Molecular basis for ADP-induced platelet activation. I. Evidence for three distinct ADP receptors on human platelets

Different G protein-coupled signaling pathways are involved in alpha granule release from human platelets

Alpha granule release plays an important role in propagating a hemostatic response upon platelet activation. We evaluated the ability of various agonists to cause alpha granule release in platelets. Alpha granule release was measured by determining P-selectin surface expression in aspirin-treated washed platelets. ADP-induced P-selectin expression was inhibited both by MRS 2179 (a P2Y1 selective antagonist) and AR-C69931MX (a P2Y12 selective antagonist), suggesting a role for both Galpha(q) and Galpha(i) pathways in ADP-mediated alpha granule release. Consistent with these observations, the combination of serotonin (a Galpha(q) pathway stimulator) and epinephrine (a Galpha(z) pathway stimulator) also caused alpha granule release. Furthermore, U46619-induced P-selectin expression was unaffected by MRS 2179 but was dramatically inhibited by AR-C69931, indicating a dominant role for P2Y12 in U46619-mediated alpha granule release. Additionally, the Galpha(12/13)-stimulating peptide YFLLRNP potentiated alpha granule secretion in combination with either ADP or serotonin/epinephrine costimulation but was unable to induce secretion by itself. Finally, costimulation of the Galpha(i) and Galpha(12/13) pathways resulted in a significant dose-dependent increase in alpha granule release. We conclude that ADP-induced alpha granule release in aspirin-treated platelets occurs through costimulation of Galpha(q) and Galpha(i) signaling pathways. The P2Y12 receptor plays an important role in thromboxane A(2)-mediated alpha granule release, and furthermore activation of Galpha(12/13) and Galpha(q) signaling pathway can cause alpha granule release.

Identification of the active metabolite of ticlopidine from rat in vitro metabolites

1 Ticlopidine is a well-known anti-platelet agent, but is not active by itself in vitro. We identified a metabolite with anti-platelet activity, which was generated after incubation of 2-oxo-ticlopidine with phenobarbital-induced rat liver homogenate in vitro. 2 An active moiety (UR-4501) was isolated by high-performance liquid chromatography after large-scale preparation of metabolites. 3 The chemical structure of UR-4501 was determined by a combination of liquid chromatography mass/mass spectrometry (LC/MS/MS) and nuclear magnetic resonance (NMR) analysis. 4 UR-4501 produced a concentration-dependent inhibition (3-100 microm) of ADP (10 microm)-induced human platelet aggregation, whereas 2-oxo-ticlopidine (3-100 microm) did not elicit inhibitory responses. 5 UR-4501 (10-100 microm) strongly inhibited ADP- and collagen-induced aggregation and slightly inhibited thrombin-induced aggregation. 6 The inhibition of rat washed platelet aggregation by UR-4501 (100 microm) persisted, even after the platelets had been washed twice. 7 These results suggest that UR-4501 is the molecule responsible for the in vivo activities of ticlopidine.

Role of G protein-gated inwardly rectifying potassium channels in P2Y12 receptor-mediated platelet functional responses

The role of the G(i)-coupled platelet P2Y(12) receptor in platelet function has been well established. However, the functional effector or effectors contributing directly to alphaIIbbeta3 activation in human platelets has not been delineated. As the P2Y(12) receptor has been shown to activate G protein-gated, inwardly rectifying potassium (GIRK) channels, we investigated whether GIRK channels mediate any of the functional responses of the platelet P2Y(12) receptor. Western blot analysis revealed that platelets express GIRK1, GIRK2, and GIRK4. In aspirin-treated and washed human platelets, 2 structurally distinct GIRK inhibitors, SCH23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride) and U50488H (trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(pyrrolidinyl)cyclohexyl] benzeneacetamide methanesulfonate), inhibited adenosine diphosphate (ADP)-, 2-methylthioADP (2-MeSADP)-, U46619-, and low-dose thrombin-mediated platelet aggregation. However, the GIRK channel inhibitors did not affect platelet aggregation induced by high concentrations of thrombin, AYPGKF, or convulxin. Furthermore, the GIRK channel inhibitors reversed SFLLRN-induced platelet aggregation, inhibited the P2Y(12)-mediated potentiation of dense granule secretion and Akt phosphorylation, and did not affect the agonist-induced G(q)-mediated platelet shape change and intracellular calcium mobilization. Unlike AR-C 69931MX, a P2Y(12) receptor-selective antagonist, the GIRK channel blockers did not affect the ADP-induced adenlylyl cyclase inhibition, indicating that they do not directly antagonize the P2Y(12) receptor. We conclude that GIRK channels are important functional effectors of the P2Y(12) receptor in human platelets.

N-linked glycosylation of platelet P2Y12 ADP receptor is essential for signal transduction but not for ligand binding or cell surface expression

P(2)Y(12) receptor is a G(i)-coupled adenosine diphosphate (ADP) receptor with a critical role in platelet aggregation. It contains two potential N-linked glycosylation sites at its extra cellular amino-terminus, which may modulate its activity. Studies of both tunicamycin treatment and site-directed mutagenesis have revealed a dispensable role of the N-linked glycosylation in the receptor's surface expression and ligand binding activity. However, the non-glycosylated P(2)Y(12) receptor is defective in the P(2)Y(12)-mediated inhibition of the adenylyl cyclase activity. Thus the study uncovers an unexpected vital role of N-linked glycans in receptor's signal transducing step but not in surface expression or ligand binding.

Phosphorylation of the vasodilator-stimulated phosphoprotein (VASP) by the anti-platelet drug, cilostazol, in platelets

Vasodilator-stimulated phosphoprotein (VASP) is a regulator of actin dynamics in platelets and a common substrate of both cAMP- and cGMP-dependent protein kinases (PKA and PKG). Elevations of the cAMP and cGMP concentration have been shown to inhibit platelet aggregation. Intracellular levels of cAMP and cGMP are regulated by the synthesizing system of adenylate cyclases, and hydrolysis by cyclic nucleotide phosphodiesterases (PDEs). The present study examined the effect of the anti-platelet drug, cilostazol, which inhibits PDE3 activity, on VASP phosphorylation in platelets. VASP phosphorylation was examined by immunoblotting with an anti-VASP antibody, M4, and an anti-phospho-VASP antibody, 16C2. Cilostazol phosphorylated VASP at both Ser157 and Ser239 in a concentration-dependent manner, but EHNA (PDE2 inhibitor), dipyridamole and zaprinast (PDE5 inhibitors) did not. Forskolin (adenylate cyclase activator) and sodium nitroprusside (SNP, NO donor) resulted in the VASP phosphorylation, with increase in the cAMP and cGMP level, respectively. Cilostazol increased cAMP, but not cGMP levels, in platelets. EHNA, zaprinast and dipyridamole, had no effect on cAMP and cGMP levels. The PKA/PKG inhibitor, H-89, inhibited VASP phosphorylation by cilostazol. These results demonstrated that cilostazol phosphorylates VASP through the PDE3 inhibition, increase of cAMP level, and PKA activation in platelets.

Effects of platelet inhibitors on propyl gallate-induced platelet aggregation, protein tyrosine phosphorylation, and platelet factor 3 activation

Propyl gallate (PG) is a platelet agonist characterized by inducing platelet aggregation, protein tyrosine phosphorylation, and platelet factor 3 activity. The mechanisms of platelet activation following PG stimulation were examined by pre-incubating platelets with well-defined platelet inhibitors using platelet aggregation, protein tyrosine phosphorylation, activated plasma clotting time, and annexin V binding by flow cytometry. PG-induced platelet aggregation and tyrosine phosphorylation of multiple proteins were substantially abolished by aspirin, apyrase, and abciximab (c7E3), suggesting that PG is associated with activation of platelet cyclooxygenase 1, adenosine phosphate receptors, and glycoprotein IIb/IIIa, respectively. The phosphorylation of the cytoskeletal enzyme pp60(c-src) increased following PG stimulation, but was blunted by pre-incubation of platelets with aspirin, apyrase, and c7E3, suggesting that tyrosine kinase is important for the signal transduction of platelet aggregation. Propyl gallate also activates platelet factor 3 by decreasing the platelet coagulation time and increasing platelet annexin V binding. Platelet incubation with aspirin, apyrase, and c7E3 did not alter PG-induced platelet coagulation and annexin V binding. The results suggest that platelet factor 3 activation and membrane phosphotidylserine expression were not involved with activation of platelet cyclooxygenase, adenosine phosphate receptors, and glycoprotein IIb/IIIa. PG is unique in its ability to stimulate platelet aggregation and coagulation simultaneously, and platelet inhibitors in this study affect only platelet aggregation but not platelet coagulation.

LDL oxidized by hypochlorous acid causes irreversible platelet aggregation when combined with low levels of ADP, thrombin, epinephrine, or macrophage-derived chemokine (CCL22)

The in vitro oxidation of low-density lipoprotein (LDL) by hypochlorous acid produces a modified form (HOCl-LDL) capable of stimulating platelet function. We now report that HOCl-LDL is highly effective at inducing platelet function, causing stable aggregation and alpha-granule secretion. Such stimulation depended on the presence of low levels of primary agonists such as adenosine diphosphate (ADP) and thrombin, or others like epinephrine (EPI) and macrophage-derived chemokine (MDC, CCL22). Agonist levels, which by themselves induced little or reversible aggregation, caused strong stable aggregation when combined with low levels of HOCl-LDL. Platelet activation by HOCl-LDL and ADP (1 microM) caused P-selectin (CD62P) exposure, without serotonin or adenosine triphosphate (ATP) secretion. Intracellular calcium levels rose slowly (from 100 to 200 nM) in response to HOCl-LDL alone and rapidly when combined with ADP to about 300 nM. p38 mitogen-activated protein kinase (MAPK) became phosphorylated in response to HOCl-LDL alone. This phosphorylation was not blocked by the protein kinase C (PKC) inhibitor bisindolylmaleimide, which reduced the extent of aggregation and calcium increase. However, the p38 MAPK inhibitor SB203580 blocked platelet aggregation and phosphorylation of p38 MAPK. These findings suggest that HOCl-LDL exposed during atherosclerotic plaque rupture, coupled with low levels of primary agonists, can rapidly induce extensive and stable thrombus formation.

Synthesis and pharmacological evaluation of 5H-[1]benzopyrano[4,3-d]pyrimidines effective as antiplatelet/analgesic agents

Synthesis and pharmacological screening of new 2-methylthio/2-methanesulfonyl/2-methoxy-5H-[1]benzopyrano[4,3-d]pyrimidines were planned in order to study the effects of the 5-substitution with alkoxy/phenoxy/alkylthio and phenylthio groups both on in vitro antiplatelet and in vivo antinociceptive activities. Antiplatelet activity was assessed in vitro against ADP, Arachidonic acid and U46619 induced aggregation, in rabbit plasma. Anti-inflammatory, analgesic and antipyretic activities were tested in rat paw edema, mouse writhing test and LPS induced rat fever, respectively. Amongst test compounds, 2-methylthio derivatives displayed an ASA-like antiplatelet activity whereas 2-methoxy and, particularly, 2-methanesulfonyl derivatives showed a broad spectrum of antiplatelet action, inhibiting both the ADP- and the AA- and U46619-induced aggregation. With regard to the in vivo pharmacological activities, mainly the 2-methoxy derivatives showed a significant analgesic effect comparable to that of indomethacin. SAR considerations, also in comparison with a number of previously described compounds, were performed.

ADP receptor antagonists as antiplatelet therapeutics

With the cloning of the P2Y12 receptor, the molecular basis for ADP-induced platelet aggregation is seemingly complete. Two platelet-bound ADP receptors, P2Y1 and P2Y12, operate through unique pathways to induce and sustain platelet aggregation via the glycoprotein (GP)IIb-IIIa integrin. P2Y1 operates via a glycoprotein q (Gq) pathway, activates phospholipase C, induces platelet shape change and is responsible for intracellular calcium mobilisation. P2Y12 inhibits adenylyl cyclase through a glycoprotein i (Gi)-dependent pathway, and is the target of the clinically used thienopyridines, ticlopidine (Ticlid, F. Hoffman-La Roche) and clopidogrel (Plavix, Bristol-Myers Squibb/Sanofi-Synthelabo). In addition, the receptor is targeted by the ADP analogue AR-C66096, which is currently in Phase IIb clinical trials, as well as other non-nucleoside-based preclinical leads.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Purification and functional reconstitution of the human P2Y12 receptor

The human P2Y12 receptor (P2Y12-R) is a member of the G protein coupled P2Y receptor family, which is intimately involved in platelet physiology. We describe here the purification and functional characterization of recombinant P2Y12-R after high-level expression from a baculovirus in Sf9 insect cells. Purified P2Y12-R, Gbeta1gamma2, and various Galpha-subunits were reconstituted in lipid vesicles, and steady-state GTPase activity was quantified. GTP hydrolysis in proteoliposomes formed with purified P2Y12-R and Galphai2beta1gamma2 was stimulated by addition of either 2-methylthio-ADP (2MeSADP) or RGS4 and was markedly enhanced by their combined presence. 2MeSADP was the most potent agonist (EC50 = 80 nM) examined, whereas ADP, the cognate agonist of the P2Y12-R, was 3 orders of magnitude less potent. ATP had no effect alone but inhibited the action of 2MeSADP; therefore, ATP is a relatively low-affinity antagonist of the P2Y12-R. The G protein selectivity of the P2Y12-R was examined by reconstitution with various G protein alpha-subunits in heterotrimeric form with Gbeta1gamma2. The most robust coupling of the P2Y12-R was to Galphai2, but effective coupling also occurred to Galphai1 and Galphai3. In contrast, little or no coupling occurred to Galphao or Galphaq. These results illustrate that the signaling properties of the P2Y12-R can be studied as a purified protein under conditions that circumvent the complications that occur in vivo because of nucleotide metabolism and interconversion as well as nucleotide release.

Molecular characterization and immunolocalization of Schistosoma mansoni ATP-diphosphohydrolase

Schistosoma mansoni, a human parasite that constitutes a major health problem in developing countries, escapes from host defenses and survives in the human bloodstream. Here, we report the cloning of a S. mansoni ATP-diphosphohydrolase ortholog (SmATPDase1). Southern blots indicated that in S. mansoni it is a single-copy gene. RT-PCR revealed that SmATPDase1 is expressed in five stages of the parasite life cycle, namely cercaria, schistosomula, adults, eggs, and miracidia. Using confocal microscopy, SmATPDase1 protein was immunolocalized on the external surface in all stages, except eggs, being conspicuously present in adults. ATPDase, which is present on the outer surface of endothelial cells lining human blood vessels, has been implicated in thromboregulation by promoting ADP hydrolysis and inhibition of platelet aggregation. The presence of an ATPDase ortholog on the surface of S. mansoni suggests that the enzyme might play a role in the escape from host defenses that would involve platelet activation.

Differential effects of phosphodiesterase inhibitors on platelet activating factor (PAF)- and adenosine diphosphate (ADP)-induced equine platelet aggregation

Compounds that activate adenylate cyclase, increasing intracellular cyclic adenosine monophosphate (cAMP), inhibit equine platelet aggregation. Cyclic AMP is broken down by phosphodiesterase (PDE) and, in the present study, the effects of theophylline, a nonselective PDE inhibitor, and selective inhibitors of PDE isoenzymes PDE3, PDE4 and PDE5, on equine platelet aggregation in response to platelet activating factor (PAF) and adenosine diphosphate (ADP) have been examined. Theophylline and the PDE3 inhibitors, trequinsin and quazinone, inhibited both PAF and ADP-induced aggregation in a concentration dependent manner. The inhibition of PAF-induced aggregation was, however, significantly greater than that of the response to ADP. The inhibitory effects of theophylline and the PDE3 inhibitors on ADP- but not PAF-, induced aggregation were reversed by addition of the calcium ionophore, A23187. Rolipram and zaprinast, inhibitors of PDE4 and PDE5, respectively, had no effect on either PAF- or ADP-induced aggregation. These results demonstrate that inhibition of aggregation caused by PAF or ADP can be achieved by selective inhibition of PDE3 but suggest that there may be agonist-specific differences in the intracellular signalling pathways that regulate equine platelet aggregation.

Inherited platelet-based bleeding disorders

Inherited platelet-based bleeding disorders include abnormalities of platelet number and function, and are generally classified based on the abnormal functions or responses. However, a clear distinction is problematic, and in this review, the classification has been based on abnormalities of platelet components that share common characteristics. Inherited thrombocytopenias are rare, but probably underdiagnosed. They are usually classified according to both platelet size and the presence or absence of clinical features other than those deriving from the platelet defect. Hereditary disorders of platelet function can be classified as resulting from: (i) abnormalities of the platelet receptors for adhesive proteins; (ii) abnormalities of the platelet receptors for soluble agonists; (iii) abnormalities of the platelet granules; (iv) abnormalities of the signal-transduction pathways; (v) abnormalities of the membrane phospholipids; and (vi) miscellaneous abnormalities of platelet function. The literature on these disorders is reviewed, and the underlying defects discussed.

Inactivation of the human P2Y12 receptor by thiol reagents requires interaction with both extracellular cysteine residues, Cys17 and Cys270

Human platelets express 2 G protein-coupled nucleotide receptors: the platelet adenosine diphosphate (ADP) receptor coupled to stimulation of phospholipase C (P2Y(1)) via heterotrimeric guanosine 5-triphosphate (GTP)-binding protein G(q), and the platelet ADP receptor coupled to inhibition of adenylyl cyclase (P2Y(12)) via heterotrimeric GTP-binding protein G(i). Although these 2 receptors are encoded on the same chromosome and have similar pharmacologic profiles, they have different reactivities toward thiol reagents. The thiol agent p-chloromercuribenzene sulfonic acid (pCMBS) and the active metabolites from antiplatelet drugs, clopidogrel and CS-747, inactivate the P2Y(12) receptor and are predicted to interact with the extracellular cysteine residues on the P2Y(12) receptor. In this study we identified the reactive cysteine residues on the human P2Y(12) receptor by site-directed mutagenesis using pCMBS as the thiol reagent. Cys97Ser and Cys175Ser mutants of the P2Y(12) receptor did not express when transfected into Chinese hamster ovary (CHO-K1) cells, indicating the essential nature of a disulfide bridge between these residues. The Cys17Ser, Cys270Ser, and Cys17Ser/Cys270Ser double mutants had similar median effective concentration (EC(50)) values for ADP and 2-methylthio-ADP (2-MeSADP) when compared with the wild-type P2Y(12). Similarly, the median inhibitory concentration (IC(50)) values for BzATP (2',3'-O-(4- benzoylbenzoyl) adenosine 5'-triphosphate), an antagonist of the P2Y(12) receptor, also did not differ dramatically among these mutants and the wild-type P2Y(12) receptor. pCMBS inactivated the wild-type P2Y(12) receptor in a concentration-dependent manner, whereas it had no effect on the P2Y(1) receptor. Finally, pCMBS partially affected the G(i) coupling of Cys17Ser or Cys270Ser receptor mutants, but had no effect on Cys17Ser/Cys270Ser P2Y(12) receptor-mediated inhibition of adenylyl cyclase. These results indicate that, unlike the P2Y(1) receptor, which has 2 essential disulfide bridges linking its extracellular domains, the P2Y(12) receptor has 2 free cysteines in its extracellular domains (Cys17 and Cys270), both of which are targets of thiol reagents. We speculate that the active metabolites of clopidogrel and CS-747 form disulfide bridges with both Cys17 and Cys270 in the P2Y(12) receptor, and thereby inactivate the receptor.

P2Y receptors modulate ion channel function through interactions involving the C-terminal domain

Nucleotide stimulation of G(q)-coupled P2Y receptors expressed in Xenopus laevis oocytes produces the activation of an endogenous voltage-gated ion channel, previously identified as the transient inward (T(in)) channel. Expression of human P2Y(1), human P2Y(2), rat P2Y(6), human P2Y(11), or skate P2Y receptors in oocytes resulted in modulation of the voltage dependence and inactivation gating of the channel. Expression of the human P2Y(4) receptor, rat M(1)-muscarinic receptor, and human B(1)-bradykinin receptor did not alter the properties of the T(in) channel. Replacement of the C-terminal domain of the human B(1)-bradykinin receptor with the C-terminal domains of either the human P2Y(1) or human P2Y(2) receptor resulted in voltage dependence and inactivation-gating properties, respectively, of the T(in) channel that were similar to those elicited by the respective native P2Y receptor. Systematic truncation of the C-terminal region of the human P2Y(1) receptor identified a short region responsible for modulation of the T(in) channel. This region contains a conserved sequence motif found in all P2Y receptors that modulates the voltage dependence of the T(in) channel. Synthetic 20-mer peptides from the C-terminal domains of human P2Y(1) and P2Y(2) receptors produced a shift in the voltage dependence and slowed inactivation gating, respectively, after injection into oocytes expressing human B(1)-bradykinin or truncated human P2Y(1) receptors. These results indicate that certain P2Y receptors are capable of modulating the voltage sensitivity and inactivation gating of an endogenous oocyte ion channel through interactions involving the C-terminal region of the receptor. Such modulation of ion channel function could also exist in native mammalian cells that express P2Y receptors.

The effect of a loading dose (300 mg) of clopidogrel on platelet function in patients with peripheral arterial disease

Clopidogrel acts on the P2Y12 adenosine diphosphate (ADP) purinergic receptors on human platelets. The aim of this study was to establish if a loading dose of clopidogrel inhibits platelet activation in patients with peripheral arterial disease (PAD). Two indices of platelet activation were considered: platelet shape change (PSC) and aggregation. Citrated blood was collected from ten PAD patients who were not on aspirin, at baseline (0 hours) and 2 and 4 hours after these patients ingested a loading dose (300 mg) of clopidogrel. ADP (5 micromo/L)-induced platelet aggregation in whole blood was inhibited after 2 hours (free platelet count, 47% +/- 19% vs. 68% +/- 15%; p < or = 0.001) and 4 hours (47% +/- 19% vs. 66% +/- 16%; p < or = 0.001). There was also a significant inhibition of 5- hydroxytryptamine (SHT, 5.0 micromol/L)-induced platelet aggregation at 2 hours. This trend was also observed for 10-micomol/L ADP-induced aggregation. ADP (0.3-0.4 micromol/L)-induced PSC was significantly inhibited at 4 hours (increase in median platelet volume, 6.3%, 1.8-10.7 vs. 1.2%, 0-5.3; p = 0.01). 5HT (0.5 micromol/L)-induced PSC at 4 hours was also significantly inhibited (8.1, 5.3-10.6 vs. 3.0, 0-8.2; p = 0.03). A loading dose of clopidogrel (300 mg) inhibits platelet activation in PAD patients, as early as 2 hours. To the authors' knowledge, no other study considered the effect of a loading dose of clopidogrel in PAD.

Molecular bases of defective signal transduction in the platelet P2Y12 receptor of a patient with congenital bleeding

We have identified structural attributes required for signal transduction through a seven-transmembrane-domain receptor. Platelets from a patient (AC) with a congenital bleeding disorder had normal shape change but reduced and reversible aggregation in response to 4 microM ADP, similar to normal platelets with blocked P2Y(12) receptor. The response to 20 microM ADP, albeit still decreased, was more pronounced and was reduced by a P2Y(12) antagonist, indicating some residual receptor function. ADP failed to lower the adenylyl cyclase activity stimulated by prostaglandin E(1) in the patient's platelets, even though the number and affinity of 2-methylthioadenosine 5'-[(33)P]diphosphate-binding sites was normal. Analysis of the patient's P2Y(12) gene revealed a G-to-A transition in one allele, changing the codon for Arg-256 in the sixth transmembrane domain to Gln, and a C-to-T transition in the other allele, changing the codon for Arg-265 in the third extracellular loop to Trp. Neither mutation interfered with receptor surface expression but both altered function, since ADP inhibited the forskolin-induced increase of cAMP markedly less in cells transfected with either mutant P2Y(12) as compared with wild-type receptor. These studies delineate a region of P2Y(12) required for normal function after ADP binding.

PKD: a new protein kinase C-dependent pathway in platelets

Protein kinase D (PKD, also known as PKCmu) is closely related to the protein kinase C superfamily but is differentially regulated and has a distinct catalytic domain that shares homology with Ca(2+)-dependent protein kinases. PKD is highly expressed in hematopoietic cells and undergoes rapid and sustained activation upon stimulation of immune receptors. PKD is regulated through phosphorylation by protein kinase C (PKC). In the present study, we show that PKD is expressed in human platelets and that it is rapidly activated by receptors coupled to heterotrimeric G-proteins or tyrosine kinases. Activation of PKD is mediated downstream of PKC. Strong agonists such as convulxin, which acts on GPVI, and thrombin cause sustained activation of PKC and PKD, whereas the thromboxane mimetic U46619 gives rise to transient activation of PKC and PKD. Activation of PKD by submaximal concentrations of phospholipase C-coupled receptor agonists is potentiated by G(i)-coupled receptors (eg, adenosine diphosphate and epinephrine). This study shows that PKD is rapidly activated by a wide variety of platelet agonists through a PKC-dependent pathway. Activation of PKD enables phosphorylation of a distinct set of substrates to those targeted by PKC in platelets.

The role of platelet receptors and adhesion molecules in coronary artery disease

Platelets play a significant role in coronary artery disease through interactions with each other and with other cell types. These interactions are mediated by certain receptors on the surface of platelets and other cells which can lead to intra-coronary thrombus formation and occlusion that may result in acute coronary syndromes. The important roles of the currently available anti-platelet therapies have been well established in many clinical outcome trials in cardiovascular patients. An understanding of these different interactions provides the clinician with a background that supports the clinical importance of currently available anti-platelet therapies. Moreover, knowledge of the mechanisms of cellular crosstalk will lead to important advances in the development of better antithrombotic therapies.

Blockade of the platelet P2Y12 receptor by AR-C69931MX sustains coronary artery recanalization and improves the myocardial tissue perfusion in a canine thrombosis model

OBJECTIVE

Reperfusion therapy for myocardial infarction is limited by a significant reocclusion rate and less optimal myocardial tissue perfusion due to excessive platelet accumulation and recruitment at the sites of vascular injury. We assessed the influence of a selective P2Y(12)-receptor antagonist (AR-C69931MX), in conjunction with thrombolytic therapy, on the prevention of platelet aggregation and thrombus formation.

METHODS AND RESULTS

A canine coronary electrolytic injury thrombosis model was used. Tissue-type plasminogen activator (t-PA; 1 mg/kg in phase I, 0.5 mg/kg in phase II in the AR-C69931MX group, and 1 mg/kg in the placebo group in phase I and II) was administered 30 minutes after thrombus formation; either saline or AR-C69931MX (4 micro g x kg(-1) x min(-1)) was given to all animals intravenously 10 minutes before t-PA administration for a total of 2 hours. All animals received heparin (80 U/kg) as an intravenous bolus followed by a continuous infusion of 17 U x kg(-1) x h(-1). Myocardial tissue perfusion was evaluated by use of the colored microsphere technique and real-time myocardial contrast echocardiography. The incidences of reocclusion and cyclic flow variation were significantly decreased in the AR-C69931MX group (P<0.05). Myocardial tissue flow with AR-C69931MX treatment improved significantly at 20 and 120 minutes after reflow, whereas tissue flow with placebo remained at a level similar to that during occlusion (P<0.05).

CONCLUSIONS

The adjunctive administration of AR-C69931MX blocked ADP-mediated platelet aggregation and recruitment and prevented platelet-mediated thrombosis, resulting in prolongation of reperfusion time and a decrease in reocclusion and cyclic flow variations. Importantly, myocardial tissue perfusion was significantly improved in the P2Y(12) antagonist group.

Effect of clopidogrel administration to healthy volunteers on platelet phosphorylation events triggered by ADP

The action of clopidogrel on platelet receptors was analysed using platelets obtained from 11 healthy volunteers given 75 mg of clopidogrel daily for 8 d. Samples of blood were taken before treatment and after 8 d of medication. Determination of 2-methylthioadenosine diphosphate trisodium (2MesADP)-induced platelet aggregation, serine/threonine and tyrosine phosphorylations were performed in the absence or presence of the P2Y1-receptor-specific antagonist: adenosine 3'-phosphate 5'-phosphate (A3P5P) or the strong inhibitor of GPIIb/IIIa activation: SR121566.

MAJOR CONCLUSIONS

1). Serine and threonine phosphorylations of the myosin light chain (P20) and pleckstrin (P47) do not behave similarly, although they are both recognized as the result of phospholipase C pathway stimulation triggered by the P2Y1 receptor. P47 is strongly affected by the A3P5P, and this appears to be highly dependent on P2Y12. However, P20 phosphorylation occurs in the presence of A3P5P, suggesting that the P2Y12 receptor signal contributes to P20 phosphorylation mediated by a calcium-independent pathway. The results suggest that P2Y1 and P2Y12 receptors interact to modulate the phosphorylation of P20 and P47. 2). The inside-out signalling dependent on both P2Y12 and P2Y1 is necessary for GPIIb/IIIa activation. 3). Clopidogrel and SR121566 inhibited the increase in tyrosine phosphorylation induced by 2MesADP and concomitantly inhibited platelet aggregation, indicating that most of the phosphorylations are GPIIb/IIIa dependent. However, neither clopidogrel nor SR121566 inhibited the first wave of 80 kDa substrate (cortactin) which is involved in the reorganization of the cytoskeleton necessary for shape change and which appeared to be essentially P2Y1 dependent.

Lineage-specific overexpression of the P2Y1 receptor induces platelet hyper-reactivity in transgenic mice

In order to investigate the role of the platelet P2Y1 receptor in several aspects of platelet activation and thrombosis, transgenic (TG) mice overexpressing this receptor specifically in the megakaryocytic/platelet lineage were generated using the promoter of the tissue-specific platelet factor 4 gene. Studies of the saturation binding of [33P]2MeSADP in the presence or absence of the selective P2Y1 antagonist MRS2179 indicated that wild-type (WT) mouse platelets bore 150 +/- 31 P2Y1 receptors and TG platelets 276 +/- 34, representing an 84% increase in P2Y1 receptor density. This led to a well defined phenotype of platelet hyper-reactivity in vitro, as shown by increased aggregations in response to adenosine 5'-diphosphate (ADP) and low concentration of collagen in TG as compared with WT platelets. Moreover, overexpression of the P2Y1 receptor enabled ADP to induce granule secretion, unlike in WT platelets, which suggests that the level of P2Y1 expression is critical for this event. Our results further suggest that the weak responses of normal platelets to ADP are due to a limited number of P2Y1 receptors rather than to activation of a specific transduction pathway. TG mice displayed a shortened bleeding time and an increased sensitivity to in vivo platelet aggregation induced by infusion of a mixture of collagen and epinephrine. Overall, these findings emphasize the importance of the P2Y1 receptor in hemostasis and thrombosis and suggest that variable expression levels of this receptor on platelets might play a role in thrombotic states in human, which remains to be assessed.

Estrogen stimulates arachidonoylethanolamide release from human endothelial cells and platelet activation

Estrogen replacement therapy has been associated with reduction of cardiovascular events in postmenopausal women, though the mechanism for this benefit remains unclear. Here we show that at physiological concentrations estrogen activates the anandamide membrane transporter of human endothelial cells and leads to rapid elevation of calcium (apparent within 5 minutes) and release of nitric oxide (within 15 minutes). These effects are mediated by estrogen binding to a surface receptor, which shows an apparent dissociation constant (K(d)) of 9.4 +/- 1.4 nM, a maximum binding (B(max)) of 356 +/- 12 fmol x mg protein(-1), and an apparent molecular mass of approximately 60 kDa. We also show that estrogen binding to surface receptors leads to stimulation of the anandamide-synthesizing enzyme phospholipase D and to inhibition of the anandamide-hydrolyzing enzyme fatty acid amide hydrolase, the latter effect mediated by 15-lipoxygenase activity. Because the endothelial transporter is shown to move anandamide across the cell membranes bidirectionally, taken together these data suggest that the physiological activity of estrogen is to stimulate the release, rather than the uptake, of anandamide from endothelial cells. Moreover, we show that anandamide released from estrogen-stimulated endothelial cells, unlike estrogen itself, inhibits the secretion of serotonin from adenosine diphosphate (ADP)-stimulated platelets. Therefore, it is suggested that the peripheral actions of anandamide could be part of the molecular events responsible for the beneficial effects of estrogen.

Protein kinase C- and calcium-regulated pathways independently synergize with Gi pathways in agonist-induced fibrinogen receptor activation

Platelet fibrinogen receptor activation is a critical step in platelet plug formation. The fibrinogen receptor (integrin alphaIIbbeta3) is activated by agonist-mediated G(q) stimulation and resultant phospholipase C activation. We investigated the role of downstream signalling events from phospholipase C, namely the activation of protein kinase C (PKC) and rise in intracellular calcium, in agonist-induced fibrinogen receptor activation using Ro 31-8220 (a PKC inhibitor) or dimethyl BAPTA [5,5'-dimethyl-bis-(o-aminophenoxy)ethane-N,N,N', N'-tetra-acetic acid], a high-affinity calcium chelator. All the experiments were performed with human platelets treated with aspirin, to avoid positive feedback from thromboxane A2. In the presence of Ro 31-8220, platelet aggregation caused by U46619 was completely inhibited while no effect or partial inhibition was seen with ADP and the thrombin-receptor-activating peptide SFLLRN, respectively. In the presence of intracellular dimethyl BAPTA, ADP- and U46619-induced aggregation and anti-alphaIIbbeta3 antibody PAC-1 binding were completely abolished. However, similar to the effects of Ro 31-8220, dimethyl BAPTA only partially inhibited SFLLRN-induced aggregation, and was accompanied by diminished dense-granule secretion. When either PKC activation or intracellular calcium release was abrogated, aggregation and fibrinogen receptor activation with U46619 or SFLLRN was partially restored by additional selective activation of the G(i) signalling pathway. In contrast, when both PKC activity and intracellular calcium increase were simultaneously inhibited, the complete inhibition of aggregation that occurred in response to either U46619 or SFLLRN could not be restored with concomitant G(i) signalling. We conclude that, while the PKC- and calcium-regulated signalling pathways are capable of inducing activating fibrinogen receptor independently and that each can synergize with G(i) signalling to cause irreversible fibrinogen receptor activation, both pathways act synergistically to effect irreversible fibrinogen receptor activation.

Human Ntera-2/D1 neuronal progenitor cells endogenously express a functional P2Y1 receptor

We report here that human Ntera-2/D1 (NT-2) cells, an undifferentiated committed neuronal progenitor cell line, endogenously express a functional P2Y(1) receptor, while other P2Y subtypes, except perhaps P2Y(4), are not functionally expressed. Quantitative RT-PCR analysis showed that NT-2 cells abundantly express mRNA for P2Y(1) and P2Y(11) receptors, while P2Y(2) and P2Y(4) receptors were detected at considerably lower levels. Western blot analysis also demonstrated expression of P2Y(1) receptors and Galpha(q/11) subunits. Various nucleotides induced intracellular Ca(2+) mobilisation in NT-2 cells in a concentration-dependent manner with a rank order potency of 2-MeSADP > 2-MeSATP > ADP > ATP > UTP > ATPgammaS, a profile resembling that of human P2Y(1) receptors. Furthermore, P2Y(1) receptor-specific (A3P5P) and P2Y-selective (PPADS, suramin) antagonists inhibited adenine nucleotide-induced Ca(2+) responses in a concentration-dependent manner, consistent with expression of a P2Y(1) receptor. Moreover, of seven adenine nucleotides tested, only Bz-ATP and ATPgammaS elicited small increases in cAMP formation suggesting that few, if any, functional P2Y(11) receptors were expressed. P2Y(1) receptor-selective adenine nucleotides, including 2-MeSADP and ADP, also induced concentration-dependent phosphorylation and hence, activation of the extracellular-signal regulated protein kinases (ERK1/2). NT-2 cells, therefore, provide a useful neuronal-like cellular model for studying the precise signalling pathways and physiological responses mediated by a native P2Y(1) receptor.

Effects of a P2Y(12) receptor antagonist on the response of equine platelets to ADP. Comparison with human platelets

Horses show susceptibility to platelet-related disorders. Equine platelets differ from human platelets in some of their responses, so information available about human platelets must be validated in the horse. Aggregation of platelets by ADP involves both P2Y(1) and P2Y(12) receptors on the platelet surface. We have compared the effect of the P2Y(12) antagonist, AR-C67085, on equine and human platelets in vitro using turbidimetric aggregometry to measure the rate and final extent of aggregation. Aggregation profiles, concentration-response curves and pA(2) values show that the rate of aggregation of equine platelets is much more susceptible to inhibition by AR-C67085 than that of human platelets. This species difference may reflect differences in the relative numbers of P2Y(1) and P2Y(12) receptors, or in intracellular signalling pathways, but will need to be considered by equine clinicians before using P2Y(12) antagonists in the treatment of thrombotic conditions.

Coordinated signaling through both G12/13 and G(i) pathways is sufficient to activate GPIIb/IIIa in human platelets

Activation of GPIIb/IIIa is known to require agonist-induced inside-out signaling through G(q), G(i), and G(z). Although activated by several platelet agonists, including thrombin and thromboxane A(2), the contribution of the G(12/13) signaling pathway to GPIIb/IIIa activation has not been investigated. In this study, we used selective stimulation of G protein pathways to investigate the contribution of G(12/13) activation to platelet fibrinogen receptor activation. YFLLRNP is a PAR-1-specific partial agonist that, at low concentrations (60 microm), selectively activates the G(12/13) signaling cascade resulting in platelet shape change without stimulating the G(q) or G(i) signaling pathways. YFLLRNP-mediated shape change was completely inhibited by the p160(ROCK) inhibitor, Y-27632. At this low concentration, YFLLRNP-mediated G(12/13) signaling caused platelet aggregation and enhanced PAC-1 binding when combined with selective G(i) or G(z) signaling, via selective stimulation of the P2Y(12) receptor or alpha(2A)-adrenergic receptor, respectively. Similar data were obtained when using low dose (10 nm), a thromboxane A(2) mimetic, to activate G(12/13) in the presence of G(i) signaling. These results suggest that selective activation of G(12/13) causes platelet GPIIb/IIIa activation when combined with G(i) signaling. Unlike either G(12/13) or G(i) activation alone, co-activation of both G(12/13) and G(i) resulted in a small increase in intracellular calcium. Chelation of intracellular calcium with dimethyl BAPTA dramatically blocked G(12/13) and G(i)-mediated platelet aggregation. No significant effect on aggregation was seen when using selective inhibitors for p160(ROCK), PKC, or MEKK1. PI 3-kinase inhibition lead to near abolishment of platelet aggregation induced by co-stimulation of G(q) and G(i) pathways, but not by G(12/13) and G(i) pathways. These data demonstrate that co-stimulation of G(12/13) and G(i) pathways is sufficient to activate GPIIb/IIIa in human platelets in a mechanism that involves intracellular calcium, and that PI 3-kinase is an important signaling molecule downstream of G(q) but not downstream of G(12/13) pathway.

Characterization and channel coupling of the P2Y(12) nucleotide receptor of brain capillary endothelial cells

Rat brain capillary endothelial (B10) cells express an unidentified nucleotide receptor linked to adenylyl cyclase inhibition. We show that this receptor in B10 cells is identical in sequence to the P2Y(12) ADP receptor ("P2Y(T)") of platelets. When expressed heterologously, 2-methylthio-ADP (2-MeSADP; EC(50), 2 nm), ADP, and adenosine 5'-O-(2-thio)diphosphate were agonists of cAMP decrease, and 2-propylthio-D-beta,gamma-difluoromethylene-ATP was a competitive antagonist (K(B), 28 nm), as in platelets. However, 2-methylthio-ATP (2-MeSATP) (EC(50), 0.4 nm), ATP (1.9 microm), and 2-chloro-ATP (190 nm), antagonists in the platelet, were also agonists. 2-MeSADP activated (EC(50), 0.1 nm) GIRK1/GIRK2 inward rectifier K(+) channels when co-expressed with P2Y(12) receptors in sympathetic neurons. Surprisingly, P2Y(1) receptors expressed likewise gave that response; however, a full inactivation followed, absent with P2Y(12) receptors. A new P2Y(12)-mediated transduction was found, the closing of native N-type Ca(2+) channels; again both 2-MeSATP and 2-MeSADP are agonists (EC(50), 0.04 and 0.1 nm, respectively). That action, like their cAMP response, was pertussis toxin-sensitive. The Ca(2+) channel inhibition and K(+) channel activation are mediated by beta gamma subunit release from a heterotrimeric G-protein. G alpha subunit types in B10 cells were also identified. The presence in the brain capillary endothelial cell of the P2Y(12) receptor is a significant extension of its functional range.

Development of platelet inhibition by cAMP during megakaryocytopoiesis

Prostacyclin is a potent inhibitor of agonist-induced Ca2+ increases in platelets, but in the megakaryocytic cell line MEG-01 this inhibition is absent. Using human megakaryocytic cell lines representing different stages in megakaryocyte (Mk) maturation as well as stem cells and immature and mature megakaryocytes, we show that the inhibition by prostacyclin develops at a late maturation stage shortly before platelets are formed. This late appearance is not caused by insufficient cAMP formation or absent protein kinase A (PKA) activity in immature cells. Instead, the appearance of Ca2+ inhibition by prostacyclin is accompanied by a sharp increase in the expression of the catalytic subunit of PKA (PKA-C) but not by changes in the expression of the PKA-regulatory subunits Ialpha/beta, IIalpha, and IIbeta. Overexpression of PKA-C in the megakaryocytic cell line CHRF-288-11 potentiates the Ca2+ inhibition by prostacyclin. Thus, up-regulation of PKA-C appears to be a key step in the development of Ca2+ inhibition by prostacyclin in platelets.

Small aggregates of platelets can be detected sensitively by a flow cytometer equipped with an imaging device: mechanisms of epinephrine-induced aggregation and antiplatelet effects of beraprost

BACKGROUND

Although cross-talks between platelets and other blood cells are important in vivo, laboratory platelet aggregation tests have been performed mainly with the use of platelet-rich plasma (PRP) as samples. Methods that enable an efficient and sensitive detection of platelet aggregates in whole blood are being developed.

METHODS

A flow cytometer equipped with an imaging device, the flow imaging cytometer 2 (FIC2), was used to detect platelet aggregates in whole blood.

RESULTS

The FIC2 provides a resolution that is high enough to differentiate platelet aggregates from single platelets or other blood cells. Epinephrine elicited platelet aggregate formation in hirudin plus argatroban-treated whole blood, but not in PRP. The reconstitution study revealed that a small amount of adenosine diphosphate (ADP) from erythrocytes may play an important role in epinephrine-induced platelet aggregation (in whole blood), through mediation of P2Y1 receptors. When the inhibitory effect of beraprost, an antiplatelet agent, on platelet aggregation was assessed, analysis of whole blood samples with FIC2 proved to be the most sensitive among the methods available.

CONCLUSIONS

FIC2 is a promising device for detection of platelet aggregates in whole blood, with wide basic and clinical applications.

NAD(P)H oxidase-dependent platelet superoxide anion release increases platelet recruitment

Platelets, although not phagocytotic, have been suggested to release O. Since O-producing reduced nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) oxidases can be specifically activated by certain agonists and are found in several nonphagocytotic tissues, we investigated whether such an enzyme is the source of platelet-derived O. We further studied which agonists cause platelet O release and whether platelet-derived O influences thrombus formation in vitro. Collagen, but not adenosine 5'-diphosphate (ADP) or thrombin, increased O formation in washed human platelets. This was a reduced nicotinamide adenine dinucleotide (NADH)-dependent process, as shown in platelet lysates. Consistent with a role of a platelet, NAD(P)H oxidase expression of its subunits p47(phox) and p67(phox) and inhibition of platelet O formation by diphenylene-iodoniumchloride (DPI) and by the specific peptide-antagonist gp91ds-tat were observed. Whereas platelet-derived O did not influence initial aggregation, platelet recruitment to a preformed thrombus following collagen stimulation was significantly attenuated by superoxide dismutase (SOD) or DPI. It was also inhibited when ADP released during aggregation was cleaved by the ectonucleotidase apyrase. ADP in supernatants of collagen-activated platelets was decreased in the presence of SOD, resulting in lower ADP concentrations available for recruitment of further platelets. Exogenous O increased ADP- concentrations in supernatants of collagen-stimulated platelets and induced irreversible aggregation when platelets were stimulated with otherwise subthreshold concentrations of ADP. These results strongly suggest that collagen activation induces NAD(P)H oxidase-dependent O release in platelets, which in turn enhances availability of released ADP, resulting in increased platelet recruitment.

Functional significance of adenosine 5'-diphosphate receptor (P2Y(12)) in platelet activation initiated by binding of von Willebrand factor to platelet GP Ibalpha induced by conditions of high shear rate

BACKGROUND

The role of the adenosine 5'-diphosphate receptor P2Y(12) in platelet activation initiated by the von Willebrand factor (VWF)-GP Ibalpha interaction under high shear rate was investigated.

METHODS AND RESULTS

Blood samples were obtained from 11 donors. Shear-induced platelet aggregation was detected by optically modified cone-plate viscometer. Shear-induced VWF binding, P-selectin expression, and microparticle release were detected by flow cytometry. Platelet interaction with immobilized VWF was also investigated by parallel-plate flow chamber equipped with epifluorescent videomicroscopy. Effects of a selective P2Y(12) antagonist AR-C69931 MX were tested. AR-C69931 MX inhibited shear-induced platelet aggregation in a dose-dependent manner, achieving the maximum inhibition at 100 nmol/L. The extent of aggregation after exposure to a shear rate of 10 800 s(-1) for 6 minutes in the presence of 100 nmol/L AR-C69931 MX was 32.4+/-8.2% (mean+/-SD), which was significantly lower than the value in the controls of 69.7+/-9.6% (P<0.01). The inhibiting effects of AR-C69931 MX were reversed by exogenous addition of adenosine 5'-diphosphate. Shear-induced VWF binding and P-selectin surface translocation, which occurred in 4696+/-911 and 5964+/-784, respectively, of 10 000 measured platelets, was also inhibited by AR-C69931 MX (100 nmol/L) to 1948+/-528 and 2797+/-718, respectively (P=0.0018 and P=0.0009). Microparticle release was similarly inhibited. In a flow chamber experiment, firm platelet attachment on immobilized VWF was inhibited by AR-C69931 MX, whereas transient interaction was not influenced. All the above reactions were completely inhibited by blocking VWF-GP Ibalpha interaction.

CONCLUSIONS

We have demonstrated that the stimulation of P2Y(12) is involved in platelet activation initiated by the binding of VWF to GP Ibalpha induced by a high shear rate.

Protease-activated receptors 1 and 4 do not stimulate G(i) signaling pathways in the absence of secreted ADP and cause human platelet aggregation independently of G(i) signaling

Thrombin is an important agonist for platelet activation and plays a major role in hemostasis and thrombosis. Thrombin activates platelets mainly through protease-activated receptor 1 (PAR1), PAR4, and glycoprotein Ib. Because adenosine diphosphate and thromboxane A(2) have been shown to cause platelet aggregation by concomitant signaling through G(q) and G(i) pathways, we investigated whether coactivation of G(q) and G(i) signaling pathways is the general mechanism by which PAR1 and PAR4 agonists also activate platelet fibrinogen receptor (alphaIIbbeta3). A PAR1-activating peptide, SFLLRN, and PAR4-activating peptides GYPGKF and AYPGKF, caused inhibition of stimulated adenylyl cyclase in human platelets but not in the presence of either Ro 31-8220, a protein kinase C selective inhibitor that abolishes secretion, or AR-C66096, a P2Y12 receptor-selective antagonist; alpha-thrombin-induced inhibition of adenylyl cyclase was also blocked by Ro 31-8220 or AR-C66096. In platelets from a P2Y12 receptor-defective patient, alpha-thrombin, SFLLRN, and GYPGKF also failed to inhibit adenylyl cyclase. In platelets from mice lacking the P2Y12 receptor, neither alpha-thrombin nor AYPGKF caused inhibition of adenylyl cyclase. Furthermore, AR-C66096 caused a rightward shift of human platelet aggregation induced by the lower concentrations of alpha-thrombin and AYPGKF but had no effect at higher concentrations. Similar results were obtained with platelets from mice deficient in the P2Y12. We conclude that (1) thrombin- and thrombin receptor-activating peptide-induced inhibition of adenylyl cyclase in platelets depends exclusively on secreted adenosine diphosphate that stimulates G(i) signaling pathways and (2) thrombin and thrombin receptor-activating peptides cause platelet aggregation independently of G(i) signaling.

Glycoprotein VI-mediated platelet fibrinogen receptor activation occurs through calcium-sensitive and PKC-sensitive pathways without a requirement for secreted ADP

Collagen activates platelets by transducing signals through glycoprotein VI (GPVI). It is not clear whether collagen can directly activate fibrinogen receptors on the adherent platelets without a role for positive feedback agonists. We investigated the contribution of secondary G protein signaling to the mechanism of GPVI-stimulated platelet aggregation using the GPVI-selective agonists, convulxin and collagen-related peptide (CRP) as well as collagen. Adenosine diphosphate (ADP) scavengers or ADP receptor antagonists shifted the concentration-response curve slightly to the right at low concentrations of convulxin, whereas platelet aggregation at higher concentrations of convulxin was unaffected by these agents. ADP receptor antagonists shifted the concentration-response curve of collagen- or CRP-induced platelet aggregation to the right at all the concentrations. Protein kinase C inhibitor, Ro 31-8220, or a calcium chelator 5,5'-dimethyl-BAPTA shifted the concentration-response curve of convulxin-induced platelet aggregation to the right. In addition, pretreatment with both Ro 31-8220 and dimethyl-BAPTA resulted in total inhibition of convulxin-mediated aggregation. Blockade of either the calcium- or protein kinase C-regulated pathway leads to inhibition of fibrinogen receptor activation on platelets adherent to collagen, but inhibition of both pathways leads to abolished fibrinogen receptor activation. We conclude that collagen-induced activation of fibrinogen receptor on adherent platelets through GPVI signaling occurs without any significant role for secreted ADP or thromboxane A(2). Furthermore, protein kinase C- and calcium-regulated pathways independently contribute to GPVI-mediated platelet aggregation.

P2Y(13): identification and characterization of a novel Galphai-coupled ADP receptor from human and mouse

We have identified an orphan G protein-coupled receptor, SP174, that shares a high degree of homology with the recently described ADP receptor P2Y(12). mRNA for SP174 is abundant in the brain and in cells of the immune system. In the present study, we demonstrate that SP174 is also a receptor for ADP, which is coupled to Galphai. ADP potently stimulates SP174 with an EC(50) of 60 nM, and other related nucleotides are active as well, with a rank order of potency 2-methylthio-ADP tetrasodium = adenosine 5'-O-2-(thio)diphosphate = 2-methylthio-ATP tetrasodium > ADP > AP3A >ATP > IDP. This pharmacological profile is similar to that for P2Y(12). We have also identified the murine homolog of SP174, which exhibits 75% homology to the human receptor. ADP is also a potent agonist at the murine receptor, and its pharmacological profile is similar to its human counterpart, but ADP and related nucleotides are more potent at the murine receptor than the human receptor. In keeping with the general nomenclature for the purinergic receptors, we propose designating this novel receptor P2Y(13).

Platelet changes and subsequent development of pre-eclampsia and fetal growth restriction in women with abnormal uterine artery Doppler screening

OBJECTIVES

To investigate whether, in women with abnormal uterine artery Doppler, platelet volume and function will identify a subgroup of women at increased risk of pre-eclampsia and intrauterine growth restriction and whether in-vitro platelet aggregation precedes the onset of clinical disease.

DESIGN

Platelet number, volume and aggregation induced by collagen or adenosine 5'-diphosphate were evaluated in 16 non-pregnant controls, 29 pregnant women with normal uterine artery Doppler and 31 pregnant women with abnormal Doppler, hence at risk of pre-eclampsia and intrauterine growth restriction at 23 weeks. Outcome of pregnancy was recorded in each case.

RESULTS

Twelve women in the group with abnormal uterine artery Doppler subsequently developed pre-eclampsia and/or intrauterine growth restriction. All women with normal uterine artery Doppler had a normal pregnancy outcome. No differences in platelet count or in vitro platelet aggregation induced by collagen were observed between the groups. Mean platelet volume was greater in those with abnormal Doppler who had intrauterine growth restriction or normal pregnancy outcome compared with normal Doppler (10.3 and 10.3 vs. 9.4 fL, P = 0.004 and P = 0.01, respectively). Aggregation induced by adenosine diphosphate was higher in women with abnormal Doppler who developed pre-eclampsia or intrauterine growth restriction compared with those with normal outcomes (66.5 and 66.5 vs. 21%, P = 0.02, P = 0.03, respectively).

CONCLUSIONS

Women with abnormal uterine artery Doppler at 23 weeks show alterations in mean platelet volume and platelet function that relate to subsequent adverse outcome.

Effects of combined therapy with clopidogrel and acetylsalicylic acid on platelet glycoprotein expression and aggregation

SUMMARY

This study aimed to compare the effects of clopidogrel, acetylsalicylic acid (ASA), and the combination of both substances on platelet aggregation and expression of platelet membrane glycoproteins in patients with chronic coronary artery disease. We investigated platelet activation by flow cytometry and by platelet aggregation and disaggregation in 60 patients randomly assigned to 3 treatment groups: ASA, clopidogrel, combination of clopidogrel and ASA, treated for 14 days. Adenosine diphosphate (ADP)-induced expression of P-selectin and of PAC-1 was significantly reduced after 2 wk of clopidogrel but not of ASA treatment. Treatment with clopidogrel reduced the ADP-induced platelet aggregation. The combination of clopidogrel and ASA did not increase the inhibition of platelet activation compared with clopidogrel alone. A significant increase in platelet disaggregation was observed with clopidogrel alone and was more pronounced with the combination of clopidogrel and ASA. ADP-induced platelet degranulation, activation of GPIIb/IIIa receptor, and aggregation in vivo are effectively inhibited by clopidogrel. The significantly increased disaggregation under clopidogrel and ASA suggests that the combined therapy may be superior to the monotherapy in patients with coronary artery disease and a high risk for vascular events.

The role of platelets in peripheral arterial disease: therapeutic implications

Peripheral arterial disease (PAD) is associated with platelet hyperaggregability as well as an increase in morbidity and mortality from myocardial infarction and stroke. Enhanced platelet activation in PAD may substantially contribute to these adverse outcomes. A relative resistance to aspirin therapy has been reported in patients with PAD. Therefore, clopidogrel may be superior to aspirin in treatment of PAD. Furthermore, the aspirin + clopidogrel combination could be more effective than monotherapy but its risk-benefit ratio has yet to be evaluated. Clopidogrel is preferable to ticlopidine because of its safer profile and the convenience of once-daily administration. The glycoprotein (Gp) IIb/IIIa inhibitors may also find a place as short-term therapy after peripheral angioplasty. There is a need to consider the use of clopidogrel in patients who cannot tolerate aspirin. Patients who have an event while taking aspirin also present a problem. One possibility here is to substitute aspirin with clopidogrel or to add clopidogrel to the aspirin. Although these options are currently not evidence based in patients with PAD, there is emerging evidence showing that they are realistic choices.

Inhibition of adenylyl cyclase by neuronal P2Y receptors

P2Y receptors inhibiting adenylyl cyclase have been found in blood platelets, glioma cells, and endothelial cells. In platelets and glioma cells, these receptors were identified as P2Y(12). Here, we have used PC12 cells to search for adenylyl cyclase inhibiting P2Y receptors in a neuronal cellular environment. ADP and ATP (0.1 - 100 microM) left basal cyclic AMP accumulation unaltered, but reduced cyclic AMP synthesis stimulated by activation of endogenous A(2A) or recombinant beta(2) receptors. Forskolin-dependent cyclic AMP production was reduced by <or=1 microM and enhanced by 10 - 100 microM ADP; this latter effect was turned into an inhibition when A(2A) receptors were blocked. The nucleotide inhibition of cyclic AMP synthesis was not altered when P2X receptors were blocked, but abolished by pertussis toxin. The rank order of agonist potencies for the reduction of cyclic AMP was (IC(50) values): 2-methylthio-ADP (0.12 nM)=2-methylthio-ATP (0.13 nM)>ADPbetaS (71 nM)>ATP (164 nM)=ADP (244 nM). The inhibition by ADP was not antagonized by suramin, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid, or adenosine-3'-phosphate-5'-phosphate, but attenuated by reactive blue 2, ATP(alpha)S, and 2-methylthio-AMP. RT - PCR demonstrated the expression of P2Y(2), P2Y(4), P2Y(6), and P2Y(12), but not P2Y(1), receptors in PC12 cells. In Northern blots, only P2Y(2) and P2Y(12) were detectable. Differentiation with NGF did not alter these hybridization signals and left the nucleotide inhibition of adenylyl cyclase unchanged. We conclude that P2Y(12) receptors are expressed in neuronal cells and inhibit adenylyl cyclase activity.

Adenosine diphosphate (ADP)-induced thromboxane A(2) generation in human platelets requires coordinated signaling through integrin alpha(IIb)beta(3) and ADP receptors

Adenosine diphosphate (ADP) is a platelet agonist that causes platelet shape change and aggregation as well as generation of thromboxane A(2), another platelet agonist, through its effects on P2Y1, P2Y12, and P2X1 receptors. It is now reported that both 2-propylthio-D-beta gamma-dichloromethylene adenosine 5'-triphosphate (AR-C67085), a P2Y12 receptor-selective antagonist, and adenosine-2'-phosphate-5'-phosphate (A2P5P), a P2Y1 receptor-selective antagonist, inhibited ADP-induced thromboxane A(2) generation in a concentration-dependent manner, indicating that coactivation of the P2Y12 and P2Y1 receptors is essential for this event. SC49992, a fibrinogen receptor antagonist, blocked ADP-induced platelet aggregation and thromboxane A(2) production in a concentration-dependent manner. Similarly, P2 receptor antagonists or SC49992 blocked ADP-induced arachidonic acid liberation. Whereas SC49992 blocked arachidonic acid-induced platelet aggregation, it failed to inhibit thromboxane A(2) generation induced by arachidonic acid. Thus, ADP-induced arachidonic acid liberation, but not subsequent conversion to thromboxane A(2), requires outside-in signaling through the fibrinogen receptor. The Fab fragment of ligand-induced binding site-6 (LIBS6) antibody, which induces a fibrinogen-binding site on the integrin alpha(IIb)beta(3), caused both platelet aggregation and thromboxane A(2) generation. Inhibitors of phosphoinositide 3-kinase, Syk, Src kinases, or protein tyrosine phosphatases inhibited platelet aggregation but not thromboxane A(2) generation, indicating that these signaling molecules have no significant role in phospholipase A(2) activation. In the presence of P2 receptor antagonists A2P5P or AR-C67085, LIBS6 failed to generate thromboxane A(2), suggesting that inside-out signaling through ADP receptors is necessary for this event. It was concluded that both outside-in signaling from the fibrinogen receptor and inside-out signaling from the P2Y1 and P2Y12 receptors are necessary for phospholipase A(2) activation, resulting in arachidonic acid liberation and thromboxane A(2) generation.

Identification, characterization, and inhibition of the platelet ADP receptors

Adenosine diphosphate (ADP) plays a crucial role in hemostasis and thrombosis, and its receptors are potential targets for antithrombotic drugs. Two G-protein-coupled P2 receptors contribute to platelet aggregation: the P2Y1 receptor initiates aggregation through mobilization of calcium stores, whereas the P2Y12 receptor coupled to adenylyl cyclase inhibition is essential for a full aggregation response to ADP and the stabilization of aggregates. The latter is defective in certain patients with a selective congenital deficiency of aggregation to ADP. It is also the target of the antithrombotic drug clopidogrel and of adenosine triphosphate analogues and other compounds currently under evaluation. In addition, the P2X1 ionotropic receptor is present in platelets, but its role is not yet completely known. Studies in P2Y1-knockout mice and experimental thrombosis models using selective P2Y1 antagonists have shown that the P2Y1 receptor, like the P2Y12 receptor, is a potential target for new antithrombotic drugs.

Mpl ligand increases P2Y1 receptor gene expression in megakaryocytes with no concomitant change in platelet response to ADP

The P2Y(1) receptor is responsible for the initiation of platelet aggregation in response to ADP and plays a key role in thrombosis. Although this receptor is expressed early in the platelet lineage, the regulation of its expression during megakaryocyte differentiation is unknown. In the mouse megakaryocytic cell line Y10/L8057, we detected P2Y(1) mRNA of three sizes (2.5, 4.4, and 7.4 kb). These cells have previously been shown to respond to Mpl ligand, the pivotal regulator of megakaryocytopoiesis, by increasing their expression of differentiation markers. Mpl ligand enhanced levels of P2Y(1) mRNAs in Y10/L8057 cells and this effect was selective: the same cytokine did not increase levels of A2a adenosine receptor mRNA. Although Mpl ligand did not affect the short half-lives of the P2Y(1) mRNAs, it enhanced transcription of the P2Y(1) gene. It also increased cell size and the number of cell surface P2Y(1) receptors, but not P2Y(1) receptor density. Injection of Mpl ligand into mice up-regulated P2Y(1) receptor mRNAs in megakaryocytes, as shown by in situ hybridization. However, platelets isolated from these mice did not exhibit a higher P2Y(1) receptor density or increased reactivity to ADP. This correlates with the finding that Mpl ligand increases GPIIb mRNA in megakaryocytes but not the density of the protein per platelet. Thus, the enhancement of P2Y(1) receptor expression induced by Mpl ligand in megakaryocytes may be an integral feature of their differentiation, whereas clinical use of this compound might not be associated with platelet hyper-reactivity to ADP.

Signalling events underlying platelet aggregation induced by the glycoprotein VI agonist convulxin

We have investigated the role of secretion and intracellular signalling events in aggregation induced by the glycoprotein (GP)VI-selective snake venom toxin convulxin and by collagen. We demonstrate that aggregation induced by threshold concentrations of convulxin undergoes synergy with ADP acting via the P2Y12 receptor whereas there is no synergy via the P2Y1 receptor or with thromboxanes. On the other hand, apyrase, the P2Y12 receptor antagonist, AR-C67085, and indomethacin only marginally inhibit aggregation induced by convulxin. In comparison, these inhibitors severely attenuate the response to collagen. In order to investigate whether the weak inhibitory action against convulxin is due to release of agonists other than ADP from dense granules, experiments were performed on murine platelets deficient in this organelle (pearl mice platelets). A slightly greater reduction in aggregation induced by convulxin was observed in pearl platelets than in the presence of inhibitors of ADP, but a maximal response was still attained. Importantly, inhibition of protein kinase C further reduced the response to convulxin in pearl platelets demonstrating a direct role for the kinase in aggregation. Chelation of intracellular Ca2+ with 1,2-bis(2-aminophenoxy)ethane-N,N,N,N',N'-tetraacetic acid (acetoxymethyl)ester (BAPTA-AM) abolished aggregation induced by convulxin under all conditions. Activation of phospholipase C by convulxin was potentiated by ADP acting through the P2Y12 receptor. In conclusion, we show that Ca2+ and protein kinase C, but not release of the secondary agonists ADP and thromboxane A2, are required for full aggregation induced by convulxin, whereas the response induced by collagen shows a much greater dependence on secretion of secondary agonists.

Pharmacological control of platelet function

Advancement in the understanding of the mechanisms of platelet activation, as well as the development of new techniques for studying platelet function, have led to the availability of new classes of platelet inhibiting drugs. Initially, characterization of arachidonic acid metabolism in platelets furthered an understanding of the utility of cyclooxygenase inhibitors, most notably aspirin. The discovery and characterization of platelet receptors such as the adenosine diphosphate (ADP) receptor and glycoprotein IIb/IIIa has been associated with the development of novel classes of anti-platelet drug, such as thienopyridine derivatives and glycoprotein IIb/IIIa receptor antagonists, respectively. Future development in receptor pathway inhibitors also includes glycoprotein Ib/IX as well as the potential use of platelet signaling pathway inhibitors.

Mechanisms of platelet aggregation

Platelet aggregation is initiated by receptor activation coupled to intracellular signaling leading to activation of integrin alphaIIbbeta3. Recent advances in the study of platelet receptors for collagen, von Willebrand factor, thrombin, and adenosine diphosphate are providing new insights into the mechanisms of platelet aggregation.

Resistance to thromboembolism in PI3Kgamma-deficient mice

Platelet aggregation and subsequent thrombosis are the major cause of ischemic diseases such as heart attack and stroke. ADP, acting via G protein-coupled receptors (GPCRs), is an important signal in thrombus formation and involves activation of phosphoinositide 3-kinases (PI3K). When platelets from mice lacking the G protein-activated PI3Kgamma isoform were stimulated with ADP, aggregation was impaired. Collagen or thrombin, however, evoked a normal response. ADP stimulation of PI3Kgamma-deficient platelets resulted in decreased PKB/Akt phosphorylation and alpha(IIb)beta(3) fibrinogen receptor activation. These effects did not influence bleeding time but protected PI3Kgamma-null mice from death caused by ADP-induced platelet-dependent thromboembolic vascular occlusion. This result demonstrates an unsuspected, well-defined role for PI3Kgamma downstream of ADP and suggests that pharmacological targeting of PI3Kgamma has a potential use as antithrombotic therapy.