Effects of DNA damage response kinase inhibitors on increased Taxol hypersensitivity leading to caspase 3 activation

Abstract

Taxol is an antitumor agent that arrests cells in the late G2/M phases of the cell cycle. Our previous research demonstrated that PARP inhibition enhances Taxol induced cell death via oxidative stress and free radical production. In this study, we hypothesized that inhibiting DNA damage response (DDR) kinases would further increase Taxol cytotoxicity by impairing the repair of Taxol-induced DNA damage. We found that inhibition of PI3K-like DDR kinases enhanced Taxol-induced apoptosis through caspase-3 activation. We used Chinese hamster V79 cells and their ATM, ATR, and Ku80-deficient mutants which exhibited hypersensitivity to Taxol. Pharmacological inhibitors, KU55933 (ATM), NU7441 (DNA-PK), and VE821 (ATR), also sensitized V79, CHO, and U2OS human cancer cells to Taxol. This sensitization was associated with increased apoptosis, confirmed by sub-G1 analysis and caspase-3/7 activity assays. Interestingly, MCF7 cells, which lack Caspase-3, did not show enhanced sensitivity to Taxol under DDR inhibition, nor did GM5400 human fibroblast cells. In contrast, MCF7-C3 cells, with restored caspase-3 expression, exhibited significant apoptosis and sensitization, confirming a caspase-3 dependent mechanism. These findings suggest that ATM, ATR, and DNA-PK not only facilitate DNA repair but also suppress Taxol-induced apoptosis via caspase-3. Their inhibition may represent a promising strategy to boost their efficacy of Taxol and potentially enhance responses to radiation therapy through combined targeting of mitotic stress and DDR pathways.