title: Inhibition of net HepG2 cell apolipoprotein B secretion by the citrus flavonoid naringenin involves activation of phosphatidylinositol 3-kinase, independent of insulin receptor substrate-1 phosphorylation.
abstract: The flavonoid naringenin improves hyperlipidemia and hyperglycemia in streptozotocin-treated rats. In HepG2 human hepatoma cells, naringenin inhibits apolipoprotein B (apoB) secretion primarily by inhibiting microsomal triglyceride transfer protein and enhances LDL receptor (LDLr)-mediated apoB-containing lipoprotein uptake. Phosphatidylinositol 3-kinase (PI3K) activation by insulin increases sterol regulatory element-binding protein (SREBP)-1 and LDLr expression and inhibits apoB secretion in hepatocytes. Thus, we determined whether naringenin activates this pathway. Insulin and naringenin induced PI3K-dependent increases in cytosolic and nuclear SREBP-1 and LDLr expression. Similar PI3K-mediated increases in SREBP-1 were observed in McA-RH7777 rat hepatoma cells, which express predominantly SREBP-1c. Reductions in HepG2 cell media apoB with naringenin were partially attenuated by wortmannin, whereas the effect of insulin was completely blocked. Both treatments reduced apoB100 secretion in wild-type and LDLr(-/-) mouse hepatocytes to the same extent. Insulin and naringenin increased HepG2 cell PI3K activity and decreased insulin receptor substrate (IRS)-2 levels. In sharp contrast to insulin, naringenin did not induce tyrosine phosphorylation of IRS-1. We conclude that naringenin increases LDLr expression in HepG2 cells via PI3K-mediated upregulation of SREBP-1, independent of IRS-1 phosphorylation. Although this pathway may not regulate apoB secretion in primary hepatocytes, PI3K activation by this novel mechanism may explain the insulin-like effects of naringenin in vivo.
mesh_terms: D000818:Animals; D001055:Apolipoproteins B / Q000037:antagonists & inhibitors*; D022762:CCAAT-Enhancer-Binding Proteins / Q000378:metabolism; D006528:Carcinoma, Hepatocellular / Q000378:metabolism* / Q000473:pathology; D045744:Cell Line, Tumor; D004268:DNA-Binding Proteins / Q000378:metabolism; D004789:Enzyme Activation / Q000502:physiology; D005227:Fatty Acids / Q000096:biosynthesis; D044950:Flavanones / Q000494:pharmacology*; D006801:Humans; D055504:Insulin Receptor Substrate Proteins; D008113:Liver Neoplasms / Q000378:metabolism* / Q000473:pathology; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D010750:Phosphoproteins / Q000378:metabolism*; D010766:Phosphorylation; D011973:Receptors, LDL / Q000172:deficiency / Q000378:metabolism; D051780:Sterol Regulatory Element Binding Protein 1; D014157:Transcription Factors; D014280:Triglycerides / Q000096:biosynthesis; D014443:Tyrosine / Q000378:metabolism
chemical_list: D001055:Apolipoproteins B; D022762:CCAAT-Enhancer-Binding Proteins; D004268:DNA-Binding Proteins; D005227:Fatty Acids; D044950:Flavanones; C526292:IRS1 protein, human; D055504:Insulin Receptor Substrate Proteins; C526295:Irs1 protein, mouse; C526296:Irs1 protein, rat; D010750:Phosphoproteins; D011973:Receptors, LDL; C497387:SREBF1 protein, human; C497388:Srebf1 protein, mouse; D051780:Sterol Regulatory Element Binding Protein 1; D014157:Transcription Factors; D014280:Triglycerides; D014443:Tyrosine; D019869:Phosphatidylinositol 3-Kinases; C005273:naringenin