title: Cytochrome P-450 induction by 3-methylcholanthrene and its antagonism by 2,2-dimethyl-5-t-butyl-1,3-benzodioxole.
abstract: Previous studies in this laboratory have shown 2,2-dimethyl-5-t-butyl-1,3-benzodioxole (DBBD) to antagonize 3-methylcholanthrene induction of cytochrome P-450 in Dub:ICR mice yet have no effect on phenobarbital induction. In the present experiments, C57BL/6 mice, an Ah responsive strain, produced a similar response under the same experimental conditions. The hypothesis that DBBD, although not a cytochrome P-450 inducer, competes with 3-methylcholanthrene for binding to the Ah receptor was tested. Using sucrose density gradients, the Ah receptor was measured in hepatic cytosol from Dub:ICR and C57BL/6 male mice. DBBD was unable to displace either 2,3,7,8-tetra-chlorodibenzo-p-dioxin or 3-methylcholanthrene from the Ah receptor, in vitro. However, in in vivo experiments, DBBD treatment of Dub:ICR mice caused Ah receptor depression at 6 and 24 hr with complete recovery in between, while 3-methylcholanthrene treatment caused a 2-fold Ah receptor reduction at 2 hr followed by complete recovery after 12 hr. When 3-methylcholanthrene and DBBD were coadministered, the depression of the Ah receptor was additive. DBBD-pretreated mice had a 2.25-fold reduction in Ah receptor level, effectively blocking the ability of 3-methylcholanthrene to increase the cytochrome P-450 content and either benzo[a]pyrene hydroxylase or ethoxyresorufin O-deethylase activities. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis confirmed that 3-methylcholanthrene induction of cytochrome P-450 was inhibited by DBBD pretreatment. Hence, although DBBD does not displace 3-methylcholanthrene from the Ah receptor in vitro, it does antagonize 3-methylcholanthrene induction of cytochrome P-450 and also reduces the amount of available receptor in vivo. This interaction may be due either to antagonism or to downregulation of the Ah receptor.
mesh_terms: D000818:Animals; D001667:Binding, Competitive; D002352:Carrier Proteins / Q000378:metabolism; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis*; D004149:Dioxoles / Q000378:metabolism / Q000494:pharmacology*; D004790:Enzyme Induction / Q000187:drug effects; D008297:Male; D008748:Methylcholanthrene / Q000037:antagonists & inhibitors / Q000378:metabolism / Q000494:pharmacology*; D051379:Mice; D008815:Mice, Inbred Strains; D008862:Microsomes, Liver / Q000378:metabolism; D000072317:Polychlorinated Dibenzodioxins / Q000378:metabolism; D018336:Receptors, Aryl Hydrocarbon; D011955:Receptors, Drug / Q000187:drug effects / Q000378:metabolism*
chemical_list: D002352:Carrier Proteins; D065607:Cytochrome P-450 Enzyme Inhibitors; D004149:Dioxoles; D000072317:Polychlorinated Dibenzodioxins; D018336:Receptors, Aryl Hydrocarbon; D011955:Receptors, Drug; C037778:2,2-dimethyl-5-tert-butyl-1,3-benzodioxole; D008748:Methylcholanthrene; D003577:Cytochrome P-450 Enzyme System