Pharmacodynamic Comparison of Ticagrelor Monotherapy Versus Ticagrelor and Aspirin in Patients After Percutaneous Coronary Intervention: The TEMPLATE (Ticagrelor Monotherapy and Platelet Reactivity) Randomized Controlled Trial

Background

To assess differences in platelet inhibition during ticagrelor monotherapy (TIC) or dual therapy with ticagrelor and aspirin (TIC+ASP) in patients after percutaneous coronary intervention using a comprehensive panel of functional tests.

Methods and Results

In a single‐center parallel group, open label, randomized controlled trial, 110 participants were randomized to receive either TIC (n=55) or TIC+ASP (n=55) for 4 weeks. The primary outcome was the platelet aggregation response with 10 μmol/L thrombin receptor activation peptide‐6 (TRAP‐6). The secondary outcomes were platelet aggregation responses and binding of surface activation markers with a panel of other activators. The mean percentage aggregation for 10 μmol/L TRAP‐6 was similar for the TIC and TIC+ASP groups (mean difference+4.29; 95% CI, −0.87 to +9.46). Aggregation was higher in the TIC group compared with the TIC+ASP group with 1 μg/mL (+6.47; +2.04 to +10.90) and 0.5 μg/mL (+14.00; +7.63 to +20.39) collagen related peptide. Aggregation responses with 5 μmol/L TRAP‐6, 5 μmol/L or 2.5 μmol/L thromboxane A₂ receptor agonist and surface activation marker binding with 5 μmol/L TRAP‐6 or 0.5 μg/mL collagen related peptide were the same between the treatment groups.

Conclusions

Patients with PCI show similar levels of inhibition of most platelet activation pathways with TIC compared with dual therapy with TIC + ASP. However, the greater aggregation response with collagen related peptide during TIC indicates incomplete inhibition of glycoprotein VI (collagen) receptor‐mediated platelet activation. This difference in pharmacodynamic response to anti‐platelet medication may contribute to the lower bleeding rates observed with TIC compared with dual antiplatelet therapy in recent clinical trials.

Registration Information

URL: https://www.isrctn.com; Unique Identifier ISRCTN84335288.

A vasoconstrictor response to COX-1-mediated prostacyclin synthesis in young rat renal arteries that increases in prehypertensive conditions

This study aimed to determine whether prostacyclin (PGI2) functions as an endothelium-derived contracting factor (EDCF) in young rat renal arteries, and, if so, we wanted to examine the underlying mechanism(s) and how it changes in prehypertensive conditions. Vessels from Wistar-Kyoto (WKY) and prehypertensive spontaneously hypertensive rats (SHRs) of 25-28 days of age were isolated for functional and biochemical analyses. Result showed that following NO synthase (NOS) inhibition PGI2 and the thromboxane-prostanoid (TP) receptor agonist U-46619 evoked contractions in young WKY renal arteries that were similar to those in prehypertensive SHRs. Meanwhile, the endothelial muscarinic receptor agonist ACh evoked an endothelium-dependent contraction under NOS-inhibited conditions and a production of the PGI2 metabolite 6-keto-PGF1α; both were sensitive to cyclooxygenase (COX) and/or COX-1 inhibition but higher in prehypertensive SHRs than in young WKYs. Interestingly, in WKY renal arteries PGI2 did not evoke relaxation even after TP receptor antagonism that diminished the contraction evoked by the agonist. Indeed, PGI2 (IP) receptors were not detected in the vessel with Western blot. Moreover, we noted that treatment with the nonselective COX inhibitor indomethacin, which was started at the prehypertensive stage, blunted the elevation of systolic blood pressure and reduced the heart-to-body ratio in SHR within 2 mo of treatment. These results demonstrate that due to scarcity of IP receptors, PGI2, which is derived mainly from COX-1-mediated metabolism, acts as an EDCF in young WKY renal arteries, and it increases in prehypertensive conditions. Also, our data revealed that COX inhibition starting from the prehypertensive stage has an antihypertensive effect in young SHRs.

Inverse agonism of SQ 29,548 and Ramatroban on Thromboxane A2 receptor

G protein-coupled receptors (GPCRs) show some level of basal activity even in the absence of an agonist, a phenomenon referred to as constitutive activity. Such constitutive activity in GPCRs is known to have important pathophysiological roles in human disease. The thromboxane A2 receptor (TP) is a GPCR that promotes thrombosis in response to binding of the prostanoid, thromboxane A2. TP dysfunction is widely implicated in pathophysiological conditions such as bleeding disorders, hypertension and cardiovascular disease. Recently, we reported the characterization of a few constitutively active mutants (CAMs) in TP, including a genetic variant A160T. Using these CAMs as reporters, we now test the inverse agonist properties of known antagonists of TP, SQ 29,548, Ramatroban, L-670596 and Diclofenac, in HEK293T cells. Interestingly, SQ 29,548 reduced the basal activity of both, WT-TP and the CAMs while Ramatroban was able to reduce the basal activity of only the CAMs. Diclofenac and L-670596 showed no statistically significant reduction in basal activity of WT-TP or CAMs. To investigate the role of these compounds on human platelet function, we tested their effects on human megakaryocyte based system for platelet activation. Both SQ 29,548 and Ramatroban reduced the platelet hyperactivity of the A160T genetic variant. Taken together, our results suggest that SQ 29,548 and Ramatroban are inverse agonists for TP, whereas, L-670596 and Diclofenac are neutral antagonists. Our findings have important therapeutic applications in the treatment of TP mediated pathophysiological conditions.

Platelet dysfunction associated with the novel Trp29Cys thromboxane A₂ receptor variant

BACKGROUND

Genetic variations that affect the structure of the thromboxane A2 receptor (TP receptor) provide insights into the function of this key platelet and vascular receptor, but are very rare in unselected populations.

OBJECTIVES

To determine the functional consequences of the TP receptor Trp29Cys (W29C) substitution.

PATIENTS/METHODS

We performed a detailed phenotypic analysis of an index case (P1) with reduced platelet aggregation and secretion responses to TP receptor pathway activators, and a heterozygous TP receptor W29C substitution. An analysis of the variant W29C TP receptor expressed in heterologous cells was performed.

RESULTS

Total TP receptor expression in platelets from P1 was similar to that of controls, but there was reduced maximum binding and reduced affinity of binding to the TP receptor antagonist [(3) H]SQ29548. HEK293 cells transfected with W29C TP receptor cDNA showed similar total TP receptor expression to wild-type (WT) controls. However, the TP receptor agonist U46619 was less potent at inducing rises in cytosolic free Ca(2+) in HEK293 cells expressing the W29C TP receptor than in WT controls, indicating reduced receptor function. Immunofluorescence microscopy and cell surface ELISA showed intracellular retention and reduced cell surface expression of the W29C TP receptor in HEK293 cells. Consistent with the platelet phenotype, both maximum binding and the affinity of binding of [(3) H]SQ29548 to the W29C TP receptor were reduced compared to WT controls.

CONCLUSION

These findings extend the phenotypic description of the very rare disorder TP receptor deficiency, and show that the W29C substitution reduces TP receptor function by reducing surface receptor expression and by disrupting ligand binding.

Activation of NAD(P)H oxidases by thromboxane A2 receptor uncouples endothelial nitric oxide synthase

OBJECTIVE

The thromboxane receptor (TPr) and multiple TPr ligands, including thromboxane A(2) (TxA(2)) and prostaglandin H(2), are elevated during vascular and atherothrombotic diseases. How TPr stimulation causes vascular injury remains poorly defined. This study was conducted to investigate the mechanism by which TPr stimulation leads to vascular injury.

METHODS AND RESULTS

Exposure of bovine aortic endothelial cells to either [1S-(1α,2β(5Z),3α(1E,3R),4α]-7-[3-(3-hydroxy-4-(d'-iodophenoxy)-1-butenyl)-7-oxabicyclo-[2.2.1] heptan-2-yl]-5'-heptenoic acid (IBOP) or U46619, 2 structurally related TxA(2) mimetics, for 24 hours markedly increased the release of superoxide anions (O(2)(·-)) and peroxynitrite (ONOO(-)) but reduced cyclic GMP, an index of nitric oxide bioactivity. IBOP also significantly suppressed activity of endothelial nitric oxide synthase (eNOS), increased enzyme-inactive eNOS monomers, and reduced levels of tetrahydrobiopterin, an essential eNOS cofactor. IBOP- and U46619-induced increases in O(2)(·-) were accompanied by the membrane translocation of the p67(phox) subunit of NAD(P)H oxidase. Pharmacological or genetic inhibition of either NAD(P)H oxidase or TPr abolished IBOP-induced O(2)(·-) formation. Furthermore, TPr activation significantly increased protein kinase C-ζ (PKC-ζ) in membrane fractions and PKC-ζ phosphorylation at Thr410. Consistently, PKC-ζ inhibition abolished TPr activation-induced membrane translocation of p67(phox) and O(2)(·-) production. Finally, exposure of isolated mouse aortae to IBOP markedly increased O(2)(·-) in wild-type but not in those from gp91(phox) knockout mice.

CONCLUSIONS

We conclude that TPr activation via PKC-ζ-mediated NAD(P)H oxidase activation increases both O(2)(·-) and ONOO(-), resulting in eNOS uncoupling in endothelial cells.

Up-regulation of G-protein-coupled receptors for endothelin and thromboxane by lipid-soluble smoke particles in renal artery of rat

Up-regulation of G-protein-coupled receptors (GPCR) plays key roles in renal hypertension and cardiovascular disease pathogenesis. The present study was designed to examine if lipid-soluble cigarette smoking particles (DSP), nicotine and endotoxin (LPS), induce GPCR up-regulation for thromboxane A(2) (TP), endothelin type A (ET(A) ) and type B (ET(B) ) receptors in renal artery, and if intracellular signal mechanisms are involved. Renal artery segments of rats were exposed to DSP, nicotine or LPS, in organ culture for up to 24 hr. The GPCR-mediated contractions were recorded by using a myograph system. Expression of the GPCR was examined by real-time PCR and immunohistochemistry at mRNA and protein levels. Sarafatoxin 6c (S6c, selective ET(B) receptor agonist), endothelin-1 (ET-1, non-selective ET(A) and ET(B) receptor agonist) and 9,11-Dideoxy-9a,11a-methanoepoxy prostaglandin F(2a) (U46619, a TP receptor agonist) induced contractions were significantly increased after the arterial segments exposed to DSP in a concentration-dependent (0.1-0.4 μl/ml) manner, and S6c also induced a time-dependent contraction, compared to control (dimethyl sulfoxide). This was in parallel with enhanced mRNA expression for ET(B) receptor but not ET(A) and TP receptors, while increased protein expression for ET(A) , ET(B) and TP receptors was seen. The specific nuclear factor-kappa B (NF-κB) signal pathway inhibitor BMS345541 was applied to link DSP effects to the GPCR up-regulation. It totally abolished ET(B) receptor up-regulation, but not ET(A) and TP receptor up-regulations. Our results suggest that DSP transcriptionally up-regulated ET(B) receptor expression in rat renal artery via NF-κB signal pathways, whereas up-regulation of ET(A) and TP receptor-mediated contraction may involve post-transcriptional mechanisms.

Piperlongumine, a constituent of Piper longum L., inhibits rabbit platelet aggregation as a thromboxane A(2) receptor antagonist

Piper longum L. has been used as a crude drug for the treatment of the disorder of peripherally poor blood circulation in Asia. In the present study, we examined the effect of piperlongumine, a constituent of P. longum L., on rabbit platelet aggregation. Piperlongumine concentration-dependently inhibited platelet aggregation induced by thromboxane A(2) receptor agonist U46619, but it only slightly inhibited thrombin-induced one. Piperlongumine also inhibited U46619-induced phosphatidylinositol hydrolysis and the binding of [(3)H]SQ29548 to thromboxane A(2) receptor with a similar concentration-dependency to the aggregation. It is assumed that piperlongumine inhibits platelet aggregation as a thromboxane A(2) receptor antagonist.

Homologous desensitization of signalling by the alpha (alpha) isoform of the human thromboxane A2 receptor: a specific role for nitric oxide signalling

Thromboxane (TX) A₂ plays a central role in hemostasis, regulating platelet activation status and vascular tone. We have recently established that the TPβ isoform of the human TXA₂ receptor (TP) undergoes rapid, agonist-induced homologous desensitization of signalling largely through a G protein-coupled receptor kinase (GRK) 2/3-dependent mechanism with a lesser role for protein kinase (PK) C. Herein, we investigated the mechanism of desensitization of signalling by the TPα isoform. TPα undergoes profound agonist-induced desensitization of signalling (intracellular calcium mobilization and inositol 1,4,5 trisphosphate generation) in response to the TXA₂ mimetic U46619 but, unlike that of TPβ, this is independent of GRKs. Similar to TPβ, TPα undergoes partial agonist-induced desensitization that occurs through a GF 109203X-sensitive, PKC mechanism where Ser¹⁴⁵ within intracellular domain (IC)₂ represents the key phospho-target. TPα also undergoes more profound sustained PKC- and PKG-dependent desensitization where Thr³³⁷ and Ser³³¹, respectively, within its unique C-tail domain were identified as the phospho-targets. Desensitization was impaired by the nitric oxide synthase (NOS), soluble guanylyl cyclase (sGC) and PKG inhibitors l-NAME, LY 83583 and KT5823, respectively, indicating that homologous desensitization of TPα involves nitric oxide generation and signalling. Consistent with this, U46619 led to rapid phosphorylation/activation of endogenous eNOS. Collectively, data herein suggest a mechanism whereby agonist-induced PKC phosphorylation of Ser¹⁴⁵ partially and transiently impairs TPα signalling while PKG- and PKC-phosphorylation at both Ser³³¹ and Thr³³⁷, respectively, within its C-tail domain profoundly desensitizes TPα, effectively terminating its signalling. Hence, in addition to the agonist-mediated PKC feedback mechanism, U46619-activation of the NOS/sGC/PKG pathway plays a significant role in inducing homologous desensitization of TPα.

Palmitoylation of the TPbeta isoform of the human thromboxane A2 receptor. Modulation of G protein: effector coupling and modes of receptor internalization

Palmitoylation is a prevalent feature amongst G protein-coupled receptors. In this study we sought to establish whether the TPα and TPβ isoforms of the human prostanoid thromboxane (TX) A₂ receptor (TP) are palmitoylated and to assess the functional consequences thereof. Consistent with the presence of three cysteines within its unique carboxyl-terminal domain, metabolic labelling and site-directed mutagenesis confirmed that TPβ is palmitoylated at Cys³⁴⁷ and, to a lesser extent, at Cys^373,377 whereas TPα is not palmitoylated. Impairment of palmitoylation did not affect TPβ expression or its ligand affinity. Conversely, agonist-induced [Ca²⁺]_i mobilization by TPβ^C347S and the non-palmitoylated TPβ^{C347,373,377S}, but not by TPβ^C373S or TPβ^C373,377S, was significantly reduced relative to the wild type TPβ suggesting that palmitoylation at Cys³⁴⁷ is specifically required for efficient Gq/phospholipase Cβ effector coupling. Furthermore, palmitoylation at Cys^373,377 is critical for TPβ internalization with TPβ^C373S, TPβ^C373,377S and TPβ^{C347,373,377S} failing to undergo either agonist-induced or temperature-dependent tonic internalization. On the other hand, whilst TPβ^C347S underwent reduced agonist-induced internalization, it underwent tonic internalization to a similar extent as TPβ. The deficiency in agonist-induced internalization by TPβ^C347S, but not by TPβ^C373,377 nor TPβ^{C347,373,377S}, was overcome by over-expression of either β-arrestin1 or β-arrestin2. Taken together, data herein suggest that whilst palmitoylation of TPβ at Cys^373,377 is critical for both agonist- and tonic-induced internalization, palmitoylation at Cys³⁴⁷ has a role in determining which pathway is followed, be it by the β-arrestin-dependent agonist-induced pathway or by the β-arrestin-independent tonic internalization pathway.

Effects of KP-496, a novel dual antagonist for leukotriene D4 and thromboxane A2 receptors, on contractions induced by various agonists in the guinea pig trachea

BACKGROUND

A dry powder inhaler of KP-496 is currently in clinical development in Japan as an anti-asthmatic agent. The aim of this study was to evaluate the in vitro pharmacological profile of KP-496.

METHODS

The antagonistic activities of KP-496 for leukotriene (LT) D(4) and thromboxane (TX) A(2) receptors were examined using the LTD(4)- and U46619-induced contractions of the isolated guinea pig trachea. The selectivity of KP-496 was examined using various agonist-induced contractions in the isolated guinea pig trachea.

RESULTS

KP-496 produced parallel rightward shifts of the LTD(4) and U46619 concentration-response curves in a concentration-dependent manner. Schild plot analyses of the antagonistic activities of KP-496 demonstrated that it is a competitive antagonist for LTD(4) and TXA(2) receptors with pA(2) values of 8.64 and 8.23, respectively. The LTD(4) antagonistic activity of KP-496 was comparable to that of pranlukast and zafirlukast but was more potent than that of montelukast. The TXA(2) antagonistic activity of KP-496 was comparable to that of seratrodast. KP-496 and seratrodast also inhibited the prostaglandin (PG) D(2)- and PGF(2alpha)-induced contractions of the isolated guinea pig trachea. KP-496 had no effect on the histamine-, acetylcholine-, serotonin- and substance P-induced contractions of the isolated guinea pig trachea.

CONCLUSIONS

These results indicate that KP-496 is a selective dual antagonist for LTD(4) and TXA(2) receptors. LTD(4) and TXA(2) play important roles in asthma, and antagonists for these mediators are being used for the treatment of asthma. Thus, KP-496 is expected to become a novel potent therapeutic agent for asthma.

Differential Mapping of the Amino Acids Mediating Agonist and Antagonist Coordination with the Human Thromboxane A2 Receptor Protein

Despite the well documented involvement of thromboxane A(2) receptor (TPR) signaling in the pathogenesis of thrombotic diseases, there are currently no rationally designed antagonists available for clinical use. To a large extent, this derives from a lack of knowledge regarding the topography of the TPR ligand binding pocket. On this basis, the purpose of the current study was to identify the specific amino acid residues in the TPR protein that regulate ligand coordination and binding. The sites selected for mutation reside within or in close proximity to a region we previously defined as a TPR ligand binding region (i.e. the C terminus of the second extracellular loop and the leading edge of the fifth transmembrane domain). Mutation of these residues caused varying effects on the TPR-ligand coordination process. Specifically, the D193A, D193Q, and D193R mutants lost SQ29,548 (antagonist) binding and exhibited a dramatically reduced calcium response, which could not be restored by elevated U46619 (agonist) doses. The F184Y mutant lost SQ29,548 binding and exhibited a reduced calcium response (which could be restored by elevated U46619); and the T186A and S191T mutants lost SQ29,548 binding and retained a normal U46619-induced calcium response. Furthermore, these last three mutants also revealed a divergence in the binding of two structurally different antagonists, SQ29,548 and BM13.505. Two separate mutants that exhibited SQ29,548 binding yielded either a normal (F196Y) or reduced (S201T) U46619 response. Finally, mutation of other residues directly adjacent to those described above (e.g. E190A and F200A) produced no detectable effects on either SQ29,548 binding or the U46619-induced response. In summary, these results identify key amino acids (in particular Asp(193)) involved in TPR ligand coordination. These findings also demonstrate that TPR-specific ligands interact with different residues in the ligand-binding pocket.

Homologous desensitization of signalling by the beta (β) isoform of the human thromboxane A2 receptor

Thromboxane (TX) A(2) is a potent stimulator of platelet activation/aggregation and smooth muscle contraction and contributes to a variety of pathologies within the vasculature. In this study, we investigated the mechanism whereby the cellular responses to TXA(2) mediated through the TPbeta isoform of the human TXA(2) receptor (TP) are dynamically regulated by examining the mechanism of agonist-induced desensitization of intracellular signalling and second messenger generation by TPbeta. It was established that TPbeta is subject to profound agonist-induced homologous desensitization of signalling (intracellular calcium mobilization and inositol 1,3,5 trisphosphate generation) in response to stimulation with the TXA(2) mimetic U46619 and this occurs through two key mechanisms: TPbeta undergoes partial agonist-induced desensitization that occurs through a GF 109203X-sensitive, protein kinase (PK)C mechanism whereby Ser(145) within intracellular domain (IC)(2) has been identified as the key phospho-target. In addition, TPbeta also undergoes more profound and sustained agonist-induced desensitization involving G protein-coupled receptor kinase (GRK)2/3-phosphorylation of both Ser(239) and Ser(357) within its IC(3) and carboxyl-terminal C-tail domains, respectively. Inhibition of phosphorylation of either Ser(239) or Ser(357), through site directed mutagenesis, impaired desensitization while mutation of both Ser(239) and Ser(357) almost completely abolished desensitization of signalling, GRK phosphorylation and beta-arrestin association, thereby blocking TPbeta internalization. These data suggest a model whereby agonist-induced PKC phosphorylation of Ser(145) partially impairs. TPbeta signalling while GRK2/3 phosphorylation at both Ser(239) and Ser(357) within its IC(3) and C-tail domains, respectively, sterically inhibits G-protein coupling, profoundly desensitizing signalling, and promotes beta-arrestin association and, in turn, facilitates TPbeta internalization. Thromboxane (TX) A(2) is a potent stimulator of platelet aggregation and smooth muscle contraction and contributes to a variety of vascular pathologies. Herein the mechanism whereby the cellular responses to TXA(2) mediated through the TPbeta isoform of the human TXA(2) receptor (TP) are dynamically regulated was investigated by examining the mechanism of its agonist-induced desensitization of intracellular signalling and second messenger generation. TPbeta is subject to profound agonist-induced homologous desensitization of signalling (intracellular calcium mobilization and inositol 1,3,5 trisphosphate generation) in response to stimulation with the TXA(2) mimetic U46619 and this occurs through two key mechanisms: TPbeta undergoes partial agonist-induced desensitization that occurs through a GF 109203X-sensitive, protein kinase (PK)C mechanism whereby Ser(145) within intracellular domain (IC)(2) was identified as the key phospho-target. In addition, TPbeta also undergoes more profound and sustained agonist-induced desensitization involving G protein-coupled receptor kinase (GRK)2/3-phosphorylation of both Ser(239) and Ser(357) within its IC(3) and carboxyl-terminal C-tail domains, respectively. Inhibition of phosphorylation of either Ser(239) or Ser(357), through site directed mutagenesis, impaired desensitization while mutation of both Ser(239) and Ser(357) almost completely abolished desensitization of signalling, GRK phosphorylation and beta-arrestin association, thereby blocking TPbeta internalization. These data suggest a model whereby agonist-induced PKC phosphorylation of Ser(145) partially impairs TPbeta signalling while GRK2/3 phosphorylation at both Ser(239) and Ser(357) within its IC(3) and C-tail domains, respectively, sterically inhibits G-protein coupling, profoundly desensitizing signalling, and promotes beta-arrestin association and, in turn, facilitates TPbeta internalization.

Mass-dependent signaling between G protein coupled receptors

The present study provides evidence that G protein coupled receptor (GPCR) signaling pathways participate in an interactive signaling network governed by the principles of mass action. Using an inducible thromboxane A2 receptor (TPR)/platelet activating factor receptor (PAFR) co-expressing cell model, TPR or PAFR expression was independently up-regulated. Immunostaining and radioligand binding experiments demonstrated that this receptor up-regulation resulted in increased GPCR:G protein mass ratios. This increase in mass ratio impacted both TPR and PAFR ligand affinity. Specifically, up-regulating TPR expression not only decreased TPR ligand affinity, but also decreased the ligand affinity of PAFRs. A similar effect on ligand affinities was observed when PAFRs were up-regulated. In addition, increasing the GPCR:G protein mass ratio for TPRs led to desensitization of the calcium mobilization response to PAFR activation, and increasing PAFR mass desensitized the TPR-mediated calcium response. Finally, it was observed that an increased TPR:G protein mass ratio was associated with a shift in the TPR signaling response, and revealed an additional TPR signaling pathway through G(S). Collectively, these results describe a novel mechanism, i.e., mass-dependent GPCR signaling, by which cells can modulate their GPCR signaling pathways and signaling priorities.

Intramural coronary artery constrictor reactivity to thromboxane is higher in male than in female rats

BACKGROUND

The prevalence of cardiovascular disorders is higher among men than in age-matched women. This is probably related, in part, to gender-dependent differences in coronary function including thromboxane-A(2) (TXA(2)) sensitivity. This question has been examined only on major, epicardial coronaries. The intramural small arteries directly responsible for supplying the myocardial arterioles with blood have been hardly accessible for investigation, owing to difficulties in their preparation. Vasoconstrictor TXA(2) excess generated by platelets and the vascular wall may play an important role in coronary ischemic events. In the present study we tested the vasoconstrictor reactivity of intramural coronary arteries to TXA(2).

METHODS

Secondary, intramural branches of the left anterior descendent coronary artery of Sprague-Dawley rats (diameter: 200 microm) were placed into a vessel chamber. TXA(2) vasoconstrictor reactivity was measured on the basis of pressure-diameter curves in normal Krebs-Ringer solution and after addition of TXA(2) receptor agonist.

RESULTS

Vasoconstrictor response induced by TXA(2) agonist was twice as strong in males compared with females for the whole pressure range ( p < 0.001).

CONCLUSIONS

A gender-dependent difference was demonstrated in TXA(2)-induced contraction of intramural coronary artery segments. In some pathologic situations the enhanced TXA(2) release from platelets and injured vascular wall may cause greater vasoconstriction of intramural coronary arteries in males than in females.

Oligomerization of the alpha and beta isoforms of the thromboxane A2 receptor: relevance to receptor signaling and endocytosis

Thromboxane A(2) (TXA(2)) is a potent mediator of inflammation, vasoconstriction and oxidative stress. The TXA(2) receptor (TP) is a G protein-coupled receptor (GPCR) that is expressed as two alternatively spliced isoforms, alpha (343 residues) and beta (407 residues) that share the first 328 residues. For many years GPCRs were assumed to exist and function as monomeric species, but increasing evidence suggests that a dimer is the minimal functional unit of GPCRs. In the present report, using co-immunoprecipitation of differentially tagged TP expressed in HEK293 cells, we demonstrate that TPalpha and TPbeta form homo- and hetero-oligomers. Immunoblotting of lysates from human platelets with an anti-TP specific antibody revealed the presence of endogenously expressed TP oligomers. We show that TP oligomerization is an agonist-independent process highly affected by the reducing agent dithiothreitol suggesting the involvement of disulfide bonds in TP oligomerization. Over-expression of G protein-coupled receptor kinases and arrestins did not modulate the extent of receptor dimerization/oligomerization. Co-expression of two TP signaling-deficient mutants, R60L and E2402R, resulted in rescuing of receptor signal transduction suggesting that dimers/oligomers constitute the functional units of this receptor. Interestingly, TPalpha which does not undergo constitutive or agonist-induced endocytosis on its own was subjected to both types of endocytosis when co-expressed with TPbeta, indicating that TPalpha can display intracellular trafficking when complexed through hetero-oligomerization with TPbeta.

Thromboxane A receptor-mediated cell proliferation, survival and gene expression in oligodendrocytes

Thromboxane A(2) receptors (TP) were previously localized to discrete regions in the rat brain on myelinated fiber tracts and oligodendrocytes (OLGs). The present studies extended these findings and investigated the effects of TP signaling on cell proliferation, survival, and gene expression in OLG progenitor cells (OPCs) and OLGs. It was found that the TP agonist, U46619 stimulated the proliferation of OPCs and promoted the survival of mature OLGs. Examination of the early gene expression events involved in OPC proliferation, revealed that c-fos expression was substantially increased by U46619 stimulation. Treatment of OPCs or OLGs with U46619 caused activation of the mitogen-activated protein kinases (MAPK) ERK 1/2. In OPCs this activation was blocked by inhibition of src. However, in OLGs this phosphorylation was not only blocked by inhibition of src but also by inhibition of protein kinase C (PKC). Furthermore, U46619 was found to increase CREB phosphorylation in both OPCs and OLGs. Similar to ERK 1/2 activation, there was a divergence in the mechanism of the TP-mediated CREB response for each cell type. Specifically, U46619 activation was attenuated by src and protein kinase A (PKA) inhibition in OPCs, whereas in OLGs this effect was blocked by inhibition of src, PKA as well as by inhibition of PKC. Collectively, these results provide the first demonstration that TP-activated nuclear signaling events are involved in the proliferation of OPCs, the survival of mature OLGs, and the stimulation of gene expression.

The isoprostane 8-iso-PGF2alpha suppresses monocyte adhesion to human microvascular endothelial cells via two independent mechanisms

Isoprostanes, produced in vivo by non-enzymatic free-radical-induced lipid peroxidation, are markers of oxidative stress. Elevated serum and urine levels of 8-iso-PGF2alpha have been reported in a variety of diseases, many of which are characterized by early perivascular inflammatory infiltrates. It has been suggested that, in addition to being markers of oxidative stress, isoprostanes may have pathogenic functions. In this study, we investigated the potential role of 8-iso-PGF2alpha in inflammation, focusing on its effects on adhesion of monocytes to microvascular endothelial cells, an early event in the inflammatory response. In monocyte adhesion assays, 8-iso-PGF2alpha (>10(-8) M) suppressed both basal and TNF-alpha-induced monocyte adhesion to quiescent or proliferating human dermal (HMEC) and rat renal microvascular endothelial cells. In contrast, 8-iso-PGF2alpha stimulated monocyte adhesion to human umbilical vein endothelial cells (HUVEC) as also reported by others. 8-Iso-PGF2alpha had no effect on the viability (Trypan Blue exclusion) of U937 monocytes or HMEC. 8-Iso-PGF2alpha also had no effect on HMEC surface expression of ICAM-1 or VCAM-1. Exposure of HMEC to 8-iso-PGF2alpha for 1-2 h was sufficient to reduce monocyte adhesion to the cell surface, and this effect was independent of de novo protein synthesis by HMEC. The effect of 8-iso-PGF2alpha was mimicked by a thromboxane receptor (TP) agonist (U46619) and blocked by a TP antagonist (SQ29548), indicating a TP-mediated process. Signal transduction pathway inhibitors (SB203580, curcumin, and PD98059) implicated p38 and JNK, but not ERK, in 8-iso-PGF2alpha-induced suppression of monocyte adhesion. In addition to a direct effect, conditioned medium (CM) transfer experiments suggest that 8-iso-PGF2alpha induces a secondary mediator, which also suppresses monocyte adhesion but via an alternative mechanism initiated between 3-4 h, which is TP-independent, requires new protein synthesis, and is primarily dependent on activation of p38. The data show that 8-iso-PGF2alpha can suppress the attachment of monocytes to HMECs via two independent pathways, indicating a potential anti-inflammatory effect of 8-iso-PGF2alpha in the microvasculature.

Amyloid beta protein impairs motor function via thromboxane A2 in the rat striatum

Amyloid beta protein (Abeta) deposits are found in the striatum of patients with Alzheimer disease (AD) showing extrapyramidal motor dysfunction, but neuronal cell loss has not yet been detected. To clarify how Abeta impairs motor function, we analyzed intrastriatally Abeta-injected rats. Unilateral injection of Abeta(25-35) enhanced apomorphine-induced circling in an ipsilateral direction, indicating ipsilateral dysfunction of dopaminergic nigrostriatal pathways. Volumes of lesion in the Abeta(25-35)-injected striata were significantly higher than those in the saline-injected ones. The correlation between lesion volume and circling behavior was close to significance, but slightly too low, suggesting the possible involvement of other factors in the striatal dysfunction. Abeta(25-35) significantly elevated the level of thromboxane A2 (TXA2). A stable TXA2 agonist, U46619, enhanced circling behavior, and TXA2 receptor antagonists attenuated U46619- and Abeta(25-35)-enhanced circling behavior. This study demonstrated that Abeta(25-35) impairs the motor function of dopaminergic neurons via neuronal cell loss and TXA2. It also sheds light on the therapeutic potential of TXA2 receptor blockers for the neurotoxicity of Abeta.

Down-regulation of lipoxin A4 receptor by thromboxane A2 signaling in RAW246.7 cells in vitro and bleomycin-induced lung fibrosis in vivo

Lipoxins (LXs) are members of eicosanoid family that can be endogenously produced during cell-to-cell interactions such as platelet-leukocyte interactions. Anti-inflammatory function of lipoxin A4 (LXA4) as "braking signals" is mediated by the receptor. On the other hand, thromboxane A2 (TXA2) produced by catalysis of cyclooxygenase and thromboxane synthetase is released during platelet aggregation as a vasoconstrictor and a pro-inflammatory factor. To investigate interaction of TXA2 receptor (TP) and LXA4 receptor, effects of a TP agonist and a thromboxane synthetase inhibitor on expression of LXA4 receptor were examined in vitro and in vivo. A TP agonist, U46619 showed a down-regulation of LXA4 receptor induced by interleukin-1beta (IL-1beta) in RAW246.7 cells. In bleomycin-induced lung fibrosis in mice, administration of a thromboxane synthetase inhibitor DP-1904 increased LXA4 receptor mRNA and decreased type I collagen mRNA. In vitro experiments indicate that LXA4 significantly prevented enhanced proliferation of NIH3T3 fibroblasts and the collagen expression by transforming growth factor-beta (TGF-beta). These results suggest that TXA2-TP signaling could cause negative regulation of lipoxin A4 receptor under the transcriptional level during inflammatory process mediated by IL-1beta and TGF-beta induce the expression of LXA4 receptor. Furthermore, the down-regulation of LXA4 receptor by TXA2 implies a possibility that a cellular signaling by TXA2 may have a novel and potential function as a pro-inflammatory factor to inhibit anti-inflammatory effect of LXA4. Concomitantly, selective blockade of TXA2-TP signaling could be suggested to lead to anti-inflammation through active role of LXA4.

Mutational analysis of the highly conserved ERY motif of the thromboxane A2 receptor: alternative role in G protein-coupled receptor signaling

The presence of highly conserved amino acid stretches in G protein-coupled receptors (GPCRs) usually predicts an important role in receptor function. Considerable attention has therefore been focused on the involvement of the highly conserved Glu/Asp-Arg-Tyr (E/DRY) motif at the cytoplasmic end of transmembrane domain 3 in the regulation of GPCR conformational states and/or the mediation of G protein activation. In the present study, we investigated the role of Glu129 and Arg130 in the ERY of thromboxane A2 receptor alpha (TPalpha) in transfected human embryonic kidney 293 cells. We show that no conservative or nonconservative substitutions of Glu129 and Arg130 generated a constitutively active TPalpha mutant, but a nonconservative mutation of Arg130 (R130V) yielded a mutant receptor with significantly impaired 9,11-dideoxy-9alpha,11alpha-methanoepoxy-prosta-5Z,13E-dien-1-oic acid (U46619)-induced accumulation of inositol phosphates (IPs). This loss-of-function phenotype seems to be caused by the uncoupling of the TPalpha receptor from Gq, as demonstrated by the loss of high-affinity agonist binding, and not by receptor internalization, as shown by localization studies with the R130V-green fluorescent protein fusion protein. It is interesting to note that U46619-induced activation of the nonconservative E129V mutant stimulated the production of IPs with a approximately 10-fold lower EC50 and a approximately 2-fold higher Emax than in the wild-type receptor. Collectively, these data demonstrate that, unlike other GPCRs, mutations of Glu129 do not induce constitutive activity, whereas Arg130 is involved in G protein coupling or recognition, and they suggest the existence within class A GPCRs of at least two different subclasses that make different uses of the highly conserved E/DRY motif.

Role of the Rab11-Associated Intracellular Pool of Receptors Formed by Constitutive Endocytosis of the β Isoform of the Thromboxane A2 Receptor (TPβ)

Intracellular trafficking pathways of G protein-coupled receptors (GPCRs), following their agonist-induced endocytosis and their consequences on receptor function, are the subject of intense research efforts. However, less is known regarding their constitutive endocytosis. We previously demonstrated that the beta isoform of the thromboxane A(2) receptor (TPbeta) undergoes constitutive and agonist-induced endocytosis. Constitutive endocytosis of GPCRs can lead to the formation of an intracellular pool of receptors from which they can recycle back to the cell surface. In the present report, we show with the help of two TPbeta mutants (TPbeta-Y339A and TPbeta-I343A) specifically deficient in constitutive endocytosis that this intracellular pool of receptors serves to maintain agonist sensitivity over prolonged receptor stimulation in HEK293 cells. Second messenger generation by the TPbeta-Y339A and TPbeta-I343A mutants was drastically reduced compared to the wild-type receptor as suggested by dose-response and time-course experiments of inositol phosphates production following agonist treatment, despite normal coupling between the receptors and the Galpha(q) protein. Moreover, second messenger production after receptor activation was dramatically reduced when cells were pretreated with monensin, a recycling inhibitor. Receptor cell surface expression and endocytosis experiments further revealed that the small GTPase Rab11 protein is a determinant factor in controlling TPbeta recycling back to the cell surface. Co-localization experiments performed by immunofluorescence microscopy indicated that both constitutive and agonist-triggered endocytosis resulted in targeting of TPbeta to the Rab11-positive recycling endosome. Thus, we provide evidence that constitutive endocytosis of TPbeta forms a pool of receptors in the perinuclear recycling endosome from which they recycle to the cell surface, a process involved in preserving receptor sensitivity to agonist stimulation.

Thromboxane A 2 Receptor Agonists Antagonize the Proangiogenic Effects of Fibroblast Growth Factor-2

Thromboxane (TX) A 2 is released from multiple cell types and is a prime mediator of the pathogenesis of many vascular events, including angiogenesis. Endothelial cells express TXA 2 receptors (TP) but the effects of TP stimulation on angiogenesis remain controversial. In this study, we show that stimulation of endothelial cell TP impairs ligand-induced FGF receptor internalization and consequently abrogates FGF-2-induced endothelial cell migration in vitro and angiogenesis in vivo. Prevention of FGF-2-induced angiogenesis was associated with expression of the TPβ isoform. The deficit in FGFR1 internalization was mediated through activation of TPβ preventing the FGF-2-mediated decrease in p53 expression, thus enhancing thrombospondin-1 (TSP-1) release from EC and reducing FGFR1 internalization. Once released TSP-1 interacted with the α v β 3 integrin on the EC surface. On stimulation, FGFR1 and α v β 3 were found to associate in a complex. We determined that complex formation was important for receptor internalization as conditions that inhibit FGFR1 internalization, such as inappropriate ligation of α v β 3 by either TSP-1 or a neutralizing antibody, disrupted the complex. These results establish a novel role for isoform specific regulation of angiogenesis by TP, provide the first functional significance for the existence of two TP isoforms in humans, and clarify the mechanism by which TP signaling regulates FGFR1 kinetics and signaling.

Cyclooxygenase 2 is a key enzyme for inflammatory cytokine-induced angiogenesis

Cyclooxygenase1 (COX1) and COX2 mediate the rate-limiting step in arachidonic acid metabolism. Expression of COX2 mRNA and protein is often enhanced in various human cell types by inflammatory cytokines such as interleukin-1beta (IL-1beta) and tumor necrosis factor alpha (TNFalpha). IL-1beta enhanced expression of various prostanoids and this expression was blocked by COX2 selective inhibitors. IL-1beta markedly induced angiogenesis in vitro and in vivo, which was significantly inhibited by COX2 selective inhibitors but not by a vascular endothelial growth factor (VEGF) receptor tyrosine kinase inhibitor. In contrast, COX2 selective inhibitors only partially blocked VEGF-induced angiogenesis. EP2, EP4 (prostaglandin E2 receptors) agonists and thromboxane A2 (TXA2) receptor agonists induced angiogenesis in vitro and in vivo; IL-1beta-induced angiogenesis was blocked by an EP4 antagonist and a TXA2 receptor antagonist. IL-1beta induced much less angiogenesis in cornea of COX2 knockout mice than that of wild-type mice. This is the first report that COX2 and some prostanoids play a key role in IL-1beta-induced angiogenesis.