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A C-terminal HSP90 inhibitor restores glucocorticoid sensitivity and relieves a mouse allograft model of Cushing disease

Abstract

One function of the glucocorticoid receptor (GR) in corticotroph cells is to suppress the transcription of the gene encoding proopiomelanocortin (POMC), the precursor of the stress hormone adrenocorticotropin (ACTH)1. Cushing disease is a neuroendocrine condition caused by partially glucocorticoid-resistant corticotroph adenomas that excessively secrete ACTH, which leads to hypercortisolism2,3,4. Mutations that impair GR function explain glucocorticoid resistance only in sporadic cases5,6. However, the proper folding of GR depends on direct interactions with the chaperone heat shock protein 90 (HSP90, refs. 7,8). We show here that corticotroph adenomas overexpress HSP90 compared to the normal pituitary. N- and C-terminal HSP90 inhibitors act at different steps of the HSP90 catalytic cycle to regulate corticotroph cell proliferation and GR transcriptional activity. C-terminal inhibitors cause the release of mature GR from HSP90, which promotes its exit from the chaperone cycle and potentiates its transcriptional activity in a corticotroph cell line and in primary cultures of human corticotroph adenomas. In an allograft mouse model, the C-terminal HSP90 inhibitor silibinin showed anti-tumorigenic effects, partially reverted hormonal alterations, and alleviated symptoms of Cushing disease. These results suggest that the pathogenesis of Cushing disease caused by overexpression of heat shock proteins and consequently misregulated GR sensitivity may be overcome pharmacologically with an appropriate HSP90 inhibitor.

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Figure 1: Human corticotroph adenomas overexpress heat shock proteins; HSP90 inhibitors reduce AtT-20 cell proliferation.
Figure 2: Silibinin promotes the release of mature GR in AtT-20 cells through direct binding to the C-terminal domain of Hsp90.
Figure 3: Silibinin enhances GR activity in AtT-20 cells.
Figure 4: Effects of silibinin on primary cultures of human corticotroph adenomas and on a mouse allograft model of Cushing disease.

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Acknowledgements

This work was partly supported by a grant from the German Research Foundation (SFB1035 to M.S.), the Bayerisches Staatsministerium für Wirtschaft, Infrastruktur, Verkehr und Technologie (m4 Award to F.H. and M.P.-P.), Federal Ministry of Education and Research (BMBF; PersoMed to M.P.-P. and F.H.); and postdoctoral fellowships to L.F. from the European Commission (FP7-PEOPLE-20112011-IIF 301193, Hsp90NMR) and the European Molecular Biology Organization (EMBO ALTF 1255-2011). The expression plasmids for the HSP90-α CTD and GR were gifts from U. Hartl (Max Planck Institute of Biochemistry) and S. Simons Jr. (US National Institutes of Health), respectively. We thank J. Stalla and T. Kloss for technical assistance. We thank M. Theodoropoulou and K. Lucia for valuable comments on the manuscript.

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M.R. and M.P.-P. conceived of and designed the experiments. M.B. collected biopsies and diagnosed subjects. M.R., C.K., L.F., and M.P.-P. carried out experiments. M.R., M.S., F.H., G.K.S., and M.P.-P. analyzed the data. M.R., F.H., G.K.S., and M.P.-P. wrote the manuscript.

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Correspondence to Marcelo Paez-Pereda.

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Riebold, M., Kozany, C., Freiburger, L. et al. A C-terminal HSP90 inhibitor restores glucocorticoid sensitivity and relieves a mouse allograft model of Cushing disease. Nat Med 21, 276–280 (2015). https://doi.org/10.1038/nm.3776

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