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RANK Signaling Pathways and Key Molecules Inducing Osteoclast Differentiation
Biomed Sci Letters 2017;23:295-302
Published online December 31, 2017;  https://doi.org/10.15616/BSL.2017.23.4.295
© 2017 The Korean Society For Biomedical Laboratory Sciences.

Na Kyung Lee

Department of Biomedical Laboratory Science, College of Medical Sciences, Soonchunhyang University, Chungnam 31538, Korea
Correspondence to: Na Kyung Lee. Department of Biomedical Laboratory Science, College of Medical Science, Soonchunhyang University, 646 Eumnae-ri, Sinchang-myeon, Asan-Si, Chungnam 31538, Korea. Tel: +82-41-530-3036, Fax: +82-41-530-3085, e-mail: nlee@sch.ac.kr
Received December 5, 2017; Accepted December 15, 2017.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
 Abstract

Mononuclear osteoclast precursors derived from hematopoietic progenitors fuse together and then become multinucleated mature osteoclasts by macrophage-colony stimulating factor (M-CSF) and receptor activator of nuclear factor-κB ligand (RANKL). Especially, the binding of RANKL to its receptor RANK provides key signals for osteoclast differentiation and bone-resorbing function. RANK transduces intracellular signals by recruiting adaptor molecules such as TNFR-associated factors (TRAFs), which then activate mitogen activated protein kinases (MAPKs), Src/PI3K/Akt pathway, nuclear factor-κB (NF-κB) and finally amplify NFATc1 activation for the transcription and activation of osteoclast marker genes. This review will briefly describe RANKL-RANK signaling pathways and key molecules critical for osteoclast differentiation.

Keywords : Osteoclast differentiation, RANK signaling, RANKL
INTRODUCTION

Bone undergoes continuous remodeling process throughout adulthood. Osteoblasts, the bone-forming cells, are derived from a mesenchymal progenitor cells, and osteoclasts, mineralized tissues-resorbing cells, are from hematopoietic progenitors of the monocyte/macrophage lineage (Suda et al., 1999; Chambers, 2000; Teitelbaum, 2000; Aubin, 2001). The functional balance between osteoblasts and osteoclasts is critical for bone homeostasis (Suda et al., 1999; Teitelbaum, 2000). The elevation of osteoclast numbers and/or activity results in bone diseases including osteoporosis, Paget’s disease and rheumatoid arthritis.

To become multinucleated mature osteoclasts, mononuclear osteoclast precursors survive and fuse together by two main cytokines, macrophage-colony stimulating factor (M-CSF) and receptor activator of nuclear factor-κB ligand (RANKL) (Chambers, 2000). M-CSF stimulates the proliferation and survival of osteoclast precursors via c-Fms, M-CSF receptor. The binding of RANKL to its receptor RANK provides key signals for osteoclast differentiation and bone-resorbing function as well as the survival of mature osteoclasts (Fuller et al., 1998). Co-stimulatory signaling induces Ca2+ oscillation for the robust production of NFATc1, which, cooperating with RANKL, finally induce complete osteoclast differentiation (Koga et al., 2004; Mócsai et al., 2004). RANKL- or RANK-deficient mice show similar osteopetrotic phenotypes due to a complete defect of osteoclasts, suggesting that RANKL-RANK signaling is essential for the osteoclast formation (Kong et al., 1999; Marx, 2004).

RANKL, also known as OPGL (osteoprotegerin ligand), ODF (osteoclast differentiation factor) and TRANCE (tumor necrosis factor-related activation-induced cytokine), is a type 2 membrane protein which belongs to the TNF superfamily and is synthesized by stromal cells/osteoblasts and activated T cells (Wong et al., 1997). RANKL has been shown to play important roles for lymph node formation, mammary gland development and the survival of immune cells as well as bone homeostasis in the 1990s (Anderson et al., 1997; Wong et al., 1997; Lacey et al., 1998; Naito et al., 1999; Fata et al., 2000). RANK, a type 1 transmembrane protein that belongs to the tumor necrosis factor receptor (TNFR) superfamily, is expressed primarily on monocytes/macrophages including osteoclastic precursors, activated T cells, dendritic cells, and mature osteoclasts (Anderson et al., 1997; Wong et al., 1997; Lacey et al., 1998). Osteoprotegerin (OPG) is a soluble decoy receptor for RANKL that is released from bone marrow stromal cells/osteoblasts as a soluble form (Simonet et al., 1997). OPG competes with RANK for RANKL, thereby inhibiting osteoclast differentiation and function. OPG-deficient knockout mice developed osteoporosis due to an excess of osteoclasts (Simonet at al., 1997; Bucay et al., 1998).

In this review, I will briefly describe RANK signaling pathways and key molecules inducing osteoclast differentiation. The elucidation on RANKL-RANK signaling pathways might give hints of potential drug targets for preventing bone-related diseases including osteoporosis.

TNF receptor-associated factors (TRAFs)

Stimulation of RANK by RANKL binding leads to trimerization of RANK and recruits TRAF adaptor proteins to the conserved TRAF domains within the C-terminal cytoplasmic tail of RANK (Darnay et al., 1998; Walsh and Choi, 2003). Whereas TRAF 2, 3, 5 and 6 can bind to RANK, TRAF4 was shown to be a nuclear protein so failed to interact with RANK (Darnay et al., 1998; Inoue et al., 2000; Walsh and Choi, 2003). Moreover, TRAF2, TRAF5 and TRAF6 activate transcription factors such as NF-κB and AP-1 that are required for osteoclast differentiation, however, TRAF3 serves as an inhibitor of NF-κB pathway (Kanazawa et al., 2003; Hauer et al., 2005; Kanazawa and Kudo, 2005). Among the TRAFs, TRAF6 seems to be a major adaptor protein of RANKL-RANK signaling pathway for osteoclast formation and function since the phenotype of TRAF6-deficient mice is nearly similar with the bone phenotypes of RANKL- or RANK-deficient mice, severe osteopetrosis due to impaired osteoclast differentiation or bone resorbing function (Lomaga et al., 1999; Naito et al., 1999).

However, it seems that RANKL could induce osteoclast differentiation through a TRAF6-independent signaling pathway since TRAF6-deficient hematopoietic precursors can differentiate into osteoclasts by RANKL if proper cofactors are provided, in vitro. Indeed, hematopoietic precursors from RANKL-, RANK-, or TRAF6-null mice can become osteoclasts in vitro when TNFα and cofactors such as TGF-β are supported (Kim et al., 2005). Nevertheless, RANKL-induced osteoclastogenesis is considerably reduced by TRAF6 deficiency, even cofactors are added, which implies that TRAF6 is a critical downstream mediator of RANKL-RANK signaling pathway to induce osteoclast differentiation (Kim et al., 2005).

RANK-TRAF6 interaction activates the IκB kinase (IKK) complex to stimulate Nuclear factor kappa B (NF-κB) via a signaling complex with TGFβ-activated kinase 1 (TAK1), TAK-1-binding protein 1 (TAB1) and TAB2 or atypical protein kinase C (aPKC) (Mizukami et al., 2002; Duran et al., 2004). Dominant-interfering mutant forms of TAK1 inhibit RANKL-mediated activation of both IκB kinase 1/2 (IKK1/2) and JNK1, leading to the inactivation of the NF-κB and AP-1, respectively (Mizukami et al., 2002). The interaction of TRAF6 with aPKCs is through the mediation of scaffolding protein, p62 (Duran et al., 2004).

Mitogen activated protein (MAP) kinases

Recruitment of TRAF6 to RANK activates ERK, JNK, and p38 through activation of MEK1/2, MKK7, and MKK6 in osteoclast precursors, respectively (Kashiwada et al., 1998; Matsumoto et al., 2000; Yamamoto et al., 2002; He et al., 2011). The receptor for activated C kinase 1 (RACK1) acts as a scaffold protein to link the TRAF6-TAK1 complex with MKK6, which selectively facilitates the activation of p38 during the RANKL-initiated differentiation of osteoclast precursor cells into osteoclasts (Lin et al., 2015). Moreover, it was reported that RANK-TRAF6-Rac1-NADPH oxidase1- dependent pathway-induced reactive oxygen species production is required for MAPK activation for osteoclastogenesis (Lee et al., 2005). TRAF6 deficiency abolishes RANKL-mediated JNK and p38 MAPKs activation (Kobayashi et al., 2001). Mice with genetic deletion of erk1 exhibited reduced osteoclast formation in vivo, suggesting that ERK1 plays an important role in osteoclast differentiation (He et al., 2011). ERK1/2 induces activation of their downstream targets such as c-Fos. The lack of Fos (encoding c-Fos) results in increased numbers of bone marrow macrophages with the decrease of osteoclasts (Grigoriadis et al., 1994). Osteoclast precursor cells derived from jnk1-lacking mice but not from jnk2- lacking mice exhibited reduced ability to differentiate to osteoclasts (David et al., 2002). JNK1/2 induces activation of their downstream targets such as activator protein-1 (AP-1) transcription factors (David et al., 2002).

A specific inhibitor of p38, SB203580, suppressed RANKL-mediated osteoclast differentiation in RAW 264.7 cells (Li et al., 2002). Stimulation of p38 results in the downstream activation of the transcriptional regulator mi/Mitf, which controls the expression of the genes encoding tartrateresistant acid phosphatase (TRAP, encoded by Acp5) and cathepsin K (CATK), a cysteine protease (Mansky et al., 2002). Mutant osteoclasts in Mitfmi/mimice are primarily mononuclear and express decreased levels of TRAP (Luchin et al., 2000; Luchin et al., 2001). Cathepsin K knockout mice develop osteopetrosis due to a deficit matrix degradation but not demineralization (Gowen et al., 1999). Similarly, mutations in the human cathepsin K gene have demonstrated an association with a rare skeletal dysplasia, pycnodysostosis (Gelb et al., 1996; Johnson et al., 1996). TRAP, an osteoclast differentiation marker, as well as cathepsin K also affect the functional activity of osteoclast by regulating bone matrix resorption and collagen turnover (Roberts et al., 2007).

Src/PI3K/Akt pathway

RANK associates with Src family kinase through interaction between TRAF6 and Cbl scaffolding proteins (Wong et al., 1999; Arron et al., 2001), activating the pro-survival factor PI3-kinase (PI3K)/Akt pathway (Wong et al., 1999). Activated PI3K generates phosphatidylinositol-(3,4,5)-phosphate (PIP3) at the plasma membrane, which leads to the recruitment of Akt/PKB via its pleckstrin homology (PH) domain and activation (Vanhaesebroeck et al., 2000). The activation of PI3K/Akt is Src-dependent because a genetic deletion of c-Src inhibits Akt activation by RANKL (Wong et al., 1999). The PI3K inhibitor LY294002 decreases osteoclast differentiation by reducing survival of osteoclast precursor cells during differentiation (Wong et al., 1999; Lee et al., 2002). Additionally, the association of the actin-binding protein gelsolin with PI3K showed that PI3K is important for the actin filament formation in osteoclasts (Chellaiah et al., 1998).

Akt induces osteoclast differentiation through regulating the GSK3β/NFATc1 signaling cascade. Akt overexpression in osteoclast precursors induces the expression and nuclear localization of NFATc1 by GSK-3β phosphorylation and inactivation (Moon et al., 2012).

Phosphatase and tensin homolog (PTEN) and SH2-containing inositol phosphatase 1 (SHIP1) negatively regulate PI3K signaling, thus reducing osteoclast differentiation (Takeshita et al., 2002; Sugatani et al., 2003). Mice lacking the SHIP-1 or tyrosine phosphatase SHP-1, both of which inhibit ITAM signaling, showed enhanced osteoclastogenesis and induced osteoporosis (Takeshita et al., 2002).

NF-κB signaling

NF-κB signaling activated by RANKL is important for osteoclast differentiation. NF-κB is including five members: c-Rel (Rel), RelA (p65), RelB, NF-κB1 (p105/p50), and NF-κB2 (p100/p52). These NF-κB proteins (p105 and p100) become shorter following post-translational processing into p50 and p52, respectively. Since p50 and p52 lack a transcription activation domain, they form dimers with Rel family members such as c-Rel (Rel), RelA (p65), RelB (Ghosh et al., 2002; Hayden et al., 2004). The NF-κB p50/p52 double-knockout (dKO) mice develop severe osteopetrosis because of a total absence of osteoclasts and show growth retardation while single knockout of p50 or p52 failed to show developmental defects (Franzoso et al., 1997; Iotsova et al., 1997). Moreover, the dKO mice showed an increase in RANK-expressing splenocytes, which suggests that p50 and p52 are necessary for osteoclastogenesis but not for RANK-expressing progenitor formation (Xing et al., 2002). Overexpression of c-Fos rescued the defect in osteoclast formation in the absence of RANKL and dKO splenocytes treated with RANKL or TNF failed to induce c-Fos, indicating that c-Fos are downstream of NF-κB (Iotsova et al., 1997; Yamashita et al., 2007).

IκB kinase (IKK) activation by RANKL stimulation induces phosphorylation and ubiquitin-dependent proteasomal degradation of IκB. NF-κB released from the NF-κB /IκB complex translocated into the nucleus and promotes transcription of target genes (Ghosh et al., 2002; Hayden et al., 2004). NF-κB-inducing kinase (NIK) and IKKα are important for the RelB:p52 complex formation but NF-κB activation via NIK is not necessary for osteoclast formation since NIK-deficient mice failed to show osteopetrosis phenotype (Ghosh et al., 2002; Novack et al., 2003; Hayden et al., 2004).

NFATc1

NFATc1 is a master transcription factor for the terminal differentiation of osteoclasts. Activated NFATc1 undergoes nuclear translocation and activates and induces osteoclast-specific genes such as tartrate-resistant acid phosphatase (TRAP), osteoclast-associated receptor (OSCAR), cathepsin K as well as NFATcl itself (Takayanagi et al., 2002; Matsumoto et al., 2004; Asagiri et al., 2005). In addition, NFATc1 regulates cell-cell fusion of osteoclasts through upregulation of the d2 isoform of vacuolar ATPase V0 domain (Atp6v0d2) and the dendritic cell-specific transmembrane protein (DC-STAMP) (Kim et al., 2008; Feng et al., 2009). The activation of NFATc1 is regulated by calcium/calmodulin signaling and RANK does not initiate calcium signaling directly in osteoclast precursors. Thus, the activation of NFATc1 is induced by RANKL, partially, and, for the robust activation of NFATc1, costimulatory signaling and RANKL signaling are collaborating (Takayanagi et al., 2002).

RANKL induces NFATc1 through NF-κB and c-Fos activation and a deficiency of p50/p52 or c-fos causes failure of NFATc1 induction (Li et al., 2004; Matsuo et al., 2004; Asagiri et al., 2005). NFATc1-deficient embryonic stem cells failed to differentiate into osteoclasts, and the overexpression of constitutively active NFATc1 in osteoclast precursors caused efficient osteoclast differentiation even in the absence of RANKL which suggests that NFATc1 is sufficient for osteoclastogenesis (Takayanagi et al., 2002).

CONCLUSION

The interest on the maintenance of bone homeostasis and bone remodeling occurring throughout adulthood as our life span is extended is increasing. Since 1990’s, the finding of RANKL and RANK, and the studies using genetic engineered mouse models boosted significant progression in bone physiology. Especially, the importance of RANKL/RANK system in osteoclast biology has revealed critical signaling pathways and molecules regulating osteoclast differentiation. Recruitment of TRAF6 to RANK activates distinct signaling cascades and molecules as following: MAP kinases including JNK1/2, ERK1/2 and p38, Src/PI3K/Akt, Nuclear factor kappa B (NFκB) and NFATc1. Thus, although key signaling pathways and molecules involved in osteoclastogenesis have already been identified, further investigations on the molecular mechanisms specific on differentiation stages, could be of interest. Especially, more studies on detailed molecular regulatory mechanism for osteoclast fusion and bone resorbing activity would help to develop effective therapeutic drugs for bone-related diseases.

ACKNOWLEDGEMENT

This work was supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (NRF-2016R1D1A1B01012205) and the Soonchunhyang University Research Fund.

References
  1. Anderson DM, Maraskovsky E, Billingsley WL, Dougall WC, Tometsko ME, Roux ER, Teepe MC, DuBose RF, Cosman D, and Galibert L. A homologue of the TNF receptor and its ligand enhance T-cell growth and dendritic-cell function. Nature 1997;390:175-179.
    CrossRef
  2. Arron JR, Vologodskaia M, Wong BR, Naramura M, Kim N, Gu H, and Choi Y. A positive regulatory role for Cbl family proteins in tumor necrosis factor-related activation-induced cytokine (trance) and CD40L-mediated Akt activation. Journal of Biological Chemistry 2001;276:30011-30017.
    CrossRef
  3. Asagiri M, Sato K, Usami T, Ochi S, Nishina H, Yoshida H, Morita I, Wagner EF, Mak TW, Serfling E, and Takayanagi H. Autoamplification of NFATc1 expression determines its essential role in bone homeostasis. Journal of Experimental Medicine 2005;202:1261-1269.
    CrossRef
  4. Aubin JE. Regulation of osteoblast formation and function. Reviews in Endocrine & Metabolic Disorders 2001;2:81-94.
    CrossRef
  5. Bucay N, Sarosi I, Dunstan CR, Morony S, Tarpley J, Capparelli C, Scully S, Tan HL, Xu W, Lacey DL, Boyle WJ, and Simonet WS. Osteoprotegerin-deficient mice develop early onset osteoporosis and arterial calcification. Genes & Development 1998;12:1260-1268.
    CrossRef
  6. Chambers TJ. Regulation of the differentiation and function of osteoclasts. Journal of Pathology 2000;192:4-13.
    CrossRef
  7. Chellaiah M, Fitzgerald C, Alvarez U, and Hruska K. C-Src is required for stimulation of gelsolin-associated phosphatidylinositol 3-kinase. Journal of Biological Chemistry 1998;273:11908-11916.
    CrossRef
  8. Darnay BG, Haridas V, Ni J, Moore PA, and Aggarwal BB. Characterization of the intracellular domain of receptor activator of NFkappaB (RANK). interaction with tumor necrosis factor receptor-associated factors and activation of NF-kappaB and c-Jun N-terminal kinase. Journal of Biological Chemistry 1998;273:20551-20555.
    CrossRef
  9. David JP, Sabapathy K, Hoffmann O, Idarraga MH, and Wagner EF. JNK1 modulates osteoclastogenesis through both c-Jun phosphorylation-dependent and -independent mechanisms. Journal of Cell Science 2002;115:4317-4325.
    CrossRef
  10. Duran A, Serrano M, Leitges M, Flores JM, Picard S, Brown JP, Moscat J, and Diaz-Meco MT. The atypical PKC-interacting protein p62 is an important mediator of RANK-activated osteoclastogenesis. Developmental Cell 2004;6:303-309.
    CrossRef
  11. 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, and Penninger JM. The osteoclast differentiation factor osteoprotegerin-ligand is essential for mammary gland development. Cell 2000;103:41-50.
    CrossRef
  12. Feng H, Cheng T, Steer JH, Joyce DA, Pavlos NJ, Leong C, Kular J, Liu J, Feng X, Zheng MH, and Xu J. Myocyte enhancer factor2 and microphthalmia-associated transcription factor cooperate with NFATc1 to transactivate the V-ATPase d2 promoter during RANKL-induced osteoclastogenesis. Journal of Biological Chemistry 2009;284:14667-14676.
    CrossRef
  13. Franzoso G, Carlson L, Xing L, Poljak L, Shores EW, Brown KD, Leonardi A, Tran T, Boyce BF, and Siebenlist U. Requirement for NF-kappaB in osteoclast and B-cell development. Genes & Development 1997;11:3482-3496.
    CrossRef
  14. Fuller K, Wong B, Fox S, Choi Y, and Chambers TJ. TRANCE is necessary and sufficient for osteoblast-mediated activation of bone resorption in osteoclasts. Journal of Experimental Medicine 1998;188:997-1001.
    CrossRef
  15. Gelb BD, Shi GP, Chapman HA, and Desnick RJ. Pycnodysostosis, a lysosomal disease caused by cathepsin K deficiency. Science 1996;273:1236-1238.
    CrossRef
  16. Ghosh S, and Karin M. Missing pieces in the NF-kappaB puzzle. Cell 2002;109:S81-96.
    CrossRef
  17. Gowen M, Lazner F, Dodds R, Kapadia R, Feild J, Tavaria M, Bertoncello I, Drake F, Zavarselk S, Tellis I, Hertzog P, Debouck C, and Kola I. Cathepsin K knockout mice develop osteopetrosis due to a deficit in matrix degradation but not demineralization. Journal of Bone and Mineral Research 1999;14:1654-1663.
    CrossRef
  18. Grigoriadis AE, Wang ZQ, Cecchini MG, Hofstetter W, Felix R, Fleisch HA, and Wagner EF. c-Fos: a key regulator of osteoclast macrophage lineage determination and bone remodeling. Science 1994;266:443-448.
    CrossRef
  19. Hauer J, Puschner S, Ramakrishnan P, Simon U, Bongers M, Federle C, and Engelmann H. TNF receptor (TNFR)-associated factor (TRAF) 3 serves as an inhibitor of TRAF2/5-mediated activation of the noncanonical NF-kappaB pathway by TRAFbinding TNFRs. Proceedings of the National Academy of Sciences of the United States of America 2005;102:2874-2879.
    CrossRef
  20. Hayden MS, and Ghosh S. Signaling to NF-kappa B. Genes & Development 2004;18:2195-2224.
    CrossRef
  21. He Y, Staser K, Rhodes SD, Liu Y, Wu X, Park SJ, Yuan J, Yang X, Li X, Jiang L, Chen S, and Yang FC. Erk1 positively regulates osteoclast differentiation and bone resorptive activity. PLOS One 2011;6:e24780.
    CrossRef
  22. Inoue JI, Ishida T, Tsukamoto N, Kobayashi N, Naito A, Azuma S, and Yamamoto T. Tumor necrosis factor receptor-associated factor (TRAF) family: adapter proteins that mediate cytokine signaling. Experimental Cell Research 2000;254:14-24.
    CrossRef
  23. Iotsova V, Caama챰o J, Loy J, Yang Y, Lewin A, and Bravo R. Osteopetrosis in mice lacking NF-kappaB1 and NF-kappaB2. Nature Medicine 1997;3:1285-1289.
    CrossRef
  24. Johnson MR, Polymeropoulos MH, Vos HL, Ortiz de Luna RI, and Francomano CA. A nonsense mutation in the cathepsin K gene observed in a family with pycnodysostosis. Genome Research 1996;6:1050-1055.
    CrossRef
  25. Kanazawa K, and Kudo A. TRAF2 is essential for TNF-alpha-induced osteoclastogenesis. Journal of Bone and Mineral Research 2005;20:840-847.
    CrossRef
  26. Kanazawa K, Azuma Y, Nakano H, and Kudo A. TRAF5 functions in both RANKL- and TNF alpha-induced osteoclastogenesis. Journal of Bone and Mineral Research 2003;18:443-450.
    CrossRef
  27. Kashiwada M, Shirakata Y, Inoue JI, Nakano H, Okazaki K, Okumura K, Yamamoto T, Nagaoka H, and Takemori T. Tumor necrosis factor receptor-associated factor 6 (TRAF6) stimulates extracellular signal-regulated kinase (ERK) activity in CD40 signaling along a ras-independent pathway. Journal of Experimental Medicine 1998;187:237-244.
    CrossRef
  28. Kim N, Kadono Y, Takami M, Lee J, Lee SH, Okada F, Kim JH, Kobayashi T, Odgren PR, Nakano H, Yeh WC, Lee SK, Lorenzo JA, and Choi Y. Osteoclast differentiation independent of the TRANCE-RANK-TRAF6 axis. Journal of Experimental Medicine 2005;202:589-595.
    CrossRef
  29. Kim K, Lee SH, Ha KJ, Choi Y, and Kim N. NFATc1 induces osteoclast fusion via up-regulation of Atp6v0d2 and the dendritic cell-specific transmembrane protein (DC-STAMP). Molecular Endocrinology 2008;22:176-185.
    CrossRef
  30. Kobayashi N, Kadono Y, Naito A, Matsumoto K, Yamamoto T, Tanaka S, and Inoue J. Segregation of TRAF6-mediated signaling pathways clarifies its role in osteoclastogenesis. The EMBO Journal 2001;20:1271-1280.
    CrossRef
  31. Koga T, Inui M, Inoue K, Kim S, Suematsu A, Kobayashi E, Iwata T, Ohnishi H, Matozaki T, Kodama T, Taniguchi T, Takayanagi H, and Takai T. Costimulatory signals mediated by the ITAM motif cooperate with RANKL for bone homeostasis. Nature 2004;428:758-763.
    CrossRef
  32. Kong YY, 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, and Penninger JM. OPGL is a key regulator of osteoclastogenesis, lymphocyte development and lymph-node organogenesis. Nature 1999;397:315-323.
    CrossRef
  33. Lacey DL, Timms E, Tan HL, Kelley MJ, Dunstan CR, Burgess T, Elliott R, Colombero A, Elliott G, Scully S, Hsu H, Sullivan J, Hawkins N, Davy E, Capparelli C, Eli A, Qian YX, Kaufman S, Sarosi I, Shalhoub V, Senaldi G, Guo J, Delaney J, and Boyle WJ. Osteoprotegerin ligand is a cytokine that regulates osteoclast differentiation and activation. Cell 1998;93:165-176.
    CrossRef
  34. Lee NK, Choi YG, Baik JY, Han SY, Jeong DW, Bae YS, Kim N, and Lee SY. A crucial role for reactive oxygen species in RANKLinduced osteoclast differentiation. Blood 2005;106:852-859.
    CrossRef
  35. Lee SE, Woo KM, Kim SY, Kim HM, Kwack K, Lee ZH, and Kim HH. The phosphatidylinositol 3-kinase, p38, and extracellular signalregulated kinase pathways are involved in osteoclast differentiation. Bone 2002;30:71-77.
    CrossRef
  36. Li F, Matsuo K, Xing L, and Boyce BF. Over-expression of activated NFATc1 plus RANKL rescues the osteoclastogenesis defect of NF-觀B p50/p52 double knockout splenocytes. Journal of Bone and Mineral Research 2004;19:S2.
  37. Li X, Udagawa N, Itoh K, Suda K, Murase Y, Nishihara T, Suda T, and Takahashi N. p38 MAPK-mediated signals are required for inducing osteoclast differentiation but not for osteoclast function. Endocrinology 2002;143:3105-3113.
    CrossRef
  38. Lin J, Lee D, Choi Y, and Lee SY. The scaffold protein RACK1 mediates the RANKL-dependent activation of p38 MAPK in osteoclast precursors. Science Signaling 2015;8:ra54.
    CrossRef
  39. 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, Goeddel DV, and Mak TW. TRAF6 deficiency results in osteopetrosis and defective interleukin-1, CD40, and LPS signaling. Genes & Development 1999;13:1015-1024.
    CrossRef
  40. Luchin A, Purdom G, Murphy K, Clark MY, Angel N, Cassady AI, Hume DA, and Ostrowski MC. The microphthalmia transcription factor regulates expression of the tartrate-resistant acid phosphatase gene during terminal differentiation of osteoclasts. Journal of Bone and Mineral Research 2000;15:451-460.
    CrossRef
  41. Luchin A, Suchting S, Merson T, Rosol TJ, Hume DA, Cassady AI, and Ostrowski MC. Genetic and physical interactions between microphthalmia transcription factor and PU.1 are necessary for osteoclast gene expression and differentiation. Journal of Biological Chemistry 2001;276:36703-36710.
    CrossRef
  42. Mansky KC, Sankar U, Han J, and Ostrowski MC. Microphthalmia transcription factor is a target of the p38 MAPK pathway in response to receptor activator of NF-kappa B ligand signaling. Journal of Biological Chemistry 2002;277:11077-11083.
    CrossRef
  43. Marx J. Coming to grips with bone loss. Science 2004;305:1420-1422.
    CrossRef
  44. Matsumoto M, Kogawa M, Wada S, Takayanagi H, Tsujimoto M, Katayama S, Hisatake K, and Nogi Y. Essential role of p38 mitogen-activated protein kinase in cathepsin K gene expression during osteoclastogenesis through association of NFATc1 and PU.1. Journal of Biological Chemistry 2004;279:45969-45979.
    CrossRef
  45. Matsumoto M, Sudo T, Saito T, Osada H, and Tsujimoto M. Involvement of p38 mitogen-activated protein kinase signaling pathway in osteoclastogenesis mediated by receptor activator of NF-kappa B ligand (RANKL). Journal of Biological Chemistry 2000;275:31155-31161.
    CrossRef
  46. Matsuo K, Galson DL, Zhao C, Peng L, Laplace C, Wang KZ, Bachler MA, Amano H, Aburatani H, Ishikawa H, and Wagner EF. Nuclear factor of activated T-cells (NFAT) rescues osteoclastogenesis in precursors lacking c-Fos. Journal of Biological Chemistry 2004;279:26475-26480.
    CrossRef
  47. Mizukami J, Takaesu G, Akatsuka H, Sakurai H, Ninomiya-Tsuji J, Matsumoto K, and Sakurai N. Receptor activator of NF-kappaB ligand (RANKL) activates TAK1 mitogen-activated protein kinase kinase kinase through a signaling complex containing RANK, TAB2, and TRAF6. Molecular and Cellular Biology 2002;22:992-1000.
    CrossRef
  48. M처csai A, Humphrey MB, Van Ziffle JA, Hu Y, Burghardt A, Spusta SC, Majumdar S, Lanier LL, Lowell CA, and Nakamura MC. The immunomodulatory adapter proteins DAP12 and Fc receptor gamma-chain (FcRgamma) regulate development of functional osteoclasts through the Syk tyrosine kinase. Proceedings of the National Academy of Sciences of the United States of America 2004;101:6158-6163.
    CrossRef
  49. Moon JB, Kim JH, Kim K, Youn BU, Ko A, Lee SY, and Kim N. Akt induces osteoclast differentiation through regulating the GSK3棺/NFATc1 signaling cascade. Journal of Immunology 2012;188:163-169.
    CrossRef
  50. Naito A, Azuma S, Tanaka S, Miyazaki T, Takaki S, Takatsu K, Nakao K, Nakamura K, Katsuki M, Yamamoto T, and Inoue J. Severe osteopetrosis, defective interleukin-1 signaling and lymph node organogenesis in TRAF6-deficient mice. Genes to Cells 1999;4:353-362.
    CrossRef
  51. Novack DV, Yin L, Hagen-Stapleton A, Schreiber RD, Goeddel DV, Ross FP, and Teitelbaum SL. The IkappaB function of NFkappaB2 p100 controls stimulated osteoclastogenesis. Journal of Experimental Medicine 2003;198:771-781.
    Pubmed KoreaMed CrossRef
  52. Roberts HC, Knott L, Avery NC, Cox TM, Evans MJ, and Hayman AR. Altered collagen in tartrate-resistant acid phosphatase (TRAP)- deficient mice: a role for TRAP in bone collagen metabolism. Calcified Tissue International 2007;80:400-410.
    Pubmed CrossRef
  53. 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, Davy E, Bucay N, Renshaw-Gegg L, Hughes TM, Hill D, Pattison W, Campbell P, Sander S, Van G, Tarpley J, Derby P, Lee R, and Boyle WJ. Osteoprotegerin: a novel secreted protein involved in the regulation of bone density. Cell 1997;89:309-319.
    CrossRef
  54. Suda T, Takahashi N, Udagawa N, Jimi E, Gillespie MT, and Martin TJ. Modulation of osteoclast differentiation and function by the new members of the tumor necrosis factor receptor and ligand families. Endocrine Review 1999;20:345-357.
    Pubmed CrossRef
  55. Sugatani T, Alvarez U, and Hruska KA. PTEN regulates RANKL- and osteopontin-stimulated signal transduction during osteoclast differentiation and cell motility. Journal of Biological Chemistry 2003;278:5001-5008.
    Pubmed CrossRef
  56. Takayanagi H, Kim S, Koga T, Nishina H, Isshiki M, Yoshida H, Saiura A, Isobe M, Yokochi T, Inoue J, Wagner EF, Mak TW, Kodama T, and Taniguchi T. Induction and activation of the transcription factor NFATc1 (NFAT2) integrate RANKL signaling in terminal differentiation of osteoclasts. Developmental Cell 2002;3:889-901.
    CrossRef
  57. Takeshita S, Namba N, Zhao JJ, Jiang Y, Genant HK, Silva MJ, Brodt MD, Helgason CD, Kalesnikoff J, Rauh MJ, Humphries RK, Krystal G, Teitelbaum SL, and Ross FP. SHIP-deficient mice are severely osteoporotic due to increased numbers of hyperresorptive osteoclasts. Nature Medicine 2002;8:943-949.
    Pubmed CrossRef
  58. Teitelbaum SL. Bone resorption by osteoclasts. Science 2000;289:1504-1508.
    Pubmed CrossRef
  59. Vanhaesebroeck B, and Alessi DR. The PI3K-PDK1 connection: more than just a road to PKB. Biochemistry Journal 2000;346:561-576.
    Pubmed KoreaMed CrossRef
  60. Walsh MC, and Choi Y. Biology of the TRANCE axis. Cytokine & Growth Factor Reviews 2003;14:251-263.
    CrossRef
  61. Wong BR, Besser D, Kim N, Arron JR, Vologodskaia M, Hanafusa H, and Choi Y. TRANCE, a TNF family member, activates Akt/ PKB through a signaling complex involving TRAF6 and c-Src. Molecular Cell 1999;4:1041-1049.
    CrossRef
  62. Wong BR, Josien R, Lee SY, Sauter B, Li HL, Steinman RM, and Choi Y. TRANCE (tumor necrosis factor [TNF]-related activationinduced cytokine), a new TNF family member predominantly expressed in T cells, is a dendritic cell-specific survival factor. Journal of Experimental Medicine 1997;186:2075-2080.
    Pubmed KoreaMed CrossRef
  63. Xing L, Bushnell TP, Carlson L, Tai Z, Tondravi M, Siebenlist U, Young F, and Boyce BF. NF-kappaB p50 and p52 expression is not required for RANK expressing osteoclast progenitor formation but is essential for RANK- and cytokine mediated osteoclastogenesis. Journal of Bone and Mineral Research 2002;17:1200-1210.
    Pubmed CrossRef
  64. Yamamoto A, Miyazaki T, Kadono Y, Takayanagi H, Miura T, Nishina H, Katada T, Wakabayashi K, Oda H, Nakamura K, and Tanaka S. Possible involvement of IkappaB kinase 2 and MKK7 in osteoclastogenesis induced by receptor activator of nuclear factor kappaB ligand. Journal of Bone and Mineral Research 2002;17:612-621.
    Pubmed CrossRef
  65. Yamashita T, Yao Z, Li F, Zhang Q, Badell IR, Schwarz EM, Takeshita S, Wagner EF, Noda M, Matsuo K, Xing L, and Boyce BF. NF-kappaB p50 and p52 regulate receptor activator of NF-kappaB ligand (RANKL) and tumor necrosis factorinduced osteoclast precursor differentiation by activating c-Fos and NFATc1. Journal of Biological Chemistry 2007;282:18245-18253.