Cushing's disease is primarily caused by autonomic hypersecretion of 
adrenocorticotropic hormone (ACTH) from a pituitary adenoma. In Cushing's 
disease, mutations in the ubiquitin-specific protease 8 (USP8) have been 
detected. These mutations are associated with hyperactivation of USP8 that 
prevent epidermal growth factor receptor (EGFR) degradation. This leads to 
increased EGFR stability and results in the maintenance of EGFR signaling in 
Cushing's disease. USP8 inhibitors can suppress the growth of various tumors. In 
this study, the effects of a potent USP8 inhibitor, DUBs-IN-2, on ACTH 
production and cell proliferation were examined in mouse corticotroph tumor 
(AtT-20) cells. Proopiomelanocortin (Pomc) mRNA levels and ACTH levels were 
decreased in AtT-20 cells by DUBs-IN-2. Further, cell proliferation was 
inhibited, and apoptosis was induced by DUBs-IN-2. Transcript levels of 
pituitary tumor-transforming gene 1 (Pttg1), a pituitary tumor growth marker, 
were increased; and transcript levels of stress response growth arrest and DNA 
damage-inducible 45 (Gadd45β) and Cdk5 and ABL enzyme substrate 1 (Cables1) mRNA 
levels were increased in response to the drug. Gadd45β or Cables1 knockdown 
partially inhibited the DUBs-IN-2-induced decrease in cell proliferation, but 
not Pomc mRNA levels. Both GADD45β and CABLES1 may be responsible, at least in 
part, for the USP8-induced suppression of corticotroph tumor cell proliferation. 
USP-8 may be a new treatment target in Cushing's disease.