Molecular target of piperine in the inhibition of lipid droplet accumulation in macrophages.

An alkaloid piperine isolated from the Piper Nigrum was found to inhibit lipid droplet accumulation in mouse macrophages, and especially inhibited cholesteryl ester (CE) synthesis (IC50: 25 microM). The metabolism of cholesterol from lysosome to lipid droplet was inhibited with a similar IC50 (18 microM), indicating that the site of inhibition is one of the steps between the lysosomes and the endoplasmic reticulum. Therefore, effects of piperine on acyl-CoA:cholesterol acyltransferase (ACAT) activity in the microsomes prepared from mouse macrophage and liver were studied, to show that the compounds inhibited the activity in both cases (IC50: 9.1, 7.0 microM, respectively). Furthermore, piperine was found to inhibit both ACAT1 and ACAT2 isozymes to a similar extent (IC50: 16, 18 microM, respectively) in cell-based assays using ACAT1- or ACAT2-expressing cells. Thus, it was suggested that piperine inhibited macrophage ACAT to decrease CE synthesis, leading to a reduction of lipid droplets.