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1 Center for Integrative Bioscience, Okazaki National Research Institutes, 5-1 Higashiyama, Myodaiji, Okazaki 444-8585, Japan
2 Core Research for Evolution Science and Technology (CREST), Japanese Science and Technology Corporation, Kawaguchi 332-0012, Japan
3 Environmental Health Sciences Division, National Institute for Environmental Studies, Onogawa, Tsukuba 305-8506, Japan
4 Frontier Collaborative Research Center, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Yokohama, Kanagawa 226-8503, Japan
(Requests for offprints should be addressed to T Iguchi; Email: taisen{at}nibb.ac.jp)
| Abstract |
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| Introduction |
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The effects of TCDD on the reproductive tract and, more specifically, comparisons of the effects of TCDD and estrogen have been reported previously. For example, the antiestrogenic effects of TCDD are well documented (Gallo et al. 1986, Gierthy et al. 1987, Romkes et al. 1987, Safe et al. 1991), and some molecular mechanisms of toxicity have been proposed (Krishnan et al. 1995, Wormke et al. 2003). Conversely, estrogenic effects of TCDD have also been reported (Nesaretnam et al. 1996, Abdelrahim et al. 2003).
TCDD binds to an aryl hydrocarbon receptor (AhR) that is associated with heat-shock protein 90. After the ligand binds, AhR dissociates from the heat-shock protein to form a heteromeric complex with AhR nuclear translator (ARNT). The complex then moves into the nucleus and binds to a xenobiotic response element (XRE) to activate the transcription of the TCDD response gene. The ligand-dependent behavior of AhR is similar to the estrogen receptor (ER) to some extent. When estrogen binds to the ER, the ER binds to the estrogen-response element (ERE) to activate the transcription of estrogen-response genes. Two models are proposed to explain the estrogenic effect of TCDD. One is direct binding of TCDD to ER (Nesaretnam et al. 1996) and the other is ligand-dependent proteinprotein interaction of AhR and ER, which has been demonstrated with 3-methylcholanthrene (Ohtake et al. 2003).
In the present study, we used DNA microarray analysis to investigate the estrogenic activity of TCDD. Accordingly, we determined the effects of TCDD and estrogen on gene expression and compared their gene expression patterns. By this approach, we estimated the estrogenic effects of TCDD.
| Materials and methods |
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Female C57/BL6/J mice were purchased from SLC, Inc. (Tokyo, Japan), housed under a 12-h light:dark cycle and given free access to food and water. Mice were ovariectomized at 8 weeks of age and, 2 weeks later, either sesame oil vehicle (Nakarai Tesque, Kyoto, Japan) or test material was administered to the animals. Whole uteri (n=4) were collected 6 h after dosing. The doses of 17ß-estradiol (E2) (Sigma, Sigma-Aldrich Japan, Tokyo, Japan) were 0, 0.05, 0.5, 5.0 and 50 µg/kg body weight (b.w.). The doses of TCDD (Cambridge Isotope Laboratory, Andover, MA, USA) were 0, 1.0 and 10 µg/kg b.w. Our institutional animal care committee approved all animal experiments.
Immunohistochemistry
Uterine tissue sections (4 µm) were prepared and incubated with anti-ER, anti-AhR or anti-ARNT antibodies. Staining was performed with a Histofine kit (Nichirei, Tokyo, Japan) according to the manufacturers protocol. Briefly, deparaffinized and rehydrated tissue sections were boiled twice in citrate buffer (10 mM citrate, pH 6.0) for 5 min and pretreated with 3% hydrogen peroxide for 10 min. The anti-ER (H-184), anti-AhR (M-20) and anti-ARNT (C-19) antibodies (Santa Cruz Biotechnologies, Santa Cruz, CA, USA) were incubated with the tissue sections at 4 °C overnight. For immunodetection, the sections were incubated with a second antibody conjugated with peroxidase for 10 min and developed with 3,3'-diaminobenzidine.
DNA microarray analysis
Total uterine RNA was extracted with TRIzol (Invitrogen, Tokyo, Japan) and purified with the RNeasy minikit (Qiagen, Tokyo, Japan). Total RNA quality was examined with a Bioanalyzer 2100 (Agilent Japan, Tokyo, Japan). Purified RNA was processed according to the manufacturers protocol to prepare the labeled cRNAs, which were hybridized to the mouse genome U74 array (Affymetrix Japan, Tokyo, Japan). Hybridization, washing and scanning were performed according to the manufacturers protocol.
Data analysis
Scanned data were analyzed with GeneChip Suit Analysis Software ver.5.0 (Affymetrix Japan, Tokyo, Japan) to obtain the average intensity of each cell corresponding to each oligonucleotide probe. The averaged fluorescence intensity (2500) of each probe was further analyzed by dChip, a model-based expression-analysis program (Li & Wong 2001), and expression levels were estimated. The PM-only model was used for the analysis, and the estimated values were transferred to the GeneSpring software program (Silicon Genetics, Redwood City, CA, USA) and analyzed. To calculate changes in expression, genes for which average expression levels were more than 1000 fluorescence intensity units under at least one experimental condition were selected, and the average expression values of the treated samples were divided by those from control samples.
Quantitative real-time PCR
Total RNA was purified as described above. cDNA was synthesized from purified total RNA with Superscript II RT(-) (Invitrogen, Tokyo, Japan), and random primers at 42 °C for 60 min. PCR reactions were performed in the Prism 7000 sequence detector (Applied Biosystems Japan, Tokyo, Japan) with SYBR-Green PCR core reagents (Applied Biosystems Japan, Tokyo, Japan) in the presence of appropriate primers, according to the manufacturers instructions. The primers were chosen to amplify short PCR products of less than 100 base pairs, and their sequences are as follows:
Each PCR amplification was performed in triplicate in the following conditions: 2 min at 50 °C and 10 min at 95 °C, followed by a total of 40 two-temperature cycles (15 s at 95 °C and 1 min at 60 °C). Model 7000 software was used to construct amplification plots from extension-phase fluorescent emission data collected during PCR amplification. Threshold (C
) values were calculated by determining the point at which fluorescence exceeds a threshold limit.
Gene expression levels were normalized to the expression levels of L8 mRNA (U67771 [GenBank] ), and changes in concentration were calculated. Gel electrophoresis and melting curve analyses were performed to confirm correct amplicon size and the absence of nonspecific bands. Quantification of mRNAs was repeated three times with independent mice, and average levels of change were calculated.
| Results |
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To examine the effects of TCDD and estradiol on uterine gene expression, we first examined the spatial expression pattern of AhR and compared it with the expression pattern of ER and ARNT. Immunohistochemistry localized AhR and ARNT within the uterine tissues and showed that the expression pattern of AhR was very similar to that of the transcription factors ER and ARNT (Fig. 1
). All three proteins were expressed to a greater degree in luminal and glandular epithelium than in other tissues, and clear differences in expression levels could not be observed between stroma and myometrium.
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To profile uterine genes affected by TCDD, mice were ovariectomized to eliminate the effect of endogenous estrogen; 2 weeks later, they were treated with 10 µg/kg TCDD. Six hours after treatment, uteri were removed and RNA was isolated. We examined the expression of about 10 000 genes by DNA microarray analysis. As shown in Fig. 2A
, the scatter plot of gene expressions treated by TCDD versus control showed a diagonal line, indicating that most gene expression levels did not change to a large extent, and that there was little bias during data acquisition and analysis. As a result, it was found that 28 annotated genes were activated and 20 annotated genes were repressed more than twofold after filtering the data (see Materials and methods). Lists of the activated and repressed genes are provided in Tables 1
and 2
respectively.
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Comparison of TCDD gene-activation pattern with estrogen effects
To determine whether the effects of TCDD on uterine gene expression are similar to estradiol, we analyzed and compared the gene expression patterns after treatment with both agents. The genes that were activated or repressed more than twofold by either TCDD or estradiol at 6 h were selected for further analysis. The 575 selected genes, which included EST genes, were analyzed by hierarchic clustering (Fig. 3
). According to the dendrogram, this activation pattern was divided into six primary clusters, as indicated in Table 3
(Fig. 3AF
, dendrogram is not shown).
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DNA microarray analysis suggested that six of the TCDD-induced genes were activated more than twofold by estradiol. Three genes were confirmed to be activated more than 1.5-fold by both TCDD and estradiol by quantitative PCR, although the changes were not consistent with those observed on the DNA microarray (Fig. 4
). These genes included vascular endothelial growth factor A (Vegfa, NM_009505
[GenBank]
), proliferating cell nuclear antigen (Pcna, NM_011045
[GenBank]
) and ADP-ribosylation factor 4 (Arf4, NM_007479
[GenBank]
) (Kim et al. 2003) (indicated in boldface in Table 1
).
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Since the number of genes activated by both TCDD and estradiol was rather small, we selected a gene set that was induced by TCDD at 6 h and examined its temporal activation pattern after estradiol treatment. If the kinetics of AhR are much slower than those of ER, gene activation through AhR may be slower than that through ER. In this case, even if the set of activated genes is similar, temporal expression patterns may differ, and genes activated by AhR at 6 h may be similar to those activated by ER at a much earlier time point. Although several TCDD-responsive genes were activated at 12 h by estradiol (Fig. 5A
), some other genes were activated much later (Fig. 5B
), and no time point was identified at which the majority of TCDD-responsive genes were activated simultaneously. This result indicates that the E2-affected genes and the TCDD-affected genes did not overlap temporally.
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| Discussion |
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To examine the estrogenic effects of TCDD, we used DNA microarray analysis to evaluate uterine gene expression after treatment with TCDD or estrogen. The number of TCDD-affected genes was rather restricted compared with estrogen-affected genes (Watanabe et al. 2002, 2003). Several hundred genes were affected by estrogen treatment, whereas a high dose of TCDD affected only 120 genes, of which 48 were known genes. CYP1A1 was induced in a TCDD dose-dependent manner (Sadek & Allen-Hoffmann 1994), and AhR repressor (AhRR) (Mimura et al. 1999) was also activated, indicating that the doses in the present study were within the AhR-effective range. Previous studies have demonstrated that cytochrome P450 1A1 gene and AhRR are activated via AhR in various tissues and cultured cells (Mimura et al. 1999, Whitlock 1999). These genes are also activated in the uterus. Although several connexin genes are downregulated in the liver in response to TCDD, in our study, connexin 26 gene (M81445 [GenBank] ) was activated in the mouse uterus.
In addition to the genes showing dose-dependent activation, such as CYP1A1, many TCDD-affected genes were not activated dose-dependently. The genes that were equally activated at either dose might reach an induction plateau by 1 µg/kg TCDD; thus, the expression levels remain constant even at the higher dose. Likewise, other transcription factors may contribute to gene activation. For example, Cyp3A gene is activated by both SXR (Rosenfeld et al. 2003) and TCDD.
Of the genes repressed by TCDD, those related to immunoglobulin were not affected by estradiol. Several studies have described the effects of TCDD on the immune system (Baccarelli et al. 2002) and suppression of NF-
B activity (Tian et al. 1999). Since NF-
B affects immunoglobulin gene expression, repression of the genes related to immunoglobulin may be related to the adverse effects of TCDD on the immune system. Alternatively, the number of immune cells that migrate to the uterus may be reduced after TCDD treatment.
TCDD and E2-induced genes
Proliferating cell nuclear antigen (Pcna) and vascular endothelial growth factor (Vegf) were commonly activated by TCDD and E2. Theoretically, if Vegf and Pcna were activated by the estrogenic activity of TCDD, other genes that were efficiently activated by estrogen should be activated by TCDD. Since not all genes activated by estradiol were activated by TCDD, this suggests that TCDD has only partial estrogenic activity. In this context, a clearer understanding of the Vegf gene activation mechanism in response to either TCDD or estradiol may provide important information on TCDDs genetic effects.
Our DNA microarray analysis indicated that only a portion of the estrogen-activated genes (cluster A in Fig. 3
) was activated by TCDD, and a larger number of genes were not activated (cluster B in Fig. 3
). Two models have been proposed to explain the estrogenic effects of TCDD. The first focuses on the induction of ER by TCDD (Nesaretnam 1996), and the second model, based on a study of 3-methylcholanthrene, suggests a direct interaction between AhR and ER (Ohtake 2003). Although our data do not directly support these models, it can be concluded that the estrogenic effects of TCDD in vivo are very limited and that TCDD mimics estrogen only under very restricted conditions in the uterus. Because AhR and ER exhibit very similar distribution throughout the uterus (Fig. 1
), the partial estrogenic activity of TCDD may not be related to the tissue-specific distribution of these receptors in the uterus.
In summary, we examined the effects of TCDD on uterine gene expression and demonstrated only weak estrogenic activity. A better understanding of the genes that are activated by both TCDD and estrogen will contribute to a more complete understanding of TCDDs effects on the uterus.
| Acknowledgements |
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Received 12 July 2004
Accepted 12 August 2004
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