Obesity increases the risk of developing pancreatic adenocarcinoma (PDAC), but the mechanisms underlying these effects and the precise role played by the tumor stroma are not well understood. Using mice with pancreas- specific expression of oncogenic mutant Kras (KC mice), obesogenic diets were found to markedly increase the number of activated stromal myofibroblasts (aka pancreatic stellate cells, PaSC) and their deposition of extracellular matrix (ECM) proteins, especially those promoting matrix stiffening. Obesity-induced changes in the pancreatic fibrotic stroma are associated with increased numbers of macrophages and levels of various cytokines, chemokines and growth factors with immunomodulation capacity, acceleration of the progression of the tumor and increased incidence of invasive PDAC. Although the data strongly suggest a pro-tumor role for stromal PaSC in obesity-induced PDAC promotion, the precise role(s) of these cells in PDAC and their phenotypic characteristics appear more complex than initially perceived. Studies provided evidence that the extracellular signals insulin, insulin-like growth factor 1 (IGF-1), leptin, LPS and selected interleukins; and intracellular signals including mTOR/Akt, STAT3, and yes-associated protein 1 (YAP) play key roles in regulating PaSC phenotypes and in promoting the pro-cancer effects of PaSC. These effects are likely mediated by PaSC fibroinflammatory signals that boost tumor cell growth and induce apoptosis resistance. Moreover, PaSC were found to regulate tumor macrophage differentiation into immunosuppressive phenotypes conducive of tumor progression. Of interest are recent findings from retrospective analysis of large databases that patients with PDAC have significantly improved outcome and longer disease-free survival if they take simvastatin. Also, pilot studies indicate that metformin can modulate PaSC responses by regulating cellular metabolism, autophagy and expression of fibro-inflammatory mediators. In this proposal, the hypothesis is that obesity produces unique signals in the microenvironment of developing PDAC that are responsible for phenotypic alterations in the PaSC so that they produce factors that promote proliferation and apoptosis resistance in the cancer cells as well as shift the immune response to a pro-tumor state. It is also anticipated that simvastatin and metformin attenuate this PaSC promotion and may be useful for prevention of obesity-induced PDAC development. This hypothesis will be tested by (1) determining the consequences of selective elimination of PaSC in KC mice on obesity-induced PDAC promotion; (2) establishing the pathways and cellular processes in the stromal PaSC responsible for the pro-cancer phenotype promoted by obesity; (3) elucidating crosstalk between PaSC, cancer cells and tumor macrophages; and (4) determining the effects of simvastatin in combination with metformin on the pro-tumor phenotype of PaSC observed in the obese mice. In sum, this proposal is designed to show the roles and mechanisms of PaSC-induced tumor promotion occurring in obesity, and to provide pre-clinical evidence for chemopreventive strategies for pancreatic cancer.
This project is designed to determine the effects of obesity on tumor promotion in experimental models of pancreatic cancer. We will test the hypothesis that obesity produces unique signals in the microenvironment of developing pancreatic cancer that are responsible for alterations in tumor fibroblastic cells called pancreatic stellate cells (PaSC) so that they produce signals that promote proliferation and apoptosis resistance in the cancer cells as well as shift the immune response in the tumor making it resistant to immune defenses. We will also test if the cholesterol lowering drug simvastatin and the anti-diabetes drug metformin prevent these PaSC promoting effects on the developing tumor.
