Biosynthesis of sulfated mucopolysaccharides in relation to vitamins A and K

The zebrafish retinol dehydrogenase, rdh1l, is essential for intestinal development and is regulated by the tumor suppressor adenomatous polyposis coli

Retinoic acid (RA) is a potent signaling molecule that plays important roles in multiple and diverse developmental processes. The contribution of retinoic acid to promoting the development and differentiation of the vertebrate intestine and the factors that regulate RA production in the gut remain poorly defined. Herein, we report that the novel retinol dehydrogenase, rdh1l, is required for proper gut development and differentiation. rdh1l is expressed ubiquitously during early development but becomes restricted to the gut by 3 days postfertilization. Knockdown of rdh1l results in a robust RA-deficient phenotype including lack of intestinal differentiation, which can be rescued by the addition of exogenous retinoic acid. We report that adenomatous polyposis coli (APC) mutant zebrafish harbor an RA-deficient phenotype including aberrant intestinal differentiation and that these mutants can be rescued by treatment with retinoic acid or injection of rdh1l mRNA. Further, we have found that although APC mutants are deficient in rdh1l expression, they harbor increased expression of raldh2 suggesting the control of RA production by APC is via retinol dehydrogenase activity. These results provide genetic evidence that retinoic acid is required for vertebrate gut development and that the tumor suppressor APC controls the production of RA in the gut by regulating the expression of the retinol dehydrogenase, rdh1l.

The tumor suppressor adenomatous polyposis coli and caudal related homeodomain protein regulate expression of retinol dehydrogenase L

Development of normal colon epithelial cells proceeds through a systematic differentiation of cells that emerge from stem cells within the base of colon crypts. Genetic mutations in the adenomatous polyposis coli (APC) gene are thought to cause colon adenoma and carcinoma formation by enhancing colonocyte proliferation and impairing differentiation. We currently have a limited understanding of the cellular mechanisms that promote colonocyte differentiation. Herein, we present evidence supporting a lack of retinoic acid biosynthesis as a mechanism contributing to the development of colon adenomas and carcinomas. Microarray and reverse transcriptase-PCR analyses revealed reduced expression of two retinoid biosynthesis genes: retinol dehydrogenase 5 (RDH5) and retinol dehydrogenase L (RDHL) in colon adenomas and carcinomas as compared with normal colon. Consistent with the adenoma and carcinomas samples, seven colon carcinoma cell lines also lacked expression of RDH5 and RDHL. Assessment of RDH enzymatic activity within these seven cell lines showed poor conversion of retinol into retinoic acid when compared with normal cells such as normal human mammary epithelial cells. Reintroduction of wild type APC into an APC-deficient colon carcinoma cell line (HT29) resulted in increased expression of RDHL without affecting RDH5. APC-mediated induction of RDHL was paralleled by increased production of retinoic acid. Investigations into the mechanism responsible for APC induction of RDHL indicated that beta-catenin fails to repress RDHL. The colon-specific transcription factor CDX2, however, activated an RDHL promoter construct and induced endogenous RDHL. Finally, the induction of RDHL by APC appears dependent on the presence of CDX2. We propose a novel role for APC and CDX2 in controlling retinoic acid biosynthesis and in promoting a retinoid-induced program of colonocyte differentiation.

Calf intestinal mucin: isolation, partial characterization, and measurement in ileal digesta with an enzyme-linked immunosorbent assay

The aim of this study was to develop a specific ELISA for calf intestinal mucin to quantify its contribution to ileal endogenous losses. Mucin was isolated from intestinal scrapings by cesium chloride density gradient ultracentrifugation. The isolated mucin had a high concentration of glutamic and aspartic acids, threonine, and serine (13.2, 11.2, 9.6, and 9.2 mol % of total amino acids assayed, respectively). The carbohydrates present were (mol % of total hexoses): galactose 42.1, N-acetylglucosamine 24.1, N-acetylgalactosamine 23.6, fucose 4.7, mannose 3.1, and sialic acids 2.4. Amino acids and carbohydrates represented 52.6 and 47.4% of the mucin by weight, respectively. A rabbit hyperimmune plasma was raised against purified mucin and used to set up an ELISA. The linear range of this assay was 20 to 640 ng/ml. The plasma cross-reacted with calf abomasal and colonic mucin. It showed no cross-reactivity with nonmucin components and no reactivity with intestinal mucin from other animal species except for the rat. Mucin accounted for approximately 3.5% of the dry matter output at the ileum of calves fed a substitute milk based on skim milk powder. This represented a flow of 3.4 g of mucin/kg of dry matter intake. Mucin flow increased when dietary protein was provided by cow's colostrum. Finally, the developed assay is a suitable tool to investigate the impact of dietary factors on the flow of mucin along the gut of preruminant calves.