title: Molecular basis of incomplete O-glycan synthesis in MCF-7 breast cancer cells: putative role of MUC6 in Tn antigen expression.
abstract: An incomplete elongation of O-glycan saccharide chains in mucins have been found in epithelial cancers, leading to the expression of shorter carbohydrate structures, such as the Tn antigen (GalNAc-O-Ser/Thr). This antigen is one of the most specific human cancer-associated structures and is capable of inducing effective immune responses against cancer cells. We aimed to investigate the causes of the expression of Tn antigen in the Tn-rich MCF-7 breast cancer cell line focusing on the first step of the O-glycosylation process. Interestingly, amino acid sequences derived from "non-mammary" apomucins (MUC5B and MUC6) were very good acceptor substrates for ppGalNAc-Ts, which are the enzymes catalyzing the Tn antigen synthesis. MUC6 peptide glycosylation with MCF-7 microsome extracts as source of ppGalNAc-T activity yielded 95% conversion of the peptide into MUC6-Tn. In addition, the MUC6-Tn glycopeptide was a poor acceptor substrate for core 1 beta3Gal-T, the next enzyme involved in the saccharide chain biosynthesis, yielding only 5% conversion of MUC6-Tn into MUC6-TF. These results indicate that non-mammary apomucin expression could be responsible, at least in part, for Tn antigen expression in MCF-7 breast cancer cells due to a combined action on glycosyltransferases: an increase of ppGalNAc-T activity and a decrease of core 1 beta3Gal-T activity. Our hypothesis is supported by experiments in vivo showing that (a) native MUC6 glycoproteins express the Tn antigen in MCF-7 cells and (b) Tn antigen expression is increased after transfection with a construct encoding for a MUC6 recombinant protein into the low Tn-expressing breast cancer cell T47D. These results open new horizons in breast cancer glycoimmunology, stressing the potential role of non-mammary apomucins.
authors.affiliation: Laboratorio de Oncología Básica y Biología Molecular, Departamento de Bioquímica, Facultad de Medicina, Universidad de la República, Montevideo, Uruguay.
authors.forename: Teresa
authors.initials: T
authors.lastname: Freire
authors.forename: Sylvie
authors.initials: S
authors.lastname: Bay
authors.forename: Silvia
authors.initials: S
authors.lastname: von Mensdorff-Pouilly
authors.forename: Eduardo
authors.initials: E
authors.lastname: Osinaga
mesh_terms: D000595:Amino Acid Sequence; D015295:Antigens, Tumor-Associated, Carbohydrate / Q000096:biosynthesis* / Q000276:immunology; D001943:Breast Neoplasms / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D045744:Cell Line, Tumor; D006020:Glycopeptides / Q000276:immunology / Q000378:metabolism; D006031:Glycosylation; D006801:Humans; D008969:Molecular Sequence Data; D055272:Mucin-6; D009077:Mucins / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D011134:Polysaccharides / Q000096:biosynthesis*; D014162:Transfection
chemical_list: D015295:Antigens, Tumor-Associated, Carbohydrate; D006020:Glycopeptides; C524734:MUC6 protein, human; D055272:Mucin-6; D009077:Mucins; D011134:Polysaccharides; C040286:Tn antigen