Mechanical stress protects against chondrocyte pyroptosis through lipoxin A4 via synovial macrophage M2 subtype polarization in an osteoarthritis model

Our previous study found that lipoxin A4 (LXA4) exerts therapeutic effects on osteoarthritis (OA). In this study, we evaluated the effects of LXA4 via synovial macrophage M1/M2 subtype polarization on chondrocyte pyroptosis and corresponding mechanisms during mechanical stimulation. Synovial macrophages were subjected to various LXA4 concentrations and cyclic tensile strain (CTS) conditions to determine optimal co-culture conditions. The effects of LXA4 on chondrocyte pyroptosis, as represented by macrophage M1/M2 subtype polarization, were detected by western blot, immunofluorescence, and flow cytometry analyses. Forty male Sprague-Dawley rats were randomly divided into four groups (n = 10): control (CG), OA (OAG), OA with moderate-intensity treadmill exercise (OAE), and OAE + BOC-2 (an LXA4 antagonist). All rats were evaluated using histology, enzyme-linked immunosorbent assay, quantitative real-time polymerase chain reaction, and western blot analyses. We found that with increasing Kellgren-Lawrence grade, LXA4 expression was downregulated in articular fluid and that CD86 and Arg1 expression was upregulated in the synovium of patients. In vitro, CTS and LXA4 both promoted M2 subtype polarization of synovial macrophages, which inhibited the nuclear translocation of NF-κB p65 and formation of NLRP3 in chondrocytes. In vivo, the OAE treatment exerted protective effects on articular cartilage and facilitated M2 polarization of synovial macrophages. These effects were suppressed by BOC-2 treatment. We concluded that moderate CTS enhances therapeutic effects of LXA4 by inhibiting the nuclear translocation of NF-κB p65 and NLRP3. Furthermore, the therapeutic effects of LXA4 during treadmill exercise in monoiodoacetate-induced OA were driven by promotion of synovial macrophage M2 subtype polarization.