3′,4′,7-Trihydroxyflavone Downregulates NO Production in LPS- or IFN-γ-Activated MG6 Microglial Cells by Attenuating the JNK–STAT1 Pathway

Neuroinflammation induced by activated microglia is a key feature of neurodegenerative diseases such as Alzheimer’s disease. The natural flavonoid 3′,4′,7-trihydroxyflavone protects nerve cells from oxidative stress-mediated apoptosis and inhibits the aggregation of amyloid β protein in vitro. However, little is known about its effects on microglial activation. In this study, we investigated the effects of 3′,4′,7-trihydroxyflavone on lipopolysaccharide (LPS)- or interferon-γ (IFN-γ)-induced neuroinflammatory responses in MG6 microglial cells. 3′,4′,7-Trihydroxyflavone inhibited LPS- or IFN-γ-mediated nitric oxide (NO) generation and the upregulation of inducible NO synthase (iNOS) in MG6 cells. 3′,4′,7-Trihydroxyflavone also suppressed LPS- or IFN-γ-mediated phosphorylation of signal transducer and activator of transcription 1 (STAT1), which is crucial for iNOS expression. LPS stimulation induced rapid phosphorylation of c-Jun N-terminal kinase (JNK), p38 mitogen-activated protein kinase (MAPK), and extracellular signal-regulated kinase (ERK) in MG6 cells. 3′,4′,7-Trihydroxyflavone significantly inhibited the LPS-mediated phosphorylation of JNK, but not that of ERK and p38 MAPK. The inhibitory effect of 3′,4′,7-trihydroxyflavone on NO generation was mimicked by pharmacological inhibition of the JNK signaling pathway with SP600125. Furthermore, SP600125 significantly inhibited LPS- or IFN-γ-mediated phosphorylation of STAT1 in MG6 cells. These results suggest that 3′,4′,7-trihydroxyflavone exerts anti-neuroinflammatory effects via inhibition of the JNK-STAT1 pathway in microglia.