Shaheen, Sameerah
(2017)
Roles & mechanisms of NANOG-mediated drug resistance in human colorectal cells.
PhD thesis, University of Nottingham.
Abstract
NANOG is a transcription factor that functions as a central regulator of pluripotency and determines the cell fate in embryonic stem cells (ESCs). Dr Nateri’s laboratory and others have reported the expression of embryonic NANOG in a small subpopulation of cells in several human cancers including human colorectal cancer (CRC). NANOG positive CRC cells express the normal intestinal stem cell marker LGR5, as well as the induced pluripotent stem (iPS)-linked transcription factors SOX2, c-MYC, and high β-catenin. These cells strongly resemble a small subpopulation of poorly differentiated cancer stem cells (CSCs). Recent studies have reported that, in CSCs, the self-renewal and survival signals are dominant over the differentiation signals. This suggests that NANOG is an essential modulator of drug resistance complexity and tumour heterogeneity in cancer cells.
Dedifferentiation is an established hallmark of carcinogenesis and is accompanied by key signalling pathways mediated in drug resistance, such as the mitogen-activated protein kinase (MAPK) and glycogen synthase kinase-3β (GSK-3β)/β-catenin pathways, via epithelial–mesenchymal transition (EMT). However, the gain of stemness and the concomitant loss of differentiation might affect and alter the signalling pathways exclusive to NANOG-expressing cells that are required for drug resistance, not fully studied. Therefore, we aimed to characterise the mechanism by which CSCs gain self-renewal ability and lose differentiation ability through NANOG.
Hence, this study was initially focused on testing the role of NANOG in chemotherapy drug resistance in human CRC cells, using 5-fluorouracil (5-FU)–treated HCT116 cells stably overexpressing exogenous human NANOG (HCT116GFP/NANOG) and control GFP expressing cells (HCT116GFP). We show that NANOG overexpression promotes 5-FU resistance in HCT116GFP/NANOG cells versus HCT116GFP cells (Chapter 3).
To define the possible downstream regulatory pathways directly or indirectly mediated by NANOG, we examined the MAPKs; JNK (Jun-N-terminal kinase), ERK1/2 (extracellular signal–regulated kinase) and Wnt/β-catenin signalling pathways through GSK-3β association with β-catenin in HCT116GFP/NANOG cells versus HCT116GFP cells, and if NANOG mediated in activation of the EMT (epithelial–mesenchymal transition). Our data show that overexpression of NANOG protein in the CRC cells mimics previously reported ESC differentiation mechanism mediated by induction of the phosphorylated ERK1/2 (p-ERK1/2) expression and phosphorylated GSK-3β (p-GSK-3β) at Ser9 (Chapter 3). Consequently, NANOG overexpression increases the β-catenin and represses the E-cadherin, while significantly increases the vimentin level, leading to EMT activation (Chapter 4).
In this study, we have also demonstrated NANOG induction of the EMT signature, results increasing activity of symmetrical cell division, while reducing differentiation of HCT116 derived colonosphere formation (Chapter 5).
Taken together, this study describes the mechanisms of NANOG induction of EMT and NANOG sustainment of CSC-like traits via activation of the ERK/GSK-3β/β-catenin pathways in CRC cells. These findings highlight the specific mechanism of action of the NANOG-CSC signalling pathways in human CRC tumour heterogeneity, in which it might eventually identify promising approaches to cancer treatment via selectively targeting of CSCs.
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