Discussion
In a study population consisting of 318 children with asthma, lower
levels of 15-HETE were found. ROC analysis of individual parameters
demonstrated similar levels of the sensitivity and specificity between
exhaled 15-HETE and two commonly used parameters in monitoring asthma,
FEV1 and FeNO16. Further, positive
correlations were found between the levels of TXB2 and
those of LTB4 and PGE2 in the exhaled
condensate of asthmatic children. Also, among the asthmatic subjects,
negative correlations were found for TXB2 and
FEV1, and for 15-HETE and LTB4. Among
those parameters analyzed, reduced levels of TXB2, but
increased levels of 15-HETE, were noted after 3 days of oral
prednisolone treatment, concomitant with the improvement of lung
function in asthmatic children. When the asthmatic population was
stratified into different severity groups, it was noted that the ratio
of 15-HETE/LTB4 was significantly lower in subjects with
severe asthma. Furthermore, when we investigated changes in the levels
of 15-HETE and TXB2 during exacerbation and
convalescence in subjects according to the top 30%, middle 40%, and
bottom 30% (as determined at the exacerbation levels), it was found
that higher initial exacerbation would have responded well to prednisone
treatment. These results, collectively, suggest their potential utility
as a new set of lipid markers for monitoring asthma and its therapeutic
outcome.
The family of eicosanoids is the most prevalent lipid mediators which
contribute to inflammation providing both pro-inflammatory signals and
terminating the inflammatory process. Eicosanoid profiling in the
exhaled breath condensate is complementary to the cellular phenotyping
of asthmatic inflammation17. Our findings revealed
that the levels of 15-HETE were significantly reduced in the EBCs of
asthmatic subjects as compared to that of healthy controls, but was
increased after treatment. Kowal et al also reported that the mean
concentration of 15-HETE in asthma patients was significantly lower than
in healthy subjects15. Song et al demonstrated that
15-HETE regulated MUC5AC expression via modulating MMP-9, MEK/ERK/Sp-1,
and PPARγ/PTEN/ Akt signaling pathways in PMA-treated respiratory
epithelial cells18. While these results appear to be
contradictory to those suggesting that high 12/15-LOX activity and
15-HETE levels are mainly indicative of pro-inflammatory responses in
asthma 19,20. However, 15-LOX-1 preferentially
metabolizes linoleic acid to 13-hydroperoxyoctadecadienoic
acid21. Moreover, 15-LOX1 has been reported to be less
efficient than 15-LOX-2 in the production of
15-HETE22. Therefore, 15-HETE has been reported to be
synthesized mainly by 15-LOX-2 rather than 15-LOX-123.
In addition, further study will be required for the identification of
the role of 15-LOX-2 in airway inflammation. Besides the
anti-inflammatory effects, 15-HETE has been shown to be an endogenous
ligand for PPARγ (peroxisome proliferator-activated receptor gamma),
which has anti-inflammatory effects such as regulating inflammatory
cytokines24, neutrophil migration and mucin
secretion18 underlying many airway
diseases25,26. For instance, the PPARγ agonist
rosiglitazone has been shown to display bronchodilator effects in a
group of patients with glucocorticoids-resistant
asthma27. The reduction of 15-HETE may, therefore,
suggest its close relationship with asthma and warrant further
investigation.
Moreover, as 15-HETE may exert their anti-inflammatory effect through
inhibiting 5-LOX–derived pro-inflammatory
leukotrienes28,29, we also calculated the ratio of
exhaled 15-HETE:LTB4 and found significantly lower in
subjects with severe asthma. The mean 15-HETE:LTB4 ratio
was 79% lower in patients with severe asthma when compared with that in
patients with moderate asthma (P <0.01). These findings
regarding 15-HETE in EBC support data suggesting that 15-HETE
biosynthetic capacity might be defective in patients with severe asthma
and thus contribute to the perpetuation of airway inflammation in these
patients. An additional key message derived from this study is the
finding that the level of exhaled TXB2 was significantly
reduced during convalescence. TXA2 is a lipid mediator
and a bronchoconstrictor contributing to the pathophysiology of
asthma7, while TXB2 is a stable
metabolite of TXA2. The reduction of
TXB2 levels might be indicative of steroid’s effect and
a marker responsive to the intervention, concomitant with improvement of
lung function.
While, consistent with a previous report30, but not
the others13,31-33, we did not find difference in the
level of exhaled TXB2 (and its metabolite,
11-dihydro-TXB2; data not shown) between asthmatic and
healthy children, but the level of TXB2 showed
significant reduction after 3 days of oral prednisolone treatment.
Further,
Dworski
et al. found that prednisone was able to reduce the synthesis of
eicosanoids, including TXB2 level, in macrophage-rich
BAL-fluid cells from 14 atopic asthmatic volunteers at baseline and
after allergen instillation34. It is also worth noting
that in double-blind, placebo-controlled trials, the thromboxane
receptor antagonist, seratrodast, and the thromboxane synthase
inhibitor, ozagrel, were proven efficacious in the treatment of patients
with asthma35. However, the effect of
TXA2 inhibitors in asthma has not been widely used
because no statistically significant difference was observed, but it has
been suggested that it might be a good disease marker of asthma only in
a certain ethnic group36. One explanation for these
conflicting results could be phenotypically different in the study
population. Nevertheless, while the level of TXB2 may be
dependent on the stage of asthma and its severity, the reduction in
TXB2 after therapy appears to be consistent. Further
independent studies are needed to confirm these results. The finding
that the level of exhaled TXB2 was significantly reduced
during convalescence is significant in and of itself, providing a basis
for further exploring its clinical utility in monitoring the therapeutic
outcome in place of FeNO.
Furthermore, it is worth noting that LTB4,
LTE4, PGE2 and LXA4 also
showed reduction in patients with the respective levels at the 30%
percentile, and, in fact, only in those who had higher levels of exhaled
eicosanoids. This could be related to the stages of asthma progression
during exacerbation, and to the phenotypic heterogeneity of asthma in
the study population in terms of its etiology and pathogenic mechanism.
Further investigation into this possibility is clearly required. In
conclusion, these results provided insight into the measurements of
exhaled eicosanoid profiles in our study population, and showed that
there was a significant difference between the levels of
TXB2 and 15-HETE during acute asthma exacerbation and
convalescence. Additional prospective studies are necessary to evaluate
the utility of the proposed discriminator in the context of the
diagnosis and monitoring of childhood asthma.