M1 macrophages regulate TLR4/AP1 via paracrine to promote alveolar bone destruction in periodontitis

M1 macrophages induce TLR4/AP1 signaling of pre-osteoblasts to inhibit the osteoblastogenesis via paracrine, at least partially contributing to alveolar bone destruction in periodontitis.

Mouse periodontitis model was established to determine the involvement of M1 macrophages in the pathogenic process. Condition medium of the M1 macrophages (M1-CM) was incubated with pre-osteoblasts to evaluate its effects on the osteoblastogenesis. Cells after treatment with CM were used for RNA-sequencing, quantitative PCR, Western blotting, and immunofluorescence staining to figure out pathways involved in the inhibition of osteoblastogenesis.

Macrophages could be fully polarized and acquire specific phenotype like M1, which considered to be essential for the alveolar bone destruction during the development of periodontitis. However, the molecular mechanisms underlying the effects of M1 macrophages on the alveolar bone destruction are still not clear yet.

Increased infiltration of M1 macrophages was associated with alveolar bone destruction in periodontitis. M1-CM markedly suppressed the generation of osteoblasts as evidenced by decreased expressions of Runx2 and Ocn, as well as reduced activity of ALP. Interestingly, RNA-sequencing indicated the activation of TLR4/AP1 signaling pathway in pre-osteoblasts treated with CM. Inhibition of TLR4 reduced the translocation of AP1 and rescued the osteoblastogenesis reduced by M1-CM.