Ah receptor

There are even receptors that are known to become activated only due to interaction with a synthetic chemical, and no physiological agonist for such a receptor has been characterized. A model receptor in this class is the so-called Ah receptor complex that becomes activated subsequent to its exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxiu (TCDD). Activation of the. Ah receptor... 

Dioxins are prominent members of the class of polychlorinated hydrocarbons that also includes diben-zofuran, biphenyls and others. Dioxins are highly toxic environmental contaminants. Like others small planar xenobiotics, some dioxins bind with high affinity to the arylhydrocarbon (Ah) receptor. Dioxins activate the receptor over a long time period, but are themselves poor substrates for the enzymes which are induced via the Ah-receptor. These properties of the dioxins and related xenobiotics may be important for the toxicity of these compounds. Dioxins like 2,3,7,8-tetrachloro-p-dibenzodioxin can cause persistent dermatosis, like chloracne and may have other neurotoxic, immunotoxic and carcinogenic effects. 

Nuclear Receptor Regulation of Hepatic Cytochrome P450 Enzymes. Figure 1 General mechanism for transcriptional activation of CYP genes by xenochemicals that activate their cognate xeno-receptor proteins. In the case of Ah receptor, the receptor s heterodimerization partner is Arnt, whereas in the case of the nuclear receptors CAR, PXR, and PPARa, the heterodimerization partner is RXR. The coactivator and basal transcription factor complexes shown are each comprised of a large number of protein components. 

Bock KW, Kohle C (2006) Ah receptor dioxin-mediated toxic responses as hints to deregulated physiologic functions. Biochem Pharmacol 72 393-404... 

Persistent metabolite of p,p - yUY implicated in the decline of certain predatory birds Mechanism by which binding to Ah receptor causes toxic effects still unclear... 

It has come to be suggested that coplanar congeners as a group express toxicity through a common mechanism interaction with the cytosolic Ah receptor (Safe 1990 Ahlborg et al. 1994) (Environmental Health Criteria 140). Although the full... 

Ah-receptor-mediated toxicity is particularly associated with the highly toxic compound 2,3,7,8-tetrachloro-dibenzo-p-dioxin (TCDD), commonly referred to as dioxin. TCDD, and the concept of toxicity equivalency factors (TEFs) based on TCDDs, will be dealt with in Chapter 7. The main point to make at this juncture is that the toxicity of each individual coplanar congener in a mixture can be expressed in terms of a toxic equivalent calculated relative to the toxicity of dioxin. Summation of the toxic equivalents of the individual coplanar PCBs gives a measure of the toxicity of the whole mixture, as expressed through the Ah receptor mechanism. 

The toxicology of PCBs is complex and not fully understood. Coplanar PCBs interact with the Ah-receptor, with consequent induction of cytochrome P4501A1/2 and Ah-receptor-mediated toxicity. Induction of P4501A1 provides the basis of valuable biomarker assays, including bioassays such as CALUX. Certain PCBs, for example, 3,3, 4,4 -TCB, are converted to monohydroxymetabolites, which act as thyroxine antagonists. PCBs can also cause immunotoxicity (e.g., in seals). 

PCDDs and PCDEs, together with coplanar PCBs, can express Ah-receptor-mediated toxicity. TCDD (dioxin) is used as a reference compound in the determination of TEFs, which can be used to estimate TEQs (toxic equivalents) for residues of PHAHs found in wildlife samples. Biomarker assays for Ah-receptor-mediated toxicity have been based on the induction of P450 lAl. TEQs measured in field samples have sometimes been related to toxic effects upon individuals and associated ecological effects (e.g., reproductive success). 

Coplanar PCBs, PCDDs, and PCDFs express Ah-receptor-mediated toxicity (Chapter 6, Section 6.2.4). Binding to the receptor leads to induction of cytochrome P4501 and a number of associated toxic effects. Again, toxic effects are related to the extent of binding to this receptor and appear to be additive, even with complex mixtures of planar polychlorinated compounds. Induction of P4501A1/2 has been widely used as the basis of a biomarker assay. Residue data can be used to estimate TEQs for dioxin (see Chapter 7, Section 7.2.4). 

Fish hepatocyte lines have also been developed, which can show cytochrome P450 lAl induction due to PAHs and planar polychlorinated aromatic compounds binding to the Ah receptor (Vaillant et al. 1989, Pesonen et al. 1992). 

Particular attention is given to the development of new mechanistic biomarker assays and bioassays that can be used as indices of the toxicity of mixtures. These biomarker assays are typically based on toxic mechanisms such as brain acetylcholinesterase inhibition, vitamin K antagonism, thyroxin antagonism, Ah-receptor-mediated toxicity, and interaction with the estrogenic receptor. They can give integrative measures of the toxicity of mixtures of compounds where the components of the mixture share the same mode of action. They can also give evidence of potentiation as well as additive toxicity. 

Interference with corticosteroid function and the stress response has been shown for a variety of chemicals, including the pharmaceutical salicylate (Gravel and Vijayan 2006) and the PAH, phenanthrene (Monteiro et al. 2000a, 2000b). Other classes of chemicals shown to have significant effects on cortisol levels include PCBs and PAHs (Hontela et al. 1992,1997). The precise mechanisms for these effects are poorly understood, but for PCBs, are believed to be via their actions through the Ah receptor (Aluru and Vijayan 2006). 

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