Antiestrogenic Effects of 2,3,7,8-Tetrachlorodibenzo-p-dioxin Are Mediated by Direct Transcriptional Interference with the Liganded Estrogen Receptor (original) (raw)
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The Journal of Steroid Biochemistry and Molecular Biology, 1998
TCDD, the most potent congener of the polychlorinated dioxins, has been shown to be an antiestrogen. The mechanisms of TCDD-induced antiestrogenicity are still under investigation. In this study, we investigated the effects of TCDD on the expression of the estrogen receptor (ER) gene. We studied the levels of un-spliced ER transcript (hnRNA) as well as the ER mRNA in ovary, uterus and liver of TCDD-treated mice with different genetic backgrounds. To quantitate the ER hnRNA levels, the intron and exon boundary of ER hnRNA was ampli®ed by competitive RT-PCR. The ER mRNA from these mice was quantitated by competitive RT-PCR amplifying exons separated by an intron. ER hnRNA and ER mRNA levels were quantitated 4 days after a single i.p. dose of TCDD (5 m mg/kg) in female C57BL/6J (B6) mice, which carry the responsive allele to TCDD. TCDD treatment signi®cantly (p < 0.05) suppressed the levels of ER hnRNA in the ovary (27.4%) and uterus (21.9%). The decreases in ER hnRNA were coordinated with signi®cant (p < 0.01) decreases in ER mRNA in ovary (57.7%) and uterus (37.6%). There was a signi®cant decrease (20.3%, p < 0.05) in liver ER mRNA, however, the changes of ER hnRNA in liver were not signi®cant. The coordinated decreases in ER hnRNA and mRNA in TCDD-treated mice suggest a suppression of transcription of the ER gene. We performed the same study on DBA/2J (D2) mice, which possess the``non-responsive'' allele of the aryl hydrocarbon receptor (AhR). These mice demonstrated no signi®cant decrease in either the ER mRNA or hnRNA after TCDD treatment. Overall, these results suggest that TCDD suppresses the gene expression of the ER receptor by decreasing its transcription, and the AhR plays an important role in mediating this response.
Toxicology and Applied Pharmacology, 1992
Tetrachlorodibenzo-p-dioxin (TCDD) is a polychlorinated aromatic hydrocarbon with teratogenic and carcinogenic properties. Previous studies in our and other laboratories have demonstrated that TCDD has antiestrogenic properties. In order to elucidate the mechanism of action of TCDD on estrogen sensitive tissues, we studied its effects on serum estradiol and estrogen receptor (ER) levels in liver and uteri of CD1 mice. Treatment with TCDD did not result in alterations of serum estradiol levels at any of the doses tested (1.0-30 pg/kg). In contrast, TCDD treatment induced a dose-dependent decrease in hepatic and uterine ER protein as determined by an enzyme immunoassay and equilibrium binding assays. A decrease in cytosolic and nuclear ER levels in uteri occurred as early as 24 hr after initial treatment with 30 pg/kg TCDD and recovery occurred by 14 days. Hepatic cytosolic and nuclear ER also decreased at a dose of 30 rglkg TCDD at 24 hr after treatment, but recovery occurred only by 2 1 days. Studies in ovariectomized mice indicate that the regulation of hepatic ER by TCDD is independent of ovarian factors, but ovariectomy inhibited the downregulation of uterine ER by TCDD. Furthermore, determination of TCDD-induced cytochrome P-450 levels indicates that the downregulation of uterine ER is uncoupled from induction of hepatic cytochrome P-450. This study indicates that the antiestrogenic effects of low doses of TCDD are mediated through its ability to decrease hepatic and uterine ER and are not due to alterations in serum estradiol levels. Our results on ovariectomized mice indicate that TCDD-induced downregulation of ER is tissue specific and may involve different mechanisms at transcriptional or posttranscriptional levels. 0 1992 Academic Press. Inc.
Toxicological Sciences, 2009
Recent studies have shown that activated aryl hydrocarbon receptor (AHR) induced the recruitment of estrogen receptor-α (ERα) to AHR-regulated genes and that AHR is recruited to ERα-regulated genes. However, these findings were limited to a small number of wellcharacterized AHR-or ERα-responsive genes with little knowledge of what was occurring at other genomic regions. In this study, we showed using chromatin immunoprecipitation followed by hybridization to promoter focused microarrays (ChIP-chip) that 2,3,7,8-tetrachlorodibenzo-pdioxin treatment significantly increased the overlap of genomic regions bound by both AHR and ERα. Conventional and sequential ChIPs confirmed the recruitment of AHR and ERα to many of the identified regions. Transcription factor binding site analysis revealed an overrepresentation of aryl hydrocarbon receptor response elements in regions bound by both AHR and ERα, suggesting that AHR was the important factor determining the recruitment of ERα to these regions. RNA interference-mediated knockdown of AHR confirmed its requirement for the recruitment of ERα to some, but not all, of the shared regions. Our findings demonstrate not only that dioxin induces the recruitment of ERα to AHR target genes but also that AHR is recruited to estrogen-responsive regions in a gene-specific manner, suggesting that AHR utilizes both of these mechanisms to modulate estrogen-dependent signaling.
Molecular and Cellular Endocrinology, 2001
Previous in-vitro investigations of rat granulosa cells (GC) have shown that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) inhibits estrogen secretion and FSH-induced aromatase activity. Although TCDD exerted no effect on basal aromatase enzyme activity, TCDD did reduce steady-state aromatase mRNA levels in GC using competitive RT-PCR. TCDD is hypothesized to induce these changes through aromatic hydrocarbon receptor(AHR)-mediated gene transcription and the modulation of the estrogen receptor (ER)-signaling pathway. In this study we show that rat GC express mRNA for AHR and the AHR nuclear translocator (ARNT) as well as biomarkers of TCDD action, CYP1A1 and CYP1B1 mRNA. Basal CYP1A1 and ERa mRNAs were present only in trace amounts. By relative RT-PCR analysis we showed that CYP1A1 and CYP1B1 mRNA were induced significantly by TCDD at 6 h and that induction of CYP1A1 was maintained throughout the experiment. Using competitive RT-PCR, we observed no significant change in the mRNA levels of ARNT between control and TCDD-treated GC. Both AHR and ER-b mRNA levels increased significantly at 48 h with TCDD compared with controls. Since ER-b mRNA was not increased significantly until 48 h in culture, we suggest that in rat GC, the observed ER-b mRNA increase by TCDD might be a result of CYP1A1/CYP1B1 catalyzed estrogen metabolism and aromatase mRNA inhibition via AHR.
Molecular and cellular endocrinology, 2007
2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) can induce estrogenic action or inhibit estrogen-induced effects in various tissues because of aryl hydrocarbon receptor (AhR)-estrogen receptor (ER) cross-talk. In order to identify the biomarkers of TCDD endocrine disruption, we screened estrogen-responsive genes modified by TCDD exposure using specific cDNA microarrays spotted with estrogen-responsive genes. MCF-7 human breast carcinoma cells and RL95-2 human endometrial carcinoma cells were exposed to TCDD, and an analysis of their gene expression revealed 32 genes exhibiting a significant change. The mRNA expression levels of 27 genes were subsequently verified using real-time RT-PCR. Among these genes, bioinformatic analyses indicated that insulin-like growth factor-binding protein 5 (IGFBP5) gene expression might be influenced by estrogen status. In our animal experiments, IGFBP5 was also shown to be responsive to TCDD exposure in mouse fetuses in utero. These results suggest that TC...
Physiological implications of estrogen receptor modulation by 2,3,7,8-tetrachlorodibenzo-p-dioxin
Toxicology Letters, 1988
The interactions of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) with hormones and hormone receptors have important implications for TCDD toxicity. Evidence suggests that TCDD modulates receptors for glucocorticoids, prolactin, thyroxine, low density lipids, epidermal growth factor, and estrogens. Estrogen receptor modulation and the animal's physiological responses to this modulation appear to be particularly important effects and can explain much of the toxicity observed in TCDD-treated animals. Susceptibility of different species to TCDD correlates with their steroid glucuronidation capacity. Because of the close interactions and interdependent regulation of hormonal systems, other hormones may have a similar role in TCDD toxicity.