Abstract 2504: Sialylation of EGFR by the glycosyltransferase ST6Gal-I results in receptor activation and resistance to gefitinib mediated apoptosis

The Epidermal Growth Factor Receptor (EGFR) is one of the most commonly altered receptors in cancer. In the present study, we report that sialylation of EGFR by the tumor-associated sialyltransferase ST6Gal-I increases receptor activation resulting in resistance to the EGFR tyrosine kinase inhibitor gefitinib. ST6Gal-I adds an α2-6 sialic acid (a bulky, negatively charged sugar) to select cell surface receptors, thereby modulating receptor function. Previously our group has shown that ST6Gal-I activity confers hallmark cancer stem cell characteristics such as invasion, chemoresistance, and resistance to microenvironmental stressors such as growth factor deprivation and hypoxia. Kinomics data collected for this study reveal that in OV4 ovarian cancer cells, where ST6Gal-I has been overexpressed using a lentiviral construct, ST6Gal-I overexpressing cells have an increase in overall tyrosine kinase activity in comparison to cells without ST6Gal-I expression, whereas the overall serine/threonine kinase activity is largely unaffected by ST6Gal-I expression. Specifically, the receptor tyrosine kinase, EGFR, is one of the most significantly altered kinases in ST6Gal-I expressing cells, with ST6Gal-I expressing cells having an increase in overall activity in comparison to non-expressing cells. Based on these findings, we further interrogated the role of ST6Gal-I on EGFR function. To this end, we first confirmed the sialylation status of EGFR in cells with either forced ST6Gal-I overexpression or shRNA knockdown. Using OV4 and Skov3 cells, ovarian cancer cells, as well as BxPC3 pancreatic cancer cells, we conclude that not only is EGFR a substrate of ST6Gal-I, but that sialylation of EGFR by ST6Gal-I results in basal activation of the receptor, independent of EGF ligand treatment. Further, upon stimulation with EGF, receptor activation is increased in ST6Gal-I expressing versus non-expressing cells. To determine the functional significance of EGFR sialylation by ST6Gal-I, we treated cells with differential ST6Gal-I expression with gefitinib, an EGFR tyrosine kinase inhibitor widely used as an anti-cancer therapy. These studies revealed that ST6Gal-I expressing cells were protected from gefitinib mediated apoptosis. Finally, given that EGFR activation has been shown to promote epithelial to mesenchymal transition (EMT), we evaluated ST6Gal-I expressing cells treated with EGF for the EMT marker Slug. We report that while ST6Gal-I expressing cells inherently have elevated Slug expression versus cells without ST6Gal-I, treatment with EGF greatly enhances Slug expression in ST6Gal-I expressing versus non-expressing cells. Collectively, these results reveal a novel functional role for sialylated EGFR and suggest that the sialylation of EGFR may facilitate EMT as well as provide cells a mechanism to overcome chemoresistance. Citation Format: Colleen Britain, Andrew Holdbrooks, Susan Bellis. Sialylation of EGFR by the glycosyltransferase ST6Gal-I results in receptor activation and resistance to gefitinib mediated apoptosis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2504.