Inducible nitric oxide synthase and nitric oxide production by oligodendrocytes.

It has been previously demonstrated that microglia and astrocytes produce micromolar amounts of nitric oxide in vitro. In this study, we demonstrate that primary rat oligodendrocytes can be stimulated to produce iNOS mRNA as detected by Northern blot and in situ hybridization analysis and a 131-kDa iNOS protein by Western blot analysis; protein was also detected in cells by single- and double-label immunohistochemistry for iNOS and the oligodendrocyte-specific marker CNPase. NO/NOS are produced as a consequence of activation of the gene encoding the inducible nitric oxide synthase as determined by inhibition with actinomycin D and cyclohexamide. The iNOS is functional, leading to calcium/calmodulin-independent NO production in these in vitro cultures.

Inhibitors of the proteasome pathway interfere with induction of nitric oxide synthase in macrophages by blocking activation of transcription factor NF-kappa B.

The objective of this study was to elucidate the role of the proteasome pathway or multicatalytic proteinase complex in the induction of immunologic nitric oxide (NO) synthase (iNOS) in rat alveolar macrophages activated by lipopolysaccharide. Macrophages were incubated in the presence of lipopolysaccharide plus test agent for up to 24 hr. Culture media were analyzed for accumulation of stable oxidation products of NO (NO2- + N03-, designated as NOX-), cellular RNA was extracted for determination of iNOS mRNA levels by Northern blot analysis, and nuclear extracts were prepared for determination of NF-kappa B by electrophoretic mobility-shift assay. Inhibitors of calpain (alpha-N-acetyl-Leu-Leu-norleucinal; N-benzyloxycarbonyl-Leu-leucinal) and the proteasome (N-benzyloxycarbonyl-Ile-Glu-(O-t-Bu)-Ala-leucinal) markedly inhibited or abolished the induction of iNOS in macrophages. The proteinase inhibitors interfered with lipopolysaccharide-induced NOX- production by macrophages, and this effect was accompanied by comparable interference with the appearance of both iNOS mRNA and NF-kappa B. Calpain inhibitors elicited effects at concentrations of 1-100 microM, whereas the proteasome inhibitor was 1000-fold more potent, producing significant inhibitory effects at 1 nM. The present findings indicate that the proteasome pathway is essential for lipopolysaccharide-induced expression of the iNOS gene in rat alveolar macrophages. Furthermore, the data support the view that the proteasome pathway is directly involved in promoting the activation of NF-kappa B and that the induction of iNOS by lipopolysaccharide involves the transcriptional action of NF-kappaB.

Serine and cysteine proteinase inhibitors prevent nitric oxide production by activated macrophages by interfering with transcription of the inducible NO synthase gene.

The objective of this study was to ascertain the mechanism by which serine and cysteine proteinase inhibitors interfere with production of NO by LPS-activated rat alveolar macrophages. Macrophages were incubated in the presence of LPS+ test agent for 24 hr. Culture media were analyzed for NOX- accumulation, harvested cells were assayed for iNOS activity, and cellular RNA was extracted for determination of iNOS mRNA by Northern blot analysis. TPCK, TLCK, calpain inhibitor 1 (CPI-1) and calpain inhibitor 2 (CPI-2) each inhibited NOX- production and inducible iNOS expression in a concentration-dependent manner at 1-100 microM. TPCK and CPI-1 were about 10-fold more potent than TLCK and CPI-2, respectively. These data suggest that a chymotrypsin-like serine or cysteine proteinase is required for the LPS-inducible expression of the iNOS gene, perhaps by mechanisms involving activation of transcription factor NF-kappa B. Accordingly, a potent inhibitor of NF-kappa B activation whose action is attributed to inhibition of the chymotrypsin-like activity of the multicatalytic proteinase complex (MPC) was tested. Z-IE(O-t-Bu)A-Leucinal abolished NOX- production and inducible iNOS expression at 1 microM and showed over 50% inhibition at 10 nM. These observations indicate that inhibitors of MPC interfere with iNOS induction and provide strong evidence that MPC functions importantly in iNOS induction in macrophages.

Co-induction of arginase and nitric oxide synthase in murine macrophages activated by lipopolysaccharide.

In view of studies showing that not only nitric oxide synthase (NOS) activity but arginase activity is induced in rodent macrophages by lipopolysaccharide (LPS), the objective of this study was to investigate the co-induction of these two enzymes and to ascertain whether common mechanisms are involved. RAW 264.7 cells were activated by 2 micrograms LPS/ml and incubated for up to 48 hr. Inducible NOS (iNOS) and inducible arginase II (AII) activities were monitored, respectively, by measuring NO2-/NO3- accumulation in cell culture media and formation of urea (as CO2) from L-arginine by cell lysates. AII activity increased linearly up to at least 48 hr, whereas NO2-/NO3- formation reached a plateau well before 48 hr. Immunoprecipitation experiments revealed that AII accounted for 90-100% of arginase activity in LPS-activated macrophages. The inhibitor of NF-kappa B activation, pyrrolidine dithiocarbamate, inhibited the induction of iNOS but not AII. Moreover, whereas IFN-gamma caused iNOS induction, AII induction was nearly abolished by IFN-gamma, perhaps by inhibiting transcription of the AII gene. These observations indicate that co-induction of iNOS and AII occurs by distinct transcriptional mechanisms, AII induction could diminish NO production by decreasing L-arginine availability, and IFN-gamma can prevent AII induction.

Nonsteroidal anti-inflammatory drugs inhibit expression of the inducible nitric oxide synthase gene.

Increased nitric oxide production is associated with acute and chronic inflammatory processes. Accordingly, we tested the hypothesis that the therapeutic action of nonsteroidal anti-inflammatory drugs could be attributed at least in part to inhibition of excess nitric oxide production. We report here that sodium salicylate, aspirin, ibuprofen, and indomethacin markedly inhibited the appearance of the inducible inflammatory nitric oxide synthase in rat alveolar macrophages activated with lipopolysaccharide and interferon gamma. We attribute the mechanism of nitric oxide synthase inhibition by nonsteroidal anti-inflammatory drugs to pretranslational control of enzyme expression and not to direct inhibition of enzymatic activity. These observations indicate that the chronic anti-inflammatory action of nonsteroidal anti-inflammatory drugs may be due not only to inhibition of prostaglandin synthesis but also to inhibition of inducible nitric oxide synthase gene expression and nitric oxide synthesis.

Negative modulation of nitric oxide synthase by nitric oxide and nitroso compounds.

These observations clearly indicate that NO inhibits NOS activity and that nNOS and eNOS are more sensitive than iNOS to the inhibitory action of NO. Not only exogenously added NO but also enzymatically generated NO inhibits the activity of nNOS and eNOS. The mechanism by which NO inhibits NOS appears to involve the heme iron prosthetic group of NOS. Moreover, the oxidation state of the heme iron is critical in determining the magnitude of inhibition of NOS by NO. Conditions that favor the higher oxidation state of FeIII markedly increase the inhibitory action of NO, whereas conditions that favor the lower oxidation state of FeII markedly decrease the inhibitory action of NO. One of the cofactor roles of tetrahydrobiopterin may be to reduce the negative-feedback effect of NO on NOS by favoring the formation of the ferrous heme state in NOS. The inhibitory influence of NO on eNOS, albeit indirectly, was also observed in vascular endothelial cells, arterial rings, and in vivo in the perfused rabbit hindquarters vascular bed. Excess NO in the form of NO donor compounds inhibited the endothelium-dependent formation of EDRF/NO in response to endothelium-dependent vasorelaxants such as acetylcholine and bradykinin without influencing the relaxant effect of NO itself. These studies are consistent with the view that enzymatically generated NO may play an important negative-feedback regulatory role on eNOS, and therefore on vascular endothelial cell function. Several biological implications of a negative-feedback modulatory effect by NO on constitutive isoforms of NOS are evident. In nonadrenergic-noncholinergic transmission, in which NO is believed to be the principal inhibitory neurotransmitter (Sanders and Ward, 1992; Rand, 1992; Rajfer et al., 1992), NO may regulate its own synthesis, and therefore the neurotransmission process. Excess NO production may be undesirable because of the potential of NO or a reaction product of NO to elicit cytotoxic effects. Many extraneuronal factors could also contribute to decreasing the potentially cytotoxic actions of NO. For example, reduced hemoproteins such as hemoglobin, myoglobin, and/or their oxygen adducts could inactivate NO, as could superoxide anion generated in the vicinity of NO. In vascular endothelial cells either enzymatically generated NO or the presence of exogenously added NO in the form of nitrovasodilator drugs could diminish the vasodilator responses to endothelium-dependent relaxants and flow or shear stress. Although iNOS is less sensitive than either eNOS or nNOS to inhibition by NO, the generation of relatively large quantities of NO by iNOS within the confines of a cell may lead to a negative-feedback effect. The concomitant generation of superoxide anion by the same or adjacent cells could result in a diminished negative-feedback effect because of the rapid reaction between NO and superoxide anion to form peroxynitrite. Thus, NO production would increase and there would be increased peroxynitrite formation as well, which would result in enhanced cytotoxicity, provided that peroxynitrite is a cytotoxic species. Alternatively, iNOS may be conveniently insensitive to NO in order to allow for the generation of large quantities of NO for the purpose of producing cytotoxic effects.

Nitric oxide inhibits neuronal nitric oxide synthase by interacting with the heme prosthetic group. Role of tetrahydrobiopterin in modulating the inhibitory action of nitric oxide.

The objective of this study was to elucidate the mechanism by which nitric oxide (NO) inhibits NO synthase. Previous studies revealed that NO inhibits unpurified preparations of NO synthase. In the present study, the mechanism by which NO inhibits purified neuronal NO synthase from rat cerebellum was examined. The rate of L-citrulline formation from L-arginine was non-linear despite the presence of excess substrate and cofactors and was further inhibited by 30% by 200 units/ml superoxide dismutase. In contrast, 30 microM oxyhemoglobin increased NO synthase activity by 2-fold and made the reaction rate linear. These observations were consistent with the hypothesis that enzymatically generated NO inhibits NO synthase activity. Exogenous NO (0.1-10 microM) (but not NO2, nitrite, or nitrate) also inhibited NO synthase, and enzyme inhibition was not competitive with L-arginine. NO synthase inhibition by NO and other heme ligands supports the view that heme is involved in the catalytic activity of NO synthase. Oxyhemoglobin prevented but could not reverse enzyme inhibition by NO. NO synthase inhibition by NO was markedly diminished and reversed, however, by tetrahydrobiopterin (50 microM) or a tetrahydrobiopterin-regenerating system, and the latter made the reaction rate linear. In contrast, NO synthase inhibition by NO was markedly enhanced by heme oxidants (10 microM methylene blue; 3 microM ferricyanide), and these oxidants directly inhibited NO synthase activity. These observations suggest that NO interacts with enzyme-bound ferric heme to inhibit NO synthase activity. In support of this view, NO inhibited enzyme activity in the absence of turnover, when the heme iron is in the ferric state, and this inhibition was reversed by tetrahydrobiopterin. Therefore, the oxidation state of heme iron appears to be one important determinant for the inhibitory action of NO, and tetrahydrobiopterin may increase NO synthase activity by diminishing the inhibitory action of NO.

Inducible nitric oxide synthase from a rat alveolar macrophage cell line is inhibited by nitric oxide.

The objective of this study was to determine whether inducible nitric oxide (NO) synthase from a rat alveolar macrophage cell line (NR8383) activated by LPS plus IFN-gamma could be regulated by NO, one of the two products of the enzymatic reaction. This study was based on previous observations in this laboratory that NO is a negative feedback modulator of constitutive NO synthase from rat cerebellum. NO synthase activity was determined by monitoring the formation of 3H-L-citrulline from 3H-L-arginine in the presence of added cofactors. NO synthase catalyzed the conversion of L-arginine to equimolar quantities of NO and L-citrulline. NO and S-nitrosothiols inhibited NO synthase activity and this effect was enhanced by superoxide dismutase and attenuated by oxyhemoglobin. Nitrite and nitrate, the oxidation products of NO, as well as L-citrulline, the amino acid end-product, produced no significant effects on NO synthase activity. The inhibitory effect of NO on NO synthase appeared to be partially reversible upon addition of oxyhemoglobin. The inhibitory effect of NO was mimicked by other heme ligands including carbon monoxide, cyanide, and manganese-protoporphyrin IX. These observations indicate that (1) enzyme-bound heme plays a mechanistic role in the catalytic conversion of L-arginine to NO plus L-citrulline; (2) NO may function as a negative feedback modulator of inducible NO synthase by interacting with enzyme-bound heme; and (3) negative feedback modulation by NO may represent a mechanism by which the potentially toxic L-arginine-NO pathway in activated alveolar macrophages is turned off.

Negative feedback regulation of endothelial cell function by nitric oxide.

The objective of this study was to determine whether nitric oxide (NO) could function as a negative feedback modulator of endothelial cell function by inhibiting NO synthase in vascular endothelial cells. The rationale for this approach was a previous study from this laboratory, which revealed that NO inhibits neuronal NO synthase from rat cerebellum. In the present study, NO and NO-donor agents noncompetitively inhibited NO synthase derived from bovine aortic endothelial cells. Oxyhemoglobin blocked the inhibitory action of NO and by itself increased NO synthase activity. This finding suggests that NO acts as a negative feedback modulator of NO synthase. In intact aortic endothelial cells grown on microcarrier beads and perfused in a bioassay cascade system, pretreatment of cells with NO-donor agents caused a marked inhibition of endothelial NO biosynthesis in response to bradykinin and increased fluid shear or flow. When isolated bovine pulmonary arterial rings precontracted by phenylephrine were used, pretreatment of arterial rings with NO-donor agents diminished endothelium-dependent arterial relaxation involving the L-arginine-NO pathway without altering endothelium-independent relaxation to NO itself. On the basis of these studies, NO is suggested to play an important negative feedback regulatory role on endothelial NO synthase and, therefore, vascular endothelial cell function.

Oxidation of nitric oxide in aqueous solution to nitrite but not nitrate: comparison with enzymatically formed nitric oxide from L-arginine.

Nitric oxide (NO) in oxygen-containing aqueous solution has a short half-life that is often attributed to a rapid oxidation to both NO2- and NO3-. The chemical fate of NO in aqueous solution is often assumed to be the same as that in air, where NO is oxidized to NO2 followed by dimerization to N2O4. Water then reacts with N2O4 to form both NO2- and NO3-. We report here that NO in aqueous solution containing oxygen is oxidized primarily to NO2- with little or no formation of NO3-. In the presence of oxyhemoglobin or oxymyoglobin, however, NO and NO2- were oxidized completely to NO3-. Methemoglobin was inactive in this regard. The unpurified cytosolic fraction from rat cerebellum, which contains constitutive NO synthase activity, catalyzed the conversion of L-arginine primarily to NO3- (NO2-/NO3- ratio = 0.25). After chromatography on DEAE-Sephacel or affinity chromatography using 2',5'-ADP-Sepharose 4B, active fractions containing NO synthase activity catalyzed the conversion of L-arginine primarily to NO2- (NO2-/NO3- ratio = 5.6) or only to NO2-, respectively. Unpurified cytosol from activated rat alveolar macrophages catalyzed the conversion of L-arginine to NO2- without formation of NO3-. Addition of 30 microM oxyhemoglobin to all enzyme reaction mixtures resulted in the formation primarily of NO3- (NO2-/NO3- ratio = 0.09 to 0.20). Cyanide ion, which displaces NO2- from its binding sites on oxyhemoglobin, inhibited the formation of NO3-, thereby allowing NO2- to accumulate. These observations indicate clearly that the primary decomposition product of NO in aerobic aqueous solution is NO2- and that further oxidation to NO3- requires the presence of additional oxidizing species such as oxyhemoproteins.

A spectrophotometric assay for nitrate using NADPH oxidation by Aspergillus nitrate reductase.

An assay based on the oxidation of NADPH during the enzymatic conversion of nitrate to nitrate by Aspergillus nitrate reductase [EC 1.6.6.2] was developed for specific quantification of nitrate. This spectrophotometric method was used to measure nitrate present in human urine, human serum, and tissue culture medium. Used as a kinetic assay, the method exhibited (1) linearity over a range of 1.25 to 40 microM nitrate, (2) an upper sensitivity of 20 microM, (3) a lower sensitivity of 1.25 microM nitrate, and (4) intraday and interday variability ranging from 0.6 to 6.1%. To judge the acceptability of this method as a kinetic assay, we determined the Km for Aspergillus nitrate reductase to be 199 microM. The Km was based on analyzing three separate lots of commercially purified enzyme. Mean nitrate content of eight urine specimens analyzed by this assay (1111 microM) was not significantly different from that determined by a chemiluminescence method (1144 microM). Analysis of serum using the two methods showed mean nitrate concentrations of 23 and 36 microM, respectively. Based on serial dilutions of serum, the lower nitrate content of serum observed with nitrate reductase assay could not be explained by the presence of inhibitors. Rat pulmonary alveolar macrophages were induced to produce nitric oxide which oxidizes to nitrite and nitrate. Nitrite and nitrate present in tissue culture medium of unactivated and activated macrophages were in proportion to total nitrogen oxides (NO(x)) determined by the chemiluminescence method. We conclude that the Aspergillus nitrate reductase assay is an accurate spectrophotometric method for determining nitrate content of human urine and tissue culture supernatants.

Pyrrolidine dithiocarbamate inhibits induction of nitric oxide synthase activity in rat alveolar macrophages.

Since nuclear factor kappa B (NF-kappa B) is activated during many inflammatory conditions, we assessed its role in expression of the L-arginine-nitric oxide pathway by rat alveolar macrophages. Pyrrolidine dithiocarbamate (PDTC), an inhibitor of NF-kappa B activation, was added to cultured macrophages stimulated with lipopolysaccharide (LPS) and interferon-gamma (IFN gamma). Inducible nitric oxide synthase (iNOS) activity was determined by measuring the stable nitrogen oxide end products of L-arginine oxidation: nitrite (NO2-) and nitrate (NO3-). Ten, 25 and 50 microM PDTC progressively inhibited iNOS activity by macrophages. When 50 microM PDTC was added 2 h before LPS + IFN gamma, L-arginine oxidation by macrophages was inhibited by > 99%; L-arginine oxidation was reduced by 70% if 50 microM PDTC and the stimuli were introduced together; NO2- and NO3- were not decreased significantly if 50 microM PDTC was added 6 h after LPS + IFN gamma. Cycloheximide added along with LPS + IFN gamma totally inhibits iNOS activity, while cycloheximide added 6 h after LPS + IFN gamma did not reduce NO2- and NO3- in tissue culture supernatants. These findings suggest iNOS activity in macrophages treated with LPS + IFN gamma requires NF-kappa B activation.

Nitric oxide-mediated neuronal injury in multiple sclerosis.

Although several explanations have been proposed for destruction of myelin and oligodendrocytes in multiple sclerosis, there is no proven mechanism of injury. We postulate that the autoimmune response seen in multiple sclerosis results in a cytokine-mediated increase in nitric oxide production by macrophages/microglia, smooth muscle cells and/or endothelium of the central nervous system. 3 mechanisms of cellular damage due to nitric oxide are proposed: 1. direct nitric oxide cytotoxicity; 2. injury due to peroxynitrite formation from superoxide anion and nitric oxide; and 3. nitric oxide-mediated elevations of cellular cGMP that enhance tumor necrosis factor-alpha toxicity. In support of these hypotheses, the anti-inflammatory effectors, dexamethasone and transforming growth factor-beta, ameliorate symptoms seen in clinical multiple sclerosis and experimental allergic encephalitis, respectively. These 2 immunomodulators also inhibit induction of cytokine-mediated nitric oxide production by macrophages. An experimental design and therapeutic interventions which will evaluate the role of nitric oxide in the pathophysiology of experimental allergic encephalitis are presented.

Nitric oxide synthase from cerebellum catalyzes the formation of equimolar quantities of nitric oxide and citrulline from L-arginine.

This study examined whether constitutive nitric oxide (NO) synthase from rat cerebellum catalyzes the formation of equimolar amounts of NO plus citrulline from L-arginine under various conditions. Citrulline was determined by monitoring the formation of 3H-citrulline from 3H-L-arginine. NO was determined by monitoring the formation of total NOx (NO+nitrite [NO2-] + nitrate [NO3-]) by chemiluminescence after reduction of NOx to NO by acidic vanadium (III). Equal quantities of NO plus citrulline were generated from L-arginine and the formation of both products was linear for about 20 min at 37 degrees C provided L-arginine was present in excess to maintain a zero order reaction rate. Deletion of NADPH, addition of the calmodulin antagonist calmidazolium, or addition of NO synthase inhibitors (NG-methyl-L-arginine, NG-amino-L-arginine) abolished or markedly inhibited the formation of both NO and citrulline. The Km for L-arginine (14 microM; 18 microM) and the Vmax of the reaction (0.74 nmol/min/mg protein; 0.67 nmol/min/mg protein) were the same whether NO or citrulline formation, respectively, was monitored. These observations indicate clearly that NO and citrulline are formed in equimolar quantities from L-arginine by the constitutive isoform of NO synthase from rat cerebellum.