Abstract
The purpose of this study was to compare the mechanisms of intestinal retinol (ROL) and carotenoid transport. When differentiated Caco-2 cells were incubated with ROL for varying times, cellular ROL plateaued within 2 h, whereas retinyl ester (RE) formation increased continuously. ROL and RE efflux into basolateral medium (BM) increased linearly with time, ROL in the nonlipoprotein fraction and REs in chylomicrons (CMs). In contrast to carotenoids, ROL uptake was proportional to ROL concentration (0.5-110 microM). ROL efflux into BM occurred via two processes: a) a saturable process at low concentrations (<10 microM) and b) a nonsaturable process at higher concentrations. When ROL-loaded cells were maintained on retinoid-free medium, free ROL, but not REs, was secreted into BM. Glyburide significantly reduced ROL efflux but not ROL uptake. Inhibition of ABCA1 protein expression by small interfering RNAs decreased ROL efflux but not carotenoid efflux. Scavenger receptor class B type I (SR-BI) inhibition did not affect ROL transport but decreased carotenoid uptake. The present data suggest that a) ROL enters intestinal cells by diffusion, b) ROL efflux is partly facilitated, probably by the basolateral transporter ABCA1, and c) newly synthesized REs, but not preformed esters, are incorporated into CM and secreted. In contrast to ROL transport, carotenoid uptake is mediated by the apical transporter SR-BI, and carotenoid efflux occurs exclusively via their secretion in CM.
Highlights
The purpose of this study was to compare the mechanisms of intestinal retinol (ROL) and carotenoid transport
An esterification rate of ROL was estimated as 70 pmol retinyl ester (RE) formed per hour per 2 3 106 Caco-2 cells when incubated with 3 mM ROL
After incubation with ROL at concentrations of 0.5–110 mM, the cellular uptake of ROL was directly proportional to the initial ROL concentration in the Apical medium (AM)
Summary
The purpose of this study was to compare the mechanisms of intestinal retinol (ROL) and carotenoid transport. ROL and RE efflux into basolateral medium (BM) increased linearly with time, ROL in the nonlipoprotein fraction and REs in chylomicrons (CMs). The present data suggest that a) ROL enters intestinal cells by diffusion, b) ROL efflux is partly facilitated, probably by the basolateral transporter ABCA1, and c) newly synthesized REs, but not preformed esters, are incorporated into CM and secreted. Mechanisms of provitamin A (carotenoid) and vitamin A (retinol) transport into and out of intestinal Caco-2 cells. Humans must obtain vitamin A from the diet, either as preformed ROL or as provitamin A carotenoid precursors. Vitamin A is present in the diet as retinyl esters (REs) found in foods of animal origin and as provitamin A carotenoids (mainly b-carotene, a-carotene, and b-cryptoxanthin) found in plant-derived products.
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