title: Transcriptional activation of c-fos protooncogene by 17beta-estradiol: mechanism of aryl hydrocarbon receptor-mediated inhibition.
abstract: 17Beta-estradiol (E2) induced c-fos protooncogene mRNA levels in MCF-7 human breast cancer cells, and maximal induction was observed within 1 h after treatment. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) inhibited the E2-induced response within 2 h. The molecular mechanism of this response was further investigated using pFC2-CAT, a construct containing a -1400 to +41 sequence from the human c-fos protooncogene linked to a bacterial chloramphenicol acetyltransferase (CAT) reporter gene. In MCF-7 cells transiently transfected with pFC2-CAT, 10 nM E2 induced an 8.5-fold increase of CAT activity, and cotreatment with 10 nM TCDD decreased this response by more than 45%. Alpha-Naphthoflavone, an aryl hydrocarbon receptor (AhR) antagonist, blocked the inhibitory effects of TCDD; moreover, the inhibitory response was not observed in variant Ah-nonresponsive MCF-7 cells, suggesting that the AhR complex was required for estrogen receptor cross-talk. The E2-responsive sequence (-1220 to -1155) in the c-fos gene promoter contains two putative core pentanucleotide dioxin-responsive elements (DREs) at -1206 to -1202 and -1163 to -1159. In transient transfection assays using wild-type and core DRE mutant constructs, the downstream core DRE (at -1163 to -1159) was identified as a functional inhibitory DRE. The results of photo-induced cross-linking, gel mobility shift, and in vitro DNA footprinting assays showed that the AhR complex interacted with the core DRE that also overlapped the E2-responsive GC-rich site (-1168 to -1161), suggesting that the mechanism for AhR-mediated inhibitory effects may be due to quenching or masking at the Sp1-binding site.
authors.affiliation: Department of Veterinary Physiology and Pharmacology, Texas A&M University, College Station 77843-4466, USA.
authors.forename: R
authors.initials: R
authors.lastname: Duan
authors.forename: W
authors.initials: W
authors.lastname: Porter
authors.forename: I
authors.initials: I
authors.lastname: Samudio
authors.forename: C
authors.initials: C
authors.lastname: Vyhlidal
authors.forename: M
authors.initials: M
authors.lastname: Kladde
authors.forename: S
authors.initials: S
authors.lastname: Safe
mesh_terms: D001571:Benzoflavones / Q000494:pharmacology; D001665:Binding Sites; D003432:Cross-Linking Reagents; D018983:DNA Footprinting; D019175:DNA Methylation; D015257:DNA-Cytosine Methylases; D004591:Electrophoresis, Polyacrylamide Gel; D004958:Estradiol / Q000031:analogs & derivatives / Q000494:pharmacology*; D004965:Estrogen Antagonists / Q000494:pharmacology; D000077267:Fulvestrant; D016762:Genes, fos / Q000235:genetics*; D006801:Humans; D000072317:Polychlorinated Dibenzodioxins / Q000494:pharmacology; D012333:RNA, Messenger / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D018336:Receptors, Aryl Hydrocarbon / Q000187:drug effects / Q000378:metabolism; D012045:Regulatory Sequences, Nucleic Acid; D015533:Transcriptional Activation / Q000187:drug effects; D014407:Tumor Cells, Cultured; D014466:Ultraviolet Rays
chemical_list: D001571:Benzoflavones; D003432:Cross-Linking Reagents; D004965:Estrogen Antagonists; D000072317:Polychlorinated Dibenzodioxins; D012333:RNA, Messenger; D018336:Receptors, Aryl Hydrocarbon; D000077267:Fulvestrant; D004958:Estradiol; C011512:alpha-naphthoflavone; C081573:DNA modification methylase SssI; D015257:DNA-Cytosine Methylases