Pharmacological characterization of angiotensin II AT(2) receptor subtype heterogeneity in the rat adrenal cortex and medulla

The Angiotensin AT2 Receptor: From a Binding Site to a Novel Therapeutic Target

Discovered more than 30 years ago, the angiotensin AT2 receptor (AT2R) has evolved from a binding site with unknown function to a firmly established major effector within the protective arm of the renin-angiotensin system (RAS) and a target for new drugs in development. The AT2R represents an endogenous protective mechanism that can be manipulated in the majority of preclinical models to alleviate lung, renal, cardiovascular, metabolic, cutaneous, and neural diseases as well as cancer. This article is a comprehensive review summarizing our current knowledge of the AT2R, from its discovery to its position within the RAS and its overall functions. This is followed by an in-depth look at the characteristics of the AT2R, including its structure, intracellular signaling, homo- and heterodimerization, and expression. AT2R-selective ligands, from endogenous peptides to synthetic peptides and nonpeptide molecules that are used as research tools, are discussed. Finally, we summarize the known physiological roles of the AT2R and its abundant protective effects in multiple experimental disease models and expound on AT2R ligands that are undergoing development for clinical use. The present review highlights the controversial aspects and gaps in our knowledge of this receptor and illuminates future perspectives for AT2R research. SIGNIFICANCE STATEMENT: The angiotensin AT2 receptor (AT2R) is now regarded as a fully functional and important component of the renin-angiotensin system, with the potential of exerting protective actions in a variety of diseases. This review provides an in-depth view of the AT2R, which has progressed from being an enigma to becoming a therapeutic target.

Novel high molecular weight albumin-conjugated angiotensin II activates β-arrestin and G-protein pathways

PURPOSE

To study the ability of a novel bovine serum albumin-angiotensin II (BSA-Ang II) conjugate to effect responses of the AT₁ angiotensin II receptor subtype mediated by the G-protein-coupled and the beta-arrestin pathways.

METHODS

Angiotensin II (Ang II) was conjugated with bovine serum albumin and compared with Ang II for competition binding to AT₁ receptors, to stimulate aldosterone release from adrenocortical cells, to promote beta-arrestin binding to AT₁ receptors, to promote calcium mobilization, and stimulate drinking of water and saline by rats.

RESULTS

The BSA-Ang II conjugate was less potent competing for AT₁R binding, but was equally efficacious at stimulating aldosterone release from H295R adrenocortical cells. Both BSA-Ang II and Ang II stimulated calcium mobilization and beta-arrestin binding to AT₁ receptors. BSA-Ang II and Ang II stimulated water appetite equivalently but BSA-Ang II stimulated saline appetite more than Ang II. Both BSA-Ang II and Ang II were considerably more potent at causing calcium mobilization than β-arrestin binding.

CONCLUSIONS

Addition of a high molecular weight molecule to Ang II reduced its AT₁ receptor binding affinity, but did not significantly alter stimulation of aldosterone release or water consumption. The BSA-Ang II conjugate caused a greater saline appetite than Ang II suggesting that it may be a more efficacious agonist of this beta-arrestin-mediated response than Ang II. The higher potency calcium signaling response suggests that the G-protein-coupled responses predominate at physiological concentrations of Ang II, while the beta-arrestin response requires pathophysiological or pharmacological concentrations of Ang II to occur.

Regulation of zonation and homeostasis in the adrenal cortex

The adult adrenal cortex is organized into concentric zones, each specialized to produce distinct steroid hormones. Cellular composition of the cortex is highly dynamic and subject to diverse signaling controls. Cortical homeostasis and regeneration rely on centripetal migration of steroidogenic cells from the outer to the inner cortex, which is accompanied by direct conversion of zona glomerulosa (zG) into zona fasciculata (zF) cells. Given the important impact of tissue structure and growth on steroidogenic function, it is essential to understand the mechanisms governing adrenal zonation and homeostasis. Towards this end, we review the distinctions between each zone by highlighting their morphological and ultra-structural features, discuss key signaling pathways influencing zonal identity, and evaluate current evidence for long-term self-renewing stem cells in the adult cortex. Finally, we review data supporting zG-to-zF transdifferentiation/direct conversion as a major mechanism of adult cortical renewal.

Steroidogenesis-adrenal cell signal transduction

The purpose of this article is to review fundamentals in adrenal gland histophysiology. Key findings regarding the important signaling pathways involved in the regulation of steroidogenesis and adrenal growth are summarized. We illustrate how adrenal gland morphology and function are deeply interconnected in which novel signaling pathways (Wnt, Sonic hedgehog, Notch, β-catenin) or ionic channels are required for their integrity. Emphasis is given to exploring the mechanisms and challenges underlying the regulation of proliferation, growth, and functionality. Also addressed is the fact that while it is now well-accepted that steroidogenesis results from an enzymatic shuttle between mitochondria and endoplasmic reticulum, key questions still remain on the various aspects related to cellular uptake and delivery of free cholesterol. The significant progress achieved over the past decade regarding the precise molecular mechanisms by which the two main regulators of adrenal cortex, adrenocorticotropin hormone (ACTH) and angiotensin II act on their receptors is reviewed, including structure-activity relationships and their potential applications. Particular attention has been given to crucial second messengers and how various kinases, phosphatases, and cytoskeleton-associated proteins interact to ensure homeostasis and/or meet physiological demands. References to animal studies are also made in an attempt to unravel associated clinical conditions. Many of the aspects addressed in this article still represent a challenge for future studies, their outcome aimed at providing evidence that the adrenal gland, through its steroid hormones, occupies a central position in many situations where homeostasis is disrupted, thus highlighting the relevance of exploring and understanding how this key organ is regulated. © 2014 American Physiological Society. Compr Physiol 4:889-964, 2014.

Immunohistochemical detection of angiotensin receptors AT1 and AT2 in adrenal tumors

Angiotensin II is well known to affect the adrenal cell growth and function. Angiotensin receptors AT1 and AT2 were found to be present in the normal adrenal gland. However, the data on the expression of the angiotensin receptors in the adrenal tumors are very scarce. To overcome this gap, the paraffin sections of the adrenal cortical tumors and of pheochromocytomas from the archival material were immunostained with antibodies raised against AT1 (sc-1173) and AT2 (sc-9040) receptor proteins. In hyperplasia of the adrenal cortex and in benign adrenocortical adenomas, both functioning and non-functioning, the AT1 immunostaining was present mainly in the cell membranes. A positive immunoreaction was also found in the subpopulation of cell nuclei and within the cytoplasm. In the adrenal cancer, as well as in pheochromocytomas, neither cell membranes nor cell nuclei were immunostained with anti-AT1 antibody. However, a weak AT1 immunostaining was present within the cytoplasm of tumoral cells. With anti-AT2 antibody, in all tumors investigated, the tumoral cells were immunonegative but moderate to strong AT2 immunostaining was observed in the walls of intratumoral blood vessels and in the interstitial tissue. Our data indicates that the expression of AT1 receptors is altered in adrenal cancer and in pheochromocytomas. The expression of AT2 receptors, in turn, may be connected with the process of tumoral neo-angiogenesis.

Sarcosine1,glycine8 angiotensin II is an AT1 angiotensin II receptor subtype selective antagonist

Studies predating the discovery of the two major subtypes of angiotensin II (Ang II) receptors, AT1 and AT2, revealed anomalous characteristics of sarcosine1,glycine8 Ang II (Sar1,Gly8 Ang II). It competed poorly for 125I-Ang II binding in bovine brain but potently antagonized dipsogenic responses to intracerebroventricularly administered Ang II. Subsequent recognition that bovine brain contains AT(2) receptors, while dipsogenic responses to Ang II are mediated by AT1 receptors, suggests that Sar1,Gly(8) Ang II is AT1 selective. Sar1,Gly8 Ang II competed for 125I-sarcosine1,isoleucine8 Ang II binding to AT1 receptors in pituitary, liver and adrenal (the latter with the AT2 selective antagonist PD 123,319) with Ki's of 0.66, 1.40 and 1.36 nM, respectively. In contrast, the Ki of Sar1,Gly8 Ang II for AT2 receptors in rat adrenal (with the selective AT1 antagonist losartan) was 52 nM. 125I-Sar1,Gly8 Ang II (0.5-3 nM) bound to AT1 receptors in pituitary, liver, heart, adrenal, and hypothalamic membranes with high affinity (Kd=0.43, 1.6, 2.3, 0.96 and 1.8 nM, respectively), but showed no saturable binding to the adrenal AT2 receptor. 125I-Sar1,Gly8 Ang II selectively labeled AT1 receptors in sections of adrenal using receptor autoradiography. Thus, binding studies reveal Sar1,Gly8 Ang II to be the first angiotensin peptide analog to show AT1 receptor selectivity. 125I-Sar1,Gly8 Ang II offers a new means to selectively radiolabel AT1 receptors and may help to characterize ligand docking sites and agonist switches for AT1 versus AT2 receptors.

Functional involvement of angiotensin AT2 receptor in adrenal catecholamine secretion in vivo

The aim of the present study was to analyse modulations of adrenal catecholamine secretion from the adrenal gland of anesthetized dogs in response to locally administered angiotensin II (AngII) in the presence of either PD 123319 or CGP 42112, both of which are highly specific and selective ligands to angiotensin AT2 receptor. Plasma concentrations of epinephrine and norepinephrine in adrenal venous and aortic blood were quantified by a high performance liquid chromatography coupled with electrochemical detection (HPLC-EC) method. Adrenal venous blood flow was measured by gravimetry. Local administration of AngII (0.05 µg, 0.1 µM) to the left adrenal gland increased adrenal gland catecholamine output more than 30 times that found in nonstimulated states. Administration of either PD 123319 (0.085 µg (0.23 µM) to 8.5 µg (23 µM)) or CGP 42112 (0.005 µg (0.01 µM) to 5 µg (10 µM)) did not affect the basal catecholamine output significantly. The increase in adrenal catecholamine output in response to AngII was inhibited by ~80% following the largest dose of PD 123319. CGP 42112 significantly attenuated the catecholamine response to AngII by ~70%. PD 123319 and CGP 42112 were devoid of any agonist actions with respect to catecholamine output by the adrenal gland in vivo. Furthermore, both PD 123319 and CGP 42112 inhibited the increase in adrenal catecholamine secretion induced by local administration of AngII. The present study suggests that AT2 receptors play a role in mediating catecholamine secretion by the adrenal medulla in response to AngII receptor agonist administration in vivo.Key words: AT1 and AT2 subtypes, PD 123319, CGP 42112, AT2 antagonist, anesthetized dog.

Adrenal angiotensin II type 1 and type 2 receptors in Cushing's and Conn's syndromes

Cushing's and Conn's syndromes are well recognised endocrine diseases, but the pathobiology of the tumors causing these disorders is unclear. In this study we examined AT1 and AT2 gene expression in adrenal adenomas of Cushing's and Conn's syndromes. AT1 and AT2 receptor mRNA, as well as alternatively spliced AT1 transcripts, were detected by RT-PCR using adjacent adrenal cortex tissue as controls. Whereas no consistent differences in AT1 mRNA were seen compared to control adrenal cortex, AT2 mRNA levels were significantly decreased in the adenomas of Cushing's and Conn's syndromes. No changes in alternative splicing of AT1 mRNA were observed in the adrenal tumors. The fact that no consistent changes were seen in AT1 mRNA or its splicing, whereas AT2 mRNA were reduced in both forms of hormone producing adrenal tumor suggests that the AT2 receptor, rather than the AT1 subtype, may be correlated with adrenal tumorigenesis.

Angiotensin II receptors in the human adrenal gland

In the present study, we have characterized distribution and pharmacological properties of angiotensin II (Ang II) receptors in human adrenals frozen immediately after removal. Autoradiographic studies indicate that Ang II receptors are present throughout the gland. Co-incubations with DUP 753, a specific antagonist of the AT1 receptor, and with PD 123319, a specific antagonist of the AT2 receptor, reveal that Ang II receptors are mainly of type 2. The AT1 receptors are detected after 16 weeks of gestation at the periphery of the gland. Competition studies and Scatchard analysis reveal a homogenous population of high affinity AT2 binding sites (Kd = 0.68 +/- 0.1 nM). Binding capacities decrease from 1080 +/- 304 fmol/mg protein at 14 weeks to 275 +/- 55 fmol/mg protein at 21 weeks. These results differ from those obtained in adult glands where autoradiographic studies reveal that the AT1 receptors are found mainly in the zona glomerulosa and AT2 receptors mainly in the medulla. These data suggest that the AT2 receptors could be involved in the morphological or functional differentiation of the human fetal adrenal gland.