Intestinal β-carotene 15,15′-dioxygenase activity is markedly enhanced in copper-deficient rats fed on high-iron diets and fructose

Mechanisms involved in the intestinal absorption of dietary vitamin A and provitamin A carotenoids

Vitamin A is an essential nutrient for humans and is converted to the visual chromophore, 11-cis-retinal, and to the hormone, retinoic acid. Vitamin A in animal-derived foods is found as long chain acyl esters of retinol and these are digested to free fatty acids and retinol before uptake by the intestinal mucosal cell. The retinol is then reesterified to retinyl esters for incorporation into chlylomicrons and absorbed via the lymphatics or effluxed into the portal circulation facilitated by the lipid transporter, ABCA1. Provitamin A carotenoids such as β-carotene are found in plant-derived foods. These and other carotenoids are transported into the mucosal cell by scavenger receptor class B type I (SR-BI). Provitamin A carotenoids are partly converted to retinol by oxygenase and reductase enzymes and the retinol so produced is available for absorption via the two pathways described above. The efficiency of vitamin A and carotenoid intestinal absorption is determined by the regulation of a number of proteins involved in the process. Polymorphisms in genes for these proteins lead to individual variability in the metabolism and transport of vitamin A and carotenoids. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

Importance of β,β-carotene 15,15'-monooxygenase 1 (BCMO1) and β,β-carotene 9',10'-dioxygenase 2 (BCDO2) in nutrition and health

In humans, varying amounts of absorbed β-carotene are oxidatively cleaved by the enzyme β,β-carotene 15,15'-monooxygenase 1 (BCMO1) into two molecules of all-trans-retinal. The other carotenoid cleavage enzyme β,β-carotene 9',10'-dioxygenase (BCDO2) cleaves β-carotene at the 9',10' double bond forming β-apo-10'-carotenal and β-ionone. Although the contribution of BCDO2 to vitamin A formation has long been debated, BCMO1 is currently considered the key enzyme for retinoid metabolism. Furthermore, BCMO1 has limited enzyme activity towards carotenoids other than provitamin A carotenoids, whereas BCDO2 exhibits a broader specificity. Both enzymes are located at different sites within the cell, with BCMO1 being a cytosolic protein and BCDO2 being located in the mitochondria. Expression of BCMO1 in tissues other than the intestine has recently revealed its function for tissue-specific retinoid metabolism with importance in embryogenesis and lipid metabolism. On the other hand, biological activity of BCDO2 metabolites has been shown to be important in protecting against carotenoid-induced mitochondrial dysfunction. Single-nucleotide polymorphisms (SNPs) such as R267S and A379V in BCMO1 can partly explain inter-individual variations observed in carotenoid metabolism. Advancing knowledge about the physiological role of these two enzymes will contribute to understanding the importance of carotenoids in health and disease.

Cadmium induces retinoic acid signaling by regulating retinoic acid metabolic gene expression

The transition metal cadmium is an environmental teratogen. In addition, cadmium and retinoic acid can act synergistically to induce forelimb malformations. The molecular mechanism underlying the teratogenicity of cadmium and the synergistic effect with retinoic acid has not been addressed. An evolutionarily conserved gene, beta,beta-carotene 15,15'-monooxygenase (BCMO), which is involved in retinoic acid biosynthesis, was studied in both Caenorhabditis elegans and murine Hepa 1-6 cells. In C. elegans, bcmo-1 was expressed in the intestine and was cadmium inducible. Similarly, in Hepa 1-6 cells, Bcmo1 was induced by cadmium. Retinoic acid-mediated signaling increased after 24-h exposures to 5 and 10 microm cadmium in Hepa 1-6 cells. Examination of gene expression demonstrated that the induction of retinoic acid signaling by cadmium may be mediated by overexpression of Bcmo1. Furthermore, cadmium inhibited the expression of Cyp26a1 and Cyp26b1, which are involved in retinoic acid degradation. These results indicate that cadmium-induced teratogenicity may be due to the ability of the metal to increase the levels of retinoic acid by disrupting the expression of retinoic acid-metabolizing genes.

Differences in expression and activity of ?,?'-carotene-15,15'- oxygenase in liver and duodenum of cattle with yellow or white fat

Pasture-fed cattle show yellow pigmentation of their fat due to beta-carotene stored in this tissue. beta,beta'-Carotene-15,15'-oxygenase (betaCO) is an enzyme expressed in different tissues, and it cleaves beta-carotene into retinal. We compared the expression and activity of betaCO in duodenum and liver of cattle with pigmented or non-pigmented fat. In the duodenum, in situ hybridizations showed expression of betaCO in epithelial cells and crypts of the mucosa that was similar in animals from pigmented and non-pigmented fat; liver showed diffuse signal at lobules, but pigmented animals showed higher signals near the portal space. Analyses by real-time reverse-transcription polymerase chain reaction also showed amplification of mRNA for betaCO in duodenum and liver, with no difference between pigmented or non-pigmented animals. Enzyme activity was similar in the duodenum, but pigmented animals had higher enzyme activity (p = 0.004) in liver. Cattle with pigmented fat had higher expression and activity of betaCO in liver, but its level was not high enough to prevent the storage of beta-carotene in adipose tissues.

Molecular cloning of the rat beta-carotene 15,15'-monooxygenase gene and its regulation by retinoic acid

BACKGROUND

beta-Carotene exhibits biological activity as provitamin A. Key step in vitamin A formation is the cleavage of beta-carotene to retinal by an enzyme designated as beta-carotene 15,15'-monooxygenase (BCM). Recently, it is reported that expression of BCM gene in the intestine is under feedback regulation by retinoic acid (RA). However, the regulation of BCM gene expression in various other tissues is still unknown.

AIM OF THE STUDY

In the present study, we identified the full-length cDNA encoding the rat BCM gene and investigated the regulation of its expression in several tissues by RA in the presence of vitamin A deficiency.

METHODS

We cloned the full-length cDNA encoding BCM gene from a rat intestinal cDNA library by hybridization screening. The BCM gene expression was examined using Northern blotting and reverse transcription-PCR analysis. We also investigated whether BCM gene expression was regulated by retinoids in several tissues of vitamin A-deficient rats.

RESULTS

Sequence analysis of this clone revealed an open reading frame of 1,701 bases encoding a protein of 566 amino acids. The predicted polypeptide showed 94%, 81%, and 66% identity with mouse, human, and chicken BCM, respectively. Rat BCM mRNA was highly expressed in the intestine and liver, while there was weak expression in the testes, kidneys, and lungs. Immunoblotting revealed that rat BCM is a 64-kDa protein. BCM gene expression was increased in the small intestine by vitamin A deficiency compared with that in rats on a control diet, while this upregulation was suppressed by all-trans RA (ATRA) or 9-cis RA (9-cis RA). BCM gene expression in the lungs and testes was also suppressed by ATRA or 9-cis RA in rats with vitamin A deficiency. However, hepatic BCM gene expression was only decreased by ATRA and renal expression was not affected by either retinoid. As the small intestine is the major site of beta-carotene conversion, intestinal BCM gene expression may be more tightly regulated.

CONCLUSION

These data suggest that BCM gene expression in several tissues may be down-regulated by RA at the level of conversion of beta-carotene to retinal. To prevent an excess of retinol, homeostasis may occur at the level of conversion of beta-carotene to retinal in several tissues.

Sialic acid-containing milk proteins show differential immunomodulatory activities independent of sialic acid

The immunomodulatory activities of four sialic acid-containing milk proteins (kappa-casein, glycomacropeptide, lactoferrin, and proteose peptone-3 component) were determined, and the role of sialic acid was evaluated. Two in vitro models were used: murine splenocyte proliferation, where the effect on LPS-, Con A-, and PHA-stimulated proliferation was studied, and cytokine production in LPS-stimulated murine dendritic cells (DC). All four proteins inhibited LPS-induced splenocyte proliferation, though to different degrees, and independently of sialic acid. kappa-Casein strongly inhibited PHA-induced proliferation and had a weak inhibitory effect on Con A-induced proliferation, whereas lactoferrin stimulated Con A-induced proliferation. kappa-Casein, glycomacropeptide, and lactoferrin differentially affected cytokine production by DC: kappa-casein significantly inhibited production of TNF-alpha, IL-10, -12, -6, and -1beta, independent of sialic acid, whereas less-marked effects of glycomacropeptide and lactoferrin were seen. These findings thus point to important immunosuppressive effects of some milk proteins and indicate that they may function via different mechanisms.

Lycopene but not lutein nor zeaxanthin decreases in serum and lipoproteins in age-related macular degeneration patients

BACKGROUND

Epidemiological studies have established that a low serum concentration of carotenoids was associated with risk of Age-Related Macular Degeneration (ARMD). The aim of this study was to determine carotenoid levels in serum and in different lipoprotein fractions in patients diagnosed for ARMD and in matched control group.

METHOD

Thirty-four ARMD patients and 21 control subjects from Brest area (France) have been included to this study. Lipoproteins have been separated from serum by gradient density ultracentrifugation. We measured concentration of carotenoids and tocopherols in serum and in different lipoprotein fractions by HPLC.

RESULTS

No difference was observed between ARMD patients and control subjects in total serum carotenoids. Individual carotenoid levels showed that only lycopene was decreased significantly in serum, LDL and HDL fractions in patients (P<0.05). Concentrations in serum and lipoparticle fractions of lutein and zeaxanthin, the major pigments present in macula were not modified between both groups.

CONCLUSIONS

Lycopene, as liposoluble antioxidant nutrient, is the only carotenoid altered in ARMD patients. It cannot be excluded that this effect is related to different dietary habits, but we hypothesise that lower lycopene status could result also from specific antioxidant protection of lutein and zeaxanthin by lycopene.

Intestinal absorption and metabolism of carotenoids: insights from cell culture

Cell culture models are useful for studying intestinal absorption and metabolism of carotenoids. The human intestinal cell line, Caco-2, has been the most widely used model for these studies. The PF11 and TC7 clones of Caco-2 exhibit beta-carotene-15,15'-oxygenase activity, a key enzyme in the conversion of carotenoids to vitamin A. Studies on the recent cloning of this enzyme are discussed. An in vitro cell culture system used to study intestinal absorption of carotenoids is presented. Under conditions mimicking the postprandial state, Caco-2 cells on membranes take up carotenoids and secrete them incorporated into chylomicrons. Both the cellular uptake and secretion of beta-carotene are saturable, concentration-dependent processes. The selective absorption of all-trans beta-carotene versus its cis isomers, the differential absorption of individual carotenoids, and the specific interactions between carotenoids during their absorption are discussed. The participation of a specific epithelial transporter in the intestinal absorption of carotenoids is proposed.

Immunoenhancing effects of bovine glycomacropeptide and its derivatives on the proliferative response and phagocytic activities of human macrophagelike cells, U937

The immunomodulatory effect of GMP and its derivates on the cell proliferative response of human macrophagelike cell, U937, and its effect on phagocytic activities via incorporation of fluorescence beads were studied. GMP was found to be a potent immunoenhancer at low concentrations, significantly enhancing the proliferation and phagocytic activities of U937. The modulatory function could be radically altered by enzymatic treatments. Pepsin digestion significantly enhanced the degree of cell proliferation and phagocytic activities, whereas trypsin had no significant effect. The immunoenhancing effects decreased significantly after sialidase treatment; however, more than 70% of activity was retained after treatment. GMP with different carbohydrate chains was shown to possess different modulatory capabilities. Sialic acid-rich GMP fractions showed an enhanced response. These findings indicate that both the carbohydrate chains compositions, including the terminal sialic acids and the polypeptide portions of GMP, are essential for the stimulatory effects of GMP on cell proliferation and phagocytic activities of U937.

beta-Carotene 15,15'-Dioxygenase activity in human tissues and cells: evidence of an iron dependency

The two objectives of this study were to investigate beta-carotene 15,15'-dioxygenase activity in human tissues and to determine the effect of desferrioxamine on the dioxygenase activity. Two human in vitro models were used: the TC7 clone of the intestinal cell line Caco-2 and small intestinal mucosa preparations. beta-Carotene 15,15'-dioxygenase activity in the small intestinal mucosa was (mean +/- SD) 97.4 +/- 39.8 pmol/h.mg protein for five adults (44-89 y) and 20 pmol/h.mg for an infant (17 months). No activity was detected in adult stomach tissue. We report for the first time the dioxygenase activity in human liver: 62 pmol/h.mg for a normal adult liver and 7 pmol/h.mg for a liver exhibiting gross pathology. The maximum capacity of beta-carotene cleavage in an adult was estimated to be 12 mg/day (one fifth by small intestine and four fifths by liver), assuming an optimal beta-carotene/retinal cleavage ratio of 1:2. The dioxygenase activity was decreased up to 80% with increasing desferrioxamine concentrations in the two in vitro models. Desferrioxamine was characterized as a noncompetitive inhibitor. In TC7 cells, the inhibitory effect of desferrioxamine was reversed by iron addition, suggesting that this effect was related to the ability of desferrioxamine to chelate iron, purported to be an obligate cofactor of the enzyme. In conclusion, these data report the presence of beta-carotene 15,15'-dioxygenase activity in human small intestine and liver and demonstrate that desferrioxamine efficiently inhibits intestinal beta-carotene cleavage in human tissues and cells.

The enzymatic cleavage of beta-carotene: end of a controversy

The central cleavage of dietary beta-carotene to retinal was found to be the predominant mechanism whereby retinoids were formed in vivo in rats; apo-carotenals, indicative of eccentric cleavage of beta-carotene, were only a minor component (<5% of retinoids). A gene from maize that codes for a plant carotenoid cleavage enzyme was used to isolate a homologous gene from Drosophila. This gene, when transfected into an E Coli strain capable of synthesizing and accumulating beta-carotene, caused the central cleavage of beta-carotene, forming exclusively retinoids. The enzyme that the gene codes for, beta-carotene-15,15'-dioxygenase, was purified and characterized.