Tetrahydrobiopterin-dependent preservation of nitric oxide-mediated endothelial function in diabetes by targeted transgenic GTP-cyclohydrolase I overexpression

Hyperglycemia adversely modulates endothelial nitric oxide synthase during anesthetic preconditioning through tetrahydrobiopterin- and heat shock protein 90-mediated mechanisms

BACKGROUND

Endothelial nitric oxide synthase activity is regulated by (6R-)5,6,7,8-tetrahydrobiopterin (BH4) and heat shock protein 90. The authors tested the hypothesis that hyperglycemia abolishes anesthetic preconditioning (APC) through BH4- and heat shock protein 90-dependent pathways.

METHODS

Myocardial infarct size was measured in rabbits in the absence or presence of APC (30 min of isoflurane), with or without hyperglycemia, and in the presence or absence of the BH4 precursor sepiapterin. Isoflurane-dependent nitric oxide production was measured (ozone chemiluminescence) in human coronary artery endothelial cells cultured in normal (5.5 mm) or high (20 mm) glucose conditions, with or without sepiapterin (10 or 100 microm).

RESULTS

APC decreased myocardial infarct size compared with control experiments (26 +/- 6% vs. 46 +/- 3%, respectively; P < 0.05), and this action was blocked by hyperglycemia (43 +/- 4%). Sepiapterin alone had no effect on infarct size (46 +/- 3%) but restored APC during hyperglycemia (21 +/- 3%). The beneficial actions of sepiapterin to restore APC were blocked by the nitric oxide synthase inhibitor N (G)-nitro-L-arginine methyl ester (47 +/- 2%) and the BH4 synthesis inhibitor N-acetylserotonin (46 +/- 3%). Isoflurane increased nitric oxide production to 177 +/- 13% of baseline, and this action was attenuated by high glucose concentrations (125 +/- 6%). Isoflurane increased, whereas high glucose attenuated intracellular BH4/7,8-dihydrobiopterin (BH2) (high performance liquid chromatography), heat shock protein 90-endothelial nitric oxide synthase colocalization (confocal microscopy) and endothelial nitric oxide synthase activation (immunoblotting). Sepiapterin increased BH4/BH2 and dose-dependently restored nitric oxide production during hyperglycemic conditions (149 +/- 12% and 175 +/- 9%; 10 and 100 microm, respectively).

CONCLUSION

The results indicate that tetrahydrobiopterin and heat shock protein 90-regulated endothelial nitric oxide synthase activity play a central role in cardioprotection that is favorably modulated by volatile anesthetics and dysregulated by hyperglycemia. Enhancing the production of BH4 may represent a potential therapeutic strategy.

Regulation of endothelial cell tetrahydrobiopterin pathophysiological and therapeutic implications

Tetrahydrobiopterin (BH(4)) is a critical cofactor for the nitric oxide synthases. In the absence of BH(4), these enzymes become uncoupled, fail to produce nitric oxide, and begin to produce superoxide and other reactive oxygen species (ROS). BH(4) levels are modulated by a complex biosynthetic pathway, salvage enzymes, and by oxidative degradation. The enzyme GTP cyclohydrolase-1 catalyzes the first step in the de novo synthesis of BH(4) and new evidence shows that this enzyme is regulated by phosphorylation, which reduces its interaction with its feedback regulatory protein (GFRP). In the setting of a variety of common diseases, such as atherosclerosis, hypertension, and diabetes, reactive oxygen species promote oxidation of BH(4) and inhibit expression of the salvage enzyme dihydrofolate reductase (DHFR), promoting accumulation of BH(2) and NOS uncoupling. There is substantial interest in therapeutic approaches to increasing tissue levels of BH(4), largely by oral administration of this agent. BH(4) treatment has proved effective in decreasing atherosclerosis, reducing blood pressure, and preventing complications of diabetes in experimental animals. While these basic studies have been very promising, there are only a few studies showing any effect of BH(4) therapy in humans in treatment of these common problems. Whether BH(4) or related agents will be useful in treatment of human diseases needs additional study.

Identification and functional characterization of phosphorylation sites on GTP cyclohydrolase I

OBJECTIVE

The posttranslational regulation of GTP cyclohydrolase I (GCH-1), the rate-limiting enzyme for tetrahydrobiopterin (BH4) synthesis, remains elusive. Here, we identified specific phosphorylation sites on GCH-1 and characterized the function of these sites.

METHODS AND RESULTS

Mass spectrometry studies showed overexpressed rat GCH-1 was phosphorylated at serine (S) 51, S167, and threonine (T) 231 in HEK293 cells, whereas a computational analysis of GCH-1 revealed 8 potential phosphorylation sites (S51, S72, T85, T91, T103, S130, S167 and T231). GCH-1 activity and BH4 were significantly decreased in cells transfected with the phospho-defective mutants (S72A, T85A, T91A, T103A, or S130A) and increased in cells transfected with the T231A mutant. BH4 and BH2 were increased in cells transfected with S51E, S72E, T85E, T91E, T103D, or T130D mutants, but decreased in cells transfected with the T231D mutant, whereas cells transfected with the S167A or the S167E mutant had increased BH2. Additionally, cells transfected with the T231A mutant had reduced GCH-1 nuclear localization and nuclear GCH-1 activity.

CONCLUSIONS

Our data suggest GCH-1 activity is regulated either positively by phosphorylation S51, S72, T85, T91, T103, and S130, or negatively at T231. Such information might be useful in designing new therapies aiming at improving BH4 bioavailability.

Adverse ventricular remodeling and exacerbated NOS uncoupling from pressure-overload in mice lacking the beta3-adrenoreceptor

Stimulation of the beta-adrenergic system is important in the pathological response to sustained cardiac stress, forming the rationale for the use of beta-blockers in heart failure. The beta3-adrenoreceptor (AR) is thought to couple to the inhibitory G-protein, G(i), with downstream signaling through nitric oxide, although its role in the heart remains controversial. In this study, we tested whether lack of beta3-AR influences the myocardial response to pressure-overload. Baseline echocardiography in mice lacking beta3-AR (beta3(-/-)) compared to wild type (WT) showed mild LV hypertrophy at 8 weeks that worsened as they aged. beta3(-/-) mice had much greater mortality after transverse aortic constriction (TAC) than WT controls. By 3 weeks of TAC, systolic function was worse. After 9 weeks of TAC, beta3(-/-) mice also had greater LV dilation, myocyte hypertrophy and enhanced fibrosis. NOS activity declined in beta3(-/-)TAC hearts after 9 weeks, and total and NOS-dependent superoxide rose, indicating heightened oxidative stress and NOS uncoupling. The level of eNOS phosphorylation in beta3(-/-)TAC hearts was diminished, and nNOS and iNOS expression levels were increased. GTP cyclohydrolase-1 expression was reduced, although total BH4 levels were not depleted. 3 weeks of BH4 treatment rescued beta3(-/-) mice from worsened remodeling after TAC, and lowered NOS-dependent superoxide. Thus, lack of beta3-AR signaling exacerbates cardiac pressure-overload induced remodeling and enhances NOS uncoupling and consequent oxidant stress, all of which can be rescued with exogenous BH4. These data suggest a cardioprotective role for the beta3-AR in modulating oxidative stress and adverse remodeling in the failing heart.

Tetrahydrobiopterin, superoxide, and vascular dysfunction

(6R)-5,6,7,8-Tetrahydrobiopterin (BH(4)) is an endogenously produced pterin that is found widely distributed in mammalian tissues. BH(4) works as a cofactor of aromatic amino acid hydroxylases and nitric oxide synthases. In the vasculature a deficit of BH(4) is implicated in the mechanisms of several diseases including atherosclerosis, hypertension, diabetic vascular disease, and vascular complications from cigarette smoking and environmental pollution. These ill-effects are connected to the ability of BH(4) to regulate reactive oxygen species levels in the endothelium. The possibility of using BH(4) as a therapeutical agent in cardiovascular medicine is becoming more compelling and many biochemical and physiological aspects involved in this application are currently under investigation. This review summarizes our current understanding of BH(4) reactivity and some aspects of cellular production and regulation.

Mesenteric nitric oxide and superoxide production in experimental necrotizing enterocolitis

BACKGROUND

A proposed mechanism of intestinal injury in necrotizing enterocolitis (NEC) involves vascular dysfunction through altered nitric oxide synthase (NOS) activity. We hypothesize that this dysfunction results in an imbalance in nitric oxide (*NO) and superoxide (O(2)(*-)) production by the intestinal vascular endothelium, which contributes to the intestinal injury seen in NEC.

MATERIALS AND METHODS

Neonatal rat pups were divided into two groups. Control pups were breast fed and housed with their mother. Experimental NEC pups were housed separately and either exposed to formula feeding and 5% to 10% hypoxia alone (FF/H) or with the addition of lipopolysaccharide (FF/H/LPS). Mesenteries from each group were analyzed for *NO and O(2)(*-) production with and without NOS inhibition by N(G)-monomethyl-L-arginine (L-NMMA). Western blot analysis for eNOS, phosphorylated eNOS (phospho-eNOS), and inducible NOS (iNOS) was performed, and each terminal ileum was graded for intestinal injury by histology.

RESULTS

Histology revealed mild intestinal injury (grade 1-2 on a 4-point scale) in the FF/H group and severe injury (grade 3-4) in the FF/H/LPS group. The FF/H cohort had significantly increased *NO and lower O(2)(*-) production, while the FF/H/LPS group shifted to significantly decreased *NO and increased O(2)(*-) production. L-NMMA inhibited >50% of O(2)(*-) production in all three groups but only inhibited *NO production in control and FF/H pups. Western blot analysis revealed increased levels of phospho-eNOS in FF/H pups and increased iNOS in FF/H/LPS pups.

CONCLUSIONS

This study demonstrates in the progression of NEC, intestinal ischemia is associated with a shift from *NO to O(2)(*-) production, which is NOS-dependent. Potentially greater injury results from impaired vasodilatation and over-production of reactive oxygen species.

Biomarkers of premature atherosclerosis

C-reactive protein (CRP) is an acute phase protein and a biochemical marker with important prognostic value for cardiovascular events. Interleukins IL-1 and IL-6 are implicated in the pathogenesis of atherosclerosis and are associated with CRP. Apolipoproteins ApoA-I and ApoB are the main lipid metabolic markers implicated in the development and progression of atherosclerosis. Fibrinogen has also been proposed to be a major independent risk factor for cardiovascular events. Because premature atherosclerosis precedes the development of cardiovascular disease, identification of the associated biomarkers is of great importance. However, further studies will be needed to determine whether or not these markers are useful predictors of future cardiovascular events. Here, we review the roles of specific biomarkers that have been implicated in premature atherosclerosis.

Endothelium-specific GTP cyclohydrolase I overexpression accelerates refractory wound healing by suppressing oxidative stress in diabetes

Refractory wound is a severe complication that leads to limb amputation in diabetes. Endothelial nitric oxide synthase (eNOS) plays a key role in normal wound repair but is uncoupled in streptozotocin (STZ)-induced type 1 diabetes because of reduced cofactor tetrahydrobiopterin (BH(4)). We tested the hypothesis that overexpression of GTP cyclohydrolase I (GTPCH I), the rate-limiting enzyme for de novo BH(4) synthesis, retards NOS uncoupling and accelerates wound healing in STZ mice. Blood glucose levels were significantly increased in both male endothelium-specific GTPCH I transgenic mice (Tg-GCH; via a tie-2 promoter) and wild-type (WT) littermates 5 days after STZ regimen. A full-thickness excisional wound was created on mouse dorsal skin by a 4-mm punch biopsy. Wound closure was delayed in STZ mice, which was rescued in STZ Tg-GCH mice. Cutaneous BH(4) level was significantly reduced in STZ mice vs. WT mice, which was maintained in STZ Tg-GCH mice. In STZ mice, constitutive NOS (cNOS) activity and nitrite levels were decreased compared with WT mice, paralleled by increased superoxide anion (O(2)(-)) level and inducible NOS (iNOS) activity. In STZ Tg-GCH mice, nitrite level and cNOS activity were potentiated and O(2)(-) level and iNOS activity were suppressed compared with STZ mice. Thus endothelium-specific BH(4) overexpression accelerates wound healing in type 1 diabetic mice by enhancing cNOS activity and suppressing oxidative stress.

Endothelial dysfunction and vascular disease

The endothelium can evoke relaxations (dilatations) of the underlying vascular smooth muscle, by releasing vasodilator substances. The best characterized endothelium-derived relaxing factor (EDRF) is nitric oxide (NO). The endothelial cells also evoke hyperpolarization of the cell membrane of vascular smooth muscle (endothelium-dependent hyperpolarizations, EDHF-mediated responses). Endothelium-dependent relaxations involve both pertussis toxin-sensitive G(i) (e.g. responses to serotonin and thrombin) and pertussis toxin-insensitive G(q) (e.g. adenosine diphosphate and bradykinin) coupling proteins. The release of NO by the endothelial cell can be up-regulated (e.g. by oestrogens, exercise and dietary factors) and down-regulated (e.g. oxidative stress, smoking and oxidized low-density lipoproteins). It is reduced in the course of vascular disease (e.g. diabetes and hypertension). Arteries covered with regenerated endothelium (e.g. following angioplasty) selectively loose the pertussis toxin-sensitive pathway for NO release which favours vasospasm, thrombosis, penetration of macrophages, cellular growth and the inflammatory reaction leading to atherosclerosis. In addition to the release of NO (and causing endothelium-dependent hyperpolarizations), endothelial cells also can evoke contraction (constriction) of the underlying vascular smooth muscle cells by releasing endothelium-derived contracting factor (EDCF). Most endothelium-dependent acute increases in contractile force are due to the formation of vasoconstrictor prostanoids (endoperoxides and prostacyclin) which activate TP receptors of the vascular smooth muscle cells. EDCF-mediated responses are exacerbated when the production of NO is impaired (e.g. by oxidative stress, ageing, spontaneous hypertension and diabetes). They contribute to the blunting of endothelium-dependent vasodilatations in aged subjects and essential hypertensive patients.

eNOS activation by physical forces: from short-term regulation of contraction to chronic remodeling of cardiovascular tissues

Nitric oxide production in response to flow-dependent shear forces applied on the surface of endothelial cells is a fundamental mechanism of regulation of vascular tone, peripheral resistance, and tissue perfusion. This implicates the concerted action of multiple upstream "mechanosensing" molecules reversibly assembled in signalosomes recruiting endothelial nitric oxide synthase (eNOS) in specific subcellular locales, e.g., plasmalemmal caveolae. Subsequent short- and long-term increases in activity and expression of eNOS translate this mechanical stimulus into enhanced NO production and bioactivity through a complex transcriptional and posttranslational regulation of the enzyme, including by shear-stress responsive transcription factors, oxidant stress-dependent regulation of transcript stability, eNOS regulatory phosphorylations, and protein-protein interactions. Notably, eNOS expressed in cardiac myocytes is amenable to a similar regulation in response to stretching of cardiac muscle cells and in part mediates the length-dependent increase in cardiac contraction force. In addition to short-term regulation of contractile tone, eNOS mediates key aspects of cardiac and vascular remodeling, e.g., by orchestrating the mobilization, recruitment, migration, and differentiation of cardiac and vascular progenitor cells, in part by regulating the stabilization and transcriptional activity of hypoxia inducible factor in normoxia and hypoxia. The continuum of the influence of eNOS in cardiovascular biology explains its growing implication in mechanosensitive aspects of integrated physiology, such as the control of blood pressure variability or the modulation of cardiac remodeling in situations of hemodynamic overload.

Interleukin-1 regulates multiple atherogenic mechanisms in response to fat feeding

Background

Atherosclerosis is an inflammatory process that develops in individuals with known risk factors that include hypertension and hyperlipidaemia, influenced by diet. However, the interplay between diet, inflammatory mechanisms and vascular risk factors requires further research. We hypothesised that interleukin-1 (IL-1) signaling in the vessel wall would raise arterial blood pressure and promote atheroma.

Methodology/Principal Findings

Apoe^−/− and Apoe^−/−/IL-1R1^−/− mice were fed high fat diets for 8 weeks, and their blood pressure and atherosclerosis development measured. Apoe^−/−/IL-R1^−/− mice had a reduced blood pressure and significantly less atheroma than Apoe^−/− mice. Selective loss of IL-1 signaling in the vessel wall by bone marrow transplantation also reduced plaque burden (p<0.05). This was associated with an IL-1 mediated loss of endothelium-dependent relaxation and an increase in vessel wall Nox 4. Inhibition of IL-1 restored endothelium-dependent vasodilatation and reduced levels of arterial oxidative stress.

Conclusions/Significance

The IL-1 cytokine system links atherogenic environmental stimuli with arterial inflammation, oxidative stress, increased blood pressure and atherosclerosis. This is the first demonstration that inhibition of a single cytokine can block the rise in blood pressure in response to an environmental stimulus. IL-1 inhibition may have profound beneficial effects on atherogenesis in man.

Guanosine triphosphate cyclohydrolase I expression and enzymatic activity are present in caveolae of endothelial cells

Tetrahydrobiopterin is an essential cofactor required for the synthesis of NO. GTP cyclohydrolase I (GTPCH I) is the rate-limiting enzyme for tetrahydrobiopterin production in endothelial cells, yet little is known about the subcellular localization of this enzyme. In this study, we demonstrated that GTPCH I is localized to caveolar membrane microdomains along with caveolin-1 and endothelial NO synthase. GTPCH I activity was detected in isolated caveolar membranes from cultured endothelial cells. Confocal and electron microscopy analyses confirmed GTPCH I colocalization with caveolin-1. Consistent with in vitro studies, GTPCH I activity was evident in isolated caveolar microdomains from lung homogenates of wild-type mice. Importantly, a 2-fold increase in GTPCH I activity was detected in the aortas of caveolin-1-deficient mice, suggesting that caveolin-1 may be involved in the control of GTPCH I enzymatic activity. Indeed, overexpression of caveolin-1 inhibits GTPCH I activity, and tetrahydrobiopterin biosynthesis is activated by the disruption of caveolae structure. These studies demonstrate that GTPCH I is targeted to caveolae microdomains in vascular endothelial cells, and tetrahydrobiopterin production occurs in close proximity to endothelial NO synthase. In addition, our findings provide new insights into the regulation of GTPCH I activity by the caveolar coat protein, caveolin-1.

Endothelial cell-selective adhesion molecule regulates albuminuria in diabetic nephropathy

Microalbuminuria is a primary manifestation of diabetic nephropathy. Endothelial cell-selective adhesion molecule (ESAM) is a new member of the immunoglobulin superfamily which is selectively expressed by vascular endothelial cells. Although ESAM mediates homophilic interaction between endothelial cells, the role of ESAM in glomerular permeability remains unknown. We examined the expression and function of ESAM in the high glucose-induced microangiopathy in the kidney. ESAM was highly expressed in the glomerular endothelial cells, and the level was significantly reduced in the streptozotocin-induced diabetic mice. Stimulation of cultured endothelial cells with high glucose (35 mmol/l) resulted in a significant decrease in the ESAM expression compared to normal glucose (5.5 mmol/l). In vitro permeability assays revealed that albumin diffusion across endothelial monolayers was significantly increased when ESAM was knocked down by siRNA, suggesting that ESAM regulates vascular permeability of the glomeruli. To confirm these results in vivo, albuminuria was assessed using ESAM-/- mice. Urinary albumin to creatinine ratio in ESAM-/- mice was significantly higher than in ESAM+/+ mice. Transmission electron microscopy revealed that glomerular endothelial fenestration was decreased and endothelial tight junction was irregular and relatively wider in ESAM-/- mice than in ESAM+/+ mice. In conclusion, hyperglycemia downregulates ESAM and increases glomerular endothelial permeability. Thus, ESAM may regulate albumin extravasation at the glomeruli and play a role in the initiation of diabetic nephropathy.

Vascular protection by tetrahydrobiopterin: progress and therapeutic prospects

Tetrahydrobiopterin (BH4) is an essential cofactor required for the activity of endothelial nitric oxide (NO) synthase. Suboptimal concentrations of BH4 in the endothelium reduce the biosynthesis of NO, thus contributing to the pathogenesis of vascular endothelial dysfunction. Supplementation with exogenous BH4 or therapeutic approaches that increase endogenous amounts of BH4 can reduce or reverse endothelial dysfunction by restoring production of NO. Improvements in formulations of BH4 for oral delivery have stimulated clinical trials that test the efficacy of BH4 in the treatment of systemic hypertension, peripheral arterial disease, coronary artery disease, pulmonary arterial hypertension, and sickle cell disease. This review discusses ongoing progress in the translation of knowledge, accumulated in preclinical studies, into the clinical application of BH4 in the treatment of vascular diseases. This review also addresses the emerging roles of BH4 in the regulation of endothelial function and their therapeutic implications.

Nitric oxide and vascular insulin resistance

Obesity and type-II diabetes are growing major health issues worldwide. They are the leading risk factors for vascular insulin resistance, which plays an important role in the pathogenesis of cardiovascular disease, the leading cause of death in developed nations. Recent studies have shown that reduced synthesis of nitric oxide (NO; a major vasodilator) from L-arginine in endothelial cells is a major factor contributing to the impaired action of insulin in the vasculature of obese and diabetic subjects. The decreased NO generation results from a deficiency of (6R)-5,6,7,8-tetrahydrobiopterin [BH4; an essential cofactor for NO synthase (NOS)], as well as increased generation of glucosamine (an inhibitor of the pentose cycle for the production of NADPH, another cofactor for NOS) from glucose and L-glutamine. Accordingly, endothelial dysfunction can be prevented by (1) enhancement of BH4 synthesis through supplementation of its precursor (sepiapterin) via the salvage pathway; (2) transfer of the gene for GTP cyclohydrolase-I (the first and key regulatory enzyme for de novo synthesis of BH4); or (3) dietary supplementation of L-arginine (which stimulates GTP cyclohydrolase-I expression and inhibits hexosamine production). Modulation of the arginine-NO pathway by BH4 and arginine is beneficial for ameliorating vascular insulin resistance in obesity and diabetes.

Effect of endothelium-specific insulin resistance on endothelial function in vivo

OBJECTIVE

Insulin resistance is an independent risk factor for the development of cardiovascular atherosclerosis. A key step in the development of atherosclerosis is endothelial dysfunction, manifest by a reduction in bioactivity of nitric oxide (NO). Insulin resistance is associated with endothelial dysfunction; however, the mechanistic relationship between these abnormalities and the role of impaired endothelial insulin signaling versus global insulin resistance remains unclear.

RESEARCH DESIGN AND METHODS

To examine the effects of insulin resistance specific to the endothelium, we generated a transgenic mouse with endothelium-targeted overexpression of a dominant-negative mutant human insulin receptor (ESMIRO). This receptor has a mutation (Ala-Thr(1134)) in its tyrosine kinase domain that disrupts insulin signaling. Humans with the Thr(1134) mutation are insulin resistant. We performed metabolic and vascular characterization of this model.

RESULTS

ESMIRO mice had preserved glucose homeostasis and were normotensive. They had significant endothelial dysfunction as evidenced by blunted aortic vasorelaxant responses to acetylcholine (ACh) and calcium ionophore. Furthermore, the vascular action of insulin was lost in ESMIRO mice, and insulin-induced endothelial NO synthase (eNOS) phosphorylation was blunted. Despite this phenotype, ESMIRO mice demonstrate similar levels of eNOS mRNA and protein expression to wild type. ACh-induced relaxation was normalized by the superoxide dismutase mimetic, Mn(III)tetrakis(1-methyl-4-pyridyl) porphyrin pentachloride. Endothelial cells of ESMIRO mice showed increased superoxide generation and increased mRNA expression of the NADPH oxidase isoforms Nox2 and Nox4.

CONCLUSIONS

Selective endothelial insulin resistance is sufficient to induce a reduction in NO bioavailability and endothelial dysfunction that is secondary to increased generation of reactive oxygen species. This arises independent of a significant metabolic phenotype.

Apelin signaling antagonizes Ang II effects in mouse models of atherosclerosis

Apelin and its cognate G protein-coupled receptor APJ constitute a signaling pathway with a positive inotropic effect on cardiac function and a vasodepressor function in the systemic circulation. The apelin-APJ pathway appears to have opposing physiological roles to the renin-angiotensin system. Here we investigated whether the apelin-APJ pathway can directly antagonize vascular disease-related Ang II actions. In ApoE-KO mice, exogenous Ang II induced atherosclerosis and abdominal aortic aneurysm formation; we found that coinfusion of apelin abrogated these effects. Similarly, apelin treatment rescued Ang II-mediated increases in neointimal formation and vascular remodeling in a vein graft model. NO has previously been implicated in the vasodepressor function of apelin; we found that apelin treatment increased NO bioavailability in ApoE-KO mice. Furthermore, infusion of an NO synthase inhibitor blocked the apelin-mediated decrease in atherosclerosis and aneurysm formation. In rat primary aortic smooth muscle cells, apelin inhibited Ang II-mediated transcriptional regulation of multiple targets as measured by reporter assays. In addition, we demonstrated by coimmunoprecipitation and fluorescence resonance energy transfer analysis that the Ang II and apelin receptors interacted physically. Taken together, these findings indicate that apelin signaling can block Ang II actions in vascular disease by increasing NO production and inhibiting Ang II cellular signaling.

Cytokines and lipopolysaccharides induce inducible nitric oxide synthase but not enzyme activity in adult rat cardiomyocytes

There is evidence that nitric oxide (NO) formation in adult cardiomyocytes stimulated with lipopolysaccharide (LPS) is not commensurate with iNOS levels. Tetrahydrobiopterin (BH(4)) is a key factor in the stabilization and NO production by iNOS homodimer. Thus we hypothesized that BH(4) is a limiting factor for NO production in adult cardiomyocytes in response to LPS and cytokines (TNF-alpha, IL-1, IFN-gamma alone, or mixed). It was verified that LPS and cytokines induced iNOS expression which did not translate into increased nitrite or [(14)C]citrulline production. This response coincided with defective BH(4) synthesis and low GTP cyclohydrolase activity. Furthermore, supplementation with BH(4) and ascorbate failed to increase iNOS activity. This effect was related to preferential accumulation of BH(2) rather than BH(4) in these cells. Uncoupled iNOS activity in stimulated cells was examined using mitochondrial aconitase activity as an endogenous marker of superoxide anion radical (O(2)(-)) formation, and found not to be significantly inhibited. 2-Hydroxyethidium also was not significantly increased. We conclude that adult cardiomyocytes are an unlikely source of NO and O(2)(-) in inflammatory conditions. This finding adds a new and unexpected layer of complexity to our understanding of the responses of the adult heart to inflammation.

CCR2-mediated antiinflammatory effects of endothelial tetrahydrobiopterin inhibit vascular injury-induced accelerated atherosclerosis

BACKGROUND

Vascular injury results in loss of endothelial nitric oxide (NO), production of reactive oxygen species (ROS), and the initiation of an inflammatory response. Both NO and ROS modulate inflammation through redox-sensitive pathways. Tetrahydrobiopterin (BH4) is an essential cofactor for endothelial nitric oxide synthase (eNOS) that regulates enzymatic synthesis of either nitric oxide or ROS. We hypothesized that endothelial BH4 is an important regulator of inflammation and vascular remodeling.

METHODS AND RESULTS

Endothelium-targeted overexpression of GTP cyclohydrolase 1 (GCH), the rate limiting enzyme in BH4 synthesis, increased levels of tetrahydrobiopterin (BH4), reduced endothelial superoxide, improved eNOS coupling, and reduced vein graft atherosclerosis in transgenic GCH/ApoE-KO mice compared to ApoE-KO controls. Immunohistochemistry using anti-MAC-3 and MAC-1 antibody staining revealed a marked reduction in vein graft macrophage content, as did RT-PCR expression of macrophage marker CD68 mRNA levels in GCH/ApoE-KO mice. When we investigated the potential mediators of this reduction, we discovered that mRNA and protein levels of MCP-1 (CCL2) but not RANTES (CCL5) were significantly reduced in GCH/ApoE-KO aortic tissue. Consistent with this finding we found a decrease in CCR2-mediated, but not CCR5-mediated, chemotaxis in vascular tissue and plasma samples from GCH/ApoE-KO animals.

CONCLUSIONS

Increased endothelial BH4 reduces vein graft neointimal hyperplasia and atherosclerosis through a reduction in vascular inflammation. These findings highlight the importance of MCP-1/CCR2 signaling in the response to vascular injury and identify novel pathways linking endothelial BH4 to inflammation and vascular remodeling.

Acute inhibition of guanosine triphosphate cyclohydrolase 1 uncouples endothelial nitric oxide synthase and elevates blood pressure

GTP cyclohydrolase 1 (GTPCH1) is the rate-limiting enzyme in de novo synthesis of tetrahydrobiopterin (BH4), an essential cofactor for endothelial NO synthase (eNOS) dictating, at least partly, the balance of NO and superoxide produced by this enzyme. The aim of this study was to determine the effect of acute inhibition of GTPCH1 on BH4, eNOS function, and blood pressure (BP) in vivo. Exposure of bovine or mouse aortic endothelial cells to GTPCH1 inhibitors (2,4-diamino-6-hydroxypyrimidine or N-acetyl-serotonin) or GTPCH1 small-interference RNA (siRNA) significantly reduced BH4 and NO levels but increased superoxide levels. This increase was abolished by sepiapterin (BH4 precursor) or N(G)-nitro-L-arginine methyl ester (nonselective NOS inhibitor). Incubation of isolated murine aortas with 2,4-diamino-6-hydroxypyrimidine or N-acetyl-serotonin impaired acetylcholine-induced endothelium-dependent relaxation but not endothelium-independent relaxation. Aortas from GTPCH1 siRNA-injected mice, but not their control-siRNA injected counterparts, also exhibited impaired endothelium-dependent relaxation. BH4 reduction induced by GTPCH1 siRNA injection was associated with increased aortic levels of superoxide, 3-nitrotyrosine, and adhesion molecules (intercellular adhesion molecule 1 and vascular cell adhesion molecule 1), as well as a significantly elevated systolic, diastolic, and mean BP in C57BL6 mice. GTPCH1 siRNA was unable to elicit these effects in eNOS(-/-) mice. Sepiapterin supplementation, which had no effect on high BP in eNOS(-/-) mice, partially reversed GTPCH1 siRNA-induced elevation of BP in wild-type mice. In conclusion, GTPCH1 via BH4 maintains normal BP and endothelial function in vivo by preserving NO synthesis by eNOS.

AT1-receptor blockade by telmisartan upregulates GTP-cyclohydrolase I and protects eNOS in diabetic rats

Several enzymatic sources of reactive oxygen species (ROS) were described as potential reasons of eNOS uncoupling in diabetes mellitus. In the present study, we investigated the effects of AT1-receptor blockade with chronic telmisartan (25 mg/kg/day, 6.5 weeks) therapy on expression of the BH4-synthesizing enzyme GTP-cyclohydrolase I (GCH-I), eNOS uncoupling, and endothelial dysfunction in streptozotocin (STZ, 60 mg/kg iv, 7 weeks)-induced diabetes mellitus (type I). Telmisartan therapy did not modify blood glucose and body weight. Aortas from diabetic animals had vascular dysfunction as revealed by isometric tension studies (acetylcholine and nitroglycerin potency). Vascular and cardiac ROS produced by NADPH oxidase, mitochondria, eNOS, and xanthine oxidase were increased in the diabetic group as was the expression of NADPH oxidase subunits at the protein level. The expression of GCH-I and the phosphorylation of eNOS at Ser1177 was decreased by STZ treatment. Therapy with telmisartan normalized these parameters. The present study demonstrates for the first time that AT1-receptor blockade by telmisartan prevents downregulation of the BH4 synthase GCH-I and thereby eNOS uncoupling in experimental diabetes. In addition, telmisartan inhibits activation of superoxide sources like NADPH oxidase, mitochondria, and xanthine oxidase. These effects may explain the beneficial effects of telmisartan on endothelial dysfunction in diabetes.

Endothelial nitric oxide (NO) and its pathophysiologic regulation

Nitric oxide (NO) is a gaseous lipophilic free radical generated by three distinct isoforms of nitric oxide synthases (NOS), type 1 or neuronal (nNOS), type 2 or inducible (iNOS) and type 3 or endothelial NOS (eNOS). Expression of eNOS is altered in many types of cardiovascular disease, such as atherosclerosis, diabetes and hypertension. The ubiquitous chaperone heat shock protein 90 (hsp90) associates with NOS and is important for its proper folding and function. Current studies point toward a therapeutic potential by modulating hsp90-NOS association in various vascular diseases. Here we review the transcriptional regulation of endothelial NOS and factors affecting eNOS activity and function, as well as the important vascular pathologies associated with altered NOS function, focusing on the regulatory role of hsp90 and other factors in NO-associated pathogenesis of these diseases.

GCH1 haplotype determines vascular and plasma biopterin availability in coronary artery disease effects on vascular superoxide production and endothelial function

OBJECTIVES

This study sought to determine the effects of endogenous tetrahydrobiopterin (BH4) bioavailability on endothelial nitric oxide synthase (eNOS) coupling, nitric oxide (NO) bioavailability, and vascular superoxide production in patients with coronary artery disease (CAD).

BACKGROUND

GTP-cyclohydrolase I, encoded by the GCH1 gene, is the rate-limiting enzyme in the biosynthesis of BH4, an eNOS cofactor important for maintaining enzymatic coupling. We examined the associations between haplotypes of the GCH1 gene, GCH1 expression and biopterin levels, and the effects on endothelial function and vascular superoxide production.

METHODS

Blood samples and segments of internal mammary arteries and saphenous veins were obtained from patients with CAD undergoing coronary artery bypass grafting (n = 347). The GCH1 haplotypes were defined by 3 polymorphisms: rs8007267G

RESULTS

Haplotype frequencies were OO 70.6%, XO 27.4%, and XX 2.0%. The X haplotype was associated with significantly lower vascular GCH1 messenger ribonucleic acid expression and substantial reductions in both plasma and vascular BH4 levels. In X haplotype carriers both vascular superoxide and L-NAME-inhibitable superoxide were significantly increased, and were associated with reduced vasorelaxations to acetylcholine.

CONCLUSIONS

GCH1 gene expression, modulated by a particular GCH1 haplotype, is a major determinant of BH4 bioavailability both in plasma and in the vascular wall in patients with CAD. Genetic variation in GCH1 underlies important differences in endogenous BH4 availability and is a determinant of eNOS coupling, vascular redox state, and endothelial function in human vascular disease.

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Key Words

• tetrahydrobiopterin
• endothelial nitric oxide synthase
• gtp-cyclohydrolase i
• superoxide
• haplotype
• atherosclerosis

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MESH Headings

• Aged
• Biopterins/analogs & derivatives
• Biopterins/biosynthesis
• Biopterins/blood
• Coronary Artery Disease/genetics
• Coronary Artery Disease/metabolism
• Endothelium, Vascular/metabolism
• Female
• GTP Cyclohydrolase/genetics
• Genetic Variation
• Haplotypes
• Humans
• Male
• Mammary Arteries/metabolism
• Multivariate Analysis
• Nitric Oxide/metabolism
• Nitric Oxide Synthase Type III/metabolism
• Saphenous Vein/metabolism
• Superoxides/metabolism

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Grants

• R01 NS058870-01A1 NINDS NIH HHS
• FS/03/105/16340 British Heart Foundation
• R01 NS058870 NINDS NIH HHS
• RG/02/006 British Heart Foundation
• RG/07/003/23133 British Heart Foundation

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Affiliation(s)

Charalambos Antoniades Department of Cardiovascular Medicine, John Radcliffe Hospital, University of Oxford, Oxford, United Kingdom
Cheerag Shirodaria Department of Cardiovascular Medicine, John Radcliffe Hospital, University of Oxford, Oxford, United Kingdom
Tim Van Assche Department of Cardiovascular Medicine, John Radcliffe Hospital, University of Oxford, Oxford, United Kingdom
Colin Cunnington Department of Cardiovascular Medicine, John Radcliffe Hospital, University of Oxford, Oxford, United Kingdom
Irmgard Tegeder Neural Plasticity Research Group, Department of Anesthesia and Critical Care, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts Pharmazentrum Frankfurt/ZAFES, Institute for Clinical Pharmacology, Johann Wolfgang Goethe University, Frankfurt am Main, Germany
Jörn Lötsch Pharmazentrum Frankfurt/ZAFES, Institute for Clinical Pharmacology, Johann Wolfgang Goethe University, Frankfurt am Main, Germany
Tomasz J. Guzik Department of Cardiovascular Medicine, John Radcliffe Hospital, University of Oxford, Oxford, United Kingdom Departments of Medicine and Pharmacology, Jagiellonian University, Cracow, Poland
Paul Leeson Department of Cardiovascular Medicine, John Radcliffe Hospital, University of Oxford, Oxford, United Kingdom
Jonathan Diesch Department of Cardiovascular Medicine, John Radcliffe Hospital, University of Oxford, Oxford, United Kingdom
Dimitris Tousoulis Athens University Medical School, 1st Cardiology Department, Hippokration Hospital, Athens, Greece
Christodoulos Stefanadis Athens University Medical School, 1st Cardiology Department, Hippokration Hospital, Athens, Greece
Michael Costigan
Clifford J. Woolf Neural Plasticity Research Group, Department of Anesthesia and Critical Care, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts
Nicholas J. Alp Department of Cardiovascular Medicine, John Radcliffe Hospital, University of Oxford, Oxford, United Kingdom
Keith M. Channon Department of Cardiovascular Medicine, John Radcliffe Hospital, University of Oxford, Oxford, United Kingdom

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Erythropoietin increases endothelial biosynthesis of tetrahydrobiopterin by activation of protein kinase B alpha/Akt1

Tetrahydrobiopterin (BH(4)) is an essential cofactor required for enzymatic activity of endothelial NO synthase. Recently, it has been shown that vascular protective effects of erythropoietin (EPO) are dependent on activation of endothelial NO synthase. Therefore, our objective was to characterize the effect of EPO on the biosynthesis of BH(4) in the vascular wall. Incubation of isolated C57BL/6J mouse aortas for 18 hours with recombinant human EPO (1 to 50 U/mL) caused a concentration-dependent increase in intracellular BH(4) levels and activity of GTP-cyclohydrolase I. Maximal biosynthesis of BH(4) was detected at therapeutic concentrations of 5 U/mL. Removal of the endothelium abolished EPO-induced biosynthesis of BH(4) demonstrating that the vascular endothelium is a major source of BH(4). Treatment with a selective phosphatidylinositol 3-kinase inhibitor wortmannin significantly reduced BH(4) biosynthesis stimulated by EPO. The stimulatory effect of EPO on vascular GTP-cyclohydrolase I activity, BH(4) production, and phosphorylation of endothelial NO synthase was also detected in vivo in mice treated with recombinant human EPO. These effects of EPO were abolished in protein kinase Balpha/Akt1-deficient mice. In addition, EPO significantly increased systolic blood pressure and the number of circulating platelets in Akt1-deficient mice. Our results demonstrate that EPO stimulates biosynthesis of BH(4) in vascular endothelium and that the increase in BH(4) levels is caused by de novo biosynthesis of BH(4) via the phosphatidylinositol 3-kinase/Akt1 pathway. This effect is most likely designed to provide optimal intracellular concentration of the cofactor necessary for EPO-induced elevation of endothelial NO synthase activity.

L-Arginine prevents metabolic effects of high glucose in diabetic mice

We tested the hypothesis that activation of the polyol pathway and protein kinase C (PKC) during diabetes is due to loss of NO. Our results show that after 4 weeks of streptozotocin-induced diabetes, treatment with L-arginine restored NO levels and prevented tissue accumulation of sorbitol in mice, which was accompanied by an increase in glutathiolation of aldose reductase. L-Arginine treatment decreased superoxide generation in the aorta, total PKC activity and PKC-beta(II) phosphorylation in the heart, and the plasma levels of triglycerides and soluble ICAM. These data suggest that increasing NO bioavailability by L-arginine corrects the major biochemical abnormalities of diabetes.

Augmentation of vascular remodeling by uncoupled endothelial nitric oxide synthase in a mouse model of diabetes mellitus

OBJECTIVE

Diabetes mellitus is associated with increased oxidative stress, which induces oxidation of tetrahydrobiopterin (BH4) in vessel wall. Without enough BH4, eNOS is uncoupled to L-arginine and produces superoxide rather than NO. We examined the role of uncoupled eNOS in vascular remodeling in diabetes.

METHODS AND RESULTS

Diabetes mellitus was produced by streptozotocin in C57BL/6J mice. Under stable hyperglycemia, the common carotid artery was ligated, and neointimal formation was examined 4 weeks later. In diabetic mice, the neointimal area was dramatically augmented. This augmentation was associated with increased aortic superoxide formation, reduced aortic BH4/dihydrobiopterin (BH2) ratio, and decreased plasma nitrite and nitrate (NOx) levels compared with nondiabetic mice. Chronic BH4 treatment (10 mg/kg/d) reduced the neointimal area in association with suppressed superoxide production and inflammatory changes in vessels. BH4/BH2 ratio in vessel wall was preserved, and plasma NOx levels increased. Furthermore, in the presence of diabetes, overexpression of bovine eNOS resulted in augmentation of neointimal area, accompanied by increased superoxide production in the endothelium.

CONCLUSIONS

In diabetes, increased oxidative stress by uncoupled NOSs, particularly eNOS, causes augmentation of vascular remodeling. These findings indicate restoration of eNOS coupling has an atheroprotective benefit in diabetes.

Reversal of cardiac hypertrophy and fibrosis from pressure overload by tetrahydrobiopterin: efficacy of recoupling nitric oxide synthase as a therapeutic strategy

BACKGROUND

Sustained pressure overload induces pathological cardiac hypertrophy and dysfunction. Oxidative stress linked to nitric oxide synthase (NOS) uncoupling may play an important role. We tested whether tetrahydrobiopterin (BH4) can recouple NOS and reverse preestablished advanced hypertrophy, fibrosis, and dysfunction.

METHODS AND RESULTS

C57/Bl6 mice underwent transverse aortic constriction for 4 weeks, increasing cardiac mass (190%) and diastolic dimension (144%), lowering ejection fraction (-46%), and triggering NOS uncoupling and oxidative stress. Oral BH4 was then administered for 5 more weeks of pressure overload. Without reducing loading, BH4 reversed hypertrophy and fibrosis, recoupled endothelial NOS, lowered oxidant stress, and improved chamber and myocyte function, whereas untreated hearts worsened. If BH4 was started at the onset of pressure overload, it did not suppress hypertrophy over the first week when NOS activity remained preserved even in untreated transverse aortic constriction hearts. However, BH4 stopped subsequent remodeling when NOS activity was otherwise declining. A broad antioxidant, Tempol, also reduced oxidant stress yet did not recouple NOS or reverse worsened hypertrophy/fibrosis from sustained transverse aortic constriction. Microarray analysis revealed very different gene expression profiles for both treatments. BH4 did not enhance net protein kinase G activity. Finally, transgenic mice with enhanced BH4 synthesis confined to endothelial cells were unprotected against pressure overload, indicating that exogenous BH4 targeted myocytes and fibroblasts.

CONCLUSIONS

NOS recoupling by exogenous BH4 ameliorates preexisting advanced cardiac hypertrophy/fibrosis and is more effective than a less targeted antioxidant approach (Tempol). These data highlight the importance of myocyte NOS uncoupling in hypertrophic heart disease and support BH4 as a potential new approach to treat this disorder.

Mechanisms underlying recoupling of eNOS by HMG-CoA reductase inhibition in a rat model of streptozotocin-induced diabetes mellitus

OBJECTIVE

HMG-CoA reductase inhibitors have been shown to upregulate GTP cyclohydrolase I (GTPCH-I), the key enzyme for tetrahydrobiopterin de novo synthesis and to normalize tetrahydrobiopterin levels in hyperglycemic endothelial cells. We sought to determine whether in vivo treatment with the HMG-CoA reductase inhibitor atorvastatin is able to upregulate the GTPCH-I, to recouple eNOS and to normalize endothelial dysfunction in an experimental model of diabetes mellitus.

METHODS AND RESULTS

In male Wistar rats, diabetes was induced by streptozotocin (STZ, 60 mg/kg). In STZ rats, atorvastatin feeding (20 mg/kg/d, 7 weeks), normalized vascular dysfunction as analyzed by isometric tension studies, levels of circulating endothelial progenitor cells (FACS-analysis), superoxide formation (assessed by lucigenin-enhanced chemiluminescence and dihydroethidium staining), vascular levels of the phosphorylated vasodilator-stimulated phosphoprotein (P-VASP), tyrosine nitration of the prostacyclin synthase, expression of GTPCH-I, dihydrofolate reductase and eNOS, translocation of regulatory NADPH oxidase subunits rac1, p47phox and p67phox (assessed by Western blot) and vascular tetrahydrobiopterin levels as measured by HPLC. Dihydroethidine staining revealed that the reduction of vascular superoxide was at least in part due to eNOS recoupling.

CONCLUSION

HMG-CoA reductase inhibition normalizes endothelial function and reduces oxidative stress in diabetes by inhibiting activation of the vascular NADPH oxidase and by preventing eNOS uncoupling due to an upregulation of the key enzyme of tetrahydrobiopterin synthesis, GTPCH-I.

Sickle cell disease vasculopathy: a state of nitric oxide resistance

Sickle cell disease (SCD) is a hereditary hemoglobinopathy characterized by microvascular vaso-occlusion with erythrocytes containing polymerized sickle (S) hemoglobin, erythrocyte hemolysis, vasculopathy, and both acute and chronic multiorgan injury. It is associated with steady state increases in plasma cell-free hemoglobin and overproduction of reactive oxygen species (ROS). Hereditary and acquired hemolytic conditions release into plasma hemoglobin and other erythrocyte components that scavenge endothelium-derived NO and metabolize its precursor arginine, impairing NO homeostasis. Overproduction of ROS, such as superoxide, by enzymatic (xanthine oxidase, NADPH oxidase, uncoupled eNOS) and nonenzymatic pathways (Fenton chemistry), promotes intravascular oxidant stress that can likewise disrupt NO homeostasis. The synergistic bioinactivation of NO by dioxygenation and oxidation reactions with cell-free plasma hemoglobin and ROS, respectively, is discussed as a mechanism for NO resistance in SCD vasculopathy. Human physiological and transgenic animal studies provide experimental evidence of cardiovascular and pulmonary resistance to NO donors and reduced NO bioavailability that is associated with vasoconstriction, decreased blood flow, platelet activation, increased endothelin-1 expression, and end-organ injury. Emerging epidemiological data now suggest that chronic intravascular hemolysis is associated with certain clinical complications: pulmonary hypertension, cutaneous leg ulcerations, priapism, and possibly stroke. New therapeutic strategies to limit intravascular hemolysis and ROS generation and increase NO bioavailability are discussed.

Endotoxemia-related acute kidney injury in transgenic mice with endothelial overexpression of GTP cyclohydrolase-1

Endotoxin-related acute kidney injury has been shown to profoundly induce nitric oxide (NO), which activates sympathetic and renin-angiotensin system, resulting in renal vasoconstriction. While vascular muscle cells are known to upregulate inducible NO synthase (iNOS), less is known about the endothelium as a source of NO during endotoxemia. Studies were, therefore, undertaken both in vitro in mouse microvascular endothelial cells and in vivo in transgenic mice with overexpression of endothelial GTP cyclohydrolase, the rate-limiting enzyme for tetrahydrobiopterin, a cofactor for NO synthase. LPS significantly induced endothelial cell iNOS expression and NO concentration in the culture media, with no change in endothelial NO synthase expression. GTP cyclohydrolase-1 transgenic (Tg) mice demonstrated a significant increase in baseline urine NO-to-creatinine ratio and a more significant increase in renal iNOS expression and serum NO levels with LPS treatment compared with the wild-type (WT) mice. Glomerular filtration rate and renal blood flow decreased significantly in Tg mice with 1.0 mg/kg LPS, while no changes were observed in WT with the same dose of LPS. Serum IL-6 levels were significantly higher in Tg compared with WT mice during endotoxemia. The antioxidant tempol improved the glomerular filtration rate in the Tg mice. Thus endothelium can be an important source of iNOS and serum NO concentration during endotoxemia, thereby increasing the sensitivity to AKI. Reactive oxygen species appear to be involved in this acute renal injury in Tg mice during endotoxemia.

Endothelial arginase II: a novel target for the treatment of atherosclerosis

Oxidized low-density lipoproteins increase arginase activity and reciprocally decrease endothelial NO in human aortic endothelial cells. Here, we demonstrate that vascular endothelial arginase activity is increased in atherogenic-prone apolipoprotein E-null (ApoE(-/-)) and wild-type mice fed a high cholesterol diet. In ApoE(-/-) mice, selective arginase II inhibition or deletion of the arginase II gene (Arg II(-/-) mice) prevents high-cholesterol diet-dependent decreases in vascular NO production, decreases endothelial reactive oxygen species production, restores endothelial function, and prevents oxidized low-density lipoprotein-dependent increases in vascular stiffness. Furthermore, arginase inhibition significantly decreases plaque burden. These data indicate that arginase II plays a critical role in the pathophysiology of cholesterol-mediated endothelial dysfunction and represents a novel target for therapy in atherosclerosis.

Endothelium-specific GTP cyclohydrolase I overexpression attenuates blood pressure progression in salt-sensitive low-renin hypertension

BACKGROUND

Tetrahydrobiopterin (BH4) is an essential cofactor of endothelial nitric oxide synthase (eNOS). When BH4 levels are decreased, eNOS becomes uncoupled to produce superoxide anion (O2(-)) instead of NO, which contributes to endothelial dysfunction. Deoxycorticosterone acetate (DOCA)-salt hypertension is characterized by a suppressed plasma renin level due to sodium retention but manifests in eNOS uncoupling; however, how endogenous BH4 regulates blood pressure is unknown. GTP cyclohydrolase I (GTPCH I) is the rate-limiting enzyme for de novo BH4 synthesis. This study tested the hypothesis that endothelium-specific GTPCH I overexpression retards the progression of hypertension through preservation of the structure and function of resistance mesenteric arteries.

METHODS AND RESULTS

During 3 weeks of DOCA-salt treatment, arterial blood pressure was increased significantly in wild-type mice, as determined by radiotelemetry, but this increase was attenuated in transgenic mice with endothelium-specific GTPCH I overexpression (Tg-GCH). Arterial GTPCH I activity and BH4 levels were decreased significantly in wild-type DOCA-salt mice, but both were preserved in Tg-GCH mice despite DOCA-salt treatment. Significant remodeling of resistance mesenteric arteries (approximately 100-microm outside diameter) in wild-type DOCA-salt mice exists, evidenced by increased medial cross-sectional area, media thickness, and media-lumen ratio and overexpression of tenascin C, an extracellular matrix glycoprotein that contributes to hypertrophic remodeling; all of these effects were prevented in DOCA-salt-treated Tg-GCH mice. Furthermore, NO-mediated relaxation in mesenteric arteries was significantly improved in DOCA-salt-treated Tg-GCH mice, in parallel with reduced O2(-) levels. Finally, phosphorylation of eNOS at serine residue 1177 (eNOS-S1177), but not its dimer-monomer ratio, was decreased significantly in wild-type DOCA-salt mice compared with sham controls but was preserved in DOCA-salt-treated Tg-GCH mice.

CONCLUSIONS

These results demonstrate that endothelium-specific GTPCH I overexpression abrogates O2(-) production and preserves eNOS phosphorylation, which results in preserved structural and functional integrity of resistance mesenteric arteries and lowered blood pressure in low-renin hypertension.

Insulin resistance and endothelial cell dysfunction: studies in mammalian models

Type 2 diabetes and obesity are major risk factors for the development of cardiovascular atherosclerosis. Resistance to the metabolic effects of insulin on its traditional target tissues (muscle, liver and adipose tissue) is a central pathogenic feature of these disorders. However, the role of insulin resistance in non-canonical tissues, such as the endothelium, is less clear. Several large studies support a role for insulin resistance in the development of premature cardiovascular atherosclerosis independent of type 2 diabetes and obesity. A key step in the initiation and progression of atherosclerosis is a reduction in the bioactivity of endothelial cell-derived nitric oxide. Nitric oxide is a signalling molecule which has a portfolio of potential antiatherosclerotic effects. The presence of insulin receptors on endothelial cells is well documented, and the endothelium has now emerged as a potentially important target tissue for insulin, with insulin-stimulated production of nitric oxide a feature of the action of insulin on endothelial cells. The role of insulin resistance at the level of the endothelial cell in vascular pathophysiology is unclear. A number of studies in humans and gene-modified mice have demonstrated a close association between insulin resistance and nitric oxide bioactivity. In this review, we discuss the link between insulin resistance and endothelial cell function in humans and demonstrate the complimentary information provided by murine models of obesity and insulin resistance in our understanding of the vasculopathy associated with type 2 diabetes and obesity.

Improvement of vascular function by acute and chronic treatment with the PDE-5 inhibitor sildenafil in experimental diabetes mellitus

BACKGROUND AND PURPOSE

Diabetes-associated vascular dysfunction contributes to increased cardiovascular risk. We investigated whether the phosphodiesterase-5 inhibitor sildenafil would improve vascular function in diabetic rats.

EXPERIMENTAL APPROACH

Male Wistar rats were injected with streptozotocin (50 mg kg(-1), i.v.) to induce insulin-deficient diabetes. Direct effects of sildenafil as well as modification of endothelium-dependent and -independent vasorelaxation were investigated in vitro. The effects of acute and chronic (2 week) treatment in vivo of sildenafil on vascular function were also characterized in isolated aortic segments in organ bath chambers 4 weeks after diabetes induction.

KEY RESULTS

Sildenafil induced a concentration-dependent vasorelaxation, which was attenuated by the nitric oxide (NO) synthase inhibitor, N(G)-nitro-L-arginine. Acetylcholine-induced endothelium-dependent as well as endothelium-independent relaxation induced by the NO donor, DEA-NONOate, was significantly reduced in aortae from diabetic rats. Incubation with sildenafil in vitro normalized both endothelium-dependent and -independent relaxation in aortae from diabetic rats. Acute as well as chronic in vivo treatment with sildenafil resulted in enhanced endothelium-dependent and -independent vasorelaxation. Superoxide formation was increased in diabetes, associated with enhanced membrane expression of the NAD(P)H oxidase subunit gp91(phox) and Rac, which were both reduced by chronic treatment with sildenafil.

CONCLUSIONS AND IMPLICATIONS

We demonstrate that sildenafil treatment rapidly and chronically improves vascular relaxation in diabetic rats. Treatment with sildenafil might provide a similarly beneficial effect in diabetic patients.

Oxygen-derived free radicals mediate endothelium-dependent contractions in femoral arteries of rats with streptozotocin-induced diabetes

BACKGROUND AND PURPOSE

The present experiments were designed to study the contribution of oxygen-derived free radicals to endothelium-dependent contractions in femoral arteries of rats with streptozotocin-induced diabetes.

EXPERIMENTAL APPROACH

Rings with and without endothelium were suspended in organ chambers for isometric tension recording. The production of oxygen-derived free radicals in the endothelium was measured with 2',7'-dichlorodihydrofluorescein diacetate using confocal microscopy. The presence of protein was measured by western blotting.

KEY RESULTS

In the presence of L-NAME, the calcium ionophore A23187 induced larger endothelium-dependent contractions in femoral arteries from diabetic rats. Tiron, catalase, deferoxamine and MnTMPyP, but not superoxide dismutase reduced the response, suggesting that oxygen-derived free radicals are involved in the endothelium-dependent contraction. In the presence of L-NAME, A23187 increased the fluorescence signal in femoral arteries from streptozotocin-treated, but not in those from control rats, confirming that the production of oxygen-derived free radicals contributes to the enhanced endothelium-dependent contractions in diabetes. Exogenous H2O2 caused contractions in femoral arterial rings without endothelium which were reduced by deferoxamine, indicating that hydroxyl radicals contract vascular smooth muscle and thus could be an endothelium-derived contracting factor in diabetes. The reduced presence of Mn-SOD and the decreased activity of catalase in femoral arteries from streptozotocin-treated rats demonstrated the presence of a redox abnormality in arteries from rats with diabetes.

CONCLUSIONS AND IMPLICATIONS

These findings suggest that the redox abnormality resulting from diabetes increases oxidative stress which facilitates and/or causes endothelium-dependent contractions.

Telmisartan improves vascular function and reduces platelet activation in rats with streptozotocin-induced diabetes mellitus

Diabetes is associated with vascular dysfunction and platelet activation, both of which may contribute to increased cardiovascular risk. We investigated whether the angiotensin II antagonist telmisartan improves vascular dysfunction and reduces platelet activation in diabetic rats. Therefore, male Wistar rats were injected with streptozotocin (50 mg kg(-1) i.v.) to induce insulin-deficient diabetes. Treatment with telmisartan (10 mg kg(-1)day(-1)) or vehicle was initiated 2 weeks after injection of streptozotocin and continued for 2 weeks. At week 4, platelet activation was assessed in fresh whole blood and vascular function was characterized in isolated aortic segments in organ bath chambers. Diabetic rats displayed severe impairment of endothelium-dependent relaxation induced by acetylcholine as well as endothelium-independent relaxation evoked by a nitric oxide donor, which were improved by treatment with telmisartan. Treatment with telmisartan also improved endogenous platelet vasodilator-stimulated phosphoprotein phosphorylation, which was reduced in platelets from diabetic rats indicating augmented intraluminal vascular nitric oxide bioavailability. Platelets from diabetic rats had increased surface-bound fibrinogen, which was attenuated by telmisartan. Telmisartan normalizes vascular dysfunction and reduces platelet activation in diabetic rats. These effects may contribute to the reduction of cardiovascular events by angiotensin II receptor blockers in diabetic patients.

Proteasome-dependent degradation of guanosine 5'-triphosphate cyclohydrolase I causes tetrahydrobiopterin deficiency in diabetes mellitus

BACKGROUND

Tetrahydrobiopterin (BH4) deficiency is reported to uncouple the enzymatic activity of endothelial nitric oxide synthase in diabetes mellitus. The mechanism by which diabetes actually leads to BH4 deficiency remains elusive. Here, we demonstrate that diabetes reduced BH4 by increasing 26S proteasome-dependent degradation of guanosine 5'-triphosphate cyclohydrolase I (GTPCH), a rate-limiting enzyme in the synthesis of BH4, in parallel with increased formation of both superoxide and peroxynitrite (ONOO-).

METHODS AND RESULTS

Exposure of human umbilical vein endothelial cells to high glucose concentrations (30 mmol/L D-glucose) but not to high osmotic conditions (25 mmol/L L-glucose plus 5 mmol/L D-glucose) significantly lowered the levels of both GTPCH protein and BH4. In addition, high glucose increased both the 26S proteasome activity and the ubiquitination of GTPCH. Inhibition of the 26S proteasome with either MG132 or PR-11 prevented the high glucose-triggered reduction of GTPCH and BH4. Exposure of human umbilical vein endothelial cells to exogenous ONOO- increased proteasome activity and 3-nitrotyrosine in 26S proteasome. Furthermore, adenoviral overexpression of superoxide dismutase and inhibition of endothelial nitric oxide synthase with N(G)-nitro-L-arginine methyl ester significantly attenuated the high glucose-induced activation of 26S proteasome and the reduction of GTPCH. Finally, administration of MG132 or a superoxide dismutase mimetic, tempol, reversed the diabetes mellitus-induced reduction of GTPCH and BH4 and endothelial dysfunction in streptozotocin-induced diabetes mellitus.

CONCLUSIONS

We conclude that diabetes mellitus triggers BH4 deficiency by increasing proteasome-dependent degradation of GTPCH.

Small-molecule therapies for cardiac hypertrophy: moving beneath the cell surface

Pathological stress from cardiovascular disease stimulates hypertrophy of heart cells, which increases the risk of cardiac morbidity and mortality. Recent evidence has indicated that inhibiting such hypertrophy could be beneficial, encouraging drug discovery and development efforts for agents that could achieve this goal. Most existing therapies that have antihypertrophic effects target outside-in signalling in cardiac cells, but their effectiveness seems limited, and so attention has recently turned to the potential of targeting intracellular signalling pathways. Here, we focus on new developments with small-molecule inhibitors of cardiac hypertrophy, summarizing both agents that have been in or are poised for clinical testing, and pathways that offer further promising potential therapeutic targets.

Low-dose treatment with atorvastatin leads to anti-oxidative and anti-inflammatory effects in diabetes mellitus

We investigated whether atorvastatin, given in a dose to low to influence the lipid profile, has any effect on oxidative stress, inflammation and endothelial function under streptozotocin-induced diabetic conditions. Diabetes mellitus was induced in male Sprague Dawley rats by a single injection of streptozotocin. Rats were treated chronically with atorvastatin (50 mg/kg/day; p.o.) or vehicle until day 48 and compared with controls. NAD(P)H activity, protein expression nuclear factor-kappaBp65 (NF-kappaBp65) and phosphorylation of the extracellular signal-regulated kinase (ERK1/2) were assessed in the quadriceps muscle. Protein and mRNA levels of intracellular and vascular adhesion molecules (ICAM-1, VCAM-1) and cytokines were measured by Taqman or immunohistochemistry staining, respectively. Endothelial function was investigated in vivo using the autoperfused hind limb model. Diabetic groups displayed similar severe hyperglycemia. Untreated diabetic rats showed enhanced NAD(P)H activity, activation of the ERK1/2/NF-kappaBp65-pathway, enhanced expression of cytokines and cellular adhesion molecules and impaired vascular function. Low-dose therapy by atorvastatin did not alter the lipid profile but led to a reduction of NAD(P)H activity (-28%, P<0.05) associated with reduced protein expression of NF-kappaBp65 (-53%, P<0.05) and phosphorylation of its regulator mitogen-activated protein kinase (MAPK) ERK1/2 in diabetic rats. Also inflammatory markers were reduced after atorvastatin treatment indexed by reduced mRNA expression of VCAM-1 (-24%), tumor necrosis factor alpha (-59%) and interleukin 1beta (-50%) and reduced ICAM-1 (-81%) and VCAM-1 (-74%) positive staining. These beneficial effects were associated with improved endothelium-dependent vasodilatation (maximal vasodilatation: +101%; P<0.05). Lipid-independent anti-oxidative and anti-inflammatory effects of low-dose atorvastatin involving the ERK1/2/NF-kappaB-pathway are sufficient to improve endothelial function under experimental diabetic conditions.

Mechanisms for the role of tetrahydrobiopterin in endothelial function and vascular disease

NO produced by eNOS (endothelial nitric oxide synthase) is a key mediator of vascular homoeostasis. NO bioavailability is reduced early in vascular disease states, such as hypercholesterolaemia, diabetes and hypertension, and throughout the progression of atherosclerosis. This is a result of both reduced NO synthesis and increased NO consumption by reactive oxygen species. eNOS enzymatic activity appears to be determined by the availability of its cofactor BH4 (tetrahydrobiopterin). When BH4 levels are adequate, eNOS produces NO; when BH4 levels are limiting, eNOS becomes enzymatically uncoupled and generates superoxide, contributing to vascular oxidative stress and endothelial dysfunction. BH4 bioavailability is determined by a balance of enzymatic de novo synthesis and recycling, versus oxidative degradation in dysfunctional endothelium. Augmenting vascular BH4 levels by pharmacological supplementation, by enhancing the rate of de novo biosynthesis or by measures to reduce BH4 oxidation have been shown in experimental studies to enhance NO bioavailability. Thus BH4 represents a potential therapeutic target for preserving eNOS function in vascular disease.

A specific role for eNOS-derived reactive oxygen species in atherosclerosis progression

OBJECTIVE

When the availability of tetrahydrobiopterin (BH4) is deficient, endothelial nitric oxide synthase (eNOS) produces superoxide rather than NO (uncoupled eNOS). We have shown that the atherosclerotic lesion size was augmented in apolipoprotein E-deficient (ApoE-KO) mice overexpressing eNOS because of the enhanced superoxide production. In this study, we addressed the specific importance of uncoupled eNOS in atherosclerosis, and the potential mechanistic role for specific versus nonspecific antioxidant strategies in restoring eNOS coupling.

METHODS AND RESULTS

We crossed mice overexpressing eNOS in the endothelium (eNOS-Tg) with mice overexpressing GTP-cyclohydrolase I (GCH), the rate-limiting enzyme in BH4 synthesis, to generate ApoE-KO/eNOS-Tg/GCH-Tg mice. As a comparison, ApoE-KO/eNOS-Tg mice were treated with vitamin C. Atherosclerotic lesion formation was increased in ApoE-KO/eNOS-Tg mice compared with ApoE-KO mice. GCH overexpression in ApoE-KO/eNOS-Tg/GCH-Tg mice increased vascular BH4 levels and reduced plaque area. This reduction was associated with decreased superoxide production from uncoupled eNOS. Vitamin C treatment failed to reduce atherosclerotic lesion size in ApoE-KO/eNOS-Tg mice, despite reducing overall vascular superoxide production.

CONCLUSION

In contrast to vitamin C treatment, augmenting BH4 levels in the endothelium by GCH overexpression reduced the accelerated atherosclerotic lesion formation in ApoE-KO/eNOS-Tg mice, associated with a reduction of superoxide production from uncoupled eNOS.

Radiochemical HPLC detection of arginine metabolism: measurement of nitric oxide synthesis and arginase activity in vascular tissue

Nitric oxide (NO) plays a key role in vascular homeostasis. Accurate measurement of NO production by endothelial nitric oxide synthase (eNOS) is critical for the investigation of vascular disease mechanisms using genetically modified animal models. Previous assays of NO production measuring the conversion of arginine to citrulline have required homogenisation of tissue and reconstitution with cofactors including NADPH and tetrahydrobiopterin. However, the activity and regulation of NOS in vivo is critically dependant on tissue levels of these cofactors. Therefore, understanding eNOS regulation requires assays of NO production in intact vascular tissue that do not depend on the addition of exogenous cofactors and have sufficient sensitivity and specificity. We describe a novel technique, using radiochemical detection of arginine to citrulline conversion, to measure NO production within intact mouse aortas, without exogenous cofactors. We demonstrate the presence of arginase activity in mouse aortas which has the potential to confound this assay. Furthermore, we describe the use of N-hydroxy-nor-L-arginine (nor-NOHA) to inhibit arginase and permit specific detection of NO production in intact mouse tissue. Using this technique we demonstrate a 2.4-fold increase in NO production in aortas of transgenic mice overexpressing eNOS in the endothelium, and show that this technique has high specificity and high sensitivity for detection of in situ NO synthesis by eNOS in mouse vascular tissue. These results have important implications for the investigation of NOS regulation in cells and tissues.

Improved endothelial function and reduced platelet activation by chronic HMG-CoA-reductase inhibition with rosuvastatin in rats with streptozotocin-induced diabetes mellitus

Diabetes is associated with endothelial dysfunction and platelet activation, both of which may contribute to increased cardiovascular risk. We investigated whether the hydroxy-3-methyl-glutaryl CoA reductase inhibitor rosuvastatin improves endothelial function and reduces platelet activation in diabetic rats. Therefore, male Wistar rats were injected with streptozotocin (STZ, 50mg/kg i.v.) to induce insulin-deficient diabetes. Treatment with rosuvastatin (20mg/[kg day]) or vehicle was initiated 2 weeks after injection of STZ and continued for 2 weeks. Thereafter, platelet activation was assessed in fresh whole blood and vascular function was characterized in isolated aortic segments in organ bath chambers. Endothelium-dependent relaxation induced by acetylcholine was significantly attenuated in diabetic rats and improved by treatment with rosuvastatin (maximum relaxation, % of precontraction-control: 99.8+/-0.2, STZ-vehicle: 80.7+/-2.9, STZ-rosuvastatin: 98.9+/-0.7; p<0.01). Similarly, treatment with rosuvastatin significantly reduced fibrinogen-binding to activated GPIIb/IIIa (mean fluorescence-control: 161.0+/-6.9, STZ-vehicle: 207.8+/-15.9, rosuvastatin: 173.6+/-5.3; p<0.05) and P-Selectin surface expression on platelets (mean fluorescence-control: 76.5+/-7.3, STZ-vehicle: 92.1+/-5.5, rosuvastatin: 75.2+/-6.5; p<0.05), while both markers of platelet activation were increased in diabetic rats. Therefore, rosuvastatin treatment normalizes endothelial function and reduces platelet activation in diabetic rats. These effects may contribute to the reduction of cardiovascular events by statins in diabetic patients.

Relationships between nitric oxide-mediated endothelial function, eNOS coupling and blood pressure revealed by eNOS-GTP cyclohydrolase 1 double transgenic mice

Endothelium-dependent relaxation in conduit vessels is mediated largely by nitric oxide (NO), produced by the enzyme endothelial nitric oxide synthase (eNOS) in the presence of the cofactor tetrahydrobiopterin (BH4) and mediated through a cGMP-dependent downstream signalling cascade. Endothelial NOS regulates blood pressure in vivo, and impaired endothelial NO bioactivity in vascular disease states may contribute to systemic hypertension. In the absence of sufficient levels of the cofactor BH4, NO becomes uncoupled from arginine oxidation and eNOS produces superoxide rather than NO. The enzymatic uncoupling of eNOS is an important feature of vascular disease states associated with increased oxidative stress. However, whether eNOS coupling, rather than overall eNOS activity, has specific effects on endothelium-dependent vasorelaxation in vitro, or on blood pressure regulation in vivo, remains unclear. In this study, we evaluate the relationships between blood pressure and endothelial function in models of eNOS uncoupling, using mice with endothelium-targeted transgenic eNOS overexpression (eNOS-Tg), in comparison with littermates in which eNOS coupling was rescued by additional endothelium-targeted overexpression of GTP cyclohydrolase 1 (eNOS/GCH-Tg) to increase endothelial BH4 levels. Despite the previously characterized differences in eNOS-dependent superoxide production between these animals, we find that blood pressure is equally reduced in both genotypes, compared with wild-type animals. Furthermore, both eNOS-Tg and eNOS/GCH-Tg mice exhibit similarly impaired endothelium-dependent vasorelaxation. We show that reduced vasorelaxation responses result from desensitization of cGMP-mediated signalling and are associated with increased NO production rather than changes in superoxide production.