The PAS domains of AHR consist of

The PAS domains of AHR consist of two regions, PAS-A and PAS-B, which function as interfaces for dimerization with ARNT and for ligand binding, respectively [14]. Although both the bHLH and the PAS-A domains have been shown to be involved in dimerization with ARNT, a recent report suggests that only the PAS-A domain of AHR is essential [15]. Of note, designation of a chemical as an AHR ligand is typically based on AHR-dependent transcription and gel-shift assays, which do not necessarily demonstrate binding to the PAS-B region [16]. AHR ligands with different structural characteristics have been reported [17], and it is unclear whether the PAS-B domain of AHR is capable of accommodating multiple different compounds as a result of structural flexibility, or whether these ligands bind to AHR at locations other than its ligand-binding pocket. Detailed structural analysis of AHR (or PAS-B) bound to physiological ligands will help to differentiate between these possibilities. Interestingly, a Dehydroandrographolide synthesis mutant of AHR lacking the PAS-B domain (ΔPAS-B) has been shown to constitutively dimerize with ARNT, bind to DNA, and activate transcription in a ligand-independent manner [i.e., constitutively active (CA)-AHR] [18]. Three groups generated transgenic mice using this construct to examine the impact of CA-AHR in vivo. Transgenic mice expressing CA-AHR under the control of the SV40 promoter developed gastric tumors, suggesting a role of AHR in oncogenesis and cell proliferation [19]. Mice bearing CA-AHR under the control of keratin 14 promoter (keratin–CA-AHR) developed inflammatory skin lesions, and exhibited high amounts of serum immunoglobulin (Ig)E and IgG1, and a dominant type 2 T helper cell (Th2) response. To examine AHR function in T cells, CA-AHR was also expressed under the regulation of human CD2 promoter. The CD2–CA-AHR transgenic mice exhibited lower numbers of total thymocytes than wild-type mice, although the percentage of CD8+CD4− thymocytes was higher in these mice; interestingly, the peripheral T cell compartments seemed to be less affected [20]. Of note, in these mice the expression of CA-AHR was controlled not by the Ahr endogenous regulatory elements but instead was expressed from an artificial human CD2 minigene, and this might result in expression of CA-AHR in a non-physiological context, thereby confounding interpretation of the phenotype of CD2–CA-AHR transgenic mice. Studying AHR function has also been achieved through administration of ligands to activate AHR in vitro or in vivo. However, complex effects of AHR activation have been reported, presumably caused by various ligands used in vitro, or in vivo through different administration routes [11,21,22]. Therefore, the development of a mouse model wherein expression of CA-AHR is controlled by endogenous regulatory elements within the Ahr locus will be important to clarify the functions of AHR in different cell types.
Transcriptional Regulation of AHR Expression AHR is expressed in barrier tissues (e.g., the gut, the skin, and the lung) by immune cells such as lymphocytes and by tissue structural cells such as epithelial and stromal cells, and also in the liver by hepatocytes, consistent with its role as a sensor for environmental stimuli. AHR expression is regulated by environmental cues, such as cytokines [e.g., interleukin (IL)-6, IL-21, transforming growth factor (TGF)-β, and others] [22–24]. The available evidence suggests that AHR expression is high in T helper (Th)17 cells, low in FOXP3+ (forkhead box P3) regulatory T cells (Treg cells), and almost undetectable in Th1 or Th2 cells (reviewed in [11]). Of note, these studies assessed Ahr mRNA expression in bulk cell population, and the absence of data on AHR expression on a per cell basis (e.g., using the AHR reporter mice or single-cell sequencing approaches) confounds the interpretation of these results. Furthermore, a given cell type may display different levels of AHR expression in different tissues, and lack of AHR may conceivably have a disparate impact on the differentiation of specific cells in these tissues, such as Treg cells in the gut (as discussed below) and lung γδ T cells [25], arguing for the importance of determining AHR expression in any particular cells within a specific tissue milieu. Furthermore, as noted earlier, cytokines can induce AHR in some cell types, such as T cells, and thus cell culture conditions may have a significant impact on the levels of AHR expression.