Functional analysis of the transcriptional promoter for the CYP1A1 gene

Aryl hydrocarbon receptor-induced activation of the human IGH hs1.2 enhancer: Mutational analysis of putative regulatory binding motifs

The human hs1.2 enhancer within the Ig heavy chain gene (IGH) is polymorphic and associated with a number of autoimmune diseases. The polymorphic region is characterized by tandem repeats of an ∼53-bp invariant sequence containing possible binding sites for several transcription factors. Our previous studies suggest the human hs1.2 enhancer is sensitive to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), an environmental toxicant and high affinity ligand of the aryl hydrocarbon receptor (AhR). TCDD induced hs1.2 enhancer activity in an AhR-dependent manner and the number of invariant sequences influenced the magnitude of activity. To better understand the regulation of human hs1.2 enhancer activity, the objective of the current study was to utilize mutational analysis and luciferase reporter constructs to evaluate the contribution of putative transcription factor binding sites to overall hs1.2 enhancer activity and modulation by TCDD. Basal and LPS-induced activity of the hs1.2 enhancer appeared to be most affected by mutation of sites outside of the invariant sequence or deletion of the entire invariant sequence; whereas sites influencing the effect of TCDD were dependent on the cellular activation state (i.e. unstimulated vs. LPS stimulation) and relatively independent of the putative AhR binding site within the invariant sequence. These results suggest that AhR activation affects human hs1.2 activity through an as yet undetermined non-canonical pathway. A better understanding regarding the role of the hs1.2 enhancer in human Ig expression and how AhR ligands modulate its activity may lead to insights into overall Ig regulation and mechanisms of dysfunction.

Cancer risk in waterpipe smokers: a meta-analysis

Objectives

To quantify by meta-analysis the relationship between waterpipe smoking and cancer, including cancer of the head and neck, esophagus, stomach, lung and bladder.

Methods

We performed a systematic literature search to identify relevant studies, scored their quality, used fixed and random-effect models to estimate summary relative risks (SRR), evaluated heterogeneity and publication bias.

Results

We retrieved information from 28 published reports. Considering only highquality studies, waterpipe smoking was associated with increased risk of head and neck cancer (SRR 2.97; 95 % CI 2.26–3.90), esophageal cancer (1.84; 1.42–2.38) and lung cancer (2.22; 1.24–3.97), with no evidence of heterogeneity or publication bias. Increased risk was also observed for stomach and bladder cancer but based mainly on poor-quality studies. For colorectum, liver and for all sites combined risk estimates were elevated, but there were insufficient reports to perform a meta-analysis.

Conclusions

Contrary to the perception of the relative safety of waterpipe smoking, this meta-analysis provides quantitative estimates of its association with cancers of the head and neck, esophagus and lung. The scarcity and limited quality of available reports point out the need for larger carefully designed studies in well-defined populations.

Functional analysis of the dioxin response elements (DREs) of the murine CYP1A1 gene promoter: beyond the core DRE sequence

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that mediates the biological and toxicological effects of halogenated aromatic hydrocarbons, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). When activated by dioxin, the cytosolic AhR protein complex translocates into the nucleus and dimerizes with the ARNT (Ah receptor nuclear translocator) protein. The heteromeric ligand:AhR/Arnt complex then recognizes and binds to its specific DNA recognition site, the dioxin response element (DRE). DREs are located upstream of cytochrome P4501A1 (CYP1A1) and other AhR-responsive genes, and binding of the AhR complex stimulates their transcription. Although CYP1A1 expression has been used as the model system to define the biochemical and molecular mechanism of AhR action, there is still limited knowledge about the roles of each of the seven DREs located in the CYP1A1 promoter. These seven DREs are conserved in mouse, human and rat. Deletion analysis showed that a single DRE at −488 was enough to activate the transcription. Truncation analysis demonstrated that the DRE at site −981 has the highest transcriptional efficiency in response to TCDD. This result was verified by mutation analysis, suggesting that the conserved DRE at site −981 could represent a significant and universal AhR regulatory element for CYP1A1. The reversed substituted intolerant core sequence (5′-GCGTG-3′ or 5′-CACGC-3′) of seven DREs reduced the transcriptional efficiency, which illustrated that the adjacent sequences of DRE played a vital role in activating transcription. The core DRE sequence (5′-TNGCGTG-3′) tends to show a higher transcriptional level than that of the core DRE sequence (5′-CACGCNA-3′) triggered by TCDD. Furthermore, in the core DRE (5′-TNGCGTG-3′) sequence, when “N” is thymine or cytosine (T or C), the transcription efficiency was stronger compared with that of the other nucleotides. The effects of DRE orientation, DRE adjacent sequences and the nucleotide “N” in the core DRE (5′-TNGCGTG-3′) sequence on the AhR-regulated CYP1A1 transcription in response to TCDD were studied systematically, and our study laid a good foundation for further investigation into the AhR-dependent transcriptional regulation triggered by dioxin and dioxin-like compounds.

A dioxin response element in the multiple cloning site of the pGL3 luciferase reporter influences transcriptional activity

Luciferase reporter plasmids (pGL3 backbone, Promega) have been utilized to characterize the transcriptional effects of the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and other aryl hydrocarbon receptor (AhR) ligands. Following ligand activation, the AhR and its dimerization partner AhR nuclear translocator (ARNT) regulate transcription by binding dioxin response elements (DREs) in regulatory regions of dioxin-sensitive genes. Upon sequencing of our luciferase reporters, we unexpectedly identified a DRE core motif within the multiple cloning site (mcsDRE) of the pGL3 luciferase plasmid backbone in a subset of our reporters. Therefore, the objective of this study was to determine if the mcsDRE inadvertently influences reporter activity. Utilizing deletional analysis we determined that the mcsDRE did significantly alter the transcriptional effect induced by TCDD. Since many chemicals have been shown to interact with the AhR and influence transcription through the DRE, the presence of the mcsDRE in the pGL3 luciferase plasmid may inappropriately influence promoter and enhancer analysis. As such, insertion of regulatory elements into pGL3 reporters should be designed to avoid retaining the mcsDRE core motif (GCGTG) and currently utilized pGL3 reporters should be evaluated for the presence of the mcsDRE.

Regulation of the activity and expression of aryl hydrocarbon receptor by ethanol in mouse hepatic stellate cells

BACKGROUND

During the course of alcohol-induced liver damage, hepatic stellate cells are transformed into proliferative, fibrogenic, and contractile myofibroblasts. Aryl hydrocarbon receptor (AhR) is a transcription factor that controls the expression of genes involved in the metabolism of xenobiotics, inflammation, cell proliferation, and death.

METHODS

Immortal mouse hepatic stellate cells (MHSCs) were isolated from transgenic mice that expressed a thermolabile SV40 tumor antigen. Quantitative real-time reverse transcription polymerase chain reaction assays, Western blot analysis, promoter activity assays, and chromatin immunoprecipitation analyses were performed for studying the effect of ethanol (EtOH) on AhR expression and transcriptional activity.

RESULTS

Treatment of MHSCs with 50 to 200 mM EtOH for 6 hours induced AhR nuclear translocation, enhanced the promoter activity of cytochrome P450 (CYP) 1A1, increased the amount of AhR bound to the promoter of CYP1A1 and 1B1, and up-regulated the mRNA expression of these AhR target genes in a dose-dependent manner. In contrast, EtOH exposure down-regulated AhR mRNA and protein expression. Similarly, benzo(a)pyrene (BaP) at 10 nM reduced AhR and increased CYP1A1 and 1B1 mRNAs. Pretreatment of MHSCs with 50 mM EtOH for 7 days diminished the capacity of MHSCs to express CYP1A1 and 1B1 induced by a 200 mM EtOH challenge, or by 10 nM BaP. However, the up-regulatory effect of EtOH on solute carrier family 16, member 6 (SLC16a6) was unaffected by EtOH pretreatment. Similar to EtOH, dimethyl sulfoxide (DMSO) at concentrations of 50 to 100 mM down-regulated AhR and up-regulated CYP1A1 mRNA expression in a dose-dependent manner.

CONCLUSIONS

These data, for the first time, demonstrate that EtOH activates MHSC AhR and down-regulates its expression. Chronic EtOH pretreatment lowers the availability of AhR, and specifically diminishes the inducibility of CYP genes. The effect on AhR appears to not be an EtOH-specific response, as DMSO alone (and possibly other organic solvents) was also able to activate AhR.

Aryl hydrocarbon receptor ligands of widely different toxic equivalency factors induce similar histone marks in target gene chromatin

Posttranslational histone modifications are a critical regulatory mechanism of gene transcription. Previous studies from our laboratory have shown that contingent on binding to its cognate promoter motifs in the Cyp1a1 gene, activation of the aryl hydrocarbon receptor (AHR) by benzo[a]pyrene (BaP) treatment induces histone modifications in the Cyp1a1 promoter that are required for activation of gene transcription. Here, we have studied different AHR ligands, including polychlorinated biphenyls (PCBs) of different toxic equivalency factors (TEF), to determine whether changes in histone modifications are linked to different levels of Cyp1a1 expression or dependent on AHR-ligand affinity. We find that all ligands lead to the same pattern of histone modifications in a relationship that parallels the strength of their AHR-ligand affinity. Thus, whereas PCB126 (TEF 0.1), 3-methylcholanthrene, β-naphthoflavone, and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) initiate a pattern of histone marks similar to those induced by BaP, PCB77 (TEF 0.0001) causes a lower level of change in the same marks and requires a longer activation time than PCB126, BaP, or TCDD. In contrast, the non-dioxin-like PCB153 recruits AHR to the Cyp1a1 enhancer causing a displacement of enhancer-associated histone H3 but does not cause the other observed histone mark changes nor does it induce transcription. These results indicate that AHR recruitment to the promoter is not sufficient to induce the histone modifications needed to activate gene expression and show that there is a good correlation between the regulatory chromatin changes associated with ligand-induced AHR target gene transcription and the resultant toxicity of the ligand.

HDAC1 bound to the Cyp1a1 promoter blocks histone acetylation associated with Ah receptor-mediated trans-activation

Metabolic bioactivation of polycyclic aromatic hydrocarbons, such as the environmental procarcinogen benzo[a]pyrene, is catalyzed by a cytochrome P450 monooxygenase encoded by the substrate-inducible Cyp1a1 gene. Cyp1a1 induction requires trans-activation by the heterodimeric transcriptional complex formed by the liganded Ah receptor (AHR) and its partner, ARNT. Previously, we showed that constitutively bound HDAC1 dissociates from Cyp1a1 promoter chromatin after ligand-mediated induction, concomitantly with the recruitment of AHR/ARNT complexes and p300. Here, we investigated the hypothesis that HDAC1 binding maintains the Cyp1a1 gene in a silenced state in uninduced cells. We find that Cyp1a1 induction by the AHR/ARNT is associated with modification of specific chromatin marks, including hyperacetylation of histone H3K14 and H4K16, trimethylation of histone H3K4, and phosphorylation of H3S10. HDAC1 and DNMT1 form complexes on the Cyp1a1 promoter of uninduced cells but HDAC1 inhibition alone is not sufficient to induce Cyp1a1 expression, although it allows for the hyperacetylation of H3K14 and H4K16 to levels similar to those found in B[a]P-induced cells. These results show that by blocking the modification of histone marks, HDAC1 plays a central role in Cyp1a1 expression and that its removal is a necessary but not sufficient condition for Cyp1a1 induction, underscoring the requirement for a concerted series of chromatin-remodeling events to complete the initial steps of gene trans-activation by the Ah receptor.

Functional analysis of cis-regulatory regions within the dioxin-inducible CYP1A promoter/enhancer region from zebrafish (Danio rerio)

In vitro mutagenesis was utilized to render the various xenobiotic response elements (XREs) within the zebrafish CYP1A promoter/enhancer region non-functional either independently or in combination. Reporter gene assays revealed that only XRE4, XRE7, and XRE8 contributed to maximal TCDD-mediated induction of luciferase and that the contribution of each XRE to maximal induction was not equal. XRE4 and XRE7 were capable of functioning independently, while XRE8 alone could not support TCDD-mediated induction but was required for the ability of XRE4 and XRE7 to support maximal induction. These results were observed in cell lines derived from human, mouse and zebrafish. Mutagenesis of 3' nucleotides flanking the non-functional XRE5, and functional XRE4 did not alter the function of these XREs in cell culture. In silico analyses revealed the presence of putative Sp1, AP2, CREB, and two HNF-3 transcription factor binding sites that were localized to common positions within the enhancer region of both the mouse and zebrafish CYP1A genes. In vitro mutagenesis of the binding sites showed that loss of the Sp1 or AP2 sites had minimal impact on TCDD-mediated gene induction while loss of the putative CREB site resulted in a modest decrease in basal and inducible activity and mutation of the HNF-3 reduced inducible activity by >90% of controls. Collectively, these findings suggest that the presence of XREs is not the sole determinant for regulation of aryl hydrocarbon receptor (AHR)-mediated gene and do not function in an additive manner.

CYP Induction-Mediated Drug Interactions: in Vitro Assessment and Clinical Implications

Cytochrome P450 (CYP) induction-mediated interaction is one of the major concerns in clinical practice and for the pharmaceutical industry. There are two major issues associated with CYP induction: a reduction in therapeutic efficacy of comedications and an induction in reactive metabolite-induced toxicity. Because CYP induction is a metabolic liability in drug therapy, it is highly desirable to develop new drug candidates that are not potent CYP inducer to avoid the potential of CYP induction-mediated drug interactions. For this reason, today, many drug companies routinely include the assessment of CYP induction at the stage of drug discovery as part of the selection processes of new drug candidates for further clinical development. The purpose of this article is to review the molecular mechanisms of CYP induction and the clinical implications, including pharmacokinetic and pharmacodynamic consequences. In addition, factors that affect the degree of CYP induction and extrapolation of in vitro CYP induction data to in vivo situations will also be discussed. Finally, assessment of the potential of CYP induction at the drug discovery and development stage will be discussed.

Co-exposure to metals modulates CYP1A mRNA inducibility in Atlantic tomcod Microgadus tomcod from two populations

Populations from urbanized and industrialized sites are often exposed to mixtures of chemical contaminants including aromatic hydrocarbons (AHs) and heavy metals. The effects of mixtures of these contaminants on these populations are largely unknown. The Hudson River Estuary is highly contaminated with a variety of AHs including, PCBs and PAHs, and metals, and its population of Atlantic tomcod Microgadus tomcod bioaccumulates those which are persistent. The Hudson River's tomcod population exhibits resistance to persistent AHs as exemplified by significantly decreased inducibility of hepatic cytochrome P4501A (CYP1A) mRNA. We used hepatic CYP1A mRNA inducibility in tomcod from the Hudson River and a sensitive population to investigate the effects of acute co-exposure to metals on aryl hydrocarbon receptor (AHR)-mediated gene expression. Adult tomcod from the Hudson River and the cleaner Miramichi River were i.p. injected with one dose of benzo[a]pyrene (B[a]P) or coplanar PCB77 and graded doses of four metals, As, Cd, Cr, and Ni, and levels of hepatic CYP1A mRNA and protein were assayed. We observed no effects of metals treatment on basal levels of hepatic CYP1A mRNA expression, but all four metals significantly reduced CYP1A mRNA inducibility in tomcod from one or both populations. The magnitude of the inhibition of CYP1A mRNA inducibility differed among the metals and fish from the two populations. Also, the profile of the metals modulation of induced CYP1A mRNA showed differences that depended on the time after treatment of sacrifice. Our results demonstrate that co-exposure to several metals can impact inducible, but not basal levels of CYP1A expression and perhaps other toxicities mediated by the AHR.

Occupational exposure to polycyclic aromatic hydrocarbons suppresses constitutive expression of CYP1B1 on the transcript level in human leukocytes

Expression patterns of the cytochromes P450 CYP1A1 and CYP1B1 have been analyzed on the transcript level in leukocytes of persons (n = 30) occupationally exposed to polycyclic aromatic hydrocarbons (PAH). To assess effects on expression levels results were compared with data obtained from a non-exposed control group (n = 68). CYP1B1 transcripts can be detected in all subjects of the control group but vary largely in their levels (factor 35). Statistical analysis shows that this variability is neither due to the age of the persons nor due to cigarette smoking. Furthermore, there is no difference in expression levels between genders. In contrast to CYP1B1, CYP1A1 is detectable in only 14% of the subjects. People involved in graphite electrode production and exposed to PAH show largely decreased CYP1B1 transcript levels. In 67% of the subjects, CYP1B1 is no more detectable at all. Vice versa, expression of CYP1A1 is increased in exposed persons so that 80% become positive for CYP1A1 vs. 14% of the control group. The results show that occupational exposure to PAH apparently leads to effect-relevant internal doses. Both, suppression of CYP1B1 and induction of CYP1A1 in leukocytes can be used as exposure parameters proving both enzymes to be suitable biomarkers of exposure. The suppression of CYP1B1 is an unexpected effect which needs further investigation. It is discussed that CYP1B1 and CYP1A1 indeed share a common Ah receptor mediated transcriptional regulation but that differences in promoter structure of the two genes and tissue-specific expression profiles of transcription factors may cause a differential expression behaviour.

3'-methoxy-4'-nitroflavone, a reported aryl hydrocarbon receptor antagonist, enhances Cyp1a1 transcription by a dioxin responsive element-dependent mechanism

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor, regulating expression of a group of specific genes including cytochrome P450 1A1 (Cyp1a1). Stably transfected luciferase with dioxin responsive elements (DRE) in its promoter region has been commonly used as a reporter gene to study the mechanism of AhR signaling and compare potencies of TCDD and related compounds. However, how these two genes might respond to structurally diverse AhR ligands was unknown. This study investigates their expression in the same cells in response to TCDD, the most potent agonist, and 3'M4'NF, a reported potent antagonist. Our data suggest that these two compounds appear to play different roles in regulating these genes. While TCDD enhanced transcription of both genes, 3'M4'NF induced the endogenous Cyp1a1, but not the reporter gene. Mechanistic studies indicated that the increase in induction of CYP1A1 protein by 3'M4'NF was mediated by AhR-dependent transcriptional activation. Further analysis of the Cyp1a1 promoter sequence did not reveal any 3'M4'NF-specific responsive elements other than DREs. Rather, the interaction between the 3'M4'NF-bound receptor complex and DREs was confirmed by the observation that a single nucleotide mutation in DRE core sequences obliterated AhR enhancer activity in response to both TCDD and 3'M4'NF. Together these data suggest that 3'M4'NF, a weak AhR agonist, activates the AhR to recognize and interact with the same DREs as TCDD. However, depending on its concentration as well as the promoter context of a particular gene, the ability of 3'M4'NF to act as an AhR antagonist or agonist may appear different for various genes.

Accessibility and activity of the promoter for a dioxin-inducible ecto-ATPase gene

We have analyzed the core promoter for a dioxin-inducible ecto-ATPase gene in mouse hepatoma cells. The transcriptional initiation site maps to a region that contains neither a TATA sequence nor a consensus initiator sequence nor a downstream promoter element. The core promoter has constitutive activity that does not require either the aromatic hydrocarbon receptor or its heterodimerization partner Arnt. Two GC-rich regions contribute approximately equally to the constitutive activity. Proteins constitutively occupy the GC-rich regions in chromatin. The promoter assumes a non-nucleosomal configuration in its native chromosomal setting in both uninduced and dioxin-induced cells. Our findings imply that the GC-rich regions together with their cognate binding proteins carry out core promoter functions for the ecto-ATPase gene. The promoter is constitutively accessible in situ, and chromatin structure is not a limiting factor for dioxin-inducible ecto-ATPase transcription in intact cells.

Induction of cytochrome P4501A1

Cytochrome P4501A1 is a substrate-inducible microsomal enzyme that oxygenates polycyclic aromatic hydrocarbons, such as the carcinogen benzo(a)pyrene, as the initial step in their metabolic processing to water-soluble derivatives. Enzyme induction reflects increased transcription of the cognate CYP1A1 gene. The environmental toxicant 2,3,7,8-tetrachlorodibenzo-p-dioxin is the most potent known cytochrome P4501A1 inducer. Two regulatory proteins, the aromatic (aryl) hydrocarbon receptor (AhR) and the AhR nuclear translocator (Arnt), mediate induction. AhR and Arnt are prototypical members of the basic helix-loop-helix/Per-Arnt-Sim class of transcription factors. Mechanistic analyses of cytochrome P4501A1 induction provide insights into ligand-dependent mammalian gene expression, basic helix-loop-helix/Per-Arnt-Sim protein function, and dioxin action; such studies also impact public health issues concerned with molecular epidemiology, carcinogenesis, and risk assessment.

The aromatic hydrocarbon receptor, transcription, and endocrine aspects of dioxin action

The widespread and persistent environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin elicits adaptive and adverse biological responses by inducing changes in gene transcription. Some of dioxin's effects reflect disruption of endocrine homeostasis. The aromatic hydrocarbon receptor protein, together with its heterodimerization partner, the aromatic hydrocarbon receptor nuclear translocator protein, mediates dioxin action. There are notable similarities between the mechanism of dioxin action and the mechanisms of steroid/retinoid/thyroid hormone action. Studies of dioxin action may provide insights into the regulation of hormone-responsive genes and endocrine physiology.

Identification and functional characterization of a conserved, nuclear factor 1-like element in the proximal promoter region of CYP1A2 gene specifically expressed in the liver and olfactory mucosa

CYP1A2 is a major cytochrome P-450 isoform in the liver and the olfactory mucosa but is essentially not expressed in other tissues. A nuclear factor 1 (NF-1) -like element was identified in the proximal promoter region of rat, mouse, rabbit, and human CYP1A2 genes through data base analysis. In vitro DNase I footprinting with a -211 to +81 probe from the rat CYP1A2 gene and nuclear extracts from rat liver and olfactory mucosa revealed a single protected region corresponding to the NF-1-like element at -129 to -111. Protein binding to this NF-1-like element was tissue-selective and was confirmed by in vivo footprinting in native chromatin from rat liver. Multiple DNA-binding complexes were detected in gel-shift assays using the CYP1A2 NF-1-like element and nuclear extracts from liver and olfactory mucosa, all of which were supershifted in the presence of an anti-NF1 antibody. The NF-1-like element was essential for transcriptional activity of the CYP1A2 gene in an in vitro transcription assay using nuclear extracts from the two tissues. Thus, members of the NF-1 family of transcription factors may play an important role in the tissue-selective expression of the CYP1A2 gene in the liver and olfactory mucosa.

Estrogen receptor reduces CYP1A1 induction in cultured human endometrial cells

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) exerts its toxic action via the aryl hydrocarbon (Ah) receptor, which induces a battery of xenobiotic-metabolizing enzymes, including the cytochrome P450 isozyme, CYP1A1. TCDD-induced 7-ethoxycoumarin-O-deethylase activity was reduced 75% in cultured human endometrial ECC-1 cells exposed to various concentrations of 17beta-estradiol for up to 72 h, with a half-maximal effective concentration (EC50) of 0.9 nM. Reduced enzyme activity was correlated with decreased CYP1A1 mRNA levels, and transcription. Exposure to TCDD plus 17beta-estradiol also reduced CYP1A1 activity in MCF-7 breast cancer cells but not in Hep-3B human liver cells or HuE primary human keratinocytes, suggesting that the effect was specific to estrogen-regulated cells. Estrogen receptor antagonists 4-hydroxytamoxifen and 7alpha-[9-(4,4, 5,5,5-pentafluoro-pentylsulfinyl)nonyl]estra-1,3,5(10)-tr iene3, 17beta-diol restored TCDD-induced CYP1A1 transcription, steady-state mRNA levels, and enzymatic activity in ECC-1 cells. Gel mobility shift assay showed that 17beta-estradiol had little effect on Ah receptor binding to its DNA-responsive element. 17beta-Estradiol did not alter the induction of another Ah receptor-regulated gene, CYP1B1, suggesting that altered Ah receptor binding to DNA does not mediate reduced CYP1A1 transcription. Transfecting ECC-1 cells with a general transcription factor involved in CYP1A1 induction, nuclear factor-1, reversed 17beta-estradiol antagonism of dioxin induced-CYP1A1. The data suggest that 17beta-estradiol reduced CYP1A1 expression at the transcriptional level by squelching available nuclear factor-1, a transcription factor that interacts with both Ah and estrogen receptors.

Down-regulation of cytochrome P450 1A1 gene promoter by oxidative stress. Critical contribution of nuclear factor 1

Oxidative stress interferes with several cellular functions, in particular transcriptional regulation. We show here that the human cytochrome P450 1A1 (CYP1A1) is down-regulated at the transcriptional level by oxidative stress. Basal as well as 2,3,7, 8-tetrachloro-p-dioxin-induced promoter activities are strongly impaired by H2O2 treatment or glutathione depletion with L-buthionine-(S,R)-sulfoximine. Tumor necrosis factor alpha inhibits CYP1A1 expression, and this inhibition is prevented by the antioxidant pyrrolidine dithiocarbamate. We show that these regulations depend on the integrity of the nuclear factor 1 (NFI) site located in the proximal promoter. We therefore examined the redox regulation of this transcription factor. Treatment of human HepG2 or rat H4 hepatoma cells with H2O2 or L-buthionine-(S, R)-sulfoximine inactivates the binding of the NFI transcription factor to its DNA consensus sequence. Furthermore, H2O2 treatment leads to a dose-dependent decrease of reporter gene expressions driven by promoters containing NFI binding sites. Glutathione depletion and catalase inhibition also repress a NFI-driven promoter. Under the same conditions, the CP-1 transcription factor activity is not affected by oxidative stress. Thus, NFI seems particularly sensitive to oxidative stress. This accounts, at least partially, for the regulation of cyp1A1 gene expression.

Transgenic fish and its application in basic and applied research

Since 1985, transgenic fish have been successfully produced by microinjecting or electroporating desired foreign DNA into unfertilized or newly fertilized eggs using many different fish species. More recently, transgenic fish have also been produced by infecting newly fertilized eggs with pantropic, defective retroviral vectors carrying desired foreign DNA. These transgenic fish can serve as excellent experimental models for basic scientific investigations as well as in biotechnological applications. In this paper, we will review the current status of the transgenic fish research and its potential application in basic and applied research.

Functional analysis of the promoter for the human CYP1B1 gene

Our laboratory has cloned the cDNA (Sutter, T. R., Tang, Y. M., Hayes, C. L., Wo, Y.-Y. P., Jabs, E. W., Li, X., Yin, H., Cody, C. W. , and Greenlee, W. F. (1994) J. Biol. Chem. 269, 13092-13099) and gene (Tang, Y. M., Wo, Y.-Y. P., Jabs, E. W., Stewart, J. C., Sutter, T. R., and Greenlee, W. F. (1996) J. Biol. Chem. 271, 28324-28330) for human CYP1B1, a new member of the cytochrome P450 superfamily. Here, we report on the mapping and function of the CYP1B1 promoter. The CYP1B1 promoter is fully functional, when it is uncoupled from upstream enhancer elements. Deletion analysis and site-directed mutagenesis identified four regulatory elements required for maximum promoter activity: two antisense Sp1 sites (-84 to -89 and -68 to -73), a TATA-like box (-34 to -29), and an initiator motif (-5 to +3). The initiator and the TATA-like elements are both required for basal promoter activity, with enhanced activity mediated by the two antisense Sp1 elements. The CYP1B1 initiator was demonstrated by in vitro transcription analysis to be a positioning element that maintained fidelity of transcription from a single site. Specific binding to a CYP1B1 initiator probe by human nuclear extract proteins was competed either by the highly homologous murine terminal deoxynucleotidyl transferase initiator or, to a lesser extent, by the adenovirus major late initiator. Taken together, these results indicate that the structure and function of the CYP1B1 promoter confers constitutive expression of the gene and assures fidelity of transcription initiation from a single site. The CYP1B1 promoter is distinct from the promoters of the closely related cytochrome P450s CYP1A1 and CYP1A2 and is structurally and functionally similar to the promoters of constitutively expressed genes and at least two viruses.

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.

Mapping sequence specific DNA-protein interactions: a versatile, quantitative method and its application to transcription factor XF1

We have developed a method for the quantitative, exhaustive sequence specificity determination of DNA-binding proteins. The QuESSD method overcomes the limitations inherent in other published in vitro selection methods, not only defining the consensus sequence, but also quantifying the effect on DNA-protein affinity of replacing each base in the recognition domain with every other base. The features distinguishing this method from other in vitro selection approaches are: (1) instead of synthesizing one target oligonucleotide population containing a long randomized domain, we synthesize several oligonucleotide populations, each randomized at two positions. (2) Instead of carrying out several cycles of selection and amplification, we carry out a single cycle. (3) We have developed data collection and analysis procedures that eliminate artifacts and allow generation of quantitative results. The QuESSD method yields accurate measures of: (a) the selectivity of the protein for each base at each position within the recognition domain (normalized relative selectivity), (b) the contributions of individual sites within the recognition domain to the binding affinity (selectivity variance), (c) the relative binding affinity of any given sequence (global selectivity). We confirmed results by (1) tabulating directly the frequency of appearance of individual species in the pool of protein-bound oligonucleotides by cloning and sequencing individual oligonucleotides, and (2) competition EMSA analysis of oligonucleotides designed on the basis of QuESSD data. We have used this method to map the sequence specificity of the nuclear protein XF1 and to distinguish the sequence specificities of XF1 and the AH receptor complex, both of which bind to XRE1, a xenobiotic responsive element (XRE) located upstream of the CYP1A1 gene. Using data obtained by the QuESSD method, we designed oligonucleotides specific for XF1 or for the AH receptor, and prepared CAT reporter gene constructs carrying these oligonucleotides, or wild-type XRE1, upstream of a minimal promoter. Transfection studies using these constructs indicated that XF1 can function as a weak activator of basal transcription, and can, under some circumstances, compete with the AH receptor for binding to XRE1.

Ah receptor signaling pathways

The aryl hydrocarbon (Ah) receptor has occupied the attention of toxicologists for over two decades. Interest arose from the early observation that this soluble protein played key roles in the adaptive metabolic response to polycyclic aromatic hydrocarbons and in the toxic mechanism of halogenated dioxins and dibenzofurans. More recent investigations have provided a fairly clear picture of the primary adaptive signaling pathway, from agonist binding to the transcriptional activation of genes involved in the metabolism of xenobiotics. Structure-activity studies have provided an understanding of the pharmacology of this receptor; recombinant DNA approaches have identified the enhancer sequences through which this factor regulates gene expression; and functional analysis of cloned cDNAs has allowed the characterization of the major signaling components in this pathway. Our objective is to review the Ah receptor's role in regulation of xenobiotic metabolism and use this model as a framework for understanding the less well-characterized mechanism of dioxin toxicity. In addition, it is hoped that this information can serve as a model for future efforts to understand an emerging superfamily of related signaling pathways that control biological responses to an array of environmental stimuli.

The aromatic hydrocarbon receptor, dioxin action, and endocrine homeostasis

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

Multiple elements regulate phosphoenolpyruvate carboxykinase gene expression in hepatoma hybrid cells

The phosphoenolpyruvate carboxykinase (PEPCK) gene is highly expressed in cultured rat hepatoma cells, but extinguished in hepatoma x fibroblast hybrids. Extinction of PEPCK gene expression in hybrids is a polygenic process that involves several fibroblast loci, only one of which (tissue-specific extinguisher-1, TSE1) has been characterized to date. To identify sequence elements of the PEPCK gene that are involved both in TSE1-mediated extinction and in TSE1-independent processes, we assayed expression of chimeric PEPCK transgenes in transiently and stably transfected hybrid cells. We report that TSE1 responsiveness mapped to the PEPCK CRE (cAMP response element), as shown previously for the tyrosine aminotransferase gene. This was expected from the recent identification of the TSE1 gene product as a regulatory subunit of protein kinase A. However, none of the transgenes we assayed were responsive to TSE1-independent extinction mechanisms, suggesting that these controls require DNA sequences and/or chromatin structures that were not present in the transfected reporters. The implications of these findings are discussed.

Differences in deoxyribonuclease I hypersensitive sites in phenobarbital-inducible and constitutive rabbit P450IIC genes

DNase I hypersensitivity of nuclear chromatin near the rabbit cytochrome P450IIC genes was investigated by indirect end-labeling genomic Southern analysis. Major DNase I hypersensitive sites were observed in proximal (-200 base pairs) and distal (-2000 to -2200 base pairs) regions of the genes. The presence of the proximal site correlated well with the expression states of the individual genes. In contrast, the distal site was present in DNA from both liver and kidney nuclei and in untreated and phenobarbital-treated animals irrespective of the expression state of the genes. However, the distal site was present only in genes that respond to phenobarbital (cytochromes P450IIC1 and P450IIC2) and was not detected in the constitutive cytochrome P450IIC3 gene. The nucleotide sequences of 500 base pairs in the distal site regions of cytochromes P450IIC1 and P450IIC2 were 67% similar, and hepatocyte nuclear factor 1 like motifs were conserved in these two sequences. These results are consistent with the hypothesis that distal regions cooperate with the proximal promoter regions in the regulation of cytochrome P450IIC gene expression.

2,3,7,8-Tetrachlorodibenzo-p-dioxin induces cytochrome P450IA1 enzyme activity by activating transcription of the corresponding gene

We have reviewed the transcriptional regulation of the CYP1A1 gene, which encodes the cytochrome P450IA1 enzyme. TCDD induces CYP1A1 transcription via a receptor- and enhancer-dependent mechanism that involves multiple protein-DNA interactions. The system provides an example of enzyme regulation at the level of gene transcription.