Bindings of the lectins Griffonia simplicifolia-1 and pokeweed mitogen mark discrete stages of myoepithelial-like differentiation of cell lines from the rat mammary gland

Rat Muc4 (sialomucin complex) reduces binding of anti-ErbB2 antibodies to tumor cell surfaces, a potential mechanism for herceptin resistance

Muc4 (also called sialomucin complex), the rat homolog of human MUC4, is a heterodimeric glycoprotein complex that consists of a peripheral O-glycosylated mucin subunit, ASGP-1, tightly but noncovalently linked to a N-glycosylated transmembrane subunit, ASGP-2. The complex is expressed in a number of normal, vulnerable epithelial tissues, including mammary gland, uterus, colon, cornea and trachea. Muc4/SMC is also overexpressed or aberrantly expressed on a number of human tumors including breast tumors. Overexpression of Muc4/SMC has been shown to block cell-cell and cell-matrix interactions, protect tumor cells from immune surveillance and promote metastasis. In addition, as a ligand for ErbB2, Muc4/SMC can potentiate phosphorylation of ErbB2 and potentially alter signals generated from this receptor. Using A375 human melanoma cells and MCF7 human breast adenocarcinoma cells stably transfected with tetracycline regulatable Muc4, we have investigated whether overexpression of Muc4/SMC can repress antibody binding to cell surface-expressed ErbB2. Overexpression of Muc4/SMC does not affect the level of ErbB2 expression in either cell line, but it does reduce binding of a number of anti-ErbB2 antibodies, including Herceptin. Interestingly, overexpression of ErbB2 does not block binding of other unrelated antibodies of the same isotype, suggesting that the reduction in ErbB2 antibody binding is due to complex formation of Muc4/SMC and ErbB2. Furthermore, capping of Muc4/SMC with anti-Muc4/SMC antibodies reduces antibody binding to ErbB2 instead of increasing binding, again suggesting that reduced antibody binding to ErbB2 is due to steric hindrance from complex formation of Muc4/SMC and ErbB2. Thus, overexpression of Muc4/SMC on tumor cells may have both prognostic and therapeutic relevance.

Molecular cloning and characterization of CFT1, a developmentally regulated avian alpha(1,3)-fucosyltransferase gene

Although coordinate expression of carbohydrate epitopes during development is well described, mechanisms which regulate this expression remain largely unknown. In this study we demonstrate that developing chicken B cells express the LewisX terminal oligosaccharide structure in a stage-specific manner. To examine regulation of this expression, we have cloned and expressed the chicken alpha(1,3)-fucosyltransferase gene involved in LewisX biosynthesis, naming it chicken fucosyltransferase 1 (CFT1). CFT1 is characterized by a single long open reading frame of 356 amino acids encoding a type II transmembrane glycoprotein. The domain structure and predicted amino acid sequence are highly conserved between CFT1 and mammalian FucTIV genes (52.8% and 46.3% identity to mouse and human respectively). In vitro CFT1 fucosyltransferase activity utilizes LacNAc > 3'sialyl-LacNAc acceptors with almost no utilization of other neutral type II (lactose, 2-fucosyllactose), or type I (lacto-N-biose I) acceptors. CFT1-transfected cells make cell surface LewisX (COS-7) and LewisX + VIM-2 structures (Chinese hamster ovary). CFT1 gene expression is tissue-specific and includes embryonic thymus and bursa. Furthermore, expression of the CFT1 gene and cell surface LewisX structures are closely linked during B cell development. These findings reveal the evolutionary conservation between nonmammalian and mammalian alpha(1,3)-fucosyltransferase genes and demonstrate a role for fucosyltransferase gene regulation in the developmental expression of oligosaccharide structures.