Biosynthesis of the Sda determinant by beta1,4 N-acetylgalactosaminyltransferase in normal and cancer colon: relationship with sialyl Lewis X expression

The histo-blood group carbohydrate determinant Sda is expressed by about 95% of the individuals of Caucasian origin. The addition of the immunodominant sugar, a beta1,4-linked N-acetylgalactosamine, is mediated by Sda beta1,4 N-acetylgalactosaminyltransferase (b4GalNAc-T). The human b4GalNAc-T encodes two transcripts diverging in exon 1: one (long form) encodes a polypeptide with an extremely long cytoplasmic domain of 67 aminoacids while the second (short form) predicts a protein with a cytoplasmic domain of 7 aminoacids. The activity of b4GalNAc-T decreases in colorectal cancer at a very variable extent among patients. Sialyl Lewis x (sLex) is a major ligand of selectins and, if ectopically expressed by tumours, can mediate metastatization. We have permanently expressed the two b4GalNAc-T forms in the human colon cancer cell line LS174T and found that the short form induces a higher level of enzyme activity than the long form. Both forms induce expression of the Sda antigen and inhibit the expression of the sLex, very likely because of the competition between b4GalNAc-T and the fucosyltransferases which synthesize the sLex. The two b4GalNAc-T forms, fused with the green fluorescent protein (GFP) localize largely to the Golgi apparatus although a part of the enzyme molecules was present also in non-Golgi compartments. In normal human colon the Sda antigen is mainly expressed by goblet cells and by the well differentiated enterocytes of the apical portion of the gland. When the level of b4GalNAc-T activity was correlated with sLex expression in colon cancer specimens, we observed a direct relationship instead of an inverse relationship, as expected on the basis of the in vitro study. These data indicate that, although b4GalNAc-T has the potential to inhibit sialyl Lewis x expression, it does not play this role in colon carcinogenesis.