Angiotensin IV induces tyrosine phosphorylation of focal adhesion kinase and paxillin in proximal tubule cells

The procognitive and synaptogenic effects of angiotensin IV-derived peptides are dependent on activation of the hepatocyte growth factor/c-met system

A subset of angiotensin IV (AngIV)-related molecules are known to possess procognitive/antidementia properties and have been considered as templates for potential therapeutics. However, this potential has not been realized because of two factors: 1) a lack of blood-brain barrier-penetrant analogs, and 2) the absence of a validated mechanism of action. The pharmacokinetic barrier has recently been overcome with the synthesis of the orally active, blood-brain barrier-permeable analog N-hexanoic-tyrosine-isoleucine-(6) aminohexanoic amide (dihexa). Therefore, the goal of this study was to elucidate the mechanism that underlies dihexa's procognitive activity. Here, we demonstrate that dihexa binds with high affinity to hepatocyte growth factor (HGF) and both dihexa and its parent compound Norleucine 1-AngIV (Nle(1)-AngIV) induce c-Met phosphorylation in the presence of subthreshold concentrations of HGF and augment HGF-dependent cell scattering. Further, dihexa and Nle(1)-AngIV induce hippocampal spinogenesis and synaptogenesis similar to HGF itself. These actions were inhibited by an HGF antagonist and a short hairpin RNA directed at c-Met. Most importantly, the procognitive/antidementia capacity of orally delivered dihexa was blocked by an HGF antagonist delivered intracerebroventricularly as measured using the Morris water maze task of spatial learning.

Proximal nephron

The kidney plays a fundamental role in maintaining body salt and fluid balance and blood pressure homeostasis through the actions of its proximal and distal tubular segments of nephrons. However, proximal tubules are well recognized to exert a more prominent role than distal counterparts. Proximal tubules are responsible for reabsorbing approximately 65% of filtered load and most, if not all, of filtered amino acids, glucose, solutes, and low molecular weight proteins. Proximal tubules also play a key role in regulating acid-base balance by reabsorbing approximately 80% of filtered bicarbonate. The purpose of this review article is to provide a comprehensive overview of new insights and perspectives into current understanding of proximal tubules of nephrons, with an emphasis on the ultrastructure, molecular biology, cellular and integrative physiology, and the underlying signaling transduction mechanisms. The review is divided into three closely related sections. The first section focuses on the classification of nephrons and recent perspectives on the potential role of nephron numbers in human health and diseases. The second section reviews recent research on the structural and biochemical basis of proximal tubular function. The final section provides a comprehensive overview of new insights and perspectives in the physiological regulation of proximal tubular transport by vasoactive hormones. In the latter section, attention is particularly paid to new insights and perspectives learnt from recent cloning of transporters, development of transgenic animals with knockout or knockin of a particular gene of interest, and mapping of signaling pathways using microarrays and/or physiological proteomic approaches.

Elevation of the antifibrotic peptide N-acetyl-seryl-aspartyl-lysyl-proline: a blood pressure-independent beneficial effect of angiotensin I-converting enzyme inhibitors

Blockade of the renin-angiotensin system (RAS) is well recognized as an essential therapy in hypertensive, heart, and kidney diseases. There are several classes of drugs that block the RAS; these drugs are known to exhibit antifibrotic action. An analysis of the molecular mechanisms of action for these drugs can reveal potential differences in their antifibrotic roles. In this review, we discuss the antifibrotic action of RAS blockade with an emphasis on the potential importance of angiotensin I-converting enzyme (ACE) inhibition associated with the antifibrotic peptide N-acetyl-seryl-aspartyl-lysyl-proline (AcSDKP).

New insights and perspectives on intrarenal renin-angiotensin system: focus on intracrine/intracellular angiotensin II

Although renin, the rate-limiting enzyme of the renin-angiotensin system (RAS), was first discovered by Robert Tigerstedt and Bergman more than a century ago, the research on the RAS still remains stronger than ever. The RAS, once considered to be an endocrine system, is now widely recognized as dual (circulating and local/tissue) or multiple hormonal systems (endocrine, paracrine and intracrine). In addition to the classical renin/angiotensin I-converting enzyme (ACE)/angiotensin II (Ang II)/Ang II receptor (AT₁/AT₂) axis, the prorenin/(Pro)renin receptor (PRR)/MAP kinase axis, the ACE2/Ang (1-7)/Mas receptor axis, and the Ang IV/AT₄/insulin-regulated aminopeptidase (IRAP) axis have recently been discovered. Furthermore, the roles of the evolving RAS have been extended far beyond blood pressure control, aldosterone synthesis, and body fluid and electrolyte homeostasis. Indeed, novel actions and underlying signaling mechanisms for each member of the RAS in physiology and diseases are continuously uncovered. However, many challenges still remain in the RAS research field despite of more than one century's research effort. It is expected that the research on the expanded RAS will continue to play a prominent role in cardiovascular, renal and hypertension research. The purpose of this article is to review the progress recently being made in the RAS research, with special emphasis on the local RAS in the kidney and the newly discovered prorenin/PRR/MAP kinase axis, the ACE2/Ang (1-7)/Mas receptor axis, the Ang IV/AT₄/IRAP axis, and intracrine/intracellular Ang II. The improved knowledge of the expanded RAS will help us better understand how the classical renin/ACE/Ang II/AT₁ receptor axis, extracellular and/or intracellular origin, interacts with other novel RAS axes to regulate blood pressure and cardiovascular and kidney function in both physiological and diseased states.

Blood pressure and renal hemodynamic effects of angiotensin fragments

Angiotensin (Ang) II, the main effector peptide of the renin-Ang system, increases arterial blood pressure through Ang II type 1A (AT(1a)) receptor-dependent arterial vasoconstriction and by decreasing renal salt and water excretion through extrarenal and intrarenal mechanisms. AT(2) receptors are assumed to oppose these responses mediated by AT(1) receptors, thereby attenuating the pressor effects of Ang II. Nevertheless, a possible role of AT(2) receptors in the regulation of renal hemodynamics and sodium homeostasis remains to be unclear. Several other Ang fragments such as Ang III, Ang IV, Ang-(1-7) and Ang A have also been shown to display biological activity. In this review, we focus on the effects of these Ang on blood pressure, renal hemodynamics and sodium water handling, and discuss the receptors involved in these actions.

Cyst formation in the PKD2 (1-703) transgenic rat precedes deregulation of proliferation-related pathways

Background

Polycystic Kidney Disease is characterized by the formation of large fluid-filled cysts that eventually destroy the renal parenchyma leading to end-stage renal failure. Although remarkable progress has been made in understanding the pathologic mechanism of the disease, the precise orchestration of the early events leading to cyst formation is still unclear. Abnormal cellular proliferation was traditionally considered to be one of the primary irregularities leading to cyst initiation and growth. Consequently, many therapeutic interventions have focused on targeting this abnormal proliferation, and some have even progressed to clinical trials. However, the role of proliferation in cyst development was primarily examined at stages where cysts are already visible in the kidneys and therefore at later stages of disease development.

Methods

In this study we focused on the cystic phenotype since birth in an attempt to clarify the temporal contribution of cellular proliferation in cyst development. Using a PKD2 transgenic rat model (PKD2 (1-703)) of different ages (0-60 days after birth) we performed gene expression profiling and phenotype analysis by measuring various kidney parameters.

Results

Phenotype analysis demonstrated that renal cysts appear immediately after birth in the PKD2 transgenic rat model (PKD2 (1-703)). On the other hand, abnormal proliferation occurs at later stages of the disease as identified by gene expression profiling. Interestingly, other pathways appear to be deregulated at early stages of the disease in this PKD model. Specifically, gene expression analysis demonstrated that at day 0 the RAS system is involved. This is altered at day 6, when Wnt signaling and focal adhesion pathways are affected. However, at and after 24 days, proliferation, apoptosis, altered ECM signaling and many other factors become involved.

Conclusions

Our data suggest that cystogenesis precedes deregulation of proliferation-related pathways, suggesting that proliferation abnormalities may contribute in cyst growth rather than cyst formation.

The angiotensin IV analog Nle-Tyr-Leu-psi-(CH2-NH2)3-4-His-Pro-Phe (norleual) can act as a hepatocyte growth factor/c-Met inhibitor

The angiotensin (Ang) IV analog norleual [Nle-Tyr-Leu-psi-(CH2-NH2)(3-4)-His-Pro-Phe] exhibits structural homology with the hinge (linker) region of hepatocyte growth factor (HGF) and is hypothesized to act as a hinge region mimic. Norleual competitively inhibited the binding of HGF to its receptor c-Met in mouse liver membranes, with an IC(50) value of 3 pM. Predictably, norleual was able to inhibit HGF-dependent signaling, proliferation, migration, and invasion in multiple cell types at concentrations in the picomolar range. Ex vivo studies demonstrated that norleual exhibited potent antiangiogenic activity, an attribute that would be predicted for a HGF/c-Met antagonist. Furthermore, norleual suppressed pulmonary colonization by B16-F10 murine melanoma cells, which are characterized by an overactive HGF/c-Met system. Together, these data suggest that AngIV analogs exert at least some of their biological activity through interference with the HGF/c-Met system and may have utility as therapeutic agents in disorders that are dependent on an intact HGF/c-Met system. Finally, the ability of norleual to induce marked biological responses in human embryonic kidney cells, which do not express insulin-responsive aminopeptidase (IRAP), coupled with the observed effects of norleual on the HGF/c-Met system, casts doubt on the physiological significance of AngIV-dependent inhibition of IRAP. [Corrected]

Angiotensin receptor subtype mediated physiologies and behaviors: new discoveries and clinical targets

The renin-angiotensin system (RAS) mediates several classic physiologies including body water and electrolyte homeostasis, blood pressure, cyclicity of reproductive hormones and sexual behaviors, and the regulation of pituitary gland hormones. These functions appear to be mediated by the angiotensin II (AngII)/AT(1) receptor subtype system. More recently, the angiotensin IV (AngIV)/AT(4) receptor subtype system has been implicated in cognitive processing, cerebroprotection, local blood flow, stress, anxiety and depression. There is accumulating evidence to suggest an inhibitory influence by AngII acting at the AT(1) subtype, and a facilitory role by AngIV acting at the AT(4) subtype, on neuronal firing rate, long-term potentiation, associative and spatial learning, and memory. This review initially describes the biochemical pathways that permit synthesis and degradation of active angiotensin peptides and three receptor subtypes (AT(1), AT(2) and AT(4)) thus far characterized. There is vigorous debate concerning the identity of the most recently discovered receptor subtype, AT(4). Descriptions of classic and novel physiologies and behaviors controlled by the RAS are presented. This review concludes with a consideration of the emerging therapeutic applications suggested by these newly discovered functions of the RAS.

Aminopeptidase N in arterial hypertension

Aminopeptidase N (APN) or CD13 is a conserved type II integral membrane zinc-dependent metalloprotease in the M1 family of ectoenzymes. APN is abundant in the kidneys and central nervous system. Identified substrates include Angiotensin III (Ang III); neuropeptides, including enkephalins and endorphins; and homones, including kallidan and somatostatin. It is developmentally expressed, a myelomonocytic marker for leukemias, and a receptor for coronovirus. There is evolving support for APN in the regulation of arterial blood pressure and the pathogenesis of hypertension. In rodent strains, intracerebraventricular (i.c.v.) infusions of APN reduces, while inhibitors of APN activity have a pressor effect on blood pressure. Dysregulation of central APN has been linked to the pathogenesis of hypertension in the spontaneously hypertensive rat. There is evidence that renal tubule APN inhibits Na flux and plays a mechanistic role in salt-adaptation. A functional polymorphism of the ANP gene has been identified in the Dahl salt-sensitive rat. Signaling by APN impacting on blood pressure is likely mediated by regulation of the metabolism of Ang III to Ang IV. Whether APN regulates arterial blood pressure in humans or is a therapeutic target for hypertension are subjects for future exploration.

Aminopeptidase N reduces basolateral Na+ -K+ -ATPase in proximal tubule cells

Aminopeptidase N/CD13 (Anpep) is a membrane-bound protein that catalyzes the formation of natriuretic hexapeptide angiotensin IV (ANG IV) from ANG III. We previously reported that Anpep is more highly expressed in the kidneys of Dahl salt-resistant (SR/Jr) than salt-sensitive (SS/Jr) rats, Anpep maps to a quantitative trait locus for hypertension, and that the Dahl SR/Jr rat contains a functional polymorphism of the gene. This suggests that renal Anpep may be linked to salt sensitivity; however, its effect on renal Na handling has not been determined. Here, we examined regulation of basolateral Na(+)-K(+)-ATPase, a preeminent basolateral Na(+) transporter in proximal tubule cells, by Anpep in LLC-PK1 cells. Treatment of the cells with Anpep siRNA increased total cellular Na(+)-K(+)-ATPase activity and basolateral Na(+)-K(+)-ATPase abundance by approximately twofold. Conversely, Anpep overexpression reduced Na(+)-K(+)-ATPase activity and basolateral abundance by approximately 50%. Similar effects were observed after treatment with ANG IV (10 nM, x30 min and 12 h). ANG IV receptor (AGTRIV) knockdown via specific siRNA relieved the decreases in basolateral Na(+)-K(+)-ATPase levels and activity induced by Anpep overexpression. In sum, these results demonstrate that Anpep reduces basolateral Na(+)-K(+)-ATPase levels via ANG IV/AGTRIV signaling. This novel pathway may be important in renal adaptation to high salt.

Small potent ligands to the insulin-regulated aminopeptidase (IRAP)/AT4 receptor

Angiotensin IV analogs encompassing aromatic scaffolds replacing parts of the backbone of angiotensin IV have been synthesized and evaluated in biological assays. Several of the ligands displayed high affinities to the insulin-regulated aminopeptidase (IRAP)/AT(4) receptor. Displacement of the C-terminal of angiotensin IV with an o-substituted aryl acetic acid derivative delivered the ligand 4, which exhibited the highest binding affinity (K(i) = 1.9 nM). The high affinity of this ligand provides support to the hypothesis that angiotensin IV adopts a gamma-turn in the C-terminal of its bioactive conformation. Ligand (4) inhibits both human IRAP and aminopeptidase N-activity and induces proliferation of adult neural stem cells at low concentrations. Furthermore, ligand 4 is degraded considerably more slowly in membrane preparations than angiotensin IV. Hence, it might constitute a suitable research tool for biological studies of the (IRAP)/AT(4) receptor.

Role of EGF Receptor Activation in Angiotensin II–Induced Renal Epithelial Cell Hypertrophy

For determination of the molecular mechanisms underlying the induction of epithelial cell hypertrophy by angiotensin II (Ang II), a well-characterized porcine renal proximal tubular cell line LLCPKcl4, which does not express endogenous Ang II receptor subtypes, was transfected with cDNA encoding Ang II subtype 1 receptor (AT1R/Cl4). Ang II transactivated the EGF receptor (EGFR) in these AT1R/Cl4 cells, which was blocked by the selective AT1R antagonist losartan but not by the selective AT2R antagonist PD123319. Ang II did not transactivate EGFR in empty vector-transfected LLCPKcl4 cells (Vector/Cl4). Ang II elicited release of soluble heparin-binding EGF-like growth factor (HB-EGF) from AT1R/Cl4 cells, and Ang II-induced EGFR activation was prevented by pretreatment with the specific HB-EGF inhibitor CRM197 or the metalloproteinase inhibitors batimastat or phenanthroline, none of which had any effect on EGFR activation by exogenously administered EGF. Ang II stimulated protein synthesis and cell hypertrophy in AT1R/Cl4 cells without increasing cell number, and signaling studies revealed that Ang II stimulated phosphorylation of the 40S ribosomal protein S6 and the eukaryotic translation initiation factor 4E-binding protein 1, the two downstream target proteins of the mammalian target of rapamycin, which is a central regulator of protein synthesis and cell size. Ang II-induced mammalian target of rapamycin activation, [3H]leucine incorporation, and cellular hypertrophy were inhibited by pretreatment with either batimastat or CRM197 or by pretreatment with rapamycin or the EGFR tyrosine kinase inhibitor AG1478. Ang II also stimulated Smad 2/3 phosphorylation, which was blocked by a selective TGF-beta receptor I kinase inhibitor but not by CRM197. With blockade of TGF-beta receptor, Ang II-mediated hypertrophy was converted into cell proliferation, which was blocked by CRM197. In summary, this is the first demonstration that HB-EGF shedding-dependent EGFR transactivation, along with activation of TGF-beta signaling pathways, mediates Ang II-induced renal tubular epithelial cell hypertrophy.

The CNS renin-angiotensin system

The renin-angiotensin system (RAS) is one of the best-studied enzyme-neuropeptide systems in the brain and can serve as a model for the action of peptides on neuronal function in general. It is now well established that the brain has its own intrinsic RAS with all its components present in the central nervous system. The RAS generates a family of bioactive angiotensin peptides with variable biological and neurobiological activities. These include angiotensin-(1-8) [Ang II], angiotensin-(3-8) [Ang IV], and angiotensin-(1-7) [Ang-(1-7)]. These neuroactive forms of angiotensin act through specific receptors. Only Ang II acts through two different high-specific receptors, termed AT1 and AT2. Neuronal AT1 receptors mediate the stimulatory actions of Ang II on blood pressure, water and salt intake, and the secretion of vasopressin. In contrast, neuronal AT2 receptors have been implicated in the stimulation of apoptosis and as being antagonistic to AT1 receptors. Among the many potential effects mediated by stimulation of AT2 are neuronal regeneration after injury and the inhibition of pathological growth. Ang-(1-7) mediates its antihypertensive effects by stimulating the synthesis and release of vasodilator prostaglandins and nitric oxide and by potentiating the hypotensive effects of bradykinin. New data concerning the roles of Ang IV and Ang-(1-7) in cognition also support the existence of complex site-specific interactions between multiple angiotensins and multiple receptors in the mediation of important central functions of the RAS. Thus, the RAS of the brain is involved not only in the regulation of blood pressure, but also in the modulation of multiple additional functions in the brain, including processes of sensory information, learning, and memory, and the regulation of emotional responses.

AT1 receptor-activated signaling mediates angiotensin IV-induced renal cortical vasoconstriction in rats

Angiotensin IV (ANG IV), an active ANG II fragment, has been shown to induce systemic and renal cortical effects by binding to ANG IV (AT(4)) receptors and activating unique signaling transductions unrelated to classical type 1 (AT(1)) or type 2 (AT(2)) receptors. We tested whether ANG IV exerts systemic and renal cortical effects on blood pressure, renal microvascular smooth muscle cells (VSMCs), and glomerular mesangial cells (MC) and, if so, whether AT(1) receptor-activated signaling is involved. In anesthetized rats, systemic infusion of ANG II, ANG III, or ANG IV (0.01, 0.1, and 1.0 nmol.kg(-1).min(-1) iv) caused dose-dependent increases in mean arterial pressure (MAP) and decreases in renal cortical blood flow (CBF; P < 0.01). ANG II also induced dose-dependent reductions in renal medullary blood flow (P < 0.01), whereas ANG IV did not. ANG IV-induced pressor and renal cortical vasoconstriction were completely abolished by AT(1) receptor blockade with losartan (5 mg/kg iv; P < 0.05). When ANG IV (1 nmol.kg(-1).min(-1)) was infused directly in the renal artery, CBF was reduced by >30%, and the response was also blocked by losartan (P < 0.01). In the renal cortex, unlabeled ANG IV displaced (125)I-labeled [Sar(1),Ile(8)]ANG II binding, whereas unlabeled ANG II (10 microM) inhibited (125)I-labeled Nle(1)-ANG IV (AT(4)) binding in a concentration-dependent manner (P < 0.01). In freshly isolated renal VSMCs, ANG IV (100 nM) increased intracellular Ca(2+) concentration, and the effect was blocked by losartan and U-73122, a selective inhibitor of phospholipase C/inositol trisphosphate/Ca(2+) signaling (1 microM). In cultured rat MCs, ANG IV (10 nM) induced mitogen-activated protein kinase extracellular/signal-regulated kinase 1/2 phosphorylation via AT(1) receptor- and phospholipase C-activated signaling. These results suggest that, at nanomolar concentrations, ANG IV can increase MAP and induce renal cortical effects by interacting with AT(1) receptor-activated signaling.

Angiotensin IV activates the nuclear transcription factor-kappaB and related proinflammatory genes in vascular smooth muscle cells

Inflammation is a key event in the development of atherosclerosis. Nuclear factor-kappaB (NF-kappaB) is important in the inflammatory response regulation. The effector peptide of the renin angiotensin system Angiotensin II (Ang II) activates NF-kappaB and upregulates some related proinflammatory genes. Our aim was to investigate whether other angiotensin-related peptides, as the N-terminal degradation peptide Ang IV, could regulate proinflammatory factors (activation of NF-kappaB and related genes) in cultured vascular smooth muscle cells (VSMCs). In these cells, Ang IV increased NF-kappaB DNA binding activity, caused nuclear translocation of p50/p65 subunits, cytosolic IkappaB degradation and induced NF-kappaB-dependent gene transcription. Ang II activates NF-kappaB via AT1 and AT2 receptors, but AT1 or AT2 antagonists did not inhibit NF-kappaB activation caused by Ang IV. In VSMC from AT1a receptor knockout mice, Ang IV also activated NF-kappaB pathway. In those cells, the AT4 antagonist divalinal diminished dose-dependently Ang IV-induced NF-kappaB activation and prevented IkappaB degradation, but had no effect on the Ang II response, indicating that Ang IV activates the NF-kappaB pathway via AT4 receptors. Ang IV also increased the expression of proinflammatory factors under NF-kappaB control, such as MCP-1, IL-6, TNF-alpha, ICAM-1, and PAI-1, which were blocked by the AT4 antagonist. Our results reveal that Ang IV, via AT4 receptors, activates NF-kappaB pathway and increases proinflammatory genes. These data indicate that Ang IV possesses proinflammatory properties, suggesting that this Ang degradation peptide could participate in the pathogenesis of cardiovascular diseases.

Growth factors stimulate kidney proximal tubule cell migration independent of augmented tyrosine phosphorylation of focal adhesion kinase

Migration of human proximal tubule cells (HKC-5) was stimulated by epidermal growth factor (EGF), hepatocyte growth factor (HGF), and insulin-like growth factor-1 (IGF-1). Integrin signaling via phosphorylation of focal adhesion kinase (FAK) appears to play a central role in cell migration. Once stimulated, FAK undergoes autophosphorylation at tyrosine (Y) 397, followed by phosphorylation of several sites including Y576/Y577 which increases FAK's kinase activity, as well as at Y407, Y861, and Y925. EGF, HGF, and IGF-1 stimulate FAK phosphorylation in various cells. We showed that endothelin stimulated phosphorylation of Y397 in fibroblasts but not HKC-5 cells. After EGF stimulation, HKC-5 cells showed no change in tyrosine phosphorylation at FAK Y397, 407, 576, 861, or 925. Similarly, HGF and IGF-1 did not stimulate the phosphorylation of FAK Y397 in HKC-5 cells. Further, after inhibition of FAK expression by siRNA, cell migration was similar to cells treated with non-target siRNA and responded to EGF with increased migration. Thus, in proximal tubule cells, stimulation of cell migration by growth factors was independent of augmented FAK tyrosine phosphorylation.

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.

AT4 receptor binding in the developing rabbit

The binding of the AT(4)-specific analog, divalinal-AngIV (Dival), was studied in rabbit fetuses of various gestational ages. Saturation isotherm and competition data from selected tissues indicate that fetal Dival binding sites are saturable and specific for AT(4) ligands. Autoradiographs revealed that binding was present in all the specimens examined. The peripheral nerves, kidneys, and heart were particularly heavily labeled. Labeling of some tissues, such as forming bones, was not constant as gestational age increased. Other tissues, including multilocular fat, sinus hairs, and enamel organs of nascent teeth, exhibited substantial binding as these tissues developed.

Effect of I.C.V. injection of AT4 receptor ligands, NLE1-angiotensin IV and LVV-hemorphin 7, on spatial learning in rats

Central administration of angiotensin IV (Ang IV) or its analogues enhance performance of rats in passive avoidance and spatial memory paradigms. The purpose of this study was to examine the effect of a single bolus injection of two distinct AT4 ligands, Nle1-Ang IV or LVV-haemorphin-7, on spatial learning in the Barnes circular maze. Mean number of days for rats treated with either Nle1-Ang IV or LVV-haemorphin-7 to achieve learner criterion is significantly reduced compared with controls (P < 0.001 and P < 0.05 respectively). This is due to enhanced ability of the peptide-treated rats to adopt a spatial strategy for finding the escape hatch. In all three measures of learning performance, (1) the number of errors made, (2) the distance travelled and (3) the latency in finding the escape hatch, rats treated with either 100 pmol or 1 nmol of Nle1-Ang IV or 100 pmol LVV-haemorphin-7 performed significantly better than the control groups. As early as the first day of testing, the rats treated with the lower dose of Nle1-Ang IV or LVV-haemorphin-7 made fewer errors (P < 0.01 and P < 0.05 respectively) and travelled shorter distances (P < 0.05 for both groups) than the control animals. The enhanced spatial learning induced by Nle1-Ang IV (100 pmol) was attenuated by the co-administration of the AT4 receptor antagonist, divalinal-Ang IV (10 nmol). Thus, administration of AT4 ligands results in an immediate potentiation of learning, which may be associated with facilitation of synaptic transmission and/or enhancement of acetylcholine release.

NO inhibits Na+-K+-2Cl- cotransport via a cytochrome P-450-dependent pathway in renal epithelial cells (MMDD1)

Nitric oxide (NO) exerts direct effects on nephron transport. We determined the effect of NO on Na(+)-K(+)-2Cl(-) cotransport in a cell line (MMDD1) with properties of macula densa. Na(+)-K(+)-2Cl(-) cotransport was measured as bumetanide-sensitive (86)Rb(+) uptake in the presence of ouabain. MMDD1 cells expressed mRNA for the neuronal isoform of nitric oxide synthase, as well as NKCC1 and NKCC2(B) isoforms of the Na(+)-K(+)-2Cl(-) cotransporter. Preincubation of cells with the NO donors sodium nitroprusside (SNP) or S-nitroso-N-acetylpenicillamine (SNAP) caused concentration-dependent inhibition of Na(+)-K(+)-2Cl(-) cotransport. Both apical and basolateral Na(+)-K(+)-2Cl(-) cotransport was inhibited by NO donors. SNP or SNAP had no significant effect on cellular levels of cGMP, cAMP, cytosolic calcium, or phosphorylation of ERK1 and ERK2. In contrast, the inhibitors of cytochrome P-450, 1-aminobenzotriazole (ABT; 10(-3) M) or ketoconazole (1.5 x 10(-5) M), completely reversed the inhibitory effect of SNAP on apical or basolateral Na(+)-K(+)-2Cl(-) cotransport [apical: control 1.18 +/- 0.15 vs. SNAP (10(-4) M) 0.41 +/- 0.05 pmol x mg(-1) x 5 min(-1); P < 0.001; SNAP (10(-4) M) + ABT 1.32 +/- 0.10 pmol x mg(-1) x 5 min(-1); P = not significant vs. control; n = 5]. The cytochrome P-450 epoxyeicosatrienoic acid (EET) metabolite 14,15-EET (5 x 10(-7) M) inhibited both apical and basolateral cotransport, whereas 8,9-EET and 11,12-EET had no significant effect. Although 20-hydroxyeicosatetraenoic acid inhibited apical cotransport, the inhibitor of omega-hydroxylase activity HET0016 did not reverse SNAP-mediated inhibition of apical cotransport. These data indicate that NO inhibits apical and basolateral Na(+)-K(+)-2Cl(-) cotransport in MMDD1 cells. The results suggest that the inhibitory pathway is independent of cGMP and might involve stimulation of a cytochrome P-450-dependent pathway.

Angiotensin IV interacts with a juxtamembrane site on AT(4)/IRAP suggesting an allosteric mechanism of enzyme modulation

Angiotensin IV (Ang IV), the 3-8 fragment of angiotensin II, binds to a specific receptor (AT(4)) that has recently been identified as the transmembrane aminopeptidase insulin-regulated aminopeptidase (IRAP) based on the fact that the two proteins share several pharmacological and biochemical properties. Our binding studies indicated that bovine heart expresses relatively large amounts (1.2 pmol/mg protein) of high-affinity binding sites for Ang IV (K(d)=1.8 nM). A photoaffinity-labeling approach combined with mild trypsin digestion revealed that the AT(4) receptor of bovine heart is a single transmembrane domain protein (153 kDa) with a large extracellular fragment (143 kDa). After alkaline denaturation of the AT(4) receptor, trypsin digestion produced two small membrane-associated fragments (16.9 and 6.6 kDa). These results suggest that Ang IV interacts with a juxtamembrane domain of AT(4) receptor. The location of the juxtamembrane site of contact was different from that of the active site of IRAP, suggesting that Ang IV uses an allosteric mechanism to modulate the activity of the AT(4)/IRAP.

AT4 receptor is insulin-regulated membrane aminopeptidase: potential mechanisms of memory enhancement

Although angiotensin IV (Ang IV) was thought initially to be an inactive product of Ang II degradation, it was subsequently shown that the hexapeptide markedly enhances learning and memory in normal rodents and reverses the memory deficits seen in animal models of amnesia. These central nervous system effects of Ang IV are mediated by binding to a specific site, known as the AT(4) receptor, which is found in appreciable levels throughout the brain and is concentrated particularly in regions involved in cognition. This field of research was redefined by the identification of the AT(4) receptor as the transmembrane enzyme, insulin-regulated membrane aminopeptidase (IRAP). Here, we explore the potential mechanisms by which Ang IV binding to IRAP leads to the facilitation of learning and memory.

Activation of multiple signaling modules is critical in angiotensin IV-induced lung endothelial cell proliferation

Signaling events involving angiotensin IV (ANG IV)-mediated pulmonary artery endothelial cell (PAEC) proliferation were examined. ANG IV significantly increased upstream phosphatidylinositide (PI) 3-kinase (PI3K), PI-dependent kinase-1 (PDK-1), extracellular signal-related kinases (ERK1/2), and protein kinase B-alpha/Akt (PKB-alpha) activities, as well as downstream p70 ribosomal S6 kinase (p70S6K) activities and/or phosphorylation of these proteins. ANG IV also significantly increased 5-bromo-2'-deoxy-uridine incorporation into newly synthesized DNA in a concentration- and time-dependent manner. Pretreatment of cells with wortmannin and LY-294002, inhibitors of PI3K, or rapamycin, an inhibitor of the mammalian target of rapamycin kinase and p70S6K, diminished the ANG IV-mediated activation of PDK-1 and PKB-alpha as well as phosphorylation of p70S6K. Although an inhibitor of mitogen-activated protein kinase kinase, PD-98059, but not rapamycin, blocked ANG IV-induced phosphorylation of ERK1/2, both PD-98059 and rapamycin independently caused partial reduction in ANG IV-mediated cell proliferation. However, simultaneous treatment with PD-98059 and rapamycin resulted in total inhibition of ANG IV-induced cell proliferation. These results demonstrate that ANG IV-induced DNA synthesis is regulated in a coordinated fashion involving multiple signaling modules in PAEC.

Autoradiographic analysis and regulation of angiotensin receptor subtypes AT(4), AT(1), and AT((1-7)) in the kidney

Receptor autoradiography revealed that angiotensin AT(4) receptors were abundantly expressed in normal mammalian (mouse, rat, gerbil, guinea pig, rabbit) and avian (sparrow, chicken, turkey) kidneys and were more extensively distributed than previously reported (including proximal and distal segments of the nephron, interstitium, renal artery, vein, and ureter). Angiotensin AT(4) receptors were generally found to be more abundant than angiotensin AT(1) receptors in mammalian kidneys, whereas angiotensin AT((1-7)) receptors were not detected in either mammalian or avian kidneys. Rats subjected to various chronic treatments were found to preferentially decrease kidney AT(4) receptor density (furosemide, puromycin aminonucleoside, nitro-L-arginine methyl ester), decrease kidney AT(1) receptor density (bilateral ureteral obstruction), or increase kidney AT(1) receptor distribution in the inner medulla (water diuresis). These results indicate that the AT(4) receptor can be expressed in numerous cell types within the normal kidney of several species. Furthermore, several models of renal dysfunction and injury have been identified that selectively alter kidney AT(4) density and may potentially aid in elucidating the role of this novel angiotensin receptor system in renal function.