Down-regulation of inositol 1,4,5-trisphosphate receptor in cells stably expressing the constitutively active angiotensin II N111G-AT(1) receptor

Beta-arrestins operate an on/off control switch for focal adhesion kinase activity

Focal adhesion kinase (FAK) regulates key biological processes downstream of G protein-coupled receptors (GPCRs) in normal and cancer cells, but the modes of kinase activation by these receptors remain unclear. We report that after GPCR stimulation, FAK activation is controlled by a sequence of events depending on the scaffolding proteins β-arrestins and G proteins. Depletion of β-arrestins results in a marked increase in FAK autophosphorylation and focal adhesion number. We demonstrate that β-arrestins interact directly with FAK and inhibit its autophosphorylation in resting cells. Both FAK-β-arrestin interaction and FAK inhibition require the FERM domain of FAK. Following the stimulation of the angiotensin receptor AT1AR and subsequent translocation of the FAK-β-arrestin complex to the plasma membrane, β-arrestin interaction with the adaptor AP-2 releases inactive FAK from the inhibitory complex, allowing its activation by receptor-stimulated G proteins and activation of downstream FAK effectors. Release and activation of FAK in response to angiotensin are prevented by an AP-2-binding deficient β-arrestin and by a specific inhibitor of β-arrestin/AP-2 interaction; this inhibitor also prevents FAK activation in response to vasopressin. This previously unrecognized mechanism of FAK regulation involving a dual role of β-arrestins, which inhibit FAK in resting cells while driving its activation at the plasma membrane by GPCR-stimulated G proteins, opens new potential therapeutic perspectives in cancers with up-regulated FAK.

Label-free cell signaling pathway deconvolution of angiotensin type 1 receptor reveals time-resolved G-protein activity and distinct AngII and AngIIIIV responses

Angiotensin II (AngII) type 1 receptor (AT₁R) is a G protein-coupled receptor known for its role in numerous physiological processes and its implication in many vascular diseases. Its functions are mediated through G protein dependent and independent signaling pathways. AT₁R has several endogenous peptidic agonists, all derived from angiotensinogen, as well as several synthetic ligands known to elicit biased signaling responses. Here, surface plasmon resonance (SPR) was used as a cell-based and label-free technique to quantify, in real time, the response of HEK293 cells stably expressing the human AT₁R. The goal was to take advantage of the integrative nature of this assay to identify specific signaling pathways in the features of the response profiles generated by numerous endogenous and synthetic ligands of AT₁R. First, we assessed the contributions of Gq, G12/13, Gi, Gβγ, ERK1/2 and β-arrestins pathways in the cellular responses measured by SPR where Gq, G12/Rho/ROCK together with β-arrestins and ERK1/2 were found to play significant roles. More specifically, we established a major role for G12 in the early events of the AT₁R-dependent response, which was followed by a robust ERK1/2 component associated to the later phase of the signal. Interestingly, endogenous AT₁R ligands (AngII, AngIII and AngIV) exhibited distinct responses signatures with a significant increase of the ERK1/2-like components for both AngIII and AngIV, which points toward possibly distinct physiological roles for the later. We also tested AT₁R biased ligands, all of which affected both the early and later events. Our results support SPR-based integrative cellular assays as a powerful approach to delineate the contribution of specific signaling pathways for a given cell response and reveal response differences associated with ligands with distinct pharmacological properties.

Monitoring individual cell-signaling activity using combined metal-clad waveguide and surface-enhanced fluorescence imaging

Validation of a combined metal-clad waveguide and surface enhanced fluorescence imaging platform for live cell imaging.

Silence of inositol 1,4,5-trisphosphate receptor expression decreases cyantraniliprole susceptibility in Bemisia tabaci

Cyantraniliprole is the second active ingredient of anthranilic diamide insecticide, and the first to control a cross-spectrum of chewing and sucking pests such as sweetpotato whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae). The inositol 1,4,5-trisphosphate receptor (IP3R) and ryanodine receptor (RyR) are two families of Ca²⁺ release channels to raise the cytoplasmic free calcium concentration when it is activated by various extracellular stimuli. Previous study proved the over-expression of ryanodine receptor (RyR) was associated with the resistance to diamide insecticides, while the roles of IP3R in diamide resistance remain unknown. In this study, a full-length cDNA sequence of IP3R was cloned from B. tabaci through RT-PCR and rapid amplification of cDNA ends (RACE). The gene (named BtIP3R) is 9922bps long, with an open reading frame (ORF) of 8202bps, encoding a predicted IP3R of 2733 amino acids. The BtIP3R shares 47-78% identity with other insect IP3Rs. Quantitative real-time PCR (qRT-PCR) analysis showed that the BtIP3R was highly expressed in larva, pseudopupa, and female adult, while lowly expressed in egg and male adult. RNA interference (RNAi) by dietary introduction of double-stranded RNA (dsRNA) of BtIP3R significantly reduced the mRNA levels of the target gene in the adult, and dramatically decreased the susceptibility of adult B. tabaci to cyantraniliprole. The results shed light on further understanding of cyantraniliprole resistance mechanisms in B. tabaci as well as in other insects.

Constitutive activity in the angiotensin II type 1 receptor: discovery and applications

The pathophysiological actions of the renin-angiotensin system hormone, angiotensin II (AngII), are mainly mediated by the AngII type 1 (AT1) receptor, a GPCR. The intrinsic spontaneous activity of the AT1 receptor in native tissues is difficult to detect due to its low expression levels. However, factors such as the membrane environment, interaction with autoantibodies, and mechanical stretch are known to increase G protein signaling in the absence of AngII. Naturally occurring and disease-causing activating mutations have not been identified in AT1 receptor. Constitutively active mutants (CAMs) of AT1 receptor have been engineered using molecular modeling and site-directed mutagenesis approaches among which substitution of Asn(111) in the transmembrane helix III with glycine or serine results in the highest basal activity of the receptor. Transgenic animal models expressing the CAM AT1 receptors that mimic various in vivo disease conditions have been useful research tools for discovering the pathophysiological role of AT1 receptor and evaluating the therapeutic potential of inverse agonists. This chapter summarizes the studies on the constitutive activity of AT1 receptor in recombinant as well as physiological systems. The impact of the availability of CAM AT1 receptors on our understanding of the molecular mechanisms underlying receptor activation and inverse agonism is described.

Protein kinase A increases the binding affinity and the Ca2+ release activity of the inositol 1,4,5-trisphosphate receptor type 3 in RINm5F cells

BACKGROUND INFORMATION

In endocrine cells, IP(3)R (inositol 1,4,5-trisphosphate receptor), a ligand-gated Ca2+ channel, plays an important role in the control of intracellular Ca2+ concentration. There are three subtypes of IP(3)R that are distributed differentially among cell types. RINm5F cells express almost exclusively the IP(3)R-3 subtype. The purpose of the present study was to investigate the effect of PKA (protein kinase A) on the activity of IP(3)R-3 in RINm5F cells.

RESULTS

We show that immunoprecipitated IP(3)R-3 is a good substrate for PKA. Using a back-phosphorylation approach, we show that endogenous PKA phosphorylates IP(3)R-3 in intact RINm5F cells. [(3)H]IP(3) (inositol 1,4,5-trisphosphate) binding affinity and IP(3)-induced Ca2+ release activity were enhanced in permeabilized cells that were pre-treated with forskolin or PKA. The PKA-induced enhancement of IP(3)R-3 activity was also observed in intact RINm5F cells stimulated with carbachol and epidermal growth factor, two agonists that use different receptor types to activate phospholipase C.

CONCLUSION

The results of the present study reveal a converging step where the cAMP and the Ca2+ signalling systems act co-operatively in endocrine cell responses to external stimuli.

Renal proximal tubule angiotensin AT1A receptors regulate blood pressure

All components of the renin angiotensin system necessary for ANG II generation and action have been reported to be present in renal proximal convoluted tubules. Given the close relationship between renal sodium handling and blood pressure regulation, we hypothesized that modulating the action of ANG II specifically in the renal proximal tubules would alter the chronic level of blood pressure. To test this, we used a proximal tubule-specific, androgen-dependent, promoter construct (KAP2) to generate mice with either overexpression of a constitutively active angiotensin type 1A receptor transgene or depletion of endogenous angiotensin type 1A receptors. Androgen administration to female transgenic mice caused a robust induction of the transgene in the kidney and increased baseline blood pressure. In the receptor-depleted mice, androgen administration to females resulted in a Cre recombinase-mediated deletion of angiotensin type 1A receptors in the proximal tubule and reduced blood pressure. In contrast to the changes observed at baseline, there was no difference in the blood pressure response to a pressor dose of ANG II in either experimental model. These data, from two separate mouse models, provide evidence that ANG II signaling via the type 1A receptor in the renal proximal tubule is a regulator of systemic blood pressure under baseline conditions.

TNFα-induced IP3R1 expression through TNFR1/PC-PLC/PKCα and TNFR2 signalling pathways in human mesangial cell

BACKGROUND

Little information is available regarding the mechanisms involved in cytokine-induced type 1 inositol 1,4,5-trisphosphate receptor (IP(3)R1) expression in human mesangial cells (HMCs) in the occurrence of hepatorenal syndrome (HRS). Over-expression of IP(3)R1 would enhance both IP(3)-binding activity and sensitivity. We hypothesize that it is possible that increased IP(3)R1, induced by TNFα, would lead to increased IP(3) sensitivity in response to a variety of vasoconstrictors, and promote HMC contraction and thus lead to reduced GFP, promoting HRS occurrence and development.

METHODS

Quantitative real-time polymerase chain reaction and immunoblot assay were used to examine the effects of TNFα on IP(3)R1 mRNA and protein expression. Several inhibitors of kinases, depletion PKC, over-expression of dominant-negative mutant of PKC and non-radioactive PKC assay were used to examine the mechanism of signal transduction of TNFα-regulated IP(3)R1 in HMCs.

RESULTS

TNFα increased IP(3)R1 mRNA and protein expression in HMCs, an effect that was blocked by prolonged incubated chronic PMA, D609, safingol and also by transfection with domain-negative PKCα construct. TNFα activated and promoted autophosphorylation of the PKCα. In addition, both anti-TNFR1 and anti-TNFR2 antibodies blocked TNFα-induced IP(3)R1 protein expression, while only anti-TNFR1 antibodies but not anti-TNFR2 antibodies attenuated TNFα-induced PKCα activity.

CONCLUSIONS

TNFα increased the expression of IP(3)R1, and this was mediated, at least in part, through the TNFR1/PC-PLC/PKCα and TNFR2 signalling pathways in HMCs.

Molecular mechanisms of the antagonistic action between AT1 and AT2 receptors

Although angiotensin II (Ang II) binds to Ang II type 1 (AT(1)) and type 2 (AT(2)) receptors, AT(1) and AT(2) receptors have antagonistic actions with regard to cell signaling. The molecular mechanisms that underlie this antagonism are not well understood. We examined AT(1) and AT(2) receptor-induced signal cross-talk in the cytoplasm and the importance of the hetero-dimerization of AT(1) receptor with AT(2) receptor on the cell surface. AT(1) and AT(2) receptors showed antagonistic effects toward inositol phosphate production. AT(1) receptors mainly formed homo-dimers, rather than hetero-dimers with AT(2) receptor, on the cell surface as determined by immunoprecipitation, and subsequently induced cell signals. AT(2) receptor mainly formed homo-dimers, rather than hetero-dimers with AT(1) receptor, on the cell surface. The expression levels of homo-dimerized AT(1) receptor or AT(2) receptor on the cell surface did not change after treatment with Ang II, the AT(1) receptor antagonist telmisartan or the AT(2) receptor antagonist PD123319. Finally, AT(1) and AT(2) receptor-induced signals antagonized phospholipase C-beta(3) phosphorylation. In conclusion, Ang II-induced AT(1) receptor signals may be mainly blocked by AT(2) receptor signals through their negative cross-talk in the cytoplasm rather than by the hetero-dimerization of both receptors on the cell surface. The proper balance of the expression levels of AT(1) and AT(2) receptors might be critical for the antagonistic action between these receptors.

Angiotensin II AT1 receptor constitutive activation: from molecular mechanisms to pathophysiology

Mutations activating the angiotensin II AT(1) receptor are important to identify and characterize because they give access to the activation mechanisms of this G protein coupled receptor and help to characterize the signaling pathways and the potential pathophysiology of this receptor. The different constitutively activated mutations of the AT(1) receptor are mostly localized in transmembrane domains (TM) and their characterization demonstrated that release of intramolecular constraints and movements among these TM are a necessary step for receptor activation. These mutations constitutively activate Gq linked signaling pathways, receptor internalization and maybe the G protein-independent signaling pathways. Expression of such mutations in mice is linked to hypertension and cardiovascular diseases, but such natural mutations have not been identified in human pathology.

Surface Plasmon Resonance Monitoring of Cell Monolayer Integrity: Implication of Signaling Pathways Involved in Actin-Driven Morphological Remodeling

Morphological changes occurring in individual cells largely influence the physiological functions of various cell layers. The control of barrier function of epithelia and endothelia is a prime example of processes highly dependent on cellular morphology and cell layer integrity. Here, we applied the surface plasmon resonance (SPR) technique to the quantification of cellular activity of an epithelial cell monolayer stimulated by angiotensin II. The analysis of the SPR signal shows reproducible concentration-dependent biphasic responses after cell activation with angiotensin II. Phase-contrast and confocal microscopy imaging was performed to link the SPR signal to molecular and global morphological remodeling. The SPR signal was observed to be in relation with the rapid cell contraction and the subsequent cell spreading observed by phase-contrast microscopy. Additionally, the temporal redistribution of actin, observed by confocal microscopy after angiotensin II stimulation, was also found to be consistent with the SPR signal variation. The modulation of signaling pathways involved in actin-myosin driven cell contraction confirms the direct implication of actin structures in the SPR response. Additionally, we show that the intracellular calcium mobilization associated with angiotensin II stimulation did not produce any significant SPR signal variation. Altogether, our results demonstrate that SPR is a rapid label-free method to study cellular activity and molecular mechanisms implicated in the modulation of the integrity of a cell monolayer in relation to cytoskeleton remodeling with associated cell morphological changes.

Ascorbic acid decreases the binding affinity of the AT1 receptor for angiotensin II

BACKGROUND

Ascorbic acid is an essential vitamin and a powerful antioxidant. Many studies have highlighted the benefits of ascorbic acid for chronic cardiovascular diseases such as hypertension in which angiotensin II (Ang II) plays an significant role. We therefore hypothesized that ascorbic acid could modify the pharmacological properties of the AT(1) receptor for Ang II.

METHODS

Binding studies and Ca(2+) mobilization studies were performed with HEK293 cells stably expressing the AT(1) receptor for Ang II. Smooth muscle contraction studies were performed with rabbit aorta strips that endogenously express the AT(1) receptor.

RESULTS

Scatchard analysis revealed that ascorbic acid decreased the binding affinity of the AT(1) receptor without modifying its maximal binding capacity. Ascorbic acid did not modify the binding affinity of the AT(2) receptor for Ang II or of the UT receptor for urotensin II. In single-cell Ca(2+) imaging assays, ascorbic acid reduced the frequency of intracellular Ca(2+) oscillations induced by a low dose of Ang II. In functional assays, ascorbic acid significantly diminished the contraction of rabbit aorta pre-contracted with Ang II but not those pre-contracted with urotensin II.

CONCLUSIONS

Ascorbic acid decreases the binding affinity of the AT(1) receptor. These results offer a mechanistic explanation for the reported blood pressure lowering effect of ascorbic acid.

Manifold active-state conformations in GPCRs: Agonist-activated constitutively active mutant AT1receptor preferentially couples to Gq compared to the wild-type AT1receptor

The angiotensin II type I (AT(1)) receptor mediates regulation of blood pressure and water-electrolyte balance by Ang II. Substitution of Gly for Asn(111) of the AT(1) receptor constitutively activates the receptor leading to Gq-coupled IP(3) production independent of Ang II binding. The Ang II-activated conformation of the AT1(N111G) receptor was proposed to be similar to that of the wild-type AT(1) receptor, although, various aspects of the Ang II-induced conformation of this constitutively active mutant receptor have not been systematically studied. Here, we provide evidence that the conformation of the active state of the wild-type and the constitutively active AT(1) receptors are different. Upon Ang II binding an activated conformation of the wild-type AT(1) receptor activates G protein and recruits beta-arrestin. In contrast, the agonist-bound AT1(N111G) mutant receptor preferentially couples to Gq and is inadequate in beta-arrestin recruitment.

Protein kinase C decreases the apparent affinity of the inositol 1,4,5-trisphosphate receptor type 3 in RINm5F cells

In non-excitable cells, the inositol 1,4,5-trisphosphate receptor (IP3R) is an intracellular Ca2+ channel which plays a major role in Ca2+ signalling. Three isoforms of IP3R have been identified (IP3R-1, IP3R-2 and IP3R-3) and most cell types express different proportions of each isoform. The differences between the pharmacological and functional properties of the various isoforms of IP3R are poorly known. RINm5F cells who express almost exclusively (approximately 90%) the IP3R-3, represent an interesting model to study this particular isoform. Here, we investigated a regulatory mechanism by which protein kinase C (PKC) may influence IP3R-3-mediated Ca2+ release. With an immunoprecipitation approach we confirmed that RINm5F cells express almost exclusively the IP3R-3 isoform. With an in vitro phosphorylation approach, we showed that the immunopurified IP3R-3 was efficiently phosphorylated by exogenous PKC. With a direct in cellulo approach and an indirect in cellulo back-phosphorylation approach we showed that phorbol-12-myristate-13-acetate (PMA) causes the phosphorylation of IP3R-3 in intact RINm5F cells. In saponin-permeabilized RINm5F cells, 3-induced Ca2+ release was reduced after a pre-treatment with PMA. PMA also reduced the Ca2+ response of intact RINm5F cells stimulated with carbachol and EGF, two agonists that use different receptor types to activate phospholipase C. These results suggest the existence of a negative feedback mechanism involving two components of the Ca2+ signalling cascade, whereby activated PKC dampens IP3R-3 activity.

Temperature-dependent variations of ligand-receptor contact points in hAT1

Photoaffinity labelling is regularly used to investigate proteins, including peptidergic G protein-coupled receptors (GPCR). To this purpose benzophenone photolabels have been widely used to identify many contact residues in ligand-binding pockets. The three-dimensional binding environment of the human angiotensin II type 1 receptor hAT(1) has been determined using an iterative methionine mutagenesis strategy based on the photochemical properties and preferential incorporation of benzophenone onto methionine. This has led to the construction of a ligand-bound receptor structure. The present study investigated the effect of temperature on the accessibility of some of these contact points. The hAT(1) receptor and two representative Met mutants (H256M-hAT(1) and F293M-hAT(1)) from the iterative mutagenesis study were photolabelled with the benzophenone-ligand (125)I-[Sar(1), Bpa(8)]AngII at temperatures ranging from - 15 degrees C to 37 degrees C. Labelled receptors were partially purified and digested with cyanogen bromide to identify the contact points or segments. There were no changes in receptor contacts or labelling in the 7th transmembrane domains (TMD) of hAT(1) and F293M-hAT(1) across the temperature range. However, a temperature-dependent change in the ligand-receptor contact of H256M-hAT(1) was observed. At - 15 degrees C, H256M labelling was identical to that of hAT(1), indicating that the interaction was specific to the 7th TMD. Significant labelling changes were observed at higher temperatures and at 37 degrees C labelling occurred almost exclusively at mutated residue H256M-hAT(1) in the 6th TMD. Simultaneous competitive labelling of different areas of this target protein indicated that the ligand-receptor structure became increasingly fluctual at physiological temperatures, while a more compact, low mobility, and low energy conformation prevailed at low temperatures.

The AT1A receptor "gain-of-function" mutant N111S/delta329 is both constitutively active and hyperreactive to angiotensin II

The renin-angiotensin-aldosterone system (RAAS) is central to cardiovascular and renal physiology. However, there is no animal model in which the activation of the RAAS only reflects the activation of the angiotensin II (ANG II) AT1 receptor. As a first step to developing such a model, we characterized a gain-of-function mutant of the mouse AT1A receptor. This mutant carries two mutations: N111S predicted to activate the receptor constitutively and a COOH-terminal deletion, delta329, expected to reduce receptor internalization and desensitization. We expressed this double mutant (AT1A-N111S/delta329) in heterologous cells. It showed a pharmacological profile consistent with that of other constitutively active mutants. Furthermore, it increased basal production of inositol phosphates, as well as basal cytosolic and nuclear ERK activities. Basal proliferation of cells expressing the mutant was also greater than that of the wild type. The double mutant was poorly internalized and failed to recruit beta-arrestin 2 in the presence of ANG II. It also showed hypersensitive and hyperreactive responses to ANG II for both inositol phosphate production and ERK activation. The additivity of the phenotypes of the two mutations makes this mutant an appropriate candidate to test the physiological consequences of the AT1A receptor activation itself in transgenic animal models.

Expression of constitutively active angiotensin receptors in the rostral ventrolateral medulla increases blood pressure

Angiotensin type 1A (AT(1A)) receptors are expressed within the rostral ventrolateral medulla, and microinjections of angiotensin II into this region increase sympathetic vasomotor tone. To determine the effect of sustained increases in AT(1A) receptor density or activity in rostral ventrolateral medulla, we used radiotelemetry to monitor blood pressure in conscious rats before and after bilateral microinjection into the rostral ventrolateral medulla of adenoviruses encoding the wild-type AT(1A) receptor or a constitutively active version of the receptor (Asn111Gly, [N111G]AT(1A)). The constitutively active receptor signals in the absence of angiotensin II. Adenovirus-directed receptor expression was extensively characterized both in vitro and in vivo. We established that adenoviral infection was limited to the rostral ventrolateral medulla and that receptor expression was sustained for > or =10 days; we also observed that adenoviral transgene expression occurs in glia, with no transgene expression observed in neurons of the rostral ventrolateral medulla. Rats receiving the wild-type AT(1A) receptor showed no change in blood pressure, whereas animals receiving the [N111G]AT(1A) receptor displayed an increase in blood pressure that persisted for 3 to 4 days before returning to basal levels. These data indicate that increased AT(1A) receptor activity (not just overexpression) is a primary determinant of efferent drive from rostral ventrolateral medulla and reveal counterregulatory processes that moderate AT(1A) receptor actions at this crucial relay point. More importantly, they imply that constitutive receptor signaling in glia of the rostral ventrolateral medulla can modulate the activity of adjacent neurons to change blood pressure.