The immunomodulatory role of irisin on osteogenesis via AMPK-mediated macrophage polarization.

Bone healing is thought to be closely related to macrophages. Irisin, a cleaved hormone-like myokine, is well known to participate in immunoregulation and regulates bone metabolism. However, whether irisin could influence osteogenesis by affecting macrophage polarization is remain unknown. Here, the present study aims to investigate the potential immunomodulatory role of irisin on macrophages polarization and its subsequent impact on osteogenesis. We demonstrated that irisin increased cell viability without toxic effect in both Raw264.7 macrophages and MC3T3-E1 cells. Furthermore, irisin treatment polarized M0 and M1 macrophages towards M2 phenotype, with increased expression of CD206-APC, ARG-1 and TGF-β1, and decreased expression of CD86-PE and TNF-α. In addition, the direct co-cultured test of Raw264.7 macrophages and pre-osteoblastic MC3T3-E1 cells showed that irisin-treated M0 and M1 macrophages promoted osteogenesis with obvious formation of mineralized particles. Interestingly, irisin exposure robustly activated AMPK-α signaling, as manifested by increased expression of phosphorylated AMPK-α. Knockdown of AMPK-α by siRNA significantly suppressed the phosphorylation of AMPK-α, abrogated irisin-induced polarization of M2 phenotype, and inhibited the osteogenic ability of Raw264.7 macrophages. Taken together, our findings showed that irisin-induced M2 polarization enhanced osteogenesis in osteoblasts, and this effect might be associated with activation of AMPK.