Abstract PD1-07: Mutant ESR1 receptors antagonize the tumor suppressor function of androgen receptors

Background: Acquired ESR1 mutations are a dominant driver of distant metastasis in metastatic breast cancer, inducing a basal-like phenotype that is relatively resistant to ER antagonists with decreased progression free survival. The majority of ER+ patients also express the androgen receptor (AR), and although AR expression is associated with better outcomes, high AR expression has also been associated with resistance to endocrine therapy (ET). Thus, there is currently a conundrum on how best to target AR, and to define when it is participating as a proliferative/metastatic driver, or when it may be a potential tumor suppressor associated with good prognosis. Methods: We generated ESR1 Y537S or D538G homozygous mutations in MCF-7 cells using CRISPR Cas-9 technology. ChIP-Seq, transcriptome analyses, and quantitative analysis of ER ChIP-Seq profiles from patient biopsies were integrated. Nuclear fractionation and immunoblot analyses were performed. Results: Correlation and gene set enrichment analyses demonstrated that the androgen response pathway was significantly reduced in ESR1 mutant, compared to wild-type (WT) ER cells. A dramatic redistribution in AR binding sites to heterochromatin was observed in ESR1 mutant cells. AR and ER co-occupied DNA binding sites were redistributed only to ERE motifs present in a restricted set of gene enhancer regions. Quantitative analysis of ER DNA binding profiles from ER+ patients showed that the highest recruitment of ER to AR/ER binding sites were in patients with metastatic breast cancer. Integration of AR/ER co-bound sites with the differentially-expressed mutant transcriptome identified a gene signature that predicted poor disease-free and overall survival in ER+ primary breast cancer patients from the METABRIC database. Elevated gene expression of these AR/ER co-regulated genes, including NCOA3, BMP7, N4BP3, and FOXA1, were validated in a cohort of metastatic tumors (N=97), compared to primary tumors (N=276). AR protein levels were elevated in ESR1 mutant tumors. Mechanistic studies demonstrated that elevated AR phosphorylation at specific sites (pS515, pS650) and decreased phosphorylation (pS308) occurs in mutant cells. These sites are known to affect AR transcriptional activity, protein stability, and nuclear export. Decreased K48-AR ubiquitination was observed, suggesting that elevated AR protein levels result from altered post-transcriptional modification of AR in mutant cells. Conclusions: We propose a two-prong genomic activation mechanism in ESR1 mutant tumors with AR redistribution to heterochromatin, and genomic co-activation of AR and ER. We hypothesize that the loss of AR tumor suppressor function may help drive metastasis in ESR1 mutant tumors via AR’s collaboration with ESR1 mutant oncogenic activity. Efficacious targeting of AR may require disruption of this driver AR/ER-regulated transcriptional program.

Citation Format: Sandra L. Grimm, Guowei Gu, Sarah K. Herzog, Thomas L. Gonzalez, Hangqing Lin, Amanda R. Beyer, Yassine Rechoum, Tasneem Bawa-Khalfe, Ashfia F. Khan, Lili Du, W. Fraser Symmans, Ralf Kittler, Cristian Coarfa, Suzanne A.W. Fuqua. Mutant ESR1 receptors antagonize the tumor suppressor function of androgen receptors [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr PD1-07.

Abstract PD8-09: Androgen and ESR1 mutant receptors mechanistically collaborate for overexpression of genes associated with poor outcomes in ER-positive metastatic breast cancer

Background: Recurrences are delayed for years with endocrine therapy (ET), but resistance ultimately evolves with the development of ER+ metastatic breast cancer (MBC). ESR1 mutations are acquired in MBC undergoing ET in about 20-40% of patients. We have previously demonstrated that AR overexpression confers resistance to both tamoxifen and aromatase inhibitor treatments. We discovered that AR protein is up-regulated in tumors expressing ESR1 mutations, and herein explore the functional consequences of AR-ESR1 mutant co-expression on metastatic progression of breast cancer. Objective: We hypothesize that ET resistance mechanisms driven by AR overexpression and acquisition of ESR1 mutations co-evolve under the selective pressure of therapy to drive distant metastasis. We investigated whether AR engages in novel genomic binding interactions with mutant ER to promote tumor progression. Methods: Expression profiling of CRISPR-Cas9 engineered cell lines was integrated with chromatin immunoprecipitation-sequencing (ChIP-Seq) to define the genomic distribution of ER and AR binding sites. Genomic distribution of AR-ER co-occupied binding sites from MBC patient tumors, and a gene expression signature associated with patient outcomes was generated using using Cox Regression analysis to calculate Hazard Ratios for recurrence free survival from gene expression in the KMPlotter Breast Cancer Dataset and signature validation by Kaplan Meier analysis in the ER+ METABRIC patient cohort. Results: AR protein was post-transcriptionally elevated in ESR1 mutant cells, including distant metastatic tumors from ESR1 mutant xengraft models. Transcriptome data showed significant elevation of Cancer Hallmark pathways, including epithelial-mesenchymal transition (EMT) and estrogen response, but down-regulation of androgen response genes. However, both ER and AR functions were constitutively activated, independent of their respective ligands in ESR1 mutant-expressing models. AR was disproportionally re-distributed to selected enhancer genomic regions, but also heterochromatin in ESR1 mutant compared to cells with WT ER in the presence of estrogen. In addition, AR-ER co-occupied sites were increased in enhancer regions in ESR1 mutant cells, and these sites best discriminated distal ER enhancer regions in non-responder patients and MBC. . The expression of genes co-bound by AR and ER were significantly associated with shorter recurrence-free and overall survival in ER+ breast cancer. Treatment with AR agonists blocked distant metastasis of ESR1 mutant PDX models in vivo, in part due to reduction in ER protein and ER binding to select genomic sites. Conclusions: Our results demonstrate that unliganded AR collaborates with ESR1 mutations to regulate genes associated with the metastasis of ER+ breast cancer. We hypothesize that in an ESR1 mutant genomic background, AR acquires ER co-activator functions that are blocked with AR agonists, suggesting a unique hormonal therapeutic vulnerability in tumors with ESR1 mutations.

Citation Format: Suzanne Fuqua, Thomas L Gonzalez, Guowei Gu, Sarah Herzog, Yassine Rechoum, Amanda Beyer, David Edwards, Hangqing Lin, BAsil Paul, Ralf Kittler, Nancy Weigel, Sandra Grimm, Cristian Coarfa, Tasneem Bawa-Khalfe. Androgen and ESR1 mutant receptors mechanistically collaborate for overexpression of genes associated with poor outcomes in ER-positive metastatic breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PD8-09.

Correction to: Targeted degradation of activating estrogen receptor α ligand-binding domain mutations in human breast cancer

In the original publication of the article, the spelling of the sixth author's given name was incorrect. The corrected author name should read as "Wadie David". The original article has been corrected.

Targeted degradation of activating estrogen receptor α ligand-binding domain mutations in human breast cancer

PURPOSE

Studies have identified several estrogen receptor α (ERα) ligand-binding domain (LBD) somatic mutations in endocrine therapy resistant, metastatic ER-positive breast cancers. The most common mutations, Tyr537Ser (Y537S) and Asp538Gly (D538G), are detected in ~ 30% of endocrine resistant metastatic breast cancer patients. These ESR1 mutations induce the agonist conformation of ERα, confer an estrogen-independent phenotype, and promote drug resistance to antiestrogens.

METHODS

ER-positive, estrogen-dependent MCF-7 cells were engineered to express either the Y537S or D538G mutants using CRISPR knock-in (cY537S and cD538G). These cells were used to screen several estrogen receptor degrader (ERD) compounds synthesized using the Proteolysis Targeting Chimeras (PROTAC) method to induce degradation of ERα via the ubiquitin-proteasome pathway.

RESULTS

Wild-type MCF-7 and ERα LBD mutant cells were treated with ERD-148 (10 pM-1 µM) and assayed for cellular proliferation using the PrestoBlue cell viability assay. ERD-148 attenuated ER-dependent growth with IC₅₀ values of 0.8, 10.5, and 6.1 nM in MCF-7, cY537S, and cD538G cells, respectively. Western blot analysis showed that MCF-7 cells treated with 1 nM ERD-148 for 24 h exhibited reduced ERα protein expression as compared to the mutants. The ER-regulated gene, GREB1, demonstrated significant downregulation in parental and mutant cells after 24 h of ERD-148 treatment at 10 nM. Growth of the ER-negative, estrogen-independent MDA-MB-231 breast cancer cells was not inhibited by ERD-148 at the ~ IC₉₀ observed in the ER-positive cells.

CONCLUSION

ERD-148 inhibits the growth of ER-positive breast cancer cells via downregulating ERα with comparable potency to Fulvestrant with marginal non-specific toxicity.

Abstract 4766: Transcriptomic profiling reveals a potential role for JAK/STAT inhibition in CDK4/6 inhibitor-resistant, ER+ breast cancers

Purpose: Specific cyclin-dependent kinase (CDK) inhibitors are standard of care for patients with metastatic, estrogen receptor-positive (ER+) breast cancer. CDK4/6 inhibitors have improved rates of progression free survival among metastatic, ER+ patients, but resistance limits their clinical efficacy. Various mechanisms of resistance to CDK4/6 inhibitors have been reported, but a comprehensive understanding of this resistance remains elusive.

Methods: We generated in vitro models of acquired (AR) and intrinsic (IR) resistance to CDK4/6 inhibitors using ER+ breast cancer cell lines (MCF-7, T47D) cultured with either continuous high dose (500nM) or dose-escalated (50nM to 500nM) CDK4/6 inhibition over three months. RNA expression and gene set enrichment analysis (GSEA) was used to nominate potential pathways associated with AR and IR palbociclib resistance. Reverse phase protein array (RPPA) and western blots were used to measure protein and phosphoprotein levels in CDK4/6 inhibitor resistant cell lines to validate nominated pathways. Cellular proliferation assays were performed to calculate the half-maximal inhibitory concentration (IC50) with inhibitors for CDK4/6 and JAK/STAT.

Results: Proliferation assays confirmed that MCF-7 AR and IR cells are resistant to palbociclib (IC50 both >1uM) compared to parental cells (60nM); similar results were observed in the T47D cell lines. Cells resistant to either palbociclib, ribociclib, or abemaciclib demonstrated cross resistance to all three inhibitors. GSEA of transcriptomic data identified 579 genes (from AR cells) and 936 genes (from IR cells) that were differentially expressed between palbociclib-resistant MCF-7s and parental controls. RPPA analyses identified several key pathways that regulate CDK4/6 inhibitor resistance in these models. From GSEA analysis, the interferon (JAK/STAT) signaling pathway was the most differentially expressed pathway identified between palbociclib-resistant and sensitive cells. Western blot analyses showed that baseline expression of phospho-STAT1 is significantly elevated in palbociclib-resistant cells. In cellular proliferation assays, palbociclib-resistant MCF-7s and T47Ds retained sensitivity to JAK/STAT inhibitors like the JAK2-selective compound AZ960.

Conclusions: Our data suggests that overactivation of JAK/STAT signaling may be directly involved in the development of CDK4/6 inhibitor resistance in ER-dependent tumors. CDK4/6 inhibitor-resistant cells retain sensitivity to single-agent JAK/STAT inhibition, suggesting that this may be a viable therapeutic option for patients with CDK4/6 inhibitor-resistant ER+ breast cancer. This work was supported in part by 5T32GM007767-40 (Pesch), the Breast Cancer Research Foundation (N003173 to JMR), the UM Rogel Cancer Center and the Taubman Emerging Scholar funds.

Citation Format: Andrea M. Pesch, Thomas L. Gonzalez, Benjamin C. Chandler, Siqi Sun, Christina L. Gersch, José M. Larios, Wadie S. David, Corey W. Speers, James M. Rae. Transcriptomic profiling reveals a potential role for JAK/STAT inhibition in CDK4/6 inhibitor-resistant, ER+ breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4766.

Metabolites of n-Butylparaben and iso-Butylparaben Exhibit Estrogenic Properties in MCF-7 and T47D Human Breast Cancer Cell Lines

Two oxidized metabolites of n-butylparaben (BuP) and iso-butylparaben (IsoBuP) discovered in human urine samples exhibit structural similarity to endogenous estrogens. We hypothesized that these metabolites bind to the human estrogen receptor (ER) and promote estrogen signaling. We tested this using models of ER-mediated cellular proliferation. The estrogenic properties of 3-hydroxy n-butyl 4-hydroxybenzoate (3OH) and 2-hydroxy iso-butyl 4-hydroxybenzoate (2OH) were determined using the ER-positive, estrogen-dependent human breast cancer cell lines MCF-7, and T47D. The 3OH metabolite induced cellular proliferation with EC50 of 8.2 µM in MCF-7 cells. The EC50 for 3OH in T47D cells could not be reached. The 2OH metabolite induced proliferation with EC50 of 2.2 µM and 43.0 µM in MCF-7 and T47D cells, respectively. The EC50 for the parental IsoBuP and BuP was 0.30 and 1.2 µM in MCF-7 cells, respectively. The expression of a pro-proliferative, estrogen-inducible gene (GREB1) was induced by these compounds and blocked by co-administration of an ER antagonist (ICI 182, 780), confirming the ER-dependence of these effects. The metabolites promoted significant ER-dependent transcriptional activity of an ERE-luciferase reporter construct at 10 and 20 µM for 2OH and 10 µM for 3OH. Computational docking studies showed that the paraben compounds exhibited the potential for favorable ligand-binding domain interactions with human ERα in a manner similar to known x-ray crystal structures of 17ß-estradiol in complex with ERα. We conclude that the hydroxylated metabolites of BuP and IsoBuP are weak estrogens and should be considered as additional components of potential endocrine disrupting effects upon paraben exposure.

Homology models of mouse and rat estrogen receptor-α ligand-binding domain created by in silico mutagenesis of a human template: molecular docking with 17ß-estradiol, diethylstilbestrol, and paraben analogs

Crystal structures exist for human, but not rodent, estrogen receptor-α ligand-binding domain (ERα-LBD). Consequently, rodent studies involving binding of compounds to ERα-LBD are limited in their molecular-level interpretation and extrapolation to humans. Because the sequences of rodent and human ERα-LBDs are > 95% identical, we expected their 3D structures and ligand binding to be highly similar. To test this hypothesis, we used the human ERα-LBD structure (PDB 3UUD) as a template to produce rat and mouse homology models. Employing the rodent models and human structure, we generated docking poses of 23 Group A ligands (17ß-estradiol, diethylstilbestrol, and 21 paraben analogs) in AutoDock Vina for interspecies comparisons. Ligand RMSDs (Å) (median, 95% CI) were 0.49 (0.21-1.82) (human-mouse) and 1.19 (0.22-1.82) (human-rat), well below the 2.0-2.5 Å range for equivalent docking poses. Numbers of interspecies ligand-receptor residue contacts were highly similar, with Sorensen Sc (%) = 96.8 (90.0-100) (human-mouse) and 97.7 (89.5-100) (human-rat). Likewise, numbers of interspecies ligand-receptor residue contacts were highly correlated: Pearson r = 0.913 (human-mouse) and 0.925 (human-rat). Numbers of interspecies ligand-receptor atom contacts were even more tightly correlated: r = 0.979 (human-mouse) and 0.986 (human-rat). Pyramid plots of numbers of ligand-receptor atom contacts by residue exhibited high interspecies symmetry and had Spearman r s = 0.977 (human-mouse) and 0.966 (human-rat). Group B ligands included 15 ring-substituted parabens recently shown experimentally to exhibit decreased binding to human ERα and to exert increased antimicrobial activity. Ligand efficiencies calculated from docking ligands into human ERα-LBD were well correlated with those derived from published experimental data (Pearson partial r p = 0.894 and 0.918; Groups A and B, respectively). Overall, the results indicate that our constructed rodent ERα-LBDs interact with ligands in like manner to the human receptor, thus providing a high level of confidence in extrapolations of rodent to human ligand-receptor interactions.

Implication of environmental estrogens on breast cancer treatment and progression

Breast cancer is the most diagnosed malignancy among women in the United States. Approximately 70% of breast tumors express estrogen receptor alpha and are deemed ER-positive. ER-positive breast tumors depend upon endogenous estrogens to promote ER-mediated cellular proliferation. Decades of research have led to a fundamental understanding of the role ER signaling in this disease and this knowledge has led to significant advancements in the clinical use of antiestrogens for breast cancer treatment. However, adjuvant breast cancer recurrence and metastatic disease progression due to endocrine therapy resistance are prominent and unresolved issues. The established role that estrogens play in breast cancer pathogenesis explains why some patients initially respond to endocrine therapy but also why a significant number of patients become refractory to antiestrogen treatment. It is been hypothesized that exposure to environmental steroid hormone mimics and/or acquired mechanisms of resistance may explain why endocrine therapy fails in a subset of breast cancer patients. This review will highlight: 1) the relationship between ER signaling and breast cancer pathogenesis, 2) the implication of environmental exposures on steroid hormone regulated processes including breast cancer, and 3) the unresolved issue of endocrine therapy resistance.