Angiotensin II Type 1 Receptor–Associated Protein Is an Endogenous Inhibitor of Angiotensin II Type 1 Receptor Action in Cardiac Hypertrophy
Role in Check and Balance
Angiotensin II (Ang II) type 1 (AT1) receptor is the primary effector of the renin-angiotensin system and mediates many key physiological and pathological actions of Ang II through a complex orchestration of intracellular signaling molecules. As a G protein–coupled receptor, AT1 receptor activation induces the canonical Gq protein–dependent inositol phosphate turnover and intracellular calcium release pathway, as well as receptor phosphorylation by G protein–coupled receptor kinases and the recruitment of β-arrestin as a G protein–independent pathway to mediate the cellular effects of Ang II.1 Recent investigations have highlighted the importance of the AT1 receptor carboxyl-terminal domain that binds to a variety of intracellular proteins, such as G protein–coupled receptor kinases and β-arrestin, and plays a pivotal role on receptor internalization, desensitization, phosphorylation, and coupling to G proteins.1 A relatively new and potentially important player in this orchestra is the AT1 receptor–associated protein (ATRAP) that interacts with the carboxyl-terminal domain of the AT1 receptor. ATRAP was first identified and isolated by yeast 2-hybrid screening from the mouse kidney cDNA library in the Dzau laboratory.2
ATRAP is a relatively small 18-kDa protein with 1 potential N-glycosylation site, 1 potential phosphorylation site for protein kinase C, 1 potential phosphorylation site for casein kinase II, and 3 hydrophobic domains for potential transmembrane binding.2,3 ATRAP localizes in intracellular trafficking vesicles and plasma membrane, including endoplasmic reticulum, Golgi, and endocytic vesicles.3 ATRAP interacts selectively with the carboxyl-terminal domain of the AT1 receptor but not with those of Ang II type 2, m3 muscarinic acetylcholine, bradykinin B2, endothelin B, and β2-adrenergic receptors.2 ATRAP enhances AT1 receptor desensitization and internalization.2–4
The key question about ATRAP is its function. Tamura’s laboratory showed that, in vascular smooth muscle cells in vitro, ATRAP colocalized with the AT1 receptor, promoted receptor internalization, and attenuated the Ang II–mediated c-fos–transforming growth factor-β pathway and proliferative response, suggesting that ATRAP functions as a negative regulator of AT1 receptor–mediated cell proliferation.5 Indeed, ATRAP inhibited inflammatory vascular remodeling in vivo. Horiuchi’s laboratory showed that ATRAP overexpressing transgenic (Tg) mice exhibited attenuations in vascular smooth muscle cell proliferation and neointimal formation in the injured artery that was associated with decreases in NADPH oxidase activity and p22phox expression, as well as reductions in the activations of extracellular signal-regulated kinase and signal transducer and activator of transcription 1 and 3.6 In addition, recent research showed that ATRAP decreased AT1 receptor–mediated vascular smooth muscle cell senescence via inactivation with the calcineurin/nuclear factor of activated T cells pathway.7
The inhibitory effect of ATRAP on Ang II action can be also demonstrated in the kidney. ATRAP knockout mice exhibited increased plasma volume and elevated arterial blood pressure. Renal cortical Ang II binding and acetazolamide-sensitive tubular function were enhanced in ATRAP knockout mice. These results demonstrate that ATRAP is a negative modulator of renal Ang II signaling and that loss of ATRAP results in enhanced renal reabsorptive function, leading to volume expansion and hypertension.8
As for the heart, ATRAP is expressed in cardiomyocytes. ATRAP promotes the downregulation of the AT1 receptor and attenuates certain Ang II–mediated hypertrophic responses in vitro.9 In this issue of Hypertension, using cardiomyocyte-specific ATRAP Tg mice, Wakui et al10 investigated the function of ATRAP in cardiac hypertrophy and tested the hypothesis that a downregulation of the cardiac ATRAP:AT1R ratio is involved in the Ang II–mediated cardiac hypertrophy in vivo. Indeed, the authors demonstrated that Ang II stimulation of wild-type mice induced cardiac hypertrophy accompanied by a significant reduction of ATRAP expression without a significant change of AT1 receptor expression. In contrast, in the cardiac-specific ATRAP Tg mice, the ATRAP:AT1 receptor ratio was increased, accompanied by a complete blockade of Ang II–induced cardiac hypertrophy. This was associated with suppression of the AT1 receptor downstream signal and hypertrophy-related gene expression. These data provide strong evidence that ATRAP is an endogenous inhibitor of AT1 receptor action on cardiac hypertrophy. It is important to note that, at baseline, there were no differences in heart weight or any physiological parameter between cardiac-specific ATRAP Tg mice and their littermate controls. In response to Ang II infusion, both groups of animals exhibited increases in arterial blood pressure but with discrepant cardiac hypertrophic responses. Thus, the inhibitory effect of cardiac-specific overexpression of ATRAP on cardiac hypertrophy was only detected in the presence of Ang II infusion and/or increased blood pressure. The effect of increased pressure (using a different pressor) versus Ang receptor (AT1 versus Ang II type 2) activation on ATRAP activity must be dissected further to understand the biological function of ATRAP on cardiac hypertrophy.
What is the molecular mechanism of ATRAP suppression on cardiac hypertrophy, and what is the relationship among the activations of ATRAP, AT1 receptor G protein–dependent pathway, and G protein–independent pathway? It is clear that future research should focus on elucidating the complex intracellular interactions of ATRAP with the other key players of the orchestra. With regard to Ang II–dependent hypertrophic effects in cardiomyocytes, Ang II acts on fetal cardiomyocytes through the classic G protein–coupled receptor pathway by activation of both AT1 and Ang II type 2 receptors, whereas Ang II acts on neonatal cardiomyocytes through stimulation of NADPH oxidase and subsequent activation of p38 mitogen-activated protein kinase, activator protein 1, and nuclear factor-κB by activation of the AT1 receptor. However, a direct hypertrophic effect of Ang II in adult cardiomyocytes is less convincing.11 In vivo evidence would suggest that, in pressure-dependent myocardial hypertrophy, Ang II plays an important amplifying role in the hypertrophic response to pressure overload. Ang II may act through tissue-specific cross-talks with other angiotensin peptides produced by angiotensin-converting enzyme isoforms, through differentially expressed AT1 and Ang II type 2 receptors, or through tissue specific networks, such as cross-activation of the epidermal growth factor receptor (for detail please see the review and the references within11). In the study by Wakui et al,10 although they demonstrated that the activation of p38 mitogen-activated protein kinase and the expression of atrial natriuretic peptide and brain natriuretic peptide by Ang II treatment were suppressed in the ATRAP Tg mice, the detailed molecular mechanisms and the relationship among the activations of ATRAP, G protein–dependent, and G protein–independent pathways need further clarification.
In summary, the study by Wakui et al10 reveals for the first time the inhibitory function of ATRAP, a unique signaling molecule for the AT1 receptor, on cardiac hypertrophy in vivo. It appears that ATRAP functions as a negative regulator in AT1 receptor–mediated action in the presence of hypertrophic stimuli. Clearly, ATRAP provides a new therapeutic target for cardiovascular disease. For example, ATRAP-activating ligand may be helpful in reducing pathological actions of the AT1 receptor. Ligand-induced functional selectivity is well established for the AT1 receptor,12 and biased ligand selection for ATRAP may be possible. Another therapeutic approach is to develop drugs that enhance ATRAP expression or increase the endogenous tissue ATRAP:AT1 receptor ratio. Indeed, in a recent study, prehypertensive preconditioning by AT1 receptor blockade associated with a reduced cardiovascular AT1 receptor expression and enhanced ATRAP expression improved cardiovascular protection of late-onset AT1 blockade in a model of spontaneous hypertensive heart-failure rats.13 Finally, to date, all of the ATRAP studies are based on in vitro or in vivo animal models, and further studies on ATRAP function in human health and diseases are clearly warranted.
Sources of Funding
This work was supported by grants from the National Heart, Lung, and Blood Institute (RO1 HL35610, HL58516, HL72010, and HL73219 to V.J.D.), the Edna and Fred L. Mandel, Jr, Foundation, and the Fondation Leducq (to V.J.D.).
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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