title: The conformation of apolipoprotein E isoforms in phospholipid complexes and their interaction with human Hep G2 cells.
abstract: The human hepatoma cell line, Hep G2, has been used to compare the metabolism by isolated liver cells of purified isoforms of human apolipoprotein E (apo E). Complexes of [125I]apo E-3/3, 2/2, 3/2 and 4/3 with dimyristoyl phosphatidylcholine (DMPC) were prepared by a detergent-dialysis method: discoidal, bilayer complexes with a stoichiometry of 125 +/- 15 mol DMPC/mol apo E resulted. The predominant phenotype apo E-3/3, and the phenotype apo E-2/2 characteristic of patients with Type III hyperlipoproteinemia, interact similarly with DMPC and adopt the same conformation with 60-70% alpha-helix, as monitored by circular dichroism spectroscopy. The uptake and degradation at 37 degrees C, and binding at 4 degrees C by Hep G2 cells, of [125I]apo E-3/3/DMPC and [125I]apo E-2/2/DMPC complexes were compared. Apo E-3/3 was degraded more rapidly than apo E-2/2 suggesting that the diminished catabolism of the latter phenotype by intact livers is due to lack of recognition by the hepatocytes. The observed degradation of apo E was 3-4 times greater than that which could be attributed to fluid phase endocytosis and low-affinity adsorptive endocytosis. The degradation of [125I]apo A-I by Hep G2 cells can be accounted for by the above endocytotic mechanisms. The distinction between apo E-3/3 and apo E-2/2 isoforms is attributed to the presence of a cell-surface receptor on Hep G2 cells which binds apo E-3/3 with a higher affinity than apo E-2/2.
mesh_terms: D001053:Apolipoproteins / Q000235:genetics / Q000378:metabolism*; D001057:Apolipoproteins E; D006528:Carcinoma, Hepatocellular / Q000378:metabolism*; D002460:Cell Line; D006801:Humans; D006952:Hyperlipoproteinemia Type III / Q000378:metabolism; D008099:Liver / Q000378:metabolism; D008113:Liver Neoplasms / Q000378:metabolism*; D010641:Phenotype; D010743:Phospholipids / Q000378:metabolism*; D011487:Protein Conformation
chemical_list: D001053:Apolipoproteins; D001057:Apolipoproteins E; D010743:Phospholipids