Nimbolide Targets RNF114 to Induce the Trapping of PARP1 and Poly-ADP-Ribosylation-Dependent DNA Repair Factors

Abstract PARP1 is an abundant nuclear enzyme that is critically involved in DNA damage response. Its main enzymatic function is to catalyze a protein post-translational modification known as poly-ADP-ribosylation (PARylation). Despite the tremendous progresses of PARP1 inhibitors (PARPi) in the clinic, the basic mechanism of action of PARPi is poorly understood. Recent studies point to PARP1 trapping as a key factor driving the cytotoxic and immunomodulatory functions of PARPi. However, the molecular underpinnings of PARP1 trapping remain elusive. Here, using an unbiased, quantitative proteomic screen, we identified RING finger protein 114 (RNF114), as a PARylation-dependent, ubiquitin E3 ligase involved in DNA damage response. Upon sensing DNA damage, RNF114 was recruited, in a PAR-dependent manner, to the DNA lesions, where it specifically targeted PARylated-PARP1 for ubiquitin-proteasomal degradation. The blockade of this pathway interfered with the removal of PARP1 from the DNA damage site, leading to profound PARP1 trapping. We show that a natural product, Nimbolide, targeted RNF114 to block the degradation and removal of PARylated-PARP1 from DNA lesions. Unlike conventional PARPi, nimbolide treatment induced the trapping of both PARP1 and PARylation-dependent DNA repair factors. This unique mechanism of action of nimbolide translated into its superior cytotoxic effects against BRCA -deficient cancers. Furthermore, we demonstrated that, as a super trapper of both PARylated-PARP1 and PARylation-dependent DNA repair factors, nimbolide was able to overcome the intrinsic and acquired resistance to PARPi. Finally, we showed that nimbolide treatment activated innate immune signaling and was able to synergize with various cytotoxic agents. These results point to the exciting possibility of targeting homologous recombination-deficient cancers using nimbolide and its analogs.