(Hypertension. 1999;34:574-579.)
© 1999 American Heart Association, Inc.
Scientific Contributions |
From the Department of Biochemistry, Whitaker Cardiovascular Institute, Boston University School of Medicine, Mass.
Correspondence to Peter Brecher, PhD, Department of Biochemistry, Whitaker Cardiovascular Institute, Boston University School of Medicine, 80 E Concord St, Boston, MA 02118. E-mail pbrecher{at}acs.bu.edu
| Abstract |
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or tumor necrosis factor-
when used alone
had no influence on nitrite production in the absence or
presence of N-acetyl-L-cysteine. Nitrite
accumulation was higher by the cells treated with interleukin-1ß
combined with either interferon-
or tumor necrosis factor-
compared with those treated with interleukin-1ß alone.
N-Acetyl-L-cysteine upregulated nitrite
production and inducible nitric oxide synthase expression
induced by combination treatment with interleukin-1ß and either
interferon-
or tumor necrosis factor-
. However,
N-acetyl-L-cysteine had no significant
influence in cytokine-induced activation of nuclear
factor-
B or signal transducer and activator of
transciption-1, as assessed by electrophoretic mobility shift assays.
These results demonstrate that
N-acetyl-L-cysteine possibly acted as a
thiol-containing reducing agent and facilitated the expression of
inducible nitric oxide synthase in rat vascular smooth muscle cells by
cytokines through a mechanism that is independent of nuclear
factor-
B or signal transducer and activator of
transciption-1.
Key Words: acetylcysteine cytokines nitric oxide RNA interleukins muscle, smooth, vascular
| Introduction |
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By altering the reduction/oxidation state of the cell, oxidative stress is thought to influence cellular responses during inflammation and is apparently involved in the pathogenesis of vascular diseases such as hypertension and atherosclerosis.13 14 15 Antioxidants have been shown to slow the atherogenic process in several experimental models.14 Evidence from human studies also suggests a beneficial effect of antioxidants in the prevention of atherosclerosis, although the overall roles of antioxidants require further elucidation.15 The expression of iNOS represents an example of cellular responses to stimulation of proinflammatory cytokines. However, whether and how the reduction/oxidation state affects iNOS expression in vascular cells under cytokine stimulation are unresolved questions. In the present study, we used N-acetyl-L-cysteine (NAC), a widely used thiol-containing antioxidant that is able to support intracellular glutathione synthesis and scavenge reactive oxygen intermediates, to examine how it might influence the cytokine induction of NO production and iNOS expression in cultured VSMCs from rat thoracic aorta.
| Methods |
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1x108 U/mg) was purchased
from Genzyme Corp. Recombinant rat interferon-
(IFN-
) (specific
activity:
1x107 U/mg) and rat tumor necrosis
factor-
(TNF-
) (specific activity:
2x107 U/mg) were purchased from Pepro Tech,
Inc. NAC was obtained from Sigma Chemical Co. NADPH, FAD, and nitrate
reductase were obtained from Boehringer Mannheim. Rabbit
anti-mouse macrophage iNOS (1131-1144) was obtained from
Calbiochem. Horseradish peroxidaselinked anti-rabbit antibody was
obtained from New England BioLabs.
[
-32P]dCTP (111 TBq/mmol) and
[
-32P]ATP (111 TBq/mmol) were purchased from
DuPont-New England Nuclear. All other materials used were commercial
products of the highest grade available.
Cell Culture
VSMCs were isolated from rat thoracic aorta of 8-week-old male
Wistar rats by an explant method16 and cultured in
DMEM/F12 that was supplemented with 10% FCS, 100 U/mL penicillin, 100
µg/mL streptomycin, and 0.25 mg/L amphotericin B at 37°C in a
humidified atmosphere of 95% air and 5% CO2.
The cells were used between passages 4 and 6.
Determination of Nitrite
Cells that were released with 0.05% trypsin/0.02% EDTA
solution (GIBCO) from stock cultures seeded on 24-well culture plates
in DMEM/F12 with 10% FCS. At confluence after 3- or 4-day incubation,
the cells were washed twice with and then cultured in phenol red-free
DMEM/F12 that contained 0.1% FCS for 24 to 48 hours. The medium was
changed, and various agents were added as indicated in Results. After
the designated time, the release of NO from cultures was assessed by
the determination of nitrite, a stable metabolite of NO in conditioned
medium, with Griess reagent (1% sulfanilamide and 0.1%
N-(1-naphthyl)ethylenediamine in 2% phosphoric acid).
The absorbance at 570 nm was read by a microplate reader (Dynatech MR
600). The nitrite concentrations of samples were calculated with a
series of known concentrations of sodium nitrite as a standard and
expressed as µmol/L. To examine whether NAC might modulate
IL-1ßinduced nitrite production by altering the ratio of
nitrite and nitrate, total nitrite/nitrate levels were determined in
some experiments by the use of nitrate reductase to convert nitrate to
nitrite. Briefly, 50 µL of conditioned medium was mixed with 50 µL
of distilled water or 50 µL of freshly prepared reaction solution
that contained 0.2 U/mL nitrate reductase, 0.2 mmol/L NADPH, and
0.02 mmol/L FAD. After incubation for 1.5 hours at room
temperature, 100 µL of Griess reagent was added and the absorbance at
570 nm was read. Nitrate concentration was obtained by subtraction of
the nitrite concentration (without conversion reaction) from the total
nitrite/nitrate concentration (with conversion reaction).
Northern Blot Analysis
VSMCs cultured in 100-mm Petri dishes at confluence were kept in
DMEM/F12 that contained 0.1% FCS for 24 to 48 hours and then incubated
in the same but fresh medium with or without other agents for 16 hours.
Total RNA was then extracted from the cells as described
previously17 with Trizol reagent (GIBCO-BRL) instead of
Isogen reagent. The concentration of RNA was estimated from the
absorbance at 260 nm. The RNA (10 µg per lane) was electrophoresed on
1% agarose-formaldehyde gels, transferred to nylon transfer membranes
(Micron Separations, Inc), and cross-linked to membranes by UV
irradiation. After prehybridization, the membranes were hybridized with
randomly primed [
-32P]dCTP-labeled cDNA
probe for iNOS (an insert of KpnI-BamHI
restriction fragment of the rat iNOS plasmid18 ) at
65°C for 16 hours. The membranes were washed in a serial dilution of
solutions starting at 1x SSC/0.5% SDS and ending at 0.25x
SSC/0.125% SDS at 65°C (1x SSC contains 150 mmol/L NaCl and
15 mmol/L sodium citrate, pH 7.0). Autoradiography
was performed by exposure of the membranes to Hyperfilm (Amersham) at
-80°C with intensifying screens. As a reference, the blot was
stripped off and reprobed with
[
-32P]dCTP-labeled cDNA probe for rat
ß-actin.18
Western Blot Analysis
For detection of iNOS protein, confluent VSMCs cultured in
100-mm Petri dishes were maintained in DMEM/F12 that contained 0.1%
FCS for 24 to 48 hours and incubated in fresh DMEM/F12 that contained
0.1% FCS, with or without other agents as indicated in Results. After
a 24-hour incubation, the cells were washed once with ice-cold
phosphate buffered saline and scraped into 0.5 mL of ice-cold cell
lysis buffer that contained 20 mmol/L Tris, pH 7.5, 150
mmol/L NaCl, 1 mmol/L EDTA, 1 mmol/L EGTA, 1% Triton X-100,
2.5 mmol/L sodium pyrophosphate, 1 mmol/L
ß-glycerolphosphate, 1 mmol/L
Na3VO4, 1 µg/mL
leupeptin, and 1 µg/mL aprotinin. After incubation at 4°C for 30
minutes, the lysates were centrifuged at 10 000g for
15 minutes. Protein content of the lysate supernatants was determined
with BCA protein assay reagent (Pierce), with bovine serum
albumin used as a standard. Protein samples (30 µg) from each
lysate were separated on 10% SDS-polyacrylamide gels (2.6% C)
and then electrotransferred to nitrocellulose membranes (BioRad).
Membranes were blocked for 1 hour at room temperature in a buffer that
contained 20 mmol/L Tris, pH 7.6, 137 mmol/L NaCl, 0.1%
Tween 20, and 5% nonfat dry milk, and then incubated at 4°C
overnight with 1:3000 diluted rabbit anti-mouse macrophage iNOS
in blocking buffer. After the membranes were washed, they were
incubated for 1 hour at room temperature with 1:2000 diluted HRP-linked
anti-rabbit antibody and visualized with a chemiluminescence-based
detection system (ECL, Amersham).
Electrophoretic Mobility Shift Assay
Nuclear extracts were prepared by the method of Schreiber et
al19 from cells cultured in 100-mm Petri dishes at the
designated time after treatment with or without stimuli. Five
micrograms of nuclear proteins were used for electrophoretic mobility
shift assay (EMSA). Both double-stranded
oligonucleotides that contained nuclear factor-
B
(NF-
B) consensus sequence (5'-AGT TGA GGG GAC TTT CCC AGG
C-3'; Promega) and sis-inducible element (SIE, 5'-GTG CAT
TTC CCG TAA ATC TTG TCT ACA-3'; Santa Cruz) were end-labeled
with [
-32P]ATP. The binding reaction was
performed for 20 minutes at room temperature in a total volume of 25
µL of binding buffer that contained 22.5 µg/mL poly(dI-dC), 20
mmol/L HEPES, pH 7.9, 1 mmol/L EDTA, 5 mmol/L
MgCl2, 10% glycerol, 1 mmol/L PMSF, 1
mmol/L dithiothreitol, and salt (final concentration, 55 mmol/L
KCl for NF-
B and 65 mmol/L NaCl for SIE). DNA-protein complexes
were resolved on 4% nondenaturing polyacrylamide gel (5% C).
The electrophoresis was for 2 hours 30 minutes at 200 volts in 0.5x
TBE (1x TBE=90 mmol/L Tris-Borate, pH 8.0, 2 mmol/L EDTA).
The gels were dried and then exposed to autoradiography
(Hyperfilm MP, Amersham) at -80°C for 1 to 3 hours.
Statistical Analysis
The nitrite data are expressed as mean±SD. Statistical
analysis was performed by 1-way ANOVA and Student's
t test with unpaired data with StatView version 4.01 (Abacus
Concepts, Inc). P<0.05 was considered to be
significant.
| Results |
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There was no significant production of nitrite by cells treated
with either IFN-
(200 U/mL) or TNF-
(1000 U/mL). However, both
IFN-
and TNF-
markedly enhanced IL-1ßinduced nitrite
production (Figure 1D). Nitrite accumulation in the
cultures treated with IL-1ß combined with either IFN-
or TNF-
was significantly higher in the presence of 1 mmol/L NAC than
those with only cytokines added. Thus, NAC enhanced nitrite
accumulation when added to cells treated either with IL-1ß alone or
with any combination of cytokines that caused nitrite
production.
NAC enhanced both nitrite and nitrate production induced by IL-1ß in a similar manner (Figure 1E). NAC at 1 or 5 mmol/L did not significantly affect the ratio of nitrite and nitrate (Figure 1F), suggesting no effect of NAC on the rates of NO breakdown to the stable end products of nitrite and nitrate.
NAC Enhances IL-1ßInduced iNOS Expression
We further assessed whether the effect of NAC on nitrite
production resulted from an increase in iNOS expression.
Northern blot analysis showed that iNOS mRNA was obviously
induced by treatment for 16 hours with IL-1ß (Figure 2A), and this induction was enhanced by a
combination of either IFN-
or TNF-
(Figure 2B). The steady
state iNOS mRNA levels were higher in the presence of NAC than those in
the absence of NAC. The iNOS mRNA was not detectable in untreated cells
or the cells treated with NAC alone. Steady state mRNA levels for
ß-actin were downregulated by cytokine treatment but not
affected by the addition of NAC (Figure 2A and 2B). Loading of
the RNA from different samples was equivalent on the basis of
visualization of the 28s and 18s ribosomal RNA.
|
Furthermore, Western blot analysis was performed with an iNOS-specific polyclonal antibody. IL-1ß treatment for 24 hours clearly induced the 130-kD iNOS protein expression as shown in Figure 2C. The iNOS protein was not detectable in untreated VSMCs. NAC added without cytokine at a concentration of 5 mmol/L did not induce iNOS, but clearly potentiated IL-1ßinduced iNOS protein expression in a dose-dependent manner.
NAC Has No Influence on NF-
B or STAT-1 Activation
To assess whether the effect of NAC on
cytokine-induced expression of iNOS was secondary to an
increase in activation of transcription factors known to be involved in
iNOS expression, we used EMSA to determine if NAC would influence the
nuclear translocation of NF-
B or signal transducer and
activator of transcription (STAT)-1. NF-
B activation and
translocation clearly occurred in response to IL-1ß or TNF-
(Figure 3A) but was unaffected by the
addition of IFN-
(data not shown). By contrast, IFN-
clearly
activated STAT-1, whereas IL-1ß and TNF-
had no effect
(Figure 3B). NF-
B and STAT-1 were not detectable in the
nuclear extracts treated with NAC alone and when NAC was added in
combination with any of the cytokines, there was no obvious
influence on the translocation of both NF-
B and STAT-1 (Figure 3A and B). Time-course study showed that NAC did not cause
obvious alteration in IL-1ßinduced activation of NF-
B between 15
minutes and 6 hours (Figure 4).
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| Discussion |
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2 cytokines are added, a
finding explained in part by the necessity for activation of a complex
of transcription factors that cannot be activated by a single
cytokine.18 20 However, VSMCs have been shown to
respond to IL-1ß alone with increased iNOS
expression.17 21 This response to IL-1ß was confirmed in
this study and used as a model system to examine how NAC, a substance
often used to examine the role of oxidative stress in a variety of cell
types, might influence the induction of iNOS in VSMCs.
The present study clearly demonstrated that NAC, a
thiol-containing antioxidant, enhanced IL-1ßinduced nitrite
production, and this effect was apparently associated with the
increased iNOS mRNA transcription and protein expression. Neither
IFN-
nor TNF-
alone could induce nitrite production at
tested concentrations. However, IFN-
or TNF-
could enhance
IL-1ßinduced nitrite production and iNOS expression, which
suggests a complementary effect of the signal transduction by
combination of the cytokines. The effect of NAC on
cytokine induction of NO was most evident when cells were
treated with IL-1ß and TNF-
, which implicated that selective
signaling pathways of IL-1ß and TNF-
were more sensitive to the
antioxidant.
It is well known that NF-
B activation is essential for the
induction of iNOS.22 23 NAC has been reported to inhibit
NF-
B activation or translocation in some cell types
(macrophages, Kupffer cells, mesangial cells, and
astrocytes),24 25 which might be a possible mechanism
involved in the inhibition of iNOS induction.24 However,
NAC failed to inhibit iNOS induction in rat
hepatocytes.26 In hepatocytes,
intracellular glutathione was reported to be required for
cytokine-induced NF-
B activation and iNOS expression
because the iNOS induction was prevented by glutathione depletion and
could be restored by addition of NAC.27 28 The results
from the present study showed no obvious effect of NAC on
cytokine activation of NF-
B, which was consistent
with previous reports29 that indicated no
inhibitory effect of NAC on IL-1 and
TNF-activated NF-
B in some cell lines. Similarly, we showed
no influence of NAC in cytokine-activated STAT-1. It
has been reported that DNA binding of NF-
B was only inhibited at
concentrations of NAC >20 mmol/L and that low-dose NAC protected
rats against endotoxin-mediated oxidative stress, but high-dose NAC
increased mortality.30 In our study, cell damage was
observed in the cultures with a high concentration of NAC (30
mmol/L). The effective concentrations of NAC on potentiating IL-1ß
induction of iNOS were relatively lower than those usually used for
inhibiting NF-
B activation. Because NAC may provide additional
cysteine to the cultured cells, it has been suggested that the
concentration of cysteine may be insufficient to ensure maximal NO
synthase activity.28 However, the addition of IFN-
or
TNF-
significantly elevated IL-1ßinduced nitrite
production and iNOS expression in the same media without the
addition of NAC or cysteine, thus it is unlikely that cysteine
concentration in the culture media was a restrictive factor.
Although the expression of iNOS is associated with infection or inflammation and is geared toward host defense, the role of iNOS expressed in the vascular wall during atherosclerotic injury remains to be clarified. A variety of evidence indicate that NO plays an important antiatherogenic role by inhibiting VSMC proliferation, platelet aggregation, lipoprotein oxidation, and expression of adhesion molecules.1 4 9 10 31 32 Long-term inhibition of NO synthesis has been shown to promote atherosclerosis,33 34 whereas supplementation with L-arginine was reported to reduce atherogenesis.35 However, if a large amount of NO reacts with superoxide anion to produce highly reactive peroxynitrite, the latter can directly modify lipids and proteins and is cytotoxic. NAC has been reported to be effective on hepatoprotection and pneumoprotection from inflammatory injury,36 37 and reduce cerebral infarction during reperfusion after temporary middle cerebral artery occlusion.38 The beneficial effects of NAC on protecting tissues from damage are most probably a result of its antioxidant properties and its influence in NO production. It is plausible to hypothesize that NAC could be useful for elevating the antiatherogenic effect of NO via the increase in cytokine-induced NO production and inhibition of peroxynitrite formation.
| Acknowledgments |
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Received April 23, 1999; first decision May 12, 1999; accepted June 8, 1999.
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B. J
Biol Chem. 1995;270:2954129547.
B expression in
vivo. Am J Pathol. 1997;151:12251229.[Abstract]
B activation in three different
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[Order article via Infotrieve]
B activation during cerebral
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