Hepatic Ah receptor binding affinity for 2,3,7,8-tetrachlorodibenzo-p-dioxin: similarity between beagle dog and cynomolgus monkey

Aromatic hydrocarbon receptors in mitochondrial biogenesis and function

Mitochondria are ubiquitous membrane-bound organelles that not only play a key role in maintaining cellular energy homeostasis and metabolism but also in signaling and apoptosis. Aryl hydrocarbons receptors (AhRs) are ligand-activated transcription factors that recognize a wide variety of xenobiotics, including polyaromatic hydrocarbons and dioxins, and activate diverse detoxification pathways. These receptors are also activated by natural dietary compounds and endogenous metabolites. In addition, AhRs can modulate the expression of a diverse array of genes related to mitochondrial biogenesis and function. The aim of the present review is to analyze scientific data available on the AhR signaling pathway and its interaction with the intracellular signaling pathways involved in mitochondrial functions, especially those related to cell cycle progression and apoptosis. Various evidence have reported the crosstalk between the AhR signaling pathway and the nuclear factor κB (NF-κB), tyrosine kinase receptor signaling and mitogen-activated protein kinases (MAPKs). The AhR signaling pathway seems to promote cell cycle progression in the absence of exogenous ligands, whereas the presence of exogenous ligands induces cell cycle arrest. However, its effects on apoptosis are controversial since activation or overexpression of AhR has been observed to induce or inhibit apoptosis depending on the cell type. Regarding the mitochondria, although activation by endogenous ligands is related to mitochondrial dysfunction, the effects of endogenous ligands are not well understood but point towards antiapoptotic effects and inducers of mitochondrial biogenesis.

Assays of PCB congeners and organochlorine insecticides with the transgenic Arabidopsis and tobacco plants carrying recombinant guinea pig AhR and GUS reporter genes

Certain congeners of polychlorinated biphenyls (PCBs) and organochlorine insecticides are ligands of aryl hydrocarbon receptors (AhRs) in animals. A recombinant guinea pig (g) AhR, XgDV, was constructed by fusing the ligand-binding domain of gAhR, the DNA-binding domain of LexA, and the transactivating domain of VP16. Then, the expression unit of β-glucuronidase (GUS) reporter gene regulated by XgDV was introduced into Arabidopsis and tobacco plants. When the transgenic Arabidopsis XgDV plants were cultured on Murashige-Skoog (MS) medium containing PCB congeners, the GUS activity in the plants increased toxic equivalent (TEQ)-dependently. The GUS activity in the transgenic Arabidopsis XgDV plants cultured on MS medium containing the organochlorine insecticide dieldrin was also induced. On the other hand, in the case of DDT, the GUS activity induced by 3-methylcholanthere in the plants decreased. The transgenic Arabidopsis XgDV plants detected 1000 ng g(-1) PCB126 in 1 g of soils. Thus the XgDV plants seemed to be useful for convenient assays of PCB congeners and organochlorine insecticides, without any extraction and purification steps.

Human response to dioxin: aryl hydrocarbon receptor (AhR) molecular structure, function, and dose-response data for enzyme induction indicate an impaired human AhR

The aryl hydrocarbon receptor (AhR) mediates nearly all studied adverse effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and many related compounds. Binding of TCDD or related ligands to AhR is the key initiating event in downstream biochemical responses. The binding affinity of AhR for TCDD is specific to species and strain, and studies of human AhR demonstrate binding affinities approximately an order of magnitude or more lower than those observed in the most sensitive laboratory strains and species. Molecular genetic studies confirmed that human AhR shares key mutations with the DBA mouse strain that result in an "impaired" AhR (with respect to TCDD binding and responsiveness). Despite a number of polymorphisms in human AhR, the key "DBA-type" mutations appear to be a constant feature of the human AhR, and no polymorphisms have been identified that compensate for the impaired binding function conferred by these mutations. Consistent with the impaired binding status of the human AhR, human cells have consistently required approximately 10-fold higher concentrations of TCDD in vitro than rodent cells to respond with enzyme induction. Recent studies of in vivo enzyme induction-related endpoints in human populations with moderately and highly increased TCDD body burdens detected no relationship between these endpoints and TCDD body burdens at body-burden levels up to 250 ng TEQ/kg body weight, or approximately 25 times above the upper range of current general population background body burdens, while marked elevations in enzyme activity were observed in persons with body burdens above 750 ng TEQ/kg. In contrast, the more sensitive laboratory rodent strains and species exposed to TCDD exhibit significant enzyme induction at body burdens below 50 ng/kg. These interspecies data on the most sensitive and best understood response to binding of TCDD and related compounds to the AhR are consistent with the binding affinity and molecular structure data and support the hypothesis that the human AhR is less functional than the AhR of the more sensitive laboratory animals at a molecular level. Quantitative risk assessments involving interspecies extrapolation from sensitive laboratory species and strains should take these fundamental differences into account when margins of exposure and safety factors are considered.

Aryl hydrocarbon receptors: diversity and evolution

Animals have evolved inducible enzymatic defenses to facilitate the biotransformation and elimination of toxic compounds encountered in the environment. The sensory component of this system consists of soluble receptors that regulate the expression of certain isoforms of cytochrome P450, other enzymes, and transporters in response to environmental chemicals. These receptors include several members of the steroid/nuclear receptor superfamily as well as the aryl hydrocarbon receptor (AHR), a member of the bHLH-PAS gene superfamily. In addition to its adaptive functions, the AHR serves poorly understood physiological roles; interference with those roles by dioxins and related chemicals causes toxicity. One approach to understanding the physiological significance of the AHR is to characterize its structure, function, and regulation in diverse species, including mammals, birds, fish, and invertebrates. These animal groups include model species with unique features that can be exploited to broaden our understanding of AHR function. Studies carried out in diverse species also provide phylogenetic information that allows inferences about the evolutionary history of the AHR. This review summarizes the current understanding of AHR diversity among animal species and the evolution of the AHR signaling pathway, as inferred from molecular studies in vertebrate and invertebrate animals. The AHR gene has undergone duplication and diversification in vertebrate animals, resulting in at least three members of an AHR gene family: AHR1, AHR2, and AHR repressor. The inability of invertebrate AHR homologs to bind dioxins and related chemicals, along with other evidence, suggests that the adaptive role of the AHR as a regulator of xenobiotic metabolizing enzymes may have been a vertebrate innovation. The physiological functions of the AHR during development appear to be ancestral to the adaptive functions. Sensitivity to the developmental toxicity of dioxins and related chemicals may have had its origin in the evolution of dioxin-binding capacity of the AHR in the vertebrate lineage.

cDNA cloning and characterization of a high affinity aryl hydrocarbon receptor in a cetacean, the beluga, Delphinapterus leucas

Some cetaceans bioaccumulate substantial concentrations of planar halogenated aromatic hydrocarbons (PHAHs) in their tissues, but little is known about the effects of such burdens on cetacean health. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related PHAHs cause toxicity via activation of the aryl hydrocarbon receptor (AHR), a member of the bHLH-PAS family of transcription factors. Differences in AHR structure and function are known to contribute to species-specific differences in susceptibility to PHAH toxicity. To ascertain the potential for PHAH effects in a cetacean, we characterized an AHR from the beluga whale, Delphinapterus leucas. The 3.2 kb cDNA encodes an 845-amino acid protein with a predicted size of 95.5 kDa. Overall, the beluga AHR shares 85% amino acid sequence identity with the human AHR and 75% identity with the mouse AHR Ah(b-1) allele. Beluga AHR protein synthesized in a rabbit reticulocyte lysate system demonstrated specific, high-affinity [(3)H]TCDD binding. Saturation binding analysis was used to compare the [(3)H]TCDD binding affinity of the in vitro-expressed beluga AHR with affinities of in vitro-expressed AHRs from a dioxin-sensitive mouse strain (Ah(b-1) allele) and humans. The beluga AHR bound [(3)H]TCDD with an affinity (K(d)= 0.43 +/- 0.16 nM) that was at least as high as that of the mouse AHR (K(d)= 0.68 +/- 0.23 nM), and significantly greater than that of the human AHR (K(d)= 1.63 +/- 0.64 nM). In electrophoretic mobility shift assays, the beluga AHR exhibited sequence-specific, Arnt-dependent binding to a dioxin responsive enhancer (DRE). Upon transient transfection into mammalian cells, the beluga AHR activated transcription of a luciferase reporter under control of a DRE-containing fragment of the mouse Cyp1a1 promoter. These results show that in an in vitro system, the beluga AHR possesses characteristics similar to those of AHRs from other mammals that are considered sensitive to toxic effects of PHAHs. Together, these results demonstrate that the use of in vitro-expressed proteins is a promising approach for addressing molecular and biochemical questions concerning PHAH toxicity in endangered or protected species.