Comparison of brush border membrane glycoproteins and glycoenzymes in the proximal and distal rat small intestine

The effects of ethanol administration on brush border membrane glycolipids in rat intestine

Ethanol ingestion is well known to induce morphological and biochemical changes in intestine and is responsible for intestinal dysfunctions. Luminal surface of enterocytes is rich in glycolipids, but the effects of ethanol ingestion on membrane glycolipids are not well characterized. In the present study, rats were given 1 mL of 30% ethanol daily for 15, 25, 35, and 56 days. Ethanol feeding for 15 days did not affect glycolipid pattern in microvillus membranes, but the levels of cerebrosides (glucosylceramide, lactosylceramide, globotriasyloceramide) were enhanced in rats fed with ethanol for 35 or 56 days compared with controls. In contrast, the content of fucolipids and gangliosides was reduced in rats on ethanol ingestion for 35 or 56 days. The observed changes in membrane glycolipids were substantiated using biotinylated lectins Jacalin (affinity for N-acetylgalactosamine) and Aleuria aurantia (affinity for α-l-fucose). The incorporation of [(14)C]-mannose and [(14)C]-glucosamine revealed an increase (P<.01) in glucosamination and reduction (P<.01) in mannosylation of glycolipids from ethanol-fed rats for 45 days compared with controls. These findings were further characterized by autoradiography of the glycolipids separated on thin layer chromatograms. These findings indicate that ethanol ingestion modulates the glycolipids composition of brush borders, resulting in generalized aberration of intestinal glycosylation in chronic alcoholism in rats.

Regulation of the intestinal glycoprotein glycosylation during postnatal development: role of hormonal and nutritional factors

This review focuses on the regulation of the glycoprotein glycosylation process in small intestine and colon during postnatal development. Glycoproteins play a prominent part in intestine as mucins secreted by the goblet cells and as molecules of biological interest largely present in the microvillus membrane of the enterocytes (digestive enzymes, transporters). The age-related changes in the intestinal glycosylation control the quality of glycan chains of glycoproteins. Postnatal maturation is observed at all stages of the glycoprotein glycosylation. But it is essentially characterised in the external glycosylation by a shift from sialylation to fucosylation depending on the transcriptional regulation of the corresponding glycosyltransferases, but also on coordinate changes in the activities of glycosyltransferases and of their regulatory proteins, in nucleotide-sugar bioavailability and in product degradation by oxidases. Many factors have been evoked to trigger these changes, among which are hormonal (glucocorticoids, insulin) and dietary factors. Changes in the structure of the glycoprotein glycans might be important for the transport, the barrier function, the implantation of the immune defences and of the microflora and even probably for the biological activity of some digestive enzymes.

The role of indigenous microflora in the development of murine intestinal fucosyl- and sialyltransferases

Most enteric bacteria use intestinal brushborder glycoconjugates as their target host cell receptors. It has been postulated that resident microbes regulate specific glycosyltransferases that are responsible for synthesizing brushborder glycoconjugates. To investigate this hypothesis, we measured glycosyltransferase enzyme activities in intestine from different regions of maturing conventional (CONV), germ-free (GF), and ex-germ-free (XGF) mice and compared them to general enzyme markers of gut development, for example, disaccharidases. High alpha2,3/6-Sialyltransferase (ST) activity and low alpha1,2-fucosyltransferase (FT) activities were detected from duodenum to colon in suckling CONV mice, but the relative levels of these activities reversed during the third postnatal wk, rapidly reaching adult levels by the fourth wk. These age-related enzyme changes were significantly attenuated in GF mice, maintaining an immature pattern well past 3 wk. Introduction of gut microflora in GF mice rapidly initiated maturation of glycosyltransferase activity but had no significant affect on developmental programming of dissacharidases. Therefore, in mice, intestinal glycosyltransferase activities are under tissue and developmental control and microflora play a major role in their specific ontogeny but not in overall development. These findings may help explain the regional specificity of commensal bacteria and of enteric pathogens and may also relate age-related changes in microflora to susceptibility to enteropathogens.

Implication of insulin and nutritional factors in the regulation of intestinal galactosyltransferase activity during postnatal development

In the rat small intestine, galactosyltransferases are the enzymes implicated in the biosynthesis of glycoproteins of the brush-border membranes and mucins. During postnatal development, the circulating insulin level increased at weaning in parallel with the activities of intestinal galactosyltransferases on O-glycans and N-glycans. This study deals with the role of insulin in the regulation of galactosyltransferase activities during postnatal development. The treatment of immature suckling rats with insulin induced a precocious increase in the activities of the O-glycan and N-glycan galactosyltransferases, partly reproducing the increase in galactosyltransferase activity normally found at weaning, since the O-glycan galactosyltransferase activity increased more quickly than the N-glycan galactosyltransferase activity. The sensitivity of the two galactosyltransferase activities to insulin disappeared after weaning, a period when drastic diet changes occur. In 22-day-old rats submitted to prolonged nursing (high-fat diet), the activities of the O-glycan and N-glycan galactosyltransferases were lower than those found in age-matched normally weaned rats (high-carbohydrate diet), indicating a delay in the maturation of the intestine of prolonged-nursing rats. The circulating insulin level of these animals stayed lower than that of the age-matched weaned rats. When the prolonged-nursing animals were treated with insulin, the O-glycan and N-glycan galactosyltransferase activities reached levels similar to those of the weaned rats. These observations suggest that insulin is one of the maturation factors for intestinal glycoprotein galactosylation and may be partly responsible for the natural enhancement of intestinal galactosyltransferase activities observed during postnatal development in relation to the dietary changes at weaning.

Lectin-mediated bioadhesion: binding characteristics of plant lectins on the enterocyte-like cell lines Caco-2, HT-29 and HCT-8

In order to take advantage of the biorecognition between lectins and specific carbohydrates for targeted drug delivery, fluorescein-labelled lectins of different carbohydrate specificities were screened for binding to human colorectal carcinoma cell lines by flow cytometry and confocal microscopy. The lectin-binding rate increased as follows: Dolichos biflorus agglutinin, DBA<peanut agglutinin, PNA<Lens culinaris agglutinin, LCA<Solanum tuberosum lectin, STL<Ulex europaeus isoagglutinin I, UEA-I<wheat germ agglutinin, WGA (Caco-2); PNA<UEA-I<WGA (HT-29); DBA<UEA-I<WGA (HCT-8), thus reflecting the glycosylation pattern of the cells. Compared to the BSA-binding capacity of the cells, the extent of nonspecific binding was strongly dependent on the type of cell line and lectin under investigation being lower than 2% in the case of WGA, STL and UEA-I/Caco-2 and HT-29 cells. Whereas 50% of DBA was bound nonspecifically to Caco-2 cells, the interactions DBA/HCT-8 and PNA/HT-29 were due to nonspecific binding. By competitive inhibition of lectin-adhesion to the cells upon addition of the complementary carbohydrate, specificity of lectin-binding was confirmed except for the interaction of DBA/HCT-8 and PNA/HT-29. Following on from this work, we consider WGA-, STL- and UEA-I-mediated drug delivery to be a promising approach for peroral bioadhesive formulations adhering to absorptive enterocytes.

The hydrocortisone-induced transcriptional down-regulation of beta-galactoside alpha2,6-sialyltransferase in the small intestine of suckling rats is suppressed by mifepristone (RU-38.486)

The progressive loss of sialic acids of the brush-border membrane glycoproteins is one of the major biochemical changes which occur in the rat small intestine during the transition from suckling to weaning, and this process is speeded up by an injection of glucocorticoids to the suckling animals. We used the rat liver beta-galactoside alpha2,6-sialyltransferase (ST6(N), EC 2.4.99.1) cDNA as a probe to examine the mRNA level of this enzyme in the small intestine of both suckling (13-day-old) and weaned (25-day-old) rats. In the ileum of suckling rats, the ST6(N) mRNA level was about four times higher than in the jejunum, whereas the membrane-bound enzyme activity was less than two times higher. In comparison with the controls, hydrocortisone treatment significantly decreased the level of this transcript and of the corresponding enzyme activity in both segments of the small intestine of suckling rats. Additionally, the antiglucocorticoid mifepristone (RU-38.486) suppressed the effect of hydrocortisone. The expression of ST6(N) mRNA in the small intestine of weaned (25-day-old) rats was several times lower than that in suckling (13-day-old) rats, and was unresponsive to hydrocortisone as well as to mifepristone. These results indicate that the glucocorticoid-induced transcriptional down-regulation of ST6(N) expression in the small intestine of suckling rats is mediated via the glucocorticoid receptor pathway, and support the notion that alterations in sialylation of brush-border membrane glycoconjugates occurring upon weaning are the result of a lower expression of ST6(N).

Glycoprotein receptors for a heat-stable enterotoxin (STh) produced by enterotoxigenic Escherichia coli

Glycoprotein receptors for heat-stable enterotoxin STh of enterotoxigenic Escherichia coli in the rat intestinal cell membrane were identified and characterized. Incubation of rat intestinal cell membranes with radioiodinated N-5-azidonitrobenzoyl-STh[5-19] (125I-ANB-STh[5-19]) followed by photolysis resulted in specific radiolabeling of two distinct proteins with M(r)s of 200,000 (designated STR-200A and STR-200B). STR-200A was found to be composed of two molecules of a protein with an M(r) of 70,000 (70-kDa protein), whereas STR-200B was composed of two different protein molecules with M(r)s of 53,000 (53-kDa protein) and 77,000 (77-kDa protein). These proteins showed no guanylate cyclase activity. The 70-kDa protein was labeled most with 125I-ANB-STh[5-19], suggesting that STR-200A is the main receptor protein in the rat intestinal cell membrane. The carbohydrate moieties of STR-200A and STR-200B were examined by enzymatic deglycosylation. The 70-kDa protein of STR-200A was found to contain N-linked high-mannose-type and/or hybrid-type oligosaccharides, and results suggested that it possesses at least three N glycosylation sites. The 53-kDa protein of STR-200B was found to have an N-linked complex-type oligosaccharide side chain. The deglycosylated 70-kDa protein retained activity for binding to STh, suggesting that the carbohydrate moieties of these receptor proteins are not important for binding with STh.

Changes in intestinal absorption of nutrients and brush border glycoproteins after total parenteral nutrition in rats

The effect of total parenteral nutrition on nutrients absorption and glycoprotein changes of brush border membrane was examined in rat small intestine. In total parenteral nutrition rats, a marked decrease in activity of brush border enzymes was observed mainly in the proximal and middle segments of the intestine. Galactose perfusion of jejunal segment showed that hexose absorption was significantly inhibited, while intestinal absorption of glycine or dipeptide, glycylglycine was not significantly affected by total parenteral nutrition treatment. When brush border membrane glycoprotein profile was examined by [3H]-glucosamine or [3H]-fucose incorporation into jejunal loops, significant changes were observed in the glycoprotein pattern of brush border membrane especially in the high molecular weight range over 120 kDa after total parenteral nutrition treatment, suggesting strong dependency of glycoprotein synthesis on luminal substances. Molecular weight of sucrase isomaltase in brush border membrane detected by specific antibody showed no significant difference, however, in total parenteral nutrition and control rats. Also, molecular weight of specific sodium glucose cotransporter of intestinal brush border membrane detected by selective photoaffinity labelling was not altered in total parenteral nutrition rats. It may be that prolonged absence of oral food intake may produce significant biochemical changes in brush border membrane glycoprotein and absorptive capacity of small intestine, but these changes were not observed in all brush border membrane glycoproteins.

Hormonal regulation of glycosylation process in rat small intestine: responsiveness of fucosyl-transferase activity to hydrocortisone during the suckling period, unresponsiveness after weaning

The aim of this study was to examine the possible role of certain hormones (especially hydrocortisone) in the developmental variations of intestinal fucosyl-transferase activity in rats. Thyroxine and insulin, injected into suckling rats, did not induce significant modifications of the fucosyl-transferase activity, under the conditions used, whereas this enzyme activity was highly enhanced after administration of glucocorticoids (cortisone and hydrocortisone). Hydrocortisone administration to suckling rats induced a precocious and progressive activation of the fucosyl-transferase activity up to adult level as a function of the duration of treatment. The responsiveness of suckling rats to hydrocortisone, as shown by increased fucosyl-transferase activity, disappeared at the end of the third week (corresponding to the weaning time). These physiological periods of responsiveness and unresponsiveness to hydrocortisone could be related to the binding of the hormones to receptors since the antiglucocorticoid RU 38486 counteracted the effect of hydrocortisone in suckling rats but did not prevent the developmental rise of the fucosyl-transferase activity, when administered in the third week of life. These results suggest that the normal developmental rise of the fucosyl-transferase activity is independent of glucocorticoids.

Glycosylation in intestinal epithelium

This chapter reviews the glycosylation reactions in the intestinal epithelium. The intestinal epithelium represents a good model system in which the glycosylation process can be studied. The intestinal epithelium is composed of two basic epithelial cell types: the absorptive enterocyte and the mucus-producing goblet cell. Gastrointestinal epithelial renewal ensues through the processes of cell proliferation, migration, and differentiation. This renewal occurs in discrete proliferative zones along the gastrointestinal tract. In the small intestine, this proliferative zone is restricted to the base of the crypts, whereas in the large intestine it is less restrictive, occurring in the basal two thirds of the crypt. A longitudinal section along the crypt-to-surface axis, cells in various degrees of differentiation is observed, providing a unique in vivo system in which to investigate differentiation-related glycosylation events. The glycoconjugate repertoire displayed by a given cell reflects its endogenous expression of glycosyltransferases. The role played by terminal oligosaccharide structures in cell–cell recognition phenomena and the expression of glycosyltransferases occupy a key position in the post-translational processing of glycoconjugates and thus influence cellular function.

Host response to Escherichia coli heat-labile enterotoxin via two microvillus membrane receptors in the rat intestine

The responsiveness of enterocytes to Escherichia coli heat-labile enterotoxin (LT) was studied in the small intestine of 6- to 7-week-old rats. Dose-effect analysis showed the dose required for a 50% maximal LT-induced secretory response to be at 8 nM. After the well-documented glycolipid GM1 receptor was blocked with the cholera toxin B subunit, LT still activated the second messenger cascade, measured in terms of heightened cellular adenylate cyclase activity, and caused fluid to be secreted into ligated intestinal loops. Furthermore, Scatchard analysis of binding kinetics suggested that LT bound to two receptor sites on the intestinal microvillus membrane. The toxin also bound to delipidated membrane but was competitively inhibited by a galactose-specific lectin, RCA60, suggesting that the additional receptor is a galactoglycoprotein. Western blot analysis of toxin binding to membrane proteins revealed a group of binding components around 85 to 150 kilodaltons. When measured at 2.2 nM LT, approximately 70% of LT-binding activity took place through a high-affinity (Kd1, 0.38 nM) GM1 receptor and 30% of LT-binding activity took place through a low-affinity (Kd2, 3.3 nM) glycoprotein receptor. These results suggest that LT functions through two microvillus membrane receptors in the mature rat small intestine.

Developmental changes in galactosyltransferase activity in the rat small intestine

During postnatal development, UDP-Gal: GlcNAc(beta 1-4)-galactosyltransferase (4 beta-GT) and UDP-Gal:GalNAc(beta 1-3)-galactosyltransferase (3 beta-GT) activities were increased by 17- and 24-fold, respectively, in the rat small intestine. The injection of cortisone into suckling rats resulted in precocious induction of distal 4 beta- and 3 beta-GT activities by 2.7- and 1.8-fold, respectively. Injection of phorbol-12-myristate-13-acetate (PMA) resulted in precocious induction of distal 3 beta-GT by 2.7-fold. These results suggest that intestinal galactosyltransferase activities are under developmental regulation and can be modified by cortisone and PMA.