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.

The Angiotensin II AT1 Receptor Structure-Activity Correlations in the Light of Rhodopsin Structure

The most prevalent physiological effects of ANG II, the main product of the renin-angiotensin system, are mediated by the AT1 receptor, a rhodopsin-like AGPCR. Numerous studies of the cardiovascular effects of synthetic peptide analogs allowed a detailed mapping of ANG II's structural requirements for receptor binding and activation, which were complemented by site-directed mutagenesis studies on the AT1 receptor to investigate the role of its structure in ligand binding, signal transduction, phosphorylation, binding to arrestins, internalization, desensitization, tachyphylaxis, and other properties. The knowledge of the high-resolution structure of rhodopsin allowed homology modeling of the AT1 receptor. The models thus built and mutagenesis data indicate that physiological (agonist binding) or constitutive (mutated receptor) activation may involve different degrees of expansion of the receptor's central cavity. Residues in ANG II structure seem to control these conformational changes and to dictate the type of cytosolic event elicited during the activation. 1) Agonist aromatic residues (Phe8 and Tyr4) favor the coupling to G protein, and 2) absence of these residues can favor a mechanism leading directly to receptor internalization via phosphorylation by specific kinases of the receptor's COOH-terminal Ser and Thr residues, arrestin binding, and clathrin-dependent coated-pit vesicles. On the other hand, the NH2-terminal residues of the agonists ANG II and [Sar1]-ANG II were found to bind by two distinct modes to the AT1 receptor extracellular site flanked by the COOH-terminal segments of the EC-3 loop and the NH2-terminal domain. Since the [Sar1]-ligand is the most potent molecule to trigger tachyphylaxis in AT1 receptors, it was suggested that its corresponding binding mode might be associated with this special condition of receptors.

Determining the Environment of the Ligand Binding Pocket of the Human Angiotensin II Type I (hAT1) Receptor Using the Methionine Proximity Assay

The peptide hormone angiotensin II (AngII) binds to the AT0 (angiotensin type 1) receptor within the transmembrane domains in an extended conformation, and its C-terminal residue interacts with transmembrane domain VII at Phe-293/Asn-294. The molecular environment of this binding pocket remains to be elucidated. The preferential binding of benzophenone photolabels to methionine residues in the target structure has enabled us to design an experimental approach called the methionine proximity assay, which is based on systematic mutagenesis and photolabeling to determine the molecular environment of this binding pocket. A series of 44 transmembrane domain III, VI, and VII X --> Met mutants photolabeled either with 125I-[Sar1,p'-benzoyl-L-Phe8]AngII or with 125I-[Sar1,p''-methoxy-p'-benzoyl-L-Phe8]AngII were purified and digested with cyanogen bromide. Several mutants produced digestion patterns different from that observed with wild type human AT1, indicating that they had a new receptor contact with position 8 of AngII. The following residues form this binding pocket: L112M and Y113M in transmembrane domain (TMD) III; F249M, W253M, H256M, and T260M in TMD VI; and F293M, N294M, N295M, C296M, and L297M in TMD VII. Homology modeling and incorporation of these contacts allowed us to develop an evidence-based molecular model of interactions with human AT1 that is very similar to the rhodopsin-retinal interaction.

Toward the active conformations of rhodopsin and the beta2-adrenergic receptor

Using sets of experimental distance restraints, which characterize active or inactive receptor conformations, and the X-ray crystal structure of the inactive form of bovine rhodopsin as a starting point, we have constructed models of both the active and inactive forms of rhodopsin and the beta2-adrenergic G-protein coupled receptors (GPCRs). The distance restraints were obtained from published data for site-directed crosslinking, engineered zinc binding, site-directed spin-labeling, IR spectroscopy, and cysteine accessibility studies conducted on class A GPCRs. Molecular dynamics simulations in the presence of either "active" or "inactive" restraints were used to generate two distinguishable receptor models. The process for generating the inactive and active models was validated by the hit rates, yields, and enrichment factors determined for the selection of antagonists in the inactive model and for the selection of agonists in the active model from a set of nonadrenergic GPCR drug-like ligands in a virtual screen using ligand docking software. The simulation results provide new insights into the relationships observed between selected biochemical data, the crystal structure of rhodopsin, and the structural rearrangements that occur during activation.

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.

Inhibition of bacterial peptide deformylase by biaryl acid analogs

Peptide deformylase is an essential eubacterial metalloenzyme involved in the maturation of proteins by cleaving the N-formyl group from N-blocked methionine polypeptides. Biaryl acid analogs containing tetrazole, acyl sulfonamide, or carboxylate pharmacophores were found to be potent inhibitors of recombinant Escherichia coli peptide deformylase. Two of these compounds, a biphenyl tetrazole, compound 1, and a biphenyl acyl sulfonamide, compound 4, were competitive inhibitors with K(i) values of 1.2 and 6.0 microM, respectively. By analogy to the binding of related compounds to other metalloenzymes such as Bacteroides fragilis metallo-beta-lactamase CcrA and human carbonic anhydrase, a mechanism of inhibition is proposed for these peptide deformylase inhibitors where the acidic moieties form direct ionic interactions with the active site metal cation.

Determination of peptide contact points in the human angiotensin II type I receptor (AT1) with photosensitive analogs of angiotensin II

To identify ligand-binding domains of Angiotensin II (AngII) type 1 receptor (AT1), two different radiolabeled photoreactive AngII analogs were prepared by replacing either the first or the last amino acid of the octapeptide by p-benzoyl-L-phenylalanine (Bpa). High yield, specific labeling of the AT1 receptor was obtained with the 125I-[Sar1,Bpa8]AngII analog. Digestion of the covalent 125I-[Sar1,Bpa8]AngII-AT1 complex with V8 protease generated two major fragments of 15.8 kDa and 17.8 kDa, as determined by SDS-PAGE. Treatment of the [Sar1,Bpa8]AngII-AT1 complex with cyanogen bromide produced a major fragment of 7.5 kDa which, upon further digestion with endoproteinase Lys-C, generated a fragment of 3.6 kDa. Since the 7.5-kDa fragment was sensitive to hydrolysis by 2-nitro-5-thiocyanobenzoic acid, we circumscribed the labeling site of 125I-[Sar1,Bpa8]AngII within amino acids 285 and 295 of the AT1 receptor. When the AT1 receptor was photolabeled with 125I-[Bpa1]AngII, a poor incorporation yield was obtained. Cleavage of the labeled receptor with endoproteinase Lys-C produced a glycopeptide of 31 kDa, which upon deglycosylation showed an apparent molecular mass of 7.5 kDa, delimiting the labeling site of 125I-[Bpa1]AngII within amino acids 147 and 199 of the AT1 receptor. CNBr digestion of the hAT1 I165M mutant receptor narrowed down the labeling site to the fragment 166-199. Taken together, these results indicate that the seventh transmembrane domain of the AT1 receptor interacts strongly with the C-terminal amino acid of [Sar1, Bpa8]AngII interacts with the second extracellular loop of the AT1 receptor.

Role of aromaticity of agonist switches of angiotensin II in the activation of the AT1 receptor

We have shown previously that the octapeptide angiotensin II (Ang II) activates the AT1 receptor through an induced-fit mechanism (Noda, K., Feng, Y. H., Liu, X. P., Saad, Y., Husain, A., and Karnik, S. S. (1996) Biochemistry 35, 16435-16442). In this activation process, interactions between Tyr4 and Phe8 of Ang II with Asn111 and His256 of the AT1 receptor, respectively, are essential for agonism. Here we show that aromaticity, primarily, and size, secondarily, of the Tyr4 side chain are important in activating the receptor. Activation analysis of AT1 receptor position 111 mutants by various Ang II position 4 analogues suggests that an amino-aromatic bonding interaction operates between the residue Asn111 of the AT1 receptor and Tyr4 of Ang II. Degree and potency of AT1 receptor activation by Ang II can be recreated by a reciprocal exchange of aromatic and amide groups between positions 4 and 111 of Ang II and the AT1 receptor, respectively. In several other bonding combinations, set up between Ang II position 4 analogues and receptor mutants, the gain of affinity is not accompanied by gain of function. Activation analysis of position 256 receptor mutants by Ang II position 8 analogues suggests that aromaticity of Phe8 and His256 side chains is crucial for receptor activation; however, a stacked rather than an amino-aromatic interaction appears to operate at this switch locus. Interaction between these residues, unlike the Tyr4:Asn111 interaction, plays an insignificant role in ligand docking.

Regulation of angiotensin II type 1 (AT1) receptor function

The type 1 angiotensin receptor (AT1) mediates the important biological actions of the peptide hormone, angiotensin II (AngII), by activating an array of intracellular signaling pathways. The unique temporal arrangement and duration of AngII-stimulated signals suggests a hierarchy of post-AT1 receptor binding events that permits activation of selective effector pathways. Moreover, it predicts that the coupling of AT1 receptors is tightly regulated, allowing cells to differentiate acute responses from those requiring longer periods of stimulation. Recent studies have concentrated on delineating the molecular processes involved in modulating AT1 receptor activity. In addition to AT1 receptor modification (phosphorylation), trafficking (internalization and degradation) and interaction with regulatory intracellular proteins, other processes may include receptor dimerization, cross-regulation by other receptor systems, and receptor isomerization between activated and non-activated forms. This review focuses on recent advances in this area of research, highlighting directions for future investigation.

Angiotensin receptors in the nervous system

In addition to its traditional role as a circulating hormone, angiotensin is also involved in local functions through the activity of tissue renin-angiotensin systems that occur in many organs, including the brain. In the brain, both systemic and presumptive neurally derived angiotensin and angiotensin metabolites act through specific receptors to modulate many functions. This review examines the distribution of these specific angiotensin receptors and discusses evidence regarding the function of angiotensin peptides in various brain regions. Angiotensin AT1 and AT2 receptors occur in characteristic distributions that are highly correlated with the distribution of angiotensin-like immunoreactivity in nerve terminals. Acting through the AT1 receptor in the brain, angiotensin has effects on fluid and electrolyte homeostasis, neuroendocrine systems, autonomic pathways regulating cardiovascular function and behavior. Angiotensin AT1 receptors are also found in many afferent and efferent components of the peripheral autonomic nervous system. The role of the AT2 receptor in the brain is less well understood, although recent knockout studies point to an involvement with behavioral and cardiovascular functions. In addition to the AT1 and AT2 receptors, receptors for other fragments of angiotensin have been proposed. The AT4 binding site, which binds angiotensin, has a widespread distribution in the brain quite distinct from that of the AT1 and AT2 receptors. It is associated with many cholinergic neuronal groups and also several sensory nuclei, but its function remains to be determined. Our discovery that another brain-derived peptide binds to the AT4 binding site in the brain and may represent the native ligand is discussed. Overall, the distribution of angiotensin receptors in the brain indicate that they play diverse and important physiological roles in the nervous system.