Identification of a domain of ETA receptor required for ligand binding

Critical role of a subdomain of the N-terminus of the V1a vasopressin receptor for binding agonists but not antagonists; functional rescue by the oxytocin receptor N-terminus

A fundamental issue in molecular pharmacology is to define how agonist:receptor interaction differs from that of antagonist:receptor. The V(1a) receptor (V(1a)R) is a member of a family of related G-protein-coupled receptors that are activated by the neurohypophysial peptide hormone arginine-vasopressin (AVP). Here we define a short subdomain of the N-terminus of the V(1a)R from Glu(37) to Asn(47) that is an absolute requirement for binding AVP and other agonists. In marked contrast to the situation for agonists, deleting this segment has little or no effect on the binding of either peptide or non-peptide antagonists. In addition, we established that this subdomain was crucial for receptor activation and second messenger generation. The oxytocin receptor (OTR) also binds AVP with high affinity but exhibits a different pharmacological profile to the V(1a)R. Substitution of the N-terminus of the V(1a)R with the corresponding sequence from the OTR generated a chimeric receptor (OTR(N)-V(1a)R). The presence of the OTR N-terminus recovered high affinity agonist binding such that the OTR(N)-V(1a)R possessed almost wild-type V(1a)R pharmacology and signaling. Consequently, a domain within the N-terminus is required for agonist binding but it does not provide the molecular discriminator for subtype-selective agonist recognition. Cotransfection and peptide mimetic studies demonstrated that this N-terminal subdomain had to be contiguous with the receptor polypeptide to be functional. This study establishes that a segment of the V(1a)R N-terminus has a pivotal role in the mechanism of agonist binding and provides molecular insight into key differences between the interaction of agonists and antagonists with a peptide receptor family.

Novel sodium binding properties of some cyclopentapeptide endothelin A selective receptor antagonists: electrospray and fast-atom-bombardment mass spectrometric studies

Electrospray ionization and fast atom bombardment mass spectrometric methods reveal novel interactions of endothelin A selective receptor antagonists, cyclo(D-Trp-D-Asp-Pro-D-Val-Leu), cyclo(D-Trp-D-Glu-Ala-D-allo-Ile-Leu) and cyclo(D-Trp-D-Asp-Pro-D-Ile-Leu) with sodium ions. The peptides have very high intrinsic affinities for sodium ions, and form multiple sodium adducts and sandwich structures: [M + Na]+, [M + 2Na - H]+, [M + 3Na - 2H]+, [M + 4Na - 3H]+, [M + 5Na - 4H]+, [2M + Na]+, [2M + 2Na - H]+, [2M + 3Na - 2H]+, [2M + 4Na - 3H]+, [2M + 5Na - 4H]+, [2M + 6Na - 5H]+, and [2M + 7Na - 6H]+. The three cyclic peptides exhibit similar sodium binding stoichiometries despite differences in their amino acids. The observed sodium binding properties may have implications in understanding their protective effects against ischemia-induced acute renal failure. Those cyclic peptides that offer protection may be those that have high affinities for multiple sodium ions.

Antisense homology box-derived peptides represent a new class of endothelin receptor inhibitors

Several peptides encoded by the sense and corresponding antisense DNA have been found to recognize and bind to each other. We developed software to search for sense-antisense regions within proteins taking into account the degeneracy of the genetic code, i.e., one amino acid can have several "antisense" counterparts. Using this approach, we searched endothelin receptor type A for intramolecular regions related in sense-antisense fashion. After locating these regions (termed "antisense homology boxes"), several corresponding peptides were synthesized. The four new ET(A) receptor fragment peptides ETR-P1 ("CALSVDRYRAVASW"), ETR-P3 ("QGIGPLITAIEI"), ETR-P4 ("IADNAERYSANLSSHV") and ETR-P6 ("LNRRNGSLRIALSEHLKNRREVA") reported here can inhibit ET-1 activity.

Human endothelin receptors ET(A) and ET(B) expressed in baculovirus-infected insect cells--direct application for signal transduction analysis

We expressed human endothelin receptors, ET(A) and ET(B), in insect Sf9 cells infected by recombinant baculoviruses that contained the respective cDNAs. Ligand-binding experiments showed that the two expressed receptors have the same affinities as observed for the receptors in mammalian cells, i.e. the ET(A) receptor showed an affinity order of ET-1 > or = ET-2 >> ET-3, and the ET(B) receptor remained nonselective for three isopeptide ligands. The ET(B) receptor was purified by affinity chromatography with K9-biotinyl-ET-1 without losing the ligand-binding activity from the membrane of infected Sf9 cells. Protein chemical analysis of the purified ET(B) receptor showed that it is glycosylated, and that the N-terminal 38-amino-acid peptide is susceptible to proteolytic digestion, resulting in a small 35-kDa receptor like that found in the human placenta. Surprisingly, the infected and unlysed cells showed a strong intracellular Ca2+ concentration increase ([Ca2+]i), which was generated by a unique signal-transduction pathway consisting of the insect GTP-binding protein and human endothelin receptors expressed in the late phase of virus infection. Due mainly to an efficient expression (over 200,000 receptors/cell), to a low background owing to no endogenous homolog receptor in insect Sf9 cells, and to a sensitive fluorescent reagent Fura-2, this insect Sf9 cell system can detect the [Ca2+]i induced by picomolar levels of endothelin-receptor. We propose that this highly sensitive system be used to screen for potential antagonists/agonists of endothelin receptors.

Molecular recognition of peptide and non-peptide ligands by the extracellular domains of neurohypophysial hormone receptors

This study was designed to ascertain whether the extracellular loops of vasopressin/oxytocin receptors bind ligands and, if so, to locate the molecular determinants of this ligand-receptor interaction. Ligand-binding studies were employed using a rat liver V1a vasopressin receptor preparation and both peptide and non-peptide receptor ligands. Synthetic peptides corresponding to defined regions of the extracellular surface of the neurohypophysial hormone receptors recognized radioligands. These receptor mimetics inhibited the binding of radioligands to the V1a receptor with apparent affinities (pKi) ranging from 3.1 to 6.75. The same mimetics had no effects on the binding of angiotensin II to the rat AT1 receptor, indicating specificity for V1a receptor ligands. A mimetic peptide (DITYRFRGPDWL) of the first extracellular loop (ECII) of the V1a vasopressin receptor also inhibited vasopressin-stimulated, but not angiotensin II-stimulated, glycogen phosphorylase in isolated rat hepatocytes. In contrast, scrambled ECII mimetics displayed greatly reduced affinity for vasopressin. In addition, the role of peptide side-chain versus main-chain atoms in the binding of ligands by vasopressin receptors was addressed using retro-inverso peptide mimetics. Our findings indicate a precise orientation of the extracellular receptor surface (particularly the ECII domain) which facilitates the initial 'capture' of both peptide and non-peptide ligands. Moreover, the data indicate that the main-chain atoms of both a major binding-site determinant in the first extracellular loop of the receptor and the neurohypophysial hormones contribute significantly to the ligand-receptor interaction. These findings also suggest that soluble receptor-binding domains have therapeutic potential.

Mutational analysis of the endothelin type A receptor (ETA): interactions and model of selective ETA antagonist BMS-182874 with putative ETA receptor binding cavity

Endothelin (ET) receptor antagonism is a potential therapeutic intervention in the treatment of vascular diseases. To elucidate the mechanism of antagonist-ET receptor complex formation, the interactions of four chemically distinct antagonists were investigated using a combination of genetic and biochemical approaches. By site-specific mutagenesis we previously demonstrated that Tyr129 in the second transmembrane domain was critical for high-affinity, subtype-selective binding to the A subtype of ET (ETA) receptors [Krystek et al. (1994) J. Biol. Chem. 269, 12383-12386]. Affinities of the constrained cyclic pentapeptide BQ-123, the pyrimidinylbenzenesulfonamide bosentan, the indancarboxlic acid SB 209670, and the naphthalenesulfonamide BMS-182874 were decreased 20-1000-fold in Tyr129Ala, Tyr129Ser, and Tyr129His ETA receptor mutants. Substitution of Tyr129 with Phe or Trp did not alter the high-affinity binding of BQ-123, bosentan, or SB 209670. BMS-182874 binding affinity was decreased 10-fold in Tyr129Phe and Tyr129trp ET receptors. These data indicate a role of aromatic interactions in the binding of these antagonists to ETA receptors an, in the case of BMS-182874, also suggested a hydrogen bond with the tyrosine hydroxyl. This hypothesis was supported by structure-activity data with analogs of BMS-182874 that varied the C-5 dimethylamino substituent on the naphthalene ring. Mutation of Asp126 and Asp133 also altered binding of BMS-182874 and C-5 analogs. In all cases, naphthalenesulfonamide binding was more severely affected by mutation of Asp133 than by mutation of Asp126. Phosphoinositide hydrolysis and extracellular acidification rate studies demonstrated the importance of Tyr129 to ETA-mediated signal transduction. On the basis of these data, two plausible models of the docked conformation of BMS-182874 in the ETA receptor are proposed as a starting point for further delineation of interactions that underlie antagonist-ETA receptor complex formation.

Aspartate mutation distinguishes ETA but not ETB receptor subtype-selective ligand binding while abolishing phospholipase C activation in both receptors

The endothelin receptors, ETA and ETB, are G protein-coupled receptors (GPCR) that show distinctively different binding profiles for the endothelin peptides and other ligands. We recently reported that Tyr129 in the second transmembrane region (TM2) of the ETA receptor was critical for subtype-specific ligand binding [Krystek, S.R. et al. (1994) J. Biol. Chem. 269, 12383-12386]. Receptor models indicated that aspartic acids located one helical turn above (Asp133) and below (Asp126) Tyr129 in ETA had their side chains directed toward the putative binding cavity. Similarly in ETB, Asp147 and Asp154 are located one turn below and above His150, the residue that corresponds to Tyr129. Asp126 in ETA and Asp147 in ETB correspond to the highly conserved aspartate present in TM2 of many GPCR that has frequently been shown to be crucial for agonist efficacy. Mutagenesis of Asp126 of the human ETA receptor to alanine resulted in an unaltered affinity for ET-1, a 160-fold increase in ET-3 affinity and a decrease in affinity for the ETA selective naphthalenesulfonamide, BMS-182874. ET-1 activation of phospholipase C was abolished. In addition, despite the gain in binding affinity, ET-3 failed to activate phospholipase C, suggesting that Asp126 is required for signal transduction. Mutagenesis of Asp133 to alanine indicated that it was critical only for the binding of BMS-182874. In the ETB receptor, mutation of His150 to alanine or tyrosine indicated that it plays a minor role in ETB subtype-selective ligand binding; mutation of the aspartates in TM2 of ETB did not alter ligand binding. As in the Asp126 Ala ETA variant, ET-1 and ET-3 failed to increase intracellular levels of inositol phosphates in the Asp147Ala ETB mutant. Taken together, these data support the hypothesis that Asp126 and Asp133 flanking Tyr129 in TM2 of the ETA receptor play a role in defining ETA subtype-selective ligand binding but Asp147 and Asp154 that flank the His150 in TM2 of the ETB receptor do not. Furthermore, these data indicate that Asp126 in ETA and Asp147 in ETB are important for transmembrane signaling via phospholipase C.

Endothelin receptor subtypes and their role in transmembrane signaling mechanisms

The endothelins (ETs) are potent vasoactive peptides that appear to be involved in diverse biological actions, for example, contraction, neuromodulation, and neurotransmission, as well as in various pathophysiological conditions, such as renal and heart failure. The diversity of actions of ETs may be explained in terms of (1) the existence of several receptor subtypes and (2) the activation of different signal transduction pathways. This review summarizes the state of the art in this intensively studied field, with particular focus on structural aspects, receptor heterogeneity, coupling of receptors to G-proteins, and signal transduction mechanisms mediated by the activation of ET-receptors.

Identification of a region of the human endothelin ETA receptor required for interaction with bosentan

Bosentan (Ro 47-0203, 4-tert-butyl-N-[6-(2-hydroxy-ethoxy)-5-(2- methoxy-phenoxy)-2,2'-bipyrimidin-4-yl]-benzenesulfonamide) is a new non-peptidic mixed antagonist of endothelin receptors whose binding activity was two orders higher for the endothelin ETA receptor than that for the endothelin ETB receptor. To identify which region of the human endothelin ETA receptor interacts with bosentan, we created various chimeric endothelin receptors containing domains from the endothelin ETA and ETB receptors in Chinese hamster ovary cells and studied the effect of bosentan on the binding of endothelin-1 to the chimeric receptors. We found that the chimeric endothelin ETB receptor containing domains from the endothelin ETA receptor, the second extracellular region including the proximal transmembrane region (B-region) revealed an affinity toward bosentan which was similar to that of the endothelin ETA receptor. In contrast, the chimeric endothelin ETA receptor, containing the B-region of the endothelin ETB receptor, reduced the binding affinity to the level of the endothelin ETB receptor. Since bosentan competes with endothelin-1 for binding to the endothelin ETA receptor, this receptor antagonist seems to interact with the (140-144) KLLAG sequence located at the carboxylterminus of the second transmembrane region of the endothelin ETA receptor, required for the natural ligand binding.

Identification of specific regions of the human endothelin-B receptor required for high affinity binding with endothelin-3

To investigate the endothelin-3 (ET-3) binding region of the endothelin-B (ETB) receptor, we have transiently produced various chimeric endothelin receptors in transfected Chinese hamster ovary cells. Using 125I-ET-1 as the radioactive ligand in the displacement experiment, the replacement of both the second and third extracellular regions including the flanking transmembranes of the ETB receptor with the corresponding domains of the endothelin-A (ETA) receptor, increased the apparent Ki value for ET-3 from 5 x 10(-11) M to 10(-8) M. The affinity of this chimeric receptor, ETB-BC, for ET-3 was about two orders lower than ETB yet one order higher than ETA. Previously we have reported the involvement of Lys-140 located in the C-terminus of the second transmembrane region of the ETA receptor for ET-1 binding (Eur. J. Biochem., 220, 37-43, 1994). To assess the importance of the corresponding Lys-161 of the ETB receptor in ET-3 binding, we have replaced it with Ile in the ETB receptor. The mutant receptor had a 5.6-fold reduction in its affinity for ET-3, but its affinity for ET-1 remained similar. These results demonstrate that Lys-161 of the receptor is important for high affinity binding with ET-3 which, in part, confers the non-selective binding characteristics of the ETB receptor for ET isopeptides.

Mutational analysis of human endothelin receptors ETA and ETB identification of regions involved in the selectivity for endothelin 3 or cyclo-(D-Trp-D-Asp-Pro-D-Val-Leu)

Two endothelin(ET)-receptor subtypes have been identified in mammals. They differ in their affinity towards the ET isopeptides with ETA displaying an ET-1-selective profile and ETB a non-selective one. To identify the regions responsible for the differential selectivity, chimeric forms were engineered by sequentially exchanging extracellular regions together with their flanking transmembrane domains. Two sets of reciprocal receptor mutants were thereby generated and analysed by expression in COS-7 cells. The recombinant receptor chimeras were characterised by direct and competitive radioligand-binding analysis. COS-7 cells transfected with vectors for the mutant receptors exhibited specific saturable [3-125I]iodotyrosyl ET-1 (125I-ET-1) binding, with affinities comparable to those of the wild-type receptors (apparent Ki approximately 1-6 x 10(-9) M). An average of 10(5)-10(6) binding sites/cell was calculated for the wild-type and mutant forms. In competition experiments using 125I-ET-1 and unlabeled ET-3, an ETB-selective agonist, we detected a clear switch from an ET-1-selective profile to a non-isopeptide-selective profile in ETA chimeras where the second extracellular loop and the flanking transmembrane domains IV and V, or the third extracellular loop and the flanking transmembrane domains VI and VII, had been exchanged for the corresponding parts of ETB. The opposite effect, namely a switch from a non-isopeptide-selective to an ET-1-selective binding, was observed for the mirror ETB chimeras where the symmetrical exchange had been operated. Using 125I-ET-1 and the ETA-specific antagonist cyclo-(D-Trp-D-Asp-Pro-D-Val-Leu) (BQ123), we were able to map the main determinants responsible for this selectivity to the N-terminal moiety of this receptor. Therefore, the ability for the interaction with ET-3 or BQ123 is governed by two different regions of the ET receptors.

Identification of a ligand-binding site of the human endothelin-A receptor and specific regions required for ligand selectivity

To investigate the ligand-binding site of the human endothelin-A-receptor subtype (ETA), we have produced various chimeric and mutated receptors in chinese hamster ovary cells. The substitution of Lys140 with Ile located in the C-terminus of the second transmembrane region caused a 13-fold reduction in affinity for endothelin-1 (ET-1) and 3.6-fold lower Bmax than those values for the original receptor. Correspondingly, the mutated ETA receptor with the Lys140-->Ile substitution failed to induce an increase in the intracellular calcium concentration in the presence of 1 nM ET-1. Thus, the Lys140 in the ETA receptor is important in ligand binding. ETA and ETB receptors possess the ET isopeptides selective and non-selective binding activities, respectively. Displacement experiments and the binding of 125I-ET-3 to various chimera receptors demonstrated that both the third and fourth extracellular regions, including the flanking transmembrane regions, are responsible for the ligand-binding selectivity of the ETA receptor.

Endothelin modulates osteopontin and osteocalcin messenger ribonucleic acid expression in rat osteoblastic osteosarcoma cells

Endothelins (ETs) are vasoconstrictive peptides produced mainly by endothelial cells. The ET receptors are expressed in many types of cells including osteoblast-like cells. The purpose of this study was to examine the effects of endothelin on the expression of osteoblastic phenotype-related genes. We found that endothelin-1 (ET-1) enhanced approximately two-fold the mRNA expression of both osteopontin and osteocalcin genes in rat osteoblastic osteosarcoma ROS17/2.8 cells. These effects were dose-dependent, peaking at 10(-7) M. The ET-1 enhancement of the abundance of osteopontin and osteocalcin mRNAs was time-dependent, with a maximal effect at 24 h. ET-1 modulation of the expression of the two phenotype-related gene products of osteoblasts suggests that endothelin is one of the cytokines which modulate osteoblastic functions and that this molecule may play a role in the regulation of bone metabolism.

Heterogeneity of endothelin/sarafotoxin receptors mediating contraction of the human isolated saphenous vein

We investigated the effect of the ETA receptor antagonist, BQ-123 (0.1 and 1 microM), on contraction of the human isolated saphenous vein induced by endothelin-1 or sarafotoxin S6b. Contraction in response to endothelin-1 was not affected by BQ-123. In contrast, BQ-123 biphasically attenuated the contractions due to sarafotoxin S6b. These data indicate that (i) endothelin-1 induces contractions of the human saphenous vein via a non-ETA receptor and (ii) contractions in response to sarafotoxin S6b are mediated in part via a receptor different from the receptor mediating contraction due to endothelin-1.