Transport of lipoprotein lipase across endothelial cells.

Abstract

Lipoprotein lipase (LPL), synthesized in muscle and fat, hydrolyzes plasma triglycerides primarily while bound to luminal endothelial cell surfaces. To obtain information about the movement of LPL from the basal to the luminal endothelial cell surface, we studied the transport of purified bovine milk LPL across bovine aortic endothelial cell monolayers. 125I-labeled LPL ('25I-LPL) added to the basal surface of the monolayers was detected on the apical side of the cells in two compartments: (i) in the medium of the upper chamber, and (ii) bound to the apical cell surface. The amount of 125I-LPL on the cell surface, but not in the medium, reached saturation with time and LPL dose. Catalytically active LPL was transported to the apical surface but very little LPL activity appeared in the medium. Heparinase treatment of the basal cell surface and addition of dextran sulfate (0.15 IAM) to the lower chamber decreased the amount of 125I-LPL appearing on the apical surface. Similarly, the presence of increasing molar ratios of oleic acid/bovine serum albumin at the basal surface decreased the transport of active LPL across the monolayer. Thus, a saturable transport system, which requires heparan sulfate proteoglycans and is inhibited by high concentrations of free fatty acids on the basal side of the cells, appears to exist for passage of enzymatically active LPL across endothelial cells. We postulate that regulation of LPL transport to the endothelial luminal surface modulates the physiologically active pool of LPL in vivo.

Oxidation of Low Density Lipoproteins Greatly Enhances Their Association with Lipoprotein Lipase Anchored to Endothelial Cell Matrix

“…The retention of LDL did not require an enzymatically active protein, since inactivated LPL (overnight incubation at 25°C) adsorbed to a microfilter plate was able to enhance LDL retention in an LPL concentrationdependent manner. 2 However, the binding of LPL to the cell proteoglycans does require the enzyme to be active (46). We previously observed that apoE-rich HDL and apoE mimetics (polyarginine or polylysine) were effective competitors of native LDL bound to LPL⅐ECM and that this interaction did not release LPL from the matrix (34).…”

ASP enhances in situ lipoprotein lipase activity by increasing fatty acid trapping in adipocytes

“…Furthermore, NEFAs interfere with the binding of LPL both to its activator apoC-II and to heparin sulfate proteoglycan anchors (16), decreasing LPL transport to endothelial cells (6) and displacing it from the endothelial cell surface (15,54,57). On the other hand, inhibition of chylomicron and VLDL hydrolysis can block the effect of NEFAs on LPL displacement from endothelial cells (15).…”

“…LPL is synthesized and secreted from underlying parenchymal cells, mainly adipose and muscle tissue, and is then translocated to the endothelial apical surface (6). Studies of factors affecting LPL activity in WAT have focused primarily on the synthesis and secretion of LPL from the adipocytes.…”

Heparan Sulfate Proteoglycans Are Primarily Responsible for the Maintenance of Enzyme Activity, Binding, and Degradation of Lipoprotein Lipase in Chinese Hamster Ovary Cells

“…In endothelial cells, LPL does not undergo a degradative pathway, yet binding to HSPGs is still critical (11). In this case, HSPGs are involved in the transcellular transport of active LPL across endothelial cells (12) and are responsible for maintaining the high concentration of active LPL at the luminal surface of the capillary bed. In addition to the receptor functions of HSPGs, there is some evidence that heparan sulfate binding might also regulate LPL enzymatic activity.…”