Circulating Proteasomes Are Functional and Have a Subtype Pattern Distinct from 20S Proteasomes in Major Blood Cells

Abstract

Background: 20S proteasomes, the proteolytic core particles of the major intracellular protein degradative pathway, are potential disease markers because they are detectable in human plasma as circulating proteasomes and their concentrations are increased in patients suffering from various diseases. To investigate the origin of circulating proteasomes, we compared some of their features with those of proteasomes isolated from major blood cells. Methods: We isolated circulating proteasomes from the plasma of 2 patients with rheumatoid arthritis and 2 with systemic lupus erythematosus and from human plasma from healthy donors. We purified the proteasomes to apparent homogeneity and then used electron microscopy for imaging and chromatography for subtype spectrum analysis. We compared subtype results with those from 20S proteasomes purified from 4 major blood cell populations. We also tested proteasomes for enzymatic activity and immunosubunit content. Results: Circulating proteasomes from plasma of healthy donors and from patients with autoimmune disease were found to have the same size and shape as erythrocyte proteasomes, be proteolytically active, and contain standard-and immunosubunits. Chromatography revealed 6 circulating proteasome subtype peaks in healthy donor plasma and 7 in patient donor plasma. Proteasomes from erythrocytes had 3 subtype peaks and

Circulating 20S Proteasome Levels in Patients with Mixed Connective Tissue Disease and Systemic Lupus Erythematosus

“…The origin of c20S is currently unclear. Our finding that c20S serum levels were similar in patients with and without hematological disease manifestations further strengthens the assumption that blood cells are probably not its source (16).…”

“…20S proteasomes are also detectable in healthy serum and plasma, and elevated circulating 20S proteasome (c20S) concentrations have been described in several diseases, such as hematologic malignancies, sepsis, trauma, and autoimmune diseases including systemic lupus erythematosus (SLE) (5,13,15,16). Although evidence for the functional role of c20S has not yet been provided, systemic concentrations of c20S are thought to reflect cellular damage and immunological activity.…”

Extracellular proteasome in the human alveolar space: a new housekeeping enzyme?

“…The fact that the majority of activity was found in BAL supernatant rather than BAL cell pellet does not imply that 20S proteasome is brought into the extracellular space exclusively by resident cells of the alveolar space. In fact, data from Zoeger et al (69) show in the blood proteasomal molecules that do not derive from platelets, red blood cells, or other blood-borne cells, suggesting that endothelial cells or other organs may secrete proteasome. Thus, extracellular alveolar proteasome could also derive from the blood stream.…”

Ubiquitination and Proteolysis in Acute Lung Injury

“…61 , 71 , 72 Despite research showing that proteasomes interact with phospholipid membranes, there is no evidence that explains how proteasomes may be secreted or released from cells, except as a result of cell death. Zoeger and colleagues 73 compared the function and subtype of circulating proteasomes in patients with rheumatoid arthritis, patients with systemic lupus erythematosus, and healthy volunteers. Circulating proteasomes from all groups were found to be intact and enzymatically active.…”

“…However, when subtypes were analyzed with high-resolution anion exchange chromatography, the subtypes from patients with rheumatoid arthritis and systemic lupus erythematosus were found to be distinct from those from proteasome recovered from healthy volunteer hematologic cells. 73 Related to the fi nding of increased circulating 20S proteasome, increased serum or plasma levels of ubiquitin have been detected in multiple conditions, including parasitic infection, renal failure and hemodialysis, cirrhosis, type 2 diabetes mellitus, hairy cell leukemia, and sepsis. 61 , 74 , 75 In addition, it has been proposed that extracellular ubiquitin and proteasome may have therapeutic potential.…”