The involvement of aryl hydrocarbon receptor in the activation of transforming growth factor-beta and apoptosis

Disrupting Development: Unraveling the Interplay of Aryl Hydrocarbon Receptor (AHR) and Wnt/β-Catenin Pathways in Kidney Development Under the Influence of Environmental Pollutants

Understanding the intricate molecular mechanisms governing aryl hydrocarbon receptor (AHR) and Wnt/β-Catenin pathways crosstalk is of paramount importance for elucidating normal development. We investigated the repercussions of aberrant activation of these signaling pathways on kidney development. HEK-293 cells were subjected to AHR and Wnt activators and inhibitors for 3 and 24 h. Subsequently, pregnant adult female BALB/c mice were administered treatments at gestation day 9 (GD-9), and embryos were analyzed at GD-18 using a combination of cellular, molecular, stereological, and histopathological techniques. Our results demonstrated a noteworthy escalation in oxidative stress and gene expression endpoints associated with apoptosis. Moreover, stereological analyses exhibited alterations in cortex, proximal tubule, and kidney tissue vessels volumes. Remarkably, co-treatment with 6-formylindolo [3,2-b] carbazole (FICZ) and cadmium (Cd) resulted in a significant reduction in glomerulus volume, while elevating the volumes of distal tubule, Henle loop, and connective tissue, compared to the control group. Histopathological investigations further confirmed structural changes in the loop of Henle and proximal tubule, alongside a decline in glomerular volume. Additionally, the expression levels of AHR and Ctnnb1 genes significantly increased in the Cd-treated group compared to the control group. Enhanced expression of apoptosis-related genes, including Bcl-x, Bax, and Caspase3, along with alterations in mitochondrial membrane potential and cytochrome C release, was observed. In contrast, Gsk3 gene expression was significantly decreased. Our findings robustly establish that chemical pollutants, such as Cd, disrupt the AHR and Wnt/β-Catenin physiological roles during developmental stages by inhibiting the metabolic degradation of FICZ.

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.

Ligand-Independent Activation of Aryl Hydrocarbon Receptor and Attenuation of Glutamine Levels by Natural Deep Eutectic Solvent

Natural deep eutectic solvents (NADESs) are emerging sustainable alternatives to conventional organic solvents. Beyond their role as laboratory solvents, NADESs are increasingly explored in drug delivery and as therapeutics. Their increasing applications notwithstanding, our understanding of how they interact with biomolecules at multiple levels - metabolome, proteome, and transcriptome - within human cell remain poor. Here, we deploy integrated metabolomics, proteomics, and transcriptomics to probe how NADESs perturb the molecular landscape of human cells. In a human cell line model, we found that an archetypal NADES derived from choline and geranic acid (CAGE) significantly altered the metabolome, proteome, and transcriptome. CAGE upregulated indole-3-lactic acid and 4-hydroxyphenyllactic acid levels, resulting in ligand-independent activation of aryl hydrocarbon receptor to signal the transcription of genes with implications for inflammation, immunomodulation, cell development, and chemical detoxification. Further, treating the cell line with CAGE downregulated glutamine biosynthesis, a nutrient rapidly proliferating cancer cells require. CAGE's ability to attenuate glutamine levels is potentially relevant for cancer treatment. These findings suggest that NADESs, even when derived from natural components like choline, can indirectly modulate cell biology at multiple levels, expanding their applications beyond chemistry to biomedicine and biotechnology.

Alcohol-associated fibrosis in females is mediated by female-specific activation of lysine demethylases KDM5B and KDM5C

Alcohol-associated liver disease is a major cause of alcohol-related mortality. However, the mechanisms underlying disease progression are not fully understood. Recently we found that liver molecular pathways are altered by alcohol consumption differently in males and females. We were able to associate these sex-specific pathways with two upstream regulators: H3K4-specific demethylase enzymes KDM5B and KDM5C. Mice were fed the Lieber-DeCarli alcohol liquid diet for 3 weeks or a combination of a high-fat diet with alcohol in water for 16 weeks (western diet alcohol model [WDA] model). To assess the role of histone demethylases, mice were treated with AAV-shControl, AAV-shKdm5b, and/or AAV-shKdm5c and/or AAV-shAhR vectors. Gene expression and epigenetic changes after Kdm5b/5c knockdown were assessed by RNA-sequencing and H3K4me3 chromatin immunoprecipitation analysis. We found that less than 5% of genes affected by Kdm5b/Kdm5c knockdown were common between males and females. In females, Kdm5b/Kdm5c knockdown prevented fibrosis development in mice fed the WDA alcohol diet for 16 weeks and decreased fibrosis-associated gene expression in mice fed the Lieber-DeCarli alcohol liquid diet. In contrast, fibrosis was not affected by Kdm5b/Kdm5c knockdown in males. We found that KDM5B and KDM5C promote fibrosis in females through down-regulation of the aryl hydrocarbon receptor (AhR) pathway components in hepatic stellate cells. Kdm5b/Kdm5c knockdown resulted in an up-regulation of Ahr, Arnt, and Aip in female but not in male mice, thus preventing fibrosis development. Ahr knockdown in combination with Kdm5b/Kdm5c knockdown restored profibrotic gene expression. Conclusion: KDM5 demethylases contribute to differences between males and females in the alcohol response in the liver. The KDM5/AhR axis is a female-specific mechanism of fibrosis development in alcohol-fed mice.

Aryl hydrocarbon receptor blocks aging-induced senescence in the liver and fibroblast cells

Aging impairs organismal homeostasis leading to multiple pathologies. Yet, the mechanisms and molecular intermediates involved are largely unknown. Here, we report that aged aryl hydrocarbon receptor-null mice (AhR-/-) had exacerbated cellular senescence and more liver progenitor cells. Senescence-associated markers β-galactosidase (SA-β-Gal), p16^Ink4a and p21^Cip1 and genes encoding senescence-associated secretory phenotype (SASP) factors TNF and IL1 were overexpressed in aged AhR-/- livers. Chromatin immunoprecipitation showed that AhR binding to those gene promoters repressed their expression, thus adjusting physiological levels in AhR+/+ livers. MCP-2, MMP12 and FGF secreted by senescent cells were overproduced in aged AhR-null livers. Supporting the relationship between senescence and stemness, liver progenitor cells were overrepresented in AhR-/- mice, probably contributing to increased hepatocarcinoma burden. These AhR roles are not liver-specific since adult and embryonic AhR-null fibroblasts underwent senescence in culture, overexpressing SA-β-Gal, p16^Ink4a and p21^Cip1. Notably, depletion of senescent cells with the senolytic agent navitoclax restored expression of senescent markers in AhR-/- fibroblasts, whereas senescence induction by palbociclib induced an AhR-null-like phenotype in AhR+/+ fibroblasts. AhR levels were downregulated by senescence in mouse lungs but restored upon depletion of p16^Ink4a-expressing senescent cells. Thus, AhR restricts age-induced senescence associated to a differentiated phenotype eventually inducing resistance to liver tumorigenesis.

Apoptosis and blood-testis barrier disruption during male reproductive dysfunction induced by PAHs of different molecular weights

The association between polycyclic aromatic hydrocarbons (PAHs) and male reproductive dysfunction has attracted increasing attention. The purpose of this study was to compare the male reproductive toxicity of multiple PAHs and to investigate the underlying molecular mechanisms. TM4 cells (mouse testicular Sertoli cells, SCs) were treated with benzo(a)pyrene (BaP), pyrene (Py), fluoranthene (Fl) and phenanthrene (Phe) (0, 0.1, 1, 10, 50, or 100 μM) for varying time points (4, 12, 24, or 48 h), and male C57BL/6 mice were administered BaP and Py (0, 10, 50, or 100 mg/kg body weight) for 14 days based on the cell experimental results. Histopathological examination, western blotting, ELISA, biochemical assays, RT-PCR, flow cytometry, JC-1 staining and trans-epithelium electrical resistance (TEER) measurements were used to assess apoptosis, blood-testis barrier (BTB) integrity, intracellular calcium ([Ca²⁺]_i) concentrations and oxidative stress (OS). The results revealed that the mRNA levels and enzymatic activities of CYP450 and GST family members; levels of ROS, MDA, cleaved caspase 3 (c-caspase 3), caspase 9, Bax, and cytochrome C (CytC); and numbers of TUNEL-positive cells were significantly increased by BaP and Py, while levels of AhR, GSH, SOD, CAT, Bcl-2 and ΔΨm were decreased. Additionally, BaP and Py notably interfered with tight junctions (TJs) and adherens junctions (AJs) in the BTB. Intriguingly, BaP, but not Py, induced [Ca²⁺]_i overload and gap junction (GJ) destruction. There was no dramatic effect of Fl and/or Phe on any of the above parameters except that slight cytotoxicity was observed with higher doses of Fl. Collectively, these findings showed that BaP and Py elicited SC apoptosis and BTB disruption involving mitochondrial dysfunction and OS, but [Ca²⁺]_i fluctuation and GJ injury were only observed with BaP-induced reproductive toxicity. The male reproductive toxicity of the selected PAHs was ranked in the order of BaP > Py > Fl > Phe.

Effects of Low-Dose Gestational TCDD Exposure on Behavior and on Hippocampal Neuron Morphology and Gene Expression in Mice

BACKGROUND

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a persistent and toxic environmental pollutant. Gestational exposure to TCDD has been linked to cognitive and motor deficits, and increased incidence of autism spectrum disorder (ASD) traits in children. Most animal studies of these neurodevelopmental effects involve acute TCDD exposure, which does not model typical exposure in humans.

OBJECTIVES

The aim of the study was to establish a dietary low-dose gestational TCDD exposure protocol and performed an initial characterization of the effects on offspring behavior, neurodevelopmental phenotypes, and gene expression.

METHODS

Throughout gestation, pregnant C57BL/6J mice were fed a diet containing a low dose of TCDD (9  ng TCDD/kg body weight per day) or a control diet. The offspring were tested in a battery of behavioral tests, and structural brain alterations were investigated by magnetic resonance imaging. The dendritic morphology of pyramidal neurons in the hippocampal Cornu Ammonis (CA)1 area was analyzed. RNA sequencing was performed on hippocampi of postnatal day 14 TCDD-exposed and control offspring.

RESULTS

TCDD-exposed females displayed subtle deficits in motor coordination and reversal learning. Volumetric difference between diet groups were observed in regions of the hippocampal formation, mammillary bodies, and cerebellum, alongside higher dendritic arborization of pyramidal neurons in the hippocampal CA1 region of TCDD-exposed females. RNA-seq analysis identified 405 differentially expressed genes in the hippocampus, enriched for genes with functions in regulation of microtubules, axon guidance, extracellular matrix, and genes regulated by SMAD3.

DISCUSSION

Exposure to 9  ng TCDD/kg body weight per day throughout gestation was sufficient to cause specific behavioral and structural brain phenotypes in offspring. Our data suggest that alterations in SMAD3-regulated microtubule polymerization in the developing postnatal hippocampus may lead to an abnormal morphology of neuronal dendrites that persists into adulthood. These findings show that environmental low-dose gestational exposure to TCDD can have significant, long-term impacts on brain development and function. https://doi.org/10.1289/EHP7352.

The Aryl Hydrocarbon Receptor in Chronic Kidney Disease: Friend or Foe?

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that promotes cell responses to small molecules derived from the diet, microorganisms, metabolism and pollutants. The AhR signal regulates many basic cellular processes, including cell cycle progression, adhesion, migration, apoptosis and cell proliferation. Many studies have shown that AhR is associated with chronic kidney disease (CKD) and its complications. This article reviews the current knowledge about the role of AhR in CKD, showing that AhR mediates CKD complications, including cardiovascular disease, anemia, bone disorders, cognitive dysfunction and malnutrition, and that it influences drug metabolism in individuals with CKD. AhR enhances the intestinal barrier function to reduce the harmful effects of uremic toxins. Therefore, understanding the complex roles of AhR during CKD is important to be able to target this transcription factor safely and effectively for CKD prevention and treatment.

The aryl hydrocarbon receptor: An environmental effector in the pathogenesis of fibrosis

The aryl hydrocarbon receptor (AhR) is a highly conserved transcription factor that can be activated by small molecules provided by dietary, plant, or microbial metabolites, and environmental pollutants. AhR is expressed in many cell types and engages in crosstalk with other signaling pathways, and therefore provides a molecular pathway that integrates environmental cues and metabolic processes. Fibrosis, which is defined as an aberrant extracellular matrix formation, is a reparative process in the terminal stage of chronic diseases. Both environmental and internal factors have been shown to participate in the pathogenesis of fibrosis; however, the underlying mechanisms still remain elusive. In this review, the potential role of AhR in the process of fibrosis, as well as potential opportunities and challenges in the development of AhR targeting therapeutics, are summarized.

TCDD inhibited the osteogenic differentiation of human fetal palatal mesenchymal cells through AhR and BMP-2/TGF-β/Smad signaling

Exposure to environmental toxicant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) causes cleft palate at high rates, but little is known about the underlying biological mechanisms. In the present study, we cultured osteoblasts from human fetal palate mesenchymal cells (hFPMCs) to explore the effects of TCDD on osteogenic differentiation. The results showed that TCDD significantly decreased cell proliferation, alkaline phosphatase (ALP) activity and calcium deposition. RNA analyses and protein detection demonstrated that TCDD downregulated a wide array of pro-osteogenic biomarkers. Further investigation of the underlying molecular mechanisms revealed that exposure to TCDD activated aryl hydrocarbon receptor (AhR) signaling and inhibited BMP-2/TGF-β1/Smad pathway molecules. The inactivation of AhR signaling using CRISPR/Cas9-mediated AhR deletion or by genetic siRNA knockdown significantly blocked the effects induced by TCDD, suggesting a critical role of AhR activation in the TCDD-mediated inhibition of hFPMC osteogenic differentiation. The cotreatment with TGF-β1 or BMP-2 and TCDD significantly relieved the activation of AhR and rescued the impairment of osteogenesis caused by TCDD. Taken together, our findings indicated that TCDD inhibited the osteogenic differentiation of hFPMCs via crosstalk between AhR and BMP-2/TGF-β1/Smad signaling pathway.

Temporal and tissue-specific activation of aryl hydrocarbon receptor in discrete mouse models of kidney disease

Emerging evidence in animal models of chronic kidney disease (CKD) implicates Aryl Hydrocarbon Receptor (AHR) signaling as a mediator of uremic toxicity. However, details about its tissue-specific and time-dependent activation in response to various renal pathologies remain poorly defined. Here, a comprehensive analysis of AHR induction was conducted in response to discrete models of kidney diseases using a transgenic mouse line expressing the AHR responsive-promoter tethered to a β-galactosidase reporter gene. Following validation using a canonical AHR ligand (a dioxin derivative), the transgenic mice were subjected to adenine-induced and ischemia/reperfusion-induced injury models representing CKD and acute kidney injury (AKI), respectively, in humans. Indoxyl sulfate was artificially increased in mice through the drinking water and by inhibiting its excretion into the urine. Adenine-fed mice showed a distinct and significant increase in β-galactosidase in the proximal and distal renal tubules, cardiac myocytes, hepatocytes, and microvasculature in the cerebral cortex. The pattern of β-galactosidase increase coincided with the changes in serum indoxyl sulfate levels. Machine-learning-based image quantification revealed positive correlations between indoxyl sulfate levels and β-galactosidase expression in various tissues. This pattern of β-galactosidase expression was recapitulated in the indoxyl sulfate-specific model. The ischemia/reperfusion injury model showed increase in β-galactosidase in renal tubules that persisted despite reduction in serum indoxyl sulfate and blood urea nitrogen levels. Thus, our results demonstrate a relationship between AHR activation in various tissues of mice with CKD or AKI and the levels of indoxyl sulfate. This study demonstrates the use of a reporter gene mouse to probe tissue-specific manifestations of uremia in translationally relevant animal models and provide hypothesis-generating insights into the mechanism of uremic toxicity that warrant further investigation.

Kynurenine signaling through the aryl hydrocarbon receptor maintains the undifferentiated state of human embryonic stem cells

Kynurenine, which is generated from tryptophan by indoleamine 2,3-dioxygenase 1 (IDO1), binds to the aryl hydrocarbon receptor (AhR). Here, we report that kynurenine was produced by undifferentiated human embryonic stem cells (hESCs) and by induced pluripotent stem cells (iPSCs). In undifferentiated hESCs, kynurenine stimulated the AhR to promote the expression of self-renewal genes. The kynurenine-AhR complex also stimulated the expression of IDO1 and AHR, activating a positive feedback loop. Inhibition of IDO1 activity reduced the proliferation of undifferentiated ESCs but did not stimulate their differentiation. Substantial amounts of free kynurenine were present in the culture medium, providing a paracrine signal for maintenance of the undifferentiated state. Kynurenine was not present in the medium of differentiated ESCs or iPSCs. When ESCs were induced to undergo ectodermal differentiation, the abundance of kynurenine in the medium was reduced through activation of the main kynurenine catabolic pathway mediated by kynurenine aminotransferase 2 (KAT2, also known as AADAT), resulting in the secretion of 2-aminoadipic acid (2-AAA) into the culture medium. Inhibition of KAT2 activity blocked ectodermal differentiation. Thus, kynurenine metabolism plays an important role in the maintenance of the undifferentiated state and in ectodermal differentiation. Furthermore, kynurenine in the culture medium is a biomarker for the undifferentiated state, whereas the presence of 2-AAA in the culture medium is a biomarker of ESCs and iPSCs that have committed to differentiate along the ectoderm lineage.

AhR signaling pathways and regulatory functions

Animals and humans are exposed each day to a multitude of chemicals in the air, water and food. They have developed a battery of enzymes and transporters that facilitate the biotransformation and elimination of these compounds. Moreover, a majority of these enzymes and transporters are inducible due to the activation of xenobiotic receptors which act as transcription factors for the regulation of their target genes (such as xenobiotic metabolizing enzymes, see below §4 for the AhR). These receptors include several members of the nuclear/steroid receptor family (CAR for Constitutive Androstane Receptor, PXR for Pregnane X Receptor) but also the Aryl hydrocarbon Receptor or AhR, a member of the bHLH-PAS family (basic Helix-Loop-Helix - Period/ARNT/Single minded). In addition to the regulation of xenobiotic metabolism, numerous alternative functions have been characterized for the AhR since its discovery. These alternative functions will be described in this review along with its endogenous functions as revealed by experiments performed on knock-out animals.

Overexpression of aryl hydrocarbon receptor (AHR) signalling pathway in human meningioma

Aryl hydrocarbon receptor (AHR) is a ligand activated transcription factor and involved in tumorigenesis of many cancers. However there are no reports on AHR in human meningioma. Therefore we examined the status of the AHR and its signalling molecules in human meningioma by using tumor biopsy samples and autopsy control meninges. We report the up regulation of AHR pathway genes like aryl hydrocarbon receptor nuclear translocator (ARNT), aldehyde dehydrogenase1family memberA3 (ALDH1A3), cytochrome P450, family1, subfamily A polypeptide1 (CYP1A1) and TCCD induced poly ADP ribose polymerase (TIPARP) gene expression in human meningioma. Further, AHR protein expression was found to be up regulated in all grades of human meningioma. We found that AHR localized in the nucleus for high grade anaplastic meningioma through immunohistochemical analysis. Since AHR signalling pathway was known to involve in inhibition of apoptosis in cancer cells, we evaluated the cyclophilin D levels which maintains mitochondrial permeability transition pore a critical event during apoptosis. We report that cyclophilin D levels were upregulated in all grades of human meningioma compared to control meninges. Finally we also evaluated c-Fos protein levels as its levels were regulated by AHR. Here we report that c-Fos protein levels were down regulated in all grades of human meningioma compared to control meninges. To sum-up we found that AHR signalling pathway components were upregulated, as the grade of the meningioma progresses from low to high grade, suggesting an important role of AHR signalling pathway in human meningioma.

The aryl hydrocarbon receptor in the crossroad of signalling networks with therapeutic value

The aryl hydrocarbon receptor (AhR) is well-known for its major contributions to the cellular responses against environmental toxins and carcinogens. Notably, AhR has also emerged as a key transcription factor controlling many physiological processes including cell proliferation and apoptosis, differentiation, adhesion and migration, pluripotency and stemness. These novel functions have broadened our understanding of the signalling pathways and molecular intermediates interacting with AhR under both homeostatic and pathological conditions. Recent discoveries link AhR with the function of essential organs such as liver, skin and gonads, and with complex organismal structures including the immune and cardiovascular systems. The identification of potential endogenous ligands able to regulate AhR activity, opens the possibility of designing ad hoc molecules with pharmacological and/or therapeutic value to treat human diseases in which AhR may have a causal role. Integration of experimental data from in vitro and in vivo studies with "omic" analyses of human patients affected with cancer, immune diseases, inflammation or neurological disorders will likely contribute to validate the clinical relevance of AhR and the possible benefits of modulating its activity by pharmacologically-driven strategies. In this review, we will highlight signalling pathways involved in human diseases that could be targetable by AhR modulators and discuss the feasibility of using such molecules in therapy. The pros and cons of AhR-aimed approaches will be also mentioned.

MicroRNA transcriptome analysis on hypertrophy of ligamentum flavum in patients with lumbar spinal stenosis

Introduction

Molecular pathways involved in ligamentum flavum (LF) hypertrophy are still unclarified. The purpose of this study was to characterize LF hypertrophy by microRNA (miRNA) profiling according to the classification of lumbar spinal stenosis (LSS).

Methods

Classification of patients with LSS into ligamentous and non-ligamentous cases was conducted by clinical observation and the morphometric parameter adopting the LF/spinal canal area ratio (LSAR) from measurements of magnetic resonance imaging (MRI) T2 weighed images. LF from patients with ligamentous stenosis (n=10) were considered as the degenerative hypertrophied samples, and those from patients with non-ligamentous LSS (n=7) and lumbar disc herniation (LDH, n=3) were used as non-hypertrophied controls. Profiling of miRNA from all samples was conducted by Agilent microarray. Microarray data analysis was performed with GeneSpring GX, and pathway analysis was performed using Ingenuity Pathway Analysis.

Results

The mean LSAR in the ligamentous group was significantly higher than that in the control group (0.662±0.154 vs 0.301±0.068, p=0.0000171). Ten significantly differentially expressed miRNA were identified and taken as a signature of LF hypertrophy: nine miRNA showed down-regulated expression, and one showed up-regulated expression in the ligamentous LF. Among those, miR-423-5p (r_s=-0.473, p<0.05), miR-4306 (r_s=-0.628, p<0.01), miR-516b-5p (r_s=-0.629, p<0.01), and miR-497-5p (r_s=0.461, p<0.05) were correlated to the LSAR. Pathway analysis predicted aryl hydrocarbon receptor signaling (p<0.01), Wnt/β-catenin signaling (p<0.01), and insulin receptor signaling (p<0.05) as canonical pathways associated with the miRNA signature.

Conclusions

Classification based on quantification of the MRI axial image is useful for studying hypertrophy of the LF. Aryl hydrocarbon receptor and Wnt/β-catenin signaling may be involved in LF hypertrophy.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) increases necroinflammation and hepatic stellate cell activation but does not exacerbate experimental liver fibrosis in mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a persistent environmental contaminant and high-affinity ligand for the aryl hydrocarbon receptor (AhR). Increasing evidence indicates that AhR signaling contributes to wound healing, which involves the coordinated deposition and remodeling of the extracellular matrix. In the liver, wound healing is attributed to the activation of hepatic stellate cells (HSCs), which mediate fibrogenesis through the production of soluble mediators and collagen type I. We recently reported that TCDD treatment increases the activation of human HSCs in vitro. The goal of this study was to determine how TCDD impacts HSC activation in vivo using a mouse model of experimental liver fibrosis. To elicit fibrosis, C57BL6/male mice were treated twice weekly for 8weeks with 0.5ml/kg carbon tetrachloride (CCl₄). TCDD (20μg/kg) or peanut oil (vehicle) was administered once a week during the last 2weeks. Results indicate that TCDD increased liver-body-weight ratios, serum alanine aminotransferase activity, and hepatic necroinflammation in CCl₄-treated mice. Likewise, TCDD treatment increased mRNA expression of HSC activation and fibrogenesis genes, namely α-smooth muscle actin, desmin, delta-like homolog-1, TGF-β1, and collagen type I. However, TCDD treatment did not exacerbate fibrosis, nor did it increase the collagen content of the liver. Instead, TCDD increased hepatic collagenase activity and increased expression of matrix metalloproteinase (MMP)-13 and the matrix regulatory proteins, TIMP-1 and PAI-1. These results support the conclusion that TCDD increases CCl₄-induced liver damage and exacerbates HSC activation, yet collagen deposition and the development of fibrosis may be limited by TCDD-mediated changes in extracellular matrix remodeling.

Aryl Hydrocarbon Receptor Activation by TCDD Modulates Expression of Extracellular Matrix Remodeling Genes during Experimental Liver Fibrosis

The aryl hydrocarbon receptor (AhR) is a soluble, ligand-activated transcription factor that mediates the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Increasing evidence implicates the AhR in regulating extracellular matrix (ECM) homeostasis. We recently reported that TCDD increased necroinflammation and myofibroblast activation during liver injury elicited by carbon tetrachloride (CCl₄). However, TCDD did not increase collagen deposition or exacerbate fibrosis in CCl₄-treated mice, which raises the possibility that TCDD may enhance ECM turnover. The goal of this study was to determine how TCDD impacts ECM remodeling gene expression in the liver. Male C57BL/6 mice were treated for 8 weeks with 0.5 mL/kg CCl₄, and TCDD (20 μg/kg) was administered during the last two weeks. Results indicate that TCDD increased mRNA levels of procollagen types I, III, IV, and VI and the collagen processing molecules HSP47 and lysyl oxidase. TCDD also increased gelatinase activity and mRNA levels of matrix metalloproteinase- (MMP-) 3, MMP-8, MMP-9, and MMP-13. Furthermore, TCDD modulated expression of genes in the plasminogen activator/plasmin system, which regulates MMP activation, and it also increased TIMP1 gene expression. These findings support the notion that AhR activation by TCDD dysregulates ECM remodeling gene expression and may facilitate ECM metabolism despite increased liver injury.

Innovative Therapies in Wound Healing

Background:

Apligraf is a bioengineered skin product composed of neonatal fibroblasts and keratinocytes. The FDA has approved Apligraf for the treatment of chronic venous ulcers and diabetic ulcers.

Objective:

We review the development of bioengineered skin, examine the cellular activities of various growth factors that may facilitate wound healing, and discuss the results of clinical trials with a particular construct, Apligraf, as proof of principle.

Conclusion:

Bioengineered skin acts as a “smart” delivery system for growth factors and other stimulatory substances. Not only does it present a novel treatment for chronic and diabetic ulcers, but it could also be considered for application to other types of acute wounds.

Environmental Ligands of the Aryl Hydrocarbon Receptor and Their Effects in Models of Adult Liver Progenitor Cells

The toxicity of environmental and dietary ligands of the aryl hydrocarbon receptor (AhR) in mature liver parenchymal cells is well appreciated, while considerably less attention has been paid to their impact on cell populations exhibiting phenotypic features of liver progenitor cells. Here, we discuss the results suggesting that the consequences of the AhR activation in the cellular models derived from bipotent liver progenitors could markedly differ from those in hepatocytes. In contact-inhibited liver progenitor cells, the AhR agonists induce a range of effects potentially linked with tumor promotion. They can stimulate cell cycle progression/proliferation and deregulate cell-to-cell communication, which is associated with downregulation of proteins forming gap junctions, adherens junctions, and desmosomes (such as connexin 43, E-cadherin, β-catenin, and plakoglobin), as well as with reduced cell adhesion and inhibition of intercellular communication. At the same time, toxic AhR ligands may affect the activity of the signaling pathways contributing to regulation of liver progenitor cell activation and/or differentiation, such as downregulation of Wnt/β-catenin and TGF-β signaling, or upregulation of transcriptional targets of YAP/TAZ, the effectors of Hippo signaling pathway. These data illustrate the need to better understand the potential role of liver progenitors in the AhR-mediated liver carcinogenesis and tumor promotion.

Ah Receptor Activation by Dioxin Disrupts Activin, BMP, and WNT Signals During the Early Differentiation of Mouse Embryonic Stem Cells and Inhibits Cardiomyocyte Functions

The AHR is a ligand-activated transcription factor that mediates gene-environment interactions. Genome-wide expression profiling during differentiation of mouse ES cells into cardiomyocytes showed that AHR activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin; Dioxin (TCDD), its prototypical ligand, disrupted the expression of multiple homeobox transcription factors and inhibited cardiomyocyte contractility. Here we treated ES cells with TCDD at daily differentiation intervals to investigate whether TCDD-induced loss of contractility had a developmental window of sensitivity. Surprisingly, contractility was an AHR-dependent TCDD target solely between differentiation days 0 and 3 during the period of panmesoderm development, when TCDD also disrupted expression of genes in the TGFβ/BMP2/4 and wingless-type MMTV integration site (WNT)signaling pathways, suppressed the secretion of bone morphogenetic protein (BMP4), WNT3a, and WNT5a and elevated the secretion of Activin A, as determined by ELISA of the secreted proteins in the culture medium. Supplementing the culture medium with BMP4, WNT3a, or WNT5a during the first 3 days of differentiation successfully countered TCDD-induced impairment of contractility, while anti-WNT3a, or anti-WNT5a antibodies or continuous Noggin (a BMP4 antagonist) or Activin A treatment inhibited the contractile phenotype. In Ahr(+/+), but not in Ahr(-) (/) (-) ES cells, TCDD treatment significantly increased mitochondrial copy number, suggestive of mitochondrial stress and remodeling. Sustained AHR activation during ES cell differentiation appears to disrupt the expression of signals critical to the ontogeny of cardiac mesoderm and cause the loss of contractility in the resulting cardiomyocyte lineage.

Hepatocyte-specific deletion of ARNT (aryl hydrocarbon Receptor Nuclear Translocator) results in altered fibrotic gene expression in the thioacetamide model of liver injury

Background & Aims

Recent studies have shown that increased expression of liver hypoxia inducible factor 2-α (HIF-2α) leads to liver inflammation and a pro-fibrotic gene expression signature. Aryl hydrocarbon Receptor Nuclear Translocator (ARNT) is required for HIF-2α transcriptional activity and has previously been shown to regulate hepatic metabolism in mice. In these studies we examined the role of hepatocyte ARNT in the thioacetamide (TAA)-induced model of liver fibrosis.

Methods

Hepatocyte-specific ARNT-null (LARNT) mice were created using an albumin promoter-driven Cre recombinase. LARNT and floxed control (FC) littermates were placed on chow diet and received twice weekly intraperitoneal injections of 0.15mg/g body weight of TAA for 13 weeks.

Results

TAA treated LARNT and FC mice had a similar pattern of fibrosis. Quantification of Sirius red histology staining and hydroxyproline content revealed mixed results in terms of collagen deposition in LARNT livers. There was no significant difference in hepatocyte apoptosis or proliferation, as assessed by cleaved Caspase-3 and Ki67 respectively. LARNT mice had decreased macrophage accumulation, and decreased liver mRNA expression of Col1A1, Col1A2, Col5A1, Tgfβ1, Tgfβ2, Timp1 and Timp2.

Conclusions

Deletion of hepatocyte ARNT leads to altered expression of collagen associated mRNA and reduced macrophage infiltration in the TAA-induced model of liver fibrosis. It appears that hepatocyte ARNT is not a requirement for initiation of liver fibrogenesis, but does regulate pro-fibrotic gene expression and macrophage accumulation.

Anti-remodeling and anti-fibrotic effects of the neuregulin-1β glial growth factor 2 in a large animal model of heart failure

BACKGROUND

Neuregulin-1β (NRG-1β) is a growth factor critical for cardiac development and repair with therapeutic potential for heart failure. We previously showed that the glial growth factor 2 (GGF2) isoform of NRG-1β improves cardiac function in rodents after myocardial infarction (MI), but its efficacy in a large animal model of cardiac injury has not been examined. We therefore sought to examine the effects of GGF2 on ventricular remodeling, cardiac function, and global transcription in post-MI swine, as well as potential mechanisms for anti-remodeling effects.

METHODS AND RESULTS

MI was induced in anesthetized swine (n=23) by intracoronary balloon occlusion. At 1 week post-MI, survivors (n=13) received GGF2 treatment (intravenous, biweekly for 4 weeks; n=8) or were untreated (n=5). At 5 weeks post-MI, fractional shortening was higher (32.8% versus 25.3%, P=0.019), and left ventricular (LV) end-diastolic dimension lower (4.5 versus 5.3 cm, P=0.003) in GGF2-treated animals. Treatment altered expression of 528 genes, as measured by microarrays, including collagens, basal lamina components, and matricellular proteins. GGF2-treated pigs exhibited improvements in LV cardiomyocyte mitochondria and intercalated disk structures and showed less fibrosis, altered matrix structure, and fewer myofibroblasts (myoFbs), based on trichrome staining, electron microscopy, and immunostaining. In vitro experiments with isolated murine and rat cardiac fibroblasts demonstrate that NRG-1β reduces myoFbs, and suppresses TGFβ-induced phospho-SMAD3 as well as αSMA expression.

CONCLUSIONS

These results suggest that GGF2/NRG-1β prevents adverse remodeling after injury in part via anti-fibrotic effects in the heart.

Aryl hydrocarbon receptor knock-out exacerbates choroidal neovascularization via multiple pathogenic pathways

The aryl hydrocarbon receptor (AhR) is a heterodimeric transcriptional regulator with pleiotropic functions in xenobiotic metabolism and detoxification, vascular development and cancer. Herein, we report a previously undescribed role for the AhR signalling pathway in the pathogenesis of the wet, neovascular subtype of age-related macular degeneration (AMD), the leading cause of vision loss in the elderly in the Western world. Comparative analysis of gene expression profiles of aged AhR^−/− and wild-type (wt) mice, using high-throughput RNA sequencing, revealed differential modulation of genes belonging to several AMD-related pathogenic pathways, including inflammation, angiogenesis and extracellular matrix regulation. To investigate AhR regulation of these pathways in wet AMD, we experimentally induced choroidal neovascular lesions in AhR^−/− mice and found that they measured significantly larger in area and volume compared to age-matched wt mice. Furthermore, these lesions displayed a higher number of ionized calcium-binding adaptor molecule 1-positive (Iba1⁺) microglial cells and a greater amount of collagen type IV deposition, events also seen in human wet AMD pathology specimens. Consistent with our in vivo observations, AhR knock-down was sufficient to increase choroidal endothelial cell migration and tube formation in vitro. Moreover, AhR knock-down caused an increase in collagen type IV production and secretion in both retinal pigment epithelial (RPE) and choroidal endothelial cell cultures, increased expression of angiogenic and inflammatory molecules, including vascular endothelial growth factor A (VEGFA) and chemokine (C–C motif) ligand 2 (CCL2) in RPE cells, and increased expression of secreted phosphoprotein 1 (SPP1) and transforming growth factor-β1 (TGFβ1) in choroidal endothelial cells. Collectively, our findings identify AhR as a regulator of multiple pathogenic pathways in experimentally induced choroidal neovascularization, findings that are consistent with a possible role of AhR in wet AMD. The data discussed in this paper have been deposited in NCBI's Gene Expression Omnibus; GEO Submission No. GSE56983, NCBI Tracking System No. 17021116.

Effect of nonpersistent pesticides on estrogen receptor, androgen receptor, and aryl hydrocarbon receptor

Nonpersistent pesticides are considered less harmful for the environment, but their impact as endocrine disruptors has not been fully explored. The pesticide Switch was applied to grape vines, and the maximum residue concentration of its active ingredients was quantified. The transactivation potential of the pesticides Acorit, Frupica, Steward, Reldan, Switch, Cantus, Teldor, and Scala and their active compounds (hexythiazox, mepanipyrim, indoxacarb, chlorpyrifos-methyl, cyprodinil, fludioxonil, boscalid, fenhexamid, and pyrimethanil) were tested on human estrogen receptor α (ERα), androgen receptor (AR) and arylhydrocarbon receptor (AhR) in vitro. Relative binding affinities of the pure pesticide constituents for AR and their effect on human breast cancer and prostate cancer cell lines were evaluated. Residue concentrations of Switch's ingredients were below maximum residue limits. Fludioxonil and fenhexamid were ERα agonists (EC50 -values of 3.7 and 9.0 μM, respectively) and had time-dependent effects on endogenous ERα-target gene expression (cyclin D1, progesterone receptor, and nuclear respiratory factor 1) in MCF-7 human breast cancer cells. Fludioxonil, mepanipyrim, cyprodinil, pyrimethanil, and chlorpyrifos-methyl were AhR-agonists (EC50 s of 0.42, 0.77, 1.4, 4.6, and 5.1 μM, respectively). Weak AR binding was shown for chlorpyrifos-methyl, cyprodinil, fenhexamid, and fludioxonil. Assuming a total uptake which does not take metabolism and clearance rates into account, our in vitro evidence suggests that pesticides could activate pathways affecting hormonal balance, even within permitted limits, thus potentially acting as endocrine disruptors.

Aryl hydrocarbon receptor-dependent regulation of miR-196a expression controls lung fibroblast apoptosis but not proliferation

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor implicated in the regulation of apoptosis and proliferation. Although activation of the AhR by xenobiotics such as dioxin inhibits the cell cycle and control apoptosis, paradoxically, AhR expression also promotes cell proliferation and survival independent of exogenous ligands. The microRNA (miRNA) miR-196a has also emerged as a regulator of proliferation and apoptosis but a relationship between the AhR and miR-196a is not known. Therefore, we hypothesized that AhR-dependent regulation of endogenous miR-196a expression would promote cell survival and proliferation. Utilizing lung fibroblasts from AhR deficient (AhR(-/-)) and wild-type (AhR(+/+)) mice, we show that there is ligand-independent regulation of miRNA, including low miR-196a in AhR(-/-) cells. Validation by qRT-PCR revealed a significant decrease in basal expression of miR-196a in AhR(-/-) compared to AhR(+/+) cells. Exposure to AhR agonists benzo[a]pyrene (B[a]P) and FICZ as well as AhR antagonist CH-223191 decreased miR-196a expression in AhR(+/+) fibroblasts concomitant with decreased AhR protein levels. There was increased proliferation only in AhR(+/+) lung fibroblasts in response to serum, corresponding to a decrease in p27(KIP1) protein, a cyclin-dependent kinase inhibitor. Increasing the cellular levels of miR-196a had no effect on proliferation or expression of p27(KIP1) in AhR(-/-) fibroblasts but attenuated cigarette smoke-induced apoptosis. This study provides the first evidence that AhR expression is essential for the physiological regulation of cellular miRNA levels- including miR-196a. Future experiments designed to elucidate the functional relationship between the AhR and miR-196a may delineate additional novel ligand-independent roles for the AhR.

Eriosema laurentii De Wild (Leguminosae) methanol extract has estrogenic properties and prevents menopausal symptoms in ovariectomized Wistar rats

ETHNOPHARMACOLOGICAL RELEVANCE

Eriosema laurentii De Wild (Leguminosae) is a medicinal plant used in West and Central Africa for different diseases. In Cameroon, this plant is used as a treatment for infertility, and various gynecological and menopausal complaints. However, despite this use as a natural remedy, the biological activity of Eriosema laurentii has not been studied until now.

AIM OF STUDY

In order to determine the potential use of this plant in gynecological conditions/disorders, we evaluated the estrogenic properties of a methanol extract of its aerial parts and its ability to prevent different menopausal health problems induced by bilateral oophorectomy.

MATERIAL AND METHODS

Two approaches were used. In vitro, recombinant yeast systems were applied, featuring either the respective human receptors (ERα, AR, and PR) or into chromosome III integrated human aryl hydrocarbon receptor (AhR) and the respective reporter plasmid. In vivo, the investigation was carried out using the 3 days uterotrophic assay and 9 weeks oral treatment in ovariectomized rats.

RESULTS

The results showed that the methanol extract of the aerial parts of Eriosema laurentii transactivated the estrogen receptor-α and displayed AhR agonistic activity but was neither androgenic nor progesteronic. In rats, the extract did not induce endometrium proliferation either in the 3-day or the 9-week treatment regimens, but induced vaginal stratification and cornification, prevented loss of femur bone mass, increased high density lipoprotein cholesterol (HDL-C), and reduced total cholesterol (TC), low density lipoprotein cholesterol (LDL-C), TC/HDL-C ratio, LDL-C/HDL-C ratio and the atherogenic index of plasma (AIP).

CONCLUSION

These results suggest that the methanol extract of the aerial parts of Eriosema laurentii does not seem to have an undesirable influence on the endometrium but might prevent vaginal dryness and bone mass loss and improve the lipid profile.

Aryl hydrocarbon receptor-dependent induction of liver fibrosis by dioxin

The contribution of environmental pollutants to liver fibrosis is an important and poorly explored issue. In vitro studies suggest that the environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and other aryl hydrocarbon receptor (AhR) ligands induce several genes that are known to be upregulated during liver fibrosis. Our aim was to determine whether exposure to such pollutants can lead to liver fibrosis and to characterize the mechanisms of action. Mice were treated for 2, 14, or 42 days, once a week with 25 µg/kg of TCDD. Gene and protein expression, in vitro and in vivo, as well as liver histology were investigated for each treatment. Treatment of mice with TCDD for 2 weeks modified the hepatic expression of markers of fibrosis such as collagen 1A1 and α-smooth muscle actin. This is not observed in AhR knockout mice. Following 6 weeks of treatment, histological features of murine hepatic fibrosis became apparent. In parallel, the levels of inflammatory cytokines (interleukin-1 beta, tumor necrosis factor α) and of markers of activated fibroblasts(fibroblast-specific protein 1) were found to be upregulated. Interestingly, we also found increased expression of genes of the TGF-β pathway and a concomitant decrease of miR-200a levels. Because the transcription factors of the Snail family were shown to be involved in liver fibrosis, we studied their regulation by TCDD. Two members of the Snail family were increased, whereas their negative targets, the epithelial marker E-cadherin and Claudin 1, were decreased. Further, the expression of mesenchymal markers was increased. Finally, we confirmed that Snai2 is a direct transcriptional target of TCDD in the human hepatocarcinoma cell line, HepG2. The AhR ligand, TCDD, induces hepatic fibrosis by directly regulating profibrotic pathways.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Activation of aryl hydrocarbon receptor (ahr) by tranilast, an anti-allergy drug, promotes miR-302 expression and cell reprogramming

MiR-302 has been shown to regulate pluripotency genes and help somatic cell reprogramming. Thus, promotion of endogenous miR-302 expression could be a desirable way to facilitate cell reprogramming. By using a luciferase reporter system of the miR-302 promoter, we screened and found that an anti-allergy drug, tranilast, could significantly promote miR-302 expression. Further experiments revealed that two aryl hydrocarbon receptor (AhR) binding motifs on the miR-302 promoter are critical and that activation of AhR is required for tranilast-induced miR-302 expression. Consistently, not only tranilast but other AhR agonists promoted miR-302 expression. Furthermore, the activation of AhR facilitated cell reprogramming in a miR-302-dependent way. These results elucidate that miR-302 expression can be regulated by AhR and thus provide a strategy for promoting somatic cell reprogramming by AhR ligands.

Dioxin receptor expression inhibits basal and transforming growth factor β-induced epithelial-to-mesenchymal transition

Recent studies have emphasized the role of the dioxin receptor (AhR) in maintaining cell morphology, adhesion, and migration. These novel AhR functions depend on the cell phenotype, and although AhR expression maintains mesenchymal fibroblasts migration, it inhibits keratinocytes motility. These observations prompted us to investigate whether AhR modulates the epithelial-to-mesenchymal transition (EMT). For this, we have used primary AhR(+/+) and AhR(-/-) keratinocytes and NMuMG cells engineered to knock down AhR levels (sh-AhR) or to express a constitutively active receptor (CA-AhR). Both AhR(-/-) keratinocytes and sh-AhR NMuMG cells had increased migration, reduced levels of epithelial markers E-cadherin and β-catenin, and increased expression of mesenchymal markers Snail, Slug/Snai2, vimentin, fibronectin, and α-smooth muscle actin. Consistently, AhR(+/+) and CA-AhR NMuMG cells had reduced migration and enhanced expression of epithelial markers. AhR activation by the agonist FICZ (6-formylindolo[3,2-b]carbazole) inhibited NMuMG migration, whereas the antagonist α-naphthoflavone induced migration as did AhR knockdown. Exogenous TGFβ exacerbated the promigratory mesenchymal phenotype in both AhR-expressing and AhR-depleted cells, although the effects on the latter were more pronounced. Rescuing AhR expression in sh-AhR cells reduced Snail and Slug/Snai2 levels and cell migration and restored E-cadherin levels. Interference of AhR in human HaCaT cells further supported its role in EMT. Interestingly, co-immunoprecipitation and immunofluorescence assays showed that AhR associates in common protein complexes with E-cadherin and β-catenin, suggesting the implication of AhR in cell-cell adhesion. Thus, basal or TGFβ-induced AhR down-modulation could be relevant in the acquisition of a motile EMT phenotype in both normal and transformed epithelial cells.

Aryl-hydrocarbon receptor activity modulates prolactin expression in the pituitary

Pituitary tumors account for 15% of intracranial neoplasms, however the extent to which environmental toxicants contribute to the proliferation and hormone expression of pituitary cells is unknown. Aryl-hydrocarbon receptor (AhR) interacting protein (AIP) loss of function mutations cause somatotrope and lactotrope adenomas in humans. AIP sequesters AhR and inhibits its transcriptional function. Because of the link between AIP and pituitary tumors, we hypothesize that exposure to dioxins, potent exogenous ligands for AhR that are persistent in the environment, may predispose to pituitary dysfunction through activation of AhR. In the present study, we examined the effect of AhR activation on proliferation and endogenous pituitary hormone expression in the GH3 rat somatolactotrope tumor cell line and the effect of loss of AhR action in knockout mice. GH3 cells respond to nM doses of the reversible AhR agonist β-naphthoflavone with a robust induction of Cyp1a1. Although mRNA levels of the anti-proliferative signaling cytokine TGFbeta1 are suppressed upon β-naphthoflavone treatment, we did not observe an alteration in cell proliferation. AhR activation with β-naphthoflavone suppresses Ahr expression and impairs expression of prolactin (PRL), but not growth hormone (GH) mRNA in GH3 cells. In mice, loss of Ahr similarly leads to a reduction in Prl mRNA at P3, while Gh is unaffected. Additionally, there is a significant reduction in pituitary hormones Lhb and Fshb in the absence of Ahr. Overall, these results demonstrate that AhR is important for pituitary hormone expression and suggest that environmental dioxins can exert endocrine disrupting effects at the pituitary.

Silencing of miR-124 induces neuroblastoma SK-N-SH cell differentiation, cell cycle arrest and apoptosis through promoting AHR

Neuroblastoma is the most common extracranial solid tumor in children. We investigate whether miR-124, the abundant neuronal miRNA, plays a pivotal role in neuroblastoma. Knockdown of miR-124 promotes neuroblastoma SK-N-SH cell differentiation, cell cycle arrest and apoptosis. Further miR-124 is predicted to target aryl hydrocarbon receptor (AHR) which may promote neuroblastoma cell differentiation. We validate that miR-124 may suppress the expression of AHR by targeting its 3'-UTR. These results suggest that miR-124 could serve as a potential therapeutic target of neuroblastoma.

Aryl hydrocarbon receptor-null allele mice have hematopoietic stem/progenitor cells with abnormal characteristics and functions

The aryl hydrocarbon receptor (AhR) belongs to the basic helix-loop-helix family of DNA-binding proteins that play a role in the toxicity and carcinogenicity of certain chemicals. The most potent ligand of the AhR known is 2,3,7,8-tetracholorodibenzo-p-dioxin. We previously reported tetrachlorodibenzo-p-dioxin-induced alterations in numbers and function of hematopoietic stem cells (HSCs). To better understand a possible role of the AhR in hematopoiesis, studies were undertaken in young adult AhR null-allele (KO) mice. These mice have enlarged spleens with increased number of cells from different lineages. Altered expression of several chemokine, cytokine, and their receptor genes were observed in spleen. The KO mice have altered numbers of circulating red and white blood cells, as well as a circadian rhythm-associated 2-fold increase in the number of HSC-enriched Lin(-)Sca-1(+)c-Kit(+) (LSK) cells in bone marrow. Primary cultures of KO HSCs and in vivo bromodeoxyuridine incorporation studies demonstrated an approximate 2-fold increased proliferative ability of these cells. More LSK cells from KO mice were in G(1) and S phases of cell cycle. Competitive repopulation studies also indicated significant functional changes in KO HSCs. LSK cells showed increased expression of Cebpe and decreased expression of several hematopoiesis-associated genes. These data indicate that AhR has a physiological and functional role in hematopoiesis. The AhR appears to play a role in maintaining the normal quiescence of HSCs.

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

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

Ube2l3 gene expression is modulated by activation of the aryl hydrocarbon receptor: implications for p53 ubiquitination

Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a halogenated aromatic hydrocarbon and environmental contaminant, results in several deleterious effects, including fetal malformation and cancer. These effects are mediated by the aryl hydrocarbon receptor (AhR), a ligand-activated receptor that regulates the expression of genes encoding xenobiotic-metabolizing enzymes. Several reports suggest that AhR function is beyond the adaptive chemical response. In the present study, we analyzed and compared gene expression profiles of C57BL/6N wild-type (WT) and Ahr-null mice. DNA microarray and quantitative RT-PCR analyses revealed changes in the expression of genes involved in the ubiquitin-proteasome system (UPS). UPS has an important role in cellular homeostasis control and dysfunction of this pathway has been implicated in the development of several human pathologies. Protein ubiquitination is a multi-step enzymatic process that regulates the stability, function, and/or localization of the modified proteins. This system is highly regulated post-translationally by covalent modifications. However, little information regarding the transcriptional regulation of the genes encoding ubiquitin (Ub) proteins is available. Therefore, we investigated the role of the AhR in modulation of the UPS and regulation of Ube2l3 transcription, an E2 ubiquitin-conjugating enzyme, as well as the effects on p53 degradation. Our results indicate that AhR inactivation decreases on liver proteasome activity, probably due to a down-regulation on the expression of several proteasome subunits. On the other hand, AhR activation increases Ube2l3 mRNA and protein levels by controlling Ube2l3 gene expression, resulting in increased p53 ubiquitination and degradation. In agreement with this, induction of apoptosis was attenuated by the AhR activation.

The aryl hydrocarbon receptor functions as a tumor suppressor of liver carcinogenesis

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that mediates the biological and toxic effects of its xenobiotic ligands. Previous cell culture studies have shown that, in addition to controlling the xenobiotic detoxification response, AHR activation leads to G0-G1 arrest, diminished capacity for DNA replication, and inhibition of cell proliferation. In fact, recent work from our own and from other laboratories suggests that AHR may function as a tumor suppressor gene that becomes silenced during the process of tumor formation. To test this hypothesis and determine whether the mouse Ahr gene acts as a tumor suppressor gene in vivo, we have examined the role of Ahr ablation in liver tumorigenesis induced by the genotoxic chemical diethylnitrosamine (DEN), a hepatic carcinogen that is not an AHR ligand. In mice given a single i.p. injection of DEN, AHR antagonized liver tumor formation and growth by regulating cell proliferation, inflammatory cytokine expression, and DNA damage, parameters which were significantly elevated in the livers of control and, more so, of DEN-exposed Ahr-/- mice. Ahr-/- hepatocytes also showed significantly higher numbers of 4N cells, increased expression of proliferative markers, and repression of tumor suppressor genes. These data support the concept that in its basal state in the absence of a xenobiotic ligand, the Ahr gene functions as a tumor suppressor gene, and that its silencing may be associated with cancer progression.

Aromatic hydrocarbons upregulate glyceraldehyde-3-phosphate dehydrogenase and induce changes in actin cytoskeleton. Role of the aryl hydrocarbon receptor (AhR)

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a multifunctional enzyme involved in several cellular functions including glycolysis, membrane transport, microtubule assembly, DNA replication and repair, nuclear RNA export, apoptosis, and the detection of nitric oxide stress. Therefore, modifications in the regulatory ability and function of GAPDH may alter cellular homeostasis. We report here that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and beta-naphthoflavone, which are well-known ligands for the aryl hydrocarbon receptor (AhR), increase GAPDH mRNA levels in vivo and in vitro, respectively. These compounds fail to induce GAPDH transcription in an AhR-null mouse model, suggesting that the increase in GAPDH level is dependent upon AhR activation. To analyse the consequences of AhR ligands on GAPDH function, mice were treated with TCDD and the level of liver activity of GAPDH was determined. The results showed that TCDD treatment increased GAPDH activity. On the other hand, treatment of Hepa-1 cells with beta-naphthoflavone leads to an increase in microfilament density when compared to untreated cultures. Collectively, these results suggest that AhR ligands, such as polycyclic hydrocarbons, can modify GAPDH expression and, therefore, have the potential to alter the multiple functions of this enzyme.

Dioxin receptor deficiency impairs angiogenesis by a mechanism involving VEGF-A depletion in the endothelium and transforming growth factor-beta overexpression in the stroma

Angiogenesis has key roles in development and in the progression of human diseases such as cancer. Consequently, identifying the novel markers and regulators of angiogenesis is a critical task. The dioxin receptor (AhR) contributes to vascular homeostasis and to the endothelial response to toxins, although the mechanisms involved are largely uncharacterized. Here, we show that AhR-null mice (AhR(-/-)) have impaired angiogenesis in vivo that compromises tumor xenograft growth. Aortic rings emigration experiments and RNA interference indicated that AhR(-/-) endothelial cells failed to branch and to form tube-like structures. Such a phenotype was found to be vascular endothelial growth factor (VEGF)-dependent, as AhR(-/-) aortic endothelial cells (MAECs) secreted lower amounts of active VEGF-A and their treatment with VEGF-A rescued angiogenesis in culture and in vivo. Further, the addition of anti-VEGF antibody to AhR(+/+) MAECs reduced angiogenesis. Treatment under hypoxic conditions with 2-methoxyestradiol suggested that HIF-1alpha modulates endothelial VEGF expression in an AhR-dependent manner. Importantly, AhR-null stromal myofibroblasts produced increased transforming growth factor-beta (TGFbeta) activity, which inhibited angiogenesis in human endothelial cells (HMECs) and AhR(-/-) mice, whereas the co-culture of HMECs with AhR(-/-) myofibroblasts or with their conditioned medium inhibited branching, which was restored by an anti-TGFbeta antibody. Moreover, VEGF and TGFbeta activities cooperated in modulating angiogenesis, as the addition of TGFbeta to AhR(-/-) MAECs further reduced their low basal VEGF-A activity. Thus, AhR modulates angiogenesis through a mechanism requiring VEGF activation in the endothelium and TGFbeta inactivation in the stroma. These data highlight the role of AhR in cardiovascular homeostasis and suggest that this receptor can be a novel regulator of angiogenesis during tumor development.

Aryl hydrocarbon receptor activation inhibits in vitro differentiation of human monocytes and Langerhans dendritic cells

The transcription factor aryl hydrocarbon receptor (AhR) represents a promising therapeutic target in allergy and autoimmunity. AhR signaling induced by the newly described ligand VAF347 inhibits allergic lung inflammation as well as suppresses pancreatic islet allograft rejection. These effects are likely mediated via alterations in dendritic cell (DC) function. Moreover, VAF347 induces tolerogenic DCs. Langerhans cells (LCs) are immediate targets of exogenous AhR ligands at epithelial surfaces; how they respond to AhR ligands remained undefined. We studied AhR expression and function in human LCs and myelopoietic cell subsets using a lineage differentiation and gene transduction model of human CD34(+) hematopoietic progenitors. We found that AhR is highly regulated during myeloid subset differentiation. LCs expressed highest AhR levels followed by monocytes. Conversely, neutrophil granulocytes lacked AhR expression. AhR ligands including VAF347 arrested the differentiation of monocytes and LCs at an early precursor cell stage, whereas progenitor cell expansion or granulopoiesis remained unimpaired. AhR expression was coregulated with the transcription factor PU.1 during myeloid subset differentiation. VAF347 inhibited PU.1 induction during initial monocytic differentiation, and ectopic PU.1 restored monocyte and LC generation in the presence of this compound. AhR ligands failed to interfere with cytokine receptor signaling during LC differentiation and failed to impair LC activation/maturation. VAF347-mediated antiproliferative effect on precursors undergoing LC lineage differentiation occurred in a clinically applicable serum-free culture model and was not accompanied by apoptosis induction. In conclusion, AhR agonist signaling interferes with transcriptional processes leading to monocyte/DC lineage commitment of human myeloid progenitor cells.

Ontogenic expression of hepatic Ahr mRNA is associated with histone H3K4 di-methylation during mouse liver development

The aryl hydrocarbon receptor (Ahr) is a xenobiotic sensor that regulates the expression of a battery of drug-metabolizing genes. However, Ahr is also important for normal liver development. The purpose of the present study was to examine the ontogeny of Ahr mRNA in mouse liver, and determine the epigenetic mechanisms regulating Ahr gene transcription during postnatal liver development. There was a 224% increase in hepatic Ahr mRNA from 2 days before birth to 45 days after birth. ChIP-on-chip analysis demonstrated that DNA methylation and histone H3K27 tri-methylation (H3K27Me3), two epigenetic marks for suppression of gene transcription, were consistently low around the Ahr gene locus. In contrast, enrichment of histone H3K4 di-methylation (H3K4Me2), a hallmark for gene activation, increased 182% from prenatal to young adult period around the Ahr gene locus. Regression analysis revealed a strong correlation between enrichment of H3K4Me2 and Ahr mRNA (r=0.91). In conclusion, postnatal H3K4Me2 enrichment positively associates with Ahr mRNA in developing mouse liver, providing a permissive chromatin state allowing Ahr gene transactivation in postnatal liver development.

Loss of dioxin-receptor expression accelerates wound healing in vivo by a mechanism involving TGFbeta

Delayed wound healing caused by inefficient re-epithelialization underlines chronic skin lesions such as those found in diabetes. The dioxin receptor (AhR) modulates cell plasticity and migration and its activation by occupational polycyclic aromatic hydrocarbons (PAHs) results in severe skin lesions such as contact hypersensitivity, dermatitis and chloracne. Using wild-type (Ahr+/+) and AhR-null (Ahr-/-) mouse primary keratinocyte cultures and tissue explants, we show that lack of AhR increases keratinocyte migration and accelerates skin re-epithelialization without affecting cell proliferation or recruitment of inflammatory cells. Wounds in Ahr-/- animals had elevated numbers of fibroblasts and increased collagen content in their granulation tissue. Importantly, Ahr-/- dermal fibroblasts secreted higher levels of active TGFbeta that increased keratinocyte migration in culture and that could account for over-activation of the TGFbeta pathway and for faster wound healing in the AhR-null neo-epithelium. Consistently, a TGFbeta neutralizing antibody decreased keratinocyte migration in culture and halted re-epithelialization in Ahr-/- mice. Moreover, in vivo treatment with an antisense oligonucleotide for AhR increased TGFbeta signaling and improved re-epithelialization in wounds of wild-type mice. These data indicate that AhR is relevant for wound repair and suggest that AhR downmodulation might be a potential new tool for the treatment of chronic, surgical or accidental wounds.

Regulation of insulin-like growth factor binding protein-1 and lipoprotein lipase by the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (Ahr), a ligand-activated transcriptional factor, mediates the transcriptional activation of a battery of genes encoding drug metabolism enzymes. In the present study, we investigated the hepatic mRNA expression profile in Ahr-null (Ahr KO) mice compared to wild-type mice by microarray analysis to find new Ahr target genes. Pooled total RNA samples of liver extracted from 7- and 60-week-old Ahr KO or wild-type mice were studied by DNA microarray representing 19,867 genes. It was demonstrated that 23 genes were up-regulated and 20 genes were down-regulated over 2 fold in Ahr KO mice compared with wild-type mice commonly within the different age groups. We focused on insulin-like growth factor binding protein-1 (Igfbp-1) and lipoprotein lipase (Lpl) that were up-regulated in Ahr KO mice. The higher expression in Ahr KO mice compared to wild-type mice were confirmed by real-time RT-PCR analysis. In the wild-type mice but not in the Ahr KO mice, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) treatment increased the Igfbp-1 and Lpl mRNA levels. The expression profile of Igfbp-1 protein was consistent with that of Igfbp-1 mRNA. Since Lpl is the primary enzyme responsible for hydrolysis of lipids in lipoproteins, the serum triglyceride levels were determined. Indeed, the serum triglyceride levels in Ahr KO mice was lower than that in wild-type mice in accordance with the Lpl mRNA levels. Contrary to our expectation, TCDD treatment significantly increased the serum triglyceride levels in wild-type, but did not in Ahr KO mice. These results suggest that serum triglyceride levels are not correlated with hepatic Lpl expression levels. In the present study, we found that Ahr paradoxically regulates Igfbp-1 and Lpl expressions in the liver.