The Novel Compound SIC-19 Triggers CUL4B-Mediated Ubiquitination and Degradation of SIK2

Small-molecule inhibitors represent a novel potential therapeutic for tumors. Our previous work identified SIC-19, a small molecule compound that induces degradation of salt-inducible kinase 2 (SIK2), leading to apoptosis and reduced proliferation in tumor, expecting to be a promising clinical treatment for ovarian cancer. While we initially observed that SIC-19 triggers SIK2 ubiquitination, the underlying mechanism was unknown. Here we elucidate this process by demonstrating that SIC-19 promotes the interaction between SIK2 and the CUL4B-DDB1 E3 ubiquitin ligase. Mass spectrometry discovered several ubiquitin ligases that bind to SIK2. Immunoprecipitation assays revealed that the CUL4B-DDB1 ligase is an interaction partner of SIK2. Through Western blot analysis, we demonstrated that CUL4B is functionally required for SIC-19-induced ubiquitination and SIK2 degradation. The ubiquitination and half-life of SIK2 are directly correlated with the dosage of SIC-19 and the expression of CUL4B. Site-directed mutagenesis and ubiquitination assays further confirmed that lysine 144 (K144) is the critical residue in SIK2 that regulates cancer behavior as cell proliferation and migration by leading to the degradation of SIK2. Our findings define a novel mechanism wherein SIC-19 induces CUL4B-dependent ubiquitination and degradation of SIK2, possesses the potential to act as an inhibitor, and may subsequently be developed as a targeted agent.