TNF-α Directly Enhances Osteocyte RANKL Expression and Promotes Osteoclast Formation

Aseel Marahleh, Hideki Kitaura, Fumitoshi Ohori, Akiko Kishikawa, Saika Ogawa, Wei Ren Shen, Jiawei Qi, Takahiro Noguchi, Yasuhiko Nara, Itaru Mizoguchi

Research output: Contribution to journalArticlepeer-review

4 Citations (Scopus)

Abstract

Osteoimmunology peeks into the interaction of bone and the immune system, which has largely proved to be a multiplex reaction. Osteocytes have been shown to regulate bone resorption through the expression of RANKL in physiologic and pathologic conditions. TNF-α, a product of the immune system, is an important cytokine regulating bone resorption in inflammatory conditions either directly or by increasing RANKL and M-CSF expressions by osteoblasts and stromal cells. The effect of TNF-α on a wide range of cell types has been documented; however, the direct effect of TNF-α on osteocytes has not been established yet. In this study, primary osteocytes were isolated by cell sorting from neonatal calvaria of Dmp1-Topaz mice, which express the green fluorescent protein under the influence of dentin matrix protein 1 promoter. The results show that osteocytes have a significantly higher RANKL mRNA expression when cultured with TNF-α. A co-culture system of osteocytes and TNF receptors I and II deficient osteoclast precursors treated with TNF-α show a significant increase in TRAP-positive cells while cultures without TNF-α failed to show TRAP-positive cells. Additionally, in vivo experiments of TNF-α injected to mouse calvaria show an increase in TRAP-positive cell number in the suture mesenchyme and an increase in the percentage of RANKL-positive osteocytes compared to PBS-injected calvaria. Osteocytes cultured with TNF-α show up-regulation of MAPKs phosphorylation measured by western blot, and adding MAPKs inhibitors to osteocytes cultured with TNF-α significantly decreases RANKL mRNA expression compared to osteocytes cultured with TNF-α alone. We also found that TNF-α activates the NF-κB pathway in osteocytes measured as a function of p65 subunit nuclear translocation. TNF-α directly affects osteocyte RANKL expression and increases osteoclastogenesis; our results demonstrate that osteocytes guard an important role in inflammatory bone resorption mediated by TNF-α.

Original languageEnglish
Article number2925
JournalFrontiers in immunology
Volume10
DOIs
Publication statusPublished - 2019 Dec 13

Keywords

  • RANKL
  • TNF-α
  • osteoclastogenesis
  • osteocyte
  • osteoimmunology

ASJC Scopus subject areas

  • Immunology and Allergy
  • Immunology

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