Ligand Selectivity and Gene Regulation by the Human Aryl Hydrocarbon Receptor in Transgenic Mice

Abstract

The aryl hydrocarbon receptor (AHR) is a ligand-inducible transcription factor that displays interspecies differences with the human and mouse AHR C-terminal region sequences sharing only 58% amino acid sequence identity. Compared with the mouse AHR (mAHR), the human AHR (hAHR) displays ϳ10-fold lower relative affinity for prototypical AHR ligands such as 2,3,7,8-tetrachlorodibenzo-p-dioxin, which has been attributed to the amino acid residue valine 381 (alanine 375 in the mAHR) in the ligand binding domain of the hAHR. We investigated whether the 10-fold difference in ligand-binding affinity between the mAHR and hAHR would be observed with a diverse range of AHR ligands. To test this hypothesis, ligand binding assays were performed using the photo-affinity ligand 2-azido-3-[125 I]iodo-7,8-dibromodibenzo-p-dioxin and liver cytosol isolated from hepatocyte-specific transgenic hAHR mice and C57BL/6J mice. It is noteworthy that competitive ligand-binding assays revealed that, compared with the mAHR, the hAHR has a higher relative affinity for certain compounds, including indirubin [(2Z)-2,3-biindole-2,3 (1ЈH,1ЈH)-dione and quercetin (2-(3,4dihydroxyphenyl)-3,5,7-trihydroxy-4H-chromen-4-one]. Electrophoretic mobility shift assays revealed that indirubin was more efficient at transforming the hAHR compared with the mAHR. Indirubin was also a more potent inducer of Cyp1a1 expression in transgenic hAHR mouse hepatocytes compared with C57BL/6J mouse hepatocytes. These observations suggest that indirubin is a potent hAHR ligand that is able to selectively bind to and activate the hAHR. These discoveries imply that there may be a significant degree of structural divergence between mAHR and hAHR ligands and highlights the importance of the hAHR transgenic mouse as a model to study the hAHR in vivo.

Antagonism of Aryl Hydrocarbon Receptor Signaling by 6,2′,4′-Trimethoxyflavone

“…Our rationale for using methoxyflavone derivatives was 3-fold. First, the flavonoid skeleton adheres to the structural and spatial requirements of the proposed AHR ligand-binding pocket (Waller and McKinney, 1995) as evidenced by the large number of polyphenolic flavonoids that exhibit disparate events on AHR activity (Lu et al, 1996;Ciolino et al, 1999;Flaveny et al, 2009). Second, modification of the flavone ring structure has been shown to alter AHR binding and/or associated agonist potential, depending on the position of the modification, suggesting the possibility of an arrangement that facilitates AHR binding but not receptor transformation and hence agonist activity (Lu et al, 1996).…”

Divergent Ah Receptor Ligand Selectivity during Hominin Evolution

“…These studies led to the concept that the human AHR possesses a global loss of ligand affinity relative to the rodent homolog. However, more recent reports have discovered that the human AHR has relatively higher affinity for a number of tryptophan-derived ligands, such as indirubin, indoxyl sufate, indole, and kynurenic acid (Flaveny, et al 2009;Hubbard, et al 2015). These latter studies would suggest that the ligand-binding pocket between rodents and humans exhibit differential ligand selectivity.…”

Role of Cytochrome P450 Monooxygenase in Carcinogen and Chemotherapeutic Drug Metabolism

“…Their respective response to the alternative ligand is far less robust than the ligand first described for each of them. Almost as striking is the fact that the mouse AhR has a higher affinity by $10-fold for 2,3,7,8-tetrachlorodibenzo-p-dioxin compared to the human AHR (Flaveny, Murray, Chiaro, & Perdew, 2009). Yet other ligands, such as indirubin and quercetin, have a higher affinity for the human receptor than the murine receptor.…”