report a fluorogenic assay for G protein-coupled receptor (GPCR)-β-arrestin interaction that is genetically encoded, generalizes to multiple GPCRs, and features high signal-to-noise because fluorescence is absent until its components interact upon GPCR activation. Fluorescence after protease-activated receptor-1 activation developed in minutes and required specific serine-threonine residues in the receptor carboxyl tail, consistent with a classical G protein-coupled receptor kinase dependent β-arrestin recruitment mechanism. This assay provides a useful complement to other in vivo assays of GPCR activation.

Coagulation and non-coagulation effects of thrombin

Thrombin is a multifunctional serine protease produced from prothrombin, and is a key regulator in hemostatic and non-hemostatic processes. It is the main effector protease in primary hemostasis by activating platelets, and plays a key role in secondary hemostasis. Besides its well-known functions in hemostasis, thrombin also plays a role in various non-hemostatic biological and pathophysiologic processes, predominantly mediated through activation of protease-activated receptors (PARs). Depending on several factors, such as the concentration of thrombin, the duration of activation, the location of PARs, the presence of coreceptors, and the formation of PAR heterodimers, activation of the receptor by thrombin can induce different cellular responses. Moreover, thrombin can have opposing effects in the same cell; it can induce both inflammatory and anti-inflammatory signals. Owing to the complexity of thrombin's signal transduction pathways, the exact mechanism behind the dichotomy of thrombin is yet still unknown. In this review, we highlight the hemostatic and non-hemostatic functions of thrombin, and specifically focus on the non-hemostatic dual role of thrombin under various conditions and in relation to cardiovascular disease.

Protease-activated receptors (PARs) in cancer: Novel biased signaling and targets for therapy

Despite the fact that G protein-coupled receptors (GPCRs) mediate numerous physiological processes and represent targets for therapeutics for a vast array of diseases, their role in tumor biology is under appreciated. Protease-activated receptors (PARs) form a family which belongs to GPCR class A. PAR1&2 emerge with a central role in epithelial malignancies. Although the part of PAR1&2 in cancer is on the rise, their underlying signaling events are poorly understood. We review hereby past, present, and future cancer-associated PAR biology. Mainly, their role in physiological (placenta-cytotophobalst) and patho-physiological invasion processes. The identification and characterization of signal pleckstrin homology (PH)-domain-binding motifs established critical sites for breast cancer growth in PAR1&2. Among the proteins found to harbor important PH-domains and are involved in PAR biology are Akt/PKB as also Etk/Bmx and Vav3. A point mutation in PAR2, H349A, but not R352A, abrogated PH-protein association and is sufficient to markedly reduce PAR2-instigated breast tumor growth in vivo as also placental extravillous trophoblast (EVT) invasion in vitro is markedly reduced. Similarly, the PAR1 mutant hPar1-7A, which is unable to bind PH-domain, inhibits mammary tumors and EVT invasion, endowing these motifs with physiological significance and underscoring the importance of these previously unknown PAR1 and PAR2 PH-domain-binding motifs in both pathological and physiological invasion processes.

PH motifs in PAR1&2 endow breast cancer growth

Although emerging roles of protease-activated receptor_1&2 (PAR_1&2) in cancer are recognized, their underlying signalling events are poorly understood. Here we show signal-binding motifs in PAR_1&2 that are critical for breast cancer growth. This occurs via the association of the pleckstrin homology (PH) domain with Akt/PKB as a key signalling event of PARs. Other PH-domain signal-proteins such as Etk/Bmx and Vav₃ also associate with PAR₁ and PAR₂ through their PH domains. PAR₁ and PAR₂ bind with priority to Etk/Bmx. A point mutation in PAR₂, H349A, but not in R352A, abrogates PH-protein association and is sufficient to markedly reduce PAR₂-instigated breast tumour growth in vivo and placental extravillous trophoblast (EVT) invasion in vitro. Similarly, the PAR₁ mutant hPar1-7A, which is unable to bind the PH domain, reduces mammary tumours and EVT invasion, endowing these motifs with physiological significance and underscoring the importance of these previously unknown PAR₁ and PAR₂ PH-domain-binding motifs in both pathological and physiological invasion processes.

Protease-activated-receptor 1 and 2 (PAR1 and PAR2) are key players in tumor growth. In this study, the authors identify PAR1 and PAR2 domains that bind oncogenic signalling proteins driving breast cancer progression in vivo and placental extravillous trophoblast invasion in vitro.

Protease Activated Receptors 1 and 2 Correlate Differently with Breast Cancer Aggressiveness Depending on Tumor ER Status

Experimental models implicate protease activated receptors (PARs) as important sensors of the proteolytic tumor microenvironment during breast cancer development. However, the role of the major PARs, PAR-1 and PAR-2, in human breast tumors remains to be elucidated. Here, we have investigated how PAR-1 and PAR-2 protein expression correlate with established clinicopathological variables and patient outcome in a well-characterized cohort of 221 breast cancer patients. Univariable and multivariable hazard ratios (HR) were estimated by the Cox proportional hazards model, distant disease-free survival (DDFS) and overall survival by the Kaplan–Meier method, and survival in different strata was determined by the log-rank test. Associations between PARs and clinicopathological variables were analyzed using Pearson’s χ²-test. We find that PAR-2 associates with DDFS (HR = 3.1, P = 0.003), whereas no such association was found with PAR-1 (HR = 1.2, P = 0.6). Interestingly, the effect of PAR-2 was confined to the ER-positive sub-group (HR = 5.5, P = 0.003 vs. HR = 1.2 in ER-negative; P = 0.045 for differential effect), and PAR-2 was an independent prognostic factor specifically in ER-positive tumors (HR = 3.9, P = 0.045). On the contrary, PAR-1 correlated with worse prognosis specifically in the ER-negative group (HR = 2.6, P = 0.069 vs. HR = 0.5, P = 0.19 in ER-positive; P = 0.026 for differential effect). This study provides novel insight into the respective roles of PAR-1 and PAR-2 in human breast cancer and suggests a hitherto unknown association between PARs and ER signaling that warrants further investigation.

Protease-activated receptor (PAR)-2 is required for PAR-1 signalling in pulmonary fibrosis

Idiopathic pulmonary fibrosis is the most devastating diffuse fibrosing lung disease of unknown aetiology. Compelling evidence suggests that both protease-activated receptor (PAR)-1 and PAR-2 participate in the development of pulmonary fibrosis. Previous studies have shown that bleomycin-induced lung fibrosis is diminished in both PAR-1 and PAR-2 deficient mice. We thus have been suggested that combined inactivation of PAR-1 and PAR-2 would be more effective in blocking pulmonary fibrosis. Human and murine fibroblasts were stimulated with PAR-1 and PAR-2 agonists in the absence or presence of specific PAR-1 or PAR-2 antagonists after which fibrotic markers like collagen and smooth muscle actin were analysed by Western blot. Pulmonary fibrosis was induced by intranasal instillation of bleomycin into wild-type and PAR-2 deficient mice with or without a specific PAR-1 antagonist (P1pal-12). Fibrosis was assessed by hydroxyproline quantification and (immuno)histochemical analysis. We show that specific PAR-1 and/or PAR-2 activating proteases induce fibroblast migration, differentiation and extracellular matrix production. Interestingly, however, combined activation of PAR-1 and PAR-2 did not show any additive effects on these pro-fibrotic responses. Strikingly, PAR-2 deficiency as well as pharmacological PAR-1 inhibition reduced bleomycin-induced pulmonary fibrosis to a similar extent. PAR-1 inhibition in PAR-2 deficient mice did not further diminish bleomycin-induced pulmonary fibrosis. Finally, we show that the PAR-1-dependent pro-fibrotic responses are inhibited by the PAR-2 specific antagonist. Targeting PAR-1 and PAR-2 simultaneously is not superior to targeting either receptor alone in bleomycin-induced pulmonary fibrosis. We postulate that the pro-fibrotic effects of PAR-1 require the presence of PAR-2.

A highly potent agonist to protease-activated receptor-2 reveals apical activation of the airway epithelium resulting in Ca2+-regulated ion conductance

The airway epithelium provides a barrier that separates inhaled air and its various particulates from the underlying tissues. It provides key physiological functions in both sensing the environment and initiating appropriate innate immune defenses to protect the lung. Protease-activated receptor-2 (PAR2) is expressed both apically and basolaterally throughout the airway epithelium. One consequence of basolateral PAR2 activation is the rapid, Ca(2+)-dependent ion flux that favors secretion in the normally absorptive airway epithelium. However, roles for apically expressed PAR2 activation have not been demonstrated, in part due to the lack of specific, high-potency PAR2 ligands. In the present study, we used the newly developed PAR2 ligand 2at-LIGRLO(PEG3-Pam)-NH2 in combination with well-differentiated, primary cultured airway epithelial cells from wild-type and PAR2 (-/-) mice to examine the physiological role of PAR2 in the conducting airway after apical activation. Using digital imaging microscopy of intracellular Ca(2+) concentration changes, we verified ligand potency on PAR2 in primary cultured airway cells. Examination of airway epithelial tissue in an Ussing chamber showed that apical activation of PAR2 by 2at-LIGRLO(PEG3-Pam)-NH2 resulted in a transient decrease in transepithelial resistance that was due to increased apical ion efflux. We determined pharmacologically that this increase in ion conductance was through Ca(2+)-activated Cl(-) and large-conductance K(+) channels that were blocked with a Ca(2+)-activated Cl(-) channel inhibitor and clotrimazole, respectively. Stimulation of Cl(-) efflux via PAR2 activation at the airway epithelial surface can increase airway surface liquid that would aid in clearing the airway of noxious inhaled agents.

Noncanonical PAR3 activation by factor Xa identifies a novel pathway for Tie2 activation and stabilization of vascular integrity

Endothelial barrier protective effects of activated protein C (APC) require the endothelial protein C receptor (EPCR), protease-activated receptor (PAR) 1, and PAR3. In contrast, PAR1 and PAR3 activation by thrombin results in barrier disruption. Noncanonical PAR1 and PAR3 activation by APC vs canonical activation by thrombin provides an explanation for the functional selectivity of these proteases. Here we found that factor Xa (FXa) activated PAR1 at canonical Arg41 similar to thrombin but cleaved PAR3 at noncanonical Arg41 similar to APC. This unique PAR1-PAR3 activation profile permitted the identification of noncanonical PAR3 activation as a novel activation pathway for barrier protective tunica intima endothelial receptor tyrosine kinase 2 (Tie2). APC, FXa, and the noncanonical PAR3 tethered-ligand peptide induced prolonged activation of Tie2, whereas thrombin and the canonical PAR3 tethered-ligand peptide did not. Tie2 activation by FXa required PAR3 and EPCR. FXa and the noncanonical PAR3 tethered-ligand peptide induced Tie2- and PAR3-dependent upregulation of tight-junction-associated protein zona occludens 1 (ZO-1), translocation of ZO-1 to cell-cell borders, and the formation of typical ZO-1 honeycomb patterns that are indicative of tight-junction stabilization. These data provide intriguing novel insights into the diversification of functional selectivity of protease signaling achievable by canonical and noncanonical PAR activation, such as the activation of vascular-protective Tie2 by noncanonical PAR3 activation.

Protease-activated-receptor-2 affects protease-activated-receptor-1-driven breast cancer

Mammalian protease-activated-receptor-1 and -2 (PAR1 and PAR2) are activated by proteases found in the flexible microenvironment of a tumor and play a central role in breast cancer. We propose in the present study that PAR1 and PAR2 act together as a functional unit during malignant and physiological invasion processes. This notion is supported by assessing pro-tumor functions in the presence of short hairpin; shRNA knocked-down hPar2 or by the use of a truncated PAR2 devoid of the entire cytoplasmic tail. Silencing of hPar2 by shRNA-attenuated thrombin induced PAR1 signaling as recapitulated by inhibiting the assembly of Etk/Bmx or Akt onto PAR1-C-tail, by thrombin-instigated colony formation and invasion. Strikingly, shRNA-hPar2 also inhibited the TFLLRN selective PAR1 pro-tumor functions. In addition, while evaluating the physiological invasion process of placenta extravillous trophoblast (EVT) organ culture, we observed inhibition of both thrombin or the selective PAR1 ligand; TFLLRNPNDK induced EVT invasion by shRNA-hPar2 but not by scrambled shRNA-hPar2. In parallel, when a truncated PAR2 was utilized in a xenograft mouse model, it inhibited PAR1-PAR2-driven tumor growth in vivo. Similarly, it also attenuated the interaction of Etk/Bmx with the PAR1-C-tail in vitro and decreased markedly selective PAR1-induced Matrigel invasion. Confocal images demonstrated co-localization of PAR1 and PAR2 in HEK293T cells over-expressing YFP-hPar2 and HA-hPar1. Co-immuno-precipitation analyses revealed PAR1-PAR2 complex formation but no PAR1-CXCR4 complex was formed. Taken together, our observations show that PAR1 and PAR2 act as a functional unit in tumor development and placenta-uterus interactions. This conclusion may have significant consequences on future breast cancer therapeutic modalities and improved late pregnancy outcome.

Development and evaluation of small peptidomimetic ligands to protease-activated receptor-2 (PAR2) through the use of lipid tethering

Protease-activated receptor-2 (PAR2) is a G-Protein Coupled Receptor (GPCR) activated by proteolytic cleavage to expose an attached, tethered ligand (SLIGRL). We evaluated the ability for lipid-tethered-peptidomimetics to activate PAR2 with in vitro physiological and Ca2+ signaling assays to determine minimal components necessary for potent, specific and full PAR2 activation. A known PAR2 activating compound containing a hexadecyl (Hdc) lipid via three polyethylene glycol (PEG) linkers (2at-LIGRL-PEG3-Hdc) provided a potent agonist starting point (physiological EC50 = 1.4 nM; 95% CI: 1.2-2.3 nM). In a set of truncated analogs, 2at-LIGR-PEG3-Hdc retained potency (EC50 = 2.1 nM; 1.3-3.4 nM) with improved selectivity for PAR2 over Mas1 related G-protein coupled receptor type C11, a GPCR that can be activated by the PAR2 peptide agonist, SLIGRL-NH2. 2at-LIG-PEG3-Hdc was the smallest full PAR2 agonist, albeit with a reduced EC50 (46 nM; 20-100 nM). 2at-LI-PEG3-Hdc retained specific activity for PAR2 with reduced EC50 (310 nM; 260-360 nM) but displayed partial PAR2 activation in both physiological and Ca2+ signaling assays. Further truncation (2at-L-PEG3-Hdc and 2at-PEG3-Hdc) eliminated in vitro activity. When used in vivo, full and partial PAR2 in vitro agonists evoked mechanical hypersensitivity at a 15 pmole dose while 2at-L-PEG3-Hdc lacked efficacy. Minimum peptidomimetic PAR2 agonists were developed with known heterocycle substitutes for Ser1 (isoxazole or aminothiazoyl) and cyclohexylalanine (Cha) as a substitute for Leu2. Both heterocycle-tetrapeptide and heterocycle-dipeptides displayed PAR2 specificity, however, only the heterocycle-tetrapeptides displayed full PAR2 agonism. Using the lipid-tethered-peptidomimetic approach we have developed novel structure activity relationships for PAR2 that allows for selective probing of PAR2 function across a broad range of physiological systems.

Differential signaling by protease-activated receptors: implications for therapeutic targeting

Protease-activated receptors (PARs) are a family of four G protein-coupled receptors that exhibit increasingly appreciated differences in signaling and regulation both within and between the receptor class. By nature of their proteolytic self-activation mechanism, PARs have unique processes of receptor activation, "ligand" binding, and desensitization/resensitization. These distinctive aspects have presented both challenges and opportunities in the targeting of PARs for therapeutic benefit-the most notable example of which is inhibition of PAR1 on platelets for the prevention of arterial thrombosis. However, more recent studies have uncovered further distinguishing features of PAR-mediated signaling, revealing mechanisms by which identical proteases elicit distinct effects in the same cell, as well as how distinct proteases produce different cellular consequences via the same receptor. Here we review this differential signaling by PARs, highlight how important distinctions between PAR1 and PAR4 are impacting on the progress of a new class of anti-thrombotic drugs, and discuss how these more recent insights into PAR signaling may present further opportunities for manipulating PAR activation and signaling in the development of novel therapies.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Cofactoring and dimerization of proteinase-activated receptors

Proteinase-activated receptors (PARs) are G protein-coupled receptors that transmit cellular responses to extracellular proteases and have important functions in vascular physiology, development, inflammation, and cancer progression. The established paradigm for PAR activation involves proteolytic cleavage of the extracellular N terminus, which reveals a new N terminus that functions as a tethered ligand by binding intramolecularly to the receptor to trigger transmembrane signaling. Most cells express more than one PAR, which can influence the mode of PAR activation and signaling. Clear examples include murine PAR3 cofactoring of PAR4 and transactivation of PAR2 by PAR1. Thrombin binds to and cleaves murine PAR3, which facilitates PAR4 cleavage and activation. This process is essential for thrombin signaling and platelet activation, since murine PAR3 cannot signal alone. Although PAR1 and PAR4 are both competent to signal, PAR1 is able to act as a cofactor for PAR4, facilitating more rapid cleavage and activation by thrombin. PAR1 can also facilitate PAR2 activation through a different mechanism. Cleavage of the PAR1 N terminus by thrombin generates a tethered ligand domain that can bind intermolecularly to PAR2 to activate signaling. Thus, PARs can regulate each other's activity by localizing thrombin when in complex with PAR3 and PAR4 or by cleaved PAR1, providing its tethered ligand domain for PAR2 activation. The ability of PARs to cofactor or transactivate other PARs would necessitate that the two receptors be in close proximity, likely in the form of a heterodimer. Here, we discuss the cofactoring and dimerization of PARs and the functional consequences on signaling.

Transactivation of the PAR1-PAR2 heterodimer by thrombin elicits β-arrestin-mediated endosomal signaling

Thrombin cleaves the N terminus of PAR1, generating a new N-terminal domain that functions as a tethered ligand that binds intermolecularly to activate PAR2 in trans. The mechanisms that regulate PAR1-PAR2 heterodimer signaling and trafficking are not known. We now report that PAR1 and PAR2 form a heterodimer that exhibits unique trafficking and signaling behaviors compared with receptor protomers. Using bioluminescence resonance energy transfer, immunofluorescence microscopy, co-immunoprecipitation, and cells expressing receptors exogenously and endogenously, we show that PAR1 and PAR2 specifically interact and form stable dimers. Intriguingly, the PAR1-PAR2 heterodimer displays constitutive internalization that is driven by PAR1 C-terminal tail sorting motifs and is a process that enhances dimer formation. Upon thrombin activation, PAR1-PAR2 dimers co-internalize and recruit β-arrestins to endosomes. Remarkably, PAR1-PAR2 heterodimers appear to utilize a distinct interface for β-arrestin interaction compared with receptor protomers. Moreover, thrombin-activated PAR1-PAR2 heterodimers enhance β-arrestin-mediated ERK1/2 activation in the cytoplasm, whereas activated ERK1/2 induced by the thrombin-activated PAR1 protomer redistributes to the nucleus. Thus, the formation of PAR1-PAR2 heterodimers provides additional modes of thrombin-stimulated signaling responses that appear to be distinctly regulated compared with the receptor protomer.

Expression, function and cooperating partners of protease-activated receptor type 3 in vascular endothelial cells and B lymphocytes studied with specific monoclonal antibody

Receptor-specific antibodies can both prevent ligand-receptor interaction and initiate receptor signaling. Previously we generated monoclonal antibody 8E8 (mAb 8E8) against protease-activated receptor type 3 (PAR3) which inhibited proliferation of B cell hybridoma. Here we used mAb 8E8 and PAR1-specific polyclonal antibody to reveal the functions and cooperating partners of PAR3 in endothelial cells and in B lymphocytes. MAb 8E8 or PAR1 agonist peptide stimulated IL-6 and IL-8 production and VCAM-1 expression in HPMEC-ST1.6R cells. PAR1 antibody stimulated only VCAM-1 expression, while ICAM-1 expression was stimulated with mAB 8E8 or PAR3 peptide. MAb 8E8 stimulated weak mitogenic response, while PAR1 antibody inhibited it in normal but not in malignant B lymphocytes. Sandwich ELISA assay demonstrated the interaction of PAR3 with PAR1 in malignant cell lines and with IgM in normal B lymphocytes. It is concluded that PAR3 cooperates with PAR1 to mediate the effect of thrombin on cytokine production and VCAM-1 expression in endothelial cells and on cell proliferation in malignant B cells. ICAM-1 expression in endothelial cells requires PAR3 without PAR1. The inhibitory effect of thrombin in normal B lymphocytes is mediated by PAR1 alone, while mitogenic and pro-survival signaling in B lymphocytes is provided through PAR3 in cooperation with BCR.

Protease-activated receptor-2 modulates protease-activated receptor-1-driven neointimal hyperplasia

OBJECTIVE

Emerging evidence suggests that protease-activated receptors-1 and -2 (PAR1 and PAR2) can signal together in response to proteases found in the rapidly changing microenvironment of damaged blood vessels. However, it is unknown whether PAR1 and PAR2 promote or mitigate the hyperplastic response to arterial injury. Using cell-penetrating PAR1 pepducins and mice deficient in PAR1 or PAR2, we set out to determine the respective contributions of the receptors to hyperplasia and phenotypic modulation of smooth muscle cells (SMCs) in response to arterial injury.

METHODS AND RESULTS

SMCs were strongly activated by PAR1 stimulation, as evidenced by increased mitogenesis, mitochondrial activity, and calcium mobilization. The effects of chronic PAR1 stimulation following vascular injury were studied by performing carotid artery ligations in mice treated with the PAR1 agonist pepducin, P1pal-13. Histological analysis revealed that PAR1 stimulation caused striking hyperplasia, which was ablated in PAR1(-/-) and, surprisingly, PAR2(-/-) mice. P1pal-13 treatment yielded an expression pattern consistent with a dedifferentiated phenotype in carotid artery SMCs. Detection of PAR1-PAR2 complexes provided an explanation for the hyperplastic effects of the PAR1 agonist requiring the presence of both receptors.

CONCLUSIONS

We conclude that PAR2 regulates the PAR1 hyperplastic response to arterial injury leading to stenosis.

Potent agonists of the protease activated receptor 2 (PAR2)

Novel peptidomimetic pharmacophores to PAR(2) were designed based on the known activating peptide SLIGRL-NH(2). A set of 15 analogues was evaluated with a model cell line (16HBE14o-) that highly expresses PAR(2). Cells exposed to the PAR(2) activating peptide with N-terminal 2-furoyl modification (2-furoyl-LIGRLO-NH(2)) initiated increases in intracellular calcium concentration ([Ca(2+)](i) EC(50) = 0.84 μM) and in vitro physiological responses as measured by the xCELLigence real time cell analyzer (RTCA EC(50) = 138 nM). We discovered two selective PAR(2) agonists with comparable potency: compound 1 (2-aminothiazol-4-yl; Ca(2+) EC(50) = 1.77 μM, RTCA EC(50) = 142 nM) and compound 2 (6-aminonicotinyl; Ca(2+) EC(50) = 2.60 μM, RTCA EC(50) = 311 nM). Unlike the previously described agonist, these novel agonists are devoid of the metabolically unstable 2-furoyl modification and thus provide potential advantages for PAR(2) peptide design for in vitro and in vivo studies. The novel compounds described herein also serve as a starting point for structure-activity relationship (SAR) design and are, for the first time, evaluated via a unique high throughput in vitro physiological assay. Together these will lead to discovery of more potent agonists and antagonists of PAR(2).

The coagulation system contributes to alphaVbeta6 integrin expression and liver fibrosis induced by cholestasis

Chronic injury to intrahepatic bile duct epithelial cells (BDECs) elicits expression of various mediators, including the αVβ6 integrin, promoting liver fibrosis. We tested the hypothesis that tissue factor (TF)-dependent thrombin generation and protease activated receptor-1 (PAR-1) activation contribute to liver fibrosis induced by cholestasis via induction of αVβ6 expression. To test this hypothesis, mice deficient in either TF or PAR-1 were fed a diet containing 0.025% α-naphthylisothiocyanate (ANIT), a BDEC-selective toxicant. In genetically modified mice with a 50% reduction in liver TF activity fed an ANIT diet, coagulation cascade activation and liver fibrosis were reduced. Similarly, liver fibrosis was significantly reduced in PAR-1(-/-) mice fed an ANIT diet. Hepatic integrin β6 mRNA induction, expression of αVβ6 protein by intrahepatic BDECs, and SMAD2 phosphorylation were reduced by TF deficiency and PAR-1 deficiency in mice fed the ANIT diet. Treatment with either an anti-αVβ6 blocking antibody or soluble transforming growth factor-β receptor type II reduced liver fibrosis in mice fed the ANIT diet. PAR-1 activation enhanced transforming growth factor-β1-induced integrin β6 mRNA expression in both transformed human BDECs and primary rat BDECs. Interestingly, TF and PAR-1 mRNA levels were increased in livers from patients with cholestatic liver disease. These results indicate that a TF-PAR-1 pathway contributes to liver fibrosis induced by chronic cholestasis by increasing expression of the αVβ6 integrin, an important regulator of transforming growth factor-β1 activation.

Influenza virus-cytokine-protease cycle in the pathogenesis of vascular hyperpermeability in severe influenza

Background. Severe influenza is characterized by cytokine storm and multiorgan failure with edema. The aim of this study was to define the impact of the cytokine storm on the pathogenesis of vascular hyperpermeability in severe influenza.

Methods. Weanling mice were infected with influenza A WSN/33(H1N1) virus. The levels of proinflammatory cytokines, tumor necrosis factor (TNF) α, interleukin (IL) 6, IL-1β, and trypsin were analyzed in the lung, brain, heart, and cultured human umbilical vein endothelial cells. The effects of transcriptional inhibitors on cytokine and trypsin expressions and viral replication were determined.

Results. Influenza A virus infection resulted in significant increases in TNF-α, IL-6, IL-1β, viral hemagglutininprocessing protease trypsin levels, and viral replication with vascular hyperpermeability in lung and brain in the first 6 days of infection. Trypsin upregulation was suppressed by transcriptional inhibition of cytokines in vivo and by anti-cytokine antibodies in endothelial cells. Calcium mobilization and loss of tight junction constituent, zonula occludens-1, associated with cytokine- and trypsin-induced endothelial hyperpermeability were inhibited by a protease-activated receptor-2 antagonist and a trypsin inhibitor.

Conclusions. The influenza virus-cytokine-protease cycle is one of the key mechanisms of vascular hyperpermeability in severe influenza.

Factor X/Xa elicits protective signaling responses in endothelial cells directly via PAR-2 and indirectly via endothelial protein C receptor-dependent recruitment of PAR-1

We recently demonstrated that the Gla domain-dependent interaction of protein C with endothelial protein C receptor (EPCR) leads to dissociation of the receptor from caveolin-1 and recruitment of PAR-1 to a protective signaling pathway. Thus, the activation of PAR-1 by either thrombin or PAR-1 agonist peptide elicited a barrier-protective response if endothelial cells were preincubated with protein C. In this study, we examined whether other vitamin K-dependent coagulation protease zymogens can modulate PAR-dependent signaling responses in endothelial cells. We discovered that the activation of both PAR-1 and PAR-2 in endothelial cells pretreated with factor FX (FX)-S195A, but not other procoagulant protease zymogens, also results in initiation of protective intracellular responses. Interestingly, similar to protein C, FX interaction with endothelial cells leads to dissociation of EPCR from caveolin-1 and recruitment of PAR-1 to a protective pathway. Further studies revealed that, FX activated by factor VIIa on tissue factor bearing endothelial cells also initiates protective signaling responses through the activation of PAR-2 independent of EPCR mobilization. All results could be recapitulated by the receptor agonist peptides to both PAR-1 and PAR-2. These results suggest that a cross-talk between EPCR and an unknown FX/FXa receptor, which does not require interaction with the Gla domain of FX, recruits PAR-1 to protective signaling pathways in endothelial cells.

Tissue Factor-Factor VIIa complex induces cytokine expression in coronary artery smooth muscle cells

OBJECTIVE

Within atherosclerotic lesions Tissue Factor (TF)-Factor VIIa (FVIIa) not only contributes to thrombotic events but also alters vascular remodeling through enhancement of migration. Moreover, the TF-FVIIa-FXa complex activates protease-activated receptors (PAR). TF/FVIIa/PAR-2 signaling has also been shown to promote proliferation and metastasis of tumor cells. Since coagulation factors promote inflammation which plays a major role during atherosclerosis as well as tumor metastasis this study sought to investigate the effects of FVIIa on the inflammatory response in vascular cells.

METHODS/RESULTS

FVIIa induces interleukin-8 (IL-8) and IL-6 in primary smooth muscle cells (SMC), which was correlated to the expression of TF and PAR-2 as shown by immunoassay and qRT-PCR. The effect was dose-dependent and required TF, the proteolytic activity of FVIIa and PAR-2. Secondary effects of downstream coagulation factors were excluded. No proinflammatory FVIIa effect was observed in endothelial cells (EC) and mononuclear cells (MNC), expressing either TF or PAR-2. In atherosclerotic lesions mRNA expression of PAR-1, PAR-2 and IL-8 was elevated compared to healthy vessels indicating a role for PAR-1 and PAR-2 signaling in atherosclerosis.

CONCLUSION

In addition to the procoagulant and promigratory role of the TF-FVIIa complex we identify a proinflammatory role of FVIIa in human SMC dependent on expression of TF and PAR-2 that provides yet another link between coagulation and inflammation.

The coagulation factor Xa/protease activated receptor-2 axis in the progression of liver fibrosis: a multifaceted paradigm

Hepatic fibrosis is a common response to virtually all forms of chronic liver injury independent of the etiologic agent. Despite the relatively large population of patients suffering from hepatic fibrosis and cirrhosis, no efficient and well-tolerated drugs are available for the treatment of this disorder. The lack of efficient treatment options is at least partly because the underlying cellular mechanisms leading to hepatic fibrosis are only partly understood. It is thus of pivotal importance to better understand the cellular processes contributing to the progression of hepatic fibrosis. Interestingly in this perspective, a common feature of fibrotic disease of various organs is the activation of the coagulation cascade and hepatic fibrosis is also accompanied by a local hypercoagulable state. Activated blood coagulation factors directly target liver cells by activating protease-activated receptors (PAR) thereby inducing a plethora of cellular responses like (among others) proliferation, migration and extracellular matrix production. Coagulation factor driven PAR activation thus establishes a potential link between activation of the coagulation cascade and the progression of fibrosis. The current review focuses on blood coagulation factor Xa and summarizes the variety of cellular functions induced by factor Xa-driven PAR-2 activation and the subsequent consequences for tissue repair and hepatic fibrosis.