Abstract 302: ECM/Integrin signaling promotes resistance to the combination of HER2 and PI3K inhibitors in HER2+, PIK3CA-mutant breast cancer
Authors: Hanker Ariella B. , Estrada Monica Valeria , Zhao Junfei , Cheng Feixiong , Moore Preston D. , Tyson Darren , Sanchez Violeta , Rexer Brent N. , Sanders Melinda , Zhao Zhongming , Stricker Thomas P. , Arteaga Carlos L. 
Published: July 2016

Abstract
HER2 amplification and activating mutations in PIK3CA, the gene encoding the p110α subunit of PI3K, often co-occur in breast cancer. We generated a transgenic mouse model of HER2-overexpressing (HER2+), PIK3CAH1047R-mutant breast cancer. In these mice, PIK3CAH1047R accelerates HER2-mediated tumor formation and promotes resistance to HER2 inhibitors (Hanker et al. PNAS 2013). HER2+/PIK3CA tumor growth was inhibited by treatment with the HER2 antibodies trastuzumab and pertuzumab in combination with the pan-PI3K inhibitor BKM120 (TPB). We sought to discover mechanisms of acquired resistance to the triple therapy by long-term treatment of established HER2+/PIK3CA tumors. Tumor transplants derived from a transgenic HER2+/PIK3CA tumor were initially growth inhibited by TPB. After several weeks, a subset of transplants (3/11) resumed growth in the presence of continuous TPB therapy. TPB-resistant tumors were cross-resistant to the combination of T + P + BYL719 (a p110α-specific inhibitor).
Whole exome sequencing did not identify acquired somatic alterations in TPB-resistant tumors, including in HER2. However, RNA-seq revealed significant transcriptional upregulation of extracellular matrix (ECM) genes and genes involved in cell adhesion, including collagens, tenascins, and thrombospondins. Likewise, trichrome staining revealed a significant increase in collagen fibers and IHC analysis confirmed increased Tenascin expression in the TPB-resistant tumor stroma. In addition, western blot analysis revealed increased expression of an activated form of integrin β1, a substrate for ECM ligands such as collagen, as well as P-SrcY416 (activated by integrins/focal adhesion). We also found that transcription of many of these genes is induced by short-term TPB treatment in human breast cancer cell lines by qRT-PCR.
Interestingly, primary tumor cells derived from TPB-resistant tumors no longer displayed resistance when grown in vitro. These cells regained TPB resistance when re-introduced into mice. Plating primary tumor cells on growth factor-reduced Matrigel or on Collagen I-coated plates restored resistance, suggesting that the ECM directly promotes TPB resistance. We are currently investigating whether inhibition of Integrin/Src signaling reverses TPB resistance. We are also exploring whether components of the ECM are altered in residual disease specimens from HER2+ breast cancer patients treated with neoadjuvant anti-HER2 therapies. Our data suggest that upregulation of ECM/integrin/Src signaling contributes to resistance to combinations of HER2 and PI3K inhibitors, and strongly support the growing body of literature indicating that components of the tumor microenvironment promote resistance to targeted therapies.
Citation Format: Ariella B. Hanker, Monica Valeria Estrada, Junfei Zhao, Feixiong Cheng, Preston D. Moore, Darren Tyson, Violeta Sanchez, Brent N. Rexer, Melinda Sanders, Zhongming Zhao, Thomas P. Stricker, Carlos L. Arteaga. ECM/Integrin signaling promotes resistance to the combination of HER2 and PI3K inhibitors in HER2+, PIK3CA-mutant breast cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 302.