A reliable and reproducible assay for determining the effect of natural product on macrophages lipid uptake and cholesterol efflux: A case study of maslinic acid

Macrophage foam cell formation represents a key feature that contributes to the development of atherosclerotic lesions. Assessment of cardioprotective natural compounds targeting macrophage foam cell formation processes including lipid uptake and cholesterol efflux could lead to the identification of potential lead compounds for development into novel anti-atherosclerotic drugs. In this case study, maslinic acid, a natural product was used to study the effect on lipid uptake and cholesterol efflux in THP-1-derived macrophages. Oil red O (ORO) staining and 1,1’-dioctadecyl-3,3,3’,3’-tetramethylindocarbocyanine perchlorate-labeled oxidized low-density lipoprotein (Dil-labeled oxLDL) uptake assays were performed to determine lipid uptake by macrophages while cholesterol efflux was assessed using 3-hexanoyl-NBD labeled cholesterol. ORO-stained images were further analyzed using ImageJ analysis software to determine intracellular lipid droplets accumulation and flow cytometric analysis of median fluorescence intensity were obtained to quantify Dil-labeled oxLDL uptake by macrophages. Meanwhile, 3-hexanoyl-NBD labeled cholesterol uptake and efflux from THP-1-derived macrophages were characterized. The fluorescence intensity values obtained from the medium and cell lysates were used to determine the cholesterol efflux. The results have shown that incubation with maslinic acid suppressed oxLDL-induced macrophage foam cell formation which may be contributed from its effect in reducing lipid uptake and enhancing cholesterol efflux. In conclusion, the optimized ORO staining, Dil-labeled oxLDL uptake, and fluorescent-labeled cholesterol efflux assays provide reproducible and reliable results for assessment of foam cells formation.