Effect of PDD00017273 and GSK2606414 on MZ1-induced cell death

Targeted protein degradation is a groundbreaking modality in drug discovery ; however, further improvement of the degradation efficacy is demanded. Here, we identify small molecules that enhance the targeted degradation of the anticancer target BRD4 and related hard-to-degrade targets BRD2/3 induced by CRL2VHL- or CRL4CRBN -based PROTACs. The chemicals include inhibitors of cellular signaling pathways such as poly-ADP ribosylation (PARG inhibitor PDD00017273), unfolded protein response (PERK inhibitor GSK2606414), and protein stabilization (HSP90 inhibitor luminespib). Mechanistically, PARG inhibition promotes TRIP12-mediated K29/K48-linked branched ubiquitylation of BRD4 by facilitating chromatin dissociation of BRD4 and formation of the BRD4–PROTAC– CRL2VHL ternary complex; by contrast, HSP90 inhibition promotes BRD4 degradation after the ubiquitylation step. Consequently, these signal inhibitors sensitize cells to the PROTAC-induced apoptosis. Combining the treatment with these chemicals further increases the efficacy of a highly potent PROTAC, SIM1. We propose that various cell intrinsic signaling pathways spontaneously counteract chemically induced target degradation, which can be liberated by specific inhibitors.