Stimulating effect of biologically modified low density lipoproteins on ADP-induced aggregation of washed platelets persists in absence of specific binding

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.

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 expression of hSR-B1 on platelets of patients with coronary artery disease (CAD)

INTRODUCTION

The human scavenger receptor class B type 1 (hSR-B1), which serves as a high affinity receptor for HDL, is expressed on platelet surface and mediates various anti-atherogenic functions. Based on the anti-thrombotic effect of HDL and the importance of HDL-SR-B1 in the formation of atherosclerotic plaque, the present study was aimed to investigate and compare the expression level of hSR-B1on platelets of CAD patients with that of normal controls.

METHODS

The expression of the hSR-B1 on platelets of 31 CAD patients with atherosclerotic plaque and 20 healthy controls were detected using flowcytometry and western blotting. Moreover, platelet function in response to the agonists was examined by aggregometry, and the lipid panel tests were assayed using chemistry autoanalyzer.

RESULTS

Our findings showed that the expression of hSR-B1 was significantly reduced on the surface of platelets from CAD patients with atherosclerotic disease, as compared with healthy controls (6/8% vs. 13/6%) (P < 0,001). Of particular of interest, we also found that the formation of aggregates after stimulation of the platelets with ADP was higher in patients with atherosclerotic disease than the controls; indicating an inverse relationship between hSR-B1 expression and the function of human platelets.

CONCLUSION

Taken together, the results of the present study raise the possibility that the measurement of hSR-B1 expression on human platelets may provide a valuable insight that reflects the status of RCT in patients with atherosclerosis.

Potassium 2-(1-hydroxypentyl)-benzoate inhibits ADP-induced rat platelet aggregation through P2Y1-PLC signaling pathways

Potassium 2-(1-hydroxypenty1)-benzoate (dl-PHPB) is a new drug candidate for treatment of ischemic stroke with antiplatelet effect. In this study, we investigated the mechanisms of dl-PHPB in inhibiting platelet aggregation. The ADP-activated P2Y1-Gq-PLC and P2Y12-Gi-AC pathways were observed, respectively. Intravenous injection of dl-PHPB (1.3, 3.9, 12.9 mg/kg) significantly inhibited ADP-, collagen-, and arachidonic acid-induced rat platelet aggregation in a dose-dependent manner, and dl-PHPB had a relatively more potent inhibitory effect on ADP-induced rat platelet aggregation than other agonists. Dl-PHPB also showed a decreased expression of CD62P (a marker for platelet activation) mediated by ADP. Both dl-PHPB and ticlopidine (P2Y12 receptor antagonist) decreased cytoplasmic Ca(2+) concentration. But, dl-PHPB did not reverse the inhibition of PGE1-induced platelet cAMP formation by ADP, which was different from ticlopidine. Further, dl-PHPB instead of ticlopidine showed increasing phospholipase C-β phosphorylation (ser(1105)). The m-3M3FBS, a phospholipase C activator, attenuated the inhibitory effect of dl-PHPB on ADP-induced platelet aggregation and enhanced IP1 accumulation in rat platelets. Dl-PHPB decreased IP1 accumulation induced by ADP but had no effect on IP1 level enhanced by m-3M3FBS. Our results suggest that dl-PHPB has a potent antiplatelet effect, which is mainly through blockade of P2Y1 receptor-PLC-IP3 pathway and decreasing cytoplasmic calcium.

Specific binding of hypochlorite-oxidized HDL to platelet CD36 triggers proinflammatory and procoagulant effects

OBJECTIVE

Oxidative stress and systemic inflammation negatively affect several protective functions of high density lipoproteins (HDL) and oxidative modification of HDL by the inflammation-derived oxidant hypochlorite converts HDL into a potent platelet agonist. Therefore it was the aim of this work to clarify if these platelet-activating effects result from specific binding of hypochlorite-oxidized HDL (hyp-OxHDL) to the platelet surface and to identify responsible receptors.

METHODS

Binding and functional studies were performed with hyp-OxHDL in absence and presence of (potential) competitors in normal and CD36-deficient human platelets. Platelet aggregation was quantified by light transmission aggregometry. Surface expression of CD62P, phosphatidylserine and CD40L was quantified by flow cytometry.

RESULTS

Binding studies reveal that hyp-OxHDL show specific and saturable high-affinity binding to the platelet surface. Hyp-OxHDL trigger platelet aggregation and in a dose dependent way provoke the release of significant amounts of CD40L as well as phosphatidylserine on the platelet surface. Blocking specific binding of hyp-OxHDL to the platelet surface interferes with the ability of hyp-OxHDL to stimulate human platelets. CD36-deficient human platelets show markedly reduced binding of hyp-OxHDL. Upon addition of hypochlorite-oxidized HDL, CD36-deficient platelets do not aggregate and completely fail to release CD40L or phosphatidylserine.

CONCLUSIONS

From these results we conclude that specific binding of hyp-OxHDL to platelet CD36 is essential for the proinflammatory and procoagulant effects of hyp-OxHDL shown within this work. The contribution of other receptors besides CD36 to specific binding of hyp-OxHDL to the platelet membrane appears to be minimal, at best.

Modified Low-Density Lipoproteins and High-Density Lipoproteins

It has long been known that the oxidative state of the various plasma lipoproteins modulates platelet aggregability, thereby contributing to atherogenesis. Low-density lipoprotein (LDL), occurring in vivo both in the native and oxidised forms, interacts directly with platelets, by binding to specific receptors. While the identity of the receptors for native LDL and some subfractions of high-density lipoproteins (HDL) remains disputed, apoE-containing HDL(2) binds to LRP8. The nature of these interactions as well as the distinction between candidate receptor proteins was elucidated using covalently modified apolipoproteins, which pointed to the participation of apolipoproteins in high affinity binding. However, the platelet effects initiated by binding of native lipoproteins remain controversial. Some of this ambiguity can be traced to the fact that native LDL inevitably undergoes substantial oxidisation upon modification, including by radiolabelling. The platelet-activating effects provoked by oxidised LDL are irrefutable, but many details remain unknown. The role of CD36 in platelet binding by oxidised LDL is well established, although additional receptors may exist. Much less is known about the interaction of oxidised HDL with platelets, since platelet activation was observed in some, but not all studies. Various frequently applied in vitro oxidation methods produce modified lipoprotein species that may not be relevant in vivo. Based on the reported modifications obtained by in vitro oxidation of LDL, early investigations focused mainly on the formation and the eventual effects of oxidised lipids. More recently, alterations to lipoproteins performed using hypochloric acid and myeloperoxidase redirected the attention to the role of modified apoproteins in triggering platelet responses.

Plasma very-low-density lipoprotein, low-density lipoprotein, and high-density lipoprotein oxidative modification induces procoagulant profiles in endogenous hypertriglyceridemia

This study was to investigate whether oxidatively modified lipoproteins were associated with changes of pro- and anticoagulant profiles in hypertriglyceridemic subjects. Plasma VLDL, LDL, and HDL were isolated with the one-step density gradient ultracentrifugation method. The oxidation of the lipoproteins was identified. Prothrombin time (PT) and activated partial thrombplastin time (APTT), tissue plasminogen activator and plasminogen activator inhibitor-1, and platelet aggregation rate were determined with a reaction system consisting of mixed fresh normal plasma, in endogenous hypertriglyceridemic (HTG) patients, in in vitro modified lipoproteins from a normolipidemic donor, and in experimental rats. The results indicated that oxVLDL, oxLDL, and oxHDL occurred in the plasma of HTG patients. Compared with the control group, PT and APTT, incubated with plasma VLDL, LDL, or HDL from HTG patients, respectively, were significantly reduced, while platelet maximal aggregation rates were significantly higher (P < 0.05-0.01). Similar procoagulant profiles were observed in in vitro modified lipoprotein components and in rats with intrinsic hypertriglyceridemia as well. These results support our previous finding that LDL, VLDL, and HDL were all oxidatively modified in vivo in the subjects with HTG, and suggest that procoagulation state may result from the abnormal plasma lipoprotein oxidative modification in vivo.

The influence of platelet-smooth muscle cell interaction on the oxidative modification of low-density lipoprotein

OBJECTIVE

In view of the recognized association between thrombosis and atherosclerosis, it is hypothesized that exposure of arterial smooth muscle cells (SMC) to thrombogenic agents such as platelets and thrombin will alter the oxidation of low-density lipoprotein (LDL) and that this effect may be diminished by thrombin inhibition.

METHODS

Quiescent human aortic SMC in culture were exposed to LDL (40 microg protein/ml) alone or with washed human platelets (5 x 10(6)/ml), thrombin (40 units/ml), or a combination of these agents for 48 h. The media were removed, and both media and cell lysate fractions were assayed for malondialdehyde (MDA) content as an index of oxidation. Isolated platelets exposed to LDL and thrombin were studied in a similar manner to determine their individual oxidative activity. Finally, SMC and platelets were incubated with LDL and varying concentrations of thrombin (10-80 units/ml), both alone and in the presence of the thrombin inhibitors hirudin (u/u), and heparin (u/u), and MDA was measured.

RESULTS

SMC and platelets each demonstrated an ability to oxidize LDL, increasing MDA concentrations by 1.8- (P < 0.05) and 4- (P < 0.01) fold, respectively, compared to lipid-free media. Both platelets (P < 0.05) and thrombin (P < 0.001) enhanced the oxidation of LDL by SMC, while a combination of these two agents resulted in an additive effect (P < 0.001). The SMC lysate fraction showed an increase in oxidative products following exposure to platelets (P < 0.01) but not thrombin, suggesting that platelets stimulated uptake of the oxidized lipid by the SMC. Isolated platelets responded to thrombin with an increase in MDA within the media (P < 0.001). Smooth muscle cells exposed to thrombin also showed a dose-dependent increase in LDL oxidation (P < 0.01). This effect was not altered by hirudin, but was significantly inhibited by heparin (P < 0.05).

CONCLUSIONS

These results indicate that the oxidative potential of SMC and platelets is enhanced by their coincubation and by their concurrent exposure to thrombin. Heparin appears to block thrombin-stimulated oxidation. This interaction could be relevant to the dynamic interaction between atherosclerosis and thrombogenesis.

Hypochlorite modified LDL are a stronger agonist for platelets than copper oxidized LDL

Experimental low density lipoprotein (LDL) oxidation is usually performed using trace copper, although the in vivo relevance of this method has been called into question. Such LDL augment adenosine 5'-diphosphate (ADP) induced platelet aggregation, presumably by the action of lipid derived compounds. In striking contrast, we find that LDL oxidized to a comparable extent by hypochlorite, an in vivo occurring oxidant, reveal themselves to be potent promoters of platelet aggregation. Interestingly, hypochlorite modified LDL seem to mediate their influence on human platelets by means of the modified apolipoprotein B-100 (apoB) moiety. Also, the finding that hypochlorite modified albumin is able to trigger platelet aggregation suggests an essential role for hypochlorite modified protein(s) in the process of platelet activation.