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Secreted Frizzled‐Related Protein‐1 Inhibits RANKL‐Dependent Osteoclast Formation
Author(s) -
Häusler Karl D,
Horwood Nicole J,
Chuman Yoshiro,
Fisher Jane L,
Ellis Jennifer,
Martin T John,
Rubin Jeffrey S,
Gillespie Matthew T
Publication year - 2004
Publication title -
journal of bone and mineral research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.882
H-Index - 241
eISSN - 1523-4681
pISSN - 0884-0431
DOI - 10.1359/jbmr.040807
Subject(s) - osteoclast , rankl , frizzled , microbiology and biotechnology , chemistry , signal transduction , wnt signaling pathway , biology , biochemistry , gene , receptor , activator (genetics)
Abstract We determined that sFRP‐1 mRNA was differentially expressed by osteoblast/stromal cell lines and that sFRP‐1 neutralizing antibodies and siRNA complementary to sFRP‐1 coding sequence enhanced, while recombinant sFRP‐1 inhibited, osteoclast formation. In studying the mechanism of action for sFRP‐1, we found that sFRP‐1 could bind recombinant RANKL. These results suggest potential cross‐talk between Wnt and RANKL pathways. Introduction: Osteoclast formation in normal bone remodeling requires the presence of osteoblast lineage cells that express RANKL and macrophage‐colony‐stimulating factor (M‐CSF), which interact with their cognate receptors on the osteoclast precursor. We identified secreted Frizzled‐related protein‐1 (sFRP‐1), which is known to bind to Wnt and inhibit the Wnt signaling pathway, as an osteoblast‐derived factor that impinges on osteoclast formation and activity. Materials and Methods: Differential display of mRNA from osteoblast lineage cell lines established sFRP‐1 to be highly expressed in an osteoclast supporting cell line. sFRP‐1 expression in bone was determined by in situ hybridization, and the effects of sFRP‐1 on osteoclast formation were determined using a neutralizing antibody, siRNA, for sFRP‐1 and recombinant protein. Results: In situ hybridization revealed sFRP‐1 mRNA expression in osteoblasts and chondrocytes in murine bone. sFRP‐1 mRNA expression could be elevated in calvarial primary osteoblasts in response to prostaglandin E 2 (PGE 2 ) or interleukin (IL)‐11, whereas many other osteotropic agents (e.g., IL‐1, IL‐6, calcitrol, parathyroid hormone) were without any effect. In vitro assays of osteoclast formation established sFRP‐1 to be an inhibitor of osteoclast formation. Neutralizing antibodies against sFRP‐1 enhanced TRACP + mononuclear and multinuclear osteoclast formation (3‐ and 2‐fold, respectively) in co‐cultures of murine osteoblasts with spleen cells, whereas siRNA complementary to sFRP‐1 coding sequence significantly enhanced osteoclast formation in co‐cultures of KUSA O (osteoblast/stromal cell line) and bone marrow cells, cultured in the presence of PGE 2 and 1,25(OH) 2 vitamin D 3 . Recombinant sFRP‐1 dose‐dependently inhibited osteoclast formation in osteoblast/spleen co‐cultures, RANKL + M‐CSF‐treated splenic cultures, and RANKL‐treated RAW264.7 cell cultures, indicating a direct action of sFRP‐1 on hematopoietic cells. Consistent with this, sFRP‐1 was found to bind to RANKL in ELISAs. Conclusion: sFRP‐1 is expressed by osteoblasts and inhibits osteoclast formation. While sFRP‐1 activity might involve the blocking of endogenous Wnt signaling, our results suggest that, alternatively, it could be because of direct binding to RANKL. This study describes a new mechanism whereby osteoblasts regulate osteoclastogenesis through the expression and release of sFRP‐1.