Induction of hepatic peroxisome proliferation in mice by lactofen, a diphenyl ether herbicide.

A technical grade of lactofen (1'[carboethoxy]ethyl 5-[2-chloro-4-[trifluoro-methyl] phenoxy]-2-nitrobenzoate) has been shown to induce liver tumors in mice. To determine a possible mechanism of action, the effect of exposure for 7 weeks to dietary concentrations of 2, 10, 50, and 250 ppm technical grade lactofen and 250 ppm of pure lactofen was studied for various liver parameters in groups of male and female CD-1 mice. Liver-weight to body-weight ratio, liver catalase, liver acyl-CoA oxidase, liver cell cytoplasmic eosinophilia, nuclear and cellular size, and peroxisomal staining were increased by the tumorigenic dose of lactofen, i.e., 250 ppm, in a fashion similar to the comparison chemical nafenopin (500 ppm), which is a peroxisome proliferator. Lower doses of lactofen that were reported as nontumorigenic had little or no effect on these parameters. Thus, pure and technical grade lactofen appear to induce murine liver tumors through a mechanism similar to epigenetic hepatocarcinogens of the peroxisome proliferating type.