Fas-mediated apoptosis is observed in synoviocytes of patients with rheumatoid arthritis (RA), but not in those of patients with osteoarthritis (OA). The present study was conducted to elucidate the mechanisms that initiate induction of Fas-mediated apoptosis in RA synoviocytes.
Cultured OA synoviocytes, which are insensitive to Fas-mediated apoptosis in spite of Fas antigen expression, were used in these experiments. Synovial cell proliferation and cytotoxicity studies were performed using MTS and lactate dehydrogenase release assays. Surface expression of Fas antigen was analyzed by flow cytometry. The expression and function of apoptosis-signaling molecules, such as caspase 8 and caspase 3, were examined by immunoblot analysis.
Tumor necrosis factor alpha (TNFalpha) induced proliferation of cultured OA synoviocytes. Fas ligation with anti-Fas monoclonal antibody (mAb) resulted in cytotoxic activity against cultured OA synoviocytes that had been pretreated with TNFalpha for 5 days, but not those pretreated for 2 days. In contrast, anti-Fas mAb did not show a cytotoxic effect against untreated cultured OA synoviocytes. A gradual up-regulation of caspase 8 and caspase 3, which played a role in the caspase cascade for Fas-mediated apoptosis, was observed in TNFalpha-treated cultured OA synoviocytes. In addition, Fas ligation to TNFalpha-treated cultured OA synoviocytes induced activation of caspase 8 and caspase 3, with subsequent cleavage of poly(ADP-ribose) polymerase (PARP), a substrate of activated caspase 3. More importantly, Z-IETD-FMK, a caspase 8 inhibitor, and Ac-DEVD-CHO, a caspase 3 inhibitor, almost completely inhibited Fas-mediated apoptosis of TNFalpha-treated cultured OA synoviocytes, whereas Ac-YVAD-CHO, a caspase 1 inhibitor, did not.
Our results clearly demonstrate that TNFalpha stimulates synovial cells to proliferate as well as sensitizes the cells for Fas-mediated apoptosis, at least in part by up-regulation and activation of caspase 8 and caspase 3. These findings suggest that TNFalpha may be one of the factors providing sensitization of synovial cells to Fas-mediated apoptosis in RA.