APE1/REF-1 down-regulation enhances the cytotoxic effects of temozolomide in a resistant glioblastoma cell line.

Temozolomide (TMZ) is widely used for patients with glioblastoma (GBM); however, tumor cells frequently exhibit drug-resistance. Base excision repair (BER) has been identified as a possible mediator of TMZ resistance, and an attractive approach to sensitizing cells to chemotherapy. Human apurinic/apyrimidinic endonuclease/redox factor-1 (APE1) is an essential enzyme with a role in the BER pathway by repairing abasic sites, and it also acts as a reduction factor, maintaining transcription factors in an active reduced state. Thus, we aimed to investigate whether the down-regulation of APE1 expression by siRNA can interfere with the resistance of GBM to TMZ, being evaluated by several cellular and molecular parameters. We demonstrated that APE1 knockdown associated with TMZ treatment efficiently reduced cell proliferation and clonogenic survival of resistant cells (T98G), which appears to be a consequence of increased DNA damage, S-phase arrest, and H2AX phosphorylation, resulting in apoptosis induction. On the contrary, for those assays, the sensitization effects of APE1 silencing plus TMZ treatment did not occur in the TMZ-sensitive cell line (U87MG). Interestingly, TMZ-treatment and APE1 knockdown significantly reduced cell invasion in both cell lines, but TMZ alone did not reduce the invasion capacity of U87MG cells, as observed for T98G. We also found that VEGF expression was down-regulated by TMZ treatment in T98G cells, regardless of APE1 knockdown, but U87MG showed a different response, since APE1 silencing counteracted VEGF induction promoted by TMZ, suggesting that the APE1-redox function may play an indirect role, depending on the cell line. The present results support the contribution of BER in the GBM resistance to TMZ, with a greater effect in TMZ-resistant, compared with TMZ-sensitive cells, emphasizing that APE1 can be a promising target for modifying TMZ tolerance. Furthermore, genetic characteristics of tumor cells should be considered as critical information to select an appropriate therapeutic strategy.