Abstract LB272: The ER partial agonist TTC-352 inhibits a novel epithelial-mesenchymal transition pathway, and induces DNA damage in endocrine-resistant breast cancer cells

Abstract Endocrine therapy (ET) is the standard of care for estrogen receptor-positive (ER+) breast cancer. The major treatment challenges are endocrine therapy resistance and metastasis. Epithelial mesenchymal transition (EMT) is integral to migration and invasion. Our lab has previously shown a correlation between protein kinase C α (PKCα) overexpression and tamoxifen resistance (TR) (Tonetti, 2003) and further reported a novel EMT signaling in TR breast cancer cells. This pathway involves FOXC2-mediated repression of the p120catenin promoter transcription, resulting in loss of E-cadherin at the plasma membrane and enhanced cell migration and invasion (Pham, 2017). Estrogen-induced apoptosis and tumor regression in long-term estrogen-deprived (LTED) breast cancer is explained by the induction of unfolded protein response (UPR), activation of the extrinsic death receptor pathway and intrinsic mitochondrial pathway, and down-regulation of pro-survival, cell growth pathways (Shete, 2023). Recently, it was reported that estrogen induces ER-dependent DNA damage in LTED breast cancer models (Traphagen, 2023). Although 17β-estradiol (E2) is efficacious following exhaustive ET, it is associated with adverse effects. A novel Selective Human Estrogen Receptor Partial Agonist (ShERPA), TTC-352 induces UPR and shrinks the TR tumors (Molloy, 2014; Abderrahman, 2021). The Phase I clinical trial results of TTC-352 demonstrated efficacy without reaching the maximum tolerated dose (Dudek, 2020). This study investigated the impact of E2 and TTC-352 treatment on the novel EMT pathway and DNA damage in TR cells. TR, PKCα-overexpressing cell lines used in this study include MCF-7/PKCα and LTED MCF-7:5C cells. They are more migratory than tam-sensitive MCF-7 cells. 3D spheroid invasion assay revealed that MCF-7/PKCα and MCF-7:5C spheroids are more invasive than MCF-7 spheroids. E2 and TTC-352 treatment lowered FOXC2 at the transcript level in MCF-7/PKCα cells compared to the control group. Chromatin immunoprecipitation with FOXC2 antibody showed that E2 and TTC-352 treatment lowered FOXC2 binding on the p120catenin promoter, resulting in increased p120catenin promoter transcription in MCF-7/PKCα and MCF-7:5C cells. This effect was correlated with decreased cell migration. Strong intensity of E-cadherin immunofluorescence staining at the plasma membrane was observed post-E2 and TTC-352 treatment in TR cells. Also, TR cells treated with E2 and TTC-352 exhibited nuclear foci positive for phospho-histone H2A.XS139 and p53 binding protein-1 indicative of DNA damage. In conclusion, E2 and TTC-352 treatments induce DNA damage, and do not activate the PKCα-FOXC2-p120catenin EMT signaling, thereby reducing the migratory potential of TR cells. Combination with Olaparib enhances E2-induced DNA damage in LTED models (Traphagen, 2023). These findings suggest that TTC-352 with a PARP inhibitor may be a potential combination therapy for endocrine-resistant ER+ breast cancer. Citation Format: Nivida Shete, Debra A. Tonetti. The ER partial agonist TTC-352 inhibits a novel epithelial-mesenchymal transition pathway, and induces DNA damage in endocrine-resistant breast cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_2):Abstract nr LB272.