Inhibition of 19S proteasome deubiquitinases as a promising strategy in cancer therapy

Abstract

The ubiquitin proteasome system (UPS) is the main degradation system in the eukaryotic cell and it is involved in the regulation of many crucial cellular pathways; such as signal transduction, proliferation, DNA repair, cell death, and cell cycle regulation. In comparison to cells in healthy tissues, cancer cells proliferate faster and have increased protein synthesis rate; therefore in order to preserve cellular fitness they are more dependent on UPS than normal cells. This therapeutic window has been investigated for more than 30 years and intensive studies led to the discovery of first and second generation 20S proteasome inhibitors that have been successfully applied in the treatment of multiple myeloma and mantle cell lymphoma. However; in spite of initial positive outcomes, patients eventually gain resistance and suffer from the disease relapse. Therefore there is a strong interest in the development of new drugs targeting the UPS through alternative mechanisms.

b-AP15/VLX1570 is a novel inhibitor of proteasome, recently discovered in our group, which does not block the activity of 20S catalytic core, but instead it inhibits two 19S deubiquitinating enzymes, USP14 and UCHL5. It is a small molecule with α,β-unsaturated carbonyl units that binds reversibly to cysteine deubiquitinases via Michael reaction. Despite reversible binding of the drug, commitment to death induced by b-AP15/VLX1570 is irreversible, which is due to the rapid uptake of the drug and subsequent retention in the cellular compartment. To some extent cellular response to the treatment with b-AP15/VLX1570 and bortezomib are similar: it results in the accumulation of polyubiquitinated proteins, induction of ER stress and eventually apoptosis and cell death. Treatment with b-AP15/VLX1570 however, induces higher level of proteotoxic stress, higher level of ROS and more rapid and potent apoptotic response. We found that VLX1570, a b-AP15 analogue optimized for clinical administration, has strong anti-cancer activity against multiple myeloma and acute lymphoblastic leukemia. In addition we found that the DUB inhibitor is active on cells resistant to bortezomib. This findings suggest that VLX1570 is a promising candidate drug and is currently investigated in Phase I clinical trials for the treatment of multiple myeloma resistant to conventional proteasome inhibitors.

Application of proteasome inhibitors in the treatment of solid tumors remains limited and primary and acquired resistance continues to be a concern during the development of new therapies. Using multicellular spheroids as solid tumor model, we discovered that overexpression of PA28, a stress-induced regulatory particle of proteasome, can moderate the response of cancer cells to proteasome inhibitors.

Taken together our data shows that proteasome DUB inhibitors have potential as a treatment option for malignancies and that differences in UPS expression may have relevance for the survival of different tumor types.