Quercetin mitigates valinomycin-induced cellular stress via stress-induced metabolism and cell uptake

Scope: Intestinal cells are constantly exposed to luminal toxins. In this study, we investigated the effect of cellular stress caused by valinomycin, which is structurally and functionally similar to the bacterial toxin cereulide, on quercetin metabolism and cellular localization in undifferentiated cells. Methods and results: Coadministration of quercetin and valinomycin (50 mu M quercetin/0.05 mu M valinomycin) reduced intracellular reactive oxygen species content and increased cell viability (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) of Caco-2 cells compared to valinomycin-only (0.05 mu M) treatment. Quercertin was effectively metabolized into methyl, glucuronide, and sulfate conjugates, which were mostly secreted into to the culture medium. Three different O-methylated quercetin isomers were detected. Two were exported from the cells and one remained intracellularly. Further, valinomycin caused an increase in the intracellular accumulation of O-methylated quercetin metabolites compared to cells treated only with quercetin. In valinomycin-untreated cells, quercetin and O-methylated quercetin metabolite were localized in the cell membrane, whereas valinomycin treatment resulted in their uptake by the cells. Conclusion: This is the first report on the change in metabolism, localization, and accumulation of O-methylated quercetin metabolites in undifferentiated Caco-2 cells as a response during stress caused by valinomycin. These results indicate a potential cellular stress response mechanism in undifferentiated Caco-2 cells, which adds novel insights into the mechanisms of flavonoid cellular bioactivity.