Abstract 3794: Mammary Cancer Stem Cells Promote an Aggressive Phenotype in Letrozole Resistant Breast Cancer

Abstract Aromatase inhibitors (AI), like letrozole, are the first-line treatment for ER+ breast cancer in post-menopausal women. Despite widespread successful usage of letrozole, resistance to therapy, tumor relapse, and metastasis remain the principal causes of death for breast cancer patients. While there are no cures for AI-resistant breast cancer, previous reports demonstrate AI resistance is associated with hormone independence, increased growth factor signaling, enhanced cellular motility and the epithelial to mesenchymal transition (EMT). Given recent evidence suggesting a convergence of EMT and cancer stem cells (CSC) and their combined implications in endocrine resistance, we hypothesized that breast cancer stem cells contribute to letrozole resistant breast cancer cells by increasing cellular motility. To test this hypothesis, ovariectomized immunocompromised female mice were inoculated in the mammary fat pad with either MCF-7 cells stably transfected with the human aromatase gene that were long term letrozole-treated (LTLT-Ca) or letrozole sensitive, MCF-7 cells stably transfected with the human aromatase gene (AC-1). Afterwards, intratumoral putative CSC marker expression was assessed by immunohistochemistry. Results indicate LTLT-Ca tumors were CD44+/CD24+ while AC-1 tumors were CD44low/CD24low. Both cell lines were cultured either adherently (2D) or as mammospheres (3D) and CD44/CD24 protein expression was assessed by immunoblots. When both cell lines were cultured three dimensionally, they expressed higher levels of CD44 and CD24 compared to their adherent counterparts, with the most robust change occurring within the LTLT-Ca cells. In order to quantitate the breast cancer stem cell activity mammosphere formation assay was performed and LTLT-Ca cells formed mammospheres at a 3.4-fold higher index compared to AC-1 cells. Additionally, we performed targeted gene expression arrays and our results demonstrated decreased expression of genes involved in cell adhesion and tumor suppression (ie., E-cadherin, caveolin 1 and β catenin). Taken together our study demonstrated a strong correlation between AI resistance, increased cancer stem cell expression, an invasive phenotype that may be associated with a poor prognosis. This work was supported by an NIH grant awarded to SL Tilghman (1SC1GM125617). Citation Format: Karen Melissa Gallegos, Jankiben Patel, Rashidra R. Walker, A Michael Davidson, Ian R. Davenport, Syreeta L. Tilghman. Mammary cancer stem cells promote an aggressive phenotype in letrozole resistant breast cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3794.