Serum amyloid A3 binds MD-2 to activate p38 and NF-κB pathways in a MyD88-dependent manner

Serum amyloid A (SAA) 3 is a major component of the acute phase of inflammation. We previously reported that SAA3 served as an endogenous peptide ligand for TLR4 to facilitate lung metastasis. Because these experiments were performed with SAA3 recombinant proteins purified from Escherichia coli or mammalian cells, we could not rule out the possibility of LPS contamination. In this study, we used SAA3 synthetic peptides to eliminate the presence of LPS in SAA3. We found that the SAA3 synthetic peptide (aa 20-86) (20-86) stimulated cell migration and activated p38 in a manner dependent on TLR4, MD-2, and MyD88. SAA3 (20-86) also activated NF-κB and Rho small GTPase. Using surface plasmon resonance analysis, the binding constant K_D values between SAA3 (20-86) or SAA3 (43-57) and TLR4/MD-2 protein highly purified by the baculovirus system were 2.2 and 30 μM, respectively. FLAG-tagged SAA3 tightly bound to protein A-tagged MD-2, but not to TLR4 in baculovirus coinfection experiments. Although SAA3 (20-86) caused a low, but appreciable level of endocytosis in TLR4, it induced the upregulation of both IL- 6 and TNF-α, but not IFN-β1. An i.v. injection of SAA3 (43-57) induced the lung recruitment of CD11b⁺Gr-1⁺ cells at an estimated serum concentration around its K_D value toward TLR4/MD-2. Taken together, these results suggest that SAA3 directly binds MD-2 and activates the MyD88-dependent TLR4/MD-2 pathway.