Abstract 6267: Evaluating estrogen receptor signaling modifications at bone marrow related stiffnesses in metastatic breast cancer

Abstract Bone metastasis is highly prevalent in breast cancer (BC) patients with metastatic disease and can increase the risk of hypercalcemia and pathological fractures. Several studies have evaluated the mechanisms of metastasized BC cells’ involvement in bone destruction, but how the bone marrow (BM) microenvironment contributes to this process remains ambiguous. Recent studies have shown that as BC cells experience increased stiffnesses outside of the primary microenvironment, there is higher expression of epithelial-to-mesenchymal transition (EMT) markers, increased expression of osteolytic proteins, and decreased drug responses. However, these changes have not been evaluated at stiffnesses relative to the BM. Estrogen receptor (ER) status also plays a pivotal role in BC patient survival. Clinical studies have indicated that the majority of ER+ patients with metastatic disease have more incidences of bone metastases compared to ER- patients. There is some evidence that suggests that the physical properties can also affect estrogen receptor signaling at the primary site, however, there is minimal work to establish how the physical factors of the BM can affect ER sensitivity after metastasis. We hypothesize that higher stiffness ranges of the BM can alter the estrogen receptor sensitivity of metastasized tumor cells and increase osteolysis. To analyze this question, we seeded ER+ BC cell lines (MCF7s and T47Ds) on CytoSoft® Rigidity Plates that mimicked the stiffness of the BM (0.5 - 32 kPa). After 2 days in culture, we probed the expression of downstream estrogen signaling factors, GREB1, PGR, and TIFF1, to evaluate estrogen activity using western blot. Our results show an increase in downstream estrogen signaling at higher stiffnesses (32 kPa) compared to lower stiffnesses (0.5 kPa) at the protein level but not at the transcriptional level. In the future, we will analyze factors associated with osteolysis (ITGB3, TGFB-RII, and PTHLH). Additionally, we plan to conduct RNA sequencing to analyze any larger transcriptional changes at different stiffnesses. Overall, this work gives insight into how estrogen signaling changes in the BM compared to the primary site. It will also give us insight into potential therapeutic targets for ER+ BC after metastasis to the bone. Citation Format: Logan Alexander Northcutt, Marjan Rafat, Julie Rhoades. Evaluating estrogen receptor signaling modifications at bone marrow related stiffnesses in metastatic breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6267.