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

Regulation of vascular angiotensin II type 1 and type 2 receptor and angiotensin-(1-7)/MasR signaling in normal and hypertensive pregnancy

Normal pregnancy (Norm-Preg) is associated with a slight reduction in blood pressure (BP) and decreased BP response to vasoconstrictor stimuli such as angiotensin II (Ang II), although the renin-angiotensin-aldosterone system (RAAS) is upregulated. Preeclampsia (PE) is a complication of pregnancy manifested as hypertension-in-pregnancy (HTN-Preg), and dysregulation of angiotensin biosynthesis and signaling have been implicated. Ang II activates vascular Ang II type-1 receptor (AT1R) and Ang II type-2 receptor (AT2R), while angiotensin-(1-7) promotes Ang-(1-7)/MasR signaling. The role of AT1R in vasoconstriction and the activated cellular mechanisms are well-characterized. The sensitivity of vascular AT1R to Ang II and consequent activation of vasoconstrictor mechanisms decrease during Norm-Preg, but dramatically increase in HTN-Preg. Placental ischemia in late pregnancy could also initiate the release of AT1R agonistic autoantibodies (AT1AA) with significant impact on endothelial dysfunction and activation of contraction pathways in vascular smooth muscle including [Ca2+]c and protein kinase C. On the other hand, the role of AT2R and Ang-(1-7)/MasR in vascular relaxation, particularly during Norm-Preg and PE, is less clear. During Norm-Preg, increases in the expression/activity of vascular AT2R and Ang-(1-7)/MasR promote the production of endothelium-derived relaxing factors such as nitric oxide (NO), prostacyclin and endothelium-derived hyperpolarizing factor leading to generalized vasodilation. Aortic segments of Preg rats show prominent endothelial AT2R staining and increased relaxation and NO production in response to AT2R agonist CGP42112A, and treatment with AT2R antagonist PD123319 enhances phenylephrine-induced contraction. Decreased vascular AT2R and Ang-(1-7)/MasR expression and receptor-mediated mechanisms of vascular relaxation have been suggested in HTN-Preg animal models, but their role in human PE needs further testing. Changes in angiotensin-converting enzyme-2 (ACE2) have been observed in COVID-19 patients, and whether ACE2 influences the course of COVID-19 viral infection/immunity in Norm-Preg and PE is an intriguing area for research.

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

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

Angiotensin receptor-neprilysin inhibitor improves coronary collateral perfusion

Background

We investigated the pleiotropic effects of an angiotensin receptor-neprilysin inhibitor (ARNi) on collateral-dependent myocardial perfusion in a rat model of coronary arteriogenesis, and performed comprehensive analyses to uncover the underlying molecular mechanisms.

Methods

A rat model of coronary arteriogenesis was established by implanting an inflatable occluder on the left anterior descending coronary artery followed by a 7-day repetitive occlusion procedure (ROP). Coronary collateral perfusion was measured by using a myocardial particle infusion technique. The putative ARNi-induced pro-arteriogenic effects were further investigated and compared with an angiotensin-converting enzyme inhibitor (ACEi). Expression of the membrane receptors and key enzymes in the natriuretic peptide system (NPS), renin-angiotensin-aldosterone system (RAAS) and kallikrein-kinin system (KKS) were analyzed by quantitative polymerase chain reaction (qPCR) and immunoblot assay, respectively. Protein levels of pro-arteriogenic cytokines were measured by enzyme-linked immunosorbent assay, and mitochondrial DNA copy number was assessed by qPCR due to their roles in arteriogenesis. Furthermore, murine heart endothelial cells (MHEC5-T) were treated with a neprilysin inhibitor (NEPi) alone, or in combination with bradykinin receptor antagonists. MHEC5-T proliferation was analyzed by colorimetric assay.

Results

The in vivo study showed that ARNis markedly improved coronary collateral perfusion, regulated the gene expression of KKS, and increased the concentrations of relevant pro-arteriogenic cytokines. The in vitro study demonstrated that NEPis significantly promoted MHEC5-T proliferation, which was diminished by bradykinin receptor antagonists.

Conclusion

ARNis improve coronary collateral perfusion and exert pro-arteriogenic effects via the bradykinin receptor signaling pathway.

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.

Epigenetic modifications of the renin-angiotensin system in cardiometabolic diseases

Cardiometabolic diseases (CMDs) are among the most prevalent and the highest mortality diseases. Single disease etiology such as gene mutation, polymorphisms, or environmental exposure has failed to explain the origin of CMD. This can be evident in the discrepancies in disease susceptibility among individuals exposed to the same environmental insult or who acquire the same genetic variation. Epigenetics is the intertwining of genetic and environmental factors that results in diversity in the disease course, severity, and prognosis among individuals. Environmental exposures modify the epigenome and thus provide a link for translating environmental impact on changes in gene expression and precipitation to pathological conditions. Renin-angiotensin system (RAS) is comprising genes responsible for the regulation of cardiovascular, metabolic, and glycemic functions. Epigenetic modifications of RAS genes can lead to overactivity of the system, increased sympathetic activity and autonomic dysfunction ultimately contributing to the development of CMD. In this review, we describe the three common epigenetic modulations targeting RAS components and their impact on the susceptibility to cardiometabolic dysfunction. Additionally, we highlight the therapeutic efforts of targeting these epigenetic imprints to the RAS and its effects.

Neuromuscular electrical stimulation but not photobiomodulation therapy improves cardiovascular parameters of rats with heart failure

The aim of the present study was to analyze the effect of neuromuscular electrical stimulation (NMES) and photobiomodulation (PBMT) on the cardiovascular parameters, hemodynamic function, arterial baroreflex sensitivity (BRS), and autonomic balance (ANS) of rats with heart failure (HF). Male Wistar rats (220-290 g) were organized into five groups: Sham (n = 6), Control-HF (n = 5), NMES-HF (n = 6), PBMT-HF (n = 6), and NMES + PBMT-HF (n = 6). Myocardial infarction (MI) was induced by left coronary artery ligation. Animals were subjected to an eight-week NMES and PBMT protocol. Statistical analysis included the General Linear Model (GLM) followed by a Bonferroni post-hoc test. Rats of the NMES-HF group showed a higher MI area than the Control-HF (P = 0.003), PBMT-HF (P = 0.002), and NMES + PBMT-HF (P = 0.012) groups. NMES-HF and NMES + PBMT-HF showed higher pulmonary congestion (P = 0.004 and P = 0.02) and lower systolic pressure (P = 0.019 and P = 0.002) than the Sham group. NMES + PBMT-HF showed lower mean arterial pressure (P = 0.02) than the Sham group. Control-HF showed a higher heart rate than the NMES-HF and NMES + PBMT-HF (P = 0.017 and P = 0.013) groups. There was no difference in the BRS and ANS variables between groups. In conclusion, eight-week NMES isolated or associated with PBMT protocol reduced basal heart rate, systolic and mean arterial pressure, without influence on baroreflex sensibility and autonomic control, and no effect of PBMT was seen in rats with HF.

Update on Angiotensin II Subtype 2 Receptor: Focus on Peptide and Nonpeptide Agonists

Angiotensin II (Ang II) is the most dominant effector component of the renin-angiotensin system (RAS) that generally acts through binding to two main classes of G protein-coupled receptors, namely Ang II subtype 1 receptor (AT1R) and angiotensin II subtype 2 receptor (AT2R). Despite some controversial reports, the activation of AT2R generally antagonizes the effects of Ang II binding on AT1R. Studying AT2R signaling, function, and its specific ligands in cell culture or animal studies has confirmed its beneficial effects throughout the body. These characteristics classify AT2R as part of the protective arm of the RAS that, along with functions of Ang (1-7) through Mas receptor signaling, modulates the harmful effects of Ang II on AT1R in the activated classic arm of the RAS. Although Ang II is the primary ligand for AT2R, we have summarized other natural or synthetic peptide and nonpeptide agonists with critical evaluation of their structure, mechanism of action, and biologic activity. SIGNIFICANCE STATEMENT: AT2R is one of the main components of the RAS and has a significant prospective for mediating the beneficial action of the RAS through its protective arm on the body's homeostasis. Targeting AT2R offers substantial clinical application possibilities for modulating various pathological conditions. This review provided concise information regarding the AT2R peptide and nonpeptide agonists and their potential clinical applications for various diseases.

The role of the brain renin-angiotensin system (RAS) in mild traumatic brain injury (TBI)

There is considerable interest in traumatic brain injury (TBI) induced by repeated concussions suffered by athletes in sports, military personnel from combat-and non-combat related activities, and civilian populations who suffer head injuries from accidents and domestic violence. Although the renin-angiotensin system (RAS) is primarily a systemic cardiovascular regulatory system that, when dysregulated, causes hypertension and cardiovascular pathology, the brain contains a local RAS that plays a critical role in the pathophysiology of several neurodegenerative diseases. This local RAS includes receptors for angiotensin (Ang) II within the brain parenchyma, as well as on circumventricular organs outside the blood-brain-barrier. The brain RAS acts primarily via the type 1 Ang II receptor (AT₁R), exacerbating insults and pathology. With TBI, the brain RAS may contribute to permanent brain damage, especially when a second TBI occurs before the brain recovers from an initial injury. Agents are needed that minimize the extent of injury from an acute TBI, reducing TBI-mediated permanent brain damage. This review discusses how activation of the brain RAS following TBI contributes to this damage, and how drugs that counteract activation of the AT₁R including AT₁R blockers (ARBs), renin inhibitors, angiotensin-converting enzyme (ACE) inhibitors, and agonists at type 2 Ang II receptors (AT₂) and at Ang (1-7) receptors (Mas) can potentially ameliorate TBI-induced brain damage.

Exploring cardiac plasticity in teleost: the role of humoral modulation

The fish heart represents an established natural model for evaluating basic mechanisms of the coordinated physiological reactions which maintain cardiac steady-state. This is due to its relatively simple design, but also to its multilevel morpho-functional flexibility which allows adequate responses to a variety of intrinsic (body size and shape, swimming performance, etc.), and extrinsic (temperature, salinity, oxygen level, water chemistry, etc.) factors related to the animal life style. Nowadays, although many gaps are still present, a huge literature is available about the mechanisms that fine-tune fish cardiac performance, particularly in relation to the influence exerted by substances possessing cardio-modulatory properties. Based on these premises, this review will provide an overview of the existing current knowledge regarding the humoral control of cardiac performance in fish. The role of both classic (i.e. catecholamines, angiotensin II and natriuretic peptides), and emerging cardioactive substances (i.e. the chromogranin-A-derived peptides vasostatins, catestatin and serpinin) will be illustrated and discussed. Moreover, an example of cardiomodulation elicited by peptides (e.g., nesfatin-1) associated to the regulation of feeding and metabolism will be provided. The picture will hopefully emphasize the complex circuits that sustain fish cardiac performance, also highliting the power of the teleost heart as an experimental model to deciphering mechanisms that could be difficult to explore in more elaborated cardiac morpho-functional designs.

Angiotensin type 2 receptor activation promotes browning of white adipose tissue and brown adipogenesis

Brown adipose tissue dissipates energy in the form of heat. Recent studies have shown that adult humans possess both classical brown and beige adipocytes (brown-like adipocytes in white adipose tissue, WAT), and stimulating brown and beige adipocyte formation can be a new avenue to treat obesity. Angiotensin II (AngII) is a peptide hormone that plays important roles in energy metabolism via its angiotensin type 1 or type 2 receptors (AT1R and AT2R). Adipose tissue is a major source of AngII and expresses both types of its receptors, implying the autocrine and paracrine role of AngII in regulating adipose functions and self-remodeling. Here, based on the in vitro studies on primary cultures of mouse white adipocytes, we report that, AT2R activation, either by AngII or AT2R agonist (C21), induces white adipocyte browning, by increasing PPARγ expression, at least in part, via ERK1/2, PI3kinase/Akt and AMPK signaling pathways. It is also found that AngII–AT2R enhances brown adipogenesis. In the in vivo studies on mice, administration of AT1R antagonist (ZD7155) or AT2R agonist (C21) leads to the increase of WAT browning, body temperature and serum adiponectin, as well as the decrease of WAT mass and the serum levels of TNFα, triglycerides and free fatty acids. In addition, AT2R-induced browning effect is also observed in human white adipocytes, as evidenced by the increased UCP1 expression and oxygen consumption. Finally, we provide evidence that AT2R plays important roles in hormone T3-induced white adipose browning. This study, for the first time, reveals the browning and brown adipogenic effects of AT2R and suggests a potential therapeutic target to combat obesity and related metabolic disorders.

Resistance training prevents the cardiovascular changes caused by high-fat diet

AIMS

Aerobic exercise is indicated for prevention and treatment of obesity-induced cardiovascular disorders. Although the resistance training (RT) may also produce effects similar to aerobic exercise, this is not completely clear yet. In the present study, we tested if RT in moderate intensity might prevent alterations in blood pressure (BP), sympathetic modulation of systolic blood pressure (SBP), baroreflex function and the changes in renin-angiotensin system (RAS) and cytokines mRNA expression within the nucleus of the tract solitary (NTS) in rats fed with high-fat diet (HFD).

MAIN METHODS

Male Holtzman rats (300-320 g) were divided into 4 groups: sedentary with standard chow diet (SED-SD); sedentary with high-fat diet (SED-HFD); RT with standard chow diet (RT-SD); and RT with high-fat diet (RT-HFD). The trained groups performed a total of 10 weeks of moderate intensity RT in a vertical ladder. In the first 3 weeks all experimental groups were fed with SD. In the next 7 weeks, the SED-HFD and RT-HFD groups were fed with HFD.

KEY FINDINGS

In SED-HFD, BP and sympathetic modulation of SBP increased, whereas baroreflex bradycardic responses were attenuated. RT prevented the cardiovascular and inflammatory responses (increases in tumoral necrosis factor-α and interleukin-1β) produced by HFD in SED rats. The anti-inflammatory interleukin-10, angiotensin type 2 receptor, Mas receptor and angiotensin converting enzyme 2 mRNA expressions in the NTS increased in the RT-HFD compared to SED-HFD.

SIGNIFICANCE

The data demonstrated that moderate intensity RT prevented obesity-induced cardiovascular disorders simultaneously with reduced inflammatory responses and modifications of RAS in the NTS.

CCL5 upregulates IL-10 expression and partially mediates the antihypertensive effects of IL-10 in the vascular smooth muscle cells of spontaneously hypertensive rats

Interleukin (IL)-10 inhibits angiotensin (Ang) II-induced vascular dysfunction and reduces blood pressure in hypertensive pregnant rats. The chemokine CCL5 has also been shown to downregulate Ang II-induced hypertensive mediators in spontaneously hypertensive rats (SHRs). This study investigated the effects of CCL5 on IL-10 expression, as well as its mechanisms of action in the vascular smooth muscle cells (VSMCs) of SHRs. CCL5 increased IL-10 expression in the VSMCs of SHRs; the s.c. injection of CCL5 (1.5 μg kg(-1), twice a day) for 3 weeks into SHRs with established hypertension upregulated IL-10 expression in both the thoracic aorta and the VSMCs and decreased systolic blood pressure. CCL5-induced the elevation of IL-10 expression, an effect mediated primarily via the activation of an Ang II subtype II receptor (AT2 R). Dimethylarginine dimethylaminohydrolase (DDAH)-1 activity also contributed to the elevation of IL-10 expression via CCL5 in the VSMCs of SHRs. Moreover, CCL5 partially mediated the inhibitory effects of IL-10 on Ang II-induced 12-lipoxygenase (LO) and endothelin (ET)-1 expression in the VSMCs of SHRs. Taken together, this study provides novel evidence that CCL5 plays a role in the upregulation of IL-10 activity in the VSMCs of SHRs.

Effects of angiotensin type 2 receptor on secretion of the locus coeruleus in stress-induced hypertension rats

Locus coeruleus (LC) has noradrenergic nerve terminals projecting to hypothalamus that modulating cardiovascular activity. To study the dynamic characteristics of norepinephrine (NE) release in hypothalamus followed by electrical stimulation in the locus coeruleus in the stress-induced hypertension (SIH) rats, we established the hypertension model rats by stimulations combining noise and foot-shock stress. After the end of modeling, NE release in the hypothalamus by electrical stimulation in LC was studied and NE signal was recorded by carbon fiber electrode. The peak value, the time to peak and half-life period of NE signal in both group rats were analyzed. Furthermore, to clarify the role of angiotensin II type 2 receptors (AT2) in norepinephrine (NE) release and the blood pressure of rat model of stress-induced hypertension, we intraperitoneally administered the AT2 receptor antagonist PD123319 (AT2 receptor antagonist, 0.3mg/kg, i.p.) and intracerebroventricularly injection of CGP42112 (AT2 receptor agonist, 6μg/5μl, i.c.v.) to adult male rats. We found the peak value of NE signal in the hypothalamus followed by electrical stimulation in the LC in SIH rats were higher than that in controls (P<0.01). Intraperitoneal injection of PD123319 (AT2 receptor antagonist) potentiated electrical stimulation in the LC induced NE release in the hypothalamus in SIH rats and elevated blood pressure (P<0.05), whereas intracerebroventricular injection of CGP42112 (AT2 receptor agonist) inhibited the NE release and reduced the heart rate (P<0.05). These results suggest that combining noise and foot-shock stresses increased the blood pressure and the secretion of NE in the hypothalamus followed by electrical stimulation in the LC in rats. AT2 receptors can inhibit the secretion of NE from the LC to the hypothalamus. The attenuation of presynaptic action of AT2 receptor may play a role in the pathophysiological mechanism of SIH in rats.

Increased expression of angiotensin II type 2 receptors in the solitary-vagal complex blunts renovascular hypertension

Angiotensin II increases and decreases arterial pressure by acting at angiotensin type 1 and type 2 receptors, respectively. Renovascular hypertensive rats exhibit a high level of activity of the peripheral and central renin-angiotensin system. Therefore, in the present study, we evaluated the effect of increasing the expression of angiotensin type 2 receptors in the solitary-vagal complex (nucleus of the solitary tract/dorsal motor nucleus of the vagus), a key brain stem region for cardiovascular regulation, on the development of renovascular hypertension. Holtzman normotensive rats were implanted with a silver clip around the left renal artery to induce 2-kidney 1-clip renovascular hypertension. Three weeks later, rats were microinjected in the solitary-vagal complex with either an adenoassociated virus to increase the expression of angiotensin type 2 receptors or with a control vector. We observed that increasing angiotensin type 2 receptor expression in the solitary-vagal complex attenuated the development of renovascular hypertension and also reversed the impairment of the baroreflex and the increase in the low-frequency component of systolic blood pressure observed in renovascular hypertensive rats. Furthermore, an observed decrease in mRNA levels of angiotensin-converting enzyme 2 in the solitary-vagal complex of renovascular hypertensive rats was restored to control levels after viral-mediated increases in angiotensin type 2 receptors at this site. Collectively, these data demonstrate specific and beneficial effects of angiotensin type 2 receptors via the brain of hypertensive rats and suggest that central angiotensin type 2 receptors may be a potential target for therapeutics in renovascular hypertension.

Humoral control of cardiac remodeling in fish: role of Angiotensin II

Angiotensin II (AngII), the principal effector of the Renin-Angiotensin-System (RAS), is a multipotent hormone whose biological actions include short-term modulation as well as long-term adjustments. In the eel heart, AngII elicits short-term inotropic and chronotropic effects. However, information regarding the influence of AngII on cardiac remodeling, expressed as morphological and hemodynamic changes, is lacking. To clarify the putative actions of AngII on eel cardiac remodeling, we used freshwater eels (Anguilla anguilla) intraperitoneally injected for 4 weeks with saline or AngII (0.4 or 1.2 nmol g BW(-1)) or AngII (1.2 nmol g BW(-1)) plus the AT₂ receptor antagonist CGP42112. Using an in vitro working heart preparation, the cardiac response (stroke volume changes) to preload and afterload increases has been evaluated. Hearts of all groups showed similar Frank-Starling responses. However, in response to afterload increases, stroke volume rapidly decreased in control hearts, while it was better maintained in AngII-treated counterparts. These effects were abolished by an antagonist of the AT₂ receptor, whose cardiac expression was revealed by western blotting analysis. We also found by immunolocalization and immunoblotting that AngII influences both expression and localization of molecules which regulate cell growth [such as c-kit, heat shock protein 90 (Hsp-90), endothelial Nitric Oxide Synthase "(eNOS)-like" isoform] and apoptosis [i.e. apoptosis repressor with CARD domain (ARC)], thus playing a role in cardiac long-term adjustments. These results point to a role of AngII in eel heart remodeling, providing new insights regarding the modulation of cardiac plasticity in fish.

Angiotensin receptor blockade has protective effects on the poststenotic porcine kidney

Angiotensin converting enzyme inhibitors (ACEI)/ angiotensin-II receptor blockers (ARBs) may induce an acute decrease of glomerular filtration rate (GFR) in the stenotic kidney in renal artery stenosis, but most patients tolerate these drugs well. We hypothesized that ACEI/ARBs stabilize stenotic kidney function during prolonged treatment by conferring protective effects. We tested this in control domestic pigs and pigs with renal artery stenosis untreated or treated with valsartan, or triple therapy (7 pigs in each group) for 4 weeks starting 6 weeks after stenosis induction. Renal function, oxygenation, tubular function, and microcirculation were assessed by multi-detector computed tomography (CT), blood-oxygen-level-dependent magnetic-resonance imaging, and micro-CT. Valsartan and triple therapy decreased blood pressure similarly, however, valsartan did not change the GFR of the stenotic kidney compared to renal artery stenosis and was similar to triple therapy. Both valsartan and triple therapy stimulated microvascular density, and improved tubular function. Valsartan also caused a greater increase of angiogenic factors and a decrease in oxidative stress, which were related to higher cortical perfusion and tubular response than triple therapy. Thus, valsartan did not decrease stenotic kidney GFR, but improved cortical perfusion and microcirculation. These beneficial effects may partly offset the hemodynamic GFR reduction in renal artery stenosis and preserve kidney function.

Angiotensin AT2 receptor activates the cyclic-AMP signaling pathway in eel

A unique angiotensin type 2 receptor (AT2) that induces a cAMP signaling pathway was cloned and characterized for the first time in fish, Anguilla japonica. Phylogeny and synteny results showed that the AT2s among fishes and tetrapods share the same origin despite a sub-cluster formation among eel, salmon, and zebrafish. The eel AT2 was expressed abundantly in the spleen and localized at straight arterioles and ellipsoid regions prior to the sinusoid, suggesting a role in the regulation of microcirculation and/or immune response. Various angiotensin (Ang) peptides, including Ang II, Ang III, and Ang IV, were detected in the spleen by a radioimmunoassay coupled with HPLC separation, and these endogenous peptides stimulated a cAMP signaling, which has no crosstalk with cGMP pathway. The common and contrasting features of AT2 between fishes and mammals imply some ancestral characters of AT2, which are important information for receptor binding and evolutionary studies.

Elevation of the antifibrotic peptide N-acetyl-seryl-aspartyl-lysyl-proline: a blood pressure-independent beneficial effect of angiotensin I-converting enzyme inhibitors

Blockade of the renin-angiotensin system (RAS) is well recognized as an essential therapy in hypertensive, heart, and kidney diseases. There are several classes of drugs that block the RAS; these drugs are known to exhibit antifibrotic action. An analysis of the molecular mechanisms of action for these drugs can reveal potential differences in their antifibrotic roles. In this review, we discuss the antifibrotic action of RAS blockade with an emphasis on the potential importance of angiotensin I-converting enzyme (ACE) inhibition associated with the antifibrotic peptide N-acetyl-seryl-aspartyl-lysyl-proline (AcSDKP).

AT₂receptors recruit c-Src, SHP-1 and FAK upon activation by Ang II in PND15 rat hindbrain

The functional role of AT(2) receptors is unclear and it activates unconventional signaling pathways, which in general do not involve a classical activation of a G-protein. In the present study, we aimed to investigate the transduction mechanism of AT(2) Ang II receptors in PND15 rat hindbrain membrane preparations, which represents a physiological developmental condition. To determine whether Ang II AT(2) receptors induced association to SHP-1 in rat hindbrain, co-immunoprecipitation assays were performed. Stimulation of Ang II AT(2) receptors induced both a transient tyr-phosphorylation and activation of SHP-1. The possible participation of c-Src in Ang II-mediated SHP-1 activation, we demonstrated by recruitment of c-Src in immunocomplexes obtained with anti AT(2) or anti-SHP-1 antibodies. The association of SHP-1 to c-Src was inhibited by PD123319 and the c-Src inhibitor PP2. Similarly, SHP-1 activity determined in AT(2)-immunocomplexes was inhibited by PD123319 and the c-Src inhibitor PP2. Following stimulation with Ang II, AT(2) receptors recruit c-Src, which was responsible for SHP-1 tyr-phosphorylation and activation. Since AT(2) receptors are involved in neuron migration, we tested the presence of FAK in immunocomplexes. Surprisingly, AT(2)-immunocomplexes contained mainly the 85kDa fragment of FAK. Besides, p125FAK associated to SHP-1. In summary, we demonstrated the presence of an active signal transduction mechanism in PND15 rat hindbrain, a developmental stage critical for cerebellar development. In this model, we showed a complex containing AT(2)/SHP-1/c-Src/p85FAK, suggesting a potential role of Ang II AT(2) receptors in cerebellar development and neuronal differentiation.

Di-acidic motifs in the membrane-distal C termini modulate the transport of angiotensin II receptors from the endoplasmic reticulum to the cell surface

The molecular mechanisms underlying the endoplasmic reticulum (ER) export and cell surface transport of nascent G protein-coupled receptors (GPCRs) have just begun to be revealed and previous studies have shown that hydrophobic motifs in the putative amphipathic 8(th) α-helical region within the membrane-proximal C termini play an important role. In this study, we demonstrate that di-acidic motifs in the membrane-distal, nonstructural C-terminal portions are required for the exit from the ER and transport to the plasma membrane of angiotensin II receptors, but not adrenergic receptors. More interestingly, distinct di-acidic motifs dictate optimal export trafficking of different angiotensin II receptors and export ability of each acidic residue in the di-acidic motifs cannot be fully substituted by other acidic residue. Moreover, the function of the di-acidic motifs is likely mediated through facilitating the recruitment of the receptors onto the ER-derived COPII transport vesicles. Therefore, the di-acidic motifs located in the membrane-distal C termini may represent the first linear motifs which recruit selective GPCRs onto the COPII vesicles to control their export from the ER.

Purkinje cells express Angiotensin II AT(2) receptors at different developmental stages

Angiotensin II (Ang II) binds and activates two major receptors subtypes, namely AT(1) and AT(2). In the fetus, AT(2) receptors predominate in all tissues and decline shortly after birth, being restricted to a few organs including brain. Interpretation of the function of Ang II in the cerebellum requires a thorough understanding of the localization of Ang II receptors. The aim of the present paper is to evaluate the localization of Ang II AT(2) receptors in the Purkinje cell (PC) layer during development. By binding autoradiography, a clear complementary pattern of AT(1) and AT(2) binding labeled by [(125)I] Ang II was observed in young rats within the cerebellar cortex. This pattern was present at the stages P8 and P15, but not at P30 and P60, where AT(2) binding appears low and superimposed with AT(1) binding. We demonstrate that AT(2) antibodies recognized postmitotic Purkinje cells, labeling the somata of these cells at all the stages studied, from P8 to P60, suggesting that PCs express these receptors from early stages of development until adulthood. In P8 and P15 animals, we observed a clear correspondence between immunolabeling and the well-defined layer observed by binding autoradiography. Confocal analysis allowed us to discard the co-localization of AT(2) receptors with glial fibrillary acidic protein (GFAP), a glial marker. Double immunolabeling allowed us to demonstrate the co-localization of Ang II AT(2) receptors with zebrin II, a specific PC marker. Since PCs are the sole output signal from the cerebellar cortex and considering the role of cerebellum in movement control, the specific receptor localization suggests a potential role for Ang II AT(2) receptors in the cerebellar function.

AT2 receptor signaling and sympathetic regulation

There is a growing consensus that the balance between Angiotensin Type 1 (AT1R) and Angiotensin Type 2 (AT2R) signaling in many tissues may determine the magnitude and, in some cases the direction, of the biological response. Sympatho-excitation in cardiovascular diseases is mediated by a variety of factors and is, in part, dependent on Angiotensin II signaling in the central nervous system. Recent data have provided evidence that the AT2R can modulate sympatho-excitation in animals with hypertension and heart failure. The evidence for this concept is reviewed and a model is put forward to support the rationale that therapeutic targeting of the central AT2R may be beneficial in the setting of chronic heart failure.

Developmental changes in AT1 and AT2 receptor-protein expression in rats

It has long been known that angiotensin type-1 receptors (AT1R) play a critical role in sympathetic regulation, cardiovascular activity, and hormone secretion under physiological and pathological states. On the other hand, the functional significance of angiotensin type-2 receptors (AT2R) is poorly understood. In a recent study we demonstrated that, in rats with chronic heart failure, AT1R protein expression was increased but AT2R expression was decreased in the rostral ventrolateral medulla (RVLM). This imbalance of angiotensin receptors contributed to sympatho-excitation in the heart failure state. In the current experiment, we measured AT1R and AT2R protein expressions in the brainstem, kidney and liver from male foetuses (3 days before birth), male neonates (3 days after birth), male and female adults (8 weeks) and male aged (28 months) rats by Western blot analysis. In the brainstem, we found that the foetuses and neonates exhibited a significantly lower AT2R protein expression compared with adult rats (foetus 0.08 ± 0.01, neonate 0.12 ± 0.01, male adult 0.25 ± 0.01, female adult 0.22 ± 0.02; n = 4 per group, p < 0.001 foetus and neonate compared with male or female adults). In contrast, the foetuses and neonates expressed significantly higher AT1R protein than that of the adults (foetus 0.64 ± 0.09, neonate 0.56 ± 0.01, male adult 0.13 ± 0.02, female adult 0.08 ± 0.02; n = 4 each group, p < 0.001 foetus and neonate compared with male and female adults). In the liver, the AT2R protein was also higher in foetus and neonate, than in adult rats. Interestingly, the foetal liver expressed higher AT1R protein compared with that of the neonate. In the kidney, AT2R expression was significantly increased with age (foetus 0.08 ± 0.01, neonate 0.19 ± 0.02, male adult 0.49 ± 0.04, female adult 0.90 ± 0.10; n = 4 per group, p < 0.01-0.001). AT1R expression, on the other hand, was higher in the foetuses than that in both neonate and male adults. This study provides data contrary to existing dogma that AT2R expression is higher in foetal life and low in adults, suggesting an involvement of a potentially important functional role for AT2R in adult animals and AT1R in foetal development and/or physiology.

Role of prolylcarboxypeptidase in angiotensin II type 2 receptor-mediated bradykinin release in mouse coronary artery endothelial cells

Activation of angiotensin II type 2 receptors (AT(2)R) causes the release of kinins, which have beneficial effects on the cardiovascular system. However, it is not clear how AT(2)R interact with the kallikrein-kinin system to generate kinins. Prolylcarboxypeptidase is an endothelial membrane-bound plasma prekallikrein activator that converts plasma prekallikrein to kallikrein, leading to generation of bradykinin from high-molecular-weight kininogen. We hypothesized that AT(2)R-induced bradykinin release is at least in part mediated by activation of prolylcarboxypeptidase. Cultures of mouse coronary artery endothelial cells were transfected with an adenoviral vector containing the AT(2)R gene (Ad-AT(2)R) or green fluorescent protein only (Ad-GFP) as control. We found that overexpression of AT(2)R increased prolylcarboxypeptidase mRNA by 1.7-fold and protein 2.5-fold compared with Ad-GFP controls. AT(2)R overexpression had no effect on angiotensin II type 1 receptor mRNA. Bradykinin release was increased 2.2-fold in AT(2)R-transfected cells. Activation of AT(2)R by CGP42112A, a specific AT(2)R agonist, increased bradykinin further in AT(2)R-transfected cells. These effects were diminished or abolished by AT(2)R blockade or a plasma kallikrein inhibitor. Furthermore, blocking prolylcarboxypeptidase with a small interfering RNA partially but significantly reduced bradykinin release by transfected AT(2)R cells either at the basal condition or when stimulated by the AT(2)R agonist CGP42112A. These findings suggest that overexpression of AT(2)R in mouse coronary artery endothelial cells increases expression of prolylcarboxypeptidase, which may contribute to kinin release.

Angiotensin II signaling through the AT1a and AT1b receptors does not have a role in the development of cerulein-induced chronic pancreatitis in the mouse

The intraorgan renin-angiotensin system (RAS) plays an important role in the pathophysiology of a variety of diseases and has been implicated in fibrogenesis. The role of RAS in the development of chronic pancreatitis is not well established. The blockade of RAS in rat models with angiotensin-converting enzyme inhibitors (ACEi) or angiotensin receptor 1 (AT1) blockers (ARBs) mostly have reduced pancreatic inflammation and fibrosis with a few exceptions. At the same time, the use of ACEi and ARBs in humans is associated with a modest risk of acute pancreatitis. The aim of this study was to elucidate the effect of the AT1 signaling pathway in the development of pancreatitis using AT1a- and AT1b-deficient mice as well as the ARB losartan. Chronic pancreatitis was induced by repetitive cerulein administration in C57BL/6J wild-type (WT) and AT1a- and AT1b-deficient mice (AT1a-/- and AT1b-/-), and pancreatic injury was assessed at day 10. Pancreatic weight of cerulein treated groups was significantly reduced. There was severe parenchymal atrophy and fibrosis assessed by histological examination. Fibrosis was accompanied by activation of pancreatic stellate cells (PSC) evaluated by Western blot analysis for alpha-smooth muscle actin. No differences were seen between cerulein-treated WT, AT1a-/- , AT1b-/- mice, or losartan treated-WT mice with regards to morphological or molecular alterations induced by cerulein. Our results demonstrate that AT1a and AT1b receptor pathways do not seem to be essential for the development of pancreatitis in the mouse model of pancreatitis induced by repetitive cerulein injury.

Targeting the angiotensin II type 2 receptor (AT2R) in colorectal liver metastases

BACKGROUND

Blockade of the angiotensin (ANG) II type 1 receptor (AT1R) inhibits tumour growth in several cancers, including colorectal cancer (CRC) liver metastases. While AT1R blockade has been extensively studied, the potential of targeting the antagonistically acting AT2R in cancer has not been investigated. This study examined the effect of AT2R activation with the agonist CGP42112A in a mouse model of CRC liver metastases.

RESULTS

In vitro, mouse CRC cell (MoCR) proliferation was inhibited by treatment with CGP42112A in a dose dependent manner while apoptosis was increased. Immunofluorescent staining for key signalling and secondary messengers, PLA2 and iNOS, were also increased by CGP42112A treatment in vitro. Immunohistochemical staining for proliferation (PCNA) and the apoptosis (active caspase 3) markers confirmed a CGP42112A-associated inhibition of proliferation and induction of apoptosis of mouse CRC cells (MoCR) in vivo. However, angiogenesis and vascular endothelial growth factor (VEGF) appeared to be increased by CGP42112A treatment in vivo. This increase in VEGF secretion by MoCRs was confirmed in vitro. Despite this apparent pro-angiogenic effect, a syngenic orthotopic mouse model of CRC liver metastases showed a reduction in liver to body weight ratio, an indication of tumour burden, following CGP42112A treatment compared to untreated controls.

CONCLUSIONS

These results suggest that AT2R activation might provide a novel target to inhibit tumour growth. Its potential to stimulate angiogenesis could be compensated by combination with anti-angiogenic agents.

Fyn is involved in angiotensin II type 2 receptor-induced neurite outgrowth, but not in p42/p44mapk in NG108-15 cells

In NG108-15 cells, activation of p42/p44(mapk) is essential for induction of neurite outgrowth by angiotensin II (Ang II) type 2 receptor (AT(2)). The aim was to verify whether Fyn, a member of the Src family kinases (SFK), is involved in neurite outgrowth induced by AT(2) activation. Preincubation of cells with PP1, a general inhibitor of the SKF, decreased activation of Rap1 and p42/p44(mapk) and abolished TrkA activation by Ang II or by the AT(2) agonist, CGP42112A. NG108-15 cells were transfected with a Fyn-WT and a Fyn-DN expressing vector. Fyn-WT was sufficient to induce neurite outgrowth, although transfection with Fyn-DN abolished neurite elongation. However, the Fyn-DN form failed to affect activation of TrkA, Rap1 or p42/p44(mapk) by Ang II. Thus, although SKF activity is required to achieve AT(2)-induced activation of TrkA, Rap1 and p42/p44(mapk), Fyn is essential for AT(2) receptor-induced neurite outgrowth, but not in AT(2) signaling leading to p42/p44(mapk) activation.

Downregulation of Angiotensin II-Induced 12-Lipoxygenase Expression and Cell Proliferation in Vascular Smooth Muscle Cells from Spontaneously Hypertensive Rats by CCL5

Angiotensin II (Ang II) plays an important role in vascular hypertension. The role of the chemokine CCL5 on Ang II-induced activities in vascular smooth muscle cells (VSMCs) has not been studied. In this study, we elucidated the effect of CCL5 on Ang II-induced 12-lipoxygenase (LO) expression and cell proliferation in spontaneously hypertensive rats (SHR) VSMCs. CCL5 decreased Ang II-induced 12-LO mRNA expression and protein production, and it increased Ang II type 2 (AT(2)) receptor expression in SHR VSMCs. The inhibitory effect of CCL5 on Ang II-induced 12-LO mRNA expression was mediated through the AT(2) receptor. Although treatment of CCL5 alone induced SHR VSMCs proliferation, CCL5 inhibited Ang II-induced VSMCs proliferation and PD123,319, an AT(2) receptor antagonist, blocked the inhibitory effect of CCL5 on Ang II-induced VSMCs proliferation. Phosphorylation of p38 was detected in VSMCs treated with Ang II or CCL5 alone. But, decrease of p38 phosphorylation was detected in VSMCs treated with Ang II and CCL5 simultaneously (Ang II/CCL5) and PD123,319 increased p38 phosphorylation in VSMCs treated with Ang II/CCL5. Therefore, these results suggest that the inhibitory effect of CCL5 on Ang II-induced VSMCs proliferation is mediated by the AT(2) receptor via p38 inactivation, and CCL5 may play a beneficial role in Ang II-induced vascular hypertension.

The angiotensin II type 2 receptor in cardiovascular disease

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

Resistance artery adaptation to pregnancy counteracts the vasoconstricting influence of plasma from normal pregnant women

Using a rat model, we investigated the effects of circulating factors in pregnancy on cerebrovascular and systemic vascular function by comparing myogenic reactivity, tone, and endothelial vasodilator production of the posterior cerebral artery (PCA) and mesenteric artery (MA) of nonpregnant (NP) animals perfused with nonpregnant and pregnant human plasma. Arteries from late pregnant (LP) animals were then perfused similarly to evaluate a potential adaptive effect of pregnancy on vessel function. A 3-hour exposure to pregnant plasma caused increased myogenic reactivity and tone in vessels from NP animals and produced a decreased endothelium-derived hyperpolarizing factor response in NP PCAs, findings that were not seen with MAs. The increased reactivity and tone noted in NP vessels was abolished when pregnant plasma was perfused through LP arteries, suggesting these vessels adapt during pregnancy to the vasoconstricting influence of pregnant plasma.

Detection of fetal cells in the maternal kidney during gestation in the mouse

It has been reported that fetal cells migrate into maternal blood and organs. Since these fetal chimeric cells could be involved in maternal allogeneic tolerance to the fetus, the fetal chimeric cells might be implicated in maternal-fetal immunology and development of maternal autoimmune diseases. However, the mechanism and role of fetal microchimerism remains unclear. We aimed to describe the mechanism by which fetal cells become associated with maternal organs during pregnancy, using a mouse fetal microchimerism model. Non-obese diabetic/severe combined immunodeficiency (NOD/SCID) female mice, which are useful for tracking the behavior of fetal cells in the maternal body, were mated with transgenic males expressing enhanced green fluorescent protein (GFP), and the presence of GFP-positive cells were examined in peripheral blood and organs of pregnant mothers. By flow cytometry, we showed that 0.95 +/- 0.48% of mononuclear cells detected in the maternal peripheral blood were GFP-positive, and thus of fetal origin, during the first gestational week. This value decreased to 0.10 +/- 0.13% during the third gestational week (p < 0.05). GFP-positive cells were detected in the extraglomerular mesangial region and among the epithelial cells of the proximal renal tubule of the maternal kidney. These GFP-positive cells also expressed angiotensin II receptor subtype 2 (AT2), which is known to participate in regulating organogenesis and vasoreactivity. Fetal cells expressing AT2 may therefore be involved in the regulation of vascular tone in the maternal kidney. These observations suggest that fetal cells could influence maternal renal function through activation of the AT2 signaling.

Prenatal blockade of Ang II receptors affects neonatal rat hindbrain structure and receptor localization

The development of knock-out mice for Angiotensin II (Ang II) AT(2) receptors, which exhibited altered exploratory behavior, prompted us to investigate the cerebellum and brainstem. We evaluated the effect of stimulation/inhibition of Ang II receptors on hindbrain development, in offspring (postnatal days P0, P8) of pregnant rats treated during late pregnancy (Ang II, Losartan or PD123319, 1 mg/kg/day). Receptor localization by autoradiography showed in P0 and P8 hindbrains, that most structures expressed AT(2) subtype: cerebellar cortex, cerebellar nuclei, genu facial nucleus, inferior colicullus, inferior olive. In the cerebellar cortex, [(125)I]Ang II AT(2) binding was predominant, while low AT(1) binding was observed in adjacent layers of the cerebellar cortex. Blockade of AT(2) receptors with PD123319 increased binding in cerebellar nuclei (p<0.05) and brainstem nuclei at P0, P8, in correlation with increased AT(2) receptor expression by RT-PCR. The enlarged external granular layer (EGL) in PD123319-treated P0 pups contrast with the significant decrease in Ang II binding (p<0.001) in the cerebellar cortex. Blockade of AT(2) receptors during late pregnancy seems to arrest cerebellar cortex development in P0 animals. On the contrary, increased AT(2) binding was observed in cerebellar cortex and DTg nucleus in PD123319-treated P8 animals (p<0.001). Ang II treatment leads to increased binding in the brainstem. In spite of the low doses of Ang II antagonists used, treatments were performed during a time-frame critical for hindbrain development, leading to remarkable effects. The present study makes a contribution to understand the role of Ang II receptors during hindbrain development.

Angiotensin II type 2 receptors have a major somatodendritic distribution in vasopressin-containing neurons in the mouse hypothalamic paraventricular nucleus

The hypothalamic paraventricular nucleus (PVN) and angiotensin II (AngII) play critical roles in cardiovascular and neurohumoral regulation ascribed in part to vasopressin (VP) release. The AngII actions in the PVN are mediated largely through angiotensin II type 1 (AT1) receptors. However, there is indirect evidence that the functionally elusive central angiotensin II type 2 (AT2) receptors are also mediators of AngII signaling in the PVN. We used electron microscopic dual immunolabeling of antisera recognizing the AT2 receptor and VP to test the hypothesis that mouse PVN neurons expressing VP are among the cellular sites where this receptor has a subcellular distribution conducive to local activation. Immunoreactivity for the AT2 receptor was detected in somatodendritic profiles, of which approximately 60% of the somata and approximately 28% of the dendrites also contained VP. In comparison with somata and dendrites, axons, axon terminals, and glia less frequently contained the AT2 receptor. Somatic labeling for the AT2 receptor was often seen in the cytoplasm near the Golgi lamellae and other endomembrane structures implicated in receptor trafficking. AT2 receptor immunoreactivity in dendrites was commonly localized to cytoplasmic endomembranes, but was occasionally observed on extra- or peri-synaptic portions of the plasma membrane apposed by astrocytic processes or by unlabeled axon terminals. The labeled dendritic plasmalemmal segments containing AT2 receptors received asymmetric excitatory-type or more rarely symmetric inhibitory-type contacts from unlabeled axon terminals containing dense core vesicles, many of which are known to store neuropeptides. These results provide the first ultrastructural evidence that AT2 receptors in PVN neurons expressing VP and other neuromodulators are strategically positioned for surface activation by AngII and/or intracellular trafficking.

Rab1 GTPase and dimerization in the cell surface expression of angiotensin II type 2 receptor

The physiological function of angiotensin II (Ang II) is mediated through the Ang II type 1 (AT1R) and type 2 (AT2R) receptors. Our previous studies have demonstrated that cell surface targeting of AT1R is regulated by Rab and Sar1 GTPases and the F(x)(6)LL motif in the membrane-proximal C terminus. However, the molecular mechanisms underlying the export of nascent AT2R remain poorly defined. In this report, we determined the role of Rab1 GTPase, which specifically controls protein transport from the endoplasmic reticulum (ER) to the Golgi, and receptor dimerization in the biosynthesis of AT2R. Cell surface expression of AT2R was augmented by transient expression of Rab1 and attenuated by dominant-negative Rab1 mutants and small interfering RNA-mediated knockdown of Rab1. Consistently, AT2R inhibition of epidermal growth factor-activated extracellular signal-regulated kinase 1/2 was significantly reduced by the Rab1 mutants, indicating that endogenous Rab1 modulates the cell surface targeting and signaling of AT2R. It is of interest to note that Rab1 augmented the overall expression of AT2R and its mRNA, whereas the Rab1 mutants attenuated the total AT2R expression and enhanced ubiquitin-dependent AT2R degradation. Furthermore, our previously characterized ER export-deficient AT1R mutant in which the F(x)(6)LL motif was mutated formed both homodimers and heterodimers with AT2R. Dimerization of the AT1R mutant with AT2R blocked AT2R trafficking to the cell surface, suggesting constitutive dimerization of both receptors in the ER and an important role of dimerization in ER export of the receptors. These data demonstrate for the first time that Rab1 GTPase and dimerization modulate export traffic from the ER to the cell surface of newly synthesized AT2R.

Angiotensin II modulates BBB permeability via activation of the AT(1) receptor in brain endothelial cells

Hypertensive encephalopathy occurs when acute changes in blood pressure cause breakdown of the blood-brain barrier (BBB). Angiotensin II (Ang II) plays a role in this pathophysiology. We determined whether Ang II directly regulates endothelial cell function at the BBB. In BBB microvessel endothelial cells (MECs), the Ang II (100 nmol/L; 0 to 6 h) effects on permeability to (125)I-albumin and transendothelial electrical resistance (TEER) were assessed. Angiotensin II (100 nmol/L) caused significant time-dependent changes in both (125)I-albumin permeability (25%) at 2 h and TEER (-8.87 Omega x cm(2)) at 6 h. Next, MECs were pretreated with the Ang II type 1 (AT(1)) receptor blocker telmisartan (1 micromol/L) or the Ang II type 2 (AT(2)) receptor blocker PD123,319 (1 micromol/L) followed by treatment with Ang II (100 nm). Telmisartan completely inhibited the Ang II-induced increase in (125)I-albumin permeability in MECs whereas PD123,319 had no effect. Using western blot analysis, we showed that MECs express AT(1) receptors but not AT(2) receptors. Treatment with Ang II (100 nmol/L; 0 to 6 h) also increased total protein kinase C activity. In contrast, Ang II had no effect on the expression of occludin, claudin 5, or actin. These results show that Ang II directly modulates transcytotic and paracellular permeability in BBB endothelial cells and could contribute to the pathophysiology of hypertensive encephalopathy.

Protective role of angiotensin II type 2 receptor signaling in a mouse model of pancreatic fibrosis

The renin-angiotensin system contributes to pathological processes in a variety of organs. In the pancreas, blocking the angiotensin II (AII) type 1 receptor (AT1) attenuates pancreatic fibrogenesis in animal models of pancreatitis. Because the role of the AII type 2 receptor (AT2) in modulating pancreatic injury is unknown we investigated the role of AT2 in pancreatic injury and fibrosis. Pancreatic fibrosis was induced by repetitive cerulein administration in C57BL/6 wild-type (WT) or AT2-deficient (AT2-/-) mice and assessed by morphology and gene expression at 10 days. There was no difference between WT and AT2-/- mice in the degree of acute pancreatic injury as assessed by amylase release at 9 and 12 h and by histological examination of the pancreas at 12 h. In contrast, parenchymal atrophy and fibrosis were more pronounced in AT2-/- mice compared with WT mice at 10 days. Fibrosis was accompanied by activation of pancreatic stellate cells (PSC) evaluated by Western blot analysis for alpha-smooth muscle actin and by immunocytochemistry; PSC activation was further increased in AT2-/- mice compared with WT mice. The level of pancreatic transforming growth factor-beta1 mRNA and protein after repetitive cerulein treatment was higher in AT2-/- mice than in WT mice. Our results demonstrate that, in contrast to AT1 receptor signaling, AT2 receptor signaling modulates protective antifibrogenic effects in a mouse model of cerulein-induced pancreatic fibrogenesis. We propose that the effects of AII on injury-induced pancreatic fibrosis may be determined by the balance between AT1 and AT2 receptor signaling.

Increased vascular angiotensin type 2 receptor expression and NOS-mediated mechanisms of vascular relaxation in pregnant rats

Normal pregnancy is associated with reduced blood pressure (BP) and decreased pressor response to vasoconstrictors, even though the renin-angiotensin system is upregulated. Angiotensin II (ANG II) activates both angiotensin type 1 receptors (AT(1)Rs) and angiotensin type 2 receptors (AT(2)Rs). Although the role of the AT(1)R in vascular contraction is well documented, the role of the AT(2)R in vascular relaxation, particularly during pregnancy, is less clear. It was hypothesized that the decreased BP and vasoconstriction during pregnancy was, at least in part, due to changes in AT(2)R amount, distribution, and/or postreceptor mechanisms of vascular relaxation. To test this hypothesis, systolic BP was measured in virgin and pregnant (day 19) Sprague-Dawley rats. Isometric contraction/relaxation was measured in isolated aortic rings, and nitric oxide (NO) production was measured using 4-amino-5-methylamino-2',7'-difluorescein fluorescence. AT(1)R and AT(2)R mRNA expression and protein amount were measured in tissue homogenates using real-time RT-PCR and Western blots, and their local distribution was visualized in cryosections using immunohistochemistry and immunofluorescence. BP was lower in pregnant than virgin rats. Phenylephrine (Phe) caused concentration-dependent contraction that was reduced in the aorta of pregnant compared with virgin rats. Treatment with the AT(2)R antagonist PD-123319 caused greater enhancement of Phe contraction, and the AT(2)R agonist CGP-42112A caused greater relaxation of Phe contraction in the aorta of pregnant than virgin rats. ANG II plus the AT(1)R blocker losartan induced greater NO production in the aorta of pregnant than virgin rats. RT-PCR revealed increased mRNA expression of vascular endothelial NO synthase (eNOS), little change in AT(1)Rs, and increased AT(2)Rs in pregnant compared with virgin rats. Western blots revealed an increased protein amount of activated phospho-eNOS, little change in AT(1)Rs, and increased AT(2)Rs in pregnant compared with virgin rats. Immunohistochemistry and immunofluorescence analysis in aortic sections of virgin rats revealed abundant AT(1)R staining in tunica media that largely colocalized with actin in vascular smooth muscle and less AT(2)Rs mainly in the tunica intima and endothelium. In pregnant rats, AT(1)R staining in the smooth muscle layer and adventitia was reduced, and endothelial AT(2)R staining was enhanced. These data suggest an enhanced AT(2)R-mediated vascular relaxation pathway involving increased expression/activity of endothelial AT(2)Rs and increased postreceptor activated phospho-eNOS, which may contribute to the decreased BP during pregnancy.

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

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

Evidence that the effect of angiotensin II on bovine oocyte nuclear maturation is mediated by prostaglandins E2 and F2α

Angiotensin II (AngII) prevents the inhibitory effect of follicular cells on oocyte maturation, but its involvement in LH-induced meiotic resumption remains unknown. The aim of this study was to assess the involvement of AngII in LH-induced meiotic resumption and of prostaglandins (PGs) in the action of AngII. In the experiment I, seven cows were superovulated, intrafollicularly injected with 10 μM saralasin (a competitive AngII antagonist) or saline when the follicles reached a diameter larger than 12 mm, and challenged with a GnRH agonist to induce an LH surge. Fifteen hours after GnRH, the animals were ovariectomized and the oocytes were recovered to determine the stage of meiosis. The oocytes from follicles that received saline were in germinal vesicle (GV) breakdown (30.8%) or metaphase I (MI; 69.2%) stage while those that received saralasin were in the GV stage (100%;P<0.001) 15 h after GnRH agonist. In another experiment, oocytes were co-cultured with follicular hemisections for 15 h to determine whether PGs mediate the effect of AngII on meiotic resumption. Indomethacin (10 μM) inhibited AngII-induced meiotic resumption (13.4 vs 77.5% MI without indomethacin;P<0.001). Furthermore, the GV oocytes progressed to MI at a similar rate when PGE2, PGF2αor AngII was present in the co-culture system with follicular cells (PGE277.4%, PGF2α70.0%, and AngII 75.0% MI). In conclusion, our results provide strong evidence that AngII mediates the resumption of meiosis induced by an LH surge in bovine oocytes and that this event is dependent on PGE2or PGF2αproduced by follicular cells.

Meconium increases type 1 angiotensin II receptor expression and alveolar cell death

The pulmonary renin-angiotensin system (RAS) contributes to inflammation and epithelial apoptosis in meconium aspiration. It is unclear if both angiotensin II receptors (ATR) contribute, where they are expressed and if meconium modifies subtype expression. We examined ATR subtypes in 2 wk rabbit pup lungs before and after meconium exposure and with and without captopril pretreatment or type 1 receptor (AT1R) inhibition with losartan, determining expression and cellular localization with immunoblots, RT-PCR and immunohistochemistry, respectively. Responses of cultured rat alveolar type II pneumocytes were also examined. Type 2 ATR were undetected in newborn lung before and after meconium instillation. AT1R were expressed in pulmonary vascular and bronchial smooth muscle and alveolar and bronchial epithelium. Meconium increased total lung AT1R protein approximately 3-fold (p = 0.006), mRNA 29% (p = 0.006) and immunostaining in bronchial and alveolar epithelium and smooth muscle, which were unaffected by captopril and losartan. Meconium also increased AT1R expression >3-fold in cultured type II pneumocytes and caused concentration-dependent cell death inhibited by losartan. Meconium increases AT1R expression in newborn rabbit lung and cultured type II pneumocytes and induces AT1R-mediated cell death. The pulmonary RAS contributes to the pathogenesis of meconium aspiration through increased receptor expression.

Estrogen reduces aldosterone, upregulates adrenal angiotensin II AT2 receptors and normalizes adrenomedullary Fra-2 in ovariectomized rats

We studied the effect of ovariectomy and estrogen replacement on expression of adrenal angiotensin II AT1 and AT2 receptors, aldosterone content, catecholamine synthesis, and the transcription factor Fos-related antigen 2 (Fra-2). Ovariectomy increased AT1 receptor expression in the adrenal zona glomerulosa and medulla, and decreased adrenomedullary catecholamine content and Fra-2 expression when compared to intact female rats. In the zona glomerulosa, estrogen replacement normalized AT1 receptor expression, decreased AT1B receptor mRNA, and increased AT2 receptor expression and mRNA. Estrogen treatment decreased adrenal aldosterone content. In the adrenal medulla, the effects of estrogen replacement were: normalized AT1 receptor expression, increased AT2 receptor expression, AT2 receptor mRNA, and tyrosine hydroxylase mRNA, and normalized Fra-2 expression and catecholamine content. We demonstrate that the constitutive adrenal expression of AT1 receptors, catecholamine synthesis and Fra-2 expression are partially under the control of reproductive hormones. Our results suggest that estrogen treatment decreases aldosterone production through AT1 receptor downregulation and AT2 receptor upregulation. AT2 receptor upregulation and modulation of Fra-2 expression may participate in the estrogen-dependent normalization of adrenomedullary catecholamine synthesis in ovariectomized rats. The AT2 receptor upregulation and the decrease in AT1 receptor function and in the production of the fluid-retentive, pro-inflammatory hormone aldosterone partially explain the protective effects of estrogen therapy.