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The Escherichia coli gpt gene coding for xanthine-guanine phosphoribosyl transferase has been stably transfected into HPRT- Chinese hamster V79 cells. Several gpt- cell lines have been established, which retain the sequence(s) even after long-term culture without selection for gpt. Each cell line exhibits a characteristic spontaneous mutation frequency (10(-5) to 10(-2)) in 6-thioguanine (6TG) selection. While spontaneous mutagenesis to gpt- occurs rather frequently for most cell lines, it cannot be correlated with either the number of plasmid integration sites or deletion of the plasmid sequence(s). One transgenic cell line (g12), which continuously maintains a low spontaneous mutation frequency (approximately 3 x 10(-5)), was used in comparative mutagenesis studies with wild-type V79 cells (gpt vs. hprt). Alkylating agents such as N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and beta-propiolactone (BPL) are shown to be equally toxic and mutagenic in both g12 and V79 cells. UV and X-rays are also equally toxic to both cell lines. The gpt locus of the g12 transfectants, however, is two to three times more sensitive to UV and 2.5-4.5 times more sensitive to X-ray mutagenesis than the endogenous hprt of wild-type V79 cells. The data presented here suggests that g12 cells may be useful to study mammalian mutagenesis by agents which yield limited response at the hprt locus. Future studies with these transgenic cells and other transgenic lines are planned to compare the mutability and repair of the same gene (gpt) at different integration sites in mammalian cells.

Exposure ofcells to carcinogenic nickel compounds induces many genes that are commonly expressed in cancer cells but not in normal cells.Nickel Carcinogenesis, Mutation, Epigenetics, or Selection Nickel compounds have been well established as human carcinogens. Investigations into the molecular mechanisms of nickel carcinogenesis have revealed that not all nickel compounds are equally carcinogenic: certain water-insoluble nickel compounds exhibit potent carcinogenic activity, whereas highly water-soluble nickel compounds exhibit less potency (1). The reason for the high carcinogenic activity of certain water-insoluble nickel compounds relates to their bioavailability and the ability of the nickel ions to enter cells and reach chromatin. The water-insoluble nickel compounds enter cells quite efficiently via phagocytic processes, and subsequent intracellular dissolution yields very high cellular levels of Ni2+. Mathematical estimations indicated that if a 1.45-pm nickel sulfide particle totally dissolved in a cell, the potential nickel concentration would be 250 mM, and if a 4.0-pm nickel sulfide particle totally dissolved in a cell, the potential nickel concentration would be 4.75 M. Thus, the process of phagocytosis represents a very efficient manner for the accumulation of nickel inside the cell.Nickel is classified as a borderline metal ion because it has both soft and hard metal properties and it can bind to sulfur, nitrogen, and oxygen groups. Nickel ions are very similar in structure and coordination properties to magnesium. Like magnesium, nickel binds to the oxygen of the DNA phosphate backbone; but like copper and cobalt, nickel enjoys a high affinity for the imidazole nitrogen of histidine in proteins.

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