Oxidized LDL and Reduction of the Antiaggregating Activity of Nitric Oxide Derived from Endothelial Cells

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.

Brachial artery endothelial function predicts platelet function in control subjects and in patients with acute myocardial infarction

Platelet activation occurs in an endothelium-dependent flow-mediated dilation (FMD) impairment environment. The aim of this study was to explore the association between platelet reactivity and brachial artery FMD in individuals without established cardiovascular disease (controls) and acute myocardial infarction (AMI) patients. We prospectively assessed brachial artery FMD in 151 consecutive subjects, 104 (69%) controls, and 47 (31%) AMI patients; 115 (76%) men, mean age 53 ± 11 years. Following overnight fasting and discontinuation of all medications for ≥ 12 h, percent change in brachial artery FMD (%FMD) and endothelium-independent, nitroglycerin-mediated vasodilation (%NTG) were assessed. Platelet aggregation was assessed by conventional aggregometry, and platelet adhesion and aggregation under flow conditions by cone-and-plate(let) technology (Impact-R). Smoking, diabetes, and hypertension were more common in AMI compared to control subjects (p < 0.01 for all). Furthermore, aspirin, clopidogrel, beta-blockers, angiotensin-converting enzyme inhibitors, and statin administration were more common in AMI compared to controls (p < 0.01 for all). %FMD but not %NTG was significantly lower in AMI patients compared to controls (10.2 ± 4.2% vs. 15.4 ± 4.4%; p < 0.001 and 17.2 ± 3.9% vs. 18.0 ± 3.7%, p = 0.803, respectively). %FMD was significantly and inversely associated with all platelet functions tests (p < 0.001) in all study participants. In a multivariate logistic regression (unadjusted and adjusted for age, gender, smoking status, diabetes mellitus, hypertension, hypercholesterolemia, overweight, family history, and concomitant medications), %FMD remained the best predictor of platelet function, irrespective of group allocation (AMI patients or controls). In conclusion, FMD is inversely correlated to platelet reactivity in both controls and AMI patients.

Oxidized low density lipoprotein (LDL) and platelet intracellular calcium: interaction with nitric oxide

The present study tested the effects of ox-low density lipoprotein (LDL) on nitric oxide (NO)-dependent decrease in agonist-stimulated [Ca2+]i. The effects of ox-LDL on platelet aggregation were also evaluated. Platelets loaded with FURA 2 AM (2 micromol/litre) were incubated with NO-donors for 2-10 min to obtain a 40-50% reduction in \[Ca2+]i and with NO-donors plus ox-LDL (100 microg of protein/ml). Thrombin (0.03 U/ml) was used as an agonist. In some experiments 8-Br-cGMP (0.5-1 mmol/l) was used to investigate the NO-dependent intraplatelet signalling system. Slightly oxidized LDL was obtained by leaving native LDL in the light at room temperature for at least 7 days. Ox-LDL did not cause any increase in thrombin-induced [Ca2+] (control: 215.4 +/- 44.3 nmol/l, ox-LDL 223.4 +/- 35.3 nmol/l, M +/- SEM; n = 8) and platelet aggregation (control: 78.7 +/- 4.9% , ox-LDL: 78.9 +/- 4.2% , n = 12). Ox-LDL antagonized the effects of NO-donors on platelet [Ca2+]i (NO-donor: 137.4 +/- 22.1 nmol/l, NO + ox-LDL: 177.3 +/- 27.6 nmol/l, n = 11; P < 0.001) and platelet aggregation (NO-donor: 15.4 +/- 3.4% , NO + ox-LDL: 28.9 +/- 3.8%, n = 24; P < 0.001). Ox-LDL did not affect the inhibitory activities of 8-Br-cGMP on platelet aggregation (8-Br-cGMP: 22.0 +/- 8.5%, 8-Br-cGMP + ox-LDL: 19.3 +/- 7.8%, n = 5) and platelet [Ca2+]i . In conclusion, slightly oxidized LDL does not directly activate platelets and does not i affect the intracellular NO-dependent signalling system. The present results suggest that LDL reduces the antiplatelet activity of NO mainly by preventing its biological effects.

Zinopin--the rationale for its use as a food supplement in Traveller's thrombosis and motion sickness

Venous thrombo-embolism (VTE) has been associated with periods of prolonged immobility during air, sea and road travel. Motion sickness (MS) has also been reported during both long and short journeys. Current prophylactic therapies for both these indications are generally associated with side effects. Physiological profiles of Pycnogenol and Standardized Ginger Root Extract (SGRE) representing active constituents of Zinopin have been described and reviewed in relation to their activities involved in the patho-physiology of VTE (Traveller's Sickness) and MS and their safe use as food supplement, in traveller's thrombosis and motion sickness. The pathophysiology of VTE and MS is discussed in light of epidemiological data and risk factors associated with these conditions. Rationale of development of Zinopin and its mechanism of action are discussed based on physiological synergy of Pycnogenol and SGRE. Conclusions are made in light of preliminary clinical findings obtained in an open controlled clinical trial. Further clinical study on Zinopin on these lines is suggested.

Platelet dysfunction in type 2 diabetes

Insulin resistance is a uniform finding in type 2 diabetes, as are abnormalities in the microvascular and macrovascular circulations. These complications are associated with dysfunction of platelets and the neurovascular unit. Platelets are essential for hemostasis, and knowledge of their function is basic to understanding the pathophysiology of vascular disease in diabetes. Intact healthy vascular endothelium is central to the normal functioning of smooth muscle contractility as well as its normal interaction with platelets. What is not clear is the role of hyperglycemia in the functional and organic microvascular deficiencies and platelet hyperactivity in individuals with diabetes. The entire coagulation cascade is dysfunctional in diabetes. Increased levels of fibrinogen and plasminogen activator inhibitor 1 favor both thrombosis and defective dissolution of clots once formed. Platelets in type 2 diabetic individuals adhere to vascular endothelium and aggregate more readily than those in healthy people. Loss of sensitivity to the normal restraints exercised by prostacyclin (PGI(2)) and nitric oxide (NO) generated by the vascular endothelium presents as the major defect in platelet function. Insulin is a natural antagonist of platelet hyperactivity. It sensitizes the platelet to PGI(2) and enhances endothelial generation of PGI(2) and NO. Thus, the defects in insulin action in diabetes create a milieu of disordered platelet activity conducive to macrovascular and microvascular events.

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.

Endothelial dysfunction, oxidation of low-density lipoprotein, and cardiovascular disease

The oxidative modification of low-density lipoprotein (LDL) may be dependent or independent of lipid peroxidation. This peroxidation may be initiated by metal ions, possibly in association with phospholipase activity or catalyzed by myeloperoxidase independent of metal ions. It results in the generation of aldehydes, which substitute lysine residues in the apolipoprotein B-100 moiety and thus in the generation of oxidized LDL. Endothelial injury, associated with increased production of free radicals during oxidative stress, is associated with increased prostaglandin synthesis and platelet adhesion/activation. These processes are associated with the release of aldehydes, which induce the oxidative modification of LDL in the absence of lipid peroxidation and thus in the generation of malondialdehyde (MDA)-modified LDL. We have demonstrated an association between coronary artery disease (CAD) and increased plasma levels of oxidized LDL. The increase of circulating oxidized LDL is most probably independent of plaque instability. Indeed, plasma levels of oxidized LDL were very similar for patients with stable CAD and for patients with acute coronary syndromes. Acute coronary syndromes, however, were associated with increased release of MDA-modified LDL that was independent of the necrosis of myocardial cells. These data suggest that oxidized LDL is a marker of coronary atherosclerosis whereas MDA-modified LDL is a marker of plaque instability. Recently, a prospective study in cardiac transplant patients suggested an active role of oxidized LDL in the development of CAD. Oxidized LDL may contribute to the progression of atherosclerosis by enhancing endothelial injury by inducing foam cell generation and smooth muscle proliferation.

Antioxidant activity and biologic properties of a procyanidin-rich extract from pine (Pinus maritima) bark, pycnogenol

There is growing interest in the biologic activities of plant extracts such as that obtained from the bark of the French maritime pine Pinus maritima, Pycnogenol. Pycnogenol (PYC) is a standardized extract composed of a mixture of flavonoids, mainly procyandins and phenolic acids. Studies indicate that PYC components are highly bioavailable. Uniquely PYC displays greater biologic effects as a mixture than its purified components do individually indicating that the components interact synergistically. PYC has been reported to have cardiovascular benefits, such as a vasorelaxant activity, angiotensin-converting enzyme (ACE) inhibiting activity, and the ability to enhance the microcirculation by increasing capillary permeability. Investigations of the cellular mechanisms of these therapeutic effects have demonstrated that PYC has strong free radical-scavenging activity against reactive oxygen and nitrogen species. The oligomeric components of PYC contribute significantly to the ESR free radical signal. PYC also participates in the cellular antioxidant network as indicated by its ability to regenerate the ascorbyl radical and to protect endogenous vitamin E and glutathione from oxidative stress. PYC modulates NO metabolism in activated macrophages by quenching the NO radical and inhibiting both iNOS mRNA expression and iNOS activity. The spectrum of different effects of NO in the circulation and the nervous system suggest the potential applications of PYC in immune and circulatory disorders as well as in neurodegenerative disease. PYC can bind to proteins, altering their structure and thereby modulating the activity of key enzymes and proteins involved in metabolic pathways. PYC effects redox-sensitive signal transduction pathways and alters gene expression. Aspects of PYC's activity are presented and discussed together with possible future implications and directions in the field of flavonoid research.

Inhibition of smoking-induced platelet aggregation by aspirin and pycnogenol

The effects of a bioflavonoid mixture, Pycnogenol, were assessed on platelet function in humans. Cigarette smoking increased heart rate and blood pressure. These increases were not influenced by oral consumption of Pycnogenol or Aspirin just before smoking. However, increased platelet reactivity yielding aggregation 2 hours after smoking was prevented by 500 mg Aspirin or 100 mg Pycnogenol in 22 German heavy smokers. In a group of 16 American smokers, blood pressure increased after smoking. It was unchanged after intake of 500 mg Aspirin or 125 mg Pycnogenol. In another group of 19 American smokers, increased platelet aggregation was more significantly reduced by 200 than either 150 mg or 100 mg Pycnogenol supplementation. This study showed that a single, high dose, 200 mg Pycnogenol, remained effective for over 6 days against smoking-induced platelet aggregation. Smoking increased platelet aggregation that was prevented after administration of 500 mg Aspirin and 125 mg Pycnogenol. Thus, smoking-induced enhanced platelet aggregation was inhibited by 500 mg Aspirin as well as by a lower range of 100-125 mg Pycnogenol. Aspirin significantly (p<0.001) increased bleeding time from 167 to 236 seconds while Pycnogenol did not. These observations suggest an advantageous risk-benefit ratio for Pycnogenol.

The F2-isoprostane 8-epiprostaglandin F2alpha increases platelet adhesion and reduces the antiadhesive and antiaggregatory effects of NO

F2-isoprostanes are prostaglandin (PG) isomers produced in vivo through free radical-catalyzed peroxidation of arachidonic acid, which may affect platelet function. The current study investigated the effects of 8-epiprostaglandin F2alpha (8-epi-PGF2alpha) on critical events of platelet activation. A dose-dependent increase in platelet adhesion to fibrinogen- and plasma-coated microwells by 8-epi-PGF2alpha (1 to 1000 nmol/L) was observed when resting platelets (plasma from 1.3+/-0.2% to 5.5+/-0.2%, EC50 of 48 nmol/L; fibrinogen from 3.3+/-0.3% to 6.4+/-0.2%, EC50 of 35 nmol/L; mean+/-SEM, n=8, P<0.001) and thrombin-stimulated human platelets were used. The expression of the adhesion molecule glycoprotein IIb/IIIa was increased by 10 to 1000 nmol/L 8-epi-PGF2alpha in resting platelets (from 64.8+/-2.1% to 83.9+/-1.3%; n=5, P<0.01) and in stimulated platelets. The secretion of the glycoprotein GMP-140 increased only in the presence of both thrombin and 10 to 1000 nmol/L 8-epi-PGF2alpha (from 48.5+/-3.1% to 63.1+/-2.0%, P<0.05). The antiaggregatory effects of both the NO donor NOR-3 (basal, 21.4+/-4.6%; with 8-epi-PGF2alpha, 30.8+/-6.9%; n=14, P<0.05) and endothelial cells that release NO (basal, 18.5+/-4.6%; with 8-epi-PGF2alpha, 30.7+/-5.3%; n=15, P<0.001) were also reduced. All of these effects were prevented by the thromboxane receptor antagonist GR32191 but not affected by acetylsalicylic acid. An increase in free intracellular calcium concentration, measured with the use of fura 2, was observed with 8-epi-PGF2alpha. In conclusion, F2-isoprostanes may participate in oxidative injury by inducing platelet activation and by reducing the antiplatelet activity of NO: increased platelet adhesiveness and expression of the fibrinogen receptor are induced by nanomolar amounts of 8-epi-PG-F2alpha. Platelet secretion and aggregation can also be induced in the presence of platelet agonists.

Effect of atherosclerosis on endothelium-dependent inhibition of platelet activation in humans

BACKGROUND

We investigated whether luminal release of nitric oxide (NO) contributes to inhibition of platelet activation and whether these effects are reduced in patients with atherosclerosis.

METHODS AND RESULTS

Femoral blood flow velocity and ex vivo whole blood platelet aggregation by impedance aggregometry were measured in femoral venous blood during femoral arterial infusion of acetylcholine (ACh; 30 microg/min) in 30 patients, 19 of whom had angiographic atherosclerosis. Measurements were repeated with sodium nitroprusside (40 microg/min), L-arginine (160 micromol/min), and N(G)-monomethyl-L-arginine (L-NMMA; 16 micromol/min). There was significant inhibition of collagen-induced platelet aggregation with ACh (45+/-9.5% lower, P<0.001), and this inhibition was greater in patients without atherosclerosis (68.7+/-10.4% reduction) than in those with atherosclerosis (32.5+/-8.1%, P=0.04). The magnitude of inhibition correlated with vasodilation with ACh, indicating an association between the smooth muscle and antiplatelet effects of endothelium-dependent stimulation. Neither L-NMMA nor sodium nitroprusside altered platelet aggregation. L-Arginine inhibited platelet aggregation equally in vitro (34+/-8% reduction, P<0.01) and in vivo (37+/-13% reduction, P<0.01).

CONCLUSIONS

Stimulation of NO release into the vascular lumen with ACh inhibits platelet aggregation, an effect that is attenuated in patients with atherosclerosis and endothelial dysfunction. Basal NO release does not appear to contribute to platelet passivation in vivo. L-Arginine inhibited platelet aggregation by its direct action on platelets. These findings provide a pathophysiological basis for the observed increase in thrombotic events in atherosclerosis. Use of L-arginine and other strategies to improve endothelial NO activity may impact favorably on thrombotic events in atherosclerosis.

Endothelial cells prevent accumulation of lipid hydroperoxides in low-density lipoprotein

A variety of cell types, including endothelial cells, oxidize low-density lipoprotein (LDL). To investigate the mechanisms by which endothelial cells modulate LDL oxidation states, endothelial cell cultures were incubated with LDL (240 mg cholesterol/dL) for 24 hours in M199 supplemented with fetal bovine serum (FBS, 16.7%). These conditions were not toxic to endothelial cells over the time frame of the study. Changes in LDL oxidation were monitored by measuring thiobarbituric acid-reactive substances (TBARS), lipid hydroperoxide (LOOH), and conjugated dienes (A234nm). LDL medium incubated in the absence of endothelial cells contained higher TBARS than did LDL medium incubated with endothelial cells (0.35 +/- 0.08 versus 0.23 +/- 0.08 nmol MDA/mg, respectively). LOOHs were higher in LDL medium incubated without endothelial cells than in LDL medium incubated with endothelial cells (6.8 +/- 4.4 versus 0.49 +/- 0.89 nmol/mg, respectively). Conjugated diene formation, based on changes in absorbance at 234 nm, increased to a greater extent in LDL medium incubated in the absence of endothelial cells than when endothelial cells were present. To increase oxidative stress on the endothelial cell cultures, increasing concentrations of Cu2+ (0 to 4 mumol/L) were added to LDL medium. Endothelial cells prevented LOOH accumulation until the concentration of Cu2+ exceeded 0.75 mumol/L. At 1.5 and 4 mumol/L Cu2+, endothelial cells enhanced LOOH formation nearly 3 and 2.5 times the LOOH values in the corresponding medium incubated in the absence of endothelial cells. This loss of protective function however, was not permanent. Endothelial cells, preincubated for 24 hours with Cu(2+)-containing LDL medium, were still able to prevent LOOH accumulation in fresh LDL medium. Endothelial cells prevented LOOH accumulation even when exposed to LDL medium that contained low concentrations of LOOHs (< 22 nmol/mg). However, endothelial cells accelerated the accumulation of LOOHs in LDL when exposed to LDL medium that contained slightly higher concentrations of preexisting LOOHs (approximately equal to 33 nmol/mg). These data indicate that endothelial cells have a limited capacity for preventing LOOH formation and that small increases in LOOHs may play a critical role in enhancing the potential of endothelial cells for oxidative modification of LDL.