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

Abstract

The genome-protecting role of poly(ADP-ribose) (PAR) has identified PAR polymerase-1 (PARP-1) and PAR glycohydrolase (PARG), two enzymes responsible for the synthesis and hydrolysis of PAR, as chemotherapeutic targets. Each has been previously individually evaluated in chemotherapy, but the effects of combination PARP-1 and PARG inhibition in cancer cells are not known. Here we determined the effects of the inhibition of PARP-1 and the absence or RNAi knockdown of PARG on PAR synthesis, cell death after chemotherapy and long-term viability. Using three experimental/clinical PARP-1 inhibitors in PARG-null cells, we show decreased levels 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-null cells, but decreased cell death when pretreated with each PARP-1 inhibitor. Similar results were observed in MNNG-treated HeLa cells, where RNAi knockdown of PARG or pretreatment with ABT-888 led to increased HeLa cell death, whereas combination PARG RNAi knockdown + ABT-888 failed to produce increased cell death. The results demonstrate the ability of the PARP-1 inhibitors to decrease PAR levels, maintain viability and decrease PAR-mediated cell death after chemotherapeutic treatment in the absence of PARG. Further, the results demonstrate that the combination of PARP-1 and PARG inhibition in chemotherapy does not produce increased HeLa cell death. Thus, the results indicate that inhibiting both PARP-1 and PARG, which both are chemotherapeutic targets that increase cancer cell death, does not lead to synergistic cell death in HeLa cells. Therefore, strategies that target PAR metabolism for the improved treatment of cancer may be required to target PARP-1 and PARG individually in order to optimize cancer cell death.

PARP and PARG inhibitors in cancer treatment

“…PARG mutations were identified in TNBC and highgrade serous ovarian cancers eligible for PARP inhibitor treatment ). On the one hand, PARG depletion was shown to reduce sensitivity of HeLa cells to PARP inhibitors combined with DNA-damaging agents or mouse Brca2 mutated cells to PARP inhibitors alone (Feng and Koh 2013;Gogola et al 2018). PARG deficiency may contribute to PARP inhibitor resistance by increasing PARP1 auto-PARylation that allows PARP1 dissociation from DNA (i.e., reduction in PARP trapping) and restoration of PARP signaling ).…”

Radiosensitization with an inhibitor of poly(ADP-ribose) glycohydrolase: A comparison with the PARP1/2/3 inhibitor olaparib

“…These defects in repair/replication suggest that PARG like PARP is a possible target as a single agent in certain genetic backgrounds [53] and for sensitizing to DNA damaging agents. The reported chemosensitizing effects are variable [6] , [52] , [57] , [58] , [59] , [60] , [61] , but gene depletion or silencing of PARG using siRNA has consistently resulted in sensitivity to ionising radiation (IR) in mouse ES cells [62] , [63] and human cancer cell lines [51] , [64] , with accumulation of mitotic defects and death occurring by mitotic catastrophe [51] , [64] .…”

PARG deficiency is neither synthetic lethal with BRCA1 nor PTEN deficiency

“…Therefore, both synthesis and degradation of PAR require tight regulation for competent repair [ 1 , 2 ]. These results define PARG, like PARP-1, as a new candidate target to potentiate chemo- and radiotherapy [ 8 , 12 , 13 ].…”