Parallel induction of nitric oxide and glucose-6-phosphate dehydrogenase in activated bone marrow derived macrophages

Modulation of nitric oxide synthase activity in macrophages

L-Arginine is converted to the highly reactive and unstable nitric oxide (NO) and L-citrulline by an enzyme named nitric oxide synthase (NOS). NO decomposes into other nitrogen oxides such as nitrite (NO₂^-) and nitrate (NO₂^-), and in the presence of superoxide anion to the potent oxidizing agent peroxynitrite (ONOO⁻). Activated rodent macrophages are capable of expressing an inducible form of this enzyme (iNOS) in response to appropriate stimuli, i.e., lipopolysaccharide (LPS) and interferon-γ (IFNγ). Other cytokines can modulate the induction of NO biosynthesis in macrophages. NO is a major effector molecule of the anti-microbial and cytotoxic activity of rodent macrophages against certain micro-organisms and tumour cells, respectively. The NO synthesizing pathway has been demonstrated in human monocytes and other cells, but its role in host defence seems to be accessory. A delicate functional balance between microbial stimuli, host-derived cytokines and hormones in the microenvironment regulates iNOS expression. This review will focus mainly on the known and proposed mechanisms of the regulation of iNOS induction, and on agents that can modulate NO release once the active enzyme has been expressed in the macrophage.

Oxygen-dependent regulation of nitric oxide production by inducible nitric oxide synthase

Inducible nitric oxide synthase (iNOS) catalyzes the reaction that converts the substrates O(2) and l-arginine to the products nitric oxide (NO) and l-citrulline. Macrophages, and many other cell types, upregulate and express iNOS primarily in response to inflammatory stimuli. Physiological and pathophysiological oxygen tension can regulate NO production by iNOS at multiple levels, including transcriptional, translational, posttranslational, enzyme dimerization, cofactor availability, and substrate dependence. Cell culture techniques that emphasize control of cellular PO(2), and measurement of NO or its stable products, have been used by several investigators for in vitro study of the O(2) dependence of NO production at one or more of these levels. In most cell types, prior or concurrent exposure to cytokines or other inflammatory stimuli is required for the upregulation of iNOS mRNA and protein by hypoxia. Important transcription factors that target the iNOS promoter in hypoxia include hypoxia-inducible factor 1 and/or nuclear factor κB. In contrast to the upregulation of iNOS by hypoxia, in most cell types NO production is reduced by hypoxia. Recent work suggests a prominent role for O(2) substrate dependence in the short-term regulation of iNOS-mediated NO production.

The microbicidal and cytoregulatory roles of NADPH oxidases

Despite their prominent microbicidal roles, NADPH oxidases (NOXs) have been recently found to regulate a wide variety of physiological activities. Through generation of reactive oxygen species (ROS), NOXs actively participate in cellular activities, including NET formation, inflammasome activation and wound sensing. The microbicidal and cytoregulatory roles of NOXs are contrasted in this review.

pO(2)-dependent NO production determines OPPC activity in macrophages

Stimulated macrophages produce nitric oxide (NO) via inducible nitric oxide synthase (iNOS) using molecular O(2), L-arginine, and NADPH. Exposure of macrophages to hypoxia decreases NO production within seconds, suggesting substrate limitation as the mechanism. Conflicting data exist regarding the effect of pO(2) on NADPH production via the oxidative pentose phosphate cycle (OPPC). Therefore, the present studies were developed to determine whether NADPH could be limiting for NO production under hypoxia. Production of NO metabolites (NOx) and OPPC activity by RAW 264.7 cells was significantly increased by stimulation with lipopolysaccharide (LPS) and interferon gamma (IFNgamma) at pO(2) ranging from 0.07 to 50%. OPPC activity correlated linearly with NOx production at pO(2)>0.13%. Increased OPPC activity by stimulated RAW 264.7 cells was significantly reduced by 1400 W, an iNOS inhibitor. OPPC activity was significantly increased by concomitant treatment of stimulated RAW 264.7 cells with chemical oxidants such as hydroxyethyldisulfide or pimonidazole, at 0.07 and 50% O(2), without decreasing NOx production. These results are the first to investigate the effect of pO(2) on the relationship between NO production and OPPC activity, and to rule out limitations in OPPC activity as a mechanism by which NO production is decreased under hypoxia.

Macrophage activation induces formation of the anti-inflammatory lipid cholesteryl-nitrolinoleate

Nitroalkene derivatives of fatty acids act as adaptive, anti-inflammatory signalling mediators, based on their high-affinity PPARgamma (peroxisome-proliferator-activated receptor gamma) ligand activity and electrophilic reactivity with proteins, including transcription factors. Although free or esterified lipid nitroalkene derivatives have been detected in human plasma and urine, their generation by inflammatory stimuli has not been reported. In the present study, we show increased nitration of cholesteryl-linoleate by activated murine J774.1 macrophages, yielding the mononitrated nitroalkene CLNO2 (cholesteryl-nitrolinoleate). CLNO2 levels were found to increase approximately 20-fold 24 h after macrophage activation with Escherichia coli lipopolysaccharide plus interferon-gamma; this response was concurrent with an increase in the expression of NOS2 (inducible nitric oxide synthase) and was inhibited by the (*)NO (nitric oxide) inhibitor L-NAME (N(G)-nitro-L-arginine methyl ester). Macrophage (J774.1 and bone-marrow-derived cells) inflammatory responses were suppressed when activated in the presence of CLNO2 or LNO2 (nitrolinoleate). This included: (i) inhibition of NOS2 expression and cytokine secretion through PPARgamma and *NO-independent mechanisms; (ii) induction of haem oxygenase-1 expression; and (iii) inhibition of NF-kappaB (nuclear factor kappaB) activation. Overall, these results suggest that lipid nitration occurs as part of the response of macrophages to inflammatory stimuli involving NOS2 induction and that these by-products of nitro-oxidative reactions may act as novel adaptive down-regulators of inflammatory responses.

Effect of tetrahydrobiopterin on nitric oxide synthase-containing cells in the rat hippocampus

We have observed that tetrahydrobiopterin (BH4), a cofactor of nitric oxide synthase (NOS), acts as a self-protection factor against nitric oxide (NO) toxicity in PC12 cells. To further investigate the self-protection action of BH4 in vivo, the effect of deletion of endogenous BH4 on NO-producing cells was examined in the rat hippocampus. Following the peripheral infusion of 50 mM 2,4-diamino-6-hydroxypyrimidine (DAHP), an inhibitor of GTP cyclohydrolase I, using a miniosmotic pump for 14 days, BH4 content in the hippocampus decreased as compared with the control group administered with vehicle solution, which had no effect on brain BH4 content. When the rats were administered with 50 mM DAHP and 10 mM BH4, the DAHP-induced decrease in BH4 content was prevented. The extracellular concentration of NO metabolites remained unchanged following DAHP administration, suggesting that DAHP-induced decrease in BH4 content had no effect on NO production. The number of NOS-positive cells decreased following DAHP administration in the hippocampal regions, while the number of NOS-negative cells remained unchanged. The DAHP-induced decrease in the NOS-positive cell number was prevented by the administration of 10 mM BH4 and DAHP. These results suggest that endogenous BH4 may affect NOS-positive cell number in the rat hippocampus.

Hofbauer cell activation and its increased glucose-6-phosphate dehydrogenase activity in second trimester-spontaneous abortion: an ultrastructural dual staining enzyme-cytochemical study

PROBLEM

While activated/phagocytosing phagocytes infiltrating to the chorioamnion are considered to be one of the causal agents of preterm labor onset, whether placental villous macrophages (Hofbauer cells) are activated/phagocytosing in this condition is not known.

METHOD OF STUDY

We concomitantly localized two important phagocytosis-related enzymes, acid phosphatase (ACP) and glucose-6-phosphate dehydrogenase (G6PD), in Hofbauer cells in second trimester placental villi, and compared them with those from infection-related second trimester-spontaneous abortion (miscarriage) placentas.

RESULTS

There were two types of Hofbauer cells. The first cells exhibited ACP stainings confined to the lysosomes, suggesting that they are dormant/non-activated cells. Approximately two-thirds of these cells showed weak G6PD labeling on the cytosolic side of endoplasmic reticula, and G6PD labeling was hardly recognizable in the remaining one-third. The second cells, possessing large phagosomes, showed marked ACP labeling in the phagosomes, suggesting that they are activated/phagocytosing cells. All these cells exhibited G6PD labeling, and in 'bursting cells' (possibly hyperactivated cells) G6PD deposits were marked. The percentage of activated cells in miscarriage placentas was significantly higher (44.8 +/- 6.0%) than that in gestational age-matched controls (17.4 +/- 5.3%).

CONCLUSIONS

These observations indicated that (1) G6PD activity increased in activated/phagocytosing Hofbauer cells, and (2) the percentage of phagocytosing cells increased in infection-related miscarriage placentas. Hofbauer activation and G6PD may play an role in the pathogenesis/pathophysiology of preterm labor onset.

Enzyme-cytochemically detectable glucose-6-phosphate dehydrogenase in human villous macrophages (Hofbauer cells)

Though various tissue macrophages possess high glucose-6-phosphate dehydrogenase (G6PD) activity, which plays an important role in their phagocytosis/bactericidal function, the presence of this enzyme in human placental villous macrophages (Hofbauer cells) has not been determined. We examined the ultrastructural localization of glucose-6-phosphate dehydrogenase (G6PD) in Hofbauer cells in first and second trimester placental villi, using a newly developed enzyme-cytochemistry (copper-ferrocyanide) method. Electron-dense deposits indicative of G6PD activity were clearly visible in the cytoplasm and on the cytosolic side of the endoplasmic reticulum of Hofbauer cells. Positive and negative cytochemical controls ensured specific detection of enzyme activity. These observations indicated that Hofbauer cells abundantly possessed enzyme-cytochemically detectable G6PD activity. Hofbauer cell G6PD may play a role in placental defense, by supplying NADPH-dependent enzymes (i.e. nitric oxide synthase or NADPH oxidase) with NADPH. This enzyme may also fuel Hofbauer cells with ribose 5-phosphate during their cell proliferation and cell division.

Protective role of nuclear factor kappa B against nitric oxide-induced apoptosis in J774 macrophages

We investigated the role of constitutive transcription factor nuclear factor kappaB (NF-kappaB) in nitric oxide (NO)-mediated apoptosis in J774 macrophages. Our results show that NF-kappaB is present in untreated J774 cells in a form constitutively active. Incubation of cells with sodium nitroprusside (SNP) and S-nitroso-glutathione (GSNO), two NO-generating compounds, caused: (a) inhibition of constitutive NF-kappaB/DNA binding activity; (b) decrease of cell viability; (c) DNA fragmentation; (d) ApopTag positivity. Pyrrolidine dithiocarbamate (PDTC) and N-alpha-para-tosyl-L-lysine chloromethyl ketone (TLCK), two inhibitors of NF-kappaB activation, showed the same effects of both NO-generating compounds. Furthermore, SNP and GSNO as well as PDTC and TLCK significantly increased the cytoplasmic level of IkappaBalpha. All together these results demonstrate that constitutive NF-kappaB protects J774 macrophages from NO-induced apoptosis. Moreover, these findings show, for the first time, that NO-generating compounds may induce apoptosis in J774 macrophages by down-regulating constitutive NF-kappaB/DNA binding activity and suggest a novel mechanism by which NO induces apoptosis.

The role of 6R-tetrahydrobiopterin in the nervous system

In addition to its cofactor activities for aromatic L-amino acid hydroxylases and nitric oxide synthase (NOS), 6R-tetrahydrobiopterin (6R-BH(4)) shows diverse actions on neurons. Dopamine release from the rat striatum or PC12 cells was stimulated by 6R-BH(4). The action of 6R-BH(4) was independent of its cofactor activities and stereospecific. Ca(2+) channels in rat brain and PC12 cells were activated by 6R-BH(4) via cAMP-protein kinase A pathway. Membrane potential of PC12 cells was deplorized by 6R-BH(4). Thus, it is assumed that 6R-BH(4) acts on its specific action site (possibly outside of the cell membrane) to stimulate dopamine release by activating Ca(2+) channels. Apoptosis induced by depletion of serum and nerve growth factor in PC12 cells was prevented by 6R-BH(4). The cell surviving effect of 6R-BH(4) was also mediated by activation of Ca(2+) channels and cAMP-protein kinase A pathway. However, since 6R-BH(4) did not activate mitogen activated protein kinase, it did not support neuronal differentiation. Nitric oxide (NO)-induced cell death was prevented by 6R-BH(4) in PC12 cells. NOS activity was not changed by exogenous 6R-BH(4), but NO metabolites in culture medium were decreased by 6R-BH(4). When endogenous 6R-BH(4) was reduced by inhibition of biosynthesis, cell death was induced in PC12 cells. Superoxide is observed to be generated during autoxidation of 6R-BH(4). Superoxide producing system mimicked the cell protective action of 6R-BH(4) against NO toxicity. Thus, it is considered that 6R-BH(4) protects PC12 cells against NO toxicity by generating superoxide during its autoxidation. These results raised the possibility that 6R-BH(4) is a self-protective factor against NO toxicity in NO producing neurons. Our findings indicate that 6R-BH(4) regulates neuronal activities in the brain and that 6R-BH(4) can be a promising drug for neurodegenerative disorders such as Parkinson's disease and Alzheimer's disease.

Self-protection of PC12 cells by 6R-tetrahydrobiopterin from nitric oxide toxicity

Nitric oxide (NO) has cytotoxic effects but NO producing neurons are resistant to NO toxicity. These results suggest the presence of self-protecting factors for NO toxicity. Recently, 6R-tetrahydrobiopterin (6R-BH4), a cofactor for NO synthase (NOS), has been reported to degrade NO raising the possibility that 6R-BH4 acts as a self-protecting factor for NO toxicity. In PC12 cells which have NOS, three-day culture with sodium nitroprusside (SNP) or NOC-12, NO generators, at 10-100 microM increased nitrite and nitrate concentrations in the culture medium and induced death of PC12 cells. Coadministration of 6R-BH4 (10 or 30 microM) with SNP or NOC-12 prevented cell death with reduction of nitrite and nitrate in the medium. Inhibition of 6R-BH4 synthesis by 2,4-diamino-6-hydroxypyrimidine (DAHP), an inhibitor for GTP cyclohydrolase I, decreased cellular 6R-BH4 content and viable cell number. The inhibiting effects of DAHP were restored by exogenous 6R-BH4. NOS activity, as estimated by nitrite concentrations in the medium, was unchanged by DAHP. Hypoxanthine and xanthine oxidase, which produce superoxide, mimicked the cell-protecting effect of 6R-BH4 which is reported to generate superoxide during its autoxidation. These results suggest that 6R-BH4 acts as a self-protecting factor for NO toxicity with generation of superoxide in NO-producing neurons.

Impaired production of nitric oxide, superoxide, and hydrogen peroxide in glucose 6-phosphate-dehydrogenase-deficient granulocytes

Since the generation of superoxide and hydrogen peroxide by NADPH oxidase and nitric oxide (NO) by NO synthase (NOS) in granulocytes is NADPH-dependent, we investigated the production of NO, superoxide and H2O2 in glucose 6-phosphate dehydrogenase (G6PD)-deficient human granulocytes. Our results showed that upon stimulation with either 5 microg/ml of lipopolysaccharide (LPS) or 10 microM of phorbol 12-myristate 13-acetate (PMA), the production of nitrite in normal granulocytes was elevated, 252 +/- 135% and 239 +/- 72%, respectively, compared to the resting stage. In contrast, G6PD-deficient granulocytes did not produce more nitrite upon stimulation with either LPS or PMA compared to the resting stage. Western blot analysis indicated a normal expression pattern of inducible NOS in G6PD-deficient granulocytes. In addition, the production of H2O2 and superoxide was also significantly impaired in G6PD-deficient granulocytes compared to control cells. These data demonstrate that G6PD deficiency causes an impairment in the production of NO, superoxide and H2O2.

Decrease in free-radical production with age in rat peritoneal macrophages

The respiratory-burst reaction has been studied in rat peritoneal macrophages of different ages (3, 12 and 24 months) using phorbol 12-myristate 13-acetate (PMA) to stimulate NADPH oxidase. Production of O2-. and H2O2 decreased with age (about 50 and 75% respectively); however, no difference in NADPH oxidase activity was found. NO. production was also reduced with age (40%). Furthermore, a progressive and significant decrease in the pentose phosphate flux was detected as a function of age in control and PMA-stimulated macrophages. The NADPH/NADP+ ratio decreased with age in control and PMA-stimulated macrophages. Glucose uptake was lower in middle-aged (12 months) and old (24 months) animals but no differences were found between these groups.

Nitric oxide. Novel biology with clinical relevance

OBJECTIVE

The author provides the reader with a view of the regulation and function of nitric oxide (NO), based on the three distinct enzyme isoforms that synthesize NO.

SUMMARY BACKGROUND DATA

Nitric oxide is a short-lived molecule exhibiting functions as diverse as neurotransmission and microbial killing. Recent advances in the characterization of the enzymes responsible for NO synthesis and in the understanding of how NO interacts with targets have led to new insights into the many facets of this diverse molecule.

METHODS

Nitric oxide is produced by one of three enzyme isoforms of NO synthesis. These enzymes vary considerably in their distribution, regulation, and function. Accordingly, the NO synthesis or lack of NO production will have consequences unique to that isoform. Therefore, this review summarizes the regulation and function of NO generated by each of the three isoforms.

RESULTS

Nitric oxide exhibits many unique characteristics that allow this molecule to perform so many functions. The amount, duration, and location of the NO synthesis will depend on the isoform of NO synthase expressed. For each isoform, there probably are disease processes in which deficiency states exist. For induced NO synthesis, states of overexpression exist.

CONCLUSIONS

Understanding the regulation and function of the enzymes that produce NO and the unique characteristics of each enzyme isoform is likely to lead to therapeutic approaches to prevent or treat a number of diseases.

Regulation and function of inducible nitric oxide synthase during sepsis and acute inflammation

During sepsis and inflammation profound changes in physiological function are induced by a variety of mediators, including endotoxin, various cytokines, and NO. Many of these mediators, in addition to their other functions, induce the synthesis of NO through the induction of iNOS within a variety of cell types. The regulation of iNOS expression is quite complex. Of interest is the fact that the functions of NO during sepsis range from modulating perfusion to mediating cytotoxicity. In addition, it is unique that many tissues not characterized as being involved in immune function express iNOS in a manner similar to that of tissues involved in immune function. The role of NO during episodes of acute inflammation appears to be a protective one; however, there are examples of chronic localized inflammation in both animal and human models which suggest that chronic iNOS expression may be detrimental. Further investigations into the regulation and function of NO in both the acute and chronic settings are necessary in order to fully understand this small yet unique molecule.