Induced NO Synthesis Research Articles

Induction of nitric oxide synthase activity in pulmonary arteries from normoxic and chronically hypoxic rats.

Chronic hypoxia has recently been shown to upregulate inducible nitric oxide synthase (iNOS) gene expression in rat lung. In the present study, we questioned whether induction of NO synthesis could alter the reactivity of pulmonary arteries (PA) from chronically hypoxic (CH) rats. Dose-response curves to phenylephrine (PE) 10(-9) to 5 x 10(-6) M) were examined in PA rings as well as response to L-arginine analogues in isolated lungs from CH or normoxic (N) rats after various incubation times. Although maximal contraction to PE did not differ in PA from CH rats compared to N rats at time 0 (361 +/- 53 vs 506 +/- 52 mg, respectively), it was markedly decreased after prolonged incubation (149 +/- 28 vs 386 +/- 47 mg, respectively, at 4 h; p < 0.001). This phenomenon persisted after endothelial-denudation, but was reversed by NG-monomethyl-L-arginine (L-NMMA) (5 x 10(-4) M) and prevented by actinomycin D (2 x 10(-6) M). In contrast, maximal contraction to PE in aorta from CH rats was similar at time 0 and 4 h. After a short incubation, PA contraction to L-NMMA was greater in CH than in N rats (96 +/- 17 vs 33 +/- 9 mg at 90 min; p < 0.05), was abolished after endothelial denudation, but persisted in CH rats in the presence of calmidazolium (5 x 10(-4) M). At 4 h, contraction to L-NMMA was abolished in endothelium-denuded PA from N rats but only attenuated in those from CH rats. In salt solution perfused lungs, L-NMMA added 30 or 90 min after isolation did not alter baseline pressure in N rats but caused its increase in CH rats. Whereas iNOS messenger ribonucleic acid (mRNA) was detectable by reverse-transcriptase polymerase chain reaction in the PA wall of N or CH rats after 4 h of incubation, it was absent in both at the time of isolation. In contrast, there was evidence of iNOS mRNA in lungs from CH rats at the time of isolation but no signal in those from N rats. In conclusion, there is induction of nitric oxide synthase activity in pulmonary arteries from normoxic and chronically hypoxic rats after prolonged incubation, but this effect is more pronounced in pulmonary arteries from chronically hypoxic rats.

Murine α-Macroglobulins Demonstrate Divergent Activities as Neutralizers of Transforming Growth Factor-β and as Inducers of Nitric Oxide Synthesis: A POSSIBLE MECHANISM FOR THE ENDOTOXIN INSENSITIVITY OF THE α2-MACROGLOBULIN GENE KNOCK-OUT MOUSE

alpha2-Macroglobulin null mice demonstrate increased resistance to endotoxin challenge (Umans, L., Serneels, L., Overbergh, L., Van Leuven, F., and Van den Berghe, H. (1995) J. Biol. Chem. 270, 19778-19785). We hypothesized that this phenotype might reflect the function of murine alpha2M (malpha2M) as a neutralizer of transforming growth factor-beta (TGF-beta) and inducer of nitric oxide synthesis in vivo. When incubated with wild-type mouse plasma, TGF-beta1 and TGF-beta2 bound only to malpha2M. Alternative TGF-beta-binding proteins were not detected in plasma from alpha2M(-/-) mice. Wild-type mouse plasma, but not plasma from alpha2M(-/-) mice, inhibited TGF-beta1 binding to TGF-beta receptors on fibroblasts. Purified malpha2M bound TGF-beta1 and TGF-beta2 with similar affinity; the KD values were 28 +/- 4 and 33 +/- 4 nM, respectively. Murinoglobulin, the second murine alpha-macroglobulin, bound both TGF-beta isoforms with 30-fold lower affinity. Malpha2M counteracted the activities of TGF-beta1 and TGF-beta2 in an endothelial cell growth assay. Malpha2M also induced NO synthesis when incubated with RAW 264.7 cells, an activity which probably results from the neutralization of autocrine TGF-beta activity. Human alpha2M induced NO synthesis comparably to malpha2M; however, MUG had no effect. These studies demonstrate that the ability to neutralize TGF-beta is a property of malpha2M, which is not redundant in the murine alpha-macroglobulin family or in murine plasma. Malpha2M is the only murine alpha-macroglobulin that promotes NO synthesis. The absence of malpha2M, in alpha2M(-/-) mice, may allow TGF-beta to more efficiently suppress excessive iNOS expression following endotoxin challenge.

Effects of nitric oxide (NO) synthesis inhibition on antitumor responses during interleukin-2 (IL-2) treatment of mice.

ObjectivesTo evaluate if L-arginine: NO pathway is activated in tumor tissues during IL-2 therapy and to evaluate whether IL-2 induced NO synthesis represents an antitumor effector muchanism or an inhibitory factor against therapeutic effects of IL-2.MethodsFour groups[untreated control, NG-monomethyl-L-arginine(MLA) therapy only, IL-2 therapy only, IL-2/MLA therapy groups] of BALB/c mice were injected intraperitoneally with 2×106 Meth A tumor cells on day 0. MLA was administered subcutaneously with Alzet continuous infusion pumps on day 2. IL-2 therapy (180,000 IU s.c. every 12h for 5 days) was started on day 3. NO production within ascites tumors was assessed by measuring nitrite concentrations in cultures of ascites cells harvested on day 8. Survival and the rate of body weight increment of the mice were measured to evaluate therapeutic responses. Daily urinary nitrate excretion was monitored to demonstrate the effectiveness of MLA in inhibiting NO synthesis.ResultsNitrite production in supernatants of Meth A ascites cell cultures was 63±14 μM in IL-2 treated mice and 3.2±1.5 μM in untreated controls (p < 0.001). MLA prevented the IL-2 therapy induced increase in nitrite production. IL-2 therapy did not decrease the rate of body weight increment and marginally prolonged mean survival to 18.2 days, compared to 16.6 days in control mice (p=0.255). MLA administration decreased the rate of body weight increment and prolonged mean survival of IL-2 treated mice (21.8 days, p=0.001 versus IL-2 alone). Interestingly, the MLA treatment increased the rate of body weight increment and diminished the survival of control mice to 11.6 days (p=0,003). MLA administration via Alzet continuous infusion pumps achieved appoximately 60% suppression of urinary nitrate excretion by control mice. Subcutaneous IL-2 treatment strongly induced nitric oxide synthesis (up to 3.5 μmoles of urinary nitrate/mouse/day). MLA also effectively suppressed IL-2 induced NO production.ConclusionL-arginine: NO pathway can be activated in malignant ascites, by IL-2 therapy and NO synthesis functions as an inhibitory mechanism against IL-2 induced anti-tumor effects.

Interleukin-13 inhibits inducible nitric oxide synthase expression in human mesangial cells.

The synthesis of nitric oxide in inflammatory situations requires the expression of an inducible isoform of nitric oxide synthase (iNOS). Human mesangial cells (HMC) express an iNOS enzyme after exposure to multiple co-stimuli. In this study we have observed that while tumour necrosis factor-alpha, interleukin (IL)-1 beta, interferon-gamma and bacterial lipopolysaccharide (LPS) were unable to significantly induce NO synthesis when used alone, they induced an evident stimulation of NO synthesis when used in various combinations. A mixture of the three cytokines (CM) and LPS resulted in a 10-15-fold stimulation of NO synthesis over control values which started to be significant after 16 h. The addition of IL-13, a cytokine with anti-inflammatory properties, inhibited CM/LPS-induced NO synthesis in a concentration-dependent manner. A marked inhibitory effect (60-65%) could be observed when HMC were treated with IL-13 (10 ng/ml) 24 h before, at the same time as, or even 4 h after the addition of CM/LPS. This inhibitory effect was still significant (25%) when IL-13 was added 16 h after CM/LPS. Northern analysis showed that IL-13-mediated iNOS inhibition was closely correlated with the suppression of iNOS mRNA expression. These results identify IL-13 as a powerful regulatory tool for the inhibition of NO synthesis in human cells, a property which may be pathophysiologically relevant in NO-related inflammatory processes.

Induction of NO synthesis in macrophages by Newcastle disease virus is associated with activation of nuclear factor-kappa B.

Newcastle disease virus (NDV) has received much attention recently because of its non-specific immune stimulating potential and its various anti-tumor activities. Here we describe that NDV induces synthesis of NO and causes an activation of nuclear factor-kappa B (NF-kappa B) in murine macrophages. These reactions were part of an activation process which included also stimulation of adenosine deaminase and inhibition of 5'-nucleotidase. NDV-mediated NO synthesis and NF-kappa B activation were blocked by an antioxidant (butylated hydroxyanisole), by an inhibitor of protein tyrosine kinase (genistein) and of protein kinase A (H-89), but not by an inhibitor of protein kinase C (staurosporin). These data suggest that signalling requirements of NF-kappa B activation and NO production in NDV-treated macrophages are similar.

A central role for IL-1 beta in the in vitro and in vivo regulation of hepatic inducible nitric oxide synthase. IL-1 beta induces hepatic nitric oxide synthesis.

We have previously demonstrated that high levels of inducible nitric oxide synthase (iNOS) expression in hepatocytes required a combination of LPS and TNF-alpha, IL-1 beta, and IFN-gamma. The need for such a complex regulatory system seemed unwarranted based on the importance of NO in the liver. Therefore, we investigated whether individual cytokines could induce NO synthesis in hepatocytes and characterized some of the mechanisms involved. Rat hepatocytes were stimulated in vitro with escalating doses of TNF-alpha, IL-1 beta, or IFN-gamma. Only IL-1 beta induced high levels of iNOS mRNA and corresponding NO2- + NO3- production, and dexamethasone and cycloheximide blocked a majority of this response. Nuclear run-on experiments revealed that IL-1 beta upregulated iNOS gene transcription. IL-1 receptor antagonist protein (IL-1ra) competitively inhibited IL-1 beta-stimulated NO synthesis, implying activation through a cell-specific receptor. Rats injected with both LPS and IL-1ra showed decreased hepatic iNOS mRNA and plasma NO2- + NO3- compared with rats given LPS alone, indicating that IL-1 beta plays a role in regulating iNOS expression within the liver in vivo during endotoxemia. The soluble TNF receptor antagonist, PEG-(rsTNF-RI)2, also suppressed hepatic iNOS mRNA levels and plasma NO2- + NO3- increases, supporting a role for this cytokine in LPS-induced iNOS expression. Finally, IL-1 beta at high doses also induced iNOS mRNA and significant NO2- + NO3- production in cultures of primary human hepatocytes. These data indicate an important role for IL-1 beta in the regulation of hepatic NO synthesis.

Products of γ-tocopherol reaction with NO 2 and their formation in rat insulinoma (RINm5F) cells

γ-Tocopherol, commonly found in seed oils, is the major tocopherol in the U.S. diet, is superior to α-tocopherol in preventing neoplastic transformation, and demonstrates unique reactivity toward NO 2. This article describes the products of reaction between γ-tocopherol and low concentrations of gaseous nitrogen dioxide (NO 2), as well as their endogenous formation in NO-producing RINm5F cells. γ-Tocopherol in hexane reacts with NO 2 to yield two products identified as 2,7,8-trimethyl-2(4,8,12-trimethyltridecyl)-5,6-chromaquinone, “tocored,” and 2,7,8 trimethyl-2(4,8,12-trimethyldecyl) 5-nitro, 6-chromanol, “tocoyellow.” Physical data for these two compounds and reaction characteristics are described. The formation of tocored is consistent with a proposed mechanism of γ-tocopherol-mediated reduction of NO 2 to NO involving initial reaction by NO 2 at the C-5 position to form an intermediate nitrite ester tocopheryl radical, which then reacts internally to release NO and form 5,6 epoxy γ-tocopherol. Tautomerization and further oxidation of the latter intermediate by NO 2 yields tocored as the main product observed. The reaction of γ-tocopherol with NO 2 to form NO occurs independently of light, whereas a-tocopherol requires light to generate NO from NO 2. γ-Tocopherol and aminoguanidine, an NO synthase inhibitor, were superior to α-tocopherol in preventing RINm5F cell toxicity induced by Interleukin-1β (IL-1β). Both tocored and tocoyellow were observed to form in RINm5F cells loaded with γ-tocopherol and producing NO constitutively, although a consistent increase in these products as a result of induced NO synthesis was not observed.

Interleukin 1-induced production of nitric oxide inhibits benzenetriol-mediated oxidative injury in rat hepatocytes

Background & Aims: Nitric oxide modifies free radical-mediated cell processes in multiple in vivo and in vitro systems. The aim of this study was to determine the role of hepatocyte production of NO in oxidative injury. Methods: Rat hepatocytes in primary culture were incubated with 1,2,3-benzenetriol, a source of superoxide. Interleukin (IL) 1 was added to induce NO synthesis. Injury was determined by aspartate aminotransferase (AST), malondialdehyde (MDA), and glutathione (GSH) levels. Results: Benzenetriol-induced injury increased AST and MDA levels and decreased GSH levels in control and IL-1-treated cells. Inhibition of NO synthesis in IL-1-treated cells significantly increased AST and MDA production while enhancing GSH depletion. In the presence of superoxide dismutase or S-nitroso-albumin, an exogenous source of NO, injury was decreased or abolished. NO production was significantly increased with oxidative stress. In benzenetriol-induced injury in IL-1-stimulated hepatocytes, reverse-transcription polymerase chain reaction showed significantly increased levels of inducible NO synthase messenger RNA, whereas immunoblot analysis showed similarly increased levels of inducible NO synthase protein. Conclusions: In this rat hepatocyte model of IL-1/benzenetriol-mediated injury, NO, derived from endogenous synthesis or an exogenous donor, is protective. Oxidative stress may have a role in the transcriptional control of NO synthesis.

Nitric oxide is a mediator of the decrease in cytochrome P450-dependent metabolism caused by immunostimulants.

Bacterial lipopolysaccharide (LPS) and a diverse array of other immunostimulants and cytokines suppress the metabolism of endogenous and exogenous substances by reducing activity of the hepatic cytochrome P450 mixed-function oxidase system. Although this effect of immunostimulants was first described almost 40 yr ago, the mechanism is obscure. Immunostimulants are now known to cause NO overproduction by cells via induction of nitric oxide synthase. We have investigated whether NO overproduction is involved in suppressing hepatic metabolism by LPS. In vitro treatment of hepatic microsomes with NO, produced by chemical decomposition of 3-morpholinosydnonimine or by nitric oxide synthase, substantially suppressed cytochrome P450-dependent oxygenation reactions. This effect of NO was seen with hepatic microsomes prepared from two species (rat and chicken) and after exposure to chemicals that induce distinct molecular isoforms of cytochromes P450 (beta-naphthoflavone, 3-methylcholanthrene, and phenobarbital). Spectral studies indicate that NO reacts in vitro with both Fe(2+)- and Fe(3+)-hemes in microsomal cytochromes P450. In vivo, LPS diminished the phenobarbital-induced dealkylation of 7-pentoxyresorufin by rat liver microsomes and reduced the apparent P450 content as measured by CO binding. These LPS effects were associated with induction of NO synthesis; LPS-induced NO synthesis showed a strong positive correlation with the severity of cytochrome P450 inhibition. The decrease in both hepatic microsomal P450 activity and CO binding caused by LPS was largely prevented by the selective NO synthase inhibitor N omega-nitro-L-arginine methyl ester. Our findings implicate NO over-production as a major factor mediating the suppression of hepatic metabolism by immunostimulants such as LPS.

GTP Cyclohydrolase I mRNA Is Induced by LPS in Vascular Smooth Muscle: Characterization, Sequence and Relationship to Nitric Oxide Synthase

GTP cyclohydrolase I (GTPCH) is the first and rate-limiting enzyme for the synthesis of tetrahydrobiopterin (BH4), a cofactor of nitric oxide synthase (NOS). As the induction of NO synthesis by immunostimulants in vascular smooth muscle (VSM) cells requires de novo synthesis of BH4, we investigated whether immunostimulants enhance the expression of GTPCH mRNA. GTPCH mRNA and BH4 were measured in rat VSM cells after exposure to bacterial lipopolysaccharide (LPS) in combination with interferon-γ(IFN). Reverse transcription polymerase chain reaction (RT-PCR) was used to amplify a predicted 372 bp fragment of GTPCH mRNA, deduced from the known nucleotide sequence of rat liver GTPCH cDNA. Dideoxynucleotide sequencing of the PCR fragment revealed 100% identity between LPS/IFN-induced GTPCH mRNA of VSM and the constitutive GTPCH mRNA of liver. Although BH4 was below our limit of detection in untreated VSM, low levels of GTPCH mRNA were detectable. LPS/IFN treatment triggered the appearance of BH4 and markedly increased GTPCH mRNA. Induction of GTPCH mRNA was apparent by 2 h, peaked at 4 h, and was sustained at high levels for at least 24 h. Induction of GTPCH mRNA by LPS/IFN was substantially enhanced by cycloheximide,suggesting that mRNA levels are depressed by a labile protein. Measurement of LPS/IFN-induced NOS mRNA by RT-PCR, demonstrated a timecourse of induction which mirrors that of GTPCH. Similarly, the timecourse of appearance of cytosolic NOS activity following exposure of VSM to LPS/IFN paralleled that of the increase in BH4 content. Our studies demonstrate that immunostimulants co-induce NOS and GTPCH gene expression: both events are necessary for induction of NO synthesis by VSM.

Differential regulation of inducible nitric oxide synthase by fibroblast growth factors and transforming growth factor beta in bovine retinal pigmented epithelial cells: inverse correlation with cellular proliferation.

Bovine retinal pigmented epithelial (RPE) cells express, after activation with interferon gamma (IFN-gamma) and lipopolysaccharide (LPS), an inducible nitric oxide synthase (NOS). Experiments were done to investigate the effects of the transforming growth factor beta 1, epidermal growth factor, and fibroblast growth factors (FGFs), which are abundant in the retina, on NOS activity. Transforming growth factor beta 1 slightly increases the production of nitrite, an oxidation product of NO, induced by LPS plus IFN-gamma, whereas acidic and basic FGFs markedly inhibit the nitrite release due to LPS/IFN-gamma in a concentration-dependent manner, and epidermal growth factor did not modify LPS/IFN-gamma-induced NOS activity. The growth factors alone did not stimulate nitrite release. We have attempted to elucidate the mechanism of FGF inhibition. Results with heparin, suramin, and tyrphostin suggest involvement of the high-affinity receptor for FGF in its inhibition of LPS/IFN-gamma-stimulated NOS activity. Continued stimulation of RPE cells with LPS/IFN-gamma was essential for the induction of NO synthesis, and maximal inhibition was obtained when FGF was present during stimulation with LPS/IFN-gamma, suggesting that FGF inhibits NOS induction. Furthermore, an antiproliferative action of NO was demonstrated by an inverse correlation between the amounts of nitrite or citrulline produced in response to different stimuli (LPS/IFN-gamma or LPS/IFN-gamma with growth factors) and the level of cellular proliferation. Similar inhibition of growth was obtained when RPE cells were incubated with an NO donor, sydnonimide. Because NO acts as a cytotoxic compound in the retina, FGF, by inhibiting the induction of NOS in RPE cells, may have beneficial effects in protecting the retina from cytokine and endotoxin-mediated tissue damage.

Cholesterol Enrichment of Arterial Smooth Muscle Cells Up Regulates Cytokine Induced Nitric Oxide Synthesis

Endothelium-derived relaxing factor/nitric oxide (EDRF/NO) is produced by the vascular wall and is a key modulator of vascular tone and blood pressure. NO is also produced by vascular smooth muscle (VSMC) where it can inhibit proliferation. Since cytokine-activated VSMC proliferation is a major event in the development of atherosclerosis, we investigated the influence of cholesterol (CE)-enrichment of VSMC on cytokine-induced NO synthesis. Treatment of VSMC with native LDL for one week did not promote CE-accretion or alter NO production following exposure to endotoxin (LPS). In contrast, CE-enrichment by cationized LDL augmented LPS-induction of NO synthesis 2-5-fold. While TNF-α promoted little NO synthesis in control VSMC, it was very potent after CE-enrichment. Similarly, CE-enrichment augmented IL-lα-induced NO synthesis. However, CE-enrichment did not affect the synergistic induction of NO synthesis by cytokines in combination with IFN-γ. Our findings suggest that CE-enrichment of VSMC upregulates signal transduction pathways which mediate cytokine and LPS induction of NO synthase activity.