Biosynthesis of the cancer-related sialyl-α2,6-lactosaminyl epitope in colon cancer cell lines expressing β-galactoside α2,6-sialyltransferase under a constitutive promoter

An elevation of β-galactoside α2,6-sialyltransferase (ST6Gal.I) enzyme activity and an increased α2,6-sialylation of cell membranes are among the most prominent glycosylation changes associated with colon cancer; both modifications correlate with a worse prognosis. In our previous studies, we have frequently observed a discrepancy between the ST6Gal.I level within a colon cancer sample or cell line and the respective level of reactivity with the α2,6-sialyl-specific lectin from Sambucus nigra (SNA). In this study, we have investigated quantitatively the biosynthesis of the sialyl-α2,6-lactosaminyl epitope in two colon cancer cell types expressing the ST6Gal.I cDNA under the control of a constitutive promoter. By measuring the amount of ST6Gal.I mRNA using competitive RT-PCR, the expression of α2,6-sialylated lactosaminic structures with SNA and anti-CDw75 Ig, and the presence of unsubstituted lactosaminic termini by Erythrina cristagalli lectin, we reached the following conclusions: (a) a high proportion of the cell surface lactosaminic termini remains unsubstituted, even in the presence of a very high ST6Gal.I activity. This proportion is strongly dependent on the cell type; (b) ST6Gal.I-transfected colon cancer cells do not express the CDw75 epitope; (c) the level of ST6Gal.I enzyme activity only partially correlates with the mRNA level; (d) despite the control by a constitutive 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 extracellular milieu. These results indicate that post-transcriptional and post-translational mechanisms play a pivotal role in the control of α2,6-sialylation in colon cancer cells.