© 1996 American Heart Association, Inc.
Articles |
Apolipoprotein A-II Influences the Substrate Properties of Human HDL2 and HDL3 for Hepatic Lipase
From the Department of Medicine, Baylor College of Medicine, Houston, Tex (H.-O.M., A.M.G.); the Department of Medicine, University of Innsbruck, Austria (J.R.P.); and the Department of Laboratory Medicine, Landeskrankenanstalten, Salzburg, Austria (W.P.).
Correspondence to Josef R. Patsch, MD, Department of Medicine, Universitätsklinik Innsbruck, Anichstr 35, A-6020 Innsbruck, Austria.
 |
Abstract |
|---|
 Top Abstract IntroductionMethodsResultsDiscussionReferences |
|---|
Abstract Hepatic lipase has a demonstrated dual role in plasma lipid transport in that it participates in the removal of remnants of triglyceride-rich lipoproteins from the circulation and in the metabolism of plasma HDL. The study presented here investigated the substrate properties for hepatic lipase of HDL differing in density and apolipoprotein (apo) composition. Rates of fatty acid liberation were twofold higher in HDL2 compared with the respective HDL3 subspecies. Within each density class, enzyme-catalyzed fatty acid release was nearly twofold higher from HDL containing apoA-II compared with HDL devoid of apoA-II. When native HDL3 devoid of apoA-II was reconstituted with dimeric apoA-II in vitro, rates of fatty acid liberation in reconstituted particles were similar to those in native HDL3 containing apoA-II. HDL containing apoA-II competed more effectively with small VLDL for binding of hepatic lipase than HDL devoid of apoA-II. HDL3, particularly apoA-II–containing HDL3, reduced lipolysis of triglyceride and total fatty acid liberation in small VLDL. We conclude that the substrate properties of HDLs for hepatic lipase are influenced by both their size and apoA-II content. Moreover, size as well as apoA-II content may indirectly affect remnant clearance.
Key Words: hepatic lipase • high-density lipoproteins • apolipoprotein A-II • remnant catabolism
|
Introduction |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Hepatic lipase hydrolyzes fatty acyl ester bonds of monoglycerides, diglycerides, triglycerides, and phospholipids and is essential for the homeostasis of plasma lipid and lipoprotein transport.1 2 Complete deficiency of this enzyme due to compound heterozygosity for two gene mutations is characterized by premature atherosclerosis, triglyceride enrichment of both VLDL and HDL, and the presence of ß-VLDL. In addition, the clearance of remnants of triglyceride-rich lipoproteins in the postprandial state is delayed.3 Similar lipid abnormalities have been described in other examples of heritable hepatic lipase deficiencies.4 5 6 7 The lipoprotein abnormalities associated with the familial disorders, data from animal studies, and in vitro experiments indicate that the enzyme plays an important role in the removal of remnants of triglyceride-rich lipoproteins from the circulation.8 9 10 The precise mechanism(s) whereby hepatic lipase facilitates the uptake of remnants by the liver has yet to be determined. However, lipolysis of both triglyceride and phospholipids of triglyceride-rich lipoproteins already processed by lipoprotein lipase may alter the surface disposition of apolipoproteins, thereby enhancing their affinity for cellular receptors.11 It is also possible that hepatic lipase binds to remnants and facilitates the removal of these particles by interacting directly with endocytotic receptors.12
In addition to its role in remnant metabolism, hepatic lipase hydrolyzes both triglyceride and phospholipids in HDL, and enzyme activities in postheparin plasma are inversely correlated with HDL2 plasma levels.13 14 15 We have recently shown that apoA-II enhances the substrate properties of postprandial HDL2 for hepatic lipase,16 a finding that helps to explain the preferential conversion of apoA-II–containing HDL2 into HDL3 in the postprandial state.17 To gain further insight into a possible effector function of apoA-II for hepatic lipase, it was essential to extend our studies to HDL3, which transports the majority of apoA-II in human plasma. Therefore, we compared the substrate properties of fasting A-I/A-II–HDL for hepatic lipase with those of A-I–HDL. Furthermore, we studied the ability of these HDL fractions to compete with small VLDL for hepatic lipase. We report here that the interaction of HDL with hepatic lipase is modulated by both its size and apoA-II content. These structural properties also influence the ability of HDL particles to compete with small VLDL for enzyme binding and enzyme-catalyzed fatty acid liberation.
|
Methods |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
Blood from fasting subjects was obtained after an overnight fast. Blood was collected into tubes containing 1.5 mg EDTA per milliliter of blood, and plasma was separated by centrifugation (1500g, 20 minutes, 4°C). For isolation of HDL2 and HDL3, plasma was subjected to zonal ultracentrifugation.18 Volume fractions containing HDL2 and HDL3 were purified further by gel permeation chromatography using Bio-gel A-5m (Bio-Rad) in 2.5x95-cm columns. For separation of A-I and A-I/A-II subfractions from HDL2 and HDL3, immunoaffinity chromatography was used19 as described previously.16 In brief, Mab 32, a monoclonal anti–apoA-II antibody,20 was purified from murine ascites fluid by immunoaffinity chromatography using HDL3 immobilized to Sepharose 4B. Affinity-purified Mab 32 was covalently coupled to CNBr-activated Sepharose 4B at a ratio of 9 to 11 mg/g of gel, washed extensively, and stored in Tris-HCl, pH 7.4, containing 0.02% NaN3 at 4°C. HDL2 or HDL3 was diluted to a concentration of 0.5 mg protein per milliliter with 10 mmol/L Tris-HCl, pH 7.4, containing 0.15 mol/L NaCl, 1 mmol/L EDTA, and 0.02% NaN3 (buffer I) and applied to Mab 32–Sepharose columns (1.5x19 cm) at a flow rate of 40 mL/h. The eluate was recycled twice through the columns before collecting unbound HDL. Bound HDL was eluted with 0.2 mol/L glycine buffer, pH 2.8, and collected into tubes containing 1 mol/L Tris-HCl, pH 8.0. The capacity of the gel to retain apoA-II–containing particles as well as the reproducibility of the procedure and recoveries of lipoprotein fractions applied have been described.21 Fractions were pooled, dialyzed against buffer I, and analyzed for chemical composition. Protein was quantified by the method of Lowry et al.22 Phospholipids were determined by the method of Bartlett.23 Triglyceride, cholesterol, and cholesteryl ester were determined by enzymatic procedures.24 25
For estimation of particle size, HDL fractions were subjected to electrophoresis in 5% to 20% polyacrylamide gels under nondenaturing conditions.26 Apolipoproteins of HDL fractions were separated by polyacrylamide gel electrophoresis in 0.1% SDS27 or by isoelectric focusing.28 Relative abundance of apoA-I and apoA-II was determined densitometrically as described.29
To reconstitute A-I–HDL3 with apoA-II, A-I–HDL3 containing 3.0 mg protein was incubated with 1.5 mg apoA-II in 3.0 mL of buffer I at 25°C for 2 hours.30 Incubation mixtures were brought to a density of 1.21 g/mL by using KBr and ultracentrifuged in an SW 40 rotor (Beckman) at 40 000 rpm, 10°C, for 24 hours. The top 1-mL fraction containing the reconstituted particles was diluted to 10 mL with buffer I and applied to affinity chromatography on Mab 32–Sepharose 4B. The retained fraction was eluted as described before, dialyzed, and used for incubation studies.
Reactivities of HDL subfractions with hepatic lipase were determined as described in a previous paper.16 Briefly, A-I and A-I/A-II subfractions of HDL2 and HDL3 were incubated with hepatic lipase at 28°C in 30 µL of 100 mmol/L Tris-HCl, pH 8.0, containing 4% (wt/vol) BSA. Hepatic lipase was isolated and purified from the postheparin plasma of healthy volunteers by adsorption of the enzyme to Intralipid (Vitrum) and subsequent affinity chromatography with heparin-Sepharose (Pharmacia) as described previously.16 31 The specific activity of the hepatic lipase preparations used in the present study was 2.6 µmol fatty acids released per minute per milligram protein. Since apoB and apoE are also bound by heparin-Sepharose, we estimated the contamination of our enzyme preparation with these apolipoproteins using polyacrylamide gel electrophoresis in 0.1% SDS. On the basis of densitometry of gels stained with Coomassie brilliant blue G-250 and assuming similar chromogenicity of proteins in our preparations, apoB and apoE accounted for less than 12% and 1.7% of total protein, respectively. Since typical incubation studies contained 1.5 µg of the enzyme preparation and 40 to 70 µg of HDL apolipoprotein, the amount of apoB and apoE was less than 0.5% and 0.08% of HDL apolipoprotein (wt/wt), respectively. Since the molecular weights of hepatic lipase and anti–thrombin III, the main contaminant of hepatic lipase preparations from postheparin plasma, are similar,31 we were unable to estimate hepatic lipase mass in our preparation using polyacrylamide gel electrophoresis in 0.1% SDS. Fatty acids liberated were measured using an enzymatic procedure (Free Fatty Acids Kit, Boehringer Mannheim).
To estimate the effects of lipolysis on particle size,
A-I–HDL3 and A-I/A-II–HDL3 from fasting
plasma were labeled by the iodine monochloride
procedure32 to a specific activity of
4x107 cpm/mg protein. After incubation with
hepatic lipase for 8 and 16 hours, changes in the size of labeled
HDL3 were determined by electrophoresis in 5% to 20%
polyacrylamide gradient gels under nondenaturing conditions.
The gels were dried and exposed to x-ray film and then analyzed
by densitometry, using a laser densitometer (LKB Ultrascan XL,
Pharmacia LKB Biotechnology Inc).
To determine the effects of HDL subfractions on lipolysis of VLDL-triglyceride by hepatic lipase, small VLDL was labeled with [3H]triolein and incubated with hepatic lipase and various HDL subfractions. VLDL fractions with Sf (26°, 1.063) rates of 80 to 125 were isolated from fasting plasma by zonal ultracentrifugation,33 concentrated by pressure filtration, and dialyzed against buffer I. Three hundred microliters of VLDL (15 mg triglyceride per milliliter) was mixed with 1 mL fasting plasma. Ethanol (5 µL) containing 20 µCi glycerol tri[9,10(n)-3H]oleate (1.0 Ci/mmol, Amersham) was injected into this mixture while stirring. After further stirring for 2 minutes at room temperature, the mixture was incubated at 37°C for 5 hours; adjusted to a density of 1.063 g/mL by addition of 1.21 g/mL KBr solution, pH 7.4, containing 0.15 mol/L NaCl, 0.01% EDTA, and 0.02% NaN3; overlaid with 10.4 mL of 1.006-g/mL–density KBr solution; and ultracentrifuged in an SW-41 rotor at 32 000 rpm and 10°C for 36 hours. VLDL was collected in the top 0.5-mL fraction. Forty micrograms triglyceride of [3H]triolein-labeled VLDL (specific activity, 6x108 cpm/mmol triglyceride) was incubated with hepatic lipase, the A-I or A-I/A-II subfraction of HDL3 or HDL2, 0.1 mol/L Tris-HCl (pH 8.0), and 4% BSA (wt/vol) in a total volume of 40 µL at 28°C for 1 hour. Liberation of [3H]oleic acid was measured by the method of Huttunen et al.34 To validate the labeling procedure, [3H]triolein-labeled VLDL was incubated with hepatic lipase as described above, but without HDL. Before and after incubation, lipids were extracted and separated by thin-layer chromatography as described previously.16 After visualization with iodine vapor, triglycerides were scraped from the plate, extracted, and analyzed for mass and radioactivity. Specific activities of triglycerides before and after incubation did not differ (6.2x108±0.7x108 versus 5.8x108±0.6x108 cpm/mmol triglyceride; mean±SD, n=3).
To study the partition of hepatic lipase among lipoproteins, highly purified hepatic lipase (kindly provided by Dr G. Bengtsson-Olivecrona, Umeå, Sweden) with a specific activity of 240 µmol·min-1·mg-1 protein, as determined in our laboratory, was used. Enzyme (242 ng) was incubated with VLDL (0.5 mg VLDL-triglyceride) and the respective HDL subspecies (0.1 mg protein) in a total volume of 500 µL containing 0.1 mol/L Tris-HCl, pH 8.0, and 4% BSA (wt/vol) at 4°C for 1 hour. The incubation mixture was then applied to a Sepharose CL-6B (Pharmacia) column (1x90 cm) and eluted with 0.15 mol/L NaCl, pH 7.4, containing 1 mmol/L EDTA and 0.02% NaN3 at a flow rate of 4.5 mL/h at 4°C. After phospholipid determination in individual 1.5-mL fractions, volume fractions containing VLDL and those containing HDL were pooled, concentrated by pressure filtration (PM-30, Amicon Cell, 5 to 12 psi), and used for hepatic lipase quantification.
Hepatic lipase was quantified by an immunoradiometric assay that was based on procedures described by Ritsch et al35 for measurement of CETP. Affinity-purified goat anti-human hepatic lipase antibody was kindly provided by Dr G. Bengtsson-Olivecrona. The antibody was labeled by the chloramin T method. Five micrograms of the antibody was incubated with 0.5 mCi of Na-125I (17.4 mCi/µg, Amersham) and 50 µg chloramin T in a total volume of 55 µL. After gentle mixing for 15 to 20 seconds at room temperature, the reaction was terminated by the addition of 100 µg sodium disulfite and 1 mg potassium iodide in 0.05 mol/L sodium phosphate buffer, pH 7.5. Free Na 125I was removed by filtration through a Sephadex G-50 column (1x14 cm) equilibrated with 0.05 mol/L sodium phosphate buffer, pH 7.5, containing 1% BSA (wt/vol). The specific activity of the labeled anti–hepatic lipase antibody was 3.5x104 cpm/ng. Flexible 96-well assay plates (Falcon 3912 MicroTestIII, Becton Dickinson) were coated with 125 µL/well of purified anti–hepatic lipase antibody containing 50 µg/mL protein and incubated in 0.01 mol/L bicarbonate buffer, pH 9.6, at room temperature for 3 hours. After three washes with PBS, 100 µL of sample in PBS containing 2% BSA and 1% Triton X-100 was added to each well and incubated at room temperature overnight. After three washes with PBS, 100 µL of 125I-labeled anti–hepatic lipase antibody in PBS (5x106 cpm/mL) was added and the sample incubated for 12 hours. After five rinses with PBS, the radioactivity bound to each well was measured in a gamma counter (Gamma 4000, Beckman Instruments, Inc). The range of the standard curve was 0.5 to 20 ng per incubation, and the intra-assay coefficient of variation was less than 8%. Mean recoveries (SD) of hepatic lipase mass in the presence of VLDL, A-I–HDL2, A-I/A-II–HDL2, A-I–HDL3, and A-I/A-II–HDL3 (50 µg phospholipid per milliliter, three different preparations each) were 110±9%, 101±9%, 91±11%, 89±13%, and 94±16%, respectively. Enzyme recoveries were not affected by the presence of the various HDL subfractions (analysis of variance).
|
Results |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
To compare substrate properties of HDL3 subspecies for hepatic lipase, rates of fatty acid liberation were measured as a function of time (Fig 1
). At all time points, mean
values of fatty acids released were approximately two times higher in
incubation mixtures containing A-I/A-II–HDL3 than in those
containing A-I–HDL3 (P<.001, analysis
of covariance). Consistent with our previous
studies,16 fatty acid liberation was higher in mixtures
containing A-I/A-II–HDL2 than in those containing
A-I–HDL2. Compared with the respective
HDL2 subspecies, HDL3 subfractions were
less susceptible to hepatic lipase–mediated fatty acid liberation.
This finding is consistent with the results reported by Shirai
et al,36 who used unfractionated HDL2
and HDL3. At all substrate concentrations studied, the
velocity of fatty acid release (Fig 2) was greater for
apoA-II–containing HDL fractions in comparison with the respective HDL
fraction devoid of apoA-II (P<.001, analysis of
covariance). At HDL-triglyceride concentrations
of 160 µg/mL, mean (SD) fatty acid liberation from
A-I/A-II–HDL2,
A-I–HDL2, A-I/A-II–HDL3,
and A-I–HDL3 was 7.93 (1.77), 4.69 (0.69), 3.36 (0.24),
and 1.62 (0.59) nmol/h per incubation mixture.
|
) and A-I/A-II–HDL3 (
) is shown in comparison
with A-I–HDL2 (
) and
A-I/A-II–HDL2 (
). HDL subfractions (2.4 µg
triglyceride) were incubated with 1.5 µg hepatic lipase
in 30 µL of 100 mmol/L Tris-HCl, pH 8.0, containing 4% BSA (wt/vol).
Points (bars) represent mean (SD) of three preparations from
different subjects.
|
In our previous studies with postprandial
HDL2,16 reconstitution of
A-I–HDL2 with apoA-II enhanced its substrate
properties for hepatic lipase. To examine the role of apoA-II in the
lipolysis of HDL3 by hepatic lipase, A-I–HDL3
was reconstituted with dimeric apoA-II. While the protein content
tended to be higher in reconstituted particles, the difference was not
statistically significant (Table 1). Mean (SD)
apoA-I/A-II dye-uptake ratios29 were 2.6 (0.6) and 3.2
(0.8) in reconstituted particles and native
A-I/A-II–HDL3, respectively (n=4, not significantly
different, analysis of variance, Fig 3A). The
reconstituted particles exhibited slightly lower electrophoretic
mobility in nondenaturing polyacrylamide gels than the
parent HDL3 particles devoid of apoA-II (Fig 3B). The
apparent mean diameter of reconstituted particles increased from
8.2±0.1 nm in parent particles to 8.5±0.1 nm in reconstituted
particles (mean±SD, n=3, P<.05). In each of the four
experiments with HDL3 preparations from different subjects,
the rate of fatty acid liberation catalyzed by hepatic lipase was
significantly higher in apoA-II–reconstituted particles than in
parent HDL3 devoid of apoA-II (P<.05,
analysis of variance). Furthermore, the rates of fatty acid
liberation in reconstituted particles did not differ from those in
native A-I/A-II–HDL3 (Table 2).
|
|
|
To determine size changes of A-I–HDL3 and
A-I/A-II–HDL3 resulting from hepatic lipase–mediated
hydrolysis of core triglyceride and surface phospholipid,
125I-labeled A-I–HDL3 and
A-I/A-II–HDL3 were incubated with the enzyme, and the
sizes of parent and product particles were estimated from
autoradiograms of nondenaturing polyacrylamide
gels. Incubation of labeled A-I/A-II–HDL3 with hepatic
lipase produced a population of particles that were smaller than the
parent particles (Fig 4). The apparent mean diameter of
these newly formed particles was 7.9 nm. Such a size change of
particles was not observed in control incubations without the enzyme.
Moreover, incubation of A-I–HDL3 with or without hepatic
lipase did not result in measurable changes of particle size under
these conditions.
|
To ascertain whether differences in substrate properties of HDL
subfractions affect the ability of hepatic lipase to hydrolyze
triglyceride-rich lipoproteins such as VLDL (or
chylomicron remnants), the enzyme-catalyzed hydrolysis of
3H-labeled triglyceride in small VLDL particles
was measured in the presence of A-I and A-I/A-II subfractions of
HDL2 and HDL3 (Fig 5).
With increasing concentrations of both A-I–HDL3 and
A-I/A-II–HDL3, lipolysis of
[3H]triglyceride decreased, while the two
HDL2 subspecies showed no significant inhibition of
lipolysis. Furthermore, A-I/A-II–HDL3 tended to inhibit
lipolysis of [3H]triglyceride more than
A-I–HDL3, but the difference was not statistically
significant. Under our experimental conditions, transfer of
[3H]triglyceride from VLDL to HDL was
negligible, as determined in incubations without hepatic lipase (not
shown). Total fatty acid liberation shown in Table 3 was
lower in mixtures containing HDL3 than in those containing
HDL2 (P<.05, analysis of
covariance). Furthermore, fatty acid liberation was
slightly, but significantly, lower in incubations containing
A-I/A-II–HDL3 than in those containing
A-I–HDL3 (P<.05, analysis of
covariance).
|
|
To determine whether the different inhibitory capacity of
HDL subfractions resulted from differences in enzyme availability for
VLDL, the partition of hepatic lipase between VLDL and the various HDL
species was measured. Hepatic lipase was incubated with VLDL and
the respective HDL subfraction at 4°C for 1 hour and subjected
to gel permeation chromatography on a Sepharose CL-6B
column. Separation of lipoprotein classes was monitored by determining
phospholipid in each volume fraction. Volume fractions containing VLDL
and those containing HDL were pooled and analyzed for hepatic
lipase immunoreactivity. Hepatic lipase mass associated with VLDL was
significantly lower in incubations with HDL containing apoA-II than in
those containing the respective HDL subfraction devoid of apoA-II
(Table 4). Conversely, hepatic lipase mass associated
with the HDL fraction was significantly greater in incubations with HDL
containing apoA-II than in those containing the respective HDL
subfraction devoid of apoA-II (Table 4). Compared with HDL size, the
presence of apoA-II in subfractions exhibited a greater influence on
the partition of hepatic lipase between VLDL and the respective HDL
subfraction. ApoA-II may therefore enhance the affinity of all HDL
fractions for the enzyme.
|
|
Discussion |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
This study shows that the substrate properties of human HDL for hepatic lipase are modulated by their apoA-II content. These experiments with postabsorptive HDL subfractions are consistent with previous studies in postprandial HDL216 and provide further support for an effector role of apoA-II for hepatic lipase. As reported by others,36 the enzyme-catalyzed liberation of fatty acids was lower for HDL3 than for HDL2. Lipid class composition, packing of surface components, surface charge, and cholesterol content are known to affect the enzyme-substrate interaction in the lipid-water interface,37 and all of these variables may contribute to the differences in fatty acid liberation from HDL2 and HDL3.
The superior substrate properties of HDL2 in comparison with HDL3 are consistent with in vivo studies showing an inverse association of hepatic lipase activity in postheparin plasma and plasma concentrations of HDL2.15 17 Studies in the postprandial state and in experimental animals have provided a metabolic framework to explain these associations.17 38 39 When triglyceride transport is impaired, as ascertained in the postprandial state, triglyceride is translocated from triglyceride-rich lipoproteins to HDL in exchange for cholesteryl ester. Triglyceride transferred to HDL is hydrolyzed by hepatic lipase, and HDL particles become smaller and more dense. Since hepatic lipase hydrolyzes lipids in HDL2 more effectively than in HDL3 and the core of HDL2 can accommodate more triglyceride than HDL3, the size reduction affects primarily HDL2, which is converted into HDL3. Only after depletion of HDL2 is a shift of HDL3 particles toward the smaller size noted.
In vivo observations are consistent with an effector function of apoA-II for hepatic lipase, since the plasma concentration of A-I/A-II–HDL2 is increased in hepatic lipase deficiency.40 Previous in vitro studies that describe the influences of apoA-II on hepatic lipase activity are, however, contradictory. Jahn et al41 have reported that apoA-II activates hydrolysis of triacylglycerol by hepatic lipase, while other investigators42 43 have reported inhibition of enzyme activity by apoA-II in bulk assays. Even in monolayer systems with well-defined interfacial properties, the influence of apoA-II on hepatic lipase activity varied. In monolayers consisting of 2 mol% triolein, nonsubstrate ether lipids, and cholesterol, hepatic lipase–catalyzed fatty acid liberation was less inhibited by apoA-II than by apoA-I, apoC-II, and apoC-III.37 In contrast, studies by Thuren et al44 showed that apoA-II inhibited the enzyme-catalyzed hydrolysis of didodecanoylphosphatidylethanolamine monolayers, while apoE, apoA-I, apoC-II, and apoC-III maximally stimulated hydrolysis. In monolayers containing 5 mol% trioctanoylglycerol, apoA-II inhibited hydrolysis of triglyceride to a much greater extent than apoA-I, apoE, and apoC. The possibility that apoA-II had interacted with the enzyme in the subphase, thereby reducing the adsorption of enzyme to the interface, appeared unlikely, since the surface excess of enzyme was only minimally affected by apoA-II in the subphase at low surface pressure.44 Hence, the difference in apoA-II effects on hepatic lipase activity in our experiments and the studies of Thuren et al must have resulted from differences in assay conditions, such as substrate presentation, lipid substrates, ionic strength, and Ca2+ content. Differences in substrate properties between monolayer experiments and studies with intact lipoproteins are well known, since phosphatidylcholine is not, or only poorly, lipolyzed in monolayers, while it represents a main substrate lipid in HDL.16 36 44
Previous studies have suggested that apoE may activate hepatic
lipase.45 46 In our HDL3 preparations, only
very small quantities of apoE were detectable (Fig 3A
). Plasma levels
of apoE range between 2 and 5 mg/dL in normolipemic subjects. About
half the apoE in plasma is transported by HDL, with HDL1
containing the bulk of it.47 48 Consequently, only 1% to
2% of HDL3 particles are expected to contain an apoE
molecule. In addition, we did not observe striking differences in the
apoE content of A-I–HDL3 and A-I/A-II–HDL3
(Fig 3A). Hence, apoE is unlikely to account for the different
substrate properties of HDL3 preparations. Moreover,
reconstitution of A-I–HDL3 with apoA-II enhanced its
substrate properties toward the enzyme, providing further support for
the idea of an effector function of apoA-II. On incubation of
A-I/A-II–HDL3 with hepatic lipase, a discrete population
of smaller particles emerged, suggesting heterogeneity
of this lipoprotein class (Fig 4). Such a subpopulation would have
escaped detection if the less than 5% of apoE-containing particles had
been converted preferentially. Hence, other sources of
heterogeneity that affect substrate availability, such
as packing of surface components and apolipoprotein conformation, may
have been involved.
Despite the differences in chemical composition and size and, thus,
surface pressure, the stimulatory influence of apoA-II on hepatic
lipase was present in HDL subfractions of both densities, even
though fatty acid liberation was much lower in HDL3 than in
HDL2 (Figs 1 and 2) or even small VLDL (Table 3).
The studies on partition of hepatic lipase among small VLDL and various
HDL fractions indicated that apoA-II enhanced the binding of enzyme to
HDL subfractions, while the effects of HDL size on enzyme binding were
not significant (Table 4). The lower lipolytic activities in incubation
mixtures containing small VLDL and HDL3 in comparison with
those containing VLDL and HDL2 (Table 3) may be
explained by similar binding affinities of HDL2 and
HDL3 but differences in their efficiency of lipolysis. This
interpretation would be consistent with the data shown in Figs 1 and 2 and the competition studies showing a greater
inhibitory effect of HDL3 than
HDL2 (Fig 4). Owing to slower rates of lipolysis in
HDL3, less enzyme would be available for
redistribution to small VLDL.
The stimulatory effect of apoA-II may be physiologically relevant for the catabolism of HDL3. A-I/A-II–HDL3 is the most abundant HDL subfraction in normolipemic subjects, and lecithin:cholesterol acyltransferase may be a major metabolic determinant of this lipoprotein species.21 49 Hepatic lipase, by virtue of its phospholipase A1 activity, may promote the delivery of esterified cholesterol to hepatocytes.50 Because of its enhanced interaction with the enzyme, A-I/A-II–HDL3 may be more effective in transferring cholesteryl esters to hepatocytes than A-I–HDL3. Since hepatic lipase is also detectable in ovaries and adrenals,51 52 apoA-II may play a role in the cholesterol homeostasis of these endocrine tissues.
Early studies in myocardial survivors suggested a protective role of apoA-II in CHD,53 but apoA-II deficiency due to a splice-site mutation had little influence on the occurrence of CHD.54 More recent studies in laboratory animals even suggest that plasma levels of A-I/A-II–HDL could be antagonistic to the putative protective role of A-I–HDL.55 56 57 Transgenic mice overexpressing human apoA-I showed enhanced protection against diet-induced atherosclerosis compared with mice overexpressing both human apoA-I and apoA-II.56 Transgenic mice that overexpress mouse apoA-II exhibited increased atherosclerotic lesion development compared with normal mice, despite an increase in HDL cholesterol.57 Even though structural differences exist between human and mouse apoA-I and apoA-II, these animal studies are consistent with some,58 59 but not all, studies60 61 addressing the relationship of HDL subpopulations with CHD. The potentially contrasting associations of LpA-I and LpA-I/A-II with atherosclerosis are not understood, but concepts have been presented. In vitro studies have shown that LpA-I/A-II may inhibit the LpA-I–stimulated cholesterol efflux from mouse adipocytes,62 but in other mammalian cells LpA-I and LpA-I/A-II did not differ in their ability to promote cholesterol efflux.63 64 Nevertheless, LpA-I may be more effective in mobilizing cholesterol in vivo, since it is primarily associated with lecithin:cholesterol acyltransferase, which drives cellular cholesterol efflux.65
While this report was being prepared, a study in transgenic mice that
expressed several human genes involved in plasma lipid transport
concluded that human apoA-II inhibits mouse hepatic lipase
activity.66 Human apoA-II did not inhibit the lipid
transfer activity of CETP, a result consistent with our earlier
studies.16 Several in vivo experiments suggested
inhibition of hepatic lipase by apoA-II. The conclusion rested,
however, on two key experiments performed in vitro. First, hydrolysis
of radiolabeled triglyceride was inhibited by any type of
HDL added, but HDL from transgenic mice expressing apoA-II had the
strongest inhibitory effect (see Fig 6 in Reference 6666 ).
These results are similar to our data shown in Fig 5 and Table 3. Since
dimeric apoA-II exchanges less among lipoproteins than other
HDL-associated apolipoproteins,67 transfer of apoA-II from
HDL to the radiolabeled bulk substrate may not have accounted for the
reduced release of radiolabeled fatty acids. Instead, the inhibition of
hydrolysis may have resulted from competition of HDL with the
emulsified radiolabeled triglyceride for hepatic lipase.
Second, Zhong et al66 incubated plasma from transgenic
mice expressing human apoA-I and apoC-III (AI-CIII) or apoA-I,
apoC-III, and apoA-II (AI-CIII/AII) with recombinant CETP in the
presence or absence of the lipase inhibitor E600. While HDL
triglyceride increased much less in the absence of E600 in
the AI-CIII transgenic mice, this inhibitor had no effect
on the increase of HDL triglyceride in the AI-CIII/AII
transgenic animals (see Fig 7 in Reference 6666 ). These data were
interpreted as an inhibition of hepatic lipase by apoA-II. On the basis
of the data of total plasma cholesterol,
triglyceride, and HDL cholesterol (Table I and
Fig 7, Reference 6666 ), the
triglyceride-to-cholesterol ratio of
lipoproteins other than HDL was two to three times higher in the
AI-CIII/AII transgenic mice than in the AI-CIII transgenic mice. Since
CETP-mediated net transfer of neutral lipids depends on the core
composition of donor and acceptor lipoproteins, excess
triglyceride transfer to HDL in the AI-CIII/AII transgenic
animals may have influenced the outcome of the experiment that was
performed with preheparin plasma containing low activities of hepatic
lipase. Hence, the proposal that human apoA-II inhibits mouse hepatic
lipase awaits additional confirmation in definitive experiments testing
the substrate properties of particles with purified enzyme.
The in vitro studies reported here demonstrate that hepatic lipase, in the presence of small triglyceride-rich lipoproteins, shows an increased association with apoA-II–containing particles compared with those devoid of apoA-II. Moreover, enzyme-mediated hydrolysis of VLDL-triglyceride was inhibited only by HDL3, in particular by A-I/A-II–HDL3. Since hepatic lipase is thought to play a critical role in remnant removal, the processes governing the partition of apoA-II between HDL density fractions and those determining plasma levels of A-I/A-II–HDL3 may indirectly affect the clearance of these atherogenic lipoproteins from the circulation by competing for limiting amounts of the enzyme.
|
Selected Abbreviations and Acronyms |
|---|
|
|
Acknowledgments |
|---|
This work was supported by grant HL-27341 from the National Institutes of Health and by grants S-46/06 and S07106-MED from the Austrian Fonds zur Förderung der Wissenschaftlichen Forschung. The authors thank I.Y. Chen and I. Thandi for expert technical assistance and S.M. Soyal for proofreading the manuscript. Highly purified hepatic lipase and antibodies against hepatic lipase were generously provided by Dr Gunilla Bengtsson-Olivecrona, Department of Medical Chemistry, University of Umeå, Sweden. The monoclonal anti–apoA-II antibody (Mab 32) was kindly provided by Dr L.C. Smith, Baylor College of Medicine, Houston, Tex.
Received February 23, 1995; accepted January 22, 1996.
|
References |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
1. Jackson RL. Lipoprotein lipase and hepatic lipase. In Boyer P, ed. The Enzymes, Vol XVI. New York, NY: Academic Press Inc; 1983:141-181.
2. Kinnunen PKJ, Virtanen JA, Vaino P. Lipoprotein lipase and hepatic endothelial lipase: their roles in lipoprotein metabolism. Atheroscler Rev. 1983;11:65-105.
3.
Hegele RA, Little A, Vezina C, Maguire GF, Tu L,
Wolever TS, Jenkins DJA, Connelly PW. Hepatic lipase deficiency:
clinical, biochemical and molecular genetic characteristics.
Arterioscler Thromb. 1993;13:720-728.
4. Breckenridge WC, Little JA, Alaupovic P, Wang CS, Kuksis A, Kakis G, Lindgren F, Gardiner G. Lipoprotein abnormalities associated with a familial deficiency of hepatic lipase. Atherosclerosis. 1982;45:161-179. [Medline] [Order article via Infotrieve]
5.
Carlson LA, Holmquist L, Nilsson-Ehle P.
Deficiency of hepatic lipase activity in post-heparin plasma in
familial hyper-
-triglyceridemia. Acta
Med Scand. 1986;219:435-447. [Medline]
[Order article via Infotrieve]
6. Demant T, Carlson LA, Holmquist L, Karpe F, Nilsson-Ehle P, Packard CJ, Shepherd J. Lipoprotein metabolism in hepatic lipase deficiency: studies on the turnover of apolipoprotein B and on the effect of hepatic lipase on high density lipoprotein. J Lipid Res. 1988;29:1603-1611. [Abstract]
7.
Auwerx JH, Marzetta CA, Hokanson JE, Brunzell
JD. Large buoyant LDL-like particles in hepatic lipase
deficiency. Arteriosclerosis. 1989;9:319-325.
8. Goldberg IJ, Le N-A, Paterniti JR Jr, Ginsberg HN, Lindgren FT, Brown WV. Lipoprotein metabolism during acute inhibition of hepatic triglyceride lipase in the cynomolgus monkey. J Clin Invest. 1982;70:1184-1192.
9. Daggy BP, Bensadoun A. Enrichment of apolipoprotein B-48 in the LDL density class following in vivo inhibition of hepatic lipase. Biochim Biophys Acta. 1986;877:252-261. [Medline] [Order article via Infotrieve]
10. Sultan F, Lagrange D, Jansen H, Griglio S. Inhibition of hepatic lipase activity impairs chylomicron remnant removal in rats. Biochim Biophys Acta. 1990;1042:150-152. [Medline] [Order article via Infotrieve]
11. Brasaemle DL, Cornely-Moss K, Bensadoun A. Hepatic lipase treatment of chylomicron remnants increases exposure of apolipoprotein E. J Lipid Res. 1993;34:455-465. [Abstract]
12. Shafi S, Brady SE, Bensadoun A, Havel RJ. Role of hepatic lipase in the uptake and processing of chylomicron remnants in rat liver. J Lipid Res. 1994;35:709-720. [Abstract]
13.
Applebaum-Bowden D, Haffner SM, Wahl PW, Hoover JJ,
Warnick GR, Albers JJ, Hazzard WR. Postheparin
plasma triglyceride lipases: relationships with very
low–density lipoprotein triglyceride and
high-density lipoprotein 2 cholesterol.
Arteriosclerosis. 1985;5:273-282.
14.
Kuusi T, Kesaniemi YA, Vuoristo M, Miettinen TA,
Koskenvuo M. Inheritance of high-density lipoprotein and
lipoprotein lipase and hepatic lipase activity.
Arteriosclerosis. 1987;7:421-425.
15. Patsch JR, Prasad S, Gotto AM Jr, Patsch W. High density lipoprotein2: relationship of the plasma levels of this lipoprotein species to its composition, to the magnitude of postprandial lipemia, and to the activities of lipoprotein lipase and hepatic lipase. J Clin Invest. 1987;80:341-347.
16. Mowri H-O, Patsch W, Smith LC, Gotto AM Jr, Patsch JR. Different reactivities of high density lipoprotein2 subfraction with hepatic lipase. J Lipid Res. 1992;33:1269-1279. [Abstract]
17. Patsch JR, Prasad S, Gotto AM Jr, Bengtsson-Olivecrona G. Postprandial lipemia: a key for the conversion of high density lipoprotein2 into high density lipoprotein3 by hepatic lipase. J Clin Invest. 1984;74:2017-2023.
18. Patsch JR, Patsch W. Zonal ultracentrifugation. Methods Enzymol. 1986;129:3-21. [Medline] [Order article via Infotrieve]
19.
Cheung MC, Albers JJ. Characterization of
lipoprotein particles isolated by immunoaffinity
chromatography: particles containing A-I but no
A-II. J Biol Chem. 1984;255:3178-3185.
20. Silberman SR, Bernini F, Sparrow JT, Gotto AM Jr, Smith LC. Monoclonal antibodies as probes of high-density lipoprotein structure: identification and localization of a lipid-dependent epitope. Biochemistry. 1987;26:5833-5843. [Medline] [Order article via Infotrieve]
21. Mowri H-O, Patsch JR, Ritsch A, Foger B, Brown S, Patsch W. High density lipoproteins with differing apolipoproteins: relationships to postprandial lipemia, cholesteryl ester transfer protein, and activities of lipoprotein lipase, hepatic lipase, and lecithin:cholesterol acyltransferase. J Lipid Res. 1994;35:291-300. [Abstract]
22.
Lowry OH, Rosebrough NJ, Farr AL, Randall RJ.
Protein measurement with the Folin phenol reagent.
J Biol Chem. 1951;193:265-275.
23.
Bartlett GR. Phosphorus assay in column
chromatography. J Biol
Chem. 1959;234:466-468.
24. Nagele U, Hagele EO, Sauer G, Wiedermann E, Lehmann P, Wahlefeld AW, Gruber W. Reagent for the enzymatic determination of serum total triglycerides with improved lipolytic efficiency. J Clin Chem Clin Biochem. 1984;22:165-174. [Medline] [Order article via Infotrieve]
25.
Siedel J, Hagele E, Ziegenhorn J, Wahlefeld AW.
Reagent for the enzymatic determination of serum total
cholesterol with improved lipolytic efficiency.
Clin Chem. 1983;29:1075-1080.
26. Blanche PJ, Gong EL, Forte T, Nichols AV. Characterization of human high density lipoproteins by gradient gel electrophoresis. Biochim Biophys Acta. 1981;381:233-241.
27.
Neville DM. Molecular weight determination of
protein dodecyl complexes by gel electrophoresis in a
discontinuous buffer system. J Biol Chem. 1971;246:6328-6334.
28.
Warnick GR, Mayfield C, Albers JJ, Hazzard WR.
Gel isoelectric focusing method for specific diagnosis of familial
hyperlipoproteinemia type 3. Clin
Chem. 1979;25:279-284.
29.
Patsch W, Kim K, Wiest W, Schonfeld G. Effects
of sex hormones on rat lipoproteins. Endocrinology. 1980;107:1085-1094.
30. Van Tornout P, Caster H, Lievens M-J, Rosseneu M, Assmann G. In vitro interaction of human HDL with human apolipoprotein A-II. Biochim Biophys Acta. 1981;663:630-636. [Medline] [Order article via Infotrieve]
31. Ehnholm C, Kuusi T. Preparation, characterization, and measurement of hepatic lipase. Methods Enzymol. 1986;129:716-738. [Medline] [Order article via Infotrieve]
32. McFarlane AS. Efficient trace-labeling of proteins with iodine. Nature. 1958;182:53-57. [Medline] [Order article via Infotrieve]
33.
Patsch W, Patsch JR, Kostner GM, Sailer S, Braunsteiner
H. Isolation of subfractions of human very low density
lipoproteins by zonal ultracentrifugation.
J Biol Chem. 1978;253:4911-4915.
34. Huttunen JK, Ehnholm C, Kinnunen PKT, Nikkila EA. An immunochemical method for the selective measurement of two triglyceride lipases in human postheparin plasma. Clin Chim Acta. 1975;63:335-347.[Medline] [Order article via Infotrieve]
35. Ritsch A, Auer B, Foger B, Schwarz S, Patsch JR. Polyclonal antibody–based immunoradiometric assay for quantification of cholesteryl ester transfer protein. J Lipid Res. 1993;34:673-679. [Abstract]
36. Shirai K, Barnhart RL, Jackson RL. Hydrolysis of human plasma high density lipoprotein2-phospholipids and triglycerides by hepatic lipase. Biochem Biophys Res Commun. 1981;100:591-599. [Medline] [Order article via Infotrieve]
37. Laboda HN, Glick JM, Phillips MC. Influence of the structure of the lipid-water interface on the activity of hepatic lipase. Biochemistry. 1988;27:2313-2319. [Medline] [Order article via Infotrieve]
38. Behr SR, Patsch JR, Forte T, Bensadoun A. Plasma lipoprotein changes resulting from immunologically blocked lipolysis. J Lipid Res. 1981;22:443-451. [Abstract]
39. Whitlock ME, Swenson TL, Ramakrishnan T, Leonard MT, Marcel YL, Milne RW, Tall AR. Monoclonal antibody inhibition of cholesteryl ester transfer protein activity in the rabbit: effects on lipoprotein composition and high-density lipoprotein cholesteryl ester metabolism. J Clin Invest. 1989;84:129-137.
40. Cheung MC, Wolf AC. In vitro transformation of apoA-I–containing lipoprotein subpopulations: role of lecithin:cholesterol acyltransferase. J Lipid Res. 1989;30:499-509. [Abstract]
41. Jahn CE, Osborne JC Jr, Schaefer EJ, Brewer HB Jr. Activation of the enzymic activity of hepatic lipase by apolipoprotein A-II: characterization of a major component of high density lipoprotein as the activating plasma component in vitro. Eur J Biochem. 1983;131:25-29. [Medline] [Order article via Infotrieve]
42. Shinomiya M, Sasaki N, Barnhart L, Shirai K, Jackson RL. Effect of apolipoproteins on the hepatic lipase–catalyzed hydrolysis of human plasma high density lipoprotein2-triacylglycerols. Biochim Biophys Acta. 1982;713:292-299. [Medline] [Order article via Infotrieve]
43. Kubo M, Matsuzawa Y, Tajima S, Ishikawa K, Yamamoto A, Tarui S. ApoA-I and apoA-II inhibit hepatic triglyceride lipase from human postheparin plasma. Biochem Biophys Res Commun. 1981;100:261-266. [Medline] [Order article via Infotrieve]
44.
Thuren T, Wilcox RW, Sisson P, Waite M. Hepatic
lipase hydrolysis of lipid monolayers: regulation by
apolipoproteins. J Biol Chem. 1991;266:4853-4861.
45. Thuren T, Weisgraber KH, Sisson P, Waite M. Role of apolipoprotein E in hepatic lipase catalyzed hydrolysis of phospholipid in high-density lipoproteins. Biochemistry. 1992;31:2332-2338. [Medline] [Order article via Infotrieve]
46.
Landis BA, Rotolo FS, Meyers WC, Clark AB, Quarfordt
SH. Influence of apolipoprotein E on soluble and
heparin-immobilized hepatic lipase. Am J
Physiol. 1987;252:G805-G810.
47. Gotto AM Jr, Pownall HJ, Havel RJ. Introduction to plasma lipoproteins. Methods Enzymol. 1986;128:3-41. [Medline] [Order article via Infotrieve]
48. Weisgraber KH, Mahley RW. Subfractionation of human high density lipoproteins by heparin-Sepharose affinity chromatography. J Lipid Res. 1980;21:316-325. [Abstract]
49. Rader DJ, Ikewaki K, Duverger N, Schmidt H, Pritchard H, Frohlich J, Clerc M, Dumon M-F, Fairwell T, Zech L, Santamarina-Fojo S, Brewer HB Jr. Markedly accelerated catabolism of apolipoprotein A-II (ApoA-II) and high density lipoproteins containing apoA-II in classic lecithin:cholesterol acyltransferase deficiency and fish-eye disease. J Clin Invest. 1994;93:321-330.
50. Marques-Vidal P, Azema C, Collet X, Vieu C, Chap H, Perret B. Hepatic lipase promotes the uptake of HDL esterified cholesterol by the perfused rat liver: a study using reconstituted HDL particles of defined phospholipid composition. J Lipid Res. 1994;35:373-384. [Abstract]
51. Jansen H, de Greef WJ. Heparin-releasable lipase activity of rat adrenals. Biochem J. 1981;196:739-745. [Medline] [Order article via Infotrieve]
52.
Hixenbaugh EZ, Sullivan TR Jr, Strauss JF, Laposata EA,
Komaromy M, Paavola LG. Hepatic lipase in rat ovary: ovaries
cannot synthesize hepatic lipase but accumulate it from the
circulation. J Biol Chem. 1989;264:4222-4230.
53.
Fager G, Wiklund O, Olofsson SO, Wilhelmsen L, Bjonders
G. Multivariate analysis of serum
apolipoproteins and risk factors in relation to acute myocardial
infarction. Arteriosclerosis. 1981;1:273-279.
54. Deeb SS, Takata K, Peng R, Kajiyama G, Albers JJ. A splice-junction mutation responsible for familial apolipoprotein A-II deficiency. Am J Hum Genet. 1990;46:822-827. [Medline] [Order article via Infotrieve]
55.
Mehrabian M, Quia H-H, Hyman R, Ruddle D, Laughton C,
Lusis AJ. Influence of apoA-II gene locus on HDL levels and
fatty streak development in mice. Arterioscler
Thromb. 1993;13:1-10.
56. Schultz JR, Verstuyft JG, Gong EL, Nichols AV, Rubin EM. Protein composition determines the anti-atherogenic properties of HDL in transgenic mice. Nature. 1993;21:762-764.
57.
Warden CH, Hedrick CC, Quiao J-H, Castellani LW, Lusis
AJ. Atherosclerosis in transgenic mice
overexpressing apolipoprotein A-II. Science. 1993;261:469-472.
58. Puchois P, Kandoussi A, Fievet P, Fourrier JL, Bertrand M, Koren E, Fruchart JC. Apolipoprotein A-I–containing lipoproteins in coronary artery disease. Atherosclerosis. 1987;68:35-40. [Medline] [Order article via Infotrieve]
59.
Parra HJ, Arveiler D, Evans AE, Cambou JP, Amouyel P,
Bingham A, McMaster D, Schaffer P, Douste-Blazy P, Luc G, Richard JL,
Ducimetière P, Fruchart JC, Cambien F. A case-control study
of lipoprotein particles in two populations at contrasting risk for
coronary heart disease: the ECTIM study.
Arterioscler Thromb. 1992;12:701-707.
60. Genest JJ, Bard JM, Fruchart J-C, Ordovas JM, Wilson PFW, Schaefer EJ. Plasma apolipoprotein A-I, A-II, B, E and C-III containing particles in men with premature coronary artery disease. Atherosclerosis. 1991;90:149-158. [Medline] [Order article via Infotrieve]
61.
Montali A, Vega GL, Grundy SM. Concentrations of
apolipoprotein A-I–containing particles in patients with
hypoalphalipoproteinemia. Arterioscler
Thromb. 1994;14:511-517.
62. Barbaras R, Puchois P, Fruchart J-C, Ailhoud G. Cholesterol efflux from cultured adipose cells is mediated by LpAI particles but not by LpAI:AII particles. Biochem Biophys Res Commun. 1987;142:63-69. [Medline] [Order article via Infotrieve]
63. Johnson WJ, Kilsdonk EPC, van Tol A, Phillips MC, Rothblat GH. Cholesterol efflux from cells to immunopurified subfractions of human high density lipoprotein: LP-AI and LP-AI/AII. J Lipid Res. 1991;32:1993-2000. [Abstract]
64. Oikawa S, Mendez AJ, Oram JF, Bierman EL. Effects of high-density lipoprotein particles containing apoA-I, with or without apoA-II, on intracellular cholesterol efflux. Biochim Biophys Acta. 1993;1165:327-334. [Medline] [Order article via Infotrieve]
65. Ohta T, Nakamura R, Ikeda Y, Shinohara M, Miyazaki A, Horiuchi S, Matsuda I. Differential effect of subspecies of lipoprotein containing apolipoprotein A-I on cholesterol efflux from cholesterol-loaded macrophages: functional correlation with lecithin:cholesterol acyltransferase. Biochim Biophys Acta. 1993;1165:119-128.
66. Zhong S, Goldberg IJ, Bruce C, Rubin E, Breslow JL, Tall A. Human apoA-II inhibits the hydrolysis of HDL triglyceride and the decrease of HDL size induced by hypertriglyceridemia and cholesteryl ester transfer protein in transgenic mice. J Clin Invest. 1994;94:2457-2467.
67. Phillips MC. Interactions of apolipoproteins at interfaces. In: Rosseneu M, ed. Structure and Function of Apolipoproteins. Boca Raton, Fla: CRC Press, Inc; 1992:185-216.
This article has been cited by other articles:
 |
|
 |
|
  J. Boucher, T. A. Ramsamy, S. Braschi, D. Sahoo, T. A-M. Neville, and D. L. Sparks Apolipoprotein A-II regulates HDL stability and affects hepatic lipase association and activity J. Lipid Res., May 1, 2004; 45(5): 849 - 858. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 V. M. Paromov and R. E. Morton Lipid Transfer Inhibitor Protein Defines the Participation of High Density Lipoprotein Subfractions in Lipid Transfer Reactions Mediated by Cholesterol Ester Transfer Protein (CETP) J. Biol. Chem., October 17, 2003; 278(42): 40859 - 40866. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. S. Deeb, A. Zambon, M. C. Carr, A. F. Ayyobi, and J. D. Brunzell Hepatic lipase and dyslipidemia: interactions among genetic variants, obesity, gender, and diet J. Lipid Res., July 1, 2003; 44(7): 1279 - 1286. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 K. A. Dugi, M. J. A. Amar, C. C. Haudenschild, R. D. Shamburek, A. Bensadoun, R. F. Hoyt Jr, J. Fruchart-Najib, Z. Madj, H. B. Brewer Jr, and S. Santamarina-Fojo In Vivo Evidence for Both Lipolytic and Nonlipolytic Function of Hepatic Lipase in the Metabolism of HDL Arterioscler Thromb Vasc Biol, March 1, 2000; 20(3): 793 - 800. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
W. Weng, N. A. Brandenburg, S. Zhong, J. Halkias, L. Wu, X.-c. Jiang, A. Tall, and J. L. Breslow ApoA-II maintains HDL levels in part by inhibition of hepatic lipase: studies in apoA-II and hepatic lipase double knockout mice J. Lipid Res., June 1, 1999; 40(6): 1064 - 1070. [Abstract] [Full Text]
|
|
|
|
|
|
E. Boisfer, G. Lambert, V. Atger, N. Q. Tran, D. Pastier, C. Benetollo, J.-F. Trottier, I. Beaucamps, M. Antonucci, M. Laplaud, et al. Overexpression of Human Apolipoprotein A-II in Mice Induces Hypertriglyceridemia Due to Defective Very Low Density Lipoprotein Hydrolysis J. Biol. Chem., April 23, 1999; 274(17): 11564 - 11572. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
N. J. Hime, P. J. Barter, and K.-A. Rye The Influence of Apolipoproteins on the Hepatic Lipase-mediated Hydrolysis of High Density Lipoprotein Phospholipid and Triacylglycerol J. Biol. Chem., October 16, 1998; 273(42): 27191 - 27198. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
R. A. Hegele, W. C. Breckenridge, D. W. Cox, G. F. Maguire, J. A. Little, and P. W. Connelly Elevated LDL Triglyceride Concentrations in Subjects Heterozygous for the Hepatic Lipase S267F Variant Arterioscler Thromb Vasc Biol, August 1, 1998; 18(8): 1212 - 1216. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. M. Durbin and A. Jonas The Effect of Apolipoprotein A-II on the Structure and Function of Apolipoprotein A-I in a Homogeneous Reconstituted High Density Lipoprotein Particle J. Biol. Chem., December 12, 1997; 272(50): 31333 - 31339. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||