Nickel(II) inhibits the repair of O6-methylguanine in mammalian cells

Nickel compounds are widespread carcinogens, and although only weakly mutagenic, interfere with nucleotide excision repair and with the repair of oxidative DNA base modifications. In the present study we investigated the effect of nickel(II) on the induction and repair of O-6-methylguanine and N7-methylguanine after treatment with N-methyl-N-nitrosourea (MNU). We applied Chinese hamster ovary cells stably transfected with human O-6-methylguanine-DNA methyl transferase (MGMT) cDNA (CHO-AT), and compared the results with the MGMT-deficient parental cell line. As determined by high-performance liquid chromatography/electrochemical detection (HPLC/ECD), there was a slight but mostly not significant reduction in the formation of both types of DNA lesions by MNU in the presence of nickel(II). Although nickel(II) did not markedly affect the repair of N7-methylguanine, it decreased the repair of O-6-methylguanine in a dose-dependent manner, starting at concentrations as low as 50 mu M While the MGMT protein level was not altered in the presence of nickel(II), the MGMT activity was diminished as demonstrated in cell extracts form nickel-treated cells. This repair inhibition was accompanied by an increase in MNU-induced cytotoxicity in nickel-treated CHO-AT cells but not in MGMT-deficient control cells. There is strong evidence that O-6-methylguanine is involved in tumour formation after exposure to alkylating agents. Thus, the finding that nickel(II) inhibits the repair of this lesion could be of major importance for risk assessment in case of combined exposures at work places and in the general environment.