Cadmium Inhibits DNA Strand Break Rejoining in Methyl Methanesulfonate-Treated CHO-K1 Cells

Abstract

The cogenotoxicity of Cd has been recognized. This effect may stem from Cd inhibition of DNA repair. We studied the effects of Cd on DNA repair of methyl methanesulfonate (MMS)-damaged Chinese hamster ovary cells (CHO-K1) by single-cell alkaline electrophoresis. The results indicate that in the presence of Cd, DNA strand breaks accumulated in MMS-treated cells. Using hydroxyurea (Hu) plus cytosine-β-d-arabinofuranoside (AraC) to block DNA polymerization, DNA strand breaks accumulated and Cd had little inhibitory effects on these accumulations. However, Cd inhibited the rejoining of these DNA strand breaks, which could be rejoined 6 hr after release from Hu plus AraC blockage. These results indicate that the potency of Cd inhibition of DNA repair replication and/or ligation may be greater than the inhibition of DNA adduct excision. To further elucidate this mechanism, we used anin vitrocell-free assay system to analyze the Cd effects on DNA repair synthesis, DNA polymerization, and DNA ligation. We have shown a dose-dependent inhibition of these three activities by Cd in CHO-K1 cell extract. The IC50s of Cd were 55, 26, and 10 μm, respectively. Moreover, Cd inhibition of DNA ligation in cell extract could be recovered partially by thiol compounds such as glutathione, β-mercaptoethanol, dithiothreitol, and metallothionein. Since bothin vivoandin vitrostudies demonstrated that Cd was more effectively involved in interfering with the DNA ligation step and that thiol agents could partially remove Cd inhibition of DNA ligation,we speculate thatpart of the Cd inhibition of DNA repair may be through binding of Cd to the proteins participating in DNA ligation.