Mutagenicity of mercury chloride and mechanisms of cellular defence: the role of metal-binding proteins

The mechanisms of toxicity and, particularly, the potential carcinogenicity of inorganic mercury are still under debate. Results of mutagenicity and g enotoxicity testing with mercury have been inconsistent: mercury induces DN A Single-strand breaks at low concentrations in mammalian cells but has not proved mutagenic in several bacterial mutagenicity assays. We investigated the muta-genicity of subtoxic concentrations of inorganic mercury and the role of metal-binding proteins and free radicals in this process. A mutagen icity assay using NIH 3T3 cells, transfected with a vector containing lacZ' as a reporter for mutational events, was applied. In this model, inorganic mercury significantly increased the mutation frequency in the lacZ gene, e ven at the lowest concentration tested. The mutation frequency was greatest at an Hg2+ concentration of 0.5 muM. To identify the mechanisms involved, different cellular responses to non-cytotoxic concentrations of HgCl2 were measured. Hg2+ increased the intracellular amount of reactive oxygen specie s. This induction of oxidative stress was observed, although the intracellu lar glutathione (GSH) and metallothionein (MT) concentrations were increase d significantly, Mercury-induced MT expression was even more pronounced aft er GSH depletion. Correspondingly, radical formation was more evident in th e presence of the GSH-depleting agent L-buthioneine-[S,R]-sulfoximine. Thes e findings suggest that the observed mutations might be a consequence of ox idative processes, rather than due to a direct interaction of mercury with nuclear DNA, The results also indicate that the auto-induction of MT by Hg2 + fails to prevent these mutational events.