Cross-coupling of signal transduction pathways: the dioxin receptor mediates induction of cytochrome P-450IA1 expression via a protein kinase C-dependent mechanism

Definition of a dioxin receptor mutant that is a constitutive activator of transcription: delineation of overlapping repression and ligand binding functions within the PAS domain

The intracellular dioxin (aryl hydrocarbon) receptor is a ligand-activated transcription factor that mediates the adaptive and toxic responses to environmental pollutants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin and structurally related congeners. Whereas the ligand-free receptor is characterized by its association with the molecular chaperone hsp90, exposure to ligand initiates a multistep activation process involving nuclear translocation, dissociation from the hsp90 complex, and dimerization with its partner protein Arnt. In this study, we have characterized a dioxin receptor deletion mutant lacking the minimal ligand-binding domain of the receptor. This mutant did not bind ligand and localized constitutively to the nucleus. However, this protein was functionally inert since it failed to dimerize with Arnt and to bind DNA. In contrast, a dioxin receptor deletion mutant lacking the minimal PAS B motif but maintaining the N-terminal half of the ligand-binding domain showed constitutive dimerization with Arnt, bound DNA, and activated transcription in a ligand-independent manner. Interestingly, this mutant showed a more potent functional activity than the dioxin-activated wild-type receptor in several different cell lines. In conclusion, the constitutively active dioxin receptor may provide an important mechanistic tool to investigate receptor-mediated regulatory pathways in closer detail.

Regulation of DNA binding activity of the ligand-activated aryl hydrocarbon receptor by tyrosine phosphorylation

Aryl hydrocarbon receptor (AhR), a member of the bHLH-PAS family, is a ligand-activated transcription factor which plays an important role in normal liver development and in mediating the toxicity of polycyclic and halogenated aromatic hydrocarbon pollutants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin. Phosphorylation is known to regulate the transformation process of unliganded AhR into functionally active AhR/ARNT heterodimer that has high affinity for dioxin-responsive elements (DRE) and transactivation activity. Here, we report that DRE binding activity of the AhR is regulated by phosphorylation on the AhR/ARNT complex itself. Studies with specific protein phosphatases indicated that tyrosine phosphorylation is involved in this modulation. In addition, the AhR is phosphorylated at tyrosine residue(s) as determined by anti-phosphotyrosine immunoblot analysis. These results suggest that tyrosine phosphorylation on the AhR is required for its DNA binding activity and may provide mammalian cells with another layer of control mechanism that allows cell type specific and developmental stage specific induction of the AhR target genes.

Induction and regulation of xenobiotic-metabolizing cytochrome P450s in the human A549 lung adenocarcinoma cell line

Several cytochrome P450 (CYP) enzymes are expressed in the human lung, where they participate in metabolic inactivation and activation of numerous exogenous and endogenous compounds. In this study, the expression pattern of all known xenobiotic-metabolizing CYP genes was characterized in the human alveolar type II cell-derived A549 adenocarcinoma cell line using qualitative reverse transcriptase/polymerase chain reaction (RT-PCR). In addition, the mechanisms of induction by chemicals of members in the CYP1 and CYP3A subfamilies were assessed by quantitative RT-PCR. The expression of messenger RNAs (mRNAs) of CYPs 1A1, 1B1, 2B6, 2C, 2E1, 3A5, and 3A7 was detected in the A549 cells. The amounts of mRNAs of CYPs 1A2, 2A6, 2A7, 2A13, 2F1, 3A4, and 4B1 were below the limit of detection. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induced CYP1A1 and CYP1B1 mRNAs 56-fold and 2.5-fold, respectively. CYP3A5 was induced 8-fold by dexamethasone and 11-fold by phenobarbital. CYP3A4 was not induced by any of the typical CYP3A4 inducers used. The tyrosine kinase inhibitor genistein and the protein kinase C inhibitor staurosporine blocked TCDD-elicited induction of CYP1A1, but they did not affect CYP1B1 induction. Protein phosphatase inhibitors okadaic acid and calyculin A enhanced TCDD-induction of CYP1B1 slightly, but had negligible effects on CYP1A1 induction. These results suggest that CYP1A1 and CYP1B1 are differentially regulated in human pulmonary epithelial cells and give the first indication of the induction of CYP3A5 by glucocorticoids in human lung cells. These results establish that having retained several characteristics of human lung epithelial cell CYP expression, the A549 lung cell line is a valuable model for mechanistic studies on induction of the pulmonary CYP system.

A redox mechanism controls differential DNA binding activities of hypoxia-inducible factor (HIF) 1alpha and the HIF-like factor

Hypoxia-inducible factor 1alpha (HIF-1alpha) and the HIF-like factor (HLF) are two highly related basic Helix-Loop-Helix/Per-Arnt-Sim (bHLH/PAS) homology transcription factors that undergo dramatically increased function at low oxygen levels. Despite strong similarities in their activation mechanisms (e.g. they both undergo rapid hypoxia-induced protein stabilization, bind identical target DNA sequences, and induce synthetic reporter genes to similar degrees), they are both essential for embryo survival via distinct functions during vascularization (HIF-1alpha) or catecholamine production (HLF). It is currently unknown how such specificity of action is achieved. We report here that DNA binding by HLF, but not by HIF-1alpha, is dependent upon reducing redox conditions. In vitro DNA binding and mammalian two-hybrid assays showed that a unique cysteine in the DNA-binding basic region of HLF is a target for the reducing activity of redox factor Ref-1. Although the N-terminal DNA-binding domain of HIF-1alpha can function in the absence of Ref-1, we found that the C-terminal region containing the transactivation domain requires Ref-1 for full activity. Our data reveal that the hypoxia-inducible factors are subject to complex redox control mechanisms that can target discrete regions of the proteins and are the first to establish a discriminating control mechanism for differential regulation of HIF-1alpha and HLF activity.

Lack of an absolute requirement for the native aryl hydrocarbon receptor (AhR) and AhR nuclear translocator transactivation domains in protein kinase C-mediated modulation of the AhR pathway

Protein kinase C (PKC)-mediated modulation of the aryl hydrocarbon receptor (AhR) pathway was examined in CHOK1-derived L10.I cells stably transfected with the pGUDLUC6.1 reporter; pGUDLUC6.1 is solely controlled by four dioxin-responsive enhancer elements. Co treatment of L10.I cells with 10 nM 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) and 81 nM phorbol 12-myristate 13-acetate (PMA), an activator of sn-1,2-diacylglyerol binding PKCs, enhanced transactivation of the reporter construct several-fold relative to cells treated with a saturating 10 nM TCDD dose alone; this effect was dubbed the "PMA effect." A domain swapping and deletional analysis of the native AhR and AhR nuclear translocator (ARNT) protein transactivation domains (TADs) was performed to determine if these domains are absolutely required for the AhR x ARNT dimer-mediated PMA effect in the L10.I model system; controls demonstrate the suitability of the L10.I model for these analyses and that endogenous AhR and ARNT levels are extremely low in this model. Transient coexpression of the AhR and ARNT-474-FLAG, an ARNT protein lacking the native ARNT TAD, in L10.I cells reveals the native ARNT TAD is not absolutely required for the AhR x ARNT-474-FLAG dimer to mediate the PMA effect. Transient coexpression of AhRDeltaCVP, a chimeric AhR protein in which the native AhR TAD has been replaced with the VP16 (herpes simplex virus protein 16) TAD (which control experiments demonstrate is unaffected by PMA), and ARNT in L10.I cells indicates that the native AhR TAD is not absolutely required for this AhRDeltaCVP x ARNT dimer to mediate the PMA effect. These observations strongly suggest that PKC-mediated modulation of the AhR pathway is not absolutely dependent on coactivators recruited to the AhR. ARNT dimer by the native TADs of the AhR and its heterodimerization partner ARNT.

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

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

Repression of dioxin signal transduction in fibroblasts. Identification Of a putative repressor associated with Arnt

Heterodimeric complexes of basic helix-loop-helix/PAS transcription factors are involved in regulation of diverse physiological phenomena such as circadian rhythms, reaction to low oxygen tension, and detoxification. In fibroblasts, the basic helix-loop-helix/PAS heterodimer consisting of the ligand-inducible dioxin receptor and Arnt shows DNA-binding activity, and the receptor and Arnt are able to activate transcription when fused to a heterologous DNA-binding domain. However, fibroblasts are nonresponsive to dioxin with regard to induction mediated by the DNA response element recognized by the receptor and Arnt. Here we demonstrate that Arnt is associated with a fibroblast-specific factor, forming a complex that is capable of binding the dioxin response element. This factor may function as a repressor since negative regulation of target gene induction appears to be abolished by inhibition of histone deacetylase activity by trichostatin A. Finally, the negative regulatory function of this factor appears to be restricted for dioxin signaling since Arnt was able to mediate, together with hypoxia-inducible factor-1alpha, transcriptional activation in hypoxic cells. Taken together, these data suggest that fibroblast-specific inhibition of dioxin responsiveness involves recruitment by Arnt of a cell type- and signaling pathway-specific corepressor associated with a histone deacetylase.

Regulation of cytochrome P4501B1 (CYP1B1) in mouse embryo fibroblast (C3H10T1/2) cells by protein kinase C (PKC)

The effects of co-treatment of C3H10T1/2 (10T1/2) cells with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 12-O-tetradecanoylphorbol-13-acetate (TPA) on the expression of the novel cytochrome P4501B1 (CYP1B1) were investigated. As monitored by CYP1B1-catalyzed 7,12-dimethylbenzanthracene (DMBA) metabolism, TPA suppressed basal and TCDD-induced DMBA metabolism in a concentration-dependent manner, with a maximum inhibitory concentration of 100 nM. The suppression of CYP1B1 catalytic activity occurred at two time points during which protein kinase C (PKC) was activated and down-regulated in these cells as judged by analyses of cellular PKC content and PKC-inhibitor (chelerythrine chloride)-influenced suppression of CYP1B1 catalytic activity. Experiments in which TCDD and benzanthracene (BA)-induced DMBA metabolism were monitored in PKCbeta1-overexpressing 10T1/2 cells revealed that the suppression of CYP1B1 activity is a consequence of cellular PKC elevation. This suppression phenomenon could be accounted for by PKC-mediated suppression of TCDD-induced CYP1B1 mRNA and apoprotein and of nuclear translocation of the Ah-receptor. In contrast, the mitogen-activated protein kinase (MAPK) proteins ERKs 1 and 2 were stimulated by TCDD under conditions in which PKC was activated. Collectively, our results suggest that PKC participates in the regulation of CYP1B1 in 10T1/2 cells, positively by directly suppressing the Ah-receptor signaling pathway, followed by an indirect or negative activation of the MAPK signaling pathway.

Differentiation of cultured human epidermal keratinocytes at high cell densities is mediated by endogenous activation of the protein kinase C signaling pathway

Normal human epidermal keratinocytes (NHEK) grown in serum-free medium on a plastic substrate spontaneously differentiate at high cell densities in vitro. Because protein kinase C (PKC) regulates murine keratinocyte differentiation triggered by a variety of stimuli, we examined the role of this signaling pathway in density-dependent activation of NHEK differentiation. Relative to subconfluent cultures, confluent NHEK expressed markedly higher levels of multiple differentiation markers assayed by immunoblotting, including keratin 1, loricrin, filaggrin, involucrin, TGK, and SPR-1. Expression of several of these markers continued to increase for several days after cells reached confluency. The total level of several PKC isoforms was not substantially altered in NHEK harvested at different cell densities, based on immunoblotting; however, subcellular fractionation revealed that PKCalpha underwent a redistribution to the particulate fraction in confluent and postconfluent NHEK cultures, suggesting that this isozyme was activated under these conditions and may be involved in triggering the terminal differentiation program. Supporting this concept, inhibition of PKC function using bryostatin 1 or GF 109203X blocked the induction of keratinocyte differentiation markers at high cell densities. These data suggest that endogenous activation of PKC is responsible for cell density-mediated stimulation of NHEK differentiation, establishing a critical role for this pathway in regulating human as well as murine keratinocyte differentiation.

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

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

Characterization of 2,3,7,8-tetrachlorodibenzofuran-dependent suppression and AH receptor pathway gene expression in the developing mouse mammary gland

The AH receptor (AHR) is a ligand-activated transcription factor and member of a growing family of homologous proteins implicated in development. In this study we have characterized the actions of 2,3, 7,8-tetrachlorodibenzofuran (TCDF), a well-studied AHR ligand, and the expression of AHR and selected AHR signal transduction pathway genes in the developing mouse mammary gland. High levels of AHR protein were observed in the mammary glands of C57Bl/6J (AHR +/+) mice during estrous-stimulated growth and branching of terminal end buds (TEBs). Comparative analysis of mammary gland development in AHR -/- and +/+ littermates revealed a 50% reduction in TEBs and an increase in blunt-ended terminal ducts in the AHR null animals. Treatment of mammary glands, removed from estrogen/progesterone-primed C57Bl/6J mice and maintained in organ culture, with TCDF suppressed lobule development (greater than twofold decreases in lobule number and size), with a concomitant suppression of DNA synthesis, as judged by a 35 to 45% decrease in [3H]thymidine incorporation in the TEBs. Immunohistochemical staining patterns for AHR, aryl hydrocarbon nuclear translocator (ARNT; the heterodimerization partner of AHR), and two AHR-regulated genes, Cyp1A1 and Cyp1B1, were similar and not altered by treatment of mammary glands in organ culture with TCDF. The observed differences in the development of mammary glands from AHR +/+ and -/- mice, associated expression of the AHR protein with hormone-dependent lobule development, and suppressive actions of TCDF support the position that, in C57Bl/6J mice, development of the mammary gland is at least in part AHR dependent. Development occurs in the absence of exogenous AHR ligand, suggesting that the unoccupied receptor may function to support the proliferative stages required for full lobule development.

Suppression of CYP1A1 transcription by H2O2 is mediated by xenobiotic-response element

We have previously demonstrated that H2O2 downregulates CYP1A1 and CYP1A2 transcription in isolated rat hepatocytes (C. W. Barker, et al., 1994, J. Biol. Chem. 269, 3985-3990). In the present study, induction of chloramphenicol acetyltransferase (CAT) expression driven by 3.1 kb of rat CYP1A1 upstream regulatory sequences was suppressed by 56% in Hepa-1 cells treated with H2O2. Similarly, H2O2 inhibited CAT expression from vectors containing two copies of either xenobiotic-response element (XRE) 1 or XRE2. H2O2 did not inhibit basal CAT expression in cells that were not treated with the inducer beta-napthoflavone. Electrophoretic mobility shift assays demonstrated that the suppression of XRE-dependent transcription by H2O2 was not due to changes in nuclear aryl hydrocarbon (Ah) receptor DNA binding activity. Several types of experiments indicated that modulation of XRE enhancer strength by various means could modify H2O2-dependent suppression of CAT expression. Conditions that increased the transactivation potential of the Ah receptor (increase in XRE copy number or shortening of the distance between XREs and the minimal CYP1A1 promoter) attenuated the action of H2O2, while conditions that reduced XRE-mediated transactivation potential (decrease in XRE copy number, increase of the distance between the XRE and the promoter, or reduction of the number of bound Ah receptors by lowering the concentration of inducer) potentiated the inhibitory action of H2O2.

Phorbol ester-induced transcription of a fibroblast growth factor-binding protein is modulated by a complex interplay of positive and negative regulatory promoter elements

Earlier studies from our laboratory showed that a secreted binding protein for fibroblast growth factors (FGF-BP) is expressed at high levels in squamous cell carcinoma (SCC) cell lines. Overexpression studies or conversely reduced expression of FGF-BP by ribozyme targeting have elucidated a direct role of this protein in angiogenesis during tumor development. We have also observed a significant up-regulation of FGF-BP during TPA (12-O-tetradecanoylphorbol-13-acetate) promotion of skin cancer. Here we investigate the mechanism of TPA induction of FGF-BP gene expression in the human ME-180 SCC cell line. We found that TPA increased FGF-BP mRNA levels in a time- and dose-dependent manner mediated via the protein kinase C signal transduction pathway. Results from actinomycin D and cycloheximide experiments as well as nuclear transcription assays revealed that TPA up-regulated the steady-state levels of FGF-BP mRNA by increasing its rate of gene transcription independently of de novo protein synthesis. We isolated the human FGF-BP promoter and determined by deletion analysis that TPA regulatory elements were all contained in the first 118 base pairs upstream of the transcription start site. Further mutational analysis revealed that full TPA induction required interplay between several regulatory elements with homology to Ets, AP-1, and CAATT/enhancer binding protein C/EBP sites. In addition, deletion or mutation of a 10-base pair region juxtaposed to the AP-1 site dramatically increased TPA induced FGF-BP gene expression. This region represses the extent of the FGF-BP promoter response to TPA and contained sequences recognized by the family of E box helix-loop-helix transcription factors. Gel shift analysis showed specific and TPA-inducible protein binding to the Ets, AP-1, and C/EBP sites. Furthermore, distinct, specific, and TPA-inducible binding to the imperfect E box repressor element was also apparent. Overall, our data indicate that TPA effects on FGF-BP gene transcription are tightly controlled by a complex interplay of positive elements and a novel negative regulatory element.

Protein kinase C activity is required for aryl hydrocarbon receptor pathway-mediated signal transduction

The role of protein kinase C (PKC) in the human aryl hydrocarbon receptor (hAhR) signal transduction pathway was examined in cell lines stably transfected with pGUDLUC6.1, in which luc+ is solely controlled by four dioxin-responsive elements (DREs). These cell lines, P5A11 and HG40/6, were derived from HeLa and HepG2 cells respectively. Simultaneous treatment of these cells with 2,3,7,8, -tetrachlorodibenzo-p-dioxin (TCDD) and phorbol-12-myristate-13-acetate (PMA) enhanced trans-activation of the reporter construct several-fold relative to cells treated with TCDD alone. PKC inhibitors block the PMA effect and hAhR-mediated signal transduction, demonstrating these processes require PKC activity. Examination of other independently generated, HeLa-derived cell lines stably transfected with pGUDLUC6.1 demonstrates the PMA effect in P5A11 cells is not a clonal artifact. Transient transfections indicate the PMA effect is not due to a luciferase message/gene product stabilization mechanism or stimulation of the basal transcription machinery. Examination of cytosolic preparations demonstrates PKC stimulation or inhibition does not alter hAhR and hAhR nuclear translocator protein levels or TCDD-induced down-regulation of hAhR levels. Similarly, examination of nuclear extracts indicated PKC stimulation or inhibition does not alter nuclear AhR levels or hAhR/hAhR nuclear translocator protein heterodimer DRE-binding activity as assessed by electrophoretic mobility shift assay. These results demonstrate a PKC-mediated event is required for the hAhR to form a functional transcriptional complex that leads to trans-activation and that the DRE is the minimal DNA element required for PMA to enhance AhR-mediated trans-activation.

PD98059 is an equipotent antagonist of the aryl hydrocarbon receptor and inhibitor of mitogen-activated protein kinase kinase

PD98059 [2-(2'-amino-3'-methoxyphenyl)-oxanaphthalen-4-one] is a flavonoid and a potent inhibitor of mitogen-activated protein kinase kinase (MEK). Concentrations of PD98059 of /=10 microM. In vivo exposure of cultures to 95%) of the dually phosphorylated forms of extracellular signal-regulated kinase (IC50 = 1 muM). Treatment of cultures with PD98059 of >/=1 muM either at the time of addition or up to 48 hr before the addition of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) suppressed in a concentration-dependent manner the accumulation of induced steady state CYP1A1, CYP1B1, and NQO1 mRNAs. The addition of PD98059 to rat liver cytosol just before the addition of TCDD suppressed TCDD binding (IC50 = 4 muM) and aryl hydrocarbon receptor (AHR) transformation (IC50 = 1 muM), as measured by sucrose gradient centrifugation and electrophoretic mobility shift assays. Flavone and flavanone, two closely related structural analogs of PD98059, inhibited AHR transformation by TCDD with IC50 values similar to that obtained with PD98059. However, neither analog was as potent as PD98059 in inhibiting MEK (IC50 approximately 190 muM for both). These results suggest that PD98059 is a ligand for the AHR and functions as an AHR antagonist at concentrations commonly used to inhibit MEK and signaling processes that entail MEK activation.

Aryl hydrocarbon receptor-inducible or constitutive expression of human UDP glucuronosyltransferase UGT1A6

Transcriptional regulation of human UGT1A6, a UDP glucuronosyltransferase isoform conjugating a wide variety of planar phenols, has been studied using transfection experiments with plasmids containing its 3-kb 5' upstream region and chloramphenicol acetyltransferase as reporter gene. Previously, two modes of expression of the isoform have been described: in colon carcinoma Caco-2 cells UGT1A6 was found to be TCDD-inducible, whereas in lung carcinoma A549 cells it was constitutively expressed. Therefore functional analysis of UGT1A6 regulation was carried out using these two cell lines. In the upstream region of human UGT1A6 one xenobiotic-responsive element (XRE) was found between-1498 and -1502 bp. In Caco-2 cells the reporter gene activity of the entire plasmid and of deletion mutants containing the XRE were TCDD-inducible, in contrast to experiments with a deletion mutant which did not contain the XRE. TCDD induction was marginal in transfection studies with A549 cells. Gel mobility shift analysis indicated that the aryl hydrocarbon receptor and its partner Arnt bind to the XRE. Furthermore, primer extension studies suggest cell-specific use of multiple TATA boxes. Hence, regulation of human UGT1A6 appears to be cell-specific including both constitutive and aryl hydrocarbon receptor-controlled expression.

Correlation of 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced apoptotic cell death in the embryonic vasculature with embryotoxicity

Vertebrate embryos are particularly sensitive to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Identification of tissues that are susceptible to the adverse effects of TCDD is requisite for understanding the embryo toxic effects of TCDD. The objective of the present study was to quantitate the temporal appearance of and dose dependence of apoptosis in TCDD-exposed medaka embryos (Oryzias latipes). A fluorescent-based DNA end-labeling assay provided a sensitive method for detection of TCDD-induced apoptosis in tissue sections of medaka embryos. Apoptotic cells were readily apparent in the medial yolk vein at all observed embryonic stages in TCDD-exposed embryos. Slope-comparison analysis indicated that TCDD-induced programmed cell death in the embryonic medial yolk vein was mechanistically linked to embryo mortality. These data are consistent with the hypothesis that vascular damage contributes to the acute embryo toxic effects of TCDD. However, as sublethal concentrations of dioxin-like compounds are more typical of environmental exposures, tissue damage was also assessed in medaka fry that were exposed to low doses of TCDD during embryonic development. Cell death was detected in gill and digestive tissues in visibly healthy medaka fry that had been exposed to low doses of TCDD during embryonic development. Increased expression of cytochrome P450 1A is a major biochemical consequence of TCDD exposure and is often used as a biomarker for exposure to dioxin-like compounds. Therefore, we compared the tissue distribution of TCDD-induced P450 1A expression and TCDD-induced programmed cell death. TCDD-induced programmed cell death co-localized with TCDD-induced P450 1A expression in both embryos and in visibly healthy post-hatch fry. Our results suggest that aberrant programmed cell death may be a suitable marker for exposure of feral organisms to dioxin-like compounds.

Expression of the human aryl hydrocarbon receptor complex in yeast. Activation of transcription by indole compounds

The human aryl hydrocarbon receptor (AHR) and aryl hydrocarbon receptor nuclear translocator protein (ARNT) were coexpressed in the yeast Saccharomyces cerevisiae to create a system for the study of this heterodimeric transcription factor. Specific transcriptional activation mediated by AHR/ARNT heterodimer, which is a functional indicator of receptor expression, was assessed by beta-galactosidase activity produced from a reporter plasmid. Yeast expressing AHR and ARNT displayed constitutive transcriptional activity that was not augmented by addition of AHR agonists in strains that required exogenous tryptophan for viability. In contrast, strains with an intact pathway for tryptophan biosynthesis responded to AHR agonists and had lower levels of background beta-galactosidase activity. Hexachlorobenzene, benzo(a)pyrene, and beta-naphthoflavone were effective AHR agonists in the yeast system, and had EC50 values of 200, 40, and 20 nM, respectively, for beta-galactosidase activity induction. Tryptophan, indole, indole acetic acid, and tryptamine activated transcription in yeast coexpressing AHR and ARNT (EC50 values approximately 300 microM). Indole-3-carbinol was an exceptionally potent AHR agonist (EC50 approximately 10 microM) in yeast. This yeast system is useful for the study of AHR/ARNT protein complexes, and may be generally applicable to the investigation of other multiprotein complexes.

A mutation in the aryl hydrocarbon receptor (AHR) in a cultured mammalian cell line identifies a novel region of AHR that affects DNA binding

Introduction of a retroviral expression vector for the aryl hydrocarbon receptor (AHR) restores CYP1A1 inducibility to a mutant derivative of the Hepa-1 cell line that is defective in induction of CYP1A1 by ligands for the receptor. An AHR protein with normal ligand binding activity is expressed in the mutant but ligand treatment of mutant cell extract fails to induce binding of the AHR. ARNT (aryl hydrocarbon receptor nuclear translocator) dimer to the xenobiotic responsive element (XRE). AHR cDNAs derived from the mutant encode a protein that is unimpaired in ligand-dependent dimerization with ARNT, but the AHR.ARNT dimer so formed is severely impaired in XRE binding activity. The mutant cDNAs contain a C to G mutation at base 648, causing a cysteine to tryptophan alteration at amino acid 216, located between the PER-ARNT-SIM homology region (PAS) A and PAS B repeats. Introduction of the same mutation in the wild-type AHR sequence by site-directed mutagenesis similarity impaired XRE binding activity. Substitution with the conservative amino acid, serine, had no effect on XRE binding. The tryptophan mutation, but not the wild-type allele, was detectable in genomic DNA of the mutant. The implication that an amino acid within the PAS region may be involved in DNA binding indicates that the DNA binding behavior of AHR may be more anomalous than previously suspected.

Differentiation status of cultured murine keratinocytes modulates induction of genes responsive to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Primary murine keratinocytes were cultured in a chemically defined, serum-free medium which facilitated manipulation of their differentiation status. Exposure of basal cell and differentiating cultures to >/= 0.1 nM 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) preferentially elevated 7-ethoxyresorufin O-deethylase specific activities in differentiating cultures (28-fold versus 4-fold increases after 36 h of exposure). Semiquantitative reverse-transcription polymerase chain reaction (RT-PCR) analyses demonstrated the presence of constitutive mRNA transcripts corresponding to four known TCDD-inducible genes (e.g., Cyp1a1, Cyp1b1, Ahd4, and Nmo1) in both differentiating and proliferating cultures of murine keratinocytes. All four genes were induced in differentiating cultures following exposure to TCDD. No induction occurred in comparably treated basal cell cultures. Indirect immunofluorescence analyses demonstrated the presence of aryl hydrocarbon receptor (AHR) and aryl hydrocarbon receptor nuclear translocator (ARNT) proteins in both basal and differentiating keratinocytes. Both proteins appeared to be associated with the nucleus and their nuclear association was independent of prior exposure to TCDD. These studies suggest that AHR activation in murine skin is regulated as a function of the keratinocyte differentiation program.

Growth-related signaling in vascular smooth muscle cells is deregulated by TCDD during the G0/G1 transition

Experiments have been conducted to examine the impact of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on growth-related signaling in vascular smooth muscle cells (SMCs). A 40% reduction of peak DNA synthesis was observed in SMCs only when TCDD was added during the G0/G1 transition of the cell cycle. Enhanced phosphorylation of several endogenous proteins during this period was coincident with increased tyrosine kinase activity as early as 15 min following TCDD challenge. No changes in protein phosphorylation status occurred in cells treated with TCDD during the G1/S transition or during S phase. Cotreatment of quiescent SMCs with 10 nM TCDD and serum for 3 h reduced serum-inducible binding activity to a 12-O-tetradecanoyl phorbol 13-acetate responsive element (TRE) by approximately 40%. No alterations of constitutive TRE binding were observed in quiescent SMCs treated with TCDD for up to 5 h. These data show that mitogen-related signaling in vascular SMCs is modulated by TCDD selectively during the G0/G1 transition, and these effects influence the growth behavior of these cells.

Running exercise may reduce risk for lung and liver cancer by inducing activity of antioxidant and phase II enzymes

The effects of exercise, ethanol, and exercise plus ethanol-treatments on activity of superoxide dismutase (SOD), catalase (CAT), glutathione-S-transferase (GST) and UDP-glucuronosyl transferase (UDP-GT) in lung and liver were investigated. All treatments induced SOD and CAT activity in the lung while CAT activity was enhanced only by the combined treatments in the liver. Ethanol reduced hepatic SOD activity, while with the combined treatment SOD was normal. Exercise enhanced UDP-GT activity in liver and lung while ethanol had no effect and GST activity was induced in the liver by the combined treatment. Thus exercise may reduce risk for lung and hepatic cancer and prevent an ethanol-induced increase in risk for hepatic cancer by enhancing activity of antioxidant and phase II enzymes.

Downregulation of aryl hydrocarbon receptor function and cytochrome P450 1A1 induction by expression of Ha-ras oncogenes

The immortalized human epithelial cell line MCF10A has the phenotypic characteristics of normal breast cells. Exposure of MCF10A cultures to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) stimulated the transcriptional activation of cytochrome P450 1A1 (CYP1A1), and CYP1B1, and NAD(P)H:quinone oxidoreductase. Northern blot hybridization and nuclear run-on assays demonstrated that transcriptional activation of these genes was suppressed in stably transfected cultures expressing an Ha-ras oncogene (the MCF10A-NeoT line). Similar suppression did not occur in stably transfected lines carrying the expression vector or a normal c-Ha-ras protooncogene. Western blot analyses and immunofluorescence microscopy demonstrated that the lack of inducibility in MDF10A-NeoT cells reflected neither reductions in aryl hydrocarbon receptor (AHR) and aryl hydrocarbon nuclear translocator protein nor prevention of TCDD-induced AHR translocation to the nucleus. Suppression did correlate with reductions in DNA-AHR complex formation, as analyzed by gel retardation assays of soluble cell extracts treated in vitro with TCDD. The induction of Cyp1a-1 by TCDD was also analyzed in transgenic mice that expressed a v-Ha-ras oncogene exclusively in their keratinocytes. Relative to littermates lacking the transgene, the induction of Cyp1a-1 by TCDD was partially suppressed (about 50%) in the epidermises of v-Ha-ras-positive transgenic mice. However, normal levels of Cyp1a-1 induction occurred in the livers of the same mice. induction of Cyp1a-1 by TCDD was also suppressed (more than 98%) in chemically induced skin papillomas having Ha-ras mutations, relative to uninvolved surrounding skin. These studies suggest that the p21-ras protein controls signal transduction pathways capable of modulating AHR function.

Regulation of dioxin receptor function by omeprazole

The intracellular dioxin (aryl hydrocarbon) receptor mediates signal transduction by dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) and related environmental pollutants and functions as a ligand-activated transcription factor. In this study we have examined the effects on dioxin receptor function of a potentially novel ligand, omeprazole, which is widely clinically used as a gastric anti-ulcer drug. In primary human hepatocytes omeprazole potently induced cytochrome P4501A1 mRNA expression, whereas this effect was not detected in mouse primary hepatocytes. In human hepatoma cells omeprazole was found to induce transcription of reporter genes via the xenobiotic response element that is recognized by the ligand-activated dioxin receptor. In contrast, the human dioxin receptor was not activated by omeprazole upon expression in a receptor-deficient mouse hepatoma cell line. In a reconstituted yeast (Saccharomyces cerevisiae) model system, however, both the mouse and human dioxin receptors were potently activated by omeprazole. Although omeprazole failed to displace dioxin in in vitro ligand binding assays, a residue within the ligand binding domain that is critical for dioxin binding in vitro was also critical for omeprazole responsiveness in vivo. Consistent with this observation, both omeprazole and dioxin responsiveness of the dioxin receptor was inhibited in mutant yeast cells expressing low levels of the molecular chaperone hsp90 that is critical for ligand binding activity. The sulfoxide group that is essential for formation of a planar conversion product of omeprazole was found to be critical for dioxin receptor activation. Taken together, these data suggest that omeprazole represents a precursor for a novel class of dioxin receptor agonists that are bona fide dioxin receptor ligands but generated in a strictly species-specific manner.

Effect of transforming growth factor-beta1 on expression of aryl hydrocarbon receptor and genes of Ah gene battery: clues for independent down-regulation in A549 cells

An inhibitory effect on both constitutive and inducible expression of cytochrome P450 isoenzymes has been shown for different cytokines and growth factors. We previously described an inhibition of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced CYP1A1 mRNA and enzyme activity by transforming growth factor-beta1 (TGF-beta1) in human lung cancer A549 cells. In the present study, we report that not only TCDD-induced expression of CYP1A1 but also basal mRNA expression of CYP1A1, CYP1B1, and aryl hydrocarbon receptor (AHR) was down-regulated by TGF-beta1 in cells not treated with TCDD. In contrast, mRNA expression of the AHR partner protein Arnt (aryl hydrocarbon receptor nuclear translocator) was not influenced. Furthermore, TCDD-induced expression of CYP1B1 and NMO-1 was inhibited, and the IC50 values of 5-10 pM TGF-beta1 were in the same range as observed for inhibition of CYP1A1 and AHR mRNA expression. Transfection studies with a plasmid containing a luciferase reporter gene under control of two dioxin-responsive elements indicate an effect on AHR protein expression. Results of time-course studies revealed a parallel inhibition of AHR and CYP1 mRNA expression, indicating that TGF-beta1 is a direct negative regulator of transcription of these genes. The treatment of cells with cycloheximide led to a superinduction of TCDD-induced CYP1A1 and CYP1B1 mRNA expression and abolished the inhibitory effect of TGF-beta1 on basal as well as TCDD-induced CYP1 and AHR mRNA expression. TGF-beta1 seems not to influence the stability of AHR mRNA. The results suggest that TGF-beta1 induces rapid transcription and translation of an as-yet-unknown negative regulatory factor or factors that may directly regulate expression of AHR and genes of Ah gene battery.

Lipid-mediated transfection of normal adult human hepatocytes in primary culture

The aim of this work was to develop a procedure for the lipid-mediated transfection of DNA into normal adult human hepatocytes in culture. Cells were plated in a serum-free culture medium at various cell densities, on plastic or collagen-coated dishes, both in the absence and in the presence of epidermal growth factor (EGF). The cells were incubated for various periods of time with mixtures of DNA-lipofectin or DNA-3 beta[N-(N',N'-dimethylaminoethane)-carbamoyl] cholesterol (DC-chol) liposomes, and the efficiency of transfection was assessed by measuring the activity of reporter genes, beta-galactosidase or chloramphenicol acetyl-transferase (CAT). For comparison, similar experiments were carried out with human cell lines including HepG2, Caco-2, and WRL68. The efficiency of transfection (in percentage of cells) was not significantly different after transfection with lipofectin or DC-chol and comprised between 0.04 and 1.7% (extreme values) for different cultures. The efficiency of transfection decreased as the age or density of the culture increased and increased in cultures treated with EGF. Direct measurement of the rate of DNA synthesis suggested that the efficiency of transfection was related to the number of cells entering the S phase. Under the same conditions, the efficiency of transfection was one to two orders of magnitude greater in the three cell lines. A plasmid harboring 660 bp of the 5'-flanking region of CYP1A1 (containing two xenobiotic enhancer elements) fused upstream of the promoter of thymidine kinase and the CAT reporter gene was constructed. When this plasmid was transfected in human hepatocytes, CAT activity was induced as expected. We conclude that normal adult human hepatocytes can be transfected with exogenous DNA and that the transfected construct is regulated in the manner expected from in vivo studies.

Inhibitors of serine/threonine-specific protein phosphatases stimulate transcription by the Ah receptor/Arnt dimer by affecting a step subsequent to XRE binding

The Ah receptor binds aryl hydrocarbons such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) with high affinity. After binding aryl hydrocarbons, the receptor releases the 90-kDa heat shock protein and forms a dimer with the Arnt protein capable of binding at xenobiotic-responsive elements (XREs) and stimulating the transcription of genes involved in the metabolism of aryl hydrocarbons. The activity of the Ah receptor/ Arnt dimer can be decreased by treatments causing the down-regulation of protein kinase C and decreasing the nuclear accumulation of the receptor. Incubation with acid phosphatase or with alkaline phosphatase has been reported to block XRE binding. Thus the literature suggests that phosphorylation regulates Ah receptor activity by affecting DNA binding and/or nuclear transport. A reporter plasmid containing two XREs was used to investigate the effects of phosphatase inhibitors on TCDD-dependent transcription by the Hepa-1 mouse liver cell line. The inhibitors calyculin A and okadaic acid caused two- to threefold increases in TCDD-dependent transcription at concentrations capable of selectively inhibiting protein phosphatase 1 and protein phosphatase 2A. The inhibitor cyclosporin A doubled TCDD-dependent transcription at a concentration capable of selectively inhibiting protein phosphatase 2B. All three of the phosphatase inhibitors increased TCDD-dependent transcription without affecting transcription in the absence of TCDD. Nuclear extracts were prepared from cells treated with concentrations of okadaic acid or cyclosporin A which substantially stimulated TCDD-dependent transcription. Neither of the inhibitors significantly increased the level of TCDD-dependent XRE binding in the extracts. GAL4-Arnt fusion proteins were used to further investigate whether the phosphatase inhibitors affected a step other than DNA binding. Okadaic acid treatment specifically increased the ability of a GAL4 fusion protein containing the Arnt PAS and transactivation domains to stimulate transcription. These results suggest that serine/threonine-specific protein phosphatases can act at a level subsequent to XRE binding to inhibit the ability of the Ah receptor/Arnt dimer to stimulate transcription.

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.

Oxygen- and dioxin-regulated gene expression in mouse hepatoma cells

The discovery that the oxygen-regulated transcription factor HIF-1 alpha and the dioxin receptor AhR share the common heterodimerization partner ARNT (HIF-1 beta) raised the question whether a cross-talk between oxygen and dioxin signal transduction pathways exists. To answer this question we investigated an ARNT-deficient mutant cell line (Hepa1C4), which has lost its capability of responding to dioxin. The results demonstrate that the presence of ARNT is indispensable for hypoxia-inducible HIF-1 DNA binding as well as for oxygen-regulated reporter gene activity mediated by the EPO 3' hypoxia response element (HRE). Hypoxic induction of the vascular endothelial growth factor (VEGF) gene, however, was only partially abrogated in Hepa1C4 cells, suggesting that HIF-1-independent oxygen signaling pathways might exist. We further studied HIF-1 and AhR/ARNT DNA binding activity as well as the regulation of oxygen- and xenobiotic-responsive genes by treating mouse Hepa1 hepatoma cells with hypoxia and/or the dioxin analogue ICZ. Hypoxia-inducible VEGF expression was found to be independent of ICZ-treatment, whereas ICZ-inducible cytochrome P-450IA1 expression was slightly reduced by hypoxic treatment of the cells. Interestingly, the enhancer function of a xenobiotic response element (XRE) linked to a reporter gene was induced by hypoxia, but expression of a HRE-containing reporter gene was not affected by ICZ treatment.

Okadaic acid treatment induces DNA adduct formation in BHK21 C13 fibroblasts and HESV keratinocytes

Okadaic acid (OA), a toxin involved in diarrhetic shellfish poisoning (DSP), has been shown to be a potent tumor promoter in mouse skin and glandular stomach. However, more recent studies tended to show that OA can also act as a genotoxic. In this study, using the 32P-postlabelling method, DNA adduct formation was obtained in two cell lines (BHK21 C13 fibroblasts and HESV keratinocytes) after treatment by OA for 24 h with a dose range between 0.01 and 5 nM. Nineteen adducts were observed with BHK21 C13 cells and 15 with HESV ones. Low doses did not show adduct formation. Intermediate doses have given the most important number of adducts and with higher doses, the number of adducts decreased dose dependently. Ten adducts were similar in the two strains while 9 were specific of BHK21 C13 cell line and 5 of HESV one. The highest total DNA adduct level from origin parts was estimated at 95.6 adducts/10(9) nucleotides for BHK21 C13 fibroblasts (1 nM OA treatment) and 31.1 adducts/10(9) nucleotides for HESV keratinocytes (0.5 nM OA treatment). In this case, the major adduct (number 3) represented 20% for the fibroblastic cell line and 30% for the keratinocytic strain. The genotoxic effect of OA showed in this study should lead to a more careful survey of DSP outbreaks.

Identification of c-Src as the integral component of the cytosolic Ah receptor complex, transducing the signal of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) through the protein phosphorylation pathway

We have shown previously that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) under cell-free conditions causes a significant rise in protein tyrosine kinase activity of cytosol from male guinea pig adipose tissue, and that such an effect of TCDD is Ah-receptor dependent. In the present study, we obtained evidence indicating that c-Src protein kinase is the protein kinase activated by TCDD and that this kinase is associated specifically with the Ah-receptor-complex proteins in cytosol from adipose tissue and liver of guinea pig and liver of C57BI/6J mouse, and in NIH 3T3 mouse fibroblast cells. Here, we present evidence that c-Src protein is functionally attached to the Ah-receptor (AhR) and is specifically activated upon ligand binding. This conclusion is based on several lines of evidence: (a) TCDD caused activation of protein tyrosine kinase activity when administered directly to purified Ah-receptor immunoprecipitate; (b) this stimulatory effect of TCDD was abolished when the cytosol was immunodepleted of c-Src protein or Ah-receptor protein by preincubating with anti-c-Src or anti-Ah-receptor antibody, followed by the addition of TCDD to the remaining portions of cytosol; (c) when Ah-receptor immunoprecipitate was incubated with TCDD, and the kinase(s) released to the supernatant was analyzed on autoradiography of two-dimensional (2D) electrophoresis, 32P-labeled c-Src protein was recognized; (d) the same 32P-labeled-phosphoprotein with M(r) = 60 kDa and pl = 6.1 was found in the immunoprecipitate with anti-c-Src antibody on 2D autoradiograms; (e) this same phosphoprotein disappeared when the supernatant of the Ah-receptor immunoprecipitate was immunodepleted of c-Src protein by anti-c-Src antibody; and (f) a structure-activity relationship study with TCDD and three dioxin-congeners revealed a rank order for their potency in activation of c-Src kinase activity to be identical to that of previously determined toxicity indices: i.e., TCDD > 1,2,3,7,8-pentachlorodibenzo-p-dioxin (1,2,3,7,8-PCDD) > 1,2,4,7,8-pentachlorodibenzo-p-dioxin (1,2,4,7,8-PCDD) > 2,7-dichlorodibenzo-p-dioxin (2,7-DCDD). Consistent with these results, TCDD-induced c-Src kinase activity was abolished when c-Src immunoprecipitate's suspension was preincubated with 0.1 or 1 microM alpha-naphthoflavone (AhR blocker) for 10 min prior to the addition of TCDD. In addition, pretreatment of 3T3 fibroblast cells with 3-methylcholanthrene abolished TCDD-induced c-Src kinase activity in AhR-immunoprecipitate. We conclude that c-Src protein kinase is associated specifically with the AhR complex along with hsp90 in the cytosol of these cells and that upon ligand binding to the Ah-receptor subunit, c-Src is activated and released from the complex.

Protein kinase C modulates regulation of the CYP1A1 gene by the aryl hydrocarbon receptor

Transcriptional activation of the human CYP1A1 gene by halogenated and polycyclic aromatic hydrocarbons is mediated by the aryl hydrocarbon receptor (AhR) complex, a ligand-dependent transcription factor. A competent AhR comprises at least two components following nuclear translocation and DNA binding, the AhR and the AhR nuclear translocator (Arnt) protein, whose combined action on human CYP1A1 gene transcription is shown to be dependent upon functional protein kinase C (PKC). In the present study, we examined the effects of phorbol 12-myristate 13-acetate, a potent PKC activator, on the ligand-induced transcriptional activation of the CYP1A1 gene and cellular function of the AhR in human HepG2 101L cells. The 101L cells carry a stable transgene consisting of 1800 bases of 5'-flanking DNA and the promoter of the human CYP1A1 gene linked to the firefly luciferase structural gene (Postlind, H., Vu, T. P., Tukey, R. H. & Quattrochi, L. C. (1993) Toxicol. Appl. Pharmacol. 118, 255-262). Pretreatment of cells with 12-myristate 13-acetate enhanced ligand-induced CYP1A1 gene expression 2-3-fold. Inhibition of PKC activity blocked directly the transcriptional activation and the transactivation of the CYP1A1 gene, indicating a role for PKC in the AhR-mediated transcriptional activation process. However, the DNA binding activities of the in vitro activated and the induced nuclear AhR as measured by electrophoretic mobility shift analysis were not affected when CYP1A1 transcription was inhibited, indicating the actions of PKC to be a nuclear event that works in concert with or precedes AhR binding to the gene. These results illustrate that PKC is absolutely essential for the cellular and molecular events that control induction of CYP1A1 gene transcription.

Phenobarbital induction of CYP1A1 gene expression in a primary culture of rainbow trout hepatocytes

In mammals, phenobarbital (PB) is an in vivo inducer of the cytochrome P4502B (CYP2B) family, whereas in teleosts PB induction of cytochrome P450 is unclear. We show that teleost cytochrome P4502K1 (CYP2K1) protein levels and 7-pentoxyresorufin-O-deethylase activity were not induced by exposure of primary cultures of rainbow trout hepatocytes to PB. Instead, cytochrome P4501A1 (CYP1A1) gene expression was strongly induced by PB, based upon observations of marked increases in CYP1A1 mRNA, CYP1A1 protein, and 7-ethoxyresorufin-O-deethylase activity. In accordance with these data we provide a temporal study employing antibodies for the aromatic hydrocarbon (Ah) receptor that showed an increase in Ah receptor in nuclear extracts prepared from cells exposed to PB. Employment of the electrophoretic mobility shift assay (EMSA) showed PB to cause activation or "transformation" of the Ah receptor in nuclear extracts. Studies employing actinomycin D and cycloheximide indicated that PB induction of CYP1A1 was regulated at both the transcriptional and post-transcriptional levels. Nuclear run-off experiments confirm that PB causes an increase in CYP1A1 transcription. Inhibition of protein synthesis led to the superinduction of CYP1A1 mRNA, suggesting the regulation of teleost CYP1A1 may involve a labile repressor protein. These findings suggest that PB induction of the CYP1A1 gene involves the Ah receptor and is via transcriptional activation.

Cooperative interaction between AhR.Arnt and Sp1 for the drug-inducible expression of CYP1A1 gene

Expression of CYP1A1 gene is regulated in a substrate-inducible manner through at least two kinds of regulatory DNA elements in addition to the TATA sequence, XRE (xenobiotic responsive element), and BTE (basic transcription element), a GC box sequence. The trans-acting factor on the XRE is a heterodimer consisting of arylhydrocarbon receptor (AhR) and AhR nuclear translocator (Arnt), while Sp1 acts as a regulatory factor on the BTE. We have investigated how these factors interact with one another to induce expression of the CYP1A1 gene. Both in vivo transfection assays using Drosophila Schneider line 2 (SL2) cells, which is devoid of endogenous Sp1, AhR, and Arnt, and in vitro transcription assays using baculovirus-expressed AhR, Arnt, and Sp1 proteins revealed that these factors enhanced synergistically expression of the reporter genes driven by a model CYP1A1 promoter, consisting of four repeated XRE sequences and a BTE sequence, in agreement with previous observation (Yanagida, A., Sogawa, K., Yasumoto, K., and Fujii-Kuriyama, Y. (1990) Mol. Cell. Biol. 10, 1470-1475). We have proved by coimmunoprecipitation assays and DNase I footprinting that both AhR and Arnt interact with the zinc finger domain of Sp1 via their basic HLH/PAS domains. When either the AhR.Arnt heterodimer of Sp1 was bound to its cognate DNA element, DNA binding of the second factor was facilitated. Survey of DNA sequences in the promoter region shows that the XRE and GC box elements are commonly found in the genes whose expressions are induced by polycyclic aromatic hydrocarbons, suggesting that the two regulatory DNA elements and their cognate trans-acting factors constitute a common mechanism for induction of a group of drug-metabolizing enzymes.

Induction of CYP1A1 gene by benzimidazole derivatives during Caco-2 cell differentiation. Evidence for an aryl-hydrocarbon receptor-mediated mechanism

The Caco-2 cell line, derived from a human colon adenocarcinoma, is unique in its property of spontaneously differentiating into a mature enterocyte cell type during its growth in culture. In this work, we compared the response of the CYP1A1 gene with the benzimidazole derivatives omeprazole and lansoprazole, and with the classical inducer beta-naphthoflavone in the Caco-2 cells at various culture stages. In addition, we characterized the Caco-2 aryl-hydrocarbon receptor. The protein-synthesis inhibitor cycloheximide led to a derepression of the CYP1A1 gene transcription, and to a superinduction when combined with either beta-naphthoflavone or benzimidazoles. Taking advantage of the spontaneous differentiation of Caco-2 cells in long-term cultures, we observed a difference in behavior between the classical inducer beta-naphthoflavone and the atypical inducer omeprazole. In the poorly differentiated cells, both compounds elicited comparable dose/response and rate of induction of CYP1A1 gene expression. In the fully differentiated cells, in contrast, the induction by omeprazole was only transient, whereas the response to beta-naphthoflavone was long lasting. The Caco-2 aryl-hydrocarbon receptor exhibited binding characteristics similar to those determined for human liver and other tissues. The induction of CYP1A1 transcription by benzimidazole derivatives in Caco-2 cells occurred with no direct binding of benzimidazole derivatives to the aryl-hydrocarbon receptor, as in human hepatocytes. However, transient transfection experiments clearly showed that the xenobiotic-responsive element enhancer, with which the activated aryl-hydrocarbon receptor interacts, could drive the induction of a heterologous promoter in the presence of benzimidazoles. Finally the presence of the activated aryl-hydrocarbon receptor in the nuclei of the Caco-2 cells exposed to these molecules was clearly demonstrated by gel-retardation experiments. These results question about the mechanism of ligand-independent activation of the aryl-hydrocarbon receptor and intracellular signaling, initiated by benzimidazole derivatives.

Identification of a novel domain in the aryl hydrocarbon receptor required for DNA binding

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that binds DNA in the form of a heterodimer with the AHR nuclear translocator protein (ARNT). Both proteins possess basic helix-loop-helix motifs. ARNT binds to the side of the xenobiotic responsive element (XRE) that resembles an E-box (the sequence recognized by the majority of other basic helix-loop-helix proteins), whereas AHR binds to the side of the XRE that does not conform to the E-box sequence. The basic region of ARNT closely resembles those of other E-box-binding proteins, whereas the "nominal basic region" of AHR (amino acids 27 39), although required for XRE binding, deviates from this consensus. By extensive mutational analysis it is shown here that an additional block of amino acids of AHR (from tyrosine 9 to lysine 20) that contains a highly basic segment is required for XRE binding and transcriptional activation. Deletion of the first nine amino acids negates XRE binding. Substitution of either tyrosine 9 or arginine 14 with alanine eliminates XRE binding, whereas alanine substitutions at certain other sites within the block reduce but do not eliminate binding. The reported absence of the first nine amino acids in the purified protein may therefore be artifactual. These results suggest that the amino acids of AHR involved in binding to the XRE constitute a novel DNA-binding domain, comprising amino acids located within and amino-terminal to the nominal basic region.

Definition of a minimal domain of the dioxin receptor that is associated with Hsp90 and maintains wild type ligand binding affinity and specificity

The dioxin receptor is a cytoplasmic basic helix-loop-helix/Per-Arnt-Sim homology (bHLH/PAS) protein known to bind planar polycyclic ligands including polycyclic aromatic hydrocarbons, benzoflavones, heterocyclic amines, and halogenated aromatic hydrocarbons, e.g. dioxins. Ligand-induced activation of the dioxin receptor initiates a process whereby the receptor is transformed into a nuclear transcription factor complex with a specific bHLH/PAS partner protein, Arnt. In analogy to the glucocorticoid receptor, the latent dioxin receptor is found associated with the molecular chaperone hsp90. We have defined and isolated a minimal ligand binding domain of the dioxin receptor from the central PAS region, comprising of amino acids 230 to 421, and found this domain to interact with hsp90 in vitro. Expression of the minimal ligand binding domain in wheat germ lysates or bacteria, systems which harbor hsp90 homologs unable to interact with the glucocorticoid or dioxin receptors, resulted in non-ligand binding forms of this minimal 230 to 421 fragment. Importantly, affinity of the minimal ligand binding domain for dioxin was similar to the affinity inherent in the full-length dioxin receptor, and a profile of ligand structures which specifically bound the minimal ligand binding domain was found to be conserved between this domain and the native receptor. These experiments show that the minimal ligand binding domain maintains the quantitative and qualitative aspects of ligand binding exhibited by the full-length receptor, implying that the central ligand binding pocket may exist to accommodate all classes of specific dioxin receptor ligands, and that this pocket is critically dependent upon hsp90 for its ligand binding conformation.

Effect of a negative regulatory element (NRE) on the human CYP1A1 gene expression in breast carcinoma MCF-7 and hepatoma HepG2 cells

The expression of the cytochrome P4501A1 gene, CYP1A1, is induced by e.g. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) mainly by transcriptional mechanisms. The inducers mediate their effect upon binding and activation of the aryl hydrocarbon receptor (AHR) transcription-factor complex. Utilizing chimeric CYP1A1/CAT constructs transient gene expression experiments indicate that the putative negative regulatory element (NRE) of CYP1A1 influence the relative TCDD induced CAT activity in HepG2 cells, whereas this effect was not observed in MCF-7 cells. Differences in the formation of cell-specific protein-DNA complexes were demonstrated by gel retardation assays suggesting a functional difference of NRE in these two cell lines.

Modulation of gene expression and endocrine response pathways by 2,3,7,8-tetrachlorodibenzo-p-dioxin and related compounds

The aryl hydrocarbon (Ah) receptor binds several different structural classes of chemicals, including halogenated aromatics, typified by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polynuclear aromatic and heteropolynuclear aromatic hydrocarbons. TCDD induces expression of several genes including CYP1A1, and molecular biology studies show that the Ah receptor acts as a nuclear ligand-induced transcription factor that interacts with xenobiotic or dioxin responsive elements located in 5'-flanking regions of responsive genes. TCDD also elicits diverse toxic effects, modulates endocrine pathways and inhibits a broad spectrum of estrogen (17 beta-estradiol)-induced responses in rodents and human breast cancer cell lines. Molecular biology studies show that TCDD inhibited 17 beta-estradiol-induced cathepsin D gene expression by targeted interaction of the nuclear Ah receptor with imperfect dioxin responsive elements strategically located within the estrogen receptor-Sp1 enhancer sequence of this gene.

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.

Aberrant CYP1A1 induction: discrepancy of CYP1A1 mRNA and aryl hydrocarbon hydroxylase activity in mutant cells of mouse hepatoma line, Hepa-1

We have isolated new benzo[a]pyrene-resistant clones, cl-21 and cl-32, of the mouse hepatoma line, Hepa-1. CYP1A1-dependent aryl hydrocarbon hydroxylase activity is not inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin or 3-methylcholanthrene in these two cell lines. However, mRNA of CYP1A1 is inducible in cl-21 and cl-32 cells, as in the wild-type cells, in spite of an undetectable level of cytosolic Ah receptor. The cl-21 cDNA of Cyp1a-1 was found to have a single mutation leading to an amino acid substitution from Leu (118) to Arg (118). However, the CYP1A1 protein band was not detected on Western immunoblots. The cDNA of cl-32 was found to have a single mutation leading to an amino acid change from Arg (359) to Trp (359). The presence of the mature protein in cl-32 was confirmed by Western blot analysis. Somatic cell hybridization experiments demonstrated that the phenotype of cl-21 and cl-32 is recessive and that these clones belong to the same complementation group. These data suggest that there may be a non-Ah receptor-mediated mechanism of CYP1A1 induction.

The aromatic hydrocarbon receptor, dioxin action, and endocrine homeostasis

The environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin acts through the aromatic hydrocarbon (AH) receptor to enhance CYP1A1 gene transcription. There are notable mechanistic analogies between dioxin action and lipophilic hormone action. In addition, some of dioxin's adverse effects are associated with disruptions in endocrine homeostasis. Thus, dioxin biology intersects several disciplines, including toxicology, endocrinology, molecular biology, and public health.

Biphasic modulation of protein kinase C (PKC) activity by polychlorinated dibenzo-p-dioxins (PCDDs) in serum-deprived rat aortic smooth muscle cells

Previous studies in this laboratory have shown that benzo(a)pyrene (BaP) modulates protein kinase C (PKC)-mediated phosphorylation of aortic smooth muscle cell (SMC) proteins. This observation is consistent with the ability of other aromatic hydrocarbons (AHs), such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), to modulate kinase activities in cells of hepatic, testicular, and thymic origin. Because all these chemicals share the ability to bind the aryl hydrocarbon receptor (AhR), the present studies were conducted to determine if changes in PKC activity by AHs conform with established structure-activity relationships. Experiments were conducted to examine the effects of TCDD, 2,3,7,8-tetrachlorodibenzofuran (TCDF), and 2,8-dichlorodibenzodioxin (DCDD) on the phosphorylation of exogenous histone type-III under basal and PKC-activating conditions. These congeners exhibit both high (TCDD and TCDF) and low (DCDD) AhR agonist activities. Measurements of kinase activity were conducted in the cytosolic and particulate fractions of growth-arrested (i.e., serum-deprived) cultured rat aortic SMCs incubated with 10 nM TCDD, TCDF, and DCDD for 0.5, 12, or 24 hours. No changes in basal kinase activity were induced by these chemicals at any of the times tested. Significant decreases in cytosolic and particulate PKC activity relative to controls were observed upon exposure of SMCs for 0.5 hours to 10 nM TCDD, TCDF, and DCDD. In contrast, SMCs exposed to TCDD and TCDF for 12 hours exhibited a significant increase in PKC activity in both cytosolic and particulate fractions. The PKC activity in cells exposed to DCDD for 12 hours was not altered.(ABSTRACT TRUNCATED AT 250 WORDS)

Transforming growth factor-beta 1 inhibits TCDD-induced cytochrome P450IA1 expression in human lung cancer A549 cells

The effect of transforming growth factor-beta 1 (TGF-beta 1) on the expression of cytochrome P450IA1 (CYPIA1) was examined in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated human lung cancer A549 cells. Using the reverse transcription-polymerase chain reaction (RT-PCR) it was demonstrated that TGF-beta 1 inhibits CYPIA1 expression in a dose dependent manner. Based on the inhibitory concentration 50 (IC50) of about 5 pM it is suggested that TGF-beta 1 has a physiological function in downregulation of this cytochrome. In the presence of cycloheximide, the effect of TGF-beta 1 on CYPIA1 mRNA disappeared. This finding indicates that protein synthesis may be required for the TGF-beta 1 mediated response of CYPIA1. The possible mechanisms by which TGF-beta 1 interacts with TCDD-responsive drug metabolizing enzymes are discussed.