Production of platelet thromboxane A2 inactivates purified human platelet thromboxane synthase.

Human platelet thromboxane synthase was partially purified by DEAE-cellulose, Affi-Gel Blue, and Sephacryl S-300 chromatography to a specific activity of 259 nmol of thromboxane B2/min per mg. Thromboxane synthase retained 75-90% of its enzymic activity when bound to phenyl-Sepharose. The immobilized enzyme was inactivated at pH 3.0 and inhibited by 1-benzylimidazole and U-63,557A. The ability of the enzyme to produce thromboxane A2 from prostaglandin H2 was dramatically reduced by multiple additions of prostaglandin H2. Our data suggest that the production of thromboxane A2 by the enzyme is self-limiting and that the enzyme is inactivated during the reaction.     

Inactivation of thromboxane synthase with hydroperoxy-fatty acids

Bovine lung thromboxane synthase was immobilized on phenyl-Sepharose beads by adsorption. The immobilized enzyme was catalytically active and synthesized both TXA2 and HHT. The structure-activity relationship of several hydroperoxy fatty acids and their ability to inactivate thromboxane synthase was investigated. Millimolar quantities of hydrogen peroxide and tert-butylperoxide were required to inactivate the enzyme: whereas micromolar quantities of C18 and C20 hydroperoxy fatty acids inactivated the enzyme. Pretreatment of the enzyme with long chain hydroperoxy-fatty acids resulted in a decreased synthesis of both TXB2 and HHT.     

Immunoaffinity purification and characterization of thromboxane synthase from porcine lung

Thromboxane synthase has been purified 620-fold from porcine lung microsomes by a three-step purification procedure including Lubrol-PX solubilization, reactive blue-agarose chromatography, and immunoaffinity chromatography. The purified enzyme exhibited a single protein band (53,000 daltons) on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Rabbit antiserum raised against the purified enzyme immunoprecipitated thromboxane synthase activity from crude enzyme preparations of porcine lung, cow lung, and human platelets, indicating the existence of structural homology of the enzyme in these species. Immunoblotting experiment identified the same polypeptide (53,000 daltons) in porcine lung and a polypeptide of 50,000 daltons in human platelets, confirming the identity of the enzyme and the specificity of the antiserum. Purified thromboxane synthase is a hemoprotein with a Soret-like absorption peak at 418 nm. The enzyme reaction has a Km for 15-hydroxy-9 alpha, 11 alpha-peroxidoprosta-5, 13-dienoic acid of 12 microM, an optimal pH of 7.5, and an optimal temperature of reaction at 30 degrees C. Purified thromboxane synthase catalyzed the formation of both thromboxane B2 and 12-hydroxy-5,8,10-heptadecatrienoic acid (HHT). The ratios of HHT to thromboxane B2 varied from 1.6 to 2.1 dependent on the reaction conditions. Except that HHT was formed at a greater rate, the formation of HHT and that of thromboxane responded identically to pH, temperature, substrate concentration, kinetics of formation, metal ions, and inhibitors suggesting that the two products are probably formed at the same active site via a common intermediate. Thromboxane synthase was irreversibly inactivated by 15-hydroxy-9 alpha, 11 alpha-peroxidoprosta-5,13-dienoic acid during catalysis and by treatment of 15-hydroperoxyeicosatetraenoic acid. The irreversible inactivation, however, could be protected by reversible inhibitors such as sodium (E)-3-[4-(1-imidazolylmethyl)phenyl]-2-propenoate and 15-hydroxy-11 alpha,9 alpha-(epoxymethano)-prosta-5,13-dienoic acid, suggesting that the inactivation occurred at the active site of the enzyme. The catalytic inactivation of thromboxane synthase and the greater rate of formation of HHT in thromboxane-synthesizing system may probably play important regulatory roles in the control of thromboxane synthesis.     

Differential regulation of prostaglandin E2 and thromboxane A2 production in human monocytes: implications for the use of cyclooxygenase inhibitors

There is an autocrine relationship between eicosanoid and cytokine synthesis, with the ratio of prostaglandin E2 (PGE2)/thromboxane A2 (TXA2) being one of the determinants of the level of cytokine synthesis. In monocytes, cyclooxygenase type 1 (COX-1) activity appears to favor TXA2 production and COX-2 activity appears to favor PGE2 production. This has led to speculation regarding possible linkage of COX isozymes with PGE and TXA synthase. We have studied the kinetics of PGE2 and TXA2 synthesis under conditions that rely on COX-1 or -2 activity. With small amounts of endogenously generated prostaglandin H2 (PGH2), TXA2 synthesis was greater than PGE2. With greater amounts of endogenously generated PGH2, PGE2 synthesis was greater than TXA2. Also, TXA synthase was saturated at lower substrate concentrations than PGE synthase. This pattern was observed irrespective of whether PGH2 was produced by COX-1 or COX-2 or whether it was added directly. Furthermore, the inhibition of eicosanoid production by the action of nonsteroidal anti-inflammatory drugs or by the prevention of COX-2 induction with the p38 mitogen-activated protein kinase inhibitor SKF86002 was greater for PGE2 than for TXA2. It is proposed that different kinetics of PGE synthase and TXA synthase account for the patterns of production of these eicosanoids in monocytes under a variety of experimental conditions. These properties provide an alternative explanation to notional linkage or compartmentalization of COX-1 or -2 with the respective terminal synthases and that therapeutically induced changes in eicosanoid ratios toward predominance of TXA2 may have unwanted effects in long-term anti-inflammatory and anti-arthritic therapy.     

Upregulation of thromboxane synthase in human colorectal carcinoma and the cancer cell proliferation by thromboxane A2

Tumor growth of colorectal cancers accompanies upregulation of cyclooxygenase-2, which catalyzes a conversion step from arachidonic acid to prostaglandin H(2) (PGH(2)). Here, we compared the expression levels of thromboxane synthase (TXS), which catalyzes the conversion of PGH(2) to thromboxane A(2) (TXA(2)), between human colorectal cancer tissue and its accompanying normal mucosa. It was found that TXS protein was consistently upregulated in the cancer tissues from different patients. TXS was also highly expressed in human colonic cancer cell lines. Depletion of TXS protein by the antisense oligonucleotide inhibited proliferation of the cancer cells. This inhibition was rescued by the direct addition of a stable analogue of TXA(2). The present results suggest that overexpression of TXS and subsequent excess production of TXA(2) in the cancer cells may be involved in the tumor growth of human colorectum.     

Products, kinetics, and substrate specificity of homogeneous thromboxane synthase from human platelets: development of a novel enzyme assay

Homogeneous thromboxane synthase from human platelets converted prostaglandin H2 (PGH2) to thromboxane A2 (measured as thromboxane B2, TxB2), 12(L)-hydroxy-5,8,10-heptadecatrienoic acid (HHT), and malondialdehyde (MDA) in equimolar amounts under a variety of experimental conditions. PGG2 was transformed to MDA and corresponding 15- and 12-hydroperoxy products. PGH1 was enzymatically transformed into 12(L)-hydroxy-8,10-heptadecadienoic acid (HHD) and PGH3 into TxB3 and 12(L)-hydroxy-5,8,10,14-heptadecatetraenoic acid (delta 14-HHT) as earlier reported for solubilized and partially purified thromboxane synthase preparations. The ratio of thromboxane to C17 hydroxy fatty acid formation was 1:1 with PGG2, PGH2, and PGH3 as substrates. These results confirm and extend earlier observations with partially purified enzyme that the three products are formed in a common enzymatic pathway (Diczfalusy, U., Falardeau, P., and Hammarstr?m, S. (1977) FEBS Lett. 84, 271-274). A convenient spectrophotometric assay for thromboxane synthase activity measuring the ultraviolet light absorption of the C17 hydroxy acid formed (e.g., HHT) was developed. The validity of the assay was determined employing specific inhibitors for thromboxane synthase. The substrate specificity of thromboxane synthase was determined using this assay. PGG2 and PGH3 showed Vmax and KM values similar to those of PGH2. The KM value of PGH1 was also identical to that of PGH2 but the Vmax value PGH1 was more than twice as high as that of PGH2.     

On the mechanism of prostacyclin and thromboxane A2 biosynthesis

The present research describes studies which address the mechanism of prostacyclin (PGI2) and thromboxane A2 (TXA2) biosynthesis. In addition to prostaglandin H1 (PGH1), PGG2, PGH2, and PGH3, also 8-iso-PGH2, 13(S)-hydroxy-PGH2, and 15-keto-PGH2 were applied to determine the substrate specificities and kinetics of prostacyclin and thromboxane synthase in more detail. Human platelet thromboxane synthase converted PGH1, 8-iso-PGH2, 13(S)-hydroxy-PGH2 and 15-keto-PGH2 into the corresponding heptadecanoic acid (C17) plus malondialdehyde, whereas the thromboxane derivative was formed only from PGG2, PGH2, and PGH3 together with the corresponding C17 metabolite and malondialdehyde in a 1:1:1 ratio. In contrast, PGG2, PGH2, 13(S)-hydroxy-PGH2, 15-keto-PGH2 and PGH3 were almost completely isomerized to the corresponding prostacyclin derivative by bovine aortic prostacyclin synthase, whereas PGH1 and 8-iso-PGH2 only produced the corresponding C17 hydroxy acid plus malondialdehyde. Isotope-labeling experiments with [5,6,8,9,11,12,14,15-2H]PGH2 revealed complete retention of label and no isotope effect in the course of thromboxane biosynthesis, but the loss of one 2H atom at C-6 with an isotope effect of 1.20 during PGI2 formation. Prostacyclin and thromboxane synthase bind both 9,11-epoxymethano-PGF2 alpha and 11,9-epoxymethano-PGF2 alpha at the heme iron, but according to their difference spectra in opposite ways with respect to the 9- and 11-position. In agreement with published model studies, a cage radical mechanism is proposed for both enzymes according to which the initial radical process is terminated through oxidation of carbon-centered radicals by the iron-sulfur catalytic site, followed by ionic rearrangement to PGI2 or TXA2. Various Fe(III) model compounds as well as liver microsomes or cytochrome P-450CAM can also form small amounts of PGI2 and TXA2, but mainly yield 12(S)-hydroxy-5,8,10-heptadecatrienoic acid plus malondialdehyde probably by a radical fragmentation pathway.     

Inhibition of PGE1-stimulated cAMP accumulation in human platelets by thromboxane a2.

The prostaglandin endoperoxide PGH2, HHT, HETE, thromboxane A2, and thromboxane B2, which are all products of arachidonic acid metabolism of human platelets, were tested for their ability to modulate platelet cyclic nucleotide levels. None of the compounds tested altered the basal level of cAMP or cGMP, and only PGH2 and thromboxane A2 inhibited PGE1-stimulated cAMP accumulation. Thromboxane A2 was found to be a more potent inhibitor of PGE1-stimulated cAMP accumulation and inducer of platelet aggregation than PGH2.     

Protein engineering of thromboxane synthase: conversion of membrane-bound to soluble form

Thromboxane A2 synthase (TXAS) binds to the endoplasmic reticulum membrane and catalyzes both an isomerization of prostaglandin H2 (PGH2) to form thromboxane A2 (TXA2) and a fragmentation of PGH2 to form 12-hydroxy-5,8,10-heptadecatrienoic acid (HHT) and malondialdehyde (MDA). TXAS is a non-classic cytochrome P450 in that it does not require molecular oxygen or an external electron donor for catalysis. Difficulty in obtaining crystals from the membrane-bound TXAS prompted us to modify the protein to a soluble form. Results from site-directed mutagenesis, hydropathy analysis, and homology modeling led us to identify a putative membrane association segment near the end of helix F in TXAS. We report here the generation of a soluble form of TXAS by deletion of the amino-terminal membrane-anchoring domain and replacement of the helix F and F-G loop region with the corresponding region of the structurally characterized microsomal P450 2C5. The resultant TXAS/2C5 chimera is expressed in bacteria as a cytosolic and monomeric protein. Addition of an amino-terminal leader sequence to enhance expression and a tetra-histidine segment at the carboxyl-terminus to facilitate purification yielded approximately 4 mg of nearly homogeneous TXAS/2C5 per liter of bacterial culture. The TXAS/2C5 chimera contains heme at nearly a 1:1 molar ratio and catalyzes the formation of TXA2, MDA, and HHT at a 1:1:1 ratio, although with a reduced catalytic activity compared to wild type TXAS. TXAS/2C5 exhibits electronic absorption spectra similar to wild type TXAS and has similar affinities toward distal heme ligands such as imidazole and U44069. The chimera was mono-dispersive and thus is promising for crystallization trials.     

Metabolism of prostaglandin endoperoxide by microsomes from cat lung

It has been reported that the prostaglandin (PG) precursor, arachidonic acid, produces divergent hemodynamic responses in the feline pulmonary vascular bed. However, the pattern of arachidonic acid products formed in the lung of this species is unknown. In order to determine the type and activity of terminal enzymes in the lung, prostaglandin biosynthesis by microsomes from cat lung was studied using the prostaglandin endoperoxide, PGH2, as a substrate. The major products of incubations of PGH2 with microsomes were thromboxane (TX) B2 (the major metabolite of TXA2), 6-keto-PGF1 alpha (the breakdown product of PGI2) and 12L-hydroxy-5,8,10-heptadecatrienoic acid (HHT). Formation of TXB2 was markedly reduced by imidazole. Tranylcypromine decreased the formation of TXB2 and HHT and inhibited the formation of 6-keto-PGF1 alpha. At low PGH2 concentrations, equal production of TXB2 and 6-keto-PGF1 alpha was observed. However, as PGH2 concentration increased, 6-keto-PGF1 alpha production approached early saturation while TXB2 production increased in a linear fashion. These results suggest that enzymatic formation of TXA2 and PGI2 is a function of substrate availability in the lung. These findings provide a possible explanation for the divergent hemodynamic responses to arachidonic acid infusions at high and low concentrations in the feline pulmonary vascular bed.     

Up-regulation of endothelial cyclooxygenase-2 and prostanoid synthesis by platelets. Role of thromboxane A2

Platelet-vascular endothelial cell interactions are central to the maintenance of vascular homeostasis. Thromboxane A2 (TXA2) and prostacyclin (prostaglandin (PG)I2) are the major products of cyclooxygenase (COX) metabolism by platelets and the vascular endothelium, respectively. Here we report the effects of platelet-endothelial interactions on human umbilical vein endothelial cells (HUVECs) COX-2 expression and prostanoid synthesis. Co-incubation of platelets with HUVECs resulted in a dose-dependent induction in COX-2 expression. This was accompanied by a relatively small increase in thromboxane B2 synthesis (2 ng) by comparison to the production of 6-keto-PGF1alpha and PGE2, which increased by approximately 14 and 12 ng, respectively. Abrogation of platelet-HUVEC interactions excluded direct cell-cell contact as a required event. Preincubation of HUVECs with SQ29548, a TXA2 receptor antagonist, dose-dependently inhibited platelet-induced COX-2 expression and prostanoid synthesis. Similarly, if platelet TXA2 synthesis was inhibited no induction of COX-2 was observed. Furthermore, a TXA2 analog, carbocyclic TXA2, induced HUVEC COX-2 expression and the synthesis of 6-keto-PGF1alpha and PGE2. This was also associated with an increase in the expression and activity of PGI synthase and PGE synthase but not TX synthase. Platelet co-incubation (or TXA2) also selectively activated the p44/42 mitogen-activated protein kinase pathway to regulate HUVEC COX-2 expression. Thus it seems that platelet-derived TXA2 can act in a paracrine manner to up-regulate endothelial COX-2 expression and PGI2 synthesis. These observations are of particular importance given the recent observations regarding selective COX-2 inhibitors and the suppression of PGI2 synthesis.     

Mechanism for antiplatelet effect of onion: AA release inhibition, thromboxane A(2)synthase inhibition and TXA(2)/PGH(2)receptor blockade

Antiplatelet actions of aqueous extract of onion were investigated in rat and human platelet. IC(50)values of onion extract for collagen-, thrombin-, arachidonic acid (AA)-induced aggregations and collagen-induced thromboxane A(2)(TXA(2)) formation were 0.17 +/- 0. 01, 0.23 + 0.03, 0.34 +/- 0.02 and 0.12 +/- 0.01 g/ml, respectively. [(3)H]-AA release induced by collagen (10 microg/ml) in rat platelet was decreased by onion compared to control (22.1 +/- 2.13 and 5.2 +/- 0.82% of total [(3)H]-AA incorporated, respectively). In fura-2 loaded platelets, the elevation of intracellular Ca(2+)concentration stimulated by collagen was inhibited by onion. Onion had no cytotoxic effect in platelet. Onion significantly inhibited TXA(2)synthase activity without influence on COX activity. Platelet aggregation induced by U46619, a stable TXA(2)mimetic, was inhibited by onion, indicating its antagonism for TXA(2)/PGH(2)receptor. These results suggest that the mechanism for antiplatelet effect of onion may, at least partly, involve AA release diminution, TXA(2)synthase inhibition and TXA(2)/PGH(2)receptor blockade.     

Evidence for a role of prostaglandin I2 and thromboxane A2 in the ductus venosus of the lamb

The prostaglandin (PG) endoperoxide, PGH2, and the thromboxane (TX) A2 analog, 9,11-epithio-11,12-methano-TXA2, were tested in vitro on the ductus venosus sphincter from fetal (premature and mature) and neonatal (1-day-old) lambs. PGH2 relaxed the indomethacin-contracted fetal ductus in a dose-dependent manner and its action was reduced after treatment with 15-hydroperoxyarachidonic acid. In contrast, reduced glutathione did not affect the PGH2 relaxation in the indomethacin-treated ductus, nor did it modify the response of the untreated ductus to constrictor stimuli. Unlike PGH2, the stable 9 alpha,11 alpha-epoxymethano-PGH2 analog contracted the vessel. Similarly, the TXA2 analog was a contractile agent, its action exceeding that of the PGH2 analog in potency and efficacy. The TXA2 analog was active on preparations from both premature (minimum 117 days gestation) and mature lambs, but a maximal effect was attained during the perinatal period. These results confirm the existence of a PG-mediated relaxing mechanism in the ductus venosus and suggest that the active compound is PGI2. This mechanism is likely responsible for keeping the ductus patent in the fetus. TXA2, formed within the liver parenchyma, is well suited for playing a role in postnatal closure of the vessel.     

A common binding site for primary prostanoids in vascular smooth muscles: a definitive discrimination of the binding for thromboxane A2/prostaglandin H2 receptor agonist from its antagonist

Differences in binding characteristics between agonists and antagonists for the thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptor were examined in rat cultured vascular smooth muscle cells (VSMC). Scatchard analysis indicated the existence of two binding sites for the TXA2/PGH2 agonist, whereas a single class of recognition sites for the receptor antagonists were observed with approximately the same maximum binding capacity (Bmax) as a high-affinity binding site of the agonist. Weak binding inhibition by approx. 100 nM of primary prostanoids (PGE1, PGF2 alpha and PGD2) was detected only with the TXA2/PGH2 agonist, and not with the antagonist. Primary prostanoids as well as TXA2/PGH2 agonists (U46619 and STA2) suppressed the [3H]PGF2 alpha and [3H]PGE1 binding with almost the same potency, whereas TXA2/PGH2 antagonists (S-145, SQ29,548 and ONO3708) did not. The Bmax value of the binding sites was roughly identical in PGF2 alpha, PGE1 and a low-affinity binding site of U46619. These results suggest the existence of two binding sites for TXA2/PGH2 in VSMC, i.e., a high-affinity binding site corresponding to that of the TXA2/PGH2 antagonists and a low-affinity binding site in common with primary prostanoids.     

Higher viscosity participates in the regulation of coronary flow via nitric oxide and indomethacin-sensitive contracting factor

Few studies have reported on the association of viscosity with coronary circulation. We evaluated the change in coronary flow after dextran was added to a perfusion solution to increase viscosity in isolated rat hearts. We also measured NOx- production induced by the change in shear stress in the coronary effluent, as a marker of NO synthesis. The baseline coronary flow was not influenced by the presence of either the cyclooxygenase inhibitor indomethacin, the thromboxane A2 (TXA2)-prostaglandin H2 (PGH2) receptor antagonist ONO-3708, or the TXA2 synthase inhibitor OKY-046. After exposure to solution containing 0.5% dextran, the coronary flow first decreased and then gradually increased until 10 min. The initial decrease in coronary flow was inhibited by indomethacin, ONO-3708, and OKY-046 individually. The gradual increase was completely inhibited by the NO inhibitor L-NAME, but not by indomethacin or ONO-3708. OKY-046 partially inhibited the increase. NOx- levels in the effluent were higher after the dextran solution was administered, and the increased NOx- levels were inhibited by L-NAME. The increased NOx- levels were not inhibited by inhibitors of the cyclooxygenase pathway. It appears that a higher viscosity of perfusion solution induced a gradual increase in NO production and was associated with increased production of indomethacin-sensitive contracting factor.     

Characterization of thromboxane A2/prostaglandin H2 receptors and histamine H1 receptors in cultured guinea-pig tracheal smooth-muscle cells.

We characterized thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptors and histamine H1 receptors in Guinea-pig cultured tracheal smooth-muscle cells (TSMC). [3H]SQ 29,548 (a TXA2 antagonist)-binding sites were saturable and a high affinity with a dissociation constant of 6.2 +/- 0.60 nM (mean +/- S.E.) and a receptor density of 46 +/- 4.6 fmol/10(6) cells. [3H]SQ 29548 binding was completely inhibited by TXA2 mimetics or antagonists. Intracellular calcium concentration ([Ca2+]i) in TSMC was increased with U46619 stimulation and the increase was attenuated by TXA2 antagonists, the potencies of which correlated with those inhibiting the activities of the [3H]SQ 29548 binding. [3H]Mepyramine (a H1 antagonist)-binding sites were also present in TSMC. [3H]Mepyramine had a single class of low-affinity-binding sites with a dissociation constant of 2.6 +/- 0.081 microM and a receptor density of 10.6 +/- 0.11 nmol/mg protein. [3H]Mepyramine binding in TSMC membrane was inhibited by H1 antagonists, but not by H2 antagonists. The inhibition constants of mepyramine in TSMC were 910-times lower than those in tracheal membranes. In contrast, the histamine-induced increase in [Ca2+]i in TSMC was inhibited in the presence of low concentrations of H1 antagonists. All these observations provide evidence that TXA2/PGH2 receptors, mepyramine-binding sites and/or H1 receptors are expressed in cultured TSMC.     

The biological role of thromboxane A2 in the process of hemostasis and thrombosis; pharmacology and perspectives of therapeutical use of thromboxane synthetase inhibitors and receptor PGH2/TXA2 antagonists

The biological role of thromboxane A2 in the process of hemostasis and thrombosis; pharmacology and perspectives of the therapeutical use of thromboxane synthetase inhibitors and receptor PGH2/TXA2 antagonists. Acta physiol. pol., 1985, 36 (3): 153-164. The biology of thromboxane A2 and pharmacology of drugs that selectively inhibit generation and action of this eicosanoid are reviewed. Author's opinion on therapeutical perspectives for thromboxane synthetase inhibitors and receptor PGH2/TXA2 antagonists is also presented.     

Intracellular Ca2+ mobilization and not calcium influx promotes phorbol ester-stimulated thromboxane A2 synthesis in human platelets.

Phorbol esters, potent activators of protein kinase C (PKC), greatly enhance the release of arachidonic acid and its metabolites (TXA2, HETES, HHT) by Ca2+ ionophores in human platelets. In this paper, we report the relationship between intracellular Ca2+ mobilization and external calcium influx into platelets and the ability of PMA plus A23187 to promote thromboxane A2 (TXA2) synthesis. The enhanced levels of TXA2 due to the synergistic stimulation of the platelets with A23187 and phorbol esters are not affected significantly by the presence of external Ca2+ or the calcium-chelator EGTA. PKC inhibitors, staurosporine and sphingosine, abolished phorbol myristate acetate (PMA) potentiation of TXA2 production which strongly supports the role of PKC in the synergism. Platelet aggregation is more sensitive to PMA and external calcium than TXA2 formation. PMA increased TXA2 production as much as 4-fold at low ionophore concentrations. The A23187-induced rise in [Ca2+]i was reduced by pretreatment of human platelets with phorbol esters, both in the presence and absence of EGTA, and staurosporine reversed this inhibitory effect. These results indicate that the synergistic stimulation of TXA2 production by A23187 and phorbol esters is promoted by intracellular Ca2+ mobilization and not by external calcium influx. Our data also suggest that PKC is involved in the regulation of Ca2+ mobilization from some specific intracellular stores and that PKC may also stimulate the Ca(2+)-dependent phospholipase A2 at suboptimal Ca2+i concentrations.