A Key Role of Adenosine Diphosphate in the Irreversible Platelet Aggregation Induced by the PAR1-Activating Peptide Through the Late Activation of Phosphoinositide 3-Kinase

Phosphoinositide phosphatases: just as important as the kinases

Phosphoinositide phosphatases comprise several large enzyme families with over 35 mammalian enzymes identified to date that degrade many phosphoinositide signals. Growth factor or insulin stimulation activates the phosphoinositide 3-kinase that phosphorylates phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P(2)] to form phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P(3)], which is rapidly dephosphorylated either by PTEN (phosphatase and tensin homologue deleted on chromosome 10) to PtdIns(4,5)P(2), or by the 5-phosphatases (inositol polyphosphate 5-phosphatases), generating PtdIns(3,4)P(2). 5-phosphatases also hydrolyze PtdIns(4,5)P(2) forming PtdIns(4)P. Ten mammalian 5-phosphatases have been identified, which regulate hematopoietic cell proliferation, synaptic vesicle recycling, insulin signaling, and embryonic development. Two 5-phosphatase genes, OCRL and INPP5E are mutated in Lowe and Joubert syndrome respectively. SHIP [SH2 (Src homology 2)-domain inositol phosphatase] 2, and SKIP (skeletal muscle- and kidney-enriched inositol phosphatase) negatively regulate insulin signaling and glucose homeostasis. SHIP2 polymorphisms are associated with a predisposition to insulin resistance. SHIP1 controls hematopoietic cell proliferation and is mutated in some leukemias. The inositol polyphosphate 4-phosphatases, INPP4A and INPP4B degrade PtdIns(3,4)P(2) to PtdIns(3)P and regulate neuroexcitatory cell death, or act as a tumor suppressor in breast cancer respectively. The Sac phosphatases degrade multiple phosphoinositides, such as PtdIns(3)P, PtdIns(4)P, PtdIns(5)P and PtdIns(3,5)P(2) to form PtdIns. Mutation in the Sac phosphatase gene, FIG4, leads to a degenerative neuropathy. Therefore the phosphatases, like the lipid kinases, play major roles in regulating cellular functions and their mutation or altered expression leads to many human diseases.

The platelet hyporesponsiveness to clopidogrel in acute coronary syndrome patients treated with 75 mg/day clopidogrel may be overcome within 1 month of treatment

Platelets are involved in thrombus formation and inflammation following vascular injury, while clopidogrel exerts antithrombotic and anti-inflammatory actions. We investigated various platelet-derived prothrombotic and proinflammatory mediators as well as the platelet aggregatory response in patients with acute coronary syndromes (ACS) receiving clopidogrel, as a function of the patient responsiveness to drug treatment. Blood samples were obtained from 40 patients with recent (<24 h) ACS before clopidogrel loading 600 mg (followed by a maintenance dose of 75 mg/day) as well as 5-days and 30-days afterwards. Twelve patients exhibited platelet reactivity index (PRI) values higher than 50% evaluated by the Vasodilator Stimulated Phosphoprotein (VASP) test at 5 days and were characterized as nonresponders. The platelet response to adenosine diphosphate (ADP) and thrombin receptor agonist peptide-14 (TRAP) was studied by flow cytometry and light transmission aggregometry. A maximum reduction of ADP- or TRAP-induced platelet aggregation in 28 clopidogrel responding patients was observed at 5 days postclopidogrel loading, whereas in nonresponders, it was achieved at 30-days along with a significant decrease in the PRI values. Similar results were obtained for the membrane expression of CD40L and the production of platelet-derived microparticles. By contrast, the maximum inhibition of P-selectin expression and platelet-leukocyte conjugate formation was observed at 30-days in both patient groups. A maintenance dose of 75 mg clopidogrel differentially affects the platelet aggregation and platelet-derived prothrombotic and proinflammatory mediators in ACS patients within the first month of the treatment, a phenomenon that is highly influenced by the drug response variability. Since these factors may be involved in the major adverse cardiovascular events in ACS patients, especially in those undergoing percutaneous coronary intervention, the above findings may be clinically important.

P2 receptors and platelet function

Following vessel wall injury, platelets adhere to the exposed subendothelium, become activated and release mediators such as TXA(2) and nucleotides stored at very high concentration in the so-called dense granules. Released nucleotides and other soluble agents act in a positive feedback mechanism to cause further platelet activation and amplify platelet responses induced by agents such as thrombin or collagen. Adenine nucleotides act on platelets through three distinct P2 receptors: two are G protein-coupled ADP receptors, namely the P2Y(1) and P2Y(12) receptor subtypes, while the P2X(1) receptor ligand-gated cation channel is activated by ATP. The P2Y(1) receptor initiates platelet aggregation but is not sufficient for a full platelet aggregation in response to ADP, while the P2Y(12) receptor is responsible for completion of the aggregation to ADP. The latter receptor, the molecular target of the antithrombotic drugs clopidogrel, prasugrel and ticagrelor, is responsible for most of the potentiating effects of ADP when platelets are stimulated by agents such as thrombin, collagen or immune complexes. The P2X(1) receptor is involved in platelet shape change and in activation by collagen under shear conditions. Each of these receptors is coupled to specific signal transduction pathways in response to ADP or ATP and is differentially involved in all the sequential events involved in platelet function and haemostasis. As such, they represent potential targets for antithrombotic drugs.

Pharmacogenomics of clopidogrel: evidence and perspectives

Clopidogrel has become the mainstay oral antiplatelet regimen to prevent recurrent ischemic events after acute coronary syndromes or stent placement. However, there is marked interindividual variability in the antiplatelet effects of clopidogrel, and a reduced response to this drug may be a risk factor for ischemic complications. Pharmacogenomic analyses, including candidate-gene and genome-wide association studies, have confirmed that genetic polymorphisms in the hepatic cytochrome P450 (CYP) 2C19 dominantly affect the antiplatelet effects of clopidogrel. CYP2C19 reduced-function alleles have been associated with a significant decrease in clopidogrel responsiveness and a higher risk of adverse cardiac events including stent thrombosis, myocardial infarction, and death in several prospective studies, although these effects were not reproduced in a recent large randomized study that included a randomized control group. The US Food and Drug Administration addressed this issue by adding a boxed warning to the clopidogrel label and suggesting that adjusting the clopidogrel dose or using alternative antiplatelet agents should be potentially implemented for high-risk individuals who are identified based on the CYP2C19 genotype. Although it is promising that CYP2C19 genotyping could be used to guide personalized antiplatelet clopidogrel therapy, currently there is insufficient evidence to recommend routine genetic testing. Prospective randomized clinical trials are necessary to validate this pharmacogenomic approach to clopidogrel therapy. In the most recent trial, paraoxonase-1 (PON1) was identified as a crucial new enzyme for clopidogrel bioactivation, with its common Q192R polymorphism determining the rate of active metabolite and the clinical activity of clopidogrel. Further studies are needed to investigate the comprehensive influence of a number of different polymorphisms of CYP2C19 and PON1 variant alleles or other genetic variants on clopidogrel in various ethnic populations.

Pharmacology and clinical trials of reversibly-binding P2Y12 inhibitors

The important role of the P2Y12 receptor in amplification of platelet activation and associated responses and the limitations associated with clopidogrel therapy have led to the development of novel inhibitors of this receptor. Three reversibly-binding P2Y12 inhibitors are in phase 3 development, ticagrelor, cangrelor and elinogrel. The pharmacology and clinical trial data for each of these inhibitors are discussed and compared with relevant data for the thienopyridines clopidogrel and prasugrel.

Bleeding manifestations of congenital and drug-induced defects of the platelet P2Y12 receptor for adenosine diphosphate

P2Y12, one of the two platelet receptors for adenosine diphosphate (ADP), plays a central role in platelet function. Defects of P2Y12 should be suspected when ADP, even at high concentrations (≥10 µM), is unable to induce full, irreversible platelet aggregation. Patients with congenital P2Y12 defects display a mild-to-moderate bleeding diathesis of variable severity, characterised by mucocutaneous bleeding and excessive post-surgical and post-traumatic blood loss. Drugs that inhibit P2Y12 are potent antithrombotic drugs, attesting the central role played by P2Y12 in platelet thrombus formation. Clopidogrel, the most widely used drug that inhibits P2Y12, is effective both in monotherapy and in combination with acetylsalicylic acid (ASA). Its most important drawback is the inability to inhibit adequately P2Y12-dependent platelet function in about 1/3 of patients, at the recommended therapeutic doses. The incidence of bleeding events is similar in ASA-treated and clopidogrel-treated patients; however, the combination of ASA and clopidogrel causes more bleeding than each drug in monotherapy. Compared to clopidogrel, new drugs inhibiting P2Y12, such as prasugrel and ticagrelor, decrease the risk of cardiovascular events and increase the risk of bleeding complications, because they adequately inhibit P2Y12-dependent platelet function in the vast majority of treated patients.

Lipopolysaccharide signaling without a nucleus: kinase cascades stimulate platelet shedding of proinflammatory IL-1β-rich microparticles

Platelets contain unspliced heteronuclear IL-1β RNA, which is rapidly spliced and translated upon activation. LPS is a superior agonist for this atypical platelet response, but how LPS induces proinflammatory cytokine production in anucleate cells lacking NF-κB is unknown. Platelets express functional TLR4, and stimulation by LPS induced rapid splicing, translation, and secretion of mature IL-1β after caspase-1 processing. LPS stimulated microparticle shedding, and secreted IL-1β was exclusively present in these particles. Microparticles from LPS-stimulated platelets induced VCAM-1 production by cultured human endothelial cells, and blockade of endothelial IL-1β receptor with IL-1 receptor antagonist completely suppressed endothelial activation. Splicing was posttranscriptional as the SR kinase inhibitor TG003 blocked IL-1β RNA production by platelets, but not by monocytes, and was dependent on exogenous CD14--a property of platelets. We used a combination of small-molecule inhibitors, cell-penetrating chimeric peptide inhibitors, and gene-targeted animals to show splicing required MyD88 and TIRAP, and IRAK1/4, Akt, and JNK phosphorylation and activation. Traf6 couples MyD88 to the Akt pathway and, remarkably, a Traf6 interacting peptide-antennapedia chimera was more effective than LPS in stimulating IL-1β splicing. The Traf6 chimera did not, however, stimulate microparticle shedding, nor was IL-1β released. We conclude LPS-induced kinase cascades are sufficient to alter cellular responses, that three signals emanate from platelet TLR4, and that Akt and JNK activation are sufficient to initiate posttranscriptional splicing while another event couples microparticle shedding to TLR4 activation. Platelets contribute to the inflammatory response to LPS through production of microparticles that promote endothelial cell activation.

Extracellular Ca(2+) modulates ADP-evoked aggregation through altered agonist degradation: implications for conditions used to study P2Y receptor activation

ADP is considered a weak platelet agonist due to the limited aggregation responses it induces in vitro at physiological concentrations of extracellular Ca²⁺ [(Ca²⁺)_o]. Lowering [Ca²⁺]_o paradoxically enhances ADP-evoked aggregation, an effect that has been attributed to enhanced thromboxane A₂ production. This study examined the role of ectonucleotidases in the [Ca²⁺]_o-dependence of platelet activation. Reducing [Ca²⁺]_o from millimolar to micromolar levels converted ADP (10 μmol/l)-evoked platelet aggregation from a transient to a sustained response in both platelet-rich plasma and washed suspensions. Blocking thromboxane A₂ production with aspirin had no effect on this [Ca²⁺]_o-dependence. Prevention of ADP degradation abolished the differences between low and physiological [Ca²⁺]_o resulting in a robust and sustained aggregation in both conditions. Measurements of extracellular ADP revealed reduced degradation in both plasma and apyrase-containing saline at micromolar compared to millimolar [Ca²⁺]_o. As reported previously, thromboxane A₂ generation was enhanced at low [Ca²⁺]_o, however this was independent of ectonucleotidase activity_. P2Y receptor antagonists cangrelor and MRS2179 demonstrated the necessity of P2Y₁₂ receptors for sustained ADP-evoked aggregation, with a minor role for P2Y₁. In conclusion, Ca²⁺-dependent ectonucleotidase activity is a major factor determining the extent of platelet aggregation to ADP and must be controlled for in studies of P2Y receptor activation.

Molecular defects of the platelet P2 receptors

Human platelets express three types of P2 receptors, which play important roles in platelet function: P2X(1), P2Y(1) and P2Y(12). Only patients with either quantitative or qualitative abnormalities of the platelet P2Y(12) receptor have been well-characterized so far. Deficiencies of P2Y(12) are associated with nucleotide deletions in the open-reading frame, frameshifts, and early truncation of the protein, or with a nucleotide substitution in the transduction initiation codon. Congenital dysfunctions of P2Y(12) are associated with molecular defects involving the sixth trans-membrane domain or the adjacent third extracellular loop of the receptor, which identify a region of the protein whose integrity is necessary for normal receptor function. A mutation, predicting a lysine to glutamate (Lys174Glu) substitution was associated with decreased ligand binding to the receptor, suggesting that it is responsible for disruption of the adenosine diphosphate (ADP)-binding site of the receptor. Patients with P2Y(12) defects display a mild-to-moderate bleeding diathesis, characterized by mucocutaneous bleedings and excessive post-surgical and post-traumatic blood loss. Defects of P2Y(12) should be suspected when ADP, even at high concentrations (≥10 μM), is unable to induce full, irreversible platelet aggregation. Tests that evaluate the degree of inhibition of adenylyl cyclase by ADP should be used to confirm the diagnosis.

The roles and mechanisms of PAR4 and P2Y12/phosphatidylinositol 3-kinase pathway in maintaining thrombin-induced platelet aggregation

BACKGROUND AND PURPOSE

Activation of human platelets by thrombin is mediated predominately through two proteinase-activated receptors (PARs), PAR1 and PAR4. Phosphatidylinositol 3-kinase (PI3K) inhibition leads to reversible PAR1-mediated platelet aggregation, but has no effect on the stability of platelet aggregation induced by thrombin. In the present study, the molecular mechanisms underlying this difference were investigated.

EXPERIMENTAL APPROACH

The functions of PI3K and PAR4 were assessed using specific inhibitors and aggregometry. The duration of platelet glycoprotein (GP) IIb/IIIa exposure was determined by flow cytometry with the antibody PAC-1. Western blotting and fluo-3 was used to evaluate the activation of Akt and protein kinase C (PKC) and intracellular Ca(2+) mobilization respectively.

KEY RESULTS

When PAR4 function was inhibited either by the PAR4 antagonist YD-3 [1-benzyl-3-(ethoxycarbonylphenyl)-indazole] or by receptor desensitization, the PI3K inhibitor wortmannin turned thrombin-elicited platelet aggregation from an irreversible event to a reversible event. Moreover, wortmannin plus YD-3 markedly accelerated the inactivation of GPIIb/IIIa in thrombin-stimulated platelets. The aggregation-reversing activity mainly resulted from inhibition of both PI3K-dependent PKC activation and PAR4-mediated sustained intracellular Ca(2+) rises. Blockade of ADP P2Y(12) receptor with 2-methylthioadenosine 5'-monophosphate triethylammonium salt mimicked the inhibitory effect of wortmannin on PI3K-dependent PKC activation and its ability to reverse PAR1-activating peptide-induced platelet aggregation. Co-administration of 2-methylthioadenosine 5'-monophosphate triethylammonium salt with YD-3 also decreased the stability of thrombin-induced platelet aggregation.

CONCLUSIONS AND IMPLICATIONS

These results suggest that PAR4 acts in parallel with the P2Y(12)/PI3K pathway to stabilize platelet aggregates, and provide new insights into the mechanisms of thrombus stabilization and potential applications for antithrombotic therapy.

Ticagrelor effectively and reversibly blocks murine platelet P2Y12-mediated thrombosis and demonstrates a requirement for sustained P2Y12 inhibition to prevent subsequent neointima

OBJECTIVE

Our goal was to study the effects of ticagrelor on murine platelet function and thrombosis and characterize the time course of P2Y(12) inhibition required to inhibit neointima formation following vascular injury.

METHODS AND RESULTS

Mice were treated with ticagrelor or vehicle. Platelet aggregation and P-selectin expression were assessed over time, and thrombus formation was assessed in laser-injured cremasteric arterioles of P2Y(12)+/+ and P2Y(12)-/- mice. Neointima formation in FeCl(3)-injured carotid artery was assessed in C57BL/6 mice treated with different regimens of ticagrelor. Ticagrelor inhibited platelet aggregation and P-selectin expression in a dose-dependent, reversible manner. Ticagrelor inhibited thrombus formation to the same extent as seen in P2Y(12)-/- mice. Neointima formation was markedly reduced in mice treated with ticagrelor before and 4 hours after injury (neointima area: control, 39 921±22 749 μm(2), versus ticagrelor, 3705±2600 μm(2); P<0.01), whereas administration of ticagrelor either before injury only or from 4 hours postinjury was ineffective.

CONCLUSIONS

Ticagrelor effectively and reversibly inhibits P2Y(12)-mediated platelet function and thrombosis in mice. P2Y(12) inhibition is required both at the time of and after injury to effectively inhibit neointima formation. Additional studies are warranted to evaluate the role of P2Y(12) inhibition in preventing restenosis.

Regulation and roles of PI3Kβ, a major actor in platelet signaling and functions

Phosphoinositide 3-kinases (PI3Ks) are important signaling enzymes involved in the regulation of a number of critical cell functions. Significant progress has been made during the last few years in defining the implication of individual PI3K isoforms. The role of the class IA PI3Kβ in different cell types has only been recently uncovered by the use of isoform-selective inhibitors and the development of mouse models harboring p110β catalytic subunit knock-out or germline knock-in of a kinase-dead allele of p110β. Although it is classically admitted that class IA PI3Ks are activated by receptor tyrosine kinases through recruitment of the regulatory subunits to specific tyrosine phosphorylated motifs via their SH2 domains, PI3Kβ is activated downstream of G protein-coupled receptors, and by co-operation between heterotrimeric G proteins and tyrosine kinases. PI3Kβ has been extensively studied in platelets where it appears to play an important role downstream of ITAM signaling, G protein-coupled receptors and aIIbβ3 integrin. Accordingly, mouse exhibiting p110β inactivation selectively in megakaryocyte/platelets are resistant to thromboembolism induced by carotid injury. The present review summarizes recent data concerning the mechanisms of PI3Kβ regulation and the roles of this PI3K isoform in blood platelet functions and other cell types.

The platelet P2Y₁₂ receptor for adenosine diphosphate: congenital and drug-induced defects

P2Y₁₂, the G(i)-coupled platelet receptor for adenosine diphosphate (ADP), plays a central role in platelet function. Patients with congenital P2Y₁₂ defects display a mild to moderate bleeding diathesis, characterized by mucocutaneous bleedings and excessive post-surgical and post-traumatic blood loss. Defects of P2Y₁₂ should be suspected when ADP, even at high concentrations (≥ 10 μM), is unable to induce full, irreversible platelet aggregation. Tests that evaluate the degree of inhibition of adenylyl cyclase by ADP should be used to confirm the diagnosis. Drugs that inhibit P2Y₁₂ are potent antithrombotic drugs, attesting the central role played by P2Y₁₂ in platelet thrombus formation. Clopidogrel, the most widely used drug that inhibits P2Y₁₂, is effective both in monotherapy and in combination with acetylsalicylic acid. The most important drawback of clopidogrel is its inability to inhibit adequately P2Y₁₂-dependent platelet function in approximately one-third of patients who are therefore not protected from major cardiovascular events. New drugs, such as prasugrel and ticagrelor, which effectively inhibit P2Y₁₂ in the majority of patients, proved to be more efficacious than clopdidogrel in preventing major cardiovascular events. Although they increase the incidence of major bleedings, the net clinical benefit is in favor of the new P2Y₁₂ inhibitors.

Novel antiplatelet therapy

Antiplatelet therapy is a cornerstone of the management of patients with acute coronary syndromes or for those undergoing percutaneous coronary intervention. As the intricacies of platelet biology and mechanisms of thrombus formation are revealed, novel antiplatelet therapies have emerged. Bleeding risk, however, has grown in concert with more potent platelet inhibition. This article reviews platelet biology and receptors to provide a foundation for understanding of antiplatelelet therapy. It also highlights recent advances in antiplatelet therapy, with a focus on mechanisms of action, pharmacodynamic data, and the balance of thrombotic versus bleeding outcomes.

Protein targets of inflammatory serine proteases and cardiovascular disease

Serine proteases are a key component of the inflammatory response as they are discharged from activated leukocytes and mast cells or generated through the coagulation cascade. Their enzymatic activity plays a major role in the body's defense mechanisms but it has also an impact on vascular homeostasis and tissue remodeling. Here we focus on the biological role of serine proteases in the context of cardiovascular disease and their mechanism(s) of action in determining specific vascular and tissue phenotypes. Protease-activated receptors (PARs) mediate serine protease effects; however, these proteases also exert a number of biological activities independent of PARs as they target specific protein substrates implicated in vascular remodeling and the development of cardiovascular disease thus controlling their activities. In this review both PAR-dependent and -independent mechanisms of action of serine proteases are discussed for their relevance to vascular homeostasis and structural/functional alterations of the cardiovascular system. The elucidation of these mechanisms will lead to a better understanding of the molecular forces that control vascular and tissue homeostasis and to effective preventative and therapeutic approaches.

Role of phosphoinositide 3-kinase beta in platelet aggregation and thromboxane A2 generation mediated by Gi signalling pathways

PI3Ks (phosphoinositide 3-kinases) play a critical role in platelet functional responses. PI3Ks are activated upon P2Y12 receptor stimulation and generate pro-aggregatory signals. P2Y12 receptor has been shown to play a key role in the platelet aggregation and thromboxane A2 generation caused by co-stimulation with Gq or Gz, or super-stimulation of Gi pathways. In the present study, we evaluated the role of specific PI3K isoforms alpha, beta, gamma and delta in platelet aggregation, thromboxane A2 generation and ERK (extracellular-signal-regulated kinase) activation. Our results show that loss of the PI3K signal impaired the ability of ADP to induce platelet aggregation, ERK phosphorylation and thromboxane A2 generation. We also show that Gq plus Gi- or Gi plus Gz-mediated platelet aggregation, ERK phosphorylation and thromboxane A2 generation in human platelets was inhibited by TGX-221, a PI3Kbeta-selective inhibitor, but not by PIK75 (a PI3Kalpha inhibitor), AS252424 (a PI3Kgamma inhibitor) or IC87114 (a PI3Kdelta inhibitor). TGX-221 also showed a similar inhibitory effect on the Gi plus Gz-mediated platelet responses in platelets from P2Y1-/- mice. Finally, 2MeSADP (2-methyl-thio-ADP)-induced Akt phosphorylation was significantly inhibited in the presence of TGX-221, suggesting a critical role for PI3Kbeta in Gi-mediated signalling. Taken together, our results demonstrate that PI3Kbeta plays an important role in ADP-induced platelet aggregation. Moreover, PI3Kbeta mediates ADP-induced thromboxane A2 generation by regulating ERK phosphorylation.

Mutational analysis of residues important for ligand interaction with the human P2Y(12) receptor

The P2Y(12) receptor, a Gi protein-coupled receptor, plays a central role in platelet activation. In this study, we did a mutational analysis of residues possibly involved in the ligand interactions with the human P2Y(12) receptor. Mutant receptors were stably expressed in CHO-K1 cells with an HA-tag at the N-terminus. Expression of wild-type and mutant receptors was confirmed by detecting the HA-tag on the cell membrane. Residues in transmembrane helical domains (TMs) 3, 5, 6, and 7, which are homologous to residues important for P2Y(1) receptor activation and ligand recognition, were replaced by site-directed mutagenesis. ADP-induced inhibition of forskolin-stimulated cAMP levels in the presence or absence of antagonist AR-C69931MX were investigated for each of the mutant receptors. F104S and S288P significantly increased agonist-induced receptor function without affecting the antagonism by AR-C69931MX. Arg256 in TM6 and Arg 265 in extracellular loop 3 (EL3) are more important for antagonist recognition than effect on agonist-mediated receptor function. Compared to wild-type P2Y(12) receptor, mutations in Arg 256 or/and Arg 265 significantly increased the sensitivity to antagonist AR-C69931MX. Our study shows that the cytosolic side of TM3 and the exofacial side of TM5 are critical for P2Y(12) receptor function, which is different from P2Y(1). Arg 256 in TM6 and Arg265 in EL3 appear to play a role in antagonist recognition rather than effects on agonist-induced receptor function.

Platelet function and HIV: a case-control study

OBJECTIVE

Cardiovascular disease and myocardial infarction are of increasing concern in HIV-infected populations. Although platelets mediate arterial thrombosis, central to myocardial infarction, data on platelet function in HIV infection are lacking. We hypothesized that HIV-infected patients would have altered platelet reactivity.

DESIGN

A case-control study of platelet reactivity in 20 HIV-infected (HIVpos) and 20 age and sex-matched HIV-negative (HIVneg) individuals.

METHODS

Time-dependent platelet aggregation was measured in response to increasing concentrations of platelet agonists: epinephrine, collagen, thrombin receptor-activating peptide and ADP using light absorbance.

RESULTS

In both groups, mean age was 34 years, and 65% were men. Sixteen out of 20 (80%) of the HIVpos patients were on antiretroviral therapy with 12 out of 20 (60%) patients having HIV RNA less than 50 copies/ml. There were significant between-group differences in platelet reactivity across all four agonists. Platelets from HIVpos patients were more reactive to epinephrine [mean (SD) log concentration required to induce 50% maximal aggregation, 1.9 (1.2) versus 3.0 (1.7) mumol/l in HIVneg individuals, P = 0.028], whereas less platelet aggregation was observed in response to submaximal concentrations of the other agonists [thrombin receptor-activating peptide 72.5 (14.5)% versus 82.2 (7.6)% at 10 mumol/l, P = 0.011; ADP 67.3 (12.1)% versus 75.2 (8.8)% at 10 mumol/l, P = 0.035; collagen 16.6 (25.1)% versus 35.4 (31.5)% at 71.25 microg/ml, P = 0.007].

CONCLUSION

Between-group differences in platelet responses to all agonists suggest multiple underlying defects in platelet function in HIV infection. Further research is required to determine the contribution of antiretroviral therapy and relationships between platelet function and the increased cardiovascular disease observed in HIV-infected populations.

Efficacy and Safety of Ticagrelor: A Reversible P2Y12 Receptor Antagonist

Objective:

To summarize the pharmacokinetic and pharmacodynamic properties of ticagrelor, a selective P2Y12 receptor antagonist, and evaluate its role in the treatment of patients with acute coronary syndromes (ACS).

Data Sources:

A literature search was conducted in MEDLINE (1966–November 2009), International Pharmaceutical Abstracts (1970–November 2009), and EMBASE (1990–November 2009) using the MeSH terms and key words AZD6140, ticagrelor, P2Y12 receptor antagonist, cardiovascular disease, ACS, atherothrombosis, and platelets.

Study Selection And Data Extraction:

Selected studies evaluated the pharmacology, pharmacokinetics, pharmacodynamics, safety, and efficacy of ticagrelor for the treatment of ACS.

Data Synthesis:

Ticagrelor selectively and reversibly blocks the P2Y12 receptor, inhibiting platelet aggregation and preventing amplification of platelet activation. Optimal dosing strategy as determined by ticagrelor's pharmacokinetic and pharmacodynamic profile is a loading dose of 180 mg followed by 90 mg by mouth twice daily. At these doses, greater platelet inhibition is observed with ticagrelor as compared to clopidogrel 75 mg once daily in both clopidogrel-experienced and -naïve patients. Studies in patients experiencing ACS concluded that ticagrelor reduced the rate of cardiovascular death, nonfatal myocardial infarction, stent thrombosis, and overall mortality compared to clopidogrel without increasing major bleeding when administered with standard therapy for ACS. There was no significant difference in the risk of stroke with ticagrelor compared to clopidogrel; however, intracranial bleeding was more common with ticagrelor. Ticagrelor is well tolerated; however, minor bleeding, dyspnea, hypotension, nausea, and ventricular pauses were reported more frequently than with clopidogrel. Reversible inhibition with ticagrelor may allow for more rapid surgical intervention after discontinuation, suggesting greater flexibility in treatment of ACS.

Conclusions:

Ticagrelor's improved pharmacokinetic and pharmacodynamic profile builds upon the limitations of currently available P2Y12 receptor antagonists. Ticagrelor represents a promising approach for the prevention of cardiovascular events in patients with ACS.

PI 3-Kinase p110β regulation of platelet integrin α(IIb)β3

Hemopoietic cells express relatively high levels of the type I phosphoinositide (PI) 3-kinase isoforms, with p110δ and γ exhibiting specialized signaling functions in neutrophils, monocytes, mast cells, and lymphocytes. In platelets, p110β appears to be the dominant PI 3-kinase isoform regulating platelet activation, irrespective of the nature of the primary platelet activating stimulus. Based on findings with isoform-selective p110β pharmacological inhibitors and more recently with p110β-deficient platelets, p110β appears to primarily signal downstream of G(i)- and tyrosine kinase-coupled receptors. Functionally, inhibition of p110β kinase function leads to a marked defect in integrin α(IIb)β₃ adhesion and reduced platelet thrombus formation in vivo. This defect in platelet adhesive function is not associated with increased bleeding, suggesting that therapeutic targeting of p110β may represent a safe approach to reduce thrombotic complications in patients with cardiovascular disease.

Platelet thrombin receptor antagonism and atherothrombosis

Clinical manifestations of atherothrombotic disease, such as acute coronary syndromes, cerebrovascular events, and peripheral arterial disease, are major causes of mortality and morbidity worldwide. Platelet activation and aggregation are ultimately responsible for the progression and clinical presentations of atherothrombotic disease. The current standard of care, dual oral antiplatelet therapy with aspirin and the P2Y(12) adenosine diphosphate (ADP) receptor inhibitor clopidogrel, has been shown to improve outcomes in patients with atherothrombotic disease. However, aspirin and P2Y(12) inhibitors target the thromboxane A(2) and the ADP P2Y(12) platelet activation pathways and minimally affect other pathways, while agonists such as thrombin, considered to be the most potent platelet activator, continue to stimulate platelet activation and thrombosis. This may help explain why patients continue to experience recurrent ischaemic events despite receiving such therapy. Furthermore, aspirin and P2Y(12) receptor antagonists are associated with bleeding risk, as the pathways they inhibit are critical for haemostasis. The challenge remains to develop therapies that more effectively inhibit platelet activation without increasing bleeding complications. The inhibition of the protease-activated receptor-1 (PAR-1) for thrombin has been shown to inhibit thrombin-mediated platelet activation without increasing bleeding in pre-clinical models and small-scale clinical trials. PAR-1 inhibition in fact does not interfere with thrombin-dependent fibrin generation and coagulation, which are essential for haemostasis. Thus PAR-1 antagonism coupled with existing dual oral antiplatelet therapy may potentially offer more comprehensive platelet inhibition without the liability of increased bleeding.

Phosphoinositide 3-kinase p110 beta regulates integrin alpha IIb beta 3 avidity and the cellular transmission of contractile forces

Phosphoinositide (PI) 3-kinase (PI3K) signaling processes play an important role in regulating the adhesive function of integrin alpha(IIb)beta(3), necessary for platelet spreading and sustained platelet aggregation. PI3K inhibitors are effective at reducing platelet aggregation and thrombus formation in vivo and as a consequence are currently being evaluated as novel antithrombotic agents. PI3K regulation of integrin alpha(IIb)beta(3) activation (affinity modulation) primarily occurs downstream of G(i)-coupled and tyrosine kinase-linked receptors linked to the activation of Rap1b, AKT, and phospholipase C. In the present study, we demonstrate an important role for PI3Ks in regulating the avidity (strength of adhesion) of high affinity integrin alpha(IIb)beta(3) bonds, necessary for the cellular transmission of contractile forces. Using knock-out mouse models and isoform-selective PI3K inhibitors, we demonstrate that the Type Ia p110 beta isoform plays a major role in regulating thrombin-stimulated fibrin clot retraction in vitro. Reduced clot retraction induced by PI3K inhibitors was not associated with defects in integrin alpha(IIb)beta(3) activation, actin polymerization, or actomyosin contractility but was associated with a defect in integrin alpha(IIb)beta(3) association with the contractile cytoskeleton. Analysis of integrin alpha(IIb)beta(3) adhesion contacts using total internal reflection fluorescence microscopy revealed an important role for PI3Ks in regulating the stability of high affinity integrin alpha(IIb)beta(3) bonds. These studies demonstrate an important role for PI3K p110 beta in regulating the avidity of high affinity integrin alpha(IIb)beta(3) receptors, necessary for the cellular transmission of contractile forces. These findings may provide new insight into the potential antithrombotic properties of PI3K p110 beta inhibitors.

Role of phosphoinositide 3-kinase beta in glycoprotein VI-mediated Akt activation in platelets

Glycoprotein (GP) VI is a critical platelet collagen receptor. Phosphoinositide 3-kinase (PI3K) plays an important role in GPVI-mediated platelet activation, yet the major PI3K isoforms involved in this process have not been identified. In addition, stimulation of GPVI results in the activation of Akt, a downstream effector of PI3K. Thus, we investigated the contribution of PI3K isoforms to GPVI-mediated platelet activation and Akt activation. A protein kinase C inhibitor GF 109203X or a P2Y(12) receptor antagonist AR-C69931MX partly reduced GPVI-induced Akt phosphorylation. Platelets from mice dosed with clopidogrel also showed partial Akt phosphorylation, indicating that GPVI-mediated Akt phosphorylation is regulated by both secretion-dependent and -independent pathways. In addition, GPVI-induced Akt phosphorylation in the presence of ADP antagonists was completely inhibited by PI3K inhibitor LY294002 and PI3Kbeta inhibitor TGX-221 indicating an essential role of PI3Kbeta in Akt activation directly downstream of GPVI. Moreover, GPVI-mediated platelet aggregation, secretion, and intracellular Ca(2+) mobilization were significantly inhibited by TGX-221, and less strongly inhibited by PI3Kalpha inhibitor PIK75, but were not affected by PI3Kgamma inhibitor AS252424 and PI3Kdelta inhibitor IC87114. Consistently, GPVI-induced integrin alpha(IIb)beta(3) activation of PI3Kgamma(-/-) and PI3Kdelta(-/-) platelets also showed no significant difference compared with wild-type platelets. These results demonstrate that GPVI-induced Akt activation in platelets is dependent in part on G(i) stimulation through P2Y(12) receptor activation by secreted ADP. In addition, a significant portion of GPVI-dependent, ADP-independent Akt activation also exists, and PI3Kbeta plays an essential role in GPVI-mediated platelet aggregation and Akt activation.

Multiple antiplatelet effects of clopidogrel are not modulated by statin type in patients undergoing percutaneous coronary intervention

We investigated whether statin type or dose influenced the inhibition of platelet function induced by clopidogrel in a prospective, open, parallel group study in patients undergoing elective percutaneous coronary intervention. Patients were taking CYP3A4 metabolised atorvastatin (n = 20) or simvastatin (n = 21), non-CYP3A4 metabolised pravastatin (n = 11) or fluvastatin (n = 2), or no statin therapy (n = 5). ADP and TRAP-induced platelet aggregation were measured using optical aggregometry, whole-blood single-platelet counting, and the Ultegra and Plateletworks point-of-care systems. Platelet pro-coagulant activity (annexin V binding and microparticle formation), P-selectin expression and platelet-leukocyte conjugate formation were assessed by flow cytometry. Platelet responses were measured at baseline, 4 h post clopidogrel 300 mg, and after 10 and 28 days with clopidogrel 75 mg daily. Clopidogrel significantly inhibited both ADP and TRAP-induced platelet responses over time, with steady state inhibition achieved by day 10. This was demonstrated by all techniques used. There was no significant effect of statin type or dose on platelet responses by any method at any time-point. In conclusion, statins do not influence the inhibitory effects of clopidogrel on multiple platelet responses, including aggregation, P-selectin expression, platelet-leucocyte conjugate formation and pro-coagulant responses, in patients undergoing elective PCI.

PAR-1 genotype influences platelet aggregation and procoagulant responses in patients with coronary artery disease prior to and during clopidogrel therapy

Genetic variations of the protease-activated receptor-1 (PAR-1) have been associated with platelet receptor density and linked to thrombin receptor-activating peptide (TRAP)-induced phenotypes of platelet aggregation and P-selectin expression. We investigated whether the PAR-1 intervening sequence-14 A>T dimorphism influences platelet procoagulant activity. We also determined whether the P2Y12 antagonist clopidogrel could offset any observed functional polymorphism of the PAR-1 receptor by inhibiting P2Y12-mediated amplification of TRAP-induced responses. We studied 54 patients listed for elective percutaneous coronary intervention assessing TRAP-induced platelet aggregation and markers of procoagulant activity. Platelet responses were measured at baseline, 4 h post clopidogrel 300 mg, and 10 and 28 days following clopidogrel 75 mg daily. Each patient was genotyped for the PAR-1 intervening sequence-14 A/T dimorphism. Increased platelet aggregation and procoagulant responses were observed with PAR-1 A allele homozygotes. Clopidogrel significantly inhibited these platelet responses regardless of PAR-1 genotype, but did not offset the hyper-reactivity associated with the A/A homozygotes. We conclude that a common sequence variation within the PAR-1 gene influences TRAP-induced platelet procoagulant activity as well as aggregation. Higher platelet reactivity associated with PAR-1 IVSn-14 A allele homozygotes persists despite clopidogrel therapy. These individuals may be at higher risk of thromboembolic events and may require additional anti-platelet medication.

The P2Y12 antagonists, 2-methylthioadenosine 5'-monophosphate triethylammonium salt and cangrelor (ARC69931MX), can inhibit human platelet aggregation through a Gi-independent increase in cAMP levels

ADP plays an integral role in the process of hemostasis by signaling through two platelet G-protein-coupled receptors, P2Y1 and P2Y12. The recent use of antagonists against these two receptors has contributed a substantial body of data characterizing the ADP signaling pathways in human platelets. Specifically, the results have indicated that although P2Y1 receptors are involved in the initiation of platelet aggregation, P2Y12 receptor activation appears to account for the bulk of the ADP-mediated effects. Based on this consideration, emphasis has been placed on the development of a new class of P2Y12 antagonists (separate from clopidogrel and ticlopidine) as an approach to the treatment of thromboembolic disorders. The present work examined the molecular mechanisms by which two of these widely used adenosine-based P2Y12 antagonists (2-methylthioadenosine 5'-monophosphate triethylammonium salt (2MeSAMP) and ARC69931MX), inhibit human platelet activation. It was found that both of these compounds raise platelet cAMP to levels that substantially inhibit platelet aggregation. Furthermore, the results demonstrated that this elevation of cAMP did not require Gi signaling or functional P2Y12 receptors but was mediated through activation of a separate G protein-coupled pathway, presumably involving Gs. However, additional experiments revealed that neither 2MeSAMP nor ARC69931MX (cangrelor) increased cAMP through activation of A2a, IP, DP, or EP2 receptors, which are known to couple to Gs. Collectively, these findings indicate that 2MeSAMP and ARC69931MX interact with an unidentified platelet G protein-coupled receptor that stimulates cAMP-mediated inhibition of platelet function. This inhibition is in addition to that derived from antagonism of P2Y12 receptors.

Safety and tolerability of SCH 530348 in patients undergoing non-urgent percutaneous coronary intervention: a randomised, double-blind, placebo-controlled phase II study

BACKGROUND

An antithrombotic drug is needed that safely reduces cardiovascular events in patients undergoing percutaneous coronary intervention (PCI). We therefore assessed the tolerability and safety of SCH 530348-an oral platelet protease-activated receptor-1 antagonist.

METHODS

We randomly assigned patients aged 45 years or older and undergoing non-urgent PCI or coronary angiography with planned PCI to an oral loading dose of SCH 530348 (10 mg, 20 mg, or 40 mg) or matching placebo in a 3:1 ratio in a multicentre international study. Those in the SCH 530348 group who subsequently underwent PCI (primary PCI cohort) continued taking an oral maintenance dose (0.5 mg, 1.0 mg, or 2.5 mg per day), and patients in the placebo group continued placebo for 60 days. The primary endpoint was the incidence of clinically significant major or minor bleeding according to the thrombolysis in myocardial infarction (TIMI) scale. Both investigators and patients were unaware of treatment allocation. Analysis was by intention to treat. This study is registered with ClinicalTrials.gov, number NCT00132912.

FINDINGS

257 patients were assigned to placebo and 773 to SCH 530348. The primary endpoint occurred in 2 (2%) of 129, 3 (3%) of 120, and 7 (4%) of 173 patients, respectively, in the SCH 530348 10 mg, 20 mg, and 40 mg groups compared with 5 (3%) of 151 patients in the placebo group (p=0.5786). TIMI major plus minor bleeding occurred in 3 (2%) of 136, 5 (4%) of 139, and 4 (3%) of 138 patients given SCH 530348 0.5 mg, 1.0 mg, and 2.5 mg once per day, respectively (p=0.7561).

INTERPRETATION

Oral SCH 530348 was generally well tolerated and did not cause increased TIMI bleeding, even when administered concomitantly with aspirin and clopidogrel. Further testing in phase III trials to accurately define the safety and efficacy of SCH 530348 is warranted.

Multiple ways to switch platelet integrins on and off

In the classical concept of platelet integrin activation, it is considered that unidirectional conformational changes of alpha(IIb)beta(3) and alpha(2)beta(1) regulate the adhesiveness of platelets for fibrin(ogen) and collagen, respectively. Here, we summarize recent evidence that these conformational changes: (i) can also occur in the reverse direction; and (ii) are not independent events. Platelet stimulation through the P2Y(12) receptors provokes only transient alpha(IIb)beta(3) activation via signaling routes involving phosphoinositide 3-kinases and Rap1b. Furthermore, alpha(IIb)beta(3) can be secondarily inactivated in platelets with prolonged high Ca(2+) rises, which expose phosphatidylserine and bind coagulation factors. Thus, platelet stimulation with strong agonists (collagen and thrombin) also results in transient integrin activation. Integrin alpha(2)beta(1) is found to be activated by a mechanism that is directly linked to alpha(IIb)beta(3) activation. Integrin alpha(2)beta(1) can adopt different activation states, depending on the trigger. Conclusively, reversibility and synchrony of platelet integrin activation are newly identified mechanisms to restrict thrombus growth and to allow optimal coagulation factor binding. Back-shifting of activated integrins towards their resting state may be a novel goal of antithrombotic medication.

Protease-activated receptor-induced Akt activation--regulation and possible function

BACKGROUND

Thrombin induces the activation of the platelet serine/threonine kinase Akt. Akt activation is dependent on its phosphorylation at Thr308 and Ser473. The mechanism by which thrombin induces Akt phosphorylation is controversial, as is the role of Akt in platelet function.

OBJECTIVES

To investigate how protease-activated receptors (PARs) stimulate Akt and the role that Akt plays in human platelet function.

METHODS

Platelets were stimulated through PAR1 or PAR4. Specific inhibitors were used to evaluate, by Western blotting, signaling pathways regulating Akt phosphorylation, and the role of activated Akt was evaluated by aggregometry and flow cytometry.

RESULTS

Phospholipase C (PLC) controls Akt phosphorylation elicited by PARs. Stimulation of PAR1 or PAR4 resulted in rapid Akt phosphorylation, independently of secreted ADP and phosphatidylinositol-3-kinase (PI3K) activation. Akt phosphorylation approximately 60 s after PAR1 stimulation became entirely dependent on the purinergic receptor P2Y(12) and the activation of PI3K. In contrast, PAR4 partially sustained Akt phosphorylation independently of P2Y(12) and PI3K for up to 300 s. Pharmacologic inhibition of Akt reduced P-selectin expression and fibrinogen binding in platelets stimulated through PAR1, and delayed platelet aggregation in response to submaximal PAR1 or PAR4 stimulation, although aggregation at 300 s was unaffected.

CONCLUSIONS

Platelet PAR stimulation causes rapid Akt phosphorylation downstream of PLC, whereas with continuous stimulation, ADP and PI3K are required for maintaining Akt phosphorylation. Activated Akt regulates platelet function by modulating secretion and alpha(IIb)beta(3) activation.

Characterization of a new peptide agonist of the protease-activated receptor-1

A new peptide (TFRRRLSRATR), derived from the c-terminal of human platelet P2Y(1) receptor, was synthesized and its biological function was evaluated. This peptide activated platelets in a concentration-dependent manner, causing shape change, aggregation, secretion and calcium mobilization. Of the several receptor antagonists tested, only BMS200261, a protease activated receptor 1 (PAR-1) specific antagonist, totally abolished the peptide-induced platelet aggregation, secretion and calcium mobilization. The TFRRR-peptide-pretreated washed platelets failed to aggregate in response to SFLLRN (10 microM) but not to AYPGKF (500 microM). In addition, in mouse platelets, peptide concentrations up to 600 microM failed to cause platelet activation, indicating that the TFRRR-peptide activated platelets through the PAR-1 receptor, rather than through the PAR-4 receptor. The shape change induced by 10 microM peptide was totally abolished by Y-27632, an inhibitor of p160(ROCK) which is a downstream mediator of G12/13 pathways. The TFRRR-peptide, YFLLRNP, and the physiological agonist thrombin selectively activated G12/13 pathways at low concentrations and began to activate both Gq and G12/13 pathways with increasing concentrations. Similar to SFLLRN, the TFRRR-peptide caused phosphorylation of Akt and Erk in a P2Y(12) receptor-dependent manner, and p-38 MAP kinase activation in a P2Y(12)-independent manner. The effects of this peptide are elicited by the first six amino acids (TFRRRL) whereas the remaining peptide (LSRATR), TFERRN, or TFEERN had no effects on platelets. We conclude that TFRRRL activates human platelets through PAR-1 receptors.

Insulin-like growth factor-1 regulates platelet activation through PI3-Kalpha isoform

Platelets release insulin-like growth factor-1 (IGF-1) from alpha granules upon activation. We have investigated the regulation of IGF-1 in G(i)-dependent pathways leading to Akt activation and the role of IGF-1 in platelet activation. IGF-1 alone failed to induce platelet aggregation, but IGF-1 potentiated 2-MeSADP-induced platelet aggregation in a concentration-dependent manner. IGF-1 triggered platelet aggregation in combination with selective P2Y(1) receptor activation. IGF-1 also caused platelet aggregation without shape change when combined with selective G(z) stimulation by epinephrine, suggesting the role of IGF-1 in platelet aggregation by supplementing G(i) pathways. The potentiating effect of IGF-1 was not affected by intracellular calcium chelation. Importantly, IGF-1 was unable to potentiate platelet aggregation by the phosphatidylinositol 3-kinase (PI3-K) inhibitor wortmannin, suggesting a critical regulation by PI3-K. Moreover, the potentiating effect of IGF-1 was abolished by the presence of PI3-K p110alpha inhibitor PIK-75. Stimulation of platelets with IGF-1 resulted in phosphorylation of Akt, a downstream effector of PI3-K, which was completely inhibited by wortmannin. IGF-1-induced Akt phosphorylation was abolished by PIK-75 suggesting the contribution of PI3-K p110alpha for activation of Akt by IGF-1. These results demonstrate that IGF-1 plays a role in potentiating platelet aggregation by complementing G(i)- but not G(q)-signaling pathways via PI3-K p110alpha.

Regulation and functional consequences of ADP receptor-mediated ERK2 activation in platelets

We have previously shown that ADP-induced thromboxane generation in platelets requires signalling events from the G(q)-coupled P2Y1 receptor (platelet ADP receptor coupled to stimulation of phospholipase C) and the G(i)-coupled P2Y12 receptor (platelet ADP receptor coupled to inhibition of adenylate cyclase) in addition to outside-in signalling. While it is also known that extracellular calcium negatively regulates ADP-induced thromboxane A2 generation, the underlying mechanism remains unclear. In the present study we sought to elucidate the signalling mechanisms and regulation by extracellular calcium of ADP-induced thromboxane A2 generation in platelets. ERK (extracllular-signal-regulated kinase) 2 activation occurred when outside-in signalling was blocked, indicating that it is a downstream event from the P2Y receptors. However, blockade of either P2Y1 or the P2Y12 receptors with corresponding antagonists completely abolished ERK phosphorylation, indicating that both P2Y receptors are required for ADP-induced ERK activation. Inhibitors of Src family kinases or the ERK upstream kinase MEK [MAPK (mitogen-activated protein kinase)/ERK kinase] abrogated ADP-induced ERK phosphorylation and thromboxane A2 generation. Finally ADP- or G(i)+G(z)-induced ERK phosphorylation was blocked in the presence of extracellular calcium. The present studies show that ERK2 is activated downstream of P2Y receptors through a complex mechanism involving Src kinases and this plays an important role in ADP-induced thromboxane A2 generation. We also conclude that extracellular calcium blocks ADP-induced thromboxane A2 generation through the inhibition of ERK activation.

Deficiency of Src homology 2 domain-containing inositol 5-phosphatase 1 affects platelet responses and thrombus growth

Platelets are critical for normal hemostasis. Their deregulation can lead to bleeding or to arterial thrombosis, a primary cause of heart attack and ischemic stroke. Src homology 2 domain-containing inositol 5-phosphatase 1 (SHIP1) is a 5-phosphatase capable of dephosphorylating the phosphatidylinositol 3,4,5-trisphosphate second messenger into phosphatidylinositol 3,4-bisphosphate. SHIP1 plays a critical role in regulating the level of these 2 lipids in platelets. Using SHIP1-deficient mice, we found that its loss affects platelet aggregation in response to several agonists with minor effects on fibrinogen binding and beta(3) integrin tyrosine phosphorylation. Accordingly, SHIP1-null mice showed defects in arterial thrombus formation in response to a localized laser-induced injury. Moreover, these mice had a prolonged tail bleeding time. Upon stimulation, SHIP1-deficient platelets showed large membrane extensions, abnormalities in the open canalicular system, and a dramatic decrease in close cell-cell contacts. Interestingly, SHIP1 appeared to be required for platelet contractility, thrombus organization, and fibrin clot retraction. These data indicate that SHIP1 is an important element of the platelet signaling machinery to support normal hemostasis. To our knowledge, this is the first report unraveling an important function of SHIP1 in the activation of hematopoietic cells, in contrast to its well-documented role in the negative regulation of lymphocytes.

Inhibition of the platelet P2Y12 receptor for adenosine diphosphate potentiates the antiplatelet effect of prostacyclin

BACKGROUND

Activation of two receptors for adenosine diphosphate (ADP), P2Y(1) and P2Y(12), is necessary for ADP-induced platelet aggregation (PA). It is generally believed that the antithrombotic effects of drugs inhibiting P2Y(12), such as clopidogrel, are uniquely mediated by inhibition of P2Y(12)-dependent PA. However, as P2Y(12) is negatively coupled to adenylyl cyclase (AC), its inhibition may also exert antithrombotic effects through the potentiation of prostacyclin (PGI(2)), which inhibit PA by stimulating AC.

OBJECTIVES

To test whether inhibition of P2Y(12) potentiates the antiplatelet effects of PGI(2).

METHODS

We measured the effects of PGI(2) (0.01-10 microm) on PA of washed human platelets induced by thrombin (0.5 U mL(-1)) in the presence or absence of ARC69931MX (anti-P2Y(12)) or MRS2500 (anti-P2Y(1)).

RESULTS

PGI(2) inhibited PA in the presence of anti-P2Y(12), but not in the presence of anti-P2Y(1) or in the absence of inhibitors. In contrast, dibutyryl-cyclicAMP inhibited PA both in the presence and absence of anti-P2Y(1) or anti-P2Y(12). PGI(2) increased platelet cyclicAMP levels only in the absence of thrombin or in the presence of thrombin plus anti-P2Y(12).

CONCLUSIONS

PGI(2) did not inhibit PA induced by thrombin, because its effect on AC was prevented by released ADP interacting with P2Y(12). Anti-P2Y(12) drugs, by rescuing AC activity, potentiate the antiplatelet effect of PGI(2), which may contribute to their antithrombotic effect.

Effects on blood compatibility in vitro by combining a direct P2Y12 receptor inhibitor and heparin coating of stents

The effect of the direct platelet P2Y12 receptor inhibitor, AR-C69931MX, on activation of blood induced by stents with and without heparin coating was investigated using a whole blood Chandler loop model in vitro. Stents were deployed in Chandler loops. Fresh human blood with heparin and AR-C69931MX was rotated for 1 h at 37 degrees C and used for measurements of platelets, microparticles, thrombin-antithrombin complex (TAT), fibrinogen binding to platelets, P-selectin expression by platelets, CD11b, Prothrombin Fragment F1+2, FXIa-AT, FXIIa-AT, C3a, sC5b-9 and stent score. In the first experiment there were four study groups with unmodified stents: 1a, no AR-C69931MX; 1b, 250 nmol/L; 1c, 750 nmol/L; 1d, 2250 nmol/L of AR-C69931MX. In the second experiment the concentration of AR-C69931MX was 500 nmol/L: 2a; tubings without stent; 2b; tubings with heparin-coated stent; 2c; tubings with unmodified stents. Heparin-coated stents were used in the third experiment: 3a; no AR-C69931MX; 3b; 500 nmol/L of AR-C69931MX. In the first experiment there were significant differences in all parameters analysed except for C3a, and stent score when the group with no AR-C69931MX was compared to all the groups with AR-C69931MX. In the second experiment there were significant differences in platelet count, TAT, FXIa-AT, FXIIa-AT and stent score when unmodified stents were compared to loops with no stents and partly to loops with heparin-coated stents. In the third experiment there was a significant reduction in generation of TAT, stent score and better preservation of platelet number by combining the platelet inhibitor and heparin-coated stents as compared to heparin-coated stents alone. The conclusion is that the direct P2Y12 receptor inhibitor AR-C69931MX reduced the different aspects of activation of blood induced by both unmodified and heparin-coated stents.

Protease-activated receptors in cardiovascular diseases

Thrombosis associated with the pathophysiological activation of platelets and vascular cells has brought thrombin and its receptors to the forefront of cardiovascular medicine. Thrombin signaling through the protease-activated receptors (PARs) has been shown to influence a wide range of physiological responses including platelet activation, intimal hyperplasia, inflammation, and maintenance of vascular tone and barrier function. The thrombin receptors PAR1 and PAR4 can be effectively targeted in animals in which acute or prolonged exposure to thrombin leads to thrombosis and/or restenosis. In the present study, we describe the molecular and pharmacological basis of small-molecule inhibitors that target PAR1. In addition, we discuss a new class of cell-penetrating inhibitors, termed pepducins, that provide insight into previously unidentified roles of PAR1 and PAR4 in protease signaling.

Haemostasis

When the continuity of the vascular endothelium is disrupted, platelets and fibrin seal off the defect. Haemostatic processes are classified as primary (mainly involving platelets) and secondary (mainly related to fibrin formation or blood coagulation). When the blood clot is no longer required for haemostasis, the fibrinolytic system will dissolve it. The pivotal ligand for initial platelet recruitment to injured vessel wall components is von Willebrand factor (vWF), a multimeric protein present in the subendothelium and in plasma, where it is conformationally activated by shear forces. Adhering activated platelets recruit additional platelets, which are in turn activated and form a platelet aggregate. Coagulation is initiated by a reaction, activating factors IX and X. Once critical amounts of factor Xa are generated, thrombin generation is initiated and soluble fibrinogen is converted into insoluble fibrin. Excessive thrombin generation is prevented via inhibition by antithrombin and also via downregulation of its further generation by activation of the protein C pathway. Activation of the fibrinolytic system results from conversion of the proenzyme plasminogen into the active serine proteinase plasmin by tissue-type or urokinase-type plasminogen activators. Plasmin digests the fibrin component of a blood clot. Inhibition of the fibrinolytic system occurs at the level of the plasminogen activator (by plasminogen activator inhibitors) or at the level of plasmin (by alpha2-antiplasmin). Together, these physiological processes act to maintain normal functioning blood vessels and a non-thrombotic state.

International Union of Pharmacology LVIII: update on the P2Y G protein-coupled nucleotide receptors: from molecular mechanisms and pathophysiology to therapy

There have been many advances in our knowledge about different aspects of P2Y receptor signaling since the last review published by our International Union of Pharmacology subcommittee. More receptor subtypes have been cloned and characterized and most orphan receptors de-orphanized, so that it is now possible to provide a basis for a future subdivision of P2Y receptor subtypes. More is known about the functional elements of the P2Y receptor molecules and the signaling pathways involved, including interactions with ion channels. There have been substantial developments in the design of selective agonists and antagonists to some of the P2Y receptor subtypes. There are new findings about the mechanisms underlying nucleotide release and ectoenzymatic nucleotide breakdown. Interactions between P2Y receptors and receptors to other signaling molecules have been explored as well as P2Y-mediated control of gene transcription. The distribution and roles of P2Y receptor subtypes in many different cell types are better understood and P2Y receptor-related compounds are being explored for therapeutic purposes. These and other advances are discussed in the present review.

Continuous signaling via PI3K isoforms beta and gamma is required for platelet ADP receptor function in dynamic thrombus stabilization

Signaling from collagen and G protein-coupled receptors leads to platelet adhesion and subsequent thrombus formation. Paracrine agonists such as ADP, thromboxane, and Gas6 are required for platelet aggregate formation. We hypothesized that thrombi are intrinsically unstable structures and that their stabilization requires persistent paracrine activity and continuous signaling, maintaining integrin alpha(IIb)beta3 activation. Here, we studied the disassembly of human and murine thrombi formed on collagen under high shear conditions. Platelet aggregates rapidly disintegrated (1) in the absence of fibrinogen-containing plasma; (2) by blocking or inhibiting alpha(IIb)beta3; (3) by blocking P2Y12 receptors; (4) by suppression of phosphoinositide 3-kinase (PI3K) beta. In murine blood, absence of PI3Kgamma led to formation of unstable thrombi, leading to dissociation of multiplatelet aggregates. In addition, blocking PI3Kbeta delayed initial thrombus formation and reduced individual platelet-platelet contact. Similarly without flow, agonist-induced aggregation was reversed by late suppression of P2Y12 or PI3K isoforms, resulting in single platelets that had inactivated alpha(IIb)beta3 and no longer bound fibrinogen. Together, the data indicate that continuous outside-in signaling via P2Y12 and both PI3Kbeta and PI3Kgamma isoforms is required for perpetuated alpha(IIb)beta3 activation and maintenance of a platelet aggregate. This novel concept of intrinsic, dynamic thrombus instability gives possibilities for the use of antiplatelet therapy.

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.

Role of the growth arrest-specific gene 6 (gas6) product in thrombus stabilization

Growth arrest-specific gene 6 (gas6) product enhances the formation of stable platelet macroaggregates in response to various agonists. To determine whether Gas6 amplifies the response to known platelet agonists through one or more of its receptor tyrosine kinases of the Tyro3 family, mice deficient in any one of the Gas6 receptors (Gas6-Rs: Tyro3, Axl, or Mer) were submitted to thrombosis challenge and their platelet function. The loss of any one of the Gas6-Rs protects mice against thromboembolism induced by collagen-epinephrine and stasis-induced thrombosis. Importantly, these mice do not suffer spontaneous bleeding and have a normal bleeding time but a tendency to repetitively re-bleed after transient hemostasis. Re-bleeding in mice lacking any one of the Gas6-Rs is not due to thrombocytopenia or coagulopathy but to a platelet dysfunction characterized by a lack of the second wave of platelet aggregation and an impaired clot retraction, at least in part by reducing outside-in alpha(IIb)beta(3) signaling and platelet granule secretion. The early release of Gas6 by agonists perpetuates platelet activation through its three receptors, reinforcing outside-in alpha(IIb)beta(3) signaling by activation of PI3K and Akt signaling and stimulation of tyrosine phosphorylation of the beta(3) integrin. Furthermore, "trapping" Gas6 prevents pathological thrombosis, which indicates that blocking this novel cross-talk between the Gas6-Rs and alpha(IIb)beta(3) integrin may constitute a novel target for antithrombotic therapy.

Evidence that the purinergic receptor P2Y12 potentiates platelet shape change by a Rho kinase-dependent mechanism

ADP activates human platelets through two G-protein coupled receptors, P2Y1 and P2Y12, to induce a range of functional responses. Here we have addressed the role and mechanism of P2Y12 in modulating ADP-induced platelet shape change. Although the response depended upon activation of P2Y1, it was potentiated by P2Y12 as the P2Y12-selective antagonists AR-C69931MX and 2MeSAMP partially inhibited shape change in the later phase of the response. This was paralleled by inhibition of pseudopod formation, platelet spheration, actin polymerisation and myosin light chain phosphorylation. P2Y12 is known to couple to activation of PI3 kinase and inhibition of adenylate cyclase, but we showed that neither of these signalling events couples to regulation of shape change by this receptor. However, by assessment of phosphorylation of its major substrate myosin light chain phosphatase, we provide direct evidence for activation of Rho kinase by ADP, and that although P2Y1 is required for activation of Rho kinase, P2Y12 is able to potentiate its activity. We conclude that P2Y12 plays a potentiatory role in ADP-induced shape change through regulation of the Rho kinase pathway, potentiating both myosin phosphorylation and actin polymerisation, and this forms part of an important signalling pathway additional to its well-established Gi-coupled pathways.