Proteins that interact with and mediate the activity of the aryl hydrocarbon receptor and its partner ARNT

Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) and related chemicals produces a diverse array of tissue- and species-specific toxic and biological effects. The majority of these responses are mediated by the aryl hydrocarbon receptor (AhR), a ligand-dependent transcription factor whose binding to DNA as a heterodimer with the AhR nuclear translocator (ARNT) protein, leads to increased gene transcription. The diversity in AhR-mediated toxic and biological effects suggests that the specificity of AhR-dependent responsiveness could be conferred by tissue-, species-, and/or cell-specific factors that bind to and regulate AhR and/or ARNT functionality.; Several coactivators and corepressors known to regulate nuclear hormone receptor activity were examined for their ability to modulate AhR signal transduction. Two factors which physically and functionally interact with the AhR were identified. One of these cofactors is SMRT (silencing mediator of retinoic acid and thyroid hormone receptors), a corepressor protein that has been shown to repress the activity of unliganded DNA-bound nuclear hormone receptors. Transient cotransfection experiments in mouse Hepa1c1c7 and human MCF-7 and BG-1 cells using a reporter plasmid containing the dioxin-responsive mouse CYP1A1 upstream region revealed that SMRT actually enhances AhR signaling. However, when a reporter plasmid containing the upstream region of the human CYP1A1 gene was used, SMRT repressed AhR signaling in MCF-7 cells, but not Hepa1c1c7. These and other results indicate that SMRT-dependent repression in human cells occurs in an AhR-independent manner, regulated by other nuclear factors. GRIP1 (glucocorticoid receptor interacting protein 1), a nuclear receptor coactivator, was also shown to interact with the AhR. In transient cotransfection experiments in Hepa1c1c7 cells, GRIP1 significantly enhances AhR-mediated induction of a reporter gene. Deletion and coimmunoprecipitation experiments not only demonstrated a direct interaction between GRIP1 and the AhR, they identified the interaction domains of each protein. In addition, deletion analysis suggested a role for the coactivator p300 in the enhancement by GRIP1. Overall, our studies demonstrated a role for nuclear hormone receptor cofactors in modulating AhR responsiveness.