Vascular protection by tetrahydrobiopterin: progress and therapeutic prospects

Transforming growth factor-β regulates endothelial function during high salt intake in rats

Previous studies have demonstrated that an increase in dietary NaCl (salt) intake stimulated endothelial cells to produce transforming growth factor-β (TGF-β). The intent of the present study was to determine the functional significance of increased TGF-β on endothelial cell function. Young Sprague-Dawley rats were fed diets containing 0.3 or 8.0% NaCl for 2 days before treatment with a specific inhibitor of the TGF-β receptor I/activin receptor-like kinase 5 kinase, or vehicle for another 2 days. At day 4 of study, endothelial phosphorylated Smad2 (S465/467) increased and phosphatase and tensin homologue deleted on chromosome 10 (PTEN) levels decreased in the high-salt-treated rats. In addition, phosphorylated Akt (S473) and phosphorylation of the endothelial isoform of NO synthase (NOS3) at S1177 increased. Treatment with the TGF-β receptor I/activin receptor-like kinase 5 inhibitor reduced Smad2 phosphorylation to levels observed in rats on the low-salt diet and prevented the downstream signaling events induced by the high-salt diet. In human umbilical vein endothelial cells, reduction in PTEN levels increased phosphorylated Akt and NOS3. Treatment of macrovascular endothelial cells with TGF-β1 increased phosphorylated NOS3 and the concentration of NO metabolites in the medium but had no effect on either of these variables in cells pretreated with small interfering RNA directed against PTEN. Thus, during high salt intake, an increase in TGF-β directly promoted a reduction in endothelial PTEN levels, which in turn regulated Akt activation and NOS3 phosphorylation. This effect closes a feedback loop that potentially mitigates the effect of TGF-β on the vasculature.

Tetrahydrobiopterin has a glucose-lowering effect by suppressing hepatic gluconeogenesis in an endothelial nitric oxide synthase-dependent manner in diabetic mice

Endothelial nitric oxide synthase (eNOS) dysfunction induces insulin resistance and glucose intolerance. Tetrahydrobiopterin (BH4) is an essential cofactor of eNOS that regulates eNOS activity. In the diabetic state, BH4 is oxidized to 7,8-dihydrobiopterin, which leads to eNOS dysfunction owing to eNOS uncoupling. The current study investigates the effects of BH4 on glucose metabolism and insulin sensitivity in diabetic mice. Single administration of BH4 lowered fasting blood glucose levels in wild-type mice with streptozotocin (STZ)-induced diabetes and alleviated eNOS dysfunction by increasing eNOS dimerization in the liver of these mice. Liver has a critical role in glucose-lowering effects of BH4 through suppression of hepatic gluconeogenesis. BH4 activated AMP kinase (AMPK), and the suppressing effect of BH4 on gluconeogenesis was AMPK-dependent. In addition, the glucose-lowering effect and activation of AMPK by BH4 did not appear in mice with STZ-induced diabetes lacking eNOS. Consecutive administration of BH4 in ob/ob mice ameliorated glucose intolerance and insulin resistance. Taken together, BH4 suppresses hepatic gluconeogenesis in an eNOS-dependent manner, and BH4 has a glucose-lowering effect as well as an insulin-sensitizing effect in diabetic mice. BH4 has potential in the treatment of type 2 diabetes.

Deficient eNOS phosphorylation is a mechanism for diabetic vascular dysfunction contributing to increased stroke size

BACKGROUND AND PURPOSE

Phosphorylation of eNOS, an important post-translational modulator of its enzymatic activity, is reduced in diabetes mellitus. We hypothesized that modulation of eNOS phosphorylation could overcome diabetic vascular dysfunction and improves the outcome to stroke.

METHODS

We used the db/db mouse model of type 2 diabetes mellitus. We mated db/db mice with eNOS knock-in mice that carry single amino acid mutations at the S1176 phosphorylation site; the phosphomimetic SD mutation (serine replaced by aspartate) shows increased eNOS enzymatic activity, whereas the unphosphorylatable SA mutation (serine replaced by alanine) shows decreased eNOS activity. We characterized the vascular anatomy, baseline physiological parameters, and vascular reactivity. We used the middle cerebral artery occlusion model of stroke and measured infarct volume and neurological deficits.

RESULTS

db/db mice showed diminished eNOS phosphorylation at S1176. eNOS SD and SA mutations do not change the vascular anatomy at the Circle of Willis, brain capillary density, heart rate, or arterial blood gases of db/db mice. The eNOS SD mutation, but not the SA mutation, lowers blood pressure and improves vascular reactivity to acetylcholine in db/db mice. The eNOS SD mutation reduces stroke size and neurological deficit after middle cerebral artery occlusion.

CONCLUSIONS

Diminished eNOS phosphorylation is a mechanism of vascular dysfunction in db/db mice. We show here that modulation of the eNOS S1176 phosphorylation site in db/db mice is associated with improved vascular reactivity and improved outcome to stroke after middle cerebral artery occlusion.

Site-specific antioxidative therapy for prevention of atherosclerosis and cardiovascular disease

Oxidative stress has been implicated in pathophysiology of aging and age-associated disease. Antioxidative medicine has become a practice for prevention of atherosclerosis. However, limited success in preventing cardiovascular disease (CVD) in individuals with atherosclerosis using general antioxidants has prompted us to develop a novel antioxidative strategy to prevent atherosclerosis. Reducing visceral adipose tissue by calorie restriction (CR) and regular endurance exercise represents a causative therapy for ameliorating oxidative stress. Some of the recently emerging drugs used for the treatment of CVD may be assigned as site-specific antioxidants. CR and exercise mimetic agents are the choice for individuals who are difficult to continue CR and exercise. Better understanding of molecular and cellular biology of redox signaling will pave the way for more effective antioxidative medicine for prevention of CVD and prolongation of healthy life span.

The regulation of vascular tetrahydrobiopterin bioavailability

6R l-erythro-5,6,7,8-tetrahydrobiopterin (BH4) is an essential cofactor for several enzymes including phenylalanine hydroxylase and the nitric oxide synthases (NOS). Oral supplementation of BH4 has been successfully employed to treat subsets of patients with hyperphenylalaninaemia. More recently, research efforts have focussed on understanding whether BH4 supplementation may also be efficacious in cardiovascular disorders that are underpinned by reduced nitric oxide bioavailability. Whilst numerous preclinical and clinical studies have demonstrated a positive association between enhanced BH4 and vascular function, the efficacy of orally administered BH4 in human cardiovascular disease remains unclear. Furthermore, interventions that limit BH4 bioavailability may provide benefit in diseases where nitric oxide over production contributes to pathology. This review describes the pathways involved in BH4 bio-regulation and discusses other endogenous mechanisms that could be harnessed therapeutically to manipulate vascular BH4 levels.

Increased production of superoxide anion contributes to dysfunction of the arteriovenous fistula

Vascular access dysfunction causes morbidity in hemodialysis patients. This study examined the generation and pathobiological significance of superoxide anion in a rat femoral arteriovenous fistula (AVF). One week after AVF creation, there was increased production of superoxide anion accompanied by decreased total superoxide dismutase (SOD) and Cu/Zn SOD activities and induction of the redox-sensitive gene heme oxygenase-1. Immunohistochemical studies of nitrotyrosine formation demonstrated that peroxynitrite, a product of superoxide anion and nitric oxide, was present in increased amounts in endothelial and smooth muscle cells in the AVF. Because uncoupled NOS isoforms generate superoxide anion, and NOS coupling requires tetrahydrobiopterin (BH(4)) as a cofactor, we assessed NOS uncoupling by determining the ratio of BH(4) to dihydrobiopterin (BH(2)); the BH(4)-to-BH(2) ratio was markedly attenuated in the AVF. Because Src is a vasculopathic signaling species upstream and downstream of superoxide anion, such expression was evaluated; expression of Src and phosphorylated Src was both markedly increased in the AVF. Expression of NADPH oxidase (NOX) 1, NOX2, NOX4, cyclooxygenase (COX) 1, COX2, p47(phox), and p67(phox) was all unchanged, as assessed by Western analyses, thereby suggesting that these proteins may not be involved in increased production of superoxide anion. Finally, administration of tempol, a superoxide anion scavenger, decreased neointima formation in the juxta-anastomotic venous segment and improved AVF blood flow. We conclude that the AVF exhibits increased superoxide anion generation that may reflect the combined effects of decreased scavenging by SOD and increased generation by uncoupled NOS, and that enhanced superoxide anion production promotes juxta-anastomotic stenosis and impairs AVF function.

Prevention of lethal murine pancreas ischemia reperfusion injury is specific for tetrahydrobiopterin

Tetrahydrobiopterin has been shown to efficiently abrogate ischemia reperfusion injury (IRI). However, it is unclear, whether its beneficial action relies on cofactor activity of one of the five known tetrahydrobiopterin-dependent reactions or on its antioxidative capacity. We therefore compared tetrahydrobiopterin with the pterin derivate tetrahydroneopterin (similar biochemical properties, but no nitric oxide synthase cofactor activity) and the antioxidants vitamin C and 5-methyltetrahydrofolate. Donor mice were pretreated with tetrahydrobiopterin, tetrahydroneopterin, vitamin C, or 5-methyltetrahydrofolate. Pancreatic grafts were subjected to 16-h cold ischemia time and implanted in syngeneic recipients. Untreated and nontransplanted animals served as controls. Following 2-h reperfusion, microcirculation was analyzed by intravital fluorescence microscopy. Graft damage was assessed by histology and nitrotyrosine immunostaining, and tetrahydrobiopterin levels were determined by HPLC. Recipient survival served as ultimate readout. Prolonged cold ischemia time resulted in microcirculatory breakdown. Only tetrahydrobiopterin pretreatment succeeded to preserve the capillary net, whereas all other compounds showed no beneficial effects. Along with increased intragraft tetrahydrobiopterin levels during recovery and implantation, only tetrahydrobiopterin pretreatment led to significant reduction of IRI-related parenchymal damage enabling recipient survival. These results show a striking superiority of tetrahydrobiopterin in preventing lethal IRI compared with related compounds and suggest nitric oxide synthases as treatment target.

Tetrahydrobiopterin protects soluble guanylate cyclase against oxidative inactivation

Tetrahydrobiopterin (BH4) is a major endogenous vasoprotective agent that improves endothelial function by increasing nitric oxide (NO) synthesis and scavenging of superoxide and peroxynitrite. Therefore, administration of BH4 is considered a promising therapy for cardiovascular diseases associated with endothelial dysfunction and oxidative stress. Here we report on a novel function of BH4 that might contribute to the beneficial vascular effects of the pteridine. Treatment of cultured porcine aortic endothelial cells with nitroglycerin (GTN) or 1H-[1,2,4]-oxadiazolo[4,3-a]quinoxaline-1-one (ODQ) resulted in heme oxidation of soluble guanylate cyclase (sGC), as evident from diminished NO-induced cGMP accumulation that was paralleled by increased cGMP response to a heme- and NO-independent activator of soluble guanylate cyclase [4-([(4-carboxybutyl)[2-(5-fluoro-2-([4'-(trifluoromethyl)biphenyl-4-yl]methoxy)phenyl)ethyl]amino]methyl)benzoic acid (BAY 60-2770)]. Whereas scavenging of superoxide and/or peroxynitrite with superoxide dismutase, tiron, Mn(III)tetrakis(4-benzoic acid)porphyrin, and urate had no protective effects, supplementation of the cells with BH4, either by application of BH4 directly or of its precursors dihydrobiopterin or sepiapterin, completely prevented the inhibition of NO-induced cGMP accumulation by GTN and ODQ. Tetrahydroneopterin had the same effect, and virtually identical results were obtained with RFL-6 fibroblasts, suggesting that our observation reflects a general feature of tetrahydropteridines that is unrelated to NO synthase function and not limited to endothelial cells. Protection of sGC against oxidative inactivation may contribute to the known beneficial effects of BH4 in cardiovascular disorders associated with oxidative stress.

Uncoupling of eNOS causes superoxide anion production and impairs NO signaling in the cerebral microvessels of hph-1 mice

In this study, we used the GTP cyclohydrolase I-deficient mice, i.e., hyperphenylalaninemic (hph-1) mice, to test the hypothesis that the loss of tetrahydrobiopterin (BH(4)) in cerebral microvessels causes endothelial nitric oxide synthase (eNOS) uncoupling, resulting in increased superoxide anion production and inhibition of endothelial nitric oxide signaling. Both homozygous mutant (hph-1(-/-)) and heterozygous mutant (hph-1(+/-) mice) demonstrated reduction in GTP cyclohydrolase I activity and reduced bioavailability of BH(4). In the cerebral microvessels of hph-1(+/-) and hph-1(-/-) mice, increased superoxide anion production was inhibited by supplementation of BH(4) or NOS inhibitor- L- N(G) -nitro arginine-methyl ester, indicative of eNOS uncoupling. Expression of 3-nitrotyrosine was significantly increased, whereas NO production and cGMP levels were significantly reduced. Expressions of antioxidant enzymes namely copper and zinc superoxide dismutase, manganese superoxide dismutase, and catalase were not affected by uncoupling of eNOS. Reduced levels of BH(4), increased superoxide anion production, as well as inhibition of NO signaling were not different between the microvessels of male and female mice. The results of our study are the first to demonstrate that, regardless of gender, reduced BH(4) bioavailability causes eNOS uncoupling, increases superoxide anion production, inhibits eNOS/cGMP signaling, and imposes significant oxidative stress in the cerebral microvasculature.

Nucleotide excision DNA repair is associated with age-related vascular dysfunction

BACKGROUND

Vascular dysfunction in atherosclerosis and diabetes mellitus, as observed in the aging population of developed societies, is associated with vascular DNA damage and cell senescence. We hypothesized that cumulative DNA damage during aging contributes to vascular dysfunction.

METHODS AND RESULTS

In mice with genomic instability resulting from the defective nucleotide excision repair genes ERCC1 and XPD (Ercc1(d/-) and Xpd(TTD) mice), we explored age-dependent vascular function compared with that in wild-type mice. Ercc1(d/-) mice showed increased vascular cell senescence, accelerated development of vasodilator dysfunction, increased vascular stiffness, and elevated blood pressure at a very young age. The vasodilator dysfunction was due to decreased endothelial nitric oxide synthase levels and impaired smooth muscle cell function, which involved phosphodiesterase activity. Similar to Ercc1(d/-) mice, age-related endothelium-dependent vasodilator dysfunction in Xpd(TTD) animals was increased. To investigate the implications for human vascular disease, we explored associations between single-nucleotide polymorphisms of selected nucleotide excision repair genes and arterial stiffness within the AortaGen Consortium and found a significant association of a single-nucleotide polymorphism (rs2029298) in the putative promoter region of DDB2 gene with carotid-femoral pulse wave velocity.

CONCLUSIONS

Mice with genomic instability recapitulate age-dependent vascular dysfunction as observed in animal models and in humans but with an accelerated progression compared with wild-type mice. In addition, we found associations between variations in human DNA repair genes and markers for vascular stiffness, which is associated with aging. Our study supports the concept that genomic instability contributes importantly to the development of cardiovascular disease.

Platelets in pulmonary hypertension: a causative role or a simple association?

PATHOPHYSIOLOGY OF PULMONARY ARTERIAL HYPERTENSION IS BASED ON THREE BASIC MECHANISMS: thrombotic pulmonary vascular lesions, vasoconstriction and vascular remodeling. Platelets are related to all of these mechanisms by their aggregation, production, storage and release of several mediators. The role of platelets is more prominent in some types of pulmonary arterial hypertension, including those which are secondary to inflammatory and infectious diseases, hemoglobinopathies, essential thrombocythemia, drugs, thromboembolism, and cardiac surgery. Most pulmonary antihypertensive drugs have a negative effect on platelets. In this review, the mechanisms of platelets association with pulmonary arterial hypertension, those types of pulmonary arterial hypertension with greatest platelet contribution to their pathophysiology, and the effects of pulmonary antihypertensive drugs on platelets are summarized.

Insulin-induced generation of reactive oxygen species and uncoupling of nitric oxide synthase underlie the cerebrovascular insulin resistance in obese rats

Hyperinsulinemia accompanying insulin resistance (IR) is an independent risk factor for stroke. The objective is to examine the cerebrovascular actions of insulin in Zucker obese (ZO) rats with IR and Zucker lean (ZL) control rats. Diameter measurements of cerebral arteries showed diminished insulin-induced vasodilation in ZO compared with ZL. Endothelial denudation revealed vasoconstriction to insulin that was greater in ZO compared with ZL. Nonspecific inhibition of nitric oxide synthase (NOS) paradoxically improved vasodilation in ZO. Scavenging of reactive oxygen species (ROS), supplementation of tetrahydrobiopterin (BH(4)) precursor, and inhibition of neuronal NOS or NADPH oxidase or cyclooxygenase (COX) improved insulin-induced vasodilation in ZO. Immunoblot experiments revealed that insulin-induced phosphorylation of Akt, endothelial NOS, and expression of GTP cyclohydrolase-I (GTP-CH) were diminished, but phosphorylation of PKC and ERK was enhanced in ZO arteries. Fluorescence studies showed increased ROS in ZO arteries in response to insulin that was sensitive to NOS inhibition and BH(4) supplementation. Thus, a vicious cycle of abnormal insulin-induced ROS generation instigating NOS uncoupling leading to further ROS production underlies the cerebrovascular IR in ZO rats. In addition, decreased bioavailability and impaired synthesis of BH(4) by GTP-CH induced by insulin promoted NOS uncoupling.

Direct detection of tetrahydrobiopterin (BH4) and dopamine in rat brain using liquid chromatography coupled electrospray tandem mass spectrometry

A simple and rapid liquid chromatography tandem mass spectrometry (LC-MS/MS) method was developed for the quantification of tetrahydrobiopterin (BH4) and dopamine in rat brain using epsilon-acetamidocaproic acid (AACA) as an internal standard. Proteins in the samples were precipitated with acetonitrile and then the supernatants were separated by a Sepax Polar-Imidazole (2.1 × 100 mm, i.d., 3 μm) column using a mixture of 10mM ammonium formate in acetonitrile/water (75:25, v/v) as the mobile phase at a flow rate of 300 μl/min. Quantification was performed on a triple quadrupole mass spectrometer employing electrospray ionization with the operating conditions as multiple reaction monitoring (MRM) and positive ion mode from m/z 242.1 → 166.0 for BH4, m/z 154.1 → 90.0 for dopamine and m/z 174.1 → 114.0 for AACA (IS). The total chromatographic run time was for 5.5 min. The method was validated for the analysis of samples: the limit of detection was 10 ng/g. The calibration curve was linear between 10-2000 ng/g for BH4 (r(2)=0.995) and 10-5000 ng/g for dopamine (r(2)=0.997) in the rat brain. Thus, good correlated LC-ESI/MS/MS results were obtained and found to be a powerful tool for the quantitative analysis of BH4 and dopamine in the rat brain.

Pulmonary hypertension in the newborn GTP cyclohydrolase I-deficient mouse

Tetrahydrobiopterin (BH4) is a regulator of endothelial nitric oxide synthase (eNOS) activity. Deficient levels result in eNOS uncoupling, with a shift from nitric oxide to superoxide generation. The hph-1 mutant mouse has deficient GTP cyclohydrolase I (GTPCH1) activity, resulting in low BH4 tissue content. The adult hph-1 mouse has pulmonary hypertension, but whether such condition is present from birth is not known. Thus, we evaluated newborn animals' pulmonary arterial medial thickness, biopterin content (BH4+BH2), H(2)O(2) and eNOS, right ventricle-to-left ventricle+septum (RV/LV+septum) ratio, near-resistance pulmonary artery agonist-induced force, and endothelium-dependent and -independent relaxation. The lung biopterin content was inversely related to age for both types, but significantly lower in hph-1 mice, compared to wild-type animals. As judged by the RV/LV+septum ratio, newborn hph-1 mice have pulmonary hypertension and, after a 2-week 13% oxygen exposure, the ratios were similar in both types. The pulmonary arterial agonist-induced force was reduced (P<0.01) in hph-1 animals and no type-dependent difference in endothelium-dependent or -independent vasorelaxation was observed. Compared to wild-type mice, the lung H(2)O(2) content was increased, whereas the eNOS expression was decreased (P<0.01) in hph-1 animals. The pulmonary arterial medial thickness, a surrogate marker of vascular remodeling, was increased (P<0.01) in hph-1 compared to wild-type mice. In conclusion, our data suggest that pulmonary hypertension is present from birth in the GTPCH1-deficient mice, not as a result of impaired vasodilation, but secondary to vascular remodeling.

Differential effects of eNOS uncoupling on conduit and small arteries in GTP-cyclohydrolase I-deficient hph-1 mice

In the present study, we used the hph-1 mouse, which displays GTP-cyclohydrolase I (GTPCH I) deficiency, to test the hypothesis that loss of tetrahydrobiopterin (BH(4)) in conduit and small arteries activates compensatory mechanisms designed to protect vascular wall from oxidative stress induced by uncoupling of endothelial nitric oxide synthase (eNOS). Both GTPCH I activity and BH(4) levels were reduced in the aortas and small mesenteric arteries of hph-1 mice. However, the BH(4)-to-7,8-dihydrobiopterin ratio was significantly reduced only in hph-1 aortas. Furthermore, superoxide anion and 3-nitrotyrosine production were significantly enhanced in aortas but not in small mesenteric arteries of hph-1 mice. In contrast to the aorta, protein expression of copper- and zinc-containing superoxide dismutase (CuZnSOD) was significantly increased in small mesenteric arteries of hph-1 mice. Protein expression of catalase was increased in both aortas and small mesenteric arteries of hph-1 mice. Further analysis of endothelial nitric oxide synthase (eNOS)/cyclic guanosine monophosphate (cGMP) signaling demonstrated that protein expression of phosphorylated Ser(1177)-eNOS as well as basal cGMP levels and hydrogen peroxide was increased in hph-1 aortas. Increased production of hydrogen peroxide in hph-1 mice aortas appears to be the most likely mechanism responsible for phosphorylation of eNOS and elevation of cGMP. In contrast, upregulation of CuZnSOD and catalase in resistance arteries is sufficient to protect vascular tissue from increased production of reactive oxygen species generated by uncoupling of eNOS. The results of our study suggest that anatomical origin determines the ability of vessel wall to cope with oxidative stress induced by uncoupling of eNOS.

Induction of vascular GTP-cyclohydrolase I and endogenous tetrahydrobiopterin synthesis protect against inflammation-induced endothelial dysfunction in human atherosclerosis

BACKGROUND

The endothelial nitric oxide synthase cofactor tetrahydrobiopterin (BH4) is essential for maintenance of enzymatic function. We hypothesized that induction of BH4 synthesis might be an endothelial defense mechanism against inflammation in vascular disease states.

METHODS AND RESULTS

In Study 1, 20 healthy individuals were randomized to receive Salmonella typhi vaccine (a model of acute inflammation) or placebo in a double-blind study. Vaccination increased circulating BH4 and interleukin 6 and induced endothelial dysfunction (as evaluated by brachial artery flow-mediated dilation) after 8 hours. In Study 2, a functional haplotype (X haplotype) in the GCH1 gene, encoding GTP-cyclohydrolase I, the rate-limiting enzyme in biopterin biosynthesis, was associated with endothelial dysfunction in the presence of high-sensitivity C-reactive protein in 440 coronary artery disease patients. In Study 3, 10 patients with coronary artery disease homozygotes for the GCH1 X haplotype (XX) and 40 without the haplotype (OO) underwent S Typhi vaccination. XX patients were unable to increase plasma BH4 and had a greater reduction of flow-mediated dilation than OO patients. In Study 4, vessel segments from 19 patients undergoing coronary bypass surgery were incubated with or without cytokines (interleukin-6/tumor necrosis factor-α/lipopolysaccharide) for 24 hours. Cytokine stimulation upregulated GCH1 expression, increased vascular BH4, and improved vasorelaxation in response to acetylcholine, which was inhibited by the GTP-cyclohydrolase inhibitor 2,4-diamino-6-hydroxypyrimidine.

CONCLUSIONS

The ability to increase vascular GCH1 expression and BH4 synthesis in response to inflammation preserves endothelial function in inflammatory states. These novel findings identify BH4 as a vascular defense mechanism against inflammation-induced endothelial dysfunction.

Targeting endothelial and myocardial dysfunction with tetrahydrobiopterin

Tetrahydrobiopterin (BH(4)) is an essential cofactor for aromatic amino acid hydroxylases and for all three nitric oxide synthase (NOS) isoforms. It also has a protective role in the cell as an antioxidant and scavenger of reactive nitrogen and oxygen species. Experimental studies in humans and animals demonstrate that decreased BH(4)-bioavailability, with subsequent uncoupling of endothelial NOS (eNOS) plays an important role in the pathogenesis of endothelial dysfunction, hypertension, ischemia-reperfusion injury, and pathologic cardiac remodeling. Synthetic BH(4) is clinically approved for the treatment of phenylketonuria, and experimental studies support its capacity for ameliorating cardiovascular pathophysiologies. To date, however, the translation of these studies to human patients remains limited, and early results have been mixed. In this review, we discuss the pathophysiologic role of decreased BH(4) bioavailability, molecular mechanisms regulating its metabolism, and its potential therapeutic use as well as pitfalls as an NOS-modulating drug. This article is part of a special issue entitled ''Key Signaling Molecules in Hypertrophy and Heart Failure.''

Oxidative stress as pathogenesis of cardiovascular risk associated with metabolic syndrome

Metabolic syndrome (MetS) is characterized by accumulation of visceral fat associated with the clustering of metabolic and pathophysiological cardiovascular risk factors: impaired glucose tolerance, dyslipidemia, and hypertension. Although the definition of MetS is different among countries, visceral obesity is an indispensable component of MetS. A growing body of evidence suggests that increased oxidative stress to adipocytes is central to the pathogenesis of cardiovascular disease in MetS. Increased oxidative stress to adipocytes causes dysregulated expression of inflammation-related adipocytokines in MetS, which contributes to obesity-associated vasculopathy and cardiovascular risk primarily through endothelial dysfunction. The purpose of present review is to unravel the mechanistic link between oxidative stress and cardiovascular risk in MetS, focusing on insulin resistance, hypertension, and atherosclerosis. Then, therapeutic opportunities translated from the bench to bedside will be provided to develop novel strategies to cardiovascular risk factors in MetS.

Reactive oxygen species and endothelial function--role of nitric oxide synthase uncoupling and Nox family nicotinamide adenine dinucleotide phosphate oxidases

The healthy endothelium prevents platelet aggregation and leucocyte adhesion, controls permeability to plasma components and maintains vascular integrity. Damage to the endothelium promotes endothelial dysfunction characterized by: altered endothelium-mediated vasodilation, increased vascular reactivity, platelet aggregation, thrombus formation, increased permeability, leucocyte adhesion and monocyte migration. Molecular processes contributing to these phenomena include increased expression of adhesion molecules, synthesis of pro-inflammatory and pro-thrombotic factors and increased endothelin-1 secretion. Decreased nitric oxide bioavailability and increased generation of reactive oxygen species (ROS) are among the major molecular changes associated with endothelial dysfunction. A critical source of endothelial ROS is a family of non-phagocytic nicotinamide adenine dinucleotide phosphate (NADPH) oxidases, including the prototypic Nox2-based NADPH oxidases, Nox1, Nox4 and Nox5. Other possible sources include mitochondrial electron transport enzymes, xanthine oxidase, cyclooxygenase, lipoxygenase and uncoupled nitric oxide synthase (NOS). Cross-talk between ROS-generating enzymes, such as mitochondrial oxidases and Noxs, is increasingly implicated in cellular ROS production. The present review discusses the importance of endothelial ROS in health and disease and focuses on the major ROS-generating systems in the endothelium, namely uncoupled endothelial nitric oxide synthase and NADPH oxidases.

Tetrahydrobiopterin: biochemistry and pathophysiology

BH4 (6R-L-erythro-5,6,7,8-tetrahydrobiopterin) is an essential cofactor of a set of enzymes that are of central metabolic importance, including four aromatic amino acid hydroxylases, alkylglycerol mono-oxygenase and three NOS (NO synthase) isoenzymes. Consequently, BH4 is present in probably every cell or tissue of higher organisms and plays a key role in a number of biological processes and pathological states associated with monoamine neurotransmitter formation, cardiovascular and endothelial dysfunction, the immune response and pain sensitivity. BH4 is formed de novo from GTP via a sequence of three enzymatic steps carried out by GTP cyclohydrolase I, 6-pyruvoyltetrahydropterin synthase and sepiapterin reductase. An alternative or salvage pathway involves dihydrofolate reductase and may play an essential role in peripheral tissues. Cofactor regeneration requires pterin-4a-carbinolamine dehydratase and dihydropteridine reductase, except for NOSs, in which the BH4 cofactor undergoes a one-electron redox cycle without the need for additional regeneration enzymes. With regard to the regulation of cofactor biosynthesis, the major controlling point is GTP cyclohydrolase I. BH4 biosynthesis is controlled in mammals by hormones and cytokines. BH4 deficiency due to autosomal recessive mutations in all enzymes, except for sepiapterin reductase, has been described as a cause of hyperphenylalaninaemia. A major contributor to vascular dysfunction associated with hypertension, ischaemic reperfusion injury, diabetes and others, appears to be an effect of oxidized BH4, which leads to an increased formation of oxygen-derived radicals instead of NO by decoupled NOS. Furthermore, several neurological diseases have been suggested to be a consequence of restricted cofactor availability, and oral cofactor replacement therapy to stabilize mutant phenylalanine hydroxylase in the BH4-responsive type of hyperphenylalaninaemia has an advantageous effect on pathological phenylalanine levels in patients.

A novel high-throughput screening assay for discovery of molecules that increase cellular tetrahydrobiopterin

Tetrahydrobiopterin (BH(4)) is an essential cofactor for the nitric oxide (NO) synthases and the aromatic amino acid hydroxylases. Insufficient BH(4) has been implicated in various cardiovascular and neurological disorders. GTP cyclohydrolase 1 (GTPCH-1) is the rate-limiting enzyme for de novo biosynthesis of BH(4). The authors have recently shown that the interaction of GTPCH-1 with GTP cyclohydrolase feedback regulatory protein (GFRP) inhibits endothelial GTPCH-1 enzyme activity, BH(4) levels, and NO production. They propose that agents that disrupt the GTPCH-1/GFRP interaction can increase cellular GTPCH-1 activity, BH(4) levels, and NO production. They developed and optimized a novel time-resolved fluorescence resonance energy transfer (TR-FRET) assay to monitor the interaction of GTPCH-1 and GFRP. This assay is highly sensitive and stable and has a signal-to-background ratio (S/B) greater than 12 and a Z' factor greater than 0.8. This assay was used in an ultra-high-throughput screening (uHTS) format to screen the Library of Pharmacologically Active Compounds. Using independent protein-protein interaction and cellular activity assays, the authors identified compounds that disrupt GTPCH-1/GFRP binding and increase endothelial cell biopterin levels. Thus, this TR-FRET assay could be applied in future uHTS of additional libraries to search for molecules that increase GTPCH-1 activity and BH(4) levels.

Increasing dihydrobiopterin causes dysfunction of endothelial nitric oxide synthase in rats in vivo

An elevation of oxidized forms of tetrahydrobiopterin (BH(4)), especially dihydrobiopterin (BH(2)), has been reported in the setting of oxidative stress, such as arteriosclerotic/atherosclerotic disorders, where endothelial nitric oxide synthase (eNOS) is dysfunctional, but the role of BH(2) in the regulation of eNOS activity in vivo remains to be evaluated. This study was designed to clarify whether increasing BH(2) concentration causes endothelial dysfunction in rats. To increase vascular BH(2) levels, the BH(2) precursor sepiapterin (SEP) was intravenously given after the administration of the specific dihydrofolate reductase inhibitor methotrexate (MTX) to block intracellular conversion of BH(2) to BH(4). MTX/SEP treatment did not significantly affect aortic BH(4) levels compared with control treatment. However, MTX/SEP treatment markedly augmented aortic BH(2) levels (291.1 ± 29.2 vs. 33.4 ± 6.4 pmol/g, P < 0.01) in association with moderate hypertension. Treatment with MTX alone did not significantly alter blood pressure or BH(4) levels but decreased the BH(4)-to-BH(2) ratio. Treatment with MTX/SEP, but not with MTX alone, impaired ACh-induced vasodilator and depressor responses compared with the control treatment (both P < 0.05) and also aggravated ACh-induced endothelium-dependent relaxations (P < 0.05) of isolated aortas without affecting sodium nitroprusside-induced endothelium-independent relaxations. Importantly, MTX/SEP treatment significantly enhanced aortic superoxide production, which was diminished by NOS inhibitor treatment, and the impaired ACh-induced relaxations were reversed with SOD (P < 0.05), suggesting the involvement of eNOS uncoupling. These results indicate, for the first time, that increasing BH(2) causes eNOS dysfunction in vivo even in the absence of BH(4) deficiency, demonstrating a novel insight into the regulation of endothelial function.

Rapid, direct effects of statin treatment on arterial redox state and nitric oxide bioavailability in human atherosclerosis via tetrahydrobiopterin-mediated endothelial nitric oxide synthase coupling

BACKGROUND

Treatment with statins improves clinical outcome, but the exact mechanisms of pleiotropic statin effects on vascular function in human atherosclerosis remain unclear. We examined the direct effects of atorvastatin on tetrahydrobiopterin-mediated endothelial nitric oxide (NO) synthase coupling in patients with coronary artery disease.

METHODS AND RESULTS

We first examined the association of statin treatment with vascular NO bioavailability and arterial superoxide (O(2)(·-)) in 492 patients undergoing coronary artery bypass graft surgery. Then, 42 statin-naïve patients undergoing elective coronary artery bypass graft surgery were randomized to atorvastatin 40 mg/d or placebo for 3 days before surgery to examine the impact of atorvastatin on endothelial function and O(2)(·-) generation in internal mammary arteries. Finally, segments of internal mammary arteries from 26 patients were used in ex vivo experiments to evaluate the statin-dependent mechanisms regulating the vascular redox state. Statin treatment was associated with improved vascular NO bioavailability and reduced O(2)(·-) generation in internal mammary arteries. Oral atorvastatin increased vascular tetrahydrobiopterin bioavailability and reduced basal and N-nitro-l-arginine methyl ester-inhibitable O(2)(·-) in internal mammary arteries independently of low-density lipoprotein lowering. In ex vivo experiments, atorvastatin rapidly improved vascular tetrahydrobiopterin bioavailability by upregulating GTP-cyclohydrolase I gene expression and activity, resulting in improved endothelial NO synthase coupling and reduced vascular O(2)(·-). These effects were reversed by mevalonate, indicating a direct effect of vascular hydroxymethylglutaryl-coenzyme A reductase inhibition.

CONCLUSIONS

This study demonstrates for the first time in humans the direct effects of statin treatment on the vascular wall, supporting the notion that this effect is independent of low-density lipoprotein lowering. Atorvastatin directly improves vascular NO bioavailability and reduces vascular O(2)(·-) through tetrahydrobiopterin-mediated endothelial NO synthase coupling. These findings provide new insights into the mechanisms mediating the beneficial vascular effects of statins in humans.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrials.gov. Unique identifier: NCT01013103.

Oxidative stress and hypertension: current concepts

Hypertension is a major contributor to the development of renal failure, cardiovascular disease, and stroke. These pathologies are associated with vascular functional and structural changes including endothelial dysfunction, altered contractility, and vascular remodeling. Central to these phenomena is oxidative stress. Factors that activate pro-oxidant enzymes, such as NADPH oxidase, remain poorly defined, but likely involve angiotensin II, mechanical stretch, and inflammatory cytokines. Reactive oxygen species influence vascular, renal, and cardiac function and structure by modulating cell growth, contraction/dilatation, and inflammatory responses via redox-dependent signaling pathways. Compelling data from molecular and cellular experiments, together with animal studies, implicate a role for oxidative stress in hypertension. However, the clinical evidence is still controversial. This review provides current insights on the mechanisms of the generation of reactive oxygen species and the vascular effects of oxidative stress and discusses the significance of oxidative damage in experimental and clinical hypertension.

Modulating endothelial nitric oxide synthase: a new cardiovascular therapeutic strategy

The pathogenesis of many cardiovascular diseases is associated with reduced nitric oxide (NO) bioavailability and/or increased endothelial NO synthase (eNOS)-dependent superoxide formation. These findings support that restoring and conserving adequate NO signaling in the heart and blood vessels is a promising therapeutic intervention. In particular, modulating eNOS, e.g., through increasing the bioavailability of its substrate and cofactors, enhancing its transcription, and interfering with other modulators of eNOS pathway, such as netrin-1, has a high potential for effective treatments of cardiovascular diseases. This review provides an overview of the possibilities for modulating eNOS and how this may be translated to the clinic in addition to describing the genetic models used to study eNOS modulation.

Endothelial nitric oxide synthase uncoupling as a key mediator of melanocyte malignant transformation associated with sustained stress conditions

Melanoma cell lines and cells corresponding to premalignant melanocytes were established by our group after subjecting a nontumorigenic murine melanocyte lineage, melan-a, to sequential cycles of anchorage blockade. Previous results showed that in melan-a cells the superoxide level increases after such procedure. Superoxide production during melanocyte de-adhesion was inhibited by L-sepiapterin, the precursor of eNOS cofactor BH4, and increased by the inhibitor of BH4 synthesis, DAHP, hence indicating a partial uncoupling state of eNOS. The eNOS uncoupling seems to be maintained in cells derived from melan-a, because they present decreased nitric oxide and increased superoxide levels. The inhibition of superoxide production in Tm5 melanoma cells with L-sepiapterin reinforces their eNOS-uncoupled state. The maintenance of oxidative stress seems to be important in melanoma apoptosis resistance because Mn(III)TBAP, a superoxide scavenger, or L-sepiapterin renders Tm5 cells more sensitive to anoikis and chemotherapy. More importantly, eNOS uncoupling seems to play a pivotal role in melanocyte malignant transformation induced by sustained anchorage impediment, because no malignant transformation was observed when L-NAME-treated melanocytes were subjected to sequential cycles of de-adhesion. Our results show that uncoupled eNOS contributes to superoxide production during melanocyte anchorage impediment, contributing to anoikis resistance and malignant transformation.

Pathogenetic role of eNOS uncoupling in cardiopulmonary disorders

The homodimeric flavohemeprotein endothelial nitric oxide synthase (eNOS) oxidizes l-arginine to l-citrulline and nitric oxide (NO), which acutely vasodilates blood vessels and inhibits platelet aggregation. Chronically, eNOS has a major role in the regulation of blood pressure and prevention of atherosclerosis by decreasing leukocyte adhesion and smooth muscle proliferation. However, a disturbed vascular redox balance results in eNOS damage and uncoupling of oxygen activation from l-arginine conversion. Uncoupled eNOS monomerizes and generates reactive oxygen species (ROS) rather than NO. Indeed, eNOS uncoupling has been suggested as one of the main pathomechanisms in a broad range of cardiovascular and pulmonary disorders such as atherosclerosis, ventricular remodeling, and pulmonary hypertension. Therefore, modulating uncoupled eNOS, in particular eNOS-dependent ROS generation, is an attractive therapeutic approach to preventing and/or treating cardiopulmonary disorders, including protective effects during cardiothoracic surgery. This review provides a comprehensive overview of the pathogenetic role of uncoupled eNOS in both cardiovascular and pulmonary disorders. In addition, the related therapeutic possibilities such as supplementation with the eNOS substrate l-arginine, volatile NO, and direct NO donors as well as eNOS modulators such as the eNOS cofactor tetrahydrobiopterin and folic acid are discussed in detail.

eNOS-uncoupling in age-related erectile dysfunction

Aging is associated with ED. Although age-related ED is attributed largely to increased oxidative stress and endothelial dysfunction in the penis, the molecular mechanisms underlying this effect are not fully defined. We evaluated whether endothelial nitric oxide synthase (eNOS) uncoupling in the aged rat penis is a contributing mechanism. Correlatively, we evaluated the effect of replacement with eNOS cofactor tetrahydrobiopterin (BH(4)) on erectile function in the aged rats. Male Fischer 344 'young' (4-month-old) and 'aged' (19-month-old) rats were treated with a BH(4) precursor sepiapterin (10 mg/kg intraperitoneally) or vehicle for 4 days. After 1-day washout, erectile function was assessed in response to electrical stimulation of the cavernous nerve. Endothelial dysfunction (eNOS uncoupling) and oxidative stress (thiobarbituric acid reactive substances, TBARS) were measured by conducting western blot in penes samples. Erectile response was significantly reduced in aged rats, whereas eNOS uncoupling and TBARS production were significantly increased in the aged rat penis compared with young rats. Sepiapterin significantly improved erectile response in aged rats and prevented increase in TBARS production, but did not affect eNOS uncoupling in the penis of aged rats. These findings suggest that aging induces eNOS uncoupling in the penis, resulting in increased oxidative stress and ED.

ACE2-angiotensin-(1-7)-Mas axis and oxidative stress in cardiovascular disease

The renin-angiotensin-aldosterone system (RAAS) is a pivotal regulator of physiological homeostasis and diseases of the cardiovascular system. Recently, new factors have been discovered, such as angiotensin-converting enzyme 2 (ACE2), angiotensin-(1-7) and Mas. This newly defined ACE2-angiotensin-(1-7)-Mas axis was shown to have a critical role in the vasculature and in the heart, exerting mainly protective effects. One important mechanism of the classic and the new RAAS regulate vascular function is through the regulation of redox signaling. Angiotensin II is a classic prooxidant peptide that increases superoxide production through the activation of NAD(P)H oxidases. This review summarizes the current knowledge about the ACE2-angiotensin-(1-7)-Mas axis and redox signaling in the context of cardiovascular regulation and disease. By interacting with its receptor Mas, angiotensin-(1-7) induces the release of nitric oxide from endothelial cells and thereby counteracts the effects of angiotensin II. ACE2 converts angiotensin II to angiotensin-(1-7) and, thus, is a pivotal regulator of the local effects of the RAAS on the vessel wall. Taken together, the ACE2-angiotensin-(1-7)-Mas axis emerges as a novel therapeutic target in the context of cardiovascular and metabolic diseases.

Tetrahydrobiopterin does not affect end-organ responsiveness to norepinephrine-mediated vasoconstriction in aged skin

We have recently demonstrated that tetrahydrobiopterin (BH(4)) augments reflex vasoconstriction (VC) in aged skin. Although this appears to occur through its role in norepinephrine (NE) biosynthesis, the extent with which vascular mechanisms are affected are unknown. We hypothesized that localized BH(4) supplementation would not affect the VC response to exogenous NE when sympathetic nerves were blocked. Two microdialysis fibers were placed in bretylium tosylate pretreated (presynaptically blocks neurotransmitter release from sympathetic adrenergic nerve terminals; iontophoresis, 200 μA for 20 min) 3-cm(2) forearm skin of 10 young (Y) and 10 older (O) subjects for perfusion of 1) Ringer (control) and 2) 5 mM BH(4). While local skin temperature was clamped at 34°C, six concentrations of NE (10(-12), 10(-10), 10(-8), 10(-6), 10(-4), 10(-2) M) were infused at each drug-treated site. Cutaneous vascular conductance (CVC) was calculated (CVC = laser Doppler flux/mean arterial pressure) and normalized to baseline (%ΔCVC(base)). Despite prejunctional adrenergic blockade, NE-mediated VC was blunted in aged skin at each NE dose (10(-12): -12 ± 2 vs. -21 ± 2; 10(-10): -15 ± 2 vs. -27 ± 1; 10(-8): -22 ± 2 vs. -32 ± 2; 10(-6): -27 ± 2 vs. -38 ± 1; 10(-4): -52 ± 3 vs. -66 ± 5; 10(-2): -62 ± 3 vs. -75 ± 4%ΔCVC(base); P < 0.01), and this response was not affected by pretreatment with BH(4) (P > 0.05). Localized BH(4) did not affect end-organ responsiveness to exogenous NE, suggesting that the effects of BH(4) on cutaneous VC are primarily isolated to the NE biosynthetic pathway.

Donor pretreatment with tetrahydrobiopterin saves pancreatic isografts from ischemia reperfusion injury in a mouse model

Depletion of the nitric oxide synthase cofactor tetrahydrobiopterin (H4B) during ischemia and reperfusion is associated with severe graft pancreatitis. Since clinically feasible approaches to prevent ischemia reperfusion injury (IRI) by H4B-substitution are missing we investigated its therapeutic potential in a murine pancreas transplantation model using different treatment regimens. Grafts were subjected to 16 h cold ischemia time (CIT) and different treatment regimens: no treatment, 160 μM H4B to perfusion solution, H4B 50 mg/kg prior to reperfusion and H4B 50 mg/kg before recovery of organs. Nontransplanted animals served as controls. Recipient survival and endocrine graft function were assessed. Graft microcirculation was analyzed 2 h after reperfusion by intravital fluorescence microscopy. Parenchymal damage was assessed by histology and nitrotyrosine immunohistochemistry, H4B tissue levels by high pressure liquid chromatography (HPLC). Compared to nontransplanted controls prolonged CIT resulted in significant microcirculatory deterioration. Different efficacy according to route and timing of administration could be observed. Only donor pretreatment with H4B resulted in almost completely abrogated IRI-related damage showing graft microcirculation comparable to nontransplanted controls and restored intragraft H4B levels, resulting in significant reduction of parenchymal damage (p < 0.002) and improved survival and endocrine function (p = 0.0002 each). H4B donor pretreatment abrogates ischemia-induced parenchymal damage and represents a promising strategy to prevent IRI following pancreas transplantation.

Is tetrahydrobiopterin a therapeutic option in diabetic hypertensive patients?

Nitric oxide (NO) is an important regulator of vascular tone, and is also an antithrombotic, anti-inflammatory, antiproliferative, and antiatherogenic factor. Endothelial function is altered in patients with coronary artery disease, stroke, and peripheral artery disease, and endothelial dysfunction correlates with the risk factor profile for a patient. Hypertension and type 2 diabetes are risk factors for vascular disease, and are both pathologies characterized by loss of NO activity. Indeed, endothelial dysfunction is usually present in diabetic and/or hypertensive patients. Tetrahydrobiopterin is an essential cofactor for the NO synthase enzyme, and insufficiency of this cofactor leads to uncoupling of the enzyme, release of superoxide, endothelial dysfunction, progression of hypertension, and finally, proatherogenic effects. Tetrahydrobiopterin is also an important mediator of NO synthase regulation in type 2 diabetes and hypertension, and may be a rational therapeutic target to restore endothelial function and prevent vascular disease in these patients. The aim of this paper is to review the rationale for therapeutic strategies directed to biopterins as a target for vascular disease in type 2 diabetic hypertensive patients.

Tetrahydrobiopterin (BH4), a cofactor for nNOS, restores gastric emptying and nNOS expression in female diabetic rats

Gastroparesis is a debilitating disease predominantly affecting young women. Recently, dysregulation of neuronal nitric oxide synthase (nNOS) in myenteric plexus neurons has been implicated for delayed solid gastric emptying/gastroparesis in diabetic patients. In this study, we have explored the role of tetrahydrobiopterin (BH4), a major cofactor for nNOS activity and NO synthesis in diabetic gastroparesis. Diabetes was induced with single injection of streptozotocin (55 mg/kg body wt, ip) in female rats, with experiments performed on week 3 or 9 following induction, with or without 3-wk BH4 supplementation. Gastric pyloric BH4 levels were significantly decreased in diabetic female rats compared with control (18.6 +/- 1.45 vs. 31.0 +/- 2.31 pmol/mg protein). In vitro studies showed that 2,4-diamino-6-hydroxypyrimidine (DAHP), an inhibitor of BH4 synthesis, significantly decreased gastric NO release and nitrergic relaxation. Three-week dietary supplementation of BH4 either from day 1 or week 6 significantly attenuated diabetes-induced delayed gastric emptying for solids (3 wk: BH4, 67 +/- 6.7 vs. diabetic, 36.05 +/- 7.09; 9 wk: BH4, 57 +/- 8.45 vs. diabetic, 33 +/- 9.91) and diabetes-induced reduction in pyloric nNOS-alpha protein expression in female rats. Supplementation of BH4 significantly restored gastric nNOS-alpha dimerization in 9-wk-old diabetic female rats. In addition, BH4 treatment reversed (17.23 +/- 5.81 vs. 42.0 +/- 2.70 mmHg x s) the diabetes-induced changes in intragastric pressures (IGP) and gastric pyloric nitrergic relaxation (-0.62 +/- 0.01 vs. -0.22 +/- 0.07). BH4 deficiency plays a critical role in diabetes-induced alterations including delayed solid gastric emptying, increased IGP, reduced pyloric nitrergic relaxation, and nNOS-alpha expression in female rats. Supplementation of BH4 accelerates gastric emptying by restoring nitrergic system in diabetic female rats. Therefore, BH4 supplementation is a potential therapeutic option for female patients of diabetic gastroparesis.

Improvement of impaired endothelial function by tetrahydrobiopterin in stroke-prone spontaneously hypertensive rats

To investigate the role of tetrahydrobiopterin (BH4), an essential cofactor of nitric oxide synthase, in endothelial function in a model of genetic hypertension, acetylcholine- and sodium nitroprusside (SNP)-induced vasodilator responses were examined in the absence and presence of BH4 in age-matched adult stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar-Kyoto (WKY) rats. Acetylcholine-induced depressor responses attenuated significantly in SHRSP compared with those in WKY rats. Acetylcholine-induced relaxations in phenylephrine-precontracted aortic rings of SHRSP were also significantly impaired as compared to those of WKY rats, while SNP-induced relaxations were similar between both strains. In SHRSP, intravenous infusion of BH4 (0.12 mg/kg per min for 20 min following a bolus injection of 0.48 mg/kg) significantly improved vasodilator responses to acetylcholine without affecting those to SNP, but in WKY rats BH4 did not influence those to acetylcholine. BH4 infusion itself had no hemodynamic effect in both strains. However, BH4 levels in plasma and thoracic aorta as well as plasma concentrations of nitrite plus nitrate, metabolites of NO, in SHRSP were all significantly greater than those in WKY rats, suggesting the occurrence of compensatory upregulation of NO synthesis in SHRSP. These results demonstrate that the impaired endothelial function in SHRSP cannot be explained simply by the decrease in absolute amount of BH4.

Investigational drugs in sickle cell anemia

Sickle cell anemia is one of the most common autosomal recessive diseases in the world. Patients with sickle cell anemia have variable penetrance and it is hard to predict the risk and timing of complications. It is characterized by a point mutation in the beta-globin gene (GAG --> GTG) and the production of hemoglobin S. The latter leads to decreased deformability of the red blood cells (RBCs) that adhere to endothelia cells culminating in vascular occlusion and its sequelae of tissue ischemia and organ damage. Moreover, sickled RBCs undergo intravascular hemolysis and accelerated erythropoesis. The hallmarks of this disease are shortened RBC survival and vaso-occlusive crises. For the past ten years, the pathophysiology of this disease has been better elucidated and has led to significant improvements in the standard of care. Vaso-occlusion is now understood to be a complex event that involves abnormal interactions between RBCs, leukocytes, endothelial cells and the coagulation pathways. The field of translational research in sickle cell anemia has expanded greatly and has led to new clinical trials with new therapeutic agents and strategies. In this paper, we review the drugs that are now being investigated in the treatment of sickle cell anemia.

Early and prominent alterations in hemodynamics, signaling, and gene expression following renal ischemia in sickle cell disease

Acute ischemic insults to the kidney are recognized complications of human sickle cell disease (SCD). The present study analyzed in a transgenic SCD murine model the early renal response to acute ischemia. Renal hemodynamics were profoundly impaired following ischemia in sickle mice compared with wild-type mice: glomerular filtration rate, along with renal plasma flow and blood flow rates, were markedly reduced, while renal vascular resistances were increased more than threefold in sickle mice following ischemia. In addition to these changes in renal hemodynamics, there were profound disturbances in renal signaling processes: phosphorylation of members of the MAPK and Akt signaling proteins occurred in the kidney in wild-type mice after ischemia, whereas such phosphorylation did not occur in the kidney in sickle mice after ischemia. ATP content in the postischemic kidney in sickle mice was less than half that observed in wild-type mice. Examination of the expression of candidate genes uncovered changes that may predispose to increased sensitivity of the kidney in sickle mice to ischemia: increased expression of inducible nitric oxide synthase and decreased expression of endothelial nitric oxide synthase, and increased expression of TNF-alpha. Inducibility of anti-inflammatory, cytoprotective genes, such as heme oxygenase-1 and IL-10, was not impaired in sickle mice after ischemia. We conclude that the kidney in SCD is remarkably vulnerable to acute ischemic insults. We speculate that such sensitivity of the kidney to ischemia in SCD may underlie the occurrence of acute kidney injury in patients with SCD and may set the stage for the emergence of chronic kidney disease in SCD.

Cellular signaling and NO production

The endothelium can evoke relaxations (dilatations) of the underlying vascular smooth muscle, by releasing vasodilator substances. The best characterized endothelium-derived relaxing factor is nitric oxide (NO), which is synthesized by the endothelial isoform of nitric oxide synthase (eNOS). Endothelium-dependent relaxations involve both pertussis-toxin-sensitive G(i) (e.g., responses to serotonin, sphingosine 1-phosphate, alpha(2)-adrenergic agonists, and thrombin) and pertussis-toxin-insensitive G(q) (e.g., adenosine diphosphate and bradykinin) coupling proteins. eNOS undergoes a complex pattern of intracellular regulation, including post-translational modifications involving enzyme acylation and phosphorylation. eNOS is reversibly targeted to signal-transducing plasmalemmal caveolae where the enzyme interacts with a number of regulatory proteins, many of which are modified in cardiovascular disease states. The release of nitric oxide by the endothelial cell can be up- (e.g., by estrogens, 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 lose the pertussis-toxin-sensitive pathway for NO release which favors vasospasm, thrombosis, penetration of macrophages, cellular growth, and the inflammatory reaction leading to atherosclerosis. The unraveling of the complex interaction of the pathways regulating the presence and the activity of eNOS will enhance the understanding of the perturbations in endothelium-dependent signaling that are seen in cardiovascular disease states, and may lead to the identification of novel targets for therapeutic intervention.

Evidence against tetrahydrobiopterin depletion of vascular tissue exposed to nitric oxide/superoxide or nitroglycerin

Several cardiovascular disorders, including atherosclerosis and tolerance to the antianginal drug nitroglycerin (GTN), may be associated with the generation of superoxide anions, which react with nitric oxide (NO) to yield peroxynitrite. According to a widely held view, oxidation of tetrahydrobiopterin (BH(4)) by peroxynitrite causes uncoupling of endothelial NO synthase (eNOS), resulting in reduced NO bioavailability and endothelial dysfunction under conditions of oxidative stress. In this study we determined the levels of reduced biopterins and endothelial function in cultured cells exposed to peroxynitrite and GTN as well as in blood vessels isolated from GTN-tolerant guinea pigs and rats. BH(4) was rapidly oxidized by peroxynitrite and 3-morpholino sydnonimine (SIN-1) in buffer, but this was prevented by glutathione and not observed in endothelial cells exposed to SIN-1 or GTN. Prolonged treatment of the cells with 0.1 mM GTN caused slow N(G)-nitro-l-arginine-sensitive formation of reactive oxygen species without affecting eNOS activity. Endothelial function and BH(4)/BH(2) levels were identical in blood vessels of control and GTN-tolerant animals. Our results suggest that peroxynitrite-triggered BH(4) oxidation does not occur in endothelial cells or GTN-exposed blood vessels. GTN seems to trigger minor eNOS uncoupling that is unrelated to BH(4) depletion and without observable consequence on eNOS function.

A systems biology consideration of the vasculopathy of sickle cell anemia: the need for multi-modality chemo-prophylaxsis

Much of the morbidity and mortality of sickle cell anemia is accounted for by a chronic vasculopathy syndrome. There is currently no identified therapy, interventional or prophylactic, for this problem. For two reasons, development of an effective therapeutic approach will require a systems biology level perspective on the vascular pathobiology of sickle disease. In the first place, multiple biological processes contribute to the pathogenesis of vasculopathy: red cell sickling, inflammation and adhesion biology, coagulation activation, stasis, deficient bioavailability and excessive consumption of NO, excessive oxidation, and reperfusion injury physiology. The probable hierarchy of involvement of these disparate sub-biologies places inflammation caused by reperfusion injury physiology as the likely, proximate, linking pathophysiological factor. In the second place, most of these sub-biologies overlap with each other and, in any case, have multiple points of potential interaction and transactivation. Consequently, an approach modeled upon chemotherapy for cancer is needed. This would be a truly multi-modality approach that hopefully could be achieved via employment of relatively few drugs. It is proposed here that the specific combination of a statin with suberoylanilide hydroxamic acid would provide a suitable, broad, multi-modality approach to chemo-prophylaxis for sickle vasculopathy.

Ascorbic acid and tetrahydrobiopterin potentiate the EDHF phenomenon by generating hydrogen peroxide

AIMS

Our objective was to investigate whether pro-oxidant properties of ascorbic acid (AA) and tetrahydrobiopterin (BH(4)) modulate endothelium-dependent, electrotonically mediated arterial relaxation.

METHODS AND RESULTS

In studies with rabbit iliac artery (RIA) rings, NO-independent, endothelium-derived hyperpolarizing factor (EDHF)-type relaxations evoked by the sarcoplasmic endoplasmic reticulum Ca(2+)-ATPase inhibitor cyclopiazonic acid and the G protein-coupled agonist acetylcholine (ACh) were enhanced by AA (1 mM) and BH(4) (200 microM), which generated buffer concentrations of H(2)O(2) in the range of 40-80 microM. Exogenous H(2)O(2) potentiated cyclopiazonic acid (CPA)- and ACh-evoked relaxations with a threshold of 10-30 microM, and potentiation by AA and BH(4) was abolished by catalase, which destroyed H(2)O(2) generated by oxidation of these agents in the organ chamber. Adventitial application of H(2)O(2) also enhanced EDHF-type dilator responses evoked by CPA and ACh in RIA segments perfused intraluminally with H(2)O(2)-free buffer, albeit with reduced efficacy. In RIA rings, both control relaxations and their potentiation by H(2)O(2) were overcome by blockade of gap junctions by connexin-mimetic peptides (YDKSFPISHVR and SRPTEK) targeted to the first and second extracellular loops of the dominant vascular connexins expressed in the RIA. Superoxide dismutase attenuated the potentiation of EDHF-type relaxations by BH(4), but not AA, consistent with findings demonstrating a differential role for superoxide anions in the generation of H(2)O(2) by the two agents.

CONCLUSION

Pro-oxidant effects of AA and BH(4) can enhance the EDHF phenomenon by generating H(2)O(2), which has previously been shown to amplify electrotonic hyperpolarization-mediated relaxation by facilitating Ca(2+) release from endothelial stores.

Translating the oxidative stress hypothesis into the clinic: NOX versus NOS

Cardiovascular diseases remain the leading cause of death in industrialised nations. Since the pathomechanisms of most cardiovascular diseases are not understood, the majority of therapeutic approaches are symptom-orientated. Knowing the molecular mechanism of disease would enable more targeted therapies. One postulated underlying mechanism of cardiovascular diseases is oxidative stress, i.e. the increased occurrence of reactive oxygen species such as superoxide. Oxidative stress leads to a dysfunction of vascular endothelium-dependent protective mechanisms. There is growing evidence that this scenario also involves impaired nitric oxide (NO)-cyclic GMP signalling. Out of a number of enzyme families that can produce reactive oxygen species, NADPH oxidases stand out, as they are the only enzymes whose sole purpose is to produce reactive oxygen species. This review focuses on the clinically validated targets of oxidative stress, NO synthase (NOS) and the NO receptor, soluble guanylate cyclase as well as the source of ROS, e.g. NADPH oxidases. We place recent knowledge in the function and regulation of these enzyme families into clinical perspective. For a comprehensive overview of the biology and pharmacology of oxidative stress and possible other sources and targets, we refer to other literature overviews.

Vasculopathy in sickle cell disease: Biology, pathophysiology, genetics, translational medicine, and new research directions

Sickle cell disease has been very well characterized as a single amino acid molecular disorder of hemoglobin leading to its pathological polymerization, with resulting red cell rigidity that causes poor microvascular blood flow, with consequent tissue ischemia and infarction. More recently, an independent spectrum of pathophysiology of blood vessel function has been demonstrated, involving abnormal vascular tone and activated, adhesive endothelium. These vasculopathic abnormalities are attributable to pathways involving hemolysis-associated defects in nitric oxide bioavailability, oxidative stress, ischemia-reperfusion injury, hemostatic activation, leukocytes and platelets. Vasculopathy of sickle cell disease has been implicated in the development of pulmonary hypertension, stroke, leg ulceration and priapism, particularly associated with hemolytic severity, and reported also in other severe hemolytic disorders. This vasculopathy might also play a role in other chronic organ dysfunction in patients with sickle cell disease. These pathways present novel targets for pharmacologic intervention, and several clinical trials are already under way. The authors present their perspectives of a workshop held at the National Institutes of Health in August 2008 on vasculopathy in sickle cell disease, along with meritorious future scientific questions on the topic of vascular complications of sickle cell disease.