AT2 receptor activation regulates myocardial eNOS expression via the calcineurin-NF-AT pathway

Alamandine treatment prevents LPS-induced acute renal and systemic dysfunction with multi-organ injury in rats via inhibiting iNOS expression

Sepsis is defined as the dysregulated immune response leading to multi-organ dysfunction and injury. Sepsis-induced acute kidney injury is a significant contributor to morbidity and mortality. Alamandine (ALA) is a novel endogenous peptide of the renin-angiotensin-aldosterone system. It is known for its anti-inflammatory and anti-apoptotic effects, but its functional and vascular effects on sepsis remain unclear. We aimed to investigate the effects of ALA, as a pre- and post-treatment agent, on lipopolysaccharide (LPS)-induced systemic and renal dysfunction and injury in the LPS-induced endotoxemia model in rats via functional, hemodynamic, vascular, molecular, biochemical, and histopathological evaluation. 10 mg/kg intraperitoneal LPS injection caused both hepatic and renal injury, decreased blood flow in several organs, and renal dysfunction at 20 h in Sprague-Dawley rats. Our results showed that ALA treatment ameliorated systemic and renal inflammation, reduced inflammatory cytokines, prevented the enhancement of the mortality rate, reversed vascular dysfunction, corrected decreased blood flows in several organs, and reduced renal and hepatic injury via inhibiting iNOS (inducible nitric oxide synthase) and caspase expressions in the kidney. In addition, expressions of different ALA-related receptors showed alterations in this model, and ALA treatment reversed these alterations. These data suggest that ALA's systemic and renal protective effects are achieved through its anti-inflammatory, anti-pyroptotic, and anti-apoptotic effects on hemodynamic and vascular functions via reduced iNOS expression.

Characterization of the First Animal Toxin Acting as an Antagonist on AT1 Receptor

The renin-angiotensin system (RAS) is one of the main regulatory systems of cardiovascular homeostasis. It is mainly composed of angiotensin-converting enzyme (ACE) and angiotensin II receptors AT1 and AT2. ACE and AT1 are targets of choice for the treatment of hypertension, whereas the AT2 receptor is still not exploited due to the lack of knowledge of its physiological properties. Peptide toxins from venoms display multiple biological functions associated with varied chemical and structural properties. If Brazilian viper toxins have been described to inhibit ACE, no animal toxin is known to act on AT1/AT2 receptors. We screened a library of toxins on angiotensin II receptors with a radioligand competition binding assay. Functional characterization of the selected toxin was conducted by measuring second messenger production, G-protein activation and β-arrestin 2 recruitment using bioluminescence resonance energy transfer (BRET) based biosensors. We identified one original toxin, A-CTX-cMila, which is a 7-residues cyclic peptide from Conus miliaris with no homology sequence with known angiotensin peptides nor identified toxins, displaying a 100-fold selectivity for AT1 over AT2. This toxin shows a competitive antagonism mode of action on AT1, blocking Gαq, Gαi3, GαoA, β-arrestin 2 pathways and ERK_1/2 activation. These results describe the first animal toxin active on angiotensin II receptors.

Angiotensin II type 1 receptor is involved in hypertension and vascular alterations caused by environmental toxicant hexachlorobenzene

Environmental hexachlorobenzene (HCB) increases blood pressure (BP) in female rats, causing alterations in arterial structure and function. Here we study the role of Angiotensin II receptor type 1 (AT1) in HCB-induced hypertension through the use of AT1 antagonist losartan. HCB-treated male rats showed a 22.7% increase in BP which was prevented by losartan. Losartan blocked HCB-induced changes in arterial morphology (decreased aorta cell number and increased wall thickness). Losartan also prevented HCB-induced alterations in artery relaxation by acetylcholine and nitroprusside but not the reduction in the maximum contraction by phenylephrine. Losartan rescued arterial molecular alterations caused by HCB (i.e. an increase in TGF-β1 and AT1 expression and a decrease in eNOS expression and nitrite levels) and reduced hydrogen sulfide plasma concentration. In conclusion: in this work we demonstrate that AT1 activity is involved in HCB effects on the vascular system leading to hypertension.

Angiotensin II receptor type 1 - An update on structure, expression and pathology

The AT₁ receptor, a major effector of the renin-angiotensin system, has been extensively studied in the context of cardiovascular and renal disease. Moreover, angiotensin receptor blockers, sartans, are among the most frequently prescribed drugs for the treatment of hypertension, chronic heart failure and chronic kidney disease. However, precise molecular insights into the structure of this important drug target have not been available until recently. In this context, seminal studies have now revealed exciting new insights into the structure and biased signaling of the receptor and may thus foster the development of novel therapeutic approaches to enhance the efficacy of pharmacological angiotensin receptor antagonism or to enable therapeutic induction of biased receptor activity. In this review, we will therefore highlight these and other seminal publications to summarize the current understanding of the tertiary structure, ligand binding properties and downstream signal transduction of the AT₁ receptor.

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

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

Identification of protein phosphatase involvement in the AT2 receptor-induced activation of endothelial nitric oxide synthase

The Angiotensin II type 2 receptor (AT₂R) promotes vasodilation by nitric oxide (NO) release from endothelial cells. However, the mechanisms underlying the AT₂R-induced stimulation of endothelial NO synthase (eNOS) is still not completely understood. Therefore, we investigated whether in addition to the known AT₂R-mediated phosphorylation of eNOS at Ser¹¹⁷⁷, activation of phosphatases and dephosphorylation of eNOS at Tyr⁶⁵⁷ and Thr⁴⁹⁵ are also involved. Human aortic endothelial cells (HAEC) were stimulated with the AT₂R-agonist Compound 21 (C21) (1 µM) in the presence or absence of either PD123319 (10 µM; AT₂R antagonist), l-NG-Nitroarginine methyl ester (l-NAME) (10 µM; eNOS inhibitor), MK-2206 (100 nM; protein kinase B (Akt) inhibitor) sodium fluoride (NaF) (1 nM; serine/threonine phosphatase inhibitor) or sodium orthovanadate (Na₃VO₄) (10 nM; tyrosine phosphatase inhibitor). NO release was estimated by quantifying 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate (DAF-FM) fluorescence. The phosphorylation status of activating (eNOS-Ser¹¹⁷⁷) or inhibitory eNOS residues (eNOS-Tyr⁶⁵⁷, eNOS-Thr⁴⁹⁵) was determined by Western blotting. Phosphorylation of Akt at Ser⁴⁷³ was measured to estimate Akt activity. AT₂R stimulation significantly increased NO release from HAEC, which was blocked by PD123319, l-NAME and both phosphatase inhibitors. Intracellular calcium transients were not changed by C21. AT₂R stimulation resulted in phosphorylation of eNOS-Ser¹¹⁷⁷ and dephosphorylation of eNOS-Tyr⁶⁵⁷ and eNOS-Thr⁴⁹⁵ Phosphorylation at eNOS-Ser¹¹⁷⁷ was prevented by inhibition of Akt with MK-2206. From these data, we conclude that AT₂R stimulation in human endothelial cells increases eNOS activity through phosphorylation of activating eNOS residues (eNOS-Ser¹¹⁷⁷) by Akt, and through dephosphorylation of inactivating eNOS residues (eNOS-Tyr⁶⁵⁷, eNOS-Thr⁴⁹⁵) by serine/threonine and tyrosine phosphatases, thus increasing NO release.

Maintenance of normal blood pressure is dependent on IP3R1-mediated regulation of eNOS

Endothelial cells (ECs) are critical mediators of blood pressure (BP) regulation, primarily via the generation and release of vasorelaxants, including nitric oxide (NO). NO is produced in ECs by endothelial NO synthase (eNOS), which is activated by both calcium (Ca(2+))-dependent and independent pathways. Here, we report that intracellular Ca(2+) release from the endoplasmic reticulum (ER) via inositol 1,4,5-trisphosphate receptor (IP3R) is required for Ca(2+)-dependent eNOS activation. EC-specific type 1 1,4,5-trisphosphate receptor knockout (IP3R1(-/-)) mice are hypertensive and display blunted vasodilation in response to acetylcholine (ACh). Moreover, eNOS activity is reduced in both isolated IP3R1-deficient murine ECs and human ECs following IP3R1 knockdown. IP3R1 is upstream of calcineurin, a Ca(2+)/calmodulin-activated serine/threonine protein phosphatase. We show here that the calcineurin/nuclear factor of activated T cells (NFAT) pathway is less active and eNOS levels are decreased in IP3R1-deficient ECs. Furthermore, the calcineurin inhibitor cyclosporin A, whose use has been associated with the development of hypertension, reduces eNOS activity and vasodilation following ACh stimulation. Our results demonstrate that IP3R1 plays a crucial role in the EC-mediated vasorelaxation and the maintenance of normal BP.

Eya4 Induces Hypertrophy via Regulation of p27kip1

BACKGROUND

E193, a heterozygous truncating mutation in the human transcription cofactor Eyes absent 4 (Eya4), causes hearing impairment followed by dilative cardiomyopathy.

METHODS AND RESULTS

In this study, we first show Eya4 and E193 alter the expression of p27(kip1) in vitro, suggesting Eya4 is a negative regulator of p27. Next, we generated transgenic mice with cardiac-specific overexpression of Eya4 or E193. Luciferase and chromatin immunoprecipitation assays confirmed Eya4 and E193 bind and regulate p27 expression in a contradictory manner. Activity and phosphorylation status of the downstream molecules casein kinase-2α and histone deacetylase 2 were significantly elevated in Eya4- but significantly reduced in E193-overexpressing animals compared with wild-type littermates. Magnetic resonance imaging and hemodynamic analysis indicate Eya4-overexpression results in an age-dependent development of hypertrophy already under baseline conditions with no obvious functional effects, whereas E193 animals develop onset of dilative cardiomyopathy as seen in human E193 patients. Both cardiac phenotypes were aggravated on pressure overload. Finally, we identified a new heterozygous truncating Eya4 mutation, E215, which leads to similar clinical features of disease and a stable myocardial expression of the mutant protein as seen with E193.

CONCLUSIONS

Our results implicate Eya4/Six1 regulates normal cardiac function via p27/casein kinase-2α/histone deacetylase 2 and indicate that mutations within this transcriptional complex and signaling cascade lead to the development of cardiomyopathy.

The effects of A2B receptor modulators on vascular endothelial growth factor and nitric oxide axis in chronic cyclosporine nephropathy

Introduction:

To investigate the actions of adenosine A_2B receptor modulators on VEGF and NO levels in CsA nephropathy.

Materials and Methods:

Nephropathy was induced by administrating 25 mg/kg (s.c) of CsA for 5 weeks. The VEGF and NO levels were measured in kidney tissue. Serum creatinine, creatinine clearance, urinary albumin excretion, blood urea nitrogen, kidney pathology score were measured to assess renal function. The analysis of mRNA expression of A_2B receptor and VEGF was performed.

Results:

Administration of CsA for 5 weeks induced adverse renal function. The mRNA expression of VEGF was reduced in renal tissue after 5 weeks of CsA treatment. The renal VEGF and NO levels were also reduced in these animals. In vivo administration of A_2B adenosine receptor agonist increased renal VEGF which was inhibited by a selective A_2B AR antagonist (MRS1754) in CsA-treated animals. The increase in VEGF was associated with reversal of adverse renal functions. The effects of A_2B AR modulators were prominent in CsA-treated animals compared with control animals suggesting CsA treatment may upregulate A_2B ARs. The mRNA expression of A_2B AR was increased after 5 weeks of CsA.

Conclusions:

A_2B AR modulators may provide new therapeutic options to retard CsA nephropathy by mediating renal VEGF and NO.

ROS and endothelial nitric oxide synthase (eNOS)-dependent trafficking of angiotensin II type 2 receptor begets neuronal NOS in cardiac myocytes

Angiotensin II (Ang II), a potent precursor of hypertrophy and heart failure, upregulates neuronal nitric oxide synthase (nNOS or NOS1) in the myocardium. Here, we investigate the involvement of type 1 and 2 angiotensin receptors (AT1R and AT2R) and molecular mechanisms mediating Ang II-upregulation of nNOS. Our results showed that pre-treatment of left ventricular (LV) myocytes with antagonists of AT1R or AT2R (losartan, PD123319) and ROS scavengers (apocynin, tiron or PEG-catalase) blocked Ang II-upregulation of nNOS. Surface biotinylation or immunocytochemistry experiments demonstrated that AT1R expression in plasma membrane was progressively decreased (internalization), whereas AT2R was increased (membrane trafficking) by Ang II. Inhibition of AT1R or ROS scavengers prevented Ang II-induced translocation of AT2R to plasma membrane, suggesting an alignment of AT1R-ROS-AT2R. Furthermore, Ang II increased eNOS-Ser(1177) but decreased eNOS-Thr(495), indicating concomitant activation of eNOS. Intriguingly, ROS scavengers but not AT2R antagonist prevented Ang II-activation of eNOS. NOS inhibitor (L-NG-Nitroarginine Methyl Ester, L-NAME) or eNOS gene deletion (eNOS(-/-)) abolished Ang II-induced membrane trafficking of AT2R, nNOS protein expression and activity. Mechanistically, S-nitrosation of AT2R was increased by sodium nitroprusside (SNP), a NO donor. Site-specific mutagenesis analysis reveals that C-terminal cysteine 349 in AT2R is essential in AT2R translocation to plasma membrane. Taken together, we demonstrate, for the first time, that Ang II upregulates nNOS protein expression and activity via AT1R/ROS/eNOS-dependent S-nitrosation and membrane translocation of AT2R. Our results suggest a novel crosstalk between AT1R and AT2R in regulating nNOS via eNOS in the myocardium under pathogenic stimuli.

Bone marrow angiotensin AT2 receptor deficiency aggravates atherosclerosis development by eliminating macrophage liver X receptor-mediated anti-atherogenic actions

BACKGROUND

Bone marrow (BM) Angiotensin II (Ang II) type 1 (AT1) receptor plays a crucial role in atherosclerosis development; however, the effect of BM Ang II type 2 (AT2) receptor on atherogenesis remains undefined.

METHODS AND RESULTS

We generated BM chimera apoE-deficient (apoE(-/-)) mice whose BM cells were repopulated with AT2-deficient (Agtr2(-/-)) or wild-type (Agtr2(+/+)) cells. After 2 months of a high-cholesterol diet, the atherosclerotic lesion area was significantly increased in the apoE(-/-)/BM-Agtr2(-/-) mice compared with the apoE(-/-)/BM-Agtr2(+/+) mice (51%, P < 0.05), accompanied by an augmented accumulation of lesion macrophages. Although phenotypic polarization in BM-derived macrophages and lipopolysaccharide-induced expression of proinflammatory cytokines in thioglycollate-induced peritoneal macrophages (TGPMs) were not affected by AT2-deficiency, mRNA and protein expression levels of macrophage liver X receptor β (LXRβ) were significantly decreased in Agtr2(-/-) TGPMs compared with Agtr2(+/+) TGPMs. Anti-inflammatory effects of LXR agonist (GW3965) were markedly inhibited in Agtr2(-/-) TGPMs. Furthermore, the expression levels of ATP-binding cassette transporter ABCA1 and CCR7 were much lower in Agtr2(-/-) TGPMs than Agtr2(+/+) TGPMs, accompanied by a significantly reduced cholesterol efflux.

CONCLUSIONS

Our findings demonstrate that BM-AT2 deficiency aggravates atherosclerosis, at least in part, by eliminating the anti-atherogenic properties of macrophages elicited by LXRβ activation.

VEGF increases the proliferative capacity and eNOS/NO levels of endothelial progenitor cells through the calcineurin/NFAT signalling pathway

We have investigated whether VEGF (vascular endothelial growth factor) regulates the proliferative capacity and eNOS (endothelial nitric oxide synthase)/NO (nitric oxide) pathway of EPCs (endothelial progenitor cells) by activating CaN (calcineurin)/NFAT (nuclear factor of activated T-cells) signalling. EPCs were obtained from cultured mononuclear cells isolated from the peripheral blood of healthy adults. Treatment with VEGF (50 ng/ml) potently promoted CaN enzymatic activity, activation of NFAT2, cell proliferation, eNOS protein expression and NO production. Pretreatment with cyclosporin A (10 μg/ml), a pharmacological inhibitor of CaN or 11R-VIVIT, a special inhibitor of NFAT, completely abrogated the aforementioned effects of VEGF treatment and increased apoptosis. The results indicate that VEGF treatment promotes the proliferative capacity of human EPCs by activating CaN/NFAT signalling leading to increased eNOS protein expression and NO production.

Angiotensin II type 2 receptor agonists: where should they be applied?

INTRODUCTION

Angiotensin II, the active endproduct of the renin-angiotensin system (RAS), exerts its effects via angiotensin II type 1 and type 2 (AT(1), AT(2)) receptors. AT(1) receptors mediate all well-known effects of angiotensin II, ranging from vasoconstriction to tissue remodeling. Thus, to treat cardiovascular disease, RAS blockade aims at preventing angiotensin II-AT(1) receptor interaction. Yet RAS blockade is often accompanied by rises in angiotensin II, which may exert beneficial effects via AT(2) receptors.

AREAS COVERED

This review summarizes our current knowledge on AT(2) receptors, describing their location, function(s), endogenous agonist(s) and intracellular signaling cascades. It discusses the beneficial effects obtained with C21, a recently developed AT(2) receptor agonist. Important questions that are addressed are do these receptors truly antagonize AT(1) receptor-mediated effects? What about their role in the diseased state and their heterodimerization with other receptors?

EXPERT OPINION

The general view that AT(2) receptors exclusively exert beneficial effects has been challenged, and in pathological models, their function sometimes mimics that of AT(1) receptors, for example, inducing vasoconstriction and cardiac hypertrophy. Yet given its upregulation in various pathological conditions, the AT(2) receptor remains a promising target for treatment, allowing effects beyond blood pressure-lowering, for example, in stroke, aneurysm formation, inflammation and myocardial fibrosis.

Interaction of plasma membrane Ca(2+)-ATPase isoform 4 with calcineurin A: implications for catecholamine secretion by PC12 cells

PMCA1-4 isoforms have been recently recognised as regulators of various signalling pathways in mammalian cells. PMCAs were found to interact with calcineurin A in an isoform specific manner. In this study we focus on the interaction of calcineurin A with PMCA4 and its effect on catecholamine secretion in PC12 cells with reduced PMCA2 or PMCA3 content. Reduction of synthesis of PMCA2 or PMCA3 led to upregulation of PMCA4 manifested by preferential interaction of PMCA4 with calcineurin A. On the other hand, we observed a significant reduction of dopamine secretion, which did not correspond with an increased [Ca(2+)](c). This result indicates that the interaction of PMCA4 with calcineurin A plays a regulatory role in the signalling during catecholamine secretion.

The differential effect of angiotensin-converting enzyme inhibitors and angiotensin receptor blockers with respect to foot ulcer and limb amputation in those with diabetes

Diabetic foot ulcers (DFU) or lower extremity amputation (LEA) are complications of diabetes. In those with diabetes, angiotensin-converting enzyme inhibitors (ACEi) and angiotensin receptor blockers (ARB) are commonly used to prevent the progression of kidney disease. Recent studies have indicated that angiotensin may affect angiogenesis and wound repair. Our goal was to evaluate in those with diabetes the likelihood of developing a DFU or LEA among users of ACEi or ARB using a retrospective cohort design of general practices in the United Kingdom. We studied 40,342 individuals at least 35 years of age with diabetes who were first prescribed ACEi or ARB between 1995 and 2006. A total of 35,153 individuals were treated with ACEi, 12,437 individuals with ARB, and 7,310 both. The hazard ratio for DFU was 0.50 (95% confidence intervals: 0.43, 0.59), showing an increased risk of DFU for those using ACEi vs. ARB. The hazard ratio for LEA was 0.72 (0.48, 1.01). However, among those with lower extremity peripheral arterial disease the hazard ratio was 0.45 (0.22, 0.91) for the new onset of a LEA. In conclusion, among those with diabetes, exposure to ACEi as compared with ARB increases the risk of developing a DFU or LEA.

Angiotensin1-9 antagonises pro-hypertrophic signalling in cardiomyocytes via the angiotensin type 2 receptor

The renin–angiotensin system (RAS) regulates blood pressure mainly via the actions of angiotensin (Ang)II, generated via angiotensin converting enzyme (ACE). The ACE homologue ACE2 metabolises AngII to Ang1-7, decreasing AngII and increasing Ang1-7, which counteracts AngII activity via the Mas receptor. However, ACE2 also converts AngI to Ang1-9, a poorly characterised peptide which can be further converted to Ang1-7 via ACE. Ang1-9 stimulates bradykinin release in endothelium and has antihypertrophic actions in the heart, attributed to its being a competitive inhibitor of ACE, leading to decreased AngII, rather than increased Ang1-7. To date no direct receptor-mediated effects of Ang1-9 have been described. To further understand the role of Ang1-9 in RAS function we assessed its action in cardiomyocyte hypertrophy in rat neonatal H9c2 and primary adult rabbit left ventricular cardiomyocytes, compared to Ang1-7. Cardiomyocyte hypertrophy was stimulated with AngII or vasopressin, significantly increasing cell size by approximately 1.2-fold (P < 0.05) as well as stimulating expression of the hypertrophy gene markers atrial natriuretic peptide, brain natriuretic peptide, β-myosin heavy chain and myosin light chain (2- to 5-fold, P < 0.05). Both Ang1-9 and Ang1-7 were able to block hypertrophy induced by either agonist (control, 186.4 μm; AngII, 232.8 μm; AngII+Ang1-7, 198.3 μm; AngII+Ang1-9, 195.9 μm; P < 0.05). The effects of Ang1-9 were not inhibited by captopril, supporting previous evidence that Ang1-9 acts independently of Ang1-7. Next, we investigated receptor signalling via angiotensin type 1 and type 2 receptors (AT1R, AT2R) and Mas. The AT1R antagonist losartan blocked AngII-induced, but not vasopressin-induced, hypertrophy. Losartan did not block the antihypertrophic effects of Ang1-9, or Ang1-7 on vasopressin-stimulated cardiomyocytes. The Mas antagonist A779 efficiently blocked the antihypertrophic effects of Ang1-7, without affecting Ang1-9. Furthermore, Ang1-7 activity was also inhibited in the presence of the bradykinin type 2 receptor antagonist HOE140, without affecting Ang1-9. Moreover, we observed that the AT2R antagonist PD123,319 abolished the antihypertrophic effects of Ang1-9, without affecting Ang1-7, suggesting Ang1-9 signals via the AT2R. Radioligand binding assays demonstrated that Ang1-9 was able to bind the AT2R (pKi = 6.28 ± 0.1). In summary, we ascribe a direct biological role for Ang1-9 acting via the AT2R. This has implications for RAS function and identifying new therapeutic targets in cardiovascular disease.

Chronic intermittent hypoxia augments chemoreflex control of sympathetic activity: role of the angiotensin II type 1 receptor

Chronic exposure to intermittent hypoxia (CIH) increases carotid sinus nerve activity in normoxia and in response to acute hypoxia. We hypothesized that CIH augments basal and chemoreflex-stimulated sympathetic outflow through an angiotensin receptor-dependent mechanism. Rats were exposed to CIH for 28 days: a subset was treated with losartan. Then, lumbar sympathetic activity was recorded under anesthesia during 20-s apneas, isocapnic hypoxia, and potassium cyanide. We measured carotid body superoxide production and expression of angiotensin II type-1 receptor, neuronal nitric oxide synthase, and NADPH oxidase. Sympathetic activity was higher in CIH vs. control rats at baseline, during apneas and isocapnic hypoxia, but not cyanide. Carotid body superoxide production and expression of angiotensin II type 1 receptor and gp91(phox) subunit of NADPH oxidase were elevated in CIH rats, whereas expression of neuronal nitric oxide synthase was reduced. None of these differences were evident in animals treated with losartan. CIH-induced augmentation of chemoreflex sensitivity occurs, at least in part, via the renin-angiotensin system.

eNOS phosphorylation and translocation are altered in male but not female mice by increased activation of the Gαq protein

Little is known about sex-dependent physiological and pathophysiological differences in cardiac endothelial nitric oxide synthase (eNOS) expression and activation. Therefore, we investigated cardiac morphology and eNOS protein expression, including its translocation-dependent activation and phosphorylation, in cardiac tissue of male and female wild-type mice and transgenic heart-failure mice having a cardiac-specific, 5-fold overexpression of the Gαq protein. In addition, we measured calcineurin protein expression. Heart-to-body weight ratio was increased in Gαq mice. Female wild-type mice showed higher eNOS protein expression and activation (translocation and phosphorylation) than did wild-type males. In cardiac tissue of Gαq mice, these sex-dependent differences remained or were enhanced. Protein expression of the catalytic subunit calcineurin A, which has been shown to dephosphorylate eNOS, was higher in wild-type males than in wild-type females. These differences were increased in the Gαq mice model. We conclude that sex differences exist in cardiac eNOS protein expression and phosphorylation. Increased activation of the Gαq protein appears to alter eNOS protein expression and phosphorylation only in males.

Protective effects of sphingosine-1-phosphate receptor agonist treatment after myocardial ischaemia-reperfusion

AIMS

Several experimental studies have demonstrated protection against cardiac ischaemia-reperfusion injury achieved by pre-treatment with exogenous sphingosine-1-phosphate (S1P). We tested the hypothesis that pharmacological S1P receptor agonists improve recovery of function when applied with reperfusion.

METHODS AND RESULTS

Isolated rat cardiomyocytes were stimulated with exogenous S1P, the selective S1P1 receptor agonist SEW2871, or the S1P1/3 receptor agonist FTY720. Western blot analysis was performed to analyse downstream signalling pathways. Ischaemia-reperfusion studies were conducted in rat cardiomyocytes, isolated Langendorff-perfused rat hearts, and in human myocardial muscle strip preparations to evaluate the effect of S1P receptor agonists on cell death and recovery of mechanical function. All S1P receptor agonists were able to activate Akt. This was associated with transactivation of the epidermal growth factor receptor. In isolated cardiomyocytes, selective stimulation of the S1P1 receptor by SEW2871 induced protection against cell death when administered either before or after ischaemia-reperfusion. In isolated rat hearts, treatment with FTY720 during reperfusion attenuated the rise in left ventricular end-diastolic pressure (LVEDP) and improved the recovery of left ventricular developed pressure without limiting infarct size. However, selective S1P1 receptor stimulation did not improve functional recovery but rather increased LVEDP. Additional experiments employing a human myocardial ischaemia-reperfusion model also demonstrated improved functional recovery induced by FTY720 treatment during reperfusion.

CONCLUSION

Pharmacological S1P receptor agonists have distinct effects on ischaemia-reperfusion injury. Their efficacy when applied during reperfusion makes them potential candidates for pharmaceutical postconditioning therapy after cardiac ischaemia.

Tacrolimus reduces nitric oxide synthase function by binding to FKBP rather than by its calcineurin effect

Hypertension develops in many patients receiving the immunosuppressive drug tacrolimus (FK506). One possible mechanism for hypertension is a reduction in vasodilatory nitric oxide. We found that tacrolimus and a calcineurin autoinhibitory peptide significantly decreased vascular calcineurin activity; however, only tacrolimus altered intracellular calcium release in mouse aortic endothelial cells. In mouse aortas, incubation with tacrolimus increased protein kinase C activity and basal endothelial nitric oxide synthase phosphorylation at threonine 495 but reduced basal and agonist-induced endothelial nitric oxide synthase phosphorylation at serine 1177, a mechanism known to inhibit synthase activity. While this decreased nitric oxide production and endothelial function, the calcineurin autoinhibitory peptide had no such effects. Inhibition of ryanodine receptor opening or protein kinase C blocked the effects of tacrolimus. Since it is known that the FK506 binding protein (FKBP12/12.6) interacts with the ryanodine receptor to regulate calcium release, we propose this as the mechanism by which tacrolimus alters intracellular calcium and endothelial nitric oxide synthase rather than by its effect on calcineurin. Our study shows that prevention of the tacrolimus-induced intracellular calcium leak may attenuate endothelial dysfunction and the consequent hypertension.

Central administration of angiotensin-(1-7) stimulates nitric oxide release and upregulates the endothelial nitric oxide synthase expression following focal cerebral ischemia/reperfusion in rats

Angiotensin-(1-7) [Ang-(1-7)] is an endogenous peptide of the renin-angiotensin system with several beneficial effects that are often opposite to those attributed to angiotensin II (Ang II). Since there are no data available so far on the role of Ang-(1-7) after cerebral ischemia/reperfusion, in this paper, we investigated the central administration of Ang-(1-7) modulates in vivo the nitric oxide(NO) release and the endothelial NO synthase (eNOS) expression following focal cerebral ischemia/reperfusion in rats. Cerebral ischemia-reperfusion injury was induced by intraluminal thread occlusion of middle cerebral artery in the adult male rats. The levels of NO in ischemic tissues were measured by NO detection kits. Reverse transcription (RT)-PCR and western blot were used to determine messenger RNA (mRNA) and protein levels of the eNOS in ischemic tissues. The cerebral ischemic lesion resulted in a significant increase of NO release at 3 and 6h compared with sham operation group in our model after reperfusion, whereas both medium and high doses Ang-(1-7) markedly enhanced NO levels at 3-24h, and 3-72h after reperfusion, respectively. In addition, NO release increased was significantly induced by high-dose Ang-(1-7) compared with medium-dose Ang-(1-7) at 24-72 h after reperfusion. Medium and high-dose Ang-(1-7) significantly stimulated eNOS activation when compared with artificial cerebrospinal fluid (aCSF) treatment group at 3, 6, 12, 24, and 48h after reperfusion, however, no significant changes in eNOS expression were found between medium and high-dose Ang-(1-7) at different times after the ischemic insult. These findings indicate that medium and high-dose Ang-(1-7) stimulate NO release and upregulate eNOS expression in ischemic tissues following focal cerebral ischemia/reperfusion in rats.

Angiotensin II: a hormone involved in and contributing to pro-hypertrophic cardiac networks and target of anti-hypertrophic cross-talks

Angiotensin II (Ang II) plays a major role in the progression of myocardial hypertrophy to heart failure. Inhibiting the angiotensin converting enzyme (ACE) or blockade of the corresponding Ang II receptors is used extensively in clinical practice, but there is scope for refinement of this mode of therapy. This review summarizes the current understanding of the direct effects of Ang II on cardiomyocytes and then focus particularly on interaction of components of the renin-angiotensin system with other hormones and cytokines. New findings described in approximately 400 papers identified in the PubMed database and published during the 2.5 years are discussed in the context of previous relevant literature. The cardiac action of Ang II is influenced by the activity of different isoforms of ACE leading to different amounts of Ang II by comparison with other angiotensinogen-derived peptides. The effect of Ang II is mediated by at least two different AT receptors that are differentially expressed in cardiomyocytes from neonatal, adult and failing hearts. The intracellular effects of Ang II are influenced by nitric oxide (NO)/cGMP-dependent cross talk and are mediated by the release of autocrine factors, such as transforming growth factor (TGF)-beta1 and interleukin (IL)-6. Besides interactions with cytokines, Ang II is involved in systemic networks including aldosterone, parathyroid hormone and adrenomedullin, which have their own effects on cardiomyocytes that modify, amplify or antagonize the primary effect of Ang II. Finally, hyperinsulemia and hyperglycaemia influence Ang II-dependent processes in diabetes and its cardiac sequelae.

Targeted deletion of the calcineurin inhibitor DSCR1 suppresses tumor growth

The NF-AT transcription factors regulated by the phosphatase calcineurin play a role in breast cancer metastasis-promoting tumor cell invasion. Metastasis is a multistep process requiring angiogenesis and endothelial activation. NF-AT is also expressed in endothelial cells, and calcineurin-NF-AT signaling is an important downstream effector of the proangiogenic cytokine VEGF. One isoform of the endogenous calcineurin regulator, Down syndrome candidate region-1 (DSCR1.Ex4), suppresses calcineurin-NFAT signaling blocking endothelial proliferation. However, overexpression of the other DSCR1 isoform (DSCR1.Ex1) may promote angiogenesis. We report that targeted deletion of both isoforms leads to hyperactivated calcineurin and precocious endothelial apoptosis, inhibiting formation of an effective tumor vasculature and suppressing tumorigenesis. Treatment with the specific pharmacological calcineurin inhibitor cyclosporin A rescues this endothelial defect in DSCR1(-/-) mice, restoring tumor growth.

Pressure overload-induced hypertrophy in transgenic mice selectively overexpressing AT2 receptors in ventricular myocytes

The role of the angiotensin II type 2 (AT2) receptor in cardiac hypertrophy remains controversial. We studied the effects of AT2 receptors on chronic pressure overload-induced cardiac hypertrophy in transgenic mice selectively overexpressing AT2 receptors in ventricular myocytes. Left ventricular (LV) hypertrophy was induced by ascending aorta banding (AS). Transgenic mice overexpressing AT2 (AT2TG-AS) and nontransgenic mice (NTG-AS) were studied after 70 days of aortic banding. Nonbanded NTG mice were used as controls. LV function was determined by catheterization via LV puncture and cardiac magnetic resonance imaging. LV myocyte diameter and interstitial collagen were determined by confocal microscopy. Atrial natriuretic polypeptide (ANP) and brain natriuretic peptide (BNP) were analyzed by Northern blot. Sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA)2, inducible nitric oxide synthase (iNOS), endothelial NOS, ERK1/2, p70S6K, Src-homology 2 domain-containing protein tyrosine phosphatase-1, and protein serine/threonine phosphatase 2A were analyzed by Western blot. LV myocyte diameter and collagen were significantly reduced in AT2TG-AS compared with NTG-AS mice. LV anterior and posterior wall thickness were not different between AT2TG-AS and NTG-AS mice. LV systolic and diastolic dimensions were significantly higher in AT2TG-AS than in NTG-AS mice. LV systolic pressure and end-diastolic pressure were lower in AT2TG-AS than in NTG-AS mice. ANP, BNP, and SERCA2 were not different between AT2TG-AS and NTG-AS mice. Phospholamban (PLB) and the PLB-to-SERCA2 ratio were significantly higher in AT2TG-AS than in NTG-AS mice. iNOS was higher in AT2TG-AS than in NTG-AS mice but not significantly different. Our results indicate that AT2 receptor overexpression modified the pathological hypertrophic response to aortic banding in transgenic mice.

Cross-talk related to insulin and angiotensin II binding on myocardial remodelling in diabetic rat hearts

This study focused on the regulation and affinity modulation of angiotensin II (Ang II) binding to its receptor subtypes (AT(1)- and AT(2)-receptor) in the coronary endothelium (CE) and cardiomyocytes (CM) of Sprague-Dawley male rats in normal (N), normal treated with losartan (NL), streptozotocin-induced diabetic (D), insulin-treated diabetic (DI), losartan-treated diabetic (DL), and diabetic co-treated with insulin and losartan (DIL). Heart perfusion was used to estimate Ang II binding affinity (tau=1/k-(n)) to its receptor subtypes on CE and CM. Diabetes decreased tau value on CE and increased it on CM as compared to normal. In DL group, the tau value decreased on CE but was normalised on CM. Insulin treatment alone (DI) or with losartan (DIL) restored t to normal on both CE and CM. Western blot results for AT(1)-receptor density showed an increase in diabetics compared to normal with no normalising effect with insulin treatment. The AT(1)-receptor density was normalised in the diabetic groups treated with losartan +/- insulin. Results for AT(2)-receptor regulation revealed a significant difference between untreated (D) and losartan-treated (DL, DIL) diabetic groups. All of these data show the interrelated pathway and cross-talk between insulin and Ang II system indicating potentially negative effects on the diabetic heart.

Uncoupling of the VEGF-endothelial nitric oxide axis in diabetic nephropathy: an explanation for the paradoxical effects of VEGF in renal disease

In many forms of experimental kidney diseases, renal VEGF is low, and administering VEGF can be shown to be protective. A paradox occurs in diabetes, in which renal VEGF levels are high and a deleterious effect of VEGF on kidney disease has been shown. We have hypothesized that endothelial dysfunction induced by hyperglycemia or other factors may underlie the pathogenic mechanisms of a high VEGF state. VEGF normally stimulates endothelial nitric oxide (NO) release and acts in concert with elevated NO levels as a trophic factor for vascular endothelium. The increased NO derived from the endothelial cell acts as an inhibitory factor that prevents excess endothelial cell proliferation, vascular smooth muscle cell proliferation, and macrophage infiltration. In the setting where NO bioavailability is reduced in diabetes, high levels of VEGF lead to excessive endothelial cell proliferation, stimulation of macrophage chemotaxis, and vascular smooth muscle cell activation. Consistent with this hypothesis is our recent observation that diabetes induced in endothelial NO-deficient mice results in clinical and histological features identical to human diabetic nephropathy. The discovery of the key role for impaired endothelial NO bioavailability in the stimulation of VEGF and VEGF-dependent disease may provide key insights into not only the pathogenesis of diabetic nephropathy but also the utility and hazard of administering VEGF as a treatment for kidney disease.

Angiotensin II mediates postischemic leukocyte-endothelial interactions: role of calcitonin gene-related peptide

Vascular inflammation and enhanced production of angiotensin II (ANG II) are involved in the pathogenesis of hypertension and diabetes, disease states that predispose the afflicted individuals to ischemic disorders. In light of these observations, we postulated that ANG II may play a role in promoting leukocyte rolling (LR) and adhesion (LA) in postcapillary venules after exposure of the small intestine to ischemia-reperfusion (I/R). Using an intravital microscopic approach in C57BL/6J mice, we showed that ANG II type I (AT1) or type II (AT2) receptor antagonism (with valsartan or PD-123319, respectively), inhibition of angiotensin-converting enzyme (ACE) with captopril, or calcitonin gene-related peptide (CGRP) receptor blockade (CGRP8-37) prevented postischemic LR but did not influence I/R-induced LA. However, both postischemic LR and LA were largely abolished by concomitant AT1 and AT2 receptor blockade or chymase inhibition (with Y-40079). Additionally, exogenously administered ANG II increased LR and LA, effects that were attenuated by pretreatment with a CGRP receptor antagonist or an NADPH oxidase inhibitor (apocynin). Our work suggests that ANG II, formed by the enzymatic activity of ACE and chymase, plays an important role in inducing postischemic LR and LA, effects that involve the engagement of both AT1 and AT2 receptors and may be mediated by CGRP and NADPH oxidase.

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

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

Interaction of Endothelial Nitric Oxide and Angiotensin in the Circulation

Discovery of the unexpected intercellular messenger and transmitter nitric oxide (NO) was the highlight of highly competitive investigations to identify the nature of endothelium-derived relaxing factor. This labile, gaseous molecule plays obligatory roles as one of the most promising physiological regulators in cardiovascular function. Its biological effects include vasodilatation, increased regional blood perfusion, lowering of systemic blood pressure, and antithrombosis and anti-atherosclerosis effects, which counteract the vascular actions of endogenous angiotensin (ANG) II. Interactions of these vasodilator and vasoconstrictor substances in the circulation have been a topic that has drawn the special interest of both cardiovascular researchers and clinicians. Therapeutic agents that inhibit the synthesis and action of ANG II are widely accepted to be essential in treating circulatory and metabolic dysfunctions, including hypertension and diabetes mellitus, and increased availability of NO is one of the most important pharmacological mechanisms underlying their beneficial actions. ANG II provokes vascular actions through various receptor subtypes (AT1, AT2, and AT4), which are differently involved in NO synthesis and actions. ANG II and its derivatives, ANG III, ANG IV, and ANG-(1-7), alter vascular contractility with different mechanisms of action in relation to NO. This review article summarizes information concerning advances in research on interactions between NO and ANG in reference to ANG receptor subtypes, radical oxygen species, particularly superoxide anions, ANG-converting enzyme inhibitors, and ANG receptor blockers in patients with cardiovascular disease, healthy individuals, and experimental animals. Interactions of ANG and endothelium-derived relaxing factor other than NO, such as prostaglandin I2 and endothelium-derived hyperpolarizing factor, are also described.

Inhibition of Nuclear Import of Calcineurin Prevents Myocardial Hypertrophy

The time that transcription factors remain nuclear is a major determinant for transcriptional activity. It has recently been demonstrated that the phosphatase calcineurin is translocated to the nucleus with the transcription factor nuclear factor of activated T cells (NF-AT). This study identifies a nuclear localization sequence (NLS) and a nuclear export signal (NES) in the sequence of calcineurin. Furthermore we identified the nuclear cargo protein importinβ 1 to be responsible for nuclear translocation of calcineurin. Inhibition of the calcineurin/importin interaction by a competitive peptide (KQECKIKYSERV), which mimicked the calcineurin NLS, prevented nuclear entry of calcineurin. A noninhibitory control peptide did not interfere with the calcineurin/importin binding. Using this approach, we were able to prevent the development of myocardial hypertrophy. In angiotensin II-stimulated cardiomyocytes, [ 3 H]-leucine incorporation (159%±9 versus 111%±11; P <0.01) and cell size were suppressed significantly by the NLS peptide compared with a control peptide. The NLS peptide inhibited calcineurin/NF-AT transcriptional activity (227%±11 versus 133%±8; P <0.01), whereas calcineurin phosphatase activity was unaffected (298%±9 versus 270%±11; P =NS). We conclude that calcineurin is not only capable of dephosphorylating NF-AT, thus enabling its nuclear import, but the presence of calcineurin in the nucleus is also important for full NF-AT transcriptional activity.

Mechanisms of the Anti-Ischemic Effect of Angiotensin II AT( 1 ) Receptor Antagonists in the Brain

1. Circulating and locally formed Angiotensin II regulates the cerebral circulation through stimulation of AT(1) receptors located in cerebrovascular endothelial cells and in brain centers controlling cerebrovascular flow. 2. The cerebrovascular autoregulation is designed to maintain a constant blood flow to the brain, by vasodilatation when blood pressure decreases and vasoconstriction when blood pressure increases. 3. During hypertension, there is a shift in the cerebrovascular autoregulation to the right, in the direction of higher blood pressures, as a consequence of decreased cerebrovascular compliance resulting from vasoconstriction and pathological growth. In hypertension, when perfusion pressure decreases as a consequence of blockade of a cerebral artery, reduced cerebrovascular compliance results in more frequent and more severe strokes with a larger area of injured tissue. 4. There is a cerebrovascular angiotensinergic overdrive in genetically hypertensive rats, manifested as an increased expression of cerebrovascular AT(1) receptors and increased activity of the brain Angiotensin II system. Excess AT(1) receptor stimulation is a main factor in the cerebrovascular pathological growth and decreased compliance, the alteration of the cerebrovascular eNOS/iNOS ratio, and in the inflammatory reaction characteristic of cerebral blood vessels in genetic hypertension. All these factors increase vulnerability to brain ischemia and stroke. 5. Sustained blockade of AT(1) receptors with peripheral and centrally active AT(1) receptor antagonists (ARBs) reverses the cerebrovascular pathological growth and inflammation, increases cerebrovascular compliance, restores the eNOS/iNOS ratio and decreases cerebrovascular inflammation. These effects result in a reduction of the vulnerability to brain ischemia, revealed, when an experimental stroke is produced, in protection of the blood flow in the zone of penumbra and substantial reduction in neuronal injury. 6. The protection against ischemia resulting is related to inhibition of the Renin-Angiotensin System and not directly related to the decrease in blood pressure produced by these compounds. A similar decrease in blood pressure as a result of the administration of beta-adrenergic receptor and calcium channel blockers does not protect from brain ischemia. 7. In addition, sustained AT(1) receptor inhibition enhances AT(2) receptor expression, associated with increased eNOS activity and NO formation followed by enhanced vasodilatation. Direct AT(1) inhibition and indirect AT(2) receptor stimulation are associated factors normalizing cerebrovascular compliance, reducing cerebrovascular inflammation and decreasing the vulnerability to brain ischemia.8. These results strongly suggest that inhibition of AT(1) receptors should be considered as a preventive therapeutic measure to protect the brain from ischemia, and as a possible novel therapy of inflammatory conditions of the brain.

AT1 receptor blockade regulates the local angiotensin II system in cerebral microvessels from spontaneously hypertensive rats

BACKGROUND AND PURPOSE

Blockade of angiotensin II AT1 receptors in cerebral microvessels protects against brain ischemia and inflammation. In this study, we tried to clarify the presence and regulation of the local renin-angiotensin system (RAS) in brain microvessels in hypertension.

METHODS

Spontaneously hypertensive rats (SHR) and Wistar Kyoto (WKY) controls were treated with an AT1 receptor antagonist (candesartan, 0.3 mg/kg per day) via subcutaneous osmotic minipumps for 4 weeks. The expression and localization of RAS components and the effect of AT1 receptor blockade were assessed by Affymetrix microarray, qRT-PCR, Western blots, immunohistochemistry and immunofluorescence.

RESULTS

We found transcripts of most of RAS components in our microarray database, and confirmed their expression by qRT-PCR. Angiotensinogen (Aogen), angiotensin-converting enzyme (ACE) and AT1 receptors were localized to the endothelium. There was no evidence of AT2 receptor localization in the microvascular endothelium. In SHR, (pro)renin receptor mRNA and AT1 receptor mRNA and protein expression were higher, whereas Aogen, ACE mRNA and AT2 receptor mRNA and protein expression were lower than in WKY rats. Candesartan treatment increased Aogen, ACE and AT2 receptor in SHR, and increased ACE and decreased Aogen in WKY rats, without affecting the (pro)renin and AT1 receptors.

CONCLUSIONS

Increased (pro)renin and AT1 receptor expression in SHR substantiates the importance of the local RAS overdrive in the cerebrovascular pathophysiology in hypertension. AT1 receptor blockade and increased AT2 receptor stimulation after administration of candesartan may contribute to the protection against brain ischemia and inflammation.

Calcineurin regulates myocardial function during acute endotoxemia

RATIONALE

Cyclosporin A (CsA) is known to preserve cardiac contractile function during endotoxemia, but the mechanism is unclear. Increased nitric oxide (NO) production and altered mitochondrial function are implicated as mechanisms contributing to sepsis-induced cardiac dysfunction, and CsA has the capacity to reduce NO production and inhibit mitochondrial dysfunction relating to the mitochondrial permeability transition (MPT).

OBJECTIVES

We hypothesized that CsA would protect against endotoxin-mediated cardiac contractile dysfunction by attenuating NO production and preserving mitochondrial function.

METHODS

Left ventricular function was measured continuously over 4 h in cats assigned as follows: control animals (n = 7); LPS alone (3 mg/kg, n = 8); and CsA (6 mg/kg, n = 7), a calcineurin inhibitor that blocks the MPT, or tacrolimus (FK506, 0.1 mg/kg, n = 7), a calcineurin inhibitor lacking MPT activity, followed in 30 min by LPS. Myocardial tissue was then analyzed for NO synthase-2 expression, tissue nitration, protein carbonylation, and mitochondrial morphology and function.

MEASUREMENTS AND MAIN RESULTS

LPS treatment resulted in impaired left ventricular contractility, altered mitochondrial morphology and function, and increased protein nitration. As hypothesized, CsA pretreatment normalized cardiac performance and mitochondrial respiration and reduced myocardial protein nitration. Unexpectedly, FK506 pretreatment had similar effects, normalizing both cardiac and mitochondrial parameters. However, CsA and FK506 pretreatments markedly increased protein carbonylation in the myocardium despite elevated manganese superoxide dismutase activity during endotoxemia.

CONCLUSIONS

Our data indicate that calcineurin is a critical regulator of mitochondrial respiration, tissue nitration, protein carbonylation, and contractile function in the heart during acute endotoxemia.

Angiotensin type 1 receptor is linked to inhibition of nitric oxide production in pulmonary endothelial cells

We previously demonstrated that angiotensin II (Ang II) stimulates an increase in nitric oxide synthase (NOS) mRNA levels, eNOS protein expression and NO production via the type 2 (AT2) receptor, whereas signaling via the type 1 (AT1) receptor negatively regulates NO production in bovine pulmonary artery endothelial cells (BPAECs). In the present study, we investigated the components of the AT1 receptor-linked signaling pathway(s) that are involved in the downregulation of eNOS protein expression in BPAECs. Treatment of BPAECs with either AT1 receptor antagonists or an anti-AT1 receptor antibody induced eNOS protein expression. Furthermore, intracellular delivery of GP-Antagonist-2A, an inhibitor of Galphaq proteins, and treatment of BPAECs with U73122, a phosphatidylinositol-phospholipase C (PLC)-specific inhibitor, enhanced eNOS protein expression. Treatment of BPAECs with the cell-permeable calcium chelator, BAPTA/AM, increased eNOS protein expression at 8 h, while increasing intracellular calcium with either thapsigargin or A23187 prevented Ang II-induced eNOS protein expression. Phorbol myristate acetate (PMA), a protein kinase C (PKC) activator, completely prevented Ang II-stimulated eNOS protein expression at 8 h, whereas depletion of PKC by long-term treatment with PMA, induced eNOS protein expression. Treatment of BPAECs with a PKCalpha-specific inhibitor or transfection of BPAECs with an anti-PKCalpha neutralizing antibody stimulated eNOS protein expression. Conversely, rottlerin, a PKCdelta specific isoform inhibitor had no effect on basal or Ang II-stimulated eNOS protein expression. Moreover, treatment of BPAECs with U73122, BAPTA/AM and PKCalpha-specific inhibitors increased NO production at 8 h. In conclusion, Ang II downregulates eNOS protein expression via an AT1 receptor-linked pathway involving Galphaq/PLC/calcium/PKCalpha signaling pathway in BPAECs.

Recent advances in the understanding of the role of nitric oxide in cardiovascular homeostasis

Nitric oxide synthases (NOS) are the enzymes responsible for nitric oxide (NO) generation. To date, 3 distinct NOS isoforms have been identified: neuronal NOS (NOS1), inducible NOS (NOS2), and endothelial NOS (NOS3). Biochemically, NOS consists of a flavin-containing reductase domain, a heme-containing oxygenase domain, and regulatory sites. NOS catalyse an overall 5-electron oxidation of one Nomega-atom of the guanidino group of L-arginine to form NO and L-citrulline. NO exerts a plethora of biological effects in the cardiovascular system. The basal formation of NO in mitochondria by a mitochondrial NOS seems to be one of the main regulators of cellular respiration, mitochondrial transmembrane potential, and transmembrane proton gradient. This review focuses on recent advances in the understanding of the role of enzyme and enzyme-independent NO formation, regulation of NO bioactivity, new aspects of NO on cardiac function and morphology, and the clinical impact and perspectives of these recent advances in our knowledge on NO-related pathways.

Angiotensin type 2 receptor-mediated phosphorylation of eNOS in the aortas of mice with 2-kidney, 1-clip hypertension

To evaluate the role of vascular angiotensin II (Ang II) type 2 (AT2) receptor in renovascular hypertension, we investigated expressions of AT2 receptor and endothelial nitric oxide synthase (eNOS) in thoracic aortas of mice with 2-kidney, 1-clip (2K1C) hypertension. The mRNA levels of AT2 receptor in aortas, but not those of AT1 and bradykinin B2 receptors, increased 14 days but not 42 days after clipping. The contractile response to Ang II (>0.1 micromol/L) was attenuated in aortic rings excised 14 days after clipping and was restored to that of rings from sham mice by antagonists of AT2 receptor (PD123319) and B2 receptor (icatibant). The aortic levels of total eNOS, phosphorylated eNOS at Ser1177 (p-eNOS), total Akt, and phosphorylated Akt at Ser473 (p-Akt) were increased in 2K1C mice on day 14, whereas only eNOS levels were increased on day 42. The aortic cGMP levels were 20-fold greater in 2K1C mice on day 14 compared with sham mice. Administration of nicardipine for 4 days before the excision of aortas 14 days after clipping not only reduced blood pressure but also decreased the aortic levels of eNOS, p-eNOS, Akt, p-Akt, and cGMP to sham levels, whereas the administration of PD123319 or icatibant to 2K1C mice decreased p-eNOS and cGMP to sham levels without affecting blood pressure and the levels of eNOS, Akt and p-Akt. These results suggest that vascular NO production is enhanced by increased eNOS phosphorylation via the activation of AT2 receptors in the course of 2K1C hypertension.

Targeted Proteolysis Sustains Calcineurin Activation

Background— Calcineurin (CnA) is important in the regulation of myocardial hypertrophy. We demonstrated that targeted proteolysis of the CnA autoinhibitory domain under pathological myocardial workload leads to increased CnA activity in human myocardium. Here, we investigated the proteolytic mechanism leading to activation of CnA.

Methods and Results— In patients with diseased myocardium, we found strong nuclear translocation of CnA. In contrast, in normal human myocardium, there was a cytosolic distribution of CnA. Stimulation of rat cardiomyocytes with angiotensin (Ang) II increased calpain activity significantly (433±11%; P <0.01; n=6) and caused proteolysis of the autoinhibitory domain of CnA. Inhibition of calpain by a membrane-permeable calpain inhibitor prevented proteolysis. We identified the cleavage site of calpain in the human CnA sequence at amino acid 424. CnA activity was increased after Ang II stimulation (310±29%; P <0.01; n=6) and remained high after removal of Ang II (214±17%; P <0.01; n=6). Addition of a calpain inhibitor to the medium decreased CnA activity (110±19%; P =NS; n=6) after removal of Ang II. Ang II stimulation of cardiomyocytes also translocated CnA into the nucleus as demonstrated by immunohistochemical staining and transfection assays with GFP-tagged CnA. Calpain inhibition and therefore suppression of calpain-mediated proteolysis of CnA enabled CnA exit from the nucleus.

Conclusions— Ang II stimulation of cardiomyocytes increased calpain activity, leading to proteolysis of the autoinhibitory domain of CnA. This causes an increase in CnA activity and results in nuclear translocation of CnA. Loss of the autoinhibitory domain renders CnA constitutively nuclear and active, even after removal of the hypertrophic stimulus.

Chronic ventricular myocyte-specific overexpression of angiotensin II type 2 receptor results in intrinsic myocyte contractile dysfunction

ANG II type 2 receptor (AT2) is upregulated in failing hearts, but its effect on myocyte contractile function is not known. We measured fractional cell shortening and intracellular Ca2+concentration transients in left ventricular myocytes derived from transgenic mice in which ventricle-specific expression of AT2was driven by the myosin light chain 2v promoter. Confocal microscopy studies confirmed upregulation of AT2in the ventricular myocytes and partial colocalization of AT2with AT1. Three components of contractile performance were studied. First, baseline measurements (0.5 Hz, 1.5 mmol/l extracellular Ca2+concentration, 25°C) and study of contractile reserve at faster pacing rates (1–5 Hz) revealed Ca2+-dependent contractile dysfunction in myocytes from AT2transgenic mice. Comparison of two transgenic lines suggested a dose-dependent relationship between magnitude of contractile dysfunction and level of AT2expression. Second, activity of the Na+/H+exchanger, a dominant transporter that regulates beat-to-beat intracellular pH, was impaired in the transgenic myocytes. Third, the inotropic response to β-adrenergic versus ANG II stimulation differed. Both lines showed impaired contractile response to β-adrenergic stimulation. ANG II elicited an increase in contractility and intracellular Ca2+in wild-type myocytes but caused a negative inotropic effect in myocytes from AT2transgenic mice. In contrast with β-adrenergic response, the depressed response to ANG II was related to level of AT2overexpression. The depressed response to ANG II was also present in myocytes from young transgenic mice before development of heart failure. Thus chronic overexpression of AT2has the potential to cause Ca2+- and pH-dependent contractile dysfunction in ventricular myocytes, as well as loss of the inotropic response to ANG II.

Down-regulation of cardiac bradykinin B2 receptors and eNOs mRNA in rats with remnant kidneys

The changes in the expression of cardiac bradykinin B2 receptors (BKB2Rs) and endogenous nitrix oxide synthase (eNOs) mRNA were studied in rats with remnant kidneys. Thirty-two rats were divided into sham-operated and experimental groups randomly (n = 16 in each group). The remnant kidney model was established by 2-stage 5/6 nephrectomy. Blood pressure and serum Cr were measured before operation and 15, 30, 60, 120 days after 5/6 nephrectomy. Eight animals in each group were killed at the first month and 4th month after the operation. The expression of BKB2Rs and eNOs mRNAs was detected by using RT-real time PCR from isolated left ventricle, and their correlation was also analyzed. The results showed that blood pressure and serum Cr were increased significantly 15 days after 5/6 nephrectomy (both P<0.01), and the hypertension and azomia existed constantly till 120 days but had no significant fluctuation. Cardiac BKB2Rs and eNOs mRNA was declined time-dependently (both P<0.05). And there was a close positive correlation between cardiac BKB2Rs and eNOs mRNA (r= 0.82, P< 0.01). It was suggested that a significant chronic renal failure can be produced at least 15 days after 5/6 nephrotomy and can sustain more than 4 months. The expression of BKB2Rs and eNOs was down-regulated time-dependently in this model, and there was a significant correlation between them.

Nitric oxide, angiotensin II, and hypertension

Although initially adaptive, the changes that accompany hypertension, namely, cell growth, endothelial dysfunction, and extracellular matrix deposition, eventually can become maladaptive and lead to end-organ disease such as heart failure, coronary artery disease, and renal failure. A functional imbalance between angiotensin II (Ang II) and nitric oxide (NO) plays an important pathogenetic role in hypertensive end-organ injury. NO, an endogenous vasodilator, inhibitor of vascular smooth muscle and mesangial cell growth, and natriuretic agent, is synthesized in the endothelium by a constitutive NO synthase. NO antagonizes the effects of Ang II on vascular tone, cell growth, and renal sodium excretion, and also down-regulates the synthesis of angiotensin-converting enzyme (ACE) and Ang II type 1 receptors. On the other hand, Ang II decreases NO bioavailability by promoting oxidative stress. A better understanding of the pathophysiologic mechanisms involved in hypertensive end-organ damage may aid in identifying markers of cardiovascular susceptibility to injury and in developing therapeutic interventions. We propose that those antihypertensive agents that lower blood pressure and concomitantly restore the homeostatic balance of vasoactive agents such as Ang II and NO within the vessel wall would be more effective in preventing or arresting end-organ disease.

Angiotensin II stimulates nitric oxide production in pulmonary artery endothelium via the type 2 receptor

We previously reported that angiotensin II stimulates an increase in nitric oxide production in pulmonary artery endothelial cells. The aims of this study were to determine which receptor subtype mediates the angiotensin II-dependent increase in nitric oxide production and to investigate the roles of the angiotensin type 1 and type 2 receptors in modulating angiotensin II-dependent vasoconstriction in pulmonary arteries. Pulmonary artery endothelial cells express both angiotensin II type 1 and type 2 receptors as assessed by RT-PCR, Western blot analysis, and flow cytometry. Treatment of the endothelial cells with PD-123319, a type 2 receptor antagonist, prevented the angiotensin II-dependent increase in nitric oxide synthase mRNA, protein levels, and nitric oxide production. In contrast, the type 1 receptor antagonist losartan enhanced nitric oxide synthase mRNA levels, protein expression, and nitric oxide production. Pretreatment of the endothelial cells with either PD-123319 or an anti-angiotensin II antibody prevented this losartan enhancement of nitric oxide production. Angiotensin II-dependent enhanced hypoxic contractions in pulmonary arteries were blocked by the type 1 receptor antagonist candesartan; however, PD-123319 enhanced hypoxic contractions in angiotensin II-treated endothelium-intact vessels. These data demonstrate that angiotensin II stimulates an increase in nitric oxide synthase mRNA, protein expression, and nitric oxide production via the type 2 receptor, whereas signaling via the type 1 receptor negatively regulates nitric oxide production in the pulmonary endothelium. This endothelial, type 2 receptor-dependent increase in nitric oxide may serve to counterbalance the angiotensin II-dependent vasoconstriction in smooth muscle cells, ultimately regulating pulmonary vascular tone.

Src kinase mediates angiotensin II-dependent increase in pulmonary endothelial nitric oxide synthase

We have previously demonstrated that angiotensin II (Ang II) stimulates nitric oxide (NO) production in bovine pulmonary artery endothelial cells (BPAECs) by increasing NO synthase (NOS) expression via the type 2 receptor. The purpose of this study was to identify the Ang II-dependent signaling pathway that mediates this increase in endothelial NOS (eNOS). The Ang II-dependent increase in eNOS expression is prevented when BPAECs are pretreated with the tyrosine kinase inhibitors, herbimycin A and 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-D]pyrimidine, which also blocked Ang II-dependent mitogen-activated protein kinase (MAPK) kinase/extracellular-regulated protein kinase (MEK)-1 and MAPK phosphorylation, suggesting that Src is upstream of MAPK in this pathway. Transfection of BPAECs with an Src dominant negative mutant cDNA prevented the Ang II-dependent Src activation and increase in eNOS protein expression. PD98059, a MEK-1 inhibitor, prevented the Ang II-dependent phosphorylation of extracellular-regulated protein kinases 1 and 2 and increase in eNOS expression. Neither AG1478, an epidermal growth factor receptor kinase inhibitor, nor AG1295, a platelet derived growth factor receptor kinase inhibitor, had any effect on Ang II-stimulated Src activity, MAPK activation, or eNOS expression. Pertussis toxin prevented the Ang II-dependent increase in Src activity, MAPK activation, and eNOS expression. These data suggest that Ang II stimulates Src tyrosine kinase via a pertussis toxin-sensitive pathway, which in turn activates the MAPK pathway, resulting in increased eNOS protein expression in BPAECs.

Nitric Oxide Mediates Benefits of Angiotensin II Type 2 Receptor Overexpression During Post-Infarct Remodeling

We hypothesized that nitric oxide (NO) mediates the benefits of cardiac angiotensin II type 2 (AT(2)-R) overexpression during postmyocardial infarction (post-MI) remodeling. Eleven wild-type (WT) C57BL/6 mice and 28 transgenic (TG) mice with AT(2)-R overexpression were studied by cardiac magnetic resonance imaging (CMR) at baseline and days 1 and 28 post-MI induced by left anterior descending artery occlusion and reperfusion. Sixteen TG mice were treated from day 1 through 28 post-MI with the NO synthase inhibitor N(G)-nitro-l-arginine methyl ester in drinking water at 1 mg/mL (TG-Rx). Left ventricular mass index (LVMI), end-diastolic volume index (EDVI) and end-systolic volume index (ESVI), wall thickness, percent thickening, and ejection fraction (EF) were measured. Infarct size on day 1 was assessed by post-contrast CMR. Interstitial collagen was quantified in noninfarcted regions. At baseline, heart rate (HR), blood pressure (BP), LVMI, EDVI, and ESVI were similar between groups, as were infarct size and weekly post-MI HR and systolic BP. By day 28 post-MI, EDVI and ESVI were similar in WT and TG-Rx, but significantly lower in TG (ESVI: 1.41+/-0.18 microL/g versus 2.53+/-0.14 microL/g in WT; 2.17+/-0.14 microL/g in TG-Rx; P<0.008 for both). At day 28, EF was higher in TG (46.3%+/-2.9%) compared with WT and TG-Rx (32.7+/-2.3% and 33.7+/-2.3, respectively; P<0.003 for both). Wall thickening at day 28 post-MI was greater in the base and mid-LV in TG than WT and TG-Rx. Noninfarcted region interstitial collagen was similar between groups. Thus, the NO pathway may mediate much of the benefits of cardiac AT(2)-R overexpression during post-MI remodeling.

Protein kinase C switches the Raf kinase inhibitor from Raf-1 to GRK-2

Feedback inhibition is a fundamental principle in signal transduction allowing rapid adaptation to different stimuli. In mammalian cells, the major feedback inhibitor for G-protein-coupled receptors (GPCR) is G-protein-coupled receptor kinase 2 (GRK-2), which phosphorylates activated receptors, uncouples them from G proteins and initiates their internalization. The functions of GRK-2 are indispensable and need to be tightly controlled. Dysregulation promotes disorders such as hypertension or heart failure. In our search for a control mechanism for this vital kinase, here we show that the Raf kinase inhibitor protein (RKIP) is a physiological inhibitor of GRK-2. After stimulation of GPCR, RKIP dissociates from its known target, Raf-1 (refs 6-8), to associate with GRK-2 and block its activity. This switch is triggered by protein kinase C (PKC)-dependent phosphorylation of the RKIP on serine 153. The data delineate a new principle in signal transduction: by activating PKC, the incoming receptor signal is enhanced both by removing an inhibitor from Raf-1 and by blocking receptor internalization. A physiological role for this mechanism is shown in cardiomyocytes in which the downregulation of RKIP restrains beta-adrenergic signalling and contractile activity.

Cardiac Hypertrophy Is Associated With Decreased eNOS Expression in Angiotensin AT 2 Receptor–Deficient Mice

Angiotensin II receptors play an essential role in cardiovascular physiology and disease. The significance of angiotensin type II (AT2) receptors in cardiac disease still remains elusive. Thus, we tested in gene-targeted mice whether AT2 receptors modulate cardiac function and remodeling after experimental myocardial injury. To generate myocardial infarcts of reproducible size, a cryolesion was generated at the free wall of the left ventricle of wild-type mice (Agtr2+/Y) and mice carrying a deletion of the AT2 receptor gene (Agtr2-/Y). Postinjury remodeling was followed up for 4 weeks after cryoinjury. The cryoprocedure led to an increased heart weight/body weight ratio and heart weight/tibia length ratio in AT2-deficient mice compared with control mice. Morphometric analysis revealed a significant increase in myocyte cross-sectional area after cardiac injury (infarct vs sham Agtr2+/Y, +53%; vs Agtr2-/Y, +95%). Expression of endothelial nitric oxide synthase (eNOS) was significantly lower in hearts from Agtr2-/Y than from Agtr2+/Y mice. eNOS downregulation was accompanied by a decrease in cardiac cGMP levels in Agtr2-/Y mice. In isolated murine cardiomyocytes, angiotensin II induced eNOS expression through AT2 receptors, and inhibition of NO production by NG-nitro-l-arginine methyl ester abolished the antihypertrophic effect of AT2 on cardiac myocytes. Our results demonstrate in a genetic mouse model that angiotensin II AT2 receptors exert an antihypertrophic effect in cardiac remodeling after myocardial cryoinjury and link the expression of cardiac eNOS to AT2 receptor activation.