The conformation of apolipoprotein E isoforms in phospholipid complexes and their interaction with human Hep G2 cells

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.