Discussion
To our knowledge, this is the first study that establishes the metabolic
signatures and underlying molecular pathways of different inflammatory
asthma phenotypes in induced sputum sample by combing untargeted and
targeted metabolomics. This study demonstrated that the unique metabolic
profiles that existed between different inflammatory asthma phenotypes
and healthy subjects with 77 differential metabolites identified.
Pathway topology analysis uncovered that 5 pathways could be involved in
the
pathogenesis
of different asthma phenotypes. Further, 24 targeted quantification
metabolites were validated significantly differentially expressed
between asthma inflammatory phenotypes, which significantly correlated
to clinical and inflammatory profiles. Finally, adenosine
5’-monophosphate, allantoin and nicotinamide were shown to predict rate
ratios of severe asthma exacerbations. These findings indicated novel
immunometabolic mechanisms in different asthma phenotypes, with
providing more phenotypic and therapeutic implications.
There have been many metabolomics studies on the difference between
asthmatics and healthy subjects or on distinct asthma severity have been
performed extensively10, 26. As for the metabolome
research on infammatory phenotype, some studies have shown that
inflammatory asthma phenotypes can be discriminated by an electronic
nose breath analyzer12. Brinkman et
al27 further demonstrated that inflammatory phenotypes
of severe asthma could be identified by unbiased clustering of exhaled
breath profiles using eNose technology. However, eNose technology seems
to be suitable for the noninvasive identification of asthma inflammatory
phenotypes but is principally unable to identify the individual
metabolic pathway and metabolites driving the distinction between the
subgroups.
This current study unraveled that histidine metabolism,
glycerophospholipid metabolism, nicotinate and nicotinamide metabolism,
linoleic acid metabolism, and phenylalanine, tyrosine and tryptophan
biosynthesis were involved in the pathogenesis of different asthma
phenotypes. In fact, previous studies supported our results as histidine
metabolism was identified as implicated pathway between asthma and
healthy subjects28, and glycerophospholipid metabolism
was also participated in the pathogenesis of asthma, especially
EA29, 30. In addition, linoleic acid metabolism was
the most significant pathway between NA and EA or PGA (all impact value
=1, P < 0.05), which indicated that the important role
of linoleic acid metabolism in the NA. Panda et al. 31had proved that linoleic acid metabolite leads to steroid resistant
asthma features partially through NF-κB. Therefore, it is speculated
that the development of targeted treatment of linoleic acid metabolism
has important implications for the individualized treatment of NA.
Notably, it found that differential metabolites correlated to clinical
and inflammatory profiles. Adenosine-5’-monophosphate was related with
poor asthma control and severe asthma exacerbation. The discovery that
adenosine-5’-monophosphate levels are increased in the bronchoalveolar
lavage fluid of patients with asthma and increase further after allergen
challenge raises the possibility that the adenosine generated in
asthmatic airways itself contributes to the pathogenesis of
asthma32, 33. It is possible that the recruitment and
activation of inflammatory cells and subsequent smooth muscle
contraction that occur during asthma exacerbation leads to an increase
in oxygen and energy demand, thus leading to an increase in the level of
adenosine in the airway in asthma34. We also
demonstrated for the first time that allantoin and nicotinamide were
involved in severe asthma exacerbation. The roles of these metabolites
in asthma pathogenesis and clinical outcomes of asthma remain unknown
and further studies are required to their involvement in asthma.
As for the inflammation markers, histamine was significantly related to
FeNO and IL-5. Histamine has been shown to play a key role in the
pathogenesis of a variety of allergic diseases, including allergic
asthma35. Except for the partial expression of
histamine by immune cells such as basophil36, mast
cells are the main source of histamine expression and express a large
number of different receptors on their cell surface, such as,FcεR1,
FcγRI, complement receptors (C3AR and C5AR), and ligand receptors such
as neurotrophic factor, substance P, vasoactive intestinal peptide, and
adenosine phosphate37-40. These ligand and allergens
can activate mast cells to release the pro-inflammatory mediators
including histamine. Furthermore, histamine has been shown to promote
TH cell differentiation and the release of type 2
inflammatory factors such as IL-5. Consistently, antihistmine has been
demonstrated to significantly reduce the risk of emergency visit and
hospitalization for asthma, especially in asthmatic patients with other
allergic diseases, such as allergic rhinitis 41,
42.Therefore, antihistamine therapy may be considered as an additive
treatment for eosinophilic asthma, as it is often associated with
allergic diseases such as allergic rhinitis, as also found in current
study.
Various amino acids and amino acid dipeptides such as taurine,
alanyl-leucine were significantly correlated to TNF-α, IL-1β, IL-8 and
IL-17, which indicated that these differential metabolites may play a
very important role in the development of NA. For example, Taurine, a
β-amino acid that is not integrated into proteins, is highly expressed
in the intracellular chambers of most tissues. Taurine has been reported
to be increased in bronchoalveolar lavage fluid in asthmatic
patients43, 44. I has been shown shown that plasma
taurine is formed with arachidonic acid in asthmatic
patients45 Animal studies have also shown that
branching-chain amino acids valine, leucine and isoleucine mediated
asthma-related airway inflammation through lipid oxidation
pathways46. Notably, this study also found a number of
amino dipeptides that had not been reported in previous studies, and
their differential expression were closely related to neutrophilic
asthma. Because induced sputum specimens are biological specimens of
open airway, they do not simply reflect the changes of human metabolism,
but also may reflect the changes of local airway microenvironment,
including local microbiota and human host. In fact, recent studies have
shown a significant correlation between neutrophilic asthma and airway
microbiology47.Therefore, this study illustrated
important differences of these amino acid dipeptides may reflect the
metabolic characteristics of airway microorganisms of NA, and the
analysis combined with microbiology may provide us with further
scientific evidence.
This study has several limitations to be addressed. First, an external
validation in a new cohort was lacking. However, the differential
metabolites were validated by exploring the relationships of
differential metabolites with clinical and inflammation profiles of
asthma in an independent cohort population would strengthen the results.
Second, as for the unavailability of reference standards, some of the
identified metabolites in the discovery set are undetected, which
somewhat limits the exploring of the potential metabolic signature.
In conclusion, different inflammatory asthma phenotypes have specific
metabolic profiles with 77 differential metabolic signatures and 5
underlying molecular pathways identified. These metabolic pathways
identified involve the histidine metabolism, glycerophospholipid
metabolism, nicotinate and nicotinamide metabolism, linoleic acid
metabolism, phenylalanine, tyrosine and tryptophan biosynthesis.
Differential metabolites identified correlate to clinical and
inflammatory profiles of asthma, which may serve as potential
therapeutic target in different inflammatory asthma phenotypes.