Discovery of SKP2-Recruiting PROTACs for Target Protein Degradation

Proteolysis targeting chimeras (PROTACs) have emerged as an intriguing therapeutic strategy for targeted protein degradation (TPD), functioning as heterobifunctional compounds that induce the redirection of E3 ligases to ubiquitinate neo-substrates for proteasomal degradation. Despite the presence of over 600 E3 ligases, only a limited subset has been successfully harnessed for TPD. This study demonstrates that S-phase kinase-associated protein 2 (SKP2), the substrate receptor of the Cullin RING ligase 1 (CRL1) subfamily, can be employed for TPD using a selective, non-covalent SKP2 recruiter, SL1. We designed and synthesized SKP2-recruiting degraders by linking SL1 to the BRD4 inhibitor JQ1. These compounds effectively induce BRD4 degradation in MV-4-11 cells, with the most potent compound 2-1 exhibiting a half-maximal degradation (DC₅₀) of 298 nM, validating their potential as PROTACs. Mechanistic investigations show that 2-1 promotes BRD4 ubiquitination and subsequent degradation in a proteasome- and neddylation-dependent manner, which can be rescued by SKP2 knockdown and knockout. We further demonstrate that SKP2-directed PROTACs effectively degrade Androgen receptor (AR) in 22RV1 cells. These findings emphasize that SKP2, frequently overexpressed in various tumor cells, can be successfully exploited for TPD through non-covalent PROTACs, expanding the pool of E3 ligases available for potential therapeutic applications.