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X. Zhang, A.F. Clark, T. Yorio; FK506–Binding Protein 51 Regulates Nuclear Transport of the Glucocorticoid Receptor Beta and Glucocorticoid Responsiveness . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4767.
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Previously, we reported that glucocorticoid receptor ß (GRß), was associated with glucocorticoid (GC) hyper–responsiveness in glaucoma. We also demonstrated that heat shock protein 90 (Hsp90) served as a chaperone in the nuclear import of GRß. FK506–binding protein 51 (FKBP51), a Hsp90 binding immunophilin believed to play a role in activation of GRα, may also participate in GC sensitivity. In this work, we investigated the effect of FKBP51 on the nuclear transport of GRß and GC responssiveness in human trabecular meshwork (TM) cell lines and in HeLa cells.
We investigated the protein–protein interactions among GRα, GRß, immunophilins, Hsp90, and dynein by co–immunoprecipitation and western immunoblotting. Co–chaperone activity of FKBP51 in the nuclear transport of GRß was studied by transient transfection in the presence or absence of FK506. Confocal microscopy was used to study the subcellular distribution of the multi–protein heterocomplex consisting of GRß–Hsp90–FKBP51, with and without the Hsp90 inhibitor 17–AAG. A GRE–luciferase reporter assay was performed to study cellular sensitivity to dexamethasone (DEX) by overexpression of GRß and FKBP51.
Our data demonstrated that both FKBP51 and FKBP52 were complexed with GRα. DEX treatment caused nuclear translocation of the GRα–FKBP52 complex. In contrast, only FKBP51 complexed with GRß, which was not regulated by DEX. Immunoprecipitation with anti–FKBP51 antibody also pulled down Hsp90 and dynein. Overexpression of FKBP51 increased the association of GRß with FKBP51. 17–AAG blocked the nuclear import of Hsp90 and also excluded FKBP51 from the nucleus. These data support the idea that FKBP51 was a co–chaperone for nuclear transport of GRß. FK506 treatment did not change DEX–induced nuclear translocation of FKBP52–GRα, while increasing nuclear accumulation of FKBP51–GRß in normal HTM–5 cells, but not in glaucomatous GTM–3 cells. Western Blot analysis showed a differential expression pattern of FKBP51 between HTM–5 and GTM–3 cells. These data are consistent with the reporter gene assay that FK506 decreased DEX– induced luciferase in HTM–5 cells, but not in GTM–3 cells.
Our data demonstrate that FKBP51, but not FKBP52, is involved in constitutive nuclear transport of GRα and GRß, which may represent a novel mechanism through which FKBP51 causes GC resistance. The differential expression of FKBP51 in glaucomatous TM cells could disrupt the constitutive nuclear import of GRß and contribute to the low nuclear expression of GRß and the hyper–responsiveness to GC that we previously reported .
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