Pro-apoptotic Proteins for the Maintenance of Homeostatic Balance
Abstract
Abstract
Apoptosis is the process of regulated cellular death that is vital in development, defense against pathogens, and maintaining cellular homeostasis. Proper regulation of apoptosis can protect against the development of autoimmunity by causing the elimination of autoreactive cells. Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic joint inflammation and the destruction of cartilage and bone of the affected joint as a result of the recruitment of leukocytes to the RA joint and activation of leukocytes, synoviocytes, and osteoclasts.1 The Bcl- 2 protein family, which mediates the intrinsic apoptosis pathway, has recently been shown to play an important role in the prevention of rheumatic diseases, including RA. This article discusses the role of the Bcl-2 family in regulating apoptosis, with a focus on the pro-apoptotic proteins, which are crucial for maintaining a proper homeostatic balance.
Intrinsic Apoptosis Pathway
There are two distinct apoptotic pathways: extrinsic and intrinsic. While both pathways conclude with the activation of caspases and the degradation of DNA and other vital cellular components, they differ in their mechanisms. The extrinsic pathway is triggered from outside the cell with the binding of a death ligand to its cognate death receptor. The intrinsic pathway is mediated from within the cell, usually in response to cellular signals resulting from cellular stresses such as DNA damage, cell-cycle dysregulation, hypoxia, or loss of cell survival factors. The Bcl-2 family mediates the intrinsic pathway, is composed of both anti- and pro-apoptotic members, and is defined by the presence of shared Bcl-2 homology (BH) domains. The pro-apoptotic family members can be further subdivided based on the number of BH domains within the protein. The multi-domain proteins include the executor proteins Bak and Bax. The BH3-only proteins include initiator proteins such as Bim and Puma, which can interact with a wide range of antiapoptotic Bcl-2 family proteins, as well as sensitizer proteins such as Bad, Bok, and Bmf, which have a much more restricted ability to interact with antiapoptotic family members.1
Upon stimulation, intrinsic pathway activator proteins such as Bim translocate to the mitochondria, where they interact with Bcl-2 and other antiapoptotic proteins.2 While the exact mechanism of this interaction is still poorly understood, it is believed that it results in antiapoptotic proteins becoming sequestered away from the executioner proteins Bak and Bax, which are then allowed to form homo- and heterodimers. The prevailing hypothesis is that these dimers act as pores, which allow for release of cytochrome-C and other apoptogenic molecules from the mitochondria. Once out of the mitochondria, cytochrome-C forms the apoptosome with Apaf-1 and pro-caspase-9, leading to the catalysis of pro-caspase-9 into caspase-9 and the resultant catalysis of procaspase- 3 to caspase-3. The ultimate result of this activation will be DNA degradation and cell death.
Cell Death and Rheumatoid Arthritis
RA can be characterized on a cellular level by an increase in the number of lymphocytes, macrophages, and synovial fibroblasts in the joint synovium. Inflammation in the joint is promoted through the production of pro-inflammatory cytokines and chemokines, including tumor necrosis factor-α (TNF-α), interleukin-1 (IL-1), IL-8, and monocyte chemoattractant protein-1 (MCP-1), produced by the expanded number of cells in the RA
joint. 3 The chemotactic joint milieu results in the recruitment of additional cells, perpetuating the damage to the joint. Hyperplasia of the synovial lining results in an increase in thickness from one to two cell layers to 10–12 cell layers in RA patients as a consequence of the increased cellularity in the joint. This increase in synovium thickness occurs in conjunction with an increase in the expression of the antiapoptotic
proteins Bcl-2,4 Mcl-1,5,6 and Bcl-xL7 in the RA patient synovium compared with control synovial tissue. Lining thickness and the inflammatory score have previously been shown to directly correlate with the frequency of Bcl-2-positive cells.4 These data suggest that decreased synovial apoptosis, as characterized by increased Bcl-2 expression, may be associated with worse patient outcomes in RA. Strengthening this hypothesis, it has been shown by in situ hybridization that messenger RNA (mRNA) levels of Bcl-2 are increased in RA compared with osteoarthritis (OA) tissue. 8 In experiments utilizing the adjuvant-induced arthritis model in rats, the data suggest that the levels of Bcl-2 are increased at sites of inflammation and erosion.9
Despite reports showing increased expression of the pro-apoptotic Bcl-2 family members Bax10 and Puma 11 in RA tissue, the clear lack of apoptotic cells suggests that these proteins are not sufficient to induce apoptosis in the RA joint. It has been reported that RNA-interference (i) of the proapoptotic executioner proteins Bak and Bax results in increased cell death in RA synovial fibroblasts.5 The level to which Bak-/- Bax-/- RNAi induces death is similar to that which has been shown following synovial fibroblast treatment with Bim-RNAi in the presence of antisense Mcl-1.5 Taken together, these data suggest a critical requirement for Bim as well as Bak/Bax in RA synovial fibroblasts.
