Inhibition of platelet function by high-density lipoprotein from a patient with apolipoprotein E deficiency

HDL-mediated mechanisms of protection in cardiovascular disease

Low plasma levels of HDL-cholesterol (HDL-C) represent a strong and independent risk factor for cardiovascular disease. HDL particles display a wide spectrum of atheroprotective activities, which include effluxing cellular cholesterol, diminishing cellular death, decreasing vascular constriction, reducing inflammatory response, protecting from pathological oxidation, combating bacterial infection, lessening platelet activation, regulating gene expression by virtue of microRNAs, and improving glucose metabolism. It remains presently indeterminate as to whether some biological activities of HDL are more relevant for the protection of the endothelium from atherogenesis when compared with others. The multitude of such activities raises the question of a proper assay to assess HDL functionality ex vivo. Together with clear understanding of molecular mechanisms underlying atheroprotective properties of HDL, such assay will provide a basis to resolve the ultimate question of the HDL field to allow the development of efficient HDL-targeting therapies.

High-density lipoproteins, platelets and the pathogenesis of atherosclerosis

1. Prospective and interventional studies demonstrate an inverse relationship between plasma high-density lipoprotein (HDL)-cholesterol and the incidence of coronary artery disease. Although the atheroprotective effects of HDL are usually attributed to the reverse cholesterol transport, in which HDL shuttles cholesterol from cells in the arterial wall to the liver, other mechanisms are also under investigation. 2. Platelets are involved in both the initiation and progression of atherosclerotic lesions. In addition, the formation of thrombi over ruptured atherosclerotic plaques results in the narrowing or complete occlusion of coronary arteries. Current experimental evidence suggests that HDL may exert antiplatelet effects and thereby counteract the development of atherothrombotic vascular disease. 3. In vitro studies show that HDL inhibits agonist-stimulated platelet aggregation, fibrinogen binding, granule secretion and liberation of thromboxane A(2). Inhibitory effects of HDL are mediated, in part, by scavenger receptor type B1 and/or the apolipoprotein E receptor apoER2/LRP8 and are linked to the induction of intracellular signalling cascades encompassing stimulation of protein kinase C, cytoplasmatic alkalization and generation of nitric oxide. 4. Populational studies demonstrate that there is an inverse association between plasma HDL levels and recurrent venous thromboembolism. In addition, HDL-cholesterol has been identified as an independent predictor of acute platelet thrombus formation. The administration of reconstituted HDL particles in humans attenuates ex vivo platelet activation. 5. The present review summarizes recent advances in understanding HDL-platelet interactions and discusses the potential use of HDL-like particles in the therapy of thrombosis.

Thrombocytopenia and platelet abnormalities in high-density lipoprotein receptor-deficient mice

OBJECTIVE

High-density lipoprotein (HDL) receptor, scavenger receptor class B, type I (SR-BI), mediated cellular uptake of lipoprotein cholesterol controls HDL structure and plasma HDL and biliary cholesterol levels. In SR-BI knockout (KO) mice, an unusually high plasma unesterified-to-total cholesterol ratio (UC:TC) and abnormally large HDL particles apparently contribute to pathology, including female infertility, susceptibility to atherosclerosis and coronary heart disease, and anemia. Here we examined the influence of SR-BI deficiency on platelets.

METHODS AND RESULTS

The high plasma UC:TC ratio in SR-BI KO mice was correlated with platelet abnormalities, including high cholesterol content, abnormal morphologies, high clearance rates, and thrombocytopenia. One day after platelets from wild-type mice were infused into SR-BI KO mice, they exhibited abnormally high cholesterol content and clearance rates similar to those of endogenous platelets. Platelets from SR-BI KO mice exhibited in vitro a blunted aggregation response to the agonist ADP but a normal response to PAR4.

CONCLUSIONS

In SR-BI KO mice abnormal circulating lipoproteins, particularly their high UC:TC ratio-rather than the absence of SR-BI in platelets themselves-induce defects in platelet structure and clearance, together with a mild defect in function.

Reconstituted high-density lipoprotein inhibits thrombin-induced endothelial tissue factor expression through inhibition of RhoA and stimulation of phosphatidylinositol 3-kinase but not Akt/endothelial nitric oxide synthase

Endothelial cells express negligible amounts of tissue factor (TF) that can be induced by thrombin, which is important for acute coronary syndromes. Recent research suggests that endothelial TF expression is positively regulated by RhoA and p38mapk, but negatively by Akt/endothelial nitric oxide synthase (eNOS) pathway. High-density lipoprotein (HDL) is atheroprotective and exerts antiatherothrombotic effect. This study investigated the effect of a reconstituted HDL (rHDL) on endothelial TF expression induced by thrombin and the underlying mechanisms. In cultured human umbilical vein and aortic endothelial cells, thrombin (4 U/mL, 4 hours) increased TF protein level, which was reduced by rHDL (0.1 mg/mL, 43% inhibition, n=3 to 7, P<0.01). Activation of RhoA but not p38mapk by thrombin was prevented by rHDL. rHDL stimulated Akt/eNOS pathway. The phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin or LY294002 abolished the activation of Akt/eNOS and reversed the inhibitory effect of rHDL on TF expression. Adenoviral expression of the active PI3K mutant (p110) reduced TF expression stimulated by thrombin without inhibiting RhoA activation, whereas expression of the active Akt mutant (m/p) further facilitated TF upregulation by thrombin. Moreover, a dominant-negative Akt mutant (KA) reduced thrombin's effect and did not reverse the rHDL's inhibitory effect on TF expression. Inhibition of eNOS by N(omega)-nitro-L-arginine methyl ester (100 micromol/L) did not affect the rHDL's effect. In conclusion, rHDL inhibits thrombin-induced human endothelial TF expression through inhibition of RhoA and activation of PI3K but not Akt/eNOS. These findings implicate a novel mechanism of antiatherothrombotic effects of HDL.

Apolipoprotein E and atherosclerosis: insight from animal and human studies

Major advances have been made in our understanding of the role of apolipoprotein E (apoE) in the onset and development of atherosclerosis. Increasing evidence from both animal and human studies suggests that apoE is able to protect against atherosclerosis by: a) promoting efficient uptake of triglyceride-rich lipoproteins from the circulation; b) maintaining normal macrophage lipid homeostasis; c) playing a role in cellular cholesterol efflux and reverse cholesterol transport; d) acting as an antioxidant; e) inhibiting platelet aggregation; and f) modulating immune function. In humans, apoE is polymorphic, and this genetic variation has a strong effect on its antiatherogenic characteristics. Thus, compared to the epsilon3 allele, the epsilon4 allele promotes atherosclerosis, whereas the epsilon2 allele is either pro- or anti-atherogenic, depending on the influence of both environmental and genetic factors. ApoE and its gene are prime targets for therapeutic intervention aimed at preventing or treating atherosclerotic vascular disease.

HDL3-mediated inhibition of thrombin-induced platelet aggregation and fibrinogen binding occurs via decreased production of phosphoinositide-derived second messengers 1,2-diacylglycerol and inositol 1,4,5-tris-phosphate

We demonstrate that physiological concentrations of HDL3 inhibit the thrombin-induced platelet fibrinogen binding and aggregation in a time- and concentration-dependent fashion. The underlying mechanism includes HDL3-mediated inhibition of phosphatidylinositol 4,5-bis-phosphate turnover, 1,2-diacylglycerol and inositol 1,4,5-tris-phosphate formation, and intracellular calcium mobilization. The inhibitory effects of HDL3 on inositol 1,4,5-tris-phosphate formation and intracellular calcium mobilization were abolished after covalent modification of HDL3 with dimethylsuberimidate. Furthermore, they could be blocked by calphostin C and bis-indolylmaleimide, 2 highly selective and structurally unrelated protein kinase C inhibitors. However, the inhibitory effects of HDL3 were not blocked by H89, a protein kinase A inhibitor. In addition, HDL3 failed to induce cAMP formation but stimulated the phosphorylation of the protein kinase C 40- to 47-kD major protein substrate. We observed a close temporal relationship between the HDL3-mediated inhibition of thrombin-induced inositol 1,4,5-tris-phosphate formation, intracellular calcium mobilization, and fibrinogen binding and the phosphorylation of the protein kinase C 40- to 47-kD major protein substrate. Taken together, these findings indicate that the HDL3-mediated inhibition of thrombin-induced fibrinogen binding and aggregation occurs via inhibition of phosphatidylinositol 4,5-bis-phosphate turnover and formation of 1,2-diacylglycerol and inositol 1,4,5-tris-phosphate. Protein kinase C may be involved in this process.

Inhibition of ADP-induced platelet aggregation by apoE is not mediated by membrane cholesterol depletion

We have previously shown that plasma HDL-E, a minor subclass of high-density lipoproteins (HDL) containing apolipoprotein (apo) E, has a potent anti-platelet effect and implicated apoE as the active constituent. Recently, apoE complexes with phospholipids (DMPC) were reported to inhibit thrombin-induced aggregation by sequestering platelet membrane cholesterol. Here we demonstrate that platelet cholesterol depletion is an improbable explanation for the suppressive effect of apoE:DMPC on ADP-mediated platelet aggregation; only 0.5% of cholesterol was released prior to addition of ADP to initiate aggregation while lactoferrin, which does not accept cellular cholesterol, was also inhibitory. Previous studies have shown that apoE and lactoferrin are both bound by platelets but whether this provides the initial stimulus for suppression of aggregation remains to be established.

Inhibition of ADP-induced platelet aggregation by apoE is not mediated by membrane cholesterol depletion

We have previously shown that plasma HDL-E, a minor subclass of high-density lipoproteins (HDL) containing apolipoprotein (apo) E, has a potent anti-platelet effect and implicated apoE as the active constituent. Recently, apoE complexes with phospholipids (DMPC) were reported to inhibit thrombin-induced aggregation by sequestering platelet membrane cholesterol. Here we demonstrate that platelet cholesterol depletion is an improbable explanation for the suppressive effect of apoE:DMPC on ADP-mediated platelet aggregation; only 0.5% of cholesterol was released prior to addition of ADP to initiate aggregation while lactoferrin, which does not accept cellular cholesterol, was also inhibitory. Previous studies have shown that apoE and lactoferrin are both bound by platelets but whether this provides the initial stimulus for suppression of aggregation remains to be established.

Remnant-like lipoproteins stimulate whole blood platelet aggregation in vitro

We have developed a simple, rapid assay method to measure remnant-like lipoproteins by using an immunoaffinity gel mixture of anti apo B-100 and apoA-1 antibodies to Sepharose 4B. Characterization of the unbound lipoproteins has shown that they represent chylomicron and VLDL remnant particles (RLP). Preincubation of whole blood with RLP resulted in the enhanced activation of aggregation with ADP and collagen. Such enhancement was not observed in the presence of lipoprotein deficient serum or albumin preparation. The extent of enhancement was 2.78 times by 7.5 microM of ADP and 44 times by 0.5 microgram/ml of collagen in the presence of RLP-preparation 1 (RLP-1), respectively. In the presence of RLP-2, the enhancement was 5.37 times by 7.5 microM of ADP and 102 times by 0.5 microgram/ml of collagen, respectively. On the other hand RLP slightly inhibited PRP aggregation by these agonists. Inhibitions were 19% by 7.5 microM of ADP and 18% by 1.0 microgram/of collagen in the presence of RLP-1, respectively. Incubation of whole blood with RLP did not result in the release of factors to stimulate platelets or ADP- or collagen-induced platelet aggregation in vitro. The extents of enhanced aggregation in whole blood or inhibition in PRP were not correlated with RLP-cholesterol nor RLP-protein concentrations of RLP preparations used. These results may indicate that RLP not only interact with platelets but with erythrocytes or leukocytes. Our findings support the hypothesis that the postprandial increase in remnant lipoproteins is an atherosclerotic risk factor and may be a part of the reasons of thrombotic complications by stimulating platelets in patients with remnant hyperlipoproteinemia.

Inhibitory effect of high-density lipoprotein on platelet function is mediated by increase in nitric oxide synthase activity in platelets

Although high-density lipoprotein (HDL) has been found to decrease platelet function per se, little is known regarding the mechanism of its platelet inhibitory effect. In this study, we confirmed the inhibitory effect of HDL on platelet aggregation and 14C-serotonin release in thrombin-activated washed human platelets. The inhibition of platelet function was associated with an increase in nitric oxide synthase activity, measured as the conversion of 3H-L-arginine to 3H-L-citrulline as well as nitrite release in the platelet supernates. The inhibition of platelet function by HDL was reversed by preincubation of washed platelets with an inhibitor of nitric oxide synthase, NG-nitro-L-arginine methyl ester (L-NAME), and potentiated by co-incubation with the precursor of nitric oxide, L-arginine. These observations suggest that HDL decreases platelet function by increasing nitric oxide synthase activity in human platelets.