Inhibition of apoB secretion from HepG2 cells by insulin is amplified by naringenin, independent of the insulin receptor

Abstract

Hepatic overproduction of apolipoprotein B (apoB)-containing lipoproteins is characteristic of the dyslipidemia associated with insulin resistance. Recently, we demonstrated that the flavonoid naringenin, like insulin, decreased apoB secretion from HepG2 cells by activation of both the phosphoinositide-3-kinase (PI3-K) pathway and the mitogen-activated protein kinase/extracellular-regulated kinase (MAPK erk ) pathway. In the present study, we determined whether naringenin-induced signaling required the insulin receptor (IR) and sensitized the cell to the effects of insulin, and whether the kinetics of apoB assembly and secretion in cells exposed to naringenin were similar to those of insulin. Immunoblot analysis revealed that insulin stimulated maximal phosphorylation of IR and IR substrate-1 after 10 min, whereas naringenin did not affect either at any time point up to 60 min. The combination of naringenin and submaximal concentrations of insulin potentiated extracellularregulated kinase 1/2 activation and enhanced upregulation of the LDL receptor, downregulation of microsomal triglyceride transfer protein expression, and inhibition of apoB-100 secretion. Multicompartmental modeling of apoB pulse-chase studies revealed that attenuation of secreted radiolabeled apoB in naringenin-or insulin-treated cells was similar under lipoprotein-deficient or oleate-stimulated conditions. Naringenin and insulin both stimulated intracellular apoB degradation via a kinetically defined rapid pathway. Therefore, naringenin, like insulin, inhibits apoB secretion through activation of both PI3-K and MAPK erk signaling, resulting in similar kinetics of apoB secretion. However, the mechanism for naringenin-induced signaling is independent of the IR. Naringenin represents a possible strategy for reduction of hepatic apoB secretion, particularly in the setting of insulin

Naringenin Decreases Progression of Atherosclerosis by Improving Dyslipidemia in High-Fat–Fed Low-Density Lipoprotein Receptor–Null Mice

“…32 Previously, it was demonstrated that the citrus flavonoid naringenin potently inhibits apoB100 secretion by activating signaling cascades in HepG2 cells, in a manner similar to insulin. [23][24][25] In contrast to insulin, signaling by naringenin occurs through a mechanism independent of phosphorylation of the insulin receptor. 24,25 Recently, it was reported that naringenin sensitizes hepatocytes to respond to low doses of insulin and potentiates the inhibition of apoB100 secretion.…”

“…[23][24][25] In contrast to insulin, signaling by naringenin occurs through a mechanism independent of phosphorylation of the insulin receptor. 24,25 Recently, it was reported that naringenin sensitizes hepatocytes to respond to low doses of insulin and potentiates the inhibition of apoB100 secretion. 24 In a mouse model of insulin resistance (ie, the Western-fed Ldlr Ϫ/Ϫ mouse), the addition of naringenin to the high-fat diet over 4 weeks ameliorated the overproduction of apoB100-containing lipoproteins, attenuated hepatic lipid accumulation, and prevented dyslipidemia.…”

“…24,25 Recently, it was reported that naringenin sensitizes hepatocytes to respond to low doses of insulin and potentiates the inhibition of apoB100 secretion. 24 In a mouse model of insulin resistance (ie, the Western-fed Ldlr Ϫ/Ϫ mouse), the addition of naringenin to the high-fat diet over 4 weeks ameliorated the overproduction of apoB100-containing lipoproteins, attenuated hepatic lipid accumulation, and prevented dyslipidemia. 26 The major findings of this study are that 6-month supplementation of the Western diet with naringenin prevented hypercholesterolemia, hypertriglyceridemia, and hyperinsulinemia; and reduced hepatic steatosis.…”

“…21 The citrusderived flavonoid, naringenin, has lipid-lowering properties 22 ; and inhibits VLDL secretion from hepatocytes in a manner resembling insulin. [23][24][25] By using a mouse model that replicates many components of metabolic syndrome, Ldlr Ϫ/Ϫ mice fed a high-fat Western diet, we were able to show in studies of 4 weeks duration that the addition of naringenin to the Western diet prevented VLDL-apoB100 overproduction, ameliorated hepatic TG accumulation, and attenuated dyslipidemia. Through a reduction in both VLDL-derived and endogenously synthesized fatty acid, naringenin also prevented muscle lipid accumulation and attenuated the development of hyperinsulinemia.…”

Inflammatory NF-κB activation promotes hepatic apolipoprotein B100 secretion: evidence for a link between hepatic inflammation and lipoprotein production

“…Although the inhibitory effect of insulin on apoB100 levels appears to involve the phosphatidylinositol 3-kinase signaling cascade, Akt1, a classical downstream effector of phosphatidylinositol 3-kinase, has been shown to have no effect on apoB100 secretion (4). Parallel signaling cascades, such as the mitogen-activated protein (MAP) kinases ERK, p38, and JNK, have been demonstrated to affect apoB secretion (2,3,21,30,37), but knowledge regarding their downstream mechanisms remains limited. Since MAP kinases can be stimulated by both insulin and inflammatory cytokines, and considerable cross talk exists between the insulin signaling cascades and inflammatory pathways, inflammation could play an important role in the overproduction of apoB100-containing lipoproteins observed in insulin-resistant states.…”