Bromodeoxyuridine resistance induced in mouse lymphoma cells by microsomal activation of dimethylnitrosamine.

Many chemicals are not mutagenic per se, but when metabolized by mammalian tissues yield mutagenic products. Dimethylnitrosamine (DMN) is such a promutagen. It has no effect on cell growth or mutant frequency when incubated alone with L5178Y mouse lymphoma cells, but exerts both mutagenic and toxic effects when incubated in a microsome reaction mixture. Microsomes were prepared from C3H/f We 16-wk-old male mice by the calcium preciptation technique. L5178Y continuously cultured mouse lymphoma cells heterozygous for thymidine kinase (TK+/-) were incubated for 15 min with calcium-precipitated microsomes and various concentrations of DMN in appropriate reaction mixtures. After a 48-hr expression time, treated cells were cloned in soft agar with and without bromodeoxyuridine (BUdR) (50 mug/ml); 10 days later colonies grown to greater than about 200 mum diameter were counted. The frequency of BUdR-resistant (mutant) colonies increased linearly with the DMN concentration. A reconstruction experiment showed that the assay conditions did not significantly alter the relationship between parent and BUdR-resistant cells in growth and cloning efficiency. The smallest dose of DMN used in these experiments was 100mumol/liter, the one-sided (100 mumol greater than control frequency) -p value is 0.036. The locus is extremely sensitive to mutagenesis by DMN compared with other known mutagens at similar levels of cell survival.