Discovery of a Natural Ent-Kaurene Diterpenoid Oridonin as an E3 Ligase Recruiter for PROTACs

PROTACs have emerged as a therapeutic modality for the targeted degradation of proteins of interest (POIs). Central to PROTAC technology are the E3 ligase recruiters, yet only a few of them have been identified due to the lack of ligandable pockets in ligases, especially among single-subunit ligases. We propose that binders of partner proteins of single-subunit ligases could be repurposed as new ligase recruiters. MDM2 is a single-subunit ligase overexpressed in tumors. Nucleolin (NCL) is an MDM2 partner protein that displays a similar tumor-specific overexpression pattern and nuclear-cytoplasmic shuttling role to MDM2. Furthermore, NCL is selectively translocated on the tumor cell surface, where it acts as an internalization receptor for its binders. We reveal that the NCL-binding Oridonin (Ori), a natural ent-kaurene diterpenoid, is capable of recruiting MDM2 by employing NCL as a molecular bridge. We design Ori-based PROTACs for modulating oncogenic POIs, including BRD4 and EGFR. These PROTACs direct the assembly of MDM2-NCL-PROTAC-POI complexes to induce proteasomal degradation of POIs and tumor shrinkage. In addition to its role as a ligase engaged by PROTACs, MDM2, along with its homologue MDMX, plays a nonredundant function in inhibiting p53 activity. Dual inhibition of MDM2/X is proposed as a promising antitumor strategy. We demonstrate that Ori also recruits MDMX in an NCL-dependent manner. Ori-based homo-PROTACs induce MDM2/X dual degradation and attenuate tumor progression. Our findings prove the feasibility of repurposing the binders of ligase partner proteins as new ligase recruiters in PROTACs and highlight the potential of Ori as an MDM2/X recruiter.