Regulation of angiotensin II type 1 (AT1) receptor function

Aerobic Exercise Training Reduces Atherogenesis Induced by Low-Sodium Diet in LDL Receptor Knockout Mice

This study investigated the efficacy of aerobic exercise training (AET) in the prevention of dyslipidemia, insulin resistance (IR), and atherogenesis induced by severe low-sodium (LS) diet. LDL receptor knockout (LDLR KO) mice were fed a low-sodium (LS) (0.15% NaCl) or normal-sodium (NS; 1.27% NaCl) diet, submitted to AET in a treadmill, 5 times/week, 60 min/day, 15 m/min, for 90 days, or kept sedentary. Blood pressure (BP), plasma total cholesterol (TC) and triglyceride (TG) concentrations, lipoprotein profile, and insulin sensitivity were evaluated at the end of the AET protocol. Lipid infiltration, angiotensin II type 1 receptor (AT1), receptor for advanced glycation end products (RAGE), carboxymethyllysine (CML), and 4-hydroxynonenal (4-HNE) contents as well as gene expression were determined in the brachiocephalic trunk. BP and TC and gene expression were similar among groups. Compared to the NS diet, the LS diet increased vascular lipid infiltration, CML, RAGE, 4-HNE, plasma TG, LDL-cholesterol, and VLDL-TG. Conversely, the LS diet reduced vascular AT1 receptor, insulin sensitivity, HDL-cholesterol, and HDL-TG. AET prevented arterial lipid infiltration; increases in CML, RAGE, and 4-HNE contents; and reduced AT1 levels and improved LS-induced peripheral IR. The current study showed that AET counteracted the deleterious effects of chronic LS diet in an atherogenesis-prone model by ameliorating peripheral IR, lipid infiltration, CML, RAGE, 4-HNE, and AT1 receptor in the intima-media of the brachiocephalic trunk. These events occurred independently of the amelioration of plasma-lipid profile, which was negatively affected by the severe dietary-sodium restriction.

Mechanisms and Clinical Implications of Endothelial Dysfunction in Arterial Hypertension

The endothelium is composed of a monolayer of endothelial cells, lining the interior surface of blood and lymphatic vessels. Endothelial cells display important homeostatic functions, since they are able to respond to humoral and hemodynamic stimuli. Thus, endothelial dysfunction has been proposed as a key and early pathogenic mechanism in many clinical conditions. Given the relevant repercussions on cardiovascular risk, the complex interplay between endothelial dysfunction and systemic arterial hypertension has been a matter of study in recent years. Numerous articles have been published on this issue, all of which contribute to providing an interesting insight into the molecular mechanisms of endothelial dysfunction in arterial hypertension and its role as a biomarker of inflammation, oxidative stress, and vascular disease. The prognostic and therapeutic implications of endothelial dysfunction have also been analyzed in this clinical setting, with interesting new findings and potential applications in clinical practice and future research. The aim of this review is to summarize the pathophysiology of the relationship between endothelial dysfunction and systemic arterial hypertension, with a focus on the personalized pharmacological and rehabilitation strategies targeting endothelial dysfunction while treating hypertension and cardiovascular comorbidities.

Association study indicates combined effect of interleukin-10 and angiotensin-converting enzyme in basal cell carcinoma development

Cytokines involved in inflammatory and immune response have been associated with risk for development of basal cell carcinoma (BCC). In this study, three functional DNA polymorphisms affecting gene expression were investigated in 54 BCC patients and 111 healthy controls: interleukin-1b (IL-1b) +3953C/T, interleukin-10 (IL-10) - 1082G/A and angiotensin-converting enzyme (ACE) insertion/deletion (I/D) polymorphisms. Significant increase of the variant alleles was observed in IL-10 - 1082G (P = 0.019) and in ACE D (P = 0.003) in BCC patients in comparison to controls. Multivariate logistic regression models evaluated the contribution of homozygous and heterozygous variant polymorphisms to the risk for BCC development. The studied polymorphisms influencing the expression of IL-10 and ACE genes were recognized as potential predictive factors for BCC. These findings suggest a possible molecular mechanism leading to BCC development that is likely to involve the activation of angiotensin receptors in combination with increased plasma levels of IL-10 in patients.

Renin-angiotensin system in vertebrates: phylogenetic view of structure and function

Renin substrate, biological renin activity, and/or renin-secreting cells in kidneys evolved at an early stage of vertebrate phylogeny. Angiotensin (Ang) I and II molecules have been identified biochemically in representative species of all vertebrate classes, although variation occurs in amino acids at positions 1, 5, and 9 of Ang I. Variations have also evolved in amino acid positions 3 and 4 in some cartilaginous fish. Angiotensin receptors, AT₁ and AT₂ homologues, have been identified molecularly or characterized pharmacologically in nonmammalian vertebrates. Also, various forms of angiotensins that bypass the traditional renin-angiotensin system (RAS) cascades or those from large peptide substrates, particularly in tissues, are present. Nonetheless, the phylogenetically important functions of RAS are to maintain blood pressure/blood volume homeostasis and ion-fluid balance via the kidney and central mechanisms. Stimulation of cell growth and vascularization, possibly via paracrine action of angiotensins, and the molecular biology of RAS and its receptors have been intensive research foci. This review provides an overview of: (1) the phylogenetic appearance, structure, and biochemistry of the RAS cascade; (2) the properties of angiotensin receptors from comparative viewpoints; and (3) the functions and regulation of the RAS in nonmammalian vertebrates. Discussions focus on the most fundamental functions of the RAS that have been conserved throughout phylogenetic advancement, as well as on their physiological implications and significance. Examining the biological history of RAS will help us analyze the complex RAS systems of mammals. Furthermore, suitable models for answering specific questions are often found in more primitive animals.

Angiotensin II (de)sensitization: Fluid intake studies with implications for cardiovascular control

Cardiovascular disease is the leading cause of death worldwide and hypertension is the most common risk factor for death. Although many anti-hypertensive pharmacotherapies are approved for use in the United States, rates of hypertension have increased over the past decade. This review article summarizes a presentation given at the 2015 meeting of the Society for the Study of Ingestive Behavior. The presentation described work performed in our laboratory that uses angiotensin II-induced drinking as a model system to study behavioral and cardiovascular effects of the renin-angiotensin system, a key component of blood pressure regulation, and a common target of anti-hypertensives. Angiotensin II (AngII) is a potent dipsogen, but the drinking response shows a rapid desensitization after repeated injections of AngII. This desensitization appears to be dependent upon the timing of the injections, requires activation of the AngII type 1 (AT1) receptor, requires activation of mitogen-activated protein (MAP) kinase family members, and involves the anteroventral third ventricle (AV3V) region as a critical site of action. Moreover, the response does not appear to be the result of a more general suppression of behavior, a sensitized pressor response to AngII, or an aversive state generated by the treatment. More recent studies suggest that the treatment regimen used to produce desensitization in our laboratory also prevents the sensitization that occurs after daily bolus injections of AngII. Our hope is that these findings can be used to support future basic research on the topic that could lead to new developments in treatments for hypertension.

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.

Properly timed exposure to central ANG II prevents behavioral sensitization and changes in angiotensin receptor expression

Previous studies show that the angiotensin type 1 receptor (AT1R) is susceptible to rapid desensitization, but that more chronic treatments that stimulate ANG II lead to sensitization of several responses. It is unclear, however, if the processes of desensitization and sensitization interact. To test for differences in AT1R expression associated with single or repeated injections of ANG II, we measured AT1R mRNA in nuclei that control fluid intake of rats given ANG II either in a single injection or divided into three injections spaced 20 min apart. Rats given a single injection of ANG II had more AT1R mRNA in the subfornical organ (SFO) and the periventricular tissue surrounding the anteroventral third ventricle (AV3V) than did controls. The effect was not observed, however, when the same cumulative dose of ANG II was divided into multiple injections. Behavioral tests found that single daily injections of ANG II sensitized the dipsogenic response to ANG II, but a daily regimen of four injections did not cause sensitization. Analysis of (125)I-Sar(1)-ANG II binding revealed a paradoxical decrease in binding in the caudal AV3V and dorsal median preoptic nucleus after 5 days of single daily injections of ANG II; however, this effect was absent in rats treated for 5 days with four daily ANG II injections. Taken together, these data suggest that a desensitizing treatment regimen prevents behavior- and receptor-level effects of repeated daily ANG II.

The vascular renin-angiotensin system contributes to blunted vasodilation induced by transient high pressure in human adipose microvessels

Increased intraluminal pressure can reduce endothelial function in resistance arterioles; however, the mechanism of this impairment is unknown. The purpose of this study was to determine the effect of local renin-angiotensin system inhibition on the pressure-induced blunting of endothelium-dependent vasodilation in human adipose arterioles. Arterioles (100-200 μm) were dissected from fresh adipose surgical specimens, cannulated onto glass micropipettes, pressurized to an intraluminal pressure of 60 mmHg, and constricted with endothelin-1. Vasodilation to ACh was assessed at 60 mmHg and again after a 30-min exposure to an intraluminal pressure of 150 mmHg. The vasodilator response to ACh was significantly reduced in vessels exposed to 150 mmHg. Exposure of the vessels to the superoxide scavenger polyethylene glycol-SOD (100 U/ml), the ANG II type 1 receptor antagonist losartan (10(-6) mol/l), or the angiotensin-converting enzyme inhibitor captopril (10(-5) mol/l) prevented the pressure-induced reduction in ACh-dependent vasodilation observed in untreated vessels. High intraluminal pressure had no effect on papaverine-induced vasodilation or ANG II sensitivity. Increased intraluminal pressure increased dihydroethidium fluorescence in cannulated vessels, which could be prevented by polyethylene glycol-SOD or losartan treatment and endothelial denudation. These data indicate that high intraluminal pressure can increase vascular superoxide and reduce nitric oxide-mediated vasodilation via activation of the vascular renin-angiotensin system. This study provides evidence showing that the local renin-angiotensin system in the human microvasculature may be pressure sensitive and contribute to endothelial dysfunction after acute bouts of hypertension.

The anteroventral third ventricle region is critical for the behavioral desensitization caused by repeated injections of angiotensin II

A single central injection of angiotensin II (AngII) potently increases water intake; however, a growing body of research suggests that repeated, acute intracerebroventricular injections of AngII cause a reduction in the dipsogenic response to subsequent AngII. This AngII-induced behavioral desensitization is specific to the effects of angiotensin and mediated by the angiotensin type-1 (AT1) receptor. The neuroanatomical substrate for this phenomenon, however, remains unknown. The anteroventral third ventricle (AV3V) region is an important site for the behavioral and physiological actions of AngII. Therefore, we hypothesized that this region also mediates the effects of repeated central AngII administration. In support of this hypothesis, we found that repeated injections of AngII into the AV3V reduced water intake stimulated by a test injection of AngII given into this region. Moreover, repeated AngII injections in the AV3V reduced water intake after AngII was injected into the lateral ventricle. These studies also demonstrate that activation of the AT1 receptor within the AV3V is required for AngII-induced behavioral desensitization because direct injection of the AT1 receptor antagonist, losartan, into the AV3V blocked the desensitizing effect of repeated AngII injections into the lateral ventricle. These findings provide additional support for a role of the AV3V in the dipsogenic actions of AngII, and suggest that this region is critical for the desensitization that occurs after acute repeated central injections of AngII.

Shear stress activates extracellular signal-regulated kinase 1/2 via the angiotensin II type 1 receptor

Mechanical factors such as strain, pressure, and shear stress are key regulators of cell function, but the molecular mechanisms underlying the detection and responses to such stimuli are poorly understood. Whether the angiotensin II (AngII) AT1 receptor (AT1R) transduces shear stress in endothelial cells (ECs) is unknown. We exposed human umbilical cord endothelial cells (HUVECs) to a shear stress of 0 (control) or 15 dyn/cm(2) for 5 or 10 min. The colocalization of AT1R with caveolin-1 (Cav1), endosomal markers Rab5, EEA1, and Rab7, and lysosomal marker Lamp-1 increased in shear stimulated cells, detected by immunocytochemistry. Shear stress reduced labeling of wild-type mouse ECs (18±3% of unsheared control, P<0.01) but not Cav1(-/-) ECs (90±10%) with fluorescent AngII, confirming that internalization of AT1R requires Cav1. Shear stress activated ERK1/2 2-fold (P<0.01), which was prevented by the AT1R blocker losartan. NADPH oxidase inhibition with apocynin prevented both the colocalization of AT1R with Cav1 and the induction of ERK1/2 by shear stress. Moreover, shear-dependent ERK1/2 activation was minimal in CHO cells expressing an AT1Ra mutant that does not internalize, compared with cells expressing wild-type AT1Ra (P<0.05). Hence, AT1R may be an important transducer of shear stress-dependent activation of ERK1/2.

Shear stress-induced Ang II AT1 receptor activation: G-protein dependent and independent mechanisms

Mechanotransduction enables cells to sense and respond to stimuli, such as strain, pressure and shear stress (SS), critical for maintenance of cardiovascular homeostasis or pathological states. The angiotensin II type 1 receptor (AT1R) was the first G protein-coupled receptor described to display stretch-induced activation in cardiomyocytes independent of its ligand Ang II. Here, we assessed whether SS (15 dynes/cm(2), 10 min), an important mechanical force present in the cardiovascular system, activates AT1R independent of its ligand. SS induced extracellular signal-regulated kinase (ERK) activation, used as a surrogate of AT1R activation, in Chinese hamster ovary cells expressing the AT1R (CHO+AT1) but not in wild type cells (CHO). AT1R dependent SS-induced ERK activation involves Ca(2+) inflow and activation of Gαq since Ca(2+) chelator EGTA or Gαq-specific inhibitor YM-254890 decreased SS-induced ERK activation. On the other hand, the activation of JAK-2 and Src, two intracellular signaling molecules independent of G protein activation, were not differently modulated in the presence of AT1R. Also, ERK activation by SS was observed in CHO cells expressing the mutated AT1R DRY/AAY, which has impaired ability to activate Gαq dependent intracellular signaling. Altogether we provided evidence that SS activates AT1R in the absence of its ligand by both a G protein-dependent and -independent pathways. The biological relevance of these observations deserves to be further investigated since the novel mechanisms described extend the knowledge of the activation of GPCRs independent of its traditional ligand.

Association of angiotensin-converting enzyme gene insertion/deletion polymorphism with decreased risk for basal cell carcinoma

The incidence of basal cell carcinoma (BCC) is significantly reduced in individuals treated with inhibitors of angiotensin I-converting enzyme (ACE) that produces angiotensin II. The objective of this study was to investigate the possible association of a functional polymorphism in the ACE gene, which affects its transcription, with risk for BCC. In DNA samples of 92 patients with BCC and 103 healthy controls of Greek origin and comparable age and gender, we studied the ACE gene insertion/deletion (I/D) polymorphism. Fisher's exact test was used for comparison of allele and genotype frequencies between the control and patients' groups. The detected low expression I allele frequency in the group of BCC patients was significantly decreased compared to controls (15.8 vs. 31.1 %, respectively; P = 0.001). ID heterozygotes exhibited 3.06 times lower BCC risk, compared with DD homozygotes (P = 0.001; OR = 0.327, 95 % CI = 0.174-0.615). The protective role of I allele was particularly prominent in women (P = 0.007, OR = 0.299, 95 % CI = 0.125-0.716), while for men it exhibited a marginal level (P = 0.041). These findings indicate that the low expression ACE I allele carriers have a decreased risk for BCC. The protective effect of the ID genotype against BCC may be explained by a possible underlying mechanism involving the effect of produced angiotensin II levels on its receptors due to putatively different binding affinity.

Mitogen-activated protein kinase is required for the behavioural desensitization that occurs after repeated injections of angiotensin II

Angiotensin II (Ang II) acts on central angiotensin type 1 (AT(1)) receptors to increase water and saline intake. Prolonged exposure to Ang II in cell culture models results in a desensitization of the AT(1) receptor that is thought to involve receptor internalization, and a behavioural correlate of this desensitization has been shown in rats after repeated central injections of Ang II. Specifically, rats given repeated injections of Ang II drink less water than control animals after a subsequent test injection of Ang II. In the same conditions, however, repeated injections of Ang II have no effect on Ang II-induced saline intake. Given earlier studies indicating that separate intracellular signalling pathways mediate Ang II-induced water and saline intake, we hypothesized that the desensitization observed in rats may be incomplete, leaving the receptor able to activate mitogen-activated protein (MAP) kinases (ERK1/2), which play a role in Ang II-induced saline intake without affecting water intake. In support of this hypothesis, we found no difference in MAP kinase phosphorylation after an Ang II test injection in rats given prior treatment with repeated injections of vehicle, Ang II or Sar(1),Ile(4),Ile(8)-Ang II (SII), an Ang II analogue that activates MAP kinase without G protein coupling. In addition, we found that pretreatment with the MAP kinase inhibitor U0126 completely blocked the desensitizing effect of repeated Ang II injections on water intake. Furthermore, Ang II-induced water intake was reduced to a similar extent by repeated injections of Ang II or SII. The results suggest that G protein-independent signalling is sufficient to produce behavioural desensitization of the angiotensin system and that the desensitization requires MAP kinase activation.

Investigation into the specificity of angiotensin II-induced behavioral desensitization

Angiotensin II (AngII) plays a key role in maintaining body fluid homeostasis. The physiological and behavioral effects of central AngII include increased blood pressure and fluid intake. In vitro experiments demonstrate that repeated exposure to AngII reduces the efficacy of subsequent AngII, and behavioral studies indicate that prior icv AngII administration reduces the dipsogenic response to AngII administered later. Specifically, rats given a treatment regimen of three icv injections of a large dose of AngII, each separated by 20 min, drink less water in response to a test injection of AngII than do vehicle-treated controls given the same test injection. The present studies were designed to test three potential explanations for the reduced dipsogenic potency of AngII after repeated administration. To this end, we tested for motor impairment caused by repeated injections of AngII, for a possible role of visceral distress or illness, and for differences in the pressor response to the final test injection of AngII. We found that repeated injections of AngII neither affected drinking stimulated by carbachol nor did they produce a conditioned flavor avoidance. Furthermore, we found no evidence that differences in the pressor response to the final test injection of AngII accounted for the difference in intake. In light of these findings, we are able to reject these three explanations for the observed behavioral desensitization, and, we suggest instead that the mechanism for this phenomenon may be at the level of the receptor.

Alan [corrected] N. Epstein award: Intracellular signaling and ingestive behaviors

Understanding the role of intracellular signaling pathways in ingestive behavior is a challenging problem in behavioral neuroscience. This review summarizes work conducted on two systems with the aim of identifying intracellular events that relate to food and fluid intake. The first set of experiments focused on melanocortin receptors and their ability to signal through members of the mitogen-activated protein (MAP) kinase family. The second set of experiments focused on the role of intracellular signaling pathways in water and saline intakes that are stimulated by angiotensin II (AngII). The initial findings in each line of research have been extended by subsequent research that is discussed in turn. The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Portland, July 2009.

Repeated administration of angiotensin II reduces its dipsogenic effect without affecting saline intake

Angiotensin II (Ang II) acts at central type 1 (AT(1)) receptors to increase intake of water and saline. In vitro studies demonstrated rapid desensitization of the AT(1) receptor after Ang II exposure, and behavioural studies in rats suggest that exposure to Ang II decreases the dipsogenic potency of subsequent Ang II. Nevertheless, the effect of repeated Ang II injections on saline intake remains untested, and a reliable protocol for examining this purported behavioural desensitization has not emerged from the literature. To address these issues, we established a reliable approach to study Ang II-induced dipsetic desensitization and used this approach to test the requirement of central AT(1) receptors and the specificity of the effect for water intake. Rats given a treatment regimen of three injections of Ang II (300 ng, intracerebroventricular), each separated by 20 min, drank less water than control rats after a subsequent test injection of Ang II. The effect was relatively short lasting, dependent on the dose and timing of Ang II, and was almost completely blocked by the AT(1) receptor antagonist losartan. In further testing, when rats were given access to both water and 1.5% saline, animals that received an Ang II treatment regimen drank less water than control animals, but saline intake was unaffected. These data support previous suggestions that Ang II-induced water and saline intakes are separable. Given the role of G protein uncoupling in desensitization of the AT(1) receptor, these data are consistent with the emerging hypothesis that AT(1) receptor G protein-dependent intracellular signalling pathways are more relevant for water, but not saline, intake.

Role of helix 8 in G protein-coupled receptors based on structure-function studies on the type 1 angiotensin receptor

G protein-coupled receptors (GPCRs) are transmembrane receptors that convert extracellular stimuli to intracellular signals. The type 1 angiotensin II receptor is a widely studied GPCR with roles in blood pressure regulation,water and salt balance and cell growth. The complex molecular and structural changes that underpin receptor activation and signaling are the focus of intense research. Increasingly, there is an appreciation that the plasma membrane participates in receptor function via direct, physical interactions that reciprocally modulate both lipid and receptor and provide microdomains for specialized activities. Reversible protein:lipid interactions are commonly mediated by amphipathic -helices in proteins and one such motif - a short helix, referred to as helix VIII/8 (H8), located at the start of the carboxyl (C)-terminus of GPCRs - is gaining recognition for its importance to GPCR function. Here, we review the identification of H8 in GPCRs and examine its capacity to sense and interact with diverse proteins and lipid environment, most notably with acidic lipids that include phosphatidylinositol phosphates.

Effect of Dominant-Negative Epidermal Growth Factor Receptors on Cardiomyocyte Hypertrophy

Angiotensin II (AngII) induces heart growth via cardiomyocyte hypertrophy, and central to this is the capacity of the type 1 AngII receptor (AT1R) to "transactivate" epidermal growth factor receptors (EGFRs)--a family with four main subtypes (HER1-4)--although the exact molecular mechanism remains unresolved. In this study, the pharmacological inhibition of AngII-stimulated ERK1/2 activation and cardiomyocyte hypertrophy by increasing concentrations of an EGFR inhibitor, AG1478, indicated that other EGFR subtypes, in addition to HER1, may be involved. We constructed expression vectors and adenoviruses expressing truncated mutant versions of HER1, HER2, and HER4 and determined their capacity to act as dominant-negative inhibitors when co-transfected with full-length EGFRs. It is surprising that adenoviral-mediated expression of these truncated EGFRs in cardiomyocytes led to paradoxical, ligand-independent increases in cardiomyocyte hypertrophy and unusual morphological changes. These results challenge our perception of AT1R-mediated EGFR transactivation and imply that truncated EGFRs may affect cell function through unconventional mechanisms.

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.

The effect of angiotensin II on intracellular pH is mediated by AT1receptor translocation

The effect of ANG II on intracellular pH (pHi) recovery rate and AT1receptor translocation was investigated in transfected MDCK cells. The pHirecovery rate was evaluated by fluorescence microscopy using the fluorescent probe BCECF-AM. The human angiotensin II receptor isoform 1 (hAT1) translocation was analyzed by immunofluorescence and confocal microscope. Our data show that transfected cells in control situation have a pHirecovery rate of 0.219 ± 0.017 pH U/min ( n = 11). This value was similar to nontransfected cells [0.211 ± 0.009 pH U/min ( n = 12)]. Both values were significantly increased with ANG II (10−9M) but not with ANG II (10−6M). Losartan (10−7M) and dimethyl-BAPTA-AM (10−7M) decreased significantly the stimulatory effect of ANG II (10−9M) and induced an increase in Na+/H+exchanger 1 (NHE-1) activity with ANG II (10−6M). Immunofluorescence studies indicated that in control situation, the hAT1receptor was predominantly expressed in cytosol. However, it was translocated to plasma membrane with ANG II (10−9M) and internalized with ANG II (10−6M). Losartan (10−7M) induced hAT1translocation to plasma membrane in all studied groups. Dimethyl-BAPTA-AM (10−7M) did not change the effect of ANG II (10−9M) on the hAT1receptor distribution but induced its accumulation at plasma membrane in cells treated with ANG II (10−6M). With ionomycin (10−6M), the receptor was accumulated in cytosol. The results indicate that, in MDCK cells, the effect of ANG II on NHE-1 activity is associated with ligand binding to AT1receptor and intracellular signaling events related to AT1translocation.

Uridine adenosine tetraphosphate induces contraction and relaxation in rat aorta

Uridine adenosine tetraphosphate (Up(4)A) has been recently reported as an endothelium-derived vasoconstrictor and plasma levels of this dinucleotide are increased in juvenile hypertensive subjects. This study aimed to evaluate the vascular actions of Up(4)A, typify the putative purinergic receptors that might mediate these effects and characterize the intracellular signaling pathways that may govern Up(4)A responses. Up(4)A induced a modest endothelium-dependent relaxation of rat aortic rings contracted with phenylephrine. From baseline, Up(4)A induced concentration-dependent contractions that were significantly potentiated by endothelium removal or nitric oxide synthase inhibition. The contractile response induced by Up(4)A was not tachyphylactic and was significantly reduced in the presence of P1 or P2X receptor antagonists, L-type Ca(2+) channel blocker and Rho-kinase inhibitor. Up(4)A-induced contraction apparently involves superoxide anion formation since it was significantly reduced by treatment with apocynin or tempol. This study presents the unique findings that the endogenous compound Up(4)A is able to induce relaxation in addition to contraction of rat aorta. Up(4)A-induced contraction is modulated by nitric oxide production, mediated by P1 and P2X receptor activation, and involves L-type Ca(2+) channels, Rho-kinase pathway and superoxide formation.

Methyl-β-cyclodextrin Prevents Angiotensin II-Induced Tachyphylactic Contractile Responses in Rat Aorta

Tachyphylaxis or desensitization is frequently observed following angiotensin II type I (AT1) receptor activation by angiotensin II. One of the possible mechanisms contributing to receptor desensitization involves receptor internalization. In addition to clathrin-coated pits/vesicles, caveolae, small invaginations in the plasma membrane rich in cholesterol, may also be involved in receptor internalization. After activation, AT1 receptor partially redistributes to lipid-enriched domains. We hypothesize that AT1 receptor internalization via caveolae contributes to the tachyphylactic response observed to angiotensin II. Endothelium-denuded rat aortic rings were exposed to increasing concentrations of angiotensin II or phenylephrine, generating two cumulative concentration-effect curves (CCEC) with a 90-min interval separating each curve (CCEC-I and CCEC-II). CCEC-II was performed in the presence of either vehicle or methyl-beta-cyclodextrin (CD), a drug that depletes cholesterol from the membrane and disassembles caveolae. CCEC-II to angiotensin II, but not to phenylephrine, was blunted in aortic rings treated with vehicle. In the presence of CD, CCEC-II did not differ significantly from CCEC-I for both agonists. CCEC-I to angiotensin II was abolished when in the presence of the AT1 receptor antagonist. The presence of AT1 receptors at the aortic smooth muscle cells' membrane treated with angiotensin II was observed by immunofluorescence only in the presence of CD. In addition, caveolin-1 coimmunoprecipitated with AT1 receptor after agonist stimulation, and this interaction was inhibited by CD. Our data suggest that caveolae are involved in the tachyphylactic contractile response induced by angiotensin II in rat aorta, and this effect is related to receptor internalization.

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.

The use of heat-induced hydrolysis in immunohistochemistry on angiotensin II (AT1) receptors enhances the immunoreactivity in paraformaldehyde-fixed brain tissue of normotensive Sprague–Dawley rats

The research on components of the renin-angiotensin system delivered a broad image of angiotensin II-binding sites. Especially, immunohistochemistry (IHC) provided an exact anatomical localization of the AT(1) receptor in the rat brain. Yet, controversial results between in vitro receptor autoradiography and IHC as well as between immunohistochemical studies using various antisera started a vehement discussion concerning specificity and cross-reactivity of these antisera. In particular the magnocellular subdivision of the paraventricular nucleus (PVN) and the supraoptic nucleus (SON) provided controversial results on the localization of AT(1) receptors. Both areas are known for angiotensin II-induced release of vasopressin (VP) and oxytocin (OXT). To evaluate the significance of the appropriate method of antigen retrieval and its relevance for the detection of AT(1) receptors we performed IHC on AT(1) receptors in paraformaldehyde-fixed and paraffin-embedded brain tissue of Sprague-Dawley rats using either the detergent Triton X-100 or microwave oven heating. This study demonstrates that heat-induced hydrolysis enhances the quality and quantity of immunoreactivity (IR) in IHC on AT(1) receptors. In the organum vasculosum lamina terminalis and in the parvocellular subdivisions of the PVN we report a distribution of AT(1)-like-IR similar to that observed with other methods. However, in addition, we provide evidence that distinct AT(1)-like-IR is also localized in few magnocellular neurons of the PVN and in few parvocellular neurons of the dorsal SON but not in magnocellular neurons of the SON. Moreover, parallel IHC indicates that few magnocellular OXT- or VP-releasing neurons of the PVN as well as parvocellular OXT-releasing neurons of the SON do also contain AT(1) receptors.

Interactions between Conserved Residues in Transmembrane Helices 2 and 7 during Angiotensin AT1Receptor Activation

Site-directed mutagenesis studies and independent molecular modeling studies were combined to investigate the network of inter-residue interactions within the transmembrane region of the angiotensin AT(1a) receptor. Site-directed mutagenesis was focused on residues Tyr292, Asn294, Asn295, and Asn298 in transmembrane helix 7, and the conserved Asp74 in helix 2 and other polar residues. Functional interactions between pairs of residues were evaluated by determining the effects of single and double-reciprocal mutations on agonist-induced AT(1a) receptor activation. Replacement of Tyr292 by aspartate in helix 7 abolished radioligand binding to both Y292D and D74Y/Y292D mutant receptors. Reciprocal mutations of Asp74/Asn294, Ser115/Asn294, Ser252/Asn294, and Asn298/Sen115 caused additive impairment of function, suggesting that these pairs of residues make independent contributions to AT(1a) receptor activation. In contrast, mutations of the Asp74/Tyr298 pair revealed that the D74N/N298D reciprocal mutation substantially increased the impaired inositol phosphate responses of the D74N and N298D receptors. Extensive molecular modeling yielded 3D models of the TM region of the AT(1) receptor and the mutants as well as of their complexes with angiotensin II, which were used to rationalize the possible reasons of impairing of function of some mutants. These data indicate that Asp74 and Asn298 are not optimally positioned for direct strong interaction in the resting conformation of the AT(1a) receptor. Balance of interactions between residues in helix 2 (as D74) and helix 7 (as N294, N295 and N298) in the AT(1) receptors, however, has a crucial role both in determining their functional activity and levels of their expression.

Type 1 angiotensin receptor pharmacology: signaling beyond G proteins

Drugs that inhibit the production of angiotensin II (AngII) or its access to the type 1 angiotensin receptor (AT₁R) are prescribed to alleviate high blood pressure and its cardiovascular complications. Accordingly, much research has focused on the molecular pharmacology of AT₁R activation and signaling. An emerging theme is that the AT₁R generates G protein dependent as well as independent signals and that these transduction systems separately contribute to AT₁R biology in health and disease. Regulatory molecules termed arrestins are central to this process as is the capacity of AT₁R to crosstalk with other receptor systems, such as the widely studied transactivation of growth factor receptors. AT₁R function can also be modulated by polymorphisms in the AGTR gene, which may significantly alter receptor expression and function; a capacity of the receptor to dimerize/oligomerize with altered pharmacology; and by the cellular environment in which the receptor resides. Together, these aspects of the AT₁R “flavour” the response to angiotensin; they may also contribute to disease, determine the efficacy of current drugs and offer a unique opportunity to develop new therapeutics that antagonize only selective facets of AT₁R function.

Characterization of two polyclonal peptide antibodies that recognize the carboxy terminus of angiotensin II AT1A and AT1B receptors

1. The differential identification of the angiotensin AT1A and AT1B receptor subtypes is impaired by the existing>96% homology of both receptors. In the present study, we characterized two polyclonal rabbit peptide antibodies, namely alpha-AT1A and alpha-AT1B, that recognize the C-terminal region of mouse AT1A and AT1B receptors, respectively. 2. In immunoblotting, both antibodies detected two major AT1 receptor-specific bands at sizes of 72.5 and 87.6 kDa in mouse tissues and in Neuro-2a cell lysates. In immunohistochemistry, antibodies demonstrated AT1 receptor-specific staining in renal proximal and distal tubules, as well as in kidney glomeruli. In addition, both antibodies stained AT1 receptors in Neuro-2a cells with G-protein receptor typical distribution. Dot-blot and ELISA analysis of the alpha-AT1A antibody showed 2.5- to fourfold higher selectivity for its AT1A receptor target peptide (1A-PEP) compared with the non-specific AT1B receptor peptide (1B-PEP). In contrast, the alpha-AT1B antibody showed high binding affinity towards its target peptide 1B-PEP, but also demonstrated high cross-reactivity for the non-specific peptide 1A-PEP (1.4- to twofold in ELISA and dot-blot analysis). In contrast with the lack of recognition by the alpha-AT1B antibody, the alpha-AT1A antibody selectively recognized the AT1A receptor fused to red fluorescence protein in transiently transfected Chinese hamster ovary cells. 3. In summary, we have generated two new peptide antibodies to the mouse AT1A and AT1B receptors (alpha-AT1A and alpha-AT1B), of which the alpha-AT1A antibody has the capability to distinguish AT1A receptor types in immunological approaches.

Role of the Cys18–Cys274 disulfide bond and of the third extracellular loop in the constitutive activation and internalization of angiotensin II type 1 receptor

An insertion of residues in the third extracellular loop and a disulfide bond linking this loop to the N-terminal domain were identified in a structural model of a G-protein coupled receptor specific to angiotensin II (AT1 receptor), built in homology to the seven-transmembrane-helix bundle of rhodopsin. Both the insertion and the disulfide bond were located close to an extracellular locus, flanked by the second extracellular loop (EC-2), the third extracellular loop (EC-3) and the N-terminal domain of the receptor; they contained residues identified by mutagenesis studies to bind the angiotensin II N-terminal segment (residues D1 and R2). It was postulated that the insertion and the disulfide bond, also found in other receptors such as those for bradykinin, endothelin, purine and other ligands, might play a role in regulating the function of the AT1 receptor. This possibility was investigated by assaying AT1 forms devoid of the insertion and with mutations to Ser on both positions of Cys residues forming the disulfide bond. Binding and activation experiments showed that abolition of this bond led to constitutive activation, decay of agonist binding and receptor activation levels. Furthermore, the receptors thus mutated were translocated to cytosolic environments including those in the nucleus. The receptor form with full deletion of the EC-3 loop residue insertion, displayed a wild type receptor behavior.

Enhanced angiotensin II-induced activation of Na+, K+-ATPase in the proximal tubules of obese Zucker rats

Renal angiotensin II (AII) is suggested to play a role in the enhanced sodium reabsorption that causes a shift in pressure natriuresis in obesity related hypertension; however, the mechanism is not known. Therefore, to assess the influence of AII on tubular sodium transport, we determined the effect of AII on the Na+, K+-ATPase activity (NKA), an active transporter regulated by the AT1 receptor activity, in the isolated proximal tubules of lean and obese Zucker rats. Also, we determined the levels of the tubular AT1 receptor and associated signal transducing G proteins, as the initial signaling components that mediate the effects of AII on Na+, K+-ATPase activity. In the isolated proximal tubules, AII produced greater stimulation of the NKA activity in obese compared with lean rats. Determination of the AT1 receptors by Scatchard analysis of the [125I] Sar-Ang II binding and Western blot analysis in the basolateral (BLM) and brush border membrane (BBM) revealed a modest but significant increase (23%) in the AT1 receptor number mainly in the BLM of obese compared with lean rats. The AII affinity for AT1 receptors, as determined by IC50 values of AII to displace [125I] Sar-Ang II binding in BLM and BBM were similar in lean and obese rats. Western blot analysis revealed significant increases in Gialpha1, Gialpha2, Gialpha3, and Gq/11alpha in BLM and Gialpha1, Gialpha3, and Gq/11alpha in BBM of obese as compared with lean rats. The increase in the levels of the AT1 receptor and G proteins, mainly in the BLM, may be contributing to the enhanced AII-induced activation of NKA in the proximal tubules of obese rats. This phenomenon, in part, may be responsible for the increased sodium reabsorption and the development of hypertension in obese Zucker rats.

Angiotensin II type 2 receptor gene deficiency attenuates susceptibility to tobacco-specific nitrosamine-induced lung tumorigenesis: involvement of transforming growth factor-beta-dependent cell growth attenuation

To clarify an involvement of angiotensin II signaling in lung neoplasia, we have examined the effect of angiotensin II receptor deficiency on 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)-induced lung tumorigenesis. Male angiotensin II type 2 receptor (AT2)-null mice with an SWR/J genetic background and control wild-type mice were treated with NNK (100 mg/kg, i.p.) or saline vehicle. NNK treatment caused the development of lung tumors in all wild-type control mice (100 % tumor prevalence), but only 85% of AT2-null mice developed tumors. The tumor multiplicity in AT2-null mice (1.9 +/- 0.3) was significantly smaller than that in wild-type mice (4.1 +/- 0.9). Primary cultured lung fibroblasts prepared from both AT2-null and wild-type mice markedly increased the colony counts of A549 lung cancer cells in soft agar, but a consistently higher colony count was observed with the wild-type fibroblasts (fold increase in colony number, 5.6 +/- 0.5) than with the AT2-null fibroblasts (3.5 +/- 0.8). The underlying mechanism by which angiotensin II regulates cancer cell growth is due to the regulation of active transforming growth factor-beta (TGF-beta) production. Although the total level of TGF-beta was significantly stimulated when A549 cells were cocultured with either type of fibroblasts, the level of active TGF-beta in the conditioned medium was consistently higher with AT2-null fibroblasts than with wild-type fibroblasts. These results imply that the AT2 receptor negatively regulates the level of active TGF-beta and thus increases NNK-induced lung tumorigenesis. The AT2 receptor function in lung stromal fibroblasts may be a potential modulator of tumor susceptibility in chemical carcinogen-induced lung tumorigenesis.

Role of Elastase-2 as an Angiotensin II-Forming Enzyme in Rat Carotid Artery

We have described the biochemical, enzymatic, and structural properties of a chymostatin-sensitive angiotensin (Ang) I-converting elastase-2 found in the rat mesenteric arterial bed perfusate. We determined the mRNA for elastase-2 and its relative role in generating Ang II in the rat isolated aorta and carotid artery rings. In carotid rings, the Ang I-induced vasoconstrictor effect was only partially inhibited by captopril or chymostatin, whereas that of tetradecapeptide renin substrate (TDP) was greatly inhibited by chymostatin but unaffected by captopril; however, Ang I- and TDP-induced effects were abolished by the combination of both inhibitors. Effects of [Pro11-D-Ala12]-Ang I (PDA), an Ang I-converting enzyme (ACE)-resistant biologically inactive precursor of Ang II were blocked by chymostatin or N-acetyl-Ala-Ala-Pro-Leu-chloromethylketone (elastase-2 inhibitor) in carotid artery. PDA failed to induce an effect in aortic rings, and Ang I-induced contractions were completely inhibited by captopril. The mRNA for rat elastase-2 was detected in aorta, carotid, and mesenteric arteries, although its expression was found to be less important in aorta. These findings indicate the presence of a functional alternative pathway to ACE for Ang II generation in rat carotid artery and represent strong evidence of a physiological role for elastase-2; however, its functional contribution to Ang II formation in aorta appears to be negligible.

Translational control of the rat angiotensin type 1a receptor by alternative splicing

The rat angiotensin type 1a receptor (AT1aR) is comprised of three exons. Two transcripts are possible due to alternative splicing of exon 2 (E1,3 and E1,2,3). Both transcripts code for identical AT1aR proteins since they differ only in the length of their 5' leader sequence (5'LS). We investigated the functional differences of these two transcripts in stably transfected Chinese hamster ovary (CHO) cells and also determined the splice variant composition in rat tissues. E1,3 expressing cells exhibited 1.8-fold higher AT1R densities and five-fold higher levels of Ang II-stimulated inositol phosphate production compared to E1,2,3 expressing cells. No differences in E1,3 and E1,2,3 mRNA levels or mRNA stability were seen. In vitro translation assays revealed 1.8-fold higher AT1aR protein levels from E1,3 compared to E1,2,3 transcripts, suggesting exon 2 reduces functional AT1R expression by inhibiting translation. Deletion of 10 nucleotides in exon 2 increased translation of the mutated E1,2,3 transcript to levels which were indistinguishable from E1,3, suggesting that this loop region of a predicted hairpin contributes to the inhibitory RNA cis element within exon 2. Comparison of AT1aR exonic composition and AT1R densities in rat tissues suggests alternative splicing is regulated in a tissue-specific manner and contributes to tissue-specific differences in AT1R density.

Evaluation of the Membrane-binding Properties of the Proximal Region of the Angiotensin II Receptor (AT1A) Carboxyl Terminus by Surface Plasmon Resonance

The proximal region of the angiotensin II receptor (AT1A) carboxyl-terminus (known as helix VIII) is important for receptor function. In this study, we used surface plasmon resonance (SPR) to examine the interaction of helix VIII-derived peptides with three model lipid membranes. The membrane-binding properties of these synthetic peptides, as well as a series of peptide analogues with modified amino acid sequences, could be explained by both amino acid sequence and kinetic binding data by SPR. The helix VIII peptides showed a higher affinity for lipid membranes that contained negatively charged phospholipid, rather than zwitterionic phospholipid. The findings of an SPR study may be useful for estimating the cooperative binding of intracellular receptor domains with G proteins and the components of the lipid bilayer.

Polymorphisms of renin-angiotensin system genes with high-altitude pulmonary edema in Japanese subjects

STUDY OBJECTIVES

The renin-angiotensin system (RAS), including angiotensin-converting enzyme (ACE) and angiotensin II type 1 receptor (AT(1)R), plays an important role in the pathogenesis of pulmonary hypertension, which is suggested to be critical in the development of high-altitude pulmonary edema (HAPE). Investigating the associations of the polymorphisms in the genes of RAS with HAPE is to elucidate the genetic background underlying this disease.

DESIGN

A cross-sectional, case-control study.

SETTING

Shinshu University Hospital, Matsumoto, Japan.

PARTICIPANTS

Forty-nine HAPE-susceptible (HAPE-s) subjects with a history of HAPE, and 55 healthy climbers with HAPE resistance (HAPE-r).

INTERVENTIONS

Twenty-one of 49 HAPE-s subjects underwent right cardiac catheterization.

MEASUREMENTS AND RESULTS

The insertion/deletion polymorphism in the ACE gene (ACE-I/D) was investigated by polymerase chain reaction (PCR). There was no significant difference of the distribution of the ACE-I/D polymorphism between the HAPE-s and HAPE-r groups. The A(1166)C and G(1517)T single-nucleotide polymorphisms (SNPs) in AT(1)R gene were investigated by the PCR following digested by corresponding restricted endonuclease enzymes. The distribution of the G(1517)T SNP was significantly different between the two groups (p = 0.012). The pulmonary hemodynamics of the 21 HAPE-s subjects were retrospectively examined. The pulmonary artery pressure (PAP), pulmonary vascular resistance (PVR), and PVR index (PVRI) were all significantly increased on hospital admission. Moreover, the PVR and PVRI were significantly higher in the HAPE-s subjects with D positivity than in the HAPE-s subjects with I positivity (PVR, p = 0.015; PVRI, p = 0.028), while the PAP did not show any significant difference between the two subgroups.

CONCLUSIONS

The ACE-I/D polymorphism is not associated with HAPE susceptibility in Japanese subjects. The AT(1)R gene polymorphisms may likely associate with HAPE susceptibility. The D allele of the ACE-I/D polymorphism probably contributes to the hyperresponsive PVR and PVRI to acute hypoxia.

Stable Interaction between β-Arrestin 2 and Angiotensin Type 1A Receptor Is Required for β-Arrestin 2-mediated Activation of Extracellular Signal-regulated Kinases 1 and 2

Binding of beta-arrestins to seven-membrane-spanning receptors (7MSRs) not only leads to receptor desensitization and endocytosis but also elicits additional signaling processes. We recently proposed that stimulation of the angiotensin type 1A (AT(1A)) receptor results in independent beta-arrestin 2- and G protein-mediated extracellular signal-regulated kinases 1 and 2 (ERK1/2) activation. Here we utilize two AT(1A) mutant receptors to study these independent pathways, one truncated at residue 324, thus removing all potential carboxyl-terminal phosphorylation sites, and the other bearing four mutations in the serine/threonine-rich clusters in the carboxyl terminus. As assessed by confocal microscopy, the two mutant receptors interacted with beta-arrestin 2-green fluorescent protein with much lower affinity than did the wild-type receptor. In addition, the mutant receptors more robustly stimulated G protein-mediated inositol phosphate production. Approximately one-half of the wild-type AT(1A) receptor-stimulated ERK1/2 activation was via a beta-arrestin 2-dependent pathway (suppressed by beta-arrestin 2 small interfering RNA), whereas the rest was mediated by a G protein-dependent pathway (suppressed by protein kinase C inhibitor). ERK1/2 activation by the mutant receptors was insensitive to beta-arrestin 2 small interfering RNA but was reduced more than 80% by a protein kinase C inhibitor. The biochemical consequences of ERK activation by the G protein and beta-arrestin 2-dependent pathways were also distinct. G-protein-mediated ERK activation enhanced the transcription of early growth response 1, whereas beta-arrestin 2-dependent ERK activation did not. In addition, stimulation of the truncated AT(1A) mutant receptor caused significantly greater early growth response 1 transcription than did the wild-type receptor. These findings demonstrate how the ability of receptors to interact with beta-arrestins determines both the mechanism of ERK activation as well as the physiological consequences of this activation.

Translational regulation of angiotensin type 1a receptor expression and signaling by upstream AUGs in the 5' leader sequence

Rat angiotensin type 1a receptor (AT(1a)R) is regulated by four upstream AUGs present in the 5' leader sequence (5'-LS). Disruption of all four upstream AUGs (QM) results in 2-3-fold higher levels of angiotensin type 1 receptor (AT(1)R) densities in transiently transfected rat aortic smooth muscle cells (A10 cells) and stably transfected Chinese hamster ovary cells. Cells expressing QM have 5-fold higher levels of angiotensin II-induced inositol phosphate production than wild type (WT). Polysome analysis showed that QM mRNA is present in heavier fractions than the WT transcript, and 5.7-fold more AT(1)R protein is produced by in vitro translation from QM transcripts compared with WT transcripts. The AT(1a)R comprises 3 exons. Exon 3 (E3) encodes the entire open reading frame and 3'-untranslated region. Exons 1 and 2 (E1 and E2) and 52 nucleotides of E3 encode the 5'-LS. The AUGs in both exons contribute to the inhibitory effect on AT(1)R expression but not to the same degree. Disruption of the AUGs in exon 2 (DM2) relieves half of the inhibition, whereas disruption of the AUGs in exon 1 (DM1) is without effect. Disruption of the AUGs in exon 2 results in levels of receptor expression and translation that are indistinguishable from the alternative splice variant E1,3, which we previously showed was more efficiently translated than the E1,2,3 transcript. Individual mutations revealed that only the fourth AUG increased AT(1)R translation. In conclusion, all four AUGs present in the 5'-LS function cumulatively to suppress AT(1a)R expression and signaling by inhibiting translation. These data also show that both AUGs in E2 contribute to the inhibitory cis element present in this alternatively spliced exon.

Localization of components of the renin-angiotensin system in the suprachiasmatic nucleus of normotensive Sprague-Dawley rats: part A. angiotensin I/II, a light and electron microscopic study

The central pacemaker of the mammalian circadian clock, identified in the suprachiasmatic nucleus (SCN), is of special interest for many chronomedical studies on neuropeptides. Based on its role in the modulation of blood pressure and vasopressin release, the distribution and function of the neuropeptide angiotensin II (ANG II) in the SCN became a target for several immunohistological studies. At the light microscopic level, the distribution of ANG II in the SCN is well known, but detailed information about the localization of ANG II in the SCN at the ultrastructural level is missing. To gain further insight in the functional aspects of ANG II in the SCN, we investigated on the subcellular localization of the neuropeptide ANG II and its precursor ANG I in the SCN. The current report presents a light and electron microscopic study on ANG I/II-immunoreactivity in the suprachiasmatic nucleus of normotensive Sprague-Dawley rats.

Localization of components of the renin–angiotensin system in the suprachiasmatic nucleus of normotensive Sprague–Dawley rats

The dominant pacemaker of the mammalian circadian clock, located in the suprachiasmatic nucleus (SCN), is of special interest for many pharmacological, physiological and immunohistological studies on angiotensins and their receptors. Based on its role in the circadian modulation of blood pressure and vasopressin release, the distribution and function of the neuropeptide angiotensin II (ANG II) and its AT1-receptors (AT1) in the SCN became a target for several immunohistological studies. Though the distribution of ANG II and vasopressin in the SCN is well known at light microscopic level, detailed data concerning the AT1-receptor distribution in the SCN is missing. To confirm the mechanisms by which ANG II exerts its actions in the SCN, it is vital to understand how the brain renin-angiotensin system is organized at the cellular level, including the distribution of ANG II and the ANG II (AT1)-receptors as well as the protein-receptor complex. The current paper presents a light- and electron microscopic study on AT1-receptor-immunolabeling in the suprachiasmatic nucleus of normotensive Sprague-Dawley rats.

Structural determinants of agonist-induced signaling and regulation of the angiotensin AT1 receptor

Angiotensin II (Ang II) regulates aldosterone secretion by stimulating inositol phosphate production and Ca(2+) signaling in adrenal glomerulosa cells via the G(q)-coupled AT(1) receptor, which is rapidly internalized upon agonist binding. Ang II also binds to the heptahelical AT(2) receptor, which neither activates inositol phosphate signaling nor undergoes receptor internalization. The differential behaviors of the AT(1) and AT(2) receptors were analyzed in chimeric angiotensin receptors created by swapping the second (IL2), the third (IL3) intracellular loops and/or the cytoplasmic tail (CT) between these receptors. When transiently expressed in COS-7 cells, the chimeric receptors showed only minor alterations in their ligand binding properties. Measurements of the internalization kinetics and inositol phosphate responses of chimeric AT(1A) receptors indicated that the CT is required for normal receptor internalization, and IL2 is a determinant of G protein activation. In addition, the amino-terminal portion of IL3 is required for both receptor functions. However, only substitution of IL2 impaired Ang II-induced ERK activation, suggesting that alternative mechanisms are responsible for ERK activation in signaling-deficient mutant AT(1) receptors. Substitution of IL2, IL3, or CT of the AT(1A) receptor into the AT(2) receptor sequence did not endow the latter with the ability to internalize or to mediate inositol phosphate signaling responses. These data suggest that the lack of receptor internalization and inositol phosphate signal generation by the AT(2) receptor is a consequence of its different activation mechanism, rather than the inability of its cytoplasmic domains to couple to intracellular effectors.

Angiotensin II subtype 1A (AT1A) receptors in the rat sensory vagal complex: subcellular localization and association with endogenous angiotensin

Angiotensin II (Ang II) type 1 (AT1) receptors are prevalent in the sensory vagal complex including the nucleus tractus solitarii (NTS) and area postrema, each of which has been implicated in the central cardiovascular effects produced by Ang II. In rodents, these actions prominently involve the AT1A receptor. Thus, we examined the electron microscopic dual immunolabeling of antisera recognizing the AT1A receptor and Ang II to determine interactive sites in the sensory vagal complex of rat brain. In both the area postrema and adjacent dorsomedial NTS, many somatodendritic profiles were dually labeled for the AT1A receptor and Ang II. In these profiles, AT1A receptor-immunoreactivity was often seen in the cytoplasm beneath labeled portions of the plasma membrane and in endosome-like granules as well as Golgi lamellae and outer nuclear membranes. In addition, AT1A receptor labeling was detected on the plasma membrane and in association with cytoplasmic membranes in many small axons and axon terminals. These terminals were morphologically heterogeneous containing multiple types of vesicles and forming either inhibitory- or excitatory-type synapses. In the area postrema, AT1A receptor labeling also was detected in many non-neuronal cells including glia, capillary endothelial cells and perivascular fibroblasts that were less prevalent in the NTS. We conclude that in the rat sensory vagal complex, AT1A receptors are strategically positioned for involvement in modulation of the postsynaptic excitability and intracrine hormone-like effects of Ang II. In addition, these receptors have distributions consistent with diverse roles in regulation of transmitter release, regional blood flow and/or vascular permeability.

Skeletal muscle RAS and exercise performance

A local renin-angiotensin system (RAS) may be suggested by evidence of gene expression of RAS components within the tissue as well as physiological responsiveness of this gene expression. This review will focus on the evidence supporting the existence of the constituent elements of a physiologically functional paracrine muscle RAS. The effect of local skeletal muscle RAS on human exercise performance will be explored via its relation with pharmacological intervention and genetic studies. The most likely configuration of the muscle RAS is a combination of in situ synthesis and uptake from the circulation of RAS components. A reduction in angiotensin-converting enzyme (ACE) activity reverses the decline in physical performance due to peripheral muscle factors in those with congestive heart failure and may halt or slow decline in muscle strength in elderly women. Genetic studies suggest that increased ACE and angiotensin II (Ang II) mediate greater strength gains perhaps via muscle hypertrophy whereas lower ACE levels and reduced bradykinin (BK) degradation mediate enhanced endurance performance perhaps via changes in substrate availability, muscle fibre type and efficiency.

Endocytosis of the AT1A angiotensin receptor is independent of ubiquitylation of its cytoplasmic serine/threonine-rich region

Agonist-induced internalisation of the rat type 1A (AT(1A)) angiotensin II receptor is associated with phosphorylation of a serine/threonine-rich region in its cytoplasmic tail. In yeast, hyperphosphorylation of the alpha-factor pheromone receptor regulates endocytosis of the receptor by facilitating the monoubiquitylation of its cytoplasmic tail on lysine residues. The role of receptor ubiquitylation in AT(1A) receptor internalisation was evaluated by deletion or replacement of lysine residues in its agonist-sensitive serine/threonine-rich region. Expression of such receptor mutants in CHO cells showed that these modifications had no detectable effect on the angiotensin II-induced endocytosis of the AT(1A) receptor. Furthermore, fusion of ubiquitin in-frame to an internalisation-deficient AT(1A) receptor mutant with a truncated carboxyl-terminal tail did not restore the endocytosis of the resulting chimeric receptor. No impairment of receptor internalisation was observed after substitution of all lysine residues in the serine/threonine-rich region at saturating angiotensin II concentrations, where endocytosis occurs by a beta-arrestin and dynamin independent mechanism. Taken together, these data demonstrate that ubiquitylation of the cytoplasmic serine/threonine-rich region of the AT(1A) receptor on lysine residues is not required for its agonist-induced internalisation, and suggest that endocytosis of mammalian G protein-coupled receptors (GPCRs) occurs by a different mechanism than that of yeast GPCRs.

Regulation of angiotensin II receptors in the prostate of the transgenic (mRen-2)27 rat: effect of angiotensin-converting enzyme inhibition

We examined the regulation/expression of angiotensin II (Ang II) receptors in the transgenic (TG) (mRen-2)27 rat compared to the normal Sprague-Dawley (SD) rat. Ang II receptor binding and mRNA expression were determined by quantitative autoradiography and real-time PCR, respectively. Ang II receptors in the rat prostate rat were of the AT(1) receptor subtype and were significantly reduced in the prostate of the TG rat compared to the normal SD rat. However, AT(1) receptor binding was significantly higher in the prostate of the TG rat treated with the ACE inhibitor lisinopril compared to the untreated TG rat and comparable to the control SD rat. In contrast to the protein, AT(1) receptor mRNA expression was not reduced in the prostate of the TG rat compared to the SD rat. However, AT(1) receptor mRNA was markedly reduced in the prostate of the lisinopril-treated TG rat compared to the untreated TG rat or control SD rat. In conclusion, the findings suggest that AT(1) receptors are present in the rat prostate at a protein level and are subject to down-regulation in the TG rat which may be due to receptor internalisation as a consequence of receptor hyper-stimulation by increased local tissue levels of Ang II. Moreover, AT(1) receptor protein and mRNA expression in the prostate may be inversely modulated.

The renin-angiotensin and the kallikrein-kinin systems

The renin-angiotensin system (RAS) and the kallikrein-kinin system (KKS) each encompasses a large number of molecules, with several participating in both systems. The RAS generates a family of bioactive angiotensin peptides with varying biological activities. These include angiotensin-(1-8) (Ang II), angiotensin-(2-8) (Ang III), angiotensin-(3-8) (Ang IV), and angiotensin-(1-7) [Ang-(1-7)]. Ang II and Ang III act on type 1 (AT(1)) and type 2 (AT(2)) angiotensin receptors, whereas, Ang IV and Ang-(1-7) act on their own receptors. The KKS also generates a family of bioactive peptides with varying biological activities. These include hydroxylated and non-hydroxylated bradykinin and kallidin peptides and their carboxypeptidase metabolites des-Arg(9)-bradykinin and des-Arg(10)-kallidin. Whereas bradykinin and kallidin act mainly via the type 2 bradykinin (B(2)) receptor, des-Arg(9)-bradykinin and des-Arg(10)-kallidin act mainly via the type 1 bradykinin (B(1)) receptor. The AT(1) receptor forms heterodimers with the AT(2) and B(2) receptors and there is cross talk between the AT(1) and epidermal growth factor receptors. The B(2) receptor also interacts with angiotensin converting enzyme and nitric oxide synthase. Both angiotensin and kinin peptides are metabolised by many different peptidases that are important determinants of the activities of the RAS and KKS, and several of which participate in both systems.

The role of a conserved region of the second intracellular loop in AT1 angiotensin receptor activation and signaling

The pleiotropic actions of angiotensin II are mediated by the primarily G(q) protein-coupled type 1 angiotensin (AT(1)) receptor. In this study a mutational analysis of the function of the conserved DRYXXV/IXXPL domain in the second intracellular loop of the rat AT(1A) receptor was performed in COS7 cells. Alanine substitution studies showed that single replacement of the highly conserved Asp(125) and Arg(126), but not Tyr(127), moderately impaired angiotensin II-induced inositol phosphate signaling. However, concomitant substitution of both Asp(125) and Arg(126) caused marked reduction of both inositol phosphate signaling and receptor internalization. Alanine scanning of the adjacent residues showed that substitution of Ile(130), His(132), and Pro(133) reduced agonist-induced inositol phosphate signal generation, whereas mutations of Met(134) also impaired receptor internalization. Expression of the D125A mutant AT(1A) receptor in COS7 cells endowed the receptor with moderate constitutive activity, as indicated by its enhanced basal Elk1 promoter activity and inositol phosphate response to partial agonists. Angiotensin II-induced stimulation of the Elk1 promoter showed parallel impairment with inositol phosphate signal generation in receptors containing mutations in this region of the AT(1A) receptor. These data confirm that Ca(2+) signal generation is required for the nuclear effects of angiotensin II-induced ERK activation. They are also consistent with the role of the conserved DRY sequence of the AT(1A) receptor in receptor activation, and of Asp(125) in constraining the receptor in its inactive conformation. Furthermore, in the cytoplasmic helical extension of the third helix, an apolar surface that includes Ile(130) and Met(134) appears to have a direct role in G protein coupling.

Conversion of brain angiotensin II to angiotensin III is critical for pressor response in rats

The present investigation measured the relative pressor potencies of intracerebroventricularly infused ANG II, ANG III, and the metabolically resistant analogs d-Asp(1)ANG II and d-Arg(1)ANG III in alert freely moving rats. The stability of these analogs was further facilitated by pretreatment with the specific aminopeptidase A inhibitor EC33 or the aminopeptidase N inhibitor PC18. The results indicate that the maximum elevations in mean arterial pressure (MAP) were very similar for each of these compounds across the dose range 1, 10, and 100 pmol/min during a 5-min infusion period. However, d-Asp(1)ANG II revealed significantly extended durations of pressor effects before return to base level MAP. Pretreatment intracerebroventricular infusion with EC33 blocked the pressor activity induced by the subsequent infusion of d-Asp(1)ANG II, whereas EC33 had no effect on the pressor response to subsequent infusion of d-Arg(1)ANG III. In contrast, pretreatment infusion with PC18 extended the duration of the d-Asp(1)ANG II pressor effect by about two to three times and the duration of d-Arg(1)ANG III's effect by approximately 10 to 15 times. Pretreatment with the specific AT(1) receptor antagonist losartan blocked the pressor responses induced by the subsequent infusion of both analogs indicating that they act via the AT(1) receptor subtype. These results suggest that the brain AT(1) receptor may be designed to preferentially respond to ANG III, and ANG III's importance as a centrally active ligand has been underestimated.

Portal hypertension is associated with increased mRNA levels of vasopressor G-protein-coupled receptors in human hepatic arteries

BACKGROUND

The contractile response of human splanchnic vessels to different vasoconstrictors is attenuated in cirrhosis. Functional studies indicate a cellular signalling defect upstream of the G-protein level. The aim of the present study was to analyze expression and mRNA levels of the following most relevant vasopressor receptors in the smooth musculature of human hepatic arteries: alpha1 adrenoceptor (AR) subtypes a, b and d, angiotensin II type 1 receptor (AT1), arginine vasopressin receptor type 1a (V1a), endothelin receptor type A (ETA) and B (ETB).

MATERIALS AND METHODS

Hepatic arteries were collected from 10 donors (noncirrhotic) and 14 recipients (cirrhotic) at liver transplantations. Real-time-PCR was performed to quantify steady-state levels of receptor mRNAs.

RESULTS

alpha 1aAR mRNA levels showed no significant difference between the cirrhotic arteries and the controls while the mRNA levels of the other vasoactive receptors were significantly higher in the cirrhotic hepatic arteries (alpha 1bAR: 4-fold, P = 0.013; AT1: 16-fold, P = 0.024; V1a: 23-fold, P = 0.001; ETA: 4-fold, P = 0.02; ETB: 8-fold, P = 0.008). No mRNA for the alpha 1dAR was detected either in the donor or recipient hepatic arteries.

CONCLUSION

We conclude that vascular hyporeactivity to the most relevant endogenous vasoconstrictors of cirrhotic hepatic arteries is not caused by a receptor down-regulation at mRNA levels. In contrast they were up-regulated.