Low-dose LPS promotes foam cell formation by facilitate Myd88-IRF1 interaction and IRF1 nuclear translocation. A, The relative mRNA levels of IRF1 in peritoneal macrophages derived from endotoxemia mice and control mice. B, Representative images of Oil Red O staining in ox-LDL (50 μg/mL) incubated peritoneal macrophages derived from wild type (WT) and IRF1-/- mice with or without endotoxemia. Scale bar = 50 μm. C, Total cholesterol content was measured in ox-LDL incubated peritoneal macrophages derived from WT and IRF1-/- mice with or without endotoxemia. D, Total cholesterol content was measured in ox-LDL incubated peritoneal macrophages treated with TLR agonists, that is, LPS (TLR4, 50 pg/mL), Pam3CSK4 (TLR2/1, 300 ng/mL) and Poly (I:C) (TLR3, 25 μg/mL). E, Effects of TLR agonists on IRF1 nuclear translocation and SR-AI expression in macrophages incubated with ox-LDL, as determined by Western blot. F, Effects of LPS on IRF1 nuclear translocation (green) and SR-AI expression (red) in macrophages incubated with ox-LDL, as determined by immunofluorescence assay. Scale bar = 50 μm. G, Effects of TLR agonists on the interaction of IRF1 with SR-AI promoter in macrophages incubated with ox-LDL, as determined by ChIP assay. H, Immunoprecipitation with the control IgG or an anti-IRF1 antibody from ox-LDL incubated macrophages with or without LPS challenge, followed by immunoblot analysis with antibody to Myd88. I, Effects of LPS on cholesterol content (left panel), SR-AI expression (middle panel) and the interaction of IRF1 with SR-AI promoter (right panel) in ox-LDL incubated macrophages silenced with si-Myd88 or si-IRF1. Data represent the mean ± SEM of three to five independent experiments. One-way ANOVA with Bonferroni test was used to produce the P values given in figure. * P < 0.05. ns, no significance. J, Ox-LDL incubated macrophages were silenced with scramble siRNA or si-Myd88 followed by LPS challenge (50 pg/mL), then the lysates were subjected to immunoprecipitation with the control IgG or an anti-IRF1 antibody and analyzed by Western blot with antibodies against Ubc9 and Sumo1. K, Schematic diagram of the molecular mechanisms of IRF1 nuclear translocation in foam cells challenged with TLR agonists.