| There is mounting evidence from epidemiological studies as well as animal in vivo and in vitro studies to suggest that the consumption of quercetin, one of the most predominant dietary flavonoids, is beneficial to human health. However, the extent to which flavonoids are absorbed and the mechanisms involved are poorly understood. This dissertation has examined the general hypothesis that human intestinal absorption of quercetin and its main dietary form, i.e. quercetin 4'-beta-glucoside is limited and regulated by multiple transport mechanisms. These studies were done using the Caco-2 cell monolayer, a validated model of human intestinal absorption.;Using Caco-2 cell monolayers, together with molecularly specific analysis by reverse-phase HPLC, we were able to demonstrate that quercetin crosses the intestinal epithelium via a transcellular route with an apparent permeability (Papp) of 5.8 +/- 1.1 x 10-6 cm sec-1, predicting approximately 60% oral absorption. In similar experiments, quercetin 4'-beta-glucoside demonstrated no absorption but surprisingly did demonstrate basolateral to apical efflux. The efflux of quercetin 4'-beta-glucoside demonstrated saturation and was competitively inhibited by MK-571, a selective inhibitor of the closely related MRP isoforms 1 and 2. Localization of MRP2 but not MRP1 at the apical membrane of the Caco-2 cell monolayer by immunofluorescent laser scanning confocal microscopy implicated a role for MRP2 in the efflux of quercetin 4'-beta-glucoside This is the first report of a neutral flavonoid or glucoside being transported by this predominantly anionic transporter. As efflux by MRP2 could have masked evidence for SGLT1-dependent absorption, we examined cellular uptake of quercetin 4'-beta-glucoside across the apical membrane of Caco-2 cells. Despite a lack of apical to basolateral absorption, quercetin 4'-beta-glucoside was transported across the apical membrane and this transport was sodium-dependent and inhibited by the presence of phloridzin, a selective SGLT1 inhibitor, and by glucose, an SGLT1 substrate. In addition, transport of the glucoside was examined in CHO cells stably transfected with rabbit SGLT1, G6D3 cells. Cellular uptake was greater in the G6D3 cells than in the parental cells, and transport in the G6D3 cells was sodium-dependent and inhibited by phloridzin and glucose, further supporting a role for SGLT1 in the absorption of quercetin 4 '-beta-glucoside.;The studies in this dissertation are the first to describe the mechanisms governing the intestinal absorption of dietary flavonoids. The studies have demonstrated that quercetin is absorbed via a facile transcellular route, and that while quercetin 4'-beta-glucoside is absorbed across the apical membrane by SGLT1, transcellular absorption of the glucoside appears to be limited, in part, by MRP2 mediated efflux. |
