Altered Glycosylation of the MUC-1 Protein Core Contributes to the Colon Carcinoma-associated Increase of Mucin-bound Sialyl-Lewis^x Expression¹

The mucin carbohydrate epitope sialyl-Le^x, detected with the monoclonal antibody AM-3, is strongly overexpressed in >90% of human colon carcinomas. We show here that in colon carcinoma one of the mucin cores bearing the sialyl-Le^x group is MUC-1, whereas sialy-Le^x present in normal colon is not detectable on MUC-1. The amounts of MUC-1 core detectable with the monoclonal antibody BC3 in extracts of tumor tissue are 60–180% of those in normal tissue. Two other carbohydrate epitopes located on MUC-1 in mucins from normal and tumor tissue have also been characterized. In contrast to sialyl-Le^x, their expression on MUC-1 is variable and does not correlate with the malignant transformation of colonic mucosa. The transfer of the sialyl-Le^x group onto the MUC-1 core contributes to the colon carcinoma-associated overexpression of the sialyl-Le^x epitope.