Interleukin-17A induces cathepsin K and MMP-9 expression in osteoclasts via celecoxib-blocked prostaglandin E₂ in osteoblasts

Interleukin-17 (IL-17) is a cytokine secreted primarily by T_H-17 cells that can stimulate the development of osteoclasts (osteoclastogenesis) in the presence of osteoblasts. IL-17, through osteoblasts, has indirect effects on the expression of bone resorption-related enzymes in osteoclasts, which have not been well clarified. Here, using MC3T3-E1 cells and RAW264.7 cells as osteoblasts and osteoclast precursors, we aimed to clarify these effects of IL-17A. MC3T3-E1 cells were cultured in the presence or absence of IL-17A for 72 h and the conditioned media collected (in the presence of soluble receptor activator of NF-B ligand) and used to culture RAW264.7 cells. To assess osteoclast differentiation, adherent cells were fixed and stained for tartrate-resistant acid phosphatase (TRAP). Our analyses demonstrated that the number of TRAP-positive multinucleated cells increases after 3 days of culture in conditioned medium from IL-17A-treated cells compared to untreated controls. In addition, we observed that the levels of cathepsin K and MMP-9 increase in the conditioned medium from IL-17A-treated cells, whereas CA II expression levels remain unaffected. PGE₂ production from MC3T3-E1 cells increased in the presence of IL-17A. Celecoxib, a specific inhibitor of cyclooxygenase-2 (COX-2), blocked both the IL-17A-stimulated increase in TRAP-positive multinucleated cells and the expression of cathepsin K and MMP-9. Furthermore, when MC3T3-E1 cells were transformed with small interfering RNA to silence COX-2 expression before IL-17A treatment, the resulting conditioned medium was less effective at inducing cathepsin K and MMP-9 expression in RAW264.7 cells. These results suggest that IL-17A induces the differentiation and function of osteoclasts via celecoxib-blocked prostaglandin, mainly PGE ₂, in osteoblasts.