The Role of Receptor Activator of Nuclear Factor-{kappa}B Ligand and Osteoprotegerin in the Pathogenesis and Treatment of Metabolic Bone Diseases1

Lorenz C. Hofbauer and Armin E. Heufelder

Division of Gastroenterology and Endocrinology, Zentrum für Innere Medizin, Philipps University, D-35033 Marburg, Germany

Address all correspondence and requests for reprints to: Lorenz C. Hofbauer, M.D., Division of Gastroenterology and Endocrinology, Zentrum für Innere Medizin, Philipps University, Baldingerstrasse, D-35033 Marburg, Germany. E-mail: hofbauer{at}post.med.uni-marburg.de


    Introduction
 Top
 Introduction
 RANKL/OPGL/ODF/TRANCE: the...
 RANK: the receptor
 OPG: the decoy receptor
 Triangular regulation by RANKL,...
 Regulation of the RANKL/OPG...
 OPG/RANKL and the pathogenesis...
 Potential therapeutic role of...
 Conclusions
 References
 
BONE IS permanently turned over by the balanced and coordinated action of bone-resorbing osteoclasts and bone-forming osteoblasts, resulting in continuous renewal, functional response to external and internal stimuli, and structural integrity (1, 2, 3). Inappropriate bone resorption caused by enhanced osteoclast formation and activity has been previously implicated in the pathogenesis of many metabolic bone diseases. Strategies to inhibit bone resorption have emerged as potent and promising tools for the prevention and treatment of bone and mineral disorders (1, 2, 3). The recent discovery and characterization of a novel cytokine system, the tumor necrosis factor (TNF) ligand family member, receptor activator of nuclear factor-{kappa}B ligand (RANKL/OPGL/ODF/TRANCE) (4, 5, 6, 7);2 its receptor, receptor activator of nuclear factor {kappa}B (RANK/ODAR) (7); and its soluble (decoy) receptor, osteoprotegerin (OPG/OCIF/TR1) (8, 9, 10, 11), has established a novel paradigm of osteoclast biology: RANKL, RANK, and OPG constitute the three essential regulatory components of osteoclast formation, fusion, survival, activation, and apoptosis (12, 13, 14). This article3 reviews the molecular aspects of this novel cytokine system and its regulation and modulation by various osteotropic agents. Moreover, we summarize the implications of this cytokine system on the pathogenesis of various bone diseases and discuss the therapeutic potential of OPG in the clinical setting.


    RANKL/OPGL/ODF/TRANCE: the ligand
 Top
 Introduction
 RANKL/OPGL/ODF/TRANCE: the...
 RANK: the receptor
 OPG: the decoy receptor
 Triangular regulation by RANKL,...
 Regulation of the RANKL/OPG...
 OPG/RANKL and the pathogenesis...
 Potential therapeutic role of...
 Conclusions
 References
 
The TNF ligand family member, osteoprotegerin ligand (OPGL)/osteoclast differentiation factor (ODF) was discovered in 1998 by two groups (4, 5) as the cognate ligand for OPG/osteoclastogenesis inhibitory factor (OCIF) (8, 9). This factor had been identified by the same two groups the year before and was shown to be identical to TNF-related activation-induced cytokine (TRANCE) (6) and receptor activator of nuclear factor-{kappa}B ligand (RANKL) (7). RANKL will be the preferred term according to a consensus nomenclature (12). RANKL is a cell-bound polypeptide of 317 amino acids (4, 5) that is cleaved by a protease, TNF{alpha}-converting enzyme-like protease (TACE) (15), at position 140 or 145, respectively, to give rise to a shorter soluble ectodomain variant of similar activity (4, 15).

The major sources of RANKL production in the bone/bone marrow microenvironment are bone marrow stromal cells, osteoblasts, chondrocytes, mesenchymal cells of the periosteum, osteoclasts, endothelial cells, and T cells (4, 5, 6, 7, 16). Detailed analyses of its extraskeletal tissue distribution revealed RANKL messenger ribonucleic acid (mRNA) and protein production in brain, heart, kidney, skeletal muscle, and skin (16). Of note, RANKL production is highest in undifferentiated stromal cells and is greatly reduced once these multipotential mesenchymal cells become terminally committed to the osteoblastic phenotype (17, 18, 19). Interestingly, the RANKL gene promoter structure contains both vitamin D and glucocorticoid response elements as well as a binding site for the essential osteoblastic transcription factor, cbfa-1 (20, 21, 22).

In a series of in vitro experiments, RANKL was consistently shown to stimulate the differentiation, survival, and fusion of osteoclastic precursor cells, to activate mature osteoclasts, and to prolong their life span by inhibiting apoptosis (4, 5, 23, 24, 25, 26, 27), thus expanding the pool of active osteoclasts capable of forming resorption lacunae (Fig. 1Go). In the presence of permissive levels of macrophage colony-stimulating factor (M-CSF), RANKL is both sufficient and necessary for all steps of osteoclast development (28) (Fig. 1Go). Of note, other factors, such as transforming growth factor-ß and PGE2, may cooperate and synergize with RANKL (29, 30), although the precise roles of these factors in osteoclast biology are not well defined.



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Figure 1. Regulation of osteoclast functions by RANKL, OPG, and RANK. Osteoblastic cells express a cell-bound and a soluble form of RANKL as well as the soluble neutralizing receptor, OPG. RANKL and OPG have opposite effects on osteoclast biology. 1) Multipotential macrophagic lineage cells are stimulated by M-CSF, which binds to its receptor c-fms to proliferate and to become osteoclast precursor cells. 2) Under permissive levels of M-CSF, binding of osteoblast-derived RANKL to the receptor RANK induces differentiation of osteoclast precursors to prefusion osteoclasts. OPG blocks this effect by neutralizing RANKL. 3) Survival and fusion are stimulated by RANKL, a process that yields multinucleated osteoclasts and is modulated by cytokines such as IL-1, TNF{alpha}, and IL-6 and is inhibited by OPG. 4) Osteoclasts are activated by RANKL (as well as IL-1, TNF{alpha}, and IL-6) and become capable of resorbing bone. This is blocked by OPG. 5) Mature osteoclasts undergo apoptosis, which is induced by OPG and prevented by RANKL.

 
Consistent with these effects, parenteral administration of recombinant RANKL to mice in vivo resulted in severe osteoporosis associated with increased osteoclastic activity (as assessed by a doubling of the resorption surface), rapid bone loss, and severe hypercalcemia (4). By contrast, mice rendered deficient in RANKL by targeted ablation have severe osteopetrosis, impaired tooth eruption, and a lack of mature osteoclasts, indicating that RANKL is required for osteoclast differentiation and activation (31). Of note, RANKL deficiency also results in defects of B and T lymphocyte maturation (lymph node agenesis, thymus hypoplasia) (31).


    RANK: the receptor
 Top
 Introduction
 RANKL/OPGL/ODF/TRANCE: the...
 RANK: the receptor
 OPG: the decoy receptor
 Triangular regulation by RANKL,...
 Regulation of the RANKL/OPG...
 OPG/RANKL and the pathogenesis...
 Potential therapeutic role of...
 Conclusions
 References
 
The effects of RANKL are mediated by binding to and activating its specific high affinity receptor, receptor activator of nuclear factor (NF)-{kappa}B (RANK)/osteoclast differentiation and activation receptor (ODAR; Fig. 1Go) (7, 32). RANK, a type I transmembrane protein composed of 616 amino acids, contains 4 cysteine-rich pseudorepeats, a characteristic feature of the TNF receptor (TNFR) superfamily (7, 32, 33). RANK expression is largely limited to osteoclasts, T and B cells, dendritic cells, and fibroblasts (7, 32, 33).

RANK activation involves RANKL-dependent interaction with TNFR-associated factor (TRAF) family members, the transcription factors NF-{kappa}B and c-Fos, c-Jun N-terminal kinase (JNK), the protooncogene c-src, and the antiapoptotic serine/threonine kinase Akt/PKB. Stimulating antibodies directed against the extracellular domains of RANK mimicked the action of RANKL and induced osteoclastogenesis in vitro, whereas inhibitory fragments of this RANK antibody or a soluble RANK variant (which compete with the cell-bound RANK for RANKL) blocked osteoclastogenesis (32, 33).

Various in vivo models support RANK as an essential requirement for normal osteoclastic differentiation and activation. Transgenic mice overexpressing soluble RANK displayed an osteopetrotic phenotype due to decreased osteo-clastogenesis (32). In addition, RANK knockout mice revealed a phenotype characterized by osteopetrosis and associated immune defects (38, 39), which is similar to the phenotype observed in RANKL-deficient mice (31). Moreover, mice subjected to targeted ablation of downstream elements of the RANK pathway, such as TRAF-6 (40) and NF-{kappa}B1/NF-{kappa}B2 (41), also have an osteopetrotic phenotype, clearly establishing the RANK signaling pathway as the key gatekeeper of osteoclast differentiation and activation.


    OPG: the decoy receptor
 Top
 Introduction
 RANKL/OPGL/ODF/TRANCE: the...
 RANK: the receptor
 OPG: the decoy receptor
 Triangular regulation by RANKL,...
 Regulation of the RANKL/OPG...
 OPG/RANKL and the pathogenesis...
 Potential therapeutic role of...
 Conclusions
 References
 
Osteoprotegerin (OPG)/osteoclastogenesis inhibitory factor (OCIF) were independently discovered by two groups in 1997 as the first component of this novel cytokine system (8, 9, 42). Other investigators have since confirmed these data and named it TNF receptor-related molecule-1 (TR1) (10, 11) and follicular dendritic cell receptor-1 (FDCR-1) (43). OPG is the only known secreted TNFR member that lacks a transmembrane domain (8, 9, 10, 11). OPG is a propeptide of 401 amino acids with a short signal peptide (21 amino acids) that forms homodimers linked through a cystein-cystein disulfide bond at position 400 (8, 9, 10, 11). One study suggests that OPG may also exist in a membrane-bound form on the surface of a follicular dendritic cell line (43). In contrast to RANKL and RANK, whose abundance is generally low and whose expression is mainly restricted to the skeletal and immune system, OPG is expressed in high concentrations by a variety of tissues and cell types (8, 9, 10, 11, 42, 43, 44, 45). In bone, OPG is mainly produced by cells of the osteoblastic lineage, with increasing production as the cells become more differentiated (16, 17, 18, 45, 46).

OPG is a decoy receptor that acts by binding to and neutralizing both the soluble and the cell-bound form of RANKL (8, 9) and that of another TNF ligand family member, TRAIL (47). In vitro, OPG inhibits the differentiation, survival, and fusion of osteoclastic precursor cells, blocks activation of mature osteoclasts, and induces osteoclast apoptosis (4, 5, 11, 26, 27, 42, 46, 47, 48, 49, 50, 51) (Fig. 1Go).

In vivo, manipulation of OPG production has generated both extremes of skeletal phenotypes, osteopetrosis and osteoporosis. Overexpression of OPG in transgenic mice (in which the transgene is activated after birth) is associated with severe osteopetrosis (8), similar to that in RANKL (31) and RANK knockout mice (38, 39), but without the associated immune abnormalities observed in the latter two animal models. In contrast, mice with targeted ablation of OPG have severe, early-onset osteoporosis due to enhanced osteoclastic activity (52, 53). Of note, one study also revealed severe arterial calcification in OPG-deficient mice (52), indicating a protective role for OPG in the vascular system. Parenteral administration of recombinant OPG to normal rodents resulted in increased bone mass (8, 9), and completely prevented ovariectomy-induced bone loss (8) without apparent adverse skeletal and extraskeletal side effects. These successful initial studies have formed the theoretical background for the potential use of OPG as an antiresorptive agent.


    Triangular regulation by RANKL, RANK, and OPG
 Top
 Introduction
 RANKL/OPGL/ODF/TRANCE: the...
 RANK: the receptor
 OPG: the decoy receptor
 Triangular regulation by RANKL,...
 Regulation of the RANKL/OPG...
 OPG/RANKL and the pathogenesis...
 Potential therapeutic role of...
 Conclusions
 References
 
Identification of the three components, RANKL, RANK, and OPG, the subsequent characterization of their effects in vitro and in vivo, and characterization of transgenic and knockout models have unambiguously established this novel cytokine system as the essential and final effector system of osteoclast biology (Fig. 1Go) (reviewed in Refs. 12, 13, 14). In the skeleton, RANKL and OPG are mainly produced by cells of the osteoblastic lineage, whereas RANK is expressed by osteoclasts. The RANKL/OPG ratio is highest in undifferentiated preosteoblastic cells and decreases with osteoblastic differentiation (17, 18, 19) (which is paralleled by a decreasing capacity to support osteoclastogenesis). Because the RANKL gene promoter contains a binding site for the osteoblastic transcription factor, cbfa-1 (20, 21, 22), it has been concluded that osteoclast formation is coupled to osteoblast differentiation through cbfa-1 and RANKL. This concept may plausibly explain the sequential cycle of bone resorption and formation during bone remodeling (Fig. 1Go).

As mentioned earlier, RANKL-induced activation of RANK and its second messenger system is both necessary and sufficient for all aspects of osteoclast ontogeny, and only the initial step (osteoclastogenesis) requires permissive levels of M-CSF (12, 13, 14). Previous studies have suggested a role for interleukin-1 (IL-1), TNF{alpha}, and IL-6 as other important proresorptive cytokines (54, 55, 56). Recent reports suggest that these cytokines, in addition to modulating RANKL, OPG, or both (see below), may directly modulate osteoclast differentiation and activation through RANKL- and RANK-independent mechanisms (57, 58, 59, 60). Further studies are required to fully understand how these various cytokines and their signaling cascades converge at or circumvent the RANKL-RANK-NF-{kappa}B pathway.


    Regulation of the RANKL/OPG system by calcitropic agents
 Top
 Introduction
 RANKL/OPGL/ODF/TRANCE: the...
 RANK: the receptor
 OPG: the decoy receptor
 Triangular regulation by RANKL,...
 Regulation of the RANKL/OPG...
 OPG/RANKL and the pathogenesis...
 Potential therapeutic role of...
 Conclusions
 References
 
Because the RANKL/OPG ratio is an essential determinant of osteoclast biology, any agent that regulates RANKL, OPG, or both, is likely to indirectly modulate osteoclast differentiation, activation, and apoptosis (12, 13, 14). Various peptide and steroid hormones, growth factors, cytokines, drugs, and ions have been shown to regulate the RANKL/OPG system in different stromal and osteoblastic cell models (Table 1Go), and changes in the RANKL/OPG ratio were found to be associated with altered osteoclast differentiation and activation.


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Table 1. Regulation of RANKL and OPG by various agents in alphabetical order

 
Among the factors that increase RANKL mRNA expression in osteoblastic cells are the proinflammatory cytokines, IL-1, IL-6, IL-11, oncostatin, TNF{alpha}, and PGE2 (5, 61, 62, 63), and the steroid hormones, dexamethasone and 1{alpha},25-dihydroxyvitamin D3 [1,25-(OH)2D3] (5, 46, 62, 64, 65). In addition, RANKL gene expression is enhanced by PTH (5, 25, 64, 66, 67) as well as by commonly used immunosuppressants (cyclosporin A, rapamycin, and tacrolimus) (68). Furthermore, some of the RANKL-stimulating factors have not been previously shown to be regulators of osteoclastogenesis and osteoclast activation. These include Indian hedgehog (69), fibroblast growth factor-2 (70), autocrine motility factor (71), bone morphogenetic protein-7 (72), and intracellular calcium cations (73). Factors that inhibit RANKL expression include transforming growth factor-ß (74), inhibin (75), and vasoactive intestinal peptide (76).

OPG production is stimulated by the proinflammatory cytokines, IL-1 and TNF (11, 45, 61, 77, 78), transforming growth factor-ß (73, 79), bone morphogenetic protein-2 (45) and -7 (72), 1{alpha},25-(OH)2D3 (45), 17ß-estradiol (80, 81), vasoactive intestinal peptide (76), and calcium cations (9). OPG production is decreased by PGE2 (82); glucocorticoids (44, 46, 65); 1{alpha},25-(OH)2D3 (64, 79); the immunosuppressants cyclosporin A, rapamycin, and tacrolimus (68); the pure estrogen receptor antagonist ICI 182,780 (80); and PTH (66, 79).

Although RANKL and OPG are regulated by various factors (Table 1Go), RANK expression on osteoclastic cells was found to be stable, with little regulation by osteotropic agents (83). However, in the immune systems, RANK expression on T cells is up-regulated by IL-4 and transforming growth factor-ß, and RANK expression on dendritic cells is down-regulated by CD40 ligand (7).


    OPG/RANKL and the pathogenesis of metabolic bone diseases
 Top
 Introduction
 RANKL/OPGL/ODF/TRANCE: the...
 RANK: the receptor
 OPG: the decoy receptor
 Triangular regulation by RANKL,...
 Regulation of the RANKL/OPG...
 OPG/RANKL and the pathogenesis...
 Potential therapeutic role of...
 Conclusions
 References
 
Osteoporosis associated with estrogen deficiency. Several cytokines (IL-1, TNF{alpha}, IL-6, and M-CSF) have been implicated in the pathogenesis of postmenopausal osteoporosis, because their expression or activity is up-regulated by estrogen deficiency and down-regulated by estrogen administration (54, 55, 56). The early phase of estrogen deficiency (5–8 yr after menopause) is associated with enhanced osteoclastogenesis and bone resorption and results in rapid bone loss (84). Several lines of evidence suggest that RANKL and OPG may mediate at least in part some of the antiresorptive effects of estrogen. First, estrogen stimulates the expression of OPG in osteoblastic (80) and stromal cells (81) via transcriptional activation of ER-{alpha}. Second, estrogen deficiency in ovariectomized rodents results in decreased OPG expression (85, 86) and increases RANKL production (86, 87) by cells of the bone and bone marrow microenvironment, whereas estrogen replacement therapy prevents it (85). As mentioned previously, OPG administration completely abolishes ovariectomy-induced bone loss in rodents (8). Third, estrogen treatment inhibits the responsiveness of osteoclastic precursor cells to the actions of RANKL (88, 89), possibly by blunting RANKL-induced JNK activation (88).

Despite this compelling experimental evidence, epidemiological studies have generated conflicting data. In one study, which observed a small increase in serum OPG levels in osteoporotic vs. nonosteoporotic postmenopausal women (90), it was hypothesized that elevated OPG levels may compensate for increased bone resorption. By contrast, another study has failed to confirm this (91).

Drug-induced osteoporosis. Glucocorticoid-induced osteoporosis is the most frequent secondary form of osteoporosis. Various cytokines have been suggested as mediators of the increase in osteoclastic bone resorption that usually occurs within the first few months after initiation of glucocorticoid treatment (92, 93). In vitro, glucocorticoids down-regulate OPG production via a transcriptional mechanism (44, 46, 65, 79) and up-regulate RANKL expression (5, 46, 62, 64, 65) in osteoblastic cells, possibly by activating the glucocorticoid response element of the RANKL gene promoter (21). This marked increase in the osteoblastic RANKL/OPG ratio after glucocorticoid exposure is associated with enhanced osteoclastogenesis (46). The inhibitory effect of glucocorticoids on serum OPG levels (94) suggests that OPG deficiency after glucocorticoid excess may promote rapid bone loss in vivo (95).

Immunosuppressants are also known to induce rapid bone loss in recipients of allogeneic organ transplants. These agents have been found to increase RANKL expression and decrease OPG production in human marrow stromal cells and to induce osteoclastogenesis (68).

Hyperparathyroidism. Continuously increased and unregulated serum PTH concentrations, as present in hyperparathyroidism, result in increased bone resorption and bone loss (96). In vitro, PTH was found to stimulate RANKL mRNA levels (5, 25, 64, 66, 67), to decrease OPG production in osteoblastic cells (66, 79), and to increase the RANKL/OPG ratio (64, 66). All of these changes were associated with increased osteoclast formation (66). It is unclear whether intermittent exposure to PTH (which may have anabolic effects on bone in vivo), rather than continuous exposure, leads to a favorable change in the RANKL/OPG ratio.

Paget’s disease. Paget’s disease is characterized by uncontrolled bone remodeling due to increased bone resorption by osteoclasts (97). A hyperosteoclastogenic bone marrow microenvironment due to abnormal production of or response to cytokines (IL-6) and hormones [1,25(OH)2D3], perhaps triggered by the presence of paramyxoviral-like nuclear inclusions, has been suggested as a potential mechanism (97). Only recently, enhanced RANKL expression was observed in a bone marrow stromal cell line developed from a patient with Paget’s disease (98). Furthermore, osteoclastic precursor cells harvested from affected lesions were found to have increased sensitivity toward the effects of RANKL (98, 99). Thus, local overexpression of RANKL and/or local hypersensitivity to the effects of RANKL may account for the highly localized pattern of Paget’s disease in bone (98).

Bone loss associated with rheumatoid arthritis. Systemic osteoporosis and local periarticular bone loss adjacent to affected joints are common in patients with rheumatoid arthritis and have been attributed to high circulating and local levels of proinflammatory cytokines (55, 100). Activated CD4+ helper cells, which constitute a major population of immune cells in the inflamed synovium in rheumatoid arthritis, have been shown to produce RANKL and to support osteoclastogenesis in vitro (101, 102). In addition, systemic activation of T cells resulted in RANKL-mediated osteoclast formation and bone loss in vivo. (101). Moreover, in rat adjuvant arthritis, a T cell-dependent animal model of rheumatoid arthritis, neutralization of RANKL by exogenous administration of OPG prevented bone loss and cartilage damage (101). Increased RANKL production has also been demonstrated in synovial fibroblasts derived from patients with rheumatoid arthritis (103, 104), but not in subjects with other forms of arthritis, and this was associated with increased osteoclastogenesis (103). In addition, OPG levels in the synovial fluid of patients with rheumatoid arthritis (but not other joint diseases) are inappropriately low (105), which may limit the capacity to counteract the detrimental effects of RANKL on enhanced osteoclastogenesis at the sites of active joint disease. Interestingly, the effects of IL-17, a potent stimulator of osteoclastogenesis in rheumatoid arthritis, could be inhibited by OPG in vitro (106), indicating a potential therapeutic avenue for severe rheumatoid arthritis.

Osteopetrosis. Osteopetrosis is a rare polygenic disorder of generalized increased bone mass due to decreased osteoclastogenesis and bone resorption (107). Although the RANKL/OPG system has not directly been implicated in human osteopetrotic syndromes, several animal models indicate that RANKL, OPG, and their signaling pathway may play a potential role in the pathogenesis of osteopetrosis. For instance, mice deficient in RANKL (31), RANK (38, 39), TRAF-6 (40), and NF-{kappa}B1/NF-{kappa}B2 (41) have severe osteopetrosis. Moreover, transgenic mice overexpressing either OPG (8) or soluble RANK (32), both of which neutralize RANKL, also have osteopetrosis. Of note, a stop mutation in the coding region of the gene for M-CSF (which is the permissive factor for the effects of RANKL; see Fig. 1Go) has been previously detected in the murine model of spontaneous osteopetrosis (108).

Bone tumors and bone metastases. Abnormalities of the RANKL/RANK/OPG system have been suggested to be involved in the evolution of benign and malignant bone tumors and in the development of osteolytic lesions by tumor metastases. Osteoclastomata (giant cell tumors) are lytic lesions of the epiphyses of long bones. Stromal cells of affected lesions (109, 110) and synovial fibroblasts of patients with giant cell tumor (103) were found to produce high levels of transcripts for RANKL as well as for M-CSF (110), whereas giant cells of affected lesions produced high levels of RANK (109, 110). Increased RANKL mRNA expression has been detected in cell lines of multiple myeloma, which is commonly associated with osteolysis and hypercalcemia (111), whereas tissue harvested from sporadic osteosarcoma revealed a loss of RANKL expression, consistent with decreased perilesional bone resorption (112).

Several groups have clearly demonstrated that breast cancer cells (by secreting PTH-related peptide) indirectly stimulate stromal cells and osteoblasts to increase RANKL production (113, 114, 115, 116) and to limit OPG production (113, 114), thus leading to increased formation, activation, and survival of osteoclasts. In addition, the propensity of prostate cancer cell lines to cause either osteolytic or osteoblastic metastases was correlated with either a higher (osteolytic) or a lower (osteoblastic) RANKL/OPG ratio produced by prostate cancer cells compared to normal prostate cells (117).

Miscellaneous. Other diseases in which the RANKL/RANK/OPG system may play a role include familial expansile osteolysis, a rare autosomal dominant disorder characterized by focal areas of enhanced bone resorption (118), and its murine equivalent, chronic multifocal osteomyelitis (119), both of which are due to a gain of function mutation, leading to constitutive activation of RANK. Of note, a similar mechanism is also present in patients with familial Paget’s disease of bone (118). Other potential RANKL/RANK/OPG-dependent bone diseases include disuse osteoporosis (up-regulation of RANKL by a lack of mechanical strain, which down-regulates RANKL) (120) and periodontal disease (modulation of RANKL and OPG by proinflammatory cytokines in dental cells) (121). RANKL and OPG may also play a role in vascular calcification and atherogenesis, because these factors are produced by vascular smooth muscle cells (68) and endothelial cells (122) and are regulated by various hormones and cytokines (68, 122). Moreover, OPG-deficient mice reveal severe arterial calcification (52).


    Potential therapeutic role of OPG in bone diseases
 Top
 Introduction
 RANKL/OPGL/ODF/TRANCE: the...
 RANK: the receptor
 OPG: the decoy receptor
 Triangular regulation by RANKL,...
 Regulation of the RANKL/OPG...
 OPG/RANKL and the pathogenesis...
 Potential therapeutic role of...
 Conclusions
 References
 
Administration of recombinant OPG to animals. The pronounced antiresorptive and bone mass-enhancing effects of OPG in vivo has been initially described by two groups (8, 9). Simonet et al. (8) reported a 3-fold increase in bone mass in young mice after treatment with OPG (10 mg/kg·day for 7 days), and Yasuda et al. (9) observed a 1.8-fold increase in bone mass in young rats after treatment with OPG (24 mg/kg·day for 14 days). Subsequent studies confirmed the antiresorptive and hypocalcemic effects of parenteral administration of OPG in normal rodents with physiological bone resorption (123, 124). In addition, OPG was also shown to suppress enhanced bone resorption in rodents. In their initial paper, Simonet et al. reported that OPG treatment (5 mg/kg·day for 14 days) completely prevented ovariectomy-induced bone loss in rats (8). Subsequently, OPG was shown to inhibit increased bone resorption during normal linear bone growth (125); after challenge with IL-1ß, TNF{alpha}, PTH, PTHrP, 1,25-(OH)2D3 (125, 126), and RANKL (4); and after challenge with PTH or 1{alpha},25-(OH)2D3 after thyroparathyroidectomy (127). Of note, in some of these studies, OPG also completely controlled associated hypercalcemia (4, 126). Similar effects have been shown for soluble RANK, which neutralizes RANKL, but not TRAIL (128).

These studies have generated the hypothesis that OPG may be useful for treatment of bone and mineral abnormalities in animal tumor models. OPG (124, 125, 129, 130) and soluble RANK (128) were found to control increased bone resorption and hypercalcemia in rodent models of humoral hypercalcemia of malignancy. Overall, treatment with OPG was well tolerated; had a rapid, marked, and long-lasting hypocalcemic effect; and was devoid of apparent adverse effects (124, 125, 128, 129, 130). OPG administration was also capable of preventing and suppressing osteolysis and hypercalcemia in animal models with bone metastases due to colonic and breast cancer (131).

Finally, OPG prevented increased bone loss associated with immobilization and disuse osteoporosis (132, 133). OPG was more potent compared to the bisphosphonates pamidronate and ibandronate in preventing bone loss in both the sciatic nerve crush model (132) and the tail suspension model (133).

Administration of recombinant OPG to humans. The beneficial effects, safety, and tolerability of OPG have been recently demonstrated in a randomized, double blind, placebo-controlled clinical study on 52 postmenopausal women (134). A single sc dose of human recombinant OPG (3 mg/kg) reduced biochemical markers of bone resorption (deoxypyridinolines, -80% by day 5) and of bone formation (osteocalcin, -20% by day 29), indicating the potential of a marked and prolonged suppression of increased bone turnover. Except for a modest decrease in serum and urinary calcium concentrations with a subsequent increase in serum PTH levels, no other laboratory abnormalities were observed (134). The pharmacokinetic profile of OPG indicates that serum levels peak between 24–48 h after sc administration and then decrease, with a terminal half-life of 38–47 h (135). The volume of distribution of recombinant human OPG is between 37–41 ml/kg after iv administration, which suggests that OPG does not distribute into the extravascular compartment (135).


    Conclusions
 Top
 Introduction
 RANKL/OPGL/ODF/TRANCE: the...
 RANK: the receptor
 OPG: the decoy receptor
 Triangular regulation by RANKL,...
 Regulation of the RANKL/OPG...
 OPG/RANKL and the pathogenesis...
 Potential therapeutic role of...
 Conclusions
 References
 
RANKL and OPG have emerged as a key agonist/antagonist cytokine system that regulates important aspects of osteoclast biology, including differentiation, fusion, survival, activation, and apoptosis. RANKL increases the pool of active osteoclasts by activating its specific receptor RANK located on osteoclastic cells, thus increasing bone resorption, whereas OPG, which neutralizes RANKL, has opposite effects. RANKL and OPG are produced by bone marrow-derived stromal cells and osteoblasts and are regulated by various calcitropic cytokines, hormones, and drugs. Alterations or abnormalities of the RANKL/OPG system have been implicated in different metabolic bone diseases characterized by increased osteoclast differentiation and activation and by enhanced bone resorption (Table 2Go). These disorders include postmenopausal osteoporosis, glucocorticoid-induced osteoporosis, hyperparathyroidism, Paget’s disease, rheumatoid arthritis, and bone malignancies. Animal studies have suggested that exogenous administration of OPG may prevent the adverse skeletal effects of ovariectomy, humoral hypercalcemia of malignancy, osteolytic metastases, and immobilization. Preliminary results from clinical studies in postmenopausal women indicate that exogenous administration of OPG is a safe and well tolerated form of therapy that results in prolonged antiresorptive effects when given once per month. In view of this rapid progress in our basic understanding of osteoclast biology and the compelling experimental and clinical evidence, recombinant OPG will expand our armamentarium of compounds to fight osteoporosis and other metabolic bone diseases in the years to come.


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Table 2. Spectrum of metabolic bone diseases with potential involvement of the RANKL/OPG system

 


    Footnotes
 
1 This work was supported by Grants Ho 1875/1–1 and Ho 1875/2–1 (to L.C.H.) from Deutsche Forschungsgemeinschaft (Bonn, Germany) and by Grant 10-1697-Ho 1 (to L.C.H.) from Deutsche Krebshilfe (Bonn, Germany). Back

2 The following abbreviations are used: NF-{kappa}B, nuclear factor-{kappa}B; OCIF, osteoclastogenesis inhibitory factor; ODAR, osteoclast differentiation and activation receptor; ODF, osteoclast differentiation factor; OPG, osteoprotegerin; OPGL, OPG ligand; RANK, receptor activator of NF-{kappa}B; RANKL; RANK ligand; TACE, TNF{alpha}-converting enzyme-like protease; TNF, tumor necrosis factor; TNFR, TNF receptor; RAF, TNFR-associated factor; TRANCE, TNF-related activation-induced cytokine. According to a future nomenclature, RANKL will be referred to as TNF superfamily member TNFSF11, and RANK and OPG will be referred to as TNF receptor superfamily members TNFRSF-11A and –11B, respectively (see http://www.hugo-international.org). Back

3 To present an inclusive and up to date review, the authors have elected to cite several studies that have only been published in a preliminary (abstract) format. Back

Received December 16, 1999.

Revised March 13, 2000.

Accepted March 21, 2000.


    References
 Top
 Introduction
 RANKL/OPGL/ODF/TRANCE: the...
 RANK: the receptor
 OPG: the decoy receptor
 Triangular regulation by RANKL,...
 Regulation of the RANKL/OPG...
 OPG/RANKL and the pathogenesis...
 Potential therapeutic role of...
 Conclusions
 References
 

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