© 1999 American Heart Association, Inc.
Original Contributions |
ApoB100 Secretion From HepG2 Cells is Decreased by the ACAT Inhibitor CI-1011
An Effect Associated With Enhanced Intracellular Degradation of ApoB
From the Departments of Medicine and Biochemistry and The John P. Robarts Research Institute, University of Western Ontario, London, Ontario, Canada (L.J.W., M.W.H.); the University of Washington, Seattle, WA (P.H.R.B.); and Parke-Davis Pharmaceutical Research, Warner Lambert Co, Ann Arbor, MI (R.S.N.).
Correspondence to Murray W. Huff, The John Robarts Research Institute, 4-16, University of Western Ontario, 100 Perth Drive, London, Ontario N6A 5K8, Canada. E-mail mhuff{at}julian.uwo.ca
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Abstract |
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Abstract—The concept that hepatic cholesteryl ester (CE) mass and the rate of cholesterol esterification regulate hepatocyte assembly and secretion of apoB-containing lipoproteins remains controversial. The present study was carried out in HepG2 cells to correlate the rate of cholesterol esterification and CE mass with apoB secretion by CI-1011, an acyl CoA:cholesterol acyltransferase (ACAT) inhibitor that is known to decrease apoB secretion, in vivo, in miniature pigs. HepG2 cells were incubated with CI-1011 (10 nmol/L, 1 µmol/L, and 10 µmol/L) for 24 hours. ApoB secretion into media was decreased by 25%, 27%, and 43%, respectively (P<0.0012). CI-1011 (10 µmol/L) inhibited HepG2 cell ACAT activity by 79% (P<0.002) and cellular CE mass by 32% (P<0.05). In contrast, another ACAT inhibitor, DuP 128 (10 µmol/L), decreased cellular ACAT activity and CE mass by 85% (P<0.002) and 42% (P=0.01), respectively, but had no effect on apoB secretion into media. To characterize the reduction in apoB secretion by CI-1011, pulse-chase experiments were performed and analyzed by multicompartmental modelling using SAAM II. CI-1011 did not affect the synthesis of apoB or albumin. However, apoB secretion into the media was decreased by 42% (P=0.019). Intracellular apoB degradation increased proportionately (P=0.019). The secretion of albumin and cellular reuptake of labeled lipoproteins were unchanged. CI-1011 and DuP 128 did not affect apoB mRNA concentrations. These results show that CI-1011 decreases apoB secretion by a mechanism that involves an enhanced intracellular degradation of apoB. This study demonstrates that ACAT inhibitors can exert differential effects on apoB secretion from HepG2 cells that do not reflect their efficacy in inhibiting cholesterol esterification.
Key Words: Acyl CoA: cholesterol acyltransferase inhibitor • apoB • HepG2 cells • cholesterol esterification • CI-1011 • DuP 128
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Introduction |
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ApoB100, synthesized in the liver, is an essential structural component of VLDL and its metabolic products, IDL and LDL.1 2 ApoB is required for the intracellular assembly and secretion of these lipoproteins, and serves as a ligand for LDL receptor-mediated clearance of these particles from the plasma.3 Hepatic overproduction of apoB-containing lipoproteins is a major risk factor for atherosclerosis. Therefore, studies designed to understand the processes regulating apoB assembly and secretion from hepatocytes are important. The production of these lipoproteins is complex and requires the coordinated synthesis and assembly of apoB, triglyceride (TG), free and esterified cholesterol (FC; cholesteryl ester, CE), phospholipids (PLs), and other components. Although considerable progress has been made in understanding these processes, many questions remain unresolved.
ApoB is thought to be regulated primarily at posttranscriptional levels. The rates of apoB translocation across the endoplasmic reticulum (ER) membrane and intracellular degradation are believed to regulate the amount of apoB-containing lipoproteins secreted from the liver.4 5 Therefore, the secretion rate of apoB results from the proportion of newly synthesized apoB molecules that associate with lipid and move through the secretory pathway, versus the proportion that are degraded within the cell shortly after, or during, their synthesis. Although hepatic regulation of apoB secretion is generally considered to occur posttranslationally, transcriptional regulation may also play a minor role. Increased delivery of 25-hydroxycholesterol6 or VLDL7 to cultured hepatocytes has been shown to affect mRNA levels and secretion efficiency of apoB.
Several factors have been suggested to play key roles in the translational and posttranslational regulation of apoB secretion, including amino acid concentration, hormonal environment, and hepatic lipid availability and composition.8 9 10 11 Of these factors, lipid availability has recently received considerable attention. The importance of hepatic triglyceride for apoB secretion has been well documented.7 8 12 13 14 15 Increased triglyceride synthesis has been shown to regulate apoB posttranscriptionally, by reducing the proportion of apoB degraded intracellularly, thus increasing secretion.7 12 Reduced availability of PL, the major surface component of VLDL, has also been shown to decrease VLDL secretion from rat hepatocytes.16
The role of hepatic CE in the assembly and secretion of apoB-containing lipoproteins remains controversial. The regulation of apoB secretion by the rate of cholesterol synthesis, esterification, and/or the mass of CE, has been documented in vitro6 17 18 19 20 21 and in vivo.22 23 24 25 In contrast, several studies argue against the regulation of apoB by CE availability.8 13 14 15 26 For example, Wu et al14 have shown that apoB secretion from HepG2 cells was unaffected by either long- or short-term changes in cellular CE content. Similar conclusions were reported by Furukawa et al13 and Sato et al26 in studies involving the inhibition of cholesterol synthesis in HepG2 cells. One possible explanation for these discrepancies may be the comparison of apoB secretion in relation to CE synthesis rates rather than cellular CE mass. Kohen Avramoglu et al20 have shown that not only newly synthesized, but also preformed CE, stimulates the secretion of apoB-containing lipoproteins. Additional discrepancies may arise from the choice of cell model used, as hepatoma cell lines and primary hepatocytes have been shown to differ in their response to modulators of FC and CE biosynthesis.21 27 Nevertheless, the role of CE in the assembly and secretion of apoB-containing lipoproteins remains unresolved.
The present study was designed to further investigate the regulation of apoB secretion by inhibition of cholesterol esterification, using 2 specific acyl CoA:cholesterol acyltransferase (ACAT) inhibitors, CI-1011 and DuP 128. Studies in chow-fed rats and rabbits fed a cholesterol-free casein diet revealed that CI-1011 is more effective in reducing plasma total cholesterol (TC) levels than equivalent doses of DuP 128.28 Previous in vivo apoB kinetic studies from this laboratory have shown that DuP 128, a potent noncompetitive inhibitor of ACAT, significantly inhibited hepatic ACAT when administered to pigs intravenously.23 29 This was associated with a significant decrease in the VLDL apoB secretion into plasma, but had no effect on LDL apoB production. A recently developed ACAT inhibitor, CI-1011, has marked lipid and apoB lowering ability in vivo,30 31 32 and displays ACAT inhibitory activity in vitro.28 In contrast to DuP 128, oral administration of CI-1011 decreased both VLDL and LDL apoB production rates in apoB kinetic studies performed in pigs.33 These 2 inhibitors differ in structure, lipid solubility, and 50% inhibitory concentration (IC50), raising the possibility that the mechanism(s) by which these inhibitors regulate apoB secretion may differ. The mechanism of CI-1011 action on apoB secretion in vitro has not been previously studied. Therefore, in the present study, the ACAT inhibitors CI-1011 and DuP 128 were used as probes to investigate the role of cholesterol esterification and CE mass, in the regulation of apoB secretion from HepG2 cells. Specifically, this study was conducted (1) to determine if the in vivo differences in apoB secretion, as stated above, would be revealed in HepG2 cells (the most widely used cell model for apoB secretion to date), thus allowing us to investigate the mechanisms for these differences, and (2) to examine whether the effect of ACAT inhibition on apoB secretion is associated with changes in apoB intracellular degradation. We explored the kinetics of cellular apoB metabolism using the novel approach of multicompartmental modelling, using SAAM II to analyze pulse-chase studies. The results of this study demonstrated that both ACAT inhibitors decreased cellular CE mass and cholesterol esterification; however, only CI-1011 decreased apoB secretion from HepG2 cells. These findings reveal potentially novel mechanisms regulating apoB secretion in HepG2 cells by CI-1011 that may be independent of its ability to inhibit cholesterol esterification.
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Methods |
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Cell Culture
HepG2 cells were obtained from the American Type Culture Collection (ATCC) and grown on 100 mm culture dishes (Falcon Scientific) as described previously.34 For experiments, HepG2 cells were plated in either 100-mm culture dishes for RNA isolation, or in 6-well (35-mm) culture plates (Falcon Scientific) for all other experiments. The cells were washed with citrate saline (15 mmol/L sodium citrate, 0.13 mol/L potassium chloride, pH 7.4) and the media replaced with MEM containing 5% human lipoprotein deficient serum (LPDS). The appropriate concentrations of ACAT inhibitors, solubilized in DMSO, were added to the dishes. The ACAT inhibitor CI-1011 was provided by Parke-Davis Pharmaceutical Research and DuP 128 was provided by DuPont Merck Pharmaceutical Co. Experimental DMSO concentrations did not exceed 0.5% and all incubation media were adjusted for the same DMSO concentration. In some experiments, 0.8 mmol/L oleic acid, complexed to fatty acid free BSA in a molar ratio of 5:1 was added to the cells when the inhibitors were added.
Unless otherwise stated, the effect of ACAT inhibition was studied over a 24-hour period. After the incubation period, the medium was collected from the 35-mm dishes and centrifuged at 2500 rpm (IEC Centra-8R centrifuge) for 10 minutes to remove detached cells and debris. The supernatant was immediately stored at -80°C for measurement of apoB100 concentrations. The cells were washed 2 times with 1 mL of buffer B (0.15 mol/L NaCl, 50 mmol/L Tris, pH 7.4, 0.2% BSA) and then washed 2 times in the same buffer without BSA. Cellular lipids were extracted with 1 mL of hexane-isopropanol, 3:2 (vol/vol). After 30 minutes the extracts were removed and the cells extracted with an additional 1 mL of hexane-isopropanol, 3:2 (vol/vol). The 2 extracts were pooled for analysis. The soluble cell protein was dissolved in 2 mL of 0.1 mol/L NaOH and collected. Cell protein content was analyzed by a modification of the Lowry method.35
ApoB Mass Quantitation
ApoB concentrations in the medium were measured by ELISA. In the
ELISA, monoclonal human apoB antibody (Calbiochem) was used as the
capture antibody, and an affinity purified polyclonal apoB antibody
conjugated to peroxidase (The Binding Site) was used for detection.
O-phenylenediamine dihydrochloride
(Sigma-Aldrich) was used as substrate for color development, and
the absorbance at 450 nm was determined using a Vmax Kinetic 96
multi-well microplate reader (Molecular Devices). A purified human LDL
standard (d=1.030 to 1.050 g/mL) was used to calibrate the
assay. Media samples were diluted 10-fold for analysis. ApoB
mass results are reported as µg per mg cell protein.
Lipid Mass Quantitation
Cellular TG, FC, and TC were quantitated by a modification of
the method of Carr et al,36 using enzymatic reagents from
Boehringer Mannheim and a Vmax Kinetic 96 multi-well microplate
reader. The extracted lipids from each sample were evaporated to
dryness under N2, resuspended in 2.4 mL of a
chloroform/Triton X-100 mixture (0.5% Triton X-100 vol/vol), and the
solvent evaporated again under N2. The samples
were resolubilized in 600 µL of deionized H2O
(final concentration, 2% Triton X-100). Cellular lipids were then
analyzed as described previously.37 For the
determination of triglyceride mass, samples were diluted
2:5 (vol/vol) with a 2% Triton X-100 (in deionized
H2O, vol/vol) solution, and analyzed as
described.37 Cellular lipid results are reported as µg
of cellular lipid (CE, TG, or FC) per mg cell protein.
Lipid Synthesis and Secretion
The incorporation of [1-14C] oleic acid
(Amersham) into cellular CE and TG was determined by a modification of
the methods described previously.34 Each dish received
0.08 µCi of [1-14C] oleic acid (50 mCi/mmol),
complexed with fatty acid-free BSA. The incorporation of
[1-14C] acetic acid (Amersham) into cellular
CE, TC, and PL was determined by the addition of 0.5 µCi acetic acid
(57 mCi/mmol) per well. Incubations were carried out for 5 hours,
either immediately after the addition of the desired
inhibitors, or after a 19-hour preincubation with the
inhibitors. The incubations were terminated by washing the
cells 3 times with buffer B without BSA. The lipids were extracted in
situ as described above. The extracted lipids from cells incubated with
[1-14C] acetic acid were divided into 2 equal
fractions. One fraction was saponified as described
previously,34 and the determination of
[1-14C] acetic acid incorporation into TC was
determined after thin layer chromatography (TLC) as
described below. The other fraction, and the lipid extract from the
[1-14C] oleic acid incorporation studies, were
not saponified; radioactivity incorporated into the various lipid
fractions was determined after TLC.34 The silica gel was
scraped into vials containing 8 mL scintillation fluid (Aquasol,
Packard) for counting (Beckman LS 3801 liquid scintillation counter).
The determination of radiolabeled CE, TG, and PL secreted into the
media was performed, as described above, after a 24-hour incubation
rather than a 5-hour incubation. The media was removed after the
24-hour incubation and the lipids recovered by the method of Folch et
al.38 The extracted lipids were separated by TLC and
analyzed as described above.
Analysis of Lipoprotein Fractions
The distribution of secreted apoB into lipoproteins with the
density of VLDL (d<1.006 g/mL), IDL (d=1.006 to
1.019 g/mL), and LDL (d=1.019 to 1.063 g/mL) was determined
after the incubation of cells with 50 µCi (150 Ci/mmol)
L-[4,5-3H] leucine
(Amersham) for 24 hours. The media was removed after the incubation and
mixed with 4 mL normal human plasma. VLDL, IDL, and LDL fractions were
isolated by ultracentrifugation, as described
previously.39 ApoB was isolated from each lipoprotein
fraction by isopropanol precipitation39 and
analyzed by scintillation counting. This precipitation method
is specific for apoB, as described by Egusa et al.40
Pulse-Chase Experiments
Cells were preincubated with CI-1011 (10 µmol/L) or DMSO
(control) for 24 hours before metabolic labeling. The cells
were washed 2 times with PBS, warmed to 37°C and incubated with
methionine-free MEM (ICN) for 20 minutes. Cells were then pulsed for 10
minutes with 100 µCi/mL Tran [35S] label
(1000 Ci/mmol, [35S]-L-methionine
and [35S]-L-cysteine; ICN), and
chased for various periods (5 to 120 minutes) in MEM containing 10
mmol/L methionine and 5 mmol/L cysteine. CI-1011 (10
µmol/L) or DMSO (control) was present for each incubation. The
media were collected, centrifuged for 2 minutes at 14 000 rpm
in a microcentrifuge, and mixed with an equal volume of
solubilization buffer (PBS containing 1% Nonidet P-40, 1%
deoxycholate, 5 mmol/L EDTA, 1 mmol/L EGTA, 2 mmol/L
PMSF, 0.1 mmol/L leupeptin and 0.5 µmol/L
N–acetyl-leucinyl-leucinyl-norleucinal [ALLN, Sigma]) and used for
immunoprecipitation. The cells were washed with ice-cold PBS, placed on
a tray of ice, and solubilized in 1 mL of solubilization buffer. Cell
extracts were centrifuged in a microcentrifuge at
14 000 rpm for 5 minutes and the supernatant was used for
immunoprecipitation.
To assess the uptake of newly secreted apoB-containing lipoproteins, untreated cells were pulsed with Tran [35S] label for 10 minutes and chased for 60 minutes as outlined above. This media, containing [35S] apoB-lipoproteins, was collected and placed on other cells that had been pretreated with CI-1011 (10 µmol/L) or DMSO alone (control), for 24 hours. After the cells were incubated in the labeled media for 60 minutes, the media were collected, and apoB immunoprecipitated as outlined below.
Immunoprecipitation of cell extracts and media was carried out by a method similar to that described by Adeli,41 with some modification. Briefly, aliquots of cell extracts and media were incubated with 10 µL preimmune rabbit serum for 1 hour. Insoluble Protein A (30 µL, Staphylococcus aureus suspension; Sigma) was added to each sample and incubated for 1 hour. Each tube was centrifuged at 14 000 rpm for 5 minutes and the supernatant was transferred to a tube containing 5 µL of anti-apoB antiserum (Boehringer-Mannheim) or human anti-albumin antibody (The Binding Site). The samples were incubated on a rocking platform overnight at 4°C. Insoluble Protein A suspension (75 µL) was added to each sample and incubated on a rocking platform for 3 hours at 4°C and centrifuged at 14 000 rpm for 2 minutes. Immunoprecipitates were washed 3 times with buffer (PBS containing 1% deoxycholate, 1% Nonidet P-40, 0.1% SDS, 5 mmol/L EDTA). Finally, the immunoprecipitates were prepared for SDS-PAGE by suspending and boiling in 80 µL of electrophoresis buffer (10% SDS, 0.5 mol/L Tris, pH 6.8, ß-mercaptoethanol, 8% glycerol and 0.1% bromophenol blue).
An aliquot of each sample, based on cell protein content, was used for SDS-PAGE (3% to 15% gradient gel) essentially as described by Laemmli.42 Molecular weight markers (Bio-Rad) were included in each gel. The gels were dried and the radioactivity in each band, corresponding to apoB or albumin, was determined using a phosphoimager system with ImageQuant software (Molecular Dynamics). Alternatively, the gels were treated with Enhance (DuPont), dried, and exposed to Kodak X-Omat AR film. The bands corresponding to apoB and albumin, as visualized by fluorography, were excised from the gel, digested, and the radioactivity determined by scintillation counting. The 2 methods of assessing apoB and albumin radioactivity gave comparable results. The apoB values reported in this study are those determined by volume analysis of the gels scanned using a phosphoimager. This method allows for the selection of the appropriate bands for analysis and correction for background radioactivity. Radioactivity associated with immunoprecipitated apoB from cells 5 minutes after the start of the chase period was used as the maximal rate of incorporation of [35S] counts into apoB; all other time points were normalized to these initial counts.
Development of a Multicompartmental Model
The pulse-chase data were analyzed by multicompartmental
modelling. A compartmental model of apoB synthesis and secretion was
developed using the SAAM II program (SAAM Inst). The model was
developed using apoB radioactivity data from HepG2 cells and media.
Figure 1
shows the compartments of the
model and the pathways that connect the compartments. The following is
a simplified description of the model development. Initially, an
intracellular compartment (compartment 1) was included to
represent the dosing compartment (ie, the
35S-methionine pool). Although the tracer was
added to the media, it was assumed that the transport of the tracer
into the cells was essentially instantaneous. A second compartment, a
delay compartment (compartment 2), was added to represent the
time from the initial pulse of radioactivity until cellular
immunoprecipitatable apoB radioactivity was detected. Two compartments
were then added to represent intracellular apoB radioactivity.
Because apoB is progressively lipidated after synthesis, the initial
compartment (compartment 3) was designated to represent newly
synthesized apoB, and a second compartment (compartment 4) was
designated to represent lipidated or partially lipidated apoB
destined for secretion. Because most experimental data suggest that
apoB degradation results from inadequately lipidated
apoB,11 43 we initially constructed the model to allow for
degradation of apoB entirely from compartment 3 (newly synthesized
apoB). Intracellular apoB radioactivity (measured experimentally) was
allowed to distribute between these 2 compartments (compartments 3 and
4) in such a manner as to best fit the experimental data. An
extracellular compartment (compartment 6) was included to
represent media apoB radioactivity, and all of the apoB
experimental data derived from media apoB radioactivity was assigned to
this compartment. The compartmental model was fit to each data set
(cell apoB and media apoB) by taking into account the pulse
administration of the 35S-methionine for 10
minutes. The input of tracer was stopped at 10 minutes to simulate the
removal of tracer from the media and was chased with unlabelled
methionine. Five minutes after the chase, the first samples were
collected for analysis. To optimize the fit, a delay
compartment was introduced to represent the time it takes for
apoB to appear in the media (compartment 5). A second, more slowly
turning over degradation compartment (compartment 7), containing apoB
derived from compartment 3 was required to fit the rate of degradation
defined by the experimental data. The addition of pathways for apoB
secretion directly from compartment 3, or for apoB degradation from
compartment 4, were tested, but neither of these pathways were required
to fit the model to the experimental data.
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RNase Protection Analysis of HepG2 mRNA
A HindIII/PstI fragment of human apoB,
cloned into psp72 (Promega) (kindly provided by Dr N. Azrolan,
Rockefeller University, NY), a HindIII/XbaI
fragment from GAPDH (ATCC) subcloned into pGEM-7Zf (Promega), a
PstI/PstI fragment from the human LDL receptor
(ATCC) subcloned into pbluescript SK+ (Stratagene), and a
HindIII/PstI fragment of the human HMG-CoA
reductase (ATCC) subcloned into pbluescript SK+, served as templates to
synthesize antisense RNA probes. These riboprobes were then used to
measure apoB, GAPDH, LDL receptor, and HMG CoA reductase mRNA in a
modification of the RNase protection/solution hybridization assay of
Azrolan and Breslow,44 as described
previously.25 The sample and standard RNAs were assayed in
duplicate and hybridization was linear to 
120 pg of cRNA.
Within-assay coefficients of variation were <10%.
Statistical Treatment of Results
All values are presented as mean±SEM. Tests for
statistical significance of differences were compared using unpaired
t test for all experiments, with the exception of
pulse-chase analysis, which was compared using paired
t test. A P value<0.05 was considered
significant.
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Results |
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Modulation of ApoB Secretion From HepG2 Cells by ACAT Inhibition
HepG2 cells were incubated with an ACAT inhibitor, CI-1011 or DuP 128, at 10 nmol/L, 1 µmol/L, and 10 µmol/L concentrations for 24 hours. The media was analyzed for apoB using a noncompetitive sandwich ELISA. Compared with control cells, apoB in the media decreased by 25% (P<0.02), 27% (P<0.02), and 43% (P<0.001) at 10 nmol/L, 1 µmol/L, and 10 µmol/L of CI-1011, respectively (Table 1
). In contrast, apoB in the media was
unchanged by DuP 128 treatment at the same inhibitor
concentrations. In preliminary studies CI-1011 and DuP 128
significantly inhibited HepG2 cell cholesterol
esterification at concentrations of 10 µmol/L. Increasing DuP
128 from 10 µmol/L to 20 µmol/L did not reveal any effect
on apoB accumulation in the media despite a >90% inhibition of
cholesterol esterification (data not shown). Furthermore,
increasing the dose of CI-1011 to 20 µmol/L did not further
enhance the inhibition of apoB secretion. In some experiments, cell
viability began to decline at the 20 µmol/L dose of both
inhibitors. Therefore, a concentration of 10 µmol/L
was used as the highest dose for the remaining experiments. The ability
of these inhibitors to decrease the stimulation of apoB
secretion induced by oleic acid treatment was also assessed.
HepG2 cells were treated with 0.8 mmol/L oleic acid (complexed to
BSA, molar ratio of 5:1) for 24 hours in the presence or absence of
CI-1011 (10 µmol/L) or DuP 128 (10 µmol/L). As shown in
Table 1, oleic acid treatment induced a 2-fold increase in apoB
secretion compared with control. Incubation of HepG2 cells with CI-1011
(10 µmol/L) decreased apoB secretion by 28.2%
(P<0.0012) compared with oleic acid-treated controls. In
contrast, DuP 128 (10 µmol/L) failed to significantly alter apoB
secretion in the presence of oleic acid.
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Effect on Cell Lipid Mass
To determine the effect of the ACAT inhibitors on the
mass of cellular TG, FC, and CE, intracellular lipids were extracted
after the 24-hour incubation. CE mass was decreased by 11.9%, 29.6%,
and 32% (P<0.05) at 10 nmol/L, 1 µmol/L, and
10 µmol/L CI-1011, respectively (Figure 2A). DuP 128 decreased CE mass by 7.4%,
40% (P<0.011), and 42% (P<0.011) at 10
nmol/L, 1 µmol/L, and 10 µmol/L, respectively (Figure 2A). There was no significant difference between the 2
inhibitors with respect to their effects on cellular CE
mass. FC mass was not significantly changed by either
inhibitor at the 3 concentrations examined (Figure 2B). A trend for a decrease in TG mass was observed for both
inhibitors at 10 nmol/L and 1 µmol/L concentrations
(Figure 2C). This decrease only reached statistical significance
for 10 nmol/L DuP 128 (24% reduction, P<0.05). At the
highest concentration of both inhibitors, there was a trend
to increased cellular TG mass. Incubation of HepG2 cells with 0.8
mmol/L oleic acid resulted in a 3-fold increase in cellular TG mass,
and a 50% increase in CE mass after a 24-hour incubation (data not
shown). CI-1011 decreased the cellular CE mass by 23.4%, 37.2%, and
47.5% (P<0.03), compared with oleic acid-treated control
cells, at 10 nmol/L, 1 µmol/L, and 10 µmol/L,
respectively (Figure 2D). DuP 128 decreased cellular CE mass by
20.9%, 42.2% (P<0.03), and 57.6% (P<0.011).
In oleic acid-treated cells, neither inhibitor
significantly altered cellular TG or FC mass at any of the
inhibitor concentrations tested (data not shown).
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P<0.05; and
P<0.03 compared with control.
Effect on Cell Lipid Biosynthesis and Secretion
Incorporation of [14C] acetic acid or
[14C] oleic acid into cellular lipids was
measured to determine the effect of CI-1011 and DuP 128 on lipid
biosynthetic rates. Cells were treated with the inhibitors
at a concentration of 10 µmol/L. Incubations with the labeled
precursors were carried out from 0 to 5 hours or from 19 to 24 hours
after the addition of the inhibitors. This protocol allowed
for the measurement of ACAT inhibition over time, and allowed us to
determine whether the difference in apoB secretion was caused by a
difference in the metabolism or clearance of
inhibitor from the hepatocyte, resulting in an
attenuation of the effect on cholesterol esterification at
later time points. As shown in Table 2, a
significant reduction in the [14C] oleic acid
incorporation into CE was found for both inhibitors during
the first 5 hours of incubation: a 61.8% decrease for CI-1011
(P<0.002) and a 90.1% for DuP 128 (P<0.002).
During this time, incorporation of [14C] oleic
acid into TG was increased 21% (P<0.05) by DuP 128
treatment, but was not altered by CI-1011. During the 19- to 24-hour
incubation period, the incorporation of oleic acid into CE was
decreased by 78.6% for CI-1011 (P<0.002) and 85.2% for
DuP 128 (P<0.002). The inhibition of
cholesterol esterification by DuP 128 was greater than that
for CI-1011 throughout the 24-hour incubation period, though only
significantly greater during the 0- to 5-hour incubation
(P<0.02). Incorporation of oleic acid into TG from 19 to 24
hours was similar to that seen during the first 5-hour incubation.
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The effects of CI-1011 (10 µmol/L) and DuP 128 (10
µmol/L) on the incorporation of [14C] acetic
acid into CE, TC, and PL are presented in Table 3. [14C] acetic
acid incorporation into CE during the first 5 hours was decreased by
both inhibitors: 89.1% (P<0.0001) for CI-1011
and 92.9% (P<0.0001) for DuP 128. During the 0- to 5-hour
period, [14C] acetic acid incorporation into
TC, as determined after saponification, was unaffected by either
inhibitor. Similarly, incorporation into PL was unchanged
by either inhibitor during this incubation period.
Inhibition of [14C] acetic acid incorporation
into CE was maintained during the second incubation period (19 to 24
hours). [14C] acetic acid incorporation (19 to
24 hours) into TC decreased 48.7% for CI-1011 (P<0.03) and
50.1% for DuP 128 (P<0.03). During this period, PL
synthesis from acetic acid was reduced 18.7% for CI-1011
(P<0.03) and 23.2% for DuP 128 (P<0.03). PL
synthesis in cells treated with CI-1011 was not significantly different
from cells incubated with DuP 128.
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The effect of CI-1011 (10 µmol/L) and DuP 128 (10 µmol/L)
on the incorporation of oleic acid and acetic acid into secreted TG,
CE, and PL was determined during a 24-hour incubation with either
[14C] oleic acid or
[14C] acetic acid. As shown in Table 4, DuP 128 treatment significantly
reduced the secretion of CE labeled with either
[14C] oleic acid (81% decrease,
P<0.01) or [14C] acetic acid (61%
decrease, P<0.005), but had no effect on the secretion of
[14C]-labeled TG or PL. Similar to DuP 128,
CI-1011 treatment caused a significant reduction in the secretion of CE
from [14C] oleic acid (70% decrease,
P<0.05) and [14C] acetic acid (57%
decrease, P<0.005). In contrast to DuP 128, CI-1011 caused
a significant reduction in the secretion of
[14C]-labeled TG and PL. The secretion of
[14C]-labeled TG was reduced 50%
(P<0.01) by CI-1011 treatment, despite an unchanged
cellular TG synthesis and mass. These results indicate that CI-1011
reduced the secretion of TG into the medium.
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Effect on ApoB Distribution in Secreted Lipoprotein
Fractions
To determine the effect of DuP 128 and CI-1011 on the distribution
of apoB in secreted lipoproteins, HepG2 cells were incubated with 1 of
the inhibitors in the presence of
L-[4,5-3H] leucine for 24 hours. ApoB from the
media lipoprotein fractions VLDL (d<1.006 g/mL), IDL
(d=1.006 to 1.019 g/mL), and LDL (d=1.019 to
1.063 g/mL), was isolated and quantitated. The results are shown in
Figure 3. The lipoprotein distribution of
apoB in control cells was similar to that reported previously in HepG2
cells that were cultured under similar conditions.45
Compared with control, DuP 128 (10 µmol/L) resulted in a 20%
decrease in the recovery of apoB in the VLDL fraction but an increase
in the recovery of apoB in the IDL (32% increase) and LDL (29%
increase) fractions. The total recovery of apoB in the 3 fractions
combined was unchanged (2.6% decrease) compared with control. In
contrast, CI-1011 (10 µmol/L) resulted in a decreased recovery
of apoB in the VLDL (26% decrease), IDL (23% decrease), and LDL (13%
decrease) fractions. The total recovery of apoB in the 3 fractions was
decreased 25.2%.
|
Effect of CI-1011 on ApoB Secretion and Intracellular
Degradation
To better define the mechanism responsible for reducing the
accumulation of apoB in the medium of CI-1011-treated HepG2 cells,
pulse-chase studies were conducted. HepG2 cells were preincubated with
10 µmol/L CI-1011 for 24 hours, then labeled with Tran
[35S] methionine for 10 minutes and chased for
various periods of time (5 to 120 minutes). Total cellular apoB
radioactivity after 5 minutes of chase was not significantly different
in CI-1011-treated cells compared with control cells, indicating that
apoB synthesis was not affected. Figure 4A and 4C shows the observed data points
for apoB secreted into the media and the line of best fit generated by
the kinetic model for 2 of 4 separate experiments performed. Inspection
of the curves shows that less apoB appears in the media of
CI-1011-treated cells. The data for all 4 experiments was
analyzed by multicompartmental modelling using SAAM II, and the
key kinetic parameters generated from the model are shown
in Table 5. The rate constant
k(4,3) represents the fraction of apoB synthesized
that enters the pathway destined for secretion, whereas
k(0,3)+k(7,3) represents the fraction of
apoB that enters the pathways destined for degradation. The percent of
synthesized apoB that is secreted was determined from these rate
constants, and was found to be decreased by 42% (P=0.019)
in CI-1011-treated cells (4.81% of apoB secreted) compared with
control cells (8.30% of apoB secreted). The time from the addition of
[35S] methionine until radiolabeled apoB was
detected in the media was not affected by CI-1011 (23.4 minutes for
CI-1011, compared with 20.8 minutes for control cells). To determine
whether the reduced secretion was caused by an enhanced intracellular
degradation, the cellular apoB immunoprecipitates were
analyzed. Figure 4B and 4D shows the observed data
points and the line of best fit generated by the kinetic model for
total apoB (media plus intracellular) in 2 of 4 experiments performed.
Inspection of the curves shows that the intracellular degradation of
apoB is enhanced by CI-1011 treatment. The percent of apoB degraded, as
determined by the kinetic analysis, was significantly greater
in CI-1011-treated cells (95.2% of apoB degraded) (Table 5)
compared with control cells (91.7% of apoB degraded,
P=0.019). The rate of degradation directly from compartment
3 (Figure 1
) was 2-fold greater than the rate of conversion
to compartment 7. The percent of apoB in compartment 3, which was
degraded either directly from compartment 3 (71%) or converted to
compartment 7 (20%), was unaffected by treatment (data not shown). The
recovery of radiolabeled albumin in cell lysates and medium was
not significantly different for CI-1011 treatment compared with control
at all of the time points (data not shown).
|
)
or without (
) CI-1011 (10 µmol/L) for 24 hours were
pulse-labeled for 10 minutes and chased for various time periods (5 to
120 minutes). ApoB radioactivity secreted into the media is shown in
panels A and C for 2 of 4 experiments performed. Total apoB, determined
as the sum of apoB in the media plus the cell, is shown in panel B
(corresponding to experiment in panel A) and panel D (corresponding to
experiment in panel C). Data points represent the observed
data, and the lines are the best fit generated by the kinetic
model.
|
Once secreted into the medium, apoB-containing lipoproteins may be rapidly taken up again by the cells, resulting in a net decrease in medium apoB. To determine the extent of apoB reuptake and whether CI-1011 treatment of HepG2 cells could affect the rate of this uptake, HepG2 cells were pulsed for 10 minutes with Tran [35S] label and chased for 60 minutes. The medium was collected and placed onto another set of HepG2 cells that had been incubated in the presence or absence of CI-1011 for 24 hours. The cells were incubated for 60 minutes and the amount of apoB remaining in the medium was determined by immunoprecipitation. There was no difference in apoB radioactivity remaining in the medium from control cells (87.4%) compared with CI-1011 treated cells (84.8%), indicating that the uptake of newly secreted apoB-containing lipoproteins was low and could not account for the reduction in medium apoB found with CI-1011 treatment.
Effect on ApoB, LDL Receptor, and HMG CoA Reductase mRNA
Content
The abundance of specific mRNAs were determined in HepG2 cells
treated with CI-1011 (10 µmol/L) and DuP 128 (10 µmol/L).
HepG2 cells were incubated with 1 of the inhibitors for 24
hours and the RNA was extracted and quantitated. As shown in Table 6, apoB and LDL receptor mRNA levels were
not changed by either CI-1011 or DuP 128, compared with control. HMG
CoA reductase mRNA significantly decreased by 39% with CI-1011 and
DuP 128, compared with control. GAPDH mRNA levels remained unchanged by
CI-1011 or DuP 128 treatment.
|
|
Discussion |
|---|
|
TopAbstractIntroductionMethodsResultsDiscussionReferences |
|---|
ACAT inhibitors may have beneficial effects in the treatment of hyperlipidemia and atherosclerosis. These compounds have been shown to prevent foam cell formation in the arterial wall,46 inhibit cholesterol absorption in the small intestine,47 48 and decrease the hepatic secretion of CEs in VLDL.22 However, the role of cholesterol esterification and the ability of ACAT inhibitors to regulate hepatic apoB secretion is controversial. In particular, the investigation of these processes in HepG2 cells has resulted in conflicting findings. For example, the Sandoz ACAT inhibitor 58-035 has been associated with decreases,19 no change,8 14 15 or increases49 in apoB secretion. Also, Kohen Avramoglu et al20 demonstrated that the increased apoB secretion in CE-loaded HepG2 cells was not inhibited by 58-035.
In HepG2 cells, we found that the ACAT inhibitor CI-1011 decreased apoB accumulation in the medium at concentrations ranging from 10 nmol/L to 10 µmol/L. In contrast, the ACAT inhibitor DuP 128 did not affect the concentration of apoB in the medium. The hypothesis that ACAT inhibitors reduce the assembly and secretion of apoB-containing lipoproteins by limiting the availability of CE to associate with nascent apoB particles was examined by determining the ability of these inhibitors to reduce cellular CE mass and/or the rate of cholesterol esterification. Based on our finding that CI-1011, but not DuP 128, reduced apoB accumulation in the medium, we expected that CI-1011 would be more effective in reducing cellular CE availability. However, we found that CI-1011 was less effective in decreasing cholesterol esterification and in reducing CE mass, relative to DuP 128, over the 24-hour incubation period. Therefore, the ability of CI-1011 to reduce apoB secretion does not appear to be related to an enhanced ability to reduce CE availability.
Several studies have shown a decrease in apoB secretion after inhibition of HMG-CoA reductase and thus cholesterol synthesis.19 21 24 39 However, the effect of CI-1011 cannot be explained by changes in FC, as neither inhibitor significantly altered cholesterol synthesis during the initial 5-hour incubation. During the last 5 hours of incubation, both inhibitors caused similar significant reductions in cholesterol synthesis. The reduction in cholesterol synthesis can be explained by a downregulation of HMG-CoA reductase caused by an increase in cellular FC levels secondary to ACAT inhibition.
ApoB secretion has been shown to be regulated by TG7 8 12 13 14 15 and PL16 availability. CI-1011 did not change either cellular TG mass or synthesis, suggesting that an altered TG availability is not responsible for the effect of this inhibitor on apoB secretion. In this study, DuP 128 (10 µmol/L) caused a significant increase in the synthesis of TG. It is possible that this increase in TG may act to compensate for a reduced CE availability, masking an effect on apoB secretion. The increased TG synthesis seen with DuP 128 was modest compared with that usually associated with altered apoB secretion.13 50 However, it is possible that DuP 128 may increase a small, but important, pool of TG required for lipoprotein assembly. CI-1011 and DuP 128 did not significantly affect PL synthesis during the 0- to 5-hour incubation. Furthermore, whereas both inhibitors caused small but significant reductions in PL synthesis when measured during the 19- to 24-hour incubation, the reductions were similar for both inhibitors. The reason for the decreased PL synthesis is unknown.
The finding of reduced apoB accumulation in the media of cells incubated with CI-1011, but not DuP 128, is supported by the analysis of the lipoprotein fraction distribution, which shows a reduced accumulation of [3H]-leucine labeled VLDL, IDL, and LDL apoB secreted from the cells incubated with CI-1011. In contrast, DuP 128 caused a shift in the distribution of lipoproteins toward a more dense particle, with no reduction in total apoB accumulation. These results indicate that CI-1011, but not DuP 128, decreased the number of apoB-containing lipoproteins secreted.
To further characterize the regulation of apoB secretion by CI-1011,
pulse-chase experiments were carried out. These studies revealed that
CI-1011 exerts its effects posttranslationally, as apoB mRNA levels and
apoB synthesis were unchanged. The posttranslational regulation by
CI-1011 resulted in a decreased secretion of apoB-containing
lipoproteins from the HepG2 cells. The decreased secretion was
associated with an enhanced intracellular degradation of apoB. The
analysis of apoB cellular pulse-chase data by
multicompartmental modelling in this study is a novel approach. We have
previously used compartmental modelling for in vivo apoB kinetic
studies,23 25 33 and have now extended this technique to
the analysis of apoB pulse-chase data in HepG2 cells. This
approach has allowed us to more precisely define the kinetics of apoB
secretion and to identify intracellular pools of apoB with distinct
kinetic characteristics. The time from the addition of the
35S-labeled methionine to the first appearance of
labeled apoB in the media was found to be 23 minutes and was not
affected by CI-1011. This time is similar to the estimates previously
reported by Bostrom et al.51 According to the model, apoB
radioactivity initially appears in 1 intracellular compartment,
designated compartment 3, and subsequently appears in compartment 4. We
speculate that compartment 3 represents newly synthesized apoB
and that compartment 4 represents lipidated or partially
lipidated apoB. ApoB appears to be primarily degraded from compartment
3, a finding that is consistent with the concept that poorly
lipidated apoB is targeted for degradation. ApoB is either degraded
directly or after transfer to a more slowly turning over degradation
compartment (compartment 7). Interestingly, the effect of CI-1011 on
degradation is mainly on the rapid, direct degradation pathway, as
evidenced by the finding that the rate of degradation directly from
compartment 3 is 2-fold greater than the conversion to compartment
7. Although the intracellular pools of apoB in this study were defined
kinetically, it is tempting to speculate that the degradation pathways
represent different degradation processes as described by
others.43 52 53 54 55 It is conceivable that the rapidly
turning over degradation pool represents cytoplasmic
degradation mediated by the proteasome43 52 53 and that
the slowly turning over pool represents apoB degradation within
the ER lumen.54 55 This study demonstrates the usefulness
of multicompartmental modelling for apoB pulse-chase studies. This
technique should prove valuable in future studies that seek to explore
the mechanisms and characteristics of multiple pathways for apoB
intracellular degradation.
This is the first report demonstrating that reduced apoB secretion in the presence of an ACAT inhibitor is associated with an increased intracellular degradation of apoB. Graham et al56 found differing affects on apoB secretion from HepG2 cells by 2 ACAT inhibitors, 447C88 and CL277,082, however the difference was only revealed at inhibitor concentrations of 100 µmol/L. Whether the reduction in apoB secretion seen with CL277,082 was caused by alterations in apoB synthesis or intracellular degradation was not determined. More recently, Ooyen at el49 examined the effect of ACAT inhibition on the intracellular degradation of apoB in HepG2 cells. However, in contrast to our findings, they show a decreased intracellular degradation and increased secretion of apoB using the ACAT inhibitor 58-035. This was thought to be because of enhanced TG synthesis caused by 58-035.
The availability of core lipids such as TG and CE to associate with apoB has been suggested to regulate the rate of apoB translocation, and thus the proportion of apoB secreted.4 57 Whether the enhanced intracellular degradation of apoB by CI-1011 is associated with a reduced translocation of apoB across the ER is currently unknown. Additional posttranslational mechanisms may play a role in the regulation of apoB secretion by CI-1011. We have provided evidence that argues against an altered uptake of newly synthesized apoB-containing lipoproteins. This is supported by an unchanged reuptake of 35S-labeled apoB from the media, as well as an unaltered LDL receptor mRNA expression. Other possible mechanisms involve the interaction of apoB with various proteases41 43 52 53 54 55 and chaperones43 during the movement of apoB through the secretory pathway. The microsomal triglyceride transfer protein (MTP) has also been shown to be essential for the secretion of apo B-containing lipoproteins,58 by mediating the transfer of TG and CE to nascent apoB59 and possibly by facilitating the translocation of apoB across the ER membrane.60 61 It is possible that CI-1011 influences these processes.
The reasons for the observed differences in apoB secretion between CI-1011 and DuP 128 are not clear. The structures of these 2 inhibitors are different.28 62 DuP 128 is a lipophilic, noncompetitive ACAT inhibitor with limited oral bioavailability23 and an in vitro IC50 of 10 nmol/L for rat liver microsomal ACAT activity.63 64 In contrast, CI-1011, is an acyl sulfamate with quite different physiochemical properties compared with the more lipophilic, typical ACAT inhibitors.28 CI-1011 has a high IC50, ranging from 12 µmol/L to 60 nmol/L depending on ACAT assay conditions.28 How these structural differences result in the differential effects on the regulation of apoB secretion are currently unknown.
Previous studies from this laboratory have shown that, in vivo, DuP 128 significantly reduced VLDL apoB secretion into plasma in pigs.23 29 CI-1011 significantly inhibited VLDL apoB secretion into plasma to a similar extent, but also decreased LDL apoB secretion.33 The reason why both ACAT inhibitors decrease apoB secretion in vivo, but not in HepG2 cells, is unknown. The HepG2 cell line has been widely used as a model to study the regulation of apoB secretion.7 8 11 13 14 19 20 56 Despite this, HepG2 cells do not assemble typical TG-rich lipoproteins.65 66 This is not caused by impaired synthesis of TG in HepG2 cells, but a reduced delivery of newly synthesized TG to a pool available for association with apoB.50 65 Although not clearly understood, it is possible that this defect in TG mobilization and lipoprotein assembly renders the HepG2 cell relatively insensitive to regulation of apoB by ACAT inhibition.8 14 15 This study confirms the results found by others,14 56 that HepG2 cells may not be an appropriate model to reflect the effect of ACAT inhibition found in vivo. However, the HepG2 cell model has allowed us to determine, using CI-1011, an alternate pathway regulating apoB secretion by mechanism(s) that do not appear to be linked to an ability to alter total cellular CE concentration or esterification rates. Thus, we speculate that, in primary hepatocytes and in vivo, at least 2 intracellular CE pools may regulate apoB secretion and that 1 of these pools may be absent in HepG2 cells, leaving this putative alternate pathway as the primary mechanism for CE-regulated apoB secretion. It is possible that this alternate CE pool is more readily inhibited by CI-1011 compared with DuP 128.
In summary, this study show that ACAT inhibitors with differing properties can exert different effects on the regulation of apoB secretion, which may not be caused by their efficacy in inhibiting cholesterol esterification. Specifically, treatment of HepG2 cells with CI-1011 has revealed potentially novel mechanisms that may contribute to the regulation of apoB. This is the first study to show that reduced apoB secretion in response to ACAT inhibition involves enhanced intracellular degradation of apoB. The nature of these mechanisms, and if they occur in primary hepatocytes or in vivo, requires further investigation.
|
Acknowledgments |
|---|
This work was supported by a grant from the Heart and Stroke Foundation of Ontario (T-3371, to M.W.H.), the National Institutes of Health (HL49110, NCRR12609, to P.H.R.B.). L.J. Wilcox is a recipient of a Medical Research Council of Canada Studentship, P.H.R. Barrett is a recipient of a fellowship from the Fogarty Center (TW02272). M.W. Huff is a Career Investigator of the Heart and Stroke Foundation of Ontario. We thank Cindy Sawyez and Dawn Telford for expert technical assistance. We acknowledge Roger S. Newton, Parke-Davis Pharmaceutical Research (Warner Lambert Co, Ann Arbor, MI), for kindly providing CI-1011, and Peter J. Gillies, DuPont Merck Pharmaceutical Co (Wilmington, DE) for kindly providing DuP 128.
Received June 1, 1998; accepted September 1, 1998.
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