Quercetin Derivatives from Siegesbeckia glabrescens Inhibit the Expression of COX-2 Through the Suppression of NF-κB Activation in Microglia

The activation of microglia induces the overproduction of inflammatory mediators that are responsible for the neurodegenerative disorders including Alzheimer's disease and Parkinson's disease. The large amounts of prostaglandin $E_2$ ($PGE_2$) produced by inducible cyclooxygenase (COX-2) is one of the main inflammatory mediators that can contribute to neurodegeneration. The inhibition of COX-2 thus may provide therapeutic strategy for the treatment of neurodegenerative diseases. From the activity-guided purification of EtOAc soluble fraction of Siegesbeckia glabrescens, four compounds were isolated as inhibitors of $PGE_2$ production in LPS-activated microglia. Their structures were determined as 3, 4'-dimethylquercetin (1), 3, 7-dimethylquercetin (2), 3-methylquercetin (3) and 3, 7, 4'-trimethylquercetin (4) by the mass and NMR spectral data analysis. The compounds 1-4 showed dose-dependent inhibition of $PGE_2$ production in LPS-activated microglia with their $IC_{50}$ values of 7.1, 4.9, 4.4, $12.4\;{\mu}M$ respectively. They reduced the expression of protein and mRNA of COX-2 through the inhibition of I-${\kappa}B{\alpha}$ degradation and NF-$\kappa}B$ activity that were correlated with the inactivation of p38 and ERK. Therefore the active compounds from Siegesbeckia glabrescens may have therapeutic effects on neuro-inflammatory diseases through the inhibition of overproduction of $PGE_2$ and suppression of COX-2 overexpression.