MEK-ERK inhibition corrects the defect in VLDL assembly in HepG2 cells: potential role of ERK in VLDL-ApoB100 particle assembly.
OBJECTIVE
Hepatic VLDL assembly is defective in HepG2 cells, resulting in the secretion of immature triglyceride-poor LDL-sized apoB particles. We investigated the mechanisms underlying defective VLDL assembly in HepG2 and have obtained evidence implicating the MEK-ERK pathway.


RESULTS
HepG2 cells exhibited considerably higher levels of the ERK1/2 mass and activity compared with primary hepatocytes. Inhibition of ERK1/2 using the MEK1/MEK2 inhibitor, U0126 (but not the inactive analogue) led to a significant increase in apoB secretion. In the presence of oleic acid, ERK1/2 inhibition caused a major shift in the lipoprotein distribution with a majority of particles secreted as VLDL, an effect independent of insulin. In contrast, overexpression of constitutively active MEK1 decreased apoB and large VLDL secretion. MEK1/2 inhibition significantly increased both cellular and microsomal TG mass, and mRNA levels for DGAT-1 and DGAT-2. In contrast to ERK, modulation of the PI3-K pathway or inhibition of the p38 MAP kinase, had no effect on lipoprotein density profile. Modulation of the MEK-ERK pathway in primary hamster hepatocytes led to changes in apoB secretion and altered the density profile of apoB-containing lipoproteins.


CONCLUSIONS
Inhibition of the overactive ras-MEK-ERK pathway in HepG2 cells can correct the defect in VLDL assembly leading to the secretion of large, VLDL-sized particles, similar to primary hepatocytes, implicating the MEK-ERK cascade in VLDL assembly in the HepG2 model. Modulation of this pathway in primary hepatocytes also regulates apoB secretion and appears to alter the formation of VLDL-1 sized particles.
Tsai
Julie
J
Division of Clinical Biochemistry, Hospital for Sick Children, University of Toronto, Ontario, Canada M5G 1X8.
Qiu
Wei
W
Kohen-Avramoglu
Rita
R
Adeli
Khosrow
K
D000818:Animals; D053299:Apolipoprotein B-100 / Q000378:metabolism*; D002070:Butadienes / Q000494:pharmacology; D045744:Cell Line, Tumor; D006224:Cricetinae; D051048:Diacylglycerol O-Acyltransferase / Q000235:genetics / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D048049:Extracellular Signal-Regulated MAP Kinases / Q000037:antagonists & inhibitors* / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D005786:Gene Expression Regulation / Q000187:drug effects / Q000502:physiology; D018197:Hepatoblastoma / Q000378:metabolism* / Q000473:pathology; D022781:Hepatocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D008079:Lipoproteins, VLDL / Q000378:metabolism*; D008113:Liver Neoplasms / Q000378:metabolism* / Q000473:pathology; D048369:MAP Kinase Kinase 1 / Q000037:antagonists & inhibitors* / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D048370:MAP Kinase Kinase 2 / Q000037:antagonists & inhibitors* / Q000187:drug effects / Q000235:genetics / Q000378:metabolism; D008647:Mesocricetus; D009570:Nitriles / Q000494:pharmacology; D019301:Oleic Acid / Q000494:pharmacology; D010316:Particle Size; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D051381:Rats; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D014280:Triglycerides / Q000378:metabolism
D053299:Apolipoprotein B-100; D002070:Butadienes; D004791:Enzyme Inhibitors; D008079:Lipoproteins, VLDL; D009570:Nitriles; D012333:RNA, Messenger; D014280:Triglycerides; C113580:U 0126; D019301:Oleic Acid; C494881:DGAT1 protein, human; C494885:DGAT2 protein, human; D051048:Diacylglycerol O-Acyltransferase; C482128:MAP2K2 protein, human; D048049:Extracellular Signal-Regulated MAP Kinases; D048369:MAP Kinase Kinase 1; D048370:MAP Kinase Kinase 2; C482125:MAP2K1 protein, human