Increased proteasome activator 28 gamma (PA28γ) levels are unspecific but correlate with disease activity in rheumatoid arthritis

Background: PA28 gamma (also known as Ki, REG gamma, PMSE3), a member of the ubiquitin-and ATP-independent proteasome activator family 11S, has been proved to show proteasome-dependent and -independent effects on several proteins including tumor suppressor p53, cyclin-dependent kinase inhibitor p21 and steroid receptor co-activator 3 (SCR-3). Interestingly, PA28 gamma is overexpressed in pathological tissue of various cancers affecting e. g. breast, bowl and thyroids. Furthermore, anti-PA28 gamma autoantibodies have been linked to several autoimmune disorders. The aim of this study was to develop and evaluate a novel and sensitive PA28 gamma sandwich ELISA for the quantification of PA28 gamma serum levels in patients with cancer and autoimmune diseases for diagnostic and prognostic purposes. Methods: PA28 gamma-specific polyclonal antibodies and recombinant His-tagged PA28 gamma were purified and used to develop a sandwich ELISA for the detection of circulating PA28 gamma. With this new assay, PA28 gamma serum levels of patients with various cancers, rheumatoid arthritis (RA), Sjogren's syndrome (SS), adult-onset Still's disease (AOSD) and different connective-tissue diseases (CTD) were compared with healthy control subjects. Anti-PA28 gamma autoantibodies were additionally confirmed using a newly developed microbead assay. Results: The developed PA28 gamma sandwich ELISA showed a high specificity with a detection limit of 3 ng/ml. A significant up-regulation of circulating PA28 gamma was detected in the sera of patients with cancer, RA, SS and CTD. A significant correlation was observed dependent on age as well as anti-PA28 gamma autoantibody levels with circulating PA28 gamma protein levels. Furthermore, PA28 gamma serum levels showed a correlation with disease activity in patients with RA under treatment with the T-cell directed biological compound abatacept according to disease activity score 28 (DAS28) and erythrocyte sedimentation rate (ESR). Conclusion: The application of PA28 gamma as a novel biomarker for diagnostic purposes of a specific disease is limited, since elevated levels were observed in different disorders. However, the correlation with disease activity in patients with RA suggests a prognostic value, which needs to be addressed by further studies. Therefore our results show that PA28 gamma is a useful marker which should be included in studies related to novel treatments, e. g. abatacept.