All ETDs from UAB

Advisory Committee Chair

Susan Bellis

Advisory Committee Members

Etty (Tika) Benveniste

Daniel Bullard

Rakesh Patel

Ralph Sanderson

Document Type

Dissertation

Date of Award

2018

Degree Name by School

Doctor of Philosophy (PhD) Heersink School of Medicine

Abstract

Among the four classes of biological macromolecules, glycans are essential for proper cellular functioning. However, our knowledge of the contribution of these sugars, including the glycosyltransferases that add them, to cell signaling is limited. This dissertation focuses on the role of the ST6Gal-I sialyltransferase in modulating receptor-mediated signal transduction in cancer and inflammation. ST6Gal-I is a Golgi enzyme that conjugates α2-6-linked sialic acids to N-linked glycans on select glycoproteins. Through its sialylation of surface receptors, ST6Gal-I activity can significantly modify their function. The first section of this dissertation concentrates on the ST6Gal-I-mediated regulation of the signaling pathway of TNFR1, which was previously identified by our group to be a ST6Gal-I substrate. Upon activation by TNF, TNFR1 is well-known to elicit cell survival or cell death signaling, however, the mechanisms regulating TNFR1 to induce these disparate outcomes are poorly understood. Using ovarian and pancreatic epithelial cancer cells with ST6Gal-I overexpression or knockdown, we demonstrate that sialylation of TNFR1 blocks TNF-induced apoptosis while simultaneously potentiating NFκB and Akt-mediated survival signaling. In examining the mechanism by which sialylation alters TNFR1 signaling, we determined that α2-6 sialylation inhibits TNF-induced internalization of TNFR1, a requirement for apoptosis initiation. Considering ST6Gal-I is highly upregulated in various cancers, these data highlight a mechanism that cancer cells may exploit to survive within the TNF-rich tumor microenvironment. The second section of this dissertation shifts attention to the ST6Gal-I-directed effects on inflammatory signaling, particularly the NFκB and JAK/STAT pathways. Like epithelial cells, TNF-induced NFκB signaling is attenuated in monocytic cell lines and murine macrophages with ST6Gal-I knockdown/knockout. Moreover, reduced ST6Gal-I expression diminishes LPS-stimulated NFκB and STAT3 activation. We also identified the LPS receptor, TLR4, as a ST6Gal-I substrate, providing a candidate receptor responsible for the sialylation-driven alterations of LPS signaling. We next evaluated the sialylation effect on JAK/STAT activation induced by cytokines, including IFNγ, IL-6 and GM-CSF. In contrast to TNF and LPS-dependent signaling, ST6Gal-I activity has a negligible effect on cytokine-stimulated STAT activation, suggesting that ST6Gal-I selectively alters inflammatory signaling through specific receptors. Collectively, the findings in this dissertation underline the potency of ST6Gal-I-mediated sialylation in altering receptor-mediated signaling.

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