Biosynthesis of two distinct types of mucin in HM3 human colon cancer cells

Microbial modulation of innate defense: goblet cells and the intestinal mucus layer

The gastrointestinal epithelium is covered by a protective mucus gel composed predominantly of mucin glycoproteins that are synthesized and secreted by goblet cells. Changes in goblet cell functions and in the chemical composition of intestinal mucus are detected in response to a broad range of luminal insults, including alterations of the normal microbiota. However, the regulatory networks that mediate goblet cell responses to intestinal insults are poorly defined. The present review summarizes the results of developmental, gnotobiotic, and in vitro studies that showed alterations in mucin gene expression, mucus composition, or mucus secretion in response to intestinal microbes or host-derived inflammatory mediators. The dynamic nature of the mucus layer is shown. Available data indicate that intestinal microbes may affect goblet cell dynamics and the mucus layer directly via the local release of bioactive factors or indirectly via activation of host immune cells. A precise definition of the regulatory networks that interface with goblet cells may have broad biomedical applications because mucus alterations appear to characterize most diseases of mucosal tissues.

Dimerization of the human MUC2 mucin in the endoplasmic reticulum is followed by a N-glycosylation-dependent transfer of the mono- and dimers to the Golgi apparatus

Pulse-chase experiments in the colon cell line LS 174T combined with subcellular fractionation by sucrose density gradient centrifugation showed that the initial dimerization of the MUC2 apomucin started directly after translocation of the apomucin into the rough endoplasmic reticulum as detected by calnexin reactivity. As the mono- and dimers were chased, O-glycosylated MUC2 mono- and dimers were precipitated using an O-glycosylation-insensitive antiserum against the N-terminal domain of the MUC2 mucin. These O-glycosylated species were precipitated from the fractions that comigrated with the galactosyltransferase activity during the subcellular fractionation, indicating that not only MUC2 dimers but also a significant amount of monomers are transferred into the Golgi apparatus. Inhibition of N-glycosylation with tunicamycin treatment slowed down the rate of dimerization and introduced further oligomerization of the MUC2 apomucin in the endoplasmic reticulum. Results of two-dimensional gel electrophoresis demonstrated that these oligomers (putative tri- and tetramers) were stabilized by disulfide bonds. The non-N-glycosylated species of the MUC2 mucin were retained in the endoplasmic reticulum because no O-glycosylated species were precipitated after inhibition by tunicamycin. This suggests that N-glycans of MUC2 are necessary for the correct folding and dimerization of the MUC2 mucin.

Mucin glycoproteins in neoplasia

Mucins are high molecular weight glycoproteins that are heavily glycosylated with many oligosaccharide side chains linked O-glycosidically to the protein backbone. With the recent application of molecular biological methods, the structures of apomucins and regulation of mucin genes are beginning to be understood. At least nine human mucin genes have been identified to date. Although a complete protein sequence is known for only three human mucins (MUC1, MUC2, and MUC7), common motifs have been identified in many mucins. The pattern of tissue and cell-specific expression of these mucin genes are emerging, suggesting a distinct role for each member of this diverse mucin gene family. In epithelial cancers, many of the phenotypic markers for pre-malignant and malignant cells have been found on the carbohydrate and peptide moieties of mucin glycoproteins. The expression of carbohydrate antigens appears to be due to modification of peripheral carbohydrate structures and the exposure of inner core region carbohydrates. The expression of some of the sialylated carbohydrate antigens appears to correlate with poor prognosis and increased metastatic potential in some cancers. The exposure of peptide backbone structures of mucin glycoproteins in malignancies appears to be due to abnormal glycosylation during biosynthesis. Dysregulation of tissue and cell-specific expression of mucin genes also occurs in epithelial cancers. At present, the role of mucin glycoproteins in various stages of epithelial cell carcinogenesis (including the preneoplastic state and metastasis), in cancer diagnosis and immunotherapy is under investigation.

Characterization of mucin in whole-gut lavage fluid obtained from patients with inflammatory bowel disease

Whole-gut lavage fluid, collected by administering an electrolyte lavage solution orally, was found to be an excellent and easily collectable source of abundant mucin. Furthermore, the biochemical features of the mucin from patients with ulcerative colitis and Crohn's disease were investigated. The mucin was separated into four fractions by Sepharose CL-4B, Sepharose CL-2B, and DEAE Sephacel chromatography. Compared with healthy subjects, the total yields of mucin from ulcerative colitis patients were low due to a deficiency of neutral mucin, whereas those from Crohn's disease patients were high, which was attributable mainly to high-molecular-weight mucin. The fucose and sulfate contents were low in ulcerative colitis, but only the former was low in Crohn's disease. The different biochemical features of the mucin obtained from whole gut lavage fluid appear to reflect mucosal pathological changes associated with inflammatory bowel disease.

Mucus glycoproteins and their role in colorectal disease

In this review, the nature and impact of progress in the study of mucins is outlined, emphasizing the current understanding of the structure and physiological function of these molecules in the colorectum. The use of new methods for preparation and separation has led to improvements in the analysis of mucins; these are detailed, as are their difficulties and pitfalls. Results obtained with these methods are correlated with long-established histochemical techniques and the use of chemical, lectin, and antibody reagents for general and specific detection of mucins in all procedures is described. Improvements in the detection and analysis of mucins in biopsy-size tissue samples and in larger numbers of individual clinical cases have now permitted a much wider approach to the pathological evaluation of mucin biology and progress with these techniques is outlined. The significance of the discovery of a family of mucin genes is presented and new concepts of mucin structure resulting from these studies are described. Bacterial degradation of the mucus layer at the surface of the colorectal mucosa is considered in line with the homeostatic relationship with mucosal mucin synthesis. Finally, the implications of abnormal mucins in colorectal disease are considered in the light of recent methodological advances.

Exogenous nitric oxide stimulates mucin secretion from LS174T colonic adenocarcinoma cells

The effect of exogenous and endogenous nitric oxide on the secretion of mucins from the human colonic adenocarcinoma cell-line LS174T was studied. Mucin secretion was followed by measuring the release of [3H]-glucosamine metabolically labelled glycoproteins eluted in the void volume of Sepharose 4B column chromatography. In response to exogenously produced nitric oxide from sodium nitroprusside, mucin secretion occurred in a time- and dose-dependent fashion that preceded epithelial cell damage. However, in the presence of the nitric oxide scavenger myoglobin, mucin secretion and cell damage were abrogated. Endogenously produced nitric oxide did not affect mucin secretion as the addition of excess L-arginine, the substrate for nitric oxide synthase, the removal of arginine from the culture medium with arginase or the inhibition of nitric oxide synthase with the competitive inhibitor NG-monomethyl-L-arginine had no effect on basal mucin release. These results suggest that exogenously produced nitric oxide can directly affect mucin secretion as a cytoprotective mechanism.

Early steps in the biosynthesis of MUC2 epithelial mucin in colon cancer cells

Expression of the MUC2 mucin has been demonstrated in normal gastrointestinal and respiratory epithelium and in carcinomas of the gastrointestinal and respiratory tracts, breast, ovary, and bladder using RNA probes and (or) monoclonal antibodies reactive with peptide epitopes on the 23 amino acid tandem repeat. Mouse monoclonal antibodies 4F1 and 3A2 were previously obtained by immunization with mucin derived from the LS174T colon cancer cell line and a KLH conjugate of a synthetic MUC2 VNTR peptide. These antibodies react with distinct epitopes on synthetic VNTR peptides and with normal and malignant epithelial tissues. In the present study, we examined the biosynthesis of MUC2 in LS174T colon cancer cells, using these antibodies to immunoprecipitate labelled mucin. A very high molecular mass protein was immunoprecipitated following 1 min pulse labelling with [3H]threonine and [3H]proline. A slight increase in molecular mass was observed over the next 16 min; however, unlike the MUC1 mucin, there was no large difference in apparent molecular mass between the MUC2 protein precursor and fully processed mucin using separation by SDS-PAGE. O-Glycosylation began within 1 h of synthesis of the protein core. Mucin secretion into the culture medium was detected in the 2nd hour following synthesis and was largely completed within 4 h of synthesis. Secreted mucin was far less reactive with these monoclonal antibodies than the precursor protein.

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.

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.

Localization of mucin (MUC2 and MUC3) messenger RNA and peptide expression in human normal intestine and colon cancer

BACKGROUND/AIMS

Several studies have reported Northern blot data showing that mucin is expressed in a tissue-specific manner. To determine whether expression is limited to specific cell types within these tissues requires histological analysis.

METHODS

Both immunocytochemistry and in situ hybridization were used to identify cell types expressing the MUC2 and MUC3 mucins in the human small intestine, colon, and colon carcinoma.

RESULTS

In the normal small intestine and colon, an antibody recognizing the MUC2 apomucin stained goblet cells. In contrast, an antibody recognizing the MUC3 apomucin stained both goblet and absorptive cells. Consistent with this, in situ hybridization showed MUC2 messenger RNA (mRNA) only in goblet cells and MUC3 mRNA in both goblet and absorptive cells. In several samples of moderately well-differentiated colon cancer, MUC2 and MUC3 showed distinct patterns of expression, but the expression level of each was reduced compared with levels in normal tissue; there was considerable tumor-to-tumor and cell-to-cell variability using both mucin antibodies and complementary DNA probes.

CONCLUSIONS

Individual mucin genes have distinct patterns of expression within mucin-producing tissues, suggesting that the various mucin gene products play distinct functional roles.