Detection and characterization of a low affinity form of cytosolic Ah receptor in livers of mice nonresponsive to induction of cytochrome P1-450 by 3-methylcholanthrene.

Ah "nonresponsive" mice (prototype, DBA/2) show no significant increase in hepatic P1-450 (P450IA1) when treated with 3-methylcholanthrene or other nonhalogenated polycyclic aromatic hydrocarbons. Potent halogenated aromatics such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induce P1-450 in liver of nonresponsive mice, but the dose required is approximately 15-fold higher than in "responsive" mice (prototype, C57BL/6). It was postulated several years ago that the genetic basis of nonresponsiveness was a "defect" in the Ah receptor, which normally binds TCDD and other inducers and mediates the induction process. Cytosolic Ah receptor hitherto had not been detectable in hepatic cytosol from nonresponsive mice. Using a modified sucrose gradient assay that we developed in studies on human tissue [Cancer Res. 47:4861-4868 (1987)], we now have detected cytosolic Ah receptor in nonresponsive mice. By saturation analysis, the concentration of specific binding sites for [3H]TCDD in hepatic cytosol from DBA/2J mice was (mean +/- SE) 55 +/- 6.6 fmol/mg of cytosolic protein (n = 21) compared with 133 +/- 7.1 fmol/mg (n = 21) in responsive C57BL/6J mice. Ah receptor also was detected in significant concentrations in other nonresponsive strains; SWR/J, AKR/J, RF/J, and DBA/2N. The sedimentation coefficient on sucrose gradients was the same (approximately 9 S) in nonresponsive as in responsive strains. The major difference in nonresponsive mice is that hepatic cytosolic Ah receptor has an apparent affinity for [3H]TCDD that is about 10-fold lower than in responsive strains; Kd in DBA/2J mice = 16 +/- 2.5 nM (n = 21) and Kd in C57BL/6J mice = 1.8 +/- 0.2 nM (n = 21). Thus, nonresponsive mice do possess the cytosolic Ah receptor in liver. However, the receptor is present in reduced concentration and appears to be a low affinity form, possibly as the result of a mutation in the gene(s) coding for the receptor protein(s).