| Co-Investigator(Kenkyū-buntansha) |
SURUGA Kazuhito University of Shizuoka, School of Food and Nutritional Sciences, Department of Nutrition, Instructor, 食品栄養科学部, 助手 (70315852)
TAKASE Sachiko University of Shizuoka, School of Food and Nutritional Sciences, Department of Nutrition, Professor, 食品栄養科学部, 教授 (10046196)
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| Research Abstract |
1. The 5' flanking regions of two representing disaccharidase genes, i.e., sucrase-isomaltase (SI) and lactase-phlorizin hydrolase (LPH) were cloned. In the 5' flanking region of both rat LPH and rat SI genes, common Cdx-2 binding elements were present and termed as CE-LPH1 and SIF1, respectively. In the 5' region of rat SI gene, a HNF1-binding element was also present which was associated with the diurnal variations in the SI gene expression.
2. Developmental changes in the expressions of glucose transporters (GLUT5, SGLT1, GLUT2) and lactase were compared with those of nuclear factors in rat jejunum. After weaning, the mRNA levels of all types of glucose transporters were elevated, which was closely related with the rise in the expression of a thyroid hormone nuclear receptor (TR α-1). The decrease in the expression of LPH was accompanied by a decrease in the amounts of the nuclear factor binding to CE-LPH1.
3. Oro-gastric feeding of a sucrose-containing diet to rats caused parallel in
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creases in SI and LPH mRNA levels within 3h, which were completely abolished by an injection of actinomycin D. A difference was observed in the site along the villus-crypt columns where the induction started ; the initial rise in SI mRNA level occurred at the lower villus, whereas that of LPH mRNA level occurred at more apical and broader locus of the villus.
4. Among the monosaccharides tested, fructose gave rise to the most prominent increases in the transcripts of SI and LPH genes, which were accompanied by a coordinate rise in the transcripts of SGLT1 and GLUT5 genes. Feeding the diet containing glucose or α-methylglucoside (a non-metabolizable sugar) did not increase the transcript levels of SI, LPH and the intestinal hexose transporters. When a solution containing either fructose or glucose was simultaneously perfused into two consecutive cannulated and irrigated loops of jejunum, the loop perfused with fructose exhibited greater SI and GLUT5 mRNA levels than that perfused with glucose.
5. Nuclear run-on assays revealed that fructose, but not glucose, increased the transcription of SI, LPH and GLUT5. Electromobility shift assays using the CE-LPH1 and the SIF1 elements revealed that nuclear extracts from the jejunum of rats fed the high-starch diet gave greater density of retarded bands than those of rats fed the low-starch diet. In addition, we found that force feeding a fructose diet gave rise to an increase in the binding of the dimeric nuclear protein (Cdx2) to the SIF1 element, which was reproduced by dephosphorylation of the nuclear protein. Less
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