Inducible nitric oxide synthase in human diseases

Complex contribution of the 3'-untranslated region to the expressional regulation of the human inducible nitric-oxide synthase gene. Involvement of the RNA-binding protein HuR

Cytokine stimulation of human DLD-1 cells resulted in a marked expression of nitric-oxide synthase (NOS) II mRNA and protein accompanied by only a moderate increase in transcriptional activity. Also, there was a basal transcription of the NOS II gene, which did not result in measurable NOS II expression. The 3'-untranslated region (3'-UTR) of the NOS II mRNA contains four AUUUA motifs and one AUUUUA motif, known to destabilize the mRNAs of proto-oncogenes, nuclear transcription factors, and cytokines. Luciferase reporter gene constructs containing the NOS II 3'-UTR showed a significantly reduced luciferase activity. The embryonic lethal abnormal vision (ELAV)-like protein HuR was found to bind with high affinity to the adenylate/uridylate-rich elements of the NOS II 3'-UTR. Inhibition of HuR with antisense constructs reduced the cytokine-induced NOS II mRNA, whereas overexpression of HuR potentiated the cytokine-induced NOS II expression. This provides evidence that NOS II expression is regulated at the transcriptional and post-transcriptional level. Binding of HuR to the 3'-UTR of the NOS II mRNA seems to play an essential role in the stabilization of this mRNA.

Dimerization of major histocompatibility complex class I on the surface of THP-1 cells stimulates the expression of inducible nitric oxide synthase and subsequent nitric oxide release

We show that dimerization of major histocompatibility complex (MHC) class I on a human monocytic cell line, THP-1, induces nitric oxide (NO) synthesis. Cells cultured in the presence of a human MHC class I-specific monoclonal antibody produced significant amounts of NO after 72 hr. Reverse transcription-polymerase chain reaction and flow cytometry analysis revealed that the cells synthesized detectable levels of inducible NO synthase mRNA and protein. These effects were not seen after treatment with monovalent Fab fragments or Fc fragments of the same antibody, or after treatment with a control antibody. These data show a link between innate and acquired immune mechanisms mediated by NO and MHC class I.

Evaluating the role of inducible nitric oxide synthase using a novel and selective inducible nitric oxide synthase inhibitor in septic lung injury produced by cecal ligation and puncture

We studied the role of inducible nitric oxide synthase (iNOS) in septic lung injury using a novel and selective iNOS inhibitor (a fused piperidine derivative; ONO-1714) following a cecal ligation and puncture (CLP) procedure. All rats that received CLP died within 48 h after the intervention. The subcutaneous injection of ONO-1714 at 0.03 mg/kg every 12 h resulted in a significantly longer survival time than the saline control only when administration was started 12 h after the CLP procedure. The other administration schedules, which started immediately or 6 h after the intervention, did not show any improvement in the survival rates in comparison with the saline control. The administration of ONO-1714 at higher (0.1 mg/ kg) or lower (0.01 mg/kg) doses when given anytime after the intervention did not improve the survival rates. The NO(x) (NO(2)(-) + NO(3)(-)) levels in the plasma significantly increased 12 h after intervention in comparison with NO(x) at 0 h and thereafter further increased in parallel with the time elapsed. The CLP rats that were initially treated with ONO-1714 (0.03 mg/kg subcutaneously every 12 h) 12 h after intervention showed significantly reduced NO(x) levels in the plasma in comparison with the saline control. The NO synthase activity in lung homogenates increased from 6 to 24 h after the CLP and thereafter decreased to 42 h. The administration of ONO-1714 inhibited iNOS activity (under calcium-free conditions) in preference to total iNOS activity (under calcium-dependent conditions) in lung homogenates, which thus suggested that this compound selectively inhibited iNOS in lung tissue. The iNOS protein expression in the lung and liver homogenates showed a similar time course with alterations of NOS activity, namely a maximum level at 24 h after the intervention followed by decreasing levels to 42 h. On the other hand, other isozymes of NOS, eNOS, and nNOS in lung homogenates, were constantly expressed over the time course after the CLP. Since the iNOS mRNA expression in lung homogenates continued to elevate until 42 h, the decrease in iNOS activity and protein expression later than 24 h after the CLP was thus considered to be due to some posttranscriptional mechanism during the late phase of sepsis. In conclusion, intervention with a potent and selective iNOS inhibitor seemed to improve survival in CLP rats when used at the appropriate doses and time points. However, the self-limited mechanism of iNOS regulation in the lungs may also indicate that iNOS plays only a limited role in sepsis and septic shock.

Varied protocols of cadmium exposure produce different effects on nitric oxide production in macrophages

Different protocols of cadmium (Cd) exposure in non-cytotoxic conditions (i.e. 10 microM Cd for 18 h), and their effect on nitric oxide (NO) generation induced by NO inductor agents (NOIA) in peritoneal macrophages (pM) were studied. In all cases, NOIA (i.e. bacterial lipopolysaccharide [LPS], phorbol ester [PMA], okadaic acid [OA] or their combinations [LPS/OA] and [LPS/PMA]) were added at the beginning of the first incubation, only. Simultaneously exposure with 10 microM Cd enhanced NO generation and inducible NO synthase (iNOS) expression evoked by LPS, OA, PMA; those induced by LPS/PMA were not modified; and those caused by LPS/OA in relation to culture without Cd (medium) decreased. Double incubation, either with or without Cd (Cd+Cd or medium+medium), or Cd added at the start of the first or second incubation only (Cd+medium or medium+Cd), were tested. After the second incubation, medium+Cd protocol produced the highest NO generation in relation to other exposure protocols. When NO production was measured at the end of the second incubation, Cd+medium protocol enhanced NO production induced by OA, and LPS/OA, while medium+Cd protocol enhanced the response to LPS, PMA, and LPS/OA, in both cases in relation to the first incubation. Cd+Cd incubation protocol decreases the response to all NOIA in relation to another protocols. Cd effect on NO generation in macrophages is dependent on protocol and timing of exposure.

Reactive oxygen and reactive nitrogen intermediates in innate and specific immunity

Nitric oxide, nitric oxide derivatives and reactive oxygen intermediates are toxic molecules of the immune system which contribute to the control of microbial pathogens and tumors. There is recent evidence for additional functions of these oxygen metabolites in innate and adaptive immunity; these functions include the modulation of the cytokine response of lymphocytes and the regulation of immune cell apoptosis, as well as immunodeviating effects. Components of several signal transduction pathways have been identified as intracellular targets for reactive nitrogen and oxygen intermediates.

Inactivation of zinc finger transcription factors provides a mechanism for a gene regulatory role of nitric oxide

Nitric oxide (NO) is known to induce Zn(2+) release from the zinc-storing protein metallothionein and to induce Zn(2+) release within the nuclei and cytoplasm of cells. This suggests that zinc finger proteins may be primary targets of NO-induced stress. In this study, the specific interaction of the heterodimeric complex of two zinc finger transcription factors, 1alpha,25-dihydroxyvitamin D(3) (1alpha,25(OH)(2)D(3)) receptor (VDR) and retinoid X receptor (RXR) with 1alpha,25(OH)(2)D(3) response elements (VDREs), was used as a model system. NO was applied to this system via the NO donors SNOC and MAMA/NO and caused a dose-dependent inhibition of VDR-RXR-VDRE complex formation (IC(50) values 0.5-0.8 mM). Ligand-bound or preformed complexes displayed less sensitivity to NO-induced stress. These in vitro effects of NO were found to be reversible. Functional assays in transiently transfected cells indicated that NO can also act in vivo as a repressor of 1alpha,25(OH)(2)D(3) signaling (IC(50) value of the slow NO donor DETA/NO, 0.5 mM). These findings suggest that NO has a modulatory role on transcription factors depending on their sensitivity to NO-induced stress, thus providing a mechanism for a gene regulatory function of NO.

Implications of inducible nitric oxide synthase expression and enzyme activity

We summarize here our current knowledge about inducible nitric oxide synthase (NOS) activity in human diseases and disorders. As basic research discovers more and more effects of low or high concentrations of NO toward molecular and cellular targets, successful therapies involving inhibition of NO synthesis or application of NO to treat human diseases are still lacking. This is in part due to the fact that the impact of NO on cell function or death are complex and often even appear to be contradictory. NO may be cytotoxic but may also protect cells from a toxic insult; it is apoptosis-inducing but also exhibits prominent anti-apoptotic activity. NO is an antioxidant but may also compromise the cellular redox state via oxidation of thiols like glutathione. NO may activate specific signal transduction pathways but is also reported to inhibit exactly these, and NO may activate or inhibit gene transcription. The situation may even be more complicated, because NO, depending on its concentration, may react with oxygen or the superoxide anion radical to yield reactive species with a much broader chemical reaction spectrum than NO itself. Thus, the action of NO during inflammatory reactions has to be considered in the context of timing and duration of its synthesis as well as stages and specific events in inflammation.

Inducible nitric oxide synthase and nitric oxide production in Leishmania infantum-infected human macrophages stimulated with interferon-gamma and bacterial lipopolysaccharide

Nitric oxide produced by an inducible nitric oxide synthase constitutes one of the main microbicidal mechanisms of murine macrophages and its importance is now being recognized for human macrophages. In this study we evaluated inducible nitric oxide synthase expression, nitric oxide release, and parasitocidal ability of Leishmania infantum-infected monocyte-derived human macrophages. The inducible nitric oxide synthase was detected by immunofluorescence and western blotting and nitric oxide production was measured by the Griess reaction for nitrites. Parasite killing was microscopically evaluated by fluorescent dyes. Experiments were performed on macrophages with or without previous stimulation with recombinant human interferon-gamma and bacterial lipopolysaccharide. Inducible nitric oxide synthase expression and nitric oxide release were higher in Leishmania-infected stimulated macrophages than in uninfected cells or infected cells without previous stimulation. Nitric oxide production and parasitocidal activity against Leishmania infantum were reduced in macrophages treated with the nitric oxide synthase inhibitor L-N(G) monomethylarginine. These results suggest a microbicidal role for nitric oxide in human leishmaniasis, with the possible practical application of immunological or pharmacological regulation of nitric oxide synthesis in the treatment of this infection.

Nitric oxide production is increased in patients with inflammatory myositis

Nitric oxide (NO) production is increased in several inflammatory disorders. We have previously demonstrated higher levels of NO production among patients with rheumatoid arthritis and systemic lupus erythematosus. In this study we measured serum levels of nitrite and citrulline using calorimetric methods as surrogate markers of NO production among patients with inflammatory myositis (IM). Twenty patients with IM and 19 age- and sex-matched controls were studied. Serum nitrite levels were significantly higher among patients than among controls (986.6 +/- 880 and 204.3 +/- 113.9 nmol/ml, respectively; P = 0.001). Serum citrulline levels, too, were significantly higher among patients than among controls (3755.7 +/- 1905.5 and 189 +/- 177.2 nmol/ml, respectively; P < 0.0001). There was a positive correlation between steroid dosage and serum citrulline levels (r = 0.51, P = 0.036) and a negative correlation between steroid dosage and disease duration (r = -0.54, P = 0.025). It was concluded that NO production is increased in patients with IM and those with more active disease, as indicated by higher steroid dosage, have higher serum citrulline levels.

Zinc finger transcription factors as molecular targets for nitric oxide-mediated immunosuppression: inhibition of IL-2 gene expression in murine lymphocytes

BACKGROUND

Nitric oxide (NO) has frequently been shown to display immunosuppressive activities. We describe here a molecular mechanism contributing to this effect.

MATERIALS AND METHODS

Murine T cell lymphoma EL4-6.1 cells were activated with the physiological stimulus interleukin (IL)-1beta to express IL-2 mRNA in the presence or absence of subtoxic concentrations of the physiological spontaneous NO donor S-nitrosocysteine (SNOC). Subsequently, semiquantitative RT-PCR and gel shift assays with nuclear extracts were performed to analyze the effects of NO on IL-2 mRNA expression and on the activity of the dominant regulating transcription factors Sp1, EGR-1, and NFATc.

RESULTS

NO inhibits IL-1beta-induced IL-2 mRNA expression in EL4-6.1 cells. The suppressive activity of NO was concentration dependent and found to be completely reversible. Importantly, NO at the concentrations used induced neither apoptosis nor necrosis. Dominant regulation of IL-2 gene expression is known to reside in the zinc finger transcription factors Sp1 or EGR-1 and in the non-zinc finger protein NFAT. NO abrogates the DNA binding activities of recombinant Sp1 and EGR-1. More importantly, gel shift assays also showed a lack of DNA binding of native Sp1 derived from NO-treated nuclear extracts and that from NO-treated viable lymphocytes. This effect is selective, as the DNA binding activity of recombinant NFATc was not affected by NO.

CONCLUSION

Inactivation of zinc finger transcription factors by NO appears to be a molecular mechanism in the immunosuppressive activity of NO in mammals, thus contributing to NO-mediated inhibition of IL-2 gene expression after physiological stimuli. The exact understanding of the molecular mechanism leading to NO-mediated, fully reversible suppression of immune reactions may lead to use of this naturally occurring tool as an aid in inflammatory diseases.

Suppression of inducible cyclooxygenase and inducible nitric oxide synthase by apigenin and related flavonoids in mouse macrophages

Prostaglandins biosynthesis and nitric oxide production have been implicated in the process of carcinogenesis and inflammation. In this study, we investigated the effect of various flavonoids and (-)-epigallocatechin-3-gallate on the activities of inducible cyclooxygenase (COX-2) and inducible nitric oxide synthase (iNOS) in lipopolysaccharide (LPS)-activated RAW 264.7 macrophages. Apigenin, genistein and kaempferol were markedly active inhibitors of transcriptional activation of COX-2, with IC(50) < 15 microM. In addition, apigenin and kaempferol were also markedly active inhibitors of transcriptional activation of iNOS, with IC(50) < 15 microM. Of those compounds tested, apigenin was the most potent inhibitor of transcriptional activation of both COX-2 and iNOS. Western and northern blot analyses demonstrated that apigenin significantly blocked protein and mRNA expression of COX-2 and iNOS in LPS-activated macrophages. Transient transfection experiments showed that LPS caused an approximately 4-fold increase in both COX-2 and iNOS promoter activities, these increments were suppressed by apigenin. Moreover, electrophoretic mobility shift assay (EMSA) experiments indicated that apigenin blocked the LPS-induced activation of nuclear factor-kB (NF-kB). The inhibition of NF-kB activation occurs through the prevention of inhibitor kB (IkB) degradation. Transient transfection experiments also showed that apigenin inhibited NF-kB-dependent transcriptional activity. Finally, we showed that apigenin could inhibit the IkB kinase activity induced by LPS or interferon-gamma. The results of further studies suggest that suppression of transcriptional activation of COX-2 and iNOS by apigenin might mainly be mediated through inhibition of IkB kinase activity. This study suggests that modulation of COX-2 and iNOS by apigenin and related flavonoids may be important in the prevention of carcinogenesis and inflammation.

Nitric oxide, skin growth and differentiation: more questions than answers?

Nitric oxide has varied effects on the skin. In this review the role of nitric oxide in cutaneous wound healing, apoptosis, carcinogenesis and psoriasis is discussed.

Microtubule dysfunction by posttranslational nitrotyrosination of alpha-tubulin: a nitric oxide-dependent mechanism of cellular injury

NO2Tyr (3-Nitrotyrosine) is a modified amino acid that is formed by nitric oxide-derived species and has been implicated in the pathology of diverse human diseases. Nitration of active-site tyrosine residues is known to compromise protein structure and function. Although free NO2Tyr is produced in abundant concentrations under pathological conditions, its capacity to alter protein structure and function at the translational or posttranslational level is unknown. Here, we report that free NO2Tyr is transported into mammalian cells and selectively incorporated into the extreme carboxyl terminus of alpha-tubulin via a posttranslational mechanism catalyzed by the enzyme tubulin-tyrosine ligase. In contrast to the enzymatically regulated carboxyl-terminal tyrosination/detyrosination cycle of alpha-tubulin, incorporation of NO2Tyr shows apparent irreversibility. Nitrotyrosination of alpha-tubulin induces alterations in cell morphology, changes in microtubule organization, loss of epithelial-barrier function, and intracellular redistribution of the motor protein cytoplasmic dynein. These observations imply that posttranslational nitrotyrosination of alpha-tubulin invokes conformational changes, either directly or via allosteric interactions, in the surface-exposed carboxyl terminus of alpha-tubulin that compromises the function of this critical domain in regulating microtubule organization and binding of motor- and microtubule-associated proteins. Collectively, these observations illustrate a mechanism whereby free NO2Tyr can impact deleteriously on cell function under pathological conditions encompassing reactive nitrogen species production. The data also yield further insight into the role that the alpha-tubulin tyrosination/detyrosination cycle plays in microtubule function.

Inhibition of Inducible Nitric Oxide Synthase Exacerbates Chronic Cerebral Toxoplasmosis in Toxoplasma gondii-Susceptible C57BL/6 Mice But Does Not Reactivate the Latent Disease in T. gondii-Resistant BALB/c Mice

Infection of C57BL/6 mice with Toxoplasma gondii leads to progressive and ultimately fatal chronic Toxoplasma encephalitis (TE). Genetic deletion or inhibition of inducible nitric oxide synthase (iNOS) from the beginning of infection increased the number of T. gondii cysts in the brain and markedly reduced the time-to-death in this mouse strain. In the present study, we addressed whether iNOS also contributes to the control of intracerebral parasites in a clinically stable latent infection that develops in T. gondii-resistant BALB/c mice after resolution of the acute phase of TE. iNOS was expressed in the inflammatory cerebral infiltrates of latently infected BALB/c mice, but the number of iNOS+ cells was significantly lower than in the brains of chronically infected T. gondii-susceptible C57BL/6 mice. In BALB/c mice with latent TE (&gt;30 days of infection), treatment with the iNOS inhibitors l-N6-iminoethyl-lysine or l-nitroarginine-methylester for ≤40 days did not result in an increase of the intracerebral parasitic load and a reactivation of the disease, despite the presence of iNOS-suppressive inhibitor levels in the brain. However, l-nitroarginine-methylester treatment had remarkably toxic effects and induced a severe wasting syndrome with high mortality. In contrast to BALB/c mice, l-N6-iminoethyl-lysine treatment rapidly exacerbated the already established chronic TE of C57BL/6 mice. Thus, the containment of latent toxoplasms in T. gondii-resistant BALB/c mice is independent of iNOS, whereas the temporary control of intracerebral parasites in T. gondii-susceptible C57BL/6 mice with chronic TE requires iNOS activity.

Pathophysiology of nitric oxide and related species: free radical reactions and modification of biomolecules

Since its initial discovery as an endogenously produced bioactive mediator, nitric oxide (.NO) has been found to play a critical role in the cellular function of nearly all organ systems. Furthermore, aberrant production of .NO or reactive nitrogen species (RNS) derived from .NO, has been implicated in a number of pathological conditions, such as acute lung disease, atherosclerosis and septic shock. While .NO itself is fairly non-toxic, secondary RNS are oxidants and nitrating agents that can modify both the structure and function of numerous biomolecules both in vitro, and in vivo. The mechanisms by which RNS mediate toxicity are largely dictated by its unique reactivity. The study of how reactive nitrogen species (RNS) derived from .NO interact with biomolecules such as proteins, carbohydrates and lipids, to modify both their structure and function is an area of active research, which is lending major new insights into the mechanisms underlying their pathophysiological role in human disease. In the context of .NO-dependent pathophysiology, these biochemical reactions will play a major role since they: (i) lead to removal of .NO and decreased efficiency of .NO as an endothelial-derived relaxation factor (e.g. in hypertension, atherosclerosis) and (ii) lead to production of other intermediate species and covalently modified biomolecules that cause injury and cellular dysfunction during inflammation. Although the physical and chemical properties of .NO and .NO-derived RNS are well characterised, extrapolating this fundamental knowledge to a complicated biological environment is a current challenge for researchers in the field of .NO and free radical research. In this review, we describe the impact of .NO and .NO-derived RNS on biological processes primarily from a biochemical standpoint. In this way, it is our intention to outline the most pertinent and relevant reactions of RNS, as they apply to a diverse array of pathophysiological states. Since reactions of RNS in vivo are likely to be vast and complex, our aim in this review is threefold: (i) address the major sources and reactions of .NO-derived RNS in biological systems, (ii) describe current knowledge regarding the functional consequences underlying .NO-dependent covalent modification of specific biomolecules, and (iii) to summarise and critically evaluate the available evidence implicating these reactions in human pathology. To this end, three areas of special interest have been chosen for detailed description, namely, formation and role of S-nitrosothiols, modulation of lipid oxidation/nitration by RNS, and tyrosine nitration mechanisms and consequences.