Doctor of Philosophy (PhD)
Michael A. Hollingsworth
The transmembrane glycoprotein MUC1 is aberrantly expressed in the majority of pancreatic ductal adenocarcinoma cases and promotes tumor progression by engaging in morphogenetic signaling through its cytoplasmic tail. Furthermore, MUC1 can translocate to the nucleus and function as a transcriptional co-regulator in conjunction with transcriptional complexes containing activator protein-1 (AP-1) and p53. The specificity of these interactions are thought to rely on specific patterning of post-translational modifications within the cytoplasmic tail of MUC1.
Within this dissertation, we examined how MUC1 influences the formation and activity of these transcription factors and the resulting impact on tumor progression and metastasis. In our first set of studies, we evaluated the global pattern of post-translational modifications present within MUC1. While previous studies have shown phosphorylation of specific residues, we found that MUC1 exists in a hyperphosphorylated state containing potentially more than 10 phosphorylated residues. Furthermore, malignant tumors exhibit a higher degree of phosphorylation and changes in the environment also drive changes in this patterning. This is also true of several MUC1 splice variants, suggesting that MUC1 can readily integrate numerous signaling pathways to initiate the appropriate signaling compartment.
We further evaluated how MUC1 expression regulates the function of AP-1 and found that MUC1 promoted the formation of AP-1 dimers consisting of c-Jun and FRA-1. We further demonstrated that FRA-1 is a novel driver of pancreatic tumor migration, invasion, and overall progression both in conjunction, and independent of MUC1 expression. In vivo studies show increased expression of FRA-1 in pancreatic cancer and increased expression of FRA-1 target genes involved in epithelial-to-mesenchymal transition suggesting FRA-1 may be a potential therapeutic target in pancreatic cancer.
We explored this possibility utilizing inhibitors of bromodomain and extraterminal domain containing proteins, which had shown inhibitory effects on expression of FRA-1. We found we were unable to reproduce these effects, however, combining FRA-1 knockdown with BET inhibition resulted in additive effects in decreasing cellular growth. These effects are seemingly due to diminished expression of pro-growth and survival genes. Future studies targeting FRA-1 may identify better therapeutic partners.
Lastly, we examined how the mutational status of p53 influenced its interaction with MUC1. As mutant p53 has been shown to exhibit gain-of-function effects in cancer, we sought to evaluate whether different mutants would preferentially interact with MUC1 and any potential transcriptional changes that result. We found that MUC1 preferentially interacted with the hotspot mutant R273H, as compared to R175H and wildtype p53 in a dox-inducible model.
Hanson, Ryan L., "Exploration into the Functional Impact of MUC1 on the Formation and Regulation of Transcriptional Complexes Containing AP-1 and p53" (2016). Theses & Dissertations. 125.