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Members of the nuclear receptor superfamily play key roles in a host of physiologic and pathologic processes from embryogenesis to cancer. Some members, including the retinoic acid receptor (RAR), are activated by ligand binding but are unaffected in their subcellular distribution, which is predominantly nuclear. In contrast, several members of the steroid receptor family, including the glucocorticoid receptor, are cytoplasmic and only translocate to the nucleus after ligand binding. We have constructed chimeras between RAR and glucocorticoid receptor that selectively respond to RAR agonists but display cytoplasmic localization in the absence of ligand. These chimeric receptors manifest both nuclear translocation and gene activation functions in response to physiological concentrations of RAR ligands. The ability to achieve regulated subcellular trafficking with a heterologous ligand binding domain has implications both for current models of receptor translocation and for structural-functional conservation of ligand binding domains broadly across the receptor superfamily. When coupled to the green fluorescent protein, chimeric receptors offer a powerful new tool to 1) study mechanisms of steroid receptor translocation, 2) detect dynamic and graded distributions of ligands in complex microenvironments such as embryos, and 3) screen for novel ligands of "orphan" receptors in vivo.
Mackem et al. (Sat,) studied this question.