Role of tetrahydrobiopterin availability in the regulation of nitric-oxide synthase expression in human mesangial cells

The intracellular environment of human macrophages that produce nitric oxide promotes growth of mycobacteria

Nitric oxide (NO) is a diffusible radical gas produced from the activity of nitric oxide synthase (NOS). NOS activity in murine macrophages has a protective role against mycobacteria through generation of reactive nitrogen intermediates (RNIs). However, the production of NO by human macrophages has remained unclear due to the lack of sensitive reagents to detect NO directly. The purpose of this study was to investigate NO production and the consequence to mycobacteria in primary human macrophages. We found that Mycobacterium bovis BCG or Mycobacterium tuberculosis infection of human macrophages induced expression of NOS2 and NOS3 that resulted in detectable production of NO. Treatment with gamma interferon (IFN-γ), l-arginine, and tetrahydrobiopterin enhanced expression of NOS2 and NOS3 isoforms, as well as NO production. Both of these enzymes were shown to contribute to NO production. The maximal level of NO produced by human macrophages was not bactericidal or bacteriostatic to M. tuberculosis or BCG. The number of viable mycobacteria was increased in macrophages that produced NO, and this requires expression of nitrate reductase. An narG mutant of M. tuberculosis persisted but was unable to grow in human macrophages. Taken together, these data (i) enhance our understanding of primary human macrophage potential to produce NO, (ii) demonstrate that the level of RNIs produced in response to IFN-γ in vitro is not sufficient to limit intracellular mycobacterial growth, and (iii) suggest that mycobacteria may use RNIs to enhance their survival in human macrophages.

Nitric oxide modulators: an emerging class of medicinal agents

Nitric oxide, a unique messenger in biological system, is ubiquitously present virtually in all tissues revealing its versatile nature of being involved in diverse physiological functions such as vascular tone, inhibition of platelet aggregation, cell adhesion, neurotransmission and enzyme and immune regulation. The tremendous advancements made in the past few decades in this area suggests that the nitric oxide modulation either by its exogenous release through nitric oxide donors or inhibition of its synthesis by nitric oxide synthase inhibitors in physiological milieu may provide newer clinical strategies for the treatment of some diseases. In this review, an attempt is made to document and understand the biological chemistry of different classes of nitric oxide modulators that would prove to be a fruitful area in the years to come.

In vivo activation of AMP-activated protein kinase attenuates diabetes-enhanced degradation of GTP cyclohydrolase I

OBJECTIVE

The activation of AMP-activated protein kinase (AMPK) has been reported to improve endothelial function. However, the targets of AMPK in endothelial cells remain poorly defined. The aim of this study was to test whether AMPK suppresses the degradation of GTP-cyclohydrolase (GTPCH I), a key event in vascular endothelial dysfunction in diabetes.

RESEARCH DESIGN AND METHODS

Both human umbilical vein endothelial cells and aortas isolated from streptozotocin-injected diabetic mice were assayed for phospho-AMPK (Thr172), GTPCH I, tetrahydrobiopterin (BH4), and endothelial functions.

RESULTS

Oral administration of metformin (300 mg x kg(-1) . day(-1), 4 weeks) in streptozotocin-injected mice significantly blunted the diabetes-induced reduction of AMPK phosphorylation at Thr172. Metformin treatment also normalized acetylcholine-induced endothelial relaxation and increased the levels of GTPCH I and BH4. The administration of AICAR, an AMPK activator, or adenoviral overexpression of a constitutively active mutant of AMPK abolished the high-glucose-induced (30 mmol/l) reduction of GTPCH I, biopeterins, and BH4 but had no effect on GTPCH I mRNA. Furthermore, AICAR or overexpression of AMPK inhibited the high-glucose-enhanced 26S proteasome activity. Consistently, inhibition of the proteasome by MG132 abolished high-glucose-induced reduction of GTPCH I in human umbilical vein endothelial cells. Further, aortas isolated from AMPKalpha2(-/-) mice, which exhibited elevated 26S proteasome activity, had reduced levels of GTPCH I and BH4. Finally, either administration of MG132 or supplementation of l-sepiapterin normalized the impaired endothelium-dependent relaxation in aortas isolated from AMPKalpha2(-/-) mice.

CONCLUSIONS

We conclude that AMPK activation normalizes vascular endothelial function by suppressing 26S proteasome-mediated GTPCH I degradation in diabetes.

Analysis of the effect of tetrahydrobiopterin on PAH gene expression in hepatoma cells

Tetrahydrobiopterin (BH4)-responsive phenylalanine hydroxylase (PAH) deficiency is a recently recognized variant of phenylketonuria, with a probable multifactorial molecular basis. In this study we have investigated the effect of BH4 on PAH gene expression in human hepatoma. Our results show that increased BH4 levels result in an enhancement of PAH activity and PAH protein, due to longer turnover rates, while PAH mRNA levels remain unchanged. This was confirmed for mutant PAH proteins (A309V, V388M and Y414C) associated to in vivo BH4 responsiveness, validating previous studies. We can conclude that there is no effect of the cofactor on PAH gene transcription, probably being the chemical chaperone effect of BH4 stabilizing mutant PAH proteins the major underlying mechanism of the response.

Cytoprotective function of tetrahydrobiopterin in rat liver ischemia/reperfusion injury

BACKGROUND

Tetrahydrobiopterin (BH(4)) is a key coenzyme of nitric oxide synthase (NOS), which is associated with a cytoprotective function in various ischemia-reperfusion (I/R) injury models. There have been a few reports on the efficacy of BH(4) in the treatment of I/R injury in other organs; therefore, the aim of this study was to investigate the effect of BH(4) related with NOS reaction in hepatic I/R injury.

METHODS

A model of 70% liver I/R injury with a 100-minute ischemic time was created in rats, and the non-ischemic lobes were then resected. The rats were given BH(4) (BH(4) group) or saline solution (saline group) before reperfusion. The specific inducible NOS blocker 1400W was used to evaluate the effect of endogenous inducible NOS in the I/R hepatic injury. Survival, nitric oxide products (nitrate and nitrite), NOS expression, and nitrotyrosine (ie, the peroxynitrite product) were measured after reperfusion.

RESULTS

On day 7, the survival rate was 62.5% in the BH(4) group, as opposed to 14.3% in the saline group (P = .0004); 1400W administration to the BH(4) group decreased the survival rate to 0% (P = .003). BH(4) prevented the significant increase in total bilirubin levels (P < .01) after 12-hour reperfusion. The increases in serum alanine transaminase levels (after 3 hours and 12 hours of reperfusion) were significantly (P < .01) attenuated in the BH(4) group. BH(4) increased the nitrate/nitrite concentrations in liver tissue (P < .05) and reduced nitrotyrosine production, and the protein assay showed that BH(4) increased inducible NOS and endothelial NOS expression. Histologic examination of the liver revealed that BH(4) mitigated the damage that was caused by liver I/R.

CONCLUSION

Exogenous BH(4) increased nitric oxide production, which attenuated the hepatic I/R injury.

Nitric oxide formation in acutely rejecting cardiac allografts correlates with GTP cyclohydrolase I activity

Inducible nitric oxide synthase (iNOS) is a prominent component of the complex array of mediators in acute graft rejection. While NO production is determined by iNOS expression, BH4 (tetrahydrobiopterin), a cofactor of iNOS synthesized by GTP cyclohydrolase I, has been considered critical in sustaining NO production. In the present study, we examined time-dependent changes in iNOS and GTP cyclohydrolase I in rat cardiac allografts. The increase in iNOS protein and mRNA in allografts was similar at POD4 (post-operative day 4) and POD6. However, the peak increase in intragraft NO level at POD4 was not sustained at POD6. This disparity could not be explained by any decrease in iNOS enzyme activity measured ex vivo with optimal amounts of substrate and cofactors. Lower iNOS activity could be explained by changes in total biopterin levels in allografts at POD4 that was decreased to baseline at POD6. Changes in biopterin production correlated with lower GTP cyclohydrolase I protein levels but not by any change in GTP cyclohydrolase I mRNA. Functionally, allografts displayed bradycardia and distended diastolic and systolic dimensions at POD6 but not at POD4. Likewise, histological rejection scores were increased at POD4 but with a secondary increased stage at POD6. It is hypothesized that the dissimilar amounts of NO at early and later stages of rejection is due to uncoupling of iNOS arising from disproportionate synthesis of BH4. These findings provide insight into a potential pathway regulating NO bioactivity in graft rejection. Such knowledge may potentially assist in the design of newer strategies to prevent acute graft rejection.

Regulation of the expression of inducible nitric oxide synthase

The role of nitric oxide (NO) generated by the inducible isoform of nitric oxide synthase (iNOS) is very complex. Induction of iNOS expression and hence NO production has been described to have beneficial antiviral, antiparasital, microbicidal, immunomodulatory, and antitumoral effects. However, induced at the wrong place or at the wrong time, iNOS has detrimental consequences and seems to be involved in the pathophysiology of different human diseases. The pathways regulating iNOS expression seem to vary in different cells or different species. In general, activation of the transcription factors nuclear factor (NF)-kappaB and signal transducer and activator of transcription (STAT)-1alpha and thereby activation of the iNOS promoter seems to be an essential step in the regulation of iNOS expression in most cells. Also, post-transcriptional mechanisms are critically involved in the regulation of iNOS expression.

The Mechanism of Potent GTP Cyclohydrolase I Inhibition by 2,4-Diamino-6-hydroxypyrimidine

Inhibition of GTP cyclohydrolase I (GTPCH) has been used as a selective tool to assess the role of de novo synthesis of (6R)-5,6,7,8-tetrahydro-L-biopterin (BH4) in a biological system. Toward this end, 2,4-diamino-6-hydroxypyrimidine (DAHP) has been used as the prototypical GTPCH inhibitor. Using a novel real-time kinetic microplate assay for GTPCH activity and purified prokaryote-expressed recombinant proteins, we show that potent inhibition by DAHP is not the result of a direct interaction with GTPCH. Rather, inhibition by DAHP in phosphate buffer occurs via an indirect mechanism that requires the presence of GTPCH feedback regulatory protein (GFRP). Notably, GFRP was previously discovered as the essential factor that reconstitutes inhibition of pure recombinant GTPCH by the pathway end product BH4. Thus, DAHP inhibits GTPCH by engaging the endogenous feedback inhibitory system. We further demonstrate that L-Phe fully reverses the inhibition of GTPCH by DAHP/GFRP, which is also a feature in common with inhibition by BH4/GFRP. These findings suggest that DAHP is not an indiscriminate inhibitor of GTPCH in biological systems; instead, it is predicted to preferentially attenuate GTPCH activity in cells that most abundantly express GFRP and/or contain the lowest levels of L-Phe.

Nitric oxide synthesis in the kidney: isoforms, biosynthesis, and functions in health

Nitric oxide (NO) is a gaseous free radical that serves cell signaling, cellular energetics, host defense, and inflammatory functions in virtually all cells. In the kidney and vasculature, NO plays fundamental roles in the control of systemic and intrarenal hemodynamics, the tubuloglomerular feedback response, pressure natriuresis, release of sympathetic neurotransmitters and renin, and tubular solute and water transport. NO is synthesized from L-arginine by NO synthases (NOS). Because of its high chemical reactivity and high diffusibility, NO production by each of the 3 major NOS isoforms is regulated tightly at multiple levels from gene transcription to spatial proximity near intended targets to covalent modification and allosteric regulation of the enzyme itself. Many of these regulatory mechanisms have yet to be tested in renal cells. The NOS isoforms are distributed differentially and regulated in the kidney, and there remains some controversy over the specific expression of functional protein for the NOS isoforms in specific renal cell populations. Mice with targeted deletion of each of the NOS isoforms have been generated, and these each have unique phenotypes. Studies of the renal and vascular phenotypes of these mice have yielded important insights into certain vascular diseases, ischemic acute renal failure, the tubuloglomerular feedback response, and some mechanisms of tubular fluid and electrolyte transport, but thus far have been underexploited. This review explores the collective knowledge regarding the structure, regulation, and function of the NOS isoforms gleaned from various tissues, and highlights the progress and gaps in understanding in applying this information to renal and vascular physiology.

Suppression of pathogen-inducible NO synthase (iNOS) activity in tomato increases susceptibility to Pseudomonas syringae

Inducible NO synthase (iNOS) activity is induced upon pathogen inoculation in resistant, but not susceptible, tobacco and Arabidopsis plants. It was shown recently that a variant form of the Arabidopsis P protein (AtvarP) has iNOS activity. P protein is part of the glycine decarboxylase complex (GDC). It is unclear whether P protein also has iNOS activity and, if so, whether AtvarP, P, or both, play a role in plant defense. Here, we show that iNOS activity is induced in both resistant and susceptible tomato leaves upon inoculation with the Pseudomonas syringae pv. tomato strain DC3000. Virus-induced gene-silencing targeting LevarP, a putative tomato ortholog of AtvarP, led to complete suppression of DC3000-induced iNOS activation and an approximately 80% reduction in GDC activity; it also increased disease-symptom severity and DC3000 growth in both resistant and susceptible tomato. To determine whether enhanced susceptibility exhibited by LevarP-silenced, susceptible tomato was due to loss of (i) iNOS activity, (ii) GDC activity, or (iii) both, GDC activity was inhibited with or without concurrent suppression of iNOS. Treatment with methotrexate inhibited both iNOS and GDC activities and resulted in increased susceptibility, comparable with that observed in LevarP-silenced plants. When normal iNOS activity was maintained in the presence of methotrexate by the addition of tetrahydrobiopterin, there was no change in susceptibility, despite a dramatic reduction in GDC activity. Together, these results indicate that iNOS contributes to host defense response against DC3000.

Diminished production of nitric oxide synthase cofactor tetrahydrobiopterin by rosiglitazone in adipocytes

Increased nitric oxide (NO) synthesis has been proposed to participate in the generation of insulin resistance in adipose and muscle tissues. Therefore, we examined the potential rate-limiting role of tetrahydrobiopterin (BH4) in cytokine-induced NO synthesis, and the effect of peroxisome proliferator activated receptor-gamma (PPARgamma) activation using the insulin-sensitizer rosiglitazone on cytokine-induced BH4 synthesis in 3T3-L1 adipocytes. Our data indicate that modulated availability of the mandatory nitric oxide synthase (NOS) cofactor BH4 affected cytokine-induced NO generation. Semiquantitative linear range reverse transcription polymerase chain reaction (RT-PCR) analysis demonstrated that rosiglitazone not only reduced inducible nitric oxide synthase (iNOS) mRNA transcription, but also guanosine triphosphate cyclohydrolase (GTPCH), the rate-limiting and controlling step of BH4 synthesis. Accordingly, intracellular BH4 concentration was reduced by 45% following rosiglitazone treatment. Furthermore, we observed a transient inhibitory effect of natural PPARgamma ligand 15-deoxy-Delta(12,14)-prostaglandin J2 (15d-PJ2) on cytokine-mediated iNOS and GTPCH induction. Thus, the inhibition of cytokine-induced NO synthesis by rosiglitazone is at least in part attributable to reduced availability of BH4, the synthesis of which might represent a potential new target in the treatment of type 2 diabetes and insulin resistance.

Increased vulnerability to ischemia/reperfusion-induced ventricular tachyarrhythmias by pre-ischemic inhibition of nitric oxide synthase in isolated rat hearts

INTRODUCTION

The relationship between vulnerability to reperfusion-induced ventricular tachyarrhythmias, such as ventricular tachycardia (VT) and fibrillation (VF), and the endogenous activity of nitric oxide synthase (NOS) has not been well documented. The objective of the present study was to clarify whether the vulnerability to reperfusion-induced VT/VF changes with preishemic, sustained inhibition of NOS.

METHODS

The experiments were performed using Langendorff-perfused isolated rat hearts, in which left ventricular pressure (LVP) and left ventricular cardiomyograms (LVCMGs) were measured.

RESULTS

A pre-ischemic, sustained inhibition of NOS resulted in an increased vulnerability to reperfusion-induced VT/VF, and the increase was markedly attenuated by co-treatment with L-arginine or by post-ischemic treatment with 2,4-diamino-6-hydroxypyrimidine (DAHP), an inhibitor of tetrahydrobiopterin (BH(4)) synthesis. We then tried to elucidate whether nitric oxide (NO) and superoxide were produced during reperfusion, and ATP-sensitive potassium channels (K(ATP)), especially mitochondrial ATP-sensitive potassium channels (mitoK(ATP)), are involved in the increased vulnerability. Post-ischemic inhibition of NOS and treatment with a NO scavenger attenuated the increased vulnerability to reperfusion-induced VT/VF, but post-ischemic treatment with a superoxide scavenger did not. In addition, post-ischemic treatment with S-nitroso-N-acetyl-DL-penicillamine (SNAP), a NO donor, or with diazoxide, a selective opener of mitoK(ATP), increased the VT/VF duration during reperfusion. The increased vulnerability to VT/VF was attenuated by the treatment with a selective mitoK(ATP) blocker.

CONCLUSION

The results suggest that a pre-ischemic, sustained inhibition of NOS increases the vulnerability to reperfusion-induced VT/VF, and the NO-mitoK(ATP) pathway is one of the possible factors contributing to the increased vulnerability to VT/VF.

Signalling mechanisms involved in the induction of inducible nitric oxide synthase by Lactobacillus rhamnosus GG, endotoxin, and lipoteichoic acid

BACKGROUND AND AIMS

Probiotic Lactobacillus rhamnosus GG (Lactobacillus GG) has been found beneficial in the treatment of viral and antibiotic-associated diarrhea. Recently, it has also been shown to induce nitric oxide (NO) production, and have some other immunostimulatory effects. The aim of the present study was to investigate the mechanisms involved in the induction of inducible nitric oxide synthase (iNOS) and NO production by Lactobacillus GG.

METHODS AND RESULTS

iNOS expression and NO production induced by Lactobacillus GG, lipopolysaccharide (LPS), and lipoteichoic acid (LTA) was abrogated by NOS inhibitors L-NMMA and 1400W, by a protein synthesis inhibitor cycloheximide, by a tyrosine kinase inhibitor genistein and by a NF-kappaB inhibitor pyrrolidinedithiocarbamate (PDTC) in J774 macrophages. Polymyxin B inhibited NO production induced by LPS, but did not inhibit Lactobacillus GG induced NO production. p42/44 MAP-kinase inhibitor PD98059, dexamethasone and cyclosporine A inhibited partially iNOS protein expression and NO formation in Lactobacillus GG, LPS and LTA treated cells. Ro 31-8220 (protein kinase C inhibitor) and SB203580 (p38 MAP-kinase inhibitor) had only a minor effect on NO production.

CONCLUSIONS

Lactobacillus GG induced NO production through iNOS pathway and the mechanisms mediating that process were very similar with those involved in LPS and LTA induced NO synthesis.

Tetrahydrobiopterin deficiency increases neuronal vulnerability to hypoxia

Tetrahydrobiopterin (BH4) is an essential co-factor for nitric oxide synthases (NOS). The aim of the present work was to study whether BH4 deficiency affects the vulnerability of neurones in primary culture to hypoxia. Intracellular BH4 levels were depleted by pre-incubating neurones with 5 mm 2,4-diamino-6-hydroxypyrimidine (DAHP) for 18 h, after which cells were exposed for 1 h to normoxic or hypoxic conditions. Our results showed that whereas neurones were resistant to hypoxia-induced cellular damage, BH4 deficiency in neurones led to oxidative stress, mitochondrial depolarization, ATP depletion and necrosis after 1 h of hypoxia. Indeed, hypoxia specifically inhibited mitochondrial complex IV activity in BH4-deficient neurones. All these effects were counteracted when neuronal BH4 levels were restored by incubating cells with exogenous BH4 during the hypoxic period. Moreover, hypoxia-induced damage in BH4-deficient neurones was prevented when Nomega-nitro-l-arginine monomethyl ester (NAME), haemoglobin or superoxide dismutase plus catalase were present during the hypoxic period, suggesting that peroxynitrite might be involved in the process. In fact, BH4 deficiency elicited neuronal NO dysfunction, resulting in an increase in peroxynitrite generation by cells, as shown by the enhancement in tyrosine nitration; this was prevented by supplements of BH4, NAME, haemoglobin or superoxide dismutase plus catalase during hypoxia. Our results suggest that BH4 deficiency converts neuronal NOS into an efficient peroxynitrite synthase, which is responsible for the increase in neuronal vulnerability to hypoxia-induced mitochondrial damage and necrosis.

cAMP inhibits cytokine-induced biosynthesis of tetrahydrobiopterin in human umbilical vein endothelial cells

We studied the effects of cAMP on cytokine (interferon-gamma plus tumor necrosis factor-alpha)-induced stimulation of tetrahydrobiopterin (BH4) synthesis in human umbilical vein endothelial cells (HUVEC). The cytokine mixture caused a marked increase in the biosynthesis and release of BH4 by HUVEC. Dibutyryl-cAMP produced a dose-dependent inhibition of this cytokine-induced stimulation of synthesis and release of BH4 by these cells. 8-Bromo-cAMP also caused a significant inhibition, although the effects were less marked than those of dibutyryl-cAMP. Both forskolin and the stable analog of prostacyclin, iloprost, caused cAMP accumulation and a concomitant diminution of the cytokine-induced BH4 synthesis in HUVEC. Dibutyryl-cAMP and iloprost also significantly inhibited the cytokine-induced stimulation of GTP cyclohydrolase I (GCHI) activity and mRNA production. We concluded that the suppression by the cAMP messenger system of cytokine-induced stimulation of synthesis and release of BH4 by HUVEC can be attributed to the inhibition of the activity of GCHI, the rate-limiting enzyme in BH4 biosynthetic pathway, in HUVEC. The data also suggest that the cAMP-mediated reduction in the GCHI mRNA level may at least partially explain the decline in GCHI activity. It is reasoned that under inflammatory conditions, cAMP-elevating agents such as prostacyclin exert regulatory effects on circulation by inhibiting cytokine-induced synthesis and release of BH4 by HUVEC.

Mechanisms of synergistic cytokine-induced nitric oxide production in human alveolar epithelial cells

Nitric oxide (NO) derived from inducible NO synthase (iNOS) at sites of inflammation is closely related to host defense against infection and airway inflammation. Cytokines are known to stimulate NO production in human alveolar epithelial cells in a synergistic (nonlinear or nonadditive) manner. The mechanism of this synergy is not known. We measured the activation of the transcription factor NF-kappaB, the iNOS protein, and NO production in A549 monolayers (human alveolar epithelial cell line) in response to different combinations of IL-1beta, INF-gamma, and TNF-alpha (100 ng/ml), and the cofactors FMN, FAD, and BH4. We found that both IL-1beta and TNF-alpha could independently activate cytosolic NF-kappaB, direct its translocation into the nucleus, and induce iNOS monomer synthesis. In addition, different combinations of cytokines produced synergistic amounts of iNOS monomers. Exogenous BH4 (0.1 microM) had no impact on NO production induced by cytokine combinations that included IL-1beta, but significantly enhanced NO production in the presence of INF-gamma and TNF-alpha, and allowed TNF-alpha independently to produce NO. We conclude that there are at least three mechanisms of synergistic cytokine-induced NO production: (1) the biosynthesis of iNOS monomer due to nonlinear interactions by transcription factors, (2) synergistic cytosolic activation of NF-kappaB, and (3) parallel biosynthesis of BH4 in the presence of cytokine combinations that include IL-1beta.

Inducible nitric oxide synthase expression

This unit contains three protocols that can be used to determine iNOS expression in mouse macrophage-like cells, RAW 264.7, by measuring end product, protein, and mRNA. A bacterial product, lipopolysaccharide (LPS), stimulates iNOS expression in RAW 264.7 cells.

Posttranscriptional regulation of human iNOS by the NO/cGMP pathway

Nitric oxide (NO) and cGMP may exert positive or negative effects on inducible NO synthase (iNOS) expression. We have explored the influence of the NO/cGMP pathway on iNOS levels in human mesangial cells. Inhibition of NOS activity during an 8-h stimulation with IL-1beta plus tumor necrosis factor (TNF)-alpha reduced iNOS levels, while NO donors amplified iNOS induction threefold. However, time-course studies revealed a subsequent inhibitory effect of NO donors on iNOS protein and mRNA levels. This suggests that NO may contribute both to iNOS induction and downregulation. Soluble guanylyl cyclase (sGC) activation may be involved in these effects. Inhibition of sGC attenuated IL-1beta/TNF-alpha-elicited iNOS induction and reduced NO-driven amplification. Interestingly, cGMP analogs also modulated iNOS protein and mRNA levels in a biphasic manner. Inhibition of transcription unveiled a negative posttranscriptional modulation of the iNOS transcript by NO and cGMP at late times of induction. Supplementation with 8-bromo-cGMP (8-BrcGMP) reduced iNOS mRNA stability by 50%. These observations evidence a complex feedback regulation of iNOS expression, in which posttranscriptional mechanisms may play an important role.

Parallel induction of nitric oxide and tetrahydrobiopterin synthesis in alveolar macrophages

BACKGROUND

Nitric oxide (NO) and an essential cofactor for both constitutive and inducible NO synthase (NOS) activity, tetrahydrobiopterin (6R-L-erythro-1',2'-dihydroxypropyl-2-amino-4-hydroxy-5,6,7,8-tetrahydropteridine; BH4), are thought to be important modulators of function in normal and inflamed airways. However, the exact pathologic roles of NO and BH4 remain obscure. Even less is known about the effects of cytokines on alveolar macrophages.

OBJECTIVE

This study was designed to determine whether NO and BH4 are induced by cytokines in mouse alveolar macrophages and to investigate whether NO synthesis is affected by changes in intracellular BH4 levels in alveolar macrophages.

METHODS

We compared the induction by lipopolysaccharide (LPS), interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha), and interleukin-2 (IL-2) of NO production and BH4 synthesis in alveolar macrophages. To determine whether NO synthesis is affected by changes in intracellular BH4 levels in alveolar macrophages, we used inhibitors of BH4 biosynthesis.

RESULTS

Activation of alveolar macrophages induced parallel increases in NO and intracellular BH4 levels, although induction of the latter appears to be somewhat more sensitive than that of the latter to diverse cytokines. Inducible NO production in alveolar macrophages was blocked by inhibitors of BH4 biosynthesis. IL-2, an important component of the immunomodulatory system, was only a weak activator of alveolar macrophages by itself but potently synergized with IFN-gamma to stimulate the production of both NO and BH4.

CONCLUSION

Our results suggest that BH4 synthesis in alveolar macrophages is a potential target for therapeutic intervention in airway inflammatory diseases, such as asthma, cystic fibrosis, and acute bronchial infections whose pathology may be mediated by overproduction of NO.

Expression of nitric oxide synthase isoforms in pregnant human myometrium

1. Endogenous nitric oxide has been proposed to play a role in the control of myometrial contractility in pregnancy. In this study, the expression, localisation and regulation of nitric oxide synthase (NOS) isoforms have been examined in human pregnant myometrium and cultured human myometrial smooth muscle cells, by immunoblotting, immunohistochemistry and reverse transcription-polymerase chain reaction. 2. Immunoblotting of extracts from freshly isolated myometrial tissue, affinity-enriched for NOS proteins by precipitation with ADP-sepharose, revealed expression of endothelial NOS (eNOS or NOS3) in tissues from preterm, term non-labour and active labour at term. Inducible NOS (iNOS or NOS2) and neuronal NOS (nNOS or NOS1) proteins were not detected at any stage of pregnancy. 3. Immunohistochemical detection showed that expression of eNOS protein was restricted to the endothelium of the myometrial vasculature, with no staining detected in myometrial smooth muscle cells. 4. Messenger RNA for all three NOS isoforms was detected, although iNOS and nNOS mRNAs were detectable only with high cycle number, implying a low copy number. 5. NOS isoforms were not detectable in human myometrial smooth muscle cells cultured from term non-labour pregnancies. Cytokine stimulation of cultured myometrial cells did not induce iNOS expression or nitrite accumulation in the culture medium, although both iNOS protein and nitrite release were detected in the human pulmonary epithelial cell line A549. 6. Levels of eNOS protein and of NOS mRNA expression were not correlated with gestational stage, suggesting that endogenously produced NO is not likely to be a modulator of myometrial tone during human pregnancy.

Dual effect of nitric oxide donors on cyclooxygenase-2 expression in human mesangial cells

Nitric oxide (NO) is emerging as a key regulator of gene expression, capable of playing either positive or negative roles. The results of this study indicate that NO exerts a dual effect on cyclooxygenase-2 (COX-2) expression in human mesangial cells (HMC). Treatment of HMC with NO synthase inhibitors attenuated interleukin-1beta (IL-1beta/tumor necrosis factor-alpha (TNF-alpha)-elicited COX-2 protein and mRNA expression, suggesting a positive role of endogenous NO on COX-2 induction. However, NO donors (sodium nitroprusside [SNP] and S-nitroso-N-acetylpenicillamine [SNAP]) amplified cytokine-elicited COX-2 expression at early time points of treatment (up to 8 h for mRNA and up to 24 h for protein expression), but were inhibitory at later times. Oligonucleotide decoy experiments confirmed the importance of nuclear factor kappaB (NF-kappaB) activation for COX-2 induction by IL-1beta/TNF-alpha. Treatment with N(G)-nitro-L-arginine methyl ester (L-NAME) did not affect initial activation of NF-kappaB by IL-1beta/TNF-alpha, but unveiled an inhibitory effect of NO generation on NF-kappaB activity after 4 h. In HMC supplemented with SNP, cytokine-induced NF-kappaB activation was potentiated at early times of induction (5 to 15 min), but inhibited at later times (1 to 4 h), suggesting a dual effect of NO donors on NF-kappaB activation. Interestingly, IkappaBalpha protein levels followed a reciprocal pattern of expression: IkappaBalpha levels were lower at early times of induction in NO donor-supplemented cells; however, after 1 h of treatment, IkappaBalpha levels became higher than in cells treated only with cytokines. In the presence of SNP, cytokine-elicited IkappaBalpha mRNA induction was initially delayed, but was amplified at later times. These changes in IkappaBalpha expression could contribute to the dual effects of NO donors on NF-kappaB activation and COX-2 expression in HMC.

Regulation of cyclooxygenase-2 expression in human mesangial cells--transcriptional inhibition by IL-13

Activated mesangial cells may play an important part in glomerulonephritis. Cytokines can modulate the release of prostanoids by human mesangial cells (HMC). We have investigated the effects of pro-inflammatory stimuli on COX-2 expression in HMC and its potential modulation by interleukin (IL)-13. HMC released increased amounts of prostaglandin E2 (PGE2) after treatment with several combinations of IL-1 beta, tumor necrosis factor (TNF)-alpha and/or lipopolysaccharide. Increases in PGE2 correlated with the induction of COX-2 protein expression. The accumulation of PGE2 elicited by a combination of IL-1 beta/TNF-alpha correlated closely with the temporal pattern of COX-2 protein expression, which reflected the induction of COX-2 mRNA. IL-13 inhibited IL-1 beta/TNF-alpha-elicited PGE2 production, as well as COX-2 protein and mRNA expression in a concentration-dependent fashion. With 50 ng.mL-1 IL-13 these parameters were inhibited by 90, 80 and 84%, respectively. In HMC transfected with the 5' regulatory region of the COX-2 gene, IL-13 suppressed cytokine-induced promoter activation. Our results suggest that COX-2 expression is a major target for IL-13-mediated abrogation of prostaglandin release by HMC and support that this process takes place by transcriptional inhibition of the COX-2 gene.

Coexpression of GTP cyclohydrolase I and inducible nitric oxide synthase mRNAs in mouse osteoblastic cells activated by proinflammatory cytokines

Proinflammatory cytokines, a combination of IL-1beta, TNF-alpha, and IFN-gamma, caused mRNA expression of GTP cyclohydrolase I (GTP-CH), the rate-limiting enzyme in tetrahydrobiopterin (BH4) biosynthesis, and of inducible nitric oxide synthase (iNOS) in a well-characterized osteoblastic clone MC3T3-E1 cell line. We found the expression of the GTP-CH gene in osteoblasts for the first time. The expression of GTP-CH and iNOS mRNAs was found to be maximal at 3 and 9 h, respectively. The expression of both genes elicited increases in BH4 and NO levels. Pharmacological studies using 2,4-diamino-6-hydroxypyrimidine, an inhibitor of GTP-CH activity, showed that BH4 is involved in the activity of iNOS, but not in the induction of iNOS mRNA. The results using an inhibitor of nuclear factor (NF)-kappaB and activating protein-1 (AP-1) activation suggested that coinduction of the two genes in response to cytokines occurred via activation of NF-kappaB and AP-1. In MC3T3-E1 cells BH4 and sepiapterin, producing BH4, could protect against apoptosis, i.e. the degradation of nuclear DNA in the cells, induced by NO derived from S-nitroso-N-acetyl-D-L-penicillamine. These results suggest that the induction of BH4 together with NO by proinflammatory cytokines could protect against NO-induced apoptosis in MC3T3-E1 cells.

Modulation of inducible nitric oxide synthase mRNA stability by tetrahydrobiopterin in vascular smooth muscle cells

Tetrahydrobiopterin (BH4) regulates inducible nitric oxide synthase (iNOS) as cofactor and allosteric effector. The present paper describes a novel function of BH4 in vascular smooth muscle cells (SMC). By varying BH4 levels with dicumarol (an inhibitor of BH4 synthesis) and sepiapterin (an exogenous source of co-factor), we investigated iNOS expression in activated rat aortic SMC. In sepiapterin-supplemented cells, iNOS protein levels were increased while in dicumarol-treated cells, iNOS levels were diminished. Time-kinetic experiments revealed that inhibition or supplementation of BH4 synthesis had no effects on iNOS induction or transcription rate. However, iNOS mRNA was present over a prolonged time in sepiapterin-supplemented SMC. Analysis of iNOS mRNA levels showed stable iNOS mRNA in sepiapterin-treated cells 8 hours after transcription inhibition, while in dicumarol-treated cells iNOS mRNA disappeared. The decrease of iNOS mRNA by dicumarol was abolished by sepiapterin. These data indicate that BH4 post-transcriptionally stabilizes iNOS mRNA in SMC. By this way BH4 modulates iNOS expression in the vascular system.

Nitric oxide production by cultured human aortic smooth muscle cells: stimulation by fluid flow

This study demonstrated that exposure of cultured human aortic smooth muscle cells (SMC) to fluid flow resulted in nitric oxide (NO) production, monitored by nitrite and guanosine 3',5'-cyclic monophosphate production. A rapid burst in nitrite production rate was followed by a more gradual increase throughout the period of flow exposure. Neither the initial burst nor the prolonged nitrite production was dependent on the level of shear stress in the range of 1.1-25 dyn/cm2. Repeated exposure to shear stress after a 30-min static period restimulated nitrite production similar to the initial burst. Ca(2+)-calmodulin antagonists blocked the initial burst in nitrite release. An inhibitor of nitric oxide synthase (NOS) blocked nitrite production, indicating that changes in nitrite reflect NO production. Treatment with dexamethasone or cycloheximide had no effect on nitrite production. Monoclonal antibodies directed against the inducible and endothelial NOS isoforms showed no immunoreactivity on Western blots, whereas monoclonal antibodies directed against the neuronal NOS gave specific products. These findings suggest that human aortic SMC express a constitutive neuronal NOS isoform, the enzymatic activity of which is modulated by flow.

Induction of tetrahydrobiopterin synthesis in human umbilical vein smooth muscle cells by inflammatory stimuli

Tetrahydrobiopterin (BH4) is an obligatory cofactor and regulator of nitric oxide synthases (NOS). We evaluated the biosynthesis of BH4 in human umbilical vein smooth muscle cells (HUVSMC). Trace amounts of BH4 were found intra- and extracellularly in untreated cells. When HUVSMC were activated by individual inflammatory stimuli (IL-1beta, TNFalpha, IFNgamma or LPS), both intra- and extracellular levels of BH4 increased significantly, with TNFalpha being the most potent single stimulus. Combined inflammatory cytokines synergized in the induction of an up to 600-fold increase of BH4 synthesis. Addition of LPS to the cytokine mixture led to a further increase of BH4 synthesis. Neopterin, a product of the first intermediate in BH4 biosynthesis, was also raised, but to a much lesser extent. The increase of BH4 synthesis was paralleled by an enhanced expression of isoform-1 (the only isoform coding for the active enzyme) of GTP cyclohydrolase I in cytokine treated cells. Our results show for the first time that BH4 biosynthesis is strongly induced by combinations of inflammatory stimuli in HUVSMC. The importance of BH4-dependent NO synthesis in HUVSMC needs, however, additional detailed studies.

Tetrahydrobiopterin modulates cyclooxygenase-2 expression in human mesangial cells

Tetrahydrobiopterin (BH4) biosynthetic pathways are stimulated under inflammatory conditions. The newly synthesized BH4 serves as a cofactor for optimal activity of inducible nitric oxide synthase (NOS2). In human mesangial cells (HMC), BH4 is also a limiting factor for NOS2 expression. In this study we show that BH4 availability can also play a modulatory role in the expression of cyclooxygenase 2 (COX-2) in HMC. Supplementing HMC with the BH4 donor sepiapterin potentiated IL-1beta/TNF-alpha-induced COX-2 expression by approximately 2-fold. This effect was abolished by methotrexate. In contrast, the NOS inhibitor L-NAME and the soluble guanylate cyclase inhibitor ODQ did not block sepiapterin amplification of COX-2 expression. Moreover, sepiapterin was found to modulate the tyrosine phosphorylation of several cellular substrates, an early event which occurred well before the induction of NOS2 could be evidenced. These findings suggest a role for BH4 in the modulation of mesangial cell responses to pro-inflammatory stimuli.

Inhalation of the nitric oxide synthase cofactor tetrahydrobiopterin in healthy volunteers

Pulmonary endothelial dysfunction is the hallmark of acute lung injury. Impaired pulmonary endothelial nitric oxide (NO) production in this event has been described. Tetrahydrobiopterin (BH4) is an essential cofactor for NO synthase and modulator of its activity. At high local concentrations, BH4 provokes local vasodilation in vivo in healthy individuals. At lower concentrations, BH4 selectively and locally restores disturbed NO-dependent vasodilation in patients with endothelial dysfunction. In this preliminary study, we therefore investigated the feasibility of BH4 inhalation in five healthy human volunteers. Inhalation of buffered, aqueous BH4-dihydrochloride solution was well tolerated; despite the buffer, BH4 stability was completely preserved. Resorption of inhaled BH4 was demonstrated by significantly increased BH4 levels in plasma and urine. Inhaled BH4 did not alter pulmonary function and had no effect on systemic hemodynamic values. Our data demonstrate that inhalation is a novel method for local BH4 administration, offering a basic therapeutic tool for investigation of restoration of impaired NO-dependent vasodilation due to pulmonary endothelial dysfunction.

Nitric oxide synthases and cardiac muscle. Autocrine and paracrine influences

The different cell types comprising cardiac muscle express one or more of the three isoforms (neuronal NOS, or nNOS; inducible NOS, or iNOS; and endothelial NOS, or eNOS) of nitric oxide synthase (NOS). nNOS is expressed in orthosympathetic nerve terminals and regulates the release of catecholamines in the heart. eNOS constitutively expressed in endothelial cells inhibits contractile tone and the proliferation of underlying vascular smooth muscle cells, inhibits platelet aggregation and monocyte adhesion, promotes diastolic relaxation, and decreases O2 consumption in cardiac muscle through paracrinally produced NO. eNOS is also constitutively expressed in cardiac myocytes from rodent and human species, where it autocrinally opposes the inotropic action of catecholamines after muscarinic cholinergic and beta-adrenergic receptor stimulation. iNOS gene transcription and protein expression are induced in all cell types after exposure to a variety of inflammatory cytokines. Aside from participating in the immune defense against intracellular microorganisms and viruses, the large amounts of NO produced autocrinally or paracrinally mediate the vasoplegia and myocardial depression characteristic of systemic immune stimulation and promote cell death through apoptosis. In cardiac myocytes, NO may regulate L-type calcium current and contraction through activation of cGMP-dependent protein kinase and cGMP-modulated phosphodiesterases. Other mechanisms independent of cGMP elevations may operate through interaction of NO with heme proteins, non-heme iron, or free thiol residues on target signaling proteins, enzymes, or ion channels. Given the multiplicity of NOS isoforms expressed in cardiac muscle and of the potential molecular targets for the NO produced, tight molecular regulation of NOS expression and activity at the transcriptional and posttranscriptional level appear to be needed to coordinate the many roles of NO in heart function in health and disease.

Characterization of a constitutive type III nitric oxide synthase in human U937 monocytic cells: stimulation by soluble CD23

The soluble cleavage fragment of the low-affinity immunoglobulin E (IgE) receptor/CD23 (sCD23 25000 MW) and antibodies directed against their receptors on monocytes, CD11b and CD11c, stimulate the production of nitric oxide (NO) by these cells and we have suggested that the enzyme involved could be related to the endothelial constitutive type III nitric oxide synthase (ecNOS). In the present work, we have analysed the characteristic properties of this NOS isoform in the model of the human promonocytic cells U937 By reverse-transcription polymerase chain reaction (RT-PCR), the presence of an mRNA coding for type III NOS was found in U937 cells and the corresponding protein was detected by immunofluorescence in permeabilized cells with a specific anti-ecNOS monoclonal antibody (mAb). Membrane extracts displayed a NOS activity dependent on the presence of calcium and calmodulin in the reaction medium and that was abrogated in the presence of EGTA. Recombinant soluble CD23 (25000 MW) was found to trigger an NO-dependent cGMP accumulation in these cells, which was abrogated by calcium chelators and inhibitors of the calcium/calmodulin complex. Moreover, sCD23 elicited a transient augmentation of intracytoplasmic free calcium concentration [Ca2+]i that was dependent on the presence of calcium in the external buffer and was prevented in the presence of EGTA, indicating that it was due to a calcium influx. In conclusion, human promonocytic cells such as U937 exhibit a functional type III NOS that can be stimulated by calcium-raising agents, such as sCD23.