Comparative functional properties of two structurally similar selective nonpeptide drug-like ligands for the angiotensin II type-2 (AT2) receptor. Effects on neurite outgrowth in NG108-15 cells

N-(Heteroaryl)thiophene sulfonamides as angiotensin AT2 receptor ligands

Two series of N-(heteroaryl)thiophene sulfonamides, encompassing either a methylene imidazole group or a tert-butylimidazolylacetyl group in the meta position of the benzene ring, have been synthesized. An AT2R selective ligand with a Ki of 42 nM was identified in the first series and in the second series, six AT2R selective ligands with significantly improved binding affinities and Ki values of <5 nM were discovered. The binding modes to AT2R were explored by docking calculations combined with molecular dynamics simulations. Although some of the high affinity ligands exhibited fair stability in human liver microsomes, comparable to that observed with C21 undergoing clinical trials, most ligands displayed a very low metabolic stability with t½ of less than 10 min in human liver microsomes. The most promising ligand, with an AT2R Ki value of 4.9 nM and with intermediate stability in human hepatocytes (t½ = 77 min) caused a concentration-dependent vasorelaxation of pre-contracted mouse aorta.

Nitric oxide-cyclic GMP role in Ang II induced hyperpolarization in bovine aortic endothelium cell line (BAE-1)

Angiotensin II (Ang II), a mitogen-activated peptide, exerts numerous effects on the cardiovascular system including the regulation of blood pressure. The current study focused on the potential mechanisms that seem to be involved in Ang II vasodilation using bovine aortic endothelial cells (BAE-1) cell lines. Expression of the Ang II receptor (AT2) in BAE-1 was checked by western blots in the presence of valsartan (AT1 inhibitor). To check if Ang II's vasodilator impact was mediated by the nitric oxide (NO) pathway, the Griess reagent was used. Furthermore, cell-attached patch-clamp and fire-polished borosilicate electrodes with a resistance of 3-5 MΩ in the working solutions was used to record membrane currents from treated BAE-1. BEA-1 revealed 50 kDa immunoreactive bands that matched AT2. The concentration of AT2 was elevated in valsartan-treated cells in comparison to control cells. The biochemical experimental data indicated that the NO level increased in a concentration-dependent manner. Meanwhile, Ang II at a concentration of 1 µM, the level of NO increased more than at 100 µM. In patch-clamp experiments, K current and chord conductance were enhanced after incubation of Ang II with valsartan. When 100 µM Ang II was added, the current peaked rapidly and after 15 min of incubation, the maximum value was obtained, as opposed to 10 min and control (110.9 ± 13.3 pA control, 141.4 ± 30.4 pA after 10 min and 174.4 ± 49.3 pA after 15 min). Ang II type two receptor inhibitor (PD1231777) reduced the current and conductance induced by Ang II. The presented data revealed that Ang II released NO via the activation of AT2. K currents were stimulated by Ang II and evoked mainly a current consistent with the activation of K channels.

Neurite outgrowth induced by stimulation of angiotensin II AT2 receptors in SH-SY5Y neuroblastoma cells involves c-Src activation

Neuroblastoma, the most common extracranial solid tumor occurring in childhood, originates from the aberrant proliferation of neural crest cells. Accordingly, the mechanism underling neuronal differentiation could provide new strategies for neuroblastoma treatment. It is well known that neurite outgrowth could be induced by Angiotensin II (Ang II) AT₂ receptors; however, the signaling mechanism and its possible interaction with NGF (neural growth factor) receptors remain unclear. Here, we show that Ang II and CGP42112A (AT₂ receptor agonist) promote neuronal differentiation by inducing neurite outgrowth and βIII-tubulin expression in SH-SY5Y neuroblastoma cells. In addition, we demonstrate that treatment with PD123319 (AT₂ receptor antagonist) reverts Ang II or CGP42112A-induced differentiation. By using specific pharmacological inhibitors we established that neurite outgrowth induced by CGP42112A requires the activation of MEK (mitogen-activated protein kinase kinase), SphK (sphingosine kinase) and c-Src but not PI3K (phosphatidylinositol 3-kinase). Certainly, CGP42112A stimulated a rapid and transient (30 s, 1 min) phosphorylation of c-Src at residue Y⁴¹⁶ (indicative of activation), following by a Src deactivation as indicated by phosphorylation of Y⁵²⁷. Moreover, inhibition of the NGF receptor tyrosine kinase A (TrkA) reduced neurite outgrowth induced by Ang II and CGP42112A. In summary, we demonstrated that AT₂ receptor-stimulated neurite outgrowth in SH-SY5Y cells involves the induction of MEK, SphK and c-Src and suggests a possible transactivation of TrkA. In that regard, AT₂ signaling pathway is a key player in neuronal differentiation and might be a potential target for therapeutic treatments.

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.

Angiotensin II AT2 receptor ligands with phenylthiazole scaffolds

The syntheses and the AT₁R and AT₂R binding data of a series of new small molecule ligands are reported. These ligands comprise a phenylthiazole scaffold rather than the biphenyl or phenylthiophene scaffolds found in essentially all of the previously described ligands originating from the nonselective AT₁R/AT₂R ligand L-162,313 and the AT₂R selective agonist C21, the latter now in Phase II/III clinical trials. A phenylthiazole rather than the phenylthiophene scaffold that is present in the AT₂R selective agonist C21 and in the AT₂R selective antagonist C38 had a deleterious effect on the affinity to AT₂R. Nevertheless, a significant improvement could be accomplished by introduction of a small bulky alkyl group in the 2-position of the imidazole ring attached through a methylene group bridge to the phenylthiazole scaffold. Hence, a combination of a 2-tert-butyl or a 2-isopropyl group and a butoxycarbonyl furnished potent AT₂R selective ligands. Furthermore, a high affinity ligand derived from L-162,313 and exhibiting a > 35 fold selectivity for AT₁R was identified (10). The ligand 21 with the 2-tert-butyl group and ∼ 35 fold selectivity for AT₂R, demonstrated high stability in human, rat and mouse liver microsomes and a very attractive profile with regard to the inhibition of common drug-metabolizing CYP enzymes. Thus, very low levels of inhibition of CYP 3A (5%), 2D6 (12%), 2C8 (26%), 2C9 (23%) and 2B6 (24%) were observed with the 2-tert-butyl derivative comprising the methoxycarbonyl sulfonamide function, levels that are significantly lower than those obtained with C21 under the same experimental conditions.

1,800 MHz Radiofrequency Electromagnetic Irradiation Impairs Neurite Outgrowth With a Decrease in Rap1-GTP in Primary Mouse Hippocampal Neurons and Neuro2a Cells

Background: With the global popularity of communication devices such as mobile phones, there are increasing concerns regarding the effect of radiofrequency electromagnetic radiation (RF-EMR) on the brain, one of the most important organs sensitive to RF-EMR exposure at 1,800 MHz. However, the effects of RF-EMR exposure on neuronal cells are unclear. Neurite outgrowth plays a critical role in brain development, therefore, determining the effects of 1,800 MHz RF-EMR exposure on neurite outgrowth is important for exploring its effects on brain development.

Objectives: We aimed to investigate the effects of 1,800 MHz RF-EMR exposure for 48 h on neurite outgrowth in neuronal cells and to explore the associated role of the Rap1 signaling pathway.

Material and Methods: Primary hippocampal neurons from C57BL/6 mice and Neuro2a cells were exposed to 1,800 MHz RF-EMR at a specific absorption rate (SAR) value of 4 W/kg for 48 h. CCK-8 assays were used to determine the cell viability after 24, 48, and 72 h of irradiation. Neurite outgrowth of primary hippocampal neurons (DIV 2) and Neuro2a cells was observed with a 20 × optical microscope and recognized by ImageJ software. Rap1a and Rap1b gene expressions were detected by real-time quantitative PCR. Rap1, Rap1a, Rap1b, Rap1GAP, and p-MEK1/2 protein expressions were detected by western blot. Rap1-GTP expression was detected by immunoprecipitation. The role of Rap1-GTP was assessed by transfecting a constitutively active mutant plasmid (Rap1-Gly_Val-GFP) into Neuro2a cells.

Results: Exposure to 1,800 MHz RF-EMR for 24, 48, and 72 h at 4 W/kg did not influence cell viability. The neurite length, primary and secondary neurite numbers, and branch points of primary mouse hippocampal neurons were significantly impaired by 48-h RF-EMR exposure. The neurite-bearing cell percentage and neurite length of Neuro2a cells were also inhibited by 48-h RF-EMR exposure. Rap1 activity was inhibited by 48-h RF-EMR with no detectable alteration in either gene or protein expression of Rap1. The protein expression of Rap1GAP increased after 48-h RF-EMR exposure, while the expression of p-MEK1/2 protein decreased. Overexpression of constitutively active Rap1 reversed the decrease in Rap1-GTP and the neurite outgrowth impairment in Neuro2a cells induced by 1,800 MHz RF-EMR exposure for 48 h.

Conclusion: Rap1 activity and related signaling pathways are involved in the disturbance of neurite outgrowth induced by 48-h 1,800 MHz RF-EMR exposure. The effects of RF-EMR exposure on neuronal development in infants and children deserve greater focus.

Facilitation of TRKB Activation by the Angiotensin II Receptor Type-2 (AT2R) Agonist C21

Blockers of angiotensin II type 1 receptor (AT1R) exert antidepressant-like effects by indirectly facilitating the activation of the angiotensin II type 2 receptor (AT2R), which leads to increased surface expression and transactivation of tropomyosin-related kinase B receptors (TRKB). Compound 21 (C21) is a non-peptide AT2R agonist that produces neuroprotective effects. However, the behavioral effects of C21 and its involvement with the brain-derived neurotrophic factor (BDNF)-TRKB system still need further investigation. The aim of the present study was to assess the effect of C21 on the activation of TRKB and its consequences on conditioned fear. The administration of C21 (0.1–10 μM/15 min) increased the surface levels of TRKB but was not sufficient to increase the levels of phosphorylated TRKB (pTRKB) in cultured cortical neurons from rat embryos. Consistent with increased TRKB surface expression, C21 (10 μM/15 min or 3 days) facilitated the effect of BDNF (0.1 ng/mL/15 min) on pTRKB in these cells. In contextual fear conditioning, the freezing time of C21-treated (administered intranasally) wild-type mice was decreased compared to the vehicle-treated group, but no effect of C21 was observed in BDNF.het animals. We observed no effect of C21 in the elevated plus-maze test for anxiety. Taken together, our results indicate that C21 facilitated BDNF effect by increasing the levels of TRKB on the cell surface and reduced the freezing time of mice in a BDNF-dependent manner, but not through a general anxiolytic-like effect.

The Angiotensin II Type 2 Receptor, a Target for Protection and Regeneration of the Peripheral Nervous System?

Preclinical evidence, accumulated over the past decade, indicates that the angiotensin II type 2 receptor (AT2R) stimulation exerts significant neuroprotective effects in various animal models of neuronal injury, notably in the central nervous system. While the atypical G protein-coupled receptor superfamily nature of AT2R and its related signaling are still under investigation, pharmacological studies have shown that stimulation of AT2R leads to neuritogenesis in vitro and in vivo. In this review, we focus on the potential neuroprotective and neuroregenerative roles of AT2R specifically in the peripheral nervous system (PNS). The first section describes the evidence for AT2R expression in the PNS and highlights current controversies concerning the cellular distribution of the receptor. The second section focuses on AT2R signaling implicated in neuronal survival and in neurite outgrowth. The following sections review the relatively few preclinical studies highlighting the putative neuroprotective and neuroregenerative effects of AT2R stimulation in the context of peripheral neuropathy.

The counter regulatory axis of the renin angiotensin system in the brain and ischaemic stroke: Insight from preclinical stroke studies and therapeutic potential

Stroke is the 2nd leading cause of death worldwide and the leading cause of physical disability and cognitive issues. Although we have made progress in certain aspects of stroke treatment, the consequences remain substantial and new treatments are needed. Hypertension has long been recognised as a major risk factor for stroke, both haemorrhagic and ischaemic. The renin angiotensin system (RAS) plays a key role in blood pressure regulation and this, plus local expression and signalling of RAS in the brain, both support the potential for targeting this axis therapeutically in the setting of stroke. While historically, focus has been on suppressing classical RAS signalling through the angiotensin type 1 receptor (AT1R), the identification of a counter-regulatory axis of the RAS signalling via the angiotensin type 2 receptor (AT2R) and Mas receptor has renewed interest in targeting the RAS. This review describes RAS signalling in the brain and the potential of targeting the Mas receptor and AT2R in preclinical models of ischaemic stroke. The animal and experimental models, and the route and timing of intervention, are considered from a translational perspective.

N-(Methyloxycarbonyl)thiophene sulfonamides as high affinity AT2 receptor ligands

A series of meta-substituted acetophenone derivatives, encompassing N-(alkyloxycarbonyl)thiophene sulfonamide fragments have been synthesized. Several selective AT2 receptor ligands were identified, among those a tert-butylimidazole derivative (20) with a K_i of 9.3 nM, that demonstrates a high stability in human liver microsomes (t_½ = 62 min) and in human hepatocytes (t_½ = 194 min). This methyloxycarbonylthiophene sulfonamide is a 20-fold more potent binder to the AT2 receptor and is considerably more stable in human liver microsomes, than a previously reported and broadly studied structurally related AT₂R prototype antagonist 3 (C38). Ligand 20 acts as an AT₂R agonist and caused an AT₂R mediated concentration-dependent vasorelaxation of pre-contracted mouse aorta. Furthermore, in contrast to imidazole derivative C38, the tert-butylimidazole derivative 20 is a poor inhibitor of CYP3A4, CYP2D6 and CYP2C9. It is demonstrated herein that smaller alkyloxycarbonyl groups make the ligands in this series of AT₂R selective compounds less prone to degradation and that a high AT2 receptor affinity can be retained after truncation of the alkyloxycarbonyl group. Binding modes of the most potent AT₂R ligands were explored by docking calculations combined with molecular dynamics simulations.

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.

High affinity rigidified AT2 receptor ligands with indane scaffolds

Rigidification of the isobutyl side chain of drug-like AT2 receptor agonists and antagonists that are structurally related to the first reported selective AT2 receptor agonist 1 (C21) delivered bioactive indane derivatives. Four enantiomer pairs were synthesized and the enantiomers were isolated in an optical purity >99%. The enantiomers 7a, 7b, 8a, 8b, 9a, 9b, 10a and 10b bind to the AT2 receptor with moderate (K i = 54-223 nM) to high affinity (K i = 2.2-7.0 nM). The enantiomer with positive optical rotation (+) exhibited the highest affinity at the receptor. The indane derivatives 7b and 10a are among the most potent AT2 receptor antagonists reported so far. As illustrated by the enantiomer pairs 7a/b and 10a/b, an alteration at the stereogenic center has a pronounced impact on the activation process of the AT2 receptor, and can convert agonists to antagonists and vice versa.

A Series of Analogues to the AT2R Prototype Antagonist C38 Allow Fine Tuning of the Previously Reported Antagonist Binding Mode

We here report on our continued studies of ligands binding to the promising drug target angiotensin II type 2 receptor (AT2R). Two series of compounds were synthesized and investigated. The first series explored the effects of adding small substituents to the phenyl ring of the known selective nonpeptide AT2R antagonist C38, generating small but significant shifts in AT2R affinity. One compound in the first series was equipotent to C38 and showed similar kinetic solubility, and stability in both human and mouse liver microsomes. The second series was comprised of new bicyclic derivatives, amongst which one ligand exhibited a five-fold improved affinity to AT2R as compared to C38. The majority of the compounds in the second series, including the most potent ligand, were inferior to C38 with regard to stability in both human and mouse microsomes. In contrast to our previously reported findings, ligands with shorter carbamate alkyl chains only demonstrated slightly improved stability in microsomes. Based on data presented herein, a more adequate, tentative model of the binding modes of ligand analogues to the prototype AT2R antagonist C38 is proposed, as deduced from docking redefined by molecular dynamic simulations.

A convenient transesterification method for synthesis of AT2 receptor ligands with improved stability in human liver microsomes

A series of AT₂R ligands have been synthesized applying a quick, simple, and safe transesterification-type reaction whereby the sulfonyl carbamate alkyl tail of the selective AT₂R antagonist C38 was varied. Furthermore, a limited number of compounds where acyl sulfonamides and sulfonyl ureas served as carboxylic acid bioisosteres were synthesized and evaluated. By reducing the size of the alkyl chain of the sulfonyl carbamates, ligands 7a and 7b were identified with significantly improved in vitro metabolic stability in both human and mouse liver microsomes as compared to C38 while retaining the AT₂R binding affinity and AT₂R/AT₁R selectivity. Eight of the compounds synthesized exhibit an improved stability in human microsomes as compared to C38.

Interconversion of Functional Activity by Minor Structural Alterations in Nonpeptide AT2 Receptor Ligands

Migration of the methylene imidazole side chain in the first reported selective drug-like AT2 receptor agonist C21/M024 (1) delivered the AT2 receptor antagonist C38/M132 (2). We now report that the AT2 receptor antagonist compound 4, a biphenyl derivative that is structurally related to 2, is transformed to the agonist 6 by migration of the isobutyl group. The importance of the relative position of the methylene imidazole and the isobutyl substituent is highlighted herein.

Saralasin and Sarile Are AT2 Receptor Agonists

Saralasin and sarile, extensively studied over the past 40 years as angiotensin II (Ang II) receptor blockers, induce neurite outgrowth in a NG108-15 cell assay to a similar extent as the endogenous Ang II. In their undifferentiated state, these cells express mainly the AT2 receptor. The neurite outgrowth was inhibited by preincubation with the AT2 receptor selective antagonist PD 123,319, which suggests that the observed outgrowth was mediated by the AT2 receptor. Neither saralasin nor sarile reduced the neurite outgrowth induced by Ang II proving that the two octapeptides do not act as antagonists at the AT2 receptor and may be considered as AT2 receptor agonists.

Electronic sculpting of ligand-GPCR subtype selectivity: the case of angiotensin II

GPCR subtypes possess distinct functional and pharmacological profiles, and thus development of subtype-selective ligands has immense therapeutic potential. This is especially the case for the angiotensin receptor subtypes AT1R and AT2R, where a functional negative control has been described and AT2R activation highlighted as an important cancer drug target. We describe a strategy to fine-tune ligand selectivity for the AT2R/AT1R subtypes through electronic control of ligand aromatic-prolyl interactions. Through this strategy an AT2R high affinity (K_i = 3 nM) agonist analogue that exerted 18,000-fold higher selectivity for AT2R versus AT1R was obtained. We show that this compound is a negative regulator of AT1R signaling since it is able to inhibit MCF-7 breast carcinoma cellular proliferation in the low nanomolar range.

N-Aryl Isoleucine Derivatives as Angiotensin II AT2 Receptor Ligands

A novel series of ligands for the recombinant human AT2 receptor has been synthesized utilizing a fast and efficient palladium-catalyzed procedure for aminocarbonylation as the key reaction. Molybdenum hexacarbonyl [Mo(CO)6] was employed as the carbon monoxide source, and controlled microwave heating was applied. The prepared N-aryl isoleucine derivatives, encompassing a variety of amide groups attached to the aromatic system, exhibit binding affinities at best with K i values in the low micromolar range versus the recombinant human AT2 receptor. Some of the new nonpeptidic isoleucine derivatives may serve as starting points for further structural optimization. The presented data emphasize the importance of using human receptors in drug discovery programs.

Synthesis and evaluation of isoleucine derived angiotensin II AT(2) receptor ligands

Sixteen new C-terminally modified analogues of 2, a previously described potent and selective AT2R ligand, were designed, synthesized and evaluated for their affinity to the AT2R receptor. The introduction of large, hydrophobic substituents was shown to be beneficial and the most active compound (17, Ki=8.5 μM) was over 12-times more potent than the lead compound 2.

Expression and role of the angiotensin II AT2 receptor in human prostate tissue: in search of a new therapeutic option for prostate cancer

BACKGROUND

Evidence shows that angiotensin II type 1 receptor (AT1R) blockers may be associated with improved outcome in prostate cancer patients. It has been proposed that part of this effect could be due to angiotensin II type 2 receptor (AT2R) activation, the only active angiotensin II receptor in this situation. This study aimed to characterize the localization and expression of AT2R in prostate tissues and to assess its role on cell morphology and number in prostatic epithelial cells in primary culture.

METHODS

AT2R and its AT2R-interacting protein (ATIP) expression were assessed on non-tumoral and tumoral human prostate using tissue microarray immunohistochemistry, binding assay, and Western blotting. AT2R effect on cell number was measured in primary cultures of epithelial cells from non-tumoral human prostate.

RESULTS

AT2R was localized at the level of the acinar epithelial layer and its expression decreased in cancers with a Gleason score 6 or higher. In contrast, ATIP expression increased with cancer progression. Treatment of primary cell cultures from non-tumoral prostate tissues with C21/M024, a selective AT2R agonist, alone or in co-incubation with losartan, an AT1R antagonist, significantly decreased cell number compared to untreated cells.

CONCLUSIONS

AT2R and ATIP are present in non-tumoral human prostate tissues and differentially regulated according to Gleason score. The decrease in non-tumoral prostate cell number upon selective AT2R stimulation suggests that AT2R may have a protective role against prostate cancer development. Treatment with a selective AT2R agonist could represent a new approach for prostate cancer prevention or for patients on active surveillance.

AT2 Receptor-Interacting Proteins ATIPs in the Brain

A complete renin-angiotensin system (RAS) is locally expressed in the brain and fulfills important functions. Angiotensin II, the major biologically active peptide of the RAS, acts via binding to two main receptor subtypes designated AT1 and AT2. The present paper focuses on AT2 receptors, which have been reported to have neuroprotective effects on stroke, degenerative diseases, and cognitive functions. Our group has identified a family of AT2 receptor interacting proteins (ATIPs) comprising three major members (ATIP1, ATIP3, and ATIP4) with different intracellular localization. Of interest, all ATIP members are expressed in brain tissues and carry a conserved domain able to interact with the AT2 receptor intracellular tail, suggesting a role in AT2-mediated brain functions. We summarize here current knowledge on the ATIP family of proteins, and we present new experimental evidence showing interaction defects between ATIP1 and two mutant forms of the AT2 receptor identified in cases of mental retardation. These studies point to a functional role of the AT2/ATIP1 axis in cognition.