Targeted protein degradation is a groundbreaking modality in drug discovery; however, its regulatory mechanisms remain incompletely understood. The degradation of the neosubstrate induced by PROTACs proceeds through unique mechanisms that are in part common to but different from the degradation mechanism of genuine substrates. We previously discovered that the targeted degradation of BRD4 involves K29/K48-linked branched ubiquitin chains and the E3 ubiquitin ligase TRIP12.
Subsequently, we screened cellular signaling pathways that modulate the degradation of BRD4 induced by CRL2VHL- or CRL4CRBN-based PROTACs. Among the chemicals identified as degradation enhancers are inhibitors of signaling pathways, including poly-ADP ribosylation (PARG inhibitor PDD00017273), the unfolded protein response (PERK inhibitor GSK2606414), and protein stabilization (HSP90 inhibitor luminespib). In particular, PARG inhibition promotes TRIP12-mediated K29/K48-linked branched ubiquitylation of BRD4. These findings suggest that various cell-intrinsic signaling pathways intrinsically counteract chemically induced target degradation at multiple steps, and that their inhibitory effects can be relieved by specific inhibitors
Based on these results, we searched for regulators that influence the efficacy of targeted degradation of chromatin-associated neosubstrates. In this presentation, I will share our unpublished findings on cellular enzymes that regulate different steps in targeted protein degradation.