Integrin alpha6beta4 contributions in pancreatic adenocarcinoma
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Pancreatic adenocarcinomas are among the most lethal human cancers due to an elevated incidence of tumor cell invasion and metastasis for reasons that are currently unclear. In this dissertation, I determined how the pro-invasive integrin alpha6beta4 expression was related to pancreatic adenocarcinoma tumor progression in tumor samples and assessed in-vitro if the expression of this integrin was required for the migration and invasion of pancreatic cancer cells. In addition, I explored the mechanisms of how the alpha6beta4 integrin could contribute to an invasive and migratory phenotype. To examine if integrin alpha6beta4 expression was related to cancer progression, immunohistochemical analysis was performed in normal pancreas, pancreatic intraepithelial neoplasia (PanIN) lesions, pancreatic adenocarcinomas, and chronic pancreatitis. In normal pancreatic ducts, integrin alpha6beta4 was noted only at the cell’s basal interface with the basement membrane. In pancreatic adenocarcinomas, 92% (104/113) demonstrated overexpression of integrin alpha6beta4 and altered localization to the cytoplasm and membranous regions. This pattern of expression was observed in all PanIN lesions as early as PanIN-1A. In contrast, 93% (13/14) of chronic pancreatitis samples resembled the staining pattern of normal pancreas. When cancer was present in areas of chronic pancreatitis, this altered expression of alpha6beta4 integrin identified the cancer. These results indicate that the early overexpression of the alpha6beta4 integrin in pancreatic adenocarcinoma progression may contribute to the malignancy of this disease. \r\nTo understand the contribution of integrin alpha6beta4 expression in cell migration and invasion, cell lines were screened for the presence of integrin alpha6beta4 by immunoblotting and fluorescence activated cell sorting and correlated with their ability to migrate towards hepatocyte growth factor (HGF), a known mitogenic and motility factor of pancreatic carcinomas. I found that cell surface expression positively correlated with the cell line ability to migrate and invade towards HGF. When cells expressing high levels of integrin alpha6beta4 were treated with siRNAs targeting the alpha6 or beta4 integrin subunits, I observed a reduction in HGF-induced migration and invasion. Furthermore, the activity of the small GTPase Rac-1 decreased when alpha6beta4 integrin expression was reduced. In addition, expression of the Rac-specific nucleotide exchange factor, Tiam-1 was upregulated by the integrin alpha6beta4 and required for Rac-1 activity. These results suggest that the integrin alpha6beta4 plays an important role in the migratory and invasive phenotype of pancreatic carcinoma cells and that the Tiam-1-Rac1 pathway is an important mediator of integrin alpha6beta4-mediated HGF-induced migration and invasion. Overall this study provided evidence that integrin alpha6beta4 is an important contributor to the migratory and invasive phenotype that characterize pancreatic adenocarcinomas.