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Updated: Jan 19, 2026
In Vitro Mouse Bone Marrow Stromal Cell Differentiation into Adipocytes
Bone Marrow Stromal Cells Transcriptionally Repress ESR1 but Cannot Overcome Constitutive ESR1 Mutant Activity
David K Lung1,2, Jay W Warrick3, Peiman Hematti4
1Department of Oncology, McArdle Laboratory for Cancer Research, University of Wisconsin-Madison, Madison, Wisconsin.
Abstract:
Estrogen receptor α (ER) is the target of endocrine therapies in ER-positive breast cancer (BC), but their therapeutic effectiveness diminishes with disease progression. Most metastatic BCs retain an ER-positive status, but ER expression levels are reduced. We asked how the bone tumor microenvironment (TME) regulates ER expression. We observed ESR1 mRNA and ER protein downregulation in BC cells treated with conditioned media (CM) from patient-derived, cancer-activated bone marrow stromal cells (BMSCs) and the BMSC cell line HS5. Decreases in ESR1 mRNA were attributed to decreases in nascent transcripts as well as decreased RNA polymerase II occupancy and H3K27Ac levels on the ESR1 promoter and/or distal enhancer (ENH1). Repression extended to neighboring genes of ESR1, including ARMT1 and SYNE1. Although ERK/MAPK signaling pathway can repress ER expression by other TME cell types, MAPK inhibition did not reverse decreases in ER expression by BMSC-CM. ESR1 mRNA and ER protein half-lives in MCF7 cells were unchanged by BMSC-CM treatment. Whereas ER phosphorylation was induced, ER activity was repressed by BMSC-CM as neither ER occupancy at known binding sites nor estrogen response element-luciferase activity was detected. BMSC-CM also repressed expression of ER target genes. In cells expressing the Y537S and D538G ESR1 mutations, BMSC-CM reduced ESR1, but expression of target genes PGR and TFF1 remained significantly elevated compared with that of control wild-type cells. These studies demonstrate that BMSCs can transcriptionally corepress ESR1 with neighboring genes and inhibit receptor activity, but the functional consequences of the BMSC TME can be limited by metastasis-associated ESR1 mutations.
Insights
Bone marrow stromal cells (BMSCs) in the tumor microenvironment (TME) can reduce estrogen receptor alpha (ER) expression and activity in breast cancer (BC) cells. However, common ER mutations may limit the impact of BMSCs on ER target gene expression.
Area of Science:
- Oncology
- Molecular Biology
- Cancer Research
Background:
- Estrogen receptor alpha (ER) is a key target for endocrine therapies in ER-positive breast cancer (BC).
- Therapeutic effectiveness of ER-targeted therapies decreases with disease progression, often associated with reduced ER expression in metastatic BC.
- The bone tumor microenvironment (TME) may play a role in regulating ER expression.
Purpose of the Study:
- To investigate how the bone TME regulates ER expression in breast cancer cells.
- To determine the mechanisms by which bone marrow stromal cells (BMSCs) affect ER expression and activity.
- To assess the impact of metastasis-associated ER mutations on BMSC-mediated repression.
Main Methods:
- Treatment of BC cells with conditioned media (CM) from patient-derived BMSCs and HS5 cell line.
- Analysis of ESR1 mRNA and ER protein levels, nascent transcripts, RNA polymerase II occupancy, and H3K27Ac levels.
- Assessment of ERK/MAPK signaling pathway activity.
- Measurement of ER phosphorylation, ER activity via DNA binding and reporter assays, and ER target gene expression.
- Evaluation of ESR1 mutations (Y537S, D538G) in response to BMSC-CM.
Main Results:
- BMSC-CM significantly downregulated ESR1 mRNA and ER protein in BC cells.
- Transcriptional corepression of ESR1 and neighboring genes (ARMT1, SYNE1) by BMSCs was observed, involving reduced nascent transcription and RNA polymerase II occupancy.
- BMSC-CM repressed ER activity and target gene expression, despite inducing ER phosphorylation.
- Metastasis-associated ESR1 mutations (Y537S, D538G) partially rescued the expression of ER target genes (PGR, TFF1) from BMSC-mediated repression.
Conclusions:
- BMSCs within the bone TME can transcriptionally repress ESR1 expression and inhibit ER activity in breast cancer cells.
- The repressive effects of the BMSC TME on ER signaling can be limited by common ESR1 mutations found in metastatic disease.
- Understanding these TME-mediated regulatory mechanisms is crucial for developing more effective breast cancer therapies.
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