Flavone potently stimulates an apical transporter for flavonoids in human intestinal Caco-2 cells

Scope Based on the studies suggesting that active transport mechanisms contribute to the absorption of flavonoids into human intestinal Caco-2 cells, we here used the structurally similar fluorescent rhodamine 123 to test a possible influence of flavonoids on its uptake. Methods and results Rhodamine absorption displayed saturation kinetics with a Km of 1.1 mu M and a pH-optimum of 8.5 and was stimulated by flavone four-fold in its Vmax. Ring C of the other 16 flavonoids tested turned out to be of special importance in order to act as potent inhibitors for rhodamine transport, with a positive charge there, as present in the anthocyanidins, or a 2,3 double bond together with an aromatic ring fused to position 2, as present in flavones and flavonols, being essential structural requirements. Flavone-stimulated rhodamine uptake was unaffected by classical substrates of organic cation transporters or inhibitors of adenosine triphosphate (ATP)-dependent efflux pumps. Also, inhibitors of mitogen-activated protein kinases or tyrosine kinases did not influence the transport, whose stimulation, however, was essentially dependent on the simultaneous presence of flavone. The existence of a flavone-activated apical flavonoid transporter in Caco-2 cells was finally associated with the potently diminished transepithelial apical to basolateral fluxes of 14C-kaempferol in the presence of competing unlabeled flavonoid substrates. Conclusion In conclusion, flavone activates an as yet unidentified transporter for flavonoids in the apical membrane of Caco-2 cells.