Abstract
Proteasome-mediated proteolysis is important for many basic cellular processes. In addition to their functions in the cell, proteasomes have been found in physiological fluids of both healthy and diseased humans including cancer patients. Higher levels of these proteasomes are associated with higher cancer burden and stage. The etiology and functions of these proteasomes, referred to as circulating, plasmatic, or extracellular proteasomes (ex-PSs), are unclear. Here we show that human cancer cell lines, as well as human endometrium-derived mesenchymal stem cells (hMESCs), release proteasome complexes into culture medium (CM). To define ex-PS composition, we have affinity purified them from CM conditioned by human leukemia cell line K562. Using matrix-assisted laser desorption/ionization (MALDI) Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS), we have identified core 20S proteasome subunits and a set of 15 proteasome-interacting proteins (PIPs), all previously described as exosome cargo proteins. Three of them, PPIase A, aldolase A, and transferrin, have never been reported as PIPs. The study provides compelling arguments that ex-PSs do not contain 19S or PA200 regulatory particles and are represented exclusively by the 20S complex.
Highlights
Proteasomes are responsible for the majority of non-lysosomal protein degradation in eukaryotic cells
We report an optimization of serum-free culture medium for cultivation of K562 cells for extracellular proteasome (ex-cell proteasome (PS)) preparation www.impactjournals.com/oncotarget (Figure 1)
Since ex-PSs were detected in physiological fluids of humans and because ex-PS concentration is increased in patients with various malignancies, the possibility arose that ex-PSs are released by damage or from abnormal cells
Summary
Proteasomes are responsible for the majority of non-lysosomal protein degradation in eukaryotic cells. They are involved in many important biological processes such as cell cycle progression, apoptosis, stress response and regulation of the immune and inflammatory responses [1, 2]. Proteasomes are localized in both the cell cytoplasm and the nucleus [3], and can be released into the extracellular space [4, 5]. The term proteasome refers to several complexes with multiple functions and specificities in a single eukaryotic cell. The 26S proteasome is an ATP-dependent protease that selectively degrades various cellular proteins carrying specific degradation signals such as a polyubiquitin chain [6, 7]. The 19S RP contains ATPases and a recognition sites for polyubiquitinated proteins which are subject to degradation
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