Large, Diverse Population Cohorts of hiPSCs and Derived Hepatocyte-like Cells Reveal Functional Genetic Variation at Blood Lipid-Associated Loci.

Genome-wide association studies have struggled to identify functional genes and variants underlying complex phenotypes. We recruited a multi-ethnic cohort of healthy volunteers (n = 91) and used their tissue to generate induced pluripotent stem cells (iPSCs) and hepatocyte-like cells (HLCs) for genome-wide mapping of expression quantitative trait loci (eQTLs) and allele-specific expression (ASE). We identified many eQTL genes (eGenes) not observed in the comparably sized Genotype-Tissue Expression project's human liver cohort (n = 96). Focusing on blood lipid-associated loci, we performed massively parallel reporter assays to screen candidate functional variants and used genome-edited stem cells, CRISPR interference, and mouse modeling to establish rs2277862-CPNE1, rs10889356-DOCK7, rs10889356-ANGPTL3, and rs10872142-FRK as functional SNP-gene sets. We demonstrated HLC eGenes CPNE1, VKORC1, UBE2L3, and ANGPTL3 and HLC ASE gene ACAA2 to be lipid-functional genes in mouse models. These findings endorse an iPSC-based experimental framework to discover functional variants and genes contributing to complex human traits.

A functional, genome-wide evaluation of liposensitive yeast identifies the "ARE2 required for viability" (ARV1) gene product as a major component of eukaryotic fatty acid resistance.

The toxic subcellular accumulation of lipids predisposes several human metabolic syndromes, including obesity, type 2 diabetes, and some forms of neurodegeneration. To identify pathways that prevent lipid-induced cell death, we performed a genome-wide fatty acid sensitivity screen in Saccharomyces cerevisiae. We identified 167 yeast mutants as sensitive to 0.5 mm palmitoleate, 45% of which define pathways that were conserved in humans. 63 lesions also impacted the status of the lipid droplet; however, this was not correlated to the degree of fatty acid sensitivity. The most liposensitive yeast strain arose due to deletion of the "ARE2 required for viability" (ARV1) gene, encoding an evolutionarily conserved, potential lipid transporter that localizes to the endoplasmic reticulum membrane. Down-regulation of mammalian ARV1 in MIN6 pancreatic ß-cells or HEK293 cells resulted in decreased neutral lipid synthesis, increased fatty acid sensitivity, and lipoapoptosis. Conversely, elevated expression of human ARV1 in HEK293 cells or mouse liver significantly increased triglyceride mass and lipid droplet number. The ARV1-induced hepatic triglyceride accumulation was accompanied by up-regulation of DGAT1, a triglyceride synthesis gene, and the fatty acid transporter, CD36. Furthermore, ARV1 was identified as a transcriptional of the protein peroxisome proliferator-activated receptor α (PPARα), a key regulator of lipid homeostasis whose transcriptional targets include DGAT1 and CD36. These results implicate ARV1 as a protective factor in lipotoxic diseases due to modulation of fatty acid metabolism. In conclusion, a lipotoxicity-based genetic screen in a model microorganism has identified 75 human genes that may play key roles in neutral lipid metabolism and disease.

Gene therapy in a humanized mouse model of familial hypercholesterolemia leads to marked regression of atherosclerosis.

BACKGROUND: Familial hypercholesterolemia (FH) is an autosomal codominant disorder caused by mutations in the low-density lipoprotein receptor (LDLR) gene. Homozygous FH patients (hoFH) have severe hypercholesterolemia leading to life threatening atherosclerosis in childhood and adolescence. Mice with germ line interruptions in the Ldlr and Apobec1 genes (Ldlr(-/-)Apobec1(-/-)) simulate metabolic and clinical aspects of hoFH, including atherogenesis on a chow diet. METHODS/PRINCIPAL FINDINGS: In this study, vectors based on adeno-associated virus 8 (AAV8) were used to deliver the gene for mouse Ldlr (mLDLR) to the livers of Ldlr(-/-)Apobec1(-/-) mice. A single intravenous injection of AAV8.mLDLR was found to significantly reduce plasma cholesterol and non-HDL cholesterol levels in chow-fed animals at doses as low as 3×10(9) genome copies/mouse. Whereas Ldlr(-/-)Apobec1(-/-) mice fed a western-type diet and injected with a control AAV8.null vector experienced a further 65% progression in atherosclerosis over 2 months compared with baseline mice, Ldlr(-/-)Apobec1(-/-) mice treated with AAV8.mLDLR realized an 87% regression of atherosclerotic lesions after 3 months compared to baseline mice. Immunohistochemical analyses revealed a substantial remodeling of atherosclerotic lesions. CONCLUSIONS/SIGNIFICANCE: Collectively, the results presented herein suggest that AAV8-based gene therapy for FH may be feasible and support further development of this approach. The pre-clinical data from these studies will enable for the effective translation of gene therapy into the clinic for treatment of FH.

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.

Complete prevention of atherosclerosis in apoE-deficient mice by hepatic human apoE gene transfer with adeno-associated virus serotypes 7 and 8.

OBJECTIVE: Using intravenous injection of adeno-associated viral (AAV) vectors based on novel serotypes 7 and 8, we examined whether liver-specific expression of human apolipoprotein E (apoE) in apoE-deficient mice would completely prevent atherosclerosis after 1 year of sustained expression. METHODS AND RESULTS: Chow-fed apoE-/- mice were injected via the tail vein with vectors based on AAV2 or novel serotypes AAV7 and AAV8 encoding human apoE3 driven by a liver-specific promoter. In contrast to the first-generation AAV2 vector, apoE levels of mice injected with chimeric AAV2/7 and AAV2/8 vectors reached approximately 2-fold greater than normal human plasma levels by week 4 and maintained therapeutic levels up to 1 year. Cholesterol levels of AAV2/7-apoE and AAV2/8-apoE-treated mice were reduced to normal murine wild-type levels and were maintained for 1 year. At termination after 1 year, extensive atherosclerosis was present in the thoracic aortas and aortic roots of control AAV2/8-lacZ and AAV2-apoE-injected mice, but was completely prevented in both the AAV2/7 and AAV2/8-apoE-treated mice. CONCLUSIONS: We demonstrate that intravenous administration of AAV2/7- and AAV2/8-apoE vectors effectively mediated robust and sustained hepatic-specific expression of apoE and completely prevented atherosclerosis at 1 year.

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.

Persistent liver expression of murine apoA-l using vectors based on adeno-associated viral vectors serotypes 5 and 1.

Plasma levels of high-density lipoprotein-cholesterol (HDL-C) and apolipoprotein A-l (apoA-l) are inversely related to risk for coronary heart disease. Overexpression of apoA-l inhibits atherosclerosis in animal models. A method of stably expressing apoA-l using somatic gene transfer would be of interest. Pseudotyped adeno-associated virus (AAV) vectors comprised of inverted terminal repeats from AAV serotype 2 have been used for liver-directed gene transfers. We hypothesized that liver-directed gene transfer of apoA-l using vectors based on AAV serotypes 1 and 5 would result in higher-level, prolonged expression of apoA-l and increased HDL-C. To test this hypothesis we injected apoA-l-/- mice via the tail vein with either AAV2, AAV1 or AAV5 vectors encoding the murine apoA-l cDNA driven by the liver-specific thyroxine binding globulin promoter. Plasma levels of murine apoA-l and HDL-C were highest in mice injected with the AAV1-based vector and lowest in mice injected with the AAV2-based vector. Expression of apoA-l was stable up to 1 year after vector injection. These results indicate that AAV5 and AAV1 are more effective vectors for achieving higher levels of stable transgene expression of apoA-l after liver-directed gene transfer than AAV2. Furthermore, AAV1-based vectors generate higher apoA-l levels than AAV5-based vectors. It is possible that the levels of expression achieved using these vectors will be therapeutic in preventing atherosclerosis.

Tissue-specific expression pattern of human endothelial lipase in transgenic mice.

Endothelial lipase (EL), a new member of the triacylglycerol lipase gene family, is a key enzyme in HDL metabolism. The EL expression pattern in humans was reported to be unique and complementary to that documented for lipoprotein lipase. The regulatory elements responsible for the tissue-specific EL expression are not identified yet. In order to confine these sequences to a defined region of the EL promoter, we analyzed EL mRNA expression in human EL transgenic mice expressing EL under the control of the endogenous human promoter. We identified small intestine, mammary gland, adipose tissue and the adrenal gland as previously unknown tissues to express EL. Our data demonstrate that regulatory elements within 11.4 kb of 5' and 9.9 kb of 3' human EL flanking region promote the expression of EL in small intestine, ovary, testis, mammary gland, brain, lung, aorta, adipose tissue and the adrenals, whereas regulatory sequences located between 27.4 and 11.4 kb of 5' or 9.9 and 48.7 kb of 3' human EL flanking region seem to be responsible for kidney-specific EL expression.

Apolipoproteins regulate the kinetics of endothelial lipase-mediated hydrolysis of phospholipids in reconstituted high-density lipoproteins.

Endothelial lipase (EL) is a newly identified member of the triglyceride lipase gene family that hydrolyzes high-density lipoprotein (HDL) phospholipids. This study investigates the ability of the major apolipoproteins of rHDL to regulate the kinetics of EL-mediated phospholipid hydrolysis in well-characterized, homogeneous preparations of spherical rHDL. The rHDL contained either apoA-I as the only apolipoprotein, (A-I)rHDL, apoA-II as the only apolipoprotein, (A-II)rHDL, or apoA-I as well as apoA-II, (A-I/A-II)rHDL. The rHDL were comparable in terms of size and lipid composition and contained cholesteryl esters (CE) as their sole core lipid. Phospholipid hydrolysis was quantitated as the mass of nonesterified fatty acids (NEFA) released from the rHDL during incubation with EL. The V(max) of phospholipid hydrolysis for (A-I/A-II)rHDL [391.9 +/- 12.9 nmol of NEFA formed (mL of EL)(-1) h(-1)] was greater than (A-I)rHDL [152.8 +/- 4.7 nmol of NEFA formed (mL of EL)(-1) h(-1)]. The energy of activation (E(a)) for the hydrolysis reactions was calculated to be 52.1 and 34.8 kJ mol(-1) for (A-I)rHDL and (A-I/A-II)rHDL, respectively. Minimal phospholipid hydrolysis was observed for the (A-II)rHDL. Kinetic analysis showed that EL has a higher affinity for the phospholipids in (A-I)rHDL [K(m)(app) = 0.10 +/- 0.01 mM] than in (A-I/A-II)rHDL [K(m)(app) = 0.27 +/- 0.03 mM]. Furthermore, (A-I)rHDL is a competitive inhibitor of the EL-mediated phospholipid hydrolysis of (A-I/A-II)rHDL. These results establish that apolipoproteins are major determinants of the kinetics of EL-mediated phospholipid hydrolysis in rHDL.

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.

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.

Evidence that hepatic lipase and endothelial lipase have different substrate specificities for high-density lipoprotein phospholipids.

Hepatic lipase (HL) and endothelial lipase (EL) are both members of the triglyceride lipase gene family. HL hydrolyzes phospholipids and triglycerides in triglyceride-rich lipoproteins and high-density lipoproteins (HDL). EL hydrolyzes HDL phospholipids and has low triglyceride lipase activity. The aim of this study was to determine if HL and EL hydrolyze different HDL phospholipids and whether HDL phospholipid composition regulates the interaction of EL and HL with the particle surface. Spherical, reconstituted HDL (rHDL) containing either 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), 1-palmitoyl-2-linoleoylphosphatidylcholine (PLPC), 1-palmitoyl-2-arachidonylphosphatidylcholine (PAPC), or 1-palmitoyl-2-docosahexanoylphosphatidylcholine (PDPC) as the only phospholipid, apolipoprotein A-I as the only apolipoprotein, and either cholesteryl esters (CE) only or mixtures of CE and triolein (TO) in their core were prepared. The rHDL were similar in size and had comparable core lipid/apoA-I molar ratios. The CE-containing rHDL were used to determine the kinetics of HL- and EL-mediated phospholipid hydrolysis. For HL the V(max) of phospholipid hydrolysis for (POPC)rHDL > (PLPC)rHDL approximately (PDPC)rHDL > (PAPC)rHDL, while the K(m)(app) for (POPC)rHDL > (PDPC)rHDL > (PLPC)rHDL > (PAPC)rHDL. For EL the V(max) for (PDPC)rHDL > (PAPC)rHDL > (PLPC)rHDL approximately (POPC)rHDL, while the K(m)(app) for (PAPC)rHDL approximately (PLPC)rHDL > (POPC)rHDL > (PDPC)rHDL. The kinetics of EL- and HL-mediated TO hydrolysis was determined using rHDL that contained TO in their core. For HL the V(max) of TO hydrolysis for (PLPC)rHDL > (POPC)rHDL > (PAPC)rHDL > (PDPC)rHDL, while the K(m)(app) for (PLPC)rHDL > (POPC)rHDL approximately (PAPC)rHDL > (PDPC)rHDL. For EL the V(max) and K(m)(app) for (PAPC)rHDL > (PDPC)rHDL > (PLPC)rHDL > (POPC)rHDL. These results establish that EL and HL have different substrate specificities for rHDL phospholipids and that their interactions with the rHDL surface are regulated by phospholipids.

Dose-dependent acceleration of high-density lipoprotein catabolism by endothelial lipase.

BACKGROUND: Factors that regulate the metabolism of HDL and apolipoprotein A-I (apoA-I) are incompletely understood. Overexpression of endothelial lipase (EL) markedly reduces plasma levels of HDL cholesterol and apoA-I in mice, but the mechanisms of this effect remain unknown. METHODS AND RESULTS: We used different doses of a recombinant adenoviral vector to overexpress human EL in mice and studied the effects on plasma phospholipase activity, plasma lipids, HDL particle size, HDL turnover, and tissue sites of HDL degradation in mice. Overexpression of EL was associated with a significant dose-dependent increase in postheparin plasma phospholipase activity. Plasma phospholipid, HDL cholesterol, and apoA-I levels were markedly decreased, even at the lowest dose of vector. Kinetic studies demonstrated a significant dose-dependent increase in the fractional catabolic rate of HDL-apolipoprotein in EL-overexpressing mice. The postheparin plasma phospholipase activity was significantly positively correlated with HDL-apolipoprotein fractional catabolic rate. The uptake of apoA-I by the kidney and the liver was significantly increased by 2.5-fold and 3-fold, respectively, in mice overexpressing EL. CONCLUSIONS: Expression of EL in mice results in a dose-dependent increase in postheparin plasma phospholipase activity, catabolic rate of HDL-apolipoprotein, and uptake of apoA-I in both kidney and liver.

Effects of nonlipolytic ligand function of endothelial lipase on high density lipoprotein metabolism in vivo.

Endothelial lipase (EL) influences high density lipoprotein (HDL) metabolism in vivo and mediates bridging and uptake of HDL particles independent of its lipolytic activity in vitro. To determine whether EL has a nonlipolytic ligand function in HDL metabolism in vivo, 1 x 1011 particles of a recombinant adenovirus encoding human EL (AdEL), catalytically inactive human EL (AdELS149A), or control (Adnull) were injected into wild-type, apoA-I transgenic, and hepatic lipase knockout mice. ELS149A protein was expressed at higher levels than wild-type EL. EL and ELS149A protein were both substantially increased in the postheparin plasma compared with preheparin, indicating that both the wild-type and mutant EL were bound to cell-surface heparan sulfate proteoglycans. Overexpression of wild-type EL was associated with a significantly increased postheparin-plasma phospholipase activity and dramatically decreased levels of total cholesterol, HDL cholesterol, phospholipids, and apoA-I. Injection of AdELS149A did not result in increased phospholipase activity confirming that ELS149A was catalytically inactive. Expression of ELS149A did not decrease lipid or apoA-I levels in wild-type and apoA-I transgenic mice yet led to an intermediate reduction of total cholesterol, HDL cholesterol, and phospholipids in hepatic lipase-deficient mice compared with control and EL-expressing mice. Our study demonstrates for the first time that EL has both a lipolytic and nonlipolytic function in HDL metabolism in vivo. Lipolytic activity of EL, however, seems to be most important for its effects on systemic HDL metabolism.

Endogenously produced endothelial lipase enhances binding and cellular processing of plasma lipoproteins via heparan sulfate proteoglycan-mediated pathway.

Endothelial lipase (EL) is a new member of the triglyceride lipase gene family, which includes lipoprotein lipase (LpL) and hepatic lipase (HL). Enzymatic activity of EL has been studied before. Here we characterized the ability of EL to bridge lipoproteins to the cell surface. Expression of EL in wild-type Chinese hamster ovary (CHO)-K1 but not in heparan sulfate proteoglycan (HSPG)-deficient CHO-677 cells resulted in 3-4.4-fold increases of 125I-low density lipoprotein (LDL) and 125I-high density lipoprotein 3 binding (HDL3). Inhibition of proteoglycan sulfation by sodium chlorate or incubation of cells with labeled lipoproteins in the presence of heparin (100 microg/ml) abolished bridging effects of EL. An enzymatically inactive EL, EL-S149A, was equally effective in facilitating lipoprotein bridging as native EL. Processing of LDL and HDL differed notably after initial binding via EL to the cell surface. More than 90% of the surface-bound 125I-LDL was destined for internalization and degradation, whereas about 70% of the surface-bound 125I-HDL3 was released back into the medium. These differences were significantly attenuated after HDL clustering was promoted using antibody against apolipoprotein A-I. At equal protein concentration of added lipoproteins the ratio of HDL3 to VLDL bridging via EL was 0.092 compared with 0.174 via HL and 0.002 via LpL. In summary, EL mediates binding and uptake of plasma lipoproteins via a process that is independent of its enzymatic activity, requires cellular heparan sulfate proteoglycans, and is regulated by ligand clustering.

Endothelial cells secrete triglyceride lipase and phospholipase activities in response to cytokines as a result of endothelial lipase.

The endothelium interacts extensively with lipids and lipoproteins, but there are very few data regarding the ability of endothelial cells to secrete lipases. In this study, we investigated the ability of endothelial cells to secrete the triglyceride lipase and phospholipase activities characteristic of endothelial lipase (EL), a recently described member of the triglyceride lipase gene family. No lipase activities were detected under basal conditions, but treatment with cytokines significantly stimulated the expression of both activities. Using antibodies to EL, we determined that both activities were primarily a result of this enzyme. In addition to the increase in lipolytic activity, cytokine treatment was demonstrated to substantially upregulate EL protein and EL mRNA in a dose-dependent manner. Cytokines did not change EL mRNA stability. Both new protein synthesis and activation of NF-kappaB influenced the induction of EL by cytokines, suggesting that multiple pathways contribute to this process. The upregulation of EL by cytokines is in sharp contrast to the downregulation by cytokines of the other two major members of this gene family, lipoprotein lipase and hepatic lipase, and has implications for the physiological role of EL in inflammatory conditions and its potential role in the modulation of lipoprotein metabolism during inflammatory conditions, including atherosclerosis.

Characterization of the lipolytic activity of endothelial lipase.

Endothelial lipase (EL) is a new member of the triglyceride lipase gene family previously reported to have phospholipase activity. Using radiolabeled lipid substrates, we characterized the lipolytic activity of this enzyme in comparison to lipoprotein lipase (LPL) and hepatic lipase (HL) using conditioned medium from cells infected with recombinant adenoviruses encoding each of the enzymes. In the absence of serum, EL had clearly detectable triglyceride lipase activity. Both the triglyceride lipase and phospholipase activities of EL were inhibited in a dose-dependent fashion by the addition of serum. The ratio of triglyceride lipase to phospholipase activity of EL was 0.65, compared with ratios of 24.1 for HL and 139.9 for LPL, placing EL at the opposite end of the lipolytic spectrum from LPL. Neither lipase activity of EL was influenced by the addition of apolipoprotein C-II (apoC-II), indicating that EL, like HL, does not require apoC-II for activation. Like LPL but not HL, both lipase activities of EL were inhibited by 1 M NaCl. The relative ability of EL, versus HL and LPL, to hydrolyze lipids in isolated lipoprotein fractions was also examined using generation of FFAs as an end point. As expected, based on the relative triglyceride lipase activities of the three enzymes, the triglyceride-rich lipoproteins, chylomicrons, VLDL, and IDL, were efficiently hydrolyzed by LPL and HL. EL hydrolyzed HDL more efficiently than the other lipoprotein fractions, and LDL was a poor substrate for all of the enzymes.

Lipases and HDL metabolism.

Plasma levels of high-density lipoprotein (HDL) cholesterol are strongly inversely associated with atherosclerotic cardiovascular disease, and overexpression of HDL proteins, such as apolipoprotein A-I in animals, reduces progression and even induces regression of atherosclerosis. Therefore, HDL metabolism is recognized as a potential target for therapeutic intervention of atherosclerotic vascular diseases. The antiatherogenic properties of HDL include promotion of cellular cholesterol efflux and reverse cholesterol transport, as well as antioxidant, anti-inflammatory and anticoagulant properties. The molecular regulation of HDL metabolism is not fully understood, but it is influenced by several extracellular lipases. Here, we focus on new developments and insights into the role of secreted lipases on HDL metabolism and their relationship to atherosclerosis.