Roles of coactivator proteins in dioxin induction of CYP1A1 and CYP1B1 in human breast cancer cells

Exploring aryl hydrocarbon receptor expression and distribution in the tumor microenvironment, with a focus on immune cells, in various solid cancer types

Introduction

Aryl hydrocarbon receptor (AhR) is a transcription factor that performs various functions upon ligand activation. Several studies have explored the role of AhR expression in tumor progression and immune surveillance. Nevertheless, investigations on the distribution of AhR expression, specifically in cancer or immune cells in the tumor microenvironment (TME), remain limited. Examining the AhR expression and distribution in the TME is crucial for gaining insights into the mechanism of action of AhR-targeting anticancer agents and their potential as biomarkers.

Methods

Here, we used multiplexed immunohistochemistry (mIHC) and image cytometry to investigate the AhR expression and distribution in 513 patient samples, of which 292 are patients with one of five solid cancer types. Additionally, we analyzed the nuclear and cytosolic distribution of AhR expression.

Results

Our findings reveal that AhR expression was primarily localized in cancer cells, followed by stromal T cells and macrophages. Furthermore, we observed a positive correlation between the nuclear and cytosolic expression of AhR, indicating that the expression of AhR as a biomarker is independent of its localization. Interestingly, the expression patterns of AhR were categorized into three clusters based on the cancer type, with high AhR expression levels being found in regulatory T cells (Tregs) in non-small cell lung cancer (NSCLC).

Discussion

These findings are anticipated to serve as pivotal evidence for the design of clinical trials and the analysis of the anticancer mechanisms of AhR-targeting therapies.

Uremic toxins mediate kidney diseases: the role of aryl hydrocarbon receptor

Aryl hydrocarbon receptor (AhR) was originally identified as an environmental sensor that responds to pollutants. Subsequent research has revealed that AhR recognizes multiple exogenous and endogenous molecules, including uremic toxins retained in the body due to the decline in renal function. Therefore, AhR is also considered to be a uremic toxin receptor. As a ligand-activated transcriptional factor, the activation of AhR is involved in cell differentiation and senescence, lipid metabolism and fibrogenesis. The accumulation of uremic toxins in the body is hazardous to all tissues and organs. The identification of the endogenous uremic toxin receptor opens the door to investigating the precise role and molecular mechanism of tissue and organ damage induced by uremic toxins. This review focuses on summarizing recent findings on the role of AhR activation induced by uremic toxins in chronic kidney disease, diabetic nephropathy and acute kidney injury. Furthermore, potential clinical approaches to mitigate the effects of uremic toxins are explored herein, such as enhancing uremic toxin clearance through dialysis, reducing uremic toxin production through dietary interventions or microbial manipulation, and manipulating metabolic pathways induced by uremic toxins through controlling AhR signaling. This information may also shed light on the mechanism of uremic toxin-induced injury to other organs, and provide insights into clinical approaches to manipulate the accumulated uremic toxins.

Nuclear localization of STING1 competes with canonical signaling to activate AHR for commensal and intestinal homeostasis

Extensive studies demonstrate the importance of the STING1 (also known as STING) protein as a signaling hub that coordinates immune and autophagic responses to ectopic DNA in the cytoplasm. Here, we report a nuclear function of STING1 in driving the activation of the transcription factor aryl hydrocarbon receptor (AHR) to control gut microbiota composition and homeostasis. This function was independent of DNA sensing and autophagy and showed competitive inhibition with cytoplasmic cyclic guanosine monophosphate (GMP)-AMP synthase (CGAS)-STING1 signaling. Structurally, the cyclic dinucleotide binding domain of STING1 interacted with the AHR N-terminal domain. Proteomic analyses revealed that STING1-mediated transcriptional activation of AHR required additional nuclear partners, including positive and negative regulatory proteins. Although AHR ligands could rescue colitis pathology and dysbiosis in wild-type mice, this protection was abrogated by mutational inactivation of STING1. These findings establish a key framework for understanding the nuclear molecular crosstalk between the microbiota and the immune system.

The evolution and structure/function of bHLH-PAS transcription factor family

Proteins that contain basic helix-loop-helix (bHLH) and Per-Arnt-Sim motifs (PAS) function as transcription factors. bHLH-PAS proteins exhibit essential and diverse functions throughout the body, from cell specification and differentiation in embryonic development to the proper function of organs like the brain and liver in adulthood. bHLH-PAS proteins are divided into two classes, which form heterodimers to regulate transcription. Class I bHLH-PAS proteins are typically activated in response to specific stimuli, while class II proteins are expressed more ubiquitously. Here, we discuss the general structure and functions of bHLH-PAS proteins throughout the animal kingdom, including family members that do not fit neatly into the class I-class II organization. We review heterodimerization between class I and class II bHLH-PAS proteins, binding partner selectivity and functional redundancy. Finally, we discuss the evolution of bHLH-PAS proteins, and why a class I protein essential for cardiovascular development in vertebrates like chicken and fish is absent from mammals.

OTUD6A promotes prostate tumorigenesis via deubiquitinating Brg1 and AR

Ovarian tumor (OTU) subfamily deubiquitinases are involved in various cellular processes, such as inflammation, ferroptosis and tumorigenesis; however, their pathological roles in prostate cancer (PCa) remain largely unexplored. In this study, we observed that several OTU members displayed genomic amplification in PCa, among which ovarian tumor deubiquitinase 6A (OTUD6A) amplified in the top around 15–20%. Further clinical investigation showed that the OTUD6A protein was highly expressed in prostate tumors, and increased OTUD6A expression correlated with a higher biochemical recurrence risk after prostatectomy. Biologically, wild-type but not a catalytically inactive mutant form of OTUD6A was required for PCa cell progression. In vivo experiments demonstrated that OTUD6A oligonucleotides markedly suppressed prostate tumorigenesis in Pten^PC−/− mice and patient-derived xenograft (PDX) models. Mechanistically, the SWI/SNF ATPase subunit Brg1 and the nuclear receptor AR (androgen receptor) were identified as essential substrates for OTUD6A in PCa cells by a mass spectrometry (MS) screening approach. Furthermore, OTUD6A stabilized these two proteins by erasing the K27-linked polyubiquitination of Brg1 and K11-linked polyubiquitination of AR. OTUD6A amplification exhibited strong mutual exclusivity with mutations in the tumor suppressors FBXW7 and SPOP. Collectively, our results indicate the therapeutic potential of targeting OTUD6A as a deubiquitinase of Brg1 and AR for PCa treatment.

OTUD6A, a deubiquitinase, is amplified in prostate cancer and correlates with poor survivability, increasing the growth of prostate cancer cell lines and PDX models. OTUD6A stabilizes the expression of Brg1 and AR through the removal of K27- and K11-linked polyubiquination.

Role of AHR Ligands in Skin Homeostasis and Cutaneous Inflammation

Aryl hydrocarbon receptor (AHR) is an important regulator of skin barrier function. It also controls immune-mediated skin responses. The AHR modulates various physiological functions by acting as a sensor that mediates environment–cell interactions, particularly during immune and inflammatory responses. Diverse experimental systems have been used to assess the AHR’s role in skin inflammation, including in vitro assays of keratinocyte stimulation and murine models of psoriasis and atopic dermatitis. Similar approaches have addressed the role of AHR ligands, e.g., TCDD, FICZ, and microbiota-derived metabolites, in skin homeostasis and pathology. Tapinarof is a novel AHR-modulating agent that inhibits skin inflammation and enhances skin barrier function. The topical application of tapinarof is being evaluated in clinical trials to treat psoriasis and atopic dermatitis. In the present review, we summarize the effects of natural and synthetic AHR ligands in keratinocytes and inflammatory cells, and their relevance in normal skin homeostasis and cutaneous inflammatory diseases.

FLII and MLL1 Cooperatively Regulate Aryl Hydrocarbon Receptor-Mediated Transcription in ARPE-19 Cells

Aryl hydrocarbon receptors (AHRs), a class of ligand-dependent nuclear receptors that regulate cellular responses by inducing the expression of various target genes in response to external signals, are implicated in maintaining retinal tissue homeostasis. Previous studies have shown that the regulation of AHR-induced gene expression requires transcriptional co-regulators. However, it is not yet clear how chromatin remodelers, histone methyltransferases and coactivators interact during AHR-mediated gene expression in human retinal cells. In this study, we reveal that the histone methyltransferase MLL1 and the coactivator FLII are involved in AHR-mediated gene expression in retinal pigment epithelial cells. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) significantly increased the expression of CYP1A1, CYP1B1 and AHRR in ARPE-19 cells, whereas FLII or MLL1 depletion significantly reduced the expression of these genes induced by TCDD. Mechanistically, FLII binds to AHR in a ligand-dependent manner in ARPE-19 cells. In particular, the binding of FLII to MLL1 occurs through the GelB domain of FLII. In addition, MLL1 binds to AHR in a ligand-independent manner. FLII is involved in the recruitment of the BRG1 chromatin remodeler and MLL1 histone methyltransferase to the AHR-regulated CYP1A1 gene region in ARPE-19 cells and consequently, plays an important role in RNA polymerase II binding and transcriptional activity by modulating chromatin accessibility. Our results identify the functions and mechanisms of action of FLII and MLL1 in AHR-induced gene expression in human retinal pigment epithelial cells.

Role of DNA methylation-related chromatin remodeling in aryl hydrocarbon receptor-dependent regulation of T-2 toxin highly inducible Cytochrome P450 1A4 gene

Mycotoxins are toxic secondary metabolites produced by food-contaminating fungi, which lead to global epigenetic changes and cause toxicity to both farm animals and humans. However, whether mycotoxins induce gene-specific epigenetic alterations associated with inducible downstream gene expression is unclear as are the underlying regulatory mechanisms. Here, we found that T-2 toxin and its deacetylated metabolites but not deoxynivalenol (DON) or other representative mycotoxins highly induced the expression of cytochrome P450 1A4 (CYP1A4) in both Leghorn male hepatoma (LMH) cells and chicken primary hepatocytes, and this effect was related to the regulation of both aryl hydrocarbon receptor (AhR) and DNA methylation. We used methylation-sensitive restriction enzyme digestion-qPCR (MSRE-qPCR) and chromatin immunoprecipitation (ChIP) assays and found that the binding of DNA methyltransferase 1 (DNMT1) and histone deacetylase 2 (HDAC2) to highly methylated CpG island 3-2 at the enhancer of CYP1A4 was accompanied by the recruitment of the repressive histone modification marker H3K27me3, inducing a silent state. In turn, T-2 toxin stimulation enriched the binding of AhR to demethylated CpG island 3-2, which facilitated p300 and H3K9ac recruitment and ultimately generated an activated chromatin structure at the enhancer by increasing the active histone modification markers, including H3K4me3, H3K27ac, and H3K14ac. Interestingly, T-2 toxin-induced AhR activation also facilitated RNA polymerase II binding to CpG island 2, which may form a transcriptionally active chromatin structure at the promoter and ultimately transactivate CYP1A4. Our findings provide novel insights into the epigenetic regulation of T-2 toxin-induced gene expression.

Epigenetic Regulations of AhR in the Aspect of Immunomodulation

Environmental factors contribute to autoimmune disease manifestation, and as regarded today, AhR has become an important factor in studies of immunomodulation. Besides immunological aspects, AhR also plays a role in pharmacological, toxicological and many other physiological processes such as adaptive metabolism. In recent years, epigenetic mechanisms have provided new insight into gene regulation and reveal a new contribution to autoimmune disease pathogenesis. DNA methylation, histone modifications, chromatin alterations, microRNA and consequently non-genetic changes in phenotypes connect with environmental factors. Increasing data reveals AhR cross-roads with the most significant in immunology pathways. Although study on epigenetic modulations in autoimmune diseases is still not well understood, therefore future research will help us understand their pathophysiology and help to find new therapeutic strategies. Present literature review sheds the light on the common ground between remodeling chromatin compounds and autoimmune antibodies used in diagnostics. In the proposed review we summarize recent findings that describe epigenetic factors which regulate AhR activity and impact diverse immunological responses and pathological changes.

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

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

TCDD Toxicity Mediated by Epigenetic Mechanisms

Dioxins are highly toxic and persistent halogenated organic pollutants belonging to two families i.e., Polychlorinated Dibenzo-p-Dioxins (PCDDs) and Polychlorinated Dibenzo Furans (PCDFs). They can cause cancer, reproductive and developmental issues, damage to the immune system, and can deeply interfere with the endocrine system. Dioxins toxicity is mediated by the Aryl-hydrocarbon Receptor (AhR) which mediates the cellular metabolic adaptation to these planar aromatic xenobiotics through the classical transcriptional regulation pathway, including AhR binding of ligand in the cytosol, translocation of the receptor to the nucleus, dimerization with the AhR nuclear translocator, and the binding of this heterodimeric transcription factor to dioxin-responsive elements which regulate the expression of genes involved in xenobiotic metabolism. 2,3,7,8-TCDD is the most toxic among dioxins showing the highest affinity toward the AhR receptor. Beside this classical and well-studied pathway, a number of papers are dealing with the role of epigenetic mechanisms in the response to environmental xenobiotics. In this review, we report on the potential role of epigenetic mechanisms in dioxins-induced cellular response by inspecting recent literature and focusing our attention on epigenetic mechanisms induced by the most toxic 2,3,7,8-TCDD.

Alterations of Histone Modifications Contribute to Pregnane X Receptor-Mediated Induction of CYP3A4 by Rifampicin

CYP3A4 is one of the major drug-metabolizing enzymes in human and is responsible for the metabolism of 60% of clinically used drugs. Many drugs are able to induce the expression of CYP3A4, which usually causes drug-drug interactions and adverse drug reactions. This study aims to explore the role of histone modifications in rifampicin-induced expression of CYP3A4 in LS174T cells. We found that the induction of CYP3A4 mRNA (4- to 15-fold) by rifampicin in LS174T cells was associated with increased levels of histone H3 lysine 4 trimethylation (H3K4me3, above 1.8-fold) and H3 acetylation (above 2-fold) and a decreased level of histone H3 lysine 27 trimethylation (H3K27me3, about 50%) in the CYP3A4 promoter. Rifampicin enhanced recruitment to the CYP3A4 promoter of nuclear receptor coactivator 6 (NCOA6, above 3-fold) and histone acetyltransferase p300 (p300, above 1.6-fold). Silencing NCOA6 or p300 by short-hairpin RNAs resulted in inhibition of the CYP3A4 induction as well as altered levels of H3K4me3, H3K27me3, or H3 acetylation in the CYP3A4 promoter. Knockdown of pregnane X receptor (PXR) expression not only suppressed the recruitment of NCOA6 and p300 but also abolished the changes caused by rifampicin in H3K4me3, H3K27me3, and H3 acetylation levels in the CYP3A4 promoter. Moreover, rifampicin treatment enhanced the nuclear accumulation and interactions between PXR and NCOA6/p300. In conclusion, we show that the alterations of histone modifications contribute to the PXR-mediated induction of CYP3A4 by rifampicin.

The aryl hydrocarbon receptor repressor - More than a simple feedback inhibitor of AhR signaling: Clues for its role in inflammation and cancer

The aryl hydrocarbon receptor repressor (AhRR) was first described as a specific competitive repressor of aryl hydrocarbon receptor (AhR) activity based on its ability to dimerize with the AhR nuclear translocator (ARNT) and through direct competition of AhR/ARNT and AhRR/ARNT complexes for binding to dioxin-responsive elements (DREs). Like AhR, AhRR belongs to the basic Helix-Loop-Helix/Per-ARNT-Sim (bHLH/PAS) protein family but lacks functional ligand-binding and transactivation domains. Transient transfection experiments with ARNT and AhRR mutants examining the inhibitory mechanism of AhRR suggested a more complex mechanism than the simple mechanism of negative feedback through sequestration of ARNT to regulate AhR signaling. Recently, AhRR has been shown to act as a tumor suppressor gene in several types of cancer cells. Furthermore, epidemiological studies have found epigenetic changes and silencing of AhRR associated with exposure to cigarette smoke and cancer development. Additional studies from our laboratories have demonstrated that AhRR represses other signaling pathways including NF-κB and is capable of regulating inflammatory responses. A better understanding of the regulatory mechanisms of AhRR in AhR signaling and adverse outcome pathways leading to deregulated inflammatory responses contributing to tumor promotion and other adverse health effects is expected from future studies. This review article summarizes the characteristics of AhRR as an inhibitor of AhR activity and highlights more recent findings pointing out the role of AhRR in inflammation and tumorigenesis.

The Aryl Hydrocarbon Receptor in Immunity: Tools and Potential

The signaling pathway of the evolutionary old transcription factor AhR is inducible by a number of small molecular weight chemicals, including toxicants such as polycyclic aromatic hydrocarbons, bacterial toxic pigments, and physiological compounds such as tryptophan derivatives or dietary indoles. AhR activation is of immunological importance, but at the same time mediates toxicity of environmental pollutants, such as immunosuppression by dioxins. Measuring AhR activity and identification of ligands is thus of great interest for a variety of research fields. In this chapter, I briefly introduce the AhR signaling pathway, its role in immunology, and the tools and assays needed to analyze AhR signaling. Both are also needed when therapeutic applications are envisioned.

The role of the aryl hydrocarbon receptor in the development of cells with the molecular and functional characteristics of cancer stem-like cells

BACKGROUND

Self-renewing, chemoresistant breast cancer stem cells are believed to contribute significantly to cancer invasion, migration and patient relapse. Therefore, the identification of signaling pathways that regulate the acquisition of stem-like qualities is an important step towards understanding why patients relapse and towards development of novel therapeutics that specifically target cancer stem cell vulnerabilities. Recent studies identified a role for the aryl hydrocarbon receptor (AHR), an environmental carcinogen receptor implicated in cancer initiation, in normal tissue-specific stem cell self-renewal. These studies inspired the hypothesis that the AHR plays a role in the acquisition of cancer stem cell-like qualities.

RESULTS

To test this hypothesis, AHR activity in Hs578T triple negative and SUM149 inflammatory breast cancer cells were modulated with AHR ligands, shRNA or AHR-specific inhibitors, and phenotypic, genomic and functional stem cell-associated characteristics were evaluated. The data demonstrate that (1) ALDH(high) cells express elevated levels of Ahr and Cyp1b1 and Cyp1a1, AHR-driven genes, (2) AHR knockdown reduces ALDH activity by 80%, (3) AHR hyper-activation with several ligands, including environmental ligands, significantly increases ALDH1 activity, expression of stem cell- and invasion/migration-associated genes, and accelerates cell migration, (4) a significant correlation between Ahr or Cyp1b1 expression (as a surrogate marker for AHR activity) and expression of stem cell- and invasion/migration-associated gene sets is seen with genomic data obtained from 79 human breast cancer cell lines and over 1,850 primary human breast cancers, (5) the AHR interacts directly with Sox2, a master regulator of self-renewal; AHR ligands increase this interaction and nuclear SOX2 translocation, (6) AHR knockdown inhibits tumorsphere formation in low adherence conditions, (7) AHR inhibition blocks the rapid migration of ALDH(high) cells and reduces ALDH(high) cell chemoresistance, (8) ALDH(high) cells are highly efficient at initiating tumors in orthotopic xenografts, and (9) AHR knockdown inhibits tumor initiation and reduces tumor Aldh1a1, Sox2, and Cyp1b1 expression in vivo.

CONCLUSIONS

These data suggest that the AHR plays an important role in development of cells with cancer stem cell-like qualities and that environmental AHR ligands may exacerbate breast cancer by enhancing expression of these properties.

Aryl hydrocarbon receptor (AHR) regulation of L-Type Amino Acid Transporter 1 (LAT-1) expression in MCF-7 and MDA-MB-231 breast cancer cells

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that is regulated by environmental toxicants that function as AHR agonists such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). L-Type Amino Acid Transporter 1 (LAT1) is a leucine transporter that is overexpressed in cancer. The regulation of LAT1 by AHR in MCF-7 and MDA-MB-231 breast cancer cells (BCCs) was investigated in this report. Ingenuity pathway analysis (IPA) revealed a significant association between TCDD-regulated genes (TRGs) and molecular transport. Overlapping the TCDD-RNA-Seq dataset obtained in this study with a published TCDD-ChIP-seq dataset identified LAT1 as a primary target of AHR-dependent TCDD induction. Short interfering RNA (siRNA)-directed knockdown of AHR confirmed that TCDD-stimulated increases in LAT1 mRNA and protein required AHR expression. TCDD-stimulated increases in LAT1 mRNA were also inhibited by the AHR antagonist CH-223191. Upregulation of LAT1 by TCDD coincided with increases in leucine uptake by MCF-7 cells in response to TCDD. Chromatin immunoprecipitation-quantitative PCR (ChIP-qPCR) assays revealed increases in AHR, AHR nuclear translocator (ARNT) and p300 binding and histone H3 acetylation at an AHR binding site in the LAT1 gene in response to TCDD. In MCF-7 and MDA-MB-231 cells, endogenous levels of LAT1 mRNA and protein were reduced in response to knockdown of AHR expression. Knockdown experiments demonstrated that proliferation of MCF-7 and MDA-MB-231 cells is dependent on both LAT1 and AHR. Collectively, these findings confirm the dependence of cancer cells on leucine uptake and establish a mechanism for extrinsic and intrinsic regulation of LAT1 by AHR.

The role of aryl hydrocarbon receptor (AhR) in the pathology of pleomorphic adenoma in parotid gland

OBJECTIVES

Pleomorphic adenoma (benign mixed tumor) is one of the most common salivary gland tumors. However, molecular mechanisms implicated in its development are not entirely defined. Therefore, the study aimed at definition of aryl hydrocarbon receptor (AhR) involvement in pleomorphic adenoma pathology, as the AhR controlled gene system was documented to play a role in development of various human tumors.

DESIGN

The study was carried out in pleomorphic adenoma and control parotid gland tissues where gene expression of AHR, AhR nuclear translocator (ARNT), AhR repressor (AHRR), as well as AhR controlled genes: CYP1A1 and CYP1B1, at mRNA and protein (immunohistochemistry) levels were studied. Functional evaluation of AhR system was evaluated in HSY cells (human parotid gland adenocarcinoma cells) using 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) as AhR specific inducer.

RESULTS

Pleomorphic adenoma specimens showed cytoplasmic and nuclear AhR expression in epithelial cells as well as in mesenchymal cells. In parotid gland AhR was expressed in cytoplasm of duct cells. Quantitative expression at mRNA level showed significantly higher expression of AHR, ARNT and CYP1B1, and comparable levels of CYP1A1 in pleomorphic adenoma tissue in comparison to healthy parotid gland. The HSY cell study revealed significantly higher expression level of AHRR in HSY as compared with MCF-7 cells (human breast adenocarcinoma cell line used as reference). Upon TCDD stimulation a drop in AHRR level in HSY cells and an increase in MCF-7 cells were observed. The HSY and MCF-7 cell proliferation rate (measured by WST-1 test) was not affected by TCDD.

CONCLUSIONS

Summarizing both in vitro and in vivo observations it can be stated that AhR system may play a role in the pathology of pleomorphic adenoma.

Development, Maintenance, and Reversal of Multiple Drug Resistance: At the Crossroads of TFPI1, ABC Transporters, and HIF1

Early detection and improved therapies for many cancers are enhancing survival rates. Although many cytotoxic therapies are approved for aggressive or metastatic cancer; response rates are low and acquisition of de novo resistance is virtually universal. For decades; chemotherapeutic treatments for cancer have included anthracyclines such as Doxorubicin (DOX); and its use in aggressive tumors appears to remain a viable option; but drug resistance arises against DOX; as for all other classes of compounds. Our recent work suggests the anticoagulant protein Tissue Factor Pathway Inhibitor 1α (TFPI1α) plays a role in driving the development of multiple drug resistance (MDR); but not maintenance; of the MDR state. Other factors; such as the ABC transporter drug efflux pumps MDR-1/P-gp (ABCB1) and BCRP (ABCG2); are required for MDR maintenance; as well as development. The patient population struggling with therapeutic resistance specifically requires novel treatment options to resensitize these tumor cells to therapy. In this review we discuss the development, maintenance, and reversal of MDR as three distinct phases of cancer biology. Possible means to exploit these stages to reverse MDR will be explored. Early molecular detection of MDR cancers before clinical failure has the potential to offer new approaches to fighting MDR cancer.

Homocitrullination Is a Novel Histone H1 Epigenetic Mark Dependent on Aryl Hydrocarbon Receptor Recruitment of Carbamoyl Phosphate Synthase 1

The aryl hydrocarbon receptor (AhR), a regulator of xenobiotic toxicity, is a member of the eukaryotic Per-Arnt-Sim domain protein family of transcription factors. Recent evidence identified a novel AhR DNA recognition sequence called the nonconsensus xenobiotic response element (NC-XRE). AhR binding to the NC-XRE in response to activation by the canonical ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin resulted in concomitant recruitment of carbamoyl phosphate synthase 1 (CPS1) to the NC-XRE. Studies presented here demonstrate that CPS1 is a bona fide nuclear protein involved in homocitrullination (hcit), including a key lysine residue on histone H1 (H1K34hcit). H1K34hcit represents a hitherto unknown epigenetic mark implicated in enhanced gene expression of the peptidylarginine deiminase 2 gene, itself a chromatin-modifying protein. Collectively, our data suggest that AhR activation promotes CPS1 recruitment to DNA enhancer sites in the genome, resulting in a specific enzyme-independent post-translational modification of the linker histone H1 protein (H1K34hcit), pivotal in altering local chromatin structure and transcriptional activation.

Ah Receptor Pathway Intricacies; Signaling Through Diverse Protein Partners and DNA-Motifs

The Ah receptor is a transcription factor that modulates gene expression via interactions with multiple protein partners; these are reviewed, including the novel NC-XRE pathway involving KLF6.

The aryl hydrocarbon receptor in barrier organ physiology, immunology, and toxicology

The aryl hydrocarbon receptor (AhR) is an evolutionarily old transcription factor belonging to the Per-ARNT-Sim-basic helix-loop-helix protein family. AhR translocates into the nucleus upon binding of various small molecules into the pocket of its single-ligand binding domain. AhR binding to both xenobiotic and endogenous ligands results in highly cell-specific transcriptome changes and in changes in cellular functions. We discuss here the role of AhR for immune cells of the barrier organs: skin, gut, and lung. Both adaptive and innate immune cells require AhR signaling at critical checkpoints. We also discuss the current two prevailing views-namely, 1) AhR as a promiscuous sensor for small chemicals and 2) a role for AhR as a balancing factor for cell differentiation and function, which is controlled by levels of endogenous high-affinity ligands. AhR signaling is considered a promising drug and preventive target, particularly for cancer, inflammatory, and autoimmune diseases. Therefore, understanding its biology is of great importance.

SIN3A, generally regarded as a transcriptional repressor, is required for induction of gene transcription by the aryl hydrocarbon receptor

CYP1A1 bioactivates several procarcinogens and detoxifies several xenobiotic compounds. Transcription of CYP1A1 is highly induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) via the aryl hydrocarbon receptor. We recently described an RNAi high throughput screening performed in the Hepa-1 mouse hepatoma cell line, which revealed that SIN3A is necessary for the induction of CYP1A1-dependent ethoxyresorufin-o-deethylase (EROD) enzymatic activity by TCDD. In the current studies, we sought to provide insight into the role of SIN3A in this process, particularly because studies on SIN3A have usually focused on its repressive activity on transcription. We report that ectopic expression of human SIN3A in Hepa-1 cells enhanced EROD induction by TCDD and efficiently rescued TCDD induction of EROD activity in cells treated with an siRNA to mouse SIN3A, thus validating a role for SIN3A in CYP1A1 induction. We demonstrate that SIN3A is required for TCDD induction of the CYP1A1 protein in Hepa-1 cells but not for expression of the aryl hydrocarbon receptor protein. In addition, siRNAs for SIN3A decreased TCDD-mediated induction of CYP1A1 mRNA and EROD activity in human hepatoma cell line Hep3B. We establish that TCDD treatment of Hepa-1 cells rapidly increases the degree of SIN3A binding to both the proximal promoter and enhancer of the Cyp1a1 gene and demonstrate that increased binding to the promoter also occurs in human Hep3B, HepG2, and MCF-7 cells. These studies establish that SIN3A physically interacts with the CYP1A1 gene and extends the transcriptional role of SIN3A to a gene that is very rapidly and dramatically induced.

IL-1 and TNF-α regulation of aryl hydrocarbon receptor (AhR) expression in HSY human salivary cells

OBJECTIVES

Recent findings demonstrate that nuclear receptor - aryl hydrocarbon receptor (AhR) may play an important role in the pathogenesis of Sjögren's syndrome (SS) via involvement in Epstein-Barr virus reactivation. In that study a reporter system was used. Therefore, it was decided to define AhR expression in human salivary cell line (HSY) and its functional regulators.

DESIGN

The expression and functional regulation of AhR was studied in HSY cells. The cells were incubated with dioxin (TCDD) - AhR model inducer, IL-1 and TNF-α. qRT-PCR was applied to assess the expression of AHR, AHRR (AhR repressor), ARNT (AhR nuclear translocator) as well as AhR dependent genes: CYP1A1 and CYP1B1. Enzymatic activity of CYP1A1 and CYP1B1 was evaluated using luciferin-labelled CYPs substrate.

RESULTS

In general, dioxin did not significantly influence the expression of AHR and ARNT, but reduced AHRR level. AhR dependent gene expression, i.e. CYP1A1 and CYP1B1 increased gradually with TCDD incubation time. TNF-α significantly induced AHR along with CYP1A1 and CYP1B1 expression. IL-1β did not affect AHR expression, and had minimal effects on CYP1 mRNA levels. Exposure of HSY cells to TCDD resulted in time-dependent induction of CYP1A1 and CYP1B1 enzymatic activity.

CONCLUSIONS

This study documents functional expression of AhR in HSY as well as induction of AhR and its dependent genes by TNF-α.

Differential suppression of the aryl hydrocarbon receptor nuclear translocator-dependent function by an aryl hydrocarbon receptor PAS-A-derived inhibitory molecule

The aryl hydrocarbon receptor (AhR) heterodimerizes with the aryl hydrocarbon receptor nuclear translocator (Arnt) for transcriptional regulation. We generated three N-terminal deletion constructs of the human AhR of 12-24 kDa in size--namely D1, D2, and D3--to suppress the Arnt function. We observed that all three deletions interact with the human Arnt with similar affinities. D2, which contains part of the AhR PAS-A domain and interacts with the PAS-A domain of Arnt, inhibits the formation of the AhR gel shift complex. D2 suppresses the 3-methylcholanthrene-induced, dioxin response element (DRE)-driven luciferase activity in Hep3B cells and exogenous Arnt reverses this D2 suppression. D2 suppresses the induction of CYP1A1 at both the message and protein levels in Hep3B cells; however, the CYP1B1 induction is not affected. D2 suppresses the recruitment of Arnt to the cyp1a1 promoter but not to the cyp1b1 promoter, partly because the AhR/Arnt heterodimer binds better to the cyp1b1 DRE than to the cyp1a1 DRE. Interestingly, D2 has no effect on the cobalt chloride-induced, hypoxia inducible factor-1 (HIF-1)-dependent expression of vegf, aldolase c, and ldh-a messages. Our data reveal that the flanking sequences of the DRE contribute to the binding affinity of the AhR/Arnt heterodimer to its endogenous enhancers and the function of AhR and HIF-1 can be differentially suppressed by the D2 inhibitory molecule.

Potential role of epigenetic mechanisms in the regulation of drug metabolism and transport

This is a report of a symposium on the potential role of epigenetic mechanisms in the control of drug disposition sponsored by the American Society for Pharmacology and Experimental Therapeutics and held at the Experimental Biology 2013 meeting in Boston, MA, April 21, 2013. Epigenetics is a rapidly evolving area, and recent studies have revealed that expression of drug-metabolizing enzymes and transporters is regulated by epigenetic factors, including histone modification, DNA methylation, and noncoding RNAs. The symposium speakers provided an overview of genetic and epigenetic mechanisms underlying variable drug metabolism and drug response, as well as the implications for personalized medicine. Considerable insight into the epigenetic mechanisms in differential regulation of the dioxin-inducible drug and carcinogen-metabolizing enzymes CYP1A1 and 1B1 was provided. The role of noncoding microRNAs in the control of drug metabolism and disposition through targeting of cytochrome P450 (P450) enzymes and ATP-binding cassette membrane transporters was discussed. In addition, potential effects of xenobiotics on chromatin interactions and epigenomics, as well as the possible role of long noncoding RNAs in regulation of P450s during liver maturation were presented.

New cytochrome P450 1B1, 1C1, 2Aa, 2Y3, and 2K genes from Chinese rare minnow (Gobiocypris rarus): Molecular characterization, basal expression and response of rare minnow CYP1s and CYP2s mRNA exposed to the AHR agonist benzo[a]pyrene

Cytochrome P450 (CYP450) genes play an important role in catalyzing oxidative metabolism of toxicants. Recently, CYP1 subfamily were discovered and reported in fish, however, little is known regarding the CYP2 isoforms in fish. In the present study, the cDNA fragments of CYP 1B1 and 1C1 and CYP2Aa, 2Y3, and 2K of rare minnow were cloned and exhibited a high amino acid sequence identity compared with their zebrafish orthologs. Basal expression showed CYP1C1 and CYP 2Aa expression were observed in all eight tissues analyzed (liver, gill, intestine, kidney, spleen, brain, skin, and muscle). CYP 1A, and 1B1 expression was found in all tissues except for muscle and skin. However, CYP 2Y3 was expressed in liver, spleen, intestine and muscle whereas CYP 2K in liver, kidney and intestine. 4 and 100μgL(-1) Benzo[a]pyrene (BaP) induced patterns showed that CYP 1A, 1B1 and 1C1 expression in liver, gill, and intestine was strongly up-regulated (p<0.05). Furthermore, CYP 2Y3 was strongly induced in liver from BaP treatments (p<0.05). The high induction on mRNA level of CYP1s and CYP 2Y3 by BaP could be associated with catalyzing detoxification and indicated that CYP2s may also be potential biomarker to screen AHR agonist. The high responsiveness of CYP1 and 2 genes suggested Chinese rare minnow is feasible to screen and assess pollution with AHR agonist.

Expression of genes involved in xenobiotic metabolism and transport in end-stage liver disease: up-regulation of ABCC4 and CYP1B1

BACKGROUND

Expression of drug-metabolizing enzymes and drug transporters in liver is mainly regulated by a system of nuclear receptors. The aim of the current study was to investigate the expression of nuclear receptors, as well as these enzymes and transporters, in liver samples from patients suffering from end-stage liver disease of various etiologies (HCV infection, alcohol liver disease, and primary sclerosis cholangitis).

METHODS

Gene expression was measured using quantitative real-time PCR with surgical specimens from livers of patients with end-stage liver disease, and non-tumoral liver tissue that served as control.

RESULTS

Our study confirmed that the expression of most phase I enzymes is suppressed in end-stage liver disease, and is correlated with a decrease in NR1I2 and NR1I3, the main regulators of xenobiotic metabolism. While mRNA levels of phase II enzymes were generally unchanged, some ABC transporters were up-regulated. The most spectacular increases in expression were observed with ABCC4 (MRP4) - at the mRNA level, and CYP1B1 - at both the mRNA and protein levels. We also demonstrated that IL-6 can induce CYP1B1 expression independently of CYP1A1, in a human hepatocellular liver carcinoma cell line.

CONCLUSIONS

As CYP1B1 is an enzyme which converts various substrates into carcinogenous metabolites, its overexpression in liver may be one of the factors increasing the risk of hepatic cancers in patients with liver disease. CYP1A1 and CYP1B1 are often referred to as model AHR target genes, but CYP1A1 was down-regulated in diseased liver samples. This points to the existence of differences in regulation of these two genes.

An ex vivo approach to the differential parenchymal responses induced by cigarette whole smoke and its vapor phase

Using a rat lung slice model, this study compared the stress responses induced by cigarette whole smoke (WS) to that induced by the vapor phase (VP) of the smoke. Following a 3-day exposure, lung slices exposed to 4, 10 and 20% WS retained 85, 42 and 16% relative survival respectively in comparison to the air-exposed ones. Consistently, histological observations revealed concentration-related alveolar damages in the lung slices. Expression of 5 stress-response genes was examined following a single 30 min exposure to 4% WS or VP. WS exposure resulted in 4, 11 and 50-fold induction of IL-1β, kinin type I receptor (B₁R) and CYP1A1 genes, respectively, while CYP1B1 and TNF-α genes expression was found only two times higher in comparison to VP group. Since cigarette WS consists of particulate and vapor phases, these results highlight the preferential or synergistic role of the particulate phase in the induction of IL-1β, B₁R and CYP1A1 genes and that VP did not have comparable effects on expression of these genes. However, both phases fairly contributed to the induction of CYP1B1 and TNF-α genes. VP was the fraction responsible for the toxic effect since WS did not produce further toxicity. The 4% whole smoke deposited about 7.1 μg/cm² of total particulate matter (TPM) to the exposure chamber which may account for observed differential stress responses in the lung slices.

Exposure to polycyclic aromatic hydrocarbons: bulky DNA adducts and cellular responses

Environmental and dietary carcinogens such as polycyclic aromatic hydrocarbons (PAHs) have been intensively studied for decades. Although the genotoxicity of these compounds is well characterized (i.e., formation of bulky PAH-DNA adducts), molecular details on the DNA damage response triggered by PAHs in cells and tissues remain to be clarified. The conversion of hazardous PAHs into carcinogenic intermediates depends on enzyme-catalyzed biotransformation. Certain cytochrome P450-dependent monooxygenases (CYPs) play a pivotal role in PAH metabolism. In particular, CYP1A1 and 1B1 catalyze oxidation of PAHs toward primary epoxide species that can further be converted into multiple follow-up products, both nonenzymatically and enzymatically. Distinct functions between these major CYP enzymes have only been appreciated since transgenic animal models had been derived. Electrophilic PAH metabolites are capable of forming stable DNA adducts or to promote depurination at damaged nucleotide sites. During the following DNA replication cycle, bulky PAH-DNA adducts may be converted into mutations, thereby affecting hot spot sites in regulatory important genes such as Ras, p53, and others. Depending on the degree of DNA distortion and cell cycle progression, PAH-DNA adducts trigger nucleotide excision repair (NER) and various DNA damage responses that might include TP53-dependent apoptosis in certain cell types. In fact, cellular responses to bulky PAH-DNA damage are complex because distinct signaling branches such as ATM/ATR, NER, TP53, but also MAP kinases, interact and cooperate to determine the overall outcome to cellular injuries initiated by PAH-DNA adducts. Further, PAHs and other xenobiotics can also confer DNA damage via an alternative route of metabolic activation, which leads to the generation of PAH semiquinone radicals and reactive oxygen species (ROS). One-electron oxidations mediated by peroxidases or other enzymes can result in PAH radical cations that mainly form unstable DNA adducts subjected to depurination. In addition, generation of ROS can also trigger multiple cellular signaling pathways not directly related to mutagenic or cytotoxic effects, including those mediated by NFκB, SAPK/JNK, and p38. In recent years, it became clear that PAHs may also be involved in inflammatory diseases, autoimmune disorders, or atherosclerosis. Further research is under way to better characterize the significance of such newly recognized systemic effects of PAHs and to reconsider risk assessment for human health.

Perspectives on the potential involvement of the AH receptor-dioxin axis in cardiovascular disease

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that mediates the induction of the CYP1 family of cytochrome P450s and of several phase II detoxification enzymes. Although induction of these genes is the best characterized AHR function, it does not adequately explain the diversity of AHR-mediated effects. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the prototypical AHR ligand and dioxin congener and a model for many environmentally relevant organochlorinated compounds. Research over the course of the last 30 years has made it evident that AHR activation in response to TCDD and other xenobiotic agonists directly affects multiple metabolic pathways, leading to the identification of many AHR-directed effects of dioxin involved in regulation of growth factor signaling, cell cycle proliferation, differentiation, arrest, and apoptosis. There is ample evidence that TCDD causes persistent cardiac defects in zebrafish, chickens, mice, and likely humans and is associated with human cardiovascular disease. The question that I address here is whether exposure to TCDD during early development perturbs the concerted differentiation patterns of cardiovascular cell lineages and tissues and leads to cardiac malformations and long-term cardiovascular disease. Research to define the mechanisms responsible for the lifelong cardiovascular malformations resulting from TCDD exposure during embryonic development will be highly significant to the prevention of environmental cardiovascular injury.

Ah receptor antagonism inhibits constitutive and cytokine inducible IL6 production in head and neck tumor cell lines

There is increasing evidence that the aryl hydrocarbon receptor (AHR) plays a role in tumor progression through numerous mechanisms. We have previously shown that, in certain cancer cell lines that are typically nonresponsive to cytokine-mediated IL6 induction, activation of the AHR with the agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin derepresses the IL6 promoter and allows for synergistic induction following IL1β treatment. The mechanism by which this occurs involves liganded AHR binding upstream from the transcription start site and dismissing HDAC-containing corepressor complexes, giving rise to a promoter structure that is more amenable to NF-κB activation. This fact, combined with observations of multiple endogenously produced chemicals activating the AHR, led us to study its role in basal expression among high cytokine-producing cancer cell lines. The current study provides evidence that several head and neck squamous cell carcinoma cell lines have a level of constitutively bound AHR at the IL6 promoter, allowing for higher basal and readily inducible IL6 transcription. Treatment of these cell lines with an AHR antagonist led to dismissal of the AHR from the IL6 promoter and recruitment of corepressor complexes, thus diminishing cytokine expression. Head and neck squamous cell carcinoma is typically a high cytokine-producing tumor type, with IL6 expression levels correlating with disease aggressiveness. For this reason, AHR antagonist treatment could represent a novel adjuvant therapy for patients, lowering pro-growth and antiapoptotic signaling with minimal systemic side effects.

Regulation of benzo[a]pyrene-mediated DNA- and glutathione-adduct formation by 2,3,7,8-tetrachlorodibenzo-p-dioxin in human lung cells

Environmental carcinogens, such as polycyclic aromatic hydrocarbons (PAHs), require metabolic activation to DNA-reactive metabolites in order to exert their tumorigenic effects. Benzo[a]pyrene (B[a]P), a prototypic PAH, is metabolized by cytochrome P450 (P450) 1A1/1B1 and epoxide hydrolase to (-)-B[a]P-7,8-dihydro-7,8-diol (B[a]P-7,8-dihydrodiol). B[a]P-7,8-dihydrodiol then undergoes further P4501A1/1B1-mediated metabolism to the ultimate carcinogen, (+)-anti-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydro-B[a]P (B[a]PDE), which forms DNA-adducts primarily with 2'-deoxyguanosine (dGuo) to form (+)-anti-trans-B[a]PDE-N(2)-dGuo (B[a]PDE-dGuo) in DNA. Pretreatment of cells with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is known to induce P4501A1/1B1 mRNA expression through the aryl hydrocarbon receptor (AhR) pathway. This causes increased B[a]PDE-dGuo formation in liver cells. In contrast, TCDD induction of H358 lung cells surprisingly caused a decrease in (-)-B[a]P-7,8-dihydrodiol-mediated (+)-B[a]PDE-dGuo adduct formation when compared with the non-TCDD-induced cells. Furthermore, treatment of the TCDD-induced cells with (±)-B[a]PDE also resulted in decreased (+)-B[a]PDE-dGuo adduct formation when compared with the non-TCDD-induced cells. These data suggested that it was a detoxification pathway that had been up-regulated rather than an activation pathway that had been down-regulated. LC-MS was used to analyze B[a]PDE-dGuo and B[a]PDE-GSH-adducts in H358 lung and HepG2 liver cells. There was a significant increase in the (-)-B[a]PDE-GSH-adduct with high enantiomeric excess after treatment of the TCDD-induced H358 cells with (±)-B[a]PDE when compared with the noninduced cells. This could explain why increased activation of (-)-B[a]P-7,8-dihydrodiol through TCDD up-regulation of P4501A1/1B1 did not lead to increased (+)-B[a]PDE-dGuo adducts in the H358 lung cells. In addition, TCDD did not induce B[a]PDE-GSH-adduct formation in HepG2 liver cells. (±)-B[a]PDE-GSH-adducts were formed at much lower levels in both TCDD-induced and noninduced HepG2 cells when compared with (-)-B[a]PDE-GSH-adducts in the H358 lung cells. Therefore, our study has revealed that there is a subtle balance between activation and detoxification of B[a]P in lung-derived compared with liver-derived cells and that this determines how much DNA damage occurs.

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.

Role of epigenetic mechanisms in differential regulation of the dioxin-inducible human CYP1A1 and CYP1B1 genes

The aryl hydrocarbon receptor (AhR) mediates induction of CYP1A1 and CYP1B1 by 2,3,7,8-tetrachlorodibenzo-ρ-dioxin (dioxin) via binding to xenobiotic-responsive elements (XREs) in their enhancer regions. CYP1A1 and CYPIB1 were both inducible by dioxin in human MCF-7 cells. However, only CYP1A1 was inducible in human HepG2 cells. Further experiments focused on providing an explanation for this last observation. Dioxin induced the recruitment of AHR and the transcriptional coactivators p300 and p300/cAMP response element-binding protein binding protein-associated factor (PCAF) to the CYP1B1 enhancer in HepG2 cells but failed to induce recruitment of RNA polymerase II (polII) or the TATA binding protein (TBP) and acetylations of histones 3 and 4 or methylation of histone 3 at the promoter. Because p300 was required for dioxin induction of the aforementioned histone modifications at the CYP1B1 promoter and for induction of CYP1B1 transcription (in MCF-7 cells), the recruitments of p300 and AhR, although necessary, are not sufficient for eliciting the above responses to dioxin. Cytosine residues within CpG dinucleotides at the enhancer, including those within the XREs, were partially methylated, whereas those at the promoter were fully methylated. Treatment of HepG2 cells with 5-aza-2'-deoxycytidine led to partial demethylation of the promoter, restored polII and TBP binding, and CYP1B1 inducibility. Thus, the deficiency of CYP1B1 induction in HepG2 cells is ascribable to cytosine methylation at the promoter, which prevents recruitment of TBP and polII. It is noteworthy that our data indicate that stable recruitment of p300 and PCAF to the CYP1B1 gene does not require their tethering to the promoter and to the enhancer.

Inhibition of aryl hydrocarbon receptor-dependent transcription by resveratrol or kaempferol is independent of estrogen receptor α expression in human breast cancer cells

Resveratrol and kaempferol are natural chemopreventative agents that are also aryl hydrocarbon receptor (AHR) antagonists and estrogen receptor (ER) agonists. In this study we evaluated the role of ERα in resveratrol- and kaempferol-mediated inhibition of AHR-dependent transcription. Kaempferol or resveratrol inhibited dioxin-induced cytochrome P450 1A1 (CYP1A1) and CYP1B1 expression levels and recruitment of AHR, ERα and co-activators to CYP1A1 and CYP1B1. Both phytochemicals induced the expression and recruitment of ERα to gene amplified in breast cancer 1 (GREB1). RNAi-mediated knockdown of ERα in T-47D cells did not affect the inhibitory action of either phytochemical on AHR activity. Both compounds also inhibited AHR-dependent transcription in ERα-negative MDA-MB-231 and BT-549 breast cancer cells. These data show that ERα does not contribute to the AHR-inhibitory activities of resveratrol and kaempferol.

3-methylcholanthrene induces differential recruitment of aryl hydrocarbon receptor to human promoters

The aryl hydrocarbon receptor (AHR) is a ligand-activated protein that mediates the toxic actions of polycyclic aromatic and halogenated compounds. Identifying genes directly regulated by AHR is important in understanding the pathways regulated by this receptor. Here we used the techniques of chromatin immunoprecipitation and DNA microarrays (ChIP-chip) to detect AHR-bound genomic regions after 3-methylcholanthrene (3MC) treatment of T-47D human breast cancer cells. We identified 241 AHR-3MC-bound regions, and transcription factor-binding site analysis revealed a strong overrepresentation of the AHR-responsive element. Conventional ChIP confirmed recruitment of AHR to 26 regions with target gene responses to 3MC varying from activation to inhibition to having no effect. A comparison of identified AHR-3MC-bound regions with AHR-2,3,7,8-tetrchlorodibenzo-p-dioxin (TCDD)-bound regions from our previous study (Ahmed, S., Valen, E., Sandelin, A., and Matthews, J. (2009). Toxicol. Sci. 111, 254-266) revealed that 127 regions were common between the data sets. Time course ChIPs for six of the regions showed that 3MC induced gene-specific changes in histone H3 acetylation and methylation and induced differential oscillatory binding of AHR, with a periodicity between 1.5 and 2 h. Re-treatment of cells with 3MC failed to alter the oscillatory binding profiles of AHR or aryl hydrocarbon receptor nuclear translocator. Cells became responsive to 3MC but not TCDD after 24 h of exposure to 3MC, highlighting important differences in AHR responsiveness between the two ligands. Our results reveal a number of novel AHR-bound promoter regions and target genes that exhibit differential kinetic binding profiles and regulation by AHR.

Repression of aryl hydrocarbon receptor transcriptional activity by epidermal growth factor

The aryl hydrocarbon receptor (AHR) mediates most, if not all, of the many toxicological effects of the environmental pollutant 2,3,7,8-tetrachlorodibenzo- p-dioxin [(TCDD) or dioxin]. The "classical" pathway of AHR action involves dimerization of the liganded AHR with the aryl hydrocarbon nuclear translocator (ARNT) protein, and the AHR-ARNT dimer specifically associates with the enhancer regions of dioxin-responsive genes, leading to their increased transcription. Sutter and coworkers recently reported that epidermal growth factor (EGF) represses the dioxin-mediated induction of CYP1A1 in cultured normal human keratinocytes by inhibiting the recruitment of the transcriptional coactivator protein p300 to the CYP1A1 gene. EGF also inhibits the dioxin-dependent induction of certain parameters in keratinocytes that are reflective of dioxin-induced chloracne. These findings point to the potential usefulness of EGF for the treatment of chloracne and also describe a novel mechanism for repression of dioxin-induced gene transcription.

Resveratrol inhibits dioxin-induced expression of human CYP1A1 and CYP1B1 by inhibiting recruitment of the aryl hydrocarbon receptor complex and RNA polymerase II to the regulatory regions of the corresponding genes

The CYP1A family of cytochrome P450s (CYPs), comprising CYP1A1, CYP1A2, and CYP1B1, plays a role in bioactivation of several procarcinogens to carcinogenic derivatives, and also in detoxification of several xenobiotic compounds. Resveratrol (3,4,5-trihydroxystelbine) is a naturally occurring compound that has been shown in a number of studies to inhibit the induction of CYP1A1 and CYP1B1 by dioxin (2,3,7,8-tetrachloro-dibenzo-p-dioxin), but the mechanism(s) of resveratrol inhibition is controversial. In the current study, 100nM dioxin treatment for 24, 48, and 72 h induced CYP1A1, CYP1A2, and CYP1B1 mRNA levels in the human breast cancer cell line MCF-7, and CYP1A1 and CYP1A2 mRNA levels in the human hepatocellular carcinoma cell line, HepG2. Simultaneous treatment with 10 microM resveratrol significantly inhibited dioxin-induced mRNA expression levels of these genes in both cell lines. Our studies are novel in that we used the chromatin immunoprecipitation assay to assay dioxin-induced recruitment of the aryl hydrocarbon receptor (AHR), and aryl hydrocarbon nuclear translocator (ARNT) to the enhancer regions and recruitment of RNA polymerase II to the promoter regions, of the CYP1A1 and CYP1B1 genes in their natural chromosomal settings. These recruitments were significantly inhibited in cells cotreated with resveratrol. Our studies thus indicate that resveratrol inhibits dioxin induction of the CYP1 family members either by directly or indirectly inhibiting the recruitment of the transcription factors AHR and ARNT to the xenobiotic response elements of the corresponding genes. The reduced transcriptional factor binding at their enhancers then results in reduced pol II recruitment at the promoters of these genes.

EGF receptor signaling blocks aryl hydrocarbon receptor-mediated transcription and cell differentiation in human epidermal keratinocytes

Dioxin is an extremely potent carcinogen. In highly exposed people, the most commonly observed toxicity is chloracne, a pathological response of the skin. Most of the effects of dioxin are attributed to its activation of the aryl hydrocarbon receptor (AHR), a transcription factor that binds to the Ah receptor nuclear translocator (ARNT) to regulate the transcription of numerous genes, including CYP1A1 and CYP1B1. In cultures of normal human epidermal keratinocytes dioxin accelerates cell differentiation, as measured by the formation of cornified envelopes. We show that this acceleration is mediated by the AHR; also, that dioxin increases the expression of several genes known to be regulated by ARNT, which have critical roles in the cornification and epidermal barrier function of the skin. Importantly, we demonstrate that all of these responses are opposed by ligand-activation of the EGF receptor (R), an important regulator of keratinocyte cell fate. In the CYP1A1 enhancer, EGFR activation prevents recruitment of the p300 coactivator, although not affecting the binding of the AHR or ARNT. The total cellular level of p300 protein does not decrease, and overexpression of p300 relieves EGFR-mediated repression of transcription, indicating that p300 is a critical target for the repression of the AHR complex by EGFR signaling. These results provide a mechanism by which 2,3,7,8-tetrachlorodibenzo-p-dioxin is able to disrupt epidermal homeostasis and identify EGFR signaling as a regulator of the AHR. This signaling may modulate the incidence and severity of chloracne and be of therapeutic relevance to human poisonings by dioxin.