Effect of chloroform on dichloroacetic acid and trichloroacetic acid-induced hypomethylation and expression of the c-myc gene and on their promotion of liver and kidney tumors in mice.

Chloroform, dichloroacetic acid (DCA) and trichloroacetic acid (TCA) are mouse liver carcinogens that are chlorine disinfection by-products found in drinking water. The effect of chloroform on DCA and TCA-induced hypomethylation and expression of the c-myc gene and on their promotion of liver and kidney tumors was determined. B6C3F1 mice were administered 0, 400, 800 and 1600 mg/l chloroform in drinking water and 500 mg/kg DCA or TCA-administered daily by gavage. DCA, TCA and to a lesser extent chloroform decreased the methylation and increased the mRNA expression of the c-myc gene. Co-administering chloroform prevented only DCA and not TCA-induced hypomethylation and increased mRNA expression of the gene. The effect of chloroform on tumor promotion by DCA and TCA was determined in female and male B6C3F1 mice initiated on day 15 of age with N-methyl-N-nitrosourea. Starting at 5 weeks of age, the mice received in their drinking water DCA (3.2 g/l) or TCA (4.0 g/l) with 0, 800 or 1600 mg/l chloroform until they were killed at 36 weeks. Liver tumors promoted by DCA and TCA were predominantly basophilic except for DCA-treated female mice that were eosinophilic. Only DCA promoted foci of altered hepatocytes and they were eosinophilic in both sexes. Chloroform prevented DCA, but not TCA promotion of liver foci and tumors. In male mice, TCA promoted kidney tumors while DCA promoted kidney tumors only when co-administered with chloroform. Hence, chloroform prevented the hypomethylation and increased mRNA expression of the c-myc gene and the promotion of liver tumors by DCA, while enhancing DCA-promotion of kidney tumors. Thus, the concurrent exposure to two carcinogens, chloroform and DCA resulted in less than additive activity in one organ and synergism in another organ.