LETTER:
To the Editor;
The receptor for advanced glycation end products (RAGE) is a protein
member of the immunoglobulin superfamily. It is constitutively expressed
in the lungs and localized in the basal membrane of type 1 and 2
alveolar epithelial cells. Also, the RAGE has been described in vascular
smooth muscle cells, endothelial cells, and immune cells.
Due to its main expression in the lungs the has been studied in several
respiratory diseasesdescribed as an important inflammatory mediator in
allergic airway inflammation, asthma, pulmonary fibrosis, lung cancer,
chronic obstructive pulmonary disease, acute lung injury, pneumonia,
cystic fibrosis, bronchopulmonary dysplasia, and
COVID-191.
The RAGE-axis is activated through the presence of a wide variety of
endogenous ligands like the advanced glycation end products (AGEs),
S100/calgranulin proteins or high mobility group box 1 protein (HMGB1).
Those RAGE-ligands, expressed in pro-inflammatory states, upregulate the
RAGE expression and induce the upregulation of soluble isoforms. The
soluble RAGE or sRAGE is the predominant RAGE isoform in plasma. It is
derived from membrane-bound RAGE or full-length RAGE (mRAGE or fl-RAGE)
by proteolytic cleavage2. Others soluble RAGE forms
are consequence of RNA alternative splicing, its main example is the
endogenous secretory RAGE (esRAGE)3. The function of
those soluble isoforms is not completely understood, and it has been
hypothesized that they have an anti-inflamatory role as RAGE decoy
receptor. Because of its similarity with mRAGE, the sRAGE may capture
and inactivate circulating ligands. This may prevent RAGE-axis
activation and therefore, stop or downregulate the proinflammatory
status4.
Cystic fibrosis (CF) is the most common life-threatening genetic
condition in Caucasian population. It is associated with bronchial
infection and airway inflammation due to an impaired mucociliary
clearance. Different inflammatory pathways play a critical role in CF
lung disease progression, making it an attractive area of research and
important therapeutic target.
Few studies have explored the RAGE-axis in CF. These studies have been
focused on CF related diabetes (CFRD) and have shown contradictory
results. Mulrennan et al5 assessed RAGE in healthy
controls, CF patients, CFRD patients and diabetics no-CF. They concluded
that sRAGE did not differ significantly among any group. In the other
side, sputum sRAGE level were significantly lower in CF and CFRD
patients compared to healthy and diabetics subjects, despite a markedly
overexpressed of mRAGE mRNA in CF and CFRD sputum. In addition, they
found a significantly positive correlation with sputum sRAGE level and
forced expiratory volume in 1 second percent predicted
(FEV1pp). In contrast, Hunt et al6 did
not find any significant relationships between plasma sRAGE levels and
absolute values of FEV1. But the plasma advanced
glycation end products (AGEs) were significantly elevated in CFRD and
correlate negatively with FEV1. This report found that
CFRD patients had high plasma levels of AGEs and S100A12, but sRAGE was
not significantly different among CF, CFRD and healthy controls.
Based on previously explained, our group present this preliminary study.
The first objective was to describe the plasma sRAGE levels in children
with cystic CF (CFp). Later we compare these values with the sRAGE
levels in a healthy cohort. Finally, we study its possible correlation
with CFp baseline clinical features.
We conducted a single-center, cross-sectional observational study in the
Pediatric CF Unit of a tertiary hospital in Madrid (Spain). The study
was carried out according to the principles of the Declaration of
Helsinki and current legislation and approved by the ethic committee. We
included consecutive clinically stable CF patients (aged < 18
years) since November 2018 to October 2019. Blood tests and clinical
data collection were performed at the same time as the annual review. We
include clinical and demographic data such age, sex, pancreatic status,
CF liver disease (CFLD), diabetes related to CF (CFRD), airway
colonization-infection, pulmonary function test, CFTR mutations, body
mass index, current treatments, respiratory support, history of
hemoptysis, history of allergic bronchopulmonary aspergillosis (ABPA).
Plasma sRAGE level were measured by Human RAGE Quantikine ELISA Kit
(R&D Systems Inc., Minneapolis, MN), using an enzyme-linked polyclonal
antibody specific for human RAGE (extracellular domain). We collected 3
ml of whole blood via venipuncture in EDTA blood tube and spun at 1500
rpm for 10 min. After that, plasma was stored at −80 °C until use with
the commercial kits. Serum sRAGE level was compared to a historical
cohort of healthy controls.
Thirty-five CFp were included, their clinical characteristics are
provided in Table 1. The healthy cohort differs significantly from our
CFp in age, with a median age of 9 and 12,04 years respectively (p =
0.003) (Figure 1). The median plasma sRAGE level in CFp was 1494,75
pg/ml [interquartile range (IQR) 708,75pg/ml], compared with 714,20
pg/ml (IQR) in the historical cohort of healthy controls, being
statistically significantly different (Wilcoxon-Mann-Whitney test, p
< 0,001). Univariate analyzes were performed (Spearman’s Rho)
to correlate clinical characteristics with plasma sRAGE levels. A
positive correlation was found between plasma sRAGE level and the
following spirometry values: forced expiratory volume in 1 second/forced
vital capacity ratio (FEV1/FVC) (0.488, p 0,004) and
forced expiratory flow between 25% and 75%
(FEF25%-75%) (0.38, p 0,032).
In this preliminary study, the plasma sRAGE level were higher in CFp
than in healthy controls. To our knowledge, our study is the largest to
describe plasma sRAGE values in CFp and the only one carried out in
pediatric CF population. Also, in contrast with previous papers, we
described a positive correlation between FEV1/FVC and
FEF25%-75% and plasma sRAGE5,6.
As said, the sRAGE levels were higher in CFp than in healthy controls.
This observation may be due to a proinflammatory status in the
respiratory airway of CFp. The sRAGE cleavaged from lung cells may act
as a decoy receptor for the different RAGE ligands. Related with this
observation we also described a positive correlation between FEV1/FVC
and FEF25%-75% and plasma sRAGE. Both spirometric
values inform about airway obstruction and early lung damage in CF. In
our cohort, those children with higher sRAGE levels showed lower values.
The possible sRAGE protective role in these children should be studied
and confirmed in future studies. Finally, we did not observe correlation
between sRAGE and FEV1. Most our patients have a normal
FEV1 baseline value and this may influence our results
which are similar to the observations done by Hunt et
al6.
Our preliminary study has several limitations. It was cross-sectional
and do not allow to study sRAGE temporal dynamic in the children
included. Related to the comparison with healthy children we observed
that they were younger than our CFp group. We compared the plasma sRAGE
level with an historical cohort, not matched controls. It may affect the
results, but we found a large difference in the absolute value of sRAGE,
hardly explainable only by age. About the RAGE-axis, we didn’t assess
RAGE-ligands or sRAGE levels from the respiratory airway so we cannot
evaluate if sRAGE showed intrinsic respiratory airway upregulation.
In conclusion, our pilot study shown that CFp have higher plasma sRAGE
level than healthy controls. Also, plasma sRAGE level is positively
correlated with spirometry less obstruction. Further multicenter studies
are needed to determine the true role of sRAGE in CFp with a larger
population and more clinical and molecular variables.