Abstract 4711: HER3 promotes triple-negative breast cancer progression by upregulating PHF8 via miR-34b-dependent mechanism

Abstract Introduction: Triple-negative breast cancer (TNBC) is a highly aggressive and heterogeneous malignancy with poor prognosis and limited treatment options, presenting a significant clinical challenge. Current therapeutic strategies are insufficient to address the complexities of TNBC. In a previous study, we identified the human epidermal growth factor receptor 3 (HER3) signaling as a key driver of TNBC progression and a potential therapeutic target. However, the exact mechanisms through which HER3 signaling contributes to TNBC remain poorly understood. Methods: To explore the molecular mechanisms underlying HER3 signaling in TNBC, we performed RNA sequencing (RNA-seq) to identify transcriptomic changes following HER3 knockdown in HCC1806 TNBC cells. Colony formation assays were used to assess cell viability, while Western blotting and quantitative real-time PCR (qRT-PCR) were employed to analyze protein and mRNA expression levels, respectively. Gene expression was modulated using lentiviral vectors carrying specific cDNAs or shRNAs. Flow cytometry was used to analyze cell cycle distribution. In vivo tumor xenograft models were established by subcutaneously inoculating HCC1806 cells into nude mice to evaluate the role of HER3-PHF8 axis in tumor progression. Immunohistochemistry (IHC) was performed to assess protein expression in both mouse tumor models and clinical TNBC specimens. Results: RNA-seq analysis revealed a significant reduction in the expression of the histone demethylase PHF8 (also known as KDM7B) following HER3 knockdown in HCC1806 cells. Activation of HER3 by Heregulin (HRG) increased PHF8 expression, while HER3 knockdown decreased PHF8 levels and upregulated p27. PHF8 was shown to be critical for HER3-driven TNBC cell proliferation and progression. Notably, overexpression of PHF8 reversed the G1-phase cell cycle arrest induced by HER3 knockdown. In vivo, increased PHF8 expression rescued tumor growth inhibition resulting from HER3 signaling blockade. Analysis of The Cancer Genome Atlas (TCGA) data and clinical TNBC samples revealed a positive correlation between HER3 and PHF8 expressions at both the mRNA and protein levels. Further investigation identified miR-34b, which was negatively regulated by HER3 signaling, as a microRNA targeting PHF8. Luciferase reporter assays confirmed that miR-34b suppressed PHF8 expression, and MiR-34b mimics and inhibitors significantly downregulated or upregulated PHF8 protein levels, respectively, compared to the negative control in TNBC cells. Conclusion: Our findings demonstrate that activated HER3 signaling suppresses miR-34b, leading to increased PHF8 expression and promoting TNBC progression. Targeting the HER3/PHF8 axis or restoring miR-34b levels offers promising therapeutic strategies to improve outcomes for TNBC patients. Citation Format: Hui Lyu, Congcong Tan, Yakun Wu, Margaret E. Larsen, Shou-ching Tang, Guobin Kang, Qingzhao Yu, Bolin Liu. HER3 promotes triple-negative breast cancer progression by upregulating PHF8 via miR-34b-dependent mechanism [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 4711.