Inhibition of apoB secretion from HepG2 cells by insulin is amplified by naringenin, independent of the insulin receptor.
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 extracellular-regulated 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 resistance.
Allister
Emma M
EM
Robarts Research Institute, Departments of Medicine and Biochemistry, University of Western Ontario, London, Ontario, Canada.
Mulvihill
Erin E
EE
Barrett
P Hugh R
PH
Edwards
Jane Y
JY
Carter
Lindsey P
LP
Huff
Murray W
MW
D048868:Adaptor Proteins, Signal Transducing / Q000378:metabolism; D000818:Animals; D053299:Apolipoprotein B-100 / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D002352:Carrier Proteins / Q000235:genetics; D002417:Cattle; D045744:Cell Line, Tumor; D004789:Enzyme Activation / Q000187:drug effects; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D044950:Flavanones / Q000494:pharmacology*; D005786:Gene Expression Regulation / Q000187:drug effects; D022781:Hepatocytes / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D007328:Insulin / Q000494:pharmacology*; D055504:Insulin Receptor Substrate Proteins; D007700:Kinetics; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D011972:Receptor, Insulin / Q000378:metabolism; D011973:Receptors, LDL / Q000235:genetics
D048868:Adaptor Proteins, Signal Transducing; D053299:Apolipoprotein B-100; D002352:Carrier Proteins; D044950:Flavanones; C526292:IRS1 protein, human; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; D011973:Receptors, LDL; C106092:microsomal triglyceride transfer protein; D019869:Phosphatidylinositol 3-Kinases; D011972:Receptor, Insulin; D048049:Extracellular Signal-Regulated MAP Kinases; C005273:naringenin