Journal Article

Sclerostin Stimulates Osteocyte Support of Osteoclast Activity by a RANKL-Dependent Pathway

Kogawa, Masakazu, Wijenayaka, Asiri R., Lim, Hui Peng, Atkins, Gerald J., Bonewald, Lynda F., Findlay, David M.

2011

Overview

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.

• Publisher: Public Library of Science,Public Library of Science (PLoS)
• Subject: Acid phosphatase (tartrate-resistant) / Acid Phosphatase - metabolism / Adult / Animals / Apoptosis / Apoptosis - drug effects / Biocompatibility / Biology / Bone density / Bone mass / Bone Morphogenetic Proteins - pharmacology / Bone resorption / Calcification, Physiologic - drug effects / Caspase / Cell Differentiation - drug effects / Cell Differentiation - genetics / Cell Survival - drug effects / Cells, Cultured / Cytokines / Discipline / Ethics / Gene expression / Gene Expression Regulation - drug effects / Genes / Genetic Markers / Genotype & phenotype / Homeostasis / Humans / Isoenzymes - metabolism / Kinases / Leukocytes (mononuclear) / Low density lipoprotein receptors / Medicine / Mice / Monoculture / Osteoblastogenesis / Osteoblasts - cytology / Osteoblasts - drug effects / Osteoblasts - metabolism / Osteoclasts / Osteoclasts - cytology / Osteoclasts - drug effects / Osteoclasts - metabolism / Osteocytes / Osteocytes - cytology / Osteocytes - drug effects / Osteocytes - metabolism / Osteogenesis - drug effects / Osteogenesis - genetics / Osteoprotegerin / Peripheral blood mononuclear cells / RANK Ligand - genetics / RANK Ligand - metabolism / Recombinant Proteins - pharmacology / RNA / Signal Transduction - drug effects / SOST protein / Splenocytes / Tartrate-Resistant Acid Phosphatase / TRANCE protein / Tumor necrosis factor-TNF