Pathways of mucin O-glycosylation in normal and malignant rat colonic epithelial cells reveal a mechanism for cancer-associated sialyl-Tn antigen expression

The Sialyl-Tn antigen (Sialyl alpha -Ser/Thr) is expressed as a cancer-asso ciated antigen on the surface of cancer cells. Its presence is associated w ith a poor prognosis in patients with colorectal and other cancers. We prev iously reported that Sialyl-Tn expression in LSC human colon cancer cells c ould be explained by a specific lack of the activity of core 1 beta3-Gal-tr ansferase (Brockhausen et al., Glycoconjugate J. 15, 595-603, 1998) and an inability to synthesize the common O-glycan core structures. To support thi s mechanism, or find other mechanisms to explain Sialyl-Tn antigen expressi on, we investigated the O-glycosylation pathways in clonal rat colon cancer cell lines that were selected for positive or negative expression of sialy l-Tn antigen, and compared these pathways to those in normal rat colonic mu cosa. Normal rat colonic mucosa had very active glycosyltransferases synthe sizing O-glycan core structures 1 to 4. Several sialyl-, sulfo- and fucosyl transferases were also active. An M type core 2 beta6-GlcNAc-transferase wa s found to be present in rat colon mucosa and all of the rat colon cancer c ells. O-glycosylation pathways in rat colon cancer cells were significantly different from normal rat colonic mucosa; for example, rat colon cancer ce lls lost the ability to synthesize O-glycan core 3. All rat colon cancer ce ll lines, regardless of the Sialyl-Tn phenotype, expressed glycosyltransfer ases assembling complex O-glycans of core 1 and core 2 structures (unlike h uman LSC colon cancer cells which lack core 1 PB-Gal-transferase activity). It was the activity of CMP-sialic acid:GalNAc-mucin alpha6-sialyltransfera se that coincided with Sialyl-Tn expression. Sialyl-Tn negative cells had a several fold higher activity of core 2 beta6-GlcNAc-transferase which synt hesizes complex O-glycans that may mask adjacent Sialyl-Tn epitopes. The re sults suggest a new mechanism controlling Sialyl-Tn expression in cancer ce lls.