Elevated binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin and 3-methylcholanthrene to the Ah receptor in hepatic cytosols from phenobarbital-treated rats and mice

Aromatic hydrocarbon receptors in mitochondrial biogenesis and function

Mitochondria are ubiquitous membrane-bound organelles that not only play a key role in maintaining cellular energy homeostasis and metabolism but also in signaling and apoptosis. Aryl hydrocarbons receptors (AhRs) are ligand-activated transcription factors that recognize a wide variety of xenobiotics, including polyaromatic hydrocarbons and dioxins, and activate diverse detoxification pathways. These receptors are also activated by natural dietary compounds and endogenous metabolites. In addition, AhRs can modulate the expression of a diverse array of genes related to mitochondrial biogenesis and function. The aim of the present review is to analyze scientific data available on the AhR signaling pathway and its interaction with the intracellular signaling pathways involved in mitochondrial functions, especially those related to cell cycle progression and apoptosis. Various evidence have reported the crosstalk between the AhR signaling pathway and the nuclear factor κB (NF-κB), tyrosine kinase receptor signaling and mitogen-activated protein kinases (MAPKs). The AhR signaling pathway seems to promote cell cycle progression in the absence of exogenous ligands, whereas the presence of exogenous ligands induces cell cycle arrest. However, its effects on apoptosis are controversial since activation or overexpression of AhR has been observed to induce or inhibit apoptosis depending on the cell type. Regarding the mitochondria, although activation by endogenous ligands is related to mitochondrial dysfunction, the effects of endogenous ligands are not well understood but point towards antiapoptotic effects and inducers of mitochondrial biogenesis.

The mycotoxin, patulin, increases the expression of PXR and AhR and their target cytochrome P450s in primary cultured human hepatocytes

The mycotoxin, patulin (PAT), which is frequently found in apples, grapes, oranges, pear, peaches, and in apple juices, has previously been shown to be cytotoxic, genotoxic, and mutagenic. In this study, we have investigated the effect of PAT on mRNA level of pregnane X receptor (PXR), constitutive androstane receptor (CAR), aryl hydrocarbon receptor (AhR), and their corresponding target cytochrome P450s. Using primary cultures of adult human hepatocytes, we evaluated PAT cytotoxicity on hepatocytes after 24 hours of treatment. Real time reverse-transcriptase polymerase chain reaction procedure was employed to determine the effect of PAT on receptors (PXR, CAR, and AhR) and cytochrome (CYP3A4, 2B6, 3A5, 2C9, 1A1, and 1A2) genes. Our results showed that PAT reduced hepatocyte viability. At a noncytotoxic range of PAT concentrations, PAT induced an upregulation of the PXR gene in the three treated hepatocytes cultures, whereas CAR was overexpressed in only 1 treated liver. PXR gene induction was accompanied by the enhancement of CYP2B6, 3A5, 2C9, and 3A4 expression. PAT was also found to induce an overexpression of AhR and CYP1A1 and CYP1A2 mRNA expression. These findings suggested that PAT may activate PXR and/or CAR and AhR. However, further investigations are needed to confirm nuclear receptor activation by PAT and to elucidate the molecular mechanism of PAT action.

The activated aryl hydrocarbon receptor synergizes mitogen-induced murine liver hyperplasia

Mechanisms of hepatocyte proliferation triggered by tissue loss are distinguishable from those that promote proliferation in the intact liver in response to mitogens. Previous studies demonstrate that exogenous activation of the aryl hydrocarbon receptor (AhR), a soluble ligand-activated transcription factor in the basic helix-loop-helix family of proteins, suppresses compensatory liver regeneration elicited by surgical partial hepatectomy. The goal of the present study was to determine how AhR activation modulates hepatocyte cell cycle progression in the intact liver following treatment with the hepatomitogen, 1,4-bis[2-(3,5-dichloropyridyloxy)] benzene (TCPOBOP). Mice were pretreated with the exogenous AhR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) 24h prior to treatment with TCPOBOP (3 mg/kg).). In contrast to the suppressive effects of AhR activation observed during compensatory regeneration, TCDD pretreatment resulted in a 30-50% increase in hepatocyte proliferation in the intact liver of TCPOBOP-treated mice. Although pretreatment with TCDD suppressed CDK2 kinase activity and increased the association of CDK2 with negative regulatory proteins p21Cip1 and p27Kip1, a corresponding increase in CDK4/cyclin D1 association and CDK4 activity which culminated in enhanced phosphorylation of retinoblastoma protein, consistent with the increased proliferative response. These findings are in stark contrast to previous observations that the activated AhR can suppress hepatocyte proliferation in vivo and reveal a new complexity to AhR-mediated cell cycle control.

Regulating the regulator: Factors that control levels and activity of the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AHR) participates in a wide range of critical cellular events in response to endogenous signals or xenobiotic chemicals. Hence, it is important that AHR levels and activity themselves be well controlled in target tissues. The AHR is essentially ubiquitous in its distribution in mammalian tissues. However, levels of the receptor vary widely across different tissues and among different cell types. AHR levels and activity are modulated by exposure to the receptor's own ligands and are influenced by other xenobiotic chemicals. Many different factors impinge on AHR levels and AHR activity. These factors may alter responsiveness of downstream pathways, thereby affecting normal physiologic functions as well as responses to toxic environmental chemicals such as dioxins. Our commentary appraises the current literature on factors that regulate AHR levels/activity and attempts to identify fruitful strategies towards discovery of key pathways by which AHR levels are modulated in response to endogenous signals and in response to xenobiotic chemicals. An extraordinarily large number of agents alter the level or activity of the AHR. We have not yet entered an age of enlightenment sufficient to achieve true understanding of the interplay of mechanisms that regulate AHR expression in space and in time.

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

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

7-12 Dimethylbenz[a]anthracene-induced bone marrow hypocellularity is dependent on signaling through both the TNFR and PKR

In addition to being carcinogenic, polycyclic aromatic hydrocarbons (PAHs) are known to cause deleterious effects on the immune system, including a marked reduction in bone marrow granulocytes and B lymphocytes. The molecular mechanisms underlying bone marrow hypocellularity are incompletely understood. Hematopoiesis is governed by the production of cytokines and the resultant signaling pathways that they initiate. Our hypothesis was that PAHs may disrupt cytokine production in the bone marrow resulting in the perturbation in bone marrow cellularity observed after PAH administration. TNF-alpha and IFN-gamma are two cytokines that are involved in the regulation of hematopoiesis. Based on observations made in previous research, we sought to determine if the effects of 7-12 dimethylbenz[a]anthracene (DMBA) on the murine bone marrow were mediated through the actions of these molecules. Transgenic mice that were null for either IFN-gamma or TNF-alpha receptors were injected with DMBA and the resulting bone marrow cellularity compared with wild-type mice. We observed that tumor necrosis factor alpha receptor (TNFR) null mice were protected against DMBA-induced bone marrow hypocellularity, while IFN-gamma null mice were not. In addition, we found that dsRNA-dependent protein kinase (PKR) null mice were also protected from DMBA-induced hypocellularity. PKR is an intracellular signaling molecule that has been demonstrated to be activated by TNFR-mediated signaling. Furthermore, we observed upregulation of PKR in the bone marrow after DMBA administration that was dependent on signaling through TNFR. These results point to a role for TNFR-dependent signaling, operating at least in part via PKR activation, as a mechanism for DMBA-induced bone marrow toxicity.

Serum withdrawal leads to reduced aryl hydrocarbon receptor expression and loss of cytochrome P4501A inducibility in PLHC-1 cells

Changes in the expression of the aryl hydrocarbon receptor (AHR) have been documented in several systems and in response to a variety of treatments. The significance of these findings is unclear, because the effects of such changes on subsequent responses to AHR ligands seldom have been measured. We tested the ability of changes in serum used in cell culture medium to alter expression of the AHR and induction of cytochrome P4501A (CYP1A) in PLHC-1 teleost hepatoma cells. Culture of early-passage cells in serum-free medium for 2 days led to a loss of CYP1A inducibility by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In contrast, culture in 10% delipidated calf serum increased the TCDD-induced levels of both CYP1A protein and enzymatic activity relative to levels in cells cultured in 10% complete calf serum. These effects were consistent between 8 and 24hr post-treatment, indicating that the kinetics of induction were unaffected. In cells cultured in serum-free medium for 1 and 2 days there was a progressive loss of CYP1A inducibility. This loss of response paralleled a time-dependent decline in AHR protein, as measured by specific binding of [3H]TCDD. Using an operational model for AHR action in PLHC-1 cells, the measured reduction in AHR could be shown to predict the loss of CYP1A induction. Expression of AHR protein was unaffected by culture in 10% delipidated serum. The effects of serum-free medium and delipidated serum were found only in early-passage cells; inducibility of CYP1A and expression of AHR protein in late-passage cells were unaffected by serum withdrawal. Comparison of early- and late-passage cells revealed a 2-fold greater rate of proliferation in the latter, suggesting that a growth advantage is coincident with loss of the serum-dependency of AHR expression. These results provide a quantitative link between changes in receptor expression and a downstream response, establishing a foundation for future studies of receptor expression and sensitivity to toxic responses in vitro and in vivo.

In vivo up-regulation of aryl hydrocarbon receptor expression by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in a dioxin-resistant rat model

The aryl hydrocarbon receptor (AHR) mediates toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and regulates expression of several genes such as CYP1A1. Little is known about what regulates expression of the AHR itself. We tested the ability of TCDD to alter in vivo expression of its own receptor in rat strains that are susceptible to TCDD lethality [Long-Evans (Turku AB) (L-E) and Sprague Dawley (SD)] and in a rat strain that is remarkably resistant to TCDD lethality [Han/Wistar (Kuopio) (H/W)]. Rats were administered a single, intragastric dose of 5 or 50 microg/kg of TCDD. Hepatic cytosol, nuclear extract, and RNA were prepared at 1, 4, and 10 days after TCDD exposure. AHR expression was assessed at three levels: ligand binding function, immunoreactive protein and mRNA. TCDD at 5 microg/kg produced a 2- to 3-fold increase in cytosolic AHR in all strains; 50 microg/kg produced depletion at day 1 followed by recovery in SD and H/W but not L-E rats. Both the increase in AHR above basal levels and the recovery from initial depletion were accompanied by elevations in steady-state AHR mRNA, suggesting a pre-translational mechanism for AHR regulation by its own ligand. This up-regulation in vivo is in contrast to the sustained depletion of AHR caused by TCDD in cell culture. There was no clear relationship between AHR regulation and strain sensitivity; thus, the large inherent strain differences in susceptibility to TCDD lethality probably are not explained by differential regulation of AHR by TCDD.

Relative contributions of affinity and intrinsic efficacy to aryl hydrocarbon receptor ligand potency

Models of receptor action are valuable for describing properties of ligand-receptor interactions and thereby contribute to mechanism-based risk assessment of receptor-mediated toxic effects. In order to build such a model for the aryl hydrocarbon receptor (AHR), binding affinities and CYP1A induction potencies were measured in PLHC-1 cells and were used to determine intrinsic efficacies for 10 halogenated aromatic hydrocarbons (HAH): 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,7, 8-tetrachlorodibenzofuran (TCDF), and eight polychlorinated biphenyls (PCB). TCDD, TCDF, and non-ortho-substituted PCBs 77, 81, 126, and 169 behaved as full agonists and displayed high-intrinsic efficacy. In contrast, the mono- and di-ortho-substituted PCBs bound to the AHR but displayed lower or no intrinsic efficacy. PCB 156 was a full agonist, but with an intrinsic efficacy 10- to 50-fold lower than non-ortho-substituted PCBs. PCB 118 was a very weak partial agonist. PCBs 105 and 128 were shown to be competitive antagonists in this system. The model was then used to predict CYP1A induction by binary mixtures. These predictions were tested with binary mixtures of PCB 126, 128, or 156 with TCDD. Both PCB 156 (a low-intrinsic efficacy agonist) and PCB 128 (a competitive antagonist) inhibited the response to TCDD, while the response to TCDD and PCB126 was additive. These data support the following conclusions: 1) only 1-2% of the receptors in the cell need be occupied to achieve 50% of maximal CYP1A induction by one of the high-intrinsic efficacy agonists, demonstrating the existence of "spare" receptors in this system; 2) the insensitivity of fish to ortho-substituted PCBs is due to both reduced affinity and reduced intrinsic efficacy compared to non-ortho-substituted PCBs; 3) PCB congeners exhibit distinct structure-affinity and structure-efficacy relationships. Separation of AHR ligand action into the properties of affinity and intrinsic efficacy allows for improved prediction of the behavior of complex mixtures of ligands, as well as mechanistic comparisons across species and toxic endpoints.

Analysis of the murine AhR gene promoter

The Ah receptor (AhR) is a ligand-dependent transcription factor that mediates biological and toxicological actions of halogenated aromatic hydrocarbons such as 2,3,7,8-tetrachlorodibenzo-p-dioxin. Although much is known about the biochemical and molecular mechanisms of AhR action, little is known about the factors and events that control expression of the AhR gene itself. The 5'-flanking region of the murine AhR gene was characterized and deletion analysis demonstrated that regulatory elements necessary for full constitutive promoter activity are contained within a fragment encompassing -184 to +380 of the AhR gene. The murine AhR gene promoter is a GC-rich, TATA-less promoter that which contains at least five putative Spl-like binding sites. Transient transfection experiments not only identified a region between -1431 and -721 that represses constitutive promoter activity by 2- to 3-fold, but also demonstrate that basal AhR promoter activity occurs in a cell- and species-specific manner. n-Butyrate, a nonspecific histone deacetylase inhibitor, increased AhR promoter activity 8-fold, suggesting a role for histone acetylation in AhR gene promoter activity. Overall, this study defines upstream regulatory regions important for constitutive AhR gene expression and identifies a novel activator of AhR gene expression.

Effects of histone deacetylase inhibitors on the Ah receptor gene promoter

The aromatic hydrocarbon receptor (AhR) is a ligand-dependent basic helix-loop-helix-PAS-containing transcription factor which is activated by chemicals such as 2,3,7,8-tetrachlorodibenzo-p-dioxin. Constitutive expression of the AhR gene occurs in a tissue- and developmentally specific manner and appears to be altered by chemicals which affect histone deacetylase (HDAC) activity in cells in culture. Here we have directly characterized the effects of two HDAC inhibitors, n-butyrate and trichostatin A, on the promoter activity of the murine AhR gene. HDAC inhibitors increased the constitutive activity of the AhR gene promoter in a luciferase reporter construct by five- to sevenfold in a dose- and time-dependent manner in several cell lines and was correlated with an increase in endogenous AhR activity in an AhR-deficient cell line. Deletion analysis of the upstream region of the AhR gene localized the HDAC inhibitor effect to a 167-bp region encompassing -77 to +90 of the AhR gene promoter. Cotransfection of an AhR promoter-luciferase reporter plasmid with a vector expressing the E1A(12s) oncoprotein, a negative regulator of p300, a protein with histone acetylase activity, decreased AhR promoter activity fivefold. Overall, our results support a role for histone acetylation in the transcriptional activity of the AhR gene promoter.

Molecular characteristics of carbaryl, a CYP1A1 gene inducer

Carbaryl belongs to a series of compounds that activate the CYP1A1 gene. This study demonstrates the inability of carbaryl to compete with 2,3,7,8-tetrachlorodibenzo-p-dioxin for binding to the rat aryl hydrocarbon (dioxin) receptor. Structural and physicochemical properties of this insecticide, in relation to the requirements for binding to the aryl hydrocarbon receptor, are described. The crystal structure was determined experimentally using X-ray diffraction. A conformational search using molecular mechanics was performed by means of a Monte-Carlo-type method and a stochastic dynamics simulation. Lipophilicity calculations, log P, and molecular lipophilicity potential are also presented. Common and discriminating properties of carbaryl and aryl hydrocarbon receptor ligands are discussed.

The aryl hydrocarbon receptor: a comparative perspective

The aryl hydrocarbon receptor (Ah receptor or AHR) is a ligand-activated transcription factor involved in the regulation of several genes, including those for xenobiotic-metabolizing enzymes such as cytochrome P450 1A and 1B forms. Ligands for the AHR include a variety of aromatic hydrocarbons, including the chlorinated dioxins and related halogenated aromatic hydrocarbons whose toxicity occurs through activation of the AHR. The AHR and its dimerization partner ARNT are members of the emerging bHLH-PAS family of transcriptional regulatory proteins. In this review, our current understanding of the AHR signal transduction pathway in non-mammalian and other non-traditional species is summarized, with an emphasis on similarities and differences in comparison to the AHR pathway in rodents and humans. Evidence and prospects for the presence of a functional AHR in early vertebrates and invertebrates are also examined. An overview of the bHLH-PAS family is presented in relation to the diversity of bHLH-PAS proteins and the functional and evolutionary relationships of the AHR and ARNT to the other members of this family. Finally, some of the most promising directions for future research on the comparative biochemistry and molecular biology of the AHR and ARNT are discussed.

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.

Induction of CYP1A1 gene expression in H4-II-E rat hepatoma cells by benzo[e]pyrene

In the rat, expression of the CYP1A1 gene is closely associated with arylhydrocarbon hydroxylase (AHH) enzyme activity. AHH is an inducile enzyme activity known to play an important role in the bioactivation of polycyclic aromatic hydrocarbons (PAHs) to mutagenic and carcinogenic metabolites. PAH-induced expression of the CYP1A1 gene appears to be regulated by several trans-acting factors, including the Ah receptor and the 4S PAH-binding protein. In this study, we used the PAH isomers benzo[a]pyrene (BaP) and benzo[e]pyrene (BeP) to further evaluate the role of the 4S PAH-binding protein in induction of the CYP1A1 gene in H4-II-E rat hepatoma cells. Although BaP is believed to bind to both the Ah receptor and the 4S protein, BeP has been reported to bind exclusively to the 4S protein. The results of the study presented here indicate that BaP and BeP induce the expression of the CYP1A1 gene, as measured by ethoxyresorufin O-deethylase (EROD) activity, in a concentration-dependent manner. However, BaP is about 25 times as potent as BeP in inducing EROD activity in these cells. Slot-blot analysis of total RNA isolated from these cells indicated that BeP, BaP, and 3-methylcholanthrene increased the level of CYP1A1 mRNA expression. Sucrose-gradient analysis of BeP binding activity indicated that BeP bound with high affinity to the 4S PAH-binding protein, but not to the Ah receptor. These results suggest that the 4S protein may play a role in the PAH-induced expression of the CYP1A1 gene in rat H4-II-E cells.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related compounds as antioestrogens: characterization and mechanism of action

In the female Sprague-Dawley rat uterus 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds exhibited a broad spectrum of antioestrogenic responses. For example 2,3,7,8-TCDD inhibited the 17 beta-oestradiol-induced uterine wet weight increase, peroxidase activity, oestrogen and progesterone receptor levels, epidermal growth factor (EGF) receptor binding, and EGF receptor and c-fos protooncogene mRNA levels. The aryl hydrocarbon (Ah) receptor was identified in the rat uterus and the antioestrogenic activities of TCDD and related compounds were structure-dependent. In parallel studies, the effects of TCDD as an antioestrogen in MCF-7 human breast cancer cells was also investigated. TCDD inhibited the 17 beta-oestradiol-induced proliferation of these cells and the secretion of the 34-, 52- and 160-kDa proteins. Treatment of MCF-7 cells with 1 nM [3H]-17 beta-oestradiol resulted in a rapid accumulation of nuclear oestrogen receptor (ER) complexes. Pretreatment of the cells with TCDD caused a rapid decrease in nuclear ER binding activity and immunoreactive protein; moreover, the structure-dependent potencies of TCDD and related compounds as antioestrogens were similar to their Ah receptor binding affinities. TCDD also caused a decrease in nuclear ER levels in wild-type Ah-responsive Hepa 1c1c7 cells but was inactive in Ah non-responsive mutant Hepa 1c1c7 cells. Moreover, in the wild-type cells, both actinomycin D and cycloheximide blocked the effects of TCDD. 6-Methyl-1,3,8-trichlorodibenzofuran (MCDF) has previously been characterized as a TCDD antagonist in rodents and in transformed rodent cell lines. However, like TCDD, MCDF also exhibited a broad spectrum of antioestrogenic activities in both the female Sprague-Dawley rat uterus and MCF-7 cells. MCDF is relatively non-toxic compared to TCDD and is being investigated as a compound which may be clinically useful for the treatment of mammary cancer.

The Ah receptor and the mechanism of dioxin toxicity

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Effect of phenobarbital on the glucocorticoid receptor in rat hepatoma cells

Phenobarbital is a potent inducer of several liver-specific genes such as those encoding detoxication enzymes, including cytochromes P450. However, the mechanisms of action of the barbiturate are poorly understood. Since both, phenobarbital and glucocorticoids, are capable of inducing the same cytochrome P450 species, we asked whether the glucocorticoid receptor could participate to the phenobarbital induced responses. The results presented here show that phenobarbital was able to induce a two-fold increase in the affinity of the glucocorticoid receptor for the binding of dexamethasone, as well as a 30% increase of the receptor number in Reuber rat hepatoma cells of the Fao line. These effects may have a biological significance since they were paralleled by an enhancement of the dexamethasone-induced tyrosine aminotransferase activity, a glucocorticoid inducible function in rat hepatoma cells and in rat liver. To our knowledge, phenobarbital is the first compound shown to be able to induce, in intact cells, an increase in the affinity of the glucocorticoid receptor for the binding of its ligand.

Chemically induced hepatic cytosol from the Sprague-Dawley rat: evidence for specific binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin to components kinetically distinct from the Ah receptor

A series of exogenous chemicals was used as potential inducers for the hepatic Ah receptor in the Sprague-Dawley rat. 2,3,7,8-Tetrachlorodibenzo-p-dioxin, 2,2',4,4',5,5'-hexachlorobiphenyl and phenobarbital all induced an elevated level of 3H-2,3,7,8-tetrachlorodibenzo-p-dioxin specific binding, while 3,3',4,4'-tetrachloroazobenzene and trans-3,3',4,4'-tetrachlorostilbene caused a depression. Mixtures of these chemicals caused additive effects. Elevated levels of specific binding appeared to be heterologous, comprising a binding species having the normal high stability of the Ah receptor in its liganded form, and another less stable substance having a half-life of approximately 2 h at 37 degrees C.

Effects of various inducers on diethylstilbestrol metabolism, drug-metabolizing enzyme activities and the aromatic hydrocarbon (Ah) receptor in male Syrian golden hamster liver

In order to elucidate the role of metabolic activation of the synthetic estrogen, diethylstilbestrol (DES), in the mechanism of liver tumor formation in male Syrian golden hamsters observed after combined treatment with DES and 7,8-benzoflavone (7,8-BF), the metabolism of DES and the concentrations and activities of various drug-metabolizing enzymes were studied in hamster liver microsomes after various pretreatments. The levels of the hepatic aromatic hydrocarbon (Ah) receptor were also determined. Pretreatment with 7,8-BF increased both P450 and cytochrome b5 levels, whereas phenobarbital (PB) and 3-methylcholanthrene (MC) induced P450 but not cytochrome b5. 7,8-BF pretreatment increased 7-ethoxyresorufin-O-deethylase (EROD) 3-fold and 7-pentoxyresorufin-O-dealkylase (PROD) 2.5-fold, whereas aromatic hydrocarbon hydroxylase (AHH) and 7-ethoxycoumarin-O-deethylase (ECOD) activities were only slightly induced by 7,8-BF. MC pretreatment increased EROD 8-fold and PROD activity 7-fold, whereas PB pretreatment enhanced AHH 4.5-fold and PROD activity 4-fold. In contrast to PB, pretreatment with 7,8-BF and MC reduced the oxidative metabolism of DES in hepatic microsomes, but the pattern of metabolites was identical with that in untreated controls. Treatment of hamsters with the inducers changed the hepatic Ah receptor level. PB and MC-pretreatment resulted in an increase of the receptor level 1.5-fold and 1.3-fold, respectively, whereas 7,8-BF-pretreatment leads to a 1.5-fold decrease. The dissociation constant Kd is 170 nM for the reaction of 7,8-BF with the hamster Ah receptor compared to 70 nM for 5,6-BF and 38 nM for 2,3,7,8-tetrachlorodibenzofuran (TCDF). The Kd-value is 3.6 nM for TCDF with the rat receptor protein. It is concluded from these data that metabolic activation of DES is not involved in the mechanism of hepatocarcinogenesis in this animal tumor model.

Dexamethasone-mediated potentiation of P450IA1 induction in H4IIEC3/T hepatoma cells is dependent on a time-consuming process and associated with induction of the Ah receptor

The synergistic effect of dexamethasone (DEX) and polycyclic aromatic hydrocarbons on the induction of cytochrome P450IA1 (P450IA1) was examined in H4IIEC3/T Reuber hepatoma cells. P450IA1 activity was determined by the hydroxylation of benzo[a]pyrene (AHH) and deethylation of 7-ethoxyresorufin (EROD). The amount of Ah receptor, i.e. the specific cytosolic binding protein of 3-methylcholanthrene or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in H4IIEC3/T cells was characterized and quantitated by high performance gel filtration. Benz[a]anthracene and TCDD induced AHH and EROD activities, respectively, about 20-fold within 4 h. The increase was about 100-fold when cells were pretreated with DEX. The glucocorticoid alone induced P450IA1 activities 3-4 fold. DEX elicited half maximum AHH induction at a concentration of 20 nM in the presence or absence of benz[a]anthracene. Maximal potentiation of AHH induction required treatment with DEX for at least 32 h prior to the exposure to benz[a]anthracene. Treatment of H4IIEC3/T cells with DEX for 20 h caused a 2-3-fold increase in the amount of Ah receptor. The results suggest that the synergistic effect of DEX and polycyclic aromatic hydrocarbons on P450IA1 induction involves a time-consuming process which may consist of the synthesis or modification of a factor, possibly the Ah receptor.

Intracellular lipoproteins as carriers for 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo(a)pyrene in rat and mouse liver

The possible role of hepatic lipoproteins as intracellular carriers in the transport of 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo(a)pyrene was assessed by in vitro and in vivo studies. Following administration of [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin or unlabelled 2,3,7,8-tetrachlorodibenzofuran to C57 BL/6 mice or Sprague-Dawley rats these compounds were bound to lipoproteins which subsequently underwent rapid and pronounced degradative processing, possibly catalysed by lipoprotein lipase, to heavier entities. At the highest doses of xenobiotics administered, an almost complete disappearance of lipoprotein particles was observed. The in vitro incubation of [3H]2,3,7,8-tetrachlorodibenzo-p-dioxin-lipoprotein and [3H]benzo(a)pyrene-lipoprotein complexes with separated Ah receptor and 4S protein, respectively, demonstrated that a passive transfer occurred; the latter was likely dependent on both the relative affinities of the ligands towards the different cellular binding components as well as on their quantitative binding capacity. Taken together, these findings support the idea of a carrier-role for lipoproteins in the intracellular transport of hydrophobic xenobiotics and it may be asked whether the widespread modulators of lipoprotein level such as fibrates or others affect drug transfer or action.

Separation of the different classes of intrahepatic lipoproteins from various animal species. Their binding with 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo(a)pyrene

Using several analytical methods, including sucrose density gradient and potassium bromide density gradient ultracentrifugations, we have demonstrated that liver cells contain a range of lipoproteins somewhat distinct from those found in plasma. In addition to very low-density lipoproteins (VLDL), low-density lipoproteins (LDL) and high-density lipoproteins (HDL), many heavier entities have been found in the cytosol of various animal species. These heavier entities might represent either anabolic or catabolic intermediates of lipoproteins. Labelled hydrophobic xenobiotics such as 2,3,7,8-tetrachlorodibenzo-p-dioxin or benzo(a)pyrene strongly bind to the various classes of lipoproteins and may be used as radioactive tracers in the analysis and possibly in the metabolic studies of intracellular lipoproteins. Moreover, this binding may be a prerequisite for a storage or/and a carrier--roles of lipoproteins in the intracellular distribution of lipophilic xenobiotics within the cells.

Interaction of hexachlorobenzene with the receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin in vitro and in vivo. Evidence that hexachlorobenzene is a weak Ah receptor agonist

Hexachlorobenzene (HCB) produces hepatic porphyria and induces the hepatic cytochrome P450 isozymes P450c (P450IA1) and P450d (P450IA2) in rodents. These and other effects of HCB resemble those of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which acts via its binding to the aromatic hydrocarbon (Ah) receptor. We therefore examined the ability of HCB to interact with this receptor in vitro and in vivo. HCB, at concentrations of 1 microM or higher, inhibited the specific binding of [3H]TCDD (0.3 nM) to the Ah receptor in vitro, whereas the solubility of [3H]TCDD was affected only at 100 microM HCB. The inhibition was competitive, with a KI of approximately 2.1 microM. In rats fed a diet containing 3000 ppm HCB for varying times (4 h to 7 days), the specific binding of [3H]TCDD in hepatic cytosol was reduced by up to 40%, as observed previously for known Ah receptor agonists. The decrease in [3H]TCDD specific binding in cytosol of HCB-treated rats was due principally to a decrease in the number of binding sites for [3H]TCDD rather than competition from residual HCB. As shown by immunoblotting and radioimmunoassay, HCB induced the cytochrome P450 isozymes P450c and P450d, which are regulated by the Ah receptor, as well as the phenobarbital-inducible isozymes P450b and P450e. Together these results indicate that HCB is a weak agonist for the Ah receptor, and suggest that some of its effects may be mediated by its interaction with this gene-regulatory protein.

Further consideration of phenobarbital effects on cytochrome P-450 activity in the killifish, Fundulus heteroclitus

1. Ethoxyresorufin O-deethylase (EROD) activity, aldrin epoxidase (AE) activity, cytochrome P-450 content, and levels of cytochrome P-450E (the major BNF-inducible P-450 form and primary EROD catalyst in scup) or its homologues were measured in hepatic microsomes isolated from Fundulus heteroclitus, scup (Stenotomus chrysops) and brook trout (Salvelinus fontinalis) treated with beta-naphthoflavone (BNF) or phenobarbital (PB). 2. In all three teleost species, BNF treatment caused expected increases in P-450 content, EROD activity and P-450E level; but either no change or a slight decrease in AE turnover rate (nmol/min/nmol P-450). 3. Polyclonal antibodies to P-450E did not inhibit AE activity in microsomes from BNF-treated scup, confirming that this major BNF-inducible P-450 form does not catalyze AE activity in fish. 4. In contrast, PB treatment did not affect hepatic AE activity, P-450 content or levels of "P-450E" in F. heteroclitus, but did variably affect EROD activity which was suppressed in one experiment and elevated in another. 5. The results indicate that (i) contrary to previous reports, neither PB nor MC-type inducers increase AE activity in F. heteroclitus, (ii) MC-type inducers do not affect AE activity in the other teleost species examined, and (iii) AE activity is not a reliable indicator of P-450 induction by environmental chemicals. 6. We emphasize the need to establish the mechanism of PB action, and the nature of any fish P-450 forms analogous to PB-inducible forms in mammals in order to conclusively evaluate PB-responses in fish.

Trout P450IA1: cDNA and deduced protein sequence, expression in liver, and evolutionary significance

The cytochrome P1(450) (P450IA1) cDNA has been isolated and sequenced from liver of 3-methylcholanthrene-treated rainbow trout (Salmo gairdneri). The cDNA hybridizes to a 2.8-kb mRNA that is induced at least 10-fold by 3-methylcholanthrene. Southern blot analysis suggests the presence of a single gene or a very small number of genes. An open reading frame of the 2573-bp cDNA encodes a 522-residue protein (Mr = 59,241) that is more similar to the mammalian P450IA1 than the mammalian P450IA2 proteins. The aromatic hydrocarbon (Ah) receptor, responsible for mammalian P450IA1 and IA2 inducibility, was detected in trout liver cytosol by specific binding to [1,6-3H]2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in vitro. Comparison of the fish P450IA1 protein with human, mouse, rat and rabbit P450IA1 and P450IA2 proteins reveals the presence of a remarkably large number of single amino acids and stretches of 2-6 residues in a row that are invariant among these nine P450 proteins. These conserved regions may participate in the binding of the NADPH-P450 oxidoreductase flavoprotein, substrate, or heme.

Phenobarbital, dexamethasone and benzanthracene induce several cytochrome P450 mRNAs in rat hepatoma cells

Hepatoma cells derived from the Reuber H35 rat hepatoma express cytochrome P450 enzymes of two major families: polycyclic aromatic hydrocarbon-inducible forms are found in both differentiated and dedifferentiated cells while phenobarbital (PB)-inducible forms are found only in differentiated cells. We report here that (i) benzanthracene and PB induce P450 c mRNA in differentiated and dedifferentiated cells and (ii) dexamethasone and PB induce P450 b/e and/or P450 PB1 mRNAs in differentiated cells but not in dedifferentiated cells.

Synthesis and aryl hydrocarbon receptor binding properties of radiolabeled polychlorinated dibenzofuran congeners

Microchlorination of 1,4,9[3H]dibenzofuran gave several polychlorinated dibenzofuran (PCDF) products and 2,3,7,8-[3H]tetrachlorodibenzofuran (TCDF), 1,2,3,7,8-[3H]pentachlorodibenzofuran (PeCDF), and 1,2,3,6,7,8-/1,2,3,4,7,8-hexachlorodibenzofuran (HCDF) of high specific activity (57, 34, and 32.5 Ci/mmol, respectively) were purified by preparative high-pressure liquid chromatography. These compounds were investigated as radioligands for the rat liver cytosolic aryl hydrocarbon (Ah) receptor protein. Like 2,3,7,8-[3H]tetrachlorodibenzo-p-dioxin (TCDD), the radiolabeled PCDF congeners exhibited saturable binding with the receptor protein and sucrose density gradient analysis of the radiolabeled ligand-receptor complexes gave specific binding peaks with comparable sedimentation profiles. The rank order of radioligand binding affinities (Kd values) was 2,3,7,8-TCDD greater than 2,3,7,8-TCDF greater than 1,2,3,6,7,8-HCDF greater than 1,2,3,7,8-PeCDF and the maximum difference in Kd values for the four radioligands was less than 13-fold (0.44-5.9 nM). The interactions of the PCDF radioligands with the cytosolic receptor all exhibited saturable binding curves and linear Scatchard plots and the slopes of their Hill plots were in the range 1.0-1.1, thus indicating that cooperativity was not a factor in these binding interactions. The relative stabilities and dissociation kinetics of the radioligand-receptor complexes were highly dependent on the structure of the radioligand. The dissociation curves of the 2,3,7,8-[3H]TCDD and PCDF receptor complexes were biphasic and this suggests that there may be a temporal shift in ligand binding affinities. However, the rates of dissociation did not correlate with the rank order of ligand binding affinities. The stabilities of the radioligand-receptor complexes were also dependent on the structures of the radioligands; for example, the 2,3,7,8-[3H]TCDD-receptor complex degraded more rapidly than the PCDF-receptor complex and these relative stabilities were clearly not related to the Kd values or the relative in vivo or in vitro biologic potencies of these halogenated aryl hydrocarbons.

Further characterization of the polycyclic aromatic hydrocarbon binding properties of the 4S protein

A 4-S protein which specifically binds [3H]benzo(a)pyrene and other polycyclic aromatic hydrocarbons has been investigated in the rat using a hydroxylapatite assay and sucrose gradient analysis. Although there was significant interanimal variation, the specific polycyclic aromatic hydrocarbon binding activity appeared to be highest in 4-week-old male rats and declined with age. The specific [3H]benzo(a)pyrene binding activity was induced after pretreatment with either phenobarbital or isosafrole as evidenced by a 72 and 61% increase, respectively, over untreated controls. No apparent increase in specific binding activity was observed after pretreatment of animals with 3-methylcholanthrene. Pretreatment with either phenobarbital or isosafrole also resulted in the appearance of a small, nonspecific, benzo(a)pyrene binding peak at the 8- to 9-S region in the sucrose density gradients. This 8-S peak was not seen in untreated control animals and represented low affinity, high capacity binding sites. In contrast to the 8-S protein, the 4-S binding protein had low affinity for polychlorinated aromatic compounds such as tetrachlorodibenzodioxin and tetrachlorodibenzofuran. The addition of a 200-fold excess of tetrachlorodibenzofuran to incubations did not displace [3H]benzo(a)pyrene from the 4-S protein. The addition of sodium molybdate to isolation buffers, known to stabilize certain hormone receptors, did not alter the sedimentation coefficient or the specific binding activity of the 4-S protein. These experiments indicate that the 4-S protein does not appear to be a subunit of the 8-S protein. We conclude that in the rat the 4-S protein is distinct from the 8-S protein and the 4-S species may regulate the polycyclic aromatic hydrocarbon-induced expression of aryl hydrocarbon hydroxylase activity.

Induction of cytochrome P-450 in congenic C57BL/6J mice by isosafrole: lack of correlation with the Ah locus

Isosafrole induction of cytochrome P-450 was compared in congenic strains of C57BL/6J mice, one of which expresses normal levels of the Ah receptor [B6(Ahb)], and another that does not contain a measurable receptor concentration [B6(Ahd)]. Using sucrose gradient analysis of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) binding, an Ah receptor concentration of 69.1 +/- 3.8 fmol/mg protein was measured in the hepatic cytosol from B6(Ahb) mice, while no receptor could be detected in the cytosol from B6(Ahd) mice. Isosafrole treatment (75 mg/kg X 3 days) increased the total hepatic microsomal cytochrome P-450 content to the same extent in the two congenic strains. The level of microsomal monooxygenase induction in the isosafrole-treated B6(Ahd) mice was greater than that of B6(Ahb) mice for ethylmorphine N-demethylase and isosafrole metabolite-complex formation, the latter a measure of cytochrome P2-450. In the case of 7-ethoxycoumarin O-deethylase only the isosafrole-treated B6(Ahd) mice had elevated microsomal activity. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) also revealed a similar induction pattern for the two congenic strains, following isosafrole treatment. Thus, the isosafrole treated B6(Ahd) mice produced an equivalent or slightly larger induction of cytochrome P-450 than the B6(Ahb) mice, suggesting that there is no direct role for the Ah receptor in the regulation of these cytochrome P-450 monooxygenase activities by isosafrole.

Elevation of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) polychlorinated biphenyls. Structure-activity relationships

Administration of the commercial polychlorinated biphenyl (PCB) Aroclor 1254 to immature male Wistar rats resulted in increased levels (80-110%) of the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) hepatic cytosolic receptor protein which remained elevated for 14 days. The effects of structure on the activity of individual PCB congeners to modulate hepatic cytosolic receptor levels were compared to the structure-activity relationships (SARs) which have been developed previously for PCBs as inducers of hepatic microsomal monooxygenases. 3,3',4,4'-Tetra- and 3,3',4,4',5-pentachlorobiphenyl induced the cytochrome P-448-dependent monooxygenase, ethoxyresorufin O-deethylase (EROD), and resembled 3-methylcholanthrene in their mode of monooxygenase enzyme induction. These congeners also bound to the receptor protein; however, neither compound increased hepatic cytosolic receptor protein levels. Several PCB congeners which exhibit low binding affinities for the cytosolic receptor protein resembled phenobarbitone (PB) in their mode of monooxygenase enzyme induction and, like PB, elevated cytosolic receptor protein levels. Nevertheless, a comparison of the time course of monooxygenase enzyme induction and receptor protein elevation by 2,2',4,4',5,5'-hexachlorobiphenyl and PB illustrated significant differences in their activities. PB-mediated elevation of receptor levels was maximized 24 hr after the last dose, and 48 hr later the receptor levels decreased to control values. In contrast, 5 days after administration of a single dose of 2,2',4,4',5,5'-hexachlorobiphenyl (300 mumoles/kg) the receptor levels were elevated significantly, and these increased levels (205-127% increases over control) persisted for 14 days. There was no correlation between increased levels of hepatic receptor protein and the induction of the cytochrome P-450-dependent monooxygenases, aldrin epoxidase or 4-dimethylaminoantipyrine N-demethylase. Two PCBs, 2,3,3',4,4',5- and 2,2',3,4,4',5-hexachlorobiphenyl, which resembled Aroclor 1254 in their mode of monooxygenase enzyme induction, also elevated hepatic receptor protein levels but were less active than the PB-type inducers. Thus, the SARs developed for PCBs which elevate cytosolic receptor levels demonstrate that the most active compounds exhibit the lowest affinity for the receptor protein and do not induce EROD. In contrast, the more toxic PCB congeners which are approximate isostereomers of 2,3,7,8-TCDD both induced EROD and bound with high affinity to the receptor protein but did not increase hepatic cytosolic receptor protein levels.

The induction of cytochrome P-450 by isosafrole and related methylenedioxyphenyl compounds

Using sucrose gradients, the Ah receptor and a 3-4S binding peak were measured in hepatic cytosol from Dub: ICR, C57BL/6, and DBA/2 male mice. Isosafrole, piperonyl butoxide, and 5-t-butyl-1,3-benzodioxole were unable to displace 2,3,7,8-tetrachlorodibenzo-p-dioxin or 3-methylcholanthrene from either the Ah receptor or the 3-4S binding peak, in vitro. In in vivo experiments, treatment of C57BL/6 mice with 3-methylcholanthrene caused a 4-fold reduction in Ah receptor binding 2 h after i.p. injection; whereas, isosafrole caused a 2-fold enhancement of the Ah receptor after 24 h. This increase in the Ah receptor binding following isosafrole treatment may be due to induction. 3-Methylcholanthrene treatment of C57BL/6 mice also caused a 3-fold reduction in the 3-4S binding peak 2 h after i.p. injection; isosafrole treatment had little or no effect on the 3-4S peak in C57BL/6 or DBA/2 mice. Both in vivo and in vitro data appear to demonstrate that there is no direct role for the Ah receptor or the 3-4S protein in the regulation of cytochrome P-450 by methylenedioxyphenyl compounds. Using Sephadex G-100 chromatography, a cytosolic protein fraction was obtained from C57BL/6 and Dub:ICR mice which was previously implicated by others as a carrier in the metabolism of benzo[a]pyrene (B[a]P). This fraction was applied to sucrose gradients and sedimented in the 3-4S region. Hence it appears that the 3-4S binding peak may be the carrier described by these workers.

Measurements of the cytosolic Ah receptor among four strains of Drosophila melanogaster

Four strains of Drosophila melanogaster exhibit differences in aryl hydrocarbon hydroxylase (AHH) inducibility by phenobarbital or Aroclor 1254, yet do not show the typical AHH induction response when exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or benzo[a]anthracene. Adult flies were nevertheless examined for the presence of cytosolic TCDD-specific binding (Ah receptor). Berlin-K and Haag 79 exhibit AHH induction by Aroclor 1254 and possess detectable amounts of Ah receptor. Hikone-R has negligible AHH inducibility by Aroclor 1254, yet possesses measurable amounts of the receptor. Oregon-K displays AHH induction by Aroclor 1254 but has no detectable levels of the cytosolic receptor. Specific (high-affinity, low-capacity and saturable) binding of [3H-1,6]TCDD to the Ah receptor in D. melanogaster was shown to be similar to that observed in C57BL/6 mouse liver. Similar specific binding of generally labeled [3H]benzo[a]anthracene in D. melanogaster cytosol was not found. These data suggest that the presence of the Ah receptor per se, or quantity of receptor, does not guarantee AHH inducibility by TCDD or benzo[a]anthracene in adults of these four fruit fly strains.

Analysis of Ah gene locus by somatic cell hybridization: expression of Ah regulatory gene product for 2,3,7,8,-tetrachlorodibenzo-p-dioxin in mouse L-cell x mouse hepatoma cell hybrids

Properties of the aryl hydrocarbon hydroxylase (AHH) enzyme system were examined in polycyclic aromatic hydrocarbon (PAH) -noninducible L-cell x PAH-inducible hepatoma (Hepa) mouse cell hybrids. In hybrids, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces AHH activity. The levels of maximal TCDD-induced AHH activity in the hybrids and the Hepa parent are similar, although a greater concentration of TCDD is required for expression in the hybrids. This concentration difference appears to reflect dilution of AHH-associated gene products by the L-cell parent rather than altered gene expression. The regulatory gene product, the Ah receptor, is expressed similarly in the hybrids and Hepa parent. Both demonstrate specific, high-affinity binding of [3H]TCDD to an equivalent number of receptor sites per cell. These results suggest that the molecular mechanism of phenotypic resemblance to the inducible Hepa parent (i.e., "dominance") in the mouse L-cell x Hepa hybrids involves expression of only the Hepa Ah gene complex.