Mechanisms of Intestinal Absorption of Carotenoids: Insights from In Vitro Systems - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 377

exclusively with chylomicrons. Recent studies were designed to compare mechanisms of retinol and
carotenoid transport (During et al., 2007). When cells were incubated with retinol for varying times
(1-24 h), cellular retinol reached plateau levels within 2 h, whereas retinyl ester formation increased
continuously. Retinol and retinyl ester efl ux into basolateral medium increased linearly with time.
Free retinol was associated with the nonlipoprotein fraction and retinyl esters with chylomicrons.
In contrast to carotenoids, retinol uptake at the apical membrane was directly proportional to initial
retinol concentration over a wide range (0.5-110 mM). However, free retinol efl ux from the basolat-
eral membrane occurred via two processes: (a) a saturable process at low concentrations ( 10 mM)

and (b) a nonsaturable process at higher concentrations. When cells were loaded with retinol and
then maintained on retinoid-free medium for 5d, free retinol, but not retinyl esters, was secreted
into the basolateral medium. Glyburide (inhibitor of ABCA1 and other transporters) signii cantly
reduced free retinol efl ux, but not cellular retinol uptake. Inhibition of ABCA1 protein expression
by siRNAs inhibited free retinol efl ux but had no effect on carotenoid efl ux from the basolateral
membrane. SR-B1 inhibition did not affect retinol transport, but decreased cellular uptake of b-C,
b-cryptoxanthin, and LUT. Importantly, the extent of inhibition of SR-BI expression correlated with
the extent of inhibition of carotenoid absorption in this system (Figure 17.5). Inhibition of NPC1L1
expression by siRNA did not affect either retinol or carotenoid uptake. These data suggest that (a)
free retinol enters intestinal cells by diffusion; (b) free retinol efl ux is partly facilitated, probably
by the basolateral transporter ABCA1; and (c) newly synthesized retinyl esters, but not preformed
esters, are incorporated into chylomicrons and secreted. In contrast to vitamin A transport, caro-
tenoid uptake is mediated by the apical transporter SR-B1 and carotenoid efl ux occurs exclusively
via their secretion in chylomicrons.

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