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

Inhibition of Heat Shock Factor 1 Enhances Repressive Molecular Mechanisms on the POMC Promoter

BACKGROUND

Cushing's disease (CD) is caused by adrenocorticotropic hormone (ACTH)-secreting pituitary tumours. They express high levels of heat shock protein 90 and heat shock factor 1 (HSF1) in comparison to the normal tissue counterpart, indicating activated cellular stress.

AIMS

Our objectives were: (1) to correlate HSF1 expression with clinical features and hormonal/radiological findings of CD, and (2) to investigate the effects of HSF1 inhibition as a target for CD treatment.

PATIENTS/METHODS

We examined the expression of total and pSer326HSF1 (marker for its transcriptional activation) by Western blot on eight human CD tumours and compared to the HSF1 status of normal pituitary. We screened a cohort of 45 patients with CD for HSF1 by immunohistochemistry and correlated the HSF1 immunoreactivity score with the available clinical data. We evaluated the effects of HSF1 silencing with RNA interference and the HSF1 inhibitor KRIBB11 in AtT-20 cells and four primary cultures of human corticotroph tumours.

RESULTS

We show that HSF1 protein is highly expressed and transcriptionally active in CD tumours in comparison to normal pituitary. The immunoreactivity score for HSF1 did not correlate with the typical clinical features of the disease. HSF1 inhibition reduced proopiomelanocortin (Pomc) transcription in AtT-20 cells. The HSF1 inhibitor KRIBB11 suppressed ACTH synthesis from 75% of human CD tumours in primary cell culture. This inhibitory action on Pomc transcription was mediated by increased glucocorticoid receptor and suppressed Nurr77/Nurr1 and AP-1 transcriptional activities.

CONCLUSIONS

These data show that HSF1 regulates POMC transcription. Pharmacological targeting of HSF1 may be a promising treatment option for the control of excess ACTH secretion in CD.

Heat Shock Protein 90 as a Prognostic Marker and Therapeutic Target for Adrenocortical Carcinoma

Background: Adrenocortical carcinoma (ACC) is a rare tumor entity with restricted therapeutic opportunities. HSP90 (Heat Shock Protein 90) chaperone activity is fundamental for cell survival and contributes to different oncogenic signaling pathways. Indeed, agents targeting HSP90 function have shown therapeutic efficacy in several cancer types. We have examined the expression of HSP90 in different adrenal tumors and evaluated the use of HSP90 inhibitors in vitro as possible therapy for ACC. Methods: Immunohistochemical expression of HSP90 isoforms was investigated in different adrenocortical tumors and associated with clinical features. Additionally, a panel of N-terminal (17-allylamino-17-demethoxygeldanamycin (17-AAG), luminespib, and ganetespib) and C-terminal (novobiocin and silibinin) HSP90 inhibitors were tested on various ACC cell lines. Results: Within adrenocortical tumors, ACC samples exhibited the highest expression of HSP90β. Within a cohort of ACC patients, HSP90β expression levels were inversely correlated with recurrence-free and overall survival. In functional assays, among five different compounds tested luminespib and ganetespib induced a significant decrease in cell viability in single as well as in combined treatments with compounds of the clinically used EDP-M scheme (etoposide, doxorubicin, cisplatin, mitotane). Inhibition of cell viability correlated furthermore with a decrease in proliferation, in cell migration and an increase in apoptosis. Moreover, analysis of cancer pathways indicated a modulation of the ERK1/2-and AKT-pathways by luminespib and ganetespib treatment. Conclusions: Our findings emphasize HSP90 as a marker with prognostic impact and promising target with N-terminal HSP90 inhibitors as drugs with potential therapeutic efficacy toward ACC.

Gene expression profiling in human corticotroph tumours reveals distinct, neuroendocrine profiles

Adrenocorticotrophic hormone (ACTH)-secreting pituitary adenomas give rise to a severe endocrinological disorder, comprising Cushing's disease, with multifaceted clinical presentation and treatment outcomes. Experimental studies suggest that the disease variability is inherent to the pituitary tumour, thus indicating the need for further studies into tumour biology. The present study evaluated transcriptome expression pattern in a large series of ACTH-secreting pituitary adenoma specimens in order to identify molecular signatures of these tumours. Gene expression profiling of formalin-fixed, paraffin-embedded specimens from 40 human ACTH-secreting pituitary adenomas revealed the significant expression of genes involved in protein biosynthesis and ribosomal function, in keeping with the neuroendocrine cell profile. Unsupervised cluster analysis identified 3 distinct gene profile clusters and several genes were uniquely overexpressed in a given cluster, accounting for different molecular signatures. Of note, gene expression profiles were associated with clinical features, such as the age and size of the tumour. Altogether, the findings of the present study show that corticotroph tumours are characterised by a neuroendocrine gene expression profile and present subgroup-specific molecular features.

Recent advances in understanding corticotroph pituitary tumor initiation and progression

Cushing’s disease is the most frequent form of hypercortisolism and is caused by hypophyseal corticotroph adenomas secreting excessive amounts of adrenocorticotropic hormone. Most of the tumors develop sporadically and only a limited number of corticotroph adenomas have been found to be associated with different neuroendocrine syndromes or with familial isolated pituitary adenomas. The pathogenic mechanisms of corticotroph adenomas are largely unknown, but the discovered aberrant chaperoning activity of heat shock protein 90 on the one hand and the presence of ubiquitin-specific protease 8 mutations on the other hand partially explained the causes of their development. Corticotroph tumors arise initially as benign microadenomas but with time form invasively growing aggressive macroadenomas which can switch to corticotroph carcinomas in extremely rare cases. The mechanisms through which corticotroph tumors escape from glucocorticoid negative feedback are still poorly understood, as are the processes that trigger the progression of benign corticotroph adenomas toward aggressive and malignant phenotypes. This review summarizes recent findings regarding initiation and progression of corticotroph pituitary tumors.

Identification of recurrent USP48 and BRAF mutations in Cushing's disease

Cushing's disease results from corticotroph adenomas of the pituitary that hypersecrete adrenocorticotropin (ACTH), leading to excess glucocorticoid and hypercortisolism. Mutations of the deubiquitinase gene USP8 occur in 35-62% of corticotroph adenomas. However, the major driver mutations in USP8 wild-type tumors remain elusive. Here, we report recurrent mutations in the deubiquitinase gene USP48 (predominantly encoding p.M415I or p.M415V; 21/91 subjects) and BRAF (encoding p.V600E; 15/91 subjects) in corticotroph adenomas with wild-type USP8. Similar to USP8 mutants, both USP48 and BRAF mutants enhance the promoter activity and transcription of the gene encoding proopiomelanocortin (POMC), which is the precursor of ACTH, providing a potential mechanism for ACTH overproduction in corticotroph adenomas. Moreover, primary corticotroph tumor cells harboring BRAF V600E are sensitive to the BRAF inhibitor vemurafenib. Our study thus contributes to the understanding of the molecular mechanism of the pathogenesis of corticotroph adenoma and informs therapeutic targets for this disease.

PATHOGENESIS OF CUSHING DISEASE: AN UPDATE ON THE GENETICS OF CORTICOTROPINOMAS

OBJECTIVE

Cushing disease is a rare severe condition caused by pituitary tumors that secrete adrenocorticotropic hormone (ACTH), leading to excessive endogenous glucocorticoid production. Tumors causing Cushing disease, also called corticotropinomas, are typically monoclonal neoplasms that mainly occur sporadically.

METHODS

Literature review.

RESULTS

Cushing disease is very rarely encountered in genetic familial syndromes. Oncogenes and tumor suppressor genes commonly associated with other tumor types are only rarely mutated in this tumor type. The advent of next-generation sequencing led to the identification of a single mutational hotspot in the ubiquitin-specific protease 8 ( USP8) gene in almost half of Cushing disease tumors.

CONCLUSION

The new discoveries showcase a novel mechanism responsible for corticotroph tumorigenesis and ACTH hypersecretion and highlight USP8 and its downstream signaling pathways as potential promising pharmacologic targets for the management of Cushing disease.

ABBREVIATIONS

ACTH = adrenocorticotropic hormone; BRG1 = Brahma-related gene 1; CABLES1 = CDK5 and ABL1 enzyme substrate 1; CD = Cushing disease; CNC = Carney complex; DICER1 = cytoplasmic endoribonuclease III; EGFR = epidermal growth factor receptor; GR = glucocorticoid receptor; IL = interleukin; MEN = multiple endocrine neoplasia; miRNA = microRNA; POMC = proopiomelanocortin; SSTR = somatostatin receptor; USP8 = ubiquitin-specific protease 8.

New and emerging drug therapies for Cushing's disease

INTRODUCTION

Cushing's disease is a rare systemic and disabling disease due to oversecretion of adrenocorticotrophic hormone (ACTH) resulting in excess cortisol levels. Diagnosis and treatment are difficult; despite the availability of various pharmaceutical treatment options, there is an ongoing, unmet need for even more effective treatment.

AREAS COVERED

The present review aims at providing an overview of available drugs and presenting new developments. Focusing on the pituitary as a target, the review covers compounds targeting pituitary cell signaling or cell cycle control such as heat shock protein inhibitors (e.g. silibinin), histone deacetylase inhibitors (trichostatin A, vorinostat), kinase inhibitors (gefitinib, seliciclib), and others (such as triptolide, AT-101). Levoketoconazole and osilodrostat are in clinical testing and inhibit steroidogenesis. Blockade of ACTH receptor binding at the adrenal level is explained as a theoretical drug target. Inhibition of binding of the glucocorticoid receptor in the peripheral tissue plays a minor role due to its lack of biomonitoring options.

EXPERT OPINION

In our opinion, further research and drug development of pituitary-directed targets are necessary. Combination therapies may exert synergistic effects and allow for smaller and better tolerated doses, but more experience and data are needed to guide such treatment schemes.

New Molecular Targets for Treatment of Cushing's Disease

Despite the best outcomes from trans-sphenoidal surgery, approximately one-third of patients with Cushing's disease will need medical therapy. Current treatments have drawbacks and there is a clear clinical need for new therapies. Recent understanding of molecular pathways leading to excess ACTH secretion has identified key components that may be targeted with the aim to provide novel effective treatment for this devastating disease. These include testicular orphan nuclear receptor 4, heat shock protein 90, and epidermal growth factor receptor. Based on data from preclinical studies, clinical trials are seeking to assess whether targeting these novel pathways can translate into patient benefit.

Proteomic characterization of Withaferin A-targeted protein networks for the treatment of monoclonal myeloma gammopathies

Withaferin A (WA), a natural steroid lactone from the plant Withania somnifera, is often studied because of its antitumor properties. Although many in vitro and in vivo studies have been performed, the identification of Withaferin A protein targets and its mechanism of antitumor action remain incomplete. We used quantitative chemoproteomics and differential protein expression analysis to characterize the WA antitumor effects on a multiple myeloma cell model. Identified relevant targets were further validated by Ingenuity Pathway Analysis and Western blot and indicate that WA targets protein networks that are specific for monoclonal gammopathy of undetermined significance (MGUS) and other closely related disorders, such as multiple myeloma (MM) and Waldenström macroglobulinemia (WM). By blocking the PSMB10 proteasome subunit, downregulation of ANXA4, potential association with HDAC6 and upregulation of HMOX1, WA puts a massive blockage on both proteotoxic and oxidative stress responses pathways, leaving cancer cells defenseless against WA induced stresses. These results indicate that WA mediated apoptosis is preceded by simultaneous targeting of cellular stress response pathways like proteasome degradation, autophagy and unfolded protein stress response and thus suggests that WA can be used as an effective treatment for MGUS and other closely related disorders.

SIGNIFICANCE

Multifunctional antitumor compounds are of great potential since they reduce the risk of multidrug resistance in chemotherapy. Unfortunately, characterization of all protein targets of a multifunctional compound is lacking. Therefore, we optimized an SILAC quantitative chemoproteomics workflow to identify the potential protein targets of Withaferin A (WA), a natural multifunctional compound with promising antitumor properties. To further understand the antitumor mechanisms of WA, we performed a differential protein expression analysis and combined the altered expression data with chemoproteome WA target data in the highly curated Ingenuity Pathway database. We provide a first global overview on how WA kills multiple myeloma cancer cells and serve as a starting point for further in depth experiments. Furthermore, the combined approach can be used for other types of cancer and/or other promising multifunctional compounds, thereby increasing the potential development of new antitumor therapies.

Multi-chaperone function modulation and association with cytoskeletal proteins are key features of the function of AIP in the pituitary gland

Despite the well-recognized role of loss-of-function mutations of the aryl hydrocarbon receptor interacting protein gene (AIP) predisposing to pituitary adenomas, the pituitary-specific function of this tumor suppressor remains an enigma. To determine the repertoire of interacting partners for the AIP protein in somatotroph cells, wild-type and variant AIP proteins were used for pull-down/quantitative mass spectrometry experiments against lysates of rat somatotropinoma-derived cells; relevant findings were validated by co-immunoprecipitation and co-localization. Global gene expression was studied in AIP mutation positive and negative pituitary adenomas via RNA microarrays. Direct interaction with AIP was confirmed for three known and six novel partner proteins. Novel interactions with HSPA5 and HSPA9, together with known interactions with HSP90AA1, HSP90AB1 and HSPA8, indicate that the function/stability of multiple chaperone client proteins could be perturbed by a deficient AIP co-chaperone function. Interactions with TUBB, TUBB2A, NME1 and SOD1 were also identified. The AIP variants p.R304* and p.R304Q showed impaired interactions with HSPA8, HSP90AB1, NME1 and SOD1; p.R304* also displayed reduced binding to TUBB and TUBB2A, and AIP-mutated tumors showed reduced TUBB2A expression. Our findings suggest that cytoskeletal organization, cell motility/adhesion, as well as oxidative stress responses, are functions that are likely to be involved in the tumor suppressor activity of AIP.

Genetics of Cushing's disease

Cushing's disease (CD) is a rare disabling condition caused by Adrenocorticotropic hormone (ACTH)-secreting adenomas of the pituitary. The majority of corticotropic adenomas are monoclonal and occur sporadically. Only rarely does CD arise in the context of genetic familial syndromes. Targeted sequencing of oncogenes and tumour suppressor genes commonly mutated in other tumours did not identify recurrent mutations. In contrast, next generation sequencing allowed us recently to clarify the genetic basis of CD: we identified somatic driver mutations in the ubiquitin-specific protease 8 (USP8) gene in a significant portion of corticotropinomas. These mutations represent a novel and unique mechanism leading to ACTH excess. Inhibition of USP8 or its downstream signalling pathways could represent a new therapeutic approach for the management of CD. In this review, we will focus on this new evidence and its implication for clinical care of affected patients.

Pituitary-Directed Therapies for Cushing's Disease

Cushing's disease (CD) is caused by a pituitary corticotroph neuroendocrine tumor inducing uncontrolled hypercortisolism. Transsphenoidal surgery is the first-line treatment in most cases. Nonetheless, some patients will not achieve cure even in expert hands, others may not be surgical candidates and a significant percentage will experience recurrence. Many patients will thus require medical therapy to achieve disease control. Pharmacologic options to treat CD have increased in recent years, with an explosion in knowledge related to pathophysiology at the molecular level. In this review, we focus on medications targeting specifically pituitary adrenocorticotropic hormone-secreting tumors. The only medication in this group approved for the treatment of CD is pasireotide, a somatostatin receptor ligand. Cabergoline and temozolomide may also be used in select cases. Previously studied and abandoned medical options are briefly discussed, and emphasis is made on upcoming medications. Mechanism of action and available data on efficacy and safety of cell cycle inhibitor roscovitine, epidermal growth factor receptor inhibitor gefitinib, retinoic acid, and silibinin, a heat shock protein 90 inhibitor are also presented.

Recent advances in understanding Cushing disease: resistance to glucocorticoid negative feedback and somatic USP8 mutations

Cushing’s disease is a rare disease with a characteristic phenotype due to significant hypercortisolism driven by over-secretion of adrenocorticotropic hormone and to high morbidity and mortality if untreated. It is caused by a corticotroph adenoma of the pituitary, but the exact mechanisms leading to tumorigenesis are not clear. Recent advances in molecular biology such as the discovery of somatic mutations of the ubiquitin-specific peptidase 8 ( USP8) gene allow new insights into the pathogenesis, which could be translated into exciting and much-needed therapeutic applications.

Proopiomelanocortin, glucocorticoid, and CRH receptor expression in human ACTH-secreting pituitary adenomas

ACTH-secreting pituitary tumors are by definition partially autonomous, i.e., secrete ACTH independent of physiological control. However, only few, small-sized studies on proopiomelanocortin (POMC) and its regulation by corticotropin-releasing hormone (CRH) or glucocorticoids are available. Objective of the present study was to report on constitutive and CRH- and dexamethasone-regulated POMC, CRH (CRH-R1), and glucocorticoid receptor (NR3C1) gene expression in a large series of human corticotrope adenomas. Fifty-three ACTH-secreting adenomas were incubated with 10 nM CRH or 10 nM dexamethasone for 24 h. POMC, CRH-R1, NR3C1, and its alpha and beta isoforms were quantified and medium ACTH measured. Constitutive POMC expression proved extremely variable, with macroadenomas exhibiting higher levels than microadenomas. POMC increased during CRH in most specimens; conversely, changes induced by dexamethasone were varied, ranging from decrease to paradoxical increase. No correlation between POMC and ACTH was detected in any experimental condition. CRH-R1 expression was not linked to the response to CRH while NR3C1 was expressed at greater levels in specimens who failed to inhibit during dexamethasone; glucocorticoid receptor α was the more abundant isoform and subject to down-regulation by dexamethasone. Our results demonstrate a considerable variability in POMC expression among tumors and no correlation between POMC and ACTH, suggesting that POMC peptide processing/transport plays a major role in modulating ACTH secretion. Further, CRH-R1 and NR3C1 expression were not linked to the expected ligand-induced outcome, indicating that receptor signaling rather than abundance determines corticotrope responses. Our findings pave the way to new avenues of research into Cushing's disease pathophysiology.

Currently used and investigational drugs for Cushing´s disease

INTRODUCTION

Cushing's disease (CD) is caused by a corticotroph adenoma of the pituitary gland that secretes excess adrenocorticotropic hormone (ACTH) causing increased morbidity and mortality. Surgery is the treatment of choice, but is not always successful. Alternatives include radiotherapy, adrenal surgery, and pharmaceutical therapy. The latter is increasingly gaining momentum due to the recent development of compounds that reduce hypercortisolaemia or its symptoms, acting through different mechanisms. Areas covered: In this article, the authors provide a complete overview of the treatment options for Cushing´s disease, including adrenal-directed, tumor-targeted, and peripheral therapies that are currently used or in development, and discuss their potential advantages and limitations. Expert opinion: Considering the lack of long-term remission in up to half of the patients after surgery, and the delayed response to radiotherapy along with potential side effects, there is a strong need for an effective pharmaceutical treatment. Pasireotide, mifepristone, ketoconazole and metyrapone have been approved by regulatory authorities but their use remains limited due to considerable costs and side effects. Research in this field has focused recently on the improvement of pre-existing drugs and the development of safe new ones. However, few approaches aim at targeting the source of the disease, the ACTH-secreting adenoma.

Different cAMP sources are critically involved in G protein-coupled receptor CRHR1 signaling

Corticotropin-releasing hormone receptor 1 (CRHR1) activates G protein-dependent and internalization-dependent signaling mechanisms. Here, we report that the cyclic AMP (cAMP) response of CRHR1 in physiologically relevant scenarios engages separate cAMP sources, involving the atypical soluble adenylyl cyclase (sAC) in addition to transmembrane adenylyl cyclases (tmACs). cAMP produced by tmACs and sAC is required for the acute phase of extracellular signal regulated kinase 1/2 activation triggered by CRH-stimulated CRHR1, but only sAC activity is essential for the sustained internalization-dependent phase. Thus, different cAMP sources are involved in different signaling mechanisms. Examination of the cAMP response revealed that CRH-activated CRHR1 generates cAMP after endocytosis. Characterizing CRHR1 signaling uncovered a specific link between CRH-activated CRHR1, sAC, and endosome-based signaling. We provide evidence of sAC being involved in an endocytosis-dependent cAMP response, strengthening the emerging model of GPCR signaling in which the cAMP response does not occur exclusively at the plasma membrane and introducing the notion of sAC as an alternative source of cAMP.

60 YEARS OF POMC: Transcriptional and epigenetic regulation of POMC gene expression

Expression of the pro-opiomelanocortin (POMC) gene integrates numerous inputs that reflect the developmental history of POMC-expressing cells of the pituitary and hypothalamus, as well as their critical role in the endocrine system. These inputs are integrated at specific regulatory sequences within the promoter and pituitary or hypothalamic enhancers of the POMC locus. Investigations of developmental mechanisms and transcription factors (TFs) responsible for pituitary activation of POMC transcription led to the discovery of the Pitx factors that have critical roles in pituitary development and striking patterning functions in embryonic development. Terminal differentiation of the two pituitary POMC lineages, the corticotrophs and melanotrophs, is controlled by Tpit; mutations of the human TPIT gene cause isolated adrenocorticotrophic hormone deficiency. Intermediate lobe and melanotroph identity is provided by the pioneer TF Pax7 that remodels chromatin to reveal a new repertoire of enhancers for Tpit action. Many signaling pathways regulate POMC transcription including activation by hypothalamic corticotrophin-releasing hormone acting through the orphan nuclear receptors of the Nur family and feedback repression by glucocorticoids and their glucocorticoid receptor. TFs of the basic helix-loop-helix, Smad, Stat, Etv, and nuclear factor-B families also mediate signals for control of POMC transcription. Whereas most of these regulatory processes are conserved in different species, there are also notable differences between specific targets for regulation of the human compared with mouse POMC genes.

Modulation of Protein-Protein Interactions for the Development of Novel Therapeutics

Protein-protein interactions (PPIs) underlie most biological processes. An increasing interest to investigate the unexplored potential of PPIs in drug discovery is driven by the need to find novel therapeutic targets for a whole range of diseases with a high unmet medical need. To date, PPI inhibition with small molecules is the mechanism that has most often been explored, resulting in significant progress towards drug development. However, also PPI stabilization is gradually gaining ground. In this review, we provide a focused overview of a number of PPIs that control critical regulatory pathways and constitute targets for the design of novel therapeutics. We discuss PPI-modulating small molecules that are already pursued in clinical trials. In addition, we review a number of PPIs that are still under preclinical investigation but for which preliminary data support their use as therapeutic targets.

Sequencing analysis of the human glucocorticoid receptor (NR3C1) gene in multiple sclerosis patients

Various specific human glucocorticoid receptor (NR3C1) gene polymorphisms have been described in multiple sclerosis (MS) patients and correlated with disease progression, susceptibility and aggressiveness. Herein, we investigated the presence of gene alterations in the entire coding region of the NR3C1 in MS patients of variable clinical status (CIS, RRMS and SPMS) and the association(s) of these alterations with severity of disease (EDSS), response to glucocorticoid (GC) treatment and clinical improvement. Sixty Caucasian Greek MS patients were included. Sequencing the coding sequences and intron-exon boundaries of the NR3C1 did not reveal the presence of mutation(s) in any of the MS patients. Three previously described polymorphisms were detected: p.N363S (rs6195), p.N766N (rs6196) and c.1469-16G>T (rs6188). None of the identified alleles/genotypes were found to be associated with the severity of disease, response to glucocorticoids and disease subtypes. Known polymorphism, such as ER22/23EK that has been previously detected in MS patients, was not detected. There is a considerable ethnicity-related variation in the frequency of the NR3C1 polymorphisms. Although a genetic basis of the glucocorticoid sensitivity exists in healthy population, in the presence of chronic inflammation and abundance of cytokines--such in MS patients--other factors appear to play a more important role in GC sensitivity.

Anticancer Inhibitors of Hsp90 Function: Beyond the Usual Suspects

The 90-kDa heat-shock protein (Hsp90) is a molecular chaperone responsible for the stability and function of a wide variety of client proteins that are critical for cell growth and survival. Many of these client proteins are frequently mutated and/or overexpressed in cancer cells and are therefore being actively pursued as individual therapeutic targets. Consequently, Hsp90 inhibition offers a promising strategy for simultaneous degradation of several anticancer targets. Currently, most Hsp90 inhibitors under clinical evaluation act by blocking the binding of ATP to the Hsp90 N-terminal domain and thereby, induce the degradation of many Hsp90-dependent oncoproteins. Although, they have shown some promising initial results, clinical challenges such as induction of the heat-shock response, retinopathy, and gastrointestinal tract toxicity are emerging from human trials, which constantly raise concerns about the future development of these inhibitors. Novobiocin derivatives, which do not bind the chaperone's N-terminal ATPase pocket, have emerged over the past decade as an alternative strategy to inhibit Hsp90, but to date, no derivative has been investigated in the clinical setting. In recent years, a number of natural or synthetic compounds have been identified that modulate Hsp90 function via various mechanisms. These compounds not only offer new chemotypes for the development of future Hsp90 inhibitors but can also serve as chemical probes to unravel the biology of Hsp90. This chapter presents a synopsis of inhibitors that directly, allosterically, or even indirectly alters Hsp90 function, and highlights their proposed mechanisms of action.

Novel and less explored chemotypes of natural origin for the inhibition of Hsp90

This review focuses on novel classes of natural products whose structures have not yet been thoroughly explored for medicinal chemistry purposes. These novel chemotypes may be useful starting points to develop compounds that alter Hsp90 functionvianovel mechanisms.

The New Molecular Landscape of Cushing's Disease

Cushing's disease (CD) is caused by corticotropin-secreting pituitary adenomas and results in substantial morbidity and mortality. Its molecular basis has remained poorly understood until the past few years, when several proteins and genes [such as testicular orphan nuclear receptor 4 (TR4) and heat shock protein 90 (HSP90)] were found to play key roles in the disease. Most recently, mutations in the gene of ubiquitin-specific peptidase 8 (USP8) increasing its deubiquination activity were discovered in a high percentage of corticotroph adenomas. Here, we will discuss emerging insights in the molecular alterations that finally result in CD. The therapeutic potential of these findings needs to be carefully evaluated in the near future, hopefully resulting in new treatment options for this devastating disorder.

Decoding the genetic basis of Cushing's disease: USP8 in the spotlight

Cushing's disease (CD) arises from pituitary-dependent glucocorticoid excess due to an ACTH-secreting corticotroph tumor. Genetic hits in oncogenes and tumor suppressor genes that afflict other pituitary tumor subtypes are not found in corticotrophinomas. Recently, a somatic mutational hotspot was found in up to half of corticotrophinomas in the USP8 gene that encodes a protein that impairs the downregulation of the epidermal growth factor receptor (EGFR) and enables its constitutive signaling. EGF is an important regulator of corticotroph function and its receptor is highly expressed in Cushing's pituitary tumors, where it leads to increased ACTH synthesis in vitro and in vivo. The mutational hotspot found in corticotrophinomas hyper-activates USP8, enabling it to rescue EGFR from lysosomal degradation and ensure its stimulatory signaling. This review presents new developments in the study of the genetics of CD and focuses on the USP8-EGFR system as trigger and target of corticotroph tumorigenesis.

Inhibitory effects of trichostatin A on adrenocorticotropic hormone production and proliferation of corticotroph tumor AtT-20 cells

Cushing's disease is primarily caused by adrenocorticotropic hormone (ACTH)-producing pituitary adenomas. Pituitary tumor-transforming gene 1 (PTTG1) expression, a hallmark of pituitary tumors, stimulates pituitary cell proliferation. Histone deacetylases (HDACs) play an important role in regulating gene transcription and HDAC inhibitors induce cellular differentiation and suppress tumor cell proliferation. HDAC inhibitors also repress PTTG1 mRNA levels. Trichostatin A (TSA) is a potent cell-permeable HDAC inhibitor that blocks cell cycle progression. In the present study, we determined the effect of TSA on ACTH production and cellular proliferation in mouse AtT-20 corticotroph tumor cells. TSA decreased proopiomelanocortin (POMC) mRNA levels in AtT-20 cells and reduced ACTH levels in the culture medium of these cells. The TSA-induced decreases in POMC mRNA levels were not modulated when TSA and dexamethasone were simultaneously administered. Drug treatment also decreased AtT-20 cell proliferation, induced apoptosis, and increased the percentage of cells in G0/G1 phase using flow cytometry. TSA decreased PTTG1 mRNA levels. Furthermore, PTTG1 knockdown inhibited cellular proliferation. Its knockdown also inhibited POMC mRNA and ACTH levels. TSA inhibits ACTH production and corticotroph tumor cell proliferation. TSA may inhibit cellular proliferation, and ACTH synthesis and secretion by decreasing PTTG1 expression.