Elevation of hepatic microsomal epoxide hydrolase activity by 2-acetylaminofluorene: strain and species differences

Hepatocarcinogens have been shown to cause marked elevation of hepatic microsomal epoxide hydrolyase activity in the rat at short intervals after administration. The present studies were designed to characterize 2-acetylaminofluorene (AAF) mediated epoxide hydrolase elevation and to investigate the relationship between epoxide hydrolase increases, AAF metabolism, and hepatocarcinogenicity. Oral or i.p. administration of AAF to F-344 rats produced log-linear dose-response curves for epoxide hydrolase elevation, measured with either benzo[a]pyrene-4,5-oxide or styrene oxide substrate. Following a single dose of AAF (35 mg/kg), epoxide hydrolase activity was maximally increased (560% of control) within 48 h, and the activity declined slowly, with a half-life of 17.5 days. Co-treatment with actinomycin D effectively blocked the AAF dependent increase in epoxide hydrolase, suggesting that de novo protein synthesis is associated with the increase in enzyme activity. Dose-response curves for epoxide hydrolase induction by AAF, N-hydroxy-2-acetylaminofluorene (N-OH-AAF), and 2-aminofluorene were compared, and the potencies for increasing epoxide hydrolase activity reflected the relative hepatocarcinogenic potentials of these agents. In mice, which are resistant to the hepatocarcinogenic action of AAF and deficient in AAF-N-hydroxylase activity, AAF caused no significant increase in hepatic microsomal epoxide hydrolase activity. Similarly, in Cotton rats and guinea pigs, which are lacking in ability to form the sulfate conjugate of N-OH-AAF, neither i.p. nor dietary administration of AAF elicited increases in epoxide hydrolase activity at doses which were maximally effective in F-344 rats. These results support the hypothesis that the ability of compounds to increase epoxide hydrolase activity is related to their carcinogenic potency. Furthermore, the results suggest that increases in epoxide hydrolase activity are associated with metabolism of AAF to the putative proximate carcinogen N-OH-AAF, and the subsequent conversion of this compound to the N-O-sulfate conjugate.