A method to quantitate the relative initiating and promoting potencies of hepatocarcinogenic agents in their dose-response relationships to altered hepatic foci

Abstract

The relative response to various initiating doses of diethylnitrosamine (DEN) and dimethylbenz[a]anthracene of the induction of numbers and size (vol. % of liver) of altered hepatic foci (AHF) in livers of adult female rats of the Sprague-Dawley and Fischer 344 (F-344) strains was studied by methods of quantitative stereology in the presence and absence of the promoting agent, phenobarbital (PB, 0.05% in the diet). In all cases, a relatively linear response with dose, even at the lowest doses employed, was obtained except for the numbers of AHF at the highest dose of DEN (30 mg/kg), which was not significantly different from that at a dose of 10 mg/kg in F-344 female rats. Similar dose-response data were obtained at various doses of two promoting agents effective in hepatocarcinogenesis, PB and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), in livers of F-344 female rats following initiation with DEN (10 mg/kg) 24 h post-70% hepatectomy. The response to these agents exhibited threshold levels below which no increase in number or vol. % of liver of AHF was noted in comparison with that in livers of animals not treated with the promoting agents. At several subthreshold doses of both PB and TCDD an inhibition of AHF formation and growth (measured as vol. % of liver) was observed. Based on quantitative stereologic calculations, parameters for the estimation for the relative potency of chemicals as initiating or promoting agents have been established. These are defined as: initiation index = no. of foci induced X liver-1 X [mmol/kg body wt]-1 and promotion index = Vf/Vc X mmol-1 X weeks-1, where Vf is the total volume fraction (%) occupied by AHF in the livers of rats treated with the test agent and Vc is the total volume of AHF in control animals which have only been initiated. These parameters were calculated for a number of agents based on data published in the literature and from those reported herein. Neither parameter varied significantly with the dose of the initiating agent based on the data in this paper. The range of promotion indices extended over more than eight orders of magnitude, whereas that of initiation indices was much less variable. Such parameters may be useful as quantitative estimates of the potency of hepatocarcinogenic agents, such values having potential application to risk estimations.

Quantitative Relationship Between Transforming Growth Factor-Alpha and Hepatic Focal Phenotype and Progression in Female Mouse Liver

“…Basophilic foci are putative precursors of DEN-initiated mouse liver tumors (55), but only a small fraction of basophilic hepatic foci appear to progress to liver tumors. We previously determined ratios of conversion from hepatic foci to tumors in DEN-initiated mice (34 (21,42). A normal rate of foci development, but decreased progression of preneoplastic lesions to hepatocellular carcinomas, occurred in DEN-initiated and PBpromoted TGF-a-targeted (knock-out) mice (46) further supports increased TGF-a as a factor in the progression of some preneoplastic hepatic lesions.…”

“…In humans and rodents, hepatocarcinogenesis is a multistep process (9,15,41,42,55). Basophilic foci are putative precursors of DEN-initiated mouse liver tumors (55), but only a small fraction of basophilic hepatic foci appear to progress to liver tumors.…”

“…Hepatic foci are putative preneoplastic lesions believed capable of progressing to hepatocellular adenomas and carcinomas (8,15,42). Numerous nongenotoxic environmental carcinogens preferentially increased the growth of DNA-damaged hepatocytes and hepatic foci in rodents.…”

“…Numerous nongenotoxic environmental carcinogens preferentially increased the growth of DNA-damaged hepatocytes and hepatic foci in rodents. These nongenotoxic agents are often operationally classified as hepatic tumor promoters (42). Hepatic foci exhibited a higher rate of DNA synthesis as measured by labeling indices (LIs) than nonfocal hepatocytes (35,49,51).…”

Prevalidation of a Rat Liver Foci Bioassay (RLFB) Based on Results from 1600 Rats: A Study Report

“…More refined stereological analysis of FAH has been proposed as a valuable tool to study the initiating and promoting potencies of liver cancer risk factors (see for example 45,54). Based on the notion that FAH are monoclonal in origin (48,65), the number of foci is regarded as a rough estimate of the initiating potency of a test agent, while changes in the size distribution by clonal expansion of the initiated cells are considered as a rough estimate of their promoting activity.…”