Leukotriene A₄ Metabolites Are Endogenous Ligands for the Ah Receptor

Abstract

In addition to orchestrating an adaptive metabolic response to xenobiotic compounds, the aryl hydrocarbon receptor (AHR) also plays a necessary role in the normal physiology of mice. The AHR is activated by a structurally diverse group of chemicals ranging from carcinogenic environmental pollutants to dietary metabolites and a number of endogenous molecules. Leukotriene A 4 (5,6-LTA 4) metabolites were identified in DRE-driven luciferase reporter assays as activators of AHR signaling. Various LTA 4 metabolites, including several 5,6- and 5,12-DiHETE products, were screened for AHR activity with 6- trans-LTB 4, 6- trans-12- epi-LTB 4, 5( S),6( S)-DiHETE, and 5( S),6( R)-DiHETE eliciting a significant level of AHR transcriptional activity. However, electrophoretic mobility shift assays (EMSAs) revealed that only 5,6-DiHETE isomers were capable of directly binding and activating the AHR to a DNA-binding species in vitro. Furthermore, ligand competition binding experiments confirm the ability of these compounds to directly bind to the AHR. Interestingly, "aged" preparations of 5,6-DiHETE isomers produced an enhanced level of AHR activation while demonstrating an increase in binding affinity for the receptor. Although the reason for this has not been fully determined, the formation of geometric isomers in the conjugated triene region of these molecules may play a role in the observed increase in AHR-mediated transcriptional activity. This work suggests a connection between AHR activation and inflammatory signaling molecules produced by the 5-lipoxygenase pathway.

12(R)-Hydroxy-5(Z),8(Z),10(E),14(Z)-eicosatetraenoic Acid [12(R)-HETE], an Arachidonic Acid Derivative, Is an Activator of the Aryl Hydrocarbon Receptor

“…This suggests that an arachidonic acid metabolite might be involved in the activation of CYP1A1. The data presented here, coupled with a recent report from our laboratory demonstrating that 5,6-DiHETE is a ligand for the AHR (Chiaro et al, 2008), suggest that the AHR may be activated under inflammatory conditions. This provides a possible link between inflammation or differentiation, production of certain lipoxygenase products, and AHR activation within a variety of tissues.…”

“…Identifying such a ligand(s) would enable the biological role(s) for this enigmatic orphan receptor to be more precisely determined, thus allowing the significance of excessive AHR activation by environmental compounds to be more thoroughly evaluated. In the past, several eicosanoid molecules, including lipoxin A 4 (Schaldach et al, 1999), 5,6-DiHETE (Chiaro et al, 2008), and prostaglandins (Seidel et al, 2001), have been reported as AHR ligands. During a screening of eicosanoids, in particular those produced by lipoxygenase or cytochrome P450 metabolic pathways, several molecules possessing AHR activity were identified.…”

Endogenous aryl hydrocarbon receptor promotes basal and inducible expression of tumor necrosis factor target genes in MCF-7 cancer cells

“…There are potential mechanisms to explain AHR activity in MCF-7 cells in the absence of TCDD. Chiaro et al 2008 discovered that the 5-lipoxygenase (5-LOX) pathway generates 5,6-dihydroxyeicosatetraenoic acid isomers (5,6- DiHETEs) that induce expression of a DRE-promoter reporter construct, the formation of AHR-DNA binding complexes in EMSA assays, and increases in CYP1A1 mRNA in hepatocytes [51]. DiNatale and colleagues reported that the tryptophan metabolite kynurenic acid induced CYP1A1 mRNA, DRE-promoter reporter activity and the formation of an AHR-DNA complex, and competitively displaced labeled AHR ligand from AHR in hepatocytes [52].…”

Role of Retinoids, Rexinoids and Thyroid Hormone in the Expression of Cytochrome P450 Enzymes

“…AhR was considered for many years as an orphan receptor, however, a number of endogenous AhR ligands were identified up to date [93,110], including. bilirubin [111], biliverdin [112], indirubin [113], indole and tryptophan derivatives [114,115] and arachidonic acid derivatives [116][117][118][119]. AhR is responsible for the regulation of cytochromes P450 CYP1A1, CYP1A2 and CYP1B1 [120], but it also plays various endogenous functions [121] such as regulating the cell cycle and proliferation [122,123], immune response [124][125][126], circadian rhythm [127], tumor promotion [128,129], the expression of lipid metabolism genes [130,131] etc.…”