Induction of the Cyp1a-1 dioxin-responsive enhancer in transgenic mice

Bioluminescence imaging of Cyp1a1-luciferase reporter mice demonstrates prolonged activation of the aryl hydrocarbon receptor in the lung

Aryl hydrocarbon receptor (AHR) signalling integrates biological processes that sense and respond to environmental, dietary, and metabolic challenges to ensure tissue homeostasis. AHR is a transcription factor that is inactive in the cytosol but upon encounter with ligand translocates to the nucleus and drives the expression of AHR targets, including genes of the cytochrome P4501 family of enzymes such as Cyp1a1. To dynamically visualise AHR activity in vivo, we generated reporter mice in which firefly luciferase (Fluc) was non-disruptively targeted into the endogenous Cyp1a1 locus. Exposure of these animals to FICZ, 3-MC or to dietary I3C induced strong bioluminescence signal and Cyp1a1 expression in many organs including liver, lung and intestine. Longitudinal studies revealed that AHR activity was surprisingly long-lived in the lung, with sustained Cyp1a1 expression evident in discrete populations of cells including columnar epithelia around bronchioles. Our data link diet to lung physiology and also reveal the power of bespoke Cyp1a1-Fluc reporters to longitudinally monitor AHR activity in vivo.

Generation and application of a novel transgenic zebrafish line Tg(GAcyp1a:eGFP/Luc) as an in vivo assay to sensitive and specific monitoring of DLCs in the environment

CYP1A is the most commonly used biomarker and transgenic fish which carrying a cyp1a promoter to drive a reporter gene can be used as reliable way to monitor dioxin/dioxin-like compounds (DLCs) in the environment. Here, we cloned the cyp1a promoter of Gambusia affinis and this promoter showed stronger transcriptional activity than that of zebrafish. Then, a Tg(GAcyp1a:eGFP/Luc) transgenic zebrafish line was first constructed with the G. affinis cyp1a promoter driving eGFP expression using meganuclease I-SceI mediated transgenesis technology. The Tg(GAcyp1a:eGFP/Luc) larvae at 72 h post-fertilization (hpf) were tested by exposing to TCDD for 72 h, and induced GFP was mainly expressed in the liver with low background. The Tg(GAcyp1a:eGFP/Luc) zebrafish showed high sensitivity (limit of detection of 0.322 ng/L TCDD and 0.7 TEQ-ng/L PCDD/Fs) and specificity (insensitive to responses to PAHs and PCBs). In addition, the transgenic line showed a low detection concentration of the DLCs contaminated environmental samples (as low as 1.8 TEQ-ng/L), and the eGFP fluorescence intensity and the chemical-TEQ values were closely correlated. In conclusion, a sensitively and specifically transgenic zebrafish line was established to convenient and effective to detect DLCs in the environment.

Dopamine D2-Receptor Antagonists Down-Regulate CYP1A1/2 and CYP1B1 in the Rat Liver

Dopaminergic systems regulate the release of several hormones including growth hormone (GH), thyroid hormones, insulin, glucocorticoids and prolactin (PRL) that play significant roles in the regulation of various Cytochrome P450 (CYP) enzymes. The present study investigated the role of dopamine D₂-receptor-linked pathways in the regulation of CYP1A1, CYP1A2 and CYP1B1 that belong to a battery of genes controlled by the Aryl Hydrocarbon Receptor (AhR) and play a crucial role in the metabolism and toxicity of numerous environmental toxicants. Inhibition of dopamine D₂-receptors with sulpiride (SULP) significantly repressed the constitutive and benzo[a]pyrene (B[a]P)-induced CYP1A1, CYP1A2 and CYP1B expression in the rat liver. The expression of AhR, heat shock protein 90 (HSP90) and AhR nuclear translocator (ARNT) was suppressed by SULP in B[a]P-treated livers, whereas the AhRR expression was increased by the drug suggesting that the SULP-mediated repression of the CYP1 inducibility is due to inactivation of the AhR regulatory system. At signal transduction level, the D₂-mediated down-regulation of constitutive CYP1A1/2 and CYP1B1 expression appears to be mediated by activation of the insulin/PI3K/AKT pathway. PRL-linked pathways exerting a negative control on various CYPs, and inactivation of the glucocorticoid-linked pathways that positively control the AhR-regulated CYP1 genes, may also participate in the SULP-mediated repression of both, the constitutive and induced CYP1 expression. The present findings indicate that drugs acting as D₂-dopamine receptor antagonists can modify several hormone systems that regulate the expression of CYP1A1, CYP1A2 and CYP1B1, and may affect the toxicity and carcinogenicity outcome of numerous toxicants and pre-carcinogenic substances. Therefore, these drugs could be considered as a part of the strategy to reduce the risk of exposure to environmental pollutants and pre-carcinogens.

Cyp1a reporter zebrafish reveals target tissues for dioxin

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is the unintentional byproduct of various industrial processes, is classified as human carcinogen and could disrupt reproductive, developmental and endocrine systems. Induction of cyp1a1 is used as an indicator of TCDD exposure. We sought to determine tissues that are vulnerable to TCDD toxicity using a transgenic zebrafish (Danio rerio) model. We inserted a nuclear enhanced green fluorescent protein gene (EGFP) into the start codon of a zebrafish cyp1a gene in a fosmid clone using DNA recombineering. The resulting recombineered fosmid was then used to generate cyp1a reporter zebrafish, embryos of which were exposed to TCDD. Expression pattern of EGFP in the reporter zebrafish mirrored that of endogenous cyp1a mRNA. In addition, exposure of the embryos to TCDD at as low as 10 pM for 72 h, which does not elicit morphological abnormalities of embryos, markedly increased GFP expression. Furthermore, the reporter embryos responded to other AhR ligands as well. Exposure of the embryos to TCDD revealed previously reported (the cardiovascular system, liver, pancreas, kidney, swim bladder and skin) and unreported target tissues (retinal bipolar cells, otic vesicle, lateral line, cloaca and pectoral fin bud) for TCDD. Transgenic cyp1a reporter zebrafish we have developed can further understanding of ecotoxicological relevance and human health risks by TCDD. In addition, they could be used to identify agonists of AhR and antidotes to TCDD toxicity.

Strategies for designing transgenic DNA constructs

Generation and characterization of transgenic mice are important elements of biomedical research. In recent years, transgenic technology has become more versatile and sophisticated, mainly because of the incorporation of recombinase-mediated conditional expression and targeted insertion, site-specific endonuclease-mediated genome editing, siRNA-mediated gene knockdown, various inducible gene expression systems, and fluorescent protein marking and tracking techniques. Site-specific recombinases (such as PhiC31) and engineered endonucleases (such as ZFN and Talen) have significantly enhanced our ability to target transgenes into specific genomic loci, but currently a great majority of transgenic mouse lines are continuingly being created using the conventional random insertion method. A major challenge for using this conventional method is that the genomic environment at the integration site has a substantial influence on the expression of the transgene. Although our understanding of such chromosomal position effects and our means to combat them are still primitive, adhering to some general guidelines can significantly increase the odds of successful transgene expression. This chapter first discusses the major problems associated with transgene expression, and then describes some of the principles for using plasmid and bacterial artificial chromosomes (BACs) for generating transgenic constructs. Finally, the strategies for conducting each of the major types of transgenic research are discussed, including gene overexpression, promoter characterization, cell-lineage tracing, mutant complementation, expression of double or multiple transgenes, siRNA knockdown, and conditional and inducible systems.

Mouse breast cancer model-dependent changes in metabolic syndrome-associated phenotypes caused by maternal dioxin exposure and dietary fat

Diets high in fat are associated with increased susceptibility to obesity and metabolic syndrome. Increased adipose tissue that is caused by high-fat diets (HFD) results in altered storage of lipophilic toxicants like 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which may further increase susceptibility to metabolic syndrome. Because both TCDD and HFD are associated with increased breast cancer risk, we examined their effects on metabolic syndrome-associated phenotypes in three mouse models of breast cancer: 7,12-dimethylbenz[a]anthracene (DMBA), Tg(MMTV-Neu)202Mul/J (HER2), and TgN(MMTV-PyMT)634Mul/J (PyMT), all on an FVB/N genetic background. Pregnant mice dosed with 1 microg/kg of TCDD or vehicle on gestational day 12.5 were placed on a HFD or low-fat diet (LFD) at parturition. Body weights, percent body fat, and fasting blood glucose were measured longitudinally, and triglycerides were measured at study termination. On HFD, all cancer models reached the pubertal growth spurt ahead of FVB controls. Among mice fed HFD, the HER2 model had a greater increase in body weight and adipose tissue from puberty through adulthood compared with the PyMT and DMBA models. However, the DMBA model consistently had higher fasting blood glucose levels than the PyMT and HER2 models. TCDD only impacted serum triglycerides in the PyMT model maintained on HFD. Because the estrogenic activity of the HFD was three times lower than that of the LFD, differential dietary estrogenic activities did not drive the observed phenotypic differences. Rather, the HFD-dependent changes were cancer model dependent. These results show that cancer models can have differential effects on metabolic syndrome-associated phenotypes even before cancers arise.

Direct, continuous monitoring of air pollution by transgenic sensor mice responsive to halogenated and polycyclic aromatic hydrocarbons

BACKGROUND

The aryl hydrocarbon receptor (AhR, also called the dioxin receptor) plays crucial roles in toxicologic responses of animals to environmental pollutants, especially to halogenated and polycyclic aromatic hydrocarbons. To achieve direct, continuous risk assessment of air pollution using biological systems, we generated transgenic sensor mice that produce secreted alkaline phosphatase (SEAP) under the control of AhR.

METHODS

To characterize responses of the mice to AhR agonists, sensor mice were orally administered 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 3-methylcholanthrene (3MC), benzo[a]pyrene (B[a]P), or beta-naphthoflavone (BNF), and serum levels of SEAP were evaluated. To monitor air pollution caused by cigarette smoke, we placed the mice each day in an experimental smoking room, and evaluated activity of serum SEAP for up to 4 days. Activation of AhR in individual organs was also examined by reverse transcription-polymerase chain reaction (RT-PCR) analysis of SEAP.

RESULTS

In response to oral exposure to TCDD, sensor mice exhibited dramatic and sustained activation of AhR. The mice also responded sensitively to 3MC, B[a]P, and BNF. Activation of AhR was dose dependent, and the liver was identified as the main responding organ. After exposure to the smoking environment, sensor mice consistently exhibited transient, reversible activation of AhR. RT-PCR analysis of SEAP revealed that activation of AhR occurred predominantly in the lung.

CONCLUSION

We are the first laboratory to demonstrate successfully direct, comprehensive monitoring of air pollution using genetically engineered mammals. The established system would be useful for real risk assessment of halogenated and polycyclic aromatic hydrocarbons in the air, especially in smoking environments.

The human CYP1A1 gene is regulated in a developmental and tissue-specific fashion in transgenic mice

Regulation and expression of human CYP1A1 is demonstrated in transgenic mice. We have developed two transgenic mouse lines. One mouse strain (CYPLucR) carries a functional human CYP1A1 promoter (-1612 to +293)-luciferase reporter gene, and the other strain (CYP1A1N) expresses CYP1A1 under control of the full-length human CYP1A1 gene and 9 kb of flanking regulatory DNA. With CYPLucR(+/-) mice, 2,3,7,8-tetrachlordibenzo-p-dioxin (TCDD) and several other aryl hydrocarbon receptor ligands induced hepatocyte-specific luciferase activity. When other tissues were examined, TCDD induced luciferase activity in brain with limited induction in lung and no detectable luciferase activity in kidney. Treatment of CYP1A1N(+/-) mice with TCDD resulted in induction of human CYP1A1 in liver and lung, while mouse Cyp1a1 was induced in liver, lung, and kidney. Although induced CYP1A1/Cyp1a1 could not be detected by Western blot analysis in brains from CYP1A1N(+/-) mice, induction in brain was verified by detection of CYP1A1/Cyp1a1 RNA. The administration of TCDD to nursing mothers to examine the effect of lactational exposure via milk demonstrated prominent induction of luciferase activity in livers of CYPLucR(+/-) newborn pups with limited induction in brain. However, TCDD treatment of adult CYPLucR(+/-) mice led to a 7-10-fold induction of brain luciferase activity. Combined these results indicate that tissue-specific and developmental factors are controlling aryl hydrocarbon receptor-mediated induction of human CYP1A1.

Hazard identification by methods of animal-based toxicology

This paper is one of several prepared under the project "Food Safety In Europe: Risk Assessment of Chemicals in Food and Diet" (FOSIE), a European Commission Concerted Action Programme, organised by the International Life Sciences Institute, Europe (ILSI). The aim of the FOSIE project is to review the current state of the science of risk assessment of chemicals in food and diet, by consideration of the four stages of risk assessment, that is, hazard identification, hazard characterisation, exposure assessment and risk characterisation. The contribution of animal-based methods in toxicology to hazard identification of chemicals in food and diet is discussed. The importance of first applying existing technical and chemical knowledge to the design of safety testing programs for food chemicals is emphasised. There is consideration of the presently available and commonly used toxicity testing approaches and methodologies, including acute and repeated dose toxicity, reproductive and developmental toxicity, neurotoxicity, genotoxicity, carcinogenicity, immunotoxicity and food allergy. They are considered from the perspective of whether they are appropriate for assessing food chemicals and whether they are adequate to detect currently known or anticipated hazards from food. Gaps in knowledge and future research needs are identified; research on these could lead to improvements in the methods of hazard identification for food chemicals. The potential impact of some emerging techniques and toxicological issues on hazard identification for food chemicals, such as new measurement techniques, the use of transgenic animals, assessment of hormone balance and the possibilities for conducting studies in which common human diseases have been modelled, is also considered.

Xenobiotic response in humanized double transgenic mice expressing tetracycline-controlled transactivator and human CYP1B1

The cytochrome P450 enzymes (P450s or CYPs) are a superfamily of hemeproteins that catalyze the monooxygenation of a wide range of endobiotic and xenobiotic substrates. A typical strategy in toxicological research and testing involves applying a toxicant at high doses for a short period to homogeneous animals under controlled conditions. However, the conditions of this approach have very little in common with actual human exposure. Transgenic (Tg) mice carrying human genes encoding a drug-metabolizing enzyme (CYP) offer a solution to many of the difficulties in the evaluation of chemical toxicity. It has been demonstrated that the expression of human CYP transgenes under the control of mammalian-inducible promoters exhibits relatively poor fold increases after induction. In this study, we used the tetracycline-regulated (tet) promoter system to increase the expression of the human CYP1B1 (hCYP1B1) gene in the tissues of transgenic mice. By mating two lineages of transgenic mice, double transgenic (dTg) mice expressing both tTA and hCYP1B1 genes under the control of the tet promoter were successfully produced, into which the two transgenes were introduced in an embryo. The expression pattern of tTA-driven hCYP1B1 transgene featured a fold induction of more than 3 to 12 in the brain, heart, and lung and 2- to 4-fold induction in the liver, kidney, and intestine upon doxycycline removal. Immunohistochemical staining with hCYP1B1 antibody was also increased by the removal of doxycycline. In addition, the activities of CYP liver microsomes in the dTg mice without doxycycline showed an increase compared to that in the dTg mice treated with doxycycline. The level of activities correspond to the levels of human CYP1B1 protein expression in the Tg mice (-dox) that was increased by 2-fold induction as compared to that of the dTg mice with doxycycline. Thus, overproduction in Tg can be purified and the activity of purified human CYP1B1 can be characterized by alterations to the coding sequence in order to solve the physiological function of this enzyme in a humanized in vivo system. It is also possible to examine the activity of purified human CYP1B1 using several environmental toxicants such as procarcinogens.

Mutagenicity of TCDD in Big Blue transgenic rats

The compound 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a significant environmental contaminant resulting from such industrial processes as pulp and paper production. TCDD is a suspected human carcinogen and its ability to induce cancer in laboratory rodents is well documented. Its mechanism of tumor initiation, however, is not well understood and in vitro mutagenicity studies have yielded inconsistent results. In this study, Big Blue lacI transgenic rats were used to assess the mutagenicity of TCDD in both male and female animals. After 6 weeks of exposure to 2 microg/kg TCDD neither an increase in mutation frequency nor any change in mutation spectrum was observed in either male or female animals.

Antisense transgenics in animals

Antisense transgenesis provides a methodology for ablating gene expression in targeted tissues through the use of tissue-specific or controllable promoters. The two major features to be considered in the design of a construct for injection are (1) the target sequence and (2) the promoter to be used. Information is provided to help the investigator make decisions in these regards. The standard methodology of making transgenics is not replicated but some hints as how best to use a transgenic facility are provided. An overview of methods for studying transgene expression and target gene suppression is given. In summary, antisense transgenesis may provide a more readily achievable method for tissue-specific ablation of a gene's function than controllable knockouts provide.

Intermittent, Repetitive Corticosteroid-Induced Upregulation of Platelet Levels After Adenovirus-Mediated Transfer to the Liver of a Chimeric Glucocorticoid-Responsive Promoter Controlling the Thrombopoietin cDNA

For many in vivo gene therapy clinical applications, it is desirable to control the expression of the transferred transgene using pharmacologic agents. To evaluate the feasibility of accomplishing this using corticosteroids, pharmacologic agents widely used in clinical medicine, we constructed replication deficient adenoviral (Ad) vectors containing an expression cassette with a chimeric promoter comprised of five glucocorticoid response elements (GRE) and the chloramphenicol acetyltransferase reporter gene (AdGRE.CAT) or the murine thrombopoietin cDNA (AdGRE.mTPO). In vitro studies showed the vectors functioned as expected, with marked glucocorticoid-induced upregulation of the CAT or mTPO transgenes. To evaluate the inducibility of the GRE promoter in vivo, the AdGRE.CAT vector was administered intravenously to C57B1/6 mice, and CAT activity was quantified in liver before and after intraperitoneal administration of dexamethasone. The GRE promoter activity was dependent on the dexamethasone dose, with a 100-fold increase in CAT expression with 50 μg dexamethasone, similar to the levels observed in vivo with the Rous sarcoma virus long terminal repeat constitutive promoter. After dexamethasone administration, maximum CAT activity was observed at day 2, with a slow decline to baseline levels by 2 weeks. Based on these observations, we hypothesized that a single administration of an Ad vector-mediated transfer of the chimeric GRE inducible promoter driving the mTPO cDNA would enable repetitive administration of corticosteroids to repetitively upregulate platelet levels for 1 to 2 weeks. The data show that this occurs, with dexamethasone administration every 3 weeks associated with 1-week elevations (at each 3-week interval) of serum mTPO levels, megakaryocyte numbers in bone marrow, and platelet levels fourfold to sixfold over baseline. Thus, with the appropriate promoter, it is possible to use a commonly used pharmacologic agent to upregulate the expression of a newly transferred gene on demand.

© 1998 by The American Society of Hematology.

Intermittent, Repetitive Corticosteroid-Induced Upregulation of Platelet Levels After Adenovirus-Mediated Transfer to the Liver of a Chimeric Glucocorticoid-Responsive Promoter Controlling the Thrombopoietin cDNA

For many in vivo gene therapy clinical applications, it is desirable to control the expression of the transferred transgene using pharmacologic agents. To evaluate the feasibility of accomplishing this using corticosteroids, pharmacologic agents widely used in clinical medicine, we constructed replication deficient adenoviral (Ad) vectors containing an expression cassette with a chimeric promoter comprised of five glucocorticoid response elements (GRE) and the chloramphenicol acetyltransferase reporter gene (AdGRE.CAT) or the murine thrombopoietin cDNA (AdGRE.mTPO). In vitro studies showed the vectors functioned as expected, with marked glucocorticoid-induced upregulation of the CAT or mTPO transgenes. To evaluate the inducibility of the GRE promoter in vivo, the AdGRE.CAT vector was administered intravenously to C57B1/6 mice, and CAT activity was quantified in liver before and after intraperitoneal administration of dexamethasone. The GRE promoter activity was dependent on the dexamethasone dose, with a 100-fold increase in CAT expression with 50 μg dexamethasone, similar to the levels observed in vivo with the Rous sarcoma virus long terminal repeat constitutive promoter. After dexamethasone administration, maximum CAT activity was observed at day 2, with a slow decline to baseline levels by 2 weeks. Based on these observations, we hypothesized that a single administration of an Ad vector-mediated transfer of the chimeric GRE inducible promoter driving the mTPO cDNA would enable repetitive administration of corticosteroids to repetitively upregulate platelet levels for 1 to 2 weeks. The data show that this occurs, with dexamethasone administration every 3 weeks associated with 1-week elevations (at each 3-week interval) of serum mTPO levels, megakaryocyte numbers in bone marrow, and platelet levels fourfold to sixfold over baseline. Thus, with the appropriate promoter, it is possible to use a commonly used pharmacologic agent to upregulate the expression of a newly transferred gene on demand.

© 1998 by The American Society of Hematology.

Use of transgenic animals in understanding molecular mechanisms of toxicity

Understanding molecular mechanisms of chemical toxicity and the potential risks of drugs to man is a pivotal part of the drug development process. With the dramatic increase in the number of new chemical entities arising from high throughput screening, there is an urgent need to develop systems for the rapid evaluation of potential drugs so that those agents which are most likely to be free of adverse effects can be identified at the earliest possible stage in drug development. The complex mechanisms of action of chemical toxins has made it extremely difficult to evaluate the precise toxic mechanism and also the relative role of specific genes in either potentiating or ameliorating the toxic effect. This problem can be addressed by the application of genetic strategies. Such strategies can exploit strain differences in susceptibility to specific toxic agents or, with the rapidly developing technologies, can exploit the use of transgenic animals where specific genes can be manipulated and subsequent effects on chemical toxicity evaluated. Transgenic animals can be exploited in a variety of ways to understand mechanisms of chemical toxicity. For example, a human gene encoding a drug metabolizing enzyme can be directly introduced and the effects on toxic response evaluated. Alternatively, specific genes can be deleted from the mouse genome and the consequences on toxicological response determined. Many toxic chemical agents modulate patterns of gene expression within target cells. This can be used to screen for responses to different types of toxic insult. In such experiments the promotor of a stress-regulated gene can be ligated to a suitable reporter gene, such as lacZ, or green fluorescent protein, and inserted into the genome of an appropriate test species. On administration of a chemical agent, cells which are sensitive to the toxic effects of that chemical will express the reporter, which can then be identified using an appropriate assay system. This latter strategy provides the potential for screening a large number of compounds rapidly for their potential toxic effects and also provides information on tissue and cellular specificity. Experiments using transgenic animals can be complex, and care must be taken to ensure that the results are not affected by background activities within the species being used. For example, the introduction of a specific human cytochrome P450 gene may have no effect on the metabolic disposition of a drug or toxin because of the background activity within the mouse. As the toxicity of a chemical agent is determined by a wide range of different factors including drug uptake, metabolism, detoxification and repair, differences between man and the species being used could potentially generate a toxic response in the animal model whereas no toxicity may be observed in man. In spite of these confounding factors, the application of transgenic animals to toxicological issues has enormous potential for speeding up the drug discovery process and will undoubtedly become part of this process in the future.

Regulation of the CYP1A1 promoter in transgenic mice: an exquisitely sensitive on-off system for cell specific gene regulation

Mammalian cytochrome P-450s in the CYP1A gene family catalyse the oxidation of a wide range of drugs and foreign compounds resulting in their excretion. These enzymes are highly inducible by a range of compounds, including polycyclic aromatic hydrocarbons such as 3-methylcholanthrene (3-MC) and dioxins. Analysis of the CYP1A1 promoter has identified dioxin responsive enhancer elements which mediate the induction response. In order to evaluate this promoter as an in vivo regulatable expression system and to gain further insights into the tissue specific regulation of this gene, an 8.5 kb genomic fragment of the rat CYP1A1 promoter was cloned upstream of the lacZ reporter gene. This construct was used to generate transgenic mice and three independent lines were expanded for further study. The regulation of beta-galactosidase expression was determined in mock and 3-MC-treated mice in an extensive range of tissues. In untreated animals no transgene expression was detectable over non-transgenic controls. Treatment with 3-MC caused a profound increase in transgene expression (> 1,000-fold) in many tissues including liver, adrenal, kidney and intestine. Inducible transgene expression was also detectable in many of the other tissues including the spleen, lung, pancreas and the reproductive organs. Although the absolute levels of induction varied, no significant differences in the pattern of transgene expression were observed between the three different transgenic mouse lines. In addition, the pattern of transgene expression correlated closely with the reported regulation of CYP1A1 protein. These results indicate that the CYP1A1 promoter can drive expression of heterologous genes in a truly on/off manner in a variety of tissues and cell types which will allow the expression of other proteins to be controlled in vivo. This reporter system also provides a model for establishing the environmental and hormonal factors regulating the expression of the CYP1A1 gene.

Regulatable promoters for use in gene therapy applications: modification of the 5'-flanking region of the CFTR gene with multiple cAMP response elements to support basal, low-level gene expression that can be upregulated by exogenous agents that raise intracellular levels of cAMP

This study focuses on the design, construction, and evaluation of a chimeric promoter for gene therapy applications where it is desirable to have low-level basal expression of the newly transferred gene, which can be induced to higher levels of expression by the administration of pharmacologic agents that can be safely used locally and/or systemically in humans. To achieve this, a chimeric promoter was constructed using fragments of the 5'-flanking region of the human cystic fibrosis transmembrane conductance regulator (CFTR) gene, and multiple tandem repeats of the consensus sequence and flanking elements of the cAMP response element (CRE), promoter sequences that support increased transcription in response to elevations in intracellular cAMP levels. Preliminary studies using plasmid vectors demonstrated that: (i) the 5'-flanking sequences from the CFTR gene have low promoter activity in the human airway epithelial cell lines; (ii) chimeras using -718 bp fragment from the 5'-flanking sequence of CFTR gene as the base, with the addition of 4-10 units of a 25-bp sequence containing the CRE consensus sequence, were all inducible by a rise in intracellular cAMP, with the chimera having eight CRE repeats the most responsive; and (iii) a CF126(CRE8) chjimera, consisting of the -126 bp fragment from the 5'-flanking region of CFTR gene together with eight CRE repeats, yielded low-level basal activity but maximal upregulation by cAMP, resulting in expression of the reporter gene that was 51-58% of an RSV-LTR control. On the basis of these observations, replication-deficient adenoviral vectors containing the CF126(CRE8) chimera and the luciferase reporter gene [AdCF126(CRE8).Luc] or the Escherichia coli lacZ (beta-Gal) reporter gene [AdCF126(CRE8). beta gal] were constructed. In several human airway epithelial cell lines, the AdCF126 (CRE). Luc vector provided low basal activity, but was significantly upregulated by agents that increase cAMP levels. Intranasal administration of the beta-Gal-expressing AdCF126(CRE8) beta gal vector into C57B1/6 mice demonstrated cAMP-induced upregulation of the reporter gene in airway epithelial cells. Quantification of the inducibility of the basal promoter activity in the airway epithelium using the AdCF126(CRE8). Luc vector demonstrated an 11-fold upregulation of the basal promoter activity in the lung with the administration of a phosphodiesterase inhibitor and a cAMP analog. These observations demonstrate the feasibility of using a chimeric promoter comprised of a minimal fragment of the CFTR 5'-flanking region, together with added multiple CRE, to control genes delivered in vivo. Importantly, because there are many drugs used in humans that raise cAMP, the concept of using a cAMP-regulatable promoter may also be a useful approach to enhance the safety, efficacy, and feasibility of a variety of human gene therapy strategies.

Creation of a reversible on/off system for site-specific in vivo control of exogenous gene activity in the renal glomerulus

Using genetically engineered glomerular mesangial cells, an in vivo gene transfer approach was developed that specifically targets the renal glomerulus. By combining this system with a tetracycline (Tc)-responsive promoter, the present study aimed to create a reversible on/off system for site-specific in vivo control of exogenous gene activity within the glomerulus. In the Tc regulatory system, a Tc-controlled transactivator (tTA) encoded by a regulator plasmid induces target gene transcription by binding to a tTA-responsive promoter located in a response plasmid. Tc inhibits this tTA-dependent transactivation via its affinity for tTA. In double-transfected cells, therefore, the activity of a transgene can be controlled by Tc. Cultured rat mesangial cells were cotransfected with a regulator plasmid and a response plasmid that introduces a beta-galactosidase gene. In vitro, stable double-transfectant MtTAG cells exhibited no beta-galactosidase activity in the presence of Tc. However, following withdrawal of Tc from culture media, expression of beta-galactosidase was induced within 24 h. When Tc was again added, the expression was rapidly resuppressed. Low concentrations of Tc were sufficient to maintain the silent state of tTA-dependent promoter. MtTAG cells were then transferred into the rat glomeruli via renal artery injection. In the isolated chimeric glomeruli, expression of beta-galactosidase was induced ex vivo in the absence of Tc, whereas it was repressed in its presence. When Tc-pretreated MtTAG cells were transferred into the glomeruli of untreated rats, beta-galactosidase expression was induced in vivo within 3 days. Oral administration of Tc dramatically suppressed this induction. These data demonstrate the feasibility of using mesangial cell vectors combined with the Tc regulatory system for site-specific in vivo control of exogenous gene expression in the glomerulus.

Cytochrome P450 1A1 promoter as a genetic switch for the regulatable and physiological expression of a plasma protein in transgenic mice

Transgenic and gene knockout techniques allow for in vivo study of the consequences of adding or subtracting specific genes. However, in some instances, such as the study of lethal mutations or of the physiological consequences of changing gene expression, turning on and off an introduced gene at will would be advantageous. We have used cytochrome p450 1A1 promoter to drive expression of the human apolipoprotein E (apoE) gene in transgenic mice. In six independent lines, robust expression of the transgene depended upon injection of the inducer beta-naphthoflavone, whereas the seventh line had high basal expression that was augmented further by the inducer. The low level of basal expression in an inducer-dependent line was confirmed upon breeding the transgene onto the hypercholesterolemic apoE-deficient background. In the basal state transgene expression was physiologically insignificant, as these mice were as hypercholesterolemic as their nontransgenic apoE-deficient littermates. When injected with the inducer, plasma cholesterol levels of the transgenic mice decreased dramatically as apoE expression was induced to yield greater than physiological levels in plasma. The inducer could pass transplacentally from an injected mother to her fetuses with concomitant induction of fetal transgene mRNA. Inducer could also pass via breast milk from an injected mother to her suckling neonatal pups, giving rise to the induction of human apoE in neonate plasma. These finding suggest a strategy to temporarily ameliorate genetic deficiencies that would otherwise lead to fetal or neonatal lethality.

A modified tetracycline-regulated system provides autoregulatory, inducible gene expression in cultured cells and transgenic mice

A system for tetracycline-regulated inducible gene expression was described recently which relies on constitutive expression of a tetracycline-controlled transactivator (tTA) fusion protein combining the tetracycline repressor and the transcriptional activation domain of VP16 [Gossen, M. & Bujard, H. (1992) Proc. Natl. Acad. Sci. USA 89, 5547-5551]. This system yielded only low levels of transactivator protein, probably because tTA is toxic. To avoid this difficulty, we placed the tTA gene under the control of the inducible promoter to which tTA binds, making expression of tTA itself inducible and autoregulatory. When used to drive expression of the recombination activating genes 1 and 2 (RAG-1 and RAG-2), the autoregulatory system yielded both substantially higher levels of variable (diversity) joining [V(D)J] recombination activity (70-fold on average) and inducible expression in a much larger fraction of transfected cells (autoregulatory, 90%, vs. constitutive, 18%). In addition, this system allowed the creation of transgenic mice in which expression of a luciferase transgene was inducible tens to hundreds of times the basal levels in most tissues examined. Induced levels of expression were highest in thymus and lung and appear to be substantially higher than in previously reported inducible luciferase transgenic mice created with the constitutive system. With the modified system, inducible transactivator mRNA and protein were easily detected in cell lines by RNA and Western blotting, and transactivator mRNA was detected by RNA blotting in some tissues of transgenic mice. This autoregulatory system represents an improved strategy for tetracycline-regulated gene expression both in cultured cells and in transgenic animals.

Antisense and differentiation

The use of antisense technology in animals has great potential for studying individual genes and for generating animal models of human disorders. Further control can be obtained by the use of inducible promoters to regulate the expression of antisense constructs, so that the timing and degree of antisense inhibition can be manipulated. In addition, tissue-specific promoters and enhancers provide the potential for targeting antisense inhibition to specific organs. Transgene expression in mice directed by the Cyp1a-1 promoter and enhancer elements can be increased in the liver up to 10,000-fold by administration to the animals of the inducer, 3-methylcholanthrene. Transfected antisense constructs containing splice site regions of the hypoxanthine guanine phosphoribosyltransferase (HPRT) gene can reduce HPRT activity to less than 1% of levels in parental NIH-3T3, COS, or HeLa cells. Antisense constructs including splice site regions and driven by the inducible Cyp1a-1 promoter should prove to be powerful tools in generating mouse models of human disorders.

Regulating gene expression in transgenic animals

Regardless of the field of application, the raison d'etre of transgenic animals is to study gene regulation and function. With increasing frequency, mammalian genes are being isolated with no concomitant knowledge of their function. The human genome mapping initiative will undoubtedly produce a cornucopia of such genes. While the merit of taking a transgenic route to study genes of unknown function is axiomatic, the choices of strategies for gene regulation in vivo may not be fully appreciated. This review will address two main points: first, the targeted and regulated expression of genes, and second, the structural and functional ablation of genes.