ABSTRACT
Nickel, cadmium, cobalt, and arsenic compounds are well-known carcinogens to humans and experimental animals. Even though their DNA-damaging potentials are rather weak, they interfere with the nucleotide and base excision repair at low, noncytotoxic concentrations. For example, both water-soluble Ni(II) and particulate black NiO greatly reduced the repair of DNA adducts induced by benzo[a]pyrene, an important environmental pollutant. Furthermore, Ni(II), As(III), and Co(II) interfered with cell cycle progression and cell cycle control in response to ultraviolet C radiation. As potential molecular targets, interactions with so-called zinc finger proteins involved in DNA repair and/or DNA damage signaling were investigated. We observed an inactivation of the bacterial formamidopyrimidine-DNA glycosylase (Fpg), the mammalian xeroderma pigmentosum group A protein (XPA), and the poly(adenosine diphosphate-ribose)polymerase (PARP). Although all proteins were inhibited by Cd(II) and Cu(II), XPA and PARP but not Fpg were inhibited by Co(II) and Ni(II). As(III) deserves special attention, as it inactivated only PARP, but did so at very low concentrations starting from 10 nM. Because DNA is permanently damaged by endogenous and environmental factors, functioning processing of DNA lesions is an important prerequisite for maintaining genomic integrity; its inactivation by metal compounds may therefore constitute an important mechanism of metal-related carcinogenicity.
Subject(s)
Cell Cycle/drug effects , DNA Repair , Metals, Heavy/adverse effects , Zinc Fingers , Animals , DNA-Binding Proteins/pharmacology , DNA-Formamidopyrimidine Glycosylase , Humans , N-Glycosyl Hydrolases/pharmacology , Poly(ADP-ribose) Polymerases/pharmacology , Xeroderma Pigmentosum Group A ProteinABSTRACT
Metal ions are essential components of biological systems; nevertheless, even essential elements may have toxic or carcinogenic properties. Thus, besides As(III) and Cd(II), also Ni(II) and Co(II) have been shown previously to disturb different types of DNA repair systems at low, non-cytotoxic concentrations. Since some metals exert high affinities for SH groups, we investigated whether zinc finger structures in DNA-binding motifs of DNA repair proteins are potential targets for toxic metal ions. The bacterial formamidopyrimidine-DNA glycosylase (Fpg protein) involved in base excision repair was inhibited by Cd(II), Cu(II) and Hg(II) with increasing efficiencies, whereas Co(II), As(III), Pb(II) and Ni(II) had no effect. Furthermore, Cd(II) still disturbed enzyme function when bound to metallothionein. Strong inhibition was also observed in the presence of phenylselenyl chloride, followed by selenocystine, while selenomethionine was not inhibitory. Regarding the mammalian XPA protein involved in the recognition of DNA lesions during nucleotide excision repair, its DNA-binding capacity was diminished by Cd(II), Cu(II), Ni(II) and Co(II), while Hg(II), Pb(II) and As(III) were ineffective. Finally, the H(2)O(2)-induced activation of the poly(ADP-ribose)polymerase (PARP) involved in DNA strand break detection and apoptosis was greatly reduced by Cd(II), Co(II), Ni(II) and As(III). Similarly, the disruption of correct p53 folding and DNA binding by Cd(II), Ni(II) and Co(II) has been shown by other authors. Therefore, zinc-dependent proteins involved in DNA repair and cell-cycle control may represent sensitive targets for some toxic metals such as Cd(II), Ni(II), Co(II) and Cu(II), as well as for some selenium compounds. Relevant mechanisms of inhibition appear to be the displacement of zinc by other transition metals as well as redox reactions leading to thiol/disulfide interchange.
Subject(s)
DNA Repair/drug effects , DNA-Binding Proteins/metabolism , Metals, Heavy/toxicity , Zinc Fingers/drug effects , Zinc/toxicity , Cations, Divalent , DNA Repair/physiology , Humans , N-Glycosyl Hydrolases/metabolism , Protein Folding , RNA-Binding Proteins/metabolism , Xeroderma Pigmentosum Group A Protein , Zinc Fingers/geneticsABSTRACT
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.
Subject(s)
DNA Repair/drug effects , DNA, Circular/metabolism , DNA-Binding Proteins/metabolism , Metals, Heavy/toxicity , N-Glycosyl Hydrolases/metabolism , RNA-Binding Proteins/metabolism , Zinc Fingers/drug effects , Arsenic/toxicity , Cations, Divalent/toxicity , DNA Repair/physiology , DNA-Formamidopyrimidine Glycosylase , Xeroderma Pigmentosum Group A ProteinABSTRACT
Compounds of nickel, cadmium, cobalt and arsenic have been shown previously to inhibit DNA repair processes at low concentrations. In the present study we investigated whether this repair inhibition may be caused by the displacement of zinc in zinc finger structures of DNA repair proteins. As models, the bacterial formamidopyrimidine-DNA glycosylase (Fpg) and the mammalian XPA protein were applied. Both proteins were inhibited by Cd(II) and Cu(II). Hg(II) strongly inhibited the Fpg protein, but did not affect the XPA protein. In contrast, the XPA protein was disturbed by Co(II) and Ni(II), while the activity of the Fpg protein was not reduced. Neither protein was inhibited by As(III) or Pb(II). Thus, each zinc finger protein appears to have its own structural features and sensitivities towards toxic metal ions. Furthermore, each metal exerts specific mechanisms leading to DNA repair inhibition.