Inhibition of Discoidin Domain Receptor 1 Reduces Collagen-Mediated Tumorigenicity in Pancreatic Ductal Adenocarcinoma

An extracellular matrix (ECM) rich in fibrillar collagens is a principal component of pancreatic ductal adenocarcinoma (PDA). The ECM provides structural support for the tumor and facilitates tumor cell survival and chemoresistance by activating cell surface receptors on tumor cells. Fibrillar collagens bind the collagen-specific receptor tyrosine kinase discoidin domain receptor 1 (DDR1), implicated in regulating cell proliferation, migration, adhesion, ECM remodeling, and response to growth factors. Additionally, collagen has been shown to promote chemoresistance in pancreatic tumor cells. I hypothesize that the regulation of collagen-mediated DDR1 signaling promotes chemoresistance. Collagen expression and deposition is a complex process that is orchestrated in part by the matricellular protein SPARC. SPARC expression in human PDA patients correlates with improved chemoresponse; however, the mechanism underlying this is unclear. I proposed that SPARC reduces collagen binding to collagen receptors. Structural studies identified that SPARC and DDR1 share the same collagen-binding site. I demonstrated that SPARC inhibited collagen binding to DDR1 via in vitro binding assays and cell-based activity assays. To determine the functional relevance of Sparc expression and collagen-mediated Ddr1 activation in PDA, Sparc-null (Sparc-/-) mice were crossed with a GEMM of PDA, KIC (LSL KrasG12D/+; Ink4aArflox/lox; p48Cre/+). Survival was reduced and tumors were more aggressive in Sparc-/-; KIC mice. Tumors from these animals also displayed elevated Ddr1-mediated signaling. Human PDA, and primary PDA cell lines isolated from Sparc+/+; KIC and Sparc-/-; KIC animals, were used to probe collagen signaling and collagen activation of DDR1 stimulated downstream intermediates including protein tyrosine kinase 2 (PYK2) and pseudopodium-enriched atypical kinase 1 (PEAK1). Furthermore, utilization of a novel DDR1 small molecule inhibitor (7rh) abrogated collagen-induced DDR1 signaling and blunted tumor cell colony formation, migration, and enhanced sensitivity to gemcitabine. Additionally, 7rh inhibited Ddr1 signaling in syngeneic, genetic, and human xenograft pancreatic tumors and was well tolerated. Therapy studies combining standard chemotherapy (gemcitabine plus nab-paclitaxel) with 7rh in vivo dramatically improved survival of mice compared to standard therapy alone. These data confirm that inhibition of collagen signaling in PDA is an attractive therapeutic strategy and demonstrate that DDR1 is a target that can be inhibited pharmacologically.