PARP-1 inhibition sensitizes temozolomide-treated glioblastoma cell lines and decreases drug resistance independent of MGMT activity andPTENproficiency

Information on the mechanisms that are associated with tumor resistance has the potential to provide the fundamental basis for novel therapeutic strategies. In glioblastoma (GBM), predictive biomarkers of cellular responses to temozolomide (TMZ) combined with poly-ADP-ribose polymerase inhibitor (PARPi) remain largely unidentified. In this context, the influence of MGMT (O-6-methylguanine DNA methyltransferase) and PTEN (phosphatase and tensin homologue deleted on chromosome ten) has been studied in addition to the occurrence of synthetic lethality involving PTEN and PARPi. The present study investigated whether PARP-1 inhibition by NU1025 may increase the cytotoxicity of TMZ-induced lesions in GBM cells, and whether these mechanisms can be influenced by MGMT andPTENstatus. The impact ofPTENdeficiency in repair pathways, and the effects of PARP-1 inhibition andPTENsilencing, in terms of synthetic lethality, were also assessed. NU1025 combined with TMZ effectively sensitized TMZ-resistant cells (T98GPTEN-mutated and LN18PTEN-wild-type) and TMZ-sensitive cells (U251MGPTEN-mutated), in contrast to NU1025 alone. However, the sensitizing effects were not observed in U87MG (PTEN-mutated) cells, suggesting that specific genetic alterations may influence the response to drug treatment. The sensitizing effects occurred independently of MGMT activity, which was evaluated in O6-BG-treated cells.PTENsilencing using small interfering (si)RNA did not sensitizePTEN-proficient cells to TMZ + NU1025, or NU1025 alone, indicating an absence of synthetic lethality. The responses to TMZ + NU1025 involved antiproliferative activity, G2/M arrest, double strand breaks and the induction of apoptosis. Following 20 days of recovery after three consecutive days of TMZ treatment, TMZ-resistant cells were observed. However, when TMZ was combined with NU1025, the viability of T98G and LN18 cells was extremely decreased, indicating a lethal drug combination. Therefore, independently of MGMT proficiency andPTENstatus, TMZ combined with PARPi may be a promising strategy that can be used to overcome TMZ acquired resistance in GBM cells. (AU)