Multiple growth factors modulate mRNA expression of angiotensin II type-2 receptor in R3T3 cells

Novel Therapeutic Approaches Targeting the Renin-Angiotensin System and Associated Peptides in Hypertension and Heart Failure

Despite the success of renin-angiotensin system (RAS) blockade by angiotensin-converting enzyme (ACE) inhibitors and angiotensin II type 1 receptor (AT1R) blockers, current therapies for hypertension and related cardiovascular diseases are still inadequate. Identification of additional components of the RAS and associated vasoactive pathways, as well as new structural and functional insights into established targets, have led to novel therapeutic approaches with the potential to provide improved cardiovascular protection and better blood pressure control and/or reduced adverse side effects. The simultaneous modulation of several neurohumoral mediators in key interconnected blood pressure-regulating pathways has been an attractive approach to improve treatment efficacy, and several novel approaches involve combination therapy or dual-acting agents. In addition, increased understanding of the complexity of the RAS has led to novel approaches aimed at upregulating the ACE2/angiotensin-(1-7)/Mas axis to counter-regulate the harmful effects of the ACE/angiotensin II/angiotensin III/AT1R axis. These advances have opened new avenues for the development of novel drugs targeting the RAS to better treat hypertension and heart failure. Here we focus on new therapies in preclinical and early clinical stages of development, including novel small molecule inhibitors and receptor agonists/antagonists, less conventional strategies such as gene therapy to suppress angiotensinogen at the RNA level, recombinant ACE2 protein, and novel bispecific designer peptides.

Pregnancy upregulates angiotensin type 2 receptor expression and increases blood flow in uterine arteries of rats

Normal pregnancy is associated with decreased uterine vascular contraction and increased blood flow even though angiotensin II (AngII) levels are increased. AngII not only activates the angiotensin type 1 receptor (AT1R) to mediate vasoconstriction but also angiotensin type 2 receptor (AT2R) to cause vasodilation. We hypothesized that upregulation of AT2R expression and function accounts for increased uterine artery blood flow during pregnancy. Virgin, pregnant (at different days of gestation) and post-partum Sprague-Dawley rats were used to determine uterine artery hemodynamics using micro ultrasound and plasma angiotensin II levels by ELISA. Isolated uterine arteries were examined for AT1R and AT2R expression and isometric contraction/relaxation. Plasma AngII levels were steady up to mid-pregnancy, increased as pregnancy advanced, reaching a peak in late pregnancy, and then restored to pre-pregnant levels after delivery. The pattern of increase in AngII levels mirrored a parallel increase in uterine blood flow. AT1R expression did not change, but AT2R expression increased during pregnancy correlating with uterine blood flow increase. Treatment with the AT2R antagonist PD123319 reduced uterine arterial blood flow. Vasoconstriction to angiotensin II was blunted in pregnant rats. Treatment with PD123319 caused greater enhancement of AngII contraction in pregnant than virgin rats. Ex vivo exposure of estradiol to uterine arterial rings dose dependently upregulated AT2R expression, that was inhibited by estrogen receptor antagonist. These results demonstrate that elevated AngII levels during gestation induce an increase in uterine blood flow via heightened AT2R-mediated signaling. Estrogens appear to directly upregulate uterine vascular AT2R independent of any endogenous factors.

Dysregulation of the Renin-Angiotensin System and the Vasopressinergic System Interactions in Cardiovascular Disorders

Purpose of Review

In many instances, the renin-angiotensin system (RAS) and the vasopressinergic system (VPS) are jointly activated by the same stimuli and engaged in the regulation of the same processes.

Recent Findings

Angiotensin II (Ang II) and arginine vasopressin (AVP), which are the main active compounds of the RAS and the VPS, interact at several levels. Firstly, Ang II, acting on AT1 receptors (AT1R), plays a significant role in the release of AVP from vasopressinergic neurons and AVP, stimulating V1a receptors (V1aR), regulates the release of renin in the kidney. Secondly, Ang II and AVP, acting on AT1R and V1aR, respectively, exert vasoconstriction, increase cardiac contractility, stimulate the sympathoadrenal system, and elevate blood pressure. At the same time, they act antagonistically in the regulation of blood pressure by baroreflex. Thirdly, the cooperative action of Ang II acting on AT1R and AVP stimulating both V1aR and V2 receptors in the kidney is necessary for the appropriate regulation of renal blood flow and the efficient resorption of sodium and water. Furthermore, both peptides enhance the release of aldosterone and potentiate its action in the renal tubules.

Summary

In this review, we (1) point attention to the role of the cooperative action of Ang II and AVP for the regulation of blood pressure and the water-electrolyte balance under physiological conditions, (2) present the subcellular mechanisms underlying interactions of these two peptides, and (3) provide evidence that dysregulation of the cooperative action of Ang II and AVP significantly contributes to the development of disturbances in the regulation of blood pressure and the water-electrolyte balance in cardiovascular diseases.

AT2 receptor activities and pathophysiological implications

Although angiotensin II subtype-2 receptor (AT2R) was discovered over 2 decades ago, its contribution to physiology and pathophysiology is not fully elucidated. Current knowledge suggests that under normal physiologic conditions, AT2R counterbalances the effects of angiotensin II subtype-1 receptor (AT1R). A major obstacle for AT2R investigations was the lack of specific agonists. Most of the earlier AT2R studies were performed using the peptidic agonist, CG42112A, or the nonpeptidic antagonist PD123319. CGP42112A is nonspecific for AT2R and in higher concentrations can bind to AT1R. Recently, the development of specific nonpeptidic AT2R agonists boosted the efforts in identifying the therapeutic potentials for AT2R stimulation. Unlike AT1R, AT2R is involved in vasodilation by the release of bradykinin and nitric oxide, anti-inflammation, and healing from injury. Interestingly, the vasodilatory effects of AT2R stimulation were not associated with significant reduction in blood pressure. In the kidney, AT2R stimulation produced natriuresis, increased renal blood flow, and reduced tissue inflammation. In animal studies, enhanced AT2R function led to reduction of cardiac inflammation and fibrosis, and reduced the size of the infarcted area. Similarly, AT2R stimulation demonstrated protective effects in vasculature and brain.

Classical Renin-Angiotensin system in kidney physiology

The renin-angiotensin system has powerful effects in control of the blood pressure and sodium homeostasis. These actions are coordinated through integrated actions in the kidney, cardiovascular system and the central nervous system. Along with its impact on blood pressure, the renin-angiotensin system also influences a range of processes from inflammation and immune responses to longevity. Here, we review the actions of the "classical" renin-angiotensin system, whereby the substrate protein angiotensinogen is processed in a two-step reaction by renin and angiotensin converting enzyme, resulting in the sequential generation of angiotensin I and angiotensin II, the major biologically active renin-angiotensin system peptide, which exerts its actions via type 1 and type 2 angiotensin receptors. In recent years, several new enzymes, peptides, and receptors related to the renin-angiotensin system have been identified, manifesting a complexity that was previously unappreciated. While the functions of these alternative pathways will be reviewed elsewhere in this journal, our focus here is on the physiological role of components of the "classical" renin-angiotensin system, with an emphasis on new developments and modern concepts.

Branched-chain amino acid supplemented diet during maternal food restriction prevents developmental hypertension in adult rat offspring

Maternal food restriction is known to cause developmental hypertension in offspring. We have previously shown that maternal high-protein diet can reverse fetal programming of hypertension and that branched-chain amino acid (BCAA) concentrations in maternal and fetal plasma were increased by maternal high-protein intake. Then, we hypothesized that isocaloric supplementation with BCAA to a maternal food restriction can reverse the adverse outcome. Pregnant rats were divided into four groups at 7.5 days postcoitum: normally nourished (NN) and 70% undernourished (UN) groups with and without BCAA supplementation (NN-standard diet (SD), NN-BCAA, UN-SD and UN-BCAA groups). Compared with pups in the NN groups, those in the UN-SD group had significantly increased systolic blood pressure (SBP) at 8 and 16 weeks of age (P < 0.05). However, the elevation of SBP was not observed in offspring in the UN-BCAA group. Offspring glomeruli number of the UN groups was significantly lower (P < 0.05) than that of the NN groups, independent of BCAA supplementation. Angiotensin II receptor type 2 (ATR2) mRNA and protein expression in the kidney was significantly augmented in the UN-BCAA group at 30 weeks of age. In conclusion, BCAA supplementation during maternal food restriction prevents developmental hypertension together with increased ATR2 expression in adult offspring kidney.

RAS-Mediated Adaptive Mechanisms in Cardiovascular Tissues: Confounding Factors of RAS Blockade Therapy and Alternative Approaches

Since the classic experiments by Tigerstedt and Bergman that established the role of renin in hypertension a century ago, aggressive efforts have been launched to effectively block the renin-angiotensin system (RAS). Blockade of RAS is advocated at multiple levels by direct renin inhibitor, angiotensin-converting enzyme inhibitor and/or angiotensin II type 1 receptor blocker, or aldosterone inhibitor (spironolactone), and has now become part of the standard of care to control hypertension and related metabolic diseases including diabetes. However, recent lessons learned from randomized clinical trials question the wisdom of blocking RAS at multiple levels. In this context, it is highly pertinent that components of RAS are evolutionarily conserved, and novel physiological/adaptive/protective roles for renin and angiotensin-converting enzyme are currently emerging. Angiotensin II, the classical RAS effector peptide responsible for hypertension, hypertrophy, fluid retention and fibrosis, manifests its cardiovascular protective effect when it activates the angiotensin II type 2 receptor. Additionally, angiotensin-converting enzyme 2 and the angiotensin II metabolite Ang-(1-7) that acts through the Mas proto-oncogene constitute the cardiovascular and renal protective branch of RAS. It is conceivable that modulating this vasodilative/anti-inflammatory branch of RAS by activation of the RAS components that constitute this branch may offer a safer long-term treatment strategy to balance RAS activity and achieve homeostasis compared to chronic multilevel RAS inhibition.

Recent advances involving the renin-angiotensin system

The renin-angiotensin system (RAS) exercises fundamental control over sodium and water handling in the kidney. Accordingly, dysregulation of the RAS leads to blood pressure elevation with ensuing renal and cardiovascular damage. Recent studies have revealed that the RAS hormonal cascade is more complex than initially posited with multiple enzymes, effector molecules, and receptors that coordinately regulate the effects of the RAS on the kidney and vasculature. Moreover, recently identified tissue-specific RAS components have pleomorphic effects independent of the circulating RAS that influence critical homeostatic mechanisms including the immune response and fetal development. Further characterization of the diverse interactions between the RAS and other signaling pathways within specific tissues should lead to novel treatments for renal and cardiovascular disease.

Effect of angiotensin type 2 receptor over-expression on the rat mesangial cell fibrotic phenotype: effect of gender

BACKGROUND AND AIM

The protective role of angiotensin type 2 receptors (AT2-Rs) is still controversial. As AT2-Rs are minimally expressed in adult tissues the aim of the current study was to over-express AT2-Rs in rat mesangial cells in order to ascertain their potential role in modulating renal scarring.

METHODS

Male and female mesangial cells were transiently transfected with AT2-R or control vector then 'injured' with macrophage-conditioned medium (MCM). Culture supernatants and extracted RNA were analysed for evidence of an anti-fibrotic phenotype.

RESULTS

Supernatant fibronectin levels in female mesangial cells treated with MCM were reduced in AT2-R transfected cells (p < 0.001) compared to controls. AT2-R transfected male cells showed a trend towards lower constitutive fibronectin levels. There was no effect of AT2-R transfection on TGF-β or TNF-α secretion; however, IL-1β levels were reduced in male cells treated with MCM. RT-PCR demonstrated that constitutive kallikrein mRNA levels were suppressed in both male and female AT2-R transfected cells. Bradykinin receptors (BkB2-R and BkB1-R) were unaffected in female cells although the BkB1-R was upregulated in male cells treated with MCM.

CONCLUSION

This data provides a case for AT2 receptors playing a protective role in rat mesangial cells independent of the effects of blood pressure control.

Role of the renin angiotensin system in diabetic nephropathy

Diabetic nephropathy has been the cause of lot of morbidity and mortality in the diabetic population. The renin angiotensin system (RAS) is considered to be involved in most of the pathological processes that result in diabetic nephropathy. This system has various subsystems which contribute to the disease pathology. One of these involves angiotensin II (Ang II) which shows increased activity during diabetic nephropathy. This causes hypertrophy of various renal cells and has a pressor effect on arteriolar smooth muscle resulting in increased vascular pressure. Ang II also induces inflammation, apoptosis, cell growth, migration and differentiation. Monocyte chemoattractant protein-1 production responsible for renal fibrosis is also regulated by RAS. Polymorphism of angiotensin converting enzyme (ACE) and Angiotensinogen has been shown to have effects on RAS. Available treatment modalities have proven effective in controlling the progression of nephropathy. Various drugs (based on antagonism of RAS) are currently in the market and others are still under trial. Amongst the approved drugs, ACE inhibitors and angiotensin receptor blockers (ARBs) are widely used in clinical practice. ARBs are shown to be superior to ACE inhibitors in terms of reducing proteinuria but the combined role of ARBs with ACE inhibitors in diabetic nephropathy is under debate.

The angiotensin II type 2 receptor and the kidney

Recent knowledge demonstrated that the renin-angiotensin system (RAS) functions as a local renal paracrine system. All components of the RAS are present within the kidney and include angiotensinogen, renin, angiotensin I, angiotensin-converting enzymes, angiotensin II, the angiotensin II type 1 (AT(1)) receptor and the angiotensin II type 2 (AT(2)) receptor. Angiotensin II is the major effector hormone of the RAS and contributes to a variety of renal and cardiovascular physiologic and pathologic mechanisms through stimulation of AT(1) and AT(2) receptors. Angiotensin receptor blockers were developed based on the advanced knowledge of the AT(1) receptor contribution to development of a variety of kidney, vascular and cardiac diseases including but not limited to hypertension, diabetic nephropathy, heart failure, myocardial infarction and atherosclerosis. In contrast, knowledge concerning the role of the AT(2) receptor in health and disease is still emerging. The AT(2) receptor is believed to counterbalance the effects of the AT(1) receptor through influencing cellular differentiation, vasodilation, inhibition of cellular proliferation and hypertrophy, nitric oxide production and natriuresis. Thus, the pursuit of a specific AT(2) receptor agonist is a potentially fruitful area for combating renal and cardiovascular diseases. This review focuses on the role of the AT(2) receptor in the kidney.

Angiotensin II and interleukin-1 interact to increase generation of dopaminergic neurons from neurospheres of mesencephalic precursors

Cultures of rat mesencephalic precursors treated with interleukin-1beta or angiotensin II contained significantly more dopaminergic neurons than controls. However, simultaneous treatment with angiotensin II and interleukin-1beta did not induce any further increase. Treatment with the angiotensin type-2 receptor antagonist PD 123319 precluded both the angiotensin- and the interleukin-induced increase. The present results indicate that angiotensin type-2 receptors and interleukin-1 cooperate to induce the dopaminergic phenotype.

Angiotensin II stimulates arachidonic acid release from bone marrow stromal cells

INTRODUCTION

Angiotensin II (Ang II) is recognised as a regulator of haematopoiesis, but its actions within the bone marrow are not fully understood. Support of haematopoiesis by bone marrow stromal cells (MSC) is dependent on factors that include arachidonic acid and macrophage colony stimulating factor (MCSF), both of which are increased by Ang II stimulation in other tissues. To further elucidate the mechanisms of Ang II-regulated haematopoiesis, we determined whether Ang II-stimulation alters arachidonic acid release and MCSF secretion from MSC.

METHODS

Cynomolgus monkey MSC isolated from bone marrow aspirates and the human HS-5 stromal cell line were studied for Ang II-mediated arachidonic acid (AA) release, while secretion of MCSF in response to Ang II was studied in HS-5 cells. Cells were labelled overnight with 3H-AA and the release of 3H-AA was measured in culture medium following 20 minutes stimulation with Ang II, alone or in combination with the AT1- or AT2-receptor antagonists, losartan and PD 123319, respectively. MCSF secretion into culture medium was measured using an enzyme immunoassay following 24 hours of treatment with Ang II alone or in combination with losartan or PD 123319. Phorbol-myristate-acetate, known to stimulate release of AA and MCSF, was used as a positive control in both experiments.

RESULTS

In response to Ang II, release of 3H-AA from monkey and human MSC was increased (p<0.05) to 147+/-4% and 124+/-3% of control, respectively. The AT1- and AT2-receptor antagonists, losartan and PD 123319, individually reduced Ang II-stimulated 3H-AA release. In contrast, Ang II had no effect on secretion of MCSF from HS-5 cells.

CONCLUSIONS

These results provide mechanistic evidence for Ang II-mediated haematopoiesis through AA release that may, in part, explain Ang II-facilitated recovery of haematopoiesis in experimental myelosuppression and the anaemias associated with Ang II receptor blockade.

Effects of adrenomedullin on cell proliferation in rat adventitia induced by lysophosphatidic acid

Lysophosphatidic acid (LPA) is a bioactive phospholipid having growth factor-like activity on fibroblasts and is involved in cardiovascular diseases such as hypertension and heart failure by inducing vascular remodeling, characterized by fibroblast proliferation and migration in adventitia. Among various bioactive factors that LPA works with, adrenomedullin (ADM) is a multiple functional peptide with an important cytoprotective effect against cardiovascular damage. We studied rat aortic adventitia to explore the possible paracrine/autocrine interaction between endogenous ADM and LPA. LPA stimulation of the adventitia to secrete ADM and express its mRNA was concentration dependent. ADM inhibited LPA-induced proliferation in adventitial cells and attenuated the activity of mitogen-activated protein kinase (MAPK) stimulated by LPA. In contrast, treatment with specific antagonists of the ADM receptor potentiated the LPA-induced proliferation in adventitial cells. We concluded that LPA stimulates the adventitia to produce and secrete ADM, which in turn regulates the vascular biological effects of LPA.

Renal expression of angiotensin type 2 (AT2) receptors during kidney damage

BACKGROUND

Activation of the renin angiotensin system has been described in pathologic conditions, including kidney damage. Angiotensin II (Ang II) acts through two receptors, AT1 and AT2. Most of the known actions of Ang II, including vasoconstriction and fibrosis, are due to AT1 activation. Recent data suggest that AT2 participates in the regulation of cell growth and renal inflammatory infiltration. Therefore, we investigated the renal expression of AT2 receptors in several models of renal injury.

METHODS

Investigations were done in the following experimental models of kidney damage: systemic infusion of Ang II (inflammation), folic acid nephropathy (tubular cell death), and protein overload proteinuria. AT2 expression was determined by immunohistochemistry (protein) and reverse transcription-polymerase chain reaction (RT-PCR) (gene).

RESULTS

In control animals, low levels of renal expression of AT2 were found. Ang II infusion resulted in an up-regulation of AT2 in tubular cells and de novo AT2 expression in glomeruli and vessels, associated with the presence of inflammatory cells. Acute tubular injury induced by folic acid was characterized by AT2 overexpression and apoptosis in tubular cells. Protein overload caused heavy proteinuria and tubular AT2 up-regulation.

CONCLUSION

AT2 is re-expressed in pathologic conditions of kidney damage, such as inflammation, apoptosis, and proteinuria, suggesting a potential role of this receptor during renal injury.

Angiotensin II AT2 receptor subtype: an uprising frontier in cardiovascular disease?

The renin-angiotensin system (RAS) plays a pivotal role in the regulation of fluid, electrolyte balance and blood pressure, and is a modulator of cellular growth and proliferation. Biological actions of RAS are linked to the binding of the effector molecule, angiotensin II (AngII), to specific membrane receptors, mostly the AT1 subtype and, to a lesser extent, other subtypes. Following the identification and characterization of the AT2 subtype receptor, it has been proposed that a complex interaction between AngII and its receptors may play an important role in the effects of RAS. In this paper current information on AngII subtype receptors--their structure, regulation and intracellular signalling--are reviewed, with a particular emphasis on the potential relevance for cardiovascular pathophysiology. In addition, we discuss modulation of expression of the AT2 receptor and its interaction with the AT1 receptor subtype, as well as the potential effects of this receptor on blood pressure regulation. A better understanding of the integrated effects of the AngII subtype receptors may help to elucidate the function of the RAS, as well as their participation in the mechanisms of cardiovascular disease and attendant therapeutic implications.

Newly recognized components of the renin-angiotensin system: potential roles in cardiovascular and renal regulation

The renin-angiotensin system (RAS) is a coordinated hormonal cascade in the control of cardiovascular, renal, and adrenal function that governs body fluid and electrolyte balance, as well as arterial pressure. The classical RAS consists of a circulating endocrine system in which the principal effector hormone is angiotensin (ANG) II. ANG is produced by the action of renin on angiotensinogen to form ANG I and its subsequent conversion to the biologically active octapeptide by ANG-converting enzyme. ANG II actions are mediated via the ANG type 1 receptor. Here, we discuss recent advances in our understanding of the components and actions of the RAS, including local tissue RASs, a renin receptor, ANG-converting enzyme-2, ANG (1-7), the function of the ANG type 2 receptor, and ANG receptor heterodimerization. The role of the RAS in the regulation of cardiovascular and renal function is reviewed and discussed in light of these newly recognized components.

Cytokines down-regulate alpha1-adrenergic receptor expression during endotoxemia

OBJECTIVE

The reduced pressure response to norepinephrine in septic patients has directed our interest to the regulation of alpha1-adrenergic receptors in vitro and in vivo during conditions mimicking acute sepsis.

DESIGN

Prospective animal trial followed by a controlled cell culture study.

SETTING

Laboratory of the Department of Anesthesiology.

SUBJECTS

Male Sprague-Dawley rats weighing 200 to 250 g and a mesangial cell line.

INTERVENTIONS

Experimental endotoxemia was induced in rats with lipopolysaccharide, and blood pressure dose-response studies with norepinephrine were performed. Alpha1-receptor gene expression was determined in various organs by a specific RNase protection assay, and tissue concentrations of the proinflammatory cytokines interleukin-1beta and tumor necrosis factor-alpha were measured. Rat renal mesangial cells were incubated with these cytokines or with nitric oxide donors to investigate the regulation of alpha1-adrenergic receptors during severe inflammation on a cellular level.

MEASUREMENTS AND MAIN RESULTS

The pressor effect of norepinephrine was markedly diminished during endotoxemia. The animals showed down-regulated mRNA levels of alpha1A-, alpha1B- and alpha1D-receptors in all organs investigated, and the tissue concentrations of interleukin-1beta and tumor necrosis factor-alpha were highly increased during experimental endotoxemia. Incubation of cultured rat renal mesangial cells with the cytokines resulted in diminished alpha -receptor gene expression and [3H]prazosin binding capacity, whereas incubation of the cells with nitric oxide donors did not affect alpha1B-receptor expression. In line, blocking of cytokine-induced nitric oxide synthesis by coincubation of mesangial cells with N(G)-nitro-L-arginine methyl ester did not influence cytokine-induced down-regulation of alpha1B-receptors.

CONCLUSIONS

Our data show that endotoxemia causes a systemic down-regulation of alpha1-receptors on the level of gene expression and suggest that this effect is likely mediated by proinflammatory cytokines in a synergistic but nitric oxide-independent fashion. We propose that this down-regulation of alpha1-adrenergic receptors contributes to the attenuated blood pressure response to norepinephrine and, therefore, to septic circulatory failure in patients.

Cytokine-mediated downregulation of vasopressin V(1A) receptors during acute endotoxemia in rats

The reduced pressure response to vasopressin during acute sepsis has directed our interest to the regulation of vasopressin V(1A) receptors. Rats were injected with lipopolysaccharide for induction of experimental gram-negative sepsis. V(1A) receptor gene expression was downregulated in the liver, lung, kidney, and heart during endotoxemia. Inasmuch as the concentrations of proinflammatory cytokines such as interleukin-1beta, tumor necrosis factor-alpha, and interferon-gamma were highly increased during sepsis, the influence of these cytokines on V(1A) receptor expression was investigated in primary cultures of hepatocytes and in the aortic vascular smooth muscle cell line A7r5. V(1A) receptor expression was downregulated by the cytokines in a nitric oxide-independent manner. Blood pressure dose-response studies after injection of endotoxin showed a diminished responsiveness to the selective V(1) receptor agonist Phe(2),Ile(3),Orn(8)-vasopressin. Our data show that sepsis causes a downregulation of V(1A) receptors and suggest that this effect is likely mediated by proinflammatory cytokines. We propose that this downregulation of V(1A) receptors contributes to the attenuated responsiveness of blood pressure in response to vasopressin and, therefore, contributes to the circulatory failure in septic shock.

Angiotensin receptors: signaling, vascular pathophysiology, and interactions with ceramide

Angiotensin II (ANG II) is a pleiotropic vasoactive peptide that binds to two distinct receptors: the ANG II type 1 (AT(1)) and type 2 (AT(2)) receptors. Activation of the renin-angiotensin system (RAS) results in vascular hypertrophy, vasoconstriction, salt and water retention, and hypertension. These effects are mediated predominantly by AT(1) receptors. Paradoxically, other ANG II-mediated effects, including cell death, vasodilation, and natriuresis, are mediated by AT(2) receptor activation. Our understanding of ANG II signaling mechanisms remains incomplete. AT(1) receptor activation triggers a variety of intracellular systems, including tyrosine kinase-induced protein phosphorylation, production of arachidonic acid metabolites, alteration of reactive oxidant species activities, and fluxes in intracellular Ca(2+) concentrations. AT(2) receptor activation leads to stimulation of bradykinin, nitric oxide production, and prostaglandin metabolism, which are, in large part, opposite to the effects of the AT(1) receptor. The signaling pathways of ANG II receptor activation are a focus of intense investigative effort. We critically appraise the literature on the signaling mechanisms whereby AT(1) and AT(2) receptors elicit their respective actions. We also consider the recently reported interaction between ANG II and ceramide, a lipid second messenger that mediates cytokine receptor activation. Finally, we discuss the potential physiological cross talk that may be operative between the angiotensin receptor subtypes in relation to health and cardiovascular disease. This may be clinically relevant, inasmuch as inhibitors of the RAS are increasingly used in treatment of hypertension and coronary heart disease, where activation of the RAS is recognized.

Nitric oxide-dependent down-regulation of angiotensin II type 2 receptors during experimental sepsis

OBJECTIVE

The systemic renin-angiotensin system is highly activated during septic shock. This has focused interest in regulation of the adrenal angiotensin II type 2 receptor (AT2) as the target thought to mediate angiotensin II-induced adrenal catecholamine release during experimental sepsis in vivo. In addition, the influence of typical endogenous mediators of sepsis, such as proinflammatory cytokines and nitric oxide, on AT2 receptor expression should be investigated in vitro.

DESIGN

Prospective animal trial followed by a controlled cell culture study.

SETTING

Laboratory of the Department of Anesthesiology.

SUBJECTS

Male Sprague-Dawley rats weighing 200-250 g, PC12 cell line.

INTERVENTIONS

Rats were injected with lipopolysaccharide to stimulate Gram-negative sepsis or lipoteichoic acid to stimulate Gram-positive sepsis. AT2 receptor expression, abundance of the proinflammatory cytokines (interleukin-1beta, tumor necrosis factor-alpha, and interferon-gamma), and nitric oxide synthase II expression have been determined in the adrenal gland. Rat adrenal pheochromocytoma cells were incubated with these cytokines or with the nitric oxide donors sodium nitroprusside or S-nitroso-N-acetylpenicillamine to investigate the regulation of AT2 receptors during severe inflammation on a cellular level.

MEASUREMENTS AND MAIN RESULTS

In the adrenal gland, AT2 receptor expression was down-regulated in both models of sepsis, whereas tissue cytokine concentrations were elevated and nitric oxide synthase II expression was induced. Incubation of PC12 cells with proinflammatory cytokines resulted in a dose-dependent diminished expression of AT2 receptors, which was mimicked by incubation with nitric oxide donors. Blocking of cytokine-induced nitric oxide synthesis by co-incubation of PC12 cells with NG-nitro-l-arginine methyl ester prevented down-regulation of AT2 receptors.

CONCLUSIONS

These findings show that in our model of sepsis, the expression of AT2 receptors in the adrenal gland is down-regulated in a nitric oxide-dependent manner. Because AT2 receptors are thought to be involved in adrenal catecholamine secretion in a stimulatory fashion, the diminished expression of AT2 receptors could play an important role in the pathogenesis of septic shock via impaired angiotensin II-induced adrenal catecholamine release, despite a strong activation of the systemic renin-angiotensin system.

Fos protein is required for the re-expression of angiotensin II type 1 receptors in the nucleus tractus solitarii after baroreceptor activation in the rat

We evaluated in Sprague--Dawley rats the hypothesis that Fos protein induced by baroreceptor activation in the nucleus tractus solitarii participates in transcriptional regulation of the expression of angiotensin receptor genes. Reverse transcription-polymerase chain reaction revealed that baroreceptor activation elicited by sustained hypertension resulted in a transient decrease in angiotensin II subtype 1, but not subtype 2, receptor messenger RNA, in the dorsomedial medulla, including the nucleus tractus solitarii. There was subsequently a transitory reduction in the pressor response elicited by microinjection bilaterally of angiotensin II (40 pmol) into the nucleus tractus solitarii, followed by an increase in c-fos messenger RNA and Fos immunoreactivity at the same nucleus. Both the re-expression of angiotensin II subtype 1 receptor messenger RNA and restoration of pressor response to angiotensin II after baroreceptor activation were significantly blunted by bilateral application into the nucleus tractus solitarii of an antisense oligonucleotide (50 pmol) that targets against the initiation codon of c-fos messenger RNA. Control pretreatment with the corresponding sense oligonucleotide (50 pmol), or an antisense c-fos oligonucleotide that targets against a different portion of the coding sequence of the c-fos messenger RNA (50 pmol), was ineffective. At the receptor level, the angiotensin II-induced pressor response was antagonized by the subtype 1 receptor antagonist losartan (1.6 nmol), but not by the subtype 2 receptor antagonist PD-123319 (1.6 nmol). These findings suggest that sustained hypertension down-regulates angiotensin II subtype 1 receptors at both messenger RNA and functional expression levels in the nucleus tractus solitarii. Furthermore, Fos protein induced in the nucleus tractus solitarii by baroreceptor activation may play a permissive role in the transcriptional regulation of the re-expression of this subtype of angiotensin receptors.

Differential role of angiotensin II receptor subtypes on endothelial superoxide formation

The physiological role of the angiotensin II AT2 receptor subtype is not fully characterized. We studied whether AT2 receptor could antagonize AT1 mediated superoxide formation in endothelial cells. In quiescent human umbilical vein endothelial cells (HUVEC) superoxide formation was measured after long-term incubation (6 h) with angiotensin II in the presence or absence of its receptor blocker candesartan (AT1) or PD123319 (AT2) using the cytochrome c assay. In separate experiments, the effects of AT2 mediated effects on activities of cellular phosphates including the src homology 2 domain containing phosphatases (SHP-1) was studied. The basal superoxide formation (0.19+/-0.03 nmol superoxide mg protein(-1) min(-1)) in HUVEC was increased by 37.1% after exposure to angiotensin II (100 nM,) which was due to an activation of a NAD(P)H oxidase. This was abolished by candesartan (1 microM) as well as the tyrosine kinase inhibitor genistein. In contrast, blockade of AT2 receptors by PD123319 enhanced the superoxide formation by 73.7% in intact cells. Stimulation of AT2 went along with an increased activity of tyrosine phosphatases in total cell lysates (29.8%) and, in particular, a marked stimulation of src homology 2 domain containing phosphatases (SHP-1, by 293.4%). The tyrosine phosphatase inhibitor vanadate, in turn, prevented the AT2 mediated effects on superoxide formation. The expression of both angiotensin II receptor subtypes AT1 and AT2 was confirmed by RT - PCR analysis. It is concluded that AT2 functionally antagonizes the AT1 induced endothelial superoxide formation by a pathway involving tyrosine phosphatases.

The angiotensin II type 2 receptor: an enigma with multiple variations

Since it was discovered ten years ago, the angiotensin II (ANG II) type 2 (AT(2)) receptor has been an enigma. This receptor binds ANG II with a high affinity but is not responsible for mediating any of the classical physiological actions of this peptide, all of which involve the ANG II type 1 (AT(1)) receptor. Furthermore, the AT(2) receptor exhibits dramatic differences in biochemical and functional properties and in patterns of expression compared with the AT(1) receptor. During the past decade, much information has been gathered about the AT(2) receptor, and the steadily increasing number of publications indicates a growing interest in this new and independent area of research. A number of studies suggest a role of AT(2) receptors in brain, renal, and cardiovascular functions and in the processes of apoptosis and tissue regeneration. Despite these advances, nothing stands out as the major singular function of these receptors. The study of AT(2) receptors has reached a crossroads, and innovative approaches must be considered so that unifying mechanisms as to the function of these unique receptors can be put forward. In this review we will discuss the advances that have been made in understanding the biology of the AT(2) receptor. Furthermore, we will consider how these discoveries, along with newer experimental approaches, may eventually lead to the elusive physiological and pathophysiological functions of these receptors.

Expression and localization of angiotensin subtype receptor proteins in the hypertensive rat heart

The cellular localization of the AT(2) receptor and the regulation of its expression in hypertrophied left ventricle are not well known. We compared the expression of the cardiac AT(1) and AT(2) receptor in spontaneously hypertensive rats/Izumo strain (SHR/Izm) and Wistar Kyoto rats/Izumo strain (WKY/Izm), ages 4, 12, and 20 wk, by means of immunohistochemistry and Western blot analysis. In SHR/Izm, compared with WKY/Izm, blood pressure (161 +/- 2 vs. 120 +/- 2 mmHg at 12 wk, P </= 0.01, and 199 +/- 3 vs. 123 +/- 3 mmHg at 20 wk, P </= 0.01) and heart-to-body weight ratio (3.76 +/- 0.07 vs. 3.06 +/- 0.06 mg/g at 12 wk, P </= 0.01, and 3.90 +/- 0.08 vs. 3.01 +/- 0.12 mg/g at 20 wk, P </= 0.01) were significantly elevated. There was no difference in these values between the two strains at 4 wk of age. Histologically, 20-wk-old SHR/Izm demonstrated myocardial hypertrophy, a thickening of the smooth muscle layer of the intracardiac arteries, and perivascular fibrosis. By immunohistochemistry, the AT(2) receptor was localized to cardiomyocytes and vascular endothelial cells, but not in the vascular smooth muscle cells. No major AT(2) receptor signal was observed in perivascular fibrosis at any age in either strain of rats. No difference was detected in this localization between the two strains. By Western blotting, a single 44-kDa band for the AT(2) receptor and a single 60-kDa band for the AT(1) receptor were detected in ventricles from both strains of rats at all ages. Densitometric analysis demonstrated that the AT(2) receptor 44-kDa band was decreased by 20% at 12 wk and 32% at 20 wk (P < 0.01) in SHR/Izm compared with WKY/Izm. The intensity of the AT(1) receptor 60-kDa band was increased by 57% in 20-wk-old SHR/Izm compared with WKY/Izm (P < 0.05). There was no significant difference in the intensity of the 44- or 60-kDa bands in 4-wk-old animals of either strain. We demonstrated a decrease in the AT(2) receptor and an increase in the AT(1) receptor protein with no change in their localizations in hypertrophied left ventricular myocytes of SHR/Izm.

Lipopolysaccharides and cytokines downregulate the angiotensin II type 2 receptor in rat cardiac fibroblasts

The present study examines the effect of lipopolysaccharides and proinflammatory cytokines on the expression of the second isoform of the angiotensin II receptor (AT(2)), which may have a role in lowering collagen deposition in cardiac tissue. Cardiac fibroblasts express high levels of both angiotensin II type 1 (AT(1)) and type 2 receptors. Incubation with lipopolysaccharides for 24 h dose- and time-dependently decreased angiotensin II AT(2) receptor expression with no apparent difference in the affinity. Actinomycin D, cycloheximide, N(omega)-nitro-L-arginine methyl ester and the protein tyrosine kinase inhibitor herbimycin A, but not the protein kinase C inhibitors bisindolylmaleimide and calphostin C, abolished the inhibitory action of lipopolysaccharides. The cytokines interleukin-1beta and tissue necrosis factor-alpha mimicked the effect of lipopolysaccharides. All three compounds induced inducible nitric oxide synthase (iNOS). The nitric oxide donor sodium nitroprusside and the cGMP analog 8-bromoguanosine cyclic monophosphate downregulated angiotensin II AT (2) receptor expression. The findings are consistent with the pathway in which lipopolysaccharides or cytokines induce iNOS. The data suggest that lipopolysaccharide- or cytokine-dependent induction of iNOS and resultant production of nitric oxide leads to the production of cGMP, which in turn downregulates expression of the angiotensin II AT (2) receptor in cardiac fibroblasts.

Renal renin-angiotensin system dysregulation caused by partial bladder outlet obstruction in fetal sheep

BACKGROUND

To determine whether fetal renal obstruction activates the renal renin-angiotensin system (RAS), an important mediator in normal kidney development and obstructive nephropathy, we used a model of fetal partial bladder outlet obstruction (PBOO).

METHODS

Total RNA and protein was extracted from kidney of sheep fetuses with partial bladder outlet obstruction created at 95 days gestation, after 2 (N = 6) and 5 weeks of obstruction (term; N = 6), and from normal fetal sheep at various time points between 60 and 135 days of gestation (total N = 19). Relative levels of mRNA for renin, angiotensinogen, type 1 and 2 angiotensin II (Ang II) receptors (AT-1 and AT-2), and transforming growth factor-beta1 (TGF-beta1) were assessed by semiquantitative reverse transcription-polymerase chain reaction. Expression levels of AT-2 receptor protein were measured by Western blot analysis.

RESULTS

Renin mRNA expression was increased (250%) after two weeks of obstruction. In normal fetuses, AT-1 expression was low at 60 to 75 days of gestation and increased toward the end of gestation, whereas AT-2 expression showed a reversed pattern. At 109 days, PBOO caused an increased expression of AT-2 mRNA compared with normals (400%). Correspondingly, AT-2 receptor protein was more abundant in obstructed kidneys. TGF-beta1 mRNA expression was significantly increased in obstructed kidneys at 109 days gestation.

CONCLUSIONS

These observations confirm the reciprocal developmental regulation of AT-1 and AT-2 receptors' expression, suggesting their functional role in renal development. Partial bladder outlet obstruction produces specific alterations: increased renin expression and altered balance of receptor subtypes, which may induce altered functional and vascular regulation of the obstructed fetal kidney. TGF-beta1, a mediator of Ang II-induced fibrosis, may play a role in inducing and propagating interstitial fibrosis.

Characterization of AT2 receptor expression in NIH 3T3 fibroblasts

1. A high expression of angiotensin II receptors and of angiotensin-converting enzyme (ACE) activity was detected in confluent NIH 3T3 fibroblasts. 2. Characterization with selective ligands, dithiothreitol, and GTP gamma S, indicated that only the AT2 subtype was expressed. 3. AT2 receptors and ACE expression were strictly dependent on the cell density and growth phase of the cells, with AT2 receptors being expressed earlier than ACE. In contrast, high expression of AT2 receptors irrespective of their growth state was observed in NIH 3T3 cells lacking contact inhibition upon neoplastic transformation with ras. 4. Our results imply a possible relation of AT2 receptors to cell growth and cell-cell contact.

The increase in angiotensin type-2 receptor mRNA level by glutamate stimulation in cultured rat cortical cells

The changes in the angiotensin type-2 (AT2) receptor mRNA level during glutamate neurotoxicity in cultured rat cortical cells are examined to assess the possible involvement of AT2 receptor in cell injury. The day 10-14 cortical neurons were exposed to glutamate at a toxic concentration of 100 microM for 15 min. The viability of the culture was reduced by 60% after 24 h. AT2 receptor mRNA was then increased 2-fold after exposure to glutamate, while the maximum increase was observed in a dose-dependent manner (50-1000 microM) 3 h after glutamate stimulation. AT2 receptor binding also increased 3-12 h after glutamate exposure. The results suggest that the increase in the AT2 receptor preceded to some extent the insult of the cell after exposure. The increase in the mRNA level was suppressed by MK-801, N-methyl-D-aspartate (NMDA) receptor antagonist, thus indicating the possible involvement of NMDA receptor. The increase in the mRNA level was also antagonized by N-nitro L-arginine methyl-ester, a nitric oxide synthase inhibitor. The hemoglobin, a nitric oxide trap, inhibited the increase in the mRNA level. These results suggest that the increase in the mRNA level is associated with the nitric oxide synthesis by glutamate exposure. The viability of cortical cells after glutamate stimulation was partially restored by the AT2 receptor antagonist and by the antisense oligonucleotide for the AT2 receptor. The present results thus suggest that the AT2 receptor may in some way be related to one of the processes in cell injury caused by glutamate.

Regulation of cell proliferation and angiotensin II type 2 receptors in R3T3 cells

The effects of insulin-like growth factor 1 (IGF-1), basic fibroblast growth factor (bFGF), transforming growth factor beta1 (TGFbeta1), fetal calf serum (FCS) and angiotensin II (AngII) on cell proliferation, (3H-thymidine incorporation and cell number) and AT2 receptor number and mRNA levels in R3T3 cells have been studied. All growth factors as well as FCS markedly increased cell proliferation, whereas AngII increased slightly 3H-thymidine incorporation, but not cell number. TGFbeta1, bFGF and FCS reduced by more than 80% both AT2 receptor number and mRNA, by inhibiting the transcription rate. In contrast, IGF-1 and AngII increased about 4-fold AT2 receptor number, but only IGF-1 increased AT2 mRNA. When added together the effects of IGF-1 and AngII were more than additive on AT2 receptor number, but not on mRNA level. None of the factors studied modified AT2 mRNA half-life. In conclusion, the present results demonstrated that: 1/ cell proliferation is not correlated with AT2 expression; 2/ growth factors regulate, positively or negatively, AT2 transcription rate, whereas AngII regulates the translation rate of AT2 mRNA; 3/ all the effects of AngII on R3T3 are mediated by AT2 receptors since they are mimicked by the AT2 agonist CGP42112 and blocked by the AT2 antagonist PD123177.

Na+,K+-ATPase inhibitors in rat urine: origins and physiological significance

To identify the origins and structures of mammalian tissue-derived Na+,K+-ATPase inhibitors, we investigated the tissue distribution of inhibitors in rats. Among many tissues tested, urine was found to contain high levels of many inhibitors. The structures of the two major inhibitors were identified as neoconvalloside and periplogenin monorhamnoside, which are derivatives of strophanthidin. Urinary levels of these inhibitors, however, decreased considerably after changing the diet from the regular diet to purified synthetic diet, suggesting that the majority of the urinary inhibitors are of dietary origin. Investigation of the ingredients of the diet further revealed that alfalfa meal and ground oats are the major sources of these cardiac glycosides. As to the physiological relevance of the cardiac glycosides, a low concentration (1-50 nM) of ouabain dose-dependently enhanced aldosterone secretion from adrenal glomerulosa cells by an increase in local renin release. Ouabain was also found to be involved in AT2 receptor-specific expression in rat PC12W cells through an increment in intracellular Na+. These results suggest that Na+,K+-ATPase inhibitors, regardless of the source, are involved in the regulation of blood pressure.

Effects of growth factors on cell proliferation and angiotensin II type 2 receptor number and mRNA in PC12W and R3T3 cells

Previous studies have suggested that the expression of angiotensin type 2 receptor was inversely related to cell proliferation. We examined the effects of insulin-like growth factor (IGF-1), basic fibroblast growth factor (bFGF), transforming growth factor beta1 (TGFbeta1) and fetal calf serum (FCS) on cell proliferation and AT2 binding sites and mRNA level in PC12W (rat pheochromocytoma cell line) and R3T3 (mouse fibroblast cell line) which express abundant AT2 receptors. In both cell lines, serum deprivation markedly increased both AT2 receptor number and mRNA. However, in the absence of serum cell proliferation continued in PC12W and R3T3 at late passages (R3T3 LP) but not at early passages (R3T3 EP). In PC12W, none of the three growth factors studied stimulated cell proliferation, but TGFbeta1 and more particularly bFGF markedly reduced AT2 expression. In R3T3 LP, IGF-1 and bFGF, but not TGFbeta1, slightly stimulated cell proliferation, but the three factors, specially bFGF, reduced AT2 expression. In contrast, in R3T3 EP, the three growth factors significantly increased cell proliferation, but whereas TGFbeta1 and bFGF markedly reduced AT2 binding sites and mRNA, IGF-1 caused the opposite effects. These results indicate that regulation of AT2 expression is not correlated with cell proliferation and appears to be more complex than initially suspected. In addition, they show that the same factor can have an opposite effect on AT2 expression in the same cell line depending upon the cell passage.

Angiotensin AT2 receptor degradation is prevented by ligand occupation

A substantial increase in [125I]Sar1, Ile-Angiotensin II binding activity can be observed 24 hours after treatment of R3T3 cells with AT2 receptor agonists and antagonists. An increase in the radioligand binding activity, although less profound, can also be observed 6 hours after AT2 receptor ligand treatment, on fetal human kidney cells expressing a recombinant human AT2 receptor. However, the increase in radioligand binding activity cannot be detected unless the ligands are removed from the cell surface by an acid-glycine (pH 3) wash, just prior to the binding assay. Interestingly, an acid-glycine wash 24 hours prior to the binding assay causes a dramatic decrease in the radioligand binding activity on untreated R3T3 cells. This decrease, which was prevented by angiotensin II treatment, suggests the existence of an unknown endogenous factor which, like the AT2 receptor ligands, seems to prevent AT2 receptor degradation.

Modulation of angiotensin II type 2 receptor mRNA in rat hypothalamus and brainstem neuronal cultures by growth factors

This study investigates the regulatory effects of growth factors upon angiotensin II type 2 (AT2) mRNA levels in neurons co-cultured from newborn rat hypothalamus and brainstem. Incubation of cultured neurons with nerve growth factor (NGF; 5-50 ng/ml) caused time-dependent changes in the steady-state levels of AT2 receptor mRNA. Short-term (0.5-1.0 h) incubations with NGF resulted in significant increases in AT2 receptor mRNA, whereas longer-term incubations (4-24 h) caused significant decreases. Activation of NGF receptors is known to stimulate phospholipase C-gamma and subsequently activate protein kinase C (PKC). Incubation of cultures with the PKC activator, phorbol-12-myristate-13-acetate (PMA; 100 nM), caused temporal changes in AT2 receptor mRNA levels similar to those observed with NGF. By contrast, insulin (0.1-10 microg/ml) elicited only significant decreases in AT2 receptor mRNA levels. The observed abilities of NGF and insulin to regulate the expression of AT2 receptor mRNA are consistent with the fact that the AT2 receptor gene promoter region contains several cis DNA regulatory elements that respond to growth factor-stimulated transcription factors. These novel observations which show that NGF and insulin can regulate AT2 receptor mRNA in neurons derived from neonatal rat CNS lend support to the idea that AT2 receptors have a role in development and differentiation.

Transient upregulation of the AT2 receptor mRNA level after global ischemia in the rat brain

The present study examined changes in angiotensin receptors (AT1 and AT2) and angiotensinogen mRNA level after global ischemia in the rat brain. The AT2 mRNA level increased by three-fold in both the cortex and hippocampus, which are known to be sensitive to ischemic injury, 3 h after ischemia. The increase thus appeared only during the early reperfusion period. In the striatum, amygdala and cerebellum, the level increased moderately 3 h and/or 24 h after ischemia; there was no change in the hypothalamus. On the other hand, the AT1A and AT1B receptor mRNA levels were not altered in the cortex or hippocampus during the early reperfusion period, even 3 h and 24 h after ischemia. There was no significant alteration in angiotensinogen mRNA level 3 h or 24 h after ischemia. These results suggest that the transient upregulation of AT2 receptor mRNA occurs in the cortex and hippocampus after injury and these changes may be in some way related to the molecular events which lead to delayed neuronal cell death.