Targeted ablation reveals a novel role of FKBP52 in gene-specific regulation of glucocorticoid receptor transcriptional activity

Abstract

FKBP52 is a tetratricopeptide repeat (TPR) protein with peptidyl-prolyl isomerase activity and is found in steroid receptor complexes, including glucocorticoid receptor (GR). It is generally accepted that FKBP52 has a stimulatory effect on GR transcriptional activity. However, the mechanism by which FKBP52 controls GR is not yet clear, with reports showing effects on GR hormone-binding affinity and/or hormone-induced nuclear translocation. To address this issue, we have generated mice with targeted ablation of the FKBP52 gene. To date, no overt defects of GR-regulated physiology have been found in these animals, demonstrating that FKBP52 is not an essential regulator of global GR activity. To better assess the impact of FKBP52 on GR, mouse embryonic fibroblasts (MEFs) were generated from wild-type (WT) and FKBP52-deficient (KO) animals. Analysis of GR activity at reporter genes showed an approximate 70% reduction of activity in 52KO MEF cells, with no effect of FKBP52 loss on thyroid receptor. Interestingly, GR activity at endogenous genes was not globally affected in 52KO cells, with reduced activity at GILZ and FKBP51, but not at SGK and p21. Thus, FKBP52 appears to be a gene-specific modulator of GR. To investigate the mechanism of this action, analyses of GR heterocomplex composition, hormone-binding affinity, and ability to undergo hormone-induced nuclear translocation and DNA-binding were performed. Interestingly, no effect of FKBP52 loss was found for any of these GR properties, suggesting that the main function of FKBP52 is a heretofore-unknown ability to control GR activity at target genes. Lastly, loss of FKBP52 did not affect the ability of GR to undergo hormone-induced autologous down-regulation, showing that FKBP52 does not contribute to all branches of GR signaling. The implications of these results to the potential actions of FKBP52 on GR activity in vivo are discussed.

Dose- and time-dependent glucocorticoid receptor signaling in podocytes

“…Although none of the treatments altered the GR mRNA level to a significant extent, the amount of GR protein decreased markedly after 1 day of either treatment, suggesting the activity of posttranscriptional processes such as GR degradation or diminished translation. Such posttranscriptionally autologous downregulation of the GR represents a negative feedback mechanism that has also been observed in other cells, e.g., in mouse embryonic fibroblasts, and for other genes, e.g., the cyclooxygenase-2 (15,41). 7.…”

“…In addition, we demonstrated the presence of mRNA encoding synaptopodin, a marker of podocyte differentiation (18,44). We also demonstrated the expression of genes known to be regulated by GC in rodents and/or primates which served as reporters for GC signaling, including the phenol sulfotransferase 1 (PST1) and the small heat shock protein ␣B-Cry, in addition to FKBP51 and the GR themselves (1,4,34,41). The expression of corresponding proteins was confirmed by SDS-PAGE followed by Western blotting (Fig.…”

The hsp90-FKBP52 Complex Links the Mineralocorticoid Receptor to Motor Proteins and Persists Bound to the Receptor in Early Nuclear Events

“…Among the members of this subfamily, FKBP52 and FKBP51 seem to compete not only for their binding to hsp90 but also for the functional consequences of that interaction. Thus, FKBP51 represses GR, PR, MR, and Aldo receptor (AR) steroid-binding capacity and transcriptional activity, whereas FKBP52 shows no significant effect on steroid receptor action or a slight activation effect if the amount of endogenous FKBP52 is sufficient (6,21,24,49,66,67). The experiment shown in Fig.…”

“…More recently, analyses of GR heterocomplex composition, hormone-binding affinity, and ability to undergo hormone-induced nuclear translocation and DNA binding were performed, and no effect of FKBP52 loss was found for these GR properties (67). Consequently, the exact mechanistic contribution of TPR proteins to steroid receptor function remains controversial.…”

Protein Phosphatase 5 Mediates Lipid Metabolism through Reciprocal Control of Glucocorticoid Receptor and Peroxisome Proliferator-activated Receptor-γ (PPARγ)

“…Hormone-free GR␣ in most cells preferen-tially interacts with FKBP51 and PP5 with a reduced presence of FKBP52 (15). Despite this, FKBP52 is still needed as a positive regulator of GR␣ to control its transcriptional activity in a genespecific manner (16), whereas most studies suggest that FKBP51 and PP5 are negative regulators of GR␣ (17)(18)(19). In contrast to GR␣, very little is known concerning Hsp90 and TPR protein involvement with PPARs.…”