Heparan Sulfate Proteoglycan-Mediated Uptake of Apolipoprotein E−Triglyceride-Rich Lipoprotein Particles: A Major Pathway at Physiological Particle Concentrations

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Abstract
We explored potential mechanisms of non-low-density lipoprotein (LDL) receptor-mediated uptake of triglyceride-rich particles (TGRP) in the presence of apolipoprotein E (apo E). Human fibroblasts were incubated with model intermediate-density lipoprotein- (IDL-) sized TGRP (10−1000 μg of neutral lipid/mL) containing apo E. The extent of receptor-mediated uptake of TGRP was assessed with (a) an anti-apo E monoclonal antibody, which blocks receptor interaction; (b) incubation with heparin; (c) normal vs LDL receptor-negative fibroblasts; and (d) receptor-associated protein (RAP) to determine the potential contribution of LDL receptor-related protein (LRP). Cell surface heparan sulfate proteoglycan- (HSPG-) mediated uptake was examined with or without the addition of heparinase and heparitinase to cell incubation mixtures. At low particle concentrations (≤100 μg of neutral lipid/mL), almost all apo E−TGRP uptake was via the LDL receptor. At higher particle concentrations, within the physiologic range (>250 μg of neutral lipid/mL), most (≥60%) particle uptake and internalization was via HSPG-mediated pathways. This HSPG pathway did not involve classical lipoprotein receptors, such as LRP or the LDL receptor. These data suggest that in peripheral tissues, such as the arterial wall, apo E may act in TGRP as a ligand for uptake not only via the LDL receptor and LRP pathways but also via HSPG pathways that are receptor-independent. Thus, at physiologic particle concentrations apo E−TGRP can be bound and internalized in certain cells by relatively low affinity but high capacity HSPG-mediated pathways.