Cloning of a unique lipase from endothelial cells extends the lipase gene family

Vertebrate endothelial lipase: comparative studies of an ancient gene and protein in vertebrate evolution

Endothelial lipase (gene: LIPG; enzyme: EL) is one of three members of the triglyceride lipase family that contributes to lipoprotein degradation within the circulation system and plays a major role in HDL metabolism in the body. In this study, in silico methods were used to predict the amino acid sequences, secondary and tertiary structures, and gene locations for LIPG genes and encoded proteins using data from several vertebrate genome projects. LIPG is located on human chromosome 18 and is distinct from other human 'neutral lipase' genes, hepatic lipase (gene: LIPC; enzyme: HL) and lipoprotein lipase (gene: LPL; enzyme: LPL) examined. Vertebrate LIPG genes usually contained 10 coding exons located on the positive strand for most primates, as well as for horse, bovine, opossum, platypus and frog genomes. The rat LIPG gene however contained only 9 coding exons apparently due to the presence of a 'stop' codon' within exon 9. Vertebrate EL protein subunits shared 58-97% sequence identity as compared with 38-45% sequence identities with human HL and LPL. Four previously reported human EL N-glycosylation sites were predominantly conserved among the 10 potential N-glycosylation sites observed for the vertebrate EL sequences examined. Sequence alignments and identities for key EL amino acid residues were observed as well as conservation of predicted secondary and tertiary structures with those previously reported for horse pancreatic lipase (PL) (Bourne et al. 1994). Several potential sites for regulating LIPG gene expression were observed including CpG islands near the LIPG gene promoter and a predicted microRNA binding site near the 3'-untranslated region. Promoter regions containing functional polymorphisms that regulate HDL cholesterol in baboons were conserved among primates but not retained between primates and rodents. Phylogenetic analyses examined the relationships and potential evolutionary origins of the vertebrate LIPG gene subfamily with other neutral triglyceride lipase gene families, LIPC and LPL. It is apparent that the triglyceride lipase ancestral gene for the vertebrate LIPG gene predated the appearance of fish during vertebrate evolution >500 million years ago.

Phospholipases A₁

Phospholipase A(1) (PLA(1)) is an enzyme that hydrolyzes phospholipids and produces 2-acyl-lysophospholipids and fatty acids. This lipolytic activity is conserved in a wide range of organisms but is carried out by a diverse set of PLA(1) enzymes. Where their function is known, PLA(1)s have been shown to act as digestive enzymes, possess central roles in membrane maintenance and remodeling, or regulate important cellular mechanisms by the production of various lysophospholipid mediators, such as lysophosphatidylserine and lysophosphatidic acid, which in turn have multiple biological functions.

O-glycosylation modulates proprotein convertase activation of angiopoietin-like protein 3: possible role of polypeptide GalNAc-transferase-2 in regulation of concentrations of plasma lipids

The angiopoietin-like protein 3 (ANGPTL3) is an important inhibitor of the endothelial and lipoprotein lipases and a promising drug target. ANGPTL3 undergoes proprotein convertase processing (RAPR(224)↓TT) for activation, and the processing site contains two potential GalNAc O-glycosylation sites immediately C-terminal (TT(226)). We developed an in vivo model system in CHO ldlD cells that was used to show that O-glycosylation in the processing site blocked processing of ANGPTL3. Genome-wide SNP association studies have identified the polypeptide GalNAc-transferase gene, GALNT2, as a candidate gene for low HDL and high triglyceride blood levels. We hypothesized that the GalNAc-T2 transferase performed critical O-glycosylation of proteins involved in lipid metabolism. Screening of a panel of proteins known to affect lipid metabolism for potential sites glycosylated by GalNAc-T2 led to identification of Thr(226) adjacent to the proprotein convertase processing site in ANGPTL3. We demonstrated that GalNAc-T2 glycosylation of Thr(226) in a peptide with the RAPR(224)↓TT processing site blocks in vitro furin cleavage. The study demonstrates that ANGPTL3 activation is modulated by O-glycosylation and that this step is probably controlled by GalNAc-T2.

Endothelial lipase (EL) and EL-generated lysophosphatidylcholines promote IL-8 expression in endothelial cells

OBJECTIVE

Previously we identified palmitoyl-lysophosphatidylcholine (LPC 16:0), as well as linoleoyl-, arachidonoyl- and oleoyl-LPC (LPC 18:2, 20:4 and 18:1) as the most prominent LPC species generated by the action of endothelial lipase (EL) on high-density lipoprotein (HDL). In the present study, the impact of EL and EL-generated LPC on interleukin-8 (IL-8) synthesis was examined in vitro in primary human aortic endothelial cells (HAEC) and in mice.

METHODS AND RESULTS

Adenovirus-mediated overexpression of the catalytically active EL, but not its inactive mutant, increased endothelial synthesis of IL-8 mRNA and protein in a time- and HDL-concentration-dependent manner. While LPC 18:2 was inactive, LPC 16:0, 18:1 and 20:4 promoted IL-8 mRNA- and protein-synthesis, differing in potencies and kinetics. The effects of all tested LPC on IL-8 synthesis were completely abrogated by addition of BSA and chelation of intracellular Ca(2+). Underlying signaling pathways also included NFkB, p38-MAPK, ERK, PKC and PKA. In mice, adenovirus-mediated overexpression of EL caused an elevation in the plasma levels of MIP-2 (murine IL-8 analogue) accompanied by a markedly increased plasma LPC/PC ratio. Intravenously injected LPC also raised MIP-2 plasma concentration, however to a lesser extent than EL overexpression.

CONCLUSION

Our results indicate that EL and EL-generated LPC, except of LPC 18:2, promote endothelial IL-8 synthesis, with different efficacy and kinetics, related to acyl-chain length and degree of saturation. Accordingly, due to its capacity to modulate the availability of the pro-inflammatory and pro-adhesive chemokine IL-8, EL should be considered an important player in the development of atherosclerosis.

Association of the LIPG 584C > T polymorphism and serum lipid levels in the Guangxi Bai Ku Yao and Han populations

BACKGROUND

Endothelial lipase (EL) is a major determinant of high-density lipoprotein-cholesterol (HDL-C) metabolism, but the association of endothelial lipase gene (LIPG) polymorphism and serum HDL-C levels is scarce and conflicting in diverse populations. Bai Ku Yao is an isolated subgroup of the Yao minority in China. This study was designed to detect the association of LIPG 584C > T (rs2000813) polymorphism and several environmental factors with serum lipid levels in the Guangxi Bai Ku Yao and Han populations.

METHODS

A total of 645 subjects of Bai Ku Yao and 638 participants of Han Chinese were randomly selected from our previous stratified randomized cluster samples. Genotyping of the LIPG 584C > T was performed by polymerase chain reaction and restriction fragment length polymorphism combined with gel electrophoresis, and then confirmed by direct sequencing.

RESULTS

The levels of serum total cholesterol (TC), HDL-C, low-density lipoprotein cholesterol (LDL-C) and apolipoprotein (Apo) AI and ApoB were lower in Bai Ku Yao than in Han (P < 0.05 - 0.001). The frequency of C and T alleles was 73.5% and 26.5% in Bai Ku Yao, and 67.9% and 32.1% in Han (P < 0.01); respectively. The frequency of CC, CT and TT genotypes was 50.4%, 46.2% and 3.4% in Bai Ku Yao, and 41.4%, 53.1% and 5.5% in Han (P < 0.01); respectively. Serum HDL-C levels in both ethnic groups were different among the three genotypes (P < 0.05 for each). Serum TC levels in both ethnic groups were also different between the CC and CT/TT genotypes (P < 0.05 for each). The T allele carriers had higher serum HDL-C and TC levels than the T allele noncarriers. Multivariate logistic regression analysis showed that the levels of HDL-C and ApoB were correlated with genotypes in Bai Ku Yao (P < 0.05 for each), whereas the levels of TC and HDL-C were associated with genotypes in Han Chinese (P < 0.05 and P < 0.01). Serum lipid parameters were also correlated with several environmental factors in the both ethnic groups.

CONCLUSIONS

The frequency of LIPG 584T allele is lower in Bai Ku Yao than in Han Chinese. The LIPG 584T allele is associated with increased serum HDL-C, TC and ApoB levels. The differences in serum HDL-C, TC and ApoB levels between the two ethnic groups might partly result from different genotypic and allelic frequencies of LIPG 584C > T or different LIPG-enviromental interactions.

Targeted deletion of endothelial lipase increases HDL particles with anti-inflammatory properties both in vitro and in vivo

Previous studies have shown that targeted deletion of endothelial lipase (EL) markedly increases the plasma high density lipoprotein cholesterol (HDL-C) level in mice. However, little is known about the functional quality of HDL particles after EL inhibition. Therefore, the present study assessed the functional quality of HDL isolated from EL(-/-) and wild-type (WT) mice. Anti-inflammatory functions of HDL from EL(-/-) and WT mice were evaluated by in vitro assays. The HDL functions such as PON-1 or PAF-AH activities, inhibition of cytokine-induced vascular cell adhesion molecule-1 expression, inhibition of LDL oxidation, and the ability of cholesterol efflux were similar in HDL isolated from WT and EL(-/-) mice. In contrast, the lipopolysaccharide-neutralizing capacity of HDL was significantly higher in EL(-/-) mice than that in WT mice. To evaluate the anti-inflammatory actions of HDL in vivo, lipopolysaccharide-induced systemic inflammation was generated in these mice. EL(-/-) mice showed higher survival rate and lower expression of inflammatory markers than WT mice. Intravenous administration of HDL isolated from EL(-/-) mice significantly improved the mortality after lipopolysaccharide injection in WT mice. In conclusion, targeted disruption of EL increased HDL particles with preserved anti-inflammatory and anti-atherosclerotic functions. Thus, EL inhibition would be a useful strategy to raise 'good' cholesterol in the plasma.

Acyl chain-dependent effect of lysophosphatidylcholine on endothelial prostacyclin production

Previously we identified palmitoyl-lysophosphatidylcholine (16:0 LPC), linoleoyl-LPC (18:2 LPC), arachidonoyl-LPC (20:4 LPC), and oleoyl-LPC (18:1 LPC) as the most prominent LPC species generated by the action of endothelial lipase (EL) on high-density lipoprotein. In the present study, the impact of those LPC on prostacyclin (PGI(2)) production was examined in vitro in primary human aortic endothelial cells (HAEC) and in vivo in mice. Although 18:2 LPC was inactive, 16:0, 18:1, and 20:4 LPC induced PGI(2) production in HAEC by 1.4-, 3-, and 8.3-fold, respectively. LPC-elicited 6-keto PGF1α formation depended on both cyclooxygenase (COX)-1 and COX-2 and on the activity of cytosolic phospholipase type IVA (cPLA2). The LPC-induced, cPLA2-dependent (14)C-arachidonic acid (AA) release was increased 4.5-fold with 16:0, 2-fold with 18:1, and 2.7-fold with 20:4 LPC, respectively, and related to the ability of LPC to increase cytosolic Ca(2+) concentration. In vivo, LPC increased 6-keto PGF(1α) concentration in mouse plasma with a similar order of potency as found in HAEC. Our results indicate that the tested LPC species are capable of eliciting production of PGI(2), whereby the efficacy and the relative contribution of underlying mechanisms are strongly related to acyl-chain length and degree of saturation.

Role of endothelial lipase in atherosclerosis

Endothelial lipase, which is a newly identified member of the lipase family, plays an important role in high-density lipoprotein metabolism, which catalyzes the hydrolysis of high-density lipoprotein phospholipids and facilitates the clearance of high-density lipoprotein from the circulation. In addition, inflammatory cytokines, including tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 beta (IL-1beta), upregulate endothelial lipase expression, and endothelial lipase also affects the expression of cytokines, which in turn play an important role in atherogenesis. Endothelial lipase expression has been associated with macrophages within human atherosclerotic lesions. However, an important challenge is to determine how endothelial lipase alters the progression of atherosclerosis. Although few data are available from human studies, it seems that plasma endothelial lipase levels in individuals with atherosclerosis might be higher than that measured in healthy individuals. Therefore, we believe that endothelial lipase might be a promising marker for atherosclerosis in clinical settings in the future.

Impact of combined deficiency of hepatic lipase and endothelial lipase on the metabolism of both high-density lipoproteins and apolipoprotein B-containing lipoproteins

RATIONALE

Hepatic lipase (HL) and endothelial lipase (EL) are extracellular lipases that both hydrolyze triglycerides and phospholipids and display potentially overlapping or complementary roles in lipoprotein metabolism.

OBJECTIVE

We sought to dissect the overlapping roles of HL and EL by generating mice deficient in both HL and EL (HL/EL-dko) for comparison with single HL-knockout (ko) and EL-ko mice, as well as wild-type mice.

METHODS AND RESULTS

Reproduction and viability of the HL/EL-dko mice were impaired compared with the single-knockout mice. The plasma levels of total cholesterol, high-density lipoprotein (HDL) cholesterol, non-HDL cholesterol, and phospholipids in the HL/EL-dko mice were markedly higher than those in the single-knockout mice. Most notably, the HL/EL-dko mice exhibited an unexpected substantial increase in small low-density lipoproteins. Kinetic studies with [(3)H]cholesteryl ether-labeled very-low-density lipoproteins demonstrated that the HL/EL-dko mice accumulated counts in the smallest low-density lipoprotein-sized fractions, as assessed by size exclusion chromatography, suggesting that it arises from lipolysis of very-low-density lipoproteins. HDL from all 3 lipase knockout models had an increased cholesterol efflux capacity but reduced clearance of HDL cholesteryl esters versus control mice. Despite their higher HDL cholesterol levels, neither HL-ko, EL-ko, nor HL/EL-dko mice demonstrated an increased rate of macrophage reverse cholesterol transport in vivo.

CONCLUSIONS

These studies reveal an additive effect of HL and EL on HDL metabolism but not macrophage reverse cholesterol transport in mice and an unexpected redundant role of HL and EL in apolipoprotein B lipoprotein metabolism.

Comparative studies of mammalian acid lipases: Evidence for a new gene family in mouse and rat (Lipo)

At least six families of mammalian acid lipases (E.C. 3.1.1.-) catalyse the hydrolysis of triglycerides in the body, designated as LIPA (lysosomal), LIPF (gastric), LIPJ (testis) and LIPK, LIPM and LIPN (epidermal), which belong to the AB hydrolase superfamily. In this study, in silico methods were used to predict the amino acid sequences, secondary and tertiary structures, and gene locations for acid lipase genes and encoded proteins using data from several mammalian genome projects. Mammalian acid lipase genes were located within a gene cluster for each of the 8 mammalian genomes examined, including human (Homo sapiens), chimpanzee (Pons troglodytes), rhesus monkey (Macacca mulatta), mouse (Mus musculus), rat (Rattus norvegicus), cow (Bos taurus), horse (Equus caballus) and dog (Canis familaris), with each containing 9 coding exons. Human and mouse acid lipases shared 44-87% sequence identity and exhibited sequence alignments and identities for key amino acid residues and conservation of predicted secondary and tertiary structures with those previously reported for human gastric lipase (LIPF) (Roussel et al., 1999). Evidence for a new family of acid lipase genes is reported for mouse and rat genomes, designated as Lipo. Mouse acid lipase genes are subject to differential mRNA tissue expression, with Lipa showing wide tissue expression, while others have a more restricted tissue expression in the digestive tract (Lipf), salivary gland (Lipo) and epidermal tissues (Lipk, Lipm and Lipn). Phylogenetic analyses of the mammalian acid lipase gene families suggested that these genes are products of gene duplication events prior to eutherian mammalian evolution and derived from an ancestral vertebrate LIPA gene, which is present in the frog, Xenopus tropicalis.

Regulated expression of endothelial lipase in atherosclerosis

Endothelial lipase (EL) is a major determinant of HDL metabolism and associated with the development of atherosclerosis, however the regulated expression of EL in atherosclerosis is unclear. In this study, we investigated EL expression in rat atherosclerosis and explored the potential mechanisms regulating EL expression by employing LPS on Raw264.7 cells in vitro. Rat atherosclerosis model was established fed on high-cholesterol diet (HCD) combined with vitamin D(2) (VD). Western blotting and immunochemistry staining revealed that EL expression was increased in the aorta, especially the atherosclerotic lesions in HCD rats. LPS increased EL expression in a time and dose dependent manner in Raw264.7 cells and NFkappaB inhibitor, PDTC attenuated the effects of LPS on EL. EMSA revealed that LPS stimulated NFkappaB binding to EL promoter. In summary, EL was upregulated in rat atherosclerosis and LPS stimulates EL expression in vitro through NFkappaB activation.

Pitavastatin decreases the expression of endothelial lipase both in vitro and in vivo

AIMS

In addition to their cholesterol-lowering effect, statins increase high-density lipoprotein cholesterol (HDL-C) levels. Endothelial lipase (EL) is a regulator of plasma HDL-C levels. In the present study, the effects of statins on EL expression were investigated.

METHODS AND RESULTS

The 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor pitavastatin suppressed basal and cytokine-treated EL expression in endothelial cells. Concomitant treatment with mevalonate or geranylgeranyl pyrophosphate completely reversed the inhibitory effect of pitavastatin, suggesting that geranylgeranylated proteins are involved in the inhibition of EL expression by statins. Inhibition of RhoA activity by overexpression of a dominant-negative mutant of RhoA or a Rho kinase inhibitor decreased EL levels. Pitavastatin reduced phospholipase activities of endothelial cells, and concomitant treatment with mevalonate reversed its inhibitory effect. Pitavastatin reduced RhoA activity and EL expression in mouse tissues. Furthermore, plasma EL concentrations in human subjects were measured by enzyme-linked immunosorbent assays. Plasma EL levels were negatively associated with plasma HDL levels in 237 patients with cardiovascular diseases, and pitavastatin treatment reduced plasma EL levels and increased HDL-C levels in 48 patients with hypercholesterolaemia.

CONCLUSION

These findings suggest that statins can reduce EL expression in vitro and in vivo via inhibition of RhoA activity. The inhibition of EL expression in the vessel wall may contribute to the anti-atherogenic effects of statins.

Role of endothelial lipase in plasma HDL levels in a murine model of hypertriglyceridemia

AIM

Hypertriglyceridemia is the most common cause of low plasma high-density lipoprotein cholesterol (HDL-C) levels; however, the correlation between high triglyceride (TG) and low HDL-C remains unclear. Endothelial lipase (EL) is a determinant of plasma HDL levels. We investigated the role of EL in HDL metabolism in a murine model of acute hypertriglyceridemia.

METHODS AND RESULTS

To establish TG-dominant hyperlipidemia, EL-/- and wild-type (WT) mice were injected with Poloxamer-407 (P-407, 0.5 g/kg, i.p.). A single injection of P-407 resulted in a marked increase in plasma TG and cholesterol levels together with a decrease in HDL-C levels. Although plasma TG levels were similar in EL-/- and WT mice after P-407 injection, HDL-C levels were 80% higher and the HDL particle size was significantly larger in EL-/- mice than in WT mice. P-407 treatment inhibited plasma lipoprotein lipase activity and EL phospholipase activity, without decreasing their expressions. Adenovirus-mediated overexpression of EL in the liver reduced plasma HDL-C levels in both normo- and hyperlipidemic mice, while overexpression of catalytically inactive EL reduced HDL-C levels in hyperlipidemic mice. Cell culture experiments revealed that both catalytically active and inactive EL promoted cellular HDL uptake to the same extent.

CONCLUSION

EL regulates plasma HDL levels in mice in the normolipidemic as well as the acute hypertriglyceridemic state. EL can modulate plasma HDL-CHOL levels through both its lipolytic and ligand-binding functions in hypertriglyceridemic mice, while lipolytic activity appears to be the main determinant for its effects on HDL metabolism in normolipidemic mice.

Comparative and functional genomics of lipases in holometabolous insects

Lipases have key roles in insect lipid acquisition, storage and mobilisation and are also fundamental to many physiological processes underpinning insect reproduction, development, defence from pathogens and oxidative stress, and pheromone signalling. We have screened the recently sequenced genomes of five species from four orders of holometabolous insects, the dipterans Drosophila melanogaster and Anopheles gambiae, the hymenopteran Apis mellifera, the moth Bombyx mori and the beetle Tribolium castaneum, for the six major lipase families that are also found in other organisms. The two most numerous families in the insects, the neutral and acid lipases, are also the main families in mammals, albeit not in Caenorhabditis elegans, plants or microbes. Total numbers of the lipases vary two-fold across the five insect species, from numbers similar to those in mammals up to numbers comparable to those seen in C. elegans. Whilst there is a high degree of orthology with mammalian lipases in the other four families, the great majority of the insect neutral and acid lipases have arisen since the insect orders themselves diverged. Intriguingly, about 10% of the insect neutral and acid lipases have lost motifs critical for catalytic function. Examination of the length of lid and loop regions of the neutral lipase sequences suggest that most of the insect lipases lack triacylglycerol (TAG) hydrolysis activity, although the acid lipases all have intact cap domains required for TAG hydrolysis. We have also reviewed the sequence databases and scientific literature for insights into the expression profiles and functions of the insect neutral and acid lipases and the orthologues of the mammalian adipose triglyceride lipase which has a pivotal role in lipid mobilisation. These data suggest that some of the acid and neutral lipase diversity may be due to a requirement for rapid accumulation of dietary lipids. The different roles required of lipases at the four discrete life stages of holometabolous insects may also contribute to the diversity of lipases required by insects. In addition, insects use lipases to perform roles for which there are no correlates in mammals, including as yolk and male accessory gland proteins.

A naturally occurring variant of endothelial lipase associated with elevated HDL exhibits impaired synthesis

Human endothelial lipase (EL) is a member of a family of lipases and phospholipases that are involved in the metabolism of plasma lipoproteins. EL displays a preference to hydrolyze lipids in HDL. We report here that a naturally occurring low frequency coding variant in the EL gene (LIPG), glycine-26 to serine (G26S), is significantly more common in African-American individuals with elevated HDL cholesterol (HDL-C) levels. To test the hypothesis that this variant results in reduced EL function, we extensively characterized and compared the catalytic and noncatalytic functions of the G26S variant and wild-type (WT) EL. While the catalytic-specific activity of G26S EL is similar to WT EL, its secretion is markedly reduced. Consistent with this observation, we found that carriers of the G26S variant had significantly reduced plasma levels of EL protein. Thus, this N-terminal variant results in reduced secretion of EL protein, plausibly leading to increased HDL-C levels.

The T111I variant in the endothelial lipase gene and risk of coronary heart disease in three independent populations

AIMS

Endothelial lipase (LIPG) is implicated in the metabolism of high-density lipoprotein cholesterol (HDL-C). Small studies in selected populations have reported higher HDL-C levels among carriers of the common T111I variant in LIPG, but whether this variant is associated with plasma lipids and risk of coronary heart disease (CHD) in the general population is unclear. The objective of this study was to address the associations of the T111I variant with plasma lipids and risk of CHD in three independent prospective studies of generally healthy men and women.

METHODS AND RESULTS

The T111I variant was genotyped in case-control studies of CHD nested within the Diet, Cancer, and Health study with 998 cases, Nurses' Health Study with 241 cases, and Health Professionals Follow-up Study with 262 cases. The minor allele frequency in the combined pool of controls was 0.29. The T111I variant was not associated with HDL-C or any other lipid and lipoprotein measures. Compared with wildtype homozygotes, the pooled estimate for risk of CHD was 0.95 (0.85-1.06) per T111I allele.

CONCLUSION

Our analysis among healthy Caucasian men and women from three independent studies does not support an association between the T111I variant and HDL-C, other plasma lipids, or risk of CHD.

Loss-of-function variants in endothelial lipase are a cause of elevated HDL cholesterol in humans

Elevated plasma concentrations of HDL cholesterol (HDL-C) are associated with protection from atherosclerotic cardiovascular disease. Animal models indicate that decreased expression of endothelial lipase (LIPG) is inversely associated with HDL-C levels, and genome-wide association studies have identified LIPG variants as being associated with HDL-C levels in humans. We hypothesized that loss-of-function mutations in LIPG may result in elevated HDL-C and therefore performed deep resequencing of LIPG exons in cases with elevated HDL-C levels and controls with decreased HDL-C levels. We identified a significant excess of nonsynonymous LIPG variants unique to cases with elevated HDL-C. In vitro lipase activity assays demonstrated that these variants significantly decreased endothelial lipase activity. In addition, a meta-analysis across 5 cohorts demonstrated that the low-frequency Asn396Ser variant is significantly associated with increased HDL-C, while the common Thr111Ile variant is not. Functional analysis confirmed that the Asn396Ser variant has significantly decreased lipase activity both in vitro and in vivo, while the Thr111Ile variant has normal lipase activity. Our results establish that loss-of-function mutations in LIPG lead to increased HDL-C levels and support the idea that inhibition of endothelial lipase may be an effective mechanism to raise HDL-C.

Hepatic SR-BI, not endothelial lipase, expression determines biliary cholesterol secretion in mice

High density lipoprotein cholesterol is thought to represent a preferred source of sterols secreted into bile following hepatic uptake by scavenger receptor class B type I (SR-BI). The present study aimed to determine the metabolic effects of an endothelial lipase (EL)-mediated stimulation of HDL cholesterol uptake on liver lipid metabolism and biliary cholesterol secretion in wild-type, SR-BI knockout, and SR-BI overexpressing mice. In each model, injection of an EL expressing adenovirus decreased plasma HDL cholesterol (P < 0.001) whereas hepatic cholesterol content increased (P < 0.05), translating into decreased expression of sterol-regulatory element binding protein 2 (SREBP2) and its target genes HMG-CoA reductase and LDL receptor (each P < 0.01). Biliary cholesterol secretion was dependent on hepatic SR-BI expression, being decreased in SR-BI knockouts (P < 0.001) and increased following hepatic SR-BI overexpression (P < 0.001). However, in each model, biliary secretion of cholesterol, bile acids, and phospholipids as well as fecal bile acid and neutral sterol content, remained unchanged in response to EL overexpression. Importantly, hepatic ABCG5/G8 expression did not correlate with biliary cholesterol secretion rates under these conditions. These results demonstrate that an acute decrease of plasma HDL cholesterol levels by overexpressing EL increases hepatic cholesterol content but leaves biliary sterol secretion unaltered. Instead, biliary cholesterol secretion rates are related to the hepatic expression level of SR-BI. These data stress the importance of SR-BI for biliary cholesterol secretion and might have relevance for concepts of reverse cholesterol transport.

High-density lipoproteins, inflammation and oxidative stress

Plasma levels of HDL (high-density lipoprotein)-cholesterol are strongly and inversely correlated with atherosclerotic cardiovascular disease. Both clinical and epidemiological studies have reported an inverse and independent association between serum HDL-cholesterol levels and CHD (coronary heart disease) risk. The cardioprotective effects of HDLs have been attributed to several mechanisms, including their involvement in the reverse cholesterol transport pathway. HDLs also have antioxidant, anti-inflammatory and antithrombotic properties and promote endothelial repair, all of which are likely to contribute to their ability to prevent CHD. The first part of this review summarizes what is known about the origins and metabolism of HDL. We then focus on the anti-inflammatory and antioxidant properties of HDL and discuss why these characteristics are cardioprotective.

Targeted inactivation of endothelial lipase attenuates lung allergic inflammation through raising plasma HDL level and inhibiting eosinophil infiltration

Endothelial lipase (EL) is a novel phospholipase that determines plasma high-density lipoprotein cholesterol (HDL-C) levels. We have investigated the role of HDL-C in lung allergic inflammation by using EL knockout (EL-KO) mice that are high in HDL-C. EL-KO and wild-type control mice were sensitized and challenged with ovalbumin to evoke eosinophilic inflammation in the lung. EL was expressed in epithelial cells, alveolar type II cells, and endothelial cells in the lung, and its expression was upregulated during inflammation. Concomitant with attenuated hyperresponsiveness of the airway smooth muscles, the number of eosinophils in bronchoalveolar lavage and the expression of VCAM-1 were lower in EL-KO mice than in control mice. HDL reduced cytokine-induced VCAM-1 expression in cultured endothelial cells. When plasma HDL levels were decreased to similar levels in both mouse groups by adenovirus-mediated overexpression of EL, however, eosinophil infiltration was still lower in EL-KO mice. In vitro adhesion assays revealed that EL expression on the cell surface promoted the interaction of eosinophils through the ligand-binding function of EL. In summary, targeted inactivation of EL attenuated allergic inflammation in the lung, and the protective effects in EL-KO mice were associated with high plasma HDL levels, downregulation of VCAM-1, and loss of the direct ligand-binding function of EL. Thus EL is a novel modulator of the progression of allergic asthma.

Scavenger receptor BI-mediated selective uptake is required for the remodeling of high density lipoprotein by endothelial lipase

Endothelial lipase (EL) is a negative regulator of high density lipoprotein (HDL) cholesterol plasma levels, and scavenger receptor BI (SR-BI) is involved in remodeling of HDL. The present study investigates the requirement of SR-BI for the effects of EL-mediated phospholipid hydrolysis on HDL metabolism in vivo. In vitro, selective uptake from EL-modified HDL was 129% higher than selective uptake from control HDL in SR-BI-overexpressing cells (p=0.01). In vivo overexpression of human EL by means of recombinant adenovirus decreased HDL plasma levels significantly (p<0.01). Fast protein liquid chromatography analysis and agarose gel electrophoresis revealed that EL expression resulted in the generation of small pre-beta HDL particles in wild-type mice, whereas in SR-BI-/- mice small HDL were preferentially removed. In kinetic experiments the fractional catabolic rate (FCR) of HDL cholesteryl ester increased by 110% (p<0.001), and the FCR of HDL apolipoproteins increased by 64% (p<0.001) in response to EL overexpression in wild-type mice. In SR-BI-/- mice a similar increase in the HDL apolipoprotein FCR occurred (p<0.001); however, there was no further increase in HDL cholesteryl ester catabolism. The apparent whole body selective uptake was increased 3-fold by EL in wild-type mice (p<0.001), whereas there was no selective uptake in SR-BI knock-out mice. EL overexpression increased hepatic selective uptake as well as holoparticle uptake (each p<0.01) in wild-type mice, whereas in SR-BI knock-out mice only holoparticle uptake increased (p<0.01). Our results indicate that SR-BI-mediated selective uptake of HDL cholesteryl ester is essential for the remodeling of large alpha-migrating HDL particles by EL.

White adipose tissue as endocrine organ and its role in obesity

Due to the public health problem represented by obesity, the study of adipose tissue, particularly of the adipocyte, is central to the understanding of metabolic abnormalities associated with the development of obesity. The concept of adipocyte as endocrine and functional cell is not totally understood and can be currently defined as the capacity of the adipocyte to sense, manage, and send signals to maintain energy equilibrium in the body. Adipocyte functionality is lost during obesity and has been related to adipocyte hypertrophy, disequilibrium between lipogenesis and lipolysis, impaired transcriptional regulation of the key factors that control adipogenesis, and lack of sensitivity to external signals, as well as a failure in the signal transduction process. Thus, dysfunctional adipocytes contribute to abnormal utilization of fatty acids causing lipotoxicity in non-adipose tissue such as liver, pancreas and heart, among others. To understand the metabolism of the adipocyte it is necessary to have an overview of the developmental process of new adipocytes, regulation of adipogenesis, lipogenesis and lipolysis, endocrine function of adipocytes and metabolic consequences of its dysfunction. Finally, the key role of adipose tissue is shown by studies in transgenic animals or in animal models of diet-induced obesity that indicate the contribution of adipose tissue during the development of metabolic syndrome. Thus, understanding of the molecular process that occurs in the adipocyte will provide new tools for the treatment of metabolic abnormalities during obesity.

Genetic-epidemiological evidence on genes associated with HDL cholesterol levels: a systematic in-depth review

High-density lipoprotein (HDL) particles exhibit multiple antiatherogenic effects. They are key players in the reverse cholesterol transport which shuttles cholesterol from peripheral cells (e.g. macrophages) to the liver or other tissues. This complex process is thought to represent the basis for the antiatherogenic properties of HDL particles. The amount of cholesterol transported in HDL particles is measured as HDL cholesterol (HDLC) and is inversely correlated with the risk for coronary artery disease: an increase of 1mg/dL of HDLC levels is associated with a 2% and 3% decrease of the risk for coronary artery disease in men and women, respectively. Genetically determined conditions with high HDLC levels (e.g. familial hyperalphalipoproteinemia) often coexist with longevity, and higher HDLC levels were found among healthy elderly individuals. HDLC levels are under considerable genetic control with heritability estimates of up to 80%. The identification and characterization of genetic variants associated with HDLC concentrations can provide new insights into the background of longevity. This review provides an extended overview on the current genetic-epidemiological evidence from association studies on genes involved in HDLC metabolism. It provides a path through the jungle of association studies which are sometimes confusing due to the varying and sometimes erroneous names of genetic variants, positions and directions of associations. Furthermore, it reviews the recent findings from genome-wide association studies which have identified new genes influencing HDLC levels. The yet identified genes together explain only a small amount of less than 10% of the HDLC variance, which leaves an enormous room for further yet to be identified genetic variants. This might be accomplished by large population-based genome-wide meta-analyses and by deep-sequencing approaches on the identified genes. The resulting findings will probably result in a re-drawing and extension of the involved metabolic pathways of HDLC metabolism.

Discovery of potent, selective sulfonylfuran urea endothelial lipase inhibitors

Endothelial lipase (EL) activity has been implicated in HDL catabolism, vascular inflammation, and atherogenesis, and inhibitors are therefore expected to be useful for the treatment of cardiovascular disease. Sulfonylfuran urea 1 was identified in a high-throughput screening campaign as a potent and non-selective EL inhibitor. A lead optimization effort was undertaken to improve potency and selectivity, and modifications leading to improved LPL selectivity were identified. Radiolabeling studies were undertaken to establish the mechanism of action for these inhibitors, which were ultimately demonstrated to be irreversible inhibitors.

Endothelial lipase promotes apolipoprotein AI-mediated cholesterol efflux in THP-1 macrophages

OBJECTIVE

Endothelial lipase (EL) is expressed by macrophages within atherosclerotic lesions. We investigated the influence of EL expression on cholesterol efflux in macrophages.

METHODS AND RESULTS

The present study used lentivirus to introduce either EL shRNA for loss-of-function studies or EL cDNA for gain-of-function studies to investigate the role of EL in apoAI-mediated cholesterol efflux. ApoAI-mediated cholesterol efflux was decreased after EL suppression, but increased with EL overexpression in free cholesterol labeled and acLDL loaded THP-1 macrophages. Similar findings were observed in THP-1 macrophages after exogenous EL addition and in transfected 293 cells. EL-related apoAI-mediated cholesterol efflux decreased after treatment with heparin or catalytic inactivation (S149A mutation or tetrahydrolipstatin) alone, and completely inhibited in combination. Furthermore, EL expression did not change ABCA1 expression, but was positively correlated with apoAI binding to macrophages and 293 cells. This effect was mitigated after heparin treatment but not influenced by catalytic inactivation via tetrahydrolipstatin or the S149A mutation. Moreover, EL expression was positively associated with lysophosphatidylcholine production and inversely with phosphatidylcholine, phosphatidylethanolamine, and sphingomyelin levels. Lysophosphatidylcholine treatment dose-dependently stimulated apoAI-mediated cholesterol efflux in THP-1 macrophages.

CONCLUSIONS

EL appears to promote apoAI-mediated cholesterol efflux through catalytic and noncatalytic-dependent mechanisms.

Long-chain polyunsaturated fatty acid (LC-PUFA) transfer across the placenta

Fetal long-chain polyunsaturated fatty acid (LC-PUFA) supply during pregnancy is of major importance, particularly with respect to docosahexaenoic acid (DHA) that is an important component of the nervous system cell membranes. Growing evidence points to direct effects of DHA status on visual and cognitive outcomes in the offspring. Furthermore, DHA supply in pregnancy reduces the risk of preterm delivery. Because of limited fetal capacity to synthesize LC-PUFA, the fetus depends on LC-PUFA transfer across the placenta. Molecular mechanisms of placental LC-PUFA uptake and transport are not fully understood, but it has been clearly demonstrated that there is a preferential DHA transfer. Thus, the placenta is of pivotal importance for the selective channeling of DHA from maternal diet and body stores to the fetus. Several studies have associated various fatty acid transport and binding proteins (FATP) with the preferential DHA transfer, but also the importance of the different lipolytic enzymes has been shown. Although the exact mechanisms and the interaction of these factors remains elusive, recent studies have shed more light on the processes involved, and this review summarizes the current understanding of molecular mechanisms of LC-PUFA transport across the placenta and the impact on pregnancy outcome and fetal development.

A high-throughput screen for endothelial lipase using HDL as substrate

Endothelial lipase (EL) is a 482-amino-acid protein from the triglyceride lipase gene family that uses a Ser-His-Asp triad for catalysis. Its expression in endothelial cells and preference for phospholipids rather than triglycerides are unique. Animal models in which it is overexpressed or knocked out indicate EL levels are inversely correlated with high-density lipoprotein cholesterol (HDL-C). HDL-C is commonly referred to as the good form of cholesterol because it is involved in the reverse cholesterol transport pathway, in which excess cholesterol is effluxed from peripheral tissues for excretion or reabsorption. Thus, EL inhibition in humans is expected to lead to increases in HDL levels and possibly a decrease in cardiovascular disease. To discover inhibitors of EL, a coupled assay for EL has been developed, using its native substrate, HDL. Hydrolysis of HDL by EL yields free fatty acids, which are coupled through acyl-CoA synthetase, acyl-CoA oxidase, and horseradish peroxidase to produce the fluorescent species resorufin. This assay was developed into a 5-microL, 1536-well assay format, and a high-throughput screen was executed against the GSK collection. In addition to describing the screening results, novel post-HTS mechanism-of-action studies were developed for EL and applied to 1 of the screening hits as an example.

Type 2 diabetes mellitus and endothelial lipase

OBJECTIVE

Endothelial lipase (EL), a new member of the triacylglycerol lipase family, modulates the metabolism of high-density lipoproteins and is upregulated by inflammatory cytokines. Since type 2 diabetes is associated with chronic subclinical inflammation, we have determined whether serum EL concentration is increased in type 2 diabetes and investigated the effect of insulin on EL.

METHODS

237 type 2 diabetic patients on oral anti-diabetic agents, 111 type 2 diabetic patients on insulin therapy and 226 non-diabetic controls were recruited. Serum EL was measured by ELISA. To investigate the effect of insulin on EL production by endothelial cells, human aortic endothelial cells were incubated with insulin and EL mRNA and protein in cell medium was measured. Serum EL was also measured in 16 diabetic subjects before and after starting insulin therapy.

RESULTS

Serum EL level was highest in patients on oral anti-diabetic agents whereas those on insulin had similar EL level as controls (oral: 26.7+/-16.1ng/ml; insulin: 23.3+/-11.6, controls: 23.9+/-12.0; ANOVA p=0.04). In both controls and patients on oral anti-diabetic agents, EL correlated with log(CRP) (r=0.20, p=0.003; r=0.23, p<0.001, respectively) but no correlation was seen in patients on insulin. In vitro experiments showed that insulin significantly reduced EL mRNA and protein in human aortic endothelial cells in a dose-dependent manner. Serum EL concentration also significantly decreased in diabetic patients after starting insulin therapy (p<0.03).

CONCLUSION

Serum EL concentration was increased in type 2 diabetic patients and was associated with the degree of subclinical inflammation and exogenous insulin therapy lowered serum EL concentration.

Gene expression analysis of human endometrial endothelial cells exposed to op'-DDT

The endocrine disrupting chemical o, p'-dichlorodiphenyltrichloroethane (DDT) can affect reproductive organs, tissues and cells in several species. Treatment of human endometrial endothelial cells (HEECs) with 50 microM o,p'-DDT decreased their proliferation compared with the control. Microarray analyses revealed that o,p'-DDT affected biological processes such as the cell cycle, cell division, defence response and lipid and steroid metabolism, in cellular components such as the plasma membrane and chromosomes, with molecular functions involved in signalling, receptor and cytokine activity, confirming the results of the proliferation assay. Expression of five of the most differentially expressed genes identified in the microarray analysis was verified by real-time quantitative reverse transcription polymerase chain reaction in five HEEC cultures obtained from women in the proliferative phase and in five cultures obtained from women in the secretory phase of the menstrual cycle after treatment with o,p'-DDT. The present study supports our previous findings of decreased proliferation and increased cell death in response to o,p'-DDT and may offer important clues to the mechanisms of action of o,p'-DDT.

Endothelial lipase gene polymorphism is associated with acute myocardial infarction, independently of high-density lipoprotein-cholesterol levels

BACKGROUND

Endothelial lipase (EL) is a major determinant of high-density lipoprotein-cholesterol (HDL-C) metabolism and promotes monocytes recruitment. The local expression of EL could influence atherogenesis directly, in addition to its systemic role in HDL metabolism. The EL gene has a common 584C/T polymorphism, but it is unclear whether this polymorphism is associated with HDL-C levels or acute myocardial infarction (AMI).

METHODS AND RESULTS

A case-control study of 107 AMI patients and 107 control subjects was conducted. T allele frequency was lower in the AMI group than in controls (0.18 vs 0.26, p<0.05). No significant association was found between the 584C/T polymorphism and HDL-C levels. Multivariate regression analyses showed that the association of the T allele with AMI was statistically significant and independent of other risk factors when age, sex, hypertension, hypercholesterolemia, and diabetes mellitus were included in the analyses (odds ratio (OR), 0.52; 95% confidence interval (95% CI) 0.28-0.98; p=0.04). However, when smoking status was included, the association of the T allele with AMI did not remain statistically significant (OR, 0.61; 95% CI 0.32-1.18; p=0.14).

CONCLUSIONS

The 584C/T polymorphism of the EL gene was associated with AMI independently of HDL-C levels and thus may be involved in the pathogenesis of AMI.

Protective effect of an endothelial lipase gene variant on coronary artery disease in a Chinese population

The aim of the present study was to assess the influence of the endothelial lipase (EL) gene 584C/T variant, which results in a change at codon 111 of the EL gene from threonine to isoleucine, on the risk of coronary artery disease (CAD) in a Chinese population. The study population consisted of 265 CAD patients and 265 age- and sex-matched control subjects. The T allele frequency was significantly lower among CAD patients than among control subjects (18.3% vs. 29.8%; P < 0.001). In both the CAD and control groups, the T allele carriers had higher high density lipoprotein cholesterol (HDL-C) levels than homozygote C allele carriers. In a multiple logistic regression model adjusted for age, sex, body mass index, smoking, hypertension, diabetes, hyperlipidemia, and low density lipoprotein cholesterol, a significantly decreased risk of developing CAD was found in subjects carrying a variant CT or TT genotype (odds ratio = 0.496, 95% confidence interval = 0.341-0.723; P < 0.001), and the significance persisted after further adjustment for HDL-C. In conclusion, our observation that the EL 584T allele was associated with protection from CAD in this Chinese population replicates the findings in a Japanese study, which found a similar association of this allele with acute myocardial infarction, independent of HDL-C levels.

Endothelial lipase: its role in cardiovascular disease

Endothelial lipase (EL) has recently been identified as a new member of the triglyceride lipase gene family. EL shares a relatively high degree of homology with lipoprotein lipase and hepatic lipase, but it appears to be more specific at hydrolyzing phospholipids than lipoprotein lipase and hepatic lipase. EL is also the only identified lipase that is synthesized and expressed by endothelial cells. Data from in vitro and in vivo animal studies have suggested that EL may play a key role in modulating the metabolism of high density lipoproteins. Data are less consistent in clarifying how EL contributes to the metabolism of apolipoprotein B-containing lipoproteins. Investigations in humans are scarce. To date, increased plasma EL concentrations have been associated with a deteriorated lipoprotein-lipid profile along with elevated plasma triglyceride and apolipoprotein B concentrations, as well as with smaller low density lipoprotein particle size. Elevated proinflammatory cytokine concentrations and an increased prevalence of the metabolic syndrome have also been observed among individuals with elevated plasma EL concentrations. Taken together, data suggest that EL is one of several key regulatory enzymes of lipoprotein-lipid metabolism and that a proinflammatory state, such as the metabolic syndrome, may be implicated in the processes relating plasma EL concentrations and lipoprotein concentrations. EL should thus be considered to play an important role in the pathophysiology of cardiovascular disease.

Endothelial lipase is increased by inflammation and promotes LDL uptake in macrophages

AIM

Endothelial lipase (EL) is a member of the lipoprotein lipase family that regulates HDL metabolism. EL is known to act as a bridging molecule for monocytes or lipoproteins in vascular endothelial cells. We investigated the role and regulatory mechanisms of EL expression in macrophages.

METHODS

Macrophages originating from wild-type (EL+/+) and EL-deficient (EL-/-) mice were stimulated with lipopolysaccharide (LPS). The expression of EL mRNA was evaluated by northern blotting. DiI-LDL was used to measure the uptake of native low-density lipoprotein (nLDL).

RESULTS

LPS increased EL mRNA levels by increasing intracellular oxidative stress in the macrophages. LPS did not affect EL expression in macrophages derived from Toll-like receptor 4 (TLR4) gene mutant mice, C3H/HeJ. The uptake of nLDL after LPS-treatment was significantly lower in macrophages from EL-/- mice than those from EL+/+ mice. Simvastatin suppressed the LPS-induced upregulation of EL expression and uptake of nLDL.

CONCLUSIONS

EL expression is upregulated by LPS via TLR4 and promotes the uptake of nLDL by macrophages. Simvastatin inhibits the LPS-induced up-regulation and uptake in macrophages. Thus, our findings provide a novel role for EL in lipoprotein metabolism and would expand the range of anti-atherogenic effects of statins.

Endothelial lipase is highly expressed in macrophages in advanced human atherosclerotic lesions

Endothelial lipase (EL) is expressed in endothelial cells, and affects plasma lipoprotein metabolism by hydrolyzing phospholipids in HDL. To determine the cellular expression of EL mRNA and protein in human atherosclerotic lesions, we performed in situ hybridization and immunohistochemical studies on sections of carotid endarterectomy specimens from patients with symptomatic cerebrovascular disease. In each of eight patients, EL mRNA and/or protein were seen in areas between the necrotic core and the fibrotic cap where they colocalized with LPL and macrophage-specific CD68. Moreover, there was a positive association between the expression of EL mRNA and CD68 mRNA in plaques from 26 patients. The impact of differentiation from monocytes into macrophages, and subsequently foam cells (by incubation with acetylated LDL) on expression was studied using THP-1 monocytes and primary human monocytes. EL mRNA expression increased markedly when either type of monocytes was differentiated into macrophages. Upon further differentiation into foam cells EL mRNA decreased whereas protein levels remained high compared to monocytes. In conclusion, macrophages in advanced human atherosclerotic lesions display high levels of EL expression, and the level of EL expression varies greatly during transformation of blood monocytes into foam cells.

N-Glycosylation regulates endothelial lipase-mediated phospholipid hydrolysis in apoE- and apoA-I-containing high density lipoproteins

Endothelial lipase (EL) is a member of the triglyceride lipase gene family with high phospholipase and low triacylglycerol lipase activities and a distinct preference for hydrolyzing phospholipids in HDL. EL has five potential N-glycosylation sites, four of which are glycosylated. The aim of this study was to determine how glycosylation affects the phospholipase activity of EL in physiologically relevant substrates. Site-directed mutants of EL were generated by replacing asparagine (N) 62, 118, 375, and 473 with alanine (A). These glycan-deficient mutants were used to investigate the kinetics of phospholipid hydrolysis in fully characterized preparations of spherical reconstituted high density lipoprotein (rHDL) containing apolipoprotein E2 (apoE2) [(E2)rHDL], apoE3 [(E3)rHDL], apoE4 [(E4)rHDL], or apoA-I [(A-I)rHDL] as the sole apolipoprotein. Wild-type EL hydrolyzed the phospholipids in (A-I)rHDL, (E2)rHDL, (E3)rHDL, and (E4)rHDL to similar extents. The phospholipase activities of EL N118A, EL N375A, and EL N473A were significantly diminished relative to that of wild-type EL, with the greatest reduction being apparent for (E3)rHDL. The phospholipase activity of EL N62A was increased up to 6-fold relative to that of wild-type EL, with the greatest enhancement of activity being observed for (E2)rHDL. These data show that individual N-linked glycans have unique and important effects on the phospholipase activity and substrate specificity of EL.

Endothelial lipase and the metabolic syndrome

PURPOSE OF REVIEW

Several in-vitro and in-vivo animal studies indicate that endothelial lipase plays a key role in the intravascular remodeling of lipoproteins, particularly HDL. This review integrates this body of knowledge with more recent data in humans linking endothelial lipase to HDL metabolism and other features of the metabolic syndrome.

RECENT FINDINGS

Human studies generally support the involvement of endothelial lipase in modulating plasma HDL. The association between endothelial lipase and metabolism of apolipoprotein B-containing lipoproteins in humans, however, has not been entirely consistent with previous findings in vitro and in animals. Finally, elevated plasma endothelial lipase has been associated with abdominal obesity and hypertension, and there is now compelling evidence that inflammation and in-vivo regulation of endothelial lipase may be intrinsically related.

SUMMARY

Accumulating evidence indicates that endothelial lipase plays a role in the etiology of the atherogenic plasma lipoprotein profile characteristic of the metabolic syndrome. Increased endothelial lipase activity is linked to the underlying proinflammatory state in this condition. Further studies are required, however, to define the extent to which endothelial lipase contributes to the dyslipidemia of the metabolic syndrome relative to other important regulating factors, such as lipoprotein lipase, hepatic lipase, and cholesterol ester transfer protein.

Dysregulation of placental endothelial lipase and lipoprotein lipase in intrauterine growth-restricted pregnancies

CONTEXT

Fetal supply of maternally derived fatty acids requires lipase-mediated hydrolysis of lipoprotein-borne triglycerides and phospholipids at the placental surface.

OBJECTIVE

The objective of the study was to test the hypothesis that members of the triglyceride lipase gene (TLG) family are expressed in the human placenta at the maternoplacental (syncytiotrophoblast) and fetoplacental (endothelial cells) interface and that their expression is altered in pregnancy pathologies.

DESIGN AND SETTING

Expression of TLG family members in primary placental cells (trophoblast and endothelial cells) and tissues of first-trimester and term human placenta was analyzed by microarrays, RT-PCR, Western blotting, and immunohistochemistry. Their expression was compared between normal pregnancies and those complicated with intrauterine growth restriction (IUGR).

PARTICIPANTS

Participants included women with uncomplicated pregnancies and pregnancies complicated by IUGR.

RESULTS

Endothelial lipase (EL) and lipoprotein lipase (LPL) were the only lipases among the TLG family expressed in key cells of the human placenta. In first trimester, EL and LPL were expressed in trophoblasts. At term, EL was detected in trophoblasts and endothelial cells, whereas LPL was absent in these cells. Both lipases were found at placental blood vessels, EL in vascular endothelial cells and LPL in the surrounding smooth muscle cells. In total placental tissue EL expression prevails in first trimester and at term. Compared with normal placentas, EL mRNA was decreased (30%; P < 0.02), whereas LPL mRNA expression was increased (2.4-fold; P < 0.015) in IUGR.

CONCLUSION

EL is the predominant TLG family member in the human placenta present at both interfaces. EL and LPL are dysregulated in IUGR.

Glycosylation of endothelial lipase at asparagine-116 reduces activity and the hydrolysis of native lipoproteins in vitro and in vivo

We previously identified that four of five putative N-linked glycosylation sites of human endothelial lipase (EL) are utilized and suggested that the substitution of asparagine-116 (Asn-116) with alanine (Ala) (N116A) increased the hydrolytic activity of EL. The current study demonstrates that mutagenesis of either Asn-116 to threonine (Thr) or Thr-118 to Ala also disrupted the glycosylation of EL and enhanced catalytic activity toward synthetic substrates by 3-fold versus wild-type EL. Furthermore, we assessed the hydrolysis of native lipoprotein lipids by EL-N116A. EL-N116A exhibited a 5-fold increase in LDL hydrolysis and a 1.8-fold increase in HDL2 hydrolysis. Consistent with these observations, adenovirus-mediated expression of EL-N116A in mice significantly reduced the levels of both LDL and HDL cholesterol beyond the reductions observed by the expression of wild-type EL alone. Finally, we introduced Asn-116 of EL into the analogous positions within LPL and HL, resulting in N-linked glycosylation at this site. Glycosylation at this site suppressed the LPL hydrolysis of synthetic substrates, LDL, HDL2, and HDL3 but had little effect on HL activity. These data suggest that N-linked glycosylation at Asn-116 reduces the ability of EL to hydrolyze lipids in LDL and HDL2.

Upregulation of Macrophage Endothelial Lipase by Toll-Like Receptors 4 and 3 Modulates Macrophage Interleukin-10 and -12 Production

Limited data suggest that endothelial lipase (EL) is synthesized not only by endothelial cells but also by macrophages. Previous studies showed that proinflammatory cytokines upregulate EL in endothelial cells, but there are very few data regarding EL expression, regulation, and functional consequences in macrophages. In the present study, RAW cells and mouse peritoneal macrophages were treated with Toll-like receptor (TLR) ligands and EL expression and its consequences were assessed. We demonstrate that lipopolysaccharide, a TLR4 ligand; and polyinosinic:polycytidylic acid (poly I:C), a TLR3 ligand; but not lipoteichoic acid, a TLR2 ligand, upregulate macrophage EL expression both ex vivo and in vivo. In contrast, macrophage lipoprotein lipase expression is significantly repressed by lipopolysaccharide or poly I:C. Using C3HJ and TLR3 knockout mice, we further show that upregulation of macrophage EL expression by lipopolysaccharide or poly I:C is TLR4 or TLR3 dependent, respectively. Furthermore, we demonstrate that lipopolysaccharide induced interleukin (IL)-10 production was significantly reduced, whereas IL-12 production is significantly increased in J744 macrophages and mouse peritoneal macrophages overexpressing human EL. Conversely, significantly increased IL-10 and significantly decreased IL-12 expression were observed in mouse peritoneal macrophages isolated from EL knockout mice. Finally we show that the catalytic activity is required for EL to modulate the balance of macrophage IL-10 and IL-12 production. These results suggest that macrophage EL may play important roles in modulating the macrophage inflammatory response through local hydrolysis of HDL.

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.

Polyacrylamide gradient gel electrophoresis of lipoprotein subclasses

High-density (HDL), low-density (LDL), and very-low-density (VLDL) lipoproteins are heterogeneous cholesterol-containing particles that differ in their metabolism, environmental interactions, and association with disease. Several protocols use polyacrylamide gradient gel electrophoresis (GGE) to separate these major lipoproteins into known subclasses. This article provides a brief history of the discovery of lipoprotein heterogeneity and an overview of relevant lipoprotein metabolism, highlighting the importance of the subclasses in the context of their metabolic origins, fates, and clinical implications. Various techniques using polyacrylamide GGE to assess HDL and LDL heterogeneity are described, and how the genetic and environmental determinations of HDL and LDL affect lipoprotein size heterogeneity and the implications for cardiovascular disease are outlined.

Molecular regulation of HDL metabolism and function: implications for novel therapies

HDL metabolism represents a major target for the development of therapies intended to reduce the risk of atherosclerotic cardiovascular disease. HDL metabolism is complex and involves dissociation of HDL apolipoprotein and HDL cholesterol metabolism. Advances in our understanding of the molecular regulation of HDL metabolism, macrophage cholesterol efflux, and HDL function will lead to a variety of novel therapeutics.

Angiopoietin-like protein3 regulates plasma HDL cholesterol through suppression of endothelial lipase

OBJECTIVE

A low level of high-density lipoprotein (HDL) in plasma has been recognized as an aspect of metabolic syndrome and as a crucial risk factor of cardiovascular events. However, the physiological regulation of plasma HDL levels has not been completely defined. Current studies aim to reveal the contribution of angiopoietin-like protein3 (angptl3), previously known as a plasma suppressor of lipoprotein lipase, to HDL metabolism.

METHODS AND RESULTS

Angptl3-deficient mice showed low plasma HDL cholesterol and HDL phospholipid (PL), and which were increased by ANGPTL3 supplementation via adenovirus. In vitro, ANGPTL3 inhibited the phospholipase activity of endothelial lipase (EL), which hydrolyzes HDL-PL and hence decreases plasma HDL levels, through a putative heparin-binding site in the N-terminal domain of ANGPTL3. Post-heparin plasma in Angptl3-knockout mice had higher phospholipase activity than did that in wild-type mice, suggesting that the activity of endogenous EL is elevated in Angptl3-deficient mice. Furthermore, we established an ELISA system for human ANGPTL3 and found that plasma ANGPTL3 levels significantly correlated with plasma HDL cholesterol and HDL-PL levels in human subjects.

CONCLUSIONS

Angptl3 acts as an inhibitor of EL and may be involved in the regulation of plasma HDL cholesterol and HDL-PL levels in humans and rodents.