Thromboxane A2 (TXA2) formation by washed human platelets under the influence of low and high density lipoproteins from healthy donors

Activation of intracellular signaling systems by high-density lipoproteins

The proteosome of high-density lipoprotein particles is quite complex and consists of up to 75 different proteins and enzymes. The specific protein cargo of HDL particles regulates their functionality. In addition to their documented capacity to engage in reverse cholesterol transport, reduce oxidized lipid, and function as apoprotein donors, HDL particles can activate a variety of signaling systems in endothelial cells, smooth muscle cells, and platelets. The HDLs can deliver sphingolipids to the surface of these cell types and activate sphingosine phosphate receptors. Sphingosine phosphate receptors are coupled to numerous different intracellular signaling cascades exerting roles in vasodilatation, inflammation, cell migration and apoptosis, inhibition of platelet activation, and endothelial adhesion molecule expression, among other functions. The ability of HDL to influence such a diverse array of cellular functions lends biological plausibility to the substantial epidemiological and clinical evidence suggesting that the HDLs are unique among lipoproteins in that they are vasculoprotective and antiatherogenic.

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.

A novel role for CD36 in VLDL-enhanced platelet activation

Type 2 diabetes is characterized by increased plasma triglyceride levels and a fourfold increase in ischemic heart disease, but the mechanism is unclear. CD36 is a receptor/transporter that binds fatty acids of lipoproteins. CD36 deficiency has been linked with insulin resistance. There is strong evidence of in vivo interaction between platelets and atherogenic lipoproteins suggesting that atherogenic triglyceride-rich lipoproteins, such as VLDL, that are increased in diabetic dyslipidemia are important in this process. This study demonstrates that VLDL binds to the platelet receptor CD36, enhances platelet thromboxane A2 production, and causes increased collagen-mediated platelet aggregation. VLDL enhanced collagen-induced platelet aggregation by 1) shortening the time taken for aggregation to begin (lag time) to 70% of control (P = 0.001); 2) increasing maximum aggregation to 170% of control (P = 0.008); and 3) increasing thromboxane production to 3,318% of control (P = 0.004), where control represents platelets stimulated with collagen (100%). A monoclonal antibody against CD36 attenuated VLDL-enhanced collagen-induced platelet aggregation by 1) inhibiting binding of VLDL to platelets by 75% (P = 0.041); 2) lengthening lag time to 190% (P < 0.001); and 3) decreasing thromboxane production to 8% of control (P < 0.001). In support of this finding, platelets from Cd36-deficient rats showed no increase in aggregation, thromboxane production, and VLDL binding in contrast to platelets from rats expressing CD36. These data suggest that platelet Cd36 has a key role in VLDL-induced collagen-mediated platelet aggregation, possibly contributing to atherothrombosis associated with increased VLDL levels.

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.

Influence of diet-modified lipoprotein fractions from rabbits on eicosanoid production and on proliferation of rabbit skin fibroblasts

Three groups of rabbits received for 8 weeks either ordinary pellets, fish oil-enriched pellets or a cholesterol-rich (0.5%) diet for modification of lipoprotein composition. At the end of the dietary period, low density lipoproteins (LDL) and high density lipoproteins (HDL) were prepared from plasma of these rabbits. Rabbit skin fibroblasts (RSF) in culture were exposed to these isolated LDL and HDL fractions or to human serum albumin (HSA), respectively. Eicosanoid release was estimated after stimulation of cells with the Ca ionophore A 23187. Exogenous added HDL from the pellet group and HDL from the cholesterol-fed group as well as HSA stimulated PGI2 production by rabbit skin fibroblasts (P < 0.05, respectively) as compared to controls. All LDL preparations investigated inhibited PGI2 release from RSF (P < 0.05, respectively). The TXA2 release from RSF was increased only by HDL from cholesterol-fed rabbits and by HSA (P < 0.05, respectively). Pretreatment of RSF with cholesterol significantly decreased the number of detectable cells. Subsequent addition of HDL from the pellet or from the fish oil group completely restored the ability of cholesterol-enriched RSF to proliferate (P < 0.05, respectively).

Effects of exercise training and deconditioning on platelet function in men

Platelets play an important role in the pathogenesis of cardiovascular disease. It has also been noticed that regular exercise can reduce the risk of cardiovascular disease. This is the first study to demonstrate that endurance exercise training may suppress platelet adhesiveness and aggregation and that deconditioning may reverse the training effects. Healthy male sedentary subjects were randomly divided into control and training groups. The trained men were trained on a bicycle ergometer at about 60% of maximal oxygen consumption for 30 minutes per day, 5 days per week for 8 weeks, then deconditioned for 12 weeks. During the experimental period, blood samples of the trained subjects were collected before and immediately after a progressive exercise test every 4 weeks. The same experiments were applied to the controls at the beginning of this study and 8 weeks thereafter. A tapered parallel-plate chamber was used to assess platelet adhesiveness. Platelet aggregation induced by ADP was evaluated by the percentage of reduction in single platelet count. Our results showed that (1) platelet adhesiveness and aggregability were increased by short-term strenuous exercise in both control and trained groups, but the enhancement of platelet aggregability was decreased after exercise training in the trained subjects; (2) at rest and immediately after strenuous exercise, platelet adhesiveness and aggregability were decreased by training, whereas they were unchanged in the control group; and (3) deconditioning reversed the training effects on resting and postexercise platelet adhesiveness and aggregability back to the pretraining state. These results suggest that platelet adhesiveness and aggregability may be depressed by exercise training but be reversed back to the pretraining state after deconditioning.

Lysine modification of LDL or lipoprotein(a) by 4-hydroxynonenal or malondialdehyde decreases platelet serotonin secretion without affecting platelet aggregability and eicosanoid formation

The effects of lysine-modified atherogenic plasma lipoproteins, known to be constituents of human atherosclerotic plaques, were studied on platelet function in vitro. LDL and lipoprotein(a) [Lp(a)] modified with secondary breakdown products of lipid peroxidation (4-hydroxy-2,3-trans-nonenal [HNE] 0.1 to 10 mmol/L or malondialdehyde [MDA] 1 to 50 mmol/L) induced neither spontaneous platelet aggregation nor secretion of 5-hydroxytryptamine (5-HT) from platelet aminestorage granules. Incubation of platelets with HNE- or MDA-modified LDL or Lp(a) (up to 1200 micrograms protein/mL) prior to thrombin (0.2 U/mL)- or collagen (2 micrograms/mL)-induced aggregation did not enhance platelet aggregability or formation of eicosanoids, ie, thromboxane A2 or prostaglandins E2 and F2 alpha. In contrast to native lipoproteins, HNE- or MDA-modified LDL and Lp(a) (approximately 20% to 30% of total apolipoprotein lysine residues modified) exerted a pronounced dose-dependent inhibition of 5-HT release from activated platelets in the following order: HNE LDL (50%) > HNE Lp(a) (40%) > MDA LDL (20%) > MDA Lp(a) (5%). Preincubation of human blood platelets with acetylated LDL or Lp(a) (approximately 60% to 70% of total lysine residues modified) prior to aggregation impaired serotonin secretion by 50% compared with native LDL or Lp(a). These findings suggest that the interaction of platelets with aldehyde-modified atherogenic plasma lipoproteins should not necessarily be considered as proatherogenic with respect to the effects observed in our in vitro studies.

von Willebrand's disease and hemophilia are associated with diminished thromboxane A2 (TXA2) formation in clotting whole blood

Von Willebrand's disease (vWd) and hemophilia are associated with hemorrhagic diathesis and disturbances in platelet aggregation to vessel wall. We compared the time course of thromboxane A2 (TXA2) formation by platelets during spontaneous clotting of blood of patients with von Willebrand syndrome and from patients with hemophilia A or B with that of healthy controls which were matched for sex, age and serum lipid status. In clotting blood of healthy females the TXA2 production rose at 37 degrees C in 60 min up to 228.2 +/- 32.3 ng/ml. In patients with vWd the TXA2 production at 60 min was significantly lower (129.1 +/- 26.7 ng/ml, p < 0/05). In hemophilia type A and B the TXA2 formation after 5-30 min was significantly diminished in comparison to healthy male controls (p < 0.05). From the diminished amount of TXA2 formed during spontaneous clotting of whole blood we conclude that the activation of platelets of patients with von Willebrand syndrome or hemophilia type A and B is diminished as compared to healthy controls possibly caused by reduced formation of thrombin in the blood coagulation process.

Lipoproteins from normolipidemic and dyslipidemic subjects modify the thromboxane A2 generation by platelets in clotting human blood

The study was performed to investigate the influence of lipoproteins (LP) on the thromboxane (TX) A2 formation capacity of platelets in clotting whole blood in vitro. The different lipoprotein fractions VLDL, LDL, HDL2 and HDL3 were isolated from blood of normo- or dyslipidemic volunteers by ultracentrifugation. These lipoproteins were incubated in blood with different levels of serum total cholesterol (TC) taken from normolipidemics (TC < 200 mg/dl), moderate hypercholesterolemics (TC: 200-250 mg/dl) or subjects with high cholesterol level (TC > 250 mg/dl), respectively. The amount of serum TXA2 formed within 60 min at 37 degrees C was measured by enzyme immunoassay. The results obtained show that the efficacy of separate LP fractions to influence the TXA2 production depends not only on the type of LP fraction but also on the source of plasma used for isolation of LP and on the cholesterol level in the blood for incubation: LDL taken from normolipidemics or moderate hyperlipidemics inhibited the TXA2 formation in blood from normolipidemics (P < 0.02, respectively), but enhanced it in blood from persons with moderate hypercholesterolemia (P < 0.05). LDL from hyperlipidemics enhanced TXA2 production in blood from hyperlipidemics (P < 0.05). The HDL2 fractions inhibited the TXA2 formation in blood from normo- and hypercholesterolemics (P < 0.02, resp.), but there was no effect of HDL2 in clotting blood from persons with moderate hypercholesterolemia. All HDL3 fractions tested inhibited the TXA2 formation in all types of blood used for clotting (P < 0.02, resp.), probably due to their great cholesterol accepting capacity.

Effect of ethanol on low density lipoprotein and platelet composition

The present study was designed to investigate the effect of ethanol (EtOH) dose on low density lipoprotein (LDL) and platelet composition. Male squirrel monkeys were divided into three groups designated Control, Low, and High EtOH, and fed isocaloric liquid diets containing 0%, 12%, and 24% of calories as EtOH, respectively. After four months of treatment, monkeys fed the 12% alcohol dose had LDL and platelet cholesterol concentrations similar to Controls. By contrast, platelet membranes from High EtOH animals contained significantly more cholesterol which was associated with higher levels of plasma LDL cholesterol and apolipoprotein B. Blood platelet count, size, and mass were similar for all groups and circulating platelet aggregates were absent in the two alcohol cohorts. Despite elevations in platelet cholesterol mass and thromboxane A2 (TXA2) precursor, phospholipid arachidonate, platelet responsiveness, measured as thromboxane formed in response to a collagen challenge in vitro, and the cholesterol/phospholipid molar ratio, were not significantly altered by high dose alcohol. Normal platelet activity in High EtOH monkeys may have resulted from a significant increase in the platelet phospholipid polyunsaturated/saturated fatty acid ratio and a non-significant increase in platelet phospholipid mass, both of which would have a fluidizing effect on platelet membranes. Our data indicate that low EtOH intake has no effect on platelet composition and function while unfavorable platelet cholesterol enrichment following consumption of high dose ethanol may arise from elevations in plasma LDL.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of TXA2 generation of platelets by human lipoproteins

The lipoprotein (LP) fractions VLDL, LDL, HDL2 and HDL3 were prepared by ultracentrifugation of plasma from healthy volunteers and from patients with coronary heart disease (CHD). We investigated the capacity of platelets from healthy volunteers and patients with atherosclerosis to generate thromboxane A2 (TXA2) during spontaneous clotting of whole blood under the influence of the lipoprotein fractions. In our experiments the serum concentration of TXB2, reflecting the capacity of platelets to generate TXA2 during clotting, depends on several factors: the type of LP fraction used, the blood used for generation of TXA2, and for the same LP fraction whether it was taken from plasma of healthy volunteers or patients with CHD. VLDL prepared from plasma of healthy volunteers inhibited but VLDL prepared from plasma of patients with CHD enhanced the TXA2 formation of platelets from healthy volunteers (p less than 0.05, resp.). LDL from CHD patients inhibited the TXA2 formation of platelets from atherosclerotic patients (p less than 0.01). The HDL subfractions HDL2 and HDL3 from healthy volunteers inhibited TXA2 formation by platelets from healthy volunteers as well as those from atherosclerotic patients (p less than 0.05; p less than 0.01, respectively). HDL2 from patients with CHD inhibited only the TXA2 formation of platelets from healthy volunteers (p less than 0.01), whereas HDL3 from CHD patients inhibited only the TXA2 formation of platelets from atherosclerotic patients (p less than 0.01).

Platelet activation by oxidatively modified low density lipoproteins

Interactions between altered lipoproteins and platelets may be important in atherosclerosis lesion formation and thrombosis. The aims of this study were to compare the effects of oxidatively modified and native low density lipoproteins (LDL) and high density lipoproteins (HDL) on platelet responses in the presence and absence of other platelet agonists, and investigate the mechanism(s) by which lipoproteins influence platelet activation. We have shown that native and oxidatively modified lipoproteins differ importantly in their effects on platelets; oxidation renders lipoproteins more reactive to platelets. Native LDL promote aggregation of human platelets, enhance the mobilization of arachidonate from phospholipids, increase thromboxane B2 production, and decrease membrane fluidity. Oxidized LDL are more reactive than native LDL and alone cause aggregation. Native HDL inhibit platelet responses and increase membrane fluidity. Oxidized HDL promote aggregation and cause spontaneous aggregation. The enhanced platelet responses cannot be attributed to increased production of thromboxane A2 since cyclooxygenase inhibitors (aspirin, indomethacin) have little inhibitory effect. The data suggest that activation of platelets by lipoproteins results from changes in membrane fluidity. These observations shed new light on the potential role of altered lipoproteins in the pathogenesis of atherosclerosis and its complications.