The effect of inducers of mixed-function oxidases on hepatic microsome-mediated aflatoxin B1 transformation in C3H/10T1/2 cells.

The potent hepatotoxin and hepatocarcinogen aflatoxin B1 (AFB1) is metabolized by different forms of cytochrome P450 associated with the hepatic mixed-function oxidase system. C3H/10T1/2(10T1/2) cells, which have limited inherent capacity to metabolize AFB1, were treated with AFB1 in the presence of hepatic microsomes isolated from chemically treated rats to investigate the effects of the induction of specific cytochromes P450 on AFB1-mediated toxicity and transformation. Relative to uninduced microsomes, phenobarbital (PB) treatment induced AFB1-DNA binding (essentially representing the formation of AFB1-8,9-oxide bound to DNA) 3.2-fold, while pretreatment with Aroclor 1254, 3-methylcholanthrene (3-MC), or 5,6-benzoflavone (beta-NF) preferentially induced aflatoxin M1 (AFM1) formation from 2- to 5-fold. 10T1/2 cells were exposed to a multiple treatment regimen with 4 microM AFB1 and hepatic microsomes from uninduced, PB-, Aroclor 1254-, 3-MC-, or beta-NF-treated rats; respective cumulative toxicities of approximately 90, 95, 70, 60, and 40% control (no microsomes) values resulted. An enhanced AFB1 transformation response correlated with the increasing toxicities observed for the different treatments, with uninduced or PB-induced microsomes yielding approximately four foci/dish, while treatments with Aroclor 1254-, 3-MC-, or beta-NF-induced microsomes resulted in only one to two foci/dish. These results demonstrate that AFB1 is a complete carcinogen in the 10T1/2 system when repetitive incubations are used in conjunction with an appropriate hepatic microsomal activation system. These data also correlate the induction of AFB1-4-hydroxylase with a decrease in AFB1-mediated toxicity and transformation of 10T1/2 cells, and support the hypothesis that the Phase I metabolic conversion of AFB1 to AFM1 in the liver represents an effective detoxification pathway per se.