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 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
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 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. Also,
high 12/15-LOX activity and 15-HETE levels have been suggested to be
indicative of pro-inflammatory responses in asthma19,20. 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
cytokines21,22, neutrophil migration and mucin
secretion18. For instance, the PPARγ agonist
rosiglitazone has been shown to display bronchodilator effects in a
group of patients with glucocorticoids-resistant
asthma23. 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
leukotrienes24, 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
suggest 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. Moreover, TXA2 is
a lipid mediator and a bronchoconstrictor contributing to the
pathophysiology of asthma7, while TXB2is a stable metabolite of TXA2. The reduction of
TXB2 levels might be indicative of steroid’s effect and
a marker responsive to the intervention.
While, consistent with a previous report25, but not
the others13,26,27, 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 instillation28. 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 asthma29. 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 group30. 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, 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 diagnosis and monitoring of childhood asthma.
Acknowledgments: This work was supported by Chang Gung Memorial
Hospital (CMRPG3G2051, CMRPVVK0161)., the Ministry of Science and
Technology, Taiwan (MOST 108-2314-B-182A-088) and, in part, by a grant
from National Health Research Institutes, Taiwan (EO-109-PP-10).
Impact statement: The exhaled 15-HETE was found to discriminate
childhood asthma while decreased levels of exhaled TXB2and increased levels of 15-HETE were prominent after treatment.
Conflict of interest : The authors declare that they have no
conflict of interests