Identification of the region of AT2 receptor needed for inhibition of the AT1 receptor-mediated inositol 1,4,5-triphosphate generation

Novel Pharmacology Following Heteromerization of the Angiotensin II Type 2 Receptor and the Bradykinin Type 2 Receptor

The angiotensin type 2 (AT₂) receptor and the bradykinin type 2 (B₂) receptor are G protein-coupled receptors (GPCRs) that have major roles in the cardiovascular system. The two receptors are known to functionally interact at various levels, and there is some evidence that the observed crosstalk may occur as a result of heteromerization. We investigated evidence for heteromerization of the AT₂ receptor and the B₂ receptor in HEK293FT cells using various bioluminescence resonance energy transfer (BRET)-proximity based assays, including the Receptor Heteromer Investigation Technology (Receptor-HIT) and the NanoBRET ligand-binding assay. The Receptor-HIT assay showed that Gα_q, GRK2 and β-arrestin2 recruitment proximal to AT₂ receptors only occurred upon B₂ receptor coexpression and activation, all of which is indicative of AT₂-B₂ receptor heteromerization. Additionally, we also observed specific coupling of the B₂ receptor with the Gα_z protein, and this was found only in cells coexpressing both receptors and stimulated with bradykinin. The recruitment of Gα_z, Gα_q, GRK2 and β-arrestin2 was inhibited by B₂ receptor but not AT₂ receptor antagonism, indicating the importance of B₂ receptor activation within AT₂-B₂ heteromers. The close proximity between the AT₂ receptor and B₂ receptor at the cell surface was also demonstrated with the NanoBRET ligand-binding assay. Together, our data demonstrate functional interaction between the AT₂ receptor and B₂ receptor in HEK293FT cells, resulting in novel pharmacology for both receptors with regard to Gα_q/GRK2/β-arrestin2 recruitment (AT₂ receptor) and Gα_z protein coupling (B₂ receptor). Our study has revealed a new mechanism for the enigmatic and poorly characterized AT₂ receptor to be functionally active within cells, further illustrating the role of heteromerization in the diversity of GPCR pharmacology and signaling.

Suppression of Inflammatory Cardiac Cytokine Network in Rats with Untreated Obesity and Pre-Diabetes by AT2 Receptor Agonist NP-6A4

Obesity affects over 42% of the United States population and exacerbates heart disease, the leading cause of death in men and women. Obesity also increases pro-inflammatory cytokines that cause chronic tissue damage to vital organs. The standard-of-care does not sufficiently attenuate these inflammatory sequelae. Angiotensin II receptor AT2R is an anti-inflammatory and cardiovascular protective molecule; however, AT2R agonists are not used in the clinic to treat heart disease. NP-6A4 is a new AT2R peptide agonist with an FDA orphan drug designation for pediatric cardiomyopathy. NP-6A4 increases AT2R expression (mRNA and protein) and nitric oxide generation in human cardiovascular cells. AT2R-antagonist PD123319 and AT2RSiRNA suppress NP-6A4-effects indicating that NP-6A4 acts through AT2R. To determine whether NP-6A4 would mitigate cardiac damage from chronic inflammation induced by untreated obesity, we investigated the effects of 2-weeks NP-6A4 treatment (1.8 mg/kg delivered subcutaneously) on cardiac pathology of male Zucker obese (ZO) rats that display obesity, pre-diabetes and cardiac dysfunction. NP-6A4 attenuated cardiac diastolic and systolic dysfunction, cardiac fibrosis and cardiomyocyte hypertrophy, but increased myocardial capillary density. NP-6A4 treatment suppressed tubulointerstitial injury marker urinary β-NAG, and liver injury marker alkaline phosphatase in serum. These protective effects of NP-6A4 occurred in the presence of obesity, hyperinsulinemia, hyperglycemia, and hyperlipidemia, and without modulating blood pressure. NP-6A4 increased expression of AT2R (consistent with human cells) and cardioprotective erythropoietin (EPO) and Notch1 in ZO rat heart, but suppressed nineteen inflammatory cytokines. Cardiac miRNA profiling and in silico analysis showed that NP-6A4 activated a unique miRNA network that may regulate expression of AT2R, EPO, Notch1 and inflammatory cytokines, and mitigate cardiac pathology. Seventeen pro-inflammatory and pro-fibrotic cytokines that increase during lethal cytokine storms caused by infections such as COVID-19 were among the cytokines suppressed by NP-6A4 treatment in ZO rat heart. Thus, NP-6A4 activates a novel anti-inflammatory network comprised of 21 proteins in the heart that was not reported previously. Since NP-6A4's unique mode of action suppresses pro-inflammatory cytokine network and attenuates myocardial damage, it can be an ideal adjuvant drug with other anti-glycemic, anti-hypertensive, standard-of-care drugs to protect the heart tissues from pro-inflammatory and pro-fibrotic cytokine attack induced by obesity.

Investigation of Receptor Heteromers Using NanoBRET Ligand Binding

Receptor heteromerization is the formation of a complex involving at least two different receptors with pharmacology that is distinct from that exhibited by its constituent receptor units. Detection of these complexes and monitoring their pharmacology is crucial for understanding how receptors function. The Receptor-Heteromer Investigation Technology (Receptor-HIT) utilizes ligand-dependent modulation of interactions between receptors and specific biomolecules for the detection and profiling of heteromer complexes. Previously, the interacting biomolecules used in Receptor-HIT assays have been intracellular proteins, however in this study we have for the first time used bioluminescence resonance energy transfer (BRET) with fluorescently-labeled ligands to investigate heteromerization of receptors on the cell surface. Using the Receptor-HIT ligand binding assay with NanoBRET, we have successfully investigated heteromers between the angiotensin II type 1 (AT1) receptor and the β2 adrenergic receptor (AT1-β2AR heteromer), as well as between the AT1 and angiotensin II type 2 receptor (AT1-AT2 heteromer).

Telmisartan Prevents Alveolar Bone Loss by Decreasing the Expression of Osteoclasts Markers in Hypertensive Rats With Periodontal Disease

Periodontal disease (PD) is a prevalent inflammatory disease with the most severe consequence being the loss of the alveolar bone and teeth. We therefore aimed to evaluate the effects of telmisartan (TELM), an angiotensin II type 1 receptor (Agtr1) antagonist, on the PD-induced alveolar bone loss, in Wistar (W) and Spontaneous Hypertensive Rats (SHRs). PD was induced by ligating the lower first molars with silk, and 10 mg/kg TELM was concomitantly administered for 15 days. The hemimandibles were subjected to microtomography, ELISA was used for detecting tumor necrosis factor (TNF-α), interleukin-1β (IL-1β), interleukin-6 (IL-6), CXCL3, and CCL2, while qRT-PCR was used for analyzing expression of components of renin-angiotensin system (RAS) (Agt, Ace, Agt1r, Agt2r, Ace2, and Masr), and bone markers (Runx2, Osx, Catnb, Alp, Col1a1, Opn, Ocn, Bsp, Bmp2, Trap, Rank, Rankl, CtsK, Mmp-2, Mmp-9, and osteoclast-associated receptor (Oscar)). The SHR + PD group showed greater alveolar bone loss than the W + PD group, what was significantly inhibited by treatment with TELM, especially in the SHR group. Additionally, TELM reduced the production of TNF-α, IL-1β, and CXCL3 in the SHR group. The expression of Agt increased in the groups with PD, while Agtr2 reduced, and TELM reduced the expression of Agtr1 and increased the expression of Agtr2, in W and SHRs. PD did not induce major changes in the expression of bone formation markers, except for the expression of Alp, which decreased in the PD groups. The bone resorption markers expression, Mmp9, Ctsk, and Vtn, was higher in the SHR + PD group, compared to the respective control and W + PD group. However, TELM attenuated these changes and increased the expression of Runx2 and Alp. Our study suggested that TELM has a protective effect on the progression of PD, especially in hypertensive animals, as evaluated by the resorption of the lower alveolar bone. This can be partly explained by the modulation in the expression of Angiotensin II receptors (AT1R and AT2R), reduced production of inflammatory mediators, the reduced expression of resorption markers, and the increased expression of the bone formation markers.

Angiotensin Type 2 Receptors: Painful, or Not?

Pain in response to various types of acute injury can be a protective stimulus to prevent the organism from using the injured part and allow tissue repair and healing. On the other hand, neuropathic pain, defined as ‘pain caused by a lesion or disease of the somatosensory nervous system’, is a debilitating pathology. The TRPA1 neurons in the Dorsal Root Ganglion (DRG) respond to reactive oxygen species (ROS) and induce pain. In acute nerve injury and inflammation, macrophages infiltrating the site of injury undergo an oxidative burst, and generate ROS that promote tissue repair and induce pain via TRPA1. The latter discourages using the injured limb, with a lack of movement helping wound healing. In chronic inflammation caused by diabetes, cancer etc., ROS levels increase systemically and modulate TRPA1 neuronal functions and cause debilitating neuropathic pain. It is important to distinguish between drug targets that elicit protective vs. debilitating pain when developing effective drugs for neuropathic pain. In this context, the connection of the Angiotensin type 2 receptor (AT₂R) to neuropathic pain presents an interesting dilemma. Several lines of evidence show that AT₂R activation promotes anti-inflammatory and anti-nociceptive signaling, tissue repair, and suppresses ROS in chronic inflammatory models. Conversely, some studies suggest that AT₂R antagonists are anti-nociceptive and therefore AT₂R is a drug target for neuropathic pain. However, AT₂R expression in nociceptive neurons is lacking, indicating that neuronal AT₂R is not involved in neuropathic pain. It is also important to consider that Novartis terminated their phase II clinical trial (EMPHENE) to validate that AT₂R antagonist EMA401 mitigates post-herpetic neuralgia. This trial, conducted in Australia, United Kingdom, and a number of European and Asian countries in 2019, was discontinued due to pre-clinical drug toxicity data. Moreover, early data from the trial did not show statistically significant positive outcomes. These facts suggest that may AT₂R not be the proper drug target for neuropathic pain in humans and its inhibition can be harmful.

Elevated blood pressure in high-fat diet-exposed low birthweight rat offspring is most likely caused by elevated glucocorticoid levels due to abnormal pituitary negative feedback

Being delivered as a low birthweight (LBW) infant is a risk factor for elevated blood pressure and future problems with cardiovascular and cerebellar diseases. Although premature babies are reported to have low numbers of nephrons, some unclear questions remain about the mechanisms underlying elevated blood pressure in full-term LBW infants. We previously reported that glucocorticoids increased miR-449a expression, and increased miR-449a expression suppressed Crhr1 expression and caused negative glucocorticoid feedback. Therefore, we conducted this study to clarify the involvement of pituitary miR-449a in the increase in blood pressure caused by higher glucocorticoids in LBW rats. We generated a fetal low-carbohydrate and calorie-restricted model rat (60% of standard chow), and some individuals showed postnatal growth failure caused by growth hormone receptor expression. Using this model, we examined how a high-fat diet (lard-based 45kcal% fat)-induced mismatch between prenatal and postnatal environments could elevate blood pressure after growth. Although LBW rats fed standard chow had slightly higher blood pressure than control rats, their blood pressure was significantly higher than controls when exposed to a high-fat diet. Observation of glomeruli subjected to periodic acid methenamine silver (PAM) staining showed no difference in number or size. Aortic and cardiac angiotensin II receptor expression was altered with compensatory responses. Blood aldosterone levels were not different between control and LBW rats, but blood corticosterone levels were significantly higher in the latter with high-fat diet exposure. Administration of metyrapone, a steroid synthesis inhibitor, reduced blood pressure to levels comparable to controls. We showed that high-fat diet exposure causes impairment of the pituitary glucocorticoid negative feedback via miR-449a. These results clarify that LBW rats have increased blood pressure due to high glucocorticoid levels when they are exposed to a high-fat diet. These findings suggest a new therapeutic target for hypertension of LBW individuals.

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.

Cell-Specific Protective Signaling Induced by the Novel AT2R-Agonist NP-6A4 on Human Endothelial and Smooth Muscle Cells

Cardiovascular disease incidence continues to rise and new treatment paradigms are warranted. We reported previously that activation of Angiotensin II receptor (encoded by the X-linked Agtr2 gene) by a new peptide agonist, NP-6A4, was more effective in protecting mouse cardiomyocyte HL-1 cells and human coronary artery vascular smooth muscle cells (hCAVSMCs) from acute nutrient deficiency than other drugs tested. To elucidate further the protective effects of NP-6A4 in human cells, we studied the effects of NP-6A4 treatment on functions of human coronary artery endothelial cells (hCAECs), and hCAVSMCs. In hCAVSMCs, NP-6A4 (1 μM) increased Agtr2 mRNA (sixfold, p < 0.05) after 12-h exposure, whereas in hCAECs, significant increase in Agtr2 mRNA (hCAECs: eightfold) was observed after prolonged exposure. Interestingly, NP-6A4 treatment (1 μM, 12 h) increased AT2R protein levels in all human cells tested. Pre-treatment with AT2R-antagonist PD123319 (20 μM) and anti-AT2R siRNA (1 μM) suppressed this effect. Thus, NP-6A4 activates a positive feedback loop for AT2R expression and signaling in hCAVSMCs and hCAECs. NP-6A4 (1–20 μM) increased cell index (CI) of hCAVSMCs as determined by real time cell analyzer (RTCA), indicating that high concentrations of NP-6A4 were not cytotoxic for hCAVSMCs, rather promoting better cell attachment and growth. Seahorse Extracellular Flux Assay revealed that NP-6A4 (1 μM) treatment for 7 days increased whole cell-based mitochondrial parameters of hCAVSMCs, specifically maximal respiration (p < 0.05), spare respiratory capacity (p < 0.05) and ATP production (p < 0.05). NP-6A4 (1 μM; 7 days) also suppressed Reactive Oxygen Species (ROS) in hCAVSMCs. Exposure to Doxorubicin (DOXO) (1 μM) increased ROS in hCAVSMCs and this effect was suppressed by NP-6A4 (1 μM). In hCAECs grown in complete medium, NP-6A4 (1 μM) and Ang II (1 μM) exerted similar changes in CI. Additionally, NP-6A4 (5 μM: 12 h) increased expression of eNOS (sixfold, p < 0.05) and generation of nitric oxide (1.3-fold, p < 0.05) in hCAECs and pre-treatment with PD123319 (20 μM) suppressed this effect partially (65%). Finally, NP-6A4 decreased phosphorylation of Jun-N-terminal kinase, implicated in apoptosis of ECs in atherosclerotic sites. Taken together, NP-6A4, through its ability to increase AT2R expression and signaling, exerts different cell-specific protective effects in human VSMCs and ECs.

Differential Effects of β-Blockers, Angiotensin II Receptor Blockers, and a Novel AT2R Agonist NP-6A4 on Stress Response of Nutrient-Starved Cardiovascular Cells

In order to determine differences in cardiovascular cell response during nutrient stress to different cardiovascular protective drugs, we investigated cell responses of serum starved mouse cardiomyocyte HL-1 cells and primary cultures of human coronary artery vascular smooth muscles (hCAVSMCs) to treatment with β-blockers (atenolol, metoprolol, carvedilol, nebivolol, 3μM each), AT1R blocker losartan (1μM) and AT2R agonists (CGP42112A and novel agonist NP-6A4, 300nM each). Treatment with nebivolol, carvedilol, metoprolol and atenolol suppressed Cell Index (CI) of serum-starved HL-1 cells (≤17%, ≤8%, ≤15% and ≤15% respectively) as measured by the Xcelligence Real-Time Cell Analyzer (RTCA). Conversely, CI was increased by Ang II (≥9.6%), CGP42112A (≥14%), and NP-6A4 (≥25%) respectively and this effect was blocked by AT2R antagonist PD123319, but not by AT1R antagonist losartan. Thus, the CI signature for each drug could be unique. MTS cell proliferation assay showed that NP-6A4, but not other drugs, increased viability (≥20%) of HL-1 and hCAVSMCs. Wheat Germ Agglutinin (WGA) staining showed that nebivolol was most effective in reducing cell sizes of HL-1 and hCAVSMCs. Myeloid Cell Leukemia 1 (MCL-1) is a protein critical for cardiovascular cell survival and implicated in cell adhesion. β-blockers significantly suppressed and NP-6A4 increased MCL-1 expression in HL-1 and hCAVSMCs as determined by immunofluorescence. Thus, reduction in cell size and/or MCL-1 expression might underlie β-blocker-induced reduction in CI of HL-1. Conversely, increase in cell viability and MCL-1 expression by NP-6A4 through AT2R could have resulted in NP-6A4 mediated increase in CI of HL-1. These data show for the first time that activation of the AT2R-MCL-1 axis by NP-6A4 in nutrient-stressed mouse and human cardiovascular cells (mouse HL-1 cells and primary cultures of hCAVSMCs) might underlie improved survival of cells treated by NP-6A4 compared to other drugs tested in this study.

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.

RAS-Mediated Adaptive Mechanisms in Cardiovascular Tissues: Confounding Factors of RAS Blockade Therapy and Alternative Approaches

Since the classic experiments by Tigerstedt and Bergman that established the role of renin in hypertension a century ago, aggressive efforts have been launched to effectively block the renin-angiotensin system (RAS). Blockade of RAS is advocated at multiple levels by direct renin inhibitor, angiotensin-converting enzyme inhibitor and/or angiotensin II type 1 receptor blocker, or aldosterone inhibitor (spironolactone), and has now become part of the standard of care to control hypertension and related metabolic diseases including diabetes. However, recent lessons learned from randomized clinical trials question the wisdom of blocking RAS at multiple levels. In this context, it is highly pertinent that components of RAS are evolutionarily conserved, and novel physiological/adaptive/protective roles for renin and angiotensin-converting enzyme are currently emerging. Angiotensin II, the classical RAS effector peptide responsible for hypertension, hypertrophy, fluid retention and fibrosis, manifests its cardiovascular protective effect when it activates the angiotensin II type 2 receptor. Additionally, angiotensin-converting enzyme 2 and the angiotensin II metabolite Ang-(1-7) that acts through the Mas proto-oncogene constitute the cardiovascular and renal protective branch of RAS. It is conceivable that modulating this vasodilative/anti-inflammatory branch of RAS by activation of the RAS components that constitute this branch may offer a safer long-term treatment strategy to balance RAS activity and achieve homeostasis compared to chronic multilevel RAS inhibition.

Angiotensin II bi-directionally regulates cyclooxygenase-2 expression in intestinal epithelial cells

We previously demonstrated that angiotensin II (Ang II) receptor signaling is involved in azoxymethane-induced mouse colon tumorigenesis. In order to clarify the role of Ang II in COX-2 expression in the intestinal epithelium, the receptor subtype-specific effect on COX-2 expression in a rat intestinal epithelial cell line (RIE-1) has been investigated. Ang II dose- and time-dependently increased the expression of COX-2, but not COX-1 mRNA and protein. This stimulation was completely blocked by the AT(1) receptor antagonist but not the AT(2) receptor antagonist. Ang II and lipopolysaccharide (LPS) additively induced COX-2 protein in RIE-1 cells, whereas the LPS-induced COX-2 expression was significantly attenuated by low concentrations of Ang II or the AT(2) agonistic peptide CGP-42112A only in AT(2) over-expressed cells. These data indicate that Ang II bi-directionally regulates COX-2 expression via both AT(1) and AT(2) receptors. Control of COX-2 expression through Ang II signaling may have significance in cytokine-induced COX-2 induction and colon tumorigenesis.

PMA- and ANG II-induced PKC regulation of the renal Na+-HCO3−cotransporter (hkNBCe1)

The renal electrogenic Na+-HCO3−cotransporter (hkNBCe1) plays a major role in the bicarbonate reabsorption by the kidney. We examined how PMA- and ANG II-activated PKCs regulate hkNBCe1 expressed with or without the ANG II receptors AT1Bin Xenopus laevis oocytes. We found that 10 nM PMA halved the hkNBCe1 current detected in voltage-clamped oocytes. A PKC-specific inhibitor GF-109203X, and a specific inhibitor of Ca-dependent conventional PKCαβγ, GÖ-6976, significantly reduced PMA inhibition. PMA did not alter surface expression of the cotransporters, but it significantly increased hkNBCe1-PKCαβγ membrane association. We found that at 10−6M, ANG II halved the hkNBCe1 current detected in oocytes coexpressing cotransporters with AT1B. A PKC-specific inhibitor GF-109203X, and a PKCε translocation inhibitor εV1–2 peptide as well as BAPTA-AM (but not GÖ-6976), significantly reduced ANG II inhibition. At 10−6M, ANG II significantly decreased surface expression of the cotransporters and increased hkNBCe1-PKCε membrane association. Additionally, we found that at 10−11and 10−10M ANG II stimulated hkNBCe1 current. This effect was blocked by BAPTA-AM and partially reduced by GF-109203X. We also found that ANG II increased intracellular Ca2+in fluo 4-loaded oocytes. Our results suggest that 1) PMA inhibition of hkNBCe1 is mediated by Ca-dependent PKCαβγ and 10 nM PMA does not induce downregulation of cotransporter surface expression. 2) ANG II (10−6M) inhibition of hkNBCe1 is mediated by both Ca-independent PKCε and downregulation of cotransporter surface expression, possibly triggered by intracellular Ca2+mobilization. 3) Similar to proximal tubule, acute ANG II has a biphasic effect on hkNBCe1 coexpressed with AT1Bin X. laevis oocytes.

Roles of the intracellular regions of angiotensin II receptor AT2 in mediating reduction of intracellular cGMP levels

We have shown previously that the angiotensin II (Ang II) receptor AT2 reduces the intracellular levels of cGMP in Xenopus oocytes when activated by ligand binding, and the C-terminal cytoplasmic tail of the AT2 acts as a negative regulator of this function. Here we report the effects of mutations in the 2nd and 3rd intracellular loops of AT2 on AT2-mediated cGMP reduction. Mutating the highly conserved DRY motif (D141G-R142G-Y143A) of the 2nd ICL implicated in activating G(alpha) subunit of trimeric G-proteins did not affect AT2-mediated cGMP reduction. Moreover, anti-Gialpha antibody or phosphodiesterase inhibitor IBMX did not inhibit AT2-mediated cGMP reduction, suggesting that Gialpha activation and subsequent phosphodiesterase activation are not involved in this function. In contrast, mutations T250R-R251N and L255F-K256R located in the C-terminus of the 3rd ICL of AT2 retained ligand-binding properties of the wild-type AT2, and its ability to interact with the ErbB3 in yeast two-hybrid assay, but abolished AT2-mediated cGMP reduction. Similarities in the roles of ICLs of AT2 in AT2-mediated cGMP reduction in oocytes, and AT2-mediated SHP1 activation in COS-7 cells, (need of 3rd ICL for both functions and lack of involvement of DRY motif), suggest that the cascade of events in these two signaling mechanisms could be similar, and that an oocyte-specific SHP1-like protein may be involved in AT2-mediated cGMP reduction in these cells.

Ligand-dependent complex formation between the Angiotensin II receptor subtype AT2 and Na+/H+ exchanger NHE6 in mammalian cells

Involvement of Angiotensin II (Ang II) in the regulation of sodium levels by modulating the Na+/H+ exchangers is demonstrated in many tissues. Screening of a mouse 17-day fetus cDNA library with the Angiotensin II receptor AT2 as the bait in yeast two-hybrid assay led us to identify an AT2-interacting mouse fetus peptide that shared 98% amino acid identity with the corresponding region of the human NHE6. NCBI Blast search showed that the clone 6430520C02 (GenBank Accession # AK032326) of the mouse genome project carried the complete sequence of this new mouse NHE6 isoform. The human and mouse NHE6 peptides share 97% overall homology. Further analysis showed that the region spanning the third intracellular loop and C-terminal cytoplasmic tail of the AT2 directly interacted with a 182 amino acid region that spans the predicted 5th intracellular loop and the initial part of the C-terminus of the mouse NHE6 in yeast two-hybrid assay. This 182-amino acid region that interacted with the AT2 also shares 98% homology with the corresponding region of rat NHE6 and therefore is highly conserved across species. We detected widespread expression of this NHE6 isoform in several rat tissues including 10-day fetus, 17-day fetus, and 30-day post-natal tissues of heart, brain, kidney and muscle. Moreover, the AT2 co-immunoiprecipitated with a hemagglutinin tagged NHE6 when expressed in human cell line MCF-7, and activated by AngII. This ligand-dependent complex formation between the AT2 and NHE6 suggests that the hormone Ang II may act as a regulator of NHE6, and Ang II-mediated direct protein-protein interaction between AT2 and NHE6 could be a mechanism for modulating the functions of the ubiquitously expressed NHE6 in different tissues.

Role of Phe308 in the seventh transmembrane domain of the AT2 receptor in ligand binding and signaling

Studies on Angiotensin II (Ang II) receptor type AT1 have suggested that interaction between the two highly conserved residues, Tyr292 in the 7th transmembrane domain (TMD) and the Asp74 in the 2nd TMD, is critical for linking the Ang II binding and AT1 receptor-Gq protein coupling. In the Ang II receptor type AT2, the Asp is conserved (Asp90 in 2nd TMD), however, there is no Tyr residue in the 7th TMD and Phe308 occupies the analogous position to Tyr292 of the AT1. Replacing this Phe308 with Ala reduced receptor affinity to peptidic ligands (125)I-Ang II (K(d) = 0.37 nM) and (125)I-CGP42112A (K(d) = 0.56 nM), but retained the ability of the AT2 to reduce cGMP levels in Xenopus oocytes. Thus, the Phe308 of the AT2 does not mimic the role of Tyr292 of the AT1 in the receptor activation upon Ang II binding. We have also shown that the M8 mutant of the AT2 with the 7th TMD similar to that of wild type AT2 can couple to PLC like the AT1 and bind the AT2-specific ligands with high affinity. Since the Ang II is shown to bind to both the AT1 and the AT2 in an identical manner, we propose that the absence of Tyr in the 7th TMD of the AT2 does not prevent the receptor from coupling to Gq-protein, rather may contribute to the freedom of the AT2 to couple to trimeric G-proteins in both G- betagamma dependent and independent manners upon Ang II binding.

G Protein–Coupled Receptor Oligomerization

The cardiovascular system is richly endowed with G protein–coupled receptors (GPCRs), members of the largest family of plasma membrane-localized receptors. During the last 10 years, it has become increasingly clear that many, if not all, GPCRs function in oligomeric complexes, as either homo- or hetero-oligomers. This review explores the mechanistic implications of GPCR dimerization and/or oligomerization on receptor activation and interactions with G proteins. The effects of GPCR oligomerization on receptor pharmacology, GPCR-mediated signaling, and potential contributions to GPCR crosstalk will be considered in the context of receptors important in the cardiovascular system. Our evolving understanding of the structural and functional consequences of GPCR oligomerization may provide novel and more selective sites for pharmacological tuning of cardiovascular function.