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Introduction. The glucocorticoid receptor is pivotal to control corticotrophin (ACTH) secretion and its function is closely linked to the heat shock protein 90 (HSP90) chaperone complex. Impaired sensitivity to glucocorticoid feedback is a hallmark of human corticotroph adenomas, i.e., Cushing’s disease, a disorder with few medical treatment options. Silibinin, a HSP90 inhibitor, has been studied in tumoral corticotroph cells and its use proposed in Cushing’s disease. Aim of the present study was to further investigate the effect of silibinin on human corticotroph adenomas in vitro. Methods. Seven human ACTH-secreting pituitary adenomas were established in culture and treated with 10 – 50 µM silibinin with/without dexamethasone for up to 72 hours. ACTH medium levels were measured and POMC and glucocorticoid receptor, i.e., NR3C1, gene expression assessed. Results. Silibinin reduced spontaneous ACTH secretion and restored sensitivity to steroid negative feedback to a different extent in individual adenomas. POMC expression was decreased both in control and dexamethasone-treated wells in specimens sensitive to silibinin. Interestingly, silibinin reduced constitutive NR3C1 expression and reversed the dexamethasone-induced inhibition. Conclusions. Our findings indicate that silibinin can inhibit ACTH synthesis and secretion in individual human corticotroph adenomas and directly affects NR3C1 gene expression. These results reveal promising effects of this HSP90 inhibitor on human corticotroph adenomas and support an innovative target treatment for patients with Cushing’s disease.

Background: Somatic mutations in the ubiquitin-specific protease 8 (USP8) gene have recently been shown to occur in ACTH-secreting pituitary adenomas, thus calling attention to the ubiquitin system in corticotrope adenomas. Objectives: Assess the consequences of USP8 mutations and establish the role of ubiquitin on ACTH turnover in human ACTH-secreting pituitary adenomas. Methods: USP8 mutation status was established in 126 ACTH-secreting adenomas. Differences in ACTH secretion and POMC expression from adenoma primary cultures and in microarray gene expression profiles from archival specimens were sought according to USP8 sequence. Ubiquitin/ACTH coimmunoprecipitation and incubation with MG132, a proteasome inhibitor, were performed in order to establish whether ubiquitin plays a role in POMC/ACTH degradation in corticotrope adenomas. Results: USP8 mutations were identified in 29 adenomas (23%). Adenomas presenting USP8 mutations secreted greater amounts of ACTH and expressed POMC at higher levels compared to USP wild-type specimens. USP8 mutant adenomas were also more sensitive to modulation by CRH and dexamethasone in vitro. At microarray analysis, genes associated with endosomal protein degradation and membrane components were downregulated in USP8 mutant adenomas as were AVPR1B, IL11RA, and PITX2. Inhibition of the ubiquitin-proteasome pathway increased ACTH secretion and POMC itself proved a target of ubiquitylation, independently of USP8 sequence status. Conclusions: Our study has shown that USP8 mutant ACTH-secreting adenomas present a more “typical” corticotrope phenotype and reduced expression of several genes associated with protein degradation. Further, ubiquitylation is directly involved in intracellular ACTH turnover, suggesting that the ubiquitin-proteasome system may represent a target for treatment of human ACTH-secreting adenomas.