Cloning and characterization of ATRAP, a novel protein that interacts with the angiotensin II type 1 receptor

Angiotensin II type 1 receptor-associated protein deletion combined with angiotensin II stimulation accelerates the development of diabetic kidney disease in mice on a C57BL/6 strain

The progress in the research field of diabetic kidney disease (DKD) has been disturbed by the lack of reliable animal models. Angiotensin II (Ang II) type 1 receptor (AT1R)-associated protein (ATRAP) promotes internalization of AT1R and selectively inhibits pathological AT1R signaling. In this study, we investigated whether overactivation of the renin-angiotensin system (RAS) through a combination of ATRAP deletion with Ang II stimulation developed a progressive DKD model in C57BL/6 mice, which are resistant to the development of kidney injury. Eight-week-old male systemic ATRAP-knockout mice on the C57BL/6 strain (KO) and their littermate wild-type mice (Ctrl) were divided into five groups: 1) Ctrl, 2) Ctrl-streptozotocin (STZ), 3) KO-STZ, 4) Ctrl-STZ-Ang II, and 5) KO-STZ-Ang II. Ang II was administered for 6 weeks from 4 weeks after STZ administration. At 10 weeks after STZ administration, mice were euthanized to evaluate kidney injuries. Neither ATRAP deletion alone nor Ang II stimulation alone developed a progressive DKD model in STZ-induced diabetic C57BL/6 mice. However, a combination of ATRAP deletion with Ang II stimulation accelerated the development of DKD as manifested by overt albuminuria, glomerular hypertrophy, podocyte loss, mesangial expansion, kidney interstitial fibrosis and functional insufficiency, concomitant with increased angiotensinogen and AT1R expression in the kidneys. In STZ-induced diabetic C57BL/6 mice that are resistant to the development of kidney injury, the combination of ATRAP deletion and Ang II stimulation accelerates the development of DKD, which may be associated with intrarenal RAS overactivation.

Enhancement of angiotensin II type 1 receptor-associated protein in the paraventricular nucleus suppresses angiotensin II-dependent hypertension

The renin-angiotensin system in the brain plays a pivotal role in modulating sympathetic nerve activity and contributes to the pathogenesis of hypertension. Angiotensin II (Ang II) type 1 receptor (AT1R)-associated protein (ATRAP) promotes internalization of AT1R while suppressing pathological overactivation of AT1R signaling. However, the pathophysiological function of ATRAP in the brain remains unknown. Therefore, this study aims to investigate whether ATRAP in the paraventricular nucleus (PVN) is involved in neurogenic hypertension pathogenesis in Ang II-infused rats. The ATRAP/AT1R ratio, which serves as an indicator of tissue AT1R hyperactivity, tended to decrease within the PVN in the Ang II group than in the vehicle group. This suggests an Ang II-induced hyperactivation of the AT1R signaling pathway in the PVN. Lentiviral vectors were generated to stimulate ATRAP expression. At 6 weeks of age, rats were microinjected with LV-Venus (Venus-expressing lentivirus) or LV-ATRAP (Venus-ATRAP-expressing lentivirus). The rats were then randomly divided into four groups: (1) Vehicle/LV-Venus, (2) Vehicle/LV-ATRAP, (3) Ang II/LV-Venus, and (4) Ang II/LV-ATRAP. Two weeks after microinjection, vehicle or Ang II was administered systemically for 2 weeks. In the Ang II/LV-ATRAP group, systolic blood pressure at 1 and 2 weeks following administration was significantly lower than that in the Ang II/LV-Venus group. Furthermore, urinary adrenaline levels tended to decrease in the Ang II/LV-ATRAP group than in the Ang II/LV-Venus group. These findings suggest that enhanced ATRAP expression in the PVN suppresses Ang II-induced hypertension, potentially by suppressing hyperactivation of the tissue AT1R signaling pathway and, subsequently, sympathetic nerve activity.

The AT1/AT2 Receptor Equilibrium Is a Cornerstone of the Regulation of the Renin Angiotensin System beyond the Cardiovascular System

The AT₁ receptor has mainly been associated with the pathological effects of the renin-angiotensin system (RAS) (e.g., hypertension, heart and kidney diseases), and constitutes a major therapeutic target. In contrast, the AT₂ receptor is presented as the protective arm of this RAS, and its targeting via specific agonists is mainly used to counteract the effects of the AT₁ receptor. The discovery of a local RAS has highlighted the importance of the balance between AT₁/AT₂ receptors at the tissue level. Disruption of this balance is suggested to be detrimental. The fine tuning of this balance is not limited to the regulation of the level of expression of these two receptors. Other mechanisms still largely unexplored, such as S-nitrosation of the AT₁ receptor, homo- and heterodimerization, and the use of AT₁ receptor-biased agonists, may significantly contribute to and/or interfere with the settings of this AT₁/AT₂ equilibrium. This review will detail, through several examples (the brain, wound healing, and the cellular cycle), the importance of the functional balance between AT₁ and AT₂ receptors, and how new molecular pharmacological approaches may act on its regulation to open up new therapeutic perspectives.

Thymus transplantation regulates blood pressure and alleviates hypertension-associated heart and kidney damage via transcription factors FoxN1 pathway

Previous studies have found that thymus is involved in the process of hypertension. However, whether thymus transplantation alleviates target organ damage in hypertensive mice remains unknown. The aim of this study was to evaluate the effects of thymus transplantation on blood pressure and target organ changes in mice with hypertension. Mice were randomly divided into normal control group (Con), hypertensive group (HTN) and thymus transplantation group (HTN + Trans). Thymus of neonatal mice was transplanted into the renal capsule of the transplantation group. After transplantation, the mouse tail noninvasive pressure was measured and heart function was evaluated weekly. Then mice were euthanized and organs or tissues were harvested at 4 weeks post-transplantation. The blood pressure of HTN + Trans group was lower than that in the HTN group. The expression of FoxN1, Aire, ATRAP, thymosin β4 and the content of sjTREC in thymus of HTN group was decreased and the number of naïve T cells in HTN group was lower compared with other two groups. The ratio of cTEC/mTEC in HTN group was higher than that in Con group and lower than that in HTN + Trans group. Cardiac pathology showed cardiac hypertrophy and fibrosis in HTN group whereas thymus transplantation improved heart function and structure. Altogether, our findings demonstrated thymus transplantation could improve thymus function of hypertensive mice, which increased the expression of thymus transcription factor FoxN1, affected the proportion of T cell subsets, and increased thymosin β4 thereby reducing blood pressure and reversing the progression of target organ damage.

Pan-cancer analysis of the angiotensin II receptor-associated protein as a prognostic and immunological gene predicting immunotherapy responses in pan-cancer

Background: Understanding interior molecular mechanisms of tumorigenesis and cancer progression contributes to antitumor treatments. The angiotensin II receptor-associated protein (AGTRAP) has been confirmed to be related with metabolic products in metabolic diseases and can drive the progression of hepatocellular carcinoma and colon carcinoma. However, functions of AGTRAP in other kinds of cancers are unclear, and a pan-cancer analysis of AGTRAP has not been carried out.

Methods and materials: We downloaded data from The Cancer Genome Atlas and Genotype-Tissue Expression dataset and The Human Protein Atlas databases and then used R software (version 4.1.1) and several bioinformatic tools to conduct the analysis.

Results: In our study, we evaluated the expression of AGTRAP in cancers, such as high expression in breast cancer, lung adenocarcinoma, and glioma and low expression in kidney chromophobe. Furthermore, our study revealed that high expression of AGTRAP is significantly related with poor prognosis in glioma, liver cancer, kidney chromophobe, and so on. We also explored the putative functional mechanisms of AGTRAP across pan-cancer, such as endoplasmic reticulum pathway, endocytosis pathway, and JAK-STAT signaling pathway. In addition, the connection between AGTRAP and tumor microenvironment, tumor mutation burden, and immune-related genes was proven.

Conclusion: Our study provided comprehensive evidence of the roles of AGTRAP in different kinds of cancers and supported the relationship of AGTRAP and tumorous immunity.

Melatonin inhibits angiotensin II-induced atrial fibrillation through preventing degradation of Ang II Type I Receptor-Associated Protein (ATRAP)

Angiotensin II (Ang II) induced Atrial fibrillation (AF) often accompanied with reduced ATRAP which is a negative modulator of Ang II type 1 receptor (AT1R). Melatonin can protect against AF, but the underlying molecular mechanism remains poorly understood. In this study, Ang II was used to induce AF, and AF inducibility and duration were documented telemetrically. Ang II-infused mice had a higher AF incidence, which was associated with atrial fibrosis, inflammation, and oxidative stress. Melatonin partially inhibited these effects, and enforced expression of siRNA-ATRAP in atria counteracted the beneficial role of melatonin. Specifically, melatonin inhibited expression of Ang II-induced proteasome and immunoproteasome subunits β2, β2i, β5, and β5i as well as their corresponding trypsin-like and chymotrypsin-like activities and blocked ATRAP degradation. In turn, this inhibited AT1R-mediated NF-κB signaling, transforming growth factor (TGF)-β1/Smad signaling in the atria, and thereby affected atrial remodeling and AF. Melatonin receptor inhibition by the chemical inhibitor luzindole partially inhibited the inhibitory effects of melatonin on proteasome activity and also Ang II-induced pathological changes in the atria. Overall, our study demonstrates that melatonin protects against Ang II-induced AF by inhibiting proteasome activity and stabilizing ATRAP expression, and these effects are partially dependent on melatonin receptor activation.

FAM114A1 influences cardiac pathological remodeling by regulating angiotensin II signaling

Cardiac pathological remodeling, a primary contributor to heart failure (HF) and death, is an important target for HF therapy. However, the signaling pathways that govern cardiac remodeling are not fully elucidated. Here, we found that a functionally unannotated human myocardial infarction–associated (MI-associated) gene, family with sequence similarity 114 member A1 (FAM114A1), is induced in failing human and mouse hearts compared with nonfailing hearts. Homozygous KO of Fam114a1 (Fam114a1^–/–) in the mouse genome reduces cardiomyocyte hypertrophy, inflammation, and cardiac fibrosis while restoring cardiac function in angiotensin II–induced (Ang II–induced) and MI-induced HF mouse models. Cardiac fibroblasts (CFs) exhibit the highest FAM114A1 expression among different cardiac cell types. FAM114A1 is a critical autonomous factor for CF proliferation, activation, and migration. Mechanistically, FAM114A1 interacts with angiotensin receptor–associated protein (AGTRAP) and regulates the expression of angiotensin type 1 receptor (AT1R) and downstream Ang II signaling transduction, and it subsequently influences profibrotic response. Our results indicate that FAM114A1 regulates Ang II signaling, thereby activating CFs and other cardiac cells and augmenting pathological cardiac remodeling. These findings provide potentially novel insights into the regulation of cardiac remodeling and identify FAM114A1 as a therapeutic target for the treatment of heart disease.

Perivascular Adipose Tissue Compensation for Endothelial Dysfunction in the Superior Mesenteric Artery of Female SHRSP.Z-<i>Lepr</i>^<i>fa</i>/IzmDmcr Rats

Regulation of arterial tone by perivascular adipose tissue (PVAT) differs between sexes. In male SHRSP.Z-<i>Lepr</i>^<i>fa</i>/IzmDmcr rats (SHRSP.ZF), PVAT exerts a compensatory relaxation effect for the loss of endothelium-mediated vasorelaxation, which occurs during the early stages of metabolic syndrome. However, this effect deteriorates by 23 weeks of age. Here, therefore, we compared the effects of PVAT in female and male SHRSP.ZF. Acetylcholine-induced relaxation in superior mesenteric artery without PVAT did not differ between 23-week-old females and males. However, the presence of PVAT enhanced relaxation in 23-week-old females, but not in males. The mRNA levels of angiotensin II type 1 receptor (AT1R) in PVAT did not differ between sexes, but AT1R-associated protein (ATRAP) and apelin levels were higher in females than in males. We observed a positive relationship between differences in artery relaxation with and without PVAT and ATRAP or apelin mRNA levels. In 30-week-old females, PVAT-enhanced relaxation disappeared, and mRNA levels of AT1R increased, while apelin levels decreased compared to 23-week-old females. These results demonstrated that in SHRSP.ZF, PVAT compensation for endothelium dysfunction extended to older ages in females than in males. Apelin and AT1R/ATRAP expression in PVAT may be predictors of favorable effects.

Deficiency of the kidney tubular angiotensin II type1 receptor-associated protein ATRAP exacerbates streptozotocin-induced diabetic glomerular injury via reducing protective macrophage polarization

Although activation of the renin-angiotensin system and of its glomerular components is implicated in the pathogenesis of diabetic nephropathy, the functional roles of the tubular renin-angiotensin system with AT1 receptor signaling in diabetic nephropathy are unclear. Tissue hyperactivity of the renin-angiotensin system is inhibited by the angiotensin II type 1 receptor-associated protein ATRAP, which negatively regulates receptor signaling. The highest expression of endogenous ATRAP occurs in the kidney, where it is mainly expressed by tubules but rarely in glomeruli. Here, we found that hyperactivation of angiotensin II type 1 receptor signaling in kidney tubules exacerbated diabetic glomerular injury in a mouse model of streptozotocin-induced diabetic nephropathy. These phenomena were accompanied by decreased expression of CD206, a marker of alternatively activated and tissue-reparative M2 macrophages, in the kidney tubulointerstitium. Additionally, adoptive transfer of M2- polarized macrophages into diabetic ATRAP-knockout mice ameliorated the glomerular injury. As a possible mechanism, the glomerular mRNA levels of tumor necrosis factor-α and oxidative stress components were increased in diabetic knockout mice compared to non-diabetic knockout mice, but these increases were ameliorated by adoptive transfer. Furthermore, proximal tubule-specific ATRAP downregulation reduced tubulointerstitial expression of CD206, the marker of M2 macrophages in diabetic mice. Thus, our findings indicate that tubular ATRAP-mediated functional modulation of angiotensin II type 1 receptor signaling modulates the accumulation of tubulointerstitial M2 macrophages, thus affecting glomerular manifestations of diabetic nephropathy via tubule-glomerular crosstalk.

ATRAP, a receptor-interacting modulator of kidney physiology, as a novel player in blood pressure and beyond

Pathological activation of kidney angiotensin II (Ang II) type 1 receptor (AT1R) signaling stimulates tubular sodium transporters, including epithelial sodium channels, to increase sodium reabsorption and blood pressure. During a search for a means to functionally and selectively modulate AT1R signaling, a molecule directly interacting with the carboxyl-terminal cytoplasmic domain of AT1R was identified and named AT1R-associated protein (ATRAP/Agtrap). We showed that ATRAP promotes constitutive AT1R internalization to inhibit pathological AT1R activation in response to certain stimuli. In the kidney, ATRAP is abundantly distributed in epithelial cells along the proximal and distal tubules. Results from genetically engineered mice with modified ATRAP expression show that ATRAP plays a key role in the regulation of renal sodium handling and the modulation of blood pressure in response to pathological stimuli and further suggest that the function of kidney tubule ATRAP may be different between distal tubules and proximal tubules, implying that ATRAP is a target of interest in hypertension.

Construction and Comprehensive Analysis of a Stratification System Based on AGTRAP in Patients with Hepatocellular Carcinoma

Background

With the development of sequencing technology, several signatures have been reported for the prediction of prognosis in patients with hepatocellular carcinoma (HCC). However, the above signatures are characterized by cumbersome application. Therefore, the study is aimed at screening out a robust stratification system based on only one gene to guide treatment.

Methods

Firstly, we used the limma package for performing differential expression analysis on 374 HCC samples, followed by Cox regression analysis on overall survival (OS) and disease-free interval (PFI). Subsequently, hub prognostic genes were found at the intersection of the above three groups. In addition, the topological degree inside the PPI network was used to screen for a unique hub gene. The rms package was used to construct two visual stratification systems for OS and PFI, and Kaplan-Meier analysis was utilized to investigate survival differences in clinical subgroups. The ssGSEA algorithm was then used to reveal the relationship between the hub gene and immune cells, immunological function, and checkpoints. In addition, we also used function annotation to explore into putative biological functions. Finally, for preliminary validation, the hub gene was knocked down in the HCC cell line.

Results

We discovered 6 prognostic genes (SKA1, CDC20, AGTRAP, BIRC5, NEIL3, and CDC25C) for constructing a PPI network after investigating survival and differential expression genes. According to the topological degree, AGTRAP was chosen as the basis for the stratification system, and it was revealed to be a risk factor with an independent prognostic value in Kaplan-Meier analysis and Cox regression analysis (P < 0.05). In addition, we constructed two visualized nomograms based on AGTRAP. The novel stratification system had a robust predictive value for PFI and OS in ROC analysis and calibration curve (P < 0.05). Meanwhile, AGTRAP upregulation was associated with T staging, N staging, M staging, pathological stage, grade, and vascular invasion (P < 0.05). Notably, AGTRAP was overexpressed in tumor tissues in all pancancers with paired samples (P < 0.05). Furthermore, AGTRAP was associated with immune response and may change immune microenvironment in HCC (P < 0.05). Next, gene enrichment analysis suggested that AGTRAP may be involved in the biological process, such as cotranslational protein targeting to the membrane. Finally, we identified the oncogenic effect of AGTRAP by qRT-PCR, colony formation, western blot, and CCK-8 assay (P < 0.05).

Conclusion

We provided robust evidences that a stratification system based on AGTRAP can guide survival prediction for HCC patients.

Therapeutic targets of natural products for the management of cardiovascular symptoms of coronavirus disease 2019

The outbreak of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was first occurred in China in December 2019 and subsequently spread all over the world with cardiovascular, renal, and pulmonary symptoms. Therefore, recognizing and treating the cardiovascular sign and symptoms that caused by coronavirus disease 2019 (COVID-19) can be effective in reducing patient mortality. To control the COVID-19-related cardiovascular symptoms, natural products are considered one of the promising choices as complementary medicine. Scientists are struggling to discover new antiviral agents specific to this virus. In this review, the natural products for management of cardiovascular symptoms of COVID-19 are categorized into three groups: (a) natural products with an impact on angiotensin II type 1 receptor; (b) natural products that inhibit angiotensin-converting enzyme activity; and (c) natural products that mimic adenosine activity. All these natural products should undergo clinical investigations to test their efficacy, safety, and toxicity in the treatment of cardiovascular symptoms of COVID-19. This article summarizes agents with potential efficacy against COVID-19-related cardiovascular symptoms.

S-adenosylmethionine upregulates the angiotensin receptor-binding protein ATRAP via the methylation of HuR in NAFLD

Nonalcoholic fatty liver disease (NAFLD) has emerged globally and is associated with inflammatory signaling. The underlying mechanisms remain poorly delineated, although NAFLD has attracted considerable attention and been extensively investigated. Recent publications have determined that angiotensin II (Ang II) plays an important role in stimulating NAFLD progression by causing lipid metabolism disorder and insulin resistance through its main receptor, Ang II type 1 receptor (AT1R). Herein, we explored the effect of supplementary S-adenosylmethionine (SAM), which is the main biological methyl donor in mammalian cells, in regulating AT1R-associated protein (ATRAP), which is the negative regulator of AT1R. We found that SAM was depleted in NAFLD and that SAM supplementation ameliorated steatosis. In addition, in both high-fat diet-fed C57BL/6 rats and L02 cells treated with oleic acid (OA), ATRAP expression was downregulated at lower SAM concentrations. Mechanistically, we found that the subcellular localization of human antigen R (HuR) was determined by the SAM concentration due to protein methylation modification. Moreover, HuR was demonstrated to directly bind ATRAP mRNA and control its nucleocytoplasmic shuttling. Thus, SAM was suggested to upregulate ATRAP protein expression by maintaining the export of its mRNA from the nucleus. Taken together, our findings suggest that SAM can positively regulate ATRAP in NAFLD and may have various potential benefits for the treatment of NAFLD.

Proinflammation, profibrosis, and arterial aging

Aging is a major risk factor for quintessential cardiovascular diseases, which are closely related to arterial proinflammation. The age-related alterations of the amount, distribution, and properties of the collagen fibers, such as cross-links and degradation in the arterial wall, are the major sequelae of proinflammation. In the aging arterial wall, collagen types I, II, and III are predominant,  and are mainly produced by stiffened vascular smooth muscle cells (VSMCs) governed by proinflammatory signaling, leading to profibrosis. Profibrosis is regulated by an increase in the proinflammatory molecules angiotensin II, milk fat globule-EGF-VIII, and transforming growth factor-beta 1 (TGF-β1) signaling and a decrease in the vasorin signaling cascade. The release of these proinflammatory factors triggers the activation of matrix metalloproteinase type II (MMP-2) and activates profibrogenic TGF-β1 signaling, contributing to profibrosis. The age-associated increase in activated MMP-2 cleaves latent TGF-β and subsequently increases TGF-β1 activity leading to collagen deposition in the arterial wall. Furthermore, a blockade of the proinflammatory signaling pathway alleviates the fibrogenic signaling, reduces profibrosis, and prevents arterial stiffening with aging. Thus, age-associated proinflammatory-profibrosis coupling is the underlying molecular mechanism of arterial stiffening with advancing age.

The pathophysiological role of angiotensin receptor-binding protein in hypertension and kidney diseases: Oshima Award Address 2019

Excessive activation of the tissue renin–angiotensin system through angiotensin II (Ang II) type 1 receptor (AT1R) plays a pivotal role in the pathogenesis of hypertension and related organ injury. AT1R-associated protein (ATRAP/Agtrap) was identified as a molecule specifically interacting with the carboxyl- terminal domain of AT1R. The results of in vitro studies showed that ATRAP suppresses Ang II-mediated pathological responses in cardiovascular cells by promoting AT1R internalization. With respect to the tissue distribution and regulation of ATRAP expression in vivo, ATRAP is broadly expressed in many tissues as is AT1R including kidney. The results of in vivo study employing genetic engineered mice with modified ATRAP expression showed that ATRAP inhibits cardiovascular injuries provoked by Ang II-induced hypertension, along with preserving physiological AT1R signaling. In addition, we have shown that ATRAP functions as an endogenous modulator so as to prevent hypertension in response to pathological stimuli, by regulating renal sodium handling. Furthermore, ATRAP may have an AT1R-independent function of renal proximal tubule to protect aging and fibrosis. These results suggest the clinical potential benefit of an ATRAP activation strategy in the treatment of hypertension and cardiorenal and vascular diseases.

Angiotensin-Receptor-Associated Protein Modulates Ca2+ Signals in Photoreceptor and Mossy Fiber cells

Fast, precise and sustained neurotransmission requires graded Ca²⁺ signals at the presynaptic terminal. Neurotransmitter release depends on a complex interplay of Ca²⁺ fluxes and Ca²⁺ buffering in the presynaptic terminal that is not fully understood. Here, we show that the angiotensin-receptor-associated protein (ATRAP) localizes to synaptic terminals throughout the central nervous system. In the retinal photoreceptor synapse and the cerebellar mossy fiber-granule cell synapse, we find that ATRAP is involved in the generation of depolarization-evoked synaptic Ca²⁺ transients. Compared to wild type, Ca²⁺ imaging in acutely isolated preparations of the retina and the cerebellum from ATRAP knockout mice reveals a significant reduction of the sarcoendoplasmic reticulum (SR) Ca²⁺-ATPase (SERCA) activity. Thus, in addition to its conventional role in angiotensin signaling, ATRAP also modulates presynaptic Ca²⁺ signaling within the central nervous system.

Angiotensin II type 1 receptor-associated protein deficiency attenuates sirtuin1 expression in an immortalised human renal proximal tubule cell line

The proximal tubule is a particularly important site for ageing-related kidney damage. Sirtuin 1 (SIRT1), an NAD⁺ (nicotinamide adenine dinucleotide)-dependent deacetylase in the proximal tubule, may be involved in renal injury associated with ageing. However, the mechanisms of SIRT1 regulation remain to be elucidated. We recently reported that angiotensin II type 1 receptor (AT1R)-associated protein (ATRAP)-deficient mice displayed age-associated renal function decline and tubulointerstitial fibrosis. Our data showed that SIRT1 protein expression was reduced in ATRAP-deficient mice, although the relationship between ATRAP deficiency and age-associated renal fibrosis is still not fully understood. It is, therefore, necessary to investigate how ATRAP affects SIRT1 protein expression to resolve ageing-associated kidney dysfunction. Here, since ageing studies are inherently lengthy, we used an ex vivo model of the proximal tubule to determine the role of ATRAP in SIRT1 protein expression. We first generated a clonal immortalised human renal proximal tubule epithelial cell line (ciRPTEC) expressing AT1R and ATRAP. Using this cell line, we demonstrated that ATRAP knockdown reduced SIRT1 protein expression in the ciRPTEC but did not alter SIRT1 mRNA expression. Thus, ATRAP likely mediates SIRT1 protein abundance in ciRPTEC.

Ablation of Immunoproteasome β5i Subunit Suppresses Hypertensive Retinopathy by Blocking ATRAP Degradation in Mice

Inflammation is associated with retinal diseases. Our recent data demonstrate that immunoproteasome catalytic subunit β2i contributes to angiotensin II (Ang II)-induced retinopathy in mice. Here, we investigated the role of another catalytic subunit β5i in regulating retinopathy and its underlying mechanisms. We induced a murine model of retinopathy by infusing Ang II (3,000 ng/kg/min) for 3 weeks into wild-type (WT) mice, β5i-knockout (KO) mice, or WT mice injected with either adenovirus-expressing β5i (Ad-β5i) or angiotensin II type 1 receptor (AT1R)-associated protein (Ad-ATRAP), which inhibits AT1R. The β5i expression and chymotrypsin-like activity were most significantly elevated in Ang II-infused retinas and serum from patients with hypertensive retinopathy. Moreover, Ang II infusion-induced retinopathy was markedly attenuated in β5i-KO mice but aggravated in Ad-β5i-injected mice. Accordingly, β5i KO markedly restored Ang II-induced downregulation of ATRAP and activation of AT1R downstream mediators, which was further enhanced in Ad-β5i-injected mice. Interestingly, overexpression of ATRAP significantly abrogated Ang II-induced retinopathy in Ad-β5i-injected mice. This study found that β5i promoted Ang II-induced retinopathy by promoting ATRAP degradation and activation of AT1R-mediated signals.

Valsartan impedes epinephrine-induced ICAM-4 activation on normal, sickle cell trait and sickle cell disease red blood cells

Abnormal red blood cell (RBC) adhesion to endothelial αvβ3 plays a crucial role in triggering vaso-occlusive episodes in sickle cell disease (SCD). It is known that epinephrine, a β-adrenergic receptor (β-AR) stimulator, increases the RBC surface density of active intercellular adhesion molecule-4 (ICAM-4) which binds to the endothelial αvβ3. It has also been demonstrated that in human embryonic kidney 293 cells, mouse cardiomyocytes, and COS-7 cell lines, the β-adrenergic and renin-angiotensin systems are interrelated and that there is a direct interaction and cross-regulation between β-AR and angiotensin II type 1 receptor (AT1R). Selective blockade of AT1R reciprocally inhibits the downstream signaling of β-ARs, similar to the inhibition observed in the presence of a β-AR-blocker. However, it is not known if this mechanism is active in human RBCs. Here, we studied the effect of valsartan, an AT1R blocker, on the surface density of active ICAM-4 receptors in normal, sickle cell trait, and homozygous sickle RBCs. We applied single molecule force spectroscopy to detect active ICAM-4 receptors on the RBC plasma membrane with and without the presence of valsartan and epinephrine. We found that epinephrine significantly increased whereas valsartan decreased their surface density. Importantly, we found that pretreatment of RBCs with valsartan significantly impeded the activation of ICAM-4 receptors induced by epinephrine. The observed reduced expression of active ICAM-4 receptors on the RBC plasma membrane leads us to conjecture that valsartan may be used as a supporting remedy for the prevention and treatment of vaso-occlusive crisis in SCD.

Effects of ATRAP in Renal Proximal Tubules on Angiotensin-Dependent Hypertension

Background We have previously shown that ATRAP (angiotensin II receptor-associated protein; Agtrap) interacts with AT1R (angiotensin II type 1 receptor) and promotes constitutive internalization of AT 1R so as to inhibit hyperactivation of its downstream signaling. In response to angiotensin II , systemic ATRAP deficiency exacerbates angiotensin II -mediated hypertension via hyperactivation of renal tubular AT 1R. Although ATRAP expression is abundant in renal proximal tubules, little is known about the actual function of renal proximal tubule ATRAP in angiotensin-mediated hypertension. Methods and Results In this study, we examined the in vivo functional role of renal proximal tubule ATRAP in angiotensin-dependent hypertension. We succeeded in generating proximal tubule-specific ATRAP knockout ( PT - KO ) mice for the first time using the Cre/loxP system with Pepck-Cre. Detailed analysis of renal ATRAP expression in PT - KO mice estimated by immunohistochemical and laser-capture microdissection analysis revealed that ATRAP mRNA expression decreased by ≈80% in proximal regions of the nephron in PT - KO mice compared with wild-type ( WT ) mice. We compared blood pressure of PT - KO and WT mice using both tail-cuff and radiotelemetric methods. Blood pressure of PT - KO mice was comparable with that of WT mice at baseline. Moreover, no significant differences were noted in pressor response to angiotensin II (600 ng/kg per min or 1000 ng/kg per minute) infusion between PT - KO and WT mice. In addition, angiotensin II -mediated cardiac hypertrophy was identical between PT - KO and WT mice. Conclusions ATRAP deficiency in proximal tubules did not exacerbate angiotensin-dependent hypertension in vivo. The results indicate that renal proximal tubule ATRAP has a minor role in angiotensin-dependent hypertension in vivo.

Immunoproteasome Subunit β5i Promotes Ang II (Angiotensin II)–Induced Atrial Fibrillation by Targeting ATRAP (Ang II Type I Receptor–Associated Protein) Degradation in Mice

Atrial fibrillation (AF) is the most common type of cardiac arrhythmia and increases the risk of stroke, heart failure, and death. Ang II (angiotensin II) triggers AF, mainly through stimulation of the AT1R (Ang II type I receptor). The immunoproteasome is a highly efficient proteolytic machine derived from the constitutive proteasome, but the role it plays in regulating AT1R activation and triggering AF remains unknown. Here, we show that among the catalytic subunits, β5i (PSMB8) expression, and chymotrypsin-like activity were the most significantly upregulated in atrial tissue of Ang II–infused mice or serum from patients with AF. β5i KO (β5i knockout) in mice markedly attenuated Ang II-induced AF incidence, atrial fibrosis, inflammatory response, and oxidative stress compared with WT (wild type) animals, but injection with recombinant adeno-associated virus serotype 9–β5i increased these effects. Moreover, we found that ATRAP (AT1R-associated protein) was a target of β5i. Overexpression of ATRAP significantly attenuated Ang II-induced atrial remodeling and AF in recombinant adeno-associated virus serotype 9–β5i-injected mice. Mechanistically, Ang II upregulated β5i expression to promote ATRAP degradation, which resulted in activation of AT1R-mediated NF-κB signaling, increased NADPH oxidase activity, increased TGF (transforming growth factor)-β1/Smad signaling, and altered the expression of Kir2.1 and CX43 (connexin 43) in the atria, thereby affecting atrial remodeling and AF. In summary, this study identifies β5i as a negative regulator of ATRAP stability that contributes to AT1R activation and to AF, highlighting that targeting β5i activity may represent a potential therapeutic approach for the treatment of hypertensive AF.

The Role of β-Arrestin Proteins in Organization of Signaling and Regulation of the AT1 Angiotensin Receptor

AT1 angiotensin receptor plays important physiological and pathophysiological roles in the cardiovascular system. Renin-angiotensin system represents a target system for drugs acting at different levels. The main effects of ATR1 stimulation involve activation of Gq proteins and subsequent IP3, DAG, and calcium signaling. It has become evident in recent years that besides the well-known G protein pathways, AT1R also activates a parallel signaling pathway through β-arrestins. β-arrestins were originally described as proteins that desensitize G protein-coupled receptors, but they can also mediate receptor internalization and G protein-independent signaling. AT1R is one of the most studied receptors, which was used to unravel the newly recognized β-arrestin-mediated pathways. β-arrestin-mediated signaling has become one of the most studied topics in recent years in molecular pharmacology and the modulation of these pathways of the AT1R might offer new therapeutic opportunities in the near future. In this paper, we review the recent advances in the field of β-arrestin signaling of the AT1R, emphasizing its role in cardiovascular regulation and heart failure.

Perivascular Adipose Tissue-Enhanced Vasodilation in Metabolic Syndrome Rats by Apelin and N-Acetyl⁻l-Cysteine-Sensitive Factor(s)

Perivascular adipose tissue (PVAT) can regulate vascular tone. In mesenteric arteries of SHRSP.Z-Lepr^fa/IzmDmcr rats (SHRSP.ZF) with metabolic syndrome, vascular dysfunction is compensated by PVAT-dependent mechanisms that disappear with increasing age. In this study, we investigated the mechanisms of the age-related changes and responsible factor(s) involved in the enhancing effects of mesenteric arterial PVAT in SHRSP.ZF. Acetylcholine- and sodium nitroprusside-induced relaxations of isolated arteries were greater with PVAT than without PVAT at 17 and 20 weeks of age (wks), and as expected, this enhancement by the presence of PVAT disappeared at 23 wks. PVAT mRNA levels of angiotensin II type 1 (AT1) receptor-associated protein was less and AT1 receptor was unchanged at 23 wks when compared to 20 wks. At 20 wks, the enhanced acetylcholine-induced relaxation by the presence of PVAT was inhibited by N-acetyl-l-cysteine (NAC). Acetylcholine-induced relaxation of arteries without PVAT was increased in the presence of exogenously added apelin. PVAT mRNA level of apelin was higher in SHRSP.ZF than in control Wistar-Kyoto rats, and the level was decreased with aging. These results suggest that AT1 receptor activation in PVAT, and changes in the regulation of apelin and a NAC-sensitive factor are related to the age-dependent deterioration of the vasodilation enhancing effects of mesenteric arterial PVAT in SHRSP.ZF.

Mechanisms of the anterograde trafficking of GPCRs: Regulation of AT1R transport by interacting proteins and motifs

Anterograde cell surface transport of nascent G protein-coupled receptors (GPCRs) en route from the endoplasmic reticulum (ER) through the Golgi apparatus represents a crucial checkpoint to control the amount of the receptors at the functional destination and the strength of receptor activation-elicited cellular responses. However, as compared with extensively studied internalization and recycling processes, the molecular mechanisms of cell surface trafficking of GPCRs are relatively less defined. Here, we will review the current advances in understanding the ER-Golgi-cell surface transport of GPCRs and use angiotensin II type 1 receptor as a representative GPCR to discuss emerging roles of receptor-interacting proteins and specific motifs embedded within the receptors in controlling the forward traffic of GPCRs along the biosynthetic pathway.

The interface between phosphatidylinositol transfer protein function and phosphoinositide signaling in higher eukaryotes

Phosphoinositides are key regulators of a large number of diverse cellular processes that include membrane trafficking, plasma membrane receptor signaling, cell proliferation, and transcription. How a small number of chemically distinct phosphoinositide signals are functionally amplified to exert specific control over such a diverse set of biological outcomes remains incompletely understood. To this end, a novel mechanism is now taking shape, and it involves phosphatidylinositol (PtdIns) transfer proteins (PITPs). The concept that PITPs exert instructive regulation of PtdIns 4-OH kinase activities and thereby channel phosphoinositide production to specific biological outcomes, identifies PITPs as central factors in the diversification of phosphoinositide signaling. There are two evolutionarily distinct families of PITPs: the Sec14-like and the StAR-related lipid transfer domain (START)-like families. Of these two families, the START-like PITPs are the least understood. Herein, we review recent insights into the biochemical, cellular, and physiological function of both PITP families with greater emphasis on the START-like PITPs, and we discuss the underlying mechanisms through which these proteins regulate phosphoinositide signaling and how these actions translate to human health and disease.

Age-Related Expression of Human AT1R Variants and Associated Renal Dysfunction in Transgenic Mice

BACKGROUND

The contribution of single nucleotide polymorphisms in transcriptional regulation of the human angiotensin receptor type I (hAT1R) gene in age-related chronic pathologies such as hypertension and associated renal disorders is not well known. The hAT1R gene has single nucleotide polymorphisms in its promoter that forms 2 haplotypes (Hap), Hap-I and Hap-II. Hap-I of AT1R gene is associated with hypertension in Caucasians. We have hypothesized here that age will alter the transcriptional environment of the cell and will regulate the expression of hAT1R gene in a haplotype-dependent manner. This could likely make subjects with Hap-I increasingly susceptible to age-associated, AT1R-mediated complications.

METHOD

We generated transgenic (TG) mice with Hap-I and Hap-II. Adults (10-12 weeks) and aged (20-24 months) TG male mice containing either Hap-I or Hap-II were divided into 4 groups to study (i) the age-associated and haplotype-specific transcriptional regulation of hAT1R gene and (ii) their physiological relevance.

RESULTS

In aged animals, TG mice with Hap-I show increased expression of hAT1R and higher blood pressure (BP); suppression of antioxidant defenses (hemoxygenase, superoxide dismutase) and antiaging molecules (ATRAP, Klotho, Sirt3); increased expression of pro-inflammatory markers (IL-6, TNFα, CRP, NOX1); and increased insulin resistance. In vivo ChIP assay shows stronger binding of transcription factor USF2 to the chromatin of Hap-I mice.

CONCLUSION

Our results suggest that in aged animals, as compared with Hap-II, the TG mice with Hap-I overexpress hAT1R gene due to the stronger transcriptional activity, thus resulting in an increase in their BP and associated renal disorders.

G Protein-Coupled Receptor Systems as Crucial Regulators of DNA Damage Response Processes

G protein-coupled receptors (GPCRs) and their associated proteins represent one of the most diverse cellular signaling systems involved in both physiological and pathophysiological processes. Aging represents perhaps the most complex biological process in humans and involves a progressive degradation of systemic integrity and physiological resilience. This is in part mediated by age-related aberrations in energy metabolism, mitochondrial function, protein folding and sorting, inflammatory activity and genomic stability. Indeed, an increased rate of unrepaired DNA damage is considered to be one of the 'hallmarks' of aging. Over the last two decades our appreciation of the complexity of GPCR signaling systems has expanded their functional signaling repertoire. One such example of this is the incipient role of GPCRs and GPCR-interacting proteins in DNA damage and repair mechanisms. Emerging data now suggest that GPCRs could function as stress sensors for intracellular damage, e.g., oxidative stress. Given this role of GPCRs in the DNA damage response process, coupled to the effective history of drug targeting of these receptors, this suggests that one important future activity of GPCR therapeutics is the rational control of DNA damage repair systems.

Mechanisms in hypertension and target organ damage: Is the role of the thymus key? (Review)

A variety of cells and cytokines have been shown to be involved in the whole process of hypertension. Data from experimental and clinical studies on hypertension have confirmed the key roles of immune cells and inflammation in the process. Dysfunction of the thymus, which modulates the development and maturation of lymphocytes, has been shown to be associated with the severity of hypertension. Furthermore, gradual atrophy, functional decline or loss of the thymus has been revealed to be associated with aging. The restoration or enhancement of thymus function via upregulation in the expression of thymus transcription factors forkhead box N1 or thymus transplantation may provide an option to halt or reverse the pathological process of hypertension. Therefore, the thymus may be key in hypertension and associated target organ damage, and may provide a novel treatment strategy for the clinical management of patients with hypertension in addition to different commercial drugs. The purpose of this review is to summarize and discuss the advances in our understanding of the impact of thymus function on hypertension from data from animal and human studies, and the potential mechanisms.

Angiotensin II Type 1 Receptor-Associated Protein Regulates Kidney Aging and Lifespan Independent of Angiotensin

Background

The kidney is easily affected by aging‐associated changes, including glomerulosclerosis, tubular atrophy, and interstitial fibrosis. Particularly, renal tubulointerstitial fibrosis is a final common pathway in most forms of progressive renal disease. Angiotensin II type 1 receptor (AT1R)‐associated protein (ATRAP), which was originally identified as a molecule that binds to AT1R, is highly expressed in the kidney. Previously, we have shown that ATRAP suppresses hyperactivation of AT1R signaling, but does not affect physiological AT1R signaling.

Methods and Results

We hypothesized that ATRAP has a novel functional role in the physiological age‐degenerative process, independent of modulation of AT1R signaling. ATRAP‐knockout mice were used to study the functional involvement of ATRAP in the aging. ATRAP‐knockout mice exhibit a normal age‐associated appearance without any evident alterations in physiological parameters, including blood pressure and cardiovascular and metabolic phenotypes. However, in ATRAP‐knockout mice compared with wild‐type mice, the following takes place: (1) age‐associated renal function decline and tubulointerstitial fibrosis are more enhanced; (2) renal tubular mitochondrial abnormalities and subsequent increases in the production of reactive oxygen species are more advanced; and (3) life span is 18.4% shorter (median life span, 100.4 versus 123.1 weeks). As a key mechanism, age‐related pathological changes in the kidney of ATRAP‐knockout mice correlated with decreased expression of the prosurvival gene, Sirtuin1. On the other hand, chronic angiotensin II infusion did not affect renal sirtuin1 expression in wild‐type mice.

Conclusions

These results indicate that ATRAP plays an important role in inhibiting kidney aging, possibly through sirtuin1‐mediated mechanism independent of blocking AT1R signaling, and further protecting normal life span.

An angiotensin II type 1 receptor binding molecule has a critical role in hypertension in a chronic kidney disease model

Angiotensin II type 1 receptor-associated protein (ATRAP) promotes AT1R internalization along with suppression of hyperactivation of tissue AT1R signaling. Here, we provide evidence that renal ATRAP plays a critical role in suppressing hypertension in a mouse remnant kidney model of chronic kidney disease. The effect of 5/6 nephrectomy on endogenous ATRAP expression was examined in the kidney of C57BL/6 and 129/Sv mice. While 129/Sv mice with a remnant kidney showed decreased renal ATRAP expression and developed hypertension, C57BL/6 mice exhibited increased renal ATRAP expression and resistance to progressive hypertension. Consequently, we hypothesized that downregulation of renal ATRAP expression is involved in pathogenesis of hypertension in the remnant kidney model of chronic kidney disease. Interestingly, 5/6 nephrectomy in ATRAP-knockout mice on the hypertension-resistant C57BL/6 background caused hypertension with increased plasma volume. Moreover, in knockout compared to wild-type C57BL/6 mice after 5/6 nephrectomy, renal expression of the epithelial sodium channel α-subunit and tumor necrosis factor-α was significantly enhanced, concomitant with increased plasma membrane angiotensin II type 1 receptor in the kidneys. Thus, renal ATRAP downregulation is involved in the onset and progression of blood pressure elevation caused by renal mass reduction, and implicates ATRAP as a therapeutic target for hypertension in chronic kidney disease.

International Union of Basic and Clinical Pharmacology. XCIX. Angiotensin Receptors: Interpreters of Pathophysiological Angiotensinergic Stimuli [corrected]

The renin angiotensin system (RAS) produced hormone peptides regulate many vital body functions. Dysfunctional signaling by receptors for RAS peptides leads to pathologic states. Nearly half of humanity today would likely benefit from modern drugs targeting these receptors. The receptors for RAS peptides consist of three G-protein-coupled receptors—the angiotensin II type 1 receptor (AT1 receptor), the angiotensin II type 2 receptor (AT2 receptor), the MAS receptor—and a type II trans-membrane zinc protein—the candidate angiotensin IV receptor (AngIV binding site). The prorenin receptor is a relatively new contender for consideration, but is not included here because the role of prorenin receptor as an independent endocrine mediator is presently unclear. The full spectrum of biologic characteristics of these receptors is still evolving, but there is evidence establishing unique roles of each receptor in cardiovascular, hemodynamic, neurologic, renal, and endothelial functions, as well as in cell proliferation, survival, matrix-cell interaction, and inflammation. Therapeutic agents targeted to these receptors are either in active use in clinical intervention of major common diseases or under evaluation for repurposing in many other disorders. Broad-spectrum influence these receptors produce in complex pathophysiological context in our body highlights their role as precise interpreters of distinctive angiotensinergic peptide cues. This review article summarizes findings published in the last 15 years on the structure, pharmacology, signaling, physiology, and disease states related to angiotensin receptors. We also discuss the challenges the pharmacologist presently faces in formally accepting newer members as established angiotensin receptors and emphasize necessary future developments.

MicroRNA network changes in the brain stem underlie the development of hypertension

Hypertension is a major chronic disease whose molecular mechanisms remain poorly understood. We compared neuroanatomical patterns of microRNAs in the brain stem of the spontaneous hypertensive rat (SHR) to the Wistar Kyoto rat (WKY, control). We quantified 419 well-annotated microRNAs in the nucleus of the solitary tract (NTS) and rostral ventrolateral medulla (RVLM), from SHR and WKY rats, during three main stages of hypertension development. Changes in microRNA expression were stage- and region-dependent, with a majority of SHR vs. WKY differential expression occurring at the hypertension onset stage in NTS versus at the prehypertension stage in RVLM. Our analysis identified 24 microRNAs showing time-dependent differential expression in SHR compared with WKY in at least one brain region. We predicted potential gene regulatory targets corresponding to catecholaminergic processes, neuroinflammation, and neuromodulation using the miRWALK and RNA22 databases, and we tested those bioinformatics predictions using high-throughput quantitative PCR to evaluate correlations of differential expression between the microRNAs and their predicted gene targets. We found a novel regulatory network motif consisting of microRNAs likely downregulating a negative regulator of prohypertensive processes such as angiotensin II signaling and leukotriene-based inflammation. Our results provide new evidence on the dynamics of microRNA expression in the development of hypertension and predictions of microRNA-mediated regulatory networks playing a region-dependent role in potentially altering brain-stem cardiovascular control circuit function leading to the development of hypertension.

Angiotensin II type 1 receptor-associated protein plays a role in regulating the local renin-angiotensin system in HSC-T6 cells

The aim of the current study was to examine the expression of the angiotensin II type 1 receptor‑associated protein (ATRAP) in the rat hepatic stellate cell line HSC‑T6 and to determine its interactions with the local renin‑angiotensin system (RAS). To achieve this goal, the effect of stimulating HSC‑T6 cells with angiotensin II (AngII) and angiotensin‑(1‑7) [Ang‑(1‑7)], on the expression of ATRAP, the angiotensin II type I receptor (AT1R), the Mas receptor and the angiotensin converting enzyme 2 (ACE2) 2, 6, 12, 18, 24 and 36 h after stimulation was investigated. Changes in expression were quantified at the gene and protein level using RT‑qPCR and western blotting, respectively. A single dose of AngII (1 µmol/l) significantly increased the gene expression of ATRAP at 12 h, whereas ACE2 gene expression levels were significantly increased at 6 h and then returned to baseline at 12 h, prior to becoming significantly lower. A single dose of Ang‑(1‑7) at the same concentration as AngII induced ATRAP gene expression, which became statistically significant at the 6 h time‑point, reached a peak at 12 h and remained elevated throughout the experimental time‑course. In addition, ACE2 mRNA expression was significantly suppressed by Ang‑(1‑7) at 6 h, reaching its lowest expression level at 24 h. The expression of AT1R and the Mas receptor were unaffected by stimulation with AngII and Ang‑(1‑7). The western blotting results were generally consistent with the mRNA expression data. In conclusion, it was identified that ATRAP is endogenously expressed in HSC‑T6 cells and therefore, may be critical in regulating the local RAS in these cells.

Renal tubule angiotensin II type 1 receptor-associated protein promotes natriuresis and inhibits salt-sensitive blood pressure elevation

BACKGROUND

Angiotensin II type 1 receptor (AT1R)-associated protein (ATRAP; Agtrap gene) promotes AT1R internalization along with suppression of pathological AT1R activation. In this study, we examined whether enhancement of ATRAP in the renal distal tubules affects sodium handling and blood pressure regulation in response to high salt (HS) loading, using ATRAP transgenic mice on a salt-sensitive C57BL/6J background.

METHODS AND RESULTS

Renal ATRAP transgenic (rATRAP-Tg) mice, which exhibit renal tubule-dominant ATRAP enhancement, and their wild-type littermate C57BL/6J mice on a normal salt diet (0.3% NaCl) at baseline were subjected to dietary HS loading (4% NaCl) for 7 days. In rATRAP-Tg mice, the dietary HS loading-mediated blood pressure elevation was suppressed compared with wild-type mice, despite similar baseline blood pressure. Although renal angiotensin II level was comparable in rATRAP-Tg and wild-type mice with and without HS loading, urinary sodium excretion in response to HS loading was significantly enhanced in the rATRAP-Tg mice. In addition, functional transport activity of the amiloride-sensitive epithelial Na(+) channel was significantly decreased under saline volume-expanded conditions in rATRAP-Tg mice compared with wild-type mice, without any evident change in epithelial Na(+) channel protein expression. Plasma membrane AT1R expression in the kidney of rATRAP-Tg mice was decreased compared with wild-type mice.

CONCLUSIONS

These results demonstrated that distal tubule-dominant enhancement of ATRAP inhibits pathological renal sodium reabsorption and blood pressure elevation in response to HS loading. The findings suggest that ATRAP-mediated modulation of sodium handling in renal distal tubules could be a target of interest in salt-sensitive blood pressure regulation.

Down-regulation of cyclooxygenase-2 by the carboxyl tail of the angiotensin II type 1 receptor

The enzyme cyclooxygenase-2 (COX-2) plays an important role in the kidney by up-regulating the production of the vasoconstrictor hormone angiotensin II (AngII), which in turn down-regulates COX-2 expression via activation of the angiotensin II type 1 receptor (AT1) receptor. Chemical inhibition of the catalytic activity of COX-2 is a well-established strategy for treating inflammation but little is known of cellular mechanisms that dispose of the protein itself. Here we show that in addition to its indirect negative feedback on COX-2, AT1 also down-regulates the expression of the COX-2 protein via a pathway that does not involve G-protein or β-arrestin-dependent signaling. Instead, AT1 enhances the ubiquitination and subsequent degradation of the enzyme in the proteasome through elements in its cytosolic carboxyl tail (CT). We find that a mutant receptor that lacks the last 35 amino acids of its CT (Δ324) is devoid of its ability to reduce COX-2, and that expression of the CT sequence alone is sufficient to down-regulate COX-2. Collectively these results propose a new role for AT1 in regulating COX-2 expression in a mechanism that deviates from its canonical signaling pathways. Down-regulation of COX-2 by a short peptide that originates from AT1 may present as a basis for novel therapeutic means of eliminating excess COX-2 protein.

Deletion of the angiotensin II type 1 receptor-associated protein enhances renal sodium reabsorption and exacerbates angiotensin II-mediated hypertension

Angiotensin II type 1 receptor (AT1R)–associated protein (ATRAP) promotes AT1R internalization along with suppression of pathological activation of tissue AT1R signaling. However, the functional significance of ATRAP in renal sodium handling and blood pressure regulation under pathological stimuli is not fully resolved. Here we show the blood pressure of mice with a gene-targeted disruption of ATRAP was comparable to that of wild-type mice at baseline. However, in ATRAP-knockout mice, angiotensin II–induced hypertension was exacerbated and the extent of positive sodium balance was increased by angiotensin II. Renal expression of the sodium-proton antiporter 3, a major sodium transporter in the proximal tubules, urinary pH, renal angiotensinogen production, and angiotensin II content was unaffected. Stimulation of the renal expression and activity of the epithelial sodium channel (ENaC), a major sodium transporter in the distal tubules, was significantly enhanced by chronic angiotensin II infusion. The circulating and urinary aldosterone levels were comparable. The blood pressure response and renal ENaC expression by aldosterone were not affected. Thus, ATRAP deficiency exacerbated angiotensin II–mediated hypertension by pathological activation of renal tubular AT1R by angiotensin II. This directly stimulates ENaC in the distal tubules and enhances sodium retention in an aldosterone-independent manner.

Activation of angiotensin II type 1 receptor-associated protein exerts an inhibitory effect on vascular hypertrophy and oxidative stress in angiotensin II-mediated hypertension

AIMS

Activation of tissue angiotensin II (Ang II) type 1 receptor (AT1R) plays an important role in the development of vascular remodelling. We have shown that the AT1R-associated protein (ATRAP/Agtrap), a specific binding protein of AT1R, functions as an endogenous inhibitor to prevent pathological activation of the tissue renin-angiotensin system. In this study, we investigated the effects of ATRAP on Ang II-induced vascular remodelling.

METHODS AND RESULTS

Transgenic (Tg) mice with a pattern of aortic vascular-dominant overexpression of ATRAP were obtained, and Ang II or vehicle was continuously infused into Tg and wild-type (Wt) mice via an osmotic minipump for 14 days. Although blood pressure of Ang II-infused Tg mice was comparable with that of Ang II-infused Wt mice, the Ang II-mediated development of aortic vascular hypertrophy was partially inhibited in Tg mice compared with Wt mice. In addition, Ang II-mediated up-regulation of vascular Nox4 and p22(phox), NADPH oxidase components, and 4-HNE, a marker of reactive oxygen species (ROS) generation, was significantly suppressed in Tg mice, with a concomitant inhibition of activation of aortic vascular p38MAPK and JNK by Ang II. This protection afforded by vascular ATRAP against Ang II-induced activation of NADPH oxidase is supported by in vitro experimental data using adenoviral transfer of recombinant ATRAP.

CONCLUSION

These results indicate that activation of aortic vascular ATRAP partially inhibits the Nox4/p22(phox)-ROS-p38MAPK/JNK pathway and pathological aortic hypertrophy provoked by Ang II-mediated hypertension, thereby suggesting ATRAP as a novel receptor-binding modulator of vascular pathophysiology.

Potential role for phosphatidylinositol transfer protein (PITP) family in lipid transfer during phospholipase C signalling

The hallmark of mammalian phosphatidylinositol transfer proteins (PITPs) is to transfer phosphatidylinositol between membrane compartments. In the mammalian genome, there are three genes that code for soluble PITP proteins, PITPα, PITPβ and RdgBβ and two genes that code for membrane-associated multi-domain proteins (RdgBαI and II) containing a PITP domain. PITPα and PITPβ constitute Class I PITPs whilst the RdgB proteins constitute Class II proteins based on sequence analysis. The PITP domain of both Class I and II can sequester one molecule of phosphatidylinositol (PI) in its hydrophobic cavity. Therefore, in principle, PITPs are therefore ideally poised to couple phosphatidylinositol delivery to the PI kinases for substrate provision for phospholipases C during cell activation. Since phosphatidylinositol (4,5)bisphosphate plays critical roles in cells, particularly at the plasma membrane, where it is a substrate for both phospholipase C and phosphoinositide-3-kinases as well as required as an intact lipid to regulate ion channels and the actin cytoskeleton, homeostatic mechanisms to maintain phosphatidylinositol(4,5)bisphosphate levels are vital. To maintain phosphatidylinositol levels, phospholipase C activation inevitably leads to the resynthesis of PI at the endoplasmic reticulum where the enzymes are located. Phosphatidic acid generated at the plasma membrane during phospholipase C activation needs to move to the ER for conversion to PI and here we provide evidence that Class II PITPs are also able to bind and transport phosphatidic acid. Thus RdgB proteins could couple PA and PI transport bidirectionally during phospholipase C signalling.

Angiotensin receptor-binding protein ATRAP/Agtrap inhibits metabolic dysfunction with visceral obesity

BACKGROUND

Metabolic disorders with visceral obesity have become a major medical problem associated with the development of hypertension, type 2 diabetes, and dyslipidemia and, ultimately, life-threatening cardiovascular and renal diseases. Adipose tissue dysfunction has been proposed as the cause of visceral obesity-related metabolic disorders, moving the tissue toward a proinflammatory phenotype.

METHODS AND RESULTS

Here we first report that adipose tissues from patients and mice with metabolic disorders exhibit decreased expression of ATRAP/Agtrap, which is a specific binding modulator of the angiotensin II type 1 receptor, despite its abundant expression in adipose tissues from normal human and control mice. Subsequently, to examine a functional role of ATRAP in the pathophysiology of metabolic disorders, we produced homozygous ATRAP deficient (Agtrap(-/-)) mice, which exhibited largely normal physiological phenotype at baseline. Under dietary high fat loading, Agtrap(-/-) mice displayed systemic metabolic dysfunction, characterized by an increased accumulation of pad fat, hypertension, dyslipidemia, and insulin resistance, along with adipose tissue inflammation. Conversely, subcutaneous transplantation of donor fat pads overexpressing ATRAP derived from Agtrap transgenic mice to Agtrap(-/-) recipient mice improved the systemic metabolic dysfunction.

CONCLUSIONS

These results demonstrate that Agtrap(-/-) mice are an effective model of metabolic disorders with visceral obesity and constitute evidence that ATRAP plays a protective role against insulin resistance, suggesting a new therapeutic target in metabolic disorders. Identification of ATRAP as a novel receptor binding modulator of adipose tissue inflammation not only has cardiovascular significance but may have generalized implication in the regulation of tissue function.

The angiotensin II AT1 receptor-associated protein Arap1 is involved in sepsis-induced hypotension

Introduction

Hypotension in septic patients results from hypovolemia, vasodilatation and hyporeactivity to vasoconstrictors, such as angiotensin II. The AT1 receptor-associated protein 1 (Arap1) is expressed in vascular smooth muscle cells and increases the surface expression of the AT1-receptor in vitro. We hypothesized that dysregulation of Arap1 may contribute to vascular hyporeactivity to angiotensin II during endotoxemia.

Methods

Arap1-deficient mice were used to assess the role of Arap1 in sepsis-induced hypotension. The isolated perfused kidney was used as an in vitro model to determine the relevance of Arap1 for vascular resistance and sensitivity to angiotensin II.

Results

During endotoxemia, mean arterial blood pressure (MAP) decreased in both genotypes, with the time course of sepsis-induced hypotension being markedly accelerated in Arap1-/- compared to +/+ mice. However, baseline MAP was similar in Arap1-/- and wildtype mice (102 ± 2 vs.103 ± 2 mmHg; telemetry measurements; n = 10; P = 0.66). Following lipopolysaccharide (LPS) injections (3 mg/kg), Arap1 expression was successively down-regulated in the wildtype mice, reaching levels below 10% of baseline expression. The endotoxemia-related decline in Arap1 expression could be recapitulated in cultured mesangial cells by incubation with pro-inflammatory cytokines, such as tumor necrosis factor α and interferon γ. Plasma renin concentration was increased in Arap1-/- mice compared to wildtype mice (66 ± 6 vs. 41 ± 4 ng AngI/ml/h; n = 23; P = 0.001), presumably contributing to preserved MAP under baseline conditions. The sensitivity of the vasculature to angiotensin II was reduced in Arap1-/- compared to +/+ mice, as determined in the isolated perfused kidney.

Conclusions

Our data suggest that down-regulation of Arap1 expression during sepsis contributes to the development of hypotension by causing reduced vascular sensitivity to angiotensin II.

Two distinct calmodulin binding sites in the third intracellular loop and carboxyl tail of angiotensin II (AT(1A)) receptor

In this study, we present data that support the presence of two distinct calmodulin binding sites within the angiotensin II receptor (AT_1A), at juxtamembrane regions of the N-terminus of the third intracellular loop (i3, amino acids 214–231) and carboxyl tail of the receptor (ct, 302–317). We used bioluminescence resonance energy transfer assays to document interactions of calmodulin with the AT_1A holo-receptor and GST-fusion protein pull-downs to demonstrate that i3 and ct interact with calmodulin in a Ca²⁺-dependent fashion. The former is a 1–12 motif and the latter belongs to 1-5-10 calmodulin binding motif. The apparent Kd of calmodulin for i3 is 177.0±9.1 nM, and for ct is 79.4±7.9 nM as assessed by dansyl-calmodulin fluorescence. Replacement of the tryptophan (W219) for alanine in i3, and phenylalanine (F309 or F313) for alanine in ct reduced their binding affinities for calmodulin, as predicted by computer docking simulations. Exogenously applied calmodulin attenuated interactions between G protein βγ subunits and i3 and ct, somewhat more so for ct than i3. Mutations W219A, F309A, and F313A did not alter Gβγ binding, but reduced the ability of calmodulin to compete with Gβγ, suggesting that calmodulin and Gβγ have overlapping, but not identical, binding requirements for i3 and ct. Calmodulin interference with the Gβγ binding to i3 and ct regions of the AT_1A receptor strongly suggests that calmodulin plays critical roles in regulating Gβγ-dependent signaling of the receptor.

Upstream stimulatory factors 1 and 2 mediate the transcription of angiotensin II binding and inhibitory protein

The angiotensin II type 1 receptor (AT1R)-associated protein (ATRAP/Agtrap) promotes constitutive internalization of the AT1R so as to specifically inhibit the pathological activation of its downstream signaling yet preserve the base-line physiological signaling activity of the AT1R. Thus, tissue-specific regulation of Agtrap expression is relevant to the pathophysiology of cardiovascular and renal disease. However, the regulatory mechanism of Agtrap gene expression has not yet been fully elucidated. In this study, we show that the proximal promoter region from −150 to +72 of the mouse Agtrap promoter, which contains the X-box, E-box, and GC-box consensus motifs, is able to elicit substantial transcription of the Agtrap gene. Among these binding motifs, we showed that the E-box specifically binds upstream stimulatory factor (Usf) 1 and Usf2, which are known E-box-binding transcription factors. It is indicated that the E-box-Usf1/Usf2 binding regulates Agtrap expression because of the following: 1) mutation of the E-box to prevent Usf1/Usf2 binding reduces Agtrap promoter activity; 2) knockdown of Usf1 or Usf2 affects both endogenous Agtrap mRNA and Agtrap protein expression, and 3) the decrease in Agtrap mRNA expression in the afflicted kidney by unilateral ureteral obstruction is accompanied by changes in Usf1 and Usf2 mRNA. Furthermore, the results of siRNA transfection in mouse distal convoluted tubule cells and those of unilateral ureteral obstruction in the afflicted mouse kidney suggest that Usf1 decreases but Usf2 increases the Agtrap gene expression by binding to the E-box. The results also demonstrate a functional E-box-USF1/USF2 interaction in the human AGTRAP promoter, thereby suggesting that a strategy of modulating the E-box-USF1/USF2 binding has novel therapeutic potential.

Shear stress activates extracellular signal-regulated kinase 1/2 via the angiotensin II type 1 receptor

Mechanical factors such as strain, pressure, and shear stress are key regulators of cell function, but the molecular mechanisms underlying the detection and responses to such stimuli are poorly understood. Whether the angiotensin II (AngII) AT1 receptor (AT1R) transduces shear stress in endothelial cells (ECs) is unknown. We exposed human umbilical cord endothelial cells (HUVECs) to a shear stress of 0 (control) or 15 dyn/cm(2) for 5 or 10 min. The colocalization of AT1R with caveolin-1 (Cav1), endosomal markers Rab5, EEA1, and Rab7, and lysosomal marker Lamp-1 increased in shear stimulated cells, detected by immunocytochemistry. Shear stress reduced labeling of wild-type mouse ECs (18±3% of unsheared control, P<0.01) but not Cav1(-/-) ECs (90±10%) with fluorescent AngII, confirming that internalization of AT1R requires Cav1. Shear stress activated ERK1/2 2-fold (P<0.01), which was prevented by the AT1R blocker losartan. NADPH oxidase inhibition with apocynin prevented both the colocalization of AT1R with Cav1 and the induction of ERK1/2 by shear stress. Moreover, shear-dependent ERK1/2 activation was minimal in CHO cells expressing an AT1Ra mutant that does not internalize, compared with cells expressing wild-type AT1Ra (P<0.05). Hence, AT1R may be an important transducer of shear stress-dependent activation of ERK1/2.

Enhanced angiotensin receptor-associated protein in renal tubule suppresses angiotensin-dependent hypertension

We have previously shown that angiotensin II type 1 receptor-associated protein (ATRAP/Agtrap) interacts with the angiotensin II type 1 receptor and promotes constitutive internalization of the receptor so as to inhibit the pathological activation of its downstream signaling but preserve baseline physiological signaling activity. The present study was designed to investigate the role of renal ATRAP in angiotensin II-dependent hypertension. We generated transgenic mice dominantly expressing ATRAP in the renal tubules, including renal distal tubules. The renal ATRAP transgenic mice exhibited no significant change in blood pressure at baseline on normal salt diet. However, in the renal ATRAP transgenic mice compared with wild-type mice, the following took place: (1) the development of high blood pressure in response to angiotensin II infusion was significantly suppressed based on radiotelemetry, (2) the extent of daily positive sodium balance was significantly reduced during angiotensin II infusion in metabolic cage analysis, and (3) the renal Na+ -Cl- cotransporter activation and α-subunit of the epithelial sodium channel induction by angiotensin II infusion were inhibited. Furthermore, adenoviral overexpression of ATRAP suppressed the angiotensin II-mediated increase in the expression of α-subunit of the epithelial sodium channel in mouse distal convoluted tubule cells. These results indicate that renal tubule-dominant ATRAP activation provokes no evident effects on blood pressure at baseline but exerts an inhibitory effect on the pathological elevation of blood pressure in response to angiotensin II stimulation, thereby suggesting that ATRAP is a potential target of interest in blood pressure modulation under pathological conditions.