Aims
Targeted protein degradation is an emerging concept in drug discovery that utilizes the ubiquitin–proteasome system to induce the degradation of disease-causing proteins. PROTACs induce proximity between the target protein and an E3 ubiquitin ligase, leading to forced ubiquitylation and subsequent proteasomal degradation of the target. A number of PROTACs have entered clinical trials for the treatment of cancer. However, the cellular signaling pathways that modulate targeted degradation remain largely unknown.In this study, we aimed to identify signaling pathways and elucidate the mechanisms that regulate PROTAC-induced protein degradation.
Results
We performed a HiBiT screening using MZ1, a CRL2VHL-based PROTAC that targets BRD4, a chromatin regulator and one of the major therapeutic targets in cancer. We identified PDD17270 (PDD), a PARG inhibitor, as a compound that enhances MZ1-induced BRD4 degradation, resulting in increased apoptotic cell death. Mechanistically, ChIP-Seq analysis revealed that PARG inhibition enhanced the dissociation of BRD4 from chromatin at specific loci. This promoted the formation of the BRD4–PROTAC–E3 ternary complex and the modification of BRD4 with K48- and K29-linked branched ubiquitin chains. Furthermore, PDD also enhanced the degradation of CDK9 induced by Thal-SNS.
Conclusion
In conclusion, we identified cellular pathways that modulate PROTAC-induced degradation of BRD family neosubstrates. Our findings suggest that cell-intrinsic pathways counteract targeted protein degradation, and that inhibition of these intrinsic inhibitory mechanisms may improve the efficacy of targeted protein degradation.