The receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin in the mouse hepatoma cell line Hepa 1c1c7. A comparison with the glucocorticoid receptor and the mouse and rat hepatic dioxin receptors

Regulation of aryl hydrocarbon receptor expression and function by glucocorticoids in mouse hepatoma cells

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that mediates most biological responses to 2,3, 7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related aromatic hydrocarbons. Although the role of the AHR in control of drug metabolism and endocrine disruption is partly understood, we know little about the regulation of the AHR itself by endocrine factors. Our work with hypophysectomized rats suggested that hepatic AHR protein level is positively regulated by pituitary-dependent factors. A current hypothesis is that adrenal glucocorticoids elevate AHR expression and enhance responsiveness to AHR agonists. Dexamethasone (DEX) at concentrations that activate the glucocorticoid receptor (GR) increased AHR mRNA, protein, and TCDD-binding by approximately 50% in Hepa-1 mouse hepatoma cells. This response was blocked by the GR antagonist 17beta-hydroxy-11beta-[4-dimethylamino phenyl]-17alpha-[1-propynyl]estra-4,9-dien-3-one (RU486), suggesting GR involvement. This small magnitude increase in AHR levels was functionally significant; pretreatment of Hepa-1 cells with DEX caused a 75% increase in the maximum induction of an AHR-activated luciferase reporter plasmid by TCDD. A luciferase reporter under control of the proximal 2.5 kilobases of the mouse Ahr 5'-flanking region and promoter was induced approximately 2.5-fold by DEX when cotransfected with a mouse GR expression plasmid. This is the first demonstration that glucocorticoids increase AHR levels in hepatoma cells via a GR-dependent transcriptional mechanism, suggesting a novel aspect of cross-talk between the AHR and the GR.

Pharmacogenetics and regulation of human cytochrome P450 1B1: implications in hormone-mediated tumor metabolism and a novel target for therapeutic intervention

Several of the hormone-mediated cancers (breast, endometrial, ovarian, and prostate) represent major cancers in both incidence and mortality rates. The etiology of these cancers is in large part modulated by the hormones estrogen and testosterone. As advanced disease develops, the common treatment for these cancers is chemotherapy. Thus, genes that can alter tissue response to hormones and alter clinical response to chemotherapy are of major interest. The cytochrome P450 1B1 (CYP1B1) may be involved in disease progression and modulate the treatment in the above hormone-mediated cancers. This review will focus on the pharmacogenetics of CYP1B1 in relation to hormone-mediated cancers and provide an assessment of cancer risk based on CYP1B1 polymorphisms and expression. In addition, it will provide a summary of CYP1B1 gene regulation and expression in normal and neoplastic tissue.

Induction of cytochrome P450 1A by cow milk-based formula: a comparative study between human milk and formula

During the treatment of neonatal apnea, formula-fed infants, compared to breastfed infants, show nearly three-fold increase in clearance of caffeine, a substrate of cytochrome P450 1A (CYP1A) and in part CYP3A4. However, human milk is known to contain higher concentrations of environmental pollutants than infant formula, which are potent CYP1A inducers. To gain insight into the mechanism underlying this apparent contradiction, we characterized CYP1A and CYP3A4 induction by human milk and cow milk-based infant formula. The mRNA and protein expression of CYP1A1/1A2 were significantly induced by cow milk-based formula, but not by human milk, in HepG2 cells. Luciferase reporter assay demonstrated that cow milk-based formula but not human milk activated aryl hydrocarbon receptor (AhR) significantly. The cotreatment of 3,4-dimethoxyflavone, an AhR antagonist, abolished the formula-induced CYP1A expression. In addition, AhR activation by dibenzo[a,h]anthracene, a potent AhR agonist, was significantly suppressed by infant formula and even more by human milk. In contrast, CYP3A4 mRNA expression was only mildly induced by formula and human milk. Consistently, neither formula nor human milk substantially activated pregnane X receptor (PXR). Effects of whey and soy protein-based formulas on the AhR-CYP1A and the PXR-CYP3A4 pathways were similar to those of cow milk-based formula. In conclusion, infant formula, but not human milk, enhances in vitro CYP1A expression via an AhR-mediated pathway, providing a potential mechanistic basis for the increased caffeine elimination in formula-fed infants.

Failure of Ah receptor to mediate induction of cytochromes P450 in the CYP1 family in the human hepatoma line SK-Hep-1

The Ah receptor mediates the induction of cytochrome P450 1A1 (CYP1A1) and toxicities of 2,3,7,8tetrachlorodibanzo-p-dioxin (TCDD). It has been detected in tissues of many species and in murine and human hepatoma lines. We show that the human hepatoma line SK-Hep-1 has cytosolic Ah receptor detectable by specific binding of [3H]TCDD. Concentrations of Ah receptor were low (mean = 43 +/- 3 fmol/mg cytosol protein compared to 430 fmol/mg protein in Hepa-1); the estimated number of receptor sites per cell is approximately 9,000, compared to 35,000 in Hepa-1. Ah receptor in SK-Hep-1 cells was physicochemically similar to Ah receptor in C57BL/6 mouse liver and in other human hepatoma lines studied to date except that binding affinity for TCDD, the most avidly bound ligand, was lower (estimated Kd was 14 nM by Woolf plot analysis). Translocation of the Ah receptor-ligand complex to the nucleus was shown; binding of the activated Ah receptor-ligand complex to an XRE in the 5'-upstream region of the CYP1A1 gene was demonstrated by gel-shift analysis. However, after SK-Hep-1 cells were incubated with typical PAHs including 3-methylcholanthrene, benzanthracene, and dibenz(a,h)anthracene, each over a wide range of concentrations, no induction of aryl hydrocarbon hydroxylase activity was detectable. On Northern analysis, no message for human CYP1A1 was detected in mRNA prepared from noninduced SK-Hep-1 cells or from cells treated for 24 h with 13 microM dibenz(a,h)anthracene. Further analysis by RT-PCR did not detect the induction of CYP1A1, CYP1A2, or CYP1B1 message in response to 10(-7) M TCDD, 10(-5) M benzanthracene, or 10(-5) M 3-methylcholanthrene. Transient transfection of reporter constructs containing either a minimal promoter or the CYP1A1 promoter fused to a reporter gene (luciferase) did not show any expression in response to increasing concentrations of TCDD up to 10(-8) M. Estimation of the size of the transcripts for AhR and ARNT protein revealed normal sizes, 2.7 and 2.4 kb, respectively. Together, these data suggest that SK-Hep-1 cells express an Ah receptor defective at the level of trans-activation of gene expression. SK-Hep-1 is the first human hepatoma line described with a demonstrable defect in CYP1A1 or its regulation.

Expression of the aryl hydrocarbon receptor/transcription factor (AhR) and AhR-regulated CYP1 gene transcripts in a rat model of mammary tumorigenesis

Exposure to ubiquitous environmental chemicals, such as polycyclic aromatic hydrocarbons (PAH), may contribute to human breast cancer. In animals, PAH induce tumors in part by activating the aryl hydrocarbon receptor (AhR)/transcription factor. Historically, investigations into AhR-regulated carcinogenesis have focused on AhR-dependent transcriptional regulation of cytochrome P450 (CYP) enzymes which oxidize PAH to mutagenic intermediates. However, recent studies suggest that the AhR directly regulates cell growth. Given the postulated role of the AhR in carcinogenesis, we predicted that: (1) tissue predisposed to PAH tumorigenesis would express the AhR and (2) aberrant AhR and/or AhR-regulated gene expression would accompany malignant transformation. To test these hypotheses, AhR and CYP1 protein and/or mRNA levels were evaluated in rat mammary tumors induced with 7, 12-dimethylbenz[a]anthracene (DMBA), a prototypic PAH and AhR ligand. Results indicate modest AhR expression in normal mammary myoepithelial and ductal epithelial cells. In contrast, high AhR levels were detected in DMBA-induced tumors. Nuclear AhR localization in tumors suggested constitutive AhR activation. In situ hybridization and quantitative RT-PCR assays indicated high AhR mRNA levels in neoplastic epithelial cells. While both AhR-regulated CYP1A1 and CYP1B1 mRNAs were induced in breast tissue within 6 h of DMBA gavage, only CYP1B1 mRNA remained elevated in tumors. These results: (1) help explain targeting of breast tissue by carcinogenic PAH, (2) imply that AhR and CYP1B1 hyper-expression represent molecular biomarkers for, at least, PAH-induced mammary cell transformation, and (3) suggest mechanisms through which the AhR may contribute to carcinogenesis well after exogenous AhR ligands have been eliminated.

Cloning and functional characterization of the bile acid-sensitive methotrexate carrier from rat liver cells

We have cloned two complementary DNAs (cDNAs), RL-Mtx-1 and RL-Mtx-2, corresponding to the bile acid- sensitive methotrexate carrier from rat liver by direct full-length rapid amplification of cDNA ends polymerase chain reaction (RACE-PCR) using degenerated primers that were deduced from published sequences of tumor cell methotrexate transporters. When expressed in Xenopus laevis oocytes and cosM6 cells, both clones mediate methotrexate and bumetanide transport. RL-Mtx-1 consists of 2,445 bp with an open reading frame of 1,536 bp. The corresponding protein with 512 amino acids has a molecular weight of 58 kd. RL-Mtx-2 (2,654 bp) differs by an additional insert of 203 bp. This insert is located in frame at position 1,196 of the RL-Mtx-1 and contains the typical splice junction sites at the 5' and 3' end, indicating that the RL-Mtx-2 messenger RNA (mRNA) is generated by alternative splicing. The insert contains a stop codon that shortens the RL-Mtx-2 protein to 330 amino acids (38 kd). Both cDNAs contain the binding site sequence for the dioxin/nuclear translocator responsive element (Ah/Arnt-receptor) in conjunction with a barbiturate recognition sequence (Barbie box). Preliminary results show that the Barbie box acts as a negative regulatory element. The two liver cDNA clones show homologies to the published sequences of folate and the reduced folate carriers, but no homology is found to the transport systems for organic anions like the Ntcp1, oatp1, OAT-K1, and OAT1. Expression of the mRNA for the methotrexate carrier is found in liver, kidney, heart, brain, spleen, lung, and skeletal muscle, but not in the testis as revealed by Northern blot analysis. The highest abundance of the mRNA is found in the kidney.

Induction of PreB cell apoptosis by 7,12-dimethylbenz[a]anthracene in long-term primary murine bone marrow cultures

Numerous studies demonstrate that polycyclic aromatic hydrocarbons (PAH) suppress immunity by modifying the function of both B and T cells. Relatively few studies have assessed the effects of these common environmental chemicals on immature lymphocytes. In the present study, long-term primary bone marrow cultures were employed to investigate the effects of a prototypic PAH and aryl hydrocarbon receptor (AhR) agonist, 7,12-dimethylbenz[a]anthracene (DMBA), on immature B lymphocytes. In this system, immature preB cells are maintained in a supportive microenvironment provided by bone marrow stromal cells. Results presented here demonstrate that (1) exposure of primary bone marrow cultures to DMBA results in preB cell death by apoptosis; (2) notably low doses of DMBA (> or = 10(-8) M) induce preB cell apoptosis; (3) in long-term cultures, bone marrow stromal cells, but not preB cells, express AhR mRNA and protein as determined by in situ hybridization, RT-PCR, and immunoblotting; (4) freshly isolated unfractionated bone marrow cells, but not purified bone marrow B cells, express AhR protein as assessed by immunohistochemistry; (5) alpha-naphthoflavone, a competitive AhR inhibitor and cytochrome P450 antagonist, completely blocks DMBA-induced preB cell apoptosis in primary bone marrow cultures; and (6) DMBA or benzo[a]pyrene injection in vivo results in bone marrow cell apoptosis consistent with the death of hematopoietic cells clustered around stromal elements. The results implicate programmed cell death as a mechanism underlying DMBA-mediated immunosuppression and suggest that preB cell death is influenced by local interactions with AhR+ bone marrow stromal cells.

Steroid receptor interactions with heat shock protein and immunophilin chaperones

We have provided a historical perspective on a body of steroid receptor research dealing with the structure and physiological significance of the untransformed 9S receptor that has often confused both novice and expert investigators. The frequent controversies and equivocations of earlier studies were due to the fact that the native, hormone-free state of these receptors is a large multiprotein complex that resisted description for many years because of its unstable and dynamic nature. The untransformed 9S state of the steroid and dioxin receptors has provided a unique system for studying the function of the ubiquitous, abundant, and conserved heat shock protein, hsp90. The hormonal control of receptor association with hsp90 provided a method of manipulating the receptor heterocomplex in a manner that was physiologically meaningful. For several steroid receptors, binding to hsp90 was required for the receptor to be in a native hormone-binding state, and for all of the receptors, hormone binding promoted dissociation of the receptor from hsp90 and conversion of the receptor to the DNA-binding state. Although the complexes between tyrosine kinases and hsp90 were discovered earlier, the hormonal regulation or steroid receptor association with hsp90 permitted much more rapid and facile study of hsp90 function. The observations that hsp90 binds to the receptors through their HBDs and that these domains can be fused to structurally different proteins bringing their function under hormonal control provided a powerful linkage between the hormonal regulation of receptor binding to hsp90 and the initial step in steroid hormone action. Because the 9S receptor hsp90 heterocomplexes could be physically stabilized by molybdate, their protein composition could be readily studied, and it became clear that these complexes are multiprotein structures containing a number of unique proteins, such as FKBP51, FKBP52, CyP-40, and p23, that were discovered because of their presence in these structures. Further analysis showed that hsp90 itself exists in a variety of native multiprotein heterocomplexes independent of steroid receptors and other 'substrate' proteins. Cell-free systems can now be used to study the formation of receptor heterocomplexes. As we outlined in the scheme of Fig. 1, the multicomponent receptor-hsp90 heterocomplex assembly system is being reconstituted, and the importance of individual proteins, such as hsp70, p60, and p23, in the assembly process is becoming recognized. It should be noted that our understanding of the mechanism and purpose of steroid receptor heterocomplex assembly is still at an early stage. We can now speculate on the roles of receptor-associated proteins in receptor action, both as individuals and as a group, but their actual functions are still vague or unknown. We can make realistic models about the chaperoning and trafficking of steroid receptors, but we don't yet know how these processes occur, we don't know where chaperoning occurs in the cell (e.g. Is it limited to the cytoplasm? Is it a diffuse process or does chaperoning occur in association with structural elements?), and, with the exception of the requirement for hormone binding, we don't know the extent to which the hsp90-based chaperone system impacts on steroid hormone action. It is not yet clear how far the discovery of this hsp90 heterocomplex assembly system will be extended to the development of a general understanding of protein processing in the cell. Because this assembly system is apparently present in all eukaryotic cells, it probably performs an essential function for many proteins. The bacterial homolog of hsp90 is not an essential protein, but hsp90 is essential in eukaryotes, and recent studies indicate that the development of the cell nucleus from prokaryotic progenitors was accompanied by the duplication of genes for hsp90 and hsp70 (698). (ABSTRACT TRUNCATED)

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.

Identification of functional aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator in murine splenocytes

The objective of the present studies was to determine whether the aryl hydrocarbon receptor (AhR) and AhR nuclear translocator (ARNT) protein are present and functional in B6C3F1 (C57BL/6 x C3H) mouse splenocytes. Northern analysis of poly(A) RNA isolated from splenocytes revealed transcripts of approximately 6.6 kb which hybridized to the AhR complementary DNA (cDNA) probe. Anti-AhR antibodies identified two major cytosolic forms of the AhR in splenocytes, approximately 95 and 104 kDa, corresponding to the codominately expressed Ahrb alleles in the B6C3F1 mice. Northern analysis utilizing an oligomer probe for ARNT identified three messenger RNA (mRNA) transcripts, approximately 5.6, 2.0, and 1.1 kb, in spleen which was consistent with the banding pattern observed in the B6C3F1 mouse liver. Western blotting confirmed the presence of the approximately 87 kDa ARNT protein in splenocytes. Protein quantitation by slot blot analysis demonstrated approximately 2.0-fold more AhR in liver than in splenocytes. Interestingly, ARNT was approximately 2.4-fold more abundant in splenocytes than in liver. Consistent with these results, comparison by quantitative reverse transcriptase-polymerase chain reaction analysis of AhR and ARNT transcripts in liver and splenocytes demonstrated approximately 2.3-fold more AhR transcripts in liver than in splenocytes and approximately 3.2-fold more ARNT transcripts in splenocytes than in liver. In addition, comparisons between AhR and ARNT transcripts isolated from the liver and splenocytes indicated a greater number of ARNT transcripts as compared with AhR in both preparations. TCDD treatment of splenocytes induced binding of the AhR nuclear complex to the dioxin-responsive enhancer (DRE) as detected by the electrophoretic mobility shift assay. These findings confirm that the AhR and ARNT are present in mouse splenocytes and are capable of binding to the DRE.

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

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor with a basic region/helix-loop-helix (bHLH) motif. AhR has been sequenced and the functional domains defined and there is information on the formation of complexes with other peptides and interactions with DNA, although these areas continue to be investigated. AhR mediates many biological effects such as developmental toxicity, including induction of cleft palate and hydronephrosis. This regulatory protein is expressed in embryonic liver and has been immunohistochemically localized in cells of human and mouse secondary palate. The expression of AhR in embryonic tissues and its ability to disrupt development suggests a significant role for this protein in development. The present study examines the pattern of AhR expression in the C57BL/6N mouse embryo from gestation days (GD) 10-16, using in situ hybridization and immunohistochemical analysis. AhR mRNA was localized with 35S-RNA antisense riboprobe (cAh1 probe, 1.8 Kb amino terminal DNA). AhR protein was localized with purified monoclonal antibody (RPT-9) raised against the N-terminal peptide sequence. AhR mRNA and protein were expressed in GD 10-13 neuroepithelium, and as development progressed the levels in brain decreased. GD 10-12 embryos also showed AhR in branchial arches, heart, somites, and liver. AhR protein and mRNA in heart were highest at GD 10-11 and decreased with age. In liver, AhR mRNA and protein levels increased and nuclear localization became more pronounced with gestational age. In GD 14-16 embryos levels in liver and adrenal were highest, but AhR was present in ectoderm, bone, and muscle. AhR expression was specific for both cell type, organ/tissue, and developmental stage, suggesting that this novel ligand-activated transcriptional regulator may be important in normal embryonic development.

A genetic analysis of processes regulating cytochrome P4501A1 expression

Cytochrome P4501A1 and its associated aryl hydrocarbon hydroxylase activity are highly inducible in the mouse hepatoma cell line, Hepa-1, by substrates of the enzyme and related compounds, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Mutants of this cell line, deficient in P4501A1 inducibility, were isolated. Some of the mutants show a dominant phenotype. Such mutants may have resulted from a genetic alteration leading to the inappropriate activation of a repressor gene that normally functions to restrict high level inducibility to the liver and certain other organs or to certain developmental stages. One dominant mutant was shown to express a protein that prevents binding of the liganded aryl hydrocarbon (Ah) receptor (which mediates induction of P4501A1) to its recognition sequence in DNA (the xenobiotic responsive element, or XRE). The majority of mutants are recessive, and were assigned to four different complementation groups (which probably correspond to four different genes). Gene A corresponds to the structural gene (Cyp1a-1) for P4501A1. Mutations in genes B, C and D all affect functioning of the Ah receptor. A cDNA for gene C was cloned. The encoded protein (ARNT) is required for ligand-dependent translocation of the Ah receptor to the nucleus and its binding to the XRE. ARNT and the Ah receptor form a heterodimeric complex which binds the XRE in a fashion such that both subunits bind the XRE directly. Both ARNT and the Ah receptor contain basic helix-loop-helix motifs. Such motifs have been identified in several transcription factors that bind DNA as heterodimers or homodimers. The roles of the proteins corresponding to the B and D genes are presently under investigation.

A comparison of the mouse versus human aryl hydrocarbon (Ah) receptor complex: effects of proteolysis

The differences in the molecular properties of the nuclear aryl hydrocarbon (Ah) receptor from human Hep G2 and mouse Hepa 1c1c7 cells were investigated by time-dependent partial proteolysis with chymotrypsin or trypsin followed by column chromatographic and velocity sedimentation analysis. The sedimentation coefficients, Stokes radii and apparent molecular weights of the untreated human and mouse Ah receptor complexes were similar. Treatment of the nuclear Ah receptor complexes from both cell lines with chymotrypsin for 10 or 60 min gave lower molecular weight proteolytic products which also exhibited comparable molecular properties and salt gradient elution profiles from Sepharose columns linked to DNA. Treatment of the human and mouse nuclear Ah receptor complexes with trypsin (5 micrograms/mg protein) for 10 or 60 min gave a minor low molecular weight (29.7- or 25.7-kDa) proteolysis product which was detected only with the mouse Hepa 1c1c7 Ah receptor complex. The time- and concentration-dependent proteolytic digest maps of the human and mouse Ah receptor were determined using receptor preparations which were photoaffinity labeled with [125I]7-iodo-2, 3-dibromodibenzo-p-dioxin. The human Ah receptor was significantly more resistant to proteolysis by trypsin or chymotrypsin than the mouse Ah receptor. At a low concentration of chymotrypsin (1 microgram/mg protein) the Hepa 1c1c7 receptor was degraded to two lower molecular weight fragments with apparent M(r) values at 71- and 48-kDa whereas the Hep G2 Ah receptor was relatively stable under these conditions. Although the human Ah receptor was more slowly hydrolyzed than the mouse receptor by trypsin, the major photolabeled breakdown products for the Ah receptor from both cell lines were observed at M(r) 48- and 45-kDa. The results of this study demonstrate that there were subtle but significant differences in the human and mouse Ah receptor complex; however, the proteolysis studies suggest that there are common structural features in their ligand binding sites.

Effects of ligand structure on the in vitro transformation of the rat cytosolic aryl hydrocarbon receptor

Incubation of radiolabeled, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF),1,2,3,7,8-pentachlorodibenzo-p-dioxin(PeCDD), 1,2,3,7,8-pentachlorodibenzofuran (PeCDF), 1,2,7,8-TCDF, and 2,3,7-trichlorodibenzo-p-dioxin (TrCDD) with rat hepatic cytosol for 2 h at 0 degrees C gave liganded aryl hydrocarbon (Ah) receptor complexes which were indistinguishable as determined by velocity sedimentation analysis and DNA-Sepharose column chromatography. Incubation of the cytosol plus the different radioligands for 2 h at 20 degrees C resulted in the formation of Ah receptor complexes which exhibited increased retention times on DNA-Sepharose columns. It was apparent that the amount of specifically bound Ah receptor complex or the levels of the transformed Ah receptor complex which eluted from the column with 0.2-0.3 M salt were dependent on the structure of the radioligand. For example, after incubation for 2 h at 20 degrees C the overall yields of the specifically bound transformed Ah receptor complex were 3.4, 2.0, 1.2, 1.9, 0.3, and 0.1%, respectively, using 2,3,7,8-TCDD, 2,3,7,8-TCDF, 1,2,3,7,8-PeCDD, 1,2,3,7,8-PeCDF, 1,2,7,8-TCDF, and 2,3,7,8-TrCDD as radioligands. A more quantitative measure of the structure-dependent transformation of the liganded cytosolic Ah receptor complex was determined using a gel retardation assay with a consensus synthetic dioxin-responsive element (DRE) (26-mer, duplex). The EC50 values obtained for the concentration-dependent formation of the retarded DRE-Ah receptor complex using 2,3,7,8-TCDD, 1,2,3,7,8-PeCDD, 2,3,7,8-TCDF, 1,2,3,7,8-PeCDF, 2,3,7-TrCDD, and 1,2,7,8-TCDF as ligands were 0.26, 0.35, 0.78, 1.75, 27.0, and 220 nM, respectively. The structure-dependent differences in these values were similar to their different potencies as Ah receptor agonists and these data suggest that the structure-dependent transformation of the liganded cytosolic Ah receptor may significantly contribute to the structure-activity relationships observed for 2,3,7,8-TCDD and related compounds.

Identification of the Ah receptor nuclear translocator protein (Arnt) as a component of the DNA binding form of the Ah receptor

The Ah (dioxin) receptor binds a number of widely disseminated environmental pollutants, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and polycyclic aromatic hydrocarbons, and mediates their carcinogenic effects. The ligand-bound receptor activates Cyp 1a1 gene transcription through interaction with specific DNA sequences, termed xenobiotic responsive elements (XREs). The Ah receptor nuclear translocator protein (Arnt) is required for Ah receptor function. Arnt is now shown to be a structural component of the XRE binding form of the Ah receptor. Furthermore, Arnt and the ligand-binding subunit of the receptor were extracted as a complex from the nuclei of cells treated with ligand. Arnt contains a basic helix-loop-helix motif, which may be responsible for interacting with both the XRE and the ligand-binding subunit.

cDNA cloning and structure of mouse putative Ah receptor

Mouse cDNA clones for a putative Ah receptor have been isolated from a cDNA library of mRNA from Hepa-1 cells by an oligonucleotide probe produced by PCR with a pair of primers which was synthesized according to the reported N-terminal sequence of 26 amino acids. The cDNA clones encode a polypeptide of 805 amino acids with a helix-loop-helix motif and with some similarity to a certain region designated PAS of Drosophila Per and Sim, and human Arnt protein. Cotransfection of an expression vector of the Ah receptor with a reporter plasmid pMC6.3k consisting of CYP1A1 promoter and CAT structural gene into CV-1 cells enhanced the CAT expression in response to added 3-methylcholanthrene.

Chemical cross-linking of the cytosolic and nuclear forms of the Ah receptor in hepatoma cell line 1c1c7

Both cytosolic and high salt nuclear extracts were isolated from Hepa 1c1c7 cells incubated with 2-azido-3[125I]iodo-7,8-dibromo-dibenzo-p-dioxin ([125I]N3Br2DpD). The [125I]N3Br2DpD-labeled cytosolic fraction was subjected to chemical cross-linking with dimethyl pimelimidate and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Chemical cross-linking of the cytosolic form of the AhR revealed monomeric (97 kDa), dimeric (185 kDa), trimeric (281 kDa), and tetrameric (327 kDa) complexes. In a time course of exposure to the cross-linking reagent, the largest form given above became the predominant AhR form observed in the cytosolic extracts. The 327 kDa cytosolic species apparently consists of a 97 kDa AhR, an approximately 88 kDa protein, an approximately 96 kDa protein, and an approximately 46 kDa protein. Nuclear extracts from [125I]N3Br2DpD-labeled Hepa 1c1c7 cells were applied to sucrose density gradients. The 6 S nuclear receptor peak fractions were pooled and subjected to chemical cross-linking. Analysis by SDS-PAGE revealed a monomeric (97 kDa) ligand binding protein and a dimeric (182 kDa) complex. This would suggest that the nuclear 6 S AhR consists of a 97 kDa AhR and an approximately 85 kDa protein. These findings would indicate that the AhR exists in cytosol as a tetrameric species, while in the nucleus the AhR exists as a heterodimer.

Comparison of the nuclear and cytosolic forms of the Ah receptor from Hepa 1c1c7 cells: charge heterogeneity and ATP binding properties

2-[125I]iodo-7,8-dibromo-p-dioxin ([125I]Br2DpD) and 2-[125I]iodo-3-azido-7,8-dibromo-p-dioxin ([125I]N3Br2-DpD) are both capable of binding to the Ah receptor (AhR) with a high degree of specificity in cultured Hepa 1c1c7 cells. After incubation with either [125I]N3Br2DpD or [125I]Br2DpD Hepa 1c1c7 cytosolic and high salt nuclear extracts were analyzed by sucrose density gradient analysis with the following results: (i) With both radioligands an approximately 9 S form of the AhR was observed in cytosolic extracts. (ii) Nuclear extracts labeled with [125I]N3Br2DpD revealed both approximately 6 S and approximately 9 S forms of the AhR. (iii) In contrast, analysis of nuclear extracts labeled with [125I]Br2DpD revealed only an approximately 6 S form of the AhR. The approximately 9 S [125I]N3Br2DpD-labeled AhR was preferentially extracted with 100 mM KCl from a nuclear fraction and mixed with monoclonal antibody 8D3, an anti-90-kDa heat shock protein antibody. Monoclonal antibody 8D3 was able to bind to the approximately 9 S nuclear form of the AhR and caused the receptor to sediment as a heavier complex on sucrose density gradients. This would indicate that the AhR can reside in the nucleus bound to 90-kDa heat shock protein. The [125I]N3Br2DpD-labeled approximately 6 S peak fractions were collected and subjected to denaturing two-dimensional gel electrophoresis. A comparison of [125I]N3Br2DpD-labeled cytosolic (9 S) AhR preparations with the nuclear (6 S) AhR by 2-D gel electrophoresis was performed. The cytosolic form of the AhR was present in the apparent pI range of 5.2-5.7; the nuclear form focused between 5.5 and 6.2. The [125I]N3Br2DpD-labeled nuclear extracts were incubated with ATP-agarose and 43% of the photoaffinity-labeled AhR bound to the affinity gel. In contrast, approximately threefold lower binding of [125I]N3Br2DpD-labeled receptor was obtained when GTP-, AMP-, or ADP-agarose was used. Only 2% of the [125I]N3Br2DpD-labeled cytosolic AhR was able to bind to ATP-agarose. These results suggest that after the AhR translocates into the nucleus the following biochemical changes occur: (i) The sedimentation value for the AhR changes from an approximately 9 S to an approximately 6 S species. (ii) The AhR attains the ability to bind with specificity to ATP. (iii) The AhR undergoes a shift to a more basic pI.

Induction of glutathione-S-transferase and heat-shock proteins in rat liver after ethylene oxide exposure

Defense mechanisms in rat liver against depletion of glutathione (GSH) and cellular injuries induced by ethylene oxide (EO) were studied. Rats were exposed to EO under either high dose (1300 ppm for 4 hr, once) or low dose (500 ppm for 6 hr, three times a week for 6 weeks) conditions. The hepatic content of GSH decreased dramatically after EO treatment, probably due to detoxication of EO. After the high dose treatment the hepatic GSH content fell by 90% of the control values but recovered within 10 to 15 hr. EO reacts directly with a variety of cellular macromolecules but all rats survived the exposure. Since the metabolites of EO are ethylene glycol and GSH-conjugates, the enzymatic activities of epoxide hydrolase and glutathione-S-transferase (GST) were determined. Only GST activity was found to occur after low dose chronic exposure. The defense mechanism at mRNA level was investigated using probes for GST and several heat-shock proteins (hsps). Enhanced accumulation of GST mRNA was detectable during the recovery period of rats after both high and low dose exposure to EO. Interestingly, both hsp32 (less than 40-fold) and hsp90 (less than 3-fold) mRNA increased after high dose exposure but the mRNA level of one of the major heat-shock proteins, hsp70, did not change under these conditions. Diethylmaleate, which is known to be a GSH depleter in liver, induced hsp32 mRNA only in rat liver, while hsp70 and hsp90 mRNA levels did not change when GSH was depleted. These results suggest that individual heat-shock proteins are induced in different ways under unphysiological conditions such as EO exposure.

Ah receptor mediating induction of cytochrome P450IA1 in a novel continuous human liver cell line (Mz-Hep-1). Detection by binding with [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin and relationship to the activity of aryl hydrocarbon hydroxylase

The Ah receptor regulates induction of cytochrome P450IA1 and mediates certain toxicities of polyhalogenated aromatics such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). It has been characterized previously in continuous cell lines, notably the mouse hepatoma line Hepa 1, the human squamous cell carcinoma line A431, and the human liver cell line Hep G2. The present work extends our knowledge of the Ah receptor in continuous human liver cell lines. Ah receptor can be detected in Mz-Hep-1, a hepatitis B virus-negative cell line derived from a Thorotrast-induced hepatocellular carcinoma. The mean concentration of Ah receptor in Mz-Hep-1 cells was 341 +/- 22 fmol/mg cytosol protein (mean +/- SEM, nine separate determinations). This is equivalent to approximately 30,000 sites per cell. The concentration of Ah receptor in Mz-Hep-1 cells is similar to that in Hepa 1 cells and approximately three times higher than that in Hep G2 cells. The Mz-Hep-1 Ah receptor sedimented in continuous sucrose gradients at approximately 9 S. Specificity of binding by [3H]TCDD was demonstrated by competitive binding of non-radiolabeled 2,3,7,8-tetrachlorodibenzofuran, 3-methylcholanthrene (MC), and dibenz[a,h]anthracene in 50-fold molar excess. Phenobarbital, which is not a substrate for P450IA1, did not compete with [3H]TCDD for binding to Mz-Hep-1 Ah receptor. Dexamethasone and estradiol also did not compete with [3H]TCDD for binding, suggesting non-identity of Ah receptor with glucocorticoid or estrogen receptor. In separate experiments, glucocorticoid receptor was identified in Mz-Hep-1 cells. By Scatchard plot analysis, the apparent equilibrium dissociation constant (Kd) for binding of [3H]TCDD to Mz-Hep-1 Ah receptor was estimated to be 4.4 nM, compared to 0.8 nM in Hepa 1 cells. By Woolf plot analysis the Kd was 5.4 nM, compared to 1.2 nM in Hepa 1 cells. The [3H]TCDD.Ah receptor complex extracted from nuclei of Mz-Hep-1 cells incubated with [3H]TCDD in culture at 37 degrees sedimented at approximately 6 S under conditions of high ionic strength. Aryl hydrocarbon hydroxylase (AHH) activity was detectable in Mz-Hep-1 cells after pretreatment with inducing chemicals. Mz-Hep-1 cells have the highest concentrations of Ah receptor in any continuous human liver cell line thus far investigated. The Mz-Hep-1 Ah receptor is similar physicochemically to that described in murine systems. AHH activity is inducible in Mz-Hep-1 cells.

Inhibition and reconstitution of Ah receptor transformation in vitro: role and partial characterization of a cytosolic factor(s)

The Ah receptor binds 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related aryl hydrocarbons and mediates their biochemical and toxic effects by modifying gene expression. In order to interact with DNA, the TCDD.receptor complex must undergo a poorly understood transformation to a form which is distinguishable by its increased affinity for DNA-Sepharose and for its specific enhancer element upstream from the cytochrome P450IA1 gene. We have found that this transformation process is inhibited in vitro by treatment of rat hepatic cytosol with activated charcoal prior to addition of [3H]TCDD. The transformed form of the receptor can be generated in the charcoal-inhibited cytosol by incubation with hepatic cytosol from either DBA/2J mice (in which [3H]TCDD-specific binding is undetectable under these conditions) or rat (in which Ah receptor was prebound with unlabeled ligand). Transformation is observed whether this addition occurs before or after [3H]TCDD is bound to the charcoal-treated receptor. Thus, transformation is (i) mediated by some additional cytosolic component(s) and (ii) separable from ligand binding. The untransformed [3H]TCDD.receptor complex, isolated by DNA-Sepharose chromatography, can also be transformed if DBA mouse hepatic cytosol is added. This partially purified untransformed receptor preparation and gel retardation analysis were used to further characterize the transforming activity in DBA cytosol. We observed that the "Ah receptor transforming factor" is heat-labile, trypsin-sensitive, removed or inactivated by charcoal, of greater than approximately 50 kDa, and elutes from Superose at a Rs of approximately 6 nm. In conjunction with our previous studies documenting the increased molecular weight of the transformed compared to the untransformed Ah receptor, and identifying the heteromeric structure of the transformed receptor, we postulate that the ligand-binding subunit (the untransformed receptor) must associate tightly with another cytosolic protein, which is also present in the receptor-defective DBA mouse, in order to transform to the transcriptionally active receptor.

Cloning of a factor required for activity of the Ah (dioxin) receptor

The aryl hydrocarbon (Ah) receptor binds various environmental pollutants, such as polycyclic aromatic hydrocarbons, heterocyclic amines, and polychlorinated aromatic compounds (dioxins, dibenzofurans, and biphenyls), and mediates the carcinogenic effects of these agents. The complementary DNA and part of the gene for an 87-kilodalton human protein that is necessary for Ah receptor function have been cloned. The protein is not the ligand-binding subunit of the receptor but is a factor that is required for the ligand-binding subunit to translocate from the cytosol to the nucleus after binding ligand. The requirement for this factor distinguishes the Ah receptor from the glucocorticoid receptor, to which the Ah receptor has been presumed to be similar. Two portions of the 87-kilodalton protein share sequence similarities with two Drosophila proteins, Per and Sim. Another segment of the protein shows conformity to the consensus sequence for the basic helix-loop-helix motif found in proteins that bind DNA as homodimers or heterodimers.

Characterization of multiple forms of the Ah receptor: recognition of a dioxin-responsive enhancer involves heteromer formation

We have employed a combination of gel retardation, protein-DNA cross-linking, and protein-protein cross-linking techniques to further examine the 2,3,7,8-tetrachlorodibenzo-p- dioxin-(TCDD-) dependent changes in the Ah receptor that result in a DNA-binding conformation. Gel retardation analysis of DNA-Sepharose chromatographic fractions of rat hepatic cytosol indicated that TCDD-dependent and sequence-specific DNA binding coeluted with a 200-kDa form of the Ah receptor (peak 2) previously characterized as being multimeric and having high affinity for calf thymus DNA. The TCDD-bound, 100-kDa form of the receptor (peak 1) bound weakly to the DNA recognition motif. These results indicated that the DNA-binding form of the Ah receptor is a multimer. SDS-polyacrylamide gel electrophoresis of peak 2 cross-linked to a bromodeoxyuridine-substituted DNA recognition motif indicated that this form of the receptor present in rat hepatic cytosol is composed of at least two DNA-binding proteins of approximately 100 and 110 kDa. Using the chemical cross-linking agent dimethyl pimelimidate, we further established that the 100-kDa form of the receptor (peak 1) associates with a different protein to generate the receptor form (peak 2) that binds to the dioxin-responsive enhancer. Photoaffinity-labeling studies indicated that only the 100-kDa protein (peak 1), and not the 110-kDa protein, binds ligand. Together, these observations imply that the DNA-binding form of the Ah receptor exists as a heteromer.

Competitive binding of 7-substituted-2,3-dichlorodibenzo-p-dioxins with human placental ah receptor--a QSAR analysis

The competitive binding affinities of thirteen 7-substituted-2,3-dichlorodibenzo-p-dioxins to the human placental cytosolic aryl hydrocarbon (Ah) receptor were determined using [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin as the radioligand. Multiple parameter linear regression analysis of the competitive binding C50 values for these compounds gave the following equation: pEC50 (M) = 6.246 + 1.632 pi - 1.764 sigma 0m + 1.282 HB where pi, sigma m and HB are the physiochemical parameters for substituent lipophilicity, meta-directing electronegativity, and hydrogen bonding capacity respectively. The 7-t-butyl- and 7-phenyl-2,3-dichlorodibenzo-p-dioxins were treated as outliers for the derivation of this equation, and these results suggest that only substituents with van der Waals' volumes less than 40 cm3/mol were accommodated in the receptor binding site. The equations previously derived from the binding of the 7-substituted-2,3-dichlorodibenzo-p-dioxins to the rat, mouse, guinea pig, and hamster hepatic cytosolic receptor were different than the correlation obtained using human placental receptor and provide further evidence for the interspecies differences in the molecular and binding properties of the Ah receptor protein.

Analysis of photoaffinity-labeled aryl hydrocarbon receptor heterogeneity by two-dimensional gel electrophoresis

The level of charge heterogeneity in the aryl hydrocarbon receptor (AhR) was examined by high-resolution denaturing two-dimensional (2D) gel electrophoresis. Hepa 1c1c7 cell cytosolic fraction was photoaffinity-labeled with 2-azido-3-[125I]iodo-7,8-dibromodibenzo-p-dioxin and applied to isoelectric focusing (IEF) tube gels. After optimization of focusing conditions a broad peak of radioactivity was detected in the apparent pI range of 5.2-5.7. IEF tube gels were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by visualization of the radiolabeled AhR by autoradiography; three distinct isoforms were detected. The same 2D electrophoretic isoform pattern was obtained when the AhR from Hepa 1c1c7 was photoaffinity-labeled in cell culture. BPrCl cells, a mutant line derived from Hepa 1c1c7 cells, contain an AhR that is unable to bind to DNA. Photoaffinity-labeled BPrCl cytosolic fractions were subjected to 2D gel electrophoretic analysis resulting in essentially the same molecular weight and isoform pattern as seen in Hepa 1c1c7 cytosol. This result would suggest that if a mutation is present in the BPrCl AhR it has not caused a significant change in its IEF pattern, although a small shift in the pI values was observed. Two-dimensional gel electrophoresis of photoaffinity-labeled cytosolic fractions from HeLa cells, the rat liver tumor cell line McA-RH7777, and buffalo rat thymus revealed three isoforms, essentially the same isoform pattern as in Hepa 1c1c7 cells. This would indicate that despite the considerable molecular weight polymorphism between species the level of charge heterogeneity is highly conserved.

Characterization of the Ah receptor mediating aryl hydrocarbon hydroxylase induction in the human liver cell line Hep G2

The Ah receptor, a soluble cytoplasmic receptor that regulates induction of cytochrome P450IA1 and mediates toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), was detected and characterized in the continuous human liver cell line Hep G2. The mean concentration of specific binding sites for TCDD was 112 +/- 26 (SEM) fmol/mg cytosol protein as determined in eight separate cytosol preparations in the presence of sodium molybdate. This is equivalent to 14,000 binding sites per cell, approximately 40% of the sites per cell found in the mouse hepatoma line Hepa-1. The cytosolic Ah receptor from Hep G2 cells sedimented at 9 S and was specific for those halogenated and nonhalogenated aromatic compounds known to be agonists for the Ah receptor in rodent tissues and cells. Specific binding in the 9 S region was detected with both [3H]TCDD and 3-[3H]methylcholanthrene. 3-[3H]Methylcholanthrene did not bind to any component besides that at approximately 9 S. Phenobarbital, dexamethasone, and estradiol did not compete with [3H]TCDD for binding to the Hep G2 Ah receptor. Specific binding of [3H]triamcinolone acetonide to glucocorticoid receptor could also be demonstrated in Hep G2 cytosol. The apparent equilibrium dissociation constant (Kd) for binding of [3H]TCDD to Hep G2 Ah receptor was 9 nM by Woolf plot analysis, about an order of magnitude weaker than the affinity of [3H]TCDD for the mouse Hepa-1 Ah receptor or for the C57BL/6 murine hepatic Ah receptor. [3H]TCDD.Ah receptor complex, which was extracted from nuclei of Hep G2 cells incubated with [3H]TCDD at 37 degrees C in culture, sedimented at approximately 6 S under conditions of high ionic strength. Aryl hydrocarbon hydroxylase (AHH) activity was significantly induced after 24 h of incubation with polycyclic aromatic hydrocarbons: the EC50 for AHH induction was 5.3 microM for benz(a)anthracene and 1.3 microM for 3-methylcholanthrene. Modification of the preparative technique for cell cytosol, especially inclusion of 20 mM sodium molybdate in homogenizing and other buffers, was necessary to detect cytosolic Hep G2 Ah receptor. Hep G2 cells appear to conserve drug-metabolizing activity associated with cytochrome P450IA1 as well as the receptor mechanism which regulates its induction.

Ah receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin in rat liver: lack of sensitivity to alkaline phosphatase when compared with that of glucocorticoid receptor

Rat hepatic cytosol was treated with alkaline phosphatase in order to determine if dephosphorylation altered the ability of Ah receptor to bind 2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin (TCDD). Glucocorticoid receptor was studied for comparison. As previously had been shown in other laboratories, treatment of cytosol with purified alkaline phosphatase dramatically reduced the subsequent ability of glucocorticoid receptor to bind hormone. However, alkaline phosphatase had no effect on the ability of Ah receptor to bind [3H]TCDD. If either glucocorticoid receptor or Ah receptor was occupied by its ligand prior to exposure to alkaline phosphatase there was no loss in ligand binding capacity. Crude alkaline phosphatase (containing some protease activity) substantially reduced the ability of glucocorticoid receptor to bind hormone and shifted the sedimentation position of the glucocorticoid receptor from approximately 8 S to approximately 2 S. Crude alkaline phosphatase did not reduce the ability of Ah receptor to bind [3H]TCDD and did not alter sedimentation of the 9 S [3H]TCDD. Ah receptor complex. Although the Ah receptor appears to be a member of the steroid receptor superfamily, the lack of effect of alkaline phosphatase on Ah receptor (compared to the sensitivity of glucocorticoid receptor) highlights another significant difference in molecular characteristics between the Ah receptor and the receptors for steroid hormones.

Induction of cytochrome P450-dependent monooxygenase activities in rat hepatoma H-4-IIE cells in culture by 2,3,7,8-tetrachlorodibenzo-p-dioxin and related compounds: mechanistic studies using radiolabeled congeners

Treatment of rat hepatoma H-4-IIE cells in culture with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF), 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), 1,2,3,7,8-pentachlorodibenzofuran (PeCDF), 1,2,7,8-TCDF, and 2,3,7-trichlorodibenzo-p-dioxin (TrCDD) resulted in the structure-dependent induction of aryl hydrocarbon hydroxylase and ethoxyresorufin O-deethylase activities. The induction potencies followed the order 2,3,7,8-TCDD greater than 2,3,7,8-TCDF greater than 1,2,3,7,8-PeCDD approximately 1,2,3,7,8-PeCDF greater than 1,2,7,8-TCDF greater than 2,3,7-TrCDD and were comparable to structure-toxicity relationships which have previously been reported. In contrast, many of the properties of these compounds were structure-independent. For example, using tritiated congeners of high specific activity (greater than 30 Ci/mmol) the sedimentation coefficients (S) for the nuclear and cytosolic aryl hydrocarbon (Ah) receptor complexes were 5-6 and 9-10 S, respectively, for all the radioligands. Moreover, examination of the processing of nuclear Ah receptor complexes for the radiolabeled congeners showed that after 6 h, the rates of nuclear processing were very low and varied between 0.006 and 0.0385 fmol degraded/mg protein/mg total DNA. These results were consistent with the reported stability and persistence of the nuclear Ah receptor complexes and in addition, there were no apparent structure-dependent differences in the processing rates. Inspection of the nuclear receptor levels and the corresponding induced enzyme activities for the congeners showed that there was a linear correlation between average nuclear receptor complex levels (18-42 h) and induced enzyme activities (32-42 h) for all six radioligands; these data indicated that the rates of cytochrome P450-dependent gene expression correlated with the levels of nuclear Ah receptor complex. In contrast, the accumulation of occupied nuclear receptor complexes in rat hepatoma H-4-IIE cells was structure-dependent and appeared to be one of the factors which governed the observed structure-induction and the previously reported structure-toxicity relationships for 2,3,7,8-TCDD and related halogenated aryl hydrocarbons.

Transient superinducibility of cytochrome P450c (CYP1A1) mRNA and transcription

Simultaneous treatment of the rat hepatocyte-derived cell line LCS7 with cycloheximide and polycyclic aromatic compounds increased CYP1A1 (cytochrome P450c) gene transcription rate four- to sixfold and mRNA levels 20-fold relative to the levels in cells treated with inducers alone. When cycloheximide was added up to 1 hr after inducer, a similar degree of superinduction occurred. However, if cycloheximide was added at 1.5 hr or later, superinduction did not occur, even though an increased transcription rate continued at these times in cells treated with inducers alone. Thus, treatment with cycloheximide revealed two phases to the response of the CYP1A1 gene to inducer. During the early phase, inhibition of protein synthesis could amplify the effect of inducer. During the later phase, transcription rate and CYP1A1 mRNA levels remained elevated due to inducer treatment, but could not be further elevated by inhibiting protein synthesis. The superinduction of CYP1A1 mRNA was also examined in primary hepatocyte cultures and in explant cultures of three tissues. These varied substantially in their superinduction response. All of these exhibited elevated levels of CYP1A1 mRNA following simultaneous treatment with inducer and cycloheximide; transcription rate was superinduced three- to fourfold in primary hepatocytes, and CYP1A1 mRNA levels were superinduced 20- to 25-fold in both kidney and lung explants. However, the delayed addition of cycloheximide had varying effects in different culture systems. In primary hepatocytes, addition of cycloheximide as long as 4 hr after addition of inducer resulted in superinduction equal to that which occurred when these agents were added together. In contrast, adding cycloheximide only 2.5 hr after adding inducer resulted in undetectable superinduction in kidney explants and diminished superinduction 70% in lung explants. Although the time course of cycloheximide responsiveness following inducer treatment has been studied in detail only in LCS7 cells, it appears that the length of the cycloheximide-responsive phase is different in different cell culture systems in in explants from different tissues. This may be related to the previously reported tissue-specific differences in CYP1A1 gene expression.

Repression of cytochrome P-450c gene expression by cotransfection with adenovirus E1a DNA

Gene expression of rat cytochrome P-450c (P-450c) depends upon inducible enhancers scattered in the 5'-upstream region of the gene. We show that expression of the P-450c gene is repressed by contransfection with adenovirus E1a DNA, regardless of the presence or absence of inducers, in a transient expression system of HeLa cells. Since cotransfection of either 13S or 12S E1a cDNA was effective in the repression, the region necessary for repression could be separated from that of transactivation of other adenovirus early genes. Moreover, we investigated the regions responsible for the inhibitory activity using in-frame deletion mutants lacking internal or external portions of the E1a proteins. The sequence responsible for the repression was located in the amino-terminal half of the E1a proteins. The inducible expression of the chimeric plasmid containing a 24-base-pair enhancer sequence of the P-450c gene placed in a heterologous promoter of SV40 was repressed by cotransfection with E1a DNA, suggesting strongly that the inhibitory effect of the E1a proteins upon P-450c gene expression was caused by blocking the enhancer activity.

Ligand-dependent interaction of the dioxin receptor with target DNA

Wild type and nuclear transfer deficient mouse hepatoma cell lines were used to study the specific DNA binding of a dioxin inducible factor. This factor interacts with XRE only after dioxin treatment and is absent in receptor mutant containing cells even after treatment. Thus, evidence is provided to substantiate the claim that the dioxin receptor is involved in the specific DNA interaction with dioxin response enhancer elements. It is also shown that the molybdate stabilised dioxin-receptor interacts with hsp90 suggesting that, in similarity to the glucocorticoid receptor, the dioxin receptor is kept in a non-transformed state in the absence of ligand.

Physicochemical characterization of the nuclear form of Ah receptor from mouse hepatoma cells exposed in culture to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Molecular properties of nuclear aromatic hydrocarbon (Ah) receptor from Hepa-1c1c9 (Hepa-1) cells were assessed by velocity sedimentation on sucrose gradients and by gel permeation chromatography on Sephacryl S-300. Nuclear Ah receptor was obtained by exposing intact cells to [3H]-2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) for 1 h at 37 degrees C in culture followed by extraction of receptor from nuclei with buffers containing 0.5 M KCl. The nuclear Ah receptor was compared to the cytosolic Ah receptor from the same cells. Under conditions of low ionic strength, the Ah receptor from Hepa-1 cytosol sedimented as a single 9.4 +/- 0.63 S binding peak that had a Stokes radius of 7.1 +/- 0.12 nm and an apparent relative molecular mass of 271,000 +/- 16,000. After prolonged (24 h) exposure to high ionic strength (0.5 M KCl), cytosol labeled with [3H]TCDD exhibited two specific binding peaks. The large form of cytosolic Ah receptor seen under high ionic strength conditions sedimented at 9.4 +/- 0.46 S, had a Stokes radius of 6.9 +/- 0.19 nm, and an apparent Mr 267,000 +/- 15,000. The smaller ligand-binding subunit generated by exposing cytosol to 0.5 M KCl sedimented at 4.9 +/- 0.62 S, had a Stokes radius of 5.0 +/- 0.14 nm, and an apparent Mr 104,000 +/- 12,000. Nuclear Ah receptor, analyzed under high ionic strength conditions, sedimented at 6.2 +/- 0.20 S, had a Stokes radius of 6.8 +/- 0.19 nm, and an apparent Mr 176,000 +/- 7000. Nuclear Ah receptor from rat H4IIE hepatoma cells was analyzed and found to have physicochemical characteristics identical to those of nuclear Ah receptor from the mouse Hepa-1 cells. The molecular mass of Hepa-1 nuclear Ah receptor was found to be statistically different from both the Mr approximately 267,000 cytosolic Ah receptor and the Mr approximately 104,000 subunit which were present in cytosol under high ionic strength conditions. Hepa-1 nuclear Ah receptor could not be converted to a smaller ligand-binding subunit by treatment with alkaline phosphatase, ribonuclease, or sulfhydryl-modifying reagents or prolonged exposure to 1.0 M KCl. Cytosolic Ah receptor from Hepa-1 cells was "transformed" by heating at 25 degrees C in vitro into a form with high affinity for DNA-cellulose. The transformed cytosolic Ah receptor, when analyzed under conditions of high ionic strength, sedimented at approximately 6 S, had a Stokes radius of approximately 6.7 nm, and an apparent Mr approximately 167,000.(ABSTRACT TRUNCATED AT 400 WORDS)

Radioligand-dependent differences in the molecular properties of the mouse and rat hepatic aryl hydrocarbon receptor complexes

A comparison of the molecular properties of the male Long-Evans rat and male C57BL/6 mouse hepatic cytosolic aryl hydrocarbon (Ah) receptor complex was determined using 2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin (TCDD) and 2,3,7,8-[3H]tetrachlorodibenzofuran (TCDF) as radioligands. In low salt buffer, the sedimentation coefficients, Stokes radii, relative molecular masses, frictional ratios, axial ratios and gel permeation chromatographic properties of the rat receptor complexes were ligand independent. In contrast, there were several ligand-dependent differences in the mouse Ah receptor complexes formed after incubation in low salt buffer and these include: sucrose density gradient analysis of the 2,3,7,8-[3H]TCDF receptor complex gave a 9.5 S specifically bound peak and a 2.6 S nonspecifically bound peak whereas the corresponding 2,3,7,8-[3H]TCDD receptor complex gave a single 9.6 S specifically bound peak; sucrose density gradient analysis of the two major peaks eluted from a Sephacryl S-300 column chromatographic separation of the 2,3,7,8-[3H]TCDF receptor complex gave two specifically bound peaks at 9.2 and 5.1 S. The molecular properties of the rat hepatic cytosolic receptor complexes incubated in high salt (0.4 M KCl) buffer were ligand independent with one exception, namely the significant difference in the sedimentation coefficient of the specifically bound disaggregated 2,3,7,8-[3H]TCDD receptor complex (6.8 S) and the corresponding 2,3,7,8-[3H]TCDF receptor complex (5.0 S). The major ligand-dependent differences in the mouse receptor complexes incubated in high salt (0.4 M KCl) were associated with the sedimentation coefficients of the complexes derived after direct incubation and after gel permeation chromatography. For example, both ligands gave two specifically bound complexes after chromatography on Sephacryl S-300 column and centrifugation of these fractions gave both the approximately 9 and approximately 5 S peaks; this suggested that there was some equilibration between the aggregated and disaggregated receptor complexes. The behavior of the 2,3,7,8-[3H]TCDF mouse receptor complex was similar after incubation in low or high salt buffer except that sucrose density gradient analysis of the gel permeation chromatographic fractions gave an additional specifically bound peak which sedimented at 7.2 S. These studies demonstrate that the molecular properties of the Ah receptor were dependent on the source of the cytosolic receptor preparation, the ionic strength of the incubation media, and the structure of the radioligand.

The dioxin receptor: a comparison with the glucocorticoid receptor

The physico-chemical properties of the dioxin and glucocorticoid receptors from rat liver and wild-type and mutant cell lines were investigated and compared. In rat liver, the receptors are virtually indistinguishable. Both are highly asymmetrical proteins with axial ratios of 12-15, have Stokes radii of 6 nm and sedimentation coefficients of approximately 4 S. This results in a calculated apparent mol. wt of approximately 100,000. The dioxin receptor from the mouse hepatoma cell line Hepa 1c1c7 represents an atypical form of the dioxin receptor with a pronounced tendency to aggregate to form Mr approximately equal to 300,000 complexes in high ionic strength and in the absence of sodium molybdate. In the presence of sulphydryl reducing agents, however, the Hepa 1c1c7 dioxin receptor dissociates to an Mr approximately 100,000 species. In analogy to the nt- mutant glucocorticoid receptor in mouse lymphoma cells, there is no gross change in the structure of the nt- dioxin mutant in mouse hepatoma cells compared with the wild-type receptor. The nt- dioxin receptor does, however, have a reduced affinity for DNA.

The dioxin receptor: characterization of its DNA-binding properties

The binding of the rat hepatic dioxin and glucocorticoid receptors to the polyanionic matrices heparin-Sepharose and DNA-cellulose in vitro and to cell nuclei in vivo was studied under various conditions. In a non-liganded and non-activated state both receptors eluted from heparin-Sepharose at a low ionic strength and were not retained on DNA-cellulose. Following ligandation and activation in vitro both receptors showed an increased affinity for heparin-Sepharose and were retained on DNA-cellulose. In analogy to these in vitro data, it was found that a high salt concentration (0.4 M KCl) was required to extract in vivo liganded dioxin receptor from purified nuclear preparations in contrast to that previously reported for non-liganded nuclear receptors. Limited proteolysis of both dioxin and glucocorticoid receptors resulted in molecular species of similar binding properties with regard to DNA-cellulose and heparin-Sepharose. We conclude that, in addition to the dioxin and glucocorticoid receptors showing considerable similarities in their physicochemical properties, they may also share a similar structural organization with regard to functional domains.