The role of nitric oxide in arthritic joints: a therapeutic target?

An Insight into the Role of Apoptosis and Autophagy in Nitric Oxide-Induced Articular Chondrocyte Cell Death

OBJECTIVE

To investigate the role and characterize the molecular mechanisms regulating apoptosis and autophagy in nitric oxide (NO)-induced chondrocyte cell death.

DESIGN

Cell apoptosis and autophagy were evaluated in chondrocytes treated with sodium nitroprusside (SNP) combined with the presence or absence of interleukin-1 beta (IL-1β) and nutrient-deprived conditions. The concentration of nitrite was determined by Griess reaction. Activation of apoptosis and autophagy were determined by immunocytochemistry, Western blot, and quantitative real-time polymerase chain reaction (qPCR) analysis. Flow cytometry and MTT assay were used to assess cell viability.

RESULTS

Cotreatment of chondrocytes with SNP and IL-1β under nutrient-deprived condition potentially enhanced the effect of NO-induced cell death. Immunocytochemistry, Western blot, and qPCR analysis indicated that treatment of chondrocytes with SNP significantly reduced autophagic activity, autophagic flux, and multiple autophagy-related (Atg) genes expression. These findings were associated with an increase in ERK, Akt, and mTOR phosphorylation, whereas autophagy induction through mTOR/p70S6K inhibition by rapamycin significantly suppressed NO-induced cell apoptosis. Furthermore, the cleavage of poly(ADP-ribose) polymerase (PARP) and caspase-3 activation in response to apoptosis was weakly detected. These results corresponded with a significant increase in apoptosis-inducing factor (AIF) expression, suggesting the involvement of the caspase-independent pathway.

CONCLUSIONS

These results demonstrate that in chondrocyte cultures with cells induced into an osteoarthritis state, NO inhibits autophagy and induces chondrocyte apoptosis mainly, but not completely through the caspase-independent pathway. Our data suggest that autophagy is a protective mechanism in the pathogenesis of osteoarthritis and could be proposed as a therapeutic target for degenerative joint diseases.

The expression of Kruppel-like factor 2 in mice with collagen-induced arthritis and its role in the induction of nitric oxide synthase production

Kruppel-like factor 2 (KLF2) is associated with acute and chronic inflammation. However, the role of KLF2 in rheumatoid arthritis (RA) remains unknown. Here, we investigated the expression of KLF2 in mice with collagen-induced arthritis (CIA) to determine whether KLF2 levels correlate with inducible nitric oxide synthase (iNOS) production in vitro. Hematoxylin and eosin staining was used to assess the synovitis and bone destruction in mice. The concentration of tumor necrosis factor (TNF)-α, interleukin (IL)-6, and nitric oxide (NO) in synovial fluid was determined by enzyme-linked immunosorbent assay, while western blot (WB) analysis was employed to detect the expression of KLF2 and iNOS in the synovium, heart, and kidneys. The expression of iNOS in MH7A cells was analyzed by quantitative polymerase chain reaction (PCR) and WB. The expression of KLF2 and iNOS was significantly elevated in the synovium, heart, and kidneys of CIA mice. This was correlated to an increase in the severity of arthritis and the concentration of inflammatory mediators including TNF-α, IL-6, and NO in joint fluid. KLF2 and iNOS expression in vitro was induced by TNF-α and KLF2 knockdown significantly reduced the TNF-α-induced iNOS expression. These findings indicate that KLF2 influences synovial inflammation in CIA mice by regulating iNOS production in synoviocytes.

Upregulated expression of inducible nitric oxide synthase plays a key role in early apoptosis after anterior cruciate ligament injury

The reason that the anterior cruciate ligament (ACL) has a very poor healing potential after injury is not well understood. In this study, we investigated the role of nitric oxide (NO) in the apoptotic cell death of ACL cells using a rabbit model and in vitro cell culture. The apoptosis of ACL cells in vivo was analyzed by TUNEL assay and electron microscopy. NO synthase (NOS) expression was observed by immunohistochemical analysis. ACL cells were cultured and the susceptibility to NO-induced apoptosis was tested. Inducible NOS (iNOS) expression after treatment with cytokines was examined by immunohistochemical and RT-PCR analyses. Mitogen-activated protein kinase (MAPK) inhibitors were used for the analysis of downstream signals. A significant number of apoptotic cells were observed on days 1 to 3 after injury; the apoptotic rate returned to the control level by day 7. Upregulation of iNOS in the ACL remnant was observed at day 1. Intraarticular injection of NOS inhibitor suppressed the apoptotic rate. Isolated ACL cells showed much higher susceptibility to NO-induced apoptosis than did medial collateral ligament cells. IL-1beta stimulated ACL cells to upregulate iNOS mRNA and increase NO production. p38 MAPK inhibitor decreased NO-induced apoptosis. Rapid iNOS induction after injury contributes to the high apoptotic rate of ACL cells, and this may partly account for the poor healing capacity of this ligament. iNOS and NO production is suggested to be stimulated by IL-1beta, and NO activates the p38 MAPK pathway and triggers an apoptotic signal in ACL cells.