Hepatitis B virus X-associated protein 2 is a subunit of the unliganded aryl hydrocarbon receptor core complex and exhibits transcriptional enhancer activity

Clinical, genetic and molecular characterization of patients with familial isolated pituitary adenomas (FIPA)

Familial pituitary adenomas can occur in MEN1 and Carney complex, as well as in the recently characterized familial isolated pituitary adenoma (FIPA) syndrome. FIPA is an autosomal dominant disease with incomplete penetrance, characterized by early-onset disease, often aggressive tumor growth and a predominance of somatotroph and lactotroph adenomas. In 20% of FIPA families, heterozygous mutations have been described in the aryl hydrocarbon receptor interacting (AIP) gene, whereas in other families the causative gene(s) are unknown. It has been suggested that AIP is a tumor suppressor gene and although experimental data support this hypothesis, the exact molecular mechanism by which its disruption leads to tumorigenesis is unclear. Here we discuss the clinical, genetic and molecular features of patients with FIPA.

MEN-4 and other multiple endocrine neoplasias due to cyclin-dependent kinase inhibitors (p27(Kip1) and p18(INK4C)) mutations

Cyclin-dependent kinase inhibitors (CDKIs) are known targets to become deregulated in various tumour types, including endocrine tumours. Typically, these cell cycle regulators are somatically inactivated in sporadic endocrine tumours. Recently, it became known that certain CDKI genes cause inherited susceptibility to endocrine neoplasia. Multiple endocrine neoplasia type 4 (MEN4) emerged as a novel form of multiple endocrine neoplasia, caused by mutations in the CDKI gene CDKN1B/p27(Kip1). The MEN4 phenotype remains unclear, but all MEN4 patients identified thus far present with parathyroid involvement, and less typically with pituitary adenomas and other endocrine features. Moreover, the CDKI gene CDKN2C/p18(INK4C) has been also implicated in endocrine neoplasia susceptibility. This review presents the recent advances in these novel MEN-related states and summarises the current knowledge of how these CDKIs may be implicated in endocrine neoplasia. In addition, it briefly presents data from Cdkn1b/p27(Kip1) and Cdkn2c/p18(INK4C) murine models, which strongly support the protective role of these inhibitors against endocrine tumourigenesis.

The inhibitory effect of aryl hydrocarbon receptor repressor (AhRR) on the growth of human breast cancer MCF-7 cells

The variant cell lines stably expressing aryl hydrocarbon receptor repressor (AhRR), MCFRR1 and MCFRR4, were established from human breast cancer MCF-7 cells by transfecting with AhRR-expression construct followed by selection, in order to analyze the effect of AhRR on the cell growth and expression of cell cycle-related genes. The variant cells showed higher levels of AhRR mRNA compared with the parental cells. MCFRR4 cells grew slowly compared with MCF-7 in both cell number and proliferation rate measured by the MTS method. Among cell cycle-related genes such as E2F, cyclin E1, cyclin D1, PCNA, p53, Rb, c-myc and p27Kip1, and estrogen responsive genes such as cathepsin D and hsp27, the expression levels of E2F, cyclin E1, PCNA and cathepsin D mRNA in MCFRR4 cells were lower than those in MCF-7 cells, while those of Rb, p27Kip1, c-myc and hsp27 mRNA were not significantly affected and that of cyclin D1 mRNA was enhanced in variant cells. Based on these results, AhRR might be suppressive on cell growth of MCF-7 by disturbing the transcriptional and/or posttranscriptional regulations of estrogen-responsive and cell cycle-related genes.

Kynurenic acid is a potent endogenous aryl hydrocarbon receptor ligand that synergistically induces interleukin-6 in the presence of inflammatory signaling

Inflammatory signaling plays a key role in tumor progression, and the pleiotropic cytokine interleukin-6 (IL-6) is an important mediator of protumorigenic properties. Activation of the aryl hydrocarbon receptor (AHR) with exogenous ligands coupled with inflammatory signals can lead to synergistic induction of IL6 expression in tumor cells. Whether there are endogenous AHR ligands that can mediate IL6 production remains to be established. The indoleamine-2,3-dioxygenase pathway is a tryptophan oxidation pathway that is involved in controlling immune tolerance, which also aids in tumor escape. We screened the metabolites of this pathway for their ability to activate the AHR; results revealed that kynurenic acid (KA) is an efficient agonist for the human AHR. Structure-activity studies further indicate that the carboxylic acid group is required for significant agonist activity. KA is capable of inducing CYP1A1 messenger RNA levels in HepG2 cells and inducing CYP1A-mediated metabolism in primary human hepatocytes. In a human dioxin response element-driven stable reporter cell line, the EC(25) was observed to be 104nM, while in a mouse stable reporter cell line, the EC(25) was 10muM. AHR ligand competition binding assays revealed that KA is a ligand for the AHR. Treatment of MCF-7 cells with interleukin-1beta and a physiologically relevant concentration of KA (e.g., 100nM) leads to induction of IL6 expression that is largely dependent on AHR expression. Our findings have established that KA is a potent AHR endogenous ligand that can induce IL6 production and xenobiotic metabolism in cells at physiologically relevant concentrations.

Molecular genetics of the aip gene in familial pituitary tumorigenesis

Pituitary adenomas usually occur as sporadic tumors, but familial cases are now increasingly identified. As opposed to multiple endocrine neoplasia type 1 and Carney complex, in familial isolated pituitary adenoma (FIPA) syndrome no other disease is associated with the familial occurrence of pituitary adenomas. It is an autosomal dominant disease with incomplete variable penetrance. Approximately 20% of patients with FIPA harbour germline mutations in the aryl hydrocarbon receptor-interacting protein (AIP) gene located on 11q13. Patients with AIP mutations have an overwhelming predominance of somatotroph and lactotroph adenomas, which often present in childhood or young adulthood. AIP, originally identified as a molecular co-chaperone of several nuclear receptors, is thought to act as a tumor suppressor gene; overexpression of wild-type, but not mutant AIP, reduces cell proliferation while knockdown of AIP stimulates it. AIP is shown to bind various proteins, including the aryl hydrocarbon receptor, Hsp90, phosphodiesterases, survivin, RET and the glucocorticoid receptor, but currently it is not clear which interaction has the leading role in pituitary tumorigenesis. This chapter summarizes the available clinical and molecular data regarding the role of AIP in the pituitary gland.

The pleiotropy of dioxin toxicity--xenobiotic misappropriation of the aryl hydrocarbon receptor's alternative physiological roles

The aryl hydrocarbon receptor is a signal regulated transcription factor that has best been characterised as regulating the xenobiotic response to a variety of planar aromatic hydrocarbons. There is compelling evidence that it mediates most, if not all, of the toxic effects of dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin). Dioxin exposure results in a wide variety of toxic outcomes including severe wasting syndrome, chloracne, thymic involution, severe immune suppression, reduced fertility, hepatotoxicity, teratogenicity, tumour promotion and death. The pleiotropy of toxic outcomes implies the disruption of a wide range of normal physiological functions. The aryl hydrocarbon receptor has developmentally restricted expression as well as developmental defects in gene-targeted mice. It has a wide range of target genes that do not fit into the classical xenobiotic metabolising gene battery and has recently been shown to interact with NF-kappa B and the estrogen receptor. There is also evidence for its activation in the absence of exogenous ligand, all of which point to various roles outside xenobiotic metabolism. Ligands so far identified display differential activation potential with respect to receptor activity. This article addresses activities of the aryl hydrocarbon receptor that are outside the xenobiotic response. Known physiological roles are discussed as well as how their disruption contributes to the pleiotropic toxicity of TCDD.

Potential roles of Arnt2 in zebrafish larval development

The aryl hydrocarbon receptor nuclear translocator (ARNT) is a basic helix-loop-helix-PAS heterodimeric transcription factor that dimerizes with other basic helix-loop-helix-PAS proteins to mediate biological responses. The function of ARNT2 is poorly understood. Here we provide an initial characterization of the zebrafish arnt2 null (arnt2(-/-)) mutant to identify functions of Arnt2 during development. Arnt2(-/-) mutant zebrafish develop normally until 120 hours postfertilization (hpf ) when morphological changes and functional deficits occur. The C-start escape response initiated by either touch or startle stimuli is absent in the mutants. Brain ventricle size is markedly increased at 120 hpf. Heart ventricles are enlarged, with decreased ventricle wall thickness. A cardiac arrhythmia, characterized by missing beats, is also observed in the mutants. This is associated with bradycardia in arnt2(-/-) larvae. Dilated liver sinusoids merge abnormally to form an extensive, labyrinth-like network of vascular channels. External appearance of arnt2(-/-) larvae at 120 hpf is indistinguishable from wild type except that the swim bladder is not inflated. The arnt2(-/-) mutants are not debilitated when phenotypic effects are first detected at 120 hpf that culminate in mortality, 4 days later around 216 hpf. Gross morphological assessment of the development of forebrain, midbrain, and hindbrain regions, neuromasts and Mauthner neurons, inner ear semicircular canals and otoliths, primary motor neurons, trigeminal ganglia, and trunk skeletal muscles, before or when the arnt2(-/-) phenotype was observed, failed to demonstrate a difference from wild type. The only effect in arnt2(-/-) larvae that occurred before 120 hpf was a decrease in expression of sim1, an Arnt2 dimerization partner, in the hypothalamus and ventral thalamus at 72 hpf. Further research is needed to determine if the primary functions of Arnt2 occur during the larval stage, when the phenotype is observed, or earlier in development.

Benzo[a]Pyrene, 3-Methylcholanthrene and ß-Naphthoflavone Induce Oxidative Stress in Hepatoma Hepa 1c1c7 Cells by an AHR-dependent Pathway

Polycyclic aromatic hydrocarbons have been shown to cause oxidative stress in vitro and in vivo in various animal models but the mechanisms by which these compounds produce oxidative stress are unknown. In the current study we have investigated the role of the aryl hydrocarbon receptor (AHR) in the production of reactive oxygen species (ROS) by its cognate ligands and the consequent effect on cyp1a1 activity, mRNA and protein expressions. For this purpose, Hepa 1c1c7 cells wild-type (WT) and C12 mutant cells, which are AHR-deficient, were incubated with increasing concentrations of the AHR-ligands, benzo[a]pyrene (B[a]P, 0.25-25 microM), 3-methylcholanthrene (3MC, 0.1-10 microM) and beta-naphthoflavone (betaNF, 1-50 microM). The studied AHR-ligands dose-dependently increased lipid peroxidation in WT but not in C12 cells. However, only B[a]P and betaNF, at the highest concentrations tested, significantly increased H2O2 production in WT but not C12 cells. The increase in lipid peroxidation and H2O2 production by AHR-ligands were accompanied by a decrease in the cyp1a1 catalytic activity but not mRNA or protein expressions, which were significantly induced in a dose-dependent manner by all AHR-ligands, suggesting a post-translational mechanism is involved in the decrease of cyp1a1 activity. The AHR-ligand-mediated decrease in cyp1a1 activity was reversed by the antioxidant N-acetylcysteine. Our results show that the AHR-ligands induce oxidative stress by an AHR-dependent pathway.

Ligand selectivity and gene regulation by the human aryl hydrocarbon receptor in transgenic mice

The aryl hydrocarbon receptor (AHR) is a ligand-inducible transcription factor that displays interspecies differences with the human and mouse AHR C-terminal region sequences sharing only 58% amino acid sequence identity. Compared with the mouse AHR (mAHR), the human AHR (hAHR) displays approximately 10-fold lower relative affinity for prototypical AHR ligands such as 2,3,7,8-tetrachlorodibenzo-p-dioxin, which has been attributed to the amino acid residue valine 381 (alanine 375 in the mAHR) in the ligand binding domain of the hAHR. We investigated whether the 10-fold difference in ligand-binding affinity between the mAHR and hAHR would be observed with a diverse range of AHR ligands. To test this hypothesis, ligand binding assays were performed using the photo-affinity ligand 2-azido-3-[(125)I]iodo-7,8-dibromodibenzo-p-dioxin and liver cytosol isolated from hepatocyte-specific transgenic hAHR mice and C57BL/6J mice. It is noteworthy that competitive ligand-binding assays revealed that, compared with the mAHR, the hAHR has a higher relative affinity for certain compounds, including indirubin [(2Z)-2,3-biindole-2,3 (1'H,1'H)-dione and quercetin (2-(3,4dihydroxyphenyl)-3,5,7-trihydroxy-4H-chromen-4-one]. Electrophoretic mobility shift assays revealed that indirubin was more efficient at transforming the hAHR compared with the mAHR. Indirubin was also a more potent inducer of Cyp1a1 expression in transgenic hAHR mouse hepatocytes compared with C57BL/6J mouse hepatocytes. These observations suggest that indirubin is a potent hAHR ligand that is able to selectively bind to and activate the hAHR. These discoveries imply that there may be a significant degree of structural divergence between mAHR and hAHR ligands and highlights the importance of the hAHR transgenic mouse as a model to study the hAHR in vivo.

The Aryl-hydrocarbon receptor does not require the p23 co-chaperone for ligand binding and target gene expression in vivo

The Aryl-hydrocarbon receptor (Ahr) is a ligand-activated transcription factor that mediates most of the toxic affects of 2,3,7,8-tetrachlorodibenzo-(p)-dioxin (TCDD) and other xenobiotic compounds. The AHR cytoplasmic complex consists of two molecules of HSP90 and at least one molecule of Hepatitis B Virus-X associated protein 2 and the co-chaperone p23. With the use of in vitro model systems, p23 has been shown previously to be important to maintaining the efficient ligand binding and subsequent downstream inducibility of the AHR. In this study we attempted to identify the role p23 plays in AHR signaling in vivo using a p23 null mouse. Ligand binding assays and western blot analysis revealed that p23 was not required for AHR protein stability and competent ligand binding in liver. Real-time RT-PCR analysis conducted on p23 null, heterozygous and homozygous mice suggested that p23 is dispensable for stable AHR protein levels, or efficient TCDD-mediated AHR activation of Cyp1a1 and Cyp1a2.

G-protein signalling negatively regulates the stability of aryl hydrocarbon receptor

Aryl hydrocarbon receptor (AhR) is a transcription factor that works as a dioxin receptor and is also involved in various physiological phenomena, including development and cell proliferation. Here, we show that the Galpha13 signal destabilizes AhR by promoting the ubiquitination of AhR. Galpha13 interacts directly with AhR-interacting protein (AIP) and inhibits the interaction between AhR and AIP, a crucial interacting protein of AhR. Strikingly, a reporter gene assay and a quantitative reverse transcription-PCR analysis indicate that the Galpha13 signal shows a potent inhibitory effect on the ligand-induced transcriptional activation of AhR. Galpha13 results in the nuclear translocation of AhR in a ligand-independent manner. However, in the presence of active Galpha13, AhR fails to form the active transcriptional complex. Taken together, we propose a new negative regulation of dioxin signalling by the G protein.

XAP2 inhibits glucocorticoid receptor activity in mammalian cells

XAP2 is member of a protein family sharing the TPR protein interaction motif. It displays close homology to the immunophilins FKBP51 and FKBP52 that act via the Hsp90 folding machinery to regulate the glucocorticoid receptor (GR). We show that XAP2 inhibits GR by reducing its responsiveness to hormone in transcriptional activation. The effect of XAP2 on GR requires its interaction with Hsp90 through the TPR motif. The PPIase-like region turned out to be enzymatically inactive. Thus, PPIase activity is not essential for the action of XAP2 on GR, similarly to FKBP51 and FKBP52.

HDAC6 modulates Hsp90 chaperone activity and regulates activation of aryl hydrocarbon receptor signaling

The aryl hydrocarbon receptor (AhR), a ligand-activated member of the basic helix-loop-helix family of transcription factors, binds with high affinity to polycyclic aromatic hydrocarbons (PAH) and the environmental toxin 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin). Most of the biochemical, biological, and toxicological responses caused by exposure to PAHs and polychlorinated dioxins are mediated, at least in part, by the AhR. The AhR is a client protein of Hsp90, a molecular chaperone that can be reversibly acetylated with functional consequences. The main objective of this study was to determine whether modulating Hsp90 acetylation would affect ligand-mediated activation of AhR signaling. Trichostatin A and suberoylanilide hydroxamic acid, two broad spectrum HDAC inhibitors, blocked PAH and dioxin-mediated induction of CYP1A1 and CYP1B1 in cell lines derived from the human aerodigestive tract. Silencing HDAC6 or treatment with tubacin, a pharmacological inhibitor of HDAC6, also suppressed the induction of CYP1A1 and CYP1B1. Inhibiting HDAC6 led to hyperacetylation of Hsp90 and loss of complex formation with AhR, cochaperone p23, and XAP-2. Inactivation or silencing of HDAC6 also led to reduced binding of ligand to the AhR and decreased translocation of the AhR from cytosol to nucleus in response to ligand. Ligand-induced recruitment of the AhR to the CYP1A1 and CYP1B1 promoters was inhibited when HDAC6 was inactivated. Mutation analysis of Hsp90 Lys(294) shows that its acetylation status is a strong determinant of interactions with AhR and p23 in addition to ligand-mediated activation of AhR signaling. Collectively, these results show that HDAC6 activity regulates the acetylation of Hsp90, the ability of Hsp90 to chaperone the AhR, and the expression of AhR-dependent genes. Given the established link between activation of AhR signaling and xenobiotic metabolism, inhibitors of HDAC6 may alter drug or carcinogen metabolism.

12(R)-Hydroxy-5(Z),8(Z),10(E),14(Z)-eicosatetraenoic acid [12(R)-HETE], an arachidonic acid derivative, is an activator of the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AHR) is a ligand-regulated transcription factor that can be activated by structurally diverse chemicals, ranging from environmental carcinogens to dietary metabolites. Evidence supporting a necessary role for the AHR in normal biology has been established; however, identification of key endogenous ligand/activator remains to be established. Here, we report the ability of 12(R)-hydroxy-5(Z),8(Z),10(E), 14(Z)-eicosatetraenoic acid [12(R)-HETE], an arachidonic acid metabolite produced by either a lipoxygenase or cytochrome P-450 pathway, to act as a potent indirect modulator of the AHR pathway. In contrast, structurally similar HETE isomers failed to demonstrate significant activation of the AHR. Electrophoretic mobility shift assays, together with ligand competition binding experiments, have demonstrated the inability of 12(R)-HETE to directly bind or directly activate the AHR to a DNA binding species in vitro. However, cell-based xenobiotic-responsive element-driven luciferase reporter assays indicate the ability of 12(R)-HETE to modulate AHR activity, and quantitation of induction of an AHR target gene confirmed 12(R)-HETE's ability to activate AHR-mediated transcription, even at high nanomolar concentrations in human hepatoma (HepG2)- and keratinocyte (HaCaT)-derived cell lines. One explanation for these results is that a metabolite of 12(R)-HETE is acting as a direct ligand for the AHR. However, several known metabolites failed to exhibit AHR activity. The ability of 12(R)-HETE to activate AHR target genes required receptor expression. These results indicate that 12(R)-HETE can serve as a potent activator of AHR activity and suggest that in normal and inflammatory disease conditions in skin, 12(R)-HETE is produced, perhaps leading to AHR activation.

Timing is everything: consequences of transient and sustained AhR activity

The aryl hydrocarbon receptor (AhR) was implicated as a mediator of xenobiotic toxicity over three decades ago. Although a complete picture continues to elude us, investigations by many laboratories during the ensuing period have revealed much about AhR biology in normal physiological processes, as well as the toxicities induced by the dioxins and related polychlorinated aromatic hydrocarbons. The findings are captured in numerous excellent reviews. This commentary attempts to inject a new perspective on some new as well as frequently overlooked observations in the context of established receptor properties. Specifically, we examine the impact of transient versus sustained receptor activation on AhR biology, and explore the potential role for cytochrome P450 expression in regulating AhR activity amongst various tissues. The growing recognition that AhR action functions through multiple mechanisms serves to further highlight the importance of limiting prolonged receptor activation.

The aryl hydrocarbon receptor is a modulator of anti-viral immunity

Although immune modulation by AhR ligands has been studied for many years, the impact of AhR activation on host defenses against viral infection has not, until recently, garnered much attention. The development of novel reagents and model systems, new information regarding anti-viral immunity, and a growing appreciation for the global health threat posed by viruses have invigorated interest in understanding how environmental signals affect susceptibility to and pathological consequences of viral infection. Using influenza A virus as a model of respiratory viral infection, recent studies show that AhR activation cues signaling events in both leukocytes and non-immune cells. Functional alterations include suppressed lymphocyte responses and increased inflammation in the infected lung. AhR-mediated events within and extrinsic to hematopoietic cells has been investigated using bone marrow chimeras, which show that AhR alters different elements of the immune response by affecting different tissue targets. In particular, suppressed CD8(+) T cell responses are due to deregulated events within leukocytes themselves, whereas increased neutrophil recruitment to and IFN-gamma levels in the lung result from AhR-regulated events extrinsic to bone marrow-derived cells. This latter discovery suggests that epithelial and endothelial cells are overlooked targets of AhR-mediated changes in immune function. Further support that AhR influences host cell responses to viral infection are provided by several studies demonstrating that AhR interacts directly with viral proteins and affects viral latency. While AhR clearly modulates host responses to viral infection, we still have much to understand about the complex interactions between immune cells, viruses, and the host environment.

Heat shock protein 90 inhibitors suppress aryl hydrocarbon receptor-mediated activation of CYP1A1 and CYP1B1 transcription and DNA adduct formation

The aryl hydrocarbon receptor (AhR), a client protein of heat shock protein 90 (HSP90), plays a significant role in polycyclic aromatic hydrocarbon (PAH)-induced carcinogenesis. Tobacco smoke, a source of PAHs, activates the AhR, leading to enhanced transcription of CYP1A1 and CYP1B1, which encode proteins that convert PAHs to genotoxic metabolites. The main objectives of this study were to determine whether HSP90 inhibitors suppress PAH-mediated induction of CYP1A1 and CYP1B1 or block benzo(a)pyrene [B(a)P]-induced formation of DNA adducts. Treatment of cell lines derived from oral leukoplakia (MSK-Leuk1) or esophageal squamous cell carcinoma (KYSE450) with a saline extract of tobacco smoke, B(a)P, or dioxin induced CYP1A1 and CYP1B1 transcription, resulting in enhanced levels of message and protein. Inhibitors of HSP90 [17-allylamino-17-demethoxygeldanamycin (17-AAG); celastrol] suppressed these inductive effects of PAHs. Treatment with 17-AAG and celastrol also caused a rapid and marked decrease in amounts of AhR protein without modulating levels of HSP90. The formation of B(a)P-induced DNA adducts in MSK-Leuk1 cells was inhibited by 17-AAG, celastrol, and alpha-naphthoflavone, a known AhR antagonist. The reduction in B(a)P-induced DNA adducts was due, at least in part, to reduced metabolic activation of B(a)P. Collectively, these results suggest that 17-AAG and celastrol, inhibitors of HSP90, suppress the activation of AhR-dependent gene expression, leading, in turn, to reduced formation of B(a)P-induced DNA adducts. Inhibitors of HSP90 may have a role in chemoprevention in addition to cancer therapy.

AhR protein trafficking and function in the skin

Because aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor, its nuclear translocation in response to ligands may be directly linked to transcriptional activation of target genes. We have investigated the biological significance of AhR from the perspective of its subcellular localization and revealed that AhR possesses a functional nuclear localization signal (NLS) as well as a nuclear export signal (NES) which controls the distribution of AhR between the cytoplasm and nucleus. The intracellular localization of AhR is regulated by phosphorylation of amino acid residues in the vicinity of the NLS and NES. In cell culture systems, cell density affects not only its intracellular distribution of AhR, but also its transactivation activity of the target genes such as transcriptional repressor Slug, which is important for the induction of epithelial-mesenchymal transitions. These effects of AhR observed in cultured cells are proposed to be reflected on the in vivo response such as morphogenesis and tumor formation. This review summarizes recent work on the control mechanism of AhR localization and progress in understanding the physiological role of AhR in the skin. We propose that AhR is involved in normal skin formation during fetal development as well as in pathological states such as epidermal wound healing and skin carcinogenesis.

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

Cytochrome P450 (CYP) 1A1 and CYP1B1 are inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin) in the human breast cancer cell line, MCF-7. Since CYP1A1 was inducible to a much greater degree than CYP1B1, we hypothesized that there may be differences in coactivator recruitment to the promoter and/or enhancer regions of these genes. Dioxin treatment leads to recruitment of the aryl hydrocarbon receptor to the enhancer regions but not to the proximal promoter regions of both the CYP1A1 and CYP1B1 genes. On the other hand, dioxin treatment facilitated recruitment of RNA polymerase II to the promoters but not the enhancer regions. Dioxin treatment also elicited recruitment of the transcriptional coactivators, steroid receptor coactivator 1 (SRC-1) and steroid receptor coactivator 2 (SRC-2) and p300, which possess intrinsic histone acetyltranferase activities, to both genes, whereas Brahma (BRM)/Switch 2-related gene 1 (BRG-1), a subunit of nucleosomal remodeling factors, was recruited more robustly to CYP1A1 relative to CYP1B1. Small inhibitory RNA-mediated knockdown of p300 and SRC-2 adversely affected dioxin induction of both genes, whereas knockdown of BRM/BRG-1 reduced CYP1A1 induction but had little, if any, effect on CYP1B1 induction. These results suggest that nucleosomal remodeling is less significant for dioxin-mediated induction of CYP1B1 than that of CYP1A1 and may be related to the more modest inducibility of the former. Interestingly, simultaneous knockdown of SRC-2 and BRM/BRG-1 had no greater effect on CYP1A1 induction than knockdown of each coactivator individually, while simultaneous knockdown of p300 and BRM/BRG-1 had a much greater effect than knockdown of each individual gene, suggesting that the recruitment of SRC-2 to CYP1A1 depends upon BRM/BRG-1, while the recruitments of p300 and BRM/BRG-1 are independent of each other. These observations provide novel insights into the functional roles of the endogenous coactivators in dioxin induction of the human CYP1A1 and CYP1B1 genes in their natural chromosomal configurations.

Large genomic deletions in AIP in pituitary adenoma predisposition

CONTEXT

Germline mutations in AIP have been recently shown to cause pituitary adenoma predisposition (PAP). Subsequently, many intragenic germline mutations have been reported, both in familial and in sporadic settings.

OBJECTIVE

Our objective was to evaluate the possible contribution of large genomic germline AIP deletions, an important mutation type in tumor predisposition syndromes, in PAP.

DESIGN

Here, we applied the multiplex ligation-dependent probe amplification assay to examine whether large genomic AIP or MEN1 alterations account for a subset of PAP cases.

PATIENTS

The study was performed on familial and sporadic pituitary adenoma cases of European origin, which had previously tested negative for germline AIP and MEN1 mutations by sequencing.

RESULTS

Two of 21 pituitary adenoma families (9.5%) were found to harbor an AIP deletion. No copy number changes were detected among 67 sporadic pituitary adenoma patients. No MEN1 deletions were found.

CONCLUSIONS

The present study shows that large genomic AIP deletions account for a subset of PAP. Therefore, in suspected PAP cases undergoing counseling and AIP genetic testing, multiplex ligation-dependent probe amplification could be considered if direct sequencing does not identify a mutation.

Cyclophilin-40 has a cellular role in the aryl hydrocarbon receptor signaling

Cyclophilin-40 (CyP40) promotes the formation of the gel shift complex that contains the aryl hydrocarbon receptor (AhR), AhR nuclear translocator (Arnt) and dioxin response element (DRE) using baculovirus expressed proteins. Here we reported that CyP40 plays a role in the AhR signaling. When the CyP40 content in MCF-7 cells is reduced, up-regulation of cyp1a1 and cyp1b1 by 3-methylchloranthrene (3MC) is also reduced, suggesting that CyP40 is essential for maximal AhR function. The CyP40 region containing amino acids 186-215, but not the peptidyl-prolyl cis-trans isomerase and tetratricopeptide repeat domains, is essential for forming the AhR/Arnt/DRE complex. CyP40 is found in the cell nucleus after 3MC treatment and appears to promote the DRE binding form of the AhR/Arnt heterodimer.

Down-regulation of the carcinogen-metabolizing enzyme cytochrome P450 1a1 by vanadium

Vanadium (V(5+)), a heavy metal contaminant with important toxicological consequences, has received considerable attention as an anticancer agent, although the mechanisms remain unknown. As a first step to investigate these mechanisms, we examined the effect of V(5+) (as ammonium metavanadate, NH(4)VO(3)) on the expression of the aryl hydrocarbon receptor (AhR)-regulated gene: cytochrome P450 1a1 (Cyp1a1) at each step of the AhR signal transduction pathway, using Hepa 1c1c7 cells. Our results showed a significant reduction in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated induction of Cyp1a1 mRNA, protein and activity levels after V(5+) treatments in a dose-dependent manner. Investigation of the effect of coexposure to V(5+) and TCDD at transcriptional levels revealed that V(5+) significantly inhibited TCDD-mediated induction of AhR-dependent luciferase reporter gene expression. Furthermore, despite not affecting the direct activation of the cytosolic AhR by TCDD and subsequently transforming it to a DNA-binding form, V(5+) inhibited the nuclear accumulation of liganded AhR and subsequent formation of the AhR/aryl hydrocarbon nuclear translocator (Arnt)/xenobiotic responsive element (XRE) complex. Importantly, the V(5+)-mediated inhibition of AhR/Arnt/XRE complex formation coincided with a significant decrease in ecto-ATPase activity. Looking at the post-transcriptional and post-translational effects of V(5+) on existing Cyp1a1 mRNA and protein levels, we showed that V(5+) did not affect Cyp1a1 mRNA or protein stability, thus eliminating possible role of V(5+) in modifying Cyp1a1 gene expression through these mechanisms. This study provides the first evidence that V(5+) down-regulates the expression of Cyp1a1 at the transcriptional level through an ATP-dependent mechanism.

Phosphodiesterases link the aryl hydrocarbon receptor complex to cyclic nucleotide signaling

The aryl hydrocarbon receptor (AHR) is a major transcription factor regulated by different mechanisms. The classical view of AHR activation by xenobiotics needs to be amended by recent findings on the regulation of AHR by endogenous ligands and by crosstalk with other signaling pathways. In the cytosol the AHR recruits a large number of binding partners, including HSP90, p23, XAP2 and the ubiquitin ligases cullin 4B and CHIP. Furthermore, XAP2 binds the cyclic nucleotide phosphodiesterases PDE2A and PDE4A5. PDE2A inhibits nuclear translocation of AHR suggesting an important regulatory role of cyclic nucleotides in AHR trafficking. Signaling involving cAMP is organized in subcellular compartments and a distinct cAMP compartment might be required for proper AHR mobility and function. We conclude that the AHR complex integrates ligand binding and cyclic nucleotide signaling to generate an adequate transcriptional response.

In vivo siRNA delivery to the mouse hypothalamus shows a role of the co-chaperone XAP2 in regulating TRH transcription

RNA interference mediated by small interfering RNAs (siRNAs) is a powerful tool for evaluating gene function in vivo. In particular it should be able to provide tissue-specific and developmental stage-specific knockdown of target genes in physiological contexts. However, there are few demonstrations of its use on neuronal specific genes in vivo. We recently developed a cationic lipid-based approach to study gene function in a neuronal context. In particular, we applied it to study how the novel partner for TRbeta1, hepatitis virus B X-associated protein 2 (XAP2), a protein first identified as a co-chaperone protein, affects T3-transcriptional repression of the hypothalamic gene, TRH. The cationic lipid-based technique used, JetSI/DOPE, was previously shown to efficiently knockdown reporter gene mRNA in vivo. Using JetSI/DOPE to vectorize siRNA against XAP2 mRNA, we show that XAP2 is needed specifically for TRbeta1-mediated (but not TRbeta2) activation of hypothalamic TRH transcription. Thus, this cationic lipid-based siRNA strategy can effectively be used to reveal fine, tissue-specific and isoform-specific effects on neuronal gene transcription in vivo.

Activation of the aryl hydrocarbon receptor is essential for mediating the anti-inflammatory effects of a novel low-molecular-weight compound

VAF347 is a low-molecular-weight compound that inhibits allergic lung inflammation in vivo. This effect is likely the result of a block of dendritic cell (DC) function to generate proinflammatory T-helper (Th) cells because VAF347 inhibits interleukin (IL)-6, CD86, and human leukocyte antigen (HLA)-DR expression by human monocyte-derived DC, 3 relevant molecules for Th-cell generation. Here we demonstrate that VAF347 interacts with the aryl hydrocarbon receptor (AhR) protein, resulting in activation of the AhR signaling pathway. Functional AhR is responsible for the biologic activity of VAF347 because (1) other AhR agonists display an identical activity profile in vitro, (2) gene silencing of wild-type AhR expression or forced overexpression of a trans-dominant negative AhR ablates VAF347 activity to inhibit cytokine induced IL-6 expression in a human monocytic cell line, and (3) AhR-deficient mice are resistant to the compound's ability to block allergic lung inflammation in vivo. These data identify the AhR protein as key molecular target of VAF347 and its essential role for mediating the anti-inflammatory effects of the compound in vitro and in vivo.

The aryl hydrocarbon receptor complex and the control of gene expression

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that controls the expression of a diverse set of genes. The toxicity of the potent AhR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin is almost exclusively mediated through this receptor. However, the key alterations in gene expression that mediate toxicity are poorly understood. It has been established through characterization of AhR-null mice that the AhR has a required physiological function, yet how endogenous mediators regulate this orphan receptor remains to be established. A picture as to how the AhR/ARNT heterodimer actually mediates gene transcription is starting to emerge. The AhR/ARNT complex can alter transcription both by binding to its cognate response element and through tethering to other transcription factors. In addition, many of the coregulatory proteins necessary for AhR-mediated transcription have been identified. Cross talk between the estrogen receptor and the AhR at the promoter of target genes appears to be an important mode of regulation. Inflammatory signaling pathways and the AhR also appear to be another important site of cross talk at the level of transcription. A major focus of this review is to highlight experimental efforts to characterize nonclassical mechanisms of AhR-mediated modulation of gene transcription.

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.

Functional drift of sequence attributes in the FK506-binding proteins (FKBPs)

Diverse members of the FK506-binding proteins (FKBPs) group and their complexes with different macrocyclic ligands of fungal origins such as FK506, rapamycin, ascomycin, and their immunosuppressive and nonimmunosuppressive derivatives display a variety of cellular and biological activities. The functional relatedness of the FKBPs was estimated from the following attributes of their aligned sequences: 1 degrees conservation of the consensus sequence; 2 degrees sequence similarity; 3 degrees pI; 4 degrees hydrophobicity; 5 degrees amino acid hydrophobicity and bulkiness profiles. Analyses of the multiple sequence alignments and intramolecular interaction networks calculated from a series of structures of the FKBPs revealed some variations in the interaction clusters formed by the AA residues that are crucial for sustaining peptidylprolyl cis/trans isomerases (PPIases) activity and binding capacity of the FKBPs. Fine diversification of the sequences of the multiple paralogues and orthologues of the FKBPs encoded in different genomes alter the intramolecular interaction patterns of their structures and allowed them to gain some selectivity in binding to diverse targets (functional drift).

The Leber congenital amaurosis protein AIPL1 functions as part of a chaperone heterocomplex

PURPOSE

AIPL1 mutations cause the severe inherited blindness Leber congenital amaurosis (LCA). The similarity of AIPL1 to tetratricopeptide repeat (TPR) cochaperones that interact with the chaperone Hsp90 and the ability of AIPL1 to suppress the aggregation of NUB1 fragments in a chaperone-like manner suggest that AIPL1 might function as part of a chaperone heterocomplex facilitating retinal protein maturation. In this study the interaction of AIPL1 with molecular chaperones is revealed and functionally characterized.

METHODS

AIPL1-interacting proteins were identified using a yeast two-hybrid system, and the effect of AIPL1 pathogenic mutations and sequence requirements mediating the identified interactions were investigated. The interactions were validated by a comprehensive set of biochemical assays, and the ability of the AIPL1-binding partners to cooperate with AIPL1 in the suppression of NUB1 fragment aggregation was assessed.

RESULTS

AIPL1 interacts with the molecular chaperones Hsp90 and Hsp70. Mutations within the TPR domain of AIPL1 or removal of the chaperone TPR acceptor site abolished the interactions. Importantly, LCA-causing mutations in AIPL1 also compromised these interactions, suggesting that the essential function of AIPL1 in photoreceptors may involve the interaction with Hsp90 and Hsp70. Examination of the role of these chaperones in AIPL1 chaperone activity demonstrated that AIPL1 cooperated with Hsp70, but not with Hsp90, to suppress the formation of NUB1 inclusions.

CONCLUSIONS

These findings suggest that AIPL1 may cooperate with both Hsp70 and Hsp90 within a retina-specific chaperone heterocomplex and that the specialized role of AIPL1 in photoreceptors may therefore be facilitated by these molecular chaperones.

Newspapers and newspaper ink contain agonists for the ah receptor

Ligand-dependent activation of the aryl hydrocarbon receptor (AhR) pathway leads to a diverse array of biological and toxicological effects. The best-studied ligands for the AhR include polycyclic and halogenated aromatic hydrocarbons, the most potent of which is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). However, as new AhR ligands are identified and characterized, their structural and physiochemical diversity continues to expand. Our identification of AhR agonists in crude extracts from diverse materials raises questions as to the magnitude and extent of human exposure to AhR ligands through normal daily activities. We have found that solvent extracts of newspapers from countries around the world stimulate the AhR signaling pathway. AhR agonist activity was observed for dimethyl sulfoxide (DMSO), ethanol, and water extracts of printed newspaper, unprinted virgin paper, and black printing ink, where activation of luciferase reporter gene expression was transient, suggesting that the AhR active chemical(s) was metabolically labile. DMSO and ethanol extracts also stimulated AhR transformation and DNA binding, and also competed with [(3)H]TCDD for binding to the AhR. In addition, DMSO extracts of printed newspaper induced cytochrome P450 1A associated 7-ethoxyresorufin-O-deethylase activity in zebrafish embryos in vivo. Although the responsible bioactive chemical(s) remain to be identified, our results demonstrate that newspapers and printing ink contain relatively potent metabolically labile agonists of the AhR. Given the large amount of recycling and reprocessing of newspapers throughout the world, release of these easily extractable AhR agonists into the environment should be examined and their potential effects on aquatic organisms assessed.

Characterization of the antiallergic drugs 3-[2-(2-phenylethyl) benzoimidazole-4-yl]-3-hydroxypropanoic acid and ethyl 3-hydroxy-3-[2-(2-phenylethyl)benzoimidazol-4-yl]propanoate as full aryl hydrocarbon receptor agonists

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates most of the toxic effects of numerous chlorinated (e.g., TCDD) and nonchlorinated polycyclic aromatic compounds (e.g., benzo[ a]pyrene). Studies in AhR null mice suggested that this receptor may also play a role in the modulation of immune responses. Recently, two drugs, namely, M50354 and M50367 (ethyl ester derivative of M50354), were described as AhR ligands with high efficacy toward reducing atopic allergic symptoms in an AhR-dependent manner by skewing T helper cell differentiation toward a T H1 phenotype [Negishi et al. (2005) J. Immunol. 175 (11), 7348-7356]. Surprisingly, these drugs were shown to have minimal activity toward inducing classical dioxin responsive element-driven AhR-mediated CYP1A1 transcription. We synthesized and reevaluated the ability of these drugs to regulate AhR activity. In contrast to previously published data, both M50354 and M50367 were found to be potent inducers of several AhR target genes, namely, CYP1A1, CYP1B1, and UGT1A2. M50367 was a more effective agonist than M50354, perhaps accounting for its higher bioavailability in vivo. However, M50354 was capable of displacing an AhR-specific radioligand more effectively than M50367. This is consistent with M50354 being the active metabolite of M50367. In conclusion, two selective inhibitors of TH2 differentiation are full AhR agonists.

A role for the aryl hydrocarbon receptor in mammary gland tumorigenesis

The aryl hydrocarbon receptor (AhR) is an evolutionarily conserved transcription factor bound and activated by ubiquitous environmental pollutants. Historically, the AhR has been studied for its transcriptional regulation of genes encoding cytochrome P450 enzymes, which metabolize many of these chemicals into mutagenic and toxic intermediates. However, recent studies demonstrate that the AhR plays an important role in the biology of several cell types in the absence of environmental chemicals. Here, this paradigm shift is discussed in the context of a putative role for the AhR in mammary gland tumorigenesis. Data demonstrating high levels of constitutively active AhR in mammary tumors are summarized. Particular focus is placed on the likelihood that the AhR contributes to ongoing mammary tumor cell growth and on the possibility that the AhR inhibits apoptosis while promoting transition to an invasive, metastatic phenotype. A working model is proposed that may help explain the sometimes contradictory outcomes observed after AhR manipulation and that serves as a blueprint for the design of therapeutics which target the AhR in breast cancer. The theme that malignant cells reveal the functions for which the AhR has been evolutionarily conserved is presented throughout this discussion.

The search for endogenous activators of the aryl hydrocarbon receptor

The primary design of this perspective is to describe the major ligand classes of the aryl hydrocarbon receptor (AHR). A grander objective is to provide models that may help define the physiological activator or "endogenous ligand" of the AHR. We present evidence supporting a developmental role for the AHR and propose mechanisms by which an endogenous ligand and consequent AHR activation might be important during normal physiology and development. From this vista, we survey the known xenobiotic, endogenous, dietary, and "unconventional" activators of the AHR, including, when possible, information about their induction potency, receptor binding affinity, and potential for exposure. In light of the essential function of the AHR in embryonic development, we discuss the candidacy of each of these compounds as physiologically important activators.

Antagonistic and agonistic effects of indigoids on the transformation of an aryl hydrocarbon receptor

Halogenated and polycyclic aromatic hydrocarbons, exogenous ligands of the aryl hydrocarbon receptor (AhR), cause various toxicological effects through the transformation of the AhR. In this study, we investigated the antagonistic effects of indigoids on the transformation in addition to their agonistic ones. In a cell-free system, indigoids induced the transformation dose-dependently, but suppressed the transformation induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin and the binding of 3-methylcholanthrene to the AhR. In mouse hepatoma Hepa-1c1c7 cells, indigoids, especially indirubin, suppressed the transformation and expression of CYP1A1 by inhibiting the translocation of AhR into the nucleus. When orally administered to mice at 10mg/kg BW/day for three successive days, indigoids did not induce AhR transformation and expression of the CYP1A subfamily in the liver, while indirubin and indigo upregulated quinone reductase activity. These results indicate that indigoids are able to bind to the AhR as ligands and exhibit antagonistic effects at lower concentrations in mammalian cells.

The aryl hydrocarbon receptor is required for optimal resistance to Listeria monocytogenes infection in mice

The aryl hydrocarbon receptor (AhR) is part of a powerful signaling system that is triggered by xenobiotic agents such as polychlorinated hydrocarbons and polycyclic aromatic hydrocarbons. Although activation of the AhR by 2,3,7,8-tetrachlorodibenzo-p-dioxin or certain polycyclic aromatic hydrocarbons can lead to immunosuppression, there is also increasing evidence that the AhR regulates certain normal developmental processes. In this study, we asked whether the AhR plays a role in host resistance using murine listeriosis as an experimental system. Our data clearly demonstrate that AhR null C57BL/6J mice (AhR(-/-)) are more susceptible to listeriosis than AhR heterozygous (AhR(+/-)) littermates when inoculated i.v. with log-phase Listeria monocytogenes. AhR(-/-) mice exhibited greater numbers of CFU of L. monocytogenes in the spleen and liver, and greater histopathological changes in the liver than AhR(+/-) mice. Serum levels of IL-6, MCP-1, IFN-gamma, and TNF-alpha were comparable between L. monocytogenes-infected AhR(-/-) and AhR(+/-) mice. Increased levels of IL-12 and IL-10 were observed in L. monocytogenes-infected AhR(-/-) mice. No significant difference was found between AhR(+/-) and AhR(-/-) macrophages ex vivo with regard to their ability to ingest and inhibit intracellular growth of L. monocytogenes. Intracellular cytokine staining of CD4(+) and CD8(+) splenocytes for IFN-gamma and TNF-alpha revealed comparable T cell-mediated responses in AhR(-/-) and AhR(+/-) mice. Previously infected AhR(-/-) and AhR(+/-) mice both exhibited enhanced resistance to reinfection with L. monocytogenes. These data provide the first evidence that AhR is required for optimal resistance but is not essential for adaptive immune response to L. monocytogenes infection.

Hepatic transcriptional networks induced by exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin

The environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) serves as a prototype for a range of environmental toxicants and as a pharmacologic probe to study signal transduction by the aryl hydrocarbon receptor (AHR). Despite a detailed understanding of how TCDD exposure leads to the transcriptional up-regulation of cytochrome P450-dependent monooxygenases, we know little about how compounds like TCDD lead to a variety of AHR-dependent toxic end points such as liver pathology, terata, thymic involution, and cancer. Using an acute exposure protocol and the toxic response of the mouse liver as a model system, we have begun a detailed microarray analysis to describe the transcriptional changes that occur after various TCDD doses and treatment times. Through correlation analysis of time- and dose-dependent toxicological end points, we are able to identify coordinately responsive transcriptional events that can be defined as primary transcriptional events and downstream events that may represent mechanistically linked sequelae or that have potential as biomarkers of toxicity.

Evidence for an aryl hydrocarbon receptor-mediated cytochrome p450 autoregulatory pathway

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor responsible for mediating the cellular response to the toxic compound 2,3,7,8,-tetrachlorodibenzo-p-dioxin. An essential role for the AhR in cellular biology has been established previously, but no high-affinity endogenous ligand has yet been identified. We have confirmed the presence of a putative endogenous ligand(s) in CV-1 cells through transient transfection with various cytochrome P450 isoforms. Expression of cytochromes P450 1A1, 1A2, or 1B1 reduced AhR-mediated luciferase reporter activity, whereas cytochrome P450 2E1 exhibited no significant effect. Studies with 2,4,3',5'-tetramethoxystilbene, a potent and specific inhibitor of cytochrome P450 1B1, was able to partially block cytochrome P450 1B1-mediated reduction in reporter gene activity. These results provide evidence of the existence of a possible feedback mechanism in which AhR-regulated cytochromes P450 from the CYP1A and CYP1B families are able to metabolically alter putative endogenous ligand(s). Several experiments were performed to provide initial characterization of these putative endogenous ligands, including electrophoretic mobility shift assay analyses, which demonstrated that these ligands directly activate the AhR. Soluble extracts from various C57BL/6J and Ahr-null mouse tissues were also analyzed for the presence of AhR activators. Studies revealed that Ahr-null mouse lung tissue had a 4-fold increase in AhR-mediated reporter activity in cells. Quantitative polymerase chain reaction analysis revealed that lung tissue exhibits relatively high constitutive CYP1A1 mRNA levels. These results suggest that there is an autoregulatory feedback loop between the AhR and cytochrome P450 1A1 in mouse lung.

The effect of aromatic hydrocarbon receptor on the phenotype of the Hepa 1c1c7 murine hepatoma cells in the absence of dioxin

The aromatic hydrocarbon receptor (AhR) mediates biological responses to certain exogenous ligands, such as the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and has also been demonstrated to modulate the cell cycle and differentiated state of several cell lines independently of exogenous ligands. In this study, we used DNA microarray analysis to elucidate the profile of genes responsive to the expression of unliganded AhR by re-introducing AhR into an AhR-deficient mouse derivative (c19) of the mouse hepatoma cell line Hepa 1c1c7. 22 gene products were up-regulated and 8 were down-regulated two-fold or more in c19 cells infected with a retroviral vector expressing mouse AhR. Surprisingly, expression of genes involved in cell proliferation or differentiation were not affected by introduction of AhR. AhR also did not restore expression of the albumin gene in c19 cells. Introduction of AhR into c12, a similar AhR-defective mouse hepatoma cell line, also did not restore albumin expression, and furthermore, did not lead to changes in cellular morphology or cell cycle parameters. These observations fail to support the notion that unliganded AhR regulates proliferation and differentiation of liver-derived cells.

Environmental toxins as modulators of antiviral immune responses

Exposure to environmental contaminants has a profound effect on immune function, yet mechanistic understanding of how pollutants deregulate immune responses has, for many chemicals, remained elusive. Available data suggest that certain pollutants alter host immune responses and increase susceptibility to viral infection. In particular, data from a combination of epidemiological and animal studies show that exposure to dioxins, cigarette smoke, diesel exhaust and other air pollutants increase pathology associated with infection. Mechanistically, some of these chemicals disrupt the kinetics and efficacy of innate and adaptive responses to infection, whereas others influence viral latency. While there remain considerable gaps in our knowledge of the complex interactions between viruses, immune cells, and the host environment, these observations indicate that pollutants are important but overlooked contributors to susceptibility and pathogenesis of viral infections.

Regulation of aryl hydrocarbon receptor activity in porcine cumulus–oocyte complexes in physiological and toxicological conditions: the role of follicular fluid

The arylhydrocarbon receptor (AhR) mediates the adverse effects of dioxin-like compounds. However, it has also been reported that the AhR may exert a role in ovarian physiology. In the present study, porcine cumulus–oocyte complexes (COCs) were maturedin vitroin the presence of 10% follicular fluid. Expression of AhR and its partner, AhR nuclear translocator occurs in immature COCs. Afterin vitromaturation (IVM), an up-regulation of AhR and cytochrome P450 1A1 (CYP1A1; the main AhR-target gene) was observed. To explore the role of the AhR during IVM, we exposed the COCs to 50 μM β-napthoflavone (βNF). The treatment induced a marked up-regulation of CYP1A1 mRNA, indicating both constitutive and inducible AhR activity. However, in contrast to what was observed in other cell types, no sign of toxicity was observed in COCs. To investigate if components of porcine follicular fluid may exert a protective role against AhR ligands, we exposed porcine COCs to βNF, in the absence of follicular fluid. In these conditions, a marked decrease in the percentage of matured oocytes, concomitant with an increase in oocyte degeneration, was observed. Furthermore, βNF increased apoptosis in cumulus cells in the absence of follicular fluid, whereas βNF has no effects when COCs were treated in the presence of porcine follicular fluid (pFF). In conclusion, these results suggest the presence of unknown endogenous AhR-ligand(s) during porcine IVM and that a dysregulation of this mechanism may result in ovotoxicity by inducing apoptosis in cumulus cells. However, this phenomenon is interrupted by the presence of follicular fluid, indicating a putative protective role for follicular fluid components against exogenous insults.

Phosphodiesterase 2A forms a complex with the co-chaperone XAP2 and regulates nuclear translocation of the aryl hydrocarbon receptor

Phosphodiesterase type 2A (PDE2A) hydrolyzes cyclic nucleotides cAMP and cGMP, thus efficiently controlling cNMP-dependent signaling pathways. PDE2A is composed of an amino-terminal region, two regulatory GAF domains, and a catalytic domain. Cyclic nucleotide hydrolysis is known to be activated by cGMP binding to GAF-B; however, other mechanisms may operate to fine-tune local cyclic nucleotide levels. In a yeast two-hybrid screening we identified XAP2, a crucial component of the aryl hydrocarbon receptor (AhR) complex, as a major PDE2A-interacting protein. We mapped the XAP2 binding site to the GAF-B domain of PDE2A. PDE assays with purified proteins showed that XAP2 binding does not change the enzymatic activity of PDE2A. To analyze whether PDE2A could affect the function of XAP2, we studied nuclear translocation of AhR, i.e. the master transcription factor controlling the expression of multiple detoxification genes. Notably, regulation of AhR target gene expression is initiated by tetrachlorodibenzodioxin (TCDD) binding to AhR and by a poorly understood cAMP-dependent pathway followed by the translocation of AhR from the cytosol into the nucleus. Binding of PDE2A to XAP2 inhibited TCDD- and cAMP-induced nuclear translocation of AhR in Hepa1c1c7 hepatocytes. Furthermore, PDE2A attenuated TCDD-induced transcription in reporter gene assays. We conclude that XAP2 targets PDE2A to the AhR complex, thereby restricting AhR mobility, possibly by a local reduction of cAMP levels. Our results provide first insights into the elusive cAMP-dependent regulation of AhR.

Carboxyl terminus of hsc70-interacting protein (CHIP) can remodel mature aryl hydrocarbon receptor (AhR) complexes and mediate ubiquitination of both the AhR and the 90 kDa heat-shock protein (hsp90) in vitro

The regulation of the aryl hydrocarbon receptor (AhR) protein levels has been an area of keen interest, given its important role in mediating the cellular adaptation and toxic response to several environmental pollutants. The carboxyl terminus of hsc70-interacting protein (CHIP) ubiquitin ligase was previously associated with the regulation of the aryl hydrocarbon receptor, although the mechanisms were not directly demonstrated. In this study, we established that CHIP could associate with the AhR at cellular levels of these two proteins, suggesting a potential role for CHIP in the regulation of the AhR complex. The analysis of the sucrose-gradient-fractionated in vitro translated AhR complexes revealed that CHIP can mediate hsp90 ubiquitination while cooperating with unidentified factors to promote the ubiquitination of mature unliganded AhR complexes. In addition, the immunophilin-like protein XAP2 was able to partially protect the AhR from CHIP-mediated ubiquitination in vitro. This protection required the direct interaction of the XAP2 with the AhR complex. Surprisingly, CHIP silencing in Hepa-1c1c7 cells by siRNA methods did not reveal the function of CHIP in the AhR complex, because it did not affect well-characterized activities of the AhR nor affect its steady-state protein levels. However, the presence of potential compensatory mechanisms may be confounding this particular observation. Our results suggest a model where the E3 ubiquitin ligase CHIP cooperates with other ubiquitination factors to remodel native AhR-hsp90 complexes and where co-chaperones such as the XAP2 may affect the ability of CHIP to target AhR complexes for ubiquitination.

Endogenous hepatic expression of the hepatitis B virus X-associated protein 2 is adequate for maximal association with aryl hydrocarbon receptor-90-kDa heat shock protein complexes

The aryl hydrocarbon receptor (AHR) is a ligand activated transcription factor that acts as an environmental sensor by binding to a variety of xenobiotics. AHR activation serves to combat xenotoxic stress by inducing metabolic enzyme expression in the liver. The hepatitis B virus X-associated protein (XAP2) is a component of the cytosolic AHR complex and modulates AHR transcriptional properties in vitro and in cell culture and yeast systems. Expression of XAP2 is low in liver compared with other nonhepatic tissues and the AHR exhibits high ligand-induced transcriptional activity. Because XAP2 has been demonstrated to repress AHR activity, we hypothesized that XAP2 may be limiting in liver and that increasing XAP2 levels would attenuate AHR transcriptional activity. To this end, transgenic mice were generated that exhibit hepatocyte-specific elevation in XAP2 expression. Transgenic XAP2 expression was restricted to liver, and its ability to complex with the AHR was verified. Gene expression experiments were performed by inducing AHR transcriptional activity with beta-naphthoflavone via intraperitoneal injection, and mRNA quantification was done by real-time polymerase chain reaction. Wild-type and transgenic animals showed little difference in constitutive or ligand-induced CYP1A1; CYP1A2; UDP glucuronosyltransferase 1A2; NAD(P)H dehydrogenase, quinone 1; constitutive androstane receptor; or nuclear factor erythroid 2-related factor 2 mRNA expression. Sucrose density fractionation and AHR immunoprecipitation experiments found little or no stoichiometric increase in bound XAP2 to the AHR between genotypes. Gene array studies were performed to identify novel XAP2-regulated targets. Taken together, this work shows that despite the relatively low level of XAP2 in liver, it is not a limiting component in AHR regulation.

Aryl Hydrocarbon Receptor Activation Impairs the Priming but Not the Recall of Influenza Virus-Specific CD8+T Cells in the Lung

The response of CD8+ T cells to influenza virus is very sensitive to modulation by aryl hydrocarbon receptor (AhR) agonists; however, the mechanism underlying AhR-mediated alterations in CD8+ T cell function remains unclear. Moreover, very little is known regarding how AhR activation affects anamnestic CD8+ T cell responses. In this study, we analyzed how AhR activation by the pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alters the in vivo distribution and frequency of CD8+ T cells specific for three different influenza A virus epitopes during and after the resolution of a primary infection. We then determined the effects of TCDD on the expansion of virus-specific memory CD8+ T cells during recall challenge. Adoptive transfer of AhR-null CD8+ T cells into congenic AhR(+/+) recipients, and the generation of CD45.2AhR(-/-)-->CD45.1AhR(+/+) chimeric mice demonstrate that AhR-regulated events within hemopoietic cells, but not directly within CD8+ T cells, underlie suppressed expansion of virus-specific CD8+ T cells during primary infection. Using a dual-adoptive transfer approach, we directly compared the responsiveness of virus-specific memory CD8+ T cells created in the presence or absence of TCDD, which revealed that despite profound suppression of the primary response to influenza virus, the recall response of virus-specific CD8+ T cells that form in the presence of TCDD is only mildly impaired. Thus, the delayed kinetics of the recall response in TCDD-treated mice reflects the fact that there are fewer memory cells at the time of reinfection rather than an inherent defect in the responsive capacity of virus-specific memory CD8+ cells.

Role of endogenous XAP2 protein on the localization and nucleocytoplasmic shuttling of the endogenous mouse Ahb-1 receptor in the presence and absence of ligand

Studies using transient expression systems have implicated the hepatitis B virus X-associated protein (XAP2) in the control of aryl hydrocarbon receptor (AHR) stability and subcellular location. Studies were performed in Hepa-1 cells to evaluate these functions of XAP2 on the mouse Ahb-1 receptor under endogenous stoichiometry. The Ahb-1 receptor is cytoplasmic, and it becomes predominantly nuclear after 30 to 60 min of ligand exposure with minimal degradation. During this time, XAP2 coprecipitates with the AHR, suggesting that it does not affect the nuclear localization of the liganded receptor. Overexpression of XAP2 in Hepa-1 cells does not result in increased association with the endogenous Ahb-1 complex or influence the receptors cytoplasmic localization. Knockdown of endogenous XAP2 by small interfering RNA results in >or=90% reduction in the amount of XAP2 associated with the endogenous Ahb-1 receptor complex. Despite the reduction in XAP2, the unliganded Ahb-1 receptor complex remains cytoplasmic, although inhibition of nuclear export results in accumulation of the receptor in the nucleus. Truncation of the C-terminal 305 amino acids of the Ahb-1 receptor (AHR500) results in proteins that exhibit a predominantly nuclear localization and remain associated with the same level of endogenous XAP2 as full-length AHRs. Together, these results support a model in which the majority of the unliganded Ahb-1 receptor complexes are associated with XAP2, and the association prevents dynamic nucleocytoplasmic shuttling in the unliganded state. After ligand binding, XAP2 remains associated with the Ahb-1 receptor complex, and it does not impair nuclear translocation but may function to limit receptor "transformation".

Involvement of some large immunophilins and their ligands in the protection and regeneration of neurons: a hypothetical mode of action

The powerful immunosuppressive drugs such as FK506 and its derivatives induce some regeneration and protection of neurons from ischaemic brain injury and some other neurological disorders. The drugs form complexes with diverse FKBPs but apparently the FKBP52/FK506 complex was shown to be involved in the protection and regeneration of neurons. We used several different sequence attributes in searching diverse genomic databases for similar motifs as those present in the FKBPs. A Fortran library of algorithms (Par_Seq) has been designed and used in searching for the similarity of sequence motifs extracted from the multiple sequence alignments of diverse groups of proteins (query motifs) and the target motifs which are encoded in various genomes. The following sequence attributes were used in the establishment of the degree of convergence between: (A) amino acid (AA) sequence similarity (ID) of the query/target motifs and (B) their: (1) AA composition (AAC); (2) hydrophobicity (HI); (3) Jensen-Shannon entropy; and (4) AA propensity to form a particular secondary structure. The sequence hallmark of two different groups of peptidylprolyl cis/trans isomerases (PPIases), namely tetratricopetide repeat (TPR) motifs, which are present in the heat-shock cyclophilins and in the large FK506-binding proteins (FKBPs) were used to search various genomic databases. The Par_Seq algorithm has revealed that the TPR motifs have similar sequence attributes as a number of hydrophobic sequence segments of functionally unrelated membrane proteins, including some of the TMs from diverse G protein-coupled receptors (GPCRs). It is proposed that binding of the FKBP52/FK506 complex to the membranes via the TPR motifs and its interaction with some membrane proteins could be in part responsible for some neuro-regeneration and neuro-protection of the brain during some ischaemia-induced stresses.