Association of FGFR1 with ERα Maintains Ligand-Independent ER Transcription and Mediates Resistance to Estrogen Deprivation in ER+ Breast Cancer

Abstract

amplification occurs in approximately 15% of estrogen receptor-positive (ER) human breast cancers. We investigated mechanisms by which amplification confers antiestrogen resistance to ER breast cancer. ER tumors from patients treated with letrozole before surgery were subjected to Ki67 IHC, FGFR1 FISH, and RNA sequencing (RNA-seq). ER/-amplified breast cancer cells, and patient-derived xenografts (PDX) were treated with FGFR1 siRNA or the FGFR tyrosine kinase inhibitor lucitanib. Endpoints were cell/xenograft growth, FGFR1/ERα association by coimmunoprecipitation and proximity ligation, ER genomic activity by ChIP sequencing, and gene expression by RT-PCR. ER/-amplified tumors in patients treated with letrozole maintained cell proliferation (Ki67). Estrogen deprivation increased total and nuclear FGFR1 and FGF ligands expression in ER/amplified primary tumors and breast cancer cells. In estrogen-free conditions, FGFR1 associated with ERα in tumor cell nuclei and regulated the transcription of ER-dependent genes. This association was inhibited by a kinase-dead FGFR1 mutant and by treatment with lucitanib. ChIP-seq analysis of estrogen-deprived ER/-amplified cells showed binding of FGFR1 and ERα to DNA. Treatment with fulvestrant and/or lucitanib reduced FGFR1 and ERα binding to DNA. RNA-seq data from -amplified patients' tumors treated with letrozole showed enrichment of estrogen response and E2F target genes. Finally, growth of ER/amplified cells and PDXs was more potently inhibited by fulvestrant and lucitanib combined than each drug alone.s These data suggest the ERα pathway remains active in estrogen-deprived ER/-amplified breast cancers. Therefore, these tumors are endocrine resistant and should be candidates for treatment with combinations of ER and FGFR antagonists. .

FGFR1 underlies obesity-associated progression of estrogen receptor–positive breast cancer after estrogen deprivation

“…Consistent with this scenario, amplification and overexpression of FGFR1 are reported to occur in up to 20% of luminal (ER-positive) subtype breast cancers (15) and are implicated in resistance to endocrine therapy (14,15,46). Recently, FGFR1 was reported to interact with ER in the cytoplasm and nucleus of endocrine-resistant breast cancer cells, suggesting that ER remains functional through growth factor pathway activation, despite estrogen deprivation (14). Taken together, these observations support a dual-requirement hypothesis that we have previously presented (38,47), in which metabolic impairment associated with obesity converges with a positive energy balance following estrogen deprivation to form a tumor-promoting environment (Figure 7).…”

“…In this environment, breast tumors expressing FGFR1 would be particularly susceptible to FGF ligand-dependent survival and growth after estrogen deprivation. Consistent with this scenario, amplification and overexpression of FGFR1 are reported to occur in up to 20% of luminal (ER-positive) subtype breast cancers (15) and are implicated in resistance to endocrine therapy (14,15,46). Recently, FGFR1 was reported to interact with ER in the cytoplasm and nucleus of endocrine-resistant breast cancer cells, suggesting that ER remains functional through growth factor pathway activation, despite estrogen deprivation (14).…”

“…Using this approach, we identified activated GFR and kinase signaling pathways that were common to tumors from patients with a high BMI and either responders or nonresponders to letrozole ( Figure 1A). Among the putatively activated pathways that were common to both obesity and a poor response to letrozole were IGF1R and FGFR1, which have been implicated in endocrine therapy resistance (14,15,21). Of the 5 upstream regulatory pathways shared by tumors from overweight patients and nonresponders, FGFR1 and, to a lesser extent, IGFR1 were predicted to be decreased in tumors that did respond to letrozole ( Figure 1B).…”

“…By investigating well-characterized and potentially novel ER-positive breast cancer models in the context of obesity, we found that breast cancers showed divergent responses to estrogen deprivation. Tumors that did not respond to treatment in the obese mice displayed evidence of ligand-dependent FGFR1 activation, which is a mediator of endocrine therapy resistance (14,15). In primary human breast tumors, high levels of pFGFR1 associated with elevated BMI (≥25 kg/m 2 ) and also with shorter disease-free and breast cancer-specific survival.…”

Mortality after breast cancer as a function of time since diagnosis by estrogen receptor status and age at diagnosis

“…They eventually become resistant to ER‐targeted therapies through several mechanisms . A novel mechanism via FGFR1 amplification, which confers resistance to antiestrogens in these breast cancers, at least partly explains the limited effects of estrogen deprivation on ER+/ FGFR1 –amplified breast cancers …”