Inhibition of proteasome deubiquitinating activity as a new cancer therapy

b-AP15 is a novel inhibitor of proteasome activity, with a different mechanism of action than the available and widely used proteasome inhibitors such as bortezomib. b-AP15 inhibits the deubiquitinating activity of the regulatory subunit of the proteasome, necessary for protein degradation, and induces cytotoxicity impairing tumor growth in mouse models. The compound may represent an alternative therapeutic approach with a broader spectrum of applicability than current proteasome inhibitors. Ubiquitin-tagged substrates are degraded by the 26S proteasome, which is a multisubunit complex comprising a proteolytic 20S core particle capped by 19S regulatory particles1,2. The approval of bortezomib for the treatment of multiple myeloma validated the 20S core particle as an anticancer drug target3. Here we describe the small molecule b-AP15 as a previously unidentified class of proteasome inhibitor that abrogates the deubiquitinating activity of the 19S regulatory particle. b-AP15 inhibited 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 induced tumor cell apoptosis that was insensitive to TP53 status and overexpression of the apoptosis inhibitor BCL2. We show that treatment with b-AP15 inhibited tumor progression in four different in vivo solid tumor models and inhibited organ infiltration in an acute myeloid leukemia model. Our results show that the deubiquitinating activity of the 19S regulatory particle is a new anticancer drug target.