Angiotensin II Receptors. In: Mukhopadhyay AK, Raizada MK, editors. Tissue Renin-Angiotensin Systems. Boston: Springer US; 1995. pp. 169-92. [DOI: 10.1007/978-1-4899-0952-7_10]

Aminopeptidases in Cardiovascular and Renal Function. Role as Predictive Renal Injury Biomarkers

Aminopeptidases (APs) are metalloenzymes that hydrolyze peptides and polypeptides by scission of the N-terminus amino acid and that also participate in the intracellular final digestion of proteins. APs play an important role in protein maturation, signal transduction, and cell-cycle control, among other processes. These enzymes are especially relevant in the control of cardiovascular and renal functions. APs participate in the regulation of the systemic and local renin-angiotensin system and also modulate the activity of neuropeptides, kinins, immunomodulatory peptides, and cytokines, even contributing to cholesterol uptake and angiogenesis. This review focuses on the role of four key APs, aspartyl-, alanyl-, glutamyl-, and leucyl-cystinyl-aminopeptidases, in the control of blood pressure (BP) and renal function and on their association with different cardiovascular and renal diseases. In this context, the effects of AP inhibitors are analyzed as therapeutic tools for BP control and renal diseases. Their role as urinary biomarkers of renal injury is also explored. The enzymatic activities of urinary APs, which act as hydrolyzing peptides on the luminal surface of the renal tubule, have emerged as early predictive renal injury biomarkers in both acute and chronic renal nephropathies, including those induced by nephrotoxic agents, obesity, hypertension, or diabetes. Hence, the analysis of urinary AP appears to be a promising diagnostic and prognostic approach to renal disease in both research and clinical settings.

New insights into the importance of aminopeptidase A in hypertension

The renin-angiotensin system (RAS) plays an important role in the maintenance of normal blood pressure and the etiology of hypertension; however, minimal attention has been paid to the degradation of the effector peptide, angiotensin II (AngII). Since aminopeptidase A (APA)-deficient mice develop hypertension APA appears to be an essential enzyme in the control of blood pressure via degradation of AngII. The robust hypertension seen in the spontaneously hypertensive rat (SHR) is due to activation of the RAS, and an accompanying decrease in kidney APA. Changes in APA have also been measured during the activation of the RAS in the Goldblatt hypertension model and Dahl salt-sensitive (DSS) rat. The DSS rat shows an elevation in renal APA activity at the onset of hypertension suggesting a protective role against elevations in circulating AngII, followed by decreased APA activity with advancing hypertension. Changes seen in human maternal serum APA activity during preeclampsia are similar to changes measured in renal APA in the DSS rat model. APA activity is higher than during normal pregnancy at the onset of preeclampsia, and with advancing preeclampsia (severe preeclampsia) declines below that seen during normal pregnancy. Serum APA activity is also increased during hormone replacement therapy (HRT), perhaps in reaction to elevated levels of AngII. Thus, it appears important to consider the relationship among activation of the RAS, circulating levels of AngII, and the availability of APA in hypertensive disorders.

The brain angiotensin system and extracellular matrix molecules in neural plasticity, learning, and memory

The brain renin-angiotensin system (RAS) has long been known to regulate several classic physiologies including blood pressure, sodium and water balance, cyclicity of reproductive hormones and sexual behaviors, and pituitary gland hormones. These physiologies are thought to be under the control of the angiotensin II (AngII)/AT1 receptor subtype system. The AT2 receptor subtype is expressed during fetal development and is less abundant in the adult. This receptor appears to oppose growth responses facilitated by the AT1 receptor, as well as growth factor receptors. Recent evidence points to an important contribution by the brain RAS to non-classic physiologies mediated by the newly discovered angiotensin IV (AngIV)/AT4 receptor subtype system. These physiologies include the regulation of blood flow, modulation of exploratory behavior, and a facilitory role in learning and memory acquisition. This system appears to interact with brain matrix metalloproteinases in order to modify extracellular matrix molecules thus permitting the synaptic remodeling critical to the neural plasticity presumed to underlie memory consolidation, reconsolidation, and retrieval. There is support for an inhibitory influence by AngII activation of the AT1 subtype, and a facilitory role by AngIV activation of the AT4 subtype, on neuronal firing rate, long-term potentiation, associative and spatial learning. The discovery of the AT4 receptor subtype, and its facilitory influence upon learning and memory, suggest an important role for the brain RAS in normal cognitive processing and perhaps in the treatment of dysfunctional memory disease states.

The Type 1 Angiotensin II Receptor Tail Affects Receptor Targeting, Internalization, and Membrane Fusion Properties

Endocytosis modulates cell responses by removing and recycling receptors from the cell surface. Type I angiotensin II receptors (AT1R) are somewhat unique in that they are expressed at apical (AP) and basolateral (BL) membranes in proximal tubule cells and both receptor sites undergo endocytosis. We analyzed AT1R cytoplasmic (-COOH) tail deletion mutants to determine whether classic AT1R endocytosis motifs functioned similarly in polarized cells and simultaneously altered receptor properties. Serially truncating the AT1R tail had little effect on AP/BL AT1R distribution as determined by 125I-angiotensin II binding in LLCPK(Cl4) cells transfected with an AT1R transcript. AP AT1R expression required the proximal 12 amino acids in the AT1R-COOH tail. Deleting all but the proximal 12 aa of the AT1R-COOH tail (T316L mutant) decreased AP AT1R internalization at 20 min (17 +/- 6%; p < 0.05 versus full-length; n = 5) and inhibited AP AT1R-stimulated arachidonic acid release (counts released per milligram of protein at 20 min: full-length, 18,762 +/- 4018; T316L, 2430 +/- 1711; n = 4; p < 0.02). Endosomal fusion assays were performed using peptide sequences of regions in the AT1R tail involved in endocytosis (YFLQLLKYIPP [LL] and LSTKMSTLSY [STL]). Peptide STL significantly inhibited endosomal fusion (22 +/- 10% of control; n = 5; p < 0.05 versus positive control). Peptide LL had no significant inhibitory effect. AT(1)R in polarized cells contain dominant endocytosis signals but these motifs do not correlate with AP or BL AT1R expression. Moreover, peptide sequences within the AT1R-COOH tail necessary for endocytosis also modulate endosomal fusion properties.

Effects of angiotensin-(1-7) and other bioactive components of the renin-angiotensin system on vascular resistance and noradrenaline release in rat kidney

OBJECTIVE

Angiotensin (Ang) is broken down enzymatically to several different metabolites which, in addition to Ang II, may have important biological effects in the kidney. This study investigates the role of Ang metabolites on vascular resistance and noradrenaline release in the rat kidney.

METHODS AND RESULTS

In rat isolated kidney Ang I, Ang II, Ang III, Ang IV and des-Asp-Ang I induced pressor responses and enhanced noradrenaline release to renal nerve stimulation (RNS) in an concentration-dependent manner, with the following rank order of potency (EC(50)): Ang II >or= Ang III > Ang I = des-Asp-Ang I > Ang IV. All effects were blocked by the AT(1)-receptor antagonist EXP 3174 (0.1 micromol/l) but not by the AT(2)-receptor antagonist PD 123319 (1 micromol/l). Angiotensin-converting enzyme (ACE) inhibition by captopril (10 micromol/l) abolished the effect of Ang I and des-Asp-Ang I but had no influence on the effect of the other metabolites. Ang-(1-7) blocked the effects of Ang I and Ang II, being 10 times more potent against Ang I than Ang II. The selective Ang-(1-7) receptor blocker d-Ala7-Ang-(1-7) (10 micromol/l) did not influence the inhibitory effects of Ang-(1-7). Ang-(1-7) (10 micromol/l) by itself had no influence on vascular resistance and RNS-induced noradrenaline release.

CONCLUSION

Ang I, Ang II, Ang III, Ang IV and des-Asp-Ang I regulate renal vascular resistance and noradrenaline release by activation of AT(1) receptors. In the case of Ang I and des-Asp-Ang I this depends on conversion by ACE. Ang-(1-7) may act as a potent endogenous inhibitor/antagonist of ACE and the AT(1)-receptors, respectively.

A role for the angiotensin IV/AT4 system in mediating natriuresis in the rat

Angiotensin II (AngII) or Angiotensin IV (AngIV) was infused into the renal artery of anesthetized rats while renal cortical blood flow was measured via laser Doppler flowmetry. The infusion of AngII produced a significant elevation in mean arterial pressure (MAP) with an accompanying decrease in cortical blood flow, glomerular filtration rate (GFR), urine volume, and urine sodium excretion. The infusion of AngIV induced significant increases in renal cortical blood flow and urine sodium excretion, without altering MAP, GFR, and urine volume. Pretreatment infusion with a specific AT1 receptor antagonist, DuP 753, blocked or attenuated the subsequent AngII effects, while pretreatment infusion with the specific AT4 receptor antagonist, Divalinal-AngIV, blocked the AngIV effects. These results support distinct and opposite roles for AngII and AngIV, i.e. AngII acts as an anti-natriuretic agent, while AngIV acts as a natriuretic agent.

The cardiac renin-angiotensin system: novel signaling mechanisms related to cardiac growth and function

Angiotensin II, the effector peptide of the renin-angiotensin system, has been demonstrated to be involved in the regulation of cellular growth of several tissues in response to developmental, physiological, and pathological processes. The recent identification of renin-angiotensin system components and localization of angiotensin II receptors in cardiac tissue suggests that locally synthesized Ang II can modulate functional and growth responses in cardiac tissue. In this review, regulation of the cardiac RAS is discussed, with an emphasis on growth-related Ang II signal transduction systems.

Evaluation of three polymorphisms in the promoter region of the angiotensin II type I receptor gene

BACKGROUND

Angiotensin II induces vasoconstriction and growth via stimulation of the AT1 receptor. A genetic variant (+1166A/C) in the 3' untranslated region of this gene had been found to be associated with arterial hypertension, aortic stiffness and coronary artery disease.

OBJECTIVE

In order to evaluate further the potential implications of the genetic variability of the AT1 gene we explored three newly characterized single nucleotide polymorphisms (SNPs) in its promoter in a Caucasian population-based sample (n = 623). One of these (-2228G/A) is in complete linkage disequilibrium with six additional SNPs in the region such that, indirectly, potential functional implications of these sites were assessed as well. For comparison, we genotyped the previously described +1166A/C variant

RESULTS

The allele frequencies of the -2228G/A, -1424C/ G and -521 C/T SNPs were 0.82/0.18, 0.963/0.037 and 0.64/0.36, respectively. Statistical analysis by one-factor ANOVA revealed no significant relationship of any allele, genotype or haplotype with age, sex, body mass index, heart rate, systolic or diastolic blood pressure, hypertension, the intake of antihypertensive medication or left ventricular mass. Likewise, renin, angiotensinogen, angiotensin-converting enzyme, aldosterone or atrial natriuretic peptide levels were not found to be associated with any of these SNPs. Surprisingly, the -2228 A allele was found to be overrepresented in subjects with diabetes mellitus (n = 25, P = 0.006). However, this result could not be confirmed when additional individuals with diabetes mellitus (n = 45) were analysed. A weak linkage disequilibrium was observed between the -2228 A allele and the +1166 C allele (chi2 1 3.1; P = 0.010).

CONCLUSION

From the present data it is unlikely that any one of the nine newly characterized SNPs in the promoter region of AT1 gene is associated with arterial hypertension.

Angiotensin-(1-7) can interact with the rat proximal tubule AT(4) receptor system

This study was undertaken to identify the non-AT(1), non-AT(2) angiotensin receptor that mediates the ANG-(1-7) inhibitory action on rat proximal tubule transport processes. ANG-(1-7) inhibited nystatin-stimulated, ouabain-suppressible O(2) consumption (QO(2)) rates in freshly isolated rat proximal tubules (reflecting reduced basolateral Na(+)-K(+)-ATPase activity). Selective angiotensin-receptor subtype antagonists revealed that AT(1) and AT(4) receptors mediated the response of ANG-(1-7). Receptor autoradiography of the rat kidney demonstrated a high density of AT(1) and AT(4) receptors and no specific (125)I-ANG(1-7) binding sites. Competition assays in rat kidney sections indicated that ANG-(1-7) competed predominantly for the AT(1) receptor site, whereas its NH(2)-terminal-deleted metabolite, ANG-(3-7), competed primarily for the AT(4)-receptor site. Metabolism of (125)I-ANG-(1-7) in rat proximal tubules generated peptide fragments that included ANG-(3-7), with the pentapeptide producing a concentration-dependent inhibition of nystatin-stimulated proximal tubule QO(2) that was abolished by AT(4)-receptor blockade. These results suggest that the generation of ANG-(3-7) from the NH(2)-terminal metabolism of ANG-(1-7) caused the interaction of the parent peptide with the proximal tubule AT(4) receptor, which elicited a decrease in energy-dependent solute transport.

Angiotensin IV AT4-receptor system in the rat kidney

Angiotensin IV, [[des-Asp1,Arg2]ANG II or ANG-(3-8)], has been shown to preferentially bind to a novel angiotensin binding site (AT4 receptor). The cellular location and function of this receptor in the rat kidney is unknown. Autoradiography localized AT4 receptors to the cell body and apical membrane of convoluted and straight proximal tubules in the cortex and outer stripe of the outer medulla. ANG IV (0.1 pM-1 microM) elicited a concentration-dependent decrease in transcellular Na+ transport (as measured by proximal tubule O2 consumption rates) in fresh suspensions of control or nystatin-stimulated (bypasses rate-limiting step of apical Na+ entry) rat proximal tubules. The inhibitory effect of 1 pM ANG IV was unaltered by either 1 microM losartan (AT1-receptor antagonist) or 1 microM PD-123319 (AT2-receptor antagonist) and yet was abolished by 1 microM divalinal-ANG IV (AT4-receptor antagonist) or ouabain pretreatment. These results demonstrate that the kidney AT4-receptor system is localized to the proximal tubule and suggests that one potential biological role of this system is in the regulation of Na+ transport by inhibiting a ouabain-sensitive component of Na(+)-K(+)-adenosinetriphosphatase activity in the rat.

Effects of age and gender on the AII-induced stimulation of prolactin release and inositol phosphate accumulation in rat anterior pituitary cells in vitro

The stimulatory effects of Angiotensin II (AII) on prolactin secretion and inositol phosphate accumulation were examined in dispersed anterior pituitary cells collected from young (3-4 month), mature (7-8 month) and old (18-20 month) male and female rats. Physiological doses of AII (0.01-10 nM) stimulated prolactin release from cells collected from mature female rats only. This effect was antagonized by pretreatment with Saralasin, an AII receptor antagonist. Significant accumulation of the inositol phosphates was observed in cells obtained from the mature, female donors and this increase preceded the prolactin response. Although there was a small increase in total inositol phosphate accumulation in cells obtained from the old female rats, this was transient and did not coincide with a similar increase in prolactin release. These results indicate that pituitary sensitivity to AII stimulation is related to the age and the gender of the donor animal. The physiological role of pituitary AII needs to be examined in sexually mature female animals.