Mutagenic responses of five independent genetic loci in CHO cells to a variety of mutagens. Development and characteristics of a mutagen screening system based on selection for multiple drug-resistant markers.

With the aim of developing a sensitive mutagen screening system, the responses of 15 different chemical mutagens at 5 independent genetic loci in Chinese hamster ovary (CHO) cells have been determined. The genetic markers which have been employed include resistance to thioguanine (Thgr), ouabain (OuaR), the protein synthesis inhibitor emetine (Emtr), the polyamine synthesis inhibitor methylglyoxal bisguanylhydrazone (Mbgr) and the nucleoside analog 5,6-dichlororibofuranosyl benzimidazole (DrbR). The optimal selection conditions for all of these genetic markers in CHO cells have been described. The chemicals whose response was investigated in these studies include direct-acting alkylating agents (ethyl methane-sulfonate, methyl methanesulfonate, beta-propiolactone, ethyleneimine, N-nitrosomethylurea and 4-nitroquinoline-N-oxide), DNA intercalating and cross-linking agents (ICR-170, acridine orange, ethidium bromide, mitomycin C and actinomycin D), polycyclic hydrocarbons (benzo[a]pyrene (B(a)P) and 7,12-dimethylbenz[a]anthracene (DMBA)) and aromatic amines (benzidine and beta-naphthylamine). Simultaneous examination of the response of the set of genetic markers to these chemicals revealed that although all of these chemicals caused a dose-dependent increase in the frequency of mutations at many of the above genetic loci, the magnitude of the mutagenic response at different genetic loci varied greatly depending upon the chemical. Of the genetic loci examined, no one single locus showed higher response to all of the above chemicals, instead, depending upon the chemical, specific loci were found to be more responsive than others. The polycyclic hydrocarbons and aromatic amines were weakly mutagenic in this system at several genetic loci even without any exogenous microsomal activation, although in the presence of a rat liver S9 fraction similar toxic and mutagenic effects of B(a)P and DMBA were observed at 5-20-fold lower concentrations. These results indicate that CHO cells may possess significant capacity for the metabolic activation of many procarcinogens, and also underscore the merits of measuring the mutagenic response at multiple genetic loci in mutagen screening studies.