Regulation of hepatic lipoprotein biogenesis, glucose homeostasis and atherosclerosis by citrus flavonoids

The metabolic pathways and molecular mechanisms whereby the citrus f1avonoids, naringenin and nobiletin, reduce apolipoprotein B100 (apoB) secretion, correct dyslipidemia, prevent atherosclerosis, and sensitize cells to insulin, were investigated in a series of in vivo and in vitro studies. In vivo experiments were conducted using several mouse models, including high fat-fed C57BL6/J low density lipoprotein receptor null (Ldlr-/-) mice, C57BL6/J wild-type mice, leptin deficient ob/ob mice, and fibroblast growth factor 21 null (Fgf21-/-) mice. In vitro experiments were performed in HepG2 human hepatoma cells. Dietary supplementation with naringenin attenuated the overproduction of apoB100 containing lipoproteins, ameliorated hepatic steatosis and attenuated dyslipidemia in high-fat fed Ldlr-/- mice. The primary mechanisms for these effects were increased hepatic fatty-acid oxidation and prevention of sterol regulatory element binding protein 1-c (SREBP1c)-mediated lipogenesis. Longer term studies in high fat-fed mice demonstrated that naringenin supplementation significantly reduced the atherosclerotic plaque development normally observed in these mice. This prevention of atherosclerosis was linked to reductions in VLDL- and LDL-cholesterol, and to correction of hyperinsulinemia. To probe the mechanism for this effect further, kinetic analyses of cellular apoB100 metabolism were performed in HepG2 cells. Multicompartmental modeling revealed that naringenin and insulin inhibited apoB100 secretion with similar kinetics. However, naringenin did so independent of signaling via the insulin receptor or insulin receptor substrate. Further studies showed that naringenin sensitized MAPKerk signaling to the extracellular stimulus of insulin to reduce microsomal triglyceride transfer protein (MTP) and increase the low density -lipoprotein receptor (LDLR), resulting in synergistic inhibition of apoB100 secretion. In further support of the insulin-sensitizing effects of naringenin, dietary supplementation reduced fasting hyperinsulinemia and improved overall insulin sensitivity and glucose tolerance in both C57BL/6 and ob/ob mice. Remarkably, in Fgf21 -/- mice, the metabolic effects of naringenin were attenuated implicating this recently characterized metabolic regulator as a potential target of citrus f1avonoids. Similar to naringenin, treatment of HepG2 cells with nobiletin activated MAPKerk signaling, increased expression of the LDLR, and decreased expression of MTP and DGAT1/2, resulting in marked inhibition of apoB100 secretion. However, the IC50 for nobiletin was 10-fold less than for naringenin. Extension of these studies to high-fat fed Ldlr-/- mice showed that nobiletin supplementation attenuated dyslipidemia, corrected hyperinsulinemia and hyperglycemia, and markedly reduced atherosclerosis. Intervention studies involving treatment of Ldlr-/- mice with pre-existing dyslipidemia, obesity and insulin resistance with either flavonoid reversed the observed metabolic abnormalities induced by a high-fat western diet. In summary, these studies provide significant mechanistic insight and support for the concept of citrus f1avonoids as novel and promising therapeutic agents to treat the dysregulation of lipid homeostasis, metabolic disease, and its cardiovascular complications.;Keywords: apolipoprotein B100, flavonoids, mouse models, atherosclerosis, insulin resistance, hepatocytes...