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.