6-Methyl-1,3,8-trichlorodibenzofuran as a 2,3,7,8-tetrachlorodibenzo-p-dioxin antagonist: Inhibition of the induction of rat cytochrome P-450 isozymes and related monooxygenase activities

In addition to being one of the most toxic chemicals known, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most potent inducer of rat liver microsomal cytochrome P-4501A1 (P-450c). Previous studies have demonstrated that a high affinity, low capacity cytosolic receptor (the Ah receptor) mediates the activity of TCDD to induce cytochrome P-4501A1, which catalyzes benzo[a]pyrene hydroxylation [aryl hydrocarbon hydroxylase (AHH)] and 7-ethoxyresorufin O-dealkylation (EROD). The results of the present study indicate that 6-methyl-1,3,8-trichlorodibenzofuran (MCDF) effectively competes with [³H]TCDD for binding to the Ah receptor in rat liver cytosol. The concentration of MCDF effecting 50% displacement of [³H]TCDD was 4.9 x 10^-8 M, which is ~50 times greater than the EC₅₀ for unlabeled TCDD (~1 x 10^-9 M). However, in contrast to TCDD, MCDF was only a weak inducer of AHH and EROD activity in rat hepatoma H-4-II cells in culture. When co-incubated, MCDF diminished in a concentration-dependent manner the ability of TCDD to induce AHH and EROD activity in vitro. Treatment of rats with 20-200 μmol/kg MCDF in vivo had little or no effect on liver microsomal AHH and EROD activity, whereas treatment of rats with 16 nmol/kg TCDD caused a 6- and a 70-fold induction of AHH and EROD activity, respectively. When co-administered, MCDF diminished by ~50% the ability of TCDD to induce AHH and EROD activity in vivo. The partial antagonism produced by 50 μmol/kg MCDF could be partially overcome by doubling the dosage of TCDD from 16 to 32 nmol/kg. Immunochemical analysis of rat liver microsomes revealed that treatment of rats with 20-200 μmol/kg MCDF caused little or no induction of cytochrome P-4501A1 and P-4501A2 (P-450d), whereas these isozymes were induced 33- and 5-fold, respectively, in rats treated with 16 nmol/kg TCDD. When co-administered, MCDF diminished by ~50% the ability of TCDD to induce cytochrome P-450A1A in vivo, which established that MCDF was not simply acting as an inhibitor of AHH and EROD activity. MCDF also antagonized the ability of TCDD to induce cytochrome P-4501A2, which suggests that the induction of both cytochromes P-4501A1 and P-4501A2 is regulated by the Ah receptor. These results indicate that MCDF binds with high affinity to the Ah receptor in rat liver cytosol and competitively blocks the binding of TCDD. However, MCDF is only a weak agonist and, as such, can antagonize the ability of TCDD to induce rat liver microsomal cytochromes P-4501A1 and P-4501A2 and associated monooxygenase activity. Its antagonistic properties may make MCDF a valuable probe to study the role of the Ah receptor in mediating the teratogenic, toxic, and other biochemical effects of TCDD and related compounds.