Elucidation of Mechanisms Underlying Proteasome Addiction in Sarcomas

Background/Objectives Proteasome inhibitors are approved to treat multiple myeloma and mantle cell lymphoma, and their efficacy in various solid tumors is under investigation. Recent reports suggest proteasome addiction in Ewing, osteo- and some soft tissue sarcomas. Mechanistic explanations generally cite proteotoxic stress. We aimed to clarify mechanism and investigate potentially safer therapeutic avenues. Methods We performed Kaplan Meier analyses of the impacts of expression level of each of the 377 human E3 ubiquitin ligases on overall survival (OS) and recurrence free survival (RFS) of sarcoma patients, identified known and predicted substrates of those E3 ligases with the most significant and robust effects, and performed enrichment analyses of these substrates. Results High level expression of 102 E3 ligases is associated with shortened OS, suggesting that they mediate proteasome addiction. Thirteen of these shorten OS by >40 months. Nineteen of these 102 E3 ligases also show correlation between increased expression and reduced RFS. Overexpression of 73 E3 ligases significantly extended OS, with 18 of these extending OS by >40 months. Elevated expression of 21 of these E3 ligases significantly extends RFS. Enrichment analyses of known and putative substrates unique to the 13 E3 ligases whose elevated expression shortens OS by >40 months and the 18 E3 ligases whose elevated expression extends OS by >40 months revealed largely non-overlapping functions. Conclusions The E3 ligases whose elevated expression shorten OS uniquely targeted cell cycle, cell-cell communication, disease, cellular responses to stimuli, DNA repair, metabolism of proteins, chromatin organization, DNA replication, reproduction, developmental biology, and gene expression functions. The E3 ligases whose elevated expression extend OS uniquely targeted extracellular matrix organization and neuronal system functions. Functions uniquely targeted by each set of ligases could reveal therapeutic targets with greater therapeutic index than the proteasome.