Bleomycin (BLM), an important anti-tumor antibiotic, enables cell death through oxidative DNA damage mediated by reactive oxygen species (ROS). However, increasing cellular resistance has become a serious limitation to its clinical application. Base excision repair (BER), the major pathway for repairing oxidative bases, is involved in resistance of DNA-damaging anticancer drugs. DNA polymerase beta (pol β), a critical BER enzyme, has been reported to play a crucial role in combating BLM-induced oxidative DNA damage, as a result, pol β inhibition may increase the sensitivity to BLM. To test this hypothesis, we evaluated the sensitivity to BLM using mouse embryo fibroblasts (MEFs) with distinct pol β expression levels (wild-type, pol β deficiency) and explored the underlying mechanisms. The results showed that cell viability of pol β-deficient MEFs was significantly lower than that of isogenic wild type when treated with the same BLM dosage. In addition, increased ROS level, DNA single strand breaks, and chromosomal breakage were observed in pol β deficient cells, indicating impaired DNA repair and enhanced oxidative DNA damage under pol β deficiency. In agreement with the findings, an enhanced hprt gene mutation frequency was also detected in pol β null cells. In summary, this study demonstrated that BLM-induced DNA damage could be repaired through BER pathway and absence of pol β allows oxidative DNA/chromosome damage and gene mutation, which contributes to BLM hypersensitivity.
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