Co-purification of soluble human galactosyltransferase and immunoglobulins

Interactions of cell-surface galactosyltransferase with immunoglobulins

Detection of the activity of beta-1,4-galactosyltransferase (beta-1,4-GT) in suspensions of viable mouse hepatocytes, the human hepatoma cell line Hep G2, the human colonic adenocarcinoma cell line HT-29, the monocyte-like cell line U937, and human splenic B and T lymphocytes suggested the presence of beta-1,4-GT, in an enzymatically active form, on plasma membranes. The presence of beta-1,4-GT on cell surfaces was also indicated from the effect of trypsinization of live cells, which significantly reduced cell surface beta-1,4-GT activity, but did not affect the activity associated with cytoplasmic membranes. Furthermore, the presence of beta-1,4-GT on the cell surface was demonstrated by indirect immunofluorescence staining of cells with anti-beta-1,4-GT antibody. The detection of radioactivity in immunoglobulins (Ig) and their component chains after incubation with suspensions of intact cells in the presence of Mn2+ and UDP-[3H]-galactose, indicated that Ig molecules were galactosylated. In the absence of UDP-[3H]-galactose, beta-1,4-GT on cell surfaces, or immobilized on Sepharose-4B, formed stable complexes with galactose acceptors, including Ig. The efficiency of binding decreased in the order: J chain > alpha chain > mu chain > polymeric IgA2 > monomeric/polymeric IgA1 > IgM > IgG. Thus, beta-1,4-GT could act as a cell-surface receptor for Ig through a cation-dependent, lectin-like association of the beta-1,4-GT with the carbohydrate moieties of the Ig. This was confirmed by indirect surface immunofluorescence and radiolabeled ligand binding assays. The binding was inhibitable by EDTA, alpha-lactalbumin (in the presence of glucose), GlcNAc, or uridine 3',5'dialdehyde. At 37 degrees C, the apparent affinity constants and association rate constants of interaction between cell surface beta-1,4-GT on glutaraldehyde-fixed HT-29 and U937 cells and alpha 2 chain or monomeric IgA1 were in the range from 7.1 x 10(7) to 4.6 x 10(8) M-1 and from 1 x 10(5) to 3 x 10(6) M-1 s-1, respectively. The dissociation rate constants and half time of dissociation calculated from these data were in the range from 2.1 x 10(-2) to 5.0 x 10(-4) s-1 and from 33 to 1380 s, respectively. The number of alpha 2 or IgA1 molecules bound per HT-29 and U937 cell were in the range from 1.9 x 10(5) to 1.3 x 10(6). The binding of IgA by the cell surface beta-1,4-GT was not associated with internalization or the catabolic degradation of the ligand.

Interactions of galactosyltransferase with serum and secretory immunoglobulins and their component chains

Assay of the activity of beta-1,4-galactosyltransferase (beta-1,4-GT) revealed that in addition to serum, milk, colostrum, amniotic and cerebrospinal fluids and malignant effusions, this enzyme is present also in tears and saliva. Molecular-sieve chromatography of human colostral whey and serum and subsequent assay of beta-1,4-GT activity have shown that beta-1,4-GT was present as a free enzyme (55 kDa) and associated with components of larger molar mass. The elution pattern did not change when the chromatography was carried out in a buffer devoid of, or enriched with, Mn2+, a cofactor of beta-1,4-GT activity. However, the activity associated with the large molar mass components was absent when the chromatography was carried out in the presence of a chelating agent (EDTA). Analyses of the eluted material by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS-PAGE), and by immunodiffusion indicated that the major colostral component in beta-1,4-GT activity-containing fractions was secretory IgA (S-IgA); in addition, the beta-1,4-GT activity was detected in fractions that contained lactoferrin and alpha-lactalbumin. Interactions of beta-1,4-GT with S-IgA and lactoferrin in colostrum were also demonstrated by the detection of radioactivity in precipitin lines obtained by immunoelectrophoresis and autoradiography of the colostral whey after it had been incubated with UDP-[3H]-galactose. Furthermore, radioactively labeled S-IgA and alpha-chain were detected when colostral whey incubated with UDP-[3H]-galactose was analyzed by SDS-PAGE under non-reducing and reducing conditions, respectively. In serum, the beta-1,4-GT-binding components identified in fractions after molecular-sieve chromatography were IgG, IgA, IgM and transferrin. The binding of beta-1,4-GT to immunoglobulins (Ig) was also demonstrated by assaying the beta-1,4-GT activity associated with Sepharose-4B-immobilized Ig of various isotypes and molecular forms, which were incubated with colostral beta-1,4-GT in the presence of Mn2+. Beta-1,4-GT measured by enzyme activity was bound to these Ig in order: polymeric IgA2 > monomeric IgA1 = polymeric IgA1 = secretory IgA = pentameric IgM > IgG. Immobilized component chains, namely alpha, mu and J chains, bound beta-1,4-GT more effectively than native Ig. Incubation of the IgA1 myeloma protein with crude human colostral galactosyltransferase in the presence of UDP[3H]-galactose and Mn2+ resulted in galactosylation of both N- and O-linked carbohydrate side chains.(ABSTRACT TRUNCATED AT 400 WORDS)

Transforming growth factor-beta enhances secretory component and major histocompatibility complex class I antigen expression on rat IEC-6 intestinal epithelial cells

Transforming growth factor-beta (TGF-beta) has been implicated as having a role in inflammatory responses by inducing cellular infiltration and the release of inflammatory cytokines. In this study, the IEC-6 rat intestinal epithelial cell line was used as a model to assess the effect of TGF-beta 1 on the expression of various plasma membrane determinants. TGF-beta 1 induced a dose-dependent increase in the percentage of cells expressing surface secretory component (SC) and class I major histocompatibility (MHC) antigens. However, the expression of class II MHC was unaffected. In contrast, epidermal growth factor had no effect on any of the surface proteins studied. The TGF-beta 1-enhanced expression of SC was accompanied by an enhanced binding of polymeric, but not monomeric, immunoglobulin A (IgA). Preincubation of the TGF-beta 1-treated cells with an anti-human beta-galactosyltransferase (beta-GT) antiserum did not block the binding of the anti-SC antibody, indicating that the TGF-beta-induced increase in SC staining was due to SC expression and not the polymeric immunoglobulin-binding enzyme, beta-GT. These results indicate that TGF-beta 1 may be important in immune functions involving intestinal epithelial cells by enhancing the expression of surface class I MHC antigens and SC, a protein responsible for the transport of polymeric IgA into the intestinal lumen.

Molecular cloning of human beta 1,4-galactosyltransferase and expression of catalytic activity of the fusion protein in Escherichia coli

1. Three groups of cDNA clones (total of six) for human UDP-galactose: beta N-acetylglucosamine galactosyltransferase (4 beta GT) were obtained by screening of a fetal liver library in lambda gt11 with an affinity purified anti4 beta GT antibody. 2. One group of clones (three clones) reacted with two distinct anti4 beta GT murine monoclonal antibodies. 3. Nucleotide sequence of this group of clones were similar to published sequence for human 4 beta GTcDNA, except the 74 nucleotides at the 5'-end. 4. Partially purified fusion protein encoded by this group of clones showed all the catalytic properties of 4 beta GT, although the cDNA was partial and the protein was probably unglycosylated.

Polymorphism and expression of the galactosyltransferase-associated protein kinase gene in normal individuals and galactosylation-defective rheumatoid arthritis patients

We used restriction endonuclease digestion of leukocyte DNA to assess the structural integrity of an N-acetylglucosamine beta 1----4 galactosyltransferase (GalTase)-associated (GTA) protein kinase gene in rheumatoid arthritis (RA) patients. This analysis provides evidence that the gross structure of the GTA protein kinase gene locus remains intact in patients with defective galactosylation and that this gene locus is polymorphic both in normal individuals and in patients with RA, although no polymorphisms unique to RA patients were observed. Initial data on the expression of this gene indicate that comparable levels of GTA protein kinase messenger RNA are present in the lymphocytes of normal individuals and RA patients, irrespective of whether lymphocytes were obtained from patients with decreased or normal levels of galactosylation.

Separation and partial characterization of two galactosyltransferase isoforms from malignant ascitic fluid

Galactosyltransferase was isolated from human malignant ascites by ammonium sulphate precipitation, alpha-lactalbumin affinity chromatography and by the removal of contaminating immunoglobulins with protein A chromatography. Two isoforms of galactosyltransferase were separated by DEAE chromatography, and partially characterised by gel filtration, non-denaturing polyacrylamide gel electrophoresis, concanavalin A binding and enzyme kinetics. The two forms isolated were compared with the isoenzymes previously described and the biochemical basis for the difference between the two forms discussed.

Blood-group-related carbohydrate antigens are expressed on human milk galactosyltransferase and are immunogenic in rabbits

Immunochemical evidence is presented for the presence of blood-group-related carbohydrate structures on human milk galactosyltransferase and for the occurrence of the corresponding specificities among rabbit antibodies to this enzyme. Although these carbohydrate specificities constitute minor populations among antisera and affinity-purified antibodies to galactosyltransferase, their presence is important in the immunohistochemical approach to enzyme localization, since they give rise to strong reactivities with epithelial cells of the gastrointestinal tract.

Cloning and sequencing of cDNA of bovine N-acetylglucosamine (beta 1-4)galactosyltransferase

Galactosyltransferases constitute a family of enzymes, each member of which transfers galactose from UDPgalactose to a specific acceptor molecule, generating a specific galactose-acceptor linkage. Two synthetic oligonucleotides, 27mer and 21mer, were synthesized, based on the amino acid sequences of two peptides derived from bovine milk N-acetylglucosaminide (beta 1-4)galactosyltransferase (EC 2.4.1.90), and used as hybridization probes to isolate cDNA clones for galactosyltransferase from a bovine mammary gland cDNA library. One of the plasmids, designated pLbGT-1, contains an insert of about 3.7 kilobases that hybridizes to both of the probes and encodes the amino acid sequences of five peptides obtained from bovine milk (beta 1-4)galactosyltransferase. A second plasmid, designated pLbGT-2, contains an insert of about 4.1 kilobases that hybridizes to only the 27mer and that encodes a polypeptide containing the sequence of the carboxyl-terminal 120 residues identical to the peptide encoded by pLbGT-1; the rest of the protein sequence, however, does not contain known sequences from bovine galactosyltransferase. The two cDNAs contain a 3'-untranslated region of about 2.7 kilobases that includes two copies of the Alu-equivalent sequences. pLbGT-1 and pLbGT-2 hybridize to mRNAs of various sizes obtained from the bovine and rat mammary gland and the human mammary tumor cell line MCF-7, with the longest mRNA from each species being around 4.5 kilobases. The results show that pLbGT-1 is a cDNA clone for bovine (beta 1-4)galactosyltransferase, and pLbGT-2 encodes a protein that is structurally and may be functionally related to transferases.

Galactosyltransferase activity is not localized to the brush border membrane of human small intestine

A recent report [Roth et al. (1985) J. Cell Biol. 100: 118-125], using immunocytochemical techniques, claimed that human duodenal galactosyltransferase is located predominantly on the external aspect of enterocyte brush border membranes. Analytical subcellular fractionation by sucrose density gradient centrifugation of human jejunum biopsy homogenates demonstrated that galactosyltransferase activity is localized to the Golgi fraction (equilibrium density of 1.14 g cm-3) and is not found in significant amounts in the brush border membrane (equilibrium density of 1.22 g cm-3).

Evaluation of galactosyltransferase isoenzyme II in a human colon carcinoma-derived cell line, HCT-8

Polyacrylamide gel electrophoresis of galactosyltransferase (GT) extracted from a human colon adenocarcinoma cell line, HCT-8, demonstrated the presence of two peaks of activity: a slow-moving peak, referred to as GT-II, and a more anodally migrating peak, designated as GT-I, which was also found for normal human serum. However, if GT solubilized from HCT-8 cells was separated by isoelectric focusing, no unique isoenzymes could be detected. Total GT activity from HCT-8 cells was purified by alpha-lactalbumin-Sepharose affinity chromatography followed by ion exchange chromatography on either DEAE-cellulose or FPLC using a Mono Q anion exchange resin. Three major peaks of activity were resolved from anion exchange chromatography. Electrophoresis of each peak revealed a GT pattern identical with that originally observed for the crude (detergent) solubilized homogenate. No enrichment of either GT-I or GT-II was observed in the three enzyme fractions. The data suggest that GT-II may be an artifactual activity of cancer cells composed of GT-I associated with some contaminating protein.

Golgi and secreted galactosyltransferase

Galactosyltransferase (GT) belongs to the glycosyltransferases. In several tissues and cell lines, the enzyme is localized by immunocytochemistry to the two to three trans cisternae of the Golgi complex and may thus be considered a specific membrane component of this type of endomembrane. As a consequence, it is the most common Golgi "marker" enzyme in cell fractionation studies. Study of its biosynthesis, membrane orientation, and turnover in several tissues and cultured cell lines has broadened our knowledge about Golgi function itself. The enzyme is oriented towards the lumen of the cisternal space. In this orientation, it catalyzes the transfer of galactose to glycoprotein-bound acetylglucosamine and, in the presence of alpha-lactalbumin, to glucose, as shown in the Golgi complex of mammary gland epithelial cells. The enzymatic properties of GT are well known. The metabolism of GT has been extensively studied in HeLa and human hepatoma cells. The enzyme is synthesized in the rough endoplasmic reticulum (RER) and provided with one N-linked oligosaccharide and palmitate residues. In the Golgi complex, terminal sugars are attached to the N-linked oligosaccharide and extensive O-glycosylation takes place. The half-life of the enzyme is about 20 hr, after which a soluble form appears in the culture medium. Release of GT into the medium is observed in all cell lines studied. This phenomenon is in accordance with the presence of soluble GT in body fluids such as serum, ascites, milk, and saliva. In patients suffering from ovarian and breast cancer, increased levels of GT enzyme activity have been reported. Whether extracellular GT is of biological significance is still a point of discussion.

Immunocytochemical demonstration of ecto-galactosyltransferase in absorptive intestinal cells

Galactosyltransferase immunoreactive sites were localized in human duodenal enterocytes by the protein A-gold technique on thin sections from low temperature Lowicryl K4M embedded biopsy specimens. Antigenic sites detected with affinity-purified, monospecific antibodies were found at the plasma membrane of absorptive enterocytes with the most intense labeling appearing along the brush border membrane. The lateral plasma membrane exhibited a lower degree of labeling at the level of the junctional complexes but the membrane interdigitations were intensely labeled. The labeling intensity decreased progressively towards the basal part of the enterocytes and reached the lowest degree along the basal plasma membrane. Quantitative evaluation of the distribution of gold-particle label proved its preferential orientation to the outer surface of the plasma membrane. In addition to this membrane-associated labeling, the glycocalyx extending from the microvillus tips was heavily labeled. Occasionally, cells without plasma membrane labeling were found adjacent to positive cells. The demonstration of ecto-galactosyltransferase on membranes other than Golgi membranes precludes its general use as a marker for Golgi membrane fractions. The possible function of galactosyltransferase on a luminal plasma membrane is unclear at present, but a role in adhesion appears possible on the basolateral plasma membrane.

Galactosyltransferases: physical, chemical, and biological aspects

Galactosyltransferases (GTs) are one of the members of a family of enzymes called glycosyltransferases involved in the biosynthesis of complex carbohydrates. These enzymes catalyze the transfer of galactose from UDP-galactose to an acceptor (glycoprotein, glycolipid) containing terminal N-acetylglucosamine or N-acetylgalactosamine residue. GTs occur in soluble (milk, serum, effusions, etc.) and insoluble (membrane) forms. The GT activities on the outer surface of the cells have been correlated with a host of cellular interactions, including fertilization, cell migration, embryonic induction, chondrogenesis, contact inhibition of growth, cell adhesion, hemostasis, intestinal cell differentiation, and immune recognition. GTs have been purified to homogeneity using affinity chromatography. Most GTs are found active in the pH range 6 to 8 and at temperatures between 35 to 40 degrees C. Manganese is an essential co-factor for GT activity. Isoenzymes of GT have been recognized, especially in tumor tissues, malignant effusions, and sera of cancer patients using polyacrylamide gel electrophoresis in the presence and absence of SDS. Depending on the source of the enzyme, the molecular weights of GTs range between 40,000 to 80,000 daltons. Carcinoma-associated GT isoenzyme has been reported to have a higher molecular weight than the normal GT isoenzyme. Development of monoclonal antibody against the cancer-specific GT isoenzyme will provide help in the development of an immunoassay for the measurement of this isoenzyme in the sera and an aid in the radioimmunolocalization of the tumors in cancer patients.

Cell surface galactosyltransferase: immunochemical localization

A cell surface UDP-galactose:N-acetylglucosamine galactosyltransferase (GT) has been directly localized on bovine cells in tissue culture by immunohistochemical techniques. A conventional rabbit heteroantiserum was prepared against an affinity-purified soluble form of GT from bovine milk, and a monospecific IgG fraction was isolated by affinity chromatography on a GT adsorbent. As demonstrated by indirect immunofluorescence, antigen to this antibody is present on the surface of all three bovine cell lines tested. It was uniformly distributed over the exposed membrane surface of fixed cells. Exposure of living cells to the anti-GT antibody resulted in its time-dependent aggregation in the plane of the membrane. Antigen (GT) was released from the membrane surface by trypsin digestion, and its reappearance required protein synthesis, since cycloheximide effectively prevented repopulation of the cell surface.

Characterization of monoclonal antibodies to serum galactosyltransferase

Monoclonal IgG and IgM antibodies (mAbs) directed against serum galactosyltransferase (GalTase) activities were prepared and characterized for their relative specificity for GalTase isoenzymes I and II (GalTase I and GalTase II). After immunization of mice with purified GalTase, 7 of 1680 fusion products screened were positive for anti-GalTase activity in a solid-phase assay; of these 7, 2 were found to bind GalTase I in a somewhat selective manner while 1 (C6) was relatively specific for GalTase II. The Ka for anti-GalTase mAbs ranged from 2.7 X 10(7) to 1.1 X 10(8) M-1. Enzymatically active GalTase could be recovered from an affinity column of C6 coupled to Sepharose 4B following application of a cell extract from a human colon cancer cell line, confirming that the mAb is directed against GalTase. A sandwich RIA was developed to measure soluble GalTase II in serum by using a combination of two isoenzyme nonspecific mAbs (F5 and V10) coated on a solid-phase support followed by the addition of antigen and GalTase II-specific 125I-labeled C6. This assay was specific for GalTase II with a sensitivity of approximately 10 ng/ml. Evaluation of 240 sera demonstrated higher levels of GalTase II in patients with gastrointestinal cancer (48 ng/ml). However, in contrast to previous results with a radiochemical assay, some normal sera contained GalTase II (mean, 14 ng/ml) and some patients with liver disease had elevated levels (mean, 23 ng/ml). These studies demonstrate the production of moderate-affinity antibodies directed to serum GalTase isoenzymes and the development of an RIA useful in the study of GalTase.

Serum galactosyltransferase isoenzymes as markers for solid tumours in humans

High resolution agarose isoelectric focusing was used to compare the galactosyltransferase isoenzyme patterns of serum from 9 healthy controls with those from 38 patients with either breast, lung, ovarian, stomach or colonic cancer. At least 12 peaks of enzyme activity were found in every sample, the healthy controls having major forms with isoelectric points of 4.74, 4.87, 4.96, 5.16 and 5.23. Thirty patients (79%) had elevated levels of at least one isoenzyme and 23 (61%) had at least 3 isoenzymes elevated compared to only 10 (26%) patients who had elevated total serum galactosyltransferase activity. The isoenzymes which were most often elevated in the cancer patient group had isoelectric points of 4.93, 5.16 and 4.61. One isoenzyme with an isoelectric point of 4.43 was preferentially elevated in patients with ovarian cancer. Those isoenzymes containing little or no sialic acid were rarely elevated in cancer patients. Although no cancer-associated isoenzyme was detected the quantitative differences observed in the cancer patient group were striking.