Submitted on June 8, 2007
Revised on July 2, 2007
Accepted on July 16, 2007
Mechanisms of provitamin a (Carotenoid) and vitamin a (Retinol) transport into and out of intestinal CACO-2 cells
Earl H. Harrison and Alexandrine During
Sciences de la Vie, Universite catholique de Louvain, Louvain-La-Neuve B-1348
Corresponding Author: during{at}bioc.ucl.ac.be
The purpose of this study was to compare mechanisms of intestinal retinol (ROL) and carotenoid transport. When differentiated Caco-2 cells were incubated with ROL for varying times, cellular ROL plateaued within 2h, whereas retinyl ester (RE) formation increased continuously. ROL and RE efflux into basolateral medium (BM) increased linearly with time; ROL in the non lipoprotein fraction and REs in chylomicrons (CM). In contrast to carotenoids, ROL uptake was proportional to ROL concentration (0.5-110M). ROL efflux into BM occurred via two processes: a) a saturable process at low concentrations (<10M) and b) a non-saturable 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 siRNAs decreased ROL efflux, but not carotenoid efflux. SR-B1 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-B1 and carotenoid efflux occurs exclusively via their secretion in CM.
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