Effect of O-Glycosylation and Sialylation Inhibitors on Classical NLS-Dependent Nuclear Protein Import in HT29-MTX Human Colon Cancer Cells

Glycosylation, including O-linked, mucin type, has an import role in the function and activity of many proteins, particularly those that are secreted or transcellular. Previous studies have indicated that mucin-type O-linked glycans may also be carried by intracellular proteins such as the stress-related protein Orp150 that is involved in classical nuclear localization sequence (NLS)-dependent nuclear protein import. This study investigated the influence on NLS-dependent nuclear protein import of the potent O-glycosylation inhibitor, Benzyl-GalNAc; the sialylation inhibitor, 5’CDP; the Golgi proton pump inhibitor, bafilomycin; and the pro-inflammatory cytokine, TNFα. Treatment of the mucus-secreting human colon cancer HT29-MTX cells with each of these agents caused a global increase of the cellular core 1 carbohydrate structure (galactose β1,3 N-acetylgalactosamine), one of the precursor structures of the complex mucin-type glycans. Benzyl-GalNAc treatment also increased the expression of the core 1 structure on cell surface glycoconjugates and caused a generalised decrease in sialylation. Hsp70 nuclear translocation upon heat stress, a process that is mediated by the classical NLS pathway, was, however, not affected by pre-treatment of the cells with any of these agents. This suggests that O-linked and/or sialylated glycans are unlikely to be involved in the classical NLS-dependent protein import mechanism.