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Transcriptome Analysis Reveals a Role for Myc in Constitutive and Antiestrogen-resistant Proliferation of Breast Cancer Cells Expressing ERαY537S and ERαD538G Mutations

ABSTRACT: Approximately 30% of metastatic breast cancers harbor estrogen receptor α (ERα/ESR1) mutations that are associated with resistance to endocrine therapy and shorter patient survival. How these ERα mutations elicit an aggressive tumor phenotype was largely unknown. Recently, we reported cell lines in which the wild-type ESR1 gene is replaced by the two most common ERα mutations, ERαY537S and ERαD538G. Here we use transcriptome analysis to demonstrate that ERαY537S cells exhibit a unique expression profile and phenotype, and identify an important role for Myc in the increased growth rate, ligand independent cell proliferation, and partial resistance to tamoxifen and fulvestrant of cells expressing the ERαY537S and ERαD538G mutations. Notably, the T47DERαY537S (TYS) cells exhibit a markedly different phenotype and transcriptome than the parental T47D cells and T47DERαD538G (TDG) cells. Consistent with their estrogen-independent proliferation, RNAseq shows the TYS and TDG cells exhibit estrogen independent gene expression patterns. Gene set enrichment analysis showed Myc target pathways are highly induced in both mutant cell lines. Unlike the parental T47D cells, in TYS and TDG cells Myc mRNA and protein exhibit constitutive, fulevestrant-resistant expression. Moreover, chromatin immunoprecipitation (ChIP) showed that ERαY537S and ERαD538G exhibit constitutive, fulvestrant-resistant, recruitment to the Myc enhancer region. Myc knockdown showed that Myc protein is required for ligand independent proliferation of TYS and TDG cells. Constitutive MYC expression in T47D cells, to levels similar to those in TYS and TDG cells, partially recapitulated the aggressive phenotypes of the TYS and TDG cells. Our study demonstrates a previously undescribed role for constitutive and antiestrogen resistant Myc expression in the aggressive phenotype exhibited by breast cancer cells expressing ERα mutations and provides powerful tools for identifying pathways that contribute to the ERα mutant phenotype.

ORGANISM(S): Homo sapiens

PROVIDER: GSE108304 | GEO | 2019/02/07

REPOSITORIES: GEO

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Project description:Background: The constitutively active ESR1 Y537S mutation is associated with endocrine therapy resistance and progression of metastatic breast cancer through its effects on estrogen receptor (ERα) gene regulatory functions. However, the complex relationship between ERα and the progesterone receptor (PR), known as ERα/PR crosstalk, has yet to be characterized in the context of the ERα Y537S mutation. This study aimed to elucidate the effects of the ERα Y537S mutation on ERα/PR crosstalk and resultant transcriptional activity and to identify potential therapeutic sensitivities that may offer novel treatment options to patients with endocrine therapy-resistant breast cancer. Methods: Proximity-based interactions of ERα and PR were assessed via proximity ligation assay (PLA). Gene expression in MCF7 and T47D cells was assessed by RNA-seq analysis with comparison to publicly available patient tumor transcriptome data. Chromatin immunoprecipitation (ChIP)-qPCR and immunoblotting were used to assess ERα/PR-associated gene expression and protein expression, respectively. Data were analyzed by ordinary two-way ANOVA (α = 0.05) with Tukey’s multiple comparisons tests. Results: Physical interaction of ERα and PR was increased in the context of the ERα Y537S mutation, including in the nucleus where this interaction may translate to altered gene expression. As such, more than 30 genes were differentially expressed in both patient tumor and cell line data (MCF7 and/or T47D cells) in the context of the ERα Y537S mutation compared to ERα WT. Of these, IRS1 stood out as a gene of interest, and ERα and PR occupancy at chromatin binding sites along IRS1 were uniquely altered in the context of ERα Y537S. Furthermore, siRNA knockdown of IRS1 or treatment with the IRS1 inhibitor NT-157 indicated a significant anti-proliferative effect of IRS1 depletion or inhibition in ERα Y537S cell lines. Conclusions: Previous research has characterized gene regulatory changes associated with the ERα Y537S mutation from the viewpoint of ERα. Here, we identify consequential changes to both ERα and PR transcription factor activity, including at chromatin binding sites for the signaling adaptor protein IRS1. We identified a significant dependence of ERα Y537S-expressing cells on IRS1 for proliferation, implicating IRS1 as a potential therapeutic target for restoring treatment sensitivity to patients with breast cancers harboring ERα Y537S mutations.

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Transcriptome Analysis Reveals a Role for Myc in Constitutive and Antiestrogen-resistant Proliferation of Breast Cancer Cells Expressing ERαY537S and ERαD538G Mutations

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