Tumor necrosis factor-alpha induced DNA cleavage in human articular chondrocytes may involve multiple endonucleolytic activities during apoptosis

Apoptosis has been documented in chondrocytes both in the growth plates of young, healthy cartilages and in osteoarthritic cartilages; little, however , is known about apoptosis in chondrocytes of normal adult articular cartil age. For the current study, apoptosis in adult chondrocytes was evaluated b y labeling DNA fragments using the ISEL in situ end labeling of 3'-recessed strand breaks) or TUNEL (5'-recessed or blunt-ended strand breaks with ter minal deoxynucleotidyl transferase-mediated nick end labeling) techniques i n primary cultures of chondrocytes in monolayer. Apoptosis was induced in t he chondrocytes by either Tumor Necrosis Factor alpha (TNF alpha), Interleu kin 1-beta (IL-1 beta), or anti-Fas antibody but only after 48 hours in cul ture. At 4 and 24 hours, there was no detectable DNA fragmentation. With TN F alpha, IL1 beta, and anti-Fas antibody, chondrocytes show evidence of at least two types of DNA strand breaks within the same cell las assessed by s imultaneous labeling with ISEL and TUNEL). Therefore, some pathways leading to apoptosis in chondrocytes appear to involve more than one type of endon uclease activity. When the chondrocytes were cultured as explants with the articular matrix intact lex vivo), neither IL-1 beta, TNF alpha, the anti-F as antibody, nor fibronectin fragments were able to induce apoptosis in the chondrocytes. In normal human adult cartilage that was untreated and uncul tured tin situ), DNA fragmentation was undetectable; however, a significant number of chondrocytes in osteoarthritic cartilage did contain strand brea ks. These data suggest that apoptosis occurs in chondrocytes in which the m atrix has been disrupted experimentally or destroyed by the osteoarthritic disease process. The results of these studies suggest that the ECM may be a n essential survival factor for chondrocytes. (C) 2000 Wiley-Liss, Inc.