Inhibition of inducible nitric oxide synthase expression by baicalein in endotoxin/cytokine-stimulated microglia.
Excessive production of nitric oxide (NO) in the central nervous system (CNS) mediated by activation of microglia has been implicated in neurotoxicity after stresses such as ischemia. Baicalein, a polyphenolic flavonoid antioxidant, is known to have anti-inflammatory, anticarcinogenic, and neuroprotective effects. In the present study, we report the inhibitory effect of baicalein on endotoxin/cytokine-induced NO production and inducible nitric oxide synthase (iNOS) gene expression in microglia. Baicalein abolished the endotoxin/cytokine-induced expression of iNOS protein, iNOS mRNA, and iNOS promoter activity in a parallel concentration-dependent manner. The suppression of iNOS expression was not mediated through the down-regulation of tumor necrosis factor-alpha (TNF-alpha) by baicalein because TNF-alpha failed to enhance endotoxin/cytokine-induced NO production in microglia. From the electrophoretic mobility shift assay (EMSA), we found that baicalein exerted a distinct inhibitory effect on the DNA binding activity of transcription factors, and this was significantly greater in nuclear factor IL-6 (NF-IL6) than in nuclear factor kappa B (NF-kappaB) and activated protein 1 (AP-1). Although extracellular signal-regulated kinase (ERK) is critical to iNOS expression, endotoxin/cytokine-stimulated phosphorylation of ERK1/2 was not significantly inhibited by baicalein. These results indicate that NF-IL6 inactivation could be the major determinant for the suppression of NO production by baicalein in microglia. Furthermore, it suggests that the inhibitory effect of baicalein on microglia activation and neurotoxic factor production is responsible for its neuroprotective action.
Chen
Chun-Jung
CJ
Department of Education and Research, Taichung Veterans General Hospital, No. 160, Section 3, Taichung-Gang Road, Taichung 407, Taiwan, ROC. cjchen@vghtc.gov.tw
Raung
Shue-Ling
SL
Liao
Su-Lan
SL
Chen
Shih-Yun
SY
D000818:Animals; D022782:CCAAT-Enhancer-Binding Protein-beta / Q000378:metabolism; D002478:Cells, Cultured; D016207:Cytokines / Q000494:pharmacology; D004247:DNA / Q000187:drug effects / Q000378:metabolism; D004347:Drug Interactions; D004731:Endotoxins / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D044950:Flavanones; D005419:Flavonoids / Q000494:pharmacology*; D008070:Lipopolysaccharides / Q000494:pharmacology; D051379:Mice; D017628:Microglia / Q000187:drug effects* / Q000201:enzymology; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D016328:NF-kappa B / Q000378:metabolism; D009569:Nitric Oxide / Q000378:metabolism; D019001:Nitric Oxide Synthase; D052247:Nitric Oxide Synthase Type II; D010766:Phosphorylation / Q000187:drug effects; D011401:Promoter Regions, Genetic / Q000187:drug effects; D051381:Rats; D018160:Receptors, Cytoplasmic and Nuclear / Q000502:physiology; D018808:Transcription Factor AP-1 / Q000378:metabolism; D014157:Transcription Factors / Q000502:physiology; D014409:Tumor Necrosis Factor-alpha / Q000502:physiology
D022782:CCAAT-Enhancer-Binding Protein-beta; D016207:Cytokines; D004731:Endotoxins; D004791:Enzyme Inhibitors; D044950:Flavanones; D005419:Flavonoids; D008070:Lipopolysaccharides; D016328:NF-kappa B; D018160:Receptors, Cytoplasmic and Nuclear; D018808:Transcription Factor AP-1; D014157:Transcription Factors; D014409:Tumor Necrosis Factor-alpha; D009569:Nitric Oxide; C006680:baicalein; D004247:DNA; D019001:Nitric Oxide Synthase; D052247:Nitric Oxide Synthase Type II; C496317:Nos2 protein, mouse; C496318:Nos2 protein, rat; D020928:Mitogen-Activated Protein Kinases