Carcinoembryonic antigen has a different molecular weight in normal colon and in cancer cells due to N-glycosylation differences

Abstract

Carcinoembryonic antigen, an apical membrane glycoprotein expressed in normal human colonic epithelial cells, colonic polyps, tumor, and tissue culture cell lines originating from colonic adenocarcinomas, is generally considered to have a molecular weight of 180,000. Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis associated with immunoprecipitation or immunoblotting with both monoclonal (Mab 517 and Mab 601) and polyclonal antibodies, we observed that carcinoembryonic antigen was actually expressed as two discrete apparent molecular weight forms in normal tissues: a broad band averaging at Mr 200,000 and a sharp band at Mr 130,000. This constituted the phenotype of the normal colon. In cancer cells we detected a single band at Mr 170,000 or lower. This variation was mainly the consequence of a modification of the glycosylation pattern of the molecule since deglycosylation by N-glycanase or biosynthesis in the presence of tunicamycin always produced a single molecular weight form, whether or not the source of tissue was normal or cancerous. By close inspection of benign, moderately transformed, and carcinomatous human colonic polyps we noticed that this shift in the molecular weight of carcinoembryonic antigen preceded the detection of other cancer markers such as nonspecific cross-reacting antigen at Mr 95,000 or the histological modifications leading to malignant diagnosis. Carcinoembryonic antigen constitutes, therefore, an important model with which to study the modifications of the glycosylation pattern induced during cancer biogenesis.