Data suggest that intestinal carotenoid absorption is a facilitated process. The present study was conducted to determine whether carotenoids and cholesterol share common pathways (transporters) for their intestinal absorption. Differentiated Caco-2 cells on membranes were incubated (16 h) with a carotenoid (1 μmol/L) with or without ezetimibe (EZ; Zetia®, an inhibitor of cholesterol transport), and with or without antibodies against the receptors, cluster determinant 36 (CD36) and scavenger receptor class B, type I (SR-BI). Carotenoid transport in Caco-2 cells (cellular uptake + secretion) was decreased by EZ (10 mg/L) as follows: β-carotene ≈ α-carotene (50% inhibition) ≫ β-cryptoxanthin ≈ lycopene (20%) ≫ lutein:zeaxanthin (1:1) (7%). EZ reduced cholesterol transport by 31%, but not retinol transport. β-Carotene transport was also inhibited by anti-SR-BI, but not by anti-CD36. The inhibitory effects of EZ and anti-SR-BI on β-carotene transport were additive, indicating that they may have different targets. Finally, differentiated Caco-2 cells treated with EZ showed a significant decrease in mRNA expression for the surface receptors SR-BI, Niemann-Pick type C1 Like 1 protein (NPC1L1), and ATP-binding cassette transporter, subfamily A (ABCA1) and for the nuclear receptors retinoid acid receptor (RAR)γ, sterol-regulatory element binding proteins (SREBP)-1 and -2, and liver X receptor (LXR)β as assessed by real-time PCR analysis. The data indicate that 1) EZ is an inhibitor of carotenoid transport, an effect that decreases with increasing polarity of the carotenoid molecule, 2) SR-BI is involved in carotenoid transport, and 3) EZ may act, not only by interacting physically with cholesterol transporters as previously suggested, but also by downregulating expression of these proteins. The cellular uptake and efflux of carotenoids, like that of cholesterol, likely involve more than one transporter.

In presence of oleate and taurocholate, differentiated CaCo-2 cell monolayers on membranes were able to assemble and secrete chylomicrons. Under these conditions, both cellular uptake and secretion into chylomicrons of β-carotene (β-C) were curvilinear, time-dependent (2–16 h), saturable, and concentration-dependent (apparent K_m of 7–10 μM) processes. Under linear concentration conditions at 16 h incubation, the extent of absorption of all-trans β-C was 11% (80% in chylomicrons), while those of 9-cis- and 13-cis-β-C were significantly lower (2–3%). The preferential uptake of the all-trans isomer was also shown in hepatic stellate HSC-T6 cells and in a cell-free system from rat liver (microsomes), but not in endothelial EAHY cells or U937 monocyte-macrophages. Moreover, extents of absorption of α-carotene (α-C), lutein (LUT), and lycopene (LYC) in CaCo-2 cells were 10%, 7%, and 2.5%, respectively. Marked carotenoid interactions were observed between LYC/β-C and β-C/α-C.

Alexandrine During, Sundari Doraiswamy, Earl H. Harrison

Recent studies support beneficial effects of polyphenols in various chronic inflammatory diseases, for example, the inflammatory bowel diseases. Inhibition of NF-κB activation by polyphenols could explain part of their anti-inflammatory properties, but few data are available on the intestine. The purpose of the present study was thus to investigate the effects of some polyphenols on NF-κB activation using human intestinal Caco-2 cells. Effects of standard polyphenols (50 μmol/l) were studied on different cellular events associated with NF-κB activation: (i) NF-κB activity using cells transiently transfected with a NF-κB–luciferase construct and stimulated by inflammatory agents (IL-1β, TNF-α or lipopolysaccharides (LPS)); (ii) phosphorylation of the inhibitor of κB (IκB-α) analysed by Western blot; (iii) secretion of IL-8 quantified by ELISA assay. Results showed that chrysin and ellagic acid inhibited NF-κB activity, whereas genistein and resveratrol increased it. These effects were independent of the nature of the inducer, indicating that polyphenols may modulate NF-κB activation by acting on a common event to the cytokine- and LPS-mediated cascades. Chrysin strongly reduced (2·5-fold) IL-1β-induced IκB-α phosphorylation, whereas ellagic acid increased it (1·7-fold). Ellagic acid, genistein and epigallocatechin gallate reduced (4- to 8-fold) IL-1β-induced IL-8 secretion, while resveratrol promoted (1·7-fold) the secretion. Chrysin also diminished IL-8 secretion by 1·6-fold (but P>0·05). The data indicate that polyphenols can modulate the NF-κB activation pathway in the intestine. Chrysin could block NF-κB activation via the inhibition of IκB-α phosphorylation. The other molecular targets of the active polyphenols are still to be identified.

Purpose. Oxidative stress has been proposed as a major pathogenic factor in age-related macular degeneration (AMD), the leading cause of vision loss among elderly people of western European ancestry. Lutein (LUT) and zeaxanthin (ZEA), major components in macular pigment, are among the retinal antioxidants. Though xanthophyll intake may reduce the likelihood of having advanced AMD, direct evidence of neuroprotection is lacking. Prior work has shown that docosahexaenoic acid (DHA), the major polyunsaturated fatty acid in the retina, delays apoptosis and promotes differentiation of photoreceptors. This study was conducted to investigate whether LUT, ZEA, and β-carotene (BC), major dietary carotenoids protect photoreceptors from oxidative stress and whether this protection is synergistic with that of DHA. methods. Pure rat retinal neurons in culture, supplemented with LUT, ZEA, or BC, with or without DHA, were subjected to oxidative stress induced with paraquat and hydrogen peroxide. Apoptosis, preservation of mitochondrial membrane potential, cytochrome c translocation, and opsin expression were evaluated. results. Pretreatment with DHA, LUT, ZEA, and BC reduced oxidative stress-induced apoptosis in photoreceptors, preserved mitochondrial potential, and prevented cytochrome c release from mitochondria. ZEA and LUT also enhanced photoreceptor differentiation. In control cultures, photoreceptors failed to grow their characteristic outer segments; addition of DHA, ZEA, or LUT increased opsin expression and promoted the development of outer-segment–like processes. conclusions. These results show for the first time the direct neuroprotection of photoreceptors by xanthophylls and suggest that ZEA and LUT, along with DHA, are important environmental influences that together promote photoreceptor survival and differentiation.