Expression and putative biological roles of lipoxygenases and leukotriene receptors in leukemia and lymphoma.

This mini-review addresses lipoxygenases and receptors for leukotrienes in hematological malignancies. Potential novel biomarkers and drug targets in leukemia and B-cell lymphoma are discussed.

Intrinsic 5-lipoxygenase activity regulates migration and adherence of mantle cell lymphoma cells.

Human B-lymphocytes express 5-lipoxygenase (5-LOX) and 5-LOX activating protein (FLAP) and can convert arachidonic acid to leukotriene B4. Mantle cell lymphoma (MCL) cells contain similar amounts of 5-LOX as human neutrophils but the function and mechanism of activation of 5-LOX in MCL cells, and in normal B-lymphocytes, are unclear. Here we show that the intrinsic 5-LOX pathway in the MCL cell line JeKo-1 has an essential role in migration and adherence of the cells, which are important pathophysiological characteristics of B-cell lymphoma. Incubation of JeKo-1 with the FLAP inhibitor GSK2190915 or the 5-LOX inhibitor zileuton, at a concentration below 1 µM, prior to stimulation with the chemotactic agent CXCL12, led to a significant reduction of migration. CRISPR/Cas9 mediated deletion of ALOX5 gene in JeKo-1 cells also led to a significantly decreased migration of the cells. Furthermore, 5-LOX and FLAP inhibitors markedly decreased the adherence of JeKo-1 cells to stromal cells. In comparison, these drugs had a similar effect on adherence of JeKo-1 cells as the Bruton tyrosine kinase inhibitor ibrutinib, which has a proven anti-tumour effect. These results indicate that inhibition of 5-LOX may be a novel treatment for MCL and certain other B-cell lymphomas.

An intronic deletion in megakaryoblastic leukemia 1 is associated with hyperproliferation of B cells in triplets with Hodgkin lymphoma.

Megakaryoblastic leukemia 1 (MKL1) is a coactivator of serum response factor and together they regulate transcription of actin cytoskeleton genes. MKL1 is associated with hematologic malignancies and immunodeficiency, but its role in B cells is unexplored. Here we examined B cells from monozygotic triplets with an intronic deletion in MKL1, two of whom had been previously treated for Hodgkin lymphoma (HL). To investigate MKL1 and B-cell responses in the pathogenesis of HL, we generated Epstein-Barr virus-transformed lymphoblastoid cell lines from the triplets and two controls. While cells from the patients with treated HL had a phenotype close to that of the healthy controls, cells from the undiagnosed triplet had increased MKL1 mRNA, increased MKL1 protein, and elevated expression of MKL1-dependent genes. This profile was associated with elevated actin content, increased cell spreading, decreased expression of CD11a integrin molecules, and delayed aggregation. Moreover, cells from the undiagnosed triplet proliferated faster, displayed a higher proportion of cells with hyperploidy, and formed large tumors in vivo This phenotype was reversible by inhibiting MKL1 activity. Interestingly, cells from the triplet treated for HL in 1985 contained two subpopulations: one with high expression of CD11a that behaved like control cells and the other with low expression of CD11a that formed large tumors in vivo similar to cells from the undiagnosed triplet. This implies that pre-malignant cells had re-emerged a long time after treatment. Together, these data suggest that dysregulated MKL1 activity participates in B-cell transformation and the pathogenesis of HL.

Targeting biosynthetic networks of the proinflammatory and proresolving lipid metabolome.

Nonsteroidal anti-inflammatory drugs interfere with the metabolism of arachidonic acid to proinflammatory prostaglandins and leukotrienes by targeting cyclooxygenases (COXs), 5-lipoxygenase (LOX), or the 5-LOX-activating protein (FLAP). These and related enzymes act in conjunction with marked crosstalk within a complex lipid mediator (LM) network where also specialized proresolving LMs (SPMs) are formed. Here, we present how prominent LM pathways can be differentially modulated in human proinflammatory M1 and proresolving M2 macrophage phenotypes that, upon exposure to Escherichia coli, produce either abundant prostaglandins and leukotrienes (M1) or SPMs (M2). Targeted liquid chromatography-tandem mass spectrometry-based metabololipidomics was applied to analyze and quantify the specific LM profiles. Besides expected on-target actions, we found that: 1) COX or 15-LOX-1 inhibitors elevate inflammatory leukotriene levels, 2) FLAP and 5-LOX inhibitors reduce leukotrienes in M1 but less so in M2 macrophages, 3) zileuton blocks resolution-initiating SPM biosynthesis, whereas FLAP inhibition increases SPM levels, and 4) that the 15-LOX-1 inhibitor 3887 suppresses SPM formation in M2 macrophages. Conclusively, interference with discrete LM biosynthetic enzymes in different macrophage phenotypes considerably affects the LM metabolomes with potential consequences for inflammation-resolution pharmacotherapy. Our data may allow better appraisal of the therapeutic potential of these drugs to intervene with inflammatory disorders.-Werner, M., Jordan, P. M., Romp, E., Czapka, A., Rao, Z., Kretzer, C., Koeberle, A., Garscha, U., Pace, S., Claesson, H.-E., Serhan, C. N., Werz, O., Gerstmeier, J. Targeting biosynthetic networks of the proinflammatory and proresolving lipid metabolome.

Human 15-lipoxygenase-1 is a regulator of dendritic-cell spreading and podosome formation.

Dendritic cells (DCs) involved in proinflammatory immune responses derive mainly from peripheral monocytes, and the cells subsequently mature and migrate into the inflammatory micromilieu. Here we report that suppressing of 15-lipoxygenase-1 led to a substantial reduction in DC spreading and podosome formation in vitro. The surface expression of CD83 was significantly lower in both sh-15-lipoxygenase-1 (15-LOX-1)-transduced cells and DCs cultivated in the presence of a novel specific 15-LOX-1 inhibitor. The T-cell response against tetanus-pulsed DCs was only affected to a minor extent on inhibition of 15-LOX-1. In contrast, endocytosis and migration ability of DCs were significantly suppressed on 15-LOX-1 inhibition. The expression of 15-LOX-1 in DCs was also demonstrated in affected human skin in atopic and contact dermatitis, showing that the enzyme is indeed expressed in inflammatory diseases in vivo. This study demonstrated that inhibiting 15-LOX-1 led to an impaired podosome formation in DCs, and consequently suppressed antigen uptake and migration capacity. These results indicated that 15-LOX-1 is a potential target for inhibiting the trafficking of DCs to lymphoid organs and inflamed tissues and decreasing the inflammatory response attenuating symptoms of certain immunologic and inflammatory disorders such as dermatitis.-Han, H., Liang, X., Ekberg, M., Kritikou, J. S., Brunnström, Å., Pelcman, B., Matl, M., Miao, X., Andersson, M., Yuan, X., Schain, F., Parvin, S., Melin, E., Sjöberg, J., Xu, D., Westerberg, L. S., Björkholm, M., Claesson, H.-E. Human 15-lipoxygenase-1 is a regulator of dendritic-cell spreading and podosome formation.

Preferential Generation of 15-HETE-PE Induced by IL-13 Regulates Goblet Cell Differentiation in Human Airway Epithelial Cells.

Type 2-associated goblet cell hyperplasia and mucus hypersecretion are well known features of asthma. 15-Lipoxygenase-1 (15LO1) is induced by the type 2 cytokine IL-13 in human airway epithelial cells (HAECs) in vitro and is increased in fresh asthmatic HAECs ex vivo. 15LO1 generates a variety of products, including 15-hydroxyeicosatetraenoic acid (15-HETE), 15-HETE-phosphatidylethanolamine (15-HETE-PE), and 13-hydroxyoctadecadienoic acid (13-HODE). In this study, we investigated the 15LO1 metabolite profile at baseline and after IL-13 treatment, as well as its influence on goblet cell differentiation in HAECs. Primary HAECs obtained from bronchial brushings of asthmatic and healthy subjects were cultured under air-liquid interface culture supplemented with arachidonic acid and linoleic acid (10 µM each) and exposed to IL-13 for 7 days. Short interfering RNA transfection and 15LO1 inhibition were applied to suppress 15LO1 expression and activity. IL-13 stimulation induced expression of 15LO1 and preferentially generated 15-HETE-PE in vitro, both of which persisted after removal of IL-13. 15LO1 inhibition (by short interfering RNA and chemical inhibitor) decreased IL-13-induced forkhead box protein A3 (FOXA3) expression and enhanced FOXA2 expression. These changes were associated with reductions in both mucin 5AC and periostin. Exogenous 15-HETE-PE stimulation (alone) recapitulated IL-13-induced FOXA3, mucin 5AC, and periostin expression. The results of this study confirm the central importance of 15LO1 and its primary product, 15-HETE-PE, for epithelial cell remodeling in HAECs.

Early growth response gene (EGR)-1 regulates leukotriene D4-induced cytokine transcription in Hodgkin lymphoma cells.

Classical Hodgkin lymphoma (cHL) has a unique pathological feature characterized by a minority of malignant Hodgkin Reed-Sternberg (H-RS) cells surrounded by numerous inflammatory cells. Cysteinyl-leukotrienes (CysLTs) are produced by eosinophils, macrophages and mast cells in the HL tumor microenvironment. In the present study we have explored the signal transduction pathways leading to leukotriene (LT) D4 induced expression of cytokines in the Hodgkin lymphoma cell line L1236 and KM-H2. Stimulation of L1236 and KM-H2 cells with LTD4 led to a concentration- and time-dependent increase at the transcriptional level of tumor necrosis factor-alpha (TNF-α), interleukin (IL)-6, IL-8, chemokine (C-C motif) ligand 3 (CCL3) and CCL4. The expression of several transcription factors was induced upon stimulation of Hodgkin cell lines with LTD4. Among these, EGR-1 was required for cytokine production. Inhibition of EGR-1 expression using shEGR-1 transduced by lentivirus led to suppression of the expression of TNF-α and IL-6. The effect of LTD4 on the expression of transcription factors and cytokines were also blocked by the specific CysLT1 receptor antagonist zafirlukast. These results demonstrate that EGR-1 plays a critical role in LTD4-induced cytokine transcription in Hodgkin cell lines.

On the biosynthesis of 15-HETE and eoxin C4 by human airway epithelial cells.

Several lines of evidence indicate that 15-lipoxygenase type 1 (15-LO-1) plays a pathophysiological role in asthma. The aim for this study was to investigate the 15-LO-1 expression and activity in primary human airway epithelial cells cultivated on micro-porous filters at air-liquid interface. Incubation of human airway epithelial cells with arachidonic acid led to the formation of 15(S)-hydroxy-eicosatetraenoic acid (15-HETE) and exposing the cells to bacteria or physical injury markedly increased their production of 15-HETE. The cells were also found to convert arachidonic acid to eoxin C4 (EXC4). Subcellular fractionation revealed that the conversion of EXA4 to EXC4 was catalyzed by a soluble glutathione transferase (GST). The GST P1-1 enzyme was found to possess the highest activity of the investigated soluble GSTs. Following IL-4 treatment of airway epithelial cells, microarray analysis confirmed high expression of 15-LO-1 and GST P1-1, and immunohistochemical staining of bronchial biopsies revealed co-localization of 15-LO-1 and GST P1-1 in airway epithelial cells. These results indicate that respiratory infection and cell injury may activate the 15-LO pathway in airway epithelial cells. Furthermore, we also demonstrate that airway epithelial cells have the capacity to produce EXC4.

N-Substituted pyrazole-3-carboxamides as inhibitors of human 15-lipoxygenase.

High-throughput screening was used to find selective inhibitors of human 15-lipoxygenase-1 (15-LOX-1). One hit, a 1-benzoyl substituted pyrazole-3-carboxanilide (1a), was used as a starting point in a program to develop potent and selective 15-LOX-1 inhibitors.

3-Substituted pyrazoles and 4-substituted triazoles as inhibitors of human 15-lipoxygenase-1.

Investigation of 1N-substituted pyrazole-3-carboxanilides as 15-lipoxygenase-1 (15-LOX-1) inhibitors demonstrated that the 1N-substituent was not essential for activity or selectivity. Additional halogen substituents on the pyrazole ring, however, increased activity. Further development led to triazole-4-carboxanilides and 2-(3-pyrazolyl) benzoxazoles, which are potent and selective 15-LOX-1 inhibitors.

The influence of aspirin on release of eoxin C4, leukotriene C4 and 15-HETE, in eosinophilic granulocytes isolated from patients with asthma.

BACKGROUND: The effect of aspirin on the release of key arachidonic acid metabolites in activated eosinophils from subjects with aspirin-intolerant asthma (AIA) has not been investigated previously, despite the characteristic eosinophilia in AIA. METHODS: Peripheral blood eosinophils were isolated from four groups of subjects: healthy volunteers (HV; n = 8), mild asthma (MA; n = 8), severe asthma (SA; n = 9) and AIA (n = 7). In the absence or presence of lysine-aspirin, eosinophils were stimulated with arachidonic acid or calcium ionophore to trigger the 15-lipoxygenase-1 (15-LO) and 5-lipoxygenase (5-LO) pathways, respectively. 15(S)-hydroxy-eicosatetraenoic acid (15-HETE) and eoxin C4 (EXC4) were measured as 15-LO products and leukotriene (LT)C4 as a product of the 5-LO pathway. RESULTS: Activated eosinophils from patients with SA and AIA produced approximately five times more 15-HETE than eosinophils from HV or MA patients. In the presence of lysine-aspirin, eosinophils from AIA, MA and SA patients generated higher levels of 15-HETE than in the absence of lysine-aspirin. Furthermore, in the presence of lysine-aspirin, formation of EXC4 was also significantly increased in eosinophils from AIA patients, and LTC4 synthesis was increased both in AIA and SA patients. CONCLUSIONS: Taken together, this study shows an increased release of the recently discovered lipid mediator EXC4, as well as the main indicator of 15-LO activity, 15-HETE, in activated eosinophils from severe and aspirin-intolerant asthmatics, and also elevated EXC4 and LTC4 formation in eosinophils from AIA patients after cellular activation in the presence of lysine-aspirin. The findings support a pathophysiological role of the 15-LO pathway in SA and AIA.

Epigenetic and transcriptional control of the 15-lipoxygenase-1 gene in a Hodgkin lymphoma cell line.

Lipoxygenases oxidatively metabolize polyunsaturated fatty acids to a rich spectrum of biologically active metabolites. The present study aimed at delineating the transcriptional and epigenetic mechanisms leading to 15-lipoxygenase-1 (15-LOX-1) expression in the Hodgkin lymphoma (HL) cell line L1236. Examination of the 15-LOX-1 5' promoter region demonstrated three putative binding sites for signal transducer and activator of transcription (STAT6) within the proximal 1200 base pairs relative to the start codon. Analysis by serial promoter deletions and STAT6 binding site mutations indicated that all three STAT6 binding sites are required for full activation of the 15-LOX-1 promoter. Chromatin immunoprecipitation assay demonstrated that these regions were occupied by STAT6 in L1236 (15-LOX-1 positive) but not in L428 (15-LOX-1 negative) cultured HL cells. Furthermore, DNA hypomethylation and histone hyperacetylation were detectable within the core promoter region of 15-LOX-1 only in L1236 cells but not L428 cells. Taken together, our data indicate that STAT6 activation and chromatin remodeling by DNA demethylation and histone acetylation are crucial for transcriptional activation of 15-LOX-1 in cultured HL cells. These prerequisites are fulfilled in the L1236 cell line, but not in the L428 cell line.

Eoxins are proinflammatory arachidonic acid metabolites produced via the 15-lipoxygenase-1 pathway in human eosinophils and mast cells.

Human eosinophils contain abundant amounts of 15-lipoxygenase (LO)-1. The biological role of 15-LO-1 in humans, however, is unclear. Incubation of eosinophils with arachidonic acid led to formation of a product with a UV absorbance maximum at 282 nm and shorter retention time than leukotriene (LT)C4 in reverse-phase HPLC. Analysis with positive-ion electrospray tandem MS identified this eosinophil metabolite as 14,15-LTC4. This metabolite could be metabolized to 14,15-LTD4 and 14,15-LTE4 in eosinophils. Because eosinophils are such an abundant source of these metabolites and to avoid confusion with 5-LO-derived LTs, we suggest the names eoxin (EX)C4, -D4, and -E4 instead of 14,15-LTC4, -D4, and -E4, respectively. Cord blood-derived mast cells and surgically removed nasal polyps from allergic subjects also produced EXC4. Incubation of eosinophils with arachidonic acid favored the production of EXC4, whereas challenge with calcium ionophore led to exclusive formation of LTC4. Eosinophils produced EXC4 after challenge with the proinflammatory agents LTC4, prostaglandin D2, and IL-5, demonstrating that EXC4 can be synthesized from the endogenous pool of arachidonic acid. EXs induced increased permeability of endothelial cell monolayer in vitro, indicating that EXs can modulate and enhance vascular permeability, a hallmark of inflammation. In this model system, EXs were 100 times more potent than histamine and almost as potent as LTC4 and LTD4. Taken together, this article describes the formation of proinflammatory EXs, in particular in human eosinophils but also in human mast cells and nasal polyps.

COX-1 dependent biosynthesis of 15-hydroxyeicosatetraenoic acid in human mast cells.

15-hydroxyeicosatetraenoic acid (15-HETE) is an arachidonic acid derived lipid mediator which can originate both from 15-lipoxygenase (15-LOX) activity and cyclooxygenase (COX) activity. The enzymatic source determines the enantiomeric profile of the 15-HETE formed. 15-HETE is the most abundant arachidonic acid metabolite in the human lung and has been suggested to influence the pathophysiology of asthma. Mast cells are central effectors in asthma, but there are contradictory reports on whether 15-HETE originates from 15-LOX or COX in human mast cells. This prompted the current study where the pathway of 15-HETE biosynthesis was examined in three human mast cell models; the cell line LAD2, cord blood derived mast cells (CBMC) and tissue isolated human lung mast cells (HLMC). Levels and enantiomeric profiles of 15-HETE and levels of the downstream metabolite 15-KETE, were analyzed by UPLC-MS/MS after stimulation with anti-IgE or calcium ionophore A23187 in the presence and absence of inhibitors of COX isoenzymes. We found that 15-HETE was produced by COX-1 in human mast cells under these experimental conditions. Unexpectedly, chiral analysis showed that the 15(R) isomer was predominant and gradually accumulated, whereas the 15(S) isomer was metabolized by the 15-hydroxyprostaglandin dehydrogenase. We conclude that during physiological conditions, i.e., without addition of exogenous arachidonic acid, both enantiomers of 15-HETE are produced by COX-1 in human mast cells but that the 15(S) isomer is selectively depleted by undergoing further metabolism. The study highlights that 15-HETE cannot be used as an indicator of 15-LOX activity for cellular studies, unless chirality and sensitivity to pharmacologic inhibition is determined.

Hodgkin Lymphoma Monozygotic Triplets Reveal Divergences in DNA Methylation Signatures.

We studied DNA methylation profiles in four different cell populations from a unique constellation of monozygotic triplets in whom two had developed Hodgkin Lymphoma (HL). We detected shared differences in DNA methylation signatures when comparing the two HL-affected triplets with the non-affected triplet. The differences were observed in naïve B-cells and marginal zone-like B-cells. DNA methylation differences were also detected when comparing each of the HL-affected triplets against each other. Even though we cannot determine whether treatment and/or disease triggered the observed differences, we believe our data are important on behalf of forthcoming studies, and that it might provide important clues for a better understanding of HL pathogenesis.

15-Lipoxygenase-1 induces expression and release of chemokines in cultured human lung epithelial cells.

15-Lipoxygenase-1 (15-LOX-1) has been proposed to be involved in various physiological and pathophysiological activities such as inflammation, atherosclerosis, cell maturation, and tumorigenesis. Asthma and chronic obstructive pulmonary disease are associated with increased expression of 15-LOX-1 in bronchial epithelial cells, but the potential functions of 15-LOX-1 in airway epithelial cells have not been well clarified. To study the function of 15-LOX-1 in bronchial epithelial cells, we ectopically expressed 15-LOX-1 in the human lung epithelial cell line A549. We found that overexpression of 15-LOX-1 in A549 cells leads to increased release of the chemokines MIP-1alpha, RANTES, and IP-10, and thereby to increased recruitment of immature dendritic cells, mast cells, and activated T cells. These results suggest that an increased expression and activity of 15-LOX-1 in lung epithelial cells is a proinflammatory event in the pathogenesis of asthma and other inflammatory lung disorders.

High expression of 5-lipoxygenase in normal and malignant mantle zone B lymphocytes.

BACKGROUND: Human B lymphocytes can produce leukotriene B4 but the biological function of the 5-lipoxygenase (5-LO) pathway in B cells is unclear. In order to better understand and define the role of 5-LO in B cells, we investigated the expression of 5-LO mRNA and protein in subsets of B cells from human tonsils and different types of B cell lymphoma. RESULTS: Based on RT-PCR and western blot/immunohistochemical staining, with a polyclonal antibody raised against 5-LO, high expression of 5-LO was found in mantle zone B cells from tonsils. By contrast, only a weak expression of 5-LO was detected in germinal centre cells and no expression in plasma cells from tonsils. This pattern of 5-LO expression was preserved in malignant lymphoma with high expression in mantle B cell lymphoma (MCL) and weak or no expression in follicular lymphoma. Primary leukemized MCL, so called B-prolymphocytic leukaemia cells, and MCL cell lines also expressed 5-LO and readily produced LTB4 after activation. CONCLUSION: The present report demonstrates the expression of 5-LO mainly in normal and malignant mantle zone B cells while the expression is low or absent in germinal centre B cells and plasma cells, indicating a role of the 5-LO pathway in B cells before the cells finally differentiate to plasma cells.

On the biosynthesis and biological role of eoxins and 15-lipoxygenase-1 in airway inflammation and Hodgkin lymphoma.

This mini-review is focused on the enzyme 15-lipoxygenase-1 (15-LO-1) and eoxins in airway inflammatory diseases and Hodgkin lymphoma. Several studies have demonstrated increased expression and activity of 15-LO-1 in the respiratory tissue from asthma patients , indicating a pathophysiological role of this enzyme in airway inflammation. Eoxins were recently identified as pro-inflammatory metabolites of arachidonic acid, formed through the 15-LO-1 pathway, in human eosinophils, mast cells, airway epithelial cells and Hodgkin lymphoma. Mice deficient of 12/15-LO, the ortholog to human 15-LO-1, had an attenuated allergic airway inflammation compared to wild type controls, also indicating a pathophysiological role of this enzyme in respiratory inflammation. The putative therapeutic implications of 15-LO-1 inhibitors in the treatment of asthma, chronic obstructive pulmonary disorder and Hodgkin lymphoma are discussed.

Mice lacking 12/15-lipoxygenase have attenuated airway allergic inflammation and remodeling.

Arachidonate 15-lipoxygenase (LO)-1 has been implicated in allergic inflammation and asthma. The overall effect of 15-LO in allergic inflammation in vivo is, however, unclear. This study investigates systemic allergen sensitization and local allergic airway inflammation and remodeling in mice lacking the murine 12/15-LO, the ortholog to human 15-LO-1. Upon systemic sensitization with intraperitoneal ovalbumin, 12/15-LO-/- mice produced elevated levels of allergen-specific immunoglobulin E compared with wild-type (Wt) controls. However, when challenged with repeated aerosolized allergen, sensitized 12/15-LO-/- mice had an impaired development of airway allergic inflammation compared with Wt controls, as indicated by reduced bronchoalveolar lavage fluid leukocytes (eosinophils, lymphocytes, macrophages) and Th2 cytokines (IL-4, IL-5, IL-13), as well as tissue eosinophils. Allergen-induced airway epithelial proliferation was also significantly attenuated in 12/15-LO-/- mice, whereas goblet cell hyperplasia was unaffected. However, 12/15-LO-/- mice had significantly reduced luminal mucus secretions compared with Wt controls. The repeated allergen challenges resulted in a dramatic increase of alpha-smooth muscle actin-positive alveolar cells in the peripheral airways, a phenomenon that was significantly less developed in 12/15-LO-/- mice. In conclusion, our data suggest that 12/15-LO-/- mice, although having a fully developed systemic sensitization, did not establish a fully developed allergic airway inflammation and associated manifestations of central and peripheral airway remodeling. These data suggest that 12/15-LO-derived metabolites play an important pathophysiologic role in allergen-induced inflammation and remodeling. Hence, pharmacologic targeting of the human 15-LO-1 may represent an attractive therapeutic strategy to control inflammation and remodeling in asthma.

Expression of 15-lipoxygenase type-1 in human mast cells.

Mast cells play a key role in the pathophysiology of asthma. These cells exert their effector functions by releasing a variety of proinflammatory and immunoregulatory compounds. Mast cells infiltrate the bronchial epithelium and smooth muscle to a higher degree in patients with asthma compared to control subjects. 15-Lipoxygenase type-1 (15-LO-1) is a prooxidant enzyme which is expressed in asthmatic lungs leading to formation of pro- and anti-inflammatory mediators. Here we report that interleukin-4 (IL-4) induced the expression of 15-LO-1 in human cord blood derived mast cells (CBMC) as demonstrated by RT-PCR, western blot and immunocytochemistry. The major metabolite of arachidonic acid formed via the 15-LO pathway in IL-4 treated CBMC was identified as 15-ketoeicosatetraenoic acid (15-KETE, also named 15-oxo-ETE) with smaller amounts of 15-hydroxyeicosatetraenoic acid (15-HETE) as identified by HPLC and mass spectrometry (MS/MS). Furthermore, immunohistochemical stainings demonstrated the expression of 15-LO-1 in mast cells in lung and skin in vivo. Osmotic activation of CBMC with mannitol resulted in activation of the 15-LO-1 pathway. In conclusion, the expression of 15-LO-1 and release of 15-LO-1 derived products by mast cells may contribute to the role of these cells in asthma and other inflammatory diseases.