Factors in Metal Mutagenesis

Studies on mutagenesis by metal compounds may be complicated by inducible tolerance mechanisms in some cells. Metallothioneins (MT) are small cysteine-rich proteins which bind a number of metals with high affinity. The reader is referred to Chap. 5 in this volume for more detail. Besides cadmium, zinc, and copper salts, a number of other metal salts have also been shown to induce MT synthesis and/or to bind to the MT protein. Hg(II), Co(II), and Ni(II), but not Pb(II), induce MT synthesis in primary cultures of rat hepatocytes (Bracken and Klaassen 1987), and may do so in the cultured cells used for mammalian mutagenesis experiments. Thus, under some experimental protocols, the metal salt being assayed may itself induce metallothionein, or the results of mutagenicity assays may be confounded by components of the medium or serum which affect the levels of metallothionein in the cell (Rossman and Goncharova, unpublished). 

The same argument can be made for other inducible proteins which may cause tolerance. Heat shock proteins are induced in human fibroblasts by Cd(II), Cu(II), As(III), Zn(II), and Hg(II), but not by Co(U), Ni(II), Fe(II), Fe(III), Mn(II), Pt(II), or Pb(II) (Levinson et al. 1980). Like metallothionein, heat shock proteins have been postulated to play a role in the acquisition of tolerance to agents which induce its synthesis. Cd(II) and arsenite also induce heme oxygenase, a protein associated with oxidative stress (Keyse and Tyrrell 1989). 

Rat kidney cells adapted to growth in 5 and 10//M Pb(N03)2 become resistant to challenge with otherwise toxic Pb(II) concentrations. This resistance is associated with de novo protein synthesis. A large number of new proteins are synthesized within 6h after the cells are exposed to Pb(II) (Hitzfeld et al. 1989). The C6 rat glioma cell line also shows an adaptive response to lead (Lake et al. 1980) and a similar response has been reported for human cells (Skreb et al. 1981). We have recently identified an inducible mechanism for arsenite resistance, with cross-resistance to arsenate and antimonite, in Chinese hamster V79 cells (Wang and Rossman 1993). This inducible response is different from the heat shock response (Wang et al. 1994). The reader is referred to Chaps. 10, 14, 15, and 17 in this volume for more information on metal-induced responses. 

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