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Objective: The present study was performed to elucidate the possible role of SIRT1 signaling in joint inflammation in human articular chondrocytes., Design: Real-time quantitative reverse transcription polymerase chain reaction (qRT-PCR) and Western blotting were performed to detect gene products and proteins involved in tumor necrosis factor α (TNF-α)-induced inflammation and cartilage degradation in human primary chondrocytes. Matrix metalloproteinase (MMP)-2 and MMP-9 activity was evaluated by gelatin zymography. Overexpression and knockdown of SIRT1 were also performed to investigate whether SIRT1 is associated with the anti-inflammatory activity of resveratrol in chondrocytes., Results: Resveratrol dose-dependently inhibited TNF-α-induced cyclooxygenase-2 (COX-2), MMP-1, MMP-3, MMP-13 and PGE(2) production in human chondrocytes. Moreover, MMP-2 and MMP-9 activity was increased by treatment with TNF-α; however, SIRT1 activation decreased the proinflammatory effects induced by TNF-α. In addition, treatment of SIRT1 activator and overexpression of SIRT1 inhibited the expression and activation of the main proinflammatory regulator NF-κB, which was increased by TNF-α. When SIRT1 was overexpressed in chondrocytes, the anti-inflammatory action of SIRT1 was similar to that exerted by resveratrol., Conclusions: SIRT1 activation deacetylates and inactivates NF-κB, and thereby, exerts an anti-inflammatory effect on chondrocytes, suggesting that SIRT1 activators could be explored as potential treatments for arthritis., (Copyright © 2012 Osteoarthritis Research Society International. Published by Elsevier Ltd. All rights reserved.)

Sphingosine-1-phosphate (S1P) is a pluripotent lipid mediator that transmits signals through a family of G-protein-coupled receptors (GPCRs) to control diverse biological processes including inflammation and wound-healing. In this study, a novel biological activity of S1P in articular chondrocytes was identified. Human primary chondrocytes were cultured in a monolayer. Reverse transcription-polymerase chain reaction (RT-PCR) and western blotting were performed to detect genes and proteins involved in inflammation and cartilage degradation when human primary chondrocytes were stimulated by interleukin (IL)-1β. Matrix metalloproteinase (MMP)-2 and MMP-9 activity was evaluated by gelatin zymography. Glycosaminoglycan (GAG) degradation was evaluated using the dimethylene blue method. Prostaglandin E2 (PGE2) was measured by enzyme-linked immunosorbent assay (ELISA). By using the S1P1 receptor agonist and antagonist, we discovered the key role played by S1P1 in the S1P-dependent inhibition of IL-1β-induced inflammation in human chondrocytes. S1P dose-dependently inhibited IL-1β-induced NF-κB p65, cyclooxygenase (COX)-2, MMP-1, MMP-3, MMP-13 and MMP-14 mRNA expression in human chondrocytes and IL-1β-induced PGE2 synthesis and GAG degradation in human cartilage explants. W146, a known S1P1 receptor antagonist, inhibited the active form of NF-κB p65 and COX-2 expression induced by IL-1β. The anti-inflammatory action of the S1P1 receptor agonist SEW2871 was similar to that of S1P. This study demonstrates that S1P has anti-inflammatory effects on chondrocytes via the S1P1 receptor. Our data suggest that targeting S1P and S1P1 may be a potential therapy for arthritis.

Hypoxia and hypoxia-related genes are important factors in articular chondrocytes during cartilage homeostasis and osteoarthritis. We have investigated the various apoptotic factors that show significance in synovial fluid obtained from normal and experimental osteoarthritic animal models and have evaluated the effect of hypoxia on articular chondrocyte apoptosis induced by these apoptotic factors. Mature beagle dogs underwent surgical transections of ligaments and medial meniscectomies to explore the underlying mechanisms of osteoarthritis. Cartilage and synovial fluid obtained from normal animals and those with osteoarthritis were evaluated via proteasome inhibition, tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) protein expression, mitochondrial transmembrane potential and levels of reactive oxygen species. Canine chondrocytes were exposed to the proteasome inhibitor N-acetyl-Leu-Leu-Norleu-al and treated with recombinant TRAIL protein under normoxic and hypoxic conditions, measuring chondrocyte cell viability, proteasome activity and levels of apoptotic factors. TRAIL protein expression and ubiquitinated proteins were increased significantly, but the proteasome activity in the synovial fluid of osteoarthritic joints relative to that in normal joints was not. Primary cultured articular chondrocytes cotreated with the proteasome inhibitor and TRAIL progressed to severe apoptosis under normoxic conditions, but the sensitization caused by the combined treatment was suppressed by exposure to hypoxia. Caspase-8 activation, c-Jun N-terminal kinase phosphorylation, the mitochondrial transmembrane potential and the generation of reactive oxygen species involved in cell death regulation were significantly inhibited under hypoxic conditions. These findings suggest that proteasome inhibition and TRAIL may be possible mechanisms in cartilage degradation and joint-related diseases. Furthermore, the maintenance of hypoxic conditions or therapy with hypoxia-related genes in the joint may be successful for the treatment of joint-related diseases, including osteoarthritis.

Tartrate-resistant acid phosphatase (TRAP) is highly expressed in osteoclasts and chondroclasts. The present study investigated changes in TRAP activity after chondrocyte death and cartilage damage, and also evaluated the possible use of TRAP as a diagnostic factor in a model of osteoarthritis. We induced experimental osteoarthritis in beagle dogs and separated chondrocytes from articular cartilage using an enzyme probe. Chondrocyte death was induced by proteasome inhibition and TRAIL treatment, and levels of lactate dehydrogenase, reactive oxygen species (ROS), caspase activation and TRAP activity were measured in the chondrocytes and synovial fluid. Proteasome inhibition and TRAIL treatment significantly enhanced chondrocyte death via caspase-8 activation and ROS generation in the primary cultured canine chondrocytes. TRAP activity was highly increased in damaged chondrocytes, but was decreased by blocking chondrocyte death using caspase inhibition or an ROS scavenger. In the synovial fluid of osteoarthritic dogs, TRAP activity as well as caspase activation and ROS levels were higher than those in the normal joint. Our study demonstrated that TRAP is activated by apoptosis and oxidative stress in primary cultured chondrocytes and osteoarthritic joints and also suggests that TRAP may be used as a diagnostic biomarker for detection of cartilage-related diseases, including osteoarthritis.

The present study investigates the effect of low oxygen concentrations on thapsigargin-induced apoptosis and reactive oxygen species (ROS)-related signaling in articular chondrocytes. Chondrocytes were obtained from normal canine knee cartilage and were treated with different concentrations of thapsigargin for 24h under normoxic (21% oxygen tension) or hypoxic (1% oxygen tension) conditions. The cells treated with thapsigargin under normoxic conditions showed a dose-dependent induction of apoptosis. However, the cellular changes and apoptotic events that occurred following thapsigargin treatment, were completely inhibited by hypoxia, including loss of mitochondrial transmembrane potential (MTP), ROS generation and JNK phosphorylation. Moreover, the cells exposed to hypoxic conditions showed increased expression of the anti-apoptotic proteins xIAP-2 and Bcl-2. We demonstrate that hypoxia inhibited thapsigargin-induced apoptosis in chondrocytes by regulating ROS-related signaling and the expression of anti-apoptotic proteins. We propose that maintaining hypoxic conditions in articular cartilage may be required for the prevention of chondrocyte and cartilage diseases such as arthritis.