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

CD36 haplotypes are associated with lipid profile in normal-weight subjects

Background

Dyslipidemia is a common metabolic disorder that may result from abnormalities in the synthesis, processing and catabolism of lipoprotein particles. Disorders of lipoprotein concentrations and elevated concentration of oxidized lipoproteins (oxLDL) are risk factors in the pathogenesis of cardiovascular diseases (CVD). CD36 plays an important role in lipid metabolism and polymorphisms in the CD36 gene are related to cardiovascular risk factors. The purpose of this study was to evaluate whether there is an association between genotypes and haplotypes of five polymorphisms in the CD36 gene with lipid levels in young normal-weight subjects.

Methods

A total of 232 unrelated subjects with normal-weight of 18 to 25 years old (157 women and 75 men) were randomly selected. The lipid profile and glucose levels were measured by enzymatic colorimetric assays. Genotyping of the polymorphisms -33137A/G (rs1984112), -31118G/A (rs1761667), -22674 T/C (rs2151916), 27645 Ins/Del (rs3840546) and 30294G/C (rs1049673) in the CD36 receptor gene was performed by polymerase chain reaction and restriction fragment length polymorphism, linkage disequilibrium analysis among the five polymorphisms and an analysis of haplotype were estimated.

Results

HDL-C levels was lower in men than in women (P = 0.03). However, the median oxLDL levels in men was higher than in women (P = 0.05). There was no significant difference in the levels of TC, TG, LDL-C and glucose (P > 0.05). HDL-C levels were lower in the subjects with TC genotype of polymorphism -22674 T/C (P = 0.04), but the carriers of TT genotype had lower oxLDL levels (P = 0.01). LDL-C levels were higher in young carriers of CC genotype for 30294G/C polymorphism than non-carriers (P = 0.03). The subjects carrying the AATDC haplotype had 3.2 times presumably higher risk of LDL-C > 100 mg/dL than the carrying the AGTIG haplotype (P = 0.02), whereas the subjects carrying the AATIC haplotype had 2.0 times presumably higher risk of TC > 200 mg/dL than the carrying the AGTIC haplotype (P = 0.02).

Conclusion

The study provides evidence of a genetic association of CD36 haplotypes with the variability in LDL-C and TC levels in a sample of normal-weight subjects.

CD36 contributes to malaria parasite-induced pro-inflammatory cytokine production and NK and T cell activation by dendritic cells

The scavenger receptor CD36 plays important roles in malaria, including the sequestration of parasite-infected erythrocytes in microvascular capillaries, control of parasitemia through phagocytic clearance by macrophages, and immunity. Although the role of CD36 in the parasite sequestration and clearance has been extensively studied, how and to what extent CD36 contributes to malaria immunity remains poorly understood. In this study, to determine the role of CD36 in malaria immunity, we assessed the internalization of CD36-adherent and CD36-nonadherent Plasmodium falciparum-infected red blood cells (IRBCs) and production of pro-inflammatory cytokines by DCs, and the ability of DCs to activate NK, and T cells. Human DCs treated with anti-CD36 antibody and CD36 deficient murine DCs internalized lower levels of CD36-adherent IRBCs and produced significantly decreased levels of pro-inflammatory cytokines compared to untreated human DCs and wild type mouse DCs, respectively. Consistent with these results, wild type murine DCs internalized lower levels of CD36-nonadherent IRBCs and produced decreased levels of pro-inflammatory cytokines than wild type DCs treated with CD36-adherent IRBCs. Further, the cytokine production by NK and T cells activated by IRBC-internalized DCs was significantly dependent on CD36. Thus, our results demonstrate that CD36 contributes significantly to the uptake of IRBCs and pro-inflammatory cytokine responses by DCs, and the ability of DCs to activate NK and T cells to produce IFN-γ. Given that DCs respond to malaria parasites very early during infection and influence development of immunity, and that CD36 contributes substantially to the cytokine production by DCs, NK and T cells, our results suggest that CD36 plays an important role in immunity to malaria. Furthermore, since the contribution of CD36 is particularly evident at low doses of infected erythrocytes, the results imply that the effect of CD36 on malaria immunity is imprinted early during infection when parasite load is low.

Low-bone-mass phenotype of deficient mice for the cluster of differentiation 36 (CD36)

Bone tissue is continuously remodeled by bone cells and maintenance of its mass relies on the balance between the processes of resorption and formation. We have reported the expression of numerous scavenger receptors, namely scavenger receptor (SR) class B type I and II (SR-BI and SR-BII), and CD36, in bone-forming osteoblasts but their physiological roles in bone metabolism are still unknown. To unravel the role of CD36 in bone metabolism, we determined the bone phenotype of CD36 knockout (CD36KO) mice and characterized the cell functions of osteoblasts lacking CD36. Weights of CD36KO mice were significantly lower than corresponding wild-type (WT) mice, yet no significant difference was found in femoral nor tibial length between CD36KO and WT mice. Analysis of bone architecture by micro-computed tomography revealed a low bone mass phenotype in CD36KO mice of both genders. Femoral trabecular bone from 1 to 6 month-old CD36KO mice showed lower bone volume, higher trabecular separation and reduced trabeculae number compared to WT mice; similar alterations were noticed for lumbar vertebrae. Plasma levels of osteocalcin (OCN) and N-terminal propeptide of type I procollagen (PINP), two known markers of bone formation, were significantly lower in CD36KO mice than in WT mice, whereas plasma levels of bone resorption markers were similar. Accordingly, histology highlighted lower osteoblast perimeter and reduced bone formation rate. In vitro functional characterization of bone marrow stromal cells and osteoblasts isolated from CD36KO mice showed reduced cell culture expansion and survival, lower gene expression of osteoblastic Runt-related transcription factor 2 (Runx2) and osterix (Osx), as well as bone sialoprotein (BSP) and osteocalcin (OCN). Our results indicate that CD36 is mandatory for adequate bone metabolism, playing a role in osteoblast functions ensuring adequate bone formation.

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.

Oxidized low-density lipoproteins induce rapid platelet activation and shape change through tyrosine kinase and Rho kinase-signaling pathways

Oxidized low-density lipoproteins (oxLDL) generated in the hyperlipidemic state may contribute to unregulated platelet activation during thrombosis. Although the ability of oxLDL to activate platelets is established, the underlying signaling mechanisms remain obscure. We show that oxLDL stimulate platelet activation through phosphorylation of the regulatory light chains of the contractile protein myosin IIa (MLC). oxLDL, but not native LDL, induced shape change, spreading, and phosphorylation of MLC (serine 19) through a pathway that was ablated under conditions that blocked CD36 ligation or inhibited Src kinases, suggesting a tyrosine kinase-dependent mechanism. Consistent with this, oxLDL induced tyrosine phosphorylation of a number of proteins including Syk and phospholipase C γ2. Inhibition of Syk, Ca(2+) mobilization, and MLC kinase (MLCK) only partially inhibited MLC phosphorylation, suggesting the presence of a second pathway. oxLDL activated RhoA and RhoA kinase (ROCK) to induce inhibitory phosphorylation of MLC phosphatase (MLCP). Moreover, inhibition of Src kinases prevented the activation of RhoA and ROCK, indicating that oxLDL regulates contractile signaling through a tyrosine kinase-dependent pathway that induces MLC phosphorylation through the dual activation of MLCK and inhibition of MLCP. These data reveal new signaling events downstream of CD36 that are critical in promoting platelet aggregation by oxLDL.

Ferric chloride-induced murine carotid arterial injury: A model of redox pathology

Ferric chloride (FeCl₃) induced vascular injury is a widely used model of occlusive thrombosis that reports platelet activation in the context of an aseptic closed vascular system. This model is based on redox-induced endothelial cell injury, which is simple and sensitive to both anticoagulant and anti-platelets drugs. The time required for platelet aggregation to occlude blood flow gives a quantitative measure of vascular damage that is pathologically relevant to thrombotic disease. We have refined the traditional FeCl₃-induced carotid artery model making the data highly reproducible with lower variation. This paper will describe our artifices and report the role of varying the oxidative damage by varying FeCl₃ concentrations and exposure. To explore a maximum difference between experimental groups, adjustment of the selected FeCl₃ dose and exposure duration may be necessary.

CD36 and Na/K-ATPase-α1 form a proinflammatory signaling loop in kidney

Proatherogenic, hyperlipidemic states demonstrate increases in circulating ligands for scavenger receptor CD36 (eg, oxidized low-density lipoprotein [oxLDL]) and the Na/K-ATPase (eg, cardiotonic steroids). These factors increase inflammation, oxidative stress, and progression of chronic kidney disease. We hypothesized that diet-induced obesity and hyperlipidemia potentiate a CD36/Na/K-ATPase-dependent inflammatory paracrine loop between proximal tubule cells (PTCs) and their associated macrophages and thereby facilitate development of chronic inflammation and tubulointerstitial fibrosis. ApoE(-/-) and apoE(-/-)/cd36(-/-) mice were fed a high-fat diet for ≤32 weeks and examined for physiologic and histologic changes in renal function. Compared with apoE(-/-), apoE(-/-)/cd36(-/-) mice had improved creatinine clearance and blood pressure which corresponded histologically with less glomerular and tubulointerstitial macrophage accumulation, foam cell formation, oxidant stress, and interstitial fibrosis. Coimmunopreciptation and a cell surface fluorescence-based crosslinking assay showed that CD36 and Na/K-ATPase α-1 colocalized in PTCs and macrophages, and this association was increased by oxLDL or the cardiotonic steroid ouabain. OxLDL and ouabain also increased activation of Src and Lyn in PTCs. Cell-free conditioned medium from PTCs treated with oxLDL or ouabain increased macrophage migration. OxLDL, ouabain, or plasma isolated from high-fat diet-fed mice stimulated reactive oxygen species production in PTCs, which was inhibited by N-acetyl-cysteine, apocynin, or Na/K-ATPase α-1 knockdown. These data suggest that ligands generated in hyperlipidemic states activate CD36 and the Na/K-ATPase and potentiate an inflammatory signaling loop involving PTCs and their associated macrophages, which facilitates the development of chronic inflammation, oxidant stress, and fibrosis underlying the renal dysfunction common to proatherogenic, hyperlipidemic states.

Platelet function in health and disease: from molecular mechanisms, redox considerations to novel therapeutic opportunities

Increased oxidative stress appears to be of fundamental importance in the pathogenesis and development of several disease processes. Indeed, it is well known that reactive oxygen species (ROS) exert critical regulatory functions within the vascular wall, and it is, therefore, plausible that platelets represent a relevant target for their action. Platelet activation cascade (including receptor-mediated tethering to the endothelium, rolling, firm adhesion, aggregation, and thrombus formation) is tightly regulated. In addition to already well-defined platelet regulatory factors, ROS may participate in the regulation of platelet activation. It is already established that enhanced ROS release from the vascular wall can indirectly affect platelet activity by scavenging nitric oxide (NO), thereby decreasing the antiplatelet properties of endothelium. On the other hand, recent data suggest that platelets themselves generate ROS, which may evoke pro-thrombotic responses, triggering many biological processes participating in atherosclerosis initiation, progression, and complication. That oxidative stress may alter platelet function is conceivable when considering that antioxidants play a role in the prevention of cardiovascular disease, although the precise mechanism accounting for changes attributable to antioxidants in atherosclerosis remains unknown. It is possible that the effects of antioxidants may be a consequence of their enhancing or promoting the antiplatelet effects of NO derived from both endothelial cells and platelets. This review focuses on current knowledge regarding ROS-dependent regulation of platelet function in health and disease, and summarizes in vitro and in vivo evidence for their physiological and potential therapeutic relevance.

7-Ketocholesteryl-9-carboxynonanoate enhances the expression of ATP-binding cassette transporter A1 via CD36

BACKGROUND

CD36 signal transductions have been reported by a variety of lipid moiety of oxidized low-density lipoprotein (oxLDL), however, CD36 signal induced by 7-ketocholesteryl-9-carboxynonanoate (oxLig-1), a lipid moiety of oxLDL has not been elucidated.

METHODS AND RESULTS

OxLig-1 leads to activation and recruitment of Src kinase Fyn, Lyn and caveolin-1 to CD36 in J774A.1 cells, but not in CD36 knockdown cells. The mitogen-activated protein (MAP) kinases c-Jun N-terminal kinase 2 (JNK2) and extracellular signal-regulated protein kinase 1 and 2 (ERK1/2) are specifically phosphorylated in J774A.1 cells after treatment with oxLig-1 and inhibited by pretreatment of Src inhibitor, AG1879. The expression of ABCA1 is up-regulated by treatment with oxLig-1in J774A.1 cells, and the increased expression of ABCA1 is dramatically down-regulated by pretreatment with pharmacological inhibitors of ERK (PD98059) and JNK (SP600125).

CONCLUSIONS

The specific CD36 signal induced by oxLig-1 initiated the activation of Fyn, Lyn, caveolin-1, JNK2 and ERK1/2, and resulted in the up-regulation of ABCA1.

Comparative Studies of Vertebrate Platelet Glycoprotein 4 (CD36)

Platelet glycoprotein 4 (CD36) (or fatty acyl translocase [FAT], or scavenger receptor class B, member 3 [SCARB3]) is an essential cell surface and skeletal muscle outer mitochondrial membrane glycoprotein involved in multiple functions in the body. CD36 serves as a ligand receptor of thrombospondin, long chain fatty acids, oxidized low density lipoproteins (LDLs) and malaria-infected erythrocytes. CD36 also influences various diseases, including angiogenesis, thrombosis, atherosclerosis, malaria, diabetes, steatosis, dementia and obesity. Genetic deficiency of this protein results in significant changes in fatty acid and oxidized lipid uptake. Comparative CD36 amino acid sequences and structures and CD36 gene locations were examined using data from several vertebrate genome projects. Vertebrate CD36 sequences shared 53-100% identity as compared with 29-32% sequence identities with other CD36-like superfamily members, SCARB1 and SCARB2. At least eight vertebrate CD36 N-glycosylation sites were conserved which are required for membrane integration. Sequence alignments, key amino acid residues and predicted secondary structures were also studied. Three CD36 domains were identified including cytoplasmic, transmembrane and exoplasmic sequences. Conserved sequences included N- and C-terminal transmembrane glycines; and exoplasmic cysteine disulphide residues; TSP-1 and PE binding sites, Thr92 and His242, respectively; 17 conserved proline and 14 glycine residues, which may participate in forming CD36 'short loops'; and basic amino acid residues, and may contribute to fatty acid and thrombospondin binding. Vertebrate CD36 genes usually contained 12 coding exons. The human CD36 gene contained transcription factor binding sites (including PPARG and PPARA) contributing to a high gene expression level (6.6 times average). Phylogenetic analyses examined the relationships and potential evolutionary origins of the vertebrate CD36 gene with vertebrate SCARB1 and SCARB2 genes. These suggested that CD36 originated in an ancestral genome and was subsequently duplicated to form three vertebrate CD36 gene family members, SCARB1, SCARB2 and CD36.

Thymidine phosphorylase inhibits vascular smooth muscle cell proliferation via upregulation of STAT3

Dysregulated growth and motility of vascular smooth muscle cells (VSMC) play important role in obstructive vascular diseases. We previously reported that gene transfer of thymidine phosphorylase (TP) into rat VSMC inhibits cell proliferation and attenuates balloon injury induced neointimal hyperplasia; however, the mechanism remains unclear. The current study identified a signaling pathway that mediates effect of TP inhibited VSMC proliferation with a TP activity-dependent manner. Rat VSMC overexpressing human TP gene (C2) or control empty vector (PC) were used. Serum stimulation induced constitutive STAT3 phosphorylation at tyrosine705 in C2 cell but not in PC, which was independent of JAK2 signaling pathway. Inhibition of Src family kinases activity inhibited STAT3 phosphorylation in C2 cells. Lyn activity was higher in C2 cell than in PC. SiRNA based gene knockdown of Lyn significantly decreased serum induced STAT3 phosphorylation in C2 and dramatically increased proliferation of this cell, suggesting that Lyn plays a pivotal role in TP inhibited VSMC proliferation. Unphosphorylated STAT3 (U-STAT3) expression was significantly increased in C2 cells, which may be due to the increased STAT3 transcription. Gene transfection of mouse wild-type or Y705F mutant STAT3 into PC cell or mouse primary cultured VSMC significantly reduced proliferation of these cells, suggesting that overexpression of U-STAT3 inhibits VSMC proliferation. We conclude that Lyn mediates TP induced STAT3 activation, which subsequently contributes to upregulate expression of U-STAT3. The U-STAT3 plays a critical role in inhibiting VSMC proliferation.

Advanced glycation end products induce a prothrombotic phenotype in mice via interaction with platelet CD36

Diabetes mellitus has been associated with platelet hyperreactivity, which plays a central role in the hyperglycemia-related prothrombotic phenotype. The mechanisms responsible for this phenomenon are not established. In the present study, we investigated the role of CD36, a class-B scavenger receptor, in this process. Using both in vitro and in vivo mouse models, we demonstrated direct and specific interactions of platelet CD36 with advanced glycation end products (AGEs) generated under hyperglycemic conditions. AGEs bound to platelet CD36 in a specific and dose-dependent manner, and binding was inhibited by the high-affinity CD36 ligand NO(2)LDL. Cd36-null platelets did not bind AGE. Using diet- and drug-induced mouse models of diabetes, we have shown that cd36-null mice had a delayed time to the formation of occlusive thrombi compared with wild-type (WT) in a FeCl(3)-induced carotid artery injury model. Cd36-null mice had a similar level of hyperglycemia and a similar level of plasma AGEs compared with WT mice under this condition, but WT mice had more AGEs incorporated into thrombi. Mechanistic studies revealed that CD36-dependent JNK2 activation is involved in this prothrombotic pathway. Therefore, the results of the present study couple vascular complications in diabetes mellitus with AGE-CD36-mediated platelet signaling and hyperreactivity.

Binding of oxidized low-density lipoprotein on circulating platelets is increased in patients with acute coronary syndromes and induces platelet adhesion to vascular wall in vivo--brief report

OBJECTIVE

Hyperlipidemia is associated with platelet hyperactivity. In the present study, we evaluated the binding of oxidized low-density lipoprotein (oxLDL) on the surface of circulating platelets in patients with stable coronary artery disease and acute coronary syndromes and its possible association with platelet activation. Furthermore, the role of oxLDL binding on platelet adhesion to collagen and endothelial cells in vitro as well as after carotid ligation in mice was investigated.

METHODS AND RESULTS

Using flow cytometry, patients with acute coronary syndromes (n=174) showed significantly enhanced oxLDL binding compared with patients with stable coronary artery disease (n=182; P=0.007). Platelet-bound oxLDL positively correlated with the degree of platelet activation (expression of P-selectin and activated fibrinogen receptor; P<0.001 for both). Plasma oxLDL was increased in patients with acute coronary syndromes compared with stable angina pectoris patients. Preincubation of isolated platelets with oxLDL, but not with native LDL, resulted in enhanced platelet adhesion to collagen and activated endothelial cells under high shear stress in vitro, as well as after carotid ligation in C57BL/6J mice and apolipoprotein E(-/-) mice fed a high cholesterol diet.

CONCLUSIONS

Increased platelet-bound oxLDL in patients with acute coronary syndromes may play an important role in atherothrombosis, thus providing a potential future therapeutic target.

Plasma levels of soluble CD36, platelet activation, inflammation, and oxidative stress are increased in type 2 diabetic patients

Inflammation, oxidative stress, and platelet activation are involved in type 2 diabetes and its complications. Soluble CD36 (sCD36) has been proposed to early identify diabetics at risk of accelerated atherothrombosis. We aimed at characterizing the platelet contribution to sCD36 in diabetes, by correlating its concentration with the extent of platelet-mediated inflammation and in vivo lipid peroxidation and investigating the effects of low-dose aspirin on these processes. A cross-sectional comparison of sCD36, soluble CD40L (sCD40L) reflecting platelet-mediated inflammation, urinary 11-dehydro-TxB(2), and 8-iso-PGF(2α), in vivo markers of platelet activation and lipid peroxidation, was performed among 200 diabetic patients (94 of them on aspirin 100mg/day) and 47 healthy controls. sCD36 levels (median [IQR]: 0.72 [0.31-1.47] vs 0.26 [0.2-0.37], P=0.003) and urinary 11-dehydro-TxB(2) levels (666 [293-1336] vs 279 [160-396], P≤0.0001) were significantly higher in diabetic patients not on aspirin (n=106) than in healthy subjects. These variables were significantly lower in aspirin-treated diabetics than untreated patients (P<0.0001). Among patients not on aspirin, those with long-standing diabetes (>1 year) had significantly higher sCD36 levels in comparison to patients with diabetes duration <1 year (1.01 [0.62-1.86] vs 0.44 [0.22-1.21], P=0.001). sCD36 linearly correlated with sCD40L (rho=0.447; P=0.0001). On multiple regression analysis, 11-dehydro-TxB(2) (β=0.360; SEM=0.0001, P=0.001), 8-iso-PGF(2α) (β=0.469; SEM=0.0001, P<0.0001), and diabetes duration (β=0.244; SEM=0.207, P=0.017) independently predicted sCD36 levels. sCD36, platelet activation, inflammation, and oxidative stress are increased in type 2 diabetes. Future studies are needed to elucidate if the incomplete down-regulation of sCD36 by low-dose aspirin implies that sCD36 may be derived from tissues other than platelets or if additional antiplatelet strategies in diabetes are necessary to interrupt CD36-dependent platelet activation.

Oxidized low-density lipoprotein-dependent platelet-derived microvesicles trigger procoagulant effects and amplify oxidative stress

The fundamental mechanisms that underlie platelet activation in atherothrombosis are still obscure. Oxidative stress is involved in central features of atherosclerosis. Platelet-derived microvesicles (PMVs) could be important mediators between oxidative stress and platelet activation. CD36 could be a receptor of PMVs, thus generating a PMV-CD36 complex. We aimed to investigate the detailed pathway by which oxidative damage contributes to platelet activation by the PMV-CD36 complex. We found that oxidized low-density lipoprotein stimulated the generation of PMVs. PMVs enhanced normal platelet activation, as assessed by the expression of integrin α(IIb)β₃, secretion of soluble P-selectin and platelet aggregation, but CD36-deficient platelets were not activated by PMVs. The function of the PMV-CD36 complex was mediated by the MKK4/JNK2 signaling axis. Meanwhile, PMVs increased the level of 8-iso-prostaglandin-F2α, a marker of oxidative stress, in a CD36- and phosphatidylserine-dependent manner. We concluded that PMVs are important mediators between oxidative stress and platelet activation. PMVs and CD36 may be effective targets for preventing platelet activation in cardiovascular diseases.

S100 proteins in cartilage: role in arthritis

S100 proteins are low molecular weight calcium binding proteins expressed in vertebrates. The family constitutes 21 known members that are expressed in several tissues and cell types and play a major role in various cellular functions. Uniquely, members of the S100 family have both intracellular and extracellular functions. Several members of the S100 family (S100A1, S100A2, S100A4, S1008, S100A9, S100A11, and S100B) have been identified in human articular cartilage, and their expression is upregulated in diseased tissue. These S100 proteins elicit a catabolic signaling pathway via receptor for advanced glycation end products (RAGE) in cartilage and may promote progression of arthritis. This review summarizes our current understanding of the role of S100 proteins in cartilage biology and in the development of arthritis.

CD9 tetraspanin interacts with CD36 on the surface of macrophages: a possible regulatory influence on uptake of oxidized low density lipoprotein

CD36 is a type 2 scavenger receptor with multiple functions. CD36 binding to oxidized LDL triggers signaling cascades that are required for macrophage foam cell formation, but the mechanisms by which CD36 signals remain incompletely understood. Mass spectrometry analysis of anti-CD36 immuno-precipitates from macrophages identified the tetraspanin CD9 as a CD36 interacting protein. Western blot showed that CD9 was precipitated from mouse macrophages by anti-CD36 monoclonal antibody and CD36 was likewise precipitated by anti-CD9, confirming the mass spectrometry results. Macrophages from cd36 null mice were used to demonstrate specificity. Membrane associations of the two proteins on intact cells was analyzed by confocal immunofluorescence microscopy and by a novel cross linking assay that detects proteins in close proximity (<40 nm). Functional significance was determined by assessing lipid accumulation, foam cell formation and JNK activation in wt, cd9 null and cd36 null macrophages exposed to oxLDL. OxLDL uptake, lipid accumulation, foam cell formation, and JNK phosphorylation were partially impaired in cd9 null macrophages. The present study demonstrates that CD9 associates with CD36 on the macrophage surface and may participate in macrophage signaling in response to oxidized LDL.

Lipid rafts and redox regulation of cellular signaling in cholesterol induced atherosclerosis

Redox mediated signaling mechanisms play crucial roles in the pathogenesis of several cardiovascular diseases. Atherosclerosis is one of the most important disorders induced mainly by hypercholesterolemia. Oxidation products and related signaling mechanisms are found within the characteristic biomarkers of atherosclerosis. Several studies have shown that redox signaling via lipid rafts play a significant role in the regulation of pathogenesis of many diseases including atherosclerosis. This review attempts to summarize redox signaling and lipid rafts in hypercholesterolemia induced atherosclerosis.

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.

Arterial thrombus formation in cardiovascular disease

The pathogenesis of arterial thrombosis is complex and dynamic. Unlike venous thrombi, arterial thrombi typically form under conditions of high blood flow and are mainly composed of platelet aggregates, giving them the appearance of 'white clots'. Strong evidence suggests that arterial thrombi originate as a consequence of an injured atherosclerotic plaque, and that their formation involves the release of prothrombotic material (such as tissue factor), platelet aggregation, and platelet adhesion to the vascular wall. The initially labile platelet plaque is then stabilized by insoluble fibrin produced upon activation of the coagulation cascade. Inherited genetic factors (gene polymorphisms) and acquired predisposing conditions (such as the concentration and activity of clotting factors) can influence both the composition and the size of an arterial thrombus. Further research is needed to elucidate the functions of blood coagulation proteins and cellular elements that are critical to the pathogenesis of arterial thrombosis. This Review explains mechanisms of pathological arterial thrombus formation and discusses genetic and acquired risk factors of atherothrombosis.

Platelet CD36 surface expression levels affect functional responses to oxidized LDL and are associated with inheritance of specific genetic polymorphisms

CD36 modulates platelet function via binding to oxidized LDL (oxLDL), cell-derived microparticles, and thrombospondin-1. We hypothesized that the level of platelet CD36 expression may be associated with inheritance of specific genetic polymorphisms and that this would determine platelet reactivity to oxLDL. Analysis of more than 500 subjects revealed that CD36 expression levels were consistent in individual donors over time but varied widely among donors (200-14,000 molecules per platelet). Platelet aggregometry and flow cytometry in a subset of subjects with various CD36 expression levels revealed a high level of correlation (r² = 0.87) between platelet activation responses to oxLDL and level of CD36 expression. A genome-wide association study of 374 white subjects from the Cleveland Clinic ASCLOGEN study showed strong associations of single nucleotide polymorphisms in CD36 with platelet surface CD36 expression. Most of these findings were replicated in a smaller subset of 25 black subjects. An innovative gene-based genome-wide scan provided further evidence that single nucleotide polymorphisms in CD36 were strongly associated with CD36 expression. These studies show that CD36 expression on platelets varies widely, correlates with functional responses to oxLDL, and is associated with inheritance of specific CD36 genetic polymorphisms, and suggest that inheritance of specific CD36 polymorphisms could affect thrombotic risk.

Contribution of the P2Y12 receptor-mediated pathway to platelet hyperreactivity in hypercholesterolemia

BACKGROUND

In hypercholesterolemia, platelets demonstrate increased reactivity and promote the development of cardiovascular disease.

OBJECTIVE

This study was carried out to investigate the contribution of the ADP receptor P2Y12-mediated pathway to platelet hyperreactivity due to hypercholesterolemia.

METHODS

Low-density lipoprotein receptor-deficient mice and C57Bl/6 wild-type mice were fed on normal chow and high-fat (Western or Paigen) diets for 8 weeks to generate differently elevated cholesterol levels. P2Y12 receptor-induced functional responses via G(i) signaling were studied ex vivo when washed murine platelets were activated by 2MeSADP and PAR4 agonist AYPGKF in the presence and absence of indomethacin. Platelet aggregation and secretion, α(IIb)β(3) receptor activation and the phosphorylation of extracellular signal-regulated protein kinase (ERK) and Akt were analyzed.

RESULTS

Plasma cholesterol levels ranged from 69 ± 10 to 1011 ± 185 mg dL(-1) depending on diet in mice with different genotypes. Agonist-dependent aggregation, dense and α-granule secretion and JON/A binding were gradually and significantly (P < 0.05) augmented at low agonist concentration in correlation with the increasing plasma cholesterol levels, even if elevated thromboxane generation was blocked. These functional responses were induced via increased levels of G(i) -mediated ERK and Akt phosphorylation in hypercholesterolemic mice vs. normocholesterolemic animals. In addition, blocking of the P2Y12 receptor by AR-C69931MX (Cangrelor) resulted in strongly reduced platelet aggregation in mice with elevated cholesterol levels compared with normocholesterolemic controls.

CONCLUSIONS

These data revealed that the P2Y12 receptor pathway was substantially involved in platelet hyperreactivity associated with mild and severe hypercholesterolemia.

Hypertension with elevated levels of oxidized low-density lipoprotein and anticardiolipin antibody in the circulation of premenopausal Indian women chronically exposed to biomass smoke during cooking

This study aims to investigate whether indoor air pollution (IAP) from biomass fuel use was associated with hypertension, platelet hyperactivity, and elevated levels of oxidized low-density lipoprotein (oxLDL) and anticardiolipin antibody (aCL). We enrolled 244 biomass fuel-using (median age 34 year) and 236 age-matched control women who cooked with liquefied petroleum gas (LPG). Enzyme-linked immunosorbent assay was used to measure oxLDL in plasma and aCL in serum, flow cytometry for P-selectin expression on platelet and reactive oxygen species (ROS) generation by leukocytes, aggregometry for platelet aggregation, spectrophotometry for superoxide dismutase (SOD) in erythrocytes, and laser photometer for particulate matter <10 and 2.5 μm in diameter (PM(10) and PM(2.5), respectively) in cooking areas. Biomass users had three times more particulate pollution in kitchen, had higher prevalence of hypertension (29.5 vs. 11.0% in control, P < 0.05), elevated oxLDL (170.6 vs. 45.9 U/l; P < 0.001), platelet P-selectin expression (9.1% vs. 2.4%), platelet aggregation (23.2 vs. 15.9 Ohm), raised aCL IgG (28.7% vs. 2.1%), IgM (8.6% of vs. 0.4%), and ROS (44%) but depleted (13%) SOD. After controlling potential confounders, the changes were positively associated with PM(10) and PM(2.5) in indoor air, suggesting a positive association between IAP and increased cardiovascular risk.

PRACTICAL IMPLICATIONS

The study showing high risk of developing cardiovascular diseases (CVD) among poor, underprivileged women in their reproductive ages in rural India is important from public health perspectives. It may motivate the government and the regulatory agencies of the country to take a serious note of the indoor air pollution (IAP) from biomass fuel use as it threatens the health of millions of women, children, and the elderly who mostly stay indoor. We hope the findings will strengthen the demand for setting up a standard for indoor air quality in the country in the line of national ambient air quality standard. The findings may also inspire the authorities to take measures for the reduction in IAP by improving housing, kitchen ventilation, and cook stoves. Moreover, the parameters used in this study can be utilized for large, population-based studies to identify women at a higher risk of developing CVD so that medical intervention can be taken at the formative stage of a disease.

Lipopolysaccharide signaling without a nucleus: kinase cascades stimulate platelet shedding of proinflammatory IL-1β-rich microparticles

Platelets contain unspliced heteronuclear IL-1β RNA, which is rapidly spliced and translated upon activation. LPS is a superior agonist for this atypical platelet response, but how LPS induces proinflammatory cytokine production in anucleate cells lacking NF-κB is unknown. Platelets express functional TLR4, and stimulation by LPS induced rapid splicing, translation, and secretion of mature IL-1β after caspase-1 processing. LPS stimulated microparticle shedding, and secreted IL-1β was exclusively present in these particles. Microparticles from LPS-stimulated platelets induced VCAM-1 production by cultured human endothelial cells, and blockade of endothelial IL-1β receptor with IL-1 receptor antagonist completely suppressed endothelial activation. Splicing was posttranscriptional as the SR kinase inhibitor TG003 blocked IL-1β RNA production by platelets, but not by monocytes, and was dependent on exogenous CD14--a property of platelets. We used a combination of small-molecule inhibitors, cell-penetrating chimeric peptide inhibitors, and gene-targeted animals to show splicing required MyD88 and TIRAP, and IRAK1/4, Akt, and JNK phosphorylation and activation. Traf6 couples MyD88 to the Akt pathway and, remarkably, a Traf6 interacting peptide-antennapedia chimera was more effective than LPS in stimulating IL-1β splicing. The Traf6 chimera did not, however, stimulate microparticle shedding, nor was IL-1β released. We conclude LPS-induced kinase cascades are sufficient to alter cellular responses, that three signals emanate from platelet TLR4, and that Akt and JNK activation are sufficient to initiate posttranscriptional splicing while another event couples microparticle shedding to TLR4 activation. Platelets contribute to the inflammatory response to LPS through production of microparticles that promote endothelial cell activation.

Vav guanine nucleotide exchange factors link hyperlipidemia and a prothrombotic state

Platelet hyperactivity associated with hyperlipidemia contributes to development of a pro-thrombotic state. We previously showed that oxidized LDL (oxLDL) formed in the setting of hyperlipidemia and atherosclerosis initiated a CD36-mediated signaling cascade leading to platelet hyperactivity. We now show that the guanine nucleotide exchange factors Vav1 and Vav3 were tyrosine phosphorylated in platelets exposed to oxLDL. Pharmacologic inhibition of src family kinases abolished Vav1 phosphorylation by oxLDL in vitro. Coimmunoprecipitations revealed the tyrosine phosphorylated form of src kinase Fyn was associated with Vav1 in platelets exposed to oxLDL. Using a platelet aggregation assay, we demonstrated that Vav1 deficiency, Fyn deficiency, or Vav1/Vav3 deficiency protected mice from diet-induced platelet hyperactivity. Furthermore, flow cytometric analysis revealed that Vav1/Vav3 deficiency significantly inhibited oxLDL-mediated integrin αIIbβIII activation of platelets costimulated with ADP. Finally, we showed with an in vivo carotid artery thrombosis model that genetic deletion of Vav1 and Vav3 together may prevent the development of occlusive thrombi in mice fed a high-fat diet. These findings implicate Vav proteins in oxLDL-mediated platelet activation and suggest that Vav family member(s) may act as critical modulators linking a prothrombotic state and hyperlipidemia.

Scavenger receptors: Implications in atherothrombotic disorders

Scavenger receptors are modified lipoprotein binding receptors, expressed on the surface of a variety of cells including endothelial, macrophages and platelets. The most extensively studied class B scavenger receptors comprise of CD36 and SR-BI and have been found to bind to native and modified LDL. Interaction of modified LDL to CD36 accelerates foam cell formation, the key step in atherosclerotic plaque deposition. Recently scavenger receptors have also been implicated in thrombosis. Platelet CD36 serves as a sensor of oxidative stress and modulator of platelet reactivity under hyperlipidemic conditions thus, inducing prothrombotic signals. In contrast, targeting platelet SR-BI corresponds to reduce platelet hyperreactivity in hyperlipidemia suggesting that targeting these receptors could be a promising strategy for the treatment of atherothrombotic disorders.

A CD36-dependent pathway enhances macrophage and adipose tissue inflammation and impairs insulin signalling

Aims

Obesity and hyperlipidaemia are associated with insulin resistance (IR); however, the mechanisms responsible remain incompletely understood. Pro-atherogenic hyperlipidaemic states are characterized by inflammation, oxidant stress, and pathophysiologic oxidized lipids, including ligands for the scavenger receptor CD36. Here we tested the hypothesis that the absence of CD36 protects mice from IR associated with diet-induced obesity and hyperlipidaemia.

Methods and results

Adipose tissue from CD36^−/− mice demonstrated a less inflammatory phenotype and improved insulin signalling in vivo and at the level of the adipocyte and macrophage. The pathophysiologic ligand oxidized low-density lipoprotein (oxLDL) activated c-Jun N-terminal kinase (JNK) and disrupted insulin signalling in both adipocytes and macrophages in a CD36-dependent manner. Macrophages isolated from CD36^−/− mice after high-fat diet feeding elicited less JNK activation and inhibition of insulin signalling in adipocytes after co-culture compared with wild-type macrophages.

Conclusion

These data suggest that a CD36-dependent inflammatory paracrine loop between adipocytes and macrophages facilitates chronic inflammation and contributes to IR common in obesity and dyslipidaemia.

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.

CD36 participates in a signaling pathway that regulates ROS formation in murine VSMCs

CD36 is a membrane glycoprotein expressed on platelets, monocytes, macrophages, and several other cell types that was recently demonstrated to be involved in platelet activation in response to oxidized phospholipids, including oxidized LDL. Although the role of CD36 in other vascular cells has not been well defined, previous studies have demonstrated that cd36-knockout (cd36-/-) mice have prolonged thrombosis times after vascular injury, which can be protective in the state of hyperlipidemia. Here, we found significantly less ROS in the vessel walls of cd36-/- mice compared with WT after chemically induced arterial injury, suggesting that CD36 may contribute to ROS generation in the VSMCs themselves. Gene expression analysis revealed that the antioxidant enzymes peroxiredoxin-2 (Prdx2) and heme oxygenase-1 were upregulated in cd36-/- VSMCs. Molecular dissection of the pathway in isolated mouse VSMCs revealed CD36 ligand-dependent induction of Fyn phosphorylation, with subsequent phosphorylation and degradation of the redox-sensitive transcription factor Nrf2. Chromatin immunoprecipitation experiments further showed that Nrf2 directly occupied the Prdx2 promoter. The importance of this pathway was evidenced by increased ROS generation in prdx2-/- mice and decreased thrombosis times in both prdx2-/- and nrf2-/- mice after vascular injury. These data suggest that CD36-mediated downregulation of antioxidant systems in VSMCs may contribute to its prothrombotic, proinflammatory, and atherogenic effects.

Serum soluble CD36, assessed by a novel monoclonal antibody-based sandwich ELISA, predicts cardiovascular mortality in dialysis patients

AIM

Accelerated atherosclerosis is a characteristic feature of chronic kidney disease (CKD). CD36 is a scavenger receptor which contributes to foam cell formation, an early crucial step in atherosclerosis development. Recently, a soluble form of CD36 (sCD36) has been discovered. The aim of the study was to develop an ELISA method for quantitative sCD36 evaluation and to measure it in a cohort of CKD stage 5 patients.

METHOD

A novel monoclonal antibody-based sandwich ELISA for sCD36 evaluation was developed and verified by repeated optimization procedures. Serum concentration of sCD36 was then analyzed in a cohort of 228 CKD stage 5 patients prior to dialysis initiation. Additionally, samples from a control group of 73 healthy, age and gender-matched subjects were evaluated.

RESULTS

The novel CD36 ELISA assay had a recovery of at least 90%, and intra- and inter-assay variability of 6 and 11%, respectively. Concentration of serum sCD36 in CKD patients was significantly increased as compared to controls, and associated with the use of HMG-CoA reductase inhibitors (statins) and the presence of diabetes mellitus (DM). Patients above the 75th percentile of sCD36 concentration were at increased risk of 3-year cardiovascular mortality, as compared to the rest of the cohort [HR 2.85 (1.09-7.59) p=0.03].

CONCLUSION

For the first time, sCD36 was assessed quantitatively in a group of patients and showed associations with DM, CKD, and statin use. Furthermore, the concentration of sCD36 predicted cardiovascular mortality in CKD stage 5 patients.

Thrombospondin-1 induces platelet activation through CD36-dependent inhibition of the cAMP/protein kinase A signaling cascade

Cyclic adenosine monophosphate (cAMP)-dependent signaling modulates platelet function at sites of vascular injury. Here we show that thrombospondin-1 (TSP-1) prevents cAMP/protein kinase A (PKA) signaling through a CD36-dependent mechanism. Prostaglandin E₁ (PGE₁) induced a robust inhibition of both platelet aggregation and platelet arrest under physiologic conditions of flow. Exogenous TSP-1 reduced significantly PGE₁-mediated inhibition of both platelet aggregation and platelet arrest. TSP-1 prevented PGE₁-stimulated cAMP accrual and phosphorylation of PKA substrates, through a mechanism requiring phosphodiesterase3A. TSP-1 also inhibited VASP phosphorylation stimulated by the nonhydrolyzable cAMP analog, 8-bromo-cAMP, indicating that it may regulate cAMP-mediated activation of PKA. The inhibitory effect of TSP-1 on cAMP signaling could be reproduced with a peptide possessing a CD36 binding sequence of TSP-1, while the effects of TSP-1 were prevented by a CD36 blocking antibody. TSP-1 and the CD36 binding peptide induced phosphorylation of Src kinases, p38 and JNK. Moreover, inhibition of Src kinases blocked TSP-1-mediated regulation of cAMP concentrations and the phosphorylation of VASP, indicating that TSP-1 modulated the cAMP/PKA signaling events through a tyrosine kinase-dependent pathway downstream of CD36. These data reveal a new role for TSP-1 in promoting platelet aggregation through modulation of the cAMP-PKA signaling pathway.

Our expanding view of platelet functions and its clinical implications

Platelets are the primary cell mediator of thrombosis. A deficiency of platelets can result in severe bleeding defects. "Overactive" platelets contribute to life-threatening outcomes in diseases such as heart attack, stroke, and cancer. The use of platelet inhibitors for thrombosis prevention must therefore seek a delicate balance between inhibiting platelet activation and an associated increased bleeding risk. There are currently few platelet inhibitors clinically available, making the search for novel anti-platelet drug targets a major research priority. Several newly identified pathways of platelet activation may hold hope in this area. In addition, important roles for platelets beyond hemostasis have been discovered. Platelets have recently been described as mediators of diverse inflammatory diseases such as arthritis, hepatitis, malaria, and atherosclerosis. This has heightened the need to broaden our understanding of platelet functions and platelet-derived inflammatory mediators. It has also heightened interest in a continued search for new platelet inhibitors and presents new opportunities for platelet inhibitors to be used in a wide array of disease treatment strategies.

Platelet JNK1 is involved in secretion and thrombus formation

The role of c-Jun NH2-terminal kinase 1 (JNK1) in hemostasis and thrombosis remains unclear. We show here, with JNK1-deficient (JNK1−/−) mice, that JNK1 plays an important role in platelet biology and thrombus formation. In tail-bleeding assays, JNK1−/− mice exhibited longer bleeding times than wild-type mice (396 ± 39 seconds vs 245 ± 32 seconds). We also carried out in vitro whole-blood perfusion assays on a collagen matrix under arterial shear conditions. Thrombus formation was significantly reduced for JNK1−/− platelets (51%). In an in vivo model of thrombosis induced by photochemical injury to cecum vessels, occlusion times were 4.3 times longer in JNK1−/− arterioles than in wild-type arterioles. Moreover, in vitro studies carried out in platelet aggregation conditions demonstrated that, at low doses of agonists, platelet secretion was impaired in JNK1−/− platelets, leading to altered integrin αIIbβ3 activation and reduced platelet aggregation, via a mechanism involving protein kinase C. JNK1 thus appears to be essential for platelet secretion in vitro, consistent with its role in thrombus growth in vivo. Finally, we showed that ERK2 and another isoform of JNK affect platelet aggregation through 2 pathways, one dependent and another independent of JNK1.

Generation and biological activities of oxidized phospholipids

Glycerophospholipids represent a common class of lipids critically important for integrity of cellular membranes. Oxidation of esterified unsaturated fatty acids dramatically changes biological activities of phospholipids. Apart from impairment of their structural function, oxidation makes oxidized phospholipids (OxPLs) markers of "modified-self" type that are recognized by soluble and cell-associated receptors of innate immunity, including scavenger receptors, natural (germ line-encoded) antibodies, and C-reactive protein, thus directing removal of senescent and apoptotic cells or oxidized lipoproteins. In addition, OxPLs acquire novel biological activities not characteristic of their unoxidized precursors, including the ability to regulate innate and adaptive immune responses. Effects of OxPLs described in vitro and in vivo suggest their potential relevance in different pathologies, including atherosclerosis, acute inflammation, lung injury, and many other conditions. This review summarizes current knowledge on the mechanisms of formation, structures, and biological activities of OxPLs. Furthermore, potential applications of OxPLs as disease biomarkers, as well as experimental therapies targeting OxPLs, are described, providing a broad overview of an emerging class of lipid mediators.

Mechanisms of cell signaling by the scavenger receptor CD36: implications in atherosclerosis and thrombosis

CD36 is a multifunctional membrane receptor present on mononuclear phagocytes, platelets, and other cells that serves as a scavenger receptor for oxidized phospholipids, apoptotic cells and certain microbial pathogens. On macrophages, CD36 interaction with oxidized LDL (oxLDL) triggers a signaling response that is pro-inflammatory and pro-atherogenic. The signaling pathway involves activation of src-family kinases, MAP kinases, and Vav family guanine nucleotide exchange factors and results in ligand internalization, foam cell formation and inhibition of migration. On platelets, CD36 interaction with oxLDL and cell-derived microparticles transduces intracellular signals that render them more reactive to low concentrations of classical agonists. In vitro studies and in vivo experiments in CD36 null mice have revealed an important mechanistic role for CD36 in atherosclerosis and thrombosis. Identification of the precise CD36 signaling pathways in specific cells elicited in response to specific ligands may yield novel targets for drug development in athero-thrombotic disorders.

Oxidative risk for atherothrombotic cardiovascular disease

In the vasculature, reactive oxidant species, including reactive oxygen, nitrogen, or halogenating species, and thiyl, tyrosyl, or protein radicals may oxidatively modify lipids and proteins with deleterious consequences for vascular function. These biologically active free radical and nonradical species may be produced by increased activation of oxidant-generating sources and/or decreased cellular antioxidant capacity. Once formed, these species may engage in reactions to yield more potent oxidants that promote transition of the homeostatic vascular phenotype to a pathobiological state that is permissive for atherothrombogenesis. This dysfunctional vasculature is characterized by lipid peroxidation and aberrant lipid deposition, inflammation, immune cell activation, platelet activation, thrombus formation, and disturbed hemodynamic flow. Each of these pathobiological states is associated with an increase in the vascular burden of free radical species-derived oxidation products and, thereby, implicates increased oxidant stress in the pathogenesis of atherothrombotic vascular disease.

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

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

Platelet kainate receptor signaling promotes thrombosis by stimulating cyclooxygenase activation

RATIONALE

Glutamate is a major signaling molecule that binds to glutamate receptors including the ionotropic glutamate receptors; kainate (KA) receptor (KAR), the N-methyl-d-aspartate receptor, and the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor. Each is well characterized in the central nervous system, but glutamate has important signaling roles in peripheral tissues as well, including a role in regulating platelet function.

OBJECTIVE

Our previous work has demonstrated that glutamate is released by platelets in high concentrations within a developing thrombus and increases platelet activation and thrombosis. We now show that platelets express a functional KAR that drives increased agonist induced platelet activation.

METHODS AND RESULTS

KAR induced increase in platelet activation is in part the result of activation of platelet cyclooxygenase in a mitogen-activated protein kinase-dependent manner. Platelets derived from KAR subunit knockout mice (GluR6(-/-)) are resistant to KA effects and have a prolonged time to thrombosis in vivo. Importantly, we have also identified polymorphisms in KAR subunits that are associated with phenotypic changes in platelet function in a large group of whites and blacks.

CONCLUSIONS

Our data demonstrate that glutamate regulation of platelet activation is in part cyclooxygenase-dependent and suggest that the KAR is a novel antithrombotic target.

Inflammation, atherosclerosis, and arterial thrombosis: role of the scavenger receptor CD36

The CD36 scavenger receptor recognizes oxidized low-density lipoprotein (LDL) and cell-derived microparticles. It is expressed on macrophages and platelets and is a mediator of both atherogenesis and thrombosis. Macrophages from CD36-null mice have a defect in foam cell formation in response to exposure to oxidized LDL, and CD36-null mice fed an atherogenic Western diet have significantly less atherosclerosis than their wild-type counterparts. On platelets, CD36 recognition of oxidized LDL contributes to their activation and provides a mechanistic link between hyperlipidemia, oxidant stress, and the prothrombotic state. Cell-derived microparticles are also major ligands for CD36 and contribute to thrombus formation in a CD36-dependent manner even in the absence of hyperlipidemia. CD36 deficiency in mice is associated with inhibition of thrombus formation and with a reduction in microparticle accumulation in thrombi. Targeting CD36 is a promising avenue for the treatment of atheroinflammatory disorders.

Dietary cholesterol plays a role in CD36-mediated atherogenesis in LDLR-knockout mice

OBJECTIVE

CD36 has been shown to play a role in atherosclerosis in the apolipoprotein E-knockout (apoE(o)) mouse. We observed no difference in aortic lesion area between Western diet (WD)-fed LDLR(o) and LDLR(o)/CD36(o) mice. The objective was to understand the mechanism of CD36-dependent atherogenesis.

METHODS AND RESULTS

ApoE(o) mice transplanted with bone marrow from LDLR(o)/CD36(o) mice had significantly less aortic lesion compared with those transplanted with LDLR(o) marrow. Reciprocal macrophage transfer into hyperlipidemic apoE(o) and LDLR(o) animals showed that foam cell formation induced by in vivo modified lipoproteins was dependent on the lipoprotein, not macrophage type. LDLR(o) and LDLR(o)/CD36(o) mice were fed a cholesterol-enriched diet (HC), and we observed significant lesion inhibition in LDLR(o)/CD36(o) mice. LDL/plasma isolated from HC-fed LDLR(o) mice induced significantly greater jnk phosphorylation, cytokine release, and reactive oxygen species secretion than LDL/plasma from WD-fed LDLR(o) mice, and this was CD36-dependent. HC-fed LDLR(o) mice had higher circulating levels of cytokines than WD-fed mice.

CONCLUSIONS

These data support the hypothesis that CD36-dependent atherogenesis is contingent on a proinflammatory milieu that promotes the creation of specific CD36 ligands, not solely hypercholesterolemia, and may explain the greater degree/accelerated rate of atherosclerosis observed in syndromes associated with inflammatory risk.

Platelet hyperreactivity, scavenger receptors and atherothrombosis

Scavenger receptors (SRs) were initially identified as macrophage receptors that recognize modified lipoproteins. The lists of SRs, their ligands and cells expressing SRs have been significantly extended during the last two decades. What has become clear is that many ligands of SRs are present in vivo only in pathologic conditions. Several SRs have been identified on platelets with the best studied being scavenger receptors CD36 and SR-BI. Platelet SRs are multiligand receptors with properties of pattern recognition receptors. CD36 and SR-BI are exposed on resting platelets, while other SRs are rapidly expressed upon platelet activation. Thus, platelets may serve as sensors of 'pathologic ligands' in circulation. The role of platelet SRs in platelet physiology is still poorly understood. However, the data are accumulating that SR ligands, present in the circulation under pathologic conditions, interact with platelet SR and modulate platelet reactivity, thereby contributing to thrombosis and cardiovascular pathology.

CD36, a scavenger receptor involved in immunity, metabolism, angiogenesis, and behavior

CD36 is a membrane glycoprotein present on platelets, mononuclear phagocytes, adipocytes, hepatocytes, myocytes, and some epithelia. On microvascular endothelial cells, CD36 is a receptor for thrombospondin-1 and related proteins and functions as a negative regulator of angiogenesis. On phagocytes, through its functions as a scavenger receptor recognizing specific oxidized phospholipids and lipoproteins, CD36 participates in internalization of apoptotic cells, certain bacterial and fungal pathogens, and modified low-density lipoproteins, thus contributing to inflammatory responses and atherothrombotic diseases. CD36 also binds long-chain fatty acids and facilitates their transport into cells, thus participating in muscle lipid utilization, adipose energy storage, and gut fat absorption and possibly contributing to the pathogenesis of metabolic disorders, such as diabetes and obesity. On sensory cells, CD36 is involved in insect pheromone signaling and rodent fatty food preference. The signaling pathways downstream of CD36 involve ligand-dependent recruitment and activation of nonreceptor tyrosine kinases, specific mitogen-activated protein kinases, and the Vav family of guanine nucleotide exchange factors; modulation of focal adhesion constituents; and generation of intracellular reactive oxygen species. CD36 in many cells is localized in specialized cholesterol-rich membrane microdomains and may also interact with other membrane receptors, such as tetraspanins and integrins. Identification of the precise CD36 signaling pathways in specific cells elicited in response to specific ligands may yield novel targets for drug development.

The role of the scavenger receptor CD36 in regulating mononuclear phagocyte trafficking to atherosclerotic lesions and vascular inflammation

AIMS

CD36 has been shown to associate with non-receptor Src kinases to activate mitogen-activated protein kinases and trigger cytoskeletal remodelling, important events in foam cell formation and macrophage migration. Yet, its role in regulating circulating mononuclear phagocyte trafficking to atherosclerotic lesions has not been investigated. The aim of the present study was to investigate the role of CD36 in modulating the recruitment of mononuclear phagocytes to the arterial wall and the associated vascular inflammation, using both pharmacological and genetic approaches.

METHODS AND RESULTS

Apolipoprotein E-deficient (apoE(-/-)) mice fed a high-fat, high-cholesterol diet were treated daily with a CD36 ligand, EP 80317 (300 microg/kg), or 0.9% NaCl for 6 or 12 weeks. Forty-eight hours before sacrifice, mice were injected iv with (111)Indium-labelled macrophages. A 65% (P < 0.001) reduction of labelled macrophage accumulation at aortic lesions was observed in EP 80317-treated mice, mainly at the level of the aortic arch and iliac arteries, correlating with a 43% reduction of atherosclerotic lesion areas. This was associated with reduced phosphorylation of the focal adhesion kinase Pyk2 following stimulation with oxidized phospholipid in a Src kinase- and CD36-dependent manner. At the vascular level, EP 80317 treatment reduced the expression of pro-inflammatory proteins, including NADPH oxidase, inducible nitric oxide synthase, vascular endothelial cell adhesion molecule-1, and CCL2 chemokine. Plasma IL-6 levels were also reduced by 40% (P < 0.05). In contrast, none of these proteins was modulated in EP 80317-treated apoE/CD36 double knockout (apoE(-/-)/CD36(-/-)) mice.

CONCLUSION

Our results support a role for CD36 signalling in the regulation of mononuclear phagocyte trafficking to atherosclerotic-prone sites and in the associated vascular wall inflammation.