Regulation of sialomucin production in colon carcinoma cells.

In this study, we investigated the biochemical basis for the induction of sialomucin production in HT-29 LMM human colon carcinoma cells. Previous studies showed that conditioned medium from organ cultures of normal human colonic connective tissues (NCCM) stimulated the biosynthesis of high molecular weight sialoglycoproteins (M(r) 740,000 and 450,000) by colon carcinoma cells in vitro. Using monoclonal antibodies specific for human milk mucin, we determined that the polypeptide core of these sialomucins was the polymorphic epithelial mucin core peptide coded by the MUC1 gene. Northern analysis showed that cells treated with NCCM expressed a higher level of MUC1 poly(A)+ mRNA than untreated cells. The UDP-GalNAC: polypeptide N-acetylgalactosaminyltransferase activity in microsomal fractions from these cells did not change upon NCCM treatment. The ratios of oligosaccharides with various negative charges secreted by HT-29 LMM cells in the presence of benzyl-alpha-D-N-acetylgalactosaminide were not affected by NCCM treatment indicating that the degree of sialylation of O-linked carbohydrate chains is not influenced by NCCM treatment. [3H]Glucosamine pulse-chase labeling studies using antibodies specific for a COOH-terminal unglycosylated region of the MUC1 mucin core peptides suggested that NCCM treatment of HT-29 LMM cells did not change the rate of MUC1-related sialomucin processing.