Abstract 2033: Protection of quiescent fibroblasts from proteasome inhibition-mediated proteotoxicity

Objectives: The aim of this study is to investigate quiescence-related resistance to proteasome-inhibition mediated proteotoxicity, and its regulatory mechanisms. Introduction: Quiescence, the reversible arrest of cellular proliferation, is critical for normal cellular development and tissue maintenance. Moreover, while proteasome inhibitors (PIs) are used to treat specific cancers, it has been shown that subsets of cells within a tumor, through quiescence, avoid PI-induced apoptosis and survive. Thus, deciphering the mechanisms through which quiescent cells resist proteasome inhibition is of basic and clinical relevance. Methods: Proliferating and quiescent primary human fibroblasts were used as a model system to study the effects of PIs (MG132, epoxomycin and Bortezomib). Flow cytometry, Western blots, microscopy and microarrays were used to identify specific pathways activated by proliferating and quiescent fibroblasts in response to proteasome inhibition. Results: We found that the proteasome-inhibited quiescent fibroblasts maintain viability while proliferating fibroblasts rapidly undergo apoptosis. We further found that proteasome-inhibited proliferating and quiescent fibroblasts up-regulate multiple protective pathways, including authophagy, chaperones and antioxidants, and evidence no observable difference in proteasome-mediated degradation pathway activity. However, we found that in proliferating fibroblasts, proteasome inhibition leads to increased reactive oxygen species (ROS), peri-nuclear protein aggresomes, and apoptotic cell death. By comparison, proteasome-inhibited quiescent cells display selective up-regulation of MnSOD (a ROS detoxifying enzyme), dispersed protein aggregates, and less apoptotic cell death. Knockdown of p62/SQSTM1 (a protein important for aggresome formation) or treatment with an antioxidant N-acetylcysteine results in reduced aggresome formation and decreased apoptosis in response to proteasome inhibition. Moreover, inhibition of autophagy sensitizes serum-starved quiescent cells to proteasome inhibition induced apoptosis. This suggests that multiple mechanisms, including autophagy, ROS detoxification and differential subcellular trafficking of ubiquitinated proteins are implicated in quiescence-related resistance to PI-induced apoptosis. These results point to possible strategies for improving the effectiveness of PIs in cancer treatment. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 10