SUMO modification regulates the transcriptional repressor function of aryl hydrocarbon receptor repressor

The complex biology of aryl hydrocarbon receptor activation in cancer and beyond

The aryl hydrocarbon receptor (AHR) signaling pathway is a complex regulatory network that plays a critical role in various biological processes, including cellular metabolism, development, and immune responses. The complexity of AHR signaling arises from multiple factors, including the diverse ligands that activate the receptor, the expression level of AHR itself, and its interaction with the AHR nuclear translocator (ARNT). Additionally, the AHR crosstalks with the AHR repressor (AHRR) or other transcription factors and signaling pathways and it can also mediate non-genomic effects. Finally, posttranslational modifications of the AHR and its interaction partners, epigenetic regulation of AHR and its target genes, as well as AHR-mediated induction of enzymes that degrade AHR-activating ligands may contribute to the context-specificity of AHR activation. Understanding the complexity of AHR signaling is crucial for deciphering its physiological and pathological roles and developing therapeutic strategies targeting this pathway. Ongoing research continues to unravel the intricacies of AHR signaling, shedding light on the regulatory mechanisms controlling its diverse functions.

The Function of Xenobiotic Receptors in Metabolic Diseases

Metabolic diseases are a series of metabolic disorders that include obesity, diabetes, insulin resistance, hypertension, and hyperlipidemia. The increased prevalence of metabolic diseases has resulted in higher mortality and mobility rates over the past decades, and this has led to extensive research focusing on the underlying mechanisms. Xenobiotic receptors (XRs) are a series of xenobiotic-sensing nuclear receptors that regulate their downstream target genes expression, thus defending the body from xenobiotic and endotoxin attacks. XR activation is associated with the development of a number of metabolic diseases such as obesity, nonalcoholic fatty liver disease, type 2 diabetes, and cardiovascular diseases, thus suggesting an important role for XRs in modulating metabolic diseases. However, the regulatory mechanism of XRs in the context of metabolic disorders under different nutrient conditions is complex and remains controversial. This review summarizes the effects of XRs on different metabolic components (cholesterol, lipids, glucose, and bile acids) in different tissues during metabolic diseases. As chronic inflammation plays a critical role in the initiation and progression of metabolic diseases, we also discuss the impact of XRs on inflammation to comprehensively recognize the role of XRs in metabolic diseases. This will provide new ideas for treating metabolic diseases by targeting XRs. SIGNIFICANCE STATEMENT: This review outlines the current understanding of xenobiotic receptors on nutrient metabolism and inflammation during metabolic diseases. This work also highlights the gaps in this field, which can be used to direct the future investigations on metabolic diseases treatment by targeting xenobiotic receptors.

MESH1 knockdown triggers proliferation arrest through TAZ repression

All organisms are constantly exposed to various stresses, necessitating adaptive strategies for survival. In bacteria, the main stress-coping mechanism is the stringent response triggered by the accumulation of “alarmone” (p)ppGpp to arrest proliferation and reprogram transcriptome. While mammalian genomes encode MESH1—the homolog of the (p)ppGpp hydrolase SpoT, current knowledge about its function remains limited. We found MESH1 expression tended to be higher in tumors and associated with poor patient outcomes. Consistently, MESH1 knockdown robustly inhibited proliferation, depleted dNTPs, reduced tumor sphere formation, and retarded xenograft growth. These antitumor phenotypes associated with MESH1 knockdown were accompanied by a significantly altered transcriptome, including the repressed expression of TAZ, a HIPPO coactivator, and proliferative gene. Importantly, TAZ restoration mitigated many anti-growth phenotypes of MESH1 knockdown, including proliferation arrest, reduced sphere formation, tumor growth inhibition, dNTP depletion, and transcriptional changes. Furthermore, TAZ repression was associated with the histone hypo-acetylation at TAZ regulatory loci due to the induction of epigenetic repressors HDAC5 and AHRR. Together, MESH1 knockdown in human cells altered the genome-wide transcriptional patterns and arrested proliferation that mimicked the bacterial stringent response through the epigenetic repression of TAZ expression.

Suppressive effects of aryl-hydrocarbon receptor repressor on adipocyte differentiation in 3T3-L1 cells

The aryl-hydrocarbon receptor repressor (AhRR) negatively regulates aryl-hydrocarbon receptor (AhR) signaling via its inhibitory transactivation. AhR is well known to suppress adipocyte differentiation, but the function of AhRR during adipogenesis is unclear. The purpose of this study was to investigate the role of AhRR in adipocyte differentiation using 3T3-L1 cells. During the early phase of differentiation, AhRR expression was transiently induced, but throughout the entire differentiation process, low levels of AhR expression were maintained. AhRR knockdown significantly increased not only glycerol-3-phosphate dehydrogenase (GPDH) activity but also lipid accumulation inside the cells. AhRR overexpression clearly reduced GPDH activity and lipid accumulation, indicating that AhRR upregulation during the early stage of adipogenesis suppresses adipocyte differentiation. Since AhRR knockdown increases the expression and activity of peroxisome proliferator-activated receptor γ (PPARγ), AhRR negatively regulates PPARγ during adipogenesis. In summary, similar to AhR, AhRR acts as an inhibitor of adipocyte differentiation. In addition to controlling the negative feedback loop of AhR, AhRR might be involved in other functions, especially in adipocyte differentiation processes.

The crystal structure of the AhRR-ARNT heterodimer reveals the structural basis of the repression of AhR-mediated transcription

2,3,7,8-Tetrachlorodibenzo-p-dioxin and related compounds are extraordinarily potent environmental toxic pollutants. Most of the 2,3,7,8-tetrachlorodibenzo-p-dioxin toxicities are mediated by aryl hydrocarbon receptor (AhR), a ligand-dependent transcription factor belonging to the basic helix-loop-helix (bHLH) Per-ARNT-Sim (PAS) family. Upon ligand binding, AhR forms a heterodimer with AhR nuclear translocator (ARNT) and induces the expression of genes involved in various biological responses. One of the genes induced by AhR encodes AhR repressor (AhRR), which also forms a heterodimer with ARNT and represses the activation of AhR-dependent transcription. The control of AhR activation is critical for managing AhR-mediated diseases, but the mechanisms by which AhRR represses AhR activation remain poorly understood, because of the lack of structural information. Here, we determined the structure of the AhRR-ARNT heterodimer by X-ray crystallography, which revealed an asymmetric intertwined domain organization presenting structural features that are both conserved and distinct among bHLH-PAS family members. The structures of AhRR-ARNT and AhR-ARNT were similar in the bHLH-PAS-A region, whereas the PAS-B of ARNT in the AhRR-ARNT complex exhibited a different domain arrangement in this family reported so far. The structure clearly disclosed that AhRR competitively represses AhR binding to ARNT and target DNA and further suggested the existence of an AhRR-ARNT-specific repression mechanism. This study provides a structural basis for understanding the mechanism by which AhRR represses AhR-mediated gene transcription.

A Molecular Mechanism To Switch the Aryl Hydrocarbon Receptor from a Transcription Factor to an E3 Ubiquitin Ligase

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that is known as a mediator of toxic responses. Recently, it was shown that the AhR has dual functions. Besides being a transcription factor, it also possesses an intrinsic E3 ubiquitin ligase function that targets, e.g., the steroid receptors for proteasomal degradation. The aim of this study was to identify the molecular switch that determines whether the AhR acts as a transcription factor or an E3 ubiquitin ligase. To do this, we used the breast cancer cell line MCF7, which expresses a functional estrogen receptor alpha (ERα) signaling pathway. Our data suggest that aryl hydrocarbon receptor nuclear translocator (ARNT) plays an important role in the modulation of the dual functions of the AhR. ARNT knockdown dramatically impaired the transcriptional activation properties of the ligand-activated AhR but did not affect its E3 ubiquitin ligase function. The availability of ARNT itself is modulated by another basic helix-loop-helix (bHLH)-Per-ARNT-SIM (PAS) protein, the repressor of AhR function (AhRR). MCF7 cells overexpressing the AhRR showed lower ERα protein levels, reduced responsiveness to estradiol, and reduced growth rates. Importantly, when these cells were used to produce estrogen-dependent xenograft tumors in SCID mice, we also observed lower ERα protein levels and a reduced tumor mass, implying a tumor-suppressive-like function of the AhR in MCF7 xenograft tumors.

The aryl hydrocarbon receptor repressor - More than a simple feedback inhibitor of AhR signaling: Clues for its role in inflammation and cancer

The aryl hydrocarbon receptor repressor (AhRR) was first described as a specific competitive repressor of aryl hydrocarbon receptor (AhR) activity based on its ability to dimerize with the AhR nuclear translocator (ARNT) and through direct competition of AhR/ARNT and AhRR/ARNT complexes for binding to dioxin-responsive elements (DREs). Like AhR, AhRR belongs to the basic Helix-Loop-Helix/Per-ARNT-Sim (bHLH/PAS) protein family but lacks functional ligand-binding and transactivation domains. Transient transfection experiments with ARNT and AhRR mutants examining the inhibitory mechanism of AhRR suggested a more complex mechanism than the simple mechanism of negative feedback through sequestration of ARNT to regulate AhR signaling. Recently, AhRR has been shown to act as a tumor suppressor gene in several types of cancer cells. Furthermore, epidemiological studies have found epigenetic changes and silencing of AhRR associated with exposure to cigarette smoke and cancer development. Additional studies from our laboratories have demonstrated that AhRR represses other signaling pathways including NF-κB and is capable of regulating inflammatory responses. A better understanding of the regulatory mechanisms of AhRR in AhR signaling and adverse outcome pathways leading to deregulated inflammatory responses contributing to tumor promotion and other adverse health effects is expected from future studies. This review article summarizes the characteristics of AhRR as an inhibitor of AhR activity and highlights more recent findings pointing out the role of AhRR in inflammation and tumorigenesis.

The Key Regulator for Language and Speech Development, FOXP2, is a Novel Substrate for SUMOylation

Transcription factor forkhead box protein P2 (FOXP2) plays an essential role in the development of language and speech. However, the transcriptional activity of FOXP2 regulated by the post-translational modifications remains unknown. Here, we demonstrated that FOXP2 is clearly defined as a SUMO target protein at the cellular levels as FOXP2 is covalently modified by both SUMO1 and SUMO3. Furthermore, SUMOylation of FOXP2 was significantly decreased by SENP2 (a specific SUMOylation protease). We further showed that FOXP2 is selectively SUMOylated in vivo on a phylogenetically conserved lysine 674 but the SUMOylation does not alter subcellular localization and stability of FOXP2. Interestingly, we observed that human etiological FOXP2 R553H mutation robustly reduces its SUMOylation potential as compared to wild-type FOXP2. In addition, the acidic residues downstream the core SUMO motif on FOXP2 are required for its full SUMOylation capacity. Finally, our functional analysis using reporter gene assays showed that SUMOylation may modulate transcriptional activity of FOXP2 in regulating downstream target genes (DISC1, SRPX2, and MiR200c). Altogether, we provide the first evidence that FOXP2 is a substrate for SUMOylation and SUMOylation of FOXP2 plays a functional role in regulating its transcriptional activity.

Role of AHR, AHRR and ARNT in response to dioxin-like PCBs in Spaurus aurata

The aryl hydrocarbon receptor (AHR) mediates a variety of biological responses to ubiquitous dioxin and PCB dioxin-like. AHR together with ARNT, AHRR, represent a novel basic helix-loop-helix/PAS family of transcriptional regulators. Their interplay may affect the xenobiotic response. The aim of this study was to investigate, by histological, immunohistochemical investigations and western-blot analysis, the expression of AHR, ARNT and AHRR in liver of seabrem (Spaurus aurata) after exposure at different time to dioxin-like PCB126 in order to deep the knowledge about their specific role. The findings showed a significant increase of AHR and ARNT expression in juvenile fishes after 12 h than control group. The induction of AHR and ARNT is also significant at 24 and 72 hours compared to the control group. Furthemore, induction of AHRR expression has proved to increase both 12 h but this induction does not seem significant to 24 and 72 hours. The most important data of this work is that the induction of AHRR, when the action of the toxic persistence substances, as dioxin and PCB-126, it is not enough to reduce AHR signaling and thus its hyperactivation leads to toxic effects in seabrem (Spaurus aurata). All this confirms the importance of AHR ligands as new class of drugs that can be directed against severe disease such as cancer.

Dioxin exposure blocks lactation through a direct effect on mammary epithelial cells mediated by the aryl hydrocarbon receptor repressor

In mammals, lactation is a rich source of nutrients and antibodies for newborn animals. However, millions of mothers each year experience an inability to breastfeed. Exposure to several environmental toxicants, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), has been strongly implicated in impaired mammary differentiation and lactation. TCDD and related polyhalogenated aromatic hydrocarbons are widespread industrial pollutants that activate the aryl hydrocarbon receptor (AHR). Despite many epidemiological and animal studies, the molecular mechanism through which AHR signaling blocks lactation remains unclear. We employed in vitro models of mammary differentiation to recapitulate lactogenesis in the presence of toxicants. We demonstrate AHR agonists directly block milk production in isolated mammary epithelial cells. Moreover, we define a novel role for the aryl hydrocarbon receptor repressor (AHRR) in mediating this response. Our mechanistic studies suggest AHRR is sufficient to block transcription of the milk gene β-casein. As TCDD is a prevalent environmental pollutant that affects women worldwide, our results have important public health implications for newborn nutrition.

Knockdown of a zebrafish aryl hydrocarbon receptor repressor (AHRRa) affects expression of genes related to photoreceptor development and hematopoiesis

The aryl hydrocarbon receptor repressor (AHRR) is a transcriptional repressor of aryl hydrocarbon receptor (AHR) and hypoxia-inducible factor (HIF) and is regulated by an AHR-dependent mechanism. Zebrafish (Danio rerio) possess two AHRR paralogs; AHRRa regulates constitutive AHR signaling during development, whereas AHRRb regulates polyaromatic hydrocarbon-induced gene expression. However, little is known about the endogenous roles and targets of AHRRs. The objective of this study was to elucidate the role of AHRRs during zebrafish development using a loss-of-function approach followed by gene expression analysis. Zebrafish embryos were microinjected with morpholino oligonucleotides against AHRRa or AHRRb to knockdown AHRR protein expression. At 72 h postfertilization (hpf), microarray analysis revealed that the expression of 279 and 116 genes was altered by knockdown of AHRRa and AHRRb, respectively. In AHRRa-morphant embryos, 97 genes were up-regulated and 182 genes were down-regulated. Among the down-regulated genes were several related to photoreceptor function, including cone-specific genes such as several opsins (opn1sw1, opn1sw2, opn1mw1, and opn1lw2), phosphodiesterases (pde6H and pde6C), retinol binding protein (rbp4l), phosducin, and arrestins. Down-regulation was confirmed by RT-PCR and with samples from an independent experiment. The four genes tested (opn1sw1, pde6H, pde6C, and arr3b) were not inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin. AHRRa knockdown also caused up-regulation of embryonic hemoglobin (hbbe3), suggesting a role for AHRR in regulating hematopoiesis. Knockdown of AHRRb caused up-regulation of 31 genes and down-regulation of 85 genes, without enrichment for any specific biological process. Overall, these results suggest that AHRRs may have important roles in development, in addition to their roles in regulating xenobiotic signaling.

bHLH-PAS proteins in cancer

Mammalian basic HLH (helix-loop-helix)-PER-ARNT-SIM (bHLH-PAS) proteins are heterodimeric transcription factors that sense and respond to environmental signals (such as pollutants) or to physiological signals (for example, hypoxia and circadian rhythms) through their two PAS domains. PAS domains form a generic three-dimensional fold, which commonly contains an internal cavity capable of small-molecule binding and outer surfaces adept at protein-protein interactions. These proteins are important in several pro-tumour and antitumour pathways and their activities can be modulated by both natural metabolites and oncometabolites. Recently determined structures and successful small-molecule screening programmes are now providing new opportunities to discover selective agonists and antagonists directed against this multitasking family of transcription factors.

SUMOylation of AhR modulates its activity and stability through inhibiting its ubiquitination

Aryl hydrocarbon receptor (AhR) is a transcription factor that belongs to the basic helix-loop-helix (bHLH) Per-Arnt-Sim homology domain (PAS) family. AhR can be activated by 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (2, 3, 7, 8-TCDD) and once activated, it promotes the abnormal expression of cytochrome P450, leading to several diseases, including cancer. In this study, we showed that AhR is subjected to post-translational modification by SUMOylation and this modification could be reversed by SENP1. Two SUMOylation sites were identified, one in the bHLH domain (K63) and the other in the TAD domain (K510) of AhR. Substitution of either K63 or K510 with arginine resulted in reduced SUMOylation for AhR. Treatment of MCF-7 cells with TCDD led to a reduced level of SUMOylated AhR in a time-dependent manner, and this occurred mainly in the nucleus. SUMOylation of AhR enhanced its stability through inhibiting its ubiquitination. Moreover, SUMOylation also repressed the transactivation activity of AhR and this could be reversed by TCDD. These results suggested that SUMOylation of AhR might play an important role in the regulation of its function, and TCDD may activate the transcriptional activity of AhR through downregulating its SUMOylation.

Cardiac function and disease: emerging role of small ubiquitin-related modifier

Small ubiquitin-related modifiers, or SUMOs, have emerged as versatile regulators of many biological functions that do so by covalent attachment to a variety of substrates via enzymatic reactions. SUMO conjugation has also been shown to be involved in a number of human pathogenic processes. More recent advances in the SUMO field have indicated a potential role for SUMO conjugation pathway in cardiogenesis. This advanced review will describe the basic features of the SUMO conjugation pathway and will summarize the most recent studies implicating the influence of the sumoylation pathway in cardiac function under both physiological and pathological conditions. WIREs Syst Biol Med 2011 3 446-457 DOI: 10.1002/wsbm.130

Single and concerted effects of benzo[a]pyrene and flavonoids on the AhR and Nrf2-pathway in the human colon carcinoma cell line Caco-2

As phytochemicals have the potential to counteract adverse effects of carcinogens we investigated the influence of the flavonoids quercetin and kaempferol on benzo[a]pyrene (BaP) mediated effects on human colon cancer cells, Caco-2. We focused on concerted effects on the expression of AhR and Nrf2 pathway components. In contrast to kaempferol, BaP and quercetin efficiently induced CYP1A1, CYP1A2 and CYP1B1-mRNA in Caco-2 cells. BaP not only acted via AhR activation but sustainably also by increasing AhR and by down-regulating AhRR mRNA. The flavonoids did not affect AhR expression but counteracted the BaP mediated AhRR repression. Only quercetin was found to induce AhRR mRNA. ARNT mRNA appeared to be slightly but significantly down-regulated by BaP as well as by flavonoids while expression of AIP was not or only slightly modulated. The Nrf2 pathway was activated by BaP and by the flavonoids shown by induction of Nrf2 and several of its target genes such as NQO1, GSTP1, GSTA1 and GCLC. Induction effects of 10 μm BaP on Nrf2, GSTP1 and NQO1 were abolished by the flavonoids. In summary, we show that quercetin supports AhR mediated effects. Both flavonoids, however, may counteract the effects of BaP on expression of AhR, AhRR, Nrf2, GSTP1 and NQO1. In conclusion, quercetin appears to have two faces, a flavonoid-like one and a PAH-like one which supports Ahr-mediated effects while kaempferol acts "just like a flavonoid". Thus, flavonoids have to be treated individually with respect to their anti-adverse activity.

Molecular and functional characterization of Aryl hydrocarbon receptor repressor from the chicken (Gallus gallus): interspecies similarities and differences

The aryl hydrocarbon receptor (AHR) repressor (AHRR) has been recognized as a negative feedback modulator of AHR-mediated responses in fish and mammals. However, the repressive mechanism by the AHRR has not been investigated in other animals. To understand the molecular mechanism of dioxin toxicity and the evolutionary history of the AHR signaling pathway in avian species, the present study addresses chicken AHRR (ckAHRR). The complementary DNA sequence of ckAHRR encodes an 84-kDa protein sharing 29-52% identities with other AHRRs. High levels of ckAHRR messenger RNA were recorded in the kidney and intestine of nontreated chicks. In hepatoma LMH cells, the 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) 50% effective concentration value for ckAHRR induction (0.0016nM) was the same as that for chicken cytochrome P450 1A5 (ckCYP1A5), implying a shared transcriptional regulation of ckAHRR and ckCYP1A5 by chicken AHR (ckAHR). In ckAHRR transient transfection assays, ckAHRR repressed both ckAHR1- and ckAHR2-mediated transcriptional activities. Deletion and mutation assays revealed that basic helix-loop-helix/Per-ARNT-Sim A domains of ckAHRR, particularly 217-402 amino acid residues, are indispensable for the repression, but the AHR nuclear translocator sequestration by ckAHRR and SUMOylation of ckAHRR are not involved in its repressive mechanism. Additionally, subcellular localization assay of ckAHR1-enhanced green fluorescent protein fusion protein showed that ckAHRR did not affect nuclear translocation of the ckAHR1. Furthermore, ckAHRR inhibited the TCDD- and 17β estradiol-enhanced ckCYP1A5 transcription through AHR-estrogen receptor α (ERα) cross talk. Taken together, the function of AHRR is conserved in chicken in terms of the negative regulation of AHR and ERα activities, but its functional mechanism is likely distinct from those of the mammalian and fish homologues.

Dioxins, the aryl hydrocarbon receptor and the central regulation of energy balance

Dioxins are ubiquitous environmental contaminants that have attracted toxicological interest not only for the potential risk they pose to human health but also because of their unique mechanism of action. This mechanism involves a specific, phylogenetically old intracellular receptor (the aryl hydrocarbon receptor, AHR) which has recently proven to have an integral regulatory role in a number of physiological processes, but whose endogenous ligand is still elusive. A major acute impact of dioxins in laboratory animals is the wasting syndrome, which represents a puzzling and dramatic perturbation of the regulatory systems for energy balance. A single dose of the most potent dioxin, TCDD, can permanently readjust the defended body weight set-point level thus providing a potentially useful tool and model for physiological research. Recent evidence of response-selective modulation of AHR action by alternative ligands suggests further that even therapeutic implications might be possible in the future.

Transcriptional repression by sumoylation of Epstein-Barr virus BZLF1 protein correlates with association of histone deacetylase

The transition from latent to lytic phases of the Epstein-Barr virus life cycle is triggered by expression of a viral transactivator, BZLF1, that then induces expression of the viral immediate-early and early genes. The BZLF1 protein is post-translationally modified by a small ubiquitin-related modifier-1 (SUMO-1). Here we found that BZLF1 is conjugated at lysine 12 not only by SUMO-1 but also by SUMO-2 and 3. The K12R mutant of BZLF1, which no longer becomes sumoylated, exhibits stronger transactivation than the wild-type BZLF1 in a reporter assay system as well as in the context of virus genome with nucleosomal structures. Furthermore, exogenous supply of a SUMO-specific protease, SENP, caused de-sumoylation of BZLF1 and enhanced BZLF1-mediated transactivation. Immunoprecipitation experiments proved that histone deacetylase 3 preferentially associated with the sumoylated form of BZLF1. Levels of the sumoylated BZLF1 increased as lytic replication progressed. Based on these observations, we conclude that sumoylation of BZLF1 regulates its transcriptional activity through histone modification during Epstein-Barr virus productive replication.

A manually curated network of the PML nuclear body interactome reveals an important role for PML-NBs in SUMOylation dynamics

Promyelocytic Leukaemia Protein nuclear bodies (PML-NBs) are dynamic nuclear protein aggregates. To gain insight in PML-NB function, reductionist and high throughput techniques have been employed to identify PML-NB proteins. Here we present a manually curated network of the PML-NB interactome based on extensive literature review including database information. By compiling 'the PML-ome', we highlighted the presence of interactors in the Small Ubiquitin Like Modifier (SUMO) conjugation pathway. Additionally, we show an enrichment of SUMOylatable proteins in the PML-NBs through an in-house prediction algorithm. Therefore, based on the PML network, we hypothesize that PML-NBs may function as a nuclear SUMOylation hotspot.

Molecular mechanisms of the physiological functions of the aryl hydrocarbon (dioxin) receptor, a multifunctional regulator that senses and responds to environmental stimuli

The aryl hydrocarbon receptor (AhR) was originally identified as a ligand-activated transcription factor that is involved in the induction of xenobiotic-metabolizing Cytochrome P4501A1 (CYP1A1). For several decades, AhR has been studied in relation to toxicology and pharmacology. With recent discoveries on novel AhR functions, AhR research has expanded into multiple aspects of physiology, such as reproduction, innate immunity and tumor suppression. In this review, we summarize and discuss recent progress in mechanistic and functional studies on AhR with particular emphasis on physiological processes.