Cloning and functional analysis of the mouse 5-lipoxygenase promoter

LPS induces ALOX5 promoter activation and 5-lipoxygenase expression in human monocytic cells

5-lipoxygenase (5-LO), coded by the ALOX5 gene, is expressed in leukocytes and catalyzes the formation of leukotrienes, pro-inflammatory lipid mediators. Leukotrienes are central to immune responses, but are also involved in inflammatory disorders and 5-LO expression is associated with leukemia stem cell survival. It is therefore important to understand mechanisms that control 5-LO expression. This study investigated the control of 5-LO expression and leukotriene biosynthesis following the maturation of human monocytic cells. MonoMac-1 (MM1) and THP-1 cells were incubated for up to 72 h with or without LPS and TGF-β. LPS, but not TGF-β, increased CD14 expression in both MM1 and THP-1 cells. Incubation with LPS (100 ng/ml) and TGF-β (1 ng/ml) synergistically increased the capacity of MM1 cells to produce 5-LO products from undetectable levels to 40±5 pmol/10⁶ cells. 5-LO product biosynthesis in THP-1 cells increased 25-fold. A synergistic effect of LPS and TGF-β was measured with increases in 5-LO mRNA of 54- and 13-fold in MM1 and THP-1 cells, respectively. 5-LO protein expression increased significantly in both MM1 and THP-1 cells. ALOX5 promoter activity was significantly elevated >2-fold in both cell lines following LPS treatment, but TGF-β was without effect. The main 5-LO products were cysteinyl-leukotrienes, however LPS and TGF-β did not impact on the capacity of the cells to metabolize leukotriene A₄. Overall, this study demonstrates that receptor-mediated stimulation of MM1 and THP-1 cells by LPS is associated with increased 5-LO expression. This represents a new mechanism by which leukotriene biosynthesis can be modulated by pathological agents.

Chronic hypoxia promotes pulmonary artery endothelial cell proliferation through H2O2-induced 5-lipoxygenase

Pulmonary Hypertension (PH) is a progressive disorder characterized by endothelial dysfunction and proliferation. Hypoxia induces PH by increasing vascular remodeling. A potential mediator in hypoxia-induced PH development is arachidonate 5-Lipoxygenase (ALOX5). While ALOX5 metabolites have been shown to promote pulmonary vasoconstriction and endothelial cell proliferation, the contribution of ALOX5 to hypoxia-induced proliferation remains unknown. We hypothesize that hypoxia exposure stimulates HPAEC proliferation by increasing ALOX5 expression and activity. To test this, human pulmonary artery endothelial cells (HPAEC) were cultured under normoxic (21% O2) or hypoxic (1% O2) conditions for 24-, 48-, or 72 hours. In a subset of cells, the ALOX5 inhibitor, zileuton, or the 5-lipoxygenase activating protein inhibitor, MK-886, was administered during hypoxia exposure. ALOX5 expression was measured by qRT-PCR and western blot and HPAEC proliferation was assessed. Our results demonstrate that 24 and 48 hours of hypoxia exposure have no effect on HPAEC proliferation or ALOX5 expression. Seventy two hours of hypoxia significantly increases HPAEC ALOX5 expression, hydrogen peroxide (H2O2) release, and HPAEC proliferation. We also demonstrate that targeted ALOX5 gene silencing or inhibition of the ALOX5 pathway by pharmacological blockade attenuates hypoxia-induced HPAEC proliferation. Furthermore, our findings indicate that hypoxia-induced increases in cell proliferation and ALOX5 expression are dependent on H2O2 production, as administration of the antioxidant PEG-catalase blocks these effects and addition of H2O2 to HPAEC promotes proliferation. Overall, these studies indicate that hypoxia exposure induces HPAEC proliferation by activating the ALOX5 pathway via the generation of H2O2.

Cooperation between RUNX1-ETO9a and novel transcriptional partner KLF6 in upregulation of Alox5 in acute myeloid leukemia

Fusion protein RUNX1-ETO (AML1-ETO, RUNX1-RUNX1T1) is expressed as the result of the 8q22;21q22 translocation [t(8;21)], which is one of the most common chromosomal abnormalities found in acute myeloid leukemia. RUNX1-ETO is thought to promote leukemia development through the aberrant regulation of RUNX1 (AML1) target genes. Repression of these genes occurs via the recruitment of the corepressors N-COR and SMRT due to their interaction with ETO. Mechanisms of RUNX1-ETO target gene upregulation remain less well understood. Here we show that RUNX1-ETO9a, the leukemogenic alternatively spliced transcript expressed from t(8;21), upregulates target gene Alox5, which is a gene critically required for the promotion of chronic myeloid leukemia development by BCR-ABL. Loss of Alox5 expression reduces activity of RUNX1-ETO9a, MLL-AF9 and PML-RARα in vitro. However, Alox5 is not essential for the induction of leukemia by RUNX1-ETO9a in vivo. Finally, we demonstrate that the upregulation of Alox5 by RUNX1-ETO9a occurs via the C₂H₂ zinc finger transcription factor KLF6, a protein required for early hematopoiesis and yolk sac development. Furthermore, KLF6 is specifically upregulated by RUNX1-ETO in human leukemia cells. This identifies KLF6 as a novel mediator of t(8;21) target gene regulation, providing a new mechanism for RUNX1-ETO transcriptional control.

The 8;21 translocation is one of the most common genetic abnormalities present in acute myeloid leukemia (AML). This translocation causes expression of the fusion gene RUNX1-ETO and its splicing isoforms. RUNX1-ETO proteins then reprogram the transcriptional landscape of the cell and cooperate with further mutations to induce leukemia development. In this study, we examine the transcriptional control of the RUNX1-ETO target gene Alox5. Although Alox5 appears to be dispensable for AML development in a mouse model, it is required for some RUNX1-ETO functions. In studying the regulation of Alox5 expression, we have discovered a novel RUNX1-ETO partner protein, KLF6, which is both upregulated by RUNX1-ETO and participates in RUNX1-ETO gene regulation. This provides new insight into the under-studied mechanisms of RUNX1-ETO target gene upregulation and identifies KLF6 as a potentially important protein for further study in t(8;21) AML development.

5-Lipoxygenase: mechanisms of regulation

5-lipoxygenase (5-LO) catalyzes two steps in biosynthesis of leukotrienes (LTs), a group of lipid mediators of inflammation derived from arachidonic acid (AA). LT antagonists are used in treatment of asthma; more recently a potential role also in atherosclerosis has raised considerable interest. Furthermore, possible effects of 5-LO metabolites in relation to tumorigenesis have emerged. Thus, an understanding of the biochemistry of this lipoxygenase has potential implications for treatment of various diseases.

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.

Suppression of 5-lipoxygenase gene is involved in triptolide-induced apoptosis in pancreatic tumor cell lines

Pancreatic adenocarcinoma is characterized by a poor prognosis and lack of response to conventional therapy. The purpose of this study was to investigate the effects of triptolide (TL) on proliferation and apoptosis of pancreatic cancer cells in vitro. We found that TL induced prominent growth inhibition and apoptosis in human pancreatic cell lines. In addition, TL treatment significantly down-regulated 5-lipoxygenase (5-LOX) expression, as well as downstream leukotriene B4 (LTB4) production, in these cell lines. Furthermore, overexpression of 5-LOX in SW1990 cell lines or exogenous LTB4 made them more resistant to TL-induced apoptosis, which was correlated with increased Bcl-2 expression. Taken together, these findings suggest that inhibition of the 5-LOX pathway of arachidonic acid metabolism is associated with the anti-proliferation activity of TL. We also provide evidence that TL has clinical therapeutic value for patients with pancreatic cancer.

PU.1 regulates cathepsin S expression in professional APCs

Cathepsin S (CTSS) is a cysteine protease that is constitutively expressed in APCs and mediates processing of MHC class II-associated invariant chain. CTSS and the Ets family transcription factor PU.1 are highly expressed in cells of both myeloid (macrophages and dendritic cells) and lymphoid (B lymphocytes) lineages. Therefore, we hypothesized that PU.1 participates in the transcriptional regulation of CTSS in these cells. In A549 cells (a human epithelial cell line that does not express either CTSS or PU.1), the expression of PU.1 enhances CTSS promoter activity approximately 5- to 10-fold. In RAW cells (a murine macrophage-like cell line that constitutively expresses both CTSS and PU.1), the expression of a dominant-negative PU.1 protein and a short-interfering RNA PU.1 construct attenuates basal CTSS promoter activity, mRNA levels, and protein expression. EMSAs show binding of PU.1 to oligonucleotides derived from the CTSS promoter at two different Ets consensus binding elements. Mutation of these sites decreases the baseline CTSS activity in RAW cells that constitutively express PU.1. Chromatin immunoprecipitation experiments show binding of PU.1 with the CTSS promoter in this same region. Finally, the expression of PU.1, in concert with several members of the IFN regulatory factor family, enhances CTSS promoter activity beyond that achieved by PU.1 alone. These data indicate that PU.1 participates in the regulation of CTSS transcription in APCs. Thus, manipulation of PU.1 expression may directly alter the endosomal proteolytic environment in these cells.

Hypoxic preconditioning protects against ischemic brain injury

Animals exposed to brief periods of moderate hypoxia (8% to 10% oxygen for 3 hours) are protected against cerebral and cardiac ischemia between 1 and 2 days later. This hypoxia preconditioning requires new RNA and protein synthesis. The mechanism of this hypoxia-induced tolerance correlates with the induction of the hypoxia-inducible factor (HIF), a transcription factor heterodimeric complex composed of inducible HIF-1alpha and constitutive HIF-1beta proteins that bind to the hypoxia response elements in a number of HIF target genes. Our recent studies show that HIF-1alpha correlates with hypoxia induced tolerance in neonatal rat brain. HIF target genes, also induced following hypoxia-induced tolerance, include vascular endothelial growth factor, erythropoietin, glucose transporters, glycolytic enzymes, and many other genes. Some or all of these genes may contribute to hypoxia-induced protection against ischemia. HIF induction of the glycolytic enzymes accounts in part for the Pasteur effect in brain and other tissues. Hypoxia-induced tolerance is not likely to be equivalent to treatment with a single HIF target gene protein since other transcription factors including Egr-1 (NGFI-A) have been implicated in hypoxia regulation of gene expression. Understanding the mechanisms and genes involved in hypoxic tolerance may provide new therapeutic targets to treat ischemic injury and enhance recovery.

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Identification and Characterization of a Phorbol Ester-responsive Element in the Murine 8S-Lipoxygenase Gene

Murine 8S-lipoxygenase (8S-LOX) is a 12-O-tetradecanoylphorbol-13-acetate (TPA)-inducible lipoxygenase. That is, it is not detected in normal mouse skin, however, a significant increase in expression is detected in the skin of TPA promotion-sensitive strains of mice after TPA treatment. In this study, we found TPA-induced 8S-LOX mRNA expression is a result of increased transcription in SSIN primary keratinocytes and further investigated transcriptional regulation of 8S-LOX expression by cloning its promoter. The cloned 8S-LOX promoter ( approximately 2 kb) in which a transcription initiation site was mapped at -27 from the ATG has neither a TATA box nor a CCAAT box. However, the promoter was highly responsive to TPA in TPA promotion-sensitive SSIN but not in TPA promotion-resistant C57BL/6J primary keratinocytes. We then identified a Sp1 binding site located -77 to -68 from the ATG that is a TPA-responsive element (TRE) of the promoter and that Sp1, Sp2, and Sp3 proteins bind to the TRE. We also found that the binding of these proteins to the TRE was significantly increased by TPA treatment and inhibition of the binding by mithramycin A decreased TPA-induced promoter activity as well as 8S-LOX mRNA expression. These data suggest that increased binding of Sp1, Sp2, and Sp3 to the TRE of the 8S-LOX promoter is a mechanism by which TPA induces 8S-LOX expression in keratinocytes.

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.