Abstract
Osteoblasts produce various types of cytokines under pathological conditions and control osteoclast differentiation. Tumor necrosis factor-α (TNF-α) 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-α through nuclear factor-κB (NF-κB) activation in MC3T3-E1 osteoblastic cells. TNF-α induced IL-34 expression in a dose- and time-dependent manner. Immunocytochemistry with an NF-κB antibody demonstrated that NF-κB was mainly localized in the cytoplasm of the untreated MC3T3-E1 cells. Rapid translocation of NF-κB from the cytoplasm to the nucleus was observed in the cells treated with TNF-α for 15 min. Translocation and transcriptional activity of NF-κB were also determined by western blotting and a luciferase reporter assay, respectively. Pretreatment with 100 µM CAPE, an inhibitor of NF-κB, significantly inhibited TNF-α-induced IL-34 expression. These results indicate that TNF-α induces IL-34 expression via NF-κB in osteoblasts.