The relative activities of the C2GnT1 and ST3Gal-I glycosyltransferases determine O-glycan structure and expression of a tumor-associated epitope on MUC1

In breast cancer, the O-glycans added to the MUC1 mucin are core 1- rather than core a-based. We have analyzed whether competition by the glycosyltran sferase, ST3Gal-I, which transfers sialic acid to galactose in the core 1 s ubstrate, is key to this switch in MUC1 glycosylation that results in the e xpression of the cancer-associated SM3 epitope. Of the three enzymes known to convert core 1 to core 2, by the addition of GlcNAc to GalNAc in core1 C 2GnT1 is the dominant enzyme expressed in normal breast tissue. Expression of C2GnT1 is low or absent in around 50% of breast cancers, whereas express ion of ST3Gal-I is consistently increased. Mapping of ST3Gal-I and C2GnT1 w ithin the Golgi pathway showed some overlap. To examine functional competit ion, the enzymes were overexpressed in T47D cells, which normally make core 1-based structures, have no detectable C2GnT1 activity and express the SM3 epitope. Overexpression of C2GnT1 resulted in loss of binding of SM3 to MU C1, accompanied by a decrease in the GalNAc/GlcNAc ratio, indicative of a s witch to core 2 structures. Transfection of a C2GnT1 expressing line with S T3Gal-I restored SM3 binding and reduced GlcNAc incorporation into MUC1 O-g lycans. Thus, even when C2GnT1 is expressed, the O-glycans added to MUC1 be come core 1-dominated structures, provided expression of ST3Gal-I is increa sed as it is in breast cancer.