Abstract P4-05-25: Impact of Estrogen Receptor Targeting Drugs on a Novel Epithelial-Mesenchymal Transition Pathway in Endocrine Resistant Breast Cancer: Implications for Metastasis

Abstract Background: Approximately 10% of breast cancer patients are metastatic at the time of diagnosis with a 27% survival rate. Currently, endocrine therapy, including tamoxifen, aromatase inhibitors (letrozole, anastrozole, and exemestane), Selective Estrogen Receptor Degraders (fulvestrant and elacestrant), is the standard of care for patients with estrogen receptor-positive (ER+) breast cancer. The major treatment challenge is endocrine therapy resistance and metastasis. Epithelial Mesenchymal Transition (EMT) is the process in which cells lose epithelial properties and acquire motility and mesenchymal phenotype, which leads to migration and invasion. EMT is characterized by the destabilization of adherens junctions by loss of E-cadherin at the plasma membrane. We previously reported a novel signaling pathway that is present in tamoxifen-resistant (TR) breast cancer cells. This pathway involves transcriptional repression of p120catenin by upstream regulators PKCa and FOXC2, resulting in the loss of E-cadherin at the plasma membrane (Pham, 2017). EMT features were characterized by decreased expression of epithelial markers and increased migration and invasion. Initially, estrogen stimulates breast cancer growth but becomes inhibitory after long-term estrogen deprivation. A novel Selective Human Estrogen Receptor Partial Agonist (ShERPA) TTC-352 is an estrogen mimic that induces unfolded protein response and shrinks the TR tumors (Molloy, 2014; Abderrahman, 2021). In this study, we investigated the impact of various ER-targeting drugs including 17b-estradiol (E2), tamoxifen, fulvestrant, and TTC-352 on the novel PKCa-FOXC2-p120catenin signaling axis. We hypothesize that ER-targeting drugs modulate the novel EMT pathway in TR breast cancer cells. Materials and Methods: TR cell lines used in this study include PKCa overexpressing cells MCF-7/PKCa, and MCF-7:5C and represent migratory ER+ breast cancer. Migration assays were performed in response to ER ligands using Falcon Permeable inserts. Migrated cells in response to chemoattractant were stained and counted after 24 hours. Luciferase reporter assays were performed to evaluate the effect of ER ligands on p120catenin transcription. Cells were transiently co-transfected with a p120catenin promoter-reporter plasmid using renilla vector for normalization. The reporter system readout is based on FOXC2 binding to the p120catenin promoter resulting in repression of luciferase transcription. Luciferase luminescence was measured on a plate reader corresponding to p120catenin transcription. Chromatin immunoprecipitation was performed using a FOXC2 antibody (Abcam-308055) to assess FOXC2 binding to the p120catenin promoter after ER ligand treatment. Results: E2 and TTC-352 treatment lowered FOXC2 binding on the p120catenin promoter, increased its transcription, and decreased the migration of MCF-7/PKCa and MCF-7:5C breast cancer cells compared to vehicle. E2 and TTC-352 treatment does not activate the PKCa-FOXC2-p120catenin EMT signaling axis and reduces the migratory potential of TR cells. Conclusion and Discussion: Our findings suggest that patients with endocrine-resistant ER+ breast cancer may benefit from E2 and TTC-352 therapy by inhibiting the activation of the PKCa-FOXC2-p120catenin signaling and cell migration identified in TR cell lines. Results of the Phase I clinical trial of TTC-352 reveal a reduced side effects profile and improved progression-free survival (Dudek, 2020). Strategizing less toxic treatment options may potentially reduce metastasis and prolong patient survival with a better quality of life. Citation Format: Nivida Shete, Debra A. Tonetti. Impact of Estrogen Receptor Targeting Drugs on a Novel Epithelial-Mesenchymal Transition Pathway in Endocrine Resistant Breast Cancer: Implications for Metastasis [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P4-05-25.