Abstract 2097: Fibroblast galectin-3 remodels the obese breast tumor microenvironment to promote cancer progression.

Abstract Background: Obesity, affecting 42.4% of U.S. adults, correlates with higher tumor grade, increased metastasis, and reduced survival in breast cancer (BC). Our prior work revealed obesity-induced secretion of collagen-crosslinking enzymes, basement membrane proteins, and the matricellular protein galectin-3 (LGALS3) in the breast. These extracellular matrix (ECM) changes were positively associated with the fibroblast population in obese breast tissue. Elevated LGALS3 expression predicts poorer overall survival in estrogen receptor-positive (ER+) BC. We hypothesize that obesity amplifies LGALS3 fibroblast signaling to disrupt ECM composition and organization, supporting BC progression. Methods: Rag1-/- mice were fed a high-fat high-sugar (HFHS) or low-fat low-sugar (LFLS) diet for 16 weeks to establish obese and lean phenotypes. ER+ UCD65 BC cells were implanted bilaterally into the mammary fat pad alone or with CD146neg (HS5) fibroblasts, generating four groups: LFLS, HFHS, LFLS + CD146neg, and HFHS + CD146neg. All mice received 1 mg of estrogen at implantation and remained on their assigned diets for eight weeks. Tumors, plasma, lungs, and mammary fat pads were collected at endpoint for analyses. LGALS3 was knocked down in CD146neg fibroblasts and overexpressed in CD146pos (HS27) fibroblasts for in vitro studies. Conditioned media was generated under starvation conditions and concentrated 20x for western blotting. Results: Obese mice implanted with CD146neg fibroblasts exhibited a 2.7-fold increase in tumor growth and 8.8-fold increase in metastasis compared with lean controls. CD146neg fibroblasts significantly enhanced angiogenesis in both lean and obese settings but did not affect lymphangiogenesis. Obesity significantly disrupted the collagen organization of the mammary fat pads. In obese mice, with CD146neg fibroblasts, increased collagen deposition, macrophage infiltration, and circulating levels of LGALS3. Genetic manipulation of LGALS3 showed that it specifically regulates collagen I secretion without altering intracellular levels. This loss of secretion triggered unfolded protein response activation and reduced TGF-β signaling, further supporting a secretion-focused mechanism. LGALS3 was diet-responsive, with time-restricted feeding and intermittent fasting lowering circulating levels by 2.1 and 2.7-fold, respectively. Conclusions: Obesity synergizes with CD146neg fibroblasts to accelerate BC aggression through LGALS3-mediated ECM remodeling, supported by a novel post-transcriptional control of collagen secretion. LGALS3 can serve as a therapeutic target and is modifiable by dietary strategies to mitigate obesity-associated BC risks, potentially improving outcomes for millions of at-risk patients. Citation Format: Ellen E. Bamberg, Kiran Vinod-Paul, Amy L. Pyo, Kirk C. Hansen, Carol A. Sartorius, Paul S. MacLean, Peter Kabos, Heather Brechbuhl. Fibroblast galectin-3 remodels the obese breast tumor microenvironment to promote cancer progression 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 2097.