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15.4 CULTURED RPE AS A MODEL OF PHYSIOLOGICAL RPE FUNCTIONS
Despite recent advances in understanding carotenoid uptake in the intestines, the specii c uptake
and retention of carotenoids in the retina is still poorly understood (Loane et al., 2008). In contrast
to photoreceptors, which in culture rapidly lose their morphology and physiological function, RPE
cells, cultured under appropriate conditions can sustain several features characteristic of them under
physiological conditions (Figure 15.4). These include phagocytosis, ability to form polarized mono-
layers, expression of growth factors, cytokines, and certain components of the complement (Chen
et al., 2007; Crane et al., 2000a; Cui et al., 2006; Ebihara et al., 2007; Ershov and Bazan, 2000;
Ishida et al., 2004; Joffre et al., 2007; Karl et al., 2007; Maminishkis et al., 2006; Momma et al.,
2003; Mukherjee et al., 2007). When isolated and cultured under appropriate conditions, RPE cells
retain the ability to synthesize melanin and sustain the retinoid cycle by synthesizing 11-
cis
-retinal
from all-
trans
-retinol (Maminishkis et al., 2006; von Recum et al., 1999).
Cultures of primary RPE and some RPE cell lines express scavenger receptors and transporters
that are deemed likely participants in carotenoid uptake and further transport. These include CD36
(Ryeom et al., 1996a,b), SR-BI and SR-BII (Duncan et al., 2002), ABCA1 (Lakkaraju et al., 2007),
and ABCB1 (MDR1/P-glycoprotein) (Aukunuru et al., 2001; Constable et al., 2006; Kennedy and
Mangini, 2002). Human RPE cell line, D407, has been shown to express fully functional BCO
(Chichili et al., 2005). Moreover cultured RPE cells express similar apo-lipoproteins to those of the
RPE
in situ
, including ApoA-I, ApoB, ApoC-I, ApoC-II, ApoE, ApoJ, and microsomal triglyceride
transfer protein (MTP), which is required for assembly of lipoproteins containing ApoB (Li et al.,
2005, 2006; Suuronen et al., 2007). In particular, cultures of ARPE-19, a spontaneously immortal-
ized human RPE cell line derived from 19-year-old human male (Dunn et al., 1996), exhibits dif-
ferentiated properties and expresses all of these scavenger receptors, transporters, apo-lipoproteins,
and MTP. Thus RPE cultures appear as an excellent model to study carotenoid uptake, dynamics of
transport, and effects of carotenoids on the RPE.
15.4.1 C
AROTENOID
U
PTAKE
, A
CCUMULATION
,
AND
S
ECRETION
IN
C
ULTURED
RPE C
ELLS
The methodology to study
in vitro
drug uptake, metabolism, and removal is well established in
extensive studies of blood-brain barrier, intestinal drug metabolism, and drug resistance in cancer
cells (Cecchelli et al., 2007; Reboul et al., 2006; Sarkadi et al., 2006; van de Kerkhof et al., 2007). In
particular, the mechanisms responsible for intestinal absorption of carotenoids have recently been a
subject of intensive investigation where the Caco-2 cell line has been used as a model of intestinal
enterocytes (O'Sullivan et al., 2007; Reboul et al., 2005; Yonekura and Nagao, 2007). In contrast
to previous beliefs of the diffusion of carotenoids and tocopherols into the enterocytes being a
passive process, it has now been shown that their uptake is mediated by SR-BI and possibly other
receptors (During and Harrison, 2007; van Bennekum et al., 2005). Study of competitive uptake
of a-tocopherol by Caco-2 cells in the presence of a mixture of carotenoids (lycopene, b-carotene,
and lutein) demonstrated that carotenoids signii cantly reduce a-tocopherol uptake, thus provid-
ing further support for an important role of SR-BI in the uptake of both carotenoids and vitamin E
Photoreceptor outer segments
Apical medium
RPE
RPE
Bruch's membrane
Chc
Basal medium
Fenestrated bed of choriocapillaris
(a)
(b)
FIGURE 15.4
Polarized cultures of RPE as a model of blood-retina barrier: (a) schematic diagram of the
RPE at the blood-retina barrier and (b) culture of polarized RPE cells as a model of the blood-retina barrier.
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