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

Cadmium is an established human and animal carcinogen. Most evidence is available for elevated risk for lung cancer after occupational exposure; however, associations between cadmium exposure and tumors at other locations including kidney, breast, and prostate may be relevant as well. Furthermore, enhanced cancer risk may not be restricted to comparatively high occupational exposure, but may also occur via environmental exposure, for example in areas in close proximity to zinc smelters. The underlying mechanisms are still a matter of manifold research activities. While direct interactions with DNA appear to be of minor importance, elevated levels of reactive oxygen species (ROS) have been detected in diverse experimental systems, presumably due to an inactivation of detoxifying enzymes. Also, the interference with proteins involved in the cellular response to DNA damage, the deregulation of cell growth as well as resistance to apoptosis appears to be involved in cadmium-induced carcinogenicity. Within this context, cadmium has been shown to disturb nucleotide excision repair, base excision repair, and mismatch repair. Particularly sensitive targets appear to be proteins with zinc-binding structures, present in DNA repair proteins such as XPA, PARP-1 as well as in the tumor suppressor protein p53. Whether or not these interactions are due to displacement of zinc or due to reactions with thiol groups involved in zinc complexation or in other critical positions under realistic exposure conditions remains to be elucidated. Further potential mechanisms relate to the interference with cellular redox regulation, either by enhanced generation of ROS or by reaction with thiol groups involved in the regulation of signaling pathways. Particularly the combination of these multiple mechanisms may give rise to a high degree of genomic instability evident in cadmium-adapted cells, relevant not only for tumor initiation, but also for later steps in tumor development.

Even though epidemiological studies have identified arsenic compounds as carcinogenic to humans, they are not mutagenic in bacterial and mammalian test systems. However, they increase the mutagenicity and clastogenicity in combination with other DNA damaging agents and there are indications of inhibition of DNA repair processes. We investigated the effect of arsenic(III) on nucleotide excision repair (NER) after UV irradiation in human fibroblasts in detail by using two repair-proficient and one partly repair-deficient xeroderma pigmentosum group C human fibroblast cell lines. The results show that two steps of NER are affected by arsenite. Most severely, the incision frequency is reduced at concentrations as low as 2.5 microM arsenic(III); at higher, cytotoxic concentrations, the ligation of repair patches is also impaired. Furthermore, our results indicate that both the global genome repair pathway and the transcription-coupled repair pathway are affected by arsenite. Repair inhibition may well explain the potentiation of genotoxic effects by arsenic in combination with other DNA damaging agents and may thus be of high relevance for the carcinogenic action of arsenic compounds.

Poly(ADP-ribosyl)ation regulates numerous cellular processes like genome maintenance and cell death, thus providing protective functions but also contributing to several pathological conditions. Poly(ADP-ribose) (PAR) molecules exhibit a remarkable heterogeneity in chain lengths and branching frequencies, but the biological significance of this is basically unknown. To unravel structure-specific functions of PAR, we used PARP1 mutants producing PAR of different qualities, i.e. short and hypobranched (PARP1\G972R), short and moderately hyperbranched (PARP1\Y986S), or strongly hyperbranched PAR (PARP1\Y986H). By reconstituting HeLa PARP1 knockout cells, we demonstrate that PARP1\G972R negatively affects cellular endpoints, such as viability, cell cycle progression and genotoxic stress resistance. In contrast, PARP1\Y986S elicits only mild effects, suggesting that PAR branching compensates for short polymer length. Interestingly, PARP1\Y986H exhibits moderate beneficial effects on cell physiology. Furthermore, different PARP1 mutants have distinct effects on molecular processes, such as gene expression and protein localization dynamics of PARP1 itself, and of its downstream factor XRCC1. Finally, the biological relevance of PAR branching is emphasized by the fact that branching frequencies vary considerably during different phases of the DNA damage-induced PARylation reaction and between different mouse tissues. Taken together, this study reveals that PAR branching and chain length essentially affect cellular functions, which further supports the notion of a ‘PAR code’.

Compounds of lead and cadmium have been shown to be carcinogenic to humans and experimental animals. However, the underlying mechanisms are still not understood. In mammalian cells in culture, lead(II) is weakly mutagenic after long incubation times and generates DNA strand breaks only after treatment with high, toxic doses. Cadmium(II) induces DNA strand breaks and chromosomal aberrations, but its mutagenic potential is rather weak. However, both metals exert pronounced indirect genotoxic effects. Lead(II) is comutagenic towards UV and N-methyl-N-nitro-N-nitrosoguanidine (MNNG) and enhances the number of UV-induced sister chromatid exchanges in V79 Chinese hamster cells. With regard to DNA repair, lead(II) causes an accumulation of DNA strand breaks after UV-irradiation in HeLa cells, indicating an interference with the polymerization or ligation step in excision repair. Cadmium(II) enhances the mutagenicity of UV light in V79 Chinese hamster cells and an increased sensitivity toward UV light is observed in various rodent and human cell lines. Furthermore, an inhibition of unscheduled DNA synthesis after UV-irradiation and a partial inhibition of the removal of UV-induced DNA lesions has been shown. For both metals, the indirect genotoxic effects are observed at low, nontoxic concentrations, suggesting that an interference with DNA repair processes may be predominant at biologically relevant concentrations. This might also explain the conflicting results of epidemiological studies obtained for both metals. Possible mechanisms of repair inhibition are discussed.