Contribution of bradykinin and nitric oxide to AT2 receptor-mediated differentiation in PC12 W cells

The AT1/AT2 Receptor Equilibrium Is a Cornerstone of the Regulation of the Renin Angiotensin System beyond the Cardiovascular System

The AT₁ receptor has mainly been associated with the pathological effects of the renin-angiotensin system (RAS) (e.g., hypertension, heart and kidney diseases), and constitutes a major therapeutic target. In contrast, the AT₂ receptor is presented as the protective arm of this RAS, and its targeting via specific agonists is mainly used to counteract the effects of the AT₁ receptor. The discovery of a local RAS has highlighted the importance of the balance between AT₁/AT₂ receptors at the tissue level. Disruption of this balance is suggested to be detrimental. The fine tuning of this balance is not limited to the regulation of the level of expression of these two receptors. Other mechanisms still largely unexplored, such as S-nitrosation of the AT₁ receptor, homo- and heterodimerization, and the use of AT₁ receptor-biased agonists, may significantly contribute to and/or interfere with the settings of this AT₁/AT₂ equilibrium. This review will detail, through several examples (the brain, wound healing, and the cellular cycle), the importance of the functional balance between AT₁ and AT₂ receptors, and how new molecular pharmacological approaches may act on its regulation to open up new therapeutic perspectives.

Update on Angiotensin II Subtype 2 Receptor: Focus on Peptide and Nonpeptide Agonists

Angiotensin II (Ang II) is the most dominant effector component of the renin-angiotensin system (RAS) that generally acts through binding to two main classes of G protein-coupled receptors, namely Ang II subtype 1 receptor (AT1R) and angiotensin II subtype 2 receptor (AT2R). Despite some controversial reports, the activation of AT2R generally antagonizes the effects of Ang II binding on AT1R. Studying AT2R signaling, function, and its specific ligands in cell culture or animal studies has confirmed its beneficial effects throughout the body. These characteristics classify AT2R as part of the protective arm of the RAS that, along with functions of Ang (1-7) through Mas receptor signaling, modulates the harmful effects of Ang II on AT1R in the activated classic arm of the RAS. Although Ang II is the primary ligand for AT2R, we have summarized other natural or synthetic peptide and nonpeptide agonists with critical evaluation of their structure, mechanism of action, and biologic activity. SIGNIFICANCE STATEMENT: AT2R is one of the main components of the RAS and has a significant prospective for mediating the beneficial action of the RAS through its protective arm on the body's homeostasis. Targeting AT2R offers substantial clinical application possibilities for modulating various pathological conditions. This review provided concise information regarding the AT2R peptide and nonpeptide agonists and their potential clinical applications for various diseases.

The counter regulatory axis of the renin angiotensin system in the brain and ischaemic stroke: Insight from preclinical stroke studies and therapeutic potential

Stroke is the 2nd leading cause of death worldwide and the leading cause of physical disability and cognitive issues. Although we have made progress in certain aspects of stroke treatment, the consequences remain substantial and new treatments are needed. Hypertension has long been recognised as a major risk factor for stroke, both haemorrhagic and ischaemic. The renin angiotensin system (RAS) plays a key role in blood pressure regulation and this, plus local expression and signalling of RAS in the brain, both support the potential for targeting this axis therapeutically in the setting of stroke. While historically, focus has been on suppressing classical RAS signalling through the angiotensin type 1 receptor (AT1R), the identification of a counter-regulatory axis of the RAS signalling via the angiotensin type 2 receptor (AT2R) and Mas receptor has renewed interest in targeting the RAS. This review describes RAS signalling in the brain and the potential of targeting the Mas receptor and AT2R in preclinical models of ischaemic stroke. The animal and experimental models, and the route and timing of intervention, are considered from a translational perspective.

Angiotensin II type II receptors and colonic dysmotility in 2,4-dinitrofluorobenzenesulfonic acid-induced colitis in rats

BACKGROUND

Angiotensin II (Ang II), the main peptide of the renin-angiotensin system (RAS), has been suggested to be involved in inflammatory bowel diseases. Since RAS has emerged as gut motility regulator, and dysmotility is associated with intestinal inflammation, our objective was to investigate in rat 2,4-dinitrobenzenesulfonic acid (DNBS)-induced colitis the functionality of RAS and its contribution to colonic motor alterations.

METHODS

The effects of Ang II on the longitudinal colonic muscular contractility of control and DNBS-treated rats were characterized in vitro. Transcripts encoding for Ang II receptors were investigated by RT-PCR.

KEY RESULTS

Inflamed preparations showed a longitudinal muscle marked hypocontractility. Angiotensin II caused contractile effects in both preparations, but the responses in DNBS preparations were reduced compared to controls. In both preparations, Losartan, AT₁ receptor antagonist, reduced Ang II effects. PD123319, AT₂ receptor antagonist, enhanced Ang II responses only in DNBS rats, as well as N^ω -Nitro-L-arginine (L-NNA), nitric oxide (NO) synthase inhibitor, or tetrodotoxin (TTX), neural toxin. The co-administration of PD123319 and TTX or L-NNA produced no additive effects. PD123319 per se improved colonic contractility in inflamed tissues. The effect was reduced in the presence of L-NNA or TTX. All Ang II receptor subtypes were expressed in both preparations.

CONCLUSIONS & INFERENCES

AT₁ receptors mediate Ang II contractile responses in rat colon. During inflammation a recruitment of Ang II AT₂ receptors would counteract AT₁ -contractile activity. A tonic activation of AT₂ receptors would contribute to the general reduction in muscle contractility during experimental inflammation. A role for enteric neurons and NO is also suggested.

Selective activation of angiotensin AT2 receptors attenuates progression of pulmonary hypertension and inhibits cardiopulmonary fibrosis

BACKGROUND AND PURPOSE

Pulmonary hypertension (PH) is a devastating disease characterized by increased pulmonary arterial pressure, which progressively leads to right-heart failure and death. A dys-regulated renin angiotensin system (RAS) has been implicated in the development and progression of PH. However, the role of the angiotensin AT2 receptor in PH has not been fully elucidated. We have taken advantage of a recently identified non-peptide AT2 receptor agonist, Compound 21 (C21), to investigate its effects on the well-established monocrotaline (MCT) rat model of PH.

EXPERIMENTAL APPROACH

A single s.c. injection of MCT (50 mg·kg(-1) ) was used to induce PH in 8-week-old male Sprague Dawley rats. After 2 weeks of MCT administration, a subset of animals began receiving either 0.03 mg·kg(-1) C21, 3 mg·kg(-1) PD-123319 or 0.5 mg·kg(-1) A779 for an additional 2 weeks, after which right ventricular haemodynamic parameters were measured and tissues were collected for gene expression and histological analyses.

KEY RESULTS

Initiation of C21 treatment significantly attenuated much of the pathophysiology associated with MCT-induced PH. Most notably, C21 reversed pulmonary fibrosis and prevented right ventricular fibrosis. These beneficial effects were associated with improvement in right heart function, decreased pulmonary vessel wall thickness, reduced pro-inflammatory cytokines and favourable modulation of the lung RAS. Conversely, co-administration of the AT2 receptor antagonist, PD-123319, or the Mas antagonist, A779, abolished the protective actions of C21.

CONCLUSIONS AND IMPLICATIONS

Taken together, our results suggest that the AT2 receptor agonist, C21, may hold promise for patients with PH.

Angiotensin II type 2 receptor regulates ROMK-like K⁺ channel activity in the renal cortical collecting duct during high dietary K⁺ adaptation

The kidney adjusts K⁺ excretion to match intake in part by regulation of the activity of apical K⁺ secretory channels, including renal outer medullary K⁺ (ROMK)-like K⁺ channels, in the cortical collecting duct (CCD). ANG II inhibits ROMK channels via the ANG II type 1 receptor (AT1R) during dietary K⁺ restriction. Because AT1Rs and ANG II type 2 receptors (AT2Rs) generally function in an antagonistic manner, we sought to characterize the regulation of ROMK channels by the AT2R. Patch-clamp experiments revealed that ANG II increased ROMK channel activity in CCDs isolated from high-K⁺ (HK)-fed but not normal K⁺ (NK)-fed rats. This response was blocked by PD-123319, an AT2R antagonist, but not by losartan, an AT1R antagonist, and was mimicked by the AT2R agonist CGP-42112. Nitric oxide (NO) synthase is present in CCD cells that express ROMK channels. Blockade of NO synthase with N-nitro-l-arginine methyl ester and free NO with 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt completely abolished ANG II-stimulated ROMK channel activity. NO enhances the synthesis of cGMP, which inhibits phosphodiesterases (PDEs) that normally degrade cAMP; cAMP increases ROMK channel activity. Pretreatment of CCDs with IBMX, a broad-spectrum PDE inhibitor, or cilostamide, a PDE3 inhibitor, abolished the stimulatory effect of ANG II on ROMK channels. Furthermore, PKA inhibitor peptide, but not an activator of the exchange protein directly activated by cAMP (Epac), also prevented the stimulatory effect of ANG II. We conclude that ANG II acts at the AT2R to stimulate ROMK channel activity in CCDs from HK-fed rats, a response opposite to that mediated by the AT1R in dietary K⁺-restricted animals, via a NO/cGMP pathway linked to a cAMP-PKA pathway.

Impact of cell type and epitope tagging on heterologous expression of G protein-coupled receptor: a systematic study on angiotensin type II receptor

Despite heterologous expression of epitope-tagged GPCR is widely adopted for functional characterization, there is lacking of systematic analysis of the impact of expression host and epitope tag on GPCR expression. Angiotensin type II (AT2) receptor displays agonist-dependent and -independent activities, coupling to a spectrum of signaling molecules. However, consensus has not been reached on the subcellular distributions, signaling cascades and receptor-mediated actions. To examine the contributions of host cell and epitope tag on receptor expression and activity, epitope-tagged AT2 receptor variants were transiently or stably expressed in HEK293, CHO-K1 and PC12 cells. The epitope-tagged AT2 receptor variants were detected both on the cell membrane and in the perinuclear region. In transiently transfected HEK293 cells, Myc-AT2 existed predominantly as monomer. Additionally, a ladder of ubiquitinated AT2 receptor proteins was detected. By contrast, stably expressed epitope-tagged AT2 receptor variants existed as both monomer and high molecular weight complexes, and the latter was enriched in cell surface. Glycosylation promoted cell surface expression of Myc-AT2 but had no effect on AT2-GFP in HEK293 cells. In cells that stably expressed Myc-AT2, serum starvation induced apoptosis in CHO-K1 cells but not in HEK293 or PC12 cells. Instead, HEK293 and PC12 cells stably expressing Myc-AT2 exhibited partial cell cycle arrest with cells accumulating at G1 and S phases, respectively. Taken together, these results suggest that expression levels, subcellular distributions and ligand-independent constitutive activities of AT2 receptor were cell type-dependent while posttranslational processing of nascent AT2 receptor protein was modulated by epitope tag and mode of expression.

How does angiotensin AT(2) receptor activation help neuronal differentiation and improve neuronal pathological situations?

The angiotensin type 2 (AT(2)) receptor of angiotensin II has long been thought to be limited to few tissues, with the primary effect of counteracting the angiotensin type 1 (AT(1)) receptor. Functional studies in neuronal cells have demonstrated AT(2) receptor capability to modulate neuronal excitability, neurite elongation, and neuronal migration, suggesting that it may be an important regulator of brain functions. The observation that the AT(2) receptor was expressed in brain areas implicated in learning and memory led to the hypothesis that it may also be implicated in cognitive functions. However, linking signaling pathways to physiological effects has always proven challenging since information relative to its physiological functions has mainly emerged from indirect observations, either from the blockade of the AT(1) receptor or through the use of transgenic animals. From a mechanistic standpoint, the main intracellular pathways linked to AT(2) receptor stimulation include modulation of phosphorylation by activation of kinases and phosphatases or the production of nitric oxide and cGMP, some of which are associated with the Gi-coupling protein. The receptor can also interact with other receptors, either G protein-coupled such as bradykinin, or growth factor receptors such as nerve growth factor or platelet-derived growth factor receptors. More recently, new advances have also led to identification of various partner proteins, thus providing new insights into this receptor's mechanism of action. This review summarizes the recent advances regarding the signaling pathways induced by the AT(2) receptor in neuronal cells, and discussed the potential therapeutic relevance of central actions of this enigmatic receptor. In particular, we highlight the possibility that selective AT(2) receptor activation by non-peptide and selective agonists could represent new pharmacological tools that may help to improve impaired cognitive performance in Alzheimer's disease and other neurological cognitive disorders.

Activation of central angiotensin type 2 receptors suppresses norepinephrine excretion and blood pressure in conscious rats

BACKGROUND

We have previously documented the finding that central angiotensin type 2 receptors (AT2R) negatively modulate sympathetic outflow and arterial blood pressure (BP). In this study, we investigated the effects of intracerebroventricular (icv) infusion of Compound 21 (C21), the first selective nonpeptide AT2R agonist, on norepinephrine (NE) excretion and BP in rats.

METHODS

C21 was infused icv for 7 days, using a micro-osmotic pump. Urinary NE concentration was measured using the NE enzyme immunoassay kit. BP was recorded by radiotelemetry. After 7 days, the rats were killed and three relevant samples from sympathetic brain regions and the cerebral cortex were obtained by micro-punching to measure neuronal nitric oxide synthase (nNOS) protein expression by western blot. In addition, the influence of C21 on neuronal potassium current (I(Kv)) was determined by whole-cell patch-clamp in a neuron cell line, CATH.a.

RESULTS

(i) Icv treatment with C21 significantly decreased both the concentration and the amount of NE in night time urine, but had no effect on daytime urine. (ii) C21-treated rats exhibited a slight but significant decrease in BP. (iii) The effects of C21 on NE excretion and BP were abolished by use of the AT2R antagonist, PD123319, and nitric oxide synthase (NOS) inhibitor, N-omega-nitro-L-arginine methyl ester (L-NAME). (iv) C21 treatment significantly upregulated nNOS expression in the paraventricular nucleus of the hypothalamus (PVN) and rostral ventrolateral medulla (RVLM), but not in the nucleus of the solitary tract (NTS) and cerebral cortex. (v) In CATH.a neurons, C21 treatment significantly increased I(Kv), and this increase was completely abolished by PD123319 and L-NAME.

CONCLUSIONS

These results demonstrate a central inhibitory influence of C21 on sympathetic outflow by means of a nNOS-dependent mechanism that might be mediated by facilitating the neuronal potassium channel.

Assessing the role of endooligopeptidase activity of Ndel1 (nuclear-distribution gene E homolog like-1) in neurite outgrowth

Ndel1 plays multiple roles in neuronal development but it is unknown whether its reported cysteine protease activity is important for these processes. Ndel1 is known to be critical for neurite outgrowth in PC12 cells where it works co-operatively in a complex with DISC1 to allow normal neuritogenesis. Through an initial interest in understanding the regulation of the expression of Ndel1 during neuronal differentiation, we have been able to show that Ndel1 expression and enzyme activity is up-regulated during neurite outgrowth in PC12 cells induced to neural differentiation. Heterologous expression of wild-type Ndel1 (Ndel1(WT)) in PC12 cells increases the percentage of cells bearing neurites in contrast to the catalytically dead mutant, Ndel1(C273A), which caused a decrease. Furthermore depletion of endogenous Ndel1 by RNAi decreased neurite outgrowth, which was rescued by transfection of the enzymatically active Ndel1(WT), but not by the Ndel1(C273A) mutant. Together these data support the notion that the endooligopeptidase activity of Ndel1 plays a crucial role in the differentiation process of PC12 cells to neurons. Genetic data and protein interaction with DISC1 might suggest a role for Ndel1 in neuropsychiatirc conditions.

AT2 receptors: functional relevance in cardiovascular disease

The renin angiotensin system (RAS) is intricately involved in normal cardiovascular homeostasis. Excessive stimulation by the octapeptide angiotensin II contributes to a range of cardiovascular pathologies and diseases via angiotensin type 1 receptor (AT1R) activation. On the other hand, tElsevier Inc.he angiotensin type 2 receptor (AT2R) is thought to counter-regulate AT1R function. In this review, we describe the enhanced expression and function of AT2R in various cardiovascular disease settings. In addition, we illustrate that the RAS consists of a family of angiotensin peptides that exert cardiovascular effects that are often distinct from those of Ang II. During cardiovascular disease, there is likely to be an increased functional importance of AT2R, stimulated by Ang II, or even shorter angiotensin peptide fragments, to limit AT1R-mediated overactivity and cardiovascular pathologies.

Estrogen elicits dorsal root ganglion axon sprouting via a renin-angiotensin system

Many painful conditions occur more frequently in women, and estrogen is a predisposing factor. Estrogen may contribute to some pain syndromes by enhancing axon outgrowth by sensory dorsal root ganglion (DRG) neurons. The objective of the present study was to define mechanisms by which estrogen elicits axon sprouting. The estrogen receptor-alpha agonist propyl pyrazole triol induced neurite outgrowth from cultured neonatal DRG neurons, whereas the estrogen receptor-beta agonist diarylpropionitrile was ineffective. 17beta-Estradiol (E2) elicited sprouting from peripherin-positive unmyelinated neurons, but not larger NF200-positive myelinated neurons. Microarray analysis showed that E2 up-regulates angiotensin II (ANGII) receptor type 2 (AT2) mRNA in vitro, and studies in adult rats confirmed increased DRG mRNA and protein in vivo. AT2 plays a central role in E2-induced axon sprouting because AT2 blockade by PD123,319 eliminated estrogen-mediated sprouting in vitro. We assessed whether AT2 may be responding to locally synthesized ANGII. DRG from adult rats expressed mRNA for renin, angiotensinogen, and angiotensin converting enzyme (ACE), and protein products were present and occasionally colocalized within neurons and other DRG cells. We determined if locally synthesized ANGII plays a role in estrogen-mediated sprouting by blocking its formation using the ACE inhibitor enalapril. ACE inhibition prevented estrogen-induced neuritogenesis. These findings support the hypothesis that estrogen promotes DRG nociceptor axon sprouting by up-regulating the AT2 receptor, and that locally synthesized ANGII can induce axon formation. Therefore, estrogen may contribute to some pain syndromes by enhancing the pro-neuritogenic effects of AT2 activation by ANGII.

Renin inhibition with aliskiren

1. Initial attempts to inhibit renin in humans have faced numerous difficulties. Molecular modelling and X-ray crystallography of the active site of renin have led to the development of new orally active renin inhibitors, such as aliskiren. 2. Aliskiren has a low bioavailability (between 2.6 and 5.0%) compensated by its high potency to inhibit renin (IC50: 0.6 nmol/L) and a long plasma half-life (23-36 h), which makes it suitable for once-daily dosing. 3. The once-daily administration of aliskiren to hypertensive patients lowers BP as strongly as standard doses of established angiotensin II type 1 (AT1) receptor blockers (losartan, valsartan, irbesartan), hydrochlorothiazide, angiotensin converting enzyme inhibitors (ramipril and lisinopril) or long acting calcium channel blockers (amlodipine). In combination therapy, aliskiren further decreases blood pressure when combined with either hydrochlorothiazide, amlodipine, irbesartan or ramipril. 4. The biochemical consequences of renin inhibition differ from those of angiotensin I-converting enzyme (ACE) inhibition and Ang II antagonism, particularly in terms of angiotensin profiles and interactions with the bradykinin-nitric oxide-cyclic guanosine monophosphate pathway and possibly the (pro)renin receptor. 5. Blockade of the renin angiotensin system (RAS) with ACE inhibitors, AT1 receptor blockers or a combination of these drugs has become one of the most successful therapeutic approaches in medicine. However, it remains unclear how to optimize RAS blockade to maximize cardiovascular and renal benefits. In this context, renin inhibition to render the RAS fully quiescent is a new possibility requiring further study.

Interaction between bradykinin subtype 2 and angiotensin II type 2 receptors during post-MI left ventricular remodeling

Angiotensin II type 2 receptor (AT2R) overexpression (AT2TG) attenuates left ventricular remodeling in a mouse model of anterior myocardial infarction (MI). We hypothesized that the beneficial effects of cardiac AT2TG are mediated via the bradykinin subtype 2 receptor (B2R). Fourteen transgenic mice overexpressing the AT2R (AT2TG mice), 10 mice with a B2R deletion (B2KO mice), 13 AT2TG mice with B2R deletion (AT2TG/B2KO mice), and 11 wild-type (WT) mice were studied. All mice were on a C57BL/6 background. Mice were studied by cardiac magnetic resonance imaging at baseline and days 1, 7, and 28 after MI induced by 1 h of occlusion of the left anterior descending artery followed by reperfusion. Short-axis images from apex to base were used to compare ventricular volumes and ejection fraction (EF). At baseline, end-diastolic volume index (EDVI) and end-systolic volume index (ESVI) were lower and EF higher in AT2TG mice compared with the other three strains. Infarct size was similar between groups. No differences were observed in global remodeling parameters at day 28 between AT2TG and AT2TG/B2KO mice; however, EDVI and ESVI were lower and EF higher in both transgenic groups than in WT or B2KO mice. Both strains lacking B2R demonstrated increased collagen content and less hypertrophy in adjacent noninfarcted regions at day 28. Attenuation of postinfarct remodeling by overexpression of AT2R is not directly mediated via a B2R pathway. However, B2R does appear to have a role in the smaller cavity size and hyperdynamic function observed at baseline in AT2TG mice and in limiting collagen deposition during postinfarct remodeling.

Comparison between early and delayed systemic treatment with candesartan of rats after ischaemic stroke

OBJECTIVE

The effects of candesartan treatment starting early (3 h) and delayed (24 h) after middle cerebral artery occlusion (MCAO) with reperfusion was investigated in normotensive rats.

METHODS

Subcutaneous treatment with candesartan (0.3 and 3 mg/kg) or vehicle was initiated 3 or 24 h after the onset of MCAO and continued for seven consecutive days (n=20 per group and timepoint). Neurological outcome was evaluated daily using two different scoring systems. Infarct and oedema volumes were determined in rats 2 or 7 days after MCAO. Mean arterial, systolic and diastolic blood pressures were recorded before and after the application of candesartan.

RESULTS

Mean arterial, systolic and diastolic blood pressures were markedly decreased with the high dose, but only moderately decreased with the low dose of candesartan. Vehicle-treated rats showed marked neurological deficits 24 h after MCAO, which gradually improved with time. Candesartan improved neurological outcomes at all timepoints only when treatment was started 3, but not 24 h after MCAO. The infarct volume was reduced on days 2 and 7 after MCAO in rats treated with the low but not the high dose of candesartan.

CONCLUSION

The present study demonstrates that only an early but not a delayed onset of treatment with candesartan exerts neuroprotection after focal ischaemia. The degree of neurological impairments did not correlate with the infarct volume, which was reduced only after the low dose of candesartan. The high dose of candesartan failed to reduce the infarct volume, probably because of an excessive blood pressure decrease.

Angiotensin II type 2 receptor-bradykinin B2 receptor functional heterodimerization

Angiotensin II type 2 (AT2R) or bradykinin B2 (B2R) receptor activation enhances NO production. Recently, we demonstrated enhancement of NO production when AT2R and B2R are simultaneously activated in vivo. However, the mechanism involved in this enhancement is unknown. Using confocal fluorescence resonance energy transfer microscopy, we report the distance between the AT2R and B2R in PC12W cell membranes to be 50+/-5 A, providing evidence and quantification of receptor heterodimerization as the mechanism for enhancing NO production. The rate of AT2R-B2R heterodimer formation is largely a function of the degree of AT2R-B2R expression. The physical association between the dimerized receptors initiates changes in intracellular phosphoprotein signaling activities leading to phosphorylation of c-Jun terminal kinase, phosphotyrosine phosphatase, inhibitory protein kappaBalpha, and activating transcription factor 2; dephosphorylation of p38 and p42/44 mitogen-activated protein kinase and signal transducer inhibitor of transcription 3; and enhancing production of NO and cGMP. Controlling the expression of AT2R-B2R, consequently influencing their biologically active dimerization, presents a potential therapeutic target for the treatment of hypertension and other cardiovascular and renal disorders.

Fundamental role of nitric oxide in neuritogenesis of PC12h cells

1 We investigated the neuritogenic action of nitric oxide (NO)-generating agents and their mechanisms of action in a subclone of rat pheochromocytoma, PC12h cells. 2 NO donors such as sodium nitroprusside (SNP, 0.05-1 microM), NOR1 (5-100 microM), NOR2 (5-20 microM), NOR3 (5-20 microM), NOR4 (5-100 microM), or S-nitroso-N-acetyl-DL-penicillamine (SNAP, 10-100 microM) significantly induced neurite outgrowth. 3 NOR4-induced neurite outgrowth was accompanied by expression of neurofilament 200 kDa subunit (NF200) protein, an axonal marker, and was significantly inhibited by an NO scavenger, a soluble GC inhibitor, and a PKG inhibitor: 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazole-1-oxyl-3-oxide (carboxy-PTIO, 20-100 microM), 1H-[1,2,4]oxadiazolo[4,3-a] quinoxalin-1-one (ODQ, 100 microM) and KT5823 (0.2-1 microM), respectively. 4 The intracellular cGMP concentration of cells was markedly increased by treatment with NOR4 (100 microM). 5 A mitogen-activated protein kinase (MAPK) kinase inhibitor, PD98059 (10-50 microM), abolished the NOR4-induced neurite outgrowth. In agreement with this observation, NOR4 did phosphorylate extracellular signal-regulated kinase (ERK) 1 and 2, substrates of MAPK kinase. 6 A membrane-permeable cGMP analog, 8-Br-cGMP (1 mM) also induced significant neurite outgrowth. The 8-Br-cGMP-induced neurite outgrowth was almost completely inhibited by both KT5823 (0.5 microM) and PD98059 (50 microM). Moreover, sustained ERK phosphorylation was observed in the 8-Br-cGMP-treated PC12h cells. 7 These results suggest that NO itself has the ability to induce neurite outgrowth and that NO-induced ERK activation involves the NO-cGMP-PKG signaling pathway in PC12h cells.

Angiotensin II induces peroxisome proliferator-activated receptor gamma in PC12W cells via angiotensin type 2 receptor activation

The angiotensin type 2 (AT2) receptor has been previously demonstrated to exert neuroprotective actions possibly by inducing neuronal cell differentiation involving neurite outgrowth. The nuclear hormone receptor peroxisome proliferator-activated receptor gamma (PPARgamma) is an important transcriptional regulator of cell differentiation. The aim of the present study was to clarify whether PPARgamma is involved in AT2-receptor-mediated morphological neuronal cell differentiation. To investigate AT2-receptor-mediated morphological neuronal cell differentiation, rat pheochromocytoma cells (PC12W cells) expressing AT2 but not AT1 receptors, were stimulated with angiotensin II (Ang II, 100 nmol/L) +/- the PPARgamma antagonists GW9662 (3 micromol/L) and bisphenol A diglycidyl ether (BADGE, 1 micromol/L), and neurite outgrowth of these cells was assessed. Ang II induced neurite outgrowth by 19 +/- 1.6-fold (p < 0.01). Antagonizing PPARgamma activity by GW9662 or BADGE potently blocked Ang II-induced neurite outgrowth (Ang II + GW9662: 6.6 +/- 1.5-fold, p < 0.05; Ang II + BADGE: 1.3 +/- 0.7-fold, p < 0.01). AT2 receptor activation by Ang II markedly induced mRNA and protein expression of the PPARgamma2 isoform and enhanced ligand-induced PPARgamma activity in transactivation assays. In conclusion, the present study demonstrates that Ang II induces PPARgamma expression and ligand-mediated PPARgamma activity via AT2 receptor activation, which appears to be a crucial process in AT2 receptor mediated neurite outgrowth. AT2 receptor/PPARgamma-dependent neurite outgrowth may play an important role during neuroprotective processes.

The AT2 receptor--a matter of love and hate

In 1989, the development of specific angiotensin receptor antagonists which distinguish between two angiotensin receptor subtypes (AT1 and AT2) led to a breakthrough in angiotensin research. It turned out, that the AT1 receptor was almost entirely responsible for the "classical" actions of angiotensin II related to the regulation of blood pressure as well as volume and electrolyte balance. However, actions and signal transduction mechanisms coupled to the AT2 receptor remained enigmatic for a long time. The present review summarizes the current knowledge of AT2 receptor distribution, signaling and function with an emphasis on growth/anti-growth, differentiation and the regeneration of neuronal tissue.

Adenoviral-mediated neuron specific transduction of angiotensin II type 2 receptors

The angiotensin II (Ang II) type 2 receptor (AT2R) is localized at specific nuclei within adult rat brain. However, a lack of specific approaches for manipulating the activity of neuronal AT2R has meant that the physiological actions of these sites in the brain remain to be established. Therefore, in this study, our aim was to develop a method by which AT2R can be specifically overexpressed in neurons and in rat brain, with the ultimate goal of a producing a system where discrete increases in AT2R levels in brain nuclei could reveal (and be linked to) physiological actions. Here, we have constructed an AT2R recombinant adenoviral vector, Ad5-SYN-AT2R-IRES-EGFP, which contains the AT2R gene and an IRES-linked EGFP reporter gene, both driven by the neuron-specific synapsin I (SYN) gene promoter. This vector efficiently transduces the AT2R into neuronal cells in culture and results in the expression of high levels of AT2R. These expressed receptors are functional in terms of inhibition of Erk mitogen activated protein kinases (Erk MAPK) and stimulation of neuronal K+ current. Furthermore, microinjection of this vector into adult rat brain elicits a long lasting ( approximately 1 month) expression of AT2R within neurons. In summary, we have developed a viral vector that can be used for the efficient transduction of AT2R into neurons both in vitro and in vivo, the use of which may help to define the physiological functions of brain AT2R in adult rats.

Candesartan prevents angiotensin II-induced facilitation of hypoxic neuronal damage through PKCdelta inhibition

To investigate the role of protein kinase Cdelta (PKCdelta) in angiotensin II-induced facilitation mechanisms of hypoxic neuronal damage and whether candesartan, an AT1 receptor antagonist, can suppress these mechanisms, we performed in vitro experiments which were free from vascular components using PC12 cells under hypoxic (12 h)/reoxygenation (0-48 h) conditions. Angiotensin II apparently increased the basal expression level of PKCdelta phosphorylated at Ser(643) before hypoxia, promoted the cleavage of PKCdelta to its catalytic fragment, and fostered the progression of DNA fragmentation after hypoxia. Candesartan inhibited both phosphorylation and cleavage of PKCdelta and suppressed the angiotensin II-induced facilitation of DNA fragmentation under hypoxic/reoxygenation conditions. However, PD123319, an AT2 receptor antagonist, influenced neither PKCdelta nor the angiotensin II-induced facilitation of DNA fragmentation. Furthermore, in PC12 cells expressing the ATP-binding mutant of PKCdelta (PKCdelta(K376R)) acting as a dominant-negative protein, both phosphorylation and cleavage of PKCdelta were attenuated and DNA fragmentation was markedly suppressed regardless of the presence of angiotensin II. These findings suggest that angiotensin II-induced facilitation of DNA fragmentation under hypoxic conditions is mediated by PKCdelta, and the mechanisms can be suppressed by the candesartan mediated blockade of the AT1 receptor.

Angiotensin AT2 receptor protects against cerebral ischemia-induced neuronal injury

Several lines of clinical and experimental evidence suggest an important role of the renin-angiotensin system in ischemic brain injury although the cellular regulation of the angiotensin AT1 and AT2 receptors and their potential relevance in this condition have not yet been clearly defined. We first assessed the regulation of brain AT1 and AT2 receptors in response to transient unilateral medial cerebral artery occlusion in rats by real-time RT-PCR, Western blot, and immunofluorescence labeling. AT2 receptors in the peri-infarct zone were significantly upregulated 2 days after transient focal cerebral ischemia. Increased AT2 receptors, which were abundantly distributed in a large number of brain regions adjacent to the infarct area including cerebral frontal cortex, piriform cortex, striatum, and hippocampus, were exclusively expressed in neurons. By contrast, AT1 receptors, which remained unaltered, were mainly expressed in astrocytes. In neurons of ischemic striatum, increased AT2 receptors were associated with intense neurite outgrowth. Blockade of central AT2 receptors with PD123177 abolished the neuroprotective effects of central AT1 receptor blockade with irbesartan on infarct size and neurological outcome. In primary cortical neurons, stimulation of AT2 receptors supported neuronal survival and neurite outgrowth. Our data indicate that cerebral AT2 receptors exert neuroprotective actions in response to ischemia-induced neuronal injury, possibly by supporting neuronal survival and neurite outgrowth in peri-ischemic brain areas.

Role of nitric oxide in the regulation of neuronal proliferation, survival and differentiation

Nitric oxide (NO), an important cellular messenger, has been linked to both neurodegenerative and neuroprotective actions. In the present review, we focus on recent data establishing a survival and differentiation role for NO in several neural in vitro and in vivo models. Nitric oxide has been found to be essential for survival of neuronal cell lines and primary neurons in culture under various death challenges. Furthermore, its lack may aggravate some neuropathological conditions in experimental animals. Several cellular pathways and signaling systems subserving this neuroprotective role of NO are considered in the review. Survey of recent data related to the developmental role of NO mainly focus on its action as a negative regulator of neuronal precursor cells proliferation and on its role of promotion of neuronal differentiation. Discussion on discrepancies arising from the literature is focused on the Janus-faced properties of the molecule and it is proposed that most controversial results are related to the intrinsic property of NO to compensate among functionally opposed effects. As an example, the increased proliferation of neural cell precursors under conditions of NO shortage may be, later on in the development, compensated by increased elimination through programmed cell death as a consequence of the lack of the survival-promoting action of the molecule. To elucidate these complex, and possibly contrasting, effects of NO is indicated as an important task for future researches.

Differential regulation by AT(1) and AT(2) receptors of angiotensin II-stimulated cyclic GMP production in rat uterine artery and aorta

1. In the present study we determined whether angiotensin II (Ang II) could increase cyclic GMP levels in two blood vessels that exhibit markedly different angiotensin II receptor subtype expression: rat uterine artery (UA; AT(2) receptor-predominant) and aorta (AT(1) receptor-predominant), and investigated the receptor subtype(s) and intracellular pathways involved. 2. UA and aorta were treated with Ang II in the absence and presence of losartan (AT(1) antagonist; 0.1 microm), PD 123319 (AT(2) antagonist; 1 microm), NOLA (NOS inhibitor; 30 microm), and HOE 140 (B(2) antagonist; 0.1 microm), or in combination. 3. Ang II (10 nm) induced a 60% increase in UA cyclic GMP content; an effect that was augmented with PD 123319 and HOE 140 pretreatment, and abolished by cotreatment with losartan, as well as by NOLA. 4. In aorta, Ang II produced concentration-dependent increases in cyclic GMP levels. Unlike effects in UA, these responses were abolished by PD 123319 and by NOLA, whereas losartan and HOE 140 caused partial inhibition. 5. Thus, in rat UA, Ang II stimulates cyclic GMP production through AT(1) and, to a less extent, AT(2) receptors. In rat aorta, the Ang II-mediated increase in cyclic GMP production is predominantly AT(2) receptor-mediated. In both preparations, NO plays a critical role in mediating the effect of Ang II, whereas bradykinin has differential roles in the two vessels. In UA, B(2) receptor blockade may result in a compensatory increase in cyclic GMP production, whilst in aorta, bradykinin accounts for approximately half of the cyclic GMP produced in response to Ang II.

Angiotensin AT2 receptors: cardiovascular hope or hype?

British Journal of Pharmacology (2003) 140, 809–824. doi:10.1038/sj.bjp.0705448