Regulation of endothelial nitric oxide synthase by tetrahydrobiopterin in vascular disease

Novel therapies targeting vascular endothelium

Endothelial dysfunction has been identified as a major mechanism involved in all the stages of atherogenesis. Evaluation of endothelial function seems to have a predictive role in humans, and therapeutic interventions improving nitric oxide bioavailability in the vasculature may improve the long-term outcome in healthy individuals, high-risk subjects, or patients with advanced atherosclerosis. Several therapeutic strategies are now available, targeting both the synthesis and oxidative inactivation of nitric oxide (NO) in human vasculature. Statins seem to be currently the most powerful category of these agents, improving endothelial function and decreasing cardiovascular risk after long-term administration. Other cardiovascular agents improving endothelial function in humans are angiotensin-converting enzyme inhibitors/angiotensin receptors blockers, which increase NO bioavailability by modifying the rennin-angiotensin-aldosterone system. Newer therapeutic approaches targeting endothelial dysfunction in specific disease states include insulin sensitizers, L-arginine (the substrate for endothelial NO synthase [eNOS]) as well as substances that target eNOS "coupling," such as folates or tetrahydrobiopterin. Although there are a variety of strategies to improve NO bioavailability in human endothelium, it is still unclear whether they have any direct benefit at a clinical level.

Tetrahydrobiopterin and cardiovascular disease

Tetrahydrobiopterin (BH4) is an essential cofactor for the aromatic amino acid hydroxylases, which are essential in the formation of neurotransmitters, and for nitric oxide synthase. It is presently used clinically to treat some forms of phenylketonuria (PKU) that can be ameliorated by BH4 supplementation. Recent evidence supports potential cardiovascular benefits from BH4 replacement for the treatment of hypertension, ischemia-reperfusion injury, and cardiac hypertrophy with chamber remodeling. Such disorders exhibit BH4 depletion because of its oxidation and/or reduced synthesis, which can result in functional uncoupling of nitric oxide synthase (NOS). Uncoupled NOS generates more oxygen free radicals and less nitric oxide, shifting the nitroso-redox balance and having adverse consequences on the cardiovascular system. While previously difficult to use as a treatment because of chemical instability and cost, newer methods to synthesize stable BH4 suggest its novel potential as a therapeutic agent. This review discusses the biochemistry, physiology, and evolving therapeutic potential of BH4 for cardiovascular disease.

Endothelial response to stress from exogenous Zn2+resembles that of NO-mediated nitrosative stress, and is protected by MT-1 overexpression

While nitric oxide (NO)-mediated biological interactions have been intensively studied, the underlying mechanisms of nitrosative stress with resulting pathology remain unclear. Previous studies have demonstrated that NO exposure increases free zinc ions (Zn2+) within cells. However, the resulting effects on endothelial cell survival have not been adequately resolved. Thus the purpose of this study was to investigate the role of altered zinc homeostasis on endothelial cell survival. Initially, we confirmed the previously observed significant increase in free Zn2+with a subsequent induction of apoptosis in our pulmonary artery endothelial cells (PAECs) exposed to the NO donor N-[2-aminoethyl]- N-[2-hydroxy-2-nitrosohydrazino]-1,2-ethylenediamine. However, NO has many effects upon cell function and we wanted to specifically evaluate the effects mediated by zinc. To accomplish this we utilized the direct addition of zinc chloride (ZnCl2) to PAEC. We observed that Zn2+-exposed PAECs exhibited a dose-dependent increase in superoxide (O2−·) generation that was localized to the mitochondria. Furthermore, we found Zn2+-exposed PAECs exhibited a significant reduction in mitochondrial membrane potential, loss of cardiolipin from the inner leaflet, caspase activation, and significant increases in TdT-mediated dUTP nick end labeling-positive cells. Furthermore, using an adenoviral construct for the overexpression of the Zn2+-binding protein, metallothionein-1 (MT-1), we found either MT-1 overexpression or coincubation with a Zn2+-selective chelator, N, N,N′, N′-tetrakis(2-pyridylmethyl)ethylene-diamide, in PAECs significantly protected the mitochondria from both NO and Zn2+-mediated disruption and induction of apoptosis and cell death. In summary, our results indicate that a loss of Zn2+homeostasis produces mitochondrial dysfunction, increased oxidative stress, and apoptotic cell death. We propose that regulation of Zn2+levels may represent a potential therapeutic target for disease associated with both nitrosative and oxidative stress.

Novel isoforms of NADPH-oxidase in cerebral vascular control

Reactive oxygen species (ROS) are thought to play an important role in the initiation and progression of a variety of vascular diseases. Furthermore, accumulating evidence indicates that ROS may also serve as important cell signalling molecules for the regulation of normal vascular function. Recently, a novel family of proteins (Nox1, 2 and 4) that act as the catalytic subunit of the superoxide (O2-) producing enzyme NADPH-oxidase has been discovered in vascular cells. There is now preliminary evidence suggesting that NADPH-oxidase-derived ROS may serve as a physiological vasodilator mechanism in the cerebral circulation. Moreover, the activity of NADPH-oxidase is profoundly greater in cerebral versus systemic arteries. Studies have shown that Nox1, Nox2 (also known as gp91phox) and Nox4 are all expressed in cerebral arteries, suggesting that multiple isoforms of NADPH-oxidase may be important for ROS production by cerebral arteries. Enhanced NADPH-oxidase activity is associated with several vascular-related diseases, including hypertension, stroke, subarachnoid haemorrhage and Alzheimer's dementia; however, the consequences of this for cerebral vascular function are controversial. For example, there is some evidence suggesting that NADPH-oxidase-derived O2- may play a role in endothelial dysfunction of cerebral arteries and a subsequent rise in cerebral vascular tone, associated with hypertension. However, activation of NADPH-oxidase elicits cerebral vasodilatation in vivo, and this mechanism is enhanced in chronic hypertension. While further supportive evidence is needed, it is an intriguing possibility that NADPH-oxidase-derived ROS may play a protective role in regulating cerebral vascular tone during disease.

Increased oxidative stress in lambs with increased pulmonary blood flow and pulmonary hypertension: role of NADPH oxidase and endothelial NO synthase

Although oxidative stress is known to contribute to endothelial dysfunction-associated systemic vascular disorders, its role in pulmonary vascular disorders is less clear. Our previous studies, using isolated pulmonary arteries taken from lambs with surgically created heart defect and increased pulmonary blood flow (Shunt), have suggested a role for reactive oxygen species (ROS) in the endothelial dysfunction of pulmonary hypertension, but in vivo data are lacking. Thus the initial objective of this study was to determine whether Shunt lambs had elevated levels of ROS generation and whether this was associated with alterations in antioxidant capacity. Our results indicate that superoxide, but not hydrogen peroxide, levels were significantly elevated in Shunt lambs. In addition, we found that the increase in superoxide generation was not associated with alterations in antioxidant enzyme expression or activity. These data suggested that there is an increase in superoxide generation rather than a decrease in scavenging capacity in the lung. Thus we next examined the expression of various subunits of the NADPH oxidase complex as a potential source of the superoxide production. Results indicated that the expression of Rac1 and p47(phox) is increased in Shunt lambs. We also found that the NADPH oxidase inhibitor diphenyliodonium (DPI) significantly reduced dihydroethidium (DHE) oxidation in lung sections prepared from Shunt but not Control lambs. As DPI can also inhibit endothelial nitric oxide synthase (eNOS) superoxide generation, we repeated this experiment using a more specific NADPH oxidase inhibitor (apocynin) and an inhibitor of NOS (3-ethylisothiourea). Our results indicated that both inhibitors significantly reduced DHE oxidation in lung sections prepared from Shunt but not Control lambs. To further investigate the mechanism by which eNOS becomes uncoupled in Shunt lambs, we evaluated the levels of dihydrobiopterin (BH(2)) and tetrahydrobiopterin (BH(4)) in lung tissues of Shunt and Control lambs. Our data indicated that although BH(4) levels were unchanged, BH(2) levels were significantly increased. Finally, we demonstrated that the addition of BH(2) produced an increase in superoxide generation from purified, recombinant eNOS. In conclusion our data demonstrate that the development of pulmonary hypertension in Shunt lambs is associated with increases in oxidative stress that are not explained by decreases in antioxidant expression or activity. Rather, the observed increase in oxidative stress is due, at least in part, to increased expression and activity of the NADPH oxidase complex and uncoupled eNOS due to elevated levels of BH(2).

Reactive oxygen species in the cerebral circulation: are they all bad?

Reactive oxygen species (ROS) are a diverse family of molecules generated by all cells. ROS may serve as important cell-signalling molecules in the cerebral circulation. Indeed, in contrast to systemic arteries, major products of superoxide metabolism, including hydrogen peroxide, are powerful cerebral vasodilators, raising the possibility that ROS represent important molecules for increasing local cerebral blood flow. Two major determinants of the overall effects of ROS on cerebrovascular tone are the rate of production of the parent molecule, superoxide, and its rate of metabolism by superoxide dismutases. Although the major enzymatic source of ROS in cerebral arteries has not been clarified, nicotinamide adenine dinucleotide phosphate, reduced form (NADPH)-oxidases, along with cyclooxygenases and lipoxygenases, are probably the primary sources. In cerebral arteries, activation of NADPH-oxidase elicits both an increase in superoxide production and vasodilatation. The identity of the ROS molecule responsible for the vasodilator effects may be hydrogen peroxide, generated from the dismutation of superoxide. NADPH-oxidase activity and function appears to be profoundly greater in cerebral versus systemic arteries. Furthermore, NADPH-oxidase-derived ROS partly contribute to flow-dependent dilatation and may offset angiotensin II-induced constriction of cerebral arteries, consistent with the hypothesis that NADPH-oxidase-derived ROS may play a physiologic role in the control of cerebrovascular tone.

Diabetic vascular disease: an experimental objective

It is well known that humans with diabetes have more atherosclerosis and its complications. The causes of this relationship are, however, unclear. Although recent data show that improved glycemic control reduces arterial disease in type 1 diabetes, other studies have shown that subjects with "prediabetes" have more cardiovascular disease before the development of hyperglycemia. Thus, either hyperglycemia and/or lack of insulin actions are toxic to arteries, or metabolic derangements exclusive of hyperglycemia are atherogenic. For >50 years animal models of diabetes and atherosclerosis have been used to uncover potential mechanisms underlying diabetes associated cardiovascular disease. Surprisingly, diabetes alone increases vascular disease in only a few select animal models. Increased atherosclerosis has been found in several animals and lines of genetically modified mice; however, diabetes often also leads to greater hyperlipidemia. This makes it difficult to separate the toxic effects of insulin lack and/or hyperglycemia from those caused by the lipids. These studies are reviewed, as well as more recent investigations using new methods to create diabetic-atherosclerotic models.

Vascular NADPH oxidases as drug targets for novel antioxidant strategies

Reactive oxygen species (ROS) play important roles in the pathogenesis of cardiovascular disease. Surprisingly, large clinical trials have shown that ROS scavenging by antioxidant vitamins is ineffective or harmful. Therefore, prevention of ROS formation, by targeting specific sources of superoxide anion and other ROS, might prove beneficial. Potential targets include the NADPH oxidases (Nox enzymes), xanthine oxidase, endothelial nitric oxide synthase and mitochondrial oxidases. Nox enzymes play a central role because they can regulate other enzymatic sources of ROS. Statins, angiotensin-converting enzyme inhibitors and angiotensin receptor antagonists block upstream signaling of Nox activation, which contributes to their clinical effectiveness. Here, we discuss novel possibilities where drugs that directly inhibit Nox activation could successfully inhibit oxidative stress.

The vascular endothelium in diabetes: a practical target for drug treatment?

Vascular disease remains a major cause of morbidity and mortality in diabetes mellitus, in spite of recent improvements in outcome, some of which may be modulated by improved endothelial function. Therapeutic strategies aimed directly at preventing, or minimising the extent of, these sequelae are required as an adjunct to treatments directed at normalising the metabolic milieu. The microvasculature, and the endothelium in particular, are early contributors to vascular dysfunction, thus raising the question as to how best to specifically target the endothelium. However, the expansive nature of the microvasculature, the varying demands that tissues have in terms of blood flow, and the heterogeneity that exists amongst cell types in different sites raises potential problems as to the practicality of such an approach. Further-more, temporal and genetic factors in the genesis of diabetic microvascular dysfunction may impact on therapeutic strategies. It is suggested that a systematic approach is required to understand the heterogeneity of the microvasculature, with particular emphasis on relating differences in gene and protein expression with functional properties. Such an approach may then provide the necessary information to allow exploitation of endothelial cell heterogeneity for unique targeted interventions, as well as providing the necessary rationale for pharmacological interventions (both prophylactic and corrective) aimed at the endothelium as a whole.

Endothelial cell superoxide anion radical generation is not dependent on endothelial nitric oxide synthase-serine 1179 phosphorylation and endothelial nitric oxide synthase dimer/monomer distribution

Tetrahydrobiopterin (BH4) and heat shock protein 90 (hsp90) have been anticipated to regulate endothelial nitric oxide synthase (eNOS)-dependent superoxide anion radical (O2*-) generation in endothelial cells. It is not known, however, whether hsp90 and BH4 increase O2*- in a synergistic manner, or whether this increase is a consequence of downstream changes in eNOS phosphorylation on serine 1179 (eNOS-S1179) and changes in dimer/monomer distribution. Here O2*- production from purified BH4 -free eNOS and eNOS:hsp90 complexes determined by spin-trapping methodology showed that hsp90 neither inhibits O2*- nor alters the requirement of BH4 to inhibit radical release from eNOS. In endothelial cells, O2*- detection with the novel high-performance liquid chromatography assay of 2-hydroxyethidium showed that inhibition of hsp90 did not increase O2*-, while a significant increase in O2*- was detected in BH4 -depleted cells. Radicicol, a hsp90 inhibitor, disrupted eNOS:hsp90 association, decreased eNOS-S1179, but increased biopterin production in a dose-dependent fashion. These changes were followed by an increase in eNOS activity, demonstrating that high biopterin levels offset inhibition of eNOS phosphorylation and diminished interaction with hsp90. In contrast, depletion of biopterin did not affect hsp90 levels or interaction with eNOS or eNOS dimer/monomer ratio in bovine aorta endothelial cells (BAECs). We conclude that low BH4 but not inhibition of hsp90 increases O2*- in BAECs by mechanism(s) that unlikely involve phosphorylation to eNOS-S1179 or eNOS monomerization.

Oxidative stress in diabetes: A mechanistic overview of its effects on atherogenesis and myocardial dysfunction

Diabetes is a major risk factor for atherosclerosis. Atherogenesis involves endothelial dysfunction, activation and injury, inflammation, and smooth muscle cell migration and proliferation. Platelet activation in the narrowed arteries is the most proximate event in the culmination of an acute event such as acute myocardial infraction and stroke. Hyperglycemia is associated with all these adverse events in the process of genesis of atherosclerosis. The effect of diabetes (hyperglycemia) is mediated in large part by the state of enhanced oxidative stress, which is not counter-balanced by endogenous antioxidants. This paper reviews the ignition of oxidative stress in diabetes and the mediation of events leading to atherogenesis.

Folic acid, inflammation, and atherosclerosis: False hopes or the need for better trials?

An increasing body of evidence supports the existence of a strong link between inflammation, endothelial dysfunction, and the onset and progression of atherosclerosis. However, a cause-effect relationship between these phenomena has not been demonstrated yet. Although some inflammatory markers, such as C-reactive protein, have been recently shown to improve risk stratification and to strongly predict cardiovascular outcome, it is largely unknown whether modifications of these markers might provide salutary effects and reduce cardiovascular morbidity and mortality. The B-vitamin folic acid has recently gained attention because of its potential to provide beneficial effects on surrogate end-points, such as endothelial function, in patients at high cardiovascular risk. However, the role of folic acid in mitigating the pro-inflammatory state associated with atherosclerosis is controversial. Despite the theoretical arguments supporting the potential anti-atherosclerotic and anti-inflammatory effects of folic acid, the current evidence is limited, deriving from small trials on different study populations, using folic acid often in combination with other vitamins and for different treatment periods. This review will consider the current evidence supporting the role of some well-established inflammatory markers in predicting cardiovascular outcomes, the mechanisms by which folic acid might exert anti-inflammatory effects, the epidemiological data relating folic acid concentrations with inflammatory markers, the published interventional studies on the effects of folic acid supplementation on these markers, and the factors that need to be considered in designing future trials.

Amino acids, arginase and nitric oxide in vascular health

1. Nitric oxide (NO) plays a fundamental role in the vasculature because of its diverse influence in vascular protection, including its well-reported antiproliferative, anti-inflammatory, antithrombotic and vasodilator effects. In many vascular disease states, NO production is reduced as a result of endothelial dysfunction, in part caused by a decrease in substrate (L-arginine) availability. 2. The role of L-arginine and other amino acids important in nitrogen balance has been re-examined in the context of their effects on vascular health. The metabolism of L-arginine is complex because it is involved in a plethora of other pathways, such as urea, creatine and agmatine production. L-Arginine supplementation in patients with vascular disease is well reported to benefit patients therapeutically because of its effect on both NO-dependent and -independent mechanisms. 3. L-Arginine availability depends on the flux of other amino acids in the body, including L-glutamine, L-glutamate, L-ornithine, L-citrulline and L-lysine. The role of L-methionine and homocystine and their effect on NO also play an influential role in the body. 4. Recent data suggest that the key enzyme involved in the L-arginine-urea cycle, arginase, is coexpressed in NO-producing cells in the vasculature. In the present review, we examine the potential role of arginase as a therapeutic target for vascular health.

Insulin resistance reduces arterial prostacyclin synthase and eNOS activities by increasing endothelial fatty acid oxidation

Insulin resistance markedly increases cardiovascular disease risk in people with normal glucose tolerance, even after adjustment for known risk factors such as LDL, triglycerides, HDL, and systolic blood pressure. In this report, we show that increased oxidation of FFAs in aortic endothelial cells without added insulin causes increased production of superoxide by the mitochondrial electron transport chain. FFA-induced overproduction of superoxide activated a variety of proinflammatory signals previously implicated in hyperglycemia-induced vascular damage and inactivated 2 important antiatherogenic enzymes, prostacyclin synthase and eNOS. In 2 nondiabetic rodent models--insulin-resistant, obese Zucker (fa/fa) rats and high-fat diet-induced insulin-resistant mice--inactivation of prostacyclin synthase and eNOS was prevented by inhibition of FFA release from adipose tissue; by inhibition of the rate-limiting enzyme for fatty acid oxidation in mitochondria, carnitine palmitoyltransferase I; and by reduction of superoxide levels. These studies identify what we believe to be a novel mechanism contributing to the accelerated atherogenesis and increased cardiovascular disease risk occurring in people with insulin resistance.

Targeting reactive oxygen species in hypertension

PURPOSE OF REVIEW

Hypertension is a major risk factor for vascular diseases such as stroke, myocardial infarction, and renal microvascular disease. The mechanism by which vascular disease develops is complex, and growing evidence suggests that an increase in reactive oxygen species during hypertension is a major contributing factor. NADPH oxidase, the primary source of reactive oxygen species in the cardiovascular system, is a strong candidate for the development of therapeutic agents to ameliorate hypertension and end-organ damage.

RECENT FINDINGS

Various scavengers and inhibitors of reactive oxygen species have been proposed for use in animal as well as human studies. While many of these agents are effective at lowering tissue reactive oxygen species levels, their specificity is a serious concern. Our laboratory has developed cell-permeant peptidic inhibitors targeting key interactions among the different NAD(P)H oxidase homologues. One of these inhibitors targeting nox2 and p47-phox interaction has proven useful in attenuating target neoplasia and hypertrophy.

SUMMARY

Strategies aimed at specifically inhibiting NAD(P)H oxidase have proven effective in attenuating cardiovascular oxidative stress. The development of new inhibitors targeting novel oxidase homologues appears to hold significant promise for clarifying the physiologic role of these homologues as well as for the development of new antioxidant therapies.

Evaluation of tetrahydrobiopterin (BH4) as a potential therapeutic agent to treat erectile dysfunction

AIM

Nitric oxide (NO)-mediated smooth muscle relaxation causes penile erections. The endothelial NO synthase (eNOS) coenzyme tetrahydrobiopterin (BH4) converts eNOS-mediated catalytic activity from oxygen radical to NO production, improving endothelial function and vascular smooth muscle relaxation.

METHODS

Using quantitative immunohistochemistry, 8-isoprostane and nitrotyrosine concentrations were compared in cavernosal tissue from 17 potent and 7 impotent men, and the effect of single oral doses of BH4 on penile rigidity and tumescence was investigated. The pharmacodynamic effect of single oral doses of BH4 on penile rigidity and tumescence was investigated in a randomized, placebo-controlled, double-blind cross-over fashion in 18 patients with erectile dysfunction (ED) while receiving visual sexual stimulation.

RESULTS

8-Isoprostane content in endothelium and smooth muscle was significantly higher in impotent patient samples; the level of nitrotyrosine was unchanged in ED patients. Relative to placebo, a single dose of 200 mg BH4 led to a mean increase in duration of > 60% penile rigidity (33.5 min [95% confidence interval (CI): 13.1-49.3] at base and 29.4 min [95% CI: 8.9-42.2] at tip). A 500-mg dose increased the relative duration of > 60% penile rigidity by 36.1 min (95% CI: 16.3-51.8) at the base and 33.7 min (95% CI: 11.4-43.9) at the tip. Treatments were well tolerated.

CONCLUSION

BH4 treatment is suggested to switch eNOS catalytic activity from super-oxide to NO formation, leading to a reduced formation of free radical reaction product 8-isoprostane without alteration of nitrotyrosine. The observed results make BH4 a suitable candidate as an ED treatment through reconstitution of altered catalytic activity of the eNOS.

Twenty-five years since the discovery of endothelium-derived relaxing factor (EDRF): does a dysfunctional endothelium contribute to the development of type 2 diabetes?

Twenty-five years ago, the discovery of endothelium-derived relaxing factor opened a door that revealed a new and exciting role for the endothelium in the regulation of blood flow and led to the discovery that nitric oxide (NO) multi-tasked as a novel cell-signalling molecule. During the next 25 years, our understanding of both the importance of the endothelium as well as NO has greatly expanded. No longer simply a barrier between the blood and vascular smooth muscle, the endothelium is now recognized as a complex tissue with heterogeneous properties. The endothelium is the source of not only NO but also numerous vasoactive molecules and signalling pathways, some of which are still not fully characterized such as the putative endothelium-derived relaxing factor. Dysfunction of the endothelium is a key risk factor for the development of macro- and microvascular disease and, by coincidence, the discovery that NO was generated in the endothelium corresponds approximately in time with the increased incidence of type 2 diabetes. Primarily linked to dietary and lifestyle changes, we are now facing a global pandemic of type 2 diabetes. Characterized by insulin resistance and hyperglycaemia, type 2 diabetes is increasingly being diagnosed in adolescents as well as children. Is there a link between dietary-related hyperglycaemic insults to the endothelium, blood flow changes, and the development of insulin resistance? This review explores the evidence for and against this hypothesis.

Folic acid reverses endothelial dysfunction induced by inhibition of tetrahydrobiopterin biosynthesis

While folic acid has been shown to reverse endothelial dysfunction, the exact underlying mechanism remains elusive. Here, folic acid reversed both the endothelial dysfunction and increased production of superoxide following depletion of rabbit aortic ring tetrahydrobiopterin (BH4) levels with 2,4-diamino-6-hydroxy-pyrimidine (DAHP) and N-acetyl-5-hydroxy-tryptamine (NAS). Incubation with l-nitroarginine methyl ester also attenuated the production of superoxide. DAHP and NAS reduced BH4 concentrations in both aorta and cultured porcine aortic endothelial cells. Folic acid had no effect on BH4 concentrations in either preparation. The superoxide anion scavenger Tiron but not folic acid reversed the endothelial dysfunction produced in aortic rings by inhibition of copper-zinc superoxide dismutase with diethyldithiocarbamic acid. Neither folic acid nor its metabolite 5-methyltetrahydrofolate prevented the in vitro oxidation of BH4. This study demonstrates that folic acid reverses the endothelial dysfunction induced by BH4 depletion independently of either the regeneration or stabilization of BH4 or an antioxidant effect.

Inhibition of brain GTP cyclohydrolase I and tetrahydrobiopterin attenuates cerebral infarction via reducing inducible NO synthase and peroxynitrite in ischemic stroke

BACKGROUND

Inducible NO synthase (NOS)-derived peroxynitrite (ONOO-) during ischemia/reperfusion contributes to ischemic brain injury. However, inducible NOS (iNOS) regulation in ischemic stroke remains unknown. Tetrahydrobiopterin (BH4) is an essential cofactor for NOS activity. The present study tested the hypothesis that inhibition of endogenous BH4 rate-limiting enzyme GTP cyclohydrolase I (GTPCH I), and thus BH4 synthesis, reduces cerebral infarction via inhibiting iNOS and ONOO- in transient focal ischemia.

METHODS

Focal ischemia (2 hours) was created in adult male Sprague-Dawley rats (250 to 300 g) by middle cerebral artery occlusion (MCAO). Rats were treated 12 hours before MCAO with vehicle or diamino-6-hydroxypyrimidine (DAHP; 0.5 g/kg IP), a selective GTPCH I inhibitor. Brains were harvested 24 hours after reperfusion for assays of infarct volume, blood-brain barrier (BBB) permeability, GTPCH I activity, BH4 levels, GTPCH I and NOS mRNA, protein expression, and superoxide anion (O2*-) and ONOO- levels.

RESULTS

Endogenous GTPCH I activity, BH4 levels, iNOS activity, and (O2*- and ONOO- levels were all augmented after ischemia/reperfusion. DAHP treatment significantly reduced GTPCH I activity, resulting in decreased BH4 levels, iNOS activity, and ONOO- levels. Consequently, DAHP treatment significantly reduced the infarct size compared with the nontreated group (22.3+/-5.6 versus 38.3+/-7.4%; n=6; P<0.05). Similarly, BBB permeability was significantly reduced after DAHP pretreatment compared with the control group (4.11+/-0.22 versus 7.78+/-0.44 microg/g tissue; n=5; P<0.05).

CONCLUSIONS

These results demonstrate that blockade of endogenous brain BH4 synthesis attenuates cerebral infarction via inhibiting iNOS and ONOO-, which may provide a mechanistic basis of novel therapeutic strategies for ischemic stroke.

Endothelin mediates superoxide production and vasoconstriction through activation of NADPH oxidase and uncoupled nitric-oxide synthase in the rat aorta

Experiments were designed to test the hypothesis that elevated levels of endothelin 1 (ET-1) in the vasculature activate NADPH oxidase and/or uncoupled nitric-oxide synthase (NOS), resulting in O2-* production, and mediate increased constriction. Rat aortic rings were incubated with ET-1 or vehicle in the presence and absence of superoxide dismutase (SOD), ebselen (glutathione peroxidase mimetic), apocynin (NADPH oxidase inhibitor), L-NAME (Nomega-nitro-L-arginine methyl ester) (NOS inhibitor), tetrahydrobiopterin (BH4) (NOS cofactor), or selective ETA and ETB receptor antagonists (BQ-123 [cyclo(D-Asp-Pro-D-Val-Leu-D-Trp)] and A-192621 [[2R-(4-propoxyphenyl)-4S-(1,3-benzodioxol-5-yl)-1-(N-(2,6-diethylphenyl)aminocarbonyl-methyl)-pyrrolidine-3R-carboxylic acid]], respectively). O2-* production was monitored by oxidized dihydroethidine staining and/or lucigenin chemiluminescence. ET-1 significantly increased O2-* production compared with vehicle. SOD, ebselen, and apocynin inhibited the ET-1-induced increase in O2-* in intact and endothelium-denuded aorta. L-NAME and BH4 inhibited the ET-1-induced increase in O2-* in intact tissue, whereas these two compounds had no effect on ET-1-induced O2-* in endothelium-denuded aorta. Preincubation with BQ-123 or A-192621, individually, had no effect on ET-1-induced O2-*; however combining both antagonists inhibited the ET-1-stimulated increase in O2-*. Rat aortic rings were incubated with ET-1 or vehicle in the presence or absence of sepiapterin (BH4 synthesis substrate) or apocynin and mounted on wire myographs to determine isometric force generation in response to increasing KCl concentrations. ET-1 increased the contractile response to KCl compared with vehicle. Treatment with either sepiapterin or apocynin attenuated the ET-1-mediated increase with no effect of sepiapterin or apocynin alone. These data support the hypothesis that ET-1 increases vascular tone, in part, through ETA/ETB receptor activation of O2-* production from NADPH oxidase and NOS uncoupling.

Endothelial dysfunction in the streptozotocin-induced diabetic apoE-deficient mouse

Endothelial dysfunction plays a role in the development of atherosclerosis and diabetes-associated vascular disease and, in the streptozotocin (STZ)-induced apoE-deficient diabetic mouse, we report that, when compared to the citrate (CIT)-treated nondiabetic apoE-deficient control, acetylcholine (Ach)-mediated endothelium-dependent relaxation was reduced in the small mesenteric arteries (SMA) and the plaque-prone regions of the aorta from the STZ-diabetic mouse. In the SMA the component of Ach-mediated relaxation that was attributed to nitric oxide (NO) from STZ-treated diabetic apoE-deficient mice was enhanced; however, the endothelium-derived hyperpolarizing factor (EDHF)-mediated component was reduced. The EDHF component was assessed by determining the component of the Ach-mediated response that was resistant to the combination of the NO synthase (NOS) inhibitor Nomega-nitro-L-arginine methyl ester, cyclooxygenase inhibitor, indomethacin, and soluble guanylate cyclase inhibitor, ODQ, and inhibited by the combination of the intermediate conductance KCa (IKCa) inhibitor TRAM-34 and the small-conductance KCa (SKCa) inhibitor apamin. Endothelial NOS was increased but SK2, SK3 and connexin (Cx) 37 mRNA expressions were significantly (P<0.05) decreased in the SMA from STZ-treated apoE-deficient mice compared to the CIT-treated controls. There was no difference in the IKCa expression or in Cx 40, 43 and 45 mRNA levels between STZ- and CIT-treated mice. The microvasculature of STZ-induced apoE-deficient mice developed endothelial dysfunction, which may be linked to a decrease in the contribution of the EDHF component due to a decrease in SK2 and 3 and Cx 37 expression.

Loss of nitric oxide and endothelial-derived hyperpolarizing factor-mediated responses in aging

BACKGROUND

Aging has considerable structural and functional effects on the vascular system of the kidney. One such effect is an alteration in vascular tone which potentially will initiate renal damage. Vascular tone is determined by the balance between vasoconstrictors and vasodilators. Therefore, we hypothesized that aging attenuates vasodilatory responses in the kidney. These changes may be mediated by a loss of nitric oxide and endothelial-derived hyperpolarizing factor (EDHF).

METHODS

The systemic and renal responses of nitric oxide and EDHF were investigated in aging (18 months old) and young (3 months old) Sprague-Dawley rats.

RESULTS

We demonstrated a general loss of vasodilatory responses in the aging kidney. In addition, nitric oxide levels were reduced in the serum and kidney cortex of aging versus young animals, although this was not accompanied with a loss of endothelial nitric oxide synthase (eNOS) protein in the kidney cortex. Aging animals also exhibited a loss in EDHF-mediated vasodilation following stimulation with either acetylcholine or bradykinin in the isolated perfused kidney.

CONCLUSION

These findings indicate that not only a defect in the nitric oxide pathway, but also a loss of EDHF-mediated responses may be responsible for impaired vasodilation in the aging kidney. This may result in enhanced vasoconstrictive responses in aging which potentially will cause renal damage and ultimately a loss in glomerular filtration rate (GFR).

Stoichiometric relationships between endothelial tetrahydrobiopterin, endothelial NO synthase (eNOS) activity, and eNOS coupling in vivo: insights from transgenic mice with endothelial-targeted GTP cyclohydrolase 1 and eNOS overexpression

Endothelial dysfunction in vascular disease states is associated with reduced NO bioactivity and increased superoxide (O2*-) production. Some data suggest that an important mechanism underlying endothelial dysfunction is endothelial NO synthase (eNOS) uncoupling, whereby eNOS generates O2*- rather than NO, possibly because of a mismatch between eNOS protein and its cofactor tetrahydrobiopterin (BH4). However, the mechanistic relationship between BH4 availability and eNOS coupling in vivo remains undefined because no studies have investigated the regulation of eNOS by BH4 in the absence of vascular disease states that cause pathological oxidative stress through multiple mechanisms. We investigated the stoichiometry of BH4-eNOS interactions in vivo by crossing endothelial-targeted eNOS transgenic (eNOS-Tg) mice with mice overexpressing endothelial GTP cyclohydrolase 1 (GCH-Tg), the rate-limiting enzyme in BH4 synthesis. eNOS protein was increased 8-fold in eNOS-Tg and eNOS/GCH-Tg mice compared with wild type. The ratio of eNOS dimer:monomer was significantly reduced in aortas from eNOS-Tg mice compared with wild-type mice but restored to normal in eNOS/GCH-Tg mice. NO synthesis was elevated by 2-fold in GCH-Tg and eNOS-Tg mice but by 4-fold in eNOS/GCH-Tg mice compared with wild type. Aortic BH4 levels were elevated in GCH-Tg and maintained in eNOS/GCH-Tg mice but depleted in eNOS-Tg mice compared with wild type. Aortic and cardiac O2*- production was significantly increased in eNOS-Tg mice compared with wild type but was normalized after NOS inhibition with Nomega-nitro-L-arginine methyl ester hydrochloride (L-NAME), suggesting O2*- production by uncoupled eNOS. In contrast, in eNOS/GCH-Tg mice, O2*- production was similar to wild type, and L-NAME had no effect, indicating preserved eNOS coupling. These data indicate that eNOS coupling is directly related to eNOS-BH4 stoichiometry even in the absence of a vascular disease state. Endothelial BH4 availability is a pivotal regulator of eNOS activity and enzymatic coupling in vivo.

Role of oxidative stress in angiotensin-II mediated contraction of human conduit arteries in patients with cardiovascular disease

BACKGROUND

Angiotensin II is a powerful vasoconstrictor involved in the development of high blood pressure and in the regulation of cardiovascular growth. Recent reports have suggested that in addition to the classical pathways involved in transducing responses to receptor activation, formation of reactive oxygen species by angiotensin II may also be involved. We investigated the importance of oxidative stress in angiotensin II induced contraction in human conduit arteries from patients with cardiovascular disease.

METHODS AND RESULTS

Isometric contraction studies using human radial arteries entailed probes modulating the redox-dependent reactions to define the oxidative pathways involved in angiotensin II contraction. In situ oxidative fluorescence was employed to detect immediate superoxide tissue production in radial and internal mammary arteries. Treatment with TEMPOL, human superoxide dismutase, diphenyleneiodonium, oxypurinol, NG-monomethyl L-arginine considerably decreased contractile response to angiotensin II in radial arteries. Similarly, angiotensin II-stimulated arterial superoxide production was reduced in the presence of the above inhibitors. On the contrary, used as controls, norepinephrine vasoconstriction was not associated with increase of superoxide and neither ciprofloxacin nor aminophylline altered basal or angiotensin II induced superoxide generation.

CONCLUSIONS

Our findings provide evidence for the role of oxidative pathways in contractile response of human conduit arteries to angiotensin II. Angiotensin II induced superoxide anion production may be mediated by multiple inter-dependent rate-limiting enzymes in both types of artery. Our studies may have important implication for future therapeutic approaches involving inhibition of angiotensin II mediated superoxide generation in hypertension and prevention of cardiovascular disease.

CONDENSED ABSTRACT

We studied the role of oxidant species in contraction responses to angiotensin II in human conduit arteries. Treating radial artery segments with the anti-oxidants with a range of inhibitors, affecting the redox dependent pathways, markedly reduced contraction to angiotensin II. In parallel experiments, oxidative fluorescence was assessed and compared in human radial and internal mammary artery. Angiotensin II induced superoxide anion production may be mediated by multiple inter-dependent rate-limiting enzymes in both types of artery.

Oxidant and redox signaling in vascular oxygen sensing mechanisms: basic concepts, current controversies, and potential importance of cytosolic NADPH

Vascular smooth muscle (VSM) derived from pulmonary arteries generally contract to hypoxia, whereas VSM from systemic arteries usually relax, indicating the presence of basic oxygen-sensing mechanisms in VSM that are adapted to the environment from which they are derived. This review considers how fundamental processes associated with the generation of reactive oxygen species (ROS) by oxidase enzymes, the metabolic control of cytosolic NADH, NADPH and glutathione redox systems, and mitochondrial function interact with signaling systems regulating vascular force in a manner that is potentially adapted to be involved in Po2 sensing. Evidence for opposing hypotheses of hypoxia, either decreasing or increasing mitochondrial ROS, is considered together with the Po2 dependence of ROS production by Nox oxidases as sensors potentially contributing to hypoxic pulmonary vasoconstriction. Processes through which ROS and NAD(P)H redox changes potentially control interactive signaling systems, including soluble guanylate cyclase, potassium channels, and intracellular calcium are discussed together with the data supporting their regulation by redox in responses to hypoxia. Evidence for hypothesized potential differences between systemic and pulmonary arteries originating from properties of mitochondrial ROS generation and the redox sensitivity of potassium channels is compared with a new hypothesis in which differences in the control of cytosolic NADPH redox by the pentose phosphate pathway results in increased NADPH and Nox oxidase-derived ROS in pulmonary arteries, whereas lower levels of glucose-6-phosphate dehydrogenase in coronary arteries may permit hypoxia to activate a vasodilator mechanism controlled by oxidation of cytosolic NADPH.

Mechanisms of Inducible Nitric Oxide Synthase–Mediated Vascular Dysfunction

Objective— Inducible nitric oxide synthase (iNOS) is expressed in arteries during inflammation and may contribute to vascular dysfunction. Effects of gene transfer of iNOS to carotid arteries were examined in vitro in the absence of systemic inflammation to allow examination of mechanisms by which iNOS impairs contraction and relaxation.

Methods and Results— After gene transfer of iNOS with an adenovirus (AdiNOS), constrictor responses to phenylephrine (PE) and U46619 were impaired. After AdiNOS, inhibition of soluble guanylate cyclase (sGC) with 1H-[1,2,4]oxadiazolo-[4,3,2]quinoxalin-1-one (ODQ) reduced the EC 50 for PE from 4.33±0.78 μmol/L to 1.15±0.43 μmol/L (mean±SEM). These results imply that iNOS impairs contraction by activation of the NO/cGMP pathway. Relaxation to acetylcholine (ACh) also was impaired after AdiNOS. Sepiapterin (300 μmol/L), the precursor for tetrahydrobiopterin (BH 4 ), improved relaxation to Ach. Because BH 4 is an essential cofactor for production of NO by both iNOS and endothelial nitric oxide synthase (eNOS), these results suggest that iNOS may reduce production of NO by eNOS by limiting availability of BH 4 . Next, we examined effects of expression of iNOS in endothelium and adventitia. Selective expression of iNOS in endothelium, but not adventitia, impaired contraction to phenylephrine and relaxation to acetylcholine.

Conclusions— We conclude that: (1) iNOS may impair contraction in part by activation of sGC; (2) iNOS impairs relaxation, at least in part, by limiting availability of BH 4 ; and (3) expression of iNOS in endothelium may be a more important mediator of vascular dysfunction than expression of iNOS in adventitia.

Myristic acid stimulates endothelial nitric-oxide synthase in a CD36- and an AMP kinase-dependent manner

Dietary free fatty acids have been reported to have various effects on the endothelium including the generation of nitric oxide. The goal of the current study was to determine the mechanism whereby free fatty acid causes an increase in nitric oxide synthesis. The specific hypothesis tested was that free fatty acid association with CD36, a class B scavenger receptor, induces the activation of endothelial nitric-oxide synthase (eNOS). A human microvascular endothelial cell line and a transfected Chinese hamster ovary cell system were used to determine which free fatty acids stimulate eNOS. Surprisingly, only myristic acid, and to a lesser extent palmitic acid, stimulated eNOS. The stimulation of eNOS was dose- and time-dependent. Competition experiments with other free fatty acids and with a CD36-blocking antibody demonstrated that the effects of myristic acid on eNOS required association with CD36. Further mechanistic studies demonstrated that the effects of myristic acid on eNOS function were not dependent on PI 3-kinase, Akt kinase, or calcium. Pharmacological studies and dominant negative constructs were used to demonstrate that myristic acid/CD36 stimulation of eNOS activity was dependent on the activation of AMP kinase. These data demonstrate an unexpected link among myristic acid, CD36, AMP kinase, and eNOS activity.

Diabetes Induces Endothelial Dysfunction but Does Not Increase Neointimal Formation in High-Fat Diet Fed C57BL/6J Mice

Studies of diabetic vascular disease have traditionally used murine models of type 1 diabetes and genetic models of type 2 diabetes. Because the majority of patients with type 2 diabetes have diet induced obesity, we sought to study the effect of diabetes on arterial disease in a mouse model of diet induced obesity/diabetes. C57Bl/6 mice fed a high-fat diet for 9 weeks developed type 2 diabetes characterized by elevated body weight, hyperglycemia, and hyperinsulinemia. Arteries from diabetic mice exhibited a marked decrease in endothelium-dependent vasodilation, a modest decrease in endothelium independent vasodilation, and an increase in sensitivity to adrenergic vasoconstricting agents. Insulin stimulated protein kinase B (akt) and endothelial nitric oxide synthase (eNOS) phosphorylation were preserved in arteries from diabetic mice; however, eNOS protein dimers were markedly diminished. Arterial nitrotyrosine staining indicated that increased levels of peroxynitrite contributed to eNOS dimer disruption in the diabetic mice. The abnormal vasomotion was not an acute response to the high-fat diet, as short term high-fat diet feeding had no effect on endothelium dependent dilation. A trend toward smaller neointimal lesions was noted in high-fat diet fed mice after femoral artery wire denudation injury. In summary, disrupted eNOS dimer formation rather than impaired insulin mediated eNOS phosphorylation contributed to the endothelial dysfunction in diet induced obese/diabetic mice. The lack of an increase in neointimal formation indicates that additional diabetes associated parameters (such as hyperlipidemia and atherosclerotic vascular disease) may need to be present to increase neointimal formation in this model.

Ventricular arterial stiffening: integrating the pathophysiology

Vascular stiffening of the large arteries is a common feature of aging and is exacerbated by many common disorders such as hypertension, diabetes, and renal disease. This change influences the phasic mechanical stresses imposed on the blood vessels that in turn is important to regulating smooth muscle tone, endothelial function, and vascular health. In addition, the heart typically adapts to confront higher and later systolic loads by both hypertrophy and ventricular systolic stiffening. This creates altered coupling between heart and vessel that importantly affects cardiovascular reserve function. In this overview, I discuss the notion of a coupling disease in which stiffness of both heart and arteries interact to limit performance and generate clinical symptoms. This involves changes in the mechanical interaction of both systems, changes in signaling within the arteries themselves, and alterations in coronary flow regulation. Lastly, I briefly review recent development in de-stiffening strategies that may pave the way to treat this syndrome and its clinical manifestations.

Pivotal role for endothelial tetrahydrobiopterin in pulmonary hypertension

BACKGROUND

Pulmonary hypertension is a fatal disease characterized by vasoconstriction and vascular remodeling. Loss of endothelial nitric oxide bioavailability is implicated in pulmonary hypertension pathogenesis. Recent evidence suggests that the cofactor tetrahydrobiopterin (BH4) is an important regulator of nitric oxide synthase enzymatic function.

METHODS AND RESULTS

In the hph-1 mouse with deficient BH4 biosynthesis, BH4 deficiency caused pulmonary hypertension, even in normoxic conditions, and greatly increased susceptibility to hypoxia-induced pulmonary hypertension. In contrast, augmented BH4 synthesis in the endothelium, by targeted transgenic overexpression of GTP-cyclohydrolase I (GCH), prevented hypoxia-induced pulmonary hypertension. Furthermore, specific augmentation of endothelial BH4 in hph-1 mice by crossing with GCH transgenic mice rescued pulmonary hypertension induced by systemic BH4 deficiency. Lung BH4 availability controlled pulmonary vascular tone, right ventricular hypertrophy, and vascular structural remodeling in a dose-dependent manner in both normoxia and hypoxia. Furthermore, BH4 availability had striking effects on the immediate vasoconstriction response to acute hypoxia. These effects of BH4 were mediated through the regulation of nitric oxide compared with superoxide synthesis by endothelial nitric oxide synthase.

CONCLUSIONS

Endothelial BH4 availability is essential for maintaining pulmonary vascular homeostasis, is a critical mediator in the pathogenesis of pulmonary hypertension, and is a novel therapeutic target.

Inhibition of endothelial nitric oxide generation by low-density lipoprotein is partially prevented by L-arginine and L-ascorbate

We evaluated, in endothelial cells, the relative effectiveness of L-arginine and L-ascorbate in preventing the decrease in nitric oxide (NO) production in response to native low-density lipoprotein (LDL) from healthy subjects (nLDL), oxidized LDL (oxLDL, formed by nLDL oxidation) or native LDL from type 2 diabetic patients (dLDL). Human umbilical vein endothelial cells (HUVEC) were exposed to nLDL, dLDL or oxLDL (100 mg protein/L), in the absence or presence of L-arginine 10(-4)mol/L and/or L-ascorbate 10(-4)mol/L; NO synthase (NOS) activity and cyclic guanosine-3',5'-monophosphate (cGMP) were measured by the conversion of L-[3H]-arginine to L-[3H]citrulline and by radioimmunoassay, respectively. Both L-arginine and L-ascorbate increased cGMP in HUVEC co-incubated with any LDL species, although to lower levels than found in the absence of LDL. L-ascorbate did not affect NOS activity, whereas L-arginine increased it, both in the absence and presence of all LDL species. The effects of combined L-arginine and L-ascorbate on NOS activity and cGMP were no greater than those of L-arginine alone. Our results suggest that L-arginine or L-ascorbate can ameliorate, but not normalize, NO production in this situation, and that combining L-arginine with L-ascorbate is unlikely to produce additional benefit as compared with L-arginine alone.

Endothelial cell dysfunction and the vascular complications associated with type 2 diabetes: assessing the health of the endothelium

Diabetes-associated vascular complications are collectively the major clinical problems facing patients with diabetes and lead to the considerably higher mortality rate than that of the general population. People with diabetes have a much higher incidence of coronary artery disease as well as peripheral vascular diseases in part because of accelerated atherogenesis. Despite the introduction of new therapies, it has not been possible to effectively reduce the high cardiovascular morbidity and mortality associated with diabetes. Of additional concern is the recognition by the World Health Organization that we are facing a global epidemic of type 2 diabetes. Endothelial dysfunction is an early indicator of cardiovascular disease, including that seen in type 2 diabetes. A healthy endothelium, as defined in terms of the vasodilator/blood flow response to an endothelium-dependent vasodilator, is an important indicator of cardiovascular health and, therefore, a goal for corrective interventions. In this review we explore the cellular basis for endothelial dysfunction in an attempt to identify appropriate new targets and strategies for the treatment of diabetes. In addition, we consider the question of biomarkers for vascular disease and evaluate their usefulness for the early detection of and their role as contributors to vascular dysfunction.

Malfunction of vascular control in lifestyle-related diseases: endothelial nitric oxide (NO) synthase/NO system in atherosclerosis

Nitric oxide (NO) from the endothelial NO synthase (eNOS) is believed to be implicated in the development and progression of atherosclerosis. The impaired endothelium-dependent vasodilatory response (EDR) has been demonstrated in vessels exposed to hypercholesterolemia and atherosclerosis. The extent of impairment serves as a predictor of future progression of atherosclerosis. As to the mechanisms of impaired EDR, increased production of superoxide is important. Recently it was revealed that eNOS becomes dysfunctional and produces superoxide rather than NO under conditions in which vascular tissue levels of tetrahydrobiopterin (BH4), a co-factor for eNOS, are deficient or lacking. Dysfunctional eNOS is closely implicated in the endothelial dysfunction represented by impaired EDR in various vascular disorders including atherosclerosis. Regarding the role of eNOS in atherogenesis, experimental studies in vitro have revealed that NO from eNOS constitutes as an anti-atherogenic molecule. In eNOS-knockout mice, eNOS deficiency augments atherosclerotic lesion formation, although the effects may be partly due to the associated hypertension. However, in eNOS-transgenic mice (eNOS-Tg) crossbred with apolipoprotein E-deficient mice (apoE-KO/eNOS-Tg), we found the accelerated lesion formation in association with increased superoxide production from vessels compared with apoE-KO mice. The vascular tissue levels of BH4 were reduced and BH2, an oxidized form, levels were increased. Chronic administration of exogenous BH4 or overexpression of GTPCH-1, a rate limiting enzyme for BH4 synthesis, restored the lesion to the levels comparable to apoE-KO mice. Therefore, eNOS may have two faces in the pathophysiology of atherosclerosis depending on tissue BH4 levels.

Endothelial cell superoxide generation: regulation and relevance for cardiovascular pathophysiology

The endothelial generation of reactive oxygen species (ROS) is important both physiologically and in the pathogenesis of many cardiovascular disorders. ROS generated by endothelial cells include superoxide (O2-*), hydrogen peroxide (H2O2), peroxynitrite (ONOO-*), nitric oxide (NO), and hydroxyl (*OH) radicals. The O2-* radical, the focus of the current review, may have several effects either directly or through the generation of other radicals, e.g., H2O2 and ONOO-*. These effects include 1) rapid inactivation of the potent signaling molecule and endothelium-derived relaxing factor NO, leading to endothelial dysfunction; 2) the mediation of signal transduction leading to altered gene transcription and protein and enzyme activities ("redox signaling"); and 3) oxidative damage. Multiple enzymes can generate O2-*, notably xanthine oxidase, uncoupled NO synthase, and mitochondria. Recent studies indicate that a major source of endothelial O2-* involved in redox signaling is a multicomponent phagocyte-type NADPH oxidase that is subject to specific regulation by stimuli such as oscillatory shear stress, hypoxia, angiotensin II, growth factors, cytokines, and hyperlipidemia. Depending on the level of oxidants generated and the relative balance between pro- and antioxidant pathways, ROS may be involved in cell growth, hypertrophy, apoptosis, endothelial activation, and adhesivity, for example, in diabetes, hypertension, atherosclerosis, heart failure, and ischemia-reperfusion. This article reviews our current knowledge regarding the sources of endothelial ROS generation, their regulation, their involvement in redox signaling, and the relevance of enhanced ROS generation and redox signaling to the pathophysiology of cardiovascular disorders where endothelial activation and dysfunction are implicated.

Tetrahydrobiopterin: regulator of endothelial nitric oxide synthase in vascular disease

In vascular disease states such as atherosclerosis and diabetes, endothelial nitric oxide (NO) bioactivity is reduced and oxidative stress is increased, resulting in endothelial dysfunction. Recent studies suggest that changes in the activity and regulation of endothelial NO synthase by its cofactor tetrahydrobiopterin (BH4) is an important contributor to endothelial dysfunction. Pharmacologic studies and more recent insights from genetically modified mouse models have improved the understanding of the mechanistic role and importance of BH4 in vascular disease pathogenesis. Targeting BH4 may provide new therapeutic strategies in vascular disease.

Coenzyme Q10 and diabetic endotheliopathy: oxidative stress and the 'recoupling hypothesis'

Increased oxidative stress in diabetes mellitus may underlie the development of endothelial cell dysfunction by decreasing the availability of nitric oxide (NO) as well as by activating pro-inflammatory pathways. In the arterial wall, redox imbalance and oxidation of tetrahydrobiopterin (BH4) uncouples endothelial nitric oxide synthase (eNOS). This results in decreased production and increased consumption of NO, and generation of free radicals, such as superoxide and peroxynitrite. In the mitochondria, increased redox potential uncouples oxidative phosphorylation, resulting in inhibition of electron transport and increased transfer of electrons to molecular oxygen to form superoxide and other oxidant radicals. Coenzyme Q10 (CoQ), a potent antioxidant and a critical intermediate of the electron transport chain, may improve endothelial dysfunction by 'recoupling' eNOS and mitochondrial oxidative phosphorylation. CoQ supplementation may also act synergistically with anti-atherogenic agents, such as fibrates and statins, to improve endotheliopathy in diabetes.

Homocysteine induces oxidative stress by uncoupling of NO synthase activity through reduction of tetrahydrobiopterin

Hyperhomocysteinemia is a risk factor for cardiovascular diseases that induces endothelial dysfunction. Here, we examine the participation of endothelial NO synthase (eNOS) in the homocysteine-induced alterations of NO/O(2)(-) balance in endothelial cells from human umbilical cord vein. When cells were treated for 24 h, homocysteine dose-dependently inhibited thrombin-activated NO release without altering eNOS phosphorylation and independently of the endogenous NOS inhibitor, asymmetric dimethylarginine. The inhibitory effect of homocysteine on NO release was associated with increased production of reactive nitrogen and oxygen species (RNS/ROS) independent of extracellular superoxide anion (O(2)(-)) and was suppressed by the NOS inhibitor L-NAME. In unstimulated cells, L-NAME markedly decreased RNS/ROS formation and the ethidium red fluorescence induced by homocysteine. This eNOS-dependent O(2)(-) synthesis was associated with reduced intracellular levels of both total biopterins (-45%) and tetrahydrobiopterin (-80%) and increased release of 7,8-dihydrobiopterin and biopterin in the extracellular medium (+40%). In addition, homocysteine suppressed the activating effect of sepiapterin on NO release, but not that of ascorbate. The results show that the oxidative stress and inhibition of NO release induced by homocysteine depend on eNOS uncoupling due to reduction of intracellular tetrahydrobiopterin availability.