Abstract Estrogen receptor (ER) drives approximately 75% of all breast cancers and serves as the primary target for endocrine therapies. However, resistance to these therapies remains a major clinical hurdle, especially in metastatic ER-positive breast cancers harboring activating ESR1 mutations (e.g., Y537S, D538G) within the ligand-binding domain. These mutations are commonly detected in metastatic lesions and circulating tumor DNA (ctDNA) from patients undergoing endocrine treatment. While combination therapies with CDK4/6 inhibitors have shown promise, the mechanisms driving resistance in ESR1-mutant tumors and their associated therapeutic vulnerabilities remain poorly defined. To address this gap, we investigated the interaction between endocrine-resistant breast cancer (ERBC) cells and the tumor microenvironment. Using co-culture systems involving seven ERBC cell lines—including genome-edited ESR1 mutants—and four stromal cell types, we profiled 28 tumor-stroma secretome combinations via cytokine antibody arrays. This approach identified CCL5 and endoglin as significantly upregulated in resistant tumor-stroma interactions. We hypothesized that CCL5 promotes endocrine resistance and metastatic progression through paracrine signaling with stromal components. To test this, we generated CCL5 knockout EO771 cells using CRISPR-Cas9 and confirmed CCL5 loss through qRT-PCR and ELISA. CCL5-deficient cells exhibited markedly reduced proliferation and migration in vitro. In vivo, orthotopic mouse models demonstrated that CCL5 knockout tumors had significantly decreased growth and metastasis. Moreover, treatment with maraviroc, a CCR5 antagonist, selectively impaired the viability of wild-type but not CCL5-deficient cells. Together, these findings identify CCL5 as a key driver of endocrine resistance and metastatic potential in ERBC, highlighting it as a promising target for therapeutic intervention. This study lays the groundwork for future clinical strategies aimed at overcoming resistance in ESR1-mutant breast cancers by disrupting CCL5 signaling. Citation Format: D. Lomonaco, K. Jin. Ccl5 and endoglin as a therapeutic target in er positive breast cancer abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS2-11-12.
Lomonaco et al. (Tue,) studied this question.