Expression of N-Acetylglucosaminyltransferase III Suppresses α2,3-Sialylation, and Its Distinctive Functions in Cell Migration Are Attributed to α2,6-Sialylation Levels

N-Acetylglucosaminyltransferase III (GnT-III), which catalyzes the addition of the bisecting GlcNAc branch on N-glycans, is usually described as a metastasis suppressor. Overexpression of GnT-III inhibited migration in multiple types of tumor cells. However, these results seem controversial to the clinical observations for the increased expression of GnT-III in human hepatomas, glioma, and ovarian cancers. Here, we present evidence that these inconsistencies are mainly attributed to the different expression pattern of cell sialylation. In detail, we show that overexpression of GnT-III significantly inhibits 2,3-sialylation but not 2,6-sialylation. The migratory ability of cells without or with a low level of 2,6-sialylation is consistently suppressed after GnT-III overexpression. In contrast, the effects of GnT-III overexpression are variable in tumor cells that are highly 2,6-sialylated. Overexpression of GnT-III promotes the cell migration in glioma cells U-251 and hepatoma cells HepG2, although it has little influence in human breast cancer cell MDA-MB-231 and gastric cancer cell MKN-45. Interestingly, up-regulation of 2,6-sialylation by overexpressing -galactoside 2,6-sialyltranferase 1 in the 2,6-hyposialylated HeLa-S3 cells abolishes the anti-migratory effects of GnT-III. Conversely, depletion of 2,6-sialylation by knock-out of -galactoside 2,6-sialyltranferase 1 in 2,6-hypersialylated HepG2 cells endows GnT-III with the anti-migratory ability. Taken together, our data clearly demonstrate that high expression of 2,6-sialylation on the cell surface could affect the anti-migratory role of GnT-III, which provides an insight into the mechanistic roles of GnT-III in tumor metastasis.