Introduction In rheumatoid arthritis joints synovial hyperplasia and inflammatory cell infiltration lead to progressive destruc tion of cartilage and bone. Although the mechanisms under lying synovial hyperplasia are not completely known, accumulating evidence suggests that alterations in the apop tosis of synoviocytes are pivotal. Interestingly, RA fibroblast like synoviocytes express death receptors. yet, they are relatively resistant to FasL, TNF, and tumor necrosis related apoptosis inducing ligand induced apoptosis. This resistance has been related to high expression of anti apop totic molecules such as Fas associated death domain like IL1 beta converting enzyme inhibitory protein , sentrin 1, Bcl 2, Mcl 1, and constitu tive activation of Akt.

Apoptosis is a process highly regulated and crucial in many physiological situations, and could involve two main pathways. the extrinsic, by activation of death receptors, and the intrinsic or mitochondrial pathway. In the extrinsic pathway, FasL, TNF, and TRAIL ligation leads to recruitment of Fas associated via death domain and procaspase 8, which form the death inducing signaling complex, where caspase 8 is activated. In turn, caspase 8 activates caspase 3, which causes DNA fragmentation and cell death. The mitochondrial pathway is induced by hypoxia, cytotoxic drugs and growth factor deprivation leading to liberation of cytochrome c and Apaf 1 mediated activation of the caspase 9. This pathway is tightly regulated by members of the Bcl 2 family with anti apoptotic function, such as Bcl 2, Bcl xL, Bcl w, Mcl 1, and A1, which prevent mitochondrial mem brane permeability and release of cyt c.

In contrast, other Bcl family members, such as Bax, Bak, Bok, BH3 interact ing domain death agonist, Bad, Bim, and Puma, are pro apoptotic and promote mitochondrial membrane per meability. In some cell types, named type II cells, the two apoptotic pathways are connected through the cleavage of Bid by activated Entinostat caspase 8. Truncated Bid translocates to the mitochondria causing release of cyt c and cell death. In contrast, in type I cells, death receptor induced apoptosis is independent of Bid. It seems that both the intrinsic and extrinsic apoptotic pathways are involved in arthritis development. There is much evidence implicating the extrinsic pathway and . How ever, support for the role of the intrinsic pathway is scant, although very convincing. For example, mice lacking Bim or Bid develop a severe synovial inflammation and bone destruction in an arthritis model. Also, evidence suggests that RA FLS are type II cells. Therefore, it is necessary to investigate the relevance of the intrinsic path way and its connection with the extrinsic pathway in the FLS resistance to apoptosis.