The silencing mediator of retinoic acid and thyroid hormone receptors can interact with the aryl hydrocarbon (Ah) receptor but fails to repress Ah receptor-dependent gene expression.
Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related chemicals causes a variety of tissue- and species-specific biological and toxicological effects, most of which are mediated by the aryl hydrocarbon receptor (AhR). The AhR complex is a ligand-dependent transcription factor that binds to its specific DNA recognition site as a dimer with the AhR nuclear translocator (ARNT) and activates gene transcription. Here, we have examined the ability of a nuclear corepressor, the silencing mediator of retinoic acid and thyroid hormone receptors (SMRT), to interact with and modulate AhR-dependent gene expression. Using glutathione S-transferase (GST) "pull-down" binding assays, we have mapped a major interaction between these factors to the silencing domain of SMRT and the PAS B ligand binding domain of AhR, and this interaction is unaffected by the addition of an AhR ligand. Association of SMRT with the AhR:ARNT:DNA complex was not detected by GST pull-down or gel retardation assays. Transient cotransfections of mammalian cells (Hepa1c1c7, MCF-7, and BG-1) with SMRT and a TCDD-inducible luciferase reporter containing the dioxin-responsive domain from the mouse CYP1A1 regulatory region revealed that SMRT does not repress, but enhances, AhR signaling. However, when a reporter containing a human CYP1A1 upstream region was cotransfected with SMRT into human MCF-7 cells, AhR-driven reporter activity was decreased by half, suggesting that SMRT acts on the human CYP1A1 promoter via a factor other than the AhR in MCF-7 cells. Furthermore, the interaction between SMRT and the AhR may have implications in pathways other than the AhR signaling pathway.
Department of Environmental Toxicology, University of California at Davis, Davis, CA 95616, USA.
S Renée
SR
Rushing
Michael S
MS
Denison
D000818:Animals; D051784:Aryl Hydrocarbon Receptor Nuclear Translocator; D001483:Base Sequence; D001943:Breast Neoplasms; D019363:Cytochrome P-450 CYP1A1 / Q000235:genetics; D004268:DNA-Binding Proteins / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D005786:Gene Expression Regulation; D005982:Glutathione Transferase / Q000235:genetics / Q000378:metabolism; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D056985:Nuclear Receptor Co-Repressor 2; D000072317:Polychlorinated Dibenzodioxins / Q000494:pharmacology; D011401:Promoter Regions, Genetic; D018336:Receptors, Aryl Hydrocarbon / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D012097:Repressor Proteins / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D020218:Response Elements / Q000187:drug effects / Q000235:genetics; D013045:Species Specificity; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D014407:Tumor Cells, Cultured
C497396:ARNT protein, human; C497397:Arnt protein, mouse; D004268:DNA-Binding Proteins; C539130:NCOR2 protein, human; C539131:Ncor2 protein, mouse; D056985:Nuclear Receptor Co-Repressor 2; D000072317:Polychlorinated Dibenzodioxins; D018336:Receptors, Aryl Hydrocarbon; D011994:Recombinant Proteins; D012097:Repressor Proteins; D014157:Transcription Factors; D051784:Aryl Hydrocarbon Receptor Nuclear Translocator; D019363:Cytochrome P-450 CYP1A1; D005982:Glutathione Transferase