Nuclear angiotensin-(1-7) receptor is functionally coupled to the formation of nitric oxide

Sex differences in the renin-angiotensin-aldosterone system and its roles in hypertension, cardiovascular, and kidney diseases

Cardiovascular disease is a pathology that exhibits well-researched biological sex differences, making it possible for physicians to tailor preventative and therapeutic approaches for various diseases. Hypertension, which is defined as blood pressure greater than 130/80 mmHg, is the primary risk factor for developing coronary artery disease, stroke, and renal failure. Approximately 48% of American men and 43% of American women suffer from hypertension. Epidemiological data suggests that during reproductive years, women have much lower rates of hypertension than men. However, this protective effect disappears after the onset of menopause. Treatment-resistant hypertension affects approximately 10.3 million US adults and is unable to be controlled even after implementing ≥3 antihypertensives with complementary mechanisms. This indicates that other mechanisms responsible for modulating blood pressure are still unclear. Understanding the differences in genetic and hormonal mechanisms that lead to hypertension would allow for sex-specific treatment and an opportunity to improve patient outcomes. Therefore, this invited review will review and discuss recent advances in studying the sex-specific physiological mechanisms that affect the renin-angiotensin system and contribute to blood pressure control. It will also discuss research on sex differences in hypertension management, treatment, and outcomes.

Mas receptor endocytosis and signaling in health and disease

The renin angiotensin system (RAS) plays a major role in blood pressure regulation and electrolyte homeostasis and is mainly composed by two axes mediating opposite effects. The pressor axis, constituted by angiotensin (Ang) II and the Ang II type 1 receptor (AT1R), exerts vasoconstrictor, proliferative, hypertensive, oxidative and pro-inflammatory actions, while the depressor/protective axis, represented by Ang-(1-7), its Mas receptor (MasR) and the Ang II type 2 receptor (AT2R), opposes the actions elicited by the pressor arm. The MasR belongs to the G protein-coupled receptor (GPCR) family. To avoid receptor overstimulation, GPCRs undergo internalization and trafficking into the cell after being stimulated. Then, the receptor may induce other signaling cascades or it may even interact with other receptors, generating distinct biological responses. Thus, control of a GPCR regarding space and time affects the specificity of the signals transduced by the receptor and the ultimate cellular response. The present chapter is focused on the signaling and trafficking pathways of MasR under physiological conditions and its participation in the pathogenesis of numerous brain diseases.

How Effective is Angiotensin II in Decreasing Mortality of Vasodilatory Shock? A Systematic Review

Background

Patients with severe vasodilation accompanied by refractory hypotension despite high doses of vasopressors were associated with a high mortality rate. The Ang-2 for the Treatment of High-Output Shock (ATHOS) 3 trial demonstrated that angiotensin 2 (Ang-2) could effectively increase MAP and blood pressure in vasodilatory shock patients. This systematic review aims to summarize the impact of Ang-2 for the treatment of vasodilatory shock on clinical outcomes, including length of stay, MAP level (before and after), and mortality also Ang-2 dose needed.

Methods

A systematic search in PubMed, Sage, ScienceDirect, Scopus and Gray literature was conducted to obtain studies about the use of Ang-2 in vasodilatory shock patients.

Results

In all of the studies that we obtained, there were different results regarding mortality in patients with vasodilatory shock with Ang-2. Mortality was significantly lower when Ang-2 was administered to patients with elevated renin. The initial dose of Ang-2 can be started at 10-20 ng/kg/min, but there is no agreement on the maximum dose. Ang-2 may be considered a third-line vasopressor if the targeted MAP has not been achieved after administration of norepinephrine >200 ng/kg/min for more than 6 hours. Although not statistically significant, the use of Ang-2 can reduce the length of stay in the ICU and in the hospital when compared to patients without Ang-2 therapy, in addition to reducing the dose of vasopressor.

Conclusion

Overall, the use of Ang-2 has potential to be a regimen for patients with vasodilatory shock. Further study is needed to obtain more data.

Functional Significance of Angiotensin Receptor Type 2 in the Neuroplasticity of Autonomic Ganglia in (mRen2)27 Transgenic Hypertensive Rats

ABSTRACT

The over-expression of Ren -2 d gene in (mRen2)27 rats leads to development of hypertension mediated by the renin-angiotensin-system axis and exaggerated sympathetic nerve activity. Exogenously applied angiotensin II (AngII) on the superior cervical ganglion evokes ganglionic compound action potentials (gCAP) and ganglionic long-term potentiation (gLTP). We studied the functional role of angiotensin receptors and expression of reactive oxygen species marker, nicotinamide adenine dinucleotide phosphate oxidase 4 (NOX4) proteins in AngII-induced postganglionic transmission. Bath-applied AngII revealed that the indices of ganglionic transmission, synaptic strength of gCAP, and decay time for gLTP are remarkably prolonged in (mRen2)27 rats and were abolished by an angiotensin receptor blocker (ARB), suggesting postganglionic AngII Type 1 (AT 1 ) receptor localization and mediation. Receptor density for AT 1 was similar in (mRen2)27 and control animals, and quantitative reverse transcription polymerase chain reaction revealed that it is consistent with the mRNA profile. Furthermore, immunocytochemistry analysis showed similar AT 1 receptor distribution and signals. However, assessment of Type 2 (AT 2 ), Ang-(1-7)-MAS and NOX4-specific proteins showed that AT 2 receptor protein expression was 4-fold lower, consistent with a low mRNA profile. MAS receptor expression was 10-fold lower and NOX4 protein was 2-fold lower. Despite similarity in the densities of AT 1 receptor, the low levels of the components of the protective arm of the renin-angiotensin system at the ganglia may contribute to the differential superior cervical ganglion sensitivity to AngII. The lower NOX4 affects reactive oxygen species balance and possibly results in activation of downstream pathways to promote increased sympathetic nerve activity. We speculate that the significant diminution in AT 2, MAS, and NOX4 protein expressions may play an indirect role in the alteration and efficacy of gCAP and gLTP in hypertension.

Consequences of hyperandrogenemia during pregnancy in female offspring: attenuated response to angiotensin II

BACKGROUND

Polycystic ovary syndrome (PCOS) is characterized by reproductive and metabolic dysfunction, and elevated blood pressure (BP). The cardiometabolic consequences of maternal hyperandrogenemia on offspring, either as adults or with aging, have not been well studied. We previously found that male offspring of hyperandrogenemic female (HAF) rats, a model of PCOS, are normotensive but have an exaggerated pressor response to angiotensin (Ang) II.

METHOD

In this study, the hypothesis was tested that adult and aging female offspring of HAF rats develop a metabolic and hypertensive phenotype. Control and HAF rats were implanted prepubertally with placebo or dihydrotestosterone pellets, which continued throughout pregnancy and lactation.

RESULTS

Female offspring of HAF dams had lower birth weight than female control offspring. Although female HAF offspring (aged 16-24 weeks) had no differences in intrarenal Ang II, plasma lipids or proteinuria, they did have lower intrarenal Ang (1-7) and lower nitrate/nitrite excretion than controls. Adult HAF offspring had similar baseline BP as controls, but had an attenuated pressor response to Ang II. With aging (16-20 months), female HAF offspring remained normotensive with an attenuated pressor response to Ang II and high salt diet but more proteinuria and higher intrarenal Ang(1-7) than controls.

CONCLUSION

Taken together, these data suggest that female HAF offspring are protected from developing hypertension, but may be at risk for renal injury with aging. Future studies are necessary to determine whether adult and postmenopausal offspring of PCOS women are at increased risk for cardiovascular dysfunction.Graphical abstract:http://links.lww.com/HJH/B820.

Renin-Angiotensin System Induced Secondary Hypertension: The Alteration of Kidney Function and Structure

Long-term hypertension is known as a major risk factor for cardiovascular and chronic kidney disease (CKD). The Renin-angiotensin system (RAS) plays a key role in hypertension pathogenesis. Angiotensin II (Ang II) enhancement in Ang II-dependent hypertension leads to progressive CKD and kidney fibrosis. In the two-kidney one-clip model (2K1C), more renin is synthesized in the principal cells of the collecting duct than juxtaglomerular cells (JGCs). An increase of renal Ang I and Ang II levels and a decrease of renal cortical and medullary Ang 1–7 occur in both kidneys of the 2K1C hypertensive rat model. In addition, the activity of the angiotensin-converting enzyme (ACE) increases, while ACE2's activity decreases in the medullary region of both kidneys in the 2K1C hypertensive model. Also, the renal prolyl carboxypeptidase (PrCP) expression and its activity reduce in the clipped kidneys. The imbalance in the production of renal ACE, ACE2, and PrCP expression causes the progression of renal injury. Intrarenal angiotensinogen (AGT) expression and urine AGT (uAGT) excretion rates in the unclipped kidney are greater than the clipped kidney in the 2K1C hypertensive rat model. The enhancement of Ang II in the clipped kidney is related to renin secretion, while the elevation of intrarenal Ang II in the unclipped kidney is related to stimulation of AGT mRNA and protein in proximal tubule cells by a direct effect of systemic Ang II level. Ang II-dependent hypertension enhances macrophages and T-cell infiltration into the kidney which increases cytokines, and AGT synthesis in proximal tubules is stimulated via cytokines. Accumulation of inflammatory cells in the kidney aggravates hypertension and renal damage. Moreover, Ang II-dependent hypertension alters renal Ang II type 1 & 2 receptors (AT₁R & AT₂R) and Mas receptor (MasR) expression, and the renal interstitial fluid bradykinin, nitric oxide, and cGMP response to AT₁R, AT₂R, or BK B₂-receptor antagonists. Based on a variety of sources including PubMed, Google Scholar, Scopus, and Science-Direct, in the current review, we will discuss the role of RAS-induced secondary hypertension on the alteration of renal function.

Intratubular, Intracellular, and Mitochondrial Angiotensin II/AT1 (AT1a) Receptor/NHE3 Signaling Plays a Critical Role in Angiotensin II-Induced Hypertension and Kidney Injury

Hypertension is well recognized to be the most important risk factor for cardiovascular diseases, stroke, and end-stage kidney failure. A quarter of the world’s adult populations and 46% of the US adults develop hypertension and currently require antihypertensive treatments. Only 50% of hypertensive patients are responsive to current antihypertensive drugs, whereas remaining patients may continue to develop cardiovascular, stroke, and kidney diseases. The mechanisms underlying the poorly controlled hypertension remain incompletely understood. Recently, we have focused our efforts to uncover additional renal mechanisms, pathways, and therapeutic targets of poorly controlled hypertension and target organ injury using novel animal models or innovative experimental approaches. Specifically, we studied and elucidated the important roles of intratubular, intracellular, and mitochondrial angiotensin II (Ang II) system in the development of Ang II-dependent hypertension. The objectives of this invited article are to review and discuss our recent findings that (a) circulating and intratubular Ang II is taken up by the proximal tubules via the (AT₁) AT_1a receptor-dependent mechanism, (b) intracellular administration of Ang II in proximal tubule cells or adenovirus-mediated overexpression of an intracellular Ang II fusion protein selectively in the mitochonria of the proximal tubules induces blood pressure responses, and (c) genetic deletion of AT₁ (AT_1a) receptors or the Na⁺/H⁺ exchanger 3 selectively in the proximal tubules decreases basal blood pressure and attenuates Ang II-induced hypertension. These studies provide a new perspective into the important roles of the intratubular, intracellular, and mitochondrial angiotensin II/AT₁ (AT_1a) receptor signaling in Ang II-dependent hypertensive kidney diseases.

Renin-angiotensin system and inflammation update

The most classical view of the renin-angiotensin system (RAS) emphasizes its role as an endocrine regulator of sodium balance and blood pressure. However, it has long become clear that the RAS has pleiotropic actions that contribute to organ damage, including modulation of inflammation. Angiotensin II (Ang II) activates angiotensin type 1 receptors (AT1R) to promote an inflammatory response and organ damage. This represents the pathophysiological basis for the successful use of RAS blockers to prevent and treat kidney and heart disease. However, other RAS components could have a built-in capacity to brake proinflammatory responses. Angiotensin type 2 receptor (AT2R) activation can oppose AT1R actions, such as vasodilatation, but its involvement in modulation of inflammation has not been conclusively proven. Angiotensin-converting enzyme 2 (ACE2) can process Ang II to generate angiotensin-(1-7) (Ang-(1-7)), that activates the Mas receptor to exert predominantly anti-inflammatory responses depending on the context. We now review recent advances in the understanding of the interaction of the RAS with inflammation. Specific topics in which novel information became available recently include intracellular angiotensin receptors; AT1R posttranslational modifications by tissue transglutaminase (TG2) and anti-AT1R autoimmunity; RAS modulation of lymphoid vessels and T lymphocyte responses, especially of Th17 and Treg responses; interactions with toll-like receptors (TLRs), programmed necrosis, and regulation of epigenetic modulators (e.g. microRNAs and bromodomain and extraterminal domain (BET) proteins). We additionally discuss an often overlooked effect of the RAS on inflammation which is the downregulation of anti-inflammatory factors such as klotho, peroxisome proliferator-activated receptor γ co-activator 1α (PGC-1α), transient receptor potential ankyrin 1 (TRPA1), SNF-related serine/threonine-protein kinase (SNRK), serine/threonine-protein phosphatase 6 catalytic subunit (Ppp6C) and n-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP). Both transcription factors, such as nuclear factor κB (NF-κB), and epigenetic regulators, such as miRNAs are involved in downmodulation of anti-inflammatory responses. A detailed analysis of pathways and targets for downmodulation of anti-inflammatory responses constitutes a novel frontier in RAS research.

Reduced urinary levels of angiotensin-converting enzyme 2 activity predict acute kidney injury in critically ill patients

Objective: Angiotensin-converting enzyme 2 activity reflects non-classical renin-angiotensin system upregulation. We assessed the association of urinary angiotensin-converting enzyme 2 (uACE2) activity with acute kidney injury (AKI). Design, setting and participants: A prospective observational study in which we measured uACE2 activity in 105 critically ill patients at risk of AKI. We report AKI stage 2 or 3 at 12 hours of urine collection (AKI12h) and AKI stage 2 or 3 at any time during intensive care unit stay in patients free from any stage of AKI at inclusion (AKIICU). AKI prediction was assessed using area under the receiver-operating characteristics curve (AUROC) and net reclassification indices (NRIs). Main outcome measure: AKI stage 2 or 3 at 12 hours of urine collection. Results: Within 12 hours of inclusion, 32 of 105 patients (30%) had developed AKI12h. Corrected uACE2 activity was significantly higher in patients without AKI12h compared with those with AKI12h (median [interquartile range], 13 [6-24] v 7 [4-10] pmol/min/mL per mmol/L of urine creatinine; P < 0.01). A 10-unit increase in uACE2 was associated with a 28% decrease in AKI12h risk (odds ratio [95% CI], 0.72 [0.46-0.97]). During intensive care unit admission, 39 of 76 patients (51%) developed AKIICU. uACE2 had an AUROC for the prediction of AKI12h of 0.68 (95% CI, 0.57-0.79), and correctly reclassified 28% of patients (positive NRI) to AKI12h. Patients with uACE2 > 8.7 pmol/min/mL per mmol/L of urine creatinine had a significantly lower risk of AKIICU on log-rank analysis (52% v 84%; P < 0.01). Conclusions: Higher uACE2 activity was associated with a decreased risk of AKI stage 2 or 3. Our findings support future evaluations of the role of the non-classical renin-angiotensin system during AKI.

Angiotensin Receptors Heterodimerization and Trafficking: How Much Do They Influence Their Biological Function?

G-protein–coupled receptors (GPCRs) are targets for around one third of currently approved and clinical prescribed drugs and represent the largest and most structurally diverse family of transmembrane signaling proteins, with almost 1000 members identified in the human genome. Upon agonist stimulation, GPCRs are internalized and trafficked inside the cell: they may be targeted to different organelles, recycled back to the plasma membrane or be degraded. Once inside the cell, the receptors may initiate other signaling pathways leading to different biological responses. GPCRs’ biological function may also be influenced by interaction with other receptors. Thus, the ultimate cellular response may depend not only on the activation of the receptor from the cell membrane, but also from receptor trafficking and/or the interaction with other receptors. This review is focused on angiotensin receptors and how their biological function is influenced by trafficking and interaction with others receptors.

Angiotensin-converting Enzyme 2 roles in the Pathogenesis of COVID-19

The new pandemic Coronavirus Disease 2019 (COVID-19) causes a wide range of clinical consequences, from asymptomatic infection to acute respiratory failure and it is very heterogeneous. The renin-angiotensin system (RAS) is well recognized as a key regulating system in circulatory homeostasis that play prominent roles in pathophysiological processes in abnormal activation for instance renal and cardiovascular diseases, obesity, and stroke. Angiotensin converting enzyme 2(ACE2) as a component of the RAS system. However, unlike the ACE, its activity is not inhibited by the ACE inhibitors. The major product of ACE2 is Ang1-7, known as a vasodilator peptide and part of the depressant arm of the RAS. There are two form of ACE2. Coronavirus cover with some proteins in order to help viral attachment to the cell membrane ACE2 as a receptor and then fuse and enter the cells. ACE2 was expressed in oral Cavity, salivary glands of the mouth, esophagus, myocardial cells, kidney, and enterocytes, along all the respiratory tract, intestine, and blood vessels. In this article, we explain the renin-angiotensin system and its components. Also, we shortly explain the organs involved in COVID-19 disease and we will talk about the possible causes of damage to these organs. We also reviewed the probable mechanism of using ACE2 in viral attachment and the probable treatment processes will also be discussed based on the surface proteins of the virus and ACE2. In addition, we briefly discuss the anti-angiotensin drugs and why patients with chronic disease are more susceptible to COVID-19 infection and show worse progression.

Classic and Nonclassic Renin-Angiotensin Systems in the Critically Ill

Classic and nonclassic renin-angiotensin systems (RAS) are 2 sides of an ubiquitous endocrine/paracrine cascade regulating blood pressure and homeostasis. Angiotensin II and angiotensin-converting enzyme (ACE) levels are associated with severity of disease in the critically ill, and are central to the physiology and the pathogenesis of circulatory shock. Angiotensin (1-7) and ACE2 act as an endogenous counterregulatory arm to the angiotensin II/ACE axis. The tissue-based RAS has paracrine effects dissociated from those of the circulating RAS. Exogenous angiotensin II or ACE2 may improve the outcome of septic shock and acute respiratory distress syndrome, respectively.

Angiotensin-(1-7) inhibits sodium transport via Mas receptor by increasing nitric oxide production in thick ascending limb

Sodium transport in the thick ascending loop of Henle (TAL) is tightly regulated by numerous factors, especially angiotensin II (Ang II), a key end-product of the renin-angiotensin system (RAS). However, an alternative end-product of the RAS, angiotensin-(1-7) [Ang-(1-7)], may counter some of the Ang II actions. Indeed, it causes vasodilation and promotes natriuresis through its effects in the proximal and distal tubule. However, its effects on the TAL are unknown. Because the TAL expresses the Mas receptor, an Ang-(1-7) ligand, which in turn may increase NO and inhibit Na+ transport, we hypothesized that Ang-(1-7) inhibits Na transport in the TAL, via a Mas receptor/NO-dependent mechanism. We tested this by measuring transport-dependent oxygen consumption (VO2 ) in TAL suspensions. Administering Ang-(1-7) decreased VO2 ; an effect prevented by dimethyl amiloride and furosemide, signifying that Ang-(1-7) inhibits transport-dependent VO2 in TAL. Ang-(1-7) also increased NO levels, known inhibitors of Na+ transport in the TAL. The effects of Ang-(1-7) on VO2 , as well as on NO levels, were ameliorated by the Mas receptor antagonist, D-Ala, in effect suggesting that Ang-(1-7) may inhibit transport-dependent VO2 in TAL via Mas receptor-dependent activation of the NO pathway. Indeed, blocking NO synthesis with L-NAME prevented the inhibitory actions of Ang-(1-7) on VO2 . Our data suggest that Ang-(1-7) may modulate TAL Na+ transport via Mas receptor-dependent increases in NO leading to the inhibition of transport activity.

Long-Term Administration of Angiotensin (1-7) to db/db Mice Reduces Oxidative Stress Damage in the Kidneys and Prevents Renal Dysfunction

Aims

The goal of this study was to evaluate the effects of long-term (16 weeks) administration of angiotensin (1–7) [A(1–7)] on kidney function in db/db mice and to identify the protective mechanisms of this therapy.

Methods

db/db mice and heterozygous controls were treated with A(1–7) or vehicle daily, subcutaneously for up to 16 weeks. Kidney injury was assessed by measuring blood flow in renal arteries, plasma creatinine levels, and proteinuria. Effects of treatment on oxidative stress were evaluated by histological staining and gene expression.

Results

16 weeks of daily administration of A(1–7) to a mouse model of severe type 2 diabetes (db/db) prevented the progression of kidney damage. Treatment with A(1–7) improved blood flow in the renal arteries, as well as decreased plasma creatinine levels and proteinuria in diabetic mice. Reduction of oxidative stress was identified as one of the mechanisms of the renoprotective action of A(1–7). Treatment prevented formation of nitrotyrosine residues, a marker of oxidative stress damage. A(1–7) also reduced the expression of two enzymes involved in formation of nitrotyrosine, namely, eNOS and NOX-4. A(1–7) regulated the phosphorylation pattern of eNOS to enhance production of NO in diabetic animals, possibly through the Akt pathway. However, these elevated levels of NO did not result in increased nitrosylation, possibly due to reduced NOX-4 levels.

Conclusions

Long-term administration of A(1–7) improved kidney function and reduced oxidative stress damage in db/db mice.

Intratubular and intracellular renin-angiotensin system in the kidney: a unifying perspective in blood pressure control

The renin-angiotensin system (RAS) is widely recognized as one of the most important vasoactive hormonal systems in the physiological regulation of blood pressure and the development of hypertension. This recognition is derived from, and supported by, extensive molecular, cellular, genetic, and pharmacological studies on the circulating (tissue-to-tissue), paracrine (cell-to-cell), and intracrine (intracellular, mitochondrial, nuclear) RAS during last several decades. Now, it is widely accepted that circulating and local RAS may act independently or interactively, to regulate sympathetic activity, systemic and renal hemodynamics, body salt and fluid balance, and blood pressure homeostasis. However, there remains continuous debate with respect to the specific sources of intratubular and intracellular RAS in the kidney and other tissues, the relative contributions of the circulating RAS to intratubular and intracellular RAS, and the roles of intratubular compared with intracellular RAS to the normal control of blood pressure or the development of angiotensin II (ANG II)-dependent hypertension. Based on a lecture given at the recent XI International Symposium on Vasoactive Peptides held in Horizonte, Brazil, this article reviews recent studies using mouse models with global, kidney- or proximal tubule-specific overexpression (knockin) or deletion (knockout) of components of the RAS or its receptors. Although much knowledge has been gained from cell- and tissue-specific transgenic or knockout models, a unifying and integrative approach is now required to better understand how the circulating and local intratubular/intracellular RAS act independently, or with other vasoactive systems, to regulate blood pressure, cardiovascular and kidney function.

The renin angiotensin system and the brain: New developments

The traditional renin-angiotensin system (RAS) is indispensable system in adjusting sodium homeostasis, body fluid volume, and controlling arterial blood pressure. The key elements are renin splitting inactive angiotensinogen to yield angiotensin (Ang-I). Ang-1 is then changed by angiotensin-1 converting enzyme (ACE) into angiotensin II (Ang-II). Using PubMed, Google Scholar, and other means, we searched the peer-reviewed literature from 1990 to 2013 for articles on newly discovered findings related to the RAS, especially focusing on how the system influences the central nervous system (CNS). The classical RAS is now considered to be only part of the picture; the discovery of additional RAS pathways in the brain and elsewhere has yielded a vastly improved understanding of how the RAS influences the CNS. Newly discovered effects of the RAS on brain tissue include neuroprotection, cognition, and cerebral vasodilation. A number of brain biochemical pathways are influenced by the brain RAS. Within various pathways, there are potential opportunities for classical pharmacologic interventions as well as the possibility of controlling gene expression.

MAS1 Receptor Trafficking Involves ERK1/2 Activation Through a β-Arrestin2-Dependent Pathway

The MAS1 receptor (R) exerts protective effects in the brain, heart, vessels, and kidney. R trafficking plays a critical function in signal termination and propagation and in R resensitization. We examined MAS1R internalization and trafficking on agonist stimulation and the role of β-arrestin2 in the activation of ERK1/2 (extracellular signal-regulated kinase 1/2) and Akt after MAS1R stimulation. Human embryonic kidney 293T cells were transfected with the coding sequence for MAS1R-YFP (MAS1R fused to yellow fluorescent protein). MAS1R internalization was evaluated by measuring the MAS1R present in the plasma membrane after agonist stimulation using a ligand-binding assay. MAS1R trafficking was evaluated by its colocalization with trafficking markers. MAS1R internalization was blocked in the presence of shRNAcaveolin-1 and with dominant negatives for Eps15 (a protein involved in endocytosed Rs by clathrin-coated pits) and for dynamin. After stimulation, MAS1R colocalized with Rab11-a slow recycling vesicle marker-and not with Rab4-a fast recycling vesicle marker-or LysoTracker-a lysosome marker. Cells transfected with MAS1R showed an increase in Akt and ERK1/2 activation on angiotensin-(1-7) stimulation, which was blocked when the clathrin-coated pits pathway was blocked. Suppression of β-arrestin2 by shRNA reduced the angiotensin-(1-7)-induced ERK1/2 activation, whereas Akt activation was not modified. We conclude that on agonist stimulation, MAS1R is internalized through clathrin-coated pits and caveolae in a dynamin-dependent manner and is then slowly recycled back to the plasma membrane. MAS1R induced Akt and ERK1/2 activation from early endosomes, and the activation of ERK1/2 was mediated by β-arrestin2. Thus, MAS1R activity and density may be tightly controlled by the cell.

Angiotensin II Type 2 Receptor and Receptor Mas Are Colocalized and Functionally Interdependent in Obese Zucker Rat Kidney

The actions of angiotensin II type 2 receptor (AT₂R) and the receptor Mas (MasR) are complex but show similar pronatriuretic function; particularly, AT₂R expression and natriuretic function are enhanced in obese/diabetic rat kidney. In light of some reports suggesting a potential positive interaction between these receptors, we tested hypothesis that renal AT₂R and MasR physically interact and are interdependent to stimulate cell signaling and promote natriuresis in obese rats. We found that infusion of AT₂R agonist C21 in obese Zucker rats (OZR) increased urine flow and urinary Na excretion which were attenuated by simultaneous infusion of the AT₂R antagonist PD123319 or the MasR antagonist A-779. Similarly, infusion of MasR agonist Ang-(1-7) in OZR increased urine flow and urinary Na excretion, which were attenuated by simultaneous infusion of A-779 or PD123319. Experiment in isolated renal proximal tubules of OZR revealed that both the agonists C21 and Ang-(1-7) stimulated NO which was blocked by either of the receptor antagonists. Dual labeling of AT₂R and MasR in OZR kidney sections and human proximal tubule epithelial cells showed that AT₂R and MasR are colocalized. The AT₂R also coimmunoprecipitated with MasR in cortical homogenate of OZR. Immunoblotting of cortical homogenate cross-linked with zero-length oxidative (sulfhydryl groups) cross-linker cupric-phenanthroline revealed a shift of AT₂R and MasR bands upward with overlapping migration for their complexes which were sensitive to the reducing β-mercaptoethanol, suggesting involvement of -SH groups in cross-linking. Collectively, the study reveals that AT₂R and MasR are colocalized and functionally interdependent in terms of stimulating NO and promoting diuretic/natriuretic response.

Recent Updates on the Proximal Tubule Renin-Angiotensin System in Angiotensin II-Dependent Hypertension

It is well recognized that the renin-angiotensin system (RAS) exists not only as circulating, paracrine (cell to cell), but also intracrine (intracellular) system. In the kidney, however, it is difficult to dissect the respective contributions of circulating RAS versus intrarenal RAS to the physiological regulation of proximal tubular Na(+) reabsorption and hypertension. Here, we review recent studies to provide an update in this research field with a focus on the proximal tubular RAS in angiotensin II (ANG II)-induced hypertension. Careful analysis of available evidence supports the hypothesis that both local synthesis or formation and AT1 (AT1a) receptor- and/or megalin-mediated uptake of angiotensinogen (AGT), ANG I and ANG II contribute to high levels of ANG II in the proximal tubules of the kidney. Under physiological conditions, nearly all major components of the RAS including AGT, prorenin, renin, ANG I, and ANG II would be filtered by the glomerulus and taken up by the proximal tubules. In ANG II-dependent hypertension, the expression of AGT, prorenin, and (pro)renin receptors, and angiotensin-converting enzyme (ACE) is upregulated rather than downregulated in the kidney. Furthermore, hypertension damages the glomerular filtration barrier, which augments the filtration of circulating AGT, prorenin, renin, ANG I, and ANG II and their uptake in the proximal tubules. Together, increased local ANG II formation and augmented uptake of circulating ANG II in the proximal tubules, via activation of AT1 (AT1a) receptors and Na(+)/H(+) exchanger 3, may provide a powerful feedforward mechanism for promoting Na(+) retention and the development of ANG II-induced hypertension.

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

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

Chronic heart failure as a state of reduced effectiveness of the natriuretic peptide system: implications for therapy

Natriuretic peptides (NPs) promote diuresis, natriuresis and vasodilation in early chronic heart failure (CHF), countering renin-angiotensin-aldosterone system (RAAS) and sympathetic nervous system (SNS) overstimulation. Despite dramatic increases in circulating NP concentrations as CHF progresses, their effects become blunted. Increases in diuresis, natriuresis, and vasodilation after administration of exogenous atrial (ANP) or brain (BNP) natriuretic peptides are attenuated in patients with advanced CHF compared with controls. Several major factors may account for the reduced effectiveness of the natriuretic peptide system (NPS) in CHF. First, there is reduced availability of active forms of NPs, namely BNP. Second, target organ responsiveness becomes diminished. Third, the counter-regulatory hormones of the RAAS and SNS, and endothelin-1 become over-activated. Therefore, pharmacological approaches to enhance the functional effectiveness of the NPS in CHF have been explored in recent years. In terms of clinical outcomes, studies of synthetic BNP, or of neprilysin inhibitors alone or associated with an angiotensin converting enzyme inhibitor, have been controversial for several reasons. Recently, however, encouraging results have been obtained with the angiotensin receptor neprilysin inhibitor sacubitril/valsartan. The available data show that treatment with sacubitril/valsartan is associated with increased levels of NPs and their intracellular mediator cyclic guanosine monophosphate, suggesting improved functional effectiveness of the NPS, in addition to beneficial effects on mortality and morbidity outcomes. Therefore, combined targeting of the NPS and RAAS with sacubitril/valsartan emerges as the current optimal approach for redressing the neurohormonal imbalance in CHF.

Immunohistochemical expression of intrarenal renin angiotensin system components in response to tempol in rats fed a high salt diet

AIM

To determine the effect of tempol in normal rats fed high salt on arterial pressure and the balance between antagonist components of the renal renin-angiotensin system.

METHODS

Sprague-Dawley rats were fed with 8% NaCl high-salt (HS) or 0.4% NaCl (normal-salt, NS) diet for 3 wk, with or without tempol (T) (1 mmol/L, administered in drinking water). Mean arterial pressure (MAP), glomerular filtration rate (GFR), and urinary sodium excretion (UVNa) were measured. We evaluated angiotensin II (Ang II), angiotensin 1-7 (Ang 1-7), angiotensin converting enzyme 2 (ACE2), mas receptor (MasR), angiotensin type 1 receptor (AT1R) and angiotensin type 2 receptor (AT2R) in renal tissues by immunohistochemistry.

RESULTS

The intake of high sodium produced a slight but significant increase in MAP and differentially regulated components of the renal renin-angiotensin system (RAS). This included an increase in Ang II and AT1R, and decrease in ACE-2 staining intensity using immunohistochemistry. Antioxidant supplementation with tempol increased natriuresis and GFR, prevented changes in blood pressure and reversed the imbalance of renal RAS components. This includes a decrease in Ang II and AT1R, as increase in AT2, ACE2, Ang (1-7) and MasR staining intensity using immunohistochemistry. In addition, the natriuretic effects of tempol were observed in NS-T group, which showed an increased staining intensity of AT2, ACE2, Ang (1-7) and MasR.

CONCLUSION

These findings suggest that a high salt diet leads to changes in the homeostasis and balance between opposing components of the renal RAS in hypertension to favour an increase in Ang II. Chronic antioxidant supplementation can modulate the balance between the natriuretic and antinatriuretic components of the renal RAS.

Angiotensinogen import in isolated proximal tubules: evidence for mitochondrial trafficking and uptake

The renal proximal tubules are a key functional component of the kidney and express the angiotensin precursor angiotensinogen; however, it is unclear the extent that tubular angiotensinogen reflects local synthesis or internalization. Therefore, the current study established the extent to which angiotensinogen is internalized by proximal tubules and the intracellular distribution. Proximal tubules were isolated from the kidney cortex of male sheep by enzymatic digestion and a discontinuous Percoll gradient. Tubules were incubated with radiolabeled ¹²⁵I-angiotensinogen for 2 h at 37°C in serum/phenol-free DMEM/F12 media. Approximately 10% of exogenous ¹²⁵I-angiotensinogen was internalized by sheep tubules. Subcellular fractionation revealed that 21 ± 4% of the internalized ¹²⁵I-angiotensinogen associated with the mitochondrial fraction with additional labeling evident in the nucleus (60 ± 7%), endoplasmic reticulum (4 ± 0.5%), and cytosol (15 ± 4%; n = 4). Subsequent studies determined whether mitochondria directly internalized ¹²⁵I-angiotensinogen using isolated mitochondria from renal cortex and human HK-2 proximal tubule cells. Sheep cortical and HK-2 mitochondria internalized ¹²⁵I-angiotensinogen at a comparable rate of (33 ± 9 vs. 21 ± 10 pmol·min^-1·mg protein^-1; n = 3). Lastly, unlabeled angiotensinogen (100 nM) competed for ¹²⁵I-angiotensinogen uptake to a greater extent than human albumin in HK-2 mitochondria (60 ± 2 vs. 16 ± 13%; P < 0.05, n = 3). Collectively, our data demonstrate angiotensinogen import and subsequent trafficking to the mitochondria in proximal tubules. We conclude that this pathway may constitute a source of the angiotensinogen precursor for the mitochondrial expression of angiotensin peptides.

Evidence for a mitochondrial angiotensin-(1-7) system in the kidney

Evidence for an intracellular renin-angiotensin system (RAS) in various cell organelles now includes the endoplasmic reticulum, nucleus, and mitochondria (Mito). Indeed, angiotensin (ANG) AT1 and AT2 receptor subtypes were functionally linked to Mito respiration and nitric oxide production, respectively, in previous studies. We undertook a biochemical analysis of the Mito RAS from male and female sheep kidney cortex. Mito were isolated by differential centrifugation followed by a discontinuous Percoll gradient and were coenriched in Mito membrane markers VDAC and ATP synthase, but not β-actin or cathepsin B. Two distinct renin antibodies identified a 37-kDa protein band in Mito; angiotensinogen (Aogen) conversion was abolished by the inhibitor aliskiren. Mito Aogen was detected by an Aogen antibody to an internal sequence of the protein, but not with an antibody directed against the ANG I N terminus. ANG peptides were quantified by three direct RIAs; mitochondrial ANG II and ANG-(1-7) contents were higher compared with ANG I (23 ± 8 and 58 ± 17 vs. 2 ± 1 fmol/mg protein; P < 0.01, n = 3). 125I-ANG I metabolism primarily revealed the formation of 125I-ANG-(1-7) in Mito that reflects the endopeptidases neprilysin and thimet oligopeptidase. Last, immunoblot studies utilizing the ANG-(1-7)/Mas receptor antibody revealed the protein in isolated Mito from sheep renal cortex. Collectively, the current data demonstrate that Mito actively metabolize the RAS precursor protein Aogen, suggesting that ANG-(1-7) may be generated within Mito to establish an intramitochondrial RAS tone and contribute to renal mitochondrial function.

Renal Blood Flow Response to Angiotensin 1-7 versus Hypertonic Sodium Chloride 7.5% Administration after Acute Hemorrhagic Shock in Rats

Background. Angiotensin 1-7 (Ang1-7) plays an important role in renal circulation. Hemorrhagic shock (HS) may cause kidney circulation disturbance, and this study was designed to investigate the renal blood flow (RBF) response to Ang1-7 after HS. Methods. 27 male Wistar rats were subjected to blood withdrawal to reduce mean arterial pressure (MAP) to 45 mmHg for 45 min. The animals were treated with saline (group 1), Ang1-7 (300 ng·kg(-1) min(-1)), Ang1-7 in hypertonic sodium chloride 7.5% (group 3), and hypertonic solution alone (group 4). Results. MAP was increased in a time-related fashion (P time < 0.0001) in all groups; however, there was a tendency for the increase in MAP in response to hypertonic solution (P = 0.09). Ang1-7, hypertonic solution, or combination of both increased RBF in groups 2-4, and these were significantly different from saline group (P = 0.05); that is, Ang1-7 leads to a significant increase in RBF to 1.35 ± 0.25 mL/min compared with 0.55 ± 0.12 mL/min in saline group (P < 0.05). Conclusion. Although Ang1-7 administration unlike hypertonic solution could not elevate MAP after HS, it potentially could increase RBF similar to hypertonic solution. This suggested that Ang1-7 recovers RBF after HS when therapeutic opportunities of hypertonic solution are limited.

Biochemical evaluation of the renin-angiotensin system: the good, bad, and absolute?

The renin-angiotensin system (RAS) constitutes a key hormonal system in the physiological regulation of blood pressure through peripheral and central mechanisms. Indeed, dysregulation of the RAS is considered a major factor in the development of cardiovascular pathologies, and pharmacological blockade of this system by the inhibition of angiotensin-converting enzyme (ACE) or antagonism of the angiotensin type 1 receptor (AT1R) offers an effective therapeutic regimen. The RAS is now defined as a system composed of different angiotensin peptides with diverse biological actions mediated by distinct receptor subtypes. The classic RAS comprises the ACE-ANG II-AT1R axis that promotes vasoconstriction; water intake; sodium retention; and increased oxidative stress, fibrosis, cellular growth, and inflammation. In contrast, the nonclassical RAS composed primarily of the ANG II/ANG III-AT2R and the ACE2-ANG-(1-7)-AT7R pathways generally opposes the actions of a stimulated ANG II-AT1R axis. In lieu of the complex and multifunctional aspects of this system, as well as increased concerns on the reproducibility among laboratories, a critical assessment is provided on the current biochemical approaches to characterize and define the various components that ultimately reflect the status of the RAS.

The ACE2-angiotensin-(1-7)-Mas axis protects against pancreatic cell damage in cell culture

OBJECTIVE

Angiotensin-converting enzyme 2 (ACE2), its product angiotensin-(1-7), and its receptor Mas have been shown to moderate the adverse effects of the ACE-angiotensin II-AT1 axis in many diseases. The aim of this study was to determine whether the ACE2-Ang-(1-7)-Mas axis could have similar effects in a cell culture model of pancreatic damage.

METHODS

AR42J cells were stimulated with 10 nmol/L cerulein to simulate acute pancreatitis. ACE2, Ang-(1-7), Mas receptor, and PI3K/AKT pathway were measured by quantitative real-time polymerase chain reaction and Western blot analysis.

RESULTS

ACE2 and Mas receptor protein levels in AR42J cells were significantly increased (P < 0.05) between 30 minutes and 6 hours postdisease induction compared with the control group. Mas receptor gene expression was significantly increased (P < 0.05) at 2 hours postdisease induction, and Ang-(1-7) was increased at 6 hours. Treatment with Ang-(1-7) in AR42J cells increased IL-10, decreased IL-6 and IL-8, and reduced the damage to pancreatic cells. Levels of IL-6 and IL-8 in AR42J cell culture were increased significantly after treatment with A779. Moreover, Ang-(1-7) increased the concentration of PI3K/AKT pathway and eNOSin AR42J cells.

CONCLUSIONS

ACE2-angiotensin-(1-7)-Mas axis significantly inhibits pancreatitis in response to decreased inflammatory factors by the activation of endothelial nitric oxide synthase and NO signaling pathways.

A possible mechanism for the progression of chronic renal disease and congestive heart failure

Chronic neurologic diseases such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis, as well as various forms of chronic renal disease and systolic congestive heart failure, are among the most common progressive degenerative disorders encountered in medicine. Each disease follows a nearly relentless course, albeit at varying rates, driven by progressive cell dysfunction and drop-out. The neurologic diseases are characterized by the progressive spread of disease-causing proteins (prion-like proteins) from cell to cell. Recent evidence indicates that cell autonomous renin angiotensin systems operate in heart and kidney, and it is known that functional intracrine proteins can also spread between cells. This then suggests that certain progressive degenerative cardiovascular disorders such as forms of chronic renal insufficiency and systolic congestive heart failure result from dysfunctional renin angiotensin system intracrine action spreading in kidney or myocardium.

Nuclear expression of renin-angiotensin system components in NRK-52E renal epithelial cells

INTRODUCTION

Isolated nuclei of sheep proximal tubules express angiotensin (Ang) receptors as well as angiotensinogen (AGT) and renin. The present study characterized the NRK-52E tubular epithelial cell line for the intracellular expression of renin-angiotensin system (RAS) components.

METHODS

RAS components were visualized by immunofluorescent staining in intact cells and protein expression in isolated nuclei.

RESULTS

An antibody to the angiotensin I (Ang I) sequence of AGT (AI-AGT) revealed only cytosolic staining, while an antibody to an internal sequence of AGT (Int-AGT) revealed primarily nuclear staining. Immunoblots of nuclear and cytosolic fractions confirmed the differential cell staining of AGT. Immunostaining for renin was present on nuclei of intact cells. Nuclear renin activity averaged 0.77±0.05 nmol/mg protein/h that was reduced by aliskiren (0.13±0.01 nmol/mg/h, n=3, p<0.01); trypsin activation increased activity three-fold. Peptide staining localized angiotensin II (Ang II) and Ang-(1-7) to the nucleus and peptide content averaged 59±2 and 57±22 fmol/mg (n=4), respectively. Peptide metabolism in isolated nuclei revealed the processing of Ang I to Ang-(1-7) by thimet oligopeptidase.

CONCLUSION

We conclude that the NRK-52E cells express an intracellular RAS localized to the nucleus and may be an appropriate cell model to elucidate the functional relevance of this system.

MAS promoter regulation: a role for Sry and tyrosine nitration of the KRAB domain of ZNF274 as a feedback mechanism

The ACE2 (angiotensin-converting enzyme 2)/Ang-(1-7) [angiotensin-(1-7)]/MAS axis of the RAS (renin-angiotensin system) has emerged as a pathway of interest in treating both cardiovascular disorders and cancer. The MAS protein is known to bind to and be activated by Ang-(1-7); however, the mechanisms of this activation are just starting to be understood. Although there are strong biochemical data regarding the regulation and activation of the AT1R (angiotensin II type 1 receptor) and the AT2R (angiotensin II type 2 receptor), with models of how AngII (angiotensin II) binds each receptor, fewer studies have characterized MAS. In the present study, we characterize the MAS promoter and provide a potential feedback mechanism that could compensate for MAS degradation following activation by Ang-(1-7). Analysis of ENCODE data for the MAS promoter revealed potential epigenetic control by KRAB (Krüppel-associated box)/KAP-1 (KRAB-associated protein-1). A proximal promoter construct for the MAS gene was repressed by the SOX [SRY (sex-determining region on the Y chromosome) box] proteins SRY, SOX2, SOX3 and SOX14, of which SRY is known to interact with the KRAB domain. The KRAB-KAP-1 complex can be tyrosine-nitrated, causing the dissociation of the KAP-1 protein and thus a potential loss of epigenetic control. Activation of MAS can lead to an increase in nitric oxide, suggesting a feedback mechanism for MAS on its own promoter. The results of the present study provide a more complete view of MAS regulation and, for the first time, suggest biochemical outcomes for nitration of the KRAB domain.

Female spontaneously hypertensive rats are more dependent on ANG (1-7) to mediate effects of low-dose AT1 receptor blockade than males

ANG (1-7) contributes to the blood pressure (BP)-lowering effect of angiotensin receptor blockers (ARBs) in male experimental animals. Females have greater ANG (1-7) concentrations than males; however, the contribution of ANG (1-7) to ARB-mediated decreases in BP in females is unknown. The current study tested the hypothesis that female spontaneously hypertensive rats (SHR) have a larger ANG (1-7) contribution to the BP-lowering effects of the ARB candesartan than male SHR. Twelve-week-old male and female SHR were randomized to receive candesartan (0.5 mg·kg(-1)·day(-1); 7 days), candesartan plus ANG II (200 ng·kg(-1)·min(-1); 7 days), the ANG (1-7) antagonist A-779 (48 μg·kg(-1)·h(-1)) plus candesartan and ANG II. Candesartan decreased basal BP in males and females (baseline vs. candesartan: 142 ± 2 vs. 122 ± 3 and 129 ± 1 vs. 115 ± 1 mmHg, respectively; P < 0.05); however, the decrease was greater in males. ANG II increased BP in males in the presence of candesartan (149 ± 2 mmHg; P < 0.05); candesartan blocked ANG II-induced increases in BP in females (116 ± 1 mmHg). Pretreatment with A-779 abolished candesartan-mediated decreases in BP in females, but not males. A-779 also exacerbated ANG II-induced proteinuria (26 ± 6 vs. 77 ± 11 μg·kg(-1)·day(-1), respectively; P < 0.05) and nephrinuria (20 ± 5 vs. 202 ± 58 μg·kg(-1)·day(-1), respectively; P < 0.05) in candesartan-treated female SHR, with no effect in males. In conclusion, females are more sensitive to the BP-lowering effect of ARBs during ANG II infusion, whereas males are more sensitive under basal conditions. In addition, ANG (1-7) has a greater contribution to ARB-mediated decreases in BP, protein, and nephrin excretion in females relative to males.

Non-canonical signalling and roles of the vasoactive peptides angiotensins and kinins

GPCRs (G-protein-coupled receptors) are among the most important targets for drug discovery due to their ubiquitous expression and participation in cellular events under both healthy and disease conditions. These receptors can be activated by a plethora of ligands, such as ions, odorants, small ligands and peptides, including angiotensins and kinins, which are vasoactive peptides that are classically involved in the pathophysiology of cardiovascular events. These peptides and their corresponding GPCRs have been reported to play roles in other systems and under pathophysiological conditions, such as cancer, central nervous system disorders, metabolic dysfunction and bone resorption. More recently, new mechanisms have been described for the functional regulation of GPCRs, including the transactivation of other signal transduction receptors and the activation of G-protein-independent pathways. The existence of such alternative mechanisms for signal transduction and the discovery of agonists that can preferentially trigger one signalling pathway over other pathways (called biased agonists) have opened new perspectives for the discovery and development of drugs with a higher specificity of action and, therefore, fewer side effects. The present review summarizes the current knowledge on the non-canonical signalling and roles of angiotensins and kinins.

Update on the Angiotensin converting enzyme 2-Angiotensin (1-7)-MAS receptor axis: fetal programing, sex differences, and intracellular pathways

The renin-angiotensin-system (RAS) constitutes an important hormonal system in the physiological regulation of blood pressure. Indeed, dysregulation of the RAS may lead to the development of cardiovascular pathologies including kidney injury. Moreover, the blockade of this system by the inhibition of angiotensin converting enzyme (ACE) or antagonism of the angiotensin type 1 receptor (AT1R) constitutes an effective therapeutic regimen. It is now apparent with the identification of multiple components of the RAS that the system is comprised of different angiotensin peptides with diverse biological actions mediated by distinct receptor subtypes. The classic RAS can be defined as the ACE-Ang II-AT1R axis that promotes vasoconstriction, sodium retention, and other mechanisms to maintain blood pressure, as well as increased oxidative stress, fibrosis, cellular growth, and inflammation in pathological conditions. In contrast, the non-classical RAS composed of the ACE2-Ang-(1-7)-Mas receptor axis generally opposes the actions of a stimulated Ang II-AT1R axis through an increase in nitric oxide and prostaglandins and mediates vasodilation, natriuresis, diuresis, and oxidative stress. Thus, a reduced tone of the Ang-(1-7) system may contribute to these pathologies as well. Moreover, the non-classical RAS components may contribute to the effects of therapeutic blockade of the classical system to reduce blood pressure and attenuate various indices of renal injury. The review considers recent studies on the ACE2-Ang-(1-7)-Mas receptor axis regarding the precursor for Ang-(1-7), the intracellular expression and sex differences of this system, as well as an emerging role of the Ang1-(1-7) pathway in fetal programing events and cardiovascular dysfunction.

New frontiers in the intrarenal Renin-Angiotensin system: a critical review of classical and new paradigms

The renin-angiotensin system (RAS) is well-recognized as one of the oldest and most important regulators of arterial blood pressure, cardiovascular, and renal function. New frontiers have recently emerged in the RAS research well beyond its classic paradigm as a potent vasoconstrictor, an aldosterone release stimulator, or a sodium-retaining hormone. First, two new members of the RAS have been uncovered, which include the renin/(Pro)renin receptor (PRR) and angiotensin-converting enzyme 2 (ACE2). Recent studies suggest that prorenin may act on the PRR independent of the classical ACE/ANG II/AT1 receptor axis, whereas ACE2 may degrade ANG II to generate ANG (1-7), which activates the Mas receptor. Second, there is increasing evidence that ANG II may function as an intracellular peptide to activate intracellular and/or nuclear receptors. Third, currently there is a debate on the relative contribution of systemic versus intrarenal RAS to the physiological regulation of blood pressure and the development of hypertension. The objectives of this article are to review and discuss the new insights and perspectives derived from recent studies using novel transgenic mice that either overexpress or are deficient of one key enzyme, ANG peptide, or receptor of the RAS. This information may help us better understand how ANG II acts, both independently or through interactions with other members of the system, to regulate the kidney function and blood pressure in health and disease.

Normal and abnormal pulmonary arteriovenous shunting: occurrence and mechanisms

Severe cyanosis due to pulmonary arteriovenous fistulas occurs often after a bidirectional superior cavopulmonary anastomosis (Glenn operation) and also in some congenital anomalies in which hepatic venous blood bypasses the lungs in the first passage. Relocation of hepatic flow into the lungs usually causes these fistulas to disappear. Similar pulmonary arteriovenous fistulas are observed in hereditary haemorrhagic telangiectasia, and in liver disease (hepatopulmonary syndrome). There is no convincing identification yet of a responsible hepatic factor that produces these lesions. Candidates for such a factor are reviewed, and the possibility of angiotensin or bradykinin contributing to the fistulas is discussed.

Angiotensin peptides and nitric oxide in cardiovascular disease

SIGNIFICANCE

The renin-angiotensin system (RAS) plays an important role in the normal control of cardiovascular and renal function in the healthy state and is a contributing factor in the development and progression of various types of cardiovascular diseases (CVD), including hypertension, diabetes, and heart failure.

RECENT ADVANCES

Evidence suggests that a balance between activation of the ACE/Ang II/AT1 receptor axis and the ACE2/Ang-(1-7)/Mas receptor axis is important for the function of the heart, kidney, and autonomic nervous system control of the circulation in the normal healthy state. An imbalance in these opposing pathways toward the ACE/Ang II/AT1 receptor axis is associated with CVD. The key component of this imbalance with respect to neural control of the circulation is the negative interaction between oxidative and NO• mechanisms, which leads to enhanced sympathetic tone and activation in disease conditions such as hypertension and heart failure.

CRITICAL ISSUES

The key mechanisms that disrupt normal regulation of Ang II and Ang-(1-7) signaling and promote pathogenesis of CVD at all organ levels remain poorly understood. The reciprocal relation between ACE and ACE2 expression and function suggests they are controlled interdependently at pre- and post-translational levels. Insights from neural studies suggest that an interaction between oxidative and nitrosative pathways may be key.

FUTURE DIRECTIONS

The role of redox mechanisms in the control of expression and activity of RAS enzymes and Ang receptors may provide important insight into the function of local tissue RAS in health and disease.

Antenatal betamethasone exposure is associated with lower ANG-(1-7) and increased ACE in the CSF of adult sheep

Antenatal betamethasone (BM) therapy accelerates lung development in preterm infants but may induce early programming events with long-term cardiovascular consequences. To elucidate these events, we developed a model of programming whereby pregnant ewes are administered BM (2 doses of 0.17 mg/kg) or vehicle at the 80th day of gestation and offspring are delivered at term. BM-exposed (BMX) offspring develop elevated blood pressure; decreased baroreflex sensitivity; and alterations in the circulating, renal, and brain renin-angiotensin systems (RAS) by 6 mo of age. We compared components of the choroid plexus fourth ventricle (ChP4) and cerebral spinal fluid (CSF) RAS between control and BMX male offspring at 6 mo of age. In the choroid plexus, high-molecular-weight renin protein and ANG I-intact angiotensinogen were unchanged between BMX and control animals. Angiotensin-converting enzyme 2 (ACE2) activity was threefold higher than either neprilysin (NEP) or angiotensin 1-converting enzyme (ACE) in control and BMX animals. Moreover, all three enzymes were equally enriched by approximately 2.5-fold in ChP4 brush-border membrane preparations. CSF ANG-(1-7) levels were significantly lower in BMX animals (351.8 ± 76.8 vs. 77.5 ± 29.7 fmol/mg; P < 0.05) and ACE activity was significantly higher (6.6 ± 0.5 vs. 8.9 ± 0.5 fmol·min(-1)·ml(-1); P < 0.05), whereas ACE2 and NEP activities were below measurable limits. A thiol-sensitive peptidase contributed to the majority of ANG-(1-7) metabolism in the CSF, with higher activity in BMX animals. We conclude that in utero BM exposure alters CSF but not ChP RAS components, resulting in lower ANG-(1-7) levels in exposed animals.

Fetal betamethasone exposure attenuates angiotensin-(1-7)-Mas receptor expression in the dorsal medulla of adult sheep

Glucocorticoids including betamethasone (BM) are routinely administered to women entering into early preterm labor to facilitate fetal lung development and decrease infant mortality; however, fetal steroid exposure may lead to deleterious long term consequences. In a sheep model of fetal programming, BM-exposed (BMX) offspring exhibit elevated mean arterial pressure (MAP) and decreased baroreflex sensitivity (BRS) for control of heart rate by 0.5-years of age associated with changes in the circulating and renal renin-angiotensin systems (RAS). In the brain solitary tract nucleus, angiotensin (Ang) II actions through the AT1 receptor oppose the beneficial actions of Ang-(1-7) at the Mas receptor for BRS regulation. Therefore, we examined Ang peptides, angiotensinogen (Aogen), and receptor expression in this brain region of exposed and control offspring of 0.5- and 1.8-years of age. Mas protein expression was significantly lower (>40%) in the dorsal medulla of BMX animals at both ages; however, AT1 receptor expression was not changed. BMX offspring exhibited a higher ratio of Ang II to Ang-(1-7) (2.30±0.36 versus 0.99±0.28; p<0.01) and Ang II to Ang I at 0.5-years. Although total Aogen was unchanged, Ang I-intact Aogen was lower in 0.5-year BMX animals (0.78±0.06 vs. 1.94±0.41; p<0.05) suggesting a greater degree of enzymatic processing of the precursor protein in exposed animals. We conclude that in utero BM exposure promotes an imbalance in the central RAS pathways of Ang II and Ang-(1-7) that may contribute to the elevated MAP and lower BRS in this model.

Podocyte ACE2 protects against diabetic nephropathy

As new components of the renin–angiotensin system (RAS) are elucidated, our understanding of the complexities of their interactions also advances. Previous studies have determined that podocytes possess a local RAS that can generate angiotensin II. Podocytes have also been shown to express angiotensin-converting enzyme 2 (ACE2), which can decrease angiotensin II levels by generation of angiotensin-(1–7). Nadarajah et al. now show that increased podocyte ACE2 activity can attenuate the development of diabetic nephropathy.

Dose-dependent effects of angiotensin-(1-7) on the NHE3 exchanger and [Ca(2+)](i) in in vivo proximal tubules

The acute direct action of angiotensin-(1-7) [ANG-(1-7)] on bicarbonate reabsorption (JHCO(3)(-)) was evaluated by stationary microperfusions on in vivo middle proximal tubules in rats using H ion-sensitive microelectrodes. The control JHCO(3)(-) is 2.82 ± 0.078 nmol·cm(-2)·s(-1) (50). ANG-(1-7) (10(-12) or 10(-9) M) in luminally perfused tubules decreases JHCO(3)(-) (36 or 60%, respectively), but ANG-(1-7) (10(-6) M) increases it (80%). A779 increases JHCO(3)(-) (30%) and prevents both the inhibitory and the stimulatory effects of ANG-(1-7) on it. S3226 decreases JHCO(3)(-) (45%) and changes the stimulatory effect of ANG-(1-7) to an inhibitory effect (30%) but does not affect the inhibitory effect of ANG-(1-7). Our results indicate that in the basal condition endogenous ANG-(1-7) inhibits JHCO(3)(-) and that the biphasic dose-dependent effect of ANG-(1-7) on JHCO(3)(-) is mediated by the Mas receptors via the Na(+)/H(+) exchanger 3 (NHE3). The control value of intracellular Ca(2+) concentration ([Ca(2+)](i)), as monitored using fura-2 AM, is 101 ± 2 nM (6), and ANG-(1-7) (10(-12), 10(-9), or 10(-6)M) transiently (3 min) increases it (by 151, 102, or 52%, respectively). A779 increases the [Ca(2+)](i) (25%) but impairs the stimulatory effect of all doses of ANG-(1-7) on it. The use of BAPTA or thapsigargin suggests a correlation between the ANG-(1-7) dose-dependent effects on [Ca(2+)](i) and JHCO(3)(-). Therefore, the interaction of the opposing dose-dependent effects of ANG II and ANG-(1-7) on [Ca(2+)](i) and JHCO(3)(-) may represent an physiological regulatory mechanism of extracellular volume and/or pH changes. However, whether [Ca(2+)](i) modification is an important direct mechanism for NHE3 activation by these peptides or is a side effect of other signaling pathways will require additional studies.

Angiotensin-converting enzyme 2, angiotensin-(1-7) and Mas: new players of the renin-angiotensin system

Angiotensin (Ang)-(1-7) is now recognized as a biologically active component of the renin-angiotensin system (RAS). Ang-(1-7) appears to play a central role in the RAS because it exerts a vast array of actions, many of them opposite to those attributed to the main effector peptide of the RAS, Ang II. The discovery of the Ang-converting enzyme (ACE) homolog ACE2 brought to light an important metabolic pathway responsible for Ang-(1-7) synthesis. This enzyme can form Ang-(1-7) from Ang II or less efficiently through hydrolysis of Ang I to Ang-(1-9) with subsequent Ang-(1-7) formation by ACE. In addition, it is now well established that the G protein-coupled receptor Mas is a functional binding site for Ang-(1-7). Thus, the axis formed by ACE2/Ang-(1-7)/Mas appears to represent an endogenous counterregulatory pathway within the RAS, the actions of which are in opposition to the vasoconstrictor/proliferative arm of the RAS consisting of ACE, Ang II, and AT(1) receptor. In this brief review, we will discuss recent findings related to the biological role of the ACE2/Ang-(1-7)/Mas arm in the cardiovascular and renal systems, as well as in metabolism. In addition, we will highlight the potential interactions of Ang-(1-7) and Mas with AT(1) and AT(2) receptors.

Upregulation of the angiotensin-converting enzyme 2/angiotensin-(1-7)/Mas receptor axis in the heart and the kidney of growth hormone receptor knock-out mice

OBJECTIVE

Growth hormone (GH) resistance leads to enhanced insulin sensitivity, decreased systolic blood pressure and increased lifespan. The aim of this study was to determine if there is a shift in the balance of the renin-angiotensin system (RAS) towards the ACE2/Ang-(1-7)/Mas receptor axis in the heart and the kidney of a model of GH resistance and retarded aging, the GH receptor knockout (GHR-/-) mouse.

DESIGN

RAS components were evaluated in the heart and the kidney of GHR-/- and control mice by immunohistochemistry and Western blotting (n=12 for both groups).

RESULTS

The immunostaining of Ang-(1-7) was increased in both the heart and the kidney of GHR-/- mice. These changes were concomitant with an increased immunostaining of the Mas receptor and ACE2 in both tissues. The immunostaining of AT1 receptor was reduced in heart and kidney of GHR-/- mice while that of AT2 receptor was increased in the heart and unaltered in the kidney. Ang II, ACE and angiotensinogen levels remained unaltered in the heart and the kidney of GH resistant mice. These results were confirmed by Western blotting and correlated with a significant increase in the abundance of the endothelial nitric oxide synthase in both tissues.

CONCLUSIONS

The shift within the RAS towards an exacerbation of the ACE2/Ang-(1-7)/Mas receptor axis observed in GHR-/- mice could be related to a protective role in cardiac and renal function; and thus, possibly contribute to the decreased incidence of cardiovascular diseases displayed by this animal model of longevity.

Subcellular characteristics of functional intracellular renin-angiotensin systems

The renin-angiotensin system (RAS) is now regarded as an integral component in not only the development of hypertension, but also in physiologic and pathophysiologic mechanisms in multiple tissues and chronic disease states. While many of the endocrine (circulating), paracrine (cell-to-different cell) and autacrine (cell-to-same cell) effects of the RAS are believed to be mediated through the canonical extracellular RAS, a complete, independent and differentially regulated intracellular RAS (iRAS) has also been proposed. Angiotensinogen, the enzymes renin and angiotensin-converting enzyme (ACE) and the angiotensin peptides can all be synthesized and retained intracellularly. Angiotensin receptors (types I and 2) are also abundant intracellularly mainly at the nuclear and mitochondrial levels. The aim of this review is to focus on the most recent information concerning the subcellular localization, distribution and functions of the iRAS and to discuss the potential consequences of activation of the subcellular RAS on different organ systems.

New insights on molecular mechanisms of renal aging

Long-term transplant outcome is importantly influenced by the age of the organ donor. The mechanisms how age carries out its pathophysiological impact on graft survival are still not understood. One major contributing factor for the observed poor performance of old donor kidneys seems in particular the age-related loss in renal regenerative capacity. In this review, we will summarize recent findings about the molecular basis of renal aging with specific focus on the potential role of somatic cellular senescence and mitochondrial aging in renal transplant outcome.

Nonclassical renin-angiotensin system and renal function

The renin-angiotensin system (RAS) constitutes one of the most important hormonal systems in the physiological regulation of blood pressure through renal and nonrenal mechanisms. Indeed, dysregulation of the RAS is considered a major factor in the development of cardiovascular pathologies, including kidney injury, and blockade of this system by the inhibition of angiotensin converting enzyme (ACE) or blockade of the angiotensin type 1 receptor (AT1R) by selective antagonists constitutes an effective therapeutic regimen. It is now apparent with the identification of multiple components of the RAS within the kidney and other tissues that the system is actually composed of different angiotensin peptides with diverse biological actions mediated by distinct receptor subtypes. The classic RAS can be defined as the ACE-Ang II-AT1R axis that promotes vasoconstriction, water intake, sodium retention, and other mechanisms to maintain blood pressure, as well as increase oxidative stress, fibrosis, cellular growth, and inflammation in pathological conditions. In contrast, the nonclassical RAS composed primarily of the AngII/Ang III-AT2R pathway and the ACE2-Ang-(1-7)-AT7R axis generally opposes the actions of a stimulated Ang II-AT1R axis through an increase in nitric oxide and prostaglandins and mediates vasodilation, natriuresis, diuresis, and reduced oxidative stress. Moreover, increasing evidence suggests that these non-classical RAS components contribute to the therapeutic blockade of the classical system to reduce blood pressure and attenuate various indices of renal injury, as well as contribute to normal renal function.

Angiotensin-(1-7) in kidney disease: a review of the controversies

Ang-(1-7) [angiotensin-(1-7)] is a biologically active heptapeptide component of the RAS (renin-angiotensin system), and is generated in the kidney at relatively high levels, via enzymatic pathways that include ACE2 (angiotensin-converting enzyme 2). The biological effects of Ang-(1-7) in the kidney are primarily mediated by interaction with the G-protein-coupled receptor Mas. However, other complex effects have been described that may involve receptor-receptor interactions with AT(1) (angiotensin II type 1) or AT(2) (angiotensin II type 2) receptors, as well as nuclear receptor binding. In the renal vasculature, Ang-(1-7) has vasodilatory properties and it opposes growth-stimulatory signalling in tubular epithelial cells. In several kidney diseases, including hypertensive and diabetic nephropathy, glomerulonephritis, tubulointerstitial fibrosis, pre-eclampsia and acute kidney injury, a growing body of evidence supports a role for endogenous or exogenous Ang-(1-7) as an antagonist of signalling mediated by AT(1) receptors and thereby as a protector against nephron injury. In certain experimental conditions, Ang-(1-7) appears to paradoxically exacerbate renal injury, suggesting that dose or route of administration, state of activation of the local RAS, cell-specific signalling or non-Mas receptor-mediated pathways may contribute to the deleterious responses. Although Ang-(1-7) has promise as a potential therapeutic agent in humans with kidney disease, further studies are required to delineate its signalling mechanisms in the kidney under physiological and pathophysiological conditions.