Abstract 4418: Epigenetic reactivation of estrogen receptor for treatment of triple-negative breast cancer

About 12% of breast cancers are invasive breast cancers, including triple negative breast cancer (TNBC), which manifest early metastasis and recurrence with limited treatment options. Therefore, a better understanding of the molecular mechanisms of TNBC holds the key for effective treatment of TNBC. Tamoxifen, a first line treatment for estrogen receptor (ERα) positive breast cancer, serves as an ERα antagonist by binding to ERα and blocking estrogen receptor signaling to suppress estrogen receptor dependent cancer cells. It is known that estrogen receptor gene (ESR1) is hypermethylated and hence ERα expression is deficient in TNBC. As a result, TNBC cells are highly resistant to tamoxifen, and the strategy used to resensitize tamoxifen response in TNBC has yet to be established. We have discovered that a DNA demethylation enzyme tet methylcytosine dioxygenase 2 (TET2) plays a critical role in reversing DNA methylation of estrogen receptor gene (ESR1) and thereby enables to reactive estrogen receptor expression in TNBC, resulting re-sensitization of TNBC cells to tamoxifen. It is also reported that a clinically used anti-diabetic drug, metformin, can enhance TET2 protein stability and expression. Therefore, we hypothesize that through TET2-mediated reactivation of estrogen receptor, metformin combined with tamoxifen can synergistically suppress TNBC growth. To test the hypothesis, we first investigated the effects of vehicle, metformin, tamoxifen, and the combination therapy (metformin + tamoxifen) on cell growth and migration/abilities of TNBC cell lines, MDA-MB-231 and BT549. We then analyzed the effects of the combination therapy on tumor growth and metastasis using a TNBC xenograft mouse model. Furthermore, immunohistochemical staining was used in patient tissue samples to analyze the correlation of TET2 and estrogen receptor expression levels to clinicopathological characteristics. Our data has revealed that metformin-tamoxifen combination therapy significantly inhibits TNBC cell growth compared to the single drug and vehicle treatment groups. Metformin-tamoxifen combination therapy more effectively suppresses TNBC cell migration compared to the single drug and vehicle treatment groups. Furthermore, using Combination Index (CI) and Dose Reduction Index analysis, we have demonstrated that metformin-tamoxifen combination therapy can synergistically inhibit TNBC cell growth compared to the single drug and vehicle treatment groups, where the metformin dose could be reduced by approximately 5-fold, and tamoxifen dose could be reduced by approximately 20-fold. Together, these findings suggest that metformin-tamoxifen combination therapy could produce a significant synergy, allowing for lowered drug doses and thus reduced toxicity, and provide clinical implications for a novel, effective, and safe TNBC treatment regimen. Citation Format: Jer-Yen Yang, Chia-Hua Cheng, Wan-Chi Lu, Yu-Wa Lei, Alice CJ Chang. Epigenetic reactivation of estrogen receptor for treatment of triple-negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4418.