Apparent absence of requirement of hydrocarbon metabolism for induction and repression of mixed-function oxidases

Presence of the 4 S polycyclic hydrocarbon-binding protein in H4-II-E cells

The 4 S polycyclic aromatic hydrocarbon (PAH)-binding protein had been implicated in regulating the expression of rat cytochrome P450IA1 which is most closely associated with aryl hydrocarbon hydroxylase (AHH). We have now investigated the presence of both the 4 S PAH-binding protein and the 8 S Ah receptor in rat hepatoma H4-II-E cells as well as the induction of P450IA1 upon their exposure to PAH's such as benzo[a]pyrene (BP) and 3-methylcholanthrene (3MC), and halogenated dioxins such as 2,3,7,8-tetrachlordibenzo-p-dioxin (TCDD) and 2,3,7,8-tetrachlorodibenzofuran (TCDBF). Sucrose density gradient analyses and hydroxylapatite assays indicate that, in addition to the 8 S protein, the 4 S PAH binding protein is present in these cells. This protein interacts in a saturable and high affinity manner with BP and 3MC, but not with TCDD or TCDBF. Using a P450IA1 probe, the induction of gene expression was observed by Northern blot analysis of total cellular RNA after exposure of the H4-II-E cells to BP, 3MC, or TCDBF. Since the 4 S protein was observed to interact only with BP and 3MC, these results suggest that this protein may also play a role in the PAH-induced expression of cytochrome P450IA1 gene expression in H4-II-E.

Dissociation of cytochrome P-450 inactivation and induction

Polycyclic aromatic hydrocarbons bind to a cytosolic receptor that translocates into the nucleus and induces the synthesis of mRNA for a subset of cytochrome P-450 isozymes. The failure to detect a "phenobarbital" receptor, however, suggests that the induction of P-450b/e, the isozymes induced by phenobarbital, is mediated by a less direct mechanism. One attractive alternative is that a receptor exists for an endogenous substance that is specifically turned over by the trace amounts of P-450b/e present in uninduced liver. Changes in the concentration of this substance caused by agents that inhibit P-450b/e would then trigger the induction response. We report here that suppressing the catalytic activities of P-450b/e for prolonged periods with 1-aminobenzotriazole, a mechanism-based irreversible inhibitor, neither induces P-450b/e nor interferes with induction of these isozymes at the level of transcription by phenobarbital. The results argue against mechanisms for the induction of P-450b/e keyed to the effective catalytic availability of these isozymes.

Identification of the Ah receptor in selected mammalian species and induction of aryl hydrocarbon hydroxylase

The Ah receptor protein, important in the mechanism of induction of aryl hydrocarbon hydroxylase activity, has been identified and partially characterized in hepatic cytosolic preparations from rat, BALB/c mouse, gerbil, hamster, rabbit, ferret and guinea-pig by means of sucrose density centrifugation analysis and hydroxyapatite binding assays. Using 2,3,7,8-tetrachloro[3H]dibenzo-p-dioxin (TCDD) as the ligand, total specific binding capacities ranged over 74-691 fmol [3H]TCDD/mg cytosolic protein and apparent dissociation constants ranged over 0.30-7.8 nM. There was no quantitative correlation between the concentration of cytosolic Ah receptors and the 3-methylcholanthrene-mediated induction of aryl hydrocarbon hydroxylase activity in the species studied. Competitive binding studies with a series of monohydroxylated benzo[a]pyrene derivatives suggested the importance of electronic character in their ability to bind to the Ah receptor and to compete with TCDD for specific binding sites on the receptor.

Compounds in urban air compete with 2,3,7,8-tetrachlorodibenzo-p-dioxin for binding to the receptor protein

Acetone extracts of filter-collected urban airborne particulate matter contain compounds which can competitively inhibit 2,3,7,8-[1,6-3H]tetrachlorodibenzo-p-dioxin (TCDD) binding to the rat liver TCDD-receptor protein. The concentration of conventional polycyclic aromatic hydrocarbons (PAHs) or chlorinated dioxins and dibenzofurans cannot account for more than 1-30% of the observed competition for [3H]TCDD binding to the receptor protein. The difference in potency between samples collected in urban areas during different periods of the year and a background sample is 25-400-fold. Collecting samples in the presence of increased concentrations of nitrogen dioxide, nitrous acid, nitric acid or ozone did not increase the amount of compounds with receptor affinity. However, with nitrogen dioxide and especially with nitric acid, a substantial increase of the mutagenic effects in the Ames Salmonella assay in the absence of mammalian activation as well as a degradation of several PAHs were noted. Affinity for the TCDD-receptor protein, mutagenicity in the absence of mammalian metabolic activation in the Ames Salmonella assay and PAH-content are characteristics of urban particulate matter showing the presence of compounds, that represent potential health risks. The compounds with affinity for the receptor may constitute a group of substances different from both conventional PAHs and direct-acting mutagens.