Role of the ubiquitin/proteasome system on ACTH turnover in rat corticotropes

Advances in Molecular Pathophysiology and Targeted Therapy for Cushing's Disease

Cushing's disease is caused by autonomous secretion of adrenocorticotropic hormone (ACTH) from corticotroph pituitary neuroendocrine tumors. As a result, excess cortisol production leads to the overt manifestation of the clinical features of Cushing's syndrome. Severe complications have been reported in patients with Cushing's disease, including hypertension, menstrual disorders, hyperglycemia, osteoporosis, atherosclerosis, infections, and mental disorders. Cushing's disease presents with a variety of clinical features, ranging from overt to subtle. In this review, we explain recent advances in molecular insights and targeted therapy for Cushing's disease. The pathophysiological characteristics of hormone production and pituitary tumor cells are also explained. Therapies to treat the tumor growth in the pituitary gland and the autonomous hypersecretion of ACTH are discussed. Drugs that target corticotroph pituitary neuroendocrine tumors have been effective, including cabergoline, a dopamine receptor type 2 agonist, and pasireotide, a multi-receptor-targeted somatostatin analog. Some of the drugs that target adrenal hormones have shown potential therapeutic benefits. Advances in potential novel therapies for Cushing's disease are also introduced.

Outer membrane protein A inhibits the degradation of caspase-1 to regulate NLRP3 inflammasome activation and exacerbate the Acinetobacter baumannii pulmonary inflammation

Acinetobacter baumannii (A. baumannii), one of the major pathogens that causes severe nosocomial infections, is characterised by a high prevalence of drug resistance. It has been reported that A. baumannii triggers the NOD-like receptor 3 (NLRP3) inflammasome, but the role of its virulence-related outer membrane protein A (ompA) remains unclear. Therefore, this study aimed to explore the effects of ompA on the NLRP3 inflammasome and its underlying molecular mechanisms. Results showed that ompA enhanced inflammatory damage, which was reduced as a result of knockout of the ompA gene. Additionally, ompA-stimulated expression of NLRP3 inflammasome was significantly blocked by silencing caspase-1, but activation of NLRP3 inflammasome was not altered after silencing ASC; this indicated that ompA was dependent on the caspase-1 pathway to activate the inflammatory response. Simultaneously, the wild-type (WT) strains triggered NLRP3 inflammasome after inhibition of caspase-1 degradation by proteasome inhibitor MG-132, aggravating tissue damage. These findings indicated that ompA may be dependent on the caspase-1 pathway to enhance inflammation and exacerbate tissue damage. Taken together, these results confirmed a novel capsase-1-modulated mechanism underpinning ompA activity, which further reveals the NLRP3 inflammasome pathway as a potential immunomodulatory target against A. baumannii infections.

Ubiquitin-Specific Protease 8 Mutant Corticotrope Adenomas Present Unique Secretory and Molecular Features and Shed Light on the Role of Ubiquitylation on ACTH Processing

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.

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.