Identification of the platelet ADP receptor targeted by antithrombotic drugs

Megakaryocytes derived from embryonic stem cells implicate CalDAG-GEFI in integrin signaling

Fibrinogen binding to integrin alphaIIbbeta3 mediates platelet aggregation and requires agonist-induced "inside-out" signals that increase alphaIIbbeta3 affinity. Agonist regulation of alphaIIbbeta3 also takes place in megakaryocytes, the bone marrow cells from which platelets are derived. To facilitate mechanistic studies of inside-out signaling, we describe here the generation of megakaryocytes in quantity from murine embryonic stem (ES) cells. Coculture of ES cells for 8-12 days with OP9 stromal cells in the presence of thrombopoietin, IL-6, and IL-11 resulted in the development of large, polyploid megakaryocytes that produced proplatelets. These cells expressed alphaIIbbeta3 and platelet glycoprotein Ibalpha but were devoid of hematopoietic stem cell, erythrocyte, and leukocyte markers. Mature megakaryocytes, but not megakaryocyte progenitors, specifically bound fibrinogen by way of alphaIIbbeta3 in response to platelet agonists. Retrovirus-mediated expression of the reporter gene, green fluorescent protein, in ES cell-derived megakaryocytes did not affect viability or alphaIIbbeta3 function. On the other hand, retroviral expression of CalDAG-GEFI, a Rap1 exchange factor identified by megakaryocyte gene profiling as a candidate integrin regulator, enhanced agonist-induced activation of Rap1b and fibrinogen binding to alphaIIbbeta3 (P < 0.01). These results establish that ES cells are a ready source of mature megakaryocytes for integrin studies and other biological applications, and they implicate CalDAG-GEFI in inside-out signaling to alphaIIbbeta3.

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.

Nucleotidase cascades are catalyzed by secreted proteins of the parasitic nematode Trichinella spiralis

Extracellular nucleotides are signaling molecules whose receptor-mediated effects are involved in a variety of physiological responses in mammalian tissues. An overwhelming body of data indicate that inflammatory and other immune responses can be modulated by the availability and local concentrations of nucleotides via nucleotide receptor signaling, but this is only just beginning to be investigated in the context of infectious disease. Evidence is provided here that the parasitic nematode Trichinella spiralis can catalyze the conversion and thus modulate both the availability and concentration of extracellular nucleotides by means of the following secreted exoenzymes: apyrase, 5'-nucleotidase, and adenosine deaminase. These enzymes were characterized in terms of substrate specificity, kinetic behavior, pH, divalent cation preferences, and response to a series of compounds. The secreted 5'-nucleotidase was identified as a protein with an apparent molecular mass of 67 kDa after N-terminal amino acid sequencing of the purified protein. The presence of adenosine deaminase was confirmed in the secreted products by Western blotting with an antibody against a mammalian enzyme, as a protein with an apparent molecular mass of 38 kDa. These secreted proteins constitute an enzymatic cascade which catalyzes the degradation of extracellular nucleotides, with a potential physiological role in the regulation of purinergic signaling.

ADP-sensitive purinoceptors induce steroidogenesis via adenylyl cyclase activation in bovine adrenocortical fasciculata cells

1. The role of P2Y receptors in the production of cAMP and the activation of protein kinase A (PKA) was studied with respect to the regulation of the steroidogenesis in primary cultures of bovine adrenocortical fasciculata cells (BAFCs). 2. ADP and ATP stimulated cAMP production with EC(50) values of 23.7+/-6.8 microM and 40.1+/-5.5 microM, respectively. In contrast, the EC(50) of BzATP for cAMP production was 153.0+/-37.4 microM. Adenosine and AMP (0.1-1000 microM) were much less effective than ADP and ATP. 2MeSADP and UTP did not exert detectable effects. ADP (10 and 100 microM) significantly stimulated steroidogenesis; the process was blocked by an adenylyl cyclase inhibitor SQ22536 (100 microM) but not by the P2Y(1) receptor antagonist MRS2179 (100 microM). 3. Real-time imaging of the PKA activity with the dye ARII, which became less fluorescent upon phosphorylation, revealed that ADP (100 microM) immediately activated PKA. These effects could be mimicked by forskolin (100 microM) and were blocked by the PKA inhibitor H89 (50 microM). UTP (100 microM) did not activate PKA. 4. The cytoplasm harvested from morphologically and electrophysiologically identified single BAFCs contained mRNA for P2Y(2) but not for P2Y(1), P2Y(4), P2Y(11) or P2Y(12) receptors, as confirmed by single-cell RT-PCR amplification (50 cycles). 5. These results suggest an expression of an ADP-sensitive G(s)-coupled purinoceptor in BAFCs. We propose that this not yet described type of P2Y receptor might mediate the extracellular purine-activated steroidogenesis via cAMP/PKA-mediated pathways, independently from the pathways involving InsP(3) production and consequent intracellular Ca(2+) increase.

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.

Discrete role for cytosolic phospholipase A(2)alpha in platelets: studies using single and double mutant mice of cytosolic and group IIA secretory phospholipase A(2)

Among several different types of phospholipase A(2) (PLA(2)), cytosolic PLA(2) (cPLA(2))alpha and group IIA (IIA) secretory PLA(2) (sPLA(2)) have been studied intensively. To determine the discrete roles of cPLA(2)alpha in platelets, we generated two sets of genetically engineered mice (cPLA(2)alpha(-/-)/sPLA(2)-IIA(-/-) and cPLA(2)alpha(-/-)/sPLA(2)-IIA(+/+)) and compared their platelet function with their respective wild-type C57BL/6J mice (cPLA(2)alpha(+/+)/sPLA(2)-IIA(-/-)) and C3H/HeN (cPLA(2)alpha(+/+)/sPLA(2)-IIA(+/+)). We found that cPLA(2)alpha is needed for the production of the vast majority of thromboxane (TX)A(2) with collagen stimulation of platelets. In cPLA(2)alpha-deficient mice, however, platelet aggregation in vitro is only fractionally decreased because small amounts of TX produced by redundant phospholipase enzymes sufficiently preserve aggregation. In comparison, adenosine triphosphate activation of platelets appears wholly independent of cPLA(2)alpha and sPLA(2)-IIA for aggregation or the production of TX, indicating that these phospholipases are specifically linked to collagen receptors. However, the lack of high levels of TX limiting vasoconstriction explains the in vivo effects seen: increased bleeding times and protection from thromboembolism. Thus, cPLA(2)alpha plays a discrete role in the collagen-stimulated production of TX and its inhibition has a therapeutic potential against thromboembolism, with potentially limited bleeding expected.

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.

Application of ex vivo flow chamber system for assessment of stent thrombosis

OBJECTIVE

Factors influencing platelet accumulation around stents were to be investigated by an ex vivo flow chamber system.

METHODS AND RESULTS

Platelet accumulations on collagen surfaces under flow conditions were augmented in the presence of stents, especially at sites downstream from coil stents. Densitometric analysis revealed that 4.9+/-0.8 times more platelets accumulated downstream from coil stents than were formed downstream from tube stents (P<0.01), suggesting that stent morphology is an important determinant factor of its thrombogenicity. Platelet accumulations around stents were significantly inhibited by a combination of ticlopidine and aspirin, whereas aspirin alone produced only modest inhibition. Anti-glycoprotein IIb/IIIa (abciximab) inhibited platelet accumulation around stents in a dose-dependent manner, whereas the antibody blocking von Willebrand factor binding to glycoprotein Ib(alpha), which had been shown to inhibit platelet thrombus formation under high shear rates, did not inhibit the accumulation downstream from the coil stents. Our results suggest that the important characteristics of in vivo stent thrombosis, ie, augmented platelet accumulation with coil stents and the strong antithrombotic effect of the combination antiplatelet agents and an anti-glycoprotein IIb/IIIa, can be reproduced in ex vivo perfusion model.

CONCLUSIONS

We conclude that an ex vivo perfusion system is useful in the assessment of the thrombogenicity of various stents and in the screening of effective antiplatelet agents.

Inhibition of platelet P2Y12 and alpha2A receptor signaling by cGMP-dependent protein kinase

The important role of cGMP and cGMP-dependent protein kinase (cGPK) for the inhibition of platelet activation and aggregation is well established and due to the inhibition of fundamental platelet responses such as agonist-stimulated calcium increase, exposure of adhesion receptors and actin polymerization. The diversity of cGMP binding proteins and their synergistic interaction with cAMP signaling in inhibiting platelets indicates that a variety of cGMP targets contribute to its antiplatelet action. Since stimulation of G(i)-proteins was recently shown to be essential for complete platelet activation/aggregation, the possibility that G(i)-signaling events are cGMP/cGPK targets was investigated. Thus, the effect of elevated cGMP levels and selective cGPK activation on purinergic and adrenergic receptor-evoked decrease of platelet cAMP content was closely examined. Experiments with a selective activator of cGPK demonstrate for the first time a cGMP-caused G(i)-protein inhibition and our data suggest that this effect is mediated by cGPK. Considering the essential role of G(i)-signaling for platelet activation, we propose that inhibition of G(i)-mediated signaling by cGMP/cGPK is an important mechanism of action underlying the platelet inhibition by cGMP-elevating endothelium derived factors and drugs.

Nucleotides induce chemotaxis and actin polymerization in immature but not mature human dendritic cells via activation of pertussis toxin-sensitive P2y receptors

Dendritic cells (DCs) are considered the principal initiators of immune response because of their ability to migrate into peripheral tissues and lymphoid organs, process antigens, and activate naive T cells. There is evidence that extracellular nucleotides regulate certain functions of DCs via G-protein-coupled P2Y receptors (P2YR) and ion-channel-gated P2X receptors (P2XR). Here we investigated the chemotactic activity and analyzed the migration-associated intracellular signaling events such as actin reorganization and Ca(++) transients induced by common P2R agonists such as adenosine 5'-triphosphate (ATP) and 2-methylthioadenosine triphosphate, the P2YR agonists UTP and adenosine 5'-diphosphate (ADP), or the P2XR agonists alphabeta-methylenadenosine-5'-triphosphate and 2',3'-(4-benzoyl)benzoyl-ATP. The common P2R agonists and the selective P2YR agonists turned out to be potent chemotactic stimuli for immature DCs, but not for mature DCs. In contrast, P2XR agonists had only marginal chemotactic activity in both DC types. Chemotaxis was paralleled by a rise in the intracellular Ca(++) concentration and by actin polymerization. Studies with pertussis toxin implicated that intracellular signaling events such as actin polymerization, mobilization of intracellular Ca(++), and migration induced by nucleotides was mediated via G(i/o) protein-coupled P2YR. Moreover, functional studies revealed selective down-regulation of this G(i/o) protein-coupled chemotactic P2YR responsiveness during maturation, although immature and mature DCs expressed similar amounts of mRNA for the P2R subtypes (P2Y(2)R, P2Y(4)R, P2Y(5)R, P2Y(7)R, P2Y(11)R and P2X(1)R, P2X(4)R, P2X(7)R), and no major differences in respect to the mRNA expression of these receptors could be observed by semiquantitative reverse transcription and polymerase chain reaction (RT-PCR). In summary, our data describe a differential chemotactic response of immature and mature DCs to nucleotides, and lend further support to the hypothesis that P2R are a novel class of immunomodulatory plasma membrane receptors suitable for pharmacological intervention.

The influence of platelet glycoprotein polymorphisms on receptor function and risk for thrombosis

With regard to hemostasis and thrombosis, collagens are among the most important physiologic components of the extracellular matrix in their role as platelet activators. Differences in the rate of platelet activation markedly influence normal hemostasis and the pathological outcome of thrombosis. Thus, collagen receptors, such as the integrin alpha2beta1 and indirectly, GPIb alpha, represent a relatively unexploited target of pharmacological control and are only recently becoming appreciated as potential factors in the generic risk for thrombosis. In general, the importance platelet glycoprotein polymorphisms as genetic risk factors for arterial thrombosis is a new area of human genomics that needs to be carefully addressed. Discrepancies in the degree to which they are reported to contribute to risk for clinical thrombosis will only be resolved once there is a universal standard for clinical study design. Most of the clinical studies differ by patient population size, ethnicity, bias in the selection of patients and controls, plurality in clinical endpoints and variation of environmental factors. Despite these differences, there is substantial evidence that the integrin beta3 PlA2 haplotype, the GPIb alpha Met145 haplotype, the GPIb alpha -5C haplotype and the integrin alpha2 haplotype 1 (807T) each contribute to the risk for and morbidity of thrombotic disease. There may remain dispute as to the extent of their contribution. However, well-designed, large, prospective, genetic and epidemiologic studies are needed to clarify the role of these and other platelet receptor polymorphisms. Most importantly, the cumulative effects of multiple platelet and plasma glycoproteins SNPs to thrombotic risk must be evaluated concurrently. Additional in vitro studies of the functional relevance underlying these polymorphisms are needed to provide a sound biological explanation for the results of clinical correlations. The opportunity now exists to make significant inroads into the development of strategies for the prevention of thrombotic disease.

P2Y and P2X purinoceptor mediated Ca2+ signalling in glial cell pathology in the central nervous system

Activation of purinoceptors by extracellular ATP is an important component of the glial response to injury in the central nervous system (CNS). ATP has been shown to evoke raised cytosolic [Ca(2+)] in astrocytes, oligodendrocytes, and microglia, the three major glial cell types in the CNS. Glial cells express a heterogenous collection of metabotropic P2Y and ionotropic P2X purinoceptors, which respectively mobilise Ca(2+) from intracellular stores and trigger Ca(2+) influx across the plasmalemma. It is likely that different receptors have distinct roles in glial cell physiology and pathology. Our studies on optic nerve glia in situ indicate that P2Y(1) and P2Y(2/4) receptors are activated at low ATP concentrations, suggesting they are the predominant purinoceptors mediating physiological Ca(2+) signalling. Glia also express P2X(1) and P2X(3) purinoceptors, which mediate fast, rapidly desensitising current and may also be important in signalling. At high concentrations, such as occur in CNS injury, ATP induces large and prolonged increases in glial [Ca(2+)](i) with a primary role for P2Y purinoceptors and inositol trisphosphate (IP(3))-dependent release of Ca(2+) from intracellular stores. In addition, we found that high concentrations of ATP activated a significant P2X component that did not desensitise or saturate and was dependent on extracellular Ca(2+). These are characteristic properties of the P2X(7) subtype, and we provide in situ evidence that application of the P2X(7) receptor agonist benzoyl-benzoyl ATP (BzATP) evokes raised [Ca(2+)](i) in optic nerve glia, and that the dye YO-PRO-1, which passes through pore-forming P2X(7) receptors, is taken up by astrocytes, oligodendrocytes and microglia. Glia also express P2X(2) and P2X(4) receptors that are also pore-forming in the presence of sustained high ATP concentrations and which may also be important in the glial injury response. There is evidence that activation of P2 purinoceptors is a key step in triggering reactive changes in glial cells, including expression of immediate early genes, induction of extracellular signal regulated kinase and cyclooxygenase-2, synthesis of phospholipase A(2), release of arachidonic acid, production of prostaglandins and release of interleukins. We show that the ATP-mediated increase in glial [Ca(2+)](i) is potentiated by arachidonic acid and reduced by the inhibition of phospholipase A(2) inhibition. Together, the results implicate ATP as a primary signalling molecule in glial cells and indicate specific roles for P2Y and P2X purinoceptors in glial cell pathology.

Extracellular adenine nucleotides inhibit the activation of human CD4+ T lymphocytes

ATP has been reported to inhibit or stimulate lymphoid cell proliferation, depending on the origin of the cells. Agents that increase cAMP, such as PGE(2), inhibit human CD4(+) T cell activation. We demonstrate that several ATP derivatives increase cAMP in both freshly purified and activated human peripheral blood CD4(+) T cells. The rank order of potency of the various nucleotides was: adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS) approximately 2'- and 3'-O-(4-benzoylbenzoyl) ATP (BzATP) > ATP > 2-methylthio-ATP >> dATP, 2-propylthio-beta,gamma-dichloromethylene-D-ATP, UDP, UTP. This effect did not involve the activation of A(2)Rs by adenosine or the synthesis of prostaglandins. ATPgammaS had no effect on cytosolic calcium, whereas BzATP induced an influx of extracellular calcium. ATPgammaS and BzATP inhibited secretion of IL-2, IL-5, IL-10, and IFN-gamma; expression of CD25; and proliferation after activation of CD4(+) T cells by immobilized anti-CD3 and soluble anti-CD28 Abs, without increasing cell death. Taken together, our results suggest that extracellular adenine nucleotides inhibit CD4(+) T cell activation via an increase in cAMP mediated by an unidentified P2YR, which might thus constitute a new therapeutic target in immunosuppressive treatments.

Activation of Rap1B by G(i) family members in platelets

It has become increasingly appreciated that receptors coupled to G(alpha)(i) family members can stimulate platelet aggregation, but the mechanism for this has remained unclear. One possible mediator is the small GTPase, Rap1, which has been shown to contribute to integrin activation in several cell lines and to be activated by a calcium-dependent mechanism in platelets. Here, we demonstrate that Rap1 is also activated by G(alpha)(i) family members in platelets. First, we show that platelets from mice lacking the G(alpha)(i) family member G(alpha)(z) (which couples to the alpha(2A) adrenergic receptor) are deficient in epinephrine-stimulated Rap1 activation. We also show that platelets from mice lacking G(alpha)(i2), which couples to the ADP receptor, P2Y12, exhibit reduced Rap1 activation in response to ADP. In contrast, platelets from mice that lack G(alpha)(q) show no decrease in the ability to activate Rap1 in response to epinephrine but show a partial reduction in ADP-stimulated Rap1 activation. This result, combined with studies of human platelets treated with ADP receptor-selective inhibitors, indicates that ADP-stimulated Rap1 activation in human platelets is dependent on both the G(alpha)(i)-coupled P2Y12 receptor and the G(alpha)(q)-coupled P2Y1 receptor. G(alpha)(i)-dependent activation of Rap1 in platelets does not appear to be mediated by enhanced intracellular calcium release because no increase in intracellular calcium concentration was detected in response to epinephrine and because the calcium response to ADP was not diminished in platelets from the G(alpha)(i2)-/- mouse. Finally, using human platelets treated with selective inhibitors of phosphatidylinositol 3-kinase (PI3K) and mouse platelets selectively lacking the G(beta)(gamma)-activated form of his enzyme (PI3Kgamma), we show that G(i)-mediated Rap1 activation is PI3K-dependent. In summary, activation of Rap1 can be stimulated by G(alpha)(i)- and PI3K-dependent mechanisms in platelets and by G(q)- and Ca(2+)-dependent mechanisms, both of which may play a role in promoting platelet activation.

Cardiopulmonary bypass induces release of soluble CD40 ligand

BACKGROUND

Cardiopulmonary bypass (CPB) is known to induce platelet activation, thrombosis, thrombocytopenia, and a systemic inflammatory response. It is known that CD40 ligand (CD40L) exists in platelets, that a soluble form of this protein (sCD40L) is released on platelet activation, that platelets are the primary source of sCD40L in blood, and that sCD40L is involved in thrombosis and inflammation. The present study was designed to determine whether sCD40L is released during CPB. Methods and Results- Blood was obtained from patients undergoing CPB-requiring surgery and analyzed for sCD40L, interleukin-6, and platelet factor 4 and beta-thromboglobulin (markers of platelet activation). Platelets were also isolated and analyzed for their levels of CD40L. Plasma levels of sCD40L increased >1.7-fold (from 0.29 to 0.51 ng/mL, P=0.001) within 1 hour on CPB and increased further to 3.7-fold (to 1.08 ng/mL, P=0.03) 2 hours after the procedure. Half of the released sCD40L was cleared in 2 hours, which allowed the sCD40L to return to approximately baseline levels 8 hours after the procedure. The platelet content of CD40L was decreased by 40% (2.675 to 1.64 ng/10(8) platelets, P=0.001) 1 hour after initiation of CPB and was similar to that observed for platelet factor 4 and beta-thromboglobulin. Interleukin-6, a marker of inflammation, also increased during CPB.

CONCLUSIONS

The present study demonstrates that CPB causes an increase in the concentration of plasma sCD40L. The corresponding decrease in platelet CD40L suggests that this prothrombotic and proinflammatory protein was derived primarily from platelets and may contribute to the thrombotic and inflammatory complications associated with CPB.

Identification of G protein-coupled receptor genes from the human genome sequence

We have identified novel G protein-coupled receptors (GPCRs) with no introns in the coding region from the human genome sequence: 322 olfactory receptors; 22 taste receptors; 128 registered GPCRs for endogenous ligands; 50 novel GPCR candidates homologous to registered GPCRs for endogenous ligands; and 59 novel GPCR candidates not homologous to registered GPCRs. The total number of GPCRs with and without introns in the human genome was estimated to be approximately 950, of which 500 are odorant or taste receptors and 450 are receptors for endogenous ligands.

Reversal of thrombin-induced deactivation of CD39/ATPDase in endothelial cells by HMG-CoA reductase inhibition: effects on Rho-GTPase and adenosine nucleotide metabolism

Adenosine triphosphate and diphosphate that activate platelet, leukocyte, and endothelium functions are hydrolyzed by endothelial CD39/ATPDase. Because CD39/ATPDase is downregulated in endothelial cells by inflammation and this may be affected by HMG-CoA reductase inhibitors, we examined the role of cerivastatin and simvastatin in regulation of endothelial CD39/ATPDase expression, metabolism of ATP/ADP, and function in platelets. Thrombin-stimulated endothelial cells in vitro were treated with the statins, and hydrolysis of exogenous ADP and ATP was assessed by high-performance liquid chromatography and malachite green assay. Platelet aggregation studies were performed with endothelial cell supernatants as triggers. CD39/ATPDase surface expression by endothelial cells was determined immunologically by fluorescence-activated cell sorter, mRNA expression by RT-PCR, and thrombin-induced dissociation of Rho-GTPases by Western blotting. Treatment by simvastatin or cerivastatin restored impaired metabolism of exogenous ATP and ADP in thrombin-activated endothelial cells by preventing thrombin-induced downregulation of CD39/ATPDase. In platelet aggregation studies, ATP and ADP supernatants of thrombin-activated endothelial cells were less stimulatory in the presence of statins than in their absence. Data show that statins preserve CD39/ATPDase activity in thrombin-treated endothelial cells involving alterations by statins of Rho-GTPase function and CD39/ATPDase expression. Preservation of adenine nucleotide metabolism may directly contribute to the observed anti-thrombotic and anti-inflammatory actions of statins.

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.

Adenine nucleotide analogues locked in a Northern methanocarba conformation: enhanced stability and potency as P2Y(1) receptor agonists

Preference for the Northern (N) ring conformation of the ribose moiety of nucleotide 5'-triphosphate agonists at P2Y(1), P2Y(2), P2Y(4), and P2Y(11) receptors, but not P2Y(6) receptors, was established using a ring-constrained methanocarba (a 3.1.0-bicyclohexane) ring as a ribose substitute (Kim et al. J. Med. Chem. 2002, 45, 208-218.). We have now combined the ring-constrained (N)-methanocarba modification of adenine nucleotides with other functionalities known to enhance potency at P2 receptors. The potency of the newly synthesized analogues was determined in the stimulation of phospholipase C through activation of turkey erythrocyte P2Y(1) or human P2Y(1) and P2Y(2) receptors stably expressed in astrocytoma cells. An (N)-methanocarba-2-methylthio-ADP analogue displayed an EC(50) at the hP2Y(1) receptor of 0.40 nM and was 55-fold more potent than the corresponding triphosphate and 16-fold more potent than the riboside 5'-diphosphate. 2-Cl-(N)-methanocarba-ATP and its N(6)-Me analogue were also highly selective, full agonists at P2Y(1) receptors. The (N)-methanocarba-2-methylthio and 2-chloromonophosphate analogues were full agonists exhibiting micromolar potency at P2Y(1) receptors, while the corresponding ribosides were inactive. Although beta,gamma-methylene-ATP was inactive at P2Y receptors, beta,gamma-methylene-(N)-methanocarba-ATP was a potent hP2Y(1) receptor agonist with an EC(50) of 160 nM and was selective versus hP2Y(2) and hP2Y(4) receptors. The rates of hydrolysis of Northern (N) and Southern (S) methanocarba analogues of AMP by rat 5'-ectonucleotidase were negligible. The rates of hydrolysis of the corresponding triphosphates by recombinant rat NTPDase1 and 2 were studied. Both isomers were hydrolyzed by NTPDase 1 at about half the rate of ATP hydrolysis. The (N) isomer was hardly hydrolyzed by NTPDase 2, while the (S) isomer was hydrolyzed at one-third of the rate of ATP hydrolysis. This suggests that new, more stable and selective nucleotide agonists may be designed on the basis of the (N)-conformation, which greatly enhanced potency at P2Y(1) receptors.

ATP release in human kidney cortex and its mitogenic effects in visceral glomerular epithelial cells

BACKGROUND

In chronic renal failure the sympathetic nervous system is activated. Sympathetic cotransmitters released within the kidney may contribute to the progression of renal disease through receptor-mediated proliferative mechanisms.

METHODS

In human renal cortex electrical stimulation induced adenosine 5'-triphosphate (ATP; luciferin-luciferase-assay) and norepinephrine (HPLC) release was measured. ATP release also was induced by alpha1- and alpha2-adrenergic agonists. [3H]-thymidine uptake was tested in human visceral glomerular epithelial cells (vGEC) and mitogen-activated protein kinase (MAPK42/44) activation in vGEC and kidney cortex. The involved P2-receptors were characterized pharmacologically and by RT-PCR.

RESULTS

Sympathetic nerve stimulation and alpha-adrenergic agonists induced release of ATP from human kidney cortex. Seventy-five percent of the ATP released originated from non-neuronal sources, mainly through activation of alpha2-adrenergic receptors. ATP (1 to 100 micromol/L) and related nucleotides (1 to 100 micromol/L) increased [3H]-thymidine uptake. The adenine nucleotides ATP, ATPgammaS, ADP and ADPbetaS were about equally potent. UTP, UDP and alpha,beta-methylene ATP had no effect. ATP, ADPbetaS but not alpha,beta-methylene ATP activated MAPK42/44. ATP induced MAPK42/44 activation, and [3H]-thymidine uptake was abolished in the presence of the MAPK inhibitor PD 98059 (100 micromol/L). mRNA for P2X4,5,6,7 and P2Y1,2,4,6,11 were detected in human vGEC by RT-PCR.

CONCLUSIONS

In human renal cortex, adrenergic stimulation releases ATP from neuronal and non-neuronal sources. ATP has mitogenic effects in vGEC and therefore the potential to contribute to progression in chronic renal disease. The pattern of purinoceptor agonist effects on DNA synthesis together with the mRNA expression suggests a major contribution of a P2Y1-like receptor.

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).

Extracellular mRNA induces dendritic cell activation by stimulating tumor necrosis factor-alpha secretion and signaling through a nucleotide receptor

We previously demonstrated that dendritic cell (DC) pulsing with antigen-encoded mRNA resulted in the loading of both major histocompatibility complex class I and II antigen presentation pathways and the delivery of an activation signal. Coculture of mRNA-pulsed DC with T cells led to the induction of a potent primary immune response. DC, in addition to recognizing foreign antigens through pattern recognition receptors, also must respond to altered self, transformed, or intracellularly infected cells. This occurs through cell surface receptors that recognize products of inflammation and cell death. In this report, we characterize two signaling pathways utilized by extracellular mRNA to activate DC. In addition, a novel ligand, poly(A), is identified that mediates signaling through a receptor that can be inhibited by pertussis toxin and suramin and can be desensitized by ATP and ADP, suggesting a P2Y type nucleotide receptor. The role of this signaling activity in vaccine design and the potential effect of mRNA released by damaged cells in the induction of immune responsiveness is discussed.

Nucleotide receptors involved in UTP-induced rat arterial smooth muscle cell migration

Many factors have been shown to be involved in the development of hyperplasic lesions of vessels, but the role of extracellular nucleotides remains largely unknown. The presence of P2Y and P2X nucleotide receptors on arterial endothelial and smooth muscle cells suggests a potential role for nucleotides in the vessel pathophysiology. Although the role of P2X in physiology of vessels is well documented, that of P2Y is not completely understood. We recently demonstrated that extracellular nucleotides, and particularly UTP, induced migration of cultured arterial smooth muscle cells (ASMCs). This migration is dependent on osteopontin expression and involves the Rho and mitogen-activated protein (MAP) kinase pathways. An important question is to determine the specific role of the different P2Y receptors of rat ASMCs in the UTP-induced migration process. Therefore, we first quantified mRNA levels of P2Y(2), P2Y(4), and P2Y(6) nucleotide receptors in cultured rat ASMCs by a competitive RT-PCR approach and demonstrated that P2Y(2) is the most highly expressed among these receptors potentially involved in the UTP-mediated response. In addition to UTP, UDP also induced ASMC migration even when UTP regeneration was inhibited, suggesting the involvement of UDP receptor P2Y(6). Moreover, suramin, a specific antagonist of rat P2Y(2) receptor, acted as an inhibitor of UTP-induced migration. Taken together, these results suggest a prominent role for the UTP receptor, P2Y(2), and for the UDP receptor, P2Y(6), in UTP-induced rat ASMC migration.

A Gi-dependent pathway is required for activation of the small GTPase Rap1B in human platelets

Stimulation of human platelets by cross-linking of the low affinity receptor for immunoglobulin, FcgammaRIIA, caused the rapid activation of the small GTPase Rap1B, as monitored by accumulation of the GTP-bound form of the protein. This process was totally dependent on the action of secreted ADP since it was completely prevented in the presence of either apyrase or creatine phosphate and creatine phosphokinase. Dose-dependent experiments revealed that the inhibitory effect of ADP scavengers was not related to the reduced increase of cytosolic Ca(2+) concentration in stimulated platelets. Activation of Rap1B induced by clustering of FcgammaRIIA was totally suppressed by AR-C69931MX, a specific antagonist of the G(i)-coupled ADP receptor P2Y12, but was not affected by blockade of the G(q)-coupled receptor, P2Y1. Similarly, direct stimulation of platelets with ADP induced the rapid activation of Rap1B. Pharmacological blockade of the P2Y1 receptor totally prevented ADP-induced Ca(2+) mobilization but did not affect activation of Rap1B. By contrast, prevention of ADP binding to the P2Y12 receptor totally suppressed activation of Rap1B without affecting Ca(2+) signaling. In platelets stimulated by cross-linking of FcgammaRIIA, inhibition of Rap1B activation by ADP scavengers could be overcome by the simultaneous recruitment of the G(i)-coupled alpha(2A)-adrenergic receptor by epinephrine. By contrast, serotonin, which binds to a G(q)-coupled receptor, could not restore activation of Rap1B. When tested alone, epinephrine was found to be able to induce GTP binding to Rap1B, whereas serotonin produced only a slight effect. Finally, activation of Rap1B induced by stimulation of the G(q)-coupled thromboxane A(2) receptor by was completely inhibited by ADP scavengers under conditions in which intracellular Ca(2+) mobilization was unaffected. Inhibition of -induced Rap1B activation was also observed upon blockade of the P2Y12 but not of the P2Y1 receptor for ADP. These results demonstrate that stimulation of a G(i)-dependent signaling pathway by either ADP of epinephrine is necessary and sufficient to activate the small GTPase Rap1B.

2-Chloro N(6)-methyl-(N)-methanocarba-2'-deoxyadenosine-3',5'-bisphosphate is a selective high affinity P2Y(1) receptor antagonist

1. We reported previously that bisphosphate derivatives of adenosine are antagonists of the P2Y(1) receptor and that modification of the ribose in these analogues is tolerated in the P2Y(1) receptor binding pharmacophore. 2. Here we delineate the pharmacological activity of one such non-nucleotide molecule, 2-chloro N(6)-methyl-(N)-methanocarba-2'-deoxyadenosine-3',5'-bisphosphate (MRS2279), in which the ribose is replaced by a cyclopentane ring constrained in the (N)-conformation by a cyclopropane moiety. 3. MRS2279 antagonized 2MeSADP-stimulated inositol phosphate formation in turkey erythrocyte membranes with competitive kinetics (pK(B)=7.75). High affinity competitive antagonism by MRS2279 was also observed at the human P2Y(1) receptor (pK(B)=8.10) stably expressed in 1321N1 human astrocytoma cells. Antagonism was specific for the P2Y(1) receptor since MRS2279 had no effect on activation of the human P2Y(2), P2Y(4), P2Y(6), or P2Y(11) receptors by their cognate agonists. 4. MRS2279 also did not block the capacity of ADP to act through the Gi/adenylyl cyclase linked P2Y receptor of platelets to inhibit cyclic AMP accumulation. 5. In contrast, the P2Y(1) receptor is known to be obligatory in the process of ADP-induced platelet aggregation, and MRS2279 competitively inhibited ADP-promoted platelet aggregation with an apparent affinity (pK(B)=8.05) similar to that observed at the human P2Y(1) receptor heterologously expressed in 1321N1 cells. 6. Taken together these results illustrate selective high affinity antagonism of the P2Y(1) receptor by a non-nucleotide molecule that should prove useful for pharmacological delineation of this receptor in various tissues.

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.

Role of ADP receptor P2Y(12) in platelet adhesion and thrombus formation in flowing blood

ADP plays a central role in regulating platelet function. It induces platelet aggregation via the activation of 2 major ADP receptors, P2Y(1) and P2Y(12). We have investigated the role of P2Y(12) in platelet adhesion and thrombus formation under physiological flow by using blood from a patient with a defect in the gene encoding P2Y(12). Anticoagulated blood from the patient and from healthy volunteers was perfused over collagen-coated coverslips. The patient's thrombi were smaller and consisted of spread platelets overlying platelets that were not spread, whereas control thrombi were large and densely packed. Identical platelet surface coverage, aggregate size, and morphology were found when a P2Y(12) antagonist, N(6)-(2-methylthioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloromethylene ATP (also known as AR-C69931 MX), was added to control blood. The addition of a P2Y(1) antagonist (adenosine-3',5'-diphospate) to control blood resulted in small, but normally structured, thrombi. Thus, the ADP-P2Y(12) interaction is essential for normal thrombus buildup on collagen. The patient's blood also showed reduced platelet adhesion on fibrinogen, which was not due to changes in morphology. Comparable results were found by using control blood with AR-C69931 MX and also with adenosine-3',5'-diphospate. This suggested that P2Y(12) and P2Y(1) were both involved in platelet adhesion on immobilized fibrinogen, thereby revealing it as ADP dependent. This was confirmed by complete inhibition on the addition of creatine phosphate/creatine phosphokinase.

Multiple regulation by external ATP of nifedipine-insensitive, high voltage-activated Ca(2+) current in guinea-pig mesenteric terminal arteriole

We investigated the receptor-mediated regulation of nifedipine-insensitive, high voltage-activated Ca(2+) currents in guinea-pig terminal mesenteric arterioles (I(mVDCC)) using the whole-cell clamp technique. Screening of various vasoactive substances revealed that ATP, histamine and substance P exert modulatory effects on I(mVDCC). The effects of ATP on I(mVDCC) after complete P2X receptor desensitization exhibited a complex concentration dependence. With 5 mM Ba(2+), ATP potentiated I(mVDCC) at low concentrations (approximately 1-100 microM), but inhibited it at higher concentrations (>100 microM). The potentiating effects of ATP were abolished by suramin (100 microM) and PPADS (10 microM) and by intracellular application of GDPbetaS (500 microM), whereas a substantial part of I(mVDCC) inhibition by milimolar concentrations of ATP remained unaffected; due probably to its divalent cation chelating actions. In divalent cation-free solution, I(mVDCC) was enlarged and underwent biphasic effects by ATPgammaS and ADP, while 2-methylthio ATP (2MeSATP) exerted only inhibition, and pyrimidines such as UTP and UDP were ineffective. ATP-induced I(mVDCC) potentiation was selectively inhibited by anti-Galpha(s) antibodies or protein kinase A (PKA) inhibitory peptides and mimicked by dibutyryl cAMP. In contrast, ATP-induced inhibition was selectively inhibited by Galpha(q/11) antibodies or protein kinase C (PKC) inhibitory peptides and mimicked by PDBu. Pretreatment with pertussis toxin was ineffective. The apparent efficacy for I(mVDCC) potentiation with PKC inhibitors was: ATPgammaS > ATP>/=ADP and for inhibition with PKA inhibitors was: 2MeSATP > ATPgammaS > ATP > ADP. Neither I(mVDCC) potentiation nor inhibition showed voltage dependence. These results suggest that I(mVDCC) is multi-phasically regulated by external ATP via P2Y(11)-resembling receptor/G(s)/PKA pathway, P2Y(1)-like receptor/G(q/11)/PKC pathway, and metal chelation.

Purinergic signaling and vascular cell proliferation and death

Evidence for the role of purinergic signaling (via P1 and P2Y receptors) in the proliferation of vascular smooth muscle and endothelial cells is reviewed. The involvement of the mitogen-activated protein kinase second-messenger cascade in this action is clearly implicated, although details of the precise intracellular pathways involved still remain to be determined. Synergistic actions of purines and pyrimidines with growth factors occur in promoting cell proliferation. Interaction between purinergic signaling for vascular cell proliferation and cell death mediated by P2X7 receptors is discussed. There is evidence of the release of ATP from endothelial cells, platelets, and sympathetic nerves as well as from damaged cells in atherosclerosis, hypertension, restenosis, and ischemia; furthermore, there is evidence that vascular smooth muscle and endothelial cells proliferate in these pathological conditions. Thus, the involvement of ATP and its breakdown product, adenosine, is implicated; it is hoped that with the development of selective P1 (A2) and P2Y receptor agonists and antagonists, new therapeutic strategies will be explored.

Adenine nucleotide-induced activation of adenosine A(2B) receptors expressed in Xenopus laevis oocytes: involvement of a rapid and localized adenosine formation by ectonucleotidases

We recently demonstrated that extracellular ATP effectively activates adenosine (Ade) A(2B) receptors indirectly through a localized rapid conversion to Ade by ectonucleotidases on the membrane surface of C6Bu-1 rat glioma cells. These responses were observed even in the presence of adenosine deaminase (ADA). Here, we demonstrate that such responses indeed occur in A(2B) receptor-expressing Xenopus laevis oocytes, which possess endogenous ectonucleotidase activity. In oocytes coexpressing the A(2B) receptor and cystic fibrosis transmembrane conductance regulator (CFTR), Ade induced a concentration-dependent increase in a cyclic AMP-activated CFTR current, a response that was inhibited by the P1 antagonist xanthine-amine congener (XAC). A brief application of ATP and beta,gamma-methylene ATP (beta,gamma-MeATP) also induced the CFTR current in a manner similar to that seen with Ade. Among several nucleotide agonists, ADP, AMP, and adenosine-5'-O-(3-thio)triphosphate induced the CFTR current. Although adenine nucleotide-induced CFTR currents were inhibited by XAC, they were highly resistant to ADA treatment; 5 U/ml ADA was required for inhibition of adenine nucleotide-induced CFTR current, whereas 1 U/ml ADA was sufficient to abolish the Ade-induced response. In addition, the ecto-5'-nucleotidase inhibitor alpha,beta-methylene ADP markedly inhibited the beta,gamma-MeATP-induced response but not the Ade-induced one. These results support our hypothesis that adenine nucleotides are rapidly and locally converted into Ade on the membrane surface, resulting in the activation of A(2B) receptors.

Enhancement of platelet aggregation and thrombus formation by arsenic in drinking water: a contributing factor to cardiovascular disease

Arsenic in drinking water is a worldwide health problem that is associated with cardiovascular disease, but the cause is currently unknown. Arsenic effects on platelets, which are important in development of cardiovascular disease, were examined in vitro and in a drinking water study using a rat animal model. Trivalent inorganic arsenic (arsenite) induced in vitro aggregation when platelets were exposed to subthreshold challenge by thrombin and several other agonists in a concentration-dependent manner, with arsenite being the most potent form tested. Arsenite also induced significant increases in serotonin secretion, thromboxane A(2) formation, and adhesion protein expression in platelets. Consistent with the in vitro studies, 4-week ingestion of arsenite-contaminated drinking water resulted in enhanced arterial thrombosis. Human platelets showed similar responses, suggesting that the effects seen in animal experiments are applicable to humans. These results will provide new insights into the mechanism of arsenic-induced cardiovascular disease. They will also allow regulatory agencies to estimate risk from arsenic-induced cardiovascular disease and to determine if drinking water regulatory levels based on human cancer studies will protect against noncancer effects associated with cardiovascular disease.

ADP-evoked phospholipase C stimulation and adenylyl cyclase inhibition in glioma C6 cells occur through two distinct nucleotide receptors, P2Y(1) and P2Y(12)

In this study we characterized the subtypes of nucleotide P2Y receptors that respond to ADP in glioma C6 cells. Direct visualization of phosphatidylinositol 4,5-bisphosphate at the cell surface revealed that extracellular ADP activates phospholipase C (PLC). Knock-down of P2Y(1) receptor with antisense oligonucleotide, as well as treatment with MRS2179 and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (P2Y(1) antagonists), attenuates receptor-mediated PLC activity. Adenylyl cyclase inhibition by ADP remains unchanged under these conditions. Reverse transcription-PCR analysis showed that P2Y(12) receptor is expressed in C6 cells. We therefore conclude that, in glioma C6 cells, two P2Y receptor subtypes are present: P2Y(1), coupled to PLC, and P2Y(12), negatively coupled to adenylyl cyclase.

Novel antagonists acting at the P2Y(1) purinergic receptor: synthesis and conformational analysis using potentiometric and nuclear magnetic resonance titration techniques

The human P2Y(1) receptor is widely distributed in many tissues and has a classical structure of a G protein-coupled receptor. Activated by adenosine-5'-diphosphate (ADP), this receptor is essential for platelet aggregation. In the present paper, we describe the synthesis of novel P2Y(1) antagonists that could be of interest at least as tools to define the physiological roles of the P2Y(1) receptor, at best as new antithrombotic agents. Thus, we prepared the 2,N(6)-dimethyl-2'-deoxyadenosine-3',5'-bisphosphate derivative, 1e. The biological activity was demonstrated by the ability of compound 1e to inhibit ADP-induced platelet aggregation, shape change, and intracellular calcium rise. This compound was a full antagonist at the P2Y(1) receptor with a pA(2) value of 7.11 +/- 0.11 and was found to be 4-fold more potent than the reference N(6)-methyl-2'-deoxyadenosine-3',5'-bisphosphate (1a, pA(2) = 6.55 +/- 0.05), revealing the potency-enhancing effects of the 2-methyl group. The better activity of 1e as compared to 1a was analyzed using both potentiometric and nuclear magnetic resonance titration techniques, which highlighted specific conformational features of this compound. These results clearly indicate the preference for both compounds for an anti conformation at the N-glycosyl linkage. Furthermore, the percentage of S conformer of 1e is close to that of 1a, which is nearly 70% at pH = 2.8 and increases dramatically when pH increases. From the macroprotonation constants, it can be noted that compound 1e is significantly more basic than 1a. This is indeed expected for the N1 adenine nitrogen due to the electron-donating character of the methyl moiety. By considering the microconstants of the phosphate groups, the higher basicity of P3 and P5 for 1e may be due to the decrease in the local dielectric constant induced by the substitution of the hydrogen atom by a more lipophilic methyl group. Thus, it may be suggested that the gain in activity of 1e when compared to the reference compound 1a would result from its gain in basicity rather than steric and conformational modifications. The synthesis of the first selective radioligand acting at the P2Y(1) receptor ([(33)P]-N(6)-methyl-2'-deoxyadenosine-3',5'-bisphosphate, 17) is also reported and will be used in the future for efficient screening needed for drug optimization.

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.

P2 receptors: new potential players in atherosclerosis

Atherosclerosis is a focal inflammatory disease of the arterial wall. It starts with the formation of fatty streaks on the arterial wall that evolve to form a raised plaque made of smooth muscle cells (SMCs), and infiltrating leukocytes surrounding a necrotic core. The pathogenesis of the atherosclerotic lesion is incompletely understood, but it is clear that a dysfunction of the endothelium, recruitment and activation of inflammatory cells and SMC proliferation have a pivotal role. Over recent years receptors for extracellular nucleotides, the P2 receptors, have been recognized as fundamental modulators of leukocytes, platelets, SMCs and endothelial cells. P2 receptors mediate chemotaxis, cytokine secretion, NO generation, platelet aggregation and cell proliferation in response to accumulation of nucleotides into the extracellular milieu. Clinical trials have shown the benefit of antagonists of the ADP platelet receptor(s) in the prevention of vascular accidents in patients with atherosclerosis. Therefore, we anticipate that a deeper understanding of the involvement of P2 receptors in atheroma formation will open new avenues for drug design and therapeutic intervention.

Methanocarba modification of uracil and adenine nucleotides: high potency of Northern ring conformation at P2Y1, P2Y2, P2Y4, and P2Y11 but not P2Y6 receptors

The potency of nucleotide antagonists at P2Y1 receptors was enhanced by replacing the ribose moiety with a constrained carbocyclic ring (Nandanan, et al. J. Med. Chem. 2000, 43, 829-842). We have now synthesized ring-constrained methanocarba analogues (in which a fused cyclopropane moiety constrains the pseudosugar ring) of adenine and uracil nucleotides, the endogenous activators of P2Y receptors. Methanocarba-adenosine 5'-triphosphate (ATP) was fixed in either a Northern (N) or a Southern (S) conformation, as defined in the pseudorotational cycle. (N)-Methanocarba-uridine was prepared from the 1-amino-pseudosugar ring by treatment with beta-ethoxyacryloyl cyanate and cyclization to form the uracil ring. Phosphorylation was carried out at the 5'-hydroxyl group through a multistep process: Reaction with phosphoramidite followed by oxidation provided the 5'-monophosphates, which then were treated with 1,1'-carbonyldiimidazole for condensation with additional phosphate groups. The ability of the analogues to stimulate phospholipase C through activation of turkey P2Y1 or human P2Y1, P2Y2, P2Y4, P2Y6, and P2Y11 receptors stably expressed in astrocytoma cells was measured. At recombinant human P2Y1 and P2Y2 receptors, (N)-methanocarba-ATP was 138- and 41-fold, respectively, more potent than racemic (S)-methanocarba-ATP as an agonist. (N)-methanocarba-ATP activated P2Y11 receptors with a potency similar to ATP. (N)-Methanocarba-uridine 5'-triphosphate (UTP) was equipotent to UTP as an agonist at human P2Y2 receptors and also activated P2Y4 receptors with an EC(50) of 85 nM. (N)-Methanocarba-uridine 5'-diphosphate (UDP) was inactive at the hP2Y6 receptor. The vascular effects of (N)-methanocarba-UTP and (N)-methanocarba-UDP were studied in a model of the rat mesenteric artery. The triphosphate was more potent than UTP in inducing a dilatory P2Y4 response (pEC(50) = 6.1 +/- 0.2), while the diphosphate was inactive as either an agonist or antagonist in a P2Y6 receptor-mediated contractile response. Our results suggest that new nucleotide agonists may be designed on the basis of the (N) conformation that favors selectivity for P2Y1, P2Y2, P2Y4, and P2Y11 receptors.

A retrospective of recombinant P2Y receptor subtypes and their pharmacology

Since the first cloning of P2Y receptor sequences in 1993 it has become apparent that this family of G-protein-coupled receptors is omnipresent. At least 25 individual sequences entered in the GenBank sequence database encode P2Y receptors from a variety of species ranging from the little skate Raja erinacea to man. In man, six receptor subtypes have been cloned and found to be functionally active (P2Y(1), P2Y(2), P2Y(4), P2Y(6), P2Y(11), and P2Y(12)). In this article a review of the P2Y receptor subtypes is presented considering both their sequences and the pharmacological profiles of the encoded receptors expressed in heterologous expression systems.

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.

Platelet endothelial cell adhesion molecule-1 signaling inhibits the activation of human platelets

Platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31) is a 130-kd transmembrane glycoprotein and a member of the growing family of receptors with immunoreceptor tyrosine-based inhibitory motifs (ITIMs). PECAM-1 is expressed on platelets, certain T cells, monocytes, neutrophils, and vascular endothelial cells and is involved in a range of cellular processes, though the role of PECAM-1 in platelets is unclear. Cross-linking of PECAM-1 results in phosphorylation of the ITIM allowing the recruitment of signaling proteins that bind by way of Src-homology domain 2 interactions. Proteins that have been implicated in the negative regulation of cellular activation by ITIM-bearing receptors include the tyrosine phosphatases SHP-1 and SHP-2. Tyrosine phosphorylation of immunoreceptor tyrosine-based activatory motif (ITAM)-bearing receptors such as the collagen receptor GPVI-Fc receptor gamma-chain complex on platelets leads to activation. Increasing evidence suggests that ITIM- and ITAM-containing receptors may act antagonistically when expressed on the same cell. In this study it is demonstrated that cross-linking PECAM-1 inhibits the aggregation and secretion of platelets in response to collagen and the GPVI-selective agonist convulxin. In these experiments thrombin-mediated platelet aggregation and secretion were also reduced, albeit to a lesser degree than for collagen, suggesting that PECAM-1 function may not be restricted to the inhibition of ITAM-containing receptor pathways. PECAM-1 activation also inhibited platelet protein tyrosine phosphorylation stimulated by convulxin and thrombin; this was accompanied by inhibition of the mobilization of calcium from intracellular stores. These data suggest that PECAM-1 may play a role in the regulation of platelet function in vivo.

A study of P2X1 receptor function in murine megakaryocytes and human platelets reveals synergy with P2Y receptors

We have examined the role of ATP-dependent P2X(1) receptors in megakaryocytes (MKs) and platelets using receptor-deficient mice and selective agonists. Alpha,beta-meATP- and ATP- evoked ionotropic inward currents were absent in whole-cell recordings from MKs of P2X(1)(-/-) mice, demonstrating that the P2X receptor phenotype in MKs, and by inference, platelets, is due to expression of homomeric P2X(1) receptors. P2X(1) receptor deficiency had no effect on MK (CD 41) numbers or size distribution, showing that it is not essential for normal MK development. P2Y receptor-stimulated [Ca(2+)](i) responses were unaffected in MKs from P2X(1)(-/-) mice, however the inward cation current associated with Ca(2+) release was reduced by approximately 50%, suggesting an interaction between the membrane conductances activated by P2X(1) and P2Y receptors. Interaction between P2X(1) and P2Y receptors in human platelets was also examined using [Ca(2+)](i) recordings from cell suspensions. Alpha,beta-meATP (10 microM) evoked a rapid transient P2X(1) receptor-mediated increase in [Ca(2+)](i), whereas ADP-(10 microM) evoked P2Y receptor responses were slower, peaked at a higher level and remained elevated for longer periods. Co-application of alpha, beta-meATP and ADP resulted in marked acceleration and amplification of the peak [Ca(2+)](i) response. We conclude that ionotropic P2X(1) receptors may play a priming role in the subsequent activation of metabotropic P2Y receptors during platelet stimulation.

Extracellular signal-regulated kinases and g protein-coupled receptors in megakaryocytic human erythroleukemia cells: selective activation, differential regulation, and dissociation from mitogenesis

Extracellular signal-regulated kinases 1 and 2 (ERK1/2) are a group of kinases that play an important role in proliferation and differentiation. In megakaryocyte-like human erythroleukemia (HEL) cells, ERK2 was found to be predominantly expressed and strongly activated by prostaglandin (PG) E(2), thrombin, and epinephrine. On the other hand, adenosine, ADP, ATP, and UTP did not significantly increase ERK1/2 phosphorylation. However, of the agonists tested, only ADP was able to stimulate thymidine uptake. Pretreatment with pertussis toxin abolished the PGE(2) response but had less of an effect on thrombin. PGE(2)- and thrombin-induced ERK1/2 activation was mimicked by 4-beta-phorbol-12-myristate-13-acetate and ionomycin and blocked by mitogen-activated protein kinase kinase inhibitor 1,4 diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene but displayed differential sensitivity to protein kinase C inhibitor bisindolylmaleimide I and Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. Analogs of cAMP or agents that stimulate cAMP production were either weak or ineffective activators. Further studies indicate that the effect of thrombin was blocked by the phosphoinositide 3-kinase inhibitor wortmannin but not by agents inhibiting tyrosine kinase activity. On the contrary, herbimycin, but not wortmannin, attenuated the effect of PGE(2). Collectively, these results indicate that ERK1/2 are selectively activated by G protein-coupled receptors and not functionally associated with proliferation in HEL cells. ERK1/2 activation in response to PGE(2) and thrombin is mediated by distinctive types of G proteins and is differentially regulated by multiple pathways, including calcium mobilization, protein kinase C, phosphoinositide 3-kinase, and tyrosine kinases.

Differential regulation of Rho and Rac through heterotrimeric G-proteins and cyclic nucleotides

Platelets were used to study the activation of Rho and Rac through G-protein-coupled receptors and its regulation by cyclic nucleotides. The thromboxane A(2) (TXA(2)) mimetic rapidly activated both small GTPases independently of integrin alpha(IIb)beta(3) activation., which leads to the activation of G(12)/G(13) and G(q) did not induce Rac activation in G alpha(q)-deficient platelets but was able to activate Rho, to stimulate actin polymerization and phosphatidylinositol 4,5-bisphosphate formation, and to induce shape change. Rac activation by in wild-type platelets could be blocked by chelation of intracellular Ca(2+) and was partially sensitive to apyrase and AR-C69931MX, an antagonist of the G(i)-coupled ADP receptor. Cyclic AMP, which completely blocks platelet function, inhibited the -induced activation of G(q) and G(12)/G(13) as well as of Rac and Rho. In contrast, cGMP, which has no effect on platelet shape change blocked only activation of G(q) and Rac. These data demonstrate that Rho and Rac are differentially regulated through heterotrimeric G-proteins. The G(12)/G(13)-mediated Rho activation is involved in the shape change response, whereas Rac is activated through G(q) and is not required for shape change. Cyclic AMP and cGMP differentially interfere with -induced Rho and Rac activation at least in part by selective effects on the regulation of individual G-proteins through the TXA(2) receptor.

Inherited defects of platelet function

Inherited platelet defects bleeding syndromes underlie of varying severity. The Bernard-Soulier syndrome and Glanzmann thrombasthenia are disorders of membrane glycoproteins. In the former, a deficiency of the GPIb-IX-V complex leads to defective platelet adhesion, while in thrombasthenia, platelet aggregation does not occur in the absence of the integrin alphaIIbbeta3. Defects of primary receptors for stimuli are increasingly being described, and include a defect of a newly cloned Gi-protein-linked, seven transmembrane domain, ADP receptor. These lead to agonist-specific deficiencies in the platelet function response, as do abnormalities in the many intracellular signaling pathways of platelets. Defects affecting secretion from dense bodies and alpha-granules, of ATP production and generation of procoagulant activity, are also encountered. Some disorders are exclusive to megakaryocytes and platelets, while in others, such as the Chediak-Higashi, Hermansky-Pudlak and Wiskott-Aldrich syndromes; the molecular lesion extends to other cell types. Disorders affecting platelet morphology, the so-called "giant platelet" syndromes should also be considered. In familial thrombocytopenias, platelets are produced in insufficient quantities to assure hemostasis. Platelet disorders are examples of rare diseases; nevertheless they have provided essential information in the elucidation of the molecular basis of platelet function.

An arginine/glutamine difference at the juxtaposition of transmembrane domain 6 and the third extracellular loop contributes to the markedly different nucleotide selectivities of human and canine P2Y11 receptors

The recently cloned canine P2Y11 receptor (cP2Y11) and its human homolog (hP2Y11) were stably expressed in Chinese hamster ovary cells (CHO-K1) and 1321N1 human astrocytoma cells, and their agonist selectivities and coupling efficiencies to phospholipase C and adenylyl cyclase were assessed. Adenosine triphosphate nucleotides were much more potent and efficacious at the hP2Y11 receptor than their corresponding diphosphates in promoting both inositol phosphate and cyclic AMP accumulation. In contrast, adenosine diphosphate nucleotides were considerably more potent at the cP2Y11 receptor than their corresponding triphosphate analogs. The tri- versus diphosphate specificity of the two receptors was further confirmed in studies using Ca(2+) mobilization as a measure of receptor activation under conditions that minimized nucleotide degradation. Moreover, 2-methylthioadenosine-5'-triphosphate and 2-methylthioadenosine-5'-diphosphate were 58- and 75-fold more potent than ATP and ADP, respectively, at the cP2Y11 receptor compared with only 2- to 3-fold more potent at the hP2Y11 receptor. Mutational analysis revealed that the change of Arg-265, which is located at the juxtaposition of transmembrane domain 6 and the third extracellular loop in the hP2Y11 receptor, to glutamine in the cP2Y11 receptor is at least partly responsible for the diphosphate selectivity but not the increased sensitivity to 2-thioether-substituted adenine nucleotides at the canine receptor. These results imply a key role for a positively charged arginine residue in contributing to the recognition of extracellular nucleotides by the P2Y11 receptor and perhaps other P2Y receptors.

Cloning, pharmacology, and tissue distribution of G-protein-coupled receptor GPR105 (KIAA0001) rodent orthologs

It has recently been shown that UDP-glucose is a potent agonist of the orphan G-protein-coupled receptor (GPCR) KIAA0001. Here we report cloning and analysis of the rat and mouse orthologs of this receptor. In accordance with GPCR nomenclature, we have renamed the cDNA clone, KIAA0001, and its orthologs GPR105 to reflect their functionality as G-protein-coupled receptors. The rat and mouse orthologs show 80% and 83% amino acid identity, respectively, to the human GPR105 protein. We demonstrate by genomic Southern blot analysis that there are no genes in the mouse or rat genomes with higher sequence similarity. Chromosomal mapping shows that the mouse and human genes are located on syntenic regions of chromosome 3. Further analyses of the rat and mouse GPR105 proteins show that they are activated by the same agonists as the human receptor, responding to UDP-glucose and closely related molecules with similar affinities. The mouse and rat receptors are widely expressed, as is the human receptor. Thus we conclude that we have identified the rat and mouse orthologs of the human gene GPR105.