Synthesis and AT2 receptor-binding properties of angiotensin II analogues

The Angiotensin AT2 Receptor: From a Binding Site to a Novel Therapeutic Target

Discovered more than 30 years ago, the angiotensin AT2 receptor (AT2R) has evolved from a binding site with unknown function to a firmly established major effector within the protective arm of the renin-angiotensin system (RAS) and a target for new drugs in development. The AT2R represents an endogenous protective mechanism that can be manipulated in the majority of preclinical models to alleviate lung, renal, cardiovascular, metabolic, cutaneous, and neural diseases as well as cancer. This article is a comprehensive review summarizing our current knowledge of the AT2R, from its discovery to its position within the RAS and its overall functions. This is followed by an in-depth look at the characteristics of the AT2R, including its structure, intracellular signaling, homo- and heterodimerization, and expression. AT2R-selective ligands, from endogenous peptides to synthetic peptides and nonpeptide molecules that are used as research tools, are discussed. Finally, we summarize the known physiological roles of the AT2R and its abundant protective effects in multiple experimental disease models and expound on AT2R ligands that are undergoing development for clinical use. The present review highlights the controversial aspects and gaps in our knowledge of this receptor and illuminates future perspectives for AT2R research. SIGNIFICANCE STATEMENT: The angiotensin AT2 receptor (AT2R) is now regarded as a fully functional and important component of the renin-angiotensin system, with the potential of exerting protective actions in a variety of diseases. This review provides an in-depth view of the AT2R, which has progressed from being an enigma to becoming a therapeutic target.

Update on Angiotensin II Subtype 2 Receptor: Focus on Peptide and Nonpeptide Agonists

Angiotensin II (Ang II) is the most dominant effector component of the renin-angiotensin system (RAS) that generally acts through binding to two main classes of G protein-coupled receptors, namely Ang II subtype 1 receptor (AT1R) and angiotensin II subtype 2 receptor (AT2R). Despite some controversial reports, the activation of AT2R generally antagonizes the effects of Ang II binding on AT1R. Studying AT2R signaling, function, and its specific ligands in cell culture or animal studies has confirmed its beneficial effects throughout the body. These characteristics classify AT2R as part of the protective arm of the RAS that, along with functions of Ang (1-7) through Mas receptor signaling, modulates the harmful effects of Ang II on AT1R in the activated classic arm of the RAS. Although Ang II is the primary ligand for AT2R, we have summarized other natural or synthetic peptide and nonpeptide agonists with critical evaluation of their structure, mechanism of action, and biologic activity. SIGNIFICANCE STATEMENT: AT2R is one of the main components of the RAS and has a significant prospective for mediating the beneficial action of the RAS through its protective arm on the body's homeostasis. Targeting AT2R offers substantial clinical application possibilities for modulating various pathological conditions. This review provided concise information regarding the AT2R peptide and nonpeptide agonists and their potential clinical applications for various diseases.

Evolution of Angiotensin Peptides and Peptidomimetics as Angiotensin II Receptor Type 2 (AT2) Receptor Agonists

Angiotensin II receptor type 1 and 2 (AT1R and AT2R) are two G-protein coupled receptors that mediate most biological functions of the octapeptide Angiotensin II (Ang II). AT2R is upregulated upon tissue damage and its activation by selective AT2R agonists has become a promising approach in the search for new classes of pharmaceutical agents. We herein analyzed the chemical evolution of AT2R agonists starting from octapeptides, through shorter peptides and peptidomimetics to the first drug-like AT2R-selective agonist, C21, which is in Phase II clinical trials and aimed for idiopathic pulmonary fibrosis. Based on the recent crystal structures of AT1R and AT2R in complex with sarile, we identified a common binding model for a series of 11 selected AT2R agonists, consisting of peptides and peptidomimetics of different length, affinity towards AT2R and selectivity versus AT1R. Subsequent molecular dynamics simulations and free energy perturbation (FEP) calculations of binding affinities allowed the identification of the bioactive conformation and common pharmacophoric points, responsible for the key interactions with the receptor, which are maintained by the drug-like agonists. The results of this study should be helpful and facilitate the search for improved and even more potent AT2R-selective drug-like agonists.

The Other Angiotensin II Receptor: AT2R as a Therapeutic Target

The active hormone of the renin-angiotensin system (RAS), angiotensin II (Ang II), is involved in several human diseases, driving the development and clinical use of several therapeutic drugs, mostly angiotensin I converting enzyme (ACE) inhibitors and angiotensin receptor type I (AT₁R) antagonists. However, angiotensin peptides can also bind to receptors different from AT₁R, in particular, angiotensin receptor type II (AT₂R), resulting in biological and physiological effects different, and sometimes antagonistic, of their binding to AT₁R. In the present Perspective, the components of the RAS and the therapeutic tools developed to control it will be reviewed. In particular, the characteristics of AT₂R and tools to modulate its functions will be discussed. Agonists or antagonists to AT₂R are potential therapeutics in cardiovascular diseases, for agonists, and in the control of pain, for antagonists, respectively. However, controlling their binding properties and their targeting to the target tissues must be optimized.

β-Pro7Ang III is a novel highly selective angiotensin II type 2 receptor (AT2R) agonist, which acts as a vasodepressor agent via the AT2R in conscious spontaneously hypertensive rats

We have synthesized a highly selective compound that is able to target a protein-binding site [called angiotensin (Ang) II type 2 receptor, AT2R] in the cardiovascular system. This research tool will enhance our ability to stimulate AT2R to produce protective effects against cardiovascular disease.

Structural determinants for binding to angiotensin converting enzyme 2 (ACE2) and angiotensin receptors 1 and 2

Angiotensin converting enzyme 2 (ACE2) is a zinc carboxypeptidase involved in the renin–angiotensin system (RAS) and inactivates the potent vasopressive peptide angiotensin II (Ang II) by removing the C-terminal phenylalanine residue to yield Ang1–7. This conversion inactivates the vasoconstrictive action of Ang II and yields a peptide that acts as a vasodilatory molecule at the Mas receptor and potentially other receptors. Given the growing complexity of RAS and level of cross-talk between ligands and their corresponding enzymes and receptors, the design of molecules with selectivity for the major RAS binding partners to control cardiovascular tone is an on-going challenge. In previous studies we used single β-amino acid substitutions to modulate the structure of Ang II and its selectivity for ACE2, AT₁R, and angiotensin type 2 (AT₂R) receptor. We showed that modification at the C-terminus of Ang II generally resulted in more pronounced changes to secondary structure and ligand binding, and here, we further explore this region for the potential to modulate ligand specificity. In this study, (1) a library of 47 peptides derived from the C-terminal tetrapeptide sequence (-IHPF) of Ang II was synthesized and assessed for ACE2 binding, (2) the terminal group requirements for high affinity ACE2 binding were explored by and N- and C-terminal modification, (3) high affinity ACE2 binding chimeric AngII analogs were then synthesized and assessed, (4) the structure of the full-length Ang II analogs were assessed by circular dichroism, and (5) the Ang II analogs were assessed for AT₁R/AT₂R selectivity by cell-based assays. Studies on the C-terminus of Ang II demonstrated varied specificity at different residue positions for ACE2 binding and four Ang II chimeric peptides were identified as selective ligands for the AT₂ receptor. Overall, these results provide insight into the residue and structural requirements for ACE2 binding and angiotensin receptor selectivity.

Similarities and differences between effects of angiotensin III and angiotensin II on human prostate cancer cell migration and proliferation

Proliferation plays a critical role in tumor growth when cell migration is essential to invasion. The effect of Ang III and Ang II was evaluated on these important processes. Changes in the migration potential of prostate cancer cells were investigated using Wound Healing Test and a Transwell Migration Chamber with a 3 μm pore size. Cell proliferation was measured with a BrdU Assay and Countess Automated Cell Counter, thus determining the influence of angiotensins on hormone-dependent (LNCaP) and hormone-independent (DU-145) human prostate cancer lines. The influence of Ang III and Ang II on classic receptors may be inhibited by Losartan or PD123319. Test peptide modulation of the AT1 and AT2 receptors was examined by Western Blot and fluorescent immunocytochemistry. The results indicate that Ang III promotes the migration of both LNCaP and DU-145 lines, whereas Ang II stimulates this process only in androgen-independent cells. Both angiotensin peptides can induce prostate cancer cell proliferation in a time- and dose-dependent manner. The obtained results show that Ang III and Ang II can modify the expression of classic receptors, particularly AT2. These results suggest that the investigated peptide can modulate cell migration and proliferation in prostate cancer cells. Angiotensins probably have a greater influence on proliferation in the early-stage prostate cancer model than hormone-independent cell lines. Assume also that Ang II can enhance the migration tendency aggressive prostate cancer cells, while Ang III does so more effective in non-metastatic cells.

Angiotensin II type 2 receptor agonists: where should they be applied?

INTRODUCTION

Angiotensin II, the active endproduct of the renin-angiotensin system (RAS), exerts its effects via angiotensin II type 1 and type 2 (AT(1), AT(2)) receptors. AT(1) receptors mediate all well-known effects of angiotensin II, ranging from vasoconstriction to tissue remodeling. Thus, to treat cardiovascular disease, RAS blockade aims at preventing angiotensin II-AT(1) receptor interaction. Yet RAS blockade is often accompanied by rises in angiotensin II, which may exert beneficial effects via AT(2) receptors.

AREAS COVERED

This review summarizes our current knowledge on AT(2) receptors, describing their location, function(s), endogenous agonist(s) and intracellular signaling cascades. It discusses the beneficial effects obtained with C21, a recently developed AT(2) receptor agonist. Important questions that are addressed are do these receptors truly antagonize AT(1) receptor-mediated effects? What about their role in the diseased state and their heterodimerization with other receptors?

EXPERT OPINION

The general view that AT(2) receptors exclusively exert beneficial effects has been challenged, and in pathological models, their function sometimes mimics that of AT(1) receptors, for example, inducing vasoconstriction and cardiac hypertrophy. Yet given its upregulation in various pathological conditions, the AT(2) receptor remains a promising target for treatment, allowing effects beyond blood pressure-lowering, for example, in stroke, aneurysm formation, inflammation and myocardial fibrosis.

Relative affinity of angiotensin peptides and novel ligands at AT1 and AT2 receptors

AT1R (angiotensin type 1 receptor) and AT2R (angiotensin type 2 receptor) are well known to be involved in the complex cardiovascular actions of AngII (angiotensin II). However, shorter peptide fragments of AngII are thought to have biological activity in their own right and elicit effects that oppose those mediated by AngII. In the present study, we have used HEK (human embryonic kidney)-293 cells stably transfected with either AT1R or AT2R to perform a systematic analysis of binding affinities of all the major angiotensin peptides. Additionally, we tested the novel AT2R agonist Compound 21, as well as the MasR (Mas receptor) agonist and antagonist AVE0991 and A-779 respectively, for their ability to bind to AT1R or AT2R. Candesartan, CGP42214 and PD123319 were used as reference compounds. Binding studies using 125I-[Sar1Ile8]AngII on the AT1R-transfected HEK-293 cells revealed only AngII, AngIII [angiotensin III; angiotensin-(2–8)] and candesartan to have high affinity for AT1R. In the AT2R-transfected HEK-293 cells, competition for 125I-[Sar1Ile8]AngII binding was observed for all ligands except candesartan, AVE0991 and A-779, the latter two compounds having negligible affinity at either AT1R or AT2R. The rank order of affinity of ligands at AT2R was CGP42112>AngII≥AngIII>Compound 21≥PD123319≫AngIV [angiotensin IV; angiotensin-(3–8)]>Ang-(1–7) [angiotensin-(1–7)]. Of note, although AngIV and Ang-(1–7) exhibited only modest affinity at AT2R compared with AngII, these two angiotensin peptides, together with AngIII, had substantial AT2R selectivity over AT1R. Collectively, our results suggest that shorter angiotensin peptides can act as endogenous ligands at AT2R.

AT2 receptors: functional relevance in cardiovascular disease

The renin angiotensin system (RAS) is intricately involved in normal cardiovascular homeostasis. Excessive stimulation by the octapeptide angiotensin II contributes to a range of cardiovascular pathologies and diseases via angiotensin type 1 receptor (AT1R) activation. On the other hand, tElsevier Inc.he angiotensin type 2 receptor (AT2R) is thought to counter-regulate AT1R function. In this review, we describe the enhanced expression and function of AT2R in various cardiovascular disease settings. In addition, we illustrate that the RAS consists of a family of angiotensin peptides that exert cardiovascular effects that are often distinct from those of Ang II. During cardiovascular disease, there is likely to be an increased functional importance of AT2R, stimulated by Ang II, or even shorter angiotensin peptide fragments, to limit AT1R-mediated overactivity and cardiovascular pathologies.

Angiotensin II and III suppress food intake via angiotensin AT(2) receptor and prostaglandin EP(4) receptor in mice

Intracerebroventricularly administered angiotensin (Ang) II and III dose-dependently suppressed food intake in mice and their anorexigenic activities were inhibited by AT(2) receptor-selective antagonist. Ang II did not suppress food intake in AT(2) receptor-knockout mice, while it did significantly in wild-type and AT(1) receptor-knockout mice. The suppression of food intake in AT(1) receptor-knockout mice was smaller than that in wild-type. The anorexigenic activities of Ang II and III were also blocked by a selective antagonist for prostaglandin EP(4) receptor. Taken together, centrally administered Ang II and III may decrease food intake through AT(2) receptor with partial involvement of AT(1) receptor, followed by EP(4) receptor activation, which is a novel pathway regulating food intake.

Design, Synthesis, and Incorporation of a β-Turn Mimetic in Angiotensin II Forming Novel Pseudopeptides with Affinity for AT1 and AT2 Receptors

A benzodiazepine-based beta-turn mimetic has been designed, synthesized, and incorporated into angiotensin II. Comparison of the mimetic with beta-turns in crystallized proteins showed that it most closely resembles a type II beta-turn. The compounds exhibited high to moderate binding affinity for the AT2 receptor, and one also displayed high affinity for the AT1 receptor. Molecular modeling showed that the high-affinity compounds could be incorporated into a previously derived model of AT2 receptor ligands.

Short pseudopeptides containing turn scaffolds with high AT2 receptor affinity

Two pentapeptides, Ac-Tyr-Ile-His-Pro-Phe/Ile, were synthesized and shown to have angiotensin II AT2 receptor affinity and agonistic activity. Based on these peptides, a new series of 13 pseudopeptides was synthesized via introduction of five different turn scaffolds replacing the Tyr-Ile amino acid residues. Pharmacological evaluation disclosed subnanomolar affinities for some of these compounds at the AT2 receptor. Substitution of Phe by Ile in this series of ligands enhanced the AT2 receptor affinity of all compounds. These results suggest that the C-terminal amino acid residues can be elaborated on to enhance the AT2 receptor affinity in truncated Ang II analogues.

A non-radioactive method for angiotensin II receptor binding studies using the rat liver

INTRODUCTION

A new non-radioactive method based on competitive ELISA has been developed for binding studies on angiotensin II (Ang II) receptors.

METHOD

Rat liver membrane was used as the source of angiotensin receptors and FITC-angiotensin II (FITC-Ang II) was used as the labeled ligand with an affinity similar to unlabeled Ang II. The effects of different concentrations of Ang II, losartan, CGP-42112A and saralasin were studied on FITC-Ang II binding.

RESULTS

The Ki values for Ang II, losartan and CGP-42112A were calculated as 0.52+/-0.22 nM, 6+/-3 nM and 0.15+/-0.07 nM, respectively. Saralasin inhibited the binding of labeled ligand biphasically, revealing two different populations of Ang receptor with different affinities for saralasin. About 74% of the binding sites were more sensitive to saralasin with a Ki value of 0.32+/-0.04 nM while saralasin showed a Ki value of 2.7+/-0.8 nM for the remaining binding sites.

DISCUSSION

The competitive ELISA method developed in this work yields Ki values for angiotensin antagonists similar to those obtained by others using radiolabeled ligands. The simplicity of this method makes it a suitable alternative to radioligand studies for routine analysis of interaction of drugs with angiotensin receptors.

Angiotensin II pseudopeptides containing 1,3,5-trisubstituted benzene scaffolds with high AT2 receptor affinity

Two 1,3,5-trisubstituted aromatic scaffolds intended to serve as gamma-turn mimetics have been synthesized and incorporated in five pseudopeptide analogues of angiotensin II (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe), replacing Val-Tyr-Ile, Val-Tyr, or Tyr-Ile. All the tested compounds exhibited nanomolar affinity for the AT2 receptor with the best compound (3) having a K(i) of 1.85 nM. Four pseudopeptides were AT2 selective, while one (5) also exhibited good affinity for the AT1 receptor (K(i) = 30.3 nM). This pseudopeptide exerted full agonistic activity in an AT2 receptor induced neurite outgrowth assay but displayed no agonistic effect in an AT1 receptor functional assay. Molecular modeling, using the program DISCOtech, showed that the high-affinity ligands could interact similarly with the AT2 receptor as other ligands with high affinity for this receptor. A tentative agonist model is proposed for AT2 receptor activation by angiotensin II analogues. We conclude that the 1,3,5-trisubstituted benzene rings can be conveniently prepared and are suitable as gamma-turn mimics.

New selective AT2 receptor ligands encompassing a gamma-turn mimetic replacing the amino acid residues 4-5 of angiotensin II act as agonists

New benzodiazepine-based gamma-turn mimetics with one or two amino acid side chains were synthesized. The gamma-turn mimetics were incorporated into angiotensin II (Ang II) replacing the Val(3)-Tyr(4)-Ile(5) or Tyr(4)-Ile(5) peptide segments. All of the resulting pseudopeptides displayed high AT(2)/AT(1) receptor selectivity and exhibited AT(2) receptor affinity in the low nanomolar range. Molecular modeling was used to investigate whether the compounds binding to the AT(2) receptor could position important structural elements in common areas. A previously described benzodiazepine-based gamma-turn mimetic with high affinity for the AT(2) receptor was also included in the modeling. It was found that the molecules, although being structurally quite different, could adopt the same binding mode/interaction pattern in agreement with the model hypothesis. The pseudopeptides selected for agonist studies were shown to act as AT(2) receptor agonists being able to induce outgrowth of neurite cells, stimulate p42/p44(mapk), and suppress proliferation of PC12 cells.