Human hepatic lipase subunit structure determination

The screening of lipase inhibitors based on the metal-organic framework Zeolitic Imidazolate Framework-8-immobilized enzyme microreactor

An online capillary electrophoresis method based-lipase immobilized enzyme microreactor was developed for lipase kinetic study and inhibitor screening from compounds from natural products. Zeolitic Imidazolate Framework-8 (ZIF-8) has the advantages of large pore size, mild synthesis conditions and good biocompatibility. Lipase was immobilized on the inner wall of capillary with the help of the metal-organic framework ZIF-8. The results of electron microscopy showed that lipase could be aggregated and fixed on the inner wall of capillary by ZIF-8. After the experimental conditions including electrophoretic separation and enzymatic reaction were optimized, the baseline separation of substrate p-nitrophenyl acetate (pNPA) and product p-nitrophenol (pNP) was achieved within 3 min. The immobilized enzyme microreactor showed good repeatability and stability, and the determined Michaelis-Menten constant (Km) of lipase was 2.75 mM, which was lower than the kinetic constant determined in off-line reaction, indicating that the immobilized enzyme had a high affinity with the substrate. In addition, the IC50 value of the positive control compound orlistat on lipase inhibition was 7.26 nM, which was consistent with the literature. Then the inhibitory activity of 10 compounds from natural products on lipase was evaluated by the ZIF-8-IMER. Among them, 7 compounds including baicalein, luteolin, epicatechin gallic acid, and chlorogenic acid, had a certain inhibitory effect on lipase. The molecular docking technology proved the interaction between the enzyme and the screened inhibitor, which provides a new method for the screening of lipase inhibitors.

Lyso-Lipid-Induced Oligodendrocyte Maturation Underlies Restoration of Optic Nerve Function

Protein hyperdeimination and deficiency of lyso-phospholipids (LPC 18:1) has been associated with the pathology of demyelinating disease in both humans and mice. We uncovered interesting biology of LPC 18:1, in which LPC 18:1 induced optic nerve function restoration through oligodendrocyte maturation and remyelination in mouse model systems. Our in vitro studies show LPC 18:1 protection against neuron-ectopic hyperdeimination and stimulation of oligodendrocyte maturation, while in vivo investigations recorded optic nerve function improvement following optic nerve injections of LPC 18:1, in contrast with LPC 18:0. Thus, just a change in a single bond renders a dramatic alternation in biological function. The incorporation of isobaric C13-histidine in newly synthesized myelin proteins and quantitative proteome shifts are consistent with remyelination underlying restoration in optic nerve function. These results suggest that exogenous LPC 18:1 may provide a therapeutic avenue for stemming vision loss in demyelinating diseases.

Capillary electrophoresis for enzyme-based studies: Applications to lipases and kinases

Capillary electrophoresis (CE) is a powerful technique continuously expanding into new application fields. One of these applications involves the study of enzymes, their catalytic activities and the alteration of this activity by specific ligands. In this review, two model enzymes, lipases and kinases, will be used since they differ substantially in their modes of action, reaction requirements and applications making them perfect subjects to demonstrate the advantages and limitations of CE-based enzymatic assays. Indeed, the ability to run CE in various operation modes and hyphenation to different detectors is essential for lipase-based studies. Additionally, the low sample consumption provided by CE promotes it as a promising technique to assay human and viral nucleoside kinases. Undeniably, these are rarely commercially available enzymes and must be frequently produced in the laboratory, a process which requires special sets of skills. CE-based lipase and kinase reactions can be performed outside the capillary (pre-capillary) where the reactants are mixed in a vial prior to their separation or, inside the capillary (in-capillary) where the reactants are mixed before the electrophoretic analysis. These enzyme-based applications of CE will be compared to those of liquid chromatography-based applications in terms of advantages and limitations. Binding assays based on affinity CE and the compelling microscale thermophoresis (MST) will be briefly presented as they allow a broad understanding of the molecular mechanism behind ligand binding and of the resulting modulation in activity.

Comparative studies of vertebrate lipoprotein lipase: a key enzyme of very low density lipoprotein metabolism

Lipoprotein lipase (LIPL or LPL; E.C.3.1.1.34) serves a dual function as a triglyceride lipase of circulating chylomicrons and very-low-density lipoproteins (VLDL) and facilitates receptor-mediated lipoprotein uptake into heart, muscle and adipose tissue. Comparative LPL amino acid sequences and protein structures and LPL gene locations were examined using data from several vertebrate genome projects. Mammalian LPL genes usually contained 9 coding exons on the positive strand. Vertebrate LPL sequences shared 58-99% identity as compared with 33-49% sequence identities with other vascular triglyceride lipases, hepatic lipase (HL) and endothelial lipase (EL). Two human LPL N-glycosylation sites were conserved among seven predicted sites for the vertebrate LPL sequences examined. Sequence alignments, key amino acid residues and conserved predicted secondary and tertiary structures were also studied. A CpG island was identified within the 5'-untranslated region of the human LPL gene which may contribute to the higher than average (×4.5 times) level of expression reported. Phylogenetic analyses examined the relationships and potential evolutionary origins of vertebrate lipase genes, LPL, LIPG (encoding EL) and LIPC (encoding HL) which suggested that these have been derived from gene duplication events of an ancestral neutral lipase gene, prior to the appearance of fish during vertebrate evolution. Comparative divergence rates for these vertebrate sequences indicated that LPL is evolving more slowly (2-3 times) than for LIPC and LIPG genes and proteins.

Vertebrate hepatic lipase genes and proteins: a review supported by bioinformatic studies

Hepatic lipase (gene: LIPC; enzyme: HL; E.C.3.1.1.3) is one of three members of the triglyceride lipase family that contributes to vascular lipoprotein degradation and serves a dual role in triglyceride hydrolysis and in facilitating receptor-mediated lipoprotein uptake into the liver. Amino acid sequences, protein structures, and gene locations for vertebrate LIPC (or Lipc for mouse and rat) genes and proteins were sourced from previous reports and vertebrate genome databases. Lipc was distinct from other neutral lipase genes (Lipg encoding endothelial lipase and Lpl encoding lipoprotein lipase [LPL]) and was located on mouse chromosome 9 with nine coding exons on the negative strand. Exon 9 of human LIPC and mouse and rat Lipc genes contained "stop codons" in different positions, causing changes in C-termini length. Vertebrate HL protein subunits shared 58%-97% sequence identities, including active, signal peptide, disulfide bond, and N-glycosylation sites, as well as proprotein convertase ("hinge") and heparin binding regions. Predicted secondary and tertiary structures revealed similarities with the three-dimensional structure reported for horse and human pancreatic lipases. Potential sites for regulating LIPC gene expression included CpG islands near the 5″-untranslated regions of the mouse and rat LIPC genes. Phylogenetic analyses examined the relationships and potential evolutionary origins of the vertebrate LIPC gene family with other neutral triglyceride lipase gene families (LIPG and LPL). We conclude that the triglyceride lipase ancestral gene for vertebrate neutral lipase genes (LIPC, LIPG, and LPL) predated the appearance of fish during vertebrate evolution.

Lysophosphatidic acid is a lipid mediator with wide range of biological activities. Biosynthetic pathways and mechanism of action

Lysophosphatidic acid (LPA) is a lipid mediator required for maintaining homeostasis of numerous physiological functions and also involved in development of some pathological processes through interactions with G protein-coupled receptors. Recently many data have appeared about the role of this phospholipid in humans, but pathways of LPA biosynthesis and mechanisms of its action remain unclear. This review presents modern concepts about biosynthesis, reception, and biological activity of LPA in humans. Natural and synthetic LPA analogs are considered in the view of their possible use in pharmacology as agonists and/or antagonists of G protein-coupled receptors of LPA.

Identification of the active form of endothelial lipase, a homodimer in a head-to-tail conformation

Endothelial lipase (EL) is a member of a subfamily of lipases that act on triglycerides and phospholipids in plasma lipoproteins, which also includes lipoprotein lipase and hepatic lipase. EL has a tropism for high density lipoprotein, and its level of phospholipase activity is similar to its level of triglyceride lipase activity. Inhibition or loss-of-function of EL in mice results in an increase in high density lipoprotein cholesterol, making it a potential therapeutic target. Although hepatic lipase and lipoprotein lipase have been shown to function as homodimers, the active form of EL is not known. In these studies, the size and conformation of the active form of EL were determined. Immunoprecipitation experiments suggested oligomerization. Ultracentrifugation experiments showed that the active form of EL had a molecular weight higher than the molecular weight of a simple monomer but less than a dimer. A construct encoding a covalent head-to-tail homodimer of EL (EL-EL) was expressed and had similar lipolytic activity to EL. The functional molecular weights determined by radiation inactivation were similar for EL and the covalent homodimer EL-EL. We previously showed that EL could be cleaved by proprotein convertases, such as PC5, resulting in loss of activity. In cells overexpressing PC5, the covalent homodimeric EL-EL appeared to be more stable, with reduced cleavage and conserved lipolytic activity. A comparative model obtained using other lipase structures suggests a structure for the head-to-tail EL homodimer that is consistent with the experimental findings. These data confirm the hypothesis that EL is active as a homodimer in head-to-tail conformation.

Angptl4 upregulates cholesterol synthesis in liver via inhibition of LPL- and HL-dependent hepatic cholesterol uptake

BACKGROUND

Dysregulation of plasma lipoprotein levels may increase the risk for atherosclerosis. Recently, angiopoietin-like protein 4, also known as fasting-induced adipose factor Fiaf, was uncovered as a novel modulator of plasma lipoprotein metabolism. Here we take advantage of the fasting-dependent phenotype of Angptl4-transgenic (Angptl4-Tg) mice to better characterize the metabolic function of Angptl4.

METHODS AND RESULTS

In 24-hour fasted mice, Angptl4 overexpression increased plasma triglycerides (TG) by 24-fold, which was attributable to elevated VLDL-, IDL/LDL- and HDL-TG content. Angptl4 overexpression decreased post-heparin LPL activity by stimulating conversion of endothelial-bound LPL dimers to circulating LPL monomers. In fasted but not fed state, Angptl4 overexpression severely impaired LPL-dependent plasma TG and cholesteryl ester clearance and subsequent uptake of fatty acids and cholesterol into tissues. Consequently, hepatic cholesterol content was significantly decreased, leading to universal upregulation of cholesterol and fatty acid synthesis pathways and increased rate of cholesterol synthesis.

CONCLUSIONS

The hypertriglyceridemic effect of Angptl4 is attributable to inhibition of LPL-dependent VLDL lipolysis by converting LPL dimers to monomers, and Angptl4 upregulates cholesterol synthesis in liver secondary to inhibition of LPL- and HL-dependent hepatic cholesterol uptake.

KIT is required for hepatic function during mouse post-natal development

Background

The Kit gene encodes a receptor tyrosine kinase involved in various biological processes including melanogenesis, hematopoiesis and gametogenesis in mice and human. A large number of Kit mutants has been described so far showing the pleiotropic phenotypes associated with partial loss-of-function of the gene. Hypomorphic mutations can induce a light coat color phenotype while complete lack of KIT function interferes with embryogenesis. Interestingly several intermediate hypomorphic mutations induced in addition growth retardation and post-natal mortality.

Results

In this report we investigated the post-natal role of Kit by using a panel of chemically-induced hypomorphic mutations recently isolated in the mouse. We found that, in addition to the classical phenotypes, mutations of Kit induced juvenile steatosis, associated with the downregulation of the three genes, VldlR, Lpin1 and Lpl, controlling lipid metabolism in the post-natal liver. Hence, Kit loss-of-functions mimicked the inactivation of genes controlling the hepatic metabolism of triglycerides, the major source of energy from maternal milk, leading to growth and viability defects during neonatal development.

Conclusion

This is a first report involving KIT in the control of lipid metabolism in neonates and opening new perspectives for understanding juvenile steatosis. Moreover, it reinforces the role of Kit during development of the liver and underscores the caution that should be exerted in using KIT inhibitors during anti-cancer treatment.

Structure and function of extracellular phospholipase A1 belonging to the pancreatic lipase gene family

Phospholipase A1 (PLA1) is an enzyme that hydrolyzes phospholipids and produces 2-acyl-lysophospholipids and fatty acids and is conserved in a wide range of organisms. Mammals have several enzymes that exhibit PLA1 activity in vitro. The extracellular PLA1s include phosphatidylserine (PS)-specific PLA1 (PS-PLA1), membrane-associated phosphatidic acid (PA)-selective PLA1s (mPA-PLA1alpha and mPA-PLA1beta), hepatic lipase (HL), endothelial lipase (EL) and pancreatic lipase-related protein 2 (PLRP2), all of which belong to the pancreatic lipase gene family. The former three PLA1s differ from other members in their substrate specificities, structural features and gene organizations, and form a subfamily in the pancreatic lipase gene family. PS-PLA1, mPA-PLA1alpha and mPA-PLA1beta exhibit only PLA1 activity, while HL, EL and PLRP2 show triacylglycerol-hydrolyzing activity in addition to PLA1 activity. The tertiary structures of lipases have two surface loops, the lid and the beta9 loop. The lid and the beta9 loop cover the active site in its closed conformation. An alignment of amino acid sequences of the pancreatic lipase gene family members revealed two molecular characteristics of PLA1s in the two surface loops. First, lipase members exhibiting PLA1 activity (PS-PLA1, mPA-PLA1alpha and mPA-PLA1beta, EL, guinea pig PLRP2 and PLA1 from hornet venom (DolmI)) have short lids. Second, PS-PLA1, mPA-PLA1alpha, mPA-PLA1beta and DolmI, which exhibit only PLA(1) activity, have short beta9 loops. Thus, the two surface loops appear to be involved in the ligand recognition. PS-PLA1 and mPA-PLA1s specifically hydrolyze PS and PA, respectively, producing their corresponding lysophospholipids. Lysophosphatidylserine and lysophosphatidic acid have been defined as lipid mediators with multiple biological functions. Thus, these PLA1s have a role in the production of these lysophospholipid mediators.

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.

Mapping the heparin-binding domain of human hepatic lipase

Human hepatic lipase (HL) is known to bind to the cell surface of hepatocytes and the sinusoidal endothelium of the liver. In each case, it appears that the enzyme remains associated with the cell surface through an ionic interaction with heparan sulfate proteoglycans. However, it remains unclear as to which residues are responsible for this critical function of the enzyme. In the present study, we have used a systematic approach to map the heparin-binding regions of human HL by utilizing peptide arrays spanning the complete sequence of the mature protein. Following probing with biotin-heparin, six peptides spanning residues 301-320 and 465-476 were identified as regions binding to heparin. Probing of an additional array containing these six parent peptides and a comprehensive series of mutant peptides identified two putative HL heparin-binding domains. The first was composed of residues R310, K312, K314, and R315 at the distal N-terminal domain and the second was composed of residues R473, K474, and R476 at the C-terminal end of the protein.

Target size analysis by radiation inactivation: the use of free radical scavengers

Several model systems were employed to assess indirect effects that occur in the process of using radiation inactivation analysis to determine protein target sizes. In the absence of free radical scavengers, such as mannitol and benzoic acid, protein functional unit sizes can be drastically overestimated. In the case of glutamate dehydrogenase, inclusion of free radical scavengers reduced the apparent target size from that of a hexamer to that of a trimer based on enzyme activity determinations. For glucose-6-phosphate dehydrogenase, the apparent target size was reduced from a dimer to a monomer. The target sizes for both glutamate dehydrogenase and glucose-6-phosphate dehydrogenase in the presence of free radical scavengers corresponded to subunit sizes when determinations of protein by sodium dodecyl sulfate-polyacrylamide gel electrophoresis or immunoblotting were done rather than enzyme activity. The free radical scavengers appear to compete with proteins for damage by secondary radiation products, since irradiation of these compounds can result in production of inhibitory species. Addition of benzoic acid/mannitol to samples undergoing irradiation was more effective in eliminating secondary damage than were 11 other potential free radical scavenging systems. Addition of a free radical scavenging system enables more accurate functional unit size determinations to be made using radiation inactivation analysis.

Severe hypoalphalipoproteinemia in mice expressing human hepatic lipase deficient in binding to heparan sulfate proteoglycan

Unlike human hepatic lipase (hHL) that is mainly cell surface-anchored via binding to heparan sulfate proteoglycans (HSPG), mouse HL (mHL) has a low affinity to HSPG and thus is largely blood-borne. The reduced HSPG binding of mHL is attributable to the C-terminal amino acids. To determine the functions of HSPG binding of hHL in vivo, we created adenovirus vectors encoding hHL or a chimeric protein (designated hHLmt) in which the C-terminal HSPG-binding sequences were replaced with the corresponding mouse sequences. Injecting hHLmt-expressing virus into C57BL/6J mice (1.8 x 10(10) virus particles/mouse) resulted in a 3-fold increase in pre-heparin HL activity, whereas infection with an identical dose of hHL virus did not change pre-heparin HL activity. In hHLmt-expressing mice, the concentration of total cholesterol and phospholipids was inversely related to the hHL activity in pre-heparin plasma in a dose- and time-dependent manner, and the decrease was mainly attributable to high density lipoproteins (HDL) cholesterol and HDL phospholipids. The expression of hHL exhibited no change in plasma total cholesterol or phospholipid levels as compared with control mice infected with luciferase or injected with saline. The reduced HDL lipids in the hHLmt-expressing mice were accompanied by markedly decreased plasma and hepatic apolipoprotein (apo) A-I. In primary hepatocytes isolated from hHLmt-expressing mice, the concentration of cell-associated and secreted apoA-I was decreased by 2-3-fold as compared with hepatocytes isolated from control mice, whereas the levels of apoB and apoE were unaltered. Infection of primary hepatocytes with hHLmt virus ex vivo also resulted in reduced apoA-I secretion but had no effect on cell-associated apoA-I. These results suggest that expression of HSPG binding-deficient hHL has a profound HDL-lowering effect.

Endothelial Lipase Promotes the Catabolism of ApoB-Containing Lipoproteins

Endothelial lipase (EL) has been found to be a key enzyme in high-density lipoprotein (HDL) metabolism in mice, leading to the concept that inhibition of EL could be a novel strategy for raising HDL cholesterol levels. However, mice are "HDL animals" and the effect of EL on atherogenic apoB-containing lipoproteins has not been elucidated. We previously found that EL is capable of hydrolyzing very low-density lipoprotein (VLDL) and LDL lipids ex vivo. To investigate the role of EL in the metabolism of apoB-containing lipoproteins in vivo, we expressed human EL in three mouse models of elevated apoB-containing lipoproteins: apoE-deficient, LDL receptor-deficient, and human apoB transgenic mice. Unexpectedly, hepatic expression of EL resulted in markedly decreased levels of VLDL/LDL cholesterol, phospholipid, and apoB accompanied by significantly increased LDL apolipoprotein and phospholipid catabolism. To determine whether lipolytic activity is required for this effect, we also expressed a catalytically inactive form of human EL (ELS149A); unexpectedly, expression of ELS149A did not lower and in fact increased plasma lipids. Coexpression and coimmunoprecipitation studies suggested that catalytically inactive ELS149A inhibits endogenous mouse EL, accounting for the increased lipid levels. We conclude that (1) in addition to its known effects on HDL metabolism, EL influences the metabolism of apoB-containing particles; (2) catalytic activity of EL is required for its effects on apoB-containing lipoproteins; and (3) overexpressed catalytically inactive EL inhibits endogenous mouse EL, resulting in increased levels of plasma lipids. In light of these results, inhibition of EL has the potential to raise levels of atherogenic lipoproteins in addition to HDL-C levels.

Maturation of hepatic lipase. Formation of functional enzyme in the endoplasmic reticulum is the rate-limiting step in its secretion

Among three lipases in the lipase gene family, hepatic lipase (HL), lipoprotein lipase, and pancreatic lipase, HL exhibits the lowest intracellular specific activity (i.e. minimal amounts of catalytic activity accompanied by massive amounts of inactive lipase mass in the endoplasmic reticulum (ER)). In addition, HL has a distinctive sedimentation profile, where the inactive mass overlaps the region containing active dimeric HL and trails into progressively larger molecular forms. Eventually, at least half of the HL inactive mass in the ER reaches an active, dimeric conformation (t(1/2) = 2 h) and is rapidly secreted. The remaining inactive mass is degraded. HL maturation occurs in the ER and is strongly dependent on binding to calnexin in the early co-/post-translational stages. Later stages of HL maturation occur without calnexin assistance, although inactive HL at all stages appears to be associated in distinct complexes with other ER proteins. Thus, unlike other lipases in the gene family, HL maturation is the rate-limiting step in its secretion as a functional enzyme.

The amino acid sequences of the carboxyl termini of human and mouse hepatic lipase influence cell surface association

Human hepatic lipase (hHL) mainly exists cell surface bound, whereas mouse HL (mHL) circulates in the blood stream. Studies have suggested that the carboxyl terminus of HL mediates cell surface binding. We prepared recombinant hHL, mHL, and chimeric proteins (hHLmt and mHLht) in which the carboxyl terminal 70 amino acids of hHL were exchanged with the corresponding sequence from mHL. The hHL, mHL, and hHLmt proteins were catalytically active using triolein and tributyrin as substrates. In transfected cells, the majority of hHLs bound to the cell surface, with only 4% of total extracellular hHL released into heparin-free media, whereas under the same conditions, 61% of total extracellular mHLs were released. Like mHL, hHLmt showed decreased cell surface binding, with 68% of total extracellular hHLmt released. To determine the precise amino acid residues involved in cell surface binding, we prepared a truncated hHL mutant (hHL471) by deleting the carboxyl terminal five residues (KRKIR). The hHL471 also retained hydrolytic activity with triolein and tributyrin, and showed decreased cell surface binding, with 40% of total extracellular protein released into the heparin-free media. These data suggest that the determinants of cell surface binding exist within the carboxyl terminal 70 amino acids of hHL, of which the last five residues play an important role.

Monomers of the catalytic domain of human neuropathy target esterase are active in the presence of phospholipid

NEST is a hydrophobic recombinant polypeptide comprising the catalytic domain (residues 727-1216) of neuropathy target esterase. NEST in bacterial lysates has potent esterase activity, which is lost after its solubilization and purification in detergent-containing solutions. Activity in purified NEST preparations was restored by the addition of phospholipids before the removal of detergent by dialysis. The pattern of digestion by proteinase K of NEST-phospholipid complexes suggested that NEST might incorporate in a topologically random fashion into nascent liposomes and that the bulk of each NEST molecule might be exposed either to the liposome lumen or the external medium. Significant quantities of NEST were liberated from NEST-phospholipid complexes by treatment with dilute acid or alkali, suggesting that charge interactions might contribute to the association; however, NEST was irreversibly denatured at these pH values. Treatment of NEST-phospholipid complexes with glutaraldehyde afforded some protection against the inactivation of esterase activity by detergent but the pattern of cross-linked forms of NEST generated did not indicate pre-existing oligomers. Similarly, the inactivation of esterase activity in NEST-phospholipid complexes by radiation indicated that NEST monomers are catalytically active. The foregoing observations are not compatible with structural algorithms predicting that the catalytic serine residue lies at the centre of one of three transmembrane helices in NEST.

Intracellular activation of rat hepatic lipase requires transport to the Golgi compartment and is associated with a decrease in sedimentation velocity

Hepatic lipase (HL) is an N-glycoprotein that acquires triglyceridase activity somewhere during maturation and secretion. To determine where and how HL becomes activated, the effect of drugs that interfere with maturation and intracellular transport of HL protein was studied using freshly isolated rat hepatocytes. Carbonyl cyanide m-chlorophenyl hydrazone (CCCP), castanospermine, monensin, and colchicin all inhibited secretion of HL without affecting its specific enzyme activity. The specific enzyme activity of intracellular HL was decreased by 25-50% upon incubation with CCCP or castanospermine, and increased 2-fold with monensin and colchicin. Glucose trimming of HL protein was not affected by CCCP, as indicated by digestion of immunoprecipitates with jack bean alpha-mannosidase. Pulse labeling experiments with [(35)S]methionine indicated that conversion of the 53-kDa precursor to the 58-kDa form, nor the development of endoglycosidase H-resistance, were essential for acquisition of enzyme activity. In sucrose gradients, HL protein from secretion media sedimented as a homogeneous band of about 5.8 S, whereas HL protein from the cell lysates migrated as a broad band extending from 5.8 S to more than 8 S. With both sources, HL activity was exclusively associated with the 5.8 S HL protein form. We conclude that glucose trimming of HL protein in the endoplasmic reticulum is not sufficient for activation; full activation occurs during or after transport from the endoplasmic reticulum to the Golgi and is associated with a decrease in sedimentation velocity.

The role of hepatic lipase in lipoprotein metabolism

Hepatic lipase (HL) is one of two major lipases released from the vascular bed by intravenous injection of heparin. HL hydrolyzes phospholipids and triglycerides of plasma lipoproteins and is a member of a lipase superfamily that includes lipoprotein lipase and pancreatic lipase. The enzyme can be divided into an NH2-terminal domain containing the catalytic site joined by a short spanning region to a smaller COOH-terminal domain. The NH2-terminal portion contains an active site serine in a pentapeptide consensus sequence, Gly-Xaa-Ser-Xaa-Gly, as part of a classic Ser-Asp-His catalytic triad, and a putative hinged loop structure covering the active site. The COOH-terminal domain contains a putative lipoprotein-binding site. The heparin-binding sites may be distributed throughout the molecule, with the characteristic elution pattern from heparin-sepharose determined by the COOH-terminal domain. Of the three N-linked glycosylation sites, Asn-56 is required for efficient secretion and enzymatic activity. HL is hypothesized to directly couple HDL lipid metabolism to tissue/cellular lipid metabolism. The potential significance of the HL pathway is that it provides the hepatocyte with a mechanism for the uptake of a subset of phospholipids enriched in unsaturated fatty acids and may allow the uptake of cholesteryl ester, free cholesterol and phospholipid without catabolism of HDL apolipoproteins. HL can hydrolyze triglyceride and phospholipid in all lipoproteins, but is predominant in the conversion of intermediate density lipoproteins to LDL and the conversion of post-prandial triglyceride-rich HDL into the post-absorptive triglyceride-poor HDL. It has been suggested that enzymatically inactive HL can play a role in hepatic lipoprotein uptake forming a 'bridge' by binding to the lipoprotein and to the cell surface. This raises the interesting possibility that production and secretion of mutant inactive HL could promote clearance of VLDL remnants. We have described a rare family with HL deficiency. Affected patients are compound heterozygotes for a mutation of Ser267Phe that causes an inactive enzyme and a mutation of Thr383Met that results in impaired secretion of HL and reduced specific activity. Human HL deficiency in the context of a second factor causing hyperlipidemia is strongly associated with premature coronary artery disease.

Hepatic lipase deficiency

Hepatic lipase (HL) is an enzyme that is made primarily by hepatocytes (and also found in adrenal gland and ovary) and hydrolyzes phospholipids and triglycerides of plasma lipoproteins. It is secreted and bound to the hepatocyte surface and readily released by heparin. It is a member of the lipase superfamily and is homologous to lipoprotein lipase and pancreatic lipase. The enzyme can be divided into an NH2-terminal domain containing the catalytic site joined by a short spanning region to a smaller COOH-terminal domain. The NH2-terminal portion contains an active site serine in a pentapeptide consensus sequence, Gly-Xaa-Ser-Xaa-Gly, as part of a classic Ser-Asp-His catalytic triad, and a putative hinged loop structure covering the active site. The COOH-terminal domain contains a putative lipoprotein-binding site. The heparin-binding sites may be distributed throughout the molecule, with the characteristic elution pattern from heparin-sepharose determined by the COOH-terminal domain. Of the three N-linked glycosylation sites, Asn-56 is required for efficient secretion and enzymatic activity. HL is hypothesized to directly couple HDL lipid metabolism to tissue/cellular lipid metabolism. The potential significance of the HL pathway is that it provides the hepatocyte with a mechanism for the uptake of a subset of phospholipids enriched in unsaturated fatty acids and may allow the uptake of cholesteryl ester, free cholesterol, and phospholipid without catabolism of HDL apolipoproteins. HL can hydrolyze triglyceride and phospholipid in all lipoproteins, but is predominant in the conversion of intermediate density lipoproteins to LDL and the conversion of post-prandial triglyceride-rich HDL into the postabsorptive triglyceride-poor HDL. HL plays a secondary role in the clearance of chylomicron remnants by the liver. Human post-heparin HL activity is inversely correlated with intermediate density lipoprotein cholesterol concentration only in subjects with a hyperlipidemia involving VLDL. This is consistent with intermediate-density lipoproteins being a substrate for HL. HDL cholesterol has been reported to be inversely correlated to HL activity, and on this basis it has been suggested that lowering HL would increase HDL cholesterol. However, the correlation could also be due to a common hormonal factor such as estrogen, which has been shown to up-regulate apoAI and HDL cholesterol and lower HL. A striking feature of severe deficiency of HL is the increase in HDL cholesterol and apolipoprotein AI and an approximately 10-fold increase in HDL triglyceride. Hyper-alpha-triglyceridemia is not a feature of antiatherogenic HDL. HL binds not only to heparan, but also to the LDL receptor-related protein. It has been suggested that enzymatically inactive HL can play a role in hepatic lipoprotein uptake, forming a "bridge" by binding to the lipoprotein and to the cell surface. This raises the interesting possibility that production and secretion of mutant inactive HL could promote clearance of VLDL remnants. We have described a rare family with HL deficiency. Affected patients are compound heterozygotes for a mutation of Ser267 to Phe that results in an inactive enzyme and a mutation of Thr383 to Met that results in impaired secretion and reduced specific activity. Human HL deficiency in the context of a second factor causing hyperlipidemia is strongly associated with premature coronary artery disease. Recently, it has been reported that mutations affecting the structure of HL (e.g., T383M) are relatively frequent in the Finnish population. A C-to-T polymorphism in the promotor region of the HL gene is associated with lowered HL activity and less strongly with increased HDL cholesterol. In summary, there is a good understanding of what HL does in lipoprotein metabolism; however, there is little understanding of its physiological importance, that is, why HL does what it does. (ABSTRACT TRUNCATED)

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.

Combined lipase deficiency (cld/cld) in mice affects differently post-translational processing of lipoprotein lipase, hepatic lipase and pancreatic lipase

Lipoprotein lipase (LPL) and hepatic lipase (HL), which act on plasma lipoproteins, belong to the same gene family as pancreatic lipase. LPL is synthesized in heart, muscle and adipose tissue, while HL is synthesized primarily in liver. LPL is also synthesized in liver of newborn rodents. The active form of LPL is a dimer, whereas that of HL has not been established. Combined lipase deficiency (CLD) is an autosomal recessive mutation (cld) in mice which impairs post-translational processing of LPL and HL. Cld/cld mice have very low LPL and HL activities (< 5% of normal), yet normal pancreatic lipase activity. They develop massive hypertriglyceridemia and die within 3 days after birth. The CLD mutation allows synthesis, glycosylation and dimerization of LPL, but blocks activation and secretion of the lipase. Thus, dimerization per se does not result in production of active LPL. Immunofluorescence studies showed that LPL is retained in endoplasmic reticulum (ER) in cld/cld cells. Translocation of Golgi components to ER by treatment with brefeldin A (BFA) enabled synthesis of active LPL in cultured cld/cld brown adipocytes. Thus, production of inactive LPL in cld/cld cells results from inability of the cells to transport LPL from ER. The CLD mutation allows synthesis and glycosylation of HL, but blocks activation of the lipase. Immunofluorescence studies located HL mostly outside of cells in liver, liver cell cultures and incubated adrenal tissue of normal and cld/cld mice and mostly inside of cells in liver cell cultures and adrenal tissues treated with monensin (to block secretion of protein). These findings demonstrate synthesis and secretion of HL by both liver and adrenal cells of normal and cld/cld mice. Thus, the CLD mutation allows secretion of inactive HL by liver and adrenals. However, it does not block synthesis or secretion of active pancreatic lipase. Our findings indicate that LPL, HL and pancreatic lipase, although closely related, are processed differently.

Oligomeric structure of hepatic lipase: evidence from a novel epitope tag technique

The subunit structure of purified rHL (rHL) was determined by gel filtration chromatography, density gradient ultracentrifugation studies and a novel approach using epitope-tagged rHL. By gel filtration studies, native rHL had an apparent molecular weight of 179 kDa whereas enzyme treated with 6 M guanidine hydrochloride (GuHCl) for 22 h at room temperature gave a protein peak at 76 kDa. Using milder conditions for denaturation of rHL, such as 1 M GuHCl for 2 h, rHL eluted in two distinct peaks, one at 179 kDa and the other at 76 kDa. In addition, both protein peaks produced under mild denaturing conditions possessed detectable catalytic activity. Consistent with studies on lipoprotein lipase, the denatured rHL eluted from heparin-Sepharose at a lower salt concentration of 0.42 M NaCl than the native rHL which eluted at 0.72 M NaCl. By density gradient ultracentrifugation studies, the estimated molecular weight of native rHL was determined to be 113 kDa. Together, the data suggest that native rHL exists as a dimer that can be denatured into monomers by GuHCl and that a fraction of the denatured enzyme has detectable enzyme activity. To confirm these results, we designed two different rHL constructs that were epitope-tagged with either the myc or flag epitope and transfected them into 293 cells. The addition of the tag was shown not to alter enzyme secretion rate or specific activity of the lipase. Partially purified lipase from media of cotransfected cells was used to establish a dimer assay which employed a sandwich ELISA. This assay firmly established the presence of a rHL species which contained both the myc and flag tags, supporting an oligomeric subunit structure for rHL. Furthermore, the data using the epitope-tagged enzyme shows that this method could be a useful tool not only in identifying the region of the lipase responsible for dimer formation but also to study other protein-protein interactions.