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

Intertwined associations between oxidative and nitrosative stress and endocannabinoid system pathways: Relevance for neuropsychiatric disorders

The endocannabinoid system (ECS) appears to regulate metabolic, cardiovascular, immune, gastrointestinal, lung, and reproductive system functions, as well as the central nervous system. There is also evidence that neuropsychiatric disorders are associated with ECS abnormalities as well as oxidative and nitrosative stress pathways. The goal of this mechanistic review is to investigate the mechanisms underlying the ECS's regulation of redox signalling, as well as the mechanisms by which activated oxidative and nitrosative stress pathways may impair ECS-mediated signalling. Cannabinoid receptor (CB)1 activation and upregulation of brain CB2 receptors reduce oxidative stress in the brain, resulting in less tissue damage and less neuroinflammation. Chronically high levels of oxidative stress may impair CB1 and CB2 receptor activity. CB1 activation in peripheral cells increases nitrosative stress and inducible nitric oxide (iNOS) activity, reducing mitochondrial activity. Upregulation of CB2 in the peripheral and central nervous systems may reduce iNOS, nitrosative stress, and neuroinflammation. Nitrosative stress may have an impact on CB1 and CB2-mediated signalling. Peripheral immune activation, which frequently occurs in response to nitro-oxidative stress, may result in increased expression of CB2 receptors on T and B lymphocytes, dendritic cells, and macrophages, reducing the production of inflammatory products and limiting the duration and intensity of the immune and oxidative stress response. In conclusion, high levels of oxidative and nitrosative stress may compromise or even abolish ECS-mediated redox pathway regulation. Future research in neuropsychiatric disorders like mood disorders and deficit schizophrenia should explore abnormalities in these intertwined signalling pathways.

Hydroxycinnamic acid-spermidine amides from Tetragonisca angustula honey as anti-Neospora caninum: In vitro and in silico studies

Tetragonisca angustula honey was fractioned in a SiO2 column to furnish three fractions (A-C) in which four hydroxycinnamic acid-Spermidine amides (HCAAs), known as N', N″, N‴-tris-p-coumaroyl spermidine, N', N″-dicaffeoyl, N‴-coumaroyl spermidine, N', N″, N‴-tris-caffeoyl spermidine and N', N″-dicaffeoyl and N‴-feruloyl spermidine were identified in the fractions B and C by electrospray ionization tandem mass spectrometry. A primary culture model previously infected with Neospora caninum (72 h) was used to evaluate the honey fractions (A-C) for two-time intervals: 24 and 72 h. Parasitic reduction ranged from 38% on fraction C (12.5 µg/ml), after 24 h, to 54% and 41% with fractions B and C (25 µg/ml) after 72 h of treatment, respectively. Additionally, HCAAs did not show any cell toxicity for 24 and 72 h. For infected cultures (72 h), the active fractions B (12.5 µg/ml) and C (25 µg/ml) decreased their NO content. In silico studies suggest that HCAAs may affect the parasite's redox pathway and improve the oxidative effect of NO released from infected cells. Here, we presented for the first time, that HCAAs from T. angustula honey have the potential to inhibit the growth of N. caninum protozoa.

Inducible nitric oxide synthase inhibitors: A comprehensive update

Inducible nitric oxide synthase (iNOS), which is expressed in response to bacterial/proinflammatory stimuli, generates nitric oxide (NO) that provides cytoprotection. Overexpression of iNOS increases the levels of NO, and this increased NO level is implicated in pathophysiology of complex multifactorial diseases like Parkinson's disease, Alzheimer's disease, multiple sclerosis, rheumatoid arthritis, and inflammatory bowel disease. Selective inhibition of iNOS is an effective approach in treatment of such complex diseases. l-Arginine, being a substrate for iNOS, is the natural lead to develop iNOS inhibitors. More than 200 research reports on development of nitric oxide synthase inhibitors by different research groups across the globe have appeared in literature so far. The first review on iNOS, in 2002, discussed the iNOS inhibitors under two classes that is, amino acid and non-amino acid derivatives. Other review articles discussing specific chemical classes of iNOS inhibitors also appeared during last decade. In the present review, all reports on both natural and synthetic iNOS inhibitors, published 2002 onwards, are studied, classified, and discussed to provide comprehensive information on iNOS inhibitors. The synthetic inhibitors are broadly classified into two categories that is, arginine and non-arginine analogs. The latter are further classified into amidines, five- or six-membered heterocyclics, fused cyclics, steroidal type, and chalcones analogs. Structures of the most/significantly potent compounds from each report are provided to know the functional groups important for incurring iNOS inhibitory activity and selectivity. This review is aimed to provide a comprehensive view to the medicinal chemists for rational designing of novel and potent iNOS inhibitors.

Inhibition of Fatty Acid Amide Hydrolase by PF-3845 Alleviates the Nitrergic and Proinflammatory Response in Rat Hippocampus Following Acute Stress

BACKGROUND

Long-term exposure to stress has been demonstrated to cause neuroinflammation through a sustained overproduction of free radicals, including nitric oxide, via an increased inducible nitric oxide synthase activity. We previously demonstrated that inducible nitric oxide synthase activity and mRNA are significantly upregulated in the rat hippocampus following just 4 hours of restraint stress. Similar to nitric oxide, endocannabinoids are synthesized on demand, with preclinical observations suggesting that cannabinoid receptor agonists and endocannabinoid enhancers inhibit nitrergic activity. Specifically, previous work has shown that enhancement of endocannabinoids via inhibition of fatty acid amide hydrolase with PF-3845 reduced inducible nitric oxide synthase-expressing microglia following traumatic brain injury. However, this describes cannabinoid modulation following physical injury, and therefore the present study aimed to examine the effects of PF-3845 in the modulation of nitrergic and inflammatory-related genes within the hippocampus after acute stress exposure.

METHODS

Following vehicle or PF-3845 injections (5 mg/kg; i.p.), male Wistar rats were exposed to 0 (control), 60, 240, or 360 minutes of restraint stress after which plasma and dorsal hippocampus were isolated for further biochemical and gene expression analysis.

RESULTS

The results demonstrate that pretreatment with PF-3845 rapidly ameliorates plasma corticosterone release at 60 minutes of stress. An increase in endocannabinoid signalling also induces an overall attenuation in inducible nitric oxide synthase, tumor necrosis factor-alpha convertase, interleukin-6, cyclooxygenase-2, peroxisome proliferator-activated receptor gamma mRNA, and the transactivation potential of nuclear factor kappa-light-chain-enhancer of activated B cells in the hippocampus.

CONCLUSIONS

These results suggest that enhanced endocannabinoid levels in the dorsal hippocampus have an overall antinitrosative and antiinflammatory effect following acute stress exposure.

Phosphodiesterase 2A is a major negative regulator of iNOS expression in lipopolysaccharide-treated mouse alveolar macrophages

PDE2A is a dual-function PDE that is stimulated by cGMP to hydrolyze cAMP preferentially. In a two-hit model of ALI, we found previously that PDE2A decreased lung cAMP, up-regulated lung iNOS, and exacerbated ALI. Recent data suggest that macrophage iNOS expression contributes to ALI but later, promotes lung-injury resolution. However, macrophage iNOS is increased by cAMP, suggesting that PDE2A could negatively regulate macrophage iNOS expression. To test this, we examined the effects of manipulating PDE2A expression and function on LPS-induced iNOS expression in a mouse AM cell line (MH-S) and primary mouse AMs. In MH-S cells, LPS (100 ng/ml) increased PDE2A expression by 15% at 15 min and 50% at 6 h before decreasing at 24 h and 48 h. iNOS expression appeared at 6 h and remained increased 48 h post-LPS. Compared with control Ad, Ad.PDE2A-shRNA enhanced LPS-induced iNOS expression further by fourfold, an effect mimicked by the PDE2A inhibitor BAY 60-7550. Adenoviral PDE2A overexpression or treatment with ANP decreased LPS-induced iNOS expression. ANP-induced inhibition of iNOS was lost by knocking down PDE2A and was not mimicked by 8-pCPT-cGMP, a cGMP analog that does not stimulate PDE2A activity. Finally, we found that in primary AMs from LPS-treated mice, PDE2A knockdown also increased iNOS expression, consistent with the MH-S cell data. We conclude that increased AM PDE2A is an important negative regulator of macrophage iNOS expression.

A kavalactone derivative inhibits lipopolysaccharide-stimulated iNOS induction and NO production through activation of Nrf2 signaling in BV2 microglial cells

Neuroinflammation and oxidative stress are involved in the pathogenesis of neurodegenerative diseases such as Alzheimer's diseases and Parkinson's disease. Naturally derived kavalactones isolated from Piper methysticum (Piperaceae) have been shown to exhibit neuroprotective effects. We have previously reported that a chemically synthesized kavalactone derivative, 2',6'-dichloro-5-methoxymethyl-5,6-dehydrokawain (compound 1) protects against oxidative stress-induced neuronal cell death through activation of Nrf2 signaling. In the present study, we examined the effect of compound 1 on neuroinflammation. In BV2 microglial cells, compound 1 strongly inhibited LPS-stimulated iNOS induction and NO production, but did not affect LPS-stimulated induction of COX2. At 6h after LPS challenge, when iNOS induction was not clearly seen, treatment with LPS or compound 1 alone increased expression of heme oxygenase 1 (HO-1) whose transcription is regulated by Nrf2. When treated with both, compound 1 enhanced LPS-stimulated HO-1 induction, which was more evident at 24h after LPS treatment. Furthermore, LPS-stimulated activation of Nrf2 signaling and nuclear translocation of Nrf2 were potentiated by compound 1. The mechanism by which compound 1 activated Nrf2 signaling was supposed to be a covalent modification of the sulfhydryl groups of Keap1 by an α,β-unsaturated carbonyl group present in the compound 1. Treatment with hemin, a HO-1 inducer, and with [Ru(CO)₃Cl₂]₂, a CO donor, decreased LPS-stimulated iNOS induction and NO production. In contrast, siRNA-mediated knockdown of HO-1 expression reduced the inhibitory effect of compound 1 on LPS-stimulated iNOS induction and NO production. The compound 1 inhibited LPS-stimulated ERK phosphorylation after LPS treatment. Finally, compound 1 suppressed LPS/IFN-γ-stimulated NO production in primary microglial cells. These results suggest that compound 1 is capable of inhibiting LPS-stimulated iNOS induction and NO production via activation of Nrf2 signaling and HO-1 induction in microglial cells. Taken together, compound 1 has a potential to reduce neuroinflammation as well as oxidative stress in neurodegenerative diseases through activation of Nrf2 signaling.

STAT1 mediates oroxylin a inhibition of iNOS and pro-inflammatory cytokines expression in microglial BV-2 cells

Microglia-mediated inflammation is implicated in pathogenesis of neurodegenerative diseases. Oroxylin A, a flavonoid isolated from Scutellariae baicalensis, has been shown to ameliorate microglia activation-mediated neurodegeneration in vivo. The molecular mechanism underlying the inhibitory effects of oroxylin A on microglia activation, however, remains unknown. In the present study, effects of oroxylin A co-treated with lipopolysaccharide (LPS, 100 ng/ml) on LPS-induced activation of cultured microglial BV-2 cells were examined. Nitric oxide (NO) production was determined by Greiss method. Expression of inducible nitric oxide synthase (iNOS), interleukin (IL)-1β and IL-6 was assessed using real-time RT-PCR or Western blot analysis. Furthermore, activation of the nuclear factor κB (NFκB) and the signal transducer and activator of transcription 1 (STAT1) was examined by Western blot analysis and transcription factor DNA-binding activity assay. Our results indicated that oroxylin A (10-100 µM) in a concentration-dependent manner inhibited LPS-induced NO production via blocking iNOS expression at both mRNA and protein levels without affecting the degradation rate of iNOS mRNA. Moreover, oroxylin A significantly attenuated LPS-induced late expression (20 hours after LPS challenge) of IL-1β and IL-6. Furthermore, oroxylin A significantly suppressed LPS-induced JAK2-mediated STAT1 phosphorylation without affecting LPS-induced NFκB-p65 nuclear translocation or NFκB-p65 DNA-binding activity. This is consistent with the finding that AG490, a specific JAK2 inhibitor, significantly inhibited LPS-induced STAT1 phosphorylation with almost completely diminished iNOS expression. These results suggest that oroxylin A, via suppressing STAT1 phosphorylation, inhibits LPS-induced expression of pro-inflammatory genes in BV-2 microglial cells.

Species-specific microRNA roles elucidated following astrocyte activation

MicroRNAs (miRNAs) are short non-coding RNAs that play a central role in regulation of gene expression by binding to target genes. Many miRNAs were associated with the function of the central nervous system (CNS) in health and disease. Astrocytes are the CNS most abundant glia cells, providing support by maintaining homeostasis and by regulating neuronal signaling, survival and synaptic plasticity. Astrocytes play a key role in repair of brain insults, as part of local immune reactivity triggered by inflammatory or pathological conditions. Thus, astrocyte activation, or astrogliosis, is an important outcome of the innate immune response, which can be elicited by endotoxins such as lipopolysaccharide (LPS) and cytokines such as interferon-gamma (IFN-γ). The involvement of miRNAs in inflammation and stress led us to hypothesize that astrogliosis is mediated by miRNA function. In this study, we compared the miRNA regulatory layer expressed in primary cultured astrocyte derived from rodents (mice) and primates (marmosets) brains upon exposure to LPS and IFN-γ. We identified subsets of differentially expressed miRNAs some of which are shared with other immunological related systems while others, surprisingly, are mouse and rat specific. Of interest, these specific miRNAs regulate genes involved in the tumor necrosis factor-alpha (TNF-α) signaling pathway, indicating a miRNA-based species-specific regulation. Our data suggests that miRNA function is more significant in the mechanisms governing astrocyte activation in rodents compared to primates.

The role of nitric oxide in osteoarthritis

Elevated levels of markers of nitric oxide (NO) production are found in osteoarthritic joints suggesting that NO is involved in the pathogenesis of osteoarthritis (OA). In OA, NO mediates many of the destructive effects of interleukin-1 (IL-1) and tumour necrosis factor-alpha (TNF-alpha) in the cartilage, and inhibitors of NO synthesis have demonstrated retardation of clinical and histological signs and symptoms in experimentally induced OA and other forms of arthritis. As an important factor in cartilage, the regulation of inducible nitric oxide synthase (iNOS) expression and activity, and the effects of NO are reviewed, especially in relation to the pathogenesis of OA.

Effect of caffeine on the secretion of peroxidase in rat submandibular gland: a study of its mechanism of action

OBJECTIVE

In this work, we analysed the mechanism of action of caffeine on peroxidase secretion in the female rat submandibular gland. The signaling molecules cAMP and nitric oxide were monitored as potential mediators.

DESIGN

The salivary gland peroxidase secretion of female albino Wistar rats was assessed by a spectroscopic method.

RESULTS

Caffeine was found to exert an increase on peroxidase secretion in a concentration-response manner: the peroxidase secretion stimulation index (SI) (secreted peroxidase from treated/secreted peroxidase from basal) for caffeine 10 microg/ml: 2.2+/-0.18 (P<0.05); caffeine 100 microg/ml alone: 3+/-0.18 (P<0.01); +LNMMA (LN monomethyl arginine): 1+/-0.1 (P<0.05); caffeine 1000 microg/ml alone: 5+/-0.35 (P<0.01); +LNMMA: 2+/-0.2 (P<0.05). These results were associated with an increase in cAMP and total nitrites production. Total nitrites, SI caffeine 100 microg/ml alone: 2.8+/-0.2 (P<0.01); +LNMMA: 1+/-0.08 (P<0.05); caffeine 1000 microg/ml alone: 4.8+/-0.3 (P<0.01); +LNMMA: 2.3+/-0.18 (P<0.05).

CONCLUSION

It could thus be concluded that cAMP and NO are involved in the mechanism of action displayed by caffeine. This is the first report on the mechanism of action of caffeine on peroxidase secretion.

Cyclic AMP: master regulator of innate immune cell function

Cyclic adenosine monophosphate (cAMP) was the original "second messenger" to be discovered. Its formation is promoted by adenylyl cyclase activation after ligation of G protein-coupled receptors by ligands including hormones, autocoids, prostaglandins, and pharmacologic agents. Increases in intracellular cAMP generally suppress innate immune functions, including inflammatory mediator generation and the phagocytosis and killing of microbes. The importance of the host cAMP axis in regulating antimicrobial defense is underscored by the fact that microbes have evolved virulence-enhancing strategies that exploit it. Many clinical situations that predispose to infection are associated with increases in cAMP, and therapeutic strategies to interrupt cAMP generation or actions have immunostimulatory potential. This article reviews the anatomy of the cAMP axis, the mechanisms by which it controls phagocyte immune function, microbial strategies to dysregulate it, and its clinical relevance.

Calcitonin gene-related peptide stimulation of nitric oxide synthesis and release from trigeminal ganglion glial cells

Clinical and basic science data support an integral role of calcitonin gene-related peptide (CGRP) in migraine pathology. Following trigeminal nerve activation, afferent release of CGRP causes vasodilation while efferent release leads to pain. Although CGRP can also be secreted from cell bodies of trigeminal neurons located within the ganglion, the function of CGRP released in the ganglion is poorly understood. Initially, we showed that SNAP-25, a protein required for CGRP release, was localized in cell bodies of trigeminal ganglia neurons. We also found that satellite glial cells in the ganglia express the CGRP1 receptor protein RAMP1. To determine whether CGRP could directly activate glial cells, primary cultures of rat trigeminal ganglia were utilized to study the effects of CGRP on glial nitric oxide (NO) synthesis and release. Under our culture conditions, >95% of the cells expressed glial fibrillary acidic protein and RAMP1. While weak iNOS staining was observed in glia under basal conditions, CGRP treatment greatly increased glial iNOS expression and NO release. This stimulatory effect was blocked by the CGRP1 receptor antagonist, CGRP(8-37) peptide. Treatment of glial cultures with forskolin or cAMP also increased iNOS expression and stimulated NO release to levels similar to CGRP. To our knowledge, this is the first evidence that activation of CGRP1 receptors regulates glial iNOS and NO release. We propose that following trigeminal nerve activation, CGRP secretion from neuronal cell bodies activates satellite glial cells that release NO and initiate inflammatory events in the ganglia that contribute to peripheral sensitization in migraine.

AH23848 accelerates inducible nitric oxide synthase degradation through attenuation of cAMP signaling in glomerular mesangial cells

Excessive release of nitric oxide (NO) by mesangial cells contributes to the pathogenesis of glomerulonephritis. Prostaglandin E(2) (PGE(2)) produced at inflammatory sites regulates the release of NO through its downstream signaling. In glomerular mesangial cells (MES-13 cells), PGE(2) modulated NO production mainly through EP4 receptor in a cAMP-dependent manner. Lipopolysaccharide and interferon-gamma (LPS+IFNgamma)-induced NO production, inducible nitric oxide synthase (iNOS) gene and protein expression were greatly inhibited by AH23848, an EP4 antagonist. Further investigation indicated that AH23848 attenuated endogenous cAMP accumulation in MES-13 cells and modulated NO production through declination of iNOS gene expression and acceleration of iNOS protein degradation. AH23848 downregulated the iNOS protein in MES-13 cells through protein kinase A (PKA) since KT5720, a PKA-specific inhibitor, reduced iNOS protein stability. A short exposure of activated MES-13 cells to okadaic acid augmented iNOS activity. AH23848 and KT5720 attenuated serine/threonine phosphorylation of iNOS protein in LPS + IFNgamma-stimulated MES-13 cells. The results of this study led us to speculate that cAMP might regulate iNOS-stimulated NO synthesis through posttranslational mechanisms. Attenuation of cAMP signaling and the phosphorylation status of the iNOS protein may account for the effect of AH23848 in accelerating iNOS protein degradation in MES-13 cells.

Branched‐chain amino acids accumulating in maple syrup urine disease induce morphological alterations in C6 glioma cells probably through reactive species

In the present study, we investigated the effects of the branched-chain amino acids (BCAA) leucine (Leu), isoleucine (Ile) and valine (Val), which accumulate in maple syrup urine disease (MSUD), on C6 glioma cell morphology and cytoskeletal reorganization by exposing the cultured cells to 1 and 5 mM BCAA. We observed that cells showed a fusiform shape with processes after 3 h treatment. Cell death was also observed when cells were incubated in the presence of the BCAA for 3 and 24 h. Val-treated cells presented the most dramatic morphological alterations. Immunocytochemistry with anti-actin and anti-GFAP antibodies revealed that all BCAA induced reorganization of actin and GFAP cytoskeleton. Although phosphorylation regulates intermediate filament (IF) assembly/disassembly, we verified that the BCAA did not change the in vitro phosphorylation of IF proteins either in C6 cells or in slices of cerebral cortex of rats during development (9-, 12-, 17- and 21-day-old). Furthermore, we observed that 3 h cell exposure to 5 mM of each BCAA resulted in a marked reduction of reduced glutathione (GSH) levels and significantly increased nitric oxide production. Finally, we observed that the morphological features caused by the BCAA on C6 cells were prevented by the use of the antioxidants GSH (1 mM) and N(omega)-nitro-L-arginine methyl ester (L-NAME, 0.5 mM). On the basis of the present results, we conclude that free radical attack might be involved in the cell morphological alterations, as well as, in the cytoskeletal reorganization elicited by the BCAA. It is therefore presumed that these findings could be involved in the neuropathological features observed in patients affected by MSUD.

Astroglial cells in primary culture: A valid model to study Neospora caninum infection in the CNS

The protozoan Neospora caninum has a veterinary importance because it causes abortion in cattle and neuromuscular alterations in dogs. We infected rat astrocytes, in vitro, with different concentrations of N. caninum. Astrocytes responded to infection by producing the pro-inflammatory cytokine TNF-alpha and the neurotoxic-free radical NO, 24 and 72 h post-infection. These data suggest that astrocytes, which are essential for brain function, are targets for the parasite and this represents a practical and valid model to study the effects of N. caninum on the CNS.

Morphological alterations and induction of oxidative stress in glial cells caused by the branched-chain alpha-keto acids accumulating in maple syrup urine disease

Maple syrup urine disease (MSUD) is an inherited neurometabolic disorder biochemically characterized by the accumulation of the branched-chain alpha-keto acids (BCKA) alpha-ketoisocaproic (KIC), alpha-keto-beta-methylvaleric (KMV) and alpha-ketoisovaleric (KIV) and their respective branched-chain alpha-amino acids in body fluids and tissues. Affected MSUD patients have predominantly neurological features, including cerebral edema and atrophy whose pathophysiology is not well established. In the present study we investigated the effects of KIC, KMV and KIV on cell morphology, cytoskeleton reorganization, actin immunocontent and on various parameters of oxidative stress, namely total antioxidant reactivity (TAR), glutathione (GSH) and nitric oxide concentrations, and on the activities of catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GPx) in C6 glioma cells. We initially observed that C6 cultivated cells exposed for 3 h to the BCKA (1 and 10 mM) changed their usual rounded morphology to a fusiform or process-bearing cell appearance, while 24 h exposure to these organic acids elicited massive cell death. Rhodamine-labelled phalloidin analysis revealed that these organic acids induced reorganization of the actin cytoskeleton with no modifications on total actin content. It was also observed that 3h cell exposure to low doses of all BCKA (1 mM) resulted in a marked reduction of the non-enzymatic antioxidant defenses, as determined by TAR and GSH measurements. In addition, KIC provoked a reduced activity of SOD and GPx, whereas KMV caused a diminution of SOD activity. In contrast, CAT activity was not modified by the metabolites. Furthermore, nitric oxide production was significantly increased by all BCKA. Finally, we observed that the morphological features caused by BCKA on C6 cells were prevented by the use of the antioxidants GSH (1.0 mM), alpha-tocopherol (trolox; 10 microM) and Nomega-nitro-L-arginine methyl ester (L-NAME; 500 microM). These results strongly indicate that oxidative stress might be involved in the cell morphological alterations and death, as well as in the cytoskeletal reorganization elicited by the BCKA. It is presumed that these findings are possibly implicated in the neuropathological features observed in patients affected by MSUD.

Signals for the induction of nitric oxide synthase in astrocytes

Nitric oxide (NO), being a double-edged sword depending on its concentration in the microenvironment, is involved in both physiological and pathological processes of many organ systems including brain and spinal cord. It is now well-documented that once inducible nitric oxide synthase (iNOS) is expressed in CNS in a signal-dependent fashion, NO in excess of physiological thresholds is produced and this excess NO then plays a role in the pathogenesis of stroke, demyelination and other neurodegenerative diseases. Therefore, a keen interest has been generated in recent years in comprehending the regulation of this enzyme in brain cells. The present review summarizes our current understanding of signaling mechanisms leading to transcription of the iNOS gene in activated astrocytes. We attempt this comprehension with a hope to identify potential targets to intervene NO-mediated CNS disorders.

Protective effect of cyclic AMP against cisplatin-induced nephrotoxicity

We reported earlier that reactive oxygen species are implicated in necrotic injury induced by a transient exposure of cultured renal tubular cells to a high concentration of cisplatin but not in apoptosis occurring after continuous exposure to a low concentration of cisplatin. We report here the protective effect of cyclic AMP against cisplatin-induced necrosis in cultured renal tubular cells as well as cisplatin-induced acute renal failure in rats. Several pharmacological agents that stimulate cyclic AMP signaling, including the nonhydrolyzable cyclic AMP analogue dibutyryl cyclic AMP, forskolin, 3-isobutyl-1-methylxanthine, and a prostacyclin analogue, beraprost, prevented cisplatin-induced cell injury in a protein kinase A-dependent manner. Cisplatin enhanced lipid peroxidation, decreased CuZn superoxide dismutase (SOD) while enhancing MnSOD activity, and increased cellular tumor necrosis factor-alpha (TNF-alpha) content. The elevation of TNF-alpha content and cell injury induced by cisplatin were attenuated by p38 mitogen-activated protein kinase (MAPK) inhibitors including SB203580 and PD169316. Indeed, cisplatin increased the number of phosphorylated p38 MAPK-like immunoreactive cells. These intracellular events were all reversed by antioxidants such as N-acetylcysteine (NAC) and glutathione or cyclic AMP analogues. The in vivo acute renal injury after cisplatin injection was associated with the elevation of renal TNF-alpha content. The cisplatin-induced renal injury and the increase in TNF-alpha content were reversed by NAC or beraprost. These findings suggest that cyclic AMP protects renal tubular cells against cisplatin-induced oxidative injury by obliterating reactive oxygen species and subsequent inhibition of TNF-alpha synthesis through blockade of p38 MAPK activation.

Regulation of inducible nitric oxide synthase gene in glial cells

Elevated levels of NO produced within the central nervous system (CNS) are associated with the pathogenesis of neuroinflammatory and neurodegenerative human diseases such as multiple sclerosis, HIV dementia, brain ischemia, trauma, Parkinson's disease, and Alzheimer's disease. Resident glial cells in the CNS (astroglia and microglia) express inducible nitric oxide synthase (iNOS) and produce high levels of NO in response to a wide variety of proinflammatory and degenerative stimuli. Although pathways resulting in the expression of iNOS may vary in two different glial cells of different species, the intracellular signaling events required for the expression of iNOS in these cells are slowly becoming clear. Various signaling cascades converge to activate several transcription factors that control the transcription of iNOS in glial cells. The present review summarizes different results and discusses current understandings about signaling mechanisms for the induction of iNOS expression in activated glial cells. A complete understanding of the regulation of iNOS expression in glial cells is expected to identify novel targets for therapeutic intervention in NO-mediated neurological disorders.

TNF-α/IFN-γ-induced iNOS expression increased by prostaglandin E2 in rat primary astrocytes via EP2-evoked cAMP/PKA and intracellular calcium signaling

Astrocytes, the most abundant glia in the central nervous system (CNS), produce a large amount of prostaglandin E(2) (PGE(2)) in response to proinflammatory mediators after CNS injury. However, it is unclear whether PGE(2) has a regulatory role in astrocytic activity under the inflamed condition. In the present work, we showed that PGE(2) increased inducible nitric oxide synthase (iNOS) production by tumor necrosis factor-alpha and interferon-gamma (T/I) in astrocytes. Pharmacological and RNA interference approaches further indicated the involvement of the receptor EP2 in PGE(2)-induced iNOS upregulation in T/I-treated astrocytes. Quantitative real-time polymerase chain reaction and gel mobility shift assays also demonstrated that PGE(2) increased iNOS transcription through EP2-induced cAMP/protein kinase A (PKA)-dependent pathway. Consistently, the effect of EP2 was significantly attenuated by the PKA inhibitor KT-5720 and partially suppressed by the inhibitor (SB203580) of p38 mitogen-activated protein kinase (p38MAPK), which serves as one of the downstream components of the PKA-dependent pathway. Interestingly, EP2-mediated PKA signaling appeared to increase intracellular Ca(2+) release through inositol triphosphate (IP3) receptor activation, which might in turn stimulate protein kinase C (PKC) activation to promote iNOS production in T/I-primed astrocytes. By analyzing the expression of astrocytic glial fibrillary acidic protein (GFAP), we found that PGE(2) alone only triggered the EP2-induced cAMP/PKA/p38MAPK signaling pathway in astrocytes. Collectively, PGE(2) may enhance T/I-induced astrocytic activation by augmenting iNOS/NO production through EP2-mediated cross-talk between cAMP/PKA and IP3/Ca(2+) signaling pathways.

Lipoic acid decreases exhaled nitric oxide concentrations in anesthetized endotoxemic rats

We recently demonstrated that lipoic acid suppresses endotoxin-stimulated expression of inducible nitric oxide synthase and nitric oxide production in mouse macrophages. In this study, we tested whether lipoic acid suppresses these inflammatory mediators in the lungs of rats. Rats were assigned to receive either no special treatment, endotoxin alone, or pretreatment with lipoic acid followed by endotoxin. After anesthetizing the rats and injecting them intraperitoneally with lipoic acid (100 mg/kg) at 4 h and again at 1 h before treatment, the rats then received either endotoxin (0.01 mg/kg) or its vehicle solution. Exhaled gas was sampled every 15 min and concentrations of nitric oxide in the samples were measured using a chemiluminescence analyzer. After 150 min of exposure to endotoxin, the lungs were harvested and snap-frozen in liquid nitrogen for subsequent analysis. Lipoic acid attenuated endotoxin-induced increases in exhaled nitric oxide concentrations (P<0.001) and iNOS (P<0.05). These findings support the hypothesis that lipoic acid inhibits endotoxin-stimulated formation of intrapulmonary nitric oxide.