Loss of Aryl Hydrocarbon Receptor Promotes Colon Tumorigenesis in ApcS580/+; KrasG12D/+ Mice

A Review of the Evidence for Tryptophan and the Kynurenine Pathway as a Regulator of Stem Cell Niches in Health and Disease

Stem cells are ubiquitously found in various tissues and organs in the body, and underpin the body's ability to repair itself following injury or disease initiation, though repair can sometimes be compromised. Understanding how stem cells are produced, and functional signaling systems between different niches is critical to understanding the potential use of stem cells in regenerative medicine. In this context, this review considers kynurenine pathway (KP) metabolism in multipotent adult progenitor cells, embryonic, haematopoietic, neural, cancer, cardiac and induced pluripotent stem cells, endothelial progenitor cells, and mesenchymal stromal cells. The KP is the major enzymatic pathway for sequentially catabolising the essential amino acid tryptophan (TRP), resulting in key metabolites including kynurenine, kynurenic acid, and quinolinic acid (QUIN). QUIN metabolism transitions into the adjoining de novo pathway for nicotinamide adenine dinucleotide (NAD) production, a critical cofactor in many fundamental cellular biochemical pathways. How stem cells uptake and utilise TRP varies between different species and stem cell types, because of their expression of transporters and responses to inflammatory cytokines. Several KP metabolites are physiologically active, with either beneficial or detrimental outcomes, and evidence of this is presented relating to several stem cell types, which is important as they may exert a significant impact on surrounding differentiated cells, particularly if they metabolise or secrete metabolites differently. Interferon-gamma (IFN-γ) in mesenchymal stromal cells, for instance, highly upregulates rate-limiting enzyme indoleamine-2,3-dioxygenase (IDO-1), initiating TRP depletion and production of metabolites including kynurenine/kynurenic acid, known agonists of the Aryl hydrocarbon receptor (AhR) transcription factor. AhR transcriptionally regulates an immunosuppressive phenotype, making them attractive for regenerative therapy. We also draw attention to important gaps in knowledge for future studies, which will underpin future application for stem cell-based cellular therapies or optimising drugs which can modulate the KP in innate stem cell populations, for disease treatment.

The aryl hydrocarbon receptor as a tumor modulator: mechanisms to therapy

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that is widely recognized to play important, but complex, modulatory roles in a variety of tumor types. In this review, we comprehensively summarize the increasingly controversial role of AhR as a tumor regulator and the mechanisms by which it alters tumor progression based on the cancer cell type. Finally, we discuss new and emerging strategies to therapeutically modulate AhR, focusing on novel agents that hold promise in current human clinical trials as well as existing FDA-approved drugs that could potentially be repurposed for cancer therapy.

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.

Tumor suppressive activity of AHR in environmental arsenic-induced carcinogenesis

The aryl hydrocarbon receptor (AHR) is a highly conserved pleiotropic transcription factor that senses environmental pollutants, microbial products, and endogenous ligands. The transcriptional targets of AHR include phase I and phase II detoxification enzymes, as well as numerous signaling molecules that affect a wide spectrum of biological and biochemical processes in a manner of cellular context-dependent. In this review, we systematically assess the latest discoveries of AHR in carcinogenesis with an emphasis on its tumor suppressor-like property that represses the expression of genes in oncogenic signaling pathways. Additionally, we outline recent progress in our studies on the interaction among AHR, TGFb and NRF2 in cellular responses to arsenic and malignant transformation. Our findings indicate that AHR antagonized TGFb and NRF2, suggesting that AHR could serve as a potential tumor suppressor in arsenic-induced carcinogenesis. Notably, while AHR can exhibit both oncogenic and tumor-suppressive properties in cancer development and the generation of the cancer stem-like cells (CSCs), the tumor suppressor-like effect of AHR warrants further extensive exploration for the prevention and clinical treatment of cancers.

Aryl hydrocarbon receptor is a tumor promoter in MYCN-amplified neuroblastoma cells through suppression of differentiation

Neuroblastoma is the most common extracranial solid tumor in children. MYCN amplification is detected in almost half of high-risk cases and is associated with poorly differentiated tumors, poor patient prognosis and poor response to therapy, including retinoids. We identify the aryl hydrocarbon receptor (AhR) as a transcription factor promoting the growth and suppressing the differentiation of MYCN-amplified neuroblastoma. A neuroblastoma specific AhR transcriptional signature reveals an inverse correlation of AhR activity with patients' outcome, suggesting AhR activity is critical for disease progression. AhR modulates chromatin structures, reducing accessibility to regions responsive to retinoic acid. Genetic and pharmacological inhibition of AhR results in induction of differentiation. Importantly, AhR antagonism with clofazimine synergizes with retinoic acid in inducing differentiation both in vitro and in vivo. Thus, we propose AhR as a target for MYCN-amplified neuroblastoma and that its antagonism, combined with current standard-of-care, may result in a more durable response in patients.

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.

Suppression of Ah Receptor (AhR) increases the aggressiveness of TNBC cells and 11-Cl-BBQ-activated AhR inhibits their growth

Triple-negative breast cancer (TNBC) represents around 15% of the 2.26 million breast cancers diagnosed worldwide annually and has the worst outcome. Despite recent therapeutic advances, there remains a lack of targeted therapies for this breast cancer subtype. The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor with biological roles in regulating development, xenobiotic metabolism, cell cycle progression and cell death. AhR activation by select ligands can promote tumor suppression in multiple cancer types. AhR can negatively regulate the activity of different oncogenic signaling pathways and can directly upregulate tumor suppressor genes such as p27Kip1. To determine the role of AhR in TNBC, we generated AhR-deficient cancer cells and investigated the impact of AhR loss on TNBC cell growth phenotypes. We found that AhR-deficient MDA-MB-468 TNBC cells have increased proliferation and formed significantly more colonies compared to AhR expressing cells. These cells without AhR expression grew aggressively in vivo. To determine the molecular targets driving this phenotype, we performed transcriptomic profiling in AhR expressing and AhR knockout MDA-MB-468 cells and identified tyrosine receptor kinases, as well as other genes involved in proliferation, survival and clonogenicity that are repressed by AhR. In order to determine therapeutic targeting of AhR in TNBC, we investigated the anti-cancer effects of the novel AhR ligand 11-chloro-7H-benzimidazo[2,1-a]benzo[de]iso-quinolin-7-one (11-Cl-BBQ), which belongs to a class of high affinity, rapidly metabolized AhR ligands called benzimidazoisoquinolines (BBQs). 11-Cl-BBQ induced AhR-dependent cancer cell-selective growth inhibition and strongly inhibited colony formation in TNBC cells.

Mutant APC reshapes Wnt signaling plasma membrane nanodomains by altering cholesterol levels via oncogenic β-catenin

Although the role of the Wnt pathway in colon carcinogenesis has been described previously, it has been recently demonstrated that Wnt signaling originates from highly dynamic nano-assemblies at the plasma membrane. However, little is known regarding the role of oncogenic APC in reshaping Wnt nanodomains. This is noteworthy, because oncogenic APC does not act autonomously and requires activation of Wnt effectors upstream of APC to drive aberrant Wnt signaling. Here, we demonstrate the role of oncogenic APC in increasing plasma membrane free cholesterol and rigidity, thereby modulating Wnt signaling hubs. This results in an overactivation of Wnt signaling in the colon. Finally, using the Drosophila sterol auxotroph model, we demonstrate the unique ability of exogenous free cholesterol to disrupt plasma membrane homeostasis and drive Wnt signaling in a wildtype APC background. Collectively, these findings provide a link between oncogenic APC, loss of plasma membrane homeostasis and CRC development.

Induction of Aryl Hydrocarbon Receptor-Mediated Cancer Cell-Selective Apoptosis in Triple-Negative Breast Cancer Cells by a High-Affinity Benzimidazoisoquinoline

Triple-negative breast cancer (TNBC) remains a disease with a paucity of targeted treatment opportunities. The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that is involved in a wide range of physiological processes, including the sensing of xenobiotics, immune function, development, and differentiation. Different small-molecule AhR ligands drive strikingly varied cellular and organismal responses. In certain cancers, AhR activation by select small molecules induces cell cycle arrest or apoptosis via activation of tumor-suppressive transcriptional programs. AhR is expressed in triple-negative breast cancers, presenting a tractable therapeutic opportunity. Here, we identify a novel ligand of the aryl hydrocarbon receptor that potently and selectively induces cell death in triple-negative breast cancer cells and TNBC stem cells via the AhR. Importantly, we found that this compound, Analog 523, exhibits minimal cytotoxicity against multiple normal human primary cells. Analog 523 represents a high-affinity AhR ligand with potential for future clinical translation as an anticancer agent.

Network pharmacology and bioinformatics were used to construct a prognostic model and immunoassay of core target genes in the combination of quercetin and kaempferol in the treatment of colorectal cancer

Purpose: CRC is a malignant tumor seriously threatening human health. Quercetin and kaempferol are representative components of traditional Chinese medicine (TCM). Previous studies have shown that both quercetin and kaempferol have antitumor pharmacological effects, nevertheless, the underlying mechanism of action remains unclear. To explore the synergy and mechanism of quercetin and kaempferol in colorectal cancer. Methods: In this study, network pharmacology, and bioinformatics are used to obtain the intersection of drug targets and disease genes. Training gene sets were acquired from the TCGA database, acquired prognostic-related genes by univariate Cox, multivariate Cox, and Lasso-Cox regression models, and validated in the GEO dataset. We also made predictions of the immune function of the samples and used molecular docking to map a model for binding two components to prognostic genes. Results: Through Lasso-Cox regression analysis, we obtained three models of drug target genes. This model predicts the combined role of quercetin and kaempferol in the treatment and prognosis of CRC. Prognostic genes are correlated with immune checkpoints and immune infiltration and play an adjuvant role in the immunotherapy of CRC. Conclusion: Core genes are regulated by quercetin and kaempferol to improve the patient's immune system and thus assist in the treatment of CRC.

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.

11-Cl-BBQ, a select modulator of AhR-regulated transcription, suppresses lung cancer cell growth via activation of p53 and p27Kip1

Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor and functions as a tumour suppressor in different cancer models. In the present study, we report detailed characterization of 11-chloro-7H-benzimidazo[2,1-a]benzo[de]iso-quinolin-7-one (11-Cl-BBQ) as a select modulator of AhR-regulated transcription (SMAhRT) with anti-cancer actions. Treatment of lung cancer cells with 11-Cl-BBQ induced potent and sustained AhR-dependent anti-proliferative effects by promoting G1 phase cell cycle arrest. Investigation of 11-Cl-BBQ-induced transcription in H460 cells with or without the AhR expression by RNA-sequencing revealed activation of p53 signalling. In addition, 11-Cl-BBQ suppressed multiple pathways involved in DNA replication and increased expression of cyclin-dependent kinase inhibitors, including p27Kip1 , in an AhR-dependent manner. CRISPR/Cas9 knockout of individual genes revealed the requirement for both p53 and p27Kip1 for the AhR-mediated anti-proliferative effects. Our results identify 11-Cl-BBQ as a potential lung cancer therapeutic, highlight the feasibility of targeting AhR and provide important mechanistic insights into AhR-mediated-anticancer actions.

Tumor-Suppressive Functions of the Aryl Hydrocarbon Receptor (AhR) and AhR as a Therapeutic Target in Cancer

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor involved in regulating a wide range of biological responses. A diverse array of xenobiotics and endogenous small molecules bind to the receptor and drive unique phenotypic responses. Due in part to its role in mediating toxic responses to environmental pollutants, AhR activation has not been traditionally viewed as a viable therapeutic approach. Nonetheless, the expression and activation of AhR can inhibit the proliferation, migration, and survival of cancer cells, and many clinically approved drugs transcriptionally activate AhR. Identification of novel select modulators of AhR-regulated transcription that promote tumor suppression is an active area of investigation. The development of AhR-targeted anticancer agents requires a thorough understanding of the molecular mechanisms driving tumor suppression. Here, we summarized the tumor-suppressive mechanisms regulated by AhR with an emphasis on the endogenous functions of the receptor in opposing carcinogenesis. In multiple different cancer models, the deletion of AhR promotes increased tumorigenesis, but a precise understanding of the molecular cues and the genetic targets of AhR involved in this process is lacking. The intent of this review was to synthesize the evidence supporting AhR-dependent tumor suppression and distill insights for development of AhR-targeted cancer therapeutics.

The Ah Receptor from Toxicity to Therapeutics: Report from the 5th AHR Meeting at Penn State University, USA, June 2022

The aryl hydrocarbon receptor (AHR) is a sensor of low-molecular-weight molecule signals that originate from environmental exposures, the microbiome, and host metabolism. Building upon initial studies examining anthropogenic chemical exposures, the list of AHR ligands of microbial, diet, and host metabolism origin continues to grow and has provided important clues as to the function of this enigmatic receptor. The AHR has now been shown to be directly involved in numerous biochemical pathways that influence host homeostasis, chronic disease development, and responses to toxic insults. As this field of study has continued to grow, it has become apparent that the AHR is an important novel target for cancer, metabolic diseases, skin conditions, and autoimmune disease. This meeting attempted to cover the scope of basic and applied research being performed to address possible applications of our basic knowledge of this receptor on therapeutic outcomes.

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.

Health Benefits of Coffee Consumption for Cancer and Other Diseases and Mechanisms of Action

Coffee is one of the most widely consumed beverages worldwide, and epidemiology studies associate higher coffee consumption with decreased rates of mortality and decreased rates of neurological and metabolic diseases, including Parkinson's disease and type 2 diabetes. In addition, there is also evidence that higher coffee consumption is associated with lower rates of colon and rectal cancer, as well as breast, endometrial, and other cancers, although for some of these cancers, the results are conflicting. These studies reflect the chemopreventive effects of coffee; there is also evidence that coffee consumption may be therapeutic for some forms of breast and colon cancer, and this needs to be further investigated. The mechanisms associated with the chemopreventive or chemotherapeutic effects of over 1000 individual compounds in roasted coffee are complex and may vary with different diseases. Some of these mechanisms may be related to nuclear factor erythroid 2 (Nrf2)-regulated pathways that target oxidative stress or pathways that induce reactive oxygen species to kill diseased cells (primarily therapeutic). There is evidence for the involvement of receptors which include the aryl hydrocarbon receptor (AhR) and orphan nuclear receptor 4A1 (NR4A1), as well as contributions from epigenetic pathways and the gut microbiome. Further elucidation of the mechanisms will facilitate the potential future clinical applications of coffee extracts for treating cancer and other inflammatory diseases.

FDI-6, a FOXM1 inhibitor, activates the aryl hydrocarbon receptor and suppresses tumorsphere formation

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that is activated by environmental contaminants such as dioxins and polycyclic aromatic hydrocarbons. Following ligand binding, AhR binds to xenobiotic responsive elements and modulates the transcription of AhR target genes. Multiple studies have shown that AhR plays important roles in a range of cancer cells and is attracting attention as a therapeutic target for cancer treatment. We have previously reported that AhR agonists inhibit tumorsphere formation in an AhR-dependent manner in the MCF-7 breast cancer cell line. In the present study, we found that FDI-6, an inhibitor of the transcription factor Forkhead Box M1 (FOXM1) induced the mRNA expression of AhR target genes, nuclear translocation of AhR, and transcriptional activity of AhR. In addition, FDI-6 dose-dependently reduced the mRNA expression of FOXM1-regulated genes in AhR-expressing MCF-7 cells, although not in AhR-deficient MCF-7 cells. Furthermore, FDI-6 was found to suppress tumorsphere formation via the AhR in MCF-7 cells and HepG2 human liver cancer cell line. On the basis of the findings of this study, we show that FDI-6, a FOXM1 inhibitor, functions as an AhR agonist, and suppresses tumorsphere formation via the AhR.

Aryl Hydrocarbon Receptor Activation Coordinates Mouse Small Intestinal Epithelial Cell Programming

In the face of mechanical, chemical, microbial, and immunologic pressure, intestinal homeostasis is maintained through balanced cellular turnover, proliferation, differentiation, and self-renewal. Here, we present evidence supporting the role of the aryl hydrocarbon receptor (AHR) in the adaptive reprogramming of small intestinal gene expression, leading to altered proliferation, lineage commitment, and remodeling of the cellular repertoire that comprises the intestinal epithelium to promote intestinal resilience. Ahr gene/protein expression and transcriptional activity exhibit marked proximal^HI to distal^LO and crypt^HI to villi^LO gradients. Genetic ablation of Ahr impairs commitment/differentiation of the secretory Paneth and goblet cell lineages and associated mucin production, restricts expression of secretory/enterocyte differentiation markers, and increases crypt-associated proliferation and villi-associated enterocyte luminal exfoliation. Ahr^-/- mice display a decrease in intestinal barrier function. Ahr^+/+ mice that maintain a diet devoid of AHR ligands intestinally phenocopy Ahr^-/- mice. In contrast, Ahr^+/+ mice exposed to AHR ligands reverse these phenotypes. Ligand-induced AHR transcriptional activity positively correlates with gene expression (Math1, Klf4, Tff3) associated with differentiation of the goblet cell secretory lineage. Math1 was identified as a direct target gene of AHR, a transcription factor critical to the development of goblet cells. These data suggest that dietary cues, relayed through the transcriptional activity of AHR, can reshape the cellular repertoire of the gastrointestinal tract.

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.

From Nucleus to Organs: Insights of Aryl Hydrocarbon Receptor Molecular Mechanisms

The aryl hydrocarbon receptor (AHR) is a markedly established regulator of a plethora of cellular and molecular processes. Its initial role in the detoxification of xenobiotic compounds has been partially overshadowed by its involvement in homeostatic and organ physiology processes. In fact, the discovery of its ability to bind specific target regulatory sequences has allowed for the understanding of how AHR modulates such processes. Thereby, AHR presents functions in transcriptional regulation, chromatin architecture modifications and participation in different key signaling pathways. Interestingly, such fields of influence end up affecting organ and tissue homeostasis, including regenerative response both to endogenous and exogenous stimuli. Therefore, from classical spheres such as canonical transcriptional regulation in embryonic development, cell migration, differentiation or tumor progression to modern approaches in epigenetics, senescence, immune system or microbiome, this review covers all aspects derived from the balance between regulation/deregulation of AHR and its physio-pathological consequences.

The AHR target gene scinderin activates the WNT pathway by facilitating the nuclear translocation of β-catenin

The ligand-activated transcription factor aryl hydrocarbon receptor (AHR) regulates cellular detoxification, proliferation and immune evasion in a range of cell types and tissues, including cancer cells. In this study, we used RNA-sequencing to identify the signature of the AHR target genes regulated by the pollutant 2,3,7,8-tetrachlorodibenzodioxin (TCDD) and the endogenous ligand kynurenine (Kyn), a tryptophan-derived metabolite. This approach identified a signature of six genes (CYP1A1, ALDH1A3, ABCG2, ADGRF1 and SCIN) as commonly activated by endogenous or exogenous ligands of AHR in multiple colon cancer cell lines. Among these, the actin-severing protein scinderin (SCIN) was necessary for cell proliferation; SCIN downregulation limited cell proliferation and its expression increased it. SCIN expression was elevated in a subset of colon cancer patient samples, which also contained elevated β-catenin levels. Remarkably, SCIN expression promoted nuclear translocation of β-catenin and activates the WNT pathway. Our study identifies a new mechanism for adhesion-mediated signaling in which SCIN, likely via its ability to alter the actin cytoskeleton, facilitates the nuclear translocation of β-catenin. This article has an associated First Person interview with the first authors of the paper.

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.

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.

Single-cell RNA Sequencing Reveals How the Aryl Hydrocarbon Receptor Shapes Cellular Differentiation Potency in the Mouse Colon

Despite recent progress recognizing the importance of aryl hydrocarbon receptor (Ahr)-dependent signaling in suppressing colon tumorigenesis, its role in regulating colonic crypt homeostasis remains unclear. To assess the effects of Ahr on intestinal epithelial cell heterogeneity and functional phenotypes, we utilized single-cell transcriptomics and advanced analytic strategies to generate a high-quality atlas for colonic intestinal crypts from wild-type and intestinal-specific Ahr knockout mice. Here we observed the promotive effects of Ahr deletion on Foxm1-regulated genes in crypt-associated canonical epithelial cell types and subtypes of goblet cells and deep crypt-secretory cells. We also show that intestinal Ahr deletion elevated single-cell entropy (a measure of differentiation potency or cell stemness) and RNA velocity length (a measure of the rate of cell differentiation) in noncycling and cycling Lgr5⁺ stem cells. In general, intercellular signaling cross-talk via soluble and membrane-bound factors was perturbed in Ahr-null colonocytes. Taken together, our single-cell RNA sequencing analyses provide new evidence of the molecular function of Ahr in modulating putative stem cell driver genes, cell potency lineage decisions, and cell-cell communication in vivo. PREVENTION RELEVANCE: Our mouse single-cell RNA sequencing analyses provide new evidence of the molecular function of Ahr in modulating colonic stemness and cell-cell communication in vivo. From a cancer prevention perspective, Ahr should be considered a therapeutic target to recalibrate remodeling of the intestinal stem cell niche.

Aryl Hydrocarbon Receptor and Its Diverse Ligands and Functions: An Exposome Receptor

The aryl hydrocarbon receptor (AhR) is a transcriptional factor that regulates multiple functions following its activation by a variety of ligands, including xenobiotics, natural products, microbiome metabolites, and endogenous molecules. Because of this diversity, the AhR constitutes an exposome receptor. One of its main functions is to regulate several lines of defense against chemical insults and bacterial infections. Indeed, in addition to its well-established detoxication function, it has several functions at physiological barriers, and it plays a critical role in immunomodulation. The AhR is also involved in the development of several organs and their homeostatic maintenance. Its activity depends on the type of ligand and on the time frame of the receptor activation, which can be either sustained or transient, leading in some cases to opposite modes of regulations as illustrated in the regulation of different cancer pathways. The development of selective modulators and their pharmacological characterization are important areas of research. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Role of the Aryl Hydrocarbon Receptor (AhR) in Mediating the Effects of Coffee in the Colon

SCOPE

This study investigates the mechanism of action and functional effects of coffee extracts in colonic cells, on intestinal stem cell growth, and inhibition of dextran sodium sulfate (DSS)-induced intestinal barrier damage in mice.

METHODS AND RESULTS

Aqueous coffee extracts induced Ah receptor (AhR) -responsive CYP1A1, CYP1B1, and UGT1A1 gene expression in colon-derived Caco2 and YAMC cells. Tissue-specific AhR knockout (AhR^f/f x Lgr5-GFP-CreERT2 x Villin-Cre), wild-type (Lgr5-CreERT2 x Villin-Cre) mice are sources of stem cell enriched organoids and both coffee extracts and norharman, an AhR-active component of these extracts inhibited stem cell growth. Coffee extracts also inhibit DSS-induced damage to intestinal barrier function and DSS-induced mucosal inflammatory genes such as IL-6 and TGF-β1 in wild-type (AhR^+/+ ) but not AhR^-/- mice. In contrast, coffee does not exhibit protective effects in intestinal-specific AhR knockout mice. Coffee extracts also enhanced overall formation of AhR-active microbial metabolites.

CONCLUSIONS

In colon-derived cells and in the mouse intestine, coffee induced several AhR-dependent responses including gene expression, inhibition of intestinal stem cell-enriched organoid growth, and inhibition of DSS-induced intestinal barrier damage. We conclude that the anti-inflammatory effects of coffee in the intestine are due, in part, to activation of AhR signaling.

Discovery and Mechanistic Characterization of a Select Modulator of AhR-regulated Transcription (SMAhRT) with Anti-cancer Effects

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor and a member of the bHLH/PAS (basic Helix-Loop-Helix/Per-Arnt-Sim) family of proteins. The AhR was cloned and characterized for its role in mediating the toxicity of dioxins. Subsequent research has identified the role of AhR in suppression of cancer cell growth. We hypothesized that the AhR is a molecular target for therapeutic intervention in cancer, and that activation of the AhR by unique AhR ligands in cancer cells could have anti-cancer effects including induction of cell death. This study describes the discovery and characterization of a new class of anti-cancer agents targeting the AhR, that we designate as Select Modulators of AhR-regulated Transcription (SMAhRTs). We employed two independent small molecule screening approaches to identify potential SMAhRTs. We report the identification of CGS-15943 that activates AhR signaling and induces apoptosis in an AhR-dependent manner in liver and breast cancer cells. Investigation of the downstream signaling pathway of this newly identified SMAhRT revealed upregulation of Fas-ligand (FasL), which is required for AhR-mediated apoptosis. Our results provide a basis for further development of a new class of anti-cancer therapeutics targeting an underappreciated molecular target, the AhR.