Inhibitors of platelet signal transduction as anti-aggregatory drugs

Platelets: Physiology and Biochemistry

This article represents a republication of an article originally published in STH in 2005. This republication is to help celebrate 50 years of publishing for STH. The original abstract follows.Platelets are specialized blood cells that play central roles in physiologic and pathologic processes of hemostasis, inflammation, tumor metastasis, wound healing, and host defense. Activation of platelets is crucial for platelet function that includes a complex interplay of adhesion and signaling molecules. This article gives an overview of the activation processes involved in primary and secondary hemostasis, for example, platelet adhesion, platelet secretion, platelet aggregation, microvesicle formation, and clot retraction/stabilization. In addition, activated platelets are predominantly involved in cross-talk to other blood and vascular cells. Stimulated "sticky" platelets enable recruitment of leukocytes at sites of vascular injury under high shear conditions. Platelet-derived microparticles as well as soluble adhesion molecules, sP-selectin and sCD40L, shed from the surface of activated platelets, are capable of activating, in turn, leukocytes and endothelial cells. This article focuses further on the new view of receptor-mediated thrombin generation of human platelets, necessary for the formation of a stable platelet-fibrin clot during secondary hemostasis. Finally, special emphasis is placed on important stimulatory and inhibitory signaling pathways that modulate platelet function.

Multifaceted effects of arachidonic acid and interaction with cyclic nucleotides in human platelets

INTRODUCTION

Arachidonic acid induced aggregation is a generally accepted test for aspirin resistance. However, doubts have been raised that arachidonic acid stimulated aggregation can be regarded as reliable testing for aspirin resistance. Arachidonic acid, in addition to platelet activation, can induce phosphatidylserine translocation on the outer surface of platelet membrane which could be mediated by apoptosis pathways or transformation of platelets to the procoagulant state.

MATERIALS AND METHODS

We explored effects of arachidonic acid over a vast range of concentrations and a wide range of read-outs for human platelet activation, procoagulant activity, and platelet viability. Additionally we tested whether cAMP- or cGMP-dependent protein kinase activation can inhibit procoagulant activity or platelet viability.

RESULTS

Arachidonic acid-induced washed platelet activation was detected at low micromolar concentrations during the first 2 min of stimulation. After longer incubation and/or at higher concentrations arachidonic acid triggered platelet procoagulant activity and reduced platelet viability. At the same time, arachidonic acid stimulated adenylate cyclase mediated protein phosphorylation which correlated with reduced platelet activation. Moreover, additional stimulation of cAMP- or cGMP-dependent protein kinase inhibited only platelet activation, but did not prevent pro-coagulant activity and platelet death.

CONCLUSIONS

While arachidonic acid induces platelet activation at low concentrations and during short incubation time, higher concentrations and lasting incubation evokes adenylate cyclase activation and subsequent protein phosphorylation corresponding to reduced platelet activation, but also enhanced pro-coagulant activity and reduced viability. Our observations provide further proof for the complex fine tuning of platelet responses in a time and agonist concentration dependent manner.

Inhibition of platelet aggregation ex vivo is repressed in apolipoprotein E deficient mice

In the present study, we assessed whether the endogenous platelet inhibitory mechanisms are altered in the early to moderate stages of the atherosclerotic process. Apolipoprotein E deficient mice (ApoE-/-), a mouse model of atherosclerosis, and their wild-type (WT) counterparts were used to assess agonist-stimulated synthesis of prostacyclin (PGI₂), inhibition of platelet aggregation ex vivo, and intra-platelet cAMP levels. Basal U46619 and ADP -induced platelet aggregation in vitro were increased in ApoE-/- mice at 18-20 weeks in comparison with 8-10 weeks of age. Systemically administered endothelin-1 (ET-1) or bradykinin (BK) inhibited platelet aggregation in a similar fashion in 8- to 10-week-old ApoE-/- and WT mice, but not in the ApoE-/- mice at 18-20 weeks of age, although both peptides maintained their capacity to increase plasma levels of the PGI₂. Intravenous infusion of PGI₂ also failed to inhibit platelet aggregation ex vivo in 18- to 20-week-old ApoE-/- mice. Interestingly, both BK and PGI₂ retained their ability to increase intraplatelet cAMP in WT and ApoE-/- mice. Our results suggest that a loss of activity of endogenous inhibitorymechanisms could contribute to the increased platelet reactivity in ApoE-/- mice, and that this phenomenon occurs early in the intermediate stage of the atherosclerotic process.

Pharmacological actions of nobiletin in the modulation of platelet function

Background and Purpose

The discovery that flavonoids are capable of inhibiting platelet function has led to their investigation as potential antithrombotic agents. However, despite the range of studies on the antiplatelet properties of flavonoids, little is known about the mechanisms by which flavonoids inhibit platelet function. In this study, we aimed to explore the pharmacological effects of a polymethoxy flavonoid, nobiletin, in the modulation of platelet function.

Experimental Approach

The ability of nobiletin to modulate platelet function was explored by using a range of in vitro and in vivo experimental approaches. Aggregation, dense granule secretion and spreading assays were performed using washed platelets. Fibrinogen binding, α-granule secretion and calcium mobilization assays were performed using platelet-rich plasma and whole blood was used in impedance aggregometry and thrombus formation experiments. The effect of nobiletin in vivo was assessed by measuring tail bleeding time using C57BL/6 mice.

Key Results

Nobiletin was shown to suppress a range of well-established activatory mechanisms, including platelet aggregation, granule secretion, integrin modulation, calcium mobilization and thrombus formation. Nobiletin extended bleeding time in mice and reduced the phosphorylation of PKB (Akt) and PLCγ2 within the collagen receptor (glycoprotein VI)-stimulated pathway, in addition to increasing the levels of cGMP and phosphorylation of vasodilator-stimulated phosphoprotein, a protein whose activity is associated with inhibitory cyclic nucleotide signalling.

Conclusions and Implications

This study provides insight into the underlying molecular mechanisms through which nobiletin modulates haemostasis and thrombus formation. Therefore, nobiletin may represent a potential antithrombotic agent of dietary origins.

Modulation of adenosine signaling prevents scopolamine-induced cognitive impairment in zebrafish

Adenosine, a purine ribonucleoside, exhibits neuromodulatory and neuroprotective effects in the brain and is involved in memory formation and cognitive function. Adenosine signaling is mediated by adenosine receptors (A1, A2A, A2B, and A3); in turn, nucleotide and nucleoside-metabolizing enzymes and adenosine transporters regulate its levels. Scopolamine, a muscarinic cholinergic receptor antagonist, has profound amnesic effects in a variety of learning paradigms and has been used to induce cognitive deficits in animal models. This study investigated the effects of acute exposure to caffeine (a non-selective antagonist of adenosine receptors A1 and A2A), ZM 241385 (adenosine receptor A2A antagonist), DPCPX (adenosine receptor A1 antagonist), dipyridamole (inhibitor of nucleoside transporters) and EHNA (inhibitor of adenosine deaminase) in a model of pharmacological cognitive impairment induced by scopolamine in adult zebrafish. Caffeine, ZM 241385, DPCPX, dipyridamole, and EHNA were acutely administered independently via i.p. in zebrafish, followed by exposure to scopolamine dissolved in tank water (200μM). These compounds prevented the scopolamine-induced amnesia without impacting locomotor activity or social interaction. Together, these data support the hypothesis that adenosine signaling may modulate memory processing, suggesting that these compounds present a potential preventive strategy against cognitive impairment.

Platelets: still a therapeutical target for haemostatic disorders

Platelets are cytoplasmatic fragments from bone marrow megakaryocytes present in blood. In this work, we review the basis of platelet mechanisms, their participation in syndromes and in arterial thrombosis, and their potential as a target for designing new antithrombotic agents. The option of new biotechnological sources is also explored.

N4-monobutyryl-cCMP activates PKA RIα and PKA RIIα more potently and with higher efficacy than PKG Iα in vitro but not in vivo

There is increasing evidence for a role of cytidine 3',5'-cyclic monophosphate (cCMP) as second messenger. In a recent study, we showed that cCMP activates both purified guanosine 3',5'-cyclic monophosphate (cGMP)-dependent protein kinase Iα (PKG Iα) and adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (PKA) isoenzymes with the regulatory subunits RIα and RIIα. Moreover, the membrane-permeant cCMP analog dibutyryl (DB)-cCMP induces effective vasodilation and inhibition of platelet aggregation via PKG Iα, but not via PKA. These data prompted us to conduct a systematic analysis of the effects of cyclic nucleotide (cNMP) analogs on purified PKG Iα and PKA RIα and RIIα We also studied the effect of DB-cCMP on PKA-dependent phosphorylation of the transcription factor cAMP response-binding protein (CREB) in S49 wild-type lymphoma cells and S49 kin(-) cells, devoid of the catalytic subunit of PKA. The major cellular metabolite of the prodrug DB-cCMP, N(4)-monobutyryl (4-MB)-cCMP, was a partial and low-potency activator of purified PKG Iα and a full and moderate-potency activator of PKA RIα and RIIα. Sp-cCMPS and Sp-cAMPS activated PKA RIα and RIIα with much higher potency and efficacy than PKG Iα. Molecular modeling suggested that the cytidine ring interacts with PKG Iα mainly via hydrophobic interactions, while the butyryl group projects away from the kinase. In contrast to DB-cAMP, DB-cCMP did not induce PKA-dependent phosphorylation in intact cells. Taken together, our data show that N(4)-monobutyryl-cCMP (4-MB-cCMP) activates PKA RIα and PKA RIIα more potently and with higher efficacy than PKG Iα in vitro but not in vivo. cNMP phosphorothioates constitute a starting point for the development of PKA activators with high selectivity relative to PKG.

Aging of the nitric oxide system: are we as old as our NO?

Background

Impaired generation and signaling of nitric oxide (NO) contribute substantially to cardiovascular (CV) risk (CVR) associated with hypertension, hyperlipidemia, and diabetes mellitus. In our rapidly aging society, advanced age is, in itself, a consistent and independent CVR factor. Many processes involved in aging are modulated by NO. We therefore postulated that aging might be independently associated with impaired NO signaling.

Methods and Results

In a prospective cohort study of 204 subjects (mean age 63±6 at study entry), we evaluated the effects of 4 years of aging on parameters of NO generation and effect, including platelet aggregability and responsiveness to NO, and plasma concentrations of the NO synthase inhibitor, asymmetric dimethylarginine (ADMA). Clinical history, lipid profile, high‐sensitivity C‐reactive protein, routine biochemistry, and 25‐hydroxyvitamin D levels were obtained at study entry and after 4 years of follow‐up. Aging was associated with marked deterioration of responsiveness of platelets to NO (P<0.0001) and increases in plasma ADMA concentrations (P<0.0001). There was a significant correlation between changes in these parameters over time (r=0.2; P=0.013). On multivariable analyses, the independent correlates of deterioration of responsiveness of platelets to NO were female gender (β=0.17; P=0.034) and low vitamin D concentrations (β=0.16; P=0.04), whereas increases in ADMA were associated with presence of diabetes (β=0.16; P=0.03) and impaired renal function (β=0.2; P=0.004).

Conclusions

Aging is associated with marked impairment of determinants of NO generation and effect, to an extent which is commensurate with adverse impact on CV outcomes. This deterioration represents a potential target for therapeutic interventions.

The inhibitory in-vitro effect of high-dose levosimendan on platelet function may be mediated through its action as a phosphodiesterase inhibitor

OBJECTIVE

Levosimendan enhances cardiac contractility by increasing myocyte sensitivity to calcium and causing vasodilatation. Although studies have evaluated the efficacy of levosimendan in heart failure, whether levosimendan produces an effect on platelets is a subject of controversy. In the present study, the in-vitro effect of levosimendan on platelet aggregation was investigated. The effect of levosimendan on the cyclic AMP concentration was determined according to its second mode of action as a selective phosphodiesterase III inhibitor.

MATERIALS AND METHODS

Platelet aggregation setting was performed using venous blood from three healthy volunteers. Different concentrations of levosimendan solution were prepared that would result in 0.04-125 µg/ml levosimendan concentrations in whole blood and in platelet-enriched plasma. After incubation for 3 min at 37°C, aggregation responses were evaluated with ADP (10 µmol/l), collagen (5 µg/ml), or NaCl. The cyclic AMP concentration was determined using the enzyme-linked immunosorbent assay technique.

RESULTS

The in-vitro results clearly showed that there was only a relationship between a high levosimendan concentration (12-125 µg/ml) and inhibition of platelet aggregation that was negatively dependent on the cAMP concentration.

CONCLUSION

Levosimendan has no significant effect as a phosphodiesterase III inhibitor on in-vitro platelet aggregation in clinically relevant doses.

Time-resolved in silico modeling of fine-tuned cAMP signaling in platelets: feedback loops, titrated phosphorylations and pharmacological modulation

Background

Hemostasis is a critical and active function of the blood mediated by platelets. Therefore, the prevention of pathological platelet aggregation is of great importance as well as of pharmaceutical and medical interest. Endogenous platelet inhibition is predominantly based on cyclic nucleotides (cAMP, cGMP) elevation and subsequent cyclic nucleotide-dependent protein kinase (PKA, PKG) activation. In turn, platelet phosphodiesterases (PDEs) and protein phosphatases counterbalance their activity. This main inhibitory pathway in human platelets is crucial for countervailing unwanted platelet activation. Consequently, the regulators of cyclic nucleotide signaling are of particular interest to pharmacology and therapeutics of atherothrombosis. Modeling of pharmacodynamics allows understanding this intricate signaling and supports the precise description of these pivotal targets for pharmacological modulation.

Results

We modeled dynamically concentration-dependent responses of pathway effectors (inhibitors, activators, drug combinations) to cyclic nucleotide signaling as well as to downstream signaling events and verified resulting model predictions by experimental data. Experiments with various cAMP affecting compounds including anti-platelet drugs and their combinations revealed a high fidelity, fine-tuned cAMP signaling in platelets without cross-talk to the cGMP pathway. The model and the data provide evidence for two independent feedback loops: PKA, which is activated by elevated cAMP levels in the platelet, subsequently inhibits adenylyl cyclase (AC) but as well activates PDE3. By multi-experiment fitting, we established a comprehensive dynamic model with one predictive, optimized and validated set of parameters. Different pharmacological conditions (inhibition, activation, drug combinations, permanent and transient perturbations) are successfully tested and simulated, including statistical validation and sensitivity analysis. Downstream cyclic nucleotide signaling events target different phosphorylation sites for cAMP- and cGMP-dependent protein kinases (PKA, PKG) in the vasodilator-stimulated phosphoprotein (VASP). VASP phosphorylation as well as cAMP levels resulting from different drug strengths and combined stimulants were quantitatively modeled. These predictions were again experimentally validated. High sensitivity of the signaling pathway at low concentrations is involved in a fine-tuned balance as well as stable activation of this inhibitory cyclic nucleotide pathway.

Conclusions

On the basis of experimental data, literature mining and database screening we established a dynamic in silico model of cyclic nucleotide signaling and probed its signaling sensitivity. Thoroughly validated, it successfully predicts drug combination effects on platelet function, including synergism, antagonism and regulatory loops.

Nitric oxide-sensitive guanylyl cyclase is the only nitric oxide receptor mediating platelet inhibition

BACKGROUND

The nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) signaling cascade is involved in the precise regulation of platelet responses. NO released from the endothelium is known to activate NO-sensitive guanylyl cyclase (NO-GC) in platelets. By the generation of cGMP and subsequent activation of cGMP-dependent protein kinase (PKG), NO-GC mediates the reduction of the intracellular calcium and inhibits platelet adhesion and aggregation. However, NO has been postulated to influence these platelet functions also via cGMP-independent mechanisms.

OBJECTIVE

We studied the effect of NO on platelets lacking NO-sensitive guanylyl cyclase with regards to aggregation, adhesion, calcium mobilization and bleeding time.

METHODS AND RESULTS

Here, we show that NO signaling leading to inhibition of agonist-induced platelet aggregation is totally abrogated in platelets from mice deficient in NO-GC (GCKO). Even at millimolar concentrations none of the several different NO donors inhibited collagen-induced aggregation of GCKO platelets. In addition, NO neither affected adenosine 5'-diphosphate (ADP)-induced adhesion nor thrombin-induced calcium release in GCKO platelets. Although the NO-induced cGMP signal transduction was totally abrogated cyclic adenosine monophosphate (cAMP) signaling was still functional; however, cGMP/cAMP crosstalk was disturbed on the level of phosphodiesterase type 3 (PDE3). These in vitro data are completed by a reduced bleeding time indicating the lack of NO effect in vivo.

CONCLUSIONS

We conclude that NO-GC is the only NO receptor in murine platelets mediating the inhibition of calcium release, adhesion and aggregation: lack of the enzyme leads to disturbance of primary hemostasis.

Platelet receptors and signaling in the dynamics of thrombus formation

Hemostasis and pathological thrombus formation are dynamic processes that require a co-ordinated series of events involving platelet membrane receptors, bidirectional intracellular signals, and release of platelet proteins and inflammatory substances. This review aims to summarize current knowledge in the key steps in the dynamics of thrombus formation, with special emphasis on the crucial participation of platelet receptors and signaling in this process. Initial tethering and firm adhesion of platelets to the exposed subendothelium is mediated by glycoprotein (GP) Ib/IX/V complex and collagen receptors, GP VI and alpha(2)beta(1) integrin, in the platelet surface, and by VWF and fibrillar collagen in the vascular site. Interactions between these elements are largely influenced by flow and trigger signaling events that reinforce adhesion and promote platelet activation. Thereafter, soluble agonists, ADP, thrombin, TxA(2), produced/released at the site of vascular injury act in autocrine and paracrine mode to amplify platelet activation and to recruit circulating platelets to the developing thrombus. Specific interactions of these agonists with their G-protein coupled receptors generate inside-out signaling leading to conformational activation of integrins, in particular alpha(IIb)beta(3), increasing their ligand affinity. Binding of alpha(IIb)beta(3) to its ligands, mainly fibrinogen, supports processes such as clot retraction and platelet aggregation. Stabilization of thrombi is supported by the late wave of signaling events promoted by close contact between aggregated platelets. The best known contact-dependent signaling is outside-in signaling through alphaIb beta(3), but new ones are being clarified such as those mediated by interaction of Eph receptors with ephrins, or by Sema 4D and Gas-6 binding to their receptors. Finally, newly identified mechanisms appear to control thrombus growth, including back-shifting of activated integrins and actuation of compensatory molecules such as ESAM or PECAM-1. The expanding knowledge of thrombotic disease is expected to translate into the development of new drugs to help management and prevention of thrombosis.

Tweaking the gain on platelet regulation: the tachykinin connection

Soluble factors such as ADP and thromboxane (TX) A(2) that are secreted or released by platelets at sites of tissue injury, mediate autocrine and paracrine regulation of platelet function, resulting in rapid localised thrombus formation. The suppression of platelet function, particularly through targeting such secondary regulatory mechanisms, that serve to 'fine-tune' the platelet response, has proven effective in the prevention of inappropriate platelet activation that results in thrombosis. The most commonly used anti-platelet approaches (ADP receptor antagonism or inhibition of TXA(2) synthesis), however, lack efficacy in many patients, suggesting the existence of additional uncharacterised mechanisms for the regulation of platelet function. Recent data, which form a focus of this review, have identified peripheral tachykinin peptide family members, such as substance P and the newly identified endokinins, as physiologically important positive feedback regulators of platelet function. The actions of tachykinins that are released from platelets during activation are mediated by the neurokinin-1 receptor. Initial analysis of the role of this receptor in platelet thrombus formation, and thrombosis in the mouse, indicate this to be a promising new target for the development of anti-thrombotic drugs.

Oncocalyxone A inhibits human platelet aggregation by increasing cGMP and by binding to GP Ibalpha glycoprotein

BACKGROUND AND PURPOSE

Oncocalyxone A (OncoA) has a concentration-dependent anti-platelet activity. The present study aimed to further understand the mechanisms related to this effect.

EXPERIMENTAL APPROACH

Human platelet aggregation was measured by means of a turbidimetric method. OncoA (32-256 microM) was tested against several platelet-aggregating agents, such as adenosine diphosphate (ADP), collagen, arachidonic acid (AA), ristocetin and thrombin.

KEY RESULTS

OncoA completely inhibited platelet aggregation with a calculated mean inhibitory concentration (IC50-microM) of 122 for ADP, 161 for collagen, 159 for AA, 169 for ristocetin and 85 for thrombin. The anti-aggregatory activity of OncoA was not inhibited by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). OncoA, at a concentration that caused no significant anti-aggregatory activity, potentiated sodium nitroprusside (SNP) anti-aggregatory activity (18.8+/-2.9%-SNP vs 85.0+/-8.2%-SNP+OncoA). The levels of nitric oxide (NO) or cAMP were not altered by OncoA while cGMP levels were increased more than 10-fold by OncoA in resting or ADP-activated platelets. Flow cytometry revealed that OncoA does not interact with receptors for fibrinogen, collagen or P-selectin. Nevertheless, OncoA decreased the binding of antibodies to GP Ibalpha, a glycoprotein that is related both to von Willebrand factor and to thrombin-induced platelet aggregation.

CONCLUSION AND IMPLICATIONS

OncoA showed anti-aggregatory activity in platelets that was associated with increased cGMP levels, not dependent on NO and with blocking GP Ibalpha glycoprotein. This new mechanism has the prospect of leading to new anti-thrombotic drugs.

Platelet integrins and immunoreceptors

Stable platelet adhesion to extracellular matrices and the formation of a hemostatic or pathological thrombus are dependent on integrin alphaIIbbeta3, also known as GPIIb-IIIa. However, maximal platelet responses to vascular injury may involve the participation of other integrins expressed in platelets (alphaVbeta3, alpha2beta1, alpha5beta1, and alpha6beta1). Platelet membrane 'immunoreceptors' contain at least one subunit with an extracellular immunoglobulin superfamily domain and/or an intracellular stimulatory immunoreceptor tyrosine-based activation motif (ITAM) or immunoreceptor tyrosine-based inhibitory motif (ITIM). Platelet ITAM receptors, such as FcgammaRIIA and the GPVI-FcRgamma complex, promote activation of integrins, while ITIM receptors, such as platelet-endothelial cell adhesion molecule-1, may promote their inhibition. This review summarizes the structure and function of platelet integrins and immunoreceptors, the emerging functional relationships between these receptor classes, and the consequences of their interaction for platelet function in hemostasis and thrombosis.

Neuro-apoptogenic and blood platelet targeting toxins in benthic marine cyanobacteria from the Portuguese coast

Six strains of marine cyanobacteria, of which five benthic, were isolated from an area of the Portuguese coast with no known apparent toxic microbial bloom. Five strains were lethal for mice. Four of them produced lethargy and four lead to bleeding. One of the toxic strains was from a genus (Aphanothece) not previously associated with toxin production. Extracts from four isolates induced SH-SY5Y-neuroblastoma cell apoptosis without affecting the viability of hepatocytes, NRK kidney cells, or fibroblasts. Aqueous extract from four isolates inhibited thrombin-induced blood platelet activation, with decreased P-selectin expression, platelet aggregation and shedding of platelet-derived micro-vesicles. Curiously, platelets treated with organic extracts from two of the cyanobacterial strains formed platelet micro-vesicles, expressed P-selectin on the surface and showed a distinct phosphotyrosine protein pattern, but failed to aggregate. We conclude that low-abundance marine cyanobacteria growing at low rates may be an important source for novel toxins that may be useful to dissect mammalian signalling pathways of apoptosis and platelet function.

Platelet adhesion signalling and the regulation of thrombus formation

Platelets perform a central role in haemostasis and thrombosis. They adhere to subendothelial collagens exposed at sites of blood vessel injury via the glycoprotein (GP) Ib-V-IX receptor complex, GPVI and integrin alpha(2)beta(1). These receptors perform distinct functions in the regulation of cell signalling involving non-receptor tyrosine kinases (e.g. Src, Fyn, Lyn, Syk and Btk), adaptor proteins, phospholipase C and lipid kinases such as phosphoinositide 3-kinase. They are also coupled to an increase in cytosolic calcium levels and protein kinase C activation, leading to the secretion of paracrine/autocrine platelet factors and an increase in integrin receptor affinities. Through the binding of plasma fibrinogen and von Willebrand Factor to integrin alpha(IIb)beta(3), a platelet thrombus is formed. Although increasing evidence indicates that each of the adhesion receptors GPIb-V-IX and GPVI and integrins alpha(2)beta(1) and alpha(IIb)beta(3) contribute to the signalling that regulates this process, the individual roles of each are only beginning to be dissected. By contrast, adhesion receptor signalling through platelet endothelial cell adhesion molecule 1 (PECAM-1) is implicated in the inhibition of platelet function and thrombus formation in the healthy circulation. Recent studies indicate that understanding of platelet adhesion signalling mechanisms might enable the development of new strategies to treat and prevent thrombosis.

Anticoagulant and antiplatelet agents: their clinical and device application(s) together with usages to engineer surfaces

An essential aspect of the treatment of patients with cardiovascular disease is the use of anticoagulant and antiplatelet agents for the prevention of further ischaemic events and vascular death resulting from thrombosis. Aspirin and heparin have been the standard therapy for the management of such conditions to date. Recently, numerous more potent platelet inhibitors together with anticoagulant agents have been developed and tested in randomized clinical trials. This article reviews the current state of the art of antiplatelet and anticoagulant therapy in light of its potential clinical efficacy. It then focuses on the usages of these agents in order to improve the performance of clinical devices such as balloon catheters, coronary stents, and femoropopliteal bypass grafting and extra corporeal circuits for cardiopulmonary bypass. The article then goes on to look at the usage of these agents more specifically heparin, heparan, hirudin, and coumarin in the development of more biocompatible scaffolds for tissue engineering.

Impaired synthesis and action of antiaggregating cyclic nucleotides in platelets from obese subjects: possible role in platelet hyperactivation in obesity

BACKGROUND

Subjects with central obesity exhibit platelet hyperactivity, which is involved in the atherosclerotic process and therefore can account for the increased risk of cardiovascular morbidity and mortality. The aim of the study was to evaluate whether alterations of platelet function in obesity involve synthesis and/or action of the two antiaggregating cyclic nucleotides adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP).

MATERIALS AND METHODS

In platelets from 16 obese and 15 control subjects we investigated the influence on platelet responses to the Adenosine-5-diphosphate sodium salt (ADP) exerted by (i) prostacyclin analogue Iloprost (0.31-5 nmol L(-1)) and the cAMP analogue 8-bromo-cAMP (10-500 micro mol L(-1)); and by (ii) nitric oxide (NO) donor sodium nitroprusside (SNP) (5-100 micro mol L(-1)) and the cGMP analogue 8-bromo-cGMP (10-500 micro mol L(-1)). IC(50) (minimal concentration of each inhibitor necessary to reduce platelet response to ADP by half) was determined. Iloprost and SNP ability to increase cyclic nucleotides was also measured.

RESULTS

Significantly greater IC(50) were observed in obese subjects than in healthy controls (1.59 +/- 0.16 vs. 0.80 +/- 0.08 nmol L(-1), P = 0.0001 for Iloprost, and 27.6 +/- 6.5 vs. 7.0 +/- 1.7 micro mol L(-1), P = 0.006, for SNP); when data from control and obese subjects were pooled together, IC(50) of Iloprost and SNP correlated with the homeostasis model assessment (HOMA IR), which is a parameter used to measure the insulin resistance (r = 0.588, P = 0.029 and r = 0.640, P = 0.006, respectively). Also the antiaggregating effect of 8-Br-cAMP and 8-Br-cGMP was smaller in the obese subjects. Finally, the ability of Iloprost to increase platelet cAMP and the ability of SNP to increase both cGMP and cAMP were reduced in obese subjects.

CONCLUSIONS

Platelet resistance to the antiaggregating effects of prostacyclin and NO in obesity is attributable to impairment of cyclic nucleotide synthesis and action. As cyclic nucleotides are the main effectors of platelet antiaggregation, the resistance to them can account for platelet hyperactivity in obesity.

Novel epinephrine and cyclic AMP-mediated activation of BCAM/Lu-dependent sickle (SS) RBC adhesion

The vasoocclusive crisis is the major clinical feature of sickle cell anemia, which is believed to be initiated or sustained by sickle (SS) red blood cell (RBC) adhesion to the vascular wall. SS RBCs, but not unaffected (AA) RBCs, adhere avidly to multiple components of the vascular wall, including laminin. Here we report a novel role for epinephrine and cyclic adenosine monophosphate (cAMP) in the regulation of human SS RBC adhesiveness via the laminin receptor, basal cell adhesion molecule/Lutheran (BCAM/Lu). Our data demonstrate that peripheral SS RBCs contain greater than 4-fold more cAMP than AA RBCs under basal conditions. Forskolin or the stress mediator epinephrine further elevates cAMP in SS RBCs and increases adhesion of SS RBCs to laminin in a protein kinase A (PKA)-dependent manner, with the low-density population being the most responsive. Epinephrine-stimulated adhesion to laminin, mediated primarily via the beta 2-adrenergic receptor, occurred in SS RBC samples from 46% of patients and was blocked by recombinant, soluble BCAM/Lu, implicating this receptor as a target of cAMP signaling. Thus, these studies demonstrate a novel, rapid regulation of SS RBC adhesion by a cAMP-dependent pathway and suggest that components of this pathway, particularly PKA, the beta 2-adrenergic receptor, and BCAM/Lu, should be further explored as potential therapeutic targets to inhibit SS RBC adhesion.