Interaction of different classes of lipoproteins with heparan sulfate, heparin, and lipoprotein lipase was studied by a surface plasmon resonance based technique on a BIAcore. The proteoglycans were covalently attached to sensor chips as previously described [Lookene, A., Chevreuil, O., Ostergaard, P., & Olivecrona, G. (1996) Biochemistry 35, 12155-12163]. Binding of all lipoproteins, except for beta-VLDL, to endothelial heparan sulfate was low. Binding of chylomicrons (from rat lymph) and of human VLDL was much increased by the presence of lipoprotein lipase. With human LDL, binding was low in the absence of lipase or at low lipase concentrations. For efficient binding, 2-4 lipase dimers per LDL particle were necessary, indicating cooperativity in the interaction. In contrast, HDL did not bind under any conditions. Heparin had higher binding capacity for lipoproteins than heparan sulfate. This was due to a higher number of binding sites on the heparin chains. Binding of LDL, VLDL, and chylomicrons to heparan sulfate-covered surfaces, both in the presence and in the absence of lipoprotein lipase, was characterized by high values for association rate constants (10(4)-10(5) M(-1) s(-1)) and low values for dissociation rate constants (10(-4)-10(-5) M(-1) s(-1)). In some experiments, rabbit beta-VLDL were directly immobilized to the sensor chips. Binding of lipoprotein lipase to these surfaces was characterized by a very high association rate constant (10(6) M(-1) s(-1)). The dissociation of triacylglycerol-rich lipoproteins was more rapid with catalytically active lipase than with active site-inhibited lipase. It was also markedly increased in the presence of free heparin, suggesting fast exchange kinetics at the surface. Based on that, we propose that lipoproteins are relatively mobile at heparan sulfate covered surfaces. Our study emphasizes the important role of lipoprotein lipase, or molecules with similar properties (apolipoprotein E, hepatic lipase), as mediators for binding of lipoproteins to proteoglycans. It also demonstrates the great potential for the use of biosensors for studies of lipoprotein interactions.
This article is available online at http://www.jlr.org regulated in a tissue-specifi c manner ( 1, 2 ). As an example, the activity in adipose tissue is relatively low between meals but rises postprandially ( 6 ). This has been suggested to govern when and to what extent lipids are delivered to the adipose tissue for storage ( 1, 2 ). On the other hand, it has been argued that the only role of the enzyme is to release the fatty acids in a form that can be transferred from lipoproteins into cells or onto albumin, and that the amount taken up by individual cells is usually governed by transport processes ( 7 ) and/or the metabolic balance within the cell ( 8 ) rather than by the rate at which the lipase releases fatty acids at endothelial sites adjacent to the cell.The first step in chylomicron catabolism is that the lipoprotein particles bind to the endothelial surface, a process named margination. Goulbourne et al. ( 9 ) have demonstrated that the margination requires LPL bound to GPIHBP1. Defi ciency of GPIHBP1, genetic or in mouse models, leads to equally severe hyperchylomicronemia as defi ciency of LPL itself ( 10 ). In this setting normal amounts of LPL are produced, but do not reach the endothelial surface. Mouse genetic models with altered expression of LPL have been generated. Total LPL defi ciency due to gene knockout leads to lethal hypertriglyceridemia within 18 h of life ( 11 ). By mating heterozygous LPL knockout mice with transgenic mice that express human LPL in either muscle ( 12 ) or heart ( 13 ), mouse lines have been generated that express the enzyme in only a single tissue. Here we have taken advantage of these mouse models to study to what extent the expression of LPL in a tissue governs initial binding and/or deposition of lipids in that tissue . For this we used rat chylomicrons, labeled in vivo with [3 H]-retinol (which is incorporated into retinyl esters and serves as label for the core lipids and for the chylomicron particles) and [ ]retinol (primarily in ester form, providing a tracer for the core lipids) were injected. TG label was cleared more rapidly than core label. There were no differences between the mouse lines in the rate at which core label was cleared. Two minutes after injection, about 5% of the core label, and hence chylomicron particles, were in the heart of WT mice. In mice that expressed LPL only in skeletal muscle, and had much reduced levels of LPL in the heart, binding of chylomicrons was reduced to 1%, whereas in mice that expressed LPL only in the heart, the binding was increased to over 10%. The same patterns of distribution were evident at 20 min when most of the label had been cleared. Thus, the amount of LPL expressed in muscle and heart governed both the binding of chylomicron particles and the assimilation of chylomicron lipids in the tissue.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.