Abstract 2435: FGFR1 is associated with resistance to interaction with estrogen receptor (ER) α endocrine therapy in ER+/FGFR1-amplified breast cancer

Proceedings: AACR 106th Annual Meeting 2015; April 18-22, 2015; Philadelphia, PA Background: Molecular alterations in the fibroblast growth factor receptor (FGFR) pathway occur in breast cancer. FGFR1 gene amplification is found in ∼10% of ER+ breast cancers, where it is associated with early recurrence on endocrine therapy. FGFR1 amplification has been reported in 30-40% of breast tumors with CCDN1 amplification; co-amplification of these genes is associated with shorter patient survival. We investigated the mechanisms by which FGFR1 amplification confers endocrine resistance. Results: We used ER+ human breast cancer cell lines with (MDA-134, CAMA-1, HCC1500) or without (MCF7, ZR75-1) FGFR1 amplification. In these cells, we investigated the effect of FGFR1 silencing with siRNA or pharmacological inhibition using the small molecule lucitanib on cell growth, ER transcriptional activity and the interaction of FGFR1 and ERα. Both FGFR1 siRNA and treatment with lucitanib reduced ER transcriptional reporter activity and proliferation of ER+/FGFR1 amplified cell lines. Conversely, addition of FGF3 and FGF19 ligands stimulated ER reporter activity in estrogen-free medium, suggesting FGFR activation regulates ER activity. FGFR1 signaling is mainly transduced to the PI3K/AKT and RAS/RAF/MEK/ERK pathways by the adaptor FRS2. Neither siRNA against FRS2 nor pharmacological inhibitors of PI3K (BKM120) and MEK (GSK1120212) reduced ER transcriptional activity in ER+/FGFR1 amplified cells, suggesting a FRS2 independent, FGFR1-ERα direct interaction explaining the inhibitory effect of FGFR1 siRNA and lucitanib on ER function. MDA-134, CAMA-1 and HCC1500 FGFR1 amplified cell lines also harbor CCND1 amplification. Further, nuclear FGFR1 has been shown to regulate target gene expression, including CCND1. Therefore, we next examined if nuclear FGFR1 and cyclin D1 interact with ER. In ER or cyclin D1 antibody pulldowns from whole cell lysates and nuclear extracts of MDA-134, CAMA-1 and HCC1500 cells, ER co-precipitated with FGFR1 and cyclin D1. The associations of ER with FGFR and of ER with cyclin D1 were not inhibited upon blockade of the FGFR1 tyrosine kinase with lucitanib. However, the association of FGFR1 with cyclin D1 was inhibited by lucitanib treatment. Chromatin immunoprecipitation of DNA protein crosslinks in CAMA-1 cells ± estradiol with a FGFR1 antibody showed that FGFR1 binds to different estrogen response elements (ERE) in DNA. This FGFR1-ERE association was abrogated by treatment with lucitanib, suggesting it depended on an active FGFR1 tyrosine kinase. Conclusions: These data suggest FGFR1 binds ER and regulates ligand-independent ER transcriptional activity. This role depends on the FGFR1 kinase activity and may involve its association with cyclin D1. These interactions may explain the endocrine resistance reported in ER+/FGFR1 amplified breast cancers and suggest these tumors should be treated with a combination of antiestrogen and FGFR inhibitors. Citation Format: Luigi Formisano, Christian D. Young, Neil Bhola, Jennifer M. Giltnane, Monica V. Estrada, Carlos L. Arteaga. FGFR1 is associated with resistance to interaction with estrogen receptor (ER) α endocrine therapy in ER+/FGFR1-amplified breast cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2435. doi:10.1158/1538-7445.AM2015-2435