Protective Effect of Thymidine on DNA Damage Induced by Hydrogen Peroxide in Human Hepatocellular Cancer Cells

Intracellular ribonucleotide (RN) and deoxyribonucleotide (dRN) pool sizes are critical for the fidelity of DNA synthesis. They are likely to be severely perturbed by many factors which disrupt the integrity and stability of DNA, leading to DNA damage. Exogenously supplied nucleosides are able to increase the deoxynucleoside triphosphate pools, then reverse the DNA damage, and decrease the oncogene-induced transformation dramatically. In this study, the impact of thymidine on the hydrogen peroxide (H₂O₂)-induced DNA damage was investigated in HepG2 liver cancer cells. From the result of the comet assay, the tail length of cells in the thymidine 600 μM + H₂O₂ 1.0 mM group was dramatically decreased from 42.1 ± 10.8 to 21.9 ± 2.4 μm compared to that exposed with 1.0 mM H₂O₂ (p < 0.05), suggesting that pretreatment of thymidine reduced the DNA damage of HepG2 cells. Although the RN and dRN contents decreased in the damage group, most of them presented increasing tendency when pretreated with thymidine, especially the key metabolites dCTP, which was mainly related with the decline in the rate of DNA synthesis. The restoration also showed a significant G0/G1 phase arrest of cell cycle progression from 44.6 ± 2.2 to 56.6 ± 0.4% after pretreated with thymidine (p < 0.05). In conclusion, our data demonstrated that the pretreatment with thymidine had a potential protective ability against oxidative damage for DNA in HepG2 cells through the perturbation of RN and dRN pools as well as cell cycle arrest, which should provide new insights into the molecular basis of preventing H₂O₂-induced oxidative DNA damage in mammalian cells.