Investigating the Role of Induced Acinar Specific Transcription Factors in Pancreatic Tumor Cells and Their Target Gene Effects on Tumorigenic Development
Pancreatic acinar cells synthesize and secrete prodigestive enzymes into the primary pancreatic duct where they traverse to the duodenum to continue the process to digestion. Due to the stress of constantly synthesizing large quantities of digestive enzymes, acinar cells must be well-equipped to address high protein production demands. The transcription factors (TFs), MIST1, PTF1a and E47, are required to maintain the pancreatic secretory homeostasis. All three are basic helix-loop-helix (bHLH) TFs which are required for the function of the pancreas. Specifically, MIST1 promotes cell polarity, cell communication, and regulated exocytosis. PTF1a dimerizes with E47, and this protein complex is required for the synthesis of digestive enzymes and the organogenesis of the pancreas. Loss of these transcriptional complexes results in the progression of acinar cells towards dysplastic lesions. These lesions can acquire oncogenic mutations which leads to pancreatic ductal adenocarcinoma (PDAC). PDAC is characterized by a loss of digestive enzyme expression, increased cell proliferation, chemotherapeutic resistance, and an increase in cancer stem cells. The overall focus of my thesis is to investigate the function of the bHLH transcriptional network in abrogating tumorigenic characteristics in tumor cells and investigate several target genes involved in the development of pretumor lesions. I hypothesized that forced expression of MIST1, E47, or PTF1a activates the acinar gene expression program in human pancreatic cancer cells. To test this hypothesis, tetracycline-inducible MIST1Myc, PTF1a Myc, and tamoxifen inducible E47ER cell lines were generated. Interestingly, MIST1 induced expression of vesicle trafficking genes and genes involved in alleviating endoplasmic reticulum stress. Importantly, PTF1a and E47 rescued digestive enzyme gene expression and induced the acinar transcription network which included the genes Mist1, NR5a2, Pdx1, Xbp1, and Gata6. RNA-Seq analysis was performed on all three bHLH factors. RNA-SEQ revealed gene networks involved in cancer stem cell maintenance, chemotherapeutic resistance, and cell proliferation were decreased. Collectively these findings suggest the bHLH factors can induce acinar gene transcription networks and decrease tumorigenesis in PDAC cells. While screening MIST1 target genes in pancreatic cancer, one gene was notably elevated, Rnd2. RND2 is a constituitively active Rho GTPase and the only Rho GTPase repressed by MIST1. Rnd2 expression is detected in the developing pancreatic tumor and in the epithelial cells of PDAC. RND2 promotes tumor cell migration and forced expression of RND2 in pancreata results in tissue dysplasia and gross structural changes in the cortical actin network. These data suggest that the Rnd2 may promote tumorigenic properties in acinar cells. My research suggests that the acinar gene program can be activated in pancreatic tumor cells and that the bHLH transcription factors MIST1, PTF1a, and E47 decrease tumorigenic qualities. Future studies are required to determine whether multiple transcription factors can further decrease tumorigenic capacity or rescue the acinar gene program. The data also implicate RND2 in tumorigenic development. Overall, the findings may have future therapeutic implications where drug treatment regimens aimed to turn on acinar transcription factors could lead to decreased tumor properties and tumor burden, possibly even reverting the cancer phenotype.
Konieczny, Purdue University.
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