A genetic dissection of intestinal fat-soluble vitamin and carotenoid absorption

Carotenoids are currently investigated regarding their potential to lower the risk of chronic disease and to combat vitamin A deficiency. Surprisingly, responses to dietary supplementation with these compounds are quite variable between individuals. Genome-wide studies have associated common genetic polymorphisms in the BCO1 gene with this variability. The BCO1 gene encodes an enzyme that is expressed in the intestine and converts provitamin A carotenoids to vitamin A-aldehyde. However, it is not clear how this enzyme can impact the bioavailability and metabolism of other carotenoids such as xanthophyll. We here provide evidence that BCO1 is a key component of a regulatory network that controls the absorption of carotenoids and fat-soluble vitamins. In this process, conversion of β-carotene to vitamin A by BCO1 induces via retinoid signaling the expression of the intestinal homeobox transcription factor ISX. Subsequently, ISX binds to conserved DNA-binding motifs upstream of the BCO1 and SCARB1 genes. SCARB1 encodes a membrane protein that facilitates absorption of fat-soluble vitamins and carotenoids. In keeping with its role as a transcriptional repressor, SCARB1 protein levels are significantly increased in the intestine of ISX-deficient mice. This increase results in augmented absorption and tissue accumulation of xanthophyll carotenoids and tocopherols. Our study shows that fat-soluble vitamin and carotenoid absorption is controlled by a BCO1-dependent negative feedback regulation. Thus, our findings provide a molecular framework for the controversial relationship between genetics and fat-soluble vitamin status in the human population.

STRA6 is critical for cellular vitamin A uptake and homeostasis

Vitamin A must be adequately distributed within the body to maintain the functions of retinoids in the periphery and chromophore production in the eyes. Blood transport of the lipophilic vitamin is mediated by the retinol-binding protein, RBP4. Biochemical evidence suggests that cellular uptake of vitamin A from RBP4 is facilitated by a membrane receptor. This receptor, identified as the Stimulated by retinoic acid gene 6 (Stra6) gene product, is highly expressed in epithelia that constitute blood-tissue barriers. Here we established a Stra6 knockout mouse model to analyze the metabolic basis of vitamin A homeostasis in peripheral tissues. These mice were viable when bred on diets replete in vitamin A, but evidenced markedly reduced levels of ocular retinoids. Ophthalmic imaging and histology revealed malformations in the choroid and retinal pigmented epithelium, early cone photoreceptor cell death, and reduced lengths of rod outer segments. Similar to the blood-retina barrier in the RPE, vitamin A transport through the blood-cerebrospinal fluid barrier in the brain's choroid plexus was impaired. Notably, treatment with pharmacological doses of vitamin A restored vitamin A transport across these barriers and rescued the vision of Stra6(-/-) mice. Furthermore, under conditions mimicking vitamin A excess and deficiency, our analyses revealed that STRA6-mediated vitamin A uptake is a regulated process mandatory for ocular vitamin A uptake when RBP4 constitutes the only transport mode in vitamin A deficiency. These findings identifying STRA6 as a bona fide vitamin A transporter have important implications for disease states associated with impaired blood vitamin A homeostasis.

Structural and functional characterization of the phytoene synthase promoter from Arabidopsis thaliana

The expression of the gene coding for the carotenogenic enzyme phytoene synthase is highly regulated. To study this, its promoter and truncated versions thereof were translationally fused to the luciferase gene as a reporter and these constructs were used to transform Arabidopsis thaliana. The full-length promoter was shown to be active in the dark, but mediated positive responses towards different light qualities (far-red, red, blue and white light). Among the herbicides tested, norflurazon and gabaculine showed no notable effects, while CPTA abolished light induction completely. Response towards different light qualities was mediated by a TATA box-proximal promoter region up to position -300, containing G-box-like elements involved in the distinction of different monochromatic light qualities applied. This is detected in electrophoretic mobility shift assays (EMSAs), which reveal differential complex formation. A TATA box distal region of the promoter was shown to be responsible for a high basal promoter activity that was not modulated by different light qualities. Using EMSAs, a novel cis-acting element ATCTA occurring in tandem between positions -854 and -841 proved to be decisive in this respect. The motif was found in several other promoter regions involved in carotenoid and tocopherol biosynthesis, as well as in the promoter regions mediating the expression of photosynthesis-related genes. The functional equivalence of the motifs was shown by successfully using the respective regions in EMSAs. We conclude that the ATCTA motif represents an element capable of mediating a coordinated regulation of these pathways at the transcriptional level.

A class B scavenger receptor mediates the cellular uptake of carotenoids in Drosophila

Carotenoids are currently being intensely investigated regarding their potential to lower the risk of chronic disease and vitamin A deficiency. Invertebrate models in which vitamin A deficiency is not lethal allow the isolation of blind but viable mutants affected in the pathway leading from dietary carotenoids to vitamin A. Using a mutant in one of these model systems, Drosophila, the vitamin A-forming enzyme has recently been molecularly identified. We now show that the molecular basis for the blindness of a different Drosophila mutant, ninaD, is a defect in the cellular uptake of carotenoids. The ninaD gene encodes a class B scavenger receptor essential for the formation of the visual chromophore. A loss of this function results in a carotenoid-free and thus vitamin A-deficient phenotype. Our investigations provide molecular insight into how carotenoids may be distributed into cells of target tissues in animals and indicate a crucial role of class B scavenger receptors rendering dietary carotenoids available for subsequent cell metabolism, as needed for their various physiological functions.

Carotene desaturation is linked to a respiratory redox pathway in Narcissus pseudonarcissus chromoplast membranes. Involvement of a 23-kDa oxygen-evolving-complex-like protein

The enzymic activity of phytoene desaturase in Narcissus pseudonarcissus chromoplast membranes depends in an essential way on the redox state of its environment. Here, the main redox-active components are quinones and tocopherols. Quinones (oxidized) act as intermediate electron acceptors in the desaturation reaction, as can be shown in reduced, hydroquinone-rich membranes. However, their complete oxidation by ferricyanide treatment of membranes leads to inhibition of the desaturation activity and, under these conditions, hydroquinones are required for reactivation. Using redox titrations, it is shown here that the optimal activity lies in the range of the midpoint potential of the plastoquinone/plastohydroquinone redox couple. For the adjustment of redox states of the redox-active lipid components in (photosynthetically inactive) chromoplasts, NADPH and oxygen are involved, the latter acting as a terminal acceptor. This results in a respiratory redox pathway in chromoplast membranes which is described here, to our knowledge, for the first time. Since phytoene desaturation responds to the redox state of quinones, which is adjusted by the respiratory redox pathway, the two reactions must be regarded as being mechanistically linked. The first protein component involved in the respiratory pathway which we have investigated molecularly is a 43-kDa NAD(P)H:quinone oxidoreductase, which is organized as a homodimer (23 +/- 3 kDa/subunit) and apparently possesses a manganese redox center. Internal protein microsequencing and cloning of the corresponding cDNA revealed a high degree of similarity to the 23-kDa protein of the oxygen-evolving complex of photosystem II, but no information about the N-terminal organization of the oxidoreductase could be obtained. During flower development, the steady-state concentration of the corresponding mRNA is up-regulated, indicating a specific function of the gene product in chlorophyll-free chromoplasts.