Abstract
2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) elicits a diverse spectrum of toxic responses that appear to be mediated through the aryl hydrocarbon receptor (AhR). TCDD also exhibits antiestrogenic activity in both the female rat uterus and MCF-7 human breast cancer cells. We used the (- 821/+14)-CAT construct from the 5' region of the Xenopus vitellogenin A2 gene which contains an estrogen responsive element (ERE) (-331/-319) and a nonhormone responsive activating binding site (AABS) (-121/-87). Transient transfection of the (-821/+14)-CAT plasmid into MCF-7 cells treated with E2, TCDD, and E2 plus TCDD demonstrated that the antiestrogenic activity of TCDD occurs at the 5'-flanking region of an estrogen-induced gene. TCDD acts as an antiestrogen through the AhR as shown by transient transfection studies in MCF-7 cells treated with E2 plus TCDD and structurally-related analogs and by transient transfection in wild type and AhR translocation deficient mutant Hepalclc7 cell lines. Alphanaphthoflavone, an AhR antagonist which inhibits formation of the nuclear AHR complex, inhibited the antiestrogenic activity of TCDD. Transfection of the plasmid into estrogen receptor (ER)- deficient HeLa cells showed that the ER was required for TCDD-induced antiestrogenicity. The results of transient transfection of deletion mutants of the (-821/+14)-CAT plasmid and electrophoretic mobility shift assays (EMSA) suggest that TCDD-mediated antiestrogenicity appears to involve the vitellogenin A2 sequence at -191/-87, which includes the AABS but not the ERE. Transient transfection assays using the AABS-tk-CAT plasmid and EMSAs with [32P]AABS oligonucleotides and nuclear extracts from MCF-7 cells indicate that TCDD acts at the AABS. An imperfect aryl hydrocarbon response element (AHRE) is located within the AABS and overlaps a tissue-specific transcription factor binding site. Some of the antiestrogenic activity of TCDD may involve altered protein-DNA interactions at the AABS which is involved in differentiation versus growth status.
Nodland, Katherine Irene (1995). Molecular mechanism of TCDD as an antiestrogen in an estrogen-induced gene. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -1562464.