Fatty acids liberated from high-density lipoprotein phospholipids by endothelial-derived lipase are incorporated into lipids in HepG2 cells

Endothelial lipase mediates efficient lipolysis of triglyceride-rich lipoproteins

Triglyceride-rich lipoproteins (TRLs) are circulating reservoirs of fatty acids used as vital energy sources for peripheral tissues. Lipoprotein lipase (LPL) is a predominant enzyme mediating triglyceride (TG) lipolysis and TRL clearance to provide fatty acids to tissues in animals. Physiological and human genetic evidence support a primary role for LPL in hydrolyzing TRL TGs. We hypothesized that endothelial lipase (EL), another extracellular lipase that primarily hydrolyzes lipoprotein phospholipids may also contribute to TRL metabolism. To explore this, we studied the impact of genetic EL loss-of-function on TRL metabolism in humans and mice. Humans carrying a loss-of-function missense variant in LIPG, p.Asn396Ser (rs77960347), demonstrated elevated plasma TGs and elevated phospholipids in TRLs, among other lipoprotein classes. Mice with germline EL deficiency challenged with excess dietary TG through refeeding or a high-fat diet exhibited elevated TGs, delayed dietary TRL clearance, and impaired TRL TG lipolysis in vivo that was rescued by EL reconstitution in the liver. Lipidomic analyses of postprandial plasma from high-fat fed Lipg-/- mice demonstrated accumulation of phospholipids and TGs harboring long-chain polyunsaturated fatty acids (PUFAs), known substrates for EL lipolysis. In vitro and in vivo, EL and LPL together promoted greater TG lipolysis than either extracellular lipase alone. Our data positions EL as a key collaborator of LPL to mediate efficient lipolysis of TRLs in humans and mice.

LIPG endothelial lipase and breast cancer risk by subtypes

Experimental data showed that endothelial lipase (LIPG) is a crucial player in breast cancer. However, very limited data exists on the role of LIPG on the risk of breast cancer in humans. We examined the LIPG-breast cancer association within our population-based case-control study from Galicia, Spain, BREOGAN (BREast Oncology GAlicia Network). Plasma LIPG and/or OxLDL were measured on 114 breast cancer cases and 82 controls from our case-control study, and were included in the present study. The risk of breast cancer increased with increasing levels of LIPG (multivariable OR for the highest category (95% CI) 2.52 (1.11-5.81), P-trend = 0.037). The LIPG-breast cancer association was restricted to Pre-menopausal breast cancer (Multivariable OR for the highest LIPG category (95% CI) 4.76 (0.94-28.77), P-trend = 0.06, and 1.79 (0.61-5.29), P-trend = 0.372, for Pre-menopausal and Post-menopausal breast cancer, respectively). The LIPG-breast cancer association was restricted to Luminal A breast cancers (Multivariable OR for the highest LIPG category (95% CI) 3.70 (1.42-10.16), P-trend = 0.015, and 2.05 (0.63-7.22), P-trend = 0.311, for Luminal A and non-Luminal A breast cancers, respectively). Subset analysis only based on HER2 receptor indicated that the LIPG-breast cancer relationship was restricted to HER2-negative breast cancers (Multivariable OR for the highest LIPG category (95% CI) 4.39 (1.70-12.03), P-trend = 0.012, and 1.10 (0.28-4.32), P-trend = 0.745, for HER2-negative and HER2-positive tumors, respectively). The LIPG-breast cancer association was restricted to women with high total cholesterol levels (Multivariable OR for the highest LIPG category (95% CI) 6.30 (2.13-20.05), P-trend = 0.018, and 0.65 (0.11-3.28), P-trend = 0.786, among women with high and low cholesterol levels, respectively). The LIPG-breast cancer association was also restricted to non-postpartum breast cancer (Multivariable OR for the highest LIPG category (95% CI) 3.83 (1.37-11.39), P-trend = 0.003, and 2.35 (0.16-63.65), P-trend = 0.396, for non-postpartum and postpartum breast cancer, respectively), although we lacked precision. The LIPG-breast cancer association was more pronounced among grades II and III than grade I breast cancers (Multivariable ORs for the highest category of LIPG (95% CI) 2.73 (1.02-7.69), P-trend = 0.057, and 1.90 (0.61-6.21), P-trend = 0.170, for grades II and III, and grade I breast cancers, respectively). No association was detected for OxLDL levels and breast cancer (Multivariable OR for the highest versus the lowest category (95% CI) 1.56 (0.56-4.32), P-trend = 0.457).

Endothelial Lipase Is Involved in Cold-Induced High-Density Lipoprotein Turnover and Reverse Cholesterol Transport in Mice

The physiologic activation of thermogenic brown and white adipose tissues (BAT/WAT) by cold exposure triggers heat production by adaptive thermogenesis, a process known to ameliorate hyperlipidemia and protect from atherosclerosis. Mechanistically, it has been shown that thermogenic activation increases lipoprotein lipase (LPL)-dependent hydrolysis of triglyceride-rich lipoproteins (TRL) and accelerates the generation of cholesterol-enriched remnants and high-density lipoprotein (HDL), which promotes cholesterol flux from the periphery to the liver. HDL is also subjected to hydrolysis by endothelial lipase (EL) (encoded by LIPG). Genome-wide association studies have identified various variants of EL that are associated with altered HDL cholesterol levels. However, a potential role of EL in BAT-mediated HDL metabolism has not been investigated so far. In the present study, we show that in mice, cold-stimulated activation of thermogenic adipocytes induced expression of Lipg in BAT and inguinal WAT but that loss of Lipg did not affect gene expression of thermogenic markers. Furthermore, in both wild type (WT) and Lipg-deficient mice, activation of thermogenesis resulted in a decline of HDL cholesterol levels. However, cold-induced remodeling of the HDL lipid composition was different between WT and Lipg-deficient mice. Notably, radioactive tracer studies with double-labeled HDL indicated that cold-induced hepatic HDL cholesterol clearance was lower in Lipg-deficient mice. Moreover, this reduced clearance was associated with impaired macrophage-to-feces cholesterol transport. Overall, these data indicate that EL is a determinant of HDL lipid composition, cholesterol flux, and HDL turnover in conditions of high thermogenic activity.

Core Binding Factor β Expression in Ovarian Granulosa Cells Is Essential for Female Fertility

Core binding factor β (CBFβ) is a non-DNA-binding partner of all RUNX proteins and critical for transcription activity of CBF transcription factors (RUNXs/CBFβ). In the ovary, the expression of Runx1 and Runx2 is highly induced by the luteinizing hormone (LH) surge in ovulatory follicles, whereas Cbfb is constitutively expressed. To investigate the physiological significance of CBFs in the ovary, the current study generated two different conditional mutant mouse models in which granulosa cell expression of Cbfb and Runx2 was reduced by Cre recombinase driven by an Esr2 promoter. Cbfbgc-/- and Cbfbgc-/- × Runx2gc+/- mice exhibited severe subfertility and infertility, respectively. In the ovaries of both mutant mice, follicles develop normally, but the majority of preovulatory follicles failed to ovulate either in response to human chorionic gonadotropin administration in pregnant mare serum gonadotropin-primed immature animals or after the LH surge at 5 months of age. Morphological and physiological changes in the corpus luteum of these mutant mice revealed the reduced size, progesterone production, and vascularization, as well as excessive lipid accumulation. In granulosa cells of periovulatory follicles and corpora lutea of these mice, the expression of Edn2, Ptgs1, Lhcgr, Sfrp4, Wnt4, Ccrl2, Lipg, Saa3, and Ptgfr was also drastically reduced. In conclusion, the current study provided in vivo evidence that CBFβ plays an essential role in female fertility by acting as a critical cofactor of CBF transcription factor complexes, which regulate the expression of specific key ovulatory and luteal genes, thus coordinating the ovulatory process and luteal development/function in mice.

HDL Cholesterol Metabolism and the Risk of CHD: New Insights from Human Genetics

PURPOSE OF REVIEW

Elevated high-density lipoprotein cholesterol levels in the blood (HDL-C) represent one of the strongest epidemiological surrogates for protection against coronary heart disease (CHD), but recent human genetic and pharmacological intervention studies have raised controversy about the causality of this relationship. Here, we review recent discoveries from human genome studies using new analytic tools as well as relevant animal studies that have both addressed, and in some cases, fueled this controversy.

RECENT FINDINGS

Methodologic developments in genotyping and sequencing, such as genome-wide association studies (GWAS), exome sequencing, and exome array genotyping, have been applied to the study of HDL-C and risk of CHD in large, multi-ethnic populations. Some of these efforts focused on population-wide variation in common variants have uncovered new polymorphisms at novel loci associated with HDL-C and, in some cases, CHD risk. Other efforts have discovered loss-of-function variants for the first time in genes previously implicated in HDL metabolism through common variant studies or animal models. These studies have allowed the genetic relationship between these pathways, HDL-C and CHD to be explored in humans for the first time through analysis tools such as Mendelian randomization. We explore these discoveries for selected key HDL-C genes CETP, LCAT, LIPG, SCARB1, and novel loci implicated from GWAS including GALNT2, KLF14, and TTC39B. Recent human genetics findings have identified new nodes regulating HDL metabolism while reshaping our current understanding of known candidate genes to HDL and CHD risk through the study of critical variants across model systems. Despite their effect on HDL-C, variants in many of the reviewed genes were found to lack any association with CHD. These data collectively indicate that HDL-C concentration, which represents a static picture of a very dynamic and heterogeneous metabolic milieu, is unlikely to be itself causally protective against CHD. In this context, human genetics represent an extremely valuable tool to further explore the biological mechanisms regulating HDL metabolism and investigate what role, if any, HDL plays in the pathogenesis of CHD.

The role of endothelial lipase in lipid metabolism, inflammation, and cancer

Endothelial lipase (LIPG) plays a critical role in lipoprotein metabolism, cytokine expression, and the lipid composition of cells. Thus far, the extensive investigations of LIPG have focused on its mechanisms and involvement in metabolic syndromes such as atherosclerosis. However, recent developments have found that LIPG plays a role in cancer. This review summarizes the field of LIPG study. We focus on the role of LIPG in lipid metabolism and the inflammatory response, and highlight the recent insights in its involvement in tumor progression. Finally, we discuss potential therapeutic strategies for targeting LIPG in cancer, and the therapeutic potential of LIPG as a drug target.

FoxA and LIPG endothelial lipase control the uptake of extracellular lipids for breast cancer growth

The mechanisms that allow breast cancer (BCa) cells to metabolically sustain rapid growth are poorly understood. Here we report that BCa cells are dependent on a mechanism to supply precursors for intracellular lipid production derived from extracellular sources and that the endothelial lipase (LIPG) fulfils this function. LIPG expression allows the import of lipid precursors, thereby contributing to BCa proliferation. LIPG stands out as an essential component of the lipid metabolic adaptations that BCa cells, and not normal tissue, must undergo to support high proliferation rates. LIPG is ubiquitously and highly expressed under the control of FoxA1 or FoxA2 in all BCa subtypes. The downregulation of either LIPG or FoxA in transformed cells results in decreased proliferation and impaired synthesis of intracellular lipids.

Deregulation of lipid metabolism in cancer cells is critical to the maintenance of certain malignant features. Here, the authors show that the proliferation of breast cancer cells depends upon the extracellular activity of the endothelial lipase enzyme LIPG whose expression is regulated by the FoxA family of transcription factors.

Metabolic Flux Distribution during Defatting of Steatotic Human Hepatoma (HepG2) Cells

Methods that rapidly decrease fat in steatotic hepatocytes may be helpful to recover severely fatty livers for transplantation. Defatting kinetics are highly dependent upon the extracellular medium composition; however, the pathways involved are poorly understood. Steatosis was induced in human hepatoma cells (HepG2) by exposure to high levels of free fatty acids, followed by defatting using plain medium containing no fatty acids, or medium supplemented with a cocktail of defatting agents previously described before. We measured the levels of 28 extracellular metabolites and intracellular triglyceride, and fed the data into a steady-state mass balance model to estimate strictly intracellular fluxes. We found that during defatting, triglyceride content decreased, while beta-oxidation, the tricarboxylic acid cycle, and the urea cycle increased. These fluxes were augmented by defatting agents, and even more so by hyperoxic conditions. In all defatting conditions, the rate of extracellular glucose uptake/release was very small compared to the internal supply from glycogenolysis, and glycolysis remained highly active. Thus, in steatotic HepG2 cells, glycolysis and fatty acid oxidation may co-exist. Together, these pathways generate reducing equivalents that are supplied to mitochondrial oxidative phosphorylation.

Eicosapentaenoic acid enhances heat stress-impaired intestinal epithelial barrier function in Caco-2 cells

Objective

Dysfunction of the intestinal epithelial tight junction (TJ) barrier is known to have an important etiologic role in the pathophysiology of heat stroke. N-3 polyunsaturated fatty acids (PUFAs), including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), play a role in maintaining and protecting the TJ structure and function. This study is aimed at investigating whether n-3 PUFAs could alleviate heat stress-induced dysfunction of intestinal tight junction.

Methods

Human intestinal epithelial Caco-2 cells were pre-incubated with EPA, DHA or arachidonic acid (AA) and then exposed to heat stress. Transepithelial electrical resistance (TEER) and Horseradish Peroxidase (HRP) permeability were measured to analyze barrier integrity. Levels of TJ proteins, including occludin, ZO-1 and claudin-2, were analyzed by Western blot and localized by immunofluorescence microscopy. Messenger RNA levels were determined by quantitative real time polymerase chain reaction (Q-PCR). TJ morphology was observed by transmission electron microscopy.

Results

EPA effectively attenuated the decrease in TEER and impairment of intestinal permeability in HRP flux induced by heat exposure. EPA significantly elevated the expression of occludin and ZO-1, while DHA was less effective and AA was not at all effective. The distortion and redistribution of TJ proteins, and disruption of morphology were also effectively prevented by pretreatment with EPA.

Conclusion

This study indicates for the first time that EPA is more potent than DHA in protecting against heat-induced permeability dysfunction and epithelial barrier damage of tight junction.

Impact of endothelial lipase on cellular lipid composition

Using mass spectrometry (MS), we examined the impact of endothelial lipase (EL) overexpression on the cellular phospholipid (PL) and triglyceride (TG) content of human aortic endothelial cells (HAEC) and of mouse plasma and liver tissue. In HAEC incubated with the major EL substrate, HDL, adenovirus (Ad)-mediated EL overexpression resulted in the generation of various lysophosphatidylcholine (LPC) and lysophosphatidylethanolamine (LPE) species in cell culture supernatants. While the cellular phosphatidylethanolamine (PE) content remained unaltered, cellular phosphatidylcholine (PC)-, LPC- and TG-contents were significantly increased upon EL overexpression. Importantly, cellular lipid composition was not altered when EL was overexpressed in the absence of HDL. [¹⁴C]-LPC accumulated in EL overexpressing, but not LacZ-control cells, incubated with [¹⁴C]-PC labeled HDL, indicating EL-mediated LPC supply. Exogenously added [¹⁴C]-LPC accumulated in HAEC as well. Its conversion to [¹⁴C]-PC was sensitive to a lysophospholipid acyltransferase (LPLAT) inhibitor, thimerosal. Incorporation of [³H]-Choline into cellular PC was 56% lower in EL compared with LacZ cells, indicating decreased endogenous PC synthesis. In mice, adenovirus mediated EL overexpression decreased plasma PC, PE and LPC and increased liver LPC, LPE and TG content. Based on our results, we conclude that EL not only supplies cells with FFA as found previously, but also with HDL-derived LPC and LPE species resulting in increased cellular TG and PC content as well as decreased endogenous PC synthesis.

Expression of candidate genes related to metabolism, immunity and cellular stress during massive mortality in the American oyster Crassostrea virginica larvae in relation to biochemical and physiological parameters

Quantification of mRNA of genes related to metabolism, immunity and cellular stress was examined in relation to a massive mortality event during the culture of American oyster larvae, Crassostrea virginica which was probably, in regard to previous microbiological analysis, induced by Vibrio infection. To document molecular changes associated with the mortality event, mRNA levels were compared to biochemical and physiological data, previously described in a companion paper. Among the 18 genes studied, comparatively to the antibiotic control, 10 showed a lower relative gene expression when the massive mortality occurred. Six of them are presumed to be related to metabolism, corroborating the metabolic depression associated with the mortality event suggested by biochemical and physiological analyses. Relationships between the regulation of antioxidant enzyme activities, lipid peroxidation, and the mRNA abundance of genes linked to oxidative stress, cytoprotection, and immune response are also discussed. Finally, we observed an increase in the transcript abundance of two genes involved in apoptosis and cell regulation simultaneously with mortality, suggesting that these processes might be linked.

Endothelial lipase mediates HDL levels in normal and hyperlipidemic rabbits

AIM

Existing evidence suggests that endothelial lipase (EL) plays an important role in high-densitylipoprotein (HDL) metabolism. Because rabbits are a useful animal model for the study of human lipid metabolism and atherosclerosis, we characterized rabbit EL (rEL) expression and investigated its relationship with plasma HDL levels in normal and hyperlipidemic rabbits.

METHODS

We cloned the rEL cDNA and analyzed the EL tissue expression using Northern blotting, real-time RT-PCR, Western blotting, and in situ hybridization. We evaluated the effects of rEL antisense on plasma HDL levels.

RESULTS

We found that rEL mRNA was highly expressed in cholesterol synthesis-related organs, including the liver, testis, and adrenal along with its expression in the lung, kidney, bone marrow, and small intestine. Interestingly, Watanabe heritable hyperlipidemic (WHHL) rabbits, a model of human familial hypercholesterolemia, had lower plasma levels of HDLs than normal rabbits. The plasma HDL levels in WHHL rabbits were inversely associated with high levels of plasma rEL proteins and hepatic expression of rEL mRNA. Injection of rEL-specific antisense oligonucleotides into rabbits resulted in the elevation of plasma large HDLs. Furthermore, we demonstrated that rEL mRNA was expressed by both endothelial cells and macrophages in the lesions of aortic atherosclerosis of WHHL rabbits.

CONCLUSIONS

rEL is expressed in multiple tissues and may have many physiological and pathophysiological functions, such as in the regulation of cholesterol metabolism and atherosclerosis. Our results suggest that EL is an important regulator of plasma HDL levels in rabbits.

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.

Application of metabolic flux analysis to identify the mechanisms of free fatty acid toxicity to human hepatoma cell line

Chronic exposure to elevated levels of free fatty acids (FFAs) has been shown to cause cell death (lipotoxicity), but the underlying mechanisms of lipotoxicity in hepatocytes remain unclear. We have previously shown that the saturated FFAs cause much greater toxicity to human hepatoma cells (HepG2) than the unsaturated ones (Srivastava and Chan, 2007). In this study, metabolic flux analysis (MFA) was applied to identify the metabolic changes associated with the cytotoxicity of saturated FFA. Measurements of the fluxes revealed that the saturated FFA, palmitate, was oxidized to a greater extent than the non-toxic oleate and had comparatively less triglyceride synthesis and reduced cystine uptake. Although fatty acid oxidation had a high positive correlation to the cytotoxicity, inhibitor experiments indicated that the cytotoxicity was not due to the higher fatty acid oxidation. Application of MFA revealed that cells exposed to palmitate also had a consistently reduced flux of glutathione (GSH) synthesis but greater de novo ceramide synthesis. These predictions were experimentally confirmed. In silico sensitivity analyses identified that the GSH synthesis was limited by the uptake of cysteine. Western blot analyses revealed that the levels of the cystine transporter xCT, but not that of the GSH-synthesis enzyme glutamyl-cysteine synthase (GCS), were reduced in the palmitate cultures, suggesting the limitation of cysteine import as the cause of the reduced GSH synthesis. Finally, supplementing with N-acetyl L-cysteine (NAC), a cysteine-provider whose uptake does not depend on xCT levels, reduced the FFA-toxicity significantly. Thus, the metabolic alterations that contributed to the toxicity and suggested treatments to reduce the toxicity were identified, which were experimentally validated.

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.

Hypochlorite-modified albumin colocalizes with RAGE in the artery wall and promotes MCP-1 expression via the RAGE-Erk1/2 MAP-kinase pathway

Signal transduction via the endothelial receptor for advanced glycation end products (RAGE) plays a key role in vascular inflammation. Recent observations have shown that the myeloperoxidase-H2O2-chloride system of activated phagocytes is highly up-regulated under inflammatory conditions where hypochlorous acid (HOCl) is formed as the major oxidant. Albumin, an in vivo carrier for myeloperoxidase is highly vulnerable to oxidation and a major representative of circulating advanced oxidized proteins during inflammatory diseases. Immunohistochemical studies performed in the present study revealed marked colocalization of HOCl-modified epitopes with RAGE and albumin in sections of human atheroma, mainly at the endothelial lining. We show that albumin modified with physiologically relevant concentrations of HOCl, added as reagent or generated by the myeloperoxidase-H2O2-chloride system, is a high affinity ligand for RAGE. Albumin, modified by HOCl in the absence of free amino acids/carbohydrates/lipids to exclude formation of AGE-like structures, induced a rapid, RAGE-dependent activation of extracellular signal-regulated kinase 1/2 and up-regulation of the proinflammatory mediator monocyte chemoattractant protein-1. Cellular activation could be blocked either by a specific polyclonal anti-RAGE IgG and/or a specific mitogen-activated protein-kinase kinase inhibitor. The present study demonstrates that HOCl-modified albumin acts as a ligand for RAGE and promotes RAGE-mediated inflammatory complications.

Regulated expression of endothelial lipase by porcine brain capillary endothelial cells constituting the blood-brain barrier

Normal neurological function depends on a constant supply of polyunsaturated fatty acids to the brain. A considerable proportion of essential fatty acids originates from lipoprotein-associated lipids that undergo uptake and/or catabolism at the blood-brain barrier (BBB). This study aimed at identifying expression and regulation of endothelial lipase (EL) in brain capillary endothelial cells (BCEC), major constituents of the BBB. Our results revealed that BCEC are capable of EL synthesis and secretion. Overexpression of EL resulted in enhanced hydrolysis of extracellular high-density lipoprotein (HDL)-associated sn-2-labeled [(14)C]20 : 4 phosphatidylcholine. [(14)C]20 : 4 was recovered in cellular lipids, indicating re-uptake and intracellular re-esterification. To investigate local regulation of EL in the cerebrovasculature, BCEC were cultured in the presence of peroxisome-proliferator activated receptor (PPAR)- and liver X receptor (LXR)-agonists, known to regulate HDL levels. These experiments revealed that 24(S)OH-cholesterol (a LXR agonist), bezafibrate (a PPARalpha agonist), or pioglitazone (a PPARgamma agonist) resulted in down-regulation of EL mRNA and protein levels. Our findings implicate that EL could generate fatty acids at the BBB for transport to deeper regions of the brain as building blocks for membrane phospholipids. In addition PPAR and LXR agonists appear to contribute to HDL homeostasis at the BBB by regulating EL expression.

Endothelial lipase is inactivated upon cleavage by the members of the proprotein convertase family

Mature endothelial lipase (EL) is a 68 kDa glycoprotein. In HepG2 cells infected with adenovirus encoding human EL, the mature EL was detectable in the cell lysates and heparin-releasable fractions. In contrast, cell media of these cells contained two EL fragments: an N-terminal 40 kDa fragment and a C-terminal 28 kDa fragment. N-terminal protein sequencing of the His-tagged 28 kDa fragment revealed that EL is cleaved on the C terminus of the sequence RNKR330, the consensus cleavage sequence for mammalian proprotein convertases (pPCs). Replacement of Arg-330 with Ser by site-directed mutagenesis totally abolished EL processing. EL processing could efficiently be attenuated by specific inhibitors of pPCs, alpha1-antitrypsin Portland (alpha1-PDX) and alpha1-antitrypsin variant AVRR. Coexpression of the pPCs furin, PC6A, and PACE4 with EL resulted in a complete conversion of the full-length EL to a truncated 40 kDa fragment. Exogenously added EL was also processed by cells, and the processing could be attenuated by alpha1-PDX. The expressed N-terminal 40 kDa fragment of EL (EL-40) harboring the catalytic site failed to hydrolyze [14C]NEFA from [14C]dipalmitoyl-PC-labeled HDL. EL-40 was incapable of bridging 125I-labeled HDL to the cells and had no impact on plasma lipid concentration when overexpressed in mice. Thus, our results demonstrate that pPCs are involved in the inactivation process of EL.

Endothelial lipase releases saturated and unsaturated fatty acids of high density lipoprotein phosphatidylcholine

We assessed the ability of endothelial lipase (EL) to hydrolyze the sn-1 and sn-2 fatty acids (FAs) from HDL phosphatidylcholine. For this purpose, reconstituted discoidal HDLs (rHDLs) that contained free cholesterol, apolipoprotein A-I, and either 1-palmitoyl-2-oleoylphosphatidylcholine, 1-palmitoyl-2-linoleoylphosphatidylcholine, or 1-palmitoyl-2-arachidonylphosphatidylcholine were incubated with EL- and control (LacZ)-conditioned media. Gas chromatography analysis of the reaction mixtures revealed that both the sn-1 (16:0) and sn-2 (18:1, 18:2, and 20:4) FAs were liberated by EL. The higher rate of sn-1 FA cleavage compared with sn-2 FA release generated corresponding sn-2 acyl lyso-species as determined by MS analysis. EL failed to release sn-2 FA from rHDLs containing 1-O-1'-hexadecenyl-2-arachidonoylphosphatidylcholine, whose sn-1 position contained a nonhydrolyzable alkyl ether linkage. The lack of phospholipase A(2) activity of EL and its ability to liberate [(14)C]FA from [(14)C]lysophosphatidylcholine (lyso-PC) led us to conclude that EL-mediated deacylation of phosphatidylcholine (PC) is initiated at the sn-1 position, followed by the release of the remaining FA from the lyso-PC intermediate. Thin-layer chromatography analysis of cellular lipids obtained from EL-overexpressing cells revealed a pronounced accumulation of [(14)C]phospholipid and [(14)C]triglyceride upon incubation with 1-palmitoyl-2-[1-(14)C]linoleoyl-PC-labeled HDL(3), indicating the ability of EL to supply cells with unsaturated FAs.

The role of endothelial lipase in high-density lipoprotein metabolism

PURPOSE OF REVIEW

Elevating high-density lipoprotein levels is increasingly being identified as an essential strategy for the prevention of atherosclerosis. Plasma levels of high-density lipoprotein cholesterol and its major protein, apoAI, are largely influenced by the rate of turnover. Lipases play an important role in modulating the metabolism of high-density lipoprotein. In particular, endothelial lipase has been shown to be an important determinant of high-density lipoprotein metabolism and levels in murine models. This article reviews new developments in our understanding of the biology of endothelial lipase and its relation to high-density lipoprotein metabolism.

RECENT FINDINGS

Inhibition of the endothelial lipase gene, either by antibody injection or by targeted gene deletion, results in an approximately 50% increase in high-density lipoprotein cholesterol levels in mice. As many as 31 single-nucleotide polymorphisms have been identified in the endothelial lipase gene. The 584 C/T mutation, which results in a threonine-to-isoleucine amino acid change, has been associated with higher high-density lipoprotein cholesterol levels in three separate studies.

SUMMARY

Increasing evidence suggests that endothelial lipase plays a significant role in high-density lipoprotein metabolism. Endothelial lipase could be an important new target for novel therapies to raise high-density lipoprotein levels.

Endothelial lipase provides an alternative pathway for FFA uptake in lipoprotein lipase-deficient mouse adipose tissue

Lipoprotein lipase (LPL) is thought to be the only enzyme responsible for the catabolism of triglycerides (TGs) associated with TG-rich lipoproteins in adipose tissue (AT). However, LPL deficiency in humans and induced mutant mice is not associated with decreased fat mass. We investigated whether endothelial lipase (EL), a recently discovered phospholipase, might represent an alternative mechanism for the uptake of phospholipid-derived fatty acids in murine lipoprotein-deficient AT. When LPL was expressed in AT and isolated murine adipocytes, EL mRNA was not detectable. In contrast, mouse AT and isolated adipocytes that lacked LPL expressed large amounts of EL mRNA. The cellular phospholipase activity in LPL-deficient fat pads was increased 4-fold compared with control fat pads and could be inhibited to control levels by a specific EL antibody. Fatty acids produced by EL activity were absorbed by adipocytes and incorporated into the TG moiety of AT. Our results suggest that EL activity in AT and other peripheral tissues might contribute to the tissue uptake of free fatty acids, which could have important implications for the metabolism of plasma lipoproteins.

Endothelial lipase-modified high-density lipoprotein exhibits diminished ability to mediate SR-BI (scavenger receptor B type I)-dependent free-cholesterol efflux

Endothelial lipase (EL) is a phospholipase with little triacylglycerol lipase activity. To assess structural and functional properties of EL-HDL (EL-modified high-density lipoprotein), HDL was incubated with conditioned medium from Cos-7 cells infected with adenovirus encoding human EL. After re-isolation of HDL by ultracentrifugation, TLC and HPLC analyses revealed that EL-HDL was markedly depleted in phosphatidylcholine and enriched in lyso-phosphatidylcholine compared with LacZ-HDL (control HDL) incubated with conditioned medium from Cos-7 cells infected with adenovirus encoding beta-galactosidase. The EL-HDL was enriched in non-esterified fatty acids and, as revealed by lipid electrophoresis, was more negatively charged than control HDL. The HDL particle size as well as the total cholesterol, free cholesterol and triacylglycerol content of HDL were not significantly altered after EL modification. The ability of EL-HDL to mediate 3H-cholesterol efflux from SR-BI (scavenger receptor B type I) overexpressing Chinese-hamster ovary cells was impaired and markedly lower compared with LacZ-HDL at HDL concentrations of 100 microg/ml and above. Studies with 125I-labelled HDL showed almost unaltered binding affinity (K(m) values) and a slightly but significantly decreased binding capacity (B(max) values) of EL-HDL to SR-BI, compared with LacZ-HDL. The ATP-binding-cassette transporter A1-dependent cholesterol and phospholipid effluxes were not affected by EL modification. From these results, we concluded that EL modification alters chemical composition and physical properties of HDL, resulting in its decreased binding capacity to SR-BI and a diminished ability to mediate SR-BI-dependent cholesterol efflux.

Endothelial lipase and HDL metabolism

PURPOSE OF REVIEW

In the past year, several laboratories taking independent approaches have provided compelling evidence that endothelial lipase, a relatively recent addition to the triglyceride lipase gene family, is a major determinant of HDL metabolism. This review summarizes recent findings from experiments in mice with altered levels of endothelial lipase, from an examination of endothelial lipase catalytic and non-catalytic functions in vitro, and from human genetic studies.

RECENT FINDINGS

An analysis of lipids and lipoproteins in endothelial lipase knockout and transgenic mice and in mice with adenovirus-driven hepatic overexpression of endothelial lipase shows, without exception, that total cholesterol, phospholipid and HDL-cholesterol all vary inversely with the endothelial lipase gene dosage, and primarily depend on endothelial lipase catalytic activity. Endothelial lipase participates in HDL metabolism by promoting the turnover of HDL components and increasing the catabolism of apolipoprotein A-I. The measurement of lipase activity on lipoprotein substrates in vitro demonstrates that endothelial lipase is distinct from other triglyceride lipases in showing the highest activity on HDL. Endothelial lipase gene polymorphisms in humans appear to be associated with HDL-cholesterol or HDL3-cholesterol concentrations.

SUMMARY

A low HDL-cholesterol level in humans is a risk factor for coronary heart disease. Although not yet demonstrated, it is possible that the action of endothelial lipase on HDL may promote atherogenesis, in which case endothelial lipase may represent an attractive target for pharmaceutical intervention.