Abstract P1-04-18: Mediator Subunit 1 (MED1) and tumor-secreted small extracellular vesicles (sEV) in breast cancer immune evasion and progression

Abstract Breast cancer is the most common malignancy in women and HER2+ breast cancer has one of the poorest prognosis. HER2-targeted therapies in combination with immune checkpoint inhibitors (ICI) have shown promising preliminary results; however, acquired HER2 resistance limited the efficacy of advanced therapeutics. MED1 is a transcription regulator co-amplified with HER2 and plays a critical role in the progression and metastasis of HER2+ breast cancer. We recently generated a MMTV-HER2/MED1 double overexpression mouse model and observed increased tumor burden, stemness, and metastasis. Single-cell sequencing revealed MMTV-HER2/MED1 tumors had lower tumor-infiltrating CD4 and CD8 T cells and increased accumulation M2 macrophage population; which was further confirmed by IHC analysis. We also found that MMTV-HER2/MED1 tumor cells secreted more and larger small extracellular vesicles (sEVs) than MMTV-HER2 tumor cells. Importantly, MMTV-HER2/MED1 but not MMTV-HER2 tumor cell-secreted sEVs promote M2 polarization of BMDMs while depletion of sEVs abolished that effect. Next, we performed sEV miRNA-seq and identified mmu-miR125b as a top upregulated miRNA involved in M2 macrophage polarization. We found that mmu-miR125b mimic treatment promotes M2 polarization of mice BMDMs in a dose-dependent manner but miR125b-depleted sEVs lose that capability. Moreover, we have carried out in vivo MED1 depletion using p-HER2apt-siMED1 RNA nanoparticles in MMTV-HER2/MED1 mice and found greatly reduced tumor burdens. Consistent with the above, IHC analysis revealed MED1 depletion reduced M2 macrophage (F4/80+ and CD206+) population with an increased accumulation of cytotoxic TIL (CD8α+ and CD4+) populations. Interestingly, we have also observed the downregulation of IRF1/PD-L1 signaling axis in MMTV-HER2/MED1 tumor tissues after the treatment. We further confirmed upregulation of IRF1/PD-L1 in MMTV-HER2/MED1 tumor cells compared to that of MMTV-HER2 tumor cells while MED1 knockdown inhibited IRF1/PD-L1 signaling axis in both MMTV-HER2/MED1 and human BT474 cells. ChIP-qPCR results further supported the direct recruitment of MED1 on IRF1 promotors to regulate PD-L1 expression. Finally, p-HER2apt-siMED1 RNA nanoparticles cotreated with anti-PD1 antibody significantly attenuated MMTV-HER2/MED1 tumor burden; highlighting potential adjuvant effect of p-HER2apt-siMED1 RNA nanoparticles. Collectively, our studies revealed the roles of MED1 in regulating breast cancer immune microenvironment and the potential use of MED1 targeted therapy alone or in combination with current regimens for better treatment of breast cancer Citation Format: Mahendra Jadhao, Yongguang Yang, Gregory Bick, Xioating Zhang. Mediator Subunit 1 (MED1) and tumor-secreted small extracellular vesicles (sEV) in breast cancer immune evasion and progression [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P1-04-18.