ReviewThe immune phenotype of AhR null mouse mutants: Not a simple mirror of xenobiotic receptor over-activation
Introduction
The aryl hydrocarbon receptor (AhR) signalling pathway is evolutionarily conserved, and can act independently or in concert with other signalling pathways. Similar to steroid hormones, the AhR molecule is a ligand-activated gene transcription factor. Residing in the cytosol chaperoned by hsp90, AIP, and p23, the AhR dissociates these proteins upon ligand binding to translocate into the nucleus. In the nucleus, the AhR dimerizes with aryl hydrocarbon nuclear translocator (ARNT), and eventually binds to small conserved promoter elements called xenobiotic response elements (XREs) for transcriptional regulation in cooperation with co-factors. The AhR is then exported to the cytosol and degraded [1].
Numerous genes contain XREs in combination with other responsive elements in promoter specific patterns, thus the ligand-bound AhR regulates a plethora of genes in a cell-, tissue- and condition-specific fashion [2], [3].
Biased by its discovery as a regulator of xenobiotic metabolizing enzymes in vertebrates more than 30 years ago [4], the AhR has long been studied for its pathological activity in response to man-made environmental pollutants. In particular halogenated polycyclic aromatic hydrocarbons (PAH), such as dioxins attracted attention and raised concern. The AhR mediates toxicity, mainly through alterations of gene expression as outlined above. The toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a prototypic ligand of the AhR, and other PAHs are far-reaching and include alterations in lipid metabolism, skin physiology, tumour promotion, and embryonic development. Last, but not least, the immune system is a very sensitive target of AhR-mediated toxicity, responding at particularly low concentrations of chemical exposure [5].
As the AhR is evolutionary old, with members of the family already present in fungi, insects, or nematodes, and expressed constitutively, but tissue-specifically, a physiological role beyond responding to man-made chemicals is commonly postulated (reviewed in [6]).
Studies with (i) persistent activation of the AhR by e.g. TCDD, (ii) with AhR null mice, hypomorphs or natural low-affinity mutants, and later (iii) with strains with cell-specific conditional AhR deletions, confirmed multiple physiological roles of the AhR (reviewed in [6]). In brief, the AhR is a regulator of cell proliferation, e.g. via induction of Cdk2, or by physical interaction with the retinoblastoma protein. It cooperates and cross-talks with other signalling pathways, shown for instance for the estrogen pathway, NFκB, or cAMP [7], [8], [9]. The AhR induces oxidative stress, and may play a role for cell migration and adhesion [6], [9], [10], [11]. AhR activity is highly cell-specific and controlled at multiple levels. Receptor affinity, expression level, signalling crosstalk, feed-back inhibition by the AhR-repressor, competition for the dimerization partner ARNT, and/or competition for transcription co-factors participate in the outcome of AhR-activation [1]. The AhR is quite promiscuous and accepts chemically very different ligands [12]. The ligand determines to a considerable extent the outcome of AhR-activation, albeit only one binding site exists [13]. Subtle changes in protein conformation, or quick degradability may be reasons [14]. Last but not least, some ligands are persistent, while others have a high metabolic turnover rate. This can result in different outcomes of AhR-activation [15], [16]; in addition to the anthropogenic chemicals such as PAHs, numerous natural ligands (i.e. not anthropogenic) and endogenous ligands (made by the organism itself) have been identified and continue to be found. Interestingly, UVB radiation present in sunlight turns tryptophan into 6-formyl-[3,2b] indolo-carbazole (FICZ), a high-affinity ligand [17], [18]. Other endogenous ligands are heme metabolites, indigo derivatives, and leukotrienes [12], [19], [20], [21], [22]. Over-activation of the AhR by various ligands, and the ensuing consequences for the immune system are the topic of a review by Nancy Kerkvliet in this issue. My review focuses on AhR-deficiency, and the outcome for the immune system, in particular, I will discuss and compare the results derived from genetically engineered null mutant mice.
Section snippets
Murine mutants of the AhR signalling pathway
In the 90s, three groups generated AhR-deficient mouse mutants independently, by either deleting exon1 or exon2. In the null mutant made in the laboratory of Frank Gonzalez, exon1 is replaced from the translational start site onwards with a neomycin gene [23]. A Japanese group around Yoshiaki Fujii-Kuriyama replaced part of exon1 with the bacterial β-galactosidase gene joined to a nuclear localization signal, allowing to screen for AhR expression [24]. In the null mutant made in the laboratory
The immune system in AhR-deficient mice
The immune system is a complex organ with highly diverse functions, short- and long-distance interactions, and memory capacities. Immune cells communicate directly with each other by cell surface structures, or over considerable distances via lymphokines and chemokines. Lymphoid organs provide relevant spatial structures for direct communication of immune cells. Immune cells follow their intrinsic programmes, and/or adapt to external cues, relayed into the cells by a number of signal
An extrinsic rather than an intrinsic role of the AhR?
Immune phenotypes of naïve mice and in infection models contributed to the understanding that AhR-over-activation is only one side of the coin, yet the better studied one. AhR over-activation by environmental pollutants is of concern for public health. Insights into AhR biology point to chances for pharmacological manipulation [61], [63]. The role of the AhR in the “untouched” state appeared unexpectedly more subtle. However, does an “untouched” state really exist? Gene expression profiling for
Summary
Three null mutants of the AhR allowed tackling the two questions, namely (i) to what extent immunotoxic events after AhR ligand exposure to environmental chemicals are AhR dependent, and (ii) whether and how the AhR plays a role for a functioning immune system. Some differences in immune phenotype were noted in the null mutant mice (such as splenocyte numbers at certain ages), but these differences reported early after generation of the mice can be explained by mixed genetic background,
Conclusion and outlook
In conclusion, the AhR seems particularly relevant for the differentiation and balance of T cell subsets in ongoing immune responses, and for the decision of the immune system to tolerate or fight antigens. The AhR thus links the immune response to environmental factors, and may help control the risk of developing adverse immune reactions.
Further research will have to focus on the role of individual ligands in shaping these responses, elucidating the environmental risks for autoimmunity,
Acknowledgements
I thank Drs. Heike Weighardt, Bettina Jux, and Nancy Kerkvliet for critical reading of the manuscript. The work in my laboratory is supported through grants of the Bundesministerium für Umwelt, the Deutsche José Carreras Stiftung für Leukämieforschung, and the Deutsche Forschungsgemeinschaft (GRK1427).
References (82)
- et al.
Microarray analysis of the AHR system: tissue-specific flexibility in signal and target genes
Toxicol Appl Pharmacol
(2007) - et al.
Stereospecific, high affinity binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin by hepatic cytosol. Evidence that the binding species is receptor for induction of aryl hydrocarbon hydroxylase
J Biol Chem
(1976) Recent advances in understanding the mechanisms of TCDD immunotoxicity
Int Immunopharmacol
(2002)- et al.
The aryl hydrocarbon receptor, more than a xenobiotic-interacting protein
FEBS Lett
(2007) - et al.
Intrinsic AhR function underlies cross-talk of dioxins with sex hormone signalings
Biochem Biophys Res Commun
(2008) - et al.
Aryl hydrocarbon receptor-deficient mice develop heightened inflammatory responses to cigarette smoke and endotoxin associated with rapid loss of the nuclear factor-kappaB component RelB
Am J Pathol
(2007) - et al.
Role of the aromatic hydrocarbon receptor and [Ah] gene battery in the oxidative stress response, cell cycle control, and apoptosis
Biochem Pharmacol
(2000) - et al.
A potential endogenous ligand for the aryl hydrocarbon receptor has potent agonist activity in vitro and in vivo
Arch Biochem Biophys
(2006) - et al.
Ligand-dependent interactions of the Ah receptor with coactivators in a mammalian two-hybrid assay
Toxicol Appl Pharmacol
(2008) - et al.
Certain photooxidized derivatives of tryptophan bind with very high affinity to the Ah receptor and are likely to be endogenous signal substances
J Biol Chem
(1987)
7-ketocholesterol is an endogenous modulator for the arylhydrocarbon receptor
J Biol Chem
Indirubin and indigo are potent aryl hydrocarbon receptor ligands present in human urine
J Biol Chem
Activation of the Ah receptor signal transduction pathway by bilirubin and biliverdin
Arch Biochem Biophys
Ahr null alleles: distinctive or different?
Biochem Pharmacol
Thymocyte development in Ah-receptor-deficient mice is refractory to TCDD-inducible changes
Int J Immunopharmacol
Ahr null alleles: distinctive or different? Biochem
Pharmacology
T lymphocytes are direct, aryl hydrocarbon receptor (AhR)-dependent targets of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD): AhR expression in both CD4+ and CD8+ T cells is necessary for full suppression of a cytotoxic T lymphocyte response by TCDD
Toxicol Appl Pharmacol
Aryl hydrocarbon receptor-deficient mice generate normal immune responses to model antigens and are resistant to TCDD-induced immune suppression
Toxicol Appl Pharmacol
Over-production of IFN-gamma and IL-12 in AhR-null mice
FEBS Lett
Data sieving analysis as a novel method to asses immunotoxic exposure to dioxins retrospectively
Int Immunopharmacol
Identification of dioxin-responsive elements (DREs) in the 5’ regions of putative dioxin-inducible genes
Chem Biol Interact
Tumor necrosis factor involvement in 2,3,7,8-tetrachlorodibenzo-p-dioxin-mediated endotoxin hypersensitivity in C57BL/6J mice congenic at the Ah locus
Toxicol Appl Pharmacol
Activation of the aryl hydrocarbon receptor is essential for mediating the anti-inflammatory effects of a novel low molecular weight compound
Blood
2,3,7,8-Tetrachlorodibenzo-p-dioxin affects the number and function of murine splenic dendritic cells and their expression of accessory molecules
Toxicol Appl Pharmacol
In vivo treatment with anti-ICAM-1 and anti-LFA-1 antibodies inhibits contact sensitization-induced migration of epidermal Langerhans cells to regional lymph nodes
Cell Immunol
Fewer CTL, not enhanced NK cells, are sufficient for viral clearance from the lungs of immunocompromised mice
Cell Immunol
Allelic variants of drug metabolizing enzymes as risk factors in psoriasis
J Invest Dermatol
Resistance to 2,3,7,8-tetrachlorodibenzo-p-dioxin toxicity and abnormal liver development in mice carrying a mutation in the nuclear localization sequence of the aryl hydrocarbon receptor
J Biol Chem
Inducibility of cytochrome P450 1A1 and chemical carcinogenesis by benzo[a]pyrene in AhR repressor-deficient mice
Biochem Biophys Res Commun
Ah receptor signaling pathways
Annu Rev Cell Dev Biol
Comparative analysis of dioxin response elements in human, mouse and rat genomic sequences
Nucleic Acids Res
Aryl hydrocarbon receptor activation by cAMP vs. dioxin: divergent signaling pathways
Proc Natl Acad Sci USA
TCDD deregulates contact inhibition in rat liver oval cells via Ah receptor, JunD and cyclin A
Oncogene
Activation of the aryl hydrocarbon receptor by structurally diverse exogenous and endogenous chemicals
Annu Rev Pharmacol Toxicol
An aryl hydrocarbon receptor conformation acts as the functional core of nuclear dioxin signaling
Nucleic Acids Res
Different structural requirements of the ligand binding domain of the aryl hydrocarbon receptor for high- and low-affinity ligand binding and receptor activation
Mol Pharmacol
Lightening up the UV response by identification of the arylhydrocarbon receptor as a cytoplasmatic target for ultraviolet B radiation
Proc Natl Acad Sci USA
Leukotriene A4 metabolites are endogenous ligands for the Ah receptor
Biochemistry
Immune system impairment and hepatic fibrosis in mice lacking the dioxin-binding Ah receptor
Science
Loss of teratogenic response to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in mice lacking the Ah (dioxin) receptor
Genes Cells
Characterization of a murine Ahr null allele: involvement of the Ah receptor in hepatic growth and development
Proc Natl Acad Sci USA
Cited by (64)
The molecular mechanisms that underpin the biological benefits of full-spectrum cannabis extract in the treatment of neuropathic pain and inflammation
2020, Biochimica et Biophysica Acta - Molecular Basis of DiseaseCitation Excerpt :Activation of AhR halts inflammation notably through induction of interleukin-22 (IL-22) [88]. Of interest, activation of AhR, dependent or independent of the gut microbiota, has been reported to limit the production of microglial pro-inflammatory mediators such as transforming growth factor alpha (TGFα) and vascular endothelial growth factor B (VEGF-B) in a mouse model of MS [90,91]. Interestingly, oral administration of Lactobacillus acidophilus was shown to combat inflammation and nociception through increasing the expression of the CB2 receptor in intestinal epithelial cells [92], suggesting that probiotics and cannabinoids might work together to halt inflammation and nociception.
2,3,7,8-Tetrachloodibenzo-p-dioxin affects the differentiation of CD4 helper T cell
2019, Toxicology LettersActivation of aryl hydrocarbon receptor regulates the LPS/IFNγ-induced inflammatory response by inducing ubiquitin-proteosomal and lysosomal degradation of RelA/p65
2018, Biochemical PharmacologyCitation Excerpt :Upon binding to its ligands, AhR translocates from the cytoplasm to the nucleus where it binds to xenobiotic response elements (XREs) located in the promoter of its target genes, such as cytochrome P450s (CYP1A1, CYP1A2, CYP1B1), NAD(P)H quinone oxidoreductase, and UDP-glucuronosyl-transferase 6 [1]. In addition to its role in xenobiotic metabolism, AhR has been implicated in several cellular processes, including liver development [2], neurogenesis [3], cholesterol and glucose metabolism [4,5], cell proliferation and apoptosis [6], and in the homeostasis of the immune system [7–9]. It was shown that AhR modulates the adaptive and innate immune responses and has been implicated in the differentiation of IL-17-producing helper T cells and Treg cells [10].
AHR and the issue of immunotoxicity
2018, Current Opinion in Toxicology