Role of carbohydrate structures in CEA-mediated intercellular adhesion

Human carcinoembryonic antigen (CEA) is a member of a family of cell surfac e glycoproteins representing a subset of the immunoglobulin superfamily and is a major tumor marker. CEA has been demonstrated to function in vitro, a t least, as a homotypic intercellular adhesion molecule. CEA can also inhib it the differentiation of several different cell types and contribute to tu morigenesis, an activity that requires CEA-CEA interactions. Post-translati onal modifications that could modulate CEA-CEA binding are therefore of int erest. CEA is heavily glycosylated with 28 consensus sites for the addition of asparagine-linked carbohydrate structures, leading to a molecule with a bottle brush-like structure. In order to modulate the glycosylation of CEA , we transfected the functional cDNA of CEA into Chinese hamster ovary (CHO ) mutant cells, Lec1, Lec2, and Lec8, which are deficient in enzymes respon sible for various steps in the glycosylation processing pathway. Aggregatio n assays of cells in suspension were performed with stable CEA transfectant s of these cell lines and showed that all of the aberrant CEA glycoforms co uld still mediate adhesion. In addition, the specificity of adhesion of the se glycoforms was unchanged, as shown by homotypic and heterotypic adhesion assays between the transfectants. Led and Lec2 transfectants did, however, show an increased speed and final extent of aggregation, which is consiste nt with models in which sugar structures interfere with binding through pro tein domains. Lec8 transfectants, on the other hand, with more truncated su gar structures than Lec2, showed less aggregation than wild type (WT) trans fectants. We therefore conclude that carbohydrates do not determine the adh esion property of CEA or its specificity, in spite of the unusually high de gree of glycosylation; they do, however, modulate the strength of adhesion.