Arthritis and Rheumatology
Arthritis and Rheumatology
5 years ago

Proteasomal function is impaired in human osteoarthritic chondrocytes and this can contribute to decreased SOX9 and aggrecan

Ramon L Serrano, Robert Terkeltaub, Martin K Lotz, Liang-Yu Chen, Ru Liu-Bryan
Objective Osteoarthritis (OA) chondrocytes have impaired autophagy, one arm of the proteostasis network that coordinates proteome and organelle quality control and degradation. Deficient proteostasis impacts differentiation and viability, and inflammatory processes in aging and disease. Studying OA chondrocytes, we assessed ubiquitin proteasome system proteasomal function. Methods We evaluated human knee cartilages by immunohistochemistry, and assessed proteasomal function, levels of proteasomal core subunits and chaperones, and autophagy in cultured chondrocytes. Assays included Western blotting, quantitative RT-PCR, proteasomal protease activity and cell immunofluorescence. Results Human knee OA cartilages demonstrated polyubiquitin accumulation, with increased ubiquitin K48-linked polyubiquitinated proteins in situ, suggesting proteasomal impairment. Cultured OA chondrocytes demonstrated accumulation of K48 polyubiquitinated proteins, significantly decreased 20S proteasome core protease activity, and decreased levels of phosphorylated FOXO4 and PSMD11, a FOXO4-inducible promoter of proteasomal activation. Levels of proteasomal core subunits PSMB3, 5, 6, and assembly chaperone PSMG1 were not decreased in OA chondrocytes. In normal chondrocytes, PSMD11 siRNA knockdown stimulated certain autophagy machinery elements, elevated extracellular nitric oxide (NO), and decreased chondrocytic master transcription factor SOX9 protein (and mRNA) and aggrecan (AGC1) mRNA. PSMD11 gain of function by transfection increased proteasomal function, raised levels of SOX9-induced AGC1 mRNA, stimulated elements of the autophagic machinery, and inhibited extracellular levels of IL-1-induced NO and MMP13 in OA chondrocytes. Conclusion Deficient PSMD11, associated with less phosphorylated FOXO4, promotes impaired proteasomal function in OA chondrocytes, dysregulated chondrocytic homeostasis, and decreased levels of SOX9 mRNA, SOX9 protein, and AGC1 mRNA. Chondrocyte proteasomal impairment may be a therapy target for OA. This article is protected by copyright. All rights reserved.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/art.40456

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