Oltipraz-mediated Changes in Aflatoxin B, Biotransformation in Rat Liver: Implications for Human Chemointervention1

Oltipraz (OPZ) is currently being considered for human use to protect against aflatoxin B, (AFB)-induced hepatocarcinogenesis based on its proven protective effect in rats. The effectiveness of this treatment pre sumes that orthologous cytochrome P450 and glutathione S-transferase (GST) isozymes metabolize AFB in humans as they do in rats. In this study, alterations in the expression of multiple forms of cytochrome P450 and GST were evaluated after treatment with OPZ, as well as other known P450 inducers, including 3-methylcholanthrene, pregnenolone16a-carbonitrile, and ciprofibrate. Evidence is presented that the malespecific rat CYP 3A2, an orthologue of human CYP 3A4, may be primar ily responsible for AFB activation in rat liver at both high and low AFB substrate concentrations. The CYP 1A2 enzyme does not appear to play a role in AFB activation in rat liver at any substrate concentration, whereas the major human P450 enzyme capable of activating AFB at a low substrate concentration has been identified as CYP 1A2. Surprisingly, we found that the CYP 1A2 steady-state mRNA level and the CYP 1A2associated methoxyresorufin-O-demethylase activity were induced ap proximately 3and 2-fold, respectively, by OPZ in rat liver. However, because CYP 1A2 does not appear to participate in AFB activation, induction of CYP 1A2 may be insignificant for AFB-induced hepatocar cinogenesis in rat models. In the rat, a heterodimeric a class GST enzyme containing the Yc2 subunit is the only polypeptide characterized to date in this species with high catalytic activity for the conjugation of activated AFB with glutathione. The GST Yc2 steady-state mRNA level was induced 5-fold by OPZ treatment. This induction was mirrored by significant increases in both the corresponding protein level and AFB-8,9-epoxideconjugating enzyme activity, which may contribute significantly to pro tection against AFB-induced carcinogenesis in the rat. Investigations from this and other laboratories have not revealed any evidence for a Yc2-like GST isozyme with high AFB-8,9-epoxide-conjugating activity in human liver. We have also been unable to demonstrate that the two major human a class GST isozymes, Al-1 and A2-2, purified from bacteria expressing the corresponding cDNAs, exhibit any significant AFB-8,9-epoxide-conjugating activity. Our results suggest that humans may not be protected to the same extent as rats against AFB-induced hepatocarcinogenesis by treatment with OPZ and that further investigations are needed to estab lish the usefulness of OPZ for protection against human exposure to AFB.