Development of radioimmunoassay for thromboxane B2

Thromboxanes: synthase and receptors

Thromboxane A2 is a biologically potent arachidonate metabolite through the cyclooxygenase pathway. It induces platelet aggregation and smooth muscle contraction and may promote mitogenesis and apoptosis of other cells. Its roles in physiological and pathological conditions have been widely documented. The enzyme that catalyzes its synthesis, thromboxane A2 synthase, and the receptors that mediate its actions, thromboxane A2 receptors, are the two key components critical for the functioning of this potent autacoid. Recent molecular biological studies have revealed the structure-function relationship and gene organizations of these proteins as well as genetic and epigenetic factors modulating their gene expression. Future investigation should shed light on detailed molecular signaling events specifying thromboxane A2 actions, and the genetic underpinning of the enzyme and the receptors in health and disease.

Differential expression of thromboxane A synthase and prostaglandin H synthase in megakaryocytic cell line

We determined the expression of isoforms of prostaglandin H synthase (PGHS) and thromboxane A synthase (TXAS) in a human megakaryocyte cell line (MEG-01. The basal levels of full-length TXAS mRNA and the 60 kDa TXAS protein were high when compared to those of PGHS-1 and PGHS-2 in uninduced cells. Despite a high TXAS level, uninduced MEG-01 cells synthesized only a small amount of thromboxane A2 (TXA2) due to limited PGHS-1 or PGHS-2 expressions. Following PMA induction there was little change in TXAS. PGHS-2 mRNA was significantly increased at only 3 h of PMA treatment and the level declined rapidly, whereas PGHS-1 mRNA and protein levels were concordantly stimulated. Induction of PGHS-1 reached plateau on day 3 of PMA treatment. Analysis of arachidonate metabolism in cells induced by PMA for 3 and 5 days showed a high level of PGH2 synthesis which exceeded the TXAS capacity for TXA2 synthesis. Only traces of PGHS-2 mRNA and alternate-spliced TXAS mRNA were detected in human platelets. We conclude that TXAS and PGHS are differentially expressed in MEG-01 during PMA-induced differentiation.

Altered hepatic eicosanoid concentrations in rats treated with the peroxisome proliferators ciprofibrate and perfluorodecanoic acid

Several hypolipidemic drugs, plasticizers, and other chemicals induce hepatic peroxisome proliferation and hepatocellular carcinomas in rodents. These agents induce and promote hepatocarcinogenesis by unknown mechanisms, since most studies have not found them to be genotoxic. Peroxisome proliferators increase the expression of several genes, including those for the enzymes of the peroxisomal beta-oxidation pathway and the cytochrome P-450 4A family, which metabolize lipids, including eicosanoids and their precursor fatty acids. The peroxisome proliferators ciprofibrate and perfluorodecanoic acid (PFDA) were therefore examined for their ability to alter hepatic eicosanoid concentrations. Rats received injections of 3 or 10 mg PFDA/kg body weight every 14 days or were fed 0.01% ciprofibrate for 10 days, 24 days, 6 weeks, 26 weeks, or 54 weeks. The activity of the peroxisomal enzyme fatty acyl CoA oxidase was significantly increased by both ciprofibrate and PFDA at all times. Hepatic concentrations of prostaglandins E2 and F2a (PGE2, PGF2a), thromboxane B2 (TXB2), and leukotriene C4 (LTC4) were measured by immunoassay. Concentrations of PGE2, PGF2a, and TXB2 were decreased in livers of rats receiving ciprofibrate or PFDA compared to livers of control rats, with ciprofibrate exerting a greater effect than PFDA at the doses used. Hepatic LTC4 concentrations were significantly increased by ciprofibrate at 10 days and PFDA at 54 weeks, and significantly decreased by PFDA at 26 weeks. These alterations in eicosanoid concentrations may be important in the natural history of peroxisome proliferator-induced hepatocarcinogenesis.

Dual inhibitory mechanism of secretin action on acid secretion in totally isolated, vascularly perfused rat stomach

BACKGROUND/AIMS

Secretin is an inhibitory hormone of gastric acid secretion. However, its inhibitory mechanism has not been well understood. Possible roles of both somatostatin and prostaglandins were investigated.

METHODS

Totally isolated rat stomachs were vascularly perfused with Krebs-Ringer buffer containing 50 mumol/L isobutyl methylxanthine at 1.4 mL.min-1. Gastric lumen was perfused with 0.15 mol/L NaCl at 1.0 min.min-1. Effect of secretin in three different doses given intra-arterially on basal acid secretion and acid secretion stimulated by pentagastrin was studied. To determine roles of somatostatin and prostaglandins in the secretin-induced inhibition, an antisomatostatin serum and indomethacin were tested, and both somatostatin and prostaglandin E2 concentrations in portal venous effluent were determined by radioimmunoassay.

RESULTS

Both basal- and pentagastrin-stimulated acid secretion were significantly inhibited by secretin. The inhibition was completely reversed by either indomethacin or antisomatostatin serum. Secretin significantly increased concentrations of both somatostatin and prostaglandin E2. Although indomethacin blocked the increase in prostaglandin E2, secretin-induced increase in prostaglandin E2 was not affected by antisomatostatin serum or was indomethacin influenced by somatostatin level. Finally, the inhibition by somatostatin of acid secretion was not affected by indomethacin.

CONCLUSIONS

The inhibition of gastric acid secretion by secretin in rats is mediated by simultaneous releases of both somatostatin and prostaglandin E2, which independently inhibit gastric acid secretion.

Agonist-induced lipoxin A4 generation: detection by a novel lipoxin A4-ELISA

Lipoxin A4 (LXA4) possesses potent bioactions. To facilitate its detection, an enzyme-linked immunosorbent assay (ELISA) was developed that proved sensitive and selective. Quantitation by ELISA of LXA4 generated from cellular sources strongly correlated (r = 0.99) with values obtained by high-pressure liquid chromatography (HPLC). We used this LXA4-ELISA to examine parameters influencing LXA4 generation from endogenous substrates during human platelet-neutrophil (PLT-PMN) interactions in vitro. Agonist-induced LXA4 production was clearly evident at a PLT-PMN ratio of 10:1, and recombinant human granulocyte/monocyte colony stimulating factor-priming of PMN augmented LXA4 generation 5-6 fold. The chemotactic peptide formylmethionyl-leucyl-phenylalanine, platelet-derived growth factor and arachidonic acid (20:4n-6) each stimulated formation of immunoreactive LXA4 (iLXA4) in these co-incubations. The presence of iLXA4 was also evaluated in vivo in aspirin-sensitive asthmatic patients who, in a randomized, double-blind crossover design, underwent nasal lavage after they each ingested a predetermined threshold dose of aspirin or placebo. Aspirin challenge provoked statistically significant increases in iLXA4 in each patient (P < 0.005). These results validate the use of a solid-phase ELISA for detection of LXA4. Furthermore, the use of this ELISA has allowed the first documentation of iLXA4 formation in human subjects with aspirin-sensitive asthma following specific antigenic challenge.

The differences in 12-hydroxyeicosatetraenoic acid and thromboxane B2 release from guinea pig platelets

Anti-12(S)-hydroxyeicosatetraenoic acid (12-HETE)-antibody and anti-thromboxane B2 (TXB2)-antibody were generated and applied to the radioimmunoassay. The detection limit for 12-HETE was 16 pg. The cross-reactivities of anti-12-HETE-antibody were 4.6% for 15-HETE, 0.18% for 5-HETE and below 0.15% for leukotrienes and prostaglandins (PGs). 12-HETE and TXB2 released from guinea pig platelets were measured by radioimmunoassay. Platelet activating factor (PAF) at 10(-9) M induced the aggregation of platelets, the releases of immunoreactive-12-HETE (1.8 +/- 1.2 ng/10(8) platelets, mean +/- S.D.) and immunoreactive-TXB2 (18.5 +/- 17.3 ng/10(8) platelets). Collagen at 1 microgram/ml also evoked platelet aggregation, the releases of immunoreactive-12-HETE (2.7 +/- 1.1 ng/10(8) platelets) and immunoreactive-TXB2 (11.8 +/- 4.6 ng/10(8) platelets). By the stimulation with these compounds, TXB2 was produced in a greater amount than 12-HETE from guinea pig platelets. Although 10(-7) M and 10(-6) M U46619, a TXA2 mimetic, caused platelet aggregation, arachidonic acid metabolites were not released. These data suggest the presence of different mechanisms of platelet activation depending on each stimulus.

Platelet-activating factor (PAF) inhibitory profile of KO-286011 on blood platelets in vitro and in vivo

A newly synthesised structural analogue of PAF, coded KO-286011 (1-O-hexadecyl-2-O-ethyl-rac-glycero-3-phosphoric acid 4-(N,N-dimethylamino)pyridinium butylester), was proved for its ability to inhibit PAF-mediated platelet responses in vitro and in vivo. The compound inhibited effectively the PAF-induced aggregation and secretion of human and rabbit platelets. In contrast, there was little influence on ADP-, collagen-, and arachidonic acid-triggered platelet responses. Schild-analysis of aggregation data ascertained in human platelet-rich plasma was consistent with a simple competitive antagonism and yielded a pA2 of 6.44. Proaggregatory activity of KO-286011 was excluded turbidimetrically as well as by means of a single cell counting technique. [3H]PAF binding studies provided evidence that KO-286011 exerts its inhibitory action at the PAF-receptor level. A significant inhibition of the ex vivo PAF-induced platelet aggregation was found after i.v. administration of 0.5 mg/kg KO-286011 to rabbits. The effect was most pronounced 5 min after dosing the inhibitor and detectable over a period of 30 min. Intravenous administration of 10 and 25 micrograms/kg KO-286011 to guinea pigs prevented dose-dependently the PAF-induced formation of thromboxane A2. The PAF-inhibitory action of KO-286011 was more potent than that of the ginkgolide BN 52021.

Measurement of PGH synthase activity using a pan-specific monoclonal antibody

A pan-specific monoclonal antibody that recognizes a variety of prostaglandin moieties and does not recognize arachidonic acid or the hydroxy-eicosatetraenoic acids was used to assess the general ability of a tissue to produce prostaglandins. Although this assay does not give a quantitative measure of PGH synthase activity, it does provide a sensitive and convenient means of screening a large number of samples for prostaglandin production.

Purification and characterization of an NAD(+)-dependent dehydrogenase that catalyzes the oxidation of thromboxane B2 at C-11 from porcine liver. Development and application of 11-dehydro-thromboxane B2 radioimmunoassay to enzyme assay

11-Dehydro-thromboxane B2 has been identified as a major metabolite of infused as well as endogenous thromboxane B2 in mammalian plasma and urine. This metabolite is derived from thromboxane B2 by enzymatic oxidation at C-11 catalyzed by 11-hydroxythromboxane B2 dehydrogenase. A radioimmunoassay for 11-dehydro-thromboxane B2 has been developed and used for enzyme assay, purification and characterization. Antibodies were generated against 11-dehydro-thromboxane B2 conjugated to bovine thyroglobulin. Labeled marker was prepared by radioiodinating 11-dehydro-thromboxane B2-tyrosine methyl ester conjugate. A sensitive radioimmunoassay capable of detecting 10 pg of 11-dehydro-thromboxane B2 per assay tube was developed. The antibodies showed minimal crossreaction with thromboxane B2 (0.03%), prostaglandin D2 (2.76%) and other eicosanoids (less than 0.03%). The enzyme activity was determined by assaying NAD(+)-dependent formation of immunoreactive 11-dehydro-thromboxane B2 from thromboxane B2. The enzyme was found to be enriched in liver although significant activity was also detected in gastrointestinal tract and kidney in pig. The enzyme was purified from porcine liver cytosol to apparent homogeneity using conventional and affinity chromatography. The purified enzyme exhibited coenzyme specificity for NAD+ and used thromboxane B2 as a substrate. The enzyme also catalyzes NADH-dependent reduction of 11-dehydro-thromboxane B2 to thromboxane B2 indicating the reversibility of the enzyme catalyzed reaction. The apparent Km values for thromboxane B2, 11-dehydro-thromboxane B2 and NAD+ are 8.1, 8.0 and 23 microM, respectively. Subunit Mr was shown to be 55,000, whereas the native enzyme Mr was found to be 110,000 indicating that the enzyme is a dimer. The enzyme is sensitive to sulfhydryl inhibitions suggesting cysteine residues are essential to enzyme activity. The availability of a homogeneous enzyme preparation should allow further studies on the substrate specificity and the structure and function of the enzyme.

Dimethylthiourea reverses sepsis-induced pulmonary hypertension in piglets

Dimethylthiourea (DMTU), a putative hydroxyl radical scavenger, attenuates thromboxane generation and pulmonary hypertension in the piglet model of group B streptococcal (GBS) sepsis. This study tested the hypothesis that DMTU reverses ongoing GBS-induced pulmonary hypertension coincident with decreased thromboxane production. Piglets (n = 15) received a 60 min infusion of GBS (10(-8) cfu/kg/min). Mean pulmonary artery pressure (Ppa), arterial blood gases (ABGs), and thromboxane B2 (TXB) levels were measured at 10 min intervals throughout the study. GBS infusion resulted in a marked increase in pulmonary artery pressure (mean delta Ppa = 31 mm Hg) and a significant decline in PaO2 (mean = -80 torr) within 10 min of beginning the infusion. pH decreased from a mean of 7.47 to 7.37. DMTU, 750 mg/kg, or normal saline vehicle was infused over 10-15 min beginning 10 min after initiating GBS. Ppa decreased significantly within 10 min of DMTU infusion. Piglets receiving vehicle had a slow decline in Ppa. Piglets receiving DMTU also had an improvement in PaO2 and showed no further drop in pH. Piglets receiving vehicle had no improvement in PaO2 and demonstrated a continued decline in pH. TXB levels did not differ between the groups at any time interval. We conclude that DMTU can partially reverse GBS-induced pulmonary hypertension, but may function through mechanisms independent of thromboxane generation.

Calcium ionophore potentiates chemotactic peptide and platelet activating factor in stimulating thromboxane B2 and leukotriene B4 biosynthesis in human neutrophils

Formyl-Met-Leu-Phe (FMLP) and platelet activating factor (PAF) stimulated the synthesis of thromboxane B2 (TXB2) and leukotriene B4 (LTB4) to a small degree in human neutrophils. Calcium ionophore A-23187 enhanced synergistically both FMLP and PAF induced eicosanoid synthesis, whereas phorbol ester PMA attenuated PAF but not FMLP stimulated arachidonate metabolism. These results suggest that calcium mobilization may be a rate limiting step in FMLP and PAF induced synthesis of TXB2 and LTB4 and that protein kinase C activation may play a negative regulatory role in PAF stimulated eicosanoid synthesis.

Phorbol ester induces inhibition of arachidonate incorporation into phospholipids in human neutrophils

PMA is known to enhance calcium ionophore A-23187 induced arachidonate release in human neutrophils. Mechanism of enhancement by PMA is not clear. We have found that neutrophils pretreated with PMA showed significant reduction in labeled arachidonate uptake. Decrease in arachidonate uptake following PMA treatment was attributed, at least in part, to inactivation of arachidonoyl CoA synthase and arachidonoyl CoA lysophosphatide acyltransferase, two key enzymes involved in arachidonate incorporation into phospholipids. These results suggest that PMA may induce protein kinase C activation which in turn may cause inactivation of the two enzymes involved in incorporation of arachidonate resulting in greater availability of arachidonate which is liberated by A-23187 for oxygenation and release into extracellular space.

ABBREVIATIONS

PMA, 4 beta-phorbol 12-myristate 13-acetate; PDD, 4 alpha-phorbol 12,13-didecanoate; TXB2, thromboxane B2; LTB4, leukotriene B4; PC, phosphatidylcholine; LPC, lysophosphatidylcholine; DMSO, dimethylsulfoxide.

Quantitation of 13-hydroxyoctadecadienoic acid (13-HODE) by radioimmunoassay

Antibodies against 13-hydroxyoctadecadienoic acid (13-HODE) were produced in rabbits by immunizing the animal with 13-HODE-thyroglobulin conjugate. The antibodies appeared to be rather specific for 13-HODE since other hydroxy fatty acids showed minimal crossreaction. The radioimmunoassay was capable of detecting 50 pg per assay tube and was applied to the study of the biosynthesis of 13-HODE in platelets and leukocytes. In contrast to reported findings from endothelial cells, A-23187, thrombin and collagen stimulated synthesis and release of 13-HODE from platelets. However, insignificant synthesis of 13-HODE was found in leukocytes following A-23187 stimulation. Exogenous addition of linoleic acid stimulated the synthesis of 13-HODE from both platelets and leukocytes. The majority of 13-HODE synthesized was found in the medium. These studies suggest that both types of blood cells possess active (omega-6) lipoxygenase. Platelets may use endogenously released linoleic acid to synthesize 13-HODE, whereas leukocytes may utilize linoleic acid released from other cell types for 13-HODE synthesis.

Alterations of arachidonate metabolism in cardiovascular system by cigarette smoking

Male rats were exposed to freshly generated cigarette smoke once daily for various lengths of time. Inhalation of smoke was verified by elevated levels of carboxyhemoglobin. Metabolism of arachidonate in the cardiovascular system to thromboxane and prostacyclin through the cyclooxygenase pathway and their further metabolism to 15-keto-derivatives, and to 12-hydroxyeicosatetraenoic acid (12-HETE) through lipoxygenase pathway was investigated. Synthesis of thromboxane and prostacyclin in platelets and aortas respectively was not changed within 8 weeks of smoke exposure. However, formation of 12-HETE in platelets was significantly increased after 4 weeks of smoke exposure. Catabolism of thromboxane and prostacyclin as determined by NAD(+)-dependent 15-hydroxyprostaglandin dehydrogenase activity was greatly decreased in lung but not in kidney and stomach following 4 weeks of smoke exposure. Increased 12-lipoxygenase activity in platelets may lead to stimulation of migration and proliferation of smooth muscle cells and to increased synthesis of leukotrienes in neutrophils. Decreased pulmonary prostaglandin catabolic activity may result in increase in circulating thromboxane/prostacyclin ratio and subsequently alteration of vascular homeostasis. The consequence of these biochemical changes may contribute to the development of atherosclerosis, thromboembolism and emphysema commonly found in smokers.

Pharmacology of UK-38485 (dazmegrel), a specific inhibitor of thromboxane A2 synthetase

Thromboxane A2 (TXA2) synthesis in rabbit and human platelet rich plasma (PRP) was inhibited in a dose-dependent manner by UK-38485 (dazmegrel) when the PRP was aggregated with collagen, arachidonic acid and ADP. The level of inhibition was time-dependent. That is, the dose-response curves shifted to lower concentrations with increasing incubation times with UK-38485 prior to addition of aggregation agents. Following bolus intravenous injections of UK-38485 in rabbits, the elimination from serum fitted a 3-exponential curve. The terminal elimination phase had a half-life of 69.8 +/- 3.8 min. Oral treatment of rabbits with UK-38485 for 2 weeks showed that animals with serum concentrations of 0.358 +/- 0.091 microgram/ml of the inhibitor had TXA2 synthesis inhibited in serum by 83.8 +/- 7.1%. This corresponded to animals which were treated with 20 mg/kg/day of the inhibitor.

Synergism between chemotactic peptide and platelet-activating factor in stimulating thromboxane B2 and leukotriene B4 biosynthesis in human neutrophils

Formyl-Met-Leu-Phe (FMLP) and platelet-activating factor (PAF) were capable of stimulating thromboxane B2 (TXB2) and leukotriene B4 (LTB4) syntheses in human neutrophils, albeit in a relatively poor degree. A combination of FMLP and PAF, however, was synergistic in stimulating TXB2 and LTB4 syntheses. Phorbol myristate acetate (PMA) appeared to attenuate PAF- but not FMLP-induced arachidonate metabolism. These results suggest that cooperative action of FMLP and PAF on arachidonate release and metabolism does exist and that PMA-mediated protein kinase C activation may regulate FMLP and PAF actions in a different manner.

Does protein kinase C activation mediate thrombin-induced arachidonate release in human platelets?

Thrombin stimulated rapid formation of diacylglycerol, inositol 1,4,5-trisphosphate (IP3) and thromboxane B2 (TXB2) in human platelets. Formation of diacylglycerol and IP3 appeared to precede that of TXB2. Activation of protein kinase C by diacylglycerol combining with Ca+2 mobilization by IP3 has been implicated in mediating arachidonate release. However, addition of the protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7) to platelet suspension did not inhibit thrombin-stimulated arachidonate release and TXB2 synthesis, whereas addition of the Ca+2 antagonist, 3,4,5-trimethoxybenzoic acid 8-(diethylamino) octyl ester (TMB-8) or the calmodulin antagonist N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) abolished arachidonate release. The correlation of IP3 production with arachidonate release on increasing the concentrations of thrombin was further examined. IP3 production reached near maximum at 0.2 U/ml, whereas TXB2 synthesis continued to increase at 1 U/ml. These results suggest that protein kinase C activation may not mediate arachidonate release and that Ca+2 mobilization by IP3 may only partially account for arachidonate release in platelets stimulated with relatively high concentrations of thrombin.

Monoclonal antibody against TXB2: its use in solid and liquid phase radioimmunoassays

Monoclonal antibodies to thromboxane B2 (TXB2) have been produced and characterized. Both liquid and solid phase radioimmunoassays have been developed using one of these monoclonal antibodies. The two assays gave similar results when used to quantitate TXB2 in 11 serum samples; however the solid phase assay was more sensitive than the liquid phase assay (i.e., 63 pg/ml vs 19 pg/ml) at a B/BO = 90%). Despite a difference in the sensitivity of the two assay systems, the cross-reactivity of the monoclonal antibody for PGD2, PGE2, PGF2 alpha and 6 keto-PGF1 alpha was the same.

Stimulations of arachidonate release and inositol-1,4,5-triphosphate formation are mediated by distinct G-proteins in human platelets

Addition of fluoroaluminate to human platelet suspension stimulated thromboxane synthesis and inositol-1,4,5-triphosphate formation in a time and dose dependent manner. Neomycin inhibited markedly fluoroaluminate induced inositol-1,4,5-triphosphate formation without significantly affecting thromboxane synthesis. Preincubation of platelets with PGE1, also inhibited significantly inositol-1,4,5-triphosphate formation with modest reduction of thromboxane synthesis. On the contrary, pretreatment of platelets with pertussis toxin inhibited fluoroaluminate stimulated thromboxane synthesis without affecting inositol-1,4,5-triphosphate formation. Similarly, preincubation of platelets with phorbol ester, PMA, inhibited markedly thromboxane synthesis with modest reduction of inositol-1,4,5-triphosphate formation. These results indicate that inositol-1,4,5-triphosphate formation and arachidonate release and thromboxane synthesis are controlled separately and are mediated by different G-proteins which are coupled to phospholipase C and phospholipase A2 respectively in platelets.

Increase in thromboxane B2 and decrease in prostaglandin E2 and 6-keto-prostaglandin F1 alpha release into rat bronchoalveolar fluid as a consequence of cigarette smoking

Rats were exposed to cigarette smoke once daily for 4 to 8 weeks. Bronchoalveolar lavage fluid was obtained from each animal and assayed for immunoreactive PGE2, TXB2 and 6-Keto-PGF1 alpha. Significant increase in TXB2 and decrease in PGE2 and 6-Keto-PGF1 a release into bronchoalveolar fluid as a consequence of cigarette smoking were observed. These changes of arachidonate metabolites in lung alveoli may account in part for bronchoconstriction induced by cigarette smoking.

Selective inhibition of 5-lipoxygenase pathway in rat pulmonary alveolar macrophages by cigarette smoking

Pulmonary alveolar macrophages from sham or cigarette-smoke-exposed rats were examined for their ability to transform exogenously added arachidonate to metabolites of lipoxygenase and cyclooxygenase pathways. Synthesis of 5-HETE and leukotriene B4 was selectively inhibited by cigarette smoke exposure, whereas the formation of prostaglandin E2 and thromboxane B2 remained unchanged. Selective inhibition of the lipoxygenase pathway was further reflected by the reduced content of leukotriene B4 in bronchoalveolar fluid of smoke-exposed rats. These results suggest that lipoxygenase-derived products may play a unique role in smoking-induced pulmonary diseases.

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.

Binding of prostacyclin by plasma glycoproteins

The binding of prostacyclin (PGI2) to plasma proteins and the resulting increase in PGI2 stability was investigated. Using gel filtration to separate bound and free PGI2, we have found that Cohn Fraction VI can bind PGI2, and retard its hydrolysis to 6-keto-PGF1 alpha (6KPGF1 alpha). The biological activity of the bound PGI2 correlated well with the quantity of bound PGI2, measured as 6KPGF1 alpha by RIA. Fraction VI bound a greater percentage of PGI2 than the other eicosanoids tested (i.e., PGI2 greater than TXB2 greater than LTB4 greater than PGE1 greater than PGF2 alpha). The PGI2 binding activity of Fraction VI was lost after neuraminidase treatment. Our data suggest that Fraction VI glycoproteins may play an important role in the binding and stabilization of PGI2 by plasma proteins.

Selective inhibition of leukotriene B4 biosynthesis in rat pulmonary alveolar macrophages by dietary selenium deficiency

Weanling male Fisher 344 rats were maintained on low selenium basal and Se-supplemented diets for 38 weeks. A several fold reduction in the glutathione peroxidase activity of the lung and liver tissues in rats maintained on low Se basal diet established their Se-deficient status. Analysis of the supernatants from resting pulmonary alveolar macrophage suspensions showed negligible extracellular release of PGE2, TXB2 and LTB4 in both diet groups. A challenge with opsonized zymosan particles increased the release of the same three arachidonic acid metabolites by several fold in both diet groups. The differences between the two diet groups with respect to the secretion of the products of the cyclooxygenase pathway, PGE2 and TXB2 were negligible. By contrast, a significant reduction in the extracellular release of LTB4 was observed in cells from animals on low selenium basal diet. These results suggest a selective inhibition of LTB4 biosynthesis in pulmonary alveolar macrophages by dietary deficiency of selenium.

Increase in enzyme activities of prostaglandin biosynthesis and catabolism by acute ureteral ligation in rat kidney

Acute ureteral obstruction increased cyclooxygenase, thromboxane and prostacyclin synthases and NAD+-linked 15-hydroxyprostaglandin dehydrogenase activities in rat kidney. Significant increase in prostaglandin biosynthetic and catabolic activities may mediate some pathological consequences found in obstructive nephropathy.

Synergistic stimulation of thromboxane biosynthesis by calcium ionophore and phorbol ester or thrombin in human platelets

Phorbol ester PMA and low concentrations of calcium ionophore A-23187, which given separately have minimal effect in stimulating thromboxane synthesis in human platelets, showed marked synergism when given simultaneously. A similar synergism can be also demonstrated between thrombin or collagen and low concentrations of A-23187 but not of PMA. Simultaneous addition of thrombin and PMA results in less synthesis of thromboxane than that of thrombin alone. These studies suggest that protein kinase C activation by agonists may not only induce but also regulate thromboxane synthesis in human platelets.

Changes in prostacyclin and thromboxane biosynthesis and their catabolic enzyme activity in kidneys of aging rats

The effects of aging on the prostacyclin and thromboxane biosynthesis and prostaglandin catabolic enzyme activity in rat kidney were investigated. The prostacyclin biosynthesis, using arachidonic acid as substrate, was the greatest in young kidneys (2 months old) and then progressively decreased in mature (12 months old) and old (24 months old) kidneys, while thromboxane biosynthetic activity showed no significant change as a function of age. When prostaglandin H2 was used as substrate, the prostacyclin and thromboxane biosynthesis showed similar results as when arachidonic acid was used as substrate; the prostacyclin biosynthesis progressively decreased and thromboxane biosynthesis showed no significant change as a function of age. The fatty acid cyclooxygenase in kidney was measured by a specific radioimmunoassay. No significant change in renal fatty acid cyclooxygenase as a function of age was found. Thus, we concluded that the progressive decrease in renal prostacyclin biosynthesis as a function of age is due to a defect in prostacyclin synthetase in aged kidneys. The prostaglandin catabolic enzyme, NAD+-dependent 15-hydroxyprostaglandin dehydrogenase, in kidneys was also investigated. The enzyme activity progressively decreased as a function of age, which suggested a decrease in the metabolism of thromboxane A2 in aged kidneys. The present results, indicating a decrease in renal prostacyclin biosynthesis and renal 15-hydroxyprostaglandin dehydrogenase activity with aging, might contribute to a plausible explanation of the progressive decrease in renal functions in the elderly.

Changes in arachidonate metabolism in aortas and platelets in aging rats

Prostacyclin formation by aortas, thromboxane biosynthesis and 12-lipoxygenase pathway in platelets as a function of age was investigated. Our results indicate an age-related decrease in prostacyclin production by aortas, while no significant change in thromboxane biosynthesis in platelets from mature (12 month old) to senescent (24 month old) rats was observed. Evidence is also presented indicating a decrease in 12-L-hydroperoxy-5,8,10,14-eicosatetraenoic acid (12-HPETE) peroxidase activity in platelets as a function of age.

Cigarette smoking stimulates lipoxygenase but not cyclooxygenase pathway in platelets

Male rats were exposed to freshly generated cigarette smoke once daily for 4 to 8 weeks. Inhalation of smoke was verified by elevated level of carboxyhemoglobin. Arachidonate metabolism through lipoxygenase and cyclooxygenase pathways in platelets was determined. Cigarette smoking increased 12-lipoxygenase activity significantly without affecting the cyclooxygenase pathway. In view of platelet-leukocyte interactions and potent chemotactic activity of 12-HETE for aortic smooth muscle cell migration, increased 12-lipoxygenase activity may predispose individuals to atherosclerosis, thromboembolism and emphysema commonly found in smokers.

Characterization of imidazo[1,5-a]pyridine-5-hexanoic acid (CGS 13080) as a selective thromboxane synthetase inhibitor using in vitro and in vivo biochemical models

CGS 13080 inhibited cell-free thromboxane synthetase with an IC50 of 3 nM. It was at least five orders of magnitude less potent toward other key enzymes involved in arachidonic acid metabolism. Submicromolar concentrations inhibited calcium ionophore-induced formation of thromboxane B2 by intact human platelets with concomitant accumulation of prostaglandin E2. Oral doses lower than 1 mg/kg in rats suppressed the elevations of plasma thromboxane B2 induced by calcium ionophore. This was attended by shunting of endoperoxide substrate to 6-keto-prostaglandin F1 alpha and prostaglandin E2. CGS 13080 is one of the most potent and selective thromboxane synthetase inhibitors yet identified.

Enzyme immunoassay of thromboxane B2

An immunoassay for thromboxane B2 was developed in which the hapten molecule was labeled with beta-galactosidase. The immunoprecipitate formed after competition between enzyme-labeled and unlabeled thromboxane B2 was subjected to a fluorometric assay of beta-galactosidase. Thromboxane B2 was detectable in the range of 0.1-30 pmol. Both enzyme immunoassay and radioimmunoassay showed essentially the same cross-reactivities with other prostaglandins and their metabolites when the same antibody was used. Known amounts of thromboxane B2 were added to human plasma, and the sample was applied to an octadecyl silica column. The extract was analyzed by enzyme immunoassay to examine the correlation between the added (x) and measured (y) thromboxane B2 (y = 1.09x + 11.07 pmol/ml, r = 0.99). A satisfactory correlation was observed between radioimmunoassay (x) and enzyme immunoassay (y) (y = 0.92x + 4.64 pmol/ml, r = 0.96). The validity of enzyme immunoassay was also confirmed by gas chromatography-mass spectrometry of a dimethylisopropylsilyl ether derivative of thromboxane B2 methyl ester. The method was applicable to the assay of thromboxane B2 produced from endogenous precursor during thrombin-induced aggregation of human platelets.

The reduction of platelet thrombi on damaged vessel wall by a thromboxane synthetase inhibitor in rabbits

The role of thromboxane A2 (TXA2) in platelet-vessel wall interaction was investigated using 1-benzylimidazole (1-BI), a selective thromboxane synthetase inhibitor. 1-BI (0.9 mM) will reduce the aggregatory response of rabbit platelets to 0.2 mM arachidonate by 50% and their production of TXA2 by 84%. The effect of 1-BI on platelet thrombus formation was evaluated in vivo on New Zealand white male rabbits using the autologous indium-111 labeled platelet technique. After injection of autologous 111In-platelets, 10 cm of the abdominal aorta was de-endothelialized with a balloon catheter. Three hours later the animals were sacrificed and injured and uninjured segments of the aorta removed. The radioactivity and dry weight of the tissue were determined. The radioactivity/gm of tissue was greater for the injured tissue than for the uninjured tissue. 1-BI at 10 mg/kg reduced the specific platelet accumulation at the injured site (n = 5; 4.8 +/- 0.3 X 10(5) cpm/gm) compared to the controls (n = 10; 11.7 +/- 2.1 X 10(5) cpm/gm). Platelet accumulation on the injured tissue was further reduced by increasing the dosage to 30 mg/kg. Thirty minutes after 1-BI administration (30 mg/kg), platelets were less sensitive to arachidonate-induced aggregation (a 67% decrease) and TXA2 production was decreased 82%. Alterations in platelet sensitivity persisted for up to 3 hours. These findings indicate that TXA2 plays an important role in platelet-vessel wall interaction.

Hypoxia stimulates prostacyclin generation by dog lung in vitro

We have investigated the possibility that pulmonary biosynthesis of prostacyclin and thromboxane A2 (TXA2) may be affected by variations in PO2. Fresh lung homogenated from 8 dogs were incubated for 1 hour at 37 degrees C in Krebs-Ringer solution, at high (492 mmHg) or low (53 mmHg) PO2. After incubation, the stable metabolites of prostacyclin (6-keto-PGF1) and of TXA2 (TXB2) were measured by radioimmunoassay. The basal ("pre-incubation") levels of these metabolites, measured in tubes in which biosynthesis was arrested by the addition of indomethacin (10 g/ml), were 0.76 +/- 0.15 and 0.76 +/- 0.11 ng/mg wet wt. for 6-keto-PGF1 alpha, in high and low PO2, respectively, and 0.97 +/- 0.09 and 0.82 +/- 0.15 x 10(-1) ng/mg wet wt. for TXB2, in high and low PO2 respectively. Synthesis during incubation in other tubes was estimated by subtracting basal values from those measured at the end of incubation. More 6-keto-PGF1 alpha was formed in homogenates exposed to low PO2 (3.01 +/- 0.45) than in those exposed kept at high PO2 (1.89 +/- 0.37, p less than 0.001), but equal amounts of TXB2 were bound under both conditions. These results suggest that hypoxia may stimulate pulmonary prostacyclin synthesis; a pulmonary vasodilator, prostacyclin may help modulate hypoxic vasoconstriction in the lung.

Dissociation of systemic and renal effects in endotoxemia. Prostaglandin inhibition uncovers an important role of renal nerves

To elucidate the mechanisms responsible for systemic and renal hemodynamic changes in early endotoxemia, the roles of prostaglandins (PG) and renal nerves were investigated. Endotoxin (E, 3 micrograms/kg i.v.) was given to two groups of anesthetized dogs that had undergone unilateral renal denervation: Group I (n = 9) E only; Group II (n = 11) E + indomethacin (10 mg/kg i.v.) or meclofenamate (5 mg/kg i.v.). A third group of dogs (Group III, n = 5) received indomethacin (10 mg/kg i.v.) only. 1 h after E group I dogs, mean arterial pressure (MAP) decreased from 126 to 94 mm Hg (P less than 0.001), and prostacyclin (6-keto-Fl alpha metabolite, PGI2) increased (from 0.64 to 2.08 ng/ml, P less than 0.005). Glomerular filtration rate (GFR) and renal blood flow (RBF) declined comparably both in innervated and denervated kidneys. In marked contrast, group II dogs had a stable MAP (136-144 mm Hg, NS) and no increase in PGI2 levels. Plasma renin activity (0.7-2.5 ng/ml per h, P less than 0.005) increased, and renin secretion was greater in innervated compared with denervated kidneys (255 vs. 74 U/min, P less than 0.01) in these PG-inhibited dogs. In addition, denervated kidneys in group II dogs had a greater GFR (42 vs. 34 ml/min, P less than 0.01) and RFB (241 vs. 182 ml/min, P less than 0.01) than innervated kidneys after E. Group III animals had no significant changes in systemic or renal hemodynamics, plasma renin activity or PGI2 during the study. These results suggest that PGI2 mediates the systemic hypotension of early endotoxemia in the PG-intact animal. Moreover, PG inhibition uncovers an important effect of E to increase efferent renal nerve activity with a consequent decline in GFR and RBF independent of changes in MAP. Finally, the results demonstrate that renal nerves are important stimuli to renin secretion in early endotoxemia via pathways that are PG-independent.

A radioimmunoassay for 6-ketoprostaglandin E1

A radioimmunoassay for 6-ketoprostaglandin E1 has been developed. 6-keto-prostaglandin E1 antibodies were produced in rabbits by repeated immunization with 6-ketoprostaglandin E1 coupled to bovine serum albumin. [125]-labeled hapten with high specific radioactivity was prepared by radioiodination of 6-ketoprostaglandin in E1-tyrosine methyl ester conjugate followed by purification with thin layer chromatography. The antibodies showed good specificity toward 6-ketoprostaglandin E1 and crossreacted only significantly with prostaglandin E1. The sensitivity of the assay was 10 pg per assay tube. Application of the radioimmunoassay was demonstrated by the detection of immunoreactive 6-ketoprostaglandin E1 from 6-ketoprostaglandin F1 alpha catalyzed by swine renal NADP+-linked 9-hydroxyprostaglandin dehydrogenase.

Purification of thromboxane synthetase and evidence of two distinct mechanisms for the formation of 12-L-hydroxy-5,8,10-heptadecatrienoic acid by porcine lung microsomes

Synthesis of thromboxane B2 and 12-L-hydroxy-5,8,10-heptadecatrienoic acid by crude microsomes and by partially purified thromboxane synthetase from porcine lung has been studied. Formation of both compounds catalyzed by the purified enzyme is inhibited similarly by imidazole or increased temperature (52 degrees C). However, during the initial stages of purification only the production of thromboxane B2 was diminished by imidazole or increased temperature, indicating the presence of an additional mechanism for the formation of 12-L-hydroxy-5,8,10-heptadecatrienoic acid. This novel mechanism is attributed to the effect of nondialyzable heat-stable factor(s) present in the crude preparation which stimulates the formation of 12-L-hydroxy-5,8,10-heptadecatrienoic acid from prostaglandin H2 in a time-dependent manner.

Abnormal platelet response to thromboxane A2

To determine the pathogenetic mechanism of a hereditary primary platelet release disorder, arachidonic acid metabolism via the cyclooxygenase pathway was investigated. The propositus' platelets exhibited defective release reaction and second-wave aggregation when stimulated by sodium arachidonate or U46619, a thromboxane A2 (TXA2) agonist. The lack of platelet response to U46619 suggested that the defect was beyond the thromboxane synthetase level. Furthermore, thromboxane B2 (TXB2) formation in the propositus' platelets (558.52 ng/10(8) platelets) was within the normal range (574.29 +/- SD 27.39 ng/10(8) platelets) and TXA2 formation appeared to be adequate for aggregating normal platelets. The results were indicative of an abnormal platelet response to TXA2. Failure of the propositus' platelets to aggregate in response to TXA2 formed in normal platelet-rich plasma induced by arachidonate confirmed this notion. To gain further insight, platelet cyclic (c) AMP content was determined. Prostacyclin induced a significant elevation of the propositus' platelet cAMP level comparable to normal values. U46619 suppressed prostaglandin I2-induced cAMP elevation in normal subjects but had no such effect in the patient. We conclude that the primary release disorder observed in this kindred is due to an abnormal platelet respnse to TXA2 possibly because of TXA2/PGH2 receptor abnormalities.