Macrophages affect the magnitude and duration of inflammatory response in a functionally heterogeneous manner. The phenotype of macrophages is maintained through a reversible homeostatic mechanism. A number of determinants that modulate macrophage plasticity have been identified, although the precise mechanisms are not fully understood. Here we report that stimulation of isolated human blood monocytes and mouse bone marrow-derived macrophages with human serum amyloid A (SAA), a major acute-phase protein, leads to induced expression of macrophage M2 markers including IL-10, Ym1, Fizz-1, MRC1, IL-1Rn and CCL17. The same effect was observed with macrophages exposed to SAA in peritoneal cavity. SAA also increases arginase 1 activity and enhances macrophage efferocytosis of apoptotic neutrophils in mouse macrophages. The induction of M2 markers requires MyD88 and the activation of multiple signaling pathways, but is independent of Stat6. SAA induces IRF4 expression and increases its DNA-binding activity. Silencing IRF4 by siRNA abrogates SAA-induced expression of the M2 markers. These results suggest a potential role for SAA to alter macrophage phenotype and modulate macrophage functions through a MyD88-dependent mechanism that involves IRF4-mediated transcription.
Serum amyloid A (SAA) is known as an acute-phase protein and a biomarker for inflammatory diseases. Published studies have shown that SAA possesses proinflammatory cytokine-like activity and is chemotactic for phagocytes, but the structural basis for these activities remains unidentified. In this article, we report that truncated SAA1 proteins lacking N- and C-terminal sequences exhibit reduced proinflammatory activity and strongly suppress LPS-induced expression of IL-1β, IL-6, and TNF-α in macrophages. A truncated SAA1 containing aa 11-58 was examined further and found to facilitate p38 MAPK phosphorylation while reducing LPS-stimulated phosphorylation of ERK and JNK. In LPS-challenged mice, aa 11-58 reduced the severity of acute lung injury, with significantly less neutrophil infiltration in the lungs and attenuated pulmonary expression of IL-1β, IL-6, and TNF-α. Coadministration of aa 11-58 markedly improved mouse survival in response to a lethal dose of LPS. A potent induction of IL-10 was observed in a TLR2-dependent, but TLR4-independent, manner in macrophages stimulated with aa 11-58. However, the aa 11-58 fragment of SAA1 was unable to induce chemotaxis or calcium flux through formyl peptide receptor 2. These results indicate that the N- and C-terminal sequences contain structural determinants for the proinflammatory and chemotactic activities of SAA1, and their removal switches SAA1 to an anti-inflammatory role. Given that proteolytic processing of SAA is associated with the pathological changes in several diseases, including secondary amyloidosis, our findings may shed light on the structure-function relationship of SAA1 with respect to its role in inflammation.
Macrophages play an important role in the quality, duration and magnitude of most inflammatory reactions in a polarized manner. However, the determinants that modulate their plasticity remain unclear. Serum amyloid A is an acute-phase protein which induces both anti-inflammatory and pro-inflammatory cytokines during inflammation. In this study, we explored the potential involvement of SAA in modulating macrophage polarization. SAA not only induced the expression of anti-inflammatory M2-specific genes such as IL-10, Arginase-1, Ym-1, Fizz-1, Mrc-1, IL-1Rn, CCL17, but also promoted efferocytosis of apoptotic neutrophils, a process that is critical for the resolution of inflammation. Through experiments with formyl peptide receptor 2 antogonist WRW4 and Fpr2 deficient macrophages, we showed that the induction of M2-specific genes and efferocytosis by SAA were mediated specifically by Fpr2. Additionally, SAA stimulation resulted in the increase expression and activation of the transcription factor interferon regulatory factor 4. Selective inhibitors of ERK activity abrogated SAA induction upon M2 macrophages and efferocytosis by decreasing the expression and activation of IRF4. Knocking down the expression of IRF4 abrogated the effect of SAA on M2 macrophage. Our results demonstrate that SAA can induce M2 macrophage-specific genes and promote efferocytosis via Fpr2-ERK-IRF4 signaling, and suggests an alternative mechanism by which SAA is involved in anti-inflammatory responses.
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