Open Journal of Respiratory Diseases, 2012, 2, 9-16 Published Online February 2012 (
Regulation of Nitric Oxide by Cigare tte Smok e in
Airway Cells
Jia Liu1,2, Jun Wang3, Ah Siew Sim3, Nitin Mohan1,2, Sharron Chow4, Deborah H. Yates5,
Xingli Wang6, Paul S. Thomas1,2,4
1Prince of Wales Clinical School, Faculty of Medicine, University of New South Wales, Sydney, Australia
2Department of Respiratory Medicine, Prince of Wales Hospital, Sydney, Australia
3Cardiovascular Genetics Laboratory, Prince of Wales Hospital, Sydney, Australia
4School of Medical Sciences, Faculty of Medicine, University of New South Wales, Sydney, Australia
5Department of Thoracic Medicine, St. Vincent’s Hospital, Sydney, Australia
6Division of Cardiothoracic Surgery, Texas Heart Institute at St. Luke’s Episcopal Hospital,
Baylor College of Medicine, Houston, USA
Received November 3, 2011; revised December 13, 2011; accepted December 23, 2011
Background and Objectives: Exhaled nitric oxide (NO) is decreased by smoking while oxides of nitrogen such as ni-
trites/nitrates (NOx) are increased. It was hypothesised that in vitro cigarette smoke extract (CSE) would either inhibit
NO generation by increasing the NO synthase inhibitor, NG, NG-dimethyl-L-arginine (ADMA) or increase NOx levels
via an oxidation pathway, which in turn could be inhibited by the antioxidant N-acetylcysteine NAC. Methods: Trans-
formed airway cells (A549) were cultured with control medium, 1.0% CSE in culture medium, or 0.8 mM NAC with
1.0% CSE. Baseline L-arginine, NOx and ADMA levels were measured in the media. Conditioned media were then
sampled at 1hour, 6 hours, 24 hours, 48 hours and 72 hours after incubation. Results: CSE induced significantly higher
NOx levels (mean (SD) peak increase of 135.8 (126.6)% after incubation for 6 hours (p < 0.0005)). NAC pre-treatment
partially reversed this effect to 35.6 (21.4)% at 6 hours (p = 0.009). ADMA levels were significantly higher in the CSE
conditioned media compared with control media (p = 0.02) while NAC pre-treatment did not affect ADMA levels.
Conclusions: CSE increased NOx which was partially reversed by NAC pre-treatment. ADMA levels were also in-
creased after CSE exposure, suggesting that it activates the NO pathway via oxidative-stress while inhibition probably
occurs via both ADMA and NOS.
Keywords: Airway; Cigarette; Nitric Oxide; Nitric Oxide Synthase; N-Acetylcysteine; NG; NG-Dimethyl-L-Arginine
1. Introduction
Smoking is known to decrease nitric oxide (NO) produc-
tion but the mechanism is unknown [1]. One possible
group of intermediaries implicated in the alteration in
NO are the methylarginines. Asymmetric methylargini-
nes are endogenous analogues of arginine, and include
NG-monomethyl-L-arginine (L-NMMA) and NG, NG-
dimethyl-L-arginine (asymmetric dimethylarginine, AD-
MA). It has been established that these analogues inhibit
the activity of nitric oxide synthase (NOS), presumably
by competitive antagonism at the binding site for L-ar-
ginine [2-4]. Thus, these molecules are capable of decre-
asing NOS-related nitric oxide (NO) production, and ha-
ve been proved to participate in a variety of physiological
and pathological processes [5,6].
Both L-NMMA and ADMA are detectable in plasma,
but ADMA concentrations are approximately 10 times
greater than that of L-NMMA [4]. ADMA was first iden-
tified in human urine in 1970 and has since been recog-
nised as an inhibitor of NOS which may contribute to en-
dothelial dysfunction [4,7,8]. Elevated ADMA levels in
plasma have been shown to be associated with chronic
renal failure, hypertension, chronic hypoxia-induced pul-
monary hypertension, acute coronary syndromes, heart
failure, stroke, alcoholic cirrhosis and Alzheimer’s dis-
ease [4,9-17]. Smoking has been reported to be associ-
ated with increased plasma ADMA levels, although this
has not been consistent [18,19].
Exhaled nitric oxide is a highly reproducible marker of
inflammation in the airways, and can be detected in ex-
pired breath. It is elevated in asthmatic patients, and may
be useful for monitoring asthmatic airway inflammation
[20,21]. Other markers of airway inflammation are ni-
trites and nitrates (NOx), the products of nitric oxide me-
tabolism, and these oxides of nitrogen can be detected in
opyright © 2012 SciRes. OJRD
exhaled breath condensate (EBC). NOx levels are raised
in the exhaled breath condensate (EBC) of smokers and
in patients with asthma and community-acquired pn-
eumonia [22-25]. Although NOx levels in the EBC of
smokers is elevated in comparison with normal subjects,
exhaled NO is reduced [24,26,27].
Smoking decreases exhaled NO levels, but the me-
chanism is unknown. The possible pathways include: 1)
cigarette smoking increases ADMA levels, thus suppre-
ssing NO production; 2) cigarette smoking could increase
NOx levels via the oxidative stress pathway which in turn
inhibits NO production by negative feedback; 3) cigarette
smoking could increase NOx levels directly in EBC by
donating oxides of nitrogen such as NO2· and NO3·, and
again these inhibit NO production by negative feedback
[24,28]. To explore the mechanism behind the cigarette
smoking altering NO metabolism, this in vitro study was
The respiratory tract is lined by airway epithelial cells
(AEC), which are not only a physical barrier, but also
play a crucial role in pulmonary host defense mecha-
nisms. AEC produce pro-inflammatory cytokines and
NO, and NO release from AEC can be decreased by glu-
cocorticosteroids (GCS) [29,30]. Also, NO production in
AEC has been demonstrated to be inhibited by cigarette
smoke extract (CSE) in a dose dependent manner, and
the effect of CSE on NOx is shown to be inhibited by N-
acetylcysteine (NAC) [1]. The mechanism is, however,
not fully understood.
NAC, an acetylated precursor of the amino acid L-
cysteine, is able to reduce the viscosity and elasticity of
mucus by reducing disulphide bonds, and it has been
used as a mucolytic for more than 30 years [31]. In addi-
tion to its clinical use as a mucolytic, NAC has also been
used for treatment of acetaminophen-induced hepatotox-
icity as well as for heavy metal poisoning. NAC contains
a sulfhydryl (SH) group and also acts as a free radical
scavenger in common with other thiol-containing com-
pounds. Recently, there has been increasing interest in
the finding that NAC has some beneficial effects in dis-
eases involving oxidative stress, including heart disease,
cancer and cigarette smoking [32,33]. Animal studies
have established that a large dose of NAC is capable of
inhibiting cigarette smoke-induced epithelial thickening
in rat models [34,35].
It was hypothesised that CSE would either inhibit NO
generation by increasing ADMA levels or increase NOx
levels via an oxidation pathway, which could be inhibited
by the antioxidant, NAC (Figure 1).
2. Methods
Reagents were purchased from Sigma-Aldrich (Sydney,
Australia) unless otherwise indicated.
Figure 1. Nitric oxi de metabolism pathway s and the effect s of
cigarette smoke. It was hypothesised that CSE would either
inhibit NO generation by increasing ADMA levels (Hypothesis
1) or increase NOx levels via an oxidation pathways (Hypothe-
sis 2), which could be inhibited by the antioxidant, NAC.
2.1. Nitrite/Nitrate Assay (NOx)
Total NOx concentration in cell media was measured by a
fluorescent modification of the Greiss method [36]. Sam-
ples were mixed with NADPH, FAD and nitrate reduc-
tase with final concentrations of 50 μM, 5 μM and 50
IU/L respectively. These were incubated at 37˚C for 1
hour, which allows nitrate to be converted to nitrite. Ni-
trite was conjugated with 0.05 mg/ml 2,3-diaminonaph-
thalene (DAN) in 0.62 M HCL to allow quantification by
fluorescence. The reaction was terminated with 2.8 M
NaOH. The resultant fluorescence was immediately read
on a CytoFluor Series 4000, Multi Well Plate Reader
(Perseptive Biosystems, Framingham, MA, USA) at ex-
citation 360/40, emission 395/25, gain 50. The assay li-
mit of detection was 2 μmol/L, mean (SD) intra-assay
coefficient of variation 3.1(3.4)%.
2.2. ADMA Measurements
All the samples were aliquoted in 250 μL portions and
kept at –70˚C. None of the samples were thawed until
processing for the ADMA assay.
ADMA concentrations in cell media were assessed by
high performance liquid chromatography (HPLC) [18,37,
38]. The chromatography equipment comprised a SCL-
10A System controller, SIL-10A-XL Auto injector, Sam-
ple cooler, LC-10ADVP Liquid Chromatograph and RF-
10AXL fluorescence detector (Shimadzu, Kyoto, Japan).
ADMA standard concentrations comprised 1.0, 0.5 and
0.1 μmol/L solutions. L-NMMA (10 μmol/L) was added
to each standard solution and sample as an internal stan-
dard. Standard solution (200 μL) or the sample were mix-
ed with 100 μL of internal standard and 700 μL pH 7.0
phosphate-buffered saline (PBS). These were applied to
Oasis MCX solid-phase extraction (SPE) columns (Wa-
ters, Rydalmere, Australia) which had been precondi-
tioned with 1mL 100% methanol and 1mL pH 7.0 PBS.
The columns were then washed with 1 mL 100 mmol/L
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J. LIU ET AL. 11
HCL and 1 mL 100% methanol. The amino acids were
eluted with 1 mL elution buffer of ammonia: water:
methanol mixture of 10:40:50. The solvent was evapo-
rated under nitrogen at 70˚C. The residue was then dis-
solved in 100 μL mobile phase A (0.05 mol/L potassium
phosphate buffer pH 6.5 with 8.7% acetonitrile (ACN)),
mixed with 100 μL ortho-phthaldialdehyde (OPA) con-
taining 0.1% 3-mercaptopropionic acid. The analytes
were separated by a Symmetry C18 5 μm 3.9 × 150 mm
column (Waters, Rydalmere, Australia). The height of
the peak at 20.5 minutes was used to calculate the AD-
MA concentration. The coefficient of variation for the
standards was <5%.
2.3. Cell Culture
The A549 cell line was cultured in F-12K Medium (Gi-
bco, Grand Island, New York, USA) supplemented with
10% heat treated fetal bovine serum (FBS) (Gibco,
Grand Island, New York, USA), 50 IU/mL penicillin and
streptomycin. 1 × 106 cells were transferred to each flask
in 2 mL medium at the concentration of 0.5 × 106 cells/
mL one day before the experiment. Viability was as-
sessed by Trypan Blue exclusion.
2.4. Preparation of the Cigarette Smoke Extract
Main stream smoke from 5 filtered commercial cigarettes
(Marlboro, Philip Morris Ltd., Australia) was drawn thr-
ough 1 mL of medium by the application of a constant
vacuum according to our published method (1). Briefly,
each cigarette was burned within 5 to 6 minutes. The pH
of the control and the CSE conditioned medium was ti-
trated to pH 7.0 after being filtered to remove insoluble
particles. Control and CSE conditioned medium were
further diluted with medium to a control medium and a
concentration of 1.0% CSE conditioned medium. CSE at
a concentration of 1.0% has been proved to be non-toxic
in previous studies and was used within 2 hours of pre-
paration [1,39]. Both CSE and control medium were ster-
ilised by filtration using a 0.22 micrometer filter (Milli-
pore, Carrigtwohill, Co. Cork, Ireland).
2.5. Preparation of NAC Solution
Thirty minutes prior to the addition of CSE to the medi-
um, NAC 200 mg/mL (Bristol-Myers Squibb, Noble Pa-
rk North, Victoria, Australia) was added to give a final
concentration of 0.8 mM.
2.6. Sampling
Control medium, 1.0% CSE, and NAC plus 1.0% CSE
medium were applied to the flasks, respectively. Baseline
NOx and ADMA levels were measured in all control, C-
SE and NAC plus CSE media. The supernatant was then
sampled at 1hour, 6 hours, 24 hours, 48 hours and 72
hours after incubation. Samples were aliquoted in 250 μL
portions and kept at –70˚C. None of the samples were
thawed until processing for the assays. Cell concentration
and viability were counted at the time of sampling.
2.7. Statistical Analysis
Quantitative variables are expressed as mean +/– SE.
After confirming the Normal distribution an unpaired
Student t test was used for between group comparisons
and univariate ANOVA was used for comparisons am-
ong three or more groups. Two-tailed p < 0.05 was re-
garded as statistically significant.
3. Results
3.1. Cell Viability and Concentration
No significant differences in the viability of the cells were
found between the control media, CSE conditioned me-
dia and the NAC pre-treated CSE conditioned media af-
ter 72 hours incubation. Mean (SD) cell viability was
95.2 (2.3)% in control group, 95.5(2.4)% in the CSE gro-
up and 95.6 (2.6)% in the NAC pre-treated CSE group
from baseline up to 72 hours after incubation. At 72 hours
the viability was 96.9% and 96.6% for control and CSE
groups respectively.
Cell number and concentration in all the groups incre-
ased significantly with time (p < 0.0005) while percent-
age change in cell concentration of CSE exposed cells
was significantly lower when compared with both control
cells (p = 0.004, univariate analysis) and the cells pre-
treated with NAC (p = 0.003). As the increase in cell nu-
mber varied, data were expressed as concentration of
each marker per million cells.
3.2. NOx
No difference in NOx was observed in CSE media com-
pared to control media. Exposure to CSE was associated
with an increase in NOx over the 72 hours study period
which became significantly greater than control at 6hours,
24 hours and 72 hours. Percentage change in NOx levels
per million cells (p < 0.0005, univariate analysis, Figure
2) was significantly higher in the CSE conditioned media
compared with the control media. NAC pre-treatment
was able to partially reverse the increase in NOx levels in
response to CSE to a level approaching that seen in the
control group (p = 0.009, Figure 2).
3.3. ADMA
No consistently detectable ADMA was present in CSE.
The addition of CSE to the culture medium demonstrated
a significantly greater percentage increase in ADMA me-
Copyright © 2012 SciRes. OJRD
dia levels per million cells when compared with the con-
trol media (p = 0.02, Figure 3). No significant difference
was found in ADMA levels between CSE conditioned
media and NAC pre-treated CSE media (p = 0.07, Figure
3.4. L-Arginine
L-arginine levels decreased significantly with time in all
three groups (p < 0.0005, Figure 4), and the percentage
Figure 2. A549 cells were incubated with control medium
(), 1.0% CSE conditioned medium () and NAC
pre-treated CSE conditioned medium (). NOx levels
were measured at baseline, 1hour, 6 hours, 24 hours, 48
hours and 72 hours after incubation. CSE () induced
significantly higher NOx levels in the conditioned medium
when compared with control medium () (p < 0.0005,
univariate analysis). Significantly lower NOx levels were
found in the NAC pre-treated CSE conditioned medium
() when compared with those without NAC pre-treat-
ment as in 2A above () (p = 0.002, univariate analysis).
Figure 3. Percentage change in ADMA levels was signify-
cantly higher in the CSE conditioned medium () com-
pared with the control medium group () (p = 0.007,
univariate analysis). No significant difference was observed
in ADMA levels between the CSE conditio ned medium group
() and the NAC pre-treated medium cells) (p =
0.07, univariate analy
Figure 4. The percentage fall in L-arginine levels in the CSE
conditioned medium was significantly less than the control
medium () (p = 0.04, univariate analysis). No signify-
cant difference was found in percentage change in L-ar-
ginine levels between the CSE conditioned medium ()
and NAC pre-treated CSE conditioned medium () (p =
0.1, univariate analysis).
fall in L-arginine levels in CSE conditioned media was
significantly less than in the control media (p = 0.04) but
there was no significant difference in percentage change
between CSE conditioned media and NAC pre-treated
CSE conditioned media (p = 0.1).
4. Discussion
Cigarette smoking is associated with many diseases, in-
cluding airway inflammation and cardiovascular diseases.
It has been demonstrated that smokers have significantly
lower exhaled NO levels compared to non-smokers [26],
and that either active or passive smoking can decrease
exhaled NO levels immediately [27,40].
In these experiments, cell numbers increased after 24
hours incubation in all three groups, but there was slower
growth in the cells exposed to CSE conditioned media
when compared with control cells, an effect which was
reversed by NAC pre-treatment. There was no significant
difference in cell viability between the three groups even
at 72 hours, but CSS may affect both cell division and
the L-arginine pathway.
ADMA has been recognised as an endogenous analog-
ue of L-arginine, and it competitively inhibits NOS [4]. It
has been reported that the lung is a major source of cir-
culating ADMA [41]. Type 1 protein-arginine methyl-
transferases (PRMTs) have been identified to play a key
role in arginine methylation to produce ADMA [42] and
ADMA is mainly metabolised by the enzyme dimethy-
larginine dimethylaminohydrolase (DDAH). Increased
PRMT levels were observed in hypoxic conditions in a
mouse model, which led to elevated ADMA levels [43].
It has been demonstrated that probucol, as a potent anti-
oxidant, is able to decrease ADMA levels by both inhib-
iting PRMT1 expression and enhancing DDAH active-
ity [44].
Copyright © 2012 SciRes. OJRD
J. LIU ET AL. 13
In this study, there was an increase in ADMA levels
observed in the cell culture medium during the first 24
hours after exposure to CSE, which could explain why
exhaled NO levels decrease after acute cigarette smoking
in humans. Current smokers in previous studies usually
have had a cigarette within 24 hours prior to their ex-
haled NO sample collection, and the increased ADMA
levels in lung epithelial cells during the first 24 hours
after cigarette smoke exposure could therefore explain
the lower exhaled NO levels observed in current smokers
when compared to non-smokers.
The end products of NO were expected to be either
decreased in the cells after exposure to CSE as the result
of suppressed NO generation via NOS pathway, or could
be increased via an oxidative stress pathway. Finding
higher NOx levels in the CSE conditioned media com-
pared with control media is, however consistent with in
vivo studies, which showed increased NOx levels in EBC
and plasma samples in healthy smokers when compared
with healthy non-smokers [25,45]. The increased NOx
levels in CSE conditioned media or EBC after exposure
to cigarette smoke may be due to activation of an oxida-
tive stress pathway. The baseline NOx levels in CSE con-
ditioned media were not higher when compared with
control media, thus, the elevated NOx levels in the CSE
conditioned media were not due to NOx-rich cigarette
smoke itself, and the dilution of the CSE was such that
any contribution of CSE-donated NOx was negligible.
The elevated NOx levels in the cell culture medium of
cells exposed to CSE were significantly reversed by NAC
pre-treatment and could be the result of a decrease in
oxidative stress induced by NAC. The protective effect
of NAC in airway epithelial cells against cigarette smoke
is consistent with a similar finding in our previous study
[1]. NAC, with its anti-oxidant properties, can both in-
crease the levels of reduced glutathione (GSH) and act as
a direct scavenger of free radicals such as OH , H2O2 and
O2 [46-48]. Elevated GSH levels have been shown to be
correlated with decreased NO levels [49-51]. NAC has
been demonstrated to have a protective effect against
oxidative stress, which is related to its inhibitory effect
on NO production in rat models of diabetes [52,53]. The
mechanism of the inhibitory effect of NAC on NO pro-
duction is not completely understood. One study, how-
ever, showed that NAC had strong inhibitory effect on
iNOS expression, which led to decreased NOx produc-
tion [54].
In this study, the CSE conditioned media was associ-
ated with significantly increased ADMA levels when
compared with control media. This is consistent with oth-
er studies [55]. No difference, however, was found betwe-
en CSE conditioned media and NAC pre-treated CSE
conditioned media, which suggests NAC is not affecting
the ADMA pathway. The fall in L-arginine levels in the
CSE conditioned media was significantly less than that in
the control media and there was no significant difference
in L-arginine levels between CSE conditioned media and
NAC pre-treated CSE media. The reason why cells ex-
posed to CSE consumed less L-arginine but produced
more ADMA is unclear. Besides being converted to AD-
MA via PRMT, L-arginine is also the substrate for NOS
to form NO and L-citrulline. In addition, L-arginine is
also converted to L-ornithine via the arginase pathway.
No studies have reported the effect of CSE on PRMT.
Since increased ADMA levels were observed in the CSE
conditioned media, more L-arginine would be expected
to be converted to ADMA in the CSE exposed cells.
Thus, either NOS activity or arginase activity was hy-
pothesised to be inhibited by the CSE. Arginase activity,
however, has been reported to be increased by cigarette
smoking in asthmatic airways, which leads to more L-
arginine consumption [56,57]. C, the metabolism of L-
arginine to both ADMA and L-ornithine might be ex-
pected. Consequently, more L-arginine would be con-
sumed in the metabolism from L-arginine to L-ornithine.
However, the NOS activity may be inhibited by elevated
levels of ADMA in the CSE conditioned media and this
would be consistent with a recent report that cigarette
smoke extract inhibits NOS activity in a healthy male
rabbit model [57].
The decreased consumption of L-arginine and elevated
ADMA levels after exposure to CSE were not reversed
by NAC pre-treatment. In this in vitro study, CSE did
show some effects on ADMA-NOS pathway although
the elevated NOx levels induced by CSE were mainly due
to the activation of oxidative stress rather than the
ADMA pathway. Thus, this study has shown that while
cigarette smoke in vitro does increase ADMA generation,
there is an associated increase in NOx via an oxidative
stress pathway.
This study has confirmed in vitro the effects seen in
the airway in vivo, namely an increase in NOx, which is
not derived directly from cigarette smoke but is a re-
sponse of airway cells to the smoke. Our hypothesis that
an increase in NOx may cause a negative feedback inhi-
bition of NOS is not substantiated, as we have shown that
there is an increase in ADMA in response to CSE. This
increase in ADMA is probably the mechanism by which
NOS inhibition occurs.
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