Regulation of nitric oxide synthesis by proinflammatory cytokines in human umbilical vein endothelial cells. Elevations in tetrahydrobiopterin levels enhance endothelial nitric oxide synthase specific activity

Vascular protection by tetrahydrobiopterin: progress and therapeutic prospects

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

Systematic evaluation of nitric oxide, tetrahydrobiopterin, and anandamide levels in a porcine model of endotoxemia

PURPOSE

Using a lipopolysaccharide (LPS)-treated porcine model, we examined: (1) whether nitric oxide (NO), anandamide, and tetrahydrobiopterin (BH4) increased or not in early endotoxic shock; and (2) the location of the major site of production of these molecules, by comparing their concentrations in arteries and the portal and hepatic veins.

METHODS

Ten pigs received an infusion of LPS at 1.7 microg x kg(-1)x h(-1) via the portal vein for 240 min. Consecutive changes in systemic hemodynamics, hepatosplanchnic circulation, and oxygen delivery were measured. Furthermore, the variable changes in the concentrations of nitrite and nitrate (NOx), anandamide, and BH4 were measured. To access the effects of surgery, anesthesia, and fluid management on BH4, an experiment without LPS infusion was performed in two other animals.

RESULTS

Mean arterial pressure and cardiac index started to decrease at 60 min after LPS infusion. However, systemic vascular resistance remained unchanged. Total hepatic blood flow and hepatic oxygen delivery also decreased significantly. NOx and anandamide did not change during LPS infusion. BH4 values did not change without LPS infusion. However, BH4 values increased significantly in the arterial, portal, and hepatic circulation during LPS infusion, especially in the hepatic vein (from 136.8 +/- 27.5 to 281.3 +/- 123.2 mol/ml; P < 0.01).

CONCLUSION

Our data suggest that the BH4 values were significantly increased in several organs, especially in the liver during endotoxic shock. Impaired cardiac output and decreased blood pressure appeared in the early phase of porcine endotoxemia. Longer-term observation of these parameters after LPS treatment should be performed as the next step in future studies.

Epigenetic regulation of vascular endothelial gene expression

Epigenetics refers to chromatin-based pathways important in the regulation of gene expression and includes 3 distinct, but highly interrelated, mechanisms: DNA methylation, histone density and posttranslational modifications, and RNA-based mechanisms. Together, they offer a newer perspective on transcriptional control paradigms in vascular endothelial cells and provide a molecular basis for how the environment impacts the genome to modify disease susceptibility. This review provides an introduction to epigenetic concepts for vascular biologists. Using endothelial nitric oxide synthase (NOS3) as an example, we examine the growing body of evidence implicating epigenetic pathways in the control of vascular endothelial gene expression in health and disease.

Oral administration of tetrahydrobiopterin attenuates testicular damage by reducing nitric oxide synthase activity in a cryptorchid mouse model

Experimental cryptorchidism has been shown to induce germ cell apoptosis. Nitric oxide (NO), a ubiquitous free radical produced by NO synthases (NOSs), has been associated with apoptosis in a number of cell types. However, the regulation of NOSs in experimental cryptorchid testes remains unknown. Tetrahydrobiopterin (BH4), an essential cofactor of NOS, plays an important role in the generation of NO. It has been reported that activation of the immune system stimulates an increase in endogenous BH4 rate-limiting enzyme GTP cyclohydrolase I (GTPCH I) activity, resulting in an increase in intracellular BH4 levels and BH4-dependent NO synthesis in various cells. We examined the effect of dietary treatment with BH4 on GTPCH I, BH4 synthesis, NO production, and testicular damage in cryptorchid model mice. Male mice were treated with oral BH4 starting from age 4 weeks or received standard diet only, and right cryptorchid testes were created surgically at age 10 weeks. The testes were evaluated 0, 3, 5, 7, and 10 days after surgery by assays of testicular weight, BH4 and dihydrobiopterin (oxidized BH4) levels, GTPCH I mRNA levels, NOS protein expression levels, NO concentration, and nitrotyrosine (product of ONOO(-); determinant of NO-dependent damage) levels. In untreated mice, GTPCH I mRNA and BH4 levels increased and eNOS protein expression, NO concentration, and nitrotyrosine levels increased gradually. BH4 treatment decreased GTPCH I mRNA and BH4 levels, with concomitant reduction of eNOS protein levels, nitrotyrosine levels, and NO concentration, resulting in reduced testicular damage. Our findings demonstrate that supplementation with BH4 could provide a new therapeutic intervention for heat stress-based testicular dysfunction.

Over-expression of GTP-cyclohydrolase 1 feedback regulatory protein attenuates LPS and cytokine-stimulated nitric oxide production

GTP-cyclohydrolase 1 (GTP-CH1) catalyses the first and rate-limiting step for the de novo production of tetrahydrobiopterin (BH(4)), an essential cofactor for nitric oxide synthase (NOS). The GTP-CH1-BH(4) pathway is emerging as an important regulator in a number of pathologies associated with over-production of nitric oxide (NO) and hence a more detailed understanding of this pathway may lead to novel therapeutic targets for the treatment of certain vascular diseases. GTP-CH1 activity can be inhibited by BH(4) through its protein-protein interactions with GTP-CH1 regulatory protein (GFRP), and transcriptional and post-translational modification of both GTP-CH1 and GFRP have been reported in response to proinflammatory stimuli. However, the functional significance of GFRP/GTP-CH1 interactions on NO pathways has not yet been demonstrated. We aimed to investigate whether over-expression of GFRP could affect NO production in living cells. Over-expression of N-terminally Myc-tagged recombinant human GFRP in the murine endothelial cell line sEnd 1 resulted in no significant effect on basal BH(4) nor NO levels but significantly attenuated the rise in BH(4) and NO observed following lipopolysaccharide and cytokine stimulation of cells. This study demonstrates that GFRP can play a direct regulatory role in iNOS-mediated NO synthesis and suggests that the allosteric regulation of GTP-CH1 activity by GFRP may be an important mechanism regulating BH(4) and NO levels in vivo.

Role of nitric oxide production in anaphylaxis and its relevance for the treatment of anaphylactic hypotension with methylene blue

OBJECTIVE

To review the role of nitric oxide production in anaphylaxis.

DATA SOURCES

We performed MEDLINE searches of the literature. In addition, some references known to the authors but not listed in MEDLINE, such as abstracts and a CD-ROM, were included. Finally, additional clinical details of the cases were provided by one of the authors.

STUDY SELECTION

Primary reports were preferentially selected for inclusion. However, some secondary publications are also cited.

RESULTS

Histamine along with other mediators, such as leukotrienes, tumor necrosis factor, and platelet-activating factor, induce the production of nitric oxide. Nitric oxide can inhibit the release and effects of catecholamines. Sympathetic amines may inhibit production of nitric oxide. Studies in animals have demonstrated the generation of nitric oxide during anaphylaxis. Inhibition of nitric oxide synthase improves survival in an animal model of anaphylaxis. Nitric oxide causes vasodilation indirectly by increasing the activation of guanylyl cyclase, which then causes smooth muscle relaxation by increasing the concentration of smooth muscle cyclic guanosine monophosphate. Methylene blue is an inhibitor of guanylyl cyclase, which increases systemic vascular resistance and reverses shock in animal studies. The previously reported successful treatment with methylene blue of 11 patients with anaphylactic hypotension is reviewed.

CONCLUSION

Nitric oxide plays a significant role in the pathophysiology of anaphylaxis. Treatment with methylene blue should be considered in patients with anaphylactic hypotension that has not responded to other interventions.

The molecules: mechanisms of arterial vasodilatation observed in the splanchnic and systemic circulation in portal hypertension

A hyperdynamic splanchnic and systemic circulation is typical of cirrhotic patients and has been observed in all experimental forms of portal hypertension. The hyperdynamic circulation is most likely initiated by arterial vasodilatation, leading to central hypovolemia, sodium retention, and an increased intravascular volume. Arterial vasodilatation is regulated by a complex interplay of various vasodilator molecules and factors that influence the production of those vasodilator molecules. Nitric oxide (NO) has been recognized as the most important vasodilator molecule that mediates the excessive arterial vasodilatation observed in portal hypertension. The aims of this review are (1) to categorize NO synthase isoforms involved in NO overproduction; (2) to explain the mechanisms of endothelial NO synthase up-regulation; and (3) to summarize other molecules involved in the arterial vasodilatation.

Splanchnic and systemic vasodilation: the experimental models

Experimental models are a sine qua non condition for unraveling the specific components and mechanisms contributing to vascular dysfunction and arterial vasodilation in portal hypertension. Moreover, a careful selection of the type of animal model, vascular bed, and methodology is crucial for any investigation of this issue. In this review, some critical aspects related to experimental models in portal hypertension and the techniques applied are highlighted. In addition, a detailed summary of the mechanisms of arterial vasodilation in portal hypertension is presented. First, humoral and endothelial vasodilators, predominantly nitric oxide but also carbon monoxide and endothelium-derived hyperpolarizing factor, and others are discussed. Second, time course and potential stimuli triggering and/or perpetuating splanchnic vasodilation are delineated. Finally, a brief general overview of vascular smooth muscle signaling sets the stage for a discussion on cotransmission, receptor desensitization, and the observed impairment in vasoconstrictor-induced smooth muscle contraction in the splanchnic and systemic circulation during portal hypertension.

Effect of dl-nebivolol, its enantiomers and metabolites on the intracellular production of superoxide and nitric oxide in human endothelial cells

Nebivolol, a third generation selective beta(1)-adrenoceptor (beta(1)-AR) antagonist, has been reported to reduce intracellular oxidative stress and to induce the release of nitric oxide (NO) from the endothelium. Nebivolol is also subjected to a complex metabolic process where glucuronidation, aromatic and alicyclic hydroxylation are the major pathways leading to several metabolites. We have studied the effect of nebivolol, its enantiomers and metabolites on intracellular oxidative stress and NO availability in human umbilical vein endothelial cells (HUVECs). Furthermore, since the receptors involved in this endothelial effect of nebivolol remain controversial, we have studied this matter by the use of antagonists of beta-AR. dl-Nebivolol, d-nebivolol and l-nebivolol significantly reduced the formation of reactive oxygen species (ROS) and superoxide induced by oxidized-low density lipoprotein (ox-LDL), although the racemic and l-form were significantly more active than d-nebivolol in this activity. A marked decrease in the availability of intracellular NO was found in HUVECs exposed to ox-LDL and this parameter was normalized by the prior incubation with dl-nebivolol, d-nebivolol and l-nebivolol; the effect of racemate was mainly mimicked by its l-enantiomer. eNOS activity significantly increased by a 5-min contact of HUVECs with dl-nebivolol and l-nebivolol, but not with d-nebivolol, and a similar pattern was observed when the intracellular calcium increase was measured. The metabolites A2, A3', A12 and A14 but not A1, A3 and R 81,928, derived from different metabolic pathways, retained the antioxidant activity of the parent racemic compound dl-nebivolol, reducing the intracellular formation of ROS and superoxide. The effects of dl-nebivolol on intracellular formation of NO, eNOS activity and intracellular Ca(2+) were partially antagonized by the antagonists of beta(1-2)-AR nadolol or by the beta(3)-AR antagonist SR59230A and further antagonized by their combination or by (beta(1-2-3)-AR antagonist bupranolol. In conclusion, this study shows that the NO releasing effect of nebivolol is mainly due to its l-enantiomer; the racemate and its enantiomers possess a remarkable antioxidant activity that contributes to its effect on the cellular NO metabolism and the activation of beta(3)-AR through a calcium dependent pathway is involved in the mechanisms leading to the NO release.

Cerebral blood perfusion changes in multiple sclerosis

The proximity of immune cell aggregations to the vasculature is a hallmark of multiple sclerosis. Furthermore, it is widely accepted that inflammation is able to modulate the microcirculation. Until recently, the detection of cerebral blood perfusion changes was technically challenging, and perfusion studies in multiple sclerosis patients yielded contradictory results. However, new developments in fast magnetic resonance imaging have enabled us to image the cerebral hemodynamics based on the dynamic tracking of a bolus of paramagnetic contrast agents (dynamic susceptibility contrast). This review discusses the technical principles, possible pitfalls, and potential for absolute quantification of cerebral blood volume and flow in a clinical setting. It also outlines recent findings on inflammation associated perfusion changes, which are inseparable from pathological considerations in multiple sclerosis.

C-Reactive protein-induced endothelial microparticle generation in HUVECs is related to BH4-dependent NO formation

BACKGROUND

C-reactive protein (CRP) has been proven to facilitate endothelial injury via reduced NO production. Endothelial microparticles (EMPs) have emerged as a novel marker of endothelial injury.

METHODS

In vitro cultured human umbilical vein endothelial cells (HUVECs) were incubated with CRP (20 mg/l) for 24 h. The numbers of EMPs with CD31- and CD51-positive staining were assessed flow-cytometrically, and NO production was measured using the Griess reaction in the presence or absence of tetrahydrobiopterin (BH(4)), respectively.

RESULTS

The number of EMPs was significantly increased in HUVECs stimulated by CRP compared with the control group and, in parallel, NO production was decreased (p < 0.05). In the presence of CRP, pretreatment with BH(4) decreased EMP counts and restored NO production to baseline levels (p < 0.05) while pretreatment with 2,4-diamino-6-hydroxypyrimidine (DAHP), a BH(4) synthesis inhibitor, further prompted EMP formation and decreased NO production (p < 0.05). However, adding exogenous BH(4) after pretreatment with DAHP suppressed EMP formation and restored NO production (p < 0.05).

CONCLUSIONS

This study demonstrates that CRP induces EMP generation in HUVECs and this effect is, at least in part, related to impaired BH(4)-dependent NO production. Augmented EMP generation in HUVECs is suggested as a novel potential mechanism contributing to the pathogenesis of vascular injury related to CRP.

Janus-faced role of endothelial NO synthase in vascular disease: uncoupling of oxygen reduction from NO synthesis and its pharmacological reversal

Endothelial NO synthase (eNOS) is the predominant enzyme responsible for vascular NO synthesis. A functional eNOS transfers electrons from NADPH to its heme center, where L-arginine is oxidized to L-citrulline and NO. Common conditions predisposing to atherosclerosis, such as hypertension, hypercholesterolemia, diabetes mellitus and smoking, are associated with enhanced production of reactive oxygen species (ROS) and reduced amounts of bioactive NO in the vessel wall. NADPH oxidases represent major sources of ROS in cardiovascular pathophysiology. NADPH oxidase-derived superoxide avidly interacts with eNOS-derived NO to form peroxynitrite (ONOO(-)), which oxidizes the essential NOS cofactor (6R-)5,6,7,8-tetrahydrobiopterin (BH(4)). As a consequence, oxygen reduction uncouples from NO synthesis, thereby rendering NOS to a superoxide-producing pro-atherosclerotic enzyme. Supplementation with BH(4) corrects eNOS dysfunction in several animal models and in patients. Administration of high local doses of the antioxidant L-ascorbic acid (vitamin C) improves endothelial function, whereas large-scale clinical trials do not support a strong role for oral vitamin C and/or E in reducing cardiovascular disease. Statins, angiotensin-converting enzyme inhibitors and AT1 receptor blockers have the potential of reducing vascular oxidative stress. Finally, novel approaches are being tested to block pathways leading to oxidative stress (e.g. protein kinase C) or to upregulate antioxidant enzymes.

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.

TNF-alpha potentiates protein-tyrosine nitration through activation of NADPH oxidase and eNOS localized in membrane rafts and caveolae of bovine aortic endothelial cells

A major source of reactive oxygen species (ROS) in endothelial cells is the NADPH oxidase enzyme complex. The selective distributions of any enzyme within cells have important implications in regulating enzyme effectiveness through facilitation of access to local substrates and/or product targets. Because membrane rafts provide a spatially preferable environment for a variety of enzyme systems, we sought to determine whether NADPH oxidase is present and functional in this plasma membrane compartment in endothelial cells. We found that, in resting endothelial cells, NADPH oxidase subunits were preassembled and the enzyme functional in membrane rafts, specifically in caveolae. Stimulation with TNF-alpha induced additional recruitment of the p47(phox) regulatory subunit to raft-localized NADPH oxidase and enhanced ROS production within raft domains. TNF-alpha also induced nitric oxide production through activation of endothelial nitric oxide synthase (eNOS) present in the same membrane compartment. The dual activation of superoxide and nitric oxide-generating systems provided a spatially favorable environment for nitration of tyrosine-containing proteins localized to rafts. Perturbation of membrane raft structural integrity with cholesterol-sequestering compounds caused the delocalization of NADPH oxidase subunits and eNOS from the rafts and inhibited TNF-alpha-induced ROS production and protein tyrosine nitration. Together, these data provide evidence that membrane rafts and caveolae play a role in the spatial regulation of NADPH oxidase and subsequent ROS/reactive nitrogen species in endothelial cells.

Stat3 mediates interleukin-6 [correction of interelukin-6] inhibition of human endothelial nitric-oxide synthase expression

Chronic activation of the acute phase response (APR) is associated with atherosclerosis. Elevated levels of interleukin-6, the major inducer of the APR, are associated with an increased risk of cardiovascular events. One of the clinical hallmarks of atherogenesis is endothelial dysfunction, characterized by a decrease in endothelial production of nitric oxide (NO). We hypothesized that interleukin-6 (IL-6) decreases endothelial NO synthase (eNOS) expression. We now show that IL-6 treatment of human aortic endothelial cells (HAEC) decreases steady-state levels of human eNOS mRNA and protein. This decrease in eNOS expression is caused in part by IL-6 inhibition of transactivation of the human eNOS promoter. To explore the mechanism by which IL-6 affects eNOS expression, we examined activation of signal transducer and transactivator-3 (Stat3). The IL-6 receptor (IL-6R) is expressed in HAEC, and Stat3 is phosphorylated in response to IL-6 stimulation of the IL-6R. We identified four consensus sequences for Stat3 binding (SIE) in the eNOS promoter at positions -1520, -1024, -840, and -540. Transfection of eNOS promoter mutants revealed that the SIE at -1024 mediates Stat3 inhibition of eNOS promoter activity. Gel-shift analysis of nuclear extracts from HAEC treated with IL-6 confirms that Stat3 binds to a complex containing the SIE at -1024. RNA silencing of STAT3 blocks the inhibitory effect of IL-6 on eNOS expression. Our data show that IL-6 has direct effects upon endothelial cells, inhibiting eNOS expression in part through Stat3. Decreased levels of eNOS may be an important component of the pro-atherogenic effect of the APR.

Inducible-NOS but not neuronal-NOS participate in the acute effect of TNF-α on hypothalamic insulin-dependent inhibition of food intake

TNF-alpha acts on the hypothalamus modulating food intake and energy expenditure through mechanisms incompletely elucidated. Here, we explore the hypothesis that, to modulate insulin-induced anorexigenic signaling in hypothalamus, TNF-alpha requires the synthesis of NO. TNF-alpha activates signal transduction through JNK and p38 in hypothalamus, peaking at 10(-8) M. This is accompanied by the induction of expression of the inducible and neuronal forms of NOS, in both cases peaking at 10(-12) M. In addition, TNF-alpha stimulates NOS catalytic activity. Pre-treatment with TNF-alpha at a low dose (10(-12) M) inhibits insulin-dependent anorexigenic signaling, and this effect is abolished in iNOS but not in nNOS knockout mice.

Increased vascular biosynthesis of tetrahydrobiopterin in apolipoprotein E-deficient mice

Previous studies suggested that loss of tetrahydrobiopterin (BH(4)) may play an important role in the pathogenesis of vascular endothelial dysfunction induced by diabetes and hypertension. In contrast, controversial results have been reported regarding BH(4) metabolism in experimental models of atherosclerosis. Therefore, the present study was designed to characterize the expression and activity of GTP-cyclohydrolase I, a rate-limiting enzyme in biosynthesis of BH(4), during atherogenesis. BH(4) levels were significantly increased in atherosclerotic aortas of apolipoprotein E (apoE)-deficient mice as compared with wild-type mice after 5 mo of Western diet treatment. This increase was further significantly enhanced in apoE-deficient mice fed for 9 and 14 mo. Removal of the endothelium almost eliminated BH(4) in wild-type mice but not in apoE-deficient mice, suggesting that a major component of increased BH(4) synthesis is localized in the vascular media of apoE-deficient mice. Oxidative products of BH(4) were low and did not differ between wild-type and apoE-deficient mice over the course of this study. Increased protein expression and enzymatic activity of GTP-cyclohydrolase I were detected in aortas of apoE-deficient mice (P < 0.05), providing molecular mechanisms responsible for elevation of vascular BH(4). In contrast to aortas, we did not detect any change in levels of BH(4) and in GTP-cyclohydrolase I expression in the brain. Our results demonstrate selective increase of intracellular BH(4) levels via elevation of GTP-cyclohydrolase I activity in vascular tissue of apoE-deficient mice.

Effect of IBD sera on expression of inducible and endothelial nitric oxide synthase in human umbilical vein endothelial cells

AIM: To study the expression of endothelial and inducible nitric oxide synthases (eNOS and iNOS) and their role in inflammatory bowel disease (IBD).

METHODS: We examined the effect of sera obtained from patients with active Crohn’s disease (CD) and ulcerative colitis (UC) on the function and viability of human umbilical vein endothelial cells (HUVEC). HUVECs were cultured for 0-48 h in the presence of a medium containing pooled serum of healthy controls, or serum from patients with active CD or UC. Expression of eNOS and iNOS was visualized by immunofluorescence, and quantified by the densitometry of Western blots. Proliferation activity was assessed by computerized image analyses of Ki-67 immunoreactive cells, and also tested in the presence of the NOS inhibitor, 10^-4 mol/L L-NAME. Apoptosis and necrosis was examined by the annexin-V-biotin method and by propidium iodide staining, respectively.

RESULTS: In HUVEC immediately after exposure to UC, serum eNOS was markedly induced, reaching a peak at 12 h. In contrast, a decrease in eNOS was observed after incubation with CD sera and the eNOS level was minimal at 20 h compared to control (18% ± 16% vs 23% ± 15% P<0.01). UC or CD serum caused a significant increase in iNOS compared to control (UC: 300% ± 21%; CD: 275% ± 27% vs 108% ± 14%, P<0.01). Apoptosis/necrosis characteristics did not differ significantly in either experiment. Increased proliferation activity was detected in the presence of CD serum or after treatment with L-NAME. Cultures showed tube-like formations after 24 h treatment with CD serum.

CONCLUSION: IBD sera evoked changes in the ratio of eNOS/iNOS, whereas did not influence the viability of HUVEC. These involved down-regulation of eNOS and up-regulation of iNOS simultaneously, leading to increased proliferation activity and possibly a reduced anti-inflammatory protection of endothelial cells.

The hyperdynamic circulation of chronic liver diseases: from the patient to the molecule

The hyperdynamic circulatory syndrome observed in chronic liver diseases is a great example of research that originated from clinical observations and progressed in the last 50 years from the patient to the experimental laboratory. Our knowledge has evolved from the patient to the molecule, using experimental models that serve as a source for understanding the complex pathophysiological mechanisms that govern this complex syndrome. We now know that progressive vasodilatation is central to the detrimental effects observed in multiple organs. Although nitric oxide has been shown to be the primary vasodilator molecule in these effects, other molecules also participate in the complex mechanisms of vasodilatation. This review summarizes three major areas: first, clinical observation in patients; second, experimental models used to study the hyperdynamic circulatory syndrome; and third, the vasodilator molecules that play roles in vascular abnormalities observed in portal hypertension.

Increased blood flow causes coordinated upregulation of arterial eNOS and biosynthesis of tetrahydrobiopterin

Shear stress, imposed on the vascular endothelium by circulating blood, critically sustains vascular synthesis of nitric oxide (NO). Endothelial NO synthase (eNOS) activity is determined by heat shock protein 90 (HSP90), caveolin-1, and the cofactor tetrahydrobiopterin (BH4). To determine whether increased blood flow concomitantly upregulates eNOS and GTP cyclohydrolase I (GTPCH I, the rate-limiting enzyme in BH4 biosynthesis), an aortocaval fistula model in the rat was employed wherein aortic blood flow is enhanced proximal but decreased distal to the fistula. Eight weeks after the creation of the aortocaval fistula, the proximal and distal aortic segments were harvested; sham-operated rats served as controls. Vasomotor function was assessed by isometric force recording. Expression of eNOS, HSP90, caveolin-1, Akt, phosphorylated eNOS (eNOS-Ser1177), and GTPCH I were determined by Western blot analysis. Biosynthesis of BH4 and GTPCH-I activity was examined by HPLC. In the aortic segments exposed to increased flow, contractions to KCl and phenylephrine were reduced, whereas endothelium-dependent relaxations were not affected compared with sham-operated or aortic segments with reduced blood flow. Expression of eNOS, caveolin-1, phosphorylated Akt, and eNOS-Ser1177 was enhanced in aortas exposed to increased blood flow. High flow augmented levels of cGMP and BH4 and increased expression of GTPCH I. In aggregate, these findings provide the first demonstration in vivo that coordinated vascular upregulation of eNOS, and GTPCH I accompanies increased blood flow. This induction of GTPCH I increases BH4 production, thereby optimizing the generation of NO by eNOS and thus the adaptive, vasorelaxant response required in sustaining increased blood flow.

Naphthazarin and methylnaphthazarin cause vascular dysfunction by impairment of endothelium-derived nitric oxide and increased superoxide anion generation

The effects of the naphthoquinone analogue, naphthazarin (Nap), and its derivative, methylnaphthazarin (MetNap), on vascular reactivity were studied using isolated rat aortic rings and human umbilical vein endothelial cells (HUVECs). In this study, we determined vessel tension, nitric oxide (NO) formation, endothelial nitric oxide synthase (eNOS) activity, eNOS protein expression, and superoxide anion (O2*-) generation in an effort to evaluate the effect of Nap and MetNap on the impairment of the NO-mediated pathway. Lower concentrations of Nap (0.01-1 microM) and MetNap (1-10 microM) concentration-dependently enhanced phenylephrine (PE)-induced vasocontraction and abrogated acetylcholine (ACh)-induced vasorelaxation in an endothelium-dependent manner. On HUVECs, both Nap and MetNap concentration-dependently inhibited NO formation induced by A23187, and also partially inhibited nitric oxide synthase (NOS) activity. eNOS protein expression by HUVECs was not affected by treatment with Nap or MetNap, even within 24h. These data suggest that Nap and MetNap might act as inhibitors of nitric oxide synthesis in the endothelium. In addition, Nap and MetNap were also shown to generate O2*- on HUVECs with short-term treatment. We concluded that Nap and MetNap inhibited agonist-induced relaxation and induced vasocontraction in an endothelium-dependent manner, and these effects might have been due to modification of the NO content by inhibition of NOS activity and bioinactivation through O2*- generation.

Nitric oxide and the vascular endothelium

The vascular endothelium synthesises the vasodilator and anti-aggregatory mediator nitric oxide (NO) from L-arginine. This action is catalysed by the action of NO synthases, of which two forms are present in the endothelium. Endothelial (e)NOS is highly regulated, constitutively active and generates NO in response to shear stress and other physiological stimuli. Inducible (i)NOS is expressed in response to immunological stimuli, is transcriptionally regulated and, once activated, generates large amounts of NO that contribute to pathological conditions. The physiological actions of NO include the regulation of vascular tone and blood pressure, prevention of platelet aggregation and inhibition of vascular smooth muscle proliferation. Many of these actions are a result of the activation by NO of the soluble guanylate cyclase and consequent generation of cyclic guanosine monophosphate (cGMP). An additional target of NO is the cytochrome c oxidase, the terminal enzyme in the electron transport chain, which is inhibited by NO in a manner that is reversible and competitive with oxygen. The consequent reduction of cytochrome c oxidase leads to the release of superoxide anion. This may be an NO-regulated cell signalling system which, under certain circumstances, may lead to the formation of the powerful oxidant species, peroxynitrite, that is associated with a variety of vascular diseases.

Epigenetic basis for the transcriptional hyporesponsiveness of the human inducible nitric oxide synthase gene in vascular endothelial cells

A marked difference exists in the inducibility of inducible NO synthase (iNOS) between humans and rodents. Although important cis and trans factors in the murine and human iNOS promoters have been characterized using episomal-based approaches, a compelling molecular explanation for why human iNOS is resistant to induction has not been reported. In this study we present evidence that the hyporesponsiveness of the human iNOS promoter is based in part on epigenetic silencing, specifically hypermethylation of CpG dinucleotides and histone H3 lysine 9 methylation. Using bisulfite sequencing, we demonstrated that the iNOS promoter was heavily methylated at CpG dinucleotides in a variety of primary human endothelial cells and vascular smooth muscle cells, all of which are notoriously resistant to iNOS induction. In contrast, in human cell types capable of iNOS induction (e.g., A549 pulmonary adenocarcinoma, DLD-1 colon adenocarcinoma, and primary hepatocytes), the iNOS promoter was relatively hypomethylated. Treatment of human cells, such as DLD-1, with a DNA methyltransferase inhibitor (5-azacytidine) induced global and iNOS promoter DNA hypomethylation. Importantly, 5-azacytidine enhanced the cytokine inducibility of iNOS. Using chromatin immunoprecipitation, we found that the human iNOS promoter was basally enriched with di- and trimethylation of H3 lysine 9 in endothelial cells, and this did not change with cytokine addition. This contrasted with the absence of lysine 9 methylation in inducible cell types. Importantly, chromatin immunoprecipitation demonstrated the selective presence of the methyl-CpG-binding transcriptional repressor MeCP2 at the iNOS promoter in endothelial cells. Collectively, our work defines a role for chromatin-based mechanisms in the control of human iNOS gene expression.

Endothelial nitric oxide synthase activation by tumor necrosis factor alpha through neutral sphingomyelinase 2, sphingosine kinase 1, and sphingosine 1 phosphate receptors: a novel pathway relevant to the pathophysiology of endothelium

OBJECTIVE

Tumor necrosis factor alpha (TNF-alpha), a key proinflammatory cytokine acting on the endothelium, activates endothelial nitric oxide synthase (eNOS). We have examined the signaling pathway leading to this activation and its biological role in endothelium, which are still unknown.

METHODS AND RESULTS

In human endothelial cells, we found that eNOS activation by TNF-alpha is time dependent and requires activation of Akt, a known eNOS activator. eNOS activation was preceded by sequential activation of neutral-sphingomyelinase-2 (N-SMase2) and sphingosine-kinase-1 (SK1) and generation of sphingosine-1-phosphate (Sph1P). Inhibition of N-SMase2 inhibited Sph1P formation, whereas inhibition of SK1 did not affect N-SMase2 activation by TNF-alpha. Blockade of N-SMase2, SK1, or the Sph1P receptors S1P1 and S1P3, either by silencing or pharmacological inhibitors, prevented eNOS activation. Thus, eNOS is activated by TNF-alpha via S1P receptors, activated by Sph1P generated through N-SMase2 and SK1 activation. We found that nitric oxide generated through this pathway has a biological role, because it inhibits the expression of E-selectin and the adhesion of dendritic cells to the endothelium stimulated by TNF-alpha.

CONCLUSIONS

This study establishes a previously undescribed link among TNF-alpha, Sph1P, and eNOS in a same signaling pathway of biological relevance in the process of endothelial cell activation by TNF-alpha.

High serum TNF-alpha level in Type 2 diabetic patients with microangiopathy is associated with eNOS down-regulation and apoptosis in endothelial cells

A high dose of tumor necrosis factor (TNF)-alpha induces endothelial dysfunction and enhances apoptosis in vitro. The present study was conducted to examine whether incubating human umbilical vein endothelial cells (HUVECs) with serum from Type 2 diabetic patients complicated with retinopathy and/or microalbuminemia demonstrate endothelial dysfunction. Serum levels of TNF-alpha and vascular endothelial growth factor (VEGF) were elevated in diabetic patients. Plasma levels of TNF-alpha, two soluble TNF-alpha receptors (sTNFR), and VEGF were assessed in diabetic patients (CD, n=21) complicated with retinopathy and/or nephropathy, uncomplicated diabetic patients (UD, n=18), and in healthy normal participants (NS, n=16). In HUVECs incubated with patient's serum, endothelial constitutive nitric oxide synthase (eNOS) protein expressions were measured by Western blot analysis. Apoptosis in HUVECs was determined by optical microscopy, DNA fragmentation, and CPP32-like protease activity. Serum TNF-alpha, sTNFR-I, and asymmetric dimethylarginine (ADMA), an endogenous inhibitor of NOS, in CD were significantly higher than in UD or NS. While, serum sTNFR-I and VEGF levels were significantly increased in the both diabetic patients, compared with those of NS, no difference was observed in the serum TNF-alpha, sTNFR-II, and ADMA levels between UD and NS. eNOS down-regulation and apoptosis were seen in HUVECs incubated with serum from CD for 24 h, but those observations were completely counteracted in the incubation by the addition of the antihuman TNF-alpha antibody. These results imply that eNOS down-regulation in CD is associated with high serum TNF-alpha levels despite of high serum of VEGF levels. Therefore, endothelial dysfunction in diabetic patients complicated with microangiopathy may, in part, be attributed to high serum TNF-alpha levels.

Tetrahydrobiopterin corrects Escherichia coli endotoxin-induced endothelial dysfunction

Acute inflammation causes endothelial dysfunction, which is partly mediated by oxidant stress and inactivation of nitric oxide. The contribution of depletion of tetrahydrobiopterin (BH(4)), the cofactor required for nitric oxide generation, is unclear. In this randomized, double-blind, three-way crossover study, forearm blood flow (FBF) responses to ACh and glyceryltrinitrate (GTN) were measured before and 3.5 h after infusion of Escherichia coli endotoxin (LPS, 20 IU/kg iv) in eight healthy men. The effect of intra-arterial BH(4) (500 microg/min), placebo, or vitamin C (24 mg/min) was studied on separate days 3.5 h after LPS infusion. In addition, human umbilical vein endothelial cells were incubated for 24 h with vitamin C and LPS. ACh and GTN caused dose-dependent forearm vasodilation. The FBF response to ACh, which was decreased by 23 +/- 17% (P < 0.05) by LPS infusion, was restored to baseline reactivity by BH(4) and vitamin C. FBF responses to GTN were not affected by BH(4) or vitamin C. LPS increased leukocyte count, high-sensitivity C-reactive protein, IL-6, IL-1beta, IFN-gamma, monocyte chemoattractant protein-1, pulse rate, and body temperature and decreased platelet count and vitamin C concentration. Vitamin C increased forearm plasma concentration of BH(4) by 32% (P < 0.02). Incubation with LPS and vitamin C, but not LPS alone, increased intracellular BH(4) concentration in human umbilical vein endothelial cells. Impaired endothelial function during acute inflammation can be restored by BH(4) or vitamin C. Vitamin C may exert some of its salutary effects by increasing BH(4) concentration.

Supplementation with tetrahydrobiopterin prevents the cardiovascular effects of angiotensin II-induced oxidative and nitrosative stress

OBJECTIVE

The pteridine cofactor tetrahydrobiopterin (BH4) has emerged as a critical determinant of endothelial nitric oxide synthase (eNOS) activity. When BH4 availability is limited, eNOS does not produce nitric oxide (NO) but instead generates superoxide. BH4 may reverse endothelial dysfunction due to cardiovascular disease, including atherosclerosis, coronary artery disease and hypertension. In this study, the influence of BH4 on cardiovascular parameters and the production of free radicals following angiotensin II (Ang II) infusion was assessed.

METHODS

BH4 (20 mg/kg per day in drinking water) was administered with Ang II (300 ng/kg per min subcutaneously, osmotic pump) for 7 days in Sprague-Dawley rats. In addition, BH4 was also given in vehicle-infused rats.

RESULTS

Treatment with BH4 significantly prevented some of the effects of Ang II, such as impaired vascular responses to acetylcholine, hypertension and increases in heart weight index values. Treatment with BH4 also significantly reduced Ang II-induced increases in inducible NO synthase expression, nitrotyrosine immunostaining, NO production and superoxide anion formation in rats.

CONCLUSION

These results indicate that BH4 might prevent the development of hypertension and myocardial hypertrophy, as well as the Ang II-induced production of superoxide and NO, thereby reducing the production of peroxynitrite. Therefore, BH4 may protect against the cardiovascular manifestations of oxidative and nitrosative stress in this experimental model of Ang II-mediated hypertension.

Nitric Oxide Derived from Human Umbilical Vein Endothelial Cells Inhibits Transendothelial Migration of Neutrophils

We evaluated the roles of nitric oxide (NO) derived from endothelial cells in neutrophil transendothelial migration (TEM). Pretreatment of human umbilical vein endothelial cells (HUVECs) with NG-nitro-L-arginine methyl ester hydrochloride (L-NAME) or NG-monomethyl L-arginine (L-NMMA), which are inhibitors of NO synthases, enhanced neutrophil TEM. Similar augmentation of TEM was observed in the presence of an NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (carboxy PTIO). Neutrophil TEM across L-NAME- or L-NMMA-treated HUVECs was inhibited by continuous NO supply by NO donors. These findings support the suggestion that continuous production of NO by endothelial cells suppresses neutrophil TEM. Flow cytometric analyses revealed that NO accumulates in neutrophils co-cultured with NO-producing HUVECs. A decreased amount of NO was detected in neutrophils co-cultured with L-NAME-treated HUVECs compared with neutrophils co-cultured with untreated HUVECs. Soluble guanylyl cyclase (sGC) is known as one of the most important targets of NO in neutrophils. 3-(53-Hydroxymethyl-23furyl)-1-benzyl indazole (YC-1), an activator of sGC, inhibited L-NAME-induced neutrophil TEM. It was interesting that inhibition of neutrophil sGC with 1-H[1,2,4-]oxadiazolo[4,3-a]quinoxalin-1-1 (ODQ) was sufficient to enhance TEM. These results suggest that NO derived from HUVECs acts on neutrophils to inhibit TEM, at least in part by activating sGC. Our findings imply the role of NO constitutively generated by HUVECs in protection against excessive neutrophil extravasation and unnecessary tissue damage under physiological conditions.

The three nitric-oxide synthases differ in their kinetics of tetrahydrobiopterin radical formation, heme-dioxy reduction, and arginine hydroxylation

The nitric-oxide synthases (NOSs) make nitric oxide and citrulline from l-arginine. How the bound cofactor (6R)-tetrahydrobiopterin (H4B) participates in Arg hydroxylation is a topic of interest. We demonstrated previously that H4B radical formation in the inducible NOS oxygenase domain (iNOSoxy) is kinetically coupled to the disappearance of a heme-dioxy intermediate and to Arg hydroxylation. Here we report single turnover studies that determine and compare the kinetics of these transitions in Arg hydroxylation reactions catalyzed by the oxygenase domains of endothelial and neuronal NOSs (eNOSoxy and nNOSoxy). There was a buildup of a heme-dioxy intermediate in eNOSoxy and nNOSoxy followed by a monophasic transition to ferric enzyme during the reaction. The rate of heme-dioxy decay matched the rates of H4B radical formation and Arg hydroxylation in both enzymes. The rates of H4B radical formation differed such that nNOSoxy (18 s(-1)) > iNOSoxy (11 s(-1)) > eNOSoxy (6 s(-1)), whereas the lifetimes of the resulting H4B radical followed an opposite rank order. 5MeH4B supported a three-fold faster radical formation and greater radical stability relative to H4B in both eNOSoxy and nNOSoxy. Our results indicate the following: (i) the three NOSs share a common mechanism, whereby H4B transfers an electron to the heme-dioxy intermediate. This step enables Arg hydroxylation and is rate-limiting for all subsequent steps in the hydroxylation reaction. (ii) A direct correlation exists between pterin radical stability and the speed of its formation in the three NOSs. (iii) Uncoupled NO synthesis often seen for eNOS at low H4B concentrations may be caused by the slow formation and poor stability of its H4B radical.

Changes in tetrahydrobiopterin levels in endothelial cells and adult cardiomyocytes induced by LPS and hydrogen peroxide--a role for GFRP?

Alterations in tetrahydrobiopterin (BH4) levels have significant consequences in vascular pathophysiology. However, the mechanisms regulating BH4 remain poorly understood. The activity of GTP cyclohydrolase I (GTPCH-I), the first enzyme in BH4 biosynthesis, is controlled by protein levels, posttranslational modifications and interaction with GTPCH-I feedback regulatory protein (GFRP). This work examined the correlation between GTPCH-I protein levels and activity and changes in BH4 in human endothelial cells (HAECs) and adult rat cardiomyocytes (ARCM). Changes in BH4 were stimulated with LPS in HAECs and ARCM, and with hydrogen peroxide in HAECs only. Biopterin production by HAECs and ARCM were attained with concentrations of LPS >>1 microg/ml and responses were nonlinear with respect to LPS concentrations. Western blot analysis demonstrated that induction of biopterin synthesis in HAECs and ARCM by LPS does not entail augmentation of constitutive GTPCH-I protein levels. However, LPS diminished GFRP mRNA, suggesting that disruption of GTPCH-I:GFRP complex enhances de novo biopterin synthesis. Conversely, treatment with hydrogen peroxide increased GTPCH-I and GFRP mRNA levels in HAECs while depleting BH4 and GSH, which was counteracted by catalase. This indicates that GFRP may override increases in GTPCH-I protein inhibiting enzyme activity. This conclusion is further supported by depletion of biopterin in cells transiently transfected with GFRP. Thus, allosteric regulation of GTPCH-I activity in the cardiovascular system maybe an important mechanism regulating BH4 levels through GFRP signaling.

Improvement of Function and Morphology of Tumor Necrosis Factor-.ALPHA. Treated Endothelial Cells With 17-.BETA. Estradiol A Preliminary Study for a Feasible Simple Model for Atherosclerosis

BACKGROUND

Dysfunction of endothelial cells (EC) to produce endothelial nitric oxide synthase (eNOS) by tumor necrosis factor-alpha (TNF-alpha) causes critical features of vascular inflammation associated with several disease states (eg, atherosclerosis including increased platelet aggregation and adhesion on EC, elevated adhesion molecules and enhanced inflammatory cells binding to EC). 17-beta estradiol (E2) can stimulate eNOS production and improve the critical features of atherosclerosis. Using TNF-alpha and E2, we attempted to develop an in vitro vascular model for studying atherosclerosis.

METHODS AND RESULTS

Human umbilical vein endothelial cells (HUVEC) grown in transwells were cocultured with smooth muscle cells in a 24-well plate to mimic the major components of the vascular wall. The model was incubated with TNF-alpha (10 ng/ml) for 12 h, prior exposed to E2 (100 pg/ml) for 6-12 h, then investigated by transmission and scanning electron microscopes. The result indicated recovered morphology with good tight junction, and decreased platelet adhesion was noted in defective HUVEC after E2 treatment.

CONCLUSION

17-beta estradiol was represented as an antiatherosclerogenic agent to demonstrate feasibility of the model. Although our finding focused only on the endothelium, this would be the basis for our future studies to develop ex vivo continuous perfusion of human vessel segments for a further atherosclerosis study.

Cytokines, nitric oxide, and cGMP modulate the permeability of an in vitro model of the human blood-brain barrier

The endothelial cells (EC) of the microvasculature in the brain form the anatomical basis of the blood-brain barrier (BBB). In the present study, the effects of agents that modify the permeability of a well-established in vitro model of the human BBB were studied. The monolayers formed by confluent human brain microvessel endothelial cell (HBMEC) cultures are impermeable to the macromolecule tracer horseradish peroxidase (HRP) and have high electrical resistance. Exposure of HBMEC to various cytokines including TNF-alpha, IL-1beta, interferon gamma (IFN-gamma), or lipopolysaccharide (LPS) decreased transendothelial electrical resistance (TEER) mainly by increasing the permeability of the tight junctions. Primary cultures of HBMEC express endothelial nitric oxide synthase (eNOS) and produce low levels of NO. Treatment with the NO donors sodium nitroprusside (SNP) and DETA NONOate or the cGMP agonist 8-Br-cGMP significantly increased monolayer resistance. Conversely, inhibition of soluble guanylyl cyclase with ODQ rapidly decreased the resistance, and pretreatment of HBMEC with Rp-8-CPT-cGMPS, an inhibitor of cGMP-dependent protein kinase, partially prevented the 8-Br-cGMP-induced increase in resistance. Furthermore, NO donors and 8-Br-cGMP could also reverse the increased permeability of the monolayers induced by IL-1beta, IFN-gamma, and LPS. These results indicate that NO can decrease the permeability of the human BBB through a mechanism at least partly dependent on cGMP production and cGMP-dependent protein kinase activation.

T cell-mediated vascular dysfunction of human allografts results from IFN-gamma dysregulation of NO synthase

Allograft vascular dysfunction predisposes to arteriosclerosis and graft loss. We examined how dysfunction develops in transplanted human arteries in response to circulating allogeneic T cells in vivo using immunodeficient murine hosts. Within 7-9 days, transplanted arteries developed endothelial cell (EC) dysfunction but remained sensitive to exogenous NO. By 2 weeks, the grafts developed impaired contractility and desensitization to NO, both signs of VSMC dysfunction. These T cell-dependent changes correlated with loss of eNOS and expression of iNOS--the latter predominantly within infiltrating T cells. Neutralizing IFN-gamma completely prevented both vascular dysfunction and changes in NOS expression; neutralizing TNF reduced IFN-gamma production and partially prevented dysfunction. Inhibiting iNOS partially preserved responses to NO at 2 weeks and reduced graft intimal expansion after 4 weeks in vivo. In vitro, memory CD4+ T cells acted on allogeneic cultured ECs to reduce eNOS activity and expression of protein and mRNA. These effects required T cell activation by class II MHC antigens and costimulators (principally lymphocyte function-associated antigen-3, or LFA-3) on the ECs and were mediated by production of soluble mediators including IFN-gamma and TNF. We conclude that IFN-gamma is a central mediator of vascular dysfunction and, through dysregulation of NOS expression, links early dysfunction with late arteriosclerosis.

Gene expression profiling of inflamed human endothelial cells and influence of activated protein C

BACKGROUND

During systemic inflammation, activation of vascular endothelium by proinflammatory cytokines leads to hypotension, microvascular thrombosis, and organ damage. Recent data suggest a link between coagulation and inflammation through the activated protein C (APC) pathway. We studied gene expression profiles in human coronary artery endothelial cells (HCAECs) exposed to proinflammatory stimuli and the influence of APC on expression of candidate genes regulated by these stimuli.

METHODS AND RESULTS

HCAECs were stimulated with interleukin-1beta, interferon-gamma, and tumor necrosis factor-alpha. In gene expression profiling, 400 of 8400 genes were regulated >2-fold. Verification of selected candidate genes was achieved by measuring expression of mRNA species by real-time polymerase chain reaction, cytokine secretion by ELISA, and metabolites of tetrahydrobiopterin (BH4) biosynthesis by high-performance liquid chromatography. BH4 synthesis, interleukin-6, interleukin-8, monocyte chemotactic protein-1 (MCP-1), and intercellular adhesion molecule-1 (ICAM-1) were downregulated by APC at the transcriptional and protein level. Endothelial nitric oxide synthase, endothelial adhesion molecule, and vascular cell adhesion molecule-1 were not affected by APC. Activities of transcription factors c-Fos, FosB, and c-Rel were inhibited by APC in inflamed HCAECs.

CONCLUSIONS

Our study revealed a novel antiinflammatory mechanism of APC-dependent gene regulation in HCAECs since c-Fos-dependent induction of MCP-1 and ICAM-1 was suppressed. APC downregulates expression and activity of genes related to inflammation, most pronounced under intermediate or mild inflammatory conditions.

Alpha-tocopherol amplifies phosphorylation of endothelial nitric oxide synthase at serine 1177 and its short-chain derivative trolox stabilizes tetrahydrobiopterin

Alpha-tocopherol has been shown to increase nitric oxide (NO)-dependent relaxation but the underlying mechanisms have not been fully characterized. The present study investigates the effect of alpha-tocopherol and its derivative trolox on the synthesis of NO in human umbilical vein endothelial cells. NO was assayed as citrulline (co-product of NO) and cGMP (product of the NO-activated soluble guanylate cyclase) on ionomycin stimulation of cells. Ionomycin induced citrulline and cGMP formation partially through phosphorylation of endothelial NO synthase (eNOS) at its serine residue 1177, which was mediated mainly by calmodulin-dependent kinase II. Preincubation of cells with alpha-tocopherol or trolox increased eNOS activity in a concentration-dependent manner without changing eNOS expression. The effect of the water-soluble trolox was due to chemical stabilization of the eNOS cofactor tetrahydrobiopterin. On the contrary, alpha-tocopherol, located mainly in cellular membranes, did not affect tetrahydrobiopterin but increased ionomycin-induced eNOS phosphorylation at serine 1177. The effects of alpha-tocopherol on citrulline and cGMP formation and eNOS phosphorylation were amplified by co-incubation with ascorbate, which is suggested to regenerate oxidized alpha-tocopherol and to act synergistically with alpha-tocopherol. Our data describe a new vasoprotective function of alpha-tocopherol that may contribute to the prevention of endothelial dysfunction in vivo.

Human endothelial progenitor cells tolerate oxidative stress due to intrinsically high expression of manganese superoxide dismutase

OBJECTIVE

Endothelial progenitor cells (EPCs) display a unique aptitude to promote angiogenesis and restore endothelial function of injured vessels. How progenitor cells can execute a regenerative program in the unfavorable environment of injury/inflammation-induced oxidative stress is poorly understood. We hypothesized that EPCs are resistant to oxidative stress and that this resistance is due to high expression and activity of antioxidant enzymes.

METHODS AND RESULTS

EPCs outgrown from human blood of healthy subjects demonstrated a marked resistance to cytotoxic effect of LY83583 (an generator), tumor necrosis factor-alpha, and serum depletion. LY83583 inhibited in vitro tube formation by human umbilical vein endothelial cells (HUVECs) and human coronary artery endothelial cells (CAECs), but not by EPCs. Compared with HUVECs and CAECs, EPCs exhibited approximately 3- to 4-fold higher expression and activity of manganese superoxide dismutase (MnSOD), but not copper zinc superoxide dismutase (CuZnSOD) or catalase. The antioxidant profile in EPCs was associated with preservation of the mitochondrial network when exposed to LY83583. Moreover, cytotoxic effects of LY83583 on CAECs and HUVECs were reversed by adenoviral overexpression of MnSOD.

CONCLUSIONS

Human EPCs are resistant to oxidative stress. High intrinsic expression of MnSOD is a critical mechanism protecting EPCs against oxidative stress.

T lymphocyte-endothelial cell interactions

Human vascular endothelial cells (EC) basally display class I and II MHC-peptide complexes on their surface and come in regular contact with circulating T cells. We propose that EC present microbial antigens to memory T cells as a mechanism of immune surveillance. Activated T cells, in turn, provide both soluble and contact-dependent signals to modulate normal EC functions, including formation and remodeling of blood vessels, regulation of blood flow, regulation of blood fluidity, maintenance of permselectivity, recruitment of inflammatory leukocytes, and antigen presentation leading to activation of T cells. T cell interactions with vascular EC are thus bidirectional and link the immune and circulatory systems.

Functional tetrahydrobiopterin synthesis in human platelets

BACKGROUND

Previous studies have provided evidence for the importance of platelet-derived nitric oxide (NO) for the regulation of hemostasis. Tetrahydrobiopterin (BH4) is an essential cofactor and regulator of NO synthase activity in the vasculature; however, it is as yet unknown whether platelets dispose over a functional BH4 synthesis.

METHODS AND RESULTS

We quantified mRNA expression of genes involved in BH4 synthesis, measured enzymatic activities, and determined intraplatelet levels of pteridines in platelets from healthy volunteers and from patients treated for prolonged periods of time with glucocorticoids. Freshly isolated platelets from healthy volunteers show functional BH4 synthesis, as evidenced by the presence of mRNA species and enzymatic activity of GTP cyclohydrolase I (GTPCH), 6-pyruvoyl tetrahydropterin synthase, and sepiapterin reductase. Biopterin was the major intraplatelet pteridine, whereas no neopterin was found. mRNA expression and enzymatic activity of GTPCH were undetectably low in platelets that had been stored for 5 days, and no pteridines were found in these platelets. Freshly isolated platelets from patients treated with glucocorticoids had decreased mRNA expression and activity of GTPCH compared with platelets from healthy volunteers.

CONCLUSIONS

Human platelets dispose over a functional de novo BH4 synthesis. Furthermore, our results indicate the potential of external factors, eg, prolonged storage or glucocorticoid therapy, to significantly affect BH4 synthesis within platelets. Together, these findings offer new insights into the biology and pathobiology of platelet function in humans.

Spectrophotometric analysis of the protective effect of ascorbate against spontaneous oxidation of tetrahydrobiopterin in aqueous solution: kinetic characteristics and potentiation by catalase of ascorbate action

Tetrahydrobiopterin (BH(4)) is oxidized by O(2) readily in aqueous solutions and physiological concentrations of ascorbate have been shown to inhibit this reaction. In order to gain insight into the mechanism of ascorbate effect, a spectrophotometric analysis was applied for the study of the time course of BH(4) oxidation in the presence of various concentrations of ascorbate and the effect of various temperatures on the apparent second-order rate constant of BH(4) oxidation (k(ox)) in the presence or absence of catalase. In 100 micromol/l concentration, ascorbate alone prolonged the half-life time of 36 micromol/l BH(4) 1.4-fold whereas in the presence of catalase 1.85-fold. In the presence of catalase ascorbate decreased the value of k(ox) to 51 +/- 0.67%, whereas in the absence of it only to 64 +/- 0.77% of control (P < 0.01). The extent of ascorbate effect was not dependent on temperature, at least between 22 and 37 degrees C, either in the presence or absence of catalase. In the absence of catalase the apparent Arrhenius activation energies: 57.02 +/- 0.09 kJ/mol (-ascorbate) and 56.77 +/- 2.21 kJ/mol (+ascorbate) whereas in the presence of catalase: 62.72 +/- 1.37 kJ/mol (-ascorbate) and 59.93 +/- 2.84 kJ/mol (+ascorbate, mean +/- S.E.M., n=3) were obtained. The study shows that catalase potentiates the BH(4)-stabilizing effect of ascorbate. It is concluded that removal of H(2)O(2) generated from BH(4) during oxidation by O(2) prevents a decrease of ascorbate concentration, and in the presence of ascorbate the pacemaker step in the overall reaction is the oxidation of BH(4) and not the reduction of the quinonoid BH(2) back to BH(4) by ascorbate.

Role of tetrahydrobiopterin in adrenocorticotropic hormone-induced hypertension in the rat

Adrenocorticotropic hormone (ACTH)-induced hypertension in the rat is characterized by nitric oxide deficiency. Tetrahydrobiopterin (BH4) is an essential cofactor for the enzyme nitric oxide synthase and glucocorticoids have been reported to reduce cytokine-induced BH4 production. Accordingly we hypothesized that ACTH-induced hypertension would be reversed by BH4 supplementation. Male Sprague-Dawley rats (n = 33) were treated with BH4 in vehicle (10 mg/kg/day i.p.) or vehicle alone (5 mg/kg/day i.p. of ascorbic acid in 4 mM HCl) for 10 days. ACTH (0.2 mg/kg s.c.) or saline daily injection was started 2 days after BH4 or vehicle treatment and continued for 8 days. Systolic blood pressure (SBP) was measured on alternate days using the tail cuff method. Treatment with HCl, ascorbic acid or BH4 alone had no effect on SBP. In saline treated rats, neither BH4 nor its vehicle modified SBP. In ACTH treated rats, SBP was increased in both BH4 (from 128 +/- 6 to 142 +/- 4 mmHg, T0 to T10, P < 0.0005, one way ANOVA) and vehicle groups (from 127 +/- 3 to 158 +/- 7 mmHg, T0 to T10, P < 0.001, one way ANOVA). There was no significant difference in SBP between BH4 + ACTH treated and vehicle + ACTH treated rats. Thus, daily injection of BH4 (10 mg/kg i.p.) failed to prevent the development of ACTH-induced hypertension in rat.

Endothelial Nitric Oxide Synthase

Advances in our understanding of the molecular mechanisms involved in the constitutive and regulated expression of endothelial nitric oxide synthase (eNOS) mRNA expression present a new level of complexity to the study of endothelial gene regulation in health and disease. Recent studies highlight the contribution of both transcription and RNA stability to net steady-state mRNA levels of eNOS in vascular endothelium, introducing a new paradigm to gene regulation in the injured blood vessel. Constitutive eNOS expression is dependent on basal transcription machinery in the core promoter, involving positive and negative protein–protein and protein–DNA interactions. Chromatin-based mechanisms and epigenetic events also regulate expression of eNOS at the transcriptional level in a cell-restricted fashion. Although constitutively active, important physiological and pathophysiologic stimuli alter eNOS gene transcription rates. For instance, eNOS transcription rates increase in response to lysophosphatidylcholine, shear stress, and TGF-β, among others. Under basal conditions, eNOS mRNA is extremely stable. Surprisingly, posttranscriptional mechanisms have emerged as important regulatory pathways in the observed decreases in eNOS expression in some settings. In models of inflammation, proliferation/injury, oxidized low-density lipoprotein treatment, and hypoxia, eNOS mRNA destabilization plays a significant role in the rapid downregulation of eNOS mRNA levels.

In vivo expression and function of recombinant GTPCH I in the rabbit carotid artery

Tetrahydrobiopterin (BH4) is an essential co-factor for endothelial nitric oxide synthase enzymatic activity. GTP cyclohydrolase I (GTPCH I) is the rate-limiting enzyme in BH4 synthesis. This study set out to test the hypothesis that in vivo gene transfer of GTPCH I to endothelial cells could increase bioavailability of BH4, enhance biosynthesis of nitric oxide and thereby enhance endothelium-dependent relaxations mediated by nitric oxide. In vivo gene transfer was carried out by adenovirus (Ad)-mediated delivery into rabbit carotid arteries. Each artery was transduced by 20-min intraluminal incubation of 10(9) plaque-forming units of Ad-encoding GTPCH I (AdGTPCH) or beta-galactosidase as a control. The rabbits were euthanized 72 h later, and vasomotor function of isolated arteries was assessed by isometric force recording. GTPCH I enzymatic activity, BH4, and oxidized biopterin levels were detected with the use of HPLC, and cGMP was measured with the use of radioimmunoassay. Expression of recombinant proteins was detected predominantly in endothelial cells. Both GTPCH I activity and BH4 levels were increased in arteries transduced with AdGTPCH. However, contraction to phenylephrine (10(-5) to 10(-9) M), endothelium-dependent relaxation to acetylcholine (10(-5) to 10(-9) M) and cGMP levels were not significantly affected by increased expression of GTPCH I. Our results suggest that expression of GTPCH I in vascular endothelium in vivo increases intracellular concentration of BH4. However, under physiological conditions, it appears that this increase does not affect nitric oxide production in endothelial cells of the carotid artery.