The gastric mucin M1 antigens, markers associated with colonic carcinogenesis, have been characterized by new antimucin monoclonal antibodies (MAbs). These MAbs, obtained against mucins isolated from a human ovarian mucinous cyst (MAbs 19M1, 21M1 and 45M1) and from a pancreatic adenocarcinoma (MAb 96RA), were compared with 5 other anti-M1 mucin MAbs described previously, which characterized the a, b, c, d and e mucin M1 epitopes. Using immunoperoxidase, these new MAbs exclusively stained the surface gastric epithelium of normal human gastro-intestinal tract and reacted with fetal, precancerous and cancerous colonic mucosa, but not with normal colon. Immunoradiofixation studies showed that these new MAbs are directed against 3 epitopes (f, g and h) which are different from the a, b, c, d and e mucin M1 epitopes, though present on the same a immunoreactive high-molecular-weight components (greater than 1,000 kDa) with a density of 1.4 by CsCl-density-gradient ultracentrifugation. M1 antigenicity is characterized by a family of 8 different M1 epitopes which were destroyed with beta-mercaptoethanol (except for the f epitope), sensitive to a 5 hr trypsin treatment and resistant to 5 mM periodate (except for the h epitope). Some epitopes (b, c and d) showed increasing immunoreactivity after 20 mM periodate treatment, suggesting cryptic location. In rat-colon adenocarcinomas, M1 mucin epitopes were masked but could be decrypted using high periodate treatment, similar to normal rat gastric mucosa, thus suggesting the absence of drastic changes in the saccharide coat of the peptide mucin portion bearing M1 epitopes. Cryptic location, periodate resistance, sensitivity to protease and conformational behavior strongly suggest that the peptidic core of gastric (or fetal colonic) mucin plays a role in M1 immunoreactivity. Indeed, the resurgence of M1 antigens during colonic carcinogenesis is due to re-expression of the peptide core of gastric (or fetal colonic) mucins.
Gastric M1 mucin and the MUC5AC gene show a similar oncofetal expression in the colon. Our aim was to determine whether M1 mucin is the product of the MUC5AC gene. A recombinant baculovirus encoding the C‐terminal portion of the MUC5AC gene as a fusion protein was isolated and the immunoreactivity of the recombinant mucin (rM) toward M1 antibodies studied. Chicken antibodies also were raised against purified rM. Besides its reactivity with L56/C, a serum recognizing the bacterially expressed MUC5AC gene product, rM was endowed with M1 immunoreactivity: (i) rM‐expressing cells were stained specifically with anti‐M1 serum and with the monoclonal antibody (MAb) 21M1, defining the M1‐f epitope; (ii) both L56/C and anti‐M1 antibodies recognized the same bands in immunoblots of rM‐containing cell extracts; (iii) the 21M1 antibody reacted with rM in an immunoradiometric assay. Among the 7 M1 epitopes, M1‐f was the only one encoded by the 3′ portion of the MUC5AC gene. It was the only epitope detected in a native mucin M1‐derived 170 kDa bromelain proteolytic fragment. Furthermore, the staining patterns of human tissues obtained with either anti‐rM chicken antibodies or anti‐M1 antibodies were identical. We conclude that M1 immunoreactivity is encoded at least in part by the MUC5AC gene. Int. J. Cancer 75:767–773, 1998.© 1998 Wiley‐Liss, Inc.
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