Biosynthesis of the cancer-related sialyl-alpha 2,6-lactosaminyl epitope in colon cancer cell lines expressing beta-galactoside alpha 2,6-sialyltransferase under a constitutive promoter

An elevation of beta -galactoside alpha2,6-sialyltransferase (ST6Gal.I) enz yme activity and an increased alpha2,6-sialylation of cell membranes are am ong the most prominent glycosylation changes associated with colon cancer; both modifications correlate with a worse prognosis. In our previous studie s, we have frequently observed a discrepancy between the ST6Gal.I level wit hin a colon cancer sample or cell line and the respective level of reactivi ty with the alpha2,6-sialyl-specific lectin from Sambucus nigra (SNA). In t his study, we have investigated quantitatively the biosynthesis of the sial yl-alpha2,6-lactosaminyl epitope in two colon cancer cell types expressing the ST6Gal.I cDNA under the control of a constitutive promoter. By measurin g the amount of ST6Gal.I mRNA using competitive RT-PCR., the expression of alpha2,6-sialylated lactosaminic structures with SNA and anti-CDw75 Ig, and the presence of unsubstituted lactosaminic termini by Erythrina cristagall i lectin, we reached the following conclusions: (a) a high proportion of th e cell surface lactosaminic termini remains unsubstituted, even in the pres ence of a very high ST6Gal.I activity. This proportion is strongly dependen t on the cell type; (b) ST6Gal.I-transfected colon cancer cells do not expr ess the CDw75 epitope; (c) the level of ST6Gal.I enzyme activity only parti ally correlates with the mRNA level; (d) despite the control by a constitut ive promoter, the ST6Gal.I mRNA is not constantly expressed over time; and (e) a very large portion of the enzyme molecules is secreted in the extrace llular milieu. These results indicate that posttranscriptional and post-tra nslational mechanisms play a pivotal role in the control of alpha2,6-sialyl ation in colon cancer cells.