Inhibition of thrombin and SFLLR-peptide stimulation of platelet aggregation, phospholipase A2 and Na+/H+ exchange by a thrombin receptor antagonist

Manipulation of tissue factor-mediated basal PAR-2 signalling on macrophages determines sensitivity for IFNγ responsiveness and significantly modifies the phenotype of murine DTH

Background

Tissue factor (TF) generates proteases that can signal through PAR-1 and PAR-2. We have previously demonstrated PAR-1 signalling primes innate myeloid cells to be exquisitely sensitive to interferon-gamma (IFNγ). In this work we explored how TF mediated PAR-2 signalling modulated responsiveness to IFNγ and investigated the interplay between PAR-1/-2 signalling on macrophages.

Methodology

We characterised how TF through PAR-2 influenced IFNγ sensitivity in vitro using PCR and flow cytometry. and how it influenced oxazolone-induced delayed type hypersensitivity (DTH) responses in vivo. We investigated how basal signalling through PAR-2 influenced PAR-1 signalling using a combination of TF-inhibitors and PAR-1 &-2 agonists and antagonists. Finally, we investigated whether this system could be targeted therapeutically using 3-mercaptopropionyl-F-Cha-Cha-RKPNDK (3-MP), which has actions on both PAR-1 and -2.

Results

TF delivered a basal signal through PAR-2 that upregulated SOCS3 expression and blunted M1 polarisation after IFNγ stimulation, opposing the priming achieved by signalling through PAR-1. PAR-1 and -2 agonists or antagonists could be used in combination to modify this basal signal in vitro and in vivo. 3-MP, by virtue of its PAR-2 agonist properties was superior to agents with only PAR-1 antagonist properties at reducing M1 polarisation induced by IFNγ and suppressing DTH. Tethering a myristoyl electrostatic switch almost completely abolished the DTH response.

Conclusions

TF-mediated signalling through PARs-1 and -2 act in a homeostatic way to determine how myeloid cells respond to IFNγ. 3-MP, an agent that simultaneously inhibits PAR-1 whilst delivering a PAR-2 signal, can almost completely abolish immune responses dependent on M1 polarisation, particularly if potency is enhanced by targeting to cell membranes; this has potential therapeutic potential in multiple diseases.

Aprotinin and the protease-activated receptor 1 thrombin receptor: antithrombosis, inflammation, and stroke reduction

Cardiopulmonary bypass, although remaining an indispensable asset in cardiac surgery, especially in more complex and repeat operations, is associated with significant thrombin generation in the bypass circuit, leading to the activation of platelets, the coagulation system, an inflammatory response, and perioperative stroke. Recent clinical studies and meta-analyses of clinical trials in coronary artery bypass grafting surgery have confirmed that aprotinin not only reduces transfusion requirements in cardiac surgery but also confers significant protection against platelet dysfunction, activation of the systemic inflammatory response, and perioperative stroke when administered at the full (or "Hammersmith") dose. This article reviews research from several independent groups to propose a novel mechanism through which the antithrombotic, anti-inflammatory, and neuroprotective mechanism might be mediated, via protection of the high-affinity thrombin receptor protease-activated receptor 1 (PAR1).

Thrombin induces expression of cytokine-induced SH2 protein (CIS) in rat brain astrocytes: involvement of phospholipase A2, cyclooxygenase, and lipoxygenase

Previously we have reported that thrombin induces inflammatory mediators in brain glial cells (Ryu et al. 2000. J Biol Chem 275:29955). In the present study, we found that thrombin induced a negative regulator of a cytokine signaling molecule, cytokine-induced SH2 protein (CIS), in rat brain astrocytes. In response to thrombin, CIS expression was increased at both the mRNA and protein levels. Although STAT5 is known to regulate CIS expression, thrombin did not activate STAT5, and inhibitors of JAK2 (AG490) and JAK3 (WHI-P97 and WHI-P154) had little effect on thrombin-induced CIS expression. In contrast, cytosolic phospholipase A(2) (cPLA(2)), cyclooxygenase (COX), and lipoxygenase (LO) play a role in CIS expression, since inhibitors of cPLA(2), cyclooxygenase (COX), and LO significantly reduced CIS expression. Reactive oxygen species (ROS) scavengers (N-acetyl-cysteine [NAC] and trolox) reduced thrombin-induced CIS expression, and inhibitors of COX and LO reduced ROS produced by thrombin. Furthermore, prostaglandin E(2) (PGE(2)) and leukotriene B(4) (LTB(4)), products of COX and LO, respectively, potentiated thrombin-induced CIS expression, indicating that ROS, and PGE(2) and LTB(4) generated by COX and LO, mediate CIS expression. Since interferon-gamma (IFN-gamma)-induced GAS-luciferase activity and tyrosine phosphorylation of STAT1 and STAT3 were lower in CIS-transfected cells compared to control vector-transfected cells, CIS could have anti-inflammatory activity. These data suggest that thrombin-stimulation of ROS and prostaglandin and leukotriene production via the cPLA(2), COX and LO pathways results in CIS expression. More importantly, CIS expression may be a negative feedback mechanism that prevents prolonged inflammatory responses.

Design and Synthesis of Novel Biologically Active Thrombin Receptor Non-Peptide Mimetics Based on the Pharmacophoric Cluster Phe/Arg/NH2 of the Ser42-Phe-Leu-Leu-Arg46 Motif Sequence: Platelet Aggregation and Relaxant Activities

The identification of the thrombin receptor has promoted the interest for the development of new therapeutic agents capable of selectively inhibiting unwanted biological effects of thrombin on various cell types. In this study we have designed and synthesized two series of new thrombin receptor antagonists based on the thrombin receptor motif sequence S42FLLR46, one possessing two (Phe/Arg) pharmacophoric groups and the other possessing three (Phe/Arg/NH2). N-(6-Guanidohexanoyl)-N'-(phenylacetyl)piperazine (1), N-(phenylacetyl)-4-(6-guanidohexanoylamidomethyl)piperidine (2), and N-(phenylacetyl)-3-(6-guanidohexanoylamido)pyrrolidine (3) (group A) carry the two pharmacophoric side chains of Phe and Arg residues incorporated on three different templates (piperazine, 4-aminomethylpiperidine, and 3-aminopyrrolidine). Compounds with three pharmacophoric groups (group B) were built similarly to group A using the same templates with the addition of an extra methylamino group leading to (S)-N-(6-guanidohexanoyl)-N'-(2-amino-3-phenylpropionyl)piperazine (4), (S)-N-(2-amino-3-phenylpropionyl)-4-(6-guanidohexanoylamidomethyl)piperidine (5), and (S)-N-(2-amino-3-phenylpropionyl)-3-(6-guanidohexanoylamido)pyrrolidine (6). Compounds were able to inhibit thrombin-induced human platelet activation even at low concentrations. In particular, among compounds in group A, compound 3 was found to be the most powerful thrombin receptor activation inhibitor, showing an IC50 of approximately 0.11 mM on platelet aggregation assay. Among compounds in group B, compound 4 was the most powerful to inhibit thrombin-induced platelet aggregation, showing an IC50 of approximately 0.09 mM. All compounds were also found to act as agonists in the rat aorta relaxation assay. Interestingly, the order of potency of these compounds as agonists of the endothelial thrombin receptor was the inverse of the order of potency of the same compounds as antagonists of the platelet thrombin receptor. Such compounds that are causing vasodilation while simultaneously inhibiting platelet aggregation would be very useful in preventing the installation of atherosclerotic lesions and deserve further investigation as potential drugs for treating cardiovascular diseases. The above findings coupled with computational analysis molecular dynamics experiments support also our hypothesis that a cluster of phenyl, guanidino, and amino groups is responsible for thrombin receptor triggering and activation.

Protease-activated receptors: contribution to physiology and disease

Proteases acting at the surface of cells generate and destroy receptor agonists and activate and inactivate receptors, thereby making a vitally important contribution to signal transduction. Certain serine proteases that derive from the circulation (e.g., coagulation factors), inflammatory cells (e.g., mast cell and neutrophil proteases), and from multiple other sources (e.g., epithelial cells, neurons, bacteria, fungi) can cleave protease-activated receptors (PARs), a family of four G protein-coupled receptors. Cleavage within the extracellular amino terminus exposes a tethered ligand domain, which binds to and activates the receptors to initiate multiple signaling cascades. Despite this irreversible mechanism of activation, signaling by PARs is efficiently terminated by receptor desensitization (receptor phosphorylation and uncoupling from G proteins) and downregulation (receptor degradation by cell-surface and lysosomal proteases). Protease signaling in tissues depends on the generation and release of proteases, availability of cofactors, presence of protease inhibitors, and activation and inactivation of PARs. Many proteases that activate PARs are produced during tissue damage, and PARs make important contributions to tissue responses to injury, including hemostasis, repair, cell survival, inflammation, and pain. Drugs that mimic or interfere with these processes are attractive therapies: selective agonists of PARs may facilitate healing, repair, and protection, whereas protease inhibitors and PAR antagonists can impede exacerbated inflammation and pain. Major future challenges will be to understand the role of proteases and PARs in physiological control mechanisms and human diseases and to develop selective agonists and antagonists that can be used to probe function and treat disease.

Proteinase-activated receptors in the nervous system

Recent data point to important roles for proteinases and their cognate proteinase-activated receptors (PARs) in the ontogeny and pathophysiology of the nervous system. PARs are a family of G-protein-coupled receptors that can affect neural cell proliferation, morphology and physiology. PARs also have important roles in neuroinflammatory and degenerative diseases such as human immunodeficiency virus-associated dementia, Alzheimer's disease and pain. These receptors might also influence the pathogenesis of stroke and multiple sclerosis, conditions in which the blood-brain barrier is disrupted. The diversity of effects of PARs on neural function and their widespread distribution in the nervous system make them attractive therapeutic targets for neurological disorders. Here, we review the roles of PARs in the central and peripheral nervous systems during health and disease, with a focus on neuroinflammatory and degenerative disorders.

Role of protease-activated receptors in airway function: a target for therapeutic intervention?

Protease-activated receptors (PARs) are G-protein-coupled, seven transmembrane domain receptors that act as cellular enzyme sensors. These receptors are activated by the proteolytic cleavage at the amino terminus, enabling interaction between the newly formed "tethered ligand" and the second extracellular loop of the receptor to confer cellular signalling. PARs can also be activated by small peptides that mimic the tethered ligand. In the respiratory tract, PARs may be regulated by endogenous proteases, such as airway trypsin and mast cell tryptase, as well as exogenous proteases, including inhaled aeroallergens such as those from house dust mite faecal pellets. Immunoreactive PARs have been identified in multiple cell types of the respiratory tract, and PAR activation has been reported to stimulate cellular mitogenesis and to promote tissue inflammation. Activation of PARs concurrently stimulates the release of bronchorelaxant and anti-inflammatory mediators, which may serve to induce cytoprotection and to minimise tissue trauma associated with severe chronic airways inflammation. Furthermore, airway inflammatory responses are associated with increased epithelial PAR expression and elevated concentrations of PAR-activating, and PAR-inactivating, proteases in the extracellular space. On this basis, PARs are likely to play a regulatory role in airway homeostasis, and may participate in respiratory inflammatory disorders, such as asthma and chronic obstructive pulmonary disease. Further studies focussing on the effects of newly developed PAR agonists and antagonists in appropriate models of airway inflammation will permit better insight into the role of PARs in respiratory pathophysiology and their potential as therapeutic targets.

Thrombin receptor (PAR-1) antagonists. Solid-phase synthesis of indole-based peptide mimetics by anchoring to a secondary amide

A novel, 10-step, solid-phase method, based on a secondary amide linker, was developed to construct a diverse library of indole-based SFLLR peptide mimetics as thrombin receptor (protease-activated receptor 1, PAR-1) antagonists. The key steps include stepwise reductive alkylation, urea formation, and Mannich reaction. Screening of the library led to a quick development of the SAR and the significant improvement of PAR-1 activity.

Protease activated receptors: theme and variations

The four PAR family members are G protein coupled receptors that are normally activated by proteolytic exposure of an occult tethered ligand. Three of the family members are thrombin receptors. The fourth (PAR2) is not activated by thrombin, but can be activated by other proteases, including trypsin, tryptase and Factor Xa. This review focuses on recent information about the manner in which signaling through these receptors is initiated and terminated, including evidence for inter- as well as intramolecular modes of activation, and continuing efforts to identify additional, biologically-relevant proteases that can activate PAR family members.

Extracellular mutations of protease-activated receptor-1 result in differential activation by thrombin and thrombin receptor agonist peptide

The protease-activated thrombin receptor-1 (PAR-1) can be activated by both the tethered ligand exposed by thrombin cleavage and a synthetic peptide having the tethered ligand sequence (thrombin receptor agonist peptide or TRAP). We conducted a mutational analysis of extracellular residues of the receptor potentially involved in interaction with both the tethered ligand and the soluble peptide agonist. Agonist-stimulated calcium efflux in X. laevis oocytes or inositol phosphate accumulation in COS-7 cells was used to assess receptor activation. We have also examined the binding of a radiolabeled TRAP for the wild-type and mutant PAR-1 receptors. Our results indicated that most of the mutations strongly affected TRAP-induced responses without significantly altering thrombin-induced responses or TRAP binding. Several point mutations and deletion of extracellular domains (DeltaEC3, DeltaNH3) drastically altered the ability of mutant receptors to respond to TRAP, but not to thrombin, and did not affect the affinity for the radiolabeled TRAP by these mutant receptors. Only mutations that disrupted the putative disulfide bond or substitution of multiple acidic residues in the second extracellular loop by alanine had a significant effect on both ligand binding and thrombin activation. These results suggest that although both agonists can activate PAR-1, there are profound differences in the ability of thrombin and TRAP to activate PAR-1. In addition, we have found PAR-1 mutants with the ability to dissociate receptor-specific binding from functional activity.

Inhibition of cellular action of thrombin by N3-cyclopropyl-7-[[4-(1-methylethyl)phenyl]methyl]-7H-pyrrolo[3, 2-f]quinazoline-1,3-diamine (SCH 79797), a nonpeptide thrombin receptor antagonist

A growing body of evidence suggests an important contribution of the cellular actions of thrombin to thrombosis and restenosis following angioplasty. Recently we reported on SCH 79797 (N3-cyclopropyl-7-¿[4-(1-methylethyl)phenyl]methyl¿-7H-pyrrolo[3, 2-f]quinazoline-1,3-diamine) and its analogs as new potent, nonpeptide thrombin receptor antagonists. This study further characterizes the biochemical and pharmacological actions of pyrroloquinazoline inhibitors of protease activated receptor-1 (PAR-1) in human platelets and coronary artery smooth muscle cells (hCASMC). SCH 79797 and its N-methyl analog (SCH 203099) inhibited binding of a high-affinity thrombin receptor-activating peptide ([(3)H]haTRAP, Ala-Phe(p-F)-Arg-ChA-HArg-[(3)H]Tyr-NH(2)) to PAR-1 with IC(50) values of 70 and 45 nM, respectively. SCH 79797 inhibited [(3)H]haTRAP binding in a competitive manner. SCH 79797 and SCH 203099 inhibited alpha-thrombin- and haTRAP-induced aggregation of human platelets, but did not inhibit human platelet aggregation induced by the tethered ligand agonist for protease-activated receptor-4 (PAR-4), gamma-thrombin, ADP, or collagen. SCH 203099 inhibited surface expression of P-selectin induced by haTRAP and thrombin, and it did not increase P-selectin expression or prevent thrombin cleavage of the receptor. Thrombin and TFLLRNPNDK-NH(2) (TK), a PAR-1-selective agonist, produced transient increases in cytosolic free Ca(2+) concentration ([Ca(2+)](i)) in hCASMC. This increase in [Ca(2+)](i) was inhibited effectively by SCH 79797. However, the Ca(2+) transients induced by SLIGKV-NH(2,) a PAR-2-selective agonist, were not inhibited by SCH 79797. Thrombin- and TK-stimulated [(3)H]thymidine incorporation also was inhibited completely by SCH 79797. The results of this study demonstrate that SCH 79797 and SCH 203099 are potent, selective antagonists of PAR-1 in human platelets and hCASMC. These data also suggest that the thrombin stimulation of Ca(2+) transients and mitogenesis in hCASMC is mediated primarily through activation of PAR-1.

Structure-function analysis of protease-activated receptor 4 tethered ligand peptides. Determinants of specificity and utility in assays of receptor function

Thrombin activates protease-activated receptors (PARs) by specific cleavage of their amino-terminal exodomains to unmask a tethered ligand that binds intramolecularly to the body of the receptor to effect transmembrane signaling. Peptides that mimic such ligands are valuable as agonists for probing PAR function, but the tethered ligand peptide for PAR4, GYPGKF, lacks potency and is of limited utility. In a structure-activity analysis of PAR4 peptides, AYPGKF was approximately 10-fold more potent than GYPGKF and, unlike GYPGKF, elicited PAR4-mediated responses comparable in magnitude to those elicited by thrombin. AYPGKF was relatively specific for PAR4 in part due to the tyrosine at position 2; substitution of phenylalanine or p-fluorophenylalanine at this position produced peptides that activated both PAR1 and PAR4. Because human platelets express both PAR1 and PAR4, it might be desirable to inhibit both receptors. Identifying a single agonist for both receptors raises the possibility that a single antagonist for both receptors might be developed. The AYPGKF peptide is a useful new tool for probing PAR4 function. For example, AYPGKF activated and desensitized PAR4 in platelets and, like thrombin, triggered phosphoinositide hydrolysis but not inhibition of adenylyl cyclase in PAR4-expressing cells. The latter shows that, unlike PAR1, PAR4 couples to G(q) and not G(i).

Shifts in the affinity distribution of one class of seven-transmembrane receptors by activation of a separate class of seven-transmembrane receptors

We have demonstrated previously that activation of thrombin receptors causes increased Galpha(q) coupling to thromboxane A(2) receptors and increased thromboxane A(2) receptor ligand affinity. These results led to the hypothesis that thrombin receptor activation stimulates Galpha(q) redistribution to thromboxane A(2) receptors, thereby shifting them to a higher affinity state. The present study investigated three questions regarding this inter-receptor signaling phenomenon: (i) does activation of thrombin receptors cause a redistribution of thromboxane A(2) receptor subpopulations; (ii) does inter-receptor signaling require that participating receptors couple to the same family of G-protein alpha-subunits; and (iii) does inter-receptor signaling occur in cell types other than platelets? It was found that thrombin receptor activation caused a shift in the thromboxane A(2) receptor binding data from a one-site model to a two-site model (K(i) = 0.5 microM vs K(i) = 10 nM and 1.1 microM for the antagonist 4-[2-[[(4-chlorophenyl)sulfonyl]amino]ethyl]benzeneacetic acid (BM13. 505) and K(i) = 2.5 microM vs K(i) = 29.5 nM and 2.6 microM for the agonist 9,11-dideoxy-9alpha,11alpha-methanoepoxy prostaglandin F(2alpha) (U46619). It also was found that activation of prostaglandin D(2) receptors also caused a shift of prostacyclin receptor binding data from a one-site model (IC(50) = 10.1 nM) to a two-site model (IC(50) = 3.3 and 12.5 nM). The physiological manifestation of this inter-receptor signaling between prostacyclin and prostaglandin D(2) receptors was a synergistic inhibition of human platelet aggregation. Finally, the present results established that activation of endothelial cell thrombin receptors shifts thromboxane A(2) receptor affinity from K(i) = 0.8 microM (control) to K(i) = 0.2 microM (thrombin receptor-activating peptide), indicating that cells other than platelets have the capability to signal between seven-transmembrane receptors.

Design, synthesis, and biological characterization of a peptide-mimetic antagonist for a tethered-ligand receptor

Protease-activated receptors (PARs) represent a unique family of seven-transmembrane G protein-coupled receptors, which are enzymatically cleaved to expose a truncated extracellular N terminus that acts as a tethered activating ligand. PAR-1 is cleaved and activated by the serine protease alpha-thrombin, is expressed in various tissues (e.g., platelets and vascular cells), and is involved in cellular responses associated with hemostasis, proliferation, and tissue injury. We have discovered a series of potent peptide-mimetic antagonists of PAR-1, exemplified by RWJ-56110. Spatial relationships between important functional groups of the PAR-1 agonist peptide epitope SFLLRN were employed to design and synthesize candidate ligands with appropriate groups attached to a rigid molecular scaffold. Prototype RWJ-53052 was identified and optimized via solid-phase parallel synthesis of chemical libraries. RWJ-56110 emerged as a potent, selective PAR-1 antagonist, devoid of PAR-1 agonist and thrombin inhibitory activity. It binds to PAR-1, interferes with PAR-1 calcium mobilization and cellular function (platelet aggregation; cell proliferation), and has no effect on PAR-2, PAR-3, or PAR-4. By flow cytometry, RWJ-56110 was confirmed as a direct inhibitor of PAR-1 activation and internalization, without affecting N-terminal cleavage. At high concentrations of alpha-thrombin, RWJ-56110 fully blocked activation responses in human vascular cells, albeit not in human platelets; whereas, at high concentrations of SFLLRN-NH(2), RWJ-56110 blocked activation responses in both cell types. Thus, thrombin activates human platelets independently of PAR-1, i.e., through PAR-4, which we confirmed by PCR analysis. Selective PAR-1 antagonists, such as RWJ-56110, should serve as useful tools to study PARs and may have therapeutic potential for treating thrombosis and restenosis.

Sodium-hydrogen exchange and platelet function

On stimulation of platelets with agonists, for example, thrombin, a rapid rise in intracellular pH is observed. This alkalinization is mediated by an increase in transport activity of the Na(+)/H(+) exchanger isoform NHE1. In addition to this Na(+)/H(+) exchange mechanism, platelets express bicarbonate/chloride exchangers, which also contribute to pH(i) homeostasis. The main functions of NHE1 in platelets include pH(i) control, volume regulation, and participation in cell signaling. The isoform NHE1 is highly sensitive toward inhibition by EIPA, Hoe694, and Hoe642. The regulation of NHE1 activity is complex and is not completely understood. It includes the MAP kinase cascade, the Ca/calmodulin system, several heterotrimeric G proteins (Galpha12, Galpha13, Galphaq, and Galphai), small G proteins (ras, cdc42, rhoA), and downstream kinases (e.g., p160ROCK). Volume challenges stimulate tyrosine phosphorylation of cytoplasmic proteins, which ultimately activate NHE1. Thrombin, thromboxane, platelet-activating factor, angiotensin II, endothelin, phorbol ester, and Ca(2+) ionophors stimulate NHE1 activity in platelets. Blockade of platelet NHE1 can inhibit platelet activation. With the development of highly specific NHE1 inhibitors, detailed investigation of the relationships between NHE1 activity and platelet activation now becomes feasible.

Design and synthesis of thrombin receptor-derived nonpeptide mimetics utilizing a piperazine scaffold

Focal thrombus formation and vasoconstriction serve to defend vessels when vascular damage occurs, but may be detrimental when an atherosclerotic plaque is disrupted. Recently, the identification of the platelet thrombin receptor opened a new area in the development of agents that may selectively inhibit the effects of thrombin on cells, without affecting fibrin formation. In this regard, we have synthesized a number of 1,4-disubstituted piperazines which are designed to be analogues of thrombin receptor activating peptides (TRAP) and carry the pharmacophoric features of Phe and Arg residues present in the active pentapeptide SFLLR. These compounds were tested in the rat aorta relaxation assay and in platelet aggregation studies and their biological activity was consistent with a direct action on thrombin receptor. Furthermore, the structure activity relationships confirmed the importance of Phe and Arg for receptor activation and the molecular modeling revealed an intriguing relationship between their amphipathic similarity with SFLLR and their biological activity.

A novel molecular design of thrombin receptor antagonist

In a computer modeling of transmembrane domains of human thrombin receptor, Lys-158 was found near the ligand binding site. To capture this basic residue, analogs of peptide ligand containing a series of acidic amino acids were synthesized and assayed for human platelet aggregation, and Ser-(p-F)Phe-Aad(= alphaaminoadipic acid)-Leu-Arg-Asn-Pro-NH2 was found to be a potent antagonist.

Phospholipase A2: its role in ADP- and thrombin-induced platelet activation mechanisms

ADP and thrombin are two of the most important agonists of platelet aggregation--a cellular response that is critical for maintaining normal hemostasis. However, aberrant platelet aggregation induced by these agonists plays a central role in the pathogenesis of cardiovascular and cerebrovascular diseases. Agonist-induced primary or secondary activation of phospholipases leads to generation of the second messengers that participate in biochemical reactions essential to a number of platelet responses elicited by ADP and thrombin. Phospholipase A2 (PLA2) has been linked to cardiovascular diseases. However, the mechanism(s) of activation of PLA2 in platelets stimulated by ADP and thrombin has remained less well defined and much less appreciated. The purpose of this review is to examine and compare the molecular mechanisms of activation of PLA2 in platelets stimulated by ADP and thrombin.

Proteinase-activated receptors: novel mechanisms of signaling by serine proteases

Although serine proteases are usually considered to act principally as degradative enzymes, certain proteases are signaling molecules that specifically regulate cells by cleaving and triggering members of a new family of proteinase-activated receptors (PARs). There are three members of this family, PAR-1 and PAR-3, which are receptors for thrombin, and PAR-2, a receptor for trypsin and mast cell tryptase. Proteases cleave within the extracellular NH2-terminus of their receptors to expose a new NH2-terminus. Specific residues within this tethered ligand domain interact with extracellular domains of the cleaved receptor, resulting in activation. In common with many G protein-coupled receptors, PARs couple to multiple G proteins and thereby activate many parallel mechanisms of signal transduction. PARs are expressed in multiple tissues by a wide variety of cells, where they are involved in several pathophysiological processes, including growth and development, mitogenesis, and inflammation. Because the cleaved receptor is physically coupled to its agonist, efficient mechanisms exist to terminate signaling and prevent uncontrolled stimulation. These include cleavage of the tethered ligand, receptor phosphorylation and uncoupling from G proteins, and endocytosis and lysosomal degradation of activated receptors.

Thrombin-induced thromboxane synthesis by human platelets. Properties of anion binding exosite I-independent receptor

These studies have examined the effects of thrombin-related agonists in stimulating thromboxane production by human platelets. The results presented show that (1) the maximal response elicited by thrombin receptor agonist peptide (TRAP) stimulation was 40% to 50% of that seen with thrombin or the thrombin mutant Thrombin Quick I; (2) pretreatment of platelets with prostaglandin E1 or genistein resulted in differential inhibition of thromboxane production in response to TRAP compared with either enzyme agonist; (3) an antibody to the thrombin receptor cleavage site that inhibits increases in intracellular [Ca2+] only partially reduced thromboxane production in response to 5 nmol+L thrombin and 15 nmol/L Thrombin Quick I; (4) preincubation with 20 mumol/L TRAP resulted in desensitization to further stimulation by 100 mumol/L TRAP, but not by 100 nmol/L thrombin; and (5) the response to thrombin after TRAP desensitization was completely inhibited by the tyrosine kinase inhibitor genistein and was independent of an intracellular [Ca2+] flux, The cumulative results may be explained by the existence of two proteolytically activated receptors that result in thromboxane production in response to thrombin. One is the thrombin receptor/substrate, PAR-1. Stimulation through the second receptor/substrate depends on a genistein-sensitive step, is independent of an intracellular Ca2+ flux, and is initiated by a thrombin-activated receptor that does not depend on interaction with anion-binding exosite I, as previously indicated by the relative activity of Thrombin Quick I in stimulating platelet aggregation and thromboxane production. The proposed second thrombin receptor on platelets represents an additional member of the class of proteolytically activated receptors.

Pharmacological characterization of protease-activated receptor (PAR-1) in rat astrocytes

The proteolytic action of thrombin on its receptor (protease-activated receptor-1 or PAR-1) results in a conformational change in which the new N-terminal sequence auto-activates the receptor. Peptide analogs of this N-terminal sequence (TRAPs) are able to mimic the effect of thrombin and an extensive search has led to the definition of the structural requirement for the agonist and antagonist activity on thrombin receptors in several peripheral systems. Thrombin plays an important role in central and peripheral nervous system development and PAR-1 is present in neurons and astrocytes. We have now characterized thrombin receptors pharmacologically in cultured rat astrocytes by using [3H]thymidine incorporation and reversal of stellation induced by Bt2cAMP as end-points. Thrombin increased [3H]thymidine incorporation into DNA with an EC50 of 1 nM and induced a complete reversion of cell stellation. The effects of thrombin on [3H]thymidine incorporation were mimicked by TRAP-14 (EC50 = 3 microM) and a peptide containing non-natural amino acids Ala-Phe(p-F)-Arg-Cha-HArg-Tyr-NH2 (A6Y; EC50 = 0.8 microM). Similarly, these two peptides reversed Bt2cAMP-induced stellation. The effect of thrombin, TRAP-14 and A6Y on [3H]thymidine incorporation into DNA was significantly prevented by L9R, a 9-amino-acid peptide (Leu-Val-Arg-D-Cys-Gly-Lys-His-Ser-Arg; IC50 = 180 microM against thrombin and TRAP-14 and 800 microM against A6Y) previously described as an antagonist in human platelet aggregation. L9R antagonized also thrombin effects on astrocyte morphology. These results demonstrate that rat astrocytes express PAR-1 receptors which are pharmacologically similar to those previously characterized in human platelets.

Characterization of in vitro and in vivo platelet responses to thrombin and thrombin receptor-activating peptides in guinea pigs

Guinea pig platelets are similar to human platelets in their responsiveness to thrombin receptor-activating peptides and other agonists. Therefore, guinea pigs anesthetized with Inactin (90 mg/kg i.p.) were used to assess in vivo activities of thrombin and thrombin receptor-activating peptides (TRAPs) using 111 In-labeled platelets and a microcomputer-based system. The aggregatory responses are expressed as percent change for a 20 min period over basal radioactivity (AUC). Reversible accumulation of platelets occurred in the pulmonary microcirculation in response to stimuli. Human thrombin (50 and 100 U/kg i.v.) caused a dose-related platelet accumulation. Responses of similar magnitude were induced by SFLLRN (TRAP-(1-6)) and Ala-Phe(p-F)-Arg-Cha-HArg-Tyr-NH2 (high-affinity thrombin receptor-activating peptide, 0.03, 0.1 and 0.3 mg/kg i.v.). High-affinity thrombin receptor-activating peptide, a new synthetic oligopeptide agonist, is about 3-fold more potent than TRAP-(1-6), a wild-type sequence. Similarly, high-affinity thrombin receptor-activating peptide is about 4 times more potent than TRAP-(1-6) in the radioligand binding study using platelet membrane. By comparison, high-affinity thrombin receptor-activating peptide manifested an aggregatory activity (EC60 = 1.2 microM) about 15 times more potent than that of TRAP-(1-6)(EC60 = 18.6 microM) in washed guinea pig platelets. The intrapulmonary platelet aggregation in response to thrombin, TRAP-(1-6) and high-affinity thrombin receptor-activating peptide was characterized by long duration (approximately 30 min); a reduction in response (18-54%) tended to occur with repeated challenges, presumably due to desensitization and consumption. The response to thrombin (100 U/kg) was greatly inhibited by (D)-Phe-Pro-Arg-chloromethyl ketone (PPACK), a potent thrombin inhibitor (250 micrograms/kg + 6 micrograms/kg per min i.v. x 30): AUC, 150 +/- 552 vs. 7171 +/- 1052 in the control period (n = 8, P < 0.05). The response to high-affinity thrombin receptor-activating peptide (0.03 mg/kg), which acts on thrombin receptor directly, was not affected by PPACK. It is concluded that guinea pigs are an appropriate preparation for evaluation of in vivo activity of thrombin inhibitors as well as thrombin receptor agonists and antagonists.

Binding of a thrombin receptor tethered ligand analogue to human platelet thrombin receptor

A thrombin receptor-radioligand binding assay was developed using [3H]A(pF-F)R(ChA)(hR)Y-NH2 ([3H]haTRAP), a high affinity thrombin receptor-activating peptide (TRAP), and human platelet membranes. Scatchard analysis of saturation binding data indicated that [3H]haTRAP bound to platelet membranes with a Kd of 15 nM and a Bmax of 5.2 pmol/mg of protein. The binding was reduced by GPPNHP, a nonmetabolizable GTP analogue. Various TRAPs and a TRAP antagonist, but not other receptor agonists, displaced [3H]haTRAP from the binding sites. SFLLRN-NH2, a thrombin receptor-tethered ligand analogue, and [3H]haTRAP exhibited competitive binding for the same binding sites. The relative affinity of these peptides for the binding site paralleled their EC50 or IC50 values for platelet aggregation. These data indicate that [3H]haTRAP binds specifically and saturably to the functioning G protein-linked thrombin (tethered ligand) receptor in human platelet membranes.

Development of potent thrombin receptor antagonist peptides

A peptide-based structure-activity study is reported leading to the discovery of novel potent thrombin receptor antagonists. Systematic substitution of nonproteogenic amino acids for the second and third residues of the human thrombin receptor "tethered ligand" sequence (SFLLR) led to a series of agonists with enhanced potency. The most potent pentapeptide agonist identified was Ser-p-fluoroPhe-p-guanidinoPhe-Leu-Arg-NH2, 9 (EC50 approximately 0.04 microM for stimulation of human platelet aggregation, approximately 10-fold more potent than the natural pentapeptide). Systematic substitution of the NH2-terminal Ser in 9 with neutral hydrophobic NH2-acyl groups led to partial agonists and eventually antagonists with unprecedented potency (greater than 1000-fold increase over the previously reported antagonist 3-mercaptopropionyl-Phe-Cha-Cha-Arg-Lys-Pro-Asn-Asp-Lys-NH2). In the series of NH2-acyl tetrapeptide antagonists, N-transcinnamoyl-p-fluoroPhe-p-guanidinoPhe-Leu-Arg-NH 2, 41 (BMS-197525), was identified as the tightest binding (IC50 approximately 8 nM) and most potent with an IC50 approximately 0.20 microM for inhibition of SFLLRNP-NH2-stimulated platelet aggregation. Systematic single substitutions in 41 indicated that, in addition to the NH2-terminal acyl group, the side chains at the second and third positions were also responsible for important and specific receptor interactions. The p-fluoroPhe and p-guanidinoPhe residues in the second and third positions of 41 were observed to be optimal in both the agonist and antagonist series. In the case of antagonists, however, an appropriately positioned positively charged group (i.e., protonated base) at the third residue was required. In contrast, such a substitution was not required for potent agonist activity. An even more potent antagonist resulted when 41 was extended at the C-terminus by a single Arg residue giving rise to analog 90 (BMS-200261) which had an IC50 approximately 20 nM for inhibition of SFLLRNP-NH2-stimulated platelet aggregation. When the C-terminal Arg of 90 was replaced by an Orn-(Ndelta-propionyl) residue, the resulting antagonist 91 (BMS-200661) was suitable for use in radioligand binding assays (Kd = 10-30 nM). Antagonist activity observed for selected compounds was verified through secondary assays in that these analogs prevented SFLLRNP-NH2-stimulated GTPase activity in platelet membranes and Ca2+ mobilization in cultured human smooth muscle cells and mouse fibroblasts. Furthermore, this inhibition occurred at concentrations that had no effect on thrombin catalytic activity indicating a specific activity attributable to receptor binding and not enzyme inhibition.