Tumor-promoting effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin and phenobarbital in initiated weanling Sprague-Dawley rats: a quantitative, phenotypic, and ras p21 protein study.

In an initiation-promotion protocol, female weanling Sprague-Dawley rats were initiated with 10 mg/kg nitrosodiethylamine and promotion was started after 30 days. Promotion regimens were as follows: 2,3,7,8-tetrachlorodibenzo-para-dioxin (TCDD; 150 ppt in diet) continuously until day 450; phenobarbital (PB; 500 ppm in diet) until day 170; PB until day 170, followed by TCDD until day 240; and PB until day 170, followed by a basal diet (BD) until day 240 and subsequently TCDD from days 240 to 450. TCDD fed to initiated rats had a promoting effect on the development of adenosine triphosphatase-negative altered hepatocellular foci (AHF). At 450 days, the volume fraction of liver occupied by AHF was increased in initiated rats given TCDD continuously and in those given PB followed by TCDD, whereas the mean volume of AHF was significantly larger in initiated rats given TCDD continuously. PB and TCDD promoted similar phenotypes of AHF as seen in hemotoxylin and eosin-stained sections, but the eosinophilic phenotype most closely correlated with the development of hepatocellular neoplasms. The protooncogene product ras p21 protein was present in the majority of PB- and TCDD-promoted AHF, hepatocellular adenomas, and hepatocellular carcinomas. Eosinophilic AHF and ras p21 protein expression most closely correlated with neoplastic development, suggesting that these cell populations, when promoted, may be at greater risks for developing into neoplasms.