Extracellular vesicle-mediated preconditioning of osteoblasts by ER+ breast cancer cells: implications for the bone metastatic niche

Breast cancer metastasis to bone most commonly occurs in late-stage disease and remains a major contributor to morbidity, reduced quality of life, and mortality, particularly in estrogen receptor-positive (ER+) tumors. Intercellular communication between tumor cells and bone-resident cells plays a critical role in promoting tumor invasion and development of metastatic bone lesions. Historically, tumor-bone interactions have been viewed largely through the lens of soluble factor-mediated signaling, particularly in the context of osteoclast-driven bone resorption and osteolytic lesion formation. However, the majority of bone metastases exhibit a mixed lytic and sclerotic phenotype, reflecting the combined involvement of osteoclast- and osteoblast-mediated remodeling. Here, we investigate how ER+ breast cancer-derived extracellular vesicle-enriched conditioned media influence osteoblast behavior and signaling in vitro, positioning osteoblasts as key intermediaries in bone metastatic progression. We first show that ER+ T47D cells release significantly higher EVs than MCF10A non-tumorigenic breast epithelial cells. Next, we demonstrate that osteoblasts actively internalize T47D-derived EVs by 24 h, resulting in significant alterations in their proliferation and signaling profiles associated with chemotaxis and vascular-associated pathways. Finally, we show that conditioned media from T47D EV-preconditioned osteoblasts did not significantly enhance breast tumor cell proliferation or overall motility outcomes. However, despite similar mean motility metrics between conditions, migration behavior exhibited distinct population-level distributions. Collectively, these findings demonstrate that ER+ breast cancer-derived EVs drive selective reprogramming of osteoblast signaling and function as an important component in tumor-bone niche conditioning.