STEM-25. ST6GAL1 MEDIATED REGULATION OF THE GLIOBLASTOMA SECRETOME

Abstract Glioblastoma (GBM) secreted factors have been identified as key mediators of cellular crosstalk enabling tumor progression. Secreted proteins may be glycosylated, and one form of glycosylation is sialylation. We recently determined that brain tumor initiating cells (BTICs) express elevated levels of the glycosyltransferase beta-galactoside α2,6-sialyltransferase 1 (ST6GAL1). ST6GAL1 adds α2,6 sialic acid to select proteins bound for the cell surface or secreted. In GBM and most cancers, the role of ST6GAL1 in regulating the secretome is completely unknown. Using proteomics screens with BTIC conditioned media, we determined that knockdown of ST6GAL1 altered 1) levels of secreted proteins and 2) levels of α2,6-sialylated proteins. In an unbiased proteomics screen of BTIC conditioned media, levels of seven secreted proteins were decreased and three were increased with two different ST6GAL1 shRNAs in comparison to a non-targeting control. In a separate experiment using a lectin that binds specifically to α2,6-sialic acid, levels of seven proteins were decreased and three were increased. Together, we identified four proteins whose secreted levels and α2,6-sialylation were both ST6GAL1 regulated. Among these, galectin-3-binding protein (LGALS3BP) was of interest because LGALS3BP is upregulated in many cancers, including GBM. LGALS3BP increases tumor growth and progression, including by promoting metastasis through increased cell adhesion, migration, and invasion. We validated by immunoblotting that the secreted form of LGALS3BP was decreased with ST6GAL1 knockdown. Consistent with the known roles of LGALS3BP, treatment of GBM cells with conditioned media from ST6GAL1 knockdown cells altered cell adhesion in comparison to media from non-targeting control cells, although we continue to investigate the impact of ST6GAL1 regulation of LGALS3BP on GBM growth. Overall, this work is the first to determine that α2,6-sialylation by ST6GAL1 is required for maintaining the GBM secretome and that ST6Gal1 plays an important role in the regulation of LGALS3BP secretion and α2,6-sialylation in GBM.