The ligand binding site of the angiotensin AT1 receptor

Targeting the Renin-angiotensin-aldosterone System (RAAS) for Cardiovascular Protection and Enhanced Oncological Outcomes: Review

OPINION STATEMENT

The renin-angiotensin-aldosterone system (RAAS) is a crucial regulator of the cardiovascular system and a target for widely used therapeutic drugs. Dysregulation of RAAS, implicated in prevalent diseases like hypertension and heart failure, has recently gained attention in oncological contexts due to its role in tumor biology and cardiovascular toxicities (CVTs). Thus, RAAS inhibitors (RAASi) may be used as potential supplementary therapies in cancer treatment and CVT prevention. Oncological treatments have evolved significantly, impacting patient survival and safety profiles. However, they pose cardiovascular risks, necessitating strategies for mitigating adverse effects. The main drug classes used in oncology include anthracyclines, anti-HER2 therapies, immune checkpoint inhibitors (ICIs), and vascular endothelial growth factor (VEGF) signaling pathway inhibitors (VSPI). While effective against cancer, these drugs induce varying CVTs. RAASi adjunctive therapy shows promise in enhancing clinical outcomes and protecting the cardiovascular system. Understanding RAAS involvement in cancer and CVT can inform personalized treatment approaches and improve patient care.

Endocytosis and signaling of angiotensin II type 1 receptor

A vasoactive octapeptide angiotensin II (Ang II) hormone is the key regulator of the renin-angiotensin system (RAS). It binds with the two different plasma membrane receptors like angiotensin II type 1 (AT1) and type 2 (AT2) and consequence various biological responses occur. Further, AT1 has two subtypes such as AT1A and AT1B. These angiotensin receptors are classified to be G protein-coupled receptors (GPCRs). The main constituent of RAS is the AT1 receptor (AT1R), and its activation, signal transduction, and regulation have been extensively studied. After Ang II stimulation, the ligand-receptor complexes internalized and trafficked through the early endosome, recycling endosome, and some receptors skipped the recycling endosome and trafficked to the lysosome for metabolic degradation. Moreover, some short sequence motifs located in the carboxyl-terminus (CT) of the receptor play a vital role in the internalization, phosphorylation, subcellular trafficking, signaling, and desensitization. Furthermore, in endocytosis, the various proteins interact with the CT region of the receptor. This chapter highlights the basic mechanism of AT1 receptor internalization, trafficking and signaling in both physiological and pathophysiological conditions.

Cartilage oligomeric matrix protein is an endogenous β-arrestin-2-selective allosteric modulator of AT1 receptor counteracting vascular injury

Compelling evidence has revealed that biased activation of G protein-coupled receptor (GPCR) signaling, including angiotensin II (AngII) receptor type 1 (AT1) signaling, plays pivotal roles in vascular homeostasis and injury, but whether a clinically relevant endogenous biased antagonism of AT1 signaling exists under physiological and pathophysiological conditions has not been clearly elucidated. Here, we show that an extracellular matrix protein, cartilage oligomeric matrix protein (COMP), acts as an endogenous allosteric biased modulator of the AT1 receptor and its deficiency is clinically associated with abdominal aortic aneurysm (AAA) development. COMP directly interacts with the extracellular N-terminus of the AT1 via its EGF domain and inhibits AT1-β-arrestin-2 signaling, but not Gq or Gi signaling, in a selective manner through allosteric regulation of AT1 intracellular conformational states. COMP deficiency results in activation of AT1a-β-arrestin-2 signaling and subsequent exclusive AAA formation in response to AngII infusion. AAAs in COMP^-/- or ApoE^-/- mice are rescued by AT1a or β-arrestin-2 deficiency, or the application of a peptidomimetic mimicking the AT1-binding motif of COMP. Explorations of the endogenous biased antagonism of AT1 receptor or other GPCRs may reveal novel therapeutic strategies for cardiovascular diseases.

The ACE2/Ang (1-7)/MasR axis as an emerging target for antihypertensive peptides

Food protein-derived bioactive peptides, particularly antihypertensive peptides, are important constituents of functional foods or nutraceuticals. Most antihypertensive are identified as the inhibitors of angiotensin converting enzyme (ACE), a key enzyme responsible for the generation of angiotensin II (Ang II), which is a vasoconstricting peptide. Hence, ACE has long been used as a universal target to identify antihypertensive peptides. Angiotensin converting enzyme 2 (ACE2), is a homolog of ACE but uses Ang II as its key substrate to produce angiotensin (1-7), exerting vasodilatory activity via the mas receptor (MasR). Therefore, ACE2 functions in the opposite way as ACE and is an emerging novel target for cardiovascular therapy. The potential of food protein-derived bioactive peptides in targeting ACE2 has been rarely explored. While, recently we found that IRW, an egg white ovotransferrin-derived antihypertensive peptide, reduced blood pressure in spontaneously hypertensive rats via the ACE2/Ang (1-7)/MasR axis, indicating a new mechanism of food protein-derived bioactive peptides in reducing blood pressure. The objectives of this review are to summarize the functions of the ACE2/Ang (1-7)/MasR axis and to examine its potential roles in the actions of food protein-derived antihypertensive peptides. The interaction between antihypertensive peptides and the ACE2/Ang (1-7)/MasR axis will also be discussed.

International Union of Basic and Clinical Pharmacology. XCIX. Angiotensin Receptors: Interpreters of Pathophysiological Angiotensinergic Stimuli [corrected]

The renin angiotensin system (RAS) produced hormone peptides regulate many vital body functions. Dysfunctional signaling by receptors for RAS peptides leads to pathologic states. Nearly half of humanity today would likely benefit from modern drugs targeting these receptors. The receptors for RAS peptides consist of three G-protein-coupled receptors—the angiotensin II type 1 receptor (AT1 receptor), the angiotensin II type 2 receptor (AT2 receptor), the MAS receptor—and a type II trans-membrane zinc protein—the candidate angiotensin IV receptor (AngIV binding site). The prorenin receptor is a relatively new contender for consideration, but is not included here because the role of prorenin receptor as an independent endocrine mediator is presently unclear. The full spectrum of biologic characteristics of these receptors is still evolving, but there is evidence establishing unique roles of each receptor in cardiovascular, hemodynamic, neurologic, renal, and endothelial functions, as well as in cell proliferation, survival, matrix-cell interaction, and inflammation. Therapeutic agents targeted to these receptors are either in active use in clinical intervention of major common diseases or under evaluation for repurposing in many other disorders. Broad-spectrum influence these receptors produce in complex pathophysiological context in our body highlights their role as precise interpreters of distinctive angiotensinergic peptide cues. This review article summarizes findings published in the last 15 years on the structure, pharmacology, signaling, physiology, and disease states related to angiotensin receptors. We also discuss the challenges the pharmacologist presently faces in formally accepting newer members as established angiotensin receptors and emphasize necessary future developments.

Multivalent targeting of AT1 receptors with angiotensin II-functionalized nanoparticles

The angiotensin II receptor type 1 (AT1R) is a G protein-coupled receptor of paramount significance since it is overexpressed in a number of diseased tissues that are highly attractive for nanoparticle targeting. However, it is also expressed at physiological levels in healthy tissue. Multivalent interactions mediated by multiple AT1R-binding moieties per nanoparticle could promote a high binding avidity to AT1R overexpressing cells and concomitantly spare off-target tissue. To investigate the feasibility of this approach, angiotensin II was thiolated and conjugated to PEGylated quantum dots. Nanoparticle binding, uptake and affinity to several cell lines was investigated in detail. The colloids were rapidly taken up by clathrin-mediated endocytosis into AT1R-expressing cells and showed no interaction with receptor negative cells. The EC50 of the thiolated angiotensin II was determined to be 261 nM, whereas the ligand-conjugated Qdots activated the receptor with an EC50 of 8.9 nM. This 30-fold higher affinity of the nanoparticles compared to the unconjugated peptide clearly demonstrated the presence of multivalent effects when using agonist-targeted nanoparticles. Our study provides compelling evidence that, despite being immediately endocytosed, Ang II-coupled nanoparticles exert potent multivalent ligand-receptor interactions that can be used to establish high affinities to an AT1R overexpressing cell and tissue.

Structural determinants for binding, activation, and functional selectivity of the angiotensin AT1 receptor

The renin-angiotensin system (RAS) plays an important role in the pathophysiology of cardiovascular disorders. Pharmacologic interventions targeting the RAS cascade have led to the discovery of renin inhibitors, angiotensin-converting enzyme inhibitors, and AT(1) receptor blockers (ARBs) to treat hypertension and some cardiovascular and renal disorders. Mutagenesis and modeling studies have revealed that differential functional outcomes are the results of multiple active states conformed by the AT(1) receptor upon interaction with angiotensin II (Ang II). The binding of agonist is dependent on both extracellular and intramembrane regions of the receptor molecule, and as a consequence occupies more extensive area of the receptor than a non-peptide antagonist. Both agonist and antagonist bind to the same intramembrane regions to interfere with each other's binding to exhibit competitive, surmountable interaction. The nature of interactions with the amino acids in the receptor is different for each of the ARBs given the small differences in the molecular structure between drugs. AT(1) receptors attain different conformation states after binding various Ang II analogues, resulting in variable responses through activation of multiple signaling pathways. These include both classical and non-classical pathways mediated through growth factor receptor transactivations, and provide cross-communication between downstream signaling molecules. The structural requirements for AT(1) receptors to activate extracellular signal-regulated kinases 1 and 2 through G proteins, or G protein-independently through β-arrestin, are different. We review the structural and functional characteristics of Ang II and its analogs and antagonists, and their interaction with amino acid residues in the AT(1) receptor.

Structural analysis of three peptides related to the transmambranic helix VI of AT1 receptor

INTRODUCTION

Angiotensin II (AII) is the main active product of the renin angiotensin system. Better known effects of AII are via AT1 receptor (AT1R). Expression of AT1R mutants (L265D and L262D) in CHO cells increased cAMP formation when compared to CHO cells expressing the wild type (WT) AT1R. Morphological transformation of CHO cells transfected with mutants correlated with their increased cAMP formation. DNA synthesis was inhibited in these cells too, indicating that cAMP promotes inhibitory effects on transfected CHO cells growth and causes their morphological change from a tumorigenic phenotype to a non-tumorigenic one.

OBJECTIVES

To assess the importance of leucine 262 and 265 in determining AT1R structure by means of a comparative structural analysis of two mutant peptides and of a wild-type fragment.

METHODOLOGY

Three peptides had their conformation compared by circular dichroism (CD): L262D(259-272), L265D(259-272) (mutants) and WT(260-277).

RESULTS

Secondary structures were: beta-turn for WT and L262D and random coil for L265D.

CONCLUSIONS

Strong correlation was found in the results of biochemical, cellular and structural approaches used to compare WT AT1R to mutant types. Random coil structure of the L265D mutant may be a key point to explain those changes observed in biochemical (binding and signal transduction) and proliferation assays (Correa et al., 2005). beta-Turn formation is an important step during early protein folding and this secondary simple structure is present in L262D and WT, but not in L265D. Therefore, leucine 265 seems to play a crucial role in determining an entirely functional AT1R.

Directed evolution of angiotensin II-inhibiting peptides using a microbead display

Angiotensin II (ang II), an octapeptide (DRVYVHPF), can regulate blood pressure by binding specifically to its receptor, AT1. A peptide (VVIVIY) in the first transmembrane of AT1 has been found, via peptide array technology, to have an affinity for ang II. In this study, the peptide P2, which contained the VVIVIY sequence, was mutated and screened using microbead display technology that utilized emulsion PCR and cell-free protein synthesis. After one round of screening, the binding activities of collected mutants were estimated using flow cytometry and a peptide array. Two of these exhibited improved association rate constants to ang II, compared to the P2 peptide.

Molecular mechanisms of vasoconstrictor action of imidazo[1,2-alpha]benzimidazole derivative RU-1117 possessing local anesthetic properties

We studied the mechanisms of action of imidazobenzimidazole derivative RU-1117 on calcium homeostasis in myocytes isolated from rat thoracic aorta. In therapeutic concentrations, RU-1117 increased the content of free calcium ions due to their mobilization from intracellular depot via the IP3-dependent mechanisms. Antagonists of angiotensin II AT1-receptors irbesartan and, to greater extent, eprosartan abolished the calcium-mobilizing action of RU-1117. Selective antagonist of endothelin ETA-receptors sitaxsentan and alpha1-adrenoceptor agonist prazosin produced no effect on calcium mobilization caused by novel local anesthetic RU-1117.

AT1 receptor blocker-insensitive mutant AT1A angiotensin receptors reveal the presence of G protein-independent signaling in C9 cells

Although mutant receptors are highly useful to dissect the signal transduction pathways of receptors, they are difficult to study in physiological target tissues, due to the presence of endogenous receptors. To study AT(1) angiotensin receptors in their physiological environment, we constructed a mutant receptor, which differs only from the AT(1A) receptor in its reduced affinity for candesartan, a biphenylimidazole antagonist. We have determined that the conserved S109Y substitution of the rat AT(1A) receptor eliminates its candesartan binding, without exerting any major effect on its angiotensin II and peptide angiotensin receptor antagonist binding, internalization kinetics, beta-arrestin binding, and potency or efficacy of the inositol phosphate response. To demonstrate the usefulness of this mutant receptor in signal transduction studies, we combined it with substitution of the highly conserved DRY sequence with AAY, which abolishes G protein activation. In rat C9 hepatocytes the S109Y receptor caused ERK activation with the same mechanism as the endogenous AT(1) receptor. After combination with the DRY/AAY mutation G protein-independent ERK activation was detected demonstrating that this approach can be used to study the angiotensin II-stimulated signaling pathways in cells endogenously expressing AT(1) receptors.

Interactions between Conserved Residues in Transmembrane Helices 2 and 7 during Angiotensin AT1Receptor Activation

Site-directed mutagenesis studies and independent molecular modeling studies were combined to investigate the network of inter-residue interactions within the transmembrane region of the angiotensin AT(1a) receptor. Site-directed mutagenesis was focused on residues Tyr292, Asn294, Asn295, and Asn298 in transmembrane helix 7, and the conserved Asp74 in helix 2 and other polar residues. Functional interactions between pairs of residues were evaluated by determining the effects of single and double-reciprocal mutations on agonist-induced AT(1a) receptor activation. Replacement of Tyr292 by aspartate in helix 7 abolished radioligand binding to both Y292D and D74Y/Y292D mutant receptors. Reciprocal mutations of Asp74/Asn294, Ser115/Asn294, Ser252/Asn294, and Asn298/Sen115 caused additive impairment of function, suggesting that these pairs of residues make independent contributions to AT(1a) receptor activation. In contrast, mutations of the Asp74/Tyr298 pair revealed that the D74N/N298D reciprocal mutation substantially increased the impaired inositol phosphate responses of the D74N and N298D receptors. Extensive molecular modeling yielded 3D models of the TM region of the AT(1) receptor and the mutants as well as of their complexes with angiotensin II, which were used to rationalize the possible reasons of impairing of function of some mutants. These data indicate that Asp74 and Asn298 are not optimally positioned for direct strong interaction in the resting conformation of the AT(1a) receptor. Balance of interactions between residues in helix 2 (as D74) and helix 7 (as N294, N295 and N298) in the AT(1) receptors, however, has a crucial role both in determining their functional activity and levels of their expression.

Ligand-supported homology modeling of the human angiotensin II type 1 (AT1) receptor: Insights into the molecular determinants of telmisartan binding

Angiotensin II type 1 (AT(1)) receptor belongs to the super-family of G-protein-coupled receptors, and antagonists of the AT(1) receptor are effectively used in the treatment of hypertension. To understand the molecular interactions of these antagonists, such as losartan and telmisartan, with the AT(1) receptor, a homology model of the human AT(1) (hAT(1)) receptor with all connecting loops was constructed from the 2.6 A resolution crystal structure (PDB i.d., 1L9H) of bovine rhodopsin. The initial model generated by MODELLER was subjected to a stepwise ligand-supported model refinement. This protocol involved initial docking of non-peptide AT(1) antagonists in the putative binding site, followed by several rounds of iterative energy minimizations and molecular dynamics simulations. The final model was validated based on its correlation with several structure-activity relationships and site-directed mutagenesis data. The final model was also found to be in agreement with a previously reported AT(1) antagonist pharmacophore model. Docking studies were performed for a series of non-peptide AT(1) receptor antagonists in the active site of the final hAT(1) receptor model. The docking was able to identify key molecular interactions for all the AT(1) antagonists studied. Reasonable correlation was observed between the interaction energy values and the corresponding binding affinities of these ligands, providing further validation for the model. In addition, an extensive unrestrained molecular dynamics simulation showed that the docking-derived bound pose of telmisartan is energetically stable. Knowledge gained from the present studies can be used in structure-based drug design for developing novel ligands for the AT(1) receptor.

ATP increases intracellular calcium in supraoptic neurons by activation of both P2X and P2Y purinergic receptors

ATP increases intracellular calcium concentration ([Ca(2+)](i)) in supraoptic nucleus (SON) neurons in hypothalamo-neurohypophyseal system explants loaded with the Ca(2+)-sensitive dye, fura 2-AM. Involvement of P2X purinergic receptors (P2XR) in this response was anticipated, because ATP stimulation of vasopressin release from hypothalamo-neurohypophyseal system explants required activation of P2XRs, and activation of P2XRs induced an increase in [Ca(2+)](i) in dissociated SON neurons. However, the ATP-induced increase in [Ca(2+)](i) persisted after removal of Ca(2+) from the perifusate ([Ca(2+)](o)). This suggested involvement of P2Y purinergic receptors (P2YR), because P2YRs induce Ca(2+) release from intracellular stores, whereas P2XRs are Ca(2+)-permeable ion channels. Depletion of [Ca(2+)](i) stores with thapsigargin (TG) prevented the ATP-induced increase in [Ca(2+)](i) in zero, but not in 2 mM [Ca(2+)](o), indicating that both Ca(2+) influx and release of intracellular Ca(2+) contribute to the ATP response. Ca(2+) influx was partially blocked by cadmium, indicating a contribution of voltage-gated Ca(2+) channels. PPADS (pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid), and iso-PPADS, P2XR antagonists, attenuated, but did not abolish, the ATP-induced increase in [Ca(2+)](i). Combined treatment with PPADS or iso-PPADS and TG prevented the response. A cocktail of P2YR agonists consisting of UTP, UDP, and 2-methylthio-ADP increased [Ca(2+)](i) (with or without tetrodotoxin) that was markedly attenuated by TG. 2-Methylthio-ADP alone induced consistent and larger increases in [Ca(2+)](i) than UTP or UDP. MRS2179, a specific P2Y(1)R antagonist, eliminated the response to ATP in zero [Ca(2+)](o). Thus, both P2XR and P2YR participate in the ATP-induced increase in [Ca(2+)](i), and the P2Y(1)R subtype is more prominent than P2Y(2)R, P2Y(4)R, or P2Y(6)R in SON.

Synthesis, binding studies and in vivo biological evaluation of novel non-peptide antihypertensive analogues

AT(1) antagonists (SARTANs) constitute the last generation of drugs for the treatment of hypertension, designed and synthesized to mimic the C-terminal segment of the vasoconstrictive hormone angiotensin II (AngII). They exert their action by blocking the binding of AngII on the AT(1) receptor. Up to date eight AT(1) antagonists have been approved for the regulation of high blood pressure. Although these molecules share common structural features and are designed to act under the same mechanism, they have differences in their pharmacological profiles and antihypertensive efficacy. Thus, there is still a need for novel analogues with better pharmacological and financial profiles. An example of a novel synthetic non peptide AT(1) antagonist which devoids the classical template of SARTANs is MM1. In vivo studies showed that MMK molecules, which fall in the same class of MM1, had a significant antihypertensive (40-80% compared to the drug losartan) activity. However, in vitro affinity studies showed that losartan has considerably higher affinity. The theoretical docking studies showed that MM1 acts on the same site of the receptor as losartan. They exert hydrophobic interactions with amino acid Val108 of the third helix of the AT(1) receptor and other hydrophobic amino acids in spatial vicinity. In addition, losartan favours multiple hydrogen bondings between its tetrazole group with Lys199. These additional interactions may in part explain its higher in vitro binding affinity.

Molecular mechanisms of constitutive activity: mutations at position 111 of the angiotensin AT1 receptor

A possible molecular mechanism for the constitutive activity of mutants of the angiotensin type 1 receptor (AT1) at position 111 was suggested by molecular modeling. This involves a cascade of conformational changes in spatial positions of side chains along transmembrane helix (TM3) from L112 to Y113 to F117, which in turn, results in conformational changes in TM4 (residues I152 and M155) leading to the movement of TM4 as a whole. The mechanism is consistent with the available data of site-directed mutagenesis, as well as with correct predictions of constitutive activity of mutants L112F and L112C. It was also predicted that the double mutant N111G/L112A might possess basal constitutive activity comparable with that of the N111G mutant, whereas the double mutants N111G/Y113A, N111G/F117A, and N111G/I152A would have lower levels of basal activity. Experimental studies of the above double mutants showed significant constitutive activity of N111G/L112A and N111G/F117A. The basal activity of N111G/I152A was higher than expected, and that of N111G/Y113A was not determined due to poor expression of the mutant. The proposed mechanism of constitutive activity of the AT(1) receptor reveals a novel nonsimplistic view on the general problem of constitutive activity, and clearly demonstrates the inherent complexity of the process of G protein-coupled receptor (GPCR) activation.

Pharmacological concentration of angiotensin-(1-7) activates NADPH oxidase after ischemia-reperfusion in rat heart through AT1 receptor stimulation

The cardiovascular role of angiotensin-(1-7), especially in the functional and metabolic alterations associated with ischemia-reperfusion (IR), is still not clearly defined. Our objective was to evaluate the cardiac effects of angiotensin-(1-7), the receptors involved, and their relationships with NADPH oxidase activation under non-ischemic conditions and, during an ischemia-reperfusion sequence. Isolated perfused rat hearts underwent 45 min of non-ischemic perfusion, or 30 min of global ischemia followed by 30 min of reperfusion. Angiotensin-(1-7) and/or AT1 receptor blocker losartan or angiotensin-(1-7) receptor antagonist (D-Ala7)-angiotensin-(1-7) were perfused. Our results showed that angiotensin-(1-7) was without effect at low concentrations (10(-10) to 10(-7) M). At a pharmacological concentration, 0.5 microM angiotensin-(1-7) induced vasoconstriction, which was antagonised by losartan. After ischemia, we noted a partial recovery of functional parameters, which was not modified by any of the treatments. The expression of AT1 receptor mRNA was increased by ischemia-reperfusion, except in (D-Ala7)-angiotensin-(1-7) treated hearts. Angiotensin-(1-7) further increased the AT1 expression. NADPH oxidase activity was enhanced in 0.5 microM angiotensin-(1-7)-treated hearts subjected to ischemia-reperfusion, this effect was totally reversed by losartan. This is the first time that it has been shown that, in the heart, angiotensin-(1-7) at pharmacological concentration activates NADPH oxidase, an enzyme thought to be involved in several angiotensin II effects.

Effect of angiotensin II type 2 receptor blockade on mitogen activated protein kinases during myocardial ischemia-reperfusion

Mitogen-activated protein kinases (MAPKs) have been implicated during ischemia-reperfusion (IR) and angiotensin II (AngII) type 2 receptor (AT2R) blockade has been shown to induce cardioprotection involving protein kinase Cepsilon (PKCepsilon) signaling after IR. We examined whether the 3 major MAPKs, p38, c-Jun NH2-terminal kinase (JNK-1 and JNK-2), and extracellular signal regulated kinases (ERK-1 and ERK-2) are activated after IR and whether treatment with the AT2R antagonist PD123,319 (PD) alters their expression. Isolated rat hearts were randomized to control (aerobic perfusion, 80 min), IR (no drug; 50 min of perfusion, 30 min global ischemia and 30 min reperfusion; working mode), and IR + PD (0.3 micromol/l) and left ventricular (LV) work was measured. We measured LV tissue content of p38, p-p38, p-JNK-1 (54 kDa), p-JNK-2 (46 kDa), p-ERK-1 (44 kDa), p-ERK-2 (42 kDa) and PKCepsilon proteins by immunoblotting and cGMP by enzyme immunoassay. IR resulted in significant LV dysfunction, increase in p-p38 and p-JNK-1/-2, no change in p-ERK-1/-2 or PKCepsilon, and decrease in cGMP. PD improved LV recovery after IR, induced a slight increase in p-p38 (p < 0.01 vs. control), normalized p-JNK-1, did not change p-ERK-1/-2, and increased PKCepsilon and cGMP. The overall results suggest that p38 and JNK might play a significant role in acute IR injury and the cardioprotective effect of AT2R blockade independent of ERK. The activation of p38 and JNKs during IR may be linked, in part, to AT2R stimulation.

Identification of Substituted 6-Amino-4-phenyltetrahydroquinoline Derivatives: Potent Antagonists for the Follicle-Stimulating Hormone Receptor

Substituted 6-amino-4-phenyl-tetrahydroquinoline derivatives are described that are antagonists for the G(s)-protein-coupled human follicle-stimulating hormone (FSH) receptor. These compounds show high antagonistic efficacy in vitro using a CHO cell line expressing the human FSH receptor. Antagonist 10 also showed a submicromolar IC(50) in a more physiologically relevant rat granulosa cell assay and was found to significantly inhibit follicle growth and ovulation in an ex vivo mouse model. This compound class may open the way toward a novel, nonsteroidal approach for contraception.

Mutagenesis of the AT1 receptor reveals different binding modes of angiotensin II and [Sar1]-angiotensin II

Homology modeling of the structure of the AT1 receptor, based on the high resolution rhodopsin crystal structure, indicated that it is unlikely that the binding of AngII to AT1 involves simultaneously all the receptor's residues reported in the literature to participate in this process. Site-directed mutagenesis using Ala substitution of charged residues Lys20, Arg23, Glu91 and Arg93 was performed to evaluate the participation of their side-chains in ligand binding and in triggering the cell's response. A comparative analysis by competition binding and functional assays using angiotensin II and the analog [Sar1]-angiotensin II suggests an important role for Arg23 of AT1 receptor in binding of the natural agonist. It is discussed whether some receptor's residues participate directly in the binding with AngII or whether they are part of a regulatory site.

Structural determinants of agonist-induced signaling and regulation of the angiotensin AT1 receptor

Angiotensin II (Ang II) regulates aldosterone secretion by stimulating inositol phosphate production and Ca(2+) signaling in adrenal glomerulosa cells via the G(q)-coupled AT(1) receptor, which is rapidly internalized upon agonist binding. Ang II also binds to the heptahelical AT(2) receptor, which neither activates inositol phosphate signaling nor undergoes receptor internalization. The differential behaviors of the AT(1) and AT(2) receptors were analyzed in chimeric angiotensin receptors created by swapping the second (IL2), the third (IL3) intracellular loops and/or the cytoplasmic tail (CT) between these receptors. When transiently expressed in COS-7 cells, the chimeric receptors showed only minor alterations in their ligand binding properties. Measurements of the internalization kinetics and inositol phosphate responses of chimeric AT(1A) receptors indicated that the CT is required for normal receptor internalization, and IL2 is a determinant of G protein activation. In addition, the amino-terminal portion of IL3 is required for both receptor functions. However, only substitution of IL2 impaired Ang II-induced ERK activation, suggesting that alternative mechanisms are responsible for ERK activation in signaling-deficient mutant AT(1) receptors. Substitution of IL2, IL3, or CT of the AT(1A) receptor into the AT(2) receptor sequence did not endow the latter with the ability to internalize or to mediate inositol phosphate signaling responses. These data suggest that the lack of receptor internalization and inositol phosphate signal generation by the AT(2) receptor is a consequence of its different activation mechanism, rather than the inability of its cytoplasmic domains to couple to intracellular effectors.

Pharmacological properties of angiotensin II receptors in cultured rabbit gingival fibroblasts

We demonstrated that angiotensin II (Ang II, 10-1000 nM) induced proliferation of cultured rabbit gingival fibroblasts in a concentration-dependent manner. The Ang II-induced proliferation was inhibited by CV-11974 (AT1 antagonist; 1 microM) and saralasin (AT1/AT2 antagonist; 1 microM), but not by PD123,319 (AT2 antagonist; 1 microM), suggesting that Ang II-induced proliferation was mediated via AT1 receptors present in and/or on gingival fibroblasts. The results of Western blot analysis indicated the presence of AT1 and AT2 receptors in/on the fibroblasts. In a subsequent radioligand binding assay, the binding of [3H]Ang II to the fibroblasts was specific and saturable with both high- and low-affinity sites. Competition binding experiments indicated that Ang II completely displaced [3H]Ang II binding, and CV-11974 and PD123,319 maximally displaced up to approximately 63% and 37% of the total binding, respectively. Ang II and CV-11974 completely displaced the [3H]DuP753 binding but PD123,319 did not, indicating a single population of binding site. These findings demonstrate that gingival fibroblasts contain both AT1 and AT2 receptor subtypes for Ang II, and support that Ang II stimulation of AT1 receptors results in proliferation of the fibroblasts.

Angiotensin II inhibitory peptide found in the receptor sequence using peptide array

Peptide array consisting of hundreds of peptides spatially addressed and synthesized on a cellulose membrane support was used to screen ligand-inhibitory peptides. As a model, angiotensin II (Ang II), a significant peptide related to the treatment of cardiovascular diseases, was chosen as the target ligand. Peptide arrays covering the Ang II receptor type 1 sequence were prepared, and peptide domains with high affinity to the Ang II fluorescein conjugate were investigated. The peptide (VVIVIY) within the first transmembrane region exhibited the highest affinity to Ang II. The synthesized soluble VVIVIY peptide had an 84% inhibitory effect on Ang II-induced aorta contraction. These results indicate that our screening strategy utilizing peptide array is an effective approach for the peptide drug development.

Altered Effects of Angiotensin II Type 1 and Type 2 Receptor Blockers on Cardiac Norepinephrine Release and Inotropic Responses During Cardiac Sympathetic Nerve Stimulation in Aorto-Caval Shunt Rats

BACKGROUND

Inhibition of the sympathetic nervous and renin - angiotensin systems has become an important strategy in the treatment of chronic heart failure. However, direct evidence of how inhibition of the renin - angiotensin system alters sympathetic activity in a diseased heart is lacking.

METHODS AND RESULTS

Four weeks after abdominal aorto-caval (AV) shunting or sham operation in rats, the hearts were retrogradely perfused in vivo and the left ventricles contracted isovolumetrically at 300 beats/min. Sympathetic nerve stimulation (SNS) was performed in the baseline state and repeated with an infusion of the angiotensin II (A-II) type 1 receptor (AT(1)-R) blocker, losartan, the A-II type 2 receptor (AT(2)-R) blocker, PD123319, or A-II. Norepinephrine (NE) overflow and left ventricular (LV) inotropic responses during baseline SNS were lower in the AV shunt rats. Losartan did not change the NE overflow or the LV inotropic responses to SNS in the sham rats, but did increase them in the AV shunt rats. PD123319 changed neither parameter in the sham rats, but decreased both in the AV shunt rats. A-II enhanced the NE overflow but attenuated the LV inotropic responses to SNS in the sham rats, but attenuated both in the AV shunt rats.

CONCLUSIONS

The effects of A-II via the AT(1)-R and AT(2)-R on the adrenergic drive in the heart were altered significantly in volume overload hypertrophy induced by AV shunting.

The constitutively active N111G-AT1 receptor for angiotensin II maintains a high affinity conformation despite being uncoupled from its cognate G protein Gq/11alpha

Asn111, localized in the third transmembrane domain of the AT1 receptor for angiotensin II, plays a critical role in stabilizing the inactive conformation of the receptor. We evaluated the functional and G protein-coupling properties of mutant AT1 receptors in which Asn111 was substituted with smaller (Ala or Gly) or larger residues (Gln or Trp). All four mutants were expressed at high levels in COS-7 cells and, except for N111W-AT1, recognized 125I-Ang II with high affinities comparable to that of the wild-type AT1 receptor. In phospholipase C assays, the four mutants encompassed the entire spectrum of functional states, ranging from constitutive activity (without agonist) for N111A-AT1 and N111G-AT1 to a significant loss of activity (upon maximal stimulation) for N111Q-AT1 and a major loss of activity for N111W-AT1. In Ca2+ mobilization studies, N111W-AT1 produced a weak Ca2+ transient and, unexpectedly, N111G-AT1 also produced a Ca2+ transient that was much weaker than that of the wild-type AT1. The agonist binding affinity of N111W-AT1 was not modified in the presence of GTPgamma S, suggesting that this receptor is not basally coupled to a G protein. GTPgamma S did not modify the high agonist-binding affinity of N111G-AT1 but abolished the coimmunoprecipitation of Gq/11alpha with this constitutively active mutant receptor. These results are a direct demonstration that the N111G-AT1 receptor maintains a high affinity conformation despite being uncoupled from the G protein Gq/11.

Tachyphylactic properties of angiotensin II analogs with bulky and hydrophobic substituents at the N-terminus

Tachyphylaxis, defined as the acute loss of response of some smooth muscles upon repeated stimulations with angiotensin II (Ang II), has been shown to be dependent mainly on the N-terminal region of the ligand. To further study the structural requirements for the induction of tachyphylaxis we have synthesized Ang II analogs containing the bulky and very lipophilic substituents 9-fluorenylmethyloxycarbonyl (Fmoc) and 9-fluorenylmethyl ester (OFm) at the alpha-amino (Nalpha-Fmoc-Ang II) or the beta-carboxyl ([Asp(OFm)1]-Ang II) groups of the Asp1 residue, respectively. In binding assays with Chinese hamster ovary cells transfected with the AT1 Ang II receptor, Nalpha-Fmoc-Ang II bound with high affinity, whereas [Asp(OFm)1]-Ang II showed lower affinity. In biological assays, these two analogs were full agonists and showed 30 and 3%, respectively, of the Ang II potency in contracting the guinea-pig ileum smooth muscle. The two analogs induced tachyphylaxis, in spite of the lack of a free amino group in Nalpha-Fmoc-Ang II. Thus, analogs with Fmoc- or OFm-type groups coupled to the Asp1 residue, whether at the amino or carboxyl functions, induce tachyphylaxis through an unreported mechanism. Based in these findings and those available from the literature, an alternate molecular interaction mode between Ang II N-terminal portion and the AT1 receptor is proposed to explain the tachyphylactic phenomenon.

Interaction of angiotensin II with the C-terminal 300-320 fragment of the rat angiotensin II receptor AT1a monitored by NMR

Interaction between angiotensin II (Ang II) and the fragment peptide 300-320 (fCT300-320) of the rat angiotensin II receptor AT1a was demonstrated by relaxation measurements, NOE effects, chemical shift variations, and CD measurements. The correlation times modulating dipolar interactions for the bound and free forms of Ang II were estimated by the ratio of the nonselective and single-selective longitudinal relaxation rates. The intermolecular NOEs observed in NOESY spectra between HN protons of 9Lys(fCT) and 6His(ang), 10Phe(fCT) and 8Phe(ang), HN proton of 3Tyr(fCT) and Halpha of 4Tyr(ang), 5Phe(fCT)Hdelta and Halpha of 4Tyr(ang) indicated that Ang II aromatic residues are directly involved in the interaction, as also verified by relaxation data. Some fCT300-320 backbone features were inferred by the CSI method and CD experiments revealing that the presence of Ang II enhances the existential probability of helical conformations in the fCT fragment. Restrained molecular dynamics using the simulated annealing protocol was performed with intermolecular NOEs as constraints, imposing an alpha-helix backbone structure to fCT300-320 fragment. In the built model, one strongly preferred interaction was found that allows intermolecular stacking between aromatic rings and forces the peptide to wrap around the 6Leu side chain of the receptor fragment.

Constitutive activation of the angiotensin II type 1 receptor alters the spatial proximity of transmembrane 7 to the ligand-binding pocket

Activation of G protein-coupled receptors by agonists involves significant movement of transmembrane domains (TM) following binding of agonist. The underlying structural mechanism by which receptor activation takes place is largely unknown but can be inferred by detecting variability within the environment of the ligand-binding pocket, which constitutes a water-accessible crevice surrounded by the seven TM helices. Using the substituted cysteine accessibility method, we initially identified those residues within the seventh transmembrane domain (TM7) of wild type angiotensin II type 1 (AT1) receptor that contribute to forming the binding site pocket. We have substituted successively TM7 residues ranging from Ile276 to Tyr302 to cysteine. Treatment of A277C, V280C, T282C, A283C, I286C, A291C, and F301C mutant receptors with the charged sulfhydryl-specific alkylating agent MTSEA significantly inhibited ligand binding, which suggests that these residues orient themselves within the water-accessible binding pocket of the AT1 receptor. Interestingly, this pattern of acquired MTSEA sensitivity was greatly reduced for TM7 reporter cysteines engineered in a constitutively active mutant of the AT1 receptor. Our data suggest that upon activation, TM7 of the AT1 receptor goes through a pattern of helical movements that results in its distancing from the binding pocket per se. These studies support accumulating evidence whereby elements of TM7 of class A GPCRs promote activation of the receptor through structural rearrangements.

Endocytosis of the AT1A angiotensin receptor is independent of ubiquitylation of its cytoplasmic serine/threonine-rich region

Agonist-induced internalisation of the rat type 1A (AT(1A)) angiotensin II receptor is associated with phosphorylation of a serine/threonine-rich region in its cytoplasmic tail. In yeast, hyperphosphorylation of the alpha-factor pheromone receptor regulates endocytosis of the receptor by facilitating the monoubiquitylation of its cytoplasmic tail on lysine residues. The role of receptor ubiquitylation in AT(1A) receptor internalisation was evaluated by deletion or replacement of lysine residues in its agonist-sensitive serine/threonine-rich region. Expression of such receptor mutants in CHO cells showed that these modifications had no detectable effect on the angiotensin II-induced endocytosis of the AT(1A) receptor. Furthermore, fusion of ubiquitin in-frame to an internalisation-deficient AT(1A) receptor mutant with a truncated carboxyl-terminal tail did not restore the endocytosis of the resulting chimeric receptor. No impairment of receptor internalisation was observed after substitution of all lysine residues in the serine/threonine-rich region at saturating angiotensin II concentrations, where endocytosis occurs by a beta-arrestin and dynamin independent mechanism. Taken together, these data demonstrate that ubiquitylation of the cytoplasmic serine/threonine-rich region of the AT(1A) receptor on lysine residues is not required for its agonist-induced internalisation, and suggest that endocytosis of mammalian G protein-coupled receptors (GPCRs) occurs by a different mechanism than that of yeast GPCRs.

The role of a conserved region of the second intracellular loop in AT1 angiotensin receptor activation and signaling

The pleiotropic actions of angiotensin II are mediated by the primarily G(q) protein-coupled type 1 angiotensin (AT(1)) receptor. In this study a mutational analysis of the function of the conserved DRYXXV/IXXPL domain in the second intracellular loop of the rat AT(1A) receptor was performed in COS7 cells. Alanine substitution studies showed that single replacement of the highly conserved Asp(125) and Arg(126), but not Tyr(127), moderately impaired angiotensin II-induced inositol phosphate signaling. However, concomitant substitution of both Asp(125) and Arg(126) caused marked reduction of both inositol phosphate signaling and receptor internalization. Alanine scanning of the adjacent residues showed that substitution of Ile(130), His(132), and Pro(133) reduced agonist-induced inositol phosphate signal generation, whereas mutations of Met(134) also impaired receptor internalization. Expression of the D125A mutant AT(1A) receptor in COS7 cells endowed the receptor with moderate constitutive activity, as indicated by its enhanced basal Elk1 promoter activity and inositol phosphate response to partial agonists. Angiotensin II-induced stimulation of the Elk1 promoter showed parallel impairment with inositol phosphate signal generation in receptors containing mutations in this region of the AT(1A) receptor. These data confirm that Ca(2+) signal generation is required for the nuclear effects of angiotensin II-induced ERK activation. They are also consistent with the role of the conserved DRY sequence of the AT(1A) receptor in receptor activation, and of Asp(125) in constraining the receptor in its inactive conformation. Furthermore, in the cytoplasmic helical extension of the third helix, an apolar surface that includes Ile(130) and Met(134) appears to have a direct role in G protein coupling.

Conformational features of a synthetic model of the first extracellular loop of the angiotensin II AT1A receptor

The angiotensin II AT1A receptor belongs to the G-protein coupled receptors (GPCRs). Like other membrane proteins, GPCRs are not easily amenable to direct structure determination by the currently available methods. The peptide encompassing the putative first extracellular loop of AT1A (residues Thr88-Leu100, el1) has been synthesized along with a cyclic model where the linear peptide has been covalently linked to a template designed to keep the distance between the peptide termini as expected in the receptor. The conformational features of the two molecules have been studied using circular dichroism and NMR techniques. The region W94PFG97 forms a type-II beta-turn and undergoes a Trp-Pro peptide bond cis-trans isomerization in both peptides confirming that these characteristics are intrinsic to el1. In addition, the presence of the spacer seems to modulate the flexibility of the peptide.

Agonist induction and conformational selection during activation of a G-protein-coupled receptor

Substitutions of Asn111 of the AT(1) angiotensin receptor and mutations of the corresponding amino acids in other G-protein-coupled receptors (GPCRs) cause constitutive receptor activation. Ligand binding and signalling of constitutively active mutant GPCRs are discussed and similarities and differences during the activation of amine and peptide GPCRs are identified. Studies using the AT(1) receptor suggest that conformational selection is not sufficient to explain the mechanism of receptor activation, and that agonist binding to the receptor provides energy to induce activation of the receptor. Because agonist binding also actively facilitates the conformational rearrangements leading to activation of other GPCRs we propose that agonist induction should be considered as a general mechanism of GPCR activation.

Stretch-inactivated cation channels: cellular targets for modulation of osmosensitivity in supraoptic neurons

Rat magnocellular neurosecretory cells (MNCs) show an intrinsic sensitivity to acute changes in fluid osmolality. Experiments in acutely isolated supraoptic MNCs have shown that these responses are due to in part to the cell volume-dependent modulation of gadolinium-sensitive 33 pS stretch-inactivated cation (SIC) channels. Previous studies in vivo have shown that the slope (i.e. gain) of the 'osmosensory' relation between VP release and plasma osmolality can be increased or decreased under various physiological and pathological conditions. Here, we review recent work that shows how changes in external [Na] and excitatory neuropeptides such as angiotensin II (Ang II), cholecystokinin (CCK) and neurotensin (NT), may influence osmosensory gain in acutely isolated MNCs. Whole-cell and single-channel recording experiments have revealed that changes in external Na cause proportional changes in osmosensory gain as a result of modified SIC channel permeability and not by affecting mechanotransduction. In contrast, Ang II, CCK, or NT appear to convergently, and directly, stimulate the osmosensory cation conductance in MNCs. Preliminary analysis in current clamp further suggests that osmosensory gain may be increased upon exposure to these excitatory peptides. Whether such mechanisms contribute to the modulation of osmosensory gain in vivo remains to be established.

Aliphatic amino acids in helix VI of the AT(1) receptor play a relevant role in agonist binding and activity

Angiotensin II (AII) AT(1) receptor mutants with replacements of aliphatic amino acids in the distal region of helix VI and the adjoining region of the third extracellular loop (EC-3) were expressed in Chinese hamster ovary (CHO) cells to determine their role in ligand binding and activation. The triple mutant [L262D, L265D, L268D]AT(1) (L3D) showed a marked reduction in affinity for AII and for non-peptide (losartan) and peptide ([Sar(1)Leu(8) ]AII) antagonists; in functional assays using inositol phosphate (IP) accumulation, the relative potency and the maximum effect of AII were reduced in L3D. Replacement of Leu(268) (in EC-3) and Leu(262) (in the transmembrane domain) by aspartyl residues did not cause significant changes in the receptor's affinity for the ligands and in IP production. In contrast, the point mutation L265D, at helix VI, markedly decreased affinity and ability to stimulate phosphatidylinositol turnover. Molecular modeling of the AT(1) receptor based on a recent crystal structure of rhodopsin, suggests that the side chain of Leu(265) but not that of Leu(262) is facing a cleft between helices V and VI and interacts with the lipid bilayer, thus helping to stabilize the receptor structure near the Lys(199) residue of helix V in the agonist binding site which is necessary for full activity.

Relevant role of Leu265 in helix VI of the angiotensin AT1 receptor in agonist binding and activity

The finding of critical residues for angiotensin II (AII) binding and receptor signalling in helices V and VI led us to assess if, in this region of the receptor, aliphatic side chains might play a role in the agonist-mediated mechanism. Two mutations of the angiotensin AT1 receptor were designed to explore a possible role of a leucine at two positions, Leu265 and Leu268. Thus two mutants, L265D and L268D, were prepared through single substitutions of Leu265, located in the C-terminal region of transmembrane VI (TM-VI), and Leu268, in the adjoining region of the third extracellular loop (EC-3), for an aspartyl residue, and were stably transfected into Chinese hamster ovary (CHO) cells. Ligand-binding experiments and the functional assays determining inositol phosphate (IP) production were performed in these cells expressing these mutants. No significant changes were found in the binding affinity for the ligands, AII, DuP753, and [Sar1Leu8]AII in the mutant L268D. Moreover, the relative potency and the maximum effect on IP production of this mutant were similar to those of the wild-type receptor. In contrast, L265D mutant AT1 receptor, located within the transmembrane domain, markedly decreased binding affinity and ability to stimulate phosphatidylinositol turnover. Our results suggest that the hydrophobic side chain of Leu265, at the C-terminal portion of the AT1's TM-VI, but not Leu268, which belongs to the EC-3 loop, might interact with the AII molecule. On the other side, the aliphatic side chain of Leu265 may be involved in the formation of the ligand binding sites through allosteric effects, thus helping to stabilize the receptor structure around the agonist binding site for full activity.

Side-chain substitutions within angiotensin II reveal different requirements for signaling, internalization, and phosphorylation of type 1A angiotensin receptors

Binding of the peptide hormone angiotensin II (AngII) to the type 1 (AT(1A)) receptor and the subsequent activation of phospholipase C-mediated signaling, involves specific determinants within the AngII peptide sequence. In contrast, the contribution of such determinants to AT(1A) receptor internalization, phosphorylation and activation of mitogen-activated protein kinase (MAPK) signaling is not known. In this study, the internalization of an enhanced green fluorescent protein-tagged AT(1A) receptor (AT(1A)-EGFP), in response to AngII and a series of substituted analogs, was visualized and quantified using confocal microscopy. AngII-stimulation resulted in a rapid, concentration-dependent internalization of the chimeric receptor, which was prevented by pretreatment with the nonpeptide AT(1) receptor antagonist EXP3174. Remarkably, AT(1A) receptor internalization was unaffected by substitution of AngII side chains, including single and double substitutions of Tyr(4) and Phe(8) that abolish phospholipase C signaling through the receptor. AngII-induced receptor phosphorylation was significantly inhibited by several substitutions at Phe(8) as well as alanine replacement of Asp(1). The activation of MAPK was only significantly inhibited by substitutions at position eight in the peptide and specific substitutions did not equally inhibit inositol phosphate production, receptor phosphorylation and MAPK activation. These results indicate that separate, yet overlapping, contacts made between the AngII peptide and the AT(1A) receptor select/induce distinct receptor conformations that preferentially affect particular receptor outcomes. The requirements for AT(1A) receptor internalization seem to be less stringent than receptor activation and signaling, suggesting an inherent bias toward receptor deactivation.

Efficacy of angiotensin II type 1 receptor blockade on reperfusion-induced arrhythmias and mortality early after myocardial infarction is increased in transgenic rats with cardiac angiotensin II type 1 overexpression

Angiotensin II induces ischemia/reperfusion (I/R)-induced arrhythmias and blockade of the angiotensin II type 1 receptor (AT1R) may therefore be beneficial in preventing arrhythmias and decreasing mortality after myocardial infarction (MI). Because the AT1R is upregulated after myocardial ischemia, it was hypothesized that the level of AT1R expression would mediate the response to AT1R blockade. Transgenic (TGR) rats that overexpress the human AT1R and Sprague-Dawley rats were used as controls. Total duration of arrhythmia (seconds) after I/R injury was similar in TGR and SD rats (433 +/- 109 vs. 376 +/- 117, p = n.s.). AT1R blockade with losartan decreased total duration of arrhythmia in the TGR rats (433 +/- 110 s-164 +/- 48 s; p < 0.05), whereas it caused a nonsignificant increase in the SD rats (376 +/- 117 s-497 +/- 97). In vivo, survival in the first 24 hours after MI was impaired in TGR rats (39%; SD, 63%). Losartan improved survival significantly in TGR rats (from 39% to 80%, p < 0.05). A smaller, nonsignificant effect was observed in SD rats (63% to 81%). AT1R blockade is beneficial only when the AT1R was overexpressed, both in reducing the reperfusion-induced arrhythmias and mortality early after MI.

Peptidergic excitation of supraoptic nucleus neurons: involvement of stretch-inactivated cation channels

Although the primary stimulus regulating vasopressin (VP) release is a change in systemic osmolality, other physiological parameters are known to affect VP secretion or modulate the osmotic control over its release. Neuropeptides feature prominently in afferents underlying the central regulation of the VP-releasing magnocellular neurosecretory cells (MNCs). Although little is yet known of the circumstances under which peptides are released onto MNCs, previous studies have shown that a common response profile to exogenous peptide application is a slow excitation that seems to result from the activation of a nonselective cation conductance. In this paper we review the basis for the excitatory effects of angiotensin II, cholecystokinin, and neurotensin in MNCs acutely isolated from the supraoptic nucleus of adult rats. Saturating concentrations of these three peptides evoked nonadditive increases in macroscopic cation conductance. During single-channel recordings Ang II, CCK, and NT caused kinetically identical increases in the probability of opening of 35-pS nonselective cation channels. Patches containing only one channel further revealed that the activity of single channels could be regulated by separate applications of all three peptides. Peptide-stimulated channels were also found to be inactivated by increases in membrane stretch and to be blocked by low concentrations of gadolinium (Gd(3+)). It is concluded that many excitatory peptides depolarize MNCs by stimulating the stretch-inactivated cation channels underlying osmoreception. Convergent regulation of these channels provides a potentially powerful mechanism for integrating signals derived from the various afferents involved in the regulation of MNCs.

3D model for TM region of the AT-1 receptor in complex with angiotensin II independently validated by site-directed mutagenesis data

A three-dimensional model of the complex of angiotensin II (AII) with the transmembrane (TM) region of the angiotensin II receptor of type 1 (the AT-1 receptor) was obtained by molecular modeling procedures employing structural homology to the X-ray structure of rhodopsin. Since the modeling procedure considered only steric and energy considerations without prior knowledge of the experimental results of site-directed mutagenesis, the results with receptor mutants could be used for independent validation of the model. Indeed, the model brings in contact the residues of AII responsible for agonistic activity, Tyr(4), His(6), and Phe(8), with many residues of AT-1 involved in signal transduction according to site-directed mutagenesis. The model also predicts the existence of several possible conformational pathways for transferring the binding signal through the TM region of AT-1 to the intracellular loops interacting with the G-protein.

Ligand-induced endocytosis and nuclear localization of angiotensin II receptors expressed in CHO cells

A construct (AT1R-NF) containing a "Flag" sequence added to the N-terminus of the rat AT1 receptor was stably expressed in Chinese hamster ovary cells and quantified in the cell membrane by confocal microscopy after reaction with a fluorescein-labeled anti-Flag monoclonal antibody. Angiotensin II bound to AT1R-NF and induced endocytosis with a half-time of 2 min. After 60-90 min, fluorescence accumulated around the cell nucleus, suggesting migration of the ligand-receptor complex to the nuclear membrane. Angiotensin antagonists also induced endocytosis, suggesting that a common step in the transduction signal mechanism occurring after ligand binding may be responsible for the ligand-receptor complex internalization.

Angiotensin II reduces infarct size and has no effect on post-ischaemic contractile dysfunction in isolated rat hearts

1. In order to test the hypothesis that angiotensin II exacerbates myocardial ischaemia-reperfusion (IR) injury, we examined the effects of graded angiotension II concentrations of angiotensin II on IR injury in both working and non-working (Langendorff) isolated rat hearts. 2. Non-working hearts were subjected to 30 min aerobic perfusion (baseline) then 25 min of global, no-flow ischaemia followed by 30 min of reperfusion either in the absence (control, n=7) or presence of 1 (n=6) or 10 nM (n=5) angiotensin II). Recoveries of LV developed pressure and coronary flow after 30 min reperfusion in control hearts (58+/-9 and 40+/-8% of baseline levels, respectively) were no different from hearts treated with 1 or 10 nM angiotensin II. Infarct size (determined at the end of reperfusion by triphenyltetrazolium chloride staining) was reduced by angiotensin II in a concentration-dependent manner (from a control value of 27+/-3 to 18+/-4% and 9+/-3% of the LV, respectively). 3. Working hearts were subjected to 50 min pre-ischaemic (pre-I) aerobic perfusion then 30 min of global, no-flow ischaemia followed by 30 min of reperfusion either in the absence (control, n=14) or presence of 1 (n=8), 10 (n=7) or 100 nM (n=7) angiotensin II). In controls, post-ischaemic (post-I) left ventricular (LV) work and efficiency of oxygen consumption were depressed (43+/-9 and 42+/-10% of pre-I levels, respectively). The presence of angiotensin II throughout IR had no effect on LV work compared with control. 4. Thus, angiotensin II reduces infarct size in a concentration-dependent manner but has no effect on contractile stunning associated with IR in isolated rat hearts.

The angiotensin II type 1 receptor and receptor-associated proteins

The mechanisms of regulation, activation and signal transduction of the angiotensin II (Ang II) type 1 (AT1) receptor have been studied extensively in the decade after its cloning. The AT1 receptor is a major component of the renin-angiotensin system (RAS). It mediates the classical biological actions of Ang II. Among the structures required for regulation and activation of the receptor, its carboxyl-terminal region plays crucial roles in receptor internalization, desensitization and phosphorylation. The mechanisms involved in heterotrimeric G-protein coupling to the receptor, activation of the downstream signaling pathway by G proteins and the Ang II signal transduction pathways leading to specific cellular responses are discussed. In addition, recent work on the identification and characterization of novel proteins associated with carboxyl-terminus of the AT1 receptor is presented. These novel proteins will advance our understanding of how the receptor is internalized and recycled as they provide molecular mechanisms for the activation and regulation of G-protein-coupled receptors.

Intracellular angiotensin II elicits Ca2+ increases in A7r5 vascular smooth muscle cells

Recent studies show that angiotensin II can act within the cell, possibly via intracellular receptors pharmacologically different from typical plasma membrane angiotensin II receptors. The signal transduction of intracellular angiotensin II is unclear. Therefore, we investigated the effects of intracellular angiotensin II in cells devoid of physiological responses to extracellular angiotensin II (A7r5 vascular smooth muscle cells). Intracellular delivery of angiotensin II was obtained by using liposomes or cell permeabilisation. Intracellular angiotensin II stimulated Ca2+ influx, as measured by 45Ca2+ uptake and single-cell fluorimetry. This effect was insensitive to extracellular or intracellular addition of losartan (angiotensin AT(1) receptor antagonist) or PD123319 ((s)-1-(4-[dimethylamino]-3-methylphenyl)methyl-5-(diphenylacetyl)-4,5,6,7-tetrahydro-1H-imidazo[4,5-c]pyridine-6-carboxylate) (angiotensin AT2 receptor antagonist). Intracellular angiotensin II stimulated inositol-1,4,5-trisphosphate (Ins(1,4,5,)P3) production and increased the size of the Ins(1,4,5,)P3 releasable 45Ca2+ pool in permeabilised cells, independent of losartan and PD123319. Small G-proteins did not participate in this process, as assessed by using GDPbetaS. Intracellular delivery of angiotensin I was unable to elicit any of the effects elicited by intracellular angiotensin II. We conclude from our intracellular angiotensin application experiments that angiotensin II modulates Ca2+ homeostasis even in the absence of extracellular actions. Pharmacological properties suggest the involvement of putative angiotensin non-AT1-/non-AT2 receptors.

Brain angiotensin receptors and sympathoadrenal regulation during insulin-induced hypoglycemia

Simultaneous blockade of systemic AT1 and AT2 receptors or converting enzyme inhibition (CEI) attenuates the hypoglycemia-induced reflex increase of epinephrine (Epi). To examine the role of brain AT1 and AT2 receptors in the reflex regulation of Epi release, we measured catecholamines, hemodynamics, and renin during insulin-induced hypoglycemia in conscious rats pretreated intracerebroventricularly with losartan, PD-123319, losartan and PD-123319, or vehicle. Epi and norepinephrine (NE) increased 60-and 3-fold, respectively. However, the gain of the reflex increase in plasma Epi (Deltaplasma Epi/Deltaplasma glucose) and the overall Epi and NE responses were similar in all groups. The ensuing blood pressure response was similar between groups, but the corresponding bradycardia was augmented after PD-123319 (P < 0.05 vs. vehicle) or combined losartan and PD-123319 (P < 0.01 vs. vehicle). The findings indicate 1) brain angiotensin receptors are not essential for the reflex regulation of Epi release during hypoglycemia and 2) the gain of baroreceptor-mediated bradycardia is increased by blockade of brain AT2 receptors in this model.

MK-954 (losartan potassium) exerts endothelial protective effects against reperfusion injury: evidence of an e-NOS mRNA overexpression after global ischemia

BACKGROUND

the cardiac Renin-Angiotensin system (RAS) plays an important role in the regulation of coronary flow and cardiac function and structure in normal and pathological conditions such as ischemia-reperfusion (I/R) injury. The aim of this study was to investigate the effects of the Angiotensin II type 1 (AT-1) receptor antagonist MK-954 (losartan potassium) on postischemic endothelial dysfunction and NOS mRNA expression (inducible nitric oxide synthase, iNOS; endothelial nitric oxide synthase, eNOS) in isolated working rat hearts.

METHODS

isolated working rat hearts were subjected to 15 min global ischemia and 180 min reperfusion. MK-954 was added to perfusion buffer (a modified Krebs-Henseleit solution) at 1 microM concentration. We assessed functional parameters, creatin kinase (CK) release, heart weight changes, microvascular postischemic hyperpermeability (FITC-albumin extravasation) and morphological ultrastructural alterations. eNOS and iNOS mRNA levels were also detected by the means of multiplex RT-PCR technique using glyceraldehyde-3-phosphate dehydrogenase (G3PDH) gene as internal control; results were expressed as densitometric ratio.

RESULTS

in Losartan-treated hearts we observed a significant reduction of postischemic contractile dysfunction, CK release and myocardial ultrastructural damage; postischemic FITC-albumin extravasation was significantly reduced respect to controls. Moreover, 1 microM Losartan produced a significant reduction of eNOS/G3PDH respect to untreated hearts submitted to I/R. Regarding iNOS/G3PDH ratio, no significant changes were detected in Losartan-treated hearts compared with controls.

CONCLUSIONS

our study revealed that Losartan treatment before ischemia, and during reperfusion, is able to reduce the reperfusion injury of the rat heart by reducing mechanical and microcirculatory dysfunction and necrotic cell death, ameliorating cardiac ultrastructure and endothelial protection, probably inducing eNOS over-expression and reducing post-ischemic hyperpermeability of coronary microcirculation.

Angiotensin II and calcium channels

Sixty years after its initial discovery, the octapeptide hormone angiotensin II (AngII) has proved to play numerous physiological roles that reach far beyond its initial description as a hypertensive factor. In spite of the host of target tissues that have been identified, only two major receptor subtypes, AT1 and AT2, are currently fully identified. The specificity of the effects of AngII relies upon numerous and complex intracellular signaling pathways that often mobilize calcium ions from intracellular stores or from the extracellular medium. Various types of calcium channels (store- or voltage-operated channels) endowed with distinct functional properties play a crucial role in these processes. The activity of these channels can be modulated by AngII in a positive and/or negative fashion, depending on the cell type under observation. This chapter reviews the main characteristics of AngII receptor subtypes and of the various calcium channels as well as the involvement of the multiple signal transduction mechanisms triggered by the hormone in the cell-specific modulation of the activity of these channels.

Mechanisms and functions of AT(1) angiotensin receptor internalization

The type 1 (AT(1)) angiotensin receptor, which mediates the known physiological and pharmacological actions of angiotensin II, activates numerous intracellular signaling pathways and undergoes rapid internalization upon agonist binding. Morphological and biochemical studies have shown that agonist-induced endocytosis of the AT(1) receptor occurs via clathrin-coated pits, and is dependent on two regions in the cytoplasmic tail of the receptor. However, it is independent of G protein activation and signaling, and does not require the conserved NPXXY motif in the seventh transmembrane helix. The dependence of internalization of the AT(1) receptor on a cytoplasmic serine-threonine-rich region that is phosphorylated during agonist stimulation suggests that endocytosis is regulated by phosphorylation of the AT(1) receptor tail. beta-Arrestins have been implicated in the desensitization and endocytosis of several G protein-coupled receptors, but the exact nature of the adaptor protein required for association of the AT(1) receptor with clathrin-coated pits, and the role of dynamin in the internalization process, are still controversial. There is increasing evidence for a role of internalization in sustained signal generation from the AT(1) receptor. Several aspects of the mechanisms and specific function of AT(1) receptor internalization, including its precise mode and route of endocytosis, and the potential roles of cytoplasmic and nuclear receptors, remain to be elucidated.