Abstract
We have analyzed structural and functional features of the human glucocorticoid receptor (hGR) for their effects on receptor subcellular distribution. COS 1 cells transiently transfected with wild type and mutant hGR cDNAs were assessed immunocytochemically using well-characterized antipeptide antibodies to the hGR. The effect of administration of steroid hormones (and the antiglucocorticoid RU486) on receptor localization was evaluated. Unliganded wild type receptors expressed in COS 1 cells were predominately cytoplasmic. Addition of glucocorticoids or the glucocorticoid receptor antagonist, RU486, resulted in complete translocation of these receptors into the nucleus whereas non-glucocorticoid steroids or dibutyryl cAMP were not effective in promoting nuclear translocation. Thus, nuclear translocation was specific for steroids capable of high affinity binding to the hGR. To elucidate the potential role of receptor domains in receptor localization, COS 1 cells transiently transfected with various receptor cDNA mutants were analyzed in a similar manner. Translocation of an hGR deletion mutant lacking the majority of the amino terminus (deletion of amino acids 77–262) was identical to the wild type receptor despite the absence of a transactivation domain. Receptors in which the DNA binding domain was either partially or totally deleted showed an impaired capacity to undergo hormone-inducible nuclear translocation. Deletion of the hinge region of the hGR (which also contains part of the nuclear localization signal, NL1) resulted in receptor localization in the cytoplasm. Mutants in the ligand binding domain exhibited two localization phenotypes, exclusively nuclear or cytoplasmic. Receptor mutants truncated after amino acid 550 were found in the nucleus in the presence and absence of hormone consistent with the existence of nuclear localization inhibitory sequences in the ligand binding domain of the receptor. However, a linker insertion mutant (at amino acid 582) which results in a receptor deficient in ligand binding did not undergo nuclear translocation indicating that nuclear localization inhibitory sequences were intact in this mutant. The role of receptor phosphorylation on hormone induced nuclear translocation was also examined. Mouse glucocorticoid receptors which contained mutations of certain hormone inducible phosphorylation sites exhibited translocation properties similar to wild type mGR indicating that these phosphorylation sites on the receptor do not play a major role in hormone inducible nuclear translocation.
