Competition Effects on Carotenoid Absorption by Caco-2 Cells - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 384

18.2.2 C OMPETITION BETWEEN C AROTENOIDS AND O THER
A NTIOXIDANT M ICRONUTRIENTS
In a normal meal containing plant-derived foods, carotenoids are necessarily ingested with other
dietary antioxidants, the main ones being vitamin C, E, and polyphenols. It is assumed that these
antioxidants may either protect carotenoids from degradation in the gastrointestinal tract before
their absorption, or compete with carotenoids for absorption. The effect of the other main dietary
antioxidants on carotenoid uptake by intestinal cells has been addressed in a recent study performed
in our laboratory (Reboul et al. 2007a). In this study, a full factorial design experiment was elabo-
rated to assess the effect of vitamin C, vitamin E (an equimolar mixture of ( R , R , R )-a-tocopherol and
( R , R , R )-g-tocopherol was used), and polyphenols (a mixture of gallic acid, caffeic acid, (+)-catechin
and naringenin was used) on the absorption of a carotenoid model: lutein. All of the above cited
antioxidants were provided to cells across a range of physiological concentrations. While the mixture
of polyphenols signii cantly ( P < 0.05) impaired lutein uptake, no signii cant effects were observed
with vitamins C or E. Also, no signii cant degradation of lutein occurred during the duration of the
experiments regardless of the presence of other antioxidant micronutrients, which indicates that the
observed effects were likely due to competition and not degradation. Additionally, no interaction
was observed between the different classes of micronutrients in terms of lutein uptake. The fact that
the polyphenol mixture signii cantly impaired lutein uptake raised the question as to which specii c
polyphenol(s) of the mixture were responsible for this effect. Therefore, we conducted a second
series of experiments to measure the individual effect of each polyphenol. These additional experi-
ments showed that naringenin was the only polyphenol able to signii cantly impair lutein uptake
(about 25% for 25 mM and 50% for 150 mM naringenin, respectively; P < 0.05). We observed that
the mixture of polyphenols containing 25 mM naringenin had a similar effect on 25 mM naringenin
alone, indicating that the other polyphenols tested (gallic acid, caffeic acid, and (+)-catechin) had
probably no signii cant effect on lutein uptake. The specii c effect exerted by naringenin needs to
be further elucidated by additional experiments, but it was highlighted that naringenin was the
most lipophilic of all the polyphenols tested (log P = 2.52 vs. 0.86, 0.82, and 0.38 for gallic acid,
caffeic acid, and (+)-catechin, respectively; Cooper et al. 1997). Therefore it was hypothesized that
naringenin affects lutein uptake through an interaction with SR-BI, which is known to transport
lipophilic molecules with low substrate specii city. A second hypothesis was that naringenin inter-
acts with membrane lipids (Tachibana et al. 2004), thereby altering the invagination of lipid raft
domains containing lutein receptors.
In the above study, vitamin E had no signii cant effect on lutein absorption. This was sur-
prising since both ( R , R , R )-a-tocopherol and ( R , R , R )-g-tocopherol have been shown to be trans-
ported through the SR-BI (Reboul et al. 2006), which is also involved in lutein uptake (Reboul
et al. 2005). We suggest that this is likely due to the fact that vitamin E was provided over a con-
centration range that was close to the normal physiological concentration (maximum 5.5 mM).
When tocopherols and carotenoids were incubated at higher concentrations (40 and 6 mM for
a-tocopherol and lutein, respectively) (Reboul et al. 2006), lutein signii cantly impaired tocoph-
erol uptake. Conversely, there was no signii cant effect of b-carotene or lycopene on tocopherol
uptake but, as discussed above this was probably due to the low concentration of these carote-
noids that could be incorporated in mixed micelles (2.8 and 0.4 mM for b-carotene and lycopene,
respectively). This result is in agreement with another study in which it was shown that a mixture
of carotenoids (lycopene, b-carotene, and lutein) signii cantly impaired a- tocopherol absorption
in Caco-2 cells (Reboul et al. 2007b). The studies described above show that carotenoids likely
impair tocopherol absorption. Therefore, although not yet observed in Caco-2 cell experiments,
it is likely that tocopherol can impair carotenoid absorption as well. This hypothesis is supported
by an in vivo study in rats where a-tocopherol decreased canthaxanthin absorption (Hageman
et al. 1999).