Sclerostin stimulates osteocyte support of osteoclast activity by a RANKL-dependent pathway

PLoS One. 2011;6(10):e25900. doi: 10.1371/journal.pone.0025900. Epub 2011 Oct 4.

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acid Phosphatase / metabolism
  • Adaptor Proteins, Signal Transducing
  • Adult
  • Animals
  • Apoptosis / drug effects
  • Bone Morphogenetic Proteins / pharmacology*
  • Calcification, Physiologic / drug effects
  • Cell Differentiation / drug effects
  • Cell Differentiation / genetics
  • Cell Survival / drug effects
  • Cells, Cultured
  • Gene Expression Regulation / drug effects
  • Genetic Markers
  • Humans
  • Isoenzymes / metabolism
  • Mice
  • Osteoblasts / cytology
  • Osteoblasts / drug effects
  • Osteoblasts / metabolism
  • Osteoclasts / cytology*
  • Osteoclasts / drug effects
  • Osteoclasts / metabolism
  • Osteocytes / cytology*
  • Osteocytes / drug effects
  • Osteocytes / metabolism
  • Osteogenesis / drug effects
  • Osteogenesis / genetics
  • RANK Ligand / genetics
  • RANK Ligand / metabolism*
  • Recombinant Proteins / pharmacology
  • Signal Transduction / drug effects*
  • Tartrate-Resistant Acid Phosphatase

Substances

  • Adaptor Proteins, Signal Transducing
  • Bone Morphogenetic Proteins
  • Genetic Markers
  • Isoenzymes
  • RANK Ligand
  • Recombinant Proteins
  • SOST protein, human
  • Acid Phosphatase
  • Acp5 protein, mouse
  • Tartrate-Resistant Acid Phosphatase