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Abstract. Both carcinoembryonic antigen (CEA) and neural cell adhesion molecule (NCAM) belong to the immunoglobulin supergene family and have been demonstrated to function as homotypic Ca ++-independent intercellular adhesion molecules. CEA and NCAM cannot associate heterotypically indicating that they have different binding specificities. To define the domains of CEA involved in homotypic interaction, hybrid cDNAs consisting of various domains from CEA and NCAM were constructed and were transfected into a CHO-derived cell line; stable transfectant clones showing cell surface expression of CEA/NCAM chimeric-proteins were assessed for their adhesive properties by homotypic and heterotypic aggregation assays. The results indicate that all five of the Ig(C)-like domains of NCAM are required for intercellular adhesion while the COOH-terminal domain containing the fibronectin-like repeats is dispensable.The results also show that adhesion mediated by CEA involves binding between the Ig(V)-like aminoterminal domain and one of the Ig(C)-like internal repeat domains: thus while transfectants expressing constructs containing either the N domain or the internal domains alone were incapable of homotypic adhesion, they formed heterotypic aggregates when mixed. Furthermore, peptides consisting of both the N domain and the third internal repeat domain of CEA blocked CEA-mediated cell aggregation, thus providing direct evidence for the involvement of the two domains in adhesion. We therefore propose a novel model for interactions between immunoglobulin supergene family members in which especially strong binding is effected by double reciprocal interactions between the V-like domains and C-like domains of antiparallel CEA molecules on apposing cell surfaces.C ELL-cell interactions mediated by intercellular adhesion molecules (CAMs) ~ are essential for numerous cellular responses, ranging from cell-mediated immune responses to the sorting of heterogeneous cell populations into organized arrays during tissue formation in organogenesis (Armstrong, 1989). A major proportion of known CAMs to date belong to the Ig gene superfamily (Williams and Barclay, 1988). This family of molecules is involved in many different types of cell surface recognition events, including specific intercellular adhesion (Edelman, 1985). Common to these molecules is the Ig homology unit which consists of two/3 sheets stabilized by disulfide bonds. Based on primary sequence similarities and secondary structure predictions, three homology unit types have been Please address all correspondence to Dr. C. P. Stanners, McGill Cancer Centre, 3655 Drummond Street, Montreal, Quebec, Canada H3G 1Y6.1. Abbreviations used in this paper: CAMs, intercellular adhesion molecules; CKS, CMP-KDO synthetase; GPI, glyco-phosphatidylinositol; GPI-PLC, glycophosphatidylinositol phospholipase C; NCAM, neural cell adhesion molecule; TFMS, trifluoromethanesulfonic acid. defined: the variable like-domain (V-set) and two constant like-domains (Cl-set or C2-set) (Williams and Barclay, 1...

Carcinoembryonic antigen (CEA) expression is perhaps the most prevalent of phenotypic changes observed in human cancer cells. The molecular genetic basis of this phenomenon, however, is completely unknown. Twenty-seven CEA cDNA clones were isolated from a human colon adenocarcinoma cell line. Most of these clones are full length and consist of a number (usually three) of surprisingly similar long (534 base pairs) repeats between a 5' end of 520 base pairs and a 3' end with three different termination points. The predicted translation product of these clones consists of a processed signal sequence of 34 amino acids, an amino-terminal sequence of 107 amino acids, which includes the known terminal amino acid sequence of CEA, three repeated domains of 178 amino acids each, and a membrane-anchoring domain of 27 amino acids, giving a total of 702 amino acids and a molecular weight of 72,813 for the mature protein. The repeated domains have conserved features, including the first 67 amino acids at their N termini and the presence of four cysteine residues. Comparisons with the amino acid sequences of other proteins reveals homology of the repeats with various members of the immunoglobulin supergene family, particularly the human T-cell receptor y chain. CEA cDNA clones in the SP-65 vector were shown to produce transcripts in vitro which could be translated in vitro to yield a protein of molecular weight 73,000 which in turn could be precipitated with CEA-specific antibodies. CEA cDNA clones were also inserted into an animal cell expression vector and introduced by transfection into mammalian cell lines. These transfectants produced a CEA-immunoprecipitable glycoprotein which could be visualized by immunofluorescence on the cell surface.Carcinoembryonic antigen (CEA), a large cell membrane glycoprotein of molecular weight about 180,000 (40), is produced in a high proportion of human tumors arising at the most common sites including colon, breast, and lung (37). CEA is also produced during human embryogenesis, while a related but distinctly different series of antigens termed CEA-cross-reactive antigens can be produced by a variety of adult normal cells (17). The consistent presence of CEA in tumors has led to its wide use as a clinical assay for prognosis and management of colon carcinomas. The normal function, role in carcinogenesis, and molecular basis of production of CEA in tumors, however, are completely unknown. As a first step in the solution of these questions, we have initiated a study of the molecular genetics of CEA, beginning with the isolation of a family of CEA cDNA clones.We now report the molecular cloning, nucleotide sequence, and structural analysis of a number of functional full-length CEA cDNA clones. These clones reveal a basic structure of common 5' and 3' ends embracing a number (usually three) of strikingly similar relatively long repeating units. The repeats include amino acid sequences which show significant homology with several members of the rat immunoglobulin family and, in particular, with...

Abstract. Human carcinoembryonic antigen (CEA), a widely used tumor marker, is a member of a family of cell surface glycoproteins that are overexpressed in many carcinomas. CEA has been shown to function in vitro as a homotypic intercellular adhesion molecule. This correlation of overproduction of an adhesion molecule with neoplastic transformation provoked a test of the effect of CEA on cell differentiation. Using stable CEA transfectants of the rat L6 myoblast cell line as a model system of differentiation, we show that fusion into myotubes and, in fact, the entire molecular program of differentiation, including creatine phosphokinase upregulation, myogenin upregulation, and ~-actin downregulation are completely abrogated by the ectopic expression of CEA. The blocking of the upregulation of myogenin, a transcriptional regulator responsible for the execution of the entire myogenic differentiation program, indicates that CEA expression intercepts the process at a very early stage. The adhesion function of CEA is essential for this effect since an adhesion-defective N domain deletion mutant of CEA was ineffective in blocking fusion and CEA transfectants treated with adhesion-blocking peptides fused normally. Furthermore, CEA transfectants maintain their high division potential, whereas control transfectants lose division potential with differentiation similarly to the parental cell line. Thus the expression of functional CEA on the surface of cells can block terminal differentiation and maintain proliferative potential.