Inhibition of Microsomal Triglyceride Transfer Protein Expression and Apolipoprotein B100 Secretion by the Citrus Flavonoid Naringenin and by Insulin Involves Activation of the Mitogen-Activated Protein Kinase Pathway in Hepatocytes

Abstract

Microsomal triglyceride transfer protein (MTP) is necessary for hepatocyte assembly and secretion of apolipoprotein (apo)B100-containing lipoproteins. The citrus flavonoid naringenin, like insulin, decreased MTP expression in HepG2 cells, resulting in inhibition of apoB100 secretion; however, the mechanism for naringenin is independent of insulin receptor substrate-1/2. Recently, it was reported that insulin decreased MTP expression in HepG2 cells via the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) (MAPK erk ) pathway. We hypothesized that naringenin acts via a similar mechanism. Inhibition of MAPK kinase (MEK) 1/2 in HepG2 cells significantly attenuated the naringenin-and insulin-induced reduction in MTP expression. Both naringenin and insulin increased ERK1/2 phosphorylation, which was completely inhibited by MEK1/2 inhibition and enhanced by inhibition of MAPK p38 , a negative regulator of MAPK erk activity. Inhibition of MEK1/2 significantly attenuated both the naringenin-and insulin-induced decrease in apoB100 secretion demonstrating a direct link between MAPK erk activation and apoB100 secretion. Furthermore, both compounds increased MAPK p38 activation, and therefore inhibition of MAPK p38 amplified the naringenin-and insulin-induced decrease in apoB100 secretion. We conclude that MAPK erk signaling in hepatocytes is critical for inhibition of apoB100 secretion by naringenin and insulin. Therefore, naringenin may prove useful for activating insulin-signaling pathways important for regulation of hepatocyte lipid homeostasis. Diabetes 54:1676 -1683, 2005 I t is well established that insulin resistance and type 2 diabetes result in overproduction of hepatic apolipoprotein (apo)B-containing lipoproteins, which confers an increased atherosclerotic risk (1,2). Microsomal triglyceride transfer protein (MTP) is rate-limiting for hepatic lipoprotein assembly. MTP catalyzes the transfer of lipids to newly synthesized apoB within the endoplasmic reticulum facilitating secretion of nascent lipoproteins (3). Recent evidence from the fructose-fed hamster model of insulin resistance has shown that an increased activity of MTP contributes to the increased secretion/overproduction of hepatic apoB100 lipoproteins (4). Furthermore, amelioration of hepatic insulin resistance in this model resulted in normalization of MTP expression and reduction of the overproduction of apoB100-containing lipoproteins (5). Under normal culture conditions, insulin negatively regulates the expression of MTP in hepatocytes (6). Therefore, other compounds that activate intracellular insulin-signaling pathways may also decrease MTP expression and potentially be used in patients to treat dyslipidemia associated with insulin resistance.Previous studies in cell culture systems and animal models have shown that the grapefruit flavonoid naringenin dramatically decreases the secretion of hepatic lipoproteins (7-14). We reported that naringenin dosedependently inhibited apoB secretion in HepG2 cells (11) resulting...

Effects of green tea on insulin sensitivity, lipid profile and expression of PPARα and PPARγ and their target genes in obese dogs

“…Our results showed for the first time that LPL expression was dramatically elevated in visceral and subcutaneous adipose tissues, and in skeletal muscle, by green tea supplementation. It has been shown that EGCG is a potent inhibitor of apoB100 secretion (40) , possibly through microsomal transfer protein inhibition, as demonstrated in vitro for other flavonoids (41) . Furthermore, EGCG promotes the accumulation of TAG in cytosolic lipid droplets, thereby diverting lipids from the assembly of VLDL to storage in the cytosol (42) .…”

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

“…The shaded compartments represent the compartments containing apoB radioactivity that were determined experimentally. expression (1,2). However, an extensive pulse-chase protocol coupled with multicompartmental modeling has not been used to characterize how insulin affects the kinetic movement of apoB-100 within hepatocytes.…”

“…Activation of these two pathways results in upregulation of the LDL receptor (LDLr) and inhibition of microsomal triglyceride transfer protein (MTP), leading to the inhibition of apoB-100 secretion (1)(2)(3)(4). Exposure of cells to insulin rapidly activates intracellular signaling by tyrosine phosphorylation of the insulin receptor (IR) and insulin receptor substrate-1 (IRS-1) within 10 min (5).…”

“…Previous pulse-chase studies in HepG2 cells analyzed by multicompartmental modeling have shown that naringenin inhibits apoB-100 secretion via increased intracellular degradation of apoB that is partially mediated through inhibition of MTP activity (12). Insulin has also been shown to inhibit the secretion of apoB-100 from hepatocytes (1,2,4,(13)(14)(15)(16), mediated in part through activation of PI3-K activity (1,2,4) and inhibition of MTP Fig. 1.…”

“…Naringenin, like insulin, modulates hepatic VLDL apolipoprotein B-100 (apoB-100) production through activation of intracellular signaling cascades, namely the phosphoinositide-3-kinase (PI3-K) and mitogen-activated protein kinase/ extracellular-regulated kinase (MAPK erk ) cascades (1,2). Activation of these two pathways results in upregulation of the LDL receptor (LDLr) and inhibition of microsomal triglyceride transfer protein (MTP), leading to the inhibition of apoB-100 secretion (1)(2)(3)(4).…”