Renal nuclear angiotensin II receptors in normal and hypertensive rats

Effects of Renal Denervation on the Enhanced Renal Vascular Responsiveness to Angiotensin II in High-Output Heart Failure: Angiotensin II Receptor Binding Assessment and Functional Studies in Ren-2 Transgenic Hypertensive Rats

Detailed mechanism(s) of the beneficial effects of renal denervation (RDN) on the course of heart failure (HF) remain unclear. The study aimed to evaluate renal vascular responsiveness to angiotensin II (ANG II) and to characterize ANG II type 1 (AT₁) and type 2 (AT₂) receptors in the kidney of Ren-2 transgenic rats (TGR), a model of ANG II-dependent hypertension. HF was induced by volume overload using aorto-caval fistula (ACF). The studies were performed two weeks after RDN (three weeks after the creation of ACF), i.e., when non-denervated ACF TGR enter the decompensation phase of HF whereas those after RDN are still in the compensation phase. We found that ACF TGR showed lower renal blood flow (RBF) and its exaggerated response to intrarenal ANG II (8 ng); RDN further augmented this responsiveness. We found that all ANG II receptors in the kidney cortex were of the AT₁ subtype. ANG II receptor binding characteristics in the renal cortex did not significantly differ between experimental groups, hence AT₁ alterations are not responsible for renal vascular hyperresponsiveness to ANG II in ACF TGR, denervated or not. In conclusion, maintained renal AT₁ receptor binding combined with elevated ANG II levels and renal vascular hyperresponsiveness to ANG II in ACF TGR influence renal hemodynamics and tubular reabsorption and lead to renal dysfunction in the high-output HF model. Since RDN did not attenuate the RBF decrease and enhanced renal vascular responsiveness to ANG II, the beneficial actions of RDN on HF-related mortality are probably not dominantly mediated by renal mechanism(s).

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.

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.

Augmentation of angiotensinogen expression in the proximal tubule by intracellular angiotensin II via AT1a/MAPK/NF-кB signaling pathways

Long-term angiotensin II (ANG II) infusion significantly increases ANG II levels in the kidney through two major mechanisms: AT1 receptor-mediated augmentation of angiotensinogen (AGT) expression and uptake of circulating ANG II by the proximal tubules. However, it is not known whether intracellular ANG II stimulates AGT expression in the proximal tubule. In the present study, we overexpressed an intracellular cyan fluorescent ANG II fusion protein (Ad-sglt2-ECFP/ANG II) selectively in the proximal tubule of rats and mice using the sodium and glucose cotransporter 2 (sglt2) promoter. AGT mRNA and protein expression in the renal cortex and 24-h urinary AGT excretion were determined 4 wk following overexpression of ECFP/ANG II in the proximal tubule. Systolic blood pressure was significantly increased with a small antinatriuretic effect in rats and mice with proximal tubule-selective expression of ECFP/ANG II (P < 0.01). AGT mRNA and protein expression in the cortex were increased by >1.5-fold and 61 ± 16% (P < 0.05), whereas urinary AGT excretion was increased from 48.7 ± 5.7 (n = 13) to 102 ± 13.5 (n = 13) ng/24 h (P < 0.05). However, plasma AGT, renin activity, and ANG II levels remained unaltered by ECFP/ANG II. The increased AGT mRNA and protein expressions in the cortex by ECFP/ANG II were blocked in AT1a-knockout (KO) mice. Studies in cultured mouse proximal tubule cells demonstrated involvement of AT1a receptor/MAP kinases/NF-кB signaling pathways. These results indicate that intracellular ANG II stimulates AGT expression in the proximal tubules, leading to increased AGT formation and secretion into the tubular fluid, which contributes to ANG II-dependent hypertension.

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.

G protein-coupled receptor signaling in cardiac nuclear membranes

G protein-coupled receptors (GPCRs) play key physiological roles and represent a significant target for drug development. However, historically, drugs were developed with the understanding that GPCRs as a therapeutic target exist solely on cell surface membranes. More recently, GPCRs have been detected on intracellular membranes, including the nuclear membrane, and the concept that intracellular GPCRs are functional is become more widely accepted. Nuclear GPCRs couple to effectors and regulate signaling pathways, analogous to their counterparts at the cell surface, but may serve distinct biological roles. Hence, the physiological responses mediated by GPCR ligands, or pharmacological agents, result from the integration of their actions at extracellular and intracellular receptors. The net effect depends on the ability of a given ligand or drug to access intracellular receptors, as dictated by its structure, lipophilic properties, and affinity for nuclear receptors. This review will discuss angiotensin II, endothelin, and β-adrenergic receptors located on the nuclear envelope in cardiac cells in terms of their origin, activation, and role in cardiovascular function and pathology.

Angiotensin II blockade: how its molecular targets may signal to mitochondria and slow aging. Coincidences with calorie restriction and mTOR inhibition

Caloric restriction (CR), renin angiotensin system blockade (RAS-bl), and rapamycin-mediated mechanistic target of rapamycin (mTOR) inhibition increase survival and retard aging across species. Previously, we have summarized CR and RAS-bl's converging effects, and the mitochondrial function changes associated with their physiological benefits. mTOR inhibition and enhanced sirtuin and KLOTHO signaling contribute to the benefits of CR in aging. mTORC1/mTORC2 complexes contribute to cell growth and metabolic regulation. Prolonged mTORC1 activation may lead to age-related disease progression; thus, rapamycin-mediated mTOR inhibition and CR may extend lifespan and retard aging through mTORC1 interference. Sirtuins by deacetylating histone and transcription-related proteins modulate signaling and survival pathways and mitochondrial functioning. CR regulates several mammalian sirtuins favoring their role in aging regulation. KLOTHO/fibroblast growth factor 23 (FGF23) contribute to control Ca(2+), phosphate, and vitamin D metabolism, and their dysregulation may participate in age-related disease. Here we review how mTOR inhibition extends lifespan, how KLOTHO functions as an aging suppressor, how sirtuins mediate longevity, how vitamin D loss may contribute to age-related disease, and how they relate to mitochondrial function. Also, we discuss how RAS-bl downregulates mTOR and upregulates KLOTHO, sirtuin, and vitamin D receptor expression, suggesting that at least some of RAS-bl benefits in aging are mediated through the modulation of mTOR, KLOTHO, and sirtuin expression and vitamin D signaling, paralleling CR actions in age retardation. Concluding, the available evidence endorses the idea that RAS-bl is among the interventions that may turn out to provide relief to the spreading issue of age-associated chronic disease.

Angiotensin II: immunohistochemical study in Sardinian pterygium

The Angiotensin II (Ang II) is the principal effector peptide of the RAS system. It has a pleiotropic effect and, beside its physiological role, it has the property to stimulate angiogenesis and activate multiple signalling pathways related to cell proliferation. The purpose of the study was to determinate the Ang II expression and localization in Sardinian pterygium and normal conjunctiva by immunohistochemistry, and its possible involvement in the development and progression of the disease. Twenty-three pterygiums and eleven normal conjunctiva specimens obtained from Sardinian patients, were processed for paraffin embedding and assessed for the immunohistochemi-cal revelation of Ang II. Significant Ang II expression was identified in pterygium and conjunctiva. Particularly, thirteen pterygium specimens (n=13) displayed exclusively moderate to strong nuclear staining; some specimens (n=5) showed exclusively a moderate cytoplasmic immunoreactivity, and few specimens (n=2) displayed moderate to strong immunoreactivity in both cytoplasm and nucleus. Only 3 specimens were negative. Statistical significance difference in respect of nuclear and cytoplasmic localization was observed between normal conjunctiva and pterygium (P=0.020). The results showed a predominant intranuclear localization of Ang II in pterygium epithelial cells, in spite of conjunctiva that mainly showed cytoplasmic localization. These findings suggest a possible role for Ang II in the development and/or progression of pterygium mediated by the activation of local RAS system.

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.

Oxidative stress in hypertension: role of the kidney

SIGNIFICANCE

Renal oxidative stress can be a cause, a consequence, or more often a potentiating factor for hypertension. Increased reactive oxygen species (ROS) in the kidney have been reported in multiple models of hypertension and related to renal vasoconstriction and alterations of renal function. Nicotinamide adenine dinucleotide phosphate oxidase is the central source of ROS in the hypertensive kidney, but a defective antioxidant system also can contribute.

RECENT ADVANCES

Superoxide has been identified as the principal ROS implicated for vascular and tubular dysfunction, but hydrogen peroxide (H2O2) has been implicated in diminishing preglomerular vascular reactivity, and promoting medullary blood flow and pressure natriuresis in hypertensive animals.

CRITICAL ISSUES AND FUTURE DIRECTIONS

Increased renal ROS have been implicated in renal vasoconstriction, renin release, activation of renal afferent nerves, augmented contraction, and myogenic responses of afferent arterioles, enhanced tubuloglomerular feedback, dysfunction of glomerular cells, and proteinuria. Inhibition of ROS with antioxidants, superoxide dismutase mimetics, or blockers of the renin-angiotensin-aldosterone system or genetic deletion of one of the components of the signaling cascade often attenuates or delays the onset of hypertension and preserves the renal structure and function. Novel approaches are required to dampen the renal oxidative stress pathways to reduced O2(-•) rather than H2O2 selectivity and/or to enhance the endogenous antioxidant pathways to susceptible subjects to prevent the development and renal-damaging effects of hypertension.

Immunohistochemical Localization of AT1a, AT1b, and AT2 Angiotensin II Receptor Subtypes in the Rat Adrenal, Pituitary, and Brain with a Perspective Commentary

Angiotensin II increases blood pressure and stimulates thirst and sodium appetite in the brain. It also stimulates secretion of aldosterone from the adrenal zona glomerulosa and epinephrine from the adrenal medulla. The rat has 3 subtypes of angiotensin II receptors: AT1a, AT1b, and AT2. mRNAs for all three subtypes occur in the adrenal and brain. To immunohistochemically differentiate these receptor subtypes, rabbits were immunized with C-terminal fragments of these subtypes to generate receptor subtype-specific antibodies. Immunofluorescence revealed AT1a and AT2 receptors in adrenal zona glomerulosa and medulla. AT1b immunofluorescence was present in the zona glomerulosa, but not the medulla. Ultrastructural immunogold labeling for the AT1a receptor in glomerulosa and medullary cells localized it to plasma membrane, endocytic vesicles, multivesicular bodies, and the nucleus. AT1b and AT2, but not AT1a, immunofluorescence was observed in the anterior pituitary. Stellate cells were AT1b positive while ovoid cells were AT2 positive. In the brain, neurons were AT1a, AT1b, and AT2 positive, but glia was only AT1b positive. Highest levels of AT1a, AT1b, and AT2 receptor immunofluorescence were in the subfornical organ, median eminence, area postrema, paraventricular nucleus, and solitary tract nucleus. These studies complement those employing different techniques to characterize Ang II receptors.

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.

Novel roles of nuclear angiotensin receptors and signaling mechanisms

The renin-angiotensin system (RAS) constitutes an important hormonal system in the physiological regulation of blood pressure. The dysregulation of the RAS is considered a major influence in the development and progression of cardiovascular disease and other pathologies. Indeed, experimental and clinical evidence indicates that blockade of this system with angiotensin-converting enzyme (ACE) inhibitors or angiotensin type 1 receptor (AT1R) antagonists is an effective therapy to attenuate hypertension and diabetic renal injury, and to improve heart failure. Originally defined as a circulating system, multiple tissues express a complete RAS, and compelling evidence now favors an intracellular system involved in cell signaling and function. Within the kidney, intracellular expression of the three predominant ANG receptor subtypes is evident in the nuclear compartment. The ANG type 1 receptor (AT1R) is coupled to the generation of reactive oxygen species (ROS) through the activation of phosphoinositol-3 kinase (PI3K) and PKC. In contrast, both ANG type 2 (AT2R) and ANG-(1-7) (AT7R) receptors stimulate nitric oxide (NO) formation, which may involve nuclear endothelial NO synthase (eNOS). Moreover, blockade of either ACE2-the enzyme that converts ANG II to ANG-(1-7)-or the AT7 receptor exacerbates the ANG II-ROS response on renal nuclei. Finally, in a model of fetal programmed hypertension, the nuclear ROS response to ANG II is enhanced, while both AT2 and AT7 stimulation of NO is attenuated, suggesting that an imbalance in the intracellular RAS may contribute to the development of programming events. We conclude that a functional intracellular or nuclear RAS may have important implications in the therapeutic approaches to cardiovascular disease.

Lessons from in vitro studies and a related intracellular angiotensin II transgenic mouse model

In the classical renin-angiotensin system, circulating ANG II mediates growth stimulatory and hemodynamic effects through the plasma membrane ANG II type I receptor, AT1. ANG II also exists in the intracellular space in some native cells, and tissues and can be upregulated in diseases, including hypertension and diabetes. Moreover, intracellular AT1 receptors can be found associated with endosomes, nuclei, and mitochondria. Intracellular ANG II can function in a canonical fashion through the native receptor and also in a noncanonical fashion through interaction with alternative proteins. Likewise, the receptor and proteolytic fragments of the receptor can function independently of ANG II. Participation of the receptor and ligand in alternative intracellular pathways may serve to amplify events that are initiated at the plasma membrane. We review historical and current literature relevant to ANG II, compared with other intracrines, in tissue culture and transgenic models. In particular, we describe a new transgenic mouse model, which demonstrates that intracellular ANG II is linked to high blood pressure. Appreciation of the diverse, pleiotropic intracellular effects of components of the renin-angiotensin system should lead to alternative disease treatment targets and new therapies.

Jak2-Independent Activation of Stat3 by Intracellular Angiotensin II in Human Mesangial Cells

Ang II is shown to mediate the stimulatory effect of high glucose on TGF-b1 and extracellular matrix proteins in glomerular mesangial cells. Also inhibition of Ang II formation in cell media (extracellular) and lysates (intracellular) blocks high-glucose effects on TGF-b1 and matrix more effectively compared to inhibition of extracellular Ang II alone. To investigate whether intracellular Ang II can stimulate TGF-b1 and matrix independent of extracellular Ang II, cultured human mesangial cells were transfected with Ang II to increase intracellular Ang II levels and its effects on TGF-b1 and matrix proteins were determined. Prior to transfection, cells were treated with candesartan to block extracellular Ang II-induced responses via cell membrane AT1 receptors. Transfection of cells with Ang II resulted in increased levels of intracellular Ang II which was accompanied by increased production of TGF-b1, collagen IV, fibronectin, and cell proliferation as well. On further examination, intracellular Ang II was found to activate Stat3 transcription factor including increased Stat3 protein expression, tyrosine 705 phosphorylation, and DNA-binding activity. Treatment with AG-490, an inhibitor of Jak2, did not block intracellular Ang II-induced Stat3 phosphorylation at tyrosine 705 residue indicating a Jak2-independent mechanism used by intracellular Ang II for Stat3 phosphorylation. In contrast, extracellular Ang II-induced tyrosine 705 phosphorylation of Stat3 was inhibited by AG-490 confirming the presence of a Jak2-dependent pathway. These findings suggest that intracellular Ang II increases TGF-b1 and matrix in human mesangial cells and also activates Stat3 transcription factor without involvement of the extracellular Ang II signaling pathway.

Skeletal muscle myoblasts possess a stretch-responsive local angiotensin signalling system

A paucity of information exists regarding the presence of local renin—angiotensin systems (RASs) in skeletal muscle and associated muscle stem cells. Skeletal muscle and muscle stem cells were isolated from C57BL/6 mice and examined for the presence of a local RAS using quantitative reverse transcriptase polymerase chain reaction (qRT-PCR), immunohistochemistry (IHC), Western blotting and liquid chromatography-mass spectrometry (LC-MS). Furthermore, the effect of mechanical stimulation on RAS member gene expression was analysed. Whole skeletal muscle, primary myoblasts and C2C12 derived myoblasts and myotubes differentially expressed members of the RAS including angiotensinogen, angiotensin-converting enzyme (ACE), angiotensin II (Ang II) type 1 (AT1) and type 2 (AT2). Renin transcripts were never detected, however, mRNA for the ‘renin-like’ enzyme cathepsin D was observed and Ang I and Ang II were identified in cell culture supernatants from proliferating myoblasts. AT1 appeared to co-localise with polymerised actin filaments in proliferating myoblasts and was primarily found in the nucleus of terminally differentiated myotubes. Furthermore, mechanical stretch of proliferating and differentiating C2C12 cells differentially induced mRNA expression of angiotensinogen, AT 1 and AT2. Proliferating and differentiated muscle stem cells possess a local stress-responsive RAS in vitro. The precise function of a local RAS in myoblasts remains unknown. However, evidence presented here suggests that Ang II may be a regulator of skeletal muscle myoblasts.

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

The kidney is an important target for the actions of the renin-angiotensin system (RAS) and this tissue contains a complete local RAS that expresses the bioactive peptides angiotensin II (ANG II) and Ang-(1-7). We find both angiotensin type 1 (AT(1)R) and type 2 (AT(2)R) receptors expressed on renal nuclei that stimulate reactive oxygen species and nitric oxide (NO), respectively. Since Ang-(1-7) also exhibits actions within the kidney and the Ang-(1-7)/Mas receptor protein contains a nuclear localization sequence, we determined the expression of Ang-(1-7) receptors in nuclei isolated from the kidneys of young adult sheep. Binding studies with (125)I-[Sar(1)Thr(8)]-ANG II revealed sites sensitive to the Ang-(1-7) antagonist [d-Ala(7)]-Ang-(1-7) (DALA, A779), as well as to AT(2) and AT(1) antagonists. Incubation of Ang-(1-7) [10(-15) to 10(-9) M] with isolated cortical nuclei elicited a dose-dependent increase in the fluorescence of the NO indicator [4-amino-5-methylamino-2',7']-difluorofluorescein diacetate. The NO response to Ang-(1-7) was abolished by the NO inhibitor N-nitro-l-arginine methyl ester and DALA, but not the AT(1) antagonist losartan or the AT(2) blocker PD123319. Immunofluorescent studies utilizing the Ang-(1-7)/Mas receptor antibody revealed immunolabeling of the proximal tubules but not staining within the glomerulus in cortical sections of the sheep kidney. In the nuclear fraction of isolated proximal tubules, immunoblots revealed the precursor angiotensinogen and renin, as well as functional activity for ACE, ACE2, and neprilysin. We conclude that renal nuclei express Ang-(1-7)/Mas receptors that are functionally linked to NO formation. The marked sensitivity of the intracellular NO response to Ang-(1-7) implicates a functional role of the Ang-(1-7) axis within the nucleus. Moreover, evidence for the precursor and enzymatic components of the RAS within the nuclear compartment of the proximal tubules provides a potential pathway for the intracellular generation of Ang-(1-7).

Angiotensin II type 1 receptor-mediated augmentation of urinary excretion of endogenous angiotensin II in Val5-angiotensin II-infused rats

Rats infused chronically with Val(5)-Angiotensin (Ang) II exhibit increased urinary excretion of endogenous Ile(5)-Ang II by the 12th day of infusion, suggesting the stimulation of endogenous Ang II formation by Val(5)-Ang II infusion. The present study determined the time course of increased urinary Ang II excretion and the effects of Ang II type 1 receptor blockade (candesartan, 2 mg/kg per day) on the urinary excretion rates of Ile(5)-Ang II in Val(5)-Ang II-infused (80 ng/min) rats. Ile(5)-Ang II was separated from Val(5)-Ang II by high-performance liquid chromatography and measured by radioimmunoassay. Systolic blood pressure increased progressively (215+/-2 mm Hg) in Val(5)-Ang II-infused rats (n=5), whereas the candesartan-treated group (n=6) remained normotensive (124+/-3 mm Hg). Candesartan treatment significantly increased the level of plasma Ile(5)-Ang II (24.0+/-7.6 versus 156.9+/-24.6 fmol/mL; P<0.01); in contrast, there was a markedly lower intrarenal Ile(5)-Ang II content (357.9+/-76.6 versus 21.1+/-2.8 fmol/g; P<0.01). Urinary Ile(5)-Ang II excretion rates were elevated by day 9 (2185.7+/-283.2 fmol/24 hours) in Val(5)-Ang II-infused rats but not in candesartan-treated rats (740.6+/-110.3 fmol/24 hours). Thus, Ang II type 1 receptor blockade prevents the increase in urinary excretion of endogenous Ang II in rats subjected to chronic Ang II infusion. These data indicate that the increased urinary excretion of endogenous Ang II in Val(5)-Ang II-infused rats is primarily attributed to Ang II type 1 receptor-dependent secretion into and/or de novo formation of Ang II within the tubular lumen.

Transgenic mice expressing an intracellular fluorescent fusion of angiotensin II demonstrate renal thrombotic microangiopathy and elevated blood pressure

We have generated transgenic mice that express angiotensin II (ANG II) fused downstream of enhanced cyan fluorescent protein, expression of which is regulated by the mouse metallothionein promoter. The fusion protein, which lacks a secretory signal, is retained intracellularly. In the present study, RT-PCR, immunoblot analyses, whole-animal fluorescent imaging, and fluorescent microscopy of murine embryonic fibroblasts confirm expression of the fusion protein in vivo and in vitro. The transgene is expressed in all tissues tested (including brain, heart, kidney, liver, lung, and testes), and radioimmunoassay of plasma samples obtained from transgenic mice indicate no increase in circulating ANG II over wild-type levels, consistent with intracellular retention of the transgene product. Kidneys from transgenic and corresponding wild-type littermates were histologically evaluated, and abnormalities in transgenic mice consistent with thrombotic microangiopathy were observed; microthrombosis was frequently observed within the glomerular capillaries and small vessels. In addition, systolic and diastolic blood pressures, measured by telemetry (n = 8 for each group), were significantly higher in transgenic mice compared with wild-type littermates. Blood pressure of line A male transgenic mice was 125 + or - 1.7 over 97 + or - 1.6 compared with 109 + or - 1.7 over 83 + or - 1.4 mmHg in wild-type littermates (systolic over diastolic). In summary, overexpression of an intracellular fluorescent fusion protein of ANG II correlates with elevated blood pressure and kidney pathology. This transgenic model may be useful to further explore the intracellular renin-angiotensin system and its implication in abnormal kidney function and hypertension.

Angiotensin II, mitochondria, cytoskeletal, and extracellular matrix connections: an integrating viewpoint

Malfunctioning mitochondria strongly participate in the pathogenesis of cardiovascular damage associated with hypertension and other disease conditions. Eukaryotic cells move, assume their shape, resist mechanical stress, accommodate their internal constituents, and transmit signals by relying on the constant remodeling of cytoskeleton filaments. Mitochondrial ATP is needed to support cytoskeletal dynamics. Conversely, mitochondria need to interact with cytoskeletal elements to achieve normal motility, morphology, localization, and function. Extracellular matrix (ECM) quantity and quality influence cellular growth, differentiation, morphology, survival, and mobility. Mitochondria can sense ECM composition changes, and changes in mitochondrial functioning modify the ECM. Maladaptive ECM and cytoskeletal alterations occur in a number of cardiac conditions and in most types of glomerulosclerosis, leading to cardiovascular and renal fibrosis, respectively. Angiotensin II (ANG II), a vasoactive peptide and growth factor, stimulates cytosolic and mitochondrial oxidant production, eventually leading to mitochondrial dysfunction. Also, by inducing integrin/focal adhesion changes, ANG II regulates ECM and cytoskeletal composition and organization and, accordingly, contributes to the pathogenesis of cardiovascular remodeling. ANG II-initiated integrin signaling results in the release of transforming growth factor-β1 (TGF-β1), a cytokine that modifies ECM composition and structure, induces reorganization of the cytoskeleton, and modifies mitochondrial function. Therefore, it is possible to hypothesize that the depression of mitochondrial energy metabolism brought about by ANG II is preceded by ANG II-induced integrin signaling and the consequent derangement of the cytoskeletal filament network and/or ECM organization. ANG II-dependent TGF-β1 release is a potential link between ANG II, ECM, and cytoskeleton derangements and mitochondrial dysfunction. It is necessary to emphasize that the present hypothesis is among many other plausible explanations for ANG II-mediated mitochondrial dysfunction. A potential limitation of this proposal is that the results compiled here were obtained in different cells, tissues, and/or experimental models.

Nuclear angiotensin II type 2 (AT2) receptors are functionally linked to nitric oxide production

Expression of nuclear angiotensin II type 1 (AT(1)) receptors in rat kidney provides further support for the concept of an intracellular renin-angiotensin system. Thus we examined the cellular distribution of renal ANG II receptors in sheep to determine the existence and functional roles of intracellular ANG receptors in higher order species. Receptor binding was performed using the nonselective ANG II antagonist (125)I-[Sar(1),Thr(8)]-ANG II ((125)I-sarthran) with the AT(1) antagonist losartan (LOS) or the AT(2) antagonist PD123319 (PD) in isolated nuclei (NUC) and plasma membrane (PM) fractions obtained by differential centrifugation or density gradient separation. In both fetal and adult sheep kidney, PD competed for the majority of cortical NUC (> or =70%) and PM (> or =80%) sites while LOS competition predominated in medullary NUC (> or =75%) and PM (> or =70%). Immunodetection with an AT(2) antibody revealed a single approximately 42-kDa band in both NUC and PM extracts, suggesting a mature molecular form of the NUC receptor. Autoradiography for receptor subtypes localized AT(2) in the tubulointerstitium, AT(1) in the medulla and vasa recta, and both AT(1) and AT(2) in glomeruli. Loading of NUC with the fluorescent nitric oxide (NO) detector DAF showed increased NO production with ANG II (1 nM), which was abolished by PD and N-nitro-l-arginine methyl ester, but not LOS. Our studies demonstrate ANG II receptor subtypes are differentially expressed in ovine kidney, while nuclear AT(2) receptors are functionally linked to NO production. These findings provide further evidence of a functional intracellular renin-angiotensin system within the kidney, which may represent a therapeutic target for the regulation of blood pressure.

Intracellular ANG II directly induces in vitro transcription of TGF-beta1, MCP-1, and NHE-3 mRNAs in isolated rat renal cortical nuclei via activation of nuclear AT1a receptors

The present study tested the hypothesis that intracellular ANG II directly induces transcriptional effects by stimulating AT(1a) receptors in the nucleus of rat renal cortical cells. Intact nuclei were freshly isolated from the rat renal cortex, and transcriptional responses to ANG II were studied using in vitro RNA transcription assays and semiquantitative RT-PCR. High-power phase-contrast micrographs showed that isolated nuclei were encircled by an intact nuclear envelope and stained strongly by the DNA marker 4',6-diamidino-2-phenylindole, but not by the membrane or endosomal markers. Fluorescein isothiocyanate-labeled ANG II and [(125)I]Val(5)-ANG II binding confirmed the presence of ANG II receptors in the nuclei with a predominance of AT(1) receptors. RT-PCR showed that AT(1a) mRNA expression was threefold greater than AT(1b) receptor mRNAs in these nuclei. In freshly isolated nuclei, ANG II increased in vitro [alpha-(32)P]CTP incorporation in a concentration-dependent manner, and the effect was confirmed by autoradiography and RNA electrophoresis. ANG II markedly increased in vitro transcription of mRNAs for transforming growth factor-beta1 by 143% (P < 0.01), macrophage chemoattractant protein-1 by 89% (P < 0.01), and the sodium and hydrogen exchanger-3 by 110% (P < 0.01). These transcriptional effects of ANG II on the nuclei were completely blocked by the AT(1) receptor antagonist losartan (P < 0.01). By contrast, ANG II had no effects on transcription of angiotensinogen and glyceraldehyde-3-phosphate dehydrogenase mRNAs. Because these transcriptional effects of ANG II in isolated nuclei were induced by ANG II in the absence of cell surface receptor-mediated signaling and completely blocked by losartan, we concluded that ANG II may directly stimulate nuclear AT(1a) receptors to induce transcriptional responses that are associated with tubular epithelial sodium transport, cellular growth and hypertrophy, and proinflammatory cytokines.

The intrarenal renin-angiotensin system: from physiology to the pathobiology of hypertension and kidney disease

In recent years, the focus of interest on the role of the renin-angiotensin system (RAS) in the pathophysiology of hypertension and organ injury has changed to a major emphasis on the role of the local RAS in specific tissues. In the kidney, all of the RAS components are present and intrarenal angiotensin II (Ang II) is formed by independent multiple mechanisms. Proximal tubular angiotensinogen, collecting duct renin, and tubular angiotensin II type 1 (AT1) receptors are positively augmented by intrarenal Ang II. In addition to the classic RAS pathways, prorenin receptors and chymase are also involved in local Ang II formation in the kidney. Moreover, circulating Ang II is actively internalized into proximal tubular cells by AT1 receptor-dependent mechanisms. Consequently, Ang II is compartmentalized in the renal interstitial fluid and the proximal tubular compartments with much higher concentrations than those existing in the circulation. Recent evidence has also revealed that inappropriate activation of the intrarenal RAS is an important contributor to the pathogenesis of hypertension and renal injury. Thus, it is necessary to understand the mechanisms responsible for independent regulation of the intrarenal RAS. In this review, we will briefly summarize our current understanding of independent regulation of the intrarenal RAS and discuss how inappropriate activation of this system contributes to the development and maintenance of hypertension and renal injury. We will also discuss the impact of antihypertensive agents in preventing the progressive increases in the intrarenal RAS during the development of hypertension and renal injury.

Novel roles of intracrine angiotensin II and signalling mechanisms in kidney cells

Angiotensin II (Ang II) has powerful sodium-retaining, growth-promoting and pro- inflammatory properties in addition to its physiological role in maintaining body salt and fluid balance and blood pressure homeostasis. Increased circulating and local tissue Ang II is one of the most important factors contributing to the development of sodium and fluid retention, hypertension and target organ damage. The importance of Ang II in the pathogenesis of hypertension and target organ injury is best demonstrated by the effectiveness of angiotensin- converting enzyme (ACE) inhibitors and AT1-receptor antagonists in treating hypertension and progressive renal disease including diabetic nephropathy. The detrimental effects of Ang II are mediated primarily by the AT1-receptor, while the AT2-receptor may oppose the AT1-receptor. The classical view of the AT1-receptor-mediated effects of Ang II is that the agonist binds its receptors at the cell surface, and following receptor phosphorylation, activates downstream signal transduction pathways and intracellular responses. However, evidence is emerging that binding of Ang II to its cell surface AT1-receptors also activates endocytotic (or internalisation) processes that promote trafficking of both the effector and the receptor into intracellular compartments. Whether internalised Ang II has important intracrine and signalling actions is not well understood. The purpose of this article is to review recent advances in Ang II research with focus on the mechanisms underlying high levels of intracellular Ang II in proximal tubule cells and the contribution of receptor-mediated endocytosis of extracellular Ang II. Further attention is devoted to the question whether intracellular and/or internalised Ang II plays a physiological role by activating cytoplasmic or nuclear receptors in proximal tubule cells. This information may aid future development of drugs to prevent and treat Ang II-induced target organ injury in cardiovascular and renal diseases by blocking intracellular and/or nuclear actions of Ang II.

Effect of ACE inhibitors on angiotensin II in rat mesangial cells cultured in high glucose

This study investigates whether angiotensin converting enzyme (ACE) inhibitors can block high glucose-induced increase of angiotensin II (Ang II) levels in cultured rat mesangial cells. Incubation of cells with high glucose for 5 days increased Ang II in the cell media (extracellular) and cell lysates (intracellular) by approximately 1.5-fold and approximately 2.2-fold, respectively. Captopril blocked high glucose-induced increase in extracellular Ang II levels but not in intracellular Ang II levels. Since the inability of captopril to inhibit intracellular Ang II in intact cells could be explained by failure of the drug to enter the cells, further studies were performed using lysates from cells pre-treated with high glucose for 5 days. Addition of captopril to cell lysates was indeed able to inhibit Ang I conversion to Ang II but only partially. These findings suggest the involvement of non-ACE mechanisms in Ang II formation from Ang I in mesangial cells.

Differential expression of nuclear AT1 receptors and angiotensin II within the kidney of the male congenic mRen2. Lewis rat

We established a new congenic model of hypertension, the mRen(2). Lewis rat and assessed the intracellular expression of angiotensin peptides and receptors in the kidney. The congenic strain was established from the backcross of the (mRen2)27 transgenic rat that expresses the mouse renin 2 gene onto the Lewis strain. The 20-wk-old male congenic rats were markedly hypertensive compared with the Lewis controls (systolic blood pressure: 195 +/- 2 vs. 107 +/- 2 mmHg, P < 0.01). Although plasma ANG II levels were not different between strains, circulating levels of ANG-(1-7) were 270% higher and ANG I concentrations were 40% lower in the mRen2. Lewis rats. In contrast, both cortical (CORT) and medullary (MED) ANG II concentrations were 60% higher in the mRen2. Lewis rats, whereas tissue ANG I was 66 and 84% lower in CORT and MED. For both strains, MED ANG II, ANG I, and ANG-(1-7) were significantly higher than CORT levels. Intracellular ANG II binding distinguished nuclear (NUC) and plasma membrane (PM) receptor using the ANG II radioligand 125I-sarthran. Isolated CORT nuclei exhibited a high density (Bmax >200 fmol/mg protein) and affinity for the sarthran ligand (KD<0.5 nM); the majority of these sites (>95%) were the AT1 receptor subtype. CORT ANG II receptor Bmax and KD values in nuclei were 75 and 50% lower, respectively, for the mRen2. Lewis vs. the Lewis rats. In the MED, the PM receptor density (Lewis: 50 +/- 4 vs. mRen2. Lewis: 21 +/- 5 fmol/mg protein) and affinity (Lewis: 0.31 +/- 0.1 vs. 0.69 +/- 0.1 nM) were lower in the mRen2. Lewis rats. In summary, the hypertensive mRen2. Lewis rats exhibit higher ANG II in both CORT and MED regions of the kidney. Evaluation of intracellular ANG II receptors revealed lower CORT NUC and MED PM AT1 sites in the mRen2. Lewis. The downregulation of AT1 sites in the mRen2. Lewis rats may reflect a compensatory response to dampen the elevated levels of intrarenal ANG II.

Why are angiotensin concentrations so high in the kidney?

PURPOSE OF REVIEW

Recent studies have reported that intrarenal angiotensin II content and angiotensin II concentrations in the proximal tubular fluid and renal interstitial fluid are much greater than the circulating angiotensin II levels. These high intrarenal angiotensin II levels are responsible for regulating renal hemodynamics and tubular transport.

RECENT FINDINGS

Intrarenal angiotensin II levels have been assessed from total tissue contents as well as renal interstitial fluid and proximal tubular fluid concentrations. Total tissue contents expressed per gram of tissue weight are greater than plasma angiotensin II concentrations; tubular fluid concentrations and renal interstitial fluid concentrations are even greater in the range of 3-10 pmoles/ml. In hypertensive states, there is also an increased intracellular accumulation of angiotensin II mediated by angiotensin type 1 receptor-dependent endocytosis. The high intrarenal angiotensin II levels are also caused by the presence of angiotensinogen messenger RNA and protein in the proximal tubule cells. Furthermore, there is positive amplification by which increases in circulating angiotensin II stimulate increased production and secretion of angiotensinogen, which is also manifested as an increased urinary excretion rate.

SUMMARY

The ability of the kidney to generate high intratubular and interstitial concentrations allows the kidney to regulate intrarenal levels in accord with the homeostatic needs for the regulation of renal hemodynamics and tubular reabsorption and the regulation of sodium balance. When inappropriately stimulated, high intrarenal angiotensin II levels contribute to excessive salt and water retention, the development of hypertension, and long-term proliferative effects leading to renal injury.

Megalin binds and internalizes angiotensin II

Megalin is an abundant membrane protein heavily involved in receptor-mediated endocytosis. The major functions of megalin in vivo remain incompletely defined as megalin typically faces specialized milieus such as glomerular filtrate, airways, epididymal fluid, thyroid colloid, and yolk sac fluid, which lack many of its known ligands. In the course of studies on ANG II internalization, we were surprised when only part of the uptake of labeled ANG II into immortalized yolk sac cells (BN-16 cells) was blocked by specific peptide inhibitors and direct competitors of the angiotensin type 1 receptor. This led us to test if megalin was a receptor for ANG II. Four lines of direct evidence demonstrate that megalin and, to a lesser extent, its chaperone protein cubilin are receptors for ANG II. First, in BN-16 cells anti-megalin and anti-cubilin antisera interfere with ANG II uptake. Second, also in BN-16 cells, pure ANG II competes for uptake of a known megalin ligand. Third, in proximal tubule cell brush-border membrane vesicles extracted from mice, anti-megalin antisera interfere with ANG II binding. Fourth, purified megalin binds ANG II directly in surface plasmon resonance experiments. The finding that megalin is a receptor for ANG II suggests a major new function for the megalin pathway in vivo. These results also indicate that ANG II internalization in some tissues is megalin dependent and that megalin may play a role in regulating proximal tubule ANG II levels.

Intrarenal angiotensin II and hypertension

Elevations in intrarenal angiotensin II (Ang II) cause reductions in renal function and sodium excretion that contribute to progressive hypertension and lead to renal and vascular injury. Augmentation of intrarenal Ang II occurs by several processes, leading to levels much greater than can be explained from the circulating levels. In Ang II-dependent hypertension, Ang II is internalized via an AT1 receptor mechanism, but there is also sustained intrarenal production of Ang II. Ang II exerts a positive feedback action on intrarenal angiotensinogen (AGT) mRNA and protein. The increased intrarenal AGT production is associated with increased intrarenal and intracellular Ang II contents and urinary AGT excretion rates. The increased urinary AGT indicates spillover of AGT into distal nephron segments supporting enhanced distal Ang II formation and sodium reabsorption. The augmentation of intrarenal Ang II provides the basis for sustained actions on renal function, sodium excretion, and maintenance of hypertension.