Aryl Hydrocarbon Receptor Function Research Articles

Dysregulation of stress erythropoiesis and enhanced susceptibility to Salmonella Typhimurium infection in AhR-deficient mice.

By acting as an environmental sensor, the ligand-induced transcription factor aryl hydrocarbon receptor (AhR) regulates acute innate and adaptive immune responses against pathogens. Here, we analyzed the function of AhR in a model for chronic systemic infection with attenuated Salmonella Typhimurium (STM). WT and AhR-deficient mice were infected with the attenuated STM strain TAS2010 and analyzed for bacterial burden, host defense functions and inflammatory stress erythropoiesis. AhR-deficient mice were highly susceptible to TAS2010 infection compared with WT mice demonstrated by reduced bacterial clearance and increased mortality. STM infection resulted in macrocytic anemia and enhanced splenomegaly along with destruction of the splenic architecture in AhR-deficient mice. In addition, AhR-deficient mice displayed a major expansion of splenic immature red blood cells, indicative of infection-induced stress erythropoiesis. Elevated serum levels of erythropoietin and interleukin-6 upon infection as well as increased numbers of splenic stress erythroid progenitors already in steady state probably drive this effect and might cause the alterations in splenic immune cell compartments, thereby preventing an effective host defense against STM in AhR-deficient mice. AhR-deficient mice fail to clear chronic TAS2010 infection due to enhanced stress erythropoiesis in the spleen and accompanying destruction of the splenic architecture.

The Role of Aryl Hydrocarbon Receptor in the Pathogenesis and Treatment of Psoriasis.

The pathogenesis of psoriasis is complex. Aryl hydrocarbon receptor (AhR) is a transcription factor that can be bound and activated by structurally diverse ligands and plays an important role in a range of biological processes and in the pathogenesis of different diseases. Recently, the role of AhR in psoriasis has attracted attention. AhR has toxicological functions and physiological functions. The overexpression and activation of AhR induced by the environmental pollutant and exogenous AhR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) can drive the development of psoriasis. This TCDD-mediated toxicological response disrupts the physiological functions of AhR resulting in skin barrier disorders and the release of inflammatory cytokines, 2 of the pivotal factors of psoriasis. In addition, highly upregulated kynureninase in psoriasis decreases endogenous AhR agonists, thereby weakening the physiological functions of AhR. Activating AhR physiological signalling should be useful in the treatment of psoriasis. Studies have demonstrated that physiological activation of AhR can dampen the severity of psoriasis. The oldest and effective treatment for psoriasis coal tar works by activating AhR, and both new anti-psoriasis drugs tapinarof and benvitimod are formulations of AhR agonist, supporting that activation of AhR can be used as a new strategy for the treatment of psoriasis. Preclinical and preliminary clinical studies have revealed the anti-psoriasis effects of a number of AhR agonists, providing potential candidates for the development of new drugs for the treatment of psoriasis.

Structural Insights into the Activation of Human Aryl Hydrocarbon Receptor by the Environmental Contaminant Benzo[a]pyrene and Structurally Related Compounds

The aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor belonging to the bHLH/PAS protein family and responding to hundreds of natural and chemical substances. It is primarily involved in the defense against chemical insults and bacterial infections or in the adaptive immune response, but also in the development of pathological conditions ranging from inflammatory to neoplastic disorders. Despite its prominent roles in many (patho)physiological processes, the lack of high-resolution structural data has precluded for thirty years an in-depth understanding of the structural mechanisms underlying ligand-binding specificity, promiscuity and activation of AHR. We recently reported a cryogenic electron microscopy (cryo-EM) structure of human AHR bound to the natural ligand indirubin, the chaperone Hsp90 and the co-chaperone XAP2 that provided the first experimental visualization of its ligand-binding PAS-B domain. Here, we report a 2.75 Å resolution structure of the AHR complex bound to the environmental pollutant benzo[a]pyrene (B[a]P). The structure substantiates the existence of a bipartite PAS-B ligand-binding pocket with a geometrically constrained primary binding site controlling ligand binding specificity and affinity, and a secondary binding site contributing to the binding promiscuity of AHR. We also report a docking study of B[a]P congeners that validates the B[a]P-bound PAS-B structure as a suitable model for accurate computational ligand binding assessment. Finally, comparison of our agonist-bound complex with the recently reported structures of mouse and fruit fly AHR PAS-B in different activation states suggests a ligand-induced loop conformational change potentially involved in the regulation of AHR function.

Aryl hydrocarbon receptor regulates IL-22 receptor expression on thymic epithelial cell and accelerates thymus regeneration

Improving regeneration of damaged thymus is important for reconstituting T-cell immunity. Interleukin-22 (IL-22) was proved to improve thymus regeneration through recovering thymic epithelial cells (TECs). The IL-22 receptor IL-22RA1 is crucial for mediating IL-22 functions. Mechanism that regulates IL-22RA1 expression is unknown. Through using TECs-conditional knockout mice, we found aryl hydrocarbon receptor (AHR) is important for thymus regeneration, because Foxn1-cre-mediated AHR knockout (AhrKO) significantly blocks recovery of thymus cells. Giving mice the AHR inhibitor CH-223191 or the AHR agonist FICZ blocks or accelerates thymus regeneration, respectively. AhrKO-mediated blockade of thymus regeneration could not be rescued by giving exogenous IL-22. Mechanistically, AhrKO mice shows decreased IL-22RA1 expression. In the murine TECs cell line mTEC1 cells, targeting AHR shows an impact on IL-22RA1 mRNA levels. Using chromatin immunoprecipitation and luciferase reporter assays, we find AHR co-operates with STAT3, binds the promotor region of IL-22RA1 gene and transcriptionally increases IL-22RA1 expression in mTEC1 cells. Foxn1-cre-mediated IL-22RA1 knockout (Il22ra1KO) blocks thymus regeneration after irradiation. Furthermore, targeting AHR or IL-22RA1 has significant impacts on severity of murine chronic graft-versus-host disease (cGVHD), which is an autoimmune-like complication following allogeneic hematopoietic cell transplantation. Giving FICZ decreases cGVHD, whereas Il22ra1KO exacerbates cGVHD. The impacts on cGVHD are associated with thymus regeneration and T-cell immune reconstitution. In conclusion, we report an unrecognized function of TECs-expressed AHR in thymus regeneration and AHR transcriptionally regulates IL-22RA1 expression, which have implications for improving thymus regeneration and controlling cGVHD.

Activation of Chaperone-Mediated Autophagy Inhibits the Aryl Hydrocarbon Receptor Function by Degrading This Receptor in Human Lung Epithelial Carcinoma A549 Cells.

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor and a substrate protein of a Cullin 4B E3 ligase complex responsible for diverse cellular processes. In the lung, this receptor is responsible for the bioactivation of benzo[a]pyrene during tumorigenesis. Realizing that the AHR function is affected by its expression level, we are interested in the degradation mechanism of AHR in the lung. Here, we have investigated the mechanism responsible for AHR degradation using human lung epithelial A549 cells. We have observed that the AHR protein levels increase in the presence of chloroquine (CQ), an autophagy inhibitor, in a dose-dependent manner. Treatment with 6-aminonicotinamide (6-AN), a chaperone-mediated autophagy (CMA) activator, decreases AHR protein levels in a concentration-dependent and time-dependent manner. This decrease suppresses the ligand-dependent activation of the AHR target gene transcription, and can be reversed by CQ but not MG132. Knockdown of lysosome-associated membrane protein 2 (LAMP2), but not autophagy-related 5 (ATG5), suppresses the chloroquine-mediated increase in the AHR protein. AHR is resistant to CMA when its CMA motif is mutated. Suppression of the epithelial-to-mesenchymal transition in A549 cells is observed when the AHR gene is knocked out or the AHR protein level is reduced by 6-AN. Collectively, we have provided evidence supporting that AHR is continuously undergoing CMA and activation of CMA suppresses the AHR function in A549 cells.

Aryl hydrocarbon receptor attenuates cholestatic liver injury by regulating bile acid metabolism

Cholestatic liver disease is defined as the bile acids (BAs) accumulation in the liver caused by impaired synthesis, and secretion, together with excretion of BAs due to a variety of factors, which, if left untreated, can result in hepatic fibrosis, cholestatic cholangitis, cholestatic cirrhosis, eventually, end-stage liver disease. Currently, modulation of BA metabolism is still a prospective therapeutic strategy for treating the cholestatic diseases. Aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor with far-reaching effects on the chronic liver disease. However, its role and mechanism in cholestatic liver damage is still unknown. Therefore, in this work, we explored the impact of AHR on the cholestatic liver injury using AHR overexpression mediated by adeno-associated viral (AAV) vectors. We found that AHR is differentially expressed in different stages of cholestatic liver disease, showing either down-regulation or an increase in protective effects. Overexpression of AHR increased body weight, decreased serum total bilirubin (TBil) and alkaline phosphatase (ALP), reduced porphyrin accumulation in liver tissue, and regulated the bile acid pool in the cholestatic mouse model induced by DDC diet. Overall, our data indicate that AHR attenuated cholestatic liver injury. AHR function indicates that it may have an action in the clinical management of cholestasis.

Constitutive aryl hydrocarbon receptor facilitates the regenerative potential of mouse bone marrow mesenchymal stromal cells.

Bone marrow mesenchymal stromal cells (BMSCs) are the commonly used seed cells in tissue engineering. Aryl hydrocarbon receptor (AhR) is a transcription factor involved in various cellular processes. However, the function of constitutive AhR in BMSCs remains unclear. To investigate the role of AhR in the osteogenic and macrophage-modulating potential of mouse BMSCs (mBMSCs) and the underlying mechanism. Immunochemistry and immunofluorescent staining were used to observe the expression of AhR in mouse bone marrow tissue and mBMSCs. The overexpression or knockdown of AhR was achieved by lentivirus-mediated plasmid. The osteogenic potential was observed by alkaline phosphatase and alizarin red staining. The mRNA and protein levels of osteogenic markers were detected by quantitative polymerase chain reaction (qPCR) and western blot. After coculture with different mBMSCs, the cluster of differentiation (CD) 86 and CD206 expressions levels in RAW 264.7 cells were analyzed by flow cytometry. To explore the underlying molecular mechanism, the interaction of AhR with signal transducer and activator of transcription 3 (STAT3) was observed by co-immunoprecipitation and phosphorylation of STAT3 was detected by western blot. AhR expressions in mouse bone marrow tissue and isolated mBMSCs were detected. AhR overexpression enhanced the osteogenic potential of mBMSCs while AhR knockdown suppressed it. The ratio of CD86+ RAW 264.7 cells cocultured with AhR-overexpressed mBMSCs was reduced and that of CD206+ cells was increased. AhR directly interacted with STAT3. AhR overexpression increased the phosphorylation of STAT3. After inhibition of STAT3 via stattic, the promotive effects of AhR overexpression on the osteogenic differentiation and macrophage-modulating were partially counteracted. AhR plays a beneficial role in the regenerative potential of mBMSCs partially by increasing phosphorylation of STAT3.

Endothelial AHR activity prevents lung barrier disruption in viral infection.

Disruption of the lung endothelial-epithelial cell barrier following respiratory virus infection causes cell and fluid accumulation in the air spaces and compromises vital gas exchange function1. Endothelial dysfunction can exacerbate tissue damage2,3, yet it is unclear whether the lung endothelium promotes host resistance against viral pathogens. Here we show that the environmental sensor aryl hydrocarbon receptor (AHR) is highly active in lung endothelial cells and protects against influenza-induced lung vascular leakage. Loss of AHR in endothelia exacerbates lung damage and promotes the infiltration of red blood cells and leukocytes into alveolar air spaces. Moreover, barrier protection is compromised and host susceptibility to secondary bacterial infections is increasedwhenendothelialAHR is missing. AHR engages tissue-protective transcriptional networks in endothelia, including the vasoactive apelin-APJ peptide system4, to prevent a dysplastic and apoptotic response in airway epithelial cells. Finally, we show that protective AHR signalling in lung endothelial cells is dampened by the infection itself. Maintenance of protective AHR function requires a diet enriched in naturally occurring AHR ligands, which activate disease tolerance pathways in lung endothelia to prevent tissue damage. Our findings demonstrate the importance of endothelial function in lung barrier immunity. We identify a gut-lung axis that affects lung damage following encounters with viral pathogens, linking dietary composition and intake to host fitness and inter-individual variations in disease outcome.

Spermidine protects intestinal mucosal barrier function in mice colitis via the AhR/Nrf2 and AhR/STAT3 signaling pathways

Aryl hydrocarbon receptor (AhR) activation promotes intestinal barrier repair and enhances the gut mucosal barrier function in inflammatory bowel diseases (IBD). Spermidine is beneficial in several murine models of IBD and may affect AhR activity. However, the precise effects of spermidine on the intestinal barrier and AhR remain unclear. This study was designed to investigate whether spermidine affects AhR and gut barrier function in IBD models as well as, its underlying mechanism. We used dextran sulfate sodium (DSS)- and 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced mice, as well as, Caco2 cells incubated with TNF-α and IFN-γ to establish multiple IBD models, followed by spermidine intervention. Alcian blue/Periodic acid-Schiff (AB/PAS) staining, Fluorescein isothiocyanate (FITC)-dextran permeability assay, transepithelial electrical resistance (TER), tight junction protein (TJs) expression, and 16S rRNA scope in situ hybridization were performed to assess intestinal barrier function. AhR expression and the associated pathways were measured. AhR-targeted adeno-associated virus (AAV) and siRNA were used to explore the related molecular mechanisms. Spermidine significantly attenuated the increased intestinal permeability, decreased TER, abnormal distribution of TJs in colitis, and bacterial translocation from the gut tract. Additionally, it significantly increased AhR and Nrf2 expression and inhibited STAT3 phosphorylation. However, the protective effects of spermidine and the related alterations in pathway proteins were largely abolished by the specific inhibition of AhR. Our study demonstrated that spermidine rescues intestinal barrier defects in mice with colitis via the AhR-Nrf2 and AhR-STAT3 pathways, providing a potential therapeutic agent for IBD and other conditions associated with dysregulated gut barrier function.

Tryptophan diet promotes myeloid-derived suppressor cell response via aryl hydrocarbon receptor in Sjogren’s syndrome

Abstract Myeloid-derived suppressor cells (MDSCs) are heterogeneous myeloid cell populations with immunosuppressive capacity, which contribute to immune regulation and tolerance induction. We previously reported impaired MDSC function in patients with primary Sjögren’s syndrome (pSS) and mice with experimental SS (ESS). In this study, we first found that aryl hydrocarbon receptor (AhR) was highly expressed by human and murine polymorphonuclear MDSCs (PMN-MDSCs). Indole-3-propionic acid (IPA), a natural AhR ligand produced from dietary tryptophan, significantly promoted PMN-MDSC differentiation and their suppressive function on CD4 +T cells. In contrast, feeding tryptophan-free diet resulted in decreased PMN-MDSC response, a phenotype that could be reversed by IPA supplementation. Notably, AhR expression was reduced in PMN-MDSCs during ESS development, while AhR antagonism resulted in exacerbated ESS pathology and dysregulated T effector cells, which could be phenocopied by tryptophan-free diet. Interferon regulatory factor 4 (IRF4), a repressive transcription factor, was upregulated in PMN-MDSCs during ESS progression. ChIP analysis revealed the binding capacity of IRF4 to the promoter region of AhR, while IRF4 deficiency markedly enhanced AhR-mediated PMN-MDSC responses. Furthermore, tryptophan-enriched diet or dietary supplementation of IPA markedly ameliorated salivary glandular pathology in ESS mice with restored MDSC immunosuppressive function. Together, our results identify a novel function of AhR in modulating PMN-MDSC response and demonstrate the therapeutic potential of targeting AhR for the treatment of pSS.

Modeling of Aryl Hydrocarbon Receptor Pathway Intrinsic Immunometabolic Role using Glioblastoma Stem Cells and Patient-Derived Organoids (P11-13.004)

<h3>Objective:</h3> To investigate the ability of patient derived glioma stem cells organoids to model the role of AHR in glioblastoma stem cells (GSCs) function and immune molecular programs in the dish as a patient’s avatar. <h3>Background:</h3> GSCs are highly self-renewing, resistant to therapy, and form lethal tumors. Organoids are defined as 3D <i>in vitro</i> tissue-like constructs derived from stem cells which mimic their corresponding <i>in vivo</i> organ. Aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor, and key regulator of infiltrating immune cells in gliomas and associated with poor prognosis, but its role in GSC and GBMO biology is unknown. <h3>Design/Methods:</h3> Generation of glioblastoma organoids (GBMOs) from patient’s resected GBM surgical tissue. A total of 555 organoids were generated for the study. We used limited dilution analysis for GSCs, in vivo transplantation of GSCs inactivated for AHR, small animal MRI. We used RNA microarray of GBMO treated with AHR agonist and antagonist, and CRISPR-Cas9 genetic ablation for AHR promote region in GSCs and GBMOs for validation of AHR immune gene targets. <h3>Results:</h3> We show that AHR is a patient-specific regulator of the glioma intrinsic gene program in GSCs and GBMO that are enriched for AHR. We find that AHR is required for GSC self-renewal, GBMO expansion, radial glia-like cell proliferation, and expression of immune mediators seen in the mesenchymal subtype. CRISPR-Cas9 genetic ablation and pharmacological inhibition revealed that AHR regulates genes linked to intrinsic immunity, proliferation, and migration in GBMO. Genomic analysis of GBMO treated with AHR inhibitors identified expression signatures and candidate markers associated with survival of gliomas. <h3>Conclusions:</h3> Our work defines the glioma intrinsic function of AHR in a model of early GBM formation, offering a rationale for clinical exploration of a potential ‘two-hit’ target of both GBM cells and infiltrating immune cells in patients with GBM expressing high levels of AHR. <b>Disclosure:</b> Fumihiro Watanabe has nothing to disclose. Ethan Hollingsworth has nothing to disclose. Dr. Bartley has nothing to disclose. Paula Schiapparelli has nothing to disclose. Alfredo Quiñones-Hinojosa has nothing to disclose. Dr. Imitola has received personal compensation in the range of $500-$4,999 for serving as a Consultant for Novartis . The institution of Dr. Imitola has received research support from biogen. Dr. Imitola has a non-compensated relationship as a Board Member with National MS Society that is relevant to AAN interests or activities. Dr. Imitola has a non-compensated relationship as a Committee Member with International Society for Stem Cell Research that is relevant to AAN interests or activities.

Cryo-EM structure of the cytosolic AhR complex

Aryl hydrocarbon receptor (AhR) is an important ligand-activated transcription factor involved in the regulation of various important physiological functions. Here, we report the cryo-EM structures of the Hsp90-AhR-p23 complex with or without bound XAP2, where the structure of the mouse AhR PAS-B domain is resolved. A highly conserved bridge motif of AhR is responsible for the interaction with the Hsp90 dimeric lumen. The ligand-free AhR PAS-B domain is attached to the Hsp90 dimer and is stabilized in the complex with bound XAP2. In addition, the DE-loop and a group of conserved pocket inner residues in the AhR PAS-B domain are found to be important for ligand binding. These results reveal the structural basis of the biological functions of AhR. Moreover, the protein purification method presented here allows the isolation of stable mouse AhR protein, which could be used to develop high-sensitivity biosensors for environmental pollutant detection.

Indoxyl Sulfate Promotes Metastatic Characteristics of Ovarian Cancer Cells via Aryl Hydrocarbon Receptor–Mediated Downregulation of the Mas Receptor

Although platinum-combination chemotherapy shows a high response rate at the primary site, epithelial ovarian cancer (EOC) treatment remains challenging because of tumor recurrence and metastasis. Recent studies have revealed that chemotherapy paradoxically promotes cancer cell survival, proliferation, and metastasis, although the reason for this remains unclear. The underlying molecular mechanisms that contribute to chemotherapy-induced metastasis need to be elucidated to establish effective therapeutic strategies. Acute kidney injury is a known side effect of cisplatin treatment, and kidney dysfunction results in the accumulation of uremic toxins in the serum. The present study aimed to investigate whether indoxyl sulfate (IS), a representative uremic toxin, affects the pathophysiology of EOC. In this study, IS reduced the expression of Mas receptor (MasR) in cultured human EOC cells. Both knockdown of the aryl hydrocarbon receptor (AhR), which is an intracellular IS receptor, and inhibition of AhR function suppressed IS-mediated downregulation of MasR in SK-OV-3 cells. IS induced the phosphorylation of signal transducer and activator of transcription 3 (STAT3) in an AhR-dependent manner. Inhibition of the STAT3 pathway or reactive oxygen species production suppressed the IS-mediated reduction of MasR. IS stimulated cell migration and invasion of SK-OV-3 cells in an AhR-dependent manner. Cisplatin-nephropathy model mice exhibited elevated levels of serum IS accompanied by elevated levels of blood urea nitrogen and serum creatinine. Furthermore, intraperitoneal administration of IS in mice promoted tumor growth and metastasis. Finally, we found that the MasR agonist Ang-(1-7) suppressed the IS-mediated effects on cell proliferation, migration, and invasion of SK-OV-3 cells. However, the knockdown of MasR expression by specific small interfering RNA in the absence of IS resulted in only minimal promotion of cell migration and invasion. These findings demonstrate that IS promotes malignancy in ovarian cancer via AhR-mediated downregulation of MasR function, whereas Ang-(1-7) attenuates this effect, thereby suggesting that Ang-(1-7) could provide a future treatment strategy for this cancer type.

Environmental exposure and the role of AhR in the tumor microenvironment of breast cancer.

Activation of the aryl hydrocarbon receptor (AhR) through environmental exposure to chemicals including polycyclic aromatic hydrocarbons (PAHs) and polychlorinated dibenzo-p-dioxins (PCDDs) can lead to severe adverse health effects and increase the risk of breast cancer. This review considers several mechanisms which link the tumor promoting effects of environmental pollutants with the AhR signaling pathway, contributing to the development and progression of breast cancer. We explore AhR's function in shaping the tumor microenvironment, modifying immune tolerance, and regulating cancer stemness, driving breast cancer chemoresistance and metastasis. The complexity of AhR, with evidence for both oncogenic and tumor suppressor roles is discussed. We propose that AhR functions as a "molecular bridge", linking disproportionate toxin exposure and policies which underlie environmental injustice with tumor cell behaviors which drive poor patient outcomes.

Proper modulation of AHR signaling is necessary for establishing neural connectivity and oligodendrocyte precursor cell development in the embryonic zebrafish brain.

2,3,7,8-tetrachlorodibenzo-[p]-dioxin (TCDD) is a persistent global pollutant that exhibits a high affinity for the aryl hydrocarbon receptor (AHR), a ligand activated transcription factor. Epidemiological studies have associated AHR agonist exposure with multiple human neuropathologies. Consistent with the human data, research studies using laboratory models have linked pollutant-induced AHR activation to disruptions in learning and memory as well as motor impairments. Our understanding of endogenous AHR functions in brain development is limited and, correspondingly, scientists are still determining which cell types and brain regions are sensitive to AHR modulation. To identify novel phenotypes resulting from pollutant-induced AHR activation and ahr2 loss of function, we utilized the optically transparent zebrafish model. Early embryonic TCDD exposure impaired embryonic brain morphogenesis, resulted in ventriculomegaly, and disrupted neural connectivity in the optic tectum, habenula, cerebellum, and olfactory bulb. Altered neural network formation was accompanied by reduced expression of synaptic vesicle 2. Loss of ahr2 function also impaired nascent network development, but did not affect gross brain or ventricular morphology. To determine whether neural AHR activation was sufficient to disrupt connectivity, we used the Gal4/UAS system to express a constitutively active AHR specifically in differentiated neurons and observed disruptions only in the cerebellum; thus, suggesting that the phenotypes resulting from global AHR activation likely involve multiple cell types. Consistent with this hypothesis, we found that TCDD exposure reduced the number of oligodendrocyte precursor cells and their derivatives. Together, our findings indicate that proper modulation of AHR signaling is necessary for the growth and maturation of the embryonic zebrafish brain.

Aryl hydrocarbon receptor activation drives polymorphonuclear myeloid-derived suppressor cell response and efficiently attenuates experimental Sjögren's syndrome.

Myeloid-derived suppressor cells (MDSCs) comprise heterogeneous myeloid cell populations with immunosuppressive capacity that contribute to immune regulation and tolerance induction. We previously reported impaired MDSC function in patients with primary Sjögren's syndrome (pSS) and mice with experimental SS (ESS). However, the molecular mechanisms underlying MDSC dysfunction remain largely unclear. In this study, we first found that aryl hydrocarbon receptor (AhR) was highly expressed by human and murine polymorphonuclear MDSCs (PMN-MDSCs). Indole-3-propionic acid (IPA), a natural AhR ligand produced from dietary tryptophan, significantly promoted PMN-MDSC differentiation and suppressive function on CD4+ T cells. In contrast, feeding a tryptophan-free diet resulted in a decreased PMN-MDSC response, a phenotype that could be reversed by IPA supplementation. The functional importance of PMN-MDSCs was demonstrated in ESS mice by using a cell-depletion approach. Notably, AhR expression was reduced in PMN-MDSCs during ESS development, while AhR antagonism resulted in exacerbated ESS pathology and dysregulated T effector cells, which could be phenocopied by a tryptophan-free diet. Interferon regulatory factor 4 (IRF4), a repressive transcription factor, was upregulated in PMN-MDSCs during ESS progression. Chromatin immunoprecipitation analysis revealed that IRF4 could bind to the promoter region of AhR, while IRF4 deficiency markedly enhanced AhR-mediated PMN-MDSC responses. Furthermore, dietary supplementation with IPA markedly ameliorated salivary glandular pathology in ESS mice with restored MDSC immunosuppressive function. Together, our results identify a novel function of AhR in modulating the PMN-MDSC response and demonstrate the therapeutic potential of targeting AhR for the treatment of pSS.

In-depth analysis of the interactions of various aryl hydrocarbon receptor ligands from a computational perspective

Aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that acts as a machinery that controls the expression of many genes, including cytochrome P450 CYP1A1, CYP1A2 and CYP1B1. It plays a principal role in numerous biological and toxicological functions, making it a promising target for developing therapeutic agents. Several novel small molecules targeting the AhR signaling pathway are currently under investigation as antitumor agents. Some have already advanced into clinical trials in patients with various tumors. Activation of AhR by diverse chemicals either endogenous or exogenous is initiated by the binding of these ligands to the PAS-B domain, which modulates AhR functions. There is, however, limited information about how various ligands interact with the PAS-B domain for activating or inhibiting the AhR. To better understand the mode of action of AhR agonists/antagonists. The current work proposes a combination of several computational tools to build dynamical models for the PAS-B domain bound to different ligands in mouse and human. Our findings reveal the essential roles of specific PAS-B residues (e.g., S365, V381& Q383), which mediate the AhR ligand-binding process. Our results also explain how these residues regulate the promiscuity of AhR in accommodating various chemicals in its binding PAS-B ligand-binding pocket.

The aryl hydrocarbon receptor directs the differentiation of murine progenitor blastomeres

Key regulatory decisions during cleavage divisions in mammalian embryogenesis determine the fate of preimplantation embryonic cells. Single-cell RNA sequencing of early-stage—2-cell, 4-cell, and 8-cell—blastomeres show that the aryl hydrocarbon receptor (AHR), traditionally considered as an environmental sensor, directs blastomere differentiation. Disruption of AHR functions in Ahr knockout embryos or in embryos from dams exposed to dioxin, the prototypic xenobiotic AHR agonist, significantly impairs blastocyst formation, causing repression and loss of transcriptional heterogeneity of OCT4 and CDX2 and incidence of nonspecific downregulation of pluripotency. Trajectory—the path of differentiation—and gene variability analyses further confirm that deregulation of OCT4 functions and changes of transcriptional heterogeneity resulting from disruption of AHR functions restrict the emergence of differentiating blastomeres in 4-cell embryos. It appears that AHR directs the differentiation of progenitor blastomeres and that disruption of preimplantation AHR functions may significantly perturb embryogenesis leading to long-lasting conditions at the heart of disease in offspring’s adulthood.

Loss of the aryl hydrocarbon receptor increases tumorigenesis in p53-deficient mice

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that regulates cell fate via activation of a diverse set of genes. There are conflicting reports describing the role of AhR in cancer. AhR-knockout mice do not develop tumors spontaneously, yet the AhR can act as a tumor suppressor in certain contexts. Loss of tumor suppression by p53 is common in human cancer. To investigate AhR function in the absence of p53, we generated mice lacking both AhR and p53. Mice deficient for AhR and p53 had shortened lifespan, increased tumorigenesis, and an altered tumor spectrum relative to control mice lacking only p53. In addition, knockout of both AhR and p53 resulted in reduced embryonic survival and neonatal fitness. We also examined the consequences of loss of AhR in p53-heterozygous mice and observed a significantly reduced lifespan and enhanced tumor burden. These findings reveal an important role for the AhR as a tumor suppressor in the absence of p53 signaling and support the development of anti-cancer therapeutics that would promote the tumor suppressive actions of the AhR.

Aryl hydrocarbon receptor and Kr\xfcppel like factor 10 mediate a transcriptional axis modulating immune homeostasis in mosquitoes

Immune responses require delicate controls to maintain homeostasis while executing effective defense. Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor. The Krüppel-like factor 10 (KLF10) is a C2H2 zinc-finger containing transcription factor. The functions of mosquito AhR and KLF10 have not been characterized. Here we show that AhR and KLF10 constitute a transcriptional axis to modulate immune responses in mosquito Anopheles gambiae. The manipulation of AhR activities via agonists or antagonists repressed or enhanced the mosquito antibacterial immunity, respectively. KLF10 was recognized as one of the AhR target genes in the context. Phenotypically, silencing KLF10 reversed the immune suppression caused by the AhR agonist. The transcriptome comparison revealed that silencing AhR and KLF10 plus challenge altered the expression of 2245 genes in the same way. The results suggest that KLF10 is downstream of AhR in a transcriptional network responsible for immunomodulation. This AhR–KLF10 axis regulates a set of genes involved in metabolism and circadian rhythms in the context. The axis was required to suppress the adverse effect caused by the overactivation of the immune pathway IMD via the inhibitor gene Caspar silencing without a bacterial challenge. These results demonstrate that the AhR–KLF10 axis mediates an immunoregulatory transcriptional network as a negative loop to maintain immune homeostasis.