Abstract 2241: Galectin 1 as a regulator of hormone driven mammary remodeling and early dissemination with therapeutic potential in breast cancer.

Abstract Galectin-1 (GAL1) is a glycan-binding protein broadly known to shape tumor progression by promoting immune evasion and metastatic dissemination. While GAL1 is well recognized as an immunomodulator in multiple cancer types, its role in hormone-driven mammary gland biology and early tumor evolution remains poorly defined. Using our established MMTV-PyMT model WT or KO for GAL1 (Lgals1-/-), we previously demonstrated that GAL1 deficiency delays tumor onset, reduces metastatic burden, and diminishes mammary branching morphogenesis. Unexpectedly, we also observed a decreased Progesterone Receptor (PR) expression in luminal cells from Lgals1-/- mice in both normal and transformed mammary glands. These findings suggested that GAL1 may act as a critical regulator of hormone-driven mammary gland development. To test this hypothesis, we stimulated C57BL/6 Lgals1+/+ and Lgals1-/- mice with a pellet containing Medroxyprogesterone acetate (MPA), a synthetic progestin, and observed that GAL1-deficient mice exhibited a significantly reduced branching index in response to MPA stimuli. RNA-seq analysis of MPA-treated or control mammary glands further revealed that canonical progesterone-responsive genes such as Tnfsf11 (Rankl) and Wnt4 were attenuated in Lgals1-/- glands. To determine if GAL1 absence perturbs epithelial cell hierarchies, we profiled mammary epithelial cell populations by spectral flow cytometry using EpCAM, CD49f, CD49b and Sca1 markers among CD45neg cells. GAL1-deficient glands exhibited reduced MaSC and ER+ luminal progenitor cell compartments, suggesting that GAL1 helps maintain lineage balance during the development of the mammary gland. This finding is particularly relevant given that this population has been recognized as an early cell of origin in neoplastic transformation. To further interrogate the therapeutic relevance of GAL1 blockade, we used an MMTV-HER2+ model of breast cancer. We developed a GAL1 (clone 3) neutralizing monoclonal antibody and treated MMTV-HER2+ mice (15mg/kg, twice a week) from week 14 of age (preneoplastic stage), through week 19. Therapeutic blockade of GAL1 reduced branching morphogenesis, curtailed the release of early circulating tumor cells (CTCs), decreased the number of transformed ducts, and reduced lung metastasis. Importantly, anti-GAL1 treatment led to reduced PR expression in mammary luminal cells and depleted GAL1 in serum, further confirming the blockade's effectiveness. Together, these findings introduce Galectin-1 as a regulator of progesterone-driven epithelial remodeling and early metastatic spreading. By linking GAL1 to hormone signaling, epithelial hierarchy, and early dissemination, our study positions GAL1 as a promising therapeutic target for treating metastatic breast cancer progression. Citation Format: Magali Berton, Ramiro Perrotta, Luis Valencia Salazar, Tomás Dalotto Moreno, Yamil D. Mahmoud, Juan Manuel Perez Saez, Sabrina Gatto, Rosa Morales, Julio Aguirre-Ghiso, Gabriel A. Rabinovich, Mariana Salatino. Galectin 1 as a regulator of hormone driven mammary remodeling and early dissemination with therapeutic potential in breast cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 2241.