Therapeutic Targeting of the FKBP52 Co-Chaperone in Steroid Hormone Receptor-Regulated Physiology and Disease.

Steroid hormone receptors are ligand-dependent transcription factors that require the dynamic, ordered assembly of multimeric chaperone complexes to reach a functional conformation. Heat shock protein (Hsp) 70 and Hsp90 serve as the central chaperones that mediate this process in conjunction with a variety of co-chaperones. Many of these cochaperones represent potential therapeutic targets for the disruption of Hsp90 client protein function. FKBP52 is an Hsp90-associated co-chaperone that has emerged as a promising therapeutic candidate due to its functional specificity for a small subset of Hsp90 client proteins including androgen (AR), glucocorticoid (GR), and progesterone (PR) receptors. Given its Hsp90-client protein specificity, the targeting of FKBP52 should be more specific and less toxic than the Hsp90- targeting drugs. Additionally, the fkbp52-deficient mice display specific phenotypes related to androgen, progesterone, and glucocorticoid insensitivity suggesting minimal off-target effects. Finally, the fact that FKBP52 is already a validated target of the clinically approved immunosuppressive drug, FK506 (Tacrolimus), indicates that FKBP52 is a "druggable" protein. Thus, the development of FKBP52-specific small molecule inhibitors is predicted to be a highly targeted strategy with potential for the treatment of any disease that is dependent on a functional AR, GR, and/or PR signaling pathway. Much progress has been made in understanding the residues and domains critical for FKBP52 function. The proline-rich loop overhanging the FKBP52 FK1 catalytic domain is functionally important and likely represents an interaction surface within the receptor-chaperone complex. Thus, the targeting of FKBP52 proline-rich loop interactions is the most attractive therapeutic approach to disrupt FKBP52 regulation of receptor activity in steroid hormone receptor-dependent physiology and disease.