Correlation of hepatocellular proliferation with hepatocarcinogenicity induced by the mutagenic noncarcinogen:carcinogen pair--2,6- and 2,4-diaminotoluene.

2,4-Diaminotoluene (2,4-DAT) and 2,6-diaminotoluene (2,6-DAT) are equally genotoxic in the Ames/Salmonella assay and are both readily absorbed, metabolized, and excreted and metabolites of both compounds are mutagenic with metabolic activation. However, there are marked differences in the results of chronic rodent bioassays with these two compounds. 2,4-DAT is a potent hepatocarcinogen whereas 2,6-DAT failed to produce an increased incidence of tumors in any tissue even when administered at a dose higher than that of 2,4-DAT. In an effort to elucidate the source of these apparently discordant results, the present studies were designed to determine the effects of these two chemicals on cell proliferation in the liver when administered at the dose levels comparable to those used in the original bioassays. This study utilized repeated oral dosing, osmotic minipumps to deliver bromodeoxyuridine (BrDU) for 8 days, and immunohistochemistry to quantitate BrDU incorporation into hepatic DNA, CCl4 (0.4 ml/rat, single ip dose) or vehicle control groups were included as positive and negative controls, respectively. The degree of cell proliferation was quantified by the labeling index from at least 1000 hepatocytes. Results from the control studies indicate that approximately 1.1% of the hepatocytes from vehicle-treated animals replicated during the exposure period whereas approximately 50% replicated in the positive controls. The carcinogen 2,4-DAT produced a dose-dependent increase in cell proliferation of approximately 10% and 20% in livers of animals exposed to 12.5 and 25.0 mg/kg/day, respectively, whereas the noncarcinogen 2,6-DAT produced no increase in cell turnover compared to vehicle control following treatment with 25.0 or 50.0 mg/kg/day. These results indicate a positive correlation between increased cell proliferation and hepatocarcinogenesis induced by these two isomers of diaminetoluene.