Flavone potently stimulates transepithelial absorption of flavonoids by activating an apical transporter in human intestinal Caco-2 cells

Based on 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 Km of 1.1 µM and a pH-optimum of 8.5 and was stimulated by flavone fourfold in its Vmax. Ring C of the other sixteen flavonoids tested turned out to be of special importance in order to act as potent competitive 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 ATP-dependent efflux pumps. Also, inhibitors of MAP- 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 evidenced by the potently diminished transepithelial apical to basolateral fluxes of 14C-kaempferol in the presence of competing unlabeled flavonoid substrates.