Significance of membrane glycoproteins in platelet interaction with oxidized low-density lipoprotein

Platelet Redox Imbalance in Hypercholesterolemia: A Big Problem for a Small Cell

The imbalance between reactive oxygen species (ROS) synthesis and their scavenging by anti-oxidant defences is the common soil of many disorders, including hypercholesterolemia. Platelets, the smallest blood cells, are deeply involved in the pathophysiology of occlusive arterial thrombi associated with myocardial infarction and stroke. A great deal of evidence shows that both increased intraplatelet ROS synthesis and impaired ROS neutralization are implicated in the thrombotic process. Hypercholesterolemia is recognized as cause of atherosclerosis, cerebro- and cardiovascular disease, and, closely related to this, is the widespread acceptance that it strongly contributes to platelet hyperreactivity via direct oxidized LDL (oxLDL)-platelet membrane interaction via scavenger receptors such as CD36 and signaling pathways including Src family kinases (SFK), mitogen-activated protein kinases (MAPK), and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. In turn, activated platelets contribute to oxLDL generation, which ends up propagating platelet activation and thrombus formation through a mechanism mediated by oxidative stress. When evaluating the effect of lipid-lowering therapies on thrombogenesis, a large body of evidence shows that the effects of statins and proprotein convertase subtilisin/kexin type 9 inhibitors are not limited to the reduction of LDL-C but also to the down-regulation of platelet reactivity mainly by mechanisms sensitive to intracellular redox balance. In this review, we will focus on the role of oxidative stress-related mechanisms as a cause of platelet hyperreactivity and the pathophysiological link of the pleiotropism of lipid-lowering agents to the beneficial effects on platelet function.

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.

Association Between Low-Density Lipoprotein Cholesterol and Platelet Distribution Width in Acute Ischemic Stroke

Objective: Elevated low-density lipoprotein cholesterol (LDL-C) is an established risk factor for ischemic stroke; however, whether LDL-C affects the platelet deformation function in the peripheral blood circulation in patients with acute ischemic stroke (AIS) is unknown. The present study aimed to investigate the relationship between LDL-C and platelet distribution width (PDW) in AIS patients. Methods: We conducted a cross-sectional hospitalized-based study of consecutive 438 patients with AIS within 24 h. Blood samples were collected upon admission and prior to drug administration, and LDL-C and PDW (a parameter that reflects the heterogeneity of platelet volume) were assessed. The relationship between LDL-C and PDW were analyzed by linear curve fitting analyses. Crude and adjusted beta coefficients of LDL-C for PDW with 95% confidence intervals were analyzed using multivariate-adjusted linear regression models. Results: The PDW was significantly higher in the high LDL-C group compared with those in the normal LDL-C group (16.28 ± 0.37 fl vs. 16.08 ± 0.37 fl, p < 0.001). Adjusted smoothed plots suggested that there are linear relationships between LDL-C and PDW, and the Pearson's correlation coefficient (95%) was 0.387 (0.304-0.464, p < 0.001). The beta coefficients (95% CI) between LDL-C and PDW were 0.15 (0.12-0.18, p < 0.001) and 0.14 (0.11-0.18, p < 0.001), respectively, in AIS patients before and after adjusting for potential confounders. Conclusion: Our study suggested that the elevated LDL-C level was related to increased PDW among AIS patients.

Association between platelet distribution width and serum uric acid in Chinese population

Platelet distribution width (PDW) is a simple and inexpensive parameter, which could predict activation of coagulation efficiently. And it has been confirmed to have a significant role in many diseases. We aimed to explore the association between PDW and hyperuricemia in a large Chinese cohort. This cross-sectional study recruited 61,091 ostensible healthy participants (29,259 males and 31,832 females) after implementing exclusion criteria. Clinical data of the enrolled population included anthropometric measurements and serum parameters. Database was sorted by gender, and the association between PDW and hyperuricemia was analyzed after dividing PDW into quartiles. Crude and adjusted odds ratios of PDW for hyperuricemia with 95% confidence intervals were analyzed using binary logistic regression models. We found no significant difference in PDW values between the genders. Males showed significantly higher incidence of hyperuricemia than females. From binary logistic regression models, significant hyperuricemia risks only were demonstrated in PDW quartiles 2 and 3 in males (P < 0.05). This study displayed close association between PDW and hyperuricemia as a risk factor. It is meaningful to use PDW as a clinical risk predictor for hyperuricemia in males. © 2019 BioFactors, 45(3):326-334, 2019.

Different Effect of Statins on Platelet Oxidized-LDLReceptor (CD36 and LOX-1) Expressionin Hypercholesterolemic Subjects

Hydroxymethyl-glutaryl-CoA-reductase inhibitors (statins) reduce cardiovascular mortality by decreasing cholesterol as well as by non-lipid-related actions. Oxidized low-density lipoproteins (ox-LDL) are pro-atherogenic molecules and potent platelet agonists. CD36 and lectin-like ox-LDL receptor-1 (LOX-1) are specific ox-LDL receptors also expressed in platelets. This study was planned to address whether treatment with atorvastatin 10 mg/day, pravastatin 40 mg/day or simvastatin 20 mg/day could affect platelet CD36 and LOX-1 expression. Twenty-four patients for each treatment were evaluated after 3, 6, and 9 days and at 6 weeks for complete lipid profile (chromogenic), ox-LDL (ELISA), platelet P-selectin (P-sel), CD36, LOX-1 (FACS), and intracellular citrullin recovery (iCit) (HPLC). Data show hyperactivated platelets (P-sel absolute values, percent variation in activated cells, all p < 0.001), and CD36 and LOX-1 overexpression (all p < 0.001) in patients at baseline. P-sel, CD36, and LOX-1 were significantly decreased by atorvastatin and simvastatin (all p < 0.01) and related with iCit increase (r = 0.58,p < 0.001) and platelet-associated ox-LDL (r = 0.51, p < 0.01) at 9 days. Pravastatin reduced LOX-1 and P-sel (p < 0.05) at 6 weeks in relation with decreased LDL and ox-LDL (r = 0.39, p < 0.01 and r = 0.37, p < 0.01, respectively). These data suggest that atorvastatin and simvastatin reduce platelet activity by exposure of CD36 and LOX-1 before significant LDL reduction, whereas pravastatin action is detected later and in relation with LDL and ox-LDL lowering. Rapid and consistent reduction of CD36 and LOX-1 could be considered a direct anti-atherothrombotic mechanism related to the role of ox-LDL in platelet activation, platelet-endothelium interactions, and NO synthase activity.

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.

High P-selectin expression and low CD36 occupancy on circulating platelets are strong predictors of restenosis after coronary stenting in patients with coronary artery disease

Recent studies have shown that circulating platelets play an important role in the development of restenosis early after coronary stent implantation. We investigated P-selectin expression and CD36 blockade on platelets by flow cytometry in 48 consecutive patients who underwent coronary stenting. P-selectin expression was significantly higher 1 day after stenting in patients who had restenosis (n = 15) than in those who had no restenosis (n = 28), and the odds ratio for restenosis in patients with high P-selectin levels (MFI > 6.5) was 11.67 (P < 0.001) as compared with patients who had intermediate and low P-selectin levels. CD36 blockade was assessed with the use of two anti-CD36 antibodies, OKM5 and GS95 (our new anti-CD36 antibody), the binding of which indicates total CD36 amount and free CD36 unoccupied by lipid-related ligands, respectively. Binding of OKM5 to platelets was similar before and after stenting in both groups. CD36 blockade on platelets was seen 1 day after stenting in the non-restenosis group, and the odds ratio for restenosis in patients without CD36 blockade [GS95 binding ratio >0.8 as compared with binding before stenting] on day 1 was 28.60 (P < 0.001). P-selectin expression and unoccupied CD36 on platelets shortly after stenting may be strong predictors of post-stent restenosis.

[Platelet activation and hypercholesterolemia in the pathogenesis of deep vein thrombosis]

Currently it is accepted that deep vein thrombosis is a multifactorial event in which the presence of activated platelets and also plasmatic lipids seems to play a pivotal role that it is not well established in the scientific bibliography. Due to the non consensus state about these topics between the different groups working in these aspects, the topic involving deep vein thrombosis-platelets-lipids, and also their interactions, still is an interesting area of investigation, in which it is necessary to carry out studies with the aim of establishing risk factors, initial diagnostic methods and clinical assays to probe the efficacy of new therapies.

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.

Atorvastatin reduces platelet-oxidized-LDL receptor expression in hypercholesterolaemic patients

BACKGROUND

Oxidized-LDL (ox-LDL) are proatherogenic and platelet-activating molecules. Atorvastatin reduces platelet activity before cholesterol-lowering action. CD36 and lectin-like oxidized-LDL receptor-1 (LOX-1) are specific ox-LDL receptors expressed also in platelets. This study was planned to address whether the possible rapid effect of atorvastatin on platelets could be related to modulation of ox-LDL receptors.

MATERIALS AND METHODS

Forty-eight hypercholesterolaemic subjects requiring statin treatment (atorvastatin 20 mg day(-1)) after an ineffective diet regimen were evaluated for complete lipid-profile (chromogenic); P-selectin (P-sel), CD36 and LOX-1 expression (cytofluorimetric detection); circulating and platelet-associated ox-LDL (ox- and Pox-LDL, ELISA); and intracellular citrullin recovery (iCit, HPLC) at baseline and 3, 6 and 9 days after inclusion in the study. Moreover, we studied 48 normal controls matched for sex and age.

RESULTS

Platelet activity expressed by P-sel (in resting and thrombin-activated cells), CD36 and LOX-1 were increased in hypercholesterolaemic subjects (all P < 0.01). Atorvastatin induced a reduction of CD36 at 6 days (P < 0.05); and P-sel in resting (P < 0.001) and activated cells (P < 0.001) and LOX-1 were reduced at 9 days (all P < 0.001) in association with decreased Pox-LDL (P < 0.001) and increased iCit (P < 0.01). All data were obtained before a significant reduction of LDL and ox-LDL was achieved (P = 0.109 and 0.113).

DISCUSSION

Present data suggest that platelet deactivation by atorvastatin is related to CD36 and LOX-1 expression reduction before significant LDL changes. Moreover, the modulation of LOX-1 can be considered a self-relevant antiatherothrombotic action of atoravastin owing to the important role of this receptor in the ox-LDL-mediated vascular damage.

LDL oxidized by hypochlorous acid causes irreversible platelet aggregation when combined with low levels of ADP, thrombin, epinephrine, or macrophage-derived chemokine (CCL22)

The in vitro oxidation of low-density lipoprotein (LDL) by hypochlorous acid produces a modified form (HOCl-LDL) capable of stimulating platelet function. We now report that HOCl-LDL is highly effective at inducing platelet function, causing stable aggregation and alpha-granule secretion. Such stimulation depended on the presence of low levels of primary agonists such as adenosine diphosphate (ADP) and thrombin, or others like epinephrine (EPI) and macrophage-derived chemokine (MDC, CCL22). Agonist levels, which by themselves induced little or reversible aggregation, caused strong stable aggregation when combined with low levels of HOCl-LDL. Platelet activation by HOCl-LDL and ADP (1 microM) caused P-selectin (CD62P) exposure, without serotonin or adenosine triphosphate (ATP) secretion. Intracellular calcium levels rose slowly (from 100 to 200 nM) in response to HOCl-LDL alone and rapidly when combined with ADP to about 300 nM. p38 mitogen-activated protein kinase (MAPK) became phosphorylated in response to HOCl-LDL alone. This phosphorylation was not blocked by the protein kinase C (PKC) inhibitor bisindolylmaleimide, which reduced the extent of aggregation and calcium increase. However, the p38 MAPK inhibitor SB203580 blocked platelet aggregation and phosphorylation of p38 MAPK. These findings suggest that HOCl-LDL exposed during atherosclerotic plaque rupture, coupled with low levels of primary agonists, can rapidly induce extensive and stable thrombus formation.

Time-dependent effect of statins on platelet function in hypercholesterolaemia

BACKGROUND

Reduction of platelet activity induced by statins has been described as a positive effect exerted by such molecules on vascular thrombotic events. However, the relations among cholesterol (LDL-C) reduction, the timing of the antiplatelet effect, the involved mechanisms and the doses of each statin able to reduce platelet function are not actually well known. The aim of our study was to evaluate the impact of simvastatin (20 mg day-1), atorvastatin (10 mg day-1), fluvastatin (40 mg day-1) and pravastatin (40 mg day-1) on platelet function in hypercholesterolaemic subjects with relation to (LDL-C), oxidized-LDL (ox-LDL) and antiport mechanism modifications.

MATERIALS AND METHODS

Sixteen subjects were assigned to each treatment (40 males, 24 females, mean age 48.7 +/- 13.4, LDL-C 5.13 +/- 0,23 mmol L-1) and evaluated for platelet surface P-selectin (P-sel), lipid profile, ox-LDL, platelet-associated ox-LDL (Pox-LDL), platelet cholesterol content, antiport mechanisms, and intracellular and systemic NO synthase every 7 days for one month.

RESULTS

Our data show a strong relation between enhanced P-sel and Pox-LDL (r = 0.68, P < 0.01). Simvastatin, atorvastatin, fluvastatin and pravastatin reduce platelet activity after 1, 2, 3 and 4 weeks of treatment, respectively (P < 0.001, P < 0.001, P < 0.01, P < 0.05). Pox-LDL are modulated early by simvastatin, atorvastatin and fluvastatin Pox-LDL (r = 0.66, 0.65 and 0.52; P < 0.001, 0.001 and 0.01, respectively) whereas LDL-C and ox-LDL reductions associated to modifications of antiport activity act later. Moreover, they are the most relevant finding in pravastatin-related subjects.

CONCLUSIONS

Our data suggest a different impact of several statins on platelet function, which is initially related to interference with Pox-LDL rather than LDL-C reduction.

Activation-dependent surface expression of LOX-1 in human platelets

Lectin-like oxidized LDL receptor-1 (LOX-1) was initially identified as an oxidized LDL receptor in aortic endothelial cells. Here we identified LOX-1 mRNA and protein in human platelets in addition to recent findings on the expression in macrophages and smooth muscle cells. The presence of LOX-1 was further confirmed in the megakaryocytic cell lines. Flow cytometric analyses revealed that LOX-1 was exposed on the surface of platelets in an activation-dependent manner. Consistently, the activation-dependent binding of OxLDL to platelets was mostly inhibited by anti-LOX-1 antibody. Immunohistochemistry of the atherosclerotic plaque from a patient with unstable angina pectoris (UAP) revealed accumulation of LOX-1 protein at the site of thrombus. As LOX-1 recognizes and binds activated platelets, exposure of LOX-1 on activated platelets surface might assist thrombosis formation.

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.

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

Oxidized low density lipoproteins are closely associated with atherosclerosis and also might be directly involved in thrombosis because they have been shown to mediate a stimulating effect on human platelets. In this work, we used biologically modified low density lipoproteins (i.e., low density lipoproteins sufficiently oxidized to show specificity for the macrophage scavenger receptor system) to examine if specific binding of the oxidized apolipoprotein moiety to the platelet surface is a prerequisite for the platelet-stimulating effects reported by other authors. We find that biologically modified low density lipoproteins show specific binding to human platelets (K(d)=5.83+/-0.4 microg/mL, 3850+/-620 sites/platelet) and strongly augment both ADP- and thrombin-induced aggregation of washed platelets. Maleylated albumin, an antagonist of oxidized low density lipoproteins binding to all currently classified scavenger receptors, is able to reduce platelet oxidized low density lipoproteins binding to background levels. Nevertheless, maleylated albumin is not able to exert any kind of normalizing effect on the augmented ADP-induced aggregation response observed in the presence of biologically modified low density lipoproteins. From these data, we conclude that specific binding of oxidatively modified apolipoprotein B to the platelet surface is not essential to the process of platelet stimulation. Therefore, we conclude that these stimulating effects may be mediated by unidentified compounds formed in the lipid phase of the lipoproteins.