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

Abstract

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. Rhodamine absorption displayed saturation kinetics with a K(m) of 1.1 μM and a pH-optimum of 8.5 and was stimulated by flavone four-fold in its V(max) . 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 (14) C-kaempferol in the presence of competing unlabeled flavonoid substrates. In conclusion, flavone activates an as yet unidentified transporter for flavonoids in the apical membrane of Caco-2 cells.