Secreted protein acidic and rich in cysteine (SPARC,BM-40) protein belongs to the family of secreted matricellular proteins with similar domain structure. Mouse SPARC protein involved an N-terminal acidic region that binds calcium, a follistatin domain containing Kazal-like sequences, and a C-terminal extracellular calcium (EC) binding domain with two EF-hand motifs. SPARC is produced by fibroblasts, capillary endothelial cells, platelets, and macrophages, especially in areas of tissue morphogenesis and remodeling. It appears to regulate cell growth through interactions with the extracellular matrix and cytokines. SPARC is expressed at high levels in tissues undergoing morphogenesis, remodeling and wound repair. The activity of SPARC is to modulate cell-cell and cell-matrix interactions, and its de-adhesive and growth inhibitory properties in non-transformed cells have led to studies to assess its role in cancer.
Osteopontin, also known as Secreted phosphoprotein 1, Bone sialoprotein 1, BSP-1, OPN, and SPP1, is a member of the osteopontin family and a SIBLING glycoprotein. Osteopontin has been classified as T-helper 1 cytokine and thus believed to exacerbate inflammation in several chronic inflammatory diseases, including atherosclerosis. Besides proinflammatory functions, physiologically Osteopontin is a potent inhibitor of mineralization, it prevents ectopic calcium deposits and is a potent inducible inhibitor of vascular calcification. Osteopontin is expressed and secreted by various cells, and has a role in cell adhesion, chemotaxis, prevention of apoptosis, invasion, migration and anchorage-independent growth of tumor cells. Osteopontin recruitment functions of inflammatory cells are thoμght to be mediated throμgh its adhesive domains, especially the arginine-glycine-aspartate (RGD) sequence that interacts with several integrin heterodimers. Osteopontin has emerged as a potential biomarker and mediator in cardiovascular disease. In the context of atherosclerosis, OPN is generally regarded as a proinflammatory and proatherogenic molecule. However, the role of OPN in vascular calcification (VC), which is closely related to chronic and active inflammation, is that of a negative regulator because it is an inhibitor of calcification and an active inducer of decalcification. Extensive research has demonstrated the pivotal participation of Osteopontin in the regulation of cell signaling which controls neoplastic and malignant transformation. The elevated expression of Osteopontin has been observed in a variety of cancers. It has been linked with tumor metastasis and signifies a poor prognosis for the patient.
Osteoprotegerin or TNFRSF11B is a member of the TNF-receptor superfamily. This protein is an osteoblast-secreted decoy receptor that functions as a negative regulator of bone resorption. This protein specifically binds to its ligand, osteoprotegerin ligand, both of which are key extracellular regulators of osteoclast development. Studies of the mouse counterpart also suggest that this protein and its ligand play a role in lymph-node organogenesis and vascular calcification. Alternatively spliced transcript variants of this gene have been reported, but their full length nature has not been determined. Osteoprotegerin/TNFRSF11B acts as decoy receptor for RANKL and thereby neutralizes its function in osteoclastogenesis. This protein may inhibit the activation of osteoclasts and promotes osteoclast apoptosis in vitro. Bone homeostasis seems to depend on the local RANKL/OPG ratio. Osteoprotegerin/TNFRSF11B also play a role in preventing arterial calcification, act as decoy receptor for TRAIL and protect against apoptosis. TRAIL binding blocks the inhibition of osteoclastogenesis.
Osteoprotegerin (OPG, Tnfrsf11b) is a secreted protein that regulates bone density. OPG is widely expressed and constitutively released as a homodimer by mesenchymal stem cells, fibroblasts and endothelial cells. Regulation of its expression by estrogen, parathyroid hormone and cytokines is complex and changes with age. OPG acts as decoy receptor for TNFSF11/RANKL and thereby neutralizes its function in osteoclastogenesis. TRAIL decreases the release of OPG from cells that express it, while OPG inhibits TRAIL-induced apoptosis. Expression of RANK L on the cell surface, and thus its ability to stimulate osteoclastogenesis, is regulated by OPG by intracellular and extracellular mechanisms. Bone homeostasis seems to depend on the local ratio between TNFSF11 and TNFRSF11B. It may also play a role in preventing arterial calcification.
OX40 ligand (OX40L), also called CD252, is a single-pass type II membrane protein of the TNF/TNF receptor superfamily. OX40L is expressed by DCs, macrophages and B cells and signals via its cognate receptor OX40 which is mainly expressed on APCs. OX40L/OX40 interactions are important in T-cell activation and survival and for the generation of memory T cells from activated effector T cells. OX40L–OX40 co-stimulation leads to activation of TNF receptor associated factor (TRAF) 2, 3 and 5. This pathway has been shown to prolong the survival of effector CD4+Th cells as well as contributes to generation of memory T cells.
OX40, also termed CD134 and TNFRSF4, is a T cell co-stimulatory molecule of the TNF receptor superfamily which plays a key role in the survival and homeostasis of effector and memory T cells. OX40 is expressed on CD4+ and CD8+ T cells upon engagement of the TCR by antigen presenting cells along with co-stimulation by CD40-CD40 Ligand and CD28-B7. The interaction between OX40 and OX40 ligand (OX40L) will occur when activated T cells bind to professional antigen-presenting cells (APCs). The T-cell functions, including cytokine production, expansion, and survival, are then enhanced by the OX40 costimulatory signals. OX40 signals are critical for controlling the function and differentiation of Foxp3+ regulatory T cells. OX40-OX40L interaction regulates T-cell tolerance, peripheral T-cell homeostasis, and T-cell-mediated inflammatory diseases.
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