Induction of the proto-oncogene c-jun by angiotensin II

Individual Measurements of Angiotensin II Concentrations in Aqueous Humor of the Eye

The presence of constituents of the renin-angiotensin system (RAS) in ocular tissues and fluids suggests this system is involved in ocular physiology. Angiotensin II (AngII) is the main biological effector of the system, so we measured AngII in plasma and in aqueous humor of the anterior ocular chamber of patients undergoing cataract extraction. Untreated normotensive patients were compared with arterial hypertensive patients taking either diuretics which stimulate the RAS or angiotensin converting enzyme (ACE) inhibitors which reduce the production of AngII. Plasma levels of AngII were higher in patients on diuretics (5.46 +/- 1.04 fmol/ml; mean +/- SEM) than in untreated cataract patients (2.28 +/- 0.32 fmol/ml, p < 0.02), and were very low with ACE inhibitors (0.51 +/- 0.18 fmol/ml). In aqueous humor, AngII was measurable in 7 of 11 patients on diuretics (median 1.1 fmol/ml), and in 6 of 16 normotensive patients (median < 0.55 fmol/ml), but not in aqueous humor of 4 patients receiving enalapril or captopril. These results demonstrate the presence of AngII in the eye but do not exclude either its sequestration in the eye or local production. The possibility of individual measurements of intraocular AngII will permit more precise determination of its role in future studies.

Krüppel-like factor 4 transcriptionally regulates TGF-β1 and contributes to cardiac myofibroblast differentiation

Angiotensin II (Ang II) plays a major role in the pathogenesis of cardiac fibrosis in hypertension. It is known that Ang II induces TGF-β1 expression. How transcription mediates Ang II-induced TGF-β1 expression, as well as its contribution to cardiac fibrosis, is unknown. We studied the role of Krüppel-like family transcription factors in Ang II-induced myofibroblast formation. We found that among the Krüppel-like family members, Krüppel-like factor 4 (Klf4) was the highest expressed form in isolated cardiac fibroblasts after Ang II treatment. Klf4 increased expression of α-SMA and collagen, as well as increased myofibroblast formation. ChIP assays showed that Klf4 specifically bound to the TGF-β1 promoter. Deletion and mutagenesis analysis showed that the sites at -184∼-180 bp and -45∼-41 bp in the TGF-β1 promoter were responsible for Klf4 transactivation of the TGF-β1 promoter. Our studies demonstrate that Klf4 plays a pivotal role in Ang II-induced cardiac myofibroblast differentiation and collagen synthesis through transcriptional upregulation of TGF-β1.

α-lipoic acid reduces hypertension and increases baroreflex sensitivity in renovascular hypertensive rats

Renovascular hypertension has robust effects on control of blood pressure, including an impairment in baroreflex mechanisms, which involves oxidative stress. Although α-lipoic acid (LA) has been described as a potent antioxidant, its effect on renovascular hypertension and baroreflex sensitivity (BRS) has not been investigated. In the present study we analyzed the effects caused by chronic treatment with LA on blood pressure, heart rate and baroreflex sensitivity (sympathetic and parasympathetic components) in renovascular hypertensive rats. Male Wistar rats underwent 2-Kidney-1-Clip (2K1C) or sham surgery and were maintained untouched for four weeks to develop hypertension. Four weeks post-surgery, rats were treated with LA (60 mg/kg) or saline for 14 days orally. On the 15th day mean arterial pressure (MAP) and heart rate (HR) were recorded. In addition, baroreflex sensitivity test using phenylephrine (8 µg/kg, i.v.) and sodium nitroprusside (25 µg/kg, i.v.) was performed. Chronic treatment with LA decreased blood pressure in hypertensive animals; however, no significant changes in baseline HR were observed. Regarding baroreflex, LA treatment increased the sensitivity of both the sympathetic and parasympathetic components. All parameters studied were not affected by treatment with LA in normotensive animals. Our data suggest that chronic treatment with LA promotes antihypertensive effect and improves baroreflex sensitivity in rats with renovascular hypertension.

Retinoid metabolism and its effects on the vasculature

Retinoids, the metabolically-active structural derivatives of vitamin A, are critical signaling molecules in many fundamental biological processes including cell survival, proliferation and differentiation. Emerging evidence, both clinical and molecular, implicates retinoids in atherosclerosis and other vasculoproliferative disorders such as restenosis. Although the data from clinical trials examining effect of vitamin A and vitamin precursors on cardiac events have been contradictory, this data does suggest that retinoids do influence fundamental processes relevant to atherosclerosis. Preclinical animal model and cellular studies support these concepts. Retinoids exhibit complex effects on proliferation, growth, differentiation and migration of vascular smooth muscle cells (VSMC), including responses to injury and atherosclerosis. Retinoids also appear to exert important inhibitory effects on thrombosis and inflammatory responses relevant to atherogenesis. Recent studies suggest retinoids may also be involved in vascular calcification and endothelial function, for example, by modulating nitric oxide pathways. In addition, established retinoid effects on lipid metabolism and adipogenesis may indirectly influence inflammation and atherosclerosis. Collectively, these observations underscore the scope and complexity of retinoid effects relevant to vascular disease. Additional studies are needed to elucidate how context and metabolite-specific retinoid effects affect atherosclerosis. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

Angiotensin II-dependent growth of vascular smooth muscle cells requires transactivation of the epidermal growth factor receptor via a cytosolic phospholipase A(2)-mediated release of arachidonic acid

Angiotensin (Ang) II stimulates vascular smooth muscle cell (VSMC) growth via activation of cytosolic phospholipase A(2) (cPLA(2)), release of arachidonic acid (ArAc) and activation of mitogen-activated protein kinase (MAPK). The mechanism linking AT(1) receptor stimulation of ArAc release with MAPK activation may involve transactivation of the epidermal growth factor receptor (EGFR). In this study, Ang II increased phosphorylation of the EGFR and MAPK in cultured VSMC and these effects were attenuated by the cPLA(2) inhibitor arachidonyl trifluoromethyl ketone (AACOCF(3)), and restored by addition of ArAc. Ang II- or ArAc-induced phosphorylation of the EGFR and MAPK were abolished by the EGFR kinase inhibitor AG1478. Ang II or ArAc also stimulated VSMC growth that was blocked by AG1478 or the MAPK kinase (MEK) inhibitor PD98059. Thus, it appears that the cPLA(2)-dependent release of ArAc may provide a mechanism for the transactivation between the AT(1) receptor and the EGFR signaling cascade.

Purification and characterization of butyrate-induced protein phosphatase involved in apoptosis of Ehrlich ascites tumor cells

Short chain fatty acids including butyrate exhibit wide variety of biological effects towards cell growth, morphology and gene expression. In this report, we study the mechanism by which butyrate (BuA) modulates the expression of protein phosphatase when treated to the cells. As a model system, we used Ehrlich Ascites Tumor (EAT) cells in which BuA-treatment induces expression of a protein phosphatase enzyme. Subsequently, BuA-induced protein phosphatase has been biochemically purified and characterized. Further, pretreatment of caspase-3 inhibitor abolished the activity of BuA-induced protein phosphatase indicating the involvement of caspase-3 in the activation of BuA-induced protein phosphatase. In addition, the relationship between BuA-induced protein phosphatase and apoptosis has been verified. Activation of endonuclease-II has been shown in BuA-treated EAT cells and that activity was completely inhibited by sodium orthovanadate, a tyrosine phosphatase inhibitor suggesting that endonuclease-II may serve as a possible down-stream target for BuA-induced protein phosphatase. Together, the data suggest that activation of protein phosphatase may be an early and essential step in BuA-mediated apoptotic signaling pathway in EAT cells.

Angiotensin II-receptor antagonist in the treatment of hypertension

Effective treatment of high blood pressure levels represents a crucial point in reducing global cardiovascular risk, and several studies have clearly demonstrated a significant reduction in cardiovascular and renal morbidity and mortality with a more intensive blood pressure-lowering treatment. Other factors beyond blood pressure control may be important in reducing the risk related to hypertension. Pharmacologic agents blocking the renin-angiotensin system, in particular the angiotensin II-receptor blocker (ARB), a novel class of antihypertensive agents, represent an important addition to the therapeutic options for hypertension management, and recent large, international, randomized, trials have demonstrated that ARBs have clinical benefits across the spectrum of disease severity. In this article, we provide some evidence derived from these trials, supporting a role for ARBs in primary and secondary prevention of cardiovascular and renal disease, beyond blood pressure control.

A novel angiotensin II type 1 receptor-associated protein induces cellular hypertrophy in rat vascular smooth muscle and renal proximal tubular cells

Angiotensin II stimulates cellular hypertrophy in cultured vascular smooth muscle and renal proximal tubular cells. This effect is believed to be one of earliest morphological changes of heart and renal failure. However, the precise molecular mechanism involved in angiotensin II-induced hypertrophy is poorly understood. In the present study we report the isolation of a novel angiotensin II type 1 receptor-associated protein. It encodes a 531-amino acid protein. Its mRNA is detected in all human tissues examined but highly expressed in the human kidney, pancreas, heart, and human embryonic kidney cells as well as rat vascular smooth muscle and renal proximal tubular cells. Protein synthesis and relative cell size analyzed by flow cytometry studies indicate that overexpression of the novel angiotensin II type 1 receptor-associated protein induces cellular hypertrophy in cultured rat vascular smooth muscle and renal proximal tubular cells. In contrast, the hypertrophic effects was reversed in renal proximal tubular cell lines expressing the novel gene in the antisense orientation and its dominant negative mutant, which lacks the last 101 amino acids in its carboxyl-terminal tail. The hypertrophic effects are at least in part mediated via protein kinase B activation or cyclin-dependent kinase inhibitor, p27(kip1) protein expression level in vascular smooth muscle, and renal proximal tubular cells. Moreover, angiotensin II could not stimulate cellular hypertrophy in renal proximal tubular cells expressing the novel gene in the antisense orientation and its mutant. These findings may provide new molecular mechanisms to understand hypertrophic agents such as angiotensin II-induced cellular hypertrophy.

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.

PKC-zeta is required for angiotensin II-induced activation of ERK and synthesis of C-FOS in MCF-7 cells

We examined the signalling pathways responsible for the Ang II induction of growth in MCF-7 human breast cancer cells. Ang II in MCF-7 cells induced: (a) the translocation from the cytosol to membrane and nucleus of atypical protein kinase C-zeta (PKC-zeta) but not of PKC-alpha, -delta, - epsilon and -eta; (b) the expression of c-fos mRNA and protein; (c) the phosphorylation of the extracellular signal-regulated protein kinases 1 and 2 (ERK1/2). All these effects were due to the activation of the Ang II type I receptor (AT1) since they were blocked by the AT1 antagonist losartan. The Ang II-stimulated ERK1/2 phosphorylation was blocked by (a) high doses of staurosporine, inhibitor of PKC-zeta, and by a synthetic myristoylated peptide with sequences based on the endogenous PKC-zeta pseudosubstrate region (zeta-PS); (b) PD098059, a mitogen-activated protein kinase kinase inhibitor (MAPKK/MEK); and, moreover, (c) the inhibitors of phosphoinositide 3-kinases (PI3K), LY294002 and wortmannin, thus indicating that PI3K may act upstream of ERK1/2. The Ang II-evoked c-fos induction was blocked only by high doses of staurosporine and by zeta-PS whilst PD098059, LY294002 and wortmannin were ineffective, thus indicating that c-fos induction is not due to ERK1/2 activity. When the epidermal growth factor-receptor (EGFR) tyrosine kinase activity was inhibited by the use of its inhibitor AG1478, Ang II was still able to induce ERK1/2 phosphorylation and c-fos expression, therefore proving that the transactivation of EGFR was not required for these Ang II effects in MCF-7 cells. The previously reported proliferation of MCF-7 cells induced by Ang II was blocked by PD098059 and by wortmannin in a dose-dependent manner, thereby indicating that in MCF-7 cells the PI3K and ERK pathways mediate the mitogenic signalling of AT1. Our results suggest that in MCF-7 cells Ang II activates multiple signalling pathways involving PKC-zeta, PI3K and MAPK; of these pathways only PKC-zeta appears responsible for the induction of c-fos.

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.

Increased local angiotensin II formation in aneurysmal aorta

We investigated the levels and locations of angiotensin II-forming enzymes, angiotensin converting enzyme (ACE) and chymase, in aneurysmal and normal aortas. Aneurysmal aortic specimens (n = 14) were obtained at the time of operative aneurysm repair from 14 patients ranging in age from 57 to 84 y. Normal aortic specimens (n = 16) were obtained from 16 patients (48 to 72 y) who underwent coronary artery bypass surgery. The ACE and chymase activities were determined using each specimen. Sections of each specimen were immunostained with antibodies for ACE and chymase. The ACE activities in the aneurysmal and normal aortas were 0.82 +/- 0.10 and 0.14 +/- 0.05 mU/mg protein, respectively, and this difference was significant. The chymase activities in the aneurysmal and normal aortas were 17.9 +/- 2.40 and 1.02 +/- 0.18 mU/mg protein, respectively, and this difference was also significant. In the aneurysmal aorta, ACE-positive cells were detected with macrophages in the intima and media and chymase-positive cells were detected with mast cells in the media and adventitia, whereas positive ACE and chymase cells in the normal aorta were located only in the endothelium and adventitia, respectively. Angiotensin II-forming enzymes, chymase and ACE, were significantly increased in the aneurysmal aorta, and increased angiotensin II may be associated with the development of aneurysmal formations.

Identification of a zinc finger homoeodomain enhancer protein after AT(2) receptor stimulation by differential mRNA display

Using differential mRNA display, we isolated differentially expressed genes after stimulation of angiotensin II (Ang II) type 2 (AT(2)) receptors in PC12w cells. Among the polymerase chain reaction products isolated, we identified Zfhep, a zinc finger homoeodomain enhancer-binding protein and the angiotensin AT(2) receptor itself, both implicated in developmental processes. In the presence of epidermal growth factor (EGF) a concordant expression pattern of Zfhep and AT(2) mRNA was observed with marked downregulation 6 hours after stimulation with EGF+Ang II. In quiescent PC12w cells, Zfhep mRNA was time-dependently induced by Ang II (10(-7) mol/L) up to 3 hours, an effect that was blocked by the selective AT(2) antagonist PD123177 (10(-6) mol/L). We extended our studies on Zfhep mRNA expression to cells of vascular origin (coronary endothelial cells [CECs]), that express AT(1) and AT(2) receptors, and that respond differently to Ang II dependent on which receptor subtype is stimulated. In CECs, Zfhep mRNA expression was induced up to 3 hours after AT(2) stimulation. Interestingly, this Zfhep induction was observed only when the AT(1) receptor was blocked by using the selective AT(1) receptor antagonist, losartan (10(-5) mol/L), suggesting a negative-regulatory influence of AT(1) receptors on AT(2)-induced Zfhep mRNA induction. In conclusion, Zfhep not only represents a suitable marker for AT(2) receptor activation, but it may also link AT(2) signaling and downstream events involved in the proposed function of the AT(2) receptor in development and regeneration.

Angiotensin II-induced transcriptional activation of the cyclin D1 gene is mediated by Egr-1 in CHO-AT(1A) cells

Cyclin D1 protein expression is regulated by mitogenic stimuli and is a critical component in the regulation of G(1) to S phase progression of the cell cycle. Angiotensin II (Ang II) binds to specific G protein-coupled receptors and is mitogenic in Chinese hamster ovary cells stably expressing the rat vascular Ang II type 1A receptor (CHO-AT(1A)). We recently reported that in these cells, Ang II induced cyclin D1 promoter activation and protein expression in a phosphatidylinositol 3-kinase (PI3K)-, SHP-2-, and mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK)-dependent manner (Guillemot, L., Levy, A., Zhao, Z. J., Béréziat, G., and Rothhut, B. (2000) J. Biol. Chem. 275, 26349-26358). In this report, transfection studies using a series of deleted cyclin D1 promoters revealed that two regions between base pairs (bp) -136 and -96 and between bp -29 and +139 of the human cyclin D1 promoter contained regulatory elements required for Ang II-mediated induction. Mutational analysis in the -136 to -96 bp region provided evidence that a Sp1/early growth response protein (Egr) motif was responsible for cyclin D1 promoter activation by Ang II. Gel shift and supershift studies showed that Ang II-induced Egr-1 binding involved de novo protein synthesis and correlated well with Egr-1 promoter activation. Both U0126 (an inhibitor of the MAPK/ERK kinase MEK) and wortmannin (an inhibitor of PI3K) abrogated Egr-1 endogenous expression and Egr-1 promoter activity induced by Ang II. Moreover, using a co-transfection approach, we found that Ang II induction of Egr-1 promoter activity was blocked by dominant-negative p21(ras), Raf-1, and tyrosine phosphatase SHP-2 mutants. Identical effects were obtained when inhibitors and dominant negative mutants were tested on the -29 to +139 bp region of the cyclin D1 promoter. Taken together, these findings demonstrate that Ang II-induced cyclin D1 up-regulation is mediated by the activation and specific interaction of Egr-1 with the -136 to -96 bp region of the cyclin D1 promoter and by activation of the -29 to +139 bp region, both in a p21(ras)/Raf-1/MEK/ERK-dependent manner, and also involves PI3K and SHP-2.

Beneficial effect of trandolapril on the lifespan of a severe hypertensive model

It remains to be clarified whether ACE Inhibitors extend the lifespan in cases of severe hypertension. In the present study, we examined whether inhibition of angiotensin-converting enzyme (ACE) would affect mortality in a very severe hypertensive model. Twelve-week-old stroke-prone spontaneously hypertensive rats (SHR-SP) and Wistar-Kyoto rats (WKY) were used, and measurements at this age were taken as the basal values. At this age, the systolic blood pressure of SHR-SP was 219+/-4 mmHg, while that of WKY was 102+/-3 mmHg. After 12 weeks of sodium loading, the systolic blood pressure in the placebo-treated SHR-SP reached to 251+/-4 mmHg. At 4 weeks of sodium loading, 85% of the placebo-treated SHR-SP had died, and the systolic blood pressure of the surviving rats was 249+/-26 mmHg. In the trandolapril-treated SHR-SP, the systolic blood pressure was gradually increased to 293+/-5 mmHg at 16 weeks, and none of the mice had died at this time point (0% mortality). The ACE activities of the aorta, brain, heart and kidney were increased in the surviving placebo-treated SHR-SP at 4 weeks compared with the basal levels, while they were significantly decreased in the trandolapril-treated SHR-SP at 16 weeks. These data demonstrate that trandolapril extends the lifespan of this severe hypertensive model, even in cases in which the blood pressure cannot be lowered.

Inhibitory Mechanism of Daphnodorins for Human Chymase

We investigated the inhibitory mechanisms of daphnodorins for human chymase using three-dimensional molecular modeling. In daphnodorin A-human chymase complex, daphnodorin A was fixed to the active site via hydrogen bonds with Ala177, Phe29, and Gly199 in human chymase, and it formed hydrogen bonds with Ser182 and Gly180, and this complex was formed stably. In daphnodorin B-human chymase complex, daphnodorin B formed hydrogen bonds with Lys28 and Phe29 in human chymase, but it could not form hydrogen bonds with Gly199, Ala177, and Lys179. The phenyl group of daphnodorin B shifted from the P1 hole in human chymase in comparison with that of daphnodorin A. For the inhibition of human chymase by daphnodorins, we indicated that it was significant whether daphnodorins formed hydrogen bonds with Ala177 located in the P1 hole, Ser182 located in the active site, Gly180 located in the anion hole, and with Gly199, Phe29, and Lys28 in human chymase.

Retinoids inhibit the actions of angiotensin II on vascular smooth muscle cells

Retinoids are derivatives of vitamin A and powerful inhibitors of cell proliferation and inflammation. Angiotensin II (Ang II) contributes to vascular lesions by promoting cell growth of vascular smooth muscle cells (VSMCs). Therefore, we examined whether retinoids interfere with the proproliferative actions of Ang II in VSMCs via AT(1) receptor-dependent or activator protein-1 (AP-1)-dependent mechanisms. VSMCs express retinoid receptor proteins, ie, retinoic acid receptor (RAR) alpha and retinoid X receptor (RXR) alpha. Long-term exposure to 1 micromol/L all-trans retinoic acid (RA) dose-dependently inhibited Ang II-induced cell proliferation (P<0.005) as well as DNA and protein synthesis (P<0.001). All-trans RA blocked Ang II stimulation of transforming growth factor-beta(1) mRNA (P<0.005). All-trans RA inhibition of vascular VSMC growth was mediated both via RAR- and RXR-dependent pathways, as shown by receptor-specific synthetic retinoids. Transfection experiments revealed that inhibition of AP-1-dependent gene transcription is one mechanism by which all-trans RA inhibits Ang II action. RARalpha cotransfection enhanced the anti-AP-1 effects of all-trans RA dose-dependently. AP-1 activity was similarly inhibited by cotransfection with either RARalpha or RXRalpha. Ang II-induced gene expression of c-fos was abrogated by all-trans RA treatment (P<0.005). In VSMCs, all-trans RA downregulated AT(1) receptor mRNA (P<0.01) and reduced B(max) (P<0.001). All-trans RA repressed Ang II-stimulated AT(1) receptor promoter activity. The all-trans RA inhibitory effect was abolished when the AP-1 consensus site on the AT(1) receptor promoter was deleted. Our findings demonstrate that retinoids are potent inhibitors of the actions of Ang II on VSMCs. The findings support the notion that retinoids may interfere with proliferative vascular disease.

Expression of cell cycle proteins in blood vessels of angiotensin II-infused rats: role of AT(1) receptors

Angiotensin II is an important modulator of cell growth through AT(1) receptors, as demonstrated both in vivo and in vitro. We investigated the role of proteins involved in the cell cycle, including cyclin D1, cyclin-dependent kinase 4 (cdk4), and cyclin-dependent kinase inhibitors p21 and p27 in blood vessels of angiotensin II-infused rats and the effect therein of the AT(1)-receptor antagonist losartan. Male Sprague-Dawley rats were infused for 7 days with angiotensin II (120 ng/kg per minute SC) and/or treated with losartan (10 mg/kg per day orally). DNA synthesis in mesenteric arteries was evaluated by radiolabeled (3)H-thymidine incorporation. The expression of cyclin D1, cdk4, p21, and p27, which play critical roles during the G(1)-phase of the cell cycle process, was examined by Western blot analysis. Tail-cuff systolic blood pressure (mm Hg) was elevated (P<0.01, n=9) in angiotensin II-infused rats (161.3+/-8.2) versus control rats (110.1+/-5.3) and normalized by losartan (104.4+/-3.2). Radiolabeled (3)H-thymidine incorporation (cpm/100 microgram DNA) showed that angiotensin II infusion significantly increased DNA synthesis (152+/-5% versus 102+/-6% of control rats, P<0.05). Expression of cyclin D1 and cdk4 was significantly increased in the angiotensin II group to 213.7+/-8% and 263.6+/-37% of control animals, respectively, whereas expression of p21 and p27 was significantly decreased in the angiotensin II group to 23.2+/-10.4% and 10.3+/-5.3% of control animals, respectively. These effects induced by angiotensin II were normalized in the presence of losartan. Thus, when AT(1) receptors are stimulated in vivo, DNA synthesis is enhanced in blood vessels by activation of cyclin D1 and cdk4. Reduction in cell cycle kinase inhibitors p21 and p27 may contribute to activation of growth induced by in vivo AT(1) receptor stimulation.

Novel signaling pathways contributing to vascular changes in hypertension

In hypertension, increased peripheral resistance maintains elevated levels of arterial blood pressure. The increase in peripheral resistance results, in part, from abnormal constrictor and dilator responses and vascular remodeling. In this review, we consider four cellular signaling pathways as possible explanations for these abnormal vascular responses: (1) augmented signaling via the epidermal growth factor receptor to cause remodeling of the cerebrovasculature; (2) reduced sphingolipid signaling leading to blunted vasodilation and increased smooth muscle proliferation; (3) increased signaling via Rho/Rho kinase leading to enhanced vasoconstriction, and (4) a relative state of microtubular depolymerization favoring vasoconstriction in hypertension. These novel cell signaling pathways provide new pharmacological targets to reduce total peripheral vascular resistance in hypertension.

The application of high density microarray for analysis of mitogenic signaling and cell-cycle in the adrenal

Angiotensin II (AII) binds to specific G-protein coupled receptors and is mitogenic in adrenal, liver epithelial, and vascular smooth muscle cells. The H295R human adrenocortical cell line, which expresses AII receptors predominantly of the AT1 subclass, proliferates in response to treatment with AII. The induction and maintenance of cellular proliferation involves a precisely coordinated induction of a variety of genes. As the human genome sequencing projects near completion a variety of high throughput technologies have been developed in order to create dynamic displays of genomic responses. One high throughput method, the gridded cDNA microarray has been developed in which immobilised DNA samples are hybridized on glass slides for the identification of global genomic responses. For this purpose high precision robotic microarrayers have been developed at AECOM. The cyclin D1 gene, which encodes the regulatory subunit of the cyclin D1-dependent kinase (CD1K) required for phosphorylation of the retinoblastoma protein (pRB), was induced by AII in H295R cells. Abundance of the cyclin D1 gene is rate-limiting in G1 phase progression of the cell-cycle in a variety of cell types. AII induced cyclin D1 promoter activity through a c-Fos and c-Jun binding sequence at -954 bp. Theabundance of c-Fos within this complex was increased by AII treatment. Analysis of AII signaling in adrenal cells by cDNA microarray demonstrated an induction of the human homologue of Xenopus XPMC2 (HXPMC2). The cDNA for XPMC2 was previously shown to rescue mitotic catastrophe in mutant S. Pombe defective in cdc2 kinase function. Further studies are required to determine the requirement for cyclin D1 and XPMC2H in AII-induced cell-cycle progression and cellular proliferation in the adrenal.

The protein-tyrosine phosphatase SHP-2 is required during angiotensin II-mediated activation of cyclin D1 promoter in CHO-AT1A cells

Angiotensin II (Ang II) binds to specific G protein-coupled receptors and is mitogenic in Chinese hamster ovary (CHO) cells stably expressing a rat vascular angiotensin II type 1A receptor (CHO-AT(1A)). Cyclin D1 protein expression is regulated by mitogens, and its assembly with the cyclin-dependent kinases induces phosphorylation of the retinoblastoma protein pRb, a critical step in G(1) to S phase cell cycle progression contributing to the proliferative responses. In the present study, we found that in CHO-AT(1A) cells, Ang II induced a rapid and reversible tyrosine phosphorylation of various intracellular proteins including the protein-tyrosine phosphatase SHP-2. Ang II also induced cyclin D1 protein expression in a phosphatidylinositol 3-kinase and mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK)-dependent manner. Using a pharmacological and a co-transfection approach, we found that p21(ras), Raf-1, phosphatidylinositol 3-kinase and also the catalytic activity of SHP-2 and its Src homology 2 domains are required for cyclin D1 promoter/reporter gene activation by Ang II through the regulation of MAPK/ERK activity. Our findings suggest for the first time that SHP-2 could play an important role in the regulation of a gene involved in the control of cell cycle progression resulting from stimulation of a G protein-coupled receptor independently of epidermal growth factor receptor transactivation.

Endothelin-1 and smooth muscle cells: induction of jun amino-terminal kinase through an oxygen radical-sensitive mechanism

Endothelin-1 (ET-1) has been proposed to contribute to atherogenesis and plaque rupture in coronary heart disease through activation of mitogen-activated protein kinases (MAPKs) in smooth muscle cells (SMCs). Reactive oxygen species (ROS) have been shown to be important signal transduction molecules in SMCs. Thus, the present study aimed to assess the role of ROS in ET-1-mediated activation of c-Jun amino-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK) 1/2. Rat SMCs were exposed to ET-1 over time at concentrations from 10(-6) to 10(-10) mol/L, and MAPK activity was quantified. Activation of JNK and ERK was observed with a maximum stimulation at 10(-7) mol/L ET-1. JNK and ERK were activated by ET-1 binding to a single receptor (ET-1A) but differed in their downstream mechanisms: only JNK activation was sensitive to the radical scavenger N-acetylcysteine and diphenylene iodonium, an inhibitor of NADPH oxidase, indicating a role for ROS. The downstream MAPK effector and proinflammatory transcription factor, the activator protein-1 complex, was maximally activated 2 hours after the addition of ET-1. It was mainly composed of the JNK substrate c-Jun, and activation was also dependent on ROS formation. We suggest that plaque activation by ET-1 can be mediated through ROS. It can be hypothesized that the clinical benefit of antioxidants in the treatment of atherogenesis may partially depend on neutralization of ET-1-mediated ROS production.

Coronary microvascular endothelial cell redox state in left ventricular hypertrophy : the role of angiotensin II

Left ventricular hypertrophy (LVH) is associated with elevated plasma angiotensin II (Ang II) levels and endothelial dysfunction. The relationship between Ang II and endothelial dysfunction remains unknown, however, but it may involve an alteration in endothelial cell redox state. We therefore investigated the effect of Ang II on NADH/NADPH oxidase-mediated superoxide anion (O(2)(-)) production by cultured guinea pig coronary microvascular endothelial cells (CMVEs) and CMVEs freshly isolated from a guinea pig, pressure-overload model of LVH. Lucigenin chemiluminescence was used to measure O(2)(-) production in the particulate fraction of CMVE lysates. In cultured cells, incubation with Ang II (0.1 nmol/L to 1 micromol/L for 18 hours) resulted in significant (P<0.01) increases in both NADH- and NADPH-dependent O(2)(-) production, with a peak effect at 1 nmol/L. The latter was significantly (P<0.01) inhibited by the AT(1) receptor antagonist losartan (1 micromol/L for 18 hours). In contrast, the O(2)(-) response to Ang II (0.1 nmol/L to 1 micromol/L for 18 hours) was largely unaffected by concomitant exposure to the AT(2) antagonist PD 123319 (1 micromol/L). In freshly isolated CMVEs from nonoperated animals, NADH- and NADPH-dependent O(2)(-) production was not different from that in sham-operated animals but was significantly (P<0.05) elevated in the aortic-banded animals. Plasma Ang II levels were significantly (P<0.001) elevated in the aortic-banded (1.25+/-0.12 microg/L, n=12) compared with sham-operated animals (0.63+/-0.06 microg/L, n=12). These data suggest that the endothelial dysfunction associated with LVH may be due, at least in part, to the Ang II-induced upregulation of NADH/NADPH oxidase-dependent O(2)(-) production.

Role of chymase on vascular proliferation

In the normal state, vascular ACE regulates local angiotensin II formation and plays a crucial role in the regulation of blood pressure, whereas chymase is stored in secretory granules in mast cells and has no enzymatic effects such as angiotensin II-forming activity. Chymase has a maximal activity immediately upon release into the extracellular matrix in vascular tissues after mast cells have been activated by a strong stimulus such as experienced by catheter-injured and grafted vessels. Therefore, chymase plays an important role in forming local angiotensin II when vascular tissues are injured, and inhibition of chymase may be useful for preventing vascular proliferation in grafted vessels and after PTCA (Figure 6).

Characterization of recombinant human chymase expressed in Escherichia coli

We compared recombinant human chymase expressed in Escherichia coli with human chymase purified from vascular tissues. The recombinant chymase, the structure of which was NH2-enterokinase cleavage site-chymase-COOH, was expressed in Escherichia coli and then was solubilized and renatured. The protein did not have a chymase activity, but gained this activity after the cleavage of the N-terminal site by enterokinase. The enzyme was purified by heparin affinity and gel filtration columns. The N-terminal sequence of the protein was identical to the sequence for human chymase. The molecular weights of the recombinant chymase and chymase purified from human vascular tissues were 26 and 30 kDa, respectively, and the 4 kDa difference was thought to be due to the presence or absence of glycan. The optimum pH of the recombinant enzyme activity was between 7.5 and 9.0. The activity of the recombinant enzyme was inhibited by chymostatin, soybean trypsin inhibitor and phenylmethylsulfonyl fluoride, but not by ethylenediaminetetraacetic acid and aprotinin. This enzyme cleaved specifically the Phe8-His9 bond of angiotensin (Ang) I to form Ang II and that of big endothelin (ET)-1 to form ET-1-(1-31). These findings demonstrated that the enzymatic characteristics of the recombinant enzyme were identical to that of native human chymase.

Anti-atherosclerotic effects of an angiotensin converting enzyme inhibitor and an angiotensin II antagonist in Cynomolgus monkeys fed a high-cholesterol diet

1. We investigated the relationship between angiotensin II formation and the development of atherosclerotic lesions in the aorta of monkeys (Macaca fascicularis) fed a high-cholesterol (4% cholesterol and 6% corn oil) diet for 6 months, and studied the effects of an angiotensin converting enzyme (ACE) inhibitor, trandolapril (10 mg kg-1 per day, p.o.), and an angiotensin II type 1 receptor antagonist, 2-butyl-4-(methylthio)-1-[[2'[[[(propylamino)carbonyl]amino]sulfonyl] (1,1'-biphenyl)-4-yl]methyl]-1H-imidazole-5-carboxylate (HR 720; 20 mg kg-1 per day, p.o.). 2. The level of low-density lipoprotein was significantly increased by the cholesterol diet, whereas that of high-density lipoprotein was significantly decreased. The relative areas of the atherosclerotic lesions in the thoracic aorta in the normal and cholesterol-diet groups were 1.3+/-0.3 and 64+/-10%, respectively. 3. Plasma renin and ACE activities showed no differences between the normal and cholesterol-diet groups. ACE activity and the concentration of angiotensin II were significantly increased in the aorta of the cholesterol-fed monkeys. 4. Trandolapril and HR 720 decreased significantly the area of the atherosclerotic lesions in the thoracic aorta of cholesterol-fed monkeys, but not the mean blood pressure and the levels of low-density and high-density lipoproteins. 5. In plasma and aorta, trandolapril, but not HR 720, decreased significantly the ACE activities in the cholesterol-fed monkeys, while both of these drugs decreased significantly the angiotensin II levels. 6. In conclusion, blockade of angiotensin II function in vascular tissues by trandolapril or HR 720 may play an important role in preventing the development of atherosclerotic lesions.

Angiotensin II activates MAPK and stimulates growth of human pulmonary artery smooth muscle via AT1 receptors

To determine a potential role for the renin-angiotensin system in the growth of human pulmonary artery (PA) smooth muscle, we studied the localization of angiotensin (ANG) II-receptor subtypes by autoradiography in sections of human PA and in cultured PA smooth muscle cells (PASMCs) and examined the growth responses to ANG II in vitro. Specific 125I-labeled [Sar1,Ile8]ANG II binding was demonstrated within the pulmonary arterial media, but binding to cultured cells varied between isolates. Binding in tissues and cells was inhibited by the ANG II type 1 (AT1) receptor antagonist losartan but not by the type 2 (AT2) receptor antagonist PD-123319. Microautoradiographic studies indicated that cultured PASMCs exhibit heterogeneity with regard to ANG II binding sites. Addition of ANG II to serum-deprived PASMCs, exhibiting a relatively high level of 125I-[Sar1,Ile8]ANG II binding, led to a dose-dependent stimulation of DNA synthesis at 24 h and protein synthesis at 48 h. ANG II led to an increase in cell size without an increase in cell number. These effects were inhibited by losartan but not by PD-123319. In addition, ANG II led to rapid activation of mitogen-activated protein kinase (MAPK), and ANG II-stimulated DNA synthesis was inhibited by the specific inhibitor of MAPK PD-98059. We conclude that the AT1 receptor is expressed by human PASMCs in vivo and in vitro and is coupled to activation of MAPK and increased DNA and protein synthesis in vitro. These results are consistent with the hypothesis that ANG II may be involved in human pulmonary vascular remodeling.

Angiotensin II activation of insulin-like growth factor 1 receptor transcription is mediated by a tyrosine kinase-dependent redox-sensitive mechanism

We have recently shown that angiotensin II activation of insulin-like growth factor 1 receptor (IGF-1R) transcription is a critical requirement for angiotensin-stimulated vascular smooth muscle cell growth; therefore, we examined the signaling pathway involved. In rat aortic smooth muscle cells, the antioxidants N-acetyl-L-cysteine (5 mmol/L) and pyrrolidine dithiocarbamate (100 micromol/L) completely inhibited angiotensin II-stimulated increases in IGF-1R mRNA and protein levels, suggesting the involvement of reactive oxygen species. Indeed, catalase abolished the Ang II-stimulated increase of IGF-1R protein expression, and accordingly, H(2)O(2) (0.2 mmol/L) or the oxidized products of linoleic acid, hydroperoxyoctadecadienoic acids (10 micromol/L), increased IGF-1R mRNA levels at 3 hours by 74+/-20% and 107+/-22% and increased receptor number at 24 hours by 51+/-6.7% and 55+/-7.4%, respectively. The protein tyrosine kinase inhibitors genistein and tyrphostin A25 also blocked angiotensin II increases in IGF-1R mRNA and protein levels and blocked the ability of hydroperoxyoctadecadienoic acids and H(2)O(2) to increase IGF-1R expression, suggesting that oxidative stress may be an early event in the angiotensin II signaling cascade. Furthermore, calcium chelation inhibited the angiotensin II effect. Transient transfection assays revealed that a (-2350)+640 IGF-1R promoter/luciferase construct was fully responsive to angiotensin II stimulation (127+/-20% increase). Ten millimoles per liter hydroperoxyoctadecadienoic acids and 0.2 mmol/L H(2)O(2) increased luciferase activity by 79+/-8.5% and 63+/-12%, respectively, and 5 mmol/L N-acetyl-L-cysteine blocked the angiotensin II-induced upregulation of luciferase activity by 70%. These data suggest that angiotensin II stimulates IGF-1R gene transcription via calcium-dependent activation of protein tyrosine kinase activity that lies downstream from an oxidant stimulus. These findings provide key insights into the signaling mechanisms whereby angiotensin II exerts its growth-promoting effects on the vasculature.

A novel role for the cyclin-dependent kinase inhibitor p27(Kip1) in angiotensin II-stimulated vascular smooth muscle cell hypertrophy

Angiotensin II (Ang II) has been shown to stimulate either hypertrophy or hyperplasia. We postulated that the differential response of vascular smooth muscle cells (VSMCs) to Ang II is mediated by the cyclin-dependent kinase (Cdk) inhibitor p27(Kip1), which is abundant in quiescent cells and drops after serum stimulation. Ang II treatment (100 nM) of quiescent VSMCs led to upregulation of the cell-cycle regulatory proteins cyclin D1, Cdk2, proliferating cell nuclear antigen, and Cdk1. p27(Kip1) levels, however, remained high, and the activation of the G1-phase Cdk2 was inhibited as the cells underwent hypertrophy. Overexpression of p27(Kip1) cDNA inhibited serum-stimulated [(3)H]thymidine incorporation compared with control-transfected cells. This cell-cycle inhibition was associated with cellular hypertrophy, as reflected by an increase in the [(3)H]leucine/[(3)H]thymidine incorporation ratio and by an increase in forward-angle light scatter during flow cytometry at 48 hours after transfection. The role of p27(Kip1) in modulating the hypertrophic response of VSMCs to Ang II was further tested by antisense oligodeoxynucleotide (ODN) inhibition of p27(Kip1) expression. Ang II stimulated an increase in [(3)H]thymidine incorporation and the percentage of S-phase cells in antisense ODN-transfected cells but not in control ODN-transfected cells. We conclude that p27(Kip1) plays a role in mediating VSMC hypertrophy. Ang II stimulation of quiescent cells in which p27(Kip1) levels are high results in hypertrophy but promotes hyperplasia when levels of p27(Kip1) are low, as in the presence of other growth factors.

Angiotensin II (ATII)-inducible platelet-derived growth factor A-chain gene expression is p42/44 extracellular signal-regulated kinase-1/2 and Egr-1-dependent and mediated via the ATII type 1 but not type 2 receptor. Induction by ATII antagonized by nitric oxide

Angiotensin II (ATII) and platelet-derived growth factor (PDGF) are two vasoconstrictors implicated in the maintenance of normal vascular homeostasis. PDGF A-chain levels increase in cultured vascular smooth muscle cells (SMCs) exposed to ATII. The molecular mechanisms underlying this induction are not known. We used transient transfection analysis to show that ATII can increase reporter gene activity driven by fragments of the PDGF-A promoter bearing recognition elements for the transcription factor, Egr-1. Nuclear run-off experiments indicate that ATII induces Egr-1 expression at the level of transcription. Gel shift and supershift studies show that Egr-1 protein accumulates in the nuclei of SMCs exposed to ATII and binds to the proximal region of the PDGF-A promoter in a specific, time-dependent manner. ATII induced extracellular-signal regulated kinase (p42/44 ERK) activity as did phorbol 12-myristate 13-acetate. The specific MEK1/2 inhibitor, PD98059, suppressed both PDGF-A and Egr-1 endogenous and promoter-dependent expression inducible by ATII. The ATII type 1 receptor (AT1) antagonist, Losartan, inhibited ATII-induction of p42/44 ERK, as well as Egr-1 and PDGF-A, whereas neither PD123319, an AT2 receptor antagonist, nor wortmannin, an inhibitor of phosphatidylinositol 3-kinase and c-Jun N-terminal kinase, had any effect. ATII-induction of Egr-1 and PDGF-A was blocked by SIN-1, a NO donor. In addition, this pathway was blocked by overexpression of NO synthase. Collectively, these findings demonstrate that ATII activation of the PDGF-A promoter is mediated via the MEK/ERK/Egr-1 pathway and AT1 receptor and that this process is antagonized by NO.

Chymase-dependent angiotensin II formation in human vascular tissue

BACKGROUND

Some reports have suggested that, in vitro, human heart chymase in homogenates contributes little to angiotensin (Ang) II formation in the presence of natural protease inhibitors such as alpha-antitrypsin. We studied whether chymase bound to heparin, resembling an in vivo form, could contribute to Ang II formation in the presence of natural protease inhibitors.

METHODS AND RESULTS

The Ang II formation was increased time-dependently after incubation in an extract (1 mg of protein/mL) of human vascular tissues containing Ang I. The concentration of Ang II in the extract after incubation for 30 minutes was 1.67+/-0.06 nmol/mL, and we regarded this quantity of Ang II as 100%. The Ang II formation was inhibited 10%, 95%, and 96% by 1 micromol/L lisinopril, 100 micromol/L chymostatin, and 0.1 g/L alpha-antitrypsin, respectively. The extract was applied to a heparin affinity column. After the column was washed with PBS, the eluted PBS contained a weak Ang II-forming activity, which was completely inhibited by lisinopril. The eluted PBS, to which >0.8 mol/L NaCl had been added, showed a strong Ang II-forming activity which was inhibited by chymostatin and alpha-antitrypsin. After the application of the extract, the column was washed with PBS and then an Ang I solution in PBS was applied to the column. The Ang II formation in the PBS eluted from the incubated column was increased time-dependently. The concentration of Ang II in the PBS (1 mL) eluted from the column after incubation for 30 minutes was 2.56+/-0.28 nmol/mL, and we regarded this quantity of Ang II as 100%. To study the effects of inhibitors, the extract (1 mg of protein/mL) was applied to a heparin affinity column (1 mL) which was preequilibrated with PBS (3 mL); 100 micromol/L chymostatin or 0.1 g/L alpha-antitrypsin in PBS (1 mL) was then applied to the column. After the column was washed with PBS (3 mL), Ang I solution (1 mg/mL) in PBS was applied to the column, and the column was incubated for 30 minutes. The Ang II formation in the PBS eluted from the column was suppressed up to 5% by application of chymostatin, although this was not affected by application of alpha-antitrypsin.

CONCLUSIONS

These findings suggest that human chymase bound to heparin plays a functional role in Ang II formation in the presence of natural protease inhibitors such as alpha-antitrypsin.

Modulation of extracellular superoxide dismutase expression by angiotensin II and hypertension

Angiotensin II and hypertension increase vascular oxidant stress. We examined how these might affect expression of the extracellular superoxide dismutase (ecSOD), a major form of vascular SOD. In mice, angiotensin II infusion (1.1 mg/kg for 7 days) increased systolic blood pressure from 107+/-3 to 152+/-9 mm Hg and caused a 3-fold increase in ecSOD, but there was no change in the cytosolic Cu/Zn SOD protein, as determined by Western blot analysis. This was associated with a similar increase in ecSOD mRNA as assessed by RNase protection assay and was prevented by losartan. Induction of ecSOD by angiotensin II was not due to hypertension alone, because hypertension caused by norepinephrine (5.6 mg. kg-1. d-1) had no effect on ecSOD. Similarly, exposure of mouse aortas to angiotensin II (100 nmol/L) in organoid culture increased ecSOD by approximately 2-fold. In the organoid culture, angiotensin II-induced upregulation of ecSOD was prevented by losartan (10 micromol/L) and PD985059 (30 micromol/L), a specific inhibitor of p42/44 MAP kinase kinase. Angiotensin II activates the NADH/NADPH oxidase; however, diphenyleneiodonium chloride (10 micromol/L), an inhibitor of this oxidase, did not prevent p42/44 MAP kinase phosphorylation or ecSOD induction by angiotensin II. Finally, in human aortic smooth muscle cells, angiotensin II moderately increased transcriptional rate (as assessed by nuclear run-on analysis) but markedly increased ecSOD mRNA stability. Thus, angiotensin II increases ecSOD expression independent of hypertension, and this increase involves both an increase in ecSOD transcription and stabilization of ecSOD mRNA. This effect of angiotensin II on ecSOD expression may modulate the oxidative state of the vessel wall in pathological processes in which the renin-angiotensin system is activated.

Practical considerations of the pharmacology of angiotensin receptor blockers

A review of the drug class of angiotensin receptor blockers (ARBs) as well as the ARBs currently available by prescription in the United States is presented. The importance of angiotensin II production by non-angiotensin-converting enzyme (non-ACE) pathways, particularly human chymase, is discussed. Emphasis is placed on the mechanism of action of ARBs and the different binding kinetics of these agents. Although all ARBs, as a group, block the AT1 receptor, they may differ in the pharmacological characteristics of their binding and be classified as either surmountable or insurmountable antagonists. Mechanisms of surmountable and insurmountable antagonism as well as possible benefits of these blocking characteristics are discussed in relation to the various ARBs. The cardiovascular effects of activation of the two main subtypes of angiotensin receptors (AT1 and AT2) are presented. In addition to their treatment of hypertension, ACE inhibitors are recognized as being effective in the management of heart failure, left ventricular hypertrophy, recurrent myocardial infarctions, and renal disease. ARBs are currently indicated only for the treatment of hypertension; however, in vitro and in vivo pharmacological studies as well as preliminary clinical data suggest that ARBs, like ACE inhibitors, may also provide effective protection against end-organ damage in these conditions.

Activation of Akt/protein kinase B after stimulation with angiotensin II in vascular smooth muscle cells

Involvement of Akt/Protein kinase B (PKB), a serine/threonine kinase with a pleckstrin-homology domain, in angiotensin II (ANG II)-induced signal transduction was investigated in cultured vascular smooth muscle cells (VSMC). Stimulation of the cells with ANG II led to a marked increase in the kinase activity of Akt/PKB, which coincided with Ser-473 phosphorylation. ANG II-stimulated Akt/PKB activation was rapid, concentration dependent, and inhibited by the AT1-receptor antagonist CV-11974, but not by pertussis toxin. Akt/PKB activity was stimulated by the Ca2+ ionophore ionomycin, suggesting the possible involvement of Ca2+ in ANG II-stimulated Akt/PKB activation. However, blockade of Ca2+ mobilization by BAPTA-AM only partially inhibited ANG II-stimulated Akt/PKB activation. ANG II-stimulated Akt/PKB activation was inhibited by the tyrosine kinase inhibitors genistein and herbimycin A and by the phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin and LY-294002. These results indicate that ANG II stimulates Akt/PKB activity via AT1 receptors in VSMC and that the activities of tyrosine kinase and PI3K are required for this activation.

Redox regulation of c-Ha-ras and osteopontin signaling in vascular smooth muscle cells: implications in chemical atherogenesis

Reduction/oxidation (redox) reactions play a central role in the regulation of vascular cell functions. Recent studies in this laboratory have identified c-Ha-ras and osteopontin genes as critical molecular targets during oxidant-induced atherogenesis. This review focuses on the deregulation of gene transcription by redox-activated trans-acting factors after benzo(a)pyrene challenge and the modulation of extracellular matrix signaling in vascular smooth muscle cells by allylamine-induced oxidative injury. The induction of atherogenic vascular smooth muscle cell phenotypes by chemical injury exhibits remarkable parallels with those seen in other forms of atherogenesis.

Distinct and common pathways in the regulation of insulin-like growth factor-1 receptor gene expression by angiotensin II and basic fibroblast growth factor

Angiotensin II (Ang II) and basic fibroblast growth factor (bFGF) are important modulators of cell growth under physiological and pathophysiological conditions. We and others have previously shown that these growth factors increase insulin-like growth factor-1 receptor (IGF-1R) number and mRNA in vascular smooth muscle cells and that this effect is transcriptionally regulated. To study the mechanisms and the signaling pathways involved, IGF-1R promoter reporter constructs were transiently transfected in CHO-AT1 cells that overexpress angiotensin AT1 receptors. Our findings indicate that Ang II and bFGF significantly increased IGF-1R promoter activity up to 7- and 3-fold, respectively. The effect induced by Ang II was mediated via a tyrosine kinase-dependent mechanism, since tyrphostin A25 largely inhibited the Ang II-induced increase in promoter activity. In addition, co-transfection of dominant negative Ras, Raf, and MEK1 or pretreatment with the MEK inhibitor PD 98059 dose-dependently decreased both the Ang II- and bFGF-induced increase in IGF-1R transcription and protein expression, suggesting that the Ras-Raf-mitogen-activated protein kinase kinase pathway is required for both growth factors. Reactive oxygen species have been shown to act as second messengers in Ang II-induced signaling, and activation of the transcription factor NF-kappaB is redox-sensitive. While co-transfection of dominant negative IkappaBalpha mutant completely inhibited the Ang II-induced increase in transcription, it had no effect on the bFGF signaling. In contrast, co-transfection studies indicated that the transcription factors STAT1, STAT3, and c-Jun and the Janus kinase 2 kinase are required in the signaling pathway of bFGF, whereas only dominant c-Jun inhibited the Ang II-induced effect. In summary, these data demonstrate that Ang II and bFGF increase IGF-1R gene transcription via distinct as well as shared pathways and have important implications for understanding growth-stimulatory effects of these growth factors on vascular cells.

Angiotensin II type 1 receptor-modulated signaling pathways in neurons

Mammalian brain contains high densities of angiotensin II (Ang II) type 1 (AT1) receptors, localized mainly to specific nuclei within the hypothalamus and brainstem regions. Neuronal AT1 receptors within these areas mediate the stimulatory actions of central Ang II on blood pressure, water and sodium intake, and vasopressin secretion, effects that involve the modulation of brain noradrenergic pathways. This review focuses on the intracellular events that mediate the functional effects of Ang II in neurons, via AT1 receptors. The signaling pathways involved in short-term changes in neuronal activity, membrane ionic currents, norepinephrine (NE) release, and longer-term neuromodulatory actions of Ang II are discussed. It will be apparent from this discussion that the signaling pathways involved in these events are often distinct.

An A/T-rich cis-element is essential for rat angiotensin II type 1A receptor transcription in vascular smooth muscle cells

Transcriptional mechanisms regulating the expression of the rat angiotensin II type 1A receptor (rAT1AR) gene were investigated in cultured rat vascular smooth muscle cells (VSMC). Transcriptional analyses of various 5'-deletion mutants of the rAT1AR promoter region, fused upstream from the firefly luciferase gene, demonstrated that a 71 base pair (bp) region (-557 to -486 bp, with respect to transcription initiation) was necessary for expression of this gene in VSMC. Electrophoretic mobility shift assays demonstrated that specific protein-DNA complexes were formed with the -516 to -486 bp region of the rAT1AR promoter when incubated with VSMC extract. Computer analysis of this region indicated the presence of an A/T-rich sequence (i.e., TTTAAAAATAAA) which is similar to a myocyte enhancer binding factor 2 (MEF2) cis-regulatory element (i.e., CTTAAAAATAAC). Site-directed mutagenesis of this A/T-rich sequence inhibited rAT1AR promoter activity in VSMC, suggesting that this region was necessary for expression of this gene in these cells. Immuno-gel shift experiments suggest that MEF2 heterodimers may interact with the A/T-rich sequence in the rAT1AR promoter. Additionally, it was demonstrated that a transcription factor non-homologous to MEF2 can also interact with this A/T-rich site in the rAT1AR promoter. Taken together, our results suggest that MEF2 heterodimers, and/or transcription factors non-homologous to MEF2, are required to regulate the expression of the rAT1AR gene in VSMC.

Functional role of chymase in angiotensin II formation in human vascular tissue

Recent reports suggested that human heart chymase contributed little to angiotensin (Ang) II formation in the presence of natural protease inhibitors such as alpha-antitrypsin. We studied whether chymase could contribute to Ang II formation in the presence of natural protease inhibitors in the homogenate, the extract, and slices of human vascular tissue, and whether these inhibitors affect Ang I-induced vasocontractile responses due to chymase. In the homogenate, lisinopril, chymostatin, and alpha-antitrypsin inhibited the formation of Ang II by 14, 92, and 74%, respectively. In the extract, the inhibition of Ang II formation by lisinopril, chymostatin, and alpha-antitrypsin was 18, 94, and 93%, respectively. In the slices, lisinopril and chymostatin inhibited Ang II formation by 5 and 90%, respectively. However, unlike the homogenate and the extract experiments, only 8% of the Ang II formation was suppressed by alpha-antitrypsin. In isolated human gastroepiploic artery, 30% of Ang I-induced vasoconstriction was blocked by lisinopril, and the rest was completely eliminated by a combination of lisinopril and chymostatin. On the other hand, alpha-antitrypsin was ineffective in blocking Ang I-induced vasoconstriction in the presence of lisinopril, which demonstrates that Ang II formation is dependent on chymase. These findings suggest that chymase in human vascular tissue plays a functional role in Ang II formation in the presence of natural protease inhibitors such as alpha-antitrypsin.

[The renin-angiotensin system in cardiovascular diseases]

BACKGROUND

The renin-angiotensin system is mainly involved in several cardiovascular diseases and in the pathophysiology of heart failure. It exists as a circulating and a local system which can be differently regulated. Interventions in this system by angiotensin-converting enzyme (ACE) antagonists or angiotensin-receptor antagonists slow the progression of heart failure and result in prolongation of life expectancy and improvement of hemodynamics.

MECHANISMS OF ACTION

The main underlying mechanisms are: 1. Heart failure results in activation of the renin-angiotensin system as a compensatory mechanism with elevation of circulating angiotensin II, norepinephrine and vasopressin. Antagonists of this compensatory mechanisms acutely result in improvement of the hemodynamic situation. 2. Elevated circulating and local renin-angiotensin systems cause chronic structural myocardial and vascular effects. Angiotensin-converting enzyme antagonists and angiotensin-receptor blockers modulate and partly antagonize these structural changes such as myocardial hypertrophy, myocardial fibrosis and vascular proliferative responses. Gene and receptor regulation of the system are currently not fully understood and are subject of intensive research. 3. The renin-angiotensin system is closely related to the bradykinin system and thus indirectly to nitric oxide and endothelial function. Bradykinin has multiple other effects on the hemostatic system as a well as on the myocardium and vascular system.

CONCLUSION

These complex interactions require further evaluation. Research with specific bradykinin antagonists will give new insights into this system.

Angiotensin II induces hypertrophy of human airway smooth muscle cells: expression of transcription factors and transforming growth factor-beta1

Increased smooth muscle mass due to hyperplasia and hypertrophy of airway smooth muscle (ASM) cells is a common feature in asthma. Angiotensin II (Ang II), a potent vasoconstrictor and mitogen for a wide variety of cells, has recently been implicated in bronchoconstriction in asthmatics. However, a possible mitogenic role as well as underlying molecular mechanisms of this octapeptide in human ASM cells are not yet known. We studied the effects of Ang II on ASM cell proliferation and growth and on the expression of three transcription factors, egr-1, c-fos, and c-jun, as well as a cytokine, transforming growth factor-beta1 (TGF-beta1). Human ASM cells were isolated by enzymatic digestion of bronchial smooth muscle obtained from lung resection tissue. Confluent cells were growth-arrested and subsequently incubated with Ang II (100 nM) for different time periods and processed for the measurement of cell growth and gene expression. Ang II significantly induced DNA and protein synthesis in human ASM cells at 8 h, resulting in a net increase in the accumulation of protein over DNA (i.e., cellular hypertrophy) at 16 h of incubation. Cell counts and MTT-reduction assay, however, showed no increase in cell number as a result of Ang II stimulation. Ang II stimulated the expression of egr-1 and c-fos as early as 15 min, reaching maximum levels at 45 min, whereas the expression of c-jun peaked at 2 h of Ang II exposure. Furthermore, steady-state mRNA levels of TGF-beta1 were upregulated by Ang II after 4 h and reached peak levels at 16 h of incubation. Secretion of biologically active TGF-beta1 from human ASM cells was significantly (P <= 0.02) enhanced by Ang II incubation after 8 h, which remained elevated until 24 h. Our results suggest that the Ang II-induced transient early expression of transcription factors may regulate autocrine genes like TGF-beta1, of which the subsequent late upregulation could contribute to cellular hypertrophy during, for example, airway remodeling in asthma.

Nephrogenesis and angiotensin II receptor subtypes gene expression in the fetal lamb

To investigate the role of angiotensin II (ANG II) in nephrogenesis, a developmental study of renal AT1 and AT2 receptor mRNA expression was performed in parallel with the quantitative and qualitative analysis of metanephros development in fetal lamb from 60 to 140 days of gestation. Both ANG II receptor subtypes were expressed early during nephrogenesis but displayed specific spatial and temporal distribution during gestation. High-AT2 mRNA expression took place in the outermost nephrogenic area and in the undifferentiated mesenchymal cells surrounding the ampulla; level of AT2 expression in this localization followed closely glomeruli proliferation rate and disappeared after nephrogenesis completion (>120 days). AT2 mRNA was also detected in the differentiated epithelial cells of macula densa of maturing glomeruli. Although most of AT1 mRNA labeling was found in the mesangial cells of maturing glomeruli, where it persisted after nephrogenesis completion, additional labeling was found in undifferentiated cells, in cells invading the inferior cleft of S-shaped bodies (80 days), and in medullar cells between tubules (120 days). Our results suggest that each receptor subtype has a specific role in renal morphogenesis, i.e., AT2 in mesenchymal proliferation or apoptosis and AT1 in vascular smooth muscle cells differentiation.

Induction of angiotensin converting enzyme and angiotensin II receptors in the atherosclerotic aorta of high-cholesterol fed Cynomolgus monkeys

Antiatherogenic effects of imidapril and involvement of renin angiotensin system were examined in experimental atherosclerosis induced by feeding a high-cholesterol diet to Cynomolgus monkeys. Eighteen male monkeys were divided into three groups and placed under (1) normal diet (normal group), (2) high-cholesterol diet (control group), (3) high-cholesterol diet with imidapril (20 mg/kg body wt/day, orally) treatment (imidapril group). At the end of the experiment, the normal group showed no apparent atherosclerosis in their aorta evaluated by oil red-O staining, while the control group exhibited marked atherosclerotic involvement of the intimal surface of the aorta (58.4 +/- 9.3%, P < 0.01). Imidapril reduced systolic blood pressure and atherosclerotic involvement (24.1 +/- 5.5%, P < 0.05). Total cholesterol content of the descending thoracic aorta was also significantly reduced in the imidapril group. In the atherosclerotic vessels, angiotensin converting enzyme (ACE) activity evaluated by quantitative in vitro autoradiography was significantly increased in the intimal lesion. Further evaluation revealed angiotensin II (Ang II) type I (AT1) receptor density was significantly increased in the medial lesion and type II (AT2) receptor density in the adventitia. When the progression of atherosclerosis was impeded by imidapril treatment, the ACE activity level as well as the AT1 and AT2 receptor density remained at normal. Expression of mRNA for fibronectin, TGF-beta1, types I and III collagen was studied by Northern blot analysis. No significant differences in types I and III collagen mRNA levels were found between the control and imidapril group. On the other hand, mRNA expression for fibronectin and TGF-beta1 were much lower in the imidapril group than in the control group. These results suggest that increased production of Ang II and activated receptors may be involved in atherosclerotic process in this model and also antiatherogenic effect of imidapril may be derived from reduction of local Ang II production as well as its hypotensive action.

Calcium-dependent epidermal growth factor receptor transactivation mediates the angiotensin II-induced mitogen-activated protein kinase activation in vascular smooth muscle cells

We have recently reported that angiotensin II (Ang II)-induced mitogen-activated protein kinase (MAPK) activation is mainly mediated by Ca2+-dependent activation of a protein tyrosine kinase through Gq-coupled Ang II type 1 receptor in cultured rat vascular smooth muscle cells (VSMC). In the present study, we found Ang II rapidly induced the tyrosine phosphorylation of the epidermal growth factor (EGF) receptor and its association with Shc and Grb2. These reactions were inhibited by the EGF receptor kinase inhibitor, AG1478. The Ang II-induced phosphorylation of the EGF receptor was mimicked by a Ca2+ ionophore and completely inhibited by an intracellular Ca2+ chelator. Thus, AG1478 abolished the MAPK activation induced by Ang II, a Ca2+ ionophore as well as EGF but not by a phorbol ester or platelet-derived growth factor-BB in the VSMC. Moreover, Ang II induced association of EGF receptor with catalytically active c-Src. This reaction was not affected by AG1478. These data indicate that Ang II induces Ca2+-dependent transactivation of the EGF receptor which serves as a scaffold for pre-activated c-Src and for downstream adaptors, leading to MAPK activation in VSMC.

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.

Vascular remodeling: the role of angiotensin-converting enzyme inhibitors

Chronic sustained hypertension leads to structural changes of the small and large arteries. These alterations consist of smooth-muscle hypertrophy, increased deposition of collagen, and "dilution" or destruction of elastin fibers. In addition, there may be no growth at all, but a "rearrangement" of vascular wall material termed "remodeling." These changes serve to increase wall thickness and the media-to-lumen ratio and to decrease the external and internal diameter of the vessel--all of which contribute to increased systemic vascular resistance by the small arteries and increased impedance by the larger arteries. It has been suggested that these structural changes are an adaptive effort by the vessel to maintain a constancy of wall tension, but the end result is detrimental in that the effect is a further increase in systemic vascular resistance and blood-flow impedance, which lead to left ventricular hypertrophy and its consequences. The stimuli for these changes are stretch stimuli, mediated through stretch receptors on the arterial wall, and trophic stimuli mediated at the tissue level through the action of angiotensin II, aldosterone, and catecholamines. Angiotensin-converting enzyme inhibitors, especially those with effective tissue penetration, are ideal drugs to reverse these structural changes ("reverse remodeling"), decrease the systemic vascular resistance, and increase the vascular compliance. These agents exert their effects through suppression, at the tissue level, of angiotensin II, aldosterone, catecholamines, endothelins (ET1, ET3), and transforming growth factor-beta1 (TGF-beta1) and through an increase in local levels of kinins, prostaglandins, and nitric oxide, which have antigrowth effects. Although this is a class effect, it appears to be stronger with those angiotensin-converting enzyme inhibitors providing the greatest tissue penetration.

Increased mRNA expression of cardiac renin-angiotensin system and collagen synthesis in spontaneously hypertensive rats

Hypertensive cardiac hypertrophy is associated with the accumulation of collagen in the myocardial interstitium. Previous studies have demonstrated that this myocardial fibrosis accounts for impaired myocardial stiffness and ventricular dysfunction. Although cardiac fibroblasts are responsible for the synthesis of fibrillar collagen, the factors that regulate collagen synthesis in cardiac fibroblasts are not fully understood. We investigated the effects of angiotensin II on cardiac collagen synthesis in cardiac fibroblasts. Cardiac fibroblasts of 10 week old spontaneously hypertensive rats and age-matched Wistar-Kyoto rats were prepared and maintained in culture medium supplemented with 10% fetal calf serum. The expression of mRNA of the renin-angiotensin system (renin, angiotensinogen, angiotensin converting enzyme) was determined by using a ribonuclease protection assay. Basal collagen synthesis in cardiac fibroblasts from spontaneously hypertensive rats was 1.6 fold greater than that in the cell of Wistar-Kyoto rats. Angiotensin II stimulated collagen synthesis in cardiac fibroblasts in a dose-dependent manner. The responsiveness of collagen production to angiotensin II was significantly enhanced in cardiac fibroblasts from spontaneously hypertensive rats (100 nM angiotensin II resulted in 185 +/- 18% increase above basal levels, 185 +/- 18 versus 128 +/- 19% in Wistar-Kyoto rats p < 0.01). This effect was receptor-specific, because it was blocked by the competitive inhibitor saralasin and MK 954. These results indicate that collagen production was enhanced in cardiac fibroblasts from spontaneously hypertensive rats, that angiotensin II had a stimulatory effect on collagen synthesis in cardiac fibroblasts, and that cardiac fibroblasts from spontaneously hypertensive rats were hyper-responsive to stimulation by angiotensin II. Level of angiotensin and renin mRNA expressed in ventricles, and angiotensinogen mRNA expressed in fibroblasts from SHR were higher than those from WKY. These findings suggest that the cardiac renin-angiotensin system may play an important role in collagen accumulation in hypertensive cardiac hypertrophy.

Cyclic AMP-mediated inhibition of angiotensin II-induced protein synthesis is associated with suppression of tyrosine phosphorylation signaling in vascular smooth muscle cells

In the present study, we have examined the effect of increased cyclic AMP (cAMP) levels on the stimulatory action of angiotensin II (Ang II) on protein synthesis. Treatment with cAMP-elevating agents potently inhibited Ang II-induced protein synthesis in rat aortic smooth muscle cells and in rat fibroblasts expressing the human AT1 receptor. The inhibition was dose-dependent and was observed at all concentrations of the peptide. To explore the mechanism of cAMP action, we have analyzed the effects of forskolin and 3-isobutyl-1-methylxanthine on various receptor-mediated responses. Elevation of cAMP did not alter the binding properties of the AT1 receptor and did not interfere with the activation of phospholipase C or the induction of early growth response genes by Ang II. Likewise, Ang II-dependent activation of the mitogen-activated protein kinases ERK1/ERK2 and p70 S6 kinase was unaffected by cAMP. In contrast, we found that increased concentration of cAMP strongly inhibited the stimulatory effect of Ang II on protein tyrosine phosphorylation. Specifically, cAMP abolished Ang II-induced tyrosine phosphorylation of the focal adhesion-associated protein paxillin and of the tyrosine kinase Tyk2. These results identify a novel mechanism by which the cAMP signaling system may exert growth-inhibitory effects in specific cell types.