OLTIPRAZ-MEDIATED CHANGES IN AFLATOXIN B-1 BIOTRANSFORMATION IN RAT-LIVER - IMPLICATIONS FOR HUMAN CHEMOINTERVENTION

Oltipraz (OPZ) is currently being considered far human use to protect against aflatoxin B-1 (AFB)-induced hepatocarcinogenesis based on its proven protective effect in rats. The effectiveness of this treatment presumes that orthologous cytochrome P450 and glutathione S-transferas e (GST) isozymes metabolize AFB in humans as they do in rats. In this study, alterations in the expression of multiple forms of cytochrome P 450 and GST were evaluated after treatment with OPZ, as well as other known P450 inducers, including 3-methylcholanthrene, pregnenolone-16 a lpha-carbonitrile, and ciprofibrate. Evidence is presented that the ma le-specific 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 A FB substrate concentrations, The CYP 1A2 enzyme does not appear to pla y a role in AFB activation in rat liver at any substrate concentration , whereas the major human p450 enzyme capable of activating AFB at a l ow substrate concentration has been identified as CYP 1A2, Surprisingl y, we found that the CYP 1A2 steady-state mRNA level and the CYP 1A2-a ssociated methoxyresorufin-O-demethylase activity were induced approxi mately 3- and 2-fold, respectively, by OPZ in rat liver. However, beca use CTP 1A2 does not appear to participate in AFB activation, inductio n of CYP 1A2 may be insignificant for AFB-induced hepatocarcinogenesis in rat models. In the rat, a heterodimeric alpha class GST enzyme con taining the Yc2 subunit is the only polypeptide characterized to date in this species with high catalytic activity for the conjugation of ac tivated AFB with glutathione, The GST Yc2 steady-state mRNA level was induced 5-fold by OPZ treatment. This induction was mirrored by signif icant increases in both the corresponding protein level and AFB-8,9-ep oxide-conjugating enzyme activity, which may contribute significantly to protection against AFB-induced carcinogenesis in the rat Investigat ions from this and other laboratories have not revealed any evidence f or a Yc2-like GST isozyme with high AFB-8,9-epoxide-conjugating activi ty in human liver, We have also been unable to demonstrate that the tw o major human alpha class GST isozymes, A1-1 and A2-2, purified from b acteria expressing the corresponding cDNas, exhibit any significant AF B-8,9-epoxide-conjugating activity. Our results suggest that humans ma y not be protected to the same extent as rats against AFB-induced hepa tocarcinogenesis by treatment with OPZ and that further investigations are needed to establish the usefulness of OPZ for protection against human exposure to AFB.