In order to test the potential of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) as a promoter of hepatocarcinogenesis, rats which had received a single 10-mg/kg dose of diethylnitrosamine (DEN) following partial hepatectomy were given TCDD (0.14 and 1.4 µg/kg s.c. once every 2 weeks) for 7 months. Animals which received (a) only a single initiating dose of DEN after partial hepatectomy and no further treatment of (b) TCDD alone with no initiating dose of DEN exhibited relatively few enzyme-altered foci and no hepatocellular carcinomas. However, animals initiated with DEN and then given TCDD had a marked increase in enzyme-altered foci. At the higher dose of TCDD, hepatocellular carcinomas were present in five of seven rats. By means of three different enzyme markers used to evaluate the phenotypes of the enzyme-altered foci, a distinct phenotype heterogeneity of the foci was noted with a shift towards phenotypes exhibiting a greater deviation from normal liver when TCDD was given following DEN-partial hepatectomy. Quantitation of the numbers of enzyme-altered foci was performed by relating measurements made from two-dimensional tissue sections to the numbers of foci per unit volume of liver using relationships established in the field of stereology. The total volume of the liver occupied by the enzyme-altered foci, but not their number, increased with the dose of TCDD administered following DEN-partial hepatectomy. These studies demonstrate that TCDD is a potent promoting agent for hepatocarcinogenesis.

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This study was supported in part by Grants CA-07175 and CA-22484 from the National Cancer Institute and Grant ES-00965 from the National Institute of Environmental Health Sciences.

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