3 Platelet and Vessel Wall Interaction and the Genesis of Atherosclerosis

The thrombotic potential of oral pathogens

In recent times the concept of infectious agents playing a role in cardiovascular disease has attracted much attention. Chronic oral disease such as periodontitis, provides a plausible route for entry of bacteria to the circulation. Upon entry to the circulation, the oral bacteria interact with platelets. It has been proposed that their ability to induce platelet aggregation and support platelet adhesion is a critical step in the pathogenesis of the infection process. Many published studies have demonstrated multiple mechanisms through which oral bacteria are able to bind to and activate platelets. This paper will review the various mechanisms oral bacteria use to interact with platelets.

Beyond high-density lipoprotein cholesterol levels evaluating high-density lipoprotein function as influenced by novel therapeutic approaches

A number of therapeutic strategies targeting high-density lipoprotein (HDL) cholesterol and reverse cholesterol transport are being developed to halt the progression of atherosclerosis or even induce regression. However, circulating HDL cholesterol levels alone represent an inadequate measure of therapeutic efficacy. Evaluation of the potential effects of HDL-targeted interventions on atherosclerosis requires reliable assays of HDL function and surrogate markers of efficacy. Promotion of macrophage cholesterol efflux and reverse cholesterol transport is thought to be one of the most important mechanisms by which HDL protects against atherosclerosis, and methods to assess this pathway in vivo are being developed. Indexes of monocyte chemotaxis, endothelial inflammation, oxidation, nitric oxide production, and thrombosis reveal other dimensions of HDL functionality. Robust, reproducible assays that can be performed widely are needed to move this field forward and permit effective assessment of the therapeutic potential of HDL-targeted therapies.

Reinforcing a continuum of care: in-hospital initiation of long-term secondary prevention following acute coronary syndromes

INTRODUCTION

Patients with a history of acute coronary syndrome are particularly susceptible to further vascular or ischemic events. Effective secondary prevention following acute coronary syndrome requires multiple medications targeting the different mechanisms of atherothrombosis. The 2002 American College of Cardiology/American Heart Association guidelines for the management of unstable angina and non ST-segment myocardial infarction and the 2004 guidelines for ST-segment myocardial infarction assign priority to the long-term administration of four critical classes of drugs: antiplatelet agents, in particular aspirin and clopidogrel, beta-blockers, angiotensin-converting enzyme inhibitors, and statins.

CONCLUSIONS

Despite clinical trial evidence demonstrating their ability to reduce cardiovascular morbidity and mortality, available preventive pharmacotherapies remain underutilized. Suboptimal compliance with current recommendations, as with other management guidelines, arises from a host of entrenched physician, patient, and system-related factors. Optimal management of acute coronary syndrome acknowledges a continuum of care in which acute stabilization represents a single important component. Early, in-hospital implementation of secondary preventive measures reinforces the continuum of care approach, promoting a successful transition from treatment to prevention, inpatient to outpatient management, and, when appropriate, subspecialist to generalist care.

Thrombosis and Thrombolysis: Platelet Membrane Glycoproteins

Over the last quarter of a century, therapy for acute coronary syndromes has rapidly evolved. The major causative factor for acute coronary syndromes, particularly acute myocardial infarction, is now recognised to be coronary thrombosis and therapies using thrombolytic agents to dissolve thrombus or percutaneous coronary interventions (angioplasty and stenting) to mechanically disrupt thrombus and restore vessel patency are now routine. The precipitant for coronary thrombus is believed to be atherosclerotic plaque rupture. Plaque rupture has been demonstrated at autopsy in humans and in vivo in an experimental animal model for atherosclerosis. Thrombus formed at these sites is platelet rich and anti-platelet therapy has an established role in the treatment of cardiovascular disease. The study of platelet membrane glycoproteins, which mediate platelet adhesion and aggregation, has resulted in specific therapies. Future directions for research with clinical relevance include the development of markers for plaque instability.

Characterization of the proteins released from activated platelets leads to localization of novel platelet proteins in human atherosclerotic lesions

Proteins secreted by activated platelets can adhere to the vessel wall and promote the development of atherosclerosis and thrombosis. Despite this biologic significance, however, the complement of proteins comprising the platelet releasate is largely unknown. Using a proteomics approach, we have identified more than 300 proteins released by human platelets following thrombin activation. Many of the proteins identified were not previously attributed to platelets, including secretogranin III, a potential monocyte chemoattractant precursor; cyclophilin A, a vascular smooth muscle cell growth factor; calumenin, an inhibitor of the vitamin K epoxide reductase-warfarin interaction, as well as proteins of unknown function that map to expressed sequence tags. Secretogranin III, cyclophilin A, and calumenin were confirmed to localize in platelets and to be released upon activation. Furthermore, while absent in normal vasculature, they were identified in human atherosclerotic lesions. Therefore, these and other proteins released from platelets may contribute to atherosclerosis and to the thrombosis that complicates the disease. Moreover, as soluble extracellular proteins, they may prove suitable as novel therapeutic targets.

Platelet factor 4 enhances the binding of oxidized low-density lipoprotein to vascular wall cells

Accumulation of low-density lipoprotein (LDL)-derived cholesterol by macrophages in vessel walls is a pathogenomic feature of atherosclerotic lesions. Platelets contribute to lipid uptake by macrophages through mechanisms that are only partially understood. We have previously shown that platelet factor 4 (PF4) inhibits the binding and degradation of LDL through its receptor, a process that could promote the formation of oxidized LDL (ox-LDL). We have now characterized the effect of PF4 on the binding of ox-LDL to vascular cells and macrophages and on the accumulation of cholesterol esters. PF4 bound to ox-LDL directly and also increased ox-LDL binding to vascular cells and macrophages. PF4 did not stimulate ox-LDL binding to cells that do not synthesize glycosaminoglycans or after enzymatic cleavage of cell surface heparan and chondroitin sulfates. The effect of PF4 on binding ox-LDL was dependent on specific lysine residues in its C terminus. Addition of PF4 also caused an approximately 10-fold increase in the amount of ox-LDL esterified by macrophages. Furthermore, PF4 and ox-LDL co-localize in atherosclerotic lesion, especially in macrophage-derived foam cells. These observations offer a potential mechanism by which platelet activation at sites of vascular injury may promote the accumulation of deleterious lipoproteins and offer a new focus for pharmacological intervention in the development of atherosclerosis.

Sequential injury of the rabbit abdominal aorta induces intramural coagulation and luminal narrowing independent of intimal mass: extrinsic pathway inhibition eliminates luminal narrowing

We hypothesized that activation of the coagulation cascade is involved in arterial remodeling in response to sequential injury. An active site-inhibited recombinant human factor VIIa (FVIIai) was used to inhibit tissue factor, the primary cofactor in the extrinsic pathway of coagulation, in a sequential balloon injury model of the rabbit abdominal aorta. Single balloon injury produced limited intimal thickening at 3 weeks (intimal area, 0.40+/-0.05 mm2) and no loss in luminal area (12.2+/-0.9 mm2 before injury and 12.1+/-0.9 mm2 at 6 weeks after injury). Sequential balloon injury, 3 weeks after the first balloon denudation, produced a progressive loss of lumen, with 22% and 47% loss of luminal area, respectively, at 3 and 6 weeks. Luminal loss could not be accounted for by intimal growth (at 3 weeks after sequential injury, the intimal area was 0.47+/-0.08 mm2, <4% of the initial luminal area). Sequential injury acutely produced extensive mural and intramural fibrin deposition. Treatment with FVIIai inhibited both the fibrin deposition and the chronic loss of lumen. At 3 weeks after sequential injury, luminal cross-sectional areas were 9.8+/-0.6 mm2 for control rabbits and 14.3+/-1.4 mm2 for FVIIai-treated rabbits. Neither neointimal area nor cell proliferation was reduced by FVIIai treatment. The intimal cell proliferation index 3 days after injury was 7.6+/-1.1% in control rabbits versus 5.8+/-1.1% in treated rabbits (P>0.05). These results indicate that tissue factor is an important mediator of coagulation in repeat injury and implicate the extrinsic coagulation cascade in a blood vessel remodeling response that is independent of neointimal growth but leads to extensive loss of lumen.

The molecular mechanism of platelet adhesion

One of the most primitive of host-defence mechanisms is haemostasis, the ability to control blood loss. In response to vascular trauma, platelets rapidly adhere to the exposed subendothelial matrix, a process that ultimately results in the sealing of the vessel by a plug of platelets stabilised by fibrin. Paradoxically, it is the same cascade of events that leads to thrombosis and vessel occlusion, resulting in heart attack and stroke. The molecular events involved in platelet adhesion have therefore been the subject of intense investigation. In all but the largest blood vessels, the initial contact adhesion of platelets is mediated by subendothelial matrix bound von Willebrand Factor (vWF) and a specific vWF receptor on platelets, the glycoprotein (GP) Ib-V-IX complex. Our understanding of this process arose from analysis of two congenital bleeding disorders, von Willebrand's disease and the Bernard-Soulier syndrome, in which vWF or the GP Ib-V-IX, respectively, are either absent or dysfunctional. This overview discusses our current molecular understanding of platelet adhesion and how engagement of vWF by the GP Ib-V-IX complex on platelets initiates the subsequent events in platelet activation leading to either haemostasis or thrombosis.

Effect of milrinone on thromboxane A2 synthesis, cAMP phosphodiesterase and 45Ca2+ uptake by human platelets

The phosphodiesterase inhibitors milrinone and isobutylmethylxanthine (IBMX) inhibited the conversion of [3H]cAMP to [3H]AMP by washed human platelets in concentration-dependent manners (IC50: milrinone, 2.6 x 10(-6) M; IBMX, 4.6 x 10(-6) M). Milrinone and IBMX increased cAMP levels when stimulated by a single concentration (0.3 microM) of iloprost. EC50:milrinone, 5.6 x 10(-5) M; IBMX, 3 x 10(-5) M. Milrinone was a potent inhibitor of platelet thromboxane A2 (TXA2) synthesis when stimulated by median stimulatory doses of collagen (IC50: 3 x 10(-7) M), sodium fluoride (NaF) (a non-specific G protein activator; IC50: 3 x 10(-7) M) and phorbol ester myristate acetate (PMA) (a protein kinase C activator; IC50: 2.2 x 10(-7) M). In contrast, at median stimulatory doses of A23187 and arachidonate there was a marked decrease in the potency of milrinone in inhibiting TXA2 synthesis. Milrinone had a weak inhibitory effect on TXA2 synthesis when elicited by freeze fracturing. In all experiments IBMX was a weaker inhibitor of TXA2 synthesis, although the general pattern of effects was similar to milrinone. Milrinone inhibited both collagen- and adrenaline-stimulated 45Ca2+ uptake by human platelets in dose-dependent manners. Since platelet TXA2 synthesis is dependent on Ca2+, and milrinone inhibited 45Ca2+ uptake, it is concluded that milrinone exerts its inhibitory effect on platelet activity, principally through an action on Ca2+ mobilisation/binding to effector proteins (protein kinase C and/or phospholipase A2).

Visualization of platelet glycoproteins Ib and IIIa by immunoenzymatic stain using avidin-biotin peroxidase complex

A method is described here for the identification and quantitation of antigens by monoclonal antibodies. This method is based upon 1) separation (crossed-immunoelectrophoresis) and immunoprecipitation (rocket immunoelectrophoresis and crossed immunoelectrophoresis) of glycoproteins Ib and IIIa with a polyspecific antiserum; 2) binding of the non precipitating monoclonal antibody to glycoproteins precipitated by the rabbit antibody; 3) visualization of the monoclonal antibody with secondary biotinylated antibody and after addition of avidin biotin peroxidase complex, the peroxidase activity is detected by 4-Cl-1-naphtol. By this technique, the agarose gel plate could be stained directly and this allowed us to eliminate electrophoretic transblotting and radioactive compounds.

Vascular responses to platelet activation in normal and atherosclerotic primates in vivo

Platelets release vasoactive substances that may contribute to augmented vasoconstriction. In this study, we examined vascular responses to activation of platelets in vivo by infusion of collagen. Purified bovine collagen was infused into the blood-perfused hind limb of normal and atherosclerotic cynomolgus monkeys. Resistance of the total limb and large arteries was measured at constant flow. In normal monkeys, collagen produced a decrease in total limb resistance, with a modest constrictor response of the large arteries. In atherosclerotic monkeys, collagen produced a transient, small decrease in total limb resistance, with pronounced constriction of large arteries. Indomethacin (5 mg/kg i.v.) and the thromboxane A2/prostaglandin H2 receptor antagonist SQ29,548 (2 mg/kg i.v.) virtually abolished the large-artery constrictor response to collagen in atherosclerotic monkeys. The 5-HT2-serotonergic receptor antagonist ketanserin (0.6 mg/kg i.v.) had no effect on the vasoconstrictor response. We conclude that 1) large arteries constrict and small vessels dilate in response to collagen-mediated activation of platelets in vivo in normal and atherosclerotic monkeys, 2) large-artery constriction in response to activation of platelets is augmented in atherosclerotic monkeys, and 3) the augmented large-artery constriction in atherosclerotic monkeys may be mediated primarily by thromboxane. The findings provide evidence that platelets may contribute to augmented constrictor responses of atherosclerotic arteries.

The effect of anaesthetic agents on platelet function

This paper reviews studies which have investigated the effect of anaesthetic agents on platelet function. The results of these studies suggest that halothane is the only agent in current use which inhibits platelet function in concentrations used clinically. Nitrous oxide appears to cause only a modest inhibition, while enflurane and isoflurane appear to have minimal or negligible effects. There is no current evidence that intravenous induction agents, opiates, or muscle relaxants affect platelet function. Reports indicate that local anaesthetic agents inhibit platelet aggregation, but only at concentrations far greater than peak plasma concentrations found during clinical use. Epidural anaesthesia may be associated with a reduction in platelet aggregation through a mechanism unrelated to direct local anaesthetic inhibition. The clinical significance of the effect of halothane on platelet function is not known. However, it is possible that halothane may affect bleeding or thrombotic complications in a similar manner to other 'anti-platelet' drugs.

Interaction of platelets with endothelial cells: activation of a novel neutral protease

The activation of a new neutral protease of MW 85,000 was demonstrated on interaction between porcine aortic endothelial cells and human and porcine platelets in culture. The activity of this enzyme, PECAP (platelet endothelial cell activated protease), was detected by electrophoresis on polyacrylamide gels impregnated with the substrate casein. Our results showed that the platelet-endothelial cell interaction did not involve induction of synthesis of de novo enzyme, but rather an activation of a latent enzyme. PECAP cleaved casein and fibrinogen, but had no activity against gelatin or elastin. It was not inhibited by inhibitors of metalloproteases (EDTA, 1.10 phenanthroline), serine proteases (phenylmethylsulfonyl fluoride, elastatinal), or cysteine proteases (iodoacetate, N-ethylmaleimide) and it seems to be unrelated to the previously known proteases of mesenchymal and hematopoietic cells.

Bernard-Soulier syndrome

Bernard-Soulier syndrome (BSS) is a rare autosomal bleeding disorder characterized clinically by prolonged skin bleeding time, normal clot retraction and thrombocytopenia with large and morphologically abnormal platelets, and biochemically by the absence of platelet membrane glycoproteins (GP) Ib, V and IX. GP Ib and GP IX exist in the platelet membrane as a heterodimer complex which acts as the major receptor mediating platelet adhesion to blood vessel subendothelium. Studies with BSS platelets have proved particularly rewarding in the investigation of the GP Ib-IX complex as a multifunctional receptor protein. The transmembrane complex contains binding domains for von Willebrand factor, thrombin, fibrin and quinine/quinidine drug-dependent antibodies as well as an attachment site on the cytoplasmic side of the membrane for a platelet cytoskeleton. In addition, the internal segment of the beta-chain of GP Ib contains a cyclic AMP-dependent protein kinase-associated phosphorylation site which appears to regulate platelet reactivity. Limited proteolytic cleavage of the complex, in particular the GP Ib alpha-chain, has allowed immunological and functional characterization of three distinct domains; a 45 kDa segment at the N-terminal end of the alpha-chain of GP Ib, which contains binding sites for von Willebrand factor and thrombin, a 90 kDa highly glycosylated region of GP Ib alpha and a membrane-associated region consisting of the remnant of GP Ib alpha disulphide-linked to GP Ib beta and non-covalently-complexed with GP IX. This membrane-associated region contains the antigenic epitope(s) for quinine/quinidine drug-dependent antibodies. It is highly probable that the future study of platelets from patients with the Bernard-Soulier syndrome will further clarify the role of the GP Ib-IX complex in platelet physiology.

In vitro studies of the interaction of heparin, low molecular weight heparin and heparinoids with platelets

Heparin, low molecular weight heparins, and heparinoids were studied for their ability to inhibit the aggregation of platelets by various agonists and for their ability to adhere to collagen. Heparin was a very effective inhibitor of aggregation with collagen and with ristocetin as it was with adhesion to collagen. The heparinoids showed little effect on aggregation or adhesion. Heparan sulfate and pentosan polysulfate did show slight inhibitory activity against collagen aggregation and adhesion and both interacted with the antibody induced by heparin therapy. It is of interest that dermatan sulfate and the pentasaccharide were almost inert in these experiments, and are unlikely to induce bleeding by inhibition of platelet function. It is highly probable that interference with the interaction of von Willebrand factor with platelets and collagen is a major mechanism for bleeding in the heparinized patient.