Dynamic shuttling and intranuclear mobility of nuclear hormone receptors

We expressed green fluorescent protein (GFP) chimeras of estrogen, retinoic acid, and thyroid hormone receptors (ERs, RARs, and TRs, respectively) in HeLa cells to examine nucleocytoplasmic shuttling and intranuclear mobility of nuclear hormone receptors (NRs) by confocal microscopy. These receptors were predominantly in the nucleus and, interestingly, underwent intranuclear reorganization after ligand treatment. Nucleocytoplasmic shuttling was demonstrated by heterokaryon experiments and energy-dependent blockade of nuclear import and leptomycin-dependent blockade of nuclear export. Ligand addition decreased shuttling by GFP-ER, whereas heterodimerization with retinoid X receptor helped maintain TR and RAR within the nucleus. Intranuclear mobility of the GFP-NRs was studied by fluorescence recovery after photo-bleaching +/- cognate ligands. Both GFP-TR and GFP-RAR moved rapidly in the nucleus, and ligand binding did not significantly affect their mobility. In contrast, estrogen binding decreased the mobility of GFP-ER and also increased the fraction of GFP-ER that was unable to diffuse. These effects were even more pronounced with tamoxifen. Co-transfection of the co-activator, SRC-1, further slowed the mobility of liganded GFP-ER. Our findings suggest estradiol and tamoxifen exert differential effects on the intranuclear mobility of GFP-ER. They also show that ligand-binding and protein-protein interactions can affect the intracellular mobility of some NRs and thereby may contribute to their biological activity.