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F Ushikubi

Publications and source records attributed to F Ushikubi.

At least 55 records · Page 3Linked to original sources
PubMed1997

Identification of domains conferring ligand binding specificity to the prostanoid receptor. Studies on chimeric prostacyclin/prostaglandin D receptors.

To identify domains conferring ligand binding specificity to prostanoid receptors, we constructed a series of chimeric receptors by successively replacing the regions from the carboxyl-terminal tail of mouse prostacyclin (prostaglandin I (PGI)) receptor (mIP) with the corresponding regions of the mouse PGD receptor (mDP). The mIP receptor expressed in COS 7 cells bound [3H]iloprost, a PGI2 analog, and [3H]PGE1 with Kd values of 13 and 27 nM, respectively. This receptor did not bind [3H]PGD2, [3H]PGE2, and [3H]PGF2alpha. The mDP receptor bound only [3H]PGD2 with a Kd value of 43 nM. The chimeric IPN-VII/DPC receptor with replacement of the carboxyl tail of the mIP receptor with that of the mDP receptor showed 12-16-fold higher affinities for [3H]iloprost and [3H]PGE1 than the mIP receptor. The region extending from the sixth transmembrane domain to the carboxyl terminus of the mIP receptor was next replaced with the corresponding region of the mDP receptor. This chimeric IPN-V/DPVI-C receptor acquired the ability to bind [3H]PGD2 and [3H]PGE2 without decreasing the affinities of the mIP receptor to [3H]iloprost and [3H]PGE1. These binding characteristics did not change when the fourth and fifth transmembrane domains of the mIP receptor were further replaced with the corresponding regions of the mDP receptor. However, when the first extracellular to second intracellular loop of the mIP receptor containing the third transmembrane domain was further replaced with those of the mDP receptor, the affinities for [3H]PGE1, [3H]PGE2, and [3H]iloprost were markedly decreased, whereas that for [3H]PGD2 was increased by about 2-fold. [3H]PGF2alpha showed no affinity for the mIP, mDP, and all the chimeric receptors. These results suggest that the sixth to seventh transmembrane domain of the mIP receptor confers the specificity of this receptor to bind selectively to PGE1 and not to PGE2 and that the third transmembrane domain of the mDP receptor confers the selective binding of PGD2 to this receptor.

Animals↗
PubMed1997

Ligand binding specificities of the eight types and subtypes of the mouse prostanoid receptors expressed in Chinese hamster ovary cells.

1. Eight types and subtypes of the mouse prostanoid receptor, the prostaglandin D (DP) receptor, the prostaglandin F (FP) receptor, the prostaglandin I (IP) receptor, the thromboxane A (TP) receptor and the EP1, EP2, EP3 and EP4 subtypes of the prostaglandin E receptor, were stably expressed in Chinese hamster ovary cells. Their ligand binding characteristics were examined with thirty two prostanoids and their analogues by determining the Ki values from the displacement curves of radioligand binding to the respective receptors. 2. The DP, IP and TP receptors showed high ligand binding specificity and only bound their own putative ligands with high affinity such as PGD2, BW245C and BW868C for DP, cicaprost, iloprost and isocabacyclin for IP, and S-145, I-BOP and GR 32191 for TP. 3. The FP receptor bound PGF2 alpha and fluprostenol with Ki values of 3-4 nM. In addition, PGD2, 17-phenyl-PGE2, STA2, I-BOP, PGE2 and M&B-28767 bound to this receptor with Ki values less than 100 nM. 4. The EP1 receptor bound 17-phenyl-PGE2, sulprostone and iloprost in addition to PGE2 and PGE1, with Ki values of 14-36 nM. 16,16-dimethyl-PGE2 and two putative EP1 antagonists, AH6809 and SC-19220, did not show any significant binding to this receptor. M&B-28767, a putative EP3 agonist, and misoprostol, a putative EP2/EP3 agonist, also bound to this receptor with Ki values of 120 nM. 5. The EP2 and EP4 receptors showed similar binding profiles. They bound 16,16-dimethyl PGE2 and 11-deoxy-PGE1 in addition to PGE2 and PGE1. The two receptors were discriminated by butaprost, AH-13205 and AH-6809 that bound to the EP2 receptor but not to the EP4 receptor, and by 1-OH-PGE1 that bound to the EP4 but not to the EP2 receptor. 6. The EP3 receptor showed the broadest binding profile, and bound sulprostone, M&B-28767, GR63799X, 11-deoxy-PGE1, 16,16-dimethyl-PGE2 and 17-phenyl-PGE2, in addition to PGE2 and PGE1, with Ki values of 0.6-3.7 nM. In addition, three IP ligands, iloprost, carbacyclin and isocarbacyclin, and one TP ligand, STA2, bound to this receptor with Ki values comparable to the Ki values of these compounds for the IP and TP receptors, respectively. 7. 8-Epi-PGF2 alpha showed only weak binding to the IP, TP, FP, EP2 and EP3 receptor at 10 microM concentration.

Animals↗
PubMed1996

Gq-coupled receptors transmit the signal for GLUT4 translocation via an insulin-independent pathway.

Guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) induces the translocation of glucose transporter type 4 (GLUT4) from an intracellular pool to the cell surface and increases glucose uptake in adipocytes. The GTP-binding protein(s) responsible for the translocation has remained to be identified. Using a sensitive and quantitative method to assess the translocation of c-MYC epitope-tagged GLUT4, we obtained evidence that the activation of receptor-coupled Gq (neither Gi nor Gs) triggered GLUT4 translocation in cells, independently of insulin signaling pathway(s). Platelet-activating factor (PAF) induced GLUT4 translocation in the cells expressing the Gi- and Gq-coupled PAF receptor, but the translocation was induced even after pretreatment with wortmannin, an islet-activating protein and phorbol 12, 13-dibutyrate. Norepinephrine triggered GLUT4 translocation in cells expressing the Gq-coupled alpha1-adrenergic receptor, but prostaglandin E2 did not cause GLUT4 translocation in cells expressing the Gs-coupled EP4 receptor or the Gi-coupled EP3alpha receptor. The norepinephrine-stimulated GLUT4 translocation and glucose uptake via Gq may possibly contribute to the fuel supply required for thermogenesis in brown adipocytes and for the enhanced contractility in cardiomyocytes, both of which have an abundant endogenous GLUT4.

3T3 Cells↗
PubMed1996

Two thromboxane A2 receptor isoforms in human platelets. Opposite coupling to adenylyl cyclase with different sensitivity to Arg60 to Leu mutation.

Thromboxane A2 (TXA2) receptor is a key molecule in hemostasis as its abnormality leads to bleeding disorders. Two isoforms of the human TXA2 receptor have been cloned; one from placenta and the other from endothelium, here referred to as TXR alpha and TXR beta, respectively. These isoforms differ only in their carboxyl-terminal tails. We report that both isoforms are present in human platelets. The two isoforms expressed in cultured cells show similar ligand binding characteristics and phospholipase C (PLC) activation but oppositely regulate adenylyl cyclase activity; TXR alpha activates adenylyl cyclase, while TXR beta inhibits it. The Arg60 to Leu mutant of TXR alpha, which has been shown to impair PLC activation (Hirata, T., A. Kakizuka, F. Ushikubi, I. Fuse, M. Okuma, and S. Narumiya. 1994. J. Clin. Invest. 94: 1662-1667), also impairs adenylyl cyclase stimulation, whereas that of TXR beta retains its activity to inhibit adenylyl cyclase. These findings suggest that the pathway linked to adenylyl cyclase inhibition might be involved in some of the TXA2-induced platelet responses such as shape change and phospholipase A2 activation which remain unaffected in the patients with this mutation.

Adenylyl Cyclases↗
PubMed1996

Sevoflurane inhibits human platelet aggregation and thromboxane A2 formation, possibly by suppression of cyclooxygenase activity.

BACKGROUND: Halothane increases bleeding time and suppresses platelet aggregation in vivo and in vitro. A previous study by the authors suggests that halothane inhibits platelet aggregation by reducing thromboxane (TX) A2 receptor-binding affinity. However, no studies of the effects of sevoflurane on platelet aggregation have been published. METHODS: The effects of sevoflurane, halothane, and isoflurane were examined at doses of 0.13-1.4 mM. Human platelet aggregation was induced by adenosine diphosphate, epinephrine, arachidonic acid, prostaglandin G2, and a TXA2 agonist ([+]-9, 11-epithia-11, 12-methano-TXA2, STA2) and measured by aggregometry. Platelet TXB2 levels were measured by radioimmunoassay, and the ligand-binding characteristics of the TXA2 receptors were examined by Scatchard analysis using a [3H]-labeled TXA2 receptor antagonist (5Z-7-(3-endo-([ring-4-[3H] phenyl) sulphonylamino-[2.2.1.] bicyclohept-2-exo-yl) heptenoic acid, [3H]S145). RESULTS: Isoflurane (0.28-0.84 mM) did not significantly affect platelet aggregation induced by adenosine diphosphate and epinephrine. Sevoflurane (0.13-0.91 mM) and halothane (0.49-1.25 mM) inhibited secondary platelet aggregation induced by adenosine diphosphate (1-10 microM) and epinephrine (1-10 microM) without altering primary aggregation. Sevoflurane (0.13 mM) also inhibited arachidonic acid-induced aggregation, but not that induced by prostaglandin G2 or STA2, although halothane (0.49 mM) inhibited the latter. Sevoflurane (3 mM) did not affect the binding of [3H]S145 to platelets, whereas halothane (3.3 mM) suppressed it strongly. Sevoflurane (0.26 mM) and halothane (0.98 mM) strongly suppressed TXB2 formation by arachidonic acid-stimulated platelets. CONCLUSIONS: The findings that sevoflurane suppressed the effects of arachidonic acid, but not those of prostaglandin G2 and STA2, suggest strongly that sevoflurane inhibited TXA2 formation by suppressing cyclooxygenase activity. Halothane appeared to suppress both TXA2 formation and binding to its receptors. Sevoflurane has strong antiaggregatory effects at subanesthetic concentrations (greater than 0.13 mM; i.e., approximately 0.5 vol/%), whereas halothane has similar effects at somewhat greater anesthetic concentrations (0.49 mM; i.e., approximately 0.54 vol/%). Isoflurane at clinical concentration (0.84 mM; i.e., approximately 1.82 vol/%) does not affect platelet aggregation significantly.

Adenosine Diphosphate↗
PubMed1995

Involvement of thromboxane A2-thromboxane A2 receptor system of the hepatic sinusoid in pathogenesis of cold preservation/reperfusion injury in the rat liver graft.

This study was designed to investigate the possible involvement of the thromboxane A2 (TXA2)-TXA2 receptor (TXA2R) system of the hepatic sinusoid in cold preservation/reperfusion injury in liver grafts. Rat livers were preserved in cold University of Wisconsin solution for either 6 or 24 hr. The number of TXA2Rs in sinusoidal endothelial cells isolated from 0-, 6-, and 24-hr preserved liver specimens was 22.50 +/- 1.80 x 10(3)/cell, 12.66 +/- 1.00 x 10(3)/cell, and 4.17 +/- 0.65 x 10(3)/cell, respectively. Kd and Bmax at 0 hr, 6 hr, and 24 hr of preservation were 8.54 +/- 1.26 nM and 37.34 +/- 3.01 fmol/10(6) cells, 7.08 +/- 1.14 nM and 12.66 +/- 1.00 fmol/10(6) cells, and 1.91 +/- 0.10 nM and 3.88 +/- 0.59 fmol/10(6) cells, respectively. The administration of OKY-046 (inhibitor of TXA2 synthesis) to the University of Wisconsin solution suppressed this reduction in TXA2R number. Furthermore, the concentration of TXA2 in hepatic sinusoid was decreased by OKY-046. In a reperfusion experiment, liver tissue preserved for 24 hr exhibited a higher reperfusion pressure, and effluent levels of both aspartate aminotransferase and lactate dehydrogenase were markedly elevated. The addition of OKY-046 to the preservation solution, however, prevented the rise in reperfusion pressure almost completely and the increase in effluent enzyme levels. This study showed that the TXA2Rs in sinusoidal endothelial cells were internalized through binding with TXA2 during cold preservation, causing activation of the TXA2-TXA2R system. This activation apparently induces an increase in reperfusion pressure, possibly due to sinusoidal contraction, resulting in microcirculatory disturbances.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1995

The effect of inhaled anesthetics on the platelet aggregation and the ligand-binding affinity of the platelet thromboxane A2 receptor.

The mechanism by which anesthetics suppress platelet aggregation has not been elucidated. We determined the effects of halothane, enflurane, and isoflurane on human platelet aggregation induced by adenosine diphosphate (ADP), epinephrine, and a thromboxane A2 (TXA2) analog, and on ligand binding to the platelet TXA2 receptor. Halothane (2.6 mM) strongly suppressed ADP- and epinephrine-induced secondary aggregation of platelets, without significant alteration of primary aggregation. Platelet aggregation induced by a specific TXA2 agonist, (+)-9,11-epithia-11,12-methano-TXA2 (STA2), was suppressed by halothane, enflurane, and isoflurane in a concentration-dependent manner; the concentration of halothane, enflurane, and isoflurane which induced 50% inhibition (IC50) were 3.2, 12.3, and 15.7 mM, respectively (or 4.7, 9.8, and 24 minimum alveolar anesthetic concentration [MAC], respectively). The binding of a specific TXA2 receptor antagonist, 3H-S145, was significantly reduced by halothane (14-28 mM), but not by enflurane (20 mM) and isoflurane (20 mM). Scatchard analysis revealed that halothane (14 mM) increased Kd from 0.53 nM to 14.3 nM but did not alter Bmax significantly. These results indicate that halothane has a stronger suppressive effect on platelet aggregation than enflurane and isoflurane, and that the effect of halothane on platelet aggregation is due to reduction of the ligand-binding affinity of the platelet TXA2 receptor.

Adenosine Diphosphate↗
PubMed1995

In situ hybridization studies of prostacyclin receptor mRNA expression in various mouse organs.

1. Expression of prostacyclin receptor (IP receptor) mRNA was examined in various mouse organs, and the cells expressing IP receptor mRNA were identified by in situ hybridization studies. Co-localization of mRNA for the IP receptor with that for preprotachykinin A (PPTA), a precursor protein for substance P, with mRNA for the prostaglandin E receptor subtypes (EP1, EP3 and EP4), and with renin mRNA, was examined by double in situ hybridization studies in the dorsal root ganglion and kidney, respectively. 2. IP receptor mRNA was expressed in the thymus and spleen. Expression in the thymus was found exclusively in the medulla, where mature thymocytes expressed transcripts for the IP receptor. Expression in the spleen was found as scattered signals over the white pulp and as punctate signals in the red pulp. The former was found in splenic lymphocytes and the latter in megakaryocytes. 3. IP receptor mRNA was also expressed in the vascular tissues of various organs such as the aorta, coronary arteries, pulmonary arteries and the cerebral arteries, where its expression was confined to smooth muscle cells. No expression was found in veins. In the kidney, IP receptor mRNA was detected in the interlobular arteries and glomerular arterioles but not in the juxtaglomerular (JG) cells which were labelled with the renin mRNA probe. 4. IP receptor mRNA was expressed in about 40% of the neurones in the dorsal root ganglion. Both small- and large-sized neurones were labelled but no labelling was found in the glia. Expression of PPTA mRNA was found in about 30% of total neurones. About 70% of these neurones expressed IP receptor mRNA, and about half of the IP receptor-positive neurones expressed PPTA mRNA. In addition to IP mRNA, mRNAs for EP1, EP3 and EP4 receptors were expressed in about 30%, 50% and 20%, respectively, of the dorsal root ganglion neurones. About 25%, 41% and 24% of the IP receptor-positive neurons co-expressed the EP1, EP3 and EP4 receptor, respectively. 5. These results not only verified IP receptor expression in various cells and tissues known to be sensitive to prostacyclin, but also revealed its expression in other systems, which urges the study of the actions of prostacyclin in these tissues. They also indicated that the actions of prostacyclin on blood vessels and platelets are mediated by the same type of receptor. Absence of IP receptor mRNA in the JG cells suggests that the action of prostacyclin on renin release may be indirect.

Animals↗
PubMed1994

Prostaglandin E2 protects cultured cortical neurons against N-methyl-D-aspartate receptor-mediated glutamate cytotoxicity.

The effects of prostaglandin (PG) E2 on glutamate-induced cytotoxicity were examined using primary cultures of rat cortical neurons. The cell viability was significantly reduced when cultures were briefly exposed to either glutamate or N-methyl-D-aspartate (NMDA) then incubated with normal medium for 1 h. Similar cytotoxicity was observed with the brief application of ionomycin, a calcium ionophore, and S-nitrosocysteine, a nitric oxide (NO)-generating agent. PGE2 at concentrations of 0.01-1 microM dose-dependently ameliorated the glutamate-induced cytotoxicity. PGE1, butaprost, an EP2 receptor agonist, and 8-bromo-cAMP were also effective in protecting cultures against glutamate cytotoxicity. By contrast, neither 17-phenyl-omega-trinor-PGE2, an EP1 receptor agonist, nor M&B 28767, an EP3 receptor agonist, affected glutamate-induced cytotoxicity. NMDA-induced cytotoxicity was ameliorated by PGE2, butaprost, MK-801, N-omega-nitro-L-arginine, a NO synthase inhibitor, and hemoglobin, which binds NO. These agents excluding MK-801 ameliorated the ionomycin-induced cytotoxicity. The cytotoxicity induced by S-nitrosocysteine was prevented only by hemoglobin but not by the other agents including PGE2. These findings indicate that PGE2 protects cultured cortical neurons against NMDA receptor-mediated glutamate neurotoxicity via EP2 receptors. EP2 receptor stimulation may suppress a step in NO formation triggered by Ca(2+)-influx through NMDA receptors.

Animals↗
PubMed1994

Molecular characterization of a mouse prostaglandin D receptor and functional expression of the cloned gene.

Prostanoid receptors belong to the family of G protein-coupled receptors with seven transmembrane domains. By taking advantage of nucleotide sequence homology among the prostanoid receptors, we have isolated and identified a cDNA fragment and its gene encoding a mouse prostaglandin (PG) D receptor by reverse transcription polymerase chain reaction and gene cloning. This gene codes for a polypeptide of 357 amino acids, with a calculated molecular weight of 40,012. The deduced amino acid sequence has a high degree of similarity with the mouse PGI receptor and the EP2 subtype of the PGE receptor, which together form a subgroup of the prostanoid receptors. Chinese hamster ovary cells stably expressing the gene showed a single class of binding sites for [#H]PGD2 with a Kd of 40 nM. This binding was displaced by unlabeled ligands in the following order: PGD2 > BW 245C (a PGD agonist) > BW A868C (a PGD antagonist) > STA2 (a thromboxane A2 agonist). PGE2, PGF2 alpha, and iloprost showed little displacement activity at concentrations up to 10 microM. PGD2 and BW 245C also increased cAMP levels in Chinese hamster ovary cells expressing the receptor, in a concentration-dependent manner. BW A868C showed a partial agonist activity in the cAMP assay. Northern blotting analysis with mouse poly(A)+ RNA identified a major mRNA species of 3.5 kb that was most abundantly expressed in the ileum, followed by lung, stomach, and uterus.

Amino Acid Sequence↗
PubMed1994

Arg60 to Leu mutation of the human thromboxane A2 receptor in a dominantly inherited bleeding disorder.

Recent advances in molecular genetics have revealed the mechanisms underlying a variety of inherited human disorders. Among them, mutations in G protein-coupled receptors have clearly demonstrated two types of abnormalities, namely loss of function and constitutive activation of the receptors. Thromboxane A2 (TXA2) receptor is a member of the family of G protein-coupled receptors and performs an essential role in hemostasis by interacting with TXA2 to induce platelet aggregation. Here we identify a single amino acid substitution (Arg60-->Leu) in the first cytoplasmic loop of the TXA2 receptor in a dominantly inherited bleeding disorder characterized by defective platelet response to TXA2. This mutation was found exclusively in affected members of two unrelated families with the disorder. The mutant receptor expressed in Chinese hamster ovary cells showed decreased agonist-induced second messenger formation despite its normal ligand binding affinities. These results suggest that the Arg60 to Leu mutation is responsible for the disorder. Moreover, dominant inheritance of the disorder suggests the possibility that the mutation produces a dominant negative TXA2 receptor.

Amino Acid Sequence↗
PubMed1994

Functional reconstitution of platelet thromboxane A2 receptors with Gq and Gi2 in phospholipid vesicles.

The partially purified thromboxane (TX) A2 receptor was reconstituted with two species of purified heterotrimeric G proteins, Gq and Gi2, in phospholipid vesicles. The receptors reconstituted with Gq and Gi2 showed a single class of [3H]S-145 binding with Kd values of 9.6 +/- 0.7 and 12.1 +/- 1.0 nM, respectively; binding was displaced by GR32191, 9,11-epithio-11, 12-methano-thromboxane A2 (STA2), and U46619, with almost identical Ki values for each compound in the two types of reconstituted vesicles. When the receptor and Gq were reconstituted, the agonist STA2 stimulated guanosine-5'-O-(3-[35S]thio)triphosphate binding. This stimulation was half-maximal at 80 nM and reached a plateau at 1 microM STA2 stimulated the initial rate by 20-30-fold, compared with the basal rate. The stimulation of guanosine-5'-O-(3-[35S]thio)triphosphate binding to Gi2 by the agonist-liganded receptor was seen in the presence of GDP. Under these conditions, 10 microM STA2 stimulated the initial rate by 1.5-2-fold, compared with the basal rate. This effect was half-maximal at 150 nM and reached a plateau at 1 microM. The agonist-liganded receptor also stimulated the GTPase activities of the reconstituted G proteins. The steady state rates of STA2-stimulated [32P]Pi release from [gamma-32P]GTP were 2.21/min.receptor and 0.87/min.receptor in the Gq- and Gi2-reconsituted vesicles, respectively, and the Kcat values of Gq and Gi2 in the presence of STA2 were 0.87 +/- 0.21 min-1 and 2.41 +/- 0.12 min-1, respectively. These results clearly show that the TXA2 receptor functionally couples to both Gq and Gi2. Consistent with this finding, STA2, by acting on the TXA2 receptor in intact platelets, inhibited prostaglandin I2-induced cAMP elevation.

Blood Platelets↗
PubMed1993

Thromboxane A2 receptor is highly expressed in mouse immature thymocytes and mediates DNA fragmentation and apoptosis.

We have recently revealed that the thymus is the organ showing the highest expression of thromboxane (TX) A2 receptor in mice. In this study, thymic cell populations expressing the receptor were identified, and the effects of a TXA2 agonist on these cells were examined. Radioligand binding using a TXA2 receptor-specific radioligand revealed a single class of binding sites in the thymocytes with an affinity and specificity identical to those reported for the TXA2 receptor. The receptor density in these cells was comparable to that seen in blood platelets. This receptor is most highly expressed in CD4-8- and CD4+8+ immature thymocytes, followed by CD4+8- and CD4-8+ cells. The receptor density in splenic T cells was less than one fifth of that in CD4+8+ cells and no binding activity was detectable in splenic B cells. The addition of a TXA2 agonist, STA2, to thymocytes induced the disappearance of the CD4+8+ cells in a time- and concentration-dependent manner and caused DNA fragmentation. These changes were blocked by a specific TXA2 antagonist, S-145. These results demonstrate that TXA2 induces apoptotic cell death in immature thymocytes by acting on the TXA2 receptor on their cell surface and suggest a role for the TXA2/TXA2 receptor system in the thymic micro-environment.

Animals↗
PubMed1993

Alternative splicing of C-terminal tail of prostaglandin E receptor subtype EP3 determines G-protein specificity.

Peptide hormones, neurotransmitters, and autacoids activate a family of seven-transmembrane-domain receptors. Each of these receptors specifically couples to one of several G proteins, Gs, Gi, G(o) and Gp, to activate a specific second messenger system. Cell surface receptors for prostanoids have been characterized pharmacologically and the complementary DNAs for thromboxane A2 receptor and the EP3 subtype of the prostaglandin (PG)E receptor reveal that they belong to the seven-transmembrane-domain receptor family. The EP3 receptor mediates the diverse physiological actions of PGE2 (ref. 3). Although most of them occur through coupling of the EP3 receptor to Gi and inhibition of adenylyl cyclase, the EP3-mediated contraction of uterine muscle can only occur by activation of another second messenger pathway. In chromaffin cells, two different second messenger pathways are activated by PGE2 binding to an apparently single EP3 receptor class. Here we show that at least four isoforms of the EP3 receptor, which differ only at their C-terminal tails and are produced by alternative splicing, couple to different G proteins to activate different second messenger systems.

Adrenal Medulla↗
PubMed1993

Defective signal transduction induced by thromboxane A2 in a patient with a mild bleeding disorder: impaired phospholipase C activation despite normal phospholipase A2 activation.

A patient with a mild bleeding disorder whose platelets responded defectively to thromboxane A2 (TXA2) was identified, and the mechanism of this dysfunction was analyzed. The platelets were defective in shape change, aggregation, and release reaction in response to synthetic TXA2 mimetic (STA2). When the platelet TXA2 receptor was examined with both a 125I-labeled derivative of a TXA2 receptor antagonist ([125I]-PTAOH) and [3H]-labeled TXA2 agonist ([3H]U-46619), the equilibrium dissociation rate constants (kd) and the maximal concentrations of binding sites (Bmax) of the platelets to both ligands were within normal ranges, suggesting that the binding capacity of their TXA2 receptor was normal. STA2 could not induce IP3 formation and intracellular Ca2+ mobilization, whereas these responses to thrombin were within normal ranges. GTPase activity was also decreased when the patient's platelet membrane was challenged with STA2. On the other hand, lysophosphatidylinositol formation, which is a direct indicator of phospholipase A2 (PLA2) activation, was found to be normal when the [3H]-inositol-labeled platelets were challenged with STA2. Thromboxane B2 (TXB2) was also produced in response to STA2. These results suggested that the abnormality in these platelets was impaired coupling between TXA2 receptor and phospholipase C (PLC) activation. Furthermore, it is also suggested that the activation of PLA2 and PLC are separable events in thromboxane-induced platelet activation.

Adult↗