SurveyThe molecular triad OPG/RANK/RANKL: involvement in the orchestration of pathophysiological bone remodeling
Introduction
Bone is a specialized connective tissue formed by a mineralized matrix that confers it with elasticity and strength. Bone remodeling allows for adaptation to mechanical constraints and maintains homeostasis of phosphorus and calcium through coordinated phases of formation and resorption. Thus, bone remodeling involves synthesis of organic matrix by osteoblasts and bone resorption by osteoclasts. Osteoblasts differentiate from mesenchymal stem cells through a series of progenitor stages to form mature matrix-secreting osteoblasts and are progressively transformed into osteocytes [2]. Osteoblasts and osteocytes connected by gap junctions then constitute a cellular network, both on the bone surface and within the bone matrix. Osteoclasts are the main protagonists of bone resorption. If they are hematopoietic in origin and are closely related to macrophages, they also possess several characteristic features (multinucleation, highly polarized morphology, and numerous mitochondria). Osteoclasts resorb bone by attaching to the surface and then secreting protons into an extracellular compartment formed under their ruffled border. This secretion is necessary for bone mineral solubilization and the digestion of the organic matrix by acid proteases [3]. The other cell protagonists present in the bone microenvironnement (monocytes/macrophages, lymphocytes, and endothelial cells) also contribute to bone remodeling by direct contact with bone cells or by the release of soluble effectors. Thus, the numerous cellular protagonists involved in bone homeostasis are responsible for the connections that are established between bone tissue, the immune system, and the vascular compartment and allow for a better understanding of the associated pathology.
In 1965, Epker and Frost [4] demonstrated that the interactions between osteoblasts and osteoclasts are essential in bone remodeling. This equilibrium between osteoblast and osteoclast activities is tightly regulated by physical parameters (that is, mechanical stimulation) and numerous polypeptides (hormones, cytokines) [5]. Any disturbance between these effectors leads to the development of skeletal abnormalities, characterized by decreased (osteoporosis) or increased (osteopetrosis) bone mass. Increased osteoclast activity is observed in many osteopathic disorders, including post-menopausal osteoporosis, Paget's disease, primary bone tumors, lytic bone metastases, multiple myeloma, and rheumatoid arthritis, leading to increased bone resorption and a loss of bone mass. Osteoblasts control bone-resorbing activities of osteoclasts and are also clearly involved in osteoclast differentiation. Thus, coculture of osteoblast (bone marrow stromal cells) and osteoclast precursors (spleen or bone marrow cells) results in the formation of functional osteoclasts. In contrast, when both cell populations are separated by a selective membrane permeable to soluble effectors but not to cells, no osteoclasts are formed [6], [7]. These studies demonstrated for the first time that cell–cell contacts are required for the induction of osteoclastogenesis by osteoblastic cells. Moreover, these data imply that osteoblasts express ligands on their membrane that are recognized by receptors on the surface of osteoclasts. All of these observations drove many scientists to look for the effectors responsible for osteoblast-osteoclast interactions. Thus, in 1997 and 1998, research led to the identification of a novel set of cytokines within the TNF family that are required for the control of bone remodeling [8], [9], [10], [11].
The present manuscript is an extension of the earlier review recently published by Kwan Tat et al. [1] and focuses on the involvement of osteoprotegerin (OPG), of the receptor activator of nuclear factor κB ligand (RANKL), and of its cognate receptor RANK in the orchestration of pathophysiological bone remodeling (structure, production, regulation, interactions, biological activities) and their potential implications in benign and malignant human pathology.
Section snippets
The discovery of the molecular triad OPG/RANK/RANKL was a major event of the past decade in bone biology
Working on the identification of TNF receptor-related molecules with potential therapeutic interests, the Amgen group demonstrated the existence of a truncated TNF receptor-like molecule responsible for the marked osteopetrosis phenotype when overexpressed in transgenic mice [8]. Complementary experiments revealed that this osteopetrosis is associated with a decrease in osteoclastogenesis and in osteoclast activation. This bone-protecting molecule was named osteoprotegerin (OPG).
The wide range of cells which produce OPG, RANK and RANKL allows definition of three main biological systems controlled by this triad
While OPG, RANK and RANKL are produced by numerous cell types and a variety of tissues (Table 2), their expression pattern targets three main biological systems where the molecular triad could be more specifically involved: the osteoarticular, immune, and vascular systems (Fig. 2). Transgenic and knockout mice clearly revealed the potential involvement of these effectors in the three biological systems. Thus, mice deficient in OPG exhibit a decrease of total bone density and a high incidence of
OPG/RANK/RANKL are key molecules for osteoclastic differentiation but can be shunted by other molecular mediators
OPG expression and RANKL expression are modulated by numerous osteotropic agents (Table 3). Among them, OPG expression is positively regulated by estrogen, TNF-α, GH and TGF-β, and negatively by PTH and glucocorticoids. RANKL is also strongly modulated by the same effectors controlling OPG. Contrasting with OPG and RANKL, the regulation of RANK expression is more restricted to immune cells. Indeed, if RANK expression is controlled on dendritic cells by CD40L and via TCR engagement on T
TRAF6 plays a key role in the control of osteoclastic biology by OPG/RANK/RANKL
Within the OPG/RANK/RANKL triad, soluble and membrane forms of RANKL expressed by osteoblasts exert their activities through binding to their RANK receptor on osteoclasts [18]. The binding of RANKL to the extracellular RANK domain leads to the expression of specific genes involved during osteoclast differentiation, bone resorption, and osteoclast survival (Fig. 3). The initial step in RANK signaling is the binding to the TNF receptor-associated factor (TRAF) adaptator proteins within the
Osteoclastic differentiation is controlled by complex interactions between OPG, RANK, RANKL, and other secondary modulators
OPG, RANK, and RANKL molecules are characterized by their capability to form homo- and hetero-multimerized complexes (Fig. 4). Thus, while the OPG monomer is biologically active, OPG dimer formation is required to elicit full biological activity in vitro and in vivo [8]. Similarly, RANKL can form a homotrimer, as revealed by the study of their crystal structures. In this context, Lam et al. [23] proposed a model in which trimeric RANKL binds to a trimeric RANK complex, in which the
The OPG/RANK/RANKL triad is implicated in many non-malignant pathological conditions
The crucial role of OPG/RANK/RANKL in bone homeostasis and in cardiovascular functions allowed the identification of genetic mutations in RANK or OPG genes, which activate RANK or alter the interactions between RANKL and OPG, leading to hyperphosphatasia or bone abnormalities and to various vascular morphologies (Table 4). Thus, osteolytic and non-osteolytic hyperphosphatasia have been associated with an activating mutation in exon 1 of the gene encoding RANK (18 bp duplication in position 84)
OPG/RANK/RANKL involvement in malignant pathology
The dysregulation of the functional equilibrium in the OPG/RANK/RANKL triad is responsible for the osteolysis associated with malignant tumors and for the development of such tumors in bone sites (Table 5). RANKL has already been detected in several tumor cells and can be considered as a key factor involved in bone remodeling associated with bone metastases [209], [217], [219], [230], [231]. Moreover, although several primary melanoma and breast cancer cells did not express RANKL transcripts
Therapeutic strategies based on the OPG/RANK/RANKL triad
The detection of human OPG in the sera of rats gavaged with human milk affirms the key role of OPG during bone growth and the relevance of its involvement for the natural prevention of immune and bone disorders [229]. In light of the data described above, novel strategic treatments for bone loss are emerging that are based on an understanding of the functional status of the OPG/RANK/RANKL triad. Three therapeutic strategies have been envisaged; these involve: (1) cytokines and their soluble
Acknowledgements
This work was supported by a CReS INSERM no. 4CR06F, by a grant from the French Ministry of Research and Technology (TS/0220044) and by a grant from the Loire-Atlantique Committee of the Ligue Contre le Cancer. S. Théoleyre and Y. Wittrant, respectively, received a fellowship from the Loire-Atlantique Committee of the Ligue Contre le Cancer and from the Région of Pays de Loire.
References (241)
- et al.
Mesenchymal stem cells and osteoblast differentiation
- et al.
Osteoclastic acidication during bone resorption
Bone
(2002) - et al.
Osteoprotegerin: a novel secreted protein involved in the regulation of bone density
Cell
(1997) - et al.
Isolation of a novel cytokine from human fibroblasts that specifically inhibits osteoclastogenesis
Biochem Biophys Res Commun
(1997) - et al.
Osteoprotegerin ligand is a cytokine that regulates osteoclast differentiation and activation
Cell
(1998) - et al.
Osteoclastogenesis inhibitory factor suppresses osteoclast survival by interfering in the interaction of stromal cells with osteoclast
Biochem Biophys Res Commun
(1998) - et al.
Structure of the mouse osteoclastogenesis inhibitory factor (OCIF) gene and its expression in embryogenesis
Gene
(1998) - et al.
The TRAF family of signal transducers mediates NF-kappaB activation by the TRANCE receptor
J Biol Chem
(1998) - et al.
Characterization of structural domains of human osteoclastogenesis inhibitory factor
J Biol Chem
(1998) - et al.
Evidence for a role of a tumor necrosis factor-alpha (TNF-alpha)-converting enzyme-like protease in shedding of TRANCE, a TNF family member involved in osteoclastogenesis and dendritic cell survival
J Biol Chem
(1999)
Osteoprotegerin is an αvβ3-induced, NF-κB-dependent survival factor for endothelial cells
J Biol Chem
Receptor activator of NF-kappa B and osteoprotegerin expression by human microvascular endothelial cells, regulation by inflammatory cytokines, and role in human osteoclastogenesis
J Biol Chem
Effects of immunosuppressants on receptor activator of NF-kappaB ligand and osteoprotegerin production by human osteoblastic and coronary artery smooth muscle cells
Biochem Biophys Res Commun
Expression of osteoprotegerin mRNA and protein in murine megakaryocytes
Exp Hematol
Localization of RANKL (receptor activator of NF-κB ligand) mRNA and protein in skeletal and extraskeletal tissues
Bone
Transforming growth factor-beta induces expression of receptor activator of NF-kappa B ligand in vascular endothelial cells derived from bone
J Biol Chem
RANK is the essential signaling receptor for osteoclast differentiation factor in osteoclastogenesis
Biochem Biophys Res Commun
Vascular endothelial growth factor up-regulates expression of receptor activator of NF-κB (RANK) in endothelial cells: Concomitant increase of angiogenic responses to RANK ligand
J Biol Chem
The osteoclast differentiation factor osteoprotegerin-ligand is essential for mammary gland development
Cell
A novel in vivo role for OPGL in activation of monocyte effector function and inflammatory response
J Biol Chem.
Biology of the TRANCE axis
Cytokine Growth Factor Rev
Osteoprotegerin production by human osteoblast lineage cells is stimulated by vitamin D, bone morphogenetic protein-2, and cytokines
Biochem Biophys Res Commun
The inhibitory effects of vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide on osteoclast formation are associated with upregulation of osteoprotegerin and downregulation of RANKL and RANK
Biochem Biophys Res Commun
Regulation of osteoprotegerin secretion from primary cultures of human bone marrow stromal cells
Biochem Biophys Res Commun
Potential role of cbfa1, an essential transcriptional factor for osteoblast differentiation, in osteoclastogenesis: regulation of mRNA expression of osteoclast differentiation factor (ODF)
Biochem Biophys Res Commun
Transcriptional mechanisms of bone morphogenetic protein-induced osteoprotegerin gene expression
J Biol Chem
Stimulation of osteoprotegerin (OPG) gene expression by transforming growth factor-beta (TGF-beta). Mapping of the OPG promoter region that mediates TGF-beta effects
J Biol Chem
Growth hormone substitution increases gene expression of members of the IGF family in cortical bone from women with adult onset growth homone deficiency-relationship with bone turn-over
Bone
Interleukin-1beta and tumor necrosis factor-alpha, but not interleukin-6, stimulate osteoprotegerin ligand gene expression in human osteoblastic cells
Bone
Protein expression and functional difference of membrane-bound and soluble receptor activator of NF-kappaB ligand: modulation of the expression by osteotropic factors and cytokines
Biochem Biophys Res Commun
Interleukin-4 reversibly inhibits osteoclastogenesis via inhibition of NF-kappa B and mitogen-activated protein kinase signaling
J Biol Chem
Interleukin-8 stimulation of osteoclastogenesis and bone resorption is a mechanism for the increased osteolysis of metastatic bone disease
Bone
Characterization of the bone-resorptive effect of interleukin-11 in cultured mouse calvarial bones
Bone
Interleukin-18 up-regulates osteoprotegerin expression in stromal/osteoblastic cells
Biochem Biophys Res Commun
PDGF induces osteoprotegerin expression in vascular smooth muscle cells by multiple signal pathways
FEBS Lett
Transforming growth factor-beta1 increases mRNA levels of osteoclastogenesis inhibitory factor in osteoblastic/stromal cells and inhibits the survival of murine osteoclast-like cells
Biochem Biophys Res Commun
Tumor necrosis factor-alpha cooperates with receptor activator of nuclear factor kappaB ligand in generation of osteoclasts in stromal cell-depleted rat bone marrow cell culture
Bone
Tumor necrosis factor-alpha (TNF) stimulates RANKL-induced osteoclastogenesis via coupling of TNF type 1 receptor and RANK signaling pathways
J Biol Chem
Macrophage inflammatory protein-1alpha is an osteoclastogenic factor in myeloma that is independent of receptor activator of nuclear factor kappaB ligand
Blood
Vector-averaged gravity regulates gene expression of receptor activator of NF-kappaB (RANK) ligand and osteoprotegerin in bone marrow stromal cells via cyclic AMP/protein kinase A pathway
Bone
Regulation of osteoprotegerin secretion from primary cultures of human bone marrow stromal cells
Biochem Biophys Res Commun
Vitamin K(2) inhibits adipogenesis, osteoclastogenesis, and ODF/RANK ligand expression in murine bone marrow cell cultures
Bone
Histamine stimulates production of osteoclast differentiation factor/receptor activator of nuclear factor-kappaB ligand by osteoblasts
Biochem Biophys Res Commun
Adrenergic stimulation of osteoclastogenesis mediated by expression of osteoclast differentiation factor in MC3T3-E1 osteoblast-like cells
Biochem Pharmacol
Activation of peroxisome proliferator-activated receptor gamma inhibits osteoprotegerin gene expression in human aortic smooth muscle cells
Biochem Biophys Res Commun
Indian hedgehog in the late-phase differentiation in mouse chondrogenic EC cells, ATDC5: upregulation of type X collagen and osteoprotegerin ligand mRNAs
Biochem Biophys Res Commun
Bisphosphonates pamidronate and zoledronic acid stimulate osteoprotegerin production by primary human osteoblasts
Biochem Biophys Res Commun
Gamma-glutamyl trasnpeptidase stimulates RANKL expression independent of its enzymatic activity and serves as a pathological bone-resorbing factor
J Biol Chem
Inhibition of osteoclast formation by 3-methylcholanthrene, a ligand for arylhydrocarbon receptor: suppression of osteoclast differentiation factor in osteogenic cells
Biochem Pharmacol
IL-6, RANKL, TNF-alpha/IL-1: interrelations in bone resorption pathophysiology
Cytokine Growth Factor Rev.
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