Atherosclerosis, the leading cause of mortality worldwide, is characterized by chronic inflammation of vascular wall with accumulation of lipid and macrophage-derived foam cells in the subendothelial space. It has been evident that Toll-like receptors (TLRs), especially TLR4, contribute to the progression of atherosclerosis1. Myeloid differentiation factor 88 (MyD88) plays a crucial role as an adaptor molecule in signal transduction of TLR superfamily in human macrophage. The feedback regulations of LDL receptor (LDLr) and 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCoAR) mediated by sterol regulatory element binding proteins (SREBPs) cleavage-activating protein (SCAP)-SREBP2 pathway are considered to be key regulatory elements for cholesterol homeostasis in human cells2. Studies3 have shown that a reduction of atherosclerosis in MyD88-/- mice but the mechanism remains unclear. This study was designed to investigate the crosstalk between TLR4-MyD88 and SCAP-SREBP2 pathways in macrophage foam cell formation under stimulation with lipopolysaccharide (LPS). Phorbol, 12-myristate, 13-acetate (PMA) activated-human monocyte cell line (THP-1) macrophages were transfected with negative control or MyD88 siRNA, and incubated with LPS in the absence or presence of LDL. Oil Red O staining and a quantitative assay were used to assess the intracellular cholesterol content. The mRNA and protein expression of LDLr, HMGCoAR, SCAP and SREBP2 were examined by real-time quantitative RT-PCR and Western blotting. The translocation of SCAP from the endoplasmic reticulum (ER) to the Golgi was detected by immunofluorescecent staining. Values are expressed as mean ± S.D., compared by ANOVA. We demonstrated that LPS induced intracellular cholesterol ester accumulation and foam cell formation in the absence (LPS vs. Control : 215.0±16.3 vs. 151.9±11.6 μg/mg protein, P<0.01, n=4) or presence of LDL (LPS+LDL vs. LDL : 284.1±26.1 vs. 205.1±12.2 μg/mg protein, P<0.01, n=4), while knocking down MyD88 attenuated the above effects by LPS (siMyD88+LPS vs LPS : 163.9±12.5 vs. 215.0±16.3 μg/mg protein ; siMyD88+LPS+LDL vs. LPS+LDL : 185.4±16.0 vs. 284.1±26.1 μg/mg protein, P<0.01, n=4). LPS increased the gene and protein expression of LDLr (2X, P<0.01, n=4) and HMGCoAR (1.8X, P<0.01, n=4) which can be ameliorated by knocking down MyD88, suggesting the involvement of MyD88 in cholesterol homeostasis. LPS also increased SCAP gene and protein expression (1.9X, p<0.01, n=4). Furthermore, immunofluorescence using antibodies to SCAP and Golgi demonstrated that LPS caused abnormal translocation of SCAP from the ER to the Golgi. Both effects were blocked by knocking down MyD88. These results suggest that MyD88 plays an important role in the regulation of cholesterol homeostasis and that inhibition of MyD88 might be a potential approach in preventing macrophage foam cell formation.