Abstract
The estrogen receptor and aryl hydrocarbon receptor (AhR) are coexpressed in several Ah and estrogen-responsive human breast cancer cell lines. However, a recent study reported that 17beta-estradiol (E2) inhibited Ah responsiveness in mouse Hepa 1c1c7 hepatoma cells (Kharat, I., and Saatcioglu, F. (1996) J. Biol. Chem. 271, 10533-10537), and therefore, estrogen receptor-AhR cross-talk was reinvestigated in MCF-7 and mouse Hepa 1c1c7 cells. Treatment of MCF-7 or Hepa 1c1c7 cells with 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) resulted in induction of CYP1A1-dependent activity and mRNA levels. Treatment of both cell lines with E2 had no effect on basal or TCDD-inducible CYP1A1-dependent activity or mRNA levels. In MCF-7 and Hepa 1c1c7 cells transiently transfected with an Ah-responsive plasmid containing the 5'-regulatory region of the human CYP1A1 gene fused to the chloramphenicol acetyltransferase reporter gene 10 nM TCDD significantly induced chloramphenicol acetyltransferase activity; in cells cotreated with TCDD plus E2 the induced response was not affected by the hormone. Nuclear extracts from cells treated with dimethyl sulfoxide, E2, TCDD, and TCDD plus E2 were incubated with the [32P]dioxin-responsive element and analyzed by gel electrophoretic mobility shift assays. A retarded band associated with formation of a [32P]dioxin-responsive element-AhR complex was observed in nuclear extracts from cells treated with TCDD or TCDD plus E2 (cotreated). Collectively these studies suggest that E2 does not modulate AhR-mediated CYP1A1 gene expression in MCF-7 or Hepa 1c1c7 cells.
Highlights
§ Sid Kyle Professor of Toxicology. 1 The abbreviations used are: TCDD, 2,3,7,8-tetrachlorodibenzo-pdioxin; AhR, aryl hydrocarbon receptor; dioxin-responsive elements (DREs), dioxin-responsive element; E2, estrogen; ER, estrogen receptor; CAT, chloramphenicol acetyltransferase; PCR, polymerase chain reaction; Ethoxyresorufin O-Deethylase (EROD), ethoxyresorufin O-deethylase
In many cells/tissues AhR agonists such as TCDD or 3-methylcholanthrene cause a marked induction of CYP1A1 gene expression, and the AhR-mediated mechanism of this response has been extensively investigated [17,18,19]
CYP1A1 and/or CYP1A2 gene expression, and they include interleukin 6 in human HepG2 cells [28], transforming growth factor- in human A549 lung cancer cells [29], interleukin-1, insulin, oxidative stress (e.g. H2O2), epidermal growth factor, transforming growth factor-␣ in mouse or rat hepatocytes (30 – 32), and the microtubule inhibitor nocodazole in mouse Hepa 1c1c7 cells [33]. Some of these effects may be cell-specific since studies in this laboratory indicate that neither insulin nor epidermal growth factor inhibited induction of EROD activity by TCDD in MCF-7 cells
Summary
§ Sid Kyle Professor of Toxicology. 1 The abbreviations used are: TCDD, 2,3,7,8-tetrachlorodibenzo-pdioxin; AhR, aryl hydrocarbon receptor; DRE, dioxin-responsive element; E2, estrogen; ER, estrogen receptor; CAT, chloramphenicol acetyltransferase; PCR, polymerase chain reaction; EROD, ethoxyresorufin O-deethylase. Breast cancer cell lines such as ER-negative MDA-MB-231 [11] and Hs578T [12] cells express the AhR and AhR nuclear translocator proteins, but TCDD did not induce CYP1A1 or reporter gene activity in cells transiently transfected with Ah-responsive constructs. In gel mobility shift assays using nuclear extracts from cells treated with TCDD or TCDD plus E2 (cotreated), they reported that hormone treatment blocked formation of the TCDD-induced retarded band These data were inconsistent with results of previous studies, and the effects of E2 on Ah responsiveness have been reinvestigated in both MCF-7 and Hepa 1c1c7 cells. The results of this study clearly demonstrate that E2 does not affect Ah responsiveness in either cell line
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