Platelet formation of 12-hydroxyeicosatetraenoic acid and thromboxane B2 is increased in type IIA hypercholesterolemic subjects

Clot or Not? Reviewing the Reciprocal Regulation Between Lipids and Blood Clotting

Both hyperlipidemia and thrombosis contribute to the risks of atherosclerotic cardiovascular diseases, which are the leading cause of death and reduced quality of life in survivors worldwide. The accumulation of lipid-rich plaques on arterial walls eventually leads to the rupture or erosion of vulnerable lesions, triggering excessive blood clotting and leading to adverse thrombotic events. Lipoproteins are highly dynamic particles that circulate in blood, carry insoluble lipids, and are associated with proteins, many of which are involved in blood clotting. A growing body of evidence suggests a reciprocal regulatory relationship between blood clotting and lipid metabolism. In this review article, we summarize the observations that lipoproteins and lipids impact the hemostatic system, and the clotting-related proteins influence lipid metabolism. We also highlight the gaps that need to be filled in this area of research.

Analysis, physiological and clinical significance of 12-HETE: a neglected platelet-derived 12-lipoxygenase product

While the importance of cyclooxygenase (COX) in platelet function has been amply elucidated, the identification of the role of 12-lipoxygenase (12-LOX) and of its stable metabolite, 12-hydroxyeicosatretraenoic acid (12-HETE), has not been clarified as yet. Many studies have analysed the implications of 12-LOX products in different pathological disorders but the information obtained from these works is controversial. Several analytical methods have been developed over the years to simultaneously detect eicosanoids, and specifically 12-HETE, in different biological matrices, essentially enzyme-linked immunosorbent assays (ELISA), radioimmunoassays (RIA), high performance liquid chromatography (HPLC) and mass spectrometry coupled with both gas and liquid chromatography methods (GC- and LC-MS). This review is aimed at summarizing the up to now known physiological and clinical features of 12-HETE together with the analytical methods used for its determination, focusing on the critical issues regarding its measurement.

Non-synonymous polymorphism (Gln261Arg) of 12-lipoxygenase in colorectal and thyroid cancers

12-lipoxygenase (12-LOX) pathway which produces 12-HETE and hepoxiline HXA3 and HXB3, and induces production of reactive oxygen species and inflammation is increasingly being implicated in variety of cancers, including those of colorectal and thyroid cancers. Hence, we examined whether the functional polymorphism of 12-LOX (mRNA A835G; Protein Gln261Arg) has any association with human colorectal and thyroid cancers. In this communication, we report that the mutation is linked to colorectal cancer and thyroid cancers. Further, we also observed that the heterozygous mutant (AG) is more prevalent in females than in males. Frequencies of AA, AG and GG, respectively were 62.5, 36.2 and 1.3 % in controls, 36.5, 61.5 and 2.0 % in colorectal cancer cases and 35.6, 62.4 and 2.0 % in thyroid cancer cases. The results obtained suggested a significant association of the heterogenous variant (AG) with the cancers. Relative risk of the cancers with the presence of the AG variant was found to be 2.9 and 4.0 for colorectal and thyroid cancers, respectively. However, the association of the variant (AG) was significant only in male colorectal cancer patients but not in female patients. On the other hand, prevalence of the AG variant is significantly higher in control females than in male control subjects. To the best of our knowledge, this is the first study that links the 12-LOX gene polymorphism with thyroid cancer and reveals a gender bias in the prevalence of the polymorphic variants in controls and colorectal cancer patients.

Anti-platelet activity of diacetylated obovatol through regulating cyclooxygenase and lipoxygenase activities

Obovatol has been reported biological activities such as muscle relaxative, anti-gastric ulcer, anti-allergic and anti-bacterial activities. The present study was undertaken to investigate the effect of diacetylated obovatol, an obovatol derivative, on rabbit platelet aggregation, and their possible molecular mechanisms. Effects of diacetylated obovatol on platelet activation including aggregation and serotonin secretion were examined. In addition, we investigated the effect of diacetylated obovatol on archidonic acid and metabolites liberation and intracellular calcium mobilization. Diacetylated obovatol concentration-dependently inhibited the washed rabbit platelet aggregation induced by collagen and arachidonic acid, suggesting that diacetylated obovatol may selectively inhibits collagen- and arachidonic acid-mediated signal transduction. In accordance with these results, diacetylated obovatol showed a concentration-dependent decrease in cytosolic Ca(2+) mobilization and serotonin secretion. However, diacetylated obovatol did not inhibit arachidonic acid liberation; on the other hand, diacetylated obovatol inhibited the formation of arachidonic acid metabolites such as thromboxane A(2), prostaglandin D(2) and 12-HETE through interfering with cyclooxygenase (COX)-1 and lipoxygenase (LOX) activities. The results demonstrated that diacetylated obovatol has antiplatelet activities through inhibition of COX-1 and LOX activities.

JJK694, a synthesized obovatol derivative, inhibits platelet activation by suppressing cyclooxygenase and lipoxygenase activities

Obovatol has various biological activities, including anti-proliferative, neurotrophic, anti-fibrillogenic, anti-platelet, anti-fungal and anti-inflammatory activities. In this study, we investigated the effects of JJK694, a synthesized obovatol derivative, on rabbit platelet activation and its molecular mechanisms. JJK694 significantly inhibited washed rabbit platelet aggregation and serotonin secretion induced by collagen and arachidonic acid, but had little effect on thrombin- or U46619-induced aggregation. These results suggest that JJK694 selectively inhibits collagen- and arachidonic acid-mediated signaling. JJK694 also showed a concentration-dependent decrease in cytosolic Ca(2+) mobilization, but it had no effect on arachidonic acid liberation. On the other hand, it significantly inhibited the formation of arachidonic acid metabolites, including thromboxane A(2) (TXA(2)), prostaglandin D(2), and 12-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE), by suppression of cyclooxygenase (COX)-1 and lipoxygenase (LOX) activities. These results indicate that JJK694 hasanti-platelet activities through inhibition of arachidonic acid metabolite production by suppression of COX-1 and LOX activities.

Basic aspects of tumor cell fatty acid-regulated signaling and transcription factors

This article reviews the current knowledge and experimental research about the mechanisms by which fatty acids and their derivatives control specific gene expression involved during carcinogenesis. Changes in dietary fatty acids, specifically the polyunsaturated fatty acids of the ω-3 and ω-6 families and some derived eicosanoids from lipoxygenases, cyclooxygenases, and cytochrome P-450, seem to control the activity of transcription factor families involved in cancer cell proliferation or cell death. Their regulation may be carried out either through direct binding to DNA as peroxisome proliferator-activated receptors or via modulation in an indirect manner of signaling pathway molecules (e.g., protein kinase C) and other transcription factors (nuclear factor kappa B and sterol regulatory element binding protein). Knowledge of the mechanisms by which fatty acids control specific gene expression may identify important risk factors for cancer and provide insight into the development of new therapeutic strategies for a better management of whole body lipid metabolism.

L-4F alters hyperlipidemic (but not healthy) mouse plasma to reduce platelet aggregation

Background and Purpose- Hyperlipidemia is associated with platelet hyperreactivity. We hypothesized that L-4F, an apolipoprotein A-I mimetic peptide, would inhibit platelet aggregation in hyperlipidemic mice.

METHODS AND RESULTS

Injecting L-4F into apolipoprotein E (apoE)-null and low-density lipoprotein receptor-null mice resulted in a significant reduction in platelet aggregation in response to agonists; however, there was no reduction in platelet aggregation after injection of L-4F into wild-type (WT) mice. Consistent with these results, injection of L-4F into apoE-null mice prolonged bleeding time; the same result was not found in WT mice. Incubating L-4F in vitro with apoE-null platelet-rich plasma also resulted in decreased platelet aggregation. However, incubating washed platelets from either apoE-null or WT mice with L-4F did not alter aggregation. Compared with WT mice, unstimulated platelets from apoE-null mice contained significantly more 12-hydroxy 5,8,10,14-eicosatetraenoic acid, thromboxane A(2), and prostaglandins D(2) and E(2). In response to agonists, platelets from L-4F-treated apoE-null mice formed significantly less thromboxane A(2), prostaglandins D(2) and E(2), and 12-hydroxy 5,8,10,14-eicosatetraenoic acid.

CONCLUSIONS

By binding plasma-oxidized lipids that cause platelet hyperreactivity in hyperlipidemic mice, L-4F decreases platelet aggregation.

An Antithrombotic Agent, NQ301, Inhibits Thromboxane A2 Receptor and Synthase Activity in Rabbit Platelets

In previous studies we have reported that NQ301, a synthetic 1,4-naphthoquinone derivative, displays a potent antithrombotic activity, and that this might be due to antiplatelet effect, which was mediated by the inhibition of cytosolic Ca(2+) mobilization in activated platelets. In the present study, the effect of NQ301 on arachidonic acid cascade in activated platelets has been examined. NQ301 concentration-dependently inhibited washed rabbit platelet aggregation induced by collagen (10 microg/ml), arachidonic acid (100 microM) and U46619 (1 microM), a thromboxane A2 receptor agonist, with IC50 values of 0.60+/-0.02, 0.78+/-0.04 and 0.58+/-0.04 microM, respectively. NQ301 also produced a shift to the right of the concentration-effect curve of U46619, indicating a competitive type of antagonism on thromboxane A2/prostaglandin H2 receptor. NQ301 slightly inhibited collagen-induced arachidonic acid liberation. In addition, NQ301 potently suppressed thromboxane B2 formation by platelets that were exposed to arachidonic acid in a concentration-dependent manner, but had no effect on the production of prostaglandin D2, indicating an inhibitory effect on thromboxane A2 synthase. This was supported by thromboxane A2 synthase activity assay that NQ301 concentration-dependently inhibited thromboxane B2 formation converted from prostaglandin H2. Moreover, NQ301 concentration-dependently inhibited 12-hydroxy-5,8,10,14-eicosatetraenoic acid formation by platelets that were exposed to arachidonic acid. Taken together, these results suggest that NQ301 has a potential to inhibit thromboxane A2 synthase activity with thromboxane A2/prostaglandin H2 receptor blockade, and modulate arachidonic acid liberation as well as 12-hydroxy-5,8,10,14-eicosatetraenoic acid formation in platelets. This may also be a convincing mechanism for the antithrombotic action of NQ301.

Platelet 12-lipoxygenase activation via glycoprotein VI: involvement of multiple signaling pathways in agonist control of H(P)ETE synthesis

Lipoxygenases (LOX) contribute to vascular disease and inflammation through generation of bioactive lipids, including 12-hydro(pero)xyeicosatetraenoic acid (12-H(P)ETE). The physiological mechanisms that acutely control LOX product generation in mammalian cells are uncharacterized. Human platelets that contain a 12-LOX isoform (p12-LOX) were used to define pathways that activate H(P)ETE synthesis in the vasculature. Collagen and collagen-related peptide (CRP) (1 to 10 microg/mL) acutely induced platelet 12-H(P)ETE synthesis. This implicated the collagen receptor glycoprotein VI (GPVI), which signals via the immunoreceptor-based activatory motif (ITAM)-containing FcRgamma chain. Conversely, thrombin only activated at high concentrations (> 0.2 U/mL), whereas U46619 and ADP alone were ineffective. Collagen or CRP-stimulated 12-H(P)ETE generation was inhibited by staurosporine, PP2, wortmannin, BAPTA/AM, EGTA, and L-655238, implicating src-tyrosine kinases, PI3-kinase, Ca2+ mobilization, and p12-LOX translocation. In contrast, protein kinase C (PKC) inhibition potentiated 12-H(P)ETE generation. Finally, activation of the immunoreceptor tyrosine-based inhibitory motif (ITIM)-containing platelet endothelial cell adhesion molecule (PECAM-1) inhibited p12-LOX product generation. This study characterizes a receptor-dependent pathway for 12-H(P)ETE synthesis via the collagen receptor GPVI, which is negatively regulated by PECAM-1 and PKC, and demonstrates a novel link between immune receptor signaling and lipid mediator generation in the vasculature.

COX and LOX eicosanoids modulate platelet activation and procoagulation induced by two murine cancer cells

Involvement of arachidonic acid cyclooxygenase (COX) and lipoxygenase (LOX) metabolites in platelet aggregation and coagulation induced by two varieties of cancer cells of murine transplantable tumors was studied. A lung alveolar carcinoma (LAC) and a fibrosarcoma (FS), induced platelet aggregation and plasma coagulation (P<0.05). Pretreatment of both tumor lines with a COX inhibitor did not block the tumor cell induced platelet aggregation (TCIPA). COX [12(S)-HTT] and LOX [12(S)-HETE], metabolites of washed platelets (WP), alone or co-incubated with LAC or FS cells, were analyzed. We observed higher 12(S)-HETE release with respect to 12(S)HHT when WP were co-incubated with LAC cells. With both neoplastic cell (NC) lines prothrombin time (PT) was shortened. Pretreatment of NC with iodoacetic acid, soybean trypsin inhibitor or Factor X-deficient plasma increased the PT. These results indicate that AA metabolites play a role on the procoagulation and platelet aggregation induced by mesenchymal and epithelial murine cancers.

Fluvastatin: effects beyond cholesterol lowering

Coronary heart disease (CHD) remains a major therapeutic challenge in the Western world, and strategies aimed at cholesterol lowering form the mainstay of treatment. Fluvastatin is an established 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor ("statin") for the treatment of hypercholesterolemia. Its efficacy and safety have been established in numerous clinical trials. Emerging evidence now indicates that treatment with fluvastatin slows the progression of atherosclerotic CHD and reduces the incidence of cardiovascular morbimortality in the secondary prevention setting. This effect of fluvastatin cannot be explained by cholesterol lowering alone; nonlipid-related mechanisms (so-called "pleiotropic effects") undoubtedly contribute to a certain extent, and are probably linked to modulation of the mevalonate pathway. This review discusses the experimental evidence regarding the antiatherosclerotic and antithrombotic effects of fluvastatin that may contribute to its beneficial action on disease progression and clinical events. Such effects include decreased expression of adhesion molecules in monocytes and leucocyte-endothelium adherence responses, immunomodulation, prevention of low-density lipoprotein oxidation, inhibition of cholesterol esterification and accumulation, along with effects on smooth muscle cell proliferation and migration. Pleiotropic actions aimed at plaque stabilization (eg, decreased secretion of matrix metalloproteinases by macrophages), together with effects on platelet activity, tissue factor expression, and endothelial function, may contribute to an antithrombotic effect of fluvastatin. Taken together, the results of these studies indicate that the effects of fluvastatin, at therapeutic doses, may extend beyond cholesterol lowering.

Reduction of platelet activity markers in type II hypercholesterolemic patients by a HMG-CoA-reductase inhibitor

We have investigated the effects of a potent lipid-lowering therapy on the activity of platelets as measured ex vivo by the surface activation markers CD62 (PADGEM, P-selectin, GMP 140) and CD63 (GP53) in a double-blind, randomized, placebo-controlled study. Treatment with the HMG-CoA-reductase inhibitor fluvastatin (40 mg) significantly reduced the serum low density lipoprotein cholesterol concentration by 30% (p<0.01) and total cholesterol by 25% (p<0.01). The platelet membrane activation markers CD62 (PADGEM, P-selectin, GMP140) and 63 (GP53) significantly decreased by 22 and 13% (in terms of the relative fluorescence intensity) under the treatment with fluvastatin (p<0.05), respectively. The cholesterol-lowering effect is accompanied by a significant reduction of the platelet membrane activation markers CD62 and CD63 reflecting a reduced platelet activity that may contribute to the vasoprotective profile of fluvasatin.

Low-density lipoproteins supply phospholipid-bound arachidonic acid for platelet eicosanoid production

After the rapid extracorporal reduction of plasma low-density lipoprotein (LDL) by LDL apheresis, the percentages of arachidonic acid (AA)-containing species of phosphatidylcholine (PC) were lowered in the plasma of patients with hypercholesterolemia. The same PC species with AA were also decreased in the patient's platelets. Thus the supply of phospholipid-bound AA from LDL to the platelets was probably diminished after the apheresis. We therefore analyzed the concentration dependence of the transfer of phospholipid-bound AA from LDL to the platelets under in vitro conditions. The amount of [14C]AA-PC transferred to platelets strongly increased upon elevation of LDL from 0.1 to 1 mg protein/ml, with a less marked elevation being noted at higher LDL concentrations. After stimulation with thrombin (0.5 U/ml), 7.1% ([14C]AA-PC) and 10.6% ([14C]AA-phosphatidylethanolamine) of the 14C transferred from LDL to the platelets were recovered in the eicosanoids [14C]thromboxane B2 (TxB2) plus 12-[14C]hydroxyeicosatetraenoic acid. Experimental increases and reductions of the [14C]AA-PC import were associated with comparable modifications in the [14C]TxB2 production of the platelets. Accordingly, the import of phospholipid-bound [14C]AA is a necessary prerequisite for the formation of 14C-labeled eicosanoids. In summary, the transfer of phospholipids from LDL to the platelets markedly varies within the physiological range of lipoprotein concentrations. LDL contributes to platelet eicosanoid formation by supplying platelets with phospholipid-bound AA.

L-arginine attenuates platelet reactivity in hypercholesterolemic rabbits

Platelets are capable of producing nitric oxide (NO) through the L-arginine-NO synthase pathway. Acute exposure to supraphysiological concentrations of L-arginine in vitro increases the production of NO by platelets and is associated with an increase in platelet cyclic GMP (cGMP) levels and a reduction in platelet aggregation. The purpose of this study was to determine if chronic oral administration of L-arginine decreases platelet aggregation in hypercholesterolemic animals and to determine if this effect is mediated by the metabolism of L-arginine to NO. Male New Zealand White rabbits were fed normal chow (Con), a 1% cholesterol diet (Chol), or a 1% cholesterol diet supplemented with a sixfold enrichment of dietary L-arginine (Arg) or L-methionine (Met). After 10 weeks, cholesterol levels were equally increased in Chol and Arg animals, whereas plasma arginine levels were doubled in the Arg group. There was no difference in maximum aggregation initiated by ADP (100 mumol/L) between washed platelets from Con, Met, and Chol animals, but aggregation of platelets from Arg animals was significantly decreased (P < .05). In aggregating platelets from Arg animals, cGMP levels were significantly higher than the other groups (P < .05). When platelets were incubated ex vivo with the NO synthase inhibitor NG-monomethyl-L-arginine, the effects of dietary L-arginine were reversed. Chronic dietary supplementation of L-arginine decreases platelet aggregation in hypercholesterolemic rabbits. This effect appears to be due to the metabolism of L-arginine to NO.

Isologous and heterologous platelet aggregation induced by tumor cells from three murine mammary gland adenocarcinomas with different metastatic abilities

The addition of neoplastic cells (NC) isolated from murine mammary gland adenocarcinomas having moderate and high metastatic ability on in vitro heparinized mice platelet rich plasma (PRP) induced platelet aggregation. One non-metastatic line did not induce aggregation. The aggregatory effect was not correlated with the metastatic abilities. None of the three types of tumors did cause aggregation on whole blood or PRP of rats. These adenocarcinomas seem to be suitable models to study in vitro the interactions between platelets and tumor cells having metastatic and non-metastatic potentials.

The actions of lovastatin on platelet function and platelet eicosanoid receptors in type II hypercholesterolaemia. A double-blind, placebo-controlled, prospective study

We have studied the effects of 12 weeks of lovastatin (20 mg per day) on platelet function and thromboxane formation in 18 patients with type II hypercholesterolaemia in a double-blind, placebo-controlled, prospective study. Lovastatin significantly reduced total serum and LDL-cholesterol by 20% and 25% respectively. Washed platelets of lovastatin-treated patients had significantly reduced collagen-induced aggregation and thromboxane formation ex vivo. There was no change in ADP-induced platelet aggregation, but a significant increase in prostacyclin (iloprost)-stimulated platelet cyclic AMP concentrations in lovastatin-treated patients. This was associated with a significant increase in the number of prostacyclin receptors in platelet membranes prepared from lovastatin-treated patients. There was also an increase in platelet thromboxane receptors. There were no such changes in the placebo group. These data confirm our original observation of normalization of platelet function in hypercholesterolaemia by HMGCoA reductase inhibitors and suggest changes in platelet membrane composition at the megakaryocyte level as a possible site of action.

Aggregatory behaviour of platelets incubated with subcellular fractions of normal and chagasic human syncytiotrophoblast

The surface of human syncytiotrophoblast does not induce maternal blood platelet aggregation even though it is not an endothelium. It can be surmised that as occurs in endothelial injury the subcellular components of the syncytiotrophoblast may have pro- or antiaggregatory activity. During congenital Chagas' disease which is associated to trophoblast lesions, platelets may play a role in the development of T. cruzi-induced placentitis. In the present work the aggregatory behaviour of normal human blood platelets was recorded after their challenging with subcellular fractions of syncytiotrophoblast isolated from normal and chagasic women. Nuclear, Mitochondrial, Microsomal and Supernatant fractions isolated from normal and chagasic syncytiotrophoblast failed to induce per se any aggregatory reaction on platelets. When samples of platelet-rich plasma (PRP) were preincubated with normal and chagasic nuclear fractions and then stimulated with collagen at threshold level (CT-PRP) an inhibition of the aggregatory response was observed. Treatment of CT-PRP with normal and chagasic mitochondrial fractions induced inhibition of platelet aggregation whereas only chagasic fraction reduced latency time. Microsomal fraction from normal placentas showed no significant effects on platelet aggregation. It is concluded that subcellular fractions of normal human syncytiotrophoblast do not exhibit any effect on platelet aggregation, whereas those subcellular fractions enriched in intracellular membrane components isolated from chagasic placentas inhibit platelet aggregation.

Altered vascular responses to platelets from hypercholesterolemic humans

Activated platelets release potent vasoactive factors. Previous studies have focused on mechanisms by which vascular abnormalities lead to altered responses of atherosclerotic arteries. We tested the hypothesis that the activation of platelets from hypercholesterolemic humans produces abnormal vascular responses. Responses to intraluminal and abluminal activation of platelets from normal subjects and type II hypercholesterolemic patients (total cholesterol, 274 +/- 16 [mean +/- SEM] mg/dl) were examined in carotid arteries from normal rabbits perfused in vitro. Intraluminal activation of normal platelets produced pronounced dilatation of arteries preconstricted with phenylephrine. Vasodilator responses to intraluminal activation of platelets from hypercholesterolemic patients were greatly impaired. Vasodilator responses to platelets from hypercholesterolemic patients were not restored to normal by LY53,857 (10(-5) M), a 5-hydroxytryptamine2-serotonergic antagonist, by SQ29,548 (10(-5) M), a thromboxane A2/prostaglandin H2 receptor antagonist, or by apyrase (1.5 units/ml), an enzyme with ADPase activity. Abluminal activation of normal platelets produced modest constriction in quiescent arteries, and abluminal activation of platelets from hypercholesterolemic patients produced augmented vasoconstrictor responses. The major finding is that vasodilator responses to platelets from hypercholesterolemic patients are profoundly impaired, and vasoconstrictor responses to platelets from hypercholesterolemic patients are augmented. Mechanisms in addition to increased release of serotonin, thromboxane, and ADP appear to contribute to impaired vasodilator responses to hypercholesterolemic platelets. Thus, alteration of platelets by hypercholesterolemia, as well as altered vascular reactivity, may contribute to abnormal vascular responses in atherosclerosis.

Platelet and neutrophil function and eicosanoid release in a subject with abetalipoproteinaemia

It has been suggested that plasma low density lipoprotein (LDL) may affect platelet and leukocyte activity and the release of biologically active eicosanoids which may contribute to the pathogenesis of coronary heart disease. To evaluate the possible biological effects of LDL further we have studied platelet aggregation, leukocyte function and eicosanoid release in a young woman with abetalipoproteinaemia (ABL) who totally lacks LDL. The subject showed decreased platelet response to threshold doses of ADP, collagen and arachidonic acid compared to a matched control measured in parallel or to a larger control group of 22 normal subjects. Aggregation was normal to higher doses of the same agonists. Thromboxane formation in clotted whole blood and in PRP stimulated with collagen was lower for the ABL subject than the control group. Neutrophil function assessed by chemiluminescence following stimulation with PMA, zymosan and FMLP was normal for the ABL subject, although the release of leukotriene B4 from calcium ionophore stimulated cells less than controls. Neutrophil platelet activating factor (PAF) formation and half life in plasma was not altered in ABL. The fatty acid composition of plasma, platelet and erythrocyte membrane phospholipids showed particularly low levels of linoleic acid in ABL compared to controls. Platelet phospholipids from the ABL subject also showed low levels of arachidonic acid. The decrease in eicosanoid formation seen in ABL may be the result of altered fatty acid composition of cell membrane phospholipids, or perhaps a defect in arachidonate mobilization. These results indicate that in a subject who lacks circulating LDL the behaviour of platelets and neutrophils in some in vitro tests is altered, although the extent to which these effects are reflected in vivo with regard to inflammatory and thrombotic mechanisms is not known.

Platelet hyperreactivity in hyperlipidaemia with specific reference to platelet lipids and fatty acid composition

Platelet function after thrombin stimulation, the fatty acid composition of individual phospholipids, and levels of lipid components (cholesterol, cholesterol ester, phospholipids and triglycerides) were determined in total membranes of platelets from hyperlipidaemic (HL) and control subjects. Platelet aggregation, thromboxane (TX) B2 production and serotonin release was significantly greater in HL patients than in controls. Levels of platelet cholesterol, total phospholipids, cholesterol ester and triglycerides, were significantly higher in the HL patients. Small differences between the two groups were observed for the phospholipid fatty acid patterns. However, levels of arachidonic acid (AA) were significantly higher in phosphatidylinositol (PI) of HL patients (40.01 +/- 6.59 mol%) as compared to the controls (31.52 +/- 9.91 mol%) (P = 0.002). The higher levels of AA in PI, which is considered a donor pool for eicosanoid synthesis, may be an additional mechanism for the well documented platelet hyperfunction and greater TXB2 production in hyperlipidaemic subjects.

Effects of long-term treatment with lovastatin on the clotting system and blood platelets

In a study of 20 patients with hypercholesterolemia (type IIa) the effects of lovastatin (20-80 mg/day) on various clotting and thrombosis parameters were monitored for 12 months. On 11 occasions various cholesterol fractions and clotting parameters were determined in each patient. In addition, the clotting inhibitors AT III, protein C, protein S, and C1-esterase inhibitor and the fibrinolysis parameters plasminogen and alpha 2-antiplasmin were examined. Platelet function was monitored on the basis of spontaneous and induced (collagen, ADP, epinephrine, ristocetin) aggregation. Lovastatin in the above dosage brought about a 66 mg/dl (from 320 +/- 12.6 to 254 +/- 12.0 mg/dl) reduction in the total cholesterol level and a 56 mg/dl (from 244 +/- 11.4 to 188 +/- 12.1 mg/dl) reduction in LDL cholesterol at the end of the study. Fibrinogen showed a significance decrease during the study period, whereas PT and aPTT remained unaffected. The initial slopes of the ADP-induced platelet aggregation revealed a significant decrease. C-reactive protein and platelet count remained within the normal range, indicating no significant change. Thrombin clotting time, AT III, C1-esterase inhibitor, plasminogen, and alpha 2-antiplasmin were not modified. Protein C and S behaved in a contradictory way, but remained within the normal range. Long-term treatment with lovastatin was associated with a significant reduction of fibrinogen levels and platelet aggregation induced by ADP in type-IIa hypercholesterolemic patients. These alterations, as well as their role in cardiovascular disease, should be the subject of further investigations.

Leukocyte and platelet function and eicosanoid production in subjects with hypercholesterolaemia

A group of 22 subjects with type IIA hypercholesterolaemia (mean serum cholesterol = 8.3 +/- 0.3 mmol/l) were sex, age and weight matched with 22 control subjects (mean serum cholesterol = 4.5 +/- 0.1 mmol/l). Diastolic blood pressure was significantly higher in hypercholesterolaemic subjects (79.2 +/- 1.4 mm Hg) than in control subjects (71.9 +/- 1.4 mm Hg). While the high cholesterol group had 52% greater thromboxane production in clotted whole blood than controls this difference was not significant, and the platelet aggregation and serotonin secretion response to doses of collagen, ADP and arachidonic acid were similar between the 2 groups. Polymorphonuclear leukocyte (PMN) chemiluminescence (used as a measure of reactive oxygen species production) in response to low doses of the chemotactic-peptide FMLP and opsonized zymosan was significantly greater in high cholesterol subjects compared to their matched controls. The production of platelet activating factor (PAF) by calcium ionophore (2.5 micrograms) stimulated PMN isolated from hypercholesterolaemic subjects (11.5 +/- 1.4 ng/10(6) cells) was significantly greater than PAF production by cells from the control group (8.3 +/- 1.0 ng/10(6) cells). Leukotriene B4 release by PMN in response to calcium ionophore did not differ between the 2 groups. These data suggest a degree of leukocyte activation in hypercholesterolaemic subjects compared to controls with normal cholesterol. In addition, plasma levels of lyso-PAF were higher in high cholesterol subjects (317 +/- 21 ng/ml) compared to their matched controls (271 +/- 18 ng/ml) perhaps indicating increased plasma acetylhydrolase activity in subjects with raised cholesterol levels. Recently described biological activity for lyso PAF suggests a possible role for this substance in atherogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of synvinolin on platelet aggregation and thromboxane B2 synthesis in type IIa hypercholesterolemic patients

An increased susceptibility of platelets to aggregation induced by various agents and a higher production of active arachidonate metabolism have been described in type IIa hypercholesterolemia. This study was designed to evaluate whether changes in platelet function could be observed in hypercholesterolemic patients after synvinolin therapy. Administration of synvinolin to 12 type IIa hypercholesterolemic patients for 24 weeks had a lipid lowering effect and resulted in a marked reduction of platelet aggregation and thromboxane formation induced by collagen and arachidonate. Maximum response was achieved at 4-8 weeks and lipid lowering effects at 2 weeks. This finding indicates that platelet changes cannot be explained by a direct effect of synvinolin on platelets, and the antiplatelet response may therefore depend on platelet membrane lipid composition changes, particularly in the platelet cholesterol content of platelet membranes, following substantial reductions of total plasma cholesterol and LDL-cholesterol.

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.

Iloprost binding and inhibition of aggregation in platelet rich plasma. Differences between normal and type IIa hypercholesterolemic subjects

Platelets from type IIa hypercholesterolemic subjects have been previously shown to be less sensitive than normal platelets to the antiaggregatory effect of PGI2. We demonstrate here that these platelets display a reduced response to iloprost, a chemically stable analogue of PGI2, as well. In fact, the concentration of iloprost yielding 50% inhibition of PRP aggregation was higher in type IIa patients (0.77 +/- 0.08 nM) than in controls (0.51 +/- 0.06 nM, P less than 0.01). In addition, an inverse relationship existed between the threshold aggregatory concentration for collagen and the concentration of iloprost yielding 50% inhibition of PRP aggregation, both in type IIa and normal individuals. In order to elucidate the mechanism of the different sensitivity of platelets to prostacyclin and its analogue, we characterized the binding of 3H-iloprost to platelet rich plasma from single individuals. The binding was rapid, reversible, inhibited by iloprost, PGI2 and PGE1 (Kd = 50.7; 346.2 and 7500 nM, respectively); no heterogeneity of sites could be demonstrated in the PRP from a single individual. When binding studies were carried out in PRP of type IIa patients and controls, it appeared that the amount of 3H-iloprost bound at a fixed (300 nM) concentration was significantly lower in platelets from type IIa individuals (0.94 +/- 0.17 vs. 1.77 +/- 0.27 fmol/10(6) platelets, for patients and controls, respectively). It is concluded that such difference in binding might represent the mechanism underlying the reduced response to PGI2 and iloprost observed in platelets from type IIa patients.

Phosphatidylinositol (PI) and PI-associated arachidonate are elevated in platelet total membranes of type IIa hypercholesterolemic subjects

The lipid composition (phospholipid distribution and fatty acid patterns of individual glycerophospholipids) and levels of lipid components (cholesterol, total and individual phospholipid classes, arachidonic acid) have been determined in total membranes of platelets from type IIa hypercholesterolemic (HC) and control (C) subjects. Levels of cholesterol and total phospholipid, relative to the protein content, were about 80% and 60% higher respectively in platelet total membranes from HC subjects. Small differences between the two groups of samples were observed for the phospholipid distribution and the fatty acid patterns. Concentrations of individual phospholipid classes, were on the average 60% higher in HC than in C platelet membranes, with an even greater difference for phosphatidylinositol (PI) and sphingomyelin. Levels of arachidonic acid, relative to the protein content, were also 60-80% higher in membranes from HC platelet with a more than 100% increase in PI. The higher levels of the eicosanoid precursor fatty acid in phospholipids and especially in PI, which is considered a donor pool for eicosanoid synthesis, may be a contributing factor for the greater thromboxane formation and enhanced aggregation, upon stimulation, of platelets from HC patients in comparison to platelets from control subjects.

Cholestyramine treatment of type IIa hypercholesterolaemia normalizes platelet reactivity against prostacyclin

The effect of lowering total plasma and low density lipoprotein (LDL) cholesterol in heterozygous familial hypercholesterolaemia type IIa (FH) on platelet function, thromboxane (TX) formation and platelet sensitivity against iloprost, a stable prostacyclin mimetic, was studied in platelet-rich plasma ex vivo. Seven FH patients were treated with cholestyramine (12 g day 1) for 8-11 months and were compared with eight untreated FH patients and 11 healthy control subjects. In comparison with platelets from healthy controls, platelets from untreated FH patients exhibited a significantly increased aggregation response and TX formation, and a reduced reactivity against inhibition of platelet aggregation by prostacyclin. Treatment with cholestyramine for 8-11 months resulted in a 21% reduction in total serum and LDL-cholesterol. This was not accompanied by any change in platelet hyperreactivity or TX formation. However, cholestyramine treatment normalized the platelet reactivity of FH patients against iloprost, being no more different from healthy controls. It is concluded that reduction in plasma cholesterol by cholestyramine results in normalization of the reduced platelet sensitivity against prostacyclin. This might contribute to beneficial effects of cholestyramine treatment in preventing thromboembolic complications of atherosclerosis.

The increase of the openings of the surface-connected canalicular system in weakly stimulated platelets is prevented by a carbochromen derivative. A computerized morphometric analysis of freeze-etched human platelet plasma-membrane

When thrombin is added to platelet rich plasma at very low concentrations (0.02 U/ml), platelets undergo an activation process. Under these conditions platelets discharge beta-thromboglobulin, platelet factor 4 and the "platelet derived growth factor". In this paper we present evidence that when human platelets are weakly stimulated with thrombin, the number of openings of the surface connected canalicular system (SCCS) increases. This was observed ultrastructurally by the freeze-etching technique. 8-chlorocarbochromen, an antiaggregating drug, seems to inhibit this increase.

Review: lipoxygenase inhibitors and the gut

Leukotriene synthesis is influenced by several drugs currently in use for the treatment of alimentary disease, including the corticosteroids, sulphasalazine and mesalazine. However, the use of selective lipoxygenase inhibitors in human gastrointestinal disease has not been investigated. The complexity of eicosanoid metabolism, and the incomplete knowledge of roles played by each metabolite in each tissue and disease condition, make rational pharmacological manipulation of arachidonate metabolism difficult. However, lipoxygenase inhibitors show promise in animal models of inflammation, including hepatitis, and studies in vitro suggest that therapeutic benefits may be achieved using inhibitors of leukotriene synthesis in other inflammatory disorders.