Half-life of aryl hydrocarbon receptor in Hepa 1 cells: evidence for ligand-dependent alterations in cytosolic receptor levels

Aromatic hydrocarbon receptors in mitochondrial biogenesis and function

Mitochondria are ubiquitous membrane-bound organelles that not only play a key role in maintaining cellular energy homeostasis and metabolism but also in signaling and apoptosis. Aryl hydrocarbons receptors (AhRs) are ligand-activated transcription factors that recognize a wide variety of xenobiotics, including polyaromatic hydrocarbons and dioxins, and activate diverse detoxification pathways. These receptors are also activated by natural dietary compounds and endogenous metabolites. In addition, AhRs can modulate the expression of a diverse array of genes related to mitochondrial biogenesis and function. The aim of the present review is to analyze scientific data available on the AhR signaling pathway and its interaction with the intracellular signaling pathways involved in mitochondrial functions, especially those related to cell cycle progression and apoptosis. Various evidence have reported the crosstalk between the AhR signaling pathway and the nuclear factor κB (NF-κB), tyrosine kinase receptor signaling and mitogen-activated protein kinases (MAPKs). The AhR signaling pathway seems to promote cell cycle progression in the absence of exogenous ligands, whereas the presence of exogenous ligands induces cell cycle arrest. However, its effects on apoptosis are controversial since activation or overexpression of AhR has been observed to induce or inhibit apoptosis depending on the cell type. Regarding the mitochondria, although activation by endogenous ligands is related to mitochondrial dysfunction, the effects of endogenous ligands are not well understood but point towards antiapoptotic effects and inducers of mitochondrial biogenesis.

A polycyclic aromatic hydrocarbon-enriched environmental chemical mixture enhances AhR, antiapoptotic signaling and a proliferative phenotype in breast cancer cells

Emerging evidence suggests the role of environmental chemicals, in particular endocrine-disrupting chemicals (EDCs), in progression of breast cancer and treatment resistance, which can impact survival outcomes. However, most research tends to focus on tumor etiology and the effect of single chemicals, offering little insight into the effects of realistic complex mixture exposures on tumor progression. Herein, we investigated the effect of a polycyclic aromatic hydrocarbon (PAH)-enriched EDC mixture in a panel of normal and breast cancer cells and in a tumor organoid model. Cells or organoids in culture were treated with EDC mixture at doses estimated from US adult intake of the top four PAH compounds within the mixture from the National Health and Nutrition Examination Survey database. We demonstrate that low-dose PAH mixture (6, 30 and 300 nM) increased aryl hydrocarbon receptor (AhR) expression and CYP activity in estrogen receptor (ER) positive but not normal mammary or ER-negative breast cancer cells, and that upregulated AhR signaling corresponded with increased cell proliferation and expression of antiapoptotic and antioxidant proteins XIAP and SOD1. We employed a mathematical model to validate PAH-mediated increases in AhR and XIAP expression in the MCF-7 ER-positive cell line. Furthermore, the PAH mixture caused significant growth increases in ER-negative breast cancer cell derived 3D tumor organoids, providing further evidence for the role of a natural-derived PAH mixture in enhancing a tumor proliferative phenotype. Together, our integrated cell signaling, computational and phenotype analysis reveals the underlying mechanisms of EDC mixtures in breast cancer progression and survival.

Therapeutic targeting of the NRF2 and KEAP1 partnership in chronic diseases

The transcription factor NF-E2 p45-related factor 2 (NRF2; encoded by NFE2L2) and its principal negative regulator, the E3 ligase adaptor Kelch-like ECH-associated protein 1 (KEAP1), are critical in the maintenance of redox, metabolic and protein homeostasis, as well as the regulation of inflammation. Thus, NRF2 activation provides cytoprotection against numerous pathologies including chronic diseases of the lung and liver; autoimmune, neurodegenerative and metabolic disorders; and cancer initiation. One NRF2 activator has received clinical approval and several electrophilic modifiers of the cysteine-based sensor KEAP1 and inhibitors of its interaction with NRF2 are now in clinical development. However, challenges regarding target specificity, pharmacodynamic properties, efficacy and safety remain.

Aryl Hydrocarbon Receptor Ligands Indoxyl 3-sulfate and Indole-3-carbinol Inhibit FMS-like Tyrosine Kinase 3 Ligand-induced Bone Marrow-derived plasmacytoid Dendritic Cell Differentiation

Aryl hydrocarbon receptor (AhR) regulates both innate and adaptive immune responses by sensing a variety of small synthetic and natural chemicals, which act as its ligands. AhR, which is expressed in dendritic cells (DCs), regulates the differentiation of DCs. However, effects of AhR on the differentiation of DCs are variable due to the heterogeneity of DCs in cell surface marker expression, anatomical location, and functional responses. The plasmacytoid DCs (pDCs), one of DC subsets, not only induce innate as well as adaptive immune responses by secreting type I interferons and pro-inflammatory cytokines, but also induce IL-10 producing regulatory T cell or anergy or deletion of antigen-specific T cells. We showed here that AhR ligands indoxyl 3-sulfate (I3S) and indole-3-carbinol (I3C) inhibited the development of pDCs derived from bone marrow (BM) precursors induced by FMS-like tyrosine kinase 3 ligand (Flt3L). I3S and I3C downregulated the expression of signal transducer and activator of transcription 3 (STAT3) and E2-2 (Tcf4). In mice orally treated with I3S and I3C, oral tolerance to dinitrofluorobenzene was impaired and the proportion of CD11c⁺B220⁺ cells in mesenteric lymph nodes was reduced. These data demonstrate that AhR negatively regulates the development of pDCs from BM precursors induced by Flt3L, probably via repressing the expression of STAT3.

Effects of environmental carcinogen benzo(a)pyrene on canine adipose-derived mesenchymal stem cells

Dogs and their owners share the same environment and are subjected to similar environmental risk factors for developing breast cancer. Adipose tissue-derived mesenchymal stem cells (ADMSCs) may affect development and progression of breast cancer. In this study, we evaluated the effects of environmental carcinogen benzo(a)pyrene (BaP) on proliferation and differentiation of ADMSCs isolated from dogs. We characterized eight canine ADMSC lines and studied the effects of BaP on cell proliferation and differentiation. BaP did not inhibit cell proliferation of ADMSCs; however, BaP significantly inhibited differentiation potential of ADMSCs into adipocytes. BaP down-regulated AhR protein levels; however, increased its translocation from the cytoplasm to nucleus and suppressed PPARγ expression during adipogenesis. BaP increased the expression of AhR signaling pathway protein, cytochrome P450 (CYP1A1) in ADMSCs. Our data suggest that canine ADMSCs are susceptible to the environmental carcinogen BaP through AhR and PPARγ signaling pathways and may contribute to canine mammary carcinogenesis.

NcoA2-Dependent Inhibition of HIF-1α Activation Is Regulated via AhR

High endogenous levels of aryl hydrocarbon receptor (AhR) contribute to hypoxia signaling pathway inhibition following exposure to the potent AhR ligand benzo[a]pyrene (B[a]P) and could alter cellular homeostasis and disease condition. Increasing evidence indicates that AhR might compete with AhR nuclear translocator (ARNT) for complex formation with hypoxia-inducible factor-1α (HIF-1α) for transactivation, which could alter several physiological variables. Nuclear receptor coactivator 2 (NcoA2) is a transcription coactivator that regulates transcription factor activation and inhibition of basic helix-loop-helix Per (Period)-ARNT-SIM (single-minded) (bHLH-PAS) family proteins, such as HIF-1α, ARNT, and AhR, through protein-protein interactions. In this study, we demonstrated that both hypoxia and hypoxia-mimic conditions decreased NcoA2 protein expression in HEK293T cells. Hypoxia response element (HRE) and xenobiotic-responsive element (XRE) transactivation also were downregulated with NcoA2 knockdown under hypoxic conditions. In addition, B[a]P significantly decreased NcoA2 protein expression be accompanied with AhR degradation. We next evaluated whether the absence of AhR could affect NcoA2 protein function under hypoxia-mimetic conditions. NcoA2 and HIF-1α nuclear localization decreased in both B[a]P-pretreated and AhR-knockdown HepG2 cells under hypoxia-mimic conditions. Interestingly, NcoA2 overexpression downregulated HRE transactivation by competing with HIF-1α and AhR to form protein complexes with ARNT. Both NcoA2 knockdown and overexpression inhibited endothelial cell tube formation in vitro. We also demonstrated using the in vivo plug assay that NcoA2-regulated vascularization decreased in mice. Taken together, these results revealed a biphasic role of NcoA2 between AhR and hypoxic conditions, thus providing a novel mechanism underlying the cross talk between AhR and hypoxia that affects disease development and progression.

Pax3 and Pax7 play essential safeguard functions against environmental stress-induced birth defects

Exposure to environmental teratogenic pollutant leads to severe birth defects. However, the biological events underlying these developmental abnormalities remain undefined. Here, we report a molecular link between an environmental stress response pathway and key developmental genes during craniofacial development. Strikingly, mutant mice with impaired Pax3/7 function display severe craniofacial defects. We show that these are associated with an upregulation of the signaling pathway mediated by the Aryl hydrocarbon receptor (AHR), the receptor to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), revealing a genetic interaction between Pax3 and AHR signaling. Activation of AHR signaling in Pax3-deficient embryos drives facial mesenchymal cells out of the cell cycle through the upregulation of p21 expression. Accordingly, inhibiting AHR activity rescues the cycling status of these cells and the facial closure of Pax3/7 mutants. Together, our findings demonstrate that the regulation of AHR signaling by Pax3/7 is required to protect against TCDD/AHR-mediated teratogenesis during craniofacial development.

Induction of CYP1 family members under low-glucose conditions requires AhR expression and occurs through the nuclear translocation of AhR

Cross-talk between the aryl hydrocarbon receptor (AhR) pathway and the typical stress response is thought to be an important signal transduction in response to nutrient-stress conditions, such as glucose deprivation in liver cells. In the present study, we demonstrate that reduction of glucose concentration in the medium of HepG2 cells, a human hepatocellular carcinoma cell line, induces the CYP1 family and Nrf2. RNAi for AhR abolishes the induction of expression of CYP1 and Nrf2. These inductions are accompanied by the translocation of AhR into the nucleus in response to low-glucose conditions. Endogenous compounds are recruited as AhR ligands to induce various gene expressions, and our present results suggest that an endogenous AhR ligand is produced under low-glucose conditions and that the role of AhR as a transcription factor is related to the low-glucose response. The recommended glucose concentration (4.5 g/L) in the medium for culture of HepG2 was used as the high-glucose concentration in this study. We adopted 1.0 g/L as the low-glucose condition for elucidation of mechanisms of the stress response. These results will be useful to understand the relationship between drug-metabolizing enzymes and mechanisms of the anti-stress response of tumor cells, and will also be useful for investigating preventive remedies against tumor angiogenesis.

Antagonism of aryl hydrocarbon receptor signaling by 6,2',4'-trimethoxyflavone

The aryl hydrocarbon receptor (AHR) is regarded as an important homeostatic transcriptional regulator within physiological and pathophysiological processes, including xenobiotic metabolism, endocrine function, immunity, and cancer. Agonist activation of the AHR is considered deleterious based on toxicological evidence obtained with environmental pollutants, which mediate toxic effects through AHR. However, a multitude of plant-derived constituents, e.g., polyphenols that exhibit beneficial properties, have also been described as ligands for the AHR. It is conceivable that some of the positive aspects of such compounds can be attributed to suppression of AHR activity through antagonism. Therefore, we conducted a dioxin response element reporter-based screen to assess the AHR activity associated with a range of flavonoid compounds. Our screen identified two flavonoids (5-methoxyflavone and 7,4'-dimethoxyisoflavone) with previously unidentified AHR agonist potential. In addition, we have identified and characterized 6,2',4'-trimethoxyflavone (TMF) as an AHR ligand that possesses the characteristics of an antagonist having the capacity to compete with agonists, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo[a]pyrene, thus effectively inhibiting AHR-mediated transactivation of a heterologous reporter and endogenous targets, e.g., CYP1A1, independent of cell lineage or species. Furthermore, TMF displays superior action by virtue of having no partial agonist activity, in contrast to other documented antagonists, e.g., alpha-napthoflavone, which are partial weak agonists. TMF also exhibits no species or promoter dependence with regard to AHR antagonism. TMF therefore represents an improved tool allowing for more precise dissection of AHR function in the absence of any conflicting agonist activity.

Coal dust alters beta-naphthoflavone-induced aryl hydrocarbon receptor nuclear translocation in alveolar type II cells

BACKGROUND

Many polycyclic aromatic hydrocarbons (PAHs) can cause DNA adducts and initiate carcinogenesis. Mixed exposures to coal dust (CD) and PAHs are common in occupational settings. In the CD and PAH-exposed lung, CD increases apoptosis and causes alveolar type II (AT-II) cell hyperplasia but reduces CYP1A1 induction. Inflammation, but not apoptosis, appears etiologically associated with reduced CYP1A1 induction in this mixed exposure model. Many AT-II cells in the CD-exposed lungs have no detectable CYP1A1 induction after PAH exposure. Although AT-II cells are a small subfraction of lung cells, they are believed to be a potential progenitor cell for some lung cancers. Because CYP1A1 is induced via ligand-mediated nuclear translocation of the aryl hydrocarbon receptor (AhR), we investigated the effect of CD on PAH-induced nuclear translocation of AhR in AT-II cells isolated from in vivo-exposed rats. Rats received CD or vehicle (saline) by intratracheal (IT) instillation. Three days before sacrifice, half of the rats in each group started daily intraperitoneal injections of the PAH, beta-naphthoflavone (BNF).

RESULTS

Fourteen days after IT CD exposure and 1 day after the last intraperitoneal BNF injection, AhR immunofluorescence indicated that proportional AhR nuclear expression and the percentage of cells with nuclear AhR were significantly increased in rats receiving IT saline and BNF injections compared to vehicle controls. However, in CD-exposed rats, BNF did not significantly alter the nuclear localization or cytosolic expression of AhR compared to rats receiving CD and oil.

CONCLUSION

Our findings suggest that during particle and PAH mixed exposures, CD alters the BNF-induced nuclear translocation of AhR in AT-II cells. This provides an explanation for the modification of CYP1A1 induction in these cells. Thus, this study suggests that mechanisms for reduced PAH-induced CYP1A1 activity in the CD exposed lung include not only the effects of inflammation on the lung as a whole, but also reduced PAH-associated nuclear translocation of AhR in an expanded population of AT-II cells.

BRCA1 transcriptional activity is enhanced by interactions between its AD1 domain and AhR

PURPOSE

We previously reported that BRCA1 interacts with aryl hydrocarbon receptor nuclear translocator (ARNT) and that this interaction affects TCDD-induced CYP1A1 gene expression (Kang et al., J Biol Chem 281:14654-14662, 2006). In this study we continue this investigation and begin to define the significance of this interaction for the regulation of stress-induced transcription.

METHODS

Immunoprecipitations (IPs), western blot (WB) analysis, GST pull-down assays and promoter reporter assays were used to investigate whether the aryl hydrocarbon receptor (AhR) can regulate transcription that is dependent on the activation domain 1 (AD1) domain of BRCA1.

RESULTS

We show that AhR, a transcription factor, can bind specifically to AD1 in the C-terminal region of BRCA1 and affect BRCA1's ability to regulate transcription activity. We found that xenobiotics that positively and negatively affect AhR's activity as a transcription factor (e.g., dioxin and alpha-naphthoflavone, respectively), have similar effects on AhR's ability to affect AD1-domain-dependent transcription. These physical and functional AhR-AD1 interactions may require the coiled-coil motif in AD1 because point-mutations in this motif reduce these interactions.

CONCLUSION

Xenobiotic-activated AhR can function in two ways, as a component of the AhR/ARNT transcription factor and a regulator of AD1-dependent transcription. Consequently, BRCA1 has two distinct mechanisms for sensing xenobiotics and regulating AhR-dependent stress responses to these xenobiotics. We speculate that the normal functioning of this interaction could play a role in BRCA1's tumor suppressing ability.

BRCA1 modulates sensitivity to 5F-203 by regulating xenobiotic stress-inducible protein levels and EROD activity

PURPOSE

We have investigated the effects of BRCA1 over-expression and knockdown on 5F-203-induced gene expression and cytotoxicity in human breast cancer cells. 5F-203 is a chemotherapeutic prodrug that both induces a p450 enzyme, CYP1A1, and is metabolically activated by CYP1A1.

METHODS

We used several molecular biological techniques to confirm our findings. BRCA1 regulates sensitivity to 5F-203 by regulating the expression of CYP1A1 mRNA and its EROD activity. XRE-Luc reporter assays, semi-quantitative RT-PCR, Western blot analysis, EROD activity measurements, gene knockdown and MTT cell survival assays were used for this study.

RESULTS

Our results show that the ability of 5F-203 treatments to increase CYP1A1 mRNA level and CYP1A1 enzymatic activity (EROD activity) are affected by BRCA1 protein levels. In addition, the ability of 5F-203 treatments to induce proteins, P53 and P53 target genes such as P21, is significantly decreased in BRCA1 knockdown cells, suggesting that BRCA1-related effects could at least partially explain why BRCA1 knockdown increases resistance to 5F-203-mediated cytotoxicity. We also observed altered expression of the two major transcription factors (AhR and ARNT) that affect CYP1A1 expression when BRCA1 protein levels are altered.

CONCLUSION

BRCA1 is an important protein, which affects 5F-203-mediated cytotoxicity. Our findings are potentially clinically significant; they suggest that those patients most likely to respond to this new prodrug will have tumors containing normal amounts of BRCA1.

Redefining the role of the endogenous XAP2 and C-terminal hsp70-interacting protein on the endogenous Ah receptors expressed in mouse and rat cell lines

Studies using transient expression systems have implicated the XAP2 protein in the control of aryl hydrocarbon receptor (AHR) stability and subcellular location. Thus, studies were performed in cell lines that expressed endogenous rat or mouse Ah(b-1) (C57BL/6) or Ah(b-2) (C3H) AHRs with similar levels of endogenous XAP2. Unliganded rat and mouse Ah(b-2) receptor complexes associated with reduced levels of XAP2 and exhibited dynamic nucleocytoplasmic shuttling in comparison with Ah(b-1) receptors. Rat and mouse Ah(b-2) receptors also exhibited a greater magnitude of ligand-induced degradation than Ah(b-1) receptors. Small interfering RNA reduction of endogenous XAP2 by >80% had minimal impact on the level of Ah(b-2) receptors but resulted in a 25-30% reduction of Ah(b-1) receptors. XAP2 reduction resulted in increased susceptibility of the Ah(b-1) receptor to ligand-induced degradation yet produced higher levels of endogenous CYP1A1 induction. Stable expression of the Ah(b-2) receptor in the C57BL/6 background resulted in a protein with reduced association with XAP2, dynamic nucleocytoplasmic shuttling, and increased levels of ligand-induced degradation. Small interfering RNA reduction of endogenous XAP2 in a C-terminal hsp70-interacting protein knockout mouse cell line, exhibited a 25-30% reduction in the level of endogenous Ah(b-1) AHR and showed high levels of ligand-induced degradation. Thus, endogenous XAP2 exerts a negative function on a small fraction of the endogenous Ah(b-1) receptor complex but appears to have a minimal impact on endogenous rat or Ah(b-2) receptors. This implies that the analysis of the AHR-mediated signaling via rat and mouse Ah(b-2) receptors may better represent the physiology of this signal transduction pathway.

ERK kinase inhibition stabilizes the aryl hydrocarbon receptor: implications for transcriptional activation and protein degradation

The ultimate carcinogen and metabolite of benzo-[a]pyrene-7,8-dihydrodiol, benzo[a]pyrene-r-7,t-8-dihydrodiol-t-9,10-epoxide (+/-), stimulates apoptosis, and this process can be blocked by extracellular signal-regulated kinase (Erk) kinase inhibitors. However, we show here that Erk kinase inhibitors were unable to prevent B[a]P-7,8-dihydrodiol-induced apoptosis, leading us to speculate that Erk kinases are linked to regulation of the aryl hydrocarbon (Ah) receptor. Cotreatment of hepa1c1c7 cells with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and Erk kinase inhibitor PD98059, U0126, or SL327 led to enhanced nuclear accumulation of Ah receptor but with a reduced capacity to complement TCDD induction of Cyp1a1. This is explained in part by the ability of Erk kinase inhibitors to alter the steady-state levels of cellular Ah receptor, a result that leads to a dramatic induction in detectable receptor levels. These changes in cellular Ah receptor levels are associated with delayed degradation of the Ah receptor because TCDD-initiated degradation is reversed when cells are co-treated with TCDD and Erk kinase inhibitors. Erk kinase is linked to Ah receptor expression, as demonstrated by reductions in total Ah receptor levels after overexpression of constitutively active MEK1. In addition, Erk kinase activity modulates the transcriptional response because MEK1 overexpression enhances TCDD-initiated transactivation potential of the receptor. Thus, Erk kinase activity facilitates ligand-initiated transcriptional activation while targeting the Ah receptor for degradation. Immunoprecipitation experiments of the Ah receptor indicate that Erk kinase activity is associated with the receptor. It is interesting that the carboxyl region of the Ah receptor is associated with the transactivation region as well as the site for ubiquitination, indicating that Erk kinase-dependent phosphorylation targets the carboxyl region of the receptor.

The role of chaperone proteins in the aryl hydrocarbon receptor core complex

The aryl hydrocarbon receptor (AhR) exists in the absence of a ligand as a tetrameric complex composed of a 95-105 kDa ligand binding subunit, a dimer of hsp90, and the immunophilin-like X-associated protein 2 (XAP2). XAP2 has a highly conserved carboxy terminal tetratricopeptide repeat domain that is required for both hsp90 and AhR binding. Hsp 90 appears to be involved in the initial folding of newly synthesized AhR, stabilization of ligand binding conformation of the receptor, and inhibition of constitutive dimerization with ARNT. XAP2 is capable of stabilizing the AhR, as well as enhancing cytoplasmic localization of the receptor. XAP2 binds to both the AhR and hsp90 in the receptor complex, and is capable of independently binding to both hsp90 and the AhR. However, the exact functional role for XAP2 in the AhR complex remains to be fully established.

The mechanism of AH receptor protein down-regulation (degradation) and its impact on AH receptor-mediated gene regulation

The proteolytic degradation of transcription factors is an established mechanism of regulating signal transduction pathways. Recent reports have suggested that the aryl hydrocarbon receptor (AHR) protein is rapidly downregulated (degraded) following ligand binding. The downregulation of AHR has been observed in nine distinct cells culture lines derived from human and rodent tissues and has also been observed in rodent models following exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The downregulation of AHR appears to be ubiquitin mediated and occurs via the 26S proteasome pathway following nuclear export of AHR. The consequence of blocking AHR degradation in cell culture appears to be an increase in both the magnitude and duration of gene regulation by the AHR.ARNT complex. Thus, the physiological role of AHR degradation may be to modulate AHR-mediated gene regulation. This review provides analysis of the studies that have focused on the degradation of AHR in vivo and in vitro and the hypothesis that the downregulation of AHR is critical in the attenuation of AHR-mediated gene regulation.

The effect of the arylhydrocarbon receptor on the human steroidogenic acute regulatory gene promoter activity

The steroidogenic acute regulatory (StAR) protein is a rate-limiting factor in steroid hormone production. The StAR protein plays a role in the movement of cholesterol from the outer membrane to the inner membrane, where cholesterol side chain cleavage enzyme exists. Dioxins, which may act as 'endocrine disruptors', mimic and antagonize endogenous hormone actions in vivo. Although the mechanism of endocrine disruption is not clear, the actions of dioxins are known to be mediated by binding to the arylhydrocarbon receptor (AhR), and it is known that dioxins act as transcription factors to endocrine-associated gene expression. In the present study, we examined the effect of the AhR on the human StAR gene promoter, and we clarified the action mechanisms of environmental endocrine disruptors. We transfected constructs containing the human StAR gene promoter sequences pGL(2) 1.3-kb StAR (nt -1293 to +39) into mouse Y-1 adrenal tumor cells and measured the promoter activity of the StAR gene. With the addition of beta-napthoflavone (betaNF), which is a ligand of AhR, to the culture medium, the activity of the StAR gene promoter increased significantly (P<0.05), and with the addition of 1 microM of betaNF, it became maximum (3.1+/-0.6-fold higher than the control value). When the AhR and ARNT were co-transfected together in Y-1 cells or human adrenocortical carcinoma H295R cells, the promoter activity of the StAR gene significantly (P<0.05) increased, to a level 1.4+/-0.01-fold higher in Y-1 cells and to a level 1.6+/-0.04-fold higher in H295R cells than the control level, when 1 microM of betaNF was added. We examined the effect of induction of cAMP with transfection with AhR or ARNT. With the addition of 1 mM 8-Br-cAMP, there were no differences between the StAR gene promoter activities in the group in which AhR and ARNT was introduced and in the group in which they were not introduced. The results suggest that AhR plays a role in the promoter activity of the human StAR gene and that the effect of AhR on StAR gene expression may cause a disturbance to the human endocrine system.

Physiological modeling of a proposed mechanism of enzyme induction by TCDD

A physiological model was previously constructed to facilitate extrapolation of surrogates for the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in rat liver to doses comparable to human environmental exposures. The model included induction of P450 isozymes and suggested the presence of multiple binding sites with different affinities for the TCDD-liganded Ah receptor at CYP1A1 dioxin responsive elements. The model also indicated that protein synthesis on the mRNA template exhibited saturation kinetics with respect to message levels. In the present work the earlier model was revised to include the increased proteolysis of the Ah receptor on binding TCDD, more realistic representations of gene transcription and mRNA translation, and different stability for each mRNA. The revised model includes multiple TCDD-liganded Ah receptor binding sites for CYP1A1 and CYP1B1 genes, a lag of 0.2 day for production of mRNA and induced proteins, and stabilization of mRNA by a poly(A) tail. The model reproduced the transient depletion of the Ah receptor subsequent to binding ligand and the dose-response of the receptor in rats treated with biweekly oral doses of TCDD in corn oil. The model reproduced tissue TCDD concentrations observed for several dosing scenarios. Such robustness indicates the utility of the model in estimating internal dose. The model also reproduced the observed dose-response patterns for mRNA and protein for CYP1A1, CYP1A2, and CYP1B1 after repeated dosing. Neither of the two dissociation constants for the Ah receptor bound to the CYP1B1 gene is negligible, supporting the assumption of multiple response elements for this gene. The poorer induction of CYP1B1 was predicted to be due to lower affinity of the dioxin responsive elements for binding the liganded Ah receptor, suggesting the involvement of other regulatory factors, and a shorter poly(A) tail on CYP1B1 mRNA, leading to a shorter lifetime. Saturation in the kinetics of protein synthesis was linked to the limited number of ribosomes that could bind to each message molecule, resulting in fewer ribosomes bound per message at higher doses. Predicted induction at low doses was found to vary widely with the assumptions used in the construction of a model. More detailed descriptions of biological processes might provide more reliable predictions of enzyme induction.

Nuclear receptor CAR as a regulatory factor for the sexually dimorphic induction of CYB2B1 gene by phenobarbital in rat livers

The nuclear receptor constitutive active receptor (CAR) translocates into liver nuclei after phenobarbital (PB) treatment, and activates the conserved enhancer called the PB-response element module (PBREM) found in CYP2B genes. We have examined whether CAR regulates the dimorphic induction by PB of the CYP2B1 gene in Wistar Kyoto (WKY) rats. Northern blot analysis showed that PB induced CYP2B1 mRNA in male WKY rats but not female rats. An in situ injected PBREM-luciferase reporter gene was activated by PB only in the male livers. Western blot analysis revealed extremely low levels of CAR in the cytosols of female livers compared with male counterparts. CAR was accumulated in the liver nucleus of male rats in response to PB treatment, whereas the receptor was barely detectable in the liver nuclei of PB-induced females. These sexually dimorphic responses of PBREM and CAR to PB treatment were not observed with Fisher 344 rats, in which CYP2B1 mRNA was induced in both sexes. Thus, these results indicate that CAR is a regulatory factor that leads to the sexual dimorphic induction of CYP2B1 gene in WKY rats.

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.

Degradation of the basic helix-loop-helix/Per-ARNT-Sim homology domain dioxin receptor via the ubiquitin/proteasome pathway

The basic helix-loop-helix/Per-ARNT-Sim homology domain dioxin receptor (DR) translocates to the nucleus upon binding of aromatic hydrocarbon ligands typified by dioxin, whereupon it partners the Ah receptor nuclear translocator and initiates transcription. Concurrently, ligand binding down-regulates receptor levels via an unknown mechanism. In this study we show that receptor levels are dependent upon cellular compartmentalization, with entry into the nucleus leading to the rapid destruction of the DR. Ligand-induced DR translocation was bypassed by adding a heterologous nuclear localization signal to the DR, creating a constitutively nuclear form of the dioxin receptor (DRNLS). The DRNLS protein was shown to be unstable with a half-life of </=1 h whether partnering ARNT or HSP90. Thus, the structural changes induced by ligand binding have no inherent effect on DR stability but are critical in transporting the receptor prior to degradation. The proteolytic pathway that degrades the nuclear receptor is suggested to involve ubiquitination as it was inhibited by the proteasome inhibitor MG132 or co-expression of DRNLS with the ubiquitin mutant UbK48R. Incubation of cells expressing DRNLS with the phosphatase inhibitor calyculin resulted in the rapid phosphorylation and ubiquitination of DRNLS, suggesting that a nuclear kinase is required to trigger receptor proteolysis. Overall, this study demonstrates a novel mechanism of proteolysis whereby the simple relocation of a transcription factor from cytoplasm to nucleus initiates its rapid destruction.

Divergent mechanisms for loss of Ah-responsiveness in benzo[a]pyrene- and adriamycinR-resistant MCF-7 cells

The intracellular aryl hydrocarbon receptor (AhR) mediates signal transduction by environmental pollutants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and benzo[a]pyrene by functioning as a ligand-activated transcription factor. We have investigated AhR signaling in sublines of the human breast cancer cell line MCF-7 selected for resistance to AdriamycinR (AdrR) and benzo[a]pyrene (BP(R)). Previously we reported that AdrR cells have a loss of estrogen receptor (ER) expression and are Ah-nonresponsive. Here we show that AhR mRNA and protein are expressed at normal levels in AdrR cells, and the activated AhR complex is functionally capable of binding a xenobiotic responsive element. In MCF-7 cells AhR was depleted to 15% of normal levels after 4 hr TCDD treatment; however, 45% of AhR remained in AdrR cells during this time course. In BP(R) cells AhR mRNA levels were found to be decreased relative to wild-type cells, which led to decreased AhR protein levels and DNA-binding activity. Cellular ER content has been shown to correlate with Ah-responsiveness in human breast cancer cell lines. BP(R) cells were found to be ER-positive, although chronic (BP(R) cells) and acute (24 hr) exposure to benzo[a]pyrene led to significantly lower ER protein levels in MCF-7 cells. We conclude that loss of Ah-responsiveness occurs by different mechanisms in xenobiotic-resistant MCF-7 sublines: AhR mRNA is down-regulated in BP(R) cells, whereas AdrR cells are deficient in AhR signaling by a mechanism unrelated to AhR expression and activity.

Ah receptor and ARNT protein and mRNA concentrations in rat prostate: effects of stage of development and 2,3,7, 8-tetrachlorodibenzo-p-dioxin treatment

Effects of stage of development and 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) exposure on aryl hydrocarbon receptor (AhR) and AhR nuclear translocator (ARNT) protein concentrations in reproductive organs of male rats were determined. AhR protein levels in developing rat ventral and dorsolateral prostate decreased with age, declining approximately 70% between Postnatal Days (PND) 1 and 21. ARNT protein levels also decreased with age in dorsolateral, but not ventral prostate. The developmental decreases in prostatic AhR and ARNT protein were associated with decreases in AhR and ARNT mRNA. AhR and ARNT protein concentrations in fetal urogenital sinus on Gestation Days (GD) 16, 18, and 20 were similar to levels in ventral prostate on PND 7. TCDD exposure of adult male rats (0.2, 1, 5, or 25 micrograms/kg po, 24 h) decreased AhR but not ARNT protein in ventral and dorsolateral prostate, vas deferens, and epididymis. In utero and lactational TCDD exposure (1.0 micrograms/kg dam po, GD 15) did not alter ARNT levels but reduced prostatic AhR protein levels on PND 7 and delayed the developmental decrease in AhR protein in ventral and dorsolateral prostate. Finally, pretreatment of rat pups for 24 h with TCDD (5 micrograms/kg ip) down-regulated prostatic AhR protein on PND 7, but not on PND 1. Thus, prostatic AhR and ARNT protein and mRNA levels are regulated with age, whereas only AhR protein concentration is altered by TCDD exposure. Because in utero and lactational TCDD exposure only decreased prostatic AhR on PND 7, it is unlikely that down-regulation of AhR is the mechanism by which perinatal TCDD exposure impairs prostate development.

Aryl hydrocarbon receptor regulation of cytochrome P4501B1 in rat mammary fibroblasts: evidence for transcriptional repression by glucocorticoids

Cytochrome P450 1B1 (CYP1B1), which actively metabolizes polycyclic aromatic hydrocarbons, is regulated by the aryl hydrocarbon receptor (AhR) in primary cultures of rat mammary fibroblasts (RMF) and rat embryo fibroblasts (REF). 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) induced the 5.2-kilobase CYP1B1 mRNA in RMF (12-fold) and REF (14-fold) after a 6-hr treatment, with comparable increases in the microsomal protein. The synthetic glucocorticoid dexamethasone (DEX) suppresses TCDD-induced expression of CYP1B1 in RMF and REF. Suppression of CYP1B1 mRNA in RMF (maximal suppression, 70%) was observed when DEX was added 2 hr before TCDD, but was not observed with co-administration. The concentration dependence (EC50 approximately 10 nM) and reversal by the antagonist, RU486, implicates the glucocorticoid receptor. DEX inhibition of TCDD-induced CYP1B1 protein needed more extensive preincubation (>6 hr). TCDD induction of CYP1B1-luciferase constructs in RMF was mediated by a 265-base-pair upstream region (-810 to -1075), which was similarly suppressed (50-70%) by a 2-hr preincubation with 10(-7) M DEX via this enhancer region. Expression of the AhR is suppressed by DEX (70% after 12 hr), but not after the 2-hr period that was sufficient for suppression of transcription. The AhR nuclear translocator is not affected by this treatment. We conclude that glucocorticoid receptor rapidly suppresses activity of the AhR/AhR nuclear translocator complex in the CYP1B1 enhancer region, even though lacking glucocorticoid responsive element(s). DEX inhibits proliferation of RMF in this same concentration range (35%, EC50 approximately 5 nM), indicating additional effects on intracellular activity that may link to this suppression.

The aryl hydrocarbon receptor: a comparative perspective

The aryl hydrocarbon receptor (Ah receptor or AHR) is a ligand-activated transcription factor involved in the regulation of several genes, including those for xenobiotic-metabolizing enzymes such as cytochrome P450 1A and 1B forms. Ligands for the AHR include a variety of aromatic hydrocarbons, including the chlorinated dioxins and related halogenated aromatic hydrocarbons whose toxicity occurs through activation of the AHR. The AHR and its dimerization partner ARNT are members of the emerging bHLH-PAS family of transcriptional regulatory proteins. In this review, our current understanding of the AHR signal transduction pathway in non-mammalian and other non-traditional species is summarized, with an emphasis on similarities and differences in comparison to the AHR pathway in rodents and humans. Evidence and prospects for the presence of a functional AHR in early vertebrates and invertebrates are also examined. An overview of the bHLH-PAS family is presented in relation to the diversity of bHLH-PAS proteins and the functional and evolutionary relationships of the AHR and ARNT to the other members of this family. Finally, some of the most promising directions for future research on the comparative biochemistry and molecular biology of the AHR and ARNT are discussed.

Mechanisms of ligand-induced aryl hydrocarbon receptor-mediated biochemical and toxic responses

The ubiquitous environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) is a member of a broad group of halogenated aromatic hydrocarbons (HAHs) that is known to induce a wide range of toxic and biochemical responses in laboratory animals and humans. The effects of HAH exposure are mediated by binding to the cytosolic aryl hydrocarbon receptor (AhR), which is expressed in a tissue- and cell type-specific manner. The AhR is a ligand-activated transcription factor belonging to the basic helix-loop-helix/Per-AhR-Arnt-Sim (bHLH/PAS) superfamily of proteins. The mechanism of induction of gene transcription by TCDD involves ligand recognition and binding by the AhR, nuclear translocation, and dimerization with the AhR cofactor, AhR nuclear translocator (Arnt). The nuclear heterodimer interacts with cognate xenobiotic responsive elements (XREs) in promoter/enhancer regions of multiple Ah-responsive genes. Subsequent changes in chromatin structure and/or interaction of the AhR complex with the basal transcriptional machinery play a significant role in AhR-mediated gene expression. Although Arnt is a necessary component of a functional nuclear AhR complex, this protein also forms transcriptionally active heterodimers with other bHLH/PAS factors, including those involved in the transcriptional response to hypoxia. Arnt is ubiquitously expressed in mammalian systems, and results from transgenic mouse studies suggest that this protein plays a vital role in early mammalian embryonic development. Similar experiments suggest that the AhR may be involved in development of various organ systems. Thus, molecular mechanistic studies of TCDD action have contributed significantly to an improved understanding of the role of at least 2 bHLH/PAS proteins, as well as organ- and tissue-specific biochemical and toxic responses to this class of environmental toxins.

Responsiveness of the adult male rat reproductive tract to 2,3,7,8-tetrachlorodibenzo-p-dioxin exposure: Ah receptor and ARNT expression, CYP1A1 induction, and Ah receptor down-regulation

Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) either in adulthood or during late fetal and early postnatal development causes a variety of adverse effects on the male rat reproductive system. It was therefore of interest to identify male rat reproductive organs and cell types within these organs that might be direct targets of TCDD exposure. Because TCDD toxicity could possibly be the result of alterations in gene transcription mediated by the TCDD/aryl hydrocarbon receptor (AhR)/AhR nuclear translocator (ARNT) complex, the presence of the AhR and ARNT in the various organs of the adult male reproductive tract was examined using Western blotting. Both proteins were detectable in all organs examined (testis, epididymis, vas deferens, ventral prostate, dorsolateral [combined dorsal and lateral] prostate, and seminal vesicle). Although technical difficulties precluded the immunohistochemical evaluation of AhR distribution in these organs, ARNT was localized in all organs in a variety of cell types, including germ cells, epithelial cells, fibroblasts, smooth muscle cells, and endothelial cells. Subcellular localization varied across organs and across cell types within these organs. In order to determine whether TCDD exposure could alter gene expression in these organs, animals were dosed with TCDD (25 micrograms/kg po) or vehicle and euthanized at 24 h, and cytochrome P4501A1 (CYP1A1) expression was evaluated. By Western blotting, only the ventral and dorsolateral prostates exhibited significant induction of CYP1A1. Immunohistochemistry confirmed this induction and localized CYP1A1 expression to epithelial cells of the ventral and lateral lobes of the prostate. Immunohistochemistry also revealed CYP1A1 induction in select epithelial cells in the epididymis and seminal vesicle, as well as endothelial cells in the vas deferens and seminal vesicle. No induction was observed in the testis. Finally, AhR and ARNT expression in TCDD-exposed and control animals was evaluated by Western blotting. Results revealed no effect of TCDD exposure on ARNT protein expression, while AhR expression was decreased to 5-51% of control in all organs examined. In summary, both AhR and ARNT were expressed in all organs of the adult male rat reproductive tract examined, and epithelial and/or endothelial cells within each of these organs (with the exception of the testis) were responsive to TCDD exposure in terms of CYP1A1 induction. In addition, all tissues exhibited marked reductions in AhR protein content after TCDD exposure that did not correlate with the magnitude of the CYP1A1 response.

Prolonged depletion of AH receptor without alteration of receptor mRNA levels after treatment of cells in culture with 2,3,7,8-tetrachlorodibenzo-p-dioxin

Previous experiments have shown that the total cellular content of the AH receptor (AHR) drops rapidly after exposure of mouse hepatoma cells (Hepa-1) to the potent AHR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD); within 6 hr after treatment, less than 20% of the original cell content of AHR can be detected by radioligand binding or by immunoblotting. The goals of our current study were to determine the duration of receptor depletion following treatment with ligand and to determine if depletion is due to decreased expression of the Ahr gene that encodes the AHR. We found that depletion of AHR persisted for at least 72 hr after exposure to TCDD. Treatment with 3-methylcholanthrene caused a transient drop in total cell AHR, but the AHR levels returned to near pretreatment levels within 72 hr after the first exposure. TCDD treatment did not alter the levels of AHR mRNA as assessed by reverse transcription-polymerase chain reaction or slot blot assays. Thus, the decrease in AHR protein cannot be attributed to depression of transcription of the Ahr gene by TCDD. TCDD treatment did not alter the levels of the dimerization partner of the AHR, the AH receptor nuclear translocator protein (ARNT), or ARNT mRNA. In the presence of TCDD, both the AHR and the ARNT protein can be maintained at high levels in the nucleus if transcription is inhibited with actinomycin-D. In the absence of actinomycin-D, the AHR protein was lost rapidly, but the ARNT protein level in the cell was maintained. Together, these results suggest that the AHR protein is degraded through a selective mechanism that spares the ARNT protein and that the degradation pathway involves a protein that itself has a short half-life.

The Ah receptor is a sensitive target of geldanamycin-induced protein turnover

Geldanamycin (GA) binds directly to hsp90 and apparently disrupts certain hsp90 heterocomplexes. We have investigated the GA-hsp90 interaction and its effect on other associated proteins. Incubation of 2-[125I]-iodo-3-azido-7,8-dibromo-p-dioxin-labeled Hepa 1c1c7 cytosol with GA-coupled beads revealed a stable association of Ah receptor (AhR)/hsp90 complex with GA. In addition, sucrose gradient sedimentation analysis demonstrated that GA does not disrupt the 9S Ah receptor complex in vitro. HeLa and Hepa 1c1c7 cells were subjected to a dose-response and time-course treatment with GA and the level of the AhR was determined. A 75% depletion in AhR levels was observed within an hour of exposure to 100 nM GA. The relative stability of other proteins that associate with hsp90 was determined with the following rank order of sensitivity to GA exposure: AhR >> c-Raf-1 > glucocorticoid receptor > CDK4 >> p50. A series of hsp90 deletion mutants were used to map the domain that interacts with GA. Deletion of the first 221 amino acids in NH2-terminal domain resulted in loss of binding to solid-phase GA. Epitopes of monoclonal antibodies specific for hsp90 were also determined by direct immunoprecipitation with hsp90 mutants. Results indicated that monoclonal antibodies 8D3 and 3G3 interact with hsp90 via the first 221 amino acids in NH2-terminal region, whereas AC88 requires a COOH-terminal region between amino acids 661-677.

Characterization of the aromatic hydrocarbon receptor gene and its expression in Atlantic tomcod

Cytochrome P4501A1 (CYP1A1) mRNA is not inducible in Atlantic tomcod from the Hudson River that are treated with halogenated aromatic hydrocarbons (HAHs). In contrast, CYP1A1 mRNA is inducible in Hudson River tomcod that are treated with polycyclic aromatic hydrocarbons (PAHs) and in tomcod that are collected from cleaner rivers and treated with HAHs or PAHs. We hypothesize that CYP1A1 transcription is inhibited in Hudson River tomcod because of down-regulation of the aromatic hydrocarbon receptor (AhR) pathway and that separate molecular pathways modulate CYP1A1 transcription in fish treated with HAHs and PAHs. We initially evaluated levels of hepatic nuclear protein binding at enhancer elements (DREs) in the regulatory region of tomcod CYP1A1. No difference in levels of protein binding was observed between tomcod from the Hudson and Miramichi (cleaner) rivers that were untreated or were treated with benzo[a]pyrene. In contrast, levels of protein binding were lower in tomcod from the Hudson River that were treated with TCB than in similarly treated fish from the Miramichi River, suggesting differences between the populations in the structure or expression of AhR pathway molecules. To address this possibility, AhR DNA sequences were characterized from tomcod cDNA and genomic DNA libraries. In tomcod and mammals, AhR is represented by 11 exons, overall peptide sizes are similar, and amino acid sequences at basic, helix-loop-helix, PAAS A, and PAAS B domains are highly conserved. In contrast, little similarity was observed between tomcod and mammals in the sizes or sequences of AhR exons 10 and 11, including the absence in tomcod of glutamine-rich domains. No differences in levels of hepatic AhR mRNA were observed between the two populations or treatment groups when tomcod were untreated or were treated with aromatic hydrocarbons. In contrast, variation in levels of AhR mRNA expression was observed among tomcod tissues; however, no relationship was observed between levels of AhR mRNAs and CYP1A1 mRNAs in tissues from chemically or vehicle control-treated fish. RFLP analysis revealed extensive variation in exons 10 and 11 of AhR cDNA among tomcod from different rivers. Our results suggest that variation between tomcod populations in CYP1A1 mRNA inducibility is reflected by differences in levels of inducible hepatic protein binding to DREs. However, levels of hepatic AhR mRNA are not down-regulated in the Hudson River population, are not affected by AH treatments, and levels of AhR mRNA expression are not responsible for the differential inducibility of CYP1A1 transcription.

Activation of hypoxia-inducible transcription factor depends primarily upon redox-sensitive stabilization of its alpha subunit

Hypoxia-inducible factor 1 (HIF-1) is a heterodimeric transcription factor that is critical for hypoxic induction of a number of physiologically important genes. We present evidence that regulation of HIF-1 activity is primarily determined by the stability of the HIF-1alpha protein. Both HIF-1alpha and HIF-1beta mRNAs were constitutively expressed in HeLa and Hep3B cells with no significant induction by hypoxia. However, the HIF-1alpha protein was barely detectable in normoxic cells, even when HIF-1alpha was overexpressed, but was highly induced in hypoxic cells, whereas HIF-1beta protein levels remained constant, regardless of pO2. Hypoxia-induced HIF-1 binding as well as the HIF-1alpha protein were rapidly and drastically decreased in vivo following an abrupt increase to normal oxygen tension. Moreover, short pre-exposure of cells to hydrogen peroxide selectively prevented hypoxia-induced HIF-1 binding via blocking accumulation of HIF-1alpha protein, whereas treatment of hypoxic cell extracts with H2O2 had no effect on HIF-1 binding. These observations suggest that an intact redox-dependent signaling pathway is required for destablization of the HIF-1alpha protein. In hypoxic cell extracts, HIF-1 DNA binding was reversibly abolished by sulfhydryl oxidation. Furthermore, the addition of reduced thioredoxin to cell extracts enhanced HIF-1 DNA binding. Consistent with these results, overexpression of thioredoxin and Ref-1 significantly potentiated hypoxia-induced expression of a reporter construct containing the wild-type HIF-1 binding site. These experiments indicate that activation of HIF-1 involves redox-dependent stabilization of HIF-1alpha protein.

Expression of the aryl hydrocarbon receptor is regulated by serum and mitogenic growth factors in murine 3T3 fibroblasts

The aryl-hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates biological responses to planar aromatic hydrocarbons such as benzo[alpha]pyrene. However, no endogenous physiological ligand for the AhR has been identified. Since the AhR regulates bioactivity to common environmental pollutants, and since it is predicted to play an important physiological function, we have investigated the expression of the AhR during the cell cycle of murine 3T3 fibroblasts. We show here that stimulation of growth-arrested 3T3 cells with serum results in increased expression of AhR protein. Serum-induced expression of AhR in synchronized, serum-stimulated cells occurs at the onset of DNA synthesis (S phase) and is maximal at time points corresponding to late S phase. Transient transfections with an AhR-promoter-luciferase construct demonstrate that reporter gene transcription from the AhR promoter is regulated in a serum-dependent manner. Serum-dependent induction of AhR expression is prevented by an inhibitor of tyrosine kinase activity. Ligand-activated growth factor receptors (platelet-derived growth factor receptor basic fibroblast growth factor receptor) as well as an ectopically expressed tyrosine kinase (the v-Src oncoprotein) induce AhR expression in the absence of serum. Therefore, tyrosine kinase signaling is both necessary and sufficient for induction of AhR expression. Studies with the G1 blocker sodium butyrate show that the signal transduction pathways mediating serum-stimulated progression through the cell cycle are distinct from those that induce AhR expression. These data suggest that transcriptional regulation of the AhR is important in determining cellular sensitivity to the actions of AhR ligand(s) and that the AhR may play a role during the cellular proliferative response.

Review of the interaction between TCDD and glucocorticoids in embryonic palate

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental contaminant that produces adverse biological effects including developmental toxicity and teratogenesis. In the mouse embryo, TCDD induces cleft palate and hydronephrosis. The synthetic glucocorticoid, hydrocortisone (HC), induces cleft palate and a potent, synergistic interaction has been observed between TCDD and HC in C57BL/6N embryonic mice. The morphology and etiology of TCDD- and HC-induced clefts are distinctly different with formation of small palatal shelves following HC exposure and failure of normally-sized shelves to fuse after TCDD treatment. Each exposure also alters expression of several growth factors. When EGF, TGF alpha, EGF receptor, and the TGF beta's are considered as a combinatorial, interacting set of regulators, TCDD and HC each produce a unique pattern of increased and/or decreased expression across the set. The interaction of HC and TCDD results in a cleft palate whose etiology most closely resembles that observed after HC exposure, i.e. small palatal shelves. HC+TCDD-exposure also produces a pattern of growth factor expression which closely resembles that seen after HC. Both TCDD and HC act through receptor-mediated mechanisms and each compound has its own receptor. The Ah receptor (AhR) binds TCDD and the glucocorticoid receptor (GR) binds HC. On gestation day (GD) 14, in the embryonic palate exposed to TCDD, the AhR was downregulated and the GR expression increased. Conversely, following HC exposure, the GR was downregulated and AhR levels were elevated. HC+TCDD produced increased expression of both receptors and this pattern would be predicted to produce HC-like clefts as the GR-mediated responses would result in small palatal shelves. The observed cross-regulation of the receptors is believed to be important in the synergistic interaction between TCDD and HC for the induction of cleft palate.

Down-regulation of nuclear aryl hydrocarbon receptor DNA-binding and transactivation functions: requirement for a labile or inducible factor

Aryl hydrocarbons (AHs) such as 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo[a]pyrene activate the sequence-specific DNA-binding activity of the AH receptor. In the rat hepatocyte-derived cell line LCS7, DNA-binding activity peaked after 30 min and was then down-regulated, reaching negligible levels by 2 h. Down-regulation could be blocked, and DNA-binding activity maintained at maximum for many hours by inhibiting protein or RNA synthesis, implying that down-regulation is a mediated process requiring a labile or inducible protein. CYP1A1 transcription and in vivo DNA-protein interactions at xenobiotic response elements were down-regulated in parallel with DNA-binding activity in nuclear extracts, and these changes could also be blocked by inhibitors of protein synthesis. The correlation between AH receptor DNA-binding activity, intensity of in vivo footprints at xenobiotic response elements, and CYP1A1 transcription rate implies that down-regulation of AH receptor DNA-binding activity is important in regulating CYP1A1 transcription and that receptor is required continuously to maintain transcription. This correlation extends to the murine hepatoma cell line Hepa-1c1c7, in which slower kinetics of activation and down-regulation of CYP1A1 transcription paralleled slower activation and down-regulation of AH receptor DNA-binding activity. The difference in kinetics between cell lines also implies that AH receptor DNA-binding activity is modulated by a mechanism that may be influenced by cell-specific regulatory pathways. The above observations in conjunction with mixing experiments and comparisons of cytoplasmic and nuclear extracts indicate that down-regulation of AH receptor DNA-binding activity is probably due either to degradation or to conversion of the receptor to form that is inactive in both DNA binding and transactivation.

Down-regulation of nuclear aryl hydrocarbon receptor DNA-binding and transactivation functions: requirement for a labile or inducible factor

Aryl hydrocarbons (AHs) such as 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo[a]pyrene activate the sequence-specific DNA-binding activity of the AH receptor. In the rat hepatocyte-derived cell line LCS7, DNA-binding activity peaked after 30 min and was then down-regulated, reaching negligible levels by 2 h. Down-regulation could be blocked, and DNA-binding activity maintained at maximum for many hours by inhibiting protein or RNA synthesis, implying that down-regulation is a mediated process requiring a labile or inducible protein. CYP1A1 transcription and in vivo DNA-protein interactions at xenobiotic response elements were down-regulated in parallel with DNA-binding activity in nuclear extracts, and these changes could also be blocked by inhibitors of protein synthesis. The correlation between AH receptor DNA-binding activity, intensity of in vivo footprints at xenobiotic response elements, and CYP1A1 transcription rate implies that down-regulation of AH receptor DNA-binding activity is important in regulating CYP1A1 transcription and that receptor is required continuously to maintain transcription. This correlation extends to the murine hepatoma cell line Hepa-1c1c7, in which slower kinetics of activation and down-regulation of CYP1A1 transcription paralleled slower activation and down-regulation of AH receptor DNA-binding activity. The difference in kinetics between cell lines also implies that AH receptor DNA-binding activity is modulated by a mechanism that may be influenced by cell-specific regulatory pathways. The above observations in conjunction with mixing experiments and comparisons of cytoplasmic and nuclear extracts indicate that down-regulation of AH receptor DNA-binding activity is probably due either to degradation or to conversion of the receptor to form that is inactive in both DNA binding and transactivation.