Abstract
Treatment of mouse astrocyte cultures with combined interleukin (IL)-1alpha and tumor necrosis factor (TNF)-alpha induced expression of inducible nitric-oxide synthase (iNOS), resulting in sustained release of large amounts of nitric oxide, whereas TNF-alpha and IL-1alpha individually were unable to induce iNOS expression in astrocytes. The role of MAPK cascades and of NF-kappaB activation in the early intracellular signal transduction involved in iNOS transcription in TNF-alpha/IL-1alpha-stimulated astrocytes was investigated. TNF-alpha and IL-1alpha activated all p42/44(MAPK), p38(MAPK), and p54(JNK) pathways as determined by immunoprecipitation kinase assays using specific antibodies and substrates. The p38(MAPK) pathway is specifically involved in TNF-alpha/IL-1alpha-induced iNOS expression, since iNOS protein and nitric oxide release in the presence of a specific inhibitor of p38(MAPK), 4-(4-fluorophenyl)-2-2-(4-hydroxyphenyl)-5-(4-pyridyl)-imidazole (FHPI), were dramatically diminished. In contrast, PD98059, a specific inhibitor of MEK1 had no effect on iNOS expression. p38(MAPK) did not couple NF-kappaB to iNOS transcription, but NF-kappaB had a clear role in iNOS transcription regulation. Northern blot analysis showed that the p38(MAPK) pathway controlled iNOS expression at the transcriptional level, since iNOS mRNA was reduced in the presence of FHPI in TNF-alpha/IL-1alpha-stimulated astrocytes. iNOS expression was investigated with TNF receptor (TNFR)-1- and TNFR-2-deficient mice. The TNF-alpha activity in TNF-alpha/IL-1alpha-stimulated astrocytes was exclusively mediated through TNFR-1, most likely because TNFR-2-mediated signals in astrocytes did not connect to the p38(MAPK) pathway. These data suggest that TNF-alpha/IL-1alpha-induced iNOS expression depends on a yet undetermined second pathway in addition to p38(MAPK).
Highlights
Synthases (NOS), convert L-arginine to L-citrulline and Nitric oxide (NO)
The results show that a p38MAPKdependent pathway transduces inducible NOS (iNOS) expression signals elicited by combined tumor necrosis factor (TNF)-␣ and IL-1␣ stimulation in mouse primary astrocytes. p38MAPK activation is necessary but not sufficient, since stimulation by TNF-␣ or IL-1␣ alone fully activated p38MAPK but was unable to induce iNOS transcription
The necessity to stimulate cells with combined cytokines such as TNF-␣ plus IL-1 or IFN-␥ or with cytokines combined with lipopolysaccharide to induce iNOS expression has been reported in several cell types such as astrocytes [45], fibroblasts [46], and endothelial cells [47], a single cytokine can induce iNOS expression in some other cell systems including hepatocytes [48], islet cells [49], and vascular smooth muscle cells [50]
Summary
Mice—The TNF receptor-deficient mice (tnfr-1°, tnfr-2°, and tnfr-1°/ tnfr-2°) have been previously reported [37,38,39]. Samples were centrifuged at 6,500 rpm for 5 min at 4 °C, and 3 volumes of low salt buffer D (20 mM Hepes, pH 7.9, 20% (v/v) glycerol, 100 mM KCl, 0.2 mM EDTA, 1% Nonidet P-40, 1 mM PMSF, 10 mM DTT) were added to the supernatants representing the nuclear extracts. Gel Shift Assays—Electrophoretic mobility shift assays were performed by incubating 6 g of nuclear extracts with 2 g of poly(dI1⁄7dC) and 30 g of bovine serum albumin in 10 l of binding buffer (100 mM Tris, pH 7.5, 500 mM NaCl, 10 mM DTT, 10 mM EDTA, pH 7.5, 50% glycerol) for 20 min at room temperature.
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