Molecular structures of citrus flavonoids determine their effects on lipid metabolism in HepG2 cells by primarily suppressing apoB secretion

This study investigated the underlying mechanisms of action for blood lipid lowering effects of citrus flavonoids and their methoxylated analogues (n = 19; dose range: 0-100 μM) in HepG2 cells. Cholesterol (CH) and triglyceride (TG) syntheses were assessed by measuring the incorporation of (14)C-acetate and (14)C-glycerol, respectively, whereas apoB secretion was determined by ELISA. Results show that two polymethoxylated citrus flavonoids (PMFs), tangeretin and nobiletin, potently inhibited apoB secretion (IC(50) = 13 and 29 μM, respectively) and modestly inhibited CH synthesis (IC(50) = 49 and 68 μM) and TG synthesis (IC(50) = 14 and 73 μM), without effecting LDL-receptor activity. Other PMFs (e.g., sinensetin) and non-PMFs (e.g., hesperetin and naringenin) had only weak effects on CH and TG syntheses and apoB secretion (IC(50) > 100 μM). The structure-activity analysis indicated that a fully methoxylated A-ring of the flavonoid structure was associated with a potent inhibitory activity on hepatic apoB secretion. In conclusion, this study using HepG2 cells indicates that citrus flavonoids with a fully methoxylated A-ring may lower blood CH and TG concentrations primarily by suppressing hepatic apoB secretion as a main underlying mode of action.