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

Earlier work in this laboratory and that carried out by others demonstrated that after a single dose of aftatoxin B1 (AFB) the resulting liver AFB-DNA adduct levels were directly proportional to dose. Earlier workalso showed that after ten daily doses the AFB dose response relationship with y-glutamyl transpeptidase (GGT)positive preneoplastic foci measured at 3 months was sublinear, with a threshold at a dose of about 150 µg/kgbody weight/day. The objective of this study is to determine the factors influencing the shift in AFB doseresponse between AFB-DNA adducts and GGT foci. Male Fisher 344 weanling rats were orally administered oneor 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 3HAFB-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 directlyproportional to dose after the first dose, but after the tenth dose were much lower in the 200-350 µg/kg groupsthan 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 higherin 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 acutecytotoxicity, altered AFB metabolism and disposition, enhanced weight gains, or shortened foci latency but notthrough 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 lowestdose in this study and thus avoided acutely toxic effects. We hypothesize that liver tumor development may mirror foci response in a IO-dose AFB regimen with doses above 100 µg/kg due to acute toxicity effect