Forskolin inhibits expression of inducible nitric oxide synthase mRNA via inhibiting the mitogen activated protein kinase in C6 cells

FTY720P inhibits the Na+/K+ ATPase in Caco-2 cells via S1PR2: PGE2 and NO are along the signaling pathway

AIMS

Sphingosine-1-phosphate (S1P) has been implicated lately in inflammatory bowel disease which has diarrhea as one of its symptoms. Diarrhea is due to altered water movements as a result of altered electrolyte transport, and in particular sodium. Sodium movements are geared by the sodium gradient established by the Na⁺/K⁺ ATPase. The aim of this work was to investigate if S1P can modulate the activity of the ATPase, using Caco-2 cells as a model and the S1P analogue, FTY720P.

MATERIALS AND METHODS

The activity of the ATPase was assayed by measuring the amount of inorganic phosphate liberated in presence and absence of ouabain. Protein expression of the various S1P receptors was studied by western blot analysis.

KEY FINDINGS

Caco-2 cells were found to express mainly S1PR2 and S1PR3. FTY720P (7.5 nM) reduced significantly the activity of the Na⁺/K⁺ ATPase when applied for 15 min. This inhibitory effect disappeared in presence of JTE-013, a specific blocker of S1PR2, and indomethacin, an inhibitor of cyclooxygenase enzymes, and was mimicked by CYM5520, a S1PR2 agonist and by exogenous PGE2. The inhibitory effect of PGE2 did not appear when EP3 receptors were blocked or when a nitric oxide scavenger was added. RpcAMP, a PKA inhibitor, reduced the activity of the Na⁺/K⁺ ATPase, while dbcAMP, a PKA activator was without any effect and when added, abrogated the effect of PGE2.

SIGNIFICANCE

It was concluded that FTY720P inhibits the Na+/K+ ATPase via activation of S1PR2 and generation of PGE2 nitric oxide.

Beta-3 Adrenoceptor Signaling Pathways in Urothelial and Smooth Muscle Cells in the Presence of Succinate

Succinate, an intermediate metabolite of the Krebs cycle, can alter the metabolomics response to certain drugs and controls an array of molecular responses in the urothelium through activation of its receptor, G-protein coupled receptor 91 (GPR91). Mirabegron, a β3-adrenergic receptor (β3-AR) agonist used to treat overactive bladder syndrome (OAB), increases intracellular cAMP in the detrusor smooth muscle cells (SMC), leading to relaxation. We have previously shown that succinate inhibits forskolin-stimulated cAMP production in urothelium. To determine whether succinate interferes with mirabegron-mediated bladder relaxation, we examined their individual and synergistic effect in urothelial-cell and SMC signaling. We first confirmed β3-AR involvement in the mirabegron response by quantifying receptor abundance by immunoblotting in cultured urothelial cells and SMC and cellular localization by immunohistochemistry in rat bladder tissue. Mirabegron increased cAMP levels in SMC but not in urothelial cells, an increase that was inhibited by succinate, suggesting that it impairs cAMP-mediated bladder relaxation by mirabegron. Succinate and mirabegron increased inducible nitric oxide synthesis and nitric oxide secretion only in urothelial cells, suggesting that its release can indirectly induces SMC relaxation. Succinate exposure decreased the expression of β3-AR protein in whole bladder in vivo and in SMC in vitro, indicating that this metabolite may lead to impaired pharmacodynamics of the bladder. Together, our results demonstrate that increased levels of succinate in settings of metabolic stress (e.g., the metabolic syndrome) may lead to impaired mirabegron and β3-AR interaction, inhibition of cAMP production, and ultimately requiring mirabegron dose adjustment for its treatment of OAB related to these conditions.

Increased resting intracellular calcium modulates NF-κB-dependent inducible nitric-oxide synthase gene expression in dystrophic mdx skeletal myotubes

Duchenne muscular dystrophy (DMD) is a genetic disorder caused by dystrophin mutations, characterized by chronic inflammation and severe muscle wasting. Dystrophic muscles exhibit activated immune cell infiltrates, up-regulated inflammatory gene expression, and increased NF-κB activity, but the contribution of the skeletal muscle cell to this process has been unclear. The aim of this work was to study the pathways that contribute to the increased resting calcium ([Ca(2+)](rest)) observed in mdx myotubes and its possible link with up-regulation of NF-κB and pro-inflammatory gene expression in dystrophic muscle cells. [Ca(2+)](rest) was higher in mdx than in WT myotubes (308 ± 6 versus 113 ± 2 nm, p < 0.001). In mdx myotubes, both the inhibition of Ca(2+) entry (low Ca(2+) solution, Ca(2+)-free solution, and Gd(3+)) and blockade of either ryanodine receptors or inositol 1,4,5-trisphosphate receptors reduced [Ca(2+)](rest). Basal activity of NF-κB was significantly up-regulated in mdx versus WT myotubes. There was an increased transcriptional activity and p65 nuclear localization, which could be reversed when [Ca(2+)](rest) was reduced. Levels of mRNA for TNFα, IL-1β, and IL-6 were similar in WT and mdx myotubes, whereas inducible nitric-oxide synthase (iNOS) expression was increased 5-fold. Reducing [Ca(2+)](rest) using different strategies reduced iNOS gene expression presumably as a result of decreased activation of NF-κB. We propose that NF-κB, modulated by increased [Ca(2+)](rest), is constitutively active in mdx myotubes, and this mechanism can account for iNOS overexpression and the increase in reactive nitrogen species that promote damage in dystrophic skeletal muscle cells.

Possible underlying influence of p38MAPK and NF-κB in the diminished anti-anxiety effect of diazepam in stressed mice

The present study was designed to explore the possible nitriergic influence and role of p38MAPK and NF-κB in the diminished anti-anxiety effect of diazepam in stressed mice, using the elevated plus maze and light/dark box to assess anxiety. Immobilization stress for 6 h enhanced an anxiety-like behavior and increased plasma nitrite levels in mice. Diazepam (2 mg/kg, i.p.) produced an anti-anxiety effect in unstressed mice, but could not produce any change in anxiety levels of stressed mice. SB-203580 (2 mg/kg, i.p.), a specific inhibitor of p38MAPK, per se produced a significant antianxiety-like activity in stressed mice. Administration of a combination of SB-203580 (2 mg/kg, i.p.) and diazepam (2 mg/kg) in stressed mice produced a significantly higher antianxiety-like activity than that produced by SB-203580 alone. Pyrrolidine dithiocarbamate (PDTC), an inhibitor of the activation of NF-κB, per se produced a significant antianxiety-like activity in stressed mice. Combination of PDTC and diazepam also served to produce a higher significant antianxiety-like activity in stressed mice than that produced by PDTC alone. Diazepam could not produce any change in plasma nitrite levels in both unstressed and stressed mice. Both SB-203580 (2 mg/kg, i.p.) and PDTC (100 mg/kg, i.p.) significantly decreased plasma nitrite levels in stressed mice. The observations indicate that the diminished anti-anxiety effect of diazepam in stressed mice may involve strong nitriergic influence and may further be p38MAPK- and NF-κB-dependent.

Beta-adrenergic receptor stimulation modulates iNOS protein levels through p38 and ERK1/2 signaling in human retinal endothelial cells

Diabetic retinopathy is the leading cause of blindness in working-age adults. Recently, data has suggested that some of the pathological changes that occur in the diabetic retina may be due to increased expression of inflammatory markers. We have previously reported that human retinal endothelial cells cultured in high glucose show increased protein levels of iNOS, which were reduced when cells were pre-treated with a beta-1-adrenergic receptor agonist, xamoterol. The cellular signaling involved in this response is not known. The hypothesis of this study was that modulation of PKA activity through beta-adrenergic receptor stimulation can alter members of the mitogen-activated protein kinase (MAP kinase) family to regulate iNOS protein levels. To address this hypothesis, human retinal endothelial cells were grown in high glucose medium, treated with xamoterol, and immunoblotting and ELISA analyses were done to evaluate cellular signaling of PKA, p38 MAP kinase, and p42/p44 MAP kinase. The results indicate that treatment with xamoterol significantly reduced PKA levels in the retinal endothelial cells cultured in high glucose. This reduction in PKA was followed by decreased ratios of phosphorylated p42/p44 and p38 MAP kinases. Blockade of p42/p44 or p38 signaling could restore iNOS protein levels to those prior to xamoterol stimulation, suggesting that beta-adrenergic receptor regulates iNOS protein levels through regulation of PKA and MAP kinase signaling. These results offer new avenues for therapeutic intervention for retinal diseases that involve altered inflammatory marker expression.

Amoeboid microglia in the periventricular white matter induce oligodendrocyte damage through expression of proinflammatory cytokines via MAP kinase signaling pathway in hypoxic neonatal rats

Hypoxic injury in the perinatal period results in periventricular white matter (PWM) lesions with axonal damage and oligodendroglial loss. It also alters macrophage function by perpetuating expression of inflammatory mediators. Relevant to this is the preponderance of amoeboid microglial cells (AMC) characterized as active macrophages in the developing PWM. This study aimed to determine if AMC produce proinflammatory cytokines that may be linked to the oligodendroglial loss observed in hypoxic PWM damage (PWMD). Wistar rats (1 day old) were subjected to hypoxia, following which upregulated expression of tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), TNF receptor 1 (TNF-R(1)) and IL-1 receptor 1 (IL-1R(1)) was observed. This was coupled with apoptosis and expression of TNF-R(1) and IL-1R(1) in oligodendrocytes. Primary cultured microglial cells subjected to hypoxia (3% oxygen, 5% CO(2) and 92% nitrogen) showed enhanced expression of TNF-alpha and IL-1beta. Furthermore, mitogen-activated protein (MAP) kinase signaling pathway was involved in the expression of TNF-alpha and IL-1beta in microglia subjected to hypoxia. Our results suggest that following a hypoxic insult, microglial cells in the neonatal rats produce inflammatory cytokines such as TNF-alpha and IL-1beta via MAP kinase signaling pathway. These cytokines are detrimental to oligodendrocytes resulting in PWM lesion.

Pro-inflammatory cytokine-induced SAPK/MAPK and JAK/STAT in rheumatoid arthritis and the new anti-depression drugs

BACKGROUND

Adult rheumatoid arthritis (RA) patients are frequently clinically depressed. Peripheral inflammation in RA may influence neurotransmitter metabolism, neuroendocrine function, synaptic plasticity, as well as growth factor production, which can modify neural circuitry and contribute to depression.

OBJECTIVE

A convergence between pro-inflammatory cytokine-induced synovial joint inflammation in RA and the effects of pro-inflammatory cytokines on the brain may occur through activation of the stress-activated/mitogen-activated protein kinases (SAPK/MAPK) and/or Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathways.

METHODS

The PubMed and Medlines databases were critically evaluated for evidence of SAPK/MAPK and/or JAK/STAT pathway activation in RA and depression.

RESULTS/CONCLUSION

Some novel anti-depression drugs that were employed in animal models of 'sickness behavior' and in human depression clinical trials suppressed clinical markers of inflammation, as well as SAPK/MAPK and/or JAK/STAT signaling in vitro. Modifying pro-inflammatory cytokine signaling pathways in the brain with antidepressants may also be useful in ameliorating peripheral inflammation in RA.

Chotosan enhances macrophage colony-stimulating factor mRNA expression in the ischemic rat brain and C6Bu-1 glioma cells

Macrophage colony stimulating factor (M-CSF) is a cytokine which has been recently reported to have a neuroprotective effect on ischemic rat brain. In this study, we investigated the effect of chotosan, an oriental medicine, which has been clinically demonstrated to be effective for the treatment of vascular dementia, on M-CSF gene expression in rats with permanent occlusion of bilateral common carotid arteries (P2VO) in vivo and in a C6Bu-1 glioma cell line in vitro. The expression level of M-CSF mRNA in the cerebral cortices of P2VO rats was significantly higher than that in the cerebral cortices of sham-operated animals. Repeated treatment of P2VO rats with chotosan (75 mg/kg per day) for 4 d after P2VO significantly increased the expression level of M-CSF mRNA in the cortex but it had no effect on the expression of beta-actin, granulocyte colony stimulating factor (G-CSF), granulocyte/macrophage colony stimulating factor (GM-CSF) mRNAs. Moreover, the present in vitro studies revealed that chotosan treatment (10-100 mug/ml) of C6Bu-1 glioma cells dose-dependently enhanced M-CSF mRNA expression without affecting the expression of G-CSF, GM-CSF, and inducible nitric oxide synthase mRNAs. The effect of chotosan was reversed by Ro 31-8220 (1 muM), a selective protein kinase C (PKC) inhibitor, but not by H-89 (10 muM), a selective protein kinase A (PKA) inhibitor. These findings suggest that the upregulatory effect of chotosan on M-CSF mRNA expression involves PKC and may play an important role in the anti-vascular dementia action of this formula.

Opposite effects of two PKA inhibitors on cAMP inhibition of IGF-I-induced oligodendrocyte development: a problem of unspecificity?

The stimulatory effect of insulin-like growth factor I (IGF-I) on myelin basic protein (MBP) expression, a parameter for oligodendrocyte development, is mediated by the MAPK and PI3K signaling pathways. We have previously shown that the second messenger cAMP inhibits IGF-I-induced MAPK activation as well as MBP expression. We also showed that the PKA inhibitor Rp-cAMPS reverted the cAMP effect on IGF-I-induced MBP without affecting the cAMP effect on IGF-I-induced MAPK activation. Here we report that, in contrast to Rp-cAMPS, H89 (a PKA inhibitor structurally non-related to Rp-cAMPS) enhances both the inhibitory effect of cAMP on IGF-I-induced MBP expression and the inhibitory effect of cAMP on IGF-I-induced MAPK activation. Likewise, H89 is capable of inhibiting the IGF-I-induced MAPK activation in the absence of PKA stimulation. Thus, we hypothesize that an unspecific action of H89 on a target located upstream MAPK could account for the discrepancies between the effects elicited by Rp-cAMPS and H89.

The effects of phorbol 12-myristate 13-acetate, cholera toxin, prostaglandin E2 and norepinephrine on inducible nitric oxide synthase activation induced by lipopolysaccharide in C6 cells

Nitric oxide (NO) plays a significant role in the pathophysiology of the central nervous system including inflammatory, ischemic and traumatic injuries. We demonstrated the possible involvement of protein kinase C (PKC) as well as protein kinase A (PKA) in the regulation of NO synthesis induced by lipopolysaccharide (LPS) treatment. In this study, the role of phorbol 12-myristate 13-acetate (PMA), cholera toxin (CTX), pertussis toxin (PTX), prostaglandin E(2) (PGE(2)) and norepinephrine (NE) in the regulation of NO synthesis was examined in C6 glioma cells. Stimulation with LPS (1 microg/ml) evoked increases in NO production in C6 glioma cells. LPS-induced NO production was enhanced by pretreatment with PMA, CTX and PGE(2). PTX pretreatment had no effect on NO production induced by LPS. In addition, NE inhibited NO production elicited by LPS treatment. These results suggest that NO production induced by LPS in C6 glioma cells is regulated by several kinds of pathways in which CTX-specific G protein, PKC, prostanoid EP(4) receptor and adrenergic receptor may play important roles.

Roles of Ras and extracellular signal-regulated kinase-dependent IkappaBalpha degradation in oridonin-enhanced phagocytosis of apoptotic cells by human macrophage-like U937 cells

Rapid recognition and ingestion of apoptotic cells by phagocytes are important for the prevention of toxic intracellular contents release, thereby attenuate inflammation and autoimmune diseases such as systemic lupus erythematosus (SLE). We have reported that oridonin isolated from Rabdosia rubescens enhanced phagocytosis of apoptotic U937 cells by macrophage-like U937 cells through TNFalpha and IL-1beta release. In this study, the molecular mechanisms involved in this phagocytic process are investigated. Inhibitors of Ras and Raf1 kinase significantly reduced oridonin-induced phagocytic stimulation as well as extracellular signal-regulated kinase (ERK) phosphorylation. Simultaneously, oridonin-enhanced engulfment was partially blocked by a nuclear factor (NF)-kappaB inhibitor PDTC or proteasome inhibitor MG132. Further studies revealed that oridonin induced IkappaBalpha degradation, which was prevented by Ras inhibitor manumycin A, ERK inhibitor PD98059, but not prevented by c-Jun N-terminal kinase (JNK) MAPK inhibitor SP600125, and up-regulated expression of IL-1beta precursor. These results demonstrate that Ras/Raf1/ERK signaling pathway-dependent IkappaBalpha degradation, resulting in NF-kappaB activation, participates in regulation of oridonin-enhanced phagocytosis, and one of its effector functions is to induce synthesis of IL-1beta, which partially contribute to phagocytic activity of oridonin.

Resveratrol inhibits nitric oxide and TNF-alpha production by lipopolysaccharide-activated microglia

Upon activation, brain macrophages, the microglia, release proinflammatory mediators that play important roles in eliciting neuroinflammatory responses associated with neurodegenerative diseases. As resveratrol, an antioxidant component of grape, has been reported to exert anti-inflammatory activities on macrophages, we investigated its effects on the production of TNF-alpha (TNF-alpha) and nitric oxide (NO) by lipopolysaccharide (LPS)-activated microglia. Exposure of cultured rat cortical microglia and a mouse microglial cell line N9 to LPS increased their release of TNF-alpha and NO, which was significantly inhibited by resveratrol. Further studies revealed that resveratrol suppressed LPS-induced degradation of IkappaBalpha, expression of iNOS and phosphorylation of p38 mitogen-activated protein kinases (MAPKs) in N9 microglial cells. These results demonstrate a potent suppressive effect of resveratrol on proinflammatory responses of microglia, suggesting a therapeutic potential for this compound in neurodegenerative diseases accompanied by microglial activation.

Involvement of phospholipase A2 and lipoxygenase in lipopolysaccharide-induced inducible nitric oxide synthase expression in glial cells

The present study underlines the importance of phospholipase A2 (PLA2)- and lipoxygenase (LO)-mediated signaling processes in the regulation of inducible nitric oxide synthase (iNOS) gene expression. In glial cells, lipopolysaccharide (LPS) induced the activities of PLA2 (calcium-independent PLA2; iPLA2 and cytosolic PLA2; cPLA2) as well as gene expression of iNOS. The inhibition of cPLA2 by methyl arachidonyl fluorophosphates (MAFP) or antisense oligomer against cPLA2 and inhibition of iPLA2 by bromoenol lactone reduced the LPS-induced iNOS gene expression and NFkappaB activation. In addition, the inhibition of LO by nordihydroguaiaretic acid (NDGA; general LO inhibitor) or MK886 (5-LO inhibitor), but not baicalein (12-LO inhibitor), completely abrogated the LPS-induced iNOS expression. Because NDGA could abrogate the LPS-induced activation of NFkappaB, while MK886 had no effect on it, LO-mediated inhibition of iNOS gene induction by LPS may involve an NFkappaB-dependent or -independent (by 5-LO) pathway. In contrast to LO, however, the cyclooxygenase (COX) may not be involved in the regulation of LPS-mediated induction of iNOS gene because COX inhibition by indomethacin (general COX inhibitor), SC560 (COX-1 inhibitor), and NS398 (COX-2 inhibitor) affected neither the LPS-induced iNOS expression nor activation of NFkappaB. These results indicate a role for cPLA2 and iPLA2 in LPS-mediated iNOS gene induction in glial cells and the involvement of LO in these reactions.

Adenosine kinase inhibitor attenuates the expression of inducible nitric oxide synthase in glial cells

The present study demonstrates the anti-inflammatory effect of adenosine kinase inhibitor (ADKI) in glial cells. Treatment of glial cells with IC51, an ADKI, stimulated the extracellular adenosine release and reduced the LPS/IFNgamma-mediated production of NO, and induction of iNOS and TNF-alpha gene expression. The recovery of IC51-mediated inhibition of iNOS expression by adenosine transport inhibitor, S-(4-nitrobenzyl)-6-thioinosine (NBTI), and the inhibition of LPS/IFNgamma-induced iNOS gene expression by exogenous adenosine indicate a role for adenosine release in IC51-mediated iNOS expression. The rescue of IC51-mediated inhibition of iNOS expression by adenosine receptor antagonist for A2A, 8-(3-chlorostyryl)caffeine (CSC) and alloxazine for A2B, further supports a role for interaction of adenosine and its receptors in anti-inflammatory activity. The IC51-mediated induction of cAMP levels, downstream target of A2A and A2B, and inhibition of LPS/IFNgamma-induced expression of iNOS by forskolin, a cAMP activator, document a role for cAMP mediated pathway in anti-inflammatory activity of IC51. Taken together, these studies document that IC51-mediated inhibition of iNOS expression is through activation of adenosine receptors, which activates A2A and A2B resulting in increased cAMP levels following LPS/IFNgamma stimulation. Moreover, the lack of effect of IC51 or adenosine on NFkappaB DNA binding activity and its transactivity indicates that the inhibition of iNOS expression mediated by IC51 may be through an NFkappaB independent pathway.

Dual role of cAMP in iNOS expression in glial cells and macrophages is mediated by differential regulation of p38-MAPK/ATF-2 activation and iNOS stability

We reported previously that cAMP analogues or cAMP synthesis activator (forskolin; FSK) inhibit lipopolysaccharide (LPS)-induced inducible nitric-oxide systase (iNOS) gene expression in astrocytes, while they enhance that in macrophages. Here, we report that the FSK-mediated inhibition of iNOS expression in C6 glial cells is due to its reduced transcriptional activity, while the FSK-mediated enhancement of iNOS expression in RAW264.7 macrophages is a result of increased stability of iNOS protein without transcriptional enhancement. The LPS/interferon-gamma (IFN)-induced iNOS transcription was inhibited by FSK via inhibition of p38-MAPK/ATF-2 activity in glial cells while it was not affected in macrophages. In both cell types, proteasome activities were required for the spontaneous degradation of iNOS protein, and the inhibition of proteasome activity by MG132 after maximum increase of iNOS protein levels further enhanced iNOS protein induction by LPS/IFN, suggesting the involvement of proteasome in iNOS degradation. More importantly, the iNOS protein levels were equalized by the MG132 posttreatment in macrophages treated with LPS/IFN alone and along with FSK, and ubiquitinated iNOS protein levels were reduced by FSK posttreatment, suggesting that the FSK-mediated inhibition of ubiquitination of iNOS protein and the following increased stability of iNOS protein are one of the mechanisms of cAMP-pathway-mediated enhancement of iNOS gene expression in macrophages. To our knowledge, this is the first evidence that cAMP regulates iNOS expression at the posttranslational level in macrophages.

Stimulation of myelin proteolipid protein gene expression by eicosapentaenoic acid in C6 glioma cells

In this study, the role of exogenous fatty acids in the regulation of proteolipid protein (PLP) gene expression was investigated using the following model culture system: C6 glioma cells expressing the green-fluorescent protein (eGFP) driven by different segments of PLP promoter. Eicosapentanoic acid (EPA; 20:5 n-3), but not arachidonic acid (AA; 20:4 n-6), induced a significant increase in medium fluorescence intensity (MFI) determined by fluorescence-activated cell sorting (FACS). The induction of PLP promoter was time-dependent showing maximal activity between 24 and 48 h after EPA exposure. PLP promoter activation was dependent on fatty acid concentration, with maximum activation at 200 microM. Northern blot analysis confirmed the fluorescence data in C6 cells incubated with EPA. Furthermore, this treatment increased the adenylyl cyclase-cyclic AMP (cAMP) levels and the mitogen-activated protein kinase (MAPK) activation in C6 cells. PLP promoter activity was inhibited by pre-treatment with H89 (protein kinase A (PKA) inhibitor), but not with PD98059 (MAPK inhibitor), suggesting that EPA stimulates the expression of PLP via cAMP-mediated pathways.

The regulation of inducible nitric oxide synthase gene expression induced by lipopolysaccharide and tumor necrosis factor-alpha in C6 cells: involvement of AP-1 and NFkappaB

The roles of AP-1 and NFkappaB in the regulation of inducible nitric oxide synthase (iNOS) mRNA expression induced by the combination of lipopolysaccharide and tumor necrosis factor-alpha (LT) in C6 cells were examined in the present study. The iNOS mRNA level and NO release were increased by several cytokines alone or combination treatments at 24 hr. LT-induced iNOS mRNA level was maximally increased at 6 hr and maintained at higher level at least up to 24 hr. At 6 hr, iNOS protein level and NO release were also increased by LT. By western blot analysis, AP-1, such as Fra-1, Jun B, and phospho-CREB protein levels were increased by LT and translocation of NFkappaB p52 from the cytoplasm to the nucleus was increased. In addition, phosphorylations of MAPKs (ERK 1/2, p38, JNK 1/2) were increased by LT. LT-induced iNOS mRNA level was inhibited by PD98059 (MEK 1/2 inhibitor), SB203580 (p38 inhibitor), and cycloheximide (a protein synthesis blocker), indicating that the phosphorylation of ERK 1/2 and p38, and on-going protein synthesis are necessary for LT-induced iNOS expression. Electrophoretic mobility shift assay (EMSA) showed that AP-1 and NFkappaB DNA binding activities were increased at 6 hr and these AP-1 and NFkappaB DNA bands increased by LT were super-shifted when Fra-1, Jun B, or NFkappaB p50 antibody was coincubated. These findings strongly suggest that, in C6 cells, Fra-1, Jun B, NFkappaB p50, and NFkappaB p52 appear to be involved in the regulation of iNOS mRNA induced by LT.

Hypoxia induces nitric oxide production in mouse microglia via p38 mitogen-activated protein kinase pathway

In vitro exposure of microglial cells to hypoxia induces cellular activation. Also, in vivo studies of glial activation following ischemic hypoxia have shown that neuronal cell death is followed by microglial activation. Thus, it is likely that toxic inflammatory mediators produced by activated microglial cells under hypoxic conditions may exacerbate neuronal injury following cerebral ischemia. Nitric oxide (NO), which is known to be produced by activated microglia, may participate in this process. In the current work, we sought to determine whether and how the production of NO and the expression of inducible NO synthase (iNOS) are triggered by hypoxia in microglial cells. Exposure of established microglial cell lines as well as primary mouse microglial cultures to mild hypoxia (8 h) followed by reoxygenation (24 h) induced the production of NO and TNFalpha, indicating that hypoxia could lead to the inflammatory activation of microglia. Hypoxic induction of NO was accompanied by iNOS induction. Moreover, hypoxia induced the activation of p38 MAPK, but not ERK or JNK/SAPK, in BV-2 mouse microglial cells. SB203580, a specific inhibitor of p38 MAPK, blocked the hypoxic induction of NO and iNOS. Taken together, our results indicated that hypoxia could induce inflammatory activation of microglia, and the hypoxic induction of NO production in microglia is mediated through p38 MAPK pathway. Thus, during cerebral ischemia, hypoxia may not only directly damage neurons, but may also promote neuronal injury indirectly via microglial activation.

The in vitro effects of Pb acetate on NO production by C6 glial cells

The CNS neurotoxic effects of lead (Pb) are well documented; however, the molecular toxicity targets have not been clearly delineated. Astroglial cells, which are the most abundant cells in the brain and provide critical support to the neurons, are known to accumulate Pb. Although NO generated by inducible NO synthase (iNOS) in glial cells has been associated with many neurotoxic events, it can also serve to protect by modulating blood flow, increase antimicrobial and tumoricidal activities, and promote immune responses following injury or insult. The present investigations were designed to test the hypothesis that Pb exposure may perturb cytokine signal transduction pathways leading to NO production by astroglial cells. Pretreatment with Pb acetate (500 nM-10 microM) attenuated the generation of NO in a concentration-dependent manner up to 90%, and suppressed iNOS protein expression, as well as interfered with the homeostatic functions of calcium in the cytokine-induced NO signal transduction pathway. In addition, pretreatment with staurosporine, a serine-threonine kinase inhibitor, or KT5720, a specific protein kinase A inhibitor (PKA), inhibited cytokine-induced NO production in a concentration-dependent manner with IC(50) values of 26.3 and 346.7 nM, respectively. Therefore, Pb may impede events within the PKA signal transduction pathway; although, based on results from a gel shift assay, Pb does not directly affect PKA enzyme activity. Taken together, these results suggest the possibility that the suppressive effect of Pb acetate on cytokine-induced NO production in glial cells may be implicated in the neurophysiologic changes noted following occupational or environmental exposure to Pb.

Stimulation of insulin secretion and associated nuclear accumulation of iPLA(2)beta in INS-1 insulinoma cells

Accumulating evidence suggests that the cytosolic calcium-independent phospholipase A(2) (iPLA(2)beta) manifests a signaling role in insulin-secreting (INS-1) beta-cells. Earlier, we reported that insulin-secretory responses to cAMP-elevating agents are amplified in iPLA(2)beta-overexpressing INS-1 cells (Ma Z, Ramanadham S, Bohrer A, Wohltmann M, Zhang S, and Turk J. J Biol Chem 276: 13198-13208, 2001). Here, immunofluorescence, immunoaffinity, and enzymatic activity analyses are used to examine distribution of iPLA(2)beta in stimulated INS-1 cells in greater detail. Overexpression of iPLA(2)beta in INS-1 cells leads to increased accumulation of iPLA(2)beta in the nuclear fraction. Increasing glucose concentrations alone results in modest increases in insulin secretion, relative to parental cells, and in nuclear accumulation of the iPLA(2)beta protein. In contrast, cAMP-elevating agents induce robust increases in insulin secretion and in time-dependent nuclear accumulation of iPLA(2)beta fluorescence, which is reflected by increases in nuclear iPLA(2)beta protein content and specific enzymatic activity. The stimulated effects are significantly attenuated in the presence of cell-permeable inhibitors of protein phosphorylation and glycosylation. These findings suggest that conditions that amplify insulin secretion promote translocation of beta-cell iPLA(2)beta to the nuclei, where it may serve a crucial signaling role.

Induction of caspase-11 by inflammatory stimuli in rat astrocytes: lipopolysaccharide induction through p38 mitogen-activated protein kinase pathway

Caspase-11 plays a crucial role in both inflammation and apoptosis. Caspase-11 not only activates caspase-1, that is required for the maturation of proinflammatory cytokines such as interleukin (IL)-1 and IL-18, but also activates caspase-3, leading to cellular apoptosis under pathological conditions. Here, we cloned the rat homolog of caspase-11, and investigated its inducibility by inflammatory stimuli and signal transduction pathways involved. Deduced amino acid sequence of rat caspase-11 showed 88.7% similarity to mouse caspase-11, and in vitro translation of rat caspase-11 cDNA yielded approximately a 43 kDa polypeptide, which was in agreement with predicted protein size generated from full-length rat caspase-11 cDNA. The expression of caspase-11 was strongly induced at both mRNA and protein levels by inflammatory stimuli such as lipopolysaccharide (LPS), interferon-gamma, and tumor necrosis factor-alpha in C6 rat glial cells as well as primary astrocytes. LPS induced activation of both p38 mitogen-activated protein kinase (MAPK) and extracellular signal-regulated kinase (ERK) in C6 cells. However, SB203580 (specific inhibitor of p38 kinase), but not PD98059 (specific inhibitor of ERK kinase), inhibited LPS induction of caspase-11, indicating that induction of caspase-11 by LPS in astrocytes was mediated through the p38 MAPK pathway. Inflammatory induction of caspase-11 in astrocytes may play an important role in both inflammatory responses involving these cells and auto-regulatory apoptosis of activated astrocytes in inflammatory sites.