Human platelets exclusively bind oxidized low density lipoprotein showing no specificity for acetylated low density lipoprotein

Platelets as an inter-player between hyperlipidaemia and atherosclerosis

Platelet hyperreactivity and hyperlipidaemia contribute significantly to atherosclerosis. Thus, it is desirable to review the platelet-hyperlipidaemia interplay and its impact on atherogenesis. Native low-density lipoprotein (nLDL) and oxidized LDL (oxLDL) are the key proatherosclerotic components of hyperlipidaemia. nLDL binds to the platelet-specific LDL receptor (LDLR) ApoE-R2', whereas oxLDL binds to the platelet-expressed scavenger receptor CD36, lectin-type oxidized LDLR 1 and scavenger receptor class A 1. Ligation of nLDL/oxLDL induces mild platelet activation and may prime platelets for other platelet agonists. Platelets, in turn, can modulate lipoprotein metabolisms. Platelets contribute to LDL oxidation by enhancing the production of reactive oxygen species and LDLR degradation via proprotein convertase subtilisin/kexin type 9 release. Platelet-released platelet factor 4 and transforming growth factor β modulate LDL uptake and foam cell formation. Thus, platelet dysfunction and hyperlipidaemia work in concert to aggravate atherogenesis. Hypolipidemic drugs modulate platelet function, whereas antiplatelet drugs influence lipid metabolism. The research prospects of the platelet-hyperlipidaemia interplay in atherosclerosis are also discussed.

The atheroprotective role of fucoidan involves the reduction of foam cell formation by altering cholesterol flux-associated factors in macrophages

Atherosclerosis is characterized by the accumulation of lipid-laden cells in the arterial walls, resulting from dysregulation of cholesterol homeostasis in the macrophage, triggered by oxidized low-density lipoprotein (oxLDL). Previous studies have shown that fucoidan, a sulfated polysaccharide from brown seaweeds, has several atheroprotective activities, however, the mechanism of fucoidan protection is not fully understood. Thus, we investigated the effect of fucoidan on atherogenesis in apolipoprotein E-deficient (ApoE-/-) mice, on oxLDL uptake by macrophages, and on the expression of the flux-associated scavenger receptors by macrophages. Also, we examined the absorption and biodistribution of orally administered fucoidan. ApoE-/- mice fed on a cholesterol-rich diet supplemented with 1% fucoidan showed reduced dyslipidemia and atherosclerosis. Fucoidan was detected in blood and peripheral tissue after gavage, suggesting that it can exert direct systemic effects. In vitro, fucoidan reduced macrophage oxLDL uptake, which resulted in lower foam cell formation. This effect was associated with downregulation of the cholesterol influx-associated scavenger receptor (SR)-A expression, and upregulation of the cholesterol efflux-associated SR-B1 expression. In conclusion, fucoidan prevented oxLDL-mediated foam cell formation in macrophages by downregulating SR-A1/2 and by up-regulating SR-B1.

LDL-Cholesterol and Platelets: Insights into Their Interactions in Atherosclerosis

Atherosclerosis and its complications, including acute coronary syndromes, are the major cause of death worldwide. The two most important pathophysiological mechanisms underlying atherosclerosis include increased platelet activation and increased low-density lipoproteins (LDL) concentration. In contrast to LDL, oxidized (ox)-LDL have direct pro-thrombotic properties by functional interactions with platelets, leading to platelet activation and favoring thrombus formation. In this review, we summarize the currently available evidence on the interactions between LDL-cholesterol and platelets, which are based on (i) the presence of ox-LDL-binding sites on platelets, (ii) generation of ox-LDL by platelets and (iii) the role of activated platelets and ox-LDL in atherosclerosis. In addition, we elaborate on the clinical implications of these interactions, including development of the new therapeutic possibilities. The ability to understand and modulate mechanisms governing interactions between LDL-cholesterol and platelets may offer new treatment strategies for atherosclerosis prevention.

Oxidized Low-Density Lipoprotein Induces Macrophage Production of Prothrombotic Microparticles

Background Activated vascular cells produce submicron prothrombotic and proinflammatory microparticle vesicles. Atherosclerotic plaques contain high levels of microparticles. Plasma microparticle levels increase during acute coronary syndromes and the thrombotic consequences of plaque rupture likely involve macrophage-derived microparticles (MΦMPs). The activation pathways that promote MΦMP production remain poorly defined. This study tested the hypothesis that signals implicated in atherogenesis also stimulate MΦMP production. Methods and Results We stimulated human primary MΦs with proinflammatory cytokines and atherogenic lipids, and measured MΦMP production by flow cytometry. Oxidized low-density lipoprotein (oxLDL; 25 µg/mL) induced MΦMP production in a concentration-dependent manner (293% increase; P<0.001), and these oxLDL MΦMP stimulatory effects were mediated by CD36. OxLDL stimulation increased MΦMP tissue factor content by 78% (P<0.05), and oxLDL-induced MΦMP production correlated with activation of caspase 3/7 signaling pathways. Salvionolic acid B, a CD36 inhibitor and a CD36 inhibitor antibody reduced oxLDL-induced MΦMP by 67% and 60%, respectively. Caspase 3/7 inhibition reduced MΦMP release by 52% (P<0.01) and caspase 3/7 activation increased MΦMP production by 208% (P<0.01). Mevastatin pretreatment (10 µM) decreased oxLDL-induced caspase 3/7 activation and attenuated oxLDL-stimulated MΦMP production and tissue factor content by 60% (P<0.01) and 43% (P<0.05), respectively. Conclusions OxLDL induces the production of prothrombotic microparticles in macrophages. This process depends on caspases 3 and 7 and CD36 and is inhibited by mevastatin pretreatment. These findings link atherogenic signaling pathways, inflammation, and plaque thrombogenicity and identify a novel potential mechanism for antithrombotic effects of statins independent of LDL lowering.

Different Effect of Statins on Platelet Oxidized-LDLReceptor (CD36 and LOX-1) Expressionin Hypercholesterolemic Subjects

Hydroxymethyl-glutaryl-CoA-reductase inhibitors (statins) reduce cardiovascular mortality by decreasing cholesterol as well as by non-lipid-related actions. Oxidized low-density lipoproteins (ox-LDL) are pro-atherogenic molecules and potent platelet agonists. CD36 and lectin-like ox-LDL receptor-1 (LOX-1) are specific ox-LDL receptors also expressed in platelets. This study was planned to address whether treatment with atorvastatin 10 mg/day, pravastatin 40 mg/day or simvastatin 20 mg/day could affect platelet CD36 and LOX-1 expression. Twenty-four patients for each treatment were evaluated after 3, 6, and 9 days and at 6 weeks for complete lipid profile (chromogenic), ox-LDL (ELISA), platelet P-selectin (P-sel), CD36, LOX-1 (FACS), and intracellular citrullin recovery (iCit) (HPLC). Data show hyperactivated platelets (P-sel absolute values, percent variation in activated cells, all p < 0.001), and CD36 and LOX-1 overexpression (all p < 0.001) in patients at baseline. P-sel, CD36, and LOX-1 were significantly decreased by atorvastatin and simvastatin (all p < 0.01) and related with iCit increase (r = 0.58,p < 0.001) and platelet-associated ox-LDL (r = 0.51, p < 0.01) at 9 days. Pravastatin reduced LOX-1 and P-sel (p < 0.05) at 6 weeks in relation with decreased LDL and ox-LDL (r = 0.39, p < 0.01 and r = 0.37, p < 0.01, respectively). These data suggest that atorvastatin and simvastatin reduce platelet activity by exposure of CD36 and LOX-1 before significant LDL reduction, whereas pravastatin action is detected later and in relation with LDL and ox-LDL lowering. Rapid and consistent reduction of CD36 and LOX-1 could be considered a direct anti-atherothrombotic mechanism related to the role of ox-LDL in platelet activation, platelet-endothelium interactions, and NO synthase activity.

Native high density lipoproteins (HDL) interfere with platelet activation induced by oxidized low density lipoproteins (OxLDL)

Platelets and lipoproteins play a crucial role in atherogenesis, in part by their ability to modulate inflammation and oxidative stress. While oxidized low density lipoproteins (OxLDL) play a central role in the development of this disease, high density lipoproteins (HDL) represent an atheroprotective factor of utmost importance. As platelet function is remarkably sensitive to the influence of plasma lipoproteins, it was the aim of this study to clarify if HDL are able to counteract the stimulating effects of OxLDL with special emphasis on aspects of platelet function that are relevant to inflammation. Therefore, HDL were tested for their ability to interfere with pro-thrombotic and pro-inflammatory aspects of platelet function. We are able to show that HDL significantly impaired OxLDL-induced platelet aggregation and adhesion. In gel-filtered platelets, HDL decreased both the formation of reactive oxygen species and CD40L expression. Furthermore, HDL strongly interfered with OxLDL-induced formation of platelet-neutrophil aggregates in whole blood, suggesting that platelets represent a relevant and sensitive target for HDL. The finding that HDL effectively competed with the binding of OxLDL to the platelet surface might contribute to their atheroprotective and antithrombotic properties.

Hypochlorite-oxidized LDL induces intraplatelet ROS formation and surface exposure of CD40L--a prominent role of CD36

OBJECTIVE

OxLDL represents a central player in atherogenesis and has been shown to activate human blood platelets. In light of the pivotal role of CD40L in inflammation, it was the aim of this work to clarify if platelet-activating effects of oxidized LDL result in surface exposure and liberation of CD40L and to explore the role of platelet scavenger receptor CD36 in this process.

METHODS

Binding and functional studies were performed with hypochlorite-oxidized LDL in absence and presence of (potential) competitors in normal and CD36-deficient human platelets. To determine functional effects of hypochlorite-oxidized LDL on human platelets, formation of reactive oxygen species, intraplatelet calcium, CD40L and CD62P as well as platelet aggregation were quantified.

RESULTS

Addition of OxLDL to resting human platelets results in intracellular calcium flux, platelet aggregation and surface expression of CD62P. OxLDL triggers the formation of intracellular reactive oxygen species and surface exposure of CD40L, with both being sensitive to the NADPH oxidase inhibitor apocynin. In CD36-deficient human platelets, functional effects as well as high affinity binding of hypochlorite-oxidized LDL appears to be significantly reduced compared with platelets positive for CD36.

CONCLUSIONS

Our results prove a prominent--however, not exclusive--role of CD36 in platelet binding of hypochlorite-oxidized LDL. CD36 appears to be the major receptor responsible for hypochlorite-oxidized LDL-induced platelet activation that accumulates in the release of CD40L. As platelets represent the major source of CD40L, our findings emphasize an important pro-inflammatory role of platelets, especially in conditions of oxidative stress.

Specific binding of hypochlorite-oxidized HDL to platelet CD36 triggers proinflammatory and procoagulant effects

OBJECTIVE

Oxidative stress and systemic inflammation negatively affect several protective functions of high density lipoproteins (HDL) and oxidative modification of HDL by the inflammation-derived oxidant hypochlorite converts HDL into a potent platelet agonist. Therefore it was the aim of this work to clarify if these platelet-activating effects result from specific binding of hypochlorite-oxidized HDL (hyp-OxHDL) to the platelet surface and to identify responsible receptors.

METHODS

Binding and functional studies were performed with hyp-OxHDL in absence and presence of (potential) competitors in normal and CD36-deficient human platelets. Platelet aggregation was quantified by light transmission aggregometry. Surface expression of CD62P, phosphatidylserine and CD40L was quantified by flow cytometry.

RESULTS

Binding studies reveal that hyp-OxHDL show specific and saturable high-affinity binding to the platelet surface. Hyp-OxHDL trigger platelet aggregation and in a dose dependent way provoke the release of significant amounts of CD40L as well as phosphatidylserine on the platelet surface. Blocking specific binding of hyp-OxHDL to the platelet surface interferes with the ability of hyp-OxHDL to stimulate human platelets. CD36-deficient human platelets show markedly reduced binding of hyp-OxHDL. Upon addition of hypochlorite-oxidized HDL, CD36-deficient platelets do not aggregate and completely fail to release CD40L or phosphatidylserine.

CONCLUSIONS

From these results we conclude that specific binding of hyp-OxHDL to platelet CD36 is essential for the proinflammatory and procoagulant effects of hyp-OxHDL shown within this work. The contribution of other receptors besides CD36 to specific binding of hyp-OxHDL to the platelet membrane appears to be minimal, at best.

Type 2 scavenger receptor CD36 in platelet activation: the role of hyperlipemia and oxidative stress

Platelet hyper-reactivity and a systemic prothrombotic state are associated with atherosclerosis and other inflammatory conditions. CD36, a member of the Type 2 scavenger receptor family, is a multiligand pattern recognition receptor that recognizes specific oxidized phospholipids, molecules expressed on microbial pathogens, apoptotic cells, and cell-derived microparticles. Recent studies have demonstrated that CD36 binding to oxidized LDL or microparticles activates a specific signaling pathway that induces platelet activation. This pathway is activated in vivo in the setting of hyperlipidemia and oxidant stress. Genetic deletion of CD36 protects mice from pathological thrombosis associated with hyperlipidemia without any apparent effect on normal hemostasis. Targeting CD36 or its signaling pathway could potentially lead to the development of novel antithrombotic therapies for patients with atheroinflammatory disorders.

A specific CD36-dependent signaling pathway is required for platelet activation by oxidized low-density lipoprotein

Platelet hyperactivity associated with hyperlipidemia may contribute to development of a prothrombotic state. We previously showed that oxidized low-density lipoprotein (oxLDL) formed in the setting of hyperlipidemia and atherosclerosis activated platelets in a CD36-dependent manner. We now show that mitogen-activated protein kinase c-Jun N-terminal kinase (JNK)2 and its upstream activator MKK4 were phosphorylated in platelets exposed to oxLDL. Using apoE(-/-) mice as a model of hyperlipidemia, we showed that JNK was constitutively phosphorylated in platelets in a CD36-dependent manner. Inhibition of src kinase activity reduced JNK phosphorylation by oxLDL. Immunoprecipitations revealed that active phosphorylated forms of src kinases Fyn and Lyn were recruited to CD36 in platelets exposed to oxLDL. Pharmacological inhibition of the mitogen-activated protein kinase JNK or src family kinases abolished platelet activation by oxLDL in vitro. Using a murine carotid artery thrombosis model we demonstrated CD36-dependent phosphorylation of platelet JNK within thrombi. Furthermore, pharmacological inhibition of JNK prolonged thrombosis times in wild-type but not cd36-null mice in vivo. These findings suggest that a specific CD36-dependent signaling pathway is required for platelet activation by oxLDL and may provide insights related to development of novel antiplatelet therapies more relevant to atherothrombosis than to normal hemostasis.

Modified Low-Density Lipoproteins and High-Density Lipoproteins

It has long been known that the oxidative state of the various plasma lipoproteins modulates platelet aggregability, thereby contributing to atherogenesis. Low-density lipoprotein (LDL), occurring in vivo both in the native and oxidised forms, interacts directly with platelets, by binding to specific receptors. While the identity of the receptors for native LDL and some subfractions of high-density lipoproteins (HDL) remains disputed, apoE-containing HDL(2) binds to LRP8. The nature of these interactions as well as the distinction between candidate receptor proteins was elucidated using covalently modified apolipoproteins, which pointed to the participation of apolipoproteins in high affinity binding. However, the platelet effects initiated by binding of native lipoproteins remain controversial. Some of this ambiguity can be traced to the fact that native LDL inevitably undergoes substantial oxidisation upon modification, including by radiolabelling. The platelet-activating effects provoked by oxidised LDL are irrefutable, but many details remain unknown. The role of CD36 in platelet binding by oxidised LDL is well established, although additional receptors may exist. Much less is known about the interaction of oxidised HDL with platelets, since platelet activation was observed in some, but not all studies. Various frequently applied in vitro oxidation methods produce modified lipoprotein species that may not be relevant in vivo. Based on the reported modifications obtained by in vitro oxidation of LDL, early investigations focused mainly on the formation and the eventual effects of oxidised lipids. More recently, alterations to lipoproteins performed using hypochloric acid and myeloperoxidase redirected the attention to the role of modified apoproteins in triggering platelet responses.

Plasma very-low-density lipoprotein, low-density lipoprotein, and high-density lipoprotein oxidative modification induces procoagulant profiles in endogenous hypertriglyceridemia

This study was to investigate whether oxidatively modified lipoproteins were associated with changes of pro- and anticoagulant profiles in hypertriglyceridemic subjects. Plasma VLDL, LDL, and HDL were isolated with the one-step density gradient ultracentrifugation method. The oxidation of the lipoproteins was identified. Prothrombin time (PT) and activated partial thrombplastin time (APTT), tissue plasminogen activator and plasminogen activator inhibitor-1, and platelet aggregation rate were determined with a reaction system consisting of mixed fresh normal plasma, in endogenous hypertriglyceridemic (HTG) patients, in in vitro modified lipoproteins from a normolipidemic donor, and in experimental rats. The results indicated that oxVLDL, oxLDL, and oxHDL occurred in the plasma of HTG patients. Compared with the control group, PT and APTT, incubated with plasma VLDL, LDL, or HDL from HTG patients, respectively, were significantly reduced, while platelet maximal aggregation rates were significantly higher (P < 0.05-0.01). Similar procoagulant profiles were observed in in vitro modified lipoprotein components and in rats with intrinsic hypertriglyceridemia as well. These results support our previous finding that LDL, VLDL, and HDL were all oxidatively modified in vivo in the subjects with HTG, and suggest that procoagulation state may result from the abnormal plasma lipoprotein oxidative modification in vivo.

Confounding effects of platelets on flow cytometric analysis and cell-sorting experiments using blood-derived cells

BACKGROUND

Flow cytometric analysis and cell-sorting of peripheral blood leukocytes is commonplace; however, platelet contamination is typically ignored during immunophenotypic analysis and sorting of blood-derived cells.

METHODS

Red blood cells, platelets, T & B lymphocytes, monocytes, and granulocytes were sorted from rat blood preparations. Presort enrichment was performed by differential centrifugation for all cell types. Additionally, leukocyte samples were prepared by ammonium chloride lysis of red blood cells.

RESULTS

Unless proper precautions were taken, significant numbers of platelets were sorted along with (nonplatelet) cells of interest. The amount of platelet contamination varied greatly from experiment to experiment with the highest level of leukocyte-platelet association observed in the neutrophil/granulocyte population in samples prepared using ammonium chloride-based red blood cell-lysing solution.

CONCLUSIONS

Addition of an immunophenotypic marker for platelet identification is a simple, yet prudent, measure to help evaluate the impact of platelets on immunophenotypic staining when performing flow cytometric analysis or sorting of blood-derived cells and should become a routine practice. Platelet presence in postsort fractions can be due to free platelets as well as target cell-associated platelets and both sources of contamination must be addressed.

Effects of malondialdehyde content in low density lipoproteins on platelet adhesion

Platelets play a key role in thrombotic vascular occlusion at the side of an atherosclerotic plaque. Malondialdehyde (MDA) derivatives are components of oxidized LDL. Recent studies suggest enhanced adhesion of platelets on oxidized LDL, but evidence of MDA-modified LDL contribution to enhanced platelet adhesion is missing. The aim of this study was to investigate platelet adhesion on LDLs with various content of MDA and to analyze its physiological relevance. LDLs were isolated by hydroxyapatite chromatography then analyzed. MDA, vitamin E and vitamin A content was estimated by HPLC methods and adhesion of platelets on modified LDLs was assessed. Both MDA and hypochlorite reactions induced an increase of both malondialdehyde content and negative charge in LDLs preparations. In LDLs of iron-overloaded patients both MDA content and negative charge was also significantly increased. The analysis of data showed that enhanced negative charge of LDLs had only negligible influence on platelet adhesion. The binding of platelets on LDL particles modified by MDA resulted in an S-shape curve and was substantially enhanced by high levels of MDA in LDL preparations. Malondialdehyde content both in hypochlorite modified LDLs and in LDLs of iron-overloaded patient matched the lower part of the binding curve. The platelet adhesion on circulating LDLs modified in in vivo conditions of oxidation stress in blood of iron-overloaded patients depends with considerable certainty preferably on non-MDA derived changes in derivatized LDLs. On the other hand, oxidized LDLs in atherosclerotic lesions can be much more extensively modified by MDA resulting in surfaces with enhanced affinity to adhering platelets. Such LDLs may be accessible to platelets, due to endothelial denudation or plaque rupture.

Fluorine-18 radiolabeling of low-density lipoproteins: a potential approach for characterization and differentiation of metabolism of native and oxidized low-density lipoproteins in vivo

Oxidative modification of low-density lipoprotein (LDL) is regarded as a crucial event in atherogenesis. Assessing the metabolic fate of oxidized LDL (oxLDL) in vivo with radiotracer techniques is hindered by the lack of suitable sensitive and specific radiolabeling methods. We evaluated an improved methodology based on the radiolabeling of native LDL (nLDL) and oxLDL with the positron emitter fluorine-18 ((18)F) by conjugation with N-succinimidyl-4-[(18)F]fluorobenzoate ([(18)F]SFB). We investigated whether radiolabeling of LDL induces adverse structural modifications. Results suggest that radiolabeling of both nLDL and oxLDL using [(18)F]SFB causes neither additional oxidative structural modifications of LDL lipids and proteins nor alteration of their biological activity and functionality, respectively. Thus, radiolabeling of LDL using [(18)F]SFB could prove to be a promising approach for studying the kinetics of oxLDL in vivo.

LDL oxidized by hypochlorous acid causes irreversible platelet aggregation when combined with low levels of ADP, thrombin, epinephrine, or macrophage-derived chemokine (CCL22)

The in vitro oxidation of low-density lipoprotein (LDL) by hypochlorous acid produces a modified form (HOCl-LDL) capable of stimulating platelet function. We now report that HOCl-LDL is highly effective at inducing platelet function, causing stable aggregation and alpha-granule secretion. Such stimulation depended on the presence of low levels of primary agonists such as adenosine diphosphate (ADP) and thrombin, or others like epinephrine (EPI) and macrophage-derived chemokine (MDC, CCL22). Agonist levels, which by themselves induced little or reversible aggregation, caused strong stable aggregation when combined with low levels of HOCl-LDL. Platelet activation by HOCl-LDL and ADP (1 microM) caused P-selectin (CD62P) exposure, without serotonin or adenosine triphosphate (ATP) secretion. Intracellular calcium levels rose slowly (from 100 to 200 nM) in response to HOCl-LDL alone and rapidly when combined with ADP to about 300 nM. p38 mitogen-activated protein kinase (MAPK) became phosphorylated in response to HOCl-LDL alone. This phosphorylation was not blocked by the protein kinase C (PKC) inhibitor bisindolylmaleimide, which reduced the extent of aggregation and calcium increase. However, the p38 MAPK inhibitor SB203580 blocked platelet aggregation and phosphorylation of p38 MAPK. These findings suggest that HOCl-LDL exposed during atherosclerotic plaque rupture, coupled with low levels of primary agonists, can rapidly induce extensive and stable thrombus formation.

Expression of human scavenger receptor B1 on and in human platelets

OBJECTIVE

The abundance of HDL particles correlates inversely with the incidence of coronary heart disease. The human scavenger receptor B1 (hSR-B1/CLA-1) is a receptor for HDL. Expression of hSR-B1/CLA-1 mRNA and protein in human platelets was determined using reverse transcriptase-polymerase chain reaction and Western blot, respectively. Presence of the protein on the surface of platelets was shown using flow cytometry.

METHODS AND RESULTS

Immunochemical staining for hSR-B1/CLA-1 showed that it was expressed in megakaryocytes, the platelet precursors of human bone marrow. These findings prompted us to ask whether hSR-B1/CLA-1 was differentially expressed on platelets obtained from patients with atherosclerotic disease compared with those in control subjects. Our findings showed that abundance of hSR-B1/CLA-1 was significantly reduced on the surface of platelets from patients with atherosclerotic disease. The reduced levels of hSR-B1/CLA-1 were associated with increased cholesterol ester content in platelets from patients with atherosclerotic disease compared with control subjects. A negative correlation existed between hSR-B1/CLA-1 expression and platelet aggregation. In summary, our studies show that the HDL receptor hSR-B1/CLA-1 is expressed in platelets and their precursor, the megakaryocyte. The levels of hSR-B1/CLA-1 expression correlate inversely with cholesterol ester content and platelet aggregation.

CONCLUSIONS

These findings suggest that determining the level of hSR-B1/CLA-1 expression on the platelets may be a useful clinical marker for atherosclerotic diseases.

The influence of platelet-smooth muscle cell interaction on the oxidative modification of low-density lipoprotein

OBJECTIVE

In view of the recognized association between thrombosis and atherosclerosis, it is hypothesized that exposure of arterial smooth muscle cells (SMC) to thrombogenic agents such as platelets and thrombin will alter the oxidation of low-density lipoprotein (LDL) and that this effect may be diminished by thrombin inhibition.

METHODS

Quiescent human aortic SMC in culture were exposed to LDL (40 microg protein/ml) alone or with washed human platelets (5 x 10(6)/ml), thrombin (40 units/ml), or a combination of these agents for 48 h. The media were removed, and both media and cell lysate fractions were assayed for malondialdehyde (MDA) content as an index of oxidation. Isolated platelets exposed to LDL and thrombin were studied in a similar manner to determine their individual oxidative activity. Finally, SMC and platelets were incubated with LDL and varying concentrations of thrombin (10-80 units/ml), both alone and in the presence of the thrombin inhibitors hirudin (u/u), and heparin (u/u), and MDA was measured.

RESULTS

SMC and platelets each demonstrated an ability to oxidize LDL, increasing MDA concentrations by 1.8- (P < 0.05) and 4- (P < 0.01) fold, respectively, compared to lipid-free media. Both platelets (P < 0.05) and thrombin (P < 0.001) enhanced the oxidation of LDL by SMC, while a combination of these two agents resulted in an additive effect (P < 0.001). The SMC lysate fraction showed an increase in oxidative products following exposure to platelets (P < 0.01) but not thrombin, suggesting that platelets stimulated uptake of the oxidized lipid by the SMC. Isolated platelets responded to thrombin with an increase in MDA within the media (P < 0.001). Smooth muscle cells exposed to thrombin also showed a dose-dependent increase in LDL oxidation (P < 0.01). This effect was not altered by hirudin, but was significantly inhibited by heparin (P < 0.05).

CONCLUSIONS

These results indicate that the oxidative potential of SMC and platelets is enhanced by their coincubation and by their concurrent exposure to thrombin. Heparin appears to block thrombin-stimulated oxidation. This interaction could be relevant to the dynamic interaction between atherosclerosis and thrombogenesis.

Modified phosphatidylethanolamine as the active component of oxidized low density lipoprotein promoting platelet prothrombinase activity

We analyzed the influence of the atherogenic oxidized low density lipoproteins (LDL) on the activity of the platelet prothrombinase complex, a major contributor to overall thrombin formation in vivo. Platelet dependent thrombin generation was found to be strongly stimulated by in vitro oxidized LDL. The enhancement was additive to that observed with the platelet agonist thrombin. Oxidized LDL increased the platelet binding of annexin-V, suggesting that the augmented surface exposure of aminophospholipids promoted the prothrombinase activity. All of the stimulatory activity of the oxidized LDL could be recovered in the microemulsions prepared from the lipid portion of the modified particles. Phospholipid vesicles were prepared containing the total lipids of the oxidized LDL but lacking specifically in one lipid component. Following the selective removal of the ethanolamine phospholipids (PE) from the LDL lipids, the platelet-dependent thrombin formation was markedly reduced. Vesicles enriched with the isolated PE fraction alone enhanced the thrombin generation. Analyses with autoxidized phospholipids indicated that oxidation products of unsaturated diacyl-PE were mainly responsible for the increased prothrombinase activity. Oxidized LDL and its PE fraction lost their stimulatory activity after treatment with NaCNBH(3), a chemical reductant of Schiff base adducts. Phospholipid vesicles supplemented with synthetic aldehyde-PE adducts largely reproduced the stimulation of the thrombin generation. We conclude that the oxidized LDL particles elicit a pronounced prothrombotic response by increasing the activity of the platelet prothrombinase complex. Specific oxidative modifications of the LDL-associated ethanolamine phospholipids are mainly responsible for this stimulation.

Activation-dependent surface expression of LOX-1 in human platelets

Lectin-like oxidized LDL receptor-1 (LOX-1) was initially identified as an oxidized LDL receptor in aortic endothelial cells. Here we identified LOX-1 mRNA and protein in human platelets in addition to recent findings on the expression in macrophages and smooth muscle cells. The presence of LOX-1 was further confirmed in the megakaryocytic cell lines. Flow cytometric analyses revealed that LOX-1 was exposed on the surface of platelets in an activation-dependent manner. Consistently, the activation-dependent binding of OxLDL to platelets was mostly inhibited by anti-LOX-1 antibody. Immunohistochemistry of the atherosclerotic plaque from a patient with unstable angina pectoris (UAP) revealed accumulation of LOX-1 protein at the site of thrombus. As LOX-1 recognizes and binds activated platelets, exposure of LOX-1 on activated platelets surface might assist thrombosis formation.

Hypochlorite modified LDL are a stronger agonist for platelets than copper oxidized LDL

Experimental low density lipoprotein (LDL) oxidation is usually performed using trace copper, although the in vivo relevance of this method has been called into question. Such LDL augment adenosine 5'-diphosphate (ADP) induced platelet aggregation, presumably by the action of lipid derived compounds. In striking contrast, we find that LDL oxidized to a comparable extent by hypochlorite, an in vivo occurring oxidant, reveal themselves to be potent promoters of platelet aggregation. Interestingly, hypochlorite modified LDL seem to mediate their influence on human platelets by means of the modified apolipoprotein B-100 (apoB) moiety. Also, the finding that hypochlorite modified albumin is able to trigger platelet aggregation suggests an essential role for hypochlorite modified protein(s) in the process of platelet activation.

Modification of protein moiety of human low density lipoprotein by hypochlorite generates strong platelet agonist

Conflicting reports exist about the effects of mildly or extensively oxidized low density lipoproteins (LDLs) on the reactivity of human platelets. This platelet response is mainly caused by modification of the protein and lipid moiety, giving rise to very differently modified species with hardly predictable properties. The aim of this study was to prepare oxidized LDL with modifications essentially restricted to the protein moiety and to determine the eventual platelet responses. We treated LDL at 0 degrees C for 10 minutes with a 60- to 1000-fold molar excess of sodium hypochlorite in borate buffer in the presence of the radical scavenger butylated hydroxytoluene. Under these conditions, neither fragmentation of apolipoprotein B-100 nor formation of LDL aggregates was observed, and lipid oxidation products did not exceed the amount present in untreated LDLs. The degree of modification and the respective effects on platelet function were highly reproducible. Hypochlorite-modified LDLs act as strong platelet agonists, inducing morphological changes, dense granule release, and irreversible platelet aggregation. The evoked platelet effects are completely suppressed by inhibitors of the phosphoinositide cycle but not by EDTA or acetylsalicylic acid. Most likely, these effects are transmitted via high-affinity binding to a single class of sites, which does not recognize native or acetylated LDL. Obviously, modified lysines, and the secondary lipid modifications derived from them, are not essential for this interaction. We conclude that bioactive oxidized lipids are not directly involved in the stimulation of platelets by hypochlorite-modified LDLs.

Stimulating effect of biologically modified low density lipoproteins on ADP-induced aggregation of washed platelets persists in absence of specific binding

Oxidized low density lipoproteins are closely associated with atherosclerosis and also might be directly involved in thrombosis because they have been shown to mediate a stimulating effect on human platelets. In this work, we used biologically modified low density lipoproteins (i.e., low density lipoproteins sufficiently oxidized to show specificity for the macrophage scavenger receptor system) to examine if specific binding of the oxidized apolipoprotein moiety to the platelet surface is a prerequisite for the platelet-stimulating effects reported by other authors. We find that biologically modified low density lipoproteins show specific binding to human platelets (K(d)=5.83+/-0.4 microg/mL, 3850+/-620 sites/platelet) and strongly augment both ADP- and thrombin-induced aggregation of washed platelets. Maleylated albumin, an antagonist of oxidized low density lipoproteins binding to all currently classified scavenger receptors, is able to reduce platelet oxidized low density lipoproteins binding to background levels. Nevertheless, maleylated albumin is not able to exert any kind of normalizing effect on the augmented ADP-induced aggregation response observed in the presence of biologically modified low density lipoproteins. From these data, we conclude that specific binding of oxidatively modified apolipoprotein B to the platelet surface is not essential to the process of platelet stimulation. Therefore, we conclude that these stimulating effects may be mediated by unidentified compounds formed in the lipid phase of the lipoproteins.