Oxidized Low Density Lipoprotein Decreases Macrophage Expression
of Scavenger Receptor B-I*
Jihong
Han,
Andrew C.
Nicholson,
Xiaoye
Zhou,
Jianwei
Feng,
Antonio
M.
Gotto Jr., and
David P.
Hajjar
From the Center of Vascular Biology, Weill Medical College of
Cornell University, New York, New York 10021
Received for publication, December 14, 2000, and in revised form, January 23, 2001
 |
ABSTRACT |
Scavenger receptor class B type I (SR-BI) has
recently been identified as a high density lipoprotein (HDL) receptor
that mediates bidirectional flux of cholesterol across the plasma
membrane. We have previously demonstrated that oxidized low density
lipoprotein (OxLDL) will increase expression of another class B
scavenger receptor, CD36 (Han, J., Hajjar, D. P., Febbraio, M.,
and Nicholson, A. C. (1997) J. Biol. Chem. 272, 21654-21659). In studies reported herein, we evaluated the effects of
OxLDL on expression of SR-BI in macrophages to determine how exposure
to this modified lipoprotein could alter SR-BI expression and cellular
lipid flux. OxLDL decreased SR-BI expression in a dose- and
time-dependent manner. Incubation with OxLDL had no effect
on the membrane distribution of SB-BI, and it decreased expression of
both cytosolic and membrane protein. Consistent with its effect on
SR-BI protein expression, OxLDL decreased SR-BI mRNA in a
dose-dependent manner. The ability of OxLDL to decrease
SR-BI expression was dependent on the degree of LDL oxidation.
OxLDL decreased both [14C]cholesteryl
oleate/HDL uptake and efflux of [14C]cholesterol
to HDL in a time-dependent manner. Incubation of macrophages with 7-ketocholesterol, but not free cholesterol, also
inhibited expression of SR-BI. Finally, we demonstrate that the effect
of OxLDL on SR-BI is dependent on the differentiation state of the
monocyte/macrophage. These results imply that in addition to its
effect in inducing foam cell formation in macrophages through increased
uptake of oxidized lipids, OxLDL may also enhance foam cell formation
by altering SR-BI-mediated lipid flux across the cell membrane.
 |
INTRODUCTION |
Reverse cholesterol transport, defined as the flux of cholesterol
from peripheral tissues to the liver, where it is excreted in the form
of bile salts, is an important mechanism in the removal of cholesterol
from sites of lipid deposition (1). The receptor mediating
HDL1 binding has now been
identified as scavenger receptor class B type I (SR-BI) (2, 3) and its
human homologue CLA-1 (CD36 and lysosomal integral membrane protein-II
Analogous-I) (4). SR-BI belongs to the family of class B scavenger
receptors that includes CD36 (5) and lysosomal integral membrane
protein-II (6). SR-BI binds HDL with high affinity (2) but can
also bind native low density lipoprotein (LDL), acetylated LDL,
oxidized LDL, and anionic phospholipid vesicles (7).
SR-BI mediates bidirectional flux of cholesterol across the plasma
membrane. SR-BI can bind HDL reversibly and mediate cholesterol efflux
(8) and cholesteryl ester uptake (2, 9). When varying cell types were
screened for the expression level of SR-BI, a direct relationship
between the efflux rate of cholesterol to HDL and the level of SR-BI
was observed (8). In studies with stably transfected Chinese hamster
ovary cells or transiently transfected COS cells, expression of SR-BI
stimulates both the efflux of cell cholesterol and influx of HDL
cholesterol (10). Studies with genetically engineered mice have
demonstrated that SR-BI has an essential role in cholesterol uptake in
liver and steroidogenic tissues (11). Overexpression of SR-BI in liver reduces HDL levels, increases reverse cholesterol transport (12, 13),
and decreases susceptibility to atherosclerosis (14). By contrast,
inhibition of SR-BI activity in apoE-null mice accelerates the onset of
atherosclerosis (15).
SR-BI/CLA-1 is highly expressed in liver, adrenal gland, ovary (16),
atherosclerotic lesions of apoE-deficient mice (8, 17), and human
atherosclerotic lesions (18). SR-BI is expressed in human monocytes
(19), macrophages (18), and monocytic cell lines (19, 20). We and
others have shown that expression of CD36 is increased in macrophages
exposed to oxidized LDL (21-23). It has been subsequently shown that
the mechanism by which oxidized low density lipoprotein (OxLDL)
up-regulates CD36 involves activation of the transcription factor,
PPAR-
(24, 25). In the present study, we evaluate expression of
SR-BI in macrophages to determine how it is regulated in response to
OxLDL.
 |
MATERIALS AND METHODS |
Cell Culture--
Raw264.7 cells, a murine macrophage cell line
(ATCC, Mnaassas, VA), were cultured in RPMI 1640 medium containing 10%
fetal calf serum, 50 µg/ml each of penicillin and streptomycin, and 2 mM glutamine. Cells were switched to serum-free medium for
3-5 h when the confluence was about 85%. Cells received treatments in
serum-free medium. Human monocytes were isolated from the pooled blood
of volunteer donors by Ficoll-Hypaque density gradient centrifugation. Monocytes were cultured in RPMI 1640 medium containing 10% human serum
(Bioreclamation, Hicksville, NY), 50 µg/ml each of penicillin and
streptomycin, and 2 mM glutamine. Nonadherent cells were
removed by washing after 2 h of incubation.
Isolation of HDL and LDL and Preparation of OxLDL--
LDL
(1.019-1.063 g/ml) and HDL (1.063-1.210 g/ml) were isolated from
normal human plasma by sequential ultracentrifugation. They were
dialyzed against PBS containing 0.3 mM EDTA,
sterilized by filtration through a 0.22-µm filter, and stored under
nitrogen gas at 4 °C. Protein content was determined by the method
of Lowry et al. (26).
OxLDL was prepared by dialysis of 500 µg/ml of LDL in PBS
containing 5 µM CuSO4 for 12 h, (or
other indicated times) at 37 °C, followed by dialysis in PBS
containing 0.3 mM EDTA for 2 × 12 h. The purity
and charge of both LDL and OxLDL were evaluated by examining
electrophoretic migration in agarose gel. The degree of oxidation of
LDL and OxLDL was determined by measuring the amount of thiobarbituric
acid-reactive substances (TBAR). LDL had TBAR values of <1 nmol/mg.
OxLDL had TBAR values of >10 and <30 nmol/mg. All lipoproteins were
used for experiments within 3 weeks after preparation.
Isolation of Total RNA, Purification of Poly(A+) RNA,
and Northern Blotting--
Cells were lysed in RNAzolTM B
(Tel-Test, Inc.) and chloroform-extracted, and total cellular RNA was
precipitated in isopropyl alcohol. After washing with 80 and
100% ethanol, the dried pellet of total RNA was dissolved in distilled
water and quantified. The poly(A+) RNA was purified from
about 80 µg of total RNA by using the Poly(A)Ttract® mRNA
Isolation System III (Promega, Madison, WI).
Poly(A+) RNA was loaded on 1% formaldehyde-agarose
gel. After electrophoresis, poly(A+) RNA was transferred to
a Zeta-probe® GT Genomic Tested Blotting Membrane (Bio-Rad) in 10×
SSC by capillary force for overnight. The blot was UV cross-linked for
2 min and then prehybridized with HybrisolTM I (Oncor,
Inc., Gaithersburg, MD) for 30 min before the addition of
32P-randomly primed labeling probe for mouse SR-BI or
glyceraldehyde-3-phosphate dehydrogenase. After overnight
hybridization, the membrane was washed for 2 × 20 min with 2×
SSC and 0.2% SDS and for 2 × 20 min with 0.2× SSC and 0.2% SDS
at 55 °C. The blot was autoradiographed by exposure to an x-ray film
(X-OmatTM AR, Eastman Kodak Co.). The semiquantitative
assay of autoradiograms was assessed by densitometric scanning using a
UMAX (Santa Clara, CA) UC630 flatbed scanner attached to a Macintosh
IIci (Apple Computer, Cupertino, CA) running NIH Image software
(National Institutes of Health, Bethesda, MD). The probe for mouse
SR-BI was generated by reverse transcription-polymerase chain reaction based on the published sequence. The sequences of 5'- and
3'-oligonucleotides used were TCGGCGTTGTCATGATCCTC (positions 121-141)
and GGTTCATAAAAGCACGCTGG (positions 551-571), respectively (2).
Analysis of SR-BI by Western Blotting--
After treatment,
macrophages were washed twice with cold PBS and then scraped and lysed
in ice-cold lysis buffer (50 mM Tris, pH 7.5, 150 mM NaCl, 1% Triton X-100, 1% sodium deoxychlorate, 1 mM phenylmethylsulfonyl fluoride, 50 mM sodium
fluoride, 1 mM sodium orthvanadate, 50 µg/ml aprotinin,
and 50 µg/ml leupeptin). Lysate was sonicated for 20 cycles and then
microcentrifugated for 15 min at 4 °C, and supernatant was
transferred to a new test tube. For the isolation of cytosolic protein,
cells were lysed in the lysis buffer in the absence of detergent
followed by sonication. To extract membrane protein, the pellet, after
removal of cytosolic protein, was relysed in lysis buffer containing
detergent and sonicated. After centrifugation, the supernatant was
collected as membrane fraction. Proteins were separated on a SDS-PAGE
and then transferred onto nylon-enhanced nitrocellulose membrane. Membranes were blocked with a solution of 0.1% Tween 20/PBS (PBS-T) containing 5% fat-free milk for 1 h and then incubated with
rabbit polyclonal anti-SR-BI (Novus Biological, Inc., Littleton, CO) for 2 h at room temperature followed by washing for 3 × 10 min with PBS-T buffer. The blot was reblocked with PBS-T containing 5%
milk followed by incubation with horseradish peroxidase-conjugated goat
anti-rabbit IgG for another 1 h at room temperature. After washing
three times for 10 min each with PBS-T, the membrane was incubated for 1 min in a mixture of equal volumes of Western blot chemiluminescence reagents 1 and 2. The membrane was then exposed to
film before development.
Determination of Efflux of Free Cholesterol and Influx of
Cholesteryl Ester--
[14C]Cholesterol was
obtained from PerkinElmer Life Sciences. For influx study,
macrophages were cultured in 24-well plates until ~85% confluence.
After treatment, cells were washed with PBS and then incubated with a
mixture of [14C]cholesteryl oleate (200 nCi/ml) and HDL
(100 µg/ml) in serum-free RPMI 1640 medium. At the indicated times,
medium was removed, and cells were washed twice with PBS and then lysed
by the addition of 0.1 N NaOH. Radioactivity and protein
content in lysate were determined. To study the efflux, macrophages
were cultured in 24-well plates in complete medium containing free
[14C]cholesterol (80 nCi/ml) until ~85% confluence.
Cells then were switched into serum-free medium or medium containing 50 µg/ml OxLDL for 24 h. After washing twice with PBS, cells were
incubated with 400 µg/ml HDL in serum-free RPMI medium. Free
[14C]cholesterol (cholesterol efflux) was determined in
the media collected at the indicated times.
Preparation of Methylated
-Cyclodextrin-Cholesterol
Complexes--
Methylated
-cyclodextrin (Me
CD), cholesterol and
7-ketocholesterol were purchased from Sigma.
To prepare the complexes of Me
CD-cholesterol, cholesterol or
7-ketocholesterol was dissolved in a mixture of methanol and chloroform
(1:1) at a concentration of 100 mM. An ~0.5-ml solution of cholesterol or 7-ketocholesterol was dried under N2, and
then 20 ml of 10 mM Me
CD solution was added. The dried
cholesterol was suspended in solution by scraping off the wall of the
tube. The suspension of cholesterol was sonicated for 5 min on ice and rotated in a 37 °C oven overnight. After adjusting the pH to 7.4, the mixture was filtered through a 0.22-µm filter. The concentration of cholesterol in the prepared solution was about 250-300 µg/ml as
determined by a cholesterol assay kit (Sigma).
 |
RESULTS |
To investigate the effects of various lipoproteins on SR-BI
expression, macrophages were treated with OxLDL (10 and 50 µg/ml), LDL (50 µg/ml), or HDL (50 µg/ml) for 16 h. The protein was
extracted in lysis buffer containing detergent and analyzed for changes of SR-BI protein expression. OxLDL, at both 10 and 50 µg/ml,
significantly decreased SR-BI protein (Fig.
1). LDL had no effect and HDL slightly increased SR-BI expression. Concentration and time course analyses showed that OxLDL decreased SR-BI expression in a
dose-dependent manner (Fig.
2A). Decreased expression of
SR-BI by OxLDL occurred by 4 h after initiation of treatment (Fig.
2B) with a maximum inhibition occurring at 8 h. OxLDL
had no effect on the membrane distribution of SB-BI and decreased
expression of both membrane and cytosolic protein (Fig.
3).

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Fig. 1.
OxLDL, but not LDL or HDL, decreases
expression of SR-BI in macrophages. Raw264.7 cells (a murine
macrophage cell line) were grown to 85% confluence in complete RPMI
1640 medium. Cells were switched to serum-free medium for 3-5 h and
then treated with OxLDL (10 and 50 µg/ml) or LDL or HDL (50 µg/ml)
for 16 h. Protein was extracted and analyzed for SR-BI protein by
Western blotting as described under "Materials and Methods."
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Fig. 2.
OxLDL decreases SR-BI expression in a dose-
and time-dependent manner. A, dose
response. Macrophages in serum-free medium were treated with OxLDL
(0-100 µg/ml) as indicated for 16 h. Whole protein was
extracted, and SR-BI expression was evaluated by Western blotting.
B, time course. Macrophages were treated with 50 µg/ml
OxLDL in serum-free medium. Cells were lysed, and protein was
extracted for Western analysis at the indicated times.
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Fig. 3.
OxLDL decreases expression of SR-BI in
both membrane and cytosolic fractions. Macrophages were treated
with OxLDL (25, 50, or 100 µg/ml) for 16 h. Cytosolic and
membrane protein was extracted and analyzed for SR-BI by Western
blotting.
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To determine the mechanism by which OxLDL down-regulated SR-BI
expression, we next evaluated steady state mRNA levels of SR-BI in
macrophages treated with varying concentrations of OxLDL. Consistent with its effect on SR-BI protein expression, OxLDL decreased SR-BI mRNA in a dose-dependent manner (Fig.
4).

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Fig. 4.
OxLDL decreases expression of SR-BI
mRNA. Macrophages were treated with OxLDL (25, 50, or 100 µg/ml) for 12 h. Total RNA was extracted and used to isolate
poly(A+) RNA. Poly(A+) RNA from 80 µg of
total RNA was analyzed by Northern blotting using a
32P-labeled cDNA probe for SR-BI as described under
"Materials and Methods." The same blot was reprobed with
32P-labeled glyceraldehyde-3-phosphate dehydrogenase
(GAPDH) cDNA.
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We next determined if the degree of LDL oxidation affected its ability
to modulate SR-BI expression. The degree of LDL oxidation was increased
with increasing times of dialysis in PBS containing 5 µM
CuSO4. Oxidation was stopped by the addition of EDTA and dialysis in cold PBS containing EDTA. Oxidation was quantified by
assessment of TBAR and migration in agarose gel (data not shown). The
ability of OxLDL to decrease SR-BI expression increased with the length
of time of LDL oxidation (Fig. 5).

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Fig. 5.
The effect of increasing time of LDL
oxidation on expression of SR-BI. Oxidized LDL was obtained by
incubating LDL in PBS containing 5 µM CuSO4
for the indicated times. Macrophages were treated with 50 µg/ml OxLDL
for 16 h. Protein was extracted, and SR-BI expression was
determined by Western blotting.
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We next evaluated the functional consequences of decreased SB-BI
expression in response to OxLDL. Both the efflux of free cholesterol
and influx of cholesteryl ester were evaluated in macrophages
following incubation with OxLDL. OxLDL decreased both [14C]cholesteryl oleate/HDL uptake (Fig.
6A) and efflux of
[14C]cholesterol to HDL in a time-dependent
manner (Fig. 6B). The relatively small changes seen in both
HDL-mediated influx and efflux in response to OxLDL do not parallel the
degree of down-regulation of SR-BI expression in response to OxLDL. The
reasons for this are not completely clear but may reflect alterations
in both receptor- and nonreceptor-mediated effects of OxLDL on
cholesterol trafficking. Without blocking SR-BI, it is not possible to
directly attribute the effects of OxLDL on either influx or efflux of
cholesterol to SR-BI.

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Fig. 6.
OxLDL decreases HDL-mediated influx of
cholesteryl ester and efflux of free cholesterol. A,
macrophages cultured in 24-well plates were incubated with 50 µg/ml
OxLDL for 24 h. After washing with PBS, cells were incubated with
[14C]cholesteryl oleate (200 nCi/ml) and HDL (100 µg/ml) in serum-free RPMI 1640 medium. At the indicated times, the
medium was removed. Cells were washed twice with PBS and then lysed by
the addition of 0.1 N NaOH. Radioactivity and protein
content were determined in the lysate (n = 4).
B, macrophages were cultured in 24-well plates in
medium containing free [14C]cholesterol (80 nCi/ml). Cells then were switched into serum-free medium with 50 µg/ml OxLDL for 24 h. After washing twice with PBS, cells were
incubated with 400 µg/ml HDL in serum-free RPMI medium. At the
indicated times, the medium was collected, and
[14C]cholesterol was quantified in the media
(n = 3).
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To investigate the mechanism(s) by which OxLDL inhibited SR-BI
expression and to determine if changes in SR-BI could be mimicked by
incubation of macrophages with cholesterol or an oxysterol, we
evaluated SR-BI expression in cells incubated with these lipids. We
have previously demonstrated that changes in cellular cholesterol levels following incubation of macrophages with
-Cyclodextrins can
alter expression of CD36 (27).
-Cyclodextrins are cyclic oligosaccharides that encapsulate insoluble hydrophobic compounds and
allow them to become soluble in aqueous solutions (28). Cyclodextrins
are efficient at removing cholesterol from cells in culture (29, 30)
and have also been used to deliver cholesterol (in the form of
-cyclodextrin-cholesterol complexes) to manipulate cellular
cholesterol content (31). We evaluated SR-BI expression following the
addition of cholesterol or oxidized cholesterol complexed with
-cyclodextrin. Incubation of macrophages with 7-ketocholesterol-
-cyclodextrin for 16 h decreased expression of SR-BI, while cholesterol-
-cyclodextrin had no effect on SR-BI expression (Fig. 7). The mechanism by
which this oxysterol inhibits SR-BI expression remains to be determined
and is currently under investigation in our laboratory.

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Fig. 7.
Effect of cholesterol or 7-ketocholesterol on
SR-BI expression. Macrophages in serum-free medium were treated
with indicated concentrations of cholesterol (CH) or
7-ketocholesterol (7-keto-CH) complexed with Me CD for
16 h. SR-BI expression was analyzed by Western blotting.
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To confirm our results in primary human cells, we evaluated the effect
of OxLDL on SR-BI expression in human monocyte-derived macrophages.
OxLDL increased SR-BI expression when added to monocyte/macrophages after three days in culture (Fig.
8A). OxLDL also increased
expression of CD14, consistent with a differentiation-related event
(25). However, when OxLDL was incubated with fully differentiated
macrophages (10 days after isolation in culture), OxLDL had no effect
on CD14 expression but decreased SR-BI expression (Fig. 8B).

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Fig. 8.
Effect of OxLDL on SR-BI expression in human
monocyte-derived macrophages. Monocytes were isolated and cultured
as described under "Materials and Methods." Three days
(A) or 10 days (B) after isolation, the cells
were treated with the indicated concentrations of OxLDL for 16 h.
SR-BI and CD14 expression were evaluated by Western blot.
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 |
DISCUSSION |
We demonstrate that macrophage expression of SR-BI is inhibited in
response to OxLDL, resulting in reduced HDL-cholesteryl ester uptake
and HDL-mediated cholesterol efflux. Our data are consistent with
previous data demonstrating that intracellular cholesterol levels
regulate SR-BI expression and that there is an inverse relationship
between SR-BI expression and cellular cholesterol pools (32). This
inverse relationship between cellular cholesterol content and SR-BI
expression was also seen in an adrenal cell line (Y1-BS1) treated with
-cyclodextrin to deplete cellular cholesterol stores (33). As
cellular cholesterol was reduced over time, SR-BI expression increased
(33). Using a similar technique, we demonstrated a direct effect
(increased cellular cholesterol was associated with increased
expression) of cholesterol levels on expression of the macrophage class
B scavenger receptor, CD36 (27). However, in the present study,
7-ketocholesterol, but not cholesterol, reduced SR-BI expression.
7-Ketocholesterol is a prominent oxysterol formed during the oxidation
of LDL (34) but is not a peroxisome proliferator-activated
receptor-
ligand (25). Further studies are needed to determine the
specific mechanism by which this oxysterol inhibits SR-BI expression.
The effects of cholesterol on SR-BI expression may be cell
type-dependent. Rats given a 2-week high cholesterol diet
had decreased SR-BI expression in hepatic parenchymal cells, while the
expression in Kupffer cells was increased (35). In steroidogenic
tissues (adrenal gland, testes), SR-BI expression is also hormonally
regulated. Both adrenocorticotropic hormone (ACTH) and human chorionic
gonadotropin increase SR-BI expression. In rats, estrogen treatment
inhibits hepatic SR-BI expression (16). The human SR-BI gene contains the consensus site for steroidogenic factor-1, an orphan member of the
nuclear hormone receptor family involved in the regulation of
steroidogenesis (36).
SR-BI is expressed in macrophage-rich areas in foam cells within
atherosclerotic lesions of apoE-deficient mice (8, 17) and in human
atherosclerotic lesions (18). Several reports document the expression
of SR-BI/CLA-1 in monocytes, macrophages, and monocytic cell lines;
however, its function in lipid metabolism in these cells and in
cholesterol accumulation or efflux in atherosclerotic lesions remains
unclear. There is conflicting data addressing the regulation of
SR-BI/CLA-1 in monocyte/macrophages. Murao et al. reported
that CLA-1 mRNA is expressed in human monocytes and THP-1 cells, a
human promonocytic leukemia cell line (19). Expression of CLA-1 was
higher in freshly isolated monocytes than in macrophages, and
expression in THP-1 cells decreased following differentiation with
phorbol 12-myristate 13-acetate (19). Contrary to these findings, other
groups find that expression of CLA-1 is low in freshly isolated human
monocytes but increased upon differentiation of these cells in tissue
culture (17, 20). CLA-1 was induced in monocytes treated with
macrophage colony-stimulating factor and fetal calf serum (20)
and was inhibited in both monocytes and macrophages by treatment
with interferon-
, lipopolysaccharide, and tumor necrosis
factor (20).
In contrast to our data, Hirano et al. (18)
demonstrated that incubation of human monocyte-derived macrophages with
either OxLDL or acetylated LDL increased SR-BI expression by 4-fold. The reason for this discrepancy with our data and with data
demonstrating an inverse relationship between cellular cholesterol
content and SR-BI expression (33) is undetermined, but it may relate to the preparation of the OxLDL or the time frame of cell treatment. In
our experiments, macrophages were treated for 2-10 h, a time frame in
which we do not observe significant lipid accumulation in our cells.
Hirano et al. (18) incubated macrophages with modified
lipoproteins for 24-48 h. Thus, their results may be the result of
lipid accumulation that may subsequently affect expression of this
receptor by other mechanisms. More likely, these differences relate to
the differentiation state of the monocyte/macrophage. Our data in
human monocyte-derived macrophages demonstrate that the OxLDL is acting
as a differentiating agent in monocyte/macrophages that are not fully
differentiated. We and others have shown that OxLDL induces expression
of a differentiation-linked surface antigen, CD14 (Fig. 8A)
(25). At this early stage of macrophage differentiation, OxLDL
up-regulates SR-BI expression (Fig. 8A). However, in fully differentiated macrophages, a state more likely to reflect macrophages in atherosclerotic tissue, OxLDL inhibits SR-BI but has no effect on
CD-14.
It is clear from many lines of evidence that oxidation of low density
lipoproteins is a critical early event in the pathogenesis of
atherosclerosis (37). OxLDL is present in human atheroma (38) and is
the proximal source of lipid that accumulates within cells of the
atherosclerotic lesion (37). Two macrophage scavenger receptors, the
type A scavenger receptor and CD36, have been implicated in the
pathogenesis of atherosclerosis based on their presence in human
lesions (22, 39) and inhibition of lesion formation when they are
deleted by homologous recombination in murine models of atherosclerosis
(40, 41). Insight into the role of SR-BI in atherosclerosis is provided
by murine models of atherosclerosis. SR-BI protects against
development of vascular lesions in atherosclerosis-prone mice; however,
the mechanism by which it functions in this role remains unclear.
Attenuation of SR-BI accelerated the development of atherosclerosis in
ApoE knockout mice (15). Conversely, atherosclerotic lesion size is
reduced in LDL receptor knockout mice overexpressing the SR-BI
transgene (14). It is tempting to speculate that atherosclerosis in
these murine models is either enhanced or repressed by altering HDL-mediated reverse cholesterol transport. However, since SR-BI can
also act as a scavenger receptor, binding oxidized LDL, acetylated LDL,
and anionic phospholipids (3, 7, 19), it is possible that alterations
in SR-BI expression may alter circulating levels and/or binding of
these proatherogenic lipids.
In conclusion, our data showing that macrophage expression of SR-BI is
inhibited in response to OxLDL have potential implications for
atherosclerosis. We believe, based on our data, that oxidized LDL may
play a duel role in macrophage lipid accumulation. Oxidized lipid
probably accumulates in macrophages through scavenger receptor-mediated binding and uptake. However, our data also imply that OxLDL may inhibit
SR-BI-mediated efflux, which would act in a synergistic manner to
promote lipid retention within macrophages.
 |
ACKNOWLEDGEMENT |
We acknowledge the excellent technical
assistance of Larcarya D. Scott.
 |
FOOTNOTES |
*
This work was supported by National Institutes of Health
Specialized Center for Research in Atherosclerosis Grant in Molecular Medicine and Atherosclerosis P50-HL56987 (to A. C. N. and D. P. H.)
and the Abercrombie Foundation (to A. M. G.).The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
To whom correspondence should be addressed: Dept. of Pathology and
Center of Vascular Biology, Weill Medical College of Cornell University, 1300 York Ave., New York, NY 10021.
Published, JBC Papers in Press, February 6, 2001, DOI 10.1074/jbc.M011302200
 |
ABBREVIATIONS |
The abbreviations used are:
HDL, high density
lipoprotein;
SR-BI, scavenger receptor class B type I;
LDL, low density
lipoprotein;
OxLDL, oxidized LDL;
PBS, phosphate-buffered saline;
TBAR, thiobarbituric acid-reactive substances;
Me
CD, methylated
-cyclodextrin.
 |
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