Human ACAT Inhibitory Effects of Flavonoids from Sophora flavescens

Acyl-coenzyme A:cholesterol acyltransferase (ACAT, EC 2.3.1.26) is the primary cellular enzyme that catalyzes the formation of cholesteryl esters from cholesterol and fatty acyl-coenzyme A. It has been identified that two isoenzymes, ACAT-1 and ACAT-2, have different cell locations, membrane orientations, and metabolic functions. Atherosclerotic lesions are associated with the accumulation of cholesteryl ester lipid droplets via ACAT-1 in macrophagesderived foam cells. In contrast, ACAT-2 plays a key role in the intestinal absorption of cholesterol and the assembly of very low density lipoprotein (VLDL) in the liver. Therefore, ACAT inhibitors may act as antihypercholesterolemic and antiathero-sclerotic agents. For the past 20 years, numerous ACAT inhibitors have been developed, and many of them showed promise in animal studies by inhibiting intestinal or hepatic ACAT and lowering plasma cholesterol levels. But several clinical studies showed that ACAT inhibitors lacked efficacy for lowering cholesterol, suggesting that ACAT inhibitors with different preferences on ACAT-1 and ACAT2 may exert more favorable effects on atherosclerosis. Recently, we reported that sesquineolignan, saucerneol B, and dineolignans, manassantin A and B, were isolated by bioassay-guided fractionation of the methanolic extracts of the root of Saururus chinensis and showed specificity of inhibitory activity against hACAT-1 and -2. More recently, we found that shikonin derivatives isolated from Lithospermum erythrorhizon roots inhibited hACAT-1 and -2 activities. During search for new ACAT inhibitors from natural sources, we found the ethyl acetate extracts of the S. flavescens exhibited significant hACAT inhibitory activity (69% inhibition at 100 μg/mL for hACAT-1, 62% inhibition at 100 μg/mL for hACAT-2, respectively). S. flavescens, which is polyphenol-rich plant from the legume family known to possess numerous medicinal properties such as antipyretic, analgesic, anthelmintic, stomachic, and glycosidase inhibitory activities, but there is no report as to inhibit ACAT activity. In this study, we isolated nine flavonoids [sophoraflavanone G (1), (−)-kurarinone (2), leachianone A (3), kushenol A (4), (2S)-2-methoxykurarinone (5), kushenol T (6), kurarinol (7), isoxanthohumol (8), and kuraridin (9)] from the roots of S. flavescens and describe hACAT-1 and -2 inhibitory activities of compounds 1-9 in vitro assay system and compounds 3, 4, and 9 in cellbased assay system.