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

Abstract

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.Alterations in N-linked glycosylation have been observed in various malignant diseases such as cancer. Certain glycan structures resulting from the dysregulated glycosyltransferases and glycosidases are well known tumor markers and closely associated with the malignant progression (1). As examples, the ␣1,6-fucosylated ␣-fetoprotein catalyzed by ␣1,6-fucosyltransferase has been developed as a biomarker for primary hepatoma by Abelev (2). An increase in ␤1,6-GlcNAc branching of N-glycans as a consequence of enhanced expression of N-acetylglucosaminyltransferase V (GnT-V) 2 is convincingly correlated with the increased frequency of tumor metastasis (3-6). N-Acetylglucosaminyltransferase III (GnT-III) is one more important glycosyltransferase that has been receiving much attention for its involvement in the biology of tumor (7,8). It catalyzes the transfer of N-acetylglucosamine (GlcNAc) in a ␤1,4 linkage to mannose on N-glycans forming a bisecting GlcNAc structure. This step plays critical roles in determining the structure of N-glycans because introduction of the bisecting GlcNAc residue could preclude further processing and elongation of N-glycans, such as the formation of ␤1,6-GlcNAc branching structures (9). Considering also that overexpression of GnT-III inhibited the metastasis of multiple types of carcinomas (10, 1...

Bisecting GlcNAc Is a General Suppressor of Terminal Modification of N-glycan*[S]

“…S9A). This is consistent with previous reports showing that overexpression of GnT-III downregulates a2,3-sialylation but not a2,6-sialylation in cancer cell lines (64,65). Although this could be explained by the action of ST6GAL1 on O-glycans, the suppressive effects of GnT-III overexpression on SSA reactivity was not observed even in the presence of the O-glycosylation inhibitor, benzyl-GalNAc (Supplemental Fig.…”

ST6GAL1-mediated heterogeneity in sialic acid levels potentiates invasion of breast cancer epithelia

“…We show that cellular populations with distinct α2,6-Sial levels coexist together within cell lines and may reflect linkage heterogeneity within tumor cell populations in vivo. The bimodal expression of α2,6-Sial shown here can also be evidenced in several reports for MDA_MB-231 as well as other cell lines but has not been investigated before (33)(34)(35)(36)(37). The α2,6-Sial heterogeneity is counterpoised with relatively homogeneous expression in the same cells for other glycans such as α2,3-Sial, fucose, T/Tn-antigen, bisecting/ biantennary complex N-glycans & tri/tetra antennary complex N-glycans.…”

3D Structure and Function of Glycosyltransferases Involved in N-glycan Maturation

“…Our MS glycan analysis revealed increases in various terminal epitopes of N-glycans in Mgat3-knockout brain, which is concomitant with complete loss of bisecting GlcNAc, suggesting that one of the physiological functions of bisecting GlcNAc is to suppress formation of mature and complex N-glycans [56]. Similarly, Dr. Gu's group reported that the level of bisecting GlcNAc is negatively correlated with the level of sialylation in various cell lines [61].…”