Tissue distribution of aryl hydrocarbon receptor in the intestine: Implication of putative roles in tumor suppression

Aryl hydrocarbon receptor activity in intestinal epithelial cells in the formation of colonic tertiary lymphoid tissues

After birth, the development of secondary lymphoid tissues (SLTs) in the colon is dependent on the expression of the aryl hydrocarbon receptor (AhR) in immune cells as a response to the availability of AhR ligands. However, little is known about how AhR activity from intestinal epithelial cells (IECs) may influence the development of tertiary lymphoid tissues (TLTs). As organized structures that develop at sites of inflammation or infection during adulthood, TLTs serve as localized centers of adaptive immune responses, and their presence has been associated with the resolution of inflammation and tumorigenesis in the colon. Here, we investigated the effect of the conditional loss of AhR activity in IECs in the formation and immune cell composition of TLTs in a model of acute inflammation. In females, loss of AhR activity in IECs reduced the formation of TLTs without significantly changing disease outcomes or immune cell composition within TLTs. In males lacking AhR expression in IECs, increased disease activity index, lower expression of functional-IEC genes, increased number of TLTs, increased T-cell density, and lower B- to T-cell ratio were observed. These findings may represent an unfavorable prognosis when exposed to dextran sodium sulfate (DSS)-induced epithelial damage compared with females. Sex and loss of IEC AhR also resulted in changes in microbial populations in the gut. Collectively, these data suggest that the formation of TLTs in the colon is influenced by sex and AhR expression in IECs.NEW & NOTEWORTHY This is the first research of its kind to demonstrate a clear connection between biological sex and the development of tertiary lymphoid tissues (TLT) in the colon. In addition, the research finds that in a preclinical model of inflammatory bowel disease, the expression of the aryl hydrocarbon receptor (AhR) influences the development of these structures in a sex-specific manner.

Aryl Hydrocarbon Receptor Activation Limits the Fatty Acid Synthesis and Subsequent "miR-193a-3p-HDAC3-FASN" Signals to Alleviate Intestinal Fibrosis

Intestinal fibrosis is a common complication of Crohn's disease and characterized by excessive extracellular matrix (ECM) deposition. The aryl hydrocarbon receptor (AhR) detects micronutrients and microbial metabolites in diet and can attenuate intestinal fibrosis with unclear mechanisms. In this study, AhR activation was demonstrated to downregulate the transcription of collagen I and fibronectin in a Sp1- but not Sp3- or AP-1-dependent manner. A suppressed fatty acid synthesis was highlighted using untargeted metabolomics analyses, and synthetic products, palmitic acid (PA), were used as the intermediary agent. After a screening study, fatty acid synthase (FASN) was identified as the main targeted protein, and AhR activation regulated "HDAC3-acetylation" signals but not glycosylation to enhance FASN degradation. Furthermore, results of bioinformatics analysis and others showed that after being activated, AhR targeted miR-193a-3p to control HDAC3 transcription. Collectively, AhR activation inhibited ECM deposition and alleviated intestinal fibrosis by limiting fatty acid synthesis subsequent to the inhibition of "miR-193a-3p-HDAC3-FASN" signals.

Tumor formation at ileocecal junction associated with interleukin-1β upregulation in aryl hydrocarbon receptor-deficient mouse

Aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor. We previously reported spontaneous ileocecal tumorigenesis in AhR-deficient mice after the age of 10 weeks, which originated in the confined area between ileum and cecum. This study aimed to investigate the underlying mechanism that causes tumor development at this particular location. To observe mucosal architecture in detail, tissues of ileocecal region were stained with methylene blue. Gene expression profile in the ileocecal tissue was compared with cecum. Immunohistochemical analysis was performed with ileocecal tissues using antibodies against ileum-specific Reg3β or cecum-specific Pitx2. In AhR+/+ mice and AhR+/- mice, that do not develop lesions, methylene blue staining revealed the gradually changing shape and arrangement of villi from ileum to cecum. It was also observed in AhR-deficient mice before developing lesions. Microarray-based analysis revealed abundant antimicrobial genes, such as Reg3, in the ileocecal tissue while FGFR2 and Pitx2 were specific to cecum. Immunohistochemical analysis of AhR-deficient mice indicated that lesions originated from the ileocecal junction, a boundary area between different epithelial types. Site-specific gene expression analysis revealed higher expression of IL-1β at the ileocecal junction compared with the ileum or cecum of 9-11-week-old AhR-deficient mice. These findings indicate that AhR plays a vital function in the ileocecal junction. Regulating AhR activity can potentially manage the stability of ileocecal tissue possessing cancer-prone characteristics. This investigation contributes to understanding homeostasis in different epithelial transitional tissues, frequently associated with pathological states.

The potential of aryl hydrocarbon receptor as receptors for metabolic changes in tumors

Cancer cells can alter their metabolism to meet energy and molecular requirements due to unfavorable environments with oxygen and nutritional deficiencies. Therefore, metabolic reprogramming is common in a tumor microenvironment (TME). Aryl hydrocarbon receptor (AhR) is a ligand-activated nuclear transcription factor, which can be activated by many exogenous and endogenous ligands. Multiple AhR ligands can be produced by both TME and tumor cells. By attaching to various ligands, AhR regulates cancer metabolic reprogramming by dysregulating various metabolic pathways, including glycolysis, lipid metabolism, and nucleotide metabolism. These regulated pathways greatly contribute to cancer cell growth, metastasis, and evading cancer therapies; however, the underlying mechanisms remain unclear. Herein, we review the relationship between TME and metabolism and describe the important role of AhR in cancer regulation. We also focus on recent findings to discuss the idea that AhR acts as a receptor for metabolic changes in tumors, which may provide new perspectives on the direction of AhR research in tumor metabolic reprogramming and future therapeutic interventions.

Aryl hydrocarbon receptor (AhR) and pregnane X receptor (PXR) play both distinct and common roles in the regulation of colon homeostasis and intestinal carcinogenesis

Both aryl hydrocarbon receptor (AhR) and pregnane X receptor (PXR) belong among key regulators of xenobiotic metabolism in the intestinal tissue. AhR in particular is activated by a wide range of environmental and dietary carcinogens. The data accumulated over the last two decades suggest that both of these transcriptional regulators play a much wider role in the maintenance of gut homeostasis, and that both transcription factors may affect processes linked with intestinal tumorigenesis. Intestinal epithelium is continuously exposed to a wide range of AhR, PXR and dual AhR/PXR ligands formed by intestinal microbiota or originating from diet. Current evidence suggests that specific ligands of both AhR and PXR can protect intestinal epithelium against inflammation and assist in the maintenance of epithelial barrier integrity. AhR, and to a lesser extent also PXR, have been shown to play a protective role against inflammation-induced colon cancer, or, in mouse models employing overactivation of Wnt/β-catenin signaling. In contrast, other evidence suggests that both receptors may contribute to modulation of transformed colon cell behavior, with a potential to promote cancer progression and/or chemoresistance. The review focuses on both overlapping and separate roles of the two receptors in these processes, and on possible implications of their activity within the context of intestinal tissue.

AhR and Wnt/β-Catenin Signaling Pathways and Their Interplay

As evolutionarily conserved signaling cascades, AhR and Wnt signaling pathways play a critical role in the control over numerous vital embryonic and somatic processes. AhR performs many endogenous functions by integrating its signaling pathway into organ homeostasis and into the maintenance of crucial cellular functions and biological processes. The Wnt signaling pathway regulates cell proliferation, differentiation, and many other phenomena, and this regulation is important for embryonic development and the dynamic balance of adult tissues. AhR and Wnt are the main signaling pathways participating in the control of cell fate and function. They occupy a central position in a variety of processes linked with development and various pathological conditions. Given the importance of these two signaling cascades, it would be interesting to elucidate the biological implications of their interaction. Functional connections between AhR and Wnt signals take place in cases of crosstalk or interplay, about which quite a lot of information has been accumulated in recent years. This review is focused on recent studies about the mutual interactions of key mediators of AhR and Wnt/β-catenin signaling pathways and on the assessment of the complexity of the crosstalk between the AhR signaling cascade and the canonical Wnt pathway.

Clinical implications of gut microbiota and cytokine responses in coronavirus disease prognosis

Objectives

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infects gut luminal cells through the angiotensin-converting enzyme-2 receptor and disrupts the gut microbiome. We investigated whether the gut microbiome in the early stage of SARS-CoV-2 infection was associated with the prognosis of coronavirus disease (COVID-19).

Methods

Thirty COVID-19 patients and 16 healthy controls were prospectively enrolled. Blood and stool samples and clinical details were collected on days 0 (enrollment), 7, 14, and 28. Participants were categorized into four groups by their clinical course.

Results

Gut microbiota composition varied during the clinical course of COVID-19 and was closely associated with cytokine levels (p=0.003). A high abundance of the genus Dialister (linear discriminant analysis [LDA] effect size: 3.97856, p=0.004), species Peptoniphilus lacrimalis (LDA effect size: 4.00551, p=0.020), and Anaerococcus prevotii (LDA effect size: 4.00885, p=0.007) was associated with a good prognosis. Starch, sucrose, and galactose metabolism was highly activated in the gut microbiota of the poor prognosis group. Glucose-lowering diets, including whole grains, were positively correlated with a good prognosis.

Conclusion

Gut microbiota may mediate the prognosis of COVID-19 by regulating cytokine responses and controlling glucose metabolism, which is implicated in the host immune response to SARS-CoV-2.

Aryl Hydrocarbon Receptor (AhR) Signaling in Colonic Cells and Tumors

The aryl hydrocarbon receptor (AhR) is overexpressed in many tumor types and exhibits tumor-specific tumor promoter and tumor suppressor-like activity. In colon cancer, most but not all studies suggest that the AhR exhibits tumor suppressor activity which is enhanced by AhR ligands acting as agonists. Our studies investigated the role of the AhR in colon tumorigenesis using wild-type and AhR-knockout mice, the inflammation model of colon tumorigenesis using mice treated with azoxymethane (AOM)/dextran sodium sulfate (DSS) and APCS580/+; KrasG12D/+ mice all of which form intestinal tumors. The effects of tissue-specific AhR loss in the intestine of the tumor-forming mice on colonic stem cells, organoid-initiating capacity, colon tumor formation and mechanisms of AhR-mediated effects were investigated. Loss of AhR enhanced stem cell and tumor growth and in the AOM/DSS model AhR-dependent suppression of FOXM1 and downstream genes was important for AhR-dependent anticancer activity. Furthermore, the effectiveness of interleukin-22 (IL22) in colonic epithelial cells was also dependent on AhR expression. IL22 induced phosphorylation of STAT3, inhibited colonic organoid growth, promoted colonic cell proliferation in vivo and enhanced DNA repair in AOM/DSS-induced tumors. In this mouse model, the AhR suppressed SOCS3 expression and enhanced IL22-mediated activation of STAT3, whereas the loss of the AhR increased levels of SOCS3 which in turn inhibited IL22-induced STAT3 activation. In the APCS580/+; KrasG12D/+ mouse model, the loss of the AhR enhanced Wnt signaling and colon carcinogenesis. Results in both mouse models of colon carcinogenesis were complemented by single cell transcriptomics on colonic intestinal crypts which also showed that AhR deletion promoted expression of FOXM1-regulated genes in multiple colonic cell subtypes. These results support the role of the AhR as a tumor suppressor-like gene in the colon.

Inhibition of Aryl Hydrocarbon Receptor (AhR) Expression Disrupts Cell Proliferation and Alters Energy Metabolism and Fatty Acid Synthesis in Colon Cancer Cells

Simple Summary

Cancer cells undergo metabolic modifications in order to meet their high energetic demand. The aryl hydrocarbon receptor (AhR) is a ligand-activated transcriptional factor primarily known as a xenobiotic sensor. However, this receptor seems to have a wide range of physiological roles in many processes including cell proliferation, migration or control of immune responses. AhR is often overexpressed in tumor cells of various tissue origin, and several studies have indicated that AhR may also contribute to regulation of cellular metabolism, including synthesis of fatty acids (FA), one of the major steps in metabolic transition. Potential links between the AhR and the control of tumor cell proliferation and metabolism thus deserve more attention.

Abstract

The aryl hydrocarbon receptor (AhR) plays a wide range of physiological roles in cellular processes such as proliferation, migration or control of immune responses. Several studies have also indicated that AhR might contribute to the regulation of energy balance or cellular metabolism. We observed that the AhR is upregulated in tumor epithelial cells derived from colon cancer patients. Using wild-type and the corresponding AhR knockout (AhR KO) variants of human colon cancer cell lines HCT116 and HT-29, we analyzed possible role(s) of the AhR in cell proliferation and metabolism, with a focus on regulation of the synthesis of fatty acids (FAs). We observed a decreased proliferation rate in the AhR KO cells, which was accompanied with altered cell cycle progression, as well as a decreased ATP production. We also found reduced mRNA levels of key enzymes of the FA biosynthetic pathway in AhR KO colon cancer cells, in particular of stearoyl-CoA desaturase 1 (SCD1). The loss of AhR was also associated with reduced expression and/or activity of components of the PI3K/Akt pathway, which controls lipid metabolism, and other lipogenic transcriptional regulators, such as sterol regulatory element binding transcription factor 1 (SREBP1). Together, our data indicate that disruption of AhR activity in colon tumor cells may, likely in a cell-specific manner, limit their proliferation, which could be linked with a suppressive effect on their endogenous FA metabolism. More attention should be paid to potential mechanistic links between overexpressed AhR and colon tumor cell metabolism.

Combinatory Exposure to Urolithin A, Alternariol, and Deoxynivalenol Affects Colon Cancer Metabolism and Epithelial Barrier Integrity in vitro

The human gastrointestinal tract is an important site of nutrient absorption and a crucial barrier against xenobiotics. It regularly faces “chemical cocktails” composed of food constituents, their human and microbial metabolites, and foodborne contaminants, such as mycotoxins. Hence, the colonic epithelium adapts to dietary molecules tuning its immune response, structural integrity, and metabolism to maintain intestinal homeostasis. While gut microbiota metabolites of berry ellagitannins, such as urolithin A (Uro A) might contribute to physiological epithelial barrier integrity, foodborne co-contaminating mycotoxins like alternariol (AOH) and deoxynivalenol (DON) could hamper epithelial function. Hence, we investigated the response of differentiated Caco-2 cells (clone C2BBe1) in vitro to the three compounds alone or in binary mixtures. In virtue of the possible interactions of Uro A, AOH, and DON with the aryl hydrocarbon receptor (AhR) pathway, potential effects on phase-I-metabolism enzymes and epithelial structural integrity were taken as endpoints for the evaluation. Finally, Liquid chromatography tandem mass spectrometry measurements elucidated the absorption, secretion, and metabolic capacity of the cells under single and combinatory exposure scenarios. Uro A and AOH as single compounds, and as a binary mixture, were capable to induce CYP1A1/1A2/1B1 enzymes triggered by the AhR pathway. In light of its ribosome inhibiting capacity, the trichothecene suppressed the effects of both dibenzo-α-pyrones. In turn, cellular responsiveness to Uro A and AOH could be sustained when co-exposed to DON-3-sulfate, instead of DON. Colonic epithelial structural integrity was rather maintained after incubation with Uro A and AOH: this was reinforced in the combinatory exposure scenario and disrupted by DON, an effect, opposed in combination. Passage through the cells as well as the metabolism of Uro A and AOH were rather influenced by co-exposure to DON, than by interaction with each other. Therefore, we conclude that although single foodborne bioactive substances individually could either support or disrupt the epithelial structure and metabolic capacity of colon cancer, exposure to chemical mixtures changes the experimental outcome and calls for the need of combinatory investigations for proper risk assessment.

New Insights Into Gut-Bacteria-Derived Indole and Its Derivatives in Intestinal and Liver Diseases

The interaction between host and microorganism widely affects the immune and metabolic status. Indole and its derivatives are metabolites produced by the metabolism of tryptophan catalyzed by intestinal microorganisms. By activating nuclear receptors, regulating intestinal hormones, and affecting the biological effects of bacteria as signaling molecules, indole and its derivatives maintain intestinal homeostasis and impact liver metabolism and the immune response, which shows good therapeutic prospects. We reviewed recent studies on indole and its derivatives, including related metabolism, the influence of diets and intestinal commensal bacteria, and the targets and mechanisms in pathological conditions, especially progress in therapeutic strategies. New research insights into indoles will facilitate a better understanding of their druggability and application in intestinal and liver diseases.

Correlation between aryl hydrocarbon receptor and IL-17+ and Foxp3+ T-cell infiltration in bladder cancer

Bladder cancer (BC) is one of the most prevalent cancers around the world and, if not treated well, has high morbidity and mortality. Many studies have indicated that there may be various roles for the aryl hydrocarbon receptor (AHR) in the immune system. The aim of this study was to determine the frequency of Foxp3⁺ regulatory T (Treg) and T helper 17 cells (Th17) in BC tissue in comparison with controls and determine the relationship between AHR, Foxp3⁺ Treg and Th17 cells in BC. A total of 40 patients with BC were enrolled in this study. The control group was selected from non-tumoural parts of bladder tissues from the patients who have undergone cystoscopy. The percentage of regulatory T cells (Foxp3⁺ /CD4⁺ ) and Th17 (IL-17⁺ /CD4⁺ ), as well as AHR⁺ cells in BC tissues and controls, were determined by immunohistochemistry. The results of this study showed that the number of Foxp3⁺ Treg and Th17 is significantly higher in bladder tumour tissues in comparison with non-tumoural tissues. Also, the percentage of AHR⁺ lymphocytes and AHR⁺ cells was increased significantly in bladder tumour tissues rather than non-tumoural tissues. This study also found a relation between AHR and Foxp3⁺ /CD4⁺ T lymphocytes ratio cells in BC. The percentage of Foxp3⁺ Tregs and AHR⁺ cells were significantly correlated with the grade and stage of BC. An increase in the percentage of Foxp3⁺ Treg and Th17 cells may play an important role in tumour immunity; and determining the relationship between AHR and differentiation of Th17/Foxp3⁺ Treg in BC can lead to a potential cancer therapeutic possibility.

Diet-Host-Microbiota Interactions Shape Aryl Hydrocarbon Receptor Ligand Production to Modulate Intestinal Homeostasis

The aryl hydrocarbon receptor (AhR) is a ligand-activated basic-helix-loop-helix transcription factor that binds structurally diverse ligands and senses cues from environmental toxicants and physiologically relevant dietary/microbiota-derived ligands. The AhR is an ancient conserved protein and is widely expressed across different tissues in vertebrates and invertebrates. AhR signaling mediates a wide range of cellular functions in a ligand-, cell type-, species-, and context-specific manner. Dysregulation of AhR signaling is linked to many developmental defects and chronic diseases. In this review, we discuss the emerging role of AhR signaling in mediating bidirectional host-microbiome interactions. We also consider evidence showing the potential for the dietary/microbial enhancement ofhealth-promoting AhR ligands to improve clinical pathway management in the context of inflammatory bowel diseases and colon tumorigenesis.

Camalexin, an indole phytoalexin, inhibits cell proliferation, migration, and mammosphere formation in breast cancer cells via the aryl hydrocarbon receptor

Breast cancer is the most commonly diagnosed cancer among women worldwide. Despite a variety of drugs available for the treatment of patients with breast cancer, drug resistance remains a significant clinical problem. Therefore, there is an urgent need to develop drugs with new mechanisms of action. Camalexin is the main indole phytoalexin in Arabidopsis thaliana and other crucifers. Camalexin inhibits the proliferation of various cancer cells. However, the mechanism by which camalexin inhibits cell proliferation remains unclear. In this study, we found that camalexin inhibited cell proliferation and migration of breast cancer cell lines. Furthermore, camalexin also suppressed breast cancer stem cell-derived mammosphere formation. We previously reported that the ligand-activated transcription factor aryl hydrocarbon receptor (AhR) agonist suppresses mammosphere formation. Several compounds with indole structures are known to act as AhR agonists. Therefore, we hypothesized that the inhibition of mammosphere formation by camalexin may involve AhR activation. We found that camalexin increased the nuclear translocation of AhR, AhR-mediated transcriptional activation, and expression of AhR target genes. In addition, camalexin suppressed mammosphere formation in AhR-expressing breast cancer cells more than in the breast cancer cells that lacked AhR expression. Taken together, the data demonstrate that camalexin is a novel AhR agonist and that the inhibition of cell proliferation, migration, and mammosphere formation by camalexin involves the activation of AhR. Our findings suggest that camalexin, an AhR agonist, may be a novel therapeutic agent for breast cancer.

AHR in the intestinal microenvironment: safeguarding barrier function

Mammalian aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor that belongs to the basic helix-loop-helix (bHLH)-PAS family of transcription factors, which are evolutionarily conserved environmental sensors. In the absence of ligands, AHR resides in the cytoplasm in a complex with molecular chaperones such as HSP90, XAP2 and p23. Upon ligand binding, AHR translocates into the nuclear compartment, where it dimerizes with its partner protein, AHR nuclear translocator (ARNT), an obligatory partner for the DNA-binding and functional activity. Historically, AHR had mostly been considered as a key intermediary for the detrimental effects of environmental pollutants on the body. However, following the discovery of AHR-mediated functions in various immune cells, as well as the emergence of non-toxic 'natural' AHR ligands, this view slowly began to change, and the study of AHR-deficient mice revealed a plethora of important beneficial functions linked to AHR activation. This Review focuses on regulation of the AHR pathway and the barrier-protective roles AHR has in haematopoietic, as well as non-haematopoietic, cells within the intestinal microenvironment. It covers the nature of AHR ligands and feedback regulation of the AHR pathway, outlining the currently known physiological functions in immune, epithelial, endothelial and neuronal cells of the intestine.

Therapeutic effect of curcumin in gastrointestinal cancers: A comprehensive review

Gastrointestinal (GI) cancers with a high global prevalence are a leading cause of morbidity and mortality. Accordingly, there is a great need to develop efficient therapeutic approaches. Curcumin, a naturally occurring agent, is a promising compound with documented safety and anticancer activities. Recent studies have demonstrated the activity of curcumin in the prevention and treatment of different cancers. According to systematic studies on curcumin use in various diseases, it can be particularly effective in GI cancers because of its high bioavailability in the gastrointestinal tract. Nevertheless, the clinical applications of curcumin are largely limited because of its low solubility and low chemical stability in water. These limitations may be addressed by the use of relevant analogues or novel delivery systems. Herein, we summarize the pharmacological effects of curcumin against GI cancers. Moreover, we highlight the application of curcumin's analogues and novel delivery systems in the treatment of GI cancers.

Effect of diet and intestinal AhR expression on fecal microbiome and metabolomic profiles

BACKGROUND

Diet, loss of aryl hydrocarbon receptor (AhR) expression and their modification of the gut microbiota community composition and its metabolites affect the development of colorectal cancer (CRC). However, the concordance between fecal microbiota composition and the fecal metabolome is poorly understood. Mice with specific AhR deletion (AhRKO) in intestinal epithelial cell and their wild-type littermates were fed a low-fat diet or a high-fat diet. Shifts in the fecal microbiome and metabolome associated with diet and loss of AhR expression were assessed. Microbiome and metabolome data were integrated to identify specific microbial taxa that contributed to the observed metabolite shifts.

RESULTS

Our analysis shows that diet has a more pronounced effect on mouse fecal microbiota composition than the impact of the loss of AhR. In contrast, metabolomic analysis showed that the loss of AhR in intestinal epithelial cells had a more pronounced effect on metabolite profile compared to diet. Integration analysis of microbiome and metabolome identified unclassified Clostridiales, unclassified Desulfovibrionaceae, and Akkermansia as key contributors to the synthesis and/or utilization of tryptophan metabolites.

CONCLUSIONS

Akkermansia are likely to contribute to the synthesis and/or degradation of tryptophan metabolites. Our study highlights the use of multi-omic analysis to investigate the relationship between the microbiome and metabolome and identifies possible taxa that can be targeted to manipulate the microbiome for CRC treatment.

Host-microbiome interactions: the aryl hydrocarbon receptor as a critical node in tryptophan metabolites to brain signaling

Tryptophan (Trp) is not only a nutrient enhancer but also has systemic effects. Trp metabolites signaling through the well-known aryl hydrocarbon receptor (AhR) constitute the interface of microbiome-gut-brain axis. However, the pathway through which Trp metabolites affect central nervous system (CNS) function have not been fully elucidated. AhR participates in a broad variety of physiological and pathological processes that also highly relevant to intestinal homeostasis and CNS diseases. Via the AhR-dependent mechanism, Trp metabolites connect bidirectional signaling between the gut microbiome and the brain, mediated via immune, metabolic, and neural (vagal) signaling mechanisms, with downstream effects on behavior and CNS function. These findings shed light on the complex Trp regulation of microbiome-gut-brain axis and add another facet to our understanding that dietary Trp is expected to be a promising noninvasive approach for alleviating systemic diseases.

The Ah Receptor: Adaptive Metabolism, Ligand Diversity, and the Xenokine Model

The Ah receptor (AHR) has been studied for almost five decades. Yet, we still have many important questions about its role in normal physiology and development. Moreover, we still do not fully understand how this protein mediates the adverse effects of a variety of environmental pollutants, such as the polycyclic aromatic hydrocarbons (PAHs), the chlorinated dibenzo-p-dioxins ("dioxins"), and many polyhalogenated biphenyls. To provide a platform for future research, we provide the historical underpinnings of our current state of knowledge about AHR signal transduction, identify a few areas of needed research, and then develop concepts such as adaptive metabolism, ligand structural diversity, and the importance of proligands in receptor activation. We finish with a discussion of the cognate physiological role of the AHR, our perspective on why this receptor is so highly conserved, and how we might think about its cognate ligands in the future.

Effects of high-fat diet and intestinal aryl hydrocarbon receptor deletion on colon carcinogenesis

Consumption of a high-fat diet has been associated with an increased risk of developing colorectal cancer (CRC). However, the effects of the interaction between dietary fat content and the aryl hydrocarbon receptor (AhR) on colorectal carcinogenesis remain unclear. Mainly known for its role in xenobiotic metabolism, AhR has been identified as an important regulator for maintaining intestinal epithelial homeostasis. Although previous research using whole body AhR knockout mice has revealed an increased incidence of colon and cecal tumors, the unique role of AhR activity in intestinal epithelial cells (IECs) and modifying effects of fat content in the diet at different stages of sporadic CRC development are yet to be elucidated. In the present study, we have examined the effects of a high-fat diet on IEC-specific AhR knockout mice in a model of sporadic CRC. Although loss of AhR activity in IECs significantly induced the development of premalignant lesions, in a separate experiment, no significant changes in colon mass incidence were observed. Moreover, consumption of a high-fat diet promoted cell proliferation in crypts at the premalignant colon cancer lesion stage and colon mass multiplicity as well as β-catenin expression and nuclear localization in actively proliferating cells in colon masses. Our data demonstrate the modifying effects of high-fat diet and AhR deletion in IECs on tumor initiation and progression.NEW & NOTEWORTHY Through the use of an intestinal-specific aryl hydrocarbon receptor (AhR) knockout mouse model, this study demonstrates that the expression of AhR in intestinal epithelial cells is required to reduce the formation of premalignant colon cancer lesions. Furthermore, consumption of a high-fat diet and the loss of AhR in intestinal epithelial cells influences the development of colorectal cancer at various stages.

Somatic Deletion in Exon 10 of Aryl Hydrocarbon Receptor Gene in Human GH-Secreting Pituitary Tumors

OBJECTIVE/PURPOSE

The aryl hydrocarbon receptor (AHR) pathway plays a critical role in the biology of Growth Hormone (GH)-secreting pituitary tumor (somatotropinoma). Germline rs2066853 AHR variant was found to be more frequent among acromegaly patients and associated with a more severe disease with larger invasive somatropinoma, and with resistance to somatostatin analogs treatment in patients living in polluted areas. However, no somatic changes in AHR gene have been reported so far in acromegaly patients. On that basis, the aim of the study was to assess at the somatic level the AHR gene status encompassing exon 10 region, also because of the high rate of variants found in this genomic region.

METHODS

A cohort of 13 patients aged 20-76 years with biochemical, clinical and histological diagnosis of somatotropinoma was studied. DNA and RNA from pituitary tumor histological samples have been extracted and analyzed by PCR and direct sequencing for AHR gene variants, and compared with corresponding patients' germline DNA as well as normal pituitary tissue as reference control.

RESULTS

A degenerated letter codes in the region corresponding to AHR exon 10 (c.1239-c.2056) was detected in somatotropinomas-derived DNA but not in that of matched germline and pituitary normal tissue. By multiple PCR and sequencing analysis, we observed amplification only before codon 1246 and after codon 1254, confirming the presence of a tumor-restricted somatic deletion in the 5' upstream region of AHR exon 10. Analysis of PCR-amplified cDNA revealed a wildtype sequence of exon 9 and 10 in normal pituitary tissue, and a wildtype sequence of exon 9 and 10 up to codon 1246 and no sequence after the deletion region (c.1246-c.1254) in 6 out of 9 tumor samples. Patients carrying the germline rs2066853 AHR variant showed no somatic LOH at the corresponding genetic locus.

CONCLUSION

This is the first demonstration of a recurrent somatic deletion in the exon 10 of the AHR gene in somatotropinomas. The functional impact of this genetic finding needs to be clarified.

The aryl hydrocarbon receptor: an environmental sensor integrating immune responses in health and disease

The environment, diet, microbiota and body's metabolism shape complex biological processes in health and disease. However, our understanding of the molecular pathways involved in these processes is still limited. The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that integrates environmental, dietary, microbial and metabolic cues to control complex transcriptional programmes in a ligand-specific, cell-type-specific and context-specific manner. In this Review, we summarize our current knowledge of AHR and the transcriptional programmes it controls in the immune system. Finally, we discuss the role of AHR in autoimmune and neoplastic diseases of the central nervous system, with a special focus on the gut immune system, the gut-brain axis and the therapeutic potential of targeting AHR in neurological disorders.

Cecal Tumorigenesis in Aryl Hydrocarbon Receptor-Deficient Mice Depends on Cecum-Specific Mitogen-Activated Protein Kinase Pathway Activation and Inflammation

The aryl hydrocarbon receptor (AhR) is a transcription factor known as a dioxin receptor. Recently, Ahr^-/- mice were revealed to develop cecal tumors with inflammation and Wnt/β-catenin pathway activation. However, whether β-catenin degradation is AhR dependent remains unclear. To determine whether other signaling pathways function in Ahr^-/- cecal tumorigenesis, we investigated histologic characteristics of the tumors and cytokine/chemokine production in tumors and Ahr^-/- peritoneal macrophages. AhR expression was also assessed in human colorectal carcinomas. Of the 28 Ahr^-/- mice, 10 developed cecal lesions by 50 weeks of age, an incidence significantly lower than previously reported. Cecal lesions of Ahr^-/- mice developed from serrated hyperplasia to adenoma/dysplasia-like neoplasia with enhanced proliferation. Macrophage and neutrophil infiltration into the lesions was also observed early in serrated hyperplasia, although adjacent mucosa was devoid of inflammation. Il1b, Il6, Ccl2, and Cxcl5 were up-regulated at lesion sites, whereas only IL-6 production increased in Ahr^-/- peritoneal macrophages after lipopolysaccharide + ATP stimulation. Neither Myc (alias c-myc) up-regulation nor β-catenin nuclear translocation was observed, unlike previously reported. Interestingly, enhanced phosphorylation of extracellular signal-regulated kinase, Src, and epidermal growth factor receptor and Amphiregulin up-regulation at Ahr^-/- lesion sites were detected. In human serrated lesions, however, AhR expression in epithelial cells was up-regulated despite morphologic similarity to Ahr^-/- cecal lesions. Our results suggest novel mechanisms underlying Ahr^-/- cecal tumorigenesis, depending primarily on cecum-specific mitogen-activated protein kinase pathway activation and inflammation.

2,3,7,8‑tetrachlorodibenzo‑p‑dioxin suppresses the growth of human colorectal cancer cells in vitro: Implication of the aryl hydrocarbon receptor signaling

Human colorectal cancer is the third most common cancer disease with a 5‑year survival rate of 55% in USA in 2016. The investigation to identify novel biomarker factors with molecular classification may provide notable clinical information to prolong the survival of patients with colorectal cancer. The aryl hydrocarbon receptor (AHR) binds the AHR nuclear translocator in the cytoplasm of various types of cells, including liver cells, and then binds to the xenobiotic responsive element on various genes. AHR was initially discovered via its ligand, the polychlorinated hydrocarbon, 2,3,7,8‑tetrachlorodibenzo‑p‑dioxin (TCDD). The present study was undertaken to determine whether TCDD, an agonist of AHR signaling, impacts the growth of RKO human colorectal cancer cells in vitro. Treatment with TCDD (0.1‑100 nM) revealed suppressive effects on colony formation and proliferation of RKO cells, and stimulated death of these cells with subconfluence. These effects of TCDD were abolished by pretreatment with CH223191, an inhibitor of AHR signaling. Western blot analysis demonstrated that TCDD treatment decreased AHR levels and elevated cytochrome P450 family 1 subfamily A member 1 (CYP1A1) levels, indicating a stimulation of AHR signaling. TCDD treatment caused an increase in nuclear factor‑κB p65 and β‑catenin levels, although it did not have an effect on Ras levels. Notably, TCDD treatment increased the levels of p53, retinoblastoma, p21 and regucalcin, which are depressors of carcinogenesis. Additionally, action of TCDD on cell proliferation and death were not revealed in regucalcin‑overexpressing RKO cells, and regucalcin overexpression depressed AHR signaling associated with CYP1A1 expression. Thus, AHR signaling suppresses the growth of colorectal cancer cells, indicating a role as a significant targeting molecule for colorectal cancer.

Negative association between serum aryl hydrocarbon receptor concentrations and β-cell function in patients with no history of diabetes undergoing coronary angiography

BACKGROUND

The aim of the present study was to investigate the association between serum aryl hydrocarbon receptor (AhR) levels and insulin resistance and β-cell function in patients undergoing coronary angiography with no history of diabetes.

METHODS

Patients with no history of diabetes who had undergone coronary angiography underwent an oral glucose tolerance test (OGTT) 2-4 weeks after discharge from hospital; blood samples were collected for measurements of glucose, insulin, and AhR. Patients' glucose regulation status was determined on the basis of the OGTT. Insulin resistance was assessed using the homeostasis model assessment of insulin resistance (HOMA-IR). β-Cell function was assessed using the insulinogenic index (IGI).

RESULTS

The study included 473 patients (mean (±SD) age 61 ±12 years, 81.8% male, mean body mass index 26.1 ±3.6 kg/m² ). Overall, mean serum AhR concentrations were 25.1 ±12.2 pg/mL. Patients with normal glucose tolerance had a lower serum AhR concentrations than patients with prediabetes or newly diagnosed diabetes (23.4 ±10.8 vs 26.2 ±13.2 and 26.9 ±12.3 pg/mL, respectively; P = 0.029). Linear regression analysis revealed that serum AhR concentrations were not associated with HOMA-IR, but were negatively associated with IGI after adjustment for several confounders, including HOMA-IR (β = -0.162; 95% confidence interval - 0.302, -0.022; P = 0.023).

CONCLUSIONS

In patients with no history of diabetes, serum AhR concentrations were negatively associated with β-cell function, independent of several confounders, including insulin resistance.

Urolithin A Is a Dietary Microbiota-Derived Human Aryl Hydrocarbon Receptor Antagonist

Urolithins (e.g., UroA and B) are gut microbiota-derived metabolites of the natural polyphenol ellagic acid. Urolithins are associated with various health benefits, including attenuation of inflammatory signaling, anti-cancer effects and repression of lipid accumulation. The molecular mechanisms underlying the beneficial effects of urolithins remain unclear. We hypothesize that some of the human health benefits of urolithins are mediated through the aryl hydrocarbon receptor (AHR). Utilizing a cell-based reporter system, we tested urolithins for the capacity to modulate AHR activity. Cytochrome P450 1A1 (CYP1A1) mRNA levels were assessed by real-time quantitative polymerase chain reaction. Competitive ligand binding assays were performed to determine whether UroA is a direct ligand for the AHR. Subcellular AHR protein levels were examined utilizing immunoblotting analysis. AHR expression was repressed in Caco-2 cells by siRNA transfection to investigate AHR-dependency. UroA and B were able to antagonize 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced AHR-mediated transcriptional activity. Furthermore, UroA and B attenuated TCDD-mediated stimulation of CYP1A1 mRNA levels. In addition, competitive ligand binding assays characterized UroA as a direct AHR ligand. Consistent with other AHR antagonists, UroA failed to induce AHR retention in the nucleus. AHR is necessary for UroA-mediated attenuation of cytokine-induced interleukin 6 (IL6) and prostaglandin-endoperoxide synthase 2 (PTGS2) expression in Caco-2 cells. Here we identified UroA as the first dietary-derived human selective AHR antagonist produced by the gut microbiota through multi-step metabolism. Furthermore, previously reported anti-inflammatory activity of UroA may at least in part be mediated through AHR.

Aryl hydrocarbon receptor and intestinal immunity

Aryl hydrocarbon receptor (AhR) is a member of the basic helix-loop-helix-(bHLH) superfamily of transcription factors, which are associated with cellular responses to environmental stimuli, such as xenobiotics and oxygen levels. Unlike other members of bHLH, AhR is the only bHLH transcription factor that is known to be ligand activated. Early AhR studies focused on understanding the role of AhR in mediating the toxicity and carcinogenesis properties of the prototypic ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In recent years, however, it has become apparent that, in addition to its toxicological involvement, AhR is highly receptive to a wide array of endogenous and exogenous ligands, and that its activation leads to a myriad of key host physiological functions. In this study, we review the current understanding of the functions of AhR in the mucosal immune system with a focus on its role in intestinal barrier function and intestinal immune cells, as well as in intestinal homeostasis.

Heregulin-induced cell migration is promoted by aryl hydrocarbon receptor in HER2-overexpressing breast cancer cells

HER2 overexpression accounts for approximately 15-20% of all breast cancers. We have shown that HER2 overexpression leads to elevated expression of the aryl hydrocarbon receptor (AhR) in breast cancer cells. In this study, firstly, we showed that AhR expression was up-regulated by treatment with the HER3 ligand heregulin (HRG) in HER2-overexpressing breast cancer cell lines. Induction of AhR was mediated by transcriptional activation of the region of AhR promoter corresponding to - 190 to - 100 bp. In addition, HRG treatment elicited nuclear translocation of AhR. To investigate the role of AhR in HRG-HER2/HER3 signaling in HER2-overexpressing cells, we established AhR knockout (KO) HER2-overexpressing cells to perform wound-healing assays. HRG-induced cell migration was markedly attenuated by AhR KO. HRG-induced cell migration was associated with increased expression of the inflammatory cytokines interleukin (IL)-6 and IL-8 in wild type cells, but not in AhR KO cells. These results elucidate that AhR is an important factor for the malignancy in HER2 overexpressing breast cancers.

Aryl hydrocarbon receptor (AHR): "pioneer member" of the basic-helix/loop/helix per-Arnt-sim (bHLH/PAS) family of "sensors" of foreign and endogenous signals

The basic-helix/loop/helix per-Arnt-sim (bHLH/PAS) family comprises many transcription factors, found throughout all three kingdoms of life; bHLH/PAS members "sense" innumerable intracellular and extracellular "signals" - including endogenous compounds, foreign chemicals, gas molecules, redox potential, photons (light), gravity, heat, and osmotic pressure. These signals then initiate downstream signaling pathways involved in responding to that signal. The term "PAS", abbreviation for "per-Arnt-sim" was first coined in 1991. Although the mouse Arnt gene was not identified until 1991, evidence of its co-transcriptional binding partner, aryl hydrocarbon receptor (AHR), was first reported in 1974 as a "sensor" of foreign chemicals, up-regulating cytochrome P450 family 1 (CYP1) and other enzyme activities that usually metabolize the signaling chemical. Within a few years, AHR was proposed also to participate in inflammation. The mouse [Ah] locus was shown (1973-1989) to be relevant to chemical carcinogenesis, mutagenesis, toxicity and teratogenesis, the mouse Ahr gene was cloned in 1992, and the first Ahr(-/-) knockout mouse line was reported in 1995. After thousands of studies from the early 1970s to present day, we now realize that AHR participates in dozens of signaling pathways involved in critical-life processes, affecting virtually every organ and cell-type in the animal, including many invertebrates.

The aryl hydrocarbon receptor as an antitumor target of synthetic curcuminoids in colorectal cancer

BACKGROUND

Curcumin has proven to be a potent antitumor agent in both preclinical and clinical models of colorectal cancer (CRC). It has also been identified as a ligand of the transcription factor known as the aryl hydrocarbon receptor (AHR). Our laboratory has identified the AHR as a mechanism which contributes to both tumorigenesis in a mouse model of inflammatory CRC as well an apoptotic target in vitro. Curcumin's role as an AHR ligand may modulate its effects to induce colon cancer cell death, and this role may be enhanced via structural modification of the curcumin backbone. We sought to determine if the two piperidone analogs of curcumin, RL66 and RL118, exhibit more robust antitumor actions than their parent compound in the context of colorectal cancer in vitro. Moreover, to ascertain the ability of curcumin, RL66 and RL118 to activate the AHR and evaluate if this activation has any effect on CRC cell death.

MATERIALS AND METHODS

DLD1, HCT116, LS513, and RKO colon cell lines were propagated in vitro. Natural curcumin was obtained commercially, whereas RL66 and RL118 were synthesized and characterized de novo. Multiwell fluorescent/luminescent signal detection was used to simultaneously ascertain cell viability, cell cytonecrosis, and relative amounts of apoptotic activity. AHR activity was measured with a dual luciferase reporter gene system. Stable expression of small interfering RNA interference was established in the HCT116 cell lines to create AHR "knock down" cell lines.

RESULTS

Both RL66 and RL118 proved to be more potent antitumor agents than their parent compound curcumin in all cell lines tested. The majority of this cell death was due to induction of apoptosis, which occurred earlier and to a greater degree following RL66 and RL118 treatment as opposed to curcumin. Also, RL66 and RL118 were found to be activators of AHR, and a portion of their ability to cause cell death was dependent on this induction. Curcumin was found unable to activate the AHR, and levels of AHR messenger RNA did not change their effects on cell death.

CONCLUSIONS

Piperidone analogs of curcumin exhibited enhanced antitumor effects in vitro as opposed to their parent compound. Even more, this enhanced cell death profile may be partially attributed to the ability of these compounds to activate the AHR. Further study of synthetic curcumin analogs as chemopreventives and chemoadjuncts in CRC is warranted. Also, more generally, the AHR may represent a potential putative target for novel anticancer agents for CRC.

Microbiome-Linked Crosstalk in the Gastrointestinal Exposome towards Host Health and Disease

The gastrointestinal exposome represents the integration of all xenobiotic components and host-derived endogenous components affecting the host health, disease progression and ultimately clinical outcomes during the lifespan. The human gut microbiome as a dynamic exposome of commensalism continuously interacts with other exogenous exposome as well as host sentineling components including the immune and neuroendocrine circuit. The composition and diversity of the microbiome are established on the basis of the luminal environment (physical, chemical and biological exposome) and host surveillance at each part of the gastrointestinal lining. Whereas the chemical exposome derived from nutrients and other xenobiotics can influence the dynamics of microbiome community (the stability, diversity, or resilience), the microbiomes reciprocally alter the bioavailability and activities of the chemical exposome in the mucosa. In particular, xenobiotic metabolites by the gut microbial enzymes can be either beneficial or detrimental to the host health although xenobiotics can alter the composition and diversity of the gut microbiome. The integration of the mucosal crosstalk in the exposome determines the fate of microbiome community and host response to the etiologic factors of disease. Therefore, the network between microbiome and other mucosal exposome would provide new insights into the clinical intervention against the mucosal or systemic disorders via regulation of the gut-associated immunological, metabolic, or neuroendocrine system.