Downregulation of ETV4 mediates trametinib resistance via AMPK-ULK1-dependent protective autophagy in breast cancer

Trametinib is a selective MEK1/2 inhibitor initially approved for melanoma and later extended to BRAF-mutant non-small cell lung cancer and thyroid cancer, either as monotherapy or in combination with a BRAF inhibitor. The favorable pharmacologic profile of trametinib has prompted broader evaluation across cancers driven by MAPK/ERK signaling. However, its efficacy as monotherapy in breast cancer (BCa) remains limited, partly due to adaptive resistance mechanisms. Therefore, we aimed to elucidate the molecular mechanisms underlying trametinib resistance in BCa and to validate therapeutic strategies that restore sensitivity. Here, we first found that trametinib responsiveness was associated with suppression of ETV4 expression. In T47D, SKBR3, and MDA-MB-453 cells, trametinib treatment suppressed ETV4 and promoted autophagic flux. Mechanistically, both trametinib treatment and ETV4 silencing activated AMPK-Thr172 phosphorylation, which in turn phosphorylated ULK1 at Ser555 while simultaneously inhibiting mTOR, thereby triggering protective autophagic flux. Combination treatment with trametinib and the autophagy inhibitors chloroquine (CQ) or 3-methyladenine (3-MA) induced apoptotic pathways in vitro and suppressed T47D xenograft tumor growth in vivo. Our results reveal that trametinib-induced downregulation of ETV4 enables cancer cells to acquire a survival advantage by activating AMPK-ULK1-mediated protective autophagy. We further identified that inhibiting autophagy reverses trametinib resistance and induces apoptotic cell death, providing a readily translatable combination therapeutic strategy to enhance MEK1/2-targeted therapy in BCa.