Segments in the C-terminal Folding Domain of Lipoprotein Lipase Important for Binding to the Low Density Lipoprotein Receptor-related Protein and to Heparan Sulfate Proteoglycans

Abstract

Chylomicron and very low density lipoprotein (VLDL)1 remnants can be taken up via the endocytic ␣ 2 -macroglobulin receptor/LDL receptor-related protein (␣ 2 MR/LRP) either mediated by apolipoprotein E, or by lipoprotein lipase (LpL) or hepatic lipase associated with the lipoproteins (1-4). Recent results provide evidence that the uptake via ␣ 2 MR/LRP occurs in vivo, since remnant lipoproteins accumulate in mice that express reduced amounts of ␣ 2 MR/LRP and also lack functional LDL receptors (5). It has been suggested that the LpL-mediated pathway may be particularly important in the vascular wall, since ␣ 2 MR/LRP is abundant in macrophages and smooth muscle cells of atherosclerotic lesions and since LpL is secreted by macrophages in the lesions (6). In addition, it was recently shown that, whereas normal macrophages incubated with VLDL accumulate massive amounts of lipids, this does not occur in LpL-deficient macrophages (7). LpL binds to cell surface heparan sulfate proteoglycans (HSPG) as reflected in its strong affinity for binding to heparin. This property is physiologically important for the docking on endothelial cells exposing the lipase to circulating lipoproteins. LpL also binds to HSPG of other cell types as recently shown directly by immunofluorescence studies (8), and studies in Chinese hamster ovary cells have shown that uptake and degradation of LpL can be mediated by HSPG (9). In addition, the docking of LpL on HSPG of cells rich in ␣ 2 MR/LRP is important for uptake of LpL and for LpL-mediated uptake of lipoproteins via this receptor (4, 10, 11).LpL is a member of the mammalian lipase family also comprising the homologous hepatic and pancreatic lipases. The crystallographic structure of pancreatic lipase (12) shows that it consists of two folding domains, a larger N-terminal and a smaller C-terminal domain. Based upon the similarities in sequence it is thought that the domain organization is similar for LpL, and a three-dimensional model was recently proposed (13). LpL circulates both as a 96-kDa homodimer, which is the normally secreted and catalytically active form, and as a catalytically inactive monomer (14 -16). Although not known, it is likely that the LpL dimer is arranged head-to-tail in a way that allows enough space for conformational changes following substrate binding (13). The dimeric structure of LpL causes an increase in the affinity for heparin, presumably because sites in both monomers can participate in binding of one heparin molecule (13), and helps efficient ␣ 2 MR/LRP-mediated lipoprotein uptake, possibly because only the dimer can bind to a lipoprotein particle and to the receptor at the same time (10,17).The N-terminal folding domain of human LpL, comprising

Molecular modeling of the dimeric structure of human lipoprotein lipase and functional studies of the carboxyl‐terminal domain

“…The investigators of the latter study concluded that HSPG binding at the cell surface is required for maintaining LPL stability. LPL's C-terminal domain also appears to be involved in the binding of the enzyme to receptors such as the LDL receptor related protein (LRP) [24,25], with the critical LRP-binding site having been demonstrated to be between residues 340 and 438. Positively charged amino acid residues in this region were replaced with Ala to test receptor binding, because LPL is expected to interact with the negative charges in LRP's cysteine-rich repeats [26].…”

High Affinity Binding between Lipoprotein Lipase and Lipoproteins Involves Multiple Ionic and Hydrophobic Interactions, Does Not Require Enzyme Activity, and Is Modulated by Glycosaminoglycans

“…Nonetheless, it is interesting to note that a similar region in the C terminus of LPL has been implicated for its binding to the LDL receptor-related protein and Sortilin/neurotensin receptor-3 (4, 36, 37). Nielsen et al (36) have shown that amino acids 308 -384 and 404 -430 in LPL bind to the LDL receptor-related protein. This region is characterized by the presence of high density of positive charges and is probably exposed on the surface of the molecule (4).…”

Isolation and Characterization of Low Sulfated Heparan Sulfate Sequences with Affinity for Lipoprotein Lipase

“…In LPL, four clusters of basic amino acids are exposed on the opposite side of the molecule compared with the entrance to the catalytic site, and at least two, located in the N-terminal domain, seem to be important for HS binding (14,16,17). Participation of positive regions also in the C-terminal domain of LPL have been demonstrated (17)(18)(19). Heparin fragments of 8 -10 monomer units have been shown to bind to LPL and to displace the enzyme from endothelial sites, from synthetic heparin-covered surfaces and from surfaces covered by lipoprotein receptors (20 -22).…”

“…4D). A loss of a sulfate group shifted the elution position of an oligosaccharide considerably (15)(16)(17)(18)(19)(20) min in the applied gradient), while the loss of an uronic acid residue had much less impact on the elution behavior of the remaining fragment due to the protonated form of the carboxyl groups at the pH used. All fragments that originally contained a 2-O-sulfated uronic acid residue in the nonreducing end shifted position (indicated by "-2S" appended to the original peak identification) as compared with the deaminated sample in Fig.…”