Mechanism of riboflavine uptake by Caco-2 human intestinal epithelial cells

The cellular and molecular regulation of intestinal absorption of the water-soluble vitamin riboflavine (RF) is poorly understood. The availability of a suitable in vitro cultured system that possesses the transport characteristics of the native intestinal absorptive cells would provide a powerful means to address this issue. In this study, we examined RF uptake by the human-derived cultured Caco-2 intestinal epithelial cells. RF uptake was Na⁺ and pH independent and occurred without metabolic alterations of the transported RF. Initial rate of RF uptake was temperature dependent and saturable as a function of concentration at 37°C but not at 4°C (apparent Michaelis constant = 0.30 ± 0.03 μM, maximal velocity = 209.90 ± 24.40 pmol · mg protein^-1 · 3 min^-1). Unlabeled RF, lumiflavine, 8-amino- riboflavine, isoriboflavine, and lumichrome in the incubation solution caused significant inhibition of RF uptake. RF uptake was also energy dependent and was sensitive to the inhibitory effect of sulfhydryl group reagents. The membrane transport inhibitor amiloride, but not 4,4'-diisothiocyanostilbene- 2,2'-disulfonic acid, 4-acetamide-4'-isothiocyanostilbene-2,2'-disulfonic acid, furosemide, or probenecid, inhibited RF uptake in a competitive (inhibitory constant = 0.48 mM) and reversible manner. Growing Caco-2 monolayers in a RF-deficient and oversupplemented media caused significant up- and downregulation of RF uptake, respectively. These results demonstrate that existence of a carrier-mediated system for RF uptake by Caco-2 cells and provide new information regarding amiloride sensitivity, involvement of sulfhydryl groups, and up- and downregulation by the substrate level and clarify the controversy regarding the role of Na⁺ in the uptake process. These results also demonstrate the suitability of Caco-2 cells as an in vitro cultured model system for studying the regulation of RF intestinal transport.