ROLE OF HEPARAN-SULFATE PROTEOGLYCANS IN THE BINDING AND UPTAKE OF APOLIPOPROTEIN-E-ENRICHED REMNANT LIPOPROTEINS BY CULTURED-CELLS

Addition of apolipoprotein (apo) E to rabbit beta-very low density lip oproteins (beta-VLDL) has been shown to result in a marked enhancement of their binding and uptake by various cell types. Apolipoprotein E b inds to lipoprotein receptors and proteoglycans. To distinguish betwee n apoE binding to these sites, cells were treated with heparinase. Hep arinase treatment of receptor-negative familial hypercholesterolemic ( FH) fibroblasts and human hepatoma cells (HepG2) released 30-40% of ne wly synthesized cell surface S-35-labeled proteoglycans and decreased the binding of beta-VLDL+apoE to FH and normal fibroblasts and HepG2 c ells by more than 80%. Furthermore, heparinase treatment significantly decreased the uptake of fluorescently labeled beta-VLDL+apoE by HepG2 cells and decreased cholesteryl ester synthesis in FH fibroblasts by 75%. Likewise, canine chylomicron remnants enriched in apoE demonstrat ed enhanced binding that was 80% inhibited by heparinase treatment of HepG2 cells. Heparinase treatment did not affect beta-VLDL (without ad ded apoE) or low density lipoprotein (LDL) binding to these cells or t he binding activity of beta-VLDL+apoE to the LDL receptor-related prot ein (LRP) or to the LDL receptor on ligand blots. Chinese hamster ovar y (CHO) mutant cells lacking the synthesis of either heparan sulfate ( pgsD-677) or all proteoglycans (pgsA-745) did not display any enhanced binding of the beta-VLDL+apoE. By comparison, wild-type CHO cells dem onstrated enhanced binding of beta-VLDL+apoE that could be abolished b y treatment with heparinase. These mutant cells and wild-type CHO cell s possessed a similar amount of LRP, as determined by ligand blot anal yses and by alpha2-macroglobulin binding, and possessed a similar amou nt of LDL receptor activity, as determined by LDL binding. Therefore, we would interpret these data as showing that heparan sulfate proteogl ycan may be involved in the initial binding of the apoE-enriched remna nts with the subsequent involvement of the LRP in the uptake of these lipoproteins. It remains to be determined whether the heparan sulfate proteoglycan can function by itself in both the binding and internaliz ation of the apoE-enriched remnants or whether the proteoglycan is par t of a complex with LRP that mediates a two-step process, i.e. binding and subsequent internalization by the receptor.