Hydrophobic Tag Degraders Overcome Endocrine-Resistant Breast Cancer by Recruiting HSP27-Mediated E3 Ligase Complex for ERα Proteasomal Degradation.

Hydrophobic tag (HyT)-mediated protein degradation has emerged as a pivotal tool for targeted protein degradation (TPD), yet its underlying degradation mechanism remains incompletely elucidated. Herein, we designed structurally optimized HyT-based degraders by covalently conjugating hydrophobic amino acid tags to ERα-targeting ligands via alkane linkers of varying lengths, identifying the lead compound VI-10h. VI-10h exhibited potent antiproliferative activity and efficient ERα degradation in endocrine-resistant breast cancer cells (LCC2, MCF-7D538G, MCF-7Y537S, and MCF-7EGFR) and superior antitumor activity compared to the clinical drug fulvestrant (Ful) in MCF-7 and tamoxifen-resistant LCC2 xenograft models. To elucidate the HyT-mediated degradation mechanism, we synthesized biotin-conjugated HyTs (biotin-Lys and biotin-Trp) and performed pull-down assays combined with mass spectrometry. Our results unveiled that VI-10h selectively recruits heat shock protein 27 (HSP27) as a non-canonical E3 ligase adaptor protein, forms an ERα-HSP27-RING1 ternary complex to promote ERα degradation, disrupts estrogen-dependent oncogenic networks, and circumvents the drug resistance associated with conventional CRBN- or VHL-dependent E3 ligase-recruiting degraders. This study clarifies a HyT-mediated ERα degradation mechanism and supports the feasibility of using HyT degraders to overcome resistance to conventional E3 ligase-recruiting strategies and endocrine-resistant breast cancer, thereby establishing a molecular design strategy for next-generation targeted degraders.