CAF-derived exosomes drive trastuzumab resistance in HER2-positive breast cancer via YAP-USP8-HER2 axis activation

BACKGROUND AND OBJECTIVE: Cancer-associated fibroblasts (CAFs), critical constituents of the tumor microenvironment, promote tumor progression and therapeutic resistance through exosome secretion. However, the mechanisms by which CAF-derived exosomes contribute to trastuzumab resistance in HER2-positive breast cancer remain unclear. This study investigated whether CAF-derived exosomes activate the YAP-USP8 axis to stabilize HER2 and induce trastuzumab resistance. METHODS: Primary CAFs and normal fibroblasts (NFs) were isolated from HER2-positive breast cancer specimens, and exosomes were characterized by TEM, nanoparticle tracking analysis, and immunoblotting of exosomal markers. HER2-positive breast cancer cells (HCC1954 and BT-474) were treated with CAF- or NF-derived exosomes in vitro. Mechanistic studies were performed mainly in HCC1954 cells, whereas BT-474 cells were used for validation. Cargo dependency was assessed using RNase/Proteinase K protection assays, and YAP pathway involvement was examined using Verteporfin. In vivo tumor growth was evaluated using xenograft models. These effects were reproduced in BT-474 cells and were reversed by YAP or USP8 silencing. Pharmacological inhibition of YAP similarly restored trastuzumab sensitivity. TCGA-BRCA analysis revealed positive correlations between YAP1 and USP8, and between USP8 and ERBB2, supporting the molecular relevance of this axis. CONCLUSION: This study identified a novel CAF-exosome-YAP-USP8-HER2 signaling axis that drives trastuzumab insensitivity in HER2-positive breast cancer. Intervening in this pathway may represent a potential treatment approach to counteract microenvironment-induced chemoresistance.