Targeting Homologous Recombination by Pharmacological Inhibitors Enhances the Killing Response of Glioblastoma Cells Treated with Alkylating Drugs

Abstract

Malignant gliomas exhibit a high level of intrinsic and acquired drug resistance and have a dismal prognosis. First- and second-line therapeutics for glioblastomas are alkylating agents, including the chloroethylating nitrosoureas (CNU) lomustine, nimustine, fotemustine, and carmustine. These agents target the tumor DNA, forming O-chloroethylguanine adducts and secondary DNA interstrand cross-links (ICL). These cross-links are supposed to be converted into DNA double-strand breaks, which trigger cell death pathways. Here, we show that lomustine (CCNU) with moderately toxic doses induces ICLs in glioblastoma cells, inhibits DNA replication fork movement, and provokes the formation of DSBs and chromosomal aberrations. Since homologous recombination (HR) is involved in the repair of DSBs formed in response to CNUs, we elucidated whether pharmacologic inhibitors of HR might have impact on these endpoints and enhance the killing effect. We show that the Rad51 inhibitors RI-1 and B02 greatly ameliorate DSBs, chromosomal changes, and the level of apoptosis and necrosis. We also show that an inhibitor of MRE11, mirin, which blocks the formation of the MRN complex and thus the recognition of DSBs, has a sensitizing effect on these endpoints as well. In a glioma xenograft model, the Rad51 inhibitor RI-1 clearly enhanced the effect of CCNU on tumor growth. The data suggest that pharmacologic inhibition of HR, for example by RI-1, is a reasonable strategy for enhancing the anticancer effect of CNUs. Mol Cancer Ther; 15(11); 2665-78. ©2016 AACR.

DNA repair genes in astrocytoma tumorigenesis, progression and therapy resistance

“…Moreover, the pharmacological inhibition or knockdown of downstream proteins in the HR pathway, such as BRCA2, RAD51, and CHK2, also increased sensitivity to treatment (Quiros et al, 2011;Eich et al, 2013). Furthermore, GBM cells treated with other alkylating agents were more sensitive after inhibition of MRE11, a component of MRN complex that recognizes DNA lesions and recruits ATM kinase (Berte et al, 2016). In contrast, some reports suggest that ATM/ATR activation is required to induce cell death after the MMR futile cycle is established (Caporali et al, 2004) and that MRE11 knockdown decreases sensitivity to TMZ (Mirzoeva et al, 2006).…”

GBM radiosensitizers: dead in the water…or just the beginning?

“…There are very few specific HR inhibitors in pre-clinical development at this time, and most of them appear to inhibit Rad51 (24). Recent studies have suggested that these molecules may be sensitize the effects of alkylators (25) and radiotherapy (26) specifically in glioma. Small molecule inhibitors of canonical NHEJ proteins, such as DNA-PK, have been developed and are now being tested as radiosensitizers in clinical trials, although currently there are no studies specifically in GBM (27).…”

Melatonin Sensitizes Hepatocellular Carcinoma Cells to Chemotherapy Through Long Non-Coding RNA RAD51-AS1-Mediated Suppression of DNA Repair

“…However, DNA repair systems in most HCC cells are abnormally active due to the accumulation of mutations, resulting in elevated DNA repair capacity and poor chemotherapy outcomes. Recent studies have demonstrated that the effects of chemotherapy and radiotherapy can be significantly enhanced when the DNA repair pathway in cancer cells is blocked, and accordingly, DNA repair blockers, such as trans-resveratrol, B02, and IBR2, have been widely used for clinical treatment to reduce the dose of chemotherapeutic drugs and improve their therapeutic effects [ 5 , 6 , 7 ].…”