Molecular mechanism of inhibitory aryl hydrocarbon receptor - Estrogen receptor/Sp1 cross talk in breast cancer cells

Shaheen Khan, Rola Barhoumi, Robert Burghardt, Shengxi Liu, Kyounghyun Kim, Stephen Safe

Research output: Contribution to journalArticlepeer-review

50 Scopus citations


The trifunctional carbamoylphosphate synthetase/aspartate transcarbamyltransferase/dihydroorotase (CAD) gene is hormone responsive in MCF-7 and ZR-75 breast cancer cells, and this response is inhibited by the aryl hydrocarbon receptor (AhR) agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Estrogen-dependent induction of CAD mRNA and reporter gene activity in cells transfected with constructs (pCAD) containing hormone-responsive GC-rich CAD promoter inserts involves estrogen receptor α (ERα)/Sp1 interactions with these proximal GC-rich motifs. TCDD also inhibits hormone-induced transactivation in MCF-7 and ZR-75 cells transfected with pCAD constructs. The mechanism of inhibitory AhR-ERα/Sp1 cross talk was further investigated by chromatin immunoprecipitation (ChIP), and the results show that ERα/Sp1 and the AhR are constitutively bound to the CAD gene promoter and only minor changes are observed after treatment with 17β-estradiol, TCDD, or their combination. However, examination of interactions of these transcription factors by fluorescence resonance energy transfer shows that E2 enhances ERα-Sp1 interactions, whereas cotreatment with TCDD significantly decreases interaction of these proteins. These results suggest that inhibitory AhR-ERα/Sp1 cross talk is due, in part, to enhanced association of AhR and ERα (also determined by fluorescence resonance energy transfer), which coordinately dissociates ER and Sp1 and decreases ERα/Sp1-mediated transactivation, whereas remaining associated with the CAD promoter. This represents a novel interaction between two ligand activated receptors where one receptor inhibits activation of the second receptor.

Original languageEnglish (US)
Pages (from-to)2199-2214
Number of pages16
JournalMolecular Endocrinology
Issue number9
StatePublished - 2006

ASJC Scopus subject areas

  • Molecular Biology
  • Endocrinology


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