Receptor-mediated regulation of leukotriene C4 synthase activity in human platelets

Pharmacogenomics of Leukotriene Modifiers: A Systematic Review and Meta-Analysis

Pharmacogenetics research on leukotriene modifiers (LTMs) for asthma has been developing rapidly, although pharmacogenetic testing for LTMs is not yet used in clinical practice. We performed a systematic review and meta-analysis on the impact of pharmacogenomics on LTMs response. Studies published until May 2022 were searched using PubMed, EMBASE, and Cochrane databases. Pharmacogenomics/genetics studies of patients with asthma using LTMs with or without other anti-asthmatic drugs were included. Statistical tests of the meta-analysis were performed with Review Manager (Revman, version 5.4, The Cochrane Collaboration, Copenhagen, Denmark) and R language and environment for statistical computing (version 4.1.0 for Windows, R Core Team, Vienna, Austria) software. In total, 31 studies with 8084 participants were included in the systematic review and five studies were also used to perform the meta-analysis. Two included studies were genome-wide association studies (GWAS), which showed different results. Furthermore, none of the SNPs investigated in candidate gene studies were identified in GWAS. In candidate gene studies, the most widely studied SNPs were ALOX5 (tandem repeats of the Sp1-binding domain and rs2115819), LTC4S-444A/C (rs730012), and SLCO2B1 (rs12422149), with relatively inconsistent conclusions. LTC4S-444A/C polymorphism did not show a significant effect in our meta-analysis (AA vs. AC (or AC + CC): −0.06, 95%CI: −0.16 to 0.05, p = 0.31). AA homozygotes had smaller improvements in parameters pertaining to lung functions (−0.14, 95%CI: −0.23 to −0.05, p = 0.002) in a subgroup of patients with non-selective CysLT receptor antagonists and patients without inhaled corticosteroids (ICS) (−0.11, 95%CI: −0.14 to −0.08, p < 0.00001), but not in other subgroups. Variability exists in the pharmacogenomics of LTMs treatment response. Our meta-analysis and systematic review found that LTC4S-444A/C may influence the treatment response of patients taking non-selective CysLT receptor antagonists for asthma, and patients taking LTMs not in combination with ICS for asthma. Future studies are needed to validate the pharmacogenomic influence on LTMs response.

Zymosan suppresses leukotriene C₄ synthase activity in differentiating monocytes: antagonism by aspirin and protein kinase inhibitors

Cysteinyl leukotrienes (cysLTs) are potent proinflammatory mediators with particular relevance for asthma. However, control of cysLT biosynthesis in the time period after onset of acute inflammation has not been extensively studied. As a model for later phases of inflammation, we investigated regulation of leukotriene (LT) C(4) synthase (LTC(4)S) in differentiating monocytes, exposed for several days to fungal zymosan. Incubations with LTA(4) revealed 20-fold increased LTC(4)S activity during differentiation of monocytic Mono Mac 6 (MM6) cells, which was reduced by 80% in the presence of zymosan (25 μg/ml, 96 h). Zymosan (48 h) similarly attenuated LTC(4)S activity of primary human monocyte-derived macrophages and dendritic cells. Several findings indicate phosphoregulation of LTC(4)S: increased activity during MM6 cell differentiation correlated with reduced phosphorylation of 70-kDa ribosomal protein S6 kinase (p70S6K), which could phosphorylate purified LTC(4)S; the p70S6K inhibitor rapamycin (20 nM) doubled LTC(4)S activity of undifferentiated MM6 cells, and protein kinase A and C inhibitors (H-89, CGP-53353, and staurosporine) reversed the zymosan-induced suppression of LTC(4)S activity. Finally, zymosan (48 h) up-regulated PGE(2) biosynthesis, and aspirin (10 μM) or prostaglandin E(2) (PGE(2)) receptor antagonists counteracted the zymosan effect. Our results suggest a late PGE(2)-mediated phosphoregulation of LTC(4)S during microbial exposure, which may contribute to resolution of inflammation, with implications for aspirin hypersensitivity.

Association of thromboxane A2 receptor (TBXA2R) gene polymorphism in patients with aspirin-intolerant acute urticaria

BACKGROUND

The thromboxane A2 receptor (TBXA2R) is a potent broncho- and vaso-constrictor and is associated with leukotriene synthesis. Polymorphisms in the TBXA2R gene have been linked to atopy, asthma, and atopic dermatitis. This study evaluated the association between genetic TBXA2R variants and the development of acetyl salicylic acid (ASA)-intolerant acute urticaria (AIAU).

METHODS

AIAU patients (n=167), ASA-intolerant chronic urticaria (AICU) patients (n=149), and healthy controls (NC) (n=265) were included. All patients were enrolled at Ajou University Hospital in Suwon, Korea. Two TBXA2R polymorphisms (-4684T>C and 795T>C) were genotyped by primer extension using a SNAPshot ddNTP primer extension kit. Luciferase activity was measured using a dual-luciferase reporter assay kit. An electrophoretic mobility shift assay (EMSA) was performed using a nuclear extract from a human mast cell line (HMC-1).

RESULTS

Genetic association data demonstrated that compared with NC subjects, AIAU patients had a significantly higher frequency of the homozygous TT genotype of TBXA2R-4684T>C (P=0.005, P(corr) =0.03). No differences were identified between the AICU and the NC groups. Luciferase activity, reflecting promoter activity, was significantly lower with the TBXA2R-4684T-containing construct than with the -4684C-containing construct (P<0.001); the activity decreased further upon co-transfection with ETS-like gene transcription factor-1 (ELK-1) (P=0.012). EMSA revealed that the -4684T allele produced a specific shifted band, with a greater affinity than that produced by the -4684C allele.

CONCLUSION AND CLINICAL RELEVANCE

These results suggest that the TBXA2R-4684T allele may be associated with lower TBXA2R expression, which may contribute to the development of the AIAU phenotype.

Abnormal LTC4 synthase RNA degradation in neutrophils from CML patients

Neutrophils from patients with chronic myeloid leukaemia (CML) have an aberrant expression of leukotriene (LT)C4 synthase. In order to learn more about the regulation of this abnormality, LTC4 synthase mRNA expression was determined by reverse transcription polymerase chain reaction. A digoxigenin (DIG)-labelled LTC4 synthase RNA was synthesized and incubated in cytosolic extracts from CML neutrophils, normal neutrophils and eosinophils. LTC4 synthase mRNA was detected in total but not cytoplasmic RNA from normal neutrophils. In contrast, LTC4 synthase mRNA was found in the cytoplasm of CML neutrophils and in normal eosinophils, which also express the enzyme. The DIG-labelled LTC4 synthase RNA was, as opposed to normal neutrophils, degraded in cytosolic extracts from CML neutrophils. The degradation was time dependent and cell concentration dependent. Degradation was also seen in eosinophils, indicating that degradation of LTC4 synthase RNA was correlated to the expression of the protein. This study showed that the difference in expression of LTC4 synthase in normal and CML neutrophils was not because of a total lack of LTC4 synthase mRNA in normal neutrophils. However normal neutrophils lack, in contrast to CML neutrophils, LTC4 synthase mRNA in the cytoplasm. This discrepancy is not caused by a stabilized LTC4 synthase RNA in the cytosol of CML neutrophils. Instead an abnormal degradation of LTC4 synthase RNA was found in the cytosol of CML neutrophils.

Allelic association and functional studies of promoter polymorphism in the leukotriene C4 synthase gene (LTC4S) in asthma

BACKGROUND

LTC4 synthase is essential for the production of cysteinyl leukotrienes (Cys-LT), critical mediators in asthma. We have identified a novel promoter polymorphism at position -1072 (G/A) and a -444 (A/C) polymorphism has previously been reported. The role of these polymorphisms in the genetic susceptibility to asthma was examined.

METHODS

To test for genetic association with asthma phenotypes, 341 white families (two asthmatic siblings) and 184 non-asthmatic control subjects were genotyped. Genetic association was assessed using case control and transmission disequilibrium test (TDT) analyses. LTC4S promoter luciferase constructs and transiently transfected human HeLa and KU812F cells were generated to determine the functional role of these polymorphisms on basal transcription.

RESULTS

No associations were observed in case control analyses (-1072 A, q=0.09; -444 C, q=0.29); the TDT identified a borderline association between the -444 C allele and bronchial responsiveness to methacholine (p=0.065). Asthmatic children with the -444 C allele had a lower mean basal forced expiratory volume in 1 second (97.4 v 92.7% predicted, p=0.005). LTC4S promoter luciferase analyses provided no evidence for a functional role of either polymorphism in determining basal transcription.

CONCLUSION

This study does not support a role for these polymorphisms in genetic susceptibility to asthma but provides evidence to suggest a role in determining lung function parameters.

Inverse relationship between myeloid maturation and leukotriene C4 synthase expression in normal and leukemic myelopoiesis-consistent overexpression of the enzyme in myeloid cells from patients with chronic myeloid leukemia

OBJECTIVE

Leukotriene (LT) C(4) synthase (LTC(4)S) is the key enzyme in the biosynthesis of LTC(4), which has been reported to stimulate the growth of human myeloid progenitor cells and is specifically overproduced in chronic myeloid leukemia (CML). The aim of this study was to clarify the expression of LTC(4)S during normal and leukemic myelopoiesis and to investigate the correlation between abnormal LTC(4)S expression in CML myeloid cells and the activity of the disease-specific tyrosine kinase p210 BCR-ABL.

MATERIALS AND METHODS

Immature and mature myeloid cell subpopulations were isolated with magnetic cell sorting from healthy volunteer bone marrow (n = 11) and CML patient peripheral blood (n = 8), respectively. The cells were subjected to analysis of LTC(4)S protein expression and activity. Expression of LTC(4)S was investigated in CD16(+) neutrophils from CML patients before and after 1 month of medication with imatinib mesylate (STI571), which is a specific inhibitor of p210 BCR-ABL.

RESULTS

Among normal cells, the highest enzyme activity was observed in the most immature, CD34(+) progenitor cell-enriched and CD15(+) myelocyte-enriched fractions. Subsequently, LTC(4)S activity decreased with increasing maturity, with only negligible amounts of LTC(4) produced in CD16(+) neutrophils. LTC(4)S was expressed at the protein level in the immature myeloid cell fractions but not in CD16(+) cells. In CML cells, LTC(4)S activity and expression were consistently elevated. Thus, the CML CD34(+) and CD15(+) cell fractions, as well as the CD11b(+) myelocyte/metamyelocyte-enriched fractions, produced 6 to 10 times as much LTC(4) as the corresponding normal cells. Again, enzyme expression was highest in the most immature cells, although evident LTC(4)S expression and activity remained in CML CD16(+) neutrophils. Interestingly, treatment of five CML patients with imatinib mesylate down-regulated the abnormal neutrophil LTC(4)S expression and activity.

CONCLUSIONS

Expression of LTC(4)S in immature myelopoid cells is in line with a role for this enzyme in myelopoiesis. In addition, consistent overexpression of LTC(4)S in CML and the correlation to p210 BCR-ABL activity suggests that LTC(4)S may be involved in leukemic pathogenesis.

Uncoupled regulation of leukotriene C4 synthase in platelets from aspirin-intolerant asthmatics and healthy volunteers after aspirin treatment

BACKGROUND

We have reported that thromboxane A2 induces suppression of leukotriene (LT) C4 synthase activity in human platelets.

AIM

In the present study, we describe a mechanism whereby aspirin treatment can lead to increased formation of LTC4, which is a potent bronchoconstrictor and inflammatory mediator. This mechanism is also demonstrated to be present in platelets from aspirin-intolerant asthmatics (AIA).

METHODS

The effect of arachidonic acid or platelet agonists on LTC4 synthase activity was investigated in platelets obtained from healthy volunteers, aspirin-intolerant asthmatics or aspirin-tolerant asthmatics after in vivo treatment or in vitro pre-incubation with aspirin.

RESULTS

Incubation of normal platelets with arachidonic acid or collagen provoked approximately 50% reduction of platelet LTC4 synthase activity, as determined by the conversion of LTA4 to LTC4. However, the inhibitory effect of arachidonic acid or collagen was not observed after oral administration of aspirin prior to collection of the platelets. Arachidonic acid-induced inhibition of LTC4 synthase activity was totally abolished in platelets collected from peripheral blood already 30 min after aspirin ingestion but was fully restored in platelets collected 3 to 7 days after the administration of aspirin. Treatment of platelet suspensions with aspirin in vitro dose-dependently counteracted the suppressive effect of arachidonic acid on LTC4 formation, with total reversal at approximately 40 microm. In contrast, the major aspirin metabolite, salicylic acid did not alter arachidonic acid-induced reduction of LTC4 synthase activity. Similarly, LTC4 synthase activity in platelets from AIA and aspirin-tolerant asthmatics (ATA) was reduced by approximately 50% after pre-treatment with arachidonic acid in vitro. Again the inhibitory effect was abolished when platelets were pre-incubated in the presence of aspirin.

CONCLUSION

The results indicate that oral aspirin administration can lead to uncoupling of thromboxane A2-dependent negative feedback mechanisms, which may normally restrict the production of cysteinyl leukotrienes. This mechanism can be of potential interest in aspirin-induced asthma.

Inhibitory effects of helenalin and related compounds on 5-lipoxygenase and leukotriene C(4) synthase in human blood cells

The sesquiterpene lactone helenalin, which can be isolated from several plant species of the Asteraceae family, is a potent anti-inflammatory and antineoplastic agent. In agreement, alcohol extracts of these plants are used for local external treatment of inflammatory conditions. Since leukotrienes are important mediators in inflammatory processes, the inhibitory effects of helenalin and some derivatives on leukotriene (LT) biosynthesis were studied. Treatment of human platelets with helenalin provoked irreversible inhibition of LTC(4) synthase in a concentration- and time-dependent manner with an IC(50) of 12 microM after a 60 min preincubation. 11alpha,13-Dihydrohelenalin acetate was less potent. Interestingly, individual donors could be divided into two distinct groups with respect to the efficacy of helenalin to suppress platelet LTC(4) synthase. In human granulocytes, helenalin inhibited both the 5-lipoxygenase (IC(50) 9 microM after 60 min preincubation) and LTC(4) synthase in a concentration- and time-dependent fashion. In contrast, the drug was without effect on LTA(4) hydrolase. The GSH-containing adducts (2beta-(S-glutathionyl)-2,3-dihydrohelenalin and 2beta-(S-glutathionyl)-2,3,11alpha,13-tetra hydrohelenalin acetate) did not significantly inhibit LTC(4) synthase. The present results indicate a mechanism for the anti-inflammatory effect of helenalin and related compounds.

Sp1 and Sp3 function as key regulators of leukotriene C(4) synthase gene expression in the monocyte-like cell line, THP-1

The goal of this study was to examine the mechanisms of leukotriene C(4) (LTC(4)) synthase gene expression in mononuclear phagocytes. Transfection of the monocyte-like cell line THP-1 with LTC(4) synthase promoter-reporter constructs demonstrated that the first 1.3 kb of the promoter mediated a 21.1-fold increase in reporter activity. Deletion analysis revealed that the region between -92 and -23 bp, which contains a signal protein (Sp)1 consensus site at -42 to -37 bp, mediated an 11.5-fold increase in reporter activity. Using a probe from -56 to -17 bp, electrophoretic mobility shift assays (EMSAs) demonstrated that Sp1 and THP-1 and HeLa nuclear extracts bind to this region. Binding was eliminated by mutation of the Sp1 consensus site. Supershift EMSAs using anti-Sp1 and anti-Sp3 antibodies demonstrated that these Sp family members bind to the region. Transfection of the Sp-deficient Drosophila SL-2 cell line with a construct containing the -92 to -23 bp promoter region and Sp expression vectors revealed that Sp1 and Sp3 transactivate gene transcription. We conclude that the Sp1 site is a necessary element for LTC(4) synthase gene transcription. Sp1 and Sp3 function through this site to positively regulate transcription. Thus, we provide evidence that the LTC(4) synthase gene is transcriptionally regulated in mononuclear phagocytes.

Impaired leukotriene C4 generation in granulocytes from protein-energy malnourished chronically ill elderly

OBJECTIVE

The dysregulation of the immune and inflammatory systems observed in protein-energy malnutrition (PEM) may be partly due to perturbation of essential fatty acid metabolism. In this study, we assessed the calcium ionophore A23187-induced generation of the arachidonate metabolites leukotriene B4 (LTB4) and leukotriene C4 (LTC4) in isolated granulocyte suspensions.

DESIGN

Case-control study.

SETTING

A university-affiliated acute care hospital in urban Stockholm.

SUBJECTS

Fourteen severely malnourished elderly subjects with stable non-malignant disorders (age 74 +/- 1 years, mean +/- SEM) and 12 healthy age-matched controls were examined.

MAIN OUTCOME MEASURES

Leukotrienes were analysed by high-performance liquid chromatography. Body mass index (BMI, kg m-2) and delayed cutaneous hypersensitivity (DCH) reaction were determined.

RESULTS

BMI was 16. 5 +/- 0.5 and 26.2 +/- 0.9 kg m-2 (mean +/- SE) in the malnourished group and controls (P < 0.001), respectively. DCH was 8.5 mm (median) in patients and 29.5 mm in controls (P < 0.001). LTC4 generation in granulocytes from PEM patients was half of that of controls (9.1 +/- 2.0 vs. 17.8 +/- 5.2 pmol mL-1, P < 0.05) when cells were stimulated with 0.2 micromol L-1 of A23187, and 13.7 +/- 2.5 and 27.2 +/- 7.5 pmol mL-1, respectively (NS), upon stimulation with 1.0 micromol L-1 of A23187. LTB4 production in PEM patients and controls did not differ at any of the two calcium ionophore concentrations. LTC4 production correlated with BMI (r = 0.41, P < 0.05), but there was no significant correlation between DCH and LTB4 or LTC4 production.

CONCLUSION

Protein-energy malnutrition is accompanied by perturbation of leukotriene synthesis, which may be one factor underlying the dysregulation of inflammatory responses in the depleted patient.

15-Lipoxygenation of leukotriene A(4). Studies Of 12- and 15-lipoxygenase efficiency to catalyze lipoxin formation

The unstable epoxide leukotriene (LT) A(4) is a key intermediate in leukotriene biosynthesis, but may also be transformed to lipoxins via a second lipoxygenation at C-15. The capacity of various 12- and 15-lipoxygenases, including porcine leukocyte 12-lipoxygenase, a human recombinant platelet 12-lipoxygenase preparation, human platelet cytosolic fraction, rabbit reticulocyte 15-lipoxygenase, soybean 15-lipoxygenase and human eosinophil cytosolic fraction, to catalyze conversion of LTA(4) to lipoxins was investigated and standardized against the ability of the enzymes to transform arachidonic acid to 12- or 15-hydroxyeicosatetraenoic acids (HETE), respectively. The highest ratio between the capacity to produce lipoxins and HETE (LX/HETE ratio) was obtained for porcine leukocyte 12-lipoxygenase with an LX/HETE ratio of 0.3. In addition, the human platelet 100000xg supernatant 12-lipoxygenase preparation and the human platelet recombinant 12-lipoxygenase and human eosinophil 100000xg supernatant 15-lipoxygenase preparation possessed considerable capacity to produce lipoxins (ratio 0.07, 0.01 and 0.02 respectively). In contrast, lipoxin formation by the rabbit reticulocyte and soybean 15-lipoxygenases was much less pronounced (LX/HETE ratios <0.002). Kinetic studies of the human lipoxygenases revealed lower apparent K(m) for LTA(4) (9-27 microM), as compared to the other lipoxygenases tested (58-83 microM). The recombinant human 12-lipoxygenase demonstrated the lowest K(m) value for LTA(4) (9 microM) whereas the porcine leukocyte 12-lipoxygenase had the highest V(max). The profile of products was identical, irrespective of the lipoxygenase used. Thus, LXA(4) and 6S-LXA(4) together with the all-trans LXA(4) and LXB(4) isomers were isolated. Production of LXB(4) was not observed with any of the lipoxygenases. The lipoxygenase inhibitor cinnamyl-3,4-dihydroxy-alpha-cyanocinnamate was considerably more efficient to inhibit conversion of LTA(4) to lipoxins, as compared to the inhibitory effect on 12-HETE formation from arachidonic acid (IC(50) 1 and 50 microM, respectively) in the human platelet cytosolic fraction.

Aberrant expression of active leukotriene C4 synthase in CD16+ neutrophils from patients with chronic myeloid leukemia

Elevated leukotriene (LT)C4 synthase activity was observed in peripheral blood granulocyte suspensions from patients with chronic myeloid leukemia (CML). Magnetic cell sorting (MACS) with CD16 monoclonal antibodies (mAbs), which were used to fractionate granulocytes from CML patients and healthy individuals, yielded highly purified suspensions of CD16+ neutrophils. The purity of these cell fractions was verified by extensive morphologic examination. Reverse transcriptase–polymerase chain reaction (RT-PCR) analyses, demonstrating the absence of interleukin-4 messenger RNA (IL-4 mRNA), further confirmed the negligible contamination of eosinophils in these fractions. Notably, purified CML CD16+ neutrophils from all tested patients transformed exogenous LTA4 to LTC4. These cells also produced LTC4 after activation with ionophore A23187 or the chemotactic peptide fMet-LeuPhe (N-formylmethionyl-leucyl-phenylalanine). Subcellular fractionation revealed that the enzyme activity was exclusively distributed to the microsomal fraction. Expression of LTC4 synthase mRNA in CML CD16+neutrophils was confirmed by RT-PCR. Furthermore, Western blot analyses consistently demonstrated expression of LTC4 synthase at the protein level in CML CD16+ neutrophils, whereas expression of microsomal glutathione S-transferase 2 occurred occasionally. Expectedly, LTC4 synthase activity or expression of the protein could not be demonstrated in CD16+ neutrophil suspensions from any of the healthy individuals. Instead, these cells, as well as CML CD16+neutrophils, transformed LTA4 to LTB4. The results indicate that aberrant expression of LTC4 synthase is a regular feature of morphologically mature CML CD16+neutrophils. This abnormality, possibly associated with malignant transformation, can lead to increased LTC4 synthesis in vivo. Such overproduction may be of pathophysiological relevance because LTC4 has been demonstrated to stimulate proliferation of human bone marrow–derived myeloid progenitor cells.

LTC4 synthase. Enzymology, biochemistry, and molecular characterization

LTC4S conjugates reduce glutathione to LTA4 and is positioned as the pivotal and only committed enzyme involved in the formation of cysteinyl LTs. Despite its function as an enzyme that conjugates glutathione to LTA4, it is abundantly clear that LTC4S differs from the classic glutathione S-transferase (GST) families. This distinction is based on narrow substrate specificity, inability to conjugate GSH to xenobiotics, differential susceptibility to inhibitors, lack of homology, and failure to be immunorecognized by specific microsomal GST antibodies. The presence of LTC4S protein is restricted to a limited number of hematopoietic cells to include mast cells, eosinophils, basophils, monocytes/macrophages, and platelets, with the platelet being unique in its lack of the complete biosynthetic pathway for cysteinyl LTs. The purification of the protein and the cloning of the cDNA have demonstrated that the kinetic parameters of LTC4S are similar for the isolated natural or recombinant proteins. The protein is an 18-kDa integral perinuclear membrane enzyme, which is functional as a homodimer. The cDNA encodes a 150 amino-acid polypeptide monomer with three hydrophobic domains interspersed by two hydrophilic loops. Homology and secondary structural predictions have revealed that LTC4S is a member of a novel gene family that includes FLAP, mGST II, and mGST III. Each of these molecules is an integral membrane protein with the capacity to participate in LT biosynthesis: LTC4S as the terminal and only committed enzyme in cysteinyl LT formation, FLAP as an arachidonic acid presentation protein, and mGST II and mGST III as unique dual-function enzymes with primary detoxification functions. Site directed mutagenic studies of LTC4S have revealed that two residues, R51 and Y93, are involved in the acid and base catalysis, respectively, of LTA4 and GSH. Alignment of molecules with LTA4 conjugating ability demonstrates conservation of amino acid residues R51 and Y93, which appear necessary for this specific enzymatic function. The 2.5-Kb gene for human LTC4S contains five small exons and four introns, and the 5' UTR contains consensus sequences for AP-1 and AP-2 sites as well as an SP-1 site. The chromosomal localization of this gene is 5q35, distal to that of cytokine, growth factor, and receptor genes that have relevance to the development of allergic inflammation. Furthermore, there is genetic linkage of this region of human chromosome 5 to atopy and asthma, whereas no linkage exists for the chromosomal localization of the other family members, FLAP and mGST II, distinguishing LTC4S as a unique member of the novel gene family. LTC4S is profoundly overexpressed in the aspirin-induced asthmatic phenotype and correlates with overproduction of cysteinyl LTs and bronchial hyperreactivity to lysine aspirin. Ongoing studies are directed to the genomic regulation and additional polymorphisms within the gene of this pivotal enzyme, as well as to further identification of the amino acid residues central to its catalytic function.

TGF-β Increases Leukotriene C4 Synthase Expression in the Monocyte-Like Cell Line, THP-1

The goal of this study was to determine whether cytokines modulate leukotriene C4 (LTC4) synthase expression in mononuclear phagocytes. A panel of cytokines was surveyed for changes in LTC4 synthase mRNA in THP-1 cells. TGF-β1, -2, and -3 had significant stimulatory effects. The addition of TGF-β resulted in a time-dependent increase in LTC4 synthase mRNA at 6 h, which persisted through 48 h. Furthermore, this conditioning resulted in an increase in immunoreactive protein for LTC4 synthase through 7 days. TGF-β conditioning of cells resulted in a time- and dose-dependent increase in stimulated LTC4 synthase activity. Following transient transfection of THP-1 cells with a promoter-reporter construct containing 1.2 kb of the LTC4 synthase promoter, TGF-β treatment resulted in a 2-fold increase in reporter activity. Conditioning with TGF-β did not prolong the half-life of LTC4 synthase mRNA, as assessed by RNase protection assays in actinomycin D-treated cells. Cycloheximide exposure experiments revealed that new protein synthesis was not required for the observed stimulatory effect of TGF-β on LTC4 synthase mRNA. We conclude that LTC4 synthase expression is increased at a transcriptional level by TGF-β in mononuclear phagocytes.

Novel enzymatic abnormalities in AML and CML in blast crisis: elevated leucocyte leukotriene C4 synthase activity paralleled by deficient leukotriene biosynthesis from endogenous substrate

Leukotrienes (LT) are inflammatory mediators which can also exert regulatory effects on human myelopoiesis. We have studied the LT-producing capacity of freshly isolated leucocyte suspensions (containing blast cells in variable proportions) from 41 patients with acute myeloid leukaemia (AML) or chronic myeloid leukaemia (CML) in blast crisis (CMLbc) at diagnosis or relapse/resistant disease. Leucocyte suspensions from 19/29 AML patients (66%), and 2/12 CMLbc patients (17%; P = 0.012) demonstrated deficient capacity to synthesize LT from endogenous substrate after ionophore A23187 stimulation. Thus, these cells produced < 8 pmol LTB4+LTC4/10(6) cells (< 20% of mean LT formation in leucocyte suspensions from 18 healthy subjects). Addition of exogenous arachidonic acid did not normalize the LT synthesis in poor-producing cell suspensions. Purified, morphologically mature granulocytes from two AML patients also failed to produce normal amounts of LT. In leucocyte suspensions from the remaining 20 AML/CMLbc patients A23187 provoked LT biosynthesis, with markedly increased production of LTC4, but decreased LTB4 formation. Furthermore, elevated conversion of exogenous LTA4 to LTC4 was noted in the patient samples, independent of their capacity to produce LT after A23187 stimulation. The percentage of blast cells in patient white blood cell differential counts correlated inversely with ionophore-induced LT synthesis, but positively with the conversion of exogenous LTA4 to LTC4. The results suggest elevated LTC4 synthase activity and suppressed 5-lipoxygenase activity as novel enzymatic features of myeloid leukaemia patients with immature phenotype.

Eicosanoid synthesis by purified thrombocytes and erythrocytes from warm- and cold-acclimated American bullfrogs (Rana catesbeiana)

Amphibian blood plays an important role in eicosanoid synthesis. Although clotting frog blood produces eicosanoids, the cellular source of prostaglandins and thromboxanes in bullfrog blood is unknown. Thromboxane (TX)B2 synthesis from purified thrombocytes was affected by 30-day cold-acclimation at 5 degrees, but not PGE2 or leukotriene (LT) synthesis. Although no cyclooxygenase activity has been found in human erythrocytes, frog erythrocytes were capable of forming cyclooxygenase products, but the amounts were lower than those produced by thrombocytes. Additionally, there was no effect of cold exposure on eicosanoid production by isolated erythrocytes. Similar to some mammalian nucleated white blood cells, nucleated bullfrog thrombocytes and erythrocytes produced leukotrienes. The production of eicosanoids by thrombocytes was stimulated by A23187 and thrombin. Erythrocytes were stimulated by A23187. Control synthesis by erythrocytes and thrombocytes was inhibited by 5 microM indomethacin (cyclooxygenase pathway) or nordihydroguaiaretic acid (5-lipoxygenase pathway) and cyclooxygenase products were increased in the presence of nordihydroguaiaretic acid. Thrombin stimulation of eicosanoid production by thrombocytes was inhibited when the inhibitors were present prior to the final centrifugation of the cell isolation. The results suggest that cold exposure can affect eicosanoid synthesis in thrombocytes, but not erythrocytes, and that thrombocytes are a major source of eicosanoids in bullfrogs. The production of cyclooxygenase and lipoxygenase products by nucleated erythrocytes and thrombocytes suggests a role for these compounds in hemostasis and inflammatory responses in these animals.

Regulation of leukotriene-biosynthetic enzymes during differentiation of myelocytic HL-60 cells to eosinophilic or neutrophilic cells

Leukotrienes (LTs) are potent mediators of bronchial inflammation and are predominantly produced by myeloid cells. As myelocytic cells differentiate towards either eosinophils or neutrophils, the profile of leukotrienes they produce upon stimulation diverges. Eosinophils produce mainly cysteinyl leukotrienes whereas neutrophils predominantly synthesize 5(S), 12(R)-dihydroxy-6,8,10,14-eicosatetraenoic acid (LTB delta). The mechanism by which this change in leukotriene composition occurs is unknown. In this study, we investigated the control of leukotriene biosynthetic enzymes during myeloid cell differentiation. Western-blot analyses of myelocytic leukemia cell lines, HL-60#7 and HL-60, differentiated towards eosinophilic or neutrophilic cell types, respectively, demonstrated that as myelocytic cells differentiate towards eosinophils or neutrophils, the protein levels of cytosolic phospholipase A2 (cPLA2) remain constant, whereas 5-lipoxygenase and 5-lipoxygenase-activating protein (FLAP) levels are simultaneously elevated. As myelocytic cells become more eosinophil-like, 5(S)-hydroxy- 6(R)-S-glutathionyl-7,9-trans-11, 14-cis-eicosatetraenoic acid (LTC delta) synthase activity and expression of both the protein and messenger RNA in the cells are dramatically increased (approximately 75-fold), while the LTC delta synthase level and activity in neutrophil-like cells remain constant at very low levels. In contrast, in neutrophilic cells, the amount of 5,6-oxido-7,9,11,14-eicosatetraenoic acid (LTA delta) hydrolase was elevated approximately 100-fold greater than the increase in LTA delta hydrolase from eosinophilic cells. These results indicate that as a myeloid cell differentiates towards a granulocyte, similar mechanisms of regulation may be applied to the leukotriene biosynthetic pathway up to the point at which the pathway diverges. At the stage in the leukotriene biosynthetic pathway where LTA delta may be converted to either LTC delta or to LTB delta, specific regulators of transcription may become activated as a myelocytic cell differentiates, thereby causing increased LTA delta hydrolase or LTC delta synthase expression.

Phorbol ester-induced suppression of leukotriene C4 synthase activity in human granulocytes

The effect of the protein kinase C activator, phorbol 12-myristate 13-acetate (PMA), on the metabolism of exogenous leukotriene (LT)A4 in human granulocytes was investigated. After incubation with LTA4 decreased levels of LTC4 but not LTB4 were observed in granulocyte suspensions pretreated with PMA. This finding could in part be ascribed to oxidative metabolism of LTC4, since PMA induced a rapid degradation of exogenously added LTC4. After blocking of LTC4 metabolism with the H2O2 scavenger catalase, a PMA-provoked suppression of the conversion of LTA4 to LTC4 was observed, indicating PKC-dependent regulation of LTC4 synthase activity. This effect, as well as PMA-induced degradation of LTC4 was prevented by specific protein kinase C inhibitors.