Abstract
Bone remodeling is tightly controlled by various factors, including hormones, autacoids and cytokines. Among them, oncostatin M (OSM) is a multifunctional cytokine produced by osteal macrophages, which serves as an essential modulator of bone remodeling. Macrophage colony-stimulating factor (M-CSF) and osteoprotegerin are secreted by osteoblasts, and also have pivotal roles in the regulation of the bone remodeling process. The binding of basic fibroblast growth factor (bFGF), a key regulator of bone remodeling, to the corresponding receptor [fibroblast growth factor receptor (FGFR)] triggers the dimerization and activation of FGFRs, which causes the phosphorylation of FGFR substrates and subsequent activation of downstream effectors, including mitogen-activated protein kinases (MAPKs), via Grb2. bFGF can activate MAPKs, resulting in the synthesis of osteoprotegerin and vascular endothelial growth factor in osteoblast-like MC3T3-E1 cells. In the present study, the effects of OSM on bFGF-induced osteoblast activation were investigated in the synthesis of osteoprotegerin and M-CSF in osteoblasts. The release of osteoprotegerin and M-CSF were analyzed using ELISA. The mRNA expression levels of osteoprotegerin and M-CSF were analyzed using reverse transcription-quantitative PCR. Phosphorylation of p38 MAPK, stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) and p44/p42 MAPK was assessed using western blotting. OSM enhanced bFGF-induced osteoprotegerin release and bFGF-stimulated mRNA expression of osteoprotegerin. By contrast, OSM suppressed the bFGF-induced release of M-CSF and bFGF-stimulated mRNA expression of M-CSF. SB203580, a p38 MAPK inhibitor, and SP600125, a SAPK/JNK inhibitor, suppressed the bFGF-stimulated M-CSF release, whereas PD98059, an upstream kinase inhibitor of p44/p42 MAPK, failed to suppress the M-CSF release stimulated by bFGF. Furthermore, OSM enhanced the bFGF-induced phosphorylation of p38 MAPK, but attenuated the bFGF-stimulated phosphorylation of SAPK/JNK. By contrast, OSM had little effect on the bFGF-induced phosphorylation of p44/p42 MAPK. SB203580 markedly reduced the amplification of bFGF-stimulated osteoprotegerin release enhanced by OSM. These results strongly suggested that OSM may possess divergent effects on bFGF-induced osteoblast activation, upregulation of p38 MAPK and downregulation of SAPK/JNK, leading to the amplification of osteoprotegerin synthesis and the attenuation of M-CSF synthesis.