Abstract B14: Targeting the intratumoral heterogeneity of receptor tyrosine kinases in breast cancer
Authors: Tovar Elizabeth A. , Linklater Erik S. , Essenburg Curt J. , Madaj Zach , Cherba David M. , Winn Mary E. , Korkaya Hasan , Boerner Julie L. , Graveel Carrie R. 
Published: May 2016

Abstract
Breast cancers display a remarkable phenotypic diversity that is exploited to promote both tumor progression and therapeutic resistance. Recent studies in several types of cancer have highlighted the significance of intratumoral heterogeneity on both innate and acquired resistance to tyrosine kinase inhibitors (TKIs). Tumor plasticity is supported by the heterogeneous expression of receptor tyrosine kinases (RTKs) and the robustness that the overlapping signaling networks provide. Therefore a thorough understanding of the intratumoral heterogeneity is necessary for the development of effective therapeutic strategies.
The receptor tyrosine kinase MET is overexpressed in 20-30% of breast cancers and correlates with poor patient outcome. Previously, we determined that high MET expression correlated with ER-/ERBB2- and basal like breast cancers. These results and the efficacy of MET inhibitors in other cancers suggest that MET may be an effective clinical target for aggressive breast cancer subtypes. Recent studies have exposed interactions between MET and the ERBB receptor family in the progression and therapeutic resistance of several cancers. Since MET, ERBB2, and EGFR are known to be highly expressed in aggressive breast cancer subtypes, it is critical that we understand the relationships between these receptors in order to develop effective treatment strategies.
We are investigating the relationship between MET and ERBB receptor signaling in the progression and resistance of ERBB2+ and triple-negative breast cancer (TNBC). We observe that there is a large subset of ERBB2+ breast cancers that express MET and contain MET+/ERBB2+ subpopulations. In a MET+/ERBB2+ breast cancer cell line, MET depletion results in increased ERBB2 activation whereas, ERBB2 depletion results in increased MET activation. Therefore, ERBB2+ breast cancers with MET+ subpopulations may have an innate resistance to ERBB2 inhibition and may benefit from combined MET and ERBB2 inhibition. In TNBC, we observe heterogeneous expression of MET and EGFR. We have developed patient-derived xenografts (PDX) from primary and metastatic TNBCs that have diverse patterns of MET and EGFR expression/activation. In these TNBC PDX models we observe varied responses to monotherapy with MET inhibitors MGCD265 and crizotinib and the EGFR inhibitor erlotinib. Interestingly, therapeutic response to MET inhibition does not correlate with protein expression levels. In all studies, we observe significantly increased efficacy of combination therapy with MET and EGFR inhibition and a decrease in response variability. Examination of phospho-MET localization in treated tumorgrafts revealed that MET and EGFR inhibition induces distinct phospho-MET localization changes. We also observe that in the residual resistant cells phospho-MET is highly expressed in cells with mitotic bodies. We are currently performing phospho-proteomic analysis to determine the effect of MET and/or EGFR inhibition on RTK signaling networks including ERK and AKT pathways. These results will identify proteomic signatures that represent MET and/or EGFR activation/inhibition and sensitivity or resistance to monotherapy or combined MET and EGFR inhibition. Overall these studies give us a more comprehensive view of TNBC network signaling, the level of RTK heterogeneity within TNBC, and the efficacy of MET and/or EGFR inhibition on TNBC progression.
Citation Format: Elizabeth A. Tovar, Erik S. Linklater, Curt J. Essenburg, Zach Madaj, David M. Cherba, Mary E. Winn, Hasan Korkaya, Julie L. Boerner, Carrie R. Graveel. Targeting the intratumoral heterogeneity of receptor tyrosine kinases in breast cancer. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr B14.