The Role of Estrogen Receptor in Regulating Breast Cancer Cell Invasion

/Abstract: Estrogen receptor-positive (ER+) breast cancer (BC) poses a significant clinical challenge due to a substantial subset of invasive ER+ tumors (>30%) that progress to incurable metastatic states. Invasion in the primary tumor microenvironment (TME) is increasingly recognized for its pivotal role in BC progression. Notably, extracellular matrix stiffness within the TME drives tumor invasion. Recent clinical studies have shown that invasive ER+ tumors are surprisingly stiffer than ER- tumors. We found that adaptation to high matrix stiffness (stiff condition- ing) enhances ER+ BC cells' ability to sense and respond to stiffness, significantly impacting durotactic invasion; conversely, soft-conditioning suppresses durotactic invasion in ER+ BC cells. Differences between soft and stiff conditioned ER- BC cells in durotactic invasion are less significant. Our goal is to determine how two crucial TME components—matrix stiffness and estrogen (E2)—act as critical regulators of ER+ BC invasion. We propose that the response of ER+ BC cells to E2 is linked to their matrix stiffness acclimation, i.e., mechanohormonal Condi- tioning. The mechanisms underlying the heightened sensitivity of ER+ BC cells to stiffness remain unknown. Our clinical data show that ER+ BC patients with tumors exhibiting stiff-conditioning signatures have poor outcomes, highlighting the significance of studying the TME's mechanical properties in ER+ BC. The ER transcriptional target, EVL, is crucial for strengthening focal adhesions (FAs) and enriching suppressive cortical actin bundles (SCABs), indicating its dual role in cell adhesion and cortical contractility. EVL appears in distinct protein com- plexes in stiff- versus soft-conditioned cells, suggesting different functional roles based on mechanical condition- ing. Moreover, stiff-conditioning of ER+ BC cells induced a fibrotic response in vivo, significantly less pronounced in tumors derived from soft-conditioned cells. This suggests a feedback loop between stiff conditioning and in- creased fibrosis, leading to greater stiffness. We hypothesize that STIFF+E2 mechanohormonal conditioning of ER+ BC cells promotes durotactic invasion through EVL-mediated focal adhesion strengthening, while SOFT+E2 conditioning suppresses durotactic invasion through EVL-mediated cortical actin bundles generated under weak adhesion; and that these EVL-mediated invasive phenotypes are amplified in the primary tumor by promoting a fibrotic response in the TME. We will investigate this hypothesis through two specific aims: SA1, determine how the balance between traction force and cortical tension establishes a durotactic invasive phenotype; and SA2, determine how mechanohormonally conditioned cells interact with the TME. We are using several in vitro models of ER+ BC to study 2D and 3D cell motility, durotactic invasion, and force measurement and an ER+ Mammary INtra-Ductal (MIND) xenograft models, together with intravital imaging (multiphoton and optical coherence elas- tography) to visualize changes in the mechanical TME in the primary tumor. This work will uncover how mecha- nohormonal conditioning by matrix stiffness and estrogen in the TME drive durotactic invasion in ER+ BC cells and remodel the TME, further amplifying mechanohormonal conditioning. Project Number: 1R01CA307225-01 | Fiscal Year: 2026 | NIH Institute/Center: National Cancer Institute (NCI) | Principal Investigator: Ghassan Mouneimne | Institution: UNIVERSITY OF ARIZONA, TUCSON, AZ | Award Amount: $544,929 | Activity Code: R01 | Study Section: Tumor Host Interactions Study Section[THI] View on NIH RePORTER: https://reporter.nih.gov/project-details/11269318