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.

5-Lipoxygenase: regulation of expression and enzyme activity

5-Lipoxygenase (5-LO) catalyzes the first two steps in the biosynthesis of leukotrienes, a group of pro-inflammatory lipid mediators derived from arachidonic acid. Leukotriene antagonists are used in the treatment of asthma, and the potential role of leukotrienes in atherosclerosis, another chronic inflammatory disease, has recently received considerable attention. In addition, some possible effects of 5-LO metabolites in tumorigenesis have emerged. Thus, knowledge of the biochemistry of this enzyme has potential implications for the treatment of various diseases. Recent advances have expanded our understanding of the regulatory mechanisms underlying the expression and control of 5-LO activity. With regard to the control of enzyme activity, many of these findings focus on the N-terminal domain of 5-LO.

Pharmacogenetic association between ALOX5 promoter genotype and the response to anti-asthma treatment

Clinically similar asthma patients may develop airway obstruction by different mechanisms. Asthma treatments that specifically interfere with the 5-lipoxygenase (ALOX5) pathway provide a method to identify those patients in whom the products of the ALOX5 pathway (that is, the leukotrienes) contribute to the expression of the asthma phenotype. Failure of an asthma patient to respond to treatment with ALOX5-pathway modifiers indicates that leukotrienes are not critical to the expression of the asthmatic phenotype in that patient. We previously defined a family of DNA sequence variants in the core promoter of the gene ALOX5 (on chromosome 10q11.2) associated with diminished promoter-reporter activity in tissue culture. Because expression of ALOX5 is in part transcriptionally regulated, we reasoned that patients with these sequence variants may have diminished gene transcription, and therefore decreased ALOX5 product production and a diminished clinical response to treatment with a drug targeting this pathway. Such an effect indicates an interaction between gene promoter sequence variants and drug-treatment responses, that is, a pharmacogenetic effect of a promoter sequence on treatment responses.

The structure of human 5-lipoxygenase

The synthesis of both proinflammatory leukotrienes and anti-inflammatory lipoxins requires the enzyme 5-lipoxygenase (5-LOX). 5-LOX activity is short-lived, apparently in part because of an intrinsic instability of the enzyme. We identified a 5-LOX-specific destabilizing sequence that is involved in orienting the carboxyl terminus, which binds the catalytic iron. Here, we report the crystal structure at 2.4 angstrom resolution of human 5-LOX stabilized by replacement of this sequence.

Role of leukotrienes revealed by targeted disruption of the 5-lipoxygenase gene

Leukotrienes constitute a class of potent biological mediators of inflammation and anaphylaxis (for reviews see refs 1 and 2). Their biosynthesis derives from 5-lipoxygenase-catalysed oxygenation of arachidonic acid in granulocytes, macrophages and mast cells. To examine the physiological importance of leukotrienes, we have disrupted the 5-lipoxygenase gene by homologous recombination in embryonic stem cells. 5-Lipoxygenase-deficient (5LX-/-) mice develop normally and are healthy. They show a selective opposition to certain inflammatory insults. Although there is no difference in their reaction to endotoxin shock, the 5LX-/- animals resist the lethal effects of shock induced by platelet-activating factor. Reaction to ear inflammation induced by phorbol ester is normal, whereas inflammation induced by arachidonic acid is markedly reduced. Contrasts were also found in two models of leukocyte chemotaxis in vivo. The phenotype of 5LX-/- mice under injurious insult identifies the role for leukotrienes in the pathophysiology of select inflammatory states.

Identification of 5-lipoxygenase as a major gene contributing to atherosclerosis susceptibility in mice

We previously reported the identification of a locus on mouse chromosome 6 that confers almost total resistance to atherogenesis, even on a hypercholesterolemic (LDL receptor-null) background. 5-Lipoxygenase (5-LO) is the rate-limiting enzyme in leukotriene synthesis and was among the chromosome 6 locus candidate genes that we examined. The levels of 5-LO mRNA were reduced about 5-fold in a congenic strain, designated CON6, containing the resistant chromosome 6 region derived from the CAST/Ei strain (CAST), as compared with the background C57BL/6J (B6) strain. 5-LO protein levels were similarly reduced in the CON6 mice. Sequencing of the 5-LO cDNA revealed several differences between CON6 and the B6 strain. To test the whether 5-LO is responsible for the resistant phenotype, we bred a 5-LO knockout allele onto an LDL receptor-null (LDLR(-/-)) background. On this background, the mice bred poorly and only heterozygous 5-LO knockout mice were obtained. These mice showed a dramatic decrease (>26-fold; P<0.0005) in aortic lesion development, similar to the CON6 mice. Immunohistochemistry revealed that 5-LO was abundantly expressed in atherosclerotic lesions of apoE(-/-) and LDLR(-/-) deficient mice, appearing to colocalize with a subset of macrophages but not with all macrophage-staining regions. When bone marrow from 5-LO(+/-) mice was transplanted into LDLR(-/-), there was a significant reduction in atherogenesis, suggesting that macrophage 5-LO is responsible, at least in part, for the effect on atherosclerosis. These results indicate that 5-LO contributes importantly to the atherogenic process and they provide strong presumptive evidence that reduced 5-LO expression is partly responsible for the resistance to atherosclerosis in CON6 mice.

Prostaglandin, leukotriene, and lipoxin balance in chronic rhinosinusitis with and without nasal polyposis

BACKGROUND

Upper airway diseases and especially the aspirin hypersensitivity syndrome have been linked to changes in the arachidonic acid cascade; however, the specificity of these changes and their relation to inflammatory reactions in these diseases still remain controversial.

OBJECTIVE

We aimed to study the tissue eicosanoid production in 3 subgroups of patients with chronic rhinosinusitis (CRS) and control subjects and to correlate it with the severity of inflammation and clinical manifestation of aspirin sensitivity.

METHODS

Samples were prepared from sinonasal tissue of patients with CRS with (CRS-NP group, n = 13) and without nasal polyposis (CRS group, n = 11), sinonasal tissue of patients with nasal polyposis and aspirin sensitivity (CRS-ASNP group, n = 13), and normal nasal mucosa from healthy subjects (NM group, n = 8). Real-time PCR was applied for mRNA quantification of COX-2, 5-lipoxygenase, leukotriene C 4 synthase, and 15-lipoxygenase. Enzyme immunoassays were used to measure IL-5, eosinophil cationic protein, and eicosanoid (leukotriene [LT] C 4 , LTD 4 , and LTE 4 ; lipoxin A 4 ; and prostaglandin E 2 [PGE 2 ]) concentrations.

RESULTS

COX-2 mRNA and PGE 2 concentrations were similar in the CRS and NM groups but significantly decreased in nasal polyp tissue, especially in the CRS-ASNP group. LTC 4 synthase, 5-lipoxygenase mRNA, LTC 4 , LTD 4 , and LTE 4 concentrations increased with disease severity among the patient groups. 15-Lipoxygenase and lipoxin A 4 concentrations were increased in all CRS groups compared with in the NM group but were significantly downregulated in the CRS-ASNP group when compared with the CRS-NP group. IL-5 and eosinophil cationic protein were increased in both groups of nasal polyp tissue compared with in the NM and CRS groups and correlated directly with LTC 4 , LTD 4 , and LTE 4 concentrations and inversely with PGE 2 concentrations.

CONCLUSION

Changes of tissue eicosanoid metabolism do occur in CRS, even in the absence of clinical aspirin sensitivity, and these changes appear to be related to the severity of eosinophilic inflammation.

Lipoxin-mediated inhibition of IL-12 production by DCs: a mechanism for regulation of microbial immunity

Lipoxins are eicosanoid mediators that show potent inhibitory effects on the acute inflammatory process. We show here that the induction of lipoxin A(4) (LXA(4)) accompanied the in vivo suppression of interleukin 12 (IL-12) responsiveness of murine splenic dendritic cells (DCs) after microbial stimulation with an extract of Toxoplasma gondii. This paralysis of DC function could not be triggered in mice that were deficient in a key lipoxygenase involved in LXA(4) biosynthesis. In addition, DCs pre-treated with LXA(4) became refractory to microbial stimulation for IL-12 production in vitro and mice injected with a stable LXA(4) analog showed reduced splenic DC mobilization and IL-12 responses in vivo. Together, these findings indicate that the induction of lipoxins in response to microbial stimulation can provide a potent mechanism for regulating DC function during the innate response to pathogens.

Enzymes involved in the biosynthesis of leukotriene B4 and prostaglandin E2 are active in sebaceous glands

The expression of enzymes involved in leukotriene and prostaglandin signalling pathways, of interleukins 6 and 8 and of peroxisome proliferator-activated receptors in sebaceous glands of acne-involved facial skin was compared with those of non-involved skin of acne patients and of healthy individuals. Moreover, 5-lipoxygenase and leukotriene A(4) hydrolase were expressed at mRNA and protein levels in vivo and in SZ95 sebocytes in vitro (leukotriene A(4) hydrolase > 5-lipoxygenase), while 15-lipoxygenase-1 was only detected in cultured sebocytes. Cyclooxygenase-1 and cyclooxygenase-2 were also present. Peroxisome proliferator-activated receptors were constitutively expressed. Enhanced 5-lipoxygenase, cyclooxygenase 2 and interleukin 6 expression was detected in acne-involved facial skin. Arachidonic acid stimulated leukotriene B(4) and interleukin 6 release as well as prostaglandin E(2) biosynthesis in SZ95 sebocytes, induced abundant increase in neutral lipids and down-regulated peroxisome proliferator-activated receptor-alpha, but not receptor-gamma1 mRNA levels, which were the predominant peroxisome proliferator-activated receptor isotypes in SZ95 sebocytes. In conclusion, human sebocytes possess the enzyme machinery for functional leukotriene and prostaglandin pathways. A comprehensive link between inflammation and sebaceous lipid synthesis is provided.

Neutrophil-mediated changes in vascular permeability are inhibited by topical application of aspirin-triggered 15-epi-lipoxin A4 and novel lipoxin B4 stable analogues

Neutrophil (PMN) activation is critical in inflammation and reperfusion injury, suggesting that PMN-directed therapies may be of clinical use. Here, leukotriene B4 (LTB4)-induced PMN influx in ear skin was equivalent between 5-lipoxygenase knockout and wild-type mice. To explore actions of lipoxin (LX) in PMN-mediated tissue injury, we prepared several novel LX stable analogues, including analogues of LXA4 and aspirin-triggered 15-epi-LXA4 as well as LXB4, and examined their impact in PMN infiltration and vascular permeability. Each applied topically to mouse ears inhibited dramatically PMN-mediated increases in vascular permeability (IC50 range of 13-26 nmol) with a rank order of 15(R/S)-methyl-LXA4 > 16-para-fluoro-phenoxy-LXA4 approximately 5(S)-methyl-LXB4 >/= 16-phenoxy-LXA4 > 5(R)-methyl-LXB4. These LX mimetics were as potent as an LTB4 receptor antagonist, yet results from microphysiometry with mouse leukocytes indicated that they do not act as LTB4 receptor level antagonists. In addition, within 24 h of delivery, > 90% were cleared from ear biopsies. Neither IL-8, FMLP, C5a, LTD4, nor platelet-activating factor act topically to promote PMN influx. When applied with LTB4, PGE2 enhanced sharply both infiltration and vascular permeability, which were inhibited by a fluorinated stable analogue of aspirin-triggered LX. These results indicate that mimetics of LXs and aspirin-triggered 15-epi-LXA4 are topically active in this model and are potent inhibitors of both PMN infiltration and PMN-mediated vascular injury.

Cloning of the cDNA for human 5-lipoxygenase

The enzyme 5-lipoxygenase (5-LO) catalyzes the first two reactions in the pathway leading to the formation of leukotrienes from arachidonic acid. Leukotrienes are potent arachidonic acid metabolites possessing biological activities that suggest a role in the pathophysiology of allergic and inflammatory diseases. To obtain structural information about 5-LO for use in developing anti-inflammatory chemotherapeutic agents, the enzyme was purified from human polymorphonuclear leukocytes and the amino acid sequences were determined for several cyanogen bromide-derived peptides. A cDNA clone encoding a 674-amino acid protein containing all of the derived peptide sequences was isolated from a dimethyl sulfoxide differentiated HL60 cell cDNA library. The mRNA encoding 5-LO was detected only in differentiated HL60 cells and not in the undifferentiated cell line, indicating that the expression of 5-LO in this cell line is transcriptionally regulated. In addition, the human protein displays some amino acid sequence homology with several lipases and significant homology with the partial sequences of soybean and reticulocyte lipoxygenases. Thus, 5-LO appears to be a member of a larger family of related enzymes.

Growth control of lung cancer by interruption of 5-lipoxygenase-mediated growth factor signaling

Signal transduction pathways shared by different autocrine growth factors may provide an efficient approach to accomplish clinically significant control of lung cancer growth. In this study, we demonstrate that two autocrine growth factors activate 5-lipoxygenase action of the arachidonic acid metabolic pathway in lung cancer cell lines. Both growth factors increased the production of 5(S)-hydrooxyeicosa-6E,8Z,11Z,14Z-tetraeno ic acid (5-HETE), a major early 5-lipoxygenase metabolic product. Exogenously added 5-HETE stimulated lung cancer cell growth in vitro. Inhibition of 5-lipoxygenase metabolism by selective antagonists resulted in significant growth reduction for a number of lung cancer cell lines. Primary clinical specimens and lung cancer cell lines express the message for the 5-lipoxygenase enzymes responsible for the generation of active metabolites. In vivo evaluation demonstrated that interruption of 5-lipoxygenase signaling resulted in enhanced levels of programmed cell death. These findings demonstrate that 5-lipoxygenase activation is involved with growth factor-mediated growth stimulation for lung cancer cell lines. Pharmacological intervention with lipoxygenase inhibitors may be an important new clinical strategy to regulate growth factor-dependent stages of lung carcinogenesis.

Integrating genotypic and expression data in a segregating mouse population to identify 5-lipoxygenase as a susceptibility gene for obesity and bone traits

Forward genetic approaches to identify genes involved in complex traits such as common human diseases have met with limited success. Fine mapping of linkage regions and validation of positional candidates are time-consuming and not always successful. Here we detail a hybrid procedure to map loci involved in complex traits that leverages the strengths of forward and reverse genetic approaches. By integrating genotypic and expression data in a segregating mouse population, we show how clusters of expression quantitative trait loci linking to regions of the genome accurately reflect the underlying perturbation to the transcriptional network induced by DNA variations in genes that control the complex traits. By matching patterns of gene expression in a segregating population with expression responses induced by single-gene perturbation experiments, we show how genes controlling clusters of expression and clinical quantitative trait loci can be mapped directly. We demonstrate the utility of this approach by identifying 5-lipoxygenase as underlying previously identified quantitative trait loci in an F(2) cross between strains C57BL/6J and DBA/2J and showing that it has pleiotropic effects on body fat, lipid levels and bone density.

5-lipoxygenase and FLAP

The initial steps in the biosynthesis of leukotrienes from arachidonic acid are carried out by the enzyme 5-lipoxygenase (5-LO). In intact cells, the helper protein 5-LO activating protein (FLAP) is necessary for efficient enzyme utilization of endogenous substrate. The last decade has witnessed remarkable progress in our understanding of these two proteins. Here we review the molecular and cellular aspects of the expression, function, and regulation of 5-LO and FLAP.

Increased susceptibility to pulmonary hypertension in heterozygous BMPR2-mutant mice

BACKGROUND

Bone morphogenetic protein receptor-2 (BMPR2)-heterozygous, mutant (BMPR2(+/-)) mice have a genetic trait similar to that of certain patients with idiopathic pulmonary arterial hypertension (IPAH). To understand the role of BMPR2 in the development of IPAH, we examined the phenotype of BMPR2(+/-) mice and their response to inflammatory stress.

METHODS AND RESULTS

BMPR2(+/-) mice were found to have the same life span, right ventricular systolic pressure (RVSP), and lung histology as those of wild-type mice under unstressed conditions. However, when treated with recombinant adenovirus expressing 5-lipoxygenase (Ad5LO), BMPR2(+/-) mice exhibited significantly higher RVSP than wild-type mice. The increase of RVSP occurred in the first 2 weeks after Ad5LO delivery. Modest but significant muscularization of distal pulmonary arterioles appeared in BMPR2(+/-) mice 4 weeks after Ad5LO treatment. Measurement of urinary metabolites of vasoactive molecules showed that cysteinyl leukotrienes, prostacyclin metabolites, and PGE2 were all increased to a similar degree in both BMPR2(+/-) and wild-type mice during 5LO transgene expression, whereas urinary endothelin-1 remained undetectable. Urinary thromboxane A2 metabolites, in contrast, were significantly higher in BMPR2(+/-) than in wild-type mice and paralleled the increase in RVSP. Platelet activation markers, serotonin, and soluble P-selectin showed a trend toward higher concentrations in BMPR2(+/-) than wild-type mice. Cell culture studies found that BMP treatment reduced interleukin-1beta-stimulated thromboxane A2 production in the pulmonary epithelial cell line A549.

CONCLUSIONS

BMPR2(+/-) mice do not develop pulmonary hypertension spontaneously; however, under inflammatory stress, they are more susceptible to an increase in RVSP, thromboxane A2 production, and vascular remodeling than wild-type mice.

Leukotriene B4 receptor transgenic mice reveal novel protective roles for lipoxins and aspirin-triggered lipoxins in reperfusion

Polymorphonuclear neutrophil (PMN) activation is pivotal in acute inflammation and injury from reperfusion. To elucidate components controlling PMNs in vivo, we prepared novel transgenic mice with the human leukotriene (LT) B4 receptor (BLTR) for functional characterization. Overexpression of BLTR in leukocytes dramatically increased PMN trafficking to skin microabscesses and lungs after ischemia-reperfusion, whereas mice deficient in 5-lipoxygenase (5-LO) showed diminished PMN accumulation in reperfused lungs. Hence, both BLTR expression and LT biosynthesis are critical for PMN infiltration in reperfusion-initiated second-organ injury. Also, in BLTR transgenic mice, 5-LO expression and product formation were selectively increased in exudates, demonstrating that receptor overexpression amplifies proinflammatory circuits. Endogenous lipoxin (LX) A4 was produced in ischemic lungs and elevated by reperfusion. Because LXA4 and aspirin-triggered 15-epimeric LXA4 (ATL) selectively regulate leukocyte responses, they were tested in BLTR transgenic mice. Despite excessive PMN recruitment in BLTR transgenic mice, intravenous injection of ATL sharply diminished reperfusion-initiated PMN trafficking to remote organs, and topical application of LX was protective in acute dermal inflammation. These results demonstrate a direct role for BLTR with positive feedback, involving BLTR and 5-LO signaling in controlling PMNs. Moreover, LXA4 and ATL counter BLTR-amplified networks, revealing a novel protective role for LX and ATL in stress responses that has applications in perioperative medicine.

5-lipoxygenase: cellular biology and molecular pharmacology

5-lipoxygenase (5-LO) plays an essential role in the biosynthesis of leukotrienes, proinflammatory mediators which are mainly released from myeloid cells. Whereas LTB(4) is a potent chemotactic and chemokinetic agent for a variety of leukocytes, the cysteinyl-leukotrienes C, D(4) and E(4) cause vascular permeability and smooth muscle contraction. In view of these properties, leukotriene synthesis inhibitors have been hypothesized to possess therapeutic potential for the treatment of asthma, allergic disorders and other inflammatory diseases. Whereas cysteinyl-leukotriene receptor antagonists possess potential for antileukotriene therapy in asthma, results from clinical trials with leukotriene synthesis inhibitors were less encouraging. The expression of 5-LO in mammals is tightly regulated. Enzymatic activity in vitro can be modulated by calcium, ATP, phosphatidylcholine and lipid hydroperoxides; nevertheless activation of cellular 5-LO in response to external stimuli is rather incompletely understood. Intensive research revealed that on cell stimulation, 5-LO redistributes to the nuclear membrane where it colocalizes with 5-lipoxygenase-activating protein and cytosolic phospholipase A(2). In addition, various cellular proteins interacting with 5-LO have been identified. It was suggested that enzyme phosphorylation could influence redistribution and cellular activity of 5-LO and that the cellular redox tone regulates 5-LO product formation. This review highlights the determinants of cellular 5-LO activity and summarizes the molecular pharmacology of 5-LO.

5-Lipoxygenase gene disruption reduces amyloid-beta pathology in a mouse model of Alzheimer's disease

5-Lipoxygenase (5LO), by producing leukotrienes, is a proinflammatory enzyme, and there is evidence suggesting that it is up-regulated with aging and may be involved in Alzheimer's disease (AD). In this paper, we studied the effect of 5LO-targeted gene disruption on the amyloid phenotype of a transgenic mouse model of AD, the Tg2576. Amyloid-beta (Abeta) deposition in the brains of Tg2576 mice lacking 5LO was reduced by 64-80% compared with Tg2576 controls. This reduction was associated with a similar significant decrease in Abeta levels measured by sandwich ELISA. Absence of 5LO did not induce any significant change in amyloid-beta precursor protein (APP) levels and processing, or Abeta catabolic pathways. Furthermore, in vitro studies showed that 5LO activation or 5LO metabolites increase, whereas 5LO inhibition decreases, Abeta formation, secondary to correspondent changes in gamma-secretase activity. These data establish for the first time a novel functional role for 5LO in the pathogenesis of AD-like amyloidosis, thereby modulating gamma-secretase activity. Our work suggests that pharmacological inhibition of 5LO could provide a novel therapeutic tool for AD.

Molecular basis of the specific subcellular localization of the C2-like domain of 5-lipoxygenase

The activation of 5-lipoxygenase (5-LO) involves its calcium-dependent translocation to the nuclear envelope, where it catalyzes the two-step transformation of arachidonic acid into leukotriene A(4), leading to the synthesis of various leukotrienes. To understand the mechanism by which 5-LO is specifically targeted to the nuclear envelope, we studied the membrane binding properties of the amino-terminal domain of 5-LO, which has been proposed to have a C2 domain-like structure. The model building, electrostatic potential calculation, and in vitro membrane binding studies of the isolated C2-like domain of 5-LO and selected mutants show that this Ca(2+)-dependent domain selectively binds zwitterionic phosphatidylcholine, which is conferred by tryptophan residues (Trp(13), Trp(75), and Trp(102)) located in the putative Ca(2+)-binding loops. The spatiotemporal dynamics of the enhanced green fluorescence protein-tagged C2-like domain of 5-LO and mutants in living cells also show that the phosphatidylcholine selectivity of the C2-like domain accounts for the specific targeting of 5-LO to the nuclear envelope. Together, these results show that the C2-like domain of 5-LO is a genuine Ca(2+)-dependent membrane-targeting domain and that the subcellular localization of the domain is governed in large part by its membrane binding properties.

Molecular cloning and amino acid sequence of human 5-lipoxygenase

5-Lipoxygenase (EC 1.13.11.34), a Ca2+-and ATP-requiring enzyme, catalyzes the first two steps in the biosynthesis of the peptidoleukotrienes and the chemotactic factor leukotriene B4. A cDNA clone corresponding to 5-lipoxygenase was isolated from a human lung lambda gt11 expression library by immunoscreening with a polyclonal antibody. Additional clones from a human placenta lambda gt11 cDNA library were obtained by plaque hybridization with the 32P-labeled lung cDNA clone. Sequence data obtained from several overlapping clones indicate that the composite cDNAs contain the complete coding region for the enzyme. From the deduced primary structure, 5-lipoxygenase encodes a 673 amino acid protein with a calculated molecular weight of 77,839. Direct analysis of the native protein and its proteolytic fragments confirmed the deduced composition, the amino-terminal amino acid sequence, and the structure of many internal segments. 5-Lipoxygenase has no apparent sequence homology with leukotriene A4 hydrolase or Ca2+ -binding proteins. RNA blot analysis indicated substantial amounts of an mRNA species of approximately equal to 2700 nucleotides in leukocytes, lung, and placenta.

Characterization of the human 5-lipoxygenase gene

The gene for human 5-lipoxygenase has been isolated from three different bacteriophage genomic libraries and a genomic cosmid library. The gene spans greater than 82 kilobases and consists of 14 exons. The size range for the exons is 82-613 base pairs, whereas that for the introns is approximately 200 bp to greater than 26 kb. A major site of transcription initiation in leukocytes was mapped to a thymidine residue 65 base pairs upstream of the ATG initiation codon by nuclease S1 protection and primer extension experiments. Other potential minor initiation sites were found. The putative promoter region contains no TATA and CCAAT sequences in the expected positions upstream of the major transcription initiation site but contains multiple GC boxes within a (G + C)-rich region, as does the immediate 5' region of the first intron. Characteristics common to the 5' end of the human 5-lipoxygenase gene and the promoter regions of the housekeeping genes raise important questions concerning the regulation of 5-lipoxygenase gene expression.

Dual inhibition of 5-LOX and COX-2 suppresses colon cancer formation promoted by cigarette smoke

Previous studies indicate that the arachidonic acid-metabolizing enzymes COX-2 and 5-LOX are overexpressed during the process of colonic adenoma formation promoted by cigarette smoke. The aims of the present study were to investigate whether there exists a relationship between COX-2 and 5-LOX, and whether dual inhibition of COX-2 and 5-LOX has an anticarcinogenic effect in the colonic tumorigenesis promoted by cigarette smoke. Results showed that pretreating colon cancer cells with cigarette smoke extract (CSE) promoted colon cancer growth in the nude mouse xenograft model. Inhibition of COX-2 or 5-LOX reduced the tumor size. In the group treated with COX-2-inhibitor, the PGE2 level decreased while the LTB4 level increased. In contrast, in the 5-LOX-inhibitor treated group, the LTB4 level was reduced and the PGE2 level was unchanged. However, combined treatment with both COX-2 and 5-LOX inhibitors further inhibited the tumor growth promoted by CSE over treatment with either COX-2-inhibitor or 5-LOX-inhibitor alone. This was accompanied by the downregulation of PGE2 and LTB4. In an in vitro study, we found that the action of CSE on colon cancer cells was mediated by 5-LOX DNA demethylation. In summary, these results indicate that inhibition of COX-2 may lead to a shunt of arachidonic acid metabolism towards the leukotriene pathway during colonic tumorigenesis promoted by CSE. Suppression of 5-LOX did not induce such a shunt and produced a better response. Therefore, 5-LOX inhibitor is more effective than COX-2 inhibitor, and blocker of both COX-2 and 5-LOX may present a superior anticancer profile in cigarette smokers.

The orphan receptor family RZR/ROR, melatonin and 5-lipoxygenase: an unexpected relationship

The orphan receptors RZR alpha, RZR beta, ROR alpha 1, RZR alpha 2, ROR alpha 3, and ROR gamma form a subfamily within the superfamily of nuclear hormone receptors. Recently, experimental evidence that the pineal gland hormone melatonin is the natural ligand for these nuclear receptors has come to light. This discovery is rather surprising, given that most people in the field believed melatonin acts exclusively through membrane receptors. However, these new findings establish a nuclear signalling pathway for melatonin, i.e., direct ligand-induced control of target gene transcription, which most probably mediates part of the physiological functions of the hormone. Interestingly, the very recently identified first RZR/melatonin responding gene, 5-lipoxygenase, is not expressed in the brain and is not involved in circadian rhythmicity, but rather acts in the periphery, mainly in myeloid cells, as one of the key enzymes of allergic and inflammatory reactions. Thus, nuclear melatonin signalling opens up a new perspective in understanding the actions of the pineal gland hormone.

Lipoxygenase inhibitors abolish proliferation of human pancreatic cancer cells

Epidemiologic and animal studies have linked pancreatic cancer growth with fat intake, especially unsaturated fats. Arachidonic acid release from membrane phospholipids is essential for tumor cell proliferation. Lipoxygenases (LOX) constitute one pathway for arachidonate metabolism, but their role in pancreatic cancer growth is unknown. The expression of 5-LOX and 12-LOX as well as their effects on cell proliferation was investigated in four human pancreatic cancer cell lines (PANC-1, MiaPaca2, Capan2, and ASPC-1). Expression of 5-LOX and 12-LOX mRNA was measured by nested RT-PCR. Effects of LOX inhibitors and specific LOX antisense oligonucleotides on pancreatic cancer cell proliferation were measured by (3)H-thymidine incorporation. Our results showed that (1) 5-LOX and 12-LOX were expressed in all pancreatic cancer cell lines tested, while they were not detectable in normal human pancreatic ductal cells; (2) both LOX inhibitors and LOX antisense markedly inhibited cell proliferation in a concentration-dependent and time-dependent manner; (3) the 5-LOX and 12-LOX metabolites 5-HETE and 12-HETE as well as arachidonic and linoleic acids directly stimulated pancreatic cancer cell proliferation; (4) LOX inhibitor-induced growth inhibition was reversed by 5-HETE and 12-HETE. The current studies indicate that both 5-LOX and 12-LOX expression is upregulated in human pancreatic cancer cells and LOX plays a critical role in pancreatic cancer cell proliferation. LOX inhibitors may be valuable for the treatment of pancreatic cancer.