Abstract A48: MET and EGFR interaction promotes acquired resistance to kinase inhibition in TNBC
Authors: Tovar Elizabeth , Essenburg Curt , Peck Anderson , Turner Lisa , Madaj Zachary , Smith Matthew , Christensen James , Melnik Marianne , Haura Eric , Steensma Matthew , Graveel Carrie 
Published: August 2018

Abstract
Triple-negative breast cancer (TNBC) has the worst prognosis of all breast cancers, and the molecular heterogeneity within TNBC heightens the challenge of developing effective targeted therapies. Receptor tyrosine kinases (RTKs), in particular MET and EGFR, are promising therapeutic targets for TNBC due to their high expression in multiple molecular TNBC subtypes and the tendency for cancers to become “kinase addicted.” We and others have demonstrated that MET is highly expressed in TNBC and its expression correlates with poor prognosis. EGFR expression is also elevated in up to 72% of TNBCs and correlates with poor prognosis in TNBC patients. Recently, we demonstrated that MET and EGFR are coordinately and highly expressed across all TNBC subtypes and the efficacy of combined MET and EGFR inhibition in TNBC patient-derived xenograft (PDX) models. Even though MET and EGFR receptors are actionable targets due to their high activity in TNBC, crosstalk between MET and EGFR has been implicated in therapeutic resistance to kinase inhibitors in several cancer types and needs to be evaluated in TNBC.
There is strong evidence demonstrating the critical role of redundant RTK signaling networks in resistance to tyrosine kinase inhibitors (TKIs). Specifically, crosstalk between MET and EGFR has been implicated in therapeutic resistance to EGFR or MET inhibitors in colon, gastric, and lung cancers. Since MET and EGFR are highly expressed in a substantial proportion of TNBCs, we used TNBC PDX models and TNBC cell lines to interrogate mechanisms of resistance to MET and EGFR kinase inhibition. We hypothesized that coexpression, interaction, and activation of MET and EGFR promote acquired TKI resistance and an adaptive kinome response in TNBC. Using TNBC PDX models that highly express both MET and EGFR, we evaluated MET and EGFR activity, localization, and interaction after 3 days or 3 weeks of treatment with MET (glesatinib, crizotinib) and/or EGFR inhibitors (erlotinib). After 3 weeks of dual MET and EGFR inhibition in TNBC PDXs, only a small population of “resistant” cells remained. In these resistant populations, we observed increased MET and EGFR colocalization and increased MET and EGFR activation. To understand how colocalization of MET and EGFR promotes signaling redundancy and crosstalk, we asked whether MET and EGFR directly interact in TNBC PDX using a proximity ligation assay (PLA). PLA uses antibody specificity to detect direct protein-protein interactions at physiologic levels in vitro and in vivo. We observed a significant increase in MET EGFR interactions only in resistant cell populations that had been treated with both MET and EGFR inhibitors. AKT and ERK signaling were also increased in TKI-resistant cells, indicating that these are key survival pathways in resistance. Interestingly, we did not observe an increase in MET-EGFR activity or interaction in PDX tumors treated for only 3 days. Even though TNBC cells (HCC70) showed increased colocalization of MET and EGFR with combined glesatinib and erlotinib treatment, there was not a significant increase in MET-EGFR interactions. Evaluation of MET-EGFR interaction in primary TNBC patient samples revealed significant MET-EGFR interactions in a MET-amplified tumor and lymph node metastases. To our knowledge, this is the first time MET-EGFR interactions have been observed in TNBC in vivo and in human patients. Moreover, MET-EGFR interactions are able to maintain MET, EGFR, ERK, and AKT activity through a yet-unknown mechanism. These results imply that MET EGFR interaction may be a unique mechanism of resistance to kinase inhibition that may be inherent in human TNBCs with genomic amplification of MET or EGFR.
Citation Format: Elizabeth Tovar, Curt Essenburg, Anderson Peck, Lisa Turner, Zachary Madaj, Matthew Smith, James Christensen, Marianne Melnik, Eric Haura, Matthew Steensma, Carrie Graveel. MET and EGFR interaction promotes acquired resistance to kinase inhibition in TNBC [abstract]. In: Proceedings of the AACR Special Conference: Advances in Breast Cancer Research; 2017 Oct 7-10; Hollywood, CA. Philadelphia (PA): AACR; Mol Cancer Res 2018;16(8_Suppl):Abstract nr A48.