The adhesion and differentiation-inhibitory activities of the immunoglobulin superfamily member,carcinoembryonic antigen,can be independently blocked

The external domains of Ig superfamily members are involved in multiple binding interactions, both homophilic and heterophilic, that initiate molecular events leading to the execution of diverse cell functions. Human carcinoembryonic antigen (CEA), an Ig superfamily cell surface glycoprotein used widely as a clinical tumor marker, undergoes homophilic interactions that mediate intercellular adhesion. Recent evidence supports the view that deregulated overexpression of CEA has an instrumental role in tumorigenesis through the inhibition of cell differentiation and the disruption of tissue architecture. The CEA-mediated block of the myogenic differentiation of rat L6 myoblasts depends on homophilic binding of its external domains. We show here that L6 transfectant cells expressing CEA can "trans-block" the myogenesis of juxtaposed differentiation-competent L6 transfectant cells expressing a deletion mutant of CEA (DeltaNCEA). This result implies the efficacy of antiparallel CEA-CEA interactions between cells in the differentiation block. In addition, DeltaNCEA can acquire differentiation blocking activity by cross-linking with specific anti-CEA antibodies, thus implying the efficacy of parallel CEA-CEA interactions on the same cell surface. The myogenic differentiation blocking activity of CEA was demonstrated by site-directed mutations to involve three subdomains of the amino-terminal domain, shown previously to be critical for its intercellular adhesion function. Monovalent Fab fragments of monoclonal antibodies binding to the region bridging subdomains 1 and 2 could both inhibit intercellular adhesion and release the myogenic differentiation block. Amino acid substitutions Q80A, Q80R, and D82N in subdomain 3, QNDTG, however, were found to completely ablate the differentiation blocking activity of CEA but had no effect on intercellular adhesion activity. A cyclized peptide representing this subdomain was the most effective at releasing the differentiation block.