title: Sclerostin stimulates osteocyte support of osteoclast activity by a RANKL-dependent pathway.
abstract: Sclerostin is a product of mature osteocytes embedded in mineralised bone and is a negative regulator of bone mass and osteoblast differentiation. While evidence suggests that sclerostin has an anti-anabolic role, the possibility also exists that sclerostin has catabolic activity. To test this we treated human primary pre-osteocyte cultures, cells we have found are exquisitely sensitive to sclerostin, or mouse osteocyte-like MLO-Y4 cells, with recombinant human sclerostin (rhSCL) and measured effects on pro-catabolic gene expression. Sclerostin dose-dependently up-regulated the expression of receptor activator of nuclear factor kappa B (RANKL) mRNA and down-regulated that of osteoprotegerin (OPG) mRNA, causing an increase in the RANK:OPG mRNA ratio. To examine the effects of rhSCL on resulting osteoclastic activity, MLO-Y4 cells plated onto a bone-like substrate were primed with rhSCL for 3 days and then either mouse splenocytes or human peripheral blood mononuclear cells (PBMC) were added. This resulted in cultures with elevated osteoclastic resorption (approximately 7-fold) compared to untreated co-cultures. The increased resorption was abolished by co-addition of recombinant OPG. In co-cultures of MLO-Y4 cells with PBMC, SCL also increased the number and size of the TRAP-positive multinucleated cells formed. Importantly, rhSCL had no effect on TRAP-positive cell formation from monocultures of either splenocytes or PBMC. Further, rhSCL did not induce apoptosis of MLO-Y4 cells, as determined by caspase activity assays, demonstrating that the osteoclastic response was not driven by dying osteocytes. Together, these results suggest that sclerostin may have a catabolic action through promotion of osteoclast formation and activity by osteocytes, in a RANKL-dependent manner.
authors: Bone Cell Biology Group, Discipline of Orthopaedics and Trauma, University of Adelaide, and the Hanson Institute, Adelaide, Australia.
authors: Asiri R
authors: AR
authors: Wijenayaka
authors: Masakazu
authors: M
authors: Kogawa
authors: Hui Peng
authors: HP
authors: Lim
authors: Lynda F
authors: LF
authors: Bonewald
authors: David M
authors: DM
authors: Findlay
authors: Gerald J
authors: GJ
authors: Atkins
mesh_terms: D000135:Acid Phosphatase / Q000378:metabolism; D048868:Adaptor Proteins, Signal Transducing; D000328:Adult; D000818:Animals; D017209:Apoptosis / Q000187:drug effects; D019485:Bone Morphogenetic Proteins / Q000494:pharmacology*; D002113:Calcification, Physiologic / Q000187:drug effects; D002454:Cell Differentiation / Q000187:drug effects / Q000235:genetics; D002470:Cell Survival / Q000187:drug effects; D002478:Cells, Cultured; D005786:Gene Expression Regulation / Q000187:drug effects; D005819:Genetic Markers; D006801:Humans; D007527:Isoenzymes / Q000378:metabolism; D051379:Mice; D010006:Osteoblasts / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D010010:Osteoclasts / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism; D010011:Osteocytes / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism; D010012:Osteogenesis / Q000187:drug effects / Q000235:genetics; D053245:RANK Ligand / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000494:pharmacology; D015398:Signal Transduction / Q000187:drug effects*; D000071681:Tartrate-Resistant Acid Phosphatase
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D019485:Bone Morphogenetic Proteins; D005819:Genetic Markers; D007527:Isoenzymes; D053245:RANK Ligand; D011994:Recombinant Proteins; C422903:SOST protein, human; D000135:Acid Phosphatase; C000605511:Acp5 protein, mouse; D000071681:Tartrate-Resistant Acid Phosphatase