Nucleocytoplasmic shuttling of the aryl hydrocarbon receptor.

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that acts in concert with the AhR nuclear translocator (ARNT), and alters gene expression in response to environmental contaminants such as 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD). We have previously shown that AhR contains both a nuclear localization signal (NLS), AhR(13-39), and a nuclear export signal (NES), AhR(55-75), in its NH(2)-terminal region. In this study, we obtained direct evidence for the nucleocytoplasmic shuttling of AhR and show the biological significance of the shuttling in terms of the transcriptional activation of its target gene, CYP1A1. When AhR(13-75) fused with glutathione S-transferase (GST)-green fluorescent protein (GFP) was microinjected into the nucleus of a polykaryotic of BHK21 cell, the GST-AhR(13-75)-GFP migrated from one nucleus to the other. This event, nucleocytoplasmic shuttling, was completely inhibited in the presence of leptomycin B (LMB). The interaction between chromosome region maintenance 1 (CRM1) and endogenous AhR was shown by immunoprecipitation with antibodies to AhR followed by immunoblot analysis with antibodies to CRM1. The inhibition of the nuclear export of AhR by LMB repressed the transcriptional activation of the CYP1A1 gene. The findings suggest that nuclear-cytoplasmic shuttling of AhR is essential for the inducible expression of the CYP1A1 protein.