Failure of BQ123, a more potent antagonist of sarafotoxin 6b than of endothelin-1, to distinguish between these agonists in binding experiments

Endothelin-1 inhibits size dependent lymphatic clearance of PEG-based conjugates after intra-articular injection into the rat knee

Clearance of particles from the knee is an essential mechanism to maintain healthy joint homeostasis and critical to the delivery of drugs and therapeutics. One of the limitations in developing disease modifying drugs for joint diseases, such as osteoarthritis (OA), has been poor local retention of the drugs. Enhancing drug retention within the joint has been a target of biomaterial development, however, a fundamental understanding of joint clearance pathways has not been characterized. We applied near-infrared (NIR) imaging techniques to assess size-dependent in vivo clearance mechanisms of intra-articular injected, fluorescently-labelled polyethylene glycol (PEG-NIR) conjugates. The clearance of 2 kDa PEG-NIR (τ = 171 ± 11 min) was faster than 40 kDa PEG-NIR (τ = 243 ± 16 min). 40 kDa PEG-NIR signal was found in lumbar lymph node while 2 kDa PEG-NIR signal was not. Thus, these two conjugates may be cleared through different pathways, i.e. lymphatics for 40 kDa PEG-NIR and venous for 2 kDa PEG-NIR. Endothelin-1 (ET-1), a potent vasoconstrictor of vessels, is elevated in synovial fluid of OA patients but, its effects on joint clearance are unknown. Intra-articular injection of ET-1 dose-dependently inhibited the clearance of both 2 kDa and 40 kDa PEG-NIR. ET-1 caused a 1.63 ± 0.17-fold increase in peak fluorescence for 2 kDa PEG-NIR and a 1.85 ± 0.15-fold increase for 40 kDa PEG-NIR; and ET-1 doubled their clearance time constants. The effects of ET-1 were blocked by co-injection of ET receptor antagonists, bosentan or BQ-123. These findings provide fundamental insight into retention and clearance mechanisms that should be considered in the development and delivery of drugs and biomaterial carriers for joint diseases. STATEMENT OF SIGNIFICANCE: This study demonstrates that in vivo knee clearance can be measured using NIR technology and that key factors, such as size of materials and biologics, can be investigated to define joint clearance mechanisms. Therapies targeting regulation of joint clearance may be an approach to treat joint diseases like osteoarthritis. Additionally, in vivo functional assessment of clearance may be used as diagnostics to monitor progression of joint diseases.

New drugs and emerging therapeutic targets in the endothelin signaling pathway and prospects for personalized precision medicine

During the last thirty years since the discovery of endothelin-1, the therapeutic strategy that has evolved in the clinic, mainly in the treatment of pulmonary arterial hypertension, is to block the action of the peptide either at the ET(A) subtype or both receptors using orally active small molecule antagonists. Recently, there has been a rapid expansion in research targeting ET receptors using chemical entities other than small molecules, particularly monoclonal antibody antagonists and selective peptide agonists and antagonists. While usually sacrificing oral bio-availability, these compounds have other therapeutic advantages with the potential to considerably expand drug targets in the endothelin pathway and extend treatment to other pathophysiological conditions. Where the small molecule approach has been retained, a novel strategy to combine two vasoconstrictor targets, the angiotensin AT(1) receptor as well as the ET(A) receptor in the dual antagonist sparsentan has been developed. A second emerging strategy is to combine drugs that have two different targets, the ET(A) antagonist ambrisentan with the phosphodiesterase inhibitor tadalafil, to improve the treatment of pulmonary arterial hypertension. The solving of the crystal structure of the ET(B) receptor has the potential to identify allosteric binding sites for novel ligands. A further key advance is the experimental validation of a single nucleotide polymorphism that has genome wide significance in five vascular diseases and that significantly increases the amount of big endothelin-1 precursor in the plasma. This observation provides a rationale for testing this single nucleotide polymorphism to stratify patients for allocation to treatment with endothelin agents and highlights the potential to use personalized precision medicine in the endothelin field.

Endothelin

The endothelins comprise three structurally similar 21-amino acid peptides. Endothelin-1 and -2 activate two G-protein coupled receptors, ETA and ETB, with equal affinity, whereas endothelin-3 has a lower affinity for the ETA subtype. Genes encoding the peptides are present only among vertebrates. The ligand-receptor signaling pathway is a vertebrate innovation and may reflect the evolution of endothelin-1 as the most potent vasoconstrictor in the human cardiovascular system with remarkably long lasting action. Highly selective peptide ETA and ETB antagonists and ETB agonists together with radiolabeled analogs have accurately delineated endothelin pharmacology in humans and animal models, although surprisingly no ETA agonist has been discovered. ET antagonists (bosentan, ambrisentan) have revolutionized the treatment of pulmonary arterial hypertension, with the next generation of antagonists exhibiting improved efficacy (macitentan). Clinical trials continue to explore new applications, particularly in renal failure and for reducing proteinuria in diabetic nephropathy. Translational studies suggest a potential benefit of ETB agonists in chemotherapy and neuroprotection. However, demonstrating clinical efficacy of combined inhibitors of the endothelin converting enzyme and neutral endopeptidase has proved elusive. Over 28 genetic modifications have been made to the ET system in mice through global or cell-specific knockouts, knock ins, or alterations in gene expression of endothelin ligands or their target receptors. These studies have identified key roles for the endothelin isoforms and new therapeutic targets in development, fluid-electrolyte homeostasis, and cardiovascular and neuronal function. For the future, novel pharmacological strategies are emerging via small molecule epigenetic modulators, biologicals such as ETB monoclonal antibodies and the potential of signaling pathway biased agonists and antagonists.

Evidence for biased agonists and antagonists at the endothelin receptors

Biased ligands represent a new strategy for the development of more effective and better tolerated drugs. To date there has been a paucity of research exploring the potential of ligands that exhibit either G protein or β-arrestin pathway selectivity at the endothelin receptors. Re-analysis of data may allow researchers to determine whether there is existing evidence that the endogenous ET peptides or currently available agonists and antagonists exhibit pathway bias in a particular physiological or disease setting and this is explored in the review. An alternative to molecules that bind at the orthosteric site of the ET receptors are cell penetrating peptides that interact with a segment of an intracellular loop of the receptor to modify signalling behaviour. One such peptide IC2B has been shown to have efficacy in a model of pulmonary arterial hypertension. Finally, understanding the molecular pathways that contribute to disease is critical to determining whether biased ligands will provide clinical benefit. The role of ET_A signalling in ovarian cancer has been delineated in some detail and this has led to the suggestion that the development of ET_A G protein biased agonists or β-arrestin biased antagonists should be explored.

The CCR5 chemokine receptor mediates vasoconstriction and stimulates intimal hyperplasia in human vessels in vitro

AIMS

The chemokine receptor CCR5 and its inflammatory ligands have been linked to atherosclerosis, an accelerated form of which occurs in saphenous vein graft disease. We investigated the function of vascular smooth muscle CCR5 in human coronary artery and saphenous vein, vascular tissues susceptible to atherosclerosis, and vasospasm.

METHODS AND RESULTS

CCR5 ligands were vasoconstrictors in saphenous vein and coronary artery. In vein, constrictor responses to CCL4 were completely blocked by CCR5 antagonists, including maraviroc. CCR5 antagonists prevented the development of a neointima after 14 days in cultured saphenous vein. CCR5 and its ligands were expressed in normal and diseased coronary artery and saphenous vein and localized to medial and intimal smooth muscle, endothelial, and inflammatory cells. [(125)I]-CCL4 bound to venous smooth muscle with KD = 1.15 ± 0.26 nmol/L and density of 22 ± 9 fmol mg(-1) protein.

CONCLUSIONS

Our data support a potential role for CCR5 in vasoconstriction and neointimal formation in vitro and imply that CCR5 chemokines may contribute to vascular remodelling and augmented vascular tone in human coronary artery and vein graft disease. The repurposing of maraviroc for the treatment of cardiovascular disease warrants further investigation.

Defining the affinity and receptor sub-type selectivity of four classes of endothelin antagonists in clinically relevant human cardiovascular tissues

AIMS

We have compared the endothelin receptor subtype affinity (K(D)) and selectivity of four structural classes of antagonists (peptide, sulphonamide-based, carboxylic acid-based, myceric acid-based) in human cardiovascular tissues to determine whether these are predicted by values reported for human cloned receptors. Additionally, affinities (K(B)) for these antagonists, determined in ET-1-mediated vasoconstriction assays in human blood vessels, were used to identify discrepancies between K(B) and K(D) determined in the same tissues.

MAIN METHODS

Competition binding experiments were carried out in sections of human left ventricle, coronary artery and homogenates of saphenous vein to determine K(D) values for structurally different ET(A)-selective (FR139317, BMS 182874, S97-139, sitaxentan, ambrisentan) and mixed (PD142893, Ro462005, bosentan, L-749329, SB209670) antagonists. Schild-derived values of antagonist affinity were obtained in vascular functional studies.

KEY FINDINGS

When compared with previously reported data in human cloned endothelin receptors, those antagonists reported to be ET(A)-selective exhibited even greater ET(A) selectivity in human ventricle (BMS 182874, sitaxentan, ambrisentan) that expressed both receptor subtypes. Those antagonists reported to have <100 fold selectivity in cloned receptors (PD142893, Ro-462005, bosentan, SB209670, L-749329) did not distinguish between receptor subtypes in human left ventricle. For antagonists where we determined affinity in vascular functional and binding assays (Ro462005, bosentan, BMS 182874, L-749329, SB209670) there was no correlation between the degree of discrepancy in K(B) and K(D) and structural class.

SIGNIFICANCE

For an antagonist to retain ET(A)-selectivity in vivo it may be necessary to identify those compounds that have at least 1000 fold ET(A):ET(B) selectivity in in vitro assays.

Comparison of human ETA and ETB receptor signalling via G-protein and β-arrestin pathways

AIMS

To determine the pharmacology of ET(A)- and ET(B)-mediated β-arrestin recruitment and compare this to established human pharmacology of these receptors to identify evidence for endothelin receptor biased signalling and pathway specific blockade by antagonists.

MAIN METHODS

The ability of ET-1, ET-2, ET-3, sarafotoxin 6b and sarafotoxin 6c to activate ET(A) and ET(B)-mediated β-arrestin recruitment was determined in CHO-K1 cells. Affinities were obtained for ET(A) selective (BQ123, sitaxentan, ambrisentan), ET(B) selective (BQ788) and mixed (bosentan) antagonists using ET-1 and compared to affinities obtained in competition experiments in human heart and by Schild analysis in human saphenous vein. Agonist dependence of affinities was compared for BQ123 and BQ788 in the ET(A) and ET(B) β-arrestin assays respectively.

KEY FINDINGS

For β-arrestin recruitment, order of potency was as expected for the ET(A) (ET-1≥ET-2>>ET-3) and ET(B) (ET-1=ET-2=ET-3) receptors. However, at the ET(A) receptor sarafotoxin 6b and ET-3 were partial agonists. Antagonism of ET peptides by selective and mixed antagonists appeared non-competitive. BQ123, but not BQ788, exhibited agonist-dependent affinities. Bosentan was significantly more effective an inhibitor of β-arrestin recruitment mediated by ET(A) compared to the ET(B) receptor. In the ET(A) vasoconstrictor assay, ET-1, ET-2 and S6b were equipotent, full agonists and antagonists tested behaved in a competitive manner, although affinities were lower than predicted from the competition binding experiments in left ventricle.

SIGNIFICANCE

These data suggest that the pharmacology of ET(A) and ET(B) receptors linked to G-protein- and β-arrestin mediated responses was different and bosentan appeared to show bias, preferentially blocking ET(A) mediated β-arrestin recruitment.

Correlation between vascular responsivensss and expression of novel transcripts of the ETA-receptor in human vascular tissue

Alternatively spliced endothelin (ET-1) receptor transcripts have been identified, but their significance to the functional effects of ET-1 has not been established. We have investigated the presence and influence of alternatively spliced ET(A) receptor transcripts on ET-1 mediated contraction of segments of human saphenous vein. The expression of ET(A) receptor transcripts was examined with quantitative reverse transcription-polymerase chain reaction (qPCR) studies, while the response of veins to ET-1 was tested with in vitro organ bath techniques. The expression of four different transcripts for the ET(A) receptor, in which either exon 3 is spliced out (Delta3), exon 4 is spliced out (Delta4), both 3 and 4 spliced out (Delta3,4) and when both exons 2 and 4 (Delta2,4) are spliced out were identified. Functional studies showed that a lack of efficacy and potency of ET-1 is associated with a significantly lower expression of the Delta3,4 transcript. ET(A) receptor antagonism was insurmountable in samples that had lower levels of the Delta3,4 transcript, while samples from patients with higher expression of the Delta3,4 showed surmountable antagonism with BQ123. These results suggest that there is a genetic basis for the variability between individuals for the contractile effect of ET-1 at ET(A) receptors.

The therapeutic potential of PD156707 and related butenolide endothelin antagonists

Plasma concentrations of the peptide endothelin (ET) are elevated in several cardiovascular diseases. Animal studies suggest that activation of ET receptors may contribute to the increase in vascular resistance and remodelling of cardiovascular tissues that are characteristic of these pathologies. Antagonists of these receptors may therefore have important clinical potential. PD156707 (Parke-Davis) is one of a series of novel, orally-active butenolide endothelin antagonists and is highly selective for the ETA receptor. In man, this subtype mediates the profound vasoconstrictor effects of the ET peptides, and blockade of the ETA receptor may therefore produce beneficial vasodilatation. The advantage of selective ETA receptor antagonism is that it leaves unaffected vascular ETB receptors, which mediate vasorelaxation, and non-vascular ETB receptors, particularly in the lung and kidneys, which act to clear ET from the plasma. PD156707 exhibits subnanomolar affinity and greater than 1000-fold selectivity for human ETA receptors and potently inhibits ET-1-mediated vasoconstriction in human isolated blood vessels. In rats, PD156707 has good oral bioavailability (41%) and a relatively short terminal t1/2 of approximately 1 h. Structural analogues of PD156707 that have comparable selectivity and potency for the ETA receptor are reported to have even better oral bioavailability and longer plasma t1/2 values. Preclinical studies with PD156707 indicate efficacy in animal models of congestive heart failure (CHF), pulmonary hypertension (PH) and cerebral ischaemia. We await data from clinical trials to confirm the therapeutic potential of the ETA-selective butenolide antagonists in man.

Endothelin isoforms and the response to myocardial stretch

Myocardial stretch elicits a biphasic increase in developed force with a first rapid force response and a second slow force response (SFR). The rapid phase is due to an increase in myofilament Ca(2+) responsiveness; the SFR, analyzed here, is ascribed to a progressive increase in Ca(2+) transients. Experiments were performed in cat papillary muscles to further elucidate the signaling pathway underlying the SFR. Although the SFR was diminished by BQ-123, a similar endothelin (ET)-1-induced increase in force was not affected: 23 +/- 2 vs. 23 +/- 3% (not significant). Instead, BQ-123 suppressed the contractile effects of ET-2 or ET-3 (21 +/- 2 and 25 +/- 3% vs. -1 +/- 1 and -7 +/- 3% respectively, P < 0.05), suggesting that ET-2 or ET-3, but not ET-1, was involved in the SFR. Each isoform activated the Na(+)/H(+) exchanger (NHE-1), increasing intracellular Na(+) concentration by 2.0 +/- 0.1, 2.3 +/- 0.1, and 2.1 +/- 0.4 mmol/l for ET-1, ET-2, and ET-3, respectively (P < 0.05). The NHE-1 inhibitor HOE-642 prevented the increases in force and intracellular Na(+) concentration induced by all the ET isoforms, but only ET-2 and ET-3 effects were sensitive to BQ-123. Real-time RT-PCR measurements of prepro-ET-1, -ET-2, and -ET-3 were performed before and 5, 15, and 30 min after stretch. No changes in ET-1 or ET-2, but an increase of approximately 60% in ET-3, mRNA after 15 min of stretch were detected. Stretch-induced ET-3 mRNA upregulation and its mechanical counterpart were suppressed by AT(1) receptor blockade with losartan. These data suggest a role for AT(1)-mediated ET-3 released in the early activation of NHE-1 that follows myocardial stretch.

Endothelin receptor in virus-like particles: ligand binding observed by fluorescence fluctuation spectroscopy

The functional analysis of transmembrane receptor proteins is frequently hampered by the difficulty to produce sufficiently homogeneous receptor preparations that preserve the physiological biomembrane integration of the receptor protein. To improve the receptor protein density in the lipid bilayer and to maintain the physiological lipid-protein environment, a novel method has been established that enables the selective integration of transmembrane receptors into a virus-like particle (VLiP). Here we have studied the binding of tetramethylrhodamine-labeled endothelin-1 (TMR-ET-1) to VLiP-integrated endothelin A receptor (ET(A)R) by fluorescence fluctuation spectroscopy. The concentration of TMR-ET-1 was determined by fluorescence correlation spectroscopy (FCS). These measurements also confirmed that the free ligand is monomeric in solution in our experiments. Fluorescence intensity distribution analysis (FIDA) was used to quantify the fraction of ligands bound to ET(A)Rs in the VLiPs. For the interaction between ET-1 and VLiP-integrated ET(A)Rs, K(D) values of 0.5 nM and 0.3 nM were determined from ligand and receptor titration experiments, respectively. For comparison, a FIDA analysis was also carried out with ET(A)Rs in membrane fragments derived from an ET(A)R-overexpressing mammalian cell line, which yielded a similar K(D) of 0.2 nM. In addition, we examined the binding competition of a set of reference compounds to VLiP-ET(A)Rs in the presence of ET-1 and obtained K(i) values similar to those reported in the literature. Our results demonstrate that integration into VLiPs does not change the binding properties of the ET(A)Rs. FIDA analysis of VLiP-integrated receptors shows great promise for highly miniaturized and fast compound testing in the pharmaceutical industry.

Thromboxane receptor density is increased in human cardiovascular disease with evidence for inhibition at therapeutic concentrations by the AT(1) receptor antagonist losartan

1. The aim of this study was to establish how thromboxane receptors (TP) respond to the increase in levels of plasma thromboxane observed in both cardiac (cardiomyopathy, ischaemic heart disease and pulmonary hypertension) and vascular disease (atherosclerosis of coronary artery disease and accelerated atherosclerosis of saphenous vein grafts). 2. The agonist radioligand [(125)I]-BOP, bound rapidly to TP receptors in normal human cardiovascular tissue, displaying high affinity in left ventricle (K(D) 0.23 +/- 0.06 nM, B(max) 28.4 +/- 5.7 fmol mg(-1) protein) and reversibility with a t(1/2) of 10 min (n = five individuals +/- s.e.mean). 3. In the heart, TP receptor density in the right ventricle of primary pulmonary hypertensive patients were significantly increased (66.6 +/- 6 fmol mg(-1) protein) compared to non-diseased right ventricle (37.9 +/- 4.1 fmol mg(-1) protein, n = six individuals +/- s.e.mean, P<0.05). 4. In diseased vessels, TP receptor densities were significantly increased (3 fold in the intimal layer) in atherosclerotic coronary arteries, saphenous vein grafts with severe intimal thickening (n = 8-12 individuals, P<0.05) and aortic tissue (n=5 - 6 individuals, P<0.05), compared with normal vessels. 5. Losartan, tested at therapeutic doses, competed for [(125)I]-BOP binding to human vascular tissue, suggesting that some of the anti-hypertensive effects of this AT(1) receptor antagonist could also be mediated by blocking human TP receptors. 6. The differential distribution of TP receptors in the human cardiovascular system and the alteration of receptor density, accompanying the increase in endogenous thromboxane levels in cardiovascular disease, suggest that TP receptors represent a significant target for therapeutic interventions and highlights the importance for the development of novel selective antagonist for use in humans.

[125I-His(9)]-ghrelin, a novel radioligand for localizing GHS orphan receptors in human and rat tissue: up-regulation of receptors with athersclerosis

1. Ghrelin is the recently identified endogenous ligand for the cloned growth hormone secretagogue receptor (GHS-R). We have characterized for the first time the binding of human [125I-His(9)]-ghrelin to normal human and rat tissue and demonstrated expression of this 'orphan' receptor that has previously been predicted to exist from mRNA. Furthermore, we have discovered that [125I-His(9)]-ghrelin density is significantly increased in atherosclerosis. 2. [125I-His(9)]-Ghrelin bound to non-diseased human heart (left ventricle) with an association rate constant (k(obs)) of 0.16+/-0.004 min(-1), a dissociation rate constant of 0.068+/-0.0005 min(-1) (kinetically derived K(D) of 0.1 nM; n=5 individuals+/-s.e.mean), a K(D) of 0.43+/-0.08 nM and B(max) of 7.8+/-0.9 fmol mg(-1) protein (n=6 individual+/-s.e.mean). 3. Specific [125I-His(9)]-ghrelin binding was to the human vasculature including aorta, coronary, pulmonary, arcuate arteries in the kidney and saphenous veins. In rat tissues, binding sites were also localized to the vasculature in peripheral tissues as well as the granular layer of the cerebellum in the CNS. 4. [125I-His(9)]-Ghrelin binding was significantly up-regulated (3 - 4 fold) in both atherosclerotic coronary arteries and saphenous vein grafts with advanced intimal thickening, compared with normal vessels (P<0.05). 5. Our results suggest that the native receptor for [125I-His(9)]-ghrelin may be widely distributed in the human cardiovascular system. Furthermore, changes in the density of this proposed ghrelin receptor implicates this new transmitter system in the development of atherosclerosis and may therefore represent a novel therapeutic target in the treatment of cardiovascular disease.

Evidence that ET-1, but not ET-3 and S6b, ET(A)-receptor mediated contractions in isolated rat mesenteric arteries are modulated by co-activation of ET(B) receptors

The effects of agonists with endothelin (ET) ET(A)-receptor activity have been analysed in relation to their interaction with ET(B) receptors in rat mesenteric arteries. ET-1, sarafotoxin 6b (S6b) and ET-3 induced large, slow-onset and sustained contractions whereas S6c induced weak transient contractions. However, following pre-contraction with U46619 and subsequent relaxation with forskolin, the effect of S6c was amplified, indicating a potential for powerful ET(B)-receptor mediated contraction. The selective ET(A)-receptor antagonist, FR139317, produced parallel rightward shifts of ET-1, S6b and ET-3 concentration-effect curves indicating that the contractions were mediated by ET(A) receptors. However, the corresponding FR139317 pK(B) values were significantly different between the agonists. As expected FR139317 had no effect on S6c responses. Pre-treatment with S6c to desensitize ET(B) receptors, increased ET-1 potency and the pK(B) value for FR139317. In contrast, neither the potency of S6b and ET-3 nor the pK(B) values for FR139317 estimated using these agonists were affected by ET(B)-receptor desensitization. Segments pre-contracted with submaximal concentrations of S6b and ET-3, but not ET-1, rapidly relaxed following wash-out or FR139317 administration. The results indicate that the small contractile response to selective ET(B) receptor activation, barely detectable under standard bioassay conditions, is greatly amplified when adenylate cyclase activity is elevated. Moreover, the response to ET(A) receptor activation by ET-1, but not ET-3 and S6b, is significantly modified by co-activation of ET(B) receptors. This interaction has a significant effect on the apparent affinity of ET(A)-receptor selective antagonists when ET-1 is used as agonist and decreases the potency of ET-1.

Endothelin receptor mediating contraction of isolated bovine coronary artery

We studied endothelin (ET) receptors and their subtypes on isolated bovine coronary arteries. Endothelin receptors that mediated contraction of isolated bovine coronary artery were characterized by the use of antagonists and agonists. Contractions induced by the nonselective agonist ET-1 (10-10-10-7 M) were not affected by the removal of the endothelium (pEC50: 8.52, maximal contraction: 105% of that induced by 60 mM KCl). BQ-123 (3 x 10-7 M) antagonized contractions of endothelium-denuded coronary rings induced by low concentrations of ET-1 (10-10 or 10-9 M), but potentiated the contractions induced by higher concentrations of ET-1 (3 x 10-8 and 10-7 M). BQ-788 (10-6 M) potentiated contractions induced by ET-1 (3 x 10-10 and 10-7 M). In the presence of BQ-788 (10-6 M), BQ-123 (3 x 10-8-3 x 10-6 M) concentration - dependently inhibited contractions induced by ET-1 (3 x 10-10 and 10-7 M) (pA2: 6.61). Sarafotoxin S6b (10-9-3 x 10-7 M) evoked contractions in the denuded coronary artery (pEC50: 8.49, maximal contraction: 139% of 60 mM KCl). The BQ-123 caused a concentration-dependent rightward shift of contractions induced by sarafotoxin S6b (pA2: 7.89). The present study indicates that ET-1 and sarafotoxin S6b contract the isolated bovine coronary artery by stimulating ETA receptors on smooth muscle cells, and that ETB receptors might suppress the ET-1-induced contractions.

Endothelin- and sarafotoxin-induced receptor-mediated calcium mobilization in a clonal murine osteoblast-like cell line, MC3T3-E1/B

Previous studies have demonstrated that, in osteoblast-like MC3T3-E1 cells, various endothelin peptides and their homologous sarafotoxins generate prostaglandin E(2) (PGE(2)) release through an ET(A) receptor subtype. In this study, biphasic Ca(2+) signals elicited with endothelin (ET)-1, ET-2, ET-3, beta-ET, S6a1, and S6b (ET/S6) were measured by microspectrofluorimetric methods in cell suspensions loaded with Fura-2 acetoxymethylester (Fura-2 AM). Phospholipase C (PLC)-dependent calcium activation mechanisms seem to be involved. We found evidence of Ca(2+) release from thapsigargin-sensitive and non-thapsigargin-sensitive intracellular Ca(2+) stores as well as Ca(2+) transmembrane inflow through multiple voltage-independent and Ni(2+)-sensitive cation channels. Using an ET(A) receptor antagonist, BQ-123, we showed that this receptor was coupled to Ca(2+) mobilization. All agonists tested, except S6c (an ET(B)-receptor-specific agonist) induced receptor desensitization. Our results demonstrate that the ET/S6-induced Ca(2+) signaling pathway is mediated via an ET(A)-receptor subtype in MC3T3-E1/B cells.

The effect of an endothelin antagonist on blood pressure in a rat model of preeclampsia

OBJECTIVE

We attempted to determine the role of endothelin in a previously characterized animal model of preeclampsia by studying the effect of a specific endothelin antagonist, BQ123, on blood pressure.

STUDY DESIGN

A preeclampsia-like condition was induced by infusing pregnant rats with the nitric oxide synthase inhibitor N(G)-nitro-L -arginine methyl ester. Osmotic minipumps were inserted subcutaneously into timed pregnant Harlan-Sprague-Dawley rats on day 17 of pregnancy (term, 22 days). The pumps were loaded to continuously deliver either vehicle (control group) or N(G)-nitro-L -arginine methyl ester 50 mg/d, either alone or with BQ123 at 0.5 mg/d. In a similar but separate experiment, the dose of BQ123 was increased to 1 mg/d. Blood pressure was measured with the tail-cuff method before pump insertion and then daily until postpartum day 2.

RESULTS

Except for a decrease on the day after pump insertion, BQ123 0.5 mg/d had no significant effect on the hypertension induced by N(G)-nitro-L -arginine methyl ester. At the higher dose, however, BQ123 significantly attenuated the increase in blood pressure induced by N(G)-nitro-L -arginine methyl ester during most of the study period.

CONCLUSION

The effect of nitric oxide inhibition can be successfully attenuated by the use of an endothelin antagonist, thereby supporting the role of endothelin in the hypertension described with the preeclampsialike condition seen in pregnant rats.

ET(A) receptors are the primary mediators of myofilament calcium sensitization induced by ET-1 in rat pulmonary artery smooth muscle: a tyrosine kinase independent pathway

We have investigated the possibility that ET-1 can induce an increase in myofilament calcium sensitivity in pulmonary artery smooth muscle. Arterial rings were permeabilized using alpha-toxin (120 microg ml(-1)), in the presence of A23187 (10 microM) to 'knock out' Ca2+ stores, and pre-constricted with pCa 6.8 (buffered with 10 mM EGTA). In the presence of this fixed Ca2+ concentration, 1 microM ET-1 induced a sustained, reversible constriction of 0.15 mN. Pulmonary arterial rings were freeze-clamped at the peak of the induced constriction (time matched). Subsequent densitometric analysis revealed that ET-1 (1 microM) increased the level of phosphorylated myosin light chains by 34% compared to an 11% increase in the presence of pCa 6.8 alone. In contrast to ET-1, the selective ET(B) receptor agonist Sarafotoxin S6C (100 nM) failed to induce a significant constriction. The constriction induced by 1 microM ET-1 was reversibly inhibited when the preparation was preincubated (15 min) with the ETA receptor antagonist BQ 123 (100 microM). The constriction measured 0.13 mN in the absence and 0.07 mN in the presence of 100 microM BQ 123. In contrast, the constriction induced by 1 microM ET-1 measured 0.19 mN in the absence and 0.175 mN following a 15 min pre-incubation with the ET(B) antagonist BQ 788 (100 microM). The constriction induced by 1 microM ET-1 measured 0.14 mN in the presence and 0.13 mN following pre-incubation with the tyrosine kinase inhibitor Tyrphostin A23 (100 microM). We conclude that ET-1 induced an increase in myofilament calcium sensitivity in rat pulmonary arteries via the activation of ET(A) receptors and by a mechanism(s) independent of tyrosine kinase.

Characterisation of constrictor endothelin receptors in the human internal thoracic artery and saphenous vein

We studied the endothelin receptors mediating contraction in the human saphenous vein (SV) and internal thoracic artery (ITA). In the SV, the ET(A)-receptor antagonist BQ123 (1 microM) did not significantly shift the ET-1 concentration-response curve but did cause a parallel shift in the ITA. In the SV, the ET(A)-receptor agonist sarafotoxin 6b (S6b) produced a monophasic concentration-response curve that was antagonised biphasically by BQ123 (0.1-1 microM). In the ITA, S6b was an ineffective agonist with contractions seen only at 3 x 10(-9) M upward. The ET(B)-receptor agonist sarafotoxin 6c (S6c) caused constrictions in only 74% of SV rings and 42% of ITA rings. In the tissues that did respond, S6c caused a monophasic concentration-response curve with a lower maximal response than ET-1. The ET(B) antagonist BQ788 did not antagonise the responses to ET-1 in either the SV or the ITA but did antagonise the responses to S6c in the SV. The results from this study suggest that mainly ET(A) receptors mediate the contractile responses in the human SV and ITA. There is also evidence for an ET(B)-mediated response, although the contractions were much smaller than those elicited by ET-1. We also conclude that the ET(A) receptors mediating responses in these human vessels are atypical because of the different effects of BQ123 on the two vessels.

Atypical antagonism observed with BQ-123 in human saphenous vein

At present, endothelin (ET) receptor classification remains controversial. We investigated the presence of atypical ETA receptors in human saphenous veins (SV). Human SV was obtained from 24 patients undergoing coronary artery bypass grafting. Vessels were set up in organ baths, stretched to a tension of 6 g, and left to relax before being challenged with 90 mM KCl. After KCl challenge, tissues were incubated with 2.8 microM indomethacin and 100 microM L-NMMA for 30 min followed by 30 min in the presence of antagonist before a concentration-response curve to ET-1 or sarafotoxin 6b (S6b) (10(-10)-10(-7) M) was constructed. In endothelium-intact vessels, incubated with indomethacin and L-NMMA, BQ-123 (1 microM) caused nonparallel shifts, with lower concentrations of ET-1 being antagonized more than higher concentrations. This antagonism with BQ-123 was unaffected by BQ-788 (0.1 microM; n = 6) or by desensitization of ETB receptors with S6c (0.1 microM; n = 8). Blocking the Ca2+ channels with nifedipine (1 microM; n = 5) did not affect the antagonism, nor did denuding the endothelium or leaving the endothelium intact (n = 5). When S6b was used as an agonist, BQ-123 (0.3-3 microM) caused concentration-dependent biphasic shifts, with low concentrations of S6b not being antagonized. In conclusion, the antagonism observed with BQ-123 is not due to the action of ET-1 at ETB receptors, changes in Ca2+ handling, or endothelium. This unusual action of BQ-123 suggests subtypes of the ETA receptor.

Radioligand binding characteristics of the endothelin receptor in the rabbit iris

We previously suggested the presence of functionally atypical endothelin (ET) A receptors in the rabbit iris sphincter. Here, we further characterized the ET receptor by a radioligand-receptor binding study utilizing a membrane fraction of the rabbit iris. In addition, we functionally confirm the presence of an atypical ET(A) receptor in the iris dilator similar to that in the iris sphincter. In binding experiments, [125I]ET-1 was completely displaced by ET-3 in a biphasic fashion, but only partially by BQ-123 and ET(B) ligands. In the presence of RES-701, ET-3 and sarafotoxin (SRTX)-b completely displaced [125I]ET-1 in a monophasic fashion, but with shallow slopes. Moreover, ET-1, ET-3 and SRTX-b completely displaced [3H]BQ-123 with IC50 values of 0.8, 81 and 4.4 nM, respectively, but with slopes of ET-3 and SRTX-b being again shallow. In iris dilator muscles, ET-3 showed lower and SRTX-b showed higher contractile activities than ET-1. SRTX-c was inactive. BQ-123 more preferentially antagonized ET-3 and SRTX-b than ET-1, with the Schild plot slope of SRTX-b being shallow. Thus, functional experiments suggested the presence of atypical ET(A) receptors in the iris dilator similar to the iris sphincter. However, the binding experiments suggested the presence of rather typical ET(A)- and ET(B)-like receptors. Therefore, we apparently failed to show ET binding sites corresponding to functionally atypical ET(A) receptors.

Prostaglandin endoperoxide synthase-2 contributes to the endothelin/sarafotoxin-induced prostaglandin E2 synthesis in mouse osteoblastic cells (MC3T3-E1): evidence for a protein tyrosine kinase-signaling pathway and involvement of protein kinase C

Endothelin (ET) peptides are potent growth factors binding to G protein-coupled receptors. Sarafotoxins (S6) isolated from Atractaspis engaddensis are highly homologous to endothelins. In this study, we have investigated the effects of endothelin/sarafotoxin peptides on the prostaglandin synthesizing system in an osteoblast-like cell line, MC3T3-E1. ET-1, ET-2, beta-ET, and S6b rapidly stimulated prostaglandin E2 production within 5 min, whereas ET-3, S6a, and S6c did not. ET-1, ET-2, beta-ET, S6b, and S6a induced prostaglandin synthesis after 3 h of incubation. Antagonizing these effects with BQ-123, PD 142893, BQ-788, and S6c suggests signaling through an ET(A) receptor subtype in osteoblasts. Long-term prostaglandin synthesis was blocked by NS-398, and reduced to short-term levels by cycloheximide and actinomycin D, indicating induction of PGHS-2. There was only minor enhancement of cAMP accumulation by the agonists, which had no effect on prostaglandin synthesis. Induction of PGHS-2 was furthermore demonstrated by Northern blot analysis of PGHS-2 messenger RNA. Depletion of protein kinase C with TPA largely blunted the response. Genistein, an inhibitor of protein tyrosine kinases, also blocked long-term prostaglandin E2 formation. We conclude that in osteoblast-like MC3T3-E1 cells, ET-1, ET-2, beta-ET, S6b, and S6a peptides induce PGHS-2 through a protein tyrosine kinase-dependent and protein kinase C-dependent pathway, signaling through ET(A) receptor occupancy.

Detection of endothelin receptors in human coronary artery vascular smooth muscle cells but not endothelial cells by using electron microscope autoradiography

Endothelin A (ET(A)) and ET(B) receptors located on vascular smooth-muscle cells mediate potent vasoconstriction in animal and human blood vessels. Although vascular endothelial ET(B) receptors mediate vasodilatation through release of nitric oxide, prostacyclin, or an endothelium-derived hyperpolarizing factor in animal vessels, there is less evidence to support ET-mediated endothelium-dependent relaxation in human arteries. We investigated the regional and subcellular localization of ET-receptor subtypes in human epicardial coronary arteries obtained from patients with congestive heart failure undergoing cardiac transplantation. Radioligand-binding studies revealed a predominance of the ET(A)-receptor subtype in the coronary artery. ET(A) and ET(B) receptors were located in the media and in perivascular structures by using macroautoradiography. Specific binding in the intimal layer was low. Autoradiography at the ultrastructural level revealed the presence of ET(A) and ET(B) receptors on medial vascular smooth-muscle cell plasmalemma and plasmalemmal vesicles with a predominance of the ET(A) subtype. ET receptors were not detected in either luminal endothelial cells of the coronary arteries or endothelial cells lining small adventitial blood vessels. This study is the first to identify the subcellular localization of ET receptors in human coronary artery vascular smooth-muscle cells. These receptors probably mediate the well-established vasoconstrictor response to ET-1. The inability to detect ET receptors on endothelial cells suggests a minor role for endothelial cells in ET-mediated vasodilatation of the coronary vasculature.

Comparison of the contractile effects and binding kinetics of endothelin-1 and sarafotoxin S6b in rat isolated renal artery

1. To date, only two mammalian endothelin (ET) receptors, termed ETA and ETB, have been cloned, sequenced and characterized. However, several functional studies of isolated blood vessels suggest that ET-1-induced contractions may be mediated by multiple ETA receptors. In this study, the ETA receptors in renal arteries isolated from Wistar rats were characterized by isometric tension recording and radioligand binding techniques. 2. ET-1, sarafotoxin S6b (StxS6b) and ET-3 produced concentration-dependent contraction with similar response maxima in endothelium-denuded arteries, whereas the ETB receptor-selective agonist StxS6c was inactive. ET-1 and StxS6b were equipotent and 30 times more potent than ET-3. This agonist profile, together with the findings that the ETA receptor-selective antagonists, BQ-123 and FR-139317 caused concentration-dependent, rightward shifts of the concentration-effect curves to each agonist indicated that ET-1-induced contractions in rat renal artery were mediated via ETA receptors. 3. BQ-123 and FR-139317 were both significantly more potent inhibitors of contractions induced by StxS6b or ET-3 than of responses to ET-1, raising the possibility that a component of ET-1-induced contraction was mediated through atypical, BQ-123 (or FR-139317)-insensitive ETA receptors. However, in competition binding studies, specific [125I]-ET-1 and [125I]-StxS6b binding to rat renal artery sections was completely abolished by BQ-123 in a manner consistent with an action at a single site. Thus, competition binding studies did not provide any supportive evidence of the existence of a BQ-123-insensitive ETA receptor. 4. Additional studies revealed marked differences in the kinetics of [125I]-ET-1 and [125I]-StxS6b binding. Following a 3 h period of association of [125I]-ET-1 with its receptors, no significant dissociation of receptor-bound [125I]-ET-1 was observed during a 4 h washout period. In stark contrast, dissociation studies revealed that specific [125I]-StxS6b binding to ETA receptors was reversible (t0.5diss, 100 min). A series of association binding studies were also consistent with the specific binding of [125I]-ET-1 and [125I]-StxS6b being irreversible and reversible processes, respectively. 5. Thus, differences in BQ-123 potency against ET-1 and StxS6b-induced contractions in rat renal arteries might be due to differences in the kinetics of agonist binding, rather than due to the existence of atypical ETA receptors.

Effect of ETA receptor antagonists on cardiovascular responses induced by centrally administered sarafotoxin 6b: role of sympathetic nervous system

The present study was carried out to investigate the cardiovascular effects of centrally administered SRT6b in saline, BQ123 and BMS182874 pretreated male Sprague-Dawley rats, using a radioactive microsphere technique. SRT6b (100 ng, ICV) produced a transient increase (40%) in blood pressure at 5 min followed by a sustained decrease (-42%) at 30 and 60 min in control rats. Total peripheral resistance and heart rate were not significantly altered. Cardiac output increased (16%) at 5 min and decreased 30 and 60 min following SRT6b administration. Central venous pressure was not affected by SRT6b. Regional blood flow and vascular resistance did not change at 5 min following administration of SRT6b. However, a significant decrease in blood flow to the brain, heart, kidneys, liver, spleen, gastrointestinal tract and mesentery and pancreas was observed 30 and 60 min following administration of SRT6b in control (saline treated) rats. Pretreatment with ETA selective receptor antagonists, BQ123 (10 micrograms, ICV) or BMS182874 (50 micrograms, ICV) significantly attenuated the pressor and depressor effects of centrally administered SRT6b. SRT6b induced decrease in blood flow was completely blocked by pretreatment with BQ123 or BMS182874. ET-1 (100 ng, ICV) produced an increase followed by a decrease similar to SRT6b. Reserpine (5 mg/kg, IP) pretreatment attenuated the cardiovascular effects of ET-1. Role of sympathetic nervous system was determined by measuring splanchnic nerve activity. SRT6b when administered in the lateral cerebral ventricle did not produce any significant effect at 5 min, however, a significant decrease in sympathetic nerve activity was observed 30 min after its administration. It is concluded that centrally administered SRT6b produces significant changes in systematic and regional blood circulation which can be completely blocked by ETA receptor antagonist. The cardiovascular effects of centrally administered SRT6b appear to be mediated through the sympathetic nervous system.