Abstract A40: Genetic ablation of Smoothened in tumor-associated fibroblasts promotes pancreatic tumorigenesis

Abstract

Abstract Purpose of Study: Pancreatic cancer is an overwhelming fatal disease with less than 5% of patients surviving beyond 5 years. The most prominent histopathological hallmark of pancreatic cancer is its uniquely dense stromal reaction as evidenced by recent reports that highlight the significant role of stromal fibroblasts on pancreatic tumor cell biology. We used novel mouse models to show that genetic inactivation of Smoothened (Smo) in stromal fibroblasts accelerated Kras-initiated tumorigenesis. Research Method: We used a genetically engineered mouse model of pancreatic cancer that relies on constitutive activation of the Kras oncogene in the epithelium. We simultaneously employed cre-loxP technology to conditionally delete Smo exclusively in the fibroblast compartment of the pancreas, thus disrupting the crucial hedgehog paracrine signaling loop between pancreatic tumor cells and fibroblasts. Novel Findings: We showed that deletion of Smo in stromal fibroblasts accelerated pancreatic tumorigenesis through a mechanism involving destabilization of fibroblast PTEN protein. Down-regulation of PTEN enhanced TGF-α production in stromal fibroblasts, and increased epithelial cell transformation and proliferation through epithelial growth factor receptor (EGFR). A selective SMO inhibitor also decreased PTEN in a Kras mouse model as well as in human primary pancreatic cancer associated fibroblasts. Importantly, in pancreatic ductal adenocarcinoma (PDAC) patient samples, low PTEN expression correlated with low SMO expression and with reduced overall survival. These results define a pathway that reprograms stromal fibroblasts from a tumor suppressive phenotype to a tumor promoting phenotype, thus highlighting the dual functions of stromal fibroblasts in pancreatic cancer and the molecular consequences of loss of the hedgehog pathway. Thus, a more comprehensive understanding of tumor-stroma interactions is required to assure effective implementation of targeted therapies. Conclusions and Implications: Recent pre-clinical reports suggest the pancreatic tumor microenvironment functions predominantly to inhibit tumor growth, challenging the concept of tumor stroma as a therapeutic target. Our results provide molecular insight into how the balance between the opposing activities of tumor stromal fibroblasts is maintained, and potentially identifies targets for restoring stromal tumor suppressive functions. In summary, we demonstrate that ablation of paracrine hedgehog signaling in SMA-positive fibroblasts leads to proteasome-mediated degradation of the PTEN tumor suppressor protein and subsequent activation of oncogenic pathways. Citation Format: Jason R. Pitarresi, Michael C. Ostrowski. Genetic ablation of Smoothened in tumor-associated fibroblasts promotes pancreatic tumorigenesis. [abstract]. In: Proceedings of the AACR Special Conference: Function of Tumor Microenvironment in Cancer Progression; 2016 Jan 7–10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2016;76(15 Suppl):Abstract nr A40.