Regulated turnover of a cell surface-associated pool of newly synthesized apolipoprotein E in HepG2 cells

Abstract

Newly synthesized apolipoprotein E (apoE) is incompletely secreted and partially degraded by hepatocytes. To evaluate current models concerning apoE's role in remnant lipoprotein clearance by the liver, we performed a detailed and quantitative pulse-chase analysis of apoE secretion, subcellular distribution, and proteolytic degradation by the human hepatoma cell line HepG2. Only about 35% of newly synthesized apoE were found to be secreted to the culture medium. As determined by a protease-protection assay, a substantial amount of newly synthesized apoE was transported to the cell surface, constituting more than half of the cellular apoE under steady-state conditions. A subpopulation representing almost 40% of newly secreted apoE was rapidly rebound to the cell surface, indicating a dynamic equilibrium between cell surface and secreted apoE. These pools of newly synthesized apoE were subject to proteolytic turnover that occurred in lysosomes, presumably after re-endocytosis. We found that the proteolytic turnover of cell surface and secreted apoE was regulated by the availability of apoE ligands, being almost completely suppressed by the presence of lipoprotein-containing human serum or isolated lipoproteins, namely LDL and HDL. The characteristics of regulated degradation of cell surface apoE shed new light on potential physiological functions of this pool of apoE. Our results provide evidence that hepatocytes are capable of forming a large pool of cell surface-associated apoE, thereby supporting the previously proposed secretion-recapture model for apoE.