| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
*
Harvard School of Public Health and
Harvard Medical School, Boston, MA 02115;
Lipid Metabolism Unit, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114
 |
Abstract |
|---|
 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
did not induce SR-A transcript
levels, indicating that a TNF- autocrine/paracrine signaling
mechanism alone is not sufficient to recapitulate the LPS induction of
SR-A transcripts. The induction of SR-A expression by LPS-stimulated
mouse macrophages is the opposite of the down-regulation of SR-A
reported in human monocyte-derived macrophages and may have
implications for the observed resistance mice show toward
endotoxemia. |
Introduction |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
and IL-1. This cytokine secretion is thought to
initiate uncontrolled cellular activation that leads to blood vessel
damage, plasma leakage into tissues, hypotension, and organ
failure. Cell surface molecules termed scavenger receptors have been implicated in host defense through the clearance of LPS and other pathogenic molecules from the circulation (2). Of the known scavenger receptors, macrophage scavenger receptor A (SR-A)3 has been most thoroughly characterized. SR-A was first cloned on the basis of its ability to bind and mediate the cellular uptake of modified LDL (3, 4). Subsequent studies showed that SR-A is a homotrimeric glycoprotein that has broad ligand specificity, with the capacity to bind a variety of polyanionic molecules (5, 6). LPS is among the pathogenic molecules recognized by SR-A, and it was found that the SR-A-mediated internalization of LPS leads to its degradation but does not induce cellular activation (7). Finally, in mice SR-A appears to protect against LPS-induced toxic shock, because SR-A knockout animals primed with Calmette-Guérin bacillus are more sensitive to the endotoxic effects of LPS (8).
Paradoxically, however, in human monocyte-derived macrophages LPS
exposure decreases scavenger receptor activity and SR-A expression
(9). This down-regulation in human macrophages has been
attributed to TNF- secretion, which is thought to destabilize SR-A
mRNA through a paracrine/autocrine mechanism (10, 11).
Additional differences in the transcriptional regulation of human and
mouse SR-A expression have been suggested by studies of the proximal
promoter elements of the SR-A gene in these two species (12, 13).
In the present study we examined the regulation of SR-A expression in mouse macrophage populations exposed to LPS. We confirmed that SR-A expression is down-regulated in a human monocyte-macrophage cell line exposed to LPS. In contrast, we found that SR-A expression was strongly up-regulated in mouse macrophages exposed to LPS. Our results indicate that the down-regulation of SR-A observed in human monocyte/macrophages is not a universal response of macrophages to LPS.
|
Materials and Methods |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
The following reagents were purchased from the indicated
vendors: cell culture grade Escherichia coli serotype
0111:B4 LPS, actinomycin D, Hoechst 33258, and cycloheximide (Sigma,
St. Louis, MO); DMEM and RPMI cell culture medium (Life
Technologies/BRL, Gaithersburg, MD);
L-[U-14C]leucine (303
mCi/mmol, 50 µCi/ml; Amersham, Arlington Heights, IL);
[-32P]dCTP (3000 Ci/mmol) and
[-32P]UTP (800 Ci/mmol; DuPont/NEN, Boston,
MA); monoclonal 2F8 anti mouse SR-A Ab (Serotec, Oxford, U.K.); mouse
TNF- (Genzyme, Boston, MA); FBS (HyClone, Logan, UT); BALB/c mice
(Taconic, Germantown, NY); CBA/CaJ mice (The Jackson Laboratory, Bar
Harbor, ME); RNAzol-B (Tel-Test, Friendswood, TX); QuikHyb (Stratagene,
La Jolla, CA), TA vector (Invitrogen, San Diego, CA), p755 plasmid
(clone 63058, American Type Culture Collection, Manassas, VA); in vitro
transcription and RNase protection kits (Ambion, Austin TX); and
Alexa488 goat anti-rat IgG and ProLong antifade mounting medium
(Molecular Probes, Eugene, OR).
Cell culture
The human THP-1 and mouse WEHI 265.1 monocytic cell lines (TIB
202 and 204, respectively, American Type Culture Collection) were
propagated in RPMI and DMEM, respectively, supplemented with 10% FBS
and 2 x 10-5 M 2-ME. After PMA
differentiation (100 nM), cells were exposed to LPS in RPMI or DMEM
(5% FBS) without PMA or 2-ME. The mouse J774A.1 and RAW264.7
monocyte/macrophage lines (TIB 67 and 71, respectively; American Type
Culture Collection) were propagated in DMEM with 5% heat-inactivated
FBS. For individual experiments, cells were seeded at 1 x
106 cells/10-cm plate in DMEM (5% FBS), grown to
75% confluence, and treated with LPS. Mouse peritoneal macrophages
were cultured as follows. Mice (8- to 12-wk-old females) of the BALB/c
or CBA/CaJ strain were sacrificed by cervical dislocation, and
unstimulated macrophages were collected by peritoneal lavage with a
sterile solution of 0.34 M sucrose. Harvested cells were plated in RPMI
(10% FBS) and enriched for macrophages by washing away nonadherent
cells after 2 h. The medium was replaced, and cells were treated
with vehicle (dH2O) or LPS. Peritoneal
macrophages were elicited from 10- to 12-wk-old C57BL/6J mice by an
i.p. injection of 1 ml of 3% thioglycolate broth (Difco, Detroit, MI)
4 days before collection of peritoneal macrophages. The peritoneal
macrophages were enriched and cultured as described above.
Metabolic labeling, immunoprecipitation, and immunofluorescent detection of SR-A protein
J774A.1 or RAW264.7 cells were plated at 3.0 x 105 cells/well in six-well plates in phenol red-free DMEM (5% FBS). After the cells had grown to 75% confluence they were treated with LPS for 24 h. To label cellular proteins, we removed the medium and washed the cells in PBS, after which they were incubated in 0.6 ml of leucine-deficient DMEM with 40 µl of L-[U-14C]leucine for 2.5 h at 37°C and chased for 2.5 h by adding 5 ml of DMEM (5% FBS). The SR-A protein was immunoprecipitated with 2F8 mAb as previously described (14), except that protein A-Sepharose was substituted for protein G-Sepharose. For immunofluorescent detection of SR-A, J774.1 cells were grown in chamber slides (Nunc, Naperville, IL) treated with LPS, fixed in 2% paraformaldehyde, blocked with 3% BSA in PBS, and incubated with the 2F8 Ab (2 µg/ml) for 1 h at room temperature. Bound Ab was detected with an Alexa488 fluorophore conjugated to goat anti-rat IgG, nuclei were counterstained with Hoechst 33258, slides were mounted with ProLong antifade mounting medium, and fluorescence was detected with a Nikon EFD-3 scope (Melville, NY).
Northern analysis and DNA probes
Total RNA was isolated with RNAzol B reagent according to the
manufacturer’s protocol. Isolated RNA (20 µg) was run on 1.0%
formaldehyde agarose gels and transferred to nitrocellulose. RNA
transfer efficiency and integrity were assessed by ethidium bromide
staining. The nitrocellulose membrane was baked at 80°C for 2 h,
and hybridization was performed with QuikHyb according to the
manufacturer’s protocol. RT-PCR was used to amplify and clone a
portion of the mouse SR-A gene (15). The sense
(5'-TTGGCTTCCCTGGAGGTCGAG-3') and antisense
(5'-ACACAGGAACCAATGTCATTTG-3') SR-A primers amplified the region
between nucleotides 949 and 1233 of the type II isoform, which contains
sequences common to both the type I and the type II messages and a
region unique to the type II message (16). Amplified
RT-PCR products were cloned into the TA vector, and the cloned products
were verified by DNA sequencing. For Northern analysis, the SR-A
restriction fragment isolated from the TA plasmid was radiolabeled by
nick translation with [-32P]dCTP and the
Klenow enzyme to a sp. act. of 2.5 x 106
cpm/ng. For RNase protection assays an antisense riboprobe was
generated using the plasmid linearized with HindIII in an in
vitro transcription reaction as recommended by the manufacturer. The
mouse c-fms probe consisted of the full open reading frame
obtained as an EcoRI fragment isolated from the p755
plasmid. The mouse TNF- probe was a gift from H. Xu and G.
Hotamisligil (Harvard University School of Public Health, Boston, MA).
An oligonucleotide probe for 18S ribosomal RNA
(5'-ACGGTATCTGATCGTCTTCGAACC-3') was used to normalize gene
expression (17).
Determination of SR-A message stability, alternative splicing, and transcriptional activity
RAW264.7 cells were cultured in 10-cm plates as described above and were treated for 16 h with 100 ng/ml LPS or an equivalent volume of vehicle (dH2O). Actinomycin D was then added (10 µg/ml), and total RNA was collected from the cells at various time points after treatment. The RNAs were subjected to Northern analysis, and SR-A transcript levels were quantitated by phosphorimage analysis (Molecular Dynamics, Sunnyvale, CA). A regression analysis (StatView 4.0 software, Abacus Concepts, Berkeley, CA) was used to determine the statistical significance of the message half-life estimates. To determine SR-A type I and type II isoform expression levels, RNase protection assays were performed using the RPA II kit according to the manufacturer’s protocol. Nuclear run-on assays were performed to measure the transcriptional activity of the SR-A promoter according to published protocols using purified DNA fragments as probes (18).
|
Results |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
In cell culture human monocytes differentiate to macrophages and
begin to express SR-A (19). During this culture-induced
differentiation of human monocytes, exposure to LPS has been shown to
inhibit expression of SR-A through a TNF--dependent mechanism
(10). Similarly, during phorbol ester-induced
differentiation, exposure of human THP-1 monocyte-like cells to TNF-
inhibits expression of SR-A (11). In both these
experiments exposure to LPS and TNF- was concomitant with induction
of differentiation; thus, these studies do not clarify whether the
observed down-regulation of SR-A was confined to the period of
differentiation from a monocyte to a macrophage phenotype. To examine
this question THP-1 cells were first differentiated by treating them
with the phorbol ester PMA for 24 h. The PMA-containing medium was
then removed, and the cells were washed with PBS and incubated for
another 24 h in the presence or the absence of LPS in a medium
that lacked PMA. Under these conditions THP-1 cells maintained a high
level of SR-A message expression 24 h after the removal of PMA
(Fig. 1
A). Treatment of these
cells with LPS still dramatically suppressed SR-A transcripts (80%
reduction in SR-A levels relative to that in PMA-treated cells; Fig. 1B). Thus, suppression of SR-A transcripts by LPS in THP-1
cells does not depend on exposure to LPS during the differentiation
process, nor does it require the continued presence of PMA.
|
, C and
D). LPS induction of SR-A transcripts is a general response in mouse macrophages
We next studied how SR-A expression responded to LPS in cultured
mouse peritoneal macrophages. Unstimulated peritoneal macrophages were
collected from wild-type BALB/c or CBA/CaJ mice. After isolated cells
had been exposed to LPS or vehicle (dH2O) for
16 h, total RNA was isolated, and SR-A mRNA was measured relative
to 18S ribosomal RNA (Fig. 2A). In macrophages from both
BALB/c and CBA/CaJ mice, SR-A mRNA increased by 3-fold (Fig. 2B). A population of mouse macrophages rich in infiltrating
monocyte-derived macrophages was elicited from C57BL/6J mice 4 days
after peritoneal injection of thioglycolate broth. As with resident
peritoneal macrophages, LPS also up-regulated SR-A transcript levels in
this population of elicited macrophages (Fig. 2, C and
D).
|
A, control samples). Exposure
to LPS for 24 h caused a significant up-regulation of SR-A mRNA
(Fig. 3A, LPS-treated samples). Relative to 18S RNA, SR-A
transcript levels increased 3- and 7-fold, respectively, in J774.A1 and
RAW264.7 cells (Fig. 3B). Thus, the LPS-induced increase in
mouse macrophage SR-A transcript levels was a general response in both
mouse cells representing the monocyte phenotype and those representing
a more differentiated tissue macrophage phenotype.
|
To test whether induction of SR-A message levels by LPS in J774.A1
and RAW264.7 cells leads to increased SR-A protein expression, we
performed pulse-chase labeling experiments with
[14C]leucine. 14C-labeled
SR-A protein (80 kDa) was immunoprecipitated from cell lysates using
the 2F8 anti-mouse SR-A mAb (Fig. 4A). The specificity of the
80-kDa band was demonstrated by its absence in the control precipitate
(Fig. 4A). SR-A protein levels increased significantly in
J774.A1 and RAW264.7 cells exposed to LPS for 24 h compared with
those in control cells (treated with dH2O; Fig. 4B). In J774.A1 cells SR-A protein levels were measured
relative to total cell-associated [14C]leucine.
Increased expression of SR-A protein was detectable after LPS treatment
at the lowest dose tested (50 ng/ml; Fig. 4C), and the
response reached saturation at approximately a 3-fold increase in
protein levels.
|
, compare
A and B). No significant cell immunofluorescence
was seen in experiments in which the anti-SR-A Ab was omitted (Fig. 5C). These results indicate that up-regulation of SR-A
protein demonstrated by metabolic labeling and immunoprecipitation was
correlated with increased cell surface expression.
|
The pulse-chase studies indicated that 50 ng/ml LPS was nearly
saturating for induction of SR-A protein. In RAW264.7 cells treated
with LPS at 10–500 ng/ml, the increase in SR-A mRNA saturated at 10
ng/ml (data not shown). An additional dose-response experiment using
doses from 0.1 to 10 ng/ml showed significant induction of SR-A message
by LPS at a concentration as low as 1 ng/ml (Fig. 6A). This increase in SR-A
message was detectable as early as 8 h after LPS exposure (100
ng/ml), peaked at 16 h, and was still significantly elevated vs
control levels at 48 h (Fig. 6B). The induction of SR-A
transcripts appears to be an intermediate response to LPS, since we
observed morphological changes in RAW264.7 cells as early as 3 h
after LPS treatment (data not shown).
|
The delayed kinetics of SR-A transcript induction (Fig. 6B) suggested that de novo protein translation may be
necessary for the response to LPS. To test this possibility, we exposed
RAW264.7 cells to LPS for 12 h in the presence or the absence of
cycloheximide, an inhibitor of eukaryotic protein synthesis. The 4-fold
increase in SR-A transcripts induced by LPS was completely blocked by
the concomitant addition of cycloheximide (Fig. 7, A and B). Thus,
translation of new proteins is required for LPS induction of SR-A
transcripts in RAW264.7 cells.
|
We used RNase protection assays to further define whether the
mechanism of SR-A transcript induction by LPS involved splicing
effects, because SR-A type I and II isoforms are alternative
transcripts derived from one open reading frame (16). We
found that in unstimulated cells the majority of transcripts coded for
the type II isoform (79%; Fig. 8A), and that LPS did not
selectively alter splicing, because SR-AI transcripts were increased to
the same extent as SR-AII transcripts (8.0- vs 7.7-fold,
respectively).
|
B). The steady state SR-A
transcripts were induced 8 ± 0.7-fold (n = 3)
during these experiments as demonstrated in Fig. 8C. We also
tested for alteration of transcriptional activity at earlier time
points during LPS exposure (3, 6, 9, and 12 h) and again found no
evidence for induction of SR-A transcriptional activity (Fig. 8D). LPS induction mildly increases SR-A message half-life and occurs in the presence of actinomycin D
To test whether LPS altered SR-A message stability, we used
actinomycin D to block transcription in RAW264.7 cells that had been
treated with 100 ng/ml LPS for 16 h. Total RNA was collected, and
SR-A mRNA, TNF- mRNA, and 18S ribosomal RNA were measured by
Northern blotting (Fig. 9A).
The rapid decay of TNF- transcripts indicates that transcription was
effectively blocked by actinomycin D in both control and LPS-treated
cells. By regression analysis the rate of decay of SR-A mRNA (Fig. 9B) was only mildly increased in the LPS-treated cells
relative to that in control cells (8.6 ± 2.4 vs 6.8 ±
1.8 h, respectively). In these studies we noted that SR-A
transcript levels did not immediately begin to decay after the addition
of actinomycin D, suggesting that LPS induction may occur in the
presence of actinomycin D when transcription is inhibited. To explore
this further we treated RAW264.7 cells with actinomycin D after only
6 h of LPS exposure and analyzed SR-A and TNF- transcripts
(Fig. 9C). Again, TNF- transcripts rapidly decayed after
actinomycin D treatment, indicating that transcription had been
effectively blocked. In contrast, SR-A mRNA levels in LPS-treated cells
continued to increase for up to 4–4.5 h after treatment with
actinomycin D (Fig. 9C). To further examine the significance
of the increase in SR-A transcripts in the presence of actinomycin D we
searched for additional genes whose transcripts were induced by LPS. We
found that Fc
RII transcripts were induced by LPS in RAW264.7 cells
with kinetics similar to those of SR-A transcripts, but to a greater
extent (maximal induction at 16 h of 25- vs 8-fold; compare Fig. 6B and Fig. 9D). In contrast to its effects on
SR-A transcripts (Fig. 9C), actinomycin blocked induction of
FcRII transcripts (in the same experiments; Fig. 9E).
|
exposure does not recapitulate the induction of SR-A mRNA
in RAW264.7 cells exposed to LPS
It has been reported that the down-regulation of SR-A expression
by LPS in human THP-1 cells occurs through an autocrine/paracrine
mechanism driven by TNF- secretion (11). We examined
whether TNF- treatment of mouse RAW264.7 cells recapitulated the
LPS-induced increase in SR-A transcripts. Cells were treated with mouse
TNF- or LPS for 24 h, and total RNA was isolated and analyzed
for SR-A transcripts (Fig. 10A,
top panel). As expected, LPS exposure resulted in a 4.5-fold
increase in SR-A transcripts relative to 18S RNA levels (Fig. 10B). TNF- exposure, however, resulted in a modest
decrease in SR-A expression. Because expression of the M-CSF receptor
gene (c-fms) responds to both TNF- and LPS, we probed for
c-fms transcripts (Fig. 10A, middle panel). The
level of c-fms transcripts relative to 18S RNA decreased by
about 40% after TNF- exposure and by >80% after LPS exposure
(Fig. 10B). Thus, although TNF- was able to modulate
c-fms transcripts, it was not able to recapitulate the LPS
induction of SR-A transcripts in RAW264.7 mouse macrophages.
|
|
Discussion |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
In contrast, peritoneal macrophages cultured from two mouse strains increased SR-A transcript levels by 3-fold when exposed to an identical serotype of LPS. This up-regulation of SR-A appears to be a general response of differentiated mouse macrophages, because J774.A1 and RAW 264.7 macrophage cells also responded to LPS by increasing steady state levels of SR-A transcripts. Because these cells may all represent resident tissue macrophages, we used the WEHI 265.1 line as a surrogate for mouse monocyte/macrophages (20, 21). After PMA exposure to induce a macrophage phenotype, these cells up-regulated SR-A expression in response to LPS. In addition, thioglycolate-elicited cells, which are enriched in infiltrating monocyte/macrophages, also up-regulate SR-A in response to LPS. The fact that these cells only up-regulated SR-A expression 2-fold in response to LPS is consistent with the observation that a variety of inflammatory stimuli, such as thioglycolate broth, elicit SR-A expression in mice (23). Thus, for resident mouse macrophages, mouse macrophages rich in infiltrating monocyte/macrophages, and three mouse macrophage cell lines, LPS induces a different pattern of SR-A expression relative to that reported for human monocyte/macrophages.
In the J774.A1 and RAW264.7 macrophage lines, LPS treatment increased protein expression by 3-fold, and this increase was associated with increased cell surface expression. These results argue for the functional significance of LPS induction, because overexpression of SR-A increases the internalization and degradation of LPS, presumably through trafficking to the lysosome (7), without activating cellular signaling (24). The increase in SR-A transcripts occurred at a low level of LPS (1 ng/ml), with the response saturating at 10 ng/ml. These levels are physiologically relevant, because the 50% lethal dose in 129 mice is 34 ± 6.6 ng/ml, while that in SR-A knockout animals is significantly lower (4.7 ± 1.5 ng/ml) (8). Interestingly, human monocyte/macrophages display a similar dose response to LPS, because 1 ng/ml LPS down-regulates SR-A transcripts, with maximal suppression seen at 10 ng/ml (9).
The induction of SR-A transcripts by LPS was seen by 8 h, peaked
at 16 h, and persisted for 48 h. These intermediate kinetics
suggested that the LPS effect on SR-A expression may require the
translation of novel protein factors. In support of this idea, we found
that the protein synthesis inhibitor cycloheximide blocked the
induction of SR-A transcripts by LPS. Studies at different times after
stimulation showed that LPS did not enhance the transcriptional
activity of the SR-A locus. To analyze LPS effects on transcript
stability, we used standard conditions of actinomycin D treatment and
found that LPS induction mildly increased mature SR-A message half-life
from 6.8 ± 1.8 to 8.6 ± 2.4 h. This slight lengthening
of half-life induced by LPS did not adequately explain the increase in
SR-A transcripts. However, the effects of LPS on SR-A transcript levels
in the presence of actinomycin D were complex. Control cells appeared
more sensitive then LPS-treated cells to actinomycin D toxicity. We do
not believe that this difference affected the half-life of SR-A
transcripts such that it was an overestimate. If actinomycin D had
artificially stabilized transcripts in a generalized manner, such an
effect should have also been operative on TNF- transcripts. This was
not the case, as TNF- transcripts rapidly decayed by 3 h (Fig. 9A). These results indicated that the increase in mature LPS
RNA transcripts was not simply explained by either transcriptional
activation or stabilization of mature mRNA.
When we tested the effect of actinomycin D at 6 h post-LPS
treatment, a time when the levels of processed transcripts were
actively increasing, we found that mature SR-A transcripts continued to
accumulate for up to 4.5 h after the addition of actinomycin D.
These results suggests three potential interpretations: 1) that
actinomycin D failed to fully inhibit transcription; 2) that SR-A
transcription was differentially sensitive to actinomycin D; or 3) that
although actinomycin D did inhibit transcription, the primary SR-A
transcripts or intermediates present at the time of addition of
actinomycin only became detectable later as a result of
post-transcriptional mRNA processing. The first interpretation seems
unlikely, because, under the same conditions, gene transcription was
inhibited for both TNF- and FcRII (which is induced with similar
kinetics, but to a greater magnitude than the SR-A gene). Although we
cannot completely rule out that the SR-A locus was differentially
sensitive to transcriptional inhibition, such a possibility appears
unlikely, because our nuclear run-on studies did not detect increased
SR-A transcript initiation in response to LPS.
Overall, our results are most consistent with the idea that LPS induced SR-A transcripts by a post-transcriptional process that affected message maturation or export. It is known that the induction of lysozyme transcripts by LPS involves regulated sequential splicing of primary transcripts (25). This type of regulated mRNA splicing and export is known to occur in response to signaling pathways involving growth factors, nutritional status, and environmental stress (25, 26, 27, 28). However, we were unable to uncover direct evidence that LPS induced an alteration in the post-transcriptional processing of SR-A transcripts. This may be because our method of isolating RNA did not recover all of the nuclear RNA, particularly the very large (60-kb) primary SR-A transcripts. Still, we found that LPS did not alter the ratio of type I or II messages, as determined by RNase protection assays. In fact, the pattern of SR-A transcript expression visualized by Northern blotting appeared to vary among macrophages of the various mouse stains and among the cell lines studied. These transcripts represent splice variants of a single gene (16), with the larger m.w. type II transcripts thought to arise from alternative termination and polyadenylation sites in the 3'-untranslated portion of the SR-A gene (16). LPS did not appear to differentially affect the use of alternative termination or polyadenylation sites, because in the BALB/c-derived peritoneal macrophages all three transcript sizes were induced to a similar extent. Further studies are needed to determine whether the LPS induction of SR-A transcripts involves regulated sequential splicing of primary transcripts (25) or alters SR-A message export.
These studies highlight the differences between SR-A expression in
mouse and human cells. In contrast to the 6-h half life of SR-A
transcripts in RAW264.7 cells, the half-life of SR-A message in
differentiated human THP-1 cells is much longer (40 h) and declines
precipitously (10 h) when THP-1 cells are exposed to TNF-
(11). A further divergence in the regulation of mouse SR-A
was found, because TNF- did not induce the expression of SR-A in
RAW264.7 cells. Because the biological activity of TNF- was
confirmed by the down-regulation of c-fms transcripts, this
experiment indicates that TNF- signaling alone is not sufficient to
recapitulate the LPS-induced increase in SR-A expression in RAW264.7
cells.
The finding that mouse macrophages up-regulate SR-A expression when exposed to LPS is consistent with the in vivo observation that SR-A protects against LPS toxicity. Injection of LPS into the hippocampus of wild-type BALB/c mice was correlated with increased SR-A expression on infiltrating macrophages and microglia (23). Furthermore, macrophages from wild-type mice primed with Calmette-Guérin bacillus expressed scavenger receptor activity, and when SR-A knockout mice were challenged with a systemic dose of LPS, they were more susceptible to endotoxic shock (8). Additional studies are needed to test whether in mice the LPS-induced up-regulation of SR-A expression increases resistance to endotoxemia (29).
|
Footnotes |
|---|
2 Address correspondence and reprint requests to Dr. Guy L. Reed, Cardiovascular Biology Laboratory, Harvard School of Public Health, 677 Huntington Avenue, Boston, MA 02115. E-mail address:
3 Abbreviation used in this paper: SR-A, scavenger receptor A.
Received for publication October 22, 1999. Accepted for publication December 22, 1999.
|
References |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
-helical and collagen-like coiled coils. Nature 343:531.[Medline]
. J. Immunol. 148:112.[Abstract]
is transcriptionally and post-transcriptionally regulated. J. Biol. Chem. 271:7767.This article has been cited by other articles:
|
|
 |
|
  M. Muthukuru and C. W. Cutler Antigen Capture of Porphyromonas gingivalis by Human Macrophages Is Enhanced but Killing and Antigen Presentation Are Reduced by Endotoxin Tolerance Infect. Immun., February 1, 2008; 76(2): 477 - 485. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 K. Jagavelu, U. J.F. Tietge, M. Gaestel, H. Drexler, B. Schieffer, and U. Bavendiek Systemic Deficiency of the MAP Kinase Activated Protein Kinase 2 Reduces Atherosclerosis in Hypercholesterolemic Mice Circ. Res., November 26, 2007; 101(11): 1104 - 1112. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. Soares-Schanoski, V. Gomez-Pina, C. del Fresno, A. Rodriguez-Rojas, F. Garcia, A. Glaria, M. Sanchez, M. T. Vallejo-Cremades, R. Baos, P. Fuentes-Prior, et al. 6-Methylprednisolone down-regulates IRAK-M in human and murine osteoclasts and boosts bone-resorbing activity: a putative mechanism for corticoid-induced osteoporosis J. Leukoc. Biol., September 1, 2007; 82(3): 700 - 709. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 S. Jozefowski, M. Arredouani, T. Sulahian, and L. Kobzik Disparate Regulation and Function of the Class A Scavenger Receptors SR-AI/II and MARCO J. Immunol., December 15, 2005; 175(12): 8032 - 8041. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. Daugherty, N. R. Webb, D. L. Rateri, and V. L. King Thematic review series: The Immune System and Atherogenesis. Cytokine regulation of macrophage functions in atherogenesis J. Lipid Res., September 1, 2005; 46(9): 1812 - 1822. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 Q. Li, Q. Lu, V. Bottero, G. Estepa, L. Morrison, F. Mercurio, and I. M. Verma Enhanced NF-{kappa}B activation and cellular function in macrophages lacking I{kappa}B kinase 1 (IKK1) PNAS, August 30, 2005; 102(35): 12425 - 12430. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C. A. Beamer and A. Holian Scavenger receptor class A type I/II (CD204) null mice fail to develop fibrosis following silica exposure Am J Physiol Lung Cell Mol Physiol, August 1, 2005; 289(2): L186 - L195. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 K. Fukuhara-Takaki, M. Sakai, Y.-i. Sakamoto, M. Takeya, and S. Horiuchi Expression of Class A Scavenger Receptor Is Enhanced by High Glucose in Vitro and under Diabetic Conditions in Vivo: ONE MECHANISM FOR AN INCREASED RATE OF ATHEROSCLEROSIS IN DIABETES J. Biol. Chem., February 4, 2005; 280(5): 3355 - 3364. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
Y. Tamura, J.-i. Osuga, H. Adachi, R.-i. Tozawa, Y. Takanezawa, K. Ohashi, N. Yahagi, M. Sekiya, H. Okazaki, S. Tomita, et al. Scavenger Receptor Expressed by Endothelial Cells I (SREC-I) Mediates the Uptake of Acetylated Low Density Lipoproteins by Macrophages Stimulated with Lipopolysaccharide J. Biol. Chem., July 23, 2004; 279(30): 30938 - 30944. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 E. Kanters, M. J.J. Gijbels, I. van der Made, M. N. Vergouwe, P. Heeringa, G. Kraal, M. H. Hofker, and M. P. J. de Winther Hematopoietic NF-{kappa}B1 deficiency results in small atherosclerotic lesions with an inflammatory phenotype Blood, February 1, 2004; 103(3): 934 - 940. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
X. Du and M. G. Low Down-Regulation of Glycosylphosphatidylinositol-Specific Phospholipase D Induced by Lipopolysaccharide and Oxidative Stress in the Murine Monocyte- Macrophage Cell Line RAW 264.7 Infect. Immun., May 1, 2001; 69(5): 3214 - 3223. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
H.-Y. Hsu and Y.-C. Twu Tumor Necrosis Factor-alpha -mediated Protein Kinases in Regulation of Scavenger Receptor and Foam Cell Formation on Macrophage J. Biol. Chem., December 22, 2000; 275(52): 41035 - 41048. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||
