Tumor necrosis factor-α induces interleukin-34 expression through nuclear factor-κB activation in MC3T3-E1 osteoblastic cells

Osteoblasts produce various types of cytokines under pathological conditions and control osteoclast differentiation. Tumor necrosis factor-alpha (TNF-alpha) has been demonstrated to exert complex effects in osteoblasts under local inflammatory conditions, including in periodontal and periapical diseases. Interleukin-34 (IL-34) has been recently identified as a novel regulatory factor for the differentiation and function of osteoclasts. The present study provides the first evidence, to the best of our knowledge, that the expression of IL-34 is induced by TNF-alpha through nuclear factor-kappa B (NF-kappa B) activation in MC3T3-E1 osteoblastic cells. TNF-alpha induced IL-34 expression in a dose- and time-dependent manner. Immunocytochemistry with an NF-kappa B antibody demonstrated that NF-kappa B was mainly localized in the cytoplasm of the untreated MC3T3-E1 cells. Rapid translocation of NF-kappa B from the cytoplasm to the nucleus was observed in the cells treated with TNF-alpha for 15 min. Translocation and transcriptional activity of NF-kappa B were also determined by western blotting and a luciferase reporter assay, respectively. Pretreatment with 100 mu M CAPE, an inhibitor of NF-kappa B, significantly inhibited TNF-alpha-induced IL-34 expression. These results indicate that TNF-alpha induces IL-34 expression via NF-kappa B in osteoblasts.