Mucin-type glycoproteins

Allelic polymorphism in the hamster oviductin gene is due to a variable number of mucin-like tandem repeats

Oviductins are high-molecular-weight glycoproteins specifically secreted by the oviduct. These proteins bind to the zona pellucida of the ovulated oocyte and remain associated with the embryo during its transit in the oviduct. They may be involved in fertilization and early embryonic development. In order to explore their putative biological function, the cDNA sequence corresponding to oviductin in the golden hamster was determined. We found that the deduced amino acid sequence of this heavily O-glycosylated protein presents characteristics typical of mucins, including serine- or threonine-rich tandem repeats. Analysis of several cDNA clones and of genomic DNA revealed the presence of a single copy gene with two frequent alleles differing in the number of repeats. Comparison with oviductin sequences from other mammals indicates a high degree of conservation amongst species, except for the repeat region which shows divergence, notably in the number of repeats. Based on its biochemical and genetic properties, hamster oviductin can now be classified as a secretory mucin. This concept provides a new insight in the elucidation of its biological role: oviductin could possibly provide the oviduct and the oocyte with a protective coating ensuring normal tubal function and embryonic development.

Morphometric analysis of intestinal mucins under different dietary conditions and gut flora in rats

Elucidation of the mechanisms that alter the biosynthesis, turnover, and degradation of intestinal mucins is relevant to the understanding of both the normal gut ecosystem and various intestinal diseases. In this study image analysis was used to quantify the effects of diet and microbial flora on the mucin composition of goblet and deep crypt cells, the number and volume density of mucin-containing cells, and the staining density of their stored mucins in the small and large intestine of germ-free and conventionally maintained rats fed two different diets. One was a coarsely ground commercial rodent diet containing crude fiber of cereal origin and the other a purified diet composed of finely powdered ingredients, including cellulose as a source of fiber. The changes in mucin production were also analyzed in germ-free rats colonized with a human flora. Feeding a commercial diet reduced the volume density of cells containing neutral and sulfomucins in the jejunum of conventional rats and the staining density of neutral and acidic mucins in the germ-free rats. Both rat and human floras reduced the number of cells containing acidic and sulfomucins and the staining density of neutral mucins in the small intestine of animals fed on a purified diet. However, inoculation of human flora increased the staining density of stored neutral and sulfated mucins in the cells of the large intestine. The results demonstrate that the dietary changes are influential in modifying the amount and proportion of mucins in the small intestine and the microbial flora in the large intestine.

Regulated and unregulated pathways for MUC2 mucin secretion in human colonic LS180 adenocarcinoma cells are distinct

We have shown previously [McCool, Forstner and Forstner (1994) Biochem. J. 302, 111-118] using pulse-chase labelling of mucin with [3H]threonine that LS180 colonic tumour cells synthesize and secrete MUC2 without the addition of secretagogues. Treatment of the LS180 cells with monensin to disrupt Golgi function was also found to inhibit baseline secretion almost completely. In this paper we show that addition of nocodazole to inhibit microtubule assembly reduced baseline secretion by 53% over a 6 h chase period. In contrast, cytochalasin D did not affect the rate of unstimulated mucin synthesis or secretion, suggesting that baseline secretion is not influenced by disruption of actin microfilaments. In addition, regulated mucin secretion by LS180 cells was studied in response to carbachol, phorbol 12-myristate 13-acetate and A23187. Mucin released in response to secretagogues behaved identically on SDS/PAGE to that secreted under baseline conditions. T84 cells and the B6 subclone of the HT29 cell line responded in a similar manner to LS180 cells and secreted high-molecular-mass mucin which included MUC2 and behaved like LS180 mucin on SDS/PAGE. Neither monensin nor nocodazole significantly affected secretagogue-stimulated mucin secretion. Since these compounds inhibited secretion of labelled mucin under baseline conditions, mucin released by secretagogues must have come from a separate, unlabelled mucin pool in stored granules. Cytochalasin D, on the other hand, caused the release of small amounts of stored mucin, suggesting that actin microfilaments participate in regulated exocytosis. Thus two kinds of mucin secretion occur in LS180 cells. Unregulated secretion depends upon continuous transport of mucin granules from Golgi vesicles to the cell surface and does not utilize stored mucin, whereas regulated secretion involves the release of mucin from storage granules and is not affected by microtubule or Golgi disruption.

A female-specific cDNA sequence of Schistosoma mansoni encoding a mucin-like protein that is expressed in the epithelial cells of the reproductive duct

Seven cDNA clones of Schistosoma mansoni containing the C-terminal part of the deduced sequence of a mucin-like protein have been identified. The protein contains 28% threonines, 20% serines, and has a pI of 3.4. On Northern blots of RNA of adult worms, the cDNA clones detect 2 transcripts of 1.65 and 4.2 kb which are expressed only in female worms. The tissue of gene expression, as revealed by in situ hybridization, is the epithelium surrounding the female reproduction duct proximal to its entrance into the ootype. Accumulation of N-glycosylation sites suggests that the protein, like other mucins, might form a protective layer, coating the lining of the duct. Regarding its acidic pI, we hypothesize a role in preventing premature egg-shell formation. This is the first female-specifically transcribed sequence, hitherto known in S. mansoni that is not expressed in the vitellaria.

Mouse gastric mucin: cloning and chromosomal localization

Mucins protect gastric epithelium by maintaining a favourable pH gradient and preventing autodigestion. The purpose of this study was to clone a mouse gastric mucin which would provide a foundation for analysis of mucin gene regulation. Mucin was purified from the glandular portion of gastric specimens and deglycosylated by HF solvolysis. Antibodies against native and deglycosylated mouse gastric mucin (MGM) were raised in chickens. Screening of a mouse stomach cDNA library with the anti-(deglycosylated MGM) antibody yielded partial clones containing a 48 bp tandem repeat and 768 bp of non-repetitive sequence. The 16-amino-acid tandem repeat has a consensus sequence of QTSSPNTGKTSTISTT with 25% serine and 38% threonine. The MGM tandem repeat sequence bears no similarity to previously identified mucins. The MGM non-repetitive region shares sequence similarity with human MUC5AC and, to a lesser extent, human MUC2 and rat intestinal mucin. Northern blot analysis reveals a polydisperse message beginning at 13.5 kb in mouse stomach with no expression in oesophagus, trachea, small intestine, large intestine, caecum, lung or kidney. Immunoreactivity of antibodies against deglycosylated MGM and against a synthetic MGM tandem repeat peptide was restricted to superficial mucous cells, antral glands and Brunner's glands in the pyloric-duodenal region. DNA analysis shows that MGM recognizes mouse and rat DNA but not hamster, rabbit or human DNA. The MGM gene maps to a site on mouse chromosome 7 homologous to the location of a human secretory mucin gene cluster on human chromosome 11p15. Due to sequence similarity and predominant expression in the stomach, the MGM gene may be considered a MUC5AC homologue and named Muc5ac.

Simple mucin-type carbohydrate antigens (T, sialosyl-T, Tn and sialosyl-Tn) in breast carcinogenesis

Immunohistochemical analysis of the expression of simple mucin-type carbohydrate antigens (Tn, sialyl-Tn and T) was performed in a series of 43 cases of intraductal hyperplasia without atypia, 9 cases of intraductal hyperplasia with atypia, 54 cases of ductal carcinoma in situ (DCIS) and 26 cases of invasive breast carcinoma. We also studied 36 cases of isolated breast normal epithelium, 20 cases of "normal" breast epithelium adjacent to neoplasms and 14 cases of apocrine metaplasia. All antigens were detected in different frequencies in normal, hyperplastic, metaplastic and neoplastic breast epithelium. Tn and sialyl-Tn are expressed more frequently in malignant than in benign breast epithelium; while Tn expression increases from normal to invasive carcinomas, sialyl-Tn increases until DCIS and drops in invasive carcinomas, suggesting that either there is a failure of a proportion of DCIS to progress to invasive carcinoma or loss of expression of sialyl-Tn when some carcinomas become invasive. The high frequency of Tn and sialyl-Tn expression in breast intraductal proliferations probably reflects incomplete glycosylation in these lesions, which is a well-known tumour-associated phenomenon and supports the assumption that such lesions are putative precursors of breast cancer. T antigen was expressed in all groups studied, but its prevalence differed significantly between normal and neoplastic epithelium. The expression of these antigens in epithelium adjacent to carcinomas is similar to that found in isolated normal breast epithelium, whereas apocrine metaplasia has a pattern of simple mucin-type glycosylation that is specific and distinct from that of the normal breast epithelium, with a high frequency of marked expression of Tn and sialyl-Tn. The similarity of the pattern of expression of simple mucin-type antigens in metaplasia and malignant neoplasia reduces the usefulness of these markers from a diagnostic standpoint.

Characterization of the let-653 gene in Caenorhabditis elegans

A mutation in the let-653 gene of Caenorhabditis elegans results in larval death. The lethal arrest is concurrent with the appearance of a vacuole anterior to the lower pharyngeal bulb. The position of the vacuole is consistent with a dysfunction of the secretory/excretory apparatus. Germline transformation rescue experiments were able to position the let-653 gene to two overlapping cosmid subclones. Sequence data generated from both cDNA and genomic DNA subclones indicated that let-653 encodes a mucin-like protein. Our characterization suggests that a mucin-like protein is essential for effective functioning of the secretory/excretory apparatus within C. elegans.

Analysis of serine/threonine-linked oligosaccharides derived by alkaline-borohydride treatment of mucin glycoproteins electroblotted onto membranes: comparison of the saccharide profiles of the 390 kDa and 350 kDa forms of epitectin

Alkaline borohydride treatment is widely used for the release of carbohydrate moieties from O-glycosylated glycoproteins and mucins. We have adapted this procedure to micro quantities of glycoproteins blotted on membranes. After electrophoresis and transfer to nitrocellulose, nylon or polyvinylidene difluoride membrane, alkaline borohydride treatment was done directly on glycoprotein containing areas of membrane which were cut out with the aid of guide strips stained with Coomassie Blue or lectin-digoxigenin. In combination with standard saccharide fractionation techniques, this procedure can be used to characterize the oligosaccharides of mucins or mucin-type glycoproteins that are separated by gel electrophoresis from crude sources. Using this approach we have characterized the saccharides derived from the two species of epitectin, a malignancy-associated mucin type glycoprotein, isolated from metabolically labelled H.Ep2 cells.

Association of bile-salt-dependent lipase with membranes of human pancreatic microsomes

Immunolocalization studies indicated that, in contrast to other enzyme markers of human pancreatic secretion, bile-salt-dependent lipase (BSDL) was partly but specifically associated with endoplasmic reticulum membranes. In microsomes, temperature-induced phase separation using Triton X-114 elucidated the partition of BSDL between the aqueous phase and the detergent-rich phase containing hydrophilic and membrane proteins, respectively. The size of the membrane-associated BSDL (approx. 100 kDa) is compatible with that of the fully processed enzyme. Fucosylated O- and N-linked oligosaccharide structures were detected by means of specific lectins. The membrane-associated BSDL might therefore be released from membranes between the trans-Golgi compartment (where terminal fucose residues were added) and the zymogen granules where BSDL was mainly found in the soluble fraction. Even though BSDL associated with membranes was enzymically active, it appeared less efficient than the soluble form. The association of BSDL with membranes was pH-dependent and optimal association occurred between pH 5-6. The membrane-associated BSDL was released by KBr which suggests that the association of BSDL with microsomal membranes involves ionic interactions. Lipid-protein interactions are probably not involved in this association as BSDL did not associate with liver microsome membranes. We attempted to characterize the putative ligand and showed that BSDL and a 94-kDa protein, immunologically related to a glucose-regulated protein of 94 kDa (Grp94), were co-immunoprecipitated by specific antibodies directed against each individual species. It is suggested that the biogenesis of the human pancreatic BSDL involves an association with intracellular membranes and that its folding may be assisted by molecular chaperones.

MUC6 apomucin shows a distinct normal tissue distribution that correlates with Lewis antigen expression in the human stomach

BACKGROUND & AIMS

Among the human mucin complementary DNAs thus far identified, two (MUC5AC and MUC6) were cloned from stomach libraries. This study examines the distribution of MUC6 in normal tissues and compares it with that of MUC5AC as well as with the expression of Lewis blood group antigens.

METHODS

Affinity-purified rabbit antibodies detecting epitopes within the repetitive sequence of MUC5AC and MUC6 were used in enzyme-linked immunosorbent assays and immunohistochemical assays. RNA expression was analyzed by in situ hybridization. Double-labeling immunofluorescence was used to study apomucin and Lewis antigen coexpression.

RESULTS

MUC6 is detected in the stomach, colon, gallbladder, and endocervix. Two patterns of staining are observed, perinuclear and diffuse cytoplasmic, possibly reflecting differences in MUC6 glycosylation. Using both immunohistochemical assays and in situ hybridization on stomach tissue sections, MUC6 is expressed mainly in antral mucous cells, whereas MUC5AC is detected mainly in the superficial epithelium and neck glands. In antral mucosa, MUC6+ cells express Lewis(y), whereas MUC5AC+ cells express Lewis(b) and sialyl-Lewis(a).

CONCLUSIONS

It was concluded that MUC6 has a distinct tissue distribution pattern, different from that of MUC1-MUC5; MUC5AC and MUC6 are expressed by different cellular populations in normal stomach; and in this tissue, MUC5AC+ cells and MUC6+ cells show different patterns of Lewis antigen expression.

Bovine gall-bladder mucin contains two distinct tandem repeating sequences: evidence for scavenger receptor cysteine-rich repeats

Gall-bladder mucin is a densely glycosylated macromolecule which is the primary secretory product of the gall-bladder epithelium. It has been shown to bind cholesterol and other biliary lipids and to promote cholesterol crystal nucleation in vitro. In order to understand the molecular basis for mucin-lipid interactions, bovine gall-bladder mucin cDNAs were identified by expression cloning and were isolated and sequenced. The nucleotide sequences of these cDNAs revealed two distinct tandem repeating domains. One of these domains contained a 20-amino acid tandem repeating sequence enriched in threonine, serine and proline. This sequence was similar to, but not identical with, the short tandem repeating sequences identified previously in other mammalian mucins. The other domain contained a 127-amino acid tandem repeating sequence enriched in cysteine and glycine. This repeat displayed considerable sequence similarity to a family of receptor- and ligand-binding proteins containing scavenger receptor cysteine-rich repeats. By analogy with other proteins containing these cysteine-rich repeats, it is possible that, in gall-bladder mucin, this domain serves as a binding site for hydrophobic ligands such as bilirubin, cholesterol and other biliary lipids.

Histological, histochemical, and ultrastructural investigations on the gastrointestinal system of Antarctic seals: Weddell seal (Leptonychotes weddellii) and crabeater seal (Lobodon carcinophagus)

The morphology of the principal sections of the gastrointestinal system of two Antarctic seals with different dietary habits, namely, the Weddell seal (Leptonychotes weddellii) and the crabeater seal (Lobodon carcinophagus), has been investigated. Histologically examined by light microscopy, the tissue layers of the gastrointestinal tract of both seals are almost identical to those observed in most other mammals and no major differences in principle organization could be found between the two seal species. The ultrastructure of the gastric and intestinal epithelial cells has been examined and is also closely comparable to that of these cells in other mammals; however, Paneth cells have not been found in our material. In general, therefore, adaptations of the gastrointestinal tract to the aquatic environment or the diet are not obvious at the morphological levels of organization studied. Histochemical differences are found between the two closely related species; mucins of the surface epithelium in the stomach of Weddell seals are highly sulfated, while those in the crabeater seal are not. Mucous neck cells in Weddell seals contain acid mucosubstances, while those of crabeater seals contain neutral ones. Goblet cells in the small and large intestine in Weddell seals contain both neutral and acid mucosubstances. Both mucin types are detected in the crabeater seal; however, the mucins of the colon in the crabeater seal are more highly sulfated than those in the Weddell seal. The ratio of goblet cells to enterocytes in the large intestine of crabeater seals is higher than that in Weddell seals.

Distinct populations of high-M(r) mucins secreted by different human salivary glands discriminated by density-gradient electrophoresis

High-M(r) mucins [mucin glycoprotein 1 (MG1)] isolated from human saliva from the individual salivary glands were chemically characterized. The carbohydrate content of MG1 derived from palatal (PAL), submandibular (SM) and sublingual (SL) saliva was typical of mucins but showed heterogeneity, especially in the amount of sialic acid and sulphated sugar residues. The physicochemical properties of native MG1s make conventional SDS/PAGE and ion-exchange chromatography unsuitable for investigating differences between individual samples. Recently a density-gradient electrophoresis (DGE) device has been developed, primarily for separation based on the charge of entire cells or cell organelles [Tulp, Verwoerd and Pieters (1993) Electrophoresis 14, 1295-1301]. We have used this apparatus to study the high-M(r) salivary mucins. Using DGE, the MG1s of individual glands were seen to have clearly distinct electrophoretic mobilities, as monitored by ELISA using MG1-specific monoclonal antibodies. Even within a particular MG1 preparation, subpopulations could be distinguished. DGE analysis of a chemically and enzymically modified MG1 series, followed by ELISA and dot-blot detection using specific monoclonal antibodies, lectins and high-iron diamine staining, suggests that the high electrophoretic mobility of PAL-MG1 is mainly the result of a high sulphate content, whereas the SL subpopulations differ mainly in binding type and amount of sialic acid. SM-MG1 most resembles the low-mobility subpopulation of SL-MG1, except that it has a lower sulphate content. In conclusion, DGE appears to be a powerful method for analysis of native mucin; it has been used to demonstrate that MG1s from the various salivary glands are biochemically much more diverse than was previously assumed.

Stratified squamous epithelia produce mucin-like glycoproteins

The stratified squamous epithelia of the ocular surface, larynx, and vagina are mucus-coated epithelia, apices of which are subject to abrasive pressure from epithelia-epithelia interactions from eyelid, vocal cords, or vaginal folds, respectively. Mucus coats on these epithelia have generally been considered to be derived from the specialized mucin-producing cells embedded either in the epithelia or in adjacent tissues. Here we report the isolation, partial characterization, and cellular localization of a mucin-like glycoprotein produced by these stratified epithelia. In all three epithelia, the mucin-like molecule is present on cytoplasmic vesicles in subapical cells. As cells differentiate to their apical-most position adjacent to their mucus coat, the mucin-like molecule moves to the cell membrane where it is particularly prominent on microplicae folds. Lectin affinity chromatography was used to isolate the molecule from rat vaginal and corneal epithelium. Isolated material was approximately 60% carbohydrate and 40% protein. The major monosaccharide was N-acetylgalactosamine with lesser amounts of N-acetylglucosamine, galactose, mannose, xylose and fucose. Amino acid analysis demonstrated the predominant amino acids to be glycine, serine, threonine and proline. These data plus PAS and Alcian blue binding to the isolate indicate a mucin-like glycoprotein.

DNA methylation status of the MUC1 gene coding for a breast-cancer-associated protein

The MUC1 gene codes for protein products that are highly expressed in human breast-cancer tissue and that serve as tumor markers for disease progression. The factors contributing to the disease-specific over-expression of the MUC1 gene are under intensive investigation and are yet to be determined. A large transcribed region of the human MUC1 gene is a CpG island that consists of 60-bp tandemly repeating units, each of which contains one SmaI restriction site. The methylation status of regulatory regions, upstream to the transcriptional start site, is essential for the regulation of gene expression. We therefore evaluated whether the methylation status of the various regions of the MUC1 gene may affect its expression. Using SmaI, and its isoschizomer XmaI endonucleases, we demonstrated that in peripheral-blood leukocytes (PBL-DNA) that do not express the MUC1 gene, the repeat array is completely methylated, whereas the same sequences are entirely non-methylated in breast-tumor-tissue DNA (BT-DNA). In contrast, sequences upstream and downstream to the repeat array showed no difference in the methylation pattern in PBL-DNA and BT-DNA. Hypomethylation within the repeat array was also observed in other epithelial tissues that express the MUC1 gene at much lower levels to those seen in breast-cancer tissue. These studies demonstrate that hypomethylation of the tandem repeat array is an absolute requirement for MUC1 gene expression in epithelial tissues, although in breast-cancer tissue additional regulatory mechanisms must pertain for its over-expression.

Expression and glycosylation of the filamentous brush border glycocalyx (FBBG) during rabbit enterocyte differentiation along the crypt-villus axis

The filamentous brush border glycocalyx forming the ‘enteric surface coat’ of the intestinal epithelium is composed in rabbits of a 400 kDa mucin-type glycoprotein, which was purified using the 3A4 monoclonal antibody. This monoclonal antibody recognizes a filamentous brush border glycocalyx-specific glycosidic structure containing an O-acetylated sialic acid, which is absent from all the other glycoproteins in the epithelium, with the exception of certain goblet cell mucins. Here we establish that only 50% of the rabbits tested synthesized this glycosidic structure. Upon immunolabeling surface epithelia and sections of jejunum from these rabbits, the carbohydrate epitope recognized by the 3A4 mAb was found to be present on the filamentous brush border glycocalyx of a variable number of enterocytes, which were patchily distributed over all the villi. This heterogeneous expression of 3A4 antigenicity, which was also observed in the crypts, suggests the existence of differences between the patterns of differentiation of enterocytes, which results in the expression of different pools of glycosyltransferases and/or acetyl transferases. In mature enterocytes, the 3A4 determinants were present only on the filamentous brush border glycocalyx, which is anchored solely to the membrane microdomain at the tip of brush border microvilli. However, expression of 3A4 antigenicity begins in the median third of crypts, in enterocytes with a short, thin brush border devoid of apical filamentous brush border glycocalyx. Here the 3A4 epitopes were present over the whole brush border membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Biosynthesis of rat MUC2 in colon and its analogy with human MUC2

In order to identify the mucins synthesized and secreted in the rat colon, we studied their biochemical characteristics and biosynthesis and evaluated their analogy to human colonic mucins. Purified mucin from both species appeared similar with respect to composition, buoyant density and mobility on SDS/PAGE. Isolated rat colonic mucin (RCM) was used to elicit a polyclonal antiserum, which was used in metabolic labelling studies to identify mucins and mucin precursors. RCM is synthesized as a 600 kDa precursor protein, which oligomerizes before O-glycosylation. The mature, high-molecular mass mucin is secreted and displays an anomalous molecular mass on SDS/PAGE of approximately 650 kDa. Polymorphism in precursor size was found among different rats, suggesting genetic heterogeneity. Molecular mass, biosynthesis and secretion of RCM appeared similar to human MUC2. Moreover, RCM precursor could be immunoprecipitated using specific anti-(human MUC2) antisera, indicating that the RCM can be designated rat MUC2. This study describes the biosynthesis of two homologous mucins in two different species. The high degree of similarity suggests functional analogy.

Oviductins possess chitinase- and mucin-like domains: a lead in the search for the biological function of these oviduct-specific ZP-associating glycoproteins

Over the last 10 years considerable progress has been made in the immunological and biochemical characterization of oviduct-specific glycoproteins. It is now well established that a subclass of these secretory products, designated as oviductins, associate with the zona pellucida of the ovulated oocyte and with the early embryo. Recent reports on the cloning of cDNAs of oviductins from various species, including that of golden hamster (Mesocricetus auratus) oviductin by our laboratory, allowed us to compare their deduced amino acid sequences with those of other proteins. Optimal alignment analysis showed that oviductins contain regions of significant similarity with catalytically inactive mammalian members of the bacterial and microfilarial chitinase protein family. Most importantly, a close examination of the hamster and human deduced amino acid sequences revealed that both glycoproteins possess contiguous Ser/Thr rich repeated units, clustered in their carboxy-terminal portions. These mucin-type motifs are similar in the hamster and human glycoprotein, although hamster oviductin contains more of these complete units. This striking feature might indicate that these molecules play a similar role to mucin-type glycoproteins, e.g., in protecting the oocyte and early embryo against attacks from their environment. We propose a model whereby oviductins are targeted to the oocyte via the interaction of their chitinase-like domains with specific oligosaccharide moieties of the zona pellucida. Once localized to this structure, oviductin molecules would act as a protective shield around the oocyte and early embryo by virtue of their densely glycosylated mucin-type domains.

Cloning and analysis of human gastric mucin cDNA reveals two types of conserved cysteine-rich domains

Human gastric mucin was isolated by successive CsCl-gradient ultracentrifugation in the presence of guanidinium hydrochloride to prevent degradation of the polypeptide moieties of the molecules. The amino acid sequence of a tryptic fragment of this molecule was identical to that of a tryptic fragment of tracheobronchial mucin. An oligonucleotide based on this sequence hybridized specifically to human stomach mRNA and was subsequently used to screen a human stomach lambda ZAPII cDNA library. The largest of 10 positive clones encoded 850 amino acid residues, including the tryptic fragment, with high amounts of threonine, serine and proline residues. Interestingly, cysteine accounted for almost 8% of the amino acid residues. The 3' part of the sequence was very similar but not identical to the 3' region of human tracheobronchial cDNA. No tandem repeated sequences were present and the deduced polypeptide sequence contained two potential N-linked glycosylation sites. Four cysteine-rich clusters were detected, one of which was apparently homologous to the D-domains present in other mucins and in von Willebrand factor. The arrangement of the cysteines in three other cysteine-rich clusters was conserved in the human gastric mucin cDNA in a similar fashion as in two domains in the MUC2 gene product. The cysteine-rich domains were separated by short stretches of non-repetitive amino acid residues with a very high content of threonine and serine residues. These data suggest that the encoded polypeptide of this clone may be involved in disulphide-bond-mediated oligomerization of the mucin, and provide new insights into the molecular organization of mammalian apomucins.

Prediction of O-glycosylation of mammalian proteins: specificity patterns of UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase

The specificity of the enzyme(s) catalysing the covalent link between the hydroxyl side chains of serine or threonine and the sugar moiety N-acetylgalactosamine (GalNAc) is unknown. Pattern recognition by artificial neural networks and weight matrix algorithms was performed to determine the exact position of in vivo O-linked GalNAc-glycosylated serine and threonine residues from the primary sequence exclusively. The acceptor sequence context for O-glycosylation of serine was found to differ from that of threonine and the two types were therefore treated separately. The context of the sites showed a high abundance of proline, serine and threonine extending far beyond the previously reported region covering positions -4 through +4 relative to the glycosylated residue. The O-glycosylation sites were found to cluster and to have a high abundance in the N-terminal part of the protein. The sites were also found to have an increased preference for three different classes of beta-turns. No simple consensus-like rule could be deduced for the complex glycosylation sequence acceptor patterns. The neural networks were trained on the hitherto largest data material consisting of 48 carefully examined mammalian glycoproteins comprising 264 O-glycosylation sites. For detection neural network algorithms were much more reliable than weight matrices. The networks correctly found 60-95% of the O-glycosylated serine/threonine residues and 88-97% of the non-glycosylated residues in two independent test sets of known glycoproteins. A computer server using E-mail for prediction of O-glycosylation sites has been implemented and made publicly available. The Internet address is NetOglyc@cbs.dtu.dk.

The human MUC2 mucin apoprotein appears to dimerize before O-glycosylation and shares epitopes with the 'insoluble' mucin of rat small intestine

Rabbit antiserum against a synthetic peptide corresponding to a tandemly repeated amino acid sequence in the human intestinal mucin apoprotein MUC2 was used in immunoprecipitation to study the biosynthesis of MUC2 in the colon-carcinoma cell line LS 174T. Under non-reducing conditions, two bands were precipitated, the smaller with an apparent size of about 700 kDa on SDS/PAGE. When analysed by two-dimensional electrophoresis after reduction, the larger band migrated to the same position as the smaller band and was interpreted as a putative disulphide-bond-stabilized dimer. Pulse-chase experiments showed only the monomer after 5 min and the appearance of the putative dimer after 30 min. The MUC2 apoprotein was also precipitated by antisera against the HF-deglycosylated peptides of the two highly glycosylated domains of the 'insoluble' mucin complex of rat small intestine [Carlstedt, Herrmann, Karlsson, Sheehan, Fransson and Hansson (1993) J. Biol. Chem. 268, [18771-18781]. Endoprotease Lys-C cleavage of the immunopurified apoprotein gave a large fragment of about 250 kDa that was detected by both the antiserum against the MUC2 tandem repeat and one of the glycopeptide antisera. This supports the view that the 'insoluble' mucin of rat small intestine is encoded by the Muc2 gene, as recently indicated by a partial cDNA sequence [Hansson, Baeckström, Carlstedt and Klinga-Levan (1994) Biochem. Biophys. Res. Commun. 198, 181-190] and that parts of the apoprotein are conserved between the species. A lectin from the snail Helix pomatia that detects terminal alpha-GalNAc residues did not bind to the monomer or putative dimer, suggesting that O-glycosylation starts after dimerization. The results indicate that the biosynthetic pathway of the MUC2 mucin may be similar to that of the von Willebrand factor with which MUC2 shares sequence similarities at its C- and N-termini.

Expression of the leukocyte-associated sialoglycoprotein CD43 by a colon carcinoma cell line

The colon adenocarcinoma cell line COLO 205 secretes L-CanAg, a mucin-like glycoprotein carrying the carcinoma-associated sialyl-Lewis a carbohydrate epitope. In an attempt to identify its apoprotein, an NH2-terminal peptide sequence was obtained from purified L-CanAg. In all interpretable positions, this sequence showed 100% identity to the NH2-terminal of human CD43 (leukosialin, sialophorin), a plasma membrane-bound sialoglycoprotein hitherto only identified in leukocytes and other hematopoietic cells. An antiserum against deglycosylated L-CanAg and an anti-CD43 antiserum both immunoprecipitated a 61-kDa band, interpreted as the CD43 precursor, from COLO 205 cells as well as from the known CD43-expressing cell line HL-60. Results from immunoprecipitations following pulse-chase experiments and tunicamycin treatments were in agreement with earlier studies on the CD43 precursor. RNA blot analysis confirmed the expression of CD43 by the COLO 205 cell line, whereas three other colon carcinoma cell lines were negative. The glycosylation-dependent monoclonal antibody Leu-22, which recognizes leukocyte CD43, failed to bind L-CanAg, probably due to its much more extensive glycosylation. We conclude that L-CanAg is the secreted extracellular domain of a novel glycoform of CD43 and that CD43, if expressed in other carcinoma cells, may have escaped notice in studies relying on glycosylation-dependent monoclonal antibodies against leukocyte CD43.

Chaperone function of a Grp 94-related protein for folding and transport of the pancreatic bile salt-dependent lipase

In its fundamental attributes, the secretion pathway of the pancreatic bile salt-dependent lipase (BSDL) followed that described for all enzymes involved in regulated secretion. This route was inhibited by drugs that affect protein synthesis and intracellular transport. In the presence of monensin, BSDL was solely detected in microsome membrane fractions. The association of BSDL with intracellular membranes involved a protein complex, formed by at least two proteins of 94 and 56 kDa. In cells experiencing the metabolic stress due to azetidine-2-carboxylic acid, BSDL was additionally associated with a protein of 46 kDa. Affinity blotting showed that BSDL bound directly to the 94-kDa protein (p94). It was suggested that p94 could be a molecular chaperone, further identified as related to the 94-kDa glucose regulated protein (Grp 94). The membrane-associated BSDL (i.e. BSDL bound to the Grp 94-related p94) was O- and N-glycosylated and consequently appeared released from membranes in the trans-Golgi compartment. Therefore and for the first time, it is suggested that a multiprotein complex including the chaperone Grp 94-related p94 protein may play an essential role in the folding and transport of BSDL. One hypothesis is that the association of BSDL with membrane via the Grp 94-related p94 along its secretion pathway is required for its complete O-glycosylation, which occurs on the extended mucin-like structures present on the C-terminal part of the protein.

Biochemical, histochemical and cell biological investigations on the actions of mistletoe lectins I, II and III with human breast cancer cell lines

Cytotoxicity of the mistletoe lectins I, II and III towards six human breast cancer cell lines was assessed using the Mossman assay. In addition, binding of the three mistletoe lectins to the separated membrane glycoproteins of these cell lines, the binding and uptake of these lectins into the cells in tissue culture and the binding of the lectins to histological preparations of these cell lines were analysed. The results indicate that there are quantitative differences concerning the toxicity of these three lectins towards the different cell lines. Furthermore, the lectin binding pattern in the cell lines differed. In Western blots, several membrane glycoproteins were labelled by the lectins. Our results indicate subtle differences between the three lectins with regard to the parameters mentioned above; however, the toxicity of all three lectins from mistletoe is so similar that they all seem suitable for the construction of immunotoxins.

MUC-1 epithelial tumor mucin-based immunity and cancer vaccines

Many obstacles still stand in the way to eliciting an effective immune response against cancer, even though several antigens and antigenic peptides have been identified as potential tumor targets. All of them, including the MUC-1 mucin, share the caveat of being normal cellular proteins. Unlike all the others, however, MUC-1 expressed on tumors can still be considered a truly tumor-specific antigen. Its expression on normal cells is hidden from the immune system, and its aberrant glycosylation on tumors creates new epitopes recognized by the immune system. Moreover, all other tumor targets identified so far are MHC-restricted peptides that can only be recognized by patients who carry a specific HLA type, or on tumors which continue to express particular HLA alleles. MUC-1 is powerfully different. Recognized as a native molecule independent of MHC, it is a universal immunogen and a universal target, and if made effectively immunogenic, it would be expected to elicit immune responses in all patients, and against numerous MUC-1 expressing human tumors. It may, in fact, be the extraordinary solution to an extraordinary problem of cancer immunity and immunotherapy.

Distribution of sialosyl Tn and Tn antigens within normal and malignant colorectal epithelium

Expression of the core blood group structures sialosyl Tn (STn) and Tn is regarded as a colorectal cancer-specific change reflecting truncated synthesis of the oligosaccharide component of goblet cell mucin. The distribution of STn and Tn in normal and malignant epithelium has been studied in detail by a combination of mucin-, lectin-, and immunohistochemistry with and without pretreatment with potassium hydroxide (KOH), neuraminidase, and KOH-neuraminidase. When O-acetylated sialic acid (neuraminidase-resistant) is converted by saponification to non-O-acetylated sialic acid (neuraminidase-sensitive), normal colorectal goblet cells (mainly of the lower two-thirds of crypts) are immunoreactive with the monoclonal antibody TKH2 (specific for STn). This immunoreactivity is abolished by the interposition of neuraminidase, but goblet cells then become immunoreactive with Hb-Tn1 (specific for Tn). While colorectal cancer mucin expresses STn, expression of Tn is not seen in either goblet cell mucin or extracellular material showing the morphological and histochemical characteristics of secretory mucin. Tn expression in cancers is mainly limited to the Golgi zone and in a proportion of cases to cytoplasm and apical membrane (glycocalyx) of columnar cells and inspissated material within lumina. The material reacting with Hb-Tn1 may be upregulated, membrane-associated MUC1 glycoprotein rather than MUC2 or MUC4 goblet cell mucin. The presence of STn and cryptic Tn within normal colorectal goblet cells and the absence of Tn expression within colorectal cancer secretory mucin contradicts the generally accepted concept of cancer-specific incomplete glycoprotein synthesis within these neoplasms.

Isolation and characterization of cDNA clones encoding pig gastric mucin

Polyclonal antibodies raised to deglycosylated pig gastric mucin were used to screen a cDNA library constructed with pig stomach mucosal mRNA. Immunocytochemistry indicated that the antibody recognizes intracellular and secreted mucin in surface mucous cells of pig gastric epithelium. A total of 70 clones producing proteins immunoreactive to this antibody were identified, two of which (PGM-2A,9B) were fully sequenced from both ends. Clone PGM-9B hybridized to a polydisperse mRNA (3-9 kb) from pig stomach, but not liver, intestine or spleen, nor to mRNA from human, mouse, rabbit or rat stomach. Sequence analysis indicated that PGM-9B encodes 33 tandem repeats of a 16-amino-acid consensus sequence rich in serine (46%) and threonine (17%). Using the restriction enzyme MwoI, which has a single target site in the repeat, it was demonstrated that PGM-9B consists entirely of this tandem repeat. Southern-blot analysis indicated that the repeat region is contained in a 20 kb HindIII-EcoRI fragment, and BamHI digestion suggested that most of the repeats are contained in a 10 kb fragment. In situ hybridization with an antisense probe to PGM-9B showed an intense signal in the entire gastric gland. Clone PGM-2A also contains the same repeat sequence as 9B, but, in addition, has a 64-amino-acid-long non-repeat region at its 5' end. Interestingly the non-repeat region of PGM-2A has five cysteine residues, the arrangement of which is identical with that reported for human intestinal mucin gene MUC2.

The filamentous brush border glycocalyx, a mucin-like marker of enterocyte hyper-polarization

The probably sole constituent of the filamentous brush border glycocalyx, which has been defined on the basis of electron microscopic data as a set of filaments radiating from the tip of rabbit intestinal brush border microvilli, has been purified. It consists of a mucin-type glycoprotein that can be solubilized by either Triton extraction or papain treatment of the brush border membrane vesicles but is insensitive to phosphatidylinositol phospholipase C. The detergent- and papain-solubilized forms both have the same apparent molecular mass of 400 kDa (SDS/PAGE). This suggests that the filamentous brush border glycocalyx may be anchored to the membrane by a small hydrophobic peptidic tail. Ser, Thr, Pro and Ala amount to 65% of the protein core amino acid residues. The glycosidic moiety, which amounts to 73% of the molecular mass, has high O-acetylated sialic acid contents. A monoclonal antibody (3A4) raised against the purified material was produced which specifically recognized the 400-kDa band by immunoprecipitation and immunoblotting, and the filamentous brush border glycocalyx of villus enterocytes when jejunum sections were immunolabelled. The 3A4 determinant was identified with a filamentous brush border glycocalyx-specific carbohydrate structure containing an O-acetylated sialic acid. The fact that the labeled glycocalyx was anchored entirely in a membrane microdomain at the tip of the microvilli shows that mature enterocytes are hyper-polarized epithelial cells.

Compaction assay: a rapid and simple in vitro method to assess the responsiveness of a biopolymer matrix to enzymatic modification

A rapid and simple in vitro method is described which measures the extent of unrecoiled solids compression when a complex biopolymer is subjected to a centrifugal force. This method, termed the compaction assay, was used to assess the response of purulent cystic fibrosis (CF) sputum samples to the addition of recombinant human deoxyribonuclease I (rhDNase). Enzyme treatment resulted in a dramatic decrease in DNA size, a redistribution of total DNA content from the pellet to supernatant, a significant decrease in that pellet volume and a decrease in elastic modulus. Sample elasticity, measured by a dynamic cone and plate viscometer, could be related to compaction assay results. These results suggest that the compaction assay may be a useful in vitro method for rapidly assessing the actions of enzymatic disruption of a complex biopolymer, such as that observed for the actions of rhDNase on purulent airway secretions.

Prostaglandin E2 alters terminal glycosylation of high molecular weight glycoproteins, released by pig gastric mucous cells in vitro

The gastric mucus layer consists of high molecular weight glycoproteins (HMG). E-Type prostaglandins (PGs) stimulate total HMG release from isolated gastric mucous cells. We determined the effects of PGE2 on HMG glycosylation. Pig gastric mucous cells were cultured for 20 h with 1 mumol/l PGE2. Released HMG were isolated by gel chromatography and periodic acid-Schiff (PAS)-positive sugars and protein-bound [14C]GlcNAc were determined. Monosaccharides terminally linked to HMG oligosaccharide chains were monitored by lectin enzyme linked immunosorbent assay (ELISA): N-acetylglucosamine (GlcNAc) with Datura stramonium agglutinin, N-acetylgalactosamine (GalNAc) with soy bean agglutinin, fucose (Fuc) with Ulex europaeus I agglutinin and sialic acids (Sial) with Sambucus nigra agglutinin. PGE2 stimulated total HMG release, indicated by an increase of PAS-positive sugars to 170% and [14C]GlcNAc to 220% of controls. Terminal GlcNAc increased to 128%, GalNAc to 133%, Fuc to 165% and Sial to 182%. In addition to stimulation of total HMG release, PGE2 caused alterations of HMG glycosylation, which may modulate HMG viscosity and microbiological barrier function.

Molecular anatomy of an endodermal gland: investigations on mucus glycoproteins and cell turnover in Brunner's glands of Didelphis virginiana using lectins and PCNA immunoreactivity

Brunner's glands are located in the submucosa of the proximal duodenum and are unique to mammalian species. The North American opossum (Didelphis virginiana) is generally regarded as a prototype marsupial that closely resembles fossil didelphids which can be placed at the beginning of mammalian evolution. The current investigation provided an opportunity for the analysis of secretory products from these glands in a species thought to be more closely related to earlier evolutionary forms. Extracts of Brunner's glands were subjected to SDS-PAGE and Western blotting. The results indicate the presence of two high molecular weight PAS-positive glycoprotein bands. In addition to these two PAS-positive bands, several other glycoprotein bands were detected in the high molecular weight range that bind several lectins which typically recognize O-linked carbohydrates indicative of mucus type glycoproteins. The same lectins bind to glandular structures in tissue sections. Comparison of lectin binding sites with the pyloric glands of the stomach and duodenal goblet cells indicates that Brunner's glands carbohydrate residues resemble those of the pyloric glands more closely than those of the duodenal goblet cells. The low cell turnover rate in Brunner's glands is in contrast to the rapid turnover rate of goblet cell precursors in the duodenal crypts. The mucus composition and the cell turnover rate correlate well with embryological data and suggest that Brunner's glands of Didelphis evolved from an epithelium more closely associated with the stomach than that of the duodenum as the topography of the gland would suggest.

Biochemical composition of human saliva in relation to other mucosal fluids

This paper describes several salivary components and their distribution in other mucosal secretions. Histatins are polypeptides which possess exceptional anti-fungal and anti-bacterial activities, but are nevertheless present only in saliva. Proline-rich proteins (PRPs) are members of a closely related family, of which the acidic PRPs are found solely in saliva, whereas the basic PRPs are also found in other secretions. Mucins are a group of glycoproteins that contribute to the visco-elastic character of the mucosal secretions. Despite the similarities in their structure and behavior, mucins have distinct tissue distributions and amino acid sequences. Other salivary proteins are present in one or more mucosal secretions. Lysozyme is an example of a component belonging to an ancient self-defense system, whereas secretory immunoglobulin A (sIgA) is the secreted part of a sophisticated adaptive immune system. Cystatins are closely related proteins which belong to a multigene family. Alpha-Amylase is a component that is believed to play a specific role in digestion, but is nevertheless present in several body fluids. Kallikrein and albumin are components of blood plasma. But whereas albumin diffuses into the different mucosal secretions, kallikrein is secreted specifically by the mucosal glands. The presence of these proteins specifically in saliva, or their distribution in other mucosal secretions as well, may provide important clues with respect to the physiology of those proteins in the oral cavity.

Characterization of glycoprotein glycan receptors for Escherichia coli F17 fimbrial lectin

The specificity of N-acetylglucosamine-binding F17 fimbrial lectin of coli (also called FY or Att25) for the oligosacharide structures was investigated by mannose-resistant hemagglutination inhibition tests and direct binding assays. The linkage position of N-acetyl-glucosamine influenced its affinity in the preferential order of beta 1-3 > beta 1-6 > beta 1-4 beta > beta 1-2. Minimal GlcNAc beta 1-3Gal beta 1-sequence strongly bound to F17 lectin, whether located in terminal nonreducing position or internally in carbohydrate moieties. F17 lectin specifically interacted with this unit in O-glycosidically linked oligosaccharides of the bovine glycophorins and intestinal mucins. On the contrary, GlcNAc beta 1-NAsn, GlcNAc beta 1-6Man beta- and GlcNAc beta 1-4GlcNAc- of N-glycosylated proteins failed to bind to the lectin. Our findings emphasized the presence of multiple F17 mucosal receptor complex in the newborn calf intestines. Furthermore, the density of receptors for F17 fimbrial lectin seemed to depend on the age of the calf and the intestinal segment.

Sulphated macromolecules produced by in vivo labelling in the rat gastric mucosa

The aim of this study was to investigate the nature and distribution of sulphated macromolecules of the extracellular matrix in rat gastric mucosa. This was achieved by developing an in vivo labelling system. An intraperitoneal injection of 1 mCi [35S]-sulphate was given for either 4 h (0.01% incorporation into macromolecular fraction) or 8 h (0.13% incorporation). At the end of the labelling period the stomach was removed and the mucosa and submucosa was either taken as a single combined sample or separated into four layers by blunt dissection. Each sample was papain digested and analysed by ion-exchange chromatography. This analysis revealed sulphated species of differing charge existing in differing proportions throughout the mucosa. These sulphated species eluted at NaCl concentrations of approximately 0 (A), 0.19 (B), 0.34 (C) and 0.78 mol/L (D) from a Q-Sepharose ion exchange column. Further analysis by size exclusion chromatography and chemical and enzymatic digestion showed that peaks B and C had molecular weights of 2.4 x 10(5) and 2.8 x 10(5), respectively and were resistant to chondroitinase ABC, heparitinase and nitrous acid digestion. Peak D was found to contain a polydisperse population of molecules with a molecular weight range of approximately 1 x 10(4) to 6 x 10(4). This sample was susceptible to nitrous acid and chondroitinase ABC digestion and was found predominantly in the sample isolated from deeper in the tissue. We have thus developed an in vivo labelling technique for sulphated macromolecules that can be used in the further study of injury to the gastric mucosa.

Strategy for the investigation of O-linked oligosaccharides from mucins based on the separation into neutral, sialic acid- and sulfate-containing species

A method for the separation of O-linked oligosaccharides into neutral, sialic acid-containing and sulfated species was applied to oligosaccharides released by alkaline borohydride from mucin glycopeptides from porcine small intestine. The released mixture of reduced oligosaccharides was applied to an anion exchange column, and the neutral oligosaccharides were collected as the unretarded fraction. A mixture of dimethyl sulfoxide and iodomethane was passed through the column to convert the sialic acid-containing oligosaccharides into methyl esters that were eluted and converted to methyl amides by methyl amine. Finally the sulfated oligosaccharide fraction was eluted with salt. The neutral and the derivatized sialic acid-containing oligosaccharides were analysed by gas chromatography-mass spectrometry after permethylation and the sulfated oligosaccharide fraction was analysed by high performance anion exchange chromatography.

Characterisation of the single copy trefoil peptides intestinal trefoil factor and pS2 and their ability to form covalent dimers

A bacterial recombinant expression system was established to produce biologically active rat Intestinal Trefoil Factor (rITF). Characterisation of purified rITF shows that both monomers and dimers can be observed under reducing and non-reducing conditions, respectively. Site-directed mutagenesis studies show that Cys57 is necessary for rITF dimer formation. Samples of human gastrointestinal tissue following biopsy also demonstrated the presence of reducible human pS2 and ITF covalent dimers. Three-dimensional models for pS2 and ITF support the hypothesis that both pS2 and ITF can exist as disulphide-linked dimers in vivo and that any proposed function for these peptides must take dimer formation into account.

Development of active specific immunotherapeutic agents based on cancer-associated mucins

As a result of aberrant glycosylation, cancer-associated mucins expose to the immune system certain carbohydrate, peptide, and possibly glycopeptide epitopes that are not exposed on the normal mucins. This provides the basis for our development of synthetic carbohydrate, peptide, and glycopeptide-based ASI agents corresponding to the cancer-associated mucin epitopes. Our studies on ASI formulations based on carbohydrate structures such as TF and STn have demonstrated their ability to induce immune response relevant to the native epitopes on the cancer cells in animal models and in cancer patients. Further, such immune responses were able to mediate cancer rejection in an animal model. Similar studies on peptide epitopes of a cancer-associated mucin, MUC1, have also shown the ability of the synthetic antigen to induce anticancer immune responses in an animal model. Ongoing studies on the carbohydrate and peptide epitopes would allow us to define the most important target structures on cancer-associated mucins that can selectively stimulate cancer-specific immune responses. Our long-term goal is to develop multiepitopic glycopeptide ASI formulations capable of stimulating strong CMI responses against common carcinomas.

Intestinal brush border glycohydrolases: structure, function, and development

The hydrolytic enzymes of the intestinal brush border membrane are essential for the degradation of nutrients to absorbable units. Particularly, the brush border glycohydrolases are responsible for the degradation of di- and oligosaccharides into monosaccharides, and are thus crucial for the energy-intake of humans and other mammals. This review will critically discuss all that is known in the literature about intestinal brush border glycohydrolases. First, we will assess the importance of these enzymes in degradation of dietary carbohydrates. Then, we will closely examine the relevant features of the intestinal epithelium which harbors these glycohydrolases. Each of the glycohydrolytic brush border enzymes will be reviewed with respect to structure, biosynthesis, substrate specificity, hydrolytic mechanism, gene regulation and developmental expression. Finally, intestinal disorders will be discussed that affect the expression of the brush border glycohydrolases. The clinical consequences of these enzyme deficiency disorders will be discussed. Concomitantly, these disorders may provide us with important details regarding the functions and gene expression of these enzymes under specific (pathogenic) circumstances.

Helix pomatia agglutinin binding is a useful prognostic indicator in colorectal carcinoma

BACKGROUND

Most deaths from colorectal carcinoma are due to metastases. A relatively reliable prognostic indicator at surgery to date is the Dukes' stage, but this is a morphologic approach that does not elucidate biochemical changes to explain why cells became metastatic. The binding sites for the lectin from the Roman snail Helix pomatia (HPA) were shown to be good prognostic indicators in breast and gastric cancer, and accordingly, this study was performed to evaluate the use of HPA binding sites as prognostic markers in colorectal carcinoma.

METHODS

The histochemically detected expression of HPA binding sites in colorectal carcinomas (n = 130) was increased. The results of the histochemical findings were correlated with patient survival and tumor recurrence.

RESULTS

The results indicated that the prognosis for the groups of patients whose colorectal cancer cells binded to HPA in tissue sections was almost as bad as those with Dukes' Stage C disease.

CONCLUSION

Because HPA binds to N-acetylgalactosamine, the authors' results indicate that this sugar residue is at least partly involved in the process of human colorectal carcinoma cells metastasizing to regional lymph nodes and possibly also to distant sites.

Biosynthesis of a human gall-bladder mucin

Mucin glycoproteins play an important role in the initial stages of gall-stone formation by a currently largely unknown mechanism. Understanding the structure of gall-bladder mucin is necessary to comprehend the mechanism by which cholesterol monohydrate crystals aggregate. Three successive CsCl-gradient-ultracentrifugation steps were used to purify human gall-bladder mucin from gall-bladder tissue. The isolated macromolecules had a typical mucin-like monosaccharide composition and appeared as heterogeneous high-M(r) glycoproteins on SDS/PAGE. A polyclonal antiserum was raised against these molecules and the specificity of the antiserum was ascertained by immunoblotting. The antiserum specifically stained mucous granules at the apical side of all gall-bladder epithelial cells in neck, fundus and body. The antibody was subsequently used to immunoprecipitate the mucin and biosynthetic intermediates from gall-bladder-tissue homogenates. An early biosynthetic precursor of the isolated mucin was identified by SDS/PAGE as a single polypeptide with an apparent M(r) of approx. 470,000. This precursor protein was converted after 1 h into a heterogeneous high-M(r) glycoconjugate with an electrophoretic mobility similar to that of the purified mucin. The mature mucin, but not the precursor, was secreted into the culture medium, starting at 1 h. As shown by SDS/PAGE under non-reducing conditions, the precursors form disulphide-linked oligomers. Using the N-glycosylation inhibitor tunicamycin, the apparent M(r) of the precursor was decreased to approx. 410,000, indicating that N-linked glycan chains are attached to the precursor polypeptide.

Identification of a human gastric mucin precursor: N-linked glycosylation and oligomerization

Gastric mucin plays an important role in the protection of the stomach wall from chemical, microbiological and mechanical damage. We have previously isolated human gastric mucus glycoproteins and raised a polyclonal antiserum against these macromolecules. This antiserum specifically reacted with gastric mucins in immunoblotting experiments and stained mucous granules at the apical side of gastric surface epithelial cells. A similar staining pattern was obtained after incubation with an antiserum against rat gastric mucin. Next we used the antiserum in pulse-chase experiments of human stomach tissue explants. After short labelling periods with [35S]methionine and [35S]cysteine, the antiserum reacted with a polypeptide with an apparent molecular mass of approx. 500 kDa as determined by SDS/PAGE, which was converted after 90 min into a heterogeneous high-molecular-mass glycoprotein. This high-molecular-mass form, but not the 500 kDa polypeptide, was detectable in the culture medium after 2 h. This strongly suggests that the 500 kDa polypeptide is the precursor of the purified gastric mucin. Analysis of pulse-chase experiments by non-reducing SDS/PAGE revealed that the precursors form disulphide-linked oligomers early in biosynthesis, before the addition of O-linked sugars. After preincubation with the N-glycosylation inhibitor, tunicamycin, the apparent molecular mass of the precursor decreased marginally but consistently, indicating that N-linked glycan chains are present on the mucin precursor.

Biosynthesis of human colonic mucin: Muc2 is the prominent secretory mucin

BACKGROUND/AIMS

Human colonic epithelium produces large amounts of mucin. The aim of this study was to examine mucin biosynthesis in the human colon.

METHODS

Human colonic mucin was isolated using CsCl density gradients, and polyclonal antiserum was raised. Biosynthesis of colonic mucins was studied by labeling colonic explants with 35S-labeled amino acids or [35S]sulfate and subsequent immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE).

RESULTS

The polyclonal antiserum specifically recognized colonic mucin, primarily reacting with peptide epitopes. Biosynthetic pulse/chase experiments showed a 35S-amino acid-labeled mucin precursor of about 600 kilodaltons, which was converted into a mature, glycosylated, and sulfated mucin and subsequently secreted into the medium. This mature mucin comigrated with isolated colonic mucin with an apparent molecular weight of 550 kilodaltons on SDS-PAGE, whereas gel filtration indicated that the molecular weight is actually much larger. Independent immunoprecipitation with an anti-Muc2 antiserum showed cross-reactivity with the 600-kilodalton precursor.

CONCLUSIONS

These results show the biosynthesis of a secretory colonic mucin for the first time. This mucin is synthesized as a precursor protein of approximately 600 kilodaltons, which, after glycosylation, is secreted as a glycoprotein with an apparent molecular weight of 550 kilodaltons on SDS-PAGE. It is very likely that this mucin is Muc2.

Glycosylation patterns of the human colon cancer cell line HT-29 detected by Helix pomatia agglutinin and other lectins in culture, in primary tumours and in metastases in SCID mice

Human colonic cancer cells (HT-29, 10(7) cells/dose) were injected subcutaneously between the scapulae of 19 severe combined immunodeficient (SCID) mice. After 19 days, large tumours had developed in 18 out of the 19 animals and the mice were then killed. Metastases were detected in the lungs of 16 animals but not in other organs investigated. Surgical removal of the primary tumour in another group of five animals led to a prolonged survival and further growth of metastases in the lungs. HT-29 injection into the tail vein (n = 5) resulted in colonization of the lungs. The tumours that developed in the animals were signet cell carcinomas; these forms are not seen in HT-29 cells in culture. Glycoconjugate expression of the tumours was assessed using several lectins. In many cases the results indicated a stability of lectin-binding patterns from cell culture conditions to implantation into the SCID mice. This was true for the lectin Helix pomatia agglutinin (HPA), the binding of which is associated with a high metastatic potential in some human tumours, including colon cancer. All the primary tumours and metastases were HPA positive. This xenograft tumour model seems to be a clinically relevant system for the study of glycoconjugate expression in human colon cancer cells and their metastases.

Monoclonal antibody TKH2 to the cancer-associated epitope sialosyl Tn shows cross-reactivity with variants of normal colorectal goblet cell mucin

It has been suggested that synthesis of mucin-type carbohydrate chains may be arrested at the core structure Tn. This may occur following premature sialylation of Tn giving the structure STn. TKH2, a monoclonal antibody to STn, is regarded as showing highly specific reactivity with colorectal and other epithelial neoplasms. In this study we have shown that TKH2 cross-reacts with normal colorectal goblet cells that are mild PAS positive (specific for non-O-acetylated sialic acid). Approximately 9% of caucasians secrete colorectal mucin in which sialic acid is non-O-acetylated. TKH2 also cross-reacts with mild PAS negative colorectal goblet cells when sections have been pre-treated with potassium hydroxide (to remove O-acetyl groups from sialic acid). These findings make it likely that the usual non-reactivity evidenced by TKH2 for normal colorectal goblet cells is due to the presence of sialic acid that is heavily O-acetylated. The reactivity of TKH2 for colorectal cancer mucin can be readily explained by the loss of sialic acid O-acetyl substituents and not necessarily by incomplete synthesis of mucin-type carbohydrate chains.

The endometrial cell surface and implantation. Expression of the polymorphic mucin MUC-1 and adhesion molecules during the endometrial cycle

The cell surface mucin MUC-1 is present in endometrial epithelial cells and their associated apical glycocalyx and is also released into gland lumens as a secretory product. MUC-1 mRNA and core protein are found at low levels in the proliferative phase of the cycle, but their abundance increases after ovulation. Endometrial MUC-1 has been found to carry sialokeratan sulphate chains and these show a dramatically increased abundance in cells and secretions in the post-ovulatory phase of the cycle, reaching a maximum in secretions 6-7 days after the LH peak. The apical epithelium also contains adhesion receptor molecules of the integrin and CD44 families. MUC-1 is large and highly glycosylated and probably extends farther from the cell surface than these 'conventional' glycoprotein receptors. It has the potential to inhibit sterically receptor-mediated cell-cell adhesion. However, it is also possible that MUC-1 displays specific (e.g., glycan) recognition structures for the initial attachment of the blastocyst or that the embryo may create a specialised microenvironment in which to implant.

Structure, biosynthesis, and function of salivary mucins

The glandular secretions of the oral cavity lining the underlying buccal mucosa are highly specialized fluids which provide lubrication, prevent mechanical damage, protect efficiently against viral and bacterial infections, and promote the clearance of external pollutants. This mucus blanket contains large glycoproteins termed mucins which contribute greatly to the viscoelastic nature of saliva and affect its complex physiological activity. The protein core of mucins consists of repetitive sequences, rich in O-glycosylated serine and threonine, and containing many helix-breaking proline residues. These features account for the extended, somewhat rigid structure of the molecule, a high hydrodynamic volume, its high buoyant density, and high viscosity. The oligosaccharide moiety of salivary mucins accounts for up to 85% of their weight. The oligosaccharide side chains exhibit an astonishing structural diversity. The isolation, composition, structure, molecular characteristics, and functional relevance of salivary mucins and their constituents is discussed in relation to recent advancements in biochemistry and molecular biology.

Synthesis and secretion of mucin by the human colonic tumour cell line LS180

Pulse-chase labelling experiments were performed using the mucin-producing colonic carcinoma cell line LS180. Cells were pulsed with [3H]threonine or [3H]glucosamine and chased in complete media without radiolabel for various lengths of time. From cell and media extracts obtained at each time point, mucin proteins were immunoprecipitated with specific anti-mucin antibodies and analysed by SDS/PAGE and fluorography. At short labelling times with [3H]threonine, without chase, a monomeric thiol-reduction-resistant mucin precursor of apparent molecular mass > 670 kDa was identified. The precursor, in contrast to oligomeric species, was not labelled by [3H]glucosamine but exhibited binding to Vicia villosa isolectin B4, suggesting the presence of some core GalNAc residues. Treatment with tunicamycin to inhibit N-glycosylation had no effect on the apparent mass of the precursor. Identity of the mucin antigen with MUC2 mucin was established by immunoprecipitation with antibodies specific for a MUC2 tandem repeat and C-terminal regions. With increasing chase time the precursor was replaced by thiol-reduction-sensitive mucin oligomers that reached peak intracellular radiolabelling with [3H]threonine by 2 h of chase, and then declined. Only oligomeric mucin was secreted into the medium. Secretion of [3H]threonine-labelled mucin was detectable after 2 h of chase and increased as the cytoplasmic mucin label declined. Monensin inhibited [3H]glucosamine incorporation, sialylation and baseline (non-regulated) mucin secretion without affecting initial [3H]threonine incorporation or oligomerization. Oligomerization and Golgi transport are therefore essential early steps in MUC2 mucin secretion. Oligomerization may follow some core O-glycosylation with GalNAc, but precedes elongation of oligosaccharide chains.

Production and characterization of monoclonal antibodies against guinea pig tracheal mucins

Thirty-five hybridomas that secrete mouse monoclonal antibodies (MAb) against guinea pig (G.P.) tracheal mucins were established. The MAbs were characterized immunologically, biochemically, and immunohistochemically at both light and electron microscopic levels. Isotyping of the MAbs revealed 14 to be IgM, 13 IgG1, 3 IgG2, and 5 IgG3. The MAbs demonstrated various patterns of binding in immunoblots against mucins derived from G.P. tracheal explants. This suggested the presence of "subpopulations" of G.P. tracheal mucins with specific MAbs binding to different epitopes on the mucin molecules. Periodate oxidation indicated that 33 of the 35 MAbs recognized carbohydrate epitopes on the mucin molecules. Ten of the MAbs also reacted with both bovine and ferret tracheal mucins, while 7 and 6 MAbs bound only to bovine and ferret tracheal mucins, respectively. The generated MAbs should be useful for immunomeasurement of mucin secretion in vivo (e.g., in bronchoalveolar or airway lavage fluid) and in vitro (e.g., cell and organ cultures) from cells of guinea pig and (with certain MAbs) bovine and ferret origin.