| Summary: | 碩士 === 國立臺灣海洋大學 === 生物科技研究所 === 99 === Cadmium (Cd) is a carcinogenic heavy metal which may exert its carcinogenicity by decreasing the efficiency of DNA repair. Our previous studies indicated the ability of Cd ion to down-regulate the gene expressions of MutS homolog 2 (MSH2) and MSH6 that initiates DNA mismatch repair (MMR) of simple mispairs and small insertion-deletion loops in vertebrates using zebrafish (Danio rerio) embryo as a model organism. This study explored the mechanisms of Cd-induced inhibition of msh2/msh6 activities and the effects of mercuric ion on the two genes. Cd at sublethal and low toxic concentrations significantly inhibited msh2/msh6 expressions in zebrafish embryos after a 9-hr exposure as shown by QRT-PCR and mismatch binding assay, while Hg imposed only medium inhibitory effects. Inhibition of msh6 mRNA production in the tissues metal-treated embryos was confirmed by whole mount in situ hybridization. Cd exposure also suppressed the gene expression of Cu-Zn superoxide dismutase (SOD), but the expression of catalase gene was hardly affected by this metal. Moreover, the inhibitory effects of 3 μM Cd on msh2/msh6 expressions were totally reversed in the presence of 10μM D-mannitol, N-acetylcysteine, or butylhydroxytoluene, reflecting the participation of reactive oxygen species (ROS), superoxide anion radical possibly the major one, in Cd-induced gene down-regulation. Gene expressions of msh2/msh6 and SOD were generally unaffected by hydrogen peroxide. The transcription of human msh6 is dependent on the Sp1 activity, but Cd or Hg exposure barely disturbed the DNA binding activity of Sp1 factor or Sp1 protein synthesis in embryos after a 9-hr exposure. Hence, Cd is believed to inhibit msh2/msh6 expressions via the production of ROS and this study also showed the potential of Hg to disturb the recognition step of MMR.
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