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.