Endocrine therapy has long been a vital treatment option for estrogen receptor-alpha (ERα)-positive breast cancer. However, the frequent clinical problem of endocrine therapy resistance has left many patients in a difficult situation. This is particularly true when resistance leads to metastatic disease, often leaving patients with no viable treatment options.
TRIM24: A Potential New Strategy to Overcome Resistance
A recent study published in PNAS titled TRIM24 as a therapeutic target in endocrine treatment-resistant breast cancer offers a new direction for solving this challenge. The research confirms that TRIM24 is a critical component of the ERα transcription complex. By targeting the degradation of TRIM24, the study demonstrated that it can effectively inhibit ERα-driven transcriptional activity and tumor growth, providing a potential new strategy for treating endocrine therapy-resistant breast cancer.
The Role of ERα and the Challenge of Resistance
ERα-positive breast cancer accounts for over 75% of all breast cancers. As a hormone-driven transcription factor, ERα is a core driver of tumor development and progression, making it the primary target for endocrine therapy. Current endocrine treatments work in various ways: aromatase inhibitors suppress ligand activation, tamoxifen blocks co-regulator recruitment, and fulvestrant induces receptor degradation.
Despite these treatments, approximately 30% of patients still experience recurrence after adjuvant endocrine therapy. In most resistant metastatic tumors, EERα remains a driving factor. Furthermore, some patients develop ESR1 gene mutations, which allow ERα to be activated independent of hormones, further compounding the resistance.
TRIM24 is a Key Player in the ERα Complex
The research team first used Rapid Immunoprecipitation Mass Spectrometry of Endogenous Proteins (RIME) analysis in MCF7 breast cancer cells and confirmed that TRIM24 is a key ERα interacting protein. This interaction was significantly enhanced after estradiol (E2) induction, indicating that TRIM24 is recruited to the EERα transcription complex in a hormone-dependent manner.
- Co-immunoprecipitation experiments in MCF7 and T47D cells further validated this interaction.
- TRIM24 RIME analysis also showed interactions with classic ERα co-regulators like GREB1, NCOA3, and GATA3.
Clinical data analysis revealed a significant correlation between TRIM24 expression levels and higher breast cancer tumor grade and increased expression of Ki-67 (a tumor proliferation marker). Crucially, TRIM24 transcription levels progressively increased from healthy breast tissue to primary tumors and then to metastases, highlighting its close relationship with disease progression.
TRIM24 Controls ERα Transcriptional Activity
To define the impact of TRIM24 on ERα transcriptional activity, the team generated a TRIM24 knockout (KO) MCF7 cell model.
- TRIM24 knockout did not affect ERα protein levels.
- However, it significantly reduced ERα chromatin binding ability at classic target gene sites like GREB1, TFF1, and IGFBP4.
- This also led to a decrease in active histone marks, H3K23ac and H3K27ac, at these sites.
- ChIP-seq analysis for phosphorylated Ser2 RNA polymerase II CTD showed reduced polymerase binding near ERα target genes after TRIM24 knockout, which translates to inhibited transcriptional activity.
- Transcriptome data further confirmed that TRIM24 knockout significantly downregulated the expression of late estrogen response signature genes. Clinical validation from GSEA analysis of the TCGA-BRCA dataset showed a positive correlation between TRIM24 expression and ERα response genes.
The findings collectively indicate that TRIM24 is critical for maintaining ERα chromatin binding and active histone marks, thereby regulating the ERα-driven transcriptional program.
TRIM24 Degraders Show Promising Anti-Tumor Activity
Based on TRIM24’s crucial role, the researchers utilized a recently developed TRIM24 heterobifunctional protein degrader (dTRIM24). This degrader works by mediating TRIM24 ubiquitination and degradation via the VHL E3 ubiquitin ligase.
- Experiments confirmed that dTRIM24 could efficiently degrade TRIM24 in MCF7 cells without affecting ERα protein levels.
- Cell proliferation assays showed dTRIM24 treatment significantly inhibited MCF7 cell growth but had no effect on ERα-negative, TRIM24-non-expressing MDA-MB-231 cells. A second degrader (dTRIM24_2) showed similar results, and dTRIM24 was able to effectively block E2-driven MCF7 cell proliferation.
- RNA-seq analysis demonstrated that dTRIM24 treatment significantly downregulated gene sets for cell cycle progression, MYC targets, and both early and late estrogen response.
- Furthermore, ERα RIME analysis found that TRIM24 degradation reduced the interaction between ERα and the chromatin remodeling-related protein BAZ1B, suggesting TRIM24 may help ERα binding by maintaining chromatin structure.
Overcoming Endocrine Resistance
Crucially, the study also investigated endocrine therapy resistance models:
- dTRIM24 effectively inhibited the proliferation of tamoxifen-resistant (MCF7 TAMR) and long-term estrogen deprived (MCF7 LTED) resistant cell lines.
- It also worked on MCF7 cells carrying activating ESR1 mutations (Y537S, D538G). The dTRIM24_2 degrader also showed dose-dependent growth inhibition in MCF7 TAMR cells.
- In patient-derived organoid models, cell viability was significantly reduced by dTRIM24 treatment in two out of three untreated ERα-positive organoids and one out of three endocrine-resistant organoids. Furthermore, protein and transcriptional levels of classic ERα response genes were significantly downregulated in the T4-1 and T347 organoids.
- Significantly, dTRIM24 had no obvious effect on triple-negative breast cancer (TNBC) organoids, further confirming the specificity of its action.
Conclusion and Future Outlook
In summary, this groundbreaking research published in PNAS clearly defines TRIM24 as a core component of the ERα transcription complex. It confirms that TRIM24 regulates the ERα-driven transcriptional program and tumor proliferation by maintaining ERα chromatin binding ability and active histone marks.
TRIM24 degraders effectively mimic the gene knockout effect and demonstrate anti-tumor activity in endocrine-sensitive, resistant (including ESR1 mutant) breast cancer cells, and patient-derived organoids. This provides a new therapeutic target and a potential solution for the clinical challenge of endocrine therapy resistance, offering new hope for ERα-positive breast cancer patients, especially those with resistant disease.
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