Description
Osteoprotegerin (OPG, osteoclastogenesis inhibitory factor, OCIF) is a product of the TNFRSF11B gene, located on chromosome 8q24. OPG belongs to the TNF (tumor necrosis factor receptor) superfamily, that plays a key role in bone remodeling. Human OPG is a secreted glycoprotein composed of 401 aminoacid residues. OPG exists as a disulfidelinked homodimer (120 kDa) or as a monomer (60 kDa). Both of these forms are active but the dimer is more bioactive than the monomer. In contrast to most members of the TNF receptor superfamily, OPG probably exists only in a soluble form. Its ligands are RANKL and TRAIL. Human OPG shares 85% aminoacid identity to mouse OPG and 86 % identity to rat OPG. In adult humans OPG mRNA is highly expressed in bones (osteblasts), endothelial vessel cells, skin, liver, stomach, intestine, heart, brain and lung and is also present in atherosclerotic plaques. OPG and RANKL are involved in bone resorption and bone formation. OPG and receptor RANK compete with each other for binding to the ligand RANKL. Binding of RANKL to RANK stimulates osteoclasts and their activity. When RANKL binds to OPG, osteoclastogenesis decreases. OPG prevents the formation of RANKL/RANK, inhibits formation of osteoclasts and suppress bone resorption. At normal physiological conditions OPG and ligand RANKL are in balance and bone resorption and bone formation are linked. This balance can be disrupted by the lack of estrogens in menopausal women, by anti-inflammatory effect of cytokines and by changes in the level of glucocorticoids, thyroid hormones, parathyroid hormone or calcitriol. Any modification in the RANKL/OPG ratio can induce either excessive bone resorption or, in contrast, excessive bone formation. This disregulation can lead to pathological conditions such as osteoporosis/osteopenia, bone tumor associated osteolysis, or cardiovascular pathology. In postmenopausal osteoporosis, OPG serum level decreases and this decrease can be an indicator of a higher risk for bones fracture. In patients with glucocorticoid induced osteoporosis the RANKL/OPG ratio was higher. In patients with chronic obstructive pulmonary disease with low bone mineral density (BMD), RANKL/OPG ratio was significantly higher compared to those with normal BMD. Patients with juvenile idiopathic arthritis had significantly lower levels of OPG in serum and lower OPG/RANKL ratio. The OPG/RANKL/RANK system affects the cardiovascular system as well. In patients with ischemic heart disease the serum concentration of OPG was higher than that of healthy people. In patients with high OPG the risk of cardiovascular mortality is three- or four-times higher than it is in the healthy population. Finally, the presence of malignant tumors leads to an inhibition of OPG production resulting in high bone resorption. The OPG/RANKL/RANK system affects bone loss in many pathological states and participates in pathogenesis of vascular diseases. Determination of OPG concentration or RANKL/OPG ratio is a clinical indicator in the diagnosis of the pathological states mentioned below. Clinical use and areas of investigation: Postmenopausal and glucocorticoid induced osteoporosis, Reumatoid arthritis, juvenile idiopathic arthritis, Ischemic heart disease, Diseases with changed bone resorption activity