Discovery of a Natural\nEnt-Kaurene Diterpenoid Oridonin\nas an E3 Ligase Recruiter for PROTACs

PROTACs have emerged as a therapeutic modality for the\ntargeted\ndegradation of proteins of interest (POIs). Central to PROTAC technology\nare the E3 ligase recruiters, yet only a few of them have been identified\ndue to the lack of ligandable pockets in ligases, especially among\nsingle-subunit ligases. We propose that binders of partner proteins\nof single-subunit ligases could be repurposed as new ligase recruiters.\nMDM2 is a single-subunit ligase overexpressed in tumors. Nucleolin\n(NCL) is an MDM2 partner protein that displays a similar tumor-specific\noverexpression pattern and nuclear-cytoplasmic shuttling role to MDM2.\nFurthermore, NCL is selectively translocated on the tumor cell surface,\nwhere it acts as an internalization receptor for its binders. We reveal\nthat the NCL-binding Oridonin (Ori), a natural ent-kaurene diterpenoid,\nis capable of recruiting MDM2 by employing NCL as a molecular bridge.\nWe design Ori-based PROTACs for modulating oncogenic POIs, including\nBRD4 and EGFR. These PROTACs direct the assembly of MDM2-NCL-PROTAC-POI\ncomplexes to induce proteasomal degradation of POIs and tumor shrinkage.\nIn addition to its role as a ligase engaged by PROTACs, MDM2, along\nwith its homologue MDMX, plays a nonredundant function in inhibiting\np53 activity. Dual inhibition of MDM2/X is proposed as a promising\nantitumor strategy. We demonstrate that Ori also recruits MDMX in\nan NCL-dependent manner. Ori-based homo-PROTACs induce MDM2/X dual\ndegradation and attenuate tumor progression. Our findings prove the\nfeasibility of repurposing the binders of ligase partner proteins\nas new ligase recruiters in PROTACs and highlight the potential of\nOri as an MDM2/X recruiter.