IL-6 Enhances Osteocyte-Mediated Osteoclastogenesis by Promoting JAK2 and RANKL Activity In Vitro.

BACKGROUND/AIMS: Evidence suggests that IL-6 affects bone mass by modulating osteocyte communication towards osteoclasts. However, the mechanism by which IL-6 enhances osteocyte-mediated osteoclastogenesis is unclear. We aimed to investigate the inflammatory factors in serum after orthodontic surgery and their relationship between osteocytes and osteoclasts.
METHODS: Serum was obtained from 10 orthognathic surgery patients, and inflammatory factors were detected by ELISA. We treated the osteocyte-like cell line MLO-Y4 with recombinant mouse IL-6 and IL-6 receptor (IL-6R), and used quantitative RT-PCR and Western blotting to explore Receptor activator of nuclear factor-κB ligand (RANKL) expression at both the mRNA and protein level. MLO-Y4 cells were co-cultured with osteoclast precursor cells, and the formation of osteoclasts was detected by tartrate-resistant acid phosphatase (TRAP) staining. To explore the role of JAK2 in the osteocyte-mediated osteoclastogenesis, AG490, a JAK2 inhibitor, was used to inhibit the JAK2-STAT3 pathway in osteocytes.
RESULTS: In our study, we found that IL-6 and RANKL were stimulated in serum 3-7 days after orthognathic surgery. Therefore, IL-6 and IL-6 receptor enhanced the expression of RANKL at both the mRNA and protein level in MLO-Y4. Furthermore, when MLO-Y4 cells were co-cultured with osteoclast precursor cells, it significantly stimulated osteoclastogenesis. Our study indicated that osteocytes could promote osteoclastic differentiation and the formation of TRAP-positive multinucleated cells after stimulation with IL-6 and IL-6R. Our results also indicated that treatment with IL-6 and IL-6R increased RANKL mRNA expression and the RANKL/OPG expression ratio. Meanwhile, the phosphorylation of Janus kinase 2 (JAK2) and Signal transducer and activator of transcription (STAT3) also correlated with RANKL levels. Furthermore, we investigated the effects of a specific JAK2 inhibitor, AG490, on the expression of RANKL in osteocyte-like MLO-Y4 cells and osteocyte-mediated osteoclastogenesis. The results showed that AG490 inhibited (p)-JAK2 and RANKL expression. Osteoclastic differentiation was decreased after pretreatment in MLO-Y4 with mouse IL-6/IL-6R and AG490; therefore, we concluded that IL-6 increased osteocyte-mediated osteoclastic differentiation by activating JAK2 and RANKL.
CONCLUSION: The effects of IL-6/il-6R and AG490 on osteocyte-mediated osteoclastogenesis contribute to our understanding of the role of inflammatory factors in the interaction between osteocytes and osteoclast precursors. IL-6 and RANKL are key factors for bone remodelling after the orthodontic surgery, and their roles in bone remodelling may be fundamental mechanisms accelerating tooth movement by orthodontic surgery.