Platelet ADP receptors contribute to the initiation of intravascular coagulation

Alpha-hemolysin from Escherichia coli uses endogenous amplification through P2X receptor activation to induce hemolysis

Escherichia coli is the dominant facultative bacterium in the normal intestinal flora. E. coli is, however, also responsible for the majority of serious extraintestinal infections. There are distinct serotypical differences between facultative and invasive E. coli strains. Invasive strains frequently produce virulence factors such as alpha-hemolysin (HlyA), which causes hemolysis by forming pores in the erythrocyte membrane. The present study reveals that this pore formation triggers purinergic receptor activation to mediate the full hemolytic action. Non-selective ATP-receptor (P2) antagonists (PPADS, suramin) and ATP scavengers (apyrase, hexokinase) concentration dependently inhibited HlyA-induced lysis of equine, murine, and human erythrocytes. The pattern of responsiveness to more selective P2-antagonists implies that both P2X(1) and P2X(7) receptors are involved in HlyA-induced hemolysis in all three species. In addition, our results also propose a role for the pore protein pannexin1 in HlyA-induced hemolysis, as non-selective inhibitors of this channel significantly reduced hemolysis in the three species. In conclusion, activation of P2X receptors and possibly also pannexins augment hemolysis induced by the bacterial toxin, HlyA. These findings potentially have clinical perspectives as P2 antagonists may ameliorate symptoms during sepsis with hemolytic bacteria.

Effect of blocking platelet activation with AZD6140 on development of abdominal aortic aneurysm in a rat aneurysmal model

BACKGROUND

Platelet activation and thrombus renewal are keys to intraluminal thrombus formation and progression of abdominal aortic aneurysms (AAA). This study explored the ability of AZD6140, a P2Y(12) receptor antagonist, to inhibit platelet activation and prevent aneurysm development in a rat experimental model of AAA.

METHOD

Aortic aneurysms were induced by implanting a segment of sodium dodecyl sulfate-decellularized guinea pig aorta in rat aortas. One day later, rats were randomized to AZD6140 (10 mg/kg twice daily by mouth) or diluent (n = 23 per group) for either 10 (n = 18) or 42 days (n = 28). Adenosine diphosphate (ADP)-mediated platelet aggregation, aneurysm expansion, intraluminal thrombus formation, inflammatory infiltration, matrix metalloproteinase-9 (MMP-9) expression, and smooth muscle cell colonization were measured.

RESULTS

AZD6140 inhibited ADP-induced platelet aggregation in vivo for 12 hours, justifying twice-daily administration in rats. The spontaneous increase in aortic diameter shown in the aneurysmal model (2.22 +/- 0.56 mm at day 10 vs 5.21 +/- 1.22 mm at day 42) was reduced with AZD6140 (3.61 +/- 1.46 mm at day 42, P < .01). This beneficial effect was associated with a significant reduction of thrombus development, platelet CD41 expression (P < .05), and leukocyte infiltration of the mural thrombus at days 10 and 42 (P < .01). MMP-9 expression correlated with mural thrombus area and was significantly reduced by AZD6140 (P < .05). AZD6140 limited elastic fiber degradation (P < .05) and enhanced progressive colonization of the thrombus by smooth muscle cells at day 42 (P < .01).

CONCLUSIONS

These data suggest that inhibition of platelet activation limits intraluminal thrombus biologic activities, thereby impairing aneurysm development.

Sex differences in thrombosis in mice are mediated by sex-specific growth hormone secretion patterns

Sex differences in thrombosis are well described, but their underlying mechanism(s) are not completely understood. Coagulation proteins are synthesized in the liver, and liver gene expression is sex specific and depends on sex differences in growth hormone (GH) secretion--males secrete GH in a pulsatile fashion, while females secrete GH continuously. Accordingly, we tested the hypothesis that sex-specific GH secretion patterns cause sex differences in thrombosis. Male mice were more susceptible to thrombosis than females in the thromboplastin-induced pulmonary embolism model and showed shorter clotting times ex vivo. GH-deficient little (lit) mice were protected from thrombosis, and pulsatile GH given to lit mice restored the male clotting phenotype. Moreover, pulsatile GH administration resulted in a male clotting phenotype in control female mice, while continuous GH caused a female clotting phenotype in control male mice. Expression of the coagulation inhibitors Proc, Serpinc1, Serpind1, and Serpina5 were strongly modulated by sex-specific GH patterns, and GH modulated resistance to activated protein C. These results reveal what we believe to be a novel mechanism whereby sex-specific GH patterns mediate sex differences in thrombosis through coordinated changes in the expression of coagulation inhibitor genes in the liver.

Platelet tissue factor: how did it get there and is it important?

Recently, the presence of functionally active tissue factor (TF) in platelets has been reported by several groups. In this location, TF is postulated to play an important role in the propagation phase of thrombus formation. Although the existence of platelet TF still remains controversial to some extent, a review of the current literature proposes at least three distinct sources of "platelet-associated TF" in those laboratories that have reported its presence: (1) TF that is taken up in the form of circulating microparticles, usually derived from monocytes; (2) TF stored in the alpha-granules of platelets that may have been taken up and/or endogenously synthesized; and (3) TF that is synthesized and expressed on the plasma membrane of mature platelets. These pathways are not mutually exclusive, and the dominant mechanism may depend on the state of platelet activation and, possibly, on other host factors that differ in physiological hemostasis versus pathological thrombosis. This brief review will summarize the state-of-the-art understanding on the origins and possible role of platelet TF.

Reduced Atherosclerotic Lesions in P2Y 1 /Apolipoprotein E Double-Knockout Mice

Background— The P2Y 1 receptor plays a key role in arterial thrombosis and is widely expressed in many cell types involved in atherosclerosis. The aim of this study was to evaluate its potential involvement in the development of atherosclerotic lesions.

Methods and Results— Apolipoprotein E–deficient ( ApoE −/− ) and P2Y 1 −/− / ApoE −/− mice were maintained on regular chow for 17 or 30 weeks before analysis of atherosclerotic lesions. At 17 weeks, lesions in the aortic sinus and entire aorta were smaller in P2Y 1 −/− / ApoE −/− compared with those in ApoE −/− animals. At 30 weeks, the aortic sinus lesions in P2Y 1 −/− / ApoE −/− mice were still diminished in size and displayed reduced inflammation, reflected by decreased macrophage infiltration and diminished VCAM-1 immunostaining, compared with those in ApoE −/− mice. They also had a lower smooth muscle cell content. Unexpectedly, bone marrow transplantation showed that the absence of the P2Y 1 receptor in blood cells only led to no significant modification of the lesion compared with control ApoE −/− reconstituted animals. Conversely, the absence of the P2Y 1 receptor except in blood cells resulted in a reduction in lesion size similar to that in control P2Y 1 −/− / ApoE −/− reconstituted mice, pointing to a role of non–hematopoietic-derived P2Y 1 receptors, most likely the endothelial or smooth muscle cell P2Y 1 receptors. In addition, although this was not statistically significant, plasma cholesterol levels were consistently decreased in P2Y 1 −/− animals, suggesting that a modification of lipid metabolism could be responsible for the observed phenotype.

Conclusion— The P2Y 1 receptor contributes to atherosclerosis, primarily through its role in non–hematopoietic-derived cells.

Protein disulfide isomerase acts as an injury response signal that enhances fibrin generation via tissue factor activation

The activation of initiator protein tissue factor (TF) is likely to be a crucial step in the blood coagulation process, which leads to fibrin formation. The stimuli responsible for inducing TF activation are largely undefined. Here we show that the oxidoreductase protein disulfide isomerase (PDI) directly promotes TF-dependent fibrin production during thrombus formation in vivo. After endothelial denudation of mouse carotid arteries, PDI was released at the injury site from adherent platelets and disrupted vessel wall cells. Inhibition of PDI decreased TF-triggered fibrin formation in different in vivo murine models of thrombus formation, as determined by intravital fluorescence microscopy. PDI infusion increased - and, under conditions of decreased platelet adhesion, PDI inhibition reduced - fibrin generation at the injury site, indicating that PDI can directly initiate blood coagulation. In vitro, human platelet-secreted PDI contributed to the activation of cryptic TF on microvesicles (microparticles). Mass spectrometry analyses indicated that part of the extracellular cysteine 209 of TF was constitutively glutathionylated. Mixed disulfide formation contributed to maintaining TF in a state of low functionality. We propose that reduced PDI activates TF by isomerization of a mixed disulfide and a free thiol to an intramolecular disulfide. Our findings suggest that disulfide isomerases can act as injury response signals that trigger the activation of fibrin formation following vessel injury.

The active metabolite of prasugrel inhibits adenosine diphosphate- and collagen-stimulated platelet procoagulant activities

BACKGROUND

Prasugrel is a novel antiplatelet prodrug of the same thienopyridine class as clopidogrel and ticlopidine. Metabolism of prasugrel generates the active metabolite R-138727, an antagonist of the platelet P2Y(12) adenosine diphosphate (ADP) receptor, leading to inhibition of ADP-mediated platelet activation and aggregation. ADP also enhances the platelet response to collagen, and these two agonists contribute to the generation of platelet procoagulant activity. We therefore examined whether R-138727 inhibits ADP- and collagen-triggered platelet procoagulant activities.

METHODS AND RESULTS

As shown by whole blood flow cytometry, R-138727 inhibited surface phosphatidylserine expression on ADP plus collagen-stimulated platelets and tissue factor (TF) expression on ADP-, collagen-, and ADP plus collagen-stimulated monocyte-platelet aggregates. R-138727 reduced monocyte-platelet aggregate formation, thereby further inhibiting TF expression. ADP, collagen, and ADP plus collagen accelerated the kinetics of thrombin generation in recalcified whole blood and R-138727 significantly inhibited this acceleration. Clot strength in a modified thromboelastograph system was also inhibited by R-138727 (IC50 0.7 +/- 0.1 microM).

CONCLUSIONS

In addition to its previously known inhibitory effects on platelet activation and aggregation, the active metabolite of prasugrel, R-138727, inhibits platelet procoagulant activity in whole blood (as determined by phosphatidylserine expression on platelets and TF expression on monocyte-platelet aggregates), resulting in the functional consequences of delayed thrombin generation and impaired clot development.

Novel uses for anti-platelet agents as anti-inflammatory drugs

An alteration in the character and function of platelets is manifested in patients with inflammatory diseases, and these alterations have been dissociated from the well-characterized involvement of platelets in thrombosis and haemostasis. Recent evidence reveals platelet activation is sometimes critical in the development of inflammation. The mechanisms by which platelets participate in inflammation are diverse, and offer numerous opportunities for future drug intervention. There is now acceptance that platelets act as innate inflammatory cells in immune responses, with roles as sentinel cells undergoing surveillance, responding to microbial invasion, orchestrating leukocyte recruitment, and migrating through tissue, causing damage and influencing repair processes in chronic disease. Some of these processes are targeted by drugs that are being developed to target platelet participation in atherosclerosis. The actions of platelets therefore influence the pathogenesis of diverse inflammatory diseases in various body compartments, encompassing parasitic and bacterial infection, allergic inflammation (especially asthma and rhinitis), and non-atopic inflammatory conditions, for example, chronic obstructive pulmonary disease (COPD), rheumatoid arthritis (RA), inflammatory bowel disease (IBD) and atherosclerosis. This review will first discuss the evidence for platelet activation in these various inflammatory diseases, and secondly discuss the mechanisms by which this pathogenesis occurs and the various anti-platelet agents which have been developed to combat platelet activation in atherosclerosis and their potential future use for the treatment of other inflammatory diseases.

The active metabolite of prasugrel inhibits adenosine diphosphate- and collagen-stimulated platelet procoagulant activities

BACKGROUND

Prasugrel is a novel antiplatelet prodrug of the same thienopyridine class as clopidogrel and ticlopidine. Metabolism of prasugrel generates the active metabolite R-138727, an antagonist of the platelet P2Y(12) adenosine diphosphate (ADP) receptor, leading to inhibition of ADP-mediated platelet activation and aggregation. ADP also enhances the platelet response to collagen, and these two agonists contribute to the generation of platelet procoagulant activity. We therefore examined whether R-138727 inhibits ADP- and collagen-triggered platelet procoagulant activities.

METHODS AND RESULTS

As shown by whole blood flow cytometry, R-138727 inhibited surface phosphatidylserine expression on ADP plus collagen-stimulated platelets and tissue factor (TF) expression on ADP-, collagen-, and ADP plus collagen-stimulated monocyte-platelet aggregates. R-138727 reduced monocyte-platelet aggregate formation, thereby further inhibiting TF expression. ADP, collagen, and ADP plus collagen accelerated the kinetics of thrombin generation in recalcified whole blood and R-138727 significantly inhibited this acceleration. Clot strength in a modified thromboelastograph system was also inhibited by R-138727 (IC50 0.7 +/- 0.1 microM).

CONCLUSIONS

In addition to its previously known inhibitory effects on platelet activation and aggregation, the active metabolite of prasugrel, R-138727, inhibits platelet procoagulant activity in whole blood (as determined by phosphatidylserine expression on platelets and TF expression on monocyte-platelet aggregates), resulting in the functional consequences of delayed thrombin generation and impaired clot development.

Gene expression profiling for the identification of G-protein coupled receptors in human platelets

INTRODUCTION AND MATERIALS AND METHODS: G-protein coupled receptors (GPCRs) play an important role in platelet aggregation. To identify new platelet GPCRs, a platelet gene expression profile was generated and validated using quantitative real-time PCR.

RESULTS

In total, mRNA of 28 GPCR genes was detected in human platelets. The 12 most abundant platelet GPCR transcripts were: thrombin receptor PAR1 (1865+/-178%), ADP receptor P2Y(12) (459+/-88%), succinate receptor 1 (257+/-48%), ADP receptor P2Y(1) (100%), orphan P2RY(10) (68.2+/-3.3%), lysophosphatidic acid receptors GPR23 (48.2+/-11%) and GPR92 (26.1+/-3.3%), adrenergic receptor alpha(2A) (18.4+/-4.4%), orphan EBI2 (6.31+/-0.42), adenosine receptors A(2A) (2.94+/-0.24%) and A(2B) (2.88+/-0.16%) and lysophosphatidic acid receptor LPA(1) (2.59+/-0.39%) (% relative to the chosen calibrator P2Y(1)). A surprising G-protein coupled receptor redundancy was found: two ADP receptors (P2Y(1) and P2Y(12)), three adenosine receptors (A(2A), A(2B), and A(1)), four lysophosphatidic acid receptors (LPA(1), LPA(3), GPR23 and GPR92), two l-glutamate receptors (mGlu(3) and mGlu(4)) and two serotonin receptors (5-HT(1F) and 5-HT(4)). The adenosine receptor A(2B) gene expression was validated with protein expression and functional studies. Western blot confirmed A(2B) receptor protein expression and platelet flow cytometry demonstrated inhibition of the effect of NECA by the adenosine A(2B)-antagonist MRS1754.

CONCLUSIONS

We have detected several GPCRs not previously known to be expressed in platelets, including a functional adenosine A(2B) receptor. The findings could improve our understanding of platelet aggregation and provide new targets for drug development.

P2 receptors in cardiovascular regulation and disease

The role of ATP as an extracellular signalling molecule is now well established and evidence is accumulating that ATP and other nucleotides (ADP, UTP and UDP) play important roles in cardiovascular physiology and pathophysiology, acting via P2X (ion channel) and P2Y (G protein-coupled) receptors. In this article we consider the dual role of ATP in regulation of vascular tone, released as a cotransmitter from sympathetic nerves or released in the vascular lumen in response to changes in blood flow and hypoxia. Further, purinergic long-term trophic and inflammatory signalling is described in cell proliferation, differentiation, migration and death in angiogenesis, vascular remodelling, restenosis and atherosclerosis. The effects on haemostasis and cardiac regulation is reviewed. The involvement of ATP in vascular diseases such as thrombosis, hypertension and diabetes will also be discussed, as well as various heart conditions. The purinergic system may be of similar importance as the sympathetic and renin-angiotensin-aldosterone systems in cardiovascular regulation and pathophysiology. The extracellular nucleotides and their cardiovascular P2 receptors are now entering the phase of clinical development.

Delayed thrombin-induced platelet–fibrin clot generation by clopidogrel: A new dose-related effect demonstrated by thrombelastography in patients undergoing coronary artery stenting

BACKGROUND

We have previously demonstrated that clopidogrel reduces platelet activation and aggregation in patients undergoing stenting. However, the effect of the clopidogrel loading dose on the rate of thrombin-induced platelet-fibrin clot formation is unknown in this patient population.

METHODS AND RESULTS

Using thrombelastography (TEG) we measured the time to platelet-fibrin clot formation (R), a marker of the speed of thrombin generation, in 120 patients undergoing elective coronary artery stenting treated with standard and high loading doses of clopidogrel. Platelet reactivity to adenosine diphosphate (ADP) by light transmittance aggregometry (LTA) was determined simultaneously. Measurements were made immediately before and at 24 h after clopidogrel treatment. Clopidogrel produced a prolongation in R (4.4+/-1.4 min pre vs. 5.4+/-1.7 min post, p<0.001) that directly correlated with the change in platelet aggregation (r=0.65, p<0.0001). Prolongation in R was greatest in patients treated with a high loading dose (p=0.004).

CONCLUSIONS

Delayed thrombin-induced platelet-fibrin clot formation as measured by TEG is a newly reported dose-related effect of clopidogrel that may contribute to the overall antithrombotic properties of the drug in patients undergoing stenting. This effect was more marked in patients loaded with 600 mg, lending further mechanistic support for this dose of clopidogrel as a more effective antithrombotic regimen than the standard 300 mg dose. Measurement of R may serve as a new indicator of clopidogrel responsiveness.

Antiplatelet Drug Resistance and Drug-Drug Interactions: Role of Cytochrome P450 3A4

Antiplatelet therapy provided pivotal advances in the treatment of cardiovascular disease. Aspirin and thienopyridine, clopidogrel, is currently the treatment of choice in acute coronary syndromes and the prevention of thrombosis after coronary stent implantation. Despite the efficacy of this dual antiplatelet therapy in reduction of adverse coronary events in patients with acute coronary syndromes, complications persist in a subgroup of these patients. Emerging causes of aspirin and clopidogrel resistance may translate to increase risk for recurrent myocardial infarction, stroke, or cardiac related mortality. However, the mechanism of antiplatelet drug resistance remains incompletely characterized, and a sensitive and specific assay of aspirin and clopidogrel effect that reliably predicts treatment failure has not emerged. To date, evidence supporting antiplatelet drug resistance are pharmacokinetic response variability, drug-drug interaction through competitive inhibition a specific enzymatic pathway, genetic variability, and variability in the induction of enzymatic pathway in metabolic activation of prodrugs, like clopidogrel. Further investigation or guidelines are needed to optimize antiplatelet treatment strategies to identify and treat patients resistant to aspirin and/or clopidogrel.

Eosinophils are a major intravascular location for tissue factor storage and exposure

Blood cell progenitors were scanned for the presence of the coagulation starter protein tissue factor (TF) by immunoelectron microscopy. Thereby, substantial TF expression was observed in the precursor cells of eosinophils. TF levels were lower in basophil precursors and barely detectable in neutrophil progenitors. In peripheral blood immediately processed to avoid activation of the TF gene, mature eosinophils were found to considerably express TF, unique among the granulocyte and monocyte fractions. TF was preferentially located in the specific granules in resting eosinophils. Platelet-activating factor (PAF), and more pronounced, granulocyte-macrophage colony-stimulating factor (GM-CSF) plus PAF, caused translocation of preformed TF to the eosinophil cell membrane. GM-CSF/PAF also increased the TF transcript levels. The activated eosinophils exhibited procoagulant activity that was abrogated by TF inhibition. Targeting the extracellular domain of TF with specific antibodies markedly suppressed the initial phase of the eosinophil passage across the IL-4-activated endothelium. Eosinophil rolling and firm adhesion remained unaffected. This suggests that TF specifically facilitates the early transendothelial migration of the eosinophils. In summary, eosinophils maintain a high TF expression during maturation, providing a main source of preformed TF in blood, which might be relevant for the thrombogenesis promoted by hypereosinophilic conditions.

Regulation of platelet functions by P2 receptors

The main role of blood platelets is to ensure primary hemostasis, which is the maintenance of vessel integrity and cessation of bleeding upon injury. While playing a major part in acute arterial thrombosis, platelets are also involved in inflammation, atherosclerosis, and angiogenesis. ADP and ATP play a crucial role in platelet activation, and their receptors are potential targets for antithrombotic drugs. The ATP-gated cation channel P2X(1) and the two G protein-coupled ADP receptors, P2Y(1) and P2Y(12), selectively contribute to platelet aggregation and formation of a thrombus. Owing to its central role in the growth and stabilization of a thrombus, the P2Y(12) receptor is an established target of antithrombotic drugs such as clopidogrel. Studies in P2Y(1) and P2X(1) knockout mice and selective P2Y(1) and P2X(1) antagonists have shown that these receptors are also attractive targets for new antithrombotic compounds. The potential role of platelet P(2) receptors in the involvement of platelets in inflammatory processes is also discussed.

Initiation of coagulation by tissue factor carriers in blood

Fibrin can be generated in blood vessels in the absence of substantial damage to the vessel wall. According to novel observations, fibrin formation within the vessel lumen could be initiated by intravascular (blood borne) tissue factor (TF), a central starter protein of blood coagulation. We have recently detected TF in platelets, principally allowing coagulation to be initiated within the developing thrombus. While TF is stored in intraplatelet compartments under resting conditions, it is rapidly translocated to the cell surface in response to platelet activation, accumulating on filopodia. Platelet TF might be acquired from extracellular sources via transfer of TF positive microparticles and/or, potentially, be generated through translational mechanisms. Exocytotic microparticles, regular blood components, share the ability of the activated platelets to coexpose TF and phosphatidylserine, allowing the assembly of the entire coagulation system on a single membrane surface. Nonetheless, the procoagulant activity of the microparticles, when present alone, is limited. However, the vesicular TF might be uncovered by platelet-microparticle interactions. Thereby, the recruitment of platelets and microparticles to the site of vascular injury could synergistically trigger fibrin generation. In summary, by utilizing differential pathways, activated platelets are mandatory for the TF-mediated coagulation start in blood.

The platelet P2 receptors in arterial thrombosis

ADP and ATP play a crucial role in hemostasis and thrombosis and their receptors are potential targets for antithrombotic drugs. The ATP-gated channel P2X1 and the two G protein-coupled P2Y1 and P2Y12 ADP receptors selectively contribute to platelet aggregation. Due to its central role in the formation and stabilization of a thrombus, the P2Y12 receptor is a well established target of antithrombotic drugs like clopidogrel which has proved efficacious in many clinical trials and experimental models of thrombosis. Competitive P2Y12 antagonists have also been shown to be effective in experimental thrombosis as well as in several clinical trials. Studies in P2Y1 and P2X1 knock-out mice and experimental thrombosis models using selective P2Y1 and P2X1 antagonists have shown that, depending on the conditions, these receptors could also be potential targets for new antithrombotic drugs. Since both P2X1 and P2Y1 receptor inhibition result in milder prolongation of the bleeding time as compared to P2Y12 inhibition, the idea is put forward that combination of P2 receptor antagonists could improve efficacy with diminished hemorrhagic risk. However, further studies are required to validate such a point of view.

Tissue Factor in Cardiovascular Diseases

Tissue factor (TF), formerly known as thromboplastin, is the key initiator of the coagulation cascade; it binds factor VIIa resulting in activation of factor IX and factor X, ultimately leading to fibrin formation. TF expression and activity can be induced in endothelial cells, vascular smooth muscle cells, and monocytes by various stimuli such as cytokines, growth factors, and biogenic amines. These mediators act through diverse signal transduction mechanisms including MAP kinases, PI3-kinase, and protein kinase C. Cellular TF is present in three pools as surface, encrypted, and intracellular protein. TF can also be detected in the bloodstream, referred to as circulating or blood-borne TF. Elevated levels of TF are observed in patients with cardiovascular risk factors such as hypertension, diabetes, dyslipidemia, and smoking as well as in those with acute coronary syndromes. TF may indeed be involved in the pathogenesis of atherosclerosis by promoting thrombus formation; in addition, it can induce migration and proliferation of vascular smooth muscle cells. As a consequence, therapeutic strategies have been developed to specifically interfere with the action of TF such as antibodies against TF, site-inactivated factor VIIa, or recombinant TF pathway inhibitor. Inhibition of TF action appears to be an attractive target for the treatment of cardiovascular diseases.

Clopidogrel induced suppression of bovine platelet activation in vitro and a preliminary study of its effect on the development of Mannheimia haemolytica induced pneumonia

We report here on the influence of the platelet antagonist clopidogrel (Plavix) on bovine platelet function. We first evaluated the capacity of clopidogrel to inhibit adenosine diphosphate (ADP)-stimulated platelet function in the bovine species, using an ex vivo approach with blood from treated animals. Platelets isolated from treated calves displayed rapid and consistent reduction in function (aggregation, thromboxane production) upon ADP, but not platelet activating factor (PAF), stimulation. We then examined the possibility that clopidogrel could influence Mannheimia haemolytica pneumonia pathobiology using an experimental challenge model. We were unable to detect significant differences between clopidogrel treated and untreated animals when challenged with intra-tracheal inoculation of M. haemolytica. There was a trend towards inhibition of platelet degranulation in the affected regions of lungs from clopidogrel treated calves, and pre-treated challenged animals had similar amounts of fibrin deposition and enhanced fibrous tissue formation in their lungs when compared with control counterparts.

Platelet inhibition by insulin is absent in type 2 diabetes mellitus

OBJECTIVE

ADP-induced P2y12 signaling is crucial for formation and stabilization of an arterial thrombus. We demonstrated recently in platelets from healthy subjects that insulin interferes with Ca2+ increases induced by ADP-P2y1 contact through blockade of the G-protein Gi, and thereby with P2y12-mediated suppression of cAMP.

METHODS AND RESULTS

Here we show in patients with type 2 diabetes mellitus (DM2) that platelets have lost responsiveness to insulin leading to increased adhesion, aggregation, and procoagulant activity on contact with collagen. Using Ser473 phosphorylation of protein kinase B as output for insulin signaling, a 2-fold increase is found in insulin-stimulated normal platelets, but in DM platelets there is no significant response. In addition, DM2 platelets show increased P2y12-mediated suppression of cAMP and decreased P2y12 inhibition by the receptor antagonist AR-C69931MX.

CONCLUSIONS

The loss of responsiveness to insulin together with increased signaling through P2y12 might explain the hyperactivity of platelets in patients with DM2.

Identification of a potent inverse agonist at a constitutively active mutant of human P2Y12 receptor

Human platelets express two P2Y receptors: G(q)-coupled P2Y(1), and G(i)-coupled P2Y(12). Both P2Y(1) and P2Y(12) are ADP receptors on human platelets and are essential for ADP-induced platelet aggregation that plays pivotal roles in thrombosis and hemostasis. Numerous constitutively active G protein-coupled receptors have been described in natural or recombinant systems, but in the P2Y receptors, to date, no constitutive activity has been reported. In our effort to identify G protein coupling domains of the human platelet ADP receptor, we constructed a chimeric hemagglutinin-tagged human P2Y(12) receptor with its C terminus replaced by the corresponding part of human P2Y(1) receptor and stably expressed it in Chinese hamster ovary-K1 cells. It is interesting that the chimeric P2Y(12) mutant exhibited a high level of constitutive activity, as evidenced by decreased cAMP levels in the absence of agonists. The constitutive activation of the chimeric P2Y(12) mutant was dramatically inhibited by pertussis toxin, a G(i) inhibitor. The constitutively active P2Y(12) mutant retained normal responses to 2-methylthio-ADP, with an EC(50) of 0.15 +/- 0.04 nM. The constitutively active P2Y(12) mutant caused Akt phosphorylation that was abolished by the addition of pertussis toxin. Pharmacological evaluation of several P2Y(12) antagonists revealed (E)-N-[1-[7-(hexylamino)-5-(propylthio)-3H-1,2,3-triazolo-[4,5-d]-pyrimidin-3-yl]-1,5,6-trideoxy-beta-d-ribo-hept-5-enofuranuronoyl]-l-aspartic acid (AR-C78511) as a potent P2Y(12) inverse agonist and 5'-adenylic acid, N-[2-(methylthio)ethyl]-2-[(3,3,3-trifluoropropyl)thio]-, monoanhydride with (dichloromethylene)bis[phosphonic acid] (AR-C69931MX) as a neutral antagonist. In conclusion, this is the first report of a cell line stably expressing a constitutively active mutant of human platelet P2Y(12) receptor and the identification of potent inverse agonist.

Inhibitory effects of P2Y12 receptor antagonists on TRAP-induced platelet aggregation, procoagulant activity, microparticle formation and intracellular calcium responses in patients with acute coronary syndromes

Thrombin induces platelet aggregation and membrane rearrangements leading to enhanced procoagulant activity and microparticle production, all of which are thought to contribute to thrombus formation in patients with acute coronary syndromes (ACS). Clopidogrel, an adenosine diphosphate (ADP) receptor antagonist acting at the P2Y(12) receptor, has been shown to provide clinical benefit in ACS. We aimed to investigate the effects of clopidogrel ex vivo and another ADP-antagonist, AR-C69931MX in vitro on thrombin receptor activating peptide (TRAP)-induced platelet aggregation, procoagulant activity, microparticle formation and [Ca(2+)]i responses in patients with ACS. Measurements were performed in platelet-rich plasma using aggregometry and flow cytometry (n = 12). Clopidogrel (300 mg loading dose plus 75 mg daily) significantly inhibited TRAP-induced aggregation, procoagulant activity (annexin V binding) and microparticle production (all P < 0.05) but not as extensively as AR-C69931MX (400 nmol/l). [Ca(2+)]i responses induced by a combination of TRAP and ADP designed to mimic the physiological effects of released ADP showed that clopidogrel partially and AR-C69931MX completely removed the ADP component of the [Ca(2+)]i responses (n = 6). The results provide new information on the mechanisms involved in the beneficial effects of P2Y(12) antagonists in patients with ACS.

Rapamycin, but not FK-506, increases endothelial tissue factor expression: implications for drug-eluting stent design

BACKGROUND

Drugs released from stents affect the biology of vascular cells. We examined the effect of rapamycin and FK-506 on tissue factor (TF) expression in human aortic endothelial cells (HAECs) and vascular smooth muscle cells (HAVSMCs).

METHODS AND RESULTS

Rapamycin enhanced thrombin- and tumor necrosis factor (TNF)-alpha-induced endothelial TF expression in a concentration-dependent manner. The maximal increase was 2.5-fold more pronounced than that by thrombin or TNF-alpha alone and was paralleled by a 1.4-fold higher TF surface activity compared with thrombin alone. Rapamycin by itself increased basal TF levels by 40%. In HAVSMCs, rapamycin did not affect thrombin- or TNF-alpha-induced TF expression. In contrast to rapamycin, FK-506 did not enhance thrombin- or TNF-alpha-induced endothelial TF expression. Thrombin induced a transient dephosphorylation of the mammalian target of rapamycin downstream target p70S6 kinase. Rapamycin completely abrogated p70S6 kinase phosphorylation, but FK-506 did not. FK-506 antagonized the effect of rapamycin on thrombin-induced TF expression. Rapamycin did not alter the pattern of p38, extracellular signal-regulated kinase, or c-Jun NH2-terminal kinase phosphorylation. Real-time polymerase chain reaction analysis revealed that rapamycin had no influence on thrombin-induced TF mRNA levels for up to 2 hours but led to an additional increase after 3 and 5 hours.

CONCLUSIONS

Rapamycin, but not FK-506, enhances TF expression in HAECs but not in HAVSMCs. This effect requires binding to FK binding protein-12, is mediated through inhibition of the mammalian target of rapamycin, and partly occurs at the posttranscriptional level. These findings may be clinically relevant for patients receiving drug-eluting stents, particularly when antithrombotic drugs are withdrawn or ineffective, and may open novel perspectives for the design of such stents.

Hämostaseologie

The design and development of new antithrombotics, i.e. anticoagulants and antiplatelet drugs, is a rapidly expanding area of pharmacological research. New anticoagulants, i.e. inhibitors of thrombin formation and action have been developed and some of them are already in clinical use. This includes hirudin and its analogs, such as bivalirudin. In contrast to heparins, these compounds as well as low-molecular weight inhibitors of thrombin and factor Xa directly inhibit thrombus-associated generation and action of thrombin, eventually associated with a reduced bleeding tendency. Orally active compounds are available and currently subject of clinical trials. It appears possible that these new agents may replace cumarins as oral anticoagulants, specifically in long-term use, in the near future. The introduction of clopidogrel marks another important development in the field of antiplatelet drugs. Synergistic actions of this compound with acetylsalicylic acid and GP-IIb/IIIa-antagonists because of their different mode of action enhance the antithrombotic potential considerably and have been clinically confirmed. Despite of this optimistic outlook, the individual risk/benefit ratio of these new drugs, in particular in the area of anticoagulants, still needs to be defined.

Clopidogrel has no effect on D-dimer and thrombin-antithrombin III levels in patients with peripheral arterial disease undergoing peripheral percutaneous transluminal angioplasty

OBJECTIVE

Coagulation activation markers are significantly elevated in patients with peripheral arterial disease compared with healthy controls. The more severe the disease, the higher the markers. Increased coagulation activation may contribute to the disease process and the risk of complications in patients with peripheral arterial disease, particularly after endovascular intervention. Animal studies have shown that clopidogrel significantly inhibits coagulation activation. The aim of this study was to determine whether combination of aspirin and clopidogrel affects thrombin-antithrombin III and D-dimer in patients with intermittent claudication undergoing angioplasty, compared with aspirin alone.

METHODS

This was a double blind, randomized placebo-controlled trial conducted in a vascular unit in a tertiary referral center. One hundred thirty-two patients with intermittent claudication were randomized to clopidogrel and aspirin or placebo and aspirin, with a loading dose 12 hours before endovascular intervention. D-dimer and thrombin-antithrombin III (TAT) levels were measured using enzyme-linked immunosorbent assay at baseline, 1 hour before, and 1 hour, 24 hours, and 30 days after intervention in 103 patients who underwent endovascular intervention.

RESULTS

There was a significant rise in D-dimer levels at 1 hour and 24 hours after angioplasty in both groups (placebo group: 63.69, 141.45, 122.18 ng/mL; clopidogrel group: 103.79, 159.95, 134.69 ng/mL), but no difference between the two groups (P = .514). Similarly there was a significant rise in TAT levels at 1 hour after angioplasty in both groups (placebo group: 2.93, 6.16 microg/L; clopidogrel group: 3.39, 5.27 microg/L), with no significant difference between the two groups (P = .746).

CONCLUSION

Endovascular intervention results in a significant increase in TAT and D-dimer. The addition of clopidogrel to aspirin has no effect on TAT and D-dimer before or after endovascular intervention.

The platelet P2 receptors as molecular targets for old and new antiplatelet drugs

Platelet activation by ADP and ATP plays a crucial role in haemostasis and thrombosis, and their so-called P2 receptors are potential targets for antithrombotic drugs. The ATP-gated channel P2X1 and the 2 G protein-coupled P2Y1 and P2Y12 ADP receptors selectively contribute to platelet aggregation. The P2Y1 receptor is responsible for ADP-induced shape change and weak and transient aggregation, while the P2Y12 receptor is responsible for the completion and amplification of the response to ADP and to all platelet agonists, including thromboxane A2 (TXA2), thrombin, and collagen. The P2X1 receptor is involved in platelet shape change and in activation by collagen under shear conditions. Due to its central role in the formation and stabilization of a thrombus, the P2Y12 receptor is a well-established target of antithrombotic drugs like ticlopidine or clopidogrel, which have proved efficacy in many clinical trials and experimental models of thrombosis. Competitive P2Y12 antagonists have also been shown to be effective in experimental thrombosis as well as in several clinical trials. Studies in P2Y1 and P2X1 knockout mice and experimental thrombosis models using selective P2Y1 and P2X1 antagonists have shown that, depending on the conditions, these receptors could also be potential targets for new antithrombotic drugs.

Effects of a novel platelet nitric oxide donor (LA816), aspirin, clopidogrel, and combined therapy in inhibiting flow- and lesion-dependent thrombosis in the porcine ex vivo model

BACKGROUND

Acetylsalicylic acid (ASA), or aspirin, plus clopidogrel is becoming the standard antithrombotic treatment in people with coronary disease. Novel approaches such as the use of platelet-selective nitric oxide (NO) donors may provide additional protection against thrombosis. We evaluated the antithrombotic properties of a novel platelet-selective NO donor (LA816) administered alone and in combination with ASA, clopidogrel, or ASA+clopidogrel.

METHODS AND RESULTS

Thrombogenicity was measured in the porcine experimental model and assessed as platelet-thrombus formation in the ex vivo Badimon perfusion chamber. Pigs were randomly divided into 4 groups: (1) placebo control, (2) clopidogrel, (3) ASA, and (4) ASA+clopidogrel (ASA and clopidogrel were given orally, 10 mg x kg(-1) x d(-1) for 3 d). The animals were anesthetized, heparinized, and catheterized, and the Badimon perfusion chamber was placed in an extracorporeal shunt. After baseline perfusions, all animal groups received the intravenous infusion of LA816 for 2 hours. Platelet aggregation, blood pressure, and heart rate also were evaluated during the experiments. LA816, clopidogrel, and ASA+clopidogrel produced a reduction of approximately 45% on thrombus mass versus placebo control perfusions (P<0.05). Combined treatment of oral ASA+clopidogrel and intravenous LA816 produced a significant further reduction of 25% in platelet deposition (70% from placebo controls; P<0.0001). LA816 intravenous treatment did not modify blood pressure or heart rate.

CONCLUSIONS

Acute NO donation with LA816, without modifying hemodynamic parameters, provides the same inhibitory effect as that obtained with chronic treatment with clopidogrel+ASA. Moreover, LA816 provides platelet inhibitory effects in addition to those of the combined blockade of cyclooxygenase and P2y(12) receptor.

Purine receptors: GPCR structure and agonist design

An integrated approach to the study of drug-receptor interactions has been applied to adenosine receptors (ARs) and P2Y nucleotide receptors. This approach includes probing the receptor structure through site-directed mutagenesis and molecular modeling, in concert with altering the structure of the agonist ligands. Goals of this structural approach are to generate a testable hypothesis for location of the binding site and subsequently to enable the rational design of new agonists and antagonists. In this manner, receptor subtype selectivity has been increased, and agonists have been converted into partial agonists and antagonists. An approach to receptor engineering (neoceptors) has been explored, in which synthetic small molecule agonists (neoligands) are specifically tailored to activate only receptors in which the putative binding sites have been modified. This orthogonal approach to receptor activation, intended for eventual gene therapy, has been demonstrated for A3 and A2A ARs.

Regulation of platelet membrane levels of glycoprotein VI by a platelet-derived metalloproteinase

Thrombosis can be initiated when activated platelets adhere to injured blood vessels via the interaction of subendothelial collagen with its platelet receptor, glycoprotein (GP) VI. Here we observed that incubation of platelets with convulxin, collagen, or collagen-related peptide (CRP) resulted in GPVI signaling-dependent loss of surface GPVI and the appearance of an approximately 55-kDa soluble fragment of GPVI as revealed by immunoblotting. Ethylenediaminetetraacetic acid (EDTA) or GM6001 (a metalloproteinase inhibitor with broad specificity) prevented this loss. In other receptor systems, calmodulin binding to membrane-proximal cytoplasmic sequences regulates metalloproteinase-mediated ectodomain shedding. In this regard, we have previously shown that calmodulin binds to a positively charged, membrane-proximal sequence within the cytoplasmic tail of GPVI. Incubation of platelets with calmodulin inhibitor W7 (150 μM) resulted in a time-dependent loss of GPVI from the platelet surface. Both EDTA and GM6001 prevented this loss. Surface plasmon resonance demonstrated that W7 specifically blocked the association of calmodulin with an immobilized synthetic peptide corresponding to the calmodulin-binding sequence of GPVI. These findings suggest that disruption of calmodulin binding to receptor cytoplasmic tails by agonist binding to the receptor triggers metalloproteinase-mediated loss of GPVI from the platelet surface. This process may represent a potential mechanism to regulate GPVI-dependent platelet adhesion.

Antiaggregatory activity in human platelets of potent antagonists of the P2Y 1 receptor

Activation of the P2Y(1) nucleotide receptor in platelets by ADP causes changes in shape and aggregation, mediated by activation of phospholipase C (PLC). Recently, MRS2500(2-iodo-N(6)-methyl-(N)-methanocarba-2'-deoxyadenosine-3',5'-bisphosphate) was introduced as a highly potent and selective antagonist for this receptor. We have studied the actions of MRS2500 in human platelets and compared these effects with the effects of two acyclic nucleotide analogues, a bisphosphate MRS2298 and a bisphosphonate derivative MRS2496, which act as P2Y(1) receptor antagonists, although less potently than MRS2500. Improved synthetic methods for MRS2500 and MRS2496 were devised. The bisphosphonate is predicted to be more stable in general in biological systems than phosphate antagonists due to the non-hydrolyzable CP bond. MRS2500 inhibited the ADP-induced aggregation of human platelets with an IC(50) value of 0.95 nM. MRS2298 and MRS2496 also both inhibited the ADP-induced aggregation of human platelets with IC(50) values of 62.8 nM and 1.5 microM, respectively. A similar order of potency was observed for the three antagonists in binding to the recombinant human P2Y(1) receptor and in inhibition of ADP-induced shape change and ADP-induced rise in intracellular Ca(2+). No substantial antagonism of the pathway linked to the inhibition of cyclic AMP was observed for the nucleotide derivatives, indicating no interaction of these three P2Y(1) receptor antagonists with the proaggregatory P2Y(12) receptor, which is also activated by ADP. Thus, all three of the bisphosphate derivatives are highly selective antagonists of the platelet P2Y(1) receptor, and MRS2500 is the most potent such antagonist yet reported.