Nitric Oxide-Mediated Inhibition of the Mitochondrial Respiratory Chain in Cultured Astrocytes

Abstract: The Ca²⁺-independent form of nitric oxide synthase was induced in rat neonatal astrocytes in primary culture by incubation with lipopolysaccharide (1 µg/ml) plus interferon-γ (100 U/ml), and the activities of the mitochondrial respiratory chain components were assessed. Incubation for 18 h produced 25% inhibition of cytochrome c oxidase activity. NADH-ubiquinone-1 reductase (complex I) and succinate-cytochrome c reductase (complex II–III) activities were not affected. Prolonged incubation for 36 h gave rise to a 56% reduction of cytochrome c oxidase activity and a 35% reduction in succinate-cytochrome c reductase activity, but NADH-ubiquinone-1 reductase activity was unchanged. Citrate synthase activity was not affected by any of these conditions. The inhibition of the activities of these mitochondrial respiratory chain complexes was prevented by incubation in the presence of the specific nitric oxide synthase inhibitor N ^G-monomethyl- l -arginine. The lipopolysaccharide/interferon-γ treatment of the astrocytes produced an increase in glycolysis and lactate formation. These results suggest that inhibition of the mitochondrial respiratory chain after induction of astrocytic nitric oxide synthase may represent a mechanism for nitric oxide-mediated neurotoxicity.     

Pretreatment of Astrocytes with Interferon-α/β Impairs Interferon-γ Induction of Nitric Oxide Synthase

Abstract: Excessive nitric oxide/peroxynitrite generation has been implicated in the pathogenesis of multiple sclerosis, and the demonstration of increased astrocytic nitric oxide synthase activity in the postmortem brain of multiple sclerosis patients supports this hypothesis. Exposure of astrocytes, in primary culture, to interferon-γ results in stimulation of nitric oxide synthase activity and increased nitric oxide release. In contrast to interferon-γ, interferon-α/β had a minimal effect on astrocytic nitric oxide formation. Furthermore, pretreatment of astrocytes with interferon-α/β inhibited (∼65%) stimulation by interferon-γ of nitric oxide synthase activity and nitric oxide release. Treatment with interferon-α/β at a concentration as low as 10 U/ml caused inhibition of mitochondrial cytochrome c oxidase. Furthermore, the damage to cytochrome c oxidase was prevented by the putative interferon-α/β receptor antagonist oxyphenylbutazone. In view of these observations, our current hypothesis is that the mitochondrial damage caused by exposure to interferon-α/β may impair the ability of astrocytes to induce nitric oxide synthase activity on subsequent interferon-γ exposure. These results may have implications for our understanding of the mechanisms responsible for the therapeutic effects of interferon-α/β preparations in multiple sclerosis.     

Pretreatment of Astrocytes with Interferon-α/β Prevents Neuronal Mitochondrial Respiratory Chain Damage

Abstract: Excessive nitric oxide/peroxynitrite generation has been implicated in the pathogenesis of multiple sclerosis, and the demonstration of increased astrocytic nitric oxide synthase activity in the postmortem brain of multiple sclerosis patients supports this hypothesis. Interferon-β is used for the treatment of multiple sclerosis, but currently little is known regarding its mode of action. Exposure of astrocytes in culture to interferon-γ plus lipopolysaccharide results in stimulation of nitric oxide release. Using a coculture system, we have been able to use astrocytes as a source of nitric oxide/peroxynitrite in an attempt to "model" the effects of raised cytokine levels observed in multiple sclerosis and to monitor the effect on neurones. Our results indicate that stimulation of astrocytic nitric oxide synthase activity causes significant damage to the mitochondrial activities of complexes II/III and IV of neighbouring neurones. This damage was prevented by a nitric oxide synthase inhibitor, suggesting that the damage was nitric oxide-mediated. Furthermore, interferon-α/β also prevented this damage. In view of these results, we suggest that a possible mechanism of action of interferon-β in the treatment of multiple sclerosis is that it prevents astrocytic nitric oxide production, thereby limiting damage to neighbouring cells, such as neurones.     

Mast Cell Activation Causes Delayed Neurodegeneration in Mixed Hippocampal Cultures via the Nitric Oxide Pathway

Abstract: Mast cells are pleiotropic bone marrow-derived cells found in mucosal and connective tissues and in close apposition to neurons, where they play important roles in tissue inflammation and in neuroimmune interactions. Connective tissue mast cells, with which intracranial mast cells share many characteristics, contain cytokines that can cause inflammation. Here, we report that myelin basic protein, a major suspected immunogen in multiple sclerosis, as well as an antigenic stimulus, provokes mast cells to trigger a delayed cytotoxicity for neurons in mixed neuron-glia cultures from hippocampus. Neurotoxicity required a prolonged period (12 h) of mast cell incubation, and appeared to depend largely on elaboration of the free radical nitric oxide by astrocytes. Activation of astrocytes was mediated, in part, by mast cell-secreted tumor necrosis factor-α. Myelin basic protein and 17β-estradiol had a synergistic action on the induction of mast cell-associated neuronal injury. The cognate mast cell line RBL-2H3, when subjected to an antigenic stimulus, released tumor necrosis factor-α which, together with exogenous interleukin-1β (or interferon-γ), induced astroglia to produce neurotoxic quantities of nitric oxide. A small but significant proportion of mast cell-derived neurotoxicity under the above conditions occurred independently of glial nitric oxide synthase induction. Further, palmitoylethanolamide, which has been reported to reduce mast cell activation by a local autacoid mechanism, decreased neuron loss resulting from mast cell stimulation in the mixed cultures but not that caused by direct cytokine induction of astrocytic nitric oxide synthase. These results support the notion that brain mast cells could participate in the pathophysiology of chronic neurodegenerative and inflammatory diseases of the nervous system, and suggest that down-modulation of mast cell activation in such conditions could be of therapeutic benefit.     

Cytokine Induction of Inducible Nitric Oxide Synthase in an Oligodendrocyte Cell Line

Abstract : The induction of inducible nitric oxide synthase (iNOS) by proinflammatory cytokines was studied in an oligodendrocyte progenitor cell line in relation to mitogen-activated protein kinase (MAPK) activation and cytokine-mediated cytotoxicity. When introduced individually to cultures of CG4 cells, the cytokines, i.e., tumor necrosis factor-α (TNFα), interleukin-1 (IL-1), and interferon-γ (IFNγ), had either minimal (TNFα) or no (IL-1 and IFNγ) detectable stimulatory effect on the production of nitric oxide. However, combinations of these factors, in particular, TNFα plus IFNγ, elicited a strong enhancement of nitric oxide synthesis and, as revealed by western blot and RT-PCR analysis, the expression of iNOS. TNFα and IL-1 were able to activate p38 MAPK in a time- and dose-dependent manner and together showed a combinatorial effect. In contrast, IFNγ neither activated on its own nor enhanced the activation of p38 MAPK in response to TNFα and IL-1. However, a specific inhibitor of p38 MAPK, i.e., SB203580, inhibited the induction of iNOS in cytokine combination-treated cells in a dose-dependent manner, thereby suggesting a role for the MAPK cascade in regulating the induction of iNOS gene expression in cytokine-treated cells. Blocking of nitric oxide production by an inhibitor of iNOS, i.e., nitro-L-arginine methyl ester, had a minimal protective effect against cytokine-mediated cytotoxicity that occurred before the elevation of nitric oxide levels, thereby indicating temporal and functional dissociation of nitric oxide production from cell killing.     

Manganese modulates pro-inflammatory gene expression in activated glia

Redox-active metals are of paramount importance for biological functions. Their impact and cellular activities participate in the physiological and pathophysiological processes of the central nervous system (CNS), including inflammatory responses. Manganese is an essential trace element and it is required for normal biological activities and ubiquitous enzymatic reactions. However, excessive chronic exposure to manganese results in neurobehavioral deficits. Recent evidence suggests that manganese neurotoxicity involves activation of microglia or astrocytes, representative CNS immune cells. In this study, we assessed the molecular basis of the effects of manganese on the modulation of pro-inflammatory cytokines and nitric oxide (NO) production in primary rat cortical glial cells. Cultured glial cells consisted of 85% of astrocytes and 15% of microglia. Within the assayed concentrations, manganese was unable to induce tumor necrosis factor alpha (TNF-alpha) and inducible nitric oxide synthase (iNOS) expression, whereas it potentiated iNOS and TNF-alpha gene expression by lipopolysaccharide/interferon-gamma-activated glial cells. The enhancement was accompanied by elevation of free manganese, generation of oxidative stress, activation of mitogen-activated protein kinases, and increased NF-kappaB and AP-1 binding activities. The potentiated degradation of inhibitory molecule IkappaB-alpha was one of underlying mechanisms for the increased activation of NF-kappaB by manganese. However, manganese decreased iNOS enzymatic activity possibly through the depletion of cofactor since exogenous tetrahydrobiopterin reversed manganese's action. These data indicate that manganese could modulate glial inflammation through variable strategies.     

Inflammatory Neurodegeneration and Mechanisms of Microglial Killing of Neurons

Inflammatory neurodegeneration contributes to a wide variety of brain pathologies. A number of mechanisms by which inflammatory-activated microglia and astrocytes kill neurons have been identified in culture. These include: (1) acute activation of the phagocyte NADPH oxidase (PHOX) found in microglia, (2) expression of the inducible nitric oxide synthase (iNOS) in glia, and (3) microglial phagocytosis of neurons. Activation of PHOX (by cytokines, β-amyloid, prion protein, lipopolysaccharide, ATP, or arachidonate) causes microglial proliferation and inflammatory activation; thus, PHOX is a key regulator of inflammation. However, activation of PHOX alone causes little or no death, but when combined with iNOS expression results in apparent apoptosis via peroxynitrite production. Nitric oxide (NO) from iNOS expression also strongly synergizes with hypoxia to induce neuronal death because NO inhibits cytochrome oxidase in competition with oxygen, resulting in glutamate release and excitotoxicity. Finally, microglial phagocytosis of these stressed neurons may contribute to their loss.     

Induction and Regulation of Nitric Oxide Synthase in Retinal Müller Glial Cells

Abstract: Müller glial cells from the rat retina were examined for their capacity to produce nitric oxide (NO). Treatment of retinal Müller glial (RMG) cells with lipopolysaccharide (LPS), interferon-γ, and tumor necrosis factor-α induced NO synthesis as determined by nitrite release in media. Simultaneous addition of LPS, interferon-γ, and tumor necrosis factor-α caused the largest increase in NO synthesis. NO biosynthesis was detected after 12 h and was dependent on the dose of LPS, interferon-γ, and tumor necrosis factor-α. Stereoselective inhibitors of NO synthase (NOS), cycloheximide and transforming growth factor-β, blocked cytokine-induced NO production. Cytosol from LPS/cytokine-treated RMG cultures, but not from unstimulated cultures, produced a calcium/calmodulin-independent conversion of l -arginine to l -citrulline that was completely blocked by NOS inhibitor. The expression of NOS in RMG cells was confirmed by northern blot analysis, in which stimulation of these cells led to an increase in NOS mRNA levels. We conclude that RMG cells can express an inducible form of NOS similar to the macrophage isoform. High NO release from activated RMG cells might represent a protection from infection but may also contribute to the development of retinal pathologies.     

Induction of Nitric Oxide Synthase in Glial Cells

Primary astrocyte cultures, C6 glioma cells, and N18 neuroblastoma cells were assayed for nitric oxide synthase (NOS) activity with a bioassay of cyclic GMP production in RFL-6 fibroblasts. Treatment of astrocyte cultures for 16-18 h with lipopolysaccharide (LPS) induced NOS-like activity that was L-arginine and NADPH dependent, Ca2+ independent, and potentiated by superoxide dismutase. Induction was evident after 4 h, was dependent on the dose of LPS, and required protein synthesis. Treatment of astrocyte cultures with leucine methyl ester reduced microglial cell contamination from 7 to 1%, with a loss of 44% of NOS-like activity. C6 cells treated with LPS also showed Ca(2+)-independent and L-arginine-dependent NOS-like activity. N18 cells demonstrated constitutive Ca(2+)-dependent NOS-like activity that was not enhanced by LPS induction. These data indicate that NOS-like activity can be induced in microglia, astrocytes, and a related glioma cell line as it can in numerous other cell types, but not in neuron-like N18 cells.     

Interaction of 5-methyltetrahydrofolate and tetrahydrobiopterin on endothelial function

The present study was designed to investigate the interaction between 5-methyltetrahydrofolate and tetrahydrobiopterin in modulating endothelial function. Tetrahydrobiopterin is a critical cofactor for nitric oxide synthase and maintains this enzyme as a nitric oxide- versus superoxide-producing enzyme. The structure of 5-methyltetrahydrofolate is similar to tetrahydrobiopterin and both agents have been shown to improve endothelium-dependent vasodilatation. We hypothesized that 5-methyltetrahydrofolate interacts with nitric oxide synthase in a fashion analogous, yet independent, of tetrahydrobiopterin to improve endothelial function. We demonstrate that 5-methyltetrahydrofolate binds the active site of nitric oxide synthase and mimics the orientation of tetrahydrobiopterin. Furthermore, 5-methyltetrahydrofolate attenuates superoxide production (induced by inhibition of tetrahydrobiopterin synthesis) and improves endothelial function in aortae isolated from tetrahydrobiopterin-deficient rats. We suggest that 5-methyltetrahydrofolate directly interacts with nitric oxide synthase to promote nitric oxide (vs. superoxide) production and improve endothelial function. 5-Methyltetrahydrofolate may represent an important strategy for intervention aimed at improving tetrahydrobiopterin bioavailability.     

Highly purified lipoteichoic acid induced pro-inflammatory signalling in primary culture of rat microglia through Toll-like receptor 2: selective potentiation of nitric oxide production by muramyl dipeptide

In contrast to the role of lipopolysaccharide from Gram-negative bacteria, the role of Gram-positive bacterial components in inducing inflammation in the CNS remains controversial. We studied the potency of highly purified lipoteichoic acid and muramyl dipeptide isolated from Staphylococcus aureus to activate primary cultures of rat microglia. Exposure of pure microglial cultures to lipoteichoic acid triggered a significant time- and dose-dependent production of pro-inflammatory cytokines (tumour-necrosis factor-alpha, interleukin-1beta, interleukin-6) and nitric oxide. Muramyl dipeptide strongly and selectively potentiated lipoteichoic acid-induced inducible nitric oxide synthase expression and nitric oxide production. However, it did not have any significant influence on the production of pro-inflammatory cytokines. As bacterial components are recognised by the innate immunity through Toll-like receptors (TLRs) we showed that lipoteichoic acid was recognised in microglia by the TLR2 and lipopolysaccharide by the TLR4, as cells isolated from mice lacking TLR2 or TLR4 did not produce pro-inflammatory cytokines and nitric oxide upon lipoteichoic acid or lipopolysaccharide stimulation, respectively. Lipoteichoic acid-induced glia activation was mediated by p38 and ERK1/2 MAP kinases, as pretreatment with inhibitor of p38 or ERK1/2 decreased lipoteichoic acid-induced cytokine release, iNOS mRNA expression and nitric oxide production. The observed pro-inflammatory response induced by lipoteichoic acid-activated microglia could play a major role in the inflammatory response of CNS induced by Gram-positive bacteria.     

Nitric oxide synthesis and TNF-alpha secretion in RAW 264.7 macrophages: mode of action of a fermented papaya preparation

Macrophage inducible nitric oxide synthase is able to generate massive amounts of nitric oxide (NO) which contributes to the host immune defense against viruses and bacteria. Monocyte-macrophages stimulated with the bacterial wall component lipopolysaccharide (LPS) and cytokines such as interferon-gamma (IFN-gamma) express the inducible form of nitric oxide synthase (iNOS). Furthermore, tumor necrosis factor-alpha (TNF-alpha) is one of the central regulatory cytokines in macrophage antimicrobial activity and synergizes with IFN-gamma in the induction of NO synthesis. Because of its pivotal role in both antimicrobial and tumoricidal activities of macrophages, a significant effort has focused on developing therapeutic agents that regulate NO production. In the present study fermented papaya preparation (FPP) is shown to exert both immunomodulatory and antioxidant activity in the macrophage cell line RAW 264.7. Interestingly, a low and a high molecular weight fraction (LMF and HMF, respectively) of FPP exhibited different activity patterns. FPP fractions alone did not affect NO production. However in the presence of IFN-gamma, both LMF and HMF significantly increased iNOS activity and nitrite as well as nitrate accumulation. NO radical formation measured in real-time by electron paramagnetic resonance spectroscopy was higher in the presence of LMF and IFN-gamma. On the contrary, iNOS mRNA levels were enhanced further with HMF than with LMF. Moreover, LMF displayed a stronger superoxide anion scavenging activity than HMF. In the presence of IFN-gamma, both FPP fractions stimulated TNF-alpha secretion. However in non-stimulated macrophages, TNF-alpha secretion was enhanced by HMF only. Since water-soluble FPP fractions contained no lipid A, present data indicate that FPP is a macrophage activator which augments nitric oxide synthesis and TNF-alpha secretion independently of lipopolysaccharides.     

Anti-Inflammatory Effects of Ginsenoside-Rh2 Inhibits LPS-Induced Activation of Microglia and Overproduction of Inflammatory Mediators Via Modulation of TGF-β1/Smad Pathway

Microglia activation plays an important role in neuroinflammation and contributes to several neurological disorders. Hence, inhibition of both microglia activation and pro-inflammatory cytokines may lead to the effective treatment of neurodegenerative diseases. In this study, we found that GRh2 inhibited the inflammatory response to lipopolysaccharide (LPS) and prevented the LPS-induced neurotoxicity in microglia cells. GRh2 significantly decreased the generation of nitric oxide production, and tumor necrosis factor-α, interleukin (IL)-6, IL-1β, cyclooxygenase-2 and inducible nitric oxide synthase in LPS-induced activated microglia cells. Furthermore, GRh2 (20 and 50 μM) significantly increased TGF-β1 expression and reduced the expression of Smad. These results suggest that GRh2 effectively inhibits microglia activation and production of pro-inflammatory cytokines via modulating the TGF-β1/Smad pathway.     

Proinflammatory cytokine interferon-γ increases induction of indoleamine 2,3-dioxygenase in monocytic cells primed with amyloid β peptide 1–42: implications for the pathogenesis of Alzheimer's disease

Indoleamine 2,3-dioxygenase (IDO) is the rate-limiting enzyme of the kynurenine pathway of tryptophan metabolism, ultimately leading to production of the excitotoxin quinolinic acid (QUIN) by monocytic cells. In the Tg2576 mouse model of Alzheimer's disease, systemic inflammation induced by lipopolysaccharide leads to an increase in IDO expression and QUIN production in microglia surrounding amyloid plaques. We examined whether the IDO over-expression in microglia could be mediated by brain proinflammatory cytokines induced during the peripheral inflammation using THP-1 cells and peripheral blood mononuclear cells (PBMC) as models for microglia. THP-1 cells pre-treated with 5–25 μM amyloid β peptide (Aβ) (1–42) but not with Aβ (1–40) or Aβ (25–35) became an activated state as indicated by their morphological changes and enhanced adhesiveness. IDO expression was only slightly increased in the reactive cells but strongly enhanced following treatment with proinflammatory cytokine interferon-γ (IFN-γ) but not with interleukin-1β, tumor necrosis factor-α, or interleukin-6 at 100 U/mL. The concomitant addition of Aβ (1–42) with IFN-γ was totally ineffective, indicating that Aβ pre-treatment is prerequisite for a high IDO expression. The priming effect of Aβ (1–42) for the IDO induction was also observed for PBMC. These findings suggest that IFN-γ induces IDO over-expression in the primed microglia surrounding amyloid plaques.     

Nitric oxide synthase induction in glial cells: Effect on neuronal survival

In primary rat cortical glial cell cultures lipopolysaccharide (LPS) induced a dose- and time-dependent increase of intracellular cyclic GMP concentration associated with a release of nitrite. The LPS-induced cyclic GMP and nitrite increase was enhanced by interferon-γ and was prevented by L-N^G- nitroarginine, dexamethasone and cycloheximide. Thus indicates that LPS effect occured via the production of nitric oxide (NO) and involved new protein synthesis suggesting the induction of NO syntahse in these cells. Furthermore this induction was Ca²⁺-independent and was blocked by an inhibitor of the synthesis of tetrahydrobiopterin. The inducible NO synthase was also expressed by C6 glioma cells. In primary mixed cultures containing both neuronal and glial cells, the effects of LPS were less important than in primary glial cell cultures suggesting that glial cells rather than neurons expressed the inducible form of NO synthase. On the other hand no change on neuronal viability was observed after NO synthase induction by LPS in this culture type. This study indicates that glial cells are able to induce NO synthase without affecting neuronal survival.