title: Insulin-mediated inhibition of apolipoprotein B secretion requires an intracellular trafficking event and phosphatidylinositol 3-kinase activation: studies with brefeldin A and wortmannin in primary cultures of rat hepatocytes.
abstract: Insulin inhibition of the secretion of apolipoprotein B (apo B) was studied in primary cultures of rat hepatocytes by using brefeldin A (BFA), an inhibitor of protein transport from the endoplasmic reticulum (ER) to the Golgi apparatus, and by using the phosphatidylinositol 3-kinase (PI 3-K) inhibitor wortmannin. Incubation of hepatocytes with BFA (10 micrograms/ml) for 1 h inhibited the subsequent secretion of apo B, albumin and transferrin for up to 3 h. BFA treatment resulted in the time-dependent accumulation in cells of [14C]leucine-labelled proteins and apo B. Under conditions where insulin decreased total apo B (cell plus secreted), BFA blocked the insulin-dependent effect. These results suggest that export of apo B from the ER is a prerequisite for the observed insulin effect. Treatment of hepatocytes with wortmannin for 20 min abolished insulin inhibition of apo B secretion, suggesting that the insulin effect on the apo B pathway involves activation of PI 3-K. Enzyme inhibitor studies indicate that chymostatin and (+)-(2S,3S)-3-[(S)-methyl-1-(3-methylbutylcarbamoyl)-butylcarba moyl]-2- oxiranecarboxylate (E-64-c) partially block insulin effects on apo B compared with leupeptin, which had no discernible effect. The cell-permeable derivative of E-64-c, EST, and N-Ac-Leu-Leu-norleucinal (ALLN) were most effective in blocking insulin effects on apo B. These results suggest that insulin action on apo B in primary rat hepatocytes involves (1) vesicular movement of apo B from the ER; (2) activation of PI 3-K and (3) a cellular protease that is either a cysteine- or calcium-activated neutral protease.
authors.affiliation: Department of Pathology and Laboratory Medicine, University of Rochester School of Medicine and Dentistry, NY 14642, USA.
authors.forename: J D
authors.initials: JD
authors.lastname: Sparks
authors.forename: T L
authors.initials: TL
authors.lastname: Phung
authors.forename: M
authors.initials: M
authors.lastname: Bolognino
authors.forename: C E
authors.initials: CE
authors.lastname: Sparks
mesh_terms: D000730:Androstadienes / Q000494:pharmacology*; D000818:Animals; D001055:Apolipoproteins B / Q000378:metabolism*; D001692:Biological Transport; D020126:Brefeldin A; D002478:Cells, Cultured; D003517:Cyclopentanes / Q000494:pharmacology*; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D006868:Hydrolysis; D007328:Insulin / Q000494:pharmacology*; D008099:Liver / Q000187:drug effects* / Q000201:enzymology / Q000378:metabolism; D019869:Phosphatidylinositol 3-Kinases; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D000077191:Wortmannin
chemical_list: D000730:Androstadienes; D001055:Apolipoproteins B; D003517:Cyclopentanes; D004791:Enzyme Inhibitors; D007328:Insulin; D020126:Brefeldin A; D019869:Phosphatidylinositol 3-Kinases; D017853:Phosphotransferases (Alcohol Group Acceptor); D000077191:Wortmannin