Angiotensin II AT1 receptor internalization, translocation and de novo synthesis modulate cytosolic and nuclear calcium in human vascular smooth muscle cells

Understanding the impact of nuclear-localized GPCRs on cellular signalling

G protein-coupled receptors (GPCRs) have historically been associated with signalling events driven from the plasma membrane. More recently, signalling from endosomes has been recognized as a feature of internalizing receptors. However, there was little consideration given to the notion that GPCRs can be targeted to distinct subcellular locations that did not involve an initial trafficking to the cell surface. Here, we focus on the evidence for and the potential impact of GPCR signalling specifically initiated from the nuclear membrane. We also discuss the possibilities for selectively targeting this and other internal pools of receptors as novel venues for drug discovery.

Pathophysiology and pharmacology of G protein-coupled receptors in the heart

G protein-coupled receptors (GPCRs), comprising the largest superfamily of cell surface receptors, serve as fundamental modulators of cardiac health and disease owing to their key roles in the regulation of heart rate, contractile dynamics, and cardiac function. Accordingly, GPCRs are heavily pursued as drug targets for a wide variety of cardiovascular diseases ranging from heart failure, cardiomyopathy, and arrhythmia to hypertension and coronary artery disease. Recent advancements in understanding the signalling mechanisms, regulation, and pharmacological properties of GPCRs have provided valuable insights that will guide the development of novel therapeutics. Herein, we review the cellular signalling mechanisms, pathophysiological roles, and pharmacological developments of the major GPCRs in the heart, highlighting the β-adrenergic, muscarinic, and angiotensin receptors as exemplar subfamilies.

Hypotension in hereditary cardiomyopathy

It is well accepted that hypertension may lead to the development of heart failure (HF). However, little is known about the development of hypotension that may contribute to the onset of hereditary cardiomyopathy (HCM), thus promoting heart failure and early death. The purpose of this study is to verify whether a decrease in blood pressure takes place during different phases of HCM (asymptomatic, necrosis, hypertrophy, and heart failure). Using the well-known animal model, the UM-X7.1 hamster strain of HCM (HCMH), our results showed the absence of a change in mean arterial pressure (MAP) during the asymptomatic phase preceding the development of necrosis in HCMHs when compared to age-matched normal hamster (NH). However, there was a progressive decrease in MAP that reached its lowest level during the heart failure phase. The MAP during the development of the necrosis phase of HCM was accompanied by a significant increase in the level of the sodium-hydrogen exchanger, NHE1. Treatments with the potent NHE1 inhibitor, EMD 87580 (rimeporide), did not affect MAP of NH. However, treatments with EMD 87580 during the three phases of the development of HCM significantly reversed the hypotension associated with HCM.Our results showed that the development of HCM is associated with hypotension. These results suggest that a decrease in blood pressure could be a biomarker signal for HCM leading to HF and early death. Since the blockade of NHE1 significantly but partially prevented the reduction in MAP, this suggests that other mechanisms can contribute to the development of hypotension in HCM.

Insulin-Induced Cardiomyocytes Hypertrophy That Is Prevented by Taurine via β-alanine-Sensitive Na+-Taurine Symporter

Although insulin-induced cardiac hypertrophy is reported, very little information is available on the hypertrophic effect of insulin on ventricular cardiomyocytes and the regulation of sodium and calcium homeostasis. Taurine is a non-essential amino acid synthesized by cardiomyocytes and the brain and is present in low quantities in many foods, particularly seafood. The purpose of this study was to investigate whether chronic exposure to insulin induces hypertrophy of ventricular cardiomyocytes that are associated with changes in Na⁺ and Ca²⁺ homeostasis and whether taurine pre-treatment prevents these effects. Our results showed that chronic treatment with insulin leads to cardiomyocyte hypertrophy that is associated with an increase in basal intracellular Na⁺ and Ca²⁺ levels. Furthermore, long-term taurine treatment prevents morphological and ionic remodeling induced by insulin. In addition, blocking the Na⁺-taurine co-transporter prevented the taurine antihypertrophic effect. Finally, the insulin-induced remodeling of cardiomyocytes was associated with a decrease in the ratio of phospho-CREB (pCREB) to total cAMP response element binding protein (CREB); taurine prevented this effect. In conclusion, our results show that insulin induces ventricular cardiomyocyte hypertrophy via downregulation of the pCREB/tCREB level and that chronic taurine treatment prevents this effect.

Biomedical nanoparticle design: What we can learn from viruses

Viruses are nanomaterials with a number of properties that surpass those of many synthetic nanoparticles (NPs) for biomedical applications. They possess a rigorously ordered structure, come in a variety of shapes, and present unique surface elements, such as spikes. These attributes facilitate propitious biodistribution, the crossing of complex biological barriers and a minutely coordinated interaction with cells. Due to the orchestrated sequence of interactions of their stringently arranged particle corona with cellular surface receptors they effectively identify and infect their host cells with utmost specificity, while evading the immune system at the same time. Furthermore, their efficacy is enhanced by their response to stimuli and the ability to spread from cell to cell. Over the years, great efforts have been made to mimic distinct viral traits to improve biomedical nanomaterial performance. However, a closer look at the literature reveals that no comprehensive evaluation of the benefit of virus-mimetic material design on the targeting efficiency of nanomaterials exists. In this review we, therefore, elucidate the impact that viral properties had on fundamental advances in outfitting nanomaterials with the ability to interact specifically with their target cells. We give a comprehensive overview of the diverse design strategies and identify critical steps on the way to reducing them to practice. More so, we discuss the advantages and future perspectives of a virus-mimetic nanomaterial design and try to elucidate if viral mimicry holds the key for better NP targeting.

The intracellular renin-angiotensin system: Friend or foe. Some light from the dopaminergic neurons

The renin-angiotensin system (RAS) is one of the oldest hormone systems in vertebrate phylogeny. RAS was initially related to regulation of blood pressure and sodium and water homeostasis. However, local or paracrine RAS were later identified in many tissues, including brain, and play a major role in their physiology and pathophysiology. In addition, a major component, ACE2, is the entry receptor for SARS-CoV-2. Overactivation of tissue RAS leads several oxidative stress and inflammatory processes involved in aging-related degenerative changes. In addition, a third level of RAS, the intracellular or intracrine RAS (iRAS), with still unclear functions, has been observed. The possible interaction between the intracellular and extracellular RAS, and particularly the possible deleterious or beneficial effects of the iRAS activation are controversial. The dopaminergic system is particularly interesting to investigate the RAS as important functional interactions between dopamine and RAS have been observed in the brain and several peripheral tissues. Our recent observations in mitochondria and nucleus of dopaminergic neurons may clarify the role of the iRAS. This may be important for the developing of new therapeutic strategies, since the effects on both extracellular and intracellular RAS must be taken into account, and perhaps better understanding of COVID-19 cell mechanisms.

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.

Nanoparticles Mimicking Viral Cell Recognition Strategies Are Superior Transporters into Mesangial Cells

Poor drug availability in the tissue of interest is a frequent cause of therapy failure. While nanotechnology has developed a plethora of nanocarriers for drug transport, their ability to unequivocally identify cells of interest remains moderate. Viruses are the ideal nanosized carriers as they are able to address their embedded nucleic acids with high specificity to their host cells. Here, it is reported that particles endowed with a virus-like ability to identify cells by three consecutive checks have a superior ability to recognize mesangial cells (MCs) in vivo compared to conventional nanoparticles. Mimicking the initial viral attachment followed by a stepwise target cell recognition process leads to a 5- to 15-fold higher accumulation in the kidney mesangium and extensive cell uptake compared to particles lacking one or both of the viral traits. These results highlight the relevance that the viral cell identification process has on specificity and its application on the targeting strategies of nanomaterials. More so, these findings pave the way for transporting drugs into the mesangium, a tissue that is pivotal in the development of diabetic nephropathy and for which currently no efficient pharmacotherapy exists.

Repeated Porphyromonas gingivalis W83 exposure leads to release pro-inflammatory cytokynes and angiotensin II in coronary artery endothelial cells

The role of Porphyromonas gingivalis (P. gingivalis) or its virulence factors, including lipopolysaccharide (LPS) not only has been related with periodontitis but also with endothelial dysfunction, a key mechanism involved in the genesis of atherosclerosis and hypertension that involving systemic inflammatory markers as angiotensin II (Ang II) and cytokines. This study compares the effect of repeated and unique exposures of P. gingivalis W83 LPS and live bacteria on the production and expression of inflammatory mediators and vasoconstrictor molecules with Ang II. Human coronary artery endothelial cells (HCAEC) were stimulated with purified LPS of P. gingivalis (1.0, 3.5 or 7.0 μg/mL) or serial dilutions of live bacteria (MOI 1: 100 - 1:0,1) at a single or repeated exposure for a time of 24 h. mRNA expression levels of AGTR1, AGTR2, IL-8, IL-1β and MCP-1 were determined by RT-qPCR, and IL-6, MCP-1, IL-8, IL-1β and GM-CSF levels were measured by flow cytometry, ELISA determined Ang II levels. Live bacteria in a single dose increased mRNA levels of AGTR1, and repeated doses increased mRNA levels of IL-8 and IL-1β (p < 0.05). Repeated exposure of live-P. gingivalis induced significant production IL-6, MCP-1 and GM-CSF (p < 0.05). Moreover, these MCP-1, IL-6 and GM-CSF levels were greater than in cells treated with single exposure (p < 0.05), The expression of AGTR1 and production of Ang II induced by live-P. gingivalis W83 showed a vasomotor effect of whole bacteria in HCAEC more than LPS. In conclusion, the findings of this study suggest that repeated exposure of P. gingivalis in HCAEC induces the activation of proinflammatory and vasoconstrictor molecules that lead to endothelial dysfunction being a key mechanism of the onset and progression of arterial hypertension and atherosclerosis.

Increase of NADPH oxidase 3 in heart failure of hereditary cardiomyopathy

During the development of heart failure in humans and animal models, an increase in reactive oxygen species (ROS) levels was observed. However, there is no information whether this increase of ROS is associated with an increase in the density of specific isoforms of NADPH oxidases (NOXs) 1–5. The objective of this study was to verify whether the densities of NOXs 1–5 change during the development of heart failure. Using the well-known model of cardiomyopathic hamsters, the UM-X 7.1 line, a model that strongly resembles the pathology observed in humans from a morphological and functional point of view, our studies showed that, as in humans, NOXs 1–5 are present in both normal and UM-X7.1 hamster hearts. Even though the densities of NOXs 2 and 5 were unchanged, the levels of both NOXs 1 and 4 significantly decreased in UM-X7.1 hamster hearts during heart failure. These changes were accompanied with a significant increase in NOX3 level. These results suggest that, during heart failure, NOX3 plays a vital role in compensating the decrease of NOXs 1 and 4. This increase in NOX3 may also be responsible, at least in part, for the reported increase in ROS levels in heart failure.

Fast relaxation and desensitization of angiotensin II contraction in the pulmonary artery via AT1R and Akt-mediated phosphorylation of muscular eNOS

Angiotensin II (AngII) triggers a transient contraction of pulmonary arteries (PAs) followed by protracted desensitization. Based on the unconventional eNOS expression in PA smooth muscle cells (PASMCs), we hypothesized that activation of smooth muscle eNOS by AngII might be responsible for fast relaxation and tachyphylaxis. Using dual-wire myograph, mechanically endothelium-denuded rat PA [E(-)PA] showed AngII concentration-dependent transient contractions (ΔT_AngII, 95% decay within 1 min), which were abolished by losartan (AT1R antagonist). Neither PD123319 (AT2R antagonist) nor A779 (MasR antagonist) affected ΔT_AngII. When the vessels were pretreated with L-NAME (NOS inhibitor), ODQ (guanylate cyclase inhibitor), or KT5823 (PKG inhibitor), ΔT_AngII of E(-)PA became larger and sustained, whereas nNOS or iNOS inhibitors had no such effect. Immunoblotting of human PASMCs (hPASMCs) also showed eNOS expression, and AngII treatment induced activating phosphorylations of Ser¹¹⁷⁷ in eNOS and of Ser⁴⁷³ in Akt (Ser/Thr protein kinase B), an upstream signal of eNOS phosphorylation. In addition, L-NAME co-treatment promoted AngII-induced Ser¹⁹ phosphorylation of myosin light chain. In hPASMCs, AngII abolished plasma membrane expression of AT1R, and recovery by washout took more than 1 h. Consistent with the data from hPASMCs, the second application of AngII to E(-)PA did not induce contraction, and significant recovery of ΔT_AngII required prolonged washout (> 2 h) in the myography study. L-NAME treatment before the second application facilitated recovery of ΔT_AngII. Muscular eNOS plays an auto-inhibitory role in ΔT_AngII of PAs. The molecular changes investigated in hPASMCs revealed eNOS phosphorylation and internalization of AT1R by AngII. We propose that the rat PA smooth muscle eNOS-induced lusitropy and slow recovery of AT1R from tachyphylaxis might counterbalance the excessive contractile response to AngII, contributing to the distinctive low-pressure pulmonary circulation.

Influenza A virus mimetic nanoparticles trigger selective cell uptake

Poor target cell specificity is currently a major shortcoming of nanoparticles (NPs) used for biomedical applications. It causes significant material loss to off-target sites and poor availability at the intended delivery site. To overcome this limitation, we designed particles that identify cells in a virus-like manner. As a blueprint, we chose a mechanism typical of influenza A virus particles in which ectoenzymatic hemagglutinin activation by target cells is a mandatory prerequisite for binding to a secondary target structure that finally confirms cell identity and allows for uptake of the virus. We developed NPs that probe mesangial cells for the presence of angiotensin-converting enzyme on their surface using angiotensin I (Ang-I) as a proligand. This initial interaction enzymatically transforms Ang-I to a secondary ligand angiotensin II (Ang-II) that has the potential to bind in a second stage to Ang-II type-1 receptor (AT1R). The presence of the receptor confirms the target cell identity and triggers NP uptake via endocytosis. Our virus-mimetic NPs showed outstanding target-cell affinity with picomolar avidities and were able to selectively identify these cells in the presence of 90% off-target cells that carried only the AT1R. Our results demonstrate that the design of virus-mimetic cell interactive NPs is a valuable strategy to enhance NP specificity for therapeutic and diagnostic applications. Our set of primary and secondary targets is particularly suited for the identification of mesangial cells that play a pivotal role in diabetic nephropathy, one of the leading causes of renal failure, for which currently no treatment exists.

Extracellular and intracellular tumor necrosis factor alpha modulates cytosolic and nuclear calcium in human cardiovascular cells 1

Tumor necrosis factor alpha (TNFα) and its type 1 receptor (TNFR1) are implicated in several autoimmune diseases, including rheumatoid arthritis, and are associated with complications at the cardiovascular level. Using human cardiomyocytes, vascular smooth muscle, vascular endothelial, and endocardial endothelial cells coupled to indirect immunofluorescence, our results showed the presence of TNFR1 at the levels of the plasma membrane (including the cytosol) and mostly at the level of the nuclear membranes (including the nucleoplasm). The distribution of the receptor is different between cell types; however, the density is significantly higher at the nuclear level in all 4 cell types. The density of the receptor was the highest in contractile cells including the cardiomyocytes and vascular smooth muscle cells, compared with endothelial cells including endocardial endothelial and vascular endothelial cells. Using the Ca²⁺ probe Fluo-3 coupled to quantitative confocal microscopy, our results showed that the cytokine induced a sustained Ca²⁺ increase in both the cytosol and nucleoplasm of all 4 cell types. This increase was more significant at the nuclear level, mainly in endothelial cells. Our results demonstrated the presence of TNFR1 at both the cell and nuclear membranes of cardiovascular cells, and that its activation modulated both cytosolic and nuclear Ca²⁺.

Angiotensin II induces apoptosis of human right and left ventricular endocardial endothelial cells by activating the AT2 receptor 1

Endocardial endothelial cells (EECs) form a monolayer lining the ventricular cavities. Studies from our laboratory and the literature have shown differences between EECs isolated from the right and left ventricles (EECRs and EECLs, respectively). Angiotensin II (Ang II) was shown to induce apoptosis of different cell types mainly via AT₁ receptor activation. In this study, we verified whether Ang II induces apoptosis of human EECRs and EECLs (hEECRs and hEECLs, respectively) and via which type of receptor. Using the annexin V labeling and in situ TUNEL assays, our results showed that Ang II induced apoptosis of both hEECRs and hEECLs in a concentration-dependent manner. Our results using specific AT₁ and AT₂ receptor antagonists showed that the Ang-II-induced apoptosis in both hEECRs and hEECLs is mediated mainly via the AT₂ receptor. However, AT₁ receptor blockade partially prevented Ang-II-induced apoptosis, particularly in hEECRs. Hence, our results suggest that mainly AT₂ receptors mediate Ang-II-induced apoptosis of hEECRs and hEECLs. The damage of EECs would affect their function as a physical barrier between the blood and cardiomyocytes, thus affecting cardiomyocyte functions.

Physical contact between human vascular endothelial and smooth muscle cells modulates cytosolic and nuclear calcium homeostasis

The interaction between vascular endothelial cells (VECs) and vascular smooth muscle cells (VSMCs) plays an important role in the modulation of vascular tone. There is, however, no information on whether direct physical communication regulates the intracellular calcium levels of human VECs (hVECs) and (or) human VSMCs (hVSMCs). Thus, the objective of the study is to verify whether co-culture of hVECs and hVSMCs modulates cytosolic ([Ca²⁺]_c) and nuclear calcium ([Ca²⁺]_n) levels via physical contact and (or) factors released by both cell types. Quantitative 3D confocal microscopy for [Ca²⁺]_c and [Ca²⁺]_n measurement was performed in cultured hVECs or hVSMCs or in co-culture of hVECs-hVSMCs. Our results show that: (1) physical contact between hVECs-hVECs or hVSMCs-hVSMCs does not affect [Ca²⁺]_c and [Ca²⁺]_n in these 2 cell types; (2) physical contact between hVECs and hVSMCs induces a significant increase only of [Ca²⁺]_n of hVECs without affecting the level of [Ca²⁺]_c and [Ca²⁺]_n of hVSMCs; and (3) preconditioned culture medium of hVECs or hVSMCs does not affect [Ca²⁺]_c and [Ca²⁺]_n of both types of cells. We concluded that physical contact between hVECs and hVSMCs only modulates [Ca²⁺]_n in hVECs. The increase of [Ca²⁺]_n in hVECs may modulate nuclear functions that are calcium dependent.

Intracrine action of angiotensin II in mesangial cells: subcellular distribution of angiotensin II receptor subtypes AT1 and AT2

Biological effects of angiotensin II (AngII) such as regulation of AngII target genes may be triggered by interaction of AngII with intracellular AngII receptor types 1 and 2 (AT₁ and AT₂), defined as intracrine response. The aim of this study was to examine the presence of AT₁ and AT₂ receptors in nuclear membrane of human mesangial cells (HMCs) and evaluate the possible biological effects mediated by intracellular AT₁ through an intracrine mechanism. Subcellular distribution of AT₁ and AT₂ was evaluated by immunofluorescence and by western blot in isolated nuclear extract. Endogenous intracellular synthesis of AngII was stimulated by high glucose (HG). Effects of HG were analyzed in the presence of candesartan, which prevents AngII internalization. Both receptors were found in nuclear membrane. Fluorescein isothiocyanate (FITC)-labeled AngII added to isolated nuclei produced a fluorescence that was reduced in the presence of losartan or PD-123319 and quenched in the presence of both inhibitors simultaneously. HG induced overexpression of fibronectin and increased cell proliferation in the presence of candesartan, indicating an intracrine action of AngII induced by HG. Results showed the presence of nuclear receptors in HMCs that can be activated by AngII through an intracrine response independent of cytoplasmic membrane AngII receptors.

Angiotensin II receptors’ modulation of calcium homeostasis in human vascular endothelial cells

Angiotensin II (AngII) plays an important role in the regulation of vascular smooth muscle function. However, little is known about AngII and its receptors AT1 (AT1R) and AT2 (AT2R) and their modulation of intracellular calcium in vascular endothelial cells (VECs) in general and more particularly of human origin. Using western blots, our results showed that AT1Rs and AT2Rs are present in human VECs (hVECs). Using quantitative 3D confocal imaging, our results showed that AngII is present at the cytoplasmic and nucleoplasmic levels and its relative density is lower in the nucleoplasm. However, both AngII receptors AT1 and AT2 are present at both the plasma and the nuclear envelope membranes (NEMs). AngII (10−10 mol/L) induces a transient decrease of the relative density of cytosolic and nuclear AT1Rs. Blockade of AT1Rs with losartan or blocking protein synthesis with cycloheximide does not prevent internalization and nuclear translocation of AT1Rs but prevents de novo AT1R synthesis. In addition, AngII induces cytosolic and nuclear increases (EC50 near 5 × 10−14 mol/L) of calcium via the activation of AT1Rs. These results demonstrate that both AT1 and AT2 receptors are present in hVECs, and that only AT1Rs seem to undergo transcellular trafficking and modulate cytosolic and nuclear calcium homeostasis.

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.

Fructose-rich diet differently affects angiotensin II receptor content in the nucleus and a plasma membrane fraction of visceral adipose tissue

The adipose tissue renin-angiotensin system (RAS) is proposed to be a pathophysiological link between adipose tissue dysregulation and metabolic disorders induced by a fructose-rich diet (FRD). RAS can act intracellularly. We hypothesized that adipocyte nuclear membranes possess angiotensin receptor types 1 and 2 (AT1R and AT2R), which couple to nuclear signaling pathways and regulate oxidative gene expression under FRD conditions. We analyzed the effect of consumption of 10% fructose solution for 9 weeks on biochemical parameters, adipocyte morphology, and expression of AT1R, AT2R, AT1R-associated protein (ATRAP), NADPH oxidase 4 (NOX4), matrix metalloproteinase-9 (MMP-9), and manganese superoxide dismutase (MnSOD) in adipose tissue of Wistar rats. We detected AT1R and AT2R in the nuclear fraction. FRD reduced the level of angiotensin receptors in the nucleus, while increased AT1R and decreased AT2R levels were observed in the plasma membrane. FRD increased the ATRAP mRNA level and decreased MnSOD mRNA and protein levels. No significant differences were observed for MMP-9 and NOX4 mRNA levels. These findings coincided with hyperleptinemia, elevated blood pressure and triglycerides, and unchanged visceral adipose tissue mass and morphology in FRD rats. Besides providing evidence for nuclear localization of angiotensin receptors in visceral adipose tissue, this study demonstrates the different effects of FRD on AT1R expression in different cellular compartments. Elevated blood pressure and decreased antioxidant capacity in visceral fat of fructose-fed rats were accompanied by an increased AT1R level in the plasma membrane, while upregulation of ATRAP and a decrease of nuclear membrane AT1R suggest an increased capacity for attenuation of excessive AT1R signaling and visceral adiposity.

Role of endothelin-1 and its receptors, ETA and ETB, in the survival of human vascular endothelial cells

Our previous work showed the presence of endothelin-1 (ET-1) receptors, ET_A and ET_B, in human vascular endothelial cells (hVECs). In this study, we wanted to verify whether ET-1 plays a role in the survival of hVECs via the activation of its receptors ET_A and (or) ET_B (ET_AR and ET_BR, respectively). Our results showed that treatment of hVECs with ET-1 prevented apoptosis induced by genistein, an effect that was mimicked by treatment with ET_BR-specific agonist IRL1620. Furthermore, blockade of ET_BR with the selective ET_BR antagonist A-192621 prevented the anti-apoptotic effect of ET-1 in hVECs. However, activation of ET_A receptor alone did not seem to contribute to the anti-apoptotic effect of ET-1. In addition, the anti-apoptotic effect of ET_BR was found to be associated with caspase 3 inhibition and does not depend on the density of this type of receptor. In conclusion, our results showed that ET-1 possesses an anti-apoptotic effect in hVECs and that this effect is mediated, to a great extent, via the activation of ET_BR. This study revealed a new role for ET_BR in the survival of hVECs.

Difference in the response to angiotensin II between left and right ventricular endocardial endothelial cells

Previous studies focused on the right ventricular endocardial endothelial cells (EECRs) and showed that angiotensin II (Ang II) induced increase in cytosolic and nuclear calcium via AT₁ receptor activation. In the present study, we verified whether the response of left EECs (EECLs) to Ang II is different than that of EECRs. Our results showed that the EC₅₀ of the Ang II-induced increase of cytosolic and nuclear calcium in EECLs was 10× higher (around 2 × 10^-13 mol/L) than in EECRs (around 8 × 10^-12 mol/L). The densities of both AT₁ and AT₂ receptors were also higher in EECLs than those previously reported in EECRs. The effect of Ang II was mediated in both cell types via the activation of AT₁ receptors. Treatment with Ang II induced a significant increase of cytosolic and nuclear AT₁ receptors in EECRs, whereas the opposite was found in EECLs. In both cell types, there was a transient increase of cytosolic and nuclear AT₂ receptors following the Ang II treatment. In conclusion, our results showed that both AT₁ and AT₂ receptors densities are higher in both EECLs compared to what was reported in EECRs. The higher density of AT₁ receptors in EECLs compared to REECs may explain, in part, the higher sensitivity of EECLs to Ang II.

New insights into mechanisms of nuclear translocation of G-protein coupled receptors

The G-protein coupled receptor (GPCR) signaling was long believed to involve activation of receptor exclusively at the cell surface, followed by its binding to heterotrimeric G-proteins and arrestins to trigger various intracellular signaling cascades, and termination of signaling by internalization of the receptor. It is now accepted that many GPCRs continue to signal after internalization in the endosomes. Since the breakthrough discoveries of nuclear binding sites for their ligands in 1980s, several GPCRs have been detected at cell nuclei. But mechanisms of nuclear localization of GPCRs, many of whom contain putative nuclear localization signals, remain poorly understood to date. Nevertheless, it is known that subcellular trafficking of GPCRs is regulated by members of Ras superfamily of small GTPases, most notably by Rab and Arf GTPases. In this commentary, we highlight several recent studies which suggest novel roles of small GTPases, importins and sorting nexin proteins in the nuclear translocation of GPCRs via vesicular transport pathways. Taken together with increasing evidence for in vivo functionality of the nuclear GPCRs, better understanding of their trafficking will provide valuable clues in cell biology.

Na+–H+ exchanger inhibitor prevents early death in hereditary cardiomyopathy

Using the UM-X7.1 hereditary cardiomyopathic and muscular dystrophy hamsters (HCMH), we tested the effects of lifelong preventive or curative treatments during the heart failure phase with the NHE-1 inhibitor EMD 87580 (EMD) or with the angiotensin-converting enzyme inhibitor cilazapril on the intracellular Na+ and Ca2+ overloads, elevated level of NHE-1, necrosis, hypertrophy, heart failure, and early death. Our results showed that 310-day pretreatment of 30-day-old HCMHs with EMD significantly prevented cardiac necrosis, cardiomyocyte hypertrophy, and reduced the heart to body mass ratio. This treatment significantly prevented Na+ and Ca2+ overloads and the increase in NHE-1 protein level observed in HCMHs. Importantly, this lifelong preventive treatment significantly decreased the levels of creatine kinase and prevented early death of HCMHs. Curative treatment of hypertrophic 275-day-old HCMHs for 85 days with EMD significantly prevented hypertrophy and early death of HCMHs. However, treatments with cilazapril did not have any significant effects on the cardiac parameters studied or on early death of HCMHs. Our results suggest that the increase in the NHE-1 level and the consequent Na+ and Ca2+ overloads are implicated in the pathological process leading to heart failure and early death in HCMHs, and treatment with the NHE-1 inhibitor is promising for preventing early death in hereditary cardiomyopathy.

Multivalent targeting of AT1 receptors with angiotensin II-functionalized nanoparticles

The angiotensin II receptor type 1 (AT1R) is a G protein-coupled receptor of paramount significance since it is overexpressed in a number of diseased tissues that are highly attractive for nanoparticle targeting. However, it is also expressed at physiological levels in healthy tissue. Multivalent interactions mediated by multiple AT1R-binding moieties per nanoparticle could promote a high binding avidity to AT1R overexpressing cells and concomitantly spare off-target tissue. To investigate the feasibility of this approach, angiotensin II was thiolated and conjugated to PEGylated quantum dots. Nanoparticle binding, uptake and affinity to several cell lines was investigated in detail. The colloids were rapidly taken up by clathrin-mediated endocytosis into AT1R-expressing cells and showed no interaction with receptor negative cells. The EC50 of the thiolated angiotensin II was determined to be 261 nM, whereas the ligand-conjugated Qdots activated the receptor with an EC50 of 8.9 nM. This 30-fold higher affinity of the nanoparticles compared to the unconjugated peptide clearly demonstrated the presence of multivalent effects when using agonist-targeted nanoparticles. Our study provides compelling evidence that, despite being immediately endocytosed, Ang II-coupled nanoparticles exert potent multivalent ligand-receptor interactions that can be used to establish high affinities to an AT1R overexpressing cell and tissue.

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.

Nuclear membrane R-type calcium channels mediate cytosolic ET-1-induced increase of nuclear calcium in human vascular smooth muscle cells

The objective of this work was to verify whether, as in the case of the plasma membrane of human vascular smooth muscle cells (hVSMCs), cytosolic ET-1-induced increase of nuclear calcium is mediated via the activation of calcium influx through the steady-state R-type calcium channel. Pharmacological tools to identify the R-type calcium channels, as well as real 3-D confocal microscopy imaging techniques coupled to calcium fluorescent probes, were used to study the effect of cytosolic ET-1 on nuclear calcium in isolated nuclei of human hepatocytes and plasma membrane perforated hVSMCs. Our results showed that pre-treatment with pertussis toxin (PTX) or cholera toxin (CTX) prevented cytosolic ET-1 (10−9 mol/L) from inducing a sustained increase in nuclear calcium. Furthermore, the L-type calcium channel blocker nifedipine did not prevent cytosolic ET-1 from inducing an increase in nuclear calcium, as opposed to the dual L- and R-type calcium channel blocker isradipine (PN200-110) (in the presence of nifedipine). In conclusion, the preventative effect with PTX and CTX, and the absence of an effect with nifedipine, as well as the blockade by isradipine on cytosolic ET-1-induced increase in nuclear calcium, suggest that this nuclear calcium influx in hVSMCs is due to activation of the steady-state R-type calcium channel. The sarcolemmal and nuclear membrane R-type calcium channels in hVSMCs are involved in ET-1 modulation of vascular tone in physiology and pathology.

Whole-cell and nuclear NADPH oxidases levels and distribution in human endocardial endothelial, vascular smooth muscle, and vascular endothelial cells

The results of our study show that whole-cell and nuclear levels of NADPH oxidase-1 (NOX1) are similar in human vascular endothelial cells (hVECs) and smooth muscle cells (hVSMCs), but lower in human endocardial endothelial cells (hEECs). NOX2 levels were higher in hVECs and lower in hVSMCs. NOX3 levels were the same in hVECs and hVSMCs, but lower in hEECs. NOX4 levels were similar in all of the cell types. NOX4 levels were higher in hVECs than in hVSMCs. NOX5 was also present throughout the 3 cell types, including their nuclei, in the following order: hEECs > hVSMCs > hVECs. The level of basal reactive oxygen species (ROS) was highest in hVECs and lowest in hVSMCs. However, the Ca2+ level was highest in hVSMCs and lowest in hVECs. These findings suggest that all types of NOXs exist in hEECs, hVECs, and hVSMCs, although their density and distribution are cell-type dependent. The density of the different NOXs correlated with the ROS level, but not with the Ca2+ level. In conclusion, NOXs, including NOX3, exist in cardiovascular cells and their nuclei. The nucleus is a major source of ROS generation. The nuclear NOXs may contribute to ROS and Ca2+ homeostasis, which may affect cell remodeling, including the formation of nuclear T-tubules in vascular diseases and aging.

Receptors and ionic transporters in nuclear membranes: new targets for therapeutical pharmacological interventions

Work from our group and other laboratories showed that the nucleus could be considered as a cell within a cell. This is based on growing evidence of the presence and role of nuclear membrane G-protein coupled receptors and ionic transporters in the nuclear membranes of many cell types, including vascular endothelial cells, endocardial endothelial cells, vascular smooth muscle cells, cardiomyocytes, and hepatocytes. The nuclear membrane receptors were found to modulate the functioning of ionic transporters at the nuclear level, and thus contribute to regulation of nuclear ionic homeostasis. Nuclear membranes of the mentioned types of cells possess the same ionic transporters; however, the type of receptors is cell-type dependent. Regulation of cytosolic and nuclear ionic homeostasis was found to be dependent upon a tight crosstalk between receptors and ionic transporters of the plasma membranes and those of the nuclear membrane. This crosstalk seems to be the basis for excitation–contraction coupling, excitation–secretion coupling, and excitation – gene expression coupling. Further advancement in this field will certainly shed light on the role of nuclear membrane receptors and transporters in health and disease. This will in turn enable the successful design of a new class of drugs that specifically target such highly vital nuclear receptors and ionic transporters.

The intracrine renin-angiotensin system

The RAS (renin-angiotensin system) is one of the earliest and most extensively studied hormonal systems. The RAS is an atypical hormonal system in several ways. The major bioactive peptide of the system, AngII (angiotensin II), is neither synthesized in nor targets one specific organ. New research has identified additional peptides with important physiological and pathological roles. More peptides also mean newer enzymatic cascades that generate these peptides and more receptors that mediate their function. In addition, completely different roles of components that constitute the RAS have been uncovered, such as that for prorenin via the prorenin receptor. Complexity of the RAS is enhanced further by the presence of sub-systems in tissues, which act in an autocrine/paracrine manner independent of the endocrine system. The RAS seems relevant at the cellular level, wherein individual cells have a complete system, termed the intracellular RAS. Thus, from cells to tissues to the entire organism, the RAS exhibits continuity while maintaining independent control at different levels. The intracellular RAS is a relatively new concept for the RAS. The present review provides a synopsis of the literature on this system in different tissues.

The zebrafish as a novel animal model to study the molecular mechanisms of mechano-electrical feedback in the heart

Altered mechanical loading of the heart leads to hypertrophy, decompensated heart failure and fatal arrhythmias. However, the molecular mechanisms that link mechanical and electrical dysfunction remain poorly understood. Growing evidence suggest that ventricular electrical remodeling (VER) is a process that can be induced by altered mechanical stress, creating persistent electrophysiological changes that predispose the heart to life-threatening arrhythmias. While VER is clearly a physiological property of the human heart, as evidenced by "T wave memory", it is also thought to occur in a variety of pathological states associated with altered ventricular activation such as bundle branch block, myocardial infarction, and cardiac pacing. Animal models that are currently being used for investigating stretch-induced VER have significant limitations. The zebrafish has recently emerged as an attractive animal model for studying cardiovascular disease and could overcome some of these limitations. Owing to its extensively sequenced genome, high conservation of gene function, and the comprehensive genetic resources that are available in this model, the zebrafish may provide new insights into the molecular mechanisms that drive detrimental electrical remodeling in response to stretch. Here, we have established a zebrafish model to study mechano-electrical feedback in the heart, which combines efficient genetic manipulation with high-precision stretch and high-resolution electrophysiology. In this model, only 90 min of ventricular stretch caused VER and recapitulated key features of VER found previously in the mammalian heart. Our data suggest that the zebrafish model is a powerful platform for investigating the molecular mechanisms underlying mechano-electrical feedback and VER in the heart.

Cysteinyl leukotriene signaling through perinuclear CysLT(1) receptors on vascular smooth muscle cells transduces nuclear calcium signaling and alterations of gene expression

Leukotrienes are pro-inflammatory mediators that are locally produced in coronary atherosclerotic plaques. The response induced by cysteinyl leukotrienes (CysLT) in human coronary arteries may be altered under pathological conditions, such as atherosclerosis. The aim of the present study was to elucidate cysteinyl leukotriene signaling in vascular smooth muscle cells (SMCs) and the effects of inflammation on this process. Immunohistochemical analysis of human carotid endarterectomy samples revealed that the CysLT₁ leukotriene receptor was expressed in areas that also stained positive for α-smooth muscle actin. In human coronary artery smooth muscle cells, lipopolysaccharide significantly upregulated the CysLT₁ receptor and significantly enhanced the changes in intracellular calcium induced by leukotriene C₄ (LTC₄). In these cells, the CysLT₁ receptor exhibited a perinuclear expression, and LTC₄ stimulation predominantly enhanced nuclear calcium increase, which was significantly inhibited by the CysLT₁ receptor antagonist MK-571. Microarray analysis revealed, among a number of significantly upregulated genes after 24 h stimulation of human coronary artery smooth muscle cells with LTC₄, a 5-fold increase in mRNA levels for plasminogen activator inhibitor (PAI)-2. The LTC₄-induced increase in PAI-2 expression was confirmed by real-time quantitative PCR and ELISA and was inhibited by the CysLT₁ receptor antagonist MK-571 and by calcium chelators. In summary, pro-inflammatory stimulation of vascular SMCs upregulated a perinuclear CysLT₁ receptor expression coupled to nuclear calcium signaling and changes in gene expression, such as upregulation of PAI-2. Taken together, these findings suggest a role of nuclear CysLT₁ receptor signaling in vascular SMCs inducing gene expression patterns associated with atherosclerosis.

G protein-coupled receptor signalling in the cardiac nuclear membrane: evidence and possible roles in physiological and pathophysiological function

G protein-coupled receptors (GPCRs) play key physiological roles in numerous tissues, including the heart, and their dysfunction influences a wide range of cardiovascular diseases. Recently, the notion of nuclear localization and action of GPCRs has become more widely accepted. Nuclear-localized receptors may regulate distinct signalling pathways, suggesting that the biological responses mediated by GPCRs are not solely initiated at the cell surface but may result from the integration of extracellular and intracellular signalling pathways. Many of the observed nuclear effects are not prevented by classical inhibitors that exclusively target cell surface receptors, presumably because of their structures, lipophilic properties, or affinity for nuclear receptors. In this topical review, we discuss specifically how angiotensin-II, endothelin, β-adrenergic and opioid receptors located on the nuclear envelope activate signalling pathways, which convert intracrine stimuli into acute responses such as generation of second messengers and direct genomic effects, and thereby participate in the development of cardiovascular disorders.

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.

Nuclear membrane receptors for ET-1 in cardiovascular function

Plasma membrane endothelin type A (ETA) receptors are internalized and recycled to the plasma membrane, whereas endothelin type B (ETB) receptors undergo degradation and subsequent nuclear translocation. Recent studies show that G protein-coupled receptors (GPCRs) and ion transporters are also present and functional at the nuclear membranes of many cell types. Similarly to other GPCRs, ETA and ETB are present at both the plasma and nuclear membranes of several cardiovascular cell types, including human cardiac, vascular smooth muscle, endocardial endothelial, and vascular endothelial cells. The distribution and density of ETARs in the cytosol (including the cell membrane) and the nucleus (including the nuclear membranes) differ between these cell types. However, the localization and density of ET-1 and ETB receptors are similar in these cell types. The extracellular ET-1-induced increase in cytosolic ([Ca]c) and nuclear ([Ca]n) free Ca2+ is associated with an increase of cytosolic and nuclear reactive oxygen species. The extracellular ET-1-induced increase of [Ca]c and [Ca]n as well as intracellular ET-1-induced increase of [Ca]n are cell-type dependent. The type of ET-1 receptor mediating the extracellular ET-1-induced increase of [Ca]c and [Ca]n depends on the cell type. However, the cytosolic ET-1-induced increase of [Ca]n does not depend on cell type. In conclusion, nuclear membranes' ET-1 receptors may play an important role in overall ET-1 action. These nuclear membrane ET-1 receptors could be targets for a new generation of antagonists.

Presence of tubular and reticular structures in the nucleus of human vascular smooth muscle cells

In recent decades, studies addressing nuclear calcium (Ca(2+)) homeostasis and signaling contributed to redefining the role of the nucleus. Yet many aspects of nuclear Ca(2+) signaling and homeostasis are only modestly understood. The present study aimed at investigating the presence of nuclear structures which could contribute to the regulation of nuclear Ca(2+) homeostasis. Using real 3D confocal microscopy, coupled to utilization of appropriate organelle probes and specific antibodies, we identified two entities in the nuclei of intact human vascular smooth muscle cells (hVSMCs) as well as in isolated hVSMCs nuclei. Our results demonstrate the presence of an ER-like nuclear reticular structure in nuclei of intact hVSMCs and in isolated nuclei. Similar to the ER/SR, this structure possesses thapsigargin binding sites, IP(3)Rs and RyRs, thus it was named nucleoplasmic reticulum (NR). Furthermore, nuclear tubular structures were also detected. The latter, similar to the nuclear envelope membranes, possess nuclear pores, thapsigargin binding sites, Angiotensin II receptor AT(2), and are associated with Lamin A/C. However, unlike the NR and the nuclear envelope membranes, these tubular structures disappeared when the nuclei were isolated from the cells. The nuclear tubular structures were called Nuclear T-Tubules (NTTs). Our calcium studies in isolated nuclei utilizing IP(3) and Ryanodine suggest that the NR may participate in nuclear Ca(2+) signaling. On the other hand, presence of nuclear pores on the NTTs suggests that these structures can play a role in cytosol-nucleus exchange. In conclusion, two distinct structures are present in the nucleus of hVSMCs and might play an important role in nuclear Ca(2+) homeostasis.

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.

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.

Nitric oxide and reactive oxygen species in the nucleus revisitedThis review is one of a selection of papers published in a Special Issue on Oxidative Stress in Health and Disease

Recent work from our group showed that the nuclear envelope membranes contain several G protein-coupled receptors, including prostaglandin E2 (EP3R) and endothelin-1 (ET-1) receptors. Activation of EP3R increased endothelial nitric oxide synthase (eNOS) RNA expression in nuclei. eNOS and inducible NOS (iNOS) are reported to also be present at the nuclear level. Furthermore, reactive oxygen species (ROS) were also localized at the nuclear level. In this review, we show that stimulation with NO donor sodium nitroprusside results in an increase of intranuclear calcium that was dependent on guanylate cyclase activation, but independent of MAPK. This increase in nuclear calcium correlated with an increase in nuclear transcription of iNOS. H2O2 and ET-1 increase both cytosolic and nuclear ROS in human endocardial endothelial cells and in human aortic vascular smooth muscle cells. This increase in ROS levels by H2O2 and ET-1 was reversed by the antioxidant glutathione. In addition, our results strongly suggest that cytosolic signalization is not only transmitted to the nucleus but is also generated by the nucleus. Furthermore, we demonstrate that oxidative stress can be sensed by the nucleus. These results highly suggest that ROS formation is also generated directly by the nucleus and that free radicals may contribute to ET-1 regulation of nuclear Ca2+ homeostasis.

Nuclear membrane receptors and channels as targets for drug development in cardiovascular diseases

The use of confocal microscopy has shown that the nucleus plays an important role in excitation-contraction and excitation-secretion coupling of several excitable and nonexcitable cardiovascular cells. It has shown that the nuclear membranes, like the sarcolemmal membrane, possess ionic transporters as well as G protein-coupled receptors (GPCRs), which play a major role in modulating both cytosolic and nuclear ionic homeostasis and nuclear signalling. During spontaneous contraction of heart cells, the increase in cytosolic Ca2+ was immediately followed by a transient increase in nuclear Ca2+. The nuclear Ca2+ rise during excitation-contraction and excitation-secretion coupling was both dependent and independent of changes in cytosolic Ca2+. Nuclear membrane GPCRs, such as those of angiotensin II, neuropeptide Y, and ET-1, were functional and contributed to modulation of nuclear ionic homeostasis via direct and (or) indirect modulation of nuclear membrane ionic transporters such as channels, pumps, and exchangers. The signalling of nuclear membrane GPCRs may also contribute to modulation of gene expression, which may regulate proliferation and remodelling of cells and, indeed, life and death. Direct or indirect targeting of nuclear membrane ionic transporters and GPCRs may constitute a new target for drug action.

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

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

Hepatocyte expression of angiotensin II type 1 receptor is downregulated in advanced human liver fibrosis

BACKGROUND

The renin-angiotensin system plays an important role in fibrosis. Angiotensin II regulates key steps in tissue remodelling processes through angiotensin II type 1 receptor (AT1R). In bile duct-occluded rats, AT1R expression is significantly decreased in advanced liver fibrosis. Therefore, we studied the AT1R expression in human liver tissue during different stages of fibrosis caused by chronic hepatitis C.

METHODS

Liver biopsy specimens from 85 patients were analysed. Real-time reverse transcription polymerase chain reaction was used to quantify AT1R mRNA. Immunohistochemical labelling of AT1R and double staining for AT1R, CD31, CD68, CD3 and fibulin-2 were performed.

RESULTS

AT1R mRNA was significantly reduced in human liver tissue with end-stage cirrhosis compared with early fibrosis. In liver cirrhosis, immunohistochemistry revealed a decreased expression of AT1R on hepatocytes, together with an increased staining intensity on myofibroblasts, vascular endothelium and bile duct epithelium.

CONCLUSION

In conclusion, AT1R expression is downregulated in human liver cirrhosis specimens because of the reduced expression levels on hepatocytes. Therefore, antifibrogenic therapy with AT1R blockers may be most promising if initiated during early stages of fibrosis.

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.

G protein-coupled receptors in and on the cell nucleus: a new signaling paradigm?

Signaling from internalizing and endosomal receptors has almost become a classic GPCR paradigm in the last several years. However, it has become clear in recent years that GPCRs also elicit signals when resident at other subcellular sites including the endoplasmic reticulum, Golgi apparatus, and the nucleus. In this review we discuss the nature, function, and trafficking of nuclear GPCR signaling complexes, as well as potential sources of endogenous and exogenous ligands. Finally, we pose a series of questions that will need to be answered in the coming years to confirm and extend this as a new paradigm for GPCR signaling.

Trafficking of tachykinin neurokinin 3 receptor to nuclei of neurons in the paraventricular nucleus of the hypothalamus following osmotic challenge

Tachykinin neurokinin 3 receptor (NK3R) is a G-protein (GTP binding protein) -coupled receptor that is heavily expressed by magnocellular neurons of the paraventricular nucleus of the hypothalamus (PVN). Osmotic challenge is reported to activate NK3R expressed by magnocellular neurons and cause the NK3R to be internalized to the cytoplasm and perhaps the cell nucleus. In this study we show using immuno-electron microscopy that isolated nuclei from neurons in the PVN of osmotic challenged animals (rats) show a robust labeling for the NK3R. NK3R immunoreactivity was detected by Western blot in isolated nuclei of PVN neurons following the 2 M NaCl injection. No nuclear NK3R immunoreactivity was detected in control animals. NK3R antibody specificity was confirmed by small interfering (SI) RNA technology. This study establishes that the NK3R is trafficked to the nucleus of PVN neurons following a peripheral osmotic challenge.

Effect of caveolin-1 scaffolding peptide and 17β-estradiol on intracellular Ca2+ kinetics evoked by angiotensin II in human vascular smooth muscle cells

Caveolae are identifiable plasma membrane invaginations. The main structural proteins of caveolae are the caveolins. There are three caveolins expressed in mammals, designated Cav-1, Cav-2, and Cav-3. It has been postulated that Cav-1 acts as a scaffold protein for signaling proteins; these include ion channels, enzymes, and other ligand receptors like membrane-associated estrogen receptor (ER)α or ERβ. Caveolae-associated membrane proteins are involved in regulating some of the rapid estrogenic effects of 17β-estradiol. One important system related to the activity of ERα and caveolae is the renin-angiotensin system. Angiotensin II (ANG II) has numerous actions in vascular smooth muscle, including modulation of vasomotor tone, cell growth, apoptosis, phosphatidylinositol 3-kinase (PI3K)/Akt activation, and others. Many proteins associated with caveolae are in close relation with the scaffolding domain of Cav-1 (82–101 amino acid residues). It has been proposed that this peptide may acts as a kinase inhibitor. Therefore, to explore the ability of Cav-1 scaffolding peptide (CSP-1) to regulate ANG II function and analyze the relationship between ERα and ANG II type 1 and 2 (AT1 and AT2) receptors, we decided to study the effects of CSP-1 on ANG II-induced intracellular Ca2+ kinetics and the effect of 17β-estradiol on this modulation using human smooth muscle cells in culture, intracellular Ca2+ concentration measurements, immuno- and double-immunocytochemistry confocal analysis of receptor expression, immunoblot analysis, and immunocoprecipitation assays to demonstrate coexpression. We hypothesized that CSP-1 inhibits ANG II-mediated increases in intracellular Ca2+ concentrations by interfering with intracellular signaling including the PI3K/Akt pathway. We also hypothesize that AT2 receptors associate with Cav-1. Our results show that there is a close association of AT1, AT2, and ERα with Cav-1 in human arterial smooth muscle cells in culture. CSP-1 inhibits ANG II-induced intracellular signaling.

An increased expression of cysteinyl leukotriene 2 receptor in colorectal adenocarcinomas correlates with high differentiation

Increased levels of inflammatory mediators such as cysteinyl leukotrienes (CysLT) have been found in and around tumors. These data, along with our previous observation that the G-protein-coupled receptor CysLT(1)R, which signals survival and proliferation, is up-regulated in colon cancer, suggest an important role for CysLT(1)R in tumor development. The objective of this study was to examine the expression and function of the low-affinity CysLT2 receptor (CysLT2R) in colon cancer. We found lower expression levels of CysLT2R compared with CysLT(1)R in cancer cell lines as well as clinical tumor material. Interestingly, CysLT2R, like CysLT(1)R, was found to be one of few G-protein-coupled receptors that are located both at the plasma membrane and the nuclear membrane. No effect of CysLT2R signaling on cell proliferation was observed, nor was there a correlation between CysLT2R and different proliferation markers such as Ki-67 and cyclooxygenase-2 in the tumor material. Instead, we found that activation of this receptor in colon cancer cells led to cellular differentiation similar to the effects of butyrate treatment. In accordance with this finding, we found that reduced expression of CysLT2R in colon cancer was associated with poor prognosis. We report the novel finding that CysLT2R signaling leads to terminal differentiation of colon carcinoma cells and growth inhibition, and that its expression is relatively high in less malignant forms of colon cancer. These data suggest that the balance between these two receptors is important for tumor progression and disease outcome.

The intracellular renin-angiotensin system: a new paradigm

More than a century after its discovery, the physiological implications of the renin-angiotensin system (RAS) continue to expand, with the identification of new components, functions and subsystems. These advancements have led to better management and understanding of a broad range of cardiovascular and metabolic disorders. The RAS has traditionally been viewed as a circulatory system, involved in the short-term regulation of volume and blood pressure homeostasis. Recently, local RASs have been described as regulators of chronic tissue effects. Most recently, studies have provided evidence of a complete, functional RAS within cells, described as an 'intracrine' or intracellular system. A more comprehensive understanding of the intracellular RAS provides for new strategies in system regulation and a more efficacious approach to the management of RAS-related diseases.

Diverse pathways for nuclear signaling by G protein‐coupled receptors and their ligands

Recent realization that plasma membrane G protein-coupled receptors (GPCRs) may translocate and establish ligand-responsive signaling complexes in other cellular structures has motivated studies of site-specific differences in transductional pathways. GPCRs and their ligands may signal transcription and other nuclear events by two basic mechanisms. The first consists of GPCR-complex activation of messengers that enter the nucleus and there initiate cell-modifying processes without the GPCR leaving the plasma membrane. The second encompasses entry into the nuclear membranes or matrix of either GPCR ligands, which bind to non-GPCR nuclear signaling proteins, proteolytic fragments of GPCRs capable of ligand-independent signaling, or intact GPCRs with transduction-competent factors that directly initiate or regulate transcriptional events. With the second mechanism, often concurrent down-regulation of plasma membrane GPCRs terminates signaling from the cell-surface and moves it into the nuclear domain. Site-dependent differences in signals from the same GPCR provide potentials for unique cellular abnormalities attributable to defective intracellular movement and distribution of a GPCR, site-specific alterations in ligand concentration, and limited intracellular bioavailability of pharmacological agents that can interact specifically with both nuclear and plasma membrane forms of a GPCR.

Intracellular angiotensin II induces cell proliferation independent of AT1 receptor

We recently reported intracrine effects of angiotensin II (ANG II) on cardiac myocyte growth and hypertrophy that were not inhibited by the ANG II type 1 receptor (AT1) antagonist, losartan. To further determine the role of AT1 in intracrine effects, we studied the effect of intracellular ANG II (iANG II) on cell proliferation in native Chinese hamster ovary (CHO) cells and those stably transfected with AT1 receptor (CHO-AT1). CHO-AT1, but not CHO cells, showed enhanced proliferation following exposure to extracellular ANG II (eANG II). However, when transiently transfected with an iANG II expression vector, both cell types showed significantly enhanced proliferation, compared with those transfected with a scrambled peptide. Losartan blocked eANG II-induced cell proliferation, but not that induced by iANG II. To further confirm these findings, CHO and CHO-AT1 cells were stably transfected for iANG II expression (CHO-iA and CHO-AT1-iA, respectively). Cells grown in serum-free medium were counted every 24 h, up to 72 h. CHO-iA and CHO-AT1-iA cells showed a steeper growth curve compared with CHO and CHO-AT1, respectively. These observations were confirmed by Wst-1 assay. The AT1 receptor antagonists losartan, valsartan, telmisartan, and candesartan did not attenuate the faster growth rate of CHO-iA and CHO-AT1-iA cells. eANG II showed an additional growth effect in CHO-AT1-iA cells, which could be selectively blocked by losartan. These data demonstrate that intracrine ANG II can act independent of AT1 receptors and suggest novel intracellular mechanisms of action for ANG II.