Date of Award
Doctor of Philosophy
Molecular Biology, Microbiology and Biochemistry
SRY (sex determining region Y)-box 2 (SOX2) is one the embryonic stem cell transcription factors that is capable of reprogramming adult differentiated cells into an induced pluripotent cell. SOX2 is amplified in various types of epithelial cancers and its high its expression correlates with poor prognosis and decreased patient survival. Aberrant Wnt signaling drives the colo-rectal carcinogenic process and is a major determinant of the disease outcome. This study demonstrates that SOX2 counteracts Wnt driven tumor cell proliferation and maintains quiescence in a sub-population of Colo-Rectal Cancer (CRC) cells. High SOX2 expression is found in a sub-group of CRC patients with advanced disease. High SOX2 expression coupled with low Wnt activity was found in SW620 metastatic CRC cell line, while the opposite was true for the isogenic SW480 primary tumor cell line. SOX2 silencing increased Wnt activity and enhanced the oncogenic potential of SW620 cells in vitro and in vivo while over-expression had opposite effects in SW480 cells. SOX2 up-regulates the expression of PTPRK and PHLPP2 protein phosphatase genes which in turn attenuates Wnt activity by interfering with Protein Kinase A, B and C mediated beta catenin phosphorylation at Serine 552 and 675 amino acid residues thereby diminishing its nuclear sequestration and transcriptional activation. Thus SOX2 mitigates growth factor mediated Wnt activation in CRC cells and inhibits cellular proliferation so that these cells are forced to change their oncogene addiction. In effect, high SOX2 expression causes clonal evolution of APC mutant CRC cells from a state of high Wnt dependency to a state of low Wnt dependency in the process making such cells resistant to Wnt inhibitor therapy. Enhanced SOX2 transcriptional activity was associated with increased proportion of cancer cells in G0-G1 phase of cell cycle. Changing SOX2 protein levels in cells had a direct correlation with mRNA levels of RBL2-HUMAN and CDKN2B genes, which serve as regulators of G0 and G1 respectively. SOX2 was shown to physically bind and to the promoter region of these two genes and enhance their transcription. Thus high SOX2 expression, up-regulates the expression of key cell cycle inhibitor genes like RBL2 and CDKN2B and keeps cells in a dormant state. This phenomenon allows colon cancer cells to escape from cytotoxic drug therapy directed at rapidly dividing cells and cause treatment failure and disease relapse.
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