The product of the MUC1 gene, the
polymorphic epithelial mucin (PEM) is aberrantly glycosylated in breast and other
carcinomas, resulting in exposure of normally cryptic
peptide epitopes. PEM expressed by
breast cancer cells contains more sialylated O-
glycans and has a lower GlcNAc content than that expressed by normal cells. The exposure of
peptide epitopes is thus thought to be due to the
sugar side chains being shorter on the tumour-associated
mucin. To investigate possible mechanisms underlying the different pattern of glycosylation in
breast cancer cells, we analysed the pathways involved in the biosynthesis of O-
glycan chains of
mucins in normal and cancerous mammary epithelial cells. An immortalized mammary epithelial cells line originating from normal human milk. MTSV1-7, and three human
breast cancer cell lines, BT20, MCF-7 and T47D, were studied.
Glycosyltransferase activities assembling, elongating and terminating O-
glycan core-1 [Gal beta 1-3GalNAc alpha-R] and core-2 [GlcNac beta 1-6 (Gal beta 1-3) GalNAc alpha-R] were present in the normal mammary cell line. Many of the
glycosyltransferase activities were also expressed at variable levels in
breast cancer cells. However, a
sialyltransferase activity (
CMP-sialic acid Gal beta 1-3GalNAc alpha 3-
sialyltransferase) was increased several fold in all three
cancer cell lines. Moreover,
mammary cancer cell lines BT20 and T47D have lost the ability to synthesize core-2, as shown by the lack of
UDP-GlcNAc: Gal beta 1-3GalNAc (GlcNAc to GalNAc) beta 6-GlcNAc-transferase activity, which corresponded to the absence of the
mRNA transcript. However, MCF-7
breast cancer cells expressed this
enzyme. Thus, the mechanism for the exposure of
peptide epitopes in BT20 and T47D cells is proposed to be the loss of core-2 branching leading to shorter, sialylated O-
glycan chains. A different mechanism is proposed for MCF-7
breast cancer cells.