Identification of Galpha13 as one of the G-proteins that couple to human platelet thromboxane A2 receptors.

Previous studies have shown that ligand or immunoaffinity chromatography can be used to purify the human platelet thromboxane A2 (TXA2) receptor-Galphaq complex. The same principle of co-elution was used to identify another G-protein associated with platelet TXA2 receptors. It was found that in addition to Galphaq, purification of TXA2 receptors by ligand (SQ31,491)-affinity chromatography resulted in the co-purification of a member of the G12 family. Using an antipeptide antibody specific for the human G13 alpha-subunit, this G-protein was identified as Galpha13. In separate experiments, it was found that the TXA2 receptor agonist U46619 stimulated [35S]guanosine 5'-O-(3-thiotriphosphate) incorporation into G13 alpha-subunit. Further evidence for functional coupling of G13 to TXA2 receptors was provided in studies where solubilized platelet membranes were subjected to immunoaffinity chromatography using an antibody raised against native TXA2 receptor protein. It was found that U46619 induced a significant decrease in Galphaq and Galpha13 association with the receptor protein. These results indicate that both Galphaq and Galpha13 are functionally coupled to TXA2 receptors and dissociate upon agonist activation. Furthermore, this agonist effect was specifically blocked by pretreatment with the TXA2 receptor antagonist, BM13.505. Taken collectively, these data provide direct evidence that endogenous Galpha13 is a TXA2 receptor-coupled G-protein, as: 1) its alpha-subunit can be co-purified with the receptor protein using both ligand and immunoaffinity chromatography, 2) TXA2 receptor activation stimulates GTPgammaS binding to Galpha13, and 3) Galpha13 affinity for the TXA2 receptor can be modulated by agonist-receptor activation.     

Expression and localization of the thromboxane A2 receptor in human nasal mucosa

Thromboxane A2 (TXA2) has been thought a potent mediator involved in allergic rhinitis, because TXA2 was recovered from the nasal lavage fluid of allergic rhinitis patients after allergen provocation and TXA2 receptor antagonists relief nasal allergic symptoms. In order to clarify the expression of TXA2 receptor in human nasal mucosa, we investigated TXA2 receptor mRNA expression and its protein localization by polymerase chain reaction (PCR) and immunohistochemistry, respectively. Human turbinates were obtained after turbinectomy from 10 patients with nasal obstruction refractory to medical therapy. RT-PCR analysis of total RNA from nasal mucosa demonstrated the expression of TXA2 receptor alpha mRNA. The immunohistochemical studies revealed that anti-TXA2 receptor alpha antibody labeled vascular smooth muscle cells, vascular endothelial cells, epithelial cells and submucosal glands in the nasal mucosa. The results may have an important clinical implication for understanding the role of TXA2 receptor on upper airway diseases such as allergic rhinitis and non-allergic rhinitis.     

Decrease in thromboxane A2 receptor expression by differentiation with dibutyryl cyclic AMP in 1321N1 human astrocytoma cells.

Thromboxane A2 (TXA2) receptor expression with its signaling was investigated in 1321N1 human astrocytoma cells differentiated with dibutyryl cyclic AMP (dbcAMP). The cells cultured in 0.5% fetal calf serum containing 0.5 mM dbcAMP for 3 days showed the star-shaped morphology, accompanied with the reduction of a TXA2 mimetic U46619-induced phosphoinositide hydrolysis and Ca2+ mobilization. Immunoblotting analysis revealed that human astrocytoma cells expressed phospholipase C (PLC)-beta1 and -beta3, but not PLC-beta2. The contents of PLC-beta1 and beta3 were not changed by the differentiation. The alpha subunit of Gq/ll bound to TXA2-receptor was reduced by the differentiation, determined by immunoblotting after immunoprecipitation with an anti-TXA2-receptor antibody. Scatchard analysis of the binding of [3H]SQ29548, a TXA2 receptor antagonist, to the membranes revealed that the maximum binding site was reduced by the differentiation. The expression of TXA2 receptor mRNA also was reduced by the differentiation, determined by reverse-transcribed-polymerase chain reaction. Although placental type of TXA2 receptor mRNA expression increased after the differentiation, endothelial type of TXA2 receptor mRNA expression slightly decreased. The results suggest that 1321N1 human astrocytoma cells differentiated with dbcAMP show impaired TXA2 receptor-mediated phosphoinositide hydrolysis and Ca2+ mobilization, due to the decrease in TXA2 receptor number.     

Genetic variants of the receptors for thromboxane A2 and IL-4 in atopic dermatitis

Thromboxane A2 (TXA2) is an arachidonate metabolite which is considered to relate to chronic inflammation in atopic diseases characterized by elevated immunoglobulin E productivity. The elevation of immunoglobulin E levels involves many molecules including interleukin-4 (IL-4) and interleukin-4 receptor alpha chain (IL-4R alpha). To assess whether genetic variants of TXA2 receptor, IL-4 and IL-4R alpha genes relate to the elevation of serum immunoglobulin E levels in patients with atopic dermatitis (AD), we conducted an association study of genetic polymorphisms of TXA2 receptor (795C/T), IL-4 (-589C/T), and IL-4R alpha (Ile50Val) in a Japanese population (n = 789). The TXA2 receptor 795TT genotype strongly related to AD with high serum immunoglobulin E concentrations. AD patients with both TXA2 receptor 795TT genotype and the IL-4R alpha Ile50/Ile50 genotype showed the greatest immunoglobulin E concentrations. These results suggest TXA2 receptor polymorphism strongly interacts with IL-4R alpha polymorphism as a major determinant of high serum immunoglobulin E levels in AD.     

The affinities of prostaglandin H2 and thromboxane A2 for their receptor are similar in washed human platelets

Both thromboxane A2 (TXA2) and its precursor prostaglandin H2 (PGH2) are labile and share a common receptor. The affinities of these two compounds for their putative common receptor are unknown. We compared the potencies of TXA2 and PGH2 to aggregate human platelets and bind to the TXA2/PGH2 receptor. TXA2 was more potent than PGH2 in initiating aggregation in platelet-rich plasma, EC50 of 66 +/- 15 nM and 2.5 +/- 1.3 microM, respectively. In washed platelets, however, PGH2 was more potent than TXA2 with EC50 values of 45 +/- 2 nM and 163 +/- 21 nM, respectively. The affinity of these two compounds in washed platelets was determined in radioligand competition binding assays employing [125I]-PTA-OH. The Kd values for PGH2 and TXA2 were 43 nM and 125 nM, respectively. The results demonstrate that the affinity of PGH2 for the platelet TXA2/PGH2 receptor is greater than previously thought. The data raise the possibility that PGH2 may significantly contribute to the responses attributed to TXA2 in vivo.     

Effects of platelet activating factor antagonists on arachidonic acid-induced platelet aggregation and TXA2 formation

The effects of the PAF receptor antagonists WEB 2086, WEB 2170, BN 50739 and BN 52021 on AA-induced platelet aggregation (PA) and TXA2 formation were investigated in comparison with the TXA2 synthetase inhibitor HOE 944 and the TXA2 receptor antagonist BM 13.177. All PAF antagonists tested were weak inhibitors of AA-induced PA and TXA2 formation (IC50 values between 80 and 2,737 mumol/l). HOE 944 was effective in concentrations 2-3 orders of magnitude lower than PAF antagonists in inhibiting TXA2 generation. These results imply that the inhibition of TXA2 formation is of minor relevance for the actions of the investigated PAF antagonists in AA-induced PA.     

Coupling of thromboxane A2 receptor isoforms to Galpha13: effects on ligand binding and signalling.

Previous subtyping of thromboxane A2 (TXA2) receptors in platelets and vascular smooth muscle cells was based on pharmacological criteria. Two distinct carboxy-terminal splice variants for TXA2 receptors exist and they couple to several different G protein alpha subunits including Galpha13, but it has not been established whether either or both isoforms interact with and signal through it. We sought to determine: (1) which TXA2 receptor isoforms exist in vascular smooth muscle, (2) if Galpha13 is present in vascular smooth muscle and (3) if Galpha13 interacts with either or both of the two TXA2 receptor isoforms as determined by changes in ligand binding properties and generation of intracellular signals. Both TXA2 receptor isoforms and Galpha13 were found in vascular smooth muscle cells. Both the alpha and beta isoforms of the TXA2 receptors were transiently transfected with or without Galpha13 into COS-7 (radioligand binding assays) or CHO cells (agonist induced Na+/H+ exchange). Co-expression of each receptor isoform with Galpha13 significantly (P<0.05) increased the affinity of each receptor for the two agonists, I-BOP and ONO11113, and decreased the affinity of the receptor for the antagonists, SQ29,548 and L657,925. I-BOP stimulated Na+/H+ exchange in vascular smooth muscle cells. Co-expression of Galpha13 with each TXA2 receptor isoform in CHO cells resulted in a significant (P<0.04) agonist induced increase in Na+/H+ exchange compared to cells not transfected with Galpha13. The results support the possibility that the previous classification of TXA2 receptor subtypes based on pharmacological criteria reflect unique interactions with specific G protein alpha subunits.     

Internalization of the TXA2 receptor alpha and beta isoforms. Role of the differentially spliced cooh terminus in agonist-promoted receptor internalization

Thromboxane A2 (TXA2) potently stimulates platelet aggregation and smooth muscle constriction and is thought to play a role in myocardial infarction, atherosclerosis, and bronchial asthma. The TXA2 receptor (TXA2R) is a member of the G protein-coupled receptor family and is found as two alternatively spliced isoforms, alpha (343 residues) and beta (407 residues), which share the first 328 residues. In the present report, we demonstrate by enzyme-linked immunosorbent assay and immunofluorescence microscopy that the TXA2Rbeta, but not the TXA2Ralpha, undergoes agonist-induced internalization when expressed in HEK293 cells as well as several other cell types. Various dominant negative mutants were used to demonstrate that the internalization of the TXA2Rbeta is dynamin-, GRK-, and arrestin-dependent in HEK293 cells, suggesting the involvement of receptor phosphorylation and clathrin-coated pits in this process. Interestingly, the agonist-stimulated internalization of both the alpha and beta isoforms, but not of a mutant truncated after residue 328, can be promoted by overexpression of arrestin-3, identifying the C-tails of both receptors as necessary in arrestin-3 interaction. Simultaneous mutation of two dileucine motifs in the C-tail of TXA2Rbeta did not affect agonist-promoted internalization. Analysis of various C-tail deletion mutants revealed that a region between residues 355 and 366 of the TXA2Rbeta is essential for agonist-promoted internalization. These data demonstrate that alternative splicing of the TXA2R plays a critical role in regulating arrestin binding and subsequent receptor internalization.     

The mechanism of oxidative stress stabilization of the thromboxane receptor in COS-7 cells

The 8-iso-prostaglandin F(2alpha), a prostanoid produced in vivo by cyclooxygenase-independent free-radical-catalyzed lipid peroxidation, acts as a partial agonist on the thromboxane receptor (TXA(2)R) and is a potent vasoconstrictor in the oxidatively stressed isolated perfused rat heart. We hypothesized that the response in the isolated heart may be due to augmentation of TXA(2)R density, which may be initiated by the presence of oxidative radicals. Previous studies have shown that TXA(2)R density is increased during atherosclerosis on both the medial and intimal smooth muscle layers in human coronary arteries. Here we describe the effect of oxidative stress on TXA(2)R. The thromboxane A(2) receptor beta isoform (TXA(2)Rbeta) was transiently expressed in COS-7 cells. Immunofluorescence suggested that the presence of H(2)O(2) increased translocation of TXA(2)Rbeta from the endoplasmic reticulum (ER) to the Golgi complex. H(2)O(2) treatment also increased binding of a TXA(2)R antagonist ([(3)H]SQ29548) to membranes. Degradation kinetics of TXA(2)Rbeta following cycloheximide treatment, a protein synthesis inhibitor, suggested not only that TXA(2)Rbeta is a short-lived protein predominantly localized to the ER but also that TXA(2)Rbeta degradation is modulated in the presence of H(2)O(2). Our results indicate that oxidative stress induces maturation and stabilization of the TXA(2)Rbeta protein probably by intracellular translocation. Importantly, these observations also suggest that TXA(2)Rbeta levels are modulated by ER-associated degradation and controlled by the efficiency of transport to post-ER compartments. Stabilization of the TXA(2)Rbeta by translocation from a degradative compartment, i.e. the ER, can account for the augmentation of receptor density observed in vivo.     

Isoelectric and mass characterization of human platelet thromboxane A2/prostaglandin H2 receptors

To further characterize the human thromboxane A2 (TXA2)/prostaglandin H2 (PGH2) receptor, preparative isoelectric focusing (IEF) was performed on solubilized platelet membranes. TXA2/PGH2 receptors, assayed by specific binding of the TXA2/PGH2 antagonist [125I]PTA-OH, were electrofocused at pH 5.6. Scatchard analysis of IEF fraction pH 5.6 revealed a 180-fold concentration of TXA2/PGH2 receptors (Bmax = 3650 +/- 228 pM/mg focused, 19 +/- 4 pM/mg unfocused) with no change in binding affinity (Kd = 47 +/- 7 nM focused, 36 +/- 14 nM unfocused). SDS-polyacrylamide gel electrophoresis of photoaffinity-labelled electrofocused receptors revealed concentration of specifically labelled proteins having molecular masses of 49,000 and 27,000 Daltons. These results suggest that the human platelet TXA2/PGH2 receptor has a pI of 5.6, molecular mass of 49,000 Daltons, and may exist as a dimer. Preparative IEF should prove useful in the eventual purification of this receptor.     

Effect of thromboxane A(2) (TXA(2)) synthase inhibitor and TXA(2) receptor antagonist alone and in combination on antigen-induced bronchoconstriction in guinea pigs

Thromboxane A(2) (TXA(2)) causes bronchoconstriction and bronchial hyperresponsiveness. Two types of TXA(2) modifiers, one synthase inhibitor and one receptor antagonist, are widely used for the treatment of asthma in Japan. Although the target of TXA(2) modifiers is to inhibit bioactivity of TXA(2), the pharmacological properties are somewhat different between these drugs. We studied the inhibitory effects of the TXA(2) synthase inhibitor CS-518 and the TXA(2) receptor antagonist S-1452 alone and in combination on antigen-induced bronchoconstriction in passively sensitized guinea pigs treated with diphenhydramine.Both CS-518 and S-1452 inhibited the antigen-induced bronchoconstriction dose-dependently with the plateau. The combination of these drugs at the maximal inhibitory doses did not have any more effect compared with each single dosing. The combination at the submaximal doses tended to show an additive effect, but the effect was not significant.These findings suggest that other prostanoids such as PGE(2), PGI(2), PGD(2) and PGF(2alpha) may not take an important role in the antiasthmatic effects of TXA(2) modifiers.     

The stable analog carbocyclic TXA2 but not platelet-released TXA2 induces osteoclast-like cell formation

Thromboxan A(2) (TXA(2)) is the main product of arachidonic acid metabolism in activated platelets. Platelet-released supernatants (PRS) can induce osteoclast-like cell formation in murine bone marrow cultures via a cyclooxygenase (COX)/receptor activator of NF-kB-ligand (RANKL)-dependent pathway. Here we investigated a possible linkage between platelet-released TXA(2) and osteoclastogenesis. The stable analog of TXA(2), carbocyclic TXA(2) (CTXA(2)) can induce the formation of tartrate-resistant acid phosphatase positive multinucleated cells in murine bone marrow cultures via a RANKL-dependent pathway and requires the presence of stromal cells. Interestingly, the platelet-released instable TXA(2) does not account for osteoclastogenic effects as: (a) PRS-induced osteoclastogenesis in the presence of the TXA(2) receptor antagonist SQ29548; (b) inhibition of platelet TXA(2) synthesis by indomethacin and acetylsalicylic acid failed to decrease the osteoclastogenic potential of the corresponding supernatants; and (c) CTXA(2)-induced osteoclast-like cell formation independent of indomethacin and the selective COX-2 inhibitor NS398.     

Retinol induces platelet aggregation via activation of phospholipase A2

All-trans-retinol induced aggregation of rabbit platelets, and this effect could be inhibited by a cyclooxygenase inhibitor and a thromboxane A2 (TXA2) receptor antagonist, indicating an essential role for endogenously produced TXA2. We found a two-phase arachidonic acid release in retinol-stimulated platelets. The first phase was induced by the action of retinol alone and not inhibited by TXA2 receptor antagonist. The second phase was induced via synergistic action of retinol and initially generated small amount of TXA2, and was inhibited by the antagonist. Moreover, we discussed that the arachidonic acid release may be mediated by the action of phospholipase A2.     

Effect of thromboxane A2 (TXA2) synthase inhibitor and TXA2 receptor antagonist alone and in combination on antigen-induced bronchoconstriction in guinea pigs

Thromboxane A2 (TXA2) causes bronchoconstriction and bronchial hyperresponsiveness. Two types of TXA2 modifiers, one synthase inhibitor and one receptor antagonist, are widely used for the treatment of asthma in Japan. Although the target of TXA2 modifiers is to inhibit bioactivity of TXA2, the pharmacological properties are somewhat different between these drugs. We studied the inhibitory effects of the TXA2 synthase inhibitor CS-518 and the TXA2 receptor antagonist S-1452 alone and in combination on antigen-induced bronchoconstriction in passively sensitized guinea pigs treated with diphenhydramine. Both CS-518 and S-1452 inhibited the antigen-induced bronchoconstriction dose-dependently with the plateau. The combination of these drugs at the maximal inhibitory doses did not have any more effect compared with each single dosing. The combination at the submaximal doses tended to show an additive effect, but the effect was not significant. These findings suggest that other prostanoids such as PGE2, PGI2, PGD2 and PGF2alpha may not take an important role in the antiasthmatic effects of TXA2 modifiers.     

Thromboxane A2 receptor linked with the Ca2+ pathway in rat colonic crypt cells.

To clarify the presence of thromboxane A2 (TXA2) receptor in the colonic epithelium, we examined the effect of 9,11-epithio-11, 12-methano-thromboxane A2 (STA2), a stable analogue of TXA2, on intracellular free Ca2+ concentration ([Ca2+]i) of indo-1-loaded single cells in isolated rat colonic crypts by laser confocal microscopy. STA2 increased [Ca2+]i in a concentration-dependent manner with a transient peak phase and a subsequent plateau phase. The EC50 values at peak and plateau phases were 1 and 32 nM, respectively. The STA2-induced increase in [Ca2+]i was completely blocked by two selective TXA2 receptor antagonists, KW-3635 and ONO-3708. These antagonists did not affect both the basal [Ca2+]i and the carbaco-induced increase in [Ca2+]i. Prostaglandin E2 did not increase [Ca2+]i. These results indicate that the STA2-elicited increase in [Ca2+]i is mediated specifically by a TXA2 receptor in colonic crypt cells This is the first report showing the presence of a TXA2 receptor that is associated with Ca2+ mobilization in the colon.     

Specific receptors for thromboxane A2 in cultured vascular smooth muscle cells of rat aorta

The specific binding site for thromboxane A2 (TXA2) was studied in cultured vascular smooth muscle cells (VSMC) of the rat aorta. [3H]SQ29,548, a potent and selective TXA2 receptor antagonist, displayed high-affinity and specificity, as well as saturable and displaceable binding to rat VSMC in culture. Scatchard analysis of equilibrium binding at 24 degrees C revealed a single class of binding sites with a Kd of 1.7 nM and a Bmax of 8.0 fmol/10(6) cells. A series of TXA2 receptor antagonists completely suppressed [3H]SQ29,548 binding to rat VSMC, and the rank order of their inhibitory potencies (Ki) correlated well with the potencies for suppression of the U46619-induced contraction of rat thoracic aorta. These results suggest that specific binding sites for [3H]SQ29,548 represent the TXA2 receptor in rat VSMC.     

Synthesis and biological characterization of sqbazide, a novel biotinylated photoaffinity probe for the study of the human platelet thromboxane A2 receptor

SQBAzide, a biotinylated, azido derivative of the TXA2 receptor antagonist, SQ31,491, was synthesized and characterized. The compound specifically inhibited human platelet aggregation mediated by TXA2 receptor activation and irreversibly labeled platelet TXA2 receptors upon exposure to ultraviolet light. This probe should prove to be of significant value for the study of the receptor-ligand binding domain.     

The effect of three novel thromboxane A2 receptor antagonists (S-1452, AA-2414 and ONO-3708) on the increase in pulmonary pressure caused by Forssman anaphylaxis in guinea-pigs.

The effects were studied of three novel thromboxane A2 (TXA2) receptor antagonists (S-1452, AA-2414 and ONO-3708) on the increase in pulmonary pressure caused by Forssman anaphylaxis in guinea-pigs. Three TXA2 antagonists at doses of between 1 and 10 mg/kg administered orally 1 h before the challenge clearly inhibited the pulmonary pressure increase. At a dose of 10 mg/kg, all three antagonists inhibited the pulmonary pressure increase caused by leukotriene D4 (LTD4) and U-46619, but not that caused by histamine. The decrease in peripheral platelet counts caused by Forssman anaphylaxis was also clearly inhibited by the three TXA2 antagonists. However, the decreased peripheral leukocyte counts were unaffected by the three agents. The decrease in serum complement activity (CH50) was inhibited by S-1452 and AA-2414 at a dose of 10 mg/kg. In bronchoalveolar lavage fluid (BALF), significant increases in eosinophils and neutrophils were observed after Forssman anaphylaxis. Three TXA2 antagonists at a dose of 10 mg/kg (except for AA-2414 on eosinophils) did not affect the changes of leukocyte counts in BALF. Moreover, increases in the TXB2 and 6-keto-PGF1 alpha levels of the BALF brought about by Forssman anaphylaxis were unaffected by the three TXA2 receptor antagonists. Histamine and LTD4 were not changed in the BALF after Forssman anaphylaxis. These results indicate the efficacy of TXA2 receptor antagonists on the increase in pulmonary pressure caused by Forssman anaphylaxis in guinea-pigs by direct antagonism to released TXA2.