Saikosaponin D Mitigates Radioresistance in Triple-Negative Breast Cancer by Inducing MRE11 De-Lactylation via HIF1α/HDAC5 Pathway

<b>Background:</b> Triple-negative breast cancer (TNBC), the most aggressive breast cancer subtype, exhibits poor prognosis due to radiotherapy resistance. However, the underlying mechanisms and effective therapeutic agents remain elusive. <b>Methods:</b> We employed lactate/oxamate to assess DNA damage/repair in irradiated TNBC cell lines. Lentiviral vectors for MRE11/HDAC5 constructs and shRNA were used to explore lactylation via Western blot/Co-IP. TCGA data mining, tissue microarrays, proteomics-MS, and gene expression profiling were used to dissect Saikosaponin D (SSD)'s radiosensitizing mechanisms. Promoter luciferase assays and ChIP-qPCR were performed to map SSD-induced HIF1α binding sites on the HDAC5 promoter. <b>Results:</b> Elevated endogenous lactate in radioresistant TNBC cells promoted DNA repair via MRE11 Lys⁶⁷³ lactylation, a critical modification conferring radioresistance. HDAC5 was identified as the key delactylase for MRE11 Lys^673, validated by HADDOCK docking (hydrogen bond between MRE11 Lys⁶⁷³ and HDAC5 Ser¹⁸) and Co-IP (HDAC5 overexpression reduced K⁶⁷³ lactylation). TCGA and clinical tissue microarrays confirmed HDAC5 downregulation in TNBC. SSD inhibits the malignant phenotype of TNBC and enhances radiotherapy efficacy by inhibition on MRE11 lactylation via upregulating HDAC5. Mechanistically, SSD upregulated HIF1α to bind the HDAC5 promoter (-342bp to -20bp region) to activate its expression. <b>Conclusion:</b> Lactate-driven MRE11 Lys⁶⁷³ lactylation mediates radioresistance, while SSD reverses this via HIF1α/HDAC5 axis activation. Our findings identify SSD as a radiosensitizer and HDAC5/MRE11 as potential therapeutic targets for TNBC.