Ah Receptor Mediating Induction of Aryl Hydrocarbon Hydroxylase: Detection in Human Lung by Binding of 2,3,7,8-[3H]Tetrachlorodibenzo-p-dioxin

In laboratory animals and in mouse hepatoma cells in culture the Ah receptor previously has been shown to mediate induction of aryl hydrocarbon hydroxylase (cytochrome P1-450) by 3-methylcholanthrene, 2,3,7,8-tetrachlorodibenzo- p -dioxin (TCDD) and related compounds. We examined human lung cytosols to determine whether the Ah receptor was present in human tissues. Cytosol was prepared from grossly normal lung tissue obtained at pulmonary lobectomy for presumed lung cancer from 53 consecutive adult patients including 32 males (42–77 years old) and 21 females (18–81 years old). Ah receptor in the cytosols was identified and quantitated by specific binding of [3H]TCDD after separation by ultracentrifugation on sucrose gradients. Specific binding of [3H]TCDD to a component which met the criteria for Ah receptor was detected in 10 of the 53 specimens. As previously established in tissues from laboratory animals, the specific [3H]TCDD-binding component sedimented ∼9S. Binding of [3H]TCDD to the 9S component was competitively inhibited by incubation in the presence of 2,3,7,8-tetrachlorodibenzofuran, dibenz( a,h )anthracene, and nonradioactive TCDD, all known to be potent agonists for Ah -receptor-mediated induction of aryl hydrocarbon hydroxylase. Specific Ah receptor also was detected in some specimens by direct binding of [3H]-3-methylcholanthrene. The human population studied exhibited striking heterogeneity in Ah receptor concentrations. Only 10 of the 53 individuals studied had detectable Ah receptor. In specimens with detectable specific binding, the mean concentration of binding sites was 6.9 ± 1.2 (SE) fmol/mg cytosolic protein. These concentrations are approximately 10–30% of the concentrations of Ah receptor found in lung cytosols from laboratory animals. Our experiments indicate that the Ah receptor can be detected in lung cytosol from some humans and suggest that the regulatory mechanism mediating human cytochrome P1-450 induction may be similar to that in the murine model. Aryl hydrocarbon hydroxylase, the major enzyme induced under control of the Ah receptor, plays an important role in the metabolism of several carcinogens including polycyclic aromatic hydrocarbons such as benzo( a )pyrene. It is possible that differences in the Ah receptor content within the human population may be genetically based and that variation at the Ah receptor level may be an important determinant of individual susceptibility to certain chemically induced cancers.