The relationship between the branch-forming glycosyltransferases and cell surface sugar chain structures

Many recombinant proteins developed or under development for clinical use are glycoproteins, and trials aimed at improving their bioactivity or pharmacokinetics in vivo by altering specific glycan structures are ongoing. For pharmaceuticals of glycoproteins, it is important to characterize and, if possible, control the glycosylation profile. However, the mechanism responsible for the regulation of sugar chain structures found on naturally occurring glycoproteins is still unclear. To clarify the relationship between glycosyltransferases and sugar chain branch structure, we estimated six glycosyltransferases' activities (N-acetylglucosaminyltransferase (GlcNAcTase)-I, -II, -III, -IV, -V, and beta-1,4-galactosyltransferase (GalT)) which control the branch formation on asparagine (Asn)-linked sugar chains in 18 human cancer cell lines derived from several tissues. To visualize the balance of glycosyltransferase activity associated with each cell line, we expressed the relative glycosyltransferase activity in comparison to the average activity among the cell lines. These cell lines were classified into five groups according to their relative glycosyltransferase balance and were termed GlcNAcTase-I/-II, GlcNAcTase-III, GlcNAcTase-IV, GlcNAcTase-V, and GalT. We also characterized the structures of Asn-linked sugar chains on the cell surface of representative cell lines of each group. The branching structure of cell surface sugar chains roughly corresponded to the glycosyltransferase balance. This finding suggests that, for the sugar chain structure remodeling of glycoproteins, attention should be focused on the glycosyltransferase balance of host cells before introducing exogenous glycosyltransferases or down-regulating the activity of intrinsic glycosyltransferases.