Augmented Pla2g4c/Ptgs2/Hpgds axis in bronchial smooth muscle tissues of experimental asthma

Rationale Augmented smooth muscle contractility of the airways is one of the causes of airway hyperresponsiveness in asthmatics. However, the mechanism of the altered properties of airway smooth muscle cells is not well understood. Objectives To identify differentially expressed genes (DEGs) related to the bronchial smooth muscle (BSM) hyper-contractility in a murine asthma model. Methods The ovalbumin (OA)-sensitized mice were repeatedly challenged with aerosolized OA to induce asthmatic reaction. Transcriptomic profiles were generated by microarray analysis of BSM tissues from the OA-challenged and control animals, and KEGG (Kyoto Encyclopedia of Genes and Genomes) Pathway Analysis was applied. Measurements and main results Tension study showed a BSM hyperresponsiveness to acetylcholine (ACh) in the OA-challenged mice. A total of 770 genes were differentially expressed between the OA-challenged and control animals. Pathway analysis showed a significant change in arachidonic acid (AA) metabolism pathway in BSM tissues of the OA-challenged mice. Validation of DEGs by quantitative RT-PCR showed a significant increase in PLA2 group 4c (Pla2g4c)/COX-2 (Ptgs2)/PGD2 synthase 2 (Hpgds) axis. PGD2 level in bronchoalveolar fluids of the OA-challenged mice was significantly increased. A 24-h incubation of BSM tissues with PGD2 caused a hyperresponsiveness to ACh in naive control mice. Conclusions AA metabolism is shifted towards PGD2 production in BSM tissues of asthma. Increased PGD2 level in the airways might be a cause of the BSM hyperresponsiveness in asthma.


Introduction
Enhanced airway responsiveness to non-specific stimuli, called airway hyperresponsiveness (AHR), is a characteristic feature of bronchial asthma. One of the causes of the AHR is hypercontraction of smooth muscle cells of the airways [1][2][3][4][5]. Rapid relief from airway limitation in asthma attack by short-acting beta 2 -stimulant inhalation may also suggest an involvement of augmented airway smooth muscle contraction in the airway obstruction. It is thus important for development of asthma therapy to understand the disease-related changes in the contractile signaling of airway smooth muscle cells.
Smooth muscle contraction is caused by the interaction of myosin and actin filaments, and regulated by various contractile and Ca 2+ -sensitizing proteins [6,7]. One possible explanation of the hyper-contraction of smooth muscle may be an up-regulation of these proteins associated with contraction. In addition to their contractile function, smooth muscle cells of the airways also have ability to generate/secrete various biologically active substances including interleukins, chemokines, and prostanoids [8][9][10][11][12][13]. The airway structural cells, such as epithelial cells (e.g., [14]), and the accumulated inflammatory cells, such as eosinophils (e.g., [15]), also release various mediators around the smooth muscle cells. Some of these autocrine/ paracrine mediators affect transcriptional signaling in the cells of airway smooth muscle, resulting in an alteration of its function [12,[16][17][18][19]. Thus, an inclusive analysis of differentially expressed genes in airway smooth muscle tissues of asthma might provide new insight into the treatment of the AHR.
In the present study, we used a well-characterized asthma model of mice, which have AHR both in vivo and in vitro [20]. A microarray analysis was applied to identify the differentially expressed genes in BSM tissues of the AHR animals. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses revealed the arachidonic acid metabolism pathway as a significantly changed pathway associated with AHR. In particular, an augmentation of phospholipase A 2 group 4c (Pla2g4c)/cyclooxygenase-2 (Ptgs2)/prostaglandin D 2 synthase 2 (Hpgds) cascade was strongly suggested. Thus, the role of prostaglandin D 2 (PGD 2 ) in the development of BSM hyperresponsiveness, one of the causes of AHR, was also investigated.

Animals and treatments
Male BALB/c mice were purchased from the Tokyo Laboratory Animals Science Co., Ltd. (Tokyo, Japan) and housed in a pathogen-free facility. All animal experiments were approved by the Animal Care Committee of the Hoshi University (Tokyo, Japan).
Preparation of a murine model of allergic bronchial asthma, which has an in vivo AHR [20], was performed as described previously [18,21]. In brief, BALB/c mice (8 weeks of age) were actively sensitized by intraperitoneal injections of 8 μg ovalbumin (OA; Seikagaku Co., Tokyo, Japan) with 2 mg Imject Alum (Pierce Biotechnology, Inc., Rockfold, IL, USA) on Day 0 and Day 5. The sensitized mice were challenged with aerosolized OA-saline solution (5 mg/mL) for 30 min on Days 12,16 and 20. A control group of mice received the same immunization procedure but inhaled saline aerosol instead of OA challenge. The aerosol was generated with a compressor nebulizer (MiniElite™: Philips Respironics, NV, USA) and introduced to a Plexiglas chamber box (130 x 200 mm, 100 mm height) in which the mice were placed. Twenty-four h after the last OA challenge, mice were sacrificed by exsanguination from abdominal aorta under urethane (1.6 g/kg, i.p.; Sigma, St. Louis, MO) anesthesia.

RNA extraction
Both left and right main bronchi were isolated, and the epithelia, capillary vessels and connective tissues were removed as much as possible (S1 Fig) by gently rubbing with sharp tweezers under a stereomicroscope [21]. Total RNA was extracted using Trizol reagent (Invitrogen, Karlsruhe, Germany) according to the manufacturer's protocol. RNA purity and integrity were evaluated by ND-1000 Spectrophotometer (Thermo Fisher Scientific, Waltham, MA) and Agilent 2100 Bioanalyzer (Agilent Technologies, Santa Clara, CA).

Microarray analysis
RNA labeling and hybridization were performed by using the Agilent One-Color Microarray-Based Gene Expression Analysis protocol (v6.5, Agilent Technologies). Briefly, total RNA (100 ng) of each sample was linearly amplified and labeled with Cy3-dCTP. The labeled cRNAs were purified using RNAeasy Mini Kit (Qiagen, Valencia, CA). The concentration and specific activity of the labeled cRNAs (pmol Cy3/μg cRNA) were measured by NanoDrop ND-1000 (Thermo Fisher Scientific Inc.). Each labeled cRNA (600 ng) was fragmented by adding 5 μL 10 x blocking agent and 1 μL of 25 x fragmentation buffer, and then heated at 60˚C for 30 min. Finally 25 μL 2 x GE hybridization buffer was added to dilute the labeled cRNA. The hybridization solution (40 μL) was dispensed into the gasket slide and assembled to the Agilent Sure-Print G3 Mouse GE 8X60K, v2 Microarrays (Agilent Technologies). The slides were incubated for 17 h at 65˚C in an Agilent hybridization oven, and then washed at room temperature by using the Agilent One-Color Microarray-Based Gene Expression Analysis protocol (v6.5, Agilent Technologies). The hybridized array was immediately scanned with an Agilent Microarray Scanner D (Agilent Technologies). The captured array images were analyzed using Agilent Feature Extraction Software (v11.0.1.1, Agilent Technologies). The raw data for same gene was then summarized automatically in Agilent Feature Extraction Protocol to generate raw data text file, providing expression data for each gene probed on the array. Array probes that have Flag A in samples were filtered out. Selected gProcessedSignal value was transformed by logarithm and normalized by quantile method. Statistical significance of the expression data was determined using fold change and local pooled error (LPE) test in which the null hypothesis was that no difference exists between the groups. Hierarchical cluster analysis was performed using complete linkage and Euclidean distance as a measure of similarity. Gene Ontology (GO) functional enrichment analysis for the differentially expressed genes was performed using Gene Set Enrichment Analysis software (http://software.broadinstitute.org/gsea/index.jsp). The gene sets were separated according to the GO terms for biological processes, cellular components, and molecular functions. Pathway analysis was performed using GeneCodis tools (http://genecodis.cnb.csic.es) based on the Kyoto Encyclopedia of Genes and Genomes (KEGG: https://www.kegg.jp) pathway database. All data analysis and visualization of differentially expressed genes was conducted using R 3.0.2 (www.r-project.org).

Quantitative RT-PCR analyses
Expression levels of mRNA transcripts were determined by quantitative RT-PCR analysis. In brief, reverse transcription reactions were performed using a cDNA synthesis kit (RR037A: TaKaRa, Shiga, Japan) according to the manufacturer's instructions. The cDNA products from each sample were then subjected to real-time PCR analyses using StepOne™ real-time PCR system (Applied Biosystems, Foster City, CA) with Fast SYBR Green Master Mix (Applied Biosystems) according to the manufacturer's instructions. The reactions were incubated in a 48-well optical plate at 95˚C for 20 seconds, following by 43 cycles of 95˚C for 3 seconds and 60˚C for 30 seconds. The PCR primer sets used are shown in Table 1, which were designed from published sequences.

Western blot analyses
Protein samples of the BSM tissues were subjected to 15% sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and the proteins were then electrophoretically transferred to a polyvinylidene fluoride (PVDF) membrane. After blocking with EzBlock Chemi (Atto, Co., Tokyo, Japan), the PVDF membrane was incubated with the primary antibody. The primary antibodies used in the present study were polyclonal rabbit antihematopoietic prostaglandin D synthase antibody (HPGDS; 1:200 dilution; Item No. 160013; Cayman Chemical; Ann Arbore, MI). Then the membrane was incubated with horseradish peroxidase-conjugated donkey anti-rabbit IgG (1:2,500 dilution; Santa Cruz Biotechnology, Inc.; Santa Cruz, CA), detected using EzWestBlue (Atto, Co.) and analyzed by a densitometry system. Detection of house-keeping gene was also performed on the same membrane by using monoclonal mouse anti-GAPDH (1:10,000 dilution; Santa Cruz Biotechnology, Inc.).

Assessment of prostaglandin D 2 (PGD 2 ) levels in BAL fluids
After the exsanguinations, the chest of each animal was opened and a 20-gauge blunt needle was tied into the proximal trachea. Bronchoalveolar lavage (BAL) fluid was obtained by intratracheal instillation of 1 mL/animal of phosphate-buffered saline (PBS; pH 7.5, room temperature) into the lung while it was kept located within the thoracic cavity. The lavage was reinfused into the lung twice before final collection. The BAL fluids were centrifuged at 500 g, and the resultant supernatants were stored at -80˚C until use. The levels of PGD 2 in BAL fluids were measured by a competitive PGD 2 ELISA system (Item No. 512011: Cayman Chemical, Ann Arbor, MI) according to the manufacturer's instructions.

Lipidomic profiling by LC mass spectrometry
Lipid mediators were analyzed by LC and MS essentially as described previously [22,23]. Briefly, BAL fluids were supplemented with a mix consisting of four deuterated internal standards (Cayman Chemical) and lipid metabolites isolated by solid phase extraction on an Oasis HLB column (Waters, Milford, MA). The extracted samples were evaporated, reconstituted in a small volume, and the eicosanoids were separated by reverse phase LC using an XBridge C18 column (Waters). The eicosanoids were analyzed by a triple quadrupole mass spectrometer (LCMS8040, Shimadzu, Kyoto, Japan) operated in the negative-ionization mode via multiplereaction monitoring (MRM) using transitions that were optimized for selectivity and sensitivity. Quantitation was performed using calibration curves constructed for each compound, and recoveries were monitored using deuterated internal standards (15-HETE-d8, LTB 4 -d4, PGE 2 -d4, arachidonic acid-d8). Data analysis was performed using LabSolutions software (Shimadzu).

Determination of bronchial smooth muscle (BSM) responsiveness
Mice were sacrificed by exsanguination from abdominal aorta under urethane (1.6 g/kg, i.p.) anesthesia and the airway tissues under the larynx to lungs were immediately removed. About 3 mm length of the left main bronchus (about 0.5 mm diameter) was isolated. The resultant tissue ring preparation was then suspended in a 5 mL-organ bath by two stainless-steel wires (0.

Data and statistical analyses
In the real-time PCR analyses, the comparative threshold cycle (C T ) method was used for relative quantification of the mRNA transcripts. Differences in the C T values (ΔC T ) between the target gene and GAPDH were calculated to determine the relative expression levels, using the following formula: ΔΔC T = (ΔC T of the treated sample)-(ΔC T of the control sample). The relative expression level between the samples was calculated according to the equation 2 -ΔΔCT .
All the data were expressed as the mean with S.E. Statistical significance of difference was determined by unpaired Student's t-test or two-way analysis of variance (ANOVA) with post hoc Bonferroni/Dunn (StatView for Macintosh ver. 5.0, SAS Institute, Inc., NC). A value of P < 0.05 was considered significant.

Identification of differentially expressed genes in BSM tissues of the antigen-challenged mice
As shown in Fig 1, the contractile responsiveness to acetylcholine (ACh) was significantly augmented in bronchial smooth muscle (BSM) tissues isolated from the repeatedly antigen-challenged mice. In the present study, total RNA was isolated from BSM tissues of the diseased animals and was used for a DNA microarray analysis.
As shown in Fig 2A, box-whisker plotting showed similar distribution of intensities among the samples used, suggesting that the array experiment was performed under an appropriate condition. Valiations of gene expression among the specimens were shown by volcano plotting ( Fig  2B) and scatter plotting (Fig 2C). Of the 56,605 probe sets represented on the Agilent SurePrint G3 Mouse GE 8X60K v2 gene chip, 770 probe sets were differentially expressed in BSM tissues of the antigen-challenged mice as compared with those of control animals (|fold change| ! 2 and adjusted P-value < 0.05; N = 4, respectively). Among them, 557 were up-regulated and 213 were down-regulated. An unsupervised hierarchical clustering analysis of the differentially expressed genes showed a distinct separation between the antigen-challenged (C) and normal control animals (N) (Fig 3). The complete data set is publicly available in the Gene Expression Omnibus (GEO) public repository (http://www.ncbi.nlm.nih.gov/geo/) (Accession No. GSE116504) in a format that complies with the Minimal Information About a Microarray Experiment guidelines.

Gene ontology analysis and KEGG pathway enrichment analysis
Gene ontology analysis of the differentially expressed genes identified a number of different processes with statistical significance.  Male BALB/c mice were actively sensitized and repeatedly challenged with ovalbumin (OA) antigen. Twenty-four hours after the last OA challenge, the left main bronchi were isolated and the ACh responsiveness was measured as described in METHODS. Each point represents the mean ± SEM from 6 animals, respectively. NC: naive control, and Chal: repeatedly antigen-challenged groups. A significant difference was observed between the groups (P < 0.001 by two-way ANOVA with post hoc Bonferroni/Dunn test).

Validation of differentially expressed genes by RT-qPCR
In order to validate the microarray data, differentially expressed mRNA transcripts included in the AA pathway were further analyzed by real-time RT-qPCR (Fig 7). Since the PCR primer design did not allow us to distinguish between Cyp4a12a and Cyp4a12b, the sum of these transcripts were measured and referred to as Cyp4a12. With the exception of Cyp4a10, all mRNA transcripts examined showed the concordant up-or down-regulation in BSM tissues of the antigen-challenged mice (Fig 7).

Prostaglandin D 2 level in bronchoalveolar lavage fluids
The results of microarray and RT-qPCR analyses revealed that the expression of phospholipase A 2 (Pla2g4c), cyclooxygenase-2 (Ptgs2) and prostaglandin D synthase (Hpgds) were significantly increased in BSM tissues of the antigen-challenged mice. Immunoblot analysis also revealed an up-regulation of HPGDS protein in BSM tissues of the diseased mice ( Fig 8A). The findings also strongly suggest that the AA metabolism is largely shifted towards prostaglandin D 2 (PGD 2 ) production in BSM tissues of the murine asthma model. To determine the changes in AA metabolism in the airways, lipid mediators in bronchoalveolar lavage (BAL) fluids were measured using LC/MS/MS and ELISA.
In BAL fluids of the mice, a total of 78 lipid species were able to measure by using LC/MS/ MS approach. Among them, a total of 27 AA-related metabolites were consistently detected in samples (Fig 9). The level of AA itself in BAL fluids of the antigen-challenged mice (284.5 ± 81.7 pg/mL) was significantly higher than that of control animals (22.2 ± 7.2 pg/mL, P < 0.05 by unpaired Student's t-test). In the repeatedly antigen-challenged group, most of the AA-related metabolites including PGD 2 were increased (Fig 9B), but the levels of 6-keto-prostaglandin F 1 alpha (6-keto-PGF 1 alpha; 952.1 ± 44.2 pg/mL in control versus 60.2 ± 27.0 pg/mL in challenged mice, P < 0.001 by unpaired Student's t-test) and 12S-hydroxy-5Z,8E,10E-heptadecatrienoic acid (12-HHT; 6.4 ± 0.9 pg/mL in control versus 0.4 ± 0.4 pg/mL in challenged mice, P < 0.001 by unpaired Student's t-test) were significantly decreased. The levels of PGD 2 in BAL fluids were further determined using an ELISA system. As shown in Fig 8B, the PGD 2 level in BAL fluids of the antigen-challenged mice was significantly higher than that of control animals (P < 0.001 by unpaired Student's t-test).

Effects of prostaglandin D 2 on BSM function
To determine the role of prostaglandin D 2 (PGD 2 ) on the AHR, effects of PGD 2 on the isometric tension of smooth muscles were examined in BSM tissues isolated from naive control mice. Application of PGD 2 (10 −9~1 0 −5 M) had no effect on basal tone of the BSM tissues (data not Transcriptomic analysis of BSMs in AHR mice shown). When the BSM tissues were pre-contracted with 10 −5 M ACh (about a half-maximal contraction) or 60 mM K + (in the presence of 10 −6 M atropine), cumulatively applied PGD 2 (10 −9~1 0 −5 M) showed a complicated response. A transient inhibitory effect was observed at the PGD 2 concentration of 10 −6 M: this was followed by an enhancing effect by the application of 10 −5 M of PGD 2 (data not shown), indicating that a higher concentration of PGD 2 , at least, could augment the BSM contraction in naive control animals. So in the present study, effects  Transcriptomic analysis of BSMs in AHR mice of pre-treatment with PGD 2 (10 −5 M) on the BSM responsiveness to ACh were determined. As shown in Fig 10A, a 15-min treatment with PGD 2 had no effect on the BSM responsiveness to ACh. However, the ACh concentration-response curve was significantly shifted upward when the BSM tissues were incubated with PGD 2 for 24 hours (Fig 10B).

Discussion
Augmented contractility of airway smooth muscles is one of the causes of the AHR in asthmatics [1][2][3][4]24]. However, the mechanism of the altered properties of airway smooth muscle cells is not fully understood now. In the present study, we focused on BSM tissues of the antigen-challenged mice that have both in vivo AHR [20] and hyper-contractility of the isolated BSM tissues (Fig 1). To screen differentially expressed genes of the diseased BSM tissues, a DNA microarray analysis was applied using total RNA extracted from the BSM tissues. Of the 56,605 probe sets represented on the gene chip used, 557 were up-regulated and 213 were down-regulated (see Results section), indicating that gene expression is abundantly changed in the BSM tissues of asthma. The KEGG pathway analysis of the microarray data revealed a significant change in the AA metabolism pathway in BSM tissues of the antigen-challenged mice (Fig 5). In particular, an augmentation of PLA 2 group 4c (Pla2g4c)/COX-2 (Ptgs2)/PGD 2 synthase 2 (Hpgds) cascade was strongly suggested (Fig 6). Expression of these genes in smooth muscle cells of the airways has also been shown by the published gene expression data (GEO accession number GSE45723) [25] and the current study (S2 Fig). Eicosanoids, including prostaglandins, thromboxanes and leukotrienes, are important signaling molecules that have been implicated in various pathological processes including asthma [26][27][28]. Their precursor AA is freed from cell membrane phospholipid by the action of PLA 2 family of enzymes. In mice, 20 genes of the PLA 2 family (EC: 3.1.1.4) are listed in the KEGG AA metabolism pathway (Map ID: mmu00590). Among them, a total of 12 genes, Pla2g2c, Pla2g2d, Pla2g4a, Pla2g5, Pla2g4b, Pla2g16, Pla2g4c, Pla2g3, Pla2g2e, Pla2g6, Pla2g12a, and Plb1, were consistently expressed in the BSM tissues by the current microarray analysis. Among the 12 genes expressed, a dramatic increase in Pla2g4c (PLA 2 group 4c) was observed Transcriptomic analysis of BSMs in AHR mice in BSM tissues of the OA-challenged mice as compared to those of control animals (Table 2 and Fig 7). The results might be consistent with previous report where an increase in the expression of Pla2g4c was demonstrated in lungs of a murine asthma model induced by Aspergillus fumigatus [29]. It is thus possible that increased expression of Pla2g4c is a ubiquitous event in the airways of allergic asthma. PLA 2 group 4c, also named as cytosolic PLA 2 gamma (cPLA 2 gamma), was first identified by Pickard and colleagues [30], and is a Ca 2+ -independent enzyme on its PLA 2 activity [31,32]. Cells overexpressing cPLA 2 gamma could cause an increase in AA release [31,33,34]. Elevated levels of AA in BAL fluids were reported in asthmatics after inhaled antigen challenge [35]. Thus, up-regulation of PLA 2 group 4c might be responsible for the increased AA level in the airways (Fig 9) and the subsequent increase in eicosanoids that are implicated in asthma pathology [26][27][28]. PGD 2 is an acidic lipid mediator derived from the metabolism of AA by the action of cyclooxygenases, COX-1 (Ptgs1) and COX-2 (Ptgs2), and downstream PGD 2 synthases, lipocalintype PGD synthase (Ptgds) and hematopoietic PGD synthase (Hpgds). Current microarray analyses revealed that these genes were expressed in BSM tissues of the mice. Among them, the mRNA expression levels of Ptgs2 and Hpgds were significantly increased in BSM tissues of the Transcriptomic analysis of BSMs in AHR mice OA-challenged animals (Fig 6 and Table 2). Likewise, an increased expression of COX-2 has been demonstrated in lungs of guinea pig asthma model [36] and airway smooth muscles of patients with asthma [37]. Cytokine stimulation could cause an induction of COX-2 in human airway smooth muscle cells [38,39]. An up-regulation of HPGDS has also been reported in airway structural cells of asthmatics [40]. It is thus possible that the AA metabolism might largely shift towards PGD 2 production in the airways of asthma. Indeed, a significant increase in PGD 2 was also observed in BAL fluids of the OA-challenged mice (Figs 8 and 9).
To determine the role of PGD 2 on smooth muscle function of the airways, the BSMs isolated from naive control mice were treated with PGD 2 . Reportedly, PGD 2 has an ability to Transcriptomic analysis of BSMs in AHR mice cause contraction of the isolated airway smooth muscles in guinea pigs [41,42], rabbits [43] and dogs [44]. The contraction seems to be mediated partly by stimulating cholinergic neurotransmission [41,44]. However, the current organ bath studies revealed that application of PGD 2 did not affect on baseline tension of the mouse BSMs (see Results), indicating that there is species differences in the effect of PGD 2 on airway smooth muscle function. On the other hand, PGD 2 might be capable of inducing BSM hyper-contractility: PGD 2 augmented the sub-maximal contraction induced by ACh (see Results) and high-K + depolarization (in the presence of 10 −6 M atropine: data not shown). Moreover, the 24-h incubation with PGD 2 caused a BSM hyperresponsiveness to ACh (Fig 10B), as if that was observed at 24 h after the antigen challenge in the asthmatic animals (Fig 1). These findings suggest that the increased PGD 2 level in the airways (Figs 8 and 9) is one of the causes of the antigen-induced BSM hyperresponsiveness.
In the present study, although the BSM contractility was significantly augmented in the antigen-challenged mice (Fig 1), no significant change in the expression levels of genes related to the smooth muscle contraction, such as myosins (Myh11, Myl6, Myl9), actins (Acta2, Actg2), myosin light chain kinases (MLCKs: Mylk, Mylk2~4), MLC phosphatases (MLCPs: Ppp1r12a, Ppp1r12b, Ppp1r12c, Ppp1ca, Ppp1cb, Ppp1cc), and caldesmon (Cald1), was detected by the current microarray analysis. In this animal model of asthma, an augmented RhoA-mediated Ca 2+ sensitization of the BSM contraction is a cause of the BSM hyper-contractility [21]. It has also been demonstrated that an inhibition of negative regulation mediated by a microRNA (miRNA), miR-133a-3p, is the main cause of the up-regulation of RhoA protein [18,45]. Similarly, miRNA regulation of airway smooth muscle function has also been reported [37,[46][47][48][49][50]. It is thus possible that changes in post-transcriptional rather than transcriptional modulations of the gene expression might be largely involved in the alteration of smooth muscle contractility of the diseased airways. PGD 2 might cause such an epigenetic change in airway smooth muscle, resulting in its augmented contractility. Further studies are needed to make clear the exact role of PGD 2 on airway smooth muscle function.
The current LC/MS/MS analysis also revealed a significant decrease in the levels of 6-keto-PGF 1 alpha, a stable metabolite of PGI 2 , in BAL fluids of the antigen-challenged mice (see Results), indicating that PGI 2 production is decreased in the airways of asthma. PGI 2 is produced from PGH 2 , an AA-derived COX metabolite, by the action of PGI 2 synthase (Ptgis). Because PGI 2 and its synthetic analogues have been suggested to induce a BSM relaxation [51] and to have an inhibitory effect of asthma including the AHR [51][52][53], the decreased PGI 2 level in the airways might also be one of the causes of the AHR. Although the mechanism of decrease in PGI 2 level is unclear now, its precursor AA was conversely increased in the diseased airways (see Results). No change in the expression of PGI 2 synthase (Ptgis) was observed in the present microarray analysis (fold change: 1.05, adjusted P-value: P > 0.05). Similarly, the expression of thromboxane A synthase (Tbxas1) in the diseased BSM tissues was within control level (fold change: 1.66, adjusted P-value: P > 0.05). However, the LC/MS/MS approach also revealed a significant decrease in 12-HHT (see Results), whereas a significant increase in TXB 2 , a stable metabolite of TXA 2 , was observed in BAL fluids of the antigenchallenged mice (15.2 ± 7.6 pg/mL in control versus 99.2 ± 15.6 pg/mL in challenged mice, P < 0.01 by unpaired Student's t-test). Both 12-HHT and TXA 2 are generated from PGH 2 by the action of Tbxas1 [54]. It is thus important to note that changes in the levels of lipid mediators in asthma might not be explained simply by changes in the expression of related enzyme genes.
In conclusion, the current study demonstrated that the AA metabolism is largely shifted towards PGD 2 production in BSM tissues of asthma. Increased PGD 2 level in the airways might be one of the causes of airway smooth muscle hyper-contractility, that is a cause of the AHR in asthmatics. The PLA 2 group 4c/COX-2/PGD 2 synthase 2 cascade might be a potential therapeutic target for AHR in asthma, although some validation studies using human tissues should be required.