Relationship of STAT3-mediated treatment resistance on activated MAPK signaling in pancreatic cancer

181 Background: Pancreatic ductal adenocarcinoma (PDAC) is one of the most difficult human malignancies to treat due to its intrinsic (de novo) and extrinsic (acquired) chemoresistance. We have previously identified constitutively activated STAT3 as a mediator of treatment resistance. Src or EGFR activate STAT3 and promote STAT3 mediated tumor progression and mediate communication within the tumor microenvironment (TME). The purpose of this study was to further understand the molecular mechanisms of stromal-mediated chemoresistance in PDAC to generate new and promising targeted therapies. Methods: We characterized the expression of total and activated STAT3 and MAPK proteins in human pancreatic tissues (n=106), PDAC cell lines (n=9) and in PanIn lesions, primary PDAC and liver metastasis cell lines generated from tumors established in genetically engineered mice. Effects of STAT3 and MAPK inhibition (drug or siRNA) were assessed for phosphorylation of STAT3, Src, MAPK, EGFR and GSK3β and expression of cyclin-D1, SPARC, VEGF, fibronectin, CD31 and tumorigenicity in vitro and in vivo. Results: STAT3 activation is necessary for the malignant phenotype and affects survival in PDAC. In both human and mouse PDAC cell lines and tissues, there is an inverse correlation between activation of STAT3 on MAPK and GSK3β signaling. Inhibition of STAT3 reciprocally activated MAPK, GSK3β, Src and EGFR which led to subsequent reactivation of STAT3. Targeting both STAT3 and MAPK inhibited activation of STAT3, MAPK, GSK3β, Src, EGFR and cyclin-D1. Combined inhibition of STAT3 and MAPK overcame STAT3 mediated resistance and resulted in synergistic inhibition of tumorigenicity as well as inhibition of the tumor stroma, angiogenesis and hypoxia within the TME. Conclusions: The mechanism of STAT3-mediated treatment resistance is dependent on activation of MAPK signaling which in turn leads to reactivation of multiple oncogenic signaling pathways. Combined inhibition of STAT3 and MAPK overcomes therapeutic resistance, targets the TME and reduces tumorigenicity in PDAC. Targeting STAT3 and MAPK is a potent treatment regimen and may be a novel approach to enhance drug delivery and improve therapeutic response in PDAC.