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 demonst rated that after a single dose of aflatoxin B-1 (AFB) the resulting li ver AFB-DNA adduct levels were directly proportional to dose. Earlier work also showed that after ten daily doses the AFB dose-response rela tionship with gamma-glutamyl transpeptidase (GGT) positive preneoplast ic foci measured at 3 months was sublinear, with a threshold at a dose of about 150 mu g/kg body weight/day. The objective of this study is to determine the factors influencing the shift in AFB dose-response be tween 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 mu g/kg body weight/day. The animals were killed 2 or 24 h after the fir st AFB dose, or after the tenth AFB dose. The first and tenth doses we re tritiated in these animals and H-3-AFB-guanine adducts isolated fro m liver DNA were measured by HPLC. Another group was killed 3 months a fter receiving ten doses in order to measure GGT foci development. AFB -guanine adduct levels were directly proportional to dose after the fi rst dose, but after the tenth dose were much lower in the 200-350 mu g /kg groups than after a single dose. The GGT foci response confirmed e arlier work concerning a sublinear response. Among the individual anim als in the 200-350 mu g/kg groups :here was a positive relationship, a fter controlling for dose, between GGT foci development and weight gai ned both during dosing (P = 0.018) and also to a lesser extent during the early promotional period (P = 0.066). Enzyme activity levels of GG T in liver homogenates were higher in the highest dose groups and refl ected biliary proliferation rather than histological CCT stained foci. Urinary levels of AFB metabolites changed proportions in the high dos age 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 cytotoxici ty, 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 or der of magnitude less than the lowest dose in this sturdy and thus avo ided acutely toxic effects. We hypothesize that liver tumor developmen t may mirror foci response in a 10-dose AFB regimen with doses above 1 00 mu g/kg due to acute toxicity effects. (C) 1997 Elsevier Science Ir eland Ltd.