Abstract 182: SPARC as a regulator of collagen signaling in pancreatic cancer

The extracellular matrix (ECM) is a principal component of pancreatic ductal adenocarcinoma (PDA), which is rich in fibrillar collagens, and provides structural support that facilitates tumor cell survival and chemoresistance. Collagen expression and deposition is a complex process that is orchestrated in part by the matricellular protein SPARC. SPARC expression in human patients with PDA correlates with improved chemoresponse; however, this mechanism is unclear. We propose that SPARC controls collagen binding to the cell surface and reduces collagen-mediated signaling via Discoidin domain receptors (DDR1, DDR2). DDR1/DDR2 are homologous receptor tyrosine kinases that bind specifically to fibrillar collagens and have been implicated regulating cell proliferation, migration, adhesion, ECM remodeling, and response to growth factors. Additionally, collagen has been shown to promote chemoresistance in pancreatic tumor cells. We hypothesize that SPARC promotes chemoresponse by reducing collagen-mediated DDR signaling. Structural studies identified that SPARC and DDRs share the same collagen binding site. We have demonstrated that SPARC inhibits collagen binding to DDR1/DDR2 via in vitro binding assays and cell based activity assays. To determine the functional relevance of SPARC expression and collagen-mediated DDR activation in PDA, Sparc-null (Sparc-/-) mice were crossed with KIC (LSL-KrasG12D; Cdkn2alox/lox; p48Cre/+) mice. Survival was reduced and tumors were more aggressive in Sparc-/- KIC along with an elevation in DDR-mediated signaling in the tumor microenvironment. Primary PDA cell lines isolated from Sparc+/+ and Sparc-/- KIC animals were used to probe collagen signaling cascades revealing that collagen activated Ddr1 and downstream intermediates including protein tyrosine kinase 2 (Pyk2) and pseudopodium-enriched atypical kinase 1 (Peak1) in Sparc-/- cell lines. Furthermore, recombinant SPARC or neutralizing antibodies to Ddr1 abrogated collagen induced Ddr1 signaling in vitro implicating Ddr1 as a putative therapeutic target in PDA. Pharmacologic inhibition of Ddr1 was evaluated with nilotinib, a BCR-Abl inhibitor that shows high activity against Ddr1, and 7rh, a novel inhibitor with high specificity for Ddr1. In vitro nilotinib and 7rh suppress collagen-induced Ddr1 signaling, cell migration, and colony formation. In vivo nilotinib reduced the growth of implanted pancreatic tumors in Sparc-/- animals with little effect on tumor growth in Sparc+/+ m