Resumen de Epithelial Cells Secrete Interferon-γ Which Suppresses Expression of Receptor Activator of Nuclear Factor Kappa-B Ligand in Human Mandibular Osteoblast-Like Cells
Pakchisa Khonsuphap, Prasit Pavasant, Rizky Aditya Irwandi, Chidchanok Leethanakul, Anjalee Vacharaksa
Background: Prostaglandin (PG)E2 accumulates in inflamed periodontal tissue and induces receptor activator of nuclear factor kappa-B ligand (RANKL)-RANK-osteoprotegerin (OPG) signaling associated with bone resorption. Although oral epithelial cells maintain tissue homeostasis, the role of these cells in RANKL regulation remains unknown.
Methods: To mimic an inflamed condition, RANKL upregulation in human mandibular osteoblast-like cells (HMOBs) were stimulated with PGE2. Effect of recombinant human interferon (IFN)-γ or epithelial-derived IFN-γ in constitutively released or Porphyromonas gingivalis lipopolysaccharide (PgLPS)-stimulated epithelial supernatant was investigated in HMOBs. Some HMOBs were pretreated with an anti-IFN-γ antibody before PGE2 stimulation. THP-1 human monocytes and HMOBs were cocultured in a transwell system to investigate RANKL-driven THP-1 osteoclastic activity.
Results: PGE2 significantly increased RANKL messenger RNA (mRNA) and protein in HMOBs in a dose-dependent manner, while OPG protein remained similar to baseline. Epithelial cells constitutively released IFN-γ, which was substantially increased by PgLPS. HMOBs treated with epithelial supernatant or recombinant IFN-γ, concurrently with PGE2 stimulation, reduced RANKL, but not OPG, expression. In contrast, anti-IFN-γ antibody reversed the effect of epithelial mediators on RANKL expression. When cocultured with THP-1, RANKL released by PGE2-stimulated HMOBs is adequate to drive THP-1 differentiation as osteoclastogenic gene expression and bone resorption pit are increased. However, recombinant IFN-γ, or IFN-γ derived from oral epithelial cells, suppressed RANKL expression at both the mRNA and protein level, resulting in decreased THP-1-derived osteoclastic activity.
Conclusion: Oral epithelial cells interact with HMOBs by releasing IFN-γ to regulate RANKL expression and contribute to osteoclastogenesis.
Host response to pathogens is tightly regulated and essential for maintaining immune homeostasis and preventing life-threatening infections at the oral mucosal surface.1,2 Activation of immune response mediates host protection; however, unresolved inflammation can result in tissue damage and bone loss. Accumulation of local inflammatory mediators, including interleukin (IL)-1,3 tumor necrosis factor (TNF)-α4, matrix metalloproteinase,5 and prostaglandin (PG)E2, stimulates expression of receptor activator of nuclear factor (NF)kappa-B ligand (RANKL) by cells of osteoblastic lineage,6 or immune cells in close proximity,7 which promotes osteoclastogenesis. Physiologic bone remodeling is a balanced process of bone resorption and formation. However, periodontal tissue inflammation can uncouple this process, resulting in excessive bone resorption. Inhibitors of inflammatory mediators reduced destructive bone loss in an experimental periodontitis model.8 PGE2 has diverse effects in bone metabolism, simultaneously serving as a potent stimulator of both osteoclastic bone resorption and of osteoblastic bone formation.9 PGE2 levels correlate with progression of periodontal inflammation in humans and animal models.8 In patients with chronic periodontitis, PGE2 concentration in gingival crevicular fluid was significantly increased and highly correlated with clinical parameters.10 PGE2 increases cyclic adenosine monophosphate and protein kinase A activation in murine osteoblasts11 and human osteoblastic cells12 through PGE2 receptors EP2R and EP4R, stimulating RANKL production. Although RANKL is an essential cytokine for osteoclast differentiation, the decoy receptor osteoprotegerin (OPG) inhibits RANKL signaling.13 Therefore, RANKL/OPG ratio is critical in promoting osteoclastogenesis, inducing bone loss, and accelerating bone resorption.
The epithelium plays a central role in maintaining oral mucosal integrity and regulates mucosal immune homeostasis to prevent onset of uncontrolled inflammation. Microbial challenge activates intracellular NF-κB signaling pathway in oral epithelial cells, triggering an adaptive immune response. This inflammatory response modulates bone resorption by regulating RANKL-RANK-OPG interaction. Epithelial-derived mediators and potent antimicrobial peptides regulate mucosal tissue homeostasis.14 However, interaction between oral epithelial and bone cells has never been reported. Whether oral epithelial cells can modulate bone cell function to maintain bone tissue homeostasis remains unknown. To investigate the role of oral epithelial cells in regulating osteoclastogenesis, this study determined whether epithelial-derived mediators could reduce expression of RANKL, the key molecule in initiating osteoclastogenesis, in human mandibular bone-derived cells, and if this reduction decreased osteoclastic cell activity.
