Differential effects of toxic metal compounds on the activities of Fpg and XPA, two zinc finger proteins involved in DNA repair

Abstract

Even though not mutagenic, compounds of the carcinogenic metals nickel, cadmium, cobalt and arsenic have been shown previously to inhibit nucleotide excision repair and base excision repair at low, non-cytotoxic concentrations. Since some toxic metals have high affinities for -SH groups, we used the bacterial formamidopyrimidine-DNA glycosylase (Fpg protein) and the mammalian XPA protein as models to investigate whether zinc finger structures in DNA repair enzymes are particularly sensitive to carcinogenic and/or toxic metal compounds. Concentrations of </=1 mM Ni(II), Pb(II), As(III) or Co(II) did not affect the activity of the Fpg protein significantly. In contrast, the enzyme was inhibited in a dose-dependent manner by Cd(II), Cu(II) or Hg(II), starting at concentrations of 50 microM, 5 microM and 50 nM, respectively. Simultaneous treatment with Cd(II) or Cu(II) and Zn(II) partly prevented the inhibitions, while no reversal of inhibition was observed when Zn(II) was added after Cd(II) or Cu(II). In the case of Hg(II), Zn(II) had no protective effect independent of the time of its addition; however, the enzyme activity was completely restored by glutathione. Regarding XPA, Hg(II), Pb(II) or As(III) did not diminish its binding to an UV-irradiated oligonucleotide, while Cd(II), Co(II), Cu(II) and Ni(II) reduced its DNA-binding ability. Simultaneous treatment with Zn(II) prevented largely the inhibition induced by Cd(II), Co(II), and Ni(II), but only slightly in the case of Cu(II). Collectively, the results indicate that both proteins were inhibited by Cd(II) and Cu(II), XPA was additionally inactivated by Ni(II) and Co(II), and Fpg but not XPA was strongly affected by Hg(II). Even though other mechanisms of protein inactivation cannot be completely excluded, zinc finger structures may be sensitive targets for toxic metal compounds, but each zinc finger protein has unique sensitivities.

Interference by toxic metal ions with DNA repair processes and cell cycle control: molecular mechanisms.

“…The results concerning the interaction of carcinogenic metal compounds with zinc finger DNA repair enzymes have been summarized recently: Fpg was inhibited by Cd(II), Cu(II), and Hg(II), whereas Co(II), As(III), Pb(II), and Ni(II) had no effect. Nevertheless, in addition to Cd(II) and Cu(II), both Ni(II) and Co(II) inhibited DNA binding of XPA, whereas Hg(II), Pb(II), and As(III) were ineffective (24,25). One other zinc finger protein activated in response to DNA damage is the poly(adenosine diphosphate [ADP]-ribose)polymerase (PARP)-1.…”

p53-dependent but ATM-independent inhibition of DNA synthesis and G2 arrest in cadmium-treated human fibroblasts

“…Cadmium has the propensity to replace zinc in biological material and several DNA repair factors including p53 (Meplan et al, 1999), XPA (Asmuss et al, 2000), and hMutS-α (Clark and Kunkel, 2004;Banerjee and Flores-Rozas, 2005) are inhibited by cadmium. Inhibition of DNA repair or other elements of DNA damage response such as apoptosis could sensitize cells to carcinogenesis by endogenous or exogenous stresses that damage DNA (Mukherjee et al, 2004;Sancar et al, 2004).…”

Intracellular Copper Accumulation Enhances the Growth of Kineococcus radiotolerans during Chronic Irradiation

“…Copper toxicity manifests itself through indiscriminant binding to cellular ligands or competitive displacement of other metal cofactors (4,11) as well in the production of intracellular reactive oxygen species, namely, hydroxyl radicals, via Fenton chemistry (23,27). While coppercatalyzed reduction of H 2 O 2 to hydroxyl radicals can be demonstrated in vitro (3,17), the significance of this reaction in mediating DNA damage in vivo remains a controversial issue.…”