Inhibition of poly(ADP-ribose) polymerase-1 or poly(ADP-ribose) glycohydrolase individually, but not in combination, leads to improved chemotherapeutic efficacy in HeLa cells

The genome-protecting role of poly(ADP-ribose) (PAR) has identified PARpolymerase-1 (PARP-1) and PAR glycohydrolase (PARG), two enzymes responsible forthe synthesis and hydrolysis of PAR, as chemotherapeutic targets. Each has beenpreviously individually evaluated in chemotherapy, but the effects of combinationPARP-1 and PARG inhibition in cancer cells are not known. Here we determined theeffects of the inhibition of PARP-1 and the absence or RNAi knockdown of PARGon PAR synthesis, cell death after chemotherapy and long-term viability. Usingthree experimental/clinical PARP-1 inhibitors in PARG-null cells, we show decreasedlevels of PAR and increased short‑term and long‑term viability with each inhibitor,with the exception of DPQ. Treatment with the experimental chemotherapeutic agent,N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), led to increased cell death in PARG-nullcells, but decreased cell death when pretreated with each PARP-1 inhibitor. Similarresults were observed in MNNG-treated HeLa cells, where RNAi knockdown of PARGor pretreatment with ABT-888 led to increased HeLa cell death, whereas combinationPARG RNAi knockdown + ABT-888 failed to produce increased cell death. The resultsdemonstrate the ability of the PARP-1 inhibitors to decrease PAR levels, maintainviability and decrease PAR-mediated cell death after chemotherapeutic treatmentin the absence of PARG. Further, the results demonstrate that the combinationof PARP-1 and PARG inhibition in chemotherapy does not produce increased HeLacell death. Thus, the results indicate that inhibiting both PARP-1 and PARG, whichboth are chemotherapeutic targets that increase cancer cell death, does not leadto synergistic cell death in HeLa cells. Therefore, strategies that target PARmetabolism for the improved treatment of cancer may be required to target PARP-1and PARG individually in order to optimize cancer cell death.