1. Introduction
Chronic rhinosinusitis (CRS) with nasal polyps (CRSwNP) is a complex inflammatory disorder characterized by diverse clinical presentations and underlying inflammatory profiles. It can be categorized into eosinophilic CRS (ECRS) and non-ECRS based on the extent of eosinophilic infiltration in sinonasal tissue [
1]. Pathological association between the inflammatory endotype of type 1, 2, and 3 and phenotypes of CRSwNP are deeply associated with the pathophysiology of CRSwNP [
2]. ECRS, a subtype characterized by Th2-dominant inflammation, often results in less favorable surgical outcomes [
1,
3], and cases of ECRS with aspirin exacerbated respiratory disease (AERD) mostly show more severe surgical outcome due to the severe type 2 inflammation [
4].
The precise mechanism behind the formation and maintenance of nasal polyps in CRSwNP remains unclear, but there's strong evidence suggesting an association with type 2 inflammation. The pathophysiology of ECRS, especially in ECRS with AERD, involves the dysregulation in the synthesis of glycerophospholipid mediators including leukotrienes, prostaglandin, thromboxane, and
PAF [
5]. In the context of type 2 inflammatory reactions, IgE-mediated hypersensitivity triggers immune cells to process allergens, prompting a B-cell mediated immune response. This leads to the cross-linking of IgE antibodies on inflammatory cells, initiating mast cell degranulation and the release of various mediators like
PAF, histamine, and pro-inflammatory cytokines [
6,
7]. These mediators induce both early and late-phase allergic responses, resulting in cellular inflammation by recruiting eosinophils, basophils, monocytes, and lymphocytes.
PAF, a phospholipid-derived inflammatory mediator released by various cell types [
8], plays a pivotal role in multiple inflammatory states, recruiting eosinophils and neutrophils, triggering leukocyte degranulation and adhesion, and generating free radicals like superoxide and hydroxyl anions in the nasal mucosa [
9]. A previous report mentioned that a large amount of
PAF was contained in nasal polyps from the patients of CRSwNP with AERD and it correlated with tissue eosinophilia [
10]. Moreover,
PAF contributes to mast cell activation, often linked to severe anaphylactic responses [
9]. Its role in inducing vascular permeability leads to nasal hyperreactivity and congestion [
11,
12,
13].
PAF is tightly regulated and produced by two distinct pathways 4 with 1) de novo pathway, which constitutively produce low levels of
PAF and 2) remodeling pathway, which rapidly synthesize the majority of
PAF removing fatty acids (typically arachidonic acid) from the sn-2 position of membrane phospholipid phosphatidylcholine (
PC), which results in
lyso-PAF, following to convert
lyso-PAF into
PAF by
LPCAT1 and
LPCAT2 [
14,
15]. The reverse reaction from
lyso-PAF to
PC is catalyzed by
LPCAT1–4 acyltransferase activity [
16].
PAF is synthesized by a variety of cells including endothelial cells, platelets, macrophages, monocytes, neutrophils, and eosinophils [
17], and shows its biological effects at high-picomolar concentrations, such as the typical serum concentration of
PAF of 400 pg/ml in healthy individuals [
18].
Produced
PAF is rapidly degraded to
lyso-PAF by
PAF acetylhydrolases (
PAFAHs), a family of Ca2+- independent lipoprotein-associated phospholipase A2. Half-life of
PAF ranges from 3–13 min because
PAFAHs regulate
PAF activity by hydrolyzing
PAF to
lyso-PAF as a biologically inactive form [
17].
PAFAHs have three isoforms and one isoform is a secreted form (Phospholipase A2 Group VII(
PLA2G7)) and two isoforms are intracellular forms (
PAFAH Ib and
PAFAH II) [
19].
PLA2G7 and
PAFAH II can non-specifically hydrolyze oxidatively fragmented phospholipids with potent biological activities, while
PAF-
AH Ib displays high specificity for the sn-2 acetyl group of glycerophosphocholine (
GPC) including
PAF [
20]. On the other hand, there was no report about the association between allergic conditions and
PLA2G7 activity, although increased
PLA2G7 activity was reported in various pathologic conditions, including ischemic stroke, myocardial infarction, familial HDL deficiency, chronic cholestasis, diabetes mellitus, rheumatoid arthritis, essential hypertension, and peripheral vascular disease [
20]. Therefore, currently
PLA2G7 is thought to be separated from the
PAF-metabolism associated genes in allergic conditions.
PAF effects are mediated by binding to the
PAF receptor (
PTAFR), leading to mobilization of intracellular calcium and activation of kinases. Activation of
PTAFR leads to the activation of cytoplasmic
PLA2 with subsequent formation of leukotrienes, prostaglandins, and thromboxane after the cleavage of arachidonic acid [
19].
PTAFR is expressed in T lymphocytes, monocytes and macrophages, platelets, tracheal epithelium, vascular endothelium, lung alveolar wall, liver, small intestine, heart, skeletal and smooth muscle, brain microgalia and neurons, myometrium, spleen, and kidney [
21,
22].
PTAFR expression is regulated by intracellular cyclic AMP, which can downregulate
PTAFR gene expression and reduce
PAF-induced arachidonic acid release [
23,
24].
Stimulation of the
PTAFR induces production of nitric oxide [
25], histamine release from basophils, activation and degranulation of mast cells, chemotaxis of mast cells and eosinophils, recruitment of neutrophils, production of
IL-4 by B lymphocytes, bronchial smooth muscle contraction, and mucus secretion as well as increase vascular permeability [
19]. The role of
PAF in the pathogenesis of allergic rhinitis was currently demonstrated [
26], but the role of
PAF in the pathogenesis of CRSwNP was underestimated. The intricate relationship between
PAF and the pathophysiology of CRSwNP, which include ECRS with AERD often showing anaphylactic reaction after administration of NSAIDS, suggests a potential link between
PAF metabolism and its pathophysiology including the formation and maintenance of nasal polyps derived from vascular permeability associated with plasma extravasation [
11,
12,
13,
27].
Current classification methods like the JESREC criteria attempt to stratify ECRS based on clinical findings and eosinophil count [
1], transcriptome analyses have revealed that the expression level of type 2 inflammatory cytokine and chemokine do not align with these clinical classifications [
28]. These analyses have unveiled significant differences between the clinical classification and the severity of type 2 inflammation. Therefore, this study aims to explore and elucidate the complexities of
PAF metabolism-associated gene expressions of key enzymes involved in
PAF synthesis (
LPCAT1, LPCAT2, LPCAT3, LPCAT4) and degradation (
PAFAH1B2, PAFAH1B3, PAFAH2), along with
PAF receptor (
PTAFR), across subtypes classified as nonECRS and ECRS including ECRS with AERD(Asp) by clinical classification and hierarchal analysis-based classification. Understanding the significance of
PAF in orchestrating eosinophil recruitment and its association with type 2 inflammation sets the stage for investigating how variations in
PAF metabolism-associated gene expressions align with these subtypes. Identifying disparities in
PAF metabolism-associated gene expressions could be crucial in comprehending the heterogeneity of CRSwNP and potentially guiding tailored therapeutic interventions. This investigation seeks to bridge the gap between transcriptomic variations, inflammatory phenotypes, and
PAF-related molecular signatures, aiming to illuminate the intricate interplay between
PAF metabolism and CRSwNP subtypes and/or cluster-based classification. Ultimately, this study aims to unravel the diverse landscape of
PAF-associated gene expressions across distinct CRSwNP subtypes and/or cluster-based classification providing deeper insights into the molecular foundations contributing to the varying inflammatory profiles observed in this condition.
3. Discussion
PAF-metabolism associated gene expression using transcriptome analysis revealed that clinical classification based on the JESREC study showed the significant upregulation of
PTAFR gene expression in nonECRS and ECRS compared to Ctrl, but the other genes did not show any difference among nonECRS, ECRS and Ctrl. Additionally, there was no difference between nonECRS and ECRS in
PTAFR gene expression. These results showing significant upregulation in nonECRS and ECRS compared to Ctrl and no difference between nonECRS and ECRS in
PTAFR gene expression were also previously demonstrated [
27] and these results suggested that clinical classification based on the JESREC criteria or the status with/without comorbid asthma and/or AERD may not detect the difference of
PAF-metabolism in each clinical classification because PCA analysis with nonECRS, ECRS, and Asp could not segregate each other. On the other hand, upregulation of
PTAFR gene expression in nasal polyp from both nonECRS and ECRS suggests that
PTAFR upregulation enhancing the possibility of
PAF-signaling upregulation associate with the formation or maintenance of nasal polyp.
However, two clusters from hierarchical analysis could clearly segregate from each other in PCA analysis and showed the clear difference in variety of cytokines expression in KEGG pathway analysis suggesting the reflection of the severity of type 2 inflammation. In the cluster analysis of PAF-associated gene expression, cluster 2 showed significant upregulation of LPCAT1, PAFAHB2, and PTAFR, and significant downregulation of PAFAH2 compared to Ctrl. In contrast, cluster 1 showed significant upregulation of PTAFR and significant downregulation of LPCAT2. Compared between cluster 1 and 2, cluster 2 was significantly upregulated in LPCAT1 and LPCAT2 expression.
LPCAT1 and
LPCAT2 are main enzymes of
PAF synthesis from
lyso-PAF [
14,
15], and
PAFAH2 is main degrading enzymes of
PAF by hydrolyzing
PAF to
lyso-PAF as a biologically inactive form [
19]. Although
PAFAH1B2 and
PAFAH1B3 are capable of hydrolyzing
PAF, previous report demonstrated that RNAi-mediated knockdown of
PAFAH1B2 or
PAFAH1B3 does not alter
PAF levels or
PAF hydrolytic activity, indicating that these enzymes may possess alternate endogenous substrates [
29]. These results suggest that cluster2, which can reflect the severity of type 2 inflammation, have high
PAF-associated pathophysiology with upregulation of
PAF synthesis and downregulation of
PAF degrading leading to the local
PAF accumulation and intensify the effects of
PAF signaling via upregulation of
PTAFR.
The result showed for the first time that severe type 2 inflammation induce the high
PAF-associated pathophysiology and the main cause of the high
PAF-associated pathophysiology assumingly comes from high
lyso-PAF and
PAF synthesis via overexpressed
LPCAT1 and
LPCAT2 compared to the cluster of moderate to low type 2 inflammatory type of CRSwNP (cluster 1). On the other hand, our result did not show any correlation between each gene expressions associating
PAF metabolism and blood/tissue eosinophil count or the clinical classification. Only the oral FeNO and total nasal FeNO showed the relationship between
LPCAT2, LPCAT3, and
PTAFR, but could not exhibit the above differences of
PAF-metabolism associated gene expression, especially in
LPCAT1 and
LPCAT2, shown in cluster analysis. The difference may come from the discrepancy between the severity of type 2 inflammation and blood/tissue eosinophilia, both FeNO level, or clinical classification because blood/tissue eosinophil count is used for the biomarker of eosinophilia [
30] and FeNO is for airway inflammation or irritation [
31] and clinical classification is for the phenotype diagnosis [
1]. Although oral FeNO and total nasal FeNO reflect the severity of allergic inflammation [
31], our result revealed that both FeNO can reflect the severity of allergic inflammation but not the severity of type 2 inflammation. Direct biomarker for the severity of type 2 inflammation, which have been underdeveloped yet, should be made for the further analysis of pathophysiology in CRSwNP.
Previous report mentioned that difference of
lyso-PAF concentration between normal nasal polyp and nasal polyp having AERD suggested the association between airway inflammation and/or disease severity and
lyso-PAF concentration [
27]. Other paper also mentioned that human nasal polyp with severe eosinophil infiltration showed high
PAF concentration [
10]. Our result also clearly showed that the group of nasal polyps with severe type 2 inflammation had the increased
PAF-metabolism, which can play an important role in the pathogenesis of CRSwNP inducing increased tissue neutrophilia and eosinophilia in nasal mucosa as a function of
PAF signaling [
19]. Therefore, these results may enhance that
PAF-metabolism associated gene expression, especially in combination of
LPCAT1 and
LPCAT2 gene expression, can become a biomarker for the severity of type 2 inflammation in CRSwNP.
In summary, this study demonstrates the upregulation of PTAFR gene expression in nasal polyp and severe type 2 inflammation leads to increase high PAF-metabolism in nasal polyp, which may induce the proliferation and maintenance of nasal polyp. However, due to the classification using hierarchical analysis, small sample size, and only from Japanese patients, further research is needed to confirm our findings and the association between type 2 inflammation and the production of PAF-metabolism associated protein including PAF and/or lyso-PAF to reach the development of anti-PAF therapies for CRSwNP especially with severe type 2 inflammation. Additionally, the research about LPCAT1 and LPCAT2 gene expression in CRSwNP should be performed to confirm whether these gene expression become a good biomarker for the severity of type 2 inflammation or not.