Background The intracellular suppressor of cytokine signaling (SOCS)-proteins are inducible and negative regulators in receptor signaling pathways of several cytokines, toll-like receptor ligands and some growth factors. We previously have shown in mice that SOCS3 is expressed in chondrocytes during arthritis . Furthermore, SOCS3 overexpression inhibits the insulin-like growth factor (IGF)-1 response, the main anabolic factor for chondrocytes.
Objectives To determine the expression and functional consequnces of SOCS3 in human articular chondrocytes.
Methods Chondrocytes were isolated from articular cartilage of patients undergoing surgical joint replacement. The human immortalized chondrocyte cell line G6, human mesenchymal stem cell (MSC)-differentiated chondrocytes, and primary bovine chondrocytes were used for comparison. SOCS3 mRNA and protein levels were measured by quantitative PCR, western blotting and immunohistochemistry, respectively. Regulation of SOCS3 expression was examined following incubation with different cytokines and Toll-like receptor (TLR) agonists. To determine the effect of SOCS3 on the chondrocyte response to various stimuli, SOCS3 was either reduced by an inhibitor of SP1 (Mithramycin) or with short interference RNA, and enhanced by adenoviral transduction.
Results The expression of SOCS3 was significantly enhanced in chondrocytes obtained from cartilage of osteoarthritis (OA) (ΔCt 3.4±1.0, n=18 patients) and rheumatoid arthritis (RA) (ΔCt 3.4±1.4, n=6) as compared to and healthy cartilage from fractures of neck of femur (NOF) patients (ΔCt 5.3±1.2, n=8). The expression of SOCS3 correlated markedly with other genes known to be expressed in arthritic chondrocytes such as RUNX2 (r=0.341), MMP13 (r=0.511), ADAMTS4 (r=0.779), and ADAMTS5 (r=0.647). No correlation was found with aggrecan expression and an inverse relationship was found with the collagen-type II gene Col2A1 (r=0.577). Western blots and imunohistochemisty confirmed the enhanced expression of SOCS3 at the protein level in arthritic cartilage. The expression of SOCS1 in chondrocytes was low and similar between the different patient groups. The expression of SOCS3 in the immortalized human chondrocyte cell-line (G6) and MSC-derived chondrocytes could be enhanced by interleukin-1 and conditioned medium of OA synovium explants. This was dependent on the transcription factor SP1 as the specific inhibitor mithramycin prevented SOCS3 upregulation in these cells. Forced expression of SOCS3 in bovine chondrocytes impairs several aspects of chondrocyte function, including nitiric oxide production and proteoglycan synthesis. Interestingly, a similar impairment of function was found in OA chondrocytes and knockdown of SOCS3 in these chondrocytes partially restored human chondrocyte function.
Conclusions This study demonstrates that SOCS3 is highly expressed in human articular chondrocytes and affects cellular responses, which may have important implications for cartilage pathology in humans.
Smeets RL, Veenbergen S, Arntz OJ, Bennink MB, Joosten LA, van den Berg WB, van de Loo FA. A novel role for suppressor of cytokine signaling 3 in cartilage destruction via induction of chondrocyte desensitization toward insulin-like growth factor. Arthritis Rheum. 2006;54(5):1518-28.
Disclosure of Interest None Declared
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