Abstract LB-206: The role of fibroblast activation protein (FAP) in lung tumorigenesis

The desmoplastic reaction, characterized by the recruitment and differentiation of a heterogeneous population of carcinoma-associated stromal cells (CASCs) as well as extensive extracellular matrix (ECM) deposition and remodeling, regulates tumorigenesis via complex and incompletely understood mechanisms. In virtually all epithelial-derived tumors, CASCs selectively express FAP, a cell surface serine protease that promotes tumorigenesis. We investigated the mechanisms that govern FAP expression as well as the mechanisms by which FAP regulates the intratumoral desmoplastic reaction and, consequently, lung tumor progression. A screen of soluble factors revealed (1) that the inducible expression and activity of FAP in lung fibroblasts depended on ECM components and (2) differential requirements for induction of FAP and αSMA (a canonical marker of myofibroblasts). Unlike TGFβ-mediated induction of αSMA expression, TGFβ-mediated induction of FAP expression depended on a collagen-rich substratum. These experiments underscored the fundamental role of ECM in mediating fibroblast differentiation. We further delineated bidirectional regulatory mechanisms between activated fibroblasts and ECM by studying FAP-dependent changes to ECM. Using two-photon second harmonic generation imaging, we found that activated FAP−/− lung fibroblasts deposited ECMs with a significant twofold enhancement of fibrillar collagen accumulation compared to WT fibroblast-derived ECMs. Immunoblots illustrated differences in type I collagen fragmentation in WT versus FAP−/− fibroblast-derived ECMs, substantiating prior evidence that FAP plays a role in the ordered proteolytic processing of fibrillar collagen. In addition, collagen contraction assays demonstrated that FAP negatively regulates fibroblast-mediated contraction of type I collagen. Collectively, these experiments highlight the importance of FAP in orchestrating fibroblast-dependent remodeling of intratumoral ECM composition and architecture. Lastly, by analyzing tumor burden in the spontaneous KrasG12D latent allele model, we found that FAP−/- mice had significantly smaller lung tumors than WT mice. However, the multiplicity of lung tumors did not differ between WT and FAP−/− mice. These results indicate that FAP promotes progression, rather than initiation, of lung tumorigenesis. Current analyses are studying differences in intratumoral ECM in this model and how these changes to lung CASC-derived ECM influence tumor cell proliferation. In summary, we have delineated bidirectional regulatory mechanisms between FAP and ECM as well as FAP-dependent regulation of the desmoplastic reaction and lung tumor progression. These studies further clarify the mechanisms by which the microenvironment promotes tumorigenesis and validate FAP as a potential therapeutic target in the treatment of cancer. Citation Format: Diana Avery, Michelle Jacob, Lisa Chang, Leslie Todd, Ellen Pure. The role of fibroblast activation protein (FAP) in lung tumorigenesis. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-206. doi:10.1158/1538-7445.AM2015-LB-206