Heparin releases newly synthesized cell surface-associated apolipoprotein E from HepG2 cells

Heparin significantly increased the amount of newly synthesized apolipoprotein E (apoE) released by HepG2 cells. Culturing cells in the presence of 10 micrograms/ml of heparin for 2-6 days caused a 1.3-fold (day 2) to 3-fold (day 6) increase of extracellular apoE without affecting the total amount of apoE synthesized by the cells. The amounts of apoA-I and apoB produced by HepG2 cells were unaffected by heparin. Surprisingly, short-term treatment with heparin (15-30 min) also increased extracellular apoE by 2- to 3-fold. In this situation, heparin exerted its effect on apoE post-translationally. Among glycosaminoglycans (GAGs), only heparan sulfate mimicked heparin at a concentration of 10 micrograms/ml; hyaluronic acid and the chondroitin sulfates were effective only at a higher concentration (100 micrograms/ml). Extracellular apoE was not increased by treating cells with anti-apoE antiserum or a heparin-binding peptide of apoE (amino acids 130-169). Removal of cell surface-associated GAGs by culturing cells in 4-methylumbelliferyl-beta-D-xyloside ablated the effect of heparin on apoE. ApoE was released from cells by treatment with heparinases I and III, but not by chondroitinase ABC. The results provide evidence that a heparin-releasable pool of newly synthesized apoE is associated with cell surface GAGs that resemble heparin and/or heparan sulfate.