b-AP15 inhibits the activity of two 19S regulatory-particle-associated deubiquitinases, ubiquitin C-terminal hydrolase 5 (UCHL5) and ubiquitin-specific peptidase 14 (USP14), resulting in accumulation of polyubiquitin. b-AP15 results in a dose-dependent accumulation of the UbG76V-YFP reporter with IC50 of 0.8 μM, indicating impaired proteasome degradation. b-AP15 (1 μM) results in rapid accumulation of polyubiquitinated proteins in human colon carcinoma HCT-116 cells. b-AP15 (2.2 μM) increases the amounts of the cyclin-dependent kinases CDKN1A and CDKNIB and the tumor suppressor TP53 in a dose-dependent manner without altering the amounts of ornithine decarboxylase 1 (ODC1) in HCT-116 cells. b-AP15 (1 μM) results in G2/M phase cell-cycle arrest in HCT-116 cells, consistent with the accumulation of cell-cycle inhibitors. b-AP15 treatment increases the number of hypodiploid cells and is associated with increased amounts of apoptotic markers, including activated caspase-3, caspase-cleaved poly-ADP ribose polymerase (PARP) and cytokeratin-18 (CK18). b-AP15 is more toxic to HCT-116 cells as compared to immortalized epithelial cells (hTERT-RPE1) or peripheral blood mononuclear cells. b-AP15 inhibits deubiquitinating activity using a variety of substrates, including Ub-AMC, Ub-GFP22, ubiquitinated p53-binding protein homolog (HDM2), and K48- and K63-linked ubiquitin tetramer chains. ^[1] b-AP15 is an inhibitor of the UPS that induced cell death via induction of the lysosomal apoptosis pathway in a cathepsin-D dependent manner. b-AP15 elicits characteristic UPS defects including the accumulation of ubiquitin conjugates and cell cycle inhibitors such as p21, p27 and the tumor suppressor p53. b-AP15 inhibits the deubiquitinase activity of both cysteine DUBs, with USP14 being slightly more sensitive than UCHL5. b-AP15 induces apoptosis in cells over-expressing the anti-apoptotic Bcl-2 protein and in cells lacking the p53 gene. ^[2] b-AP15 (1 μM) inhibits ATP-induced IL-1β release from LPS-primed peritoneal macrophages. b-AP15 (1 μM) reduces the levels of cell death induced by nigericin treatment in THP-1 cells. b-AP15 (1 μM) significantly reduces the numbers of ASC specks formed after nigericin treatment in LPS-primed THP-1 cells. ^[3]