2,3,7,8-Tetrachlorodibenzo-p-dioxin induces the nuclear translocation of two XRE binding proteins in mice

Regulation of pulmonary and hepatic cytochrome P4501A expression in the rat by hyperoxia: implications for hyperoxic lung injury

Supplemental oxygen therapy is frequently used in the treatment of pulmonary insufficiency, as is encountered in premature infants, and in patients with acute respiratory distress syndrome. However, hyperoxia causes lung damage in experimental animals and may do so in humans. Cytochrome P4501A enzymes have been implicated in hyperoxic lung injury. In this study, we investigated the mechanisms of CYP1A1 regulation by hyperoxia and tested the hypothesis that aryl hydrocarbon receptor (AHR)-dependent mechanisms contribute to induction of CYP1A1 and that modulation of CYP1A by hyperoxia may have implications for lung injury. Exposure of adult male Sprague-Dawley rats to hyperoxia for 24 to 48 h led to increased expression of pulmonary CYP1A1 enzyme, which was preceded by enhancement of the corresponding mRNA, followed by decline of induction at 60 h, when the animals displayed severe respiratory distress and lung inflammation. Similarly, hepatic CYP1A1/1A2 mRNAs were markedly induced between 24 and 48 h of hyperoxia, with induction declining by 60 h. Electrophoretic mobility shift assays (EMSA) and experiments with AHR (-/-) mice indicated that AHR-dependent mechanisms contributed to CYP1A induction. The AHR (-/-) mice were refractory to CYP1A1 induction by hyperoxia and were more sensitive to lung injury than wild-type mice. Lungs of hyperoxic rats showed increase in the expression of CYP1A1 in airway epithelial cells, type II pneumocytes, and endothelial cells. In conclusion, our results suggest that induction of CYP1A1 by hyperoxia is mediated by AHR-dependent mechanisms and that modulation of CYP1A enzymes by hyperoxia may have implications for hyperoxic lung injury.

Aryl hydrocarbon receptor-mediated signal transduction

The aryl hydrocarbon (or dioxin) receptor (AhR) is a ligand-activated basic helix-loop-helix (bHLH) protein that heterodimerizes with the bHLH protein ARNT (aryl hydrocarbon nuclear translocator) forming a complex that binds to xenobiotic regulatory elements in target gene enhancers. Genetic, biochemical, and molecular biology studies have revealed that the AhR mediates the toxic and biological effects of environmentally persistent dioxins and related compounds. Cloning of the receptor and its DNA-binding partner, ARNT, has facilitated detailed efforts to understand the mechanisms of AhR-mediated signal transduction. These studies have determined that this unique receptor consists of several functional domains and belongs to a subfamily of bHLH proteins that share a conserved motif termed the PAS domain. In addition, recent genetic studies have revealed that expression of the AhR is a requirement for proper embryonal development, which appears to be a common function shared by many other bHLH proteins. This review is a summary of recent molecular studies of AhR-mediated gene regulation.

Developmental expression of two members of a new class of transcription factors: II. Expression of aryl hydrocarbon receptor nuclear translocator in the C57BL/6N mouse embryo

The aryl hydrocarbon receptor (AhR) and the AhR nuclear translocator protein (ARNT) are basic-helix-loop-helix (bHLH) proteins involved in transcriptional regulation. The AhR is a ligand-activated partner of the ARNT protein. Both proteins are required to transcriptionally regulate gene expression. ARNT must be complexed to AhR to permit binding to the regulatory DNA sequence. The AhR-ligand complex is known to mediate a range of biological responses, such as developmental toxicity, induction of cleft palate, and hydronephrosis. AhR and ARNT are expressed in human embryonic palatal cells and AhR was recently shown to have a specific developmental pattern of expression in the mouse embryo. In the present study, expression of ARNT is characterized in C57Bl/6N mouse embryos from gestation day (GD) 10-16 using immunohistochemistry and in situ hybridization. Af affinity purified antibody against human ARNT (1.1 micrograms/ml) was detected with an avidin-biotin-peroxidase complex. ARNT mRNA was localized with a 35S-RNA probe from pBM5/NEO-M1-1. Specific spatial and temporal patterns of ARNT expression emerged and mRNA and protein expression correlated. The GD 10-11 embryos showed highest levels of ARNT in neuroepithelial cells of the neural tube, visceral arches, otic and optic placodes, and preganglionic complexes. The heart also had significant expression of ARNT with strong nuclear localization. After GD11, expression in heart and brain declined. In GD 12-13 embryos expression was highest in the liver where expression increased from GD 12-16. At GD 15-16 the highest levels of ARNT occurred in adrenal gland and liver, although ARNT was also detected in submandibular gland, ectoderm, tongue, bone, and muscle. In all of these tissues ARNT was cytoplasmic as well as nuclear, except in some of the cortical adrenal cells in which ARNT was strongly cytoplasmic with little or no nuclear localization. These specific patterns of ARNT expression, which differ in certain tissues from the expression of AhR, suggest that ARNT may have additional roles in normal embryonic development.

Dioxins: model chemicals for assessing receptor-mediated toxicity

Dioxins and related compounds are chlorinated aromatic hydrocarbons that are persistent in both environmental and biological samples. Many members of this class of compounds produce a similar spectrum of toxicity which is mediated by interaction with the Ah receptor. The toxic effects of these chemicals can best be described by their actions as growth dysregulators. Dioxins disrupt normal homeostatic processes that tightly regulate cellular growth and differentiation. Disruption in these processes produce a variety of toxicities and pathologies. The available data indicate that humans are sensitive to the toxic effects of these chemicals. Clearer definition of human responses and the body burdens associated with such effects requires more research. Comprehensive risk assessments of dioxins should include all Ah receptor ligands such as the halogenated dibenzofurans and biphenyls.

Alterations in splenocyte protein kinase C (PKC) activity by 2,3,7,8-tetrachlorodibenzo-p-dioxin in vivo

The effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on growth factor-coupled activation of nuclear protein kinase C (nPKC) and on the subcellular distribution of PKC activity in rat splenocytes were investigated. Seven days after a single injection of TCDD (50 micrograms/kg body weight), cytosolic and particulate PKC activity was significantly higher in splenocytes from TCDD-treated rats or pair-fed control rats compared to ad libitum-fed animals. In a separate experiment, purified splenocyte nuclei from TCDD-treated animals and controls were used to study activation of nPKC by growth factors and other trophic agents. Growth factor-stimulated nPKC activation was attenuated in splenic nuclei from TCDD-treated rats compared to vehicle-treated controls. Evidence presented here suggests that the cellular mechanism of TCDD toxicity leading to immunosuppression in rodents may be mediated in part by uncoupling of growth factor receptors linked to PKC activation at the level of the nucleus. However, changes in total splenocyte PKC activity appear to be correlated with hypophagia since cytosolic and particulate PKC levels were elevated in TCDD-treated rats and their pair-fed partners.

Cloning and characterization of cDNAs encoding mouse Ugt1.6 and rabbit UGT1.6: differential induction by 2,3,7,8-tetrachlorodibenzo-p-dioxin

In this report, cDNAs for mouse liver Ugt1.6 and rabbit liver UGT1.6 have been cloned and characterized. The predicted amino acid sequence of mouse Ugt1.6 is 93% and 78% similar to the rat and human UGT1.6 sequences, respectively, while the rabbit UGT1.6 is 79% and 83% similar to the rat and human UGT1.6 sequences, respectively. To examine the substrate specificities of the proteins encoded by the mouse Ugt1.6 and rabbit UGT1.6 cDNAs, the recombinants were expressed in monkey kidney COS-1 cells. Transfection of the mouse and rabbit recombinants allowed for the expression of the UGT1.6 proteins as determined by immunoprecipitation of newly synthesized protein. The expressed UGTs conjugated small planar phenolic molecules such as 4-nitrophenol, 1-naphthol, and 4-methylumbelliferone. While the bulky phenol 4-hydroxybiphenyl was not a substrate for the enzymes, 2-hydroxybiphenyl was an excellent substrate. Androgens and estrogens were not conjugated by either mouse Ugt1.6 or rabbit UGT1.6. In rodents, UGT1.6 mRNA is expressed constitutively and induced when the animals are treated with the Ah receptor ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Using wild-type mouse hepatoma cells and the Ah receptor deficient class II cells, it was demonstrated that induction of mouse Ugt1.6 was dependent upon a functional Ah receptor complex. However, when New Zealand white rabbits were treated with TCDD and liver mRNA was examined by Northern blot analysis, it was shown that TCDD had no effect on the induction of UGT1.6 mRNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular biology of the aromatic hydrocarbon (dioxin) receptor

The aromatic hydrocarbon (AH) (dioxin) receptor was discovered almost 20 years ago and achieved notoriety as the front-line site of action of highly toxic environmental chemicals such as halogenated dioxins and polychlorinated biphenyls. Increasing evidence suggests that the AH receptor plays a key role in proliferation and differentiation of cells exposed to dioxins and, perhaps, to endogenous ligands. Recent cloning of the AH receptor and its indispensable partner, the AH-receptor-nuclear-translocator protein, has opened new opportunities to determine how the AH receptor functions, how it evolved and what its multiple roles might be in normal physiology as well as in toxicology. This review by Allan Okey, David Riddick and Patricia Harper aims to provide a brief history of AH receptor research and gives a timely summary of what is known and what is not known about the structure and function of this fascinating protein.

Aryl hydrocarbon or dioxin receptor: biologic and toxic responses

1. The AhR represents a ligand-activated transcription factor. Receptor agonists include planar aromatic compounds, a variety of heterocyclic plant constituents, and PCDD/PCDF. The latter lead to persistent activation of the receptor due to their strong binding affinity and long biologic half-life of over 10 years in human blood and fat. Practically every person on earth is exposed to these compounds via the diet (> 90%) and by high concentrations in mother's milk. PCDD/PCDF produced toxic responses in exposed people (primarily chloracne and immunosuppression) in the past. However, the present PCDD/PCDF levels (basal levels) in the general population are below those warranting toxicologic concern. 2. The AhR has been characterized as a helix-loop-helix transcription factor related to the Drosophila developmental genes sim and per. The cytosolic form of the receptor is present as an inactive complex with two subunits of HSP90. After ligand binding HSP90 is released and the receptor enters the nucleus as a heterodimer together with a related protein ARNT. It binds with high affinity to certain enhancer elements in the upstream region of several genes such as cytochrome P4501A1 (CYP1A1). The AhR transcriptionally activates several drug-metabolizing enzymes and proteins involved in growth/differentiation, such as the plasminogen activator inhibitor PAI-2 and IL-1 beta. In addition, it modulates the action of a number of other nuclear transcription factors such as receptors of the steroid hormone receptor superfamily and of cell surface receptors such as EGF. With the exception of CYP1A1 induction, little is known about the mechanism of transcriptional activation of the AhR-controlled genes. Many AhR-modulated biologic responses (such as modulation of the estrogen and EGF receptor) appear to be indirect. 3. Persistent activation of the AhR is probably responsible for toxic responses in experimental animals and humans. They are markedly tissue and species specific. In rodents a wasting syndrome, immunosuppression, teratogenicity, chloracne, and carcinogenicity/tumor promotion have been well studied. There is good evidence for an involvement for the AhR in these responses. However, the chain of events from receptor activation to the diverse toxic endpoints is largely unknown. Alteration of growth and differentiation of epithelial tissues may underlie most of the toxic responses. A lot has already been achieved, mostly by characterizing the AhR and transcriptional activation of CYP1A1. Still more work lies ahead of us, for example, elucidation of the physiologic roles of the AhR and of the chains of events from receptor activation to the various biologic and toxic endpoints.