Abstract 4825: Inhibition of AKT abrogates resistance to endocrine therapy in estrogen receptor positive breast cancer

Abstract

Abstract Estrogen receptor α-positive (ER+) breast cancers eventually adapt to hormone deprivation and acquire resistance to aromatase inhibitors. Upon acquisition of hormone independence, ER+ breast cancer cells increase their dependence on the phosphatidylinositol-3 kinase (PI3K)/AKT signaling pathway. Herein we investigated the effects of AKT inhibition on the hormone-independent growth of four ER+ breast cancer cell lines (MCF-7, ZR75-1, MDA-MB-361 and HCC-1428) with acquired resistance to estrogen deprivation (termed long-term estrogen-deprived cells, LTED). Treatment with the ATP-competitive AKT inhibitor AZD5363 inhibited phosphorylation of the AKT/mTOR substrates PRAS40, GSK3β/α, and S6K while inducing hyperphosphorylation of AKT at S473 and T308. AZD5363 suppressed the hormone-independent growth of 3/4 LTED lines. AZD5363 prevented the emergence of hormone-independent MCF-7, ZR75-1, and MDA-361 cells. Further, treatment of ovariectomized athymic mice with AZD5363 suppressed the hormone-independent growth of ER+/PI3K mutant MCF-7 xenografts. Combined treatment with AZD5363 and the ER downregulator fulvestrant more effectively suppressed xenograft growth than either drug alone. AZD5363 also caused dose-dependent inhibition of an ER+ breast cancer explant model unresponsive to fulvestrant or tamoxifen. We next examined whether AKT inhibition results in feedback upregulation of mitogenic signaling pathways. Treatment with AZD5363 resulted in upregulation and phosphorylation of the HER3 receptor tyrosine kinase (RTK), as well as upregulation of insulin receptor (InsR) protein levels and phosphorylation of Src at Y416. A Src family kinase inhibitor suppressed AZD5363-induced upregulation of P-HER3 in MCF-7/LTED cells. Src inhibition or siRNA-mediated knockdown of InsR or IGF-IR significantly enhanced the growth inhibitory effects of AZD5363. Following treatment with AZD5363, phospho-RTK array analysis revealed increased phosphorylation of multiple RTKs in MCF-7/LTED and/or ZR75-1/LTED cells, including InsR, IGF-IR, EGFR, HER2, HER3, HER4, Dtk, and FGFR3. Treatment with AZD5363 also upregulated RTK mRNA levels. Further, treatment of MCF-7/LTED cells with AZD5363 resulted in marked translocation of AKT PH-GFP to the membrane, reflective of increased PIP3 production and, thus, AKT phosphorylation at T308. Pre-treatment with the PI3K inhibitor BKM120 or the IGF-IR/InsR TKI AEW541 prevented AZD5363-induced membrane localization of PH-GFP. These results suggest that 1) AKT signaling is critical for hormone-independent growth of ER+ breast cancer cells in vitro and in vivo; and 2) AKT inhibition in these cells induces feedback upregulation of RTK expression and activity. Thus, inhibitors of AKT merit evaluation as a potential treatment for endocrine-resistant breast cancer. The combination of these agents with inhibitors of upstream RTKs are under evaluation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4825. doi:1538-7445.AM2012-4825