A study of the intracellular and secreted forms of the MUC2 mucin from the PC/AA intestinal cell line

Mucin 2 (MUC2) modulates the aggressiveness of breast cancer

PURPOSE

Tumors that secrete large volumes of mucus are chemotherapy resistant, however, mechanisms underlying this resistance are unknown. One protein highly expressed in mucin secreting breast cancers is the secreted mucin, Mucin 2 (MUC2). While MUC2 is expressed in some breast cancers it is absent in normal breast tissue, implicating it in breast cancer. However, the effects of MUC2 on breast cancer are largely unknown. This study examined the role of MUC2 in modulating breast cancer proliferation, response to chemotherapy and metastasis.

METHODS

Using patient derived xenografts we developed two novel cell lines, called BCK4 and PT12, which express high levels of MUC2. To modulate MUC2 levels, BCK4 and PT12 cells were engineered to express shRNA targeted to MUC2 (shMUC2, low MUC2) or a non-targeting control (shCONT, high MUC2) and proliferation and apoptosis were measured in vitro and in vivo. BCK4 cells with shCONT or shMUC2 were labeled with GFP-luciferase and examined in an experimental metastasis model; disease burden and site specific dissemination were monitored by intravital imaging and fluorescence guided dissection, respectively.

RESULTS

Proliferation decreased in BCK4 and PT12 shMUC2 cells versus control cells both in vitro and in vivo. Chemotherapy induced minimal apoptosis in control cells expressing high MUC2 but increased apoptosis in shMUC2 cells containing low MUC2. An experimental metastasis model showed disease burden decreased when breast cancer cells contained low versus high MUC2. Treatment with Epidermal Growth Factor (EGF) increased MUC2 expression in BCK4 cells; this induction was abolished by the EGF-receptor inhibitor, Erlotinib.

CONCLUSIONS

MUC2 plays an important role in mediating proliferation, apoptosis and metastasis of breast cancer cells. MUC2 may be important in guiding treatment and predicting outcomes in breast cancer patients.

Modeling the Effect of Mucin Binding in the Gut on Drug Delivery

An important part the absorption, distribution, metabolism and excretion of an oral therapeutic is the flux rate of drug compound crossing the mucus lining of the gut. To understand this part of the absorption process, we develop a mathematical model of advection, diffusion and binding of drug compounds within the mucus layer of the intestines. Analysis of this model yields simple, measurable criteria for the successful mucin layer traversal of drug compound.

Use of Recombinant Mucin Glycoprotein to Assess the Interaction of the Gastric Pathogen Helicobacter pylori with the Secreted Human Mucin MUC5AC

There is intense interest in how bacteria interact with mucin glycoproteins in order to colonise mucosal surfaces. In this study, we have assessed the feasibility of using recombinant mucin glycoproteins to study the interaction of the gastric pathogen Helicobacter pylori with MUC5AC, a mucin which the organism exhibits a distinct tropism for. Stable clonal populations of cells expressing a construct encoding for a truncated version of MUC5AC containing N- and C-termini interspersed with two native tandem repeat sequences (N + 2TR + C) were generated. Binding of H. pylori to protein immunoprecipitated from cell lysates and supernatants was assessed. High molecular weight mucin could be detected in both cell lysates and supernatants of transfected cells. Recombinant protein formed high molecular weight oligomers, was both N and O glycosylated, underwent cleavage similar to native MUC5AC and was secreted from the cell. H. pylori bound better to secreted mucin than intracellular mucin suggesting that modifications on extracellular MUC5AC promoted binding. Lectin analysis demonstrated that secreted mucin was differentially glycosylated compared to intracellular mucin. H. pylori also bound to a recombinant C-terminus MUC5AC protein, but binding to this protein did not inhibit binding to the N + 2TR + C protein. This study demonstrates the feasibility of using recombinant mucins containing tandem repeat sequences to assess microbial mucin interactions.

Biosynthesis of the polymeric gel-forming mucin MUC5B

MUC5B is a major polymeric mucin in the airway mucus gel and is an essential component of innate defense of the respiratory epithelium. Knowledge of the synthesis and intracellular processing of MUC5B is incomplete. We investigated the molecular details of MUC5B assembly in primary human bronchial epithelial cells (HBECs) grown at an air-liquid interface (ALI). Electrophoretic and centrifugal separations of intracellular forms of MUC5B probed with antibodies specific for non-O-glycosylated and O-glycosylated forms of the mucin identified three major intracellular populations of MUC5B (non-O-glycosylated monomer and dimer, and O-glycosylated polymers). Biophysical analysis of recombinant MUC5B COOH-terminus (CT5B; D4-B-C-CK) expressed in 293-EBNA cells showed that MUC5B dimerizes by disulfide linkage. Pulse-chase studies in the HBEC ALI cultures showed that non-O-glycosylated MUC5B was synthesized within 20 min of metabolic labeling and O-glycosylated, polymeric mucin within 2 h. Radiolabeled O-glycosylated mucin polymers were secreted within 2 h and the majority were released by 48 h. These data indicate that MUC5B follows a similar assembly to the related glycoprotein, von Willebrand factor (vWF); however, unlike vWF the MUC5B polypeptide shows no evidence of major proteolytic processing of D-domains during the production of the mature secreted polymeric mucin in normal and cystic fibrosis (CF) primary bronchial epithelial cells. In contrast, MUC5B D-domains were modified by neutrophil elastase, a protease commonly found in CF sputum, demonstrating that proteolytic degradation of MUC5B is an extracellular event in CF sputum. These results define the pathway for synthesis of MUC5B in primary human goblet cells.

Characterization of neopeptides in equine articular cartilage degradation

Osteoarthritis is characterized by a loss of extracellular matrix that leads to cartilage degradation and joint space narrowing. Specific proteases, including the aggrecanases ADAMTS-4 and matrix metalloproteinase 3, are important in initiating and promoting cartilage degradation in osteoarthritis. This study investigated protease-specific and disease-specific cleavage patterns of particular extracellular matrix proteins by comparing new peptide fragments, neopeptides, in specific exogenous protease-driven digestion of a crude cartilage proteoglycan extract and an in-vitro model of early osteoarthritis. Additionally, equine cartilage explants were treated with interleukin-1 and the media collected. Proteolytic cleavage products following trypsin digestion were then identified using tandem mass spectrometry. Complete sequences of proteolytically cleaved neopeptides were determined for the major cartilage proteoglycans aggrecan, biglycan, decorin, fibromodulin plus cartilage oligomeric matrix protein. The generation of neopeptides varied with enzyme specificity; however, some peptides were common to all samples. Previous known and novel cleavage sites were identifies. The identification of novel peptide fragments provides a platform for the development of antibodies that could assist in the identification of biomarkers for osteoarthritis (OA), as well as the identification of basic biochemical processes underlying OA.

Cranberry proanthocyanidins improve the gut mucous layer morphology and function in mice receiving elemental enteral nutrition

BACKGROUND

Lamina propria Th2 cytokines, interleukin (IL)-4 and IL-13, stimulate goblet cell (GC) proliferation and MUC2 production, which protect the intestinal mucosa. Elemental enteral nutrition (EEN) reduces tissue IL-4 and impairs barrier function. Proanthocyanidins (PACs) stimulate oral mucin levels. We hypothesized that adding PAC to EEN would maintain Th2-without stimulating Th1-cytokines and preserve luminal MUC2 vs EEN alone.

MATERIALS AND METHODS

Seventy mice were randomized to 5 diet groups-standard chow, intragastric EEN, or EEN with lowPAC, midPAC (50 mg), or highPAC (100 mg PAC/kg BW)-for 5 days, starting 2 days after gastric cannulation. Ileal tissue was analyzed for histomorphology and the cytokines IL-4, IL-13, IL-1β, IL-6, and TNF-α by enzyme-linked immunosorbent assay. MUC2 was measured in intestinal washes.

RESULTS

EEN lowered IL-13 (P < .05) compared with standard chow, whereas IL-4 was not significant (P < .07). LowPAC and midPAC increased IL-13 (P < .05), whereas highPAC increased both IL-4 and IL-13 (P < .05) compared with EEN. All EEN diets reduced (P < .05) crypt depth compared with the chow group. Compared with standard chow, GC numbers and luminal MUC2 were reduced with EEN (P < .05). These effects were attenuated (P < .05) with midPAC and highPAC. No changes were observed in tissue Th1 cytokines.

CONCLUSIONS

Adding PACs to EEN reverses impaired intestinal barrier function following EEN by improving the gut mucous layer and function through increased GC size and number as well as levels of MUC2 and ileal IL-4 and IL-13.

Mathematical model of colitis-associated colon cancer

As a result of chronic inflammation of their colon, patients with ulcerative colitis or Crohn's disease are at risk of developing colon cancer. In this paper, we consider the progression of colitis-associated colon cancer. Unlike normal colon mucosa, the inflammed colon mucosa undergoes genetic mutations, affecting, in particular, tumor suppressors TP53 and adenomatous polyposis coli (APC) gene. We develop a mathematical model that involves these genes, under chronic inflammation, as well as NF-κB, β-catenin, MUC1 and MUC2. The model demonstrates that increased level of cells with TP53 mutations results in abnormal growth and proliferation of the epithelium; further increase in the epithelium proliferation results from additional APC mutations. The model may serve as a conceptual framework for further data-based study of the early stage of colon cancer.

Assessment of intracellular mucin content in vivo

Airway mucus presents a first line of defense against inhaled materials. It also, however, is a significant pathological contributor to chronic lung diseases, such as asthma, cystic fibrosis, and chronic obstructive pulmonary disease. Thus, gaining a better understanding of the mechanisms of mucus production and secretion is an important goal for improving respiratory health. Mucins, the chief glycoprotein components of airway mucus, are very large polymeric glycoproteins, and measuring their production and secretion in experimental animals presents significant technical challenges. Over the past several years, we have developed assays for accurately quantifying mucin production and secretion using histological and biochemical assays. These methods are described here.

Potential application of alternatively glycosylated serum MUC1 and MUC5AC in gastric cancer diagnosis

Post-transcription modification of proteins can be altered during carcinogenesis. In this study, quantitative sandwich enzyme immunoassays were utilized to explore the clinical diagnostic value of the alternatively glycosylated MUC1 and MUC5AC. Four pairs of antibodies were selected to construct quantitative sandwich enzyme immunoassay. Serum mucin levels of 104 primary gastric cancer patients and 120 healthy individuals were measured using the four antibody pairs. The detection sensitivities of each antibody pair against gastric cancers were 42.31%, 25.00%, 38.46% and 30.77% respectively, with a specificity of 90.00%, significantly higher than widely used tumor markers CEA (21.15%) and CA19-9 (18.27%). When monitoring in parallel with all of the four antibody pairs, the detection sensitivity increased to 75.00%, with the same 90.00% specificity. Immunoblotting of the serum samples using the anti-mucin antibodies revealed highly variable glycosylation patterns among gastric cancer patients. In addition, real-time PCR indicated the elevated mRNA levels of MUC1 and/or MUC5AC in gastric cancers. The cancer-specific epitopes were also detected in other alimentary canal epithelium cancers such as colonic, nasopharyngeal and esophageal cancers, but with much lower sensitivities. Our results suggested that alternatively glycosylated MUC1 and MUC5AC could be of significant potential as effective tumor markers in gastric cancer diagnosis.

MUC2, MUC5AC and MUC5B in the mucus of a patient with pseudomyxoma peritonei: biochemical and immunohistochemical study

A 58-year-old man with a 1 year history of progressive abdominal distension underwent a laparotomy for pseudomyxoma peritonei. The mucin was identified and characterized in the present study. Approximately 6 L of crude mucus in the sol (highly viscous) and gel (semisolid) phases was obtained from the patient's peritoneal cavity. The sol material was briefly homogenized followed by slow stirring at dilutions of up to 1:10 with 6 mol/L guanidinium chloride and proteolytic inhibitors for periods of up to 48 h. Preparative and analytical gel filtration on Sepharose 2B showed some PAS-positive material eluting in the void volume accompanied by equal or larger amounts of protein in the void and included volumes of the columns. Sodium dodecylsulfate-polyacrylamide gel electrophoresis of purified mucin on a 4-20% gradient gel showed PAS-positive material on the top of the running gel and a distinct smaller-sized species of mucin of higher electrophoretic mobility with background material in between the large and small mucin. Western blot (confirmed by immunohistochemical analysis) after agarose gel electrophoresis showed the presence of MUC2, MUC5AC and MUC5B in the mucus. There was no MUC1, MUC1core or MUC6 in the tissue. Histopathological examination confirmed a mucinous appendicular adenocarcinoma. Histology showed the mucin to be predominantly of the sulfated and non-sulfated acidic type. Serine, threonine and proline comprised 21.6% of the total amino acid composition of the sample. The viscous nature of the material is due to the presence of three gel-forming mucins and possibly to its high content of protein.

DMBT1 expression and glycosylation during the adenoma-carcinoma sequence in colorectal cancer

The gene DMBT1 (deleted in malignant brain tumour-1) has been proposed to play a role in brain and epithelial cancer, but shows unusual features for a classical tumour-suppressor gene. On the one hand, DMBT1 has been linked to mucosal protection, whereas, on the other, it potentially plays a role in epithelial differentiation. Thus its function in a particular tissue is of mechanistic importance for its role in cancer. Because the former function requires secretion to the lumen and the latter function may depend on its presence in the extracellular matrix, we decided to investigate DMBT1 expression, location and its mode of secretion during malignant transformation in colorectal cancer. Using human colorectal PC/AA cell lines and tissue sections from individual patients, we have examined the expression of DMBT1 and its glycosylation in the adenoma-carcinoma sequence leading to the adenocarcinoma phenotype.

Action of a library of O-glycosylation inhibitors on the growth of human colorectal cancer cells in culture

O-glycosylation is thought to play a significant role in the regulation of cell growth. However, only limited information is available, and few specific and selective inhibitors have been found. We have synthesized a library of O-glycosylation inhibitors based on benzyl-O-N-acetyl-D-galactosamine. These inhibitors were tested with an established series of human colorectal cancer cell lines, which model the adenoma-carcinoma sequence. Cancer cells were incubated with the inhibitors, and examined for cell growth patterns, and cellular and subcellular glycosylation using a range of lectins with confocal microscopy. The specificity of O-glycan inhibition was confirmed for the library, relative to other forms of glycosylation. All inhibitors tested resulted in smaller cell yields. However, a differential effect on O-glycosylation was detected using the lectins showing variation of localization at a subcellular level in the various cell lines. Further differential action of the inhibitor library was observed for apoptosis and on the cell cycle with the cell lines tested. This work demonstrates that O-glycosylation is closely involved in the regulation of cell growth in colorectal cancer cells and that the generation of a library of low-molecular-mass inhibitors offers a valuable means of examining this regulation at the molecular level.

A preliminary approach to the separation ofLeishmaniacell-surface antigens

The purpose of the current study was to characterize Leishmania cell-surface antigens by two different methods established for the purification of glycoproteins and proteins, and to point out a useful approach to define their size and mass heterogeneity. L. tropica parasites were initially isolated from patients with active cutaneous leishmaniasis and were then cultured in vitro. The parasite-cell layer was solubilised with 6 M guanidinium chloride (GuHCl) and subsequently prepared for the purification procedure. The methods used in this work were gel filtration chromatography and isopycnic density-gradient centrifugation. Because of the presence of a substantial amount of non-specific proteins in the culture medium, these methods were not effective alone in distinguishing these antigens. However, a good idea of their N-glycosylated structures could be obtained by using Periodic acid-Schiffs (PAS) and Con A lectin, and also size and mass heterogeneity. A combination of these methods effected a clear separation of the antigens. Amino acid analysis of the purified antigens was performed to positively identify them as well-known Leishmania cell-surface antigen gene products. The results confirmed the presence of more than one cell-surface antigen on the Leishmania parasite and the combination of gel chromatography and density-gradient centrifugation could be useful for their isolation.

Identification of Molecular Intermediates in the Assembly Pathway of the MUC5AC Mucin

MUC5AC mucins secreted by HT-29 cells in culture are oligomeric glycoproteins with characteristics similar to the MUC5AC mucins isolated from human airway sputum (Sheehan, J. K., Brazeau, C., Kutay, S., Pigeon, H., Kirkham, S., Howard, M., and Thornton, D. J. (2000) Biochem. J. 347, 37-44). Therefore we have used this cell line as a model system to investigate the biosynthesis of this major airway mucin. Initial experiments showed that the MUC5AC mucins isolated from the cells were liable to depolymerization depending on the conditions used for their solubilization. Prevention against reduction resulted in large oligomers associated with the cells, similar to those secreted into the medium. Using a combination of density gradient centrifugation and agarose gel electrophoresis coupled with probes specific for different forms of the mucin we identified five major intracellular populations of the MUC5AC polypeptide (unglycosylated monomer and dimer, GalNAc-substituted dimer, fully glycosylated dimer, and higher order oligomers). Pulse-chase studies were performed to follow the flow of radioactivity through these various intracellular forms into the mature oligomeric mucin secreted into the medium (a process taking approximately 2-4 h). The results show that the mucin polypeptide undergoes dimerization and then becomes substituted with GalNAc residues prior to glycan elaboration to produce a mature mucin dimer, which then undergoes multimerization. These data indicate that this oligomeric mucin follows a similar assembly to the von Willebrand factor glycoprotein to yield long linear disulfide-linked chains.

Partial characterisation of high-molecular weight glycoconjugates in the trail mucus of the freshwater pond snail Lymnaea stagnalis

We have studied the glycoconjugates in trail mucus of the pond snail Lymnaea stagnalis. The mucus was dissolved with 6 M guanidinium hydrochloride (GuHCl) and the major component was comprised of very high-M(r) glycoconjugates that were eluted in the void volume of a Sepharose CL-4B gel-filtration column. This high-M(r) material was pooled and thereafter subjected to density gradient centrifugation first in 4 M GuHCl/CsCl and subsequently 0.2 M GuHCl/CsCl to further remove non-glycosylated proteins and DNA. The harvested glycoconjugate pool chromatographed in the void volume of Sepharose CL-2B. However, reduction of disulfide bonds lowered the molecular size of approximately 80% of the void material yielding a major fragment and some minor smaller fragments in gel chromatography. The reduced glycoconjugates were digested with papain and yielded high molecular weight, proteinase-resistant glycopeptides. This fragmentation pattern is similar to that found for oligomeric gel-forming mucins in mammals and the amino acid composition (60% Ser/Thr) and sugar analysis of the glycopeptides is consistent with mucin-like molecules, there being no significant amounts of xylose or uronic acids. The residual 20% of the preparation, which apparently resisted reduction and protease digestion, had a similar amino acid composition to the bulk, but was somewhat different in sugar composition, containing some xylose and a significant amount of glucuronic acid. The two groups of molecules had very different morphologies in the electron microscope. Taken together, these data suggest that trail mucus is a complex mixture of at least two families of protein-glycoconjugate molecules based upon the gel-forming mucin and proteoglycan families, though we cannot rule out that polysaccharides may also be present.

Human preocular mucins reflect changes in surface physiology

BACKGROUND/AIMS

Mucin function is associated with both peptide core and glycosylation characteristics. The authors assessed whether structural alterations occurring during mucin residence in the tear film reflect changes in ocular surface physiology.

METHODS

Ocular surface mucus was collected from normal volunteers as N-acetyl cysteine (NAcCys) washes or directly from the speculum after cataract surgery. To assess the influence of surface health on mucins, NAcCys washings were also obtained from patients with symptoms, but no clinical signs of dry eye (symptomatics). Mucins were extracted in guanidine hydrochloride (GuHCl) with protease inhibitors. Buoyant density of mucin species, a correlate of glycosylation density, was followed by reactivity with anti-peptide core antibodies. Mucin hydrodynamic volume was assessed by gel filtration on Sepharose CL2B.

RESULTS

Surface fluid and mucus contained soluble forms of MUC1, MUC2, MUC4, and MUC5AC and also the same species requiring DTT solubilisation. Reactivity with antibodies to MUC2 and MUC5AC peaked at 1.3-1.5 g/ml in normals, while dominated by underglycosylated forms in symptomatics. Surface mucins were predominantly smaller than intracellular species. MUC2 size distributions were different in symptomatics and normals, while those of MUC5AC were similar in these two groups.

CONCLUSIONS

A reduction in surface mucin size indicates post-secretory cleavage. Dissimilarities in surface mucin glycosylation and individual MUC size distributions in symptomatics suggest changes in preocular mucin that might precede dry eye signs.

Abnormal subcellular distribution of mature MUC2 and de novo MUC5AC mucins in adenomas of the rectum: immunohistochemical detection using non-VNTR antibodies to MUC2 and MUC5AC peptide

Anti-mucin variable number tandem repeat (VNTR) antibodies have been used previously to demonstrate the de novo presence of MUC5AC and MUC6 mucin in colorectal adenomas and increased synthesis of MUC2, the major secreted mucin in normal colorectal mucosa. Here we examined secreted mucins in tubular, tubulovillous and villous adenomas of the rectum using non-VNTR antibodies designed to assess mature mucin. Mucin gene messenger RNAs were detected by in situ hybridization. The anti-MUC2 non-VNTR antibody in the goblet cells of adenomas revealed a staining pattern of increased cytoplasmic, Golgi and membrane staining with no change in goblet vesicle reactivity compared with normal controls. In addition, blank goblet cell vesicle immunostaining for MUC2 was found in the transitional mucosa adjacent to all types of adenoma. Although a trend to overexpression of MUC2 was observed with in situ hybridization this was not detected with immunohistology. De novo synthesis of MUC5AC, but not MUC5B or MUC6 mucin was seen in all adenomas and transitional mucosa using immunohistochemistry. There was no correlation of MUC2 or MUC5AC mucin with polyp size or the grade of dysplasia using the non-VNTR antibodies. This study demonstrates that anti-mucin non-VNTR antibodies reveal a different subcellular-localization in rectal adenomas compared with normal colorectal mucosa. Further, this pattern is in contrast to that reported for anti-mucin VNTR antibodies. Combined use of these reagents may benefit future assessment of these cancers.

Differential expression of the chromosome 11 mucin genes in colorectal cancer

The four secretory mucin genes clustered on chromosome 11, MUC2, MUC5AC, MUC5B and MUC6, were screened in 37 patients with cancers in the left hemi-colon or rectum and 10 normal rectal controls. The mucin genes were detected by in situ hybridization using oligonucleotide probes to the variable number tandem repeat (VNTR) sequences, while the proteins were stained with non-VNTR (MUC2, MUC5AC and MUC5B) or VNTR (MUC6) antibodies. Low levels of MUC2 mRNA were detected in non-mucinous adenocarcinomas (5/27) while a higher proportion of mucinous carcinomas (4/9) was positive. All 25 cases of adjacent normal tissue expressed MUC2 mRNA. No transcripts for MUC5AC, MUC5B or MUC 6 were detected in any of these specimens. MUC2 protein product was detected immunohistochemically in 34/36 carcinoma specimens, with no change from normal controls. There was de novo expression of MUC5AC in 23/36 carcinomas. No MUC5B or MUC6 protein was detected. No difference in MUC2 and MUC5AC protein was found between mucinous and non-mucinous carcinomas. The level of MUC2 was increased in moderately differentiated cancers compared with normal controls and decreased in the poorly differentiated group. Decreased MUC2 was found in poorly differentiated compared with moderately differentiated tumours. More MUC5AC protein was detected in well and moderately differentiated tumours than in poorly differentiated tumours and in all tumours relative to controls. The pattern of MUC2 staining in cancers was different from control tissue, with strong staining in the perinuclear region and none in goblet cell vesicles. MUC5AC staining was mainly detected in the cytoplasm. Poor detection of MUC2 and MUC5AC mRNA and associated strong staining for the total protein suggests altered biosynthesis and processing, leading to the characteristic subcellular distribution. Hence, change in the synthesis of MUC2 and the de novo appearance of MUC5AC in colorectal carcinomas may be significant events in the adenoma-carcinoma sequence, with possible implications for tumour prognosis.

Evolution and classification of cystine knot-containing hormones and related extracellular signaling molecules

The cystine knot three-dimensional structure is found in many extracellular molecules and is conserved among divergent species. The identification of proteins with a cystine knot structure is difficult by commonly used pairwise alignments because the sequence homology among these proteins is low. Taking advantage of complete genome sequences in diverse organisms, we used a complementary approach of pattern searches and pairwise alignments to screen the predicted protein sequences of five model species (human, fly, worm, slime mold, and yeast) and retrieved proteins with low sequence homology but containing a typical cystine knot signature. Sequence comparison between proteins known to have a cystine knot three-dimensional structure (transforming growth factor-beta, glycoprotein hormone, and platelet-derived growth factor subfamily members) identified new crucial amino acid residues (two hydrophilic amino acid residues flanking cysteine 5 of the cystine knot). In addition to the well known members of the cystine knot superfamily, novel subfamilies of proteins (mucins, norrie disease protein, von Willebrand factor, bone morphogenetic protein antagonists, and slit-like proteins) were identified as putative cystine knot-containing proteins. Phylogenetic analysis revealed the ancient evolution of these proteins and the relationship between hormones [e.g. transforming growth factor-beta (TGFbeta)] and extracellular matrix proteins (e.g. mucins). They are absent in the unicellular yeast genome but present in nematode, fly, and higher species, indicating that the cystine knot structure evolved in extracellular signaling molecules of multicellular organisms. All data retrieved by this study can be viewed at http://hormone.stanford.edu/.

Characterization of mucins from cultured normal human tracheobronchial epithelial cells

Early-passage normal human tracheobronchial epithelial (NHTBE) cells grown in air-liquid interface cultures in medium containing retinoids differentiate into a mucociliary epithelium over a 2- to 3-wk period and express increasing mRNA levels of the airway mucin genes MUC5AC and MUC5B as the cultures age; the levels of MUC2 mRNA were very low throughout the study. Using specific antibodies to MUC5AC and MUC5B mucins, we noted a gradual increase in these two mucins in the intracellular and apically secreted pools as a function of time. A low level of MUC2 mucin was detected, which did not change with time. The intracellular and apically secreted mucins isolated from day 14 and day 21 cultures by density gradient centrifugation were similar in density to those previously isolated from human respiratory mucus secretions. The sedimentation rate of the apically secreted mucins indicated that they were highly oligomerized, polydisperse macromolecules similar to those previously documented from in vivo secretions. In contrast, the cell-associated mucins from the cultured NHTBE cells were much smaller, possibly only monomers and dimers. Anion-exchange chromatography detected no differences in charge density between the reduced and carboxymethylated cell-associated and secreted forms of the MUC5AC and MUC5B mucins. The MUC5AC mucin was of similar charge density to its in vivo counterpart; however, MUC5B was more homogeneous than that found in vivo. Finally, evidence is presented for an intracellular NH(2)-terminal cleavage of the MUC5B mucins. These studies indicate that the mucins produced by cultured NHTBE cells are similar to those found in human airways, suggesting that this cell culture model is suited for studies of respiratory mucin biosynthesis, processing, and assembly.

Preliminary study pointing out a significant alteration in the biochemical composition of MUC2 in colorectal mucinous carcinoma

OBJECTIVES

In this study, we characterized colonic MUC2 mucin from a mucinous carcinoma cell line and tried to find out carcinoma-associated alterations by comparing the results with those obtained from its benign phenotype previously.

DESIGN AND METHODS

The molecular size distribution of the extracted molecules and their reactivity with two different MUC2 polypeptide antibodies indicated the presence of precursor and mature forms of the mucin in both cell lines. Isopycnic density gradient centrifugation gave good resolution of mature and precursor forms of MUC2 as assessed by agarose gel electrophoresis. Using this approach, we compared the different forms of MUC2 between benign and malign colonic cells.

RESULTS

In the comparison, we detected some aberrant glycosylated MUC2 molecules in mucinous carcinoma cell line. Agarose gel electrophoretic analysis of the low-density fractions indicated that these molecules are more charged than precursors, however, they are smaller and/or less glycosylated than mature MUC2 molecules.

CONCLUSION

The identification of unusual partially glycosylated forms of the major colonic mucin MUC2 is novel and unexpected. Implication of defective processes in the post translational modification/ processing of MUC2 opens a new field in the cancer mucin biology.