All ETDs from UAB

Advisory Committee Chair

Susan L Bellis

Advisory Committee Members

Daniel C Bullard

Charles N Landen

Rakesh P Patel

Ralph D Sanderson

Document Type

Dissertation

Date of Award

2015

Degree Name by School

Doctor of Philosophy (PhD) Heersink School of Medicine

Abstract

Alterations in tumor cell glycosylation have been observed for decades, but the functional consequences of specific glycosyltransferase activity are not well understood. This dissertation focuses on the role of the sialyltransferase ST6Gal-I in driving ovarian and pancreatic cancer development. ST6Gal-I adds a negatively-charged sialic acid sugar in an alpha 2-6 linkage to select receptors, which can alter their function. We show that ST6Gal-I protein is upregulated in ovarian and pancreatic cancers but not expressed in normal epithelial tissue from these organs. ST6Gal-I expression in ovarian cancer correlates with decreased progression-free and overall survival, and we present evidence that ST6Gal-I expression is enriched in metastases. Functionally, ST6Gal-I expressing ovarian cells are resistant to cisplatin, and are selected for during prolonged cisplatin treatment. ST6Gal-I expression also confers resistance to gemcitabine in pancreatic cells, demonstrating chemoresistance to multiple agents. Futhermore, in patient-derived xenograft models, gemcitabine treatment selects for ST6Gal-I expressing tumor cells demonstrating the first in vivo evidence of ST6Gal-I-chemoresistance. Chemotherapy resistance is a hallmark of cancer stem cells (CSCs) so we hypothesized that ST6Gal-I may confer additional CSC characteristics. We find that forced knockdown or overexpression of ST6Gal-I controls spheroid growth. Furthermore, growth as tumorspheres enriches for ST6Gal-I expressing cells. Notably, alpha 2-6 sialic acid expressing primary cancer cells form spheroids in culture, in contrast to low alpha 2-6 expressing tumor cells. Tumor initiating potential is impaired in tumor cells by knockdown of ST6Gal-I in limiting dilution, subcutaneous tumor formation assays. Furthermore, we show that chemically-induced carcinogenesis results in greater tumor initiation on a ST6Gal-I overexpressing genetic mouse model. Similarly, mouse genetic models reveal early pancreatic cancer precursors express ST6Gal-I. This led us to test the influence of ST6Gal-I activity on initiating events in tumors. We present a novel regulatory mechanism for controlling key cancer transcription factor expression centering on ST6Gal-I. The phenotype induced by ST6Gal-I activity results in the upregulation of Sox9 and Slug, transcription factors shown to reprogram cells to a less differentiated state. These findings provide novel evidence that tumor glycosylation can promote a stem-like phenotype and provides a plausible mechanism for ST6Gal-I driving CSC characteristics.

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