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Airway inflammation has been identified with increased levels
of nitric oxide (NO) and its oxidation products in expired breath (1, 2) and breath condensates (3). The airways of asthmatics are
chronically inflamed and the approach to treatment of these patients is impacted by the intensity of the inflammatory process. A noninvasive method that could serially survey the airways for inflammation would be ideally suited for clinical practice and might improve the care of such patients. The potential ramifications for serial monitoring of exhaled NO in asthma, a disease
now reaching near epidemic proportions, are therefore enormous
only a few months ago this journal carried recommendations for standardized procedures for on line and offline measurements of exhaled NO in adults and children (4).
If treatment of patients with asthma is to be guided by levels of expired NO then it goes without saying that these levels should accurately predict the presence of airway inflammation. In other words, one might reasonably expect the source of NO to be the inflamed airway epithelium and/or the inflammatory cells that infiltrate the airways of patients with asthma. The evidence in support of this common perception, however, is not as strong as one might think. It is heavily based on one study that showed expression of the inducible nitric oxide synthase (NOS2) in the respiratory epithelium of asthmatic patients, whereas the enzyme was not found in nonasthmatic controls (5). This made good sense because cytokines, which are elevated in the airway fluid of asthmatics, stimulate NOS2 activity in respiratory epithelial cells in vitro (6). Moreover, these results seemed in keeping with the increased immunostaining for the NO-related "footprint" nitrotyrosine, whose presence in airway passages of asthmatics was putatively implicated in pathophysiology (7), and the finding that glucocorticoids, which inhibit NOS2 transcription and expression (8, 9), reduced the immunostaining (7). We have come to appreciate, however, that NOS2 is in fact expressed constitutively in normal human airways (10, 11) and that alternative isoforms of NOS may have more important roles in airway hyperresponsiveness (12, 13). That is, neither the presence of NOS2 nor footprints of its activity (6, 10, 11, 14, 15) imply pathophysiology.
If an increase in NOS2 activity in the respiratory epithelium
is not the reason for the elevation in expired NO, what is? In
this issue of the Journal, Hunt and colleagues (pp. 694-699) provide evidence that a drop in the airway lining fluid pH (from 7.65 in control subjects to 5.2 in cases) can account for much if
not all of the increase in expired NO in asthma (16). Expired NO is shown to derive from the reservoir of micromolar nitrite (NO2
), which is present in the airway lining fluid of all subjects, albeit at slightly elevated concentrations in asthmatics.
The biochemical explanation is that nitrite becomes protonated
under acidic conditions to form nitrous acid (HNO2). This
HNO2 then disproportionates to release NO into the airway:
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(1) |
Intriguingly, acidification of nitrite also introduces chemistry that results in tyrosine nitrationa reaction pathway that was shown over a quarter a century ago to promote the formation of nitrotyrosine under physiological conditions (for
instance, the equation below) (17, 18) and one which would
explain the increased immunostaining for nitrotyrosine in airways of asthmatics:
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(2) |
Hunt and coworkers additionally demonstrate that glucocorticoid treatment normalizes the airway fluid pH in asthmatic patients, usually within 48 h (16). Glucocorticoid-mediated decreases in both exhaled NO and nitrotyrosine staining of the epithelium (7) are therefore well rationalized by normalization of airway pH.
A remaining puzzle is why the NO generated by nitrite acidification is not immediately sequestered by the high concentrations of airway lining fluid glutathione (GSH) that act to buffer NO-related activity. Previous studies make it clear that a significant fraction of NO generated by NOS (or for that matter administered by inhalation) reacts oxidatively with glutathione to fortify the micromolar concentrations of S-nitrosoglutathione (GSNO) that are found within the lining fluid of our airways (19, 20). Moreover, in the presence of low pH, an additional GSNO-yielding reaction pathway is available
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(3) |
that should effectively compete with the disproportionation reaction that generates NO (Equation 1). Recent studies have shown, however, that levels of GSNO are actually reduced in the airway lining fluid of asthmatics as compared with normal subjects (21). One attractive explanation that would reconcile this paradoxical set of data is that GSNO is metabolized in the asthmatic airway to NO, whose levels are thereby increased in expired breath.
S-nitrosylation reactions, such as those generating GSNO in the airways, are of particular importance in NO biology as they serve to conserve, rather than consume, NO bioactivity. In particular, GSNO is considered to be a major source of bronchodilator activity in the airway lining fluid (19, 22, 23) and, more generally, may contribute to airway homeostasis through its antimicrobial and anti-inflammatory properties (24, 25). Hunt and colleagues (16) additionally emphasize the anti-apoptotic activity of endogenous S-nitrosothiols (SNOs) (26), which may protect the epithelial lining of the airway from inflammatory or chemical insult. It is important to recognize, however, that it cannot be assumed that the salutary effects of GSNO are shared by NO. On the contrary, NO is a weaker bronchodilator and antimicrobial agent than GSNO, it is present in airways at much lower concentrations, and it is more disposed to toxic reaction pathways in O2 rich enviroments (19, 24, 27). Moreover, even the elevated NO concentrations in asthmatic airways (~ 20 ppb) are several orders of magnitude lower than those shown to clinically improve airway obstruction (28). Taken as a whole, the studies by Gaston's group (3, 16, 21) highlight a metabolic defect in the asthmatic airway, in which increases in expired NO are evidently occurring at the expense of endogenous S-nitrosothiols that function to maintain airway patency. This picture represents a change from the conventional viewpoint on the role of NO in asthma, where increases in NO have been thought of either in terms of a compensatory mechanism that counters airway hyperreactivity or, conversely, as directly contributing to disease pathogenesis (5, 7). The new perspective on free NO as a metabolic byproduct that reflects a depletion in NO-related bioactivity in asthma would be consistent with the modest effects that NOS inhibitors have on asthmatic responses (29, 30).
The key discovery of Hunt and coworkers (16) is not the defect in NO metabolism that is illuminated by their studies, but rather the finding that pH is low in the airway lining fluid. The lung, like most other tissues, does not handle acid well. Remarkably, lowering of airway pH into the asthmatic range produces bronchospasm, impairs ciliary motility, increases mucus viscosity, damages the epithelium and causes eosinophil spillage of broncoconstrictor and proinflammatory substances (16 [and references therein]). That is, airway acidification recapitulates many of the classical manifestations of reactive airway disease. It would be of interest to know whether airway acidification occurs in other obstructive lung diseases.
The study of Hunt and colleagues raises many new questions. For instance, what is the activity that depletes GSNO in asthma? What is the defect that causes the disturbance in acid base homeostasis? How closely linked is acidification to inflammation of the airways? Is the fall in pH a consequence of the inflammatory process or the cause of it? To what degree does induction of NOS2 contribute to expired NO in general and is the cellular origin of the enzyme an important determinant of the expired NO level? And, ultimately, would normalization of airway pH and restoration of GSNO represent novel therapeutic approaches in asthma? Answers to these questions could shape a new understanding of reactive airway disease and have a strong bearing on the management of patients with asthma.
Hunt and colleagues have, at the very least, opened a new
area of research. At this time of fundamental reassessment and
new possibilities, it would seem premature to advocate the
monitoring of expired NO in patientsit remains an appealing
technique, but one whose interpretation is in question. Where
it turns out to be a marker of airway pH, we should probably
measure pH directly in breath condensates. Mechanism-based
approaches to correct the pH of the asthmatic airway may then
be called for. Enter the era of antacid therapy of the lung?
Acknowledgments: Supported by Grants HL52529 and HL59130 from the NHLBI.
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REFERENCES
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1. Kharitonov, S. A., D. Yates, R. A. Robbins, R. Logan-Sinclair, E. A. Shinebourne, and P. J. Barnes. 1994. Increased nitric oxide in exhaled air of asthmatic patients. Lancet 343: 133-135 [Medline].
2. Gaston, B., J. M. Drazen, C. B. Chee, M. E. Wohl, and J. S. Stamler. 1994. Expired nitric oxide (NO) concentrations are elevated in patients with reactive airway disease. In Biology of Nitric Oxide: Clinical and Physiological Implications. Portland Press, London. 497-500.
3. Hunt, J., R. E. Byrns, L. J. Ignarro, and B. Gaston. 1995. Condensed expirate nitrite as a home marker for acute asthma. Lancet 346: 1235-1236 [Medline].
4.
American Thoracic Society.
1999.
Recommendations for standardized
procedures for the online and offline measurement of exhaled lower
respiratory nitric oxide and nasal nitric oxide in adults and children
1999.
Am. J. Respir. Crit. Care Med.
160:
2104-2117
5. Hamid, Q., D. R. Springall, V. Riveros-Moreno, P. Chanez, P. Howarth, A. Redington, J. Bousquet, P. Godard, S. Holgate, and J. M. Polak. 1993. Induction of nitric oxide synthase in asthma. Lancet 342: 1510-1513 [Medline].
6.
Asano, K.,
C. B. Chee,
B. Gaston,
C. M. Lilly,
C. Gerard,
J. M. Drazen, and
J. S. Stamler.
1994.
Constitutive and inducible nitric oxide synthase gene expression, regulation, and activity in human lung epithelial cells.
Proc. Natl. Acad. Sci. U.S.A.
91:
10089-10093
7.
Saleh, D.,
P. Ernst,
S. Lim,
P. J. Barnes, and
A. Giaid.
1998.
Increased
formation of the potent oxidant peroxynitrite in the airways of asthmatic patients is associated with induction of nitric oxide synthase: effect of inhaled glucocorticoid.
FASEB J.
12:
929-937
8.
Geller, D. A.,
A. K. Nussler,
M. Di Silvio,
C. J. Lowenstein,
R. A. Shapiro,
S. C. Wang,
R. L. Simmons, and
T. R. Billiar.
1993.
Cytokines,
endotoxin, and glucocorticoids regulate the expression of inducible nitric oxide synthase in hepatocytes.
Proc. Natl. Acad. Sci. U.S.A.
90:
522-526
9. Kleinert, H., C. Euchenhofer, I. Ihrig-Biedert, and U. Forstermann. 1996. Glucocorticoids inhibit the induction of nitric oxide synthase II by down-regulating cytokine-induced activity of transcription factor nuclear factor-kappa B. Molec. Pharmacol. 49: 15-21 [Abstract].
10. Kobzik, L., D. S. Bredt, C. J. Lowenstein, J. Drazen, B. Gaston, D. Sugarbaker, and J. S. Stamler. 1993. Nitric oxide synthase in human and rat lung: immunocytochemical and histochemical localization. Am. J. Respir. Cell Mol. Biol. 9: 371-377 .
11.
Guo, F. H.,
H. R. De Raeve,
T. W. Rice,
D. J. Stuehr,
F. B. Thunnissen, and
S. C. Erzurum.
1995.
Continuous nitric oxide synthesis by inducible nitric oxide synthase in normal human airway epithelium in vivo.
Proc. Natl. Acad. Sci. U.S.A.
92:
7809-7813
12.
De Sanctis, G. T.,
S. Mehta,
L. Kobzik,
C. Yandava,
A. Jiao,
P. L. Huang, and
J. M. Drazen.
1997.
Contribution of type I NOS to expired gas NO and bronchial responsiveness in mice.
Am. J. Physiol.
273:
L883-L888
13.
De Sanctis, G. T.,
J. A. MacLean,
K. Hamada,
S. Mehta,
J. A. Scott,
A. Jiao,
C. N. Yandava,
L. Kobzik,
W. W. Wolyniec,
A. J. Fabian,
C. S. Venugopal,
H. Grasemann,
P. L. Huang, and
J. M. Drazen.
1999.
Contribution of nitric oxide synthases 1, 2, and 3 to airway hyperresponsiveness and inflammation in a murine model of asthma.
J. Exper.
Med.
189:
1621-1630
14.
Day, B. J.,
M. Patel,
L. Calavetta,
L. Y. Chang, and
J. S. Stamler.
1999.
A
mechanism of paraquat toxicity involving nitric oxide synthase.
Proc.
Natl. Acad. Sci. U.S.A.
96:
12760-12765
15. Eu, J. P., L. Liu, M. Zeng, and J. S. Stamler. 2000. An apoptotic model for nitrosative stress. Biochemistry 39: 1040-1047 [Medline].
16.
Hunt, J. F.,
K. Fang,
R. Malik,
A. Snyder,
N. Malhotra,
T. A. E. Platts-Mills, and
B. Gaston.
2000.
Endogenous airway acidification: implications for asthma pathophysiology.
Am. J. Respir. Crit. Care Med.
161:
694-699
17. Knowles, M. E., J. Gilbert, and D. J. McWeeny. 1974. Nitrosation of phenols in smoked bacon. Nature 249: 672-673 [Medline].
18. Challis, B. C.. 1973. Rapid nitrosation of phenols and its implications for health hazards from dietary nitrites. Nature 244: 466 [Medline].
19.
Gaston, B.,
J. Reilly,
J. M. Drazen,
J. Fackler,
P. Ramdev,
D. Arnelle,
M. E. Mullins,
D. J. Sugarbaker,
C. Chee,
D. J. Singel, and
J. S. Stamler.
1993.
Endogenous nitrogen oxides and bronchodilator S-nitrosothiols in human airways.
Proc. Natl. Acad. Sci. U.S.A.
90:
10957-10961
20.
Simon, D. I.,
M. E. Mullins,
L. Jia,
B. Gaston,
D. J. Singel, and
J. S. Stamler.
1996.
Polynitrosylated proteins: characterization, bioactivity, and
functional consequences.
Proc. Natl. Acad. Sci. U.S.A.
93:
4736-4741
21. Gaston, B., S. Sears, J. Woods, J. Hunt, M. Ponaman, T. McMahon, and J. S. Stamler. 1998. Bronchodilator S-nitrosothiol deficiency in asthmatic respiratory failure. Lancet 351: 1317-1319 [Medline].
22.
Gaston, B.,
J. M. Drazen,
A. Jansen,
D. A. Sugarbaker,
J. Loscalzo,
W. Richards, and
J. S. Stamler.
1994.
Relaxation of human bronchial
smooth muscle by S-nitrosothiols in vitro.
J. Pharmacol. Exp. Therap.
268:
978-984
23.
Bannenberg, G.,
J. Xue,
L. Engman,
I. Cotgreave,
P. Moldeus, and
A. Ryrfeldt.
1995.
Characterization of bronchodilator effects and fate of
S-nitrosothiols in the isolated perfused and ventilated guinea pig lung.
J. Pharmacol. Exp. Therap.
272:
1238-1245
24. Fang, F. C.. 1997. Perspectives series. Host/pathogen interactions: mechanisms of nitric oxide-related antimicrobial activity. J. Clin. Invest. 99: 2818-2825 [Medline].
25.
Grisham, M. B.,
D. Jourd'Heuil, and
D. A. Wink.
1999.
Nitric oxide: I. Physiological chemistry of nitric oxide and its metabolites. Implications in inflammation.
Am. J. Physiol.
276:
G315-G321
26. Liu, L. M., and J. S. Stamler. 1999. NO: an inhibitor of cell death. Cell Death Differen. 6: 937-942 . [Medline]
27.
Lamb, N. J.,
G. J. Quinlan,
S. T. Westerman,
J. M. Gutteridge, and
T. W. Evans.
1999.
Nitration of proteins in bronchoalveolar lavage fluid
from patients with acute respiratory distress syndrome receiving inhaled nitric oxide.
Am. J. Respir. Crit. Care Med.
160:
1031-1034
28. Kacmarek, R. M., R. Ripple, B. A. Cockrill, K. J. Bloch, W. M. Zapol, and D. C. Johnson. 1996. Inhaled nitric oxide: a bronchodilator in mild asthmatics with methacholine-induced bronchospasm. Am. J. Respir. Crit. Care Med. 153: 128-135 [Abstract].
29. Yates, D. H., S. A. Kharitonov, P. S. Thomas, and P. J. Barnes. 1996. Endogenous nitric oxide is decreased in asthmatic patients by an inhibitor of inducible nitric oxide synthase. Am. J. Respir. Crit. Care Med. 154: 247-250 [Abstract].
30.
Taylor, D. A.,
J. L. McGrath,
B. J. O'Connor, and
P. J. Barnes.
1998.
Allergen-induced early and late asthmatic responses are not affected by
inhibition of endogenous nitric oxide.
Am. J. Respir. Crit. Care Med.
158:
99-106
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