Cyclic AMP-independent involvement of Rap1/B-Raf in the angiotensin II AT2 receptor signaling pathway in NG108-15 cells

AT2 activation does not influence brain damage in the early phase after experimental traumatic brain injury in male mice

Antagonism of the angiotensin II type 1 receptor (AT1) improves neurological function and reduces brain damage after experimental traumatic brain injury (TBI), which may be partly a result of enhanced indirect angiotensin II type 2 receptor (AT2) stimulation. AT2 stimulation was demonstrated to be neuroprotective via anti-inflammatory, vasodilatory, and neuroregenerative mechanisms in experimental cerebral pathology models. We recently demonstrated an upregulation of AT2 after TBI suggesting a protective mechanism. The present study investigated the effect of post-traumatic (5 days after TBI) AT2 activation via high and low doses of a selective AT2 agonist, compound 21 (C21), compared to vehicle-treated controls. No differences in the extent of the TBI-induced lesions were found between both doses of C21 and the controls. We then tested AT2-knockdown animals for secondary brain damage after experimental TBI. Lesion volume and neurological outcomes in AT2-deficient mice were similar to those in wild-type control mice at both 24 h and 5 days post-trauma. Thus, in contrast to AT1 antagonism, AT2 modulation does not influence the initial pathophysiological mechanisms of TBI in the first 5 days after the insult, indicating that AT2 plays only a minor role in the early phase following trauma-induced brain damage.

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.

Promoter methylation of galanin receptors as epigenetic biomarkers for head and neck squamous cell carcinomas

INTRODUCTION

While remarkable progress has been made in standard treatments for head and neck squamous cell carcinomas (HNSCCs), the long-term survival remains at an unsatisfactory 40-50%. To improve the survival rate, biomarkers for optimal treatment selection and prognostic prediction, as well as novel, low-toxicity treatment strategies, are required. Galanin receptor (GALR) 1 and GALR2 are well-studied tumor suppressors in HNSCCs. Compared with other clinicopathological factors, the epigenetic variants of GALRs have been found to be the most powerful markers to predict the prognosis of HNSCC patients. Areas covered: This review outlines the functions and signaling pathways of GALRs and explains the potential of GALR promoter methylation as a biomarker for HNSCC prognosis. We also summarize recent developments in promoter methylation studies in HNSCC and indicate future directions for GALR promoter methylation studies. Expert commentary: GALR studies have highlighted two major aspects with implications in HNSCC - that G-protein coupled receptors (GPCRs) act as tumor suppressor genes and that GALR promoter methylation is significantly related to the carcinogenesis of HNSCC. The findings of GALR studies can be applied to studies on other GPCRs and further in-depth DNA methylation studies. Deeper insights into GPCR epigenetics are expected to markedly improve HNSCC treatment.

Angiotensin II-triggered kinase signaling cascade in the central nervous system

Recent studies have projected the renin-angiotensin system as a central component of the physiological and pathological processes of assorted neurological disorders. Its primary effector hormone, angiotensin II (Ang II), not only mediates the physiological effects of vasoconstriction and blood pressure regulation in cardiovascular disease but is also implicated in a much wider range of neuronal activities and diseases, including Alzheimer's disease, neuronal injury, and cognitive disorders. Ang II produces different actions by acting on its two subtypes of receptors (AT1 and AT2); however, the well-known physiological actions of Ang II are mainly mediated through AT1 receptors. Moreover, recent studies also suggest the important functional role of AT2 receptor in the brain. Ang II acts on AT1 receptors and conducts its functions via MAP kinases (ERK1/2, JNK, and p38MAPK), glycogen synthase kinase, Rho/ROCK kinase, receptor tyrosine kinases (PDGF and EGFR), and nonreceptor tyrosine kinases (Src, Pyk2, and JAK/STAT). AT1R-mediated NADPH oxidase activation also leads to the generation of reactive oxygen species, widely implicated in neuroinflammation. These signaling cascades lead to glutamate excitotoxicity, apoptosis, cerebral infarction, astrocyte proliferation, nociception, neuroinflammation, and progression of other neurological disorders. The present review focuses on the Ang II-triggered signal transduction pathways in central nervous system.

Role of Lipotoxicity and Contribution of the Renin-Angiotensin System in the Development of Polycystic Ovary Syndrome

Polycystic ovary syndrome (PCOS) is a common and significant condition associated with hyperandrogenism, infertility, low quality of life, and metabolic comorbidities. One possible explanation of PCOS development is cellular dysfunction induced by nonesterified fatty acids (NEFAs), that is, lipotoxicity, which could explain both the hyperandrogenemia and insulin resistance that characterize women with PCOS. The literature suggests that androgen biosynthesis may be induced by overexposure of androgen-secreting tissues to NEFA and/or defective NEFA metabolism, leading to lipotoxic effects. Indeed, lipotoxicity could trigger androgenic hyperresponsiveness to insulin, LH, and ACTH. In most PCOS women, lipotoxicity also causes insulin resistance, inducing compensatory hyperinsulinemia, and may thus further increase hyperandrogenemia. Many approaches aimed at insulin sensitization also reduce lipotoxicity and have been shown to treat PCOS hyperandrogenemia. Furthermore, our group and others found that angiotensin II type 2 receptor (AT2R) activation is able to improve lipotoxicity. We provided evidence, using C21/M24, that AT2R activation improves adipocytes' size and insulin sensitivity in an insulin-resistant rat model, as well as androgen levels in a PCOS obese rat model. Taken together, these findings point toward the important role of lipotoxicity in PCOS development and of the RAS system as a new target for the treatment of PCOS.

Angiotensin II AT2 receptors regulate NGF-mediated neurite outgrowth via the NO-cGMP pathway

We investigated whether Angiotensin II type 2 (AT2) receptor activation was involved in NGF-induced nerve regeneration. NGF-mediated neurite outgrowth in cultured dorsal root ganglia (DRG) cells was significantly inhibited by AT2 receptor antagonist (PD123,319) treatment. AT2 receptor knockdown also inhibited NGF-mediated neurite outgrowth. To determine the mechanisms, we analyzed the NO-cGMP pathway. The cGMP analog increased NGF-mediated nerve elongation, which inhibited by PD123,319. Furthermore, soluble guanylate cyclase expression was significantly less in NGF and PD123,319 treatment DRG than in NGF treatment alone. These results suggest that NGF-mediated neurite outgrowth is suppressed by AT2 receptor signaling via the NO-cGMP-PKG pathway.

Angiotensin II analogs comprised of Pro-Amb (γ-turn scaffold) as angiotensin II type 2 (AT2) receptor agonists

We describe herein the design, synthesis and conformational investigation of Pro-Amb (proline-3-amino-2-methoxybenzoic acid) incorporated Angiotensin II and its truncated analogues.

G-Protein-Coupled Receptors: Next Generation Therapeutic Targets in Head and Neck Cancer?

Therapeutic outcome in head and neck squamous cell carcinoma (HNSCC) is poor in most advanced cases. To improve therapeutic efficiency, novel therapeutic targets and prognostic factors must be discovered. Our studies have identified several G protein-coupled receptors (GPCRs) as promising candidates. Significant epigenetic silencing of GPCR expression occurs in HNSCC compared with normal tissue, and is significantly correlated with clinical behavior. Together with the finding that GPCR activity can suppress tumor cell growth, this indicates that GPCR expression has potential utility as a prognostic factor. In this review, we discuss the roles that galanin receptor type 1 (GALR1) and type 2 (GALR2), tachykinin receptor type 1 (TACR1), and somatostatin receptor type 1 (SST1) play in HNSCC. GALR1 inhibits proliferation of HNSCC cells though ERK1/2-mediated effects on cell cycle control proteins such as p27, p57, and cyclin D1, whereas GALR2 inhibits cell proliferation and induces apoptosis in HNSCC cells. Hypermethylation of GALR1, GALR2, TACR1, and SST1 is associated with significantly reduced disease-free survival and a higher recurrence rate. Although their overall activities varies, each of these GPCRs has value as both a prognostic factor and a therapeutic target. These data indicate that further study of GPCRs is a promising strategy that will enrich pharmacogenomics and prognostic research in HNSCC.

Angiotensin AT2 receptor stimulation is anti-inflammatory in lipopolysaccharide-activated THP-1 macrophages via increased interleukin-10 production

Macrophages have an important role in the pathogenesis of hypertension and associated end-organ damage via the activation of the Toll-like receptors, such as Toll-like receptor-4 (TLR4). Accumulating evidence suggests that the angiotensin AT2 receptor (AT2R) has a protective role in pathological conditions involving inflammation and tissue injury. We have recently shown that AT(2)R stimulation is renoprotective, which occurs in part via increased levels of anti-inflammatory interleukin-10 (IL-10) production in renal epithelial cells; however, the role of AT(2)R in the inflammatory activity of macrophages is not known. The present study was designed to investigate whether AT(2)R activation stimulates an anti-inflammatory response in TLR4-induced inflammation. The effects of the anti-inflammatory mechanisms that occurred following pre-treatment with the AT(2)R agonist Compound 21 (C21) (1 μmol ml(-1)) on the cytokine profiles of THP-1 macrophages after activation by lipopolysaccharide (LPS) (1 μg ml(-1)) were studied. The AT(2)R agonist dose-dependently attenuated LPS-induced tumor necrosis factor-α (TNF-α) and IL-6 production but increased IL-10 production. IL-10 was critical for the anti-inflammatory effects of AT(2)R stimulation because the IL-10-neutralizing antibody dose-dependently abolished the AT(2)R-mediated decrease in TNF-α levels. Further, enhanced IL-10 levels were associated with a sustained, selective increase in the phosphorylation of extracellular signal-regulated kinase (ERK1/2) but not p38 mitogen-activated protein kinase (MAPK). Blocking the activation of ERK1/2 before C21 pre-treatment completely abrogated this increased IL-10 production in response to the AT(2)R agonist C21, while there was a partial reduction in IL-10 levels following the inhibition of p38. We conclude that AT(2)R stimulation exerts a novel anti-inflammatory response in THP-1 macrophages via enhanced IL-10 production as a result of sustained, selective ERK1/2 phosphorylation, which may have protective roles in hypertension and associated tissue injury.

Rap1b in smooth muscle and endothelium is required for maintenance of vascular tone and normal blood pressure

OBJECTIVE

Small GTPase Ras-related protein 1 (Rap1b) controls several basic cellular phenomena, and its deletion in mice leads to several cardiovascular defects, including impaired adhesion of blood cells and defective angiogenesis. We found that Rap1b(-/-) mice develop cardiac hypertrophy and hypertension. Therefore, we examined the function of Rap1b in regulation of blood pressure.

APPROACH AND RESULTS

Rap1b(-/-) mice developed cardiac hypertrophy and elevated blood pressure, but maintained a normal heart rate. Correcting elevated blood pressure with losartan, an angiotensin II type 1 receptor antagonist, alleviated cardiac hypertrophy in Rap1b(-/-) mice, suggesting a possibility that cardiac hypertrophy develops secondary to hypertension. The indices of renal function and plasma renin activity were normal in Rap1b(-/-) mice. Ex vivo, we examined whether the effect of Rap1b deletion on smooth muscle-mediated vessel contraction and endothelium-dependent vessel dilation, 2 major mechanisms controlling basal vascular tone, was the basis for the hypertension. We found increased contractility on stimulation with a thromboxane analog or angiotensin II or phenylephrine along with increased inhibitory phosphorylation of myosin phosphatase under basal conditions consistent with elevated basal tone and the observed hypertension. Cyclic adenosine monophosphate-dependent relaxation in response to Rap1 activator, Epac, was decreased in vessels from Rap1b(-/-) mice. Defective endothelial release of dilatory nitric oxide in response to elevated blood flow leads to hypertension. We found that nitric oxide-dependent vasodilation was significantly inhibited in Rap1b-deficient vessels.

CONCLUSIONS

This is the first report to indicate that Rap1b in both smooth muscle and endothelium plays a key role in maintaining blood pressure by controlling normal vascular tone.

The Angiotensin II Type 2 Receptor in Brain Functions: An Update

Angiotensin II (Ang II) is the main active product of the renin-angiotensin system (RAS), mediating its action via two major receptors, namely, the Ang II type 1 (AT₁) receptor and the type 2 (AT₂) receptor. Recent results also implicate several other members of the renin-angiotensin system in various aspects of brain functions. The first aim of this paper is to summarize the current state of knowledge regarding the properties and signaling of the AT₂ receptor, its expression in the brain, and its well-established effects. Secondly, we will highlight the potential role of the AT₂ receptor in cognitive function, neurological disorders and in the regulation of appetite and the possible link with development of metabolic disorders. The potential utility of novel nonpeptide selective AT₂ receptor ligands in clarifying potential roles of this receptor in physiology will also be discussed. If confirmed, these new pharmacological tools should help to improve impaired cognitive performance, not only through its action on brain microcirculation and inflammation, but also through more specific effects on neurons. However, the overall physiological relevance of the AT₂ receptor in the brain must also consider the Ang IV/AT₄ receptor.

From the first selective non-peptide AT(2) receptor agonist to structurally related antagonists

A para substitution pattern of the phenyl ring is a characteristic feature of the first reported selective AT(2) receptor agonist M024/C21 (1) and all the nonpeptidic AT(2) receptor agonists described so far. Two series of compounds structurally related to 1 but with a meta substitution pattern have now been synthesized and biologically evaluated for their affinity to the AT(1) and AT(2) receptors. A high AT(2)/AT(1) receptor selectivity was obtained with all 41 compounds synthesized, and the majority exhibited K(i) ranging from 2 to 100 nM. Five compounds were evaluated for their functional activity at the AT(2) receptor, applying a neurite outgrowth assay in NG108-15 cells. Notably, four of the five compounds, with representatives from both series, acted as potent AT(2) receptor antagonists. These compounds were found to be considerably more effective than PD 123,319, the standard AT(2) receptor antagonist used in most laboratories. No AT(2) receptor antagonists were previously reported among the derivatives with a para substitution pattern. Hence, by a minor modification of the agonist 1 it could be transformed into the antagonist, compound 38. These compounds should serve as valuable tools in the assessment of the role of the AT(2) receptor in more complex physiological models.

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.

Angiotensin II, a Neuropeptide at the Frontier between Endocrinology and Neuroscience: Is There a Link between the Angiotensin II Type 2 Receptor and Alzheimer's Disease?

Amyloid-β peptide deposition, abnormal hyperphosphorylation of tau, as well as inflammation and vascular damage, are associated with the development of Alzheimer's disease (AD). Angiotensin II (Ang II) is a peripheral hormone, as well as a neuropeptide, which binds two major receptors, namely the Ang II type 1 receptor (AT1R) and the type 2 receptor (AT2R). Activation of the AT2R counteracts most of the AT1R-mediated actions, promoting vasodilation, decreasing the expression of pro-inflammatory cytokines, both in the brain and in the cardiovascular system. There is evidence that treatment with AT1R blockers (ARBs) attenuates learning and memory deficits. Studies suggest that the therapeutic effects of ARBs may reflect this unopposed activation of the AT2R in addition to the inhibition of the AT1R. Within the context of AD, modulation of AT2R signaling could improve cognitive performance not only through its action on blood flow/brain microcirculation but also through more specific effects on neurons. This review summarizes the current state of knowledge and potential therapeutic relevance of central actions of this enigmatic receptor. In particular, we highlight the possibility that selective AT2R activation by non-peptide and highly selective agonists, acting on neuronal plasticity, could represent new pharmacological tools that may help improve impaired cognitive performance in AD and other neurological cognitive disorders.

Exploring the Backbone of Enkephalins To Adjust Their Pharmacological Profile for the δ-Opioid Receptor

The role of each of the four amide bonds in Leu(5)-enkephalin was investigated by systematically and sequentially replacing each with its corresponding trans-alkene. Six Leu(5)-enkephalin analogs based on six dipeptide surrogates and two Met(5)-enkephalin analogs were synthesized and thoroughly tested using a δ-opioid receptor internalization assay, an ERK1/2 activation assay, and a competition binding assay to evaluate their biological properties. We observed that an E-alkene can efficiently replace the first amide bond of Leu(5)- and Met(5)-enkephalin without significantly affecting biological activity. By contrast, the second amide bond was found to be highly sensitive to the same modification, suggesting that it is involved in biologically essential intra- or intermolecular interactions. Finally, we observed that the affinity and activity of analogs containing an E-alkene at either the third or fourth position were partially reduced, indicating that these amide bonds are less important for these intra- or intermolecular interactions. Overall, our study demonstrates that the systematic and sequential replacement of amide bonds by E-alkene represents an efficient way to explore peptide backbones.

Fyn is involved in angiotensin II type 2 receptor-induced neurite outgrowth, but not in p42/p44mapk in NG108-15 cells

In NG108-15 cells, activation of p42/p44(mapk) is essential for induction of neurite outgrowth by angiotensin II (Ang II) type 2 receptor (AT(2)). The aim was to verify whether Fyn, a member of the Src family kinases (SFK), is involved in neurite outgrowth induced by AT(2) activation. Preincubation of cells with PP1, a general inhibitor of the SKF, decreased activation of Rap1 and p42/p44(mapk) and abolished TrkA activation by Ang II or by the AT(2) agonist, CGP42112A. NG108-15 cells were transfected with a Fyn-WT and a Fyn-DN expressing vector. Fyn-WT was sufficient to induce neurite outgrowth, although transfection with Fyn-DN abolished neurite elongation. However, the Fyn-DN form failed to affect activation of TrkA, Rap1 or p42/p44(mapk) by Ang II. Thus, although SKF activity is required to achieve AT(2)-induced activation of TrkA, Rap1 and p42/p44(mapk), Fyn is essential for AT(2) receptor-induced neurite outgrowth, but not in AT(2) signaling leading to p42/p44(mapk) activation.

Processing of peptide and hormone precursors at the dibasic cleavage sites

Many functionally important cellular peptides and proteins, including hormones, neuropeptides, and growth factors, are synthesized as inactive precursor polypeptides, which require post-translational proteolytic processing to become biologically active polypeptides. This is achieved by the action of a relatively small number of proteases that belong to a family of seven subtilisin-like proprotein convertases (PCs) including furin. In view of this, this review focuses on the importance of privileged secondary structures and of given amino acid residues around basic cleavage sites in substrate recognition by these endoproteases. In addition to their participation in normal cell functions, PCs are crucial for the initiation and progress of many important diseases. Hence, these proteases constitute potential drug targets in medicine. Accordingly, this review also discusses the approaches used to shed light on the cleavage preference and the substrate specificity of the PCs, a prerequisite to select which PCs are promising drug targets in each disease.

Selective angiotensin II AT2 receptor agonists: Benzamide structure-activity relationships

In the investigation of the structure-activity relationship of nonpeptide AT(2) receptor agonists, a series of substituted benzamide analogues of the selective nonpeptide AT(2) receptor agonist M024 have been synthesised. In a second series, the biphenyl scaffold was compared to the thienylphenyl scaffold and the impact of the isobutyl substituent and its position on AT(1)/AT(2) receptor selectivity was also investigated. Both series included several compounds with high affinity and selectivity for the AT(2) receptor. Three of the compounds were also proven to function as agonists at the AT(2) receptor, as deduced from a neurite outgrowth assay, conducted in NG108-15 cells.

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.

Angiotensin II type 2 receptor stimulation increases the rate of NG108-15 cell migration via actin depolymerization

Angiotensin II (Ang II) has been reported to induce migration in neuronal cell types. Using time-lapse microscopy, we show here that Ang II induces acceleration in NG108-15 cell migration. This effect was antagonized by PD123319, a selective AT2 receptor antagonist, but not by DUP753, a selective AT1 receptor antagonist, and was mimicked by the specific AT2 receptor agonist CGP42112. This Ang II-induced acceleration was not sensitive to the inhibition of previously described signaling pathways of the AT2 receptor, guanylyl cyclase/cyclic GMP or p42/p44 mapk cascades, but was abolished by pertussis toxin treatment and involved PP2A activation. Immunofluorescence studies indicate that Ang II or CGP42112 decreased the amount of filamentous actin at the leading edge of the cells. This decrease was accompanied by a concomitant increase in globular actin levels. Regulation of actin turnover in actin-based motile systems is known to be mainly under the control of the actin depolymerizing factor and cofilin. Basal migration speed decreased by 77.2% in cofilin-1 small interfering RNA-transfected NG108-15 cells, along with suppression of the effect of Ang II. In addition, the Ang II-induced increase in cell velocity was abrogated in serum-free medium as well as by genistein or okadaic acid treatment in a serum-containing medium. Such results indicate that the AT2 receptor increases the migration speed of NG108-15 cells and involves a tyrosine kinase activity, followed by phosphatase activation, which may be of the PP2A type. Therefore, the present study identifies actin depolymerization and cofilin as new targets of AT2 receptor action, in the context of cellular migration.

An ab initio study of AT2 antagonists

In this study we report ab initio molecular orbital calculations on the natural hormone angiotensin II, which induces activity at AT1/AT2 receptor subtypes leading to vasoconstriction and subsequent hypertension, and AT2 antagonists. Pharmacophoric features of AT2 antagonists have been studied. A model of AT2 receptor has been made, and angiotensin II as well as antagonists has been systematically docked and their interactions with the receptor analyzed. Calculated ligand-receptor interaction energies have been correlated with experimentally observed biological potency data. Our studies indicate that antagonists retain sufficient interactions to block the receptor but may not be adequate to induce activity at the receptor. A poor antagonist is, therefore, proposed as a close mimic of angiotensin II in terms of interacting with the receptor. These studies further explore the mechanistic aspects of this important class of drugs.

Angiotensin II type 2 receptor-dependent increases in nitric oxide synthase expression in the pulmonary endothelium is mediated via a Gαi3/Ras/Raf/MAPK pathway

We have previously reported that angiotensin II (ANG II) stimulated Src tyrosine kinase via a pertussis toxin-sensitive type 2 receptor, which, in turn, activates MAPK, resulting in an increase in nitric oxide synthase (NOS) expression in pulmonary artery endothelial cells (PAECs). The present study was designed to investigate the pathway by which ANG II activates Src leading to an increase in ERK1/ERK2 phosphorylation and an increase in NOS protein in PAECs. Transfection of PAECs with Gαi3dominant negative (DN) cDNA blocked the ANG II-dependent activation of Src, ERK1/ERK2 phosphorylation, and increase in NOS expression. ANG II stimulated an increase in tyrosine phosphorylation of sequence homology of collagen (Shc; 15 min) that was prevented when PAECs were pretreated with 4-amino-5-(4-chlorophenyl)-7-( t-butyl)pyrazolo-[3,4-d]pyrimidine (PP2), a Src inhibitor. ANG II induced a Src-dependent association between Shc and growth factor receptor-bound protein 2 (Grb2) and between Grb2 and son of sevenless (Sos), both of which were maximal at 15 min. The ANG II-dependent increase in Ras GTP binding was prevented when PAECs were pretreated with the AT2antagonist PD-123319 or with PP2 or were transfected with Src DN cDNA. ANG II-dependent activation of MAPK and the increase in endothelial NOS (eNOS) were prevented when PAECs were transfected with Ras DN cDNA or treated with FTI-277, a farnesyl transferase inhibitor. ANG II induction of Raf-1 phosphorylation was prevented when PAECs were pretreated with PD-123319 and PP2. Raf kinase inhibitor 1 prevented the ANG II-dependent increase in eNOS expression. Collectively, these data suggest that Gαi3, Shc, Grb2, Ras, and Raf-1 link Src to activation of MAPK and to the AT2-dependent increase in eNOS expression in PAECs.

Galanin and galanin receptor type 1 suppress proliferation in squamous carcinoma cells: activation of the extracellular signal regulated kinase pathway and induction of cyclin-dependent kinase inhibitors

Galanin receptor 1 (GALR1) maps to a common region of 18q loss in head and neck squamous cell carcinomas and is frequently inactivated by methylation. To investigate effects of GALR1 and its signaling pathways, we stably expressed hemaglutinin-tagged GALR1 in a human oral carcinoma cell line (UM-SCC-1-GALR1) that expresses no endogenous GALR1. In transfected cells, galanin induced activation of the extracellular-regulated protein kinase-1/2 (ERK1/2) and suppressed proliferation. Galanin stimulation mediated decreased expression of cyclin D1 and increased expression of the cyclin-dependent kinase inhibitors (CKI), p27(Kip1) and p57(Kip2). Pretreatment with the ERK1/2-specific inhibitor U0126 prevented these galanin-induced effects. Phosphatidylinositol 3-kinase (PI3K) pathway activation did not differ in UM-SCC-1-GALR1 and UM-SCC-1-mock cells after galanin treatment. Pertussis toxin and LY294002 inhibition demonstrated that galanin and GALR1 induce ERK1/2 activation via Galphai, not the PI3K pathway-linked to the Gbetagamma subunit. Galanin and GALR1 also inhibit colony formation and tumor growth in vivo. Our results implicate GALR1, a Gi protein-coupled receptor, as a tumor suppressor gene that inhibits cell proliferation via ERK1/2 activation.

Role of tyrosine kinase receptors in angiotensin II AT2 receptor signaling: involvement in neurite outgrowth and in p42/p44mapk activation in NG108-15 cells

NG108-15 cells, which have a rounding-up morphology when cultured in serum-supplemented medium, extend neurites when stimulated for 3 d with angiotensin II (Ang II). The aim of the present study was to investigate whether growth factor receptors are necessary for mediating the effects of Ang II. A 3-d treatment with AG879, an inhibitor of nerve growth factor receptor TrkA, strongly affected neurite outgrowth and phosphorylation of p42/p44(mapk) induced by Ang II. PD168393, an inhibitor of epidermal growth factor (EGF) receptor slightly decreased Ang II-induced neurite outgrowth, whereas AG213, an inhibitor of both platelet-derived growth factor receptor and EGF receptor, stimulated neurite outgrowth and p42/p44(mapk) phosphorylation on its own, without affecting further stimulation with Ang II. Moreover, Ang II induced the phosphorylation of TrkA (maximum at 5 min of incubation in the presence of serum or at 20 min in cells depleted in serum for 2 h) and a rapid increase in Rap1 activity, both effects abolished in cells preincubated with 10 microm AG879. In summary, the present results demonstrate that AT(2) receptor-induced sustained activation of p42/p44(mapk) and corresponding neurite outgrowth are mediated by phosphorylation of the nerve growth factor TrkA receptor. However, the results also point out that the presence of other growth factors, such as EGF or PDFG, may interfere with the effect of Ang II. Altogether, the current findings clearly indicate that the effects of the AT(2) receptor on neurite outgrowth dynamics are modulated by the presence of growth factors in the culture medium.

Involvement of protein kinase C alpha (PKC alpha) in the early action of angiotensin II type 2 (AT2) effects on neurite outgrowth in NG108-15 cells: AT2-receptor inhibits PKC alpha and p21ras activity

The aim of the present study was to investigate whether protein kinase C (PKC) isoforms may be among the putative candidates implicated in the primary effects of the Ang II type 2 (AT2) receptor. Western blot analyses revealed the presence of PKC alpha,epsilon, iota, and zeta in NG108-15 cells. After a 3-d treatment with 3 nm Gö6976, a specific inhibitor of classical PKC isoforms, cells were characterized by the presence of one elongated process similar to that observed after treatment with Ang II or with CGP42112, a selective AT2 receptor agonist. Similar findings were observed in cells expressing a dominant-negative mutant of PKC alpha (K368A). Inhibition of PKC alpha in NG108-15 cells also decreased cell number and proliferation. In conditions of acute stimulation, Ang II induced a time-dependent and transient inhibition of PKC alpha activity, as well as a decrease in PKC alpha levels associated with the membrane. Treatment of cells with Gö6976 was also found to inhibit p21(ras) (between 1-10 min) but stimulated Rap1 activity (1-5 min) in a time-course similar to that of Ang II. Incubation of NG108-15 cells with Gö6976 (3 nm) inhibited basal p42/p44(mapk) phosphorylation, but failed to interfere with its activation by the AT(2) receptor, indicating that inhibition of PKC alpha is not directly involved in the Rap1-MEK-p42/p44(mapk) cascade. Taken together, these results indicate that PKC alpha is a primary target of the AT2 receptor. Inhibition of PKC alpha leads to a decrease in both p21(ras) activity and cell proliferation, which may facilitate AT2 receptor signaling through p42/p44(mapk), thereby leading to neurite outgrowth.

Short pseudopeptides containing turn scaffolds with high AT2 receptor affinity

Two pentapeptides, Ac-Tyr-Ile-His-Pro-Phe/Ile, were synthesized and shown to have angiotensin II AT2 receptor affinity and agonistic activity. Based on these peptides, a new series of 13 pseudopeptides was synthesized via introduction of five different turn scaffolds replacing the Tyr-Ile amino acid residues. Pharmacological evaluation disclosed subnanomolar affinities for some of these compounds at the AT2 receptor. Substitution of Phe by Ile in this series of ligands enhanced the AT2 receptor affinity of all compounds. These results suggest that the C-terminal amino acid residues can be elaborated on to enhance the AT2 receptor affinity in truncated Ang II analogues.

Epigallocathechin-3 gallate inhibits cardiac hypertrophy through blocking reactive oxidative species-dependent and -independent signal pathways

Cardiac hypertrophy is a major cause of morbidity and mortality worldwide. Recent in vitro and in vivo studies have suggested that reactive oxygen species (ROS) may play an important role in cardiac hypertrophy. It was therefore thought to be of particular value to examine the effects of antioxidants on cardiac hypertrophy. Epigallocatechin-3-gallate (EGCG) is a major bioactive polyphenol present in green tea and a potent antioxidant. The current study was designed to test the hypothesis that EGCG inhibits cardiac hypertrophy in vitro and in vivo. In this study, we investigated the effects of EGCG on angiotensin II- (Ang II) and pressure-overload-induced cardiac hypertrophy. Our results showed that EGCG attenuated Ang II- and pressure-overload-mediated cardiac hypertrophy. Both reactive oxygen species generation and NADPH oxidase expressions induced by Ang II and pressure overload were suppressed by EGCG. The increased hypertension by pressure overload was almost completely blocked after EGCG treatment. Further studies showed that EGCG inhibited Ang II-induced NF-kappaB and AP-1 activation. Inhibition of the activity of NF-kappaB was through blocking ROS-dependent p38 and JNK signaling pathways, whereas inhibition of AP-1 activation was via blocking EGFR transactivation and its downstream events ERKs/PI3K/Akt/mTOR/p70(S6K). The combination of these actions resulted in repressing the reactivation of ANP and BNP, and ultimately preventing the progress of cardiac hypertrophy. These findings indicated that EGCG prevents the development of cardiac hypertrophy through ROS-dependent and -independent mechanisms involving inhibition of different intracellular signaling transductional pathways.

The scaffold protein CNK1 interacts with the angiotensin II type 2 receptor

The scaffold protein CNK1 mediates proliferative as well as antiproliferative responses including differentiation and apoptosis. The angiotensin II type 2 (AT2) receptor belongs to the class of G protein-coupled receptors and also promotes antiproliferative effects. Here we report that CNK1 binds through the sterile alpha motif (SAM) and the conserved region in CNK (CRIC) to the AT2 receptor. The exchange of a conserved leucine residue with arginine in the CRIC domain increases the binding affinity of CNK1 to the AT2 receptor. The insertion of a negatively charged amino acid stretch into the linker region between the N- and the C-terminal part of CNK1 strengthens the interaction between CNK1 and the AT2 receptor in a Ras-regulated manner. The biological significance of the interaction was supported by coprecipitation of CNK1 and the AT2 receptor in mouse heart extracts. Thus, CNK1 may play a role in the AT2 receptor-mediated signaling pathways.

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.

RA-RhoGAP, Rap-activated Rho GTPase-activating protein implicated in neurite outgrowth through Rho

Rap1 and Rho small G proteins have been implicated in the neurite outgrowth, but the functional relationship between Rap1 and Rho in the neurite outgrowth remains to be established. Here we identified a potent Rho GTPase-activating protein (GAP), RA-RhoGAP, as a direct downstream target of Rap1 in the neurite outgrowth. RA-RhoGAP has the RA and GAP domains and showed GAP activity specific for Rho, which was enhanced by the binding of the GTP-bound active form of Rap1 to the RA domain. Overexpression of RA-RhoGAP induced inactivation of Rho for promoting the neurite outgrowth in a Rap1-dependent manner. Knockdown of RA-RhoGAP reduced the Rap1-induced neurite outgrowth. These results indicate that RA-RhoGAP transduces a signal from Rap1 to Rho and regulates the neurite outgrowth.

The EphA8 receptor induces sustained MAP kinase activation to promote neurite outgrowth in neuronal cells

Recent studies in our laboratory demonstrate that ligand-mediated activation of the EphA8 receptor critically regulates cell adhesion and migration. In this report, we show that the EphA8 receptor induces neurite outgrowth in NG108-15 cells in the absence of ligand stimulation. Using various deletion mutants lacking specific intracytoplasmic regions, we confirm that the tyrosine kinase domain of EphA8 is important for inducing neurite outgrowth. However, the tyrosine kinase activity of EphA8 is not crucial for neurite outgrowth induction. Treatment with various inhibitors further reveals that the mitogen-activated protein kinase (MAPK) signaling pathway is critical for neurite outgrowth induced by EphA8. Consistent with these results, EphA8 expression induced a sustained increase in the activity of MAPK, whereas ligand-mediated EphA8 activation had no further modulatory effects on MAP kinase activity. Additionally, activated MAPK relocalized from the cytoplasm to the nucleus in response to EphA8 transfection. These results collectively suggest that the EphA8 receptor is capable of inducing a sustained increase in MAPK activity, thereby promoting neurite outgrowth in neuronal cells.

Tempol augments angiotensin II-induced AT2 receptor-mediated relaxation in diabetic rat thoracic aorta

OBJECTIVE

To assess angiotensin II type 2 receptor-mediated responses in thoracic aorta of streptozotocin-induced diabetic rats.

METHODS

The concentration-dependent relaxation response (in the presence of an AT1 receptor blocker) to angiotensin II (Ang II) was studied in phenylephrine (PE) or potassium chloride (KCl) precontracted rat thoracic aortic rings isolated from male Sprague-Dawley rats pretreated with streptozotocin (65 mg/kg i.p.) or vehicle 8 weeks prior to the study.

RESULTS

Ang II-induced relaxation response (% relaxation), evident only in the presence of an AT1 receptor blocker, was significantly enhanced in aortic rings isolated from diabetic (55%) compared to control (25%) rats. Tempol (100 micromol/l) augmented the relaxation response in aortic rings isolated from diabetic (80%) but not control (28%) rats. N-nitro-l-arginine methyl ester (L-NAME) (100-300 micromol/l) [a nitric oxide (NO) synthase inhibitor] partially inhibited the relaxation response in diabetic (25%) and control (15%) rats. However, l-NAME (100 micromol/l) and glipizide or butanedione monoxime (1 micromol/l) (ATP-sensitive K channel blockers) together completely blocked the relaxation response. [H]Ang II saturation binding at the AT2 receptor was enhanced in aortic membranes from diabetic [maximum binding capacity, (Bmax)=1.14 +/- 0.06 fmol/mg protein] compared to control rats (Bmax=0.75 +/- 0.03 fmol/mg protein), with no change in the dissociation equilibrium constant (Kd) value (2.55 +/- 0.12 versus 2.22 +/- 0.15 nmol/l).

CONCLUSIONS

The results suggest enhanced AT2-receptor density and function [mediated by a nitric oxide and ATP-sensitive K channel-dependent relaxation response (in presence of an AT1 receptor blocker)] in thoracic aorta isolated from diabetic rats. This could be a compensatory mechanism, which may be therapeutically exploited.

Low-affinity neurotensin receptor (NTS2) signaling: internalization-dependent activation of extracellular signal-regulated kinases 1/2

The role and signaling properties of the low-affinity neurotensin receptor (NTS2) are still controversial. In particular, it is unclear whether neurotensin acts as an agonist, inverse agonist, or antagonist at this site. In view of the growing evidence for a role of NTS2 in antinociception, the elucidation of the pharmacological and coupling properties of this receptor is particularly critical. In the present study, we demonstrate that in Chinese hamster ovary (CHO) cells expressing the rat NTS2 receptor, neurotensin (NT), levocabastine, neuromedin N, and the high-affinity NT receptor antagonist SR48692 [2-[[1-(-7-chloroquinolin-4-yl)-5-(2,6-dimethoxyphenyl)-1H-pyrazole-3-carbonyl]amino]adamantane-2-carboxylic acid] all bind to and activate the NTS2 receptor. This activation is followed by ligand-induced internalization of receptor-ligand complexes, as evidenced by confocal microscopy using a fluorescent NT analog. All compounds tested produced a rapid and sustained activation of extracellular signal-regulated kinases 1/2 (ERK1/2) but were without specific effect on Ca(2+) mobilization. The agonist-induced activation of ERK1/2 was completely abolished by preincubation of the cells with the endocytosis inhibitors phenylarsine oxide and monodansylcadaverine as well as overexpression of a dominant-negative mutant of dynamin 1 (DynK44A), indicating that receptor internalization was required for ERK1/2 activation. NTS2-induced activation of ERK1/2 was not species-specific, because the same agonistic effects of NT and analogs were observed in CHO cells transfected with the human NTS2 receptor. In conclusion, this study demonstrates that NTS2 is a bona fide NT receptor and that activation of this receptor by NT or NT analogs results in an internalization-dependent activation of the ERK1/2 signaling cascade.

Angiotensin II increases differentiation of dopaminergic neurons from mesencephalic precursors via angiotensin type 2 receptors

In addition to the well-known actions of the humoral renin-angiotensin system, all components of this system are present in many tissues, including the brain, and may play a major role in brain development and differentiation. We investigated the possible effects of angiotensin II on the generation of dopaminergic phenotype neurons from proliferating neurospheres of mesencephalic precursors. We observed immunoreactivity for both angiotensin type 1 and type 2 (AT(1) and AT(2)) receptors in the cell aggregates. Double immunolabeling studies revealed that both receptor types are located in neurons and astrocytes. Interestingly, neurons with a dopaminergic phenotype (i.e. tyrosine hydroxylase activity) showed double labeling for AT(1) and AT(2) receptors although the labeling for AT(2) was more intense. Treatment of the neurospheres with angiotensin II (100 nm) during the differentiation period induced a marked increase (about 400%) in the generation of dopaminergic neurons. This was not affected by treatment with the AT(1) antagonist ZD 7155 but was blocked by treatment with the AT(2) antagonist PD 123319. This suggests that AT(2) receptors mediate the stimulatory effect of angiotensin II on the generation of dopaminergic neurons. Apoptotic cell death studies and bromodeoxyuridine immunohistochemistry indicated that the increase in generation of dopaminergic neurons is not due to increased survival or proliferation of dopaminergic cells during treatment with angiotensin and suggested that angiotensin induces increased differentiation of mesencephalic precursors towards the dopaminergic phenotype. Manipulation of the renin-angiotensin system may be useful for increasing production of dopaminergic neurons for transplantation in Parkinson's disease.

cAMP-dependent protein kinase type I regulates ethanol-induced cAMP response element-mediated gene expression via activation of CREB-binding protein and inhibition of MAPK

We have shown that the two types of cAMP-dependent protein kinase (PKA) in NG108-15 cells differentially mediate forskolin- and ethanol-induced cAMP response element (CRE)-binding protein (CREB) phosphorylation and CRE-mediated gene transcription. Activated type II PKA is translocated into the nucleus where it phosphorylates CREB. By contrast, activated type I PKA does not translocate to the nucleus but is required for CRE-mediated gene transcription by inducing the activation of other transcription cofactors such as CREB-binding protein (CBP). We show here that CBP is required for forskolin- and ethanol-induced CRE-mediated gene expression. Forskolin- and ethanol-induced CBP phosphorylation, demonstrable at 10 min, persists up to 24 h. CBP phosphorylation requires type I PKA but not type II PKA. In NG108-15 cells, ethanol and forskolin activation of type I PKA also inhibits several components of the MAPK pathway including B-Raf kinase, ERK1/2, and p90RSK phosphorylation. As a result, unphosphorylated p90RSK no longer binds to nor inhibits CBP. Moreover, MEK inhibition by PD98059 induces a significant increase of CRE-mediated gene activation. Taken together, our findings suggest that inhibition of the MAPK pathway enhances cAMP-dependent gene activation during exposure of NG108-15 cells to ethanol. This mechanism appears to involve type I PKA-dependent phosphorylation of CBP and inhibition of MEK-dependent phosphorylation of p90RSK. Under these conditions p90RSK is no longer bound to CBP, thereby promoting CBP-dependent CREB-mediated gene expression.

Effects of ras and rap1 on electrical excitability of differentiated ng108-15 cells

Effects of two small G-proteins, Rap1 and Ras, on the sodium channel activity in NG108-15 cells were studied using sindbis virus-mediated gene transfer. When an activated Rap1A mutant (Rap1-12V, the activated mutant of Rap1 carrying glycine to valine substitution at codon 12) or a dominant-negative H-Ras mutant (Ras-17N, carrying serine to asparagine substitution at codon 17) was expressed in differentiated NG108-15 cells, the proportion of cells generating action potential decreased and the amplitudes of sodium current diminished. This effect was sensitive to an inhibitor of protein kinase A. The effects of a cyclic AMP (cAMP) analog (dibutyl cAMP) on sodium current in these cells were biphasic: inhibitory at lower concentrations (<100 microM) and enhancing at higher concentrations (200-500 microM). The inhibitory phase of cAMP effect was suppressed by an activated Ras mutant (Ras-12V) while the enhancing phase was suppressed by Rap1-12V. These data are consistent with the model that Rap1 and Ras function as counteracting regulators of voltage-gated sodium current through cAMP-dependent mechanisms.

Dual role of cyclic AMP-dependent protein kinase in neuritogenesis and synaptogenesis during neuronal differentiation

To create precise neural circuits in the nervous system, neuritogenesis and synaptogenesis are the critical cellular processes during neuronal differentiation. We examined the cyclic AMP (cAMP)-responsible signaling pathways for regulating neuritogenesis and synaptogenesis in NG108-15 cells. A rise in intracellular cAMP concentration by a membrane-permeable cAMP analog, dibutyryl cAMP (DBcAMP), led to an increase in the number of neurites and varicosities. Inhibition of cAMP-dependent protein kinase (PKA) activity by a PKA inhibitor (H89) accelerated this neuritogenesis and neurite outgrowth rate. Treatment with H89, however, decreased the number of varicosities and the frequency of postsynaptic miniature current recorded in the cultured cells, resulting in suppression of synaptogenesis. Immunoblot analyses revealed that PKA activity mediates phosphorylation of a gene transcription factor, cAMP-response element binding protein (CREB). On the other hand, inhibition of a mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) pathway by a MAPK/ERK kinase (MEK) inhibitor (PD98059) suppressed both neuritogenesis and neurite outgrowth without CREB phosphorylation. These results suggest strongly that PKA simultaneously plays two different roles in neuronal differentiation: inhibition of neuritogenesis and stimulation of synaptogenesis, via CREB-mediated gene expression.

sst2 Somatostatin receptor inhibits cell proliferation through Ras-, Rap1-, and B-Raf-dependent ERK2 activation

The G protein-coupled sst2 somatostatin receptor is a critical negative regulator of cell proliferation. sstII prevents growth factor-induced cell proliferation through activation of the tyrosine phosphatase SHP-1 leading to induction of the cyclin-dependent kinase inhibitor p27Kip1. Here, we investigate the signaling molecules linking sst2 to p27Kip1. In Chinese hamster ovary-DG-44 cells stably expressing sst2 (CHO/sst2), the somatostatin analogue RC-160 transiently stimulates ERK2 activity and potentiates insulin-stimulated ERK2 activity. RC-160 also stimulates ERK2 activity in pancreatic acini isolated from normal mice, which endogenously express sst2, but has no effect in pancreatic acini derived from sst2 knock-out mice. RC-160-induced p27Kip1 up-regulation and inhibition of insulin-dependent cell proliferation are both prevented by pretreatment of CHO/sst2 cells with the MEK1/2 inhibitor PD98059. In addition, using dominant negative mutants, we show that sst2-mediated ERK2 stimulation is dependent on the pertussis toxin-sensitive Gi/o protein, the tyrosine kinase Src, both small G proteins Ras and Rap1, and the MEK kinase B-Raf but is independent of Raf-1. Phosphatidylinositol 3-kinase (PI3K) and both tyrosine phosphatases, SHP-1 and SHP-2, are required upstream of Ras and Rap1. Taken together, our results identify a novel mechanism whereby a Gi/o protein-coupled receptor inhibits cell proliferation by stimulating ERK signaling via a SHP-1-SHP-2-PI3K/Ras-Rap1/B-Raf/MEK pathway.

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.

Does Rap1 deserve a bad Rap?

The Ras superfamily of small G proteins is remarkable for both its diversity and physiological functions. One member, Rap1, has been implicated in a particularly wide range of biological processes, from cell proliferation and differentiation to cell adhesion. But the diversity of Rap1 has lead to contradictory reports of its effects. Originally identified as an antagonist of Ras-induced transformation, Rap1 can oppose other actions of Ras including regulation of cell growth and differentiation, integrin-dependent responses and synaptic plasticity. Furthermore, recent evidence confirms that Rap1, like Ras, can activate the MAP kinase cascade (ERK) in several cell types. These diverse functions of Rap1 underscore that the activation and action of Rap1 are regulated by complex factors that are cell-type specific.