Why ARNT Prostate Tumors Responding to Enzalutamide?

SUMMARY

Prostate tumors can develop resistance to androgen receptor (AR)-targeted therapies through treatment-induced changes in transcription factor activity that promote transcriptional and morphologic features of a neuroendocrine lineage. This study identifies an unexpected role for the circadian protein ARNTL in resistance to enzalutamide, a second-generation AR-targeted therapy. See related article by Linder et al., p. 2074 (4).

CYP1A1, VEGFA and Adipokine Responses of Human Adipocytes Co-exposed to PCB126 and Hypoxia

It is increasingly recognized that hypoxia may develop in adipose tissue as its mass expands. Adipose tissue is also the main reservoir of lipophilic pollutants, including polychlorinated biphenyls (PCBs). Both hypoxia and PCBs have been shown to alter adipose tissue functions. The signaling pathways induced by hypoxia and pollutants may crosstalk, as they share a common transcription factor: aryl hydrocarbon receptor nuclear translocator (ARNT). Whether hypoxia and PCBs crosstalk and affect adipokine secretion in human adipocytes remains to be explored. Using primary human adipocytes acutely co-exposed to different levels of hypoxia (24 h) and PCB126 (48 h), we observed that hypoxia significantly inhibits the PCB126 induction of cytochrome P450 (CYP1A1) transcription in a dose-response manner, and that Acriflavine (ACF)—an HIF1α inhibitor—partially restores the PCB126 induction of CYP1A1 under hypoxia. On the other hand, exposure to PCB126 did not affect the transcription of the vascular endothelial growth factor-A (VEGFA) under hypoxia. Exposure to hypoxia increased leptin and interleukin-6 (IL-6), and decreased adiponectin levels dose-dependently, while PCB126 increased IL-6 and IL-8 secretion in a dose-dependent manner. Co-exposure to PCB126 and hypoxia did not alter the adipokine secretion pattern observed under hypoxia and PCB126 exposure alone. In conclusion, our results indicate that (1) hypoxia inhibits PCB126-induced CYP1A1 expression at least partly through ARNT-dependent means, suggesting that hypoxia could affect PCB metabolism and toxicity in adipose tissue, and (2) hypoxia and PCB126 affect leptin, adiponectin, IL-6 and IL-8 secretion differently, with no apparent crosstalk between the two factors.

The molecular mechanism of AhR-ARNT-XREs signaling pathway in the detoxification response induced by polycyclic aromatic hydrocarbons (PAHs) in clam Ruditapes philippinarum

The aryl hydrocarbon receptor (AhR) has been known primarily for its role in the regulation of several drug and xenobiotic metabolizing enzymes to mitigate environmental stresses. In this study, we interfere the expression of AhR gene to investigate the mechanism of AhR signaling pathway in the detoxification and antioxidation defense system that induced by Polycyclic Aromatic Hydrocarbons (PAHs) exposure by RNA interference (RNAi). The gene expressions of aryl hydrocarbon receptor nuclear translocator (ARNT), heat shock protein 90 (Hsp90) were evaluated after being exposed to benzo(a)pyrene (BaP) (4 μg/L) for 5 days and the positive correlations between AhR, ARNT, HSP90 indirectly indicating that AhR may have the ability to bind to ligands such as PAHs in Ruditapes philippinarum (R. philippinarum). Besides, the activities of detoxification enzymes were determined to investigate the role of AhR signaling pathway played in the metabolic detoxification. What's more, the gene expressions of protein kinase C (PKC) signaling pathway, mitogen-activated protein kinase (MAPKs) signaling pathway, NF-E2-related factor 2 (Nrf2) signaling pathway and antioxidant defense system indicated that AhR may regulate the Nrf2-Keap1 signaling pathway through Kelch-like ECH-associated protein-1 (Keap1) and MAPKs, PKC signaling pathways. In conclusion, adoption of RNA interference technology to explore the role of RpAhR gene played in the detoxification and antioxidation defense system under the PAHs stress at different time points can informe molecular endpoints for application towards ecotoxicology monitoring of bivalves.

A geospatial model to determine the spatial cost-efficiency of anticoagulation drug therapy: Patients' perspective

Most atrial fibrillation (AF) patients need anticoagulation management to reduce the risk of thromboembolic events and stroke. Currently, two major drug therapies are available: warfarin and direct oral anticoagulant (DOAC). This study examined the spatial costs of these therapies and derived the least-cost market areas for both therapies in the study area. The concepts of spatial costs and the principles of forming market areas were used as theoretical starting points, and the patients' travel, time-loss, and medication cost parameters combined with geographical information systems methods were incorporated into the geospatial model. Results showed that for AF patients who live near the international normalized ratio (INR) monitoring sample collection point and have less than 15 annual INR monitoring visits, warfarin therapy resulted in the lowest cost regardless of patient's travel mode and their assumed working or retirement status. If the AF patient needs more frequent INR monitoring visits or lives farther from the nearest sample collection point, DOAC would be the least costly option. The modelled results reveal the variety and importance of patients' cost of time loss and travel costs when a physician selects the appropriate anticoagulation therapy.

Transcription factors CncC/Maf and AhR/ARNT coordinately regulate the expression of multiple GSTs conferring resistance to chlorpyrifos and cypermethrin in Spodoptera exigua

BACKGROUND

Glutathione S-transferases (GSTs) are a superfamily of multifunctional dimeric proteins existing in both prokaryotic and eukaryotic organisms. They are involved in the detoxification of both endogenous and exogenous electrophiles, including insecticides. However, the molecular mechanisms underlying the regulation of GST genes in insects are poorly understood.

RESULTS

We first identified at least three GST genes involved in resistance to the insecticides chlorpyrifos and cypermethrin. Analysis of upstream sequences revealed that three GSTs (SeGSTo2, SeGSTe6 and SeGSTd3) harbor the same cap 'n' collar C/muscle aponeurosis fibromatosis (CncC/Maf) binding site, and SeGSTo2 and SeGSTe6 contain the aryl hydrocarbon receptor/aryl hydrocarbon receptor nuclear translocator (AhR/ARNT) binding site. Luciferase reporter assay showed co-transfection of reporter plasmid containing the SeGSTe6 promoter with CncC and/or Maf expressing constructs significantly boosted transcription. Similarly, AhR and/or ARNT expressing constructs also significantly increased the promoter activities. The co-transfection of mutated reporter plasmid with CncC/Maf or AhR/ARNT did not increase transcription activity anymore. Constitutive over-expression of CncC, Maf and AhR was also found in the HZ16 strain, which might be the molecular mechanism for up-regulated expression of multiple detoxification genes conferring resistance to insecticides.

CONCLUSION

These results suggest that CncC/Maf and AhR/ARNT coordinately regulate the expression of multiple GST genes involved in insecticide resistance in Spodoptera exigua. © 2019 Society of Chemical Industry.

Transcriptomic analysis of Anabas testudineus and its defensive mechanisms in response to persistent organic pollutants exposure

The freshwater climbing perch (Anabas testudineus) can tolerate water environments contaminated with persistent organic pollutants (POPs). The mechanisms underlying this tolerance are unknown. We used de novo transcriptomic analysis to investigate the defensive mechanisms of A. testudineus against POPs based on its genetic features and biological responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure. Our results revealed a specific expansion of cytochrome P450 (CYP) 3A subfamily, which may be involved in the elimination of certain POPs. In xenobiotic responses, the aryl-hydrocarbon receptor (AhR) pathway represents a critical signaling mechanism, and we characterized four AhR and two AhR nuclear translocator homologs and one AhR repressor (AhRR) gene in A. testudineus. TCDD-induced AhRR and CYP1A mRNA upregulation suggests that negative-feedback regulation of AhR signaling through AhRR helps avoid excessive xenobiotic responses. Furthermore, liver and gill transcriptomic profiles were markedly altered after TCDD exposure, with some of the altered genes being related to common defensive responses reported in other species. Based on the newly identified TCDD-altered genes, several A. testudineus-specific responses are proposed, such as enhanced fatty acid β-oxidation. The genetic features of CYP3A subfamily and AhR pathway and the TCDD-induced defensive biological processes elucidated here enhance our understanding of A. testudineus defensive responses against POPs.

Fine Particulate Matter (PM2.5) Promoted the Invasion of Lung Cancer Cells via an ARNT2/PP2A/STAT3/MMP2 Pathway

Accumulating evidence indicates that fine particulate matter (PM2.5) exposure is associated with many cardiopulmonary diseases, particularly lung carcinoma. Nevertheless, the underlying biological mechanisms by which PM_2.5 exposure initiates and aggravates lung carcinoma remain elusive. In the present study, we collected PM_2.5 in Nanjing and explored the mechanisms underlying the oncogenic roles of PM_2.5 in the murine lung carcinoma cell line LLC in vitro and in vivo. PM_2.5 was closely attached to and internalized by lung cancer cells. Moreover, PM_2.5 increased the production of ARNT2 and the inactivation of the tumor suppressor B56γ-PP2A, which was followed by the activation of ^ps727STAT3 and the enhancement of invasive ability by MMP-2. Furthermore, we took advantage of an orthotopic lung carcinoma metastasis mouse model to illustrate the prometastatic effect of PM_2.5 in vivo; our results suggested that the ARNT2/PP2A/STAT3/MMP-2 cascade played a key role in PM_2.5-related oncogenicity. Finally, we observed that PM_2.5 was deposited in human lung carcinoma tissues, indicating that this potential pathway may also be involved in human lung carcinoma. These findings demonstrated that fine particulate matter, or PM_2.5, promoted the invasion of lung cancer cells via an ARNT2/PP2A/STAT3/MMP2 pathway, which may be targeted to alleviate the tumorigenic effect of PM_2.5 in lung cancer.

Nuclear receptor coactivator SRC-1 interacts with the Q-rich subdomain of the AhR and modulates its transactivation potential

The aryl hydrocarbon receptor (AhR), a soluble cytosolic protein, mediates many of the toxic effects of TCDD and related chemicals. The toxic effects are largely cell, tissue, and promoter context dependent. Although many details of the overall dioxin signal transduction have been elucidated, the transcriptional regulation of dioxin-induced genes like cyp1A1 is not yet completely understood. Previously, we have shown that the co-regulator RIP140 is a potential AhR coactivator. In this report, the role of coactivator, SRC-1, in AhR-mediated transcriptional regulation was examined. SRC-1 increased AhR-mediated, TCDD-dependent reporter gene activity three-fold in Hepa-1 and COS-1 cells. In in vitro interaction assays, SRC-1 was found to interact with AhR but not with ARNT. SRC-1 interacted weakly with AhR in the absence of TCDD and the addition of ligand further increased SRC-1 binding to AhR. Deletional mapping studies of the AhR revealed that SRC-1 binds to the AhR transactivation domain. Finer mapping of the SRC-1-interacting subdomains in the AhR transactivation domain suggested that the Q-rich subdomain was necessary and sufficient for interaction, similar to that seen with RIP140. Using GFP-tagged constructs, SRC-1 was shown to interact with AhR in cells. Unlike RIP140, LXXLL motifs in SRC-1 were necessary for interaction with AhR in vitro and for coactivation in Hepa-1 cells. The recruitment of certain coactivators by a variety of receptors suggests possible common coactivator pools and competition among receptors for limiting coactivators. Examination of the role of SRC-1 in AhR/ARNT transactivation in ARNT-deficient mutant Hepa-1 c4 cells demonstrates that the AhR transactivation domain is sufficient for enhanced coactivation mediated by SRC-1 in the presence of a transactivation domain deleted ARNT protein.

Ranking stressor impacts on periphyton structure and function with mesocosm experiments and environmental-change forecasts

Streams are being subjected to physical, chemical, and biological stresses stemming from both natural and anthropogenic changes to the planet. In the face of limited time and resources, scientists, resource managers, and policy makers need ways to rank stressors and their impacts so that we can prioritize them from the most to least important (i.e., perform 'ecological triage'). We report results from an experiment in which we established a periphyton community from the Huron River (Michigan, USA) in 84 experimental 'flumes' (stream mesocosms). We then dosed the flumes with gradients of six common stressors (increased temperature, taxa extinctions, sedimentation, nitrogen, phosphorus, and road salt) and monitored periphyton structure and function. A set of a priori deterministic functions were fit to each stressor-endpoint response and model averaging based on AICc weights was used to develop concentration-response best-fit predictions. Model predictions from different stressors were then compared to forecasts of future environmental change to rank stressors according to the potential magnitude of impacts. All of the stressors studied altered at least one characteristic of the periphyton; however, the extent (i.e., structural and functional changes) and magnitude of effects expected under future forecasts differed significantly among stressors. Elevated nitrogen concentrations are projected to have the greatest combined effect on stream periphyton structure and function. Extinction, sediment, and phosphorus all had similar but less substantial impact on the periphyton (e.g., affected only structure not function, smaller magnitude change). Elevated temperature and salt both had measurable effects on periphyton, but their overall impacts were much lower than any of the other stressors. For periphyton in the Huron River, our results suggest that, among the stressors examined, increased N pollution may have the greatest potential to alter the structure and function of the periphyton community, and managers should prioritize reducing anthropogenic sources of nitrogen. Our study demonstrates an experimental approach to ecological triage that can be used as an additional line of evidence to prioritize management decisions for specific ecosystems in the face of ecological change.

Metabolic adaptation of cancer and immune cells mediated by hypoxia-inducible factors

Cancer cells are characterized by high metabolic demand. The substrates in demand include oxygen, glucose, glutamine and lipids. Oxygen serves as a key substrate in cellular metabolism and bioenergetics. Hypoxia or low oxygen abundance is a common feature of the tumor microenvironment that occurs due to an imbalance in supply and demand. Many of the metabolic responses to hypoxia in both cancer cells and stromal cells are orchestrated by hypoxia-inducible factors (HIFs). In this review we summarize our current understanding of how HIFs modulate the metabolism of hypoxic cancer cells and immune cells, and how altered metabolism plays a role in cancer progression.

Ah receptor: a novel ligand-activated transcription factor

Two years have passed since the Ah receptor (AhR) and the AhR nuclear translocator (Arnt) were revealed to constitute a novel ligand-activated transcription factor by molecular biological studies on cloned cDNAs. Many interesting properties of the AhR/Arnt system have begun to be elucidated. We present here a brief summary of the molecular biology of AhR and Arnt.

Effects of hydroxy-polychlorinated biphenyl (OH-PCB) congeners on the xenobiotic biotransformation gene expression patterns in primary culture of Atlantic salmon (Salmo salar) hepatocytes

Hydroxylated metabolites of PCBs [OH-PCBs] represent new health and environmental concern because they have been shown to have agonist or antagonist interactions with hormone receptors (HRs) or hormone-receptor mediated responses. The present study was designed to investigate the estrogenic potency based on anti-AhR signalling effect of three 4-OH substituted PCB congeners (#107, #146 and #187), one 3-OH substituted congener (#138), and the pharmaceutical synthetic estrogen, ethynylestradiol (EE2) in fish in vitro system using primary culture of Atlantic salmon hepatocytes. The effects were studied by quantifying changes in transcripts with gene-sequence primer pairs for a suite of gene responses (AhRalpha, ARNT, CYP1A1, CYP3A, UGT and GST) belonging to the xenobiotic biotransformation system. Our data show that OH-PCB congeners and EE2, decreased AhRalpha and ARNT transcript levels, and CYP1A1, UGT and GST gene expressions, together with CYP3A gene expression. The decreased expression of transcripts for xenobiotic biotransformation system is related to the concentration of individual OH-PCB congener and these responses are typical of reported estrogenic and estrogen-like effects on the CYP system. Modulation of biotransformation pathways by OH-PCBs may alter xenobiotic metabolism leading to the production of toxic reactive molecules, altering pharmacokinetics and diminishing the clearance rate of individual chemicals from the organism.

The role of carbonic anhydrase IX overexpression in kidney cancer

Carbonic anhydrase IX (CA IX) is a membrane isoenzyme, the overexpression of which is associated with clear cell carcinoma of the kidney. Its overexpression is restricted mainly to cancer, as it is absent in corresponding normal tissues making it a potential cancer biomarker. Several recent studies have shown that CA IX, apart from its classical enzyme activity of reversibly hydrating carbon dioxide extracellularly to facilitate the net extrusion of protons from inside to outside the cell, it can also be a key player in the modulation of cell adhesion processes and participate in the regulation of cell proliferation in response to hypoxic environment to ultimately contribute to tumour progression. Here, we have shown that the sole tyrosine moiety of CA IX present in its intracellular domain can be phosphorylated in an epidermal growth factor dependent manner, suggesting that it can feed into the growth factor receptor dependent signalling pathways. Our studies suggest that the tyrosine phosphorylated CA IX can interact with the regulatory subunit of PI-3-Kinase, contributing to Akt activation. These studies have revealed a positive feed back loop that can form the basis of a vicious cycle that could contribute to the progression of clear cell renal carcinoma and poor prognosis. These studies show that CA IX signalling may be a part of both the hypoxia driven and hypoxia independent pathways that occur in the cancer cell. Finally, our studies emphasize the need for a more refined strategy using signal transduction therapeutics to inhibit the cell surface carbonic anhydrases for the management of this malignancy.

Binding and regulation of hypoxia-inducible factor-1 by the inhibitory PAS proteins

Hypoxia-inducible factor-1 (HIF-1), which consists of oxygen-sensitive HIF-1alpha and constitutively expressed HIF-1beta subunits, activates transcription of genes encoding proteins that mediate adaptive responses to reduced oxygen availability. The mouse inhibitory PAS (Per/Arnt/Sim) domain protein (IPAS) functions as a negative regulator in HIF-mediated gene expression. In this report, we cloned the human orthologs of the mouse IPAS gene, IPASH1 and IPASH2, to further study the regulatory mechanism of HIF-1 by the IPAS proteins. The human IPAS proteins inhibited the transactivation function of HIF-1alpha under hypoxic conditions. In addition, human IPAS proteins blocked the hypoxia-induced VEGF expression and inhibited cell migration and tube formation of human umbilical vein endothelial cells. Interestingly, both HIF-1alpha and HIF-1beta interacted with the IPAS proteins. Collectively, these results suggest that human IPAS proteins inhibit angiogenesis by binding to and inhibiting HIF-1alpha and HIF-1beta.

Investigation of the relationship between protein-protein interaction and catalytic activity of a heme-regulated phosphodiesterase from Escherichia coli (Ec DOS) by protein microarray

Ec DOS, a heme-regulated phosphodiesterase from Escherichia coli, is composed of an N-terminal heme-bound PAS domain and a C-terminal phosphodiesterase domain. The heme redox state in the PAS domain regulates Ec DOS phosphodiesterase activity. Interestingly, the isolated heme-bound PAS fragment enhances phosphodiesterase activity of full-length Ec DOS. The enhancement is also regulated by the heme redox state of the isolated PAS domain. In the present study, we used a newly developed protein microarray system to examine the relationship between catalytic activity and the interaction of full-length Ec DOS and the isolated PAS fragment. Adenosine 3',5'-cyclic monophosphate (cAMP), a substrate of the Ec DOS phosphodiesterase, was found to be indispensable for the interaction between Ec DOS and the PAS fragment, and two phosphodiesterase inhibitors, 3-isobutyl-methyl-xanthine and etazolate hydrochloride, hindered the interaction. In addition, an enzyme with a mutation in the putative cAMP-binding sites (H590 and H594) was unable to interact with Ec DOS and lacked enzymatic activity. These results strongly suggest a close relationship between Ec DOS phosphodiesterase activity and interaction with the isolated PAS fragment. Therefore, this study provides insights into the mechanism of how the isolated PAS domain activates Ec DOS, which has important implications for the general role of the isolated PAS domain in cells. Moreover, we found that multiple microscale analyses using the protein microarray system had several advantages over conventional affinity column methods, including the quantity of protein needed, the sensitivity, the variability of immobilized protein, and the time required for the experiment.

Loss of ARNT/HIF1beta mediates altered gene expression and pancreatic-islet dysfunction in human type 2 diabetes

beta cell dysfunction is a central component of the pathogenesis of type 2 diabetes. Using oligonucleotide microarrays and real-time PCR of pancreatic islets isolated from humans with type 2 diabetes versus normal glucose-tolerant controls, we identified multiple changes in expression of genes known to be important in beta cell function, including major decreases in expression of HNF4alpha, insulin receptor, IRS2, Akt2, and several glucose-metabolic-pathway genes. There was also a 90% decrease in expression of the transcription factor ARNT. Reducing ARNT levels in Min6 cells with small interfering RNA (siRNA) resulted in markedly impaired glucose-stimulated insulin release and changes in gene expression similar to those in human type 2 islets. Likewise, beta cell-specific ARNT knockout mice exhibited abnormal glucose tolerance, impaired insulin secretion, and changes in islet gene expression that mimicked those in human diabetic islets. Together, these data suggest an important role for decreased ARNT and altered gene expression in the impaired islet function of human type 2 diabetes.

The aryl hydrocarbon receptor is a regulator of cigarette smoke induction of the cyclooxygenase and prostaglandin pathways in human lung fibroblasts

Cigarette smoking can lead to chronic lung inflammation and lung cancer. Chronic inflammation, associated with expression of cyclooxygenase-2 (COX-2) and prostaglandins, predisposes to malignancy. We recently demonstrated that human lung fibroblasts are activated by cigarette smoke to express COX-2 and prostaglandin E(2) (PGE(2)). Little is known about the mechanism whereby smoke activates human lung fibroblasts to produce proinflammatory mediators. Herein, we report the central role of the aryl hydrocarbon receptor (AHR) in cigarette smoke extract (CSE)-induced COX-2, microsomal PGE(2) synthase (mPGES), and PGE(2) production in human lung fibroblasts. Western blot analysis revealed that primary strains of human lung fibroblasts express AHR and aryl hydrocarbon nuclear translocator protein, supporting the possibility that smoke activates lung fibroblasts through this pathway. Experiments were subsequently performed to determine whether the AHR was activated by CSE. Immunocytochemistry and EMSA analysis revealed that CSE induced nuclear translocation of the AHR in human lung fibroblasts. CSE decreased protein levels of the AHR, consistent with AHR ligand-induced proteosome-mediated degradation. CSE also induced mPGES-1 and COX-2 protein and increased PGE(2) production. Treatment of human fibroblasts with AHR antagonists in the presence of CSE inhibited AHR nuclear translocation as well as COX-2, mPGES-1, and PGE(2) production. These data indicate that the AHR pathway plays an important role in cigarette smoke-mediated COX-2 and PG production in human lung fibroblasts and may contribute to tobacco-associated inflammation and lung disease.

Differentiation of pluripotent C3H10T1/2 cells rapidly elevates CYP1B1 through a novel process that overcomes a loss of Ah Receptor

Stimulation of C3H10T1/2 cells by an adipogenic hormonal mixture (IDM) consisting of insulin (I), dexamethasone (D), and methylisobutylxanthine (M) substantially induces cytochrome P450 (CYP) 1B1 expression. This stimulation represents up to 40% of the level produced by maximum activation of the arylhydrocarbon receptor (AhR) with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Dexamethasone and methylisobutylxanthine in combination produced near maximum elevation of CYP1B1 along with a subsequent decline in AhR that paralleled the rise in peroxisome proliferator-activated receptorgamma1 (PPARgamma1). Inhibitors of AhR activity, which block TCDD induction, did not affect this increase of CYP1B1 expression, which was, therefore, independent of AhR activity. These responses were unaffected by inhibition of DNA synthesis, which was required for PPARgamma1 induction and terminal differentiation. Induction of CYP1B1 mRNA was paralleled by increased CYP1B1 promoter-luciferase reporter activity. The initial 0.8kb of promoter region, which was sufficient for 24h near maximum stimulation, did not contain either the key AhR-responsive elements that mediate the TCDD response or CREB and SF1 elements that mediate cAMP stimulation of rat CYP1B1 in steroidogenic cells. This reporter response to IDM stimulation, but not to TCDD, was maintained in AhR-null fibroblasts. CYP1B1 expression, unlike TCDD induction, was stimulated by IDM in only about half the cells. CYP1B1 expression partially overlapped with PPARgamma expression, which was also inversely related in clonal sub-lines. CYP1B1 expression may, therefore, represent an early stage of differentiation that requires factors associated with DNA synthesis to subsequently generate PPARgamma1.

Hypoxic preconditioning protects against ischemic brain injury

Animals exposed to brief periods of moderate hypoxia (8% to 10% oxygen for 3 hours) are protected against cerebral and cardiac ischemia between 1 and 2 days later. This hypoxia preconditioning requires new RNA and protein synthesis. The mechanism of this hypoxia-induced tolerance correlates with the induction of the hypoxia-inducible factor (HIF), a transcription factor heterodimeric complex composed of inducible HIF-1alpha and constitutive HIF-1beta proteins that bind to the hypoxia response elements in a number of HIF target genes. Our recent studies show that HIF-1alpha correlates with hypoxia induced tolerance in neonatal rat brain. HIF target genes, also induced following hypoxia-induced tolerance, include vascular endothelial growth factor, erythropoietin, glucose transporters, glycolytic enzymes, and many other genes. Some or all of these genes may contribute to hypoxia-induced protection against ischemia. HIF induction of the glycolytic enzymes accounts in part for the Pasteur effect in brain and other tissues. Hypoxia-induced tolerance is not likely to be equivalent to treatment with a single HIF target gene protein since other transcription factors including Egr-1 (NGFI-A) have been implicated in hypoxia regulation of gene expression. Understanding the mechanisms and genes involved in hypoxic tolerance may provide new therapeutic targets to treat ischemic injury and enhance recovery.

The Transactivation Domain of the Ah Receptor Is a Key Determinant of Cellular Localization and Ligand-Independent Nucleocytoplasmic Shuttling Properties†

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that regulates transcription of a number of target genes upon binding ligands such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Large intra- and interspecies variations exist with respect to sensitivity to TCDD, and this could, at least in part, be due to a considerable variation in the AhR amino acid sequence between species. The N-terminal half of the AhR is well-conserved across species, whereas the C-terminal half exhibits a considerable degree of degeneracy. It has previously been shown that there are differences between the mouse (mAhR) and human AhR (hAhR) in terms of cellular localization, nucleocytoplasmic shuttling, the effect of chaperone proteins on these properties, and differences in relative ligand affinity. In this study, two chimeras were generated such that each had the N-terminal half of one receptor and the C-terminal half of the other receptor. The C-terminal half of the receptor, containing the transactivation domain, determines the cellular localization of the transiently transfected receptor and regulates the ability of hepatitis B virus X-associated protein 2 (XAP2) to inhibit ligand-independent nuclear import of AhR. In addition, the transactivation domain (TAD) appears to determine the presence of XAP2 in the nuclear ligand-bound AhR/hsp90 complex prior to association with the AhR nuclear translocator protein (ARNT). However, the transactivation domain does not appear to play a role in determining relative ligand affinity of the receptor, and mAhR and hAhR have similar overall transactivation potential in a cell-based reporter system at a saturating dose of ligand. This study demonstrates for the first time that the transactivation domain of the AhR influences important biochemical properties of the N-terminal half of the AhR, and the degeneracy in the transactivation domain between the mAhR and the hAhR results in species-specific differences in receptor properties.

Diabetic pancreatic beta cells ARNT all they should be

A complex network of interacting transcription factors plays a critical role in normal pancreatic beta cell function, with mutations in certain transcription factor genes known to cause diabetes. In a recent issue of Cell, Gunton et al.(2005) demonstrate a role for the transcription factor ARNT/HIF1beta (hydrocarbon nuclear receptor translocator/hypoxia-inducible factor 1 beta) in normal beta cell function. ARNT expression is reduced in diabetic human islets and beta cell-specific ARNT knockout mice show the impaired glucose tolerance and abnormal insulin secretion that are characteristic of type 2 diabetes.

Hypoxia-inducible factor-dependent histone deacetylase activity determines stem cell fate in the placenta

Hypoxia-inducible factor (HIF) is a heterodimeric transcription factor composed of HIFα and the arylhydrocarbon receptor nuclear translocator(ARNT/HIF1β). Previously, we have reported that ARNT function is required for murine placental development. Here, we used cultured trophoblast stem (TS)cells to investigate the molecular basis of this requirement. In vitro, wild-type TS cell differentiation is largely restricted to spongiotrophoblasts and giant cells. Interestingly, Arnt-null TS cells differentiated into chorionic trophoblasts and syncytiotrophoblasts, as demonstrated by their expression of Tfeb, glial cells missing 1 (Gcm1) and the HIV receptor CXCR4. During this process, a region of the differentiating Arnt-null TS cells underwent granzyme B-mediated apoptosis,suggesting a role for this pathway in murine syncytiotrophoblast turnover. Surprisingly, HIF1α and HIF2α were induced during TS cell differentiation in 20% O2; additionally, pVHL levels were modulated during the same time period. These results suggest that oxygen-independent HIF functions are crucial to this differentiation process. As histone deacetylase(HDAC) activity has been linked to HIF-dependent gene expression, we investigated whether ARNT deficiency affects this epigenetic regulator. Interestingly, Arnt-null TS cells had reduced HDAC activity,increased global histone acetylation, and altered class II HDAC subcellular localization. In wild-type TS cells, inhibition of HDAC activity recapitulated the Arnt-null phenotype, suggesting that crosstalk between the HIFs and the HDACs is required for normal trophoblast differentiation. Thus, the HIFs play important roles in modulating the developmental plasticity of stem cells by integrating physiological, transcriptional and epigenetic inputs.

Topotecan blocks hypoxia-inducible factor-1alpha and vascular endothelial growth factor expression induced by insulin-like growth factor-I in neuroblastoma cells

The extent of angiogenesis and/or vascular endothelial growth factor (VEGF) expression in neuroblastoma tumors correlates with metastases, N-myc amplification, and poor clinical outcome. Understanding the mechanisms regulating VEGF expression in neuroblastoma cells provides additional therapeutic options to control neuroblastoma tumor growth. VEGF mRNA is controlled by growth factors and hypoxia via the transcription factor hypoxia-inducible factor (HIF-1alpha). HIF-1alpha protein levels are regulated by the von Hippel Lindau tumor suppressor gene, VHL, which targets HIF-1alpha degradation. To determine whether the levels of VEGF in neuroblastomas are due to mutations in VHL, we evaluated genomic DNA from 15 neuroblastoma cell lines using PCR. We found no mutations in exons 1, 2, or 3 of the VHL gene. VEGF mRNA levels in neuroblastoma cells cultured in serum-free medium increased after 8 to 16 hours in serum, insulin-like growth factor-I (IGF-I), epidermal growth factor, or platelet-derived growth factor. Serum/IGF-I induced increases in HIF-1alpha protein that temporally paralleled increases in VEGF mRNA, whereas HIF-1beta levels were unaffected. VEGF and HIF-1alpha levels were blocked by inhibitors of phosphatidylinositol 3-kinase and mammalian target of rapamycin. Furthermore, we confirmed that HIF-1alpha mediates approximately 40% of the growth factor activity stimulating VEGF protein expression. Topotecan blocked the IGF-I-stimulated increase in HIF-1alpha but not HIF-1beta, and this resulted in a decrease in VEGF in four neuroblastoma cell lines tested. These data indicate that growth factors in an autocrine or paracrine manner play a major role in regulating VEGF levels in neuroblastoma cells and that targeted therapies to phosphatidylinositol 3-kinase, mammalian target of rapamycin, and/or HIF-1alpha have the potential to inhibit VEGF expression and limit neuroblastoma tumor growth.

Aryl hydrocarbon receptor (AHR) and AHR nuclear translocator (ARNT) expression in Baikal seal (Pusa sibirica) and association with 2,3,7,8-TCDD toxic equivalents and CYP1 expression levels

Most toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related planar halogenated aromatic hydrocarbons (PHAHs) are mediated by ligand-activated aryl hydrocarbon receptor (AHR) signaling pathway. To understand the regulation mechanism of AHR and AHR nuclear translocator (ARNT) expression in wild Baikal seal (Pusa sibirica) population contaminated by PHAHs, the present study investigated hepatic mRNA expression levels of AHR and its heterodimer, ARNT genes, in association with biological index (age, gender and body weight), PHAH accumulation and expression levels of cytochrome P450 (CYP) 1A and 1B. While there was no gender difference, the AHR mRNA expression levels were increased with ages (p = 0.014) and body weights (p = 0.015), indicating that AHR expression might be affected by these biological factors. The AHR mRNA expression levels exhibited significant positive correlations with total TEQs and most of individual congener TEQs derived from polychorinated dibenzo-p-dioxins, dibenzofurans and non-ortho coplanar polychorinated biphenyls (PCBs), indicating the transcriptional up-regulation of AHR expression by these congeners. On the other hand, there was no significant correlation between individual TEQs from mono-ortho coplanar PCBs and AHR expression. These results imply the structure-related transcriptional activity of AHR among PHAHs congeners. AHR mRNA levels showed positive correlations with both CYP1A protein (p = 0.039) and CYP1A1 mRNA expression levels (p = 0.046). In contrast to AHR expression, neither the total nor individual congener TEQs influenced ARNT at the transcriptional level. ARNT mRNA showed significant negative correlations with CYP1A/1B protein (p = 0.027 and p = 0.006) and CYP1A1 mRNA expression levels (p = 0.039), implying the existence of different transcriptional regulation between AHR and ARNT genes and negative regulation by CYP1A/1B-mediated signaling pathways. The present findings may render significant insight on the basic mechanisms underlying regulation of AHR and ARNT expressions associated with biological factors and PHAH exposure in wild mammalian populations.

ERα-AHR-ARNT Protein-Protein Interactions Mediate Estradiol-dependent Transrepression of Dioxin-inducible Gene Transcription

The aryl hydrocarbon receptor (AHR) and the aryl hydrocarbon receptor nuclear translocator (ARNT) form a heterodimeric transcription factor upon binding a wide variety of environmental pollutants, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). AHR target gene activation can be repressed by estrogen and estrogen-like compounds. In this study, we demonstrate that a significant component of TCDD-inducible Cyp1a1 transcription is the result of recruitment of estrogen receptor (ER)-alpha by AHR/ARNT as a transcriptional co-repressor. Both AHR and ARNT were capable of interacting directly with ER alpha, as ascertained by glutathione S-transferase pull-down. 17Beta-estradiol repressed TCDD-activated Cyp1a1 and Cyp1b1 gene transcription in MCF-7 cells in the presence of cycloheximide, as determined by reverse transcription/real-time PCR. Furthermore, chromatin immunoprecipitation (ChIP) assays have shown that ER alpha is present at the Cyp1a1 enhancer only after co-treatment with E2 and TCDD, in MCF-7 cells. Sequential two-step ChIP assays were performed which demonstrate that AHR and ER alpha are present together at the same time on the Cyp1a1 enhancer during transrepression. Taken together these data support a role for ER-mediated transrepression of AHR-dependent gene regulation.

Bacterially produced human HIF-1α is competent for heterodimerization and specific DNA-binding

Hypoxia-inducible factor 1alpha (HIF-1alpha) is the regulatory subunit of HIF-1, the transcriptional activator and key mediator of the cellular response to hypoxia. Regulation of HIF-1alpha occurs at multiple levels and involves several different post-translational modifications. In order to examine the importance of these modifications for the basic function of HIF-1alpha, we have produced in bacteria recombinant full-length human HIF-1alpha using different expression systems. We show that this unmodified form of HIF-1alpha is able to form a stable heterodimer with the second subunit of HIF-1 (HIF-1beta or ARNT) when both proteins are co-expressed in Escherichia coli. Furthermore, this bacterially reconstituted heterodimer exhibits specific DNA-binding activity. These data indicate that post-translational modification of HIF-1alpha is not essential for its interaction with ARNT and DNA, and provide an in vitro system for the characterization of the molecular properties of HIF-1alpha.

Inactivation of the arylhydrocarbon receptor nuclear translocator (Arnt) suppresses von Hippel-Lindau disease-associated vascular tumors in mice

Patients with germ line mutations in the VHL tumor suppressor gene are predisposed to the development of highly vascularized tumors within multiple tissues. Loss of pVHL results in constitutive activation of the transcription factors HIF-1 and HIF-2, whose relative contributions to the pathogenesis of the VHL phenotype have yet to be defined. In order to examine the role of HIF in von Hippel-Lindau (VHL)-associated vascular tumorigenesis, we utilized Cre-loxP-mediated recombination to inactivate hypoxia-inducible factor-1alpha (Hif-1alpha) and arylhydrocarbon receptor nuclear translocator (Arnt) genes in a VHL mouse model of cavernous liver hemangiomas and polycythemia. Deletion of Hif-1alpha did not affect the development of vascular tumors and polycythemia, nor did it suppress the increased expression of vascular endothelial growth factor (Vegf) and erythropoietin (Epo). In contrast, phosphoglycerokinase (Pgk) expression was substantially decreased, providing evidence for target gene-dependent functional redundancy between different Hif transcription factors. Inactivation of Arnt completely suppressed the development of hemangiomas, polycythemia, and Hif-induced gene expression. Here, we demonstrate genetically that the development of VHL-associated vascular tumors in the liver depends on functional ARNT. Furthermore, we provide evidence that individual HIF transcription factors may play distinct roles in the development of specific VHL disease manifestations.

Hypoxia inducible factor pathways as targets for functional foods

The etiology of most chronic angiogenic diseases such as rheumatoid arthritis, atherosclerosis, diabetes complications, and cancer includes the presence of pockets of hypoxic cells growing behind aerobic cells and away from blood vessels. Hypoxic cells are the result of uncontrolled growth and insufficient vascularization and have undergone a shift from aerobic to anaerobic metabolism. Cells respond to hypoxia by stimulating the expression of hypoxia inducible factor (HIF), which is critical for survival under hypoxic conditions and in embryogenesis. HIF is a heterodimer consisting of the O2-regulated subunit, HIF-1alpha, and the constitutively expressed aryl hydrocarbon receptor nuclear translocator, HIF-1beta. Under hypoxic conditions, HIF-1alpha is stable, accumulates, and migrates to the nucleus where it binds to HIF-1beta to form the complex (HIF-1alpha + HIF-1beta). Transcription is initiated by the binding of the complex (HIF-1alpha + HIF-1beta) to hypoxia responsive elements (HREs). The complex [(HIF-1alpha + HIF-1beta) + HREs] stimulates the expression of genes involved in angiogenesis, anaerobic metabolism, vascular permeability, and inflammation. Experimental and clinical evidence show that these hypoxic cells are the most aggressive and difficult angiogenic disease cells to treat and are a major reason for antiangiogenic and conventional treatment failure. Hypoxia occurs in early stages of disease development (before metastasis), activates angiogenesis, and stimulates vascular remodeling. HIF-1alpha has also been identified under aerobic conditions in certain types of cancer. This review summarizes the role of hypoxia in some chronic degenerative angiogenic diseases and discusses potential functional foods to target the HIF-1alpha pathways under hypoxic and normoxic conditions. It is reported that dietary quinones, semiquinones, phenolics, vitamins, amino acids, isoprenoids, and vasoactive compounds can down-regulate the HIF-1 pathways and therefore the expression of several proangiogenic factors. Considering the lack of efficiency or the side effects of synthetic antiangiogenic drugs at clinical trials, down-regulation of hypoxia-induced angiogenesis by use of naturally occurring functional foods may provide an effective means of prevention.

Novel function of neuronal PAS domain protein 1 in erythropoietin expression in neuronal cells

The basic helix loop helix-PAS (bHLH-PAS) transcription factors have diverse roles in physiologic responses to the environment and in early development. One bHLH-PAS protein, neuronal PAS domain protein 1 (NPAS1), is reported to be expressed only in the central nervous system beginning at the late embryonic stage, but its function is unknown. Using an anti-NPAS1 antibody, we have shown that NPAS1 expression in cerebral cortex was observed first around embryonic Day 16.5 (E16.5) and was then dispersed throughout the region as cortical development progressed. From the similarity with hypoxia-inducible factor-1alpha (HIF-1alpha), we investigated whether NPAS1 regulates transcription of erythropoietin (EPO), which is the target of HIF-1alpha and is expressed in the brain during the early developmental stage. In the present study, we have shown that NPAS1 binds to the enhancer region of Epo in vivo. A luciferase reporter assay revealed a repressive effect of NPAS1 on hypoxia-responsive element-regulated gene expression. These results raise the possibility that NPAS1 plays a role in late central nervous system development by modulating EPO expression in response to cellular oxygen level.

A Hypoxia-Independent Hypoxia-Inducible Factor-1 Activation Pathway Induced by Phosphatidylinositol-3 Kinase/Akt in HER2 Overexpressing Cells

HER2 overexpression, a known prognostic factor in many human cancers, can activate phosphatidylinositol-3 kinase (PI-3K)/Akt pathways and plays an important role in mediating cell survival and tumor development. Hypoxia-inducible factors (HIFs) promote angiogenesis and energy metabolism and thereby enhance tumor growth and metastasis. HIFs, composed of alpha and beta subunits, are activated in most human cancers, including those that overexpress HER2. Previous reports have suggested that increased PI-3K/Akt or decreased PTEN activity may activate the HIF pathway in various tumors, but the detailed mechanism is still not completely understood. Here we reported an interaction between the HIF and PI-3K/Akt pathways in HER2-overexpressing cancer cells. Our results indicate that HER2 overexpression, which results in constitutively active Akt, turns on HIF-1alpha independently of hypoxia, and this activation is weaker than that under hypoxic condition. Further investigation showed that Akt is required for the hypoxia-independent HIF activity. The PI-3K/Akt pathway did not affect the HIF-1alpha binding with its E3 ligase von Hippel-Lindau but enhanced the binding affinity between the HIF-1 alpha and beta subunits. Furthermore, we found that Akt interacts with HIF-1beta and regulates HIF activity. Our results indicated that HER2 can induce HIF activation via the activation of Akt suggesting that activation of HER2/Akt pathway may promote angiogenesis independent of hypoxia, which may have important implications for the oncogenic activity of HER2 and Akt.

Analysis of the transcriptional activation domain of the Drosophila tango bHLH-PAS transcription factor

Basic-helix-loop-helix-PAS transcription factors play important roles in diverse biological processes including cellular differentiation and specification, oxygen tension regulation and dioxin metabolism. Drosophila tango is orthologous to mammalian Arnt and acts as a common dimerization partner for bHLH-PAS proteins during embryogenesis. A transient transfection assay using Drosophila S2 tissue culture cells and wild-type and mutant Drosophila tango cDNAs was used to localize the activation domain of the Tango protein. An activation domain was identified in the C-terminus of TGO consisting of poly-glutamine and histidine-proline repeats. Transcriptional activation of the fibroblast growth factor receptor (breathless) gene required an intact TGO C-terminus, in vitro. Co-expression assays of trachealess and tgo in the developing eye imaginal disc showed a requirement for the C-terminal transactivation domain of TGO for a cellular response. Genetic analysis of tgo(3) shows that the paired repeat is necessary for tracheal tubule formation in all branches. Lastly, expression of a C-terminal truncated tgo transgene specifically in the CNS midline and trachea resulted in reductions in the number of breathless-expressing cells. These results together identify TGO's transactivation domain and establish its importance for proper target gene regulation and cellular specification.

Sanguinarine activates polycyclic aromatic hydrocarbon associated metabolic pathways in human oral keratinocytes and tissues

Sanguinarine's use in human clinical applications is currently controversial. While some studies have demonstrated sanguinarine's anti-inflammatory and anti-oxidant properties, other investigations reported sanguinarine's procarcinogenic effects. Like the tobacco-associated carcinogen, benzo(a)pyrene (B(a)P), sanguinarine is a polycyclic aromatic hydrocarbon (PAH). PAH exposure activates the aryl hydrocarbon transcription activating factor (AhR), resulting in nuclear translocation, binding to the aryl hydrocarbon nuclear translocator (ARNT), which thereby increases expression of a pool of carcinogen metabolizing enzymes. The goal of this study was to investigate whether sanguinarine activates this PAH-associated signaling cascade in human oral cells and tissues. Our results demonstrate that sanguinarine: (i) results in formation of the AhR-ARNT complex, (ii) induces AhR-associated gene expression, (iii) inhibits cytochrome P450 1A1 (CYP 1A1) microsomal oxidative activity and (iv) pretreatment upregulates CYP 1A1 function. Collectively, these data provide evidence that sanguinarine activates PAH-associated signaling and metabolic pathways. Notably, previous studies have demonstrated that mammalian hepatic microsomes metabolize sanguinarine to a mutagenic epoxide. Persons who respond to sanguinarine exposure with induction of primarily Phase I relative to Phase II enzymes are, therefore, at risk for sanguinarine bioactivation and its potential mutagenic effects.

Intestinal glucose-dependent expression of glucose-6-phosphatase: involvement of the aryl receptor nuclear translocator transcription factor

Glucose-6-phosphatase (G6Pase) catalyzes the release of glucose from glucose 6-phosphate. This enzyme was mainly studied in the liver, but while detected in the small intestine little is known about the regulation of its intestinal expression. This study describes the mechanisms of the glucose-dependent regulation of G6Pase expression in intestinal cells. Results obtained in vivo and in Caco-2/TC7 enterocytes showed that glucose increases the G6Pase mRNA level. In Caco-2/TC7 cells, glucose stabilized G6Pase mRNA and activated the transcription of the gene, meaning that glucose-dependent G6Pase expression involved both transcriptional and post-transcriptional mechanisms. Reporter-gene studies showed that, although the -299/+57 region of the human G6Pase promoter was sufficient to trigger the glucose response in the hepatoma cell line HepG2, the -1157/-1133 fragment was required for maximal activation of glucose-6-phosphatase gene transcription in Caco-2/TC7 cells. This fragment binds the aryl receptor nuclear translocator (ARNT), cAMP-responsive element-binding protein, and upstream stimulatory factor transcription factors. The DNA binding activity of these transcription factors was increased in nuclear extracts of differentiated cells from the intestinal villus of mice fed sugar-rich diets as compared with mice fed a no-sugar diet. A direct implication of ARNT in the activation of G6Pase gene transcription by glucose has been observed in Caco-2/TC7 cells using RNA interference experiments. These results support a physiological role for G6Pase in the control of nutrient absorption in the small intestine.

Interaction of the PAS B domain with HSP90 accelerates hypoxia-inducible factor-1alpha stabilization

Hypoxia-inducible factor (HIF) alpha subunits are induced under hypoxic conditions, when limited oxygen supply prevents prolyl hydroxylation-dependent binding of the ubiquitin ligase pVHL and subsequent proteasomal degradation. A short normoxic half-life of HIF-alpha and a very rapid hypoxic protein stabilization are crucial to the cellular adaptation to changing oxygen supply. However, the molecular requirements for the unusually rapid mechanisms of protein synthesis, folding and nuclear translocation are not well understood. We and others previously found that the chaperone heat-shock protein 90 (HSP90) can interact with HIF-1alpha in vitro. Here we show that HSP90 also interacts with HIF-2alpha and HIF-3alpha, suggesting a general involvement of HSP90 in HIF-alpha stabilization. The PAS B domain, common to all three alpha subunits, was required for HSP90 interaction. ARNT competed with HSP90 for binding to the PAS B domain since an excess of either component inhibited the activity of the other. HSP90 as well as the heterocomplex members HSP70 and p23, but not HSP40, were detected in immunoprecipitations of endogenous cellular HIF-1alpha. While HSP90 and HSP70 bound to HIF-1alpha predominantly under normoxic conditions, ARNT bound to HIF-1alpha primarily under hypoxic conditions, suggesting that ARNT displaced HSP90 from HIF-1alpha following nuclear translocation. Hypoxic accumulation of HIF-1alpha was delayed in a novel cell model deficient for HSP90beta as well as after treatment of wild-type cells with the HSP90 inhibitor geldanamycin, suggesting that HSP90 activity is involved in the rapid HIF-1alpha protein induction.

Hypoxia-inducible factor regulates alphavbeta3 integrin cell surface expression

Hypoxia-inducible factor (HIF)-deficient placentas exhibit a number of defects, including changes in cell fate adoption, lack of fetal angiogenesis, hypocellularity, and poor invasion into maternal tissue. HIF is a heterodimeric transcription factor consisting of alpha and beta aryl hydrocarbon receptor nuclear translocator or ARNT) subunits. We used undifferentiated trophoblast stem (TS) cells to characterize HIF-dependent adhesion, migration, and invasion. Arnt(-/-) and Hifalpha(-/-) TS cells exhibit reduced adhesion and migration toward vitronectin compared with wild-type cells. Furthermore, this defect is associated with decreased cell surface expression of integrin alphavbeta3 and significantly decreased expression of this integrin in focal adhesions. Because of the importance of adhesion and migration in tumor progression (in addition to placental development), we examined the affect of culturing B16F0 melanoma cells in 1.5% oxygen (O(2)). Culturing B16F0 melanoma cells at 1.5% O(2) resulted in increased alphavbeta3 integrin surface expression and increased adhesion to and migration toward vitronectin. Together, these data suggest that HIF and O(2) tension influence placental invasion and tumor migration by increasing cell surface expression of alphavbeta3 integrin.

Rat hepatic CYP1A1 and CYP1A2 induction by menadione

The effects of menadione on activities and expression of cytochrome P450 (CYP) 1A subfamily (CYP1A) isozymes in rat hepatic tissue were examined. When rats were treated orally with 15 mg/kg menadione for 4 days, the elevation of hepatic CYP1A1/1A2 specific activities in microsomal preparations was detected with approximately 5.4- and 2.5-fold increase over control values for ethoxyresorufin-O-deethylase (EROD, CYP1A1) and methoxyresorufin-O-demethylase (MROD, CYP1A2) activities, respectively. CYP1A1 and CYP1A2 mRNA levels in the liver of menadione-treated rats were approximately 11.8- and 1.8-fold higher than in controls, respectively, whereas the expression of the CYP1A regulatory proteins aryl hydrocarbon-receptor (AhR) and AhR nuclear translocator (Arnt) was not changed at the mRNA level. The result of this study demonstrates that menadione induces CYP1A1/1A2 expression in vivo through either transcriptional activation and/or mRNA stabilization.

Identification of aldehyde oxidase 1 and aldehyde oxidase homologue 1 as dioxin-inducible genes

Aldehyde oxidases are a family of highly related molybdo-flavoenzymes acting upon a variety of compounds of industrial and medical importance. We have identified aldehyde oxidase 1 (AOX1) as a 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin) inducible gene in the mouse hepatoma cell line Hepa-1. AOX1 mRNA levels were not increased by dioxin in mutant derivatives of the Hepa-1 cell line lacking either functional aryl hydrocarbon receptor (AHR) or aryl hydrocarbon receptor nuclear translocator (ARNT) proteins, thus demonstrating that transcriptional induction of AOX1 in response to dioxin occurs through the AHR pathway. Dioxin induction of AOX1 mRNA was also observed in mouse liver. In addition, levels of AOX1 protein as well as those of aldehyde oxidase homologue 1 (AOH1), a recently identified homolog of AOX1, were elevated in mouse liver in response to dioxin. Employing an aldehyde oxidase specific substrate, AOX1/AOH1 activity was shown to be induced by dioxin in mouse liver. This activity was inhibited by a known inhibitor of aldehyde oxidases, and eliminated by including tungstate in the mouse diet, which is known to lead to inactivation of molybdoflavoenzymes, thus confirming that the enzymatic activity was attributable to AOX1/AOH1. Our observations thus identify two additional xenobiotic metabolizing enzymes induced by dioxin.

LE-PAS, a novel Arnt-dependent HLH-PAS protein, is expressed in limbic tissues and transactivates the CNS midline enhancer element

A growing family of helix-loop-helix PAS (HLH-PAS) transcription factors has emerged recently. These proteins form heterodimers and are involved in adaptation to environmental or physiologic stresses as well as in embryonic development. We describe the cloning and characterization of a novel HLH-PAS protein termed LE-PAS (limbic-enriched PAS) in mouse. LE-PAS consists of 802 amino acids and has a predicted molecular mass of 87.4 kDa. In adult mice, it is expressed exclusively in brain. Northern blot and in situ hybridization analysis indicate that LE-PAS is expressed in the limbic system and olfactory bulb. By in vitro overexpression in COS-7, cells we found that LE-PAS is a nuclear protein. Reporter gene analysis indicated that LE-PAS transactivates the CNS midline enhancer (CME) motif but not the xenobiotic response element in an Arnt-dependent fashion and without prior activation of LE-PAS protein. Our observation suggests that LE-PAS shares a similar mode of function with HLH-PAS proteins such as single minded or trachealess indicating that LE-PAS also has constitutive or developmental functions which may be critical for regulating the transcriptional control of limbic patterning and function.

Regulation of theCyp2a5Gene Involves an Aryl Hydrocarbon Receptor-Dependent Pathway

We have investigated the role of the aryl hydrocarbon receptor (AHR) in the regulation of the Cyp2a5 gene. The C57BL/6 and DBA/2 mouse strains with a genetically determined difference in AHR function were used to study the CYP2A5 induction by typical AHR ligands, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 3-methylcholanthrene. The CYP2A5 mRNA up-regulation in these mouse strains showed a difference in response, typical for AHR-regulated genes, both by TCDD in cultured primary hepatocytes and by 3-methylcholanthrene in vivo. In primary hepatocytes, TCDD caused a 3-fold elevation of the CYP2A5 protein level and a similar induction of the CYP2A5-catalyzed coumarin 7-hydroxylation activity. In reporter gene assays, the Cyp2a5 promoter region -3033 to +10 mediated a 2- to 5-fold induction of luciferase activity by TCDD treatment in primary hepatocytes and in Hepa-1 hepatoma cells with an intact AHR/AHR nuclear translocator (ARNT) complex. In Hepa-1 variant cell lines with deficiencies in the AHR/ARNT complex, the absence of ARNT abolished the induction. A putative AHR response element (XRE) was identified in the Cyp2a5 promoter at the position -2514 to -2492 and found to interact with the AHR/ARNT heterodimer. Transfection experiments combined with mutation of the XRE site indicated that the site partly mediates the TCDD induction of Cyp2a5. An additional AHR-dependent mechanism also regulates the proximal promoter of the Cyp2a5 gene. In conclusion, our studies showed that AHR ligands up-regulate Cyp2a5 transcriptionally by an AHR/ARNT-dependent mechanism and established Cyp2a5 as a novel AHR-regulated gene.

Hypoxia-responsive element-mediated soluble Tie2 vector exhibits an anti-angiogenic activity in vitro under hypoxic condition

Hypoxia-inducible factor-1 (HIF-1) is one of the key mammalian transcription factors and shows increased levels in both protein stability and intrinsic transcriptional activity during low oxygen tension. Hypoxia-activated functional HIF-1 protein binds to hypoxia-responsive elements (HRE) in the enhancers of several genes including VEGF, the major player in angiogenesis, and initiates their mRNA expression. The molecular mechanisms regulating the gene expression under hypoxic conditions could increase the therapeutic window of tumor-specific delivery systems. In this study, to examine hypoxia-specific production of anti-angiogenic therapeutic gene, we constructed 5 copies of HRE (5xHRE) of human VEGF linked to soluble Tie2 (sTie2) driven by minimal SV40 promoter (5xHRE/SV40/sTie2). Our data showed that under hypoxia the secreted sTie2 selectively inhibited tube formation and migration capacities of endothelial cells in vitro. Hence, we propose that the vector system, 5xHRE/SV40/sTie2, might be a useful tool for down-regulating tumor angiogenesis under hypoxic condition.

Hypoxic inhibition of 3-methylcholanthrene-induced CYP1A1 expression is independent of HIF-1alpha

Hypoxia-inducible factor-1alpha (HIF-1alpha) and aryl hydrocarbon receptor (AhR) both require dimerization with AhR nuclear translocator (ARNT) to initiate transcription of their respective target genes. It has been proposed that competition for ARNT results in decreased targeting of AhR to cytochrome P450 1A1 (CYP1A1) under hypoxia. We established primary cultures of HIF-1alpha null hepatocytes to examine the interaction between HIF-1alpha and AhR signaling. Gene expression of known HIF targets phosphoglycerate kinase (PGK), vascular endothelial growth factor (VEGF) and glucose transporter-1 (GLUT-1) increased under hypoxia, but was reduced in the HIF null cultures. Concomitant treatment of cultures with hypoxia (1% O2) and 3-methylcholanthrene (an AhR ligand) did not significantly alter HIF target gene expression. Furthermore, enzymatic activity and transcription of CYP1A1 was inhibited by hypoxia in HIF-1alpha null cultures, indicating that HIF-1alpha is not directly involved in negative regulation of AhR signaling.

Deferoxamine induces prolonged cardiac preconditioning via accumulation of oxygen radicals

Iron chelation by deferoxamine (DFO) blocks the Fenton reaction, but also inhibits prolyl hydroxylases and thereby activates certain hypoxia-inducible transcription factors (HIFs) that trigger cellular adaptation to hypoxia. Because both mechanisms may alleviate tissue damage in ischemia and reperfusion, we tried to differentiate their contribution to DFO-induced cardioprotection. Myocardial ischemia and reperfusion were induced in anesthetized Wistar rats. Infarct size was related to the ischemic area. Myocardial mRNA expression was determined by real-time PCR. Radical reactivity was probed in myocardial tissue slices with the redox-sensitive dye CM-H(2)DCFDA. Single ip applications of DFO (200 mg/kg) administered 2 h to 3 days before infarction reduced infarct size from 55 +/- 7% to 22-26%. Protection was abolished by the radical scavenger N-(2-mercaptopropionyl)glycine and the protein kinase C inhibitor chelerythrine when either was given 30 min before DFO, whereas subsequent application was ineffective. DFO did not alter the expression of various HIF target genes, whereas mRNAs of HIF-independent genes, aldose reductase and glucose transporter-4, were increased in infarcted myocardium 2 days after DFO treatment. Enhancement of superoxide activity by DFO could be demonstrated in vitro. Acute and prolonged myocardial preconditioning is triggered by DFO in response to accumulation of oxygen radicals and activation of protein kinase C.

Role of CBP in regulating HIF-1-mediated activation of transcription

The hypoxia-inducible factor-1 (HIF-1) is a key regulator of oxygen homeostasis in the cell. We have previously shown that HIF-1alpha and the transcriptional coactivator CBP colocalize in accumulation foci within the nucleus of hypoxic cells. In our further exploration of the hypoxia-dependent regulation of HIF-1alpha function by transcriptional coactivators we observed that coexpression of SRC-1 (another important coactivator of the hypoxia response) and HIF-1alpha did not change the individual characteristic nuclear distribution patterns. Colocalization of both these proteins proved to be mediated by CBP. Biochemical assays showed that depletion of CBP from cell extracts abrogated interaction between SRC-1 and HIF-1alpha. Thus, in contrast to the current model for the assembly of complexes between nuclear hormone receptors and coactivators, the present data suggest that it is CBP that recruits SRC-1 to HIF-1alpha in hypoxic cells. We also observed that CBP, HIF-1alpha/Arnt and HIF-1alpha/CBP accumulation foci partially overlap with the hyperphosphorylated form of RNA polymerase II, and that CBP had a stabilizing effect on the formation of the complex between HIF-1alpha and its DNA-binding partner, Arnt. In conclusion, CBP plays an important role as a mediator of HIF-1alpha/Arnt/CBP/SRC-1 complex formation, coordinating the temporally and hierarchically regulated intranuclear traffic of HIF-1alpha and associated cofactors in signal transduction in hypoxic cells.

Modulating the hypoxia-inducible factor signaling pathway: applications from cardiovascular disease to cancer

Humans, like other complex aerobic organisms, possess highly evolved systems for the delivery of dioxygen to all the cells of the body. These systems are regulated since excessive levels of dioxygen are toxic. In animals hypoxia causes an increase in the transcription levels of specific genes, including those encoding for vascular endothelial growth factor and erythropoietin. At the transcriptional level, the hypoxic response is mediated by hypoxia-inducible factor (HIF), an alpha,beta-heterodimeric protein. HIF-beta is constitutively present, but HIF-alpha levels are regulated by dioxygen. Under hypoxic conditions, levels of HIF-alpha rise, allowing its dimerization with HIF-beta and enabling transcriptional activation. Under normoxic conditions both the level of HIF-alpha and its ability to enable transcription are directly controlled by its post-translational oxidation by oxygenases. Hydroxylation of HIF-alpha at either of two conserved prolyl residues enables its recognition by the von Hippel-Lindau tumour suppressor protein which targets it for proteasomal degradation. Hydroxylation of an asparaginyl residue in the C-terminal transactivation domain of HIF-alpha directly prevents its interaction with the coactivator p300 from the transcription complex. Hydroxylation of HIF-alpha is catalysed by members of the iron (II) and 2-oxoglutarate dependent oxygenase family. In humans, three prolyl-hydroxylase isozymes (PHD1-3, for prolyl hydroxylase domain enzymes) and an asparaginyl hydroxylase (FIH, for factor inhibiting HIF) have been identified. Recent studies have identified additional post-translational modifications of HIF-alpha including acetylation and phosphorylation. Modulation of the HIF mediated hypoxic response is of potential use in a wide range of disease states including cardiovascular disease and cancer. Here we review current knowledge of the HIF pathway focusing on its regulation by dioxygen and discussion of potential targets and challenges in attempts to modulate the pathway for medicinal application.

Recruitment of Thyroid Hormone Receptor/Retinoblastoma-interacting Protein 230 by the Aryl Hydrocarbon Receptor Nuclear Translocator Is Required for the Transcriptional Response to Both Dioxin and Hypoxia

The aryl hydrocarbon receptor nuclear translocator/hypoxia-inducible factor (ARNT/HIF-1 beta) mediates an organism's response to various environmental cues, including those to chemical carcinogens, such as 2,3,7,8-tetrachlorodibenzo-rho-dioxin (TCDD or dioxin), via its formation of a functional transcription factor with the ligand activated aryl hydrocarbon receptor (AHR). Similarly, tissue responses to hypoxia are largely mediated through the HIF-1 heterodimeric transcription factor, comprising hypoxia-inducible factor-1 alpha (HIF-1 alpha) and ARNT. The latter response is essential for a metabolic switch from oxidative phosphorylation to glycolytic anaerobic metabolism as well as for angiogenesis and has been implicated as necessary for growth in many solid tumors. In this report, we demonstrate that the thyroid hormone receptor/retinoblastoma-interacting protein 230 (TRIP230) interacts directly with ARNT and is essential for both hypoxic and TCDD-mediated transcriptional responses. We initially identified TRIP230 as an ARNT-interacting protein in a yeast two-hybrid assay screen. This interaction was confirmed in mammalian cell systems using co-immunoprecipitation and in mammalian two-hybrid assays. Furthermore, TRIP230 could be recorded at sites of activated transcription of either TCDD- or hypoxia-inducible genes in a stimulus-dependent fashion by chromatin immunoprecipitation analysis. Finally, using single-cell microinjection and RNA interference assays, we demonstrate that TRIP230 is indispensable for TCDD- and hypoxia-dependent gene transcription.

Gestational exposure of Ahr and Arnt hypomorphs to dioxin rescues vascular development

The aryl hydrocarbon receptor (AHR) is commonly known for its role in the adaptive metabolism of xenobiotics and in the toxic events that follow exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin). Previously, we have demonstrated that the AHR and its heterodimeric partner, the AHR nuclear translocator (ARNT), play a role in the developmental closure of a hepatic vascular shunt known as the ductus venosus (DV). To investigate the mechanism of DV closure, we generated hypomorphic alleles of the Ahr and Arnt loci. Using these models, we then asked whether this vascular defect could be rescued by receptor activation during late development. By manipulating gestational exposure, the patent DV in AHR or ARNT hypomorphs could be efficiently closed by dioxin exposure as early as embryonic day 12.5 and as late as embryonic day 18.5. These findings define the temporal regulation of receptor activation during normal ontogeny and provide evidence to support the idea that receptor activation and AHR-ARNT heterodimerization are essential for normal vascular development. Taken in the broader context, these data demonstrate that similar AHR signaling steps govern all major aspects of AHR biology.

Aryl hydrocarbon receptor expression and activity in cerebellar granule neuroblasts: implications for development and dioxin neurotoxicity

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent teratogen that produces neurobehavioral abnormalities associated with both cognitive and locomotor systems, yet the precise regional and cellular targets of developmental neurotoxicity remain largely unknown. Most, if not all, TCDD-induced pathology is mediated via binding to the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor that belongs to the basic helix-loop-helix/Per-Arnt-Sim (bHLH/PAS) superfamily. Upon ligand binding, AhR translocates to the nucleus, dimerizes with the AhR nuclear translocator protein (Arnt), and regulates transcription by interaction with dioxin-response elements (DREs) in target genes, most notably specific cytochrome P450 (CYP) family members. To assess whether developing cerebellar granule neuroblasts are potential direct targets for TCDD toxicity, AhR expression and transcriptional activity were examined. AhR and Arnt proteins were present in mouse cerebellum from birth throughout postnatal development. AhR protein levels peaked between postnatal day (PND) 3-10, a critical period for granule neuroblast growth and maturation. Transcriptionally active AhR was detected in immature cerebellar granule cells in a transgenic dioxin-responsive lacZ mouse model after acute TCDD exposure. AhR and Arnt were also expressed in cerebellar granule neuroblast cultures. AhR localized to the nucleus in granule cells 15 min after TCDD treatment. TCCD elicited time-dependent and concentration-dependent increases in CYP1A1 and 1B1 mRNA and protein levels. Moreover, TCDD treatment reduced both thymidine incorporation and granule neuroblast survival in a concentration-dependent manner. These data suggest that (1) granule neuroblasts are direct targets for developmental AhR-mediated TCDD neurotoxicity and (2) TCDD exposure may disrupt granule cell neurogenesis.

Hypoxia affects mesoderm and enhances hemangioblast specification during early development

Hypoxia Inducible Factor (HIF), consisting of HIF1alpha and ARNT (HIF1beta) subunits, activates multiple genes in response to oxygen (O(2)) deprivation. Arnt(-/-) mice exhibit substantial defects in blood cell and vessel development. We demonstrate that hypoxia accelerates the expression of Brachyury (a mesoderm-specific transcription factor), BMP4 (a mesoderm-promoting growth factor) and FLK1 (a marker of hemangioblasts, the bipotential progenitor of endothelial and hematopoietic cells) in differentiating ES cell cultures. Significantly, proliferation of embryonic hemangioblasts (BL-CFCs) is regulated by hypoxia, as Arnt(+/+) ES cells generate increased numbers of FLK1(+) cells, and BL-CFCs with accelerated kinetics in response to low O(2). This response is HIF-dependent as Arnt(-/-) ES cells produce fewer FLK1(+) cells and BL-CFCs, under both normoxic and hypoxic conditions. Interestingly, this defect is rescued when Arnt(-/-) ES cells are co-cultured with Arnt(+/+) ES cells. Vegf(+/-)or Vegf(-/-) ES cells generate proper numbers of FLK1(+) cells but fewer BL-CFCs, suggesting that additional factors regulated by HIF (other than VEGF) are involved in these early events. Thus, hypoxic responses are important for the establishment of various progenitor cells, including early mesoderm and its differentiation into hemangioblasts. Together these data suggest that ineffective responses to hypoxia in Arnt(-/-) embryos abrogate proper cardiovascular development during early embryogenesis, including the pathways controlling hemangioblast differentiation.

Dioxin-responsive AHRE-II gene battery: identification by phylogenetic footprinting

We identified a set of genes that respond to dioxins through the recently discovered AHRE-II ("XRE-II") enhancer element. A total of 36 genes containing AHRE-II motifs conserved across human, mouse, and rat gene orthologs were identified by genome-wide transcription-factor binding-site searches and phylogenetic footprinting. Microarray experiments on liver from rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin revealed statistically significant changes in mRNA levels for 13 of these 36 genes after three hours and 15 genes after 19h. The set of responsive genes was functionally characterized by ontological analysis and found to be enriched in ion-channels and transporters. Our identification of 36 putatively AHRE-II-regulated genes highlights the regulatory versatility of the aryl hydrocarbon receptor (AHR) and the ability of the AHR and its dimerization partner, ARNT, to act both as a ligand-activated transcription-factor (on AHRE-I) and as a ligand-activated coactivator (on AHRE-II). Collectively, these results demonstrate that the AHRE-II induction mechanism is employed by multiple genes and provide the first categorization of the gene battery of a ligand-activated coactivator.