CD95-induced osteoarthritic chondrocyte apoptosis and necrosis: dependency on p38 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.