Dioxin-dependent activation of murine Cyp1a-1 gene transcription requires protein kinase C-dependent phosphorylation

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.

Cadmium interference with ERK1/2 and AhR signaling without evidence for cross-talk

The possibility that Cd may activate AhR indirectlyviaERK1/2 phosphorylation was tested as a function of enterocytic differentiation status in the human Caco-2 cells.

Genome-wide RNAi high-throughput screen identifies proteins necessary for the AHR-dependent induction of CYP1A1 by 2,3,7,8-tetrachlorodibenzo-p-dioxin

The aryl hydrocarbon receptor (AHR) has a plethora of physiological roles, and upon dysregulation, carcinogenesis can occur. One target gene of AHR encodes the xenobiotic and drug-metabolizing enzyme CYP1A1, which is inducible by the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) via the AHR. An siRNA library targeted against over 5600 gene candidates in the druggable genome was used to transfect mouse Hepa-1 cells, which were then treated with TCDD, and subsequently assayed for CYP1A1-dependent ethoxyresorufin-o-deethylase (EROD) activity. Following redundant siRNA activity (RSA) statistical analysis, we identified 93 hits that reduced EROD activity with a p value ≤ .005 and substantiated 39 of these as positive hits in a secondary screening using endoribonuclease-prepared siRNAs (esiRNAs). Twelve of the corresponding gene products were subsequently confirmed to be necessary for the induction of CYP1A1 messenger RNA by TCDD. None of the candidates were deficient in aryl hydrocarbon nuclear translocator expression. However 6 gene products including UBE2i, RAB40C, CRYGD, DCTN4, RBM5, and RAD50 are required for the expression of AHR as well as for induction of CYP1A1. We also found 2 gene products, ARMC8 and TCF20, to be required for the induction of CYP1A1, but our data are ambiguous as to whether they are required for the expression of AHR. In contrast, SIN3A, PDC, TMEM5, and CD9 are not required for AHR expression but are required for the induction of CYP1A1, implicating a direct role in Cyp1a1 transcription. Our methods, although applied to Cyp1a1, could be modified for identifying proteins that regulate other inducible genes.

Role of Scaffold Architecture and Mechanical Properties of Electrospun Scaffolds in Cell Seeding

Seeding a layer of cells at specific depths within scaffolds is an important optimization parameter for bi-layer skin models. The work presented investigated the effect of fiber diameter and its mechanical property on the depth of cell seeding for electro-spun fiber scaffold. Polycaprolactone (PCL) is used to generate scaffolds that are submicron (400nm) to micron (1100nm) using electro-spinning. 3T3 fibroblasts were seeded on the electro-spun fiber scaffold mat of 50-70 microns thickness in this study. In order to investigate the effect of fiber diameter on cell migration, first, the electrospun fiber scaffold was studied for variation of mechanical properties as a function of fiber diameters. Atomic force microscopy (AFM) was used to investigate the Young’s modulus (E) values as a function of fiber diameter. It was identified that as the fiber diameter increases, the Young’s modulus values decreases considerably from 1.1GPa to 200MPa. The variation in E is correlated with cell seeding depth as a function of vacuum pressure. A higher E value led to a lower depth of cell seeding (closer to the surface) indicating that nanofibrous scaffolds offer larger resistance to cell movement compared to microfibrous scaffolds.

Amino acid substitutions in the aryl hydrocarbon receptor ligand binding domain reveal YH439 as an atypical AhR activator

The aryl hydrocarbon receptor (AhR) is traditionally defined as a transcription factor activated by exogenous polyaromatic and halogenated aromatic hydrocarbon (PAH/HAH) ligands. Active AhR induces genes involved in xenobiotic metabolism, including cytochrome P4501A1, which function to metabolize activating ligands. However, recent studies implicate AhR in biological events that are apparently unrelated to the xenobiotic response, implying that endogenous activation mechanisms exist. Three AhR genes in zebrafish (Danio rerio) encode proteins that demonstrate differential activation in response to PAH/HAHs, with the nonresponsive drAhR1a having some sequence divergence from the PAH/HAH-responsive AhRs in the ligand binding domain (LBD). We used these differences to guide the mutagenesis of mouse AhR (mAhR), aiming to generate variants that functionally discriminate between activation mechanisms. We found substitution of histidine 285 in the LBD with tyrosine gave a receptor that could be activated by isopropyl-2-(1,3-dithietane-2-ylidene)-2-[N-(4-methylthiazol-2-yl)carbamoyl]acetate (YH439), a potential AhR ligand chemically distinct from classic PAH/HAH-type ligands, but prevented activation by both exogenous PAH/HAH ligands and the endogenous activation mimics of suspension culture and application of shear-stressed serum. The differential response of H285Y mAhR to YH439 suggests that this activator has a novel mode of interaction that tolerates tyrosine at position 285 in the LBD and is distinct from the binding mode of the well characterized PAH/HAH ligands. In support of this, the PAH-type antagonist 3',4'-dimethoxyflavone blocked mAhR activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin but not YH439. Furthermore, the strict correlation between response to exogenous PAH/HAH ligands and mimics of endogenous activation suggests that a PAH-type ligand may underpin endogenous mechanisms of activation.

Controlled vacuum seeding as a means of generating uniform cellular distribution in electrospun polycaprolactone (PCL) scaffolds

A major challenge encountered in using electrospun scaffolds for tissue engineering is the non-uniform cellular distribution in the scaffold with increasing depth under normal passive seeding conditions. Because of the small surface pores, typically few microns in diameter, cells tend to congregate and proliferate on the surface much faster compared to penetrating the scaffold interior. In order to overcome this problem, we used a vacuum seeding technique on polycaprolactone electrospun scaffolds while using NIH 3T3 fibroblasts as the model cell system. This serves as a precursor to the bilayer skin model where the fibroblasts would be residing at an intermediate layer and the keratinocytes would be on the top. Vacuum seeding was used in this study to enhance fibroblasts seeding and proliferation at different depths. Our results show that the kinetics of cell attachment and proliferation were a function of varying vacuum pressure as well as fiber diameter. Cell attachment reached a maxima somewhere between 2-8 in. Hg vacuum pressure and fell for lower vacuum pressures presumably because of cell loss through the filtration process. Cell proliferation and collagen secretion over five days indicated that vacuum pressure did not affect cellular function adversely. We also compared the combined impact of scaffold architecture (400 nm versus 1100 nm average diameter fiber scaffolds) and vacuum pressure. At a given pressure, more cells were retained in the 400 nm scaffolds compared to 1100 nm scaffolds. In addition, the cell intensity profile shows cell intensity peak shift from the top to the inner layers of the scaffold by lowering the vacuum pressure from 0 in. Hg to 20 in. Hg. For a given vacuum pressure the cells were seeded deeper within the 1100 nm scaffold. The results indicate that cells can be seeded in electrospun scaffolds at various depths in a controlled manner using a simple vacuum seeding technique. The depth of seeding is a function of pressure and scaffold fiber diameter.

Cytochrome P450 CYP1A1: wider roles in cancer progression and prevention

CYP1A1 is one of the main cytochrome P450 enzymes, examined extensively for its capacity to activate compounds with carcinogenic properties. Continuous exposure to inhalation chemicals and environmental carcinogens is thought to increase the level of CYP1A1 expression in extrahepatic tissues, through the aryl hydrocarbon receptor (AhR). Although the latter has long been recognized as a ligand-induced transcription factor, which is responsible for the xenobiotic activating pathway of several phase I and phase II metabolizing enzymes, recent evidence suggests that the AhR is involved in various cell signaling pathways critical to cell cycle regulation and normal homeostasis. Disregulation of these pathways is implicated in tumor progression. In addition, it is becoming increasingly evident that CYP1A1 plays an important role in the detoxication of environmental carcinogens, as well as in the metabolic activation of dietary compounds with cancer preventative activity. Ultimately the contribution of CYP1A1 to cancer progression or prevention may depend on the balance of procarcinogen activation/detoxication and dietary natural product extrahepatic metabolism.

The aryl hydrocarbon receptor at the crossroads of multiple signaling pathways

The aryl hydrocarbon receptor (AHR) has long been recognized as a ligand-activated transcription factor responsible for the induction of drug-metabolizing enzymes. Its role in the combinatorial matrix of cell functions was established long before the first report of an AHR cDNA sequence was published. It is only recently that other functions of this protein have begun to be recognized, and it is now clear that the AHR also functions in pathways outside of its well-characterized role in xenobiotic enzyme induction. Perturbation of these pathways by xenobiotic ligands may ultimately explain much of the toxicity of these compounds. This chapter focuses on the interactions of the AHR in pathways critical to cell cycle regulation, mitogen-activated protein kinase cascades, differentiation and apoptosis. Ultimately, the effect of a particular AHR ligand on the biology of the organism will depend on the milieu of critical pathways and proteins expressed in specific cells and tissues with which the AHR itself interacts.

Knock-in mouse lines expressing either mitochondrial or microsomal CYP1A1: differing responses to dietary benzo[a]pyrene as proof of principle

In the past, CYP1A1 protein was known to be located in the endoplasmic reticulum (ER; microsomes). More recently, CYP1A1 was shown also to be targeted to the inner mitochondrial membrane; mitochondrial import is dependent on NH(2)-terminal processing that exposes a cryptic targeting signal. It is interesting that microsomal and mitochondrial CYP1A1 enzymes exhibit different substrate specificities, electron donors, and inducer properties. To understand the physiological functions of microsomal versus mitochondrial CYP1A1, we have generated three knock-in lines by altering the CYP1A1 NH(2) terminus. Cyp1a1(mtt/mtt) mice encode an NH(2)-terminal 31-amino acid-truncated protein, deleting the ER-targeting signal and exposing the cryptic mitochondrial-targeting signal. Cyp1a1(mtp/mtp) mice encode a protein carrying L7N and L17N mutations; this mutant lacks the signal recognition particle (SRP)-binding site and subsequent ER-targeting, but requires proteolysis by a cytosolic peptidase for mitochondrial import. Cyp1a1(mc/mc) mice encode a microsomal protein having R34D and K39I mutations, which abolish the mitochondrial targeting signal. After dioxin or beta-naphthoflavone treatment of these mouse lines, the CYP1A1 protein was shown to be located in the mitochondria of the Cyp1a1(mtp/mtp) and Cyp1a1(mtt/mtt) lines and in microsomes of the Cyp1a1(mc/mc) line. To test for differences in function, we compared the response to dietary benzo[a]pyrene (BaP). After 18 days of daily oral BaP, wild-type and Cyp1a1(mc/mc) mice were completely protected, whereas Cyp1a1(-/-) and Cyp1a1(mtp/mtp) mice showed striking toxicity and compensatory up-regulation of CYP1A2 and CYP1B1 mRNA in several tissues. Our data support the likelihood that it is the microsomal rather than mitochondrial CYP1A1 enzyme that protects against oral BaP toxicity.

Role of cAMP in mediating AHR signaling

Regulation of the nuclear import of many transcription factors represents a step in gene regulation which is crucial for a number of cellular processes. The aryl hydrocarbon receptor (AHR), a basic helix-loop-helix protein of the PAS (PER-ARNT-SIM) family of transcriptional regulators is a cytosol-associated and ligand-activated receptor. The environmental toxin dioxin binds with high affinity to AHR rendering it nuclear and leading to the activation of AHR sensitive genes. However, the fact, that the AHR mediates a large variety of physiological events without the involvement of any known exogenous ligand, including liver and vascular system development, maturation of the immune system, regulation of genes involved in cellular growth, cell differentiation and circadian rhythm, speaks for an important role of AHR in cell biology independent of the presence of an exogenous ligand. Different approaches were applied to study mechanism(s) which render AHR nuclear and design its function in absence of exogenous ligands. We found that AHR is sensitive to cAMP signaling mediated by cAMP-dependent protein kinase (PKA) which fundamentally differs from AHR signaling mediated by the exogenous ligand dioxin. It has been shown that PKA mediated signaling can be confined by compartmentalization of signaling components in microdomains conferring specificity to signaling by the ubiquitous second messenger cAMP. Moreover, A-kinase-anchoring proteins (AKAPs) and newly discovered cAMP receptors, Epac (exchange protein directly activated by cAMP), may give us a further chance to enter into new dimensions of cAMP signal transmissions that potentially may bring us closer to AHR physiology.

The aryl hydrocarbon receptor cross-talks with multiple signal transduction pathways

Exposure to toxic polycyclic aromatic hydrocarbons raises a number of toxic and carcinogenic responses in experimental animals and humans mediated for the most part by the aryl hydrocarbon -- or dioxin -- receptor (AHR). The AHR is a ligand-activated transcription factor whose central role in the induction of drug-metabolizing enzymes has long been recognized. For quite some time now, it has become clear that the AHR also functions in pathways outside of its role in detoxification and that perturbation of these pathways by xenobiotic ligands may be an important part of the toxicity of these compounds. AHR activation by some of its ligands participates among others in pathways critical to cell cycle regulation, mitogen-activated protein kinase cascades, immediate-early gene induction, cross-talk within the RB/E2F axis and mobilization of crucial calcium stores. Ultimately, the effect of a particular AHR ligand may depend as much on the adaptive interactions that it established with pathways and proteins expressed in a specific cell or tissue as on the toxic responses that it raises.

Phosphorylation of xenobiotic-metabolizing cytochromes P450

The regulation of cytochromes P450 (CYPs) by induction mediated by xenobiotics is well known. Our team has discovered an additional important regulation of xenobiotic-metabolizing CYPs by phosphorylation. Individual CYPs are phosphorylated by different protein kinases, leading to CYP isoenzyme-selective changes in the metabolism of individual substrates and consequent profound changes in the control of mutagenic and cytotoxic metabolites. Some CYPs are phosphorylated by protein kinase C and some by the cyclic adenosine monophosphate (cAMP) dependent protein kinase A. We found that cAMP not only leads to drastic changes in the activity of individual CYPs, but also drastic changes in the nuclear localization of the CYP-related transcription factor Ah receptor (AHR). The consequences are very different from those of AHR nuclear translocation mediated by its classic ligands (such as dioxin and many polycyclic aromatic hydrocarbons) and may represent the long-sought physiological function of the AHR. The disturbance of this physiological function of AHR by extremely persistent high-affinity xenobiotic ligands such as dioxin may represent the most important contributing factor for their potent toxicity.

Constitutive regulation of CYP1B1 by the aryl hydrocarbon receptor (AhR) in pre-malignant and malignant mammary tissue

The aryl hydrocarbon receptor (AhR) is a receptor/transcription factor which regulates cytochrome P450 (CYP) gene transcription and which is activated by environmental carcinogens, some of which are associated with increased breast cancer risk. Here, we show that the AhR is over-expressed and constitutively active in human and rodent mammary tumors, suggesting its ongoing contribution to tumorigenesis regardless of tumor etiology. AhR regulation of CYP1A1 and CYP1B1 was studied to determine if constitutively active AhR effects the same transcriptional outcomes as environmental chemical-activated AhR. Elevated AhR and CYP1B1 but not CYP1A1 before tumor formation in a rat model of mammary tumorigenesis suggested differential CYP1B1 regulation by a constitutively active AhR. This hypothesis was tested with human mammary gland cell lines which hyper-express AhR and CYP1B1 but which express little or no CYP1A1. CYP1B1 expression was diminished by repression of AhR activity or by AhR knockdown, demonstrating AhR control of basal CYP1B1 levels. ChIP assays demonstrated constitutive AhR binding to both CYP1A1 and CYP1B1 promoters, demonstrating that differential CYP1A1 and CYP1B1 regulation by constitutively active AhR does not result from different amounts of promoter-bound AhR. While increasing AhR binding to both CYP1A1 and CYP1B1, 2,3,7,8-tetrachlorodibenzo-p-dioxin induced CYP1A1 mRNA in both a malignant and non-malignant line but increased only CYP1B1 mRNA in the malignant line, again demonstrating that the level of promoter binding does not necessarily correlate with gene mRNA levels. These studies suggest that constitutively active AhR mediates different molecular outcomes than environmental chemical-activated AhR, and further implicate the AhR in mammary tumorigenesis.

Dietary curcumin modulates transcriptional regulators of phase I and phase II enzymes in benzo[a]pyrene-treated mice: mechanism of its anti-initiating action

Curcumin has been shown to possess anti-initiating and anti-promoting activity in experimental systems. However, the mechanisms of its actions are not fully elucidated in vivo. In the present study, mechanisms of curcumin-mediated anti-initiation were investigated in mice employing benzo[a]pyrene (B[a]P) as a model carcinogen. Dietary pretreatment of mice with chemopreventive doses of curcumin showed significant inhibition of B[a]P-induced enzyme activity, protein and messenger RNA (mRNA) levels of cytochrome P450 1A1/1A2 in liver and lungs. Although curcumin alone did not alter the basal levels of aryl hydrocarbon receptor (AhR), it significantly decreased the B[a]P-induced AhR protein levels, its phosphorylation, nuclear translocation and subsequent binding to DNA, thereby decreasing the transactivation of CYP1A. Dietary curcumin led to increase in NF-E2-related factor-2 (Nrf2) protein levels and enhanced its nuclear translocation in liver and lungs of mice as compared with controls. Additionally, increased binding of Nrf2 to antioxidant response element occurred in nuclear extracts from liver and lungs of mice pretreated with dietary curcumin. Induction of activity, protein and mRNA levels of glutathione S-transferase, its isoforms and NAD(P)H:quinone oxidoreductase-1 by dietary curcumin in mice paralleled the curcumin-mediated activation of Nrf2, leading to increased detoxification of B[a]P. In agreement with the observed curcumin-mediated decrease in B[a]P-induced phase I enzyme and concomitant induction of phase II enzymes, pretreatment with dietary curcumin resulted in significant reduction of B[a]P-induced DNA adduct, oxidative damage and inflammation. To conclude, curcumin exhibits anti-initiating effects via modulating the transcriptional regulators of phase I and phase II enzymes in mice.

HDAC1 bound to the Cyp1a1 promoter blocks histone acetylation associated with Ah receptor-mediated trans-activation

Metabolic bioactivation of polycyclic aromatic hydrocarbons, such as the environmental procarcinogen benzo[a]pyrene, is catalyzed by a cytochrome P450 monooxygenase encoded by the substrate-inducible Cyp1a1 gene. Cyp1a1 induction requires trans-activation by the heterodimeric transcriptional complex formed by the liganded Ah receptor (AHR) and its partner, ARNT. Previously, we showed that constitutively bound HDAC1 dissociates from Cyp1a1 promoter chromatin after ligand-mediated induction, concomitantly with the recruitment of AHR/ARNT complexes and p300. Here, we investigated the hypothesis that HDAC1 binding maintains the Cyp1a1 gene in a silenced state in uninduced cells. We find that Cyp1a1 induction by the AHR/ARNT is associated with modification of specific chromatin marks, including hyperacetylation of histone H3K14 and H4K16, trimethylation of histone H3K4, and phosphorylation of H3S10. HDAC1 and DNMT1 form complexes on the Cyp1a1 promoter of uninduced cells but HDAC1 inhibition alone is not sufficient to induce Cyp1a1 expression, although it allows for the hyperacetylation of H3K14 and H4K16 to levels similar to those found in B[a]P-induced cells. These results show that by blocking the modification of histone marks, HDAC1 plays a central role in Cyp1a1 expression and that its removal is a necessary but not sufficient condition for Cyp1a1 induction, underscoring the requirement for a concerted series of chromatin-remodeling events to complete the initial steps of gene trans-activation by the Ah receptor.

Decreased PXR and CAR inhibit transporter and CYP mRNA Levels in the liver and intestine of mice with collagen-induced arthritis

Nuclear receptors, such as pregnane X receptor (PXR) and constitutive androstane receptor (CAR), regulate the transcription of transporters and cytochrome P450s (CYPs). We investigated whether quantitative and functional changes in PXR and CAR affected the transporters and CYPs in a mouse model of chronic arthritis. The mRNA levels of PXR were significantly decreased in the intestine of mice with collagen-induced arthritis (CIA) compared with control mice. The mRNA levels of CAR were significantly decreased in both the liver and intestine of CIA mice. The mRNA levels of Mdr1a/1b, Mrp3, BCRP and Cyp2b10 were decreased in the liver of CIA mice, while little change in the mRNA levels was observed for Cyp3a11 in the liver and the transporters in the intestine. Taken together, the present results reveal that the effects of CAR mRNA suppression on the regulation of transporters and CYPs differ between the liver and intestine in chronic arthritis.

The dioxin (aryl hydrocarbon) receptor as a model for adaptive responses of bHLH/PAS transcription factors

This review examines the common theme of adaptive responses of bHLH/PAS proteins, using the dioxin receptor as a prototype. The bHLH/PAS family of transcriptional regulators are a group of key developmental and environmental stress sensing proteins. They employ a variety of post-translational control mechanisms to regulate their transcriptional output. Amongst this family, the dioxin receptor is best known for its ability to elicit toxic responses to dioxin and dioxin like chemicals even though it mediates more benign adaptive responses to non-toxic xenobiotics. We discuss what is known about dioxin receptor physiology, both adaptive and inherent, along with its molecular regulation and put this into the context of the wider bHLH/PAS family. We also raise the issue of its toxic responses, in particular the idea that it is the dysregulation of its poorly characterised housekeeping functions that leads to these outcomes.

A rapid seeding technique for the assembly of large cell/scaffold composite constructs

These studies address critical technical issues involved in creating human mesenchymal stem cell (hMSC)/ scaffold implants for cartilage repair. These issues include obtaining a high cell density and uniform spatial cell distribution within the scaffold, factors that are critical in the initiation and homogeneity of chondrogenic differentiation. For any given scaffold, the initial seeding influences cell density, retention, and spatial distribution within the scaffold, which eventually will affect the function of the construct. Here, we discuss the development of a vacuum-aided seeding technique for HYAFF -11 sponges which we compared to passive infiltration. Our results show that, under the conditions tested, hMSCs were quantitatively and homogeneously loaded into the scaffolds with 90+% retention rates after 24 h in perfusion culture with no negative effect on cell viability or chondrogenic potential. The retention rates of the vacuum-seeded constructs were at least 2 times greater than those of passively seeded constructs at 72 h. Histomorphometric analysis revealed that the core of the vacuum-seeded constructs contained 240% more cells than the core of passively infiltrated scaffolds. The vacuum seeding technique is safe, rapid, reproducible, and results in controlled quantitative cell loading, high retention, and uniform distribution.

Growth of a human mammary tumor cell line is blocked by galangin, a naturally occurring bioflavonoid, and is accompanied by down-regulation of cyclins D3, E, and A

Introduction

This study was designed to determine if and how a non-toxic, naturally occurring bioflavonoid, galangin, affects proliferation of human mammary tumor cells. Our previous studies demonstrated that, in other cell types, galangin is a potent inhibitor of the aryl hydrocarbon receptor (AhR), an environmental carcinogen-responsive transcription factor implicated in mammary tumor initiation and growth control. Because some current breast cancer therapeutics are ineffective in estrogen receptor (ER) negative tumors and since the AhR may be involved in breast cancer proliferation, the effects of galangin on the proliferation of an ER^-, AhR^high line, Hs578T, were studied.

Methods

AhR expression and function in the presence or absence of galangin, a second AhR inhibitor, α-naphthoflavone (α-NF), an AhR agonist, indole-3-carbinol, and a transfected AhR repressor-encoding plasmid (FhAhRR) were studied in Hs578T cells by western blotting for nuclear (for instance, constitutively activated) AhR and by transfection of an AhR-driven reporter construct, pGudLuc. The effects of these agents on cell proliferation were studied by ³H-thymidine incorporation and by flow cytometry. The effects on cyclins implicated in mammary tumorigenesis were evaluated by western blotting.

Results

Hs578T cells were shown to express high levels of constitutively active AhR. Constitutive and environmental chemical-induced AhR activity was profoundly suppressed by galangin as was cell proliferation. However, the failure of α-NF or FhAhRR transfection to block proliferation indicated that galangin-mediated AhR inhibition was either insufficient or unrelated to its ability to significantly block cell proliferation at therapeutically relevant doses (IC₅₀ = 11 μM). Galangin inhibited transition of cells from the G₀/G₁ to the S phases of cell growth, likely through the nearly total elimination of cyclin D3. Expression of cyclins A and E was also suppressed.

Conclusion

Galangin is a strong inhibitor of Hs578T cell proliferation that likely mediates this effect through a relatively unique mechanism, suppression of cyclin D3, and not through the AhR. The results suggest that this non-toxic bioflavonoid may be useful as a chemotherapeutic, particularly in combination with agents that target other components of the tumor cell cycle and in situations where estrogen receptor-specific therapeutics are ineffective.

The aryl hydrocarbon receptor constitutively represses c-myc transcription in human mammary tumor cells

The aryl hydrocarbon receptor (AhR) is an environmental carcinogen-activated transcription factor associated with tumorigenesis. High levels of apparently active AhR characterize a variety of tumors, even in the absence of environmental ligands. Despite this association between transformation and AhR upregulation, little is known of the transcriptional consequences of constitutive AhR activation. Here, the effects of constitutively active and environmental ligand-induced AhR on c-myc, an oncogene whose promoter contains six AhR-binding sites (AhREs (aryl hydrocarbon response elements)), were investigated. A reporter containing the human c-myc promoter, with its six AhREs and two NF-kappaB-binding sites, was constructed. This vector, and variants with deletions in the NF-kappaB and/or AhR-binding sites, was transfected into a human breast cancer cell line, Hs578T, which expresses high levels of apparently active, nuclear AhR. Results indicate that: (1) the AhR constitutively binds the c-myc promoter; (2) there is a low but significant baseline level of c-myc promoter activity, which is not regulated by NF-kappaB and is not affected by an environmental AhR ligand; (3) deletion of any one of the AhREs has no effect on constitutive reporter activity, while deletion of all six increases reporter activity approximately fivefold; (4) a similar increase in reporter activity occurs when constitutively active AhR is suppressed by transfection with an AhR repressor plasmid (AhRR); (5) AhRR transfection significantly increases background levels of endogenous c-myc mRNA and c-Myc protein. These results suggest that the AhR influences the expression of c-Myc, a protein critical to malignant transformation.

Probing the Control Elements of the CYP1A1 Switching Module in H4IIE Hepatoma Cells

Previous research from our laboratory has shown a switch-like response to PCB 126 mediated CYP1A1 induction in primary rat hepatocytes and in H4IIE rat hepatoma cells. On a single cell level, cells appear to be either "on" or "off" for CYP1A1 induction at a given dose; some cells never respond to PCB 126. These cells represent a non-responding population. Cells that are switched "on" by PCB 126 display varying levels of induction, much like the dimmer on a light switch. The goal of the present research is to begin to uncover the mechanism for this switch-like response to CYP1A1 induction in H4IIE rat hepatoma cells. The AhR pathway is modulated by multiple co-activators and by phosphorylation. This research focuses on the phosphorylation cascades initiated by PCB 126 and the role they play in CYP1A1 induction. Our research reveals a likely role for protein kinase C (PKC) in this switch response. Inhibition of PKC by H-7 dramatically reduced the percent of cells that express CYP1A1 in response to PCB 126 treatment, as determined by flow cytometry. The effect of H-7 was concentration dependent, decreasing the number of cells expressing CYP1A1 rather than decreasing the level of CYP1A1 in all cells. This finding provides further evidence for the switch-like behavior of CYP1A1 induction and implicates PKC in this response to PCB126. The protein kinase inhibitor, HA-1004, had only a minor effect on CYP1A1 induction. A high-throughput immunoblot screen for 40 proteins revealed the regulation of several proteins/phosphoproteins by PCB 126. Most importantly, two proteins containing phosphoserine/phoshothreonine residues were increased by PCB126 treatment. However, PKC translocation studies and activity studies failed to verify that PCB126 activates PKC. It is possible that constitutive PKC activity is sufficient to maintain phosphorylation of critical components of the AhR pathway. Immunoblotting studies showed that MAP kinases ERK and JNK are not activated by PCB 126 in H4IIE cells and the ERK inhibitor U0126 did not impair CYP1A1 induction. Additional studies are planned to further investigate the role of PKC in the switch-like response to PCB 126.

Butylhydroquinone protects cells genetically deficient in glutathione biosynthesis from arsenite-induced apoptosis without significantly changing their prooxidant status

Arsenic, first among the top environmentally hazardous substances, is associated with skin, lung, liver, kidney, prostate, and bladder cancer. Arsenic is also a cardiovascular and a central nervous system toxicant, and it has genotoxic and immunotoxic effects. Paradoxically, arsenic trioxide is used successfully in the treatment of acute promyelocytic leukemia and multiple myeloma. Arsenic induces oxidative stress, and its toxicity is decreased by free thiols and increased by glutathione depletion. To further characterize the role of glutathione and oxidative stress in the toxicity of arsenic, we have used fetal fibroblasts from Gclm(-/-) mice, which lack the modifier subunit of glutamate-cysteine ligase, the rate-limiting enzyme in glutathione biosynthesis. Gclm(-/-) mouse embryo fibroblasts (MEFs) are eight times more sensitive to arsenite-induced apoptotic death. Because of a dramatic decrease in glutathione levels, Gclm(-/-) MEFs have a high prooxidant status that is not significantly relieved by treatment with the phenolic antioxidant tBHQ; however, tBHQ blocks arsenite-induced apoptosis in both Gclm(+/+) and Gclm(-/-) cells, although it raises a significant antioxidant response only in Gclm(+/+) cells. Global gene expression profiles indicate that tBHQ is significantly effective in reversing arsenite-induced gene deregulation in Gclm(+/+) but not in Gclm(-/-) MEFs. This effect of tBHQ is evident in the expression of metalloproteases and chaperones, and in the expression of genes involved in DNA damage and repair, protein biosynthesis, cell growth and maintenance, apoptosis, and cell cycle regulation. These results suggest that regulation of glutathione levels by GCLM determines the sensitivity to arsenic-induced apoptosis by setting the overall ability of the cells to mount an effective antioxidant response.

Immortalized mouse mammary fibroblasts lacking dioxin receptor have impaired tumorigenicity in a subcutaneous mouse xenograft model

Although the dioxin receptor, the aryl hydrocarbon receptor (AhR), is considered a major regulator of xenobiotic-induced carcinogenesis, its role in tumor formation in the absence of xenobiotics is still largely unknown. Trying to address this question, we have produced immortalized cell lines from wild-type (T-FGM-AhR+/+) and mutant (T-FGM-AhR-/-) mouse mammary fibroblasts by stable co-transfection with the simian virus 40 (SV-40) large T antigen and proto-oncogenic c-H-Ras. Both cell lines had a myofibroblast phenotype and similar proliferation, doubling time, SV-40 and c-H-Ras expression and activity, and cell cycle distribution. AhR+/+ and AhR-/- cells were also equally able to support growth factor- and anchorage-independent proliferation. However, the ability of T-FGM-AhR-/- to induce subcutaneous tumors (leimyosarcomas) in NOD/SCID-immunodeficient mice was close to 4-fold lower than T-FGM-AhR+/+. In culture, T-FGM-AhR-/- had diminished migration in collagen-I and decreased lamellipodia formation. VEGFR-1/Flt-1, a VEGF receptor that regulates cell migration and blood vessel formation, was also down-regulated in AhR-/- cells. Signaling through the ERK-FAK-PKB/AKT-Rac-1 pathway, which contributes to cell motility and invasion, was also significantly inhibited in T-FGM-AhR-/-. Thus, the lower tumorigenic potential of T-FGM-AhR-/- could result from a compromised adaptability of these cells to the in vivo microenvironment, possibly because of an impaired ability to migrate and to respond to angiogenesis.

The Role of Protein Kinase C in Regulation of TCDD-Mediated CYP1A1 Gene Expression

Cytochrome P450 1A1 (CYP1A1) is induced by halogenated and polycyclic aromatic hydrocarbons following activation of the aryl hydrocarbon receptor (AhR). Protein kinase C (PKC) has been implicated in the regulation of this response. In tissue culture, induction of PKC activity with phorbol esters synergizes the actions of TCDD-induced CYP1A1, while PKC inhibitors block induction of CYP1A1 by TCDD. Here, the actions of specific PKC inhibitors on CYP1A1 induction were examined using a HepG2 human cell line (TV101L) that carries a stably integrated firefly luciferase gene under control of the human CYP1A1 promoter (-1612/+293). TV101 cells were treated with TCDD and either the kinase inhibitor staurosporine or one of the PKC inhibitors GF109203X, Gö6983, or Gö6976. Aryl hydrocarbon receptor-dependent activation of CYP1A1-luciferase and cellular PKC activity were measured. TCDD treatment induced CYP1A1-luciferase activity in an AhR-dependent manner, as determined by binding of nuclear AhR to xenobiotic response elements (XREs). Dose-dependent inhibition of PKC activity by staurosporine was concordant with inhibition of TCDD-induced CYP1A1-luciferase activity. However, the PKC inhibitors GF109203X, Gö6983, and Gö6976 blocked PKC activity at concentrations independent of those necessary to block TCDD induction of CYP1A1-luciferase activity. For all inhibitors, reduction in CYP1A1-luciferase activity was independent of AhR activation, as determined by electrophoretic mobility shift analysis of TCDD-activated nuclear AhR. The specific PKC inhibitors did not significantly alter cytosolic or nuclear levels of AhR protein, whether alone or in combination with TCDD. These results suggested that PKC was not the sole factor responsible for regulation of CYP1A1.

Regulation of aryl hydrocarbon receptor signal transduction by protein tyrosine kinases

The involvement of protein tyrosine kinases (PTKs) in aryl hydrocarbon receptor (AhR)-mediated signalling by omeprazole and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was investigated in hepatoma cells. Both omeprazole- and TCDD-dependent AhR signalling was attenuated by inhibition of c-src kinase, either by using pyrazolopyrimidine 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4 ]pyrimidine (PP1) and 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) inhibitors or by expression of dominant-negative c-src. These results indicate that the overall AhR function is modulated by c-src kinase activity. In contrast, a selective inhibition of omeprazole-mediated AhR signalling was revealed by tyrosine kinase inhibitors, tyrphostins AG17 and AG879. Furthermore, omeprazole-dependent AhR activation was abolished by mutation of Tyr320 to Phe, suggesting that this residue is a putative phosphorylation site. TCDD-dependent AhR signalling was neither affected by tyrphostins nor by this mutation. Our results are consistent with activation of the AhR by omeprazole in a ligand-independent manner, via a signal transduction pathway that involves protein tyrosine kinases, and are different from the mechanism exerted by high-affinity ligands.

Protein kinase C? activity is involved in the 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced signal transduction pathway leading to apoptosis in L-MAT, a human lymphoblastic T-cell line

The aromatic hydrocarbon receptor (AhR)-dependent pathway involved in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced immunotoxicity has been studied extensively, but the AhR-independent molecular mechanism has not. In previous studies we found that the AhR is not expressed in L-MAT, a human lymphoblastic T-cell line. In this report, we provide the following evidence that the protein kinase C (PKC)theta activity is functionally involved in the AhR-independent signal transduction mechanism that participates in the TCDD-induced L-MAT cell apoptosis. First, only rottlerin, a novel PKC (nPKC)-selective inhibitor, blocked the apoptosis completely, in a dose-dependent manner. Second, PKCtheta was the major nPKC isoform (compared to PKCdelta) expressed in the L-MAT cell line. Third, a cell-permeable myristoylated PKCtheta pseudosubstrate peptide inhibitor also blocked the apoptosis completely, in a dose-dependent manner. Fourth, both rottlerin and myristoylated PKCtheta pseudosubstrate peptide inhibitor completely inhibited PKCtheta kinase activity in vitro at doses that effectively blocked TCDD-induced L-MAT cell apoptosis. TCDD treatment induced a time-dependent activation of nPKC kinase activity in L-MAT cells, and moreover, TCDD induced a translocation of PKCtheta from the cytosolic fraction to the particulate fraction in L-MAT cells. Finally, transient over-expression of a dominant negative PKCtheta (a kinase-dead mutant, K/R 409) in L-MAT cells conferred significant protection against TCDD-induced apoptosis.

Aryl hydrocarbon receptor, cell cycle regulation, toxicity, and tumorigenesis

Most effects of exposure to halogenated and polycyclic aromatic hydrocarbons are mediated by the aryl hydrocarbon receptor (AHR). It has long been recognized that the AHR is a ligand-activated transcription factor that plays a central role in the induction of drug-metabolizing enzymes and hence in xenobiotic detoxification. Of late, it has become evident that outside this well-characterized role, the AHR also functions as a modulator of cellular signaling pathways. In this Prospect, we discuss the involvement of the AHR in pathways critical to cell cycle regulation, mitogen-activated protein kinase cascades, immediate-early gene induction, and the functions of the RB protein. Ultimately, the toxicity of AHR xenobiotic ligands may be intrinsically connected with the perturbation of these pathways and depend on the many critical signaling pathways and effectors with which the AHR itself interacts.

Phenylthiourea as a weak activator of aryl hydrocarbon receptor inhibiting 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced CYP1A1 transcription in zebrafish embryo

The aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor that can be activated by a diverse synthetic and naturally-occurring chemicals, such as the halogenated aromatic hydrocarbons (HAHs) and the non-halogenated polycyclic aromatic hydrocarbons (PAHs). The liganded AHR modulates the genetic activity of a variety of xenobiotic-responsive genes, including cytochrome P4501A1 (CYP1A1). The tyrosinase inhibitor 1-phenyl-2-thiourea (PTU) is widely used in zebrafish research to suppress pigmentation in developing embryos/fry. Here we showed that 0.2 mM PTU induced a basal level of CYP1A1 transcription in zebrafish embryonic integument as early as 24 h postfertilization (hpf) stage. Subsequently, PTU induced CYP1A1 transcription in blood vessels at 36 hpf. During larval stage, the liver and all pharyngeal arch vessels of PTU-treated embryos exhibited CYP1A1 transcription as well. Comparing to TCDD, PTU induces CYP1A1 transcription with much lower efficacy in zebrafish embryos. Coincubating the embryos with PTU and TCDD led to repressing TCDD-induced CYP1A1 transcription. Mechanistic studies indicated that both of PTU- and TCDD-mediated CYP1A1 transcriptions are modulated by the same AHR-ARNT signaling pathway.

Phosphorylation of nuclear localization signal inhibits the ligand-dependent nuclear import of aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor which plays a role as an intracellular mediator of the xenobiotic signaling pathway. We previously identified the minimum nuclear localization signal (NLS) of AhR(13-39): it is composed of two basic amino acid segments, AhR(13-16:RKRR) and AhR(37-39:KRH). In this study, we showed that the two protein kinase C (PKC) sites of Ser-12 and Ser-36 are located one amino acid upstream from each of the two segments, and that a ligand-dependent nuclear import of AhR is inhibited by substitution of aspartic acid for Ser-12 (S12D) or Ser-36 (S36D), which mimics the negative charge of phosphorylation. This observation was supported by microinjection analysis, an in vitro nuclear transport assay, and a luciferase reporter assay, suggesting a two-step mechanism in the ligand-dependent nuclear translocation of AhR.

The human CYP1A1 gene is regulated in a developmental and tissue-specific fashion in transgenic mice

Regulation and expression of human CYP1A1 is demonstrated in transgenic mice. We have developed two transgenic mouse lines. One mouse strain (CYPLucR) carries a functional human CYP1A1 promoter (-1612 to +293)-luciferase reporter gene, and the other strain (CYP1A1N) expresses CYP1A1 under control of the full-length human CYP1A1 gene and 9 kb of flanking regulatory DNA. With CYPLucR(+/-) mice, 2,3,7,8-tetrachlordibenzo-p-dioxin (TCDD) and several other aryl hydrocarbon receptor ligands induced hepatocyte-specific luciferase activity. When other tissues were examined, TCDD induced luciferase activity in brain with limited induction in lung and no detectable luciferase activity in kidney. Treatment of CYP1A1N(+/-) mice with TCDD resulted in induction of human CYP1A1 in liver and lung, while mouse Cyp1a1 was induced in liver, lung, and kidney. Although induced CYP1A1/Cyp1a1 could not be detected by Western blot analysis in brains from CYP1A1N(+/-) mice, induction in brain was verified by detection of CYP1A1/Cyp1a1 RNA. The administration of TCDD to nursing mothers to examine the effect of lactational exposure via milk demonstrated prominent induction of luciferase activity in livers of CYPLucR(+/-) newborn pups with limited induction in brain. However, TCDD treatment of adult CYPLucR(+/-) mice led to a 7-10-fold induction of brain luciferase activity. Combined these results indicate that tissue-specific and developmental factors are controlling aryl hydrocarbon receptor-mediated induction of human CYP1A1.

Protein kinase C delta is activated in mouse ovarian surface epithelial cancer cells by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

Interactions between the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and protein kinase C (PKC) signaling pathways are governed in cell and tissue-specific manners, albeit the physiological significance of which is unclear. This research sought to define the effects of TCDD on the PKC pathway using a mouse ovarian surface epithelial cancer cell line (ID8). Phorbol-12-myristate-13-acetate (PMA) potentiated (1 nM) TCDD-induced 7-ethoxyresorufin-O-deethylase (EROD) activity after 24h of treatment, and pre-treatment with (1 microM) of either a general PKC inhibitor (BisI) or PKCdelta-specific inhibitor (Rotterlin) abolished the potentiation indicating that activation of PKC enhances TCDD signal transduction. Western blot analysis revealed that unstimulated ID8 cells express PKCalpha, beta, epsilon, tau, lambda and RACK1. PKCgamma, eta, theta and DGKtheta were not detected. TCDD (1 nM) increased PKCdelta protein approximately eight-fold after 24h of treatment and this effect was dose-dependent (0.1-100 nM); other PKC isoforms and related signaling proteins tested were unaffected by TCDD treatment. Immunofluorescent microscopy revealed that TCDD (1 nM) promoted the subcellular redistribution of PKCdelta, from the cytoplasm and the nucleus to the perinuclear area after 2h of treatment, however, after 24h of treatment PKCdelta was observed in nuclear structures that resembled nucleoli. TCDD (1 nM) also increased total PKC and PKCdelta-specific kinase activities in biphasic time-responsive manners. Total PKC and PKCdelta-specific activities increased after 1-2h of treatment. Then TCDD increased the total PKC activity again after 12h of treatment, whereas, PKCdelta-specific activity resurged at 24h and remained elevated at 48 h after treatment. The results indicate that TCDD preferentially induces PKCdelta protein expression and phosphotransferase activity, and its membrane translocation, indicating a potential intracellular role for PKCdelta as an effector molecule for TCDD-mediated biological events in this ovarian cancer cell line.

The aryl hydrocarbon receptor (AhR) tyrosine 9, a residue that is essential for AhR DNA binding activity, is not a phosphoresidue but augments AhR phosphorylation

We delineate a mechanism by which dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin or TCDD)-mediated formation of the aryl hydrocarbon receptor (AhR) DNA binding complex is disrupted by a single mutation at the conserved AhR tyrosine 9. Replacement of tyrosine 9 with the structurally conservative phenylalanine (AhRY9F) abolished binding to dioxin response element (DRE) D, E, and A and abrogated DRE-driven gene induction mediated by the AhR with no effect on TCDD binding, TCDD-induced nuclear localization, or ARNT heterodimerization. The speculated role for phosphorylation at tyrosine 9 was also examined. Anti-phosphotyrosine immunoblotting could not detect a major difference between the AhRY9F mutant and wild-type AhR, but a basic isoelectric point shift was detected by two-dimensional gel electrophoresis of AhRY9F. However, an antibody raised to recognize only phosphorylated tyrosine 9 (anti-AhRpY9) confirmed that AhR tyrosine 9 is not a phosphorylated residue required for DRE binding. Kinase assays using synthetic peptides corresponding to the wild-type and mutant AhR residues 1-23 demonstrated that a tyrosine at position 9 is important for substrate recognition at serine(s)/threonine(s) within this sequence by purified protein kinase C (PKC). Also, compared with AhRY9F, immunopurified full-length wild-type receptor was more rapidly phosphorylated by PKC. Furthermore, co-treatment of AhR-deficient cells that expressed AhRY9F and a DRE-driven luciferase construct with phorbol 12-myristate 13-acetate and TCDD resulted in a 30% increase in luciferase activity compared with AhRY9F treated with TCDD alone. Overall, AhR tyrosine 9, which is not a phosphorylated residue itself but is required for DNA binding, appears to play a crucial role in AhR activity by permitting proper phosphorylation of the AhR.

Proteasome Inhibition Induces Nuclear Translocation of the Dioxin Receptor Through an Sp1 and Protein Kinase C-Dependent Pathway

The dioxin receptor (AhR), in addition to its role in xenobiotic-induced carcinogenesis, appears to participate in cell proliferation, differentiation and organ homeostasis. Understanding potential mechanisms of activation of this receptor in the absence of exogenous ligands is therefore important to study its contribution to endogenous cellular functions. Using mouse embryo primary fibroblasts, we have previously shown that proteasome inhibition increased AhR transcriptional activity in the absence of xenobiotics. We suggested that proteasome inhibition-dependent AhR activation could involve an increase in the expression of the partner protein dioxin receptor nuclear translocator (ARNT). Since ARNT over-expression induced nuclear translocation of the AhR, and ARNT-deficient cells were unable to translocate this receptor to the nucleus upon proteasome inhibition, we have analyzed the effect of proteasome inhibition on the expression of regulatory proteins controlling ARNT levels. Treatment with the proteasome inhibitor MG132 increased endogenous Sp1 phosphorylation and its DNA-binding activity to the ARNT promoter. Sp1 phosphorylation and binding to the ARNT promoter, ARNT over-expression and AhR nuclear translocation were inhibited by GF109203X, a protein kinase C-specific inhibitor. In addition, MG132 stimulated protein kinase C activity in MEF cells with a pattern similar to that observed for ARNT expression. These data suggest that cellular control of protein kinase C activity, through Sp1 and ARNT, could regulate AhR transcriptional activity in the absence of xenobiotics.

A dynamic role for the Ah receptor in cell signaling? Insights from a diverse group of Ah receptor interacting proteins

The aryl hydrocarbon (Ah) receptor (AhR) is a member of the basic helix-loop-helix PER-ARNT-SIM (PAS) transcription factor family. Consistent with the notion that PAS proteins are biological sensors, AhR binding to Ah toxicants induces or represses transcription of a wide range of genes and results in a cascade of toxic responses. However, an endogenous role for AhR in development and homeostasis is supported by (1) the discovery of low affinity, endogenous ligands; (2) studies demonstrating a role for the receptor in development of liver and vascular systems, that were established using mice lacking AhR expression; and (3) the presence of functional dioxin-responsive elements in promoter regions of genes involved in cellular growth and differentiation. A large body of recent literature has implicated AhR in multiple signal transduction pathways. AhR is known to interact with signaling pathways that are mediated by estrogen receptor and other hormone receptors, hypoxia, nuclear factor kappaB, and retinoblastoma protein. In addition, AhR complexes may affect cellular signaling through interactions with various other regulatory and signaling proteins, including PAS heterodimerization partners (ARNT), chaperone and immunophilin-like proteins (e.g. HSP90, XAP2/ARA9/AIP, p23), protein kinases and phosphatases (e.g. tyrosine kinases, casein kinase 2, protein kinase C), and coactivators (e.g. SRC-1, RIP 140, CBP/p300). Here we summarize the types of molecular cross talk that have been identified between AhR and cell signaling pathways.

Identification of a putative calcium-binding protein as a dioxin-responsive gene in zebrafish and rainbow trout

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD, dioxin) is a widespread environmental contaminant that causes multiple effects in vertebrates. TCDD elicits its toxicity through aryl hydrocarbon receptor (AhR)-mediated modulation of gene regulation, increasing intracellular free calcium, and inducing calcium-mediated apoptosis in cell culture. Two TCDD-responsive cDNAs, which encode putative calcium-binding proteins, have been isolated from zebrafish and rainbow trout. The zebrafish and rainbow trout sequences are 88% similar to each other at the amino acid level and are orthologs of the human S100A4 calcium-binding protein. In zebrafish liver cell culture, treatment with TCDD increases S100A4a mRNA abundance. In juvenile rainbow trout, S100A4 mRNA was constitutively expressed in the heart, kidney, intestine, and spleen, but not in the liver. Exposure to TCDD significantly increased rainbow trout S100A4 mRNA abundance in the rainbow trout kidney. Taken together, these findings demonstrate in zebrafish and rainbow trout that dioxin increases expression of this EF-hand calcium-binding protein gene in a tissue-dependent fashion. However, demonstration that the encoded S100A4 proteins actually bind calcium and play a role in dioxin toxicity will require further study.

DNA binding and protein interactions of the AHR/ARNT heterodimer that facilitate gene activation

Gene activation by the aryl hydrocarbon receptor (AHR) and its DNA binding partner, the aryl hydrocarbon receptor nuclear translocator (ARNT) requires a number of sequential steps that occur following the binding of ligand and entry of the AHR into the nuclear compartment. This includes heterodimerization of the AHR and ARNT, formation of the appropriate amino acid/nucleotide contacts at the GCGTG recognition site and interactions between either the AHR or ARNT with proteins that facilitate changes in chromatin structure. The majority of these steps are likely modulated by changes in both phosphorylation and oxidation status of the AHR, ARNT and associated proteins. Studies of both the basic helix-loop-helix transcription factors and the nuclear hormone receptor family can provide significant insights into how this unique signaling pathway activates its target genes.

Role of the aryl hydrocarbon receptor in cell cycle regulation

Traditionally, the aryl hydrocarbon receptor (AHR) is considered to be a ligand-activated receptor and transcription factor responsible for the induction of drug-metabolizing enzymes. Its role in the combinatorial matrix of cell functions was neatly established long before the first report of an AHR cDNA sequence was published. Only recently, other functions of this protein have begun to be recognized. This review addresses novel findings relating to AHR functions that have resulted from experimental approaches markedly outside traditional receptor analyses. Here we examine the aspects of AHR biology relevant to its role in cell cycle regulation, from the activation of mitogen-activated protein kinases to the cross-talk between AHR and the RAS pathway and the functional significance of the interaction between AHR and the retinoblastoma protein. We have attempted to provide the reader with a balanced interpretation of the evidence, highlighting areas of consensus as well as areas still being contested.

Activation of mitogen-activated protein kinases (MAPKs) by aromatic hydrocarbons: role in the regulation of aryl hydrocarbon receptor (AHR) function

The aromatic hydrocarbon (Ah) receptor (AHR) is the only known cellular receptor of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and of many other widespread environmental contaminants that cause diverse toxic effects in animals and humans. Most, if not all, the biological effects of TCDD are mediated by the activation of AHR, which is a ligand-activated transcription factor required for ligand-induced expression of several detoxification genes, including those encoding for cytochrome P450 enzymes CYP1A1, CYP1A2, and CYP1B1. Environmental agents also activate several mitogen-activated protein kinase (MAPK) pathways, believed to modulate transcription factor function and to regulate gene expression. However, the contribution to TCDD toxicity resulting from cross-talk between AHR and MAPK pathways has yet to be determined. In this study, we show that TCDD and other AHR ligands induced the immediate activation of the extracellular signal-regulated kinases and the Jun N-terminal kinases, but not the p38 MAPKs. MAPK activation by TCDD did not require the AHR, since it occurred equally well in AHR-negative CV-1 cells and in Ahr (-/-) mouse embryonic fibroblasts as in AHR-positive cells. Distinct from serum factors and the tumor promoter TPA-induced MAPKs, which resulted in transcriptional activation of ELK or c-JUN, TCDD-stimulated MAPKs were critical for the induction of AHR-dependent gene transcription and CYP1A1 expression. These data indicate that AHR ligands elicit AHR-independent non-genomic events that are essential for AHR activation and function.

Regulatory interactions among three members of the vertebrate aryl hydrocarbon receptor family: AHR repressor, AHR1, and AHR2

The effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds occur via the aryl hydrocarbon receptor (AHR), a member of the basic helix-loop-helix-Per-ARNT-Sim homology (bHLH-PAS) protein superfamily. A single AHR gene has been identified in mammals, whereas many fish species, including the Atlantic killifish (Fundulus heteroclitus) possess two distinct AHR genes (AHR1 and a novel form, AHR2). A mouse bHLH-PAS protein closely related to AHR and designated AHR repressor (AHRR) is induced by 3-methylcholanthrene and represses the transcriptional activity of the AHR. To determine whether AHRR is the mammalian ortholog of fish AHR2 and to investigate the mechanisms by which AHRR regulates AHR function, we cloned an AHRR ortholog in F. heteroclitus with high sequence identity to the mouse and human AHRRs. Killifish AHRR encodes a 680-residue protein with a predicted molecular mass of 75.2 kDa. We show that in vitro expressed AHRR proteins from human, mouse, and killifish all fail to bind [(3)H]TCDD or [(3)H]beta-naphthoflavone. In transient transfection experiments using a luciferase reporter gene under control of AHR response elements, killifish AHRR inhibited the TCDD-dependent transactivation function of both AHR1 and AHR2. AHRR mRNA is widely expressed in killifish tissues and is inducible by TCDD or polychlorinated biphenyls, but its expression is not altered in a population of fish exhibiting genetic resistance to these compounds. The F. heteroclitus AHRR promoter contains three putative AHR response elements. Both AHR1 and AHR2 activated transcription of luciferase driven by the AHRR promoter, and AHRR could repress its own promoter. Thus, AHRR is an evolutionarily conserved, TCDD-inducible repressor of AHR1 and AHR2 function. Phylogenetic analysis shows that AHRR, AHR1, and AHR2 are distinct genes, members of an AHR gene family; these three vertebrate AHR-like genes descended from a single invertebrate AHR.

Tyrphostin [correction of Tryphostin] AG879, a tyrosine kinase inhibitor: prevention of transcriptional activation of the electrophile and the aromatic hydrocarbon response elements

To investigate a possible role of phosphorylation in the signal transduction pathways responsible for transcriptional regulation of drug-metabolizing enzymes, we tested seven specific tyrosine kinase inhibitors (tyrphostins) for their effects on NAD(P)H:quinone oxidoreductase-1 (NQO1) mRNA levels in mouse hepatoma Hepa-1c1c7 (Hepa-1) cells and chose to study AG879 further. The potent electrophile tert-butylhydroquinone (tBHQ) is known to activate NQO1 gene transcription via the electrophile response element (EPRE). Among the tyrphostins tested, tyrphostin AG879 was unique in preventing the accumulation of tBHQ-induced NQO1 mRNA; this effect was dependent on the AG879 dose and was also sensitive to the time when AG879 was added relative to the beginning of tBHQ treatment. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (dioxin; TCDD) is known to activate Cyp1a1 gene transcription by way of aromatic hydrocarbon response elements (AHREs). We found that AG879 also prevents, to a lesser extent, the AHRE-mediated induction of CYP1A1 and NQO1 mRNA by dioxin. Zinc or cadmium is known to activate metallothionein (Mt1) gene transcription via the metal response element (MRE). AG879 induced MT1 mRNA, and AG879 did not block zinc- or cadmium-induced MT1 mRNA, indicating that the effects of AG879 on NQO1 or CYP1A1 mRNA levels cannot be generalized to all transcripts. Using transient transfection of EPRE-, AHRE-, or MRE-driven luciferase reporter gene constructs in Hepa-1 cells, we showed that the inhibitory effects of AG879 occurred at the level of EPRE- and AHRE-mediated transcription, but that AG879 did not affect the MRE-driven transcriptional response. These data suggest that AG879 might inhibit an unknown tyrosine kinase(s) whose activity is essential for EPRE- and AHRE-mediated trans-activation of certain mammalian genes. These results also indicate that some sharing of common signal transduction pathways might exist in the regulation of genes involved in drug metabolism that also respond to oxidative stress.

The essential role of phosphatidylinositol 3-kinase and of p38 mitogen-activated protein kinase activation in the antioxidant response element-mediated rGSTA2 induction by decreased glutathione in H4IIE hepatoma cells

The protective adaptive response to electrophiles and reactive oxygen species is mediated by the enhanced expression of the phase II detoxifying genes through antioxidant response elements (AREs). The current study was designed to identify the signaling pathways responsible for the expression of rGSTA2 in response to cellular oxidative stress and to establish the molecular mechanistic basis. Deprivation of cystine and methionine caused oxidative stress in H4IIE hepatoma cells as evidenced by a marked decrease in the reduced glutathione (first order rate constant = 0.056 h(-1); t(1/2) = 12.6 h) and an increase in pro-oxidant production. Electrophoretic mobility shift assay revealed that the ARE complex, consisting of Nrf-1/2 and Maf proteins, was activated 12 to 48 h after sulfur amino acid deprivation (SAAD). The rGSTA2 mRNA level was elevated by SAAD beginning at 24 h, whereas the rGSTA2 subunit was maximally induced at 48 h. Nuclear ARE activation and rGSTA2 mRNA increase were both completely inhibited by wortmannin or LY294002, the phosphatidylinositol 3-kinase (PI3-kinase) inhibitors. The p38 mitogen-activated protein (MAP) kinase was activated at 0.5 to 3 h after SAAD, followed by sustained diminished activation up to 12 h. Inhibition of p38 MAP kinase by SB203580 prevented the ARE-mediated rGSTA2 induction. The activation of p38 MAP kinase, however, failed to be inhibited by wortmannin or LY294002, showing that PI3-kinase is not involved in the activation of p38 MAP kinase. Data showed that PI3-kinase plays an essential role in the ARE-mediated rGSTA2 induction by oxidative stress after SAAD, which activates the p38 MAP kinase and leads to rGSTA2 induction.

Regulation of DNA binding activity of the ligand-activated aryl hydrocarbon receptor by tyrosine phosphorylation

Aryl hydrocarbon receptor (AhR), a member of the bHLH-PAS family, is a ligand-activated transcription factor which plays an important role in normal liver development and in mediating the toxicity of polycyclic and halogenated aromatic hydrocarbon pollutants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin. Phosphorylation is known to regulate the transformation process of unliganded AhR into functionally active AhR/ARNT heterodimer that has high affinity for dioxin-responsive elements (DRE) and transactivation activity. Here, we report that DRE binding activity of the AhR is regulated by phosphorylation on the AhR/ARNT complex itself. Studies with specific protein phosphatases indicated that tyrosine phosphorylation is involved in this modulation. In addition, the AhR is phosphorylated at tyrosine residue(s) as determined by anti-phosphotyrosine immunoblot analysis. These results suggest that tyrosine phosphorylation on the AhR is required for its DNA binding activity and may provide mammalian cells with another layer of control mechanism that allows cell type specific and developmental stage specific induction of the AhR target genes.

The PAS superfamily: sensors of environmental and developmental signals

Over the past decade, PAS domains have been identified in dozens of signal transduction molecules and various forms have been found in animals, plants, and prokaryotes. In this review, we summarize this rapidly expanding research area by providing a detailed description of three signal transduction pathways that utilize PAS protein heterodimers to drive their transcriptional output. It is hoped that these model pathways can provide a framework for use in understanding the biology of the less well-understood members of this emerging superfamily, as well as of those to be characterized in the days to come. We use this review to develop the idea that most eukaryotic PAS proteins can be classified by functional similarities, as well as by predicted phylogenetic relationships. We focus on the alpha-class proteins, which often act as sensors of environmental signals, and the beta-class proteins, which typically act as broad-spectrum partners that target these heterodimers to their genomic targets.

Inhibition of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-stimulated Cyp1a1 promoter activity by hypoxic agents

Since hypoxia-inducible factor-1alpha (HIF-1alpha) and the arylhydrocarbon receptor (AhR) shared the AhR nuclear translocator (Arnt) for hypoxia- and AhR-mediated signaling, respectively, it was possible to establish the hypothesis that hypoxia could regulate cytochrome P450 1a1 (Cyp1a1) expression. In order to test this hypothesis, we undertook to examine the effect of hypoxia on Cyp1a1 transcription in Hepa-I cells. Mouse Cyp1a1 5'-flanking DNA, 1.6 kb was cloned into pGL3 expression vector in order to construct pmCyp1a1-Luc. Hepa-I cells were transfected with pmCyp1a1-Luc and treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the presence or absence of various hypoxic agents such as 1-100 microM cobalt chloride, 1-100 microM picolinic acid, and 1-100 microM desferrioxamine. Luciferase activity of the reporter gene was measured from pmCyp1a1-Luc-transfected Hepa-I cell lysate which contains 2 microgram total protein using luciferin as a substrate. Hypoxic agents such as cobalt chloride, picolinic acid, and desferrioxamine showed inhibition of luciferase activity that was induced by 1-nM TCDD treatment in a dose-and time-dependent manner. Concomitant treatment of 150 microM ferrous sulfate with 1-100 microM desferrioxamine or 1-100 microM picolinic acid recovered luciferase activity from that inhibited by hypoxic agents or induced by TCDD. These data demonstrated that iron-chelating and hypoxic agents inhibited dioxin-induced Cyp1a1 transcription in Hepa-I cells. Thus, we might suggest that hypoxia inhibits TCDD-induced Cyp1a1 expression due to the competition between HIF-1alpha and the AhR for the Arnt in Hepa-I cells.

Activation of transcription factors activator protein-1 and nuclear factor-kappaB by 2,3,7,8-tetrachlorodibenzo-p-dioxin

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD; dioxin), the prototype agonist of the aromatic hydrocarbon (Ah) receptor, is a potent tumor promoter as well as a complete liver carcinogen that produces an oxidative stress response in rodents and in cultured cell lines. It has been proposed that TCDD promotes neoplastic transformation through oxidative signal transduction pathways, which results in activation of immediate-early response transcription factors. To set the stage for a test of this hypothesis, we evaluated the effect of TCDD treatment on the activation of several transcription factors, including those in the nuclear factor-kappaB (NF-kappaB) and activator protein-1 (AP-1) families, which are activated by changes in the redox state of cells. In an extension of prior results, we found that TCDD treatment produced a sustained overexpression of AP-1 for at least 72 hr in wild-type mouse hepatoma Hepa-1 cells, but not in the Ah receptor-deficient derivative c35 or in cytochrome P450-1A1 (CYP1A1)-negative c37 cells. In addition, TCDD treatment caused a significant increase in the DNA binding activity of NF-kappaB, but not in the activities of the other transcription factors tested. AP-1 and NF-kappaB activation were blocked by the thiol antioxidant N-acetylcysteine and by nordihydroguaiaretic acid, an antioxidant and lipooxygenase inhibitor and an inhibitor of the epoxygenase activity of CYP1A1, and did not take place in c35, c37, or in Ah nuclear translator-deficient c4 cells. Hence, sustained activation of these two transcription factors by TCDD is likely to result from a CYP1A1-dependent and Ah receptor complex-dependent oxidative signal. Electrophoretic mobility supershift analyses with specific antibodies showed that most of the increase in NF-kappaB binding activity could be accounted for by increases in p50/p50 complexes. Since these complexes are known to repress NF-kappaB-dependent gene transcription, our results delineate a second molecular mechanism, in addition to the recently found block of tumor necrosis factor-alpha-mediated p50/p65 activation, that may be responsible for the immunosuppresive effects of TCDD.

Induction and regulation of xenobiotic-metabolizing cytochrome P450s in the human A549 lung adenocarcinoma cell line

Several cytochrome P450 (CYP) enzymes are expressed in the human lung, where they participate in metabolic inactivation and activation of numerous exogenous and endogenous compounds. In this study, the expression pattern of all known xenobiotic-metabolizing CYP genes was characterized in the human alveolar type II cell-derived A549 adenocarcinoma cell line using qualitative reverse transcriptase/polymerase chain reaction (RT-PCR). In addition, the mechanisms of induction by chemicals of members in the CYP1 and CYP3A subfamilies were assessed by quantitative RT-PCR. The expression of messenger RNAs (mRNAs) of CYPs 1A1, 1B1, 2B6, 2C, 2E1, 3A5, and 3A7 was detected in the A549 cells. The amounts of mRNAs of CYPs 1A2, 2A6, 2A7, 2A13, 2F1, 3A4, and 4B1 were below the limit of detection. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induced CYP1A1 and CYP1B1 mRNAs 56-fold and 2.5-fold, respectively. CYP3A5 was induced 8-fold by dexamethasone and 11-fold by phenobarbital. CYP3A4 was not induced by any of the typical CYP3A4 inducers used. The tyrosine kinase inhibitor genistein and the protein kinase C inhibitor staurosporine blocked TCDD-elicited induction of CYP1A1, but they did not affect CYP1B1 induction. Protein phosphatase inhibitors okadaic acid and calyculin A enhanced TCDD-induction of CYP1B1 slightly, but had negligible effects on CYP1A1 induction. These results suggest that CYP1A1 and CYP1B1 are differentially regulated in human pulmonary epithelial cells and give the first indication of the induction of CYP3A5 by glucocorticoids in human lung cells. These results establish that having retained several characteristics of human lung epithelial cell CYP expression, the A549 lung cell line is a valuable model for mechanistic studies on induction of the pulmonary CYP system.