Platelet activation

Pyridazines part 41: Synthesis, antiplatelet activity and SAR of 2,4,6-substituted 5-(3-oxo-3-phenylprop-1-en-1-yl)- or 5-(3-phenylprop-2-enoyl)pyridazin-3(2H)-ones

As part of the optimization process of the lead compound I a focussed library of diversely substituted pyridazin-3(2H)-ones containing a 3-oxo-3-phenylprop-1-en-1-yl or 3-phenylprop-2-enoyl fragment at position 5 has been obtained and evaluated as antiplatelet agents. The structural modification at positions 2, 6 and 4 of the heterocyclic moiety allowed us to obtain preliminary information on the structure-activity relationship in this family.

Red blood cells, platelets and polymorphonuclear neutrophils of patients with sickle cell disease exhibit oxidative stress that can be ameliorated by antioxidants

Sickle cell disease (SCD) is basically a red blood cell (RBC) disorder characterised by sickling and haemolysis, but platelets and polymorphonuclear neutrophils (PMN) are also involved. Oxidative damage may play a role in the pathogenesis of SCD. Using flow cytometry, we measured oxidative-state markers simultaneously in RBC, platelets and PMN obtained from 25 normal donors, nine homozygous (SS) patients and six SS/beta-thalassaemia patients. Reactive oxygen species (ROS) and reduced glutathione (GSH) were measured following staining of blood samples with fluorescence probes and gating on specific subpopulations based on size and granularity. Ten- to 30-fold higher ROS production and 20-50% lower GSH content were found in RBC, platelets and PMN from SCD patients versus those of their normal counterparts. This could in part account for the clinical manifestations, such as haemolysis, a hypercoagulable state, recurrent bacterial infections and vaso-occlusive incidences, in SCD. We further showed that exposure of SCD samples to antioxidants, such as N-acetyl-cysteine, vitamin C and vitamin E, decreased their oxidative stress. These results suggest that antioxidant treatment of patients with SCD could reduce oxidative damage to RBC, PMN and platelets, thereby alleviating symptoms associated with their pathology. The flow cytometry techniques presented herein could assist in monitoring the efficacy of such treatment.

Platelet release of β-thromboglobulin and platelet factor 4 and serotonin in plasma samples

OBJECTIVES

Platelet release of alpha granule-derived CXC chemokines and dense granule-derived serotonin in plasma samples was evaluated.

METHODS

Concentrations of the CXC chemokines beta-TG and PF4 were assayed by an enzyme immunoassay; serotonin was measured by an HPLC method.

RESULTS

Beta-TG and PF4 were more easily released than serotonin by in vitro procedures. Use of the anti-platelet cocktail CTAD and preservation of the samples at 4 degrees C were necessary to accurately measure beta-TG and PF4, but not serotonin.

CONCLUSIONS

Assaying serotonin may be useful for assessing platelet activation in vivo as a laboratory test because of facile preparation of plasma samples.

Cardamom extract as inhibitor of human platelet aggregation

The inhibitory activity of cardamom extract was studied on human platelets. Platelet aggregation and lipid peroxidation were evaluated with platelet rich plasma (PRP) and platelet membranes, respectively, obtained from blood of healthy volunteers. Human platelets were subjected to stimulation with a variety of agonists including ADP (2.5 mM), epinephrine (2.5 mM), collagen (10 mM), calcium ionophore A 23187 (6 microM) and ristocetin (1.25 microg/mL). The IC50 were 0.49, 0.21, 0.55 and 0.59 mg with ADP, epinephrine, collagen and calcium ionophore A 23187, respectively, and no inhibition with ristocetin. The inhibitory effect was dose dependent with concentrations varying between 0.14 and 0.70 mg and time dependent at IC50. Lipid peroxidation induced by iron--ascorbic acid system in platelet membranes was analysed with malondialdehyde (MDA) as an index. An increase in concentration of cardamom has decreased the MDA formation significantly. Hence, it may be said that aqueous extract of cardamom may have component(s), which protect platelets from aggregation and lipid peroxidation.

Inhibition of human platelet aggregation and membrane lipid peroxidation by food spice, saffron

The inhibitory activity of saffron extract was studied on human platelets. Platelet aggregation and lipid peroxidation were evaluated with platelet rich plasma (PRP) and platelet membranes respectively obtained from blood of healthy human volunteers. Human platelets were subjected to stimulation with a variety of agonists like ADP (61 microM), epinephrine (76 microM), collagen (11 microg/ml), calcium ionophore A 23187 (6 microM) and ristocetin (1.25 microg/ml) in the presence and absence of saffron extract with IC50 being 0.66, 0.35, 0.86 and 0.59 mg respectively and no inhibition with ristocetin. The inhibitory effect was dose dependent with concentrations varying between 0.16 to 0.80 mg and time dependent at IC50. A significant decrease was observed in malondialdehyde (MDA) formed, one of the end products of arachidonic acid metabolism and of serotonin released from dense granules of platelets at respective IC50. Lipid peroxidation in platelet membranes induced by iron-ascorbic acid system was inhibited by saffron extract significantly with IC50 of 0.33 mg. Hence, it may be said that aqueous extract of saffron may have component(s), which protect platelets from aggregation and lipid peroxidation.

A potential role for extracellular nitric oxide generation in cGMP-independent inhibition of human platelet aggregation: biochemical and pharmacological considerations

1. Nitric oxide (NO) is a potent inhibitor of platelet activation, that inhibits the agonist-induced increase in cytosolic Ca2+ concentration through both cGMP-dependent and independent pathways. However, the NO-related (NOx) species responsible for cGMP-independent signalling in platelets is unclear. We tested the hypothesis that extracellular NO, but not NO+ or peroxynitrite, generated in the extracellular compartment is responsible for cGMP-independent inhibition of platelet activation via inhibition of Ca2+ signalling. 2. Concentration-response curves for diethylamine diazeniumdiolate (DEA/NO; a spontaneous NO generator), S-nitroso-N-valerylpenicillamine (SNVP; an S-nitrosothiol) and 3-morpholinosydnonomine (SIN-1; a peroxynitrite generator) were generated in platelet-rich plasma (PRP) and washed platelets (WP) in the presence and absence of a supramaximal concentration of the soluble guanylate cyclase inhibitor, ODQ (20 microM). All three NOx donors displayed cGMP-independent inhibition of platelet aggregation in PRP, but only DEA/NO exhibited cGMP-independent inhibition of aggregation in WP. 3. Analysis of NO generation using an isolated NO-electrode revealed that cGMP-independent effects coincided with the generation of substantial levels of extracellular NO (>40 nM) from the NOx donors. 4. Reconstitution of WP with plasma factors indicated that the copper-containing plasma protein, caeruloplasmin (CP), catalysed the release of NO from SNVP, while Cu/Zn superoxide dismutase (SOD) unmasked NO generated from SIN-1. The increased generation of extracellular NO correlated with a switch to cGMP-independent effects with both NOx donors. 5. Analysis of Fura-2 loaded WP revealed that only DEA/NO inhibited Ca2+ signalling in platelets via a cGMP-independent mechanism. However, preincubation of SNVP and SIN-1 with CP and SOD, respectively, induced cGMP-independent inhibition of intraplatelet Ca2+ trafficking by the NOx donors. 6. Taken together, our data suggest that extracellular NO (>40 nM) is required for cGMP-independent inhibition of platelet activation. Plasma constituents may play an important pharmacological role in activating cGMP-independent signalling by S-nitrosothiols or peroxynitrite generators.

Inhibition of blood platelet adhesion and secretion by different phenolics from Yucca schidigera Roezl. bark

OBJECTIVE

Yucca schidigera is a plant that grows in Mexico, and it has a very high level of saponins and phenolic compounds with antioxidant action. The products of Y. schidigera are used as food additives and have a generally recognized as safe label. This study investigated the antiplatelet mechanisms of four phenolic compounds.

METHODS

We investigated antiplatelet mechanisms of the phenolic compounds trans-3,4',5-trihydroxystilbene (trans-resveratrol), trans-3,3',5,5'-tetrahydroxy-4'-methoxystilbene, and yuccaols A and C that had been isolated from the bark of Y. schidigera by studying their effects on the first step of platelet activation, i.e., platelet adhesion to collagen and fibrinogen. The effects of these compounds on the release of adenine nucleotides, proteins, and beta-N-acetyl-glycosaminidase (a marker of lysosomal secretion) from blood platelets activated by thrombin were also studied.

RESULTS

These different phenolic compounds (1 to 25 microg/mL) and their extracts decreased platelet adhesion and secretion.

CONCLUSIONS

Resveratrol and yucca extract were more reactive in decreasing these processes than were other tested phenolic compounds.

Biomaterial-associated thrombosis: roles of coagulation factors, complement, platelets and leukocytes

Our failure to produce truly non-thrombogenic materials may reflect a failure to fully understand the mechanisms of biomaterial-associated thrombosis. The community has focused on minimizing coagulation or minimizing platelet adhesion and activation. We have infrequently considered the interactions between the two although we are generally familiar with these interactions. However, we have rarely considered in the context of biomaterial-associated thrombosis the other major players in blood: complement and leukocytes. Biomaterials are known agonists of complement and leukocyte activation, but this is frequently studied only in the context of inflammation. For us, thrombosis is a special case of inflammation. Here we summarize current perspectives on all four of these components in thrombosis and with biomaterials and cardiovascular devices. We also briefly highlight a few features of biomaterial-associated thrombosis that are not often considered in the biomaterials literature: The importance of tissue factor and the extrinsic coagulation system. Complement activation as a prelude to platelet activation and its role in thrombosis. The role of leukocytes in thrombin formation. The differing time scales of these contributions.

Rod mutations associated with MYH9-related disorders disrupt nonmuscle myosin-IIA assembly

MYH9-related disorders are autosomal dominant syndromes, variably affecting platelet formation, hearing, and kidney function, and result from mutations in the human nonmuscle myosin-IIA heavy chain gene. To understand the mechanisms by which mutations in the rod region disrupt nonmuscle myosin-IIA function, we examined the in vitro behavior of 4 common mutant forms of the rod (R1165C, D1424N, E1841K, and R1933Stop) compared with wild type. We used negative-stain electron microscopy to analyze paracrystal morphology, a model system for the assembly of individual myosin-II molecules into bipolar filaments. Wild-type tail fragments formed ordered paracrystal arrays, whereas mutants formed aberrant aggregates. In mixing experiments, the mutants act dominantly to interfere with the proper assembly of wild type. Using circular dichroism, we find that 2 mutants affect the alpha-helical coiled-coil structure of individual molecules, and 2 mutants disrupt the lateral associations among individual molecules necessary to form higher-order assemblies, helping explain the dominant effects of these mutants. These results demonstrate that the most common mutations in MYH9, lesions in the rod, cause defects in nonmuscle myosin-IIA assembly. Further, the application of these methods to biochemically characterize rod mutations could be extended to other myosins responsible for disease.

Protein kinase A mediates inhibition of the thrombin-induced platelet shape change by nitric oxide

The thrombin-induced platelet shape change was blocked by nitric oxide (NO), as revealed by scanning electron microscopy, light transmission, and resistive-particle volume determination. The inhibitory effect of NO was accompanied by an increase in levels of both cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP) and phosphorylation of the vasodilator-stimulated phosphoprotein (VASP). However, the inhibition of the shape change was only mimicked by cAMP analogs (Sp-5,6-DClcBIMPS, 8-AHA-cAMP, and 8-CPT-cAMP) and not by cGMP analogs (8-Br-PET-cGMP, 8-Br-cGMP, and 8-pCPT-cGMP). The effect of NO on the thrombin-induced shape change was prevented by the protein kinase A (PKA) antagonists Rp-8-Br-cAMPS and Rp-cAMPS. The protein kinase G (PKG) antagonist Rp-8-CPT-cGMPS strongly inhibited PKG-mediated 46-kDa VASP Ser239 phosphorylation, but did not inhibit the thrombin-induced shape change or the PKA-mediated VASP Ser157 phosphorylation. Whereas an inhibitor of cyclic nucleotide phosphodiesterase (PDE) 3A (milrinone) mimicked the effect of NO, inhibitors of PDE2 (erythro-9-(2-hydroxy-3-nonyl)adenine) and PDE5 (dipyridamole) were poorly effective. We concluded that (1) NO was a potent and reversible inhibitor of the platelet shape change, (2) the shape change was reversible, (3) the inhibitory effect of NO was mediated through activation of PKA, (4) the onset of the NO effect coincided with VASP Ser157 phosphorylation, and (5) removal of NO and platelet shape change coincided with VASP Ser157 dephosphorylation. These findings are compatible with elevation of cGMP by NO in a compartment close to PDE3A, PKA, and VASP, leading to a local increase of cAMP able to block thrombin-induced shape change.

Peroxynitrite-induced changes of thiol groups in human blood platelets

In this study we investigated sulfhydryl and disulfide metabolism in blood platelets treated with peroxynitrite (ONOO-) at concentrations of 10, 100, and 1000 microM. In platelets incubated with peroxynitrite, a decrease of both protein and low molecular weight thiols (glutathione, cysteine and cysteinylglycine) was observed. The reduction of platelet free thiols caused by ONOO- was dose dependent. We found that peroxynitrite induces the oxidation of platelet free SH-groups. Platelets contain a large fraction of glutathione in reduced form (GSH). We showed that after ONOO- treatment, GSH/GSSG ratio in platelets was distinctly decreased. The presented results suggest that oxidation of thiols and depletion of GSH in platelets caused by peroxynitrite may be responsible for cytotoxic effects of this compound.

Effect of hydrogen peroxide on Ca2+ mobilisation in human platelets through sulphydryl oxidation dependent and independent mechanisms

Using Fura-2-loaded human platelets we studied the nature of the mechanisms involved in Ca2+ signalling mediated by H2O2. In a Ca2+-free medium, H2O2 (10 microM-100 mM) induced a concentration-dependent increase in [Ca2+]i. Depletion of either agonist-sensitive or mitochondrial Ca2+ pools reduced this effect while depletion of both stores abolished it. Xestospongin C, an inositol 1,3,5-trisphosphate (IP3) receptor inhibitor, reduced Ca2+ release evoked by 1 mM H2O2 by 45%, indicating that H2O2-induced Ca2+ release involves interaction with IP3 receptors. Blockade of the IP3 turnover by lithium or treatment with U-73122 did not modify H2O2-induced Ca2+ release from the agonist-sensitive pool, suggesting the involvement of a mechanism independent of IP3 generation. H2O2 inhibited Ca2+ reuptake into the agonist-sensitive stores mediated by the sarcoendoplasmic reticulum Ca2+ ATPase (SERCA). Thimerosal (5 microM), a sulphydryl reagent, induced Ca2+ release from the agonist-sensitive stores. This event was impaired by treatment with 2 mM DTT, which also inhibited H2O2-induced Ca2+ release from the agonist-sensitive pool but not from mitochondria. H2O2 reduced the ability of the plasma membrane Ca2+ ATPase (PMCA) to extrude Ca2+ by 75%, an effect that was unaffected by DTT. Consistent with this, thimerosal did not modify the PMCA activity. Finally, exposure to H2O2 triggered platelet aggregation, which was slower than that observed after agonist stimulation. We conclude that H2O2 induced Ca2+ release from agonist-sensitive stores by oxidation of sulphydryl groups in SERCA and the IP3 receptors independently of IP3 generation. In addition, H2O2 induced Ca2+ release from mitochondria and inhibited the PMCA activity by different mechanisms in human platelets.

Cadaveric versus living-donor livers: differences in inflammatory markers after transplantation

BACKGROUND

Prolonged cold storage of organs for transplantation may lead to inflammatory damage upon reperfusion. The aim of this study was to investigate whether organs from living donors experience less damage upon reperfusion than those retrieved from cadaver donors, where cold ischemia times are significantly longer.

METHODS

Biopsies were obtained from cadaveric (n=23) and living-related donor (LRD) (n=10) liver transplants before and 2 hours after reperfusion. Cryosections were stained with antibodies against neutrophils, platelets, activated platelets, and endothelium.

RESULTS

LRD liver allografts showed minimal changes postreperfusion. In contrast, after reperfusion of cadaver allografts, neutrophil infiltration was detected in 22% and increased expression of von Willebrand factor (vWF), CD41, and P-selectin in 48%, 30%, and 13% of allografts, respectively. In cadaver allografts with deposition of activated platelets expressing either P-selectin or vWF, the cold ischemia time was significantly longer (885 +/- 123 min vs. 608 +/- 214 min, P=0.04; 776.8 +/- 171 min vs. 559.3 +/- 216 min, P=0.01, respectively). Increases in neutrophils and platelets after reperfusion were not significantly associated with clinical events posttransplant. However, in cadaver transplants that experienced early acute rejection, the mean cold ischemia time was significantly longer than in allografts with no rejection (732 +/- 174 min vs. 480 +/- 221 min, P=0.006).

CONCLUSIONS

This study demonstrates that in the clinical situation, cold ischemia causes platelet deposition and neutrophil infiltration after reperfusion of cadaveric liver allografts. These early inflammatory events may contribute to make the graft more susceptible to acute rejection.

Function and regulation of Ena/VASP proteins

Regulation of cytoskeletal dynamics is required to coordinate cell movement, adhesion and shape change. The Ena/VASP protein family is thought to play an important role in linking signaling pathways to remodeling of the actin cytoskeleton. This review will examine the mechanisms by which Ena/VASP function might control actin dynamics and how these proteins are linked to various signaling pathways.

Biology and pathogenesis of thrombosis and procoagulant activity in invasive infections caused by group A streptococci and Clostridium perfringens

Group A streptococcal necrotizing fasciitis/myonecrosis and Clostridium perfringens gas gangrene are two of the most fulminant gram-positive infections in humans. Tissue destruction associated with these infections progresses rapidly to involve an entire extremity. Multiple-organ failure is common, and morbidity and mortality remain high. Systemic activation of coagulation and dysregulation of the anticoagulation pathways contribute to the pathogenesis of many diverse disease entities of infectious etiology, and it has been our hypothesis that microvascular thrombosis contributes to reduced tissue perfusion, hypoxia, and subsequent regional tissue necrosis and organ failure in these invasive gram-positive infections. This article reviews the coagulation, anticoagulation, and fibrinolytic systems from cellular players to cytokines to novel antithrombotic therapies and discusses the mechanisms contributing to occlusive microvascular thrombosis and tissue destruction in invasive group A streptococcal and C. perfringens infections. A thorough understanding of these mechanisms may suggest novel therapeutic targets for patients with these devastating infections.

Inhibition of oxidative stress in blood platelets by different phenolics from Yucca schidigera Roezl. bark

OBJECTIVE

We investigated the comparative effects of resveratrol (trans-3,4',5-trihydroxystilbene), trans-3,3',5,5'tetrahydroxy-4'-methoxystilbene, and yuccaols A and C isolated from the bark of Yucca schidigera on oxidative stress in resting blood platelets and blood platelets activated by different agonists (thrombin or thrombin receptor activating peptide).

METHODS AND RESULTS

Tested phenolics (1-25 microgram/mL) reduced, to different degrees, the level of reactive oxygen species measured by the luminol-dependent chemiluminescence and changed the production of O(2)(-) measured by the reduction of cytochrome c in resting blood platelets. They also inhibited the generation of free radicals in blood platelets activated by thrombin (P < 0.05) or thrombin receptor activating peptide (P < 0.05). Treatment of platelets with resveratrol or yuccaols A and C at the concentration of 25 microgram/mL increased (statistically non-significant) the level of thiobarbituric acid reactive substances in these cells (P > 0.05), whereas trans-3,3',5,5'tetrahydroxy-4'-methoxystilbene and the alcohol yucca extract reduced lipid peroxidation in blood platelets (P < 0.05).

CONCLUSIONS

Resveratrol and other phenolic compounds from the bark of Yucca schidigera inhibiting free radical generation in blood platelets may be beneficial in protecting against cardiovascular diseases when hyperactivity of platelets is observed.

Dietary flavanols and procyanidin oligomers from cocoa (Theobroma cacao) inhibit platelet function

BACKGROUND

Flavonoids may be partly responsible for some health benefits, including antiinflammatory action and a decreased tendency for the blood to clot. An acute dose of flavanols and oligomeric procyanidins from cocoa powder inhibits platelet activation and function over 6 h in humans.

OBJECTIVE

This study sought to evaluate whether 28 d of supplementation with cocoa flavanols and related procyanidin oligomers would modulate human platelet reactivity and primary hemostasis and reduce oxidative markers in vivo.

DESIGN

Thirty-two healthy subjects were assigned to consume active (234 mg cocoa flavanols and procyanidins/d) or placebo (< or = 6 mg cocoa flavanols and procyanidins/d) tablets in a blinded parallel-designed study. Platelet function was determined by measuring platelet aggregation, ATP release, and expression of activation-dependent platelet antigens by using flow cytometry. Plasma was analyzed for oxidation markers and antioxidant status.

RESULTS

Plasma concentrations of epicatechin and catechin in the active group increased by 81% and 28%, respectively, during the intervention period. The active group had significantly lower P selectin expression and significantly lower ADP-induced aggregation and collagen-induced aggregation than did the placebo group. Plasma ascorbic acid concentrations were significantly higher in the active than in the placebo group (P < 0.05), whereas plasma oxidation markers and antioxidant status did not change in either group.

CONCLUSIONS

Cocoa flavanol and procyanidin supplementation for 28 d significantly increased plasma epicatechin and catechin concentrations and significantly decreased platelet function. These data support the results of acute studies that used higher doses of cocoa flavanols and procyanidins.

Molecular cloning and characterization of a platelet glycoprotein Ib-binding protein from the venom of Trimeresurus stejnegeri

A platelet glycoprotein Ib-binding protein, termed TSV-GPIb-BP, was isolated from the venom of Trimeresurus stejnegeri. On SDS-polyacrylamide gel electrophoresis, TSV-GPIb-BP showed a single band with an apparent molecular weight of 28,000 and two distinct bands with apparent molecular weights of 16,000 and 15,000 under non-reducing and reducing conditions, respectively. cDNA clones containing the coding sequences for both TSV-GPIb-BP subunits were isolated and sequenced. The deduced amino acid sequences of TSV-GPIb-BP subunits were confirmed by N-terminal protein sequencing and trypsin-digested peptide mass fingerprinting. Interestingly, the alpha subunit of TSV-GPIb-BP is identical to that of alboaggregin-B, and the sequence identity of their beta subunits is 94.3%. TSV-GPIb-BP inhibited ristocetin-induced human platelet agglutination in platelet-rich plasma under lower dosages (<5 microg/ml). On the other hand, it directly aggregated washed human platelets in the absence of additional Ca2+ or any other cofactors under higher dosages (>5 microg/ml). This platelet aggregation activity was dose-dependently inhibited by specific GPIbalpha antibodies, but not by those antibodies against platelet GPIa, GPIIa, GPIIb and GPIIIa.

Acute antithrombotic effects of ximelagatran, an oral direct thrombin inhibitor, and r-hirudin in a human ex vivo model of arterial thrombosis

BACKGROUND

Thrombin plays a major role in thrombus formation through activation of platelets and conversion of fibrinogen to fibrin.

OBJECTIVES

To investigate the antithrombotic effects of the oral direct thrombin inhibitor (DTI) ximelagatran and the parenteral DTI r-hirudin in humans.

SUBJECTS AND METHODS

Healthy male volunteers randomized into four parallel groups each with 15 subjects received either ximelagatran (20, 40 or 80 mg orally) or r-hirudin (0.4 mg kg-1 intravenous bolus + infusion of 0.15 mg kg-1 h-1 for 2 h and 0.075 mg kg-1 h-1 for 3 h). Antithrombotic effects were assessed as changes in total thrombus area (TTA) and total fibrin area (TFA) from baseline, using the Badimon perfusion chamber model at baseline and 2 h and 5 h after drug administration.

RESULTS

Two hours postdosing, ximelagatran showed antithrombotic effects at both high and low shear rates (TTA% of mean baseline value +/- SEM was 76 +/- 13% and 71 +/- 17% [both P < 0.05] for the 20-mg dose, 85 +/- 11% [P > 0.05] and 62 +/- 15% [P < 0.05] for the 40-mg dose and 60 +/- 11% and 26 +/- 7% [both P < 0.05] for the 80-mg dose, respectively). r-Hirudin also showed a significant antithrombotic effect at high and low shear rates (76 +/- 11% [P = 0.05] and 57 +/- 17% [P < 0.05] of baseline values, 2 h postdosing, respectively). The inhibitory effects on TFA were similar to those on TTA.

CONCLUSIONS

The oral DTI ximelagatran shows antithrombotic effects under both high and low shear conditions. The antithrombotic effect of 40-80 mg ximelagatran appeared comparable to that of parenterally administered r-hirudin, which has been previously demonstrated to be clinically effective in acute coronary syndromes.

An intronic polymorphism in the PAR-1 gene is associated with platelet receptor density and the response to SFLLRN

Protease-activated receptor 1 (PAR-1), the main thrombin receptor on vascular cells, plays a key role in platelet activation. We examined the range of PAR-1 expression on platelets, obtained twice, 1 week apart, from 100 healthy subjects and found a 2-fold interindividual variation in receptor numbers (95% CI = 858-1700). Because PAR-1 density was stable with time (r(2) = 76%, P <.001), we sought a genetic explanation for the observed variability. To validate this approach, we also analyzed the alpha(2)beta(1) genotype according to receptor density and platelet mRNA expression data. We found that the number of PAR-1 receptors on the platelet surface is associated with the intervening sequence IVSn-14 A/T intronic variation. The number of receptors was also found to govern the platelet response to the SFLLRN agonist, in terms of aggregation and P-selectin expression. The T allele (allelic frequency, 0.14) can be considered as an allele with decreased expression, because it was associated with lower PAR-1 expression on the platelet surface and with a lower response to SFLLRN. The IVSn-14 A/T intronic variation may therefore be clinically relevant.

Equine platelet CD62P (P-selectin) expression: a phenotypic and morphologic study

Acute inflammatory diseases, such as colic, septicemia and endotoxemia are common in equines and have been shown to be correlated to vascular injury and thrombosis. In humans with similar thrombotic conditions, P-selectin and P-selectin glycoprotein ligand-1 (PSGL-1)-mediated platelet-leukocyte adhesion contributes to the pathogenesis of these disorders through the generation of inflammatory mediators and tissue factor. As such, we hypothesized that a P-selectin-PSGL-1 (platelet-leukocyte) interaction, similar to that in humans, may also exist in the horse. The objective of this study was to investigate phenotypic and morphological properties of equine platelet activation with a focus on CD62P (P-selectin) expression and CD62P mediated platelet-leukocyte interactions. To study high levels of platelet activation, we used 1 U/ml thrombin to induce secondary, irreversible aggregation in both human and equine platelets. Addition of glycyl-L-prolyl-L-arginyl-L-proline amide (GPRP) prior to thrombin activation blocked fibrin polymerization, allowing the use of flow cytometry to study alpha-granule expression as a measure of platelet activation. Thrombin activation resulted in high levels of activation, measured as P-selectin expression, in both humans and equines. Interestingly, our research illustrates that in healthy horses, P-selectin is also constitutively expressed on 20-25% of resting platelets. This finding is in direct contrast to humans, in which P-selectin expression is negligible (<5%) in the absence of agonist activation. The high baseline level of P-selectin expression among equine platelets may suggest that they are primed for leukocyte adhesion, possibly resulting in prothrombotic conditions. This phenomenon could be of significant clinical relevance, as it may be related to the rapid clinical decline often seen in equine patients with colic and endotoxemia, where vascular injury and thrombotic complications compromise patient survival. Based on these findings, further investigation into the mechanisms of platelet P-selectin-mediated inflammation and platelet-leukocyte mediated vascular injury in the horse appears warranted.

Engineering a bioactive matrix by modifications of calcium sulfate

The goal of this study was to define the conditions for the fabrication of a bioactive matrix that induces and supports cell proliferation and tissue regeneration. The proposed hypothesis was that a composite graft could be engineered by the absorption of platelet-rich plasma (PRP) onto calcium sulfate (CS). Evaluation of the biological activity of the engineered grafts was based on osteoblast proliferation studies and scanning electron microscopy (SEM) analyses. Graft samples were created in a standard size and shape so that the surface available for attachment and cell proliferation was always identical. Proliferation data were expressed as counts per minute per group and differences among groups were statistically analyzed by analysis of variance followed by the Scheffé test (alpha = 0.1). SEM analysis showed that the combination of CS and PRP presents a preserved crystalline structure well integrated by organic matrix. This combination showed the highest cell proliferation levels (p < 0.001). Further evaluations demonstrated that PRP is activated when combined with CS. When tested as a possible carrier for biologically active molecules such as platelet-derived growth factor (PDGF), CS showed increased cell proliferation (p < 0.001). SEM revealed adherent osteoblasts with broad flattened edges on CS-PRP. This study proposes CS as an efficient carrier for PRP or PDGF and supports the use of these combinations as bioactive matrices in clinical or laboratory applications.

SNAP-23 is a target for calpain cleavage in activated platelets

The role of calpain in platelet function is generally associated with aggregation and clot retraction. In this report, data are presented to show that one component of the platelet secretory machinery, SNAP-23, is specifically cleaved by calpain in activated cells. Other proteins of the membrane fusion machinery, e.g. syntaxins 2 and 4 and alpha-SNAP, are not affected. In vitro studies, using permeabilized platelets, demonstrate that cleavage is time- and calcium-dependent. Analysis of SNAP-23 cleavage products suggests that the calpain cleavage site(s) is in the C-terminal third of the molecule potentially between the cysteine-rich acyl attachment sites and the C-terminal coiled-coil domain. The time course of cleavage is most consistent with late calpain-mediated events such as pp60(c-src) cleavage, but not early events such as protein-tyrosine phosphatase-1B activation. SNAP-23 cleavage is inhibited by calpeptin, calpastatin, calpain inhibitor IV, and E-64d, but not by caspase 3 inhibitor III or cathepsin inhibitor I. When tested for their effect on secretion, none of the calpain-specific inhibitors significantly affected release of soluble components from any of the three platelet granule storage pools. These results indicate that SNAP-23 cleavage occurs after granule release and therefore may play a role in affecting granule membrane exteriorization. This is consistent with the ultrastructural morphology of calpeptin-treated platelets after activation.

Relationship between platelet phospholipid FA and mean platelet volume in healthy men

Increased mean platelet volume (MPV) has been suggested as an independent risk factor for acute myocardial infarction and the increased reactivity of large platelets. The aim of this study was to investigate the correlation between platelet phospholipid (PL) PUFA composition and MPV in 139 free-living healthy men ages 20-55 yr (vegans, n = 18; ovolacto vegetarians, n = 43; moderate meat-eaters, n = 60; and high meateaters, n = 18). Each subject completed a semiquantitative Food Frequency Questionnaire and gave a blood sample. Platelet PL FA composition and MPV were determined by standard methods. MPV was significantly greater in the vegans than in the ovolacto vegetarian, moderate, or high meat-eater groups (P < 0.01). Both vegan and ovolacto vegetarian groups had significantly higher platelet PL 18:2n-6 and 22:4n-6, and lower 20:5n-3 and 22:6n-3 compared with the moderate and high meat-eater groups. The vegans demonstrated a significant reduction in 20:4n-6 and 22:5n-3 compared with the ovolacto vegetarian, high meat-eater, and moderate meat-eater groups. Bivariate analysis results showed that MPV was significantly positively correlated with platelet PL 18:2n-6 (P = 0.048) and negatively correlated with 20:3n-6 (P = 0.02), 20:5n-3 (P = 0.005), and 22:5n-3 (P< 0.0001), respectively. In a multiple linear regression analysis, after controlling for potential confounding factors such as dietary group, age, exercise, body mass index, and dietary polyunsaturated and saturated fat, cholesterol, carbohydrate, and fiber intake, the MPV was still strongly negatively correlated with platelet PL 20:3n-6 (P = 0.003) and 22:5n-3 (P = 0.001). The present data suggest that 22:5n-3 and 20:3n-6 may play a role in the structural function of the platelet membrane.

Integrin-linked kinase phosphorylates the myosin phosphatase target subunit at the inhibitory site in platelet cytoskeleton

The myosin phosphatase (MP) composed of the catalytic subunit of type 1 protein phosphatase and myosin phosphatase target subunit isoform 1 (MYPT1) was identified as the major serine/threonine phosphatase component in the platelet-cytoskeleton fraction. MYPT1 was phosphorylated by cytoskeletal kinase(s), but the identity of the kinase(s) and the effect of phosphorylation were not established. Incubation of platelet-cytoskeletal fraction with MgATP or MgATP[S] (magnesium adenosine 5'-[gamma-thio]triphosphate) caused a decrease in the 20 kDa light-chain of smooth-muscle myosin (MLC20) phosphatase and phosphorylase phosphatase activities. MYPT1 contains a phosphorylation site, Thr-695, involved in the inhibition of MP in a RhoA/Rho kinase-dependent manner. The cytoskeletal kinase(s) phosphorylated Thr-695 of glutathione S-transferase (GST)-MYPT1, as determined with an antibody specific for phosphorylated Thr-695. The level of Rho kinase was low in the cytoskeletal fraction and was detected primarily in the membrane and cytosolic fractions. The phosphorylation of Thr-695 by the cytoskeletal kinase(s) was not affected by Rho kinase inhibitor, Y-27632, suggesting that kinase(s) other than Rho kinase were involved. In-gel kinase assay identified a kinase at 54-59 kDa that phosphorylated the C-terminal fragment of MYPT1 (GST-MYPT1(667-1004)). Western blots detected both zipper-interacting protein kinase (ZIPK) and integrin-linked kinase (ILK) at 54-59 kDa in the cytoskeleton and membrane fractions. Cytoskeletal ZIPK and ILK were separated and partially purified by chromatography on SP-Sepharose and on MonoQ. ZIPK preferentially phosphorylated MLC20 and had low activity on MYPT1. ILK phosphorylated both MLC20 and MYPT1 and phosphorylation of MYPT1 occured on Thr-695. The above results raise the potential for regulation of MP activity in platelet cytoskeleton by ILK and suggest an alternative to the Rho-linked pathway.

Inhibition of Ca2+/calmodulin-dependent protein kinase blocks morphological differentiation of plasmodium gallinaceum zygotes to ookinetes

Once ingested by mosquitoes, malaria parasites undergo complex cellular changes. These include zygote formation, transformation of zygote to ookinete, and differentiation from ookinete to oocyst. Within the oocyst, the parasite multiplies into numerous sporozoites. Modulators of intracellular calcium homeostasis, MAPTAM, and TMB-8 blocked ookinete development as did the calmodulin (CaM) antagonists W-7 and calmidazolium. Ca(2+)/CaM-dependent protein kinase inhibitor KN-93 also blocked zygote elongation, while its ineffective analog KN-92 did not have such effect. In vitro both zygote and ookinete extracts efficiently phosphorylated autocamtide-2, a classic CaM kinase substrate, which could be blocked by calmodulin antagonists W-7 and calmidazolium and CaM kinase inhibitor KN-93. These results demonstrated the presence of calmodulin-dependent CaM kinase activity in the parasite. KN-93-treated parasites, however, expressed the ookinete-specific enzyme chitinase and the ookinete surface antigen Pgs28 normally, suggesting that the morphologically untransformed parasites are biochemically mature ookinetes. In mosquitoes, KN-93-treated parasites did not develop as oocysts, while KN-92-treated parasites produced similar numbers of oocysts as controls. These data suggested that in Plasmodium gallinaceum morphological development of zygote to ookinete, but not its biochemical maturation, relies on Ca(2+)/CaM-dependent protein kinase activity and demonstrated that the morphological differentiation is essential for the further development of the parasite in infected blood-fed mosquitoes.

Resveratrol and vitamin C as antioxidants in blood platelets

We investigated the effects of two different compounds with potential antioxidative action (found in many fruits, which can be used in therapy of cardiovascular disorders): trans-resveratrol (3,4',5-trihydroxystilbene) and vitamin C on oxidative stress in blood platelets. Oxidative stress in blood platelets was estimated by the measurement of: (1) the generation of superoxide radicals (O(2)(-*)) (reduction of cytochrome c) and other reactive oxygen species--ROS: H(2)O(2), singlet oxygen and organic radicals (chemiluminescence), (2) the production of thiobarbituric acid reactive substances (TBARS) and the level of conjugate dienes as markers of lipid peroxidation. We have shown that vitamin C at the concentrations of 750-3000 microM, but not at the concentration of 100 microM (prooxidative action) significantly inhibited peroxidation of lipids (measured by TBARS and conjugate dienes) and the production of ROS in blood platelets. After the incubation of blood platelets for 30 min at 37 degrees C with vitamin C at the concentration of 3000 microM, inhibition of ROS generation (above 90%) was achieved. The inhibition of ROS production caused by resveratrol at physiological plasma concentrations 0.05-2 microM was lower than by vitamin C. At the highest concentration of vitamin C used (3000 microM), the inhibition of O(2)(-*) generation was about 40%. Moreover, we did not observe any synergistic action of resveratrol and vitamin C at antioxidative dose (3000 microM) on the inhibition of lipid peroxidation and the production of O(2)(-*) or ROS in blood platelets. Resveratrol as an antioxidant reduced oxidative stress in blood platelets caused by vitamin C at prooxidative dose (100 microM).

Cationic amphiphilic drugs and platelet phospholipase A(2) (cPLA(2))

The aim of the study was to verify and compare the effect of cationic amphiphilic drugs (CAD) from different pharmacological groups on activation of platelet phospholipase A2 (PLA2)--the essential enzyme of arachidonic pathway in blood platelets. Beta-adrenoceptor-blocking (BAB) drugs inhibited platelet aggregation in the rank order of potency: propranolol>alprenolol>metipranolol>atenolol. The higher the inhibition of arachidonic acid (AA) liberation by BAB drugs, the higher the inhibition of aggregation. Similarly did the H1-histamine antagonists bromadryl (BRO) and dithiaden (DIT) as well as the antimalarial chloroquine (CQ) show antiplatelet effect in vitro in the rank order of potency: DIT>BRO>CQ. Dose-dependent inhibition of aggregation was followed by the inhibition of AA liberation from membrane phospholipids of platelets stimulated either at the receptor site (thrombin) or by a stimulus bypassing membrane receptors (Ca2+ ionophore A23187). The rank order potency for inhibition of stimulated 3H-AA liberation from membrane phospholipids was: (a) for BAB drugs: propranolol>alprenolol>metipranolol, (b) for other drugs: DIT>BRO>CQ. The investigated drugs' interference with stimulated liberation of AA showed nonspecific inhibition of platelet cytosolic PLA2 (cPLA2) by these drugs at intracellular level. The results revealed that besides the inhibition of cyclooxygenase pathway and receptors for adenosine diphosphate (ADP) and glycoproteins Gp IIbIIIa, the interaction of drugs with cPLA2 may represent a further site for antiplatelet action.

The role of platelets in blood coagulation--effects of platelet agonists and GPIIb/IIIa inhibitors studied by free oscillation rheometry

We have studied the contribution of platelets to the coagulation of plasma and the effects of activation or inhibition of platelets on the coagulation process in unanticoagulated fresh whole blood (subsequently termed native blood). For this purpose, we have used a free oscillation rheometer (FOR), the ReoRox4, a new instrument that enables noninvasive viscoelastic measurements of clot formation in plasma and whole blood. Platelets appear essential for the initiation of coagulation if no activating surface is present. We prepared platelet-free plasma by quick centrifugation and filtration of native blood, which did not coagulate if stored in plastic containers at 37 degrees C but clotted if transferred to glass containers. Addition of platelet agonists, such as collagen or the thrombin receptor agonist peptide, SFLLRN, significantly accelerated the clotting of native blood and also changed the rheometer curve appearance, accelerating both onset and completion of clot formation (i.e. fibrin gel formation). Inhibition of platelet glycoprotein (GP) IIb/IIIa with the peptide-derived compound MK-852 or the antibody-derived abciximab (Reopro) prevented clot retraction and prolonged the clotting time with SFLLRN. In collagen-stimulated samples, MK-852 accelerated clotting but delayed completion of clotting while abciximab prolonged both clotting time and completion of clotting. To our knowledge, this is the first report showing that activation of platelets in native whole blood shortens the coagulation time ex vivo. It also describes a new instrument that enables studies of the viscoelastic properties of a forming whole blood clot.

The roles of ATP and calcium in morphological changes and cytotoxicity induced by 1,4-benzoquinone in platelets

To understand the mechanism of 1,4-benzoquinone-induced cytotoxicity in platelets, the roles of ATP and calcium in platelet toxicity and morphological changes were investigated. Using scanning electron microscopy, morphological changes including membrane blebbing were observed in rat platelets 5 min after exposure to 1,4-benzoquinone, which were significantly different from shape changes (pseudopod formation) observed in response to physiological agonists. Benzoquinone-induced membrane blebbing of platelets was associated with rapid depletion of intracellular ATP and was independent of the presence of extracellular calcium. Benzoquinone-induced platelet lysis observed between 20 and 30 min was dependent on extracellular calcium and associated with increased cytosolic calcium. Cytotoxicity induced by 1,4-benzoquinone was inhibited by antagonists of calmodulin, suggesting that calmodulin could play an important role in platelet toxicity. These results suggested that the progression of events for benzoquinone-induced cytotoxicity in platelets was as follows: 1,4-benzoquinone depletes intracellular ATP; membrane blebbing occurs; calcium homeostasis is disrupted, activation of calmodulin-dependent processes results; finally cytotoxicity occurs.

Synergistic action of beta-glucan and platelets on interleukin-8 production by human peripheral blood leukocytes

The effects of human platelets on interleukin (IL)-8 production from human peripheral blood mononuclear cells (PBMCs) and polymorphonuclear leukocytes (PMNs) stimulated with the fungal (1-->3)-beta-D-glucan schizophyllan (SPG) were examined using ELISA. PBMCs/PMNs in the presence of platelets and SPG enhanced IL-8 production in comparison with those in the presence of either platelets or SPG. IL-8 production was dependent on the concentration of platelets and incubation time, and the activity reached the maximal level at 18 h of incubation. These activities were also observed with the addition of platelets prestimulated with SPG to PBMCs. Addition of SPG directly enhanced expression of P-selectin on platelet membrane surfaces. These results suggest that platelets play a key role in the cytokine production of leukocytes induced by fungal (1-->3)-beta-D-glucans and might be mediated, at least in part, by P-selectin.

Phosphatidylserine expression on the platelet membrane of patients with myeloproliferative disorders and its effect on platelet-dependent thrombin formation

Recently, the asymmetric distribution of phospholipids in eukaryotic cell membranes has been appreciated and been found to be dependent on the activity of a number of enzymes. The expression of phosphatidylserine (PS), a negatively charged phospholipid, on the platelets of patients with polycythemia vera (P vera) and essential thrombocythemia (ET) was compared to that in normal individuals. The effect of platelet aggregation on PS expression was determined. Exposure of PS on platelets obtained from patients with P vera and ET and from age- and sex-matched healthy volunteers was measured by fluorescein-labeled Annexin V binding to platelets and by the platelets' thrombin-generating capacity determined by the prothrombinase assay. PLatelet prothrombinase activity (mean +/- standard deviation [SD]), as measured by thrombin generation, was 2.32+/-2.2 micro/mL in the P vera group and 1.55+/-1.0 micro/mL in the control group (p=0.3). PS expression as measured by Annexin V binding (mean +/- SD) was 2.6+/-2.4 % in the P vera group versus 1.55+/-1.2% among controls (p=0.03). In the ET group, prothrombinase activity (mean +/- SD) was 1.0+/-0.6 micro/mL and 2.1+/-0.9 micro/mL in the control group (p=0.006). Annexin V binding (mean +/- SD) was 4.8+/-4.2% in the ET group and 2.77+/-2.1% among control subjects (p=0.09). When the prothrombinase assay was performed after addition of adenosine diphosphate (ADP) to the platelets, there was a significant increase in thrombin generation in the myeloproliferative disorder (MPD) group (3.1+/-2.0 micro/mL) compared to the thrombin generated by unstimulated myeloproliferative disorder platelets (2.07+/-1.69 micro/mL) (p=0.0006). An increase in thrombin generation was seen in the ADP-stimulated platelet samples in all ten paired samples studied. Likewise, the addition of ADP to control platelets increased thrombin generation from 2.0+/-1.0 micro/mL in unstimulated platelets to 4.3+/-1.6 micro/mL in ADP-treated platelets (p=0.0006). Thrombin generation increased in all of the ADP-stimulated platelet samples compared to the untreated platelets. There was however, no difference in the increased thrombin generation when ADP-stimulated platelets from MPD patient and control subjects were compared (p=0.3). Results indicate that some patients with MPDs may show increased PS expression on platelet surface. When analyzed overall, there was a tendency toward greater PS expression in the P vera and ET patient groups; however, the increase did not reach statistical significance. This increase was noted in both the prothrombinase assay the Annexin V binding assay. We have also shown that stimulation of platelets by addition of the agonist ADP results in enhanced PS expression, which appears increase the thrombogenic potential of the platelets as demonstrated by the enhanced thrombin generation demonstrated by these platelets in the prothrombinase assay. There was no difference in the degree of PS expression in response to ADP stimulation between MPD and control platelets. Results show that PS expression and platelet-dependent thrombin generation is variable in patients with MPDs. This expression is increased after platelet aggregation occurs. The role of PS expression in the thromboembolic complications of MPD patients should be studied further.

Therapeutic potential of GP IIb/IIIa receptor antagonists in acute myocardial infarction

The management of acute myocardial infarction (AMI) has changed dramatically over the last two decades, with the addition of fibrinolytic agents and primary coronary intervention (PCI). The more recent development of the glycoprotein (GP) IIb/IIIa antagonists, a new class of potent antiplatelet drugs, has the potential to considerably enhance the treatment of AMI patients. A number of recent studies have highlighted the potential incremental benefits with adjunctive IIb/IIIa-targeted therapy. In this review, we will discuss the pathophysiology of myocardial infarction (MI), the physiology and role of platelets in thrombosis and describe the currently available drugs. We will briefly summarise the results of recent clinical trials, discuss some key forthcoming trials and attempt to describe how GP IIb/IIIa antagonists may directly impact the immediate and near future day-to-day care of patients with AMI.

Radiolabeled peptides in diagnosis and therapy

During the past few years, there has been exponential growth in the development of radiolabeled peptides for diagnosis and therapy. This is because the peptides can be synthesized easily and inexpensively, they have fast clearance and rapid tissue penetration, and they are less likely to be immunogenic. More importantly, most peptides have a high affinity for characteristic receptor molecules that are overexpressed on malignant mammalian cells. Peptides can be labeled with a variety of radionuclides intended for specific applications, diagnostic or therapeutic, by using both conventional and novel chelating moieties, many of which can be incorporated during the solid state synthesis of a chosen peptide. High specific-activity peptides can be prepared and used to minimize unwanted physiologic effects, and known sequences of amino acids can be modified to slow their in vivo catabolic rate. These characteristics have paved the way for the preparation of a large number of radiolabeled peptides for a variety of clinical and experimental applications. This article briefly discusses the peptide chemistry; it also summarizes the preparation of radiolabeled peptides and outlines their applications in imaging vascular thrombosis, detecting infection and inflammation, and localizing tumors. Their therapeutic applications in oncology are also presented and the future directions outlined. Peptides that have been approved for human use, such as AcuTect (Diatide, Londonderry, NH) or OctreoScan (Mallinckrodt, St. Louis, MO), or those that have made it to clinical trials, are emphasized. Also discussed are selected promising agents that are still in preclinical investigation.

Cancer procoagulant and blood platelet activation

The effects of cancer procoagulant (CP), cysteine protease (EC 3.4.22.26), on the pig blood platelet secretory process and platelet aggregation have been studied. The response of platelets to CP was compared with the response of these cells to thrombin. The obtained results show that blood platelets treated with CP (0.5, 1, 2.5, and 5 microg/ml, 2-30 min, 37 degrees C) released adenine nucleotides (P < 0.05) and proteins (P < 0.05). The secretion of compounds from blood platelets after incubation with CP does not correlate with the release of platelet lactic dehydrogenase activity (marker of cell lysis) into the extracellular medium. In comparison with thrombin action, CP stimulates secretory process to a smaller extent than thrombin alone. In the presence of CP, the thrombin action is suppressed (P < 0.05). We noticed that CP does not induce platelet aggregation.

Antiplatelet effect of 2-chloro-3-(4-acetophenyl)-amino-1,4-naphthoquinone (NQ301): a possible mechanism through inhibition of intracellular Ca2+ mobilization

The effects of 2-chloro-3-(4-acetophenyl)-amino-1,4-naphthoquinone (NQ301), an antithrombotic agent, on aggregation, binding of fibrinogen to glycoprotein (GP)IIb/IIIa complex and intracellular signals were investigated using human platelets. NQ301 significantly inhibited the collagen-, thrombin-, arachidonic acid-, thapsigargin- and calcium ionophore A23187-induced aggregation of washed human platelets with IC50 values of 13.0+/-0.1, 11.2+/-0.5, 21.0+/-0.9, 3.8+/-0.1 and 46.2+/-0.8 microM, respectively. NQ301 also significantly inhibited FITC-conjugated fibrinogen binding to human platelet surface GPIIb/IIIa complex, but failed to inhibit the fibrinogen binding to purified GPIIb/IIIa complex. These data demonstrate that NQ301 inhibits platelet aggregation by suppression of the intracellular pathway, rather than by direct inhibition of fibrinogen-GPIIb/IIIa complex binding. NQ301 significantly inhibited the increase of cytosolic Ca2+ concentration and ATP secretion, and also significantly increased platelet cAMP levels in the activated platelets. These results suggest that the antiplatelet activity of NQ301 may be mediated by inhibition of cytosolic Ca2+ mobilization, enhancement of cAMP production and inhibition of ATP secretion in activated platelets.

NQ-Y15 inhibits the calcium mobilization by elevation of cyclic AMP in rat platelets

2-1(4-Cyanophenyl)aminol-3-chloro-1,4-naphthalenedione (NQ-Y15) is a dual action drug which acts as a thromboxane A2 (TXA2) synthase inhibitor and TXA2/PGH2 receptor antagonist. In the present study, we examined the effects of NQ-Y15 on Ca2+ mobilization, which is the common event in various types of platelet activation, in arachidonic acid (AA)-stimulated rat platelets. The elevation of cytosolic Ca2+ concentration ([Ca2+]i) induced by AA was inhibited by NQ-Y15 in a concentration-dependent manner. This inhibition-effect of NQ-Y15 was found to be based on the suppression of the rise in [Ca2+]i by the inhibition of both Ca2+ release from internal stores and Ca2+ influx from the extracellular space. Our successive trial was focused on the role of cyclic AMP (cAMP) in the action of NQ-Y15, because cAMP was reported to be increased by dual action drugs such as picotamide and to inhibit the increase in [Ca2+]i. NQ-Y15 was confirmed to increase cAMP in AA-stimulated rat platelets. These results suggested that NQ-Y15 might inhibit the rise in [Ca2+]i in AA-treated rat platelets by increasing cAMP, which is involved in the inhibition of platelet activation.

Inhibitors of platelet signal transduction as anti-aggregatory drugs

In the treatment and prevention of cardiovascular diseases, inhibition of platelet aggregation is of fundamental importance. Inhibition of platelet aggregation can be achieved by either inhibition of membrane receptors or by interception of signalling pathways. While receptor antagonism provides high specificity, the inhibition of platelet signal transduction is more effective. The effectiveness results from the inhibition of platelets, regardless of the cause of activation. These common pathway inhibitors are either intercepting platelet activating mechanisms or amplifying the action of endogenous platelet inhibitors. The physiological anti-aggregants are the endothelial factors NO and prostacyclin, which elevate intracellular cGMP or cAMP content, respectively. By administration of NO-releasing agents, prostacyclin analogues or other cyclic nucleotide elevating drugs the platelet anti-aggregatory action of endothelial factors can be effectively mimicked. Besides antiplatelet activity these drugs also act on vascular smooth muscle causing relaxation and therefore vasodilation, an additional beneficial effect. Inhibition of phosphodiesterases causes elevation of platelet cyclic nucleotide content and thus inhibits platelet aggregation and causes vasodilation. Another relevant target for anti-aggregatory treatment is the arachidonic acid metabolic pathway. This pathway can be intercepted by blockade of either cyclooxygenase-1 (COX-1) or thromboxane synthase. Inhibition of these enzymes may be further amplified by additional antagonism of the thromboxane receptor thus not only preventing formation of thromboxane but also activation of thromboxane receptor by thromboxane precursors, which were particularly effective in clinical trials. In vivo these precursors may be metabolised to prostacyclin in the endothelium and consequently provide additional platelet anti-aggregatory activity. A rather new target for platelet anti-aggregatory treatment is the ecto-nucleotidase CD-39 which limits the plasma level of nucleotides. While several of the novel anti-aggregatory drugs were disappointing in clinical studies combinations of drugs with different effector enzymes showed potent antithrombotic efficacy.

Characterization of the effects of isoprostanes on platelet aggregation in human whole blood

We tested the effects of 11 commercially-available isoprostanes on platelet aggregation directly or when triggered by the thromboxane receptor agonist U46619 or collagen in healthy human citrated blood using a whole blood aggregometer. None of the isoprostanes tested triggered aggregation alone, nor facilitated aggregation by a sub-threshold dose of U46619 or collagen. Five isoprostanes inhibited aggregation (rank order of potency 8-iso PGE(1)>8-iso PGE(2)>8-iso PGF(2alpha)>8-iso PGF(3alpha)>8-iso-13,14-dihydro-15-keto PGF(2alpha)). Blood incubated with LPS to induce a gross inflammatory response exhibited a time dependent (2 - 12 h) reduction in aggregation to U46619 but maintained a consistent response to collagen. Under these conditions, as in control blood, none of the isoprostanes tested induced aggregation. In fact, the inhibitory actions of isoprostanes on U46619-induced aggregation were enhanced in blood treated with LPS. L-NAME inhibited aggregation induced by U46619 in fresh blood and in blood treated with LPS. In the presence of L-NAME, (with or without LPS) none of the isoprostanes tested induced aggregation but retained their inhibitory action. Thus, in human whole blood the action of 8-iso PGE(1), 8-iso PGE(2), 8-iso PGF(2alpha), 8-iso PGF(3alpha), and 8-iso-13,14-dihydro-15-keto PGF(2alpha) is antiaggregatory. Moreover, this inhibitory capacity is still apparent and may be enhanced in blood subjected to inflammatory stimulation.

Integrin-independent tyrosine phosphorylation of p125(fak) in human platelets stimulated by collagen

Collagen fibers or a glycoprotein VI-specific collagen-related peptide (CRP-XL) stimulated tyrosine phosphorylation of the focal adhesion kinase, p125(fak) (FAK), in human platelets. An integrin alpha(2)beta(1)-specific triple-helical peptide ligand, containing the sequence GFOGER (single-letter nomenclature, O = Hyp) was without effect. Antibodies to the alpha(2) and beta(1) integrin subunits did not inhibit platelet FAK tyrosine phosphorylation caused by either collagen fibers or CRP-XL. Tyrosine phosphorylation of FAK caused by CRP-XL or thrombin, but not that caused by collagen fibers, was partially inhibited by GR144053F, an antagonist of integrin alpha(IIb)beta(3). The intracellular Ca(2+) chelator, BAPTA, and the protein kinase C inhibitor, Ro31-8220, were each highly effective inhibitors of the FAK tyrosine phosphorylation caused by collagen or CRP-XL. These data suggest that, in human platelets, 1) occupation or clustering of the integrin alpha(2)beta(1) is neither sufficient nor necessary for activation of FAK, 2) the fibrinogen receptor alpha(IIb)beta(3) is not required for activation of FAK by collagen fibers, and 3) both intracellular Ca(2+) and protein kinase C activity are essential intermediaries of FAK activation.

Structure, function, and control of phosphoinositide-specific phospholipase C

Phosphoinositide-specific phospholipase C (PLC) subtypes beta, gamma, and delta comprise a related group of multidomain phosphodiesterases that cleave the polar head groups from inositol lipids. Activated by all classes of cell surface receptor, these enzymes generate the ubiquitous second messengers inositol 1,4, 5-trisphosphate and diacylglycerol. The last 5 years have seen remarkable advances in our understanding of the molecular and biological facets of PLCs. New insights into their multidomain arrangement and catalytic mechanism have been gained from crystallographic studies of PLC-delta(1), while new modes of controlling PLC activity have been uncovered in cellular studies. Most notable is the realization that PLC-beta, -gamma, and -delta isoforms act in concert, each contributing to a specific aspect of the cellular response. Clues to their true biological roles were also obtained. Long assumed to function broadly in calcium-regulated processes, genetic studies in yeast, slime molds, plants, flies, and mammals point to specific and conditional roles for each PLC isoform in cell signaling and development. In this review we consider each subtype of PLC in organisms ranging from yeast to mammals and discuss their molecular regulation and biological function.

The effect of storage on the expression of platelet membrane phosphatidylserine and the subsequent impacton the coagulant function of stored platelets

BACKGROUND

Platelet concentrates (PCs) derived from whole blood and stored under standard blood bank conditions undergo changes that are referred to as the platelet storage lesion. This study assesses the effect of PC preparation and storage on the distribution of phosphatidylserine (PS) in the platelet membrane and the effect that this distribution may have on the thrombogenic potential of stored PCs.

STUDY DESIGN AND METHODS

Fresh platelets and PCs donated by healthy donors were obtained. PCs derived from platelet-rich plasma were studied on Day 1, Day 3, and Day 6 of storage under blood bank conditions.

RESULTS

Platelet aggregation after exposure to the platelet agonists ADP and epinephrine singly declined progressively, but, when ADP and epinephrine in combination and collagen and thrombin in combination were used as agonists, the decline in platelet aggregation was less marked. PS expression as measured by Annexin V binding (mean and SD) was 2.02 +/- 0.93 percent in fresh platelet samples and increased to 5.39 +/- 4.2 percent on Day 1, 22. 1 +/- 7.1 percent on Day 3, and 39.5 +/- 12.1 percent on Day 6. Platelet prothrombinase activity (mean +/- SD) as measured by thrombin generation increased from 1.49 +/- 0.7 micro per mL in fresh platelet samples to 3.68 +/- 1.1 micro per mL in Day 1 platelets (p<0.001), 5.15 +/- 2.5 micro per mL in Day 3 platelets (p<0.001), and 4.65 +/- 2.48 micro per mL in Day 6 platelets (p<0. 001).

CONCLUSION

These results show that PS expression increases after preparation of PCs from platelet-rich plasma and rises progressively during platelet storage under blood bank conditions. Furthermore, the greater PS expression is associated with increased platelet- dependent thrombin-generating capacity.