C-terminal Hsp90-inhibitors restore glucocorticoid sensitivity in Cushing's disease

Cushing's disease is a severe neuroendocrine condition caused by partially glucocorticoid (Gc) resistant corticotroph adenomas of the anterior pituitary which express a functional glucocorticoid receptor (GR). The adenomas excessively secrete adrenocorticotropin (ACTH) into the bloodstream, and the patients suffer from chronic hypercortisolism. The pathomechanism of the partial Gc-resistance in Cushing's disease is scarcely understood and no safe and efficacious pharmacologic treatment to restore Gc-sensitivity in corticotroph adenomas exists. The molecular chaperone Hsp90 is essential for the function of the GR, and aberrant expression levels of Hsp90 have a negative impact on GR-signaling. We show by immunohistochemical staining that the inducible Hsp90α-isoform is strongly overexpressed in biopsy specimens of corticotroph adenomas from patients with Cushing's disease as compared to normal pituitary tissue. To investigate the role of Hsp90 in the GR-signaling of corticotroph cells, we pharmacologically inhibited Hsp90 with small-molecule compounds. The N-terminal Hsp90-inhibitor 17-AAG induces GR protein degradation and abolishes all aspects of GR-function. In sharp contrast, Novobiocin and the recently identified C-terminal Hsp90-inhibitor MPP-482 potentiate the activity of the agonist-bound GR. This effect correlates with the dissociation of the GR::Hsp90-complex by the C-terminal Hsp90-inhibitors at the biochemical level, while the GR protein level and the Hsp90-dependent nucleocytoplasmatic transport of the agonist-bound GR remain unaffected. Finally, MPP-482 enhances the suppression of ACTH-production by the GR in primary cultures of human corticotroph adenomas, restoring Gc-sensitivity. We show here that C-terminal Hsp90-inhibitors potentially reverse the Gc-resistance in human corticotroph adenoma cells by a novel mechanism of action. This work presents the proof-of-concept for the treatment of patients with Cushing's disease with the clinically safe Hsp90-inhibitor MPP-482 in the future.