Abstract C77: Oncogenic drivers reactivate kinase-inhibited focal adhesion kinase (FAK) through phosphorylation of effector residues

Abstract

Abstract Introduction: Focal Adhesion Kinase (FAK) is a critical signaling molecule showing elevated expression in nearly all tumor types. Phosphorylation of key tyrosine residues within FAK underlies its dual roles as both a kinase and a molecular scaffold that integrates signals from major oncogenes. Though FAK has emerged as a major drug target, inhibitors developed to date singularly target its kinase activity, leaving the scaffolding of oncogenic drivers intact. Our studies demonstrate that cells treated with FAK kinase inhibitors PF-573228, PF-562271, and defactinib retain phosphorylation of several key tyrosine residues downstream of oncogenic receptor tyrosine kinases (RTKs). Methods: We examined the phosphorylation status of key FAK scaffolding sites Y861 and Y925 in RTK expressing normal and cancer cells treated with FAK kinase inhibitors PF-573228, PF-562271, and defactinib. As Src kinase is known to directly phosphorylate Y861 and Y925, we also tested phosphorylation of these sites by RTKs in Src/Yes/Fyn (SYF) MEFs, which lack the major Src family kinases. Results: In cancer cells treated with FAK kinase inhibitors, multiple RTKs (HER2/3, PDGFR, and EGFR) activated FAK through the phosphorylation of effector tyrosines. Activation of HER2/3 in MDA-MB-453 breast cancer cells, PDGFR in U-87 glioblastoma cells, and EGFR in A549 lung cancer cells all stimulated Y861/Y925 phosphorylation in the presence of each of the three FAK kinase inhibitors. Moreover, Y861/Y925 phosphorylation occurred downstream of PDGFR and EGFR in Src-null SYF MEFs, demonstrating that RTK FAK targeting does not require Src. Finally, in vitro studies demonstrated that RTKs can directly phosphorylate these sites on FAK. Conclusion: We have identified a novel mechanism through which FAK retains signaling activity in the presence of kinase inhibitors. Continued FAK signaling in tumor cells which overexpress HER2, EGFR or PDGFR may therefore be a mode of clinical resistance to FAK kinase inhibitors. Citation Format: Sheila Figel, Felicia Lenzo, Timothy Marlowe, William Cance. Oncogenic drivers reactivate kinase-inhibited focal adhesion kinase (FAK) through phosphorylation of effector residues. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C77.