Abstract A81: Investigation of possible cell state dependent differential protein homeostasis mechanism(s)

Abstract

Proteasome inhibition has been shown to be an important strategy for treating many types of cancer. Although proteasome inhibitors are effective at eradicating rapidly proliferating cancer cells, a subset of cancer cells enter quiescence and survive, contributing to tumor recurrence. Using primary human fibroblasts as a model system we have shown that quiescent cells have decreased reactive oxygen species levels and protein aggregates, and undergo apoptosis less frequently than proliferating cells in response to proteasome inhibition. However, the precise mechanism by which quiescent cells become resistant to proteasome inhibition remains largely unknown. Using a GFP-tagged proteasome reporter, we investigated mechanisms protecting quiescent cells from proteasome inhibition-induced apoptosis. We found that, in proliferating cells, proteasome inhibition by MG132 led to high levels of a GFP-tagged proteasome reporter with a visible ubiquitination pattern. In contrast, MG132 treatment of quiescent cells resulted in reduced levels of the ubiquitinated form of this reporter as observed by Western blot analysis. Interestingly, preliminary immunoprecipitation followed by mass-spectrometry analysis revealed none canonical ubiquitin modification on lysine 27 of the reporter in quiescent cells. These findings suggested that quiescent cells may have one or more alternative pathways preventing specific proteasome substrates accumulation in response to proteasome inhibition. Moreover, our data has shown that, upon proteasome inhibition, a GFP-tagged proteasome reporter was secreted into the condition medium. Secretion of cytoplasmic proteins in response to proteasome inhibition is one possible mechanism for reducing protein aggregation and cytotoxicity. Identification of regulatory mechanisms that are activated in quiescent cells in response to proteasome inhibition may help to suggest strategies for improving the effectiveness of proteasome inhibitors in cancer treatment. Citation Format: Gordon-Victor Fon, Chandler R. Keller, David H. Perlman, Aster Legesse-Miller. Investigation of possible cell state dependent differential protein homeostasis mechanism(s). [abstract]. In: Proceedings of the Seventh AACR Conference on The Science of Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; Nov 9-12, 2014; San Antonio, TX. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2015;24(10 Suppl):Abstract nr A81.