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Minireview: The OPG/RANKL/RANK System

Endocrine Research Unit, Division of Endocrinology, Metabolism, and Nutrition, Mayo Clinic and Foundation, Rochester, Minnesota 55905

Address all correspondence and requests for reprints to: Sundeep Khosla, M.D., Mayo Clinic, 200 First Street SW, 5-194 Joseph, Rochester, Minnesota 55905. E-mail: khosla.sundeep{at}mayo.edu

	Abstract

Top Abstract Introduction OPG: The Key to... Identification of the Ligand... RANK—The Final Piece of... A Complete Picture of... Implications for the... Conclusions References

 
The identification of the OPG/RANKL/RANK system as the dominant, final mediator of osteoclastogenesis represents a major advance in bone biology. It ended a long-standing search for the specific factor produced by preosteoblastic/stromal cells that was both necessary and sufficient for osteoclast development. The initial cloning and characterization of OPG as a soluble, decoy receptor belonging to the TNF receptor superfamily was the first step that eventually led to an unraveling of this system. Soon thereafter, the molecule blocked by OPG, initially called OPG-ligand/osteoclast differentiating factor (ODF) and subsequently RANKL, was identified as the key mediator of osteoclastogenesis in both a membrane-bound form expressed on preosteoblastic/stromal cells as well as a soluble form. RANKL, in turn, was shown to bind its receptor, RANK, on osteoclast lineage cells. The decisive role played by these factors in regulating bone metabolism was demonstrated by the findings of extremes of skeletal phenotypes (osteoporosis vs. osteopetrosis) in mice with altered expression of these molecules. Over the past several years, work has focused on identifying the factors regulating this system, the signaling mechanisms involved in the RANKL/RANK pathway, and finally, potential alterations in this system in metabolic bone disorders, from the extremely common (i.e. postmenopausal osteoporosis) to the rare (i.e. familial expansile osteolysis).

	Introduction

Top Abstract Introduction OPG: The Key to... Identification of the Ligand... RANK—The Final Piece of... A Complete Picture of... Implications for the... Conclusions References

 
THE PAST 15 yr have witnessed an explosion in the field of bone biology. Indeed, the scientific view of bone has evolved from considering it as a relatively uninteresting scaffold for the rest of the body to an extremely complex organ regulated by a host of systemic and local factors. The landscape of bone biology over this time period has also been marked by a number of key discoveries that led to opening up entirely new areas for investigation. These include (among others) the identification of sex steroid receptors in bone cells (1, 2); the cloning and characterization of PTH-related peptide (PTHrP) as the major factor responsible for the syndrome of humoral hypercalcemia of malignancy (3, 4) and as an important locally active cytokine in bone, cartilage, and other tissues (5); the identification of core binding factor 1 (Cbfa1) as the (or at least a) master gene controlling osteoblast differentiation (6, 7); and the important role of apoptosis in regulating osteoblast and osteoclast number (8). Certainly, the identification and characterization of the OPG/RANK-L/RANK system, which began with a seminal paper published in 1997 (9), is a major addition to this list of momentous events in bone biology.

	OPG: The Key to the Puzzle

Top Abstract Introduction OPG: The Key to... Identification of the Ligand... RANK—The Final Piece of... A Complete Picture of... Implications for the... Conclusions References

 
OPG was discovered independently by two groups, although the manner in which these groups identified it differed markedly. The Amgen, Inc. group in the US initially described a fascinating molecule that they had happened upon as part of a project characterizing cDNAs in rat intestine (9). Indeed, it is unlikely that this group was actually searching for OPG, and the project apparently focused on identifying TNF receptor-related molecules that may potentially have therapeutic utility. One particular cDNA encoded a truncated TNF receptor-like protein that contained no hydrophobic transmembrane-spanning sequences. This clone would have been no different from other interesting cDNAs were it not for the fact that transgenic mice overexpressing this gene had a remarkable skeletal phenotype—they had marked osteopetrosis. Further analysis of these mice revealed that the osteopetrosis was due to a profound decrease in osteoclasts, indicating that OPG (short for osteoprotegerin, i.e. to protect bone) clearly must play a key role in regulating osteoclastogenesis.

Independently, the Snow Brand Milk Group in Japan reported the identification of the identical molecule (10), but the route by which they got there was clearly different. In 1981, Rodan and Martin (11) had proposed a novel hypothesis wherein the osteoblast played a central role in mediating the hormonal control of osteoclastogenesis and bone resorption. Subsequently, there was accumulating experimental evidence that osteoblastic/stromal cells were essential for in vitro osteoclastogenesis and that these cells regulated osteoclast differentiation both by producing soluble factors and also by signaling to osteoclast progenitors through cell-to-cell contact (12, 13). Thus, the Snow Brand group was systematically searching for both osteoclast stimulatory and inhibitory factors. Indeed, they had previously reported the purification of an osteoclastogenesis inhibitory factor (OCIF) from the conditioned medium of human embryonic fibroblasts (14). They subsequently used the partial protein sequence to isolate cDNA clones encoding OCIF, which turned out to be identical with the protein that had been reported by the Amgen, Inc. group (9).

These initial reports were followed by numerous studies further characterizing OPG. It was found to be initially synthesized as a 401 amino acid peptide, with a 21-amino acid propeptide that was cleaved, resulting in a mature protein of 380 amino acids (9, 10). As noted earlier, in contrast to all other TNF receptor superfamily members, OPG lacked transmembrane and cytoplasmic domains and was secreted as a soluble protein. The N-terminal region contained four cysteine-rich domains (D1–D4) and was most closely related to TNF receptor-2 and CD40. The C-terminal region contained two death domain homologous regions (D5 and D6) as well as a region (D7) containing a heparin binding site and a cysteine residue necessary for homodimerization (9, 10, 15).

OPG mRNA was found to be expressed in a number of tissues, including lung, heart, kidney, liver, stomach, intestine, brain and spinal cord, thyroid gland, and bone (9, 10). Because the major biologic action of OPG described to date has been to inhibit osteoclast differentiation and activity (9, 10), the potential role of OPG in these other tissues remains to be established. However, mice with targeted ablation of OPG not only develop severe osteoporosis due to markedly increased osteoclast formation and subsequent bone resorption (16, 17), but also have profound calcification of the large arteries, marked intimal and medial proliferation, and partial aortic dissection by the age of 4 months (16). Thus, OPG likely also plays a significant role in the vasculature, and indeed, a recent study has found that OPG may be an important survival factor for endothelial cells (18).

	Identification of the Ligand for OPG (OPG-L), RANKL

Top Abstract Introduction OPG: The Key to... Identification of the Ligand... RANK—The Final Piece of... A Complete Picture of... Implications for the... Conclusions References

 
As soon as OPG was characterized, it became clear that it likely was going to be the key to identifying the long-sought osteoclast differentiation factor expressed on osteoblastic/stromal cells that was essential for osteoclast development. Indeed, soon after the identification of OPG, both groups had used expression cloning using OPG as a probe and had identified its ligand (initially termed OPG-L/ODF) (19, 20), which turned out to be identical with two previously known members of the TNF ligand family—TNF-related activation-induced cytokine (TRANCE) (a gene induced by activation of the T cell receptor) and RANKL, a factor known to stimulate dendritic cells (21, 22).

Human RANKL is a 317-amino acid peptide that has approximately 30% homology to the TNF-related apoptosis-inducing ligand and to CD40, and approximately 20% homology to Fas ligand (19, 20, 21, 22). It has now been shown to exist in two forms: a 40- to 45-kDa cellular, membrane-bound form and a 31-kDa soluble form derived by cleavage of the full-length form at position 140 or 145 (19). RANKL mRNA is expressed at highest levels in bone and bone marrow, as well as in lymphoid tissues (lymph node, thymus, spleen, fetal liver, and Peyer’s patches) (19, 20, 21, 22). Its major role in bone is the stimulation of osteoclast differentiation (18, 19), activity (19), and inhibition of osteoclast apoptosis (23). Indeed, in the presence of low levels of macrophage-colony stimulating factor (M-CSF), RANKL appears to be both necessary and sufficient for the complete differentiation of osteoclast precursor cells into mature osteoclasts (19, 20). In addition, it is clear that RANKL (or, as it was originally identified, TRANCE) has a number of effects on immune cells, including activation of c-Jun N-terminal kinase (JNK) in T cells (21), inhibition of apoptosis of dendritic cells (24), induction of cluster formation by dendritic cells, and effects on cytokine-activated T cell proliferation (22).

Consistent with these findings, RANKL knockout mice have severe osteopetrosis with defects in tooth eruption (25). They also have a complete absence of osteoclasts. In addition, they exhibit defects in early differentiation of T and B cells, lack lymph nodes, have defects in thymic differentiation, but have a normal splenic structure and Peyer’s patches (25). A somewhat unexpected finding in these mice is that they also have defects in mammary gland development (26). In particular, they fail to form lobulo-alveolar structures during pregnancy, resulting in death of the newborns (26).

	RANK—The Final Piece of the Puzzle

Top Abstract Introduction OPG: The Key to... Identification of the Ligand... RANK—The Final Piece of... A Complete Picture of... Implications for the... Conclusions References

 
With the identification of the ligand for OPG as being identical with TRANCE/RANKL, the final piece of the puzzle fell into place relatively easily, because the receptor for this had already been identified as RANK (22). Hsu et al. (27) subsequently demonstrated that RANK mRNA was highly expressed by isolated bone marrow-derived osteoclast progenitors and by mature osteoclasts in vivo and that transgenic mice expressing a soluble RANK-Fc fusion protein had severe osteopetrosis and a skeletal phenotype similar to OPG transgenic mice. The ultimate proof that RANK expressed on preosteoclastic cells was the sole receptor on these cells for RANKL came with the demonstration that RANK knockout mice had profound osteopetrosis due to an absence of osteoclasts (28). Moreover, osteoclastogenesis could be initiated in these mice by transfer of the RANK cDNA back into hematopoietic precursors. Interestingly, similar to the RANKL knockout mice, RANK knockout mice also lacked peripheral lymph nodes and had defective B and T cell maturation but differed from the RANKL knock out mice in having normal thymic development (28).

Human RANK is a 616-amino acid peptide, with a 28-amino acid signal peptide, an N-terminal extracellular domain, a short transmembrane domain of 21 amino acids, and a large C-terminal cytoplasmic domain (22). It is expressed primarily on cells of the macrophage/monocytic lineage, including preosteoclastic cells, T and B cells, dendritic cells, and fibroblasts (22, 27). The RANKL/RANK signaling pathway has also been extensively studied in recent years. Thus, RANK activation by RANKL is followed by its interaction with TNF receptor-associated (TRAF) family members, activation of nuclear factor (NF)-B and c-Fos, JNK, c-src, and the serine/threonine kinase Akt/PKB (22, 27). Consistent with these findings, mice with various components of this signaling pathway ablated [i.e. TRAF-6 (29) or NF-B1/NF-B2 (30)] have an osteopetrotic phenotype.

	A Complete Picture of Osteoclastogenesis Emerges

Top Abstract Introduction OPG: The Key to... Identification of the Ligand... RANK—The Final Piece of... A Complete Picture of... Implications for the... Conclusions References

 
The unraveling of the OPG/RANKL/RANK system, which occurred over a span of approximately 2 yr or less, solved a long standing unresolved question in bone biology, namely the precise mechanisms by which preosteoblastic/stromal cells controlled osteoclast development. Given what we know about the process of bone remodeling, it is easy to understand why these early osteoblastic cells need to have the critical say in whether osteoclasts are formed or not. Thus, bone is constantly being resorbed and formed at specific sites in the skeleton, termed basic multicellular units. The process begins by migration of osteoclasts to these sites (activation), resorption of a packet of bone by these cells, a reversal phase characterized by apoptosis of the osteoclasts, followed by a phase of bone formation by newly formed osteoblasts. As such, it makes sense that the critical, initial step in this process, the development of osteoclasts, should be under the control of preosteoblastic/stromal cells: this ensures that the processes of bone resorption and formation will be tightly coupled, allowing for a wave of bone formation to follow each cycle of bone resorption, thus maintaining skeletal integrity. Further coupling between osteoblastogenesis and osteoclastogenesis is ensured by the fact that the osteoblast differentiation factor, cbfa1, is necessary for adequate expression of the osteoclast differentiation factor, RANKL, on the surface of preosteoblastic/stromal cells (31).

Figure 1 summarizes the current picture of the control of osteoclastogenesis that has emerged in the post OPG/RANKL/RANK era. RANKL, expressed on the surface of preosteoblastic/stromal cells, binds to RANK on the osteoclastic precursor cells. M-CSF, which binds to its receptor, c-Fms, on preosteoclastic cells, appears to be necessary for osteoclast development because it is the primary determinant of the pool of these precursor cells (32). RANKL, however, is critical for the differentiation, fusion into multinucleated cells, activation, and survival of osteoclastic cells. OPG puts a brake on the entire system by blocking the effects of RANKL. A number of proresorptive cytokines, such as TNF- and IL-1, modulate this system primarily by stimulating M-CSF production (thereby increasing the pool of preosteoclastic cells) and by directly increasing RANKL expression (33). In addition, a number of other cytokines and hormones, such as TGF-ß (increased OPG production) (34), PTH (increased RANKL/decreased OPG production) (35), 1,25-dihydroxyvitamin D₃ (increased RANKL production) (36), glucocorticoids (increased RANKL/decreased OPG production) (37), and estrogen (increased OPG production) (38, 39) exert their effects on osteoclastogenesis by regulating osteoblastic/stromal cell production of OPG and RANKL. However, not all regulation of the osteoclast is exclusively via the osteoblast because calcitonin acts directly on osteoclastic cells (40), and estrogen has been shown to induce apoptosis of osteoclasts (41) as well as inhibit osteoclast differentiation by interfering with RANK signaling, principally RANKL-induced JNK activation and c-Jun activity and expression (42, 43). Moreover, TGF-ß can also stimulate RANK expression on preosteoclastic cells, and thus enhance osteoclastic sensitivity to RANKL (44).

View larger version (43K): [in this window] [in a new window]   Figure 1. Current understanding of preosteoblastic/stromal cell regulation of osteoclastogenesis. See text for details.

	Implications for the Pathogenesis and Treatment of Disorders of Bone Metabolism

Top Abstract Introduction OPG: The Key to... Identification of the Ligand... RANK—The Final Piece of... A Complete Picture of... Implications for the... Conclusions References

 
As with any breakthrough in basic science, the onus rapidly falls on the clinical investigator to translate the findings into a better understanding of disease as well as potentially new treatment approaches. This process has just begun for the OPG/RANKL/RANK system and will likely accelerate as the tools to assess gene expression in smaller and smaller amounts of biological samples with greater reliability continue to evolve.

The disorders most clearly related to alterations in this system are familial expansile osteolysis, a rare autosomal dominant disorder characterized by focal areas of enhanced bone resorption, and familial Paget’s disease, both of which are due to mutations in the signal peptide region of the RANK protein (46). These mutations may lead to an accumulation of defective RANK translation products in the secretion pathway, resulting perhaps in receptor self-association and increased constitutive RANK signal transduction.

The role of the OPG/RANKL/RANK system in the pathogenesis of more common disorders, such as postmenopausal or age-related osteoporosis, remains controversial. As noted earlier, estrogen does increase OPG production by osteoblastic (38) and marrow stromal cells (39). However, serum OPG levels are, if anything, higher in postmenopausal women with osteoporosis and increased bone turnover (47), perhaps as a homeostatic mechanism limiting their more rapid bone loss. In addition, although OPG production by marrow stromal cells appears to decline with age (48), serum OPG levels have consistently been found to increase with age in women and in men (47).

Although depressed bone formation is the major abnormality in glucocorticoid-induced osteoporosis (50), bone resorption is often increased or at the least, inappropriately normal for the depressed level of bone formation (50). Because glucocorticoids are potent inhibitors of OPG production and also stimulate RANKL levels in osteoblastic cells (37), this decrease in the OPG/RANKL ratio may well account for an enhanced ability of preosteoblastic cells (reduced in number as they may be) to support osteoclast development, leading to the observed marked imbalance between bone formation and resorption and rapid bone loss in this condition.

As noted earlier, PTH increases RANKL and decreases OPG expression by osteoblastic cells, leading to a net catabolic effect on bone (35). However, intermittent PTH clearly has anabolic effects on bone, and recent data indicate that, in contrast to continuous exposure, intermittent exposure of marrow stromal cells to PTH does not lead to a significant alteration in the OPG/RANKL ratio, while still stimulating markers of bone formation (50). Thus, differential effects of PTH on the OPG/RANKL system, depending on whether it is administered continuously or intermittently, may well explain the catabolic vs. anabolic effects of PTH on bone.

In addition to the RANK activating mutations in familial Paget’s disease, bone marrow stromal cells in Paget’s disease have been shown to have enhanced RANKL expression, and preosteoclastic cells from affected lesions have increased sensitivity to RANKL (51). This combination of abnormalities may explain, at least in part, the increased numbers of osteoclasts in Pagetic bone. Finally, activated T cells (as in rheumatoid arthritis) have increased levels of RANKL expression (52) and RANKL, in the presence of M-CSF, can induce synovial macrophages to differentiate into osteoclastic bone-resorbing cells (53), thus potentially leading to the periarticular bone loss commonly seen in various forms of inflammatory arthritis.

OPG or other components of this system may also have therapeutic utility in conditions associated with accelerated bone resorption, including skeletal metastases from multiple myeloma or other tumors, and postmenopausal osteoporosis. Indeed, OPG has been shown to block skeletal destruction and pain in a mouse model of sarcoma-induced bone destruction (54). In addition, a RANK-Fc fusion protein was effective in suppressing bone resorption and hypercalcemia in a murine model of humoral hypercalcemia of malignancy due to xenografts of human lung cancer (55). Finally, a single dose of a OPG-Fc fusion protein resulted in a profound (by up to 80%) and sustained (for up to 3 wk) suppression of bone resorption in postmenopausal women (56). However, whether these approaches will translate into viable new therapies for these disorders or whether alternate approaches (such as the development of small molecules to locally regulate OPG/RANKL production in the bone microenvironment) will be needed remains to be seen.

	Conclusions

Top Abstract Introduction OPG: The Key to... Identification of the Ligand... RANK—The Final Piece of... A Complete Picture of... Implications for the... Conclusions References

 
The pace of developments resulting in an unraveling of the OPG/RANKL/RANK system over the past few years, leading to a nearly complete understanding of osteoblastic regulation of osteoclastogenesis, is truly breathtaking. In addition to providing fundamental insights in bone biology, these events have identified an entirely new set of candidate factors that may be involved in the pathogenesis of a number of rare and common metabolic bone diseases. Although only time will tell whether these advances will translate into new therapeutic approaches, clearly the detailed characterization of this system has opened up entirely new areas for basic and clinical investigation in bone biology and diseases, respectively.

	Acknowledgments

 
I would like to thank Drs. B. L. Riggs and T. C. Spelsberg for helpful suggestions and comments.

	Footnotes

 
This work was supported by Research Grant AG04875 from the National Institute on Aging, United States Public Health Service.

Abbreviations: Cbfa1, Core binding factor 1; JNK, c-Jun N-terminal kinase; M-CSF, macrophage-colony stimulating factor; NF, nuclear factor; OCIF, osteoclastogenesis inhibitory factor; OPG-L, ligand for OPG; PTHrP, PTH-related peptide; TRAF, TNF receptor-associated family; TRANCE, TNF-related activation-induced cytokine.

Received August 21, 2001.

Accepted for publication August 31, 2001.

	References

Top Abstract Introduction OPG: The Key to... Identification of the Ligand... RANK—The Final Piece of... A Complete Picture of... Implications for the... Conclusions References

 

1. Eriksen EF, Colvard DS, Berg NJ, Graham ML, Mann KG, Spelsberg TC, Riggs BL 1988 Evidence of estrogen receptors in normal human osteoblast-like cells. Science 241:84–86[Abstract/Free Full Text]
2. Komm BS, Terpening CM, Benz DJ, Graeme KA, O’Malley BW, Haussler MR 1988 Estrogen binding receptor mRNA, and biologic response in osteoblast-like osteosarcoma cells. Science 241:81–84[Abstract/Free Full Text]
3. Suva LJ, Winslow GA, Wettenhall REH, Hammonds RG, Moseley JM, Diefenbach-Jagger H, Rodda CP, Kemp BE, Rodriguez H, Chen EY 1987 A parathyroid hormone-related protein implicated in malignant hypercalcemia: cloning and expression. Science 237:893–896[Abstract/Free Full Text]
4. Mangin M, Webb AC, Dreyer BE, Posillico JT, Ikeda K, Weir EC, Stewart AF, Bander NH, Milstone L, Barton DE 1988 Identification of a cDNA encoding a parathyroid hormone-like peptide from a human tumor associated with humoral hypercalcemia of malignancy. Proc Natl Acad Sci USA 85:597–601[Abstract/Free Full Text]
5. Lanske B, Karpalis AC, Lee K, Luz A, Vortkamp A, Pirro A, Karperien M, Defize LH, Ho C, Mulligan RC, Abou-Samra AB, Juppner H, Segre GV, Kronenberg HM 1996 PTH/PTHrP receptor in early development and Indian hedgehog-regulated bone growth. Science 273:663–666[Abstract]
6. Ducy P, Zhang R, Geoffroy V, Ridall AL, Karsenty G 1997 Osf2/Cbfa1: a transcriptional activator of osteoblast differentiation. Cell 89:747–754[CrossRef][Medline]
7. Komori T, Yagi H, Nomura S, Yamaguchi A, Sasaki K, Deguchi K, Shimizu Y, Bronson RT, Gao YH, Inada M, Sato M, Okamoto R, Kitamura Y, Yoshiki S, Kishimoto T 1997 Targeted disruption of Cbfa1 results in a complete lack of bone formation owing to maturational arrest of osteoblasts. Cell 89:755–764[CrossRef][Medline]
8. Manolagas SC 2000 Birth and death of bone cells: basic regulatory mechanisms and implications for the pathogenesis and treatment of osteoporosis. Endocr Rev 21:115–137[Abstract/Free Full Text]
9. Simonet WS, Lacey DL, Dunstan CR, Kelley M, Chang MS, Luthy R, Nguyen HQ, Wooden S, Bennett L, Boone T, Shimamoto G, DeRose M, Elliott R, Colombero A, Tan HL, Trail G, Sullivan J, Davey E, Bucay N, Renshaw-Gregg L, Hughes TM, Hill D, Pattison W, Campbell P, Boyle WJ 1997 Osteoprotegerin: a novel secreted protein involved in the regulation of bone density. Cell 89:309–319[CrossRef][Medline]
10. Yasuda H, Shima N, Nakagawa N, Mochizucki SI, Yano K, Fujise N, Sato Y, Goto M, Yamaguchi K, Kuriyama M, Kanno T, Murakami A, Tsuda E, Morinaga T, Higashio K 1998 Identity of osteoclastogenesis inhibitory factor (OCIF) and osteoprotegerin (OPG): a mechanism by which OPG/OCIF inhibits osteoclastogenesis in vitro. Endocrinology 39:1329–1337
11. Rodan GA, Martin TJ 1981 Role of osteoblasts in hormonal control of bone resorption—a hypothesis. Calcif Tissue Int 33:349–351[CrossRef][Medline]
12. Takahashi N, Akatsu T, Uadagawa N, Sasaki T, Yamaguchi A, Moseley JM, Martin TJ, Suda T 1988 Osteoblastic cells are involved in osteoclast formation. Endocrinology 123:2600–2602[Abstract]
13. Suda T, Takahashi N, Martin TJ 1992 Modulation of osteoclast differentiation. Endocr Rev 13:66–80[CrossRef][Medline]
14. Tsuda E, Goto M, Mochizuki S, Yano K, Kobayashi F, Morinaga T, Higashio K 1997 Isolation of a novel cytokine from human fibroblasts that specifically inhibits osteoclastogenesis. Biochem Biophys Res Commun 234:137–142[CrossRef][Medline]
15. Yamaguchi K, Kinosaki M, Goto M, Kobayashi F, Tsuda E, Morinaga T, Higashio K 1998 Characterization of structural domains of human osteoclastogenesis inhibitory factor. J Biol Chem 273:5117–5123[Abstract/Free Full Text]
16. Bucay N, Sarosi I, Dunstan CR, Morony S, Tarpley J, Capparelli C, Scully S, Tan HL, Xu W, Lacey DL, Boyle WJ, Simonet WS 1998 Osteoprotegerin-deficient mice develop early onset osteoporosis and arterial calcification. Genes Dev 12:1260–1268[Abstract/Free Full Text]
17. Mizuno A, Amizuka N, Irie K, Murakami A, Fujise N, Kanno T, Sato Y, Nakagawa N, Yasuda H, Mochizuki S, Gomibuchi T, Yano K, Shima N, Washida N, Tsuda E, Morinaga T, Higashio K, Ozawa H 1998 Severe osteoporosis in mice lacking osteoclastogenesis inhibitory factor/osteoprotegerin. Biochem Biophys Res Commun 247:610–615[CrossRef][Medline]
18. Malyankar UM, Scatena M, Suchland KL, Yun TJ, Clark EA, Giachelli CM 2000 Osteoprotegerin is an v ß3-induced, NF-B-dependent survival factor for endothelial cells. J Biol Chem 275:20959–20962[Abstract/Free Full Text]
19. Lacey DL, Timms E, Tan H-L, Kelly MJ, Dunstan CR, Burgess T, Elliott R, Colombero A, Elliott G, Scully S, Hsu H, Sullivan J, Hawkins N, Davy F, Capparelli C, Eli A, Qian YX, Kaufman S, Sarosi I, Shalboub V, Senaldi G, Guo J, Delaney J, Boyle WJ 1998 Osteoprotegerin (OPG) ligand is a cytokine that regulates osteoclast differentiation and activation. Cell 93:165–176[CrossRef][Medline]
20. Yasuda H, Shima N, Nakagawa N, Yamaguchi K, Kinosaki M, Mochizuki S, Tomoyasu A, Yano K, Goto M, Murakami A, Tsuda E, Morinaga T, Higashio K, Udagawa N, Takahashi N, Suda T 1998 Osteoclast differentiation factor is a ligand for osteoprotegerin/osteoclastogenesis-inhibitory factor and is identical to TRANCE/RANKL. Proc Natl Acad Sci USA 95:3597–3602[Abstract/Free Full Text]
21. Wong BR, Rho J, Arron J, Robinson E, Orlinick J, Chao M, Kalachikov S, Cayani E, Bartlett 3rd FS, Frankel WN, Lee SY, Cho Y 1997 TRANCE is a novel ligand of the tumor necrosis factor receptor family that activates c-jun N-terminal kinase in T cells. J Biol Chem 272:25190–25194[Abstract/Free Full Text]
22. Anderson MA, Maraskovsky E, Billingsley WL, Dougall WC, Tometsko ME, Roux ER, Teepe MC, DuBose RF, Cosman D, Galibert L 1997 A homologue of the TNF receptor and its ligand enhance T-cell growth and dendritic-cell function. Nature 390:175–179[CrossRef][Medline]
23. Fuller K, Wong B, Fox S, Choi Y, Chambers TJ 1998 TRANCE is necessary and sufficient for osteoblast-mediated activation of bone resorption in osteoclasts. J Exp Med 188:997–1001[Abstract/Free Full Text]
24. Wong BR, Josien R, Young Lee S, Sauter B, Li H-L, Steinman RM, Choi Y 1997 TRANCE (tumor necrosis factor [TNF]-related activation-induced cytokine), a new TNF family member predominantly expressed in T cells, is a dendritic cell-specific survival factor. J Exp Med 186:2075–2080[Abstract/Free Full Text]
25. Kong Y-Y, Yoshida H, Sarosi I, Tan HL, Timms E, Capparelli C, Morony S, Oliveira-dos-Santos AJ, Van G, Itie A, Khoo W, Wakeham A, Dunstan CR, Lacey DL, Mak TW, Boyle WJ, Penninger JM 1999 OPGL is a key regulator of osteoclastogenesis, lymphocyte development and lymph-node organogenesis. Nature 397:315–323[CrossRef][Medline]
26. Fata JE, Kong YY, Li J, Sasaki T, Irie-Sasaki J, Moorehead RA, Elliott R, Scully S, Voura EB, Lacey DL, Boyle WJ, Khokha R, Penninger JM 2000 The osteoclast differentiation factor osteoprotegerin-ligand is essential for mammary gland development. Cell 103:41–50[CrossRef][Medline]
27. Hsu H, Lacey DL, Dunstan CR, Solovyev I, Colombero A, Timms E, Tan HL, Elliott G, Kelley MJ, Sarosi I, Wang L, Xia XZ, Elliott R, Chiu L, Black T, Scully S, Capparelli C, Morony S, Shimamoto G, Bass MB, Boyle WJ 1999 Tumor necrosis factor receptor family member RANK mediates osteoclast differentiation and activation induced by osteoprotegerin ligand. Proc Natl Acad Sci USA 96:3540–3545[Abstract/Free Full Text]
28. Li J, Sarosi I, Yan XQ, Morony S, Capparelli C, Tan HL, McCabe S, Elliott R, Scully S, Van G, Kaufman S, Juan SC, Sun Y, Tarpley J, Martin L, Christensen K, McCabe J, Kostenuik P, Hsu H, Fletcher F, Dunstan CR, Lacey DL, Boyle WJ 2000 RANK is the intrinsic hematopoietic cell surface receptor that controls osteoclastogenesis and regulation of bone mass and calcium metabolism. Proc Natl Acad Sci USA 97:1566–1571[Abstract/Free Full Text]
29. Lomaga MA, Yeh WC, Sarosi I, Duncan GS, Furlonger C, Ho A, Morony S, Capparelli C, Van G, Kaufman S, van der Heiden A, Itie A, Wakeham A, Khoo W, Sasaki T, Cao Z, Penninger JM, Paige CJ, Lacey DL, Dunstan CR, Boyle WJ, Goedde DV, Mak TW 1999 TRAF6 deficiency results in osteopetrosis and defective interleukin-1, CD40, and LPS signaling. Genes Dev 13:1015–1024[Abstract/Free Full Text]
30. Iotsova V, Caamano J, Loy J, Yang Y, Lewin A, Bravo R 1997 Osteopetrosis in mice lacking NF-B1 and NF-B2. Nature Med 3:1285–1289[CrossRef][Medline]
31. Gao YH, Shinki T, Yuasa T, Kataoka-Enomoto H, Komori T, Suda T, Yamaguchi A 1998 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 252:697–702[CrossRef][Medline]
32. Udagawa N, Takahashi N, Akatsu T, Tanaka H, Sasaki T, Nishihara T, Koga T, Martin TJ, Suda T 1990 Origin of osteoclasts: mature monocytes and macrophages are capable of differentiating into osteoclasts under a suitable microenvironment prepared by bone marrow-derived stromal cells. Proc Natl Acad Sci USA 87:7260–4726[Abstract/Free Full Text]
33. Hofbauer LC, Lacey DL, Dunstan CR, Spelsberg TC, Riggs BL, Khosla S 1999 Interleukin-1ß and tumor necrosis factor-, but not interleukin-6, stimulate osteoprotegerin ligand gene expression in human osteoblastic cells. Bone 25:255–259[Medline]
34. Takai H, Kanematsu M, Yano K, Tsuda E, Higashio K, Ikeda K, Watanabe K, Yamada Y 1998 Transforming growth factor-ß stimulates the production of osteoprotegerin/osteoclastogenesis inhibitory factor by bone marrow stromal cells. J Biol Chem 273:27091–27096[Abstract/Free Full Text]
35. Lee S-K, Lorenzo JA 1999 Parathyroid hormone stimulates TRANCE and inhibits osteoprotegerin messenger ribonucleic acid expression in murine bone marrow cultures: correlation with osteoclast-like cell formation. Endocrinology 140:3552–3561[Abstract/Free Full Text]
36. Kitazawa R, Kitazawa S, Maeda S 1999 Promotor structure of mouse RANKL/TRANCE/OPGL/ODF gene. Biochim Biophys Acta 1445:134–141[Medline]
37. Hofbauer LC, Gori F, Riggs BL, Lacey DL, Dunstan CR, Spelsberg TC, Khosla S 1999 Stimulation of osteoprotegerin ligand and inhibition of osteoprotegerin production by glucocorticoids in human osteoblastic lineage cells: potential paracrine mechanisms of glucocorticoid-induced osteoporosis. Endocrinology 140:4382–4389[Abstract/Free Full Text]
38. Hofbauer LC, Khosla S, Dunstan CR, Lacey DL, Spelsberg TC, Riggs BL 1999 Estrogen stimulates gene expression and protein production of osteoprotegerin in human osteoblastic cells. Endocrinology 140:4367–4370[Abstract/Free Full Text]
39. Saika M, Inoue D, Kido S, Matsumoto T 2001 17ß-Estradiol stimulates expression of osteoprotegerin by a mouse stromal cell line, ST-2, via estrogen receptor-. Endocrinology 142:2205–2212[Abstract/Free Full Text]
40. Nicholson GC, Moseley JM, Sexton PM, Mendelsohn FA, Martin TJ 1986 Abundant calcitonin receptors in isolated rat osteoclasts. Biochemical and autoradiographic characterization. J Clin Invest 78:355–360
41. Hughes DE, Dai A, Tiffee JC, Li HH, Mundy GR, Boyce BF 1996 Estrogen promotes apoptosis of murine osteoclasts mediated by TGF-ß. Nat Med 2:1132–1136[CrossRef][Medline]
42. Shevde NK, Bendixen AC, Dienger KM, Pike JW 2000 Estrogens suppress RANK ligand-induced osteoclast differentiation via a stromal cell independent mechanism involving c-Jun repression. Proc Natl Acad Sci USA 97:7829–7834[Abstract/Free Full Text]
43. Srivastava S, Toraldo G, Weitzmann MN, Cenci S, Ross FP, Pacifici R 2001 Estrogen decreases osteoclast formation by down-regulating receptor activator of NF-B ligand (RANKL)-induced JNK activation. J Biol Chem 276:8836–8840[Abstract/Free Full Text]
44. Yan T, Riggs BL, Boyle WJ, Khosla S 2000 Regulation of osteoclastogenesis and RANK expression by TGF-ß₁. J Cell Biochem 83:320–325
45. Gori F, Hofbauer LC, Dunstan CR, Spelsberg TC, Khosla S, Riggs BL 2000 The expression of osteoprotegerin and RANK ligand and the support of osteoclast formation by stromal-osteoblast lineage cells is developmentally regulated. Endocrinology 141:4768–4776[Abstract/Free Full Text]
46. Hughes AE, Ralston SH, Marken J, Bell C, MacPherson H, Wallace RG, van Hul W, Whyte MP, Nakatsuka K, Hovy L, Anderson DM 2000 Mutations in TNFRSF11A, affecting the signal peptide of RANK, cause familial expansile osteolysis. Nat Genet 24:45–48[CrossRef][Medline]
47. Yano K, Tsuda N, Kobayashi F, Kobayashi F, Goto M, Harada A, Ikeda K, Higashio K, Yamada Y 1999 Immunological Characterization of Circulating Osteoprotegerin/osteoclastogenesis inhibitory factor: increased serum concentrations in postmenopausal women with osteoporosis. J Bone Miner Res 14:518–527[CrossRef][Medline]
48. Makhluf HA, Mueller SM, Mizuno S, Glowacki J 2000 Age-related decline in osteoprotegerin expression by human bone marrow cells cultured in three-dimensional collagen sponges. Biochem Biophys Res Commun 268:669–672[CrossRef][Medline]
49. Lukert BP, Raisz LG 1990 Glucocorticoid-induced osteoporosis: pathogenesis and management. Ann Intern Med 112:352–364
50. Locklin RM, Khosla S, Riggs BL 2001 Mechanisms of biphasic anabolic and catabolic effects of parathyroid hormone (PTH) on bone cells. Bone 28(Suppl):S80
51. Menaa C, Reddy SV, Kurihara N, Maeda H, Anderson D, Cundy T, Cornish J, Singer FR, Bruder JM, Roodman GD 2000 Enhanced RANK ligand expression and responsivity of bone marrow cells in Paget’s disease of bone. J Clin Invest 2105:1833–1838
52. Kong YY, Feige U, Sarosi I, Bolon B, Tafuri A, Morony S, Capparelli C, Li J, Elliott R, McCabe S, Wong T, Campagnuolo G, Moran E, Bogoch ER, Van G, Nguyen LT, Ohashi PS, Lacey DL, Fish E, Boyle WJ Penninger JM 1999 Activated T cells regulate bone loss and joint destruction in adjuvant arthritis through osteoprotegerin ligand. Nature 402:304–309[CrossRef][Medline]
53. Itonaga I, Fujikawa Y, Sabokbar A, Murray DW, Athanasou NA 192 2000 Rheumatoid arthritis synovial macrophage-osteoclast differentiation is osteoprotegerin ligand-dependent. J Pathol 192:97–104[CrossRef][Medline]
54. Honore P, Luger NM, Sabino MC, Schwei MJ, Rogers SD, Mach DB, O’Keefe PF, Ramnaraine ML, Clohisy DR, Mantyh PW 2000 Osteoprotegerin blocks bone cancer-induced skeletal destruction, skeletal pain and pain-related neurochemical reorganization of the spinal cord. Nat Med 6:521–528[CrossRef][Medline]
55. Oyajobi BO, Anderson DM, Traianedes K, Williams PJ, Yoneda T, Mundy GR 2001 Therapeutic efficacy of a soluble receptor activator of nuclear factor B-IgG Fc fusion protein in suppressing bone resorption and hypercalcemia in a model of humoral hypercalcemia of malignancy. Cancer Res 61:2572–2578[Abstract/Free Full Text]
56. Bekker PJ, Holloway D, Nakanishi A, Arrighi M, Leese PT, Dunstan CR 2001 The effect of a single dose of osteoprotegerin in postmenopausal women. J Bone Miner Res 16:348–360[CrossRef][Medline]

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