Platelet Activation by Oxidized Low Density Lipoprotein Is Mediated by Cd36 and Scavenger Receptor-A

Abstract

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 2Ј (apoER2Ј)-mediated signaling to p38 MAPK and by oxidized LDL (oxLDL) through lysophosphatidic acid (LPA) signaling to Rho A and Ca 2ϩ . Here we report a new mechanism for platelet activation by oxLDL. Methods and Results-Oxidation of nLDL increases p38MAPK 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 receptorassociated 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 Ͼ40% and platelet adhesion to fibrinogen under flow by Ͼ60%. 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 Ͼ70%. Key Words: platelets Ⅲ LDL Ⅲ oxidized LDL Ⅲ CD36 Ⅲ scavenger receptor-A A n elevated level of native low density lipoprotein (nLDL) is a risk factor for arterial thrombosis and atherosclerosis as demonstrated in familial hypercholesterolemia, where defective apoB/E receptors fail to remove nLDL from the circulation. Atherogenesis starts when nLDL accumulates in the vessel wall at sites of injury and is oxidized by products from macrophages, smooth muscle cells, and endothelial cells. 1 Oxidized LDL (oxLDL) accumulates in monocytes that have infiltrated the subendothelium and differentiated into macrophages. The resulting foam cells are characteristic for the early atherosclerotic lesion. 2 OxLDL further contributes to atherosclerosis because it contains lysophosphatidic acid (LPA), which starts platelet shape change and aggregation. 3 In healthy individuals, the concentration of oxLDL is low. The normal intima contains little oxLDL (1.86Ϯ0.59 ng/g apolipoprotein B100 [apoB100]) but levels increase 6-fold in atherosclerotic lesions (11.9Ϯ1.7 ng/g apoB100). 4 Blood from atherosclerotic patients contains autoantibodies that react with oxidation-specific epitopes in both the lipid and protein moiety of oxLDL, 5,6 indicating that oxLDL is also present in the circulation. Hence, in the circulation, platelets can come into contact with oxLDL and become activated, thereby contributing to thrombotic occlusion. Conclusion-TheseThe oxidation of nLDL in vivo can be mimicked in vitro by treatment of nLDL with FeSO 4 . These oxLDL preparations resemble in vivo oxidized LDL with respect to electrophoretic mobility, density, LPA content, fragmentation of apoB100, chemotactic activity for monocytes, and susceptibility to degradation by macrophages. 3,7-9 LPA makes oxLDL a potent platelet activating agent 3 through activation of its LPA 1 and LPA 3 receptors, 10 which are members of the endothelial differentiation gene receptor family. At low concentrations, LPA stimulates Rho, Rho-ki...

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

“…Interestingly, our experiments with TSP-1 demonstrate that JNK lies downstream of Src family kinases, since JNK phosphorylation was blocked by PP1, which is consistent with other studies demonstrating that oxLDL activates a CD36-Src family kinase-JNK pathway in platelets. 25,40 However, our data suggests a divergence in the mechanisms through which Src kinases regulate cAMP/PKA and JNK. It is possible that activation of Src family kinases downstream of CD36 ligation could therefore reduce platelet sensitivity to inhibition by the cAMP/PKA pathway but also increase platelet activation through phosphorylation of JNK.…”

“…We show for the first time that TSP-1 stimulates CD36-mediated signaling events in platelets without inducing aggregation, which is consistent with recent studies demonstrating CD36-dependent stimulation of MAPKinases by oxLDL, without causing platelet aggregation. 25,40 Together, these data suggest CD36-mediated signaling in plays a secondary rather than primary role in platelet function. Based on our data, it is possible that previous studies highlighting the activatory and potentiating effects of CD36 ligands such as TSP-derived peptides, 7,8 oxLDL, 24 and VLDL 25,42 on platelet function could be due, at least in part, to their ability to modulate cAMP-PKA signaling events.…”

Vav guanine nucleotide exchange factors link hyperlipidemia and a prothrombotic state

“…4,6,7,20 Through this mechanism pathologic ligands generated in the context of hyperlipidemia and/or inflammation and oxidant stress enhance platelet reactivity and contribute to a prothrombotic state. Using oxLDL as a model ligand, we found that the initial signaling events triggered by CD36 involve liganddependent recruitment and activation of specific src family kinases Fyn and Lyn.…”

“…3,5 OxLDL and oxPC CD36 also bind to platelets via CD36 leading to platelet activation. 4,6,7 This process may contribute mechanistically to the well known clinical association between hyperlipidemia, oxidant stress, enhanced platelet reactivity, and the prothrombotic state. 4 The mechanism by which CD36-oxLDL interactions promotes platelet reactivity is not completely understood.…”

Arachidonic acid depletion extends survival of cold-stored platelets by interfering with the [glycoprotein Ib - 14-3-3 ] association

“…Human platelets were isolated 14 while maximally preventing their activation using free-flow blood collection and discarding the first 2 mL of blood and all collections that showed microaggregates as determined by particle sizing. The procedures were approved by the Medical Ethical Committee of our hospital; the laboratory is certified for ISO-9001:2008.…”