Upregulation of miR-99b-5p Modulates ESR1 Expression as an Adaptive Mechanism to Circumvent Drug Response via Facilitating ER/HER2 Crosstalk.

Background: Endocrine resistance remains a significant therapeutic challenge in estrogen receptor-positive (ER+) breast cancer, the most common subtype, contributing to increased morbidity and mortality. The interaction between ER and HER family receptors, particularly HER2 and epidermal growth factor receptor (EGFR), drives resistance to standard therapies such as tamoxifen and trastuzumab by activating key signaling pathways, including PI3K/AKT and RAS/MAPK. Dysregulated miRNAs, which are non-coding gene expression regulators, have been linked to therapy response. Aims: To investigate the role of miR-99b-5p in ER-HER2/EGFR crosstalk in BT-474 cells. Study Design: Experimental study. Methods: The expression profile and prognostic significance of miR- 99b-5p in breast cancer were analyzed using The Cancer Genome Atlas (TCGA) database. BT-474 cells were transfected with miR-99b-5p mimics and inhibitors, followed by treatment with tamoxifen and trastuzumab to assess their impact on cell proliferation and ER-HER2/EGFR crosstalk. Western blotting was performed to quantify EGFR, HER2, and ESR1 protein levels. Real-time proliferation analysis evaluated changes in cell growth following miRNA transfection and drug treatment. Results: The study revealed that miR-99b-5p is significantly overexpressed in tumors compared to normal tissues and is associated with poor patient survival and enhanced ER signaling. Transfection with miR-99b-5p mimics increased ESR1 expression and cell proliferation, even in the presence of tamoxifen or trastuzumab, indicating that miR-99b-5p contributes to therapy resistance through receptor crosstalk. Conversely, miR-99b-5p inhibition significantly restored drug sensitivity, reducing proliferation and enhancing the effectiveness of tamoxifen and trastuzumab. Conclusion: These findings establish miR-99b-5p as a key regulator of endocrine and HER2-targeted therapy resistance. Targeting miR-99b-5p could represent a potential therapeutic strategy to improve treatment outcomes in ER+/HER2+ breast cancer. Further research is needed to clarify the underlying molecular mechanisms and validate the therapeutic potential of miR-99b-5p inhibition in clinical applications.