Impaired Proteasomal Function in Human Osteoarthritic Chondrocytes Can Contribute to Decreased Levels of SOX9 and Aggrecan

Abstract

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 nitric oxide (NO) release, and decreased levels of 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 IL-1-induced NO and MMP13 release 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.

BMSC-derived exosomes from congenital polydactyly tissue alleviate osteoarthritis by promoting chondrocyte proliferation

“…Collagen type II (COL2A1), as a downstream target protein Sox9, is an important component of the cartilage matrix and is thought to be the basis of structural strength against outside pressure 40 . Similarly, aggrecan (Acan) is one of the main components of the ECM of articular cartilage and is necessary to maintain the normal shape of cartilage tissue 41 . In our study, we found that, compared with the levels in the BMSCs, the expression levels of COL2A1 and Acan were higher in the pBMSCs 14 days after chondrogenic differentiation, findings that confirm the predominance of the chondrogenic differentiation ability of pBMSCs.…”

TGFβ attenuates cartilage extracellular matrix degradation via enhancing FBXO6-mediated MMP14 ubiquitination

“…Recently, it is suggested that proteasome ubiquitination plays a role in arthritis diseases. 28 Proteasome inhibitor MG132 29 or Ub K48R mutation 30 protected the cartilage from cytokine-mediated degradation and against DMM-induced OA in mice. FBXO6, one of the five F-box proteins predicted to bind glycoprotein substrates through FBA domain, is shown to bind high mannose N-linked glycoproteins and functions as ubiquitin ligase subunits.…”

MicroRNAs and Osteoarthritis

“…However, the complexity inherent in these responses, which resulted from identification of the numerous miRs involved in the pathogenesis and progression of human OA, created uncertainty as to how employing exogenous miRs could alter articular cartilage ECM aberrant turnover and ECM degradation, all of which are consistent characteristics of OA progression to joint failure. Perhaps more germane to their potential as translational medicines is that miRs were employed to identify additional targets for intervention in OA, including those involved in autophagy and proteasome-mediated protein degradation [ 89 ]. Furthermore, results of experimental studies performed in well-validated animal models of OA in which miRs were administered to these animals confirmed that miRs were active in vivo T .…”