Carotenoid Uptake and Protection in Cultured RPE - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 326

stimulate the carotenoid efl ux from RPE cells. If such a transport pathway exists, then, how that
transport depends on the type of the carotenoid and whether it is greater on the basal side or the
apical side in polarized RPE monolayers would be informative.
Altogether, there are many unknowns about carotenoid transport in the retina. However, present
knowledge on carotenoid uptake in other cell types and the i nding of multiple proteins poten-
tially involved in carotenoid transport in the RPE and adjacent neural retina leads to the suggestion
that several hypothetical pathways exist (Figure 15.3). Many such pathways can be easily tested in
cultured RPE.
15.5 CAROTENOID PROTECTION IN THE RPE
In the retina, RPE cells are under constant oxidative stress: (1) they are exposed to high oxygen
tensions; (2) include abundant intracellular polyunsaturated lipids, including docosahexaenoate with
six unsaturated double bonds extremely susceptible to peroxidation and are exposed to polyunsatu-
rated lipids present in POS and Bruch's membrane from both apical and basal sides, respectively;
(3) are involved in transport of iron between the retina and choroidal blood supply; (4) are exposed
to a high intensity of visible light; and (5) potent photosensitizers present both inside and outside
the cell that can activate oxygen in the presence of light (Boulton et al., 2001; He et al., 2007;
Ró . anowska and Ró . anowski, 2008; Rozanowska and Sarna, 2005; Wong et al., 2007).
As discussed earlier, due to their antioxidant properties carotenoids have a potential to provide
protection against oxidative damage. However, it has been demonstrated that at least a part of the
lutein and zeaxanthin in the retina is bound to glutathione transferase GSTP1 and other as yet to
be identii ed XBPs with high afi nity for xanthophylls, and tubulin (Bernstein et al., 1997; Bhosale
et al., 2004; Yemelyanov et al., 2001). It has been shown that the binding of zeaxanthin or meso-
zeaxanthin to GSTP1 enhances their antioxidant action against lipid peroxidation induced by ther-
molabile azo-compounds as a source of peroxyl radicals (Bhosale and Bernstein, 2005). It remains
to be determined how the binding to these proteins affects xanthophyll ability to quench excited
states of photosensitizers and singlet oxygen.
Moreover, carotenoids themselves are very susceptible to oxidative damage and their oxidation
products include deleterious aldehydes (Failloux et al., 2003; Hurst et al., 2005; Rozanowski and
Rozanowska, 2005; Siems et al., 2000, 2002; Sommerburg et al., 2003). Therefore it is of interest to
i nd out how carotenoids can offer antioxidant protection in cellular systems, how stable the carote-
noids are within cells, and what the fate of the carotenoid degradation products is.
15.5.1 E FFECTS OF C AROTENOIDS ON O XIDATIVE S TRESS IN C ULTURED RPE C ELLS
As mentioned previously, the ability of carotenoids to inhibit oxidative stress was tested in vitro
in many different cell types. In the retina only lutein and zeaxanthin accumulate in sufi cient con-
centrations to exert direct antioxidant effects, therefore our further discussion of these antioxidant
effects will be focused mainly on those two xanthophylls.
Importantly, several studies have shown that cultured cells such as dermal i broblasts, mela-
noma cells, or ARPE-19 cells can accumulate substantial amounts of lutein and zeaxanthin without
deleterious effects on cell viability (Lornejad-Schafer et al., 2007; Philips et al., 2007; Roberts
et al., 2002; Rozanowska et al., 2004b). However, the exposure of ARPE-19 cells to higher concen-
trations of lutein, such as 10 mM over a period of 19 h which has led to internalization of 470 pmol
of lutein/million cells, has been shown to result in a small 9% decrease in their mitochondrial
activity (Kanofsky and Sima, 2006). This effect increased upon exposure of lutein-laden cells to
blue (430 nm; 8.4 J/cm 2 ) or green light (502 nm; 32 8.4 J/cm 2 ), where the mitochondrial activity has
decreased by about 15% (Kanofsky and Sima, 2006). It should be noted that lutein used in these
studies was of low purity of 70% and therefore it is unclear whether the cytotoxic effects can be
ascribed to lutein or other contaminants.