Effects of carcinoembryonic antigen (CEA) and MYC on cell state transitions

Human carcinoembryonic antigen (CEA) is a glycosylphosphatidylinositol (GPI)-Iinked cell surface glycoprotein which is overexpressed in several human cancers. CEA, and the CEA family members, non-specific crossreacting antigen (NCA) and biliary glycoprotein (BGP) are members of the immunoglobulin superfamily and all mediate homotypic adhesion. Ectopic expression of CEA in rat L6 and mouse C2 myoblasts can completely block myogenic differentiation. It is demonstrated here that CEA alone and in cooperation with avian v-Myc and human Bcl-2, can result in malignant cellular transformation, effects which are dependent on CEA's differentiation-blocking activity. Ectopic expression of v-Myc and c-Myc proteins in L6 transfectant cells were shown to induce programmed cell death (apoptosis) and surprisingly, myogenic differentiation, a novel function for Myc proteins. Induction of both of these cellular programs required the highly conserved subdomain, Myc homology box 2, in the Myc transactivation domain. L6 cells expressing v-Myc and Bcl-2 displayed reduced apoptosis, yet were limited in their transformation capacity owing to the persistence of v-Myc-induced differentiation. Cells expressing v-Myc Bcl-2, and CEA in combination were completely inhibited in executing their differentiation program, displayed an increased capacity to grow when deprived of anchorage to a substratum, and rapidly formed tumours in nude mice. Significantly, the critical prerequisite for the generation of transformed cells was the presence of CEA.The structural requirements for the differentiation blocking activity of CEA were also investigated. Like CEA, GPI-linked NCA also inhibits myogenic differentiation, whereas a transmembrane family member, biliary glycoprotein, BGP has no effect. Using cDNA constructs encoding chimeric proteins of CEA, BGP, and a GPI-linked musclespecific isoform of human neural cell adhesion molecule, which accelerates differentiation, it is demonstrated that the differentiation-blocking activity of CEA absolutely requires the 32 C-terminal amino acid residues of CEA which form the signal sequence for GPI-membrane attachment