Effect of oxidation on the platelet-activating properties of low-density lipoprotein

Antiplatelet Effects of DMPC-Based Synthetic High-Density Lipoproteins: Exploring Particle Structure and Noncholesterol Efflux Mechanisms

Platelet activation is a key factor in the development of cardiovascular diseases. High-density lipoprotein (HDL) is known for its cardioprotective activities including antithrombotic actions. While HDL mimetics have been explored for their potential to regulate thrombosis, their influence on platelet activity remains unclear. This study explores the capacity of synthetic HDL (sHDL) to modulate platelet function and investigates the underlying mechanisms. We examined the effects of sHDL, formulated with various ApoA1 mimetic peptides (18A, 5A, and 22A) and full-length ApoA1 protein, all complexed with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), on platelet function. DMPC-based sHDL demonstrated pronounced antiplatelet effects across all formulations. Comparison with DMPC micelles showed that all sHDL molecules were more effective, highlighting the crucial role of the protein-phospholipid complex in reducing platelet reactivity. Further analysis revealed that DMPC sHDL dose-dependently inhibited various platelet functions, including aggregation, integrin activation, α-granule secretion, protein kinase C (PKC) activation, and platelet spreading. Mechanistic studies demonstrated that DMPC sHDL's antiplatelet effects are not entirely dependent on cholesterol efflux, despite effectively reducing total platelet cholesterol. Furthermore, sHDL's activity was found to be independent of scavenger receptor BI (SR-BI). Notably, inhibition of the CD36 receptor markedly attenuated sHDL's antiplatelet activity and uptake, suggesting a novel mechanism distinct from that of native HDL. In summary, DMPC sHDL modulates platelet function through a synergistic action between protein and phospholipid components, primarily via CD36 receptor engagement. These insights pave the way for novel antiplatelet therapies utilizing sHDL's distinct properties.

Proteomic biomarkers for noninvasive left atrial appendage thrombus prediction in patients with atrial fibrillation

INTRODUCTION AND OBJECTIVES

The CHA2DS2-VASc score, used to assess the risk of left atrial appendage thrombus (LAAT) formation in patients with atrial fibrillation (AF), has limited predictive value. Moreover, transesophageal echocardiography imaging, the gold standard diagnostic method to identify thrombi, is semi-invasive. Consequently, there is a need for alternative and noninvasive diagnostic methods for LAAT risk assessment.

METHODS

Deep proteomic analysis was conducted in plasma samples from 8 patients with nonvalvular AF, divided into thrombus and control groups (4 patients in each group) based on the presence or absence of LAAT. Biomarkers associated with LAAT were validated using an enzyme-linked immunosorbent assay in a cohort of 179 patients with available clinical, transthoracic, and transesophageal echocardiography data. Predictive models were developed to assess the improvement in LAAT identification.

RESULTS

The LAAT group had higher CHA2DS2-VASc scores, larger LA diameter, and lower LAA flow velocities. Deep proteomic analysis identified 30 differentially expressed proteins, including myosin light chain 4, prenylcysteine oxidase 1 (PCYOX1), and decorin as potential diagnostic biomarkers of LAAT. The model showed that PCYOX1 and decorin provided an area under the curve (AUC) of 0.970 for LAAT prediction compared with 0.672 in a model including the CHA2DS2-VASc score and LAA cauliflower morphology. The incremental value of proteomic biomarkers for LAAT in patients with nonvalvular AF was further confirmed with the net reclassification improvement and integrated discrimination improvement indices.

CONCLUSIONS

Protein levels of PCYOX1 and decorin improve the predictive performance for LAAT in patients with nonvalvular AF.

Effect of lipid-lowering therapy on platelet reactivity in patients treated with and without antiplatelet therapy

Circulating lipoproteins may interact with platelets, increasing platelet sensitivity to aggregating agonists and their tendency towards activation and thrombus formation. In particular, patients with hypercholesterolemia exhibit a higher degree of platelet reactivity compared to normolipidemic. Moreover, accruing evidence report that lipid-lowering therapies can reduce thrombus formation, particularly in the absence of concomitant antiplatelet therapy. However, the underlying biological mechanism(s) explaining these clinical observations are not completely understood. Baseline platelet reactivity and high on-treatment platelet reactivity while on antiplatelet therapy (e.g., aspirin and clopidogrel) are associated with poor clinical outcomes. Therefore, strategies to reduce baseline platelet reactivity or improve the pharmacodynamic profile of antiplatelet therapies are an unmet clinical need. The potential use of lipid-lowering therapies for optimizing platelet reactivity provides several advantages as there is strong evidence that reducing circulating lipoproteins can improve clinical outcomes, and they may avoid the need for potent antiplatelet therapies that, although more effective, are associated with increased bleeding risk. This review will provide a systematic overview of the effects of lipid-lowering therapy on platelet reactivity in patients treated with and without antiplatelet therapy. We will focus on the potential biological mechanism(s) of action and the effect of statins, ezetimibe, proprotein convertase subtilisin/kexin 9 inhibitors, omega-3 fatty acids, and recombinant high-density lipoprotein on platelet reactivity. Ultimately, we will assess the current gaps in the literature and future perspective in the field.

Plasma proteomes of acute myeloid leukemia patients treated with transfusions reveal signatures of inflammation and hemostatic dysregulation

Background

Bone marrow aplasia is a common feature in acute myeloid leukemia (AML) patients during their remission induction treatment, and is associated with potential complications such as bleeding, infection and anemia. Frequent platelet and red cell transfusions are administered to prevent and treat these complications. However, platelet counts are poorly associated with bleeding events in this population. Therefore, plasma protein levels could add valuable insights to improve our understanding of the patient's health state. In this study, we aimed to delineate the plasma proteome, including inflammatory pathways, hemostatic and immune components, of AML patients during treatment with intensive transfusion support.

Methods

We employed unbiased mass spectrometry (MS)-based proteomics on longitudinal plasma samples from 10 AML patients during intensive-transfusion treatment phase with healthy individuals as baseline control.

Results

A total of 450 proteins were quantified in plasma samples from AML patients and healthy controls. Alteration in proteins levels were mainly observed for proteins involved in inflammation (e.g. SAA1 and CRP), and complement (e.g. C9 and MASP2) when comparing AML versus healthy individuals. Correlation analysis revealed additional affected protein dynamics, including proteins associated with coagulation cascade, endopeptidase inhibitors activity and lipoprotein remodeling.

Conclusion

The plasma proteome from AML patients during intensive treatment shows a disbalance in inflammation, endopeptidase inhibitors activity, lipoprotein remodeling, coagulation and complement. These effects and potential associations with bleeding risk will be further studied in a bigger cohort.

Supplementary Information

The online version contains supplementary material available at 10.1186/s41231-024-00189-5.

The Role of CD36/GPIV in Platelet Biology

CD36 (also known as platelet glycoprotein IV) is expressed by a variety of different cell entities, where it possesses functions as a signaling receptor, but additionally acts as a transporter for long-chain fatty acids. This dual function of CD36 has been investigated for its relevance in immune and nonimmune cells. Although CD36 was first identified on platelets, the understanding of the role of CD36 in platelet biology remained scarce for decades. In the past few years, several discoveries have shed a new light on the CD36 signaling activity in platelets. Notably, CD36 has been recognized as a sensor for oxidized low-density lipoproteins in the circulation that mitigates the threshold for platelet activation under conditions of dyslipidemia. Thus, platelet CD36 transduces atherogenic lipid stress into an increased risk for thrombosis, myocardial infarction, and stroke. The underlying pathways that are affected by CD36 are the inhibition of cyclic nucleotide signaling pathways and simultaneously the induction of activatory signaling events. Furthermore, thrombospondin-1 secreted by activated platelets binds to CD36 and furthers paracrine platelet activation. CD36 also serves as a binding hub for different coagulation factors and, thus, contributes to the plasmatic coagulation cascade. This review provides a comprehensive overview of the recent findings on platelet CD36 and presents CD36 as a relevant target for the prevention of thrombotic events for dyslipidemic individuals with an elevated risk for thrombosis.

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.

Independent association of Lp(a) with platelet reactivity in subjects without statins or antiplatelet agents

The physiological effect of Lp(a) on platelet activity is unclear. Previous studies explored the relationship between Lp(a) and platelet aggregation in patients taking statins and antiplatelet agents, but few was conducted in individuals without the bias of those drugs that either influence Lp(a) or platelet activity. The aim of this study was to assess the relationship between Lp(a) levels and platelet aggregation in subjects not taking statins or antiplatelet drugs. A hospital-based cross-sectional study was conducted to investigate the independent contribution of Lp(a) to platelet activity by controlling the effects of potential confounding factors including lipoprotein-associated phospholipase A2 [Lp-PLA2]. Blood samples were collected from 92 subjects without statins or antiplatelet agents from the Second Xiangya Hospital. The univariate correlation analysis showed a significant correlation between AA-induced average aggregation rate [AAR] and ApoB (r = 0.324, P = 0.002), ApoA1 (r = 0.252, P = 0.015), Lp(a) (r = 0.370, P < 0.001), Lp-PLA2 (r = 0.233, P = 0.025) and platelet counts [PLT] (r = 0.389, P < 0.001). Multivariate regression analysis suggested that Lp(a) contributed independently to AA-induced average aggregation rate (β = 0.023, P = 0.027) after controlling for the effects of ApoB, Lp-PLA2 and platelet counts. Lp(a) is positively associated with platelet aggregation independent of Lp-PLA2, which may partly account for the atherothrombotic effect of Lp(a).

Oxidised Low-Density Lipoprotein-Induced Platelet Hyperactivity—Receptors and Signalling Mechanisms

Dyslipidaemia leads to proatherogenic oxidative lipid stress that promotes vascular inflammation and thrombosis, the pathologies that underpin myocardial infarction, stroke, and deep vein thrombosis. These prothrombotic states are driven, at least in part, by platelet hyperactivity, and they are concurrent with the appearancxe of oxidatively modified low-density lipoproteins (LDL) in the circulation. Modified LDL are heterogenous in nature but, in a general sense, constitute a prototype circulating transporter for a plethora of oxidised lipid epitopes that act as danger-associated molecular patterns. It is well-established that oxidatively modified LDL promote platelet activation and arterial thrombosis through a number of constitutively expressed scavenger receptors, which transduce atherogenic lipid stress to a complex array of proactivatory signalling pathways in the platelets. Stimulation of these signalling events underlie the ability of modified LDL to induce platelet activation and blunt platelet inhibitory pathways, as well as promote platelet-mediated coagulation. Accumulating evidence from patients at risk of arterial thrombosis and experimental animal models of disease suggest that oxidised LDL represents a tangible link between the dyslipidaemic environment and increased platelet activation. The aim of this review is to summarise recent advances in our understanding of the pro-thrombotic signalling events induced in platelets by modified LDL ligation, describe the contribution of individual platelet scavenger receptors, and highlight potential future challenges of targeting these pathways.

Prenylcysteine Oxidase 1 (PCYOX1), a New Player in Thrombosis

Prenylcysteine Oxidase 1 (PCYOX1) is an enzyme involved in the degradation of prenylated proteins. It is expressed in different tissues including vascular and blood cells. We recently showed that the secretome from Pcyox1-silenced cells reduced platelet adhesion both to fibrinogen and endothelial cells, suggesting a potential contribution of PCYOX1 into thrombus formation. Here, we show that in vivo thrombus formation after FeCl₃ injury of the carotid artery was delayed in Pcyox1^−/− mice, which were also protected from collagen/epinephrine induced thromboembolism. The Pcyox1^−/− mice displayed normal blood cells count, vascular procoagulant activity and plasma fibrinogen levels. Deletion of Pcyox1 reduced the platelet/leukocyte aggregates in whole blood, as well as the platelet aggregation, the alpha granules release, and the α_IIbβ₃ integrin activation in platelet-rich plasma, in response to adenosine diphosphate (ADP) or thrombin receptor agonist peptide (TRAP). Washed platelets from the Pcyox1^−/− and WT animals showed similar phosphorylation pathway activation, adhesion ability and aggregation. The presence of Pcyox1^−/− plasma impaired agonist-induced WT platelet aggregation. Our findings show that the absence of PCYOX1 results in platelet hypo-reactivity and impaired arterial thrombosis, and indicates that PCYOX1 could be a novel target for antithrombotic drugs.

Effect of Oxidized LDL on Platelet Shape, Spreading, and Migration Investigated with Deep Learning Platelet Morphometry

Platelets are functionally versatile blood cells involved in thrombosis, hemostasis, atherosclerosis, and immune response. Platelet interaction with the immediate microenvironment in blood, vasculature, and tissues alters platelet morphology. The quantification of platelet morphodynamics by geometrical parameters (morphometry) can provide important insights into how platelets sense and respond to stimulatory cues in their vicinity. However, the extraction of platelet shapes from phase contrast microscopy images by conventional image processing is difficult. Here, we used a convolutional neural network (CNN) to develop a deep-learning-based approach for the unbiased extraction of information on platelet morphodynamics by phase contrast microscopy. We then investigated the effect of normal and oxidized low-density lipoproteins (LDL, oxLDL) on platelet morphodynamics, spreading, and haptotactic migration. Exposure of platelets to oxLDL led to a decreased spreading area and rate on fibrinogen, accompanied by increased formation of filopodia and impaired formation of lamellipodia. Haptotactic platelet migration was affected by both LDL and oxLDL in terms of decreased migration velocity and reduced directional persistence. Our results demonstrate the use of deep learning in investigating platelet morphodynamics and reveal differential effects of LDL and oxLDL on platelet morphology and platelet-matrix interaction.

Plasmin-Induced Activation of Human Platelets Is Modulated by Thrombospondin-1, Bona Fide Misfolded Proteins and Thiol Isomerases

Inflammatory processes are triggered by the fibrinolytic enzyme plasmin. Tissue-type plasminogen activator, which cleaves plasminogen to plasmin, can be activated by the cross-β-structure of misfolded proteins. Misfolded protein aggregates also represent substrates for plasmin, promoting their degradation, and are potent platelet agonists. However, the regulation of plasmin-mediated platelet activation by misfolded proteins and vice versa is incompletely understood. In this study, we hypothesize that plasmin acts as potent agonist of human platelets in vitro after short-term incubation at room temperature, and that the response to thrombospondin-1 and the bona fide misfolded proteins Eap and SCN^--denatured IgG interfere with plasmin, thereby modulating platelet activation. Plasmin dose-dependently induced CD62P surface expression on, and binding of fibrinogen to, human platelets in the absence/presence of plasma and in citrated whole blood, as analyzed by flow cytometry. Thrombospondin-1 pre-incubated with plasmin enhanced these plasmin-induced platelet responses at low concentration and diminished them at higher dose. Platelet fibrinogen binding was dose-dependently induced by the C-terminal thrombospondin-1 peptide RFYVVMWK, Eap or NaSCN-treated IgG, but diminished in the presence of plasmin. Blocking enzymatically catalyzed thiol-isomerization decreased plasmin-induced platelet responses, suggesting that plasmin activates platelets in a thiol-dependent manner. Thrombospondin-1, depending on the concentration, may act as cofactor or inhibitor of plasmin-induced platelet activation, and plasmin blocks platelet activation induced by misfolded proteins and vice versa, which might be of clinical relevance.

Highly oxidized low-density lipoprotein does not facilitate platelet aggregation

OBJECTIVE

This study aimed to examine whether oxidized low-density lipoprotein (oxLDL) facilitates platelet aggregation, which is one cause for development of cardiovascular disease.

METHODS

The susceptibility of platelets to aggregation was monitored by light transmittance aggregometry and a laser light scattering method using low-density lipoprotein (LDL) and oxLDL as agonists. β-thromboglobulin (β-TG) levels released from platelets were also measured after incubation with or without oxLDL.

RESULTS

Platelet aggregation was suppressed by oxLDL as estimated by maximum light transmission. Additionally, adenosine diphosphate-induced further aggregation was slightly reduced by the presence of oxLDL. Aggregation levels of a low number of platelets, which was determined by the laser light scattering method, were lower upon addition of oxLDL compared with unoxidized LDL. After a short time of incubation, oxLDL increased secreted β-TG levels in platelet-rich plasma. However, further incubation with oxLDL caused relatively lower secreted β-TG levels compared with incubation with unoxidized LDL. This fluctuation was not due to β-TG degradation by oxLDL.

CONCLUSIONS

Levels of oxLDL in vitro weakly activate platelets at an early stage, but then inhibit platelet function, such as aggregation and β-TG secretion.

Platelet lipidome: Dismantling the "Trojan horse" in the bloodstream

The platelet-lipid chapter in the story of atherothrombosis is an old one, recapitulated and revised in many contexts. For decades several stimulating facets have been added to it, both unraveling and increasing the perplexity of platelet-lipid interplay and its pathophysiological consequences. The recent paradigm shift in our perspective has evolved with lipidomic analysis of the intraplatelet compartment and platelet releasate. These investigations have disclosed that platelets are in constant interaction with circulatory lipids, often reflected in their lipid repertoire. In addition, they offer a shielded intracellular space for oxidative lipid metabolism generating "toxic" metabolites that escape degradation by plasma lipases and antioxidant defense, circulate undetected by conventional plasma lipid profile, and deposited at atherosclerotic lesions or thrombus. Lipidomics divulges this silent invader in platelet vehicles, thereby providing potential biomarkers of pathologic manifestations and therapeutic targets to be exploited, which is surmised in this review.

Dyslipidemia-associated atherogenic oxidized lipids induce platelet hyperactivity through phospholipase Cγ2-dependent reactive oxygen species generation

Oxidized low-density lipoprotein (oxLDL) and associated oxidized phosphocholine-headgroup phospholipids (oxPCs) activate blood platelets through ligation of the scavenger receptor CD36. Previously, we found that oxLDL stimulated phosphorylation of phospholipase Cγ2 (PLCγ2). However, the functional relevance of PLCγ2 phosphorylation in oxLDL-mediated platelet hyperactivity remained elusive. Here, we set out to explore the functional importance of PLCγ2 in oxLDL-mediated platelet activation using human and genetically modified murine platelets. The CD36-specific oxidized phospholipid (oxPC_CD36) triggered the generation of reactive oxygen species (ROS) in platelets under static and arterial flow conditions. The ROS generation in response to oxPC_CD36 was sustained for up to 3 h but ablated in CD36- and PLCγ2-deficient platelets. The functional importance of ROS generation in response to atherogenic lipid stress was examined through measurement of P-selectin expression. OxPC_CD36 induced P-selectin expression, but required up to 60 min incubation, consistent with the timeline for ROS generation. P-selectin expression was not observed in CD36- and PLCγ2-deficient mice. The ability of oxPC_CD36 and oxLDL to stimulate P-selectin expression was prevented by incubation of platelets with the ROS scavenger N-acetyl-cysteine (NAC) and the NOX-2 inhibitor gp91ds-tat, but not with the NOX-1 inhibitor ML171. In summary, we provide evidence that prolonged exposure to oxLDL-associated oxidized phospholipids induces platelet activation via NOX-2-mediated ROS production in a CD36- and PLCγ2-dependent manner.

The potential of chitosan and its derivatives in prevention and treatment of age-related diseases

Age-related, diet-related and protein conformational diseases, such as atherosclerosis, diabetes mellitus, cancer, hypercholesterolemia, cardiovascular and neurodegenerative diseases are common in the elderly population. The potential of chitosan, chitooligosaccharides and their derivatives in prevention and treatment of age-related dysfunctions is reviewed and discussed in this paper. The influence of oxidative stress, low density lipoprotein oxidation, increase of tissue stiffness, protein conformational changes, aging-associated chronic inflammation and their pathobiological significance have been considered. The chitosan-based functional food also has been reviewed.

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.

Oxidized low-density lipoprotein-induced CD147 expression and its inhibition by high-density lipoprotein on platelets in vitro

INTRODUCTION

Matrix metalloproteinases (MMPs) are believed to progressively degrade the collagenous components of the protective fibrous cap, leading to atherosclerotic plaque rupture or destabilization. Oxidized low-density lipoprotein (ox-LDL) enhances the release of CD147, known as the extracellular MMP inducer, from coronary smooth muscle cells. However, whether ox-LDL can induce platelet CD147 expression is unknown. Therefore, we investigated the influence of ox-LDL and high-density lipoprotein (HDL) on CD147 expression on human platelets.

MATERIALS AND METHODS

Washed platelets were incubated with ox-LDL (or native LDL) and HDL or anti-LOX-1 monoclonal antibody prior to incubation with ox-LDL. In parallel, buffer (PBS) was added to washed platelets as a control. The expression levels of CD147, CD62P, CD63 and Annexin V were assessed by flow cytometry, and soluble CD147 from the platelets was assessed by an enzyme-linked immunosorbent assay. Laser scanning microscopy (LSM) and transmission electron microscopy (TEM) were used to visualize the morphological changes and granule release, respectively, from the platelets.

RESULTS

Platelets treated with ox-LDL exhibited a significant increase in the expression of CD147 (or Annexin V), followed by increases in CD62P and CD63, compared with the control group. In contrast, HDL or anti-LOX-1 monoclonal antibody decreased these effects. The expression of soluble CD147 increased as the concentration of ox-LDL used to treat the platelets increased. After exposure to ox-LDL, morphological changes and granule release in the platelets were visualized by LSM and TEM. Additionally, the TEM revealed that HDL inhibits alpha-granule release.

CONCLUSIONS

In platelets, ox-LDL stimulates the release of CD147 via binding to LOX-1, whereas HDL inhibits this effect. This finding could provide new insights concerning the influence of ox-LDL and HDL on plaque stability by the up-regulation of CD147 on platelets.

Signaling role of CD36 in platelet activation and thrombus formation on immobilized thrombospondin or oxidized low-density lipoprotein

BACKGROUND AND OBJECTIVE

Platelets abundantly express glycoprotein CD36 with thrombospondin-1 (TSP1) and oxidized low-density lipoprotein (oxLDL) as proposed ligands. How these agents promote platelet activation is still poorly understood.

METHODS AND RESULTS

Both TSP1 and oxLDL caused limited activation of platelets in suspension. However, immobilized TSP1 and oxLDL, but not LDL, strongly supported platelet adhesion and spreading with a major role of CD36. Platelet spreading was accompanied by potent Ca(2+) rises, and resulted in exposure of P-selectin and integrin activation, all in a CD36-dependent manner with additional contributions of α(IIb) β(3) and ADP receptor stimulation. Signaling responses via CD36 involved activation of the protein tyrosine kinase Syk. In whole blood perfusion, co-coating of TSP1 or oxLDL with collagen enhanced thrombus formation at high-shear flow conditions, with increased expression on platelets of activated α(IIb) β(3), P-selectin and phosphatidylserine, again in a CD36-dependent way.

CONCLUSIONS

Immobilized TSP1 and oxLDL activate platelets partly via CD36 through a Syk kinase-dependent Ca(2+) signaling mechanism, which enhances collagen-dependent thrombus formation under flow. These findings provide novel insight into the role of CD36 in hemostasis.

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.

Molecular and cellular aspects of protein misfolding and disease

Proteins are essential elements for life. They are building blocks of all organisms and the operators of cellular functions. Humans produce a repertoire of at least 30,000 different proteins, each with a different role. Each protein has its own unique sequence and shape (native conformation) to fulfill its specific function. The appearance of incorrectly shaped (misfolded) proteins occurs on exposure to environmental changes. Protein misfolding and the subsequent aggregation is associated with various, often highly debilitating, diseases for which no sufficient cure is available yet. In the first part of this review we summarize the structural composition of proteins and the current knowledge of underlying forces that lead proteins to lose their native structure. In the second and third parts we describe the molecular and cellular mechanisms that are associated with protein misfolding in disease. Finally, in the last part we portray recent efforts to develop treatments for protein misfolding diseases.

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.

Molecular pathways used by platelets to initiate and accelerate atherogenesis

PURPOSE OF REVIEW

The response to injury model in the development of atherosclerosis is broadly accepted by the scientific audience. Platelets are generally not believed to be involved in the initiation of atherosclerosis. New data imply, however, that the response to injury model is too simple for a complete understanding of the inflammatory disease atherosclerosis. The involvement of platelets in the initiation of atherosclerotic lesion formation is critical in directing the atherosclerotic process into regeneration or ongoing vascular injury.

RECENT FINDINGS

Platelets internalize oxidized phospholipids and promote foam cell formation. Platelets also recruit circulating blood cells including progenitor cells to the vessel, that are able to differentiate into foam cells or endothelial cells depending on conditions. Platelets express various scavenger receptors that are able to regulate LDL-uptake. LDL-laden platelets are internalized by adherent progenitor cells that in turn differentiate into macrophages and foam cells.

SUMMARY

An expanding body of evidence continues to build on the role of platelets as initial actors in the development of atherosclerotic lesions. Platelets bind to leukocytes, endothelial cells, and circulating progenitor cells and initiate monocyte transformation into macrophages. Therefore platelets regulate the initiation, development and total extent of atherosclerotic lesions.

Platelet activation by oxidized low density lipoprotein is mediated by CD36 and scavenger receptor-A

OBJECTIVE

The interaction of platelets with low density lipoprotein (LDL) contributes to the development of cardiovascular disease. Platelets are activated by native LDL (nLDL) through apoE Receptor 2' (apoER2')-mediated signaling to p38(MAPK) and by oxidized LDL (oxLDL) through lysophosphatidic acid (LPA) signaling to Rho A and Ca2+. Here we report a new mechanism for platelet activation by oxLDL.

METHODS AND RESULTS

Oxidation of nLDL increases p38(MAPK) activation through a mechanism that is (1) independent of LPA, and (2) unlike nLDL-signaling not desensitized by prolonged platelet-LDL contact or inhibited by receptor-associated protein or chondroitinase ABC. Antibodies against scavenger receptors CD36 and SR-A alone fail to block p38(MAPK) activation by oxLDL but combined blockade inhibits p38(MAPK) by >40% and platelet adhesion to fibrinogen under flow by >60%. Mouse platelets deficient in either CD36 or SR-A show normal p38(MAPK) activation by oxLDL but combined deficiency of CD36 and SR-A disrupts oxLDL-induced activation of p38(MAPK) by >70%.

CONCLUSION

These findings reveal a novel platelet-activating pathway stimulated by oxLDL that is initiated by the combined action of CD36 and SR-A.

Platelets, inflammation and atherosclerosis

An expanding body of evidence continues to build on the role of platelets as initial actors in the development of atherosclerotic lesions. Platelets bind to leukocytes and endothelial cells, and initiate monocyte transformation into macrophages. Platelets internalize oxidized phospholipids and promote foam cell formation. Platelets also recruit progenitor cells to the scene that are able to differentiate into foam cells or endothelial cells depending on conditions. Platelets tip the scales in the initiation, development and total extent of atherosclerotic lesions.

Activation of Human Platelets by Misfolded Proteins

OBJECTIVE

Protein misfolding diseases result from the deposition of insoluble protein aggregates that often contain fibrils called amyloid. Amyloids are found in Alzheimer disease, atherosclerosis, diabetes mellitus, and systemic amyloidosis, which are diseases where platelet activation might be implicated.

METHODS AND RESULTS

We induced amyloid properties in 6 unrelated proteins and found that all induced platelet aggregation in contrast to fresh controls. Amyloid-induced platelet aggregation was independent of thromboxane A2 formation and ADP secretion but enhanced by feedback stimulation through these pathways. Treatments that raised cAMP (iloprost), sequestered Ca2+ (BAPTA-AM) or prevented amyloid-platelet interaction (sRAGE, tissue-type plasminogen activator [tPA]) induced almost complete inhibition. Modulation of the function of CD36 (CD36-/- mice), p38(MAPK) (SB203580), COX-1 (indomethacin), and glycoprotein Ib alpha (Nk-protease, 6D1 antibody) induced approximately 50% inhibition. Interference with fibrinogen binding (RGDS) revealed a major contribution of alphaIIb beta3-independent aggregation (agglutination).

CONCLUSIONS

Protein misfolding resulting in the appearance of amyloid induces platelet aggregation. Amyloid activates platelets through 2 pathways: one is through CD36, p38(MAPK), thromboxane A2-mediated induction of aggregation; the other is through glycoprotein Ib alpha-mediated aggregation and agglutination. The platelet stimulating properties of amyloid might explain the enhanced platelet activation observed in many diseases accompanied by the appearance of misfolded proteins with amyloid.

Effects of rosiglitazone on native low-density-lipoprotein-induced respiratory burst in circulating monocytes and on the leukocyte-endothelial interaction in cholesterol-fed rats

Low-density lipoprotein (LDL) has been implicated in the initiation and progression of atherosclerotic vascular disease. But whether LDL can elicit similar effects in the microcirculation remain unexplored. To approach this issue, the hypothesis that LDL promotes oxidative stress in monocytes and results in microvascular inflammation was tested. Native LDL was capable of stimulating respiratory burst in rat monocytes, and this was blocked by BAPTA, cytochalasin B, apocynin, and diphenyliodonium. In monocytes from rats on a high-cholesterol (4%) diet, increased intracellular calcium, actin polymerization, respiratory burst, and surface CD18 expression were found. Concurrently, leukocyte-endothelial interaction was enhanced in the cremaster microcirculation. Rosiglitazone, an insulin-sensitizing agent with antiinflammatory properties, was found to suppress native-LDL-induced actin polymerization and respiratory burst in monocytes. It also improved leukocytes activation and leukocyte-endothelial interaction due to the high cholesterol intake. Hence, native LDL stimulation of monocytes contributed to hypercholesterolemia-associated microvascular inflammation, which could be treated by rosiglitazone.

Platelet activation by low density lipoprotein and high density lipoprotein

Cardiovascular disease is the main cause of death and disability in the Western society. Lipoproteins are important in the development of cardiovascular disease since they change the properties of different cells involved in atherosclerosis and thrombosis. The interaction of platelets with lipoproteins has been under intense investigation. Particularly the initiation of platelet signaling pathways by low density lipoprotein (LDL) has been studied thoroughly, since platelets of hypercholesterolemic patients, whose plasma contains elevated LDL levels due to absent or defective LDL receptors, show hyperaggregability in vitro and enhanced activity in vivo. These observations suggest that LDL enhances platelet responsiveness. Several signaling pathways induced by LDL have been revealed in vitro, such as signaling via p38 mitogen-activated protein kinase and p125 focal adhesion kinase. High density lipoprotein (HDL) consists of two subtypes, HDL(2) and HDL(3), which have opposing effects on platelet activation. This review provides a summary of the activation of signaling pathways after platelet-LDL and platelet-HDL interaction, with special emphasis on their role in the development of thrombosis and atherosclerosis.

Platelets induce differentiation of human CD34+ progenitor cells into foam cells and endothelial cells

Recruitment of human CD34+ progenitor cells toward vascular lesions and differentiation into vascular cells has been regarded as a critical initial step in atherosclerosis. Previously we found that adherent platelets represent potential mediators of progenitor cell homing besides their role in thrombus formation. On the other hand, foam cell formation represents a key process in atherosclerotic plaque formation. To investigate whether platelets are involved in progenitor cell recruitment and differentiation into endothelial cells and foam cells, we examined the interactions of platelets and CD34+ progenitor cells. Cocultivation experiments showed that human platelets recruit CD34+ progenitor cells via the specific adhesion receptors P-selectin/PSGL-1 and beta1- and beta2-integrins. Furthermore, platelets were found to induce differentiation of CD34+ progenitor cells into mature foam cells and endothelial cells. Platelet-induced foam cell generation could be prevented partially by HMG coenzyme A reductase inhibitors via reduction of matrix metalloproteinase-9 (MMP-9) secretion. Finally, agonists of peroxisome proliferator-activated receptor-alpha and -gamma attenuated platelet-induced foam cell generation and production of MMP-9. The present study describes a potentially important mechanism of platelet-induced foam cell formation and generation of endothelium in atherogenesis and atheroprogression. The understanding and modulation of these mechanisms may offer new treatment strategies for patients at high risk for atherosclerotic diseases.

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.

Lipoprotein-associated proteins involved in platelet signaling

Platelets and lipoproteins are both key elements in the development of atherosclerosis and thrombosis. Based on their density, five classes of lipoproteins have been identified which all influence platelets via distinct mechanisms. The activation of platelets starts with binding of apolipoproteins to different platelet receptors and is followed by the activation of signaling pathways resulting in activation or inhibition of platelet functions like aggregation or secretion. In addition to apolipoproteins, lipoproteins are also associated to a large amount of proteins, enzymes and lipids that also can induce platelet activation or inhibition. This review provides a summary of the activation of signaling pathways after platelet-lipoprotein interactions initiated by lipoprotein-associated proteins and lipids.