Dissimilarity in aflatoxin dose-response relationships between DNA adduct formation and development of preneoplastic foci in rat liver

Abstract

Earlier work in this laboratory and that carried out by others demonstrated that after a single dose of aflatoxin B 1 (AFB) the resulting liver AFB-DNA adduct levels were directly proportional to dose. Earlier work also showed that after ten daily doses the AFB dose-response relationship with γ-glutamyl transpeptidase (GGT) positive preneoplastic foci measured at 3 months was sublinear, with a threshold at a dose of about 150 μg/kg body weight/day. The objective of this study is to determine the factors influencing the shift in AFB dose-response between AFB-DNA adducts and GGT foci. Male Fisher 344 weanling rats were orally administered one or ten doses of AFB ranging from 50 to 350 μg/kg body weight/day. The animals were killed 2 or 24 h after the first AFB dose, or after the tenth AFB dose. The first and tenth doses were tritiated in these animals and 3H-AFB-guanine adducts isolated from liver DNA were measured by HPLC. Another group was killed 3 months after receiving ten doses in order to measure GGT foci development. AFB-guanine adduct levels were directly proportional to dose after the first dose, but after the tenth dose were much lower in the 200–350 μg/kg groups than after a single dose. The GGT foci response confirmed earlier work concerning a sublinear response. Among the individual animals in the 200–350 μg/kg groups there was a positive relationship, after controlling for dose, between GGT foci development and weight gained both during dosing ( P = 0.018) and also to a lesser extent during the early promotional period ( P = 0.066). Enzyme activity levels of GGT in liver homogenates were higher in the highest dose groups and reflected biliary proliferation rather than histological GGT stained foci. Urinary levels of AFB metabolites changed proportions in the high dosage multiply dosed animals reflecting alteration in AFB metabolism or excretion. The differences between the linear adduct and the sublinear foci dose-response curves may be the result of non-adduct effects of higher multiple AFB doses on foci formation including acute cytotoxicity, altered AFB metabolism and disposition, enhanced weight gains, or shortened foci latency but not through enhanced guanine adduct levels. Other studies that showed a linear relationship between AFB dose and liver tumor development used continuous feeding of maximal doses an order of magnitude less than the lowest dose in this study and thus avoided acutely toxic effects. We hypothesize that liver tumor development may mirror foci response in a 10-dose AFB regimen with doses above 100 μ/kg due to acute toxicity effects.