Mechanism of induction of cytochrome p450 enzymes by the proestrogenic endocrine disruptor pesticide-methoxychlor: interactions of methoxychlor metabolites with the constitutive androstane receptor system.

Methoxychlor, a structural analog of the DDT pesticide, was previously shown to induce rat hepatic CYP2B and -3A mRNAs and the corresponding proteins [J Biochem Mol Toxicol 1998;12:315-323], Additionally, methoxychlor was found to activate the constitutive androstane receptor (CAR) system and induce CYP2B6 (J Biol Chem 1999;274:6043-6046), suggesting a mechanism for methoxychlor-mediated cytochrome P450 (P450) 2B induction. However, it has not been established whether CAR activation and P450 induction was due to methoxychlor per se and/or due to its metabolites. Also, a possible link between the estrogenic potency of methoxychlor metabolites and CAR activation or P450 induction was not investigated. The current study explores the ability of methoxychlor and its metabolites to activate CAR and whether their potency of CAR activation correlates with their respective estrogenicity. Methoxychlor and its metabolites [mono-OH-M [1,1,1-trichloro-2 (4-hydroxyphenyl)-2'-(4-methoxyphenyl)ethane]; bis-OH-M [1,1,1-trichloro-2,2'-bis(4-hydroxyphenyl)ethane]; ring-OH-M [1,1,1-trichloro-2(4-methoxyphenyl)-2'-(3-hydroxy-4-methoxyphenyl)ethane]; and tris-OH-M [1,1,1-trichloro-2(4-hydroxyphenyl)-2'-(3,4-dihydroxyphenyl)ethane]] were found to be potent activators of CAR. Dose response curves indicated that tris-OH-M is a more potent CAR activator than methoxychlor, mono-OH-M, and bis-OH-M. Since tris-OH-M is a much weaker estrogen receptor-alpha agonist than mono-OH-M and bis-OH-M, it seems that estrogenicity is not a significant factor in CAR activation. These findings indicate that alteration of methoxychlor-benzene rings, i.e., generation of phenolic constituents, does not appreciably alter CAR activation and suggest that a common structural motif in the methoxychlor class of compounds controls CAR activation. Studies are needed to identify the structural motif necessary for CAR activation and CYP2B induction.