Constitutively Active Homo-oligomeric Angiotensin II Type 2 Receptor Induces Cell Signaling Independent of Receptor Conformation and Ligand Stimulation

Molecular mechanisms of the antagonistic action between AT1 and AT2 receptors

Although angiotensin II (Ang II) binds to Ang II type 1 (AT(1)) and type 2 (AT(2)) receptors, AT(1) and AT(2) receptors have antagonistic actions with regard to cell signaling. The molecular mechanisms that underlie this antagonism are not well understood. We examined AT(1) and AT(2) receptor-induced signal cross-talk in the cytoplasm and the importance of the hetero-dimerization of AT(1) receptor with AT(2) receptor on the cell surface. AT(1) and AT(2) receptors showed antagonistic effects toward inositol phosphate production. AT(1) receptors mainly formed homo-dimers, rather than hetero-dimers with AT(2) receptor, on the cell surface as determined by immunoprecipitation, and subsequently induced cell signals. AT(2) receptor mainly formed homo-dimers, rather than hetero-dimers with AT(1) receptor, on the cell surface. The expression levels of homo-dimerized AT(1) receptor or AT(2) receptor on the cell surface did not change after treatment with Ang II, the AT(1) receptor antagonist telmisartan or the AT(2) receptor antagonist PD123319. Finally, AT(1) and AT(2) receptor-induced signals antagonized phospholipase C-beta(3) phosphorylation. In conclusion, Ang II-induced AT(1) receptor signals may be mainly blocked by AT(2) receptor signals through their negative cross-talk in the cytoplasm rather than by the hetero-dimerization of both receptors on the cell surface. The proper balance of the expression levels of AT(1) and AT(2) receptors might be critical for the antagonistic action between these receptors.

The angiotensin II type 2 receptor in cardiovascular disease

Angiotensin II (Ang II) is considered the major final mediator of the renin-angiotensin system. The actions of Ang II have been implicated in many cardiovascular conditions, such as hypertension, atherosclerosis, coronary heart disease, restenosis, and heart failure. Ang II can act through two different receptors: Ang II type 1 (AT(1)) receptor and Ang II type 2 (AT(2)) receptor. The AT(1) receptor is ubiquitously expressed in the cardiovascular system and mediates most of the physiological and pathophysiological actions of Ang II. The AT(2) receptor is highly expressed in the developing foetus, but its expression is very low in the cardiovascular system of the normal adult. Expression of the AT(2) receptor can be modulated by pathological states associated with tissue remodelling or inflammation such as hypertension, atherosclerosis, and myocardial infarction. The precise role of the AT(2) receptor remains under debate. However, it appears that the AT(2) receptor plays a vasodilatory role, and may be enhanced as a countervailing mechanism in cardiac hypertrophy, and in presence of vascular injury in hypertension and atherosclerosis. Signalling pathways induced by the stimulation of the AT(2) receptor are poorly understood, but three main mechanisms have been described: (a) activation of protein phosphatases causing protein dephosphorylation; (b) activation of bradykinin/nitric oxide/cyclic guanosine 3',5'-monophosphate pathway; and (c) stimulation of phospholipase A(2) and release of arachidonic acid. Vasodilatory effects of the AT(2) receptor, probably the only well-established role of the AT(2) receptor, have been attributed to the second of these mechanisms. The participation of the AT(2) receptor in cardiovascular remodelling and inflammation is more controversial. In vitro, AT(2) receptor stimulation clearly inhibits cardiac and vascular smooth muscle growth and proliferation, and stimulates apoptosis. In vivo, the situation is less clear, and depending on the studies, the AT(2) receptor appears to be required for cardiac hypertrophic growth or contrariwise, the AT(2) receptor has demonstrated no effects on cardiac hypertrophy. Similar controversial findings have been reported in atherosclerosis. Here we discuss the role of the AT(2) receptor on cardiovascular structure and disease, and the signalling pathways induced by its activation.

The past, present and future of angiotensin II type 2 receptor stimulation

Studying the angiotensin type 2 receptor (AT2) has been problematic in the past because a pharmacological tool for direct, specific in vitro and in vivo stimulation of the receptor has been lacking. Consequently, current knowledge about AT2 receptor signalling and function had to be obtained by indirect approaches, like studying animals or cells with genetically altered AT2 receptor expression levels, inhibitory experiments using specific AT2 receptor antagonists, stimulation with angiotensin II under concomitant angiotensin II type 1 receptor blockade or stimulation with the peptide agonist CGP42112A, which has additional AT2 receptor antagonistic properties. The recently developed non-peptide AT2 receptor agonist Compound 21 now, for the first time, allows direct, selective and specific AT2 receptor stimulation in vitro and in vivo . This new tool will certainly revolutionise AT2 receptor research, enable many new insights into AT2 receptor function and may also have the potential to become a future medical drug. This article reviews milestone findings about AT2 receptor functional properties obtained by ‘conventional’ experimental approaches within the last 20 years. Moreover, it provides an overview of the first results obtained by direct AT2 receptor stimulation with Compound 21, comprising effects on alkaline secretion, neurite outgrowth, blood pressure and post-infarct cardiac function.

Adapter proteins and promoter regulation of the angiotensin AT2 receptor — implications for cardiac pathophysiology

The angiotensin AT 2 receptor (AT2R) represents an important component of the renin-angiotensin system since it is involved in the (patho) physiology of different cardiovascular and neuronal diseases. Furthermore, AT2 receptors can partly mediate beneficial effects of angiotensin AT 1 receptor (AT1R) blockers, and direct pharmacological AT 2 receptor agonism emerges as a novel therapeutic strategy. This review discusses the constitutive and ligand-mediated activity as well as the signal transduction of the AT2 receptor, focusing on adapter proteins which directly bind to this receptor. Direct protein-protein interaction partners of the AT2 receptor described so far include the transcription factor promyelocytic zinc finger protein, AT2 receptor binding protein and the AT1 receptor. In addition, the putative crosstalk of the AT2 receptor with the renin/ prorenin receptor (RER) via the promyelocytic zinc finger protein (PLZF) and the role of oestrogens on the regulation of the AT2 receptor are presented. Conceiving the coupling of the AT2 receptor to different adapter proteins with distinct and partly opposing cellular effects and the implications of its constitutive activity might help to overcome the current controversies on the (patho)physiological role of the AT2 receptor.

Functional interactions between 7TM receptors in the renin-angiotensin system--dimerization or crosstalk?

The Renin-Angiotensin System (RAS) is important for the regulation of cardiovascular physiology, where it controls blood pressure, and salt- and water homeostasis. Dysregulation of RAS can lead to severe diseases including hypertension, diabetic nephropathy, and cardiac arrhythmia, and -failure. The importance of the RAS is clearly emphasised by the widespread use of drugs targeting this system in clinical practice. These include, renin inhibitors, angiotensin II receptor type I blockers, and inhibitors of the angiotensin converting enzyme. Some of the important effectors within the system are 7 transmembrane (7TM) receptors (or G-protein-coupled receptors) such as the angiotensin II Receptors type I and II (AT1R and AT2R) and the MAS-oncogene receptor. Several findings indicate that the 7TM receptors can form both homo- and heterodimers, or higher orders of oligomers. Furthermore, dimerization may be important for receptor function, and in the development of cardiovascular diseases. This is very significant, since "dimers" may provide pharmacologists with novel targets for improved drug therapy. However, we know that 7TM receptors can mediate signals as monomeric units, and so far it has been very difficult to establish if our observations reflect actual well-defined dimerization or merely reflect close proximity between the receptors and/or various types of functional interaction. In this review, we will present and critically discuss the current data on 7TM receptor dimerization with a clear focus on the RAS, and delineate future challenges within the field.

Involvement of c-Src tyrosine kinase in SHP-1 phosphatase activation by Ang II AT2 receptors in rat fetal tissues

Angiotensin II (Ang II) AT(2) receptors are abundantly expressed in rat fetal tissues where they probably contribute to development. In the present study we examine the effects of Ang II type 2 receptor stimulation on SHP-1 activation. Ang II (10(-7) M) elicits a rapid and transient tyrosine phosphorylation of SHP-1, maximal at 1 min, in a dose-dependent form, blocked by the AT(2) antagonist, PD123319. SHP-1 phosphorylation is followed in time by tyrosine dephosphorylation of different proteins, suggesting a sequence of events. Ang II induces association of SHP-1 to AT(2) receptors as shown by co-immunoprecipitation, Western blot and binding assays. SHP-1 activity was determined in immunocomplexes obtained with either anti-AT(2) or anti-SHP-1 antibodies, after Ang II stimulation (1 min), in correlation with the maximal level of SHP-1 phosphorylation. Interestingly, following receptor stimulation (1 min) c-Src was associated to AT(2) or SHP-1 immunocomplexes. Preincubation with the c-Src inhibitor PP2 inhibited SHP-1 activation and c-Src association, thus confirming the participation of c-Src in this pathway. We demonstrated here for the first time the involvement of c-Src in SHP-1 activation via AT(2) receptors present in an ex vivo model expressing both receptor subtypes. In this model, AT(2) receptors are not constitutively associated to SHP-1 and SHP-1 is not constitutively activated. Thus, we clearly establish that SHP-1 activation, mediated by the AT(2) subtype, involves c-Src and precedes protein tyrosine dephosphorylation, in rat fetal membranes.

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 type 2 receptor stimulation: a novel option of therapeutic interference with the renin-angiotensin system in myocardial infarction?

BACKGROUND

This study is the first to examine the effect of direct angiotensin II type 2 (AT(2)) receptor stimulation on postinfarct cardiac function with the use of the novel nonpeptide AT(2) receptor agonist compound 21 (C21).

METHODS AND RESULTS

Myocardial infarction (MI) was induced in Wistar rats by permanent ligation of the left coronary artery. Treatment with C21 (0.01, 0.03, 0.3 mg/kg per day IP) was started 24 hours after MI and was continued until euthanasia (7 days after MI). Infarct size was assessed by magnetic resonance imaging, and hemodynamic measurements were performed via transthoracic Doppler echocardiography and intracardiac Millar catheter. Cardiac tissues were analyzed for inflammation and apoptosis markers with immunoblotting and real-time reverse transcription polymerase chain reaction. C21 significantly improved systolic and diastolic ventricular function. Scar size was smallest in the C21-treated rats. In regard to underlying mechanisms, C21 diminished MI-induced Fas-ligand and caspase-3 expression in the peri-infarct zone, indicating an antiapoptotic effect. Phosphorylation of the p44/42 and p38 mitogen-activated protein kinases, both involved in the regulation of cell survival, was strongly reduced after MI but almost completely rescued by C21 treatment. Furthermore, C21 decreased MI-induced serum monocyte chemoattractant protein-1 and myeloperoxidase as well as cardiac interleukin-6, interleukin-1beta, and interleukin-2 expression, suggesting an antiinflammatory effect.

CONCLUSIONS

Direct AT(2) receptor stimulation may be a novel therapeutic approach to improve post-MI systolic and diastolic function by antiapoptotic and antiinflammatory mechanisms.

Conformational switch of angiotensin II type 1 receptor underlying mechanical stress-induced activation

The angiotensin II type 1 (AT(1)) receptor is a G protein-coupled receptor that has a crucial role in the development of load-induced cardiac hypertrophy. Here, we show that cell stretch leads to activation of the AT(1) receptor, which undergoes an anticlockwise rotation and a shift of transmembrane (TM) 7 into the ligand-binding pocket. As an inverse agonist, candesartan suppressed the stretch-induced helical movement of TM7 through the bindings of the carboxyl group of candesartan to the specific residues of the receptor. A molecular model proposes that the tight binding of candesartan to the AT(1) receptor stabilizes the receptor in the inactive conformation, preventing its shift to the active conformation. Our results show that the AT(1) receptor undergoes a conformational switch that couples mechanical stress-induced activation and inverse agonist-induced inactivation.

Differential bonding interactions of inverse agonists of angiotensin II type 1 receptor in stabilizing the inactive state

Although the sartan family of angiotensin II type 1 (AT(1)) receptor blockers (ARBs), which includes valsartan, olmesartan, and losartan, have a common pharmacophore structure, their effectiveness in therapy differs. Although their efficacy may be related to their binding strength, this notion has changed with a better understanding of the molecular mechanism. Therefore, we hypothesized that each ARB differs with regard to its molecular interactions with AT(1) receptor in inducing inverse agonism. Interactions between valsartan and residues Ser(105), Ser(109), and Lys(199) were important for binding. Valsartan is a strong inverse agonist of constitutive inositol phosphate production by the wild-type and N111G mutant receptors. Substituted cysteine accessibility mapping studies indicated that valsartan, but not losartan, which has only weak inverse agonism, may stabilize the N111G receptor in an inactive state upon binding. In addition, the inverse agonism by valsatan was mostly abolished with S105A/S109A/K199Q substitutions in the N111G background. Molecular modeling suggested that Ser(109) and Lys(199) bind to phenyl and tetrazole groups of valsartan, respectively. Ser(105) is a candidate for binding to the carboxyl group of valsartan. Thus, the most critical interaction for inducing inverse agonism involves transmembrane (TM) V (Lys(199)) of AT(1) receptor although its inverse agonist potency is comparable to olmesartan, which bonds with TM III (Tyr(113)) and TM VI (His(256)). These results provide new insights into improving ARBs and development of new G protein-coupled receptor antagonists.

Emerging Concepts of Regulation of Angiotensin II Receptors

Angiotensin (Ang) II exerts its important physiological functions through 2 distinct receptor subtypes, type 1 (AT 1 ) and type 2 (AT 2 ) receptors. Recently, new evidence has accumulated showing the existence of several novel receptor interacting proteins and various angiotensin II receptor activation mechanisms beyond the classical actions of receptors for Ang II. These associated proteins could contribute not only to Ang II receptors’ functions, but also to influencing pathophysiological states. Receptor dimerization of Ang II receptors such as homodimer, heterodimer, and complex formation with other G protein-coupled receptors has also been focused on as a new mechanism of their activation or inactivation. Moreover, ligand-independent receptor activation systems such as mechanical stretch for the AT 1 receptor have also been revealed. These emerging concepts of regulation of Ang II receptors and a new insight into future drug discovery are discussed in this review.

The angiotensin II receptor 2 is expressed and mediates angiotensin II signaling in lung fibrosis

Idiopathic pulmonary fibrosis (IPF) is a severe interstitial lung disease unresponsive to currently available therapies. In IPF, initial alveolar epithelial cell damage leads to activation of fibroblast-(myo)fibroblasts, which deposit an increased amount of a collagen-rich extracellular matrix. Angiotensin II (ANGII) signaling, mediated via angiotensin II receptor type 1 (AGTR1) or type 2 (AGTR2), controls tissue remodeling in fibrosis, but the relevance of AGTR2 remains elusive. In the present study, we demonstrated increased expression of AGTR1 und AGTR2 in human and rodent lung tissues from patients with IPF and mice subjected to bleomycin-induced fibrosis, respectively. Both AGTR1 und AGTR2 localized to interstitial fibroblasts. Quantitative analysis of cell surface expression in primary mouse fibroblasts revealed a significant increase of AGTR2 surface expression in fibrotic fibroblasts, whereas AGTR1 surface expression levels remained similar. ANGII treatment of normal fibroblasts led to enhanced migration and proliferation, which was abrogated after pretreatment with losartan (LOS), an AGTR1 inhibitor. In contrast, in fibrotic fibroblasts, migration and proliferation was modified only by AGTR2, but not AGTR1 inhibition (using PD123319). ANGII-induced effects were mediated via phosphorylation of the mitogen-activated protein kinases p38 and p42/44, which was blocked via LOS and PD123319, respectively. Similar effects of AGTR1 and AGTR2 inhibition were observed using conditioned media of alveolar epithelial cells, a prominent source of ANGII in the lung in vivo. In summary, we conclude that ANGII signaling occurs primarily via AGTR1 in normal fibroblasts, while AGTR2-mediated effects are dominant on activated (myo)-fibroblasts, a receptor switch that may perturb epithelial-mesenchymal interaction, thereby further perpetuating fibrogenesis.

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.

Two transmembrane Cys residues are involved in 5-HT4 receptor dimerization

The 5-HT(4) receptor (5-HT(4)R) belongs to the G-protein-coupled receptor (GPCR) family and is of considerable interest for the development of new drugs to treat gastrointestinal diseases and memory disorders. The 5-HT(4)R exists as a constitutive dimer but its molecular determinants are still unknown. Using co-immunoprecipitation and Bioluminescence Resonance Energy Transfer (BRET) techniques, we show here that 5-HT(4)R homodimerization but not 5-HT(4)R-beta(2) adrenergic receptor (beta(2)AR) heterodimerization is largely decreased under reducing conditions suggesting the participation of disulfide bonds in 5-HT(4)R dimerization. Molecular modeling and protein docking experiments identified four cysteine (Cys) residues potentially involved in the dimer interface through intramolecular or intermolecular disulfide bonds. We show that disulfide bridges between Cys112 and Cys145 located within TM3 and TM4, respectively, are of critical importance for 5-HT(4)R dimer formation. Our data suggest that two disulfide bridges between two transmembrane Cys residues are involved in the dimerization interface of a GPCR.

Technology Insight: modern methods to monitor protein-protein interactions reveal functional TSH receptor oligomerization

The formation of supramolecular structures (dimers or oligomers) is emerging as an important aspect of G-protein-coupled receptor (GPCR) biology. In some cases, GPCR oligomerization is a prerequisite for membrane targeting or function; in others, the relevance of the phenomenon is presently unknown. Although supramolecular structures of GPCRs were initially documented by classical biochemical techniques such as coimmunoprecipitation, many recent advances in the field of GPCR oligomerization have been prompted by the introduction of two new biophysical assays based on Förster's resonance energy transfer-fluorescence resonance energy transfer and bioluminescence resonance energy transfer. These modern techniques allow the study of protein-protein interaction in intact cells, and can be used to monitor monomer association and dissociation in vivo. Recently, oligomerization has also been reported in the case of the TSH receptor (TSHR). This review will focus on the previously unsuspected implications that oligomerization has in TSHR physiology and pathology. It is now clear that TSHR oligomerization is constitutive, occurs early during post-translational processing, and may be involved in membrane targeting and activation by the hormone or by stimulating antibodies. Oligomerization between inactive mutants and wild-type TSHR provides a molecular explanation for the dominant forms of TSH resistance.

Effects of losartan on angiotensin receptors in the hypertrophic rat heart

The effects of losartan on angiotensin receptors in hypertrophic rat hearts were studied. The study was prompted by inconsistent findings of either an increase or decrease in the mRNA of the AT1 receptor in the hearts of cardiac hypertrophic rats treated with losartan, and a paucity of information on the effects of losartan on functional angiotensin receptors in the heart. Losartan, administered i.p. to aortic coarcted rats, dose-dependently attenuated the cardiac hypertrophy. Significant effect was observed with a dose of 2.72 micromol/kg/day for four days. Hypertrophy was accompanied by an increase in [125I]-Sar1-Ile8-angiotensin II binding sites (due mainly to an increase in AT2 binding) and AT2 receptor protein in cardiac ventricles of aortic coarcted rats. Treatment with effective anti-hypertrophic doses of losartan dose-dependently downregulated the [125I]-Sar1-Ile8-angiotensin II binding sites, constitutive AT1 receptor protein, and the over expressed AT2 receptor protein. It was suggested that the anti-cardiac hypertrophic action of losartan resulted from its ability to suppress the expression of both the basal and enhanced cardiac angiotensin receptors. This raises the question as to whether such drastic action could form the therapeutic basis for the use of losartan in cardiac pathologies.

The Angiotensin II Type 2 Receptor Causes Constitutive Growth of Cardiomyocytes and Does Not Antagonize Angiotensin II Type 1 Receptor–Mediated Hypertrophy

Angiotensin II (Ang II) has important actions on the heart via type 1 (AT 1 ) and type 2 (AT 2 ) receptors. The link between AT 1 receptor activation and the hypertrophy of cardiomyocytes is accepted, whereas the contribution of the AT 2 receptor, which reportedly antagonizes the AT 1 receptor, is contentious. This ambiguity is primarily based on in vivo approaches, in which the direct effect of the AT 2 receptor and its modulation of the AT 1 receptor (at the level of the cardiomyocyte) are difficult to establish. In this study, we used adenoviruses encoding AT 1 and AT 2 to coexpress these receptors in isolated cardiomyocytes, allowing a direct examination of the consequence of varying AT 1 /AT 2 stoichiometry on cardiomyocyte hypertrophy. In myocytes expressing only the AT 1 receptor, Ang II stimulation promoted robust hypertrophy (increased protein:DNA ratio and phenotypic changes) via activation of mitogen-activated protein kinases (MAPKs). Titration of the AT 2 receptor against the AT 1 receptor did not inhibit Ang II–mediated cardiomyocyte hypertrophy. Instead, basal and Ang II–mediated hypertrophy was increased in line with the amplified expression of the AT 2 receptor, indicating a capacity for the AT 2 receptor to enhance basal cardiomyocyte growth. Indeed, expression of the AT 2 receptor alone resulted in hypertrophy; remarkably, this was unaffected by Ang II stimulation or the AT 2 receptor–specific ligands PD123319 and CGP42112. Although previous studies have indicated that the AT 2 receptor can antagonize MAPK activation via the AT 1 receptor, we found no evidence for this in cardiomyocytes. Thus, the AT 2 receptor promotes ligand-independent, constitutive cardiomyocyte hypertrophy and does not directly antagonize the AT 1 receptor in this setting.