Cloning and characterization of a human angiotensin II type 1 receptor

Cloning and characterization of a novel integrin beta3 subunit

We have identified a novel integrin beta3 subunit, termed beta3C, from a human osteoclast cDNA library. The COOH-terminal sequence and 3'-untranslated region of the beta3C subunit differs from the previously reported beta3A (platelet) and beta3B (placenta) sequences, while the regions coding for the transmembrane and extracellular domains are identical. The beta3C cytoplasmic domain contains 37 amino acids, the last 17 of which are encoded by a novel exon located about 6 kilobase pairs downstream of exon 14 of the beta3A gene. HEK 293 cells were stably co-transfected with alphaV and either beta3C (HEKbeta3C) or beta3A (HEKbeta3A). The viability of HEKbeta3C cells was lower than that of HEKbeta3A cells, and HEKbeta3C cells in culture grew as clusters rather than as a monolayer. The novel cytoplasmic domain did not affect receptor binding affinity; both alphaVbeta3A and alphaVbeta3C isoforms exhibited high affinity binding to 125I-echistatin and cyclic and linear RGD peptides. However, in contrast to HEKbeta3A, HEKbeta3C cells failed to adhere to osteopontin, an alphaVbeta3 matrix protein. The data provide further support for the key role of the cytoplasmic domain of the beta3 integrin in cell adhesion and suggest a potential role for the beta3C integrin subunit in modulating cell-matrix interactions.

Angiotensin II receptors, AT1 and AT2 in the rat epididymis. Immunocytochemical and electrophysiological studies

Previous work from our laboratory has provided evidence for the presence of a tissue renin-angiotensin system in the rat epididymis. In the current investigation, the regional localization of angiotensin II receptors, type I (AT1) and type II (AT2) was studied immunocytochemically using specific anti-peptide antibodies against the second extracellular loops of AT1 and AT2 receptors, and pharmacologically using specific receptor antagonists in conjunction with the short-circuit current technique. The immunocytochemical results showed that AT1 and AT2 immunoreactivities were predominantly localized in the basal region of the epididymal epithelium. Electrophysiological studies using the short-circuit current technique demonstrated a stimulatory effect of basolaterally applied angiotensin II on the epididymal electrogenic ion transport. This effect was inhibitable by the addition of AT1 antagonist, losartan but not by AT2 antagonist, PD123177, indicating a functional role of AT1 in epididymal electrolyte transport. The present finding suggests that angiotensin II receptors may play an important role in the regulation of epididymal function.

Angiotensin II receptors: localization of type I and type II in rat epididymides of different developmental stages

Previous studies from our laboratory have provided evidence for the existence of a local renin-angiotensin system in the rat epididymis. Evidence has also accumulated, indicating that locally formed angiotensin II from the rat epididymis may play a paracrine and/or autocrine role in regulating epididymal electrolyte and fluid transport. In the present study, specific anti-peptide antibodies against the second extracellular loops of angiotensin II type I (AT1) and type II (AT2) receptors were used to localize immunocytochemically these receptors in the rat cauda epididymides of three developmental stages, namely, immature (2-week), early mature (6-week) and fully mature (10-week). The immunostaining intensity for AT1 receptors was found to be stronger than that for AT2 receptors throughout rat epididymides of all stages. However, the immunostaining for both AT1 and AT2 receptors observed in the fully mature rat epididymis was much more intense than that observed in the epididymides of the two younger stages. While the immunostaining for both AT1 and AT2 receptors in the younger rat epididymides appeared to be distributed in both basal and apical regions, the immunostaining in the fully mature epididymis was predominantly localized in the basal region. The present finding of the differential patterns of angiotensin II receptor immunoreactivity in three different stages of the rat epididymis may reflect the fine tuning of rat epididymal function by angiotensin II, acting as a paracrine or autocrine agent, during the course of development.

Cloning and expression of angiotensin II type 2 (AT2) receptors from murine neuroblastoma N1E-115 cells: evidence for AT2 receptor heterogeneity

Homology-based PCR was used to isolate angiotensin II type 2 (AT2) receptor cDNA from murine neuroblastoma N1E-115 cells. Despite subtle differences in the nucleotide sequence (the N1E-115 clone coded for Phe133 as TTC and Gln326 as CAG; base substitutions are in bold-italics), the AT2 receptor protein was identical to other reported murine AT2 clones. When transfected into COS-1 cells, the expressed AT2 receptor displayed high affinity for AngII and for AT2-selective compounds, GTP gamma S-insensitive agonist binding and enhanced agonist binding by dithiothreitol. Previously, we have demonstrated that N1E-115 cells possess two distinct subpopulations of AT2 receptors, defined as peak I and peak III receptors, that can be separated by heparin-sepharose chromatography. The two subpopulations differ pharmacologically, biochemically and immunologically. The binding properties of the cloned AT2 receptor closely resembled that of peak III receptors. Moreover, antisera raised against peak I AT2 receptors failed to immunoreact to either peak III receptors or cloned AT2 receptors expressed in COS-1 cells. Collectively, these data suggest that the cloned AT2 receptor is identical to peak III receptors from N1E-115 cells and that a novel AT2 receptor (peak I) remains to be cloned.

Effects of age and gender on the AII-induced stimulation of prolactin release and inositol phosphate accumulation in rat anterior pituitary cells in vitro

The stimulatory effects of Angiotensin II (AII) on prolactin secretion and inositol phosphate accumulation were examined in dispersed anterior pituitary cells collected from young (3-4 month), mature (7-8 month) and old (18-20 month) male and female rats. Physiological doses of AII (0.01-10 nM) stimulated prolactin release from cells collected from mature female rats only. This effect was antagonized by pretreatment with Saralasin, an AII receptor antagonist. Significant accumulation of the inositol phosphates was observed in cells obtained from the mature, female donors and this increase preceded the prolactin response. Although there was a small increase in total inositol phosphate accumulation in cells obtained from the old female rats, this was transient and did not coincide with a similar increase in prolactin release. These results indicate that pituitary sensitivity to AII stimulation is related to the age and the gender of the donor animal. The physiological role of pituitary AII needs to be examined in sexually mature female animals.

The ligand binding site of the angiotensin AT1 receptor

The angiotensin AT1 receptor mediates numerous physiological actions of the octapeptide hormone, angiotensin II, in its cardiovascular and other target tissues. The binding of angiotensin II to the AT1 receptor is dependent on both intramembrane and extracellular regions of the receptor molecule. Non-peptide antagonists that block angiotensin binding and action interact exclusively with residues located within the intramembrane binding pocket of the AT1 receptor. However, peptide ligands also interact with extracellular residues to form additional bonds that stabilize their binding to the receptor. Here, László Hunyady, Tamás Balla and Kevin Catt describe how these and other studies have shown that interaction of ligands with residues in the intramembrane binding pocket is the conserved mechanism required for agonist activation of G protein-coupled receptors.

Angiotensin II and its different receptor subtypes in placenta and fetal membranes

The recent discovery of a local renin-angiotensin system in trophoblastic tissues has raised many questions regarding its role in the physiology of normal gestation and its implications in the pathophysiology of hypertension during pregnancy. In this article, the authors first review the most interesting aspects of the chorioplacental renin-angiotensin system, dwelling on the tissue distribution of angiotensin II and its receptor subtypes in the placenta and fetal membranes of different species. The relationship between angiotensin II and other locally synthesized chorioplacental substances is also analysed and the therapeutic implications of phenomena observed in pregnancy-associated hypertension are discussed.

Molecular cloning of a novel platelet protein showing homology to the angiotensin II receptor C-terminal domain

Oligoscreening of a cDNA library obtained from 4 beta-phorbol 12-myristate 13-acetate-stimulated human erythroleukemia (HEL) cells resulted in the isolation of a novel clone coding for a protein with a calculated molecular mass of 8110 Da. This protein of 71 amino acids shows significant homology to the carboxyl-terminal regulatory domain of angiotensin II type 1 receptors. The homology encompasses four regions of amino acid residues thought to serve as consensus sequences for phosphorylation by serine/threonine kinases such as protein kinase C, which are key mediators of intracellular signaling. Reverse transcription-polymerase chain reaction identified the transcript in human platelets, human megakaryocytic DAMI cells, and HEL cells. High stringency Northern blotting revealed a tissue-specific distribution of three transcript species, with predominant expression in skeletal muscle and pancreas. Rabbit anti-peptide antiserum was used to immunoblot protein lysates from washed resting platelets and from 4 beta-phorbol 12-myristate 13-acetate-stimulated DAMI and HEL cells. These immunoblots revealed the presence of an intense approximately 8-kDa protein band in platelets and HEL cells and a faint band of identical size in DAMI cells.

Variation of intrarenal angiotensin II and angiotensin II receptors by acute renal ischemia in the aged rat

The properties of Ang II receptors in the aged kidney and their changes after acute renal ischemia are rarely known. The aim of this study was to examine the expression of Ang II receptor mRNA and characteristics of Ang II receptors in the aged kidney and to compare different responsiveness to 45 min of acute renal ischemia in young (3-4 months) and aged (23-24 months) rats. In the normal condition, AT1 mRNA expression was much lower in the aged than that in the young rats. Maximal binding (Bmax) was also lower in the aged (1315 +/- 48 vs. 2035 +/- 257 fmol/mg, p < 0.05). The dissociation constant (KD) of glomerular Ang II receptors, however, was significantly lower in the aged rats compared to the young (6.8 +/- 1.6 vs. 17.4 +/- 2.5 nM). After acute ischemia, the expression of AT1 mRNA decreased in the young rats but increased in the aged rats. Interestingly, Bmax of glomerular Ang II receptors was significantly increased in the aged ischemic rats (1852 +/- 94 vs. 1315 +/- 48 fmol/mg) with unchanged KD. These results show that: (a) the AT1 mRNA expression and the Ang II receptor binding site are decreased with the aging process in the rat kidney; (b) the acute renal ischemia effect on different age groups has a greatly discrepant pattern in respect of Ang II receptor modulation, which may provide a potential therapeutic future for the receptor antagonists in acute renal ischemia in the aged.

Binding of valsartan to mammalian angiotensin AT1 receptors

The binding characteristics of the angiotensin AT1 receptor antagonist valsartan were investigated in different animal species and tissues. Using [125I](Sar1,Ile8) angiotensin II as radioligand, affinity constants were determined in liver and adrenal rat and marmoset, human adrenal and in rat aortic smooth muscle cells. In all tissues tested, valsartan had a greater affinity for the AT1 receptor than losartan (on average 5-fold). The affinities of both antagonists were up to 30 times weaker in the dog tissues [3H]Valsartan bound with high affinity (Kd 1.44 nmol/l) to the rat aortic smooth muscle cell AT1 receptor. Binding was saturable and reversible. Non-specific binding was low (10%). Reports that [3H]losartan binds to a non-angiotensin II binding site in rat liver and in other tissues could be confirmed. [3H]Valsartan on the other hand bound only to the AT1 receptor. Using a competition binding assay with [3H]losartan on rat liver membranes it could be shown that valsartan can bind to the 'losartan binding site', but at a 10,000-fold less affinity than for the AT1 receptor. Valsartan is therefore a highly specific and selective antagonist of the AT1 receptor. Due to its high affinity and low non-specific binding it is a suitable radioactive antagonist for the study of the distribution and function of the angiotensin AT1 receptor.

Developmental consequences of the renin-angiotensin system

Molecular, cellular, and physiological studies indicate that the renin-angiotensin system (RAS) is highly expressed during early kidney development. We propose that a major function of the RAS during early embryonic development is the modulation of growth processes that lead the primitive kidney into a properly differentiated and architecturally organized organ suited for independent extrauterine life. As development progresses, the RAS acquires new and overlapping functions such as the endocrine and paracrine regulation of blood pressure and renal hemodynamics. Disease states in adult mammals often result in expression of RAS genes and phenotypic changes resembling the embryonic pattern, emphasizing the importance of undertaking developmental studies. Because of their importance in health and disease, the immediate challenge is to identify the mechanisms that regulate the unique development of the RAS and its role(s) in normal and abnormal growth processes.

Localization of the angiotensin II and its receptor subtype expression in human endometrium and identification of a novel high-affinity angiotensin II binding site

Angiotensin (ANG) II is not only a potent vasoconstrictor but may also be involved in the regeneration of new blood vessels. In proliferative endometrium, ANG II-like immunoreactivity was detected in glandular epithelium and stroma with negligible staining around the vascular endothelium. In contrast, in secretory endometrium intense immunostaining was seen in the perivascular stromal cells around the endometrial spiral arterioles with negligible staining of the other cell types. Quantitative receptor autoradiography using the nonselective radioligand [125I]-ANG II and subtype selective competing compounds showed that endometrium contained predominantly AT2 receptors, with relatively low expression of AT1 receptors and a novel non-AT1/non-AT2 angiotensin II recognition site that was insensitive to AT1 or AT2 selective ligands. Levels of specific [125I]-ANG II receptor binding displayed cyclic changes during the menstrual cycle, reaching a maximum in early secretory endometrium and then decreasing in mid to late secretory endometrium to levels seen in early to mid proliferative endometrium. In situ hybridization showed AT1 receptor mRNA expression in the glands and in the endometrial blood vessels. The cyclic changes in ANG II-like immunoreactivity together with expression of both the known and the novel AT receptor subtypes imply that this octopeptide may play a dual role both in the control of the uterine vascular bed and also in the regeneration of the endometrium after endometrial shedding, acting as an angiogenic and mitogenic mediator.

Mutagenesis and the molecular modeling of the rat angiotensin II receptor (AT1)

The molecular interaction involved in the ligand binding of the rat angiotensin II receptor (AT1A) was studied by site-directed mutagenesis and receptor model building. The three-dimensional structure of AT1A was constructed on the basis of a multiple amino acid sequence alignment of seven transmembrane domain receptors and angiotensin II receptors and after the beta 2 adrenergic receptor model built on the template of the bacteriorhodopsin structure. These data indicated that there are conserved residues that are actively involved in the receptor-ligand interaction. Eleven conserved residues in AT1, His166, Arg167, Glu173, His183, Glu185, Lys199, Trp253, His256, Phe259, Thr260, and Asp263, were targeted individually for site-directed mutation to Ala. Using COS-7 cells transiently expressing these mutated receptors, we found that the binding of angiotensin II was not affected in three of the mutations in the second extracellular loop, whereas the ligand binding affinity was greatly reduced in mutants Lys199-->Ala, Trp253-->Ala, Phe259-->Ala, Asp263-->Ala, and Arg167-->Ala. These amino acid residues appeared to provide binding sites for Ang II. The molecular modeling provided useful structural information for the peptide hormone receptor AT1A. Binding of EXP985, a nonpeptide angiotensin II antagonist, was found to be involved with Arg167 but not Lys199.

Hormonal regulation of angiotensin II type 1 receptor expression and AT1-R mRNA levels in human adrenocortical cells

Human adrenocortical H295R cells express AII receptors which are predominantly of the AT1 but not AT2 subclass. These receptors are functionally coupled to phosphoinositidase C in a manner similar to that seen in fetal human, sheep and bovine adrenocortical cells. Treatment of H295R cells with forskolin or dbcAMP to activate the protein kinase A pathway caused a rapid (maximal by 3 h) and sustained decrease in AT1-R mRNA levels which in turn preceded a time-dependent (maximal by 12 h) and dose-dependent loss of [125I]AII binding and phosphoinositidase C activation on subsequent AII challenge. Thus, both decreased AT1-R mRNA levels and functional receptor expression appear to parallel each other in response to activation of protein kinase A. Activation of the Ca2+/protein kinase C pathways by treatment with AII also caused a rapid (maximal by 3 h) and dose-dependent loss in AT1-R mRNA, but mRNA levels subsequently rose again, approaching control levels by 36 h. Treatment with AII for 48 h had little effect on either [125I]AII binding or the subsequent phosphoinositidase C response. The effect of AII, but not forskolin, was blocked by the presence of cycloheximide. The action of AII on AT1-R mRNA was probably mediated through both protein kinase C and Ca(2+)-sensitive protein kinases as the effect at 4 h was not completely reproduced by phorbol ester alone, but was fully reproduced by a combination of phorbol ester and Ca2+ ionophore. However, increased Ca2+ influx alone, due to treatment with BAYK8644 or elevated extracellular K+, also resulted in a decrease in AT1-R mRNA levels. Thus in the H295R cell, control of AT1-R expression appears to be complex, being achieved at least in part through control of the level of AT1-R mRNA by multiple independent signaling pathways including protein kinase A, protein kinase C and Ca2+.

Non-peptide angiotensin agonist. Functional and molecular interaction with the AT1 receptor

Non-peptide ligands for peptide receptors for the G-protein-coupled type are generally antagonists, except in the opiate system. Recently, it was observed that a subset of biphenylimidazole derivatives surprisingly possessed angiotensin-like activity in vivo. In COS-7 cells transfected with the rat AT1 receptor a prototype of these compounds, L-162,313 stimulated phosphoinositide hydrolysis with an EC50 of 33 +/- 11 nM. The maximal response to the compound was 50% of that of angiotensin II in COS-7 cells but only 3% in stably transfected Chinese hamster ovary cells. The agonistic effect of L-162,313 was blocked by the AT1-specific antagonist L-158,809 and was not observed in untransfected cells. In Chinese hamster ovary cells, L-162,313 also acted as an insurmountable antagonist of the angiotensin stimulated phosphoinositide hydrolysis. In contrast to previously tested non-peptide ligands, L-162,313 bound with reasonably high affinity to the Xenopus laevis AT1 receptor. In the human receptor, the binding of L-162,313 was found to be unaffected by point mutations in transmembrane segments III and VII, which impaired the binding of biphenylimidazole antagonists. Substitutions in the extracellular domains of the human and rat receptor, which impaired the binding of angiotensin II, did not affect the binding of L-162,313. It is concluded that a subset of biphenylimidazole compounds can act as high affinity partial agonists on the AT1 receptor. These compounds have molecular interactions with the receptor which appear to differ both from that of the structurally similar non-peptide antagonists and from that of their functional counterpart, the peptide agonist.

Dilated cardiomyopathy defines serum autoantibodies against G-protein-coupled cardiovascular receptors

In order further to identify the prevalence of anti-receptor autoantibodies in the sera of patients with dilated cardiomyopathy (DCM), we attempted to detect autoantibodies against a series of G-protein-coupled cardiovascular receptors in a well-defined population of DCM patients from Japan. Peptides corresponding to the sequences of the second extracellular loops of the human beta 1 and beta 2 adrenoceptors, alpha 1 adrenoceptors, M2 muscarinic acetylcholine receptors and angiotensin II-1 (AT1) receptors were used as antigens in an enzyme immunoassay to screen the sera from patients with DCM (n = 28). Nine sera from patients with DCM (32%) and 2 sera from healthy subjects (9%) recognized the beta 1 adrenoceptor peptide. Ten sera from patients (36%) and 3 sera from healthy subjects (13%) recognized the M2 receptor peptide. Thirty-six per cent of the patients with autoantibody against the beta 1 adrenoceptor peptide. Ten sera from patients (36%) and 3 sera from healthy subjects (13%) recognized the M2 receptor peptide. Thirty-six per cent of the patients with autoantibody against the beta 1 adrenoceptor had autoantibody against the M2 receptor. However, no significantly high frequencies of autoantibodies against the beta 2 adrenoceptor, alpha 1 adrenoceptor and AT1 receptor were found in DCM patients. Our results demonstrate that a subgroup of patients with DCM have a specific spectrum of autoantibodies which are specifically directed against the second extracellular loops of the beta 1 adrenoceptors and M2 muscarinic receptors rather than other cardiovascular receptors.

The angiotensin type 1 and type 2 receptor families. Siblings or cousins?

The diverse actions of angiotensin II (AngII) are mediated by cell surface receptors. Molecular cloning techniques have identified two distinct subtypes of AngII receptors referred to as AT1 and AT2. It is now well accepted that multiple forms of the AT1 receptor exist, but similar diversity of the AT2 subtype has not been conclusively demonstrated. Nonetheless, several converging lines of evidence do suggest that multiple AT2 receptors may be present in brain and cultured neuron-like cells lines. For instance, some AT2 receptors are regulated by guanine nucleotides and sulfhydryl-reducing agents, whereas others are insensitive. AT2 receptor populations also exhibit differing pharmacological profiles particularly with respect to their affinity for peptidic and non-peptidic ligands. Moreover, a recently developed anti-AT2 polyclonal antisera reveals a unique pattern of immunohistochemical staining in brain and it does not immunoreact with the recently cloned AT2 receptor. Collectively, these results support the hypothesis of multiple AT2 receptors at least within the CNS. Future studies should reveal whether these putative AT2 receptor subtypes result from unique genes or cell-specific post-translational modifications of a single gene product.

Lysine 182 of endothelin B receptor modulates agonist selectivity and antagonist affinity: evidence for the overlap of peptide and non-peptide ligand binding sites

The potent vasoactive peptide hormone endothelin (ET) binds to receptors which belong to the G-protein coupled receptor family. The availability of non-peptide antagonists for ET receptors allows investigation of the relationship among the binding sites for peptide and non-peptide ligands. In this study, a lysine residue, conserved within transmembrane domain 3 (TM3) of the ETA and ETB receptor subtypes, is implicated in agonist and antagonist binding by its analogous position within TM3 to a binding site aspartate residue conserved within bioactive amine receptors. Replacement of this lysine within hETB by arginine, alanine, methionine, aspartate, or glutamate results in hETB variants with unaltered affinities for agonist peptide ET-1 but which have affinities for peptide agonists ET-2, ET-3, sarafotoxin 6C, and TRL 1736 which are between 1-3 orders of magnitude lower than their corresponding wild-type hETB values. Significantly, the affinities of non-peptide antagonists, (+/-)-SB 209670 and its analogs as well as Ro 46-2005, are abrogated. The results suggest that an interaction of K182 of hETB with the indan 2-carboxyl of (+/-)-SB 209670 may contribute to the high-affinity binding of the diarylindan antagonists. The results indicate that TM3 of hETB is a region of overlap among the binding sites of non-peptide antagonists and the affected peptide agonists.

Pharmacology of angiotensin II receptors in the kidney

Within the kidney angiotensin II (Ang II) exerts potent effects on renal function. The intrarenal actions of Ang II include modulation of renal blood flow, glomerular filtration rate, tubular epithelial transport, renin release and cellular growth. The actions of Ang II on the kidney are mediated by specific intrarenal receptors which, based upon physical characteristics and the selective binding of non-peptide and peptide analogs may be divided into two main subtypes, termed AT1 and AT2. AT1 receptors are present within the kidneys of all species and are located predominantly in the glomerulus, the renal tubules and the renal vasculature, including the afferent and efferent arterioles. Modulation of AT1 receptors within the kidney has been shown to mediate essentially all of the known intrarenal effects of Ang II. AT1 receptors and particularly AT2 receptors are expressed in large numbers in fetal kidney where they may play a role in development and maturation. In some species, intrarenal AT2 receptors disappear shortly after birth. In those species where AT2 receptors are present in the adult kidney their role in the control of renal function has not yet been clearly defined.

Structural model of antagonist and agonist binding to the angiotensin II, AT1 subtype, G protein coupled receptor

BACKGROUND

The family of G protein coupled receptors is the largest and perhaps most functionally diverse class of cell-surface receptors. Due to the difficulty of obtaining structural data on membrane proteins there is little information on which to base an understanding of ligand structure-activity relationships, the effects of receptor mutations and the mechanism(s) of signal transduction in this family. We therefore set out to develop a structural model for one such receptor, the human angiotensin II receptor.

RESULTS

An alignment between the human angiotensin II (type 1; hAT1), human beta 2 adrenergic, human neurokinin-1, and human bradykinin receptors, all of which are G protein coupled receptors, was used to generate a three-dimensional model of the hAT1 receptor based on bacteriorhodopsin. We observed a region within the model that was congruent with the biogenic amine binding site of beta 2, and were thus able to dock a model of the hAT1 antagonist L-158,282 (MK-996) into the transmembrane region of the receptor model. The antagonist was oriented within the helical domain by recognising that the essential acid functionality of this antagonist interacts with Lys199. The structural model is consistent with much of the information on structure-activity relationships for both non-peptide and peptide ligands.

CONCLUSIONS

Our model provides an explanation for the conversion of the antagonist L-158,282 (MK-996) to an agonist by the addition of an isobutyl group. It also suggests a model for domain motion during signal transduction. The approach of independently deriving three-dimensional receptor models and pharmacophore models of the ligands, then combining them, is a powerful technique which helps validate both models.

Mutations in transmembrane segment VII of the AT1 receptor differentiate between closely related insurmountable and competitive angiotensin antagonists

Chimeric constructs between the human and the Xenopus laevis AT1 receptor have demonstrated, that the binding of non-peptide angiotensin antagonists is dependent on non-conserved residues located deep in transmembrane segment VII of the AT1 receptor. Here we have studied four pairs of closely related antagonists each consisting of a competitive and an insurmountable compound differentiated by one out of three different types of minor chemical modifications. None of the antagonists bound to the Xenopus receptor and the binding of all of the compounds to the human receptor was severely impaired by the introduction of non-conserved residues from transmembrane segment VII of the Xenopus receptor. In all four pairs of antagonists the competitive compound was affected more by these substitutions than the corresponding insurmountable one (209 vs. 22, 281 vs. 29, 290 vs. 29 and 992 vs. 325-fold increase in Ki values). A similar pattern was observed in response to substitution of a single non-conserved residue in transmembrane segment VII, Asn295 to Ser. These results indicate that a common molecular mechanism distinguishes the interaction of insurmountable and competitive antagonists with the AT1 receptor.

Differentiation between binding sites for angiotensin II and nonpeptide antagonists on the angiotensin II type 1 receptors

To characterize binding sites for nonpeptide angiotensin antagonists on the human angiotensin II receptor type 1 (AT1 receptor) we have systematically exchanged segments of the human receptor with corresponding segments from a homologous Xenopus laevis receptor, which does not bind the nonpeptide compounds. Substitution of transmembrane segment VII of the human AT1 receptor dramatically reduced the binding affinity of all of the 11 nonpeptide antagonists tested (55- to > 2000-fold) with no effect on the binding of angiotensin. The affinity for the nonpeptide compounds decreased additionally one order of magnitude when transmembrane segment VI and the connecting extracellular loop 3 from the Xenopus receptor were also introduced into the human AT1 receptor. Exchanges of smaller segments and single residues in transmembrane segments VI and VII and extracellular loop 3 revealed that the binding of nonpeptide antagonists was dependent on nonconserved residues located deep within the transmembrane segments VI and VII, in particular Asn295 in transmembrane segment VII. Surprisingly, all exchanges in transmembrane segment VII, including the Asn295 to Ser substitution, had a more pronounced effect on the binding of the competitive antagonists relative to the insurmountable antagonists. It is concluded that the binding mode for peptide and nonpeptide ligands on the AT1 receptor is rather different and that competitive and insurmountable antagonists presumably bind to overlapping but distinct sites located in transmembrane segments VI and VII.

Epidermal growth factor-enhanced human angiotensin II type 1 receptor

The human angiotensin II type 1 (AT1) receptor gene was isolated and its promoter function analyzed by deletion mutant promoter/luciferase constructs in transfected Cos 7 cells. We found that epidermal growth factor enhanced the human AT1 promoter activity twofold to threefold. The region between -227 and -366 bp from the 5' end of the cDNA was mapped for a base sequence responsive to the epidermal growth factor stimulation. By computer analysis, PEA3 transcription factor was located in this region and was shown to bind to the promoter by gel shift assay in Cos 7 and HepG2 cells. These results indicated that the human AT1 receptor enhanced by epidermal growth factor may be due to PEA3 binding to the human AT1 promoter.

AT1 receptor characteristics of angiotensin analogue binding in human synovium

1. Angiotensin II (AII) reduces blood flow, modulates vascular remodelling and is a growth factor. Human inflammatory arthritides are characterized by synovial hypoperfusion, hypoxia and proliferation. We aimed to localize and characterize receptors for AII in human synovium. 2. We used quantitative in vitro receptor autoradiography with [125I]-(Sar1, Ile8)AII and [125I]-AII on human synovium from patients with chondromalacia patellae, osteoarthritis and rheumatoid arthritis. 3. [125I]-(Sar1, Ile8)AII and [125I]-AII bound to similar sites on synovial blood vessels, lining cells and stroma. Binding to microvessels (< 100 microns diameter) was more dense than to arteriolar media, and vascular binding was more dense than that to lining cells and stroma. 4. Microvessels and arterioles which displayed angiotensin converting enzyme-like immunoreactivity also displayed specific binding of [125I]-(Sar1, Ile8)AII. 5. Specific binding of [125I]-(Sar1, Ile8)AII to each structure was completely inhibited by 10 microM dithiothreitol and was inhibited by unlabelled ligands with the rank order of potency (Sar1, Ile8)AII > AII > losartan = SKF108566 > PD123319 indicating an AT1 subclass of angiotensin receptor. 6. GTP gamma S (1 microM) abolished specific binding of [125I]-AII and abolished the high affinity component of the binding inhibition curve for AII against [125I]-(Sar1, Ile8)AII, indicating G protein coupling. 7. The distribution of [125I]-(Sar1, Ile8)AII binding sites was similar in all disease groups and no significant differences in binding densities, affinities or specificities were observed between disease groups. 8. Locally generated AII may act on synovial AT1 receptors to modulate synovial perfusion and growth. Specific AT1 receptor antagonists should help elucidate the role of angiotensins in human arthritis.

Angiotensin II receptor antagonists: a new approach to blockade of the renin-angiotensin system

A-II exerts its activity on various target tissues by binding to its receptors. The discovery of local RASs and A-II receptors within various tissues has generated interest in the clinical usefulness of RAS inhibition by directly blocking the action of A-II at the receptor level. Different A-II receptor subtypes have been identified and subsequently termed AT1 and AT2. AT1-receptor subtypes are the predominant receptor subtypes existing in most organs and, by coupling to a transmembrane G protein, seem to be the main subtypes participating in the vasoactive responses of A-II. Saralasin, a peptide with specific A-II receptor-antagonistic activity, had limited practical long-term usefulness as a result of its short half-life, significant agonistic properties, and lack of oral bioavailability. The discovery of simple benzyl-substituted imidazoles, which possess weak but highly selective A-II receptor antagonistic properties, led to the development of losartan (DuP 753). Losartan is a potent, orally active, specific, competitive nonpeptide A-II receptor antagonist that appears to be an effective antihypertensive agent both in animal studies and in preliminary clinical trials. The therapeutic usefulness of losartan, however, is not limited to its antihypertensive effects. The potential benefits of A-II receptor antagonists include roles in postmyocardial infarction therapy, slowing A-II-induced cardiac hypertrophy, 154, 155 slowing the progression of heart failure, preventing postangioplasty restenosis, and in slowing the progression of renal disease. Furthermore, losartan, a selective A-II type 1 (AT1) receptor antagonist, has also been a valuable pharmacologic probe for studying the mechanism of A-II stimulation of its receptors. A-II receptor antagonism appears to be as effective as ACE inhibition in the treatment of hypertension and other pathologic processes that involve the RAS and may offer an alternative to those patients who cannot tolerate ACE inhibitors because of their side effects.

Molecular cloning of the canine angiotensin II receptor. An AT1-like receptor with reduced affinity for DuP753

Canine aortic strip studies revealed insensitivity of angiotensin II (AII)-induced aortic contraction to inhibition by the non-peptide antagonist DuP753 (pKB = 6.7 +/- 0.1). In order to determine the origin of this phenomenon we cloned the canine homologue of the AT1 AII receptor. Expression of this cDNA in COS-7 cells indicated a low affinity of DuP753 for the cloned receptor (KD = 92 nM). The predicted amino acid sequence is highly homologous to other mammalian AT1 receptors; sequence comparisons suggest the pharmacological difference may be the result of a threonine residue in position 163 in the IVth transmembrane domain.

Molecular cloning and functional characterization of a human liver vasoactive intestinal peptide receptor

We have isolated a cDNA from a human liver library which is 2349 base pairs in length and encodes a near-full length seven transmembrane receptor (432 amino acids), 85% homologous to the amino acid sequence for the rat vasoactive intestinal peptide (VIP) receptor. Northern blot analysis identifies a major species at 3.3 kb in lung, and to a lesser extent in brain, heart and liver. In order to confirm the identity of this human clone, double-stranded oligonucleotides encoding the signal peptide of the rat VIP receptor were constructed by polymerase chain reaction and attached to the 5' end of the human clone. COS cells transiently transfected with this human VIP receptor chimera, express a single binding site for 125I-VIP with a Kd of 9.2 +/- 2 nM. Related peptides displace 125I-VIP with a relative potency of VIP = PACAP > helodermin >> PHM > secretin, which is similar to the binding profile seen in human tissues. This human chimeric receptor is functionally coupled to the stimulation of adenylyl cyclase in transfected COS cells, as evidenced by a dose-dependent increase in intracellular cAMP accumulation. These studies indicate that this cDNA encodes a human liver VIP receptor which is functionally coupled to the activation of adenylyl cyclase.

Human AT1 receptor is a single copy gene: characterization in a stable cell line

To address conflicting reports concerning the number of angiotensin II (AII) receptor type 1 (AT1) coding loci in vertebrates, Southern blot analysis was used to determine the genomic representation of AT1 receptor genes in animals comprising a divergent evolutionary spectrum. The data demonstrate that the AT1 receptor gene is present as a single genomic copy in a broad spectrum of animals including human, monkey, dog, cow, rabbit, and chicken. In contrast, members of the rodent taxonomic order contain two genes in their genomes. These two genes may have arisen in rodents as a consequence of a gene duplication event that occurred during evolution following the branching of rodents from the mammalian phylogenetic tree. In order to investigate the properties of the human AT1 receptor in a pure cell system, the recombinant human AT1 receptor was stably expressed in mouse L cells. An isolated cell line, designated LhAT1-D6, was found to express abundant levels of recombinant receptor [430 +/- 15 fmol/mg] exhibiting high affinity [KD = 0.15 +/- 0.02 nM] for [125I][SAR1, Ile8] angiotensin II (SIA). The pharmacological profile of ligands competing for [125I] SIA binding to the expressed receptor was in accordance with that of the natural receptor. Radioligand binding of the expressed receptor was decreased in the presence of the non-hydrolyzable analog of GTP, guanosine 5'-(gamma-thio) triphosphate [GTP gamma S]. Angiotensin II evoked a rapid efflux of 45Ca2+ from LhAT1-D6 cells that was blocked by AT1 receptor specific antagonists. In addition, AII inhibited forskolin-stimulated cAMP accumulation in these cells which was blocked by the AT-1 antagonist. Thus, the LhAT1-D6 cell line provides a powerful tool to explore the human AT1 receptor regulation.

Structural analysis and regulation of angiotensin II receptors

Angiotensin (Ang II) is an octapeptide hormone that plays a crucial role in the maintenance of electrolyte homeostasis and cardiovascular function. The hemodynamic and cardiovascular effects o f Ang II are mediated by high-affinity cell-surface receptors of the AT(1) pharmacologic class. The mammalian AT(1) receptor has recently been cloned and found to encode a 359-amino-acid protein of 41,000 molecular weight. The AT, receptor belongs to the guanine nucleotide regulatory-proteincoupled receptor family and is coupled to the phospholipase C signal transduction pathway as evidenced by intracellular calcium mobilization and inositol trisphosphate production upon receptor activation. Cloning of the AT(1) receptor has facilitated the study of structure-function correlates and molecular mechanisms of receptor regulation, and will lead to substantial progress in elucidating the mechanisms governing Ang II actions.

Angiotensin II receptor blockade: an innovative approach to cardiovascular pharmacotherapy

Through the multiple actions of angiotensin II (AII), the renin-angiotensin system (RAS) participates in cardiovascular homeostasis. Angiotensin II acts by binding to specific membrane-bound receptors, which are coupled to one of several signal transduction pathways. These AII receptors exhibit heterogeneity, represented by AT1 and AT2 receptor subtypes. The AT1 receptor mediates the major cardiovascular action of the RAS. This receptor has been cloned from multiple species, disclosing features consistent with a transmembrane, G-protein-linked receptor. Further AII receptor heterogeneity is evident by the cloning of isotypes of the AT1 receptor. Blocking the interaction of AII with its receptor is the most direct site to inhibit the actions of the RAS. Many AII receptor antagonists, including peptide analogs of AII and antibodies directed against AII, possess unfavorable properties that have limited their clinical utility. The discovery and further development of imidazole compounds with AII antagonist properties and favorable characteristics, however, has promise for clinical utility. The leader in this field is a selective AT1 receptor antagonist losartan (previously known as DuP 753 or MK-954). Losartan was demonstrated to be an effective antagonist of many AII-induced actions and an effective antihypertensive agent in many animal models of hypertension (HTN). Losartan also demonstrated secondary benefits in preventing stroke, treating congestive heart failure (CHF), and delaying the progression of renal disease in animal models. Clinical studies confirm the AII antagonist action of losartan and suggest that losartan will be effective in the treatment of essential HTN. AII antagonism is likely to provide useful treatment in essential HTN and CHF, conditions in which the RAS is known to play a major role. The utility of AII antagonism may extend beyond that of HTN and CHF, as suggested by the potential usefulness of angiotensin-converting enzyme (ACE) inhibition in the treatment or prevention of many other diseases. The key advantage AII antagonists provide over ACE inhibitors is that they may avoid unwanted side effects, related to bradykinin potentiation with the latter drugs. The AII antagonists will help determine the role of the RAS in physiologic regulation and in the pathophysiology of various disease states.

The biology of angiotensin II receptors

Angiotensin II is very important in the regulation of blood pressure. This small peptide binds to cell surface receptors, initiating a wide diversity of physiologic responses. There are two major subtypes of angiotensin II receptors referred to as AT1 and AT2. In this article we describe the cloning and the biochemical characterization of the AT1 receptor. Antibodies against this receptor have been used to define its tissue distribution. The AT1 receptor is a member of the seven transmembrane spanning class of receptors. It initiates a complex series of signaling events, including activation of membrane phospholipases and intracellular kinases. In the human a single AT1 receptor protein mediates virtually all the effects of angiotensin II, suggesting that tissue specificity of angiotensin II must be due to organ-specific intracellular signaling.

A proposed bovine neuropeptide Y (NPY) receptor cDNA clone, or its human homologue, confers neither NPY binding sites nor NPY responsiveness on transfected cells

Receptors with seven transmembrane domains (7TM) constitute a large family of structurally and functionally related proteins which respond to various types of ligands. We describe here the cloning and expression of a human 7TM receptor, denoted hFB22 (human Fetal Brain 22), which is the homologue (92% amino acid identity) of a bovine receptor (LCR1) reported by others to bind neuropeptide Y (NPY) with a pharmacological profile of the Y3 receptor subtype. However, upon expression in COS1 (confirmed by Northern analysis), COS7 or CHO-K1 cells, the hFB22 receptor did not confer specific 125I-Bolton-Hunter-NPY, 3H-propionyl-NPY or 125I-peptide YY (PYY) binding sites, in either intact cells or in membrane preparations. Similarly, cells transfected with the corresponding bovine clone (LCR1) did not show specific NPY/PYY binding exceeding that resulting from endogenous binding sites; mock-transfected COS7 cells, used frequently for heterologous expression of receptors, were found to have endogenous specific 125I-NPY binding sites (Bmax = 112 fmol/mg protein; Kd = 0.25 nM). Moreover, the hFB22 transfected cells, when compared to control transfected cells, did not display de novo NPY- or PYY-induced second messenger responses, i.e., (1) inhibition of forskolin-stimulated cAMP accumulation or (2) 45Ca2+ influx. The presence of hFB22 mRNA was detected in several human neuroblastoma cell lines, none of which was found to express Y3-like NPY binding sites. hFB22 displays 39% amino acid sequence identity (in the transmembrane regions) to the human interleukin-8 receptor, and 32-36% amino acid identity to the human receptors of angiotensin II, bradykinin, and n-formylpeptide, but only 23% amino acid identity to the previously described human NPY/PYY receptor of the Y1 receptor subtype. Our results show that hFB22 and LCR1 do not encode NPY receptors, and their true ligand(s) remains to be identified.

Mechanism of pindolol-induced vasoconstriction in isolated and perfused dog coronary arteries

The mechanism of pindolol-induced vasoconstriction in isolated and perfused dog coronary arteries was studied. Single injections of pindolol (1-100 micrograms), propranolol (1-30 micrograms), and 5-hydroxytryptamine (5-HT, 0.001-1 microgram) produced a dose-related vasoconstriction in dog coronary arteries which were dilated by acetylcholine. l-Pindolol constricted coronary arteries, but d-pindolol did not. The responses to pindolol and propranolol were not affected by any of the following compounds (100 micrograms): bunazosin (a selective alpha 1-adrenergic antagonist), DG 5128 (a selective alpha 2-adrenergic antagonist), atropine (a muscarinic antagonist), chlorpheniramine (a selective H1-antagonist), cimetidine (a selective H2-antagonist), and ketanserin (a selective 5-HT2 antagonist). Methysergide (10 micrograms, a 5-HT1 and 5-HT2 antagonist) significantly reduced pindolol- and propranolol-induced vasoconstrictions, although it did not reduce norepinephrine-induced vasoconstriction in the presence of 5 microM propranolol. Methysergide (10 micrograms) and ketanserin (100 micrograms) significantly suppressed 5-HT-induced vasoconstriction. Diltiazem (100 micrograms, a calcium antagonist) and the incubation in Ca(2+)-free solution containing 1 mM EGTA for 1 hr significantly reduced the vasoconstrictions induced by pindolol and propranolol. The Ca(2+)-free solution containing 1 mM EGTA abolished the vasoconstriction induced by 5-HT in the presence of 1 microM ketanserine. In a solution containing 20 mM KCl, the vasoconstrictions caused by pindolol and propranolol were enhanced in dog coronary arteries. These results indicate that the direct contractile effects of pindolol on dog coronary arteries are mediated, at least partly, through 5-HT1-like receptors, but not through alpha-adrenergic receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization and regulation of the angiotensin II type-1 receptor (binding and mRNA) in human adrenal fasciculata-reticularis cells

The classical concept of human adrenal physiology indicates that only glomerulosa cells are the target of A-II. Herein, we demonstrated that cultured human adrenal fasciculata-reticularis cells were also responsive to this hormone. Indeed, these cells contained high affinity (Kd = 0.9-1.1 nM) and low capacity (8,000-13,000 sites/cell) A-II receptors, and more than 95% of them were of the type-1. These AT1 receptors are functional since A-II was able to increase cortisol production after 48 h of treatment. These effects were inhibited by losartan, an AT1 antagonist, but not by CGP42112A, an AT2 antagonist. The expression of the type-1 A-II receptor mRNA was detected in the whole adrenal in both adult and fetus, and in cultured human adrenal fasciculata-reticularis cells. In these cells A-II negatively regulated AT1 receptor mRNA, and this effect was also mediated through the AT1 receptor subtype.

Characterization of [3H]SK&F 108566 as a radioligand for angiotensin type-1 receptor

Rat aortic smooth muscle cells were used as a model system to characterize the binding properties of [3H]SK&F 108566, an angiotensin type-1 (AT1) receptor antagonist. The binding was specific, saturable and reversible. The association and dissociation rates of [3H]SK&F 108566 binding to smooth muscle cells were monophasic and Scatchard analysis of equilibrium binding data yielded a linear plot indicating a homogenous population of binding sites. The maximum binding (Bmax) and apparent dissociation constant (Kd) were 22,000 +/- 6000 sites/cell and 0.83 +/- 0.08nM respectively. The pharmacological specificity of [3H]SK&F 108566 binding to smooth muscle cells is consistent with that observed for AT1 and confirms AT1 receptor specificity of this radioligand. High affinity binding was observed in membranes prepared from bovine adrenal cortex, rat liver and rat kidney glomeruli. COS cells transfected with cDNA encoding human AT1 angiotensin II receptors also displayed high affinity binding site for [3H]SK&F 108566. No specific binding could be detected on membranes prepared from bovine cerebellum, a tissue rich in the angiotensin type-2 (AT2) receptor. These observations indicate that [3H]SK&F 108566 binds to sites which have pharmacological characteristics of angiotensin II AT1 subtype receptors and can be used as a subtype-selective radioligand to characterize AII receptors in various systems.

Studies on the peptides encoded by rat and human angiotensin II complementary RNA

Some evidence suggests that RNA complementary to the messenger RNA encoding a peptide hormone encodes a complementary peptide that binds the original peptide hormone. The objective of this investigation was to assess in vivo the ability of complementary angiotensin II (II Ang) peptides to block the biological effects of angiotensin II (Ang II). Increasing concentrations of rat or human II Ang were preincubated with Ang II for 2 hours, and this solution was then infused intra-arterially into the superior mesenteric artery. Human, but not rat, II Ang dose-dependently inhibited Ang II-induced mesenteric vasoconstriction. The in vivo inhibitory potencies of human II Ang and [Sar1,Ile8]Ang II, with respect to inhibition of the pressor response to Ang II, were compared by infusing intravenously increasing doses of each blocker and determining their effects on a fixed intravenous dose of Ang II. Although human II Ang could abolish the pressor response to Ang II, [Sar1,Ile8]Ang II was approximately 100 times more potent in this regard. A fixed dose of human II Ang (150 micrograms/min i.v.) inhibited the effects of increasing doses of Ang II on mesenteric vascular resistance, arterial blood pressure, and aldosterone secretion. The 1H nuclear magnetic resonance spectra of human II Ang and Ang II were determined both separately and when combined in the same cuvette. The spectrum obtained by overlaying the separate spectra for these two peptides was the same as the spectrum obtained from the mixture of these two peptides in the same cuvette.(ABSTRACT TRUNCATED AT 250 WORDS)