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

Panton-Valentine Leukocidin associated with S. aureus osteomyelitis activates platelets via neutrophil secretion products

Globalization and migration promote the spread of Panton-Valentine leukocidin (PVL)-positive Staphylococcus aureus strains. The toxin PVL is linked to the development of thrombosis in association with osteomyelitis. The mechanisms by which PVL drives thrombosis development are however still unknown. We demonstrate that PVL-damaged neutrophils activate platelets via neutrophil secretion products, such as α-defensins and the myeloperoxidase product HOCl, as well as the formation of HOCl-modified proteins. Neutrophil damage by PVL is blocked by anti-PVL-antibodies, explaining why especially young osteomyelitis patients with a low antibody titre against PVL suffer from thrombotic complications. Platelet activation in the presence of PVL-damaged neutrophils is prevented by α-defensin inhibitors and by glutathione and resveratrol, which are both inhibitors of HOCl-modified protein-induced platelet activation. Remarkably, intravenously infused glutathione also prevents activation of human platelets in an ex vivo assay. We here describe a new mechanism of PVL-neutrophil-platelet interactions, which might be extrapolated to other toxins that act on neutrophils. Our observations may make us think about new approaches to treat and/or prevent thrombotic complications in the course of infections with PVL-producing S. aureus strains.

Apolipoprotein B100 danger-associated signal 1 (ApoBDS-1) triggers platelet activation and boosts platelet-leukocyte proinflammatory responses

Low-density lipoproteins (LDL), occurring in vivo in both their native and oxidative form, modulate platelet function and thereby contribute to atherothrombosis. We recently identified and demonstrated that 'ApoB100 danger-associated signal 1' (ApoBDS-1), a native peptide derived from Apolipoprotein B-100 (ApoB100) of LDL, induces inflammatory responses in innate immune cells. Platelets are critically involved in the development as well as in the lethal consequences of atherothrombotic diseases, but whether ApoBDS-1 has also an impact on platelet function is unknown. In this study we examined the effect of ApoBDS-1 on human platelet function and platelet-leukocyte interactions in vitro. Stimulation with ApoBDS-1 induced platelet activation, degranulation, adhesion and release of proinflammatory cytokines. ApoBDS-1-stimulated platelets triggered innate immune responses by augmenting leukocyte activation, adhesion and transmigration to/through activated HUVEC monolayers, under flow conditions. These platelet-activating effects were sequence-specific, and stimulation of platelets with ApoBDS-1 activated intracellular signalling pathways, including Ca2+, PI3K/Akt, PLC, and p38- and ERK-MAPK. Moreover, our data indicates that ApoBDS-1-induced platelet activation is partially dependent of positive feedback from ADP on P2Y1 and P2Y12, and TxA2. In conclusion, we demonstrate that ApoBDS-1 is an effective platelet agonist, boosting platelet-leukocyte's proinflammatory responses, and potentially contributing to the multifaceted inflammatory-promoting effects of LDL in the pathogenesis of atherothrombosis.

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.

Hypochlorite-oxidized low density lipoproteins reduce production and bioavailability of nitric oxide in RAW 264.7 macrophages by distinct mechanisms

Oxidative modification of low density lipoproteins is thought to play a pivotal role in the development and exacerbation of atherosclerosis and atherogenesis, and is believed to be closely associated with alterations in the vascular production of nitric oxide (NO). Previous work has shown that several products emerging from lipid peroxidation (e.g. lipid hydroperoxides, lysophospholipids, oxidized cholesterol) are able to reduce NO production in macrophages. The naturally occurring oxidant hypochlorite has been shown to be responsible for the in vivo formation of hypochlorite-oxidized LDL and such OxLDL are known to lack lipid peroxidation products. In this work we demonstrate that hypochlorite-oxidized LDL mediate profound effects on lipopolysaccharide-induced nitric oxide production in RAW 264.7 macrophages. By means of the membrane-permeable NO indicator 4,5-diaminofluorescein diacetate, we are able to show decreased levels of intracellular authentic nitric oxide following incubation with hypochlorite-oxidized LDL. The observed effects are dose-dependent and comparable to results obtained in the presence of the NOS inhibitor NG-monomethyl-L-arginine. This marked reduction of intracellular NO is accompanied by a dose-dependent inhibition of inducible nitric oxide synthase (iNOS) protein and mRNA expression. Furthermore, hyp-OxLDL lead to the generation of peroxynitrite, thereby also reducing bioavailability of NO. By mediating these effects on production and bioavailability of NO, hyp-OxLDL might also contribute to atherogenesis by reducing the antiatherogenic effects of nitric oxide.

Platelet-stimulating effects of oxidized LDL are not attributable to toxic properties of the lipoproteins

One prominent feature of oxidized LDL (OxLDL) is their ability to activate human platelets and effects of OxLDL on platelet function have been shown to depend on the chemical mechanisms that form the basis for the oxidation process. In this regard, the possibility that the observed platelet-stimulating properties of OxLDL might be a direct consequence of cytotoxic effects mediated by these lipoproteins merits further investigation, as experimental strategies to overcome the toxic effects of OxLDL towards a variety of different cell types did not yield conclusive results. In the present work, we show that copper-oxidized LDL mediate severe toxic effects towards a macrophage cell line (decrease in both the number of adherent cells and the amount of incorporated tritiated thymidine, induction of apoptosis and subsequent loss of membrane integrity)--effects that are presumably attributable to products emerging from lipid peroxidation. When added to resting human platelets, copper oxidized LDL stimulate platelets but are not able to trigger an aggregation response on their own. In contrast, hypochlorite-oxidized LDL are able to trigger platelet aggregation, but do not mediate toxic effects towards nucleated cells. Even in the absence of exogenous antioxidants, these lipoproteins mediate cytostatic effects but do not negatively affect cell viability. As a conclusion, platelet-activating effects of oxidatively modified LDL are not attributable to toxic properties of the lipoproteins and this finding might expand possibilities for therapeutical intervention.

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.

Modification of low-density lipoprotein by myeloperoxidase-derived oxidants and reagent hypochlorous acid

Substantial evidence supports the notion that oxidative processes contribute to the pathogenesis of atherosclerosis and coronary heart disease. The nature of the oxidants that give rise to the elevated levels of oxidised lipids and proteins, and decreased levels of antioxidants, detected in human atherosclerotic lesions are, however, unclear, with multiple species having been invoked. Over the last few years, considerable data have been obtained in support of the hypothesis that oxidants generated by the heme enzyme myeloperoxidase play a key role in oxidation reactions in the artery wall. In this article, the evidence for a role of myeloperoxidase, and oxidants generated therefrom, in the modification of low-density lipoprotein, the major source of lipids in atherosclerotic lesions, is reviewed. Particular emphasis is placed on the reactions of the reactive species generated by this enzyme, the mechanisms and sites of damage, the role of modification of the different components of low-density lipoprotein, and the biological consequences of such oxidation on cell types present in the artery wall and in the circulation, respectively.

Structural identification of oxidized acyl-phosphatidylcholines that induce platelet activation

Oxidation of low-density lipoprotein (LDL) generates proinflammatory and prothrombotic mediators that may play a crucial role in cardiovascular and inflammatory diseases. In order to study platelet-activating components of oxidized LDL 1-stearoyl-2-arachidonoyl-sn-glycero-3-phosphocholine, a representative of the major phospholipid species in LDL, the 1-acyl-phosphatidylcholines (PC), was oxidized by CuCl(2) and H(2)O(2). After separation by high-performance liquid chromatography, three compounds were detected which induced platelet shape change at low micromolar concentrations. Platelet activation by these compounds was distinct from the pathways stimulated by platelet-activating factor, lyso-phosphatidic acid, lyso-PC and thromboxane A(2), as evidenced by the use of specific receptor antagonists. Further analyses of the oxidized phospholipids by electrospray ionization mass spectrometry structurally identified them as 1-stearoyl-2-azelaoyl-sn-glycero-3-phosphocholine (m/z 694; SAzPC), 1-stearoyl-2-glutaroyl-sn- glycero-3-phosphocholine (m/z 638; SGPC), and 1-stearoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphocholine (m/z 622; SOVPC). These observations demonstrate that novel 1-acyl-PC which had previously been found to stimulate interaction of monocytes with endothelial cells also induce platelet activation, a central step in acute thrombogenic and atherogenic processes.

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.

CXC-chemokines in coronary artery disease: possible pathogenic role of interactions between oxidized low-density lipoprotein, platelets and peripheral blood mononuclear cells

CXC-chemokines may be involved in atherogenesis. Herein we examined the possible role of CXC-chemokines in the inflammatory interactions between oxidized (ox-) low-density lipoprotein (LDL), platelets and peripheral blood mononuclear cells (PBMC) in 15 patients with coronary artery disease (CAD) without 'traditional' risk factors and 15 carefully matched controls. Our main findings were: (a) ox-LDL stimulated the release of the CXC-chemokines interleukin (IL)-8, ENA-78 and GRO-alpha from PBMC, particularly in CAD. (b) In platelets, ox-LDL induced release of ENA-78 and, when combined with SFLLRN, also of GRO-alpha, with significantly higher response in CAD. (c) Platelet-rich plasma, especially when costimulated with ox-LDL, enhanced the release of IL-8 from PBMC, particularly in CAD patients. (d) Freshly isolated PBMC showed markedly increased IL-8 mRNA expression in CAD patients. Our findings suggest enhanced inflammatory interactions between ox-LDL, platelets and PBMC in CAD patients involving CXC-chemokine related mechanisms, possible contributing to atherogenesis in these and other CAD patients.