Metabolism of Flavonoids via Enteric Recycling: Mechanistic Studies of Disposition of Apigenin in the Caco-2 Cell Culture Model

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Abstract
The purpose of this study was to determine the mechanisms responsible for intestinal disposition of apigenin in the human Caco-2 cell culture model. The results indicated that most of the absorbed apigenin (10 μM) were conjugated and only a small fraction was transported intact. The amounts of conjugates excreted, especially that of the sulfate, were dependent on days-post-seeding. Apical efflux of apigenin sulfate did not change with concentration of apigenin (4 to 40 μM), whereas its basolateral efflux increased (p < 0.01) with concentration and plateaued at about 25 μM. In contrast, sulfate formation rates in cell lysate increased with concentration and plateaued at 25 μM and were 4 to 6 times faster than the corresponding excretion rates. Formation and polarized excretion rates of glucuronidated apigenin increased with apigenin concentration but formation rates were usually 2.5 to 6 times faster than the corresponding excretion rates. Inhibitors of multidrug resistance-related proteins (MRPs) such as leukotriene C4 and MK-571, which inhibited glucuronidation of apigenin at a high concentration (≥25 μM), significantly decreased excretion of both apigenin conjugates, and higher concentrations of MK-571 increased the extent of inhibition. In contrast, an organic anion transporter (OAT) inhibitor estrone sulfate only inhibited excretion of apigenin sulfate. In conclusion, we have shown for the first time that intestinal efflux is the rate-limiting step in the intestinal excretion of phase II conjugates of flavones. Furthermore, MRP and OAT are involved in the intestinal efflux of these hydrophilic phase II conjugates.