Molecular cloning and expression of the gene encoding human angiotensin II type 2 receptor

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 Receptors Contribute to Regulate the Sympathoadrenal and Hormonal Reaction to Stress Stimuli

Angiotensin II, through AT1 receptor stimulation, mediates multiple cardiovascular, metabolic, and behavioral functions including the response to stressors. Conversely, the function of Angiotensin II AT2 receptors has not been totally clarified. In adult rodents, AT2 receptor distribution is very limited but it is particularly high in the adrenal medulla. Recent results strongly indicate that AT2 receptors contribute to the regulation of the response to stress stimuli. This occurs in association with AT1 receptors, both receptor types reciprocally influencing their expression and therefore their function. AT2 receptors appear to influence the response to many types of stressors and in all components of the hypothalamic-pituitary-adrenal axis. The molecular mechanisms involved in AT2 receptor activation, the complex interactions with AT1 receptors, and additional factors participating in the control of AT2 receptor regulation and activity in response to stressors are only partially understood. Further research is necessary to close this knowledge gap and to clarify whether AT2 receptor activation may carry the potential of a major translational advance.

The vasoprotective axes of the renin-angiotensin system: Physiological relevance and therapeutic implications in cardiovascular, hypertensive and kidney diseases

The renin-angiotensin system (RAS) is undisputedly one of the most prominent endocrine (tissue-to-tissue), paracrine (cell-to-cell) and intracrine (intracellular/nuclear) vasoactive systems in the physiological regulation of neural, cardiovascular, blood pressure, and kidney function. The importance of the RAS in the development and pathogenesis of cardiovascular, hypertensive and kidney diseases has now been firmly established in clinical trials and practice using renin inhibitors, angiotensin-converting enzyme (ACE) inhibitors, type 1 (AT₁) angiotensin II (ANG II) receptor blockers (ARBs), or aldosterone receptor antagonists as major therapeutic drugs. The major mechanisms of actions for these RAS inhibitors or receptor blockers are mediated primarily by blocking the detrimental effects of the classic angiotensinogen/renin/ACE/ANG II/AT₁/aldosterone axis. However, the RAS has expanded from this classic axis to include several other complex biochemical and physiological axes, which are derived from the metabolism of this classic axis. Currently, at least five axes of the RAS have been described, with each having its key substrate, enzyme, effector peptide, receptor, and/or downstream signaling pathways. These include the classic angiotensinogen/renin/ACE/ANG II/AT₁ receptor, the ANG II/APA/ANG III/AT₂/NO/cGMP, the ANG I/ANG II/ACE2/ANG (1-7)/Mas receptor, the prorenin/renin/prorenin receptor (PRR or Atp6ap2)/MAP kinases ERK1/2/V-ATPase, and the ANG III/APN/ANG IV/IRAP/AT₄ receptor axes. Since the roles and therapeutic implications of the classic angiotensinogen/renin/ACE/ANG II/AT₁ receptor axis have been extensively reviewed, this article will focus primarily on reviewing the roles and therapeutic implications of the vasoprotective axes of the RAS in cardiovascular, hypertensive and kidney diseases.

Expression and localization of the diacylglycerol kinase family and of phosphoinositide signaling molecules in adrenal gland

Adrenal glands play a central role in the secretion of steroid hormones and catecholamines. Previous studies have revealed that molecules engaged in phosphoinositide (PI) turnover are expressed in the adrenal gland, suggesting the importance of PI signaling in adrenal signal transduction. Diacylglycerol kinase (DGK) catalyzes the phosphorylation of diacylglycerol (DG), a major second messenger in the PI signaling cascade. The DGK family is expressed in distinct patterns in endocrine organs at the mRNA and protein levels. Nevertheless, little is known about the characteristics and morphological aspects of DGKs in the adrenal gland. We have performed immunohistochemical analyses to investigate the expression and localization of DGK isozymes, together with PI signaling molecules, in the adrenal gland at the protein level. Our results show that the DGK family and a set of PI signaling molecules are expressed intensely in zona glomerulosa cells and medullary chromaffin cells in the adrenal gland. In adrenal cells, DGKγ localizes to the Golgi complex, DGKε to the plasma membrane, and DGKζ to the nucleus. These findings show the distinct expression and subcellular localization of DGK isozymes and PI signaling molecules in the adrenal gland, suggesting that each DGK isozyme has a role in signal transduction in adrenal cells, especially in the zona glomerulosa and medulla.

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.

Ontogeny of angiotensin type 2 and type 1 receptor expression in mice

In the current experiment, we determined angiotensin type 2 receptor (AT2R) and angiotensin type 1 receptor (AT1R) protein expression by western blot analysis in developing normal mice. The results indicate that: (1) in all detected brain regions and in the spinal cord, adult mice exhibited significantly higher AT2R expression and lower AT1R expression in total protein extracts compared to fetuses and neonates; (2) other major organs, including heart, lung, liver and kidney, exhibited the same expression pattern as the brain and spinal cord; (3) reciprocal changes in AT2R and AT1R expression were found in the total protein extracts from the brainstems of mice from one-day prenatal to six weeks of age, and there was a negative correlation between AT2R and AT1R protein expression; (4) in both membrane and cytosolic fractions from the brainstem, adult mice exhibited higher AT2R and lower AT1R expression than did fetuses and neonates; and (5) in the brainstem, there were no significant differences in AT2R and AT1R messenger RNA (mRNA) levels among fetal, neonatal and adult mice. The above results reconfirmed our previous finding in rats that adult animals have higher AT2R and lower AT1R expression compared to fetuses and neonates. These data imply an involvement of AT1R in fetal development and of AT2R in adult function.

Differential expression of angiotensin II type 1 and type 2 receptors at the maternal–fetal interface: potential roles in early placental development

Angiotensin II (Ang II) is locally generated in the placenta and regulates syncytial transport, vascular contractility and trophoblast invasion. It acts through two receptor subtypes, AGTR1 and AGTR2 (AT1 and AT2), which typically mediate antagonising actions. The objectives of this study are to characterise the cellular distribution of AGTR1 and AGTR2 at the maternal–fetal interface and explore the effects on cytotrophoblast turnover. Low levels ofAGTR2mRNA were detected in first trimester placental homogenates using real-time PCR. Immunohistochemistry using polyclonal antibodies against AGTR1 and AGTR2 detected the receptors in first trimester placenta, decidua basalis and villous tip outgrowths in culture. Serial staining with cytokeratin-7 was used to identify extravillous trophoblasts (EVTs). AGTR1 was found in the syncytiotrophoblast microvillous membrane, in a subpopulation of villous cytotrophoblasts, and in Hofbauer cells. AGTR1 was strongly upregulated in cytotrophoblasts in cell columns and villous tip outgrowths, but was absent in interstitial and endovascular EVTs within the decidua. AGTR2 immunostaining was present in Hofbauer cells and villous cytotrophoblasts, but was absent from syncytiotrophoblast. Faint staining was detected in cell column cytotrophoblasts and villous outgrowths, but not in EVTs within the decidua. Both receptors were detected in placental homogenates by western blotting. Ang II significantly increased proliferation of cytotrophoblasts in both villous explants and villous tip outgrowths, but did not affect apoptosis. Blockade of AGTR1 and AGTR2 together abrogated this effect. This study shows specific expression patterns for AGTR1 and AGTR2 in distinct trophoblast populations at the maternal–fetal interface and suggests that Ang II plays a role in placental development and generation of EVTs.

Over-expression of angiotensin II type 2 receptor gene induces cell death in lung adenocarcinoma cells

The endogenous angiotensin II (Ang II) type 2 receptor (AT 2) has been shown to mediate apoptosis in cardiovascular tissues. Thus, the aim of this study was to explore the anti-cancer effect of AT 2 over-expression on lung adenocarcinoma cells in vitro using adenoviral (Ad), FuGENE, and nanoparticle vectors. All three gene transfection methods efficiently transfected AT 2 cDNA into lung cancer cells but caused minimal gene transfection in normal lung epithelial cells. Ad-AT 2 significantly attenuated multiple human lung cancer cell growth (A549 and H358) as compared to the control viral vector, Ad-LacZ, when cell viability was examined by direct cell count. Examination of annexin V by flow cytometry revealed the activation of the apoptotic pathway via AT 2 over-expression. Western Blot analysis confirmed the activation of caspase-3. Similarly, poly (lactide-co-glycolic acid) (PLGA) biodegradable nanoparticles encapsulated AT 2 plasmid DNA were shown to be effectively taken up into the lung cancer cell. Nanoparticle-based AT 2 gene transfection markedly increased AT 2 expression and resultant cell death in A549 cells. These results indicate that AT 2 over-expression effectively attenuates growth of lung adenocarcinoma cells through intrinsic apoptosis. Our results also suggest that PLGA nanoparticles can be used as an efficient gene delivery vector for lung adenocarcinoma targeted therapy.

Increased angiotensin II type 1 receptor expression in temporal arteries from patients with giant cell arteritis

PURPOSE

Currently, giant cell arteritis (GCA) is primarily treated with corticosteroids or immunomodulating agents, but there is interest in identifying other noncorticosteroid alternatives. Similarities exist in the injury pathways between GCA and atherosclerosis. Angiotensin II is a vasoactive peptide involved in vessel inflammation during atherosclerosis, and angiotensin II receptor inhibitors are effective in preventing atherosclerosis. The present study was performed to elucidate the role of angiotensin type 1 (AT(1)) and type 2 (AT(2)) receptors in GCA.

DESIGN

Experimental retrospective immunohistochemical study of temporal arteries using archival formalin-fixed, paraffin-embedded tissue.

PARTICIPANTS

Ten patients with GCA and 10 control patients, who were clinically suspected of having GCA but were diagnosed as not having GCA, were included.

METHODS

Immunohistochemistry, using anti-AT(1) and anti-AT(2) antibodies, was performed on formalin-fixed and paraffin-embedded temporal arteries.

MAIN OUTCOME MEASURES

AT(1) and AT(2) receptor immunostaining intensity was quantified.

RESULTS

Hematoxylin-eosin-stained sections of temporal arteries from patients with GCA showed intimal hyperplasia, internal elastic lamina degeneration, and band-shaped infiltrates of inflammatory cells, including lymphocytes, histocytes, and multinucleated giant cells. AT(1) receptor staining was primarily observed in the medial layer of the temporal arteries and was higher in the patients with GCA than in the control patients. This was a result of increased AT(1) receptor immunostaining of both vascular smooth muscle cells and infiltrating inflammatory cells. Only faint immunostaining was seen for AT(2) receptors, primarily in the endothelial cells, and to a lesser extent on the smooth muscle cells. Immunostaining with antibodies for the AT(2) receptor was similar in the patients with GCA and in controls.

CONCLUSIONS

These results suggest that AT(1) receptors play a role in the development of GCA. Inhibition of the angiotensin system may thus provide a noncorticosteroid alternative for the treatment of GCA.

FINANCIAL DISCLOSURE(S)

The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Ligand-supported homology modeling of the human angiotensin II type 1 (AT1) receptor: Insights into the molecular determinants of telmisartan binding

Angiotensin II type 1 (AT(1)) receptor belongs to the super-family of G-protein-coupled receptors, and antagonists of the AT(1) receptor are effectively used in the treatment of hypertension. To understand the molecular interactions of these antagonists, such as losartan and telmisartan, with the AT(1) receptor, a homology model of the human AT(1) (hAT(1)) receptor with all connecting loops was constructed from the 2.6 A resolution crystal structure (PDB i.d., 1L9H) of bovine rhodopsin. The initial model generated by MODELLER was subjected to a stepwise ligand-supported model refinement. This protocol involved initial docking of non-peptide AT(1) antagonists in the putative binding site, followed by several rounds of iterative energy minimizations and molecular dynamics simulations. The final model was validated based on its correlation with several structure-activity relationships and site-directed mutagenesis data. The final model was also found to be in agreement with a previously reported AT(1) antagonist pharmacophore model. Docking studies were performed for a series of non-peptide AT(1) receptor antagonists in the active site of the final hAT(1) receptor model. The docking was able to identify key molecular interactions for all the AT(1) antagonists studied. Reasonable correlation was observed between the interaction energy values and the corresponding binding affinities of these ligands, providing further validation for the model. In addition, an extensive unrestrained molecular dynamics simulation showed that the docking-derived bound pose of telmisartan is energetically stable. Knowledge gained from the present studies can be used in structure-based drug design for developing novel ligands for the AT(1) receptor.

Tissue-specific renin-angiotensin system in pulmonary lymphangioleiomyomatosis

Lymphangioleiomyomatosis (LAM), a multisystem disease found in middle-aged women, is characterized by cystic lung destruction and abdominal tumors (e.g., angiomyolipomas, lymphangioleimyomas), resulting from proliferation of abnormal-appearing, smooth muscle-like cells (LAM cells). The LAM cells, in combination with other cells, form nodular structures within the lung interstitium and in the walls of the cysts. LAM cells contain mutations in the tuberous sclerosis complex TSC1 and/or TSC2 genes, which lead to dysregulation of the mammalian target of rapamycin, affecting cell growth and proliferation. Proliferation and migration of vascular smooth muscle cells and production of angiogenic factors are regulated, in part, by angiotensin II. To determine whether a LAM-specific renin-angiotensin system might play a role in the pathogenesis of LAM, we investigated the expression of genes and gene products of this system in LAM nodules. mRNA for angiotensinogen was present in RNA isolated by laser-captured microdissection from LAM nodules. Angiotensin I-converting enzyme and chymase-producing mast cells were present within the LAM nodules. We detected renin in LAM cells, as determined by the presence of mRNA and immunohistochemistry. Angiotensin II type 1 and type II receptors were identified in LAM cells by immunohistochemistry and immunoblotting of microdissected LAM nodules. Angiotensin II is localized in cells containing alpha-smooth muscle actin (LAM cells). A LAM-specific renin-angiotensin system appears to function within the LAM nodule as an autocrine system that could promote LAM cell proliferation and migration, and could represent a pharmacologic target.

Ischemic heart disease down-regulates angiotensin type 1 receptor mRNA in human coronary arteries

Angiotensin II is important in the development of cardiovascular disease. In the present study, angiotensin II receptor mRNA levels were quantified by real-time polymerase chain reaction (real-time PCR) in human coronary arteries from patients with ischemic heart disease and controls. Furthermore, the suitability of artery culture for studying angiotensin receptor changes was evaluated by in vitro pharmacology and real-time PCR. The angiotensin type 1 (AT1) receptor mRNA levels were down-regulated in human coronary arteries from patients with ischemic heart disease as compared to controls (P<0.05). Culture of coronary arteries for 48 h induced down-regulation of the angiotensin AT1 and AT2 receptor mRNA levels and also a less efficacious angiotensin II-induced vasoconstriction (Emax=103+/-2% before and 23+/-7% after artery culture, P<0.001). Artery culture may thus be a suitable method for studying angiotensin receptor regulation.

Angiotensin II Receptor mRNA Expression and Vasoconstriction in Human Coronary Arteries: Effects of Heart Failure and Age

Angiotensin II is a potent vasoconstrictor that is implicated in the pathogenesis of hypertension, heart failure and atherosclerosis. In the present study, angiotensin II receptor mRNA expression levels were quantified by real-time polymerase chain reaction and the vasocontractile responses to angiotensin II were characterised by in vitro pharmacology in endothelium-denuded human coronary arteries. Angiotensin II type 1 (AT(1)) and type 2 (AT(2)) receptor mRNA expression levels were significantly down-regulated in arteries from patients with heart failure as compared to controls. The angiotensin II-induced vasoconstriction diminished with increasing age in patients with heart failure (r(2)=0.31, P<0.05). Also, the AT(1) receptor mRNA expression levels decreased with increasing age in patients with heart failure (r(2)=0.74, P<0.05), while no such correlation could be shown in the control group (r(2)=0.04, P=n.s.). The AT(2) receptor mRNA expression levels did not correlate with age in patients with heart failure or controls. In conclusion, the diminished angiotensin II vasoconstriction with age in heart failure patients is most likely due to a lower density of AT(1) receptors and may result from a longer period of exposure to heart failure in older patients.

Immunohistochemical and mRNA localization of the angiotensin II receptor subtype 2 (AT2) in follicular granulosa cells of the rat ovary

A local renin-angiotensin system (RAS), including specific angiotensin II receptor subtypes, is present in the rat ovary. Immunohistochemistry using a polyclonal antibody and mRNA in situ hybridization were performed on perfusion-fixed, paraffin-embedded ovaries obtained from untreated sexually mature, normally cycling rats. Immunofluorescent staining revealed an exclusive and distinct labeling of follicular granulosa cells showing a plaque-like expression pattern at the cell borders, being detectable in different stages of atretic degeneration. On adjacent sections the expression of the respective mRNA could be shown in granulosa cells of the same follicle. The AT2 receptor may be implicated in the ovarian atretic process by influencing follicular cell-cell communication.

Molecular cloning, characterization, and distribution of the gerbil angiotensin II AT2 receptor

We isolated a cDNA clone encoding the gerbil AT2 receptor (gAT2) gene from a gerbil adrenal gland cDNA library. The full-length cDNA contains a 1,089-bp open reading frame encoding 363 amino acid residues with 90.9, 96.1, and 95.6% identity with the human (hAT2), rat (rAT2), and mouse AT2 (mAT2) receptors, respectively. There are at least seven nonconserved amino acids in the NH2-terminal domain and in positions Val196, Val217, and Met293, important for angiotensin (ANG) II but not for CGP-42112 binding. Displacement studies in adrenal sections revealed that affinity of the gAT2 receptor was 10-20 times lower for ANG II, ANG III, and PD-123319 than was affinity of the rAT2 receptor. The affinity of each receptor remained the same for CGP-42112. When transfected into COS-7 cells, the gAT2 receptor shows affinity for ANG II that is three times lower than that shown by the hAT2 receptor, whereas affinities for ANG III and the AT2 receptor ligands CGP-42112 and PD-123319 were similar. Autoradiography in sections of the gerbil head showed higher binding in muscles, retina, skin, and molars at embryonic day 19 than at 1 wk of age. In situ hybridization and emulsion autoradiography revealed that at embryonic day 19 the gAT2 receptor mRNA was highly localized to the base of the dental papilla of maxillary and mandibular molars. Our results suggest selective growth-related functions in late gestation and early postnatal periods for the gAT2 receptor and provide an essential basis for future mutagenesis studies to further define structural requirements for agonist binding.

Comparison of the antagonistic effects of different angiotensin II receptor blockers in human coronary arteries

BACKGROUND

Angiotensin II (Ang II) is a potent vasoconstrictor and a deleterious factor in cardiovascular pathophysiology. Ang II receptor blockers (ARBs) have recently been introduced into clinical practice for treatment of hypertension and congestive heart failure.

AIMS

This study was undertaken to evaluate the inhibitory effects of ARBs on vasoconstriction in humans.

METHODS

Vasomotor tone was analyzed in endothelium denuded, human coronary artery (HCA) segments. Ang II type 1 (AT(1)) and type 2 (AT(2)) receptor mRNA expression was examined by reverse transcriptase-polymerase chain reaction (RT-PCR).

RESULTS

Ang II was a potent vasoconstrictor (pEC(50) = 7.7). At 1 nM of the AT(1) receptor antagonists, candesartan and valsartan, the maximum contraction was depressed to 57 and 50% of Ang II, respectively, indicating insurmountability. Although generally considered surmountable, the presence of 100 nM losartan elicited a depression of the Ang II response to 32%. Its active metabolite, EXP 3174 (1 nM), abolished the Ang II contraction. The AT(1) receptor antagonists had the following order of blocking effect; EXP 3174 > candesartan = valsartan > losartan. The AT(2) receptor antagonist, PD 123319 (100 nM) significantly attenuated the Ang II contraction (E(max) = 62% of Ang II). RT-PCR of HCA smooth muscle cells demonstrated expression of both AT(1) and AT(2) receptor mRNA.

CONCLUSIONS

Ang II contraction in HCA is mediated mainly by AT(1) but also involves AT(2) receptors. The active metabolite of losartan, EXP 3174, is the most efficacious AT(1) receptor antagonist in HCA.

Binding of KRH-594, an antagonist of the angiotensin II type 1 receptor, to cloned human and rat angiotensin II receptors

We studied the binding properties of KRH-594, a new selective antagonist of angiotensin II (AII) type 1 (AT1) receptors, to rat liver membranes and to recombinant AT1 and AT2 receptors. Preincubation of rat liver membranes with KRH-594 produced maximal inhibition of [125I]-AII binding when the preincubation time was 1-2 h. Preincubation with KRH-594 for 2 h decreased the B(max) value and increased the Kd value. For human AT1, human AT2, rat AT1A and rat AT1B receptors, the Ki values for KRH-594 were 1.24, 9360, 0.67, and 1.02 nm, respectively. The rank order of K1 values for human AT1 receptors was KRH-594 >> EXP3174 > candesartan = AII. The order of specificities for human AT1 and AT2 receptors was candesartan > EXP3174 > KRH-594. Although a 2-h preincubation of human AT2 receptors with KRH-594 (30 microM) or CGP 42112 (a selective AT2 receptor antagonist; 0.3 nM) inhibited binding of [125I]-AII, the suppression by KRH-594 was not significant. These results indicate that KRH-594 binds potently to AT1 receptors in an insurmountable manner, and that at a very high dose (30 microM) it may also bind to AT2 receptors, but in a surmountable manner.

125I[Sar(1)Ile(8)] angiotensin II has a different affinity for AT(1) and AT(2) receptor subtypes in ovine tissues

Iodinated angiotensin II (Ang II) and its analogues are often assumed to have equal affinities for AT(1) and AT(2) receptor subtypes. However, using saturation and competition binding assays in several tissues from pregnant, nonpregnant, and fetal sheep, we found the affinity of 125I[Sar(1)Ile(8)] Ang II for Ang II receptors was different (P<0.05) between tissue types. The dissociation constants (Kd) and half maximal displacements of [Sar(1)Ile(8)] Ang II (Sar IC(50)) were directly related (P<0.05) to proportions of AT(1) receptors, and inversely related (P<0.05) to proportions of AT(2) receptors in tissues from all groups combined, in tissues from individual groups (pregnant, nonpregnant or fetal), and in some individual tissues (uterine arteries and aortae). This suggests that 125I[Sar(1)Ile(8)] Ang II has a different affinity for AT(1) and AT(2) receptors in ovine tissues. The Kds of 125I[Sar(1)Ile(8)] Ang II for "pure" populations of AT(1) and AT(2) receptors were 1.2 and 0.3 nM, respectively, i.e. affinity was four-fold higher for AT(2) receptors. We corrected the measured proportions of the receptor subtypes using their fractional occupancies. In tissues which contained at least 10% of each receptor subtype, the corrected proportions were significantly altered (P<0.05), even in some tissues, to the extent of being reversed.

Involvement of Rho and tyrosine kinase in angiotensin II-induced actin reorganization in mesothelial cells

We investigated the role of angiotensin II type 1 (AT(1)) receptors in angiotensin II-induced actin reorganization and the signaling pathways of the response in pleural mesothelial cells. The effects of angiotensin II on actin reorganization in pleural mesothelial cells were evaluated by dual fluorescence labeling of filamentous (F) and monomeric (G) actin with fluorescein isothiocyanate (FITC)-labeled phalloidin and Texas Red-labeled DNase I, respectively. Angiotensin II (10 microM) induced actin reorganization in the presence and the absence of extracellular Ca(2+). An angiotensin AT(1) receptor antagonist ([Sar(1),Ile(8)]angiotensin II) inhibited angiotensin II-induced actin reorganization. Pretreatment with C3 exoenzyme or tyrosine kinase inhibitors significantly reduced angiotensin II-induced actin reorganization. However, pertussis toxin, phosphatidylinositol-3-kinase and protein kinase C inhibitors had no effect on these responses. These results suggest that angiotensin II-induced actin reorganization in pleural mesothelial cells is extremely dependent on the angiotensin AT(1) receptor coupled with pertussis toxin-insensitive heterotrimeric G proteins, Rho GTPases and tyrosine phosphorylation pathways.

Role of Asp297 of the AT2 receptor in high-affinity binding to different peptide ligands

To determine how ligand-receptor interaction is affected by the charges of the amino acids at position 2 of the ligands and position 297 of the AT2 receptor, we generated the Asp297Lys mutant of AT2 and a ligand SarAsp(2)Ile. Asp297Lys mutant lost affinity to Ang II and SarIle however retained partial affinity to 125I-CGP42112A. The SarAsp(2)Ile had high affinity to Asp297Lys (IC(50)3.5nM) and partial affinity to the AT2 (IC(50)15nM). Therefore, not only the charge, but also the length of the side arms of the amino acids at position 2 of the ligand and position 297 of the receptor affect their interaction.

Activation of angiotensin II subtype 2 receptor induces catecholamine release in an extracellular Ca(2+)-dependent manner through a decrease of cyclic guanosine 3',5'-monophosphate production in cultured porcine adrenal medullary chromaffin Cells

We have previously demonstrated that CGP 42112 (AT(2) agonist > or =1 nM) markedly reduces catecholamine biosynthesis through AT(2), which is the major angiotensin II (AngII) receptor subtype in cultured porcine chromaffin cells. Also, we have shown that CGP 42112 (> or =1 nM) induces a reduction in cGMP production in these cells. The present study showed that AngII reduced cGMP production via AT(2) in a manner similar to that found with CGP 42112. AngII (1 nM) significantly increased catecholamine secretion from cultured porcine adrenal medullary chromaffin cells. The stimulation was significantly inhibited by PD 123319 (AT(2) antagonist). The stimulation was moderately, but significantly, attenuated by CV-11974 (AT(1) antagonist, > or =10 nM), suggesting an involvement of AT(1). Moreover, CGP 42112 (> or =10 nM) markedly increased catecholamine release from these cells. The stimulation by CGP 42112 was abolished by PD 123319, whereas CV-11974 had no effect, indicating that this response is also mediated by AT(2). We further examined whether extracellular Ca(2+) is involved in the stimulatory effect of AT(2) on catecholamine secretion. Removal of external Ca(2+) significantly suppressed either AngII plus CV-11974 (100 nM; which simulates specific AT(2) stimulation) or CGP 42112- induced catecholamine secretion. AngII plus CV-11974 or CGP 42112 caused a sustained increase in intracellular Ca(2+) ([Ca(2+)](i)), as determined in fura-2-loaded chromaffin cells in an extracellular Ca(2+)-dependent manner. In the presence of EGTA, the subsequent addition of AngII with CV-11974 and CGP 42112 did not cause any increase in [Ca(2+)](i) levels. Consistent with this finding, CGP 42112 (10 nM to 1 microM) did not alter inositol triphosphate (IP(3)) production, a messenger for mobilization of Ca(2+) from intracellular storage sites. In addition, the intracellular Ca(2+) chelator 1,2-bis(2-amino-phenoxy)ethane-N,N,N',N'- tetraacetic acid acetoxymethylester (BAPTA) did not affect CGP 42112-induced catecholamine release. We tested whether a decrease in cGMP was the cause of the stimulatory effect of AT(2) on catecholamine secretion. Pretreatment with 8-bromo-cGMP (1 mM) prevented the stimulatory effect of AngII plus CV-11974 and CGP 42112 on both catecholamine secretion and [Ca(2+)](i). When 8-bromo-cGMP was added after application of AngII plus CV-11974 or CGP 42112, [Ca(2+)](i) induced by these agents was gradually reduced toward the baseline values. Similarly, guanylin completely abolished the AngII- plus CV-11974-induced increase in both NE secretion and [Ca(2+)](i). The Ca(2+) channel blockers, nicardipine and omega-conotoxin G VIA, at 1 microM in both cases, were also effective in inhibiting AT(2) stimulation-induced secretion. On the other hand, neither T-type voltage-dependent Ca(2+) channel blockers, flunarizine, nor Ni(2+) affected catecholamine release caused by AT(2) stimulation. These findings demonstrate that AT(2) stimulation induces catecholamine secretion by mobilizing Ca(2+) through voltage-dependent Ca(2+) channels without affecting intracellular pools and that these effects could be mediated by a decrease in cGMP production.

Angiotensin-(1-7) regulates the levels of angiotensin II receptor subtype AT1 mRNA differentially in a strain-specific fashion

Ang-(1-7) is an effector peptide of the renin-angiotensin system with several distinct actions that are likely mediated by a specific receptor. Regulatory effects of angiotensin (Ang) peptides, Ang-(1-7) and Ang II, on Ang receptor subtype 1 (AT1) mRNA expression were investigated in vascular smooth muscle cells (VSMC) from four University of Akron (Akr) rat strains (WKY, SHR and two backcross consomic lines SHR/y and SHR/a), and in SHR and WKY cells from Charles River Laboratories (Crl). In WKY/Akr and SHR/Akr, Ang-(1-7) treatment increased the levels of AT1 mRNA. This effect was inhibited by the specific Ang-(1-7) antagonist, A-779, in WKY/Akr but not SHR/Akr. Ang II had no effect in Akr cells, but it down-regulated AT1 mRNA in WKY/Crl and SHR/Crl VSMC. Ang-(1-7) did not affect AT1 mRNA levels in Crl lines. In conclusion, Ang-(1-7) regulates the AT1 receptor either directly or indirectly in a strain-specific fashion. The Ang-(1-7) antagonist, A-779, blocks the actions of Ang-(1-7) only in VSMC from WKY/Akr rats, suggesting either that the binding sites for Ang-(1-7) have different properties in SHR/Akr and WKY/Akr cell lines, or that some of the effects of Ang-(1-7) are not receptor mediated. Further, we found differences between Akr cells and Crl cells that are consistent with their genetic heterogeneity.

Angiotensin-II subtype 2 receptor agonist (CGP-42112) inhibits catecholamine biosynthesis in cultured porcine adrenal medullary chromaffin cells

Angiotensin II subtype 2 receptor (AT(2)-R) is abundantly expressed in adrenal medullary chromaffin cells. However, the physiological roles of AT(2)-R in chromaffin cells remain to be clarified. Therefore, we investigated the effects of CGP42112 (AT(2)-R agonist) on catecholamine biosynthesis in cultured porcine adrenal medullary cells. We initially confirmed AT(2)-R was predominantly expressed in porcine adrenal medullary cells by [(125)I]-Ang II binding studies. CGP42112 (>==1 nM) significantly inhibited cGMP production from the basal value. Tyrosine hydroxylase (TH) is a rate-limiting enzyme in the biosynthesis of catecholamine, and its activity is regulated by both TH-enzyme activity and TH-synthesis. CGP42112 (>==1 nM) significantly inhibited TH-enzyme activity from the basal value. These inhibitory effects of CGP42112 on TH-enzyme activity and-cGMP production were abolished by PD123319 (AT(2)-R antagonist) while CV-11974 (AT(1)-R antagonist) was ineffective. We also tested whether decrease of cGMP is involved in the inhibitory effect of CGP42112 on TH-enzyme activity. Pretreatment of 8-Br-cGMP (membrane-permeable cGMP analogue) prevented the inhibitory effect of CGP 42112 on TH-enzyme activity. Similar to that of TH-enzyme activity, CGP42112 (>==1 nM) significantly reduced TH-mRNA and TH-protein level from the basal value, and these inhibitory effects were abolished by PD123319 but not CV-11974. These findings demonstrate that CGP 42112 reduces both TH-enzyme activity and TH-synthesis and that these inhibitory effects could be mediated by decrease of cGMP production.

The angiotensin II type 2 receptor: an enigma with multiple variations

Since it was discovered ten years ago, the angiotensin II (ANG II) type 2 (AT(2)) receptor has been an enigma. This receptor binds ANG II with a high affinity but is not responsible for mediating any of the classical physiological actions of this peptide, all of which involve the ANG II type 1 (AT(1)) receptor. Furthermore, the AT(2) receptor exhibits dramatic differences in biochemical and functional properties and in patterns of expression compared with the AT(1) receptor. During the past decade, much information has been gathered about the AT(2) receptor, and the steadily increasing number of publications indicates a growing interest in this new and independent area of research. A number of studies suggest a role of AT(2) receptors in brain, renal, and cardiovascular functions and in the processes of apoptosis and tissue regeneration. Despite these advances, nothing stands out as the major singular function of these receptors. The study of AT(2) receptors has reached a crossroads, and innovative approaches must be considered so that unifying mechanisms as to the function of these unique receptors can be put forward. In this review we will discuss the advances that have been made in understanding the biology of the AT(2) receptor. Furthermore, we will consider how these discoveries, along with newer experimental approaches, may eventually lead to the elusive physiological and pathophysiological functions of these receptors.

Angiotensin II type 1 receptor-mediated increase in cytosolic Ca(2+) and proliferation in mesothelial cells

We investigated the Ca(2+) signaling pathways of the response to angiotensin II in pleural mesothelial cells and the role of these Ca(2+) signaling pathways in mesothelial cell proliferation. Rat pleural mesothelial cells were maintained in vitro, and the Ca(2+) movement to angiotensin II was evaluated using the fluorescent Ca(2+) indicator fura 2. Furthermore, proliferation of mesothelial cells was assessed using a spectrophotometric 3-(4, 5-dimethylthazol-2-yl)-2,5-diphenyl-2H-tetrasodium bromide (MTT) assay. Angiotensin II (1 pM-100 microM) induced in mesothelial cells a biphasic elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)) that consisted of a transient initial component, followed by a sustained component. Neither removal of extracellular Ca(2+) nor inhibition of Ca(2+) influx by 1 microM nifedipine affected the angiotensin II-induced initial transient elevation of [Ca(2+)](i) in mesothelial cells. Nifedipine did not block angiotensin II-induced sustained elevation of [Ca(2+)](i). Angiotensin II (1 pM-100 microM) had a proliferative effect on mesothelial cells in a dose-dependent manner. Angiotensin II type 1 (AT(1)) receptor antagonist ([Sar(1), Ile(8)]angiotensin II) inhibited both angiotensin II-induced elevation of [Ca(2+)](i) and proliferation of mesothelial cells. Pertussis toxin did not affect angiotensin II-induced responses. These results suggest that angiotensin II-induced responses to mesothelial cells are extremely dependent on the angiotensin AT(1) receptor coupled with pertussis toxin-insensitive G protein.

The angiotensin type 2 receptor: variations on an enigmatic theme

Since its discovery and molecular characterization, the angiotensin AT2.receptor has been enigmatic with respect to signalling pathways and function. Evidence now emerges that angiotensin II exerts actions through the AT2 receptor which are directly opposed to those mediated by the AT1 receptor. This can be exemplified e.g. by mutually antagonizing effects on cell growth. Upregulated by the endogenous agonist itself, as well as by several growth- and differentiating factors in development and tissue injury, the AT2 receptor appears to act as a modulator of complex biological programmes involved in embryonic development, cell differentiation, tissue protection and regeneration, as well as in programmed cell death. Research on the AT2 receptor has thus unveiled hitherto unknown functions of the renin-angiotensin system extending far beyond the classical role of this old hormonal system in cardiovascular control.

Emerging features of brain angiotensin receptors

In mammalian brain, angiotensin II AT1 and AT2 receptor subtypes are apparently expressed only in neurons and not in glia. AT1 and AT2 receptor subtypes are sometimes closely associated, but apparently expressed in different neurons. Brain AT1/AT2 interactions may occur in selective cases as inter-neuron cross talk. There are two AT1 isoforms in rodents. AT1A, which predominates, and AT1B. There are also important inter-species differences in receptor expression. Relative lack of amino acid conservation in the gerbil gAT1A receptor substantially decreases affinity for the AT1 antagonists. AT1 receptors are expressed in brain areas regulating autonomic and hormonal responses. AT1A receptors are heterogeneously regulated in a number of experimental conditions. In specific areas, AT1A receptors are not normally expressed, but are induced under influence of reproductive hormones in dopaminergic neurons. There are AT1 and AT2 receptors also in areas related to limbic, sensory and motor functions and their expression is developmentally regulated. A picture is emerging of widespread, neuronally localized, heterogeneously regulated, closely associated brain angiotensin receptor subtypes, modulating multiple functions including neuroendocrine and autonomic responses, stress, cerebrovascular flow, and perhaps brain maturation, neuronal plasticity, memory and behavior.

Role of Arg182 in the second extracellular loop of angiotensin II receptor AT2 in ligand binding

The phenolic side chain of Tyr(4) present in Ang II is proposed to interact with the side chain of Arg 167 of the AT1 receptor. To determine the contribution of the analogous Arg182 in the ligand-binding properties of the AT2, we replaced the Arg182 with Glu and Ala, and analyzed the ligand-binding properties. Our results suggest that replacing Arg182 with either Glu or Ala abolished the ability of the AT2 receptor to bind the nonspecific peptidic ligands, (125)I-Ang II and [(125)I-Sar(1)-Ile(8)]Ang II, as well as the AT2 receptor-specific peptidic ligand (125)I-CGP42112A. We have shown previously that replacing the positively charged side chain of Lys215 with the negatively charged side chain of Glu in the fifth TMD did not alter the high affinity binding of (125)I-CGP42112A to the AT2 receptor. However, ligand-binding properties of the Arg182Glu mutant suggest that positively charged side chain of Arg182 located in the junction of second ECL and the fourth TMD is critical for high affinity binding of all three peptidic ligands to the AT2 receptor.

Modulation and function of extrarenal angiotensin receptors

Historically, physiological modulation of the activity of the renin-angiotensin system (RAS) was thought to be mediated only by changes in renin secretion. Hence, altered dietary sodium (Na) intake, changes in renal perfusion pressure, and/or renal adrenoreceptor activity would lead to changes in renin release and plasma angiotensin II (Ang II) concentration, which in turn contribute to regulation of blood pressure and sodium balance. Later, it became apparent that angiotensinogen availability and Ang-converting enzyme activity are also rate-limiting factors that influence the activity of RAS. Finally, over the past few years, evidence has accumulated that indicates the number of Ang II receptors and their subtypes are of great importance in regulating the activity and function of RAS. Cloning of the Ang II receptor genes, development of specific receptor-antagonist ligands, and establishment of genetically mutated animal models have led to greater understanding of the role of Ang II receptors in the regulation of RAS function and activity. This review focuses on the functions and regulation of Ang II receptors in vascular tissues and in the adrenal gland. The authors suggest that identification of control elements for Ang II receptor expression, which are tissue-specific, may provide a basis for future therapeutic manipulation of Ang II receptors in cardiovascular disease states.

Upregulation of endothelial receptor for oxidized low-density lipoprotein (LOX-1) in cultured human coronary artery endothelial cells by angiotensin II type 1 receptor activation

Cross talk between oxidized LDL (ox-LDL) and angiotensin II (Ang II) may be relevant in atherosclerosis. In this study, we examined the presence of a specific endothelial receptor for ox-LDL (LOX-1) and Ang II receptors in human coronary artery endothelial cells (HCAECs). In addition, we studied the effect of Ang II on LOX-1 gene and protein expression. LOX-1 was consistently identified in HCAECs by reverse transcriptase-polymerase chain reaction (RT-PCR), cDNA sequence, Western blot, and 125I-labeled ox-LDL binding assay (Bmax, 29.7 ng/mg protein). The HCAECs also exhibited Ang II receptors (AT1>AT2), as determined by RT-PCR and 125I-labeled Ang II binding assay (Bmax, 2.21 and 1.19 fmol/mg protein, respectively). Incubation of HCAECs with Ang II markedly increased LOX-1 mRNA (RT-PCR) and protein (Western blot) expression. The increase in LOX-1 expression was dependent on Ang II concentration (10(-12) to 10(-6) mol/L). Ang II caused a concentration-dependent increase in 125I-labeled ox-LDL uptake by HCAECs and enhanced ox-LDL-mediated cell injury, as evident from an increase in LDH release and a decrease in cell viability. These effects of Ang II were completely blocked by pretreatment of HCAECs with losartan, a specific AT1 blocker, but not by PD123319, a specific AT2 blocker. These observations indicate the following: (1) HCAECs possess abundant LOX-1 as well as Ang II (AT1>AT2) receptors, (2) Ang II upregulates LOX-1 receptor and ox-LDL uptake, (3) the effects of Ang II are mediated by AT1 activation, and (4) Ang II enhances ox-LDL-mediated injury to HCAECs.

Role of the His273 located in the sixth transmembrane domain of the angiotensin II receptor subtype AT2 in ligand-receptor interaction

Angiotensin II receptor subtypes AT1 and AT2 are proteins with seven transmembrane domain (TMD) topology and share 34% homology. It was shown that His256, located in the sixth TMD of the AT1 receptor, is needed for the agonist activation by the Phe8 side chain of angiotensin II, although replacing this residue with arginine or glutamine did not significantly alter the affinity binding of the receptor. We hypothesized that the His273 located in the sixth transmembrane domain of the AT2 receptor may play a similar role in the functions of the AT2 receptor, although this residue was not identified as a conserved residue in the initial homology comparisions. Therefore, we replaced His273 of the AT2 receptor with arginine or glutamine and analyzed the ligand-binding properties of the mutant receptors using Xenopus oocytes as an expression system. Our results suggested that the AT2 receptor mutants His273Arg and His273 Glu have lost their affinity to [125I-Sar1-Ile8]Ang II, a peptidic ligand that binds both the AT1 and AT2 receptors and to 125I-CGP42112A, a peptidic ligand that binds specifically to the AT2 receptor. Thus, His273 located in the sixth TMD of the AT2 receptor seems to play an important role in determining the binding properties of this receptor. Moreover, these results along with our previous observation that the Lys215 located in the 5th TMD of the AT2 receptor is essential for its high affinity binding to [125I-Sar1-Ile8]Ang II indicate that key amino acids located in the 5th and 6th TMDs of the AT2 receptor are needed for high affinity binding of the AT2 to its ligands.

Upregulation of renin-angiotensin system during differentiation of monocytes to macrophages

BACKGROUND

We have demonstrated that accumulated macrophages in human coronary arteries strongly express angiotensin converting enzyme in accordance with the development of atheromatous plaques. However, there are few reports on the regulation of the renin-angiotensin system in macrophages and in monocytes as their source.

OBJECTIVE

To examine whether the renin-angiotensin system is upregulated during the differentiation of monocytes to macrophages, and whether it is further regulated by angiotensin II and cytokines.

MATERIALS AND METHODS

We used a human leukemia cell line, THP-1, for monocytes. Differentiated THP-1, induced by adding phorbol 12-myristate 13-acetate for 24 h, were used as macrophages. Expression of messenger RNA of the renin-angiotensin system components was measured by quantitative reverse-transcriptase polymerase chain reaction. Angiotensin converting enzyme activity and subtype-specific angiotensin-binding sites of cultured cells, and angiotensin II production in the culture medium were measured.

RESULTS

Macrophages expressed all components of the renin-angiotensin system except chymase. Cellular angiotensin converting enzyme activity and angiotensin II in the medium were increased 3.2- and 4.5-fold during differentiation, respectively. Expression of angiotensin II type 1 (AT1) and type 2 (AT2) receptors was increased 6.2-and 6.4-fold during differentiation, and was sustained for 7 days. Incubation with angiotensin II for 24 h caused downregulation of both AT1 and AT2 receptor messenger RNA, but the expression levels were still more than threefold higher compared with monocytes. The density of binding sites of AT1 and AT2 receptors in macrophages was 0.26 +/- 0.02 and 0.15 +/- 0.01 fmol/10(6) cells, respectively.

CONCLUSION

The renin-angiotensin system is markedly activated during monocyte/macrophage differentiation, and may participate in the development of atherosclerosis.

Proapoptotic effects of ANG II in human coronary artery endothelial cells: role of AT1 receptor and PKC activation

Anoxia-reoxygenation, tumor necrosis factor-alpha (TNF-alpha), and angiotensin II (ANG II) have been shown to induce apoptosis in myocytes. However, the role of these mediators in causing apoptosis of human coronary artery endothelial cells (HCAEC) is not known. This study was designed to examine the interaction of these mediators in induction of apoptosis in HCAEC. Cultured HCAEC were exposed to anoxia-reoxygenation, TNF-alpha, and ANG II. TNF-alpha enhanced apoptosis of HCAEC (determined by DNA nick-end labeling in situ and DNA laddering) caused by anoxia-reoxygenation. ANG II increased apoptosis beyond that caused by anoxia-reoxygenation and TNF-alpha. Apoptosis caused by ANG II was reduced by losartan, a specific ANG II type 1 receptor (AT1R) blocker, whereas PD-123,177, a specific ANG II type 2 receptor blocker, under identical conditions had minimal effect. The proapoptotic effects of ANG II were associated with the activation of protein kinase C (PKC). The importance of PKC activation as a signal transduction mechanism became evident in experiments wherein treatment of HCAEC with a specific inhibitor of PKC activation decreased ANG II-mediated apoptosis. Thus AT1R activation appears to be responsible for apoptosis caused by ANG II in HCAEC, and AT1R activation-mediated apoptosis involves activation of PKC.

Role of the third intracellular loop of the angiotensin II receptor subtype AT2 in ligand-receptor interaction

Angiotensin II (Ang II) receptor subtypes AT1 and AT2 share 34% overall homology, but the least homology is in their third intracellular loop (3rd ICL). In an attempt to elucidate the role of the 3rd ICL in determining the similarities and differences in the functions of the AT1 and the AT2 receptors, we generated a chimeric receptor in which the 3rd ICL of the AT2 receptor was replaced with that of the AT1 receptor. Ligand-binding properties and signaling properties of this receptor were assayed by expressing this receptor in Xenopus oocytes. Ligand-binding studies using [125I-Sar1-Ile8] Ang II, a peptidic ligand that binds both the AT1 and the AT2 receptor subtypes, and 125I-CGP42112A, a peptidic ligand that is specific for the AT2 receptor, showed that the chimeric receptor has lost affinity to both ligands. However, IP3 levels of the oocytes expressing the chimeric receptor were comparable to the IP3 levels of the oocytes expressing the AT1 receptor, suggesting that the chimeric receptors could couple to phospholipase C pathway in response to Ang II. We have shown previously that the nature of the amino acid present in the position 215 located in the fifth transmembrane domain (TMD) of the AT2 receptor plays an important role in determining its affinity to different ligands. Our results from the ligand-binding studies of the chimeric receptor further support the idea that the structural organization of the region spanning the 5th TMD and the 3rd ICL of the AT2 receptor has an important role in determining the ligand-binding properties of this receptor.

Absence of effect of chronic angiotensin II type 1 receptor blockade on endogenous kinin concentrations-induced paw edema model in the rat

The effects of chronic treatment with losartan. an AT1 receptor antagonist, on the tissue content of bradykinin (BK) and des-Arg9-BK and on their pharmacological effects were examined in the carrageenan-induced paw edema model (0.5% solution, 50 microl/paw) in the rat. These effects were compared with those of angiotensin-converting enzyme inhibitors (ACEi). For this purpose, rats were chronically treated with losartan (3, 10 and 30 mg/kg/day) and enalapril or quinapril (1 mg/kg/day). Endogenous BK and des-Arg9-BK tissue contents at the site of local inflammation were measured by highly sensitive and specific enzyme immunoassays. Losartan 3 mg/kg/day for 7, 14 and 28 days had no significant effect on carrageenan-induced paw edema, but both losartan 10 and 30 mg/kg/day for 14 days significantly increased the hindpaw volume by 50% at 3 h and by 59% at 5 h. These effects, similar to those measured for ACEi, were inhibited by icatibant, a B2 kinin receptor antagonist (32.5 nmol/paw), that reduced carrageenan-induced paw edema to the level seen in vehicle-treated rats. In the same model, and contrary to ACEi, losartan 3, 10 and 30 mg/kg/day for 14 days had no significant effect on endogenous BK and des-Arg9-BK levels in the local inflammatory site or on circulating and tissue ACE activities. These results show, at least in that model, that the potentiating effects of losartan on carrageenan-induced paw edema are independent of the concentrations of endogenous kinins.

Pathophysiological role of angiotensin II type 2 receptor in cardiovascular and renal diseases

Since the discovery of nonpeptidic ligands, the receptors for angiotensin (Ang) II have been classified into 2 subtypes (Ang II type 1 receptor [AT1-R] and Ang II type 2 receptor [AT2-R]). AT1-R mediates most of the cardiovascular actions of Ang II. AT2-R is expressed at very high levels in the developing fetus. Its expression is very low in the cardiovascular system of the adult. The expression of AT2-R can be modulated by pathological states associated with tissue remodeling or inflammation. In failing hearts or neointima formation after vascular injury, AT2-R is reexpressed in cells proliferating in interstitial regions or neointima and exerts an inhibitory effect on Ang II-induced mitogen signals or synthesis of extracellular matrix proteins, resulting in attenuation of the tissue remodeling. An extreme form of cell growth inhibition ends in programmed cell death, and this process, which is initiated by the withdrawal of growth factors, is also enhanced by AT2-R. Cardiac myocyte- or vascular smooth muscle-specific mice that overexpress AT2-R display an inhibition of Ang II-induced chronotropic or pressor actions, suggesting the role of AT2-R on the activity of cardiac pacemaker cells and the maintenance of vascular resistance. AT2-R also activates the kinin/nitric oxide/cGMP system in the cardiovascular and renal systems, resulting in AT2-R-mediated cardioprotection, vasodilation, and pressure natriuresis. These effects, transmitted by AT2-R, are mainly exerted by stimulation of protein tyrosine or serine/threonine phosphatases in a Gi protein-dependent manner. The expression level of AT2-R is much higher in human hearts than in rodent hearts, and the AT2-R-mediated actions are likely enhanced, especially by clinical application of AT1-R antagonists. Thus, in this review, the regulation of AT2-R expression, its cellular localization, its pathological role in cardiovascular and kidney diseases, and pharmacotherapeutic effects of AT2-R stimulation are discussed.

Deferential roles of angiotensin receptor subtypes in adrenocortical function in mice

The functional significance of angiotensin II (Ang II) receptor subtypes in adrenals remains unknown. Ang II receptor type 1a (AT1a) expression was localized by in situ hybridization to the zona glomerulosa and zona fasciculata, while AT1b was localized to the zona glomerulosa. Plasma aldosterone and corticosterone levels were measured after injection with Ang II or the type 2 receptor (AT2) agonist CGP-42112 in wild-type and AT1a deficient mice. Aldosterone and corticosterone levels were lower in AT1a deficient mice. Ang II increased plasma aldosterone levels in AT1a deficient mice, but to a lesser extent in mice pretreated with nonselective AT1a/AT1b antagonist, CV-11974. CGP-42112 did not affect aldosterone levels. Ang II increased corticosterone levels in wild-type mice but not in AT1a deficient mice. Results suggest Ang II stimulates aldosterone secretion via AT1a and AT1b in the zona glomerulosa and corticosterone secretion via AT1a in the zona fasciculata, and provide first evidence for differential roles of AT1a and AT1b in the adrenals.

Angiotensin II stimulates both aldosterone secretion and DNA synthesis via type 1 but not type 2 receptors in bovine adrenocortical cells

Angiotensin II (Ang II) type 2 receptor (AT2) has been shown to counteract the type 1 receptor (AT1)-mediated biological actions of Ang II in the cardiovascular system. The biological significance of AT2 receptor in the adrenals however remains unknown. In the present study, we investigated the roles of AT1 and AT2 receptor subtypes in the regulation of aldosterone secretion and DNA synthesis in bovine adrenocortical zona glomerulosa cells in vitro. Ang II (1 mumol/l)-stimulated aldosterone secretion was completely suppressed by AT1 antagonist CV-11974 but not affected by AT2 receptor antagonist PD-123319. Effects on DNA synthesis were investigated by determining the incorporation of BrdU into the nuclei of the cultured zona glomerulosa cells. Ang II (1 mumol/l)-stimulated DNA synthesis of the cells was also completely suppressed by CV-11974 but not by PD-123319. These results suggest that AT1 receptor but not AT2 receptor is the predominant receptor subtype which mediates the Ang II-stimulated aldosterone secretion and cell growth in bovine adrenocortical cells.