Developmental regulation of beta-galactoside alpha 2,6-sialyltransferase in small intestine epithelium

Sialic acid in the regulation of blood cell production, differentiation and turnover

Sialic acid is a unique sugar moiety that resides in the distal and most accessible position of the glycans on mammalian cell surface and extracellular glycoproteins and glycolipids. The potential for sialic acid to obscure underlying structures has long been postulated, but the means by which such structural changes directly affect biological processes continues to be elucidated. Here, we appraise the growing body of literature detailing the importance of sialic acid for the generation, differentiation, function and death of haematopoietic cells. We conclude that sialylation is a critical post-translational modification utilized in haematopoiesis to meet the dynamic needs of the organism by enforcing rapid changes in availability of lineage-specific cell types. Though long thought to be generated only cell-autonomously within the intracellular ER-Golgi secretory apparatus, emerging data also demonstrate previously unexpected diversity in the mechanisms of sialylation. Emphasis is afforded to the mechanism of extrinsic sialylation, whereby extracellular enzymes remodel cell surface and extracellular glycans, supported by charged sugar donor molecules from activated platelets.

Bacterial colonization and TH17 immunity are shaped by intestinal sialylation in neonatal mice

Interactions between the neonate host and its gut microbiome are central to the development of a healthy immune system. However, the mechanisms by which animals alter early colonization of microbiota for their benefit remain unclear. Here, we investigated the role of early-life expression of the α2,6-sialyltransferase ST6GAL1 in microbiome phylogeny and mucosal immunity. Fecal, upper respiratory, and oral microbiomes of pups expressing or lacking St6gal1 were analyzed by 16S rRNA sequencing. At weaning, the fecal microbiome of St6gal1-KO mice had reduced Clostridiodes, Coprobacillus, and Adlercreutzia, but increased Helicobacter and Bilophila. Pooled fecal microbiomes from syngeneic donors were transferred to antibiotic-treated wild-type mice, before analysis of recipient mucosal immune responses by flow cytometry, RT-qPCR, microscopy, and ELISA. Transfer of St6gal1-KO microbiome induced a mucosal Th17 response, with expression of T-bet and IL-17, and IL-22-dependent gut lengthening. Early life intestinal sialylation was characterized by RT-qPCR, immunoblot, microscopy, and sialyltransferase enzyme assays in genetic mouse models at rest or with glucocorticoid receptor modulators. St6gal1 expression was greatest in the duodenum, where it was mediated by the P1 promoter and efficiently inhibited by dexamethasone. Our data show that the inability to produce α2,6-sialyl ligands contributes to microbiome-dependent Th17 inflammation, highlighting a pathway by which the intestinal glycosylation regulates mucosal immunity.

Disruption of hepatocyte Sialylation drives a T cell-dependent pro-inflammatory immune tone

Through the catalysis of α2,6-linked sialylation, the enzyme ST6Gal1 is thought to play key roles in immune cell communication and homeostasis. Of particular importance, glycans with terminal α2,6-sialic acids are known to negatively regulate B cell receptor signaling and are associated with an immunosuppressive tumor microenvironment that promotes T cell anergy, suggesting that α2,6-sialic acids are a key immune inhibitory signal. Consistent with this model, mice harboring a hepatocyte-specific ablation of ST6Gal1 (H-cKO) develop a progressive and severe non-alcoholic fatty liver disease characterized by steatohepatitis. Using this H-cKO mouse, we have further discovered that loss of hepatocyte α2,6-sialylation not only increases the inflammatory state of the local tissue microenvironment, but also systemic T cell-dependent immune responses. H-cKO mice responded normally to innate and passively induced inflammation, but showed significantly increased morbidity in T cell-dependent house dust mite-antigen (HDM)-induced asthma and myelin oligodendrocyte glycoprotein (MOG) peptide-induced experimental autoimmune encephalomyelitis (EAE). We further discovered that H-cKO mice have a profound shift toward effector/memory T cells even among unchallenged mice, and that macrophages from both the liver and spleen expressed the inhibitory and α2,6-sialic acid-specific glycan binding molecule CD22. These findings align with previously reported pro-inflammatory changes in liver macrophages, and support a model in which the liver microenvironment sets a systemic immune tone that is regulated by tissue α2,6-sialylation and mediated by liver macrophages and systemic T cells.

Conservation of the ST6Gal I gene and its expression in the mammary gland

Milk sialoglycoconjugates can protect the gastrointestinal tract of the suckling neonate by competitively binding to invading pathogens and promoting growth of beneficial flora, and their potential role in postnatal brain development is of particular interest in human infant nutrition. Although the concentration and the distribution of sialoglycoconjugates have been extensively studied in the milk of various species, the investigation of sialyltransferase gene expression in the mammary gland, in the context of lactation, has been limited. The sialyltransferase enzyme ST6Gal I transfers sialic acid from CMP-sialic acid to type 2 (Galβ1,4GlcNAc) free disaccharides or the termini of N- or O-linked oligosaccharides using an α2,6-linkage. Expression of the ST6Gal I gene is primarily regulated at the level of transcription through the use of several cell and development-specific promoters, producing transcripts with divergent 5' untranslated regions (UTR). In the mouse mammary gland, the novel 5'UTR exon (L) appears to be associated with a drastic increase in ST6Gal I gene expression during lactation. We find that rats also possess an exon (L), suggesting conservation of this regulatory mechanism in rodents. In contrast, an exon (L)-containing transcript was not detected in the lactating bovine or human mammary gland. We also observed a trend of increasing ST6Gal I gene expression in the bovine mammary gland, culminating in involution. This is in contrast to species such as mice where the greatest change in ST6Gal I gene expression occurs between pregnancy and lactation, suggesting different roles in rodents vs. other mammals for α2,6-sialylated oligosaccharides present in milk.

Altered granulopoietic profile and exaggerated acute neutrophilic inflammation in mice with targeted deficiency in the sialyltransferase ST6Gal I

Elevation of serum sialic acid and the ST6Gal-1 sialyltransferase is part of the hepatic system inflammatory response, but the contribution of ST6Gal-1 has remained unclear. Hepatic ST6Gal-1 elevation is mediated by P1, 1 of 6 promoters regulating the ST6Gal1 gene. We report that the P1-ablated mouse, Siat1DeltaP1, and a globally ST6Gal-1-deficient mouse had significantly increased peritoneal leukocytosis after intraperitoneal challenge with thioglycollate. Exaggerated peritonitis was accompanied by only a modest increase in neutrophil viability, and transferred bone marrow-derived neutrophils from Siat1DeltaP1 mice migrated to the peritonea of recipients with normal efficiency after thioglycollate challenge. Siat1DeltaP1 mice exhibited 3-fold greater neutrophilia by thioglycollate, greater pools of epinephrine-releasable marginated neutrophils, greater sensitivity to G-CSF, elevated bone marrow CFU-G and proliferative-stage myeloid cells, and a more robust recovery from cyclophosphamide-induced myelosuppression. Bone marrow leukocytes from Siat1DeltaP1 are indistinguishable from those of wild-type mice in alpha2,6-sialylation, as revealed by the Sambucus nigra lectin, and in the expression of total ST6Gal-1 mRNA. Together, our study demonstrated a role for ST6Gal-1, possibly from extramedullary sources (eg, produced in liver) in regulating inflammation, circulating neutrophil homeostasis, and replenishing granulocyte numbers.

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.

Glucocorticoid-induced maturation of glycoprotein galactosylation and fucosylation processes in the rat small intestine

We determined the role of glucocorticoids in the maturation of glycoprotein galactosylation and fucosylation processes in the rat small intestine during postnatal development. Treatment of suckling rats with hydrocortisone (HC) increased activities of an O-glycan: galactosyltransferase, and of an alpha-1,2-fucosyltransferase, through transcriptional regulation of the FTB gene. The activities of a fucosyltransferase inhibitor and of the enzymes responsible for the synthesis and degradation of GDP-fucose were unaffected by the treatment, whereas a fall in the activity of alpha-L-fucosidase was observed. These changes were accompanied by the precocious appearance of alpha-1,2-fucose residues in complex glycan chains of brush-border membrane glycoproteins that normally appear after weaning, and with a trend to increase in alpha-1,2-fucose residues in mucins. Thus, treatment of suckling rats with hydrocortisone speeds up the maturation of glycoprotein galactosylation and fucosylation processes in the small intestine. The delayed increase in glucocorticoid levels induced by prolonged nursing, or the suppression of glucocorticoids by adrenalectomy (AD) before the normal rise in the hormone, both induced a delay in the increases in activities of the O-glycan: galactosyltransferase and alpha-1,2-fucosyltransferase observed normally after glucocorticoid enhancement. Thus, glucocorticoids might play at least a partial role in the maturation of glycoprotein glycosylation observed at weaning.

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.

Defining the glycophenotype of squamous epithelia using plant and mammalian lectins. Differentiation-dependent expression of alpha2,6- and alpha2,3-linked N-acetylneuraminic acid in squamous epithelia and carcinomas, and its differential effect on binding of the endogenous lectins galectins-1 and -3

A thorough characterization of the properties of squamous epithelial cells is necessary in order to improve our understanding of the functional aspects of normal development and malignant aberrations. Up to now, studies have focused almost exclusively on monitoring distinct protein markers. With our growing awareness of the coding function of glycan chains of cellular glycoconjugates and their interaction with receptors (lectins) in situ, defining the glycophenotype of these cells has become an important issue. Whereas the commonly applied plant lectins are tools used to map the presence and localization of biochemically defined saccharide epitopes, the introduction of endogenous (mammalian) lectins to this analysis enables us to take the step from monitoring the presence of glycan to understanding the functional implications by revealing ligand properties of the detected epitope for tissue lectin. Thus, in this study we investigated a distinct aspect of glycosylation using plant and mammalian lectins, i.e. the linkage type of sialylation. We first mapped the expression profile of the type of sialylation (alpha2,3- or alpha2,6-linked) by plant lectins. Based on the hypothesis that this factor regulates accessibility of ligands for endogenous lectins we introduced two labeled galectins to this study. Galectin-3 (but not galectin-1) binding was related to cell differentiation in normal adult and developing epithelia, cultured epidermal cells, and carcinomas derived from these epithelia. The presented data suggest that alpha2,6-linked N-acetyl-D-neuraminic acid moieties could serve to mask galectin-3-reactive glycoepitopes. As a consequence, monitoring of the linkage type of sialic acid in glycans by plant lectins therefore has implications for the extent of glycan reactivity with endogenous lectins, pointing to a potential function of changes in sialylation type beyond these cell and lectin systems.

Hyposialylation of integrins stimulates the activity of myeloid fibronectin receptors

Despite numerous reports suggesting that beta(1) integrin receptors undergo differential glycosylation, the potential role of N-linked carbohydrates in modulating integrin function has been largely ignored. In the present study, we find that beta(1) integrins are differentially glycosylated during phorbol ester (PMA)-stimulated differentiation of myeloid cells along the monocyte/macrophage lineage. PMA treatment of two myeloid cell lines, U937 and THP-1, induces a down-regulation in expression of the ST6Gal I sialyltransferase. Correspondingly, the beta(1) integrin subunit becomes hyposialylated, suggesting that the beta(1) integrin is a substrate for this enzyme. The expression of hyposialylated beta(1) integrin isoforms is temporally correlated with enhanced binding of myeloid cells to fibronectin, and, importantly, fibronectin binding is inhibited when the Golgi disrupter, brefeldin A, is used to block the expression of the hyposialylated form. Consistent with the observation that cells with hyposialylated integrins are more adhesive to fibronectin, we demonstrate that the enzymatic removal of sialic acid residues from purified alpha(5)beta(1) integrins stimulates fibronectin binding by these integrins. These data support the hypothesis that unsialylated beta(1) integrins are more adhesive to fibronectin, although desialylation of alpha(5) subunits could also contribute to increased fibronectin binding. Collectively our results suggest a novel mechanism for regulation of the beta(1) integrin family of cell adhesion receptors.

Polyamine participation in the maturation of glycoprotein fucosylation, but not sialylation, in rat small intestine

The aim of this study was to determine the role of polyamines in the diet-related maturation of the intestinal glycoprotein glycosylation during postnatal development in the rat. The activity of alpha-2,6-sialyltransferase and the sialylated forms of glycoproteins in the intestinal brush-border membranes were found to decrease considerably after weaning, in parallel with the intestinal level of putrescine. By contrast, the activity of alpha-1,2-fucosyltransferases, the mRNA levels for two alpha-1,2-fucosyltransferase genes, FTA and FTB, and the fucosylated forms of glycoproteins all increased after weaning, in parallel with the levels of spermidine and spermine. These results suggest a possible role of polyamines in the evolution of glycosylation. The treatment of suckling rats with spermidine or spermine reproduced the high intestinal levels of these polyamines corresponding to those normally found after weaning. After these treatments, a rise in the activity of the alpha-1,2-fucosyltransferase was observed, associated with a fall in alpha-L-fucosidase activity. The alpha-1,2-fucosyltransferase FTB gene was found to be regulated at the transcriptional level, but not by its inhibitor, fuctinin. The result of these variations was the precocious appearance of several alpha-1,2-fucoproteins, which are normally found in brush-border membranes after weaning. The treatment of suckling rats with putrescine, which induced only a transitory rise in intestinal putrescine, had a similar but weaker effect on the fucosylation process than spermidine or spermine, and treatment with ornithine was ineffective. alpha-2,6-Sialylation was not affected by any of the treatments. Spermidine and spermine turned out to be more effective than putrescine for intestinal glycoprotein fucosylation, but did not affect their sialylation. Spermidine and spermine, whose intestinal levels where found to increase at weaning time, may have been partly responsible for the natural evolution of the intestinal glycoprotein fucosylation that occurred during this period.

Region-specific ontogeny of alpha-2,6-sialyltransferase during normal and cortisone-induced maturation in mouse intestine

Regional differences in the ontogeny of mouse intestinal alpha-2,6-sialyltransferase activities (alpha-2,6-ST) and the influence of cortisone acetate (CA) on this expression were determined. High ST activity and alpha-2,6-ST mRNA levels were detected in immature small and large intestine, with activity increasing distally from the duodenum. As the mice matured, ST activity (predominantly alpha-2,6-ST) in the small intestine decreased rapidly to adult levels by the fourth postnatal week. CA precociously accelerated this region-specific ontogenic decline. A similar decline of ST mRNA levels reflected ST activity in the small, but not the large, intestine. Small intestinal sialyl alpha-2,6-linked glycoconjugates displayed similar developmental and CA induced-precocious declines when probed using Sambucus nigra agglutinin (SNA) lectin. SNA labeling demonstrated age-dependent diminished sialyl alpha2,6 glycoconjugate expression in goblet cells in the small (but not large) intestine, but no such regional specificity was apparent in microvillus membrane. This suggests differential regulation of sialyl alpha-2,6 glycoconjugates in absorptive vs. globlet cells. These age-dependent and region-specific differences in sialyl alpha-2,6 glycoconjugates may be mediated in part by altered alpha-2,6-ST gene expression regulated by trophic factors such as glucocorticoids.

Dietary spermidine and spermine participate in the maturation of galactosyltransferase activity and glycoprotein galactosylation in rat small intestine

This study considered the role of dietary polyamines in the maturation of intestinal glycoprotein galactosylation during postnatal development. In the rat small intestine, O-glycan: beta-1,3-galactosyltransferase and N-glycan: beta-1,4-galactosyltransferase are, respectively, involved in the glycan chain biosynthesis of mucins and of glycoproteins in the brush border membranes. Their activities increase significantly at weaning, in parallel with a rise in the intestinal content of spermidine and spermine (as determined by high performance liquid chromatography) and in proportion to the polyamine increase in food intake. The oral ingestion of spermidine or spermine (at 0.4 micromol/g body) by immature suckling rats for 4 d reproduced the levels of spermine and spermidine in their intestines at the time of weaning and induced precocious and significant rises in O-glycan: and N-glycan: galactosyltransferase activities to those normally found after weaning. In parallel, more galactose residues (detected in the complex oligosaccharide chains of glycoproteins by specific lectins after electrophoresis and transfer to nitrocellulose membranes) were observed in the brush border membranes of spermidine- and spermine-treated rats. In contrast, the ingestion of putrescine or ornithine had no effect. Diets with different levels of polyamines (milks and commercial diet), when given at weaning, induced variable evolutions of the galactosylation process, partly in relation to the amounts of polyamines ingested. These results indicate that spermidine and spermine are maturation factors that can reproduce, in immature rats, the same increase in intestinal glycoprotein galactosylation that is normally observed during weaning. They also suggest that the maturation of glycoprotein galactosylation may be a multifactorial event in which spermidine and spermine are both involved.

Mouse ST6Gal sialyltransferase gene expression during mammary gland lactation

The sialyltransferase ST6Gal mediates the biosynthetic addition of sialic acid, via an alpha2,6 linkage, to the nonreducing end of terminal lactosamine structures. Transcription of the murine ST6Gal gene, Siat1, is regulated by the selective use of multiple promoters in a tissue- and development-specific manner. Here we report that Siat1 mRNA expression is dramatically elevated in lactating (relative to virgin) mouse mammary gland. The predominant ST6Gal mRNA species expressed in lactating mammary gland is a heretofore undocumented isoform containing a unique 5'-untranslated region originating from the mouse Siat1 genetic region, now defined as Exon L, residing 549-bp 5' of the previously characterized Exon X(2). Thus, the novel ST6Gal mRNA form initiates transcription from the region designated as p4 and incorporates the unique sequence from Exon L in 5'-juxtaposition to commonly shared sequences encoded on Exon I to Exon VI. In contrast, cells derived from virgin mammary tissue expressed only the housekeeping mRNA form derived from p3, with Exon O sequence preceding Exons I-VI. The Exon L-containing, p4 class of mRNA was also not detected in a survey of eight other mouse tissues. Previous reports have indicated a strong correlation between mammary cancers and elevated ST6Gal expression in rats and in human patients. However, we uncovered neither elevated expression of ST6Gal mRNA nor appearance of p4 class in mouse breast carcinomas experimentally induced by transformation with the polyoma-middle T oncogene. A number of established breast carcinoma cell lines were also examined, with ST6Gal mRNA and activity generally low. Moreover, with the exception of the Shionogi cell line, p4 class of ST6Gal mRNA was not expressed in any of the mouse breast carcinoma specimens examined. Taken together, our data indicate that murine ST6Gal induction during lactation is achieved by de novo recruitment of a normally silent promoter. Furthermore, the data provide no support for elevated Siat1 expression on the mRNA level in association with murine mammary gland carcinogenesis. With the single exception of the Shionogi cell line, the p3 class remains the predominant ST6Gal mRNA expressed in all other murine mammary carcinoma cells examined.

The sialyl-alpha2,6-lactosaminyl-structure: biosynthesis and functional role

Sialylation represents one of the most frequently occurring terminations of the oligosaccharide chains of glycoproteins and glycolipids. Sialic acid is commonly found alpha2,3- or alpha2,6-linked to galactose (Gal), alpha2,6-linked to N-acetylgalactosamine (GalNAc) or alpha2,8-linked to another sialic acid. The biosynthesis of the various linkages is mediated by the different members of the sialyltransferase family. The addition of sialic acid in alpha2,6-linkage to the galactose residue of lactosamine (type 2 chains) is catalyzed by beta-galactoside alpha2,6-sialyltransferase (ST6Gal.I). Although expressed by a single gene, this enzyme shows a complex pattern of regulation which allows its tissue- and stage-specific modulation. The cognate oligosaccharide structure, NeuAcalpha2,6Galbeta1,4GlcNAc, is widely distributed among tissues and is involved in biological processes such as the regulation of the immune response and the progression of colon cancer. This review summarizes the current knowledge on the biochemistry of ST6Gal.I and on the functional role of the sialyl-alpha2,6-lactosaminyl structure.

Beta-galactoside alpha2,6 sialyltransferase in human colon cancer: contribution of multiple transcripts to regulation of enzyme activity and reactivity with Sambucus nigra agglutinin

Colon cancer tissues display an increased activity of beta-galactoside alpha2,6 sialyltransferase (ST6Gal.I) and an increased reactivity with the lectin from Sambucus nigra (SNA), specific for alpha2,6-sialyl-linkages. Experimental and clinical studies indicate a contribution of these alterations to tumor progression, but their molecular bases are largely unknown. In many tissues, ST6Gal.I is transcriptionally regulated through the usage of different promoters that originate mRNAs diverging in the 5;-untranslated regions. RT-PCR analysis of 14 carcinoma samples, all expressing an increased ST6Gal.I enzyme activity, and of the corresponding normal mucosa revealed the presence of at least 2 transcripts. One, containing the 5;-untranslated exons, Y+Z, is thought to represent the "housekeeping" expression, and another previously described in hepatic tissues. Both the Y+Z and the hepatic transcripts were detectable in normal and cancer tissues but that latter form had a marked tendency to accumulate in cancer. The extent of alpha2,6-sialylation of glycoconjugates, as determined by SNA-dot blot analysis, was markedly enhanced in all cancer specimens, but the level of reactivity only partially correlated with the level of enzyme expression. Western blot analysis revealed a strikingly heterogeneous pattern of SNA reactivity among cancer tissues. These data indicate that: i) during neoplastic transformation of colonic cells, ST6Gal.I expression may be modulated through a differential promoter usage; ii) the extent of alpha2,6-sialylation of cancer cell membranes is not a direct function of the ST6Gal.I activity, strongly suggesting the existence of other, more complex mechanisms of regulation.

Clinical correlations of alpha2,6-sialyltransferase expression in colorectal cancer patients

We have previously demonstrated a link between alpha2,6-Sialyltransferase (alpha2,6-ST; E.C. 2.4.99.1) expression and differentiation of colon tumors. So far, information is not available relative to the expression of alpha2,6-ST in tumors and the survival of patients with colorectal cancer. We have examined the expression of alpha2,6-ST in a variety of colorectal adenocarcinomas (n = 46) at different stages of differentiation (G1 to G3) by immunoperoxidase assay using monoclonal antibody (MAb) 6B9. Clinical outcome of the patients in a 5-year follow-up study has been correlated with the expression of alpha2,6-ST in tumors surgically removed from the same patients. No significant difference in the alpha2,6-ST expression was noted when age, sex, and tumor locations (colon, rectum) were included as parameters. However, 52% of the moderate (G2) and well-differentiated (G1) adenocarcinomas showed stronger alpha2,6-ST expression compared with poorly differentiated (G3) adenocarcinomas. Notably, absence to moderate levels of tumor alpha2,6-ST expression was correlated with 100% survival in patients with stage I and II tumors compared with 64% survival in patients with strong tumor alpha2,6-ST expression (p < 0.01). These studies suggest a negative correlation between the expression of alpha2,6-ST in tumors and a good clinical outcome in colorectal cancer patients.

Differential expression of sialic acid on porcine organs during the maturation process

Sialylated structures play important roles in cell communication, and change in a regulated manner during development and differentiation. In this work, we report the main glycosidic modifications that occur during the maturation of porcine tissues, involving the sialylation process as determined with lectins. Sialic acids were identified at several levels in a broad range of cell types of nervous, respiratory, genitourinary and lymphoid origin. Nevertheless, the most contrasting was the type of glycosidic linkage between 5-N-acetyl-neuraminic acid (Neu5Ac) and galactose (Gal) expressed in central nervous system (CNS). Newborn CNS abundantly expressed Neu5Acalpha2,3Gal, but weakly or scarcely expressed Neu5Acalpha2,6Gal/GalNAc. Maturation of CNS induced drastic changes in sialic acid expression. These changes include decrease or complete loss of NeuAcalpha2,3Gal residues, mainly in olfactory structures and brain cortex, which were replaced by their isomers Neu5Acalpha2,6Gal/GalNAc. In the brain cortex and cerebellum, the increase of Neu5Acalpha2,6Gal/GalNAc molecules was paralleled by an increase of 5-N-acetyl-9-O-acetyl-neuraminic acid (Neu5,9Ac2). In addition, terminal Gal and N-acetyl-D-galactosamine (GalNAc) residues also increased their expression in adult CNS tissues, but this was more significant in structures forming the encephalic trunk. Our results show that sialylation of porcine CNS is finely modulated throughout the maturation process.

Transcriptional regulation of human beta-galactoside alpha2, 6-sialyltransferase (hST6Gal I) gene during differentiation of the HL-60 cell line

We have previously shown that the expression of beta-galactoside alpha2,6-sialyltransferase (hST6Gal I) mRNA decreases during HL-60 differentiation induced with dimethyl sulfoxide (DMSO) and that transcriptional regulation depends on the P3 promoter that exists 5'-upstream of exon Y (A. Taniguchi et al., FEBS Lett.,441, 191-194, 1998). The regulation of hST6Gal I may be important for the expression of sialyl-Le(x)in HL-60 cells. In the present report, we studied the transcriptional regulation of hST6Gal I gene during DMSO-induced differentiation of HL-60 cells. To elucidate the molecular basis of hST6Gal I gene expression, the genomic region containing the P3 promoter of hST6Gal I was isolated and functionally characterized. Using a luciferase assay, we identified a functional DNA portion that confers an enhancer, located at nucleotide number (nt) -317 to -174 within the P3 promoter of hST6Gal I genomic DNA. This element contains two sequences similar to Sp1 (GC-box) and one sequence similar to Oct-1 recognition motifs (octamer sequence). Site-directed mutagenesis of Sp1 and Oct-1 sites showed that two Sp1 motifs and one Oct-1 motif are essential for transcriptional activity in HL-60 cells. Enhancer activity is suppressed during HL-60 cell differentiation induced with DMSO. These results suggest that GC-box and octamer sequence may play a critical role in the transcriptional regulation of the hST6Gal I gene during HL-60 cell differentiation.

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.

Molecular cloning, expression and exon/intron organization of the bovine beta-galactoside alpha2,6-sialyltransferase gene

In this study, we report the first isolation and characterization of a bovine sialyltransferase gene. Bovine cDNAs prepared from different tissues contain an open-reading frame encoding a 405 amino acid sequence showing 83%, 75%, and 60% identity with human, murine, and chicken ST6Gal I (beta-galactoside alpha2,6-sialyltransferase) sequences, respectively. When transfected into COS-7 cells, a recombinant enzyme was obtained which catalyzed the in vitro alpha2, 6-sialylation of LacNAc (NeuAcalpha2-6Galbeta1-4GlcNAc) and LacdiNAc (NeuAcalpha2-6GalNAcbeta1-4GlcNAc) acceptor substrates. The K (m) values were 2.8 and 6.9 mM, respectively. Different relative efficiencies (Vmax/Km) for the two precursors (36 for LacNAc and 4.3 for LacdiNAc) were observed. Bovine ST6Gal I gene consists of four 5'-untranslated exons E(-2) to E(1), and five coding exons from E(2) to E(6). This later carries a 3'-untranslated region of 2. 7 kb. Gene sequence spans at least 80 kb of genomic DNA. Two processed pseudogenes have been identified. They are 94.3 and 95.6% similar to the bovine cDNA, respectively. Three families of mRNA isoforms were isolated. They differed by their 5'-untranslated regions and could be generated by three tissue-specific promoters. Family 1 is made up of exons E(-2) and E(1) to E(6), family 2 of exons E(-1) to E(6), and family 3 of exons E(1) to E(6). Tissular distribution of transcript families appears noticeably different than those described in human and rat.

Down-regulation of human Galbeta1,3GalNAc/Galbeta1,4GlcNAc alpha2,3-sialyltransferase (hST3Gal IV) gene during differentiation of the HL-60 cell line

We studied the regulation of the human Galbeta1, 3GalNAc/Galbeta1,4GlcNAc alpha2,3-sialyltransferase (hST3Gal IV) gene during HL-60 cell differentiation induced by dimethyl sulfoxide (DMSO), all trans-retinoic acid (ATRA), and phorbol myristate acetate (PMA). During differentiation, levels of hST3Gal IV mRNA dramatically decreased after 1 day of stimulation. Reverse-transcription PCR identified two mRNA isoforms, types B1 and BX, in HL-60 cells. The results of luciferase assays showed that the level of B3 promoter activity is high, whereas A1/2 and B2 promoter activities are low in HL-60 cells, suggesting that type B1, BX, and B3 mRNA isoforms are expressed in HL-60 cells. A luciferase assay identified a functional DNA portion within the proximal region of the B3 promoter that confers negative transcriptional regulation on the hST3Gal IV B3 promoter during HL-60 differentiation. These results suggest that this element plays a critical role in down-regulating the B3 promoter activity during HL-60 cell differentiation.

Transcription of the beta-galactoside alpha2,6-sialyltransferase gene (SIAT1) in B-lymphocytes: cell type-specific expression correlates with presence of the divergent 5'-untranslated sequence

A single gene, SIAT1, encodes ST6Gal I, the sialyltransferase that mediates transfer of alpha2,6-linked sialic acids to Galbeta1, 4GlcNAc termini of N-linked glycoproteins. In vivo, multiple SIAT1 mRNA forms, differing only in the 5'-untranslated region, are expressed in a tissue-specific manner. This mRNA heterogeneity has been attributed, at least in part, to transcription from a number of physically distinct promoter regions. In mature B-lymphocytes, SIAT1 transcription initiates at P2, a regulatory region known to function only in B-lineage cells. Bacterial chloramphenicol acetyltransferase (CAT) under the control of the P2 region encompassing 415 bp 5'- and 125 bp 3' of the transcriptional initiation site is efficiently expressed in Louckes, a mature B-lymphoblastoid cell line. In contrast, CAT expression in Reh, a T-null/B-null precursor line, and in HepG2, a hepatoma line, are 14-fold and >25-fold less than in Louckes, respectively. The data is consistent with the presence of cis -acting regulatory elements residing both 5' and 3' of the P2 transcriptional initiation site. At least 370 bp of 5'-flanking sequence, coinciding with the inclusion of AP2 and NF-kappaB sites, is necessary for high level expression in Louckes. Exon sequences 3' of the transcription start site are also important for expression. A segment from(+)32 to(+)125 (position(+)1 is transcription start site) is capable of exerting promoter-like activity in Louckes, but not in Reh or HepG2. CAT expression by P2 is negligible in Reh cells. However, enhanced CAT activity is not accompanied by elevated mRNA levels. This observation is consistent with the relief of translational restraints imposed by the(+)32 to(+)125 region. Together, the data demonstrate that efficient and cell-specific transcription regulation in mature B lymphocytes is contained in a 495 bp P2 segment that is comprised of 370 bp of 5'-flanking region and 125 bp of transcribed region of Exon X.

Evolutionary considerations in relating oligosaccharide diversity to biological function

The oligosaccharide chains (glycans) attached to cell surface and extracellular proteins and lipids are known to mediate many important biological roles. However, for many glycans, there are still no evident functions that are of obvious benefit to the organism that synthesizes them. There is also no clear explanation for the extreme complexity and diversity of glycans that can be found on a given glycoconjugate or cell type. Based on the limited information available about the scope and distribution of this diversity among taxonomic groups, it is difficult to see clear trends or patterns consistent with different evolutionary lineages. It appears that closely related species may not necessarily share close similarities in their glycan diversity, and that more derived species may have simpler as well as more complex structures. Intraspecies diversity can also be quite extensive, often without obvious functional relevance. We suggest one general explanation for these observations, that glycan diversification in complex multicellular organisms is driven by evolutionary selection pressures of both endogenous and exogenous origin. We argue that exogenous selection pressures mediated by viral and microbial pathogens and parasites that recognize glycans have played a more prominent role, favoring intra- and interspecies diversity. This also makes it difficult to appreciate and elucidate the specific endogenous roles of the glycans within the organism that synthesizes them.

Spermidine-induced glycoprotein fucosylation in immature rat intestine

In rat small intestine, during postnatal development, the glycoprotein fucosylation is markedly increased at weaning. At the same time, a rise in the intestinal spermidine level was observed, partly due to the increase in the spermidine content of solid food given to animals at this period as compared to the spermidine content of milk. In order to mimic the spermidine increase observed in weanling rat intestines, we had treated suckling rats with spermidine by oral ingestion to study its role as maturation factor of the small intestine. In spermidine-treated suckling rats, the spermidine and N-acetyl-spermidine contents were highly increased. Spermidine treatment induced the rise in alpha-1,2-fucosyltransferase activity and the precocious appearance in the brush-border membrane of some alpha-1,2-fucoproteins in weaned rats. Such results indicate that spermidine could be a maturation factor implicated in the appearance of alpha-1,2-fucoproteins naturally observed at weaning time.

Differential expression of the hepatic transcript of beta-galactoside alpha2,6-sialyltransferase in human colon cancer cell lines

The activity of beta-galactoside alpha2,6-sialyltransferase (ST6Gal.1), the enzyme responsible for the addition of sialic acid in alpha2,6-linkage to N-acetyllactosaminic (Gal beta1,4GlcNAc) units of glycoconjugates, is increased in the vast majority of colon cancer specimens, and a positive correlation with an invasive phenotype has been suggested by several studies. In many tissues, ST6Gal.1 is regulated mainly at the transcriptional level through the use of different cell-specific promoters which generate transcripts differing in their 5'-untranslated regions. With the aim of understanding the molecular bases of the increased ST6Gal.1 expression in colon cancer, we investigated the expression of mRNA species in colon cancer cell lines and the relationship with enzyme activity and extent of alpha2,6-sialylation of cell glycoproteins. All cell lines examined express the form containing the 5'-untranslated exons Y and Z, typical of the "basal" expression of the gene, while others express also the liver transcript. This indicates that colon cancer cell lines can be grouped according to expression of the liver transcript of ST6Gal.1. The cell lines expressing only the Y+Z form display, in general, a lower activity:mRNA ratio, which might indicate reduced translational efficiency. The level of alpha2,6-sialylation of cell glycoproteins, as determined by reactivity with the Sambucus nigra lectin, is closely associated with the level of enzyme activity.

Differentiation elicits negative regulation of human beta-galactoside alpha2,6-sialyltransferase at the mRNA level in the HL-60 cell line

We studied the regulation of the beta-galactoside alpha2,6-sialyltransferase (hST6Gal I) gene during HL-60 cell differentiation induced with dimethyl sulfoxide (DMSO), all transretinoic acid (ATRA), and phorbol myristate acetate (PMA). During HL-60 cell line differentiation, cell surface levels of alpha2,6-sialic acids expression decreased, as measured by flow cytometric analysis using Sambucus nigra agglutinin (SNA). Activities of hST6Gal I and levels of hST6Gal I mRNA dramatically decreased after 1 day of stimulation. Using reverse transcription polymerase chain reaction (PT-PCR), we found the major hST6Gal I mRNA isoform in HL-60 cells contains 5'-untranslated exons Y and Z. These results suggest that the expression of cell surface alpha2,6-sialic acids is controlled at the mRNA level, which is regulated by a promoter located 5'-upstream of exon Y.

Differential expression of alpha2,6-sialyltransferase in colon tumors recognized by a monoclonal antibody

It has been reported that alpha2,6-sialyltransferase (alpha2,6-ST; E.C. 2.4.99.1) activity is associated with cellular differentiation. To define its role in colon carcinoma differentiation, we have generated murine monoclonal antibodies (MAb) against alpha2,6-sialyltransferase. The MAb, designated 6B9 of IgM isotype, showed strong reactivity with the purified and crude alpha2,6-ST by ELISA and dot blot assays. Western blotting with MAb 6B9 identified purified alpha2,6-ST of MW 47 kDa and the same MW protein from rat and human liver extracts. The MAb also reacted with two other liver proteins of approximate MW 65 and 100 kDa. Immunoperoxidase studies with formalin-fixed paraffin-embedded tissues showed that MAb 6B9 reacts with liver tissues, the staining of hepatocytes was granular and cytoplasmic. There was a distinct pattern of zonal distribution of this enzyme in hepatocytes located particularly in the portal areas of the liver corresponding to zone 1. Normal colon (100%) and hyperplastic polyps (100%) showed very weak to no reactivity. Adenomas (100%) demonstrated moderate reactivity, while the poor (33%), moderate (100%) and well-differentiated (80%) colon adenocarcinomas showed strong reactivity. Results suggest that alpha2,6-ST is associated with the differentiation state of colon tumors.

Multiplex RT-PCR method for the analysis of the expression of human sialyltransferases: application to breast cancer cells

In many cases of human cancer, the appearance of hypersialylated glycan structures is related to a precise stage of the disease; this may depend on altered regulation of one or more sialyltransferases genes. Since several distinct sialyltransferase enzymes arising from different unique genes transfer sialic acid residues in the same linkage onto the same acceptor, it is impossible to precisely determine which enzyme is involved in the observed phenotype based on enzymatic assays. We have developed a very sensitive and highly reproducible multiplex reverse transcriptase-polymerase chain reaction technique in order to monitor the expression of four human sialyltransferases genes ST6Gal I, ST3Gal I, ST3Gal III and ST3Gal IV in small cell samples. Multiplex PCR amplification using specific primers for each sialyltransferase and detection of amplification products by polyacrylamide gel electrophoresis is a method that is fast and easy to handle and has proven to be useful for establishing sialyltransferase patterns of expression in breast immortalized cell line HBL100 as well as in breast cancer cell lines MCF-7/6, MCF-7/AZ and MDA.

Murine hepatic beta-galactoside alpha 2,6-sialyltransferase gene expression involves usage of a novel upstream exon region

ST6Gal I (beta-galactoside alpha 2,6-sialyltransferase, SiaT-1, ST6N, EC 2.4.99.1) mediates the attachment of the alpha 2,6-sialyl linkage common on N-linked glycans. Previous work suggests substantial inter-species conservation in SIAT1, the gene encoding ST6Gal I. In human and in rat, hepatic-specific SIAT1 transcription is initiated at Exon I. Here we report a surprising departure in the structural organization of the murine ST6Gal I gene. By a combination of primer extension analysis, 5'-RACE analysis, and analysis of genomic sequences, we show that the murine hepatic ST6Gal I mRNA contains a novel region 5' of Exon I. This novel sequence is encoded on a discrete upstream exon, Exon H. In contrast to human and rat hepatic ST6Gal I, the murine mRNA is transcriptionally initiated at the start of Exon H. Differential mRNA blot analysis indicates that transcripts containing Exon H sequences are preferentially expressed in liver.

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).

Linkage-specific action of endogenous sialic acid O-acetyltransferase in Chinese hamster ovary cells

9-O-Acetylation of sialic acids shows cell type-specific and developmentally regulated expression in various systems. In a given cell type, O-acetylation can also be specific to a particular type of glycoconjugate. It is assumed that this regulation is achieved by control of expression of specific 9-O-acetyltransferases. However, it has been difficult to test this hypothesis, as these enzymes have so far proven intractable to purification or molecular cloning. During a cloning attempt, we discovered that while polyoma T antigen-positive Chinese hamster ovary cells (CHO-Tag cells) do not normally express cell-surface 9-O-acetylation, they do so when transiently transfected with a cDNA encoding the lactosamine-specific alpha2-6-sialyltransferase (Galbeta1-4GlcNAc:alpha2-6-sialyltransferase (ST6Gal I); formerly ST6N). This phenomenon is reproducible by stable expression of ST6Gal I in parental CHO cells, but not upon transfection of the competing lactosamine-specific alpha2-3-sialyltransferase (Galbeta1-(3)4GlcNAc:alpha2-3-sialyltransferase; (ST6Gal III) formerly ST3N) into either cell type. Further analyses of stably transfected parental CHO-K1 cells indicated that expression of the ST6Gal I gene causes selective 9-O-acetylation of alpha2-6-linked sialic acid residues on N-linked oligosaccharides. In a similar manner, while the alpha2-3-linked sialic acid residue of the endogenous GM3 ganglioside of CHO cells is not O-acetylated, transfection of an alpha2-8-sialyltransferase (GM3:alpha2-8-sialyltransferase (ST8Sia I); formerly GD3 synthase) caused expression of 9-O-acetylation of the alpha2-8-linked sialic acid residues of newly synthesized GD3. These data indicate either that linkage-specific sialic acid O-acetyltransferase(s) are constitutively expressed in CHO cells or that expression of these enzymes is secondarily induced upon expression of certain sialyltransferases. The former explanation is supported by a low level of background 9-O-acetylation found in parental CHO-K1 cells and by the finding that O-acetylation is not induced when the ST6Gal I or ST8Sia I cDNAs are overexpressed in SV40 T antigen-expressing primate (COS) cells. Taken together, these results indicate that expression of sialic acid 9-O-acetylation can be regulated by the action of specific sialyltransferases that alter the predominant linkage of the terminal sialic acids found on specific classes of glycoconjugates.

Coordinate expression of beta-galactoside alpha 2,6-sialyltransferase mRNA and enzyme activity in suckling rat jejunum cultured in different media: transcriptional induction by dexamethasone

In attempting to elucidate the molecular basis of the expression of alpha 2,6-sialyltransferase (alpha 2,6-ST) in jejunal explants of 7-day-old rats during cultivation, the total jejunal RNA was analysed by hybridization using a cDNA clone encoding rat liver alpha 2,6-ST. Under cultivation in both serum-free and serum-containing media jejunal alpha 2,6-ST mRNA closely paralleled the bound (100,000 g pellet) as well as the soluble (100,000 g supernatant) alpha 2,6-ST activity, the correlation coefficients being 0.976 and 0.816, respectively. Dexamethasone (Dx) treatment enhanced alpha 2,6-ST mRNA and membrane-bound alpha 2,6-ST activity in close correlation. Jejunal alpha 2,6-ST mRNA is sensitive to actinomycin D and is lost with apparently identical kinetics in Dx-stimulated and control explants, suggesting that regulation by Dx may be exerted by altering the rate of mRNA synthesis. Dx-dependent activation resulted in elevation of the 4.3-Kb mRNA and can be inhibited by the antiglucocorticoid onapristone, demonstrating the participation of the glucocorticoid receptor pathway.

Differentiation -dependent expression of human beta-galactoside alpha 2,6-sialyltransferase mRNA in colon carcinoma CaCo-2 cells

We have previously documented a dramatic elevation in the activity of alpha 2,6-sialytransferase towards Gal beta 1,4GlcNAc (EC 2.4.99.1) (alpha 2,6ST) in CaCo-2 cells maintained in culture for several days after confluence to elicit a high degree of enterocytic differentiation phenotype. Northern analysis performed with a probe complementary to a region of human alpha 2,6ST mRNA common to all known transcripts demonstrated that the expression of alpha 2,6ST mRNA in CaCo-2 cells increased with the degree with the degree of differentiation. When probes complementary to 5'-untranslated exons (Y + Z or X) previously identified in transcripts isolated from human placenta and from several human lymphoblastoid cell lines were used, no hybridization signal with mRNA of CaCo-2 cells was found, as reported for the mRNA of hepatoma cell line HepG2 (Wang XC, Vertino A, Eddy RL, Byers MG, Jani-Sait SN, Shows TB, Lau JTY (1993) J Biol Chem 268: 4355-61). These results support the notion that the major alpha 2,6ST transcript of CaCo-2 cells was the hepatoma isoform or a new one, so far unreported. Consistent with the differentiation-dependent increase in alpha 2,6ST-mRNA expression, an elevation of the reactivity with Sambucus nigra agglutinin of differentiated CaCo-2 cell-surface was observed, indicating an enhanced alpha 2,6-sialylation of membrane glycoconjugates.