Regulation of angiotensin type 1 receptor and its gene expression: role in renal growth

RAAS-deficient organoids indicate delayed angiogenesis as a possible cause for autosomal recessive renal tubular dysgenesis

Autosomal Recessive Renal Tubular Dysgenesis (AR-RTD) is a fatal genetic disorder characterized by complete absence or severe depletion of proximal tubules (PT) in patients harboring pathogenic variants in genes involved in the Renin-Angiotensin-Aldosterone System. To uncover the pathomechanism of AR-RTD, differentiation of ACE-/- and AGTR1-/- induced pluripotent stem cells (iPSCs) and AR-RTD patient-derived iPSCs into kidney organoids is leveraged. Comprehensive marker analyses show that both mutant and control organoids generate indistinguishable PT in vitro under normoxic (21% O2) or hypoxic (2% O2) conditions. Fully differentiated (d24) AGTR1-/- and control organoids transplanted under the kidney capsule of immunodeficient mice engraft and mature well, as do renal vesicle stage (d14) control organoids. By contrast, d14 AGTR1-/- organoids fail to engraft due to insufficient pro-angiogenic VEGF-A expression. Notably, growth under hypoxic conditions induces VEGF-A expression and rescues engraftment of AGTR1-/- organoids at d14, as does ectopic expression of VEGF-A. We propose that PT dysgenesis in AR-RTD is primarily a non-autonomous consequence of delayed angiogenesis, starving PT at a critical time in their development.

Preliminary study of ovariectomy and chronic losartan-induced alterations in brain AT1 receptors

Women who undergo oophorectomy prior to the age of natural menopause have a higher risk of neurological and psychological impairment. Treatment with the angiotensin receptor blocker (ARB) losartan for 10 weeks following ovariectomy of Long-Evans rats at 3 months of age reduced the ovariectomy-induced cognitive decrements. Following completion of the behavioral experiments, (Campos et al., 2019), the brains were harvested for preliminary receptor autoradiographic studies of AT₁ receptor (AT₁R) binding in selected brain regions using quantitative densitometric analysis of autoradiograms of ¹²⁵I-sarcosine¹, isoleucine⁸ angiotensin II binding. Four of the brain regions (amygdala, ventral subiculum, piriform cortex, and cingulate cortex) are associated with cognitive and emotional behavior while one (lateral hypothalamus) is associated with homeostasis. The density of AT₁R varied by region: ventral subiculum > amygdala and cingulate cortex, and piriform cortex > cingulate cortex. Losartan treatment decreased AT₁R binding in the ventral subiculum of sham and ovariectomized rats by 41.6%, and 46% in the piriform cortex of the sham rats, but tended to increase AT₁R binding in the piriform cortex and cingulate cortex 77% and 107%, respectively, in the ovariectomized rats. AT₁R binding did not differ significantly between intact male and sham-vehicle female rats among surveyed brain regions. These results suggest that losartan-induced changes in brain AT₁R expression may contribute to the reduced anxiety-like behavior and memory impairments seen in ovariectomized rats, but replication of these observations will be needed to determine the extent to which brain AT₁R changes mediate the adverse behavioral effects of ovariectomy.

Divergent effects of ERα and ERβ on fluid intake by female rats are not dependent on concomitant changes in AT1R expression or body weight

Estradiol (E2) decreases both water and saline intakes by female rats. The ERα and ERβ subtypes are expressed in areas of the brain that control fluid intake; however, the role that these receptors play in E2's antidipsogenic and antinatriorexigenic effects have not been examined. Accordingly, we tested the hypothesis that activation of ERα and ERβ decreases water and saline intakes by female rats. We found a divergence in E2's inhibitory effect on intake: activation of ERα decreased water intake, whereas activation of ERβ decreased saline intake. E2 decreases expression of the angiotensin II type 1 receptor (AT1R), a receptor with known relevance to water and salt intakes, in multiple areas of the brain where ERα and ERβ are differentially expressed. Therefore, we tested for agonist-induced changes in AT1R mRNA expression by RT-PCR and protein expression by analyzing receptor binding to test the hypothesis that the divergent effects of these ER subtypes are mediated by region-specific changes in AT1R expression. Although we found no changes in AT1R mRNA or binding in areas of the brain known to control fluid intake associated with agonist treatment, the experimental results replicate and extend previous findings that body weight changes mediate alterations in AT1R expression in distinct brain regions. Together, the results reveal selective effects of ER subtypes on ingestive behaviors, advancing our understanding of E2's inhibitory role in the controls of fluid intake by female rats.

Properly timed exposure to central ANG II prevents behavioral sensitization and changes in angiotensin receptor expression

Previous studies show that the angiotensin type 1 receptor (AT1R) is susceptible to rapid desensitization, but that more chronic treatments that stimulate ANG II lead to sensitization of several responses. It is unclear, however, if the processes of desensitization and sensitization interact. To test for differences in AT1R expression associated with single or repeated injections of ANG II, we measured AT1R mRNA in nuclei that control fluid intake of rats given ANG II either in a single injection or divided into three injections spaced 20 min apart. Rats given a single injection of ANG II had more AT1R mRNA in the subfornical organ (SFO) and the periventricular tissue surrounding the anteroventral third ventricle (AV3V) than did controls. The effect was not observed, however, when the same cumulative dose of ANG II was divided into multiple injections. Behavioral tests found that single daily injections of ANG II sensitized the dipsogenic response to ANG II, but a daily regimen of four injections did not cause sensitization. Analysis of (125)I-Sar(1)-ANG II binding revealed a paradoxical decrease in binding in the caudal AV3V and dorsal median preoptic nucleus after 5 days of single daily injections of ANG II; however, this effect was absent in rats treated for 5 days with four daily ANG II injections. Taken together, these data suggest that a desensitizing treatment regimen prevents behavior- and receptor-level effects of repeated daily ANG II.

Immunohistochemical Localization of AT1a, AT1b, and AT2 Angiotensin II Receptor Subtypes in the Rat Adrenal, Pituitary, and Brain with a Perspective Commentary

Angiotensin II increases blood pressure and stimulates thirst and sodium appetite in the brain. It also stimulates secretion of aldosterone from the adrenal zona glomerulosa and epinephrine from the adrenal medulla. The rat has 3 subtypes of angiotensin II receptors: AT1a, AT1b, and AT2. mRNAs for all three subtypes occur in the adrenal and brain. To immunohistochemically differentiate these receptor subtypes, rabbits were immunized with C-terminal fragments of these subtypes to generate receptor subtype-specific antibodies. Immunofluorescence revealed AT1a and AT2 receptors in adrenal zona glomerulosa and medulla. AT1b immunofluorescence was present in the zona glomerulosa, but not the medulla. Ultrastructural immunogold labeling for the AT1a receptor in glomerulosa and medullary cells localized it to plasma membrane, endocytic vesicles, multivesicular bodies, and the nucleus. AT1b and AT2, but not AT1a, immunofluorescence was observed in the anterior pituitary. Stellate cells were AT1b positive while ovoid cells were AT2 positive. In the brain, neurons were AT1a, AT1b, and AT2 positive, but glia was only AT1b positive. Highest levels of AT1a, AT1b, and AT2 receptor immunofluorescence were in the subfornical organ, median eminence, area postrema, paraventricular nucleus, and solitary tract nucleus. These studies complement those employing different techniques to characterize Ang II receptors.

Endogenous angiotensin II has fewer effects but neuronal nitric oxide synthase has excitatory effects on renal sympathetic nerve activity in salt-sensitive hypertension-induced heart failure

The effects of endogenous angiotensin II (Ang II) and neuronal nitric oxide synthase (nNOS) on tonic sympathetic activity were studied in salt-sensitive hypertension-induced heart failure. Dahl salt-sensitive rats were fed 8% NaCl diet for 9 weeks to induce chronic heart failure (CHF-DSS). The effects of intravenous administration of a selective nNOS inhibitor, S-methyl-L: -thiocitrulline (SMTC), and an Ang II type 1-receptor blocker, losartan, on renal sympathetic nerve activity (RSNA) were examined in chronically instrumented conscious rats. Baroreceptor (baro)-unloaded RSNA was obtained by decreasing arterial pressure with caval occlusion to determine tonic RSNA. SMTC significantly decreased baro-unloaded RSNA, and subsequent losartan recovered baro-unloaded RSNA to the control level in CHF-DSS rats. To compare the effects of the inhibitors between low- and high-activity states of the renin-angiotensin system (RAS), Sprague-Dawley rats were fed low (0.04%)- or high (8%)-salt diets. A significant difference was found in the effects of SMTC and/or losartan on RSNA between the high- and low-RAS states, which suggested that there is a difference in the effect of endogenous Ang II on RSNA between salt-induced and other-type heart failure. To examine the effects of heart failure on brain-tissue nNOS activity, we measured the activities of the diencephalon in heart-failure rats. Heart failure significantly suppressed diencephalon nNOS activity, which was significantly different from the results in salt-sensitive hypertension without heart failure. These results suggest that endogenous Ang II has fewer effects, but nNOS has excitatory effects on tonic RSNA in salt-sensitive hypertension-induced heart failure.

Regulation of angiotensin II type I receptor (AT1R) protein levels in the obese Zucker rat kidney and urine

Increased renal expression of the angiotensin II, type-1 receptor (AT1R) has been associated with increased blood pressure (BP) and progression of renal disease. We tested whether common medications used to treat hypertension and the metabolic syndrome alter renal AT1R; and whether urine AT1R can be used as a reasonable noninvasive marker of renal levels in the obese Zucker rat, a model for human metabolic syndrome. Immunoblotting revealed that renal and urinary levels of AT1R were significantly higher in obese versus lean rats and correlated (R = 0.62, p < 0.05). Chronic treatment with BP lowering, candesartan, an AT1R antagonist, increased renal levels of AT1R in both lean (282% of lean controls) and obese (178% of obese controls) rats, but decreased urine AT1R levels in obese rats (72% of obese controls). Similarly, chronic treatment with rosiglitazone (RGZ), a peroxisome proliferator activated receptor (subtype gamma) agonist, significantly decreased urine (43% of obese controls) but not renal AT1R (105%) in obese rats. Blood pressure, measured by radiotelemetry, was significantly correlated in untreated and RGZ-treated rats to renal AT1R (R = 0.57, p = 0.0035). Finally, high- (4%) and medium- (0.4%) NaCl diets increased excretion of AT1R in obese rats to approximately 400% of low- (0.04%) NaCl diet. This effect was markedly blunted in lean rats. Overall, we demonstrate increased renal AT1R levels in obese rats. Urine AT1R correlated with renal levels only in the untreated state. Relative salt-sensitivity of AT1R excretion in obese, relative to lean rats, may have implications for both BP and renal disease in the metabolic syndrome.

Contribution of angiotensin II internalization to intrarenal angiotensin II levels in rats

This study was designed to determine the involvement of AT(1) receptors in the uptake of ANG II in the kidney of rats exposed to differing salt intake. Male Wistar-Kyoto rats were treated with a normal-salt (NS; 1% NaCl, n = 7) or a low-salt (LS; 0.025% NaCl, n = 7) diet combined with (LS+Los, n = 7; NS+Los, n = 7) or without losartan (30 mg. kg(-1). day(-1)), an AT(1) receptor antagonist. Renin (RA) and angiotensin-converting enzyme (ACE) activities and angiotensinogen, ANG I, and ANG II levels were measured in plasma, renal cortex, and medulla. In LS rats, in both plasma and renal cortex, the increase in RA was associated with an increase in ANG I and ANG II levels compared with NS rats, but intrarenal ANG II levels increased more than ANG I levels. In NS+Los rats, the increase in RA in plasma was followed by a marked increase in plasma ANG I and ANG II levels compared with NS rats whereas in the kidney the increase of renal RA was followed by a decrease of the levels of these peptides. The same pattern was observed in LS+Los rats, but the decrease in renal ANG II levels was much more pronounced in LS+Los rats than in NS+Los rats. Our results suggest that the increase in renal ANG II levels after salt restriction results mainly from an uptake of ANG II, via AT(1) receptors. Such elevated intrarenal ANG II levels could contribute to maintain sodium and fluid balance and arterial blood pressure during salt-deficiency states.

Role of renin-angiotensin-aldosterone system in salt-sensitive hypertension induced by sensory denervation

To define the role of the renin-angiotensin-aldosterone system in a novel salt-sensitive model, neonatal Wistar rats were given capsaicin (50 mg/kg sc) on the first and second days of life. After weaning, male rats were divided into the following six groups and treated for 3 wk with: control + normal sodium diet (CON-NS), CON + high-sodium diet (CON-HS), CON + HS + spironolactone (50 mg x kg(-1) x day(-1), CON-HS-SP), capsaicin pretreatment + NS (CAP-NS), CAP-HS, and CAP-HS-SP. Radioimmunoassay shows that plasma renin activity (PRA) and plasma aldosterone level (PAL) were suppressed by HS, but they were higher in CAP-HS than in CON-HS and CON-HS-SP (P < 0.05). Both tail-cuff systolic blood pressure and mean arterial pressure were higher in CAP-HS than in all other groups (P < 0.05). Urine water and sodium excretion were increased with HS intake, but they were lower in CAP-HS than in CON-HS (P < 0.05). Western blot did not detect differences in adrenal AT1 receptor content. Therefore, insufficiently suppressed PRA and PAL in response to HS intake by sensory denervation may contribute to increased salt sensitivity and account for effectiveness of spironolactone in lowering blood pressure in this model.

Role of AT1 and AT2 receptor subtypes in salt-sensitive hypertension induced by sensory nerve degeneration

OBJECTIVE

To define the role of the type 1 angiotensin II (AT1) and type II (AT2) receptors in the development of salt-sensitive hypertension induced by sensory nerve degeneration.

DESIGN AND METHODS

Neonatal Wistar rats were given capsaicin 50 mg/kg s.c. on the first and second days of life. After weaning, male rats were divided into six groups and treated for 3 weeks with: control + high sodium diet (4%, CON-HS), capsaicin pretreatment + normal sodium diet (0.5%, CAP-NS), CAP-HS, CAP + HS + candesartan (10 mg/kg per day) (CAP-HS-CAN), CAP + HS + PD 123319 (30 mg/kg per day) (CAP-HS-PD), and capsaicin pretreatment + high sodium diet + candesartan + PD 123319 (CAP-HS-CAN-PD). Mean arterial pressure (MAP) was measured by carotid arterial catheterization. Urinary Na+ concentrations were determined by using a flame atomic absorption spectrophotometer. Levels of calcitonin gene-related peptide (CGRP) in dorsal root ganglia (DRG) and plasma renin activity (PRA) were determined by radioimmunoassay.

RESULTS

CGRP contents in DRG were decreased by capsaicin (P < 0.05). MAP was higher in CAP-HS rats compared with all the other groups (P < 0.05). The 24 h urine and sodium excretion increased when a high salt diet was given, but they were lower in CAP-HS and CAP-HS-CAN than in CON-HS (P < 0.05). PRA was suppressed in CON-HS and CAP-HS compared with CAP-NS, but it was higher in CAP-HS than in CON-HS (P < 0.05).

CONCLUSION

Insufficiently suppressed PRA by high salt intake may contribute to increased salt sensitivity and account for effectiveness of candesartan in lowering blood pressure in this model. Furthermore, PD 123319 attenuates the development of hypertension in salt-loaded rats neonatally treated with capsaicin, indicating that the AT2 receptor contributes to the increase in blood pressure.

Differential activation of the STAT pathway by angiotensin II via angiotensin type 1 and type 2 receptors in cultured human fetal mesangial cells

The vasoactive peptide angiotensin II is the principal effector of the renin-angiotensin system. It exerts mitogenic and growth-inhibiting effects in many target tissues, including renal mesangial cells. To investigate mechanisms of angiotensin II signaling in human mesangial cells, we explored the signal transducer and activator of transcription (STAT) pathway as a possible regulator of angiotensin II receptor-specific signaling. We tested whether angiotensin II could induce STAT activation and nuclear translocation of STAT proteins in human mesangial cells by electromobility shift assays and by immunostaining and confocal microscopy. We found that fetal human mesangial cells express STAT1,2,3,5, and 6 and that stimulation of these cells by angiotensin II results in rapid induction of STAT1 and STAT5 DNA-binding activity. This DNA-binding activity was identified as STAT5 for angiotensin receptor type 1 activation and STAT1 for angiotensin receptor type 2-mediated activation, as induction of STAT-DNA binding by angiotensin II could be differentially blocked by the angiotensin receptor type 1 blocker losartan and by angiotensin II receptor type 2 blocker PD 123,319. Angiotensin II also induced STAT1 and STAT5 tyrosine phosphorylation and nuclear translocation of activated STATs in a receptor subtype-specific manner. STAT activation thus appears to provide an important signaling pathway for angiotensin II-induced cellular responses.

Angiotensin II releases 20-HETE from rat renal microvessels

We studied hydroxyeicosatetraenoic acid (HETE) release in response to ANG II from preglomerular microvessels (PGMVs), the vascular segment governing changes in renal vascular resistance. PGMVs were isolated from Sprague-Dawley rats and incubated with NADPH and hormones at 37 degrees C. Eicosanoids were extracted, and cytochrome P-450 (CYP)-derived HETEs were purified and quantitated by negative chemical ionization gas chromatography-mass spectroscopy. PGMVs produced primarily 20- and 19-HETEs, namely, 7.9 +/- 1.7 and 2.2 +/- 0.5 ng/mg protein, respectively. ANG II (5 nM) increased CYP-HETE release by two- to threefold; bradykinin, phenylephrine, and Ca(2+) ionophore were without effect. [Sar(1)]ANG II (0.1-100 microM) dose dependently stimulated 19- and 20-HETEs, an effect blocked by the AT(2)-receptor antagonist PD-123319 as well as by U-73122, a phospholipase C inhibitor. Microvascular 20-HETE release was increased more than twofold by the third day in response to ANG II (120 ng. kg(-1). min(-1)) infused subcutaneously for 2 wk; it was not further enhanced after 14 days, although blood pressure continued to rise. Thus an AT(2)-phospholipse C effector unit is associated with synthesis of a vasoconstrictor product, 20-HETE, in a key renovascular segment.

Renal effects of renin-angiotensin system blockade

Pharmacological interruption of the renin-angiotensin system at different pathway levels has extended our knowledge on the distribution of angiotensin receptors in different nephron segments, its regulation and tubular cell responses. Novel beneficial effects obtained with blockade of this peptide on cellular proliferation and its interaction with other vasoactive systems are particularly important for preventing renal damage.