Isolation and characterization of revertants from four different classes of aryl hydrocarbon hydroxylase-deficient hepa-1 mutants

The aryl hydrocarbon receptor interacts with ATP5α1, a subunit of the ATP synthase complex, and modulates mitochondrial function

Dioxins, including 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD), produce a wide range of toxic effects in mammals. Most, if not all, of these toxic effects are regulated by the aryl hydrocarbon receptor (AHR). The AHR is a ligand activated transcription factor that has been shown to interact with numerous proteins capable of influencing the receptor's function. The ability of secondary proteins to alter AHR-mediated transcriptional events, a necessary step for toxicity, led us to determine whether additional interacting proteins could be identified. To this end, we have employed tandem affinity purification (TAP) of the AHR in Hepa1c1c7 cells. TAP of the AHR, followed by mass spectrometry (MS) identified ATP5α1, a subunit of the ATP synthase complex, as a strong AHR interactor in the absence of ligand. The interaction was lost upon exposure to TCDD. The association was confirmed by co-immunoprecipitation in multiple cell lines. In addition, cell fractionation experiments showed that a fraction of the AHR is found in the mitochondria. To ascribe a potential functional role to the AHR:ATP5α1 interaction, TCDD was shown to induce a hyperpolarization of the mitochondrial membrane in an AHR-dependent and transcription-independent manner. These results suggest that a fraction of the total cellular AHR pool is localized to the mitochondria and contributes to the organelle's homeostasis.

Aryl hydrocarbon nuclear translocator (hypoxia inducible factor 1beta) activity is required more during early than late tumor growth

c4 is a derivative of the mouse hepatoma cell line, Hepa-1, that harbors a mutation in the aryl hydrocarbon receptor nuclear translocator gene (Arnt, or hypoxia inducible factor 1beta [HIF-1beta]) leading to loss of activity. Clone 3 cells were generated by introducing a doxycycline-repressible Arnt expression vector into c4 cells. Clone 3 cells were injected subcutaneously into immunosuppressed mice, which were treated with doxycyline (a) throughout the growth of the subsequent tumor xenografts, or (b) from day 7 through to the end of the experiment (day 30), or not treated (c). Tumors in all groups grew exponentially between days 14 and 30, and at rates that were indistinguishable from each other. However, tumors in group a were smaller than those of the other two groups throughout the measurable growth period, while tumor volumes in groups b and c were not significantly different from each other. The degrees of vascularity and apoptosis did not correlate with the differences in degrees of growth between the different groups. Thus, Arnt is required during the early stages of growth of the tumors but less in later stages. Since Arnt does not detectably effect the growth kinetics of Hepa-1 cells either during hypoxia or normoxia, this requirement is unlikely to reflect a direct effect of Arnt on cell proliferation, and is therefore probably a consequence of altered interaction(s) between the tumor cells and the host. These studies suggest that Arnt (and HIF-1alpha/HIF-2alpha) inhibitors will be particularly effective against smaller tumors, including micrometastases.

Evidence that ligand binding is a key determinant of Ah receptor-mediated transcriptional activity

The aryl hydrocarbon receptor (AhR) mediates the biological activity of 2,3,7,8-tetrachlorodibenzo-p-dioxin. Whether the AhR can mediate enhanced transcriptional activity in the absence of ligand binding has not been established. Hepatocytes from AhR-null (AhR-KO) and wild-type (AhR-WT) neonatal mice were immortalized with Simian virus 40. Two point mutants of the AhR, A375I and A375F, were generated to test the hypothesis that the AhR requires ligand binding to exhibit significant transcriptional activity, both mutants fail to bind ligand or exhibit enhanced activity in cells exposed to AhR ligands. Upon transient, co-expression of ARNT with AhR-A375I or AhR-A375F in AhR-KO cells, these mutants exhibited significant ligand-independent transcriptional activity. However, in CV-1 cells, which others have previously shown to contain relatively high levels of AhR ligand(s), these AhR mutants exhibit essentially no constitutive activity. These results indicate that while the AhR can potentially exhibit activity in the absence of ligand binding, the high constitutive receptor activity observed in many cell lines appears to be due to the presence of endogenous AhR ligands.

Comparative microarray analysis of basal gene expression in mouse Hepa-1c1c7 wild-type and mutant cell lines

Hepa-1c1c7 wild-type and benzo[a]pyrene-resistant derived mutant cell lines have been used to elucidate pathways and mechanisms involving the aryl hydrocarbon receptor (AhR). However, there has been little focus on other biological processes which may differ between the isolated lines. In this study, mouse cDNA microarrays representing 4858 genes were used to examine differences in basal gene expression between mouse Hepa-1c1c7 wild-type and c1 (truncated Cyp1a1 protein), c4 (AhR nuclear translocator, ARNT, deficient), and c12 (low AhR levels) mutant cell lines. Surprisingly, c1 mutants exhibited the greatest number of gene expression changes compared to wild-type cells, followed by c4 and c12 lines, respectively. Differences in basal gene expression were consistent with cell line specific variations in morphology, mitochondrial activity, and proliferation rate. MTT and direct cell count assays indicate both c4 and c12 mutants exhibit increased proliferative activity when compared to wild-type cells, while the c1 mutants exhibited decreased activity. This study further characterizes Hepa-1c1c7 wild-type and mutant cells and identifies significant differences in biological processes that should be considered when conducting comparative mechanistic studies with these lines.

Identification of hypoxically inducible mRNAs in HeLa cells using differential-display PCR. Role of hypoxia-inducible factor-1

Oxygen is an important regulator of gene expression in mammalian cells, though the extent of operation and the organization of the inducible mechanisms involved are still largely undetermined. To define better the response to hypoxia, we have used differential display PCR to identify genes whose expression is induced in HeLa cells exposed to 1% oxygen. Among six genes whose induction by hypoxia was newly defined in this way, three were of known function, encoding the glucose transporter isoform 3 (Glut-3), adenylate kinase isoenzyme 3 (AK-3), and tissue factor, two were expressed sequence tags (ESTs), and one corresponded to a new sequence. One regulator of the transcriptional response to hypoxia has recently been identified as a heterodimeric DNA-binding complex termed hypoxia-inducible factor-1 (HIF-1), which is also inducible by the iron chelator, desferrioxamine. Of the six hypoxically regulated genes, at least four were also induced by exposure of the cells to desferrioxamine. To analyse further the mechanisms underlying induction of the genes identified in the differential display, inducible expression was compared in wild-type mouse hepatoma cells (Hepa-1), and mutant derivatives (c4) which fail to generate HIF-1, due to a functional defect in one component, HIF-1 beta. Two types of response were defined. For Glut-3 and AK-3, mutant (c4) cells showed almost complete loss of the inducible response to both hypoxia and desferrioxamine. In contrast, tissue factor mRNA was more inducible by both stimuli in c4 than wild-type cells. These studies demonstrate the critical importance of HIF-1 beta in newly recognized responses to hypoxia, and provide further evidence of the importance of this system of gene regulation in mammalian cells; they also demonstrate responses to both hypoxia and desferrioxamine which are independent of HIF-1 beta and which appear exaggerated in HIF-1 beta-deficient cells.

The role of the aryl hydrocarbon receptor nuclear translocator (ARNT) in hypoxic induction of gene expression. Studies in ARNT-deficient cells

Hypoxia-inducible factor-1 (HIF-1), a DNA-binding complex implicated in the regulation of gene expression by oxygen, has been shown to consist of a heterodimer of two basic helix-loop-helix Per-AHR-ARNT-Sim (PAS) proteins, HIF-1alpha, and HIF-1beta. One partner, HIF-1beta, had been recognized previously as the aryl hydrocarbon receptor nuclear translocator (ARNT), an essential component of the xenobiotic response. In the present work, ARNT-deficient mutant cells, originally derived from the mouse hepatoma line Hepa1c1c7, have been used to analyze the role of ARNT/HIF-1beta in oxygen-regulated gene expression. Two stimuli were examined: hypoxia itself and desferrioxamine, an iron-chelating agent that also activates HIF-1. Induction of the DNA binding and transcriptional activity of HIF-1 was absent in the mutant cells, indicating an essential role for ARNT/HIF-1beta. Analysis of deleted ARNT/HIF-1beta genes indicated that the basic, helix-loop-helix, and PAS domains, but not the amino or carboxyl termini, were necessary for function in the response to hypoxia. Comparison of gene expression in wild type and mutant cells demonstrated the critical importance of ARNT/HIF-1beta in the hypoxic induction of a wide variety of genes. Nevertheless, for some genes a reduced response to hypoxia and desferrioxamine persisted in these mutant cells, clearly distinguishing ARNT/HIF-1beta-dependent and ARNT/HIF-1beta-independent mechanisms of gene activation by both these stimuli.

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.

DNA binding properties of the Ah receptor in wild-type and variant mouse hepatoma cells

Mouse hepatoma Hepa 1c1c7 cells and nonresponsive mutants have been extensively used as models for investigating the molecular mechanism of induction of CYP1A1 gene transcription by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds. Incubation of cytosolic [3H]TCDD-aryl hydrocarbon (Ah) receptor from wild-type Hepa 1c1c7 cells for 16 h at 4 degrees C in 0.4 M KCl resulted in the formation of transformed liganded receptor which exhibited increased binding affinity on DNA-Sepharose columns. The elution properties of the peak with the highest DNA binding affinity were similar to the elution profiles of the nuclear receptor complex isolated from wild-type cells. TAOc1BPrcl (class I) nonresponsive mutant cells were characterized by relatively low levels of the cytosolic and nuclear Ah receptor complex. The BPrcl (class II) variant cell line contained levels of cytosolic receptor which were comparable to those observed in the wild-type cells; however, significantly reduced levels of nuclear receptor complex were observed in the class II variant cell line. Incubation of the nuclear or transformed liganded cytosolic Ah receptor from wild-type cells with a consensus 32P-labeled dioxin responsive element (DRE) in a gel shift assay gave a retarded band associated with the receptor-DRE complex. Incubation of the cytosolic receptor complex from the class I and II mutant cells for 16 h at 4 degrees C in 0.4 M KCl or for 2 h at 20 degrees C did not yield complexes with increasing binding affinities on DNA-Sepharose columns. Moreover, incubation of these complexes with 32P-labeled DRE did not give a retarded band in a gel shift assay. However, coincubation of the liganded class II mutant cytosol with cytosol from class I cells resulted in transformation of the liganded receptor and this was confirmed in both the DNA-Sepharose and gel retardation assays. These results suggest that the failure of class II mutant cells to respond to TCDD is due to a defect in the factors responsible for transformation of the cytosolic receptor complex.

Genetic and molecular analysis of the Ah receptor and of Cyp1a1 gene expression

The Ah receptor is a soluble protein complex that mediates carcinogenesis by a wide range of environmental pollutants, including polycyclic aromatic hydrocarbons, heterocyclic amines, and polychlorinated aromatic compounds. The best understood activity of the receptor concerns its role in the induction of cytochrome P450IA1. We undertook a somatic cell genetic analysis of P450IA1 induction using the mouse hepatoma cell line, Hepa-1. Clones of Hepa-1 were isolated that are defective in induction of P450IA1. Evidence was obtained that the clones are mutational in origin. Cell fusion experiments demonstrated that a few of the mutants are dominant, while the majority are recessive. The dominant mutants were shown to synthesize a repressor of P450IA1 transcription. The recessive mutants were assigned to 4 complementation groups (probably corresponding to 4 different genes). Complementation group A corresponds to the P450IA1 structural gene. Mutations in the B, C and D genes all affect functioning of the Ah receptor. A 'reverse selection procedure', whereby cells that express P450IA1 inducibility can be selected from a majority population of cells lacking inducibility, was developed. The reverse selection procedure was used to isolate transfectants of representative recessive mutants in which the mutational defects are complemented by exogenously applied genomic DNA. A human DNA-derived transfectant of a C- mutant was used to clone the human C gene. The C gene is not the ligand-binding subunit of the Ah receptor but is a protein that is required for translocation of Ah receptor-ligand complexes from cytoplasm to nucleus. In analogous experiments the dominant gene from one of the dominant mutants was transfected into wild-type Hepa-1 cells. Success in transfecting the dominant gene should provide the means for cloning it.

Polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and related compounds: environmental and mechanistic considerations which support the development of toxic equivalency factors (TEFs)

Halogenated aromatic compounds, typified by the polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), biphenyls (PCBs), and diphenylethers (PCDEs), are industrial compounds or byproducts which have been widely identified in the environment and in chemical-waste dumpsites. Halogenated aromatics are invariably present in diverse analytes as highly complex mixtures of isomers and congeners and this complicates the hazard and risk assessment of these compounds. Several studies have confirmed the common receptor-mediated mechanism of action of toxic halogenated aromatics and this has resulted in the development of structure-activity relationships for this class of chemicals. The most toxic halogenated aromatic is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and based on in vivo and in vitro studies the relative toxicities of individual halogenated aromatics have been determined relative to TCDD (i.e., toxic equivalents). The derived toxic equivalents can be used for hazard and risk assessment of halogenated aromatic mixtures; moreover, for more complex mixtures containing congeners for which no standards are available (e.g., bromo/chloro mixtures), several in vitro or in vivo assays can be utilized for hazard or risk assessment.

Genes for cytochrome P-450 and their regulation

The capacity of the liver microsomal mixed-function oxidase system to metabolize a wide variety of exogenous as well as endogenous compounds reflects the participation of multiple forms of the terminal oxidase, cytochrome P-450, which have different broad, but overlapping, substrate specificities. Several of these isozymes accumulate in the liver after exposure of animals to specific inducing agents. Recent studies employing recombinant DNA techniques to investigate the genetic and evolutionary relatedness of various cytochrome P-450 isozymes as well as the molecular basis for the induction phenomenon are described. The conclusions from these investigations are presented in the context of the substantial body of data obtained from the characterization of specific cytochrome P-450 isozymes and from studies on the induction of specific isozymes or enzymatic activities during development or after treatment of animals with various inducing agents.