The characterization of Fas non-apoptotic signaling and identification of mir-200c as a regulator of Fas signaling

Aberrant cellular signaling is tantamount to cancer development. Signaling miscues can lead to dysregulation of homeostatic cellular activity, promoting tumor growth, survival, and metastasis. Three hallmarks of tumor progression are the acquired ability to avoid cell death, expression of pro-survival factors, and epithelial to mesenchymal transition (EMT). The Fas signaling pathway is synonymous for signal transduction mediated activation of apoptosis and its dysregulation leads to an increased resistance to cell death. Furthermore, early molecular events of Fas apoptotic signaling are different between epithelial and mesenchymal cells. However, Fas mediated signaling is no longer considered solely in the scope of apoptotic activation. Fas signaling is shown to be critical in liver regeneration and certain types of neurite growth, to drive motility and invasiveness, and to activate NF-kappaB, Erk, p38, and Jun kinase pathways. Moreover, there are different thresholds for activation of Fas apoptotic and non-apoptotic pathways depending on ligand type and ligand concentration, which result in non-apoptotic pathway activation as an early event in Fas signaling. These findings place Fas at three critical points in tumorigenesis and potentially implicate Fas dysregulation as an early contributor and potential driver of tumor progression. MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression at the translational level. Dysregulation of miRNAs plays a major role in tumorigenesis, and miRNAs have been shown to contribute to both increasing and decreasing sensitivity to apoptosis. Recently the loss of part or all one family of miRNAs, the mir-200 family, was found to drive EMT. However, a link between EMT and changes to Fas signaling has not been previously established. This thesis shows that reduced Fas apoptotic sensitivity is a direct consequence of miRNA mediated EMT through targeting Fas Associated Phosphatase -- 1 (FAP-1), a new marker for mesenchymal cells and novel target of miR-200c. EMT mediated loss of Fas apoptotic activation potentially implicates Fas non-apoptotic pathways in further promoting tumorigenesis. Therefore, I explored and described the upregulation of SNARK, an anti-apoptotic AMP-kinase that is critical for Fas mediated motility and invasiveness as one consequence of Fas non-apoptotic activation. Furthermore, I found that activation of nonapoptotic Fas signaling is dependent on the level of receptor aggregation. As a result of this thesis I advance a hypothesis that miRNA mediated EMT switches Fas from activator of apoptosis to a driver of tumorigenesis.