Angiotensin II-induced cyclooxygenase 2 expression in rat aorta vascular smooth muscle cells does not require heterotrimeric G protein activation

Endocytosis and signaling of angiotensin II type 1 receptor

A vasoactive octapeptide angiotensin II (Ang II) hormone is the key regulator of the renin-angiotensin system (RAS). It binds with the two different plasma membrane receptors like angiotensin II type 1 (AT1) and type 2 (AT2) and consequence various biological responses occur. Further, AT1 has two subtypes such as AT1A and AT1B. These angiotensin receptors are classified to be G protein-coupled receptors (GPCRs). The main constituent of RAS is the AT1 receptor (AT1R), and its activation, signal transduction, and regulation have been extensively studied. After Ang II stimulation, the ligand-receptor complexes internalized and trafficked through the early endosome, recycling endosome, and some receptors skipped the recycling endosome and trafficked to the lysosome for metabolic degradation. Moreover, some short sequence motifs located in the carboxyl-terminus (CT) of the receptor play a vital role in the internalization, phosphorylation, subcellular trafficking, signaling, and desensitization. Furthermore, in endocytosis, the various proteins interact with the CT region of the receptor. This chapter highlights the basic mechanism of AT1 receptor internalization, trafficking and signaling in both physiological and pathophysiological conditions.

β-Arrestin-biased AT1R stimulation promotes extracellular matrix synthesis in renal fibrosis

The renin-angiotensin system plays a critical role in the progression of renal fibrosis. Angiotensin II type 1 receptor (AT1R) belongs to the B family of the G protein-coupled receptor (GPCR) family. β-Arrestins are known as negative regulators of GPCRs. Recently, β-arrestins have been found to regulate multiple intracellular signaling pathways independent of G proteins. In this study we investigated the role of β-arrestins in regulating extracellular matrix (ECM) synthesis in renal fibrosis. The rat kidney fibroblast cell line (NRK-49F) was treated with the β-arrestin biased agonist [1-sar, 4, 8-ile]angiotensin II (SII), which does not initiate AT1R-G protein signaling. The cells were transfected with recombinant adenoviruses expressing β-arrestin-2 gene or small-interfering RNA (siRNA) targeting β-arrestin-2. The unilateral ureteral obstruction (UUO) model was used in vivo. mRNA and protein levels of β-arrestin-2, not β-arrestin-1, were significantly upregulated in the UUO kidney tissues. SII induced the tight binding of β-arrestin-2 with AT1R. SII increased the synthesis of collagen I and fibronectin in NRK-49F, which were abolished when pretreated with candesartan (AT1R blocker). Transfection of siRNA targeting β-arrestin-2 decreased the effects of SII on ECM synthesis. Overexpression of β-arrestin-2 enhanced SII-stimulated ECM synthesis. SII induced ERK1/2 phosphorylation in NRK-49F. Transfection of siRNA targeting β-arrestin-2 inhibited ERK phosphorylation. Overexpression of β-arrestin-2 increased ERK1/2 phosphorylation. Our study first showed that AT1R-β-arrestin-2 pathway signaling plays an important role in renal fibrosis, although it was previously believed that the AT1R-G protein pathway plays a major role. Targeting β-arrestin-2 may be a potential therapeutic agent for renal fibrosis.

Clathrin-dependent internalization of the angiotensin II AT₁A receptor links receptor internalization to COX-2 protein expression in rat aortic vascular smooth muscle cells

The major effects of Angiotensin II (AngII) in vascular tissue are mediated by AngII AT1A receptor activation. Certain effects initiated by AT1A receptor activation require receptor internalization. In rat aortic vascular smooth muscle cells (RASMC), AngII stimulates cyclooxygenase 2 protein expression. We have previously shown this is mediated by β-arrestin-dependent receptor internalization and NF-κB activation. In this study, a specific inhibitor of clathrin-mediated endocytosis (CME), pitstop-2, was used to test the hypothesis that clathrin-dependent internalization of activated AT1A receptor mediates NF-κB activation and subsequent cyclooxygenase 2 expression. Radioligand binding assays, real time qt-PCR and immunoblotting were used to document the effects of pitstop-2 on AngII binding and signaling in RASMC. Laser scanning confocal microscopy (LSCM) was used to image pitstop-2׳s effects on AT1 receptor/GFP internalization in HEK-293 cells and p65 NF-κB nuclear localization in RASMC. Pitstop-2 significantly inhibited internalization of AT1A receptor (44.7% ± 3.1% Control vs. 13.2% ± 8.3% Pitstop-2; n=3) as determined by radioligand binding studies in RASMC. Studies utilizing AT1A receptor/GFP expressed in HEK 293 cells and LSCM confirmed these findings. Pitstop-2 significantly inhibited AngII-induced p65 NF-κB phosphorylation and nuclear localization, COX-2 message and protein expression in RASMC without altering activation of p42/44 ERK or TNFα signaling. Pitstop-2, a specific inhibitor of clathrin-mediated endocytosis, confirms that internalization of activated AT1A receptor mediates AngII activation of cyclooxygenase 2 expression in RASMC. These data provide support for additional intracellular signaling pathways activated through β-arrestin mediated internalization of G protein-coupled receptors, such as AT1A receptors.

Angiotensin II activates NF-κB through AT1A receptor recruitment of β-arrestin in cultured rat vascular smooth muscle cells

Activation of the angiotensin type 1A receptor (AT1AR) in rat aorta vascular smooth muscle cells (RASMC) results in increased synthesis of the proinflammatory enzyme cyclooxygenase-2 (COX-2). We previously showed that nuclear localization of internalized AT1AR results in activation of transcription of the gene for COX-2, i.e., prostaglandin-endoperoxide synthase-2. Others have suggested that ANG II stimulation of COX-2 protein synthesis is mediated by NF-κB. The purpose of the present study was to examine the interrelationship between AT1AR activation, β-arrestin recruitment, and NF-κB activation in the ability of ANG II to increase COX-2 protein synthesis in RASMC. In the present study we utilized RASMC, inhibitors of the NF-κB pathway, β-arrestin knockdown, radioligand binding, immunoblotting, and immunofluorescence to characterize the roles of AT1AR internalization, NF-κB activation, and β-arrestin in ANG II-induced COX-2 synthesis. Ro-106-9920 or parthenolide, agents that inhibit the initial steps of NF-κB activation, blocked ANG II-induced p65 NF-κB nuclear localization, COX-2 protein expression, β-arrestin recruitment, and AT1AR internalization without inhibiting ANG II-induced p42/44 ERK activation. Curcumin, an inhibitor of NF-κB-induced transcription, blocked ANG II-induced COX-2 protein expression without altering AT1AR internalization, ANG II-induced p65 NF-κB nuclear localization, or p42/44 ERK activation. Small interfering RNA-induced knockdown of β-arrestin-1 and -2 inhibited ANG II-induced p65 NF-κB nuclear localization. In vascular smooth muscle cells, internalization of the activated AT1AR mediated by β-arrestins activates the NF-κB pathway, producing nuclear localization of the transcription factor and initiation of COX-2 protein synthesis, thereby linking internalization of the receptor with the NF-κB pathway.

β-Arrestin-2 deficiency attenuates abdominal aortic aneurysm formation in mice

RATIONALE

Abdominal aortic aneurysms (AAAs) are a chronic inflammatory vascular disease for which pharmacological treatments are not available. A mouse model of AAA formation involves chronic infusion of angiotensin II (AngII), and previous studies indicated a primary role for the AngII type 1a receptor in AAA formation. β-arrestin (βarr)-2 is a multifunctional scaffolding protein that binds G-protein-coupled receptors such as AngII type 1a and regulates numerous signaling pathways and pathophysiological processes. However, a role for βarr2 in AngII-induced AAA formation is currently unknown.

OBJECTIVE

To determine whether βarr2 played a role in AngII-induced AAA formation in mice.

METHODS AND RESULTS

Treatment of βarr2(+/+) and βarr2(-/-) mice on the hyperlipidemic apolipoprotein E-deficient (apoE(-/-)) background or on normolipidemic C57BL/6 background with AngII for 28 days indicated that βarr2 deficiency significantly attenuated AAA formation. βarr2 deficiency attenuated AngII-induced expression of cyclooxygenase-2, monocyte chemoattractant protein-1, macrophage inflammatory protein 1α, and macrophage infiltration. AngII also increased the levels of phosphorylated extracellular signal-regulated kinase 1/2 in apoE(-/-)/βarr2(+/+) aortas, whereas βarr2 deficiency diminished this increase. Furthermore, inhibition of extracellular signal-regulated kinase 1/2 activation with CI1040 (100 mg/kg per day) reduced the level of AngII-induced cyclooxygenase-2 expression in apoE(-/-)/βarr2(+/+) mice to the level observed in apoE(-/-)/βarr2(-/-) mice. AngII treatment also increased matrix metalloproteinase expression and disruption of the elastic layer in apoE(-/-)/βarr2(+/+) aortas, and βarr2 deficiency reduced these effects.

CONCLUSIONS

βarr2 contributes to AngII-induced AAA formation in mice by phosphorylated extracellular signal-regulated kinase 1/2-mediated cyclooxygenase-2 induction and increased inflammation. These studies suggest that for the AngII type 1a receptor, G-protein-independent, βarr2-dependent signaling plays a major role in AngII-induced AAA formation.

Angiotensin II type 1 receptor signalling regulates microRNA differentially in cardiac fibroblasts and myocytes

BACKGROUND AND PURPOSE

The angiotensin II type 1 receptor (AT(1)R) is a key regulator of blood pressure and cardiac contractility and is profoundly involved in development of cardiac disease. Since several microRNAs (miRNAs) have been implicated in cardiac disease, we determined whether miRNAs might be regulated by AT(1)R signals in a Gαq/11-dependent or -independent manner.

EXPERIMENTAL APPROACH

We performed a global miRNA array analysis of angiotensin II (Ang II)-mediated miRNA regulation in HEK293N cells overexpressing the AT(1)R and focused on separating the role of Gαq/11-dependent and -independent pathways. MiRNA regulation was verified with quantitative PCR in both HEK293N cells and primary cardiac myocytes and fibroblasts.

KEY RESULTS

Our studies revealed five miRNAs (miR-29b, -129-3p, -132, -132* and -212) that were up-regulated by Ang II in HEK293N cells. In contrast, the biased Ang II analogue, [Sar1, Ile4, Ile8] Ang II (SII Ang II), which selectively activates Gαq/11-independent signalling, failed to regulate miRNAs in HEK293N cells. Furthermore, Ang II-induced miRNA regulation was blocked following Gαq/11 and Mek1 inhibition. The observed Ang II regulation of miRNA was confirmed in primary cultures of adult cardiac fibroblasts. Interestingly, Ang II did not regulate miRNA expression in cardiac myocytes, but SII Ang II significantly down-regulated miR-129-3p.

CONCLUSIONS AND IMPLICATIONS

Five miRNAs were regulated by Ang II through mechanisms depending on Gαq/11 and Erk1/2 activation. These miRNAs may be involved in Ang II-mediated cardiac biology and disease, as several of these miRNAs have previously been associated with cardiovascular disease and were found to be regulated in cardiac cells.

Functional relevance of biased signaling at the angiotensin II type 1 receptor

Angiotensin II type 1 receptor antagonists (AT1R blockers, or ARBs) are used commonly in the treatment of cardiovascular disorders such as heart failure and hypertension. Their clinical success arises from their ability to prevent deleterious Gα(q) protein activation downstream of AT1R, which leads to a decrease in morbidity and mortality. Recent studies have identified AT1R ligands that concurrently inhibit Gα(q) protein-dependent signaling and activate Gα(q) protein-independent/β-arrestin-dependent signaling downstream of AT1R, events that may actually improve cardiovascular performance more than conventional ARBs. The ability of such ligands to induce intracellular signaling events in an AT1R-β-arrestin-dependent manner while preventing AT1R-Gα(q) protein activity defines them as biased AT1R ligands. This mini-review will highlight recent studies that have defined biased signaling at the AT1R and discuss the possible clinical relevance of β-arrestin-biased AT1R ligands in the cardiovascular system.

The beta-arrestin pathway-selective type 1A angiotensin receptor (AT1A) agonist [Sar1,Ile4,Ile8]angiotensin II regulates a robust G protein-independent signaling network

The angiotensin II peptide analog [Sar(1),Ile(4),Ile(8)]AngII (SII) is a biased AT(1A) receptor agonist that stimulates receptor phosphorylation, β-arrestin recruitment, receptor internalization, and β-arrestin-dependent ERK1/2 activation without activating heterotrimeric G-proteins. To determine the scope of G-protein-independent AT(1A) receptor signaling, we performed a gel-based phosphoproteomic analysis of AngII and SII-induced signaling in HEK cells stably expressing AT(1A) receptors. A total of 34 differentially phosphorylated proteins were detected, of which 16 were unique to SII and eight to AngII stimulation. MALDI-TOF/TOF mass fingerprinting was employed to identify 24 SII-sensitive phosphoprotein spots, of which three (two peptide inhibitors of protein phosphatase 2A (I1PP2A and I2PP2A) and prostaglandin E synthase 3 (PGES3)) were selected for validation and further study. We found that phosphorylation of I2PP2A was associated with rapid and transient inhibition of a β-arrestin 2-associated pool of protein phosphatase 2A, leading to activation of Akt and increased phosphorylation of glycogen synthase kinase 3β in an arrestin signalsome complex. SII-stimulated PGES3 phosphorylation coincided with an increase in β-arrestin 1-associated PGES3 and an arrestin-dependent increase in cyclooxygenase 1-dependent prostaglandin E(2) synthesis. These findings suggest that AT(1A) receptors regulate a robust G protein-independent signaling network that affects protein phosphorylation and autocrine/paracrine prostaglandin production and that these pathways can be selectively modulated by biased ligands that antagonize G protein activation.

AT(1) receptor Gαq protein-independent signalling transcriptionally activates only a few genes directly, but robustly potentiates gene regulation from the β2-adrenergic receptor

The angiotensin II type 1 receptor (AT(1)R) is known to signal through heterotrimeric G proteins, and Gαq protein-independent signalling has only recently gained appreciation for profound impact on a diverse range of biological functions. β-Arrestins, among other central mediators of Gαq protein-independent signalling from the AT(1)R interact with transcriptional regulators and promote phosphorylation of nuclear proteins. However, the relative contribution of Gαq protein-independent signalling in AT(1)R mediated transcriptional regulation remains elusive. We here present a comprehensive comparative analysis of Gαq protein-dependent and -independent regulation of AT(1)R mediated gene expression. We found angiotensin II to regulate 212 genes, whereas Gαq-independent signalling obtained with the biased agonist, SII angiotensin II only regulated few genes. Interestingly, SII angiotensin II, like Ang II vastly potentiated β2-adrenergic receptor-stimulated gene expression. These novel findings indicate that the Gαq protein-independent signalling mainly modifies the transcriptional response governed by other signalling pathways, while direct induction of gene expression by the AT(1)R is dependent on classical Gαq protein activation.

Therapeutic potential of β-arrestin- and G protein-biased agonists

Members of the seven-transmembrane receptor (7TMR), or G protein-coupled receptor (GPCR), superfamily represent some of the most successful targets of modern drug therapy, with proven efficacy in the treatment of a broad range of human conditions and disease processes. It is now appreciated that β-arrestins, once viewed simply as negative regulators of traditional 7TMR-stimulated G protein signaling, act as multifunctional adapter proteins that regulate 7TMR desensitization and trafficking and promote distinct intracellular signals in their own right. Moreover, several 7TMR biased agonists, which selectively activate these divergent signaling pathways, have been identified. Here we highlight the diversity of G protein- and β-arrestin-mediated functions and the therapeutic potential of selective targeting of these in disease states.

Augmented cyclooxygenase-2 effects on renal function during varying states of angiotensin II

Nonsteroidal anti-inflammatory drug usage has long revealed renoprotective prostaglandin actions on the renal microvasculature during increased pressor hormone influence, but whether increased cyclooxygenase (COX)-2 expression supports prostaglandin vasodilatory influence by interfering with the actions of ANG II remains unresolved. Therefore, we tested the hypothesis that COX-2 inhibition causes hemodynamic and excretory effects that are increased in proportion to ANG II activity. In anesthetized Sprague-Dawley rats having augmented cortical COX-2 expression but different ANG II activity, we conducted renal clearance experiments during acute inhibition of COX-2 with nimesulide (NMSLD) and inhibition of COX-1 with SC-560. In one series of experiments, acute captopril [acute angiotensin-converting enzyme (ACE) inhibitor (aACEi)] was administered alone (n = 13) or in combination with chronic captopril [chronic ACEi (cACEi)] pretreatment (n = 19). In another series of experiments, rats were fed a normal-sodium [0.4% (NS), n = 12] or a low-sodium [0.03% (LS), n = 18] diet. NMSLD did not alter mean arterial blood pressure in any group but, in the LS and cACEi groups, decreased renal plasma flow (from 3.99 ± 0.33 to 2.85 ± 0.26 and from 4.30 ± 0.19 to 3.22 ± 0.21 ml·min(-1)·g(-1)), cortical blood flow (-12 ± 8% and -13 ± 4%), and glomerular filtration rate (from 0.88 ± 0.04 to 0.65 ± 0.05 and from 0.95 ± 0.07 to 0.70 ± 0.05 ml·min(-1)·g(-1)). In contrast, medullary blood flow (MBF) was significantly decreased by COX-2 inhibition in NS (-24 ± 5%), LS (-27 ± 8%), aACEi (-16 ± 3.8%), and cACEi (-24 ± 4.2%) groups. Absolute and fractional sodium excretion rates were unchanged by NMSLD, except in the LS group (0.75 ± 0.05 μeq/min and 0.43 ± 0.15% and 0.51 ± 0.06 μeq/min and 0.26 ± 0.10%). SC-560 did not augment the effects of NMSLD. These results demonstrate an augmented COX-2-mediated vasodilation that is not contingent on ANG II, in contrast to COX-2-mediated augmented sodium excretion, where ANG II activity is requisite. Furthermore, the COX-2 effects on MBF are not contingent on ANG II or changes in cortical microvascular responses. These results reflect COX-2 continual regulation of MBF and adaptive opposition to ANG II prohypertensinogenic effects on renal plasma flow, cortical blood flow, glomerular filtration rate, and absolute and fractional sodium excretion.