Chemoprevention of aflatoxin B-1 hepatocarcinogenesis by coumarin, a natural benzopyrone that ls a potent inducer of aflatoxin B-1-aldehyde reductase, the glutathione S-transferase A5 and P1 subunits, and NAD(P)H : quinone oxidoreductase in rat liver

Structurally diverse compounds can confer resistance to aflatoxin B-1 (AFB( 1)) hepatocarcinogenesis in the rat. Treatment with either phytochemicals [ benzyl isothiocyanate, coumarin (CMRN), or indole-3-carbinol] or synthetic antioxidants and other drugs (butylated hydroxyanisole, diethyl maleate, et hoxyquin, beta-naphthoflavone, oltipraz, phenobarbital, or trans-stilbene o xide) has been found to increase hepatic aldo-keto reductase activity towar d AFB(1)-dialdehyde and glutathione S-transferase (GST) activity toward AFB (1)-8,9-epoxide in both male and female rats. Under the conditions used, th e natural benzopyrone CMRN was a major inducer of the AFB(1) aldehyde reduc tase (AFAR) and the aflatoxin-conjugating class-alpha GST A5 subunit in rat liver, causing elevations of between 25- and 35-fold in hepatic levels of these proteins. Induction was not limited to AFAR and GSTA5: treatment with CMRN caused similar increases in the amount of the class-pi GST P1 subunit and NAD(P)H: quinone oxidoreductase in rat Liver. Immunohistochemistry dem onstrated that the overexpression of AFAR, GSTA5, GSTP1, and NAD(P)H:quinon e oxidoreductase affected by CMRN is restricted to the centrilobular (peri- acinar) zone of the lobule, sometimes extending almost as far as the portal tract. This pattern of induction was also observed with ethoxyquin, oltipr az, and trans-stilbene oxide. By contrast, induction of these proteins by b eta-naphthoflavone and diethyl maleate was predominantly periportal. Northe rn blotting showed that induction of these phase XI drug-metabolizing enzym es by CMRN was accompanied by similar increases in the levels of their mRNA s. To assess the biological significance of enzyme induction by dietary CMR N, two intervention studies were performed in which the ability of the benz opyrone to inhibit either AFB(1)-initiated preneoplastic nodules (at 13 wee ks) or AFB(1)-initiated liver tumors (at 50 weeks) was investigated. Animal s pretreated with CMRN for 2 weeks prior to administration of AFB(1), and w ith continued treatment during exposure to the carcinogen for a further 11 weeks, were protected completely from development of hepatic preneoplastic lesions by 13 weeks. In the longer-term dietary intervention, treatment wit h CMRN before and during exposure to AFB(1) for a total of 24 weeks was fou nd to significantly inhibit the number and size of tumors that subsequently developed by 50 weeks. These data suggest that consumption of a CMRN-conta ining diet provides substantial protection against the initiation of AFB(1) hepatocarcinogenesis in the rat.