Small-Molecule Inhibition of Axl Targets Tumor Immune Suppression and Enhances Chemotherapy in Pancreatic Cancer.

Activation of the receptor tyrosine kinase Axl is associated with poor outcomes in pancreatic cancer (PDAC), where it coordinately mediates immune evasion and drug resistance. Here, we demonstrate that the selective Axl kinase inhibitor BGB324 targets the tumor-immune interface to blunt the aggressive traits of PDAC cells in vitro and enhance gemcitibine efficacy in vivo Axl signaling stimulates the TBK1-NFκB pathway and innate immune suppression in the tumor microenvironment. In tumor cells, BGB324 treatment drove epithelial differentiation, expression of nucleoside transporters affecting gemcitabine response, and an immune stimulatory microenvironment. Our results establish a preclinical mechanistic rationale for the clinical development of Axl inhibitors to improve the treatment of PDAC patients.Significance: These results establish a preclinical mechanistic rationale for the clinical development of AXL inhibitors to improve the treatment of PDAC patients. Cancer Res; 78(1); 246-55. ©2017 AACR.

Hypoxia and Transforming Growth Factor ß Cooperate to Induce Fibulin-5 Expression in Pancreatic Cancer.

The deposition of extracellular matrix (ECM) is a defining feature of pancreatic ductal adenocarcinoma (PDA), where ECM signaling can promote cancer cell survival and epithelial plasticity programs. However, ECM signaling can also limit PDA tumor growth by producing cytotoxic levels of reactive oxygen species. For example, excess fibronectin stimulation of α5ß1 integrin on stromal cells in PDA results in reduced angiogenesis and increased tumor cell apoptosis because of oxidative stress. Fibulin-5 (Fbln5) is a matricellular protein that blocks fibronectin-integrin interaction and thus directly limits ECM-driven reactive oxygen species production and supports PDA progression. Compared with normal pancreatic tissue, Fbln5 is expressed abundantly in the stroma of PDA; however, the mechanisms underlying the stimulation of Fbln5 expression in PDA are undefined. Using in vitro and in vivo approaches, we report that hypoxia triggers Fbln5 expression in a TGF-ß- and PI3K-dependent manner. Pharmacologic inhibition of TGF-ß receptor, PI3K, or protein kinase B (AKT) was found to block hypoxia-induced Fbln5 expression in mouse embryonic fibroblasts and 3T3 fibroblasts. Moreover, tumor-associated fibroblasts from mouse PDA were also responsive to TGF-ß receptor and PI3K/AKT inhibition with regard to suppression of Fbln5. In genetically engineered mouse models of PDA, therapy-induced hypoxia elevated Fbln5 expression, whereas pharmacologic inhibition of TGF-ß signaling reduced Fbln5 expression. These findings offer insight into the signaling axis that induces Fbln5 expression in PDA and a potential strategy to block its production.

Matrix control of pancreatic cancer: New insights into fibronectin signaling.

Pancreatic ductal adenocarcinoma (PDA) is a highly metastatic disease that resists most current therapies. A defining characteristic of PDA is an intense fibrotic response that promotes tumor cell invasion and chemoresistance. Efforts to understand the complex relationship between the tumor and its extracellular network and to therapeutically perturb tumor-stroma interactions are ongoing. Fibronectin (FN), a provisional matrix protein abundant in PDA stroma but not normal tissues, supports metastatic spread and chemoresistance of this deadly disease. FN also supports angiogenesis, which is required for even hypovascular tumors such as PDA to develop and progress. Targeting components of the tumor stroma, such as FN, can effectively reduce tumor growth and spread while also enhancing delivery of chemotherapy. Here, we review the molecular mechanisms by which FN drives angiogenesis, metastasis and chemoresistance in PDA. In light of these new findings, we also discuss therapeutic strategies to inhibit FN signaling.

Fibulin-5 Blocks Microenvironmental ROS in Pancreatic Cancer.

Elevated oxidative stress is an aberration seen in many solid tumors, and exploiting this biochemical difference has the potential to enhance the efficacy of anticancer agents. Homeostasis of reactive oxygen species (ROS) is important for normal cell function, but excessive production of ROS can result in cellular toxicity, and therefore ROS levels must be balanced finely. Here, we highlight the relationship between the extracellular matrix and ROS production by reporting a novel function of the matricellular protein Fibulin-5 (Fbln5). We used genetically engineered mouse models of pancreatic ductal adenocarcinoma (PDAC) and found that mutation of the integrin-binding domain of Fbln5 led to decreased tumor growth, increased survival, and enhanced chemoresponse to standard PDAC therapies. Through mechanistic investigations, we found that improved survival was due to increased levels of oxidative stress in Fbln5-mutant tumors. Furthermore, loss of the Fbln5-integrin interaction augmented fibronectin signaling, driving integrin-induced ROS production in a 5-lipooxygenase-dependent manner. These data indicate that Fbln5 promotes PDAC progression by functioning as a molecular rheostat that modulates cell-ECM interactions to reduce ROS production, and thus tip the balance in favor of tumor cell survival and treatment-refractory disease.

Neutralizing murine TGFßR2 promotes a differentiated tumor cell phenotype and inhibits pancreatic cancer metastasis.

Elevated levels of TGFß are a negative prognostic indicator for patients diagnosed with pancreatic cancer; as a result, the TGFß pathway is an attractive target for therapy. However, clinical application of pharmacologic inhibition of TGFß remains challenging because TGFß has tumor suppressor functions in many epithelial malignancies, including pancreatic cancer. In fact, direct neutralization of TGFß promotes tumor progression of genetic murine models of pancreatic cancer. Here, we report that neutralizing the activity of murine TGFß receptor 2 using a monoclonal antibody (2G8) has potent antimetastatic activity in orthotopic human tumor xenografts, syngeneic tumors, and a genetic model of pancreatic cancer. 2G8 reduced activated fibroblasts, collagen deposition, microvessel density, and vascular function. These stromal-specific changes resulted in tumor cell epithelial differentiation and a potent reduction in metastases. We conclude that TGFß signaling within stromal cells participates directly in tumor cell phenotype and pancreatic cancer progression. Thus, strategies that inhibit TGFß-dependent effector functions of stromal cells could be efficacious for the therapy of pancreatic tumors. Cancer Res; 74(18); 4996-5007. ©2014 AACR.