Chondrocyte apoptosis determined by caspase-3 expression varies with fibronectin distribution in equine articular cartilage

AIM: The purpose of this study was to investigate associations between the extent of chondrocyte apoptosis and expression of the articular cartilage (AC) extracellular matrix (ECM) molecules, cartilage oligomeric matrix protein (COMP) and fibronectin. METHOD: Cartilage from four sites (when available) on equine left middle carpal joints (n = 12) were used. Expression of COMP and fibronectin was determined using specific polyclonal antibodies and a biotin-streptavidin/peroxidase method. The intensity of staining for matrix molecules was graded (none, mild, moderate, strong) in each cartilage zone. Apoptosis of chondrocytes in AC sections was assessed by their expression of active caspase-3 using immunohistochemistry. RESULTS: The intensity of fibronectin expression varied significantly according to cartilage depth, with greater expression in the deep zone than in either the superficial or middle layers (P <0.001). A significant positive association was found overall between intensity of fibronectin expression and chondrocyte apoptosis (r = 0.44, P = 0.0187). The data were also significant for superficial and deep zones (r = 0.44, P = 0.0239 and r = 0.42, P = 0.0279 respectively). Conversely, intensity of COMP expression did not show zonal differences and was un-associated with degree of apoptosis. However, COMP expression was significantly more intense in cartilage than fibronectin (P = 0.0007), and the correlation between overall intensity of COMP and fibronectin was statistically significant (r = 0.56, P = 0.0018). CONCLUSION: The positive correlation between the incidence of apoptosis and expression of fibronectin, a key ECM molecule involved in communication between the chondrocyte and surrounding matrix, suggests that chondrocyte death by apoptosis may alter cartilage metabolism, supporting the role of this process in the pathogenesis of osteoarthritis.