Biosynthesis of high molecular weight breast carcinoma associated mucin glycoproteins

Structure, Function and Gene Expression of Epithelial Mucins

In this review the main characteristics, i.e., structure, function and gene expression, of the different mucins are discussed. Mucin-type molecules consist of a core protein moiety (apomucin) where a number of carbohydrate chains are attached to serines and threonines by glycosidic bonds. O-linked carbohydrates form up to 80% of the molecule and the length of the glucidic side chains varies from one to more than 20 residues. At least eight mucin-like genes have been isolated so far, and the main characteristic is the presence of a central domain composed of a variable number of “tandem repeats”. The sequence homology of the central domain among the different members of the mucin-type family is limited, indicating that this internal domain is unique for each mucin. Thanks to the integrated results of genetic, immunological and biochemical studies, it is now possible to identify eight apomucin genes, namely MUC1, MUC2, MUC3, MUC4, MUC5AC, MUC5B, MUC6 and MUC7. MUC1 is the best characterized mucin and it is expressed on the apical surface of most polarized epithelial cells. The MUC1 gene has been cloned and sequenced. The MUC2 gene encodes a typical secretory gel-forming mucin which represents the predominant form in human intestinal and colon tissues. Another intestinal mucin is MUC3. The MUC4, MUC5AC and MUC5B genes have been isolated from a bronchial tissue cDNA library. The MUC4 and MUC5AC genes are mainly expressed in the respiratory tract, in gastric and reproductive mucosa, while MUC5B is highly detectable only in the bronchial glands. The MUC6 gene is expressed by gastric tissue and, recently, MUC7 has been cloned and sequenced using a salivary cDNA library.

Hormonal Regulation of MUC1 Expression

Several circulating mucinous markers, including CA 15.3, MCA, CA 459, CASA, and Truquant BR, are secreted products of the polymorphic MUC1 gene, and are used as diagnostic tools in patients with breast cancer. In clinical practice the measurement of the levels of these markers in the blood can give important information on the tumor's response to treatment and its biological behavior during disease monitoring. Since the marker levels reflect the activity of the tumor, it is important to know all factors influencing the production/secretion and the blood concentrations of MUC1 mucin. Recent findings suggest that MUC1 gene expression is regulated by steroid hormones and other substances present in the serum. Such observations are very important not only because of their biological significance but also for their clinical implications, as one approach to breast cancer therapy is based on chemical hormone manipulation. Nevertheless, we have preliminarily demonstrated that endocrine treatment in breast cancer patients does not influence the circulating CA 15.3 serum levels, so changes in marker levels are related only to the clinical evolution of the tumor.

Isolation and characterization of antibodies against three consecutive Tn-antigen clusters from a phage library displaying human single-chain variable fragments

The Tn-antigen, GalNAcalpha-Ser/Thr, is a tumour-associated carbohydrate antigen that may provide a sensitive and specific marker for pre-clinical detection of carcinoma and a target for cancer therapies. We recently reported that MLS128 monoclonal antibody treatment significantly inhibited colon and breast cancer cell growth. On the basis of our observations, the present study aimed to produce human anti-Tn-antigen antibodies with specificity similar to that of MLS128 monoclonal antibody, which recognizes a structure of three consecutive Tn-antigens (Tn3). Six phage clones displaying human single-chain variable fragments (scFvs) were isolated from a newly constructed phage library by panning and screening with a synthetic Tn3-peptide. Deduced amino-acid sequences of six anti-Tn3 scFvs exhibited a high degree of homology. Of those, anti-Tn3 4E10 and 4G2 scFv proteins were successfully purified from phage-infected Escherichia coli to near homogeneity. Surface plasmon resonance analyses revealed a K(D) of purified scFv proteins for Tn3 on an order of 10(-7) M, which is high for carbohydrate-specific monovalent antibodies. Further analyses suggested that both scFv proteins also bind to Tn2 and cultured colon and breast cancer cells. These results demonstrated the potential for use of these scFvs in developing antibody therapeutics targeting colon and breast cancer.

Autoproteolysis coupled to protein folding in the SEA domain of the membrane-bound MUC1 mucin

The single cell layer of the lungs and the gastrointestinal tract is protected by the mucus formed by large glycoproteins called mucins. Transmembrane mucins typically contain 110-residue SEA domains located next to the membrane. These domains undergo post-translational cleavage between glycine and serine in a characteristic GSVVV sequence, but the two peptides remain tightly associated. We show that the SEA domain of the human MUC1 transmembrane mucin undergoes a novel type of autoproteolysis, which is catalyzed by conformational stress and the conserved serine hydroxyl. We propose that self-cleaving SEA domains have evolved to dissociate as a result of mechanical rather than chemical stress at the apical cell membrane and that this protects epithelial cells from rupture. We further suggest that the cell can register mechanical shear at the mucosal surface if the dissociation is signaled via loss of a SEA-binding protein.

A review and proposed nomenclature for major proteins of the milk-fat globule membrane

The characteristics and possible functions of the most abundant proteins associated with the bovine milk-fat globule membrane are reviewed. Under the auspices of the Milk Protein Nomenclature Committee of the ADSA, a revised nomenclature for the major membrane proteins is proposed and discussed in relation to earlier schemes. We recommend that proteins be assigned specific names as they are identified by molecular cloning and sequencing techniques. The practice of identifying proteins according to their Mr, electrophoretic mobility, or staining characteristics should be discontinued, except for uncharacterized proteins. The properties and amino acid sequences of the following proteins are discussed in detail: MUC1, xanthine dehydrogenase/oxidase, CD36, butyrophilin, adipophilin, periodic acid Schiff 6/7 (PAS 6/7), and fatty acid binding protein. In addition, a compilation of less abundant proteins associated with the bovine milk-fat globule membrane is presented.

CFTR expression does not influence glycosylation of an epitope-tagged MUC1 mucin in colon carcinoma cell lines

The cause of the mucus clearance problems associated with cystic fibrosis remains poorly understood though it has been suggested that mucin hypersecretion, dehydration of mucins, and biochemical abnormalities in the glycosylation of mucins may be responsible. Since the biochemical and biophysical properties of a mucin are dependent on O-glycosylation, our aim was to evaluate the O-glycosylation of a single mucin gene product in matched pairs of cells that differed with respect to CFTR expression. An epitope-tagged MUC1 mucin cDNA (MUC1F) was used to detect variation in mucin glycosylation in stably transfected colon carcinoma cell lines HT29 and Caco2. The glycosylation of MUC1F mucin was evaluated in matched pairs of Caco2 cell lines that either express wild-type CFTR or have spontaneously lost CFTR expression. The general glycosylation pattern of MUC1F was evaluated by determining its reactivity with a series of monoclonal antibodies against known blood group and tumor-associated carbohydrate antigens. Metabolic labeling experiments were used to estimate the gross levels of glycosylation and sulfation of MUC1F mucin in these matched pairs of cell lines. Expression of CFTR in this experimental system did not affect the gross levels of glycosylation or sulfation of the MUC1F mucin nor the types of carbohydrates structures attached to the MUC1F protein.

Cloning, overexpression, and purification of cytosine deaminase from Saccharomyces cerevisiae

Cytosine deaminase is an enzyme which has been investigated for cancer chemotherapy as a result of its ability to convert the relatively nontoxic prodrug 5-fluorocytosine into the anticancer drug 5-fluorouracil. To facilitate investigations of the utility of cytosine deaminase for cancer chemotherapy, we have cloned and expressed the enzyme from Saccharomyces cerevisiae. The DNA sequence translates into a protein of 158 amino acids in length, with a predicted molecular weight of 17,563 kilodaltons. Alignment of the cytosine deaminase protein sequence from yeast with a variety of proteins defines a novel sequence motif of cytosine or cytidine binding enzymes. Recombinant expression cassettes encoding cytosine deaminase were transfected into monkey kidney COS cells, which lack endogenous cytosine deaminase, to test for production of a functional protein. Cell extracts from these transfectants contained detectable levels of enzyme activity capable of converting 5-fluorocytosine to 5-fluorouracil. Cytosine deaminase was expressed in yeast from a cDNA cassette under the control of an inducible promoter, increasing expression 250- to 300-fold relative to wild-type strains. A purification protocol has been developed which permits recovery of 60% of cytosine deaminase in active form from induced cell lysates after two purification steps. This protocol will be useful for isolating large quantities of pure enzyme which are required for the preclinical evaluation of monoclonal antibody-cytosine deaminase conjugates in combination with 5-fluorocytosine.

Synthesis and intracellular trafficking of Muc-1 and mucins by polarized mouse uterine epithelial cells

Mucins function as a protective layer rendering the apical surface of epithelial cells nonadhesive to a variety of microorganisms and macromolecules. Muc-1 is a transmembrane mucin expressed at the apical cell surface of mouse uterine epithelial cells (UEC) that disappears as UEC become receptive for embryo implantation (Surveyor, G. A., Gendler, S. J., Pemberton, L., Das, S. K., Chakraborty, I., Julian, J., Pimental, R. A., Wegner, C. W., Dey, S. K., and Carson, D. D. (1995) Endocrinology 136, 3639-3647). In the present study, the kinetics of Muc-1 assembly, cell surface expression, release, and degradation were examined in polarized mouse UEC in vitro. Mucins were identified as the predominant glycoconjugates synthesized, apically expressed, and vectorially released in both wild-type and Muc-1 null mice. When mucins were released, greater than 95% were directed to the apical compartment. Approximately half of the cell-associated mucins lost during a 24-h period were found in the apical compartment. Vectorial biotinylation detected apically disposed, cell-surface mucin and indicated that at least 34% of these mucins are released apically within 24 h. This suggests that release of mucin ectodomains is part of the mechanism of mucin removal from the apical cell surface of UEC. The half-lives of total cell-associated mucins and Muc-1 were 19.5 +/- 1 and 16.5 +/- 0.8 h, respectively. Muc-1 represented approximately 10% of the [3H]glucosamine-labeled, cell-associated mucins. Studies of the kinetics of intracellular transport of Muc-1 indicated transit times of 21 +/- 15 min from the rough endoplasmic reticulum to Golgi apparatus and 111 +/- 28 min from the Golgi apparatus to the cell surface. Collectively, these studies provide the first comprehensive description of Muc-1 and mucin maturation, metabolism, and release by polarized cells, as well as defining a major metabolic fate for mucins expressed by UEC. Normal metabolic processing appears to be sufficient to account for the removal of Muc-1 protein during the transition of UEC to a receptive state.

Inhibition by tunicamycin of mucin synthesis, not morphological changes, in epidermis during retinol-induced mucous metaplasia of chick embryonic cultured skin

BACKGROUND

Our previous studies have shown that epidermal mucous metaplasia of chick embryonic skin can be induced by culture in medium containing 20 microM retinol for only 8 hr and then in a chemically defined medium without retinol for 2 days and that retinol primarily affects the dermal cells, which then transform the epithelial cells into mucus-secreting cells.

METHODS

Tarsometatarsal skin of 13-day-old chick embryo was cultured with 20 microM retinol for 1 day and then without the vitamin but with 0.1 microgram/ml tunicamycin for 5 days. Effect of tunicamycin on epidermal mucous metaplasia was studied biochemically and morphologically.

RESULTS

Tunicamycin, which prevents the formation of N-glycans and inhibits maturation or morphological organization of various epithelial cells, irreversibly inhibited the synthesis of sulfated glycoproteins (O-glycans, mucin) in the epidermis only when applied to retinol-pretreated skin. Microvilli on the surface of the cells were well developed, but mucous granules surrounded by a limiting membrane were not observed in the upper cell layer of the epidermis, and many vesicles without electron-dense materials (mucin) and dilated rough endoplasmic reticulum were seen in the intermediate cell layers of the epidermis. When recombinants of 13-day-old normal epidermis and cultured dermis, which had been treated with retinol for 24 hr and with only tunicamycin for 2 days, were cultured without the antibiotic for 5 days, epidermal mucous metaplasia was induced.

CONCLUSION

These results suggest that tunicamycin did not prevent morphological changes induced by retinol but inhibited mucin synthesis by a direct action on the epidermis of retinol-pretreated skin. Because in some cell-line mucin precursors contain high mannose N-linked oligosaccharides side chains, tunicamycin may have inhibited mucin synthesis. Interaction between epidermal basal cells and retinol-pretreated dermal fibroblasts is prerequisite for epidermal mucous metaplasia. Thus, the present study suggests that N-linked protein glycosylation is not required for this interaction.

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.

The epithelial mucin, MUC1, of milk, mammary gland and other tissues

MUC1 is a mucin-type glycoprotein that is integrally disposed in the apical plasma membrane of the lactating epithelial cell and protrudes from the cell surface into the alveolar lumen where milk is stored. Envelopment of milk fat globules by this membrane accomplishes their secretion and conveys MUC1 into milk. The human form of this mucin has been detected in many other organs, tissues and body fluids. It projects from the cell surface as long filaments. In the human and a number of other species, MUC1 is polymorphic due to variable numbers of a tandemly repeated segment 20 amino acids in length. The individual codominantly expresses two alleles for the mucin so that differences in its size among individuals and between the two forms of an individual are observed. The tandem repeats are rich in serines and threonines which serve as O-glycosylation sites. Carbohydrate content of MUC1, as isolated from milk of human, bovine and guinea pig, is approximately 50%. The oligosaccharides carry substantial sialic acid at their termini and this accounts for two putative functions of this mucin, i.e., to keep ducts and lumens open by creating a strong negative charge on the surface of epithelial cells which would repel opposite sides of a vessel, and to bind certain pathogenic microorganisms. MUC1 is protease resistant (trypsin, chymotrypsin and pepsin) and large fragments of it can be found in the feces of some but not all breast-fed infants. MUC1 has a highly varied structure because of its polymorphism, qualitative and quantitative variations in its glycosylation between tissues, individuals and species, and differences due to divergence in the nucleotide sequences among species. Sequencing of the MUC1 gene for various species is showing promise of revealing unique evolutionary relationships and has already indicated conserved aspects of the molecule that may be functionally important. Among these are positions of serine, threonine and proline in the tandem repeats and a high degree of homology in the transmembrane and cytoplasmic segments of the molecule.

Mucins: structure, function, and associations with malignancy

Mucins are a family of high molecular weight, highly glycosylated glycoproteins found in the apical cell membrane of human epithelial cells from the mammary gland, salivary gland, digestive tract, respiratory tract, kidney, bladder, prostate, uterus and rete testis. Increased synthesis of the core protein and alterations in the carbohydrates attached to these glycoproteins are believed to play important roles in the function and proliferation of tumour cells. Aberrant glycosylation leads not only to the production of novel carbohydrate structures, but also to the exposure of the core peptide. These novel epitopes may be candidates for diagnosis or therapy, by using either synthetic mucin fragments as vaccines, or monoclonal antibody-based reagents which detect these structures.

Biosynthesis of mucin derived from a 60-kDa precursor protein in the human stomach

We studied the biosynthesis of mucin in the human stomach using an anti-mucin core peptide monoclonal antibody, 3G12. Human stomach mucosa was labeled with [35S]methionine, and chased for 3 h. An approximately 60-kDa subunit of human gastric mucin precursor protein was detected in the intracellular product. Under nonreducing conditions, dimer, trimer, and tetramer mucin precursor protein (120, 180, 240 kDa) were detected. Treatment with tunicamycin or endo-beta-N-acetylglucosaminidase H had no effect on the 60-kDa subunit and its oligomers. Extracellular products contained only the high molecular weight mucin, and the secretion was not affected by tunicamycin. By treatment with monensin or brefeldin A, the mature mucin was not secreted extracellularly. These findings suggested that a 60-kDa subunit of the mucin precursor protein was biosynthesized into mature mucin after oligomerization to tetramers, and that neither the oligomerization nor the intracellular transport of the mucin in the human stomach was associated with N-glycosylation.

Biosynthesis and secretion of mucin-related products in Hs746T gastric cancer cells

We have previously studied the biosynthesis and secretion of mucin in the normal human stomach and reported that the tetramer of the 60-kDa subunit of mucin core protein was synthesized and highly glycosylated, and that only high molecular weight mucin was secreted. In this study, we investigated the mucin-related products of a gastric cancer cell line (Hs746T). Analysis of intracellular and extracellular products immunoprecipitated with an anti-apomucin monoclonal antibody revealed that a 110-kDa protein, the dimer of the 55-kDa subunit, was synthesized and secreted. Tunicamycin treatment inhibited the secretion of the 110-kDa protein. These findings suggest that N-glycosylation may be involved in the secretory mechanism of the mucin-related product.

Antigenic similarities between respiratory and reproductive tract mucins: heterogeneity of mucin expression by human endocervix and endometrium

OBJECTIVES

To determine whether common epitopes are shared by respiratory and reproductive tract mucins and to compare the expression of cross-reactive mucin subtypes in human endocervix and endometrium.

DESIGN

An immunohistochemical study of mucin expression using a panel of monoclonal and polyclonal antimucin antibodies and timed endocervical and endometrial biopsies.

SETTING

University of Oklahoma Health Sciences Center, a tertiary care referral center.

PATIENTS

Twenty-eight women who underwent laparoscopy, laparotomy, or hysterectomy.

MAIN OUTCOME MEASURES

The expression of human tracheal mucin subspecies (types I to V) in endocervix (n = 3) and endometria (n = 25).

RESULTS

Of the five mucin subspecies, type I mucin was localized to the squamous epithelium of endocervix and both glands and stroma of endometrium. Both tissues failed to react with type II mucin. Type III mucin was localized to differentiated cells of the squamous epithelium of endocervix and the glandular endometrium. Type IV mucin was specific to endometrium and was localized both in endometrial glands and stroma with no reactivity with endocervix. Type V mucin was expressed in both cervical and endometrial stroma and glands.

CONCLUSIONS

Human respiratory and reproductive tract mucins share common peptide and carbohydrate epitopes. Human endocervix and endometria express a unique pattern of mucin antigens. Because of their restricted specificity, these monoclonal antibodies could provide new tools to investigate normal and aberrant expression of reproductive tract mucin subtypes in tissues and secretions.

Glycoconjugate composition of mammalian parotid glands elucidated in situ by lectins and glycosidases

Sugar specific lectins (PNA, RCA I, LPA, SBA, DBA, GSA IB4, GSA II, WGA, LTA, UEA I, Con A, LCA) with and without prior selective glycosidase digestion (sialidase, alpha-fucosidase, alpha-mannosidase, beta-N-acetylglucosaminidase, alpha- and beta-galactosidase, beta-glucosidase) were used in order to investigate the distribution of native accessible carbohydrates and obtain information dealing with the composition of terminal disaccharides within glycoconjugates present in acinar compartments and ductal segments of mammalian (mouse, rat, hare, and rabbit) parotid glands. Glycoconjugates containing variable amounts of mannose, glucose, N-acetylgalactosamine and N-acetylglucosamine were present in the parotid glands of all species. However, these carbohydrate chains exhibited a different composition of terminal sequences within each type of gland. For example, sialylated components having the terminal dimers sialic acid-galactose and sialic acid-N-acetylgalactosamine were found in all acinar cells, whereas fucoglycoconjugates with terminal disaccharide fucose-galactose were localized in the rat striated ducts and hare acinar cells. The terminal sequence alpha-galactose-beta-galactose was demonstrated in the mouse acinar cells. Finally, glycoconjugates characterized by the terminal dimer beta-galactose-N-acetylgalactosamine were demonstrated in the mouse acinar and ductal cells and the rat ductal ones. Thus, present findings outlined and further confirmed the possibility to elucidate the oligosaccharide structure in situ using lectin histochemistry combined with enzymatic degradation.

Mucin-type glycoproteins

Considerable advances have been made in recent years in our understanding of the biochemistry of mucin-type glycoproteins. This class of compounds is characterized mainly by a high level of O-linked oligosaccharides. Initially, the glycoproteins were solely known as the major constituents of mucus. Recent studies have shown that mucins from the gastrointestinal tract, lungs, salivary glands, sweat glands, breast, and tumor cells are structurally related to high-molecular-weight glycoproteins, which are produced by epithelial cells as membrane proteins. During mucin synthesis, an orchestrated sequence of events results in giant molecules of Mr 4 to 6 x 10(6), which are stored in mucous granules until secretion. Once secreted, mucin forms a barrier, not only to protect the delicate epithelial cells against the extracellular environment, but also to select substances for binding and uptake by these epithelia. This review is designed to critically examine relations between structure and function of the different compounds categorized as mucin glycoproteins.

Short term retinol treatment in vitro induces stable transdifferentiation of chick epidermal cells into mucus-secreting cells

Epidermal mucous metaplasia of cultured skin can be induced by treatment with excess retinol for several days (Fell 1957). In the induction of mucous metaplasia, retinol primarily affects the dermal cells and retinol-pretreated dermis can alter epidermal differentiation towards secretory epithelium (Obinata et al. 1987). In this work, we found that mucous metaplasia could be induced by culturing 13-day-old chick embryonic tarsometatarsal skin in medium containing retinol (20 μM) for only 8-24 h, followed by culture in a chemically defined medium (BGJb) without retinol or serum for 6 days. The application of cycloheximide together with retinol during the first 8 h of culture inhibited epidermal mucous metaplasia during subsequent culture for 6 days in BGJb, indicating that induction of a signal(s) in the dermis by excess retinol requires protein synthesis. However, the presence of 20 nM hydrocortisone (Takata et al. 1981) throughout the culture period did not inhibit retinol-induced epidermal mucous metaplasia of the epidermis. This indicates that a brief treatment of the skin with excess retinol determines the direction of epithelial differentiation toward secretory epithelium; this is a simpler in vitro system for the induction of epidermal mucous metaplasia than those established before.

Biochemical characterization and serological immunoassay of a pancreatic carcinoma-associated antigen defined by monoclonal antibody LD-B1

Several glycosylated macromolecules associated with normal and malignant pancreatic ductal cells have been described. We have generated a monoclonal antibody, LD-B1, by immunizing Balb/c mice with the postmicrosomal extract of fresh human pancreatic ductal carcinoma tissue and used it in this study to characterize the nature of the target antigen. The antigen detected by LD-B1 antibody was purified to homogeneity by affinity chromatography. Enzymatic and biochemical analysis showed it to be a nonsialylated glycoprotein that on Western blotting and immunoprecipitation analyses had an apparent molecular weight of 300-400 kDa. The mobility on gels was not affected by reducing or denaturing conditions. Competitive inhibition assays with various MoAbs and lectins indicated that the epitope recognized by LD-B1 antibody involves the carbohydrate sequence Gal beta 1----3Gal beta 1----3(or 4)GlcNAc beta 1----3Gal. Using a double determinant sandwich ELISA, elevated antigen levels were detected in the sera of 5 of 6 patients with pancreatic carcinoma, 0 of 3 patients with chronic pancreatitis, and 12 of 137 normal controls. These results suggest that patients with pancreatic carcinoma exhibit altered expression of a heavily glycosylated molecule related to a blood group precursor immunodeterminant.

O-glycosylation pathway for mucin-type glycoproteins

O-glycosylation is the post-translational process whereby carbohydrate is added to hydroxylated amino acids of proteins. The major O-glycosylation pathway in animal cells is involved in the synthesis of oligosaccharides linked by N-acetylgalactosamine to serine or threonine residues in 'mucin-type' proteins or their analogs. In this review, we discuss the evidence for the cellular localization of the biosynthetic steps in this pathway and propose a simplified, consensus version. We also propose variations of the simple pathway to account for its heterogeneity and variability in different cell types and differentiation states.