Abstract P6-02-10: Targeting protein-protein interactions in the proteasome assemblies as a novel strategy to treat triple negative breast cancers

The ubiquitin-proteasome pathway is responsible for the most of regulated intracellular protein degradation in human cells. The proteasome, a multifunctional, multisubunit, modular proteolytic enzyme is an essential part of the pathway. The proteasome, which comprises several proteolytic assemblies sharing a catalytic core, is a recognized anti-cancer drug target. Competitive inhibitors such as bortezomib and carfilzomib bind to the active centers of the core and are successfully used to treat aggressive blood cancers, most notably refractory/relapsed multiple myeloma and lymphomas. Unfortunately, so far breast cancers performed disappointingly in clinical trials with the proteasome inhibitors, even if a recent genome-wide screen identified proteasome addiction as vulnerability of triple negative breast cancers. Triple negative breast cancers are among the most deadly and difficult to treat and such vulnerability is more than worth to explore. Here we aimed at searching for new concepts in proteasome targeting, better suited for breast cancers than active-centers blocking drugs. The sophisticated structure of the proteasome offers ample opportunities to design noncompetitive inhibitors. The most physiologically relevant 26S proteasome is an assembly of a 20S catalytic core and a 19S regulatory particle "cap". The active sites bearing core is the canonical target of proteasome targeting anti-cancer drugs. On the other hand, the 19S protein complex is responsible for recognizing and processing the majority of intracellular protein substrates tagged for degradation by polyubiquitin. We designed small molecules that instead of binding to the active sites of the core, target protein-protein interactions between the core and the cap. The lead compound B1 in vitro prevents assembly of 20S and 19S components, destabilizes 26S proteasome and noncompetitively, most likely allosterically, inhibits the 20S core at low-nanomolar concentrations. We tested the performance of B1 in the culture of triple negative breast cancer MDA-MB-231 cells. Apparently, B1 at nanomolar concentrations suppresses the cell culture growth, lowers the content of 26S proteasome and compromises activity of the ubiquitin-proteasome system, as manifested by the accumulation of polyubiquitinated protein substrates in the cytosol of B1 treated cells. Importantly, B1 strongly synergizes with bortezomib, even if these cells are relatively resistant to the treatment with bortezomib alone. The cytotoxic effect of the combined treatment on the MDA-MB-231 cell culture is apparent with single-digit nanomolar concentrations of both drugs. Summarizing, targeting protein-protein interactions in the proteasome assemblies with small molecules, alone or in combination with competitive inhibitors, seems to provide a promising strategy to treat proteasome addicted triple negative breast cancers. Citation Format: Maria Gaczynska, Pawel A Osmulski. Targeting protein-protein interactions in the proteasome assemblies as a novel strategy to treat triple negative breast cancers [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P6-02-10.