CD 95-induced osteoarthritic chondrocyte apoptosis and necrosis : dependency on p 38 mitogen-activated protein kinase

One of the hallmarks of osteoarthritic cartilage is the loss of chondrocyte cellularity due to cell death. However, considerable controversy has recently arisen surrounding the extent of apoptotic cell death involved in development of osteoarthritis (OA). To shed light on this issue, we characterized cell death in primary OA chondrocytes mediated by the CD95 (Fas) pathway. Treatment of chondrocytes with anti-CD95 not only increased the rate of cell death but also increased the production of CD95 ligand by chondrocytes. This reveals a novel autocrine regulatory loop whereby activated chondrocytes may amplify CD95 signals by inducing synthesis of CD95 ligand. Multiple morphologic detection analyses indicated that apoptosis accounted for only a portion of chondrocyte death, whereas the other chondrocytes died by necrosis. Both chondrocyte apoptosis and necrosis depended on the activity of p38 mitogen-activated protein kinase (MAPK) within chondrocytes. Treatment of chondrocytes with the p38 MAPK inhibitor SB203580 abolished anti-CD95 induced cell death by inhibiting the activities of activating transcription factor-2 and caspase-3. In addition, inhibition of p38 MAPK activity in chondrocytes stimulated chondrocyte proliferation, as indicated by 5-bromo-2-deoxyuridine (BrdU) index. Thus, p38 MAPK is a potential therapeutic target, inhibition of which may maintain the cellularity of articular chondrocytes by inhibiting cell death and its amplification signal and by increasing cell proliferation. Introduction Chondrocytes are the only cells in articular cartilage, and thus they are responsible for its structural integrity by maintaining its extracellular matrix. Osteoarthritis (OA) is characterized by destruction of extracellular matrix and loss of chondrocyte function. Chondrocyte depletion was found to be a persistent and important event in OA [1-3], and apoptosis was believed to be a major cause of such cell depletion [4-6]. However, in a recent study [7], although a significant increase in lacunar emptying was observed in human OA cartilage, apoptotic cell death could not fully account for the loss of cells in lacunae. This raises an important question regarding the extent of the contribution of apoptotic cell death to the loss of chondrocytes during OA progression. If apoptosis does not fully account for cell loss in OA cartilage, then what else is involved? More importantly, what are the mechanisms that underlie such loss of chondrocytes? The present study was designed to address these questions by characterizing cell death in primary OA chondrocytes induced by activation of CD95 (Fas). A significant amount of CD95 ligand (CD95L) has been found in synovial fluid from patients with OA and those with rheumatoid arthritis [8]. Furthermore, in human articular cartilage, CD95 expression in close proximity to OA lesions was found to be increased relative to that further from the lesion [9]. Expression of CD95 and CD95L was higher in aged cartilage than in mature cartilage, which correlated with the decrease in viable cell density in rabbit articular cartilage during aging [10]. This in vivo evidence suggests an important role for CD95 in joint cartilage degeneration, although the precise mechanisms are unclear.