Quercetin inhibits hydrogen peroxide (H2O2)-induced NF-κB DNA binding activity and DNA damage in HepG2 cells

We have investigated the effect of the plant-derived flavonoid quercetin in relation to potential oxidant and antioxidant activity on nuclear factor kappa B (NF-kappa B) binding activity and DNA integrity in HepG2 cells. Gel mobility shift assays using a gamma-P-32-labelled NF-kappa B oligonucleotide probe showed that treatment of KepG2 cells with quercetin (up to 10 mu M, sub-cytotoxic) did not elevate NF-kappa B binding activity of nuclear extract protein but did inhibit binding activity of an extract from cells treated with the oxidant H2O2. A similar inhibition by quercetin of H2O2-induced NF-KB transcriptional activation was demonstrated using a cat reporter gene assay. Considering oxidative DNA damage, using single cell gel electrophoresis (comet) assay we have demonstrated that quercetin (10 mu M and below) did not induce DNA strand breaks. However, a marked and statistically significant (P < 0.01 at 10 mu M) inhibition of strand breakage produced by H2O2 was detected. The specific formation of 8-oxo-2'-deoxyguanosine (8-oxodG) in calf thymus DNA exposed to either gamma-irradiation or the Fenton reaction system was also inhibited (P < 0.01 at 10 mu M) by quercetin in a dose-dependent manner, This was not accompanied by formation of 8-oxodG by quercetin itself. The inhibition of 8-oxodG formation by gamma-irradiation was more potent (IC50 = 0.05 mu M) than that by the Fenton reaction(IC50 = 0.5 mu M), implying that the mechanism of protection may be different between the two systems. The inhibition of both NF-KB binding activity and oxidative DNA damage suggests that its antioxidant potential outweighs its oxidative potential in a cellular environment, which may contribute to anticarcinogenic and antiinflammatory effects.