Chemical and immunological differences between normal and tumoral colonic mucoprotein antigen

Solution structure of the all l- and d-amino acid-substituted mucin 2 epitope peptides

High-molecular-weight mucin 2 (MUC2) glycoproteins show an aberrant glycosylation pattern when expressed in human colon carcinoma: the oligosaccharide chains are shorter and some are missing. In our ongoing effort of MUC2 vaccine development, we have solved the NMR structure of the all L-amino acid and various D-amino acid-substituted derivatives of the peptide TPTPTGTQTPT, previously identified as an epitope within the tandem repeat unit of the MUC2 glycoprotein. In the all L-amino acid containing peptide and in peptide tpTPTGTQtpt (where lowercase letters mark the position of D-amino acids) we identified a type I beta-turn spanning through residues (3)TPTG(6) and (5)TGTQ(8), respectively. Our structural findings are in good agreement with the antibody recognition properties of the investigated peptides and demonstrate that peptides with good stability against enzymatic degradation can be designed with good antibody binding characteristics.

Partial D-amino acid substitution: Improved enzymatic stability and preserved Ab recognition of a MUC2 epitope peptide

The stability of an immunogen against enzymatic degradation is considered an important factor for the design of synthetic vaccines. For our studies, we have selected an epitope from the tandem-repeat unit of the high-molecular-weight MUC2 mucin glycoprotein, which can be underglycosylated in case of colon cancer. In this study, we prepared a MUC2 peptide containing the PTGTQ epitope of a MUC2 protein backbone-specific mAb 996 and its derivatives. In these peptides, the N- and C-terminal flanking regions were systematically substituted by up to three d-amino acids. Peptides prepared by solid-phase synthesis were tested for their mAb 996 binding in competitive ELISA experiments, and their stability was studied in serum and lysosomal preparation. Our data show that the epitope function of peptide (15)TPTPTGTQTPT(25) is retained even in the presence of two d-amino acid residues at its N-terminal flanking region and up to three at its C-terminal flanking region (tpTPTGTQtpt). Also, this partly d peptide shows high resistance against proteolytic degradation in diluted human serum and in lysosomal preparation. These findings suggest that, by appropriate combination of structural modifications (namely, d-amino acid substitution) in the flanks of an Ab epitope, it is feasible to construct a synthetic antigen with preserved recognition properties and high stability against enzymatic degradation. Peptides tPTPTGTQTpt and tpTPTGTQTpt derived from this study can be used for immunization experiments and as potential components of synthetic vaccines for tumor therapy.

Alkali-catalyzed beta-elimination of periodate-oxidized glycans: a novel method of chemical deglycosylation of mucin gene products in paraffin embedded sections

Altered expression of mucin gene products has been described in many epithelial cancers including colorectal cancer. However, mucins are heavily O-glycosylated making the study of apomucin expression difficult. In this study, we describe a novel method of chemical deglycosylation of mucin gene products on paraffin embedded formalin-fixed tissue sections. In the normal and cancerous colorectum, our results suggest that alkali-catalyzed beta-elimination of periodate oxidized glycan method of chemical deglycosylation modifies the structure of carbohydrates sensitive to mild periodate oxidation resulting in less steric hindrance and selectively removes Tn and sialyl-Tn structures, partially exposing the underlying apomucin epitopes. Using this method, we have demonstrated that the MUC1 tandem repeat epitope recognized by MAb 139H2 is masked predominantly due to steric hindrance by carbohydrate structures whereas the MUC2 tandem repeat epitope recognized by MAb CCP58 and pAb MRP and the MUC3 tandem repeat epitope recognized by pAb M3P are masked by the presence of carbohydrate side chains O-linked to Ser/Thr residues within the epitope. Considerable differences in the level and pattern of expression of the epitopes in the tandem repeat region of apomucins of MUC1, MUC2, and MUC3 were observed between normal and cancerous colorectal cancer tissues. We conclude that this novel chemical deglycosylation method that causes selective cleavage of distinct glycans will be useful in unmasking various mucin gene products and glycoproteins containing similar O-glycosidic linkages in the tissue sections of formalin-fixed paraffin embedded normal and pathological tissues.

Effect of D-amino acid substitution in a mucin 2 epitope on mucin-specific monoclonal antibody recognition

We have studied the influence of D-amino acid substitution in the flanking region on the antibody recognition of the 19TGTQ22 epitope core in the tandem repeat of mucin 2 (MUC2) glycoprotein. Analogue peptides corresponding to the optimal epitope sequence (16PTPTGTQ22) have been prepared by the replacement of single or multiple L-amino acid residues at the N-terminal part of the molecule. According to previous studies, this portion of the all-L 16PTPTGTQ22 peptide possesses a beta-turn secondary structure important for efficient monoclonal antibody interaction. The binding properties of sequentially modified peptides (pTPTGTQ, ptPTGTQ, ptpTGTQ, and ptptGTQ) have been analyzed by a MUC2 glycoprotein specific monoclonal antibody (MAb 996) using RIA inhibition assay and characterized by IC50 values. At the same time, we have investigated the secondary structure of the compounds by circular dichroism and Fourier transform infrared spectroscopy in solution. Our data showed that the presence of D-amino acid residue(s) at position(s) 16P, 16PT17, or 16PTP18 resulted in gradually decreasing antibody binding, but the replacement of the L-Thr at position 19 almost abolished activity. Parallel with this reduction, changes in the conformer population have been detected. The propensity of the pTPTGTQ peptide to adopt folded, most probably beta-turn, structure in water can be in correlation with its essentially preserved antibody recognition. After further substitution, the peptide still contained beta- and/or gamma-turn folded secondary structural elements. The conformation of peptide ptptGTQ could be characterized mostly by semiextended (polyproline II) and probably classic gamma-turn conformers built up from D residues.

Colonic mucin-carbohydrate components in colorectal tumors and their possible relationship to MUC2, p53 and DCC immunoreactivities

To clarify changes in mucus components during colorectal tumorigenesis, we developed novel monoclonal antibodies (Abs) against carbohydrate chains of human colorectal mucin (HCM) obtained from normal sigmoid and rectal mucosae. A hundred and ninety-nine cases of colorectal carcinoma and 67 cases of tubular adenoma, along with 250 normal colonic tissue samples, were investigated immunohistochemically. The results were compared with clinical stage, survival and MUC2 (core protein of the intestinal type mucin) expression, as well as with the status of the p53 and DCC (deleted in colorectal carcinomas) genes. In the normal colonic epithelium, HCM14 Ab reacted with the cytoplasmic regions of the goblet cells and enterocytes, while HCM21 Ab bound to mucous droplets in the former, suggesting a more mature carbohydrate structure. Both HCM14 and 21 scores were significantly decreased in adenomas and carcinomas. This is in line with an altered PAS-Alcian blue staining, indicating accumulation of mucins with incomplete or abnormal glycosylation in tumors. Levels of HCM14 and 21 binding tended to show a positive correlation with expression of MUC2 and DCC, and a negative association with p53 protein accumulation in carcinomas, although there was no apparent link to Duke's stage or the prognostic outcome. These findings suggest a possible involvement of alterations in mucin carbohydrate in colorectal tumor development. The observed changes may be associated with loss of MUC2 and DCC expression, as well as with p53 protein accumulation.

The carbohydrate composition of mucin in colonic cancer

Mucins synthesized in colonic cancer are known to be different from those in the normal colon; however, the biochemical differences between these mucins have not been defined. We have purified mucins from samples of nonneoplastic (normal) human colon and colon cancer and found that the carbohydrate content of the cancer-associated mucins is 48% of that in the normal colon, including significant reductions in galactose, N-acetylglucosamine, N-acetylgalactosamine, and fucose. By subjecting the mucins to alkaline degradation, we determined that there are 19% fewer oligosaccharide chains per milligram of cancer-associated colonic mucin than there are in mucins from normal colons. We also found a reduction in mean oligosaccharide chain length in cancer-associated mucin (5.83 carbohydrate residues per chain) compared with those derived from normal colons (10.2 residues). Total and individual amino acid contents were greater in cancer-associated mucins, with the exception of three amino acids (threonine, serine, and proline), two of which represent the O-linked glycosylation sites for glycoproteins. Thus, mucins are aberrantly glycosylated in colon cancer, both in terms of the number and mean chain length of the oligosaccharide moiety. Because of their relative abundance in colonic tissue, mucins appear to be useful molecular species in the study of the derangements in protein glycosylation that occur during neoplasia.

Analysis of cancer-associated colonic mucin by ion-exchange chromatography: evidence for a mucin species of lower molecular charge and weight in cancer

Cancer-associated mucins in the colon are antigenically distinct and glycosylated differently from their normal counterparts. Mucin-rich glycoconjugate preparations were made from nine non-neoplastic colons, seven colon cancers, and two different xenografts from mucin-producing human colon cancer cell lines, and radiolabeled with 3H. The preparation was applied to a DEAE-cellulose ion-exchange column, and eluted with a discontinuous ascending NaCl gradient resulting in seven discrete fractions or 'species'. Over half of the 3H-labeled glycoconjugates from specimens of non-neoplastic colonic epithelium eluted in fraction V (eluted with 0.25 NaCl). Significantly less of the 3H-labeled glycoconjugates from specimens of colon cancer eluted in fraction V (34%, P less than 0.0005), and there were significant increases in glycoconjugates eluted in fractions IV (P less than 0.008), III (P less than 0.0005), and II (P less than 0.028). Additional samples were prepared without the radiolabeling procedures, chromatographed on a DEAE-cellulose ion-exchange column, and analyzed for monosaccharide content. Each of the fractions contained the monosaccharides expected in mucin-type glycoproteins, but only sialic acid was differentially expressed in the seven fractions or 'species', occurring principally in the more charged species. However, differences in sialic acid content were not sufficient to explain the differences in retention on the ion-exchange column, nor were differences in O-acetylation of the mucins. Mucin-type glycoconjugates from colon cancers are relatively less charged than those from the normal colon, and elute at lower ionic strengths. Of interest, cancer-associated mucins appear to be of lower molecular weight than their normal counterparts. Additional studies of oligosaccharide and apomucin structure will be required to explain the molecular basis of these differences in charge.

Immunological characterization of a novel human colon-associated antigen (CAA) by a monoclonal antibody

We have generated a monoclonal antibody (MAb), designated anti-CAA (colon-associated antigen), using as immunogen a membrane-enriched fraction of a biopsy from a moderately-differentiated human colonic adenocarcinoma. The molecular weight of this reactive antigen was determined by Western blotting to be greater than 200 kDa. When immunohistochemical techniques were used, MAb anti-CAA reacted with epithelium in the majority of normal, dysplastic and malignant colon specimens tested (greater reactivity was observed in normal colon than in benign or malignant lesions). Cell sorter analyses demonstrated a heterogeneous distribution of CAA on the cell surface of the well-differentiated LS-174T cell line. Antigen positive and antigen-negative cells were separated by means of flow cytometric techniques. These two subpopulations were then inoculated into immunosuppressed rats, resulting in xenograft tumors which differed significantly in their degree of histologic differentiation. Antigen-positive cells developed into well-differentiated adenocarcinomas, while antigen-negative cells developed into poorly differentiated adenocarcinomas. These results, along with immunohistochemical studies, indicate that the antigen detected by MAb anti-CAA has characteristics of a colon-associated antigen whose expression correlates with cellular differentiation. Moreover, differences in molecular weight as well as tissue distribution indicate that CAA may be a novel antigen different from those previously described.

Mucin histochemistry in colonic polyps and cancer

Mucins are the predominant glycoproteins found in gastrointestinal epithelia, and their structures differ according to the location in the GI tract and the state of cellular differentiation. Different forms of mucin are secreted in colonic polyps and cancers compared to those found in the normal colon. This paper reviews the methods available to probe mucin structure and the state of knowledge regarding the structures of neoplasia-associated mucins. Many of the assumptions made in the interpretation of classical histochemical stains on tissue sections have recently been questioned, and new insight into mucin structure has been gained from the development of newer methodologies such as lectin histochemistry and immunocytochemistry that use well-defined monoclonal antibodies.

Physicochemical purification and immunological characteristics of ductal carcinoma antigen

Ductal carcinoma antigen (DCA), as recognized by monoclonal antibody (MAb) F36/22, was purified in high yield and to homogeneity from malignant effusions by means of physicochemical techniques. Fractionation procedures were monitored by immunoenzymometric assays. Preparations purified over 2,000-fold were obtained through acid-extraction, lectin-chromatographies and gel filtration steps. Purified antigen demonstrated a single component upon electrophoretic examination. This material had a high molecular weight and high immunoreactivity. Neuraminidase treatments failed to perturb the antigenicity of the component, indicating an absence of sialic acid residues at the antibody-combining site. Further, extensive proteolytic digestion effected the release of heterogeneous glycopeptides which retained immunologic activity, suggesting the presence of carbohydrate at the active site. Immunologically, DCA was compared with other tumor-associated mucin-like glycoproteins which have been detected in the circulation of patients. Results indicated no cross-reactivity between DCA and other mucins, including CA19-9 and CA-125. Further, no antigenic relationship was noted for purified carcinoembryonic antigen (CEA), also a heavily-glycosylated structure. These data thus suggest that immunological monitoring of disease may be approached using a selected panel of antigenically distinct reagents.

Immunoaffinity isolation of ductal carcinoma antigen using monoclonal antibody F36/22

Monoclonal antibody (McAb) F36/22, raised against a human breast tumor line, identifies an antigen found in the circulation of cancer patients. Antigen was purified from malignant effusions using McAb-affinity chromatography followed by adsorption-desorption from immobilized wheat germ lectin. Electrophoretic analysis demonstrated the isolation of a single high mol. wt glycoprotein exhibiting an isoionic point near pH 4.2 and a density of approx. 1.45 g/ml. Although highly reactive with wheat germ lectin, a negligible or weak interaction was observed with concanavalin A, lentil and peanut agglutinin. The antigen was immune-precipitable, indicating the occurrence of multiple McAb-binding sites, and was resistant to heat and acid treatments. Antigenicity was not perturbed following protease or neuraminidase treatments, but was affected upon exposure to alkaline conditions. Taken together, these data suggest that McAb F36/22 recognizes a high mol. wt component occurring in circulation as a mucin-like glycoprotein.

Chemical and histochemical studies of normal and diseased human gastrointestinal tract. I. A comparison between histologically normal colon, colonic tumours, ulcerative colitis and diverticular disease of the colon

Chemical and histochemical methods were used to compare the epithelial glycoproteins from formalin-fixed surgical specimens of normal human large intestine, colonic tumours, ulcerative colitis and diverticular disease. All the epithelial glycoproteins contained fucose, galactose, glucosamine, galactosamine and, in addition, sialic acids both with and without O-acyl substituents in the side chain and/or at position C4. The glycoproteins of the normal ascending and descending colons differed significantly with respect to the percentage of the sialic acids released following digestion of the de-O-acylated glycoprotein with Vibrio cholera neuraminidase and to the molar fucose-sialic acid ratio. Statistical analysis of the chemical data showed that (a) compared to normal, the sialic acids of the tumour and ulcerative colitis glycoproteins from the descending colon were significantly less substituted in the side chain and at position C4; (b) the O-acetyl substitution pattern of the sialic acids of the ulcerative colitis glycoproteins from the ascending colon and the quantitative composition of the carbohydrate prosthetic groups of the ulcerative colitis glycoproteins from both ascending and descending colons differed from normal; (c) it was not always possible to distinguish between the ulcerative colitis and tumour glycoproteins on the basis of the O-acetyl substitution pattern of their sialic acids; and (d), there were minor differences between normal glycoproteins and those from cases of diverticular disease.

Monoclonal antibodies to human colorectal tumor-associated antigens: improved elicitation and subclass restriction

Monoclonal antibodies to tumor-associated antigens (TAA) of human colorectal cancer were elicited using immunosorbents of lectins combined with peripheral protein extracts of xenografted colon adenocarcinoma. This method of immunization was compared with whole cells from surgical specimens and to crude membranes from xenografted tumors. The immunosorbent immunogens were superior to the other immunogens in three ways: (1) the number of hybrids reactive with colon tumor cells or extracts, but not with lymphoid cells or extracts, (2) the number of stable hybrids after cloning, and (3) the number of hybridoma clones reactive with tissue sections of colon tumors, but not normal colonic mucosa. In addition, lectin immunosorbents elicited primarily IgG antibodies, especially IgG3, with almost 50% of the clones of interest reacting to seemingly less immunogenic glycoproteins. The improved elicitation of monoclonal antibodies to TAA by the use of lectin immunosorbents and peripheral protein extracts has considerable potential for generating reagents useful in diagnosis and therapy of human tumors.

Alterations in human colonic mucin occurring with cellular differentiation and malignant transformation

The binding of fluorescein isothiocyanate (FITC)-conjugated lectins to mucin in the human colon was studied by using fluorescence microscopy. In normal mucosa, lectins that preferentially bind to exposed N-acetyl-galactosamine residues (Dolichos biflorus agglutinin and soybean agglutinin) bound selectively to the goblet cell mucin of well-differentiated cells in the upper colonic crypt. By contrast, lectins that require exposed non-reducing galactose residues for binding (Ricinus communis agglutinin1 and Bauhinia purpurea agglutinin) preferentially labeled the mucin of less-differentiated goblet cells located in the lower portion of the colonic crypt. The lectin derived from Arachis hypogaea (peanut agglutinin) has a high affinity for a carbohydrate structure not normally exposed in human tissues. This lectin did not label the goblet cell mucin in the normal colon. However, the mucin was labeled in all 21 colon cancer specimens examined. Additionally, the nonmalignant epithelium immediately adjacent to colon cancer (termed "transitional mucosa") also contained goblet cell mucin that was labeled by FITC-peanut agglutinin. Three conclusions may be drawn from the selective binding characteristics of FITC-lectins to colonic mucins. First, an alteration in the exposed, nonreducing carbohydrate residues occurs in human colonic mucin during the process of goblet cell differentiation. Second, an exposed carbohydrate structure that is not normally present in human tissues is expressed in the mucin produced by malignant colonic epithelium. Third, the presence of the cancer-associated carbohydrate structure in the mucin of transitional mucosa suggests that this tissue may be in the process of early malignant transformation.

Clinical aspects of gastrointestinal mucus

In such a bird's eye view of a very complicated and complex literature it is inevitable that significant contributions, particularly from earlier investigators, will have been overlooked. We have tried, however, to provide a reasonable framework for the many presentations and discussions which will take place at this conference. As in the past, it is evident that much needs to be done to reconcile the excellent histochemical studies of gastrointestinal mucus in many diseases with the increasing knowledge of mucin structure and composition. This will only be achieved by extraction of highly pure mucin from diseased bowel. In this regard, post mortem material provides an opportunity for mucin researchers which is not available to workers in other fields because of relative freedom from decomposition. The wedding of mucin technology with immunology is also a major priority. Immunoassay techniques provide the answer to quick and accurate product identification in secretion studies. Specific monoclonal antibodies will provide the route to structural differentiation of antigens in complex mixtures. It is also evident that we must seek to develop a variety of controllable models for the study of functional parameters of mucus in physiological conditions, parasite rejection, neoplasia and inflammatory states. Promising initiatives have been touched upon in this review, but these must only be the beginning. One must conclude, however, by recognizing that advances in knowledge have been truly remarkable since the last conference in 1976. One senses that some of the keys which will one day unlock the gates of this "ill-defined" kingdom are already in hand, while the remainder are at least within reach.

A mucoprotein with colon-specific determinants

The tissue specificity of colonic mucoprotein antigen (CMA) was examined to determine whether the antigen might have a role as a tissue marker. The immunofluorescent technique, using a rabbit anti-CMA antiserum, was employed to examine a wide range of tissues. Gastrointestinal mucosae as well as non-gastrointestinal mucin producing tissues were positive; however, non-gastrointestinal reactivity could be eliminated by the appropriate absorptions. Mucoproteins were purified separately from each gastrointestinal anatomic region and used to absorb the antiserum. This analysis demonstrated the existence of a family of mucoproteins with a common gastrointestinal specific determinant(s). Mucoproteins from neighboring regions, as seen by this antiserum, showed a greater structural similarity than did mucoproteins from distant regions. Absorption of the antiserum with ileal or cecal mucoprotein allowed the detection of a colon specific determinant present on CMA.