Aryl Hydrocarbon Receptor Signaling Synergizes with TLR/NF-κB-Signaling for Induction of IL-22 Through Canonical and Non-Canonical AhR Pathways

Mechanisms underlying aryl hydrocarbon receptor-driven divergent macrophage function

Macrophages play an essential role in the innate immune system by differentiating into functionally diverse subsets in order to fight infection, repair damaged tissues, and regulate inappropriate immune responses. This functional diversity stems from their ability to adapt and respond to signals in the environment, which is in part mediated through aryl hydrocarbon receptor (AHR)-signaling. AHR, an environmental sensor, can be activated by various ligands, ranging from environmental contaminants to microbially derived tryptophan metabolites. This review discusses what is currently known about how AHR-signaling influences macrophage differentiation, polarization, and function. By discussing studies that are both consistent and divergent, our goal is to highlight the need for future research on the mechanisms by which AHR acts as an immunological switch in macrophages. Ultimately, understanding the contexts in which AHR-signaling promotes and/or inhibits differentiation, proinflammatory functions, and immunoregulatory functions, will help uncover functional predictions of immunotoxicity following exposure to environmental chemicals as well as better design AHR-targeted immunotherapies.

Gene-Environment Interactions: My Unique Journey

I am deeply honored to be invited to write this scientific autobiography. As a physician-scientist, pediatrician, molecular biologist, and geneticist, I have authored/coauthored more than 600 publications in the fields of clinical medicine, biochemistry, biophysics, pharmacology, drug metabolism, toxicology, molecular biology, cancer, standardized gene nomenclature, developmental toxicology and teratogenesis, mouse genetics, human genetics, and evolutionary genomics. Looking back, I think my career can be divided into four distinct research areas, which I summarize mostly chronologically in this article: (a) discovery and characterization of the AHR/CYP1 axis, (b) pharmacogenomics and genetic prediction of response to drugs and other environmental toxicants, (c) standardized drug-metabolizing gene nomenclature based on evolutionary divergence, and (d) discovery and characterization of the SLC39A8 gene encoding the ZIP8 metal cation influx transporter. Collectively, all four topics embrace gene-environment interactions, hence the title of my autobiography.

Connecting gut microbiome changes with fish health conditions in juvenile Atlantic cod (Gadus morhua) exposed to dispersed crude oil

Polycyclic aromatic hydrocarbons found in crude oil can impair fish health following sublethal exposure. However, the dysbiosis of microbial communities within the fish host and influence it has on the toxic response of fish following exposure has been less characterized, particularly in marine species. To better understand the effect of dispersed crude oil (DCO) on juvenile Atlantic cod (Gadus morhua) microbiota composition and potential targets of exposure within the gut, fish were exposed to 0.05 ppm DCO for 1, 3, 7, or 28 days and 16 S metagenomic and metatranscriptomic sequencing on the gut and RNA sequencing on intestinal content were conducted. In addition to assessing species composition, richness, and diversity from microbial gut community analysis and transcriptomic profiling, the functional capacity of the microbiome was determined. Mycoplasma and Aliivibrio were the two most abundant genera after DCO exposure and Photobacterium the most abundant genus in controls, after 28 days. Metagenomic profiles were only significantly different between treatments after a 28-day exposure. The top identified pathways were involved in energy and the biosynthesis of carbohydrates, fatty acids, amino acids, and cellular structure. Biological processes following fish transcriptomic profiling shared common pathways with microbial functional annotations such as energy, translation, amide biosynthetic process, and proteolysis. There were 58 differently expressed genes determined from metatranscriptomic profiling after 7 days of exposure. Predicted pathways that were altered included those involved in translation, signal transduction, and Wnt signaling. EIF2 signaling was consistently dysregulated following exposure to DCO, regardless of exposure duration, with impairments in IL-22 signaling and spermine and spermidine biosynthesis in fish after 28 days. Data were consistent with predictions of a potentially reduced immune response related to gastrointestinal disease. Herein, transcriptomic-level responses helped explain the relevance of differences in gut microbial communities in fish following DCO exposure.

Deciphering the roles of aryl hydrocarbon receptor (AHR) in regulating carcinogenesis

Aryl hydrocarbon receptor (AHR) is a ligand-dependent receptor that belongs to the superfamily of basic helix-loop-helix (bHLH) transcription factors. The activation of the canonical AHR signaling pathway is known to induce the expression of cytochrome P450 enzymes, facilitating the detoxification metabolism in the human body. Additionally, AHR could interact with various signaling pathways such as epidermal growth factor receptor (EGFR), signal transducer and activator of transcription 3 (STAT3), hypoxia-inducible factor-1α (HIF-1α), nuclear factor ekappa B (NF-κβ), estrogen receptor (ER), and androgen receptor (AR) signaling pathways. Over the past 30 years, several studies have reported that various chemical, physical, or biological agents, such as tobacco, hydrocarbon compounds, industrial and agricultural chemical wastes, drugs, UV, viruses, and other toxins, could affect AHR expression or activity, promoting cancer development. Thus, it is valuable to overview how these factors regulate AHR-mediated carcinogenesis. Current findings have reported that many compounds could act as AHR ligands to drive the expressions of AHR-target genes, such as CYP1A1, CYP1B1, MMPs, and AXL, and other targets that exert a pro-proliferation or anti-apoptotic effect, like XIAP. Furthermore, some other physical and chemical agents, such as UV and 3-methylcholanthrene, could promote AHR signaling activities, increasing the signaling activities of a few oncogenic pathways, such as the phosphatidylinositol 3-kinase/protein kinase B (PI3K/AKT) and mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathways. Understanding how various factors regulate AHR-mediated carcinogenesis processes helps clinicians and scientists plan personalized therapeutic strategies to improve anti-cancer treatment efficacy. As many studies that have reported the roles of AHR in regulating carcinogenesis are preclinical or observational clinical studies that did not explore the detailed mechanisms of how different chemical, physical, or biological agents promote AHR-mediated carcinogenesis processes, future studies should focus on conducting large-scale and functional studies to unravel the underlying mechanism of how AHR interacts with different factors in regulating carcinogenesis processes.

Aryl hydrocarbon receptor (AHR): towards understanding intestinal microbial ligands including vitamin B12 and folic acid as natural antagonists

AHR has been identified as ligand-modulated transcription factor and environmental sensor. However, explanation of its multiple agonistic and antagonistic ligands is far from complete. Studies of AHR's role in host-microbiome interaction are currently a fruitful area of research. Microbial products and virulence factors have been identified as AHR agonists. In steady state they are involved in safeguarding intestinal barrier integrity. When virulence factors from pathogenic bacteria are identified by AHR of intestinal immune cells, anti-microbial defense mechanisms are activated by generating reactive oxygen species (ROS) in intestinal epithelial cells and recruited immune cells. ROS production has to be strictly controlled, and anti-inflammatory responses have to be initiated timely in the resolution phase of inflammation to avoid tissue damage and chronic inflammatory responses. Surprisingly, bacteria-generated vitamin B12/cobalamin and vitamin B9/folic acid have been identified as natural AHR antagonists, stimulating the interest of biochemists. Hints for AHR-cobalamin antagonism are pointing to cobalamin-dependent enzymes leading to alterations of TCA cycle intermediates, and TCDD-mediated loss of serum cobalamin. Although we are still at the beginning to understand mechanisms, it is likely that scientific efforts are on a rewarding path to understand novel AHR functions.

Transcriptomic signatures reveal a shift towards an anti-inflammatory gene expression profile but also the induction of type I and type II interferon signaling networks through aryl hydrocarbon receptor activation in murine macrophages

Introduction

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that regulates a broad range of target genes involved in the xenobiotic response, cell cycle control and circadian rhythm. AhR is constitutively expressed in macrophages (Mϕ), acting as key regulator of cytokine production. While proinflammatory cytokines, i.e., IL-1β, IL-6, IL-12, are suppressed through AhR activation, anti-inflammatory IL-10 is induced. However, the underlying mechanisms of those effects and the importance of the specific ligand structure are not yet completely understood.

Methods

Therefore, we have compared the global gene expression pattern in activated murine bone marrow-derived macrophages (BMMs) subsequently to exposure with either benzo[a]pyrene (BaP) or indole-3-carbinol (I3C), representing high-affinity vs. low-affinity AhR ligands, respectively, by means of mRNA sequencing. AhR dependency of observed effects was proved using BMMs from AhR-knockout (Ahr^-/-) mice.

Results and discussion

In total, more than 1,000 differentially expressed genes (DEGs) could be mapped, covering a plethora of AhR-modulated effects on basal cellular processes, i.e., transcription and translation, but also immune functions, i.e., antigen presentation, cytokine production, and phagocytosis. Among DEGs were genes that are already known to be regulated by AhR, i.e., Irf1, Ido2, and Cd84. However, we identified DEGs not yet described to be AhR-regulated in Mϕ so far, i.e., Slpi, Il12rb1, and Il21r. All six genes likely contribute to shifting the Mϕ phenotype from proinflammatory to anti-inflammatory. The majority of DEGs induced through BaP were not affected through I3C exposure, probably due to higher AhR affinity of BaP in comparison to I3C. Mapping of known aryl hydrocarbon response element (AHRE) sequence motifs in identified DEGs revealed more than 200 genes not possessing any AHRE, and therefore being not eligible for canonical regulation. Bioinformatic approaches modeled a central role of type I and type II interferons in the regulation of those genes. Additionally, RT-qPCR and ELISA confirmed a AhR-dependent expressional induction and AhR-dependent secretion of IFN-γ in response to BaP exposure, suggesting an auto- or paracrine activation pathway of Mϕ.

Aryl Hydrocarbon Receptor Activation Ameliorates Acute Respiratory Distress Syndrome through Regulation of Th17 and Th22 Cells in the Lungs

Acute respiratory distress syndrome (ARDS) is triggered by a variety of insults, including bacterial and viral infections, and this leads to high mortality. While the role of the aryl hydrocarbon receptor (AhR) in mucosal immunity is being increasingly recognized, its function during ARDS is unclear. In the current study, we investigated the role of AhR in LPS-induced ARDS. AhR ligand, indole-3-carbinol (I3C), attenuated ARDS which was associated with a decrease in CD4+ RORγt +IL-17a+IL-22+ pathogenic Th17 cells, but not CD4+RORγt +IL-17a+IL-22- homeostatic Th 17 cells, in the lungs. AhR activation also led to a significant increase in CD4+IL-17a-IL-22+ Th22 cells. I3C-mediated Th22 cell expansion was dependent on the AhR expression on RORγt+ cells. AhR activation downregulated miR-29b-2-5p in immune cells from the lungs, which in turn downregulated RORc expression and upregulated IL-22. Collectively, the current study suggests that AhR activation can attenuate ARDS and may serve as a therapeutic modality by which to treat this complex disorder. IMPORTANCE Acute respiratory distress syndrome (ARDS) is a type of respiratory failure that is triggered by a variety of bacterial and viral infections, including the coronavirus SARS-CoV2. ARDS is associated with a hyperimmune response in the lungs that which is challenging to treat. Because of this difficulty, approximately 40% of patients with ARDS die. Thus, it is critical to understand the nature of the immune response that is functional in the lungs during ARDS as well as approaches by which to attenuate it. AhR is a transcription factor that is activated by a variety of endogenous and exogenous environmental chemicals as well as bacterial metabolites. While AhR has been shown to regulate inflammation, its role in ARDS is unclear. In the current study, we provide evidence that AhR activation can attenuate LPS-mediated ARDS through the activation of Th22 cells in the lungs, which are regulated through miR-29b-2-5p. Thus, AhR can be targeted to attenuate ARDS.

Polycyclic aromatic hydrocarbons in urban particle matter exacerbate movement disorder after ischemic stroke via potentiation of neuroinflammation

BACKGROUND

A recent epidemiological study showed that air pollution is closely involved in the prognosis of ischemic stroke. We and others have reported that microglial activation in ischemic stroke plays an important role in neuronal damage. In this study, we investigated the effects of urban aerosol exposure on neuroinflammation and the prognosis of ischemic stroke using a mouse photothrombotic model.

RESULTS

When mice were intranasally exposed to CRM28, urban aerosols collected in Beijing, China, for 7 days, microglial activation was observed in the olfactory bulb and cerebral cortex. Mice exposed to CRM28 showed increased microglial activity and exacerbation of movement disorder after ischemic stroke induction. Administration of core particles stripped of attached chemicals from CRM28 by washing showed less microglial activation and suppression of movement disorder compared with CRM28-treated groups. CRM28 exposure did not affect the prognosis of ischemic stroke in null mice for aryl hydrocarbon receptor, a polycyclic aromatic hydrocarbon (PAH) receptor. Exposure to PM2.5 collected at Yokohama, Japan also exacerbated movement disorder after ischemic stroke.

CONCLUSION

Particle matter in the air is involved in neuroinflammation and aggravation of the prognosis of ischemic stroke; furthermore, PAHs in the particle matter could be responsible for the prognosis exacerbation.

A Novel Aryl Hydrocarbon Receptor Antagonist HBU651 Ameliorates Peripheral and Hypothalamic Inflammation in High-Fat Diet-Induced Obese Mice

Obesity is a chronic peripheral inflammation condition that is strongly correlated with neurodegenerative diseases and associated with exposure to environmental chemicals. The aryl hydrocarbon receptor (AhR) is a ligand-activated nuclear receptor activated by environmental chemical, such as dioxins, and also is a regulator of inflammation through interacting with nuclear factor (NF)-κB. In this study, we evaluated the anti-obesity and anti-inflammatory activity of HBU651, a novel AhR antagonist. In BV2 microglia cells, HBU651 successfully inhibited lipopolysaccharide (LPS)-mediated nuclear localization of NF-κB and production of NF-κB-dependent proinflammatory cytokines, such as tumor necrosis factor (TNF)-α, interleukin (IL)-1β, and IL-6. It also restored LPS-induced mitochondrial dysfunction. While mice being fed a high-fat diet (HFD) induced peripheral and central inflammation and obesity, HBU651 alleviated HFD-induced obesity, insulin resistance, glucose intolerance, dyslipidemia, and liver enzyme activity, without hepatic and renal damage. HBU651 ameliorated the production of inflammatory cytokines and chemokines, proinflammatory Ly6c^high monocytes, and macrophage infiltration in the blood, liver, and adipose tissue. HBU651 also decreased microglial activation in the arcuate nucleus in the hypothalamus. These findings suggest that HBU651 may be a potential candidate for the treatment of obesity-related metabolic diseases.

Dietary Tryptophan-Mediated Aryl Hydrocarbon Receptor Activation by the Gut Microbiota Alleviates Escherichia coli-Induced Endometritis in Mice

Intestinal microbiota-mediated aryl hydrocarbon receptor (AhR) activation plays an important role in host-microbiota interactions and disease development. However, whether AhR activation mediates infection-induced inflammation in remote organs is not clear. The purpose of this study is to assess the effects and underlying mechanism of AhR activation and gut microbiota-mediated dietary tryptophan (Trp) metabolism on infection-induced inflammation using an Escherichia coli (E. coli)-induced endometritis model in mice. We found that AhR activation by 6-formylindolo (3,2-b) carbazole (Ficz), which is an AhR agonist derived from the photooxidation of Trp, alleviated E. coli-induced endometritis by repairing barrier function and inhibiting inflammatory responses, while inhibition of AhR by CH223191, which is a synthetic AhR antagonist, aggravated E. coli-induced endometritis. Gut dysbiosis damaged AhR activation and exacerbated E. coli-induced endometritis in mice, which responded to the reduced abundance of AhR ligand producers, such as Lactobacillus spp. Supplementation with dietary Trp ameliorated E. coli-induced endometritis in a microbiota-dependent manner, which was associated with the production of AhR ligands. Administration of AhR ligands, including indole and indole aldehyde, but not indole-3-propionic acid, rescued the protective effect of Trp on E. coli-induced endometritis in dysbiotic mice. Moreover, consumption of Lactobacillus reuteri (L. reuteri) containing AhR ligand-producing capability also alleviated E. coli-induced endometritis in mice in an AhR-dependent manner. Our results demonstrate that microbiota-mediated AhR activation is a key factor in fighting pathogen-caused inflammation, which leads to a potential strategy to regulate the gut microbiota and metabolism by dietary Trp or probiotics for the intervention of infectious diseases and reproductive health. IMPORTANCE Infection-induced endometritis is a common and frequently occurring disease in humans and animals. Accumulating evidence suggests an important role of the gut microbiota in the development of infection-induced inflammation. Whether and how gut microbiota-mediated AhR activation regulates the pathogenesis of pathogen-induced endometritis remains unknown. The current study found that AhR activation ameliorated E. coli-induced endometritis, and inhibition of AhR produced negative results. Gut dysbiosis reduced the abundance of AhR ligand producers including Lactobacillus spp., damaged AhR activation, and exacerbated E. coli-induced endometritis. Supplementation with dietary Trp, AhR ligands, and L. reuteri containing AhR ligand-producing capability alleviated E. coli-induced endometritis in mice. Our results suggest an important role of microbiota-mediated AhR activation in the pathogenesis of endometritis and provide potential strategies for the intervention of infectious diseases and reproductive health by regulating the gut microbiota and metabolism.

Aryl Hydrocarbon Receptor (AhR)-Mediated Signaling in iPSC-Derived Human Motor Neurons

Exposure to environmental pollutants and endogenous metabolites that induce aryl hydrocarbon receptor (AhR) expression has been suggested to affect cognitive development and, particularly in boys, also motor function. As current knowledge is based on epidemiological and animal studies, in vitro models are needed to better understand the effects of these compounds in the human nervous system at the molecular level. Here, we investigated expression of AhR pathway components and how they are regulated by AhR ligands in human motor neurons. Motor neurons generated from human induced pluripotent stem cells (hiPSCs) were characterized at the molecular level and by electrophysiology. mRNA levels of AhR target genes, CYP1A1 and CYP1B1 (cytochromes P450 1A1/1B1), and AhR signaling components were monitored in hiPSCs and in differentiated neurons following treatment with AhR ligands, 2,3,7,8,-tetrachlodibenzo-p-dioxin (TCDD), L-kynurenine (L-Kyn), and kynurenic acid (KA), by RT-qPCR. Changes in AhR cellular localization and CYP1A1 activity in neurons treated with AhR ligands were also assessed. The neurons we generated express motor neuron-specific markers and are functional. Transcript levels of CYP1B1, AhR nuclear translocators (ARNT1 and ARNT2) and the AhR repressor (AhRR) change with neuronal differentiation, being significantly higher in neurons than hiPSCs. In contrast, CYP1A1 and AhR transcript levels are slightly lower in neurons than in hiPSCs. The response to TCDD treatment differs in hiPSCs and neurons, with only the latter showing significant CYP1A1 up-regulation. In contrast, TCDD slightly up-regulates CYP1B1 mRNA in hiPSCs, but downregulates it in neurons. Comparison of the effects of different AhR ligands on AhR and some of its target genes in neurons shows that L-Kyn and KA, but not TCDD, regulate AhR expression and differently affect CYP1A1 and CYP1B1 expression. Finally, although TCDD does not significantly affect AhR transcript levels, it induces AhR protein translocation to the nucleus and increases CYP1A1 activity. This is in contrast to L-Kyn and KA, which either do not affect or reduce, respectively, CYP1A1 activity. Expression of components of the AhR signaling pathway are regulated with neuronal differentiation and are differently affected by TCDD, suggesting that pluripotent stem cells might be less sensitive to this toxin than neurons. Crucially, AhR signaling is affected differently by TCDD and other AhR ligands in human motor neurons, suggesting that they can provide a valuable tool for assessing the impact of environmental pollutants.