The nitrotoluenes failed to induce unscheduled DNA synthesis in in vitro cultures of rat hepatocytes. Because intestinal bacteria are known to be involved in the metabolic activation of other nitroaromatic compounds, the genotoxicity of the nitrotoluenes was evaluated using an in vivo-in vitro hepatocyte DNA repair assay. 2-Nitrotoluene (2NT), 3-nitrotoluene, or 4-nitrotoluene was administered by gavage to male F344 rats. At selected times after treatment, primary hepatocyte cultures were prepared and incubated with [3H]thymidine, and unscheduled DNA synthesis was assessed by quantitative autoradiography. Corn oil controls ranged from -6 to -3 net grains/nucleus (NG). Only 2NT at 12 hr after treatment induced DNA repair (200 mg/kg: 15.4 NG). Twenty-four hr following treatment with 2NT, a 50-fold increase in the number of hepatocytes in S phase was observed and indicated that 2NT induces cell division in addition to DNA repair. To examine the influence of intestinal bacteria on the hepatic genotoxicity of 2NT, germ-free rats and germ-free rats inoculated with Charles River Altered Schaedler Flora were gavaged with 2NT. The cecal bacterial status was confirmed at sacrifice. 2NT did not induce DNA repair in germ-free animals (200 mg/kg: -3.8 NG), whereas DNA repair was induced in Charles River Altered Schaedler Flora-associated animals (200 mg/kg: 5.4 NG). When F344 females with conventional intestinal microflora were gavaged with 2NT and primary hepatocyte cultures were prepared, no unscheduled DNA synthesis was observed (200 mg/kg: -2.6 NG). Male and female F344 rats were shown to have similar populations of intestinal bacteria. These results demonstrate that the mononitrotoluenes display marked isomeric differences in their genotoxic potential, indicate the obligatory role of intestinal bacteria in the metabolic activation of 2NT, and show that the genotoxic potential of 2NT is dependent upon the sex of the animal under study.
A preliminary report of this work was presented at the Second Joint Meeting of the American Society for Pharmacology and Experimental Therapeutics and the Society of Toxicology, Louisville, Ky., August 15 to 19, 1982 (12).