Coactivation of two different G protein-coupled receptors is essential for ADP-induced platelet aggregation

P2 receptors and platelet function

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

Effects of antiphospholipid antibodies on in vitro platelet aggregation

Antiphospholipid antibodies contribute to the development of thrombosis, although precise mechanisms remain to be elucidated. We determined the effects of affinity-purified anti-beta(2)-glycoprotein 1 (anti-β(2)GP1) and anti-prothrombin (anti-PT) antibodies on in vitro platelet aggregation. Adenosine diphosphate (ADP) and collagen-induced platelet aggregation were performed using platelet-rich plasma ([PRP] 250 × 10(9)/L). Antiphospholipid antibodies (1.25-10 μg/mL) were preincubated with PRP for 10 minutes at 37°C prior to the addition of agonist. Anti-β(2)GP1 antibodies significantly reduced platelet aggregation (percentage area under the curve; %AUC) in a concentration-dependent manner using both 5 μmol/L (P < .001) and 2.5 μmol/L (P = .038) ADP but did not significantly affect the rate of aggregation. Anti-PT antibodies significantly enhanced 5 µg/mL collagen-induced platelet aggregation (%AUC; P = .034) but did not affect ADP-induced platelet aggregation. These results suggest (1) interactions and effects of antiphospholipid antibodies on platelets are agonist and concentration dependent and (2) anti-β(2)GP1 antibodies may inhibit dense granule release and/or inhibition of the arachidonic acid pathway.

P2Y12 receptor in platelet activation

ADP plays an important role in hemostasis and thrombosis. The P2Y12 receptor, activated by ADP, plays a central role in platelet activation and thrombus formation. Thus, the P2Y12 receptor has been an effective target for antithrombotic drugs.

Negative regulation of Gq-mediated pathways in platelets by G(12/13) pathways through Fyn kinase

Platelets contain high levels of Src family kinases (SFKs), but their functional role downstream of G protein pathways has not been completely understood. We found that platelet shape change induced by selective G(12/13) stimulation was potentiated by SFK inhibitors, which was abolished by intracellular calcium chelation. Platelet aggregation, secretion, and intracellular Ca(2+) mobilization mediated by low concentrations of SFLLRN or YFLLRNP were potentiated by SFK inhibitors. However, 2-methylthio-ADP-induced intracellular Ca(2+) mobilization and platelet aggregation were not affected by PP2, suggesting the contribution of SFKs downstream of G(12/13), but not G(q)/G(i), as a negative regulator to platelet activation. Moreover, PP2 potentiated YFLLRNP- and AYPGKF-induced PKC activation, indicating that SFKs downstream of G(12/13) regulate platelet responses through the negative regulation of PKC activation as well as calcium response. SFK inhibitors failed to potentiate platelet responses in the presence of G(q)-selective inhibitor YM254890 or in G(q)-deficient platelets, indicating that SFKs negatively regulate platelet responses through modulation of G(q) pathways. Importantly, AYPGKF-induced platelet aggregation and PKC activation were potentiated in Fyn-deficient but not in Lyn-deficient mice compared with wild-type littermates. We conclude that SFKs, especially Fyn, activated downstream of G(12/13) negatively regulate platelet responses by inhibiting intracellular calcium mobilization and PKC activation through G(q) pathways.

Docking-based virtual screening of potential human P2Y12 receptor antagonists

Platelet plays essential roles in hemostasis and its dysregulation can lead to arterial thrombosis. P2Y12 is an important platelet membrane adenosine diphosphate receptor, and its antagonists have been widely developed as anti-coagulation agents. The current P2Y12 inhibitors available in clinical practice have not fully achieved satisfactory anti-thrombotic effects, leaving room for further improvement. To identify new chemical compounds as potential anti-coagulation inhibitors, we constructed a three-dimensional structure model of human P2Y12 by homology modeling based on the recently reported G-protein coupled receptor Meleagris gallopavo β1 adrenergic receptor. Virtual screening of the modeled P2Y12 against three subsets of small molecules from the ZINC database, namely lead-like, fragment-like, and drug-like, identified a number of compounds that might have high binding affinity to P2Y12. Detailed analyses of the top three compounds from each subset with the highest scores indicated that all of these compounds beard a hydrophobic bulk supplemented with a few polar atoms which bound at the ligand binding site via largely hydrophobic interactions with the receptor. This study not only provides a structure model of P2Y12 for rational design of anti-platelet inhibitors, but also identifies some potential chemicals for further development.

Submaximal inhibition of protein kinase C restores ADP-induced dense granule secretion in platelets in the presence of Ca2+

Protein kinase C (PKC) is a family of serine/threonine kinases that play isoform-specific inhibitory and stimulatory roles in platelet activation. We show here that the pan-PKC inhibitor Ro31-8220 can be used to dissect these events following platelet activation by ADP. Submaximal concentrations of Ro31-8220 potentiated aggregation and dense granule secretion to ADP in plasma anticoagulated with citrate, in d-Phe-Pro-Arg-chloromethyl ketone-anticoagulated plasma, which has physiological levels of Ca²⁺, and in washed platelets. Potentiation was retained on inhibition of cyclooxygenase and was associated with an increase in intracellular Ca²⁺. Potentiation of aggregation and secretion was abolished by a maximally effective concentration of Ro31-8220, consistent with a critical role of PKC in secretion. ADP-induced secretion was potentiated in the presence of an inhibitor of PKCβ but not in the presence of available inhibitors of other PKC isoforms in human and mouse platelets. ADP-induced secretion was also potentiated in mouse platelets deficient in PKCϵ but not PKCθ. These results demonstrate that partial blockade of PKC potentiates aggregation and dense granule secretion by ADP in association with increased Ca²⁺. This provides a molecular explanation for the inability of ADP to induce secretion in plasma in the presence of physiological Ca²⁺ concentrations, and it reveals a novel role for PKC in inhibiting platelet activation by ADP in vivo. These results also demonstrate isoform-specific inhibitory effects of PKC in platelets.

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

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

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

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

G(12/13) signaling pathways substitute for integrin αIIbβ3-signaling for thromboxane generation in platelets

BACKGROUND

We have previously shown that ADP-induced TXA(2) generation requires signaling from αIIbβ3 integrin in platelets. Here we observed that, unlike ADP, protease-activated receptor (PAR)-mediated TXA(2) generation occurs independently of αIIbβ3. PAR agonists, but not ADP, activate G(12/13) signaling pathways. Hence, we evaluated the role of these pathways in TXA(2) generation.

PRINCIPAL FINDINGS

Inhibition of ADP-induced thromboxane generation by fibrinogen receptor antagonist SC57101 was rescued by co-stimulation of G(12/13) pathways with YFLLRNP. This observation suggested an existence of a common signaling effector downstream of integrins and G(12/13) pathways. Hence, we evaluated role of three potential tyrosine kinases; c-Src, Syk and FAK (Focal Adhesion Kinase) that are known to be activated by integrins. c-Src and Syk kinase did not play a role in ADP-induced functional responses in platelets. Selective activation of G(12/13) pathways resulted in the activation of FAK, in the absence of integrin signaling. Interestingly, αIIbβ3-mediated FAK activation occurred in a Src family kinase (SFK)-independent manner whereas G(12/13) pathway caused FAK activation in a SFK and RhoA-dependent manner. A FAK selective inhibitor TAE-226, blocked TXA(2) generation. However, in comparison to WT mice, Pf4-Cre/Fak-Floxed mice did not show any difference in platelet TXA(2) generation.

CONCLUSIONS

Therefore, we conclude that differential activation of FAK occurs downstream of Integrins and G(12/13) pathways. However, the common effector molecule, possibly a tyrosine kinase downstream of integrins and G(12/13) pathways contributing to TXA(2) generation in platelets remains elusive.

Dimerization of G protein-coupled purinergic receptors: increasing the diversity of purinergic receptor signal responses and receptor functions

It is well accepted that G protein-coupled receptors (GPCRs) arrange into dimers or higher-order oligomers that may modify various functions of GPCRs. GPCR-type purinergic receptors (i.e. adenosine and P2Y receptors) tend to form heterodimers with GPCRs not only of the different families but also of the same purinergic receptor families, leading to alterations in functional properties. In the present review, we focus on current knowledge of the formation of heterodimers between metabotropic purinergic receptors that activate novel functions in response to extracellular nucleosides/nucleotides, revealing that the dimerization seems to be employed for 'fine-tuning' of purinergic signaling. Thus, the relationship between adenosine and adenosine triphosphate is likely to be more and more intimate than simply being a metabolite of the other.

Identification of P2Y12 single-nucleotide polymorphisms and their influences on the variation in ADP-induced platelet aggregation

INTRODUCTION

Although P2Y12 has a significant role in normal hemostasis and thrombosis, no genetic study has been described about the association between P2Y12 variants and the extent of ADP-induced platelet activation in the Korean population.

MATERIALS AND METHODS

The expression levels of two reference sequences of P2Y12 mRNA transcripts (variants 1 and 2) were examined in the whole blood before direct DNA sequencing. The subjects were screened for single-nucleotide polymorphisms (SNPs) in P2Y12 by direct DNA sequencing (n=50). Frequencies of P2Y12 single nucleotide polymorphisms (SNPs), linkage disequilibrium blocks, haplotype structures, and haplotype-tagging SNPs were determined. The effects of genetic variation in the P2Y12 gene on the extent of ADP-induced platelet aggregation were studied in healthy Korean men (n=40).

RESULTS

Variant 2 (NM 176876.1) was the predominantly expressed form in all subjects, but variant 1 was also weakly expressed in all cases (n=10). A total of 20 SNPs were identified: 2 in exons, 5 in introns, and 8 and 5 in the 5'-untranslated regions of the known P2Y12 RNA variants 1 and 2, respectively. Genetic analysis of the P2Y12 SNPs and haplotypes revealed a statistically significant association between P2Y12 haplotype, denoted H3, and an increase in the ADP-induced platelet aggregation response relative to that for the reference haplotype H1 (P=0.01).

CONCLUSIONS

Application of these findings to the development of a multivariate model might be useful in explaining the variable outcome of antiplatelet drug therapy in Asian populations.

P2Y receptors in health and disease

The purine- and pyrimidine-sensitive P2Y receptors belong to the large group of G-protein-coupled receptors that are the target of approximately one-third of the pharmaceutical drugs used in the clinic today. It is therefore not unexpected that the P2Y receptors could be useful targets for drug development. This chapter will discuss P2Y receptor-based therapies currently used, in development and possible future developments. The platelet inhibitors blocking the ADP-receptor P2Y(12) reduce myocardial infarction, stroke, and mortality in patients with cardiovascular disease. Clopidogrel (Plavix) was for many years the second most selling drug in the world. The improved P2Y(12) inhibitors prasugrel, ticagrelor, and elinogrel are now entering the clinic with even more pronounced protective effects. The UTP-activated P2Y(2) receptor stimulates ciliary movement and secretion from epithelial cells. Cystic fibrosis is a monogenetic disease where reduced chloride ion secretion results in a severe lung disease and early death. No specific treatment has been available, but the P2Y(2) agonist Denufosol has been shown to improve lung function and is expected to be introduced as treatment for cystic fibrosis soon. In preclinical studies, there are indications that P2Y receptors can be important for diabetes, osteoporosis, cardiovascular, and atherosclerotic disease. In conclusion, P2Y receptors are important for the health of humans for many diseases, and we can expect even more beneficial drugs targeting P2Y receptors in the future.

Functionalized congeners of P2Y1 receptor antagonists: 2-alkynyl (N)-methanocarba 2'-deoxyadenosine 3',5'-bisphosphate analogues and conjugation to a polyamidoamine (PAMAM) dendrimer carrier

The P2Y(1) receptor is a prothrombotic G protein-coupled receptor (GPCR) activated by ADP. Preference for the North (N) ring conformation of the ribose moiety of adenine nucleotide 3',5'-bisphosphate antagonists of the P2Y(1) receptor was established by using a ring-constrained methanocarba (a bicyclo[3.1.0]hexane) ring as a ribose substitute. A series of covalently linkable N(6)-methyl-(N)-methanocarba-2'-deoxyadenosine-3',5'-bisphosphates containing extended 2-alkynyl chains was designed, and binding affinity at the human (h) P2Y(1) receptor determined. The chain of these functionalized congeners contained hydrophilic moieties, a reactive substituent, or biotin, linked via an amide. Variation of the chain length and position of an intermediate amide group revealed high affinity of carboxylic congener 8 (K(i) 23 nM) and extended amine congener 15 (K(i) 132 nM), both having a 2-(1-pentynoyl) group. A biotin conjugate 18 containing an extended epsilon-aminocaproyl spacer chain exhibited higher affinity than a shorter biotinylated analogue. Alternatively, click coupling of terminal alkynes of homologous 2-dialkynyl nucleotide derivatives to alkyl azido groups produced triazole derivatives that bound to the P2Y(1) receptor following deprotection of the bisphosphate groups. The preservation of receptor affinity of the functionalized congeners was consistent with new P2Y(1) receptor modeling and ligand docking. Attempted P2Y(1) antagonist conjugation to PAMAM dendrimer carriers by amide formation or palladium-catalyzed reaction between an alkyne on the dendrimer and a 2-iodopurine-derivatized nucleotide was unsuccessful. A dialkynyl intermediate containing the chain length favored in receptor binding was conjugated to an azide-derivatized dendrimer, and the conjugate inhibited ADP-promoted human platelet aggregation. This is the first example of attaching a strategically functionalized P2Y receptor antagonist to a PAMAM dendrimer to produce a multivalent conjugate exhibiting a desired biological effect, i.e., antithrombotic action.

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

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

Differential phosphorylation of myosin light chain (Thr)18 and (Ser)19 and functional implications in platelets

BACKGROUND

Myosin IIA is an essential platelet contractile protein that is regulated by phosphorylation of its regulatory light chain (MLC) on residues (Thr)18 and (Ser)19 via the myosin light chain kinase (MLCK).

OBJECTIVE

The present study was carried out to elucidate the mechanisms regulating MLC (Ser)19 and (Thr)18 phosphorylation and the functional consequence of each phosphorylation event in platelets.

RESULTS

Induction of 2MeSADP-induced shape change occurs within 5s along with robust phosphorylation of MLC (Ser)19 with minimal phosphorylation of MLC (Thr)18. Selective activation of G(12/13) produces both slow shape change and comparably slow MLC (Thr)18 and (Ser)19 phosphorylation. Stimulation with agonists that trigger ATP secretion caused rapid MLC (Ser)19 phosphorylation while MLC (Thr)18 phosphorylation was coincident with secretion. Platelets treated with p160(ROCK) inhibitor Y-27632 exhibited a partial inhibition in secretion and had a substantial inhibition in MLC (Thr)18 phosphorylation without effecting MLC (Ser)19 phosphorylation. These data suggest that phosphorylation of MLC (Ser)19 is downstream of Gq/Ca(2+) -dependent mechanisms and sufficient for shape change, whereas MLC (Thr)18 phosphorylation is substantially downstream of G(12/13) -regulated Rho kinase pathways and necessary, probably in concert with MLC (Ser)19 phosphorylation, for full contractile activity leading to dense granule secretion. Overall, we suggest that the amplitude of the platelet contractile response is differentially regulated by a least two different signaling pathways, which lead to different phosphorylation patterns of the myosin light chain, and this mechanism results in a graded response rather than a simple on/off switch.

RGS/Gi2alpha interactions modulate platelet accumulation and thrombus formation at sites of vascular injury

Although much is known about extrinsic regulators of platelet function such as nitric oxide and prostaglandin I(2) (PGI(2)), considerably less is known about intrinsic mechanisms that prevent overly robust platelet activation after vascular injury. Here we provide the first evidence that regulators of G-protein signaling (RGS) proteins serve this role in platelets, using mice with a G184S substitution in G(i2α) that blocks RGS/G(i2) interactions to examine the consequences of lifting constraints on G(i2)-dependent signaling without altering receptor:effector coupling. The results show that the G(i2α)(G184S) allele enhances platelet aggregation in vitro and increases platelet accumulation after vascular injury when expressed either as a global knock-in or limited to hematopoietic cells. Biochemical studies show that these changes occur in concert with an attenuated rise in cyclic adenosine monophosphate levels in response to prostacyclin and a substantial increase in basal Akt activation. In contrast, basal cyclic adenosine monophosphate (cAMP) levels, agonist-stimulated increases in [Ca(++)](i), Rap1 activation, and α-granule secretion were unaffected. Collectively, these observations (1) demonstrate an active role for RGS proteins in regulating platelet responsiveness, (2) show that this occurs in a pathway-selective manner, and (3) suggest that RGS proteins help to prevent unwarranted platelet activation as well as limiting the magnitude of the normal hemostatic response.

Relation of CYP2C19 loss-of-function polymorphism to the occurrence of stent thrombosis

IMPORTANCE OF THE FIELD

Major adverse cardiovascular events including stent thrombosis associated with residual platelet reactivity on antiplatelet treatment in high risk vascular patients is a hot issue that needs a strong effort to be solved. Dual antiplatelet therapy with clopidogrel and aspirin prevents ischemic events and improves outcomes following acute coronary syndromes and percutaneous coronary intervention. However, adverse cardiovascular events occur in these patients, and several studies have shown that patients who suffer cardiovascular complications have high post-treatment platelet reactivity despite antiplatelet treatment. Clopidogrel requires conversion to active metabolite by CYP isoenzymes. Recently, CYP2C19*2 polymorphism (G681A nucleotide substitution) has been shown to be associated with decreased metabolisation of clopidogrel, poor antiaggregant effect and increased adverse cardiovascular events.

AREAS COVERED IN THIS REVIEW

This review summarises the principal studies contributing to establish the relationship between CYP2C19*2 polymorphism and adverse outcomes in high risk patients on clopidogrel treatment.

TAKE HOME MESSAGE

Prospective studies are urgently needed to determine the clinical impact of a score that takes into account individual characteristics of patients - CYP2C19*2 genotypes, residual platelet reactivity, drug-drug interaction, as well as traditional and procedural risk factors - for the identification of the therapeutic strategy that provides the best benefit for the single subject.

SCH 602539, a protease-activated receptor-1 antagonist, inhibits thrombosis alone and in combination with cangrelor in a Folts model of arterial thrombosis in cynomolgus monkeys

OBJECTIVE

To determine the antithrombotic effects of SCH 602539, an analog of the selective protease-activated receptor (PAR)-1 antagonist vorapaxar (formerly SCH 530348) currently in advanced clinical development, and the P2Y(12) ADP receptor antagonist cangrelor, alone and in combination.

METHODS AND RESULTS

Multiple platelet activation pathways contribute to thrombosis. The effects of SCH 602539 and cangrelor alone and in combination on cyclic flow reductions were evaluated in a Folts model of thrombosis in cynomolgus monkeys. The effects of these treatments on ex vivo platelet aggregation and coagulation parameters were also monitored. Dose-dependent inhibition of cyclic flow reductions was observed after treatment with SCH 602539 alone and cangrelor alone (P<0.05 versus vehicle for the 2 highest concentrations of each agent). The combination of SCH 602539 and cangrelor was associated with synergistic antithrombotic effects (P<0.05 versus vehicle for all combinations tested). The 2 highest doses of SCH 602539 inhibited platelet aggregation in response to PAR-1-selective high-affinity thrombin receptor agonist peptide by greater than 80% but did not affect platelet aggregation induced by other agonists; also, they did not affect any coagulation parameters.

CONCLUSIONS

The combined inhibition of the PAR-1 and the P2Y(12) ADP platelet activation pathways had synergistic antithrombotic and antiplatelet effects. The addition of a PAR-1 antagonist to a P2Y(12) ADP receptor antagonist may provide incremental clinical benefits in patients with atherothrombotic disease, both in short- and long-term settings. These hypotheses need to be tested clinically.

Recent therapeutic approaches to platelet activation in coronary artery disease

We have examined the role of platelets and platelet activation, and related emerging therapeutic approaches, in acute coronary syndromes (ACS). The role of platelets in atherothrombosis and ACS is critical, since platelet activation is a key step in the manifestation of these syndromes. Adhesion of sub-endothelial collagen and von Willebrand Factor (vWF) to the glycoprotein (GP) platelet receptors (GPIaIIa and GPIb/IX/V respectively) stimulates platelet activation. During activation, platelets present pseudopodia, which ensures a tighter adhesion to the sub-endothelial matrix and, via GPIIbIIIa receptors, facilitates platelet aggregation and platelet binding to fibrinogen and vWF. Although all laboratory methods estimating platelet activation and antiplatelet therapy have specific limitations, the use of antiplatelet agents such as aspirin, clopidogrel, and GPIIbIIIa inhibitors remains essential in ACS prevention and treatment. Platelet-related genetic polymorphisms can modulate the response to these agents. Presently, antiplatelet intervention remains an important therapeutic modality, with novel antiplatelet therapies, such as prasugrel and ticagrelor under clinical investigation.

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

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

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

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

P2Y nucleotide receptors: promise of therapeutic applications

Extracellular nucleotides, such as ATP and UTP, have distinct signaling roles through a class of G-protein-coupled receptors, termed P2Y. The receptor ligands are typically charged molecules of low bioavailability and stability in vivo. Recent progress in the development of selective agonists and antagonists for P2Y receptors and study of knockout mice have led to new drug concepts based on these receptors. The rapidly accelerating progress in this field has already resulted in drug candidates for cystic fibrosis, dry eye disease and thrombosis. On the horizon are novel treatments for cardiovascular diseases, inflammatory diseases and neurodegeneration.

Determinants to optimize response to clopidogrel in acute coronary syndrome

The inhibition of platelet function by antiplatelet therapy determines the improvement of the survival of patients with clinically evident cardiovascular disease. Clopidogrel in combination with aspirin is the recommended standard of care for reducing the occurrence of cardiovascular events in patients with acute coronary syndromes undergoing percutaneous coronary intervention. However, major adverse cardiovascular events including stent thrombosis occur in patients taking clopidogrel and aspirin. A growing body of evidence demonstrates that high post-treatment platelet reactivity on antiplatelet treatment is associated with increased risk of adverse clinical events. Clopidogrel requires conversion to active metabolite by cytochrome P450 isoenzymes. The active metabolite inhibits ADP-stimulated platelet activation by irreversibly binding to P2Y₁₂ receptors. Recently, the loss-of-function CYP2C19*2 allele has been associated with decreased metabolization of clopidogrel, poor antiaggregant effect, and increased cardiovascular events. In high risk vascular patients, the CYP2C19*2 polymorphism is a strong predictor of adverse cardiovascular events and particularly of stent thrombosis. Prospective studies evaluating if an antiplatelet treatment tailored on individual characteristics of patients, CYP2C19*2 genotypes, platelet phenotype, drug–drug interaction, as well as traditional and procedural risk factors, are now urgently needed for the identification of therapeutic strategies providing the best benefit for the single subject.

Platelet function and HIV: a case-control study

OBJECTIVE

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

DESIGN

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

METHODS

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

RESULTS

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

CONCLUSION

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

Platelet thrombin receptor antagonism and atherothrombosis

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

Current status of thoracic endografting and its adjunctive pharmacology

BACKGROUND

Thoracic endovascular aortic repair is a promising treatment modality. The advantages in the short term include shorter hospital stay, avoidance of lengthy invasive procedures, and decreased anesthesia time. It has made possible the treatment of elderly patients with coexisting comorbidities who otherwise would be difficult candidates for open surgery.

OBJECTIVE

Although the technical aspects of thoracic endovascular procedures have been described in the literature, the adjunct pharmacology has not been discussed. The aim of this study is therefore to review the clinical pharmacology necessary for procedural details in repair of patients with aortic aneurysms and aortic dissection.

METHODS

A literature search was performed using PUBMED by combining relevant Medical Subject Heading key words. The query was subsequently limited to English language, and involving "human". The articles were assessed for their validity, importance, and applicability. The publications were analyzed and clinically important data were collected and incorporated.

RESULTS/CONCLUSION

Thoracic endovascular aortic repair is a complex procedure on a morbid cohort of patients with aortic pathologies. The pharmacotherapy is a crucial component of the procedure: It is aimed at facilitating the endovascular procedure and improving outcomes.

Epinephrine induces intracellular Ca2+mobilization in thrombin-desensitized platelets: a role for GPIb-IX-V

In this work we have investigated the ability of epinephrine to trigger the release of intracellular Ca2+ in thrombin-desensitized platelets. Addition of thrombin to platelets in the presence of extracellular EGTA caused a rapid and transient release of Ca2+ from intracellular stores and rendered platelets unresponsive to a second addition of the same agonist. Although epinephrine alone had no effect on intracellular Ca2+ mobilization, its addition to thrombin-desensitized platelets was associated to a rapid and evident secondary release of intracellular Ca2+. This effect of epinephrine was not observed when platelets were desensitized with other agonists able to induce phospholipase C activation, including convulxin, U46619, and ADP. Although the platelet receptor for epinephrine is coupled to the Gi family member Gz, no secondary Ca2+ release was seen in thrombin-desensitized platelets upon stimulation of other Gi-coupled receptors, including the P2Y12 receptor and the CXCR4. Addition of hirudin to thrombin-desensitized platelets prevented epinephrine-promoted secondary release of Ca2+, indicating that thrombin, rather than epinephrine itself, is actually responsible for this event as a consequence of thrombin receptors resensitization. Studies with platelets stimulated with specific PAR1- and PAR4- activating peptides proved that neither one of these thrombin receptors were involved in the secondary epinephrine-assisted Ca2+ release. Moreover, we found that thrombin was still able to induce a reduced, but evident release of Ca2+ from internal stores in PAR1- and PAR4-desensitized platelets, which could be followed by a secondary Ca2+ release upon subsequent addition of epinephrine. Importantly, both the primary and the secondary Ca2+ release induced by thrombin and epinephrine in PAR1- and PAR4-desensitized platelets were abrogated upon cleavage of GPIbalpha by the metalloproteinase mocarhagin. These results demonstrate a direct role of thrombin binding to GPIb-IX-V in the mobilization of Ca2+ from intracellular stores, and reveal that epinephrine can restore this process in desensitized platelets, thus prolonging the effect of thrombin stimulation.

Human platelet pathology related to defects in the G-protein signaling cascade

Platelets are highly responsive to signals from their environment. The sensing and processing of some of these stimuli are mediated by G-protein signal transduction cascades. It is well established that proteins involved in signal transduction may be targets for naturally occurring mutations resulting in human diseases. The best-studied molecules in platelets in relation to disease are the G-protein coupled receptors being the most platelet-specific. Many of the other signal transduction genes are often not only present in platelets but also in other tissues. Therefore, the clinical phenotype of signaling defects in platelets, apart from the membrane receptor defects, is seldom isolated to a hemostatic phenotype. Moreover, as platelets are easily accessible cells, and one of the best-studied models regarding signaling, platelets are easily applicable to investigate defects in ubiquitously expressed genes. Apart from a discussion on classical thrombopathies, this review will also deal with the less commonly known relation between platelet signaling defects and disorders with a broader clinical phenotype.

Mechanism of activation and functional role of protein kinase Ceta in human platelets

The novel class of protein kinase C (nPKC) isoform eta is expressed in platelets, but not much is known about its activation and function. In this study, we investigated the mechanism of activation and functional implications of nPKCeta using pharmacological and gene knock-out approaches. nPKCeta was phosphorylated (at Thr-512) in a time- and concentration-dependent manner by 2MeSADP. Pretreatment of platelets with MRS-2179, a P2Y1 receptor antagonist, or YM-254890, a G(q) blocker, abolished 2MeSADP-induced phosphorylation of nPKCeta. Similarly, ADP failed to activate nPKCeta in platelets isolated from P2Y1 and G(q) knock-out mice. However, pretreatment of platelets with P2Y12 receptor antagonist, AR-C69331MX did not interfere with ADP-induced nPKCeta phosphorylation. In addition, when platelets were activated with 2MeSADP under stirring conditions, although nPKCeta was phosphorylated within 30 s by ADP receptors, it was also dephosphorylated by activated integrin alpha(IIb)beta3 mediated outside-in signaling. Moreover, in the presence of SC-57101, a alpha(IIb)beta3 receptor antagonist, nPKCeta dephosphorylation was inhibited. Furthermore, in murine platelets lacking PP1cgamma, a catalytic subunit of serine/threonine phosphatase, alpha(IIb)beta3 failed to dephosphorylate nPKCeta. Thus, we conclude that ADP activates nPKCeta via P2Y1 receptor and is subsequently dephosphorylated by PP1gamma phosphatase activated by alpha(IIb)beta3 integrin. In addition, pretreatment of platelets with eta-RACK antagonistic peptides, a specific inhibitor of nPKCeta, inhibited ADP-induced thromboxane generation. However, these peptides had no affect on ADP-induced aggregation when thromboxane generation was blocked. In summary, nPKCeta positively regulates agonist-induced thromboxane generation with no effects on platelet aggregation.

Unique ligand selectivity of the GPR92/LPA5 lysophosphatidate receptor indicates role in human platelet activation

Lysophosphatidic acid (LPA) is a ligand for LPA(1-3) of the endothelial differentiation gene family G-protein-coupled receptors, and LPA(4-8) is related to the purinergic family G-protein-coupled receptor. Because the structure-activity relationship (SAR) of GPR92/LPA(5) is limited and whether LPA is its preferred endogenous ligand has been questioned in the literature, in this study we applied a combination of computational and experimental site-directed mutagenesis of LPA(5) residues predicted to interact with the headgroup of LPA. Four residues involved in ligand recognition in LPA(5) were identified as follows: R2.60N mutant abolished receptor activation, whereas H4.64E, R6.62A, and R7.32A greatly reduced receptor activation. We also investigated the SAR of LPA(5) using LPA analogs and other non-lysophospholipid ligands. SAR revealed that the rank order of agonists is alkyl glycerol phosphate > LPA > farnesyl phosphates >> N-arachidonoylglycine. These results confirm LPA(5) to be a bona fide lysophospholipid receptor. We also evaluated several compounds with previously established selectivity for the endothelial differentiation gene receptors and found several that are LPA(5) agonists. A pharmacophore model of LPA(5) binding requirements was developed for in silico screening, which identified two non-lipid LPA(5) antagonists. Because LPA(5) transcripts are abundant in human platelets, we tested its antagonists on platelet activation and found that these non-lipid LPA(5) antagonists inhibit platelet activation. The present results suggest that selective inhibition of LPA(5) may provide a basis for future anti-thrombotic therapies.

Identification and characterization of a novel P2Y 12 variant in a patient diagnosed with type 1 von Willebrand disease in the European MCMDM-1VWD study

We investigated whether defects in the P2Y(12) ADP receptor gene (P2RY12) contribute to the bleeding tendency in 92 index cases enrolled in the European MCMDM-1VWD study. A heterozygous mutation, predicting a lysine to glutamate (K174E) substitution in P2Y(12), was identified in one case with mild type 1 von Willebrand disease (VWD) and a VWF defect. Platelets from the index case and relatives carrying the K174E defect changed shape in response to ADP, but showed reduced and reversible aggregation in response to 10 muM ADP, unlike the maximal, sustained aggregation observed in controls. The reduced response was associated with an approximate 50% reduction in binding of [(3)H]2MeS-ADP to P2Y(12), whereas binding to the P2Y(1) receptor was normal. A hemagglutinin-tagged K174E P2Y(12) variant showed surface expression in CHO cells, markedly reduced binding to [(3)H]2MeS-ADP, and minimal ADP-mediated inhibition of forskolin-induced adenylyl cyclase activity. Our results provide further evidence for locus heterogeneity in type 1 VWD.

Role of platelets in atherothrombosis

Platelets play a pivotal role in atherothrombosis and therefore are primary targets of antithrombotic therapy. They release an array of agonists, such as adenosine diphosphate (ADP); adhesive molecules, such as P-selectin, thrombospondin, fibrinogen, and von Willebrand factor; coagulation factors; and growth factors. In turn, they present transmembrane receptors for a plethora of agonists and ligands. Heterodimeric glycoproteins of the integrin family bind extracellular matrix and plasma proteins; mediate adhesion, activation, spreading, and aggregation; and facilitate intercellular bidirectional signal transduction. Glycoprotein IIb/IIIa is the most abundant platelet integrin and membrane surface glycoprotein. Glycolipids, heparins, proteoglycans, tetraspanins, and a multitude of other molecules, such as tumor necrosis factor-alpha, CD40L, growth arrest-specific 6, Eph receptor tyrosine kinases, and signaling lymphocytic activation molecule receptors, have been implicated in atherothrombosis. ADP promotes platelet aggregation by binding to platelet surface receptors P2Y(1) and P2Y(12); the thienopyridines inhibit aggregation by binding covalently to P2Y(12). Thrombin, a potent initiator of platelet aggregation, activates platelets by cleaving protease-activated receptors (PARs) PAR-1 and PAR-4 and further propagates its effect by activating nearby platelets. A number of pharmacologic agents with antiplatelet actions have been developed, but the search continues for agents that strike an optimal balance between control of thrombosis and serious bleeding.

Platelet P2Y(12) receptor influences the vessel wall response to arterial injury and thrombosis

BACKGROUND

Platelets are believed to play an important role in atherogenesis and the vessel response to vascular injury. The P2Y(12) receptor (P2Y(12)) plays a central role in amplifying platelet aggregation, dense granule and alpha-granule secretion, P-selectin expression, microparticle formation, and procoagulant membrane changes, regardless of the activating stimulus. We hypothesized that P2Y(12) deficiency might reduce the vessel wall response to vascular injury as well as thrombosis in murine vascular injury models.

METHODS AND RESULTS

P2Y(12)-deficient (-/-) mice and littermate controls (+/+) were bred on a C57 BL/6 background. In vivo murine models of arterial injury were employed alone and in combination with bone marrow transplantation to investigate the role of P2Y(12) in the vessel wall response to arterial injury and thrombosis. At 21 days after ferric chloride injury, neointima formation in P2Y(12)(-/-) arteries was significantly less than that observed in control strain arteries (P<0.025). In agreement with this, the intima-media ratio was significantly greater in femoral wire-injured arteries from P2Y(12)(+/+) compared with P2Y(12)(-/-) animals (P<0.05). Bone marrow transplantation was used to examine the importance of vessel wall P2Y(12) versus platelet P2Y(12). Analysis of arterial sections from chimeric animals at 21 days after injury revealed a smaller intima-media ratio in -/- to +/+ animals than in the positive (+/+ to +/+) control group (P<0.01).

CONCLUSIONS

These data demonstrate a role for platelet P2Y(12) in the vessel wall response to arterial injury and thrombosis. This illustrates the manner in which platelets may contribute to atherogenesis and restenosis.

RhoA downstream of G(q) and G(12/13) pathways regulates protease-activated receptor-mediated dense granule release in platelets

Platelet secretion is an important physiological event in hemostasis. The protease-activated receptors, PAR 1 and PAR 4, and the thromboxane receptor activate the G(12/13) pathways, in addition to the G(q) pathways. Here, we investigated the contribution of G(12/13) pathways to platelet dense granule release. 2MeSADP, which does not activate G(12/13) pathways, does not cause dense granule release in aspirin-treated platelets. However, supplementing 2MeSADP with YFLLRNP (60muM), as selective activator of G(12/13) pathways, resulted in dense granule release. Similarly, supplementing PLC activation with G(12/13) stimulation also leads to dense granule release. These results demonstrate that supplemental signaling from G(12/13) is required for G(q)-mediated dense granule release and that ADP fails to cause dense granule release because the platelet P2Y receptors, although activate PLC, do not activate G(12/13) pathways. When RhoA, downstream signaling molecule in G(12/13) pathways, is blocked, PAR-mediated dense granule release is inhibited. Furthermore, ADP activated RhoA downstream of G(q) and upstream of PLC. Finally, RhoA regulated PKCdelta T505 phosphorylation, suggesting that RhoA pathways contribute to platelet secretion through PKCdelta activation. We conclude that G(12/13) pathways, through RhoA, regulate dense granule release and fibrinogen receptor activation in platelets.

Platelet ADP-receptor antagonists for cardiovascular disease: past, present and future

Aspirin is the foundation antiplatelet therapy for patients at risk of cardiovascular events. The thienopyridine, clopidogrel, is modestly more effective than aspirin and in patients with stroke seems to be as effective as the combination of aspirin and dipyridamole. The addition of clopidogrel to aspirin further reduces the risk of cardiovascular events in patients with acute coronary syndromes and those who undergo percutaneous coronary intervention, but uncertainty remains about whether this combination has incremental efficacy over clopidogrel monotherapy in patients with stroke or peripheral arterial disease. Clopidogrel has pharmacological limitations that have prompted the search for more effective ADP-receptor antagonists. Promising results have been achieved with the thienopyridine, prasugrel, which has been compared with clopidogrel in patients treated with aspirin. The nonthienopyridine P2Y(12) inhibitors AZD6140 and cangrelor are presently being evaluated in phase III, randomized, controlled trials.

Inhibitory effects of the antiepileptic drug ethosuximide on G protein-activated inwardly rectifying K+ channels

Antiepileptic drugs protect against seizures by modulating neuronal excitability. Ethosuximide is selectively used for the treatment of absence epilepsy, and has also been shown to have the potential for treating several other neuropsychiatric disorders in addition to several antiepileptic drugs. Although ethosuximide inhibits T-type Ca(2+), noninactivating Na(+), and Ca(2+)-activated K(+) channels, the molecular mechanisms underlying the effects of ethosuximide have not yet been sufficiently clarified. G protein-activated inwardly rectifying K(+) channels (GIRK, or Kir3) play an important role in regulating neuronal excitability, heart rate and platelet aggregation. In the present study, the effects of various antiepileptic drugs on GIRK channels were examined first by using the Xenopus oocyte expression assay. Ethosuximide at clinically relevant concentrations inhibited GIRK channels expressed in Xenopus oocytes. The inhibition was concentration-dependent, but voltage-independent, and time-independent during each voltage pulse. However, the other antiepileptic drugs tested: phenytoin, valproic acid, carbamazepine, phenobarbital, gabapentin, topiramate and zonisamide, had no significant effects on GIRK channels even at toxic concentrations. In contrast, Kir1.1 and Kir2.1 channels were insensitive to all of the drugs tested. Ethosuximide also attenuated ethanol-induced GIRK currents. These inhibitory effects of ethosuximide were not observed when ethosuximide was applied intracellularly. In granule cells of cerebellar slices, ethosuximide inhibited GTPgammaS-activated GIRK currents. Moreover, ADP- and epinephrine-induced platelet aggregation was inhibited by ethosuximide, but not by charybdotoxin, a platelet Ca(2+)-activated K(+) channel blocker. These results suggest that the inhibitory effects of ethosuximide on GIRK channels may affect some of brain, heart and platelet functions.

The Gi-coupled P2Y12 receptor regulates diacylglycerol-mediated signaling in human platelets

Stimulation of G(q)-coupled receptors activates phospholipase C and is supposed to promote both intracellular Ca(2+) mobilization and protein kinase C (PKC) activation. We found that ADP-induced phosphorylation of pleckstrin, the main platelet substrate for PKC, was completely inhibited not only by an antagonist of the G(q)-coupled P2Y1 receptor but also upon blockade of the G(i)-coupled P2Y12 receptor. The role of G(i) on PKC regulation required stimulation of phosphatidylinositol 3-kinase rather than inhibition of adenylyl cyclase. P2Y12 antagonists also inhibited pleckstrin phosphorylation, Rap1b activation, and platelet aggregation induced upon G(q) stimulation by the thromboxane A(2) analogue U46619. Importantly, activation of phospholipase C and intracellular Ca(2+) mobilization occurred normally. Phorbol 12-myristate 13-acetate overcame the inhibitory effect of P2Y12 receptor blockade on PKC activation but not on Rap1b activation and platelet aggregation. By contrast, inhibition of diacylglycerol kinase restored both PKC and Rap1b activity and caused platelet aggregation. Stimulation of P2Y12 receptor or direct inhibition of diacylglycerol kinase potentiated the effect of membrane-permeable sn-1,2-dioctanoylglycerol on platelet aggregation and pleckstrin phosphorylation, in association with inhibition of its phosphorylation to phosphatidic acid. These results reveal a novel and unexpected role of the G(i)-coupled P2Y12 receptor in the regulation of diacylglycerol-mediated events in activated platelets.

Effects of depressive and anxious symptoms on norepinephrine and platelet P-selectin responses to acute psychological stress among elderly caregivers

BACKGROUND

Caring for a spouse with Alzheimer's disease is associated with increased psychological distress, impaired immunity, and heightened cardiovascular risk. Hyperreactivity of sympathetic and platelet activation responses to acute psychological stress, or the failure to recover quickly from stressful events, may constitute an important pathway linking stress and negative affect with cardiovascular disease (CVD).

OBJECTIVES

(1) To evaluate associations between negative affect (i.e., depressive and anxious symptoms) with increased norepinephrine and P-selectin responses to an acute psychological stress task. (2) To establish whether these associations are augmented among elderly spousal caregivers (CG) compared to non-caregivers (NC).

METHODS

Depressive (DEP) and anxious (ANX) symptoms from the Brief Symptom Inventory were assessed among 39 CG and 31 NC. Plasma norepinephrine levels (NE) and percent platelet P-selectin (PSEL) expression were assayed at three time-points: rest, immediately following a laboratory speech test (reactivity), and after 14 min of recovery.

RESULTS

Among CG, but not NC, increased symptoms of depression and anxiety were associated with delayed NE recovery (DEP: beta=.460, p=.008; ANX: beta=.361, p=.034), increased PSEL reactivity (DEP: beta=.703, p<.001; ANX: beta=.526, p=.002), and delayed PSEL recovery (DEP: beta=.372, p=.039; ANX: beta=.295, p=.092), while controlling for age, gender, aspirin use, antidepressant use, and preexisting CVD. Bivariate correlations showed delayed NE recovery was also associated with increased PSEL reactivity (r=.416) and delayed PSEL recovery (r=.372; all ps<.05) among CG but not NC.

DISCUSSION

Among chronically stressed caregivers, increased levels of depressive and anxious symptoms are associated with prolonged sympathetic activation and pronounced platelet activation. These changes may represent one pathway linking caregiving stress to cardiovascular risk.

Angiogenic and cell proliferating action of the natural diarylnonanoids, malabaricone B and malabaricone C during healing of indomethacin-induced gastric ulceration

PURPOSE

To evaluate the plant phenolics, malabaricone B (mal B) and malabaricone C (mal C) in healing stomach ulcer by modulating angiogenesis.

MATERIALS AND METHODS

Male Swiss albino mice, ulcerated with indomethacin (18 mg/kg, p. o., single dose) were treated up to 7 days with different doses of mal B or mal C. The healing capacities of the drugs and their effects on the angiogenic parameters were assessed.

RESULTS

Maximum ulceration, observed on the 3rd day after indomethacin administration was effectively healed by mal B and mal C (each 10 mg/kg, p. o. x 3 days), the latter showing equivalent potency (~78% p < 0.001) as that of Omez (3 mg/kg, p. o. x 3 days) and misoprostol (10 mug/kg, p. o. x 3 days). Compared to the untreated mice, those treated with mal B or mal C respectively for 3 days increased the mucosal EGF level (139 and 178%, p < 0.001), the serum VEGF level (56%, p < 0.01 and 95%, p < 0.001) and microvessels formation (37%, p < 0.05 and 62%, p < 0.01), while reducing the serum endostatin level (37%, p < 0.05 and 61%, p < 0.01). The relative healing capacities of mal B and mal C correlated well with their respective abilities to modulate the angiogenic factors. The healing by Omez and misoprostol was not due to improved angiogenesis.

CONCLUSIONS

The drugs, mal B and mal C could effectively heal indomethacin-induced stomach ulceration in mice by promoting angiogenesis.

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

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

Combination of caregiving stress and hormone replacement therapy is associated with prolonged platelet activation to acute stress among postmenopausal women

OBJECTIVE

To investigate the combined effects of caregiving and hormone replacement therapy (HRT) on platelet hyperactivity to acute psychological stress. Both HRT and the chronic stress of caregiving have been associated with increased cardiovascular risk, potentially through a mechanism of platelet hyperactivity.

METHODS

A total of 78 elderly postmenopausal women (51 caregivers (CG) and 27 noncaregivers (NC)) were assessed for platelet activation in response to a laboratory speech test. Half the sample was taking HRT. Blood was sampled at baseline, post speech, and after 14 minutes of recovery. Platelet activation was assessed through whole blood flow cytometry assays of % aggregates (Agg), and expression of % fibrinogen receptors (FbR) and % P-selectin (P-sel) on platelet surface.

RESULTS

Multivariate repeated-measures analysis of variance revealed that CG taking HRT exhibited significantly prolonged platelet activation in response to acute stress. There was an interaction between HRT and CG on recovery from stress for Agg (F (1,71) = 5.260, p = .025), P-Sel (F(1,71 = 6.426, p = .013), and FbR (F(1,71 = 6.653, p = .012), controlling for age, cardiovascular disease, and aspirin. Among HRT users, regression analysis revealed that CG had delayed recovery of Agg (beta = 0.354, t(34) = 2.154, p = .038) and P-sel (beta = 0.498, t(34)=3.126, p = .004) from stress relative to NC. No caregiving effects on recovery were present among non-HRT users. In addition, these effects were maintained after controlling for health behaviors, medications, and medical conditions.

CONCLUSION

Chronic dementia caregiving stress in combination with HRT may impair recovery of platelet activation after acute mental stress (i.e., activation levels do not quickly return to resting levels), thereby potentially increasing cardiovascular risk among CG who take HRT.