SPECIFICITY OF LIPOPROTEIN-LIPASE BINDING TO ENDOTHELIAL-CELLS

Lipoprotein lipase (LPL) hydrolyzes circulating lipoprotein triglyceri de molecules while it is associated with the luminal surface of capill ary endothelial cells. The precise molecular mechanism by which LPL at taches to these cells is unknown. LPL and a number of other molecules, including growth factors and clotting factors, bind to heparin-affini ty gels and are eluted using high concentrations of salt. Of these mol ecules, antithrombin III and basic fibroblast growth factor have been shown to bind to specific cell surface heparan sulfate proteoglycans. Recent data from our laboratory (Sivaram et al. 1992. J. Biol. Chem. 2 67: 16517-16522) have shown that a heparin-sensitive, non-proteoglycan 116-kDa LPL-binding protein is present on cultured bovine aortic endo thelial cells (BAEC). A series of experiments was performed to study t he specificity of LPL binding to BAEC and to this 116-kDa protein. At low amounts of LPL (1 mug) I-125-labeled LPL binding to the cells was inhibited up to 82% by the addition of a 20-fold excess of unlabeled L PL. LPL binding to the BAEC was not decreased by the addition of simil ar amounts of either antithrombin or thrombin. Specific LPL binding wa s eliminated by incubating the BAEC at 4-degrees-C with heparin contai ning buffer prior to the addition of LPL. Although cellular internaliz ation of I-125-labeled LPL at 37-degrees-C was decreased when an exces s of each of the three proteins was added to the culture medium, LPL w as most effective. Furthermore, when LPL interaction with the 116-kDa binding protein was studied using ligand blots, I-125-labeled LPL bind ing was blocked only by unlabeled LPL. Low concentrations of heparin r eleased LPL bound to endothelial surfaces and also decreased the numbe r of LPL binding sites on the cells. Therefore, heparin might dissocia te both LPL and its binding protein from the cells. To determine wheth er a heparin-sensitive LPL binding site was also present on aorta, LPL binding to control and heparin-treated pieces of dog aorta was assess ed; 45% less LPL bound to the heparin-treated aorta. Thus our data sup port the hypothesis that LPL binds to two different types of endotheli al cell surface proteins, heparan sulfate proteoglycans and a specific 116-kDa, heparin-sensitive binding protein.