Native lipoproteins inhibit platelet activation induced by oxidized lipoproteins

OxLDL sensitizes platelets for increased formation of extracellular vesicles capable of finetuning macrophage gene expression

Platelet extracellular vesicles (PEVs) generated upon platelet activation may play a role in inflammatory pathologies such as atherosclerosis. Oxidized low-density lipoprotein (oxLDL), a well-known contributor to atherogenesis, activates platelets and presensitizes them for activation by other agonists. We studied the effect of oxLDL on the secretion, composition, and inflammatory functions of PEVs using contemporary EV analytics. Platelets were activated by co-stimulation with thrombin (T) and collagen (C) ± oxLDL and characterized by high-resolution flow cytometry, nanoparticle tracking analysis, proximity extension assay, western blot, and electron microscopy. The effect of PEVs on macrophage differentiation and functionality was examined by analyzing macrophage surface markers, cytokine secretion, and transcriptome. OxLDL upregulated TC-induced formation of CD61⁺, P-selectin⁺ and phosphatidylserine⁺ PEVs. Blocking the scavenger receptor CD36 significantly suppressed the oxLDL+TC-induced PEV formation, and HDL caused a slight but detectable suppression. The inflammatory protein cargo differed between the PEVs from stimulated and unstimulated platelets. Both oxLDL+TC- and TC-induced PEVs enhanced macrophage HLA-DR and CD86 expression and decreased CD11c expression as well as secretion of several cytokines. Pathways related to cell cycle and regulation of gene expression, and immune system signaling were overrepresented in the differentially expressed genes between TC PEV -treated vs. control macrophages and oxLDL+TC PEV -treated vs. control macrophages, respectively. In conclusion, we speculate that oxLDL and activated platelets contribute to proatherogenic processes by increasing the number of PEVs that provide an adhesive and procoagulant surface, contain inflammatory mediators, and subtly finetune the macrophage gene expression.

Binding of adenosine derivatives to carrier proteins may reduce their antiplatelet activity

Adenosine analogues have high affinity and selectivity for adenosine receptors (AR), and exhibit anti-platelet activity. Plasma proteins play an important role in the regulation of platelet function and may influence the action of anti-platelet compounds. Little is known about the interactions of AR agonists with plasma proteins. This study investigates the interplay between AR agonists and plasma proteins and the consequences of those interactions. Surface plasmon resonance was employed together with molecular docking study to determine the binding kinetics of four selected ARagonists (PSB0777, Cl-Ado, MRE0094, UK432097) to several carrier proteins and to clarify the nature of these interactions. The influence of a whole plasma and of some plasma components on the effectiveness of ARagonists in the inhibition of platelet function was assessed by flow cytometry (platelet activation) and ELISA (platelet adhesion). Plasma proteins remarkably diminished the effectiveness of ARagonists in inhibiting platelet activation and adhesion in vitro. ARagonists were found to strongly bind to human serum albumin (HSA) and the protein components of lipoproteins - apolipoproteins; HSA was essential for the binding of water-soluble PSB0777, whereas apolipoproteins were needed for interactions with poorly-water soluble compounds such as UK432097 and MRE0094. In addition, HSA was shown to significantly reduce the effectiveness of PSB0777 in inhibiting ADP-induced platelet activation. In conclusion, HSA and lipoproteins are important carriers for ARagonists, which can affect pharmacodynamics of ARagonists used as platelet inhibitors.

Plasma disturbance of phospholipid metabolism in major depressive disorder by integration of proteomics and metabolomics

INTRODUCTION

Major depressive disorder (MDD) is a highly prevalent mental disorder affecting millions of people worldwide. However, a clear causative etiology of MDD remains unknown. In this study, we aimed to identify critical protein alterations in plasma from patients with MDD and integrate our proteomics and previous metabolomics data to reveal significantly perturbed pathways in MDD. An isobaric tag for relative and absolute quantification (iTRAQ)-based quantitative proteomics approach was conducted to compare plasma protein expression between patients with depression and healthy controls (CON).

METHODS

For integrative analysis, Ingenuity Pathway Analysis software was used to analyze proteomics and metabolomics data and identify potential relationships among the differential proteins and metabolites.

RESULTS

A total of 74 proteins were significantly changed in patients with depression compared with those in healthy CON. Bioinformatics analysis of differential proteins revealed significant alterations in lipid transport and metabolic function, including apolipoproteins (APOE, APOC4 and APOA5), and the serine protease inhibitor. According to canonical pathway analysis, the top five statistically significant pathways were related to lipid transport, inflammation and immunity.

CONCLUSION

Causal network analysis by integrating differential proteins and metabolites suggested that the disturbance of phospholipid metabolism might promote the inflammation in the central nervous system.

Secretory phospholipase A2 modified HDL rapidly and potently suppresses platelet activation

Levels of secretory phospholipases A₂ (sPLA₂) highly increase under acute and chronic inflammatory conditions. sPLA₂ is mainly associated with high-density lipoproteins (HDL) and generates bioactive lysophospholipids implicated in acute and chronic inflammatory processes. Unexpectedly, pharmacological inhibition of sPLA₂ in patients with acute coronary syndrome was associated with an increased risk of myocardial infarction and stroke. Given that platelets are key players in thrombosis and inflammation, we hypothesized that sPLA₂-induced hydrolysis of HDL-associated phospholipids (sPLA₂-HDL) generates modified HDL particles that affect platelet function. We observed that sPLA₂-HDL potently and rapidly inhibited platelet aggregation induced by several agonists, P-selectin expression, GPIIb/IIIa activation and superoxide production, whereas native HDL showed little effects. sPLA₂-HDL suppressed the agonist-induced rise of intracellular Ca²⁺ levels and phosphorylation of Akt and ERK1/2, which trigger key steps in promoting platelet activation. Importantly, sPLA₂ in the absence of HDL showed no effects, whereas enrichment of HDL with lysophosphatidylcholines containing saturated fatty acids (the main sPLA₂ products) mimicked sPLA₂-HDL activities. Our findings suggest that sPLA₂ generates lysophosphatidylcholine-enriched HDL particles that modulate platelet function under inflammatory conditions.

Mildly oxidized HDL decrease agonist-induced platelet aggregation and release of pro-coagulant platelet extracellular vesicles

Stored platelet concentrates (PLCs) for therapeutic purpose, develop a platelet storage lesion (PSL), characterized by impaired platelet (PLT) viability and function, platelet extracellular vesicle (PL-EV) release and profound lipidomic changes. Whereas oxidized low-density lipoprotein (oxLDL) activates PLTs and promotes atherosclerosis, effects linked to oxidized high-density lipoprotein (oxHDL) are poorly characterized. PLCs from blood donors were treated with native (nHDL) or mildly oxidized HDL (moxHDL) for 5days under blood banking conditions. Flow cytometry, nanoparticle tracking analysis (NTA), aggregometry, immunoblot analysis and mass spectrometry were carried out to analyze PL-EV and platelet exosomes (PL-EX) release, PLT aggregation, protein expression, and PLT and plasma lipid composition. In comparison to total nHDL, moxHDL significantly decreased PL-EV release by -36% after 5days of PLT storage and partially reversed agonist-induced PLT aggregation. PL-EV release positively correlated with PLT aggregation. MoxHDL improved PLT membrane lipid homeostasis through enhanced uptake of lysophospholipids and their remodeling to corresponding phospholipid species. This also appeared for sphingomyelin (SM) and d18:0/d18:1 sphingosine-1-phosphate (S1P) at the expense of ceramide (Cer) and hexosylceramide (HexCer) leading to reduced Cer/S1P ratio as PLT-viability indicator. This membrane remodeling was associated with increased content of CD36 and maturation of scavenger receptor-B1 (SR-B1) protein in secreted PL-EVs. MoxHDL, more potently than nHDL, improves PLT-membrane lipid homeostasis, partially antagonizes PL-EV release and agonist-induced PLT aggregation. Altogether, this may be the result of more efficient phospho- and sphingolipid remodeling mediated by CD36 and SR-B1 in the absence of ABCA1 on PLTs. As in vitro supplement in PLCs, moxHDL has the potential to improve PLC quality and to prolong storage.

Identification of molecular species of cholesteryl ester hydroperoxides in very low-density and intermediate-density lipoproteins

Background

Oxidation of lipoproteins is thought to play a crucial role in atherogenesis. Role for triglyceride-rich lipoproteins in atherogenesis is unclear. Thus, we aimed to investigate whether cholesteryl ester hydroperoxides (CEOOH) are present in very low-density lipoproteins (VLDL) and intermediate-density lipoproteins (IDL) by using highly sensitive liquid chromatography/mass spectrometry.

Methods

Total lipids were extracted from the plasma of healthy donors ( n = 6) and their fractions of VLDL and IDL. Additional three plasma samples were analysed freshly for CEOOH. Detection and identification of CEOOH was conducted by liquid chromatography/LTQ ion trap mass spectrometry/Orbitrap high mass accuracy mass spectrometry. Authentic standards of CEOOH were used for unequivocal identification on the basis of their mass spectra.

Results

We identified six molecular CEOOH species overall, namely, Ch18:1-OOH, Ch18:2-OOH, Ch18:3-OOH, Ch20:4-OOH, Ch20:5-OOH and Ch22:6-OOH. Of them, Ch18:2-OOH, Ch20:5-OOH, Ch20:4-OOH and Ch22:6-OOH were detected in all IDL samples, while only Ch20:4-OOH was detected in all VLDL samples. All of CEOOH species except for Ch18:3-OOH were detected in plasma, with constant detection of Ch20:5-OOH, and Ch22:6-OOH in all plasma samples.

Conclusion

The presence of CEOOH species in VLDL and IDL was confirmed with the analytical sensitivity of 0.1 pmol, showing the constant appearance of more CEOOH species in IDL than VLDL. This finding might add biochemical evidences of atherogenicity of these lipoproteins. Clinical utility of measuring CEOOH level in these lipoproteins need to be investigated for the risk assessment of the cardiovascular disease.

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.

Inhibitory effects of in vivo oxidized high-density lipoproteins on platelet aggregation: evidence from patients with abetalipoproteinemia

There is evidence that high-density lipoproteins (HDLs) may regulate platelet function, but disparate results exist regarding the effects of oxidized HDLs on platelets. The objective of our study was to determine the role of in vivo oxidized HDLs on platelet aggregation. Platelet aggregation and redox status were investigated in 5 patients with abetalipoproteinemia (ABLP) or homozygous hypobetalipoproteinemia, two rare metabolic diseases characterized by the absence of apolipoprotein B-containing lipoproteins, compared to 5 control subjects. Platelets isolated from plasma of patients with ABLP aggregated 4 to 10 times more than control platelets, depending on the agonist. By contrast, no differences in the extent of platelet aggregation were observed between ABLP platelet-rich plasma (PRP) and control PRP, suggesting the presence of a protective factor in ABLP plasma. ABLP HDLs inhibited agonist-induced platelet aggregation by binding to SR-BI, while control HDLs had no effect. On the other hand, lipoprotein-deficient plasma from patients with ABLP did not inhibit platelet aggregation. Severe oxidative stress was evidenced in patients with ABLP. Compared to control HDLs, ABLP HDLs showed a 40% decrease of α-tocopherol and an 11-fold increased malondialdehyde concentration. These results demonstrate that in vivo oxidized HDLs do not lose their antiaggregatory properties despite oxidation.

Vasculitis, Atherosclerosis, and Altered HDL Composition in Heme-Oxygenase-1-Knockout Mice

To elucidate roles of heme oxygenase-1 (HO-1) in cardiovascular system, we have analyzed one-year-old HO-1-knockout mice. Homozygous HO-1-knockout mice had severe aortitis and coronary arteritis with mononuclear cellular infiltration and fatty streak formation even on a standard chow diet. Levels of plasma total cholesterol and HDL were similar among the three genotypes. However, homozygous HO-1-knockout mice had lower body weight and plasma triglyceride. HO-1-deficiency resulted in alteration of the composition of HDL. The ratio of apolipoprotein AI to AII in HO-1-knockout mice was reduced about 10-fold as compared to wild-type mice. In addition, paraoxonase, an enzyme against oxidative stress, was reduced less than 50% in HO-1-knockout mice. The knockout mice also exhibited significant elevation of plasma lipid hydroperoxides. This study using aged HO-1-knockout mice strengthened the idea that HO-1 functions to suppress systemic inflammation in artery wall and prevents plasma lipid peroxidation.

A novel murine model for non-alcoholic steatohepatitis developed by combination of a high-fat diet and oxidized low-density lipoprotein

Non-alcoholic steatohepatitis (NASH) is the hepatic manifestation of metabolic syndrome that is characterized by steatosis, inflammation, and fibrosis, and may progress to cirrhosis and carcinoma. To investigate its pathogenic processes, we established a novel murine model for NASH by combination of a high-fat diet (HFD) and oxidized low-density lipoprotein (oxLDL). Mice that received HFD for 23 weeks showed hepatic steatosis, slight fibrosis, and a high level of lipid peroxidation compared with a regular diet (RD)-fed mice. Hepatic injury and elevated tumor necrosis factor (TNF)-α mRNA expression were also detected in these mice. Moreover, oxLDL administration to HFD-fed mice during weeks 21-23 not only aggravated hepatic steatosis, fibrosis, and lipid metabolism, but also resulted in intense inflammation, including severe hepatic injury and inflammatory cell infiltration, which are the typical histological features of NASH. Inflammation was accompanied by increased gene expression of TNF-α and interleukin (IL)-6. Additionally, the livers of RD-fed animals treated with oxLDL during weeks 21-23 were characterized by foamy macrophages and inflammatory cell infiltration along with an elevated IL-6 mRNA level. These results suggest that an increased oxidative state, including HFD-induced intracellular lipid peroxidation and its extracellular source from oxLDL, is the actual trigger for hepatic inflammation in which liver injury is mediated by TNF-α and inflammatory cell accumulation is dependent on IL-6. HFD and oxLDL also induced insulin resistance in mice; additionally, oxLDL downregulated insulin secretion. In this model, CD36 overexpression was observed in the hepatocytes of HFD-fed mice and those treated with HFD and oxLDL, and in the hepatic macrophages of RD-fed mice immediately after oxLDL treatment. In vitro experiments indicated a rapid and transient elevation of CD36 on macrophage plasma membrane in response to oxLDL. Our findings demonstrate that CD36 expressed on hepatocytes and hepatic macrophages mediates the pathophysiology of NASH.

HDL scavenger receptor class B type I and platelet function

PURPOSE OF REVIEW

HDL cholesterol levels have been inversely correlated with thrombosis and HDL has been shown to mediate various antithrombotic effects. However, molecular mechanisms underlying the suppressing effect of HDL on platelet reactivity are not completely understood. The present review summarizes the recent advancements in understanding the role played by scavenger receptor class B type I (SR-BI) - an HDL receptor - in modulating platelet function and mediating platelet-HDL interactions.

RECENT FINDINGS

SR-BI is expressed on platelet surface and platelets from SR-BI knockout animals are characterized by increased free-to-total cholesterol ratio, abnormal morphology, increased reactivity to strong platelets agonists, enhanced adherence to immobilized fibrinogen, and a propensity to form arterial thrombi. Crossover incubation experiments and a bone marrow transplantation approach reveal increased wild-type platelet reactivity in plasma from SR-BI mice and normal or decreased SR-BI-deficient platelet reactivity in wild-type plasma. A similar functional platelet phenotype has been observed in human carriers of an SR-BI genetic variant. SR-BI ligands interfere with HDL binding to murine platelets and impede the agonist-induced platelet activation as effectively as native HDL. The inhibitory effects of native HDL, moderately oxidized HDL, and SR-BI ligands are abolished in SR-BI-deficient platelets but not in CD36-deficient platelets.

SUMMARY

SR-BI exerts an indirect influence on platelet reactivity via maintaining normal plasma cholesterol homeostasis. In addition, SR-BI is a functional receptor for native and moderately oxidized HDL on platelets that generates an inhibitory signal for platelet activation.

Regulation of platelet function by class B scavenger receptors in hyperlipidemia

Platelets constitutively express class B scavenger receptors CD36 and SR-BI, 2 closely related pattern recognition receptors best known for their roles in lipoprotein and lipid metabolism. The biological role of scavenger receptors in platelets is poorly understood. However, in vitro and in vivo data suggest that class B scavenger receptors modulate platelet function and contribute significantly to thrombosis by sensing pathological or physiological ligands, inducing prothrombotic signaling, and increasing platelet reactivity. Platelet CD36 recognizes a novel family of endogenous oxidized choline phospholipids that accumulate in plasma of hyperlipidemic mice and in plasma of subjects with low high-density lipoprotein levels. This interaction leads to the activation of specific signaling pathways and promotes platelet activation and thrombosis. Platelet SR-BI, on the other hand, plays a critical role in the induction of platelet hyperreactivity and accelerated thrombosis under conditions associated with increased platelet cholesterol content. Intriguingly, oxidized high-density lipoprotein, an SR-BI ligand, can suppress platelet function. These recent findings demonstrate that platelet class B scavenger receptors play roles in thrombosis in dyslipidemia and may contribute to acute cardiovascular events in vivo in hypercholesterolemia.

High-density lipoproteins, platelets and the pathogenesis of atherosclerosis

1. Prospective and interventional studies demonstrate an inverse relationship between plasma high-density lipoprotein (HDL)-cholesterol and the incidence of coronary artery disease. Although the atheroprotective effects of HDL are usually attributed to the reverse cholesterol transport, in which HDL shuttles cholesterol from cells in the arterial wall to the liver, other mechanisms are also under investigation. 2. Platelets are involved in both the initiation and progression of atherosclerotic lesions. In addition, the formation of thrombi over ruptured atherosclerotic plaques results in the narrowing or complete occlusion of coronary arteries. Current experimental evidence suggests that HDL may exert antiplatelet effects and thereby counteract the development of atherothrombotic vascular disease. 3. In vitro studies show that HDL inhibits agonist-stimulated platelet aggregation, fibrinogen binding, granule secretion and liberation of thromboxane A(2). Inhibitory effects of HDL are mediated, in part, by scavenger receptor type B1 and/or the apolipoprotein E receptor apoER2/LRP8 and are linked to the induction of intracellular signalling cascades encompassing stimulation of protein kinase C, cytoplasmatic alkalization and generation of nitric oxide. 4. Populational studies demonstrate that there is an inverse association between plasma HDL levels and recurrent venous thromboembolism. In addition, HDL-cholesterol has been identified as an independent predictor of acute platelet thrombus formation. The administration of reconstituted HDL particles in humans attenuates ex vivo platelet activation. 5. The present review summarizes recent advances in understanding HDL-platelet interactions and discusses the potential use of HDL-like particles in the therapy of thrombosis.

Oxidized high-density lipoprotein inhibits platelet activation and aggregation via scavenger receptor BI

Numerous studies have reported the presence of oxidatively modified high-density lipoprotein (OxHDL) within the intima of atheromatous plaques as well as in plasma; however, its role in the pathogenesis of thrombotic disease is not established. We now report that OxHDL, but not native HDL, is a potent inhibitor of platelet activation and aggregation induced by physiologic agonists. This antithrombotic effect was concentration and time dependent and positively correlated with the degree of lipoprotein oxidation. Oxidized lipoproteins are known ligands for scavenger receptors type B, CD36 and scavenger receptor B type I (SR-BI), both of which are expressed on platelets. Studies using murine CD36(-/-) or SR-BI(-/-) platelets demonstrated that the antithrombotic activity of OxHDL depends on platelet SR-BI but not CD36. Binding to SR-BI was required since preincubation of human and murine platelets with anti-SR-BI blocking antibody abrogated the inhibitory effect of OxHDL. Agonist-induced aggregation of platelets from endothelial nitric oxide synthase (eNOS)(-/-), Akt-1(-/-), and Akt-2(-/-) mice was inhibited by OxHDL to the same degree as platelets from wild-type (WT) mice, indicating that the OxHDL effect is mediated by a pathway different from the eNOS/Akt pathway. These novel findings suggest that contrary to the prothrombotic activity of oxidized low-density lipoprotein (OxLDL), HDL upon oxidation acquires antithrombotic activity that depends on platelet SR-BI.

Relationship between metabolic syndrome and platelet responsiveness to leptin in overweight and obese patients

OBJECTIVE

To verify whether platelet responsiveness to leptin is associated with metabolic syndrome risk factors.

DESIGN

Cross-sectional study.

SUBJECTS

We studied 169 consecutive patients, mean age=43.6+/-9.9 years, with overweight (N=57) or obesity (N=112).

MEASUREMENTS

Cluster analysis was used to generate three clusters based on platelet responsiveness to increasing doses of leptin. Profiles of metabolic syndrome risk factors of the three clusters were compared by discriminant analysis.

RESULTS

Platelet responsiveness to leptin was absent in cluster 1, whereas cluster 3 had the greatest platelet aggregation response to leptin pre-incubation. Plasma leptin levels significantly decreased from cluster 1 to cluster 3 in both gender. Patients in cluster 2 had an intermediate profile of leptin responsiveness. Highest body mass index (BMI) values were more frequent in non-responders, whereas the prevalence of high waist circumference, as well as hypertriglyceridemia and hypertension, increased with increasing responsiveness to leptin from cluster 1 to cluster 3. Pattern of metabolic syndrome risk factors qualified as group specific in 69.0% of the cluster 1, 54.9% of the cluster 2 and 55.8% of the cluster 3. Circulating leptin, waist circumference, plasma triglycerides and BMI defined distinctive patterns of metabolic syndrome risk factors in the clusters.

CONCLUSIONS

In overweight and obese outpatients, metabolic syndrome risk factors parallel to some extent platelet responsiveness to leptin. Such a correlation involves plasma leptin levels, waist circumference, plasma triglycerides and BMI, and may contribute to the excess risk of cardiovascular events in overweight and obese patients.

Platelet Interaction with Bioactive Lipids Formed by Mild Oxidation of Low-Density Lipoprotein

Oxidation of low-density lipoprotein (LDL) generates pro-inflammatory and pro-thrombotic mediators that play a crucial role in cardiovascular and inflammatory diseases. Mildly oxidized LDL (mox-LDL) and minimally modified LDL (mm-LDL) which escape the uptake of macrophage scavenger receptors accumulate in the atherosclerotic intima. Oxidatively modified LDL is also present within the electronegative LDL fraction in blood, which is elevated in patients at high risk for cardiovascular diseases. Mox-LDL and mm-LDL, but not native LDL are able to induce platelet shape change and aggregation. LDL oxidation generates lipids with platelet stimulatory properties such as lysophosphatidylcholine, certain oxidized phosphatidylcholine molecules, F(2)-isoprostanes and lysophosphatidic acid (LPA). Mox-LDL and mm-LDL are like a Trojan horse carrying these biologically active lipids and attacking cells through activation of physiological receptors and signaling mechanisms. LPA has been identified as the lipid responsible for platelet stimulation by mox-LDL, mm-LDL and also mox-HDL. These lipoproteins activate platelets by stimulating G-protein coupled LPA receptors and a Rho/Rho kinase signaling pathway leading to platelet shape change and subsequent aggregation. LPA-mediated platelet activation might contribute to arterial thrombus formation after rupture of atherosclerotic plaques and to the increased blood thrombogenicity of patients with cardiovascular diseases.

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.

Inhibitory mechanisms of low concentrations of oxidized low-density lipoprotein on platelet aggregation

The intracellular mechanisms underlying oxidized low-density lipoprotein (oxLDL)-signaling pathways in platelets are not yet completely understood. Therefore, the aim of this study was to further examine the effects of oxLDL in prevention of platelet aggregation. In this study, oxLDL concentration-dependently (40-120 microg/ml) inhibited platelet aggregation in human platelet-rich plasma stimulated by agonists. Moreover, oxLDL (40 and 80 microg/ml) markedly decreased the fluorescence intensity of platelet membranes tagged with diphenylhexatriene. Rapid phosphorylation of a protein of Mr 47,000 (P47), a marker of protein kinase C activation, was triggered by PDBu (150 nM). This phosphorylation was markedly inhibited by oxLDL (40 and 80 microg/ml) in phosphorus-32-labeled platelets. In addition, oxLDL (40 and 80 microg/ml) markedly increased levels of cyclic AMP and cyclic AMP-induced vasodilator-stimulated phosphoprotein (VASP) Ser(157) phosphorylation. The thrombin-evoked increase in pHi was inhibited in the presence of oxLDL (40 and 80 microg/ml). These results indicate that the antiplatelet activity of oxLDL may involve the following pathways. (1) oxLDL may initially induce conformational changes in platelet membranes, leading to inhibition of the activation of protein kinase C, followed by inhibition of P47 protein phosphorylation, and intracellular Ca(2+) mobilization. (2) oxLDL also activated formation of cyclic AMP and cyclic AMP-induced VASP Ser(157) phosphorylation, resulting in inhibition of the Na(+)/H(+)exchanger; this leads to reduced intracellular Ca(2+) mobilization, and ultimately to inhibition of platelet aggregation. This study further provides new insights concerning the effects of low concentrations of oxLDL on platelet aggregation.

Low concentration of oxidized low density lipoprotein suppresses platelet reactivity in vitro: an intracellular study

The intracellular mechanisms underlying oxidized low density lipoprotein (oxLDL)-signaling pathways in platelets remain obscure and findings have been controversial. Therefore, we examined the influence of oxLDL in washed human platelets. In this study, oxLDL concentration-dependently (20-100 microg/mL) inhibited platelet aggregation in human platelets stimulated by collagen (1 microg/mL) and arachidonic acid (60 microM), but not by thrombin (0.02 U/mL). The activity of oxLDL was greater at 24 h in inhibiting platelet aggregation than at 12 h. At 24 h, oxLDL concentration-dependently inhibited intracellular Ca2+ mobilization and thromboxane B2 formation in human platelets stimulated by collagen. In addition, at 24 h oxLDL (40 and 80 microg/mL) significantly increased the formation of cyclic AMP, but not cyclic GMP or nitrate. In an ESR study, 24 h-oxLDL (40 microg/mL) markedly reduced the ESR signal intensity of hydroxyl radicals (OH(-)) in both collagen (2 microg/mL)-activated platelets and Fenton reaction (H2O2 + Fe2+). The inhibitory effect of oxLDL may induce radical-radical termination reactions by oxLDL-derived lipid radical interactions with free radicals (such as hydroxyl radicals) released from activated platelets, with a resultant lowering of intracellular Ca2+ mobilization, followed by inhibition of thromboxane A2 formation, thereby leading to increased cyclic AMP formation and finally inhibited platelet aggregation. This study provides new insights concerning the effect of oxLDL in platelet aggregation.

Evaluation of risk for atherosclerosis in Alagille syndrome and progressive familial intrahepatic cholestasis: two congenital cholestatic diseases with different lipoprotein metabolisms

OBJECTIVES

To evaluate the risk for atherosclerosis in Alagille syndrome (AGS) and progressive familial intrahepatic cholestasis (PFIC) on the basis of lipoprotein metabolism and by ultrasonography.

STUDY DESIGN

Five patients with AGS and 5 with PFIC, ages 3 to 4 years, were enrolled. Intimal-medial thickness and wall stiffness of the common carotid artery were examined by ultrasonography. Serum levels of lipids and lipoproteins were determined. Further, the chemical composition of LDL and its ability to transform macrophages into foam cells were determined.

RESULTS

Intimal-medial thickness and wall stiffness were increased in patients with PFIC but not in patients with AGS. Total cholesterol, LDL cholesterol, HDL cholesterol, and lipoprotein X were remarkably increased in patients with AGS, whereas in patients with PFIC, an increase in triglyceride and a decrease in HDL cholesterol were the prominent findings. However, despite the normal LDL cholesterol level, oxidized LDL level was strikingly high in patients with PFIC. LDLs from patients with PFIC had high TG contents and exhibited high abilities to transform macrophages into foam cells.

CONCLUSIONS

These findings suggest that patients with PFIC are at high risk for cardiovascular disorders involving atherosclerosis.

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.

HDL and arteriosclerosis: beyond reverse cholesterol transport

The inverse correlation between serum levels of high density lipoprotein (HDL) cholesterol and the risk of coronary heart disease, the protection of susceptible animals from atherosclerosis by transgenic manipulation of HDL metabolism, and several potentially anti-atherogenic in vitro-properties have made HDL metabolism an interesting target for pharmacological intervention in atheroslcerosis. We have previously reviewed the concept of reverse cholesterol transport, which describes both the metabolism and the classic anti-atherogenic function of HDL (Arterioscler. Thromb. Vasc. Biol. 20 2001 13). We here summarize the current understanding of additional biological, potentially anti-atherogenic properties of HDL. HDL inhibits the chemotaxis of monocytes, the adhesion of leukocytes to the endothelium, endothelial dysfunction and apoptosis, LDL oxidation, complement activation, platelet activation and factor X activation but also stimulates the proliferation of endothelial cells and smooth muscle cells, the synthesis of prostacyclin and natriuretic peptide C in endothelial cells, and the activation of proteins C and S. These anti-inflammatory, anti-oxidative, anti-aggregatory, anti-coagulant, and pro-fibrinolytic activities are exerted by different components of HDL, namley apolipoproteins, enzymes, and even specific phospholipids. This complexity further emphasizes that changes in the functionality of HDL rather than changes of plasma HDL-cholesterol levels determine the anti-atherogenicity of therapeutic alterations of HDL metabolism.

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.

Oxidized low-density lipoprotein (LDL) enhances thromboxane A(2) synthesis by platelets, but lysolecithin as a product of LDL oxidation has an inhibitory effect

Oxidation of low-density lipoprotein (LDL) by copper sulfate led to a significant increase in lysophosphatidylcholine (lyso PC) at the expense of phosphatidylcholine. Incubation of different concentrations of oxidized LDL (oxLDL) (32-650 microg protein/ml) with platelets for 1 h at 37 degrees C increased lyso PC content. The increase was dependent on oxLDL concentration. Incubation of platelets with various concentrations of lyso PC in solution for 5 or 15 min showed that lyso PC percentage was increased in the platelet membrane and the increase was dose dependent. Platelets incubated with various concentrations of lyso PC (2-100 microM) for 5 or 15 min and then triggered with thrombin also showed a significant decrease of thromboxane A(2) (TXA(2)) release as lyso PC concentration reached 10 microM or 6 microM, respectively. The decrease of TXA(2) release was more significant as lyso PC concentration was increased. The present study showed that this inhibition of TXA(2) release by lyso PC was due to 1) inhibition of phospholipase A(2) and the decrease of free arachidonic acid liberation from platelet phospholipid and 2) inhibition of cyclooxygenase. These inhibitory effects of lyso PC were discussed in relation to its effect on membrane fluidity. Lyso PC at concentrations of 30, 50, and 100 microM caused a sudden drop in TXA(2) release and a sudden increase of lactic dehydrogenase loss from the platelets due to their lysis and inhibition of cyclooxygenase enzyme. The present study shows that oxLDL contains high levels of lyso PC that are transferable to the platelets and can weaken their responsiveness to thrombin and decrease TXA(2) release. In our previous study, we found that oxLDL also contained high levels of oxysterols and thiobarbituric acid reactive substances (TBARS), which enhanced platelet reactivity to thrombin and increased TXA(2) release. We conclude that the net effect of oxLDL on platelets will depend on its degree of oxidation and the ratio between oxysterols plus TBARS/lyso PC. Variations in this ratio may explain some of the contradictions cited in the literature concerning the effect of oxLDL on platelet activation.

Exercise shifts the platelet aggregation modulatory role from native to mildly oxidized low-density lipoprotein

PURPOSE

The role of low-density lipoprotein (LDL) lipid peroxides in strenuous exercise-induced changes in platelet function was studied in 30 patients (male/female = 22/8) aged 30-62 yr (mean +/- SD = 508).

METHODS

All subjects were subjected to a treadmill exercise test, using the standard Bruce protocol. Blood samples were collected pre-, peak, and 10 min postexercise to assess hematological and biochemical parameters and platelet aggregation. Ex vivo whole blood platelet aggregation during treadmill exercise was assessed in 10 subjects by adding mildly oxidized LDL.

RESULTS

Preexercise, a correlation existed between plasma thromboxane (TX) levels and plasma LDL cholesterol or beta-thromboglobulin (beta-TG) levels (r = 0.48, P < 0.05: r = 0.47, P < 0.05, respectively), whereas, at peak exercise, TX and beta-TG levels increased, but no correlation was seen. At peak exercise, platelets showed hyperaggregability in terms of maximal amplitude and reaction slope (P < 0.001 and P < 0.01, respectively). In contrast to the increase in plasma lipid peroxide levels seen during peak exercise (P < 0.05), LDL lipid peroxides decreased during exercise, this decrease reaching a statistical significance at 10 min postexercise (P < 0.05). In addition, the ex vivo addition of mildly oxidized LDL (10 mg protein x L(-1)) to peak exercise blood resulted in a significant attenuation of platelet aggregation and a decrease in TX release. At 10 min postexercise, a correlation was seen between LDL lipid peroxides and TX levels (r = 0.78, P < 0.001) or beta-TG levels (r = 0.68, P < 0.005).

CONCLUSION

These results suggest that LDL lipid peroxides play a role in modulating and attenuating platelet aggregation during strenuous exercise.

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.

High density lipoprotein oxidation: in vitro susceptibility and potential in vivo consequences

Elevated levels of plasma high density lipoprotein (HDL) are strongly predictive of protection against atherosclerotic vascular disease. HDL particles likely have several beneficial actions in vivo, including the initiation of reverse cholesterol transport. The apparent importance of oxidative modification of low density lipoprotein in atherogenesis raises the question of how oxidative modification of HDL might affect its cardioprotective actions. HDL is readily oxidized using numerous models of lipoprotein oxidation. In vitro evidence suggests oxidation might impair some protective actions, but actually enhance other mechanisms induced by HDL that prevent the accumulation of cholesterol in the artery wall. This article reviews the current literature concerning the relative oxidizability of HDL, the structural changes induced in HDL by oxidation in vitro, and the potential consequences of oxidative modification on the protective actions of HDL in vivo.

Effects of hypochlorite-modified low-density and high-density lipoproteins on intracellular Ca2+ and plasma membrane Ca(2+)-ATPase activity of human platelets

The presence of hypochlorite-modified lipoproteins in atherosclerotic lesions suggests that HOCl, a naturally occurring oxidant formed by the myeloperoxidase-catalyzed reaction of H2O2 and Cl-, is a candidate for generation of modified lipoproteins in vivo. We have previously demonstrated that Cu(2+)-oxidized LDL inhibits platelet plasma membrane Ca(2+)-ATPase (PMCA) in isolated membranes and causes an increase in cytosolic Ca2+ in resting whole platelets. However, Cu(2+)-oxidized LDL may not be identical in structure and function to the physiologically modified lipoprotein. Since platelet function may be affected by native and modified lipoproteins, the effect of HOCl-modified LDL and HDL3 on platelet PMCA and on the free intracellular Ca2+ concentration ([Ca2+]i) of whole platelets has been investigated. We demonstrate that in contrast to Cu(2+)-oxidized LDL, HOCl-modified LDL and HDL3 stimulate platelet PMCA activity in isolated membranes and that this effect results in a decrease of [Ca2+]i in vivo. Thus, HOCl-oxidation produces modified lipoproteins with the potential for altering platelet function and with properties different from those of the Cu(2+)-oxidized counterparts.

Oxysterols and TBARS are among the LDL oxidation products which enhance thromboxane A2 synthesis by platelets

In this study, we compared the effects of normal LDL (nLDL) and oxidized LDL (oxLDL) on thromboxane (TXA2) release by platelets triggered by low concentration of thrombin, and we determined which component of oxLDL is responsible for that activation. After oxidation of LDL with copper sulfate, the small molecular weight fraction (< 10 kDa) which was high in TBARS was removed; using Amicon Centriprep-10 concentrator membrane. More than 67% of TBARS in the oxLDL preparation was found in solution while the remaining was covalently attached to the oxLDL particles. OxLDL contained significantly higher levels of oxysterols and TBARS than the nLDL. Platelets preincubated with low concentrations of oxLDL (33-132 micrograms protein/mL) produced significantly higher TXA2 than platelets preincubated with equivalent concentrations of nLDL when triggered with thrombin. Platelets treated with oxLDL also contained significantly higher levels of oxysterols than platelets treated with nLDL. Platelets preincubated with pure cholestanetriol (10 micrograms/mL) contained a high level of cholestanetriol in the membrane, and TXA2 release was significantly increased in these platelets compared to the control platelets. The TBARS in solution also was very potent in enhancing TXA2 release by thrombin-treated platelets. These results indicate that oxysterols and the free TBARS either in solution or covalently attached to the oxLDL particles are partly responsible for the stimulatory effect of oxLDL on TXA2 release by platelets. The present study also showed that this enhancement of TXA2 release was due to activation of phospholipase A2 and to the increase of arachidonic acid liberation from the platelet phospholipids.

Lipid oxidation products and vascular function

The endothelium and blood platelets are intimately involved in both the maintenance of vascular tone and in haemostasis. They are also exposed to high concentrations of lipoproteins, either in the plasma or in the sub-endothelial region of the artery wall, and the biological activity of these cells has been shown to be modulated. Oxidative modification of these lipoproteins results in further variations in the properties of these particles in relation to the activities of the endothelium and platelets. These effects and how the work of Hermann Esterbauer on the details of lipoprotein oxidation permitted rapid progress in understanding these phenomena are discussed in this review.