Identification of 2,3,7,8-tetrachlorodibenzo-p-dioxin-responsive genes in mouse liver by serial analysis of gene expression

Sexual Dimorphism in Adipose-Hypothalamic Crosstalk and the Contribution of Aryl Hydrocarbon Receptor to Regulate Energy Homeostasis

There are fundamental sex differences in the regulation of energy homeostasis. Better understanding of the underlying mechanisms of energy balance that account for this asymmetry will assist in developing sex-specific therapies for sexually dimorphic diseases such as obesity. Multiple organs, including the hypothalamus and adipose tissue, play vital roles in the regulation of energy homeostasis, which are regulated differently in males and females. Various neuronal populations, particularly within the hypothalamus, such as arcuate nucleus (ARC), can sense nutrient content of the body by the help of peripheral hormones such leptin, derived from adipocytes, to regulate energy homeostasis. This review summarizes how adipose tissue crosstalk with homeostatic network control systems in the brain, which includes energy regulatory regions and the hypothalamic–pituitary axis, contribute to energy regulation in a sex-specific manner. Moreover, development of obesity is contingent upon diet and environmental factors. Substances from diet and environmental contaminants can exert insidious effects on energy metabolism, acting peripherally through the aryl hydrocarbon receptor (AhR). Developmental AhR activation can impart permanent alterations of neuronal development that can manifest a number of sex-specific physiological changes, which sometimes become evident only in adulthood. AhR is currently being investigated as a potential target for treating obesity. The consensus is that impaired function of the receptor protects from obesity in mice. AhR also modulates sex steroid receptors, and hence, one of the objectives of this review is to explain why investigating sex differences while examining this receptor is crucial. Overall, this review summarizes sex differences in the regulation of energy homeostasis imparted by the adipose–hypothalamic axis and examines how this axis can be affected by xenobiotics that signal through AhR.

Agent Orange and dioxin-induced myeloid leukemia: a weaponized vehicle of leukemogenesis

Agent Orange (AO) was the dominant weaponized herbicide employed by the United States (US) military during the Vietnam war. AO, however, was found to be regularly contaminated by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the most toxic dioxin known; furthermore, AO was commonly diluted in the field with other aromatic hydrocarbons to assist with delivery mechanisms. Unbeknownst to the US military and the millions exposed, these events have likely contributed to the development of acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) that has affected many veterans. Null studies regarding an association between AO exposure and AML/MDS are limited in their methodology and application. The acknowledgement that the known carcinogen TCDD was a contaminant in AO when paired with a strong biological plausibility for its leukemogenicity and an observed increased risk of AML/MDS in TCDD-exposed individuals should suffice to establish causal association and that veterans to whom this might apply should be awarded appropriate indemnity.

The Aryl Hydrocarbon Receptor in Energy Balance: The Road from Dioxin-Induced Wasting Syndrome to Combating Obesity with Ahr Ligands

The aryl hydrocarbon receptor (AHR) has been studied for over 40 years, yet our understanding of this ligand-activated transcription factor remains incomplete. Each year, novel findings continually force us to rethink the role of the AHR in mammalian biology. The AHR has historically been studied within the context of potent activation via AHR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), with a focus on how the AHR mediates TCDD toxicity. Research has subsequently revealed that the AHR is actively involved in distinct physiological processes ranging from the development of the liver and reproductive organs, to immune system function and wound healing. More recently, the AHR was implicated in the regulation of energy metabolism and is currently being investigated as a potential therapeutic target for obesity. In this review, we re-trace the steps through which the early toxicological studies of TCDD led to the conceptual framework for the AHR as a potential therapeutic target in metabolic disease. We additionally discuss the key discoveries that have been made concerning the role of the AHR in energy metabolism, as well as the current and future directions of the field.

Mixture toxicity effects and uptake of titanium dioxide (TiO2) nanoparticles and 3,3',4,4'-tetrachlorobiphenyl (PCB77) in juvenile brown trout following co-exposure via the diet

Titanium dioxide nanoparticles (n-TiO₂) are among the man-made nanomaterials that are predicted to be found at high concentrations in the aquatic environment. There, they likely co-exist with other chemical pollutants. Thus, n-TiO₂ and other chemical pollutants can be taken up together or accumulate independently from each other in prey organisms of fish. This can lead to dietary exposure of fish to n-TiO₂-chemical pollutant mixtures. In this study, we examine if simultaneous dietary exposure to n-TiO₂ and 3,3',4,4'-Tetrachlorobiphenyl (PCB77) -used as a model compound for persistent organic pollutants with dioxin-like properties- can influence the uptake and toxicological response elicited by the respective other substance. Juvenile brown trout (Salmo trutta) were fed custom-made food pellets containing n-TiO_2, PCB77 or n-TiO₂+PCB77 mixtures for 15 days. Ti and PCB77 concentrations in the liver were measured by ICP-MS and GC-MS, respectively. Besides, n-TiO₂ uptake was assessed using TEM. Combination effects on endpoints specific for PCB77 (i.e., cytochrome P450 1A (CYP1A) induction) and endpoints shared by both PCB77 and n-TiO₂ (i.e., oxidative stress-related parameters) were measured in intestine and liver using RT-qPCR and enzyme activity assays. The results show that genes encoding for proteins/enzymes essential for tight junction function (zo-1) and ROS elimination (sod-1) were significantly upregulated in the intestine of fish exposed to n-TiO₂ and PCB77 mixtures, but not in the single-substance treatments. Besides, n-TiO₂ had a potentiating effect on PCB77-induced CYP1A and glutathione reductase (GR) expression/enzyme activity in the liver. This study shows that simultaneous dietary exposure to nanomaterials and traditional environmental pollutants might result in effects that are larger than observed for the substances alone, but that understanding the mechanistic basis of such effects remains challenging.

From Classical Toxicology to Tox21: Some Critical Conceptual and Technological Advances in the Molecular Understanding of the Toxic Response Beginning From the Last Quarter of the 20th Century

Toxicology has made steady advances over the last 60+ years in understanding the mechanisms of toxicity at an increasingly finer level of cellular organization. Traditionally, toxicological studies have used animal models. However, the general adoption of the principles of 3R (Replace, Reduce, Refine) provided the impetus for the development of in vitro models in toxicity testing. The present commentary is an attempt to briefly discuss the transformation in toxicology that began around 1980. Many genes important in cellular protection and metabolism of toxicants were cloned and characterized in the 80s, and gene expression studies became feasible, too. The development of transgenic and knockout mice provided valuable animal models to investigate the role of specific genes in producing toxic effects of chemicals or protecting the organism from the toxic effects of chemicals. Further developments in toxicology came from the incorporation of the tools of "omics" (genomics, proteomics, metabolomics, interactomics), epigenetics, systems biology, computational biology, and in vitro biology. Collectively, the advances in toxicology made during the last 30-40 years are expected to provide more innovative and efficient approaches to risk assessment. A goal of experimental toxicology going forward is to reduce animal use and yet be able to conduct appropriate risk assessments and make sound regulatory decisions using alternative methods of toxicity testing. In that respect, Tox21 has provided a big picture framework for the future. Currently, regulatory decisions involving drugs, biologics, food additives, and similar compounds still utilize data from animal testing and human clinical trials. In contrast, the prioritization of environmental chemicals for further study can be made using in vitro screening and computational tools.

The aryl hydrocarbon receptor and glucocorticoid receptor interact to activate human metallothionein 2A

Although the aryl hydrocarbon receptor (AHR) and glucocorticoid receptor (GR) play essential roles in mammalian development, stress responses, and other physiological events, crosstalk between these receptors has been the subject of much debate. Metallothioneins are classic glucocorticoid-inducible genes that were reported to increase upon treatment with AHR agonists in rodent tissues and cultured human cells. In this study, the mechanism of human metallothionein 2A (MT2A) gene transcription activation by AHR was investigated. Cotreatment with 3-methylcholanthrene and dexamethasone, agonists of AHR and GR respectively, synergistically increased MT2A mRNA levels in HepG2 cells. MT2A induction was suppressed by RNA interference against AHR or GR. Coimmunoprecipitation experiments revealed a physical interaction between AHR and GR proteins. Moreover, chromatin immunoprecipitation assays indicated that AHR was recruited to the glucocorticoid response element in the MT2A promoter. Thus, we provide a novel mechanism whereby AHR modulates expression of human MT2A via the glucocorticoid response element and protein-protein interactions with GR.

Carbon tetrachloride affects inflammation-related biochemical networks in the mouse liver as identified by a customized cDNA microarray system

OBJECTIVES

We have attempted to upgrade and validate an in-house cDNA microarray system developed by our group for the evaluation of chemical toxicity.

METHODS

To establish an in-house microarray, we selected genes that play pivotal roles in detoxifying exogenous substances and maintaining homeostasis in the liver. To validate the system, we examined gene expression profiles in mouse liver following treatment with different doses of carbon tetrachloride (CCl(4)). The data were also analyzed by pathway analysis tools.

RESULTS

We upgraded our array system by collecting genes that are responsive to xenobiotic receptors, apoptosis-related genes, and stress-responsive genes. The acute toxicity of CCl(4) was confirmed by elevated levels of serum transaminase and histopathological findings. The microarray data showed the CCl(4) treatment induced significant changes in gene expression in the mouse liver, and the ingenuity pathways analysis revealed alterations in gene expression in inflammation-related networks.

CONCLUSIONS

We have established a focused microarray system that may be useful for use in toxicogenomics studies. Using this array system, we gained insight into the mechanisms by which CCl(4) exerts its toxic effects. The results of our study also indicate that the combination of focused arrays and bioinformatics tools is helpful in the mechanistic analysis of chemical toxicity.

Hepcidin gene expression induced in the developmental stages of fish upon exposure to Benzo[a]pyrene (BaP)

Hepcidin is known to be expressed in fish with bacterial challenge and iron overload. Here we first report the hepcidin expression induced in the developmental stages from embryo to fry of red sea bream (Pagarus major) and in juvenile black porgy (Acanthopagrus schlegelii B.) upon continuous waterborne exposure to BaP. The gene expression of CYP1A1 and IgL (immunoglobulin light chain) were both measured. Expression of the Pagarus major hepcidin gene (PM-hepc) was increased in post hatch fry at 24 h and 120 h exposure to BaP at concentrations of 0.1, 0.5 and 1.0 microg/l, respectively. The gene expression pattern was comparable to that of CYP1A1 but different from that of IgL. In addition, a high number of AS-hepc2 transcripts (Acanthopagrus schlegelii B. hepcidin gene) were detected in the liver upon exposure to 1.0 microg/l BaP. This study demonstrates that hepcidin gene expression is significantly induced in BaP-exposed red sea bream and black porgy.

Molecular Mechanisms of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin Cardiovascular Embryotoxicity

2,3,7,8 Tetrachlorodibenzo-p-dioxin (TCDD) and related planar halogenated aromatic hydrocarbons are widespread environmental contaminants and potent developmental toxicants. Hallmarks of embryonic exposure include edema, hemorrhage, and mortality. Recent studies in zebrafish and chicken have revealed direct impairment of cardiac muscle growth that may underlie these overt symptoms. TCDD toxicity is mediated by the aryl hydrocarbon receptor, but downstream targets remain unclear. Oxidative stress and growth factor modulation have been implicated in TCDD cardiovascular toxicity. Gene expression profiling is elucidating additional pathways by which TCDD might act. We review our understanding of the mechanism of TCDD embryotoxicity at morphological and molecular levels.

Transcriptomic responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in liver: comparison of rat and mouse

BACKGROUND

Mouse and rat models are mainstays in pharmacology, toxicology and drug development -- but differences between strains and between species complicate data interpretation and application to human health. Dioxin-like polyhalogenated aromatic hydrocarbons represent a major class of environmentally and economically relevant toxicants. In mammals dioxin exposure leads to a broad spectrum of adverse affects, including hepatotoxicity of varying severity. Several studies have shown that dioxins extensively alter hepatic mRNA levels. Surprisingly, though, analysis of a limited portion of the transcriptome revealed that rat and mouse responses diverge greatly (Boverhof et al. Toxicol Sci 94:398-416, 2006).

RESULTS

We employed oligonucleotide arrays to compare the response of 8,125 rat and mouse orthologs. We confirmed that there is limited inter-species overlap in dioxin-responsive genes. Rat-specific and mouse-specific genes are enriched for specific functional groups which differ between species, conceivably accounting for species-specificities in liver histopathology. While no evidence for the involvement of copy-number variation was found, extensive inter-species variation in the transcriptional-regulatory network was identified; Nr2f1 and Fos emerged as candidates to explain species-specific and species-independent responses, respectively.

CONCLUSION

Our results suggest that a small core of genes is responsible for mediating the similar features of dioxin hepatotoxicity in rats and mice but non-overlapping pathways are simultaneously at play to result in distinctive histopathological outcomes. The extreme divergence between mouse and rat transcriptomic responses appears to reflect divergent transcriptional-regulatory networks. Taken together, these data suggest that both rat and mouse models should be used to screen the acute hepatotoxic effects of drugs and toxic compounds.

Brain transcriptomics in response to beta-naphthoflavone treatment in rainbow trout: the role of aryl hydrocarbon receptor signaling

Polychlorinated biphenyls (PCBs) exposure disrupts steroid production in teleostean fishes. While this suppression of plasma steroid levels is thought to involve aryl hydrocarbon receptor (AhR) signaling, the target tissues impacted and the molecular mechanisms involved have rarely been addressed. We tested the hypothesis that AhR activation downregulates genes involved in neuroendocrine function, including the control of brain-pituitary-interrenal (BPI) and -gonadal (BPG) axes in rainbow trout. To elucidate receptor-specific signaling, we utilized a pharmacological approach using beta-naphthoflavone (BNF) and resveratrol (RVT) as AhR agonist and antagonist, respectively. The gene expression pattern in the brain was analysed using a low-density targeted trout cDNA array enriched with genes encoding proteins involved in endocrine signaling, stress response and metabolic adjustments. Upregulation of AhR and CYP1A1 gene expression with BNF and the inhibition of this response by RVT confirmed AhR-dependent signaling. RVT by itself impacted only a few genes, while BNF treatment significantly modulated the transcript level of 49 genes, many of which are involved in the neuroendocrine control of stress and reproduction. Of these, only 27% of the BNF-mediated transcriptional response was blocked by RVT, suggesting molecular regulation of neuroendocrine pathways that are also AhR-independent. Gene expression pattern for select genes seen with the microarray analysis was also confirmed using quantitative real-time PCR. Overall, our results reveal for the first time that BNF disrupts several key genes involved in the neuroendocrine control of stress and sex steroid biosynthesis, while the mode of action involves both AhR-dependent and -independent pathways in trout.

Expression profile of liver genes in response to hepatotoxicants identified using a SAGE-based customized DNA microarray system

Gene expression analysis using customized or focused DNA microarrays is favorable because of a reduction in the cost and time needed for the analysis. To examine the effect of chemicals on the liver, we developed an in-house cDNA microarray system, mouse Liver Stress Array ver. 1.0, containing 355 unique genes involved in drug metabolism, inflammation and liver-related proteins. These genes were selected for sensing the homeostasis of the liver and based on the information of liver transcriptome revealed by serial analysis of gene expression. By using this customized microarray, we analyzed gene expression changes in the mouse liver treated by 11 known hepatotoxicants. Gene expression measurements corresponding to the in vivo response to known hepatotoxicants revealed that profiles of chemicals with similar mechanisms clustered together. For each of the chemicals tested, several genes that were induced or repressed were common in each chemical exposure, whereas other genes were unique for the specific class compound. Ingenuity pathways analysis revealed that significant alterations in gene expression occurred in a number of biological networks by these treatments. Although the genes spotted on our array was limited to a highly focused set for toxicity classification, this work provides proof of concept that patterns of gene regulation assessed by a focused array system are useful to classify unknown chemicals.

Analysis of 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced proteome changes in 5L rat hepatoma cells reveals novel targets of dioxin action including the mitochondrial apoptosis regulator VDAC2

As part of a comprehensive survey of the impact of the environmental pollutant and hepatocarcinogen 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the proteome of hepatic cells, we have performed a high resolution two-dimensional gel electrophoresis study on the rat hepatoma cell line 5L. 78 protein species corresponding to 73 different proteins were identified as up- or down-regulated following exposure of the cells to 1 nm TCDD for 8 h. There was an overlap of only nine proteins with those detected as altered by TCDD in our recent study using the non-gel-based isotope-coded protein label method (Sarioglu, H., Brandner, S., Jacobsen, C., Meindl, T., Schmidt, A., Kellermann, J., Lottspeich, F., and Andrae, U. (2006) Quantitative analysis of 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced proteome alterations in 5L rat hepatoma cells using isotope-coded protein labels. Proteomics 6, 2407-2421) indicating a strong complementarity of the two approaches. For the majority of the altered proteins, an effect of TCDD on their abundance or posttranslational modifications had not been known before. Several observations suggest that a sizable fraction of the proteins with altered abundance was induced as an adaptive response to TCDD-induced oxidative stress that was demonstrated using the fluorescent probe dihydrorhodamine 123. A prominent group of these proteins comprised various enzymes for which there is evidence that their expression is regulated via the Keap1/Nrf2/antioxidant response element pathway. Other proteins included several involved in the maintenance of mitochondrial energy production and the regulation of the mitochondrial apoptotic pathway. A particularly intriguing finding was the up-regulation of the mitochondrial outer membrane pore protein, voltage-dependent anion channel-selective protein 2 (VDAC2), which was dependent on the presence of a functional aryl hydrocarbon receptor. The regulatability of VDAC2 protein abundance has not been described previously. In view of the recently discovered central role of VDAC2 as an inhibitor of the activation of the proapoptotic protein BAK and the mitochondrial apoptotic pathway, the present data point to a hitherto unrecognized mechanism by which TCDD may affect cellular homeostasis and survival.

Genomic profiling in nuclear receptor-mediated toxicity

Nuclear receptors (NRs) are attractive drug targets due to their role in regulation of a wide range of physiologic responses. In addition to providing therapeutic value, many pharmaceutical agents along with environmental chemicals are ligands for NRs and can cause adverse health effects that are directly related to activation of NRs. Identifying the molecular events that produce a toxic response may be confounded by the fact that there is a significant overlap in the biological processes that NRs regulate. Microarrays and other methods for gene expression profiling have served as useful, sensitive tools for discerning the mechanisms by which therapeutics and environmental chemicals invoke toxic effects. The capability to probe thousands of genes simultaneously has made genomics a prime technology for identifying drug targets, biomarkers of exposure/toxicity and key players in the mechanisms of disease. The complex intertwining networks regulated by NRs are hard to probe comprehensively without global approaches and genomics has become a key technology that facilitates our understanding of NR-dependent and -independent events. The future of drug discovery, design and optimization, and risk assessment of chemical toxicants that activate NRs will inevitably involve genomic profiling. This review will focus on genomics studies related to PPAR, CAR, PXR, RXR, LXR, FXR, and AHR.

Identification of potential biomarkers for diazinon exposure to Japanese Medaka (Oryzias latipes) using annealing control primers

A new differential display-polymerase chain reaction (PCR) method based on annealing control primers was used to screen and identify potential biomarkers from differentially expressed genes (DEGs) in medaka exposed to sub-lethal concentration of diazinon (100 ppb). Among the differentially expressed genes identified, the majority were in functional categories of protein biosynthesis, transport and metabolism according to the gene ontology classification. The differential expression of ribosomal protein genes was quantified by real time PCR. The genes encoding ribosomal proteins including L3 and S17 were selected as potential biomarkers for diazinon exposure in medaka fish.

Quantitative analysis of 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced proteome alterations in 5L rat hepatoma cells using isotope-coded protein labels

In an effort to contribute to a better understanding of the hepatic toxicity of the ubiquitous environmental pollutant and hepatocarcinogen 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a comprehensive quantitative proteome analysis was performed on 5L rat hepatoma cells exposed to 1 nM TCDD for 8 h. Changes in the abundances of individual protein species in TCDD-treated cells as compared to untreated cells were analysed using the nongel-based isotope-coded protein label (ICPL) method [Schmidt, A., Kellermann, J., Lottspeich, F., Proteomics 2005, 5, 4-15]. 89 proteins were identified as up- or down-regulated by TCDD. For the majority of the altered proteins, an impact of TCDD on their abundance had not been known before. Due to the physicochemical properties or the translational regulation of a large number of the affected proteins, their alteration would have escaped detection by gel-based methods for proteome analysis and by standard mRNA expression profiling, respectively. The identified proteins with TCDD-altered abundance include several proteins implicated in cell cycle regulation, growth factor signalling and the control of apoptosis. The results thus provide new starting-points for the investigation of specific aspects of the toxicity and carcinogenicity of dioxin in liver.

Suppression of carbonic anhydrase III mRNA level by an aryl hydrocarbon receptor ligand in primary cultured hepatocytes of rat

The effect of an aryl hydrocarbon receptor (AhR) ligand on the carbonic anhydrase III (CAIII) mRNA level was studied using primary cultured hepatocytes of rats. CAIII gene which is highly suppressible by dioxins in vivo, was also suppressible in primary cultured hepatocytes of rats by an AhR ligand, 3-methylchlanthrene (3MC). The suppression of CAIII by 3MC was observed in a dose-dependent fashion. The suppression was marked at 10 microM MC. It is likely that AhR is involved in the suppression of the CAIII gene. The transcriptional regulation region of rat CAIII gene was cloned by polymerase chain reaction on the basis of the similarity to the mouse and human CAIII genes. A 1.5 kb section upstream of rat CAIII was sequenced and the transcription initiation site of this gene was mapped to 58 bases upstream of the initiation codon. A xenobiotic responsive element (XRE)-like sequence was found at -555 to -549 bp of the transcription initiation site. The location of XRE-like element was conserved between rats and mice those CAIIIs in liver were shown as dioxins-suppressible. Although the roles of the XRE have not been clarified, these results suggest that the AhR ligands could elicit the suppressive effect on hepatic CAIII and the effect on the factors from extrahepatic tissues is not required for the suppression.

Gene Expression profile by 2,3,7,8-Tetrachlorodibenzo-p-Dioxin in the Liver of Wild-Type (AhR+/+) and Aryl Hydrocarbon Receptor-Deficient (AhR-/-) Mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is one of the most toxic environmental pollutants that cause various biological effects on mammals. The purpose of our study was to identify the genes involved in hepatotoxicity and hepatocarcinogenesis caused by TCDD. C57BL/6 (AhR+/+, wild type) and B6.129-AhR<tm1Bra>/J (AhR-/-, knock out) mice were injected i.p. with a single treatment of TCDD at the dose of 100 microg/kg body weight. Relative liver weight was significantly increased at 72 hr after TCDD treatment without an apparent histopathological change in AhR+/+ mice (p<0.05). TCDD treatment also significantly increased activity of serum alanine aminotransferase in AhR-/- mice (p<0.05). The liver was analyzed for gene expression profiles 72 hr later. As compared with AhR-/- mice, the expression of 51 genes (>3-fold) was changed in AhR+/+ mice; 28 genes were induced, while 23 genes were repressed. Most of the genes were associated with chemotaxis, inflammation, carcinogenesis, acute-phase response, immune responses, cell metabolism, cell proliferation, signal transduction, and tumor suppression. This study suggests that the microarray analysis of genes in the liver of AhR+/+ and AhR-/- mice may help to clarify the mechanism of AhR-mediated hepatotoxicity and hepatocarcinogenesis by TCDD.

Transcriptional signatures of immune cells in aryl hydrocarbon receptor (AHR)-proficient and AHR-deficient mice

The ligand-activated aryl hydrocarbon receptor (AHR) is known to modulate many genes in a highly cell-specific manner, either directly or indirectly via secondary effects. In contrast, little is known about the effects of AHR deficiency on gene expression balance. We compared the transcriptome of CD4 T cells from AHR-/- mice and wild-type mice; 390 genes, many of them immunotypic, were deregulated in AHR-deficient CD4 cells. TCDD-induced transcriptome changes correlated with the AHR expression level in immune cells. However, there was little overlap in AHR-dependent transcripts found in T lineage cells or dendritic cells. Our results demonstrate flexible gene accessibility for the AHR in immune cells. The idea of a universal battery of AHR-responsive genes is not tenable.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) alters the mRNA expression of critical genes associated with cholesterol metabolism, bile acid biosynthesis, and bile transport in rat liver: a microarray study

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent hepatotoxin that exerts its toxicity through binding to the aryl hydrocarbon receptor (AhR) and the subsequent induction or repression of gene transcription. In order to further identify novel genes and pathways that may be associated with TCDD-induced hepatotoxicity, we investigated gene changes in rat liver following exposure to single oral doses of TCDD. Male Sprague-Dawley rats were administered single doses of 0.4 microg/kg bw or 40 microg/kg bw TCDD and killed at 6 h, 24 h, or 7 days, for global analyses of gene expression. In general, low-dose TCDD exposure resulted in greater than 2-fold induction of genes coding for a battery of phase I and phase II metabolizing enzymes including CYP1A1, CYP1A2, NADPH quinone oxidoreductase, UGT1A6/7, and metallothionein 1. However, 0.4 microg/kg bw TCDD also altered the expression of Gadd45a and Cyclin D1, suggesting that even low-dose TCDD exposure can alter the expression of genes indicative of cellular stress or DNA damage and associated with cell cycle control. At the high-dose, widespread changes were observed for genes encoding cellular signaling proteins, cellular adhesion, cytoskeletal and membrane transport proteins as well as transcripts coding for lipid, carbohydrate and nitrogen metabolism. In addition, decreased expression of cytochrome P450 7A1, short heterodimer partner (SHP; gene designation nr0b2), farnesyl X receptor (FXR), Ntcp, and Slc21a5 (oatp2) were observed and confirmed by RT-PCR analyses in independent rat liver samples. Altered expression of these genes implies major deregulation of cholesterol metabolism and bile acid synthesis and transport. We suggest that these early and novel changes have the potential to contribute significantly to TCDD induced hepatotoxicity and hypercholesterolemia.

The dioxin/aryl hydrocarbon receptor mediates downregulation of osteopontin gene expression in a mouse model of gastric tumourigenesis

The dioxin/aryl hydrocarbon receptor functions as a ligand-activated transcription factor regulating transcription of a battery of genes encoding primarily drug-metabolizing enzymes. Expression of a constitutively active mutant of the aryl hydrocarbon receptor (CA-AhR) in transgenic mice results in development of stomach tumours, correlating with increased mortality. We have used suppression subtractive hybridization techniques followed by macroarray analysis to elucidate which genes are differentially expressed during this process. In the glandular stomach of CA-AhR mice, we observed decreased mRNA expression of osteopontin (OPN), a noncollagenous protein of bone matrix that is also involved in several important functions including regulation of cytokine production, macrophage accumulation, cell motility and adhesion. Downregulated expression of OPN during tumour development was confirmed by RT-PCR and RNA blot analysis. Immunohistochemical analysis showed that this decrease was confined to the corpus region, correlating with the restricted localization of the tumours. Decreased OPN mRNA expression was also observed in other organs of CA-AhR mice. Taken together, these results show that OPN is negatively regulated by the dioxin receptor, and that downregulation of its expression correlates with development of stomach tumours in mice expressing a constitutively active mutant of dioxin receptor.

Cardiovascular gene expression profiles of dioxin exposure in zebrafish embryos

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a widespread environmental contaminant that causes altered heart morphology, circulatory impairment, edema, hemorrhage, and early life stage mortality in fish. TCDD toxicity is dependent, in large part, on the aryl hydrocarbon receptor (AHR), but understanding of the molecular mechanism of cardiovascular embryotoxicity remains incomplete. To identify genes potentially involved in cardiovascular effects, we constructed custom cDNA microarrays consisting of 4896 zebrafish adult heart cDNA clones and over 200 genes with known developmental, toxicological and housekeeping roles. Gene expression profiles were obtained for 3-day-old zebrafish after early embryonic exposure to either 0.5 or 5.0 nM TCDD. In all, 516 clones were significantly differentially expressed (p < 0.005) under at least one treatment condition; 123 high-priority clones were selected for further investigation. Cytochromes P450 1A and 1B1, and other members of the AHR gene battery, were strongly and dose-dependently induced by TCDD. Importantly, altered expression of cardiac sarcomere components, including cardiac troponin T2 and multiple myosin isoforms, was consistent with the hypothesis that TCDD causes dilated cardiomyopathy. Observed increases in expression levels of mitochondrial energy transfer genes also may be related to cardiomyopathy. Other TCDD-responsive genes included fatty acid and steroid metabolism enzymes, ribosomal and signal-transduction proteins, and 18 expressed sequence tags (ESTs) with no known protein homologs. As the first broad-scale study of TCDD-modulated gene expression in a non-mammalian system, this work provides an important perspective on mechanisms of TCDD toxicity.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) induces organ- specific differential gene expression in male Japanese medaka (Oryzias latipes)

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a ubiquitous environmental contaminant with well-known adverse effects in fish. In this study, we initially exploited suppression subtractive hybridization (SSH) as a screening tool to assess qualitative gene expression changes in whole brain, liver, and testis of adult male Japanese medaka (Oryzias latipes) exposed for 48 h to a single intraperitoneal-injected dose of TCDD (10 microg TCDD/kg body weight). Across these three organs, SSH identified a total of 335 unique genes. Each set of forward- and reverse-subtracted organ cDNA libraries consisted of a distinct gene list and corresponding distribution of biological processes, suggesting that transcript profiles of these libraries were highly organ-specific. Based on sequence match significance and frequencies within each set of organ libraries, genes hypothesized to be strongly responsive (42 total) within male medaka brain, liver, or testis were semi-quantitatively screened with replicate cDNA nylon membrane arrays. In addition, TCDD-treated male medaka were surveyed for gross histological analysis of brain, liver, and testis. In general, adverse histopathological changes were not observed in the brain, and glycogen depletion was observed only in the liver. However, significant histological changes occurred in the testis, and included disorganization of spermatogenesis at the testis periphery, disruption of the interstitium, Leydig cell swelling, and Sertoli cell vacuolation. Of the 42 genes screened by cDNA array analysis, cytochrome P450 1A (CYP1A) mRNA was the only transcript significantly higher in TCDD-exposed brain, whereas 12 transcripts (including CYP1A) were significantly higher in TCDD-exposed liver, and 34 transcripts were significantly lower in TCDD-exposed testis. Therefore, the degree of TCDD-induced alterations observed in each organ at a gross histological level corresponded well with the number and ontology of gene transcripts affected on the array. Based on real-time reverse transcription polymerase chain reaction (RT-PCR), relative CYP1A (but not AHR1) transcript levels were confirmed to be significantly higher in TCDD-treated brain and liver. However, CYP1A was not significantly induced in TCDD-exposed testis, suggesting that gene expression and histopathological responses observed in the testis at 48 h may be CYP1A-independent. Based on these data, unique liver-specific and testis-specific mRNA-level targets in male medaka were identified as promising biomarkers of acute TCDD-induced toxicity.

Search for the target genes involved in the suppression of antibody production by TCDD in C57BL/6 mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) suppresses antibody production through activation of a transcription factor, the aryl hydrocarbon receptor (AhR). To explore the genes that are involved in the suppression of antibody production by TCDD, we investigated TCDD-induced changes in gene expression in the CD4 T cells and B cells of C57BL/6 mice immunized with ovalbumin (OVA) plus alum as an adjuvant. Changes in gene expression were analyzed with Affymetrix oligonucleotide microarrays. The results showed that OVA-immunization alone up-regulated expression levels of many genes in the CD4 T cells as early as 3 h after immunization, with 55 up-regulated and 5 down-regulated. At 24 h, 42 genes were found to be up-regulated and 30 down-regulated. Fewer genes were affected in the B cells than in the CD4 T cells. In contrast to the up-regulation of genes induced by immunization in the CD4 T cells, administration of TCDD to mice 3 h prior to the immunization mainly caused down-regulation of genes in the CD4 T cells when compared with immunization alone, with 1 being up-regulated and 4 down-regulated at 3 h after immunization and 3 up-regulated and 34 down-regulated at 24 h. In particular, at 3 and 24 h, TCDD suppressed expression of three and seven genes, respectively, that were up-regulated by immunization. Another characteristic of the TCDD-induced changes in gene expression was the suppression of many genes encoding proteins that are involved in GTP-binding protein-linked signaling in CD4 T cells. These results suggest that the inhibition of immunization-induced gene expression and modulation of G-protein-linked signaling in CD4 T cells are responsible for the TCDD-induced suppression of antibody production.

Differential signatures of protein expression in marmoset liver and thymus induced by single-dose TCDD treatment

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an ubiquitously distributed environmental pollutant. Health effects have been studied intensively, but low-dose effects are quite complex and not yet fully understood. In many studies, the immune system was identified as the most sensitive target. Here, we demonstrate changes of protein expression in liver and thymus of male marmosets (Callithrix jacchus) which were subjected to a single dose of a subcutaneous injection of 100 ng/kg body weight TCDD. Histopathological examination revealed myocardial fibrosis, but there were no significant findings in pathology and histopathology of liver and thymus. In order to detect more subtle treatment-related changes, we performed a comparative proteomic investigation of liver and thymus using a 2-D gel electrophoresis based proteomics approach. Fluorescence labeling and automated image analysis was used to enhance sensitivity and reproducibility. In both organs, distinct changes of protein expression were detected which were more pronounced in thymus, where the pattern of deregulated proteins could be clearly related to immune responses. In the thymus of treated animals, several toxicologically relevant factors were increased, including chaperones, glycerol-3-phosphate dehydrogenase, and adseverin. Among others, vimentin, Ca-dependent protease and protein disulfide isomerase were downregulated. In the liver, transferrins, lamin A and HSP70 were upregulated, whereas thymidine phosphorylase (synonyms: endothelial cell growth factor, PD-ECGF, gliostatin) was significantly reduced. Comparative analysis of deregulated proteins in both organs revealed a pattern of related functions, which fits well into the existing knowledge of the toxic processes and mechanisms underlying TCDD-mediated toxicity.

Complementary roles for toxicologic pathology and mathematics in toxicogenomics, with special reference to data interpretation and oscillatory dynamics

Toxicogenomics is an emerging multidisciplinary science that will profoundly impact the practice of toxicology. New generations of biologists, using evolving toxicogenomics tools, will generate massive data sets in need of interpretation. Mathematical tools are necessary to cluster and otherwise find meaningful structure in such data. The linking of this structure to gene functions and disease processes, and finally the generation of useful data interpretation remains a significant challenge. The training and background of pathologists make them ideally suited to contribute to the field of toxicogenomics, from experimental design to data interpretation. Toxicologic pathology, a discipline based on pattern recognition, requires familiarity with the dynamics of disease processes and interactions between organs, tissues, and cell populations. Optimal involvement of toxicologic pathologists in toxicogenomics requires that they communicate effectively with the many other scientists critical for the effective application of this complex discipline to societal problems. As noted by Petricoin III et al (Nature Genetics 32, 474-479, 2002), cooperation among regulators, sponsors and experts will be essential for realizing the potential of microarrays for public health. Following a brief introduction to the role of mathematics in toxicogenomics, "data interpretation" from the perspective of a pathologist is briefly discussed. Based on oscillatory behavior in the liver, the importance of an understanding of mathematics is addressed, and an approach to learning mathematics "later in life" is provided. An understanding of pathology by mathematicians involved in toxicogenomics is equally critical, as both mathematics and pathology are essential for transforming toxicogenomics data sets into useful knowledge.

A constitutively active dioxin/aryl hydrocarbon receptor promotes hepatocarcinogenesis in mice

The dioxin/aryl hydrocarbon receptor (AhR) functions as a ligand-activated transcription factor regulating transcription of a battery of genes encoding enzymes involved in drug metabolism. Known ligands include polycyclic aromatic hydrocarbons, certain polychlorinated biphenyls, and the polyhalogenated dioxins including 2,3,7,8-tetrachlorodibenzo-p-dioxin. Both polyhalogenated biphenyls and 2,3,7,8-tetrachlorodibenzo-p-dioxin are potent promoters of rodent hepatocarcinogenesis in two-stage initiation-promotion experiments. Although several lines of evidence indicate the involvement of the AhR in toxic effects mediated by polyhalogenated biphenyls and dioxins, its involvement in tumor promotion has not been unequivocally proven. In the present study, a transgenic mouse line expressing a constitutively active AhR (CA-AhR) has been used to investigate the role of the AhR in hepatocarcinogenesis. Male AhR wild-type and CA-AhR-transgenic B6C3F1-mice were treated with a single injection of the hepatocarcinogen N-nitrosodiethylamine at 6 weeks of age and were subsequently kept untreated on control diet. Thirty five weeks after carcinogen treatment, mice were sacrificed, and the prevalence and multiplicity of liver tumors were determined. Whereas only 1 small liver tumor was observed in 15 AhR-wild-type mice, 19 tumors (two >1 cm in diameter) were present in 18 CA-AhR-transgenic mice. This result demonstrates the oncogenic potential of the activated AhR and implicates an important role of the receptor in promotion of hepatocarcinogenesis. A microarray-based gene expression-profiling analysis revealed down-regulation in the liver of CA-AhR-transgenic mice of a cluster of genes encoding heat shock proteins, including GRP78/BiP, Herp1, Hsp90, DnaJ (Hsp40) homologue B1, and Hsp105, which are important for protein folding and quality control.

Functional alterations in CD11b(+)Gr-1(+) cells in mice injected with allogeneic tumor cells and treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) exposure results in an increased percentage of CD11b(+) (Mac-1(+)) cells in the spleens of mice challenged with P815 tumor cells, coincident with a failure of the mice to generate allospecific CD8(+) CTL activity. Since CD11b(+)Gr-1(+) myeloid suppressor cells (MSC) have been described as that which prevent cytotoxic T lymphocyte (CTL) development in a variety of disease states, we hypothesized that TCDD promoted MSC development, leading to suppression of CTL activity. The purpose of the present studies was to compare the phenotypic and functional characteristics of CD11b(+) cells in vehicle- and TCDD-treated mice during the P815 tumor allograft response to determine their potential to function as MSC. Initial studies showed that virtually all splenic CD11b(+) cells in both vehicle- and TCDD-treated mice co-expressed Gr-1. Consistent with MSC activity, CD11b(+)Gr-1(+) cells isolated from TCDD- but not vehicle-treated mice suppressed the development of CTL activity when added in vitro to mixed lymphocyte-P815 tumor cell cultures. Also consistent with MSC activity, this suppressive effect in vitro required cell-to-cell contact. Surprisingly, however, in vivo depletion of CD11b(+)Gr-1(+) cells failed to affect TCDD-induced suppression of the CTL response, arguing against an immunoregulatory role for the cells in vivo. Immunohistochemical analysis of the spleen showed that CD11b(+)Gr-1(+) cells were localized in the red pulp, and physically separated from the T cells in the white pulp. The localization of CD11b(+)Gr-1(+) cells in the red pulp was indicative of extramedullary myelopoiesis and suggested that TCDD enhanced myelopoiesis. A significantly enhanced neutrophilia in the blood of TCDD-treated mice supported this conclusion. CD11b(+)Gr-1(+) cells isolated from the blood or spleen of TCDD-treated mice produced up to fivefold higher levels of superoxide following PMA stimulation when compared with cells from vehicle-treated mice. However, unlike vehicle-treated mice, CD11b(+)Gr-1(+) cells from TCDD-treated mice were unable to kill YAC-1 target cells. These results indicate that TCDD exposure alters the host response to allogeneic tumor growth, resulting in enhanced myelopoiesis perhaps as a compensatory response to the suppressed T cell-mediated immunity in the face of an increasing P815 tumor burden. Furthermore, within the context of the P815 response, TCDD appears to alter the functional capabilities of mature neutrophils, by enhancing their oxidative burst capacity but reducing their tumoricidal response.

Gene expression profile analysis of the mouse liver during bacteria-induced fulminant hepatitis by a cDNA microarray system

Fulminant hepatic failure (FHF) is a disease characterized by sudden and severe impairment of liver function. To elucidate the mechanism involved in FHF, we adopted a murine model of FHF by administrating mice with heat-killed Propionibacterium acnes (P. acnes), followed by a low dose of lipopolysaccharide (LPS), and analyzed the dynamic change of gene expression profile of the murine liver using an in-house cDNA microarray system which contained most of the cDNAs encoding chemokines/cytokines and their receptors (33 chemokines/21 chemokine receptors, 28 cytokines/35 cytokine receptors) as well as 230 liver related proteins mostly selected by serial analysis of gene expression (SAGE). Among them, 335 genes were found to differ by more than 2-fold in at least one time point comparing with normal liver. Hierarchical cluster analysis revealed that except for a few genes, such as heme oxygenase (HO)-1 and nicotinamide N-methyltransferase (NNMT) of which expression increased, the expression of most of the genes encoding drug metabolizing enzymes decreased with the progress of the disease. The expression of the genes encoding chemokines/cytokines was dramatically changed, such as Mig, IP-10, RANTES, TNF-alpha, and IFN-gamma. In addition, the expression of those that were not previously linked to this murine model was also identified to be changed. These include endogenous IL-18 binding protein (IL-18BP), CXCL16 (the ligand of Bonzo, CXCR6) as well as ESTs. Taken together this study has shown the systemic and comprehensive gene expression profile during FHF and may contribute to better understanding of the mechanism of FHF.

Toxicogenomics: an opportunity to optimise drug development and safety evaluation

'Toxicogenomics', the use of complex populations of mRNA to understand toxicity, is a relatively new field which combines the wealth of gene sequence information with advances in miniaturisation technology. In a parallel evolutionary path that the broader field of toxicology has encountered, toxicogenomics is steadily changing from descriptive to mechanistic research and will ultimately progress to a predictive science. This review focuses on the application of microarray research tools to toxicology studies in preclinical development. In particular, the 'testing funnel' approach to candidate selection in drug development is used to discuss the strategic implementation of toxicogenomics to help develop drugs that harbour less toxicity. Examples of where toxicogenomics has increased mechanistic and descriptive understanding of cellular toxicity are discussed, as are early efforts to develop molecular biomarkers that are predictive of longer-term toxicity.