Chymotryptic cleavage of lipoprotein lipase. Identification of cleavage sites and functional studies of the truncated molecule

GPIHBP1 and Lipoprotein Lipase, Partners in Plasma Triglyceride Metabolism

Lipoprotein lipase (LPL), identified in the 1950s, has been studied intensively by biochemists, physiologists, and clinical investigators. These efforts uncovered a central role for LPL in plasma triglyceride metabolism and identified LPL mutations as a cause of hypertriglyceridemia. By the 1990s, with an outline for plasma triglyceride metabolism established, interest in triglyceride metabolism waned. In recent years, however, interest in plasma triglyceride metabolism has awakened, in part because of the discovery of new molecules governing triglyceride metabolism. One such protein-and the focus of this review-is GPIHBP1, a protein of capillary endothelial cells. GPIHBP1 is LPL's essential partner: it binds LPL and transports it to the capillary lumen; it is essential for lipoprotein margination along capillaries, allowing lipolysis to proceed; and it preserves LPL's structure and activity. Recently, GPIHBP1 was the key to solving the structure of LPL. These developments have transformed the models for intravascular triglyceride metabolism.

Coexpression of novel furin-resistant LPL variants with lipase maturation factor 1 enhances LPL secretion and activity

LPL is a secreted enzyme that hydrolyzes triglycerides from circulating lipoproteins. Individuals lacking LPL suffer from severe hypertriglyceridemia, a risk factor for acute pancreatitis. One potential treatment is to administer recombinant LPL as a protein therapeutic. However, use of LPL as a protein therapeutic is limited because it is an unstable enzyme that is difficult to produce in large quantities. Furthermore, these considerations also limit structural and biochemical studies that are needed for large-scale drug discovery efforts. We demonstrate that the yield of purified LPL can be dramatically enhanced by coexpressing its maturation factor, LMF1, and by introducing novel mutations into the LPL sequence to render it resistant to proteolytic cleavage by furin. One of these mutations introduces a motif for addition of an N-linked glycan to the furin-recognition site. Furin-resistant LPL has previously been reported, but is not commonly used. We show that our modifications do not adversely alter LPL's enzymatic activity, stability, or in vivo function. Together, these data show that furin-resistant LPL is a useful reagent for both biochemical and biomedical studies.

Lipoprotein lipase activity and interactions studied in human plasma by isothermal titration calorimetry

LPL hydrolyzes triglycerides in plasma lipoproteins. Due to the complex regulation mechanism, it has been difficult to mimic the physiological conditions under which LPL acts in vitro. We demonstrate that isothermal titration calorimetry (ITC), using human plasma as substrate, overcomes several limitations of previously used techniques. The high sensitivity of ITC allows continuous recording of the heat released during hydrolysis. Both initial rates and kinetics for complete hydrolysis of plasma lipids can be studied. The heat rate was shown to correspond to the release of fatty acids and was linearly related to the amount of added enzyme, either purified LPL or postheparin plasma. Addition of apoC-III reduced the initial rate of hydrolysis by LPL, but the inhibition became less prominent with time when the lipoproteins were triglyceride poor. Addition of angiopoietin-like protein (ANGPTL)3 or ANGPTL4 caused reduction of the activity of LPL via a two-step mechanism. We conclude that ITC can be used for quantitative measurements of LPL activity and interactions under in vivo-like conditions, for comparisons of the properties of plasma samples from patients and control subjects as substrates for LPL, as well as for testing of drug candidates developed with the aim to affect the LPL system.

Apolipoprotein B-100-containing lipoprotein metabolism in subjects with lipoprotein lipase gene mutations

OBJECTIVE

We investigated the impact of lipoprotein lipase (LPL) gene mutations on apolipoprotein B (apoB)-100 metabolism.

METHODS AND RESULTS

We studied 3 subjects with familial LPL deficiency; 14 subjects heterozygous for the LPL gene mutations Gly188Glu, Trp64Stop, and Ile194Thr; and 10 control subjects. Very-low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), and low-density lipoprotein (LDL)-apoB-100 kinetics were determined in the fed state using stable isotope methods and compartmental modeling. Compared with controls, familial LPL deficiency had markedly elevated plasma triglycerides and lower VLDL-apoB-100 fractional catabolic rate (FCR), IDL-apoB-100 FCR, VLDL-to-IDL conversion, and VLDL-apoB-100 production rate (P<0.01). Compared with controls, Gly188Glu had higher plasma triglyceride and VLDL- and IDL-apoB-100 concentrations and lower VLDL- and IDL-apoB-100 FCR (P<0.05). Plasma triglycerides were not different, but IDL-apoB-100 concentration and production rate and VLDL-to-IDL conversion were lower in Trp64Stop compared with controls (P<0.05). No differences between controls and Ile194Thr were observed.

CONCLUSIONS

Our results confirm that hypertriglyceridemia is a key feature of familial LPL deficiency. This is due to impaired VLDL- and IDL-apoB-100 catabolism and VLDL-to-IDL conversion. Single-allele mutations of the LPL gene result in modest to elevated plasma triglycerides. The changes in plasma triglycerides and apoB-100 kinetics are attributable to the effects of the LPL genotype.

Convergence of genes implicated in Alzheimer's disease on the cerebral cholesterol shuttle: APP, cholesterol, lipoproteins, and atherosclerosis

Polymorphic genes associated with Alzheimer's disease (see ) delineate a clearly defined pathway related to cerebral and peripheral cholesterol and lipoprotein homoeostasis. They include all of the key components of a glia/neurone cholesterol shuttle including cholesterol binding lipoproteins APOA1, APOA4, APOC1, APOC2, APOC3, APOD, APOE and LPA, cholesterol transporters ABCA1, ABCA2, lipoprotein receptors LDLR, LRP1, LRP8 and VLDLR, and the cholesterol metabolising enzymes CYP46A1 and CH25H, whose oxysterol products activate the liver X receptor NR1H2 and are metabolised to esters by SOAT1. LIPA metabolises cholesterol esters, which are transported by the cholesteryl ester transport protein CETP. The transcription factor SREBF1 controls the expression of most enzymes of cholesterol synthesis. APP is involved in this shuttle as it metabolises cholesterol to 7-betahydroxycholesterol, a substrate of SOAT1 and HSD11B1, binds to APOE and is tethered to LRP1 via APPB1, APBB2 and APBB3 at the cytoplasmic domain and via LRPAP1 at the extracellular domain. APP cleavage products are also able to prevent cholesterol binding to APOE. BACE cleaves both APP and LRP1. Gamma-secretase (PSEN1, PSEN2, NCSTN) cleaves LRP1 and LRP8 as well as APP and their degradation products control transcription factor TFCP2, which regulates thymidylate synthase (TS) and GSK3B expression. GSK3B is known to phosphorylate the microtubule protein tau (MAPT). Dysfunction of this cascade, carved out by genes implicated in Alzheimer's disease, may play a major role in its pathology. Many other genes associated with Alzheimer's disease affect cholesterol or lipoprotein function and/or have also been implicated in atherosclerosis, a feature of Alzheimer's disease, and this duality may well explain the close links between vascular and cerebral pathology in Alzheimer's disease. The definition of many of these genes as risk factors is highly contested. However, when polymorphic susceptibility genes belong to the same signaling pathway, the risk associated with multigenic disease is better related to the integrated effects of multiple polymorphisms of genes within the same pathway than to variants in any single gene [Wu, X., Gu, J., Grossman, H.B., Amos, C.I., Etzel, C., Huang, M., Zhang, Q., Millikan, R.E., Lerner, S., Dinney, C.P., Spitz, M.R., 2006. Bladder cancer predisposition: a multigenic approach to DNA-repair and cell-cycle-control genes. Am. J. Hum. Genet. 78, 464-479.]. Thus, the fact that Alzheimer's disease susceptibility genes converge on a clearly defined signaling network has important implications for genetic association studies.

Exploring the specific features of interfacial enzymology based on lipase studies

Many enzymes are active at interfaces in the living world (such as in the signaling processes at the surface of cell membranes, digestion of dietary lipids, starch and cellulose degradation, etc.), but fundamental enzymology remains largely focused on the interactions between enzymes and soluble substrates. The biochemical and kinetic characterization of lipolytic enzymes has opened up new paths of research in the field of interfacial enzymology. Lipases are water-soluble enzymes hydrolyzing insoluble triglyceride substrates, and studies on these enzymes have led to the development of specific interfacial kinetic models. Structure-function studies on lipases have thrown light on the interfacial recognition sites present in the molecular structure of these enzymes, the conformational changes occurring in the presence of lipids and amphiphiles, and the stability of the enzymes present at interfaces. The pH-dependent activity, substrate specificity and inhibition of these enzymes can all result from both "classical" interactions between a substrate or inhibitor and the active site, as well as from the adsorption of the enzymes at the surface of aggregated substrate particles such as oil drops, lipid bilayers or monomolecular lipid films. The adsorption step can provide an alternative target for improving substrate specificity and developing specific enzyme inhibitors. Several data obtained with gastric lipase, classical pancreatic lipase, pancreatic lipase-related protein 2 and phosphatidylserine-specific phospholipase A1 were chosen here to illustrate these specific features of interfacial enzymology.

Substrate specificity of lipoprotein lipase and endothelial lipase: studies of lid chimeras

The triglyceride (TG) lipase gene subfamily, consisting of LPL, HL, and endothelial lipase (EL), plays a central role in plasma lipoprotein metabolism. Compared with LPL and HL, EL is relatively more active as a phospholipase than as a TG lipase. The amino acid loop or "lid" covering the catalytic site has been implicated as the basis for the difference in substrate specificity between HL and LPL. To determine the role of the lid in the substrate specificity of EL, we studied EL in comparison with LPL by mutating specific residues of the EL lid and exchanging their lids. Mutation studies showed that amphipathic properties of the lid contribute to substrate specificity. Exchanging lids between LPL and EL only partially shifted the substrate specificity of the enzymes. Studies of a double chimera possessing both the lid and the C-terminal domain (C-domain) of EL in the LPL backbone showed that the role of the lid in determining substrate specificity does not depend on the nature of the C-domain of the lipase. Using a kinetic assay, we showed an additive effect of the EL lid on the apparent affinity for HDL(3) in the presence of the EL C-domain.

Role of N-linked glycosylation in the secretion and activity of endothelial lipase

Human endothelial lipase (EL), a member of the triglyceride lipase gene family, has five potential N-linked glycosylation sites, two of which are conserved in both lipoprotein lipase and hepatic lipase. Reduction in molecular mass of EL after treatment with glycosidases and after treatment of EL-expressing cells with the glycosylation inhibitor tunicamycin demonstrated that EL is a glycosylated protein. Each putative glycosylation site was examined by site-directed mutagenesis of the asparagine (Asn). Mutation of Asn-60 markedly reduced secretion and slightly increased specific activity. Mutation of Asn-116 did not influence secretion but increased specific activity. In both cases, this resulted from decreased apparent K(m) and increased apparent V(max). Mutation of Asn-373 did not influence secretion but significantly reduced specific activity, as a result of a decrease in apparent V(max). Mutation of Asn-471 resulted in no reduction in secretion or specific activity. Mutation of Asn-449 resulted in no change in secretion, activity, or molecular mass, indicating that the site is not utilized. The ability of mutants secreted at normal levels to mediate bridging between LDL and cell surfaces was examined. The Asn-373 mutant demonstrated a 3-fold decrease in bridging compared with wild-type EL, whereas Asn-116 and Asn-471 were similar to wild-type EL.

Structural basis of endothelial lipase tropism for HDL

Lipoprotein lipase (LPL) and endothelial lipase (EL), the most closely related enzymes among the members of the triglyceride lipase gene family with regard to primary sequence, have distinct lipolytic properties (triglyceride lipase vs. phospholipase) as well as different preferences for specific types of lipoproteins [triglyceride-rich lipoproteins vs. high density lipoprotein (HDL)] Lipid substrate specificity is believed to be conferred by the lid region located in the amino-terminal domain of the enzymes, whereas surprisingly little work has been done to identify the region mediating lipoprotein substrate specificity. To determine the domain responsible for lipoprotein preference within each enzyme, we generated the domain chimeric enzyme LPL-EL. The heterologous carboxy-terminal (C terminal) domain did not change lipid substrate preference (triglyceride vs. phospholipase) as determined by using artificial substrates. The EL C-terminal domain, however, enabled LPL-EL to bridge HDL particles like wild-type EL, whereas LPL only mediated binding of very low density lipoprotein. Unlike wild-type LPL, LPL-EL had substantial ability to hydrolyze HDL lipids similar to that of wild-type EL. Overexpression of LPL-EL in wild-type mice resulted in significantly reduced levels of HDL cholesterol and phospholipids by 93 and 85%, respectively, similar to the extent seen in EL-expressing mice, whereas no reduction of these parameters was observed in LPL-expressing mice. We conclude that the C-terminal domain of EL is crucial for the ability of EL to bind and to hydrolyze HDL and converts LPL to an enzyme fully capable of hydrolyzing HDL, highlighting the importance of the C-terminal lipase domain in lipoprotein substrate preference.

Rapid subunit exchange in dimeric lipoprotein lipase and properties of the inactive monomer

Lipoprotein lipase (LPL), a key enzyme in the metabolism of triglyceride-rich plasma lipoproteins, is a homodimer. Dissociation to monomers leads to loss of activity. Evidence that LPL dimers rapidly exchange subunits was demonstrated by fluorescence resonance energy transfer between lipase subunits labeled with Oregon Green and tetrametylrhodamine, respectively, and also by formation of heterodimers composed of radiolabeled and biotinylated lipase subunits captured on streptavidine-agarose. Compartmental modeling of the inactivation kinetics confirmed that rapid subunit exchange must occur. Studies of activity loss indicated the existence of a monomer that can form catalytically active dimers, but this intermediate state has not been possible to isolate and remains hypothetical. Differences in solution properties and conformation between the stable but catalytically inactive monomeric form of LPL and the active dimers were studied by static light scattering, intrinsic fluorescence, and probing with 4,4'-dianilino-1,1'-binaphtyl-5,5'-disulfonic acid and acrylamide. The catalytically inactive monomer appeared to have a more flexible and exposed structure than the dimers and to be more prone to aggregation. By limited proteolysis the conformational changes accompanying dissociation of the dimers to inactive monomers were localized mainly to the central part of the subunit, probably corresponding to the region for subunit interaction.

1,1'-bis(anilino)-4-,4'-bis(naphtalene)-8,8'-disulfonate acts as an inhibitor of lipoprotein lipase and competes for binding with apolipoprotein CII

Lipoprotein lipase (LPL) is dependent on apolipoprotein CII (apoCII), a component of plasma lipoproteins, for function in vivo. The hydrophobic fluorescent probe 1,1'-bis(anilino)-4,4'-bis(naphthalene)-8,8'-disulfonate (bis-ANS) was found to be a potent inhibitor of LPL. ApoCII prevented the inhibition by bis-ANS, and was also able to restore the activity of inhibited LPL in a competitive manner, but only with triacylglycerols with acyl chains longer than three carbons. Studies of fluorescence and surface plasmon resonance indicated that LPL has an exposed hydrophobic site for binding of bis-ANS. The high affinity interaction was characterized by an equilibrium constant Kd of 0.10-0.26 microm and by a relatively high on rate constant kass = 2.0 x 10(4) m(-1) s(-1) and a slow off-rate with a dissociation rate constant kdiss = 1.2 x 10(-4) s(-1). The high affinity binding of bis-ANS did not influence interaction of LPL with heparin or with lipid/water interfaces and did not dissociate the active LPL dimer into monomers. Analysis of fragments of LPL after photoincorporation of bis-ANS indicated that the high affinity binding site was located in the middle part of the N-terminal folding domain. We propose that bis-ANS binds to an exposed hydrophobic area that is located close to the active site. This area may be the binding site for individual substrate molecules and also for apoCII.

The triglyceride lipases of the pancreas

Pancreatic triglyceride lipase (PTL) and its protein cofactor, colipase, are required for efficient dietary triglyceride digestion. In addition to PTL, pancreatic acinar cells synthesize two pancreatic lipase related proteins (PLRP1 and PLRP2), which have a high degree of sequence and structural homology with PTL. PLRP1 has no known activity. PTL and PLRP2 differ in substrate specificity, behavior in bile salts and dependence on colipase. Each protein has a globular amino-terminal (N-terminal) domain, which contains the catalytic site for PTL and PLRP2, and a beta-sandwich carboxyl-terminal (C-terminal) domain, which includes the predominant colipase-binding site for PTL. Inactive and active conformations of PTL have been described. They differ in the position of a surface loop, the lid domain, and of the beta5-loop. In the inactive conformation, the lid covers the active site and, upon activation by bile salt micelles and colipase or by lipid-water interfaces, the lid moves dramatically to open and configure the active site. After the lid movement, PTL and colipase create a large hydrophobic plateau that can interact with the lipid-water interface. A hydrophobic surface loop in the C-terminal domain, the beta5' loop, may also contribute to the interfacial-binding domain of the PTL-colipase complex.

The beta 5' loop of the pancreatic lipase C2-like domain plays a critical role in the lipase-lipid interactions

The structural similarities between the C-terminal domain of human pancreatic lipase (C-HPL) and C2 domains suggested a similar function, the interaction with lipids. The catalytic N-terminal domain (N-HPL) and C-HPL were produced as individual proteins, and their partitioning between the water phase and the triglyceride-water interface was assessed using trioctanoin emulsions (TC8). N-HPL did not bind efficiently to TC8 and was inactive. C-HPL did bind to TC8 and to a phospholipid monolayer with a critical surface pressure of penetration similar to that of HPL (15 mN m(-1)). These experiments, performed in the absence of colipase and bile salts, support an absolute requirement of C-HPL for interfacial binding of HPL. To refine our analysis, we determined the contribution to lipid interactions of a hydrophobic loop (beta 5') in C-HPL by investigating a HPL mutant in which beta 5' loop hydrophobicity was increased by introducing the homologous lipoprotein lipase (LPL) beta 5' loop. This mutant (HPL-beta 5'LPL) penetrated into phospholipid monolayers at higher surface pressures than HPL, and its level of binding to TC8 was higher than that of HPL in the presence of serum albumin (BSA), an inhibitory protein that competes with HPL for interfacial adsorption. The beta 5' loop of LPL is therefore tailored for an optimal interaction with the surface of triglyceride-rich lipoproteins (VLDL and chylomicrons) containing phospholipids and apoproteins. These observations support a major contribution of the beta 5' loop in the interaction of LPL and HPL with their respective substrates.

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

Lipoprotein lipase (LPL) plays a key role in lipid metabolism. Molecular modeling of dimeric LPL was carried out using insight ii based upon the crystal structures of human, porcine, and horse pancreatic lipase. The dimeric model reveals a saddle-shaped structure and the key heparin-binding residues in the amino-terminal domain located on the top of this saddle. The models of two dimeric conformations - a closed, inactive form and an open, active form - differ with respect to how surface-loop positions affect substrate access to the catalytic site. In the closed form, the surface loop covers the catalytic site, which becomes inaccessible to solvent. Large conformational changes in the open form, especially in the loop and carboxyl-terminal domain, allow substrate access to the active site. To dissect the structure-function relationships of the LPL carboxyl-terminal domain, several residues predicted by the model structure to be essential for the functions of heparin binding and substrate recognition were mutagenized. Arg405 plays an important role in heparin binding in the active dimer. Lys413/Lys414 or Lys414 regulates heparin affinity in both monomeric and dimeric forms. To evaluate the prediction that LPL forms a homodimer in a 'head-to-tail' orientation, two inactive LPL mutants - a catalytic site mutant (S132T) and a substrate-recognition mutant (W390A/W393A/W394A) - were cotransfected into COS7 cells. Lipase activity could be recovered only when heterodimerization occurred in a head-to-tail orientation. After cotransfection, 50% of the wild-type lipase activity was recovered, indicating that lipase activity is determined by the interaction between the catalytic site on one subunit and the substrate-recognition site on the other.

Lipoprotein lipase in the arterial wall: linking LDL to the arterial extracellular matrix and much more

For low density lipoprotein (LDL) particles to be atherogenic, increasing evidence indicates that their residence time in the arterial intima must be sufficient to allow their modification into forms capable of triggering extracellular and intracellular lipid accumulation. Recent reports have confirmed the longstanding hypothesis that the major determinant(s) of initial LDL retention in the preatherosclerotic arterial intima is the proteoglycans. However, once the initial atherosclerotic lesions have formed, a shift to retention facilitated by macrophage-derived lipoprotein lipase (LPL) appears, leading to the progression of the lesions. Here, we review recent findings on the mechanisms enabling LPL to promote LDL retention and extracellular lipid accumulation in the arterial intima, and we describe the structures in the extracellular matrix that are held to be important in this process. Finally, the potentially harmful consequences of LDL linking by LPL and of other LPL actions in the arterial intima are briefly reviewed.

Phage display combinatorial libraries of short peptides: ligand selection for protein purification

A library of heptapeptides displayed on the surface of filamentous phage M13 was evaluated as a potential source of affinity ligands for the purification of Rhizomucor miehei lipase. Two independent selection (biopanning) protocols were employed: the enzyme was either physically adsorbed on polystyrene or chemically immobilized on small magnetic beads. From screening with the polystyrene-adsorbed lipase it was found that there was a rapid enrichment of the library with "doublet" clones i.e. the phage species which carried two consecutive sequences of heptapeptides, whilst no such clones were observed from the screening using lipase attached to magnetic beads. The binding of the best clones to the enzyme was unambiguously confirmed by ELISA. However the synthetic heptapeptide of identical sequence to the best "monomeric" clone did not act as a satisfactory affinity ligand after immobilization on Sepharose. This indicated that the interaction with lipase was due to both the heptapeptide and the presence of a part of the phage coat protein. This conclusion was further verified by immobilizing the whole phage on the surface of magnetic beads and using the resulting conjugate as an affinity adsorbent. The scope of application of this methodology and the possibility of preparing phage-based affinity materials are briefly discussed.

Contribution of the carboxy-terminal domain of lipoprotein lipase to interaction with heparin and lipoproteins

The C-terminal domain of lipoprotein lipase (LPL) is involved in several important interactions. To assess its contribution to the binding ability of full-length LPL we have determined kinetic constants using biosensor technique. The affinity of the C-terminal domain for heparin was about 500-fold lower than that of full-length LPL (K(d) = 1.3 microM compared to 3.1 nM). Replacement of Lys403, Arg405 and Lys407 by Ala abolished the heparin affinity, whereas replacement of Arg420 and Lys422 had little effect. The C-terminal domain increased binding of chylomicrons and VLDL to immobilized heparin relatively well, but was less than 10% efficient in binding of LDL compared to full-length LPL. Deletion of residues 390-393 (WSDW) did not change the affinity to heparin and only slightly decreased the affinity to lipoproteins. We conclude that the C-terminal folding domain contributes only moderately to the heparin affinity of full-length LPL, whereas the domain appears important for tethering triglyceride-rich lipoproteins to heparin-bound LPL.

Endothelial lipase: a new member of the triglyceride lipase gene family

The triglyceride lipase gene family plays a central role in intestinal lipid absorption, energy homeostasis, lipoprotein metabolism, and atherosclerosis. A new member of this gene family, termed endothelial lipase, was recently reported. The presence of key functional motifs, the endothelial synthesis, the enzymatic profile, and the in-vivo metabolic effects of endothelial lipase suggest that, like other members of this gene family, endothelial lipase may play a role in energy delivery to tissues and in modulating lipoprotein metabolism, and could impact on atherogenesis.

Colipase: structure and interaction with pancreatic lipase

Colipase is a small protein cofactor needed by pancreatic lipase for the efficient dietary lipid hydrolysis. It binds to the C-terminal, non-catalytic domain of lipase, thereby stabilising an active conformation and considerably increasing the overall hydrophobic binding site. Structural studies of the complex and of colipase alone have clearly revealed the functionality of its architecture. Interestingly, a structural analogy has recently been discovered between colipase and a domain in a developmental protein (Dickkopf), based on sequence analogy and homology modeling. Whether this structural analogy implies a common function (lipid interaction) remains to be clarified. Structural analogies have also been recognised between the pancreatic lipase C-terminal domain, the N-terminal domains of lipoxygenases and the C-terminal domain of alpha-toxin. These non-catalytic domains in the latter enzymes are important for interaction with membranes. It has not been established if these domains are also involved in eventual protein cofactor binding as is the case for pancreatic lipase.

Subdomain chimeras of hepatic lipase and lipoprotein lipase. Localization of heparin and cofactor binding

To specify and localize carboxyl-terminal domain functions of human hepatic lipase (HL) and human lipoprotein lipase (LPL), two subdomain chimeras were created in which portions of the carboxyl-terminal domain were exchanged between the two lipases. The first chimera (HL-LPLC1) was composed of residues 1-344 of human HL, residues 331-388 of human LPL, and residues 415-476 of human HL. The second chimera (HL-LPLC2) consisted of just two segments, residues 1-414 of human HL and residues 389-448 of human LPL. These chimeric constructs effectively divided the HL C-terminal domain into halves, with corresponding LPL sequences either in the first or second portion of that domain. Both chimeras were lipolytically active and hydrolyzed triolein emulsions to a similar extent compared with native HL and LPL. Heparin-Sepharose chromatography demonstrated that HL-LPLC1 and HL-LPLC2 eluted at 0.80 and 1.3 M NaCl, respectively, elution positions that corresponded to native HL and LPL. Hence, substitution of LPL sequences into the HL carboxyl-terminal domain resulted in the production of functional lipases, but with distinct heparin binding properties. In addition, HL-LPLC2 trioleinase activity was responsive to apoC-II activation, although the -fold stimulation was less than that observed with native LPL. Moreover, an apoC-II fragment (residues 44-79) was specifically cross-linked to LPL and HL-LPLC2, but not to HL or HL-LPLC1. Finally, both chimeras hydrolyzed phospholipid with a specific activity similar to that of HL, which was unaffected by the presence of apoC-II. These findings indicated that in addition to a region found within the amino-terminal domain of LPL, apoC-II also interacted with the last half of the carboxyl-terminal domain (residues 389-448) to achieve maximal lipolytic activation. In addition, the relative heparin affinity of HL and LPL was determined by the final 60 carboxyl-terminal residues of each enzyme.

Human pancreatic lipase: an exposed hydrophobic loop from the C-terminal domain may contribute to interfacial binding

Epitope mapping was performed using four anti-HPL monoclonal antibodies (mAb's 81-23, 146-40, 315-25, and 320-24) directed against human pancreatic lipase (HPL). Three HPL mutants produced in insect cells were tested for this purpose: (i) N-HPL, which consists of only the N-terminal domain of HPL, (ii) HPL(-lid), in which a short loop consisting of 5 amino acid residues replaces the full-length 23-residue lid domain present in HPL, and (iii) N-GPLRP2/C-HPL chimera, a chimeric mutant consisting of the N-terminal domain of the guinea pig pancreatic lipase related protein 2 (GPLRP2) fused to the C-terminal domain of HPL. The C-terminal domain of HPL (C-HPL) was prepared in a pure form after performing chymotryptic digestion of HPL. The mAb 146-40 recognizes HPL, HPL(-lid), and N-HPL but not GPLRP2, N-GPLRP2/C-HPL chimera, or the C-HPL. The antibody mAb 146-40 therefore specifically recognizes the N-terminal domain of HPL, and the epitope recognized does not include the amphiphilic lid. On the other hand, mAb's 81-23, 315-25, and 320-24 react specifically to the C-terminal domain of HPL, since they recognize HPL, HPL(-lid), the N-GPLRP2/C-HPL chimera, and the C-HPL but not N-HPL or GPLRP2. It was further established that these three mAb's recognize the same conformational epitope, the structure of which is stabilized by the N-terminal domain in the presence of SDS at concentrations greater than its critical micellar concentration. This conformational epitope was found to be located in the vicinity of Met 397 and Arg 414. These two residues delineate a highly exposed peptide stretch extending from the HPL C-terminal domain, which includes a hydrophobic surface loop (beta5'). Kinetic studies on the HPL/mAb's complexes showed that the lipase activity was much lower in these complexes than in HPL. The results of the present study suggest for the first time that the beta5' loop from the C-terminal domain may be involved in the interaction of HPL with a lipid/water interface.

Interaction of lipoproteins with heparan sulfate proteoglycans and with lipoprotein lipase. Studies by surface plasmon resonance technique

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.

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

Lipoprotein lipase (LpL) can mediate cellular uptake of chylomicron and VLDL remnants via binding to heparan sulfate proteoglycans (HSPG) and the endocytic alpha2-macroglobulin receptor/low density lipoprotein receptor-related protein (alpha2MR/LRP). Whereas it is established that the C-terminal folding domain binds to alpha2MR/LRP, it remains uncertain whether it binds to heparin and to HSPG. To identify segments important for binding to alpha2MR/LRP and to clarify possible binding to heparin, we produced constructs of the human C-terminal folding domain, LpL-(313-448), and of the fragment LpL-(347-448) in Escherichia coli. In addition to binding to alpha2MR/LRP, LpL-(313-448) displayed binding to heparin with an affinity similar to that of the LpL monomer, whereas it bound poorly to lipoprotein particles. Moreover, LpL-(313-448) displayed heparin sensitive binding to normal, but not to HSPG deficient Chinese hamster ovary cells. LpL-(313-448) and LpL-(347-448) showed similar affinities for binding to both purified alpha2MR/LRP and to heparin. Deletion of LpL residues 380-384 abolished the binding to LRP, whereas binding to heparin was unperturbed. The binding to both heparin and alpha2MR/LRP was essentially abolished following deletion of residues 404-430, and pretreatment of CHO cells with the peptide comprising aa 402-423 inhibited the binding of LpL-(313-448). We conclude that the C-terminal folding domain of human LpL has a site for binding to heparin and to HSPG, presumably involving amino acids within residues 404-430. Two segments of the domain are necessary for efficient binding to alpha2MR/LRP, one comprising residues 380-384 and another overlapping the segment important for binding to heparin.

Mutation of tryptophan residues in lipoprotein lipase. Effects on stability, immunoreactivity, and catalytic properties

Previous studies had pointed to an important function of a putative exposed loop in the C-terminal domain of lipoprotein lipase for activity against emulsified lipid substrates. This loop contains 3 tryptophan residues (Trp390, Trp393, and Trp394). We have expressed and characterized lipase mutants with tryptophan to alanine substitutions at positions 55, 114, 382, 390, 393, and 394 and a double mutant at residues 393 and 394. The substitutions in the N-terminal domain (W55A and W114A) led to poor expression of completely inactive lipase variants. Heparin-Sepharose chromatography showed that mutant W114A eluted at the same salt concentration as inactive wild-type monomers, indicating that this substitution prevented subunit interaction or led to an unstable dimer. In contrast, all mutants in the C-terminal domain were expressed as mixtures of monomers and dimers similarly to the wild-type. The dimers displayed at least some catalytic activity and had the same apparent heparin affinity as the active wild-type dimers. The mutants W390A, W393A, W394A, and W393A/W394A had decreased reactivity with the monoclonal antibody 5D2, indicating that the 5D2 epitope is longer than was reported earlier, or that conformational changes affecting the epitope had occurred. The mutants W390A, W393A, W394A, and W393A/W394A had decreased catalytic activity against a synthetic lipid emulsion of long-chain triacylglycerols (IntralipidR) and in particular against rat lymph chylomicrons. The most pronounced decrease of activity was found for the double mutant W393A/W394A which retained only 6% of the activity of the wild-type lipase, while 70% of the activity against water-soluble tributyrylglycerol was retained. In the case of chylomicrons also the affinity for the substrate particles was lowered, as indicated by severalfold higher apparent Km values. This effect was less prominent with the synthetic lipid emulsion. We conclude that the tryptophan cluster Trp390-Trp393-Trp394 contributes to binding of lipoprotein lipase to lipid/water interfaces. Utilizing different lipid substrates in different physical states, we have demonstrated that the tryptophan residues in the C-terminal domain may have a role also in the productive orientation of the enzyme at the lipid/water interface.

Interaction of lipoprotein lipase with heparin fragments and with heparan sulfate: stoichiometry, stabilization, and kinetics

The interaction of lipoprotein lipase (LPL) with heparan sulfate and with size-fractionated fragments of heparin was characterized by several approaches (stabilization, sedimentation, surface plasmon resonance, circular dichroism, fluorescence). The results show that heparin decasaccharides form a 1:1 complex with dimeric LPL and that decasaccharides are the shortest heparin fragments which can completely satisfy the heparin binding regions in dimeric LPL. Equimolar concentrations of octasaccharides also stabilized dimeric LPL, while shorter fragments (hexa- and tetrasaccharides) were less efficient. Binding of heparin did not induce major rearrangements in the conformation of LPL, supporting the view that the heparin binding region is preformed in the native structure. Interaction of LPL with heparan sulfate, as studied by surface plasmon resonance, was found to be a fast exchange process characterized by a high value for the association rate constant, 1.7 x 10(8) M-1 s-1, a relatively high dissociation rate constant, 0.05 s-1, and as a result a very low equilibrium dissociation constant equal to 0.3 nM at 0.15 M NaCl. The contribution of electrostatics was estimated to be 44% for the binding of LPL to heparan sulfate, 49% for the binding of LPL to unfractionated heparin, and 60% for the binding of LPL to affinity-purified heparin decasaccharides at 0.15 M NaCl. The number of ionic interactions between LPL and high-affinity decasaccharides was estimated to be 10. We propose an essential role of electrostatic steering in the association. Monomeric LPL had 6000-fold lower affinity for heparin than dimeric LPL had, expressed as a ratio of equilibrium dissociation constants. A model for binding of LPL to heparan sulfate-covered surfaces is proposed. Due to the fast rebinding, LPL is concentrated to the close proximity of the heparan sulfate surface. As the dissociation is also fast, the enzyme exchanges rapidly between specific binding sites on the immobilized heparan sulfate, without leaving the surface. This model may also apply to LPL at the endothelium of blood vessels.

New aspects on the role of plasma lipases in lipoprotein catabolism and atherosclerosis

The function of lipoprotein lipase (LpL) and hepatic lipase (HL) has been related to atherogenesis by several authors in the past, but convincing experimental and epidemiological evidence to support this hypothesis has been obtained only in the last years. For both enzymes, next to their role in the hydrolysis of triglyceride-rich lipoproteins, a second important function has been described recently. Both lipases can mediate the binding and subsequent uptake of lipoproteins into cells. Although this function has been clearly demonstrated in vitro for various cell types, the physiological or pathophysiological relevance remains hypothetical until final elucidation in vivo.

Construction and functional characterization of recombinant fusion proteins of human lipoprotein lipase and apolipoprotein CII

The hydrolysis of triacylglycerols of chylomicrons and very low density lipoproteins by lipoprotein lipase (LPL) requires the presence of apolipoprotein (apo) CII as a cofactor. To obtain further information on the interaction of apo CII and LPL, we generated two fusion proteins consisting of the complete LPL molecule and the mature form of apo CII. The cDNAs of both proteins were either connected directly or by a segment encoding a 16-amino-acid linker peptide. The fused cDNAs were stably expressed in human embryonic kidney (HEK) 293 cells and the enzymic properties of the recombinant proteins were examined. The fusion proteins hydrolysed both emulsified long-chain (lipase) triacyglycerol substrate and a water-soluble short-chain (esterase) fatty acid ester substrate (p-nitrophenylbutyrate), regardless of whether or not they contained the linker peptide. In the absence of exogenous apo CII, the fusion proteins had up to 3.5-times higher basal activity than wild-type LPL. Similar to wild-type LPL, the fusion proteins were inhibited by 1 M NaCl, however less than wild-type LPL. A polyclonal antibody specific for apo CII impaired their ability to hydrolyse triacylglycerol emulsions. A similar effect was seen when the tetrapeptide KGEE was used as inhibitor, which corresponds to the carboxy-terminal four amino acids of apo CII.

Molecular pathobiology of the human lipoprotein lipase gene

Lipoprotein lipase (LPL; E.C. 3.1.1.34) is a key enzyme in the metabolism of lipids. Many diseases, including obesity, coronary heart disease, chylomicronemia (pancreatitis), and atherosclerosis, appear to be directly or indirectly related to abnormalities in LPL function. Human LPL is a member of a superfamily of lipases that includes hepatic lipase and pancreatic lipase. These lipases are characterized by extensive homology, both at the level of the gene and the mature protein, suggesting that they have a common evolutionary origin. A large number of natural mutations have been discovered in the human LPL gene, which are located at different sites in the gene and affect different functions of the mature protein. There is a high prevalence of two of these mutations (207 and 188) in the Province of Québec, and one of them (207) is almost exclusive to the French-Canadian population. A study of these and other naturally occurring mutant LPL molecules, as well as those created in vitro by site-directed mutagenesis, indicate that the sequence of LPL is organized into multiple structural and functional units that act in concert in the normal enzyme. In this review, we discuss the interrelationships of LPL structure and its function, the molecular etiology of abnormal LPL in humans, and the clinical and therapeutic aspects of LPL deficiency.

Human hepatic and lipoprotein lipase: the loop covering the catalytic site mediates lipase substrate specificity

Hepatic lipase (HL) and lipoprotein lipase (LPL) are key enzymes that mediate the hydrolysis of triglycerides (TG) and phospholipids (PL) present in circulating plasma lipoproteins. Relative to triacylglycerol hydrolysis, HL displays higher phospholipase activity than LPL. The structural basis for this difference in substrate specificity has not been definitively established. We recently demonstrated that the 22-amino acid loops ("lids") covering the catalytic sites of LPL and HL are critical for the interaction with lipid substrate (Dugi, K.A., Dichek, H.L., Talley, G.D., Brewer, H.B., Jr., and Santamarina-Fojo, S. (1992) J. Biol. Chem. 267, 25086-25091). To determine whether the lipase lid plays a role in conferring the different substrate specificities of HL and LPL, we have generated four chimeric lipases. Characterization of these chimeric enzymes using TG (triolein and tributyrin) or PL (dioleoylphosphatidylcholine (DOPC) vesicles, DOPC proteoliposomes, and DOPC-mixed liposomes) substrates demonstrated marked differences between their relative PL/TG hydrolyzing activities. Chimeric LPL containing the lid of HL had reduced triolein hydrolyzing activity (49% of the wild type), but increased phospholipase activity in DOPC vesicle, DOPC proteoliposome, and DOPC-mixed liposome assay systems (443, 628, and 327% of wild-type LPL, respectively). In contrast, chimeric HL containing the LPL lid was more active against triolein (123% of the wild type) and less active against DOPC (23, 0, and 30%, respectively) than normal HL. Similar results were obtained when the lipase lids were exchanged in chimeric enzymes containing the NH2-terminal end of LPL and the COOH-terminal domain of HL. Exchange of the LPL and HL lids resulted in a reversal of the phospholipase/neutral lipase ratio, establishing the important role of this region in mediating substrate specificity. In summary, the lid covering the catalytic domains in LPL and HL plays a crucial role in determining lipase substrate specificity. The lid of LPL confers preferential triglyceride hydrolysis, whereas the lid of HL augments phospholipase activity. This study provides new insight into the structural basis for the observed in vivo differences in LPL and HL function.

Lipase structures at the interface between chemistry and biochemistry

In this chapter we review recent molecular knowledge on two structurally related mammalian triglyceride lipases which have evolved from a common ancestral gene. The common property of the lipase family members is that they interact with non-polar substances. Pancreatic lipase hydrolyzes triglycerides in the small intestine in the presence of many dietary components, other digestive enzymes and high concentrations of detergents (bile salts). Lipoprotein lipase acts at the vascular side of the blood vessels where it hydrolyses triglycerides and some phospholipids of the circulating plasma lipoproteins. A third member of the gene family, hepatic lipase, is found in the liver of mammals. Also, this lipase is involved in lipoprotein metabolism. The three lipases are distantly related to some non-catalytic yolk proteins from Drosophila (Persson et al., 1989; Kirchgessner et al., 1989; Hide et al., 1992) and to a phospholipase A1 from hornet venom (Soldatova et al., 1993).

The role of alpha 2M receptor/LRP in chylomicron remnant metabolism

A strong candidate for the long-searched CR receptor might be the alpha 2MR/LRP. Presently, we are overseeing a whole series of in vitro experiments from different laboratories that show that LRP expresses all the features for being such a receptor protein. LRP is localized on the liver cell surface, as well as on most other animal cells. It recognizes apo E-enriched lipoproteins as beta-VLDL and CR. There is evidence that CR contain LPL and it has been demonstrated that LPL binds with high affinity to LRP. This has been shown in cell binding experiments with subsequent cross-linking and in direct assays on purified receptor protein. HL, which is expressed in liver cells and localized at the liver cell surface, is also able to bind to LRP. Moreover, LRP is found in endosomes and can mediate the uptake of beta-VLDL and CR. Further studies are necessary to evaluate its role in vivo as well as its regulation. The interplay between the different ligands of this large multifunctional receptor protein needs to be clarified. It should be emphasized here that, by describing LPL as a new mediator of CR uptake in the liver and by providing evidence for a direct interaction between LPL and LRP, the role of LRP in the remnant catabolism has become even more likely.

Lipoprotein lipase: structure, function and mechanism of action

Lipoprotein lipase (LPL) plays a central role in the hydrolysis of circulating triglycerides present in chylomicrons, and very low density lipoproteins. The active form of the enzyme is a non-covalent homodimer which contains multiple functional domains required for normal hydrolytic activity including a catalytic domain, as well as sites involved in co-factor, heparin and lipid binding. Recent studies involving site-directed mutagenesis, the elucidation of the three dimensional crystallographic structure of different lipases, as well as analysis of the molecular defects that result in the expression of the familial chylomicronemia syndrome have provided new insights into the structure-function relationship of LPL. As a result, our understanding of structural domains involved in catalysis, heparin, lipid binding, and enzyme-cofactor interaction as well as the mechanism of action of LPL as an acylglycerol hydrolase has been greatly enhanced.

Interactions of lipoprotein lipase with the active-site inhibitor tetrahydrolipstatin (Orlistat)

Lipoprotein lipase (LPL) was rapidly inactivated by low concentrations of the active-site inhibitor tetrahydrolipstatin (THL). The presence of amphiphils (e.g. long-chain fatty acids) or of lipid/water interfaces (lipid emulsions) was required for inhibition to occur. Apolipoprotein CII increased the maximal inactivation rate constant by 1.8-fold in the presence of an emulsion of long-chain triacylglycerols, but had no effect in the presence of an emulsion of tributyrylglycerol. The fully inhibited enzyme had a ratio of THL/LPL of nearly 2, indicating that both subunits of the LPL homo-dimer bound THL. The THL-LPL complex was stable below pH 7.5. At higher pH reactivation occurred indicating that THL was slowly turned over by the enzyme. The apparent reactivation rate constant was increased about threefold by the presence of lipid/water interfaces. Sucrose density gradient centrifugation revealed that THL induces tetramerisation of LPL. This aggregation was reversible on reactivation of the inhibited enzyme. Binding to heparin was not affected by THL. In contrast, binding to lipid droplets and to lipoproteins was increased, indicating exposure of hydrophobic regions in the inhibited LPL. It is suggested that THL induces local conformational changes in LPL, which may involve opening of the putative surface lid structure which covers the active-site.

The C-terminus of lipoprotein lipase is essential for biological function but contains no domain for glycosylphosphatidylinositol anchoring

In this study we present evidence that the C-terminus of lipoprotein lipase contains no glycosylphosphatidylinositol addition signal and is therefore not a glycosylphosphatidylinositol-anchored protein. Furthermore, we present additional evidence that the C-terminus of lipoprotein lipase is essential for biological function. Flow cytometric analysis and enzyme-activity monitoring experiments revealed no pool of lipoprotein lipase releasable by phosphatidylinositol-specific phospholipase present on the membrane of COS cells transfected with the human lipoprotein lipase gene while, in contrast, a heparin-releasable pool could be demonstrated. [14C]Ethanolamine, a constituent of the glycosylphosphatidylinositol anchor, was not incorporated into lipoprotein lipase during metabolic labeling. C-terminal deletion mutants were constructed and expressed in COS cells to investigate the presence of glycosylphosphatidylinositol addition signal on the C-terminus of human lipoprotein lipase (LPL). The specific activities of the mutants M442 [des-(Leu443-Gly448)-LPL] and M437 [des-(Cys438-Gly448)-LPL] were 78% and 59%, respectively, less than the wild type, while the M432 mutant [des-(Ala433-Gly449)-LPL] was catalytically inactive. Determination of the stability of the mutants revealed a decreased stability of the M437, compared with wild-type, whereas M442 showed the same stability. Flow cytometric analysis showed sustained membrane expression for all mutants including the inactive M432 mutant. These results suggest that the C-terminus of lipoprotein lipase is essential for maintaining intact catalytic activity but is not involved in any posttranslational proteolytic processing, including cleavage of a glycosylphosphatidylinositol addition signal. We therefore conclude that membrane-binding of the lipase is not mediated by such anchoring.