Abstract 3138: EGR1 is a master regulator of tumor growth and macrophage polarization in inflammatory breast cancer

Abstract Background: Inflammatory breast cancer (IBC) is one of the most aggressive forms of breast cancer, accounting for 2-4% of cases but 8-10% of breast cancer-related deaths in the United States. Patients with IBC face poor prognoses and limited treatment options, highlighting the urgent need to understand the mechanisms driving this disease. The transcription factor early growth response 1 (EGR1) is highly dysregulated in various cancers, including IBC, where its expression is significantly elevated compared to stage-matched non-IBC samples. Lower EGR1 expression correlates with improved survival outcomes. Our previous research has identified EGR1 as a key mediator of the EGFR-regulated immunosuppressive tumor microenvironment (TME), a critical driver of IBC’s aggressiveness. The molecular mechanisms through which EGR1 modulates the IBC TME remain poorly understood. This study aims to fill this significant knowledge gap by exploring EGR1’s role in IBC and testing the hypothesis that targeting EGR1 is a novel therapeutic strategy for IBC. Methods: We depleted EGR1 expression in IBC cells using CRISPR/Cas9 knockout (KO) or shRNA knockdown (KD) and examined the effect on cell proliferation, migration, and invasion in vitro and tumor growth in vivo. We depleted EGR1 in THP-1 monocytes to test the impact on M2 polarization and subsequent effect on IBC cell migration. Coculture experiments were performed to investigate the crosstalk between IBC cells and M2 macrophages. RNA-sequencing (RNA-seq), real-time PCR, and Western blot analyses were performed to determine the molecular mechanisms by which EGR1 regulates the IBC TME. Results: EGR1 KO significantly reduced proliferation, colony formation, and migration of IBC cells in vitro. In vivo tumor xenograft models confirmed that EGR1 KD reduced tumor burden, decreased M2 macrophages, and increased M1 macrophages, indicating EGR1's role in modulating the TME. Depleting EGR1 in macrophages reduced M2 polarization and their migratory and phagocytic abilities, suggesting that EGR1 impacts both IBC cells and macrophages. In vitro coculture experiments showed that EGR1 KO in IBC cells reduced M2 macrophage recruitment, while EGR1 KO in macrophages attenuated IBC cell migration. RNA-seq analysis of IBC Ctrl and EGR1 KO cells showed gene downregulation in cell signaling, cancer progression, and metabolism. In EGR1 Ctrl and KO M2 macrophages, RNA-seq indicated downregulation of immunosuppressive macrophage signature genes. Conclusion: Our findings establish EGR1’s dual role in promoting cancer cell proliferation and shaping an immunosuppressive TME in IBC. Targeting EGR1 impairs M2 macrophage polarization and fosters an anti-inflammatory TME, positioning EGR1-targeted therapy as a novel IBC treatment strategy. These results warrant further investigation to elucidate the underlying molecular mechanisms and validate its therapeutic potential. Citation Format: Verena Martinez-Rodriguez, Van Ngu Trinh, Suguru Ogata, Manuel Van Gijsel-Bonnello, Andrew C. Montgomery, Naoto T. Ueno, Xiaoping Wang. EGR1 is a master regulator of tumor growth and macrophage polarization in inflammatory breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 3138.