Role of lipoxygenase metabolism in platelet function: effect of aspirin and salicylate

N-acetylcysteine Amide AD4/NACA and Thioredoxin Mimetic Peptides Inhibit Platelet Aggregation and Protect against Oxidative Stress

In the present study, we tested the effect of small-molecular-weight redox molecules on collagen-induced platelet aggregation. We used N-acetylcysteine amide (AD4/NACA), the amide form of N-acetylcysteine (NAC), a thiol antioxidant with improved lipophilicity and bioavailability compared to NAC, and the thioredoxin-mimetic (TXM) peptides, TXM-CB3, TXM-CB13, and TXM-CB30. All compounds significantly inhibited platelet aggregation induced by collagen, with TXM-peptides and AD4 being more effective than NAC. The levels of TxB₂ and 12-HETE, the main metabolites derived from the cyclooxygenase and lipoxygenase pathways following platelet activation, were significantly reduced in the presence of AD4, TXM peptides, or NAC, when tested at the highest concentration (0.6 mM). The effects of AD4, TXM-peptides, and NAC were also tested on the clotting time (CT) of whole blood. TXM-CB3 and TXM-CB30 showed the greatest increase in CT. Furthermore, two representative compounds, TXM-CB3 and NAC, showed an increase in the anti-oxidant free sulfhydryl groups of plasma detected via Ellman's method, suggesting a contribution of plasma factors to the antiaggregating effects. Our results suggest that these small-molecular-weight redox peptides might become useful for the prevention and/or treatment of oxidative stress conditions associated with platelet activation.

12-hydroxyeicosatetraenoic acid is associated with variability in aspirin-induced platelet inhibition

Background

Aspirin is one of the most widely used non-steroidal anti-inflammatory drugs (NSAIDs). It is also a commonly used anti-platelet drug, which inhibits the formation of the platelet activator, thromboxane A₂ (TxA₂) via inhibition of cyclooxygenase-1 (COX-1). However, the presence of a patient subset that fails to respond to aspirin despite reduced TxA₂ concentrations suggests that the effect of aspirin might be more complex than exclusive COX-1 inhibition.

Methods

In this study we evaluated the impact of in vivo oral administration of a standard anti-platelet dose (75 mg) of aspirin in healthy volunteers on the acute impact of in vitro collagen-mediated platelet aggregation and generation of platelet-derived TxA₂ and the 12-lipoxygenase (LOX) metabolite 12-hydroxyeicosatetraenoic acid (12-HETE). The eicosanoids were quantified using liquid chromatography-tandem mass spectrometry (LC-MS/MS).

Results

Low-dose aspirin administration not only inhibited TxA₂ generation but also decreased the production of 12-HETE. Furthermore, a significant correlation was observed between the levels of 12-HETE and collagen-induced platelet aggregation. Pre-treatment of platelets with the 12-LOX inhibitor, baicalein, prior to activation attenuated platelet aggregation.

Conclusions

These findings support a role for 12-HETE as a pro-aggregatory eicosanoid in platelet function and suggest a role for 12-HETE in variable sensitivity to aspirin. The study also highlights a potentially important mechanism by which aspirin impacts upon eicosanoid generation.

Aspirin in pregnancy

Aspirin, acetylsalicylic acid, is the most frequently consumed drug in pregnancy, taken mostly without a prescription because of headache or a minor ailment. Numerous preparations containing acetylsalicylic acid are freely available over the counter under a variety of proprietary names, and in many cases pregnant women and their doctors may be unaware that aspirin is being taken.

Gender differences in the activities of aspirin-esterases in rat tissues

The activities of aspirin (acetylsalicylic acid)-esterases were measured in several tissues (liver, kidney, adrenal glands, brain and serum) from adult male and female Wistar rats. In males, both aspirin-esterase I (assayed at pH 5.5) and II (assayed at pH 7.4) activities were higher in liver homogenates when compared to females (aspirin-esterase I: males 48.9 +/- 4.8 (N = 8) and females 29.3 +/- 4.2 (N = 8) nmol of salicylic acid formed min-1 mg protein-1; aspirin-esterase II: males 41.4 +/- 4.1 (N = 8) and females 26.1 +/- 4.5 (N = 8) nmol of salicylic acid formed min-1 mg protein-1, P < 0.001). In serum, enzyme activity was higher in females than in males (aspirin-esterase I: males 0.85 +/- 0.06 (N = 6) and females 1.18 +/- 0.11 (N = 6) nmol of salicylic acid formed min-1 mg protein-1, aspirin-esterase II: males 1.03 +/- 0.13 (N = 6) and females 1.34 +/- 0.11 (N = 6) nmol of salicylic acid formed min-1 mg protein-1, P < 0.001). In the other tissues assayed, no statistically significant difference between males and females was found. There were no statistically significant differences when the enzymes were assayed in different phases of the estrous cycle in liver and serum. These results show that the differences in aspirin-esterase activity observed between males and females are not due to the estrous cycle. The gender difference obtained in our study may indicate an involvement of gonadal hormones in the control of the hydrolysis of aspirin. This possibility is currently under investigation.

Aspirin increases the bleeding side effects in essential thrombocythemia independent of the cyclooxygenase pathway: role of the lipoxygenase pathway

Acetylsalicylic acid (ASA) is currently recommended as an antithrombotic for patients with essential thrombocythemia (ET) who are at an increased risk of thrombotic events. However, ASA is also associated with an increased risk of bleeding in these patients as compared to the risk of bleeding in other patients treated with ASA. Recent data suggest that while ASA inhibits platelet thromboxane A2 (TxA2) synthesis in all individuals, ASA has little effect or inhibits the lipoxygenase pathway (i.e., 12-hydroxyeicosatetranoic acid or 12-HETE synthesis) in some individuals, and enhances 12-HETE synthesis in others. These differential effects are associated with a pronounced prolongation of the bleeding time vs. no prolongation of the bleeding time, respectively, i.e., in ASA responders and ASA nonresponders, respectively. To determine if the increased risk of ASA-induced bleeding seen in ET patients is associated with an effect on 12-HETE synthesis, we compared the relative effects of ASA on the bleeding time, platelet TxA2 and 12-HETE synthesis, and platelet aggregation and adhesion in ET patients and healthy volunteers. ASA (300 mg, taken orally) prolonged the bleeding time in 82% of the ET patients but only 27% of the healthy volunteers although platelet TxA2 synthesis and ADP- and collagen-induced aggregation were inhibited significantly in both groups. In contrast, platelet 12-HETE synthesis was unchanged and platelet adhesion was decreased in those patients and volunteers whose bleeding times were prolonged by ASA, whereas platelet 12-HETE synthesis was increased significantly and platelet adhesion was unaffected in those patients and volunteers whose bleeding times were not prolonged, and in some cases shortened by ASA. These results confirm previous data that demonstrate that ASA has different effects on platelet 12-HETE synthesis and platelet adhesion in different individuals, i.e., inhibitory or no effect in ASA responders (in whom ASA prolonged bleeding) vs. enhancing effects in ASA nonresponders (in whom ASA did not prolong bleeding). These results also indicate that there is a greater percentage of ASA responders in patients with ET than that seen in the general population, a difference that is associated with an effect of ASA on the lipoxygenase pathway. This may explain the increased bleeding side effects seen in the ET patient population.

p-Aminobenzoic acid, but not its metabolite p-acetamidobenzoic acid, inhibits thrombin induced thromboxane formation in human platelets in a non NSAID like manner

We have previously shown that p-aminobenzoic acid (PABA) is acetylated by several cell lines and most peripheral blood cells, including platelets, to p-acetamidobenzoic acid (PACBA). The structural similarity of PABA and PACBA to local anesthetics and some non steroidal anti inflammatory drugs urged us to perform the present investigation. When human platelets were stimulated with thrombin to liberate AA, we found that PABA inhibited the production of thromboxane (TxB2) as measured with enzyme-linked immunosorbent assay. The inhibition was reversible and observed at PABA concentrations ranging between 55 and 1000 microM. At 328 microM PABA the production of TxB2 diminished by 87% (p = 0.013). PACBA in the same doses did not affect the production of TxB2. When platelets were incubated with [1-14C]AA, in the presence of PABA, the production of [1-14C]TxB2 was only slightly inhibited, according to analysis by high pressure liquid chromatography. Obviously PABA is not mainly acting as a prostaglandin H (cyclooxygenase) or Tx synthase inhibitor. It is rather affecting a step prior to thromboxane production, most likely the liberation of the precursor AA. In conclusion, our results demonstrate for the first time that PABA, a substance occurring in nature, inhibits endogenous TxB2 synthesis in human platelets and might thus exert profound effects on platelet AA metabolism.

The kinetic behaviour of a two-enzyme system in biphasic media: coupling hydrolysis and lipoxygenation

Analysis of the kinetic behaviour of a two-enzyme-system carrying out two consecutive reactions was investigated in macroheterogeneous biphasic media (octane/buffer pH 9.6, v/v = 1:1). The lipase-catalysed hydrolysis of trilinolein and the subsequent lipoxygenation of the liberated linoleic acid, were coupled in a modified Lewis cell with a well-defined liquid/liquid interfacial area. Trilinolein was dissolved in the organic phase and hydrolysed in the presence of Mucor javanicus lipase at the organic/aqueous interface. Linoleic acid, liberated after hydrolysis was transferred to the aqueous phase and reacted with lipoxygenase. This reaction consumed linoleic acid and produced hydroperoxides, which favoured the transfer of residual linoleic acid, since they possess surface active properties. Catalysis and transfer influenced each other reciprocally. At low substrate concentrations, cooperativity phenomena were observed in the experimental and also the modelled two-enzyme systems. When the initial substrate concentration was high, the kinetic behaviour of the two-enzyme system in a compartmentalised medium, seemed to be independent of the substrate concentration, unlike that observed in homogeneous monophasic enzymology. The numerical integration program used to model the two-enzyme system was based on results obtained in separate studies of the following three phenomena: (1) trilinolein hydrolysis in biphasic medium. (2) linoleic acid transfer across a liquid/liquid interface and (3) lipoxygenation in an aqueous media. Results obtained by modelling were similar to the results observed experimentally.

Use of enzymes as catalysts to promote key transformations in organic synthesis

The field of biotransformations has developed rapidly over the past eight years. The use of esterases and lipases is now widespread; these enzymes are of particular importance in the production of optically active building blocks for organic synthesis as well as in large-scale processes involving the transesterification of fats. The latter area (i.e. the catalysis of esterification processes) has stimulated research into the properties of immobilized enzymes and the use of enzymes in low-water systems. In related work, enzymes have been used for the preparation of peptides and small proteins. Redox enzymes have been investigated extensively, particularly with regard to the stereocontrolled reduction of ketones to secondary alcohols. The methods for using commercially available enzymes of this type have become increasingly ‘userfriendly’. The controlled oxidation of hydrocarbon units is another area that has deserved increased attention. For example, oxidation of benzene and simple derivatives by Pseudomonas sp. has been researched by a number of U.K. groups. These recent advances in enzyme-catalysed reactions (using both whole-cell systems and partly purified protein) for the transformation of unnatural substrates is discussed and some areas of interest for the future are outlined.

Bleeding time and platelet function in essential thrombocythemia and other myeloproliferative syndromes

Bleeding time (BT) and platelet function tests have been widely used in patients with essential thrombocythemia (ET), with the aim to support diagnosis and to identify laboratory predictors of haemorrhagic and thrombotic complications. BT is significantly prolonged in 7-19% of ET patients and several functional abnormalities have been observed in platelet structure, biochemistry and survival. However, the attempt to relate these in vivo and in vitro platelet dysfunctions with diagnosis or clinical sequelae has been generally disappointing. Therefore, BT and platelet function tests are currently not recommended in the initial evaluation or during the follow-up of patients with ET, unless in the setting of a clinical or biological study. A noteworthy exception is represented by a subset of patients characterized by very high platelet count (> 1500 x 10(9)/L) and bleeding symptoms, who can have an acquired von Willebrand disease. In these cases, prolonged BT and abnormal multimeric pattern of von Willebrand factor are useful for diagnosing and monitoring this acquired hemorrhagic disease. BT and platelet function tests should be included in the baseline evaluation of ET patients enrolled in prospective clinical trials aiming assess their predictive role on clinical end-points.

Low-dose aspirin in the prevention of preeclampsia and fetal growth retardation: rationale, mechanisms, and clinical trials

Preeclampsia is characterized by a functional imbalance between vascular prostacyclin and thromboxane A2 production. On the basis of the hypothesis that preeclampsia is at least partially caused by an increase in thromboxane A2, some studies attempted to correct this pathologic condition by pharmacologic manipulation with low-dose aspirin. The current literature suggests that the use of low-dose aspirin during pregnancy is safe with regard to congenital anomalies and fetal, neonatal, and maternal cardiovascular physiologic state and hemostasis. Aspirin at least partially corrects the pathologic increase in angiotensin II sensitivity that precedes the clinical development of preeclampsia. In addition, some clinical trials have demonstrated that low-dose aspirin is effective in reducing the incidence of preeclampsia and/or fetal growth retardation in selected high-risk women. Currently, large clinical trials are in progress to evaluate the effectiveness and side effects of the use of low-dose aspirin in preventing preeclampsia and/or fetal growth retardation. Until these studies have been completed, it will remain unclear whether antiplatelet therapy, such as low-dose aspirin, should be adopted for the prevention of either preeclampsia or fetal growth retardation.

Anti-platelet action of isoliquiritigenin, an aldose reductase inhibitor in licorice

The mechanism was studied by which isoliquiritigenin, a new aldose reductase inhibitor purified from licorice (Glycyrrhizae radix), inhibits platelet aggregation. This new agent significantly inhibited the phosphorylation of 40,000- and 20,000-dalton proteins, and inhibited the formation of 12 (S)-hydroxy-5,8,10-heptadecatrienoic acid, 12-hydroxyeicosatetraenoic acid and thromboxane B2. The inhibitory effect of isoliquiritigenin on platelet aggregation in vitro was comparable to that of aspirin. Our findings may indicate that isoliquiritigenin elicits an anti-platelet action by inhibiting not only cyclooxygenase but also lipoxygenase or peroxidase activity in platelets. Isoliquiritigenin also showed an anti-platelet action in vivo. Isoliquiritigenin appears to be the only aldose reductase inhibitor with a significant anti-platelet action. Since the hyperaggregability of platelets has been implicated in the pathogenesis of diabetic complications, isoliquiritigenin may offer a unique benefit as an aldose reductase inhibitor.

Eicosanoids, other fatty acid metabolites and the cardiovascular system: are the present antithrombotic approaches rational?

Antiplatelet +/- anticoagulant drugs are currently used as the standard treatment to prevent and treat thrombosis. While this approach is beneficial, it is not optimal. Recent evidence suggests that constituents of the vascular endothelium play an important role in regulating vessel wall thrombogenecity, thereby inhibiting thrombogenesis. These include constituents such as PGI2, tissue plasminogen activator, thrombomodulin and the lipoxygenase fatty acid metabolite derived from linoleic acid, 13-hydroxyoctadecadienoic acid (13-HODE). Consequently, new strategies have been developed to maximize the use of these agents for antithrombotic therapy. We will review these different approaches, discuss their rationale, and based upon recent experimental data, introduce an alternative approach for antithrombotic therapy which may circumvent a number of limitations and side-effect of the currently used drugs.

Relationship between vessel wall 13-HODE synthesis and vessel wall thrombogenicity following injury: influence of salicylate and dipyridamole treatment

We performed studies to determine the relationship between injured vessel wall thrombogenicity, vessel wall 13-hydroxyoctadecadienoic acid (13-HODE) synthesis and cAMP levels in rabbit treated with salicylate or dipyridamole. Injured vessel wall thrombogenicity was measured as the number of 3H-adenine labelled platelets adhered to the subendothelial basement membrane exposed by air injury in carotid arteries of rabbits treated orally with salicylate or dipyridamole. Vessel wall 13-HODE was measured by HPLC and vessel wall cAMP was measured by RIA. Vessel wall thrombogenicity was increased two-fold in rabbits treated with salicylate and decreased by half in rabbits treated with dipyridamole. The levels of vessel wall cAMP levels were correlated both with the plasma dipyridamole levels and increases in 13-HODE synthesis. cAMP levels were unaffected by salicylate treatment, but 13-HODE synthesis was decreased. We conclude that there is a significant relationship between vessel wall cAMP levels and 13-HODE synthesis, which in turn, influences subsequent vessel wall thrombogenicity.

Hydrogen peroxide and methyl mercury are primary stimuli of eicosanoid release in human platelets

Hydrogen peroxide (H2O2) and methyl mercury induced the liberation of arachidonate and its metabolites from human washed platelets. [14C]Eicosanoids were extracted from the supernatants of [14C] arachidonate-prelabelled platelets and analysed by thin layer chromatography and radioscanning. Thromboxane B2 (TXB2), 12(S)-hydroxy-5,8,10-heptadecatrienoic acid (HHT) and 12(S)-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE) were found as stable metabolites, together with unreacted arachidonate. In the presence of dazoxiben, a shift in eicosanoid metabolism was observed towards prostaglandin E2 (PGE2), prostaglandin D2 (PGD2) and prostaglandin F2 alpha (PGF2 alpha), while in the presence of indomethacin there was a shift towards 12-HETE and unmetabolized arachidonate. The concentration pattern of those metabolites resembled that found with the physiological agonist, thrombin. H2O2 and methyl mercury also induced platelet shape change, aggregation and secretion. The EC50 values for the induction of shape change and aggregation were 27 and 850 mumol/l for H2O2 and 0.33 and 2.7 mumol/l for methyl mercury, respectively. The [3H]serotonin release required higher stimulus concentrations and amounted to 45% with 2 mumol/l H2O2 and to 16% with 3 mumol/l methyl mercury. These effects on platelet function were absent in platelets exposed to acetylsalicylic acid and prevented by indomethacin, the prostaglandin H2 (PGH2)/thromboxane A2 (TXA2) receptor antagonist, daltroban, and the functional antagonist, iloprost. In contrast, none of these drugs suppressed the formation of [14C]eicosanoids, indicating that the platelet activation by H2O2 and methyl mercury essentially requires previous PGH2/TXA2 formation. As expected, the thromboxane synthase inhibitor, dazoxiben, did not prevent, but instead potentiated the activation by H2O2 and methyl mercury through accumulated PGH2.(ABSTRACT TRUNCATED AT 250 WORDS)

Relative effects of flurbiprofen on platelet 12-hydroxy-eicosatetraenoic acid and thromboxane A2 production: influence on collagen-induced platelet aggregation and adhesion

Flurbiprofen has been shown to inhibit cyclo-oxygenase metabolism of arachidonic acid to thromboxane A2 (TxA2), resulting in the inhibition of platelet aggregation. Recently, our laboratory reported that the "irreversible" phase of platelet aggregation and adhesion were regulated, in part, by the lipoxygenase metabolism of arachidonic acid to 12-hydroxy-eicosatetraenoic acid (12-HETE) in platelets, and that selective inhibition of one enzyme i.e. either cyclo-oxygenase or lipoxygenase, resulted in paradoxical effects on the metabolism of arachidonic acid and platelet response related to the other pathway. Therefore, we performed experiments to assess the relative effects of flurbiprofen on TxA2 and 12-HETE synthesis, and on collagen-induced platelet aggregation and platelet adhesion to collagen-coated surfaces. "Irreversible" collagen-induced platelet aggregation was only partially inhibited by pre-incubation with 1 x 10(-6) M flurbiprofen, while TxA2 production was elevated and 12-HETE production was maximally inhibited in these platelets. At this concentration of flurbiprofen (1 x 10(-6)M), collagen-induced platelet adhesion was also reduced by 50%. At higher concentrations of flurbiprofen, both platelet aggregation and adhesion were further reduced, with a corresponding inhibition of TxA2 production. Thus it appears that the lipoxygenase pathway of arachidonic acid metabolism in platelets is not only inhibited by flurbiprofen, but is more sensitive to inhibition by flurbiprofen than the cyclo-oxygenase pathway. This differential effect of flurbiprofen on arachidonic acid metabolism in the platelet is related to differential effects on platelet function.

Interactions between cancer cells and the microvasculature: a rate-regulator for metastasis

Hematogenous metastasis is a major consideration in the staging, treatment and prognosis of patients with cancer. Key events affecting hematogeneous metastasis occur in the microvasculature. This is a brief, selective review of some interactions involving cancer cells and the microvasculature in pathologic sequence, specifically: (1) intravasation of cancer cells; (2) the arrest of circulating cancer cells in the microvasculature; (3) cancer cell trauma associated with arrest; (4) microvascular trauma; (5) the inflammatory; and (6) the hemostatic coagulative responses associated with arrest, and finally (7) angiogenesis, leading to tumor vascularization. The evidence shows that through a series of complex interactions with cancer cells, the microvasculature acts as a rate-regulator for the metastatic process, in addition to providing routes for cancer cell dissemination and arrest sites for cancer cell emboli.

Endothelium and underlying membrane reactivity with platelets, leukocytes and tumor cells: regulation by the lipoxygenase-derived fatty acid metabolites, 13-HODE and HETES

We hypothesize that the ratio of intracellular 13-hydroxy-octadeca-dienoic acid (13 HODE) and hydroxy-eicosatetraenoic acid (5-, 12- and/or 15-HETE) influences the expression or presentation of adhesive moieties on platelets, leukocytes, malignant cells and endothelial cells, thereby influencing their subsequent adhesive interactions. Thus, we demonstrate that under unstimulated conditions, these cells preferentially synthesize linoleic acid via their lipoxygenase enzymes into 13-HODE, the intracellular level of which is associated with limited or no cell adhesion, while following stimulation, the same cells preferentially metabolize arachidonic acid via the lipoxygenase enzyme into HETEs, the production of which is associated with enhanced adhesion. Which metabolite is synthesized by these cells and the subsequent adhesivity of these cells appear to be dependent upon both the intracellular level of cAMP and the ratio of linoleic and arachidonic acid substrates. This suggests that manipulation of this ratio will have significant effects on the adhesive events involved in the pathogenesis of thrombosis, inflammation and metastasis.

Binding of 13-HODE and 5-, 12- and 15-HETE to endothelial cells and subsequent platelet, neutrophil and tumor cell adhesion

Some studies report that endothelial cells preferentially take up the lipoxygenase-derived arachidonic acid metabolite, 5-hydroxyeicosatetraenoic acid (5-HETE), released from stimulated leukocytes (polymorphonuclear leukocytes, PMNs), whereas others report that endothelial cells preferentially take up 12-HETE released from platelets. The biological relevance of these observations, however, is unknown. Recently, we and others have found that, under basal conditions, endothelial cells, PMNs and tumor cells metabolize linoleic acid via the lipoxygenase enzyme to 13-hydroxyoctadecadienoic acid (13-HODE). We propose that endogenous levels of these metabolites regulate blood-vessel wall cell adhesion. In this study, we have measured (1) the relative binding of 5-, 12- and 15-HETE, and 13-HODE to endothelial cell monolayers, and (2) their effects on endothelial cell adhesivity with platelets, PMNs and tumor cells. There was a dose-related and specific binding of 5-[3H]HETE to endothelial cells but no binding of 12- or 15-HETE or 13-HODE. Platelet or PMN adhesion to endothelial cells was unaffected by the 5-HETE binding, but tumor cell adhesion was blocked by 40% (P less than 0.01). Interestingly, preincubation of endothelial cells with 13-HODE, 12-HETE or 15-HETE decreased platelet adhesion to endothelial cells (P less than 0.05), even though these metabolites did not bind to the endothelial cells. We conclude that 5-HETE preferentially binds to endothelial cells and interferes with a specific receptor for tumor cells, whereas the other metabolites neither bind to cells nor affect cell adhesion.

Effects and interaction studies of triflusal and other salicylic derivatives on cyclooxygenase in rats

Triflusal (TR) is a new salicylic acid derivative used clinically as an antiplatelet drug. Both aspirin (ASA) and TR inhibit platelet cyclooxygenase but the effects of these drugs are different. TR (0.5-2 mM) strongly inhibited platelet aggregation and malondialdehyde formation induced by arachidonic acid. The IC50 was 0.8 mM for TR and less than 0.1 mM for ASA. Deacetylated compounds, salicylic acid (SA) and HTB (the main metabolite of TR) were apparently competitive and reversible inhibitors of cyclooxygenase and HTB was 15 times more potent than SA. They did, however, partially prevent the inhibitory effects of ASA and TR in vitro. A similar effect was observed ex vivo in rats treated with HTB (100 mg/k i.p.) before TR or ASA (20 and 5 mg/kg i.v., respectively). Moreover, TR at 10 and 20 mg/kg i.v., inhibited thromboxane production by more than 50% while its effect on vascular cyclooxygenase was negligible. These findings indicated that TR is a weaker inhibitor of cyclooxygenase than ASA, and that HTB interferes with the effect of TR and ASA, despite the fact that HTB is a more potent reversible inhibitor than SA with probably a higher affinity for this enzyme.

Tumor-cell-induced platelet aggregation is a glycoprotein-dependent and lipoxygenase-associated process

To characterize the platelet receptor sites and the platelet metabolic pathways involved in tumor-cell-induced platelet aggregation, we have used a homologous system consisting of human platelets and 2 tumor cell lines of human origin, which activate platelets through different mechanisms. Preincubation of platelets with an MAb against platelet glycoprotein Ib partially blocked tumor-cell-induced platelet aggregation, and preincubation of platelets with an MAb against the glycoprotein complex GPIIb/IIIa totally blocked the aggregation induced by the 2 tumor-cell lines. No inhibitory effect was found when platelets were treated with PAF-receptor antagonists or with specific peptides which block the platelet sites involved in bacterially induced platelet aggregation. Compounds which raised intra-platelet cAMP levels inhibited tumor-cell-induced platelet aggregation in a dose-related manner. Inhibition of cyclo-oxygenase by aspirin which blocked TxB2 formation by platelets did not inhibit platelet aggregation induced by tumor cells whereas the BW755 compound which inhibits cyclo- and lipoxygenase blocked platelet aggregation. These results demonstrate that tumor-cell-induced platelet aggregation is a glycoprotein-dependent and a lipoxygenase-associated phenomenon.

Effect of nafazatrom on platelet function and release: relationship to symptomatic episodes in patients with peripheral vascular disease

We studied the effect of nafazatrom on plasma prostacyclin (PGI2) levels, platelet function, and thromboxane B2 (TxB2), and 12-hydroxy-eicosatetraenoic acid (12-HETE) production and clinical improvement in 12 patients with peripheral vascular disease (PVD) by means of a double-blind crossover trial of placebo, 800 or 1600 mg of nafazatrom four times daily for 1 week, with intervening 2-week washout periods. Plasma PGI2 levels were measured as 6-keto-PGF1 alpha by radioimmunoassay. Platelet function ex vivo was measured as collagen and adenosine diphosphate (ADP)-induced platelet aggregation, release of 12-HETE and thromboxane A2 (measured as TxB2), and was determined by high-pressure liquid chromatography (HPLC) and radioimmunoassay, respectively. The plasma 6-keto-PGF1 alpha levels were unaffected by nafazatrom treatment (p greater than 0.25). Nafazatrom treatment had no effect on TxB2 production, but significantly altered the production of the platelet 12-HETE (p less than 0.05). There was a significant association between the changes in 12-HETE production and clinical improvement. These results suggest that the mechanism of action of nafazatrom is in part related to the inhibition of platelet function via the lipoxygenase pathway, independent of PGI2 stimulation.

Primary thrombocythaemia in childhood: symptomatic episodes and their relationship to thromboxane A2, 6-keto-PGE1 and 12-hydroxy-eicosatetraenoic acid production: a case report

In a child with primary thrombocythaemia, observations have been made over a period of five years, during which, transient apparently thrombotic events occurred in the central nervous system on several occasions. Spontaneous platelet aggregation was noted and deaggregation took place even after exposure to ADP in vitro. Associated findings included pronounced elevation in plasma levels of 6-keto-PGF1 alpha and 6-keto-PGE1, the latter described for the first time. Production of 12-HETE by platelets was markedly reduced, probably reflecting lipoxygenase deficiency which has been reported in other myeloproliferative disorders. It has been suggested that 12-HETE is a natural inhibitor of thromboxane synthetase, so the further finding of enormous generation of TxA2, measured as TxB2, by this patient's platelets may be explicable. It is suggested that the increase in TxA2 is responsible for spontaneous platelet aggregation. In response to these massive events, there is a production of 6-keto-PGE1 which in turn, promotes platelet deaggregation. Administration of aspirin resulted in symptomatic relief and complete inhibition of TxB2 production.