Phosphorylation of the thromboxane receptor alpha, the predominant isoform expressed in human platelets

A single gene encodes the human thromboxane receptor (TP), of which there are two identified splice variants, alpha and beta. Both isoforms are rapidly phosphorylated in response to thromboxane agonists when overexpressed in human embryonic kidney 293 cells; this phenomenon is only slightly altered by inhibitors of protein kinase C. Pharmacological studies have defined two classes of TP in human platelets; sites that bind the agonist I-BOP with high affinity support platelet shape change. Low affinity sites, which irreversibly bind the antagonist GR 32191, transduce platelet activation and aggregation. Isoform-specific antibodies permitted detection of TPalpha, but not TPbeta, from human platelets, although mRNA for both isoforms is present. A broad protein band of 50-60 kDa, reflecting the glycosylated receptor, was phosphorylated upon activation of platelets for 2 min with I-BOP. This was a rapid ( approximately 30 s) and transient (maximum, 2-4 min) event and was inhibited by TP antagonists. Both arachidonic acid and low concentrations of collagen stimulated TPalpha phosphorylation, which was blocked by cyclooxygenase inhibition or TP antagonism. Blockade of the low affinity TP sites with GR 32191 prevented I-BOP-induced TPalpha phosphorylation. This coincided with agonist-induced platelet aggregation and activation but not shape change. Also, activation of these sites with the isoprostane iPF2alpha-III induced platelet shape change but not TPalpha phosphorylation. Heterologous TP phosphorylation was observed in aspirin-treated platelets exposed to thrombin, high concentrations of collagen, and the calcium ionophore A 23187. Both homologous and heterologous agonist-induced phosphorylation of endogenous TPalpha was blocked by protein kinase C inhibitors. TPalpha was the only isoform detectably translated in human platelets. This appeared to correspond to the activation of the low affinity site defined by the antagonist GR 32191 and not activated by the high affinity agonist, iPF2alpha-III. Protein kinase C played a more important role in agonist-induced phosphorylation of native TPalpha in human platelets than in human embryonic kidney 293 cells overexpressing recombinant TPalpha.     

Inhibition of platelet aggregation by S-nitroso-cysteine via cGMP-independent mechanisms: evidence of inhibition of thromboxane A2 synthesis in human blood platelets

S-Nitroso-cysteine (SNC), a putative endothelium-derived relaxing factor, potently inhibited collagen- and arachidonic acid-induced platelet aggregation (IC50=100 nM) and thromboxane A2 (TxA2) synthesis of human blood platelets. ODQ, a selective inhibitor of the soluble guanylyl cyclase, inhibited SNC-induced formation of cGMP but did not reverse inhibition by SNC of collagen- and arachidonic acid-induced platelet aggregation. Combination of ODQ with SQ-29548, a specific platelet TxA2 receptor antagonist, did not modify the antiaggregatory action of SNC. Our study shows that SNC inhibits platelet aggregation by cGMP-independent mechanisms that may involve inhibition of TxA2 synthesis in human platelets.     

On the formation and effects of thromboxane A2 in human platelets

Incubation of arachidonic acid and prostaglandin G2 with a suspension of human platelets led to formation of an unstable (t1/2, 41+/-7 s) compound, thromboxane A2. Thromboxane A2 induced irreversible aggregation of washed platelets and of platelets in platelet-rich plasma and caused release of serotonin and ADP from platelets in platelet-rich plasma.     

Oxysterols and TBARS are among the LDL oxidation products which enhance thromboxane A2 synthesis by platelets

In this study, we compared the effects of normal LDL (nLDL) and oxidized LDL (oxLDL) on thromboxane (TXA2) release by platelets triggered by low concentration of thrombin, and we determined which component of oxLDL is responsible for that activation. After oxidation of LDL with copper sulfate, the small molecular weight fraction (< 10 kDa) which was high in TBARS was removed; using Amicon Centriprep-10 concentrator membrane. More than 67% of TBARS in the oxLDL preparation was found in solution while the remaining was covalently attached to the oxLDL particles. OxLDL contained significantly higher levels of oxysterols and TBARS than the nLDL. Platelets preincubated with low concentrations of oxLDL (33-132 micrograms protein/mL) produced significantly higher TXA2 than platelets preincubated with equivalent concentrations of nLDL when triggered with thrombin. Platelets treated with oxLDL also contained significantly higher levels of oxysterols than platelets treated with nLDL. Platelets preincubated with pure cholestanetriol (10 micrograms/mL) contained a high level of cholestanetriol in the membrane, and TXA2 release was significantly increased in these platelets compared to the control platelets. The TBARS in solution also was very potent in enhancing TXA2 release by thrombin-treated platelets. These results indicate that oxysterols and the free TBARS either in solution or covalently attached to the oxLDL particles are partly responsible for the stimulatory effect of oxLDL on TXA2 release by platelets. The present study also showed that this enhancement of TXA2 release was due to activation of phospholipase A2 and to the increase of arachidonic acid liberation from the platelet phospholipids.     

Specific 3H-imipramine binding in human platelets. Influence of age and sex

Human platelets have been shown to possess high-affinity binding sites for 3H-imipramine. These binding sites have a similar affinity and drug specificity to those already described in rat brain. The platelets from healthy volunteers show no difference in 3H-imipramine binding between the sexes but there is a decrease in maximal 3H-imipramine binding with increasing age of the donor.     

Increased protein synthesis by human platelets during phagocytosis of latex particles in vitro

A three-fold increase of protein synthesis by human platelets during in vitro phagocytosis of polystyrene latex particles was detected. During the first two hours of incubation, the percentage of phagocytizing platelets and the number of latex particles per platelet increased; by the end of the third hour, the first parameter remained stable, while the number of latex particles per cell had decreased. Vincristine (20 mug/ml of cell suspension) inhibited platelet protein synthesis. This effect was both time- and dose-dependent. The drug also caused a decrease in the number of phagocytizing cells, as well as in their phagocytotic activity.     

Some biochemical properties of the components of Staphylococcus aureus binding to human platelets

The purpose of this study was to ascertain whether pulmonary function in children who were lifetime residents of the highly polluted district of Teplice in northern Bohemia was lower than that for children who were lifetime residents of the cleaner district of Prachatice in southern Bohemia. Forced expiratory spirometry was measured twice (February/March and April) in approximately 235 eighth-grade students in each district. On both testing occasions, height-adjusted forced expiratory volume in 1 s and forced expiratory flow between 25% and 75% forced vital capacity were significantly lower (p < .001) in children from Teplice than in those from Prachatice. These differences were not associated with parental smoking habits, presence of pets, heating/cooking fuels, private home/apartment residency, or rural/urban residency. In Teplice, no differences were observed between lung functions measured at the end of the high pollution season (February/March) and those measured after the children breathed much cleaner air for a 4-wk period (April). This result was suggestive of a condition of chronically depressed lung function. No differences across times were observed in Prachatice, indicating that our measurements were reliable.     

Induction of cytokine expression in leukocytes by binding of thrombin-stimulated platelets

BACKGROUND: Activated platelets tether and activate myeloid leukocytes. To investigate the potential relevance of this mechanism in acute myocardial infarction (AMI), we examined cytokine induction by leukocyte-platelet adhesion and the occurrence of leukocyte-platelet conjugates in patients with AMI. METHODS AND RESULTS: We obtained peripheral venous blood samples in 20 patients with AMI before and daily for 5 days after direct percutaneous transluminal coronary angioplasty (PTCA) and in 20 patients undergoing elective PTCA. Throughout the study period, CD41 immunofluorescence of leukocytes (flow cytometry) revealed increased leukocyte-platelet adhesion in patients with AMI compared with control patients (mean +/- SE of fluorescence [channels] before PTCA: 77 +/- 16 versus 35 +/- 9; P = .003). In vitro, thrombin-stimulated fixed platelets bound to neutrophils and monocytes. Within 2 hours, this resulted in increased mRNA for interleukin (IL),1 beta, IL-8, and monocyte chemoattractant protein (MCP)-1 in unfractionated leukocytes. After 4 hours, IL-1 beta and IL-8 concentration of the cell-free supernatant had increased by 268 +/- 36% and 210 +/- 7%, respectively, and cellular MCP-1 content had increased by 170 +/- 8%. Addition of activated platelets to adherent monocytes had a similar effect and was associated with nuclear factor-kappa B activation. Inhibition of binding by anti-P selectin antibodies reduced the effect of activated platelets on cytokine production. CONCLUSIONS: In patients with AMI, leukocyte-platelet adhesion is increased. Binding of activated platelets induces IL-1 beta, IL-8, and MCP-1 in leukocytes. Our findings suggest that leukocyte-platelet adhesion contributes to the regulation of inflammatory responses in AMI.     

Stereoselective synthesis and thromboxane A2 (TXA2) receptor antagonistic activity of optically active phenol derivatives

At the present time monotherapy is preferable to polytherapy if the epilepsy can be controlled with a single drug. However, persons with intractable epilepsy may need more than one antiepileptic drug to attain the best seizure control with the fewest side-effects. Although clinical studies have not been performed to identify the best combinations, rational polytherapy would imply combining antiepileptic drugs with different mechanisms of action.     

Inhibition of platelet-vessel wall interactions by thromboxane receptor antagonism in a human in vitro system: potentiation of antiplatelet effects of aspirin

BACKGROUND: Pharmacological inhibition of arachidonic acid metabolism has proven therapeutically useful in the prevention of cardiovascular events. METHODS: We have investigated the ability of Bay u 3405, a synthetic thromboxane antagonist, to interfere with platelet aggregation and arachidonic acid metabolism. The antiplatelet action was also analysed in a perfusion system in which vascular subendothelium was exposed to circulating human blood (10 min; shear rate = 800 s-1). Platelet interactions were morphometrically analysed and results compared with those obtained in studies with blood from donors taking aspirin (acetylsalicylic acid, ASA) (500 mg day-1). The additional effect of Bay u 3405 on the antiplatelet action of ASA was also evaluated. RESULTS: Bay u 3405 caused a dose-dependent inhibition of platelet aggregation induced by U46619 with a maximal effect at concentrations > or = 0.01 microgram mL-1. Higher concentrations (> or = 0.05 micrograms mL-1) also inhibited aggregations induced by ADP or collagen. Bay u 3405 did not interfere with platelet arachidonic acid metabolism. In perfusion studies, Bay u 3405 (0.01 microgram mL-1) significantly decreased the total surface of the vessel covered by platelets (%CS = 18.7 +/- 1.09 vs. 24.4 +/- 1.94; P < 0.05) and the formation of large aggregates %T = 7.5 +/- 0.87 vs. 19.3 +/- 1.61; P < 0.01). ASA treatment reduced platelet aggregate formation (%T = 13.7 +/- 2.06; P < 0.05) but did not affect the total surface covered by platelets. The in vitro addition of Bay u 3405 to blood from ASA-treated donors further reduced the formation of large aggregates (%T = 2.7 +/- 0.79; P < 0.01 vs. ASA). CONCLUSIONS: In vitro effect of Bay u 3405 on platelet function were superior to those observed with ASA. The thromboxane antagonism antagonism provided by Bay u 3405 further enhanced the inhibition of platelet aggregate formation found after ASA treatment.     

Ticlopidine facilitates the deaggregation of human platelets aggregated by thrombin

Eleven patients with moderate to severe hypertension were studied at the Vargas Hospital of Caracas. The patients were pretreated with labetalol, 800 to 1200 mg/day, orally, over a period of 1 week, after which an intravenous infusion of dopamine, .5 to 3 micrograms/kg/minute, was given. Two intravenous dopamine infusions (30 minutes each) were performed before and after the injection of metoclopramide (30 mg, intravenous bolus). Two washout periods were also included before and after metoclopramide administration. Dopamine induced a decrease of blood pressure from 171.9 + 6.35/103.6 +/- 3.12 to 152.7 +/- 7.55/93.8 +/- 2.97 mm Hg (P < .001) without altering heart rate, and it increased plasma insulin levels from 8.29 +/- .70 microU/mL to 12.09 +/- 1.83 microU/mL (P < .01). Metoclopramide caused no changes of blood pressure or plasma insulin levels. Hypotensive responses and plasma insulin increases due to dopamine were blocked by metoclopramide, however. The authors conclude that a dopaminergic receptor may be involved in some cardiovascular responses and in modulating insulin secretion in humans.     

Increased synthesis of prostaglandin-E-like material by platelets from patients with diabetes mellitus

We studied the effects of ADP, epinephrine, collagen and arachidonic acid on platelet production of immunoreactive prostaglandin-E-like material and aggregation in 17 subjects with diabetes mellitus and 21 matched controls. Plateletrich plasma obtained from patients synthesized significantly (P less than 0.05) greater quantities of the prostaglandin-E-like material after exposure to 1 muM ADP, 1, 2 and 5 muM epinephrine and 1 microgram per milliliter of collagen than platelet-rich plasma obtained from controls. That obtained from the diabetic patients was significantly more sensitive (P less than 0.001) to the aggregating effects of the prostaglandin precursor, arachidonic acid, in vitro as compared to controls. Diabetic platelet-rich plasma metabolized arachidonic acid (0.5 mM) to immunoreactive prostaglandin-E-like material at a significantly greater rate (P less than 0.05) and extent (P less than 0.001) than that of controls. Thus, platelets obtained from diabetic patients possess increased activity of the prostaglandin synthetase system, and this characteristic may be related to the increased platelet aggregation associated with the disease.     

Correlative alteration of thromboxane A2 with antigen-induced bronchoconstriction and the role of platelets as a source of TXA2 synthesis in guinea pigs: effect of DP-1904, an inhibitor of thromboxane synthetase

Pulmonary mast cells (PMC) are important components of the inflammatory process in equine allergic lung diseases such as heaves. Very little, however, is known of the degranulation kinetics of these cells and thus, their pathophysiologic role remains largely speculative. The purpose of this study was to develop a repeatable protocol for in vitro equine PMC degranulation. Five mature horses (sex: 2 M, 3 F; age: 8.8 +/- 6.5 y), historically free of pulmonary disease and normal on clinical respiratory examination, arterial blood gas analysis, pulmonary mechanics testing and histamine inhalation challenge, were studied. Bronchoalveolar lavage was performed on 4 separate occasions, at least 2 d apart, in a different lung lobe on each occasion. The lavage fluid was concentrated by centrifugation. Cells were resuspended in modified HEPES/Tyrode, assessed for viability by Trypan blue exclusion, and PMC concentration determined. Cell inocula containing 30,000 PMC were incubated with 10(-8) to 6 x 10(-5) M A23187. Cells were then separated by centrifugation and histamine release (HR) was determined by fluorometric assay. The procedure was readily performed and yielded sufficient PMC for 30 to 60 inocula per lavage. Maximal HR (34.4 +/- 16.1%) was obtained with 10(-5) M A23187. The degranulation process was largely complete by 20 min but cell lysis was negligible. The challenge was repeatable within horse and produced a mean coefficient of variability of 23.0% following 20 min incubation with 10(-5) M A23187. We conclude that equine PMC degranulation can be repeatably performed in vitro and speculate that this protocol may be useful in further studies on the pathophysiology and treatment of equine allergic lung diseases.     

Antibodies to protease-activated receptor 3 inhibit activation of mouse platelets by thrombin

Recent studies of mice deficient in the thrombin receptor, protease-activated receptor 1 (PAR1), provided definitive evidence for the existence of a second thrombin receptor in mouse platelets. We recently identified a new thrombin receptor designated protease-activated receptor 3 (PAR3). The mRNA encoding a mouse homologue of PAR3 was highly expressed in mouse splenic megakaryocytes, making it a good candidate for the missing mouse platelet thrombin receptor. We now report that PAR3 protein is expressed on the surface of mouse platelets and that PAR3 antibodies partially inhibit activation of mouse platelets by thrombin but not U46619, a thromboxane receptor agonist. These observations suggest that PAR3 contributes to mouse platelet activation by thrombin.     

Oxygen dependency of epidermal growth factor receptor binding and DNA synthesis of rat hepatocytes

BACKGROUND/AIMS: Changes in oxygen availability modulate replicative responses in several cell types, but the effects on hepatocyte replication remain unclear. We have studied the effects of transient nonlethal hypoxia on epidermal growth factor receptor binding and epidermal growth factor-induced DNA synthesis of rat hepatocytes. METHODS: Lactate dehydrogenase activity in culture supernatant, intracellular adenosine triphosphate content, 125I-epidermal growth factor specific binding, epidermal growth factor receptor protein expression, and 3H-thymidine incorporation were compared between hepatocytes cultured in hypoxia and normoxia. RESULTS: Hypoxia up to 3 h caused no significant increase in lactate dehydrogenase activity in the culture supernatant, while intracellular adenosine triphosphate content decreased time-dependently and was restored to normoxic levels by reoxygenation (nonlethal hypoxia). Concomitantly, 125I-epidermal growth factor specific binding to hepatocytes decreased time-dependently (to 54.1% of normoxia) and was restored to control levels by reoxygenation, although 125I-insulin specific binding was not affected. The decrease in 125I-epidermal growth factor specific binding was explained by the decrease in the number of available epidermal growth factor receptors (21.37+/-3.08 to 12.16+/-1.42 fmol/10(5) cells), while the dissociation constant of the receptor was not affected. The change in the number of available receptors was not considered to be due to receptor degradation-resynthesis, since immunodetection of the epidermal growth factor receptor revealed that the receptor protein expression did not change during hypoxia and reoxygenation, and since neither actinomycin D nor cycloheximide affected the recovery of 125I-epidermal growth factor binding by reoxygenation. Inhibition of epidermal growth factor-induced DNA synthesis after hypoxia (to 75.4% of normoxia by 3 h hypoxia) paralleled the decrease in 125I-epidermal growth factor binding. CONCLUSIONS: Transient hypoxia, which caused no increase in lactate dehydrogenase leakage but affected intracellular adenosine triphosphate levels, did, however, modulate the number of available epidermal growth factor receptors without affecting the receptor protein expression, and inhibit the epidermal growth factor-induced DNA synthesis of hepatocytes. This suggests that even transient nonlethal hypoxia affects the epidermal growth factor-induced DNA synthesis of rat hepatocytes through reversible changes in the epidermal growth factor receptor molecule, which depends on oxygen availability.     

Influence of medium tonicity on respiration by suspensions of human platelets

We describe the use of graded decrements of medium osmolarity to progressively unmask respiratory capacities of whole human platelets in suspension. This departure led to the first demonstration that human platelet mitochondria are capable of tightly coupled respiration that responds to addition of mitochondrial substrates, ADP, and inhibitors in a way that other mammalian mitochondria are expected to behave. In 300 mosM media added alpha-glycerophosphate (G3P), succinate, or ADP effected only slight stimulation of base-line O2 consumption. At 180 mosM O2 consumption peaked and was not significantly affected by succinate or ADP. At 80 mosM base-line O2 consumption fell precipitously and was restored by G3P or succinate prior to being raised to its highest levels by ADP. Added NADH had no effect on O2 consumption at 80 mosM but sharply stimulated it when platelet suspensions were exposed to 60 mosM media by pretreatment with distilled water. At 80 mosM, selected compounds that inhibit or uncouple oxidative phosphorylation of isolated mammalian mitochondria from a variety of cells exerted similar influences on while platelets.     

Structural studies of receptor binding by cholera toxin mutants

The wide range of receptor binding affinities reported to result from mutations at residue Gly 33 of the cholera toxin B-pentamer (CTB) has been most puzzling. For instance, introduction of an aspartate at this position abolishes receptor binding, whereas substitution by arginine retains receptor affinity despite the larger side chain. We now report the structure determination and 2.3-A refinement of the CTB mutant Gly 33-->Arg complexed with the GM1 oligosaccharide, as well as the 2.2-A refinement of a Gly 33-->Asp mutant of the closely related Escherichia coli heat-labile enterotoxin B-pentamer (LTB). Two of the five receptor binding sites in the Gly 33-->Arg CTB mutant are occupied by bound GM1 oligosaccharide; two other sites are involved in a reciprocal toxin:toxin interaction; one site is unoccupied. We further report a higher resolution (2.0 A) determination and refinement of the wild-type CTB:GM1 oligosaccharide complex in which all five oligosaccharides are seen to be bound in essentially identical conformations. Saccharide conformation and binding interactions are very similar in both the CTB wild-type and Gly 33-->Arg mutant complexes. The protein conformation observed for the binding-deficient Gly 33-->Asp mutant of LTB does not differ substantially from that seen in the toxin:saccharide complexes. The critical nature of the side chain of residue 33 is apparently due to a limited range of subtle rearrangements available to both the toxin and the saccharide to accommodate receptor binding. The intermolecular interactions seen in the CTB (Gly 33-->Arg) complex with oligosaccharide suggest that the affinity of this mutant for the receptor is close to the self-affinity corresponding to the toxin:toxin binding interaction that has now been observed in crystal structures of three CTB mutants.