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HOXB7 is an ERα cofactor in the activation of HER2 and multiple ER target genes leading to endocrine resistance

ABSTRACT: Why breast cancers become resistant to tamoxifen despite continued expression of the estrogen receptor alpha (ERα) and what factors are responsible for high HER2 expression in these tumors remains an enigma. HOXB7 ChIP analysis followed by validation showed that HOXB7 physically interacts with ERα, and that the HOXB7-ERα complex enhances transcription of many ERα target genes including HER2. Investigating strategies for controlling HOXB7, our studies revealed that MYC, stabilized via phosphorylation mediated by EGFR-HER2 signaling, inhibits transcription of miRNA-196a, a HOXB7 repressor. This leads to increased expression of HOXB7, ER-target genes and HER2. Repressing MYC using small molecule inhibitors reverses these events, and causes regression of breast cancer xenografts. The MYC-HOXB7-HER2 signaling pathway is eminently targetable in endocrine-resistant breast cancer.

ORGANISM(S): Homo sapiens

PROVIDER: GSE63607 | GEO | 2014/11/25

SECONDARY ACCESSION(S): PRJNA268440

REPOSITORIES: GEO

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Project description: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.

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HOXB7 is an ERα cofactor in the activation of HER2 and multiple ER target genes leading to endocrine resistance

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