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

Leukotriene biosynthetic enzymes as therapeutic targets

Leukotrienes are powerful immune-regulating lipid mediators with established pathogenic roles in inflammatory allergic diseases of the respiratory tract - in particular, asthma and hay fever. More recent work indicates that these lipids also contribute to low-grade inflammation, a hallmark of cardiovascular, neurodegenerative, and metabolic diseases as well as cancer. Biosynthesis of leukotrienes involves oxidative metabolism of arachidonic acid and proceeds via a set of soluble and membrane enzymes that are primarily expressed by cells of myeloid origin. In activated immune cells, these enzymes assemble at the endoplasmic and perinuclear membrane, constituting a biosynthetic complex. This Review describes recent advances in our understanding of the components of the leukotriene-synthesizing enzyme machinery, emerging opportunities for pharmacological intervention, and the development of new medicines exploiting both antiinflammatory and pro-resolving mechanisms.

The leukotrienes: immune-modulating lipid mediators of disease

The leukotrienes are important lipid mediators with immune modulatory and proinflammatory properties. Classical bioactions of leukotrienes include chemotaxis, endothelial adherence, and activation of leukocytes, chemokine production, as well as contraction of smooth muscles in the microcirculation and respiratory tract. When formed in excess, these compounds play a pathogenic role in several acute and chronic inflammatory diseases, such as asthma, rheumatoid arthritis, and inflammatory bowel disease. An increasing number of diseases have been linked to inflammation implicating the leukotrienes as potential mediators. For example, recent investigations using genetic, morphological, and biochemical approaches have pointed to the involvement of leukotrienes in cardiovascular diseases including atherosclerosis, myocardial infarction, stroke, and abdominal aortic aneurysm. Moreover, new insights have changed our previous notion of leukotrienes as mediators of inflammatory reactions to molecules that can fine-tune the innate and adaptive immune response. Here, we review the most recent understanding of the leukotriene cascade with emphasis on recently identified roles in immune reactions and pathophysiology.

Mechanisms of induction of cytosolic and microsomal glutathione transferase (GST) genes by xenobiotics and pro-inflammatory agents

Glutathione transferase (GST) isoezymes are encoded by three separate families of genes (designated cytosolic, microsomal and mitochondrial transferases), with distinct evolutionary origins, that provide mammalian species with protection against electrophiles and oxidative stressors in the environment. Members of the cytosolic class Alpha, Mu, Pi and Theta GST, and also certain microsomal transferases (MGST2 and MGST3), are up-regulated by a diverse spectrum of foreign compounds typified by phenobarbital, 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene, pregnenolone-16α-carbonitrile, 3-methylcholanthrene, 2,3,7,8-tetrachloro-dibenzo-p-dioxin, β-naphthoflavone, butylated hydroxyanisole, ethoxyquin, oltipraz, fumaric acid, sulforaphane, coumarin, 1-[2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oyl]imidazole, 12-O-tetradecanoylphorbol-13-acetate, dexamethasone and thiazolidinediones. Collectively, these compounds induce gene expression through the constitutive androstane receptor (CAR), the pregnane X receptor (PXR), the aryl hydrocarbon receptor (AhR), NF-E2-related factor 2 (Nrf2), peroxisome proliferator-activated receptor-γ (PPARγ) and CAATT/enhancer binding protein (C/EBP) β. The microsomal T family includes 5-lipoxygenase activating protein (FLAP), leukotriene C(4) synthase (LTC4S) and prostaglandin E(2) synthase (PGES-1), and these are up-regulated by tumour necrosis factor-α, lipopolysaccharide and transforming growth factor-β. Induction of genes encoding FLAP, LTC4S and PGES-1 is mediated by the transcription factors C/EBPα, C/EBPδ, C/EBPϵ, nuclear factor-κB and early growth response-1. In this article we have reviewed the literature describing the mechanisms by which cytosolic and microsomal GST are up-regulated by xenobiotics, drugs, cytokines and endotoxin. We discuss cross-talk between the different induction mechanisms, and have employed bioinformatics to identify cis-elements in the upstream regions of GST genes to which the various transcription factors mentioned above may be recruited.

Tristetraprolin-dependent post-transcriptional regulation of inflammatory cytokine mRNA expression by apolipoprotein A-I: role of ATP-binding membrane cassette transporter A1 and signal transducer and activator of transcription 3

Atherosclerosis is an inflammatory disease characterized by the accumulation of macrophages in the arterial intima. The activated macrophages secreted more pro-inflammatory cytokines, such as tumor necrosis factor (TNF)-α, which promote the development of the disease. Apolipoprotein A-I (apoA-I), the major component of high density lipoprotein, is involved in reverse cholesterol transport of lipid metabolism. Recently, it has been found that apoA-I suppresses inflammation via repression of inflammatory cytokine expression; the mechanisms of the apoA-I-suppressive action, however, are not yet well characterized. In this study, we have for the first time found that apoA-I suppresses the expression of some inflammatory cytokines induced by lipopolysaccharide via a specific post-transcriptional regulation process, namely mRNA destabilization, in macrophages. Our further studies have also shown that AU-rich elements in the 3'-untranslated region of TNF-α mRNA are responsive to the apoA-I-mediated mRNA destabilization. The apoA-I-induced inflammatory cytokine mRNA destabilization was associated with increased expression of mRNA-destabilizing protein tristetraprolin through a JAK2/STAT3 signaling pathway-dependent manner. When blocking interaction of apoA-I with ATP-binding membrane cassette transporter A1 (ABCA1), a major receptor for apoA-I in macrophages, it would almost totally abolish the effect of apoA-I on tristetraprolin expression. These results present not only a novel mechanism for the apoA-I-mediated inflammation suppression in macrophages but also provide new insights for developing strategies for modulating vascular inflammation and atherosclerosis.

A functional Sp1/Egr1-tandem repeat polymorphism in the 5-lipoxygenase gene is not associated with myocardial infarction

Arachidonate 5-lipoxygenase is an enzyme encoded by the ALOX5 gene, and plays an important role in the synthesis of leukotrienes. These are inflammatory mediators, and have been involved in atherosclerosis and other pathological processes that require proinflammatory activities. Human and animal studies have suggested a role for the ALOX5 gene in atherosclerosis, including a significant association between a promoter polymorphism and a carotid intimal-medial thickness in response to dietary fat. This polymorphism was three- to six-tandem repeats of a Sp1/Egr1 binding motif (GGGCGG)(n), and the number of repeats has been linked with the amount of gene expression. We hypothesized that this ALOX5 polymorphism could influence the risk for myocardial infarction (MI). First, we analysed the effect of the four alleles on gene expression by transfecting the HEK-293 cell line with luciferase reporter-constructs. We found that luciferase activities are dependent on the number of the Sp1/Egr1 repeats, with the three and six repeats having the lowest and highest values. We genotyped 312 male MI survivors, aged < 55 years, and 376 healthy controls matched with patients for sex, age, and ethnicity. Ninety-six per cent of the patients were smokers, compared to only 42% among the controls (P < 0.001; OR = 31.84). The 55 + 56 repeat genotypes were less frequent in patients (55 = 56%, 56 = 0.6%) compared to controls (55 = 60%, 56 = 3%). However, these were non-significantly different frequencies. In addition, no difference in MI-onset age and biochemical values was found between the allele and genotypes. In conclusion, we confirmed the effect of the ALOX5-promoter polymorphism on gene expression, but our data did not support a significant effect of this functional variation on MI risk.

TNF-alpha downregulates the leukotriene C4 synthase gene in mononuclear phagocytes

We studied the effect of tumor necrosis factor (TNF)-alpha exposure on cysteinyl leukotriene (LT) synthesis by cells of monocyte/macrophage lineage. TNF-alpha conditioning of monocytic THP-1 cells and primary human monocytes resulted in a decreased capacity for LTC(4) release. TNF-alpha exposure (for 16-24 h) decreased LTC(4) synthase mRNA in THP-1 cells, primary mouse bone marrow-derived macrophages, and eosinophilic AML14.3D10 cells. TNF-alpha downregulated LTC(4) synthase mRNA in THP-1 cells in a dose- and time-dependent manner, with downregulation observed as early as 4 h. The effect of TNF-alpha on LTC(4) synthase mRNA expression was mediated via the MEK/ERK pathway, but not via cyclooxygenase or nitric oxide synthase pathways. Conditioning of actinomycin D-treated cells with TNF-alpha did not accelerate degradation of LTC(4) synthase mRNA. TNF-alpha produced sustained activation of p50 and p65, which were previously reported by our group to decrease LTC(4) synthase promoter activity. In transiently transfected THP-1 cells, TNF-alpha decreased promoter activity via an element located within the first 620 bp of the promoter. We conclude that TNF-alpha exposure downregulates the synthetic capacity for cysteinyl LT release and LTC(4) synthase gene expression in monocytes/macrophages via a transcriptional mechanism.

Lipopolysaccharide induces 5-lipoxygenase-activating protein gene expression in THP-1 cells via a NF-κB and C/EBP-mediated mechanism

We examined induced expression of the 5-lipoxygenase-activating protein (FLAP), which is critical for leukotriene synthesis in mononuclear phagocytes. Prolonged exposure to the bacterial component, lipopolysaccharide (LPS), increased FLAP gene transcription, mRNA expression, and protein expression in the human monocyte-like THP-1 cell line. Activation and inhibition of the NF-κB pathway modulated LPS induction of FLAP gene expression. An NF-κB-mediated mechanism of action was supported by overexpression of dominant-negative IκBα and p50/p65 proteins. EMSA/supershift and DNase I footprint analyses revealed that p50 binds to an NF-κB site located in the proximal FLAP promoter, while chromatin immunoprecipitation assays demonstrated that LPS induced binding of p50 but not of p65. Moreover, EMSA/supershift analyses demonstrated that LPS induced time-dependent binding of THP-1 nuclear extracts (containing p50) to this promoter region. Mutation of the NF-κB site decreased basal promoter activity and abolished the p50- and p65-associated induction. EMSA/supershift analyses also demonstrated that LPS induced binding of THP-1 nuclear extracts [containing CCAAT/enhancer binding protein (C/EBP)-α, -δ, and -ε] to a C/EBP site located adjacent to the NF-κB site in the FLAP promoter. We conclude that LPS enhances FLAP gene expression via both NF-κB- and C/EBP-mediated transcriptional mechanisms in mononuclear phagocytes.

Arachidonate 5-lipoxygenase promoter genotype, dietary arachidonic acid, and atherosclerosis

BACKGROUND

Leukotrienes are inflammatory mediators generated from arachidonic acid (polyunsaturated n-6 fatty acid) by the enzyme 5-lipoxygenase. Since atherosclerosis involves arterial inflammation, we hypothesized that a polymorphism in the 5-lipoxygenase gene promoter could relate to atherosclerosis in humans and that this effect could interact with the dietary intake of competing 5-lipoxygenase substrates.

METHODS

We determined 5-lipoxygenase genotypes, carotid-artery intima-media thickness, and markers of inflammation in a randomly sampled cohort of 470 healthy, middle-aged women and men from the Los Angeles Atherosclerosis Study. Dietary arachidonic acid and marine n-3 fatty acids (including a competing 5-lipoxygenase substrate that reduces the production of inflammatory leukotrienes) were measured with the use of six 24-hour recalls of food intake.

RESULTS

Variant 5-lipoxygenase genotypes (lacking the common allele) were found in 6.0 percent of the cohort. Mean (+/-SE) intima-media thickness adjusted for age, sex, height, and racial or ethnic group was increased by 80+/-19 microm (95 percent confidence interval, 43 to 116; P<0.001) among carriers of two variant alleles, as compared with carriers of the common (wild-type) allele. In multivariate analysis, the increase in intima-media thickness among carriers of two variant alleles (62 microm, P<0.001) was similar in this cohort to that associated with diabetes (64 microm, P=0.01), the strongest common cardiovascular risk factor. Increased dietary arachidonic acid significantly enhanced the apparent atherogenic effect of genotype, whereas increased dietary intake of n-3 fatty acids blunted the effect. Finally, the plasma level of C-reactive protein, a marker of inflammation, was increased by a factor of 2 among carriers of two variant alleles as compared with that among carriers of the common allele.

CONCLUSIONS

Variant 5-lipoxygenase genotypes identify a subpopulation with increased atherosclerosis. The observed diet-gene interactions further suggest that dietary n-6 polyunsaturated fatty acids promote, whereas marine n-3 fatty acids inhibit, leukotriene-mediated inflammation that leads to atherosclerosis in this subpopulation.

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.

5-lipoxygenase-activating protein gene expression. Key role of CCAAT/enhancer-binding proteins (C/EBP) in constitutive and tumor necrosis factor (TNF) alpha-induced expression in THP-1 cells

We examined expression of the 5-lipoxygenase activating protein (FLAP), which is critical for inflammatory cell leukotriene synthesis. A 3.4-kb segment of the FLAP gene 5'-untranslated region accounted for a 22-fold increase in promoter activity when transfected into the monocyte-like cell line, THP-1, and demonstrated no activity in non-inflammatory cells. Virtually all of the promoter activity was mediated by the first 134 bp upstream of the transcription start site, a region that contains CCAAT/enhancer-binding proteins (C/EBP) consensus binding sites, at -36 to -28 bp (distal) and -25 to -12 bp (proximal). DNase I footprint analyses demonstrated THP-1 nuclear extract proteins bind to the proximal site. Electrophoretic mobility shift assay analyses revealed that C/EBP alpha, delta, and epsilon bind to the proximal site and C/EBP alpha and epsilon bind to the distal site, constitutively. Transfection studies indicated that mutation of both the proximal and distal sites decreased constitutive FLAP promoter activity. Overexpression of C/EBP alpha, beta, and delta transactivated promoter activity and increased native FLAP mRNA accumulation. Mutation of both C/EBP sites essentially abolished promoter induction by C/EBP overexpression. Tumor necrosis factor (TNF) alpha induced FLAP mRNA expression, FLAP promoter activity, and C/EBP alpha, delta, and epsilon binding to the proximal and distal promoter consensus sites. Chromatin immunoprecipitation assays demonstrated that C/EBP alpha, delta, and epsilon bound to this region of the 5'-untranslated region, whereas C/EBP beta does not bind even under conditions of overexpression and stimulation. We conclude that the FLAP gene is transactivated by members of the C/EBP family of transcription factors in inflammatory cells and that these factors play an important role in FLAP gene induction by TNFalpha.

Lipopolysaccharide down-regulates the leukotriene C4 synthase gene in the monocyte-like cell line, THP-1

We studied the effects of LPS on cysteinyl leukotriene (LT) synthesis and LTC(4) synthase expression in mononuclear phagocytes. Conditioning of the monocyte-like cell line, THP-1, with LPS for 7 days resulted in significantly decreased ionophore-stimulated LTC(4) release. The putative LPS receptor, Toll-like receptor 4, was expressed in THP-1 cells. LPS down-regulated LTC(4) synthase mRNA in THP-1 cells in a dose- and time-dependent manner, with down-regulation observed as early as 4 h. Conditioning of actinomycin D-treated cells with LPS resulted in no change in the rate of LTC(4) synthase mRNA decay. LPS treatment of THP-1 cells, transiently transfected with a LTC(4) synthase promoter (1.35 kb)-reporter construct, decreased promoter activity. Neutralization of TNF-alpha and inhibition of mitogen-activated protein kinase kinase/extracellular signal-regulated kinase did not inhibit the effect of LPS. Treatment of cells with a Toll-like receptor 4-blocking Ab and an inhibitor of NF-kappaB activation resulted in inhibition of the LPS effect, while activation of NF-kappaB and p50/p65 overexpression down-regulated the LTC(4) synthase gene. LPS down-regulates cysteinyl LT release and LTC(4) synthase gene expression in mononuclear phagocytes by an NF-kappaB-mediated mechanism.

Extending the understanding of leukotrienes in asthma

PURPOSE OF REVIEW

Leukotriene modifiers have recognized utility in the management of asthma. The aim of this review is to put into context recent research findings that extend our understanding of cysteinyl leukotriene synthesis and actions in the pathogenesis of asthma and allergic disease.

RECENT FINDINGS

Previous literature has shown that T helper type 2 cytokines thought to favor asthma and allergic diseases upregulate leukotriene synthesis. Recent findings show that interleukins-4 and -13 also upregulate cysteinyl leukotriene 1 receptor expression. Conversely, the regulation of cytokine expression by leukotrienes has also been explored: cysteinyl leukotrienes upregulate type 2 cytokine expression and decrease type 1 cytokine expression, favoring an allergic phenotype. Genetic determinants of the expression of leukotriene-forming enzymes include polymorphisms of the 5-lipoxygenase and LTC(4) synthase promoters. Novel actions of leukotrienes continue to be recognized, and a role for leukotrienes in the development of airway remodeling accompanying chronic asthma is discussed. Mounting evidence implicates leukotrienes in the pathogenesis of asthma following viral infections. Finally, advances in the measurement of leukotrienes are reviewed.

SUMMARY

Leukotrienes and their receptors play an important role in the pathogenesis of asthma. Advances in our understanding of the synthesis and actions of these lipid mediators provide the scientific rationale for appropriate utilization of leukotriene modifiers and for envisioning novel leukotriene-based therapeutic approaches in the clinical management of asthma.

A highly active homeobox gene promoter regulated by Ets and Sp1 family members in normal granulosa cells and diverse tumor cell types

One mechanism by which normal cells become converted to tumor cells involves the aberrant transcriptional activation of genes that are normally silent. We characterize a promoter that normally exhibits highly tissue- and stage-specific expression but displays ubiquitous expression when cells become immortalized or malignant, regardless of their lineage or tissue origin. This promoter normally drives the expression of the Pem homeobox gene in specific cell types in ovary and placenta but is aberrantly expressed in lymphomas, neuroblastomas, retinoblastomas, carcinomas, and sarcomas. By deletion analysis we identified a region between nucleotides -80 and -104 that was absolutely critical for the expression from this distal Pem promoter (Pem Pd). Site-specific mutagenesis and transfection studies revealed that this region contains two consensus Ets sites and a single Sp1 site that were necessary for Pem Pd expression. Gel shift analysis showed that Ets and Sp1 family members bound to these sites. Transfection studies demonstrated that the Ets family members Elf1 and Gabp and the Sp1 family members Sp1 and Sp3 transactivated the Pem Pd. Surprisingly, we found that Sp3 was a more potent activator of the Pem Pd than was Sp1; this is unusual, because Sp3 is either a weak activator or a repressor of most other promoters. Activation by either Elf1 or Gabp required an intact Sp1 family member binding site, suggesting that Ets and Sp1 family members cooperate to activate Pem Pd transcription. Expression from the Pem Pd (either transiently transfected or endogenous) depended on the Ras pathway, which could explain both its Ets- and Sp1-dependent expression in normal cells and its aberrant expression in tumor cells, in which ras protooncogenes are frequently mutated. We suggest that the Pem Pd may be a useful model system to understand the molecular mechanism by which a tissue-specific promoter can be corrupted in tumor cells.