It has been estimated that around 10% of total bone mass is renewed per year leading to a full replacement of the bone tissue every 10 years. This biological mechanism called “bone remodeling” plays a central role in calcium and phosphorus homeostasis and is the result of a balance between osteoclast and osteoblast activities. Indeed, osteoclasts originating from hematopoietic stem cells resorb bone (Rousselle and Heymann 2002); and in contrast, osteoblasts, which come from mesenchymal stem cells, are specialized in the formation of new mineralized extracellular matrix (Dechaseaux et al. 2009). For many years, a central regulation based on hormone network including parathyroid hormone (PTH) or calcitonin has been considered as the key system modulating the bone remodeling in addition to the local homeostasis driven by interleukin-6 or prostaglandin E2 (PGE2). Thus, osteoclast and osteoblast activities are able to control each other, and mesenchymal stem cells and osteoblasts have been well known to modulate osteoclast differentiation and bone degradation. The late 1990s have been marked by the identification of the key master proteins Receptor Activator of Nuclear Factor Kappa B Ligand (RANKL), RANK, and Osteoprotegerin (OPG), which control the intimate communications between osteoblasts and osteoclasts. OPG was originally discovered in 1997 by two research teams independently. Boyle’s group generated transgenic mice overexpressing various TNF receptor cDNAs and one of them exhibited a huge osteoporotic phenotype associated with a defect of osteoclastogenesis (Simonet et al. 1997). The corresponding protein encoded by this cDNA was named osteoprotegerin from the latin “osteo” (bone) and “protegere” (to protect). On the same time, Tsuda’s group identified a repressor of osteoclast differentiation, which they named Osteoclast Inhibitory Factor (OCIF) (Tsuda et al. 1997; Yasuda et al. 1998a). OCIF and OPG were in fact the same protein belonging to the TNF receptor superfamily and its international name according to the TNF nomenclature is TNFRSF11B. One year later, the first ligand of OPG was identified and called OPG Ligand (OPGL) and Osteoclast Differentiation Factor (ODF) by both groups (Yasuda et al. 1998b; Lacey et al. 1998). OPG ligand was similar to a soluble protein previously named TNF-related activation-induced cytokine (TRANCE) and which was expressed after activation of T-cell receptor. RANKL/TRANCE was a member of the TNF superfamily, which bound to RANK, a transmembrane trimeric receptor (Anderson et al. 1997). Since this time, RANKL/RANK/OPG constitute the master molecular triad controlling bone resorption and this discovery has led to opening up entirely new areas in bone field (Baud’huin et al. 2013), especially the development of therapies (e.g., anti-RANKL antibody Denosumab) (Bekker et al. 2004).
Molecular Characteristics of OPG, Its Pattern of Expression, and Its Multiple Ligands
OPG characteristics and its patterns of expression
Soluble monomer and dimer
Main pattern of cellular expression
Bone marrow stromal cells, immune and hematopoietic cells (dendritic cells, follicular dendritic cells, lymphoid cells, monocytes, B and T lymphocytes, megakaryocytes), endothelial cells, fibroblasts, osteoblast, epithelial intestinal cells
Main pattern of tissue expression
Smooth muscle cells, bone, bone marrow, brain, calvaria, heart, intestines, kidney, liver, lung, mammary gland, placenta, prostate, skin, spinal cord, spleen, stomach, thyroid, cartilage
PGE2, Glucocorticoid, PTH, PTHrP, IGF1, Indomethacin, cyclosporin A, tacrolimus, dexamethasone, ionomycin, calcium, indian hedgehog
17P-estradiol, MP-2, GH, IL-1b, IL13, IL-18, IL-6, LIF, OSM, PDGF, TGFP, IFNy, RA.NKL, TNFa, FGF2, MIPla, cAMP, forskolin, inorganic phosphate, activitin A, epinephrine, leptin, triiodothyronine, _ ethanol, nitric oxide, bisphophonate
Based on these molecular features, OPG cannot only bind to RANKL but can interact with numerous ligands (Fig. 1b). OPG can interact with TRAIL with an equilibrium dissociation constant (KD) around 24 × 10 −9M (Emery et al. 1998) and is physically colocalized with von Willebrand factor into the Weibel-Palade bodies located in endothelial cells (Baud’huin et al. 2013) (Fig. 1c). The interaction between vWF and Factor VIII (FVII)/vWF do not affect the coagulation parameters. Glycosaminoglycans bind to OPG, and dermatan sulfate and chondroitin sulfate showed the highest binding affinity (Fig. 1c). The functional aspects of these interactions will be described in the paragraphs below.
OPG Is a Powerful Inhibitor of Osteoclastogenesis and Bone Resorption
The role of OPG has been established initially by the differential bone phenotype exhibited by transgenic and knockout mice. Indeed, OPG knockout mice were osteoporotic with a marked decrease of their bone mineral density and with a high incidence of spontaneous fractures and spine deformities (Bucay et al. 1998). This phenotype was totally reversed by the administration of recombinant OPG. In contrast, OPG transgenic mice showed an osteopetrosis associated with a strong decrease of osteoclastogenesis and an impairment of thymocyte development (Simonet et al. 1997). As expected, RANKL knockout mice were osteopetrotic with a defect in tooth eruption explained by a total absence of osteoclasts due to the osteoblast inability to support osteoclastogenesis. RANKL transgenic mice were osteoporotic with an exacerbated activation of osteoclast differentiation. In bone tissue, OPG is mainly secreted by osteoblasts and mesenchymal stromal cells, which produced membrane-bound and soluble RANKL (Fig. 1d). RANK is expressed by osteoclast precursors and mature osteoclasts, and the binding of RANKL to the extracellular RANK domain leads to the expression of specific genes involved in osteoclast differentiation, bone resorption, and osteoclast survival. OPG prevents the binding of RANKL to RANK and consequently inhibits osteoclast differentiation and bone resorption. OPG acts as a decoy receptor for RANKL.
OPG Presents a Large Panel of Biological Functions More Specifically in the Control of Immune and Vascular Systems
In vascular system, OPG promotes endothelial cell survival through αvβ3 and NF-κB pathways (Pritzker et al. 2004; Baud’huin et al. 2007). OPG stimulates the proliferation of endothelial colony-forming cells in vitro and the formation of microvessel in vivo (Benslimane-Ahmin et al. 2011). Furthermore, OPG bound to the vWF in endothelial cells is secreted in response to inflammatory signals. More recently, Kondegowda et al. (2015) demonstrated that OPG enhanced mouse and human β-cell proliferation, and OPG activation resulted in an increase of β-cell mass in healthy and diabetic mice. In this system, OPG abrogates the inhibitory effects of RANKL on β-cell proliferation. This last observation underlines the potential interest of OPG in diabetes treatment.
OPG and Its Dysregulation Are Related to Various Disorders
According to its inhibitory effect on bone resorption, OPG has been strongly related to the bone loss associated diseases (e.g., postmenopausal osteoporosis, bone loss in Crohn’s disease, bone destruction in rheumatoid arthritis) (Théoleyre et al. 2004). Osteoporosis is a multifactorial disease including genetic susceptibility and environmental factors. In this context, polymorphisms of OPG correlating with the risk of osteoporosis were identified (Zofkova et al. 2015). In rheumatoid arthritis, OPG acts as expected as a bone protector but has no effect on inflammation.
Growing evidence indicates that OPG has a strong impact directly and indirectly in oncology (Théoleyre et al. 2004; Lamoureux et al. 2010). Indeed, OPG binding to TRAIL leads to the neutralization of TRAIL apoptotic activity, a natural inducer of tumor cell apoptosis. In this case, OPG can act as a survival factor in several cancer types and could have a deleterious impact (Emery et al. 1998). Myeloma cells express CD138 (syndecan-1), which can bind OPG, and then controls its bioavailability by increasing its internalization and degradation. This mechanism is associated with an increase of local RANKL concentration and a high osteolysis level, which establishes a favorable niche for tumor growth. OPG expression by tumor endothelial cells also correlates with a proangiogenic phenotype and with clinical data in human tumors. Then, OPG is able to block the vicious cycle established between cancer cells and their bone microenvironment, especially osteoclasts and blood vessels (Lamoureux et al. 2010). Jones et al. (2006) demonstrated the role of RANKL in the control of the metastatic process. Indeed, numerous cancer cells express RANK and are sensitive to RANKL. RANKL triggers the migration of RANK expressing cancer cells in vitro and in vivo, and OPG abolishes RANKL activities and strongly reduces the tumor burden in bone exclusively. More recently, Yoldi et al. (2016) showed that the inhibition of RANK signaling markedly decreases the cancer stem cell pool in breast cancer as well as the tumor and metastasis initiation and enhances sensitivity to chemotherapy.
The role of OPG in vascular diseases has been provided by the vascular phenotype of OPG-deficient mice, which exhibited medial calcification of the aorta and renal arteries (Bucay et al. 1998). OPG is expressed more frequently in atheromatous carotid plaques than in femoral arteries and correlates with the macrophage infiltration (Davaine et al. 2014, 2016). Diabetic patients with uncontrolled glycemia showed increased levels of OPG associated with atherosclerosis and increased morbidity/mortality linked to vascular diseases. Interestingly, OPG administration in mice prevents vascular calcifications induced by warfarin or high vitamin D doses (Baud’huin et al. 2013). This observation suggested that OPG and its molecular partners could play a role in association between osteoporosis and vascular calcifications. Numerous epidemiological studies report the association between circulating OPG and incident of cardiovascular diseases. However, it remains unclear if the impact of OPG on cardiovascular system results from an active pathway (e.g., signaling through proteoglycans) or from a passive mechanism (e.g., TRAIL blockade or inhibition of RANKL signaling) (Bernardi et al. 2016).
In 1997, the identification of OPG, a powerful inhibitor of osteoclast differentiation and bone resorption, was the first signal of a scientific revolution in the field of bone biology. With RANKL, its main ligand OPG/RANKL is the master proteins at the origin of a new topic named osteoimmunology. Indeed, OPG/RANKL is functionally involved in bone remodeling and in immune system by controlling dendritic cells survival and B-lymphocyte maturation. However, OPG activities were not restricted to bone and immunity, and OPG delivered progressively its secrets and multiple functional facets. OPG showed its ability to bind to numerous ligands such as TRAIL, proteoglycans, glycoaminoglycans, vWF, and FVIII/vWF complex and plays consequently functional activities in diverse biological systems such as endothelial cells or β-cell proliferation. OPG is clearly related to (i) osteoporosis with identified polymorphisms; (ii) cancer development by modulating tumor cell survival and migration and by regulating the tumor niches (e.g., bone, immune, and vascular niches); (iii) cardiovascular diseases by active (e.g., OPG binding to syndecan-1) or passive mechanisms (e.g., inhibition of TRAIL activities or RANKL signaling). Even if OPG by itself remains difficult to manipulate as therapeutic tool due to its multiple ligands, OPG has led to the development of RANKL inhibitors used in clinical trials, and OPG remains a fascinating therapeutic target/agent in numerous benign and malignant diseases.
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