Ah receptor in mice genetically "nonresponsive" for cytochrome P4501A1 induction: cytosolic Ah receptor, transformation to the nuclear binding state, and induction of aryl hydrocarbon hydroxylase by halogenated and nonhalogenated aromatic hydrocarbons in embryonic tissues and cells

Abstract

The aromatic hydrocarbon (Ah) receptor mediates induction of cytochrome P4501A1 and associated aryl hydrocarbon hydroxylase (AHH) activity in tissues or cells exposed to polycyclic aromatic hydrocarbons. Strains of mice designated "nonresponsive" do not show increased hepatic AHH activity when exposed in vivo to nonhalogenated aromatic hydrocarbons such as 3-methylcholanthrene, benz[a]anthracene (BA), or benzo[a]pyrene and have reduced sensitivity to halogenated inducers such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Recently, with a modified assay, we detected Ah receptor in hepatic cytosols from adult nonresponsive mice [Mol. Pharmacol. 35:823-830 (1989)]; the receptor was present in reduced amount, and the apparent affinity for TCDD was lower than in hepatic cytosol from responsive C57BL/6J mice. Using the same assay procedure, we now report detection of Ah receptor in cytosols prepared from embryonic tissue and from cultured embryo cells of both responsive (C57BL/6J) and nonresponsive mice (DBA/2J, AKR/J, and SWR/J). Cytosolic receptor in embryonic cells from nonresponsive as well as responsive strains was detectable both with [3H]TCDD and with [3H]3-methylcholanthrene. In addition, the receptor-ligand complex could be extracted from nuclei of embryo cells exposed to [3H]TCDD in culture. AHH activity was induced in embryo cell cultures incubated with either TCDD or BA. The EC50 values for AHH induction were virtually identical in cell cultures from nonresponsive (DBA/2J) and responsive (C57BL/6J) strains, using either TCDD or BA as the inducer. Moreover, the affinity with which [3H]TCDD bound to cytosolic Ah receptor was much more similar in cytosols from cell cultures from the two strains than in cytosols prepared from adult liver. Thus, embryonic cell cultures differ in at least three respects from the adult liver, as follows: (i) Ah receptor can be detected with [3H]3-methylcholanthrene in embryonic cell cytosols but not in cytosols from adult liver; (ii) the degree of difference between nonresponsive and responsive strains in the affinity with which [3H]TCDD binds to receptor is only about 2-fold in cytosol from embryonic cells, whereas it is almost 10-fold in adult liver; and (iii) induction of AHH activity (by either TCDD or by the nonhalogenated inducer BA) shows no significant difference between strains in embryonic cell culture, whereas there is at least a 15-fold difference in responsiveness between C57BL/6J and DBA/2J mice in adult liver in vivo. The mechanistic reason for the diminished degree of difference between responsive and nonresponsive mice during embryonic cell culture (compared with adult tissues) is not yet known.