Angiotensin II induces tyrosine phosphorylation of insulin receptor substrate 1 and its association with phosphatidylinositol 3-kinase in rat heart

Modulation of the action of insulin by angiotensin-(1-7)

The prevalence of Type 2 diabetes mellitus is predicted to increase dramatically over the coming years and the clinical implications and healthcare costs from this disease are overwhelming. In many cases, this pathological condition is linked to a cluster of metabolic disorders, such as obesity, systemic hypertension and dyslipidaemia, defined as the metabolic syndrome. Insulin resistance has been proposed as the key mediator of all of these features and contributes to the associated high cardiovascular morbidity and mortality. Although the molecular mechanisms behind insulin resistance are not completely understood, a negative cross-talk between AngII (angiotensin II) and the insulin signalling pathway has been the focus of great interest in the last decade. Indeed, substantial evidence has shown that anti-hypertensive drugs that block the RAS (renin-angiotensin system) may also act to prevent diabetes. Despite its long history, new components within the RAS continue to be discovered. Among them, Ang-(1-7) [angiotensin-(1-7)] has gained special attention as a counter-regulatory hormone opposing many of the AngII-related deleterious effects. Specifically, we and others have demonstrated that Ang-(1-7) improves the action of insulin and opposes the negative effect that AngII exerts at this level. In the present review, we provide evidence showing that insulin and Ang-(1-7) share a common intracellular signalling pathway. We also address the molecular mechanisms behind the beneficial effects of Ang-(1-7) on AngII-mediated insulin resistance. Finally, we discuss potential therapeutic approaches leading to modulation of the ACE2 (angiotensin-converting enzyme 2)/Ang-(1-7)/Mas receptor axis as a very attractive strategy in the therapy of the metabolic syndrome and diabetes-associated diseases.

Phosphorylation of myocardial eNOS is altered in patients suffering from type 2 diabetes

The present study investigated whether endothelial nitric oxide synthase (eNOS) activation may be dysregulated in cardiac tissue of patients suffering from type 2 diabetes (T2D). We performed immunohistochemical measurements of translocated eNOS activation as well as eNOS phosphorylation at Ser1177, Thr495, Ser 635, Ser114, and of the protein kinase B (Akt) in isolated right atrial trabeculae of patients undergoing cardiac bypass or valve surgery with (n = 12, 68.1 ± 2.5 yr) and without T2D (n = 12, 64.7 ± 2.7 yr). In addition, we investigated oxidative (8-isoprostane) and nitrosative stress markers (nitrotyrosine) as well as the effect of pharmacological stimulation of angiotensin (AT)-receptors on eNOS-phosphorylation. Translocation-dependent eNOS activation was similar in both groups. The same holds true for eNOS phosphorylation at Ser114. eNOS phosphorylation at Ser635 was significantly increased, whereas eNOS phosphorylation of Ser1177 was significantly decreased in the diabetic group paralleled by a decrease in phosphorylation of Akt and Thr495. These alterations were accompanied by a significant decrease in nitrotyrosine. After application of angiotensin II (10 μM, 2 min) for investigation of the AT-receptor-dependent eNOS stimulation, we did not find differences between the increases in eNOS Ser1177-phosphorylation in the nondiabetic (+39.7 ± 23.5%) and in the diabetic group (32.22 ± 11.45%). A simultaneous increase in Akt phosphorylation could not be observed. The present study indicates that T2D goes along with a decrease in eNOS phosphorylation at Ser1177 under basal conditions in cardiac tissue. Whether this may be attributed to the insulin resistance of cardiac muscle has to be further investigated. Receptor-stimulated eNOS activation still works at least for angiotensin II-dependent eNOS activation.

[Metabolic syndrome in children and adolescents: doubts about terminology but not about cardiometabolic risks]

Metabolic syndrome (MS) has been a condition involved in considerable controversy, starting with the terminology. Gerald Reaven himself, the author who proposed the term MS, advised against the use of this terminology because the definition implies in at least three metabolic alterations, and it is never clear to which group of alterations we are referring to when we say that a patient has MS. In children, the problem is even more complicated, since there are many different adaptations to the criteria used in adults. On the other hand, independent of the terminology, cardiovascular risks are well-established and it is very clear that even children may present metabolic disturbances which predict future metabolic problems. The role of the pediatric endocrinologist or the general pediatrician is to investigate, especially in overweight/obese children, conditions that if treated early, may prevent future complications that today, unfortunately, are being diagnosed only in adult life. In this review, we discuss problems on the definition, epidemiology, pathophysiology, and complications of MS in children and adolescents.

Arctium lappa ameliorates endothelial dysfunction in rats fed with high fat/cholesterol diets

Background

Arctium lappa L. (Asteraceae), burdock, is a medicinal plant that is popularly used for treating hypertension, gout, hepatitis, and other inflammatory disorders. This study was performed to test the effect of ethanol extract of Arctium lappa L. (EAL) seeds on vascular reactivity and inflammatory factors in rats fed a high fat/cholesterol diet (HFCD).

Method

EAL-I (100 mg·kg⁻¹/day), EAL-II (200 mg·kg⁻¹/day), and fluvastatin (3 mg·kg⁻¹/day) groups initially received HFCD alone for 8 weeks, with EAL supplementation provided during the final 6 weeks.

Results

Treatment with low or high doses of EAL markedly attenuated plasma levels of triglycerides and augmented plasma levels of high-density lipoprotein (HDL) in HFCD-fed rats. Chronic treatment with EAL markedly reduced impairments of acetylcholine (ACh)-induced relaxation of aortic rings. Furthermore, chronic treatment with EAL significantly lowered systolic blood pressure (SBP) and maintained smooth and flexible intimal endothelial layers in HFCD-fed rats. Chronic treatment with EAL suppressed upregulation of intercellular adhesion molecule (ICAM)-1, vascular cell adhesion molecule (VCAM)-1, and E-selectin in the aorta. Chronic treatment with EAL also suppressed increases in matrix metalloproteinase (MMP)-2 expression. These results suggested that EAL can inhibit HFCD-induced vascular inflammation in the rat model.

Conclusion

The present study provides evidence that EAL ameliorates HFCD-induced vascular dysfunction through protection of vascular relaxation and suppression of vascular inflammation.

Cynanchum wilfordii ameliorates hypertension and endothelial dysfunction in rats fed with high fat/cholesterol diets

Hypercholesterolemia increases the incidence of atherosclerosis and its pathologic complications. This study was performed to test the effect of an ethanol extract of Cynanchum wilfordii (ECW) on vascular dysfunction in rats fed with high fat/cholesterol diets (HFCD). Male rats were fed a HFCD consisting of 7.5% cocoa butter and 1.25% cholesterol, with or without 100, 200 mg/day/kg ECW. Rats fed with HFCD increased body weight associated with an increase in plasma low-density lipoprotein (LDL) cholesterol level. Chronic ECW treatment in HFCD-fed rats lessened LDL cholesterol and triglyceride levels as well as elevated high-density lipoprotein (HDL) cholesterol. Chronic ECW treatment recovered the HFCD-induced increase in systolic blood pressure, maintained smooth and soft intima endothelial layers by the decrease of intima-media thickness. ECW significantly recovered the diet-induced decrease in vasorelaxation to acetylcholine, high-dose ECW apparently increased vasorelaxation response to sodium nitroprusside in rats fed with HFCD. ECW clearly restored the HFCD-induced reduction in endothelial nitric oxide (NO) synthase expression and Akt expression levels in aortic tissue, leading to improve endothelial function through an increase in endothelium-derived NO production. Furthermore, treatment of ECW significantly recovered the HFCD-induced decrease in aortic cGMP levels in rats. These findings suggest that ECW ameliorates hypertension and endothelial dysfunction via improvement of NO/cGMP signaling pathway in aortic tissue of rats fed with HFCD, suggesting a vascular protective role for this herb in the treatment and prevention of atherosclerotic vascular disease.

Remodeling of atrial ATP-sensitive K⁺ channels in a model of salt-induced elevated blood pressure

Hypertension is associated with the development of atrial fibrillation; however, the electrophysiological consequences of this condition remain poorly understood. ATP-sensitive K(+) (K(ATP)) channels, which contribute to ventricular arrhythmias, are also expressed in the atria. We hypothesized that salt-induced elevated blood pressure (BP) leads to atrial K(ATP) channel activation and increased arrhythmia inducibility. Elevated BP was induced in mice with a high-salt diet (HS) for 4 wk. High-resolution optical mapping was used to measure atrial arrhythmia inducibility, effective refractory period (ERP), and action potential duration at 90% repolarization (APD(90)). Excised patch clamping was performed to quantify K(ATP) channel properties and density. K(ATP) channel protein expression was also evaluated. Atrial arrhythmia inducibility was 22% higher in HS hearts compared with control hearts. ERP and APD(90) were significantly shorter in the right atrial appendage and left atrial appendage of HS hearts compared with control hearts. Perfusion with 1 μM glibenclamide or 300 μM tolbutamide significantly decreased arrhythmia inducibility and prolonged APD(90) in HS hearts compared with untreated HS hearts. K(ATP) channel density was 156% higher in myocytes isolated from HS animals compared with control animals. Sulfonylurea receptor 1 protein expression was increased in the left atrial appendage and right atrial appendage of HS animals (415% and 372% of NS animals, respectively). In conclusion, K(ATP) channel activation provides a mechanistic link between salt-induced elevated BP and increased atrial arrhythmia inducibility. The findings of this study have important implications for the treatment and prevention of atrial arrhythmias in the setting of hypertensive heart disease and may lead to new therapeutic approaches.

Renin-angiotensin-aldosterone blockade for cardiovascular disease prevention

The renin-angiotensin-aldosterone system (RAAS) plays a significant role in pathophysiology of multiple disease states. RAAS blockade is beneficial in patients with hypertension, acute myocardial infarction, chronic heart failure, stroke, and diabetic renal disease. RAAS blockade with the combination angiotensin-converting enzyme inhibitors (ACEIs) and angiotensin receptor blockers (ARBs) has demonstrated conflicting results in recent clinical trials. This article reviews the latest evidence of isolated ACEI or ARB use, their combination, and the role of aldosterone blockers and direct renin inhibitors in patients at risk, and makes recommendations for their use in the prevention of morbidity and mortality in cardiovascular disease.

Forkhead class O transcription factor 3a activation and Sirtuin1 overexpression in the hypertrophied myocardium of the diabetic Goto-Kakizaki rat

BACKGROUND

Ventricular remodeling in type 2 diabetes predisposes to fatal coronary heart disease. The proapoptotic forkhead class O transcription factor 3a (FOXO3a) and its modulator, the cardioprotective longevity factor and class III histone deacetylase Sirtuin1 (Sirt1), have been implicated in the regulation of the cardiomyocyte lifespan and hypertrophy.

OBJECTIVE

To examine whether FOXO3a-Sirt1 activation is involved in diabetes-induced cardiomyocyte apoptosis and ventricular hypertrophy.

METHODS

The blood pressure, cardiac functions, cardiomyocyte size, neurohumoral markers, cardiomyocyte apoptosis, nuclear binding of FOXO3a, and Sirt1 expression were determined for 12-week-old spontaneously diabetic Goto-Kakizaki rats and the nondiabetic Wistar control rats.

RESULTS

Goto-Kakizaki rats showed a modest increase in blood pressure, pronounced cardiac hypertrophy, impaired systolic function, and increased plasma brain natriuretic peptide level without changes in plasma renin activity, serum aldosterone or urinary noradrenaline excretion. The cardiomyocyte cross-sectional area was increased by 22%. Phosphorylation of FOXO3a was decreased with a concomitant increase in its nuclear translocation. The myocardial expression of the antiapoptotic FOXO3a modulator Sirt1 was increased two-fold. Acetylation of p53 at the Sirt1-specific lysine 373/382 site was markedly decreased. Myocardial caspase-3 and Bax expression were increased, indicating increased apoptotic signaling; however, terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling staining did not reveal any significant increase in cardiomyocyte apoptosis.

CONCLUSIONS

Diabetes-induced cardiac remodeling in Goto-Kakizaki rats is associated with cardiac hypertrophy, systolic dysfunction, increased apoptotic signaling and activation of the FOXO3a pathway. The present study also suggests that antiapoptotic Sirt1 protects against cardiomyocyte apoptosis and acts as a novel regulator of cardiomyocyte growth.

Angiotensin-(1–7) stimulates the phosphorylation of JAK2, IRS-1 and Akt in rat heart in vivo: role of the AT1 and Mas receptors

Angiotensin (ANG) II exerts a negative modulation on insulin signal transduction that might be involved in the pathogenesis of hypertension and insulin resistance. ANG-(1–7), an endogenous heptapeptide hormone formed by cleavage of ANG I and ANG II, counteracts many actions of ANG II. In the current study, we have explored the role of ANG-(1–7) in the signaling crosstalk that exists between ANG II and insulin. We demonstrated that ANG-(1–7) stimulates the phosphorylation of Janus kinase 2 (JAK2) and insulin receptor substrate (IRS)-1 in rat heart in vivo. This stimulating effect was blocked by administration of the selective ANG type 1 (AT1) receptor blocker losartan. In contrast to ANG II, ANG-(1–7) stimulated cardiac Akt phosphorylation, and this stimulation was blunted in presence of the receptor Mas antagonist A-779 or the phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin. The specific JAK2 inhibitor AG-490 blocked ANG-(1–7)-induced JAK2 and IRS-1 phosphorylation but had no effect on ANG-(1–7)-induced phosphorylation of Akt, indicating that activation of cardiac Akt by ANG-(1–7) appears not to involve the recruitment of JAK2 but proceeds through the receptor Mas and involves PI3K. Acute in vivo insulin-induced cardiac Akt phosphorylation was inhibited by ANG II. Interestingly, coadministration of insulin with an equimolar mixture of ANG II and ANG-(1–7) reverted this inhibitory effect. On the basis of our present results, we postulate that ANG-(1–7) could be a positive physiological contributor to the actions of insulin in heart and that the balance between ANG II and ANG-(1–7) could be relevant for the association among insulin resistance, hypertension, and cardiovascular disease.

Defective insulin and acetylcholine induction of endothelial cell-nitric oxide synthase through insulin receptor substrate/Akt signaling pathway in aorta of obese rats

The actions of acetylcholine (ACh) on endothelium mainly are mediated through muscarinic receptors, which are members of the G protein-coupled receptor family. In the present study, we show that ACh induces rapid tyrosine phosphorylation and activation of Janus kinase 2 (JAK2) in rat aorta. Upon JAK2 activation, tyrosine phosphorylation of insulin receptor substrate (IRS)-1 is detected. In addition, ACh induces JAK2/IRS-1 and IRS-1/phosphatidylinositol (PI) 3-kinase associations, downstream activation of Akt/protein kinase B, endothelial cell-nitric oxide synthase (eNOS), and extracellular signal-regulated kinase (ERK)-1/2. The pharmacological blockade of JAK2 or PI 3-kinase reduced ACh-stimulated eNOS phosphorylation, NOS activity, and aorta relaxation. These data indicate a new signal transduction pathway for IRS-1/PI 3-kinase/Akt/eNOS activation and ERK1/2 by means of JAK2 tyrosine phosphorylation stimulated by ACh in vessels. Moreover, we demonstrate that in aorta of obese rats (high-fat diet), there is an impairment in the insulin- and ACh-stimulated IRS-1/PI 3-kinase pathway, leading to reduced activation with lower protein levels of eNOS associated with a hyperactivated ERK/mitogen-activated protein kinase pathway. These results suggest that in aorta of obese rats, there not only is insulin resistance but also ACh resistance, probably mediated by a common signaling pathway that controls the activity and the protein levels of eNOS.

[Insulin and angiotensin II signaling pathways cross-talk: implications with the association between diabetes mellitus, arterial hypertension and cardiovascular disease]

Insulin (Ins) and angiotensin II (AII) play pivotal roles in the control of two vital and closely related systems: the metabolic and the circulatory, respectively. A failure in the proper action of each of these hormones results, to a variable degree, in the development of two highly prevalent and commonly overlapping diseases--diabetes mellitus (DM) and hypertension (AH). In recent years, a series of studies has revealed a tight connection between the signal transduction pathways that mediate Ins and AII actions in target tissues. This molecular cross-talk occurs at multiple levels and plays an important role in phenomena that range from the action of anti-hypertensive drugs to cardiac hypertrophy and energy acquisition by the heart. At the extracellular level, the angiotensin-converting enzyme controls AII synthesis but also interferes with Ins signaling through the proper regulation of AII and the accumulation of bradykinin. At an early intracellular level, AII, acting through JAK-2/IRS-1/PI3-kinase, JNK and ERK, may induce the serine phosphorylation and inhibition of key elements of the Ins-signaling pathway. Finally, by inducing the expression of the regulatory protein SOCS-3, AII may impose a late control on the Ins signal. This review will focus on the main advances obtained in this field and will discuss the implications of this molecular cross-talk in the common clinical association between DM and AH.

The multi-faceted cross-talk between the insulin and angiotensin II signaling systems

Insulin and angiotensin II are hormones that play pivotal roles in the control of two vital and closely related systems, the metabolic and the circulatory systems, respectively. A failure in the proper action of each of these hormones results, to a variable degree, in the development of two highly prevalent and commonly overlapping diseases-diabetes mellitus and hypertension. In recent years, a series of studies has revealed a tight connection between the signal transduction pathways that mediate insulin and angiotensin II actions in target tissues. This molecular cross-talk occurs at multiple levels and plays an important role in phenomena that range from the action of anti-hypertensive drugs to cardiac hypertrophy and energy acquisition by the heart. At the extracellular level, the angiotensin-converting enzyme controls angiotensin II synthesis but also interferes with insulin signaling through the proper regulation of angiotensin II and through the accumulation of bradykinin. At an early intracellular level, angiotensin II, acting through JAK-2/IRS-1/PI3-kinase, JNK and ERK, may induce the serine phosphorylation and inhibition of key elements of the insulin-signaling pathway. Finally, by inducing the expression of the regulatory protein SOCS-3, angiotensin II may impose a late control on the insulin signal. This review will focus on the main advances obtained in this field and will discuss the implications of this molecular cross-talk in the common clinical association between diabetes mellitus and hypertension.

Proteomic Identification of Insulin-like Growth Factor-binding Protein-6 Induced by Sublethal H2O2 Stress from Human Diploid Fibroblasts

Fibroblasts are the most ubiquitous cell types within our body. They produce various factors to maintain the texture and structure of a particular organ or tissue. To identify protein factors secreted by fibroblasts and alteration of these protein factors upon oxidative stress, HCA3 human skin diploid fibroblasts were exposed to a sublethal dose of H2O2, which induces a prematurely senescent phenotype. Conditioned media from prematurely senescent cells versus control cells were analyzed for proteins using an LC-MS/MS-based proteomic technique. Collagen alpha1(VI), collagen alpha2(I), fibronectin, lumican, and matrix metalloproteinase 2 were among the proteins consistently detected from control and H2O2-treated cells. Insulin-like growth factor-binding protein-6 (IGFBP-6) consistently showed up in the conditioned medium of H2O2-treated cells but not from untreated cells. Increased IGFBP-6 production due to H2O2 treatment was confirmed by RT-PCR and Western blot analyses. While H2O2 induced a dose-dependent elevation of IGFBP-6 mRNA, Western blot analyses detected elevated levels of IGFBP-6 protein in the conditioned medium of H2O2-treated cells. In comparison, fibronectin or matrix metalloproteinase 2 did not show changes at the mRNA level in cell lysates or at the protein level in the conditioned medium by H2O2 treatment. Using several types of toxins at sublethal doses, including cis-platin, hydroxyurea, colchicine, L-mimosine, rhodamine, dithiothreitol, or N-ethylmaleimide, we found that these agents induced increases of IGFBP-6 at mRNA and protein levels. An increased level of IGFBP-6 protein was detected in the plasma of aging mice and of young mice treated with doxorubicin. These data suggest that IGFBP-6 may serve as a sensitive biomarker of cell degeneration or injury in vitro and in vivo.

The renin angiotensin system as a therapeutic target to prevent diabetes and its complications

The role of the RAAS in development and maintenance of blood pressure is well established. In addition, the deleterious effects of angiotensin II on the heart, vasculature, and kidneys have been clearly defined. There seems to be a close relationship between endothelial dysfunction, insulin resistance (a precursor to diabetes and coronary artery disease) and angiotensin II. The signaling pathways for insulin in the vascular wall interacts with the angiotensin signaling, giving rise to potential mechanisms for development of diabetes and resulting harmful effects. A large number of clinical trials using ACE inhibitors or ARBs have shown significant reduction in secondary endpoints in the development of new onset of diabetes. Ongoing prospective studies involving ARBs (eg, the Nateglinide and Valsartan Impaired Glucose Tolerance Outcomes Research trial) and ACE inhibitors (eg, the Diabetes Re-duction Assessment with Ramipril and Rosiglita-zone Medication trial) are testing the ability of certain agents to prevent type 2 diabetes. In the meantime, it is important to recognize insulin resistance and metabolic syndrome as entities that increase the risk for cardiovascular disease. In addition to lifestyle modifications, managing endothelial dysfunction and protecting the vasculature will help prevent diabetes and cardiovascular disease.

The FOXO3a transcription factor regulates cardiac myocyte size downstream of AKT signaling

Although signaling mechanisms inducing cardiac hypertrophy have been extensively studied, little is known about the mechanisms that reverse cardiac hypertrophy. Here, we describe the existence of a similar Akt/forkhead signaling axis in cardiac myocytes in vitro and in vivo, which is regulated by insulin, insulin-like growth factor (IGF), stretch, pressure overload, and angiotensin II stimulation. FOXO3a gene transfer prevented both IGF and stretch-induced hypertrophy in rat neonatal cardiac myocyte cultures in vitro. Transduction with FOXO3a also caused a significant reduction in cardiomyocyte size in mouse hearts in vivo. Akt/FOXO signaling regulated the expression of multiple atrophy-related genes "atrogenes," including the ubiquitin ligase atrogin-1 (MAFbx). In cardiac myocyte cultures, transduction with constitutively active Akt or treatment with IGF suppressed atrogin-1 mRNA expression, whereas transduction with FOXO3a stimulated its expression. FOXO3a transduction activated the atrogin-1 promoter in both cultured myocytes and mouse heart. Thus, in cardiomyocytes, as in skeletal muscle, FOXO3a activates an atrogene transcriptional program, which retards or prevents hypertrophy and is down-regulated by multiple physiological and pathological stimuli of myocyte growth.

Suppressor of cytokine signaling-3 Provides a novel interface in the cross-talk between angiotensin II and insulin signaling systems

Angiotensin II inhibits insulin-induced activation of phosphatidylinositol 3-kinase through a mechanism, at least in part, dependent on serine phosphorylation of the insulin receptor and insulin receptor substrates (IRS)-1/2. Recent evidence shows that suppressor of cytokine signaling-3 (SOCS-3) is induced by insulin and angiotensin II and participates in the negative control of further stimulation of each of these signaling systems independently. In the present study, we evaluated the interaction of angiotensin II-induced SOCS-3 with the insulin signaling pathway in the heart of living rats. A single iv dose of angiotensin II promotes a significant increase of SOCS-3 in heart, an effect that lasts up to 180 min. Once induced, SOCS-3 interacts with the insulin receptor, JAK-2, IRS-1, and IRS-2. The inhibition of SOCS-3 expression by a phosphorthioate-modified antisense oligonucleotide partially restores angiotensin II-induced inhibition of insulin-induced insulin receptor, IRS-1 and IRS-2 tyrosine phosphorylation, and IRS-1 and IRS-2 association with p85-phosphatidylinositol 3-kinase and [Ser473] phosphorylation of Akt. Moreover, the inhibition of SOCS-3 expression partially reverses angiotensin II-induced inhibition of insulin-stimulated glucose transporter-4 translocation to the cell membrane. These results are reproduced in isolated cardiomyocytes. Thus, SOCS-3 participates, as a late event, in the negative cross-talk between angiotensin II and insulin, producing an inhibitory effect on insulin-induced glucose transporter-4 translocation.

Angiotensin II regulation of the Na+ pump involves the phosphatidylinositol-3 kinase and p42/44 mitogen-activated protein kinase signaling pathways in vascular smooth muscle cells

This investigation used primary cultured rat vascular smooth muscle cells to examine angiotensin II (Ang II) regulation of Na(+), K(+)-ATPase (Na(+) pump) activity, and Na(+) pump alpha(1)- and beta(1)-subunit gene transcription. This regulation was mediated through both phosphatidylinositol-3 kinase (PI3K) and p42/44 mitogen-activated protein kinase (p42/44(MAPK)) signaling pathways. Both acute (10 min) and prolonged (24 h) treatment with Ang II stimulated Na(+) pump activity. Also, prolonged exposure to Ang II (24 h) increased promoter transcription of the Na(+) pump alpha(1)- and beta(1)-subunits. Furthermore, PI3K activities because well because p42/44(MAPK) phosphorylation were increased within 10 min after Ang II treatment. To determine whether these stimulatory activities of Ang II are acting through Ang II receptors 1 and/or 2 (AT(1), AT(2)), cells were pretreated with either AT(1) receptor blocker losartan or the AT(2) receptor blocker PD 123,319. Indeed, these treatments prevented the stimulatory effect of Ang II on Na(+) pump activity at both acute and 24-h time points. Furthermore, the Ang II-stimulated alpha(1)-subunit promoter transcription was inhibited by losartan but not by the AT(2) receptor blocker. These results indicate that Ang II acts through both the AT(1) and AT(2) receptor to up-regulate Na(+) pump activity; however, Ang II regulates alpha(1)-gene transcription through AT(1) but not AT(2) receptors. It was also observed that the Ang II-stimulated beta(1)-subunit gene transcription is not mediated through either AT(1) or AT(2) receptors. To examine whether the Na(+)/H(+) exchanger is involved in Ang II-stimulated Na(+) pump activity, cells were pretreated with amiloride, a specific inhibitor of the Na(+)/H(+) exchanger. This pretreatment prevented 24 h, but not acute, Ang II-stimulated Na(+) pump activity. The 24-h Ang II-stimulated alpha(1)-subunit promoter transcription was also inhibited by amiloride. This suggests that the prolonged effect of Ang II on Na(+) pump activity is dependent on increased Na(+)/H(+) exchange. Because Ang II treatment for 10 min increased PI3K activity because well because p42/44(MAPK) phosphorylation, studies were performed to determine the involvement of PI3K and p42/44(MAPK) signaling pathways in both Ang II-stimulated Na(+) pump activity and alpha(1)- and beta(1)-gene transcription. Cells were pretreated with either the PI3K inhibitor wortmannin or the p42/44(MAPK) inhibitor PD 98059. Ang II-stimulated PI3K or p42/44(MAPK) activity was inhibited by these pretreatments. Furthermore, pretreatment of cells with the PI3K inhibitors wortmannin and LY29404 or the MAPK inhibitors U0126 and PD 98059 were all observed to inhibit Ang II-stimulated Na(+) pump activity. To more specifically determine the role of PI3K in Ang II-regulation of alpha(1)-and beta(1)-gene transcription, cells were cotransfected with a dominant-negative p85 construct. Cotransfection with dominant-negative p85 reduced Ang II-stimulated alpha(1)-but not beta(1)-gene transcription in vascular smooth muscle cells. These results indicate that Ang II acts through PI3K/p42/44(MAPK) signaling pathways to up-regulate Na(+) pump activity and alpha(1)-gene transcription and that Ang II-regulated beta(1)-gene transcription is not mediated through either PI3K or p42/44 (MAPK) signaling pathways.

The cross-talk between angiotensin and insulin differentially affects phosphatidylinositol 3-kinase- and mitogen-activated protein kinase-mediated signaling in rat heart: implications for insulin resistance

Insulin and angiotensin II (AngII) may act through overlapping intracellular pathways to promote cardiac myocyte growth. In this report insulin and AngII signaling, through the phosphatidylinositol 3-kinase (PI 3-kinase) and MAPK pathways, were compared in cardiac tissues of control and obese Zucker rats. AngII induced Janus kinase 2 tyrosine phosphorylation and coimmunoprecipitation with insulin receptor substrate 1 (IRS-1) and IRS-2 as well as an increase in tyrosine phosphorylation of IRS and its association with growth factor receptor-binding protein 2. Simultaneous treatment with both hormones led to marked increases in the associations of IRS-1 and -2 with growth factor receptor-binding protein 2 and in the dual phosphorylation of ERK1/2 compared with the administration of AngII or insulin alone. In contrast, an acute inhibition of both basal and insulin-stimulated PI 3-kinase activity was induced by both hormones. Insulin stimulated the phosphorylation of MAPK equally in lean and obese rats. Conversely, insulin-induced phosphorylation of Akt in heart was decreased in obese rats. Pretreatment with losartan did not change insulin-induced activation of ERK1/2 and attenuated the reduction of Akt phosphorylation in the heart of obese rats. Thus, the imbalance between PI 3-kinase-Akt and MAPK signaling pathways in the heart may play a role in the development of cardiovascular abnormalities observed in insulin-resistant states, such as in obese Zucker rats.

Suppressor of cytokine signaling 3 is induced by angiotensin II in heart and isolated cardiomyocytes, and participates in desensitization

Angiotensin II (Ang II) exerts a potent growth stimulus on the heart and vascular wall. Activation of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) intracellular signaling pathway by Ang II mediates at least some of the mitogenic responses to this hormone. In other signaling systems that use the JAK/STAT pathway, proteins of the suppressor of cytokine signaling (SOCS) family participate in signal regulation. In the present study it is demonstrated that SOCS3 is constitutively expressed at a low level in rat heart and neonatal rat ventricular myocytes. Ang II at a physiological concentration enhances the expression of SOCS3 mRNA and protein, mainly via AT1 receptors. After induction, SOCS3 associates with JAK2 and impairs further activation of the JAK2/STAT1 pathway. Pretreatment of rats with a specific phosphorthioate antisense oligonucleotide to SOCS3, reverses the desensitization to angiotensin signaling, as detected by a fall in c-Jun expression after repetitive infusions of the hormone. Thus, SOCS3 is induced by Ang II in rat heart and neonatal rat ventricular myocytes and participates in the modulation of the signal generated by this hormone.

Role of phosphatidylinositol 3-kinase/Akt pathway in angiotensin II and insulin-like growth factor-1 modulation of nitric oxide synthase in vascular smooth muscle cells

The aim of this study was to examine the role of the phosphatidylinositol 3-kinase (PI3K)/serine/threonine kinase Akt signaling pathway in mediating interactions between angiotensin II (Ang II) and insulin-like growth factor-1 (IGF-1) in regulation of inducible nitric oxide synthase (iNOS) in vascular smooth muscle cells (VSMCs). Exposure to 100 nM IGF-1 for 10 min resulted in increased insulin-receptor substrate-1 associated PI3K activity and Akt kinase activity, whereas 100 nM Ang II pretreatment for 5 min strikingly decreased these IGF-1 effects. NOS activity was also increased in VSMCs following exposure to IGF-1 (10 min up to 24 h). Pretreatment with Ang II for 5 min reduced IGF-1-induced NOS activity. IGF-1 treatment for 24 hr increased iNOS gene transcription, and Ang II pretreatment reduced this stimulation of iNOS gene expression by attenuating PI3K/Akt signaling. These results implicate PI3K/ Akt pathways in Ang II/IGF-1 regulation of iNOS in VSMCs.

The role of the angiotensin system in cardiac glucose homeostasis: therapeutic implications

Resistance to the metabolic actions of insulin is thought to play a determining role in the aetiology of a great variety of disorders, including essential hypertension, accelerated atherosclerosis and cardiomyopathies. ACE inhibitors are recognised as being highly effective therapy for hypertension and cardiac insufficiency, and have a more beneficial effect on survival rate than expected on the basis of known mechanisms of action. The mechanism responsible for these extremely positive effects are just beginning to be understood and appear to be linked to the effects these drugs have on metabolism. The relationship between the insulin and angiotensin II (Ang II) signalling pathways needs to be fully clarified in order to prevent or correct the target organ damage resulting from changes in the cross-talk of these two hormonal systems. In recent years, Ang II has been shown to play a central role in cardiovascular and neuroendocrine physiology as well as in cellular cycle control. Moreover, the fact that Ang II utilises the insulin-receptor substrate (IRS)-1 to relay signals towards their intracellular destination, provides the biochemical explanation of how these two systems interact in a healthy organism and in a diseased one. Since it is overactivity of the renin-angiotensin system that seems to impair the intracellular response to insulin signalling, cardiovascular drugs that modulate the cellular transmission of Ang II have attracted particular interest. As well as the already widely-used ACE inhibitors, selective blockers of the Ang II type 1 receptor (AT(1)) have been shown to be clinically effective in the control of haemodynamic parameters, but with perhaps a less striking effect on glucose homeostasis. Many trials have investigated the effect of Ang II blockade on systemic glucose homeostasis. The inhibition of Ang II by ACE-inhibitors frequently showed a positive effect on glycaemia and insulin sensitivity, while information on the effects of AT(1) receptor antagonists on glucose homeostasis is more limited and controversial. An important limitation of these studies has been the short treatment and follow-up periods, even for the 'so called' long-term studies which were only 6 months. Several investigators have focused on the effects of the nuclear factors involved in gene transcriptions, especially with respect to the agonists/antagonists of peroxisome proliferator-activated receptors (PPARs) and their intriguing interconnections with the insulin and Ang II subcellular pathways. In fact, in vitro and in vivo experimental studies have shown that thiazolidinediones (selective PPAR-gamma ligands) are not only powerful insulin sensitisers, but also have anti-hypertensive and anti-atherosclerotic properties. In addition to conventional pharmacological approaches, attempts have been made to use genetic transfer in the treatment of cardiovascular and metabolic disorders. The development of powerful viral vectors carrying target genes has allowed us to restore the expression/function of specific proteins involved in the cellular mechanism of insulin resistance, and research now needs to move beyond animal models. Although a clearer picture is now emerging of the pathophysiological interaction between insulin and Ang II, especially from pre-clinical studies, there is much to be done before experimental findings can be used in daily clinical practice.

Angiotensin II inhibits insulin-stimulated phosphorylation of eukaryotic initiation factor 4E-binding protein-1 in proximal tubular epithelial cells

Interaction between angiotensin II, which binds a G-protein-coupled receptor, and insulin, a ligand for receptor tyrosine kinase, was examined in renal proximal tubular epithelial cells. Augmented protein translation by insulin involves activation of eukaryotic initiation factor 4E (eIF4E) which follows the release of the factor from a heterodimeric complex by phosphorylation of its binding protein, 4E-BP1. Angiotensin II (1 nM) or insulin (1 nM) individually stimulated 4E-BP1 phosphorylation. However, pre-incubation with angiotensin II abrogated insulin-induced phosphorylation of 4E-BP1, resulting in persistent binding to eIF4E. Although angiotensin II and insulin individually activated phosphoinositide 3-kinase and extracellular signal-regulated kinase (ERK)-1/-2-type mitogen-activated protein (MAP) kinase, pre-incubation with angiotensin II abolished insulin-induced stimulation of these kinases, suggesting more proximal events in insulin signalling may be intercepted. Pretreatment with angiotensin II markedly inhibited insulin-stimulated tyrosine phosphorylation of insulin-receptor beta-chain and insulin-receptor substrate 1. Losartan prevented angiotensin II inhibition of insulin-induced ERK-1/-2-type MAP kinase activation and 4E-BP1 phosphorylation, suggesting mediation of the effect of angiotensin II by its type 1 receptor. Insulin-stimulated de novo protein synthesis was also abolished by pre-incubation with angiotensin II. These data show that angiotensin II inhibits 4E-BP1 phosphorylation and stimulation of protein synthesis induced by insulin by interfering with proximal events in insulin signalling. Our data provide a mechanistic basis for insulin insensitivity induced by angiotensin II.

Regulation of insulin-stimulated tyrosine phosphorylation of Shc and Shc/Grb2 association in liver, muscle, and adipose tissue of epinephrine- and streptozotocin-treated rats

Shc protein phosphorylation has been extensively characterized as the initial step that activates a complex mitogenic pathway through its association with Grb2. In the present study, we investigated the adrenergic control of insulin-induced Shc phosphorylation and Shc-Grb2 association, and the modulating effect of streptozotocin-induced diabetes mellitus on Shc phosphorylation and Shc/Grb2 association. Acute treatment with epinephrine, which leads to a normoglycemic insulin-resistant state, does not affect insulin-induced Shc tyrosine phosphorylation or Shc-Grb2 association in liver, muscle, or fat. By contrast, a significant increase in insulin-induced Shc phosphorylation is observed in liver and muscle of rats treated with streptozotocin. The association of Shc/Grb2 is also increased in both tissues following insulin treatment. These data suggest that while epinephrine preserves the insulin-induced phosphorylation of Shc and the mitogenic pathway stimulated by Shc-Grb2 association, treatment with streptozotocin leads to a tissue-specific increase in the activity of the initial step that ultimately results in the activation of the Shc/Grb2 mitogenic pathway.

Stimulation of 90- and 70-kDa ribosomal protein S6 kinases by arginine vasopressin and lysophosphatidic acid in rat cardiomyocytes

Arginine vasopressin (AVP) and lysophosphatidic acid (LPA) have been shown to stimulate protein kinase C (PKC) and mitogen-activated protein (MAP) kinases and the proliferation of vascular smooth muscle cells. However, the actions of these two agents in cardiomyocytes are less well understood. To investigate the signal transduction pathways of AVP and LPA, freshly isolated adult rat cardiomyocytes were examined. Both AVP and LPA induced concentration- and time-dependent stimulation of the phosphotransferase activities of p90 ribosomal S6 kinases (RSK) and their upstream activators, extracellularly regulated kinases (ERK) 1 and 2. The activation of ERK1 and ERK2 by LPA was PKC- and phosphatidylinositol 3-kinase (PI 3-kinase)-dependent. However, AVP-induced activation of RSK2, a downstream substrate of ERK1 and ERK2, was PKC-dependent and PI 3-kinase-independent. AVP and LPA were also observed to increase the phosphotransferase activity of p70 ribosomal protein S6 kinase (p70 S6K) in a time- and concentration-dependent manner. The activation of p70 S6K by LPA and AVP was PI 3-kinase-dependent. PKC was necessary in AVP- but not in LPA-induced activation of p70 S6K. Since RSK and p70 S6K have been implicated in the regulation of translational control of protein synthesis, we concluded that AVP and LPA may stimulate the growth of cardiomyocytes through these two protein kinase cascades.

Angiotensin II inhibits insulin-induced egr-1 expression in mesangial cells

The gene early growth response gene-1 (egr-1) encodes a zinc transcription factor involved in cell proliferation. Increased expression of egr-1 has been linked to heart and kidney disease. In mouse mesangial cells, insulin stimulated egr-1 expression more than angiotensin II, suggesting that insulin may play an important role in stimulating cell proliferation, leading to glomerulonephritis and diabetic nephropathy. Angiotensin II inhibited insulin-induced egr-1 expression but not c-fos expression, and the decrease in egr-1 expression was concurrent with a decrease in insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation. These results suggest that insulin-induced egr-1 expression in mouse mesangial cells is downstream of tyrosine phosphorylation of IRS-1 and activation of the MAP kinase pathway and that crosstalk between angiotensin II and insulin signaling pathways led to an inhibition of IRS-1 tyrosine phosphorylation and egr-1 expression.

Phosphatidylinositol 3-kinase is required for growth factor-induced amino acid uptake by vascular smooth muscle cells

Although accumulating evidence suggests that phosphatidylinositol 3-kinase (PI3K) is a common signaling molecule for growth factor-induced amino acid uptake by the cell, the role of PI3K in the uptake of different amino acids was not tested under the same conditions. In this study, we asked whether PI3K mediates platelet-derived growth factor (PDGF) -stimulated uptake of different amino acids that are taken up through 3 major amino acid transporters expressed in rat vascular smooth muscle cells and other cell types and whether PI3K mediates amino acid uptake stimulated with different growth factors and vasoactive substances. PDGF increased the uptake of [(3)H]leucine, [(3)H]proline, and [(3)H]arginine in a dose- and time-dependent fashion. Two different PI3K inhibitors, wortmannin (100 nmol/L) and LY294002 (10 micromol/L), completely inhibited the amino acid uptake stimulated by PDGF. Chinese hamster ovary cells expressing both PDGF receptor-beta and a dominant-negative PI3K did not increase their leucine uptake when stimulated with PDGF, whereas the same cells expressing only PDGF receptor-beta did. Transforming growth factor-beta, as well as insulin-like growth factor-I and angiotensin II, increased leucine uptake by vascular smooth muscle cells. Wortmannin and LY294002 inhibited this increase. We also found that transforming growth factor-beta stimulated PI3K activity and the phosphorylation of Akt, a downstream signaling molecule of PI3K. A similar effect of PI3K inhibitors on amino acid uptake was observed in Swiss 3T3 cells. We conclude that PI3K mediates the uptake of different amino acids by vascular smooth muscle cells and other cell types stimulated with a variety of growth factors, including transforming growth factor-beta. Our findings suggest that PI3K may play an important role in vascular pathophysiology by regulating amino acid uptake.

Convergence and divergence of the signaling pathways for insulin and phosphoinositolglycans

Phosphoinositolglycan molecules isolated from insulin-sensitive mammalian tissues have been demonstrated in numerous in vitro studies to exert partial insulin-mimetic activity on glucose and lipid metabolism in insulin-sensitive cells. However, their ill-defined structures, heterogeneous nature, and limited availability have prohibited the analysis of the underlying molecular mechanism. Phosphoinositolglycan-peptide (PIG-P) of defined and homogeneous structure prepared in large scale from the core glycan of a glycosyl-phosphatidylinositol-anchored membrane protein from Saccharomyces cerevisiae has recently been shown to stimulate glucose transport as well as a number of glucose-metabolizing enzymes and pathways to up to 90% (at 2 to 10 microns) of the maximal insulin effect in isolated rat adipocytes, cardiomyocytes, and diaphragms (G. Müller et al., 1997, Endocrinology 138: 3459-3476). Consequently, we used this PIG-P for the present study in which we compare its intracellular signaling with that of insulin. The activation of glucose transport by both PIG-P and insulin in isolated rat adipocytes and diaphragms was found to require stimulation of phosphatidylinositol (PI) 3-kinase but to be independent of functional p70S6kinase and mitogen-activated protein kinase. The increase in glycerol-3-phosphate acyltransferase activity in rat adipocytes in response to PIG-P and insulin was dependent on both PI 3-kinase and p70S6kinase. This suggest that the signaling pathways for PIG-P and insulin to glucose transport and metabolism converage at the level of PI 3-kinase. A component of the PIG-P signaling pathway located up-stream of PI 3-kinase was identified by desensitization of isolated rat adipocytes for PIG-P action by combined treatment with trypsin and NaCl under conditions that preserved cell viability and the insulin-mimetic activity of sodium vanadate but completely blunted the insulin response. Incubation of the cells with either trypsin or NaCl alone was ineffective. The desensitized adipocytes were reconstituted for stimulation of lipogenesis by PIG-P by addition of the concentrated trypsin/salt extract. The reconstituted adipocytes exhibited 65-75% of the maximal PIG-P response and similar EC50 values for PIG-P (2 to 5 microns) compared with control cells. A proteinaceous N-ethylmaleimide (NEM)-sensitive component contained in the trypsin/salt extract was demonstrated to bind in a functional manner to the adipocyte plasma membrane of desensitized adipocytes via bipolar interactions. An excess of trypsin/salt extract inhibited PIG-P action in untreated adipocytes in a competitive fashion compatible with a receptor function for PIG-P of this protein. The presence of the putative PIG-P receptor protein in detergent-insoluble complexes prepared from isolated rat adipocytes suggests that caveolae/detergent-insoluble complexes of the plasma membrane may play a role in insulin-mimetic signaling by PIG-P. Furthermore, treatment of isolated rat diaphragms and adipocytes with PIG-P as well as with other agents exerting partially insulin-mimetic activity, such as PI-specific phospholipase C (PLC) and the sulfonylurea glimepiride, triggered tyrosine phosphorylation of the caveolar marker protein caveolin, which was apparently correlated with stimulation of lipogenesis. Strikingly, in adipocytes subjected to combined trypsin/salt treatment, PIG-P, PI-specific PLC, and glimepiride failed completely to provoke insulin-mimetic effects. A working model is presented for a signaling pathway in insulin-sensitive cells used by PIG(-P) molecules which involves GPI structures, the trypsin/salt- and NEM-sensitive receptor protein for PIG-P, and additional proteins located in caveolae/detergent-insoluble complexes.

Angiotensin II activates RhoA in cardiac myocytes: a critical role of RhoA in angiotensin II-induced premyofibril formation

The organization of actin into striated fibers (myofibrils) is one of the major features of cardiac hypertrophy. However, its signal transduction mechanism is not well understood. Although Rho-family small G proteins have been implicated in actin organization in many cell types, it is not fully elucidated whether Rho mediates the organization of actin fibers by hypertrophic stimuli in cardiac myocytes. Therefore, we examined (1) whether Rho is activated by the hypertrophic stimulus, angiotensin II (Ang II), and (2) whether Rho mediates the Ang II-induced organization of actin fibers in cultured neonatal rat cardiac myocytes. Treatment of myocytes with Ang II caused a rapid formation of both striated (mature myofibrils) and nonstriated (premyofibrils) actin fibers within 30 minutes, as determined by phalloidin stainings of the polymerized actin and troponin T stainings. Immunoblot analyses and immunostainings have indicated that cardiac myocytes express RhoA, but RhoB is undetectable. In the control state, RhoA was observed predominantly in the cytosolic fraction, but it was translocated in part to the particulate fraction in response to Ang II, consistent with activation of RhoA by Ang II. Incubation of myocytes with exoenzyme C3 for 48 hours completely ADP-ribosylated Rho in vivo. The C3 treatment abolished formation of premyofibrils induced by Ang II, suggesting that Ang II causes premyofibril formation via a Rho-dependent mechanism. The Ang II-induced mature myofibril formation was only partly abolished by C3. Expression of constitutively active RhoA (V14RhoA) caused the formation of premyofibrils but not mature myofibrils. The C3 treatment inhibited Ang II-induced atrial natriuretic factor induction, whereas it had no effect on c-fos induction. These results indicate that RhoA is activated by Ang II and mediates the Ang II-induced formation of premyofibrils and induction of a subset of genes. Distinct signaling mechanisms seem to be responsible for striated mature myofibril formation by Ang II.

Signalling pathways of an insulin-mimetic phosphoinositolglycan-peptide in muscle and adipose tissue

A novel phosphoinositolglycan-peptide (PIG-P) from the yeast Saccharomyces cerevisiae potently mimicks insulin action on glucose transport and metabolism in rat muscle and adipose tissue. The aim of the present study was to elucidate the cellular signalling pathways of this insulin-mimetic compound. Rapid onset and reversibility of PIG-P action on glucose transport were observed in isolated adipocytes with a half-time of transport stimulation of 6-8 min (insulin less than 5 min). Combined treatment with PIG-P and insulin indicated additive stimulation of glucose transport at submaximal concentrations and non-additive action of both agents at maximal doses. The tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) was markedly increased in response to PIG-P in rat cardiomyocytes without any effect on the tyrosine phosphorylation of the insulin receptor beta-subunit. PIG-P action in these cells was accompanied by phosphorylation/dephosphorylation of several proteins with molecular masses of 15-30 kDa, a response not detected with insulin. Downstream signalling of IRS-1 was then analysed by monitoring IRS-1-associated phosphatidylinositol 3-kinase (PI 3-kinase) activity in cardiomyocytes. A stable (2 and 15 min incubation with PIG-P) 7-fold stimulation corresponding to about 50% of insulin action could be detected. Increased tyrosine phosphorylation of IRS-1 and enhanced PI 3-kinase activity in response to PIG-P independent of the insulin receptor was also observed in isolated adipocytes. Involvement of PI 3-kinase in PIG-P action was subsequently confirmed by the dose-dependent inhibition of PIG-P-activated glucose transport in rat diaphragm and adipocytes by the PI 3-kinase inhibitors wortmannin and LY294002. These data suggest divergent upstream signalling by insulin and PIG-P involving phosphoproteins not affected by insulin. However, PIG-P and insulin action converge at the level of IRS-1 inducing insulin-independent PI 3-kinase-mediated signalling to glucose transport.

The biochemical and physiological characteristics of receptors

Cell surface receptors play a central role in the regulation of both cellular and systemic physiology by mediating intercellular communication, facilitating protein trafficking, and regulating virtually all intracellular processes. Receptor expression is often cell specific and is determined by cellular lineage, genetics, and a variety of factors in the extracellular milieu. As receptors are generally localized on the plasma membrane and differentially expressed in certain cell types and tissues, they provide a potential target for drug delivery. However, since most receptors are integrally connected with intracellular signal transduction networks, targeting via these receptors may elicit a biological response. This review describes some established and emerging concepts regarding the structure and functions of receptors. In addition, some aspects related to the regulation and crosstalk between receptors are discussed.

Angiotensin II inhibits insulin signaling in aortic smooth muscle cells at multiple levels. A potential role for serine phosphorylation in insulin/angiotensin II crosstalk

To investigate potential interactions between angiotensin II (AII) and the insulin signaling system in the vasculature, insulin and AII regulation of insulin receptor substrate-1 (IRS-1) phosphorylation and phosphatidylinositol (PI) 3-kinase activation were examined in rat aortic smooth muscle cells. Pretreatment of cells with AII inhibited insulin-stimulated PI 3-kinase activity associated with IRS-1 by 60%. While AII did not impair insulin-stimulated tyrosine phosphorylation of the insulin receptor (IR) beta-subunit, it decreased insulin-stimulated tyrosine phosphorylation of IRS-1 by 50%. AII inhibited the insulin-stimulated association between IRS-1 and the p85 subunit of PI 3-kinase by 30-50% in a dose-dependent manner. This inhibitory effect of AII on IRS-1/PI 3-kinase association was blocked by the AII receptor antagonist saralasin, but not by AT1 antagonist losartan or AT2 antagonist PD123319. AII increased the serine phosphorylation of both the IR beta-subunit and IRS-1. In vitro binding experiments showed that autophosphorylation increased IR binding to IRS-1 from control cells by 2.5-fold versus 1.2-fold for IRS-1 from AII-stimulated cells, suggesting that AII stimulation reduces IRS-1's ability to associate with activated IR. In addition, AII increased p85 serine phosphorylation, inhibited the total pool of p85 associated PI 3-kinase activity, and decreased levels of the p50/p55 regulatory subunit of PI 3-kinase. These results suggest that activation of the renin-angiotensin system may lead to insulin resistance in the vasculature.

Tissue-specific regulation of IRS-1 in unilaterally nephrectomized rats

Insulin stimulates the tyrosine kinase activity of its receptor, resulting in the phosphorylation of its cytosolic substrate, insulin receptor substrate 1 (IRS-1). IRS-1 is also a substrate for different peptides and growth factors, and a transgenic mouse "knockout" for this protein does not have normal growth. However, the role of IRS-1 in kidney hypertrophy and/or hyperplasia was not investigated. In the present study we investigated IRS-1 protein and tyrosine phosphorylation levels in the remnant kidney after unilateral nephrectomy (UNX) in 6-week-old male Wistar rats. After insulin stimulation the levels of insulin receptor and IRS-1 tyrosine phosphorylation were reduced to 79 +/- 5% (P < 0.005) and 58 +/- 6% (P < 0.0001), respectively, of the control (C) levels, in the remnant kidney. It is possible that a circulating factor and/or a local (paracrine) factor playing a role in kidney growth can influence the early steps of insulin action in parallel. To investigate the hypothesis of a circulating factor, we studied the early steps of insulin action in liver and muscle of unilateral nephrectomized rats. There was no change in pp185 tyrosine phosphorylation levels in liver (C 100 +/- 12% vs UNX 89 +/- 9%, NS) and muscle (C 100 +/- 22% vs UNX 91 +/- 17%, NS), and also there was no change in IRS-1 phosphorylation levels in both tissues. These data demonstrate that after unilateral nephrectomy there is a decrease in insulin-induced insulin receptor and IRS-1 tyrosine phosphorylation levels in kidney but not in liver and muscle. It will be of interest to investigate which factors, probably paracrine ones, regulate these early steps of insulin action in the contralateral kidney of unilaterally nephrectomized rats.

Angiotensin II activates phosphatidylinositol 3-kinase in vascular smooth muscle cells

Phosphatidylinositol 3-kinase (PI3K) is an important component of the signal transduction systems activated by tyrosine kinase receptors. It has not been established, however, whether PI3K is also an essential mediator for G protein-coupled receptors. The potential involvement of PI3K in G protein-linked angiotensin II (Ang II)-dependent signaling was assessed in a primary cell culture system of porcine coronary artery smooth muscle cells (SMCs). Treatment of quiescent SMCs with Ang II (10(-5) to 10(-8) mol/L) resulted in a dose-dependent activation of PI3K when assayed in vivo and in vitro. The Ang II receptor antagonists losartan and PD123319 were used to establish that Ang II stimulates PI3K through the Ang II type-1 (AT1) receptor. Immunofluorescent microscopy revealed that Ang II (10(-6) mol/L) stimulated the translocation of p85, the regulatory subunit of PI3K, from the perinuclear region to distinct foci throughout the cell within 15 minutes. Western blot analysis of p85 subcellular distribution demonstrated that p85 concentrations were also increased within 15 minutes in the membrane fraction and concomitantly decreased in the cytoskeletal and nuclear fractions. These changes in PI3K location and activity were paralleled by increased tyrosine phosphorylation of p85. A potential correlation between angiotensin-mediated PI3K activation and SMC growth was found using LY294002, a specific inhibitor of PI3K, which blocked the increase in DNA and RNA synthesis as well as cellular hyperplasia generated by Ang II (10(-6) mol/L) stimulation of quiescent SMCs. These data indicate that PI3K may operate as a mediator of vascular SMC growth after stimulation with Ang II.

Angiotensin II induces activation of phosphatidylinositol 3-kinase in cardiomyocytes

BACKGROUND

Phosphatidylinositol 3-kinase phosphorylates membrane lipids at the third position of the inositol ring producing phosphoinositides, not on the pathway for production of 1,4,5-triphosphate.

OBJECTIVE

To test the hypotheses that angiotensin II (Ang II) activates phosphatidylinositol 3-kinase in cardiomyocytes and that this pathway is involved in Ang II-induced protein synthesis.

METHODS

Cardiomyocytes, in culture, from 7-day-old chick embryonic hearts were treated with Ang II and the activation of phosphatidylinositol 3-kinase was assessed after immunoprecipitation with antibodies to the p85 subunit of phosphatidylinositol 3-kinase by the conversion of PI (phosphatidylinositol) to phosphatidylinositol 3-monophosphate (PIP) in the presence of gamma-[32P]-ATP and analyzed by thin-layer chromatography. Western blotting was performed after antiphosphotyrosine immunoprecipitation with antibodies to the p85 subunit of phosphatidylinositol 3-kinase. Protein synthesis was assessed by [35S]-methionine incorporation and polyacrylamide gel electrophoresis.

RESULTS

Ang II stimulated phosphatidylinositol 3-kinase activity dramatically, with 4.5- and 3.5-fold increases in PIP formation after 1 and 5 min, respectively. The involvement of tyrosine kinases was demonstrated by Western blotting in which Ang II increased tyrosine phosphorylation of a protein recognized by antibodies to the 85 kDa subunit of phosphatidylinositol 3-kinase. Furthermore, the tyrosine kinase inhibitor lavendustin A blocked Ang II-stimulated phosphatidylinositol 3-kinase activity and conversion of phosphatidylinositol to PIP. Ang II increased new protein synthesis as reflected by the significantly (P < 0.05) greater incorporation of [35S]-methionine into cardiomyocytes treated with Ang II. The link between Ang II and protein synthesis was mediated in part through phosphatidylinositol 3-kinase because the phosphatidylinositol 3-kinase inhibitor wortmannin blocked the effect of Ang II on protein synthesis. Increased production both of nuclear and of cytosolic proteins was demonstrated by agarose gel electrophoresis of these cellular components of Ang II-treated cardiomyocytes. Wortmannin produced a general inhibition of the synthesis of nuclear and cytosolic proteins, with a greater effect on nuclear proteins. The action of wortmannin on nuclear protein synthesis was confirmed by similar findings with another phosphatidylinositol 3-kinase inhibitor, LY294002.

CONCLUSION

Phosphatidylinositol 3-kinase activation by Ang II occurs through a pathway utilizing tyrosine phosphorylation. Furthermore, this pathway is involved in cardiomyocyte protein synthesis and the possibility that it is operative in Ang II-mediated cardiac hypertrophy arises.

Tyrosine phosphorylation of insulin receptor substrate-1 and activation of the PI-3-kinase pathway by glycine-extended gastrin precursors

Glycine-extended gastrin precursors (G-Gly) were considered as processing intermediates devoid of biological activity. However, we have recently identified selective receptors for G-Gly which mediate the proliferative effects of this precursor. Little is known about the signaling pathways activated by G-Gly. In the present study, we demonstrate that PI-3-kinase is rapidly and transiently activated by G-Gly. We also observed a rapid increase in the tyrosine phosphorylation of IRS-1 and an activation of the PI-3-kinase in anti-IRS-1 immunoprecipitates, suggesting that PI-3-kinase may be activated by association with tyrosine phosphorylated IRS-1. We also demonstrated that gastrin precursors activate the serine/threonine kinase, p70 kDa S6 kinase (p70S6K), through a wortmannin sensitive pathway.

The IRS-signalling system during insulin and cytokine action

The discovery of the first intracellular substrate for insulin, IRS-1, redirected the field of diabetes research and has led to many important advances in our understanding of insulin action. Detailed analysis of IRS-1 demonstrates structure/function relationships for this modular docking molecule, including mechanisms of substrate recognition and signal propagation. Recent work has also identified other structurally similar molecules, including IRS-2, the Drosophila protein, DOS, and the Grb2-binding protein, Gab1, suggesting that this intracellular signalling strategy is conserved evolutionarily and is utilized by an expanding number of receptor systems. In fact, IRS-1 itself has been shown to be important in other growth factor and cytokine signalling systems, including growth hormone and several interleukins. Analysis of mice lacking IRS-1 confirms an important physiological role for this protein in glucose metabolism and general cell growth in the intact animal. Disregulation of the signalling pathways integrated by the IRS proteins may contribute to the pathophysiology of non-insulin-dependent diabetes mellitus or other diseases.

G protein-coupled receptors control vascular smooth muscle cell proliferation via pp60c-src and p21ras

The binding of vasoactive peptides to their respective G protein-coupled receptors has been implicated in the pathogenesis of vascular smooth muscle cell proliferation, leading to the development of hypertension, arteriosclerosis, and restenosis after vascular injury. We previously showed that the cytosolic tyrosine kinase pp60c-src is crucial for angiotensin II (ANG II)-induced activation of the protooncogene p21ras. Therefore, we investigated the role of pp60c-src and p21ras in rat aortic smooth muscle cell proliferation induced by several G protein-coupled receptors. ANG II, endothelin-1, or thrombin increased cell proliferation and DNA synthesis. Electroporation of anti-pp60c-src antibodies into cells abolished proliferation in response to these G protein-coupled receptor ligands but not in response to platelet-derived growth factor-BB (PDGF-BB). In contrast, electroporation of anti-p21ras antibody completely blocked DNA synthesis and cell proliferation in response to ANG II, endothelin-1, thrombin, and PDGF-BB. Our data indicate that the pp60c-src tyrosine kinase is necessary and specific for vascular smooth muscle cell proliferation and DNA synthesis in response to G protein-coupled receptors but not classic growth factor receptors.

Glucose-induced insulin secretion is impaired and insulin-induced phosphorylation of the insulin receptor and insulin receptor substrate-1 are increased in protein-deficient rats

Malnutrition is related to diabetes in tropical countries. In experimental animals, protein deficiency may affect insulin secretion. However, the effect of malnutrition on insulin receptor phosphorylation and further intracellular signaling events is not known. Therefore, we decided to evaluate the rate of insulin secretion and the early molecular steps of insulin action in insulin-sensitive tissues of an animal model of protein deficiency. Pancreatic islets isolated from rats fed a standard (17%) or a low (6%) protein diet were studied for their secretory response to increasing concentrations of glucose in the culture medium. Basal as well as maximal rates of insulin secretion were significantly lower in the islets isolated from rats fed a low protein diet. Moreover, the dose-response curve to glucose was significantly shifted to the right in the islets from malnourished rats compared with islets from control rats. During an oral glucose tolerance test, there were significantly lower circulating concentrations of insulin in the serum of rats fed a low protein diet in spite of no difference in serum glucose concentration between the groups, suggesting an increased peripheral insulin sensitivity. Immunoblotting and immunoprecipitation were used to study the phosphorylation of the insulin receptor and the insulin receptor substrate-1 as well as the insulin receptor substrate-1-p85 subunit of phosphatidylinositol 3-kinase association in response to insulin. Values were greater in hind-limb muscle from rats fed a low protein diet compared with controls. No differences were detected in the total amount of protein corresponding to the insulin receptor or insulin receptor substrate-1 between muscle from rats fed the two diets. Therefore, we conclude that a decreased glucose-induced insulin secretion in pancreatic islets from protein-malnourished rats is responsible, at least in part, for an increased phosphorylation of the insulin receptor, insulin receptor substrate-1 and its association with phosphatidylinositol 3-kinase. These might represent some of the factors influencing the equilibrium in glucose concentrations observed in animal models of malnutrition and undernourished subjects.

Cross-talk between the insulin and angiotensin signaling systems

Angiotensin II (AII), acting via its G-protein linked receptor, is an important regulator of cardiac, vascular, and renal function. Following injection of AII into rats, we find that there is also a rapid tyrosine phosphorylation of the major insulin receptor substrates 1 and 2 (IRS-1 and IRS-2) in the heart. This phenomenon appears to involve JAK2 tyrosine kinase, which associates with the AT1 receptor and IRS-1/IRS-2 after AII stimulation. AII-induced phosphorylation leads to binding of phosphatidylinositol 3-kinase (PI 3-kinase) to IRS-1 and IRS-2; however, in contrast to other ligands, AII injection results in an acute inhibition of both basal and insulin-stimulated PI 3-kinase activity. The latter occurs without any reduction in insulin receptor or IRS phosphorylation or in the interaction of the p85 and p110 subunits of PI 3-kinase with each other or with IRS-1/IRS-2. These effects of AII are inhibited by AT1 receptor antagonists. Thus, there is direct cross-talk between insulin and AII signaling pathways at the level of both tyrosine phosphorylation and PI 3-kinase activation. These interactions may play an important role in the association of insulin resistance, hypertension, and cardiovascular disease.

Gastrin stimulates tyrosine phosphorylation of insulin receptor substrate 1 and its association with Grb2 and the phosphatidylinositol 3-kinase

The growth-promoting effects of gastrin on normal and neoplastic gastrointestinal tissues have been shown to be mediated by the gastrin/CCKB receptor, which belongs to the family of G protein-coupled receptors. However, the downstream signaling pathways activated by gastrin are not well characterized. In the present study, we demonstrate that gastrin stimulates tyrosine phosphorylation of insulin receptor substrate 1 (IRS-1), the major cytoplasmic substrate of the insulin receptor. The gastrin-induced phosphorylation of IRS-1 was rapid and transient, occurring within 30 s of treatment and diminishing thereafter. IRS-1 binds several proteins containing Src homology 2 domains through its multiple tyrosine phosphorylation sites. Following gastrin stimulation, we observed a time- and dose-dependent association of IRS-1 with the p85 regulatory subunit of phosphatidylinositol 3-kinase (PI 3-kinase). In addition, activation of PI 3-kinase was detected in anti-IRS-1 immunoprecipitates from gastrin-treated cells, suggesting that tyrosine phosphorylation of IRS-1, which leads to the rapid recruitment of p85, might be one mechanism used by gastrin to activate PI 3-kinase. We have previously reported that tyrosine phosphorylation of Shc and its association with the Grb2-Sos complex may contribute to the activation of the mitogen-activated protein kinase pathway by gastrin. We report here that Grb2 also interacts with tyrosine-phosphorylated IRS-1 in response to gastrin. Taken together, our results suggest that IRS-1 may serve as a converging target in the signaling pathways stimulated by receptors that belong to different families, such as the gastrin/CCKB G protein-coupled receptor and the insulin receptor.

Angiotensin II controls p21ras activity via pp60c-src

Angiotensin II is the major effector peptide of the renin-angiotensin system, and it exerts its physiologic functions via a G protein-coupled cell surface receptor called AT1. We found that in rat aortic smooth muscle cells, angiotensin II stimulated the formation of Ras-GTP, Ras-Raf-1 complex formation, and the tyrosine phosphorylation of two important Ras GTPase-activating proteins (GAPs), p120 Ras-GAP and p190 Rho-GAP. Electroporation of anti-pp60c-src antibody into cultured, adherent smooth muscle cells blocked the angiotensin II stimulation of Ras-GAP and Rho-GAP tyrosine phosphorylation. In contrast electroporation of antibodies against c-Yes or c-Fyn had no effect. Anti-pp60c-src antibody also blocked angiotensin II-stimulated Ras activation and Ras-Raf-1 complex formation. These data strongly suggest that a G protein-coupled receptor such as the AT1 receptor can activate the Ras protein cascade via the tyrosine kinase pp60c-src.