Use of [125I]4'-iodoflavone as a tool to characterize ligand-dependent differences in Ah receptor behavior

Decabromobiphenyl (PBB-209) activates the aryl hydrocarbon receptor while decachlorobiphenyl (PCB-209) is inactive: experimental evidence and computational rationalization of the different behavior of some halogenated biphenyls

In rat H4IIE cells permanently transfected with a luciferase gene under the control of AhR, incubation with PBB-209 led to a statistically significant increase of luminescence. In this system, PCB-209 only caused a small induction of luciferase activity. In a fish cell line, only PBB-209 was able to provoke an induction of ethoxyresorufin- O-deethylase activity. Ligand binding to the AhR was studied by means of a cell-free in vitro system in which the activation of AhR is very unlikely to occur without ligand binding. None of the biphenyls studied provoked any activation of AhR in this system. To rationalize the results and to get insight into the molecular mechanism of activation of AhR by PBB-209 as compared with PCB-209, a comprehensive computational study was carried out on these congeners as well as on PCB-126 and PCB-169, two potent AhR activators through ligand binding. The calculations include (i) conformational analysis and dipole moments of each conformer, (ii) aromaticity indices, (iii) molecular electrostatic potentials, (iv) quadrupole moments, (v) electronic and reactivity descriptors, and (vi) dissociation energies of C-Cl and C-Br bonds in model aromatic compounds. It was found that some molecular features of PBB-209, such as the electrostatic potential (EP) and EP-derived descriptors (Politzer's parameters), indicate that PBB-209 is more similar to PCB-126 and PCB-169 than to PCB-209, which share quite similar geometries based on the substitution pattern. The similarity between PBB-209, PCB-126, and PCB-169 seems to hint that these three compounds can share, at least partially, similar mechanisms of activation of AhR. It is unquestionable that PCB-126 and PCB-169 directly bind AhR and PBB-209 does not. We hypothesize that there are several simultaneous mechanisms for activation of AhR, and the most active compounds act for more than one mechanism.

Evidence that ligand binding is a key determinant of Ah receptor-mediated transcriptional activity

The aryl hydrocarbon receptor (AhR) mediates the biological activity of 2,3,7,8-tetrachlorodibenzo-p-dioxin. Whether the AhR can mediate enhanced transcriptional activity in the absence of ligand binding has not been established. Hepatocytes from AhR-null (AhR-KO) and wild-type (AhR-WT) neonatal mice were immortalized with Simian virus 40. Two point mutants of the AhR, A375I and A375F, were generated to test the hypothesis that the AhR requires ligand binding to exhibit significant transcriptional activity, both mutants fail to bind ligand or exhibit enhanced activity in cells exposed to AhR ligands. Upon transient, co-expression of ARNT with AhR-A375I or AhR-A375F in AhR-KO cells, these mutants exhibited significant ligand-independent transcriptional activity. However, in CV-1 cells, which others have previously shown to contain relatively high levels of AhR ligand(s), these AhR mutants exhibit essentially no constitutive activity. These results indicate that while the AhR can potentially exhibit activity in the absence of ligand binding, the high constitutive receptor activity observed in many cell lines appears to be due to the presence of endogenous AhR ligands.

The aryl hydrocarbon receptor directly regulates expression of the potent mitogen epiregulin

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known to cause a large number of adverse effects, mediated largely by its binding to the aryl-hydrocarbon receptor (AhR) and subsequent modulation of gene expression. It is thought that AhR mediates these effects through the untimely and disproportionate expression of specific genes. However, the exact mechanism, or the genes involved, through which TCDD leads to these effects is still unknown. This study reports the discovery of a novel target gene, epiregulin, which is regulated by TCDD-activated AhR. Epiregulin is a growth regulator which belongs to the epidermal growth factor (EGF) family. Using real time quantitative PCR (qPCR), it was established that TCDD upregulates epiregulin gene expression. The promoter region of epiregulin has a dioxin responsive element (DRE) 56 nucleotides upstream of the transcription start site, along with three potential Sp1 binding sites. Chromatin immunoprecipitation (ChIP) assays with an anti-AhR antibody showed promoter occupancy upon TCDD treatment. Luciferase reporter assays using a vector harboring the first 125 base pairs of the epiregulin rat promoter revealed an increase in signal on TCDD treatment, which was lost upon mutation of the DRE. Epiregulin and TCDD treatment mediated a dose-dependent increase in primary mouse keratinocyte growth. These results demonstrate that AhR directly increases epiregulin expression, which could play an important role in TCDD mediated tumor promotion observed in rodent models.