Abstract 1010: Extracellular matrix signaling modulates estrogen receptor activity in breast cancer

The tumor microenvironment plays a crucial role in cancer progression. In breast cancer, the extracellular matrix (ECM) is especially important as it contributes to mammographic breast density. Women with higher mammographic breast densities are at a greater risk for the development of breast cancer. Despite this, much knowledge of the ECM’s effects on breast cancer progression remains elusive. To better understand how ECM signaling affects breast cancer, we utilize fibroblast-deposited, decellularized ECM scaffolds. Scaffolds are produced for in vitro use by culturing a confluent layer of fibroblast cells over six days followed by decellularization with a detergent solution. RNA sequencing analysis of MCF7 human breast cancer cells cultured on ECM scaffolds versus uncoated dishes revealed differential expression of hundreds of genes. Most notably, genes within hallmark sets for early and late responses to estrogen were significantly enriched when cells were cultured on ECM. We confirm the expression of estrogen-responsive genes in breast cancer cells cultured on ECM even in the complete absence of estrogen, and also note that the estrogen receptor (ER) itself is downregulated at both the RNA and protein-level, consistent with results of estrogen stimulation. Treatment of breast cancer cells with ER inhibitors hydroxytamoxifen and fulvestrant reduces ECM-induced expression of estrogen-responsive genes. Likewise, culturing breast cancer cells on ECM enhances phosphorylation of Erk. Inhibition of phospho-Erk also reduces the expression of the same estrogen-responsive genes in cells cultured on ECM. Through luciferase, conditioned media, and estrogen-starvation assays, we demonstrate that ECM signaling activates estrogen receptor signaling in an estrogen-independent, nonclassical mechanism. In addition, we show that ER positive cells cultured on ECM are more resistant to ER-targeted therapeutics, but not other chemotherapeutic agents like paclitaxel or doxorubicin. Ultimately, we describe a pathway in which ECM can induce estrogen-independent activation of the ER as well as resistance to estrogen-targeted therapies in breast cancer. Clinical research into the effects of mammographic breast density beyond breast cancer predisposition is sparse, and this work provides incentive for investigators to better understand the relationship between breast fibrosis and clinical outcome. Citation Format: Josh W. DiGiacomo, Ines Godet, Michael Trautmann Rodriguez, I Chae Ye, Daniele M. Gilkes. Extracellular matrix signaling modulates estrogen receptor activity in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1010.