Transcriptional activation of c-fos protooncogene by 17 beta-estradiol: Mechanism of aryl hydrocarbon receptor-mediated inhibition

17 beta-Estradiol (E-2) induced c-fos protooncogene mRNA levels in MCF-7 hu man breast cancer cells, and maximal induction was observed within 1 h afte r treatment. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) inhibited the E-2-i nduced response within 2 h. The molecular mechanism of this response was fu rther investigated using pFC2-CAT, a construct containing a -1400 to +41 se quence from the human c-fos protooncogene linked to a bacterial chloramphen icol acetyltransferase (CAT) reporter gene. In MCF-7 cells transiently tran sfected with pFC2-CAT, 10 nM E-2 induced an 8.5-fold increase of CAT activi ty, and cotreatment with 10 nM TCDD decreased this response by more than 45 %. alpha-Naphthoflavone, an aryl hydrocarbon receptor (AhR) antagonist, blo cked the inhibitory effects of TCDD; moreover, the inhibitory response was not observed in variant Ah-nonresponsive MCF-7 cells, suggesting that the A hR complex was required for estrogen receptor cross-talk. The E-2-responsiv e sequence (-1220 to -1155) in the c-fos gene promoter contains two putativ e core pentanucleotide dioxin-responsive elements (DREs) at -1206 to -1202 and -1163 to -1159. In transient transfection assays using wild-type and co re DRE mutant constructs, the downstream core DRE (at -1163 to -1159) was i dentified as a functional inhibitory DRE. The results of photo-induced cros s-linking, gel mobility shift, and in vitro DNA footprinting assays showed that the AhR complex interacted with the core DRE that also overlapped the E-2-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.