Targeting the ERα DBD-LBD Interface with Mitoxantrone Disrupts Receptor Function through Proteasomal Degradation

The estrogen receptor (ER or ERα) remains the primary therapeutic target for luminal breast cancer, with current treatments centered on competitive antagonists, receptor downregulators, and aromatase inhibitors. Despite these options, resistance frequently emerges, highlighting the need for alternative targeting strategies. We discovered a novel mechanism of ER inhibition that targets the previously unexplored interface between the DNA-binding domain (DBD) and ligand-binding domain (LBD) of the receptor. Through computational screening and functional assays, we identified mitoxantrone (MTO), an FDA-approved topoisomerase II inhibitor, as a specific ligand for this DBD-LBD interface. Comprehensive biophysical, biochemical, and cellular analyses demonstrate that MTO binding induces distinct conformational changes in the ER, triggering rapid cytoplasmic redistribution and proteasomal degradation through mechanisms independent of its DNA damage activity. Critically, MTO effectively inhibits constitutively active ER mutants (Y537S and D538G) associated with endocrine therapy resistance, suppressing both wild-type and mutant ER-dependent gene expression and tumor growth more potently than fulvestrant in cellular and xenograft models. These findings establish the DBD-LBD interface as a druggable allosteric site that can overcome conventional resistance mechanisms, providing a new therapeutic paradigm for targeting nuclear receptor function through disruption of interdomain communication rather than hormone-binding competition.