Activation of the Ah receptor signaling pathway by prostaglandins

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that mediates many of the biological and toxicological actions of a diverse range of chemicals, including the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin). Although no endogenous physiological ligand for the AhR has yet been described, numerous studies support the existence of such a ligand(s). Here we have examined the ability of prostaglandins and related chemicals to activate the AhR signaling system. Using two AhR-based bioassay systems we report that relatively high concentrations of several prostaglandins (namely, PGB3, PGD3, PGF3alpha, PGG2, PGH1, and PGH2) can not only stimulate AhR transformation and DNA binding in vitro, but also induce AhR-dependent reporter gene expression in mouse hepatoma cells in culture. PGG2 also induced AhR-dependent reporter gene expression to a level three-to four fold greater than that observed with a maximal inducing dose of TCDD. Sucrose gradient ligand binding analysis revealed that PGG2 could competitively displace [3H]TCDD from the AhR. Overall, our results demonstrate that selected prostaglandins are weak agonists for the AhR and they represent a structurally distinct and novel class of activator of the AhR signal transduction pathway.

Ah receptor-based chemical screening bioassays: application and limitations for the detection of Ah receptor agonists

The aromatic hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates many of the biologic and toxicologic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) and related chemicals. Here we utilized two AhR-dependent bioassay systems as screening tools to identify novel AhR agonists and to detect the presence of AhR agonists in sample extracts. These assays measure the ability of a chemical to activate AhR DNA binding in vitro (GRAB bioassay) or AhR-dependent (luciferase) gene expression in cultured cells (CALUX bioassay). Known AhR agonists (halogenated and nonhalogenated aromatic hydrocarbons) were positive in both assays, whereas the AhR antagonist alpha-naphthoflavone exhibited agonist activity only in the GRAB assay. In vitro GRAB analysis has identified several imidazoline receptor ligands and beta-carbolines as AhR agonists and also revealed the presence of AhR agonist activity in crude DMSO extracts of commercial newspapers. In contrast to their positive activity in the GRAB assay, the majority of these chemicals/extracts were only weakly active or inactive in the cell-based CALUX assay. Our results not only reveal that the ability of a chemical to activate the AhR in vitro does not necessarily correlate with its ability to induce gene expression in intact cells, but the high level of false positives obtained with the GRAB assay clearly demonstrates its inability to accurately identify AhR agonists or agonist activity. Screening of unknown chemicals, chemical classes, and samples for AhR agonist activity will require the use of intact cell bioassays.

Differential gene expression in wild-type and arnt-defective mouse hepatoma (Hepa1c1c7) cells

The aryl hydrocarbon nuclear translocator (Arnt) is a basic helix-loop-helix (per/Arnt/Ahr/sim) PAS-containing protein that can heterodimerize with the aryl hydrocarbon receptor (AhR), the hypoxia-inducible factor-1 alpha, and other PAS-containing proteins to form transcriptionally active complexes. To identify the genes whose expression is modulated by Arnt, we used the technique of differential display to compare the expression of genes in wild-type and Arnt-defective (BPRc1) mouse hepatoma (Hepa1c1c7) cells. Here we report two gene products whose expression was reduced in BPRc1 cells (a WW domain-binding, protein-like factor and one unknown gene product) when compared to wild-type cells, and two that were elevated (Steel factor and a serpin-like protein). Comparison of the relative expression of these gene products between two independently-derived, Arnt-defective cell lines, as well as in BPRc1 cells in which Arnt expression was restored by a stably integrated Arnt-expression plasmid, revealed that each gene was expressed in an Arnt-independent manner. Our results clearly demonstrate that gene expression in the variant cell clones is distinctly different from that of the parental wild-type Hepa1c1c7 cells from which they were derived and involves genes in addition to, and unrelated to, that of Arnt. The identification of these differentially expressed gene products suggests that caution should be exercised when using these variant cell lines to confirm the role of the AhR/Arnt-signaling pathway in a given cellular response.

Activation of the Ah receptor by tryptophan and tryptophan metabolites

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that mediates many of the biological and toxicological actions of a variety of hydrophobic natural and synthetic chemicals, including the environmental contaminant 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin). A variety of indole-containing chemicals, such as indole-3-carbinol, indolo[3, 2-b]carbazole, and UV photoproducts of tryptophan (TRP), have previously been identified as ligands for AhR. Here we have examined the ability of endogenous metabolites of tryptophan (TRP) to bind to and activate AhR in vitro and in cells in culture. Although hydroxylated TRP metabolites were inactive, two metabolites, namely tryptamine (TA) and indole acetic acid (IAA), were shown to be AhR agonists. Not only do TA and IAA bind competitively to AhR, but they also can stimulate AhR transformation and DNA binding and induce expression of an AhR-dependent reporter gene in cells. In addition to being an AhR ligand, TA is also a competitive substrate for cytochrome P4501A1, a well-characterized AhR- and TCDD-inducible gene product. Although these compounds are relatively weak ligands, compared to TCDD, they represent some of the first endogenous hydrophilic AhR agonists identified to date.