Role of hepatic lipase and endothelial lipase in high-density lipoprotein-mediated reverse cholesterol transport.
ABSTRACT Reverse cholesterol transport (RCT) constitutes a key part of the atheroprotective properties of high-density lipoproteins (HDL). Hepatic lipase (HL) and endothelial lipase (EL) are negative regulators of plasma HDL cholesterol levels. Although overexpression of EL decreases overall macrophage-to-feces RCT, knockout of both HL and EL leaves RCT essentially unaffected. With respect to important individual steps of RCT, current data on the role of EL and HL in cholesterol efflux are not conclusive. Both enzymes increase hepatic selective cholesterol uptake; however, this does not translate into altered biliary cholesterol secretion, which is regarded the final step of RCT. Also, the impact of HL and EL on atherosclerosis is not clear cut; rather it depends on respective experimental conditions and chosen models. More mechanistic insights into the diverse biological properties of these enzymes are therefore required to firmly establish EL and HL as targets for the treatment of atherosclerotic cardiovascular disease.
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ABSTRACT: The ATP-binding cassette transporter A1 (ABCA1) mediates the efflux of excess cholesterol from foam cells to lipid-poor apolipoprotein A-I, in a process called reverse cholesterol transport. Lipoprotein lipase (LPL) is a lipolytic enzyme expressed by macrophages within atherosclerotic lesions. Lentivirus-mediated RNA interference was used to genetically knock-down (KD) the expression of LPL in THP-1 macrophages. Silencing of the LPL gene was confirmed by end-point PCR, real time PCR, and protein analysis. Suppression of LPL expression correlated with a 1.6-fold up-regulation of ABCA1 mRNA levels, and resulted in a 4.5-fold increase in ABCA1-dependent cholesterol efflux. Replenishing LPL by addition of purified bovine LPL to the cell culture media resulted in down-regulation of ABCA1-mediated cholesterol efflux in both wild-type and LPL knockdown cells. These finding suggests an inverse correlation between macrophage LPL levels and ABCA1 cholesterol transport activity.Biochemical and Biophysical Research Communications 07/2014; · 2.28 Impact Factor
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ABSTRACT: In recent years, the high-density lipoprotein (HDL) hypothesis has been challenged. Several completed randomized clinical trials continue to fall short in demonstrating HDL, or at least HDL-cholesterol (HDL-C) levels, as being a consistent target in the prevention of cardiovascular diseases. However, population studies and findings in lipid modifying trials continue to strongly support HDL-C as a superb risk predictor. It is increasingly evident that the complexity of HDL metabolism confounds the use of HDL-C concentration as a unified target. However, important insights continue to emerge from the post hoc analyses of recently completed (i) fibrate-based FIELD and ACCORD trials, including the unexpected beneficial effect of fibrates in microvascular diseases, (ii) the niacin-based AIM-HIGH and HPS2-THRIVE studies, (iii) recombinant HDL-based as well as (iv) the completed CETP inhibitor-based trials. These together with on-going mechanistic studies on novel pathways, which include the unique roles of microRNAs, post-translational remodeling of HDL and novel pathways related to HDL modulators will provide valuable insights to guide how best to refocus and redesign the conceptual framework for selecting HDL-based targets.Critical Reviews in Clinical Laboratory Sciences. 08/2014;
- Revista medica de Chile 03/2012; 140(3):373-378. · 0.36 Impact Factor
Role of Hepatic Lipase and Endothelial Lipase
in High-Density Lipoprotein–Mediated Reverse
Wijtske Annema & Uwe J. F. Tietge
Published online: 22 March 2011
# The Author(s) 2011. This article is published with open access at Springerlink.com
Abstract Reverse cholesterol transport (RCT) constitutes a
key part of the atheroprotective properties of high-density
lipoproteins (HDL). Hepatic lipase (HL) and endothelial
lipase (EL) are negative regulators of plasma HDL
cholesterol levels. Although overexpression of EL
decreases overall macrophage-to-feces RCT, knockout of
both HL and EL leaves RCT essentially unaffected. With
respect to important individual steps of RCT, current data
on the role of EL and HL in cholesterol efflux are not
conclusive. Both enzymes increase hepatic selective cho-
lesterol uptake; however, this does not translate into altered
biliary cholesterol secretion, which is regarded the final
step of RCT. Also, the impact of HL and EL on
atherosclerosis is not clear cut; rather it depends on
respective experimental conditions and chosen models.
More mechanistic insights into the diverse biological
properties of these enzymes are therefore required to firmly
establish EL and HL as targets for the treatment of
atherosclerotic cardiovascular disease.
Keywords Atherosclerosis.Bile.Cardiovascular disease.
lipase.High density lipoproteins.Inflammation.Liver.
Reverse cholesterol transport.Risk.Therapy.Triglycerides
Despite the widespread use of low-density lipoprotein
(LDL) reduction therapies, atherosclerotic cardiovascular
disease (CVD) remains a leading cause of morbidity and
mortality in developed countries. Treatment with statins
(HMG-CoA reductase inhibitors), for example, only
decreases the risk of major coronary events by 15% to
40% even though these drugs consistently and efficiently
lower LDL cholesterol . This substantial residual risk
indicates a potential clinical benefit of novel therapeutic
strategies that look beyond LDL and focus on high-density
lipoproteins (HDL). Plasma HDL cholesterol levels are
strongly, inversely, and independently related to cardiovas-
cular disease risk . A major part of this atheroprotective
potential of HDL conceivably consists of its key role in
reverse cholesterol transport (RCT, see below for details)
. Among the factors modulating HDL metabolism,
lipases such as hepatic lipase (HL) and endothelial lipase
(EL) are of prime importance. HL and EL both belong to
the triacylglycerol lipase family, which also includes
lipoprotein lipase (LPL), but members of this family differ
in their respective hydrolytic activities. Whereas LPL is
almost exclusively a triglyceridase, HL uses both phospho-
lipids and triglycerides as substrates, whereas EL possesses
predominantly phospholipase and very little triglyceridase
activity (Table 1) . By decreasing the triglyceride and
phospholipid content of HDL, HL and EL are both
significant negative regulators of plasma HDL levels .
Therefore, pharmacologic inhibition of these lipases might
represent a valid approach to increase HDL cholesterol.
This review focuses on the impact of both HL and EL on
HDL metabolism and RCT, but also addresses the con-
sequences of modulating HL and EL expression and
activity for atherosclerotic lesion development.
W. Annema:U. J. F. Tietge (*)
Department of Pediatrics, Center for Liver, Digestive,
and Metabolic Diseases, University Medical Center Groningen,
University of Groningen,
9713 GZ, Groningen, The Netherlands
Curr Atheroscler Rep (2011) 13:257–265
Role of High-Density Lipoproteins in Reverse
HDL plays a central role in RCT (Fig. 1) . The first step
in RCT involves the efflux of cholesterol from peripheral
cells, most importantly macrophage foam cells within
atherosclerotic plaques, toward either apolipoprotein (apo)
A-I, which is mainly ATP-binding cassette transporter A1
(ABCA1)-mediated, or HDL, a process mainly driven by
ABCG1. Lecithin:cholesterol acyltransferase (LCAT) then
catalyzes the formation of mature spherical HDL by
converting free cholesterol to apolar cholesterol ester,
which moves to the core of the HDL particle. In addition
to ABCA1 and ABCG1, the scavenger receptor class B
type I (SR-BI) might also contribute to net cholesterol
efflux. The cholesterol originating from peripheral cells is
subsequently taken up into the liver in several distinct
pathways that are all relevant for RCT: 1) selective uptake
mediated by SR-BI, 2) holoparticle uptake involving the
extracellularly localized β-chain of the mitochondrial F1
ATPase as well as P2Y13 receptors, and 3) in species such
as humans, rabbits, hamsters, and monkeys, but not mice
and rats, transfer to apoB-containing lipoproteins by
cholesteryl ester transfer protein (CETP) followed by
uptake via LDL receptors. The major pathway for disposal
of hepatic cholesterol from the body is generally believed to
be its secretion into bile in the form of either free
cholesterol or bile acids. Eventually, neutral sterols (cho-
lesterol and its metabolites) and bile acids are excreted via
the feces. The RCT pathway has been reviewed in more
detailed elsewhere .
HL is a 65-kD glycoprotein synthesized and secreted
primarily by hepatocytes and to a lesser extent by macro-
phages(Table1) . In the liver, HL is anchored via heparan
sulfate proteoglycans to hepatocytes in the Space of Disse
and to the sinusoidal endothelial cell surface . Virtually all
lipoprotein subclasses are substrates for HL, resulting in the
generation of smaller and denser particles (Table 1) . In
addition, independent of its catalytic activity, HL anchored to
cell surface proteoglycans also has a bridging function
promoting receptor-mediated uptake of lipoproteins .
HL and HDL Metabolism
By hydrolyzing triglycerides and phospholipids, HL con-
verts lipid-rich large HDL2to smaller, more dense HDL3.
Mice lacking HL characteristically display increased plas-
ma levels of larger, phospholipid- and triglyceride-enriched
Table 1 Characteristics of hepatic lipase and endothelial lipase
Hepatic lipaseEndothelial lipase
Endothelial cells, macrophages, thyroid gland,
hepatocytes, kidney, lung, ovary, testis, and placenta
PL > TG
Knockout ↓ ↑
Overexpression ↑ ↓
Knockout ↓ ↔
Impact on plasma HDL cholesterol levels
TG > PL
Knockout ↑ ↔
Impact on cholesterol efflux
Impact on hepatic uptake of HDL cholesterolKnockout ↓
Impact on biliary cholesterol secretion
Impact on fecal neutral sterol and bile acid excretion
Impact on macrophage-to-feces RCT
Impact on atherosclerosisKnockout ↑ ↓
Macrophage expression ↑
Macrophage knockout ↑
TG triglycerides, PL phospholipids, RCT reverse cholesterol transport; ↑ = increased; ↓ = decreased; ↔ = not affected. Multiple arrows per
condition indicate divergent reports in literature
258Curr Atheroscler Rep (2011) 13:257–265
HDL (Table 1) [6•]. In turn, in mice and in rabbits,
overexpression of human HL resulted in a marked decrease
in HDL cholesterol due to an increased catabolic rate .
HL can also impact on HDL metabolism independent of its
lipolytic properties, as adenovirus-mediated overexpression
of catalytically inactive HL in HL knockout mice led to a
reduction in plasma HDL levels, though to a lesser extent
than observed in mice overexpressing similar levels of
wild-type HL . HL deficiency in humans also results in
elevated levels of larger HDL2particles, an enrichment of
HDL with triglycerides, and hyperalphalipoproteinemia
attributable to slower catabolism of apoA-I . Consistent-
ly, certain polymorphisms in the HL gene such as S267F,
C-480T, and C-514T that result in lower HL expression and
activity are firmly associated with higher plasma HDL
cholesterol [8, 9]. These combined data support a key role
for HL as a negative regulator of plasma HDL levels.
Impact of HL on Selected Key Steps in RCT
Cholesterol efflux is a critical first step in RCT . Plasma
as well as HDL from HL-deficient mice has an increased
capacity to elicit cholesterol efflux from lipid-laden macro-
phages in vitro (Table 1), which was ascribed to phospho-
lipid enrichment of HDL from the HL knockouts [6•].
However, in humans with a complete deficiency of HL, the
ability of plasma or isolated HDL to promote cholesterol
efflux from normal human skin fibroblasts was not altered
. Thus, the impact of HL on macrophage cholesterol
efflux remains to be elucidated. It would be interesting to
also study the effect of HL overexpression, because HL
increases the formation of preβ-HDL, an important medi-
ator of ABCA1-mediated macrophage cholesterol efflux
. In addition, the impact of macrophage HL on
cholesterol efflux has not been explored yet.
Fig. 1 Schematic depicting the impact of hepatic lipase (HL) and
endothelial lipase (EL) on reverse cholesterol transport (RCT). Small
discoidal high-density lipoprotein (HDL) particles are generated by
the liver (about 70% contribution) and by the intestine (about 30%
contribution). Free cholesterol (FC) from macrophage foam cells is
effluxed toward these particles by ATP-binding cassette transporter A1
(ABCA1). Through the action of lecithin:cholesterol acyltransferase
(LCAT), these particles mature and become spherical; ABCG1 and
scavenger receptor class B type I (SR-BI) add more cholesterol onto
these larger HDL. EL and HL hydrolyze HDL phospholipids and
phospholipids/triglycerides (TG), respectively, thereby destabilizing
the particle, resulting in shedding of poorly lipidated apolipoprotein
(apo) A-I that is subject to clearance by the kidneys. Cholesteryl ester
from these HDL particles becomes more susceptible towards SR-BI—
mediated selective uptake. In turn, selective uptake is also required for
HDL remodeling by these lipases to continue. Via SR-BI, HDL
cholesterol enters the hepatic cholesterol pool and can either be
directly secreted as free cholesterol into bile and feces or after
metabolic conversion into bile acids. Cholesteryl ester transfer protein
(CETP), expressed in humans but not in mice and rats, mediates the
hetero-exchange of cholesteryl ester (CE) originating from HDL with
triglycerides originating from apoB-containing lipoproteins. TG-
enrichment of HDL makes the particle a better substrate for HL. On
the other hand, cholesterol transferred to apoB-containing lipoproteins
can be taken up into the liver via low-density lipoprotein receptors
(LDLR) and then also enters the hepatic cholesterol pool. EL and HL
expression consistently results in lower plasma HDL levels, increased
hepatic cholesterol content, but unchanged biliary cholesterol secre-
tion rates. Results regarding the impact of these lipases on cholesterol
efflux are variable. The net effect of the absence of HL and EL on
overall RCT is minimal, whereas overexpression of EL has been
shown to decrease RCT. VLDL—very low-density lipoprotein
Curr Atheroscler Rep (2011) 13:257–265 259
Another important step in RCT is hepatic uptake of HDL
cholesterol, and several lines of evidence indicate that HL
stimulates this process (Table 1). HL increases the selective
uptake of HDL cholesteryl ester (CE) in hepatic and non-
hepatic cell lines . Moreover, in vivo hepatic selective
uptake of HDL-CE was impaired in HL knockout mice [6•]
and significantly higher in mice overexpressing human HL
. Both the hydrolytic activity of HL and its heparan
sulfate proteoglycan-dependent bridging properties contrib-
ute to the stimulating effect of HL on SR-BI—mediated
selective uptake .
Although HL facilitates the uptake of HDL cholesterol
into the liver, this does not appear to translate into increased
biliary sterol secretion (Table 1), a third important step in
RCT. The biliary secretion rates of cholesterol and bile
acids were identical in HL-deficient mice and wild-type
controls, even when fed cholesterol-enriched or lithogenic
diets . Consequently, HL deficiency did not change
fecal neutral sterol or bile acid excretion (Table 1) .
HL and Macrophage-Specific RCT
Even though RCT from macrophages to feces represents
only a small fraction of the total cholesterol pool, this flux
is nevertheless considered highly relevant for the develop-
ment of atherosclerosis. When mice were injected with3H
cholesterol-labeled macrophages into the peritoneal cavity
and the appearance of label in plasma and feces was
followed for 48 h, plasma levels of3H-sterol were higher in
mice deficient in HL compared to controls. However,
overall macrophage-to-feces RCT determined by excretion
of macrophage-derived label into the feces over 48 h
remained essentially unaffected in HL knockouts (Table 1)
[6•]. On the other hand, the effects of HL overexpression
have not been addressed thus far. Also, HL expression by
macrophages might have an impact on RCT that has not
been experimentally explored yet.
HL and Atherosclerosis
Several studies have thus far addressed the role of HL in
atherosclerosis development in different rodent models
(Table 1) and humans but without reaching conclusive
results. In LDL receptor–knockout mice, aortic atheroscle-
rotic lesion formation was accelerated when endogenous
HL was also absent , whereas transgenic overexpres-
sion of human HL reduced atherosclerosis by 40% to 70%
[12, 13]. Overexpressing the catalytically inactive variant of
human HL was atheroprotective in chow-fed LDL recep-
tor–knockout mice lacking endogenous HL [13, 14],
whereas no effect on atherosclerosis development was seen
in the same model on a Western diet . In contrast, HL
deficiency in apoE knockout mice on a regular chow diet
reduced aortic lesion size by 75% despite increased
circulating levels of atherogenic apoB-containing lip-
oproteins. Decreased atherosclerosis development in this
mouse model was proposed to be due to higher HDL
levels and an increased capacity of HDL to elicit
cholesterol efflux . Furthermore, HL deficiency
delayed the onset of occlusive atherosclerosis, resulting
in myocardial infarction and cardiac dysfunction in SR-BI/
apoE double-knockout mice . However, the impact of
HL on atherosclerotic lesion formation in an experimental
animal model expressing CETP has not been reported.
Bone marrow transplantation studies showed that restrict-
ing HL expression to macrophages enhanced the forma-
tion of early atherosclerotic lesions in both apoE knockout
mice and LCAT transgenic mice without changing plasma
lipoprotein levels . On the other hand, a more recent
publication demonstrated that deficiency of leukocyte-
derived HL reduced circulating HDL cholesterol levels
and increased the extent of aortic atherosclerosis in LDL
receptor—knockout mice expressing human CETP .
Whether the experimental design or the mouse model used
offers the explanation for these divergent results remains
to be seen.
Complete HL deficiency in humans is a rare condition;
however, some of these patients develop premature coro-
nary artery disease despite increased plasma HDL choles-
terol levels . Likewise, a lower post-heparin plasma HL
activity has been related to a higher extent of coronary
artery disease in 200 men undergoing coronary angiogra-
phy . In contrast, human studies investigating the
relationship between polymorphisms in the HL gene, which
supposedly resulted in reduced HL activity, and atheroscle-
rotic cardiovascular disease did not provide a clear picture
(summarized in Table 2). In an autopsy cohort of 700
middle-aged Finnish men, the HL C-480T promoter
polymorphism was related to a larger coronary atheroscle-
rotic plaque area , and in a large population-based
Danish study homozygosity for three common single
nucleotide polymorphisms in the HL promoter
−480T, and −729G) was associated with an increased risk
of ischemic heart disease despite higher plasma HDL
cholesterol . Furthermore, the prevalence of the C to T
substitution at position 514 of the HL gene was significantly
higher in 490 middle-aged male Caucasian Australian
patients with coronary heart disease compared with 330
controls . Controversially, in the almost 9000 partic-
ipants of the Copenhagen City Heart study, no association
between risk for ischemic cardiovascular disease and six
genetic variants of the HL gene was detectable .
Importantly, the frequency of the −514T allele of HL was
identical between men with documented coronary artery
disease and healthy controls . Other researchers found
in a multiethnic population that individuals with the CC
260 Curr Atheroscler Rep (2011) 13:257–265
genotype of the C-480T variant, who have higher HL
activity, had a 13% greater carotid intima wall-thickness
. Taken together, these data stress the need for further
carefully conducted studies that, in addition to genotyping
the participating subjects, also aim to correlate HL mass as
well as activity with relevant hard clinical end points of
EL, encoded by the LIPG gene, is a 68-kD glycoprotein that
was cloned independently in 1999 by two groups (Table 1)
[26, 27]. The amino acid sequence of EL is 41%
homologous to HL. Nevertheless, EL is distinct from other
members of the triacylglycerol family because it is synthe-
sized and secreted by endothelial cells. In addition, expres-
sion of EL has been detected in human placenta, thyroid
gland, lung, liver, kidney, ovary, testis, and macrophages in
vitro as well as in foam cells within human atherosclerotic
plaques (Table 1) [3, 26, 27]. EL has primarily sn-1-
phospholipase activity toward all lipoprotein subclasses,
however, the preferred substrate is apparently phospholipids
within HDL (Table 1) . Similar to HL, EL mediates
bridging between lipoproteins and cell surface heparin
sulfate proteoglycans .
EL and HDL Metabolism
EL is a negative regulator of plasma HDL cholesterol levels
(Table 1). Adenovirus-mediated overexpression of EL
resulted in a marked decrease in plasma HDL cholesterol
and apoA-I levels due to a dose-dependent increase in the
catabolic rate of HDL apolipoproteins as well as HDL-CE
[27, 28•, 29]. These metabolic effects are almost entirely
dependent on the catalytic activity of EL and are not so
much mediated by its bridging function, as overexpression
of catalytically inactive EL had discernable effects on
plasma HDL cholesterol only in HL-deficient mice but not
in wild-type or apoA-I transgenic mice . Conversely,
loss-of-function studies in mice using either specific EL
knockout models or antibody-mediated EL inhibition
showed increased plasma levels of phospholipids, apoA-I,
and HDL cholesterol due to a slower catabolic rate [3, 6•].
Also a shift in the HDL size toward larger particles was
noted . Interestingly, inhibition of EL activity  or EL
knockout [6•] in HL-deficient mice resulted in a further
significant increase in plasma HDL cholesterol levels,
indicating an independent and additive role of these lipases
in HDL metabolism.
Based on these experimental findings, we have proposed
a model in which hydrolysis of HDL phospholipids by EL
destabilizes the HDL particle, resulting in shedding of
Table 2 Selected hepatic lipase and endothelial lipase gene polymorphisms and their association with atherosclerosis and cardiovascular disease
Impact on lipase
Impact on HDL
Outcome for atherosclerosis or CVDReference
2879 cases and
1741 cases and
ND IncreasedNo association with risk of ischemic CVD
Increased coronary plaque area in carriers
Increased risk of ischemic heart disease in
homozygotes for all 3 variants
562 cases and
496 cases and
NDIncreased Increased risk of cardiovascular heart disease in
male carriers but not in female carriers
No association with risk of CVD
C-514T Decreased HL
Increased or not
C-480TIncreased carotid intima-media wall thickness
in homozygous carriers
Decreased risk of acute myocardial infarction
Decreased risk of coronary artery disease in
No association with risk of coronary heart
No association with risk of coronary artery
107 cases and
265 cases and
1501 cases and
607 cases and
ND Not affected
Thr111Ile ND Not affected
CVD cardiovascular disease, HL hepatic lipase, EL endothelial lipase, ND not determined
aPlease note that functional studies indicated that the Thr111Ile variant of endothelial lipase does not differ in expression and activity from wild-type
endothelial lipase [34••]
Curr Atheroscler Rep (2011) 13:257–265261
poorly lipidated apoA-I molecules that are rapidly cleared
by the kidneys (Fig. 1) [3, 29]. Concomitantly, the CE
within the remainder of the HDL particle is rendered more
susceptible toward SR-BI—mediated selective uptake, a
mechanism that is also a prerequisite for EL-induced HDL
remodeling to continue [28•, 29]. This mechanism results in
an increased hepatic cholesterol content without translating
into altered biliary cholesterol secretion .
To put these data from experimental mouse models into
perspective, an important question is if EL has a similar
impact on HDL metabolism in humans and thereby might
represent a potential therapeutic target. In a sample of 510
healthy individuals with a family history of premature
coronary artery disease, post-heparin plasma mass levels of
EL were positively correlated with small HDL and
negatively with large HDL particles . Selected common
polymorphisms of EL were also found to be associated with
HDL phenotypes [32, 33]. However, the strongest data
supporting a negative impact of EL on plasma HDL
cholesterol in humans come from a recent report demon-
strating that the low-frequency Asn396Ser LIPG variant
results in reduced lipolytic activity and is strongly associ-
ated with increased plasma HDL cholesterol levels [34••].
Another rare mutation in the EL gene, G26S, impairs
secretion of the EL protein, leading to reduced EL plasma
levels and thereby also to elevated plasma HDL cholesterol
in African-American carriers . However, the more
common Thr111Ile EL variant, which had previously been
variably and inconsistently associated with plasma HDL
cholesterol phenotypes [36, 37], did not impact on EL
function in vitro and failed to correlate with plasma HDL
cholesterol levels in a comprehensive meta-analysis of five
cohort studies [34••].
Impact of EL on Selected Key Steps of RCT
Only a few studies have examined the effect of EL on
HDL-mediated cellular cholesterol efflux (the first step in
RCT), and none have achieved conclusive results (Table 1).
Decreased EL expression in vitro in macrophages caused a
decline in efflux toward apoA-I (i.e., the part of the
pathway mainly mediated by ABCA1), whereas over-
expression of EL or addition of exogenous EL had an
opposite effect . Notably, both abolishing the catalytic
activity or the bridging function of EL partially inhibited
macrophage cholesterol efflux , indicating that these
two properties of EL are relevant in this process. In
contrast, using modification of the efflux acceptors by EL,
one study noted no effect on the ABCA1-dependent
pathway , whereas another group reported a 63%
increase in ABCA1-mediated efflux toward HDL from
EL-overexpressing mice . However, increased efflux
toward plasma and HDL from EL knockout mice has also
been observed, and an additive effect was seen in EL/HL
double-knockout mice [6•]. With respect to SR-BI–mediated
efflux, a reduced capacity of EL-modified HDL  or of
serum from EL-overexpressing mice  was described.
Although it variably modulates cholesterol efflux, EL
was consistently reported to enhance the second step in the
RCT pathway, which is the uptake of HDL cholesterol into
the liver (Table 1). While diminished selective uptake of CE
was observed using HDL3from EL-knockout mice [6•],
SR-BI—mediated selective uptake was enhanced from
HDL from EL-overexpressing mice in vitro and in vivo
[28•, 30]. This increase in selective uptake is dependent
upon both the catalytic activity and the bridging function of
EL . Consequently, these properties of EL resulted in
vivo in a significant increase in hepatic cholesterol content
following EL overexpression [28•, 30].
However, the major pathway of cholesterol excretion
from the body, namely biliary sterol secretion either as free
cholesterol or after metabolic conversion into bile acids,
remained essentially unchanged in EL-overexpressing mice
(Table 1). Correspondingly, EL overexpression did not
affect the mass fecal output of neutral sterols and bile acids
(Table 1) .
EL and Macrophage-Specific RCT
To date, two studies have explored the effect of EL on
macrophage-specific RCT using
macrophage foam cells (Table 1). EL overexpression
significantly decreased in vivo RCT and the same effect
was observed with overexpression of profurin .
Profurin inhibits proprotein convertases, which in general
mediate site-specific proteolysis and have specifically
been shown to impact plasma HDL cholesterol levels via
specific cleavage of EL resulting in inactivation of this
lipase ; therefore, profurin overexpression has a
similar metabolic effect than EL overexpression. In
addition, angiopoietin-like 3 (ANGPTL3) has been dem-
onstrated to be an endogenous inhibitor of EL, and
consequently ANGPTL3 knockout mice have low plasma
HDL cholesterol levels . Therefore, the effect on RCT
in ANGPTL3 knockouts would be expected to be the same
as in mice overexpressing EL (namely, decreased), but this
has not been experimentally tested thus far. On the other
hand, overall RCT is unchanged in EL knockout mice [6•].
However, EL knockouts had increased plasma levels of
cholesterol originating from macrophages during the time
course of the study, and this increase was significantly
greater than in HL knockout mice [6•]. No additive effect
of the absence of both EL and HL on plasma counts or on
overall RCT was observed [6•], indicating that the effects
262Curr Atheroscler Rep (2011) 13:257–265
of absence of EL and HL on overall RCT in mice are
EL and Atherosclerosis
Studies exploring the role of EL in experimental athero-
genesis have not yielded conclusive results thus far
(Table 1). One study reported that targeted disruption of
EL in apoE knockout mice attenuated atherosclerotic lesion
formation by approximately 70% on chow and to a lesser
degree on a Western-type diet . Interestingly, the lack of
EL resulted in increased plasma levels of antiatherogenic
HDL as well as proatherogenic very low-density lipoprotein
and LDL cholesterol . Therefore, the authors rather
proposed reduced monocyte adhesion to the vascular wall
via heparan sulfate proteoglycans as an underlying mech-
anism for decreased atherosclerosis development in apoE/
EL double knockout mice . On the contrary, in another
comprehensive study, no differences in atherosclerotic lesion
size and lesion macrophage content were detected between
mice expressing or lacking EL either on the apoE knockout
or the LDL-receptor knockout background . Also, this
report noted a slight but consistent increase in plasma levels
of both cholesterol within HDL as well as apoB-containing
Inflammation plays an important role in the development
of atherosclerosis, and a link between EL and inflammation
has been suggested. Expression of EL is substantially
upregulated in cultured endothelial cells in response to
proinflammatory cytokines and in humans during acute
experimental inflammation . Although EL has been
shown to enhance monocyte adhesion to the vessel wall
, another report suggested that EL decreases endothelial
adhesion molecule expression by generating peroxisome
proliferator—activated receptor α (PPARα) ligands from
HDL phospholipids . Therefore, although EL expression
is upregulated by inflammatory stimuli, it is currently unclear
if EL itself exerts anti- or proinflammatory activity. In
analogy to other secreted phospholipases , however, it
could also be conceivable that EL expression induces
endothelial dysfunction and thereby the susceptibility to
atherosclerotic CVD, although this concept has not been
experimentally addressed thus far.
In humans, the association between atherosclerotic
cardiovascular disease and EL levels or variants of the EL
gene has been investigated (summarized in Table 2). As
indirect evidence, a significant positive correlation was found
between coronary artery calcification scores and EL mass
levels in pre-heparin (odds ratio=1.67) and post-heparin
(odds ratio=2.42) plasma among apparently healthy indi-
viduals that persisted even after adjustment for plasma
lipids, vasoactive medication, and established risk factors
. A report on a relatively small group of patients with
end-stage renal disease on hemodialysis suggested that
during a 2-year follow-up, patients experiencing CVD
events had higher serum EL levels compared with event-
free patients . Two small case-control studies from Japan
and China noted that the common Thr111Ile EL variant was
related to a decreased risk of acute myocardial infarction and
coronary artery disease, respectively [47, 48]. However, the
Thr111Ile variant was shown in in vitro assays to be
indiscernible from wild-type EL [34••]. Subsequently, three
larger Caucasian study populations could not confirm an
impact of the Thr111Ile variant on cardiovascular risk .
More recent data from two independent, prospective, nested-
control studies reported that out of five single nucleotide
polymorphism in LIPG, only the Thr111Ile variant was
modestly associated with higher HDL cholesterol, higher
apoA-I levels, and a higher concentration of larger HDL
particles, whereas none of the EL variants tested proved to
be a risk factor for atherosclerotic cardiovascular disease
. Clearly, more studies are needed that relate plasma
levels and/or activity of EL to hard endpoints of CVD.
HL and EL are both negative regulators of plasma HDL
cholesterol levels. For both lipases, variable effects on
cholesterol efflux, which is the first step in RCT, have been
reported. HL and EL increase hepatic cholesterol content,
but this does not impact biliary sterol secretion. Overall, EL
overexpression decreases macrophage-to-feces RCT,
whereas EL as well as HL knockout did not affect RCT.
In terms of establishing these lipases as targets for
therapeutic intervention, increases in RCT upon inhibition
are not likely to be expected; however, a decrease in RCT
might not occur either. More work seems required on the
impact of HL and EL on atherosclerotic disease, including
measurements of plasma mass and activity. In addition,
exploring other biological properties of these lipases, such
as their role in modulating inflammatory pathways, might
prove useful in deciding whether HL and EL are suitable
targets for pharmacologic inhibition as a therapeutic
strategy against atherosclerotic CVD.
relevant to this article.
The authors report no potential conflicts of interest
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Curr Atheroscler Rep (2011) 13:257–265263
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