Potentiation by high potassium of lipopolysaccharide-induced nitric oxide production from cultured astrocytes

Uptake of K+ is an important role of astrocytes to maintain physiological lower extracellular K+ concentration in the CNS. In this study, the effect of high K+ concentration was examined on the cellular function of astrocytes from embryonic rat brain in primary culture. Nitric oxide (NO) production induced by lipopolysaccharide (LPS) was measured as an index of cellular function of astrocytes. Increasing KCl concentration to about 40 mM did not directly evoke NO production, but doubled the level of LPS (1 ng/ml)-induced NO production. K-gluconate showed a similar enhancing effect although the degree of enhancement was about half of that of KCl. Neither NaCl nor Na-gluconate showed any effect. The K(+)-channel blocker, 4-aminopyridine, but not tetraethylammonium or apamin, inhibited the enhancing effect of KCl. The LPS-induced iNOS protein expression determined by immunoblotting analysis was enhanced by high K+ treatment. The level of iNOS mRNA determined by real-time RT-PCR technique was also augmented by the presence of 40 mM KCl. These results indicate that the elevation of extracellular K+ concentration regulates astrocytic cell functions through a mechanism involving K(A)-type K(+)-channels and that potentiation of NO production by high K+ is due to the augmentation of iNOS mRNA and iNOS protein levels.     

Role of the cyclic AMP-protein kinase A pathway in lipopolysaccharide-induced nitric oxide synthase expression in RAW 264.7 macrophages. Involvement of cyclooxygenase-2.

The signaling pathway for lipopolysaccharide (LPS)-induced nitric oxide (NO) release in RAW 264.7 macrophages involves the protein kinase C and p38 activation pathways (Chen, C. C., Wang, J. K., and Lin, S. B. (1998) J. Immunol. 161, 6206-6214; Chen, C. C., and Wang, J. K. (1999) Mol. Pharmacol. 55, 481-488). In this study, the role of the cAMP-dependent protein kinase A (PKA) pathway was investigated. The PKA inhibitors, KT-5720 and H8, reduced LPS-induced NO release and inducible nitric oxide synthase (iNOS) expression. The direct PKA activator, Bt(2)cAMP, caused concentration-dependent NO release and iNOS expression, as confirmed by immunofluorescence studies. The intracellular cAMP concentration did not increase until after 6 h of LPS treatment. Two cAMP-elevating agents, forskolin and cholera toxin, potentiated the LPS-induced NO release and iNOS expression. Stimulation of cells with LPS or Bt(2)cAMP for periods of 10 min to 24 h caused nuclear factor-kappaB (NF-kappaB) activation in the nuclei, as shown by detection of NF-kappaB-specific DNA-protein binding. The PKA inhibitor, H8, inhibited the NF-kappaB activation induced by 6- or 12-h treatment with LPS but not that induced after 1, 3, or 24 h. The cyclooxygenase-2 (COX-2) inhibitors, NS-398 and indomethacin, attenuated LPS-induced NO release, iNOS expression, and NF-kappaB DNA-protein complex formation. LPS induced COX-2 expression in a time-dependent manner, and prostaglandin E(2) production was induced in parallel. These results suggest that 6 h of treatment with LPS increases intracellular cAMP levels via COX-2 induction and prostaglandin E(2) production, resulting in PKA activation, NF-kappaB activation, iNOS expression, and NO production.     

Penehyclidine hydrochloride attenuates LPS-induced iNOS production by inhibiting p38 MAPK activation in endothelial cells

The aim of this study was to investigate the inhibitory effect of penehyclidine hydrochloride (PHC) on lipopolysaccharide (LPS)-induced nitric oxide (NO) and inducible nitric oxide synthase (iNOS) production in human endothelial cell. Cultured endothelial cells were pretreated with PHC, followed by LPS treatment. NO activity were determined. iNOS expression and p38 mitogen-activated protein kinase (p38 MAPK) protein expression were measured by Western blot analysis. LPS treatment significantly induced p38 MAPK activation, iNOS expression, and NO production, which could be attenuated by 2 μg/ml PHC pretreatment. Furthermore, our study showed LPS-induced NO production and iNOS expression were suppressed by p38 MAPK inhibitor SB203580 pretreatment. We concluded that PHC attenuates NO production and iNOS expression by suppressing the activation of p38 MAPK pathway, thereby implicating a mechanism by which PHC may exert its protective effects against LPS-induced endothelial cell injury.     

Zinc Potentiates Lipopolysaccharide-induced Nitric Oxide Production in Cultured Primary Rat Astrocytes

Zn²⁺ plays a crucial role in the CNS where it accumulates in synaptic vesicles and is released during neurotransmission. Synaptically released Zn²⁺ is taken up by neurons and astrocytes. The majority of previous work has focused on neuronal damage caused by excess Zn²⁺. However, its effect on astrocyte function is not well understood. We examined the effect of extracellularly applied Zn²⁺ on nitric oxide (NO) production in primary cultured rat astrocytes, which were experimentally activated by lipopolysaccharide (LPS). Zn²⁺, at a concentration up to 125 μM, augmented LPS-induced NO production without affecting cell viability. LPS induced expression of both mRNA and protein of inducible NO synthase; this expression was enhanced by 125 µM Zn²⁺. Zn²⁺ also increased LPS-induced production of intracellular reactive oxygen species. Zn²⁺ enhanced the phosphorylation of p38-mitogen-activated protein kinase (MAPK) at 1–6 h after LPS treatment. The LPS-induced nuclear factor-kappaB (NFκB) activation was sustained for 6 h by Zn²⁺. Intracellular Zn²⁺ chelation with N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) or inhibition of p38-MAPK diminished the Zn²⁺ enhancement of LPS-induced NO production. These findings suggest that activation of MAPK and NFκB is important for mediating Zn²⁺enhancement of LPS-induced NO production in astrocytes. Such changes may exacerbate glial and neuronal damage during neuroinflammation.     

Lipopolysaccharide-induced dopaminergic cell death in rat midbrain slice cultures: role of inducible nitric oxide synthase and protection by indomethacin

Glial cell activation associated with inflammatory reaction may contribute to pathogenic processes of neurodegenerative disorders, through production of several cytotoxic molecules. We investigated the consequences of glial activation by interferon-gamma (IFN-gamma)/lipopolysaccharide (LPS) in rat midbrain slice cultures. Application of IFN-gamma followed by LPS caused dopaminergic cell death and accompanying increases in nitrite production and lactate dehydrogenase release. Aminoguanidine, an inhibitor of inducible nitric oxide synthase (iNOS), or SB203580, an inhibitor of p38 mitogen-activated protein kinase, prevented dopaminergic cell loss as well as nitrite production. SB203580 also suppressed expression of iNOS and cyclooxygenase-2 (COX-2) induced by IFN-gamma/LPS. A COX inhibitor indomethacin protected dopaminergic neurons from IFN-gamma/LPS-induced injury, whereas selective COX-2 inhibitors such as NS-398 and nimesulide did not. Notably, indomethacin was able to attenuate neurotoxicity of a nitric oxide (NO) donor. Neutralizing antibodies against tumour necrosis factor-alpha and interleukin-1beta did not inhibit dopaminergic cell death caused by IFN-gamma/LPS, although combined application of these antibodies blocked lactate dehydrogenase release and decrease in the number of non-dopaminergic neurons. These results indicate that iNOS-derived NO plays a crucial role in IFN-gamma/LPS-induced dopaminergic cell death, and that indomethacin exerts protective effect by mechanisms probably related to NO neurotoxicity rather than through COX inhibition.     

Anti-inflammatory effect of heme oxygenase 1: glycosylation and nitric oxide inhibition in macrophages

Flavonoids including the aglycones, hesperetin (HT; 5,7,3'-trihydroxy-4'-methoxy-flavanone), and naringenin (NE; 5,7,4'-trihydroxy flavanone) and glycones, hesperidin (HD; 5,7,3'-trihydroxy-4'-methoxy-flavanone 7-rhamnoglucoside) and naringin (NI; 5,7,4'-trihydroxy flavanone 7-rhamno glucoside), were used to examine the importance of rutinose at C7 on the inhibitory effects of flavonoids on lipopolysaccharide (LPS)-induced nitric oxide production in macrophages. Both HT and NE, but not their respective glycosides HD and NI, induced heme oxygenase 1 (HO-1) protein expression in the presence or absence of LPS and showed time and dose-dependent inhibition of LPS-induced nitric oxide (NO) production and inducible nitric oxide synthase (iNOS) expression in RAW264.7, J774A.1, and thioglycolate-elicited peritoneal macrophages. Additive inhibitory effect of an HO-1 inducer hemin and NE or NI on LPS-induced NO production and iNOS expression was identified, and HO enzyme inhibitor tin protoporphyrin (SnPP) attenuated the inhibitory effects of HT, NE, and hemin on LPS-induced NO production. Both NE and HT showed no effect on iNOS mRNA and protein stability in RAW264.7 cells. Removal of rutinose at C7 of HD and NI by enzymatic digestion using hesperidinase (HDase) and naringinase (NIase) produce inhibitory activity on LPS-induced NO production, according to the production of the aglycones, HT and NE, by high-performance liquid chromatography (HPLC) analysis. Furthermore, the amount of NO produced by LPS or lipoteichoic acid (LTA) was significantly reduced in HO-1-overexpressing cells (HO-1/RAW264.7) compared to that in parental cells (RAW264.7). Results of the present study provide scientific evidence to suggest that rutinose at C7 is a negative moiety in flavonoid inhibition of LPS-induced NO production, and that HO-1 is involved in the inhibitory mechanism of flavonoids on LPS-induced iNOS and NO production.     

Inhibition of oocyte maturation by theophylline: possible mechanism of action

Theophylline, a competitive inhibitor of cAMP phosphodiesterase, inhibits the maturation of oocytes previously exposed to progesterone. Cyclic AMP levels remain constant both during normal maturation and in response to theophylline, even though the latter effectively inhibits phosphodiesterase activity in the oocyte. Thus, the inhibition is not attributable to elevated cAMP levels. Rather, we suggest that theophylline may exert its inhibitory effects on maturation either by reducing rates of protein synthesis or possibly through effects at the oocyte surface.     

Saccharated colloidal iron enhances lipopolysaccharide-induced nitric oxide production in vivo

We investigated the effects of iron on the production of nitric oxide (NO), inducible NO synthase (iNOS), and plasma cytokines induced by lipopolysaccharide (LPS) in vivo. Male Wistar rats were preloaded with a single intravenous injection of saccharated colloidal iron (Fesin, 70 mg iron/kg body weight) or normal saline as a control, and then given an intraperitoneal injection of LPS (5.0 mg/kg body weight). Rats, preloaded with iron, had evidence of both iron deposition and strong iNOS induction in liver Kupffer cells upon injection of LPS; phagocytic cells in the spleen and lung had similar findings. LPS-induced NO production in iron-preloaded rats was significantly higher than control rats as accessed by NO-hemoglobin levels measured by ESR (electron spin resonance) and NOx (nitrate plus nitrite) levels. Western blot analysis showed that iron preloading significantly enhanced LPS-induced iNOS induction in the liver, but not in the spleen or lung. LPS-induced plasma levels of IL-6, IL-1beta, and TNF-alpha were also significantly higher in iron-preloaded rats as shown by ELISA, but IFN-gamma levels were unchanged. We conclude that colloidal-iron phagocytosed by liver Kupffer cells enhanced LPS-induced NO production in vivo, iNOS induction in the liver, and release of IL-6, IL-1beta, and TNF-alpha.     

Inducible nitric oxide synthase in the rat testis: evidence for potential roles in both normal function and inflammation-mediated infertility

In vitro data have indicated that nitric oxide (NO) inhibits Leydig cell testosterone production, suggesting that NO may play a role in the suppression of steroidogenesis and spermatogenic function during inflammation. Consequently, we investigated expression of the inflammation-inducible isoform of NO synthase (iNOS) in the inflamed adult rat testis and the ability of a broad-spectrum inhibitor of NO production, L-nitro-L-arginine methyl ester, to prevent Leydig cell dysfunction during inflammation. Unexpectedly, immunohistochemical and mRNA data established that iNOS is expressed constitutively in Leydig cells and in a stage-specific manner in Sertoli, peritubular, and spermatogenic cells in the normal testis. Expression was increased in a dose-dependent manner in all these cell types during lipopolysaccharide (LPS)-induced inflammation. In noninflamed testes, treatment with the NO synthase inhibitor reduced testicular interstitial fluid formation and testosterone production without any effect on serum LH levels. Administration of the inhibitor did not prevent the suppression of testicular interstitial fluid and testosterone production that occurs within 6 h after LPS treatment. Collectively, these data indicate a novel role for iNOS in autocrine or paracrine regulation of the testicular vasculature, Leydig cell steroidogenesis, and spermatogenesis in the normal testis. The data suggest that increased NO is not the major cause of acute Leydig cell dysfunction in the LPS-treated inflammation model, although a role for NO in this process cannot be excluded, particularly at other time points. Moreover, up-regulation of iNOS may contribute to the seminiferous epithelium damage caused by LPS-induced inflammation.     

Inducible nitric oxide synthase modulates lipolysis in adipocytes

The role of inducible nitric oxide synthase (iNOS) in the modulation of adipocyte lipolysis was investigated. Treatment of white and brown adipose cell lines and mouse adipose explants with a mixture of tumor necrosis factor-alpha, interferon-gamma, and lipopolysaccharide (LPS) doubled the lipolytic rate, and this was associated with marked induction of iNOS expression and nitric oxide (NO) production. iNOS inhibition by 1400W, aminoguanidine, or L-NIL pretreatment further increased the cytokine/LPS-mediated lipolysis by 30% (P < 0.05) in cultured adipocytes and in adipose explants. However, this potentiating effect of iNOS inhibition was abolished in adipose explants isolated from iNOS knockout mice. Pharmacological inhibitors of adenylyl cyclase or protein kinase A reduced cytokine/LPS-induced lipolysis and also blunted the potentiating effect of iNOS inhibition on the lipolytic rate. Furthermore, addition of the antioxidants l-cystine and l-glutathione to cytokine/LPS-stimulated adipocytes mimicked the lipolytic effect of iNOS inhibition. In conclusion, inhibition of iNOS activity in adipocytes potentiates cytokine/LPS-induced lipolysis. This effect was fully reversed by adenylyl cyclase and protein kinase A inhibitors but was mimicked by cellular antioxidants. These data suggest that iNOS-mediated NO production counteracts cytokine/LPS-mediated lipolysis in adipocytes and that this feedback mechanism involves an oxidative process upstream of cAMP production in the signaling pathway.     

Lipopolysaccharide-induced monocyte chemotactic protein-1 is enhanced by suppression of nitric oxide production, which depends on poor CD14 expression on the surface of skeletal muscle

It is known that lipopolysaccharide (LPS)-induced monocyte chemotactic protein (MCP)-1 secretion from tissues recruits monocytes from the circulation, but the mechanism of the LPS-induced MCP-1 production in skeletal muscle is largely unexplained. To clarify the effect of LPS on MCP-1 production in skeletal muscle cells, C2C12 cells from a mouse skeletal muscle cell line, and RAW 264.7 cells from a mouse macrophage cell line, were used to assess production of LPS-induced MCP-1, nitric oxide (NO) and interferon (IFN)-beta. In addition, we evaluated inducible NO synthases (iNOS) mRNA expression using RT-PCR, and cell surface expression of CD14 and toll-like receptor (TLR) 4 using flow cytometry. In C2C12 cells, LPS stimulation increased MCP-1 production (p < 0.01), but combined treatment with LPS and NO inducer, diethylammonium (Z)-1-(N,N-diethylamino) diazen-1-ium-1,2-diolate (NONOate), significantly inhibited its production (p < 0.01). LPS stimulation neither induced production of NO nor of IFN-beta, which is an NO inducer. Recombinant IFN-beta stimulation, on the other hand, enhanced LPS-induced NO production (p < 0.01). Interestingly, we found that surface expression of CD14, which regulates IFN-beta production, in C2C12 cells was much lower than that in RAW 264.7 cells, although TLR4 expression on C2C12 cells was similar to that on RAW 264.7 cells. These data suggest that the reduced NO production in response to LPS may depend on low expression of CD14 on the cell surface of skeletal muscle, and that it may enhance LPS-induced MCP-1 production. Together, these functions of skeletal muscle could decrease the risk of bacterial infection by recruitment of monocytes.     

Antiviral activity of lambda-, beta-, and gamma-interferons in the presence of theophylline: involvement of 2',5'-oligo(A) synthetase

Theophylline, an inhibitor of cAMP phosphodiesterase, induces in human ovary carcinoma cells (CaOv) a 2-2.5-fold elevation of intracellular cAMP. This rise in the cAMP level is followed by an increase of the activity of 2',5'-oligo(A) synthetase in CaOv cells -insignificant (1.5-fold) after 16 hr incubation, and substantial (3.7-fold) after 30 hr incubation, as well as the development of antiviral resistance. Once CaOv cells have been incubated with the mixtures containing theophylline (2 mM) and lambda-, beta-, and gamma-interferon preparations (0.5-13 IU/ml), the total antiviral effect of the mixtures exceeds that generated by interferon or theophylline separately; the action of the above agents being additive. These data agree with the previously obtained results and support the suggestion that cAMP phosphodiesterase inhibitors partially mimic the antiviral action of interferon.     

Statin blocks Rho/Rho-kinase signalling and disrupts the actin cytoskeleton: relationship to enhancement of LPS-mediated nitric oxide synthesis in vascular smooth muscle cells

We previously demonstrated statins to enhance cytokine-mediated nitric oxide (NO) synthesis in vascular smooth muscle cells (VSMC). To clarify the mechanism by which this occurs, we evaluated the effects of fluvastatin in lipopolysaccharide (LPS)-stimulated VSMC. NO production induced by LPS was dose-dependently enhanced by fluvastatin, as were iNOS mRNA levels and iNOS protein expression. Exogenous mevalonate and geranylgeranylpyrophosphate (GGPP) dampened the stimulatory effect of fluvastatin. A pull-down assay demonstrated fluvastatin to decrease levels of GTP-bound Rho A. Moreover, a Rho-kinase inhibitor, Y-27632, was observed to enhance LPS-induced NO production. We recently demonstrated that disrupting F-actin formation dramatically potentiates the ability of LPS to induce iNOS mRNA and protein expression. In the present study, staining of F-actin with nitrobenzoxadiazole (NBD)-phallacidin demonstrated that fluvastatin significantly impairs F-actin stress fiber formation. In light of these results, the ability of statins to increase NO production is due, at least in part, to their ability to block the biosynthesis of mevalonate, thereby preventing isoprenoid biosynthesis. This inhibits Rho/Rho-kinase signalling and, in turn, disrupts the actin cytoskeleton. Further analysis of the signalling pathway by which the actin cytoskeleton affects iNOS expression might yield new insight into mechanisms of regulation of NO production.     

Distinct signaling pathways for induction of type II NOS by IFNgamma and LPS in BV-2 microglial cells

Nitric oxide (NO) release upon microglial cell activation has been implicated in the tissue injury and cell death in many neurodegenerative diseases. Recent studies have indicated the ability of interferon-gamma (IFNgamma) and lipopolysaccharides (LPS) to independently induce type II nitric oxide synthase (iNOS) expression and NO production in BV-2 microglial cells. However, a detailed comparison between the signaling pathways activating iNOS by these two agents has not been accomplished. Analysis of PKC isoforms revealed mainly the presence of PKCdelta, iota and lambda in BV-2 cells. Although both IFNgamma and LPS could specifically enhance the tyrosine phosphorylation of PKCdelta, treatment with IFNgamma induced a steady increase of phospho-PKCdelta for up to 1h, whereas treatment with LPS elevated phospho-PKCdelta levels only transiently, with peak activity at 5 min. Rottlerin, a specific inhibitor for PKCdelta, dose-dependently inhibited IFNgamma- and LPS-induced NO production. Despite the common involvement of PKCdelta, IFNgamma- but not LPS-induced NO production involved extracellular signal-regulated kinases (ERK1/2) cascade and IFNgamma-induced phosphorylation of ERK1/2 was mediated through PKC. On the other hand, LPS- but not IFNgamma-induced NO production was through stimulation of NF-kappaB activation and nuclear translocation to interact with DNA. These results demonstrated distinct signaling pathways for induction of iNOS by IFNgamma and LPS in BV-2 microglial cells.     

Acute glucose overload potentiates nitric oxide production in lipopolysaccharide-stimulated macrophages: the role of purinergic receptor activation

The positive effects of high glucose on the cellular productivity of nitric oxide (NO), and the mechanisms of the enhancement, were investigated. Macrophages were shifted from normal-glucose medium (5.5 mM) to high-glucose medium (25 mM) and immediately treated with lipopolysaccharide (LPS). Inducible nitric oxide synthase (iNOS) expression was expressed significantly more quickly, and NO production also increased. High-glucose conditions reduced cell viability at 48 h. Pretreatment with oxidized adenosine triphosphate (o-ATP), the selective purinergic receptor antagonist, strongly reduced LPS-induced iNOS expression, NO production and cell death in cells exposed to high levels of glucose. Apyrase, an ATP-hydrolyzing enzyme, also reduced the effects of high-glucose content. High-glucose content promoted the LPS-induced release of endogenous ATP from RAW 264.7 cells, as measured by luciferin-luciferase assay. In summary, the results revealed that purinergic receptor is important in responding to LPS challenge, increasing LPS-induced NO production and cell death under high-glucose conditions, and promoting the release of ATP from macrophages in high-glucose medium.