Bcl-2 expression in synovial fibroblasts is essential for maintaining mitochondrial homeostasis and cell viability

The regulation of proliferation and cell death is vital for homeostasis, bu t the mechanism that coordinately balances these events in rheumatoid arthr itis (RA) remains largely unknown. In RA, the synovial lining thickens in p art through increased proliferation and/or decreased synovial fibroblast ce ll death. Here we demonstrate that the anti-apoptotic protein, Bcl-2, is hi ghly expressed in RA compared with osteoarthritis synovial tissues, particu larly in the CD68-negative, fibroblast-like synoviocyte population. To dete rmine the importance of endogenous Bcl-2, an adenoviral vector expressing a hammerhead ribozyme to Bcl-2 (Ad-Rbz-Bcl-2) mRNA was employed. Ad-Rbz-Bcl- 2 infection resulted in reduced Bcl-2 expression and cell viability in syno vial fibroblasts isolated from RA and osteoarthritis synovial tissues. In a ddition, Ad-Rbz-Bcl-2-induced mitochondrial permeability transition, cytoch rome c release, activation of caspases 9 and 3, and DNA fragmentation. The general caspase inhibitor zVAD.fmk blocked caspase activation, poly(ADP-rib ose) polymerase cleavage, and DNA fragmentation, but not loss of transmembr ane potential or viability, indicating that cell death was independent of c aspase activation. Ectopically expressed Bcl-x(L) inhibited Ad-Rbz-Bcl-2-in duced mitochondrial permeability transition and apoptosis in Ad-Rbz-Bcl-2-t ransduced cells. Thus, forced down-regulation of Bcl-2 does not induce a co mpensatory mechanism to prevent loss of mitochondrial integrity and cell de ath in human fibroblasts.