Distinct mechanisms of upregulation of type 1A angiotensin II receptor gene expression in kidney and adrenal gland

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.

Acute and chronic regulation of aldosterone production

Aldosterone is the major mineralocorticoid synthesized by the adrenal and plays an important role in the regulation of systemic blood pressure through the absorption of sodium and water. Aldosterone production is regulated tightly by selective expression of aldosterone synthase (CYP11B2) in the adrenal outermost zone, the zona glomerulosa. Angiotensin II (Ang II), potassium (K(+)) and adrenocorticotropin (ACTH) are the main physiological agonists which regulate aldosterone secretion. Aldosterone production is regulated within minutes of stimulation (acutely) through increased expression and phosphorylation of the steroidogenic acute regulatory (StAR) protein and over hours to days (chronically) by increased expression of the enzymes involved in the synthesis of aldosterone, particularly CYP11B2. Imbalance in any of these processes may lead to several disorders of aldosterone excess. In this review we attempt to summarize the key molecular events involved in the acute and chronic phases of aldosterone secretion.

Angiotensin AT1 receptor-associated protein Arap1 in the kidney vasculature is suppressed by angiotensin II

Arap1 is a protein that interacts with angiotensin II type 1 (AT(1)) receptors and facilitates increased AT(1) receptor surface expression in vitro. In the present study, we assessed the tissue localization and regulation of Arap1 in vivo. Arap1 was found in various mouse organs, with the highest expression in the heart, kidney, aorta, and adrenal gland. Renal Arap1 protein was restricted to the vasculature and to glomerular mesangial cells and was absent from tubular epithelia. A similar localization was found in human kidneys. To test the hypothesis that angiotensin II may control renal Arap1 expression, mice were subjected to various conditions to alter the activity of the renin-angiotensin system. A high-salt diet (4% NaCl, 7 days) upregulated Arap1 expression in mice by 47% compared with controls (0.6% NaCl, P = 0.03). Renal artery stenosis (7 days) or water restriction (48 h) suppressed Arap1 levels compared with controls (-64 and -62% in the clipped and contralateral kidney, respectively; and -50% after water restriction, P < 0.01). Angiotensin II infusion (2 μg·kg(-1)·min(-1), 7 days) reduced Arap1 mRNA levels compared with vehicle by 29% (P < 0.01), whereas AT(1) antagonism (losartan, 30 mg·kg(-1)·day(-1), 7 days) enhanced Arap1 mRNA expression by 52% (P < 0.01); changes in mRNA were paralleled by Arap1 protein abundance. Experiments with hydralazine and epithelial nitric oxide synthase-/- mice further suggested that Arap1 expression changed in parallel with angiotensin II, rather than with blood pressure per se. Similar to in vivo, Arap1 mRNA and protein were suppressed by angiotensin II in a time- and dose-dependent manner in cultured mesangial cells. In summary, Arap1 is highly expressed in the renal vasculature, and its expression is suppressed by angiotensin II. Thus Arap1 may serve as a local modulator of vascular AT(1) receptor function in vivo.

Atrap deficiency increases arterial blood pressure and plasma volume

The angiotensin receptor-associated protein (Atrap) interacts with angiotensin II (AngII) type 1 (AT1) receptors and facilitates their internalization in vitro, but little is known about the function of Atrap in vivo. Here, we detected Atrap expression in several organs of wild-type mice; the highest expression was in the kidney where it localized to the proximal tubule, particularly the brush border. There was no Atrap expression in the renal vasculature or juxtaglomerular cells. We generated Atrap-deficient (Atrap-/-) mice, which were viable and seemed grossly normal. Mean systolic BP was significantly higher in Atrap-/- mice compared with wild-type mice. Dose-response relationships of arterial BP after acute AngII infusion were similar in both genotypes. Plasma volume was significantly higher and plasma renin concentration was markedly lower in Atrap-/- mice compared with wild-type mice. (125)I-AngII binding showed enhanced surface expression of AT1 receptors in the renal cortex of Atrap-/- mice, accompanied by increased carboanhydrase-sensitive proximal tubular function. In summary, Atrap-/- mice have increased arterial pressure and plasma volume. Atrap seems to modulate volume status by acting as a negative regulator of AT1 receptors in the renal tubules.

Angiotensin II regulation of adrenocortical gene transcription

Angiotensin II (Ang II) is the key peptide hormone in the renin-angiotensin-aldosterone system (RAAS). Its ability to regulate levels of circulating aldosterone relies on actions on adrenal glomerulosa cells. Many of the Ang II effects on glomerulosa cells involve a precisely coordinated regulation of signaling cascades and gene expression. The development of genome-wide gene arrays has allowed the definition of transcriptome-wide effects of Ang II in adrenocortical cells. Analysis of the Ang II gene targets reveals broad effects on cellular gene expression, particularly the rapid induction of numerous transcription factors that may regulate long-term steroid metabolism and cell growth/proliferation. Herein we discuss the Ang II-induced genes in adrenocortical cells and review the progress in defining the role of these genes in zona glomerulosa function.

Renocortical mRNA expression of vasoactive factors during spironolactone protective effect in chronic cyclosporine nephrotoxicity

We showed that spironolactone reduced structural damage and prevented renal dysfunction in chronic cyclosporine (CsA) nephrotoxicity. These findings evidenced an aldosterone renal vascular effect under this condition. To investigate aldosterone’s role in modulating renal vascular tone, renocortical vasoactive pathways mRNA levels in chronic CsA nephrotoxicity as well as spironolactone’s effect on renal function in acute CsA nephrotoxicity were evaluated. Two experimental sets were designed. For chronic nephrotoxicity, rats fed with low-sodium diet were divided into groups receiving vehicle, spironolactone (Sp), CsA, and CsA+Sp, for 21 days. Creatinine clearance, survival percentage, and renocortical mRNA levels of pro-renin, angiotensinogen (Ang), angiotensin receptors (AT1A, AT1B, and AT2), preproendothelin, endothelin receptors (ETA, ETB), cyclooxygenase-2 (COX-2), and adenosine receptors (Ad1, Ad2A, Ad2B, and Ad3) were analyzed. For acute nephrotoxicity, similar groups fed with a standard chow diet for 7 days were included. Serum potassium and sodium, glomerular filtration rate (GFR), and renal blood flow (RBF) were determined. In chronic model, CsA produced pro-renin and ET upregulation, altered adenosine receptors expression, and reduced Ang, AT1A, AT1B, ETB, and COX-2 mRNA levels. Spironolactone protective effect in chronic nephrotoxicity was associated with prevention of pro-renin upregulation and increased AT2, together with ETBreduction. In acute nephrotoxicity, spironolactone completely prevented GFR and RBF reduction induced by CsA. Our results suggest that aldosterone contributes to renal vasoconstriction observed in CsA nephrotoxicity and that renoprotection conferred by spironolactone was related to modification of renocortical vasoactive pathways expression, in which pro-renin normalization was the most evident change in chronic nephropathy. Finally, our data point to spironolactone as a potential treatment to reduce CsA nephrotoxicity in transplant patients.

Renal angiotensin II receptors, hyperinsulinemia, and obesity

Angiotensin II, via activation of AT1 receptors in the kidney regulates sodium/fluid homeostasis and blood pressure. An exaggerated action of angiotensin II mediated via activation of AT1 receptors has been implicated in the increased renal sodium retention and the resetting of the pressure natriuresis in obesity related hypertension. Treatment of obese Zucker rats with AT1 receptor blockers reduces blood pressure to a greater extent and produces greater natriuresis. Also, there is an increased membranal AT1 receptor numbers and angiotensin II produces greater activation of sodium transporters in the isolated tubules from obese Zucker rats. Interestingly, AT2 receptors, which are believed to be beneficial to the renal and cardiovascular function in terms of their action on kidney and blood vessels, are greatly increased in proximal tubular membranes of obese Zucker rats. Whole animal and in vitro studies indicate that higher plasma insulin level, generally associated with obesity, is responsible for the up-regulation of both AT1 and AT2 receptors in the kidney. Determining the consequence of selective blocking of AT1 receptors and/or activation of the AT2 receptors on renal and cardiovascular function, and the effect of lowering insulin on these receptors present an important area of further investigation in obesity.

Increased renovascular response to angiotensin II in persons genetically predisposed to arterial hypertension disappears after chronic angiotensin-converting enzyme inhibition

OBJECTIVE AND METHODS

Functional changes in the kidneys of healthy men with (FH+) (n = 15) and without (FH-) (n = 15) family history of primary arterial hypertension were examined during administration of low-dose exogenous angiotensin II (A2) (1 ng/kg per min) before and after acute (1 mg intravenous enalaprilat) and chronic (7 days oral enalapril, 30 mg/day) angiotensin-converting enzyme (ACE) inhibition.

RESULTS

Before chronic ACE inhibition, A2 increased mean arterial blood pressure (FH+, 8.7 +/- 0.8 mmHg; FH-, 8.9 +/- 0.9 mmHg), plasma immunoreactive A2 (FH+, 21 +/- 2 pg/ml; FH-, 18 +/- 3 pg/ml) and plasma aldosterone (FH+, 64 +/- 7 pg/ml; FH-, 56 +/- 6 pg/ml) to a similar degree in both groups. Chronic ACE inhibition had no impact on A2 blood pressure, plasma A2, or plasma aldosterone effects. A2 significantly increased renal vascular resistance in both groups (FH+, 3956 +/- 462 dyne s cm(-5); FH-, 2219 +/- 550 dyne s cm(-5)), but the effect was more pronounced in FH+ (P = 0.02). Glomerular hemodynamics, estimated by a modified Gomez model, revealed increased afferent and efferent responsiveness to A2 in FH+ subjects. These differences disappeared after chronic ACE inhibition when total, afferent and efferent sensitivities to A2 were similar in both groups.

CONCLUSIONS

Systemic blood pressure and plasma aldosterone responses to A2 were similar in men with or without a genetic disposition to primary arterial hypertension. However, our data demonstrate that men with a family history of hypertension have increased renovascular sensitivity to A2, and that chronic ACE inhibition normalizes their sensitivity.

Regulation by sodium intake of type 1 angiotensin II receptor mRNAs in the kidney of Sabra rats

OBJECTIVE

To study the relationship between the sensitivity to sodium content of the diet in terms of development of hypertension and the regulation of the expression of type 1 angiotensin II receptor subtypes by such a diet.

METHODS

The expression of angiotensin II receptor subtype (AT1A and AT1B) mRNAs was studied by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) in the four zones of the kidneys of Sabra rats, sensitive or resistant to DOCA salt-induced hypertension (SBH/y and SBN/y, respectively). Rats were fed a high (8%) or normal (0.4%) NaCl diet. As vasopressin is known to be elevated in SBH/y rats and to be involved in DOCA-salt hypertension, we studied an additional group of SBH/y rats, fed a high sodium diet, enriched in water.

RESULTS

With the absence of DOCA, SBH/y rats did not develop hypertension. The high sodium diet induced a greater fall in the plasma renin activity in the SBH/y (-95%) than in the SBN/y (-63%). In the cortex (C) and inner stripe (IS), the high sodium diet decreased AT1A and AT1B mRNAs in SBH/y and SBN/y, with a higher magnitude for SBH/y, than for SBN/y (C, -28 versus -20%; IS, -42 versus -20%). The addition of water to the high sodium diet lessened the effect of sodium in the C and IS, although the plasma renin activity (PRA) was not altered.

CONCLUSION

A high sodium diet significantly decreases both AT1A and AT1B gene expression in two specific zones of the rat kidney containing the target cells of angiotensin II (C and IS). This down-regulation is organ-specific since it was observed in the kidney and adrenals, but not in the liver. Finally, SBH/y and SBN/y rats differ in the basal level of AT1 mRNA expression in the IS, and in the ability to modulate AT1 mRNA level under sodium intake.

Effects of angiotensin II and antagonists on AT(1) receptor expression in mesangial cells

Rat mesangial cells were exposed to angiotensin II, angiotensin AT(1) receptor antagonists such as losartan, EXP 3174 and candesartan, or dexamethasone for increasing periods (1-24 h). Angiotensin AT(1A) and AT(1B) receptor mRNA were measured by reverse transcription-polymerase chain reaction (RT-PCR). Angiotensin II, losartan and EXP 3174 did not modify significantly angiotensin AT(1A) and AT(1B) receptor mRNA. Candesartan increased angiotensin AT(1B) receptor mRNA and, to a lesser extent, angiotensin AT(1A) receptor mRNA. In contrast, dexamethasone decreased mainly angiotensin AT(1B) receptor mRNA. As shown by Western blot analysis, exposure of mesangial cells to angiotensin II, losartan or EXP 3174 did not produce any change in angiotensin AT(1) receptor protein, whereas dexamethasone and candesartan exerted inhibitory effects. In conclusion, the angiotensin AT(1B) receptor subtype, the most abundantly distributed in rat mesangial cells, is inhibited by glucocorticoids. The effect of candesartan is more complex with a slight stimulation of angiotensin AT(1B) mRNA and a marked inhibition of angiotensin AT(1) receptor protein. In contrast, angiotensin II and the other angiotensin AT(1) receptor antagonists studied are inactive on angiotensin AT(1) mRNA and protein.

Renal endosomes contain angiotensin peptides, converting enzyme, and AT(1A) receptors

Kidney cortex and proximal tubular angiotensin II (ANG II) levels are greater than can be explained on the basis of circulating ANG II, suggesting intrarenal compartmentalization of these peptides. One possible site of intracellular accumulation is the endosomes. In the present study, we tested for endosomal ANG I, ANG II, angiotensin type 1A receptor (AT(1A)), and angiotensin converting enzyme (ACE) activity and determined whether these levels are regulated by salt intake. Male Sprague-Dawley rats were fed chow containing either high or low dietary sodium for 10-14 days. Blood and kidneys were harvested and processed for measurement of plasma, kidney, and renal intermicrovillar cleft and endosomal angiotensin levels. Kidney ANG I averaged 179 +/- 20 fmol/g and ANG II averaged 258 +/- 36 fmol/g in rats fed a high-sodium diet and were significantly higher, averaging 347 +/- 58 fmol/g and 386 +/- 55 fmol/g, respectively, in rats fed a low-salt diet. Renal intermicrovillar clefts and endosomes contained ANG I and ANG II. Intermicrovillar cleft ANG I and ANG II levels averaged 8.4 +/- 2.6 and 74 +/- 26 fmol/mg, respectively, in rats fed a high-salt diet and 7.6 +/- 1.7 and 70 +/- 25 fmol/mg in rats fed a low-salt diet. Endosomal ANG I and ANG II levels averaged 12.3 +/- 4.4 and 43 +/- 19 fmol/mg, respectively, in rats fed a high-salt diet, and these levels were similar to those observed in rats fed a low-salt diet. Renal endosomes from rats fed a low-salt diet demonstrated significantly more AT(1A) receptor binding compared with rats fed a high-salt diet. ACE activity was detectable in renal intermicrovillar clefts and was 2.5-fold higher than the levels observed in renal endosomes. Acute enalaprilat treatment decreased ACE activity in renal intermicrovillar clefts by 90% and in renal endosomes by 84%. Likewise, intermicrovillar cleft and endosomal ANG II levels decreased by 61% and 52%, respectively, in enalaprilat-treated animals. These data demonstrate the presence of intact angiotensin peptides and ACE activity in renal intermicrovillar clefts and endosomes, indicating that intact angiotensin peptides are formed and/or trafficked through intracellular endosomal compartments and are dependent on ACE activity.

Dietary salt intake modulates angiotensin II type 1 receptor gene expression

This study aimed to characterize the influence of dietary salt intake on the gene expression of angiotensin II type 1 (AT1) receptor subtypes in different organs. Male Sprague-Dawley rats were fed low salt (0.2 mg/g), normal salt (6 mg/g), or high salt (40 mg/g) diets for 5, 10, and 20 days. mRNA levels for the two AT1 receptor subtypes were determined in adrenal gland, kidney, liver, and lung. In all of the organs examined, with the exception of the adrenal glands, low salt diet led to a transient decrease in the abundance of AT1A receptor mRNA but not of AT1B mRNA, which reached their nadirs between days 5 and 10 of feeding. In the adrenal gland, in which the AT1B receptor is predominant, low salt diet led to a transient increase in the expression of this receptor gene, with a maximum around day 10 of feeding. High salt diet exerted no significant influence on AT1 receptor gene expression in these organs. These findings indicate that the rate of salt intake, in particular, a reduction of salt intake, significantly influences AT1 receptor gene expression in an organ-, time-, and subtype-dependent fashion. It appears that AT1 receptor subtypes are differentially influenced by low salt intake, in that AT1B receptor gene expression increases and AT1A receptor gene expression decreases in this situation. This differential response of AT1 receptor gene expression may be relevant for the organism to be able to adapt to a reduction in oral salt intake.

Regulation of angiotensin II receptor AT1 subtypes in renal afferent arterioles during chronic changes in sodium diet

Studies determined the effects of chronic changes in sodium diet on the expression, regulation, and function of different angiotensin II (ANG II) receptor subtypes in renal resistance vessels. Rats were fed low- or high-sodium diets for 3 wk before study. Receptor function was assessed in vivo by measuring transient renal blood flow responses to bolus injections of ANG II (2 ng) into the renal artery. ANG II produced less pronounced renal vasoconstriction in rats fed a low- compared with high-sodium diet (16% vs. 56% decrease in renal blood flow, P < 0.001). After acute blockade of ANG II formation by iv enalaprilat injection in sodium-restricted animals, ANG II produced a 40% decrease in renal blood flow, a level between untreated dietary groups and less than high salt diet. Intrarenal administration of angiotensin II receptor type 1 (AT1) receptor antagonists losartan or EXP-3174 simultaneously with ANG II caused dose-dependent inhibition of ANG II responses. Based on maximum vasoconstriction normalized to 100% ANG II effect in each group, AT1 receptor antagonists produced the same degree of blockade in all groups, with an apparent maximum of 80-90%. In contrast, similar doses of the angiotensin II receptor type 2 (AT2) receptor ligand CGP-42112 had only a weak inhibitory effect. In vitro equilibrium-saturation 125I-ANG II binding studies on freshly isolated afferent arterioles indicated that ANG II receptor density was lower in the low- vs. high-sodium animals (157 vs. 298 fmol/mg, P < 0.04); affinity was similar (0.65 nM). Losartan and EXP-3174 displaced up to 80-90% of the ANG II binding; fractional displacement was similar in both diet groups. In contrast, the AT2 receptor analogues PD-123319 and CGP-42112 at concentrations < 10(-6) M had no effect on ANG II binding. RT-PCR assays revealed the expression of both angiotensin II receptor type 1A (AT(1A)) and angiotensin II receptor type 1B (AT(1B)) subtypes in freshly isolated afferent arterioles, while there was very little AT2 receptor expression. Total AT1 receptor mRNA expression was suppressed by low sodium intake to 66% of control levels, whereas it was increased to 132% of control by high-sodium diet, as indicated by ribonuclease protection assay. Receptor regulation was associated with parallel changes in AT(1A) and AT(1B) expression; the AT(1A)/AT(1B) ratio was stable at 3.7. We conclude that AT1 receptors are the predominant ANG II receptor type in renal resistance vessels of 7-wk-old rats. Chronic changes in sodium intake caused parallel regulation of expression and amount of receptor protein of the two AT1 receptor genes that modulate receptor function and altered reactivity of renal vessels to ANG II.