ORMDL3 and allergic asthma: From physiology to pathology

Abstract

There is a strong genetic component to asthma, and numerous genome-wide association studies have identified ORM1 (yeast)–like protein 3 (ORMDL3) as a gene associated with asthma susceptibility. However, how ORMDL3 contributes to asthma pathogenesis and its physiologic functions is not well understood and a matter of great debate. This rostrum describes recent advances and new insights in understanding of the multifaceted functions of ORMDL3 in patients with allergic asthma. We also suggest a potential unifying paradigm and discuss molecular mechanisms for the pathologic functions of ORMDL3 in asthma related to its evolutionarily conserved role in regulation of sphingolipid homeostasis. Finally, we briefly survey the utility of sphingolipid metabolites as potential biomarkers for allergic asthma.

Asthma, a syndrome with substantial heterogeneity, is a chronic airway inflammatory disease in which exposure to allergens causes intermittent attacks of breathlessness, airway hyperreactivity, wheezing, and coughing. It is one of the most prevalent diseases, and in developed countries especially, asthma has become a growing concern. Despite its prevalence, this respiratory disease still has no cure. The most effective treatments are corticosteroids to reduce inflammation and β₂-adrenergic agonists that open constricted bronchial smooth muscles. For more severe cases, anti-IgE, anti–IL-5, and anti–IL-4 antibodies are used but might not help all asthmatic patients. What makes asthma difficult to treat is its heterogeneous nature.

It is well known that a significant genetic component modulates susceptibility to asthma. Large-scale molecular and genetic studies applied to clinically characterized asthmatic patients have enhanced understanding of asthma phenotypes, potentially leading to more targeted and personalized asthma therapies. Despite extensive efforts, genome-wide association studies have not identified genetic origins of asthma. Intriguingly, however, multiple genome-wide association studies have convincingly and repeatedly identified association of the 17q21 locus and, within it, the ORM1 (yeast)–like protein 3 (ORMDL3), gasdermin B (GSDMB), and zona pellucida binding protein 2 (ZPBP2) genes.

Of the above genes, ORMDL3 has received the greatest attention for its potential functional link with asthma.^1,2 The initial strong association between single nucleotide polymorphisms (SNPs) in the ORMDL3 gene and childhood-onset asthma¹ has been subsequently extended in many epidemiologic studies of diverse ethnicities. Moreover, ORMDL3 SNPs associated with asthma are all located in noncoding regions, and several appear to be linked to increased transcription of ORMDL3.^1–3 For example, 17q21 asthma risk variants switch binding of the transcriptional regulator insulator protein CCCTC binding factor (CTCF) to the ORMDL3 promoter region in CD4 T cells⁴ or cause changes in binding of upstream stimulatory transcription factors,² and both could contribute to increased ORMDL3 expression. A change in CpG island methylation has also been reported.⁵ However, how ORMDL3 contributes to asthma pathogenesis and its functions in the lung are not well understood and are a matter of great debate.^6–8 Further work is needed to understand the mechanisms that drive this early susceptibility to asthma and to determine the relationship between ORMDL3 function and asthma exacerbation.

ORMDL FAMILY

ORMDL3 is one member of a family of 3 endoplasmic reticulum (ER)–localized proteins that are highly homologous with the yeast orthologs ORM1 and ORM2.⁹ In addition to ORMDL3, it has been suggested that SNPs in ORMDL1 and ORMDL2 are also associated with childhood asthma.¹⁰ Extensive studies in yeast have elegantly demonstrated that both ORMs negatively regulate sphingolipid synthesis by decreasing the activity of serine palmitoyltransferase (SPT), the rate-limiting enzyme in the de novo biosynthesis of sphingolipids (Fig 1, A).⁹ Sphingolipid synthesis within the ER begins with condensation of serine and palmitoyl-CoA catalyzed by SPT to produce 3-keto-dihydrosphingosine. This is reduced to the long-chain sphingoid base dihydrosphingosine.

FIG 1.

Dual roles of ORMDL3 in sphingolipid metabolism. The de novo sphingolipid biosynthesis pathway in the ER begins with condensation of serine and palmitoyl-CoA by SPT to form 3-keto-dihydrosphingosine (3-KdHSph). A, Physiologically, ORMDL3 is a negative regulator of SPT. 3-KdHSph is reduced to dihydrosphingosine (dHSph), N-acylated to dihydroceramide (dHCer), and then desaturated into the central sphingolipid metabolite ceramide (Cer). Ceramide is trafficked to the Golgi, where it is metabolized into complex sphingolipids, including sphingomyelin (SM) and glycosphingolipids (GSL), and transferred from there to the plasma membrane (PM). Complex sphingolipids are recycled in the lysosome, where they are degraded first to ceramides and then to sphingosine (Sph). Sphingosine can be phosphorylated into S1P, which is irreversibly degraded in the ER, leading ultimately to formation of phosphatidylethanolamine (PE). B, Under pathologic conditions, upregulation of ORMDL3 enhances ceramide formation through the recycling/salvage pathway. See abbreviations listed in Fig 1, A.

Ceramide synthases (CerSs), of which there are 6 with different acyl chain specificities, N-acylate dihydrosphingosine to dihydroceramides, which are then desaturated to ceramides. Ceramides are precursors of various complex sphingolipids, including sphingomyelins and glycosphingolipids (Fig 1, A). On the other hand, degradation of complex sphingolipids in the lysosome produces ceramides, which are hydrolyzed by ceramidase to sphingosine that can then be phosphorylated by sphingosine kinase to form the bioactive sphingolipid metabolite sphingosine-1-phosphate (S1P). S1P is irreversibly cleaved by S1P lyase to hexadecenal and ethanolamine phosphate, which feed into phosphatidylethanolamine synthesis or are dephosphorylated back to sphingosine by S1P-specific ER phosphatases for reuse.

ORMDLs IN REGULATION OF EUKARYOTIC SPHINGOLIPID HOMEOSTASIS

Similar to their yeast homologs, mammalian ORMDLs are ER transmembrane proteins consisting of 3 highly homologous 17-kDa proteins, ORMDL1 to ORMDL3, which also bind to SPT and inhibit its activity and de novo sphingolipid biosynthesis (Fig 1, A).^9–11 This conclusion was reached mainly based on observations that downregulation of all 3 ORMDLs in cultured cells increased dihydroceramide and ceramide levels.^9,12–14 However, there are some inconsistencies between different studies regarding the roles of individual ORMDLs.^12–14

In HepG2 liver cells pulse labeled with [¹³C]palmitate, downregulation of ORMDL3 increased the ceramide precursors dihydrosphingosine and dihydroceramide primarily from de novo biosynthesis based on [U-¹³C]palmitate incorporation into base-labeled and dual-labeled dihydroceramides, whereas downregulation of each ORMDL alone increased levels of dihydroceramides that were [¹³C]labeled only in the amide-linked fatty acid.¹⁴ These results indicate that ORMDL1 or ORMDL2, in contrast to ORMDL3, might not regulate bulk sphingolipid levels but could play a role in regulation of specific pools of dihydroceramides, recycling, or both. In contrast, it was suggested that all 3 ORMDLs in HEK 293 cells must be downregulated to relieve the inhibition of SPT.¹² Therefore it is possible that each ORMDL has distinct functions in different cell types or that their relative expression levels are different in different cell types. However, in contrast to the overwhelming data supporting a role for ORMDL3 in regulation of de novo ceramide biosynthesis, it was reported that ceramide levels were not significantly altered in Ormdl3 knockout mice and that SPT activity was not changed in cells in which Ormdl3 was deleted or overexpressed.¹⁵ Although the explanation for this discrepancy is not clear, it was speculated that decreasing one member of this highly homologous protein family is insufficient to achieve maximum inhibition of SPT activity because of overlapping biological roles.¹⁵ Alternatively, it might be due to the less sensitive methods used by this group to measure sphingolipid level. In this regard we advocate the use of highly sensitive and precise liquid chromatography–electrospray ionization–tandem mass spectrometry methods that have been developed in the last few years that more accurately measure small changes in sphingolipid levels, as well as stable isotope pulse-labeling methods for measurements of de novo sphingolipid biosynthesis.^13,14

A clearer picture is now also emerging from studies of overexpression of ORMDLs in vitro.^11,13,16 Although transfection with each of the 3 human ORMDLs reduced the increase in long-chain base synthesis in cells overexpressing SPT,¹⁶ others found that overexpression of ORMDL3 did not affect SPT activity, but that augmentation of all 3 ORMDLs effectively decreased ceramide production.¹² An explanation for this difference was furnished by an elegant study showing that complexes of ORMDLs normally interact with and regulate SPT in a stoichiometric manner that determines effects of ORMDL3 expression on sphingolipid biosynthesis.¹¹ Intriguingly, inhibition of ceramide biosynthesis by short-chain ceramides or by exogenous sphingosine was mitigated by ORMDL knockdown, supporting the notion that ORMDLs mediate feedback regulation of ceramide biosynthesis in mammalian cells. Based on binding of ceramides to the membrane-bound components of the SPT regulatory apparatus, a novel mechanism was proposed whereby ceramides directly bind to the SPT/ORMDL complex to induce a conformational change that inhibits SPT activity.¹⁷

Consistent with the notion that the stoichiometry of ORMDL3 expression is crucial for its function, we found that although modest overexpression of ORMDL3 decreased de novo ceramide biosynthesis as expected, high expression of ORMDL3 enhanced rather than reduced ceramide levels.¹³ Our stable isotope-labeling studies further indicated that this was mainly due to increased salvage/recycling of sphingolipids to ceramides rather than de novo biosynthesis (Fig 1, B).¹³ Consistent with this observation, in yeast ORM1 interacts with the CerS homolog Lac1, leading to increased rather than decreased ceramide biosynthesis.¹⁸ Likewise, activation of yeast ORM1/2 stimulates de novo synthesis of complex sphingolipids downstream of SPT.¹⁹ Thus several reports support the notion that in physiologic conditions ORMDLs suppress SPT activity and thus de novo biosynthesis of sphingolipids (Fig 1, A). However, we suggest that in pathologic conditions, such as asthma, there are much greater levels of ORMDL3, causing increased ceramide levels mainly by enhancing the salvage/recycling pathway (Fig 1. B). Future studies are needed to determine the mechanism by which greater levels of ORMDLs affect CerS activity, the recycling/salvage pathway, or both. See more discussion below on the ORMDL3/ceramide axis in patients with allergic asthma.

REGULATION OF ORMDLs

Inhibition of SPT by yeast ORMs is controlled by their phosphorylation by the serine/threonine-protein kinase YPK1, which plays an important role in protecting cells from stress.¹⁹ ORM1/2 phosphorylation disrupts their interactions with SPT, relieving inhibition of de novo sphingolipid synthesis and leading to increased formation of ceramides and complex sphingolipids.⁹ Mammalian ORMDLs all have truncated N-termini and lack the residues that are phosphorylated in yeast ORM1/2 that control their interactions with SPT.^9,16 This opens the possibility that other types of interactions might be involved in formation and disassembly of the ORMDL/SPT complex.

As mentioned above, ORMDL3 SNPs associated with asthma are located in noncoding regions and can enhance transcription of ORMDL3, which correlates with changes in T_H2 cytokines levels.² Similarly, ORMDL3 expression was greatly increased during T_H2-mediated allergic asthma in mouse models that recapitulate the cardinal features of human allergic asthma.^7,8,13

Studies with signal transducer and activator of transcription 6 (STAT6) knockout mice demonstrated that Ormdl3 mRNA induction by allergen challenge was highly dependent on STAT6 expression.⁷ Although it has been speculated that STAT6 directly binds to the Ormdl3 promoter, it was suggested that the promoter of human ORMDL3 does not have STAT6 binding sequences. Therefore it was proposed that resident lung cells produce STAT6-dependent T_H2 cytokines or chemokines in response to allergens that in turn regulate ORMDL3 expression.⁷ It is also conceivable that ORMDL3 gene transcription is cooperatively regulated by multiple transcription factors.

However, in addition to effects on transcription, several studies have shown that ORMDL levels are also regulated posttranscriptionally.^14,16,20 For example, ORMDLs are degraded during autophagy induced by cholesterol loading.²⁰ Furthermore, low doses of staurosporine, which have no effects on protein kinase C, decreased levels of ORMDLs and increased sphingomyelin synthesis, resulting in depletion of plasmalemmal phosphatidylserine.²¹ In addition, ORMDL proteins, but not their mRNA levels, are decreased by the inflammatory cytokines IL-6 and oncostatin M concomitantly with increased de novo ceramide levels.¹⁴ Moreover, during irritant-induced sterile inflammation in mice leading to induction of the acute-phase response, which is dependent on IL-1, ORMDL expression in the liver was greatly reduced and accompanied by increased ceramide levels in the liver and blood.¹⁴

These observations support the notion that physiologically, ORMDL3 is a negative regulator of ceramide de novo biosynthesis and suggest that ORMDLs are involved in regulation of ceramides during sterile inflammation (Fig 1, A). Further studies are needed to understand the molecular mechanisms by which ORMDL levels are regulated by inflammation in general and in patients with allergic asthma in particular. It is also important to elucidate how changes in ORMDL levels contribute to regulation of ceramide and sphingolipid homeostasis and their physiologic and pathologic functions.

ROLE OF ORMDL3 IN REGULATION OF THE UNFOLDED PROTEIN RESPONSE AND ER STRESS

Despite extensive efforts, the molecular mechanisms by which increased ORMDL3 expression regulates allergic asthma are still unclear. Because of its ER localization, several mechanisms have been proposed that are focused especially on ER stress and the unfolded protein response (UPR), an evolutionarily conserved adaptive response triggered by accumulation of unfolded proteins in the ER to restore its normal function. UPR and ER stress have been shown to be involved in asthma pathogenesis. The UPR has 3 major signaling branches: activating transcription factor 6 (ATF6); protein kinase R like endoplasmic reticulum kinase (PERK) and its substrate, eukaryotic translation initiation factor 2α (eIF2α); and inositol-requiring enzyme 1 (IRE1), the endonuclease activity of which alternatively splices X-box binding protein 1 (XBP1), a potent transcription factor. Interest in these was raised by the demonstration that overexpressed ORMDL3 binds and inhibits the sarcoendoplasmic reticulum calcium ATPase (SERCA) 2b, a downstream target of the ATF6 pathway, resulting in reduction of ER–mediated calcium release and activation of the PERK/eIF2α arm of the UPR.²² It was subsequently suggested that ORMDL3 inhibits mitochondrial calcium influx independently of SERCA, leading to inhibition of store-operated calcium entry and consequently affecting nuclear factor of activated T-cell nuclear translocation and IL-2 production, key steps in T-lymphocyte activation (Fig 2).⁶ However, other studies have focused on the role of ORMDL3 in regulation of ATF6, which activates target genes for the UPR during ER stress, particularly in lung epithelial cells, which are the first line of defense against irritants and pathogens, initiate airway inflammation, and produce mucus. Overexpression of ORMDL3 in human epithelial cells increased ATF6α levels, with subsequent upregulation of SERCA2b, which has been implicated in airway remodeling and enhanced expression of chemokines and metalloproteinases that contribute to asthma pathogenesis (Fig 2).⁷ Follow-up studies with transgenic mice overexpressing human ORMDL3 showed increased airway responsiveness and airway remodeling, potentially through both ATF6α target genes, such as SERCA2b, which increases airway smooth muscle cell (ASM) proliferation and contractility and by the ATF6α-independent genes TGFB1 and the metallopeptidase a disintegrin and metalloproteinase 8 (ADAM8).^23,24 Conversely, Ormdl3 knockout mice were protected from allergic airway disease induced by the fungal allergen Alternaria species. This protection correlated with decreased activation of the ATF6 arm, as demonstrated by reduced expression of the transcription factor XBP1, but not SERCA2b, and downstream activation of the ER–associated protein degradation pathway (Fig 2).²⁵ Reconstitution of ORMDL3 in the bronchial epithelium of these mice restored susceptibility to Alternaria species, suggesting that cellular stress promoted by ORMDL3 orchestrates airway hyperresponsiveness (AHR) during allergic immune responses.²⁵ ORMDL3 is also upregulated in patients with the autoimmune disease systemic lupus erythematosus.²⁶

FIG 2.

Cellular mechanism of involvement of ORMDL3 in asthma pathogenesis. Allergen challenge augments ORMDL3 expression in lung epithelial cells, which increases levels of ceramides, the ER stress signal ATF6, and the calcium pump SERCA2b, inducing airway remodeling and hyperresponsiveness. Eotaxin-1 and IL-3 upregulate ORMDL3 expression in eosinophils, activate nuclear factor κB (NF-κB), and enhance expression of integrin and CD48, promoting their trafficking, migration, and degranulation. In B cells ORMDL3 protects against apoptosis by promoting ATF6-dependent activation of autophagy. This leads to increased B-cell survival, hyperdifferentiation, and autoantibody production. Ceramides might be involved in induction of autophagy and plasma cell differentiation. In T lymphocytes overexpression of ORMDL3 negatively modulates store-operated calcium entry (SOCE) and suppresses their activation and increased T_H2 cytokine levels. Increased ORMDL3 expression in airway smooth muscle can promote airway smooth muscle cell proliferation, calcium oscillation, and contractility. ICAM1, Intercellular adhesion molecule 1; NFAT, nuclear factor of activated T cells.

In this study a global Ormdl3 knockout mouse was also used to uncover a key role for ORMDL3 in fine-tuning of B-cell development.²⁶ It was suggested that high levels of ORMDL3 in B cells induce ER stress through ATF6, resulting in increased Beclin-1 levels and autophagy that suppresses apoptosis.²⁶ Hence ORMDL3 promotes splenic B-cell survival, hyperdifferentiation, and autoantibody production that might be involved in autoimmune diseases²⁶ and also potentially in allergic responses. However, OVA allergen–challenged mice with Ormdl3 specifically deleted in airway epithelium had a significant increase, rather than decrease, in AHR compared with wild-type mice.²⁷ This increase in AHR independent of inflammation might be associated with increases in sphingolipids, particularly ceramide or S1P (see the next section for more explanation). Taken together, these studies illustrate that despite extensive research, the mechanisms by which ORMDL3 regulate ER stress are still not entirely clear.

In eosinophils ORMDL3 expression is induced by IL-3 and eotaxin-1 to promote their trafficking, recruitment, and degranulation through regulation of nuclear factor κB and levels of integrins and CD48 (Fig 2).⁸ Also, ORMDL3 can regulate other adhesion molecules because a recent study showed in alveolar epithelial cells that ORMDL3 regulates expression of intercellular adhesion molecule 1, the receptor for rhinovirus.²⁸ In this regard rhinovirus-induced ORMDL3 expression in B and T lymphocytes required cell-cell contact and plasmacytoid dendritic cells.²⁹ These studies might explain the association between ORMDL3 and susceptibility to rhinovirus-induced asthma.

ORMDL3/CERAMIDE AXIS IN PATIENTS WITH ALLERGIC ASTHMA

Although several potential mechanisms of action of ORMDL3 in asthma pathogenesis have been proposed, as discussed above, very few are related to its evolutionarily conserved role as an endogenous modulator of SPT activity and biosynthesis of ceramide that was originally uncovered in yeast and later demonstrated in eukaryotic cells. In mice treated intranasally with myriocin, an inhibitor of SPT, and in SPT heterozygous knockout mice, decreased ceramide biosynthesis induced AHR without inflammation, mucus production, or airway remodeling.³⁰ However, the role of ORMDL3 was not examined.³⁰ On the other hand, ceramide levels were increased in airway epithelia of OVA-sensitized guinea pigs³¹ and house dust mite–challenged mice.¹³ Moreover, inhibition of ceramide biosynthesis by intraperitoneal injections of fumonisin B1, an inhibitor of CerS, as well as myriocin, markedly reduced airway inflammation and hyperreactivity in these animals.^13,31 The discrepancy between these reports^13,30,31 might be due to different routes of administration of the inhibitors that might influence their effects on the airways. In addition, intratracheal delivery of ceramide caused lung inflammation, tissue remodeling, and airway flow obstruction.³² Moreover, nasal administration of the immunosuppressant drug FTY720/fingolimod reduced ORMDL3 expression and ceramide levels and mitigated airway inflammation and hyperreactivity and mucus hypersecretion in house dust mite–challenged mice.¹³ Thus, although there is general agreement that ORMDL3 is a natural negative regulator of ceramide biosynthesis that is increased in asthmatic patients, as mentioned above, it seems paradoxical that increased ceramide contributes to the immunopathology of asthma.

Recent work provided one potential solution for this contradiction and a molecular mechanism for the pathologic functions of ORMDL3 in asthmatic patients related to its evolutionarily conserved role in regulation of ceramide homeostasis.¹³ As mentioned above, ORMDL3 expression is increased in asthmatic patients. This is based on many reports that determined correlations between asthma susceptibility in human subjects and ORMDL3 expression,^1–4 as well as on increases in ORMDL3 in several models of allergic asthma in mice.^7,8,13 We propose that physiologically, ORMDL3 acts in concert with the other ORMDLs to negatively regulate SPT and ceramide de novo biosynthesis.¹³ However, highly increased pathologic ORMDL3 expression, such as that observed in patients with allergic asthma, enhanced ceramide levels primarily because of increased sphingolipid recycling in the salvage pathway.¹³ Increased ceramide levels in turn exacerbated inflammatory responses, mucus production, ER stress, and AHR.^13,31–35

UNIFYING HYPOTHESIS: CERAMIDE INVOLVEMENT IN UPR AND ER STRESS

The roles of ORMDL3 in UPR and ER stress and in regulation of ceramide homeostasis are not mutually exclusive. Cellular stress increases formation of ceramides through sphingomyelin hydrolysis or the salvage pathway. Conversely, increased ceramide levels have been implicated in ER stress and activation of several UPR pathways (Fig 3).^33–35 Alteration of CerS6 and its product, C16-ceramide, induces ATF6-mediated ER stress through release of calcium from ER stores.³⁶ Others have shown that ceramides induce SERCA inhibition and ER calcium depletion, leading to upregulation of GRP78 and activation of PERK/eIF2α and IRE1α/XBP1 arms of ER stress.³³ Furthermore, increased expression of CerS5 and CerS6 and their product in the salvage pathway, C14-ceramide, activates IRE1 and induces XBP1, which results in increased expression of IL-6.³⁵ In addition, ceramide synthesis in B cells is dependent on XBP1 and inhibition of CerS mitigated plasma cell expansion.³⁷ It seems that the effects of ceramides on the UPR might be evolutionarily conserved because increasing ceramide levels in yeast also activates the UPR³⁴ and defective ceramide homeostasis leads to UPR failure.³⁸

FIG 3.

Unifying hypothesis of ORMDL3 functions. In patients with allergic asthma, upregulation of ORMDL3 disrupts homeostatic levels of ceramides within the ER. Increased ceramide levels lead to increased levels of its downstream metabolite S1P. Increases in ceramide levels increase inflammation and mucus production and also interfere with the activity of the SERCA pump, inducing ER stress and initiating the UPR. These in turn act in a positive feed-forward amplification loop, further increasing formation of ceramides and amplifying their effects. Activation of 1 or more of the 3 UPR pathways, ATF6, PERK, or IRE1, can promote airway smooth muscle remodeling and hypersensitivity and increase production of chemokines and cytokines contributing to inflammation. Excess ceramide and S1P are secreted into the lung airways, perhaps serving as potential biomarkers of this pathology. Thus we suggest that abnormal levels of ORMDL3 have a pathogenic role in asthma exacerbation through dysregulation of sphingolipids and the ER stress responses.

Ceramides also can enhance cytokine and chemokine production in a murine asthma model.¹³ Hence ceramides produced by upregulation of ORMDL3 can function in a positive feedforward loop to amplify and perpetuate key pathways that promote changes in airway physiology during allergic immune responses. Although this hypothesis potentially explains contradictory reports in the literature, it needs to be further verified by additional human studies. For example, because airway epithelial cells derived from children with asthma with or without ORMDL3 SNPs have not been studied, whether ORMDL3 regulation of ceramides, UPR, cytokines/chemokines, or adhesion molecules in lung epithelium contribute to the development of childhood-onset asthma is still not known. Moreover, ORMDL3 is not only expressed in epithelial cells but also in T cells, B cells, eosinophils, airway smooth muscle, and other cells in which regulation of ceramide and its importance to asthma might not be the same as in the epithelium.

SPHINGOLIPIDS AS POTENTIAL BIOMARKERS FOR ALLERGIC ASTHMA

Using sophisticated mass spectrometry, we observed that C16:0 ceramide levels were also significantly increased in exhaled breath condensates from patients with severe asthma compared with those from healthy control subjects,¹³ raising the possibility that ceramides might be a useful biomarker. Specific ceramide species were also increased in sera of children with exercise-induced wheeze, which is often associated with childhood asthma.³⁹ However, it is possible that a further metabolite of ceramide, S1P, might also contribute to asthma pathogenesis. We previously showed that S1P secreted by mast cells plays important roles in mast cell–dependent, allergen-induced allergic inflammation and AHR.⁴⁰ Mice globally overexpressing human ORMDL3 had spontaneously increased airway responsiveness in the absence of inflammation, with significantly reduced S1P levels in serum but not in lung tissue. Administration of S1P to these mice further increased their AHR.⁴¹ Conversely, OVA antigen–challenged mice with specific deletion of Ormdl3 in lung epithelial cells had decreased rather than increased AHR that was associated with increased serum S1P levels,²⁷ further supporting a role for this sphingolipid metabolite. S1P levels in sera from patients have also been associated with asthma severity⁴² and aspirin-exacerbated respiratory disease.⁴³ However, in another study no significant association of the annotated ORMDL3 asthma SNPs with plasma long-chain sphingoid base levels was observed.¹⁵ Nevertheless, it is not clear how changes in levels of any sphingolipid metabolite in the circulation and not in the lung could play an important role in asthma. Targeted sphingolipidomics of bronchoalveolar lavage fluid, exhaled breath condensates, or sputum from larger cohorts of asthmatic patients compared with age/sex-matched control subjects should help determine whether ceramides or other sphingolipids can serve as biomarkers.

Asthma is a very heterogeneous disease with multiple phenotypes and different onset, course, and treatment responses. The different underlying pathophysiologic mechanisms suggest the need for additional molecular and genetic characterizations and development of targeted customized treatment modalities and the value of biomarkers to improve diagnosis and treatment. Additional molecular and genetic characterizations of cohorts of asthmatic patients are needed to better understand the different phenotypes, leading to more personalized targeted approaches to asthma therapy. Further studies can help to determine the utility of ceramides or S1P as clinical biomarkers of specific asthma phenotypes and/or to optimize treatment and monitor and improve clinical outcomes in patients with this disease.

Abbreviations used

AHR

Airway hyperresponsiveness

ATF6

Activating transcription factor 6

CerS

Ceramide synthase

eIF2α

Eukaryotic translation initiation factor 2α

ER

Endoplasmic reticulum

IRE1

Inositol-requiring enzyme 1

ORMDL3

ORM1 (yeast)–like protein 3

PERK

Protein kinase R like endoplasmic reticulum kinase

SERCA

Sarcoendoplasmic reticulum calcium ATPase

SNP

Single nucleotide polymorphism

S1P

Sphingosine-1-phosphate

SPT

Serine palmitoyltransferase

STAT6

Signal transducer and activator of transcription 6

UPR

Unfolded protein response

XBP1

X-box binding protein 1