CHYLOMICRON REMNANT UPTAKE IS REGULATED BY THE EXPRESSION AND FUNCTION OF HEPARAN-SULFATE PROTEOGLYCAN IN HEPATOCYTES

Chylomicron remnants transport cholesterol from the intestine, and are removed from the circulation principally by the liver. While hepatic receptors, including the low density lipoprotein (LDL) receptor accoun t for endocytosis, heparan sulfate proteoglycans (HSPG) participate in the initial binding of remnants to liver cells. To explore the intera ctions between HSPG and endocytosis of remnants, in the present study the expression of HSPG was inhibited in HepG2 cells transfected by a s ynthetic antisense oligodeoxynucleotide SYN5. Immunofluorescent staini ng by a monoclonal anti-syndecan antibody showed significant reduction in the expression of syndecan in SYN5-treated cells compared with con trol cells. Remnant binding decreased by about 50-70% in SYN5-transfec ted cells. Monoclonal antibodies to either heparan sulphate or the LDL receptor decreased binding by about 60-65%. The glycosylation inhibit or beta-nitrophenylxylopyranoside inhibited remnant uptake by 25%, whe reas 4-nitrophenyl-beta-D-galactopyranoside had no effect on remnant b inding. Heparinase completely abolished binding at appropriate concent rations. Heparitinase was less effective than hep arinase in inhibitin g remnant binding. Suramin completely abolished the remnant binding. P oly-arginine, poly-lysine, and protamine all reduced remnant uptake by the cells, as did polybrene, a synthetic polycation, suggesting a rol e of cation-anion interactions in remnant binding. Brefeldin A, colchi cine, and monensin caused the fluorescence associated with remnants to persist within the cells, confirming that blockers of tubulovesicular processes and Golgi function inhibit the intracellular transport and degradation of the remnants. Our results show that remnant binding to liver cells depends on the LDL receptor, on the expression of HSPG cor e proteins, and on the functionality of heparan sulfate in HSPG.