Regulation of glucocorticoid receptor function by associated TPR-domain proteins

We have identified a cooperative relationship between the glucocorticoid receptor (GR) and the TPR-domain proteins associated with the cytoplasmic GR heterocomplex. Binding of hormone induced substitution of one immunophilin (FKBP51) for another (FKBP52). Immunofluorescence and fractionation revealed hormone-induced translocation of the hormone-generated complex, consisting of GR, Hsp90, FKBP52, and dynein, from cytoplasm to nucleus. This nuclear complex can then dissociate and convert to the DNA-binding form. These studies identify immunophilin interchange as the earliest known event in steroid receptor signaling and provide the first evidence of differential roles for FKBP51 and FKBP52 immunophilins in the control of steroid receptor subcellular localization and transport. Having demonstrated GR control of associated TPR proteins, we used immunoligands to affect them directly. FK506 was found to stimulate GR transactivity beyond the effect of this ligand on hormone retention at equal intracellular steroid concentration. In contrast, CsA only affected the GR through up-regulation of hormone retention and no GR-associated Cyp40 was detected. FK506 was found to increase GR hormone-binding affinity while decreasing total binding sites. This result correlated with replacement of GR-associated FKBPs with PP5, suggesting that FK506 can target GR signaling by alterating TPR protein composition. Up-regulation of FKBP52 was found to increase GR hormone-binding affinity to a greater degree than FK506 alone, while preventing the decrease in total binding sites. FKBP52 only increased GR transactivity at intermediate hormone concentrations. Taken as a whole, these results show that immunosuppressive ligands and TPR proteins can regulate GR control of TPR protein composition of receptor complexes.