Cell-based and in silico evidence against quercetin and structurally-related flavonols as activators of vitamin D receptor

It has been reported that quercetin is an activator of rat vitamin D receptor (rVDR). However, the conclusion was based on experiments performed without all the appropriate control groups, raising the possibility of a false-positive finding. Furthermore, distinct differences exist in the chemical structures of quercetin and 1α,25-dihydroxyvitamin D3, which is a prototypic agonist of VDR. Therefore, we investigated systematically whether quercetin and other flavonols are agonists of rVDR, mouse VDR (mVDR), or human VDR (hVDR). Quercetin, 3-hydroxyflavone, galangin, datiscetin, kaempferol, morin, isorhamnetin, tamarixetin, myricetin, and syringetin did not activate rVDR, mVDR, or hVDR in HEK-293 and HepG2 cells transfected with the corresponding receptor expression plasmid and either the secreted phosphoprotein 1 (Spp1) or cytochrome P450 24A1 (CYP24A1) reporter plasmid, when compared to the respective empty vector control group transfected with one or the other reporter plasmid and treated with one of the flavonols. Control analysis indicated that lithocholic acid and 1α,25-dihydroxyvitamin D3, but not rifampicin, activated rVDR, mVDR, and hVDR. As shown in transfected HEK293 and HepG2 cells, the flavonols did not influence hVDR ligand binding domain transactivation, steroid receptor coactivator-1 recruitment, or hVDR target gene expression (transient receptor potential cation channel 6 and CYP24A1) in hVDR-expressing Caco-2 or LS180 cells. The cumulative data from the cell-based experiments were corroborated by results obtained from molecular docking analysis. In conclusion, quercetin, 3-hydroxyflavone, galangin, datiscetin, kaempferol, morin, isorhamnetin, tamarixetin, myricetin, and syringetin are not agonists of rVDR, mVDR, or hVDR, as judged by cell-based and in silico evidence.