Regulatory gene product of the Ah locus. Characterization of receptor mutants among mouse hepatoma clones.

Abstract

The major regulatory gene product of the Ah locus is a cytosolic receptor which binds avidly to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD).Ah receptor levels in the cytosol and accumulation of the inducer-receptor complex in the nucleus have been examined in the mouse hepatoma cell culture parent line Hepa-lclc7 and in six mutant clones that exhibit varying degrees of deficiency in aryl hydrocarbon (benzo[u]pyrene) hydroxylase (EC 1.14.14.1) inducibility.One of these clones (c3) is dominant, whereas the others are known to be recessive and to belong to three distinct complementation groups (and, therefore, reflect mutations in three different genes): c l and c5, c2 and c6, and c4.Clones cl, c3, and c5 have essentially normal Ah receptor levels, compared with the wild type Hepa-lclc7 parent, possess normal kinetics for translocation of the inducer-receptor complex into the nucleus, and yet exhibit very low or nondetectable basal or inducible aryl hydrocarbon hydroxylase activity; these clones could represent a mutation in the P1-450 structural gene or other genes responsible for the induced hydroxylase activity.Clones c2 and c6 are receptor-defieient mutants, having no more than 10% of wild type Ah receptor levels, normal kinetics of nuclear translocation of the inducer-receptor complex, and no more than 20% of wild type hydroxylase inducibility by either TCDD or benzo[a]anthracene.The defect in c2 cells was shown by both in vitro and in vivo TCDD-binding studies not to reflect a lack of receptor saturability.Clone c4 has normal cytosolic levels of Ah receptor, is defective in nuclear translocation of the inducer-receptor complex, and lacks any detectable basal or inducible aryl hydrocarbon hydroxylase activity. ~ ~The Ah locus controls the induction (by polycyclic aromatic compounds such as 3-methylcholanthrene, benzo[a]anthracene, and TCDD') of many drug-metabolizing enzyme activities in virtually all of the tissues examined (reviewed in Ref. 1).The Ah system is believed to comprise regulatory, structural, and probably temporal genes which may or may not be linked.Multiple forms of cytochrome P-450 are among the many structural gene products "turned on" during the sequence of events following exposure of the animal or cells in * The costs of publication of this article were defrayed in part by the payment of page charges.