2,3,7,8-Tetrachlorodibenzo-p-dioxin: examination of biochemical effects involved in the proliferation and differentiation of XB cells

Cross-talk between the aryl hydrocarbon receptor and hypoxia inducible factor signaling pathways. Demonstration of competition and compensation

The aryl hydrocarbon receptor (AHR) and the alpha-class hypoxia inducible factors (HIF1alpha, HIF2alpha, and HIF3alpha) are basic helix-loop-helix PAS (bHLH-PAS) proteins that heterodimerize with ARNT. In response to 2,3,7,8-tetrachlorodibenzo-p-dioxin, the AHR. ARNT complex binds to "dioxin responsive enhancers" (DREs) and activates genes involved in the metabolism of xenobiotics, e.g. cytochrome P4501A1 (Cyp1a1). The HIF1alpha.ARNT complex binds to "hypoxia responsive enhancers" and activates the transcription of genes that regulate adaptation to low oxygen, e.g. erythropoietin (Epo). We postulated that activation of one pathway would inhibit the other due to competition for ARNT or other limiting cellular factors. Using pathway specific reporters in transient transfection assays, we observed that DRE driven transcription was markedly inhibited by hypoxia and that hypoxia responsive enhancer driven transcription was inhibited by AHR agonists. When we attempted to support this cross-talk model using endogenous loci, we observed that activation of the hypoxia pathway inhibited Cyp1a1 up-regulation, but that activation of the AHR actually enhanced the induction of Epo by hypoxia. To explain this unexpected additivity, we examined the Epo gene and found that its promoter harbors DREs immediately upstream of its transcriptional start site. These experiments outline conditions where inhibitory and additive cross-talk occur between the hypoxia and dioxin signal transduction pathways and identify Epo as an AHR-regulated gene.

Characterization of protein phosphorylation by 2,3,7,8-tetrachlorodibenzo-p-dioxin in murine lymphocytes: indirect evidence for a role in the suppression of humoral immunity

Studies were undertaken to more thoroughly characterize 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced stimulation of kinase activity in murine lymphocytes. In female B6C3F1 mice, TCDD-induced phosphorylation of 29, 45, 52 and 63 KDa proteins was selective for B cells, with little or no enhancement observed in T cells. When B cells were purified and separated by density on a percoll gradient, phosphorylation was only observed in the band composed of activated B cells, and was not enhanced in the band composed of resting B cells. TCDD-stimulated phosphorylation was associated with both the cytosol (45 and 52 KDa species) and membrane (52 KDa species) fractions. Purified B cells from both DBA/2 (Ahdd) and C57B16 (Ahbb) mice demonstrated equivalent enhancement of phosphorylation in response to TCDD. Administration of human gamma interferon (Hu-IFNg) at concentrations from 0.5 to 500 Units/ml produced a dose-related reversal of TCDD-induced suppression of in vitro antibody responses to both the polyclonal B cell activator, LPS, and the T-dependent antigen, sRBC in whole splenocytes isolated from female B6C3F1 mice. These concentrations of Hu-IFNg did not affect the magnitude of either response in the absence of TCDD, and did not reverse dexamethasone-induced suppression of either in vitro antibody response. TCDD-induced suppression of the T-dependent response was reversed only when Hu-IFNg was added to culture within the first 18 hours after treatment with TCDD and sRBC. These studies demonstrate that Hu-IFNg can reverse TCDD-induced in vitro Ab response suppression if it is administered during the period of susceptibility to TCDD. TCDD-induced phosphorylation in isolated B cells was also antagonized following co-incubation with Hu-IFNg. The profile of TCDD-induced increases in protein phosphorylation, including the selective effect on activated B cells, the general involvement of both cytosolic and membrane proteins, the lack of segregation with the Ah-dependent processes, and the ability of Hu-IFNg to reverse both the suppression of the Ab response and the increase in phosphorylation, supports the interpretation that such phosphorylation is involved in TCDD-induced suppression of the Ab response.

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.

Protein tyrosine phosphorylation as an indicator of 2,3,7,8-tetrachloro-p-dioxin exposure in vivo and in vitro

A dose-dependent increase in tyrosine phosphorylation of five hepatic intracellular proteins with approximate molecular weights of 17, 21, 27, 29, and 34 kDa was seen 24 h after administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to C57BL/6J female mice. The ED50 values for tyrosylphosphorylation of these five proteins, respectively, were 0.26, 0.21, 0.26, 0.31, and 0.38 micrograms TCDD/Kg. TCDD induction of 7-ethoxyresorufin O-deethylase activity (EROD) was characterized by an ED50 of 2.5 micrograms/Kg. An eighteen h exposure of a human lymphoblastoma cell line (X3) to TCDD increased tyrosylphosphorylation status of ten proteins with approximate molecular weights of 16, 17, 24, 26, 27, 32, 33, 34, 35, and 36 kDa in a dose-dependent manner. The EC50 values for these TCDD-dependent tyrosylphosphorylation ranged from 0.01 to 0.07 nM TCDD. EROD induction by TCDD in X3 cells exhibited an EC50 of 0.14 nM. These data indicate that TCDD alters intracellular protein tyrosine phosphorylation and these changes are more sensitive biological indicators of TCDD exposure than induction of EROD.

The Ah receptor and the mechanism of dioxin toxicity

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Development and validation of in vitro induction assays for toxic halogenated aromatic mixtures: a review

Halogenated aromatic industrial compounds, typified by the polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs) and biphenyls (PCBs) have been identified as residues in almost every component of the global ecosystem. Risk assessment of the complex mixtures of halogenated aromatics found in environmental samples is complicated by analytical problems and the lack of toxicological information on individual compounds and mixtures. Research in our laboratory has focused on the development and vadidation of the in vitro aryl hydrocarbon hydroxylase (AHH) induction assay in rat hepatoma H-4-II E cells in culture for quantitating individual toxic halogenated aryl hydrocarbons and their mixtures. For several PCB, PCDD, PCDF congeners, their mixed bromo/chloro analogs and reconstituted mixtures there was an excellent linear correlation between their -log ED50 values for AHH induction in rat hepatoma cells and their -log ED50 values for in vivo hepatic microsomal AHH induction, inhibition of body weight gain and thymic atrophy in the rat. It has also been shown for selected compounds that there was a good correlation between their in vitro AHH induction potencies and their effects in guinea pigs (AHH induction, inhibition of body weight gain) and mice (immunotoxicity). This assay system has been utilized to quantitative the "2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) equivalents" present in extracts from diverse sources including fly ash from a municipal incinerator and pyrolyzed brominated flame retardants which contain a complex mixture of halogenated dibenzo-p-dioxins and dibenzofurans.

Effect of TCDD on the density of Langerhans cells in murine skin

Tetrachlorodibenzo-p-dioxin (TCDD) is a prototype for a group of toxic polyhalogenated aromatic hydrocarbons. We have studied the effect of TCDD on skin, specifically the difference in cutaneous response of congenic haired (hr/+) and hairless (hr/hr) mice. Topical application of 0.6 microgram of TCDD induces epidermal hyperplasia/hyperkeratinization in the skin of hr/hr mice, but does not affect the epidermis of congenic hr/+ littermates. Suppression of various parameters of the immune response has been found to be another effect of TCDD exposure in experimental animals. In the present study, we investigated the effect of topical treatment with TCDD on the density of epidermal immune cells, the Langerhans cells (LC), in the skin of hr/hr and hr/+ mice. Results showed that TCDD-induced epidermal hyperplasia/hyperkeratinization in skin of hr/hr mice is accompanied by an increase in the density of LC. In the skin of hr/+ mice, in which TCDD exposure does not induce hyperplastic changes, LC densities are not affected. The increase in LC densities in TCDD-treated hr/hr mouse skin did not result in increased sensitivity of the skin to contact hypersensitization with dinitrofluorobenzene, as measured by changes in ear thickness. When hr/hr murine skin was grafted into skin of hr/+ mice and the entire dorsal skin (including the graft) treated with TCDD, LC were increased in the grafted skin, but not in the surrounding hr/+ skin. Conversly, when hr/+ murine skin was grafted into hr/hr mice and both treated with TCDD, there was no increase in the density of LC in the grafted hr/+ skin. Concomitant treatment of hairless mice with TCDD and with indomethacin did not affect the increase in the density of LC induced by TCDD treatment alone. These findings suggest that TCDD-induced epidermal changes in hr/hr murine skin involve production of factors which mediate the increase in epidermal LC.

Effects of TCDD on the EGF receptor of XB mouse keratinizing epithelial cells

TCDD was found to cause a marked inhibition of 125I-epidermal growth factor (EGF) binding to its receptor on the cell surface of XB mouse keratinizing epithelial cells (XB cells) cultured in vitro. The EC50 concentration was estimated to be on the order of 3 x 10(-11) M 24 hours after TCDD administration. As early as 12 hours after the addition of 10(-9) M of TCDD, XB cells showed signs of a decline in 125I-EGF binding levels. The level of such EGF receptor downregulation reached a maximum at 24 hours, continued until day 2, but completely recovered by day 3. This was accompanied by a rise in protein kinase activities, particularly those of the protein tyrosine kinases during the initial period of 6-24 hours. To test the hypothesis that the EGF receptors of the cells, by showing TCDD-induced symptoms of downregulation, actually are being activated and triggering EGF-like signals, we examined the effects of both TCDD and exogenously added EGF on cell morphology, colony formation degree of keratinization, the pattern of activation of protein kinases and de novo protein synthesis, and EGF receptor phosphorylation. Based on the similarity of cell responses to these between TCDD- and EGF-treated cells, we concluded that TCDD, directly or indirectly, causes activation of the EGF receptor. In contrast, 12-O-tetradencanoylphorbol-13-acetate (TPA), which is known to downregulate EGF receptors by blocking their protein tyrosine kinase, produced dissimilar end results. The balance of evidence support the notion that the action of TCDD in this cell line is tightly coupled to the activation of the EGF receptor and that one of the key consequences of such a biochemical change is that it signals these cells to commit to terminal differentiation.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin on murine skin

The effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on skin of congenic haired and hairless newborn and adult HRS/J mice was studied. In all adult animals topical application of TCDD caused an involution of sebaceous glands. Epidermal/epithelial hyperplasia and hyperkeratinization was induced in the hairless, but not the haired mice. Trans-glutaminase (TG) activity was stimulated in both haired and hairless animals. A single application of 1 microgram of TCDD did not stimulate significant ornithine decarboxylase activity in the skin in either strain. Other than a reduction in the density of the inflammatory cell infiltrate in the dermis, topical treatment with antiinflammatory agents fluocinolone acetonide and indomethacin did not affect the cutaneous response to TCDD. Skin of newborn mice treated topically with TCDD over a 2-wk period reacted much the same as adult skin in that sebaceous glands were reduced in size and TG activity was stimulated in both haired and hairless neonates; but epidermal hyperplasia occurred only in the hairless, not the haired newborns.

2,3,7,8-Tetrachlorodibenzo-p-dioxin causes an increase in protein kinases associated with epidermal growth factor receptor in the hepatic plasma membrane

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), administered to male rats at a single intraperitoneal (IP) injection dose of 25 micrograms/kg causes down-regulation of epidermal growth factor (EGF) receptor in the plasma membrane of rat liver which starts after two days and continues throughout the experimental period (20 days). Using monoclonal antibody to EGF receptor, it was determined that TCDD-caused EFG receptor down-regulation in the rat liver was accompanied by increased protein kinase activity. Such an increase in the protein kinase activity involves, at least in part, an activation of protein tyrosine kinase. Examination of serum samples from control and treated rats revealed no detectable difference in the level of EGF itself or EGF receptor-reacting substances (eg, hormones and other growth factors). In vivo TCDD caused early eye opening and tooth eruption and poor body weight gain and hair growth in mouse neonates similar to those observed with exogenously administered EGF. The results indicate that such EGF receptor-mediated effect of TCDD has some toxocilogical significance in vivo. Although TCDD causes significant reduction in [125I]-EGF binding in the hepatic plasma membrane in susceptible strains of mice, it has only modest effects in tolerant strains. The results are consistent with the idea that the action of TCDD on the EGF receptor is mediated through the cytosolic/nuclear TCDD receptor, which is known to be regulated by the Ah locus.

Prostaglandin release by the chick embryo heart is increased by 2,3,7,8-tetrachlorodibenzo-p-dioxin and by other cytochrome P-448 inducers

Exposure of chick embryos in ovo to cytochrome P-448 inducers 2,3,7,8-tetrachlorodibenzo-p-dioxin, 3,4,3',4'-tetrachlorobiphenyl, 3,4,5,3',4',5'-hexachlorobiphenyl and beta-napthoflavone, increased cardiac prostaglandins in vitro. The dose response relationships were biphasic with prostaglandin release increasing at the low doses and returning to basal levels at higher doses. Phenobarbital was ineffective. Increased cardiac prostaglandin release was detected at doses that induced hepatic 7-ethoxyresorufin deethylase (7-ER) but which were below the threshold for cardiac induction. The fall in prostaglandin release coincided with induction of cardiac 7-ER and therefore may be attributable to increased prostaglandin metabolism. These studies show that the P-450 system may interact with the arachidonic acid metabolizing system to increase PG release and that this effect may be part of the pleiotypic response to Ah-receptor activation.