Insights into the Role of Macrophage Polarization in the Pathogenesis of Osteoporosis

Figure 1

Schematic diagram shows the relationship between M1 macrophages, M2 macrophages, osteoblasts, osteoclasts, and osteocytes. (1) The macrophage polarization state is “fluid,” and macrophage phenotypes can be changed depending on microenvironment. (2) M1 macrophage cytokines such as TNF-α downregulate osteogenic genes and suppress bone mineralization. (3) M1 macrophage cytokines such as TNF-α contribute to enhanced osteoclast activity and bone resorption. (4) M2 macrophage cytokines such as IL-4 upregulate osteogenic genes and promote bone mineralization. (5) M2 macrophage cytokines such as IL-4 inhibit osteoclast activity and bone resorption. (6) Both M1 and M2 macrophages are reported to be the precursors of osteoclasts. (7) M1 macrophages reduce osteocyte viability through M1 cytokines such as IL-1 and affect the normal physiological functions of osteocytes. (8) Osteoblasts progressively form into osteocytes.