FK506-binding protein 51 regulates nuclear transport of the glucocorticoid receptor beta and glucocorticoid responsiveness.
OBJECTIVE
A spliced variant of the human glucocorticoid receptor GRbeta has been implicated in glucocorticoid responsiveness in glaucoma. Over-expression of the FK506-binding immunophilin FKBP51 also causes a generalized state of glucocorticoid resistance. In the present study, the roles of FKBP51 in the nuclear transport of GRbeta and glucocorticoid responsiveness were investigated.


METHODS
Human trabecular meshwork cells (GTM3 and TM5) and HeLa cells were treated with dexamethasone (DEX) and FK506 and transfected with GRbeta and FKBP51 expression vectors. Coimmunoprecipitation and Western blot analyses were performed to study interactions of FKBP51 and FKBP52 with GRalpha, GRbeta, Hsp90, or dynein. The cells were transfected with a GRE-luciferase reporter to evaluate the effects of DEX and FK506 and the overexpression of GRbeta and FKBP51 on glucocorticoid-mediated gene expression.


RESULTS
FKBP51 was involved in constitutive nuclear transport of both GRalpha and -beta in the absence of ligands. FKBP52 appeared to be solely responsible for the nuclear transport of ligand-activated GRalpha. DEX stimulated the translocation of GRalpha but not GRbeta. Overexpression of either GRbeta or FKBP51 stimulated GRbeta translocation and reduced DEX-induced luciferase in HeLa cells. FK506 did not alter DEX-induced translocation of GRalpha. However, FK506 increased the association of FKBP51 with GRbeta and stimulated DEX-induced translocation of GRbeta in normal TM cells, but not in glaucoma TM cells. Increased nuclear GRbeta significantly inhibited glucocorticoid responsiveness in TM cells.


CONCLUSIONS
Nuclear transport of GRbeta represents a novel mechanism through which FKBP51 alters GC sensitivity. GRbeta and FKBP51 may be responsible for increased responsiveness in steroid-induced ocular hypertensive individuals as well as in patients with glaucoma.
Zhang
Xinyu
X
Department of Pharmacology and Neuroscience, University of North Texas Health Science Center, Fort Worth, Texas 76107, USA.
Clark
Abbot F
AF
Yorio
Thomas
T
D021581:Active Transport, Cell Nucleus; D015153:Blotting, Western; D002467:Cell Nucleus / Q000378:metabolism*; D003907:Dexamethasone / Q000494:pharmacology*; D004398:Dyneins / Q000378:metabolism; D019084:Fluorescent Antibody Technique, Indirect; D005822:Genetic Vectors; D005938:Glucocorticoids / Q000494:pharmacology*; D018841:HSP90 Heat-Shock Proteins / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D047468:Immunoprecipitation; D010957:Plasmids; D011965:Receptors, Glucocorticoid / Q000378:metabolism*; D016559:Tacrolimus / Q000494:pharmacology; D022021:Tacrolimus Binding Proteins / Q000502:physiology*; D014129:Trabecular Meshwork / Q000187:drug effects* / Q000378:metabolism; D014162:Transfection; D000099159:Tacrolimus Binding Protein 5
D003907:Dexamethasone; D004398:Dyneins; D005938:Glucocorticoids; D018841:HSP90 Heat-Shock Proteins; D011965:Receptors, Glucocorticoid; D016559:Tacrolimus; D022021:Tacrolimus Binding Proteins; D000099159:Tacrolimus Binding Protein 5; C411470:glucocorticoid receptor alpha; C121943:glucocorticoid receptor beta; C403485:tacrolimus binding protein 4