Differential characteristics of human 15-lipoxygenase isozymes and a novel splice variant of 15S-lipoxygenase

Murine Alox8 versus the human ALOX15B ortholog: differences and similarities

Human arachidonate 15-lipoxygenase type B is a lipoxygenase that catalyzes the peroxidation of arachidonic acid at carbon-15. The corresponding murine ortholog however has 8-lipoxygenase activity. Both enzymes oxygenate polyunsaturated fatty acids in S-chirality with singular reaction specificity, although they generate a different product pattern. Furthermore, while both enzymes utilize both esterified fatty acids and fatty acid hydro(pero)xides as substrates, they differ with respect to the orientation of the fatty acid in their substrate-binding pocket. While ALOX15B accepts the fatty acid "tail-first," Alox8 oxygenates the free fatty acid with its "head-first." These differences in substrate orientation and thus in regio- and stereospecificity are thought to be determined by distinct amino acid residues. Towards their biological function, both enzymes share a commonality in regulating cholesterol homeostasis in macrophages, and Alox8 knockdown is associated with reduced atherosclerosis in mice. Additional roles have been linked to lung inflammation along with tumor suppressor activity. This review focuses on the current knowledge of the enzymatic activity of human ALOX15B and murine Alox8, along with their association with diseases.

Lipoxygenases at the Intersection of Infection and Carcinogenesis

The persisting presence of opportunistic pathogens like Pseudomonas aeruginosa poses a significant threat to many immunocompromised cancer patients with pulmonary infections. This review highlights the complexity of interactions in the host's defensive eicosanoid signaling network and its hijacking by pathogenic bacteria to their own advantage. Human lipoxygenases (ALOXs) and their mouse counterparts are integral elements of the innate immune system, mostly operating in the pro-inflammatory mode. Taking into account the indispensable role of inflammation in carcinogenesis, lipoxygenases have counteracting roles in this process. In addition to describing the structure-function of lipoxygenases in this review, we discuss their roles in such critical processes as cancer cell signaling, metastases, death of cancer and immune cells through ferroptosis, as well as the roles of ALOXs in carcinogenesis promoted by pathogenic infections. Finally, we discuss perspectives of novel oncotherapeutic approaches to harness lipoxygenase signaling in tumors.

Arachidonate 15-lipoxygenase type B: Regulation, function, and its role in pathophysiology

As a lipoxygenase (LOX), arachidonate 15-lipoxygenase type B (ALOX15B) peroxidizes polyenoic fatty acids (PUFAs) including arachidonic acid (AA), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and linoleic acid (LA) to their corresponding fatty acid hydroperoxides. Distinctive to ALOX15B, fatty acid oxygenation occurs with positional specificity, catalyzed by the non-heme iron containing active site, and in addition to free PUFAs, membrane-esterified fatty acids serve as substrates for ALOX15B. Like other LOX enzymes, ALOX15B is linked to the formation of specialized pro-resolving lipid mediators (SPMs), and altered expression is apparent in various inflammatory diseases such as asthma, psoriasis, and atherosclerosis. In primary human macrophages, ALOX15B expression is associated with cellular cholesterol homeostasis and is induced by hypoxia. Like in inflammation, the role of ALOX15B in cancer is inconclusive. In prostate and breast carcinomas, ALOX15B is attributed a tumor-suppressive role, whereas in colorectal cancer, ALOX15B expression is associated with a poorer prognosis. As the biological function of ALOX15B remains an open question, this review aims to provide a comprehensive overview of the current state of research related to ALOX15B.

The enzymology of human eicosanoid pathways: the lipoxygenase branches

Eicosanoids are short-lived derivatives of polyunsaturated fatty acids that serve as autocrine and paracrine signaling molecules. They are involved numerous biological processes of both the well state and disease states. A thorough understanding of the progression the disease state and homeostasis of the well state requires a complete evaluation of the systems involved. This review examines the enzymology for the enzymes involved in the production of eicosanoids along the lipoxygenase branches of the eicosanoid pathways with particular emphasis on those derived from arachidonic acid. The enzymatic parameters, protocols to measure them, and proposed catalytic mechanisms are presented in detail.

Characterization of Epidermal Lipoxygenase Expression in Normal Human Skin and Tissue-Engineered Skin Substitutes

Lipoxygenases (LOXs) are enzymes likely to be involved in corneocyte lipid envelope formation and skin barrier function. In humans, mutations in epidermis-type lipoxygenase 3 ( eLOX-3) and 12R-lipoxygenase ( 12R-LOX) genes are associated with autosomal recessive congenital ichthyosis (ARCI), whereas deletion of these genes in mice causes epidermal defects. LOXs also represent a matter of interest in psoriasis as well as in cancer research. However, their expression as well as the exact role of these enzymes in normal human skin have not been fully described. Our goal was to characterize the expression of epidermal LOXs in both normal human skin and Tissue-Engineered Skin Substitutes (TESS) and to consider TESS as a potential model for LOX functional studies. Staining for epidermal differentiation markers and LOXs was performed, in parallel, on normal human skin and TESS. Our results showed similar expression profiles in TESS when compared with native skin for e-LOX3, 12R-LOX, 12S-lipoxygenase (12S-LOX), and 15-lipoxygenase 2 (15-LOX-2) but not for 15-lipoxygenase 1 (15-LOX-1). Because of their appropriate epidermal differentiation and LOX expression, TESS represent an alternative model for future studies on LOX function.

Safer anti-inflammatory therapy through dual COX-2/5-LOX inhibitors: A structure-based approach

Inflammatory mediators of the arachidonic acid cascade from cyclooxygenase (COX) and lipoxygenase (LOX) pathways are primarily responsible for many diseases in human beings. Chronic inflammation is associated with the pathogenesis and progression of cancer, arthritis, autoimmune, cardiovascular and neurological diseases. Traditional non-steroidal anti-inflammatory agents (tNSAIDs) inhibit cyclooxygenase pathway non-selectively and produce gastric mucosal damage due to COX-1 inhibition and allergic reactions and bronchospasm resulting from increased leukotriene levels. 'Coxibs' which are selective COX-2 inhibitors cause adverse cardiovascular events. Inhibition of any of these biosynthetic pathways could switch the metabolism to the other, which can lead to fatal side effects. Hence, there is undoubtedly an urgent need for new anti-inflammatory agents having dual mechanism that prevent release of both prostaglandins and leukotrienes. Though several molecules have been synthesized with this objective, their unfavourable toxicity profile prevented them from being used in clinics. Here, this integrative review attempts to identify the promising pharmacophore that serves as dual inhibitors of COX-2/5-LOX enzymes with improved safety profile. A better acquaintance of structural features that balance safety and efficacy of dual inhibitors would be a different approach to the process of understanding and interpreting the designing of novel anti-inflammatory agents.

Structural Dynamics of 15-Lipoxygenase-2 via Hydrogen-Deuterium Exchange

Eicosanoids are inflammatory signaling lipids that are biosynthesized in response to cellular injury or threat. They were originally thought to be pro-inflammatory molecules, but members of at least one subclass, the lipoxins, are able to resolve inflammation. One step in lipoxin synthesis is the oxygenation of arachidonic acid by 15-lipoxygenase (15-LOX). 15-LOX contains two domains: a Ca²⁺ binding PLAT domain and a catalytic domain. 15-LOX is a soluble cytosolic protein until binding of Ca²⁺ to the PLAT domain promotes translocation to the membrane surface. The role of 15-LOX structural dynamics in this translocation has remained unclear. We investigated the dynamics of 15-LOX isoform B (15-LOX-2) upon binding of Ca²⁺ and ligands, as well as upon membrane association using hydrogen-deuterium exchange mass spectrometry (HDX-MS). We used HDX-MS to probe the solvent accessibility and backbone flexibility of 15-LOX-2, revealing significant differences in deuterium incorporation between the PLAT and catalytic domains, with the PLAT domain demonstrating higher flexibility. Comparison of HDX for 15-LOX-2 in the presence and absence of Ca²⁺ indicates there are few differences in structural dynamics. Furthermore, our HDX results involving nanodisc-associated 15-LOX-2 suggest that significant structural and dynamic changes in 15-LOX-2 are not required for membrane association. Our results also show that a substrate lipid binding to the active site in the catalytic domain does induce changes in incorporation of deuterium into the PLAT domain. Overall, our results challenge the previous hypothesis that Ca²⁺ binding induces major structural changes in the PLAT domain and support the hypothesis that is interdomain communication in 15-LOX-2.

Membrane-dependent Activities of Human 15-LOX-2 and Its Murine Counterpart: IMPLICATIONS FOR MURINE MODELS OF ATHEROSCLEROSIS

The enzyme encoded by the ALOX15B gene has been linked to the development of atherosclerotic plaques in humans and in a mouse model of hypercholesterolemia. In vitro, these enzymes, which share 78% sequence identity, generate distinct products from their substrate arachidonic acid: the human enzyme, a 15-S-hydroperoxy product; and the murine enzyme, an 8-S-product. We probed the activities of these enzymes with nanodiscs as membrane mimics to determine whether they can access substrate esterified in a bilayer and characterized their activities at the membrane interface. We observed that both enzymes transform phospholipid-esterified arachidonic acid to a 15-S-product. Moreover, when expressed in transfected HEK cells, both enzymes result in significant increases in the amounts of 15-hydroxyderivatives of eicosanoids detected. In addition, we show that 15-LOX-2 is distributed at the plasma membrane when the HEK293 cells are stimulated by the addition Ca(2+) ionophore and that cellular localization is dependent upon the presence of a putative membrane insertion loop. We also report that sequence differences between the human and mouse enzymes in this loop appear to confer distinct mechanisms of enzyme-membrane interaction for the homologues.

Tumor-suppressive functions of 15-Lipoxygenase-2 and RB1CC1 in prostate cancer

15-Lipoxygenase-2 (15-LOX2) is a human-specific lipid-peroxidizing enzyme most prominently expressed in epithelial cells of normal human prostate but downregulated or completely lost in>70% of prostate cancer (PCa) cases. Transgenic expression of 15-LOX2 in the mouse prostate surprisingly causes hyperplasia. Here we first provide evidence that 15-LOX2-induced prostatic hyperplasia does not progress to PCa even in p53(+/-) or p53(-/-) background. More important, by generating 15-LOX2; Hi-Myc double transgenic (dTg) mice, we show that 15-LOX2 expression inhibits Myc-induced PCa development, such that in the 3-month- and 6-month-old dTg mice, there is a significant reduction in prostate intraneoplasia (PIN) and PCa prevalent in age-matched Hi-Myc prostates. The dTg prostates show increased cell senescence and expression of several senescence-associated molecules, including p27, phosphorylated Rb, and Rb1cc1. We further show that in HPCa, 15-LOX2 and c-Myc manifest reciprocal protein expression patterns. Moreover, RB1CC1 accumulates in senescing normal human prostate (NHP) cells, and in both NHP and RWPE-1 cells, the 15-LOX2 metabolic products 15(S)-HPETE and 15(S)-HETE induce RB1CC1. We finally show that unlike 15-LOX2, RB1CC1 is not lost but rather frequently overexpressed in PCa samples. RB1CC1 knockdown in PC3 cells enhances clonal growth in vitro and tumor growth in vivo. Together, our present studies provide evidence for tumor-suppressive functions for both 15-LOX2 and RB1CC1.

The structure of human 15-lipoxygenase-2 with a substrate mimic

Atherosclerosis is associated with chronic inflammation occurring over decades. The enzyme 15-lipoxygenase-2 (15-LOX-2) is highly expressed in large atherosclerotic plaques, and its activity has been linked to the progression of macrophages to the lipid-laden foam cells present in atherosclerotic plaques. We report here the crystal structure of human 15-LOX-2 in complex with an inhibitor that appears to bind as a substrate mimic. 15-LOX-2 contains a long loop, composed of hydrophobic amino acids, which projects from the amino-terminal membrane-binding domain. The loop is flanked by two Ca(2+)-binding sites that confer Ca(2+)-dependent membrane binding. A comparison of the human 15-LOX-2 and 5-LOX structures reveals similarities at the active sites, as well striking differences that can be exploited for design of isoform-selective inhibitors.

Preeclampsia activates 15-lipoxygenase and its metabolite 15-hydroxyeicosatetraenoic acid enhances constriction in umbilical arteries

OBJECTIVE

To compare the differential expression of 15-lipoxygenase (15-LO) isoenzymes, 15-LO-1 and 15-LO-2 in preeclampsia (PE), and normal pregnancy and its metabolite 15-hydroxyeicosatetraenoic acid (15-HETE) on the vasoconstriction of human umbilical artery (HUA) rings.

STUDY DESIGN

We performed western blotting and isometric tension studies and t-test analysis on data from 6 women with normal pregnancy and 8 women with PE.

RESULTS

Expressions of 15-LO-1 and 15-LO-2 in placentas and HUA rings in PE increased more than that in normal groups (P<0.01). 15-HETE increased HUA rings tension in a dose-dependence manner, which were significantly greater in PE than in normal pregnant controls (P<0.01). However, the constriction of HUA rings was completely eliminated by 2-aminoethoxydiphenyl borate (2-APB) in both normal pregnancy and PE (P<0.01) and attenuated partly by nifedipine in dose-dependence in normal pregnancy (10(-8)mol/L P>0.05; 10(-7), 10(-6)mol/L P<0.05) and in PE (P<0.01).

CONCLUSION

PE upregulates 15-LO pathway via 15-HETE, which increased intercellular calcium level to cause constriction of HUA rings.

12- and 15-lipoxygenases in human carotid atherosclerotic lesions: associations with cerebrovascular symptoms

Lipoxygenase (ALOX) enzymes are implicated in both pro- and anti-atherogenic processes. The aim of this study was to investigate mRNA expression of 12- and 15-lipoxygenases (ALOX12, ALOX12B, ALOX15, ALOX15B) and the atypical ALOXE3 in human carotid atherosclerotic lesions, in relation to cerebrovascular symptoms and risk factors. The Biobank of Karolinska Endarterectomies (BiKE) collection of human carotid plaque tissue and associated clinical data was utilized (n=132). Lesion mRNA levels were analyzed by TaqMan qPCR (n=132) and microarray hybridization (n=77). Of the investigated mRNAs, only ALOX15B (15-LOX-2; epidermis-type 15-LOX) was readily detected in all plaque samples by qPCR, and thus suitable for quantitative statistical evaluation. ALOX12, ALOX12B, ALOX15 and ALOXE3 were detected with lower frequency and at lower levels, or virtually undetected. Microarray analysis confirmed ALOX15B as the most abundant 12- or 15-lipoxygenase mRNA in carotid lesions. Comparing plaques with or without attributable cerebrovascular symptoms (amaurosis fugax, transient ischemic attack, or stroke), ALOX15B mRNA levels were higher in symptomatic than asymptomatic plaques (1.31 [1.11-1.56], n=102; and 0.79 [0.55-1.15], n=30, respectively; p=0.008; mean [95% CI], arbitrary units). Multiple regression analysis confirmed symptomatic/asymptomatic status as a significant determinant of ALOX15B mRNA levels, independently of potentially confounding factors. Immunohistochemical analyses showed abundant ALOX15B expression in macrophage-rich areas of carotid lesions, and lipidomic analyses demonstrated the presence of typical ALOX15B products in plaque tissue. In summary, we observed associations between high ALOX15B expression in carotid lesions and a history of cerebrovascular symptoms. These findings suggest a link between ALOX15B and atherothrombotic events that merits further investigation.

Transgenic expression of 15-lipoxygenase 2 (15-LOX2) in mouse prostate leads to hyperplasia and cell senescence

15-Lipoxygenase 2 (15-LOX2), a lipid-peroxidizing enzyme, is mainly expressed in the luminal compartment of the normal human prostate, and is often decreased or lost in prostate cancer. Previous studies from our lab implicate 15-LOX2 as a functional tumor suppressor. To better understand the biological role of 15-LOX2 in vivo, we generated prostate-specific 15-LOX2 transgenic mice using the ARR2PB promoter. Unexpectedly, transgenic expression of 15-LOX2 or 15-LOX2sv-b, a splice variant that lacks arachidonic acid-metabolizing activity, resulted in age-dependent prostatic hyperplasia and enlargement of the prostate. Prostatic hyperplasia induced by both 15-LOX2 and 15-LOX2sv-b was associated with an increase in luminal and Ki-67(+) cells; however, 15-LOX2-transgenic prostates also showed a prominent increase in basal cells. Microarray analysis revealed distinct gene expression profiles that could help explain the prostate phenotypes. Strikingly, 15-LOX2, but not 15-LOX2sv-b, transgenic prostate showed upregulation of several well-known stem or progenitor cell molecules including Sca-1, Trop2, p63, Nkx3.1 and Psca. Prostatic hyperplasia caused by both 15-LOX2 and 15-LOX2sv-b did not progress to prostatic intraprostate neoplasia or carcinoma and, mechanistically, prostate lobes (especially those of 15-LOX2 mice) showed a dramatic increase in senescent cells as revealed by increased SA-betagal, p27(Kip1) and heterochromatin protein 1gamma staining. Collectively, our results suggest that 15-LOX2 expression in mouse prostate leads to hyperplasia and also induces cell senescence, which may, in turn, function as a barrier to tumor development.

Src-dependent STAT-3-mediated expression of monocyte chemoattractant protein-1 is required for 15(S)-hydroxyeicosatetraenoic acid-induced vascular smooth muscle cell migration

To understand the role of human 15-lipoxygenase 1 (15-LOX1) in vascular wall remodeling, we have studied the effect of the major 15-LOX1 metabolite of arachidonic acid, 15(S)-hydroxyeicosatetraenoic acid (15(S)-HETE), on vascular smooth muscle cell (VSMC) migration both in vitro and in vivo. Among 5(S)-HETE, 12(S)-HETE, and 15(S)-HETE, 15(S)-HETE potentially stimulated more vascular smooth muscle cell (VSMC) migration. In addition, 15(S)-HETE-induced VSMC migration was dependent on Src-mediated activation of signal transducer and activator of transcription-3 (STAT-3). 15(S)-HETE also induced monocyte chemoattractant protein-1 (MCP-1) expression via Src-STAT-3 signaling, and neutralizing anti-MCP-1 antibodies completely negated 15(S)-HETE-induced VSMC migration. Cloning and characterization of a 2.6-kb MCP-1 promoter revealed the presence of four putative STAT-binding sites, and the site that is proximal to the transcription start site was found to be essential for 15(S)-HETE-induced Src-STAT-3-mediated MCP-1 expression. Rat carotid arteries that were subjected to balloon injury and transduced with Ad-15-LOX1 upon exposure to [(3)H]arachidonic acid ex vivo produced 15-HETE as a major eicosanoid and enhanced balloon injury-induced expression of MCP-1 in smooth muscle cells in Src and STAT-3-dependent manner in vivo. Adenovirus-mediated delivery of 15-LOX1 into rat carotid artery also led to recruitment and homing of macrophages to medial region in response to injury. In addition, transduction of Ad-15-LOX1 into arteries enhanced balloon injury-induced smooth muscle cell migration from media to intima and neointima formation. These results show for the first time that 15-LOX1-15(S)-HETE axis plays a major role in vascular wall remodeling after balloon angioplasty.

Mechanistic investigations of human reticulocyte 15- and platelet 12-lipoxygenases with arachidonic acid

Human reticulocyte 15-lipoxygenase-1 (15-hLO-1) and human platelet 12-lipoxygenase (12-hLO) have been implicated in a number of diseases, with differences in their relative activity potentially playing a central role. In this work, we characterize the catalytic mechanism of these two enzymes with arachidonic acid (AA) as the substrate. Using variable-temperature kinetic isotope effects (KIE) and solvent isotope effects (SIE), we demonstrate that both k(cat)/K(M) and k(cat) for 15-hLO-1 and 12-hLO involve multiple rate-limiting steps that include a solvent-dependent step and hydrogen atom abstraction. A relatively low k(cat)/K(M) KIE of 8 was determined for 15-hLO-1, which increases to 18 upon the addition of the allosteric effector molecule, 12-hydroxyeicosatetraenoic acid (12-HETE), indicating a tunneling mechanism. Furthermore, the addition of 12-HETE lowers the observed k(cat)/K(M) SIE from 2.2 to 1.4, indicating that the rate-limiting contribution from a solvent sensitive step in the reaction mechanism of 15-hLO-1 has decreased, with a concomitant increase in the C-H bond abstraction contribution. Finally, the allosteric binding of 12-HETE to 15-hLO-1 decreases the K(M)[O(2)] for AA to 15 microM but increases the K(M)[O(2)] for linoleic acid (LA) to 22 microM, such that the k(cat)/K(M)[O(2)] values become similar for both substrates (approximately 0.3 s(-1) microM(-1)). Considering that the oxygen concentration in cancerous tissue can be less than 5 microM, this result may have cellular implications with respect to the substrate specificity of 15-hLO-1.

CREB-mediated IL-6 expression is required for 15(S)-hydroxyeicosatetraenoic acid-induced vascular smooth muscle cell migration

OBJECTIVE

Migration of vascular smooth muscle cells (VSMCs) from media to intima is a key event in the pathophysiology of atherosclerosis and restenosis. The lipoxygenase products of polyunsaturated fatty acids (PUFA) were shown to play a role in these diseases. cAMP response element binding protein (CREB) has been implicated in the regulation of VSMC growth and motility in response to thrombin and angiotensin II. The aim of the present study was to test the role of CREB in an oxidized lipid molecule, 15(S)-HETE-induced VSMC migration and neointima formation.

METHODS AND RESULTS

15(S)-HETE stimulated VSMC migration in CREB-dependent manner, as measured by the modified Boyden chamber method. Blockade of MEK1, JNK1, or p38MAPK inhibited 15(S)-HETE-induced CREB phosphorylation and VSMC migration. 15(S)-HETE induced expression and secretion of interleukin-6 (IL-6), as analyzed by RT-PCR and ELISA, respectively. Neutralizing anti-IL-6 antibodies blocked 15(S)-HETE-induced VSMC migration. Dominant-negative mutant-mediated blockade of ERK1/2, JNK1, p38MAPK, or CREB suppressed 15(S)-HETE-induced IL-6 expression in VSMCs. Serial 5' deletions and site-directed mutagenesis of IL-6 promoter along with chromatin immunoprecipitation using anti-CREB antibodies showed that cAMP response element is essential for 15(S)-HETE-induced IL-6 expression. Dominant-negative CREB also suppressed balloon injury-induced IL-6 expression, SMC migration from media to intimal region, and neointima formation. Adenovirus-mediated transduction of 15-lipoxygenase 2 (15-LOX2) caused increased production of 15-HETE in VSMCs and enhanced IL-6 expression, SMC migration from media to intimal region, and neointima formation in response to arterial injury.

CONCLUSIONS

The above results suggest a role for 15-LOX2-15-HETE in the regulation of VSMC migration and neointima formation involving CREB-mediated IL-6 expression.

A novel role for activating transcription factor-2 in 15(S)-hydroxyeicosatetraenoic acid-induced angiogenesis

To investigate the mechanisms underlying 15(S)-HETE-induced angiogenesis, we have studied the role of the small GTPase, Rac1. We find that 15(S)-HETE activated Rac1 in human retinal microvascular endothelial cells (HRMVEC) in a time-dependent manner. Blockade of Rac1 by adenovirus-mediated expression of its dominant negative mutant suppressed HRMVEC migration as well as tube formation and Matrigel plug angiogenesis. 15(S)-HETE stimulated Src in HRMVEC in a time-dependent manner and blockade of its activation inhibited 15(S)-HETE-induced Rac1 stimulation in HRMVEC and the migration and tube formation of these cells as well as Matrigel plug angiogenesis. 15(S)-HETE stimulated JNK1 in Src-Rac1-dependent manner in HRMVEC and adenovirus-mediated expression of its dominant negative mutant suppressed the migration and tube formation of these cells and Matrigel plug angiogenesis. 15(S)-HETE activated ATF-2 in HRMVEC in Src-Rac1-JNK1-dependent manner and interference with its activation via adenovirus-mediated expression of its dominant negative mutant abrogated migration and tube formation of HRMVEC and Matrigel plug angiogenesis. In addition, 15(S)-HETE-induced MEK1 stimulation was found to be dependent on Src-Rac1 activation. Blockade of MEK1 activation inhibited 15(S)-HETE-induced JNK1 activity and ATF-2 phosphorylation. Together, these findings show that 15(S)-HETE activates ATF-2 via the Src-Rac1-MEK1-JNK1 signaling axis in HRMVEC leading to their angiogenic differentiation.

Substrate specificity changes for human reticulocyte and epithelial 15-lipoxygenases reveal allosteric product regulation

Human reticulocyte 15-lipoxygenase (15-hLO-1) and epithelial 15-lipoxygenase (15-hLO-2) have been implicated in a number of human diseases, with differences in their substrate specificity potentially playing a central role. In this paper, we present a novel method for accurately measuring the substrate specificity of the two 15-hLO isozymes and demonstrate that both cholate and specific LO products affect substrate specificity. The linoleic acid (LA) product, 13-hydroperoxyoctadienoic acid (13-HPODE), changes the ( k cat/ K m) (AA)/( k cat/ K m) (LA) ratio more than 5-fold for 15-hLO-1 and 3-fold for 15-hLO-2, while the arachidonic acid (AA) product, 12-( S)-hydroperoxyeicosatetraenoic acid (12-HPETE), affects only the ratio of 15-hLO-1 (more than 5-fold). In addition, the reduced products, 13-( S)-hydroxyoctadecadienoic acid (13-HODE) and 12-( S)-hydroxyeicosatetraenoic acid (12-HETE), also affect substrate specificity, indicating that iron oxidation is not responsible for the change in the ( k cat/ K m) (AA)/( k cat/ K m) (LA) ratio. These results, coupled with the dependence of the 15-hLO-1 k cat/ K m kinetic isotope effect ( (D) k cat/ K m) on the presence of 12-HPETE and 12-HETE, indicate that the allosteric site, previously identified in 15-hLO-1 [Mogul, R., Johansen, E., and Holman, T. R. (1999) Biochemistry 39, 4801-4807], is responsible for the change in substrate specificity. The ability of LO products to regulate substrate specificity may be relevant with respect to cancer progression and warrants further investigation into the role of this product-feedback loop in the cell.

Dietary n-3 polyunsaturated fatty acids enhance hormone ablation therapy in androgen-dependent prostate cancer

Hormone ablation therapy typically causes regression of prostate cancer and represents an important means of treating this disease, particularly after metastasis. However, hormone therapy inevitably loses its effectiveness as tumors become androgen-independent, and this conversion often leads to death because of subsequent poor responses to other forms of treatment. Because environmental factors, such as diet, have been strongly linked to prostate cancer, we examined the affects of dietary polyunsaturated fatty acids (PUFAs; at 1.5 wt%) on growth of androgen-dependent (CWR22) and androgen-independent (CWR22R) human prostatic cancer xenografts, the acute response of CWR22 tumors to ablation therapy, and their progression to androgen independence. Significant diet-induced changes in tumor n-3 or n-6 PUFA content had no affect on CWR22 or CWR22R tumors growing with or without androgen support, respectively. However, dietary changes that increased tumor eicosapentaenoic acid and linoleic acid content enhanced responses to ablation therapy, measured by cancer cell apoptosis and mitosis. In addition, relapse to androgen-independent growth (measured by renewed increases in tumor volume and serum prostate-specific antigen after ablation) positively correlated with tumor arachidonic acid content. There was no correlation between expression of 15-lipoxygenase isozymes or their products and tumor growth or responses to ablation. In conclusion, dietary n-3 PUFA may enhance the response of prostate cancer to ablation therapy and retard progression to androgen-independent growth by altering tumor PUFA content.

Isotope sensitive branching and kinetic isotope effects in the reaction of deuterated arachidonic acids with human 12- and 15-lipoxygenases

Lipoxygenases (LOs) catalyze lipid peroxidation and have been implicated in a number of human diseases connected to oxidative stress and inflammation. These enzymes have also attracted considerable attention due to large kinetic isotope effects (30-80) for the rate-limiting hydrogen abstraction step with linoleic acid (LA) as substrate. Herein, we report kinetic isotope effects (KIEs) in the reactions of three human LOs (platelet 12-hLO, reticulocyte 15-hLO-1, and epithelial 15-hLO-2) with arachidonic acid (AA). Surprisingly, the observed KIEs with AA were much smaller than the previously reported values with LA. Investigation into the origins for the smaller KIEs led to the discovery of isotope sensitive branching of the reaction pathways. Product distribution analysis demonstrated an inversion in the regioselectivity of 15-hLO-1, with hydrogen abstraction from C13 being the major pathway with unlabeled AA but abstraction from C10 predominating when the methylene group at position 13 was deuterated. Smaller but clear changes in regioselectivity were also observed for 12-hLO and 15-hLO-2.

Oxidative metabolism of a fatty acid amide hydrolase-regulated lipid, arachidonoyltaurine

A novel class of lipids, N-acyltaurines, was recently discovered in fatty acid amide hydrolase knockout mice. In some peripheral tissues, such as liver and kidney, N-acyltaurines with long, polyunsaturated acyl chains are most prevalent. Polyunsaturated fatty acids are converted to a variety of signaling molecules by cyclooxygenases (COXs) and lipoxygenases (LOXs). The ability of COXs and LOXs to oxygenate arachidonoyltaurine was evaluated to gain insight into the potential metabolic fate of N-acyltaurines. Although arachidonoyltaurine was a poor substrate for COXs, mammalian 12 S- and 15 S-LOXs oxygenated arachidonoyltaurine with similar or better efficiency than arachidonic acid. Products of arachidonoyltaurine oxygenation were characterized by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The positional specificity of single oxygenation was retained for 15 S-LOXs. However, platelet-type 12 S-LOX produced 12- and 15-hydroxyeicosatetraenoyltaurines (HETE-Ts). Furthermore, LOXs generated dihydroxyeicosatetraenoyltaurines (diHETE-Ts). Metabolism of arachidonoyltaurine by murine resident peritoneal macrophages (RPMs) was also profiled. Arachidonoyltaurine was rapidly taken up and converted primarily to 12-HETE-T. Over prolonged incubations, RPMs also generated small amounts of diHETE-T. Oxidative metabolism of polyunsaturated N-acyltaurines may represent a pathway for the generation or termination of novel signaling molecules.

15-Lipoxygenase-2 expression in human macrophages induces chemokine secretion and T cell migration

BACKGROUND

We determined previously that hypoxia results in increased 15-lipoxygenase type 2 (15-LOX-2) expression and CXCL8 secretion in macrophages. This study sought to determine whether 15-LOX-2 expression links directly with the secretion of inflammatory molecules in macrophages and also investigated its subsequent effects on T cell migration.

METHODS

Adenovirus-mediated gene delivery caused overexpression of 15-LOX-2 in human macrophages. We used cytometric bead array to measure chemokine secretion, and assessed T cell migration by counting cells in chemotaxis chambers. Expression of chemokine receptors was determined by FACS analysis. Using siRNA, we reduced 15-LOX-2 expression in human macrophages. We used scrambled siRNA as control.

RESULTS

Macrophages that overexpress 15-LOX-2 showed increased secretion of chemokine CXCL10 after 24h incubation. In addition, preconditioned medium from 15-LOX-2-overexpressing cells increased T cell migration and surface expression of CXCR3, the CXCL10 receptor. Knockdown of 15-LOX-2 expression decreased CXCL10 secretion from hypoxic macrophages and also reduced T cell migration.

CONCLUSION

In macrophages, overexpression of 15-LOX-2 results in increased secretion of CXCL10 and CCL2. Products released in response to increased 15-LOX-2 activation lead to increased expression of CD69, the T cell activation marker as well as increased T cell migration. Therefore, increased expression of 15-LOX-2 induced by hypoxia may participate in T cell recruitment in diseases such as atherosclerosis.

Cyclooxygenase-2-mediated metabolism of arachidonic acid to 15-oxo-eicosatetraenoic acid by rat intestinal epithelial cells

Rat intestinal epithelial cells that permanently express the cyclooxygenase-2 (COX-2) gene (RIES cells) were used to investigate COX-2-mediated arachidonic acid (AA) metabolism. A targeted chiral lipidomics approach was employed to quantify AA metabolites that were secreted by the cells into the culture media. When intact RIES cells were treated with calcium ionophore A-23187 (1 microM) for 1 h, 11-(R)-hydroxyeicosatetraenoic acid (HETE) was the most abundant metabolite, followed by prostaglandin (PG) E 2, 15-(S)-HETE, 15-oxo-eicosatetraenoic acid (ETE), and 15-(R)-HETE. Incubation for a further 23 h after the calcium ionophore was removed resulted in a substantial increase in PGE 2 concentrations while HETE and 15-oxo-ETE concentrations decreased to almost undetectable levels. A similar metabolic profile was observed when RIES cells were treated with increasing concentrations of AA for 24 h. Incubation of the RIES cells with 10 microM AA revealed that maximal concentrations of 11-(R)-HETE, 15-(S)-HETE, and 15-oxo-ETE occurred after 10 min of incubation when the 15-( S)-HETE concentrations were approximately twice that of PGE 2. There was a gradual decrease in the concentrations of HETE and 15-oxo-ETE over time, whereas PGE 2 concentrations increased steadily until they reached a maximum after 24 h of incubation. The ratio of PGE 2 to 15-(S)-HETE was then approximately 20:1. 15-(S)-HETE and 15-oxo-ETE concentrations declined in the cell media during prolonged incubations with pseudo-first-order rate constants of 0.0121 and 0.0073 min(-1), respectively. 15-(S)-HETE was shown to undergo metabolism primarily to 15-oxo-ETE, which was further metabolized to a glutathione (GSH) adduct. The GSH adduct of 15-oxo-ETE was further metabolized in the extracellular milieu to a cysteinylglycine adduct. Thus, we have established for the first time that 15-oxo-ETE can be formed biosynthetically from AA, that 15-(S)-HETE is its immediate precursor, and that 15-oxo-ETE forms a GSH adduct. For ionophore-A-23187-stimulated cells and at early time points for AA-stimulated cells, 11-(R)-HETE was the major eicosanoid to be secreted into the media. Adding increasing concentrations of AA to cells in culture made it possible to estimate with surprising accuracy endogenous eicosanoid production using regression analyses. Thus, after 24 h in the absence of added AA, 11-(R)-HETE and 15-(R)-HETE were estimated to be present at concentrations close to the detection limit of our very sensitive assay. These data further highlight the importance of endogenous COX-2-mediated lipid peroxidation and illustrate the necessity to monitor eicosanoid formation from endogenous stores of AA in cell culture experiments.

Structure-activity relationship studies of flavonoids as potent inhibitors of human platelet 12-hLO, reticulocyte 15-hLO-1, and prostate epithelial 15-hLO-2

Human lipoxygenase (hLO) isozymes have been implicated in a number of disease states and have attracted much attention with respect to their inhibition. One class of inhibitors, the flavonoids, have been shown to be potent lipoxygenase inhibitors but their study has been restricted to those compounds found in nature, which have limited structural variability. We have therefore carried out a comprehensive study to determine the structural requirements for flavonoid potency and selectivity against platelet 12-hLO, reticulocyte 15-hLO-1, and prostate epithelial 15-hLO-2. We conclude from this study that catechols are essential for high potency, that isoflavones and isoflavonones tend to select against 12-hLO, that isoflavons tend to select against 15-hLO-1, but few flavonoids target 15-hLO-2.

Oxidative metabolism of lipoamino acids and vanilloids by lipoxygenases and cyclooxygenases

The lipoamino acids and endovanilloids have multiple roles in nociception, pain, and inflammation, yet their biological reactivity has not been fully characterized. Cyclooxygenases (COXs) and lipoxygenases (LOs) oxygenate polyunsaturated fatty acids to generate signaling molecules. The ability of COXs and LOs to oxygenate arachidonyl-derived lipoamino acids and vanilloids was investigated. COX-1 and COX-2 were able to minimally metabolize many of these species. However, the lipoamino acids were efficiently oxygenated by 12S- and 15S-LOs. The kinetics and products of oxygenation by LOs were characterized. Whereas 15S-LOs retained positional specificity of oxygenation with these novel substrates, platelet-type 12S-LO acted as a 12/15-LO. Fatty acid oxygenases may play an important role in the metabolic inactivation of lipoamino acids or vanilloids or may convert them to bioactive derivatives.

15-Lipoxygenase 2 (15-LOX2) is a functional tumor suppressor that regulates human prostate epithelial cell differentiation, senescence, and growth (size)

15-Lipoxygenase 2 (15-LOX2) is the major mammalian lipoxygenase expressed in normal human adult prostate and its expression is decreased or lost in high-grade prostate intraepithelial neoplasia (HGPIN) and prostate cancer (PCa). Our recent work has demonstrated that (1) 15-LOX2 has multiple alternatively spliced isoforms and is a negative cell-cycle regulator in normal human prostate (NHP) epithelial cells; (2) 15-LOX2 in NHP cells is positively regulated by Sp1 and negatively regulated by Sp3; (3) 15-LOX2 in NHP cells may be partially involved in cell differentiation; (4) 15-LOX2 is cell-autonomously upregulated in cultured NHP cells and its induction is associated with NHP cell senescence; and (5) 15-LOX2 is a functional prostate tumor suppressor. Here we summarize these new findings to provide a concise view of the potential biological functions of 15-LOX2 in NHP cells and of its deregulation in PCa development.

Reduced expression of 15-lipoxygenase 2 in human head and neck carcinomas

Nonsteroidal anti-inflammatory drugs (NSAIDs) have demonstrated cancer chemoprevention effects associated with their ability to modulate polyunsaturated fatty acid metabolism. In the present study, we report a significant reduction of 15-lipoxygenase 2 (15-LOX-2) in seven carcinoma cell lines of the human head and neck when compared with normal primary cultured keratinocytes, and 18 primary head and neck squamous cell carcinomas (HNSCC) when compared with matched normal mucosa. 15-LOX-2 is mainly expressed in the mature cells of the benign squamous epithelium, but not in the basal layer cells of benign epithelium, suggesting a role of 15-LOX-2 in cell differentiation. We further found that 15-lipoxygenase activity was reduced in carcinoma cells when compared with normal primary cultured keratinocytes. When the effects of NSAIDs were examined on cell proliferation and regulation of 15-LOX-2 in the carcinoma cells, NS398 treatment resulted in significant growth inhibition associated with upregulation of 15-LOX-2 and its major metabolite 15-S-HETE. Finally, restoration of 15-LOX-2 expression into these carcinoma cells significantly inhibited cell proliferation. Our results demonstrate that 15-LOX-2 expression is significantly reduced and this reduction may promote proliferation in human head and neck carcinoma. 15-LOX-2 may be a possible biomarker in human head and neck malignancy.

Interleukin-4 Induces 15-Lipoxygenase-1 Expression in Human Orbital Fibroblasts from Patients with Graves Disease

Orbital fibroblasts orchestrate tissue remodeling in Graves disease, at least in part, because they exhibit exaggerated responses to proinflammatory cytokines. A hallmark of late stage orbital disease is vision-threatening fibrosis, the molecular basis of which remains uncertain. We report here that the Th2 cytokines, interleukin (IL)-4 and IL-13, can induce in these cells the expression of 15-lipoxygenase-1 (15-LOX-1) and in so doing up-regulate the production of 15-hydroxyeicosatetraenoic acid. IL-4 increases 15-LOX-1 protein levels through pretranslational actions. The increased steady-state 15-LOX-1 mRNA is independent of ongoing protein synthesis and involves very modestly increased gene promoter activity. Importantly, IL-4 substantially enhances 15-LOX-1 transcript stability, activity that localizes to a 293-bp sequence of the 3'-untranslated region. IL-4 activates Jak2 in orbital fibroblasts. Interrupting signaling through that pathway, either with the specific chemical inhibitor, AG490, or by transiently transfecting the cells with a Jak2 dominant negative mutant kinase, attenuates the 15-LOX-1 induction. Interferongamma, a Th1 cytokine, could block this induction by attenuating IL-4-dependent mRNA stabilization. 15-LOX-1 protein and its mRNA were undetectable in IL-4-treated dermal fibroblasts, despite comparable levels of cell surface IL-4 receptor and phosphorylated Jak2 and STAT6. Our findings suggest that orbital connective tissues may represent a site of localized 15-hydroxyeicosatetraenoic acid generation resulting from cell type-specific 15-LOX-1 mRNA stabilization by IL-4. These results may have relevance to the pathogenesis of orbital Graves disease, an inflammatory autoimmune condition that gives way to extensive fibrosis associated with a Th2 response.

15-Lipoxygenase-2 gene regulation by its product 15-(S)-hydroxyeicosatetraenoic acid through a negative feedback mechanism that involves peroxisome proliferator-activated receptor gamma

An inverse relationship exists between the expression of 15-lipoxygenase-2 (15-LOX-2) and peroxisome proliferator-activated receptor gamma (PPARgamma) in normal prostate epithelial cells (PrECs) compared with their expression in prostate carcinoma cells (PC-3). The reason for this difference, however, is unknown. We hypothesized that this inverse expression partly involves the 15-LOX-2 promoter and 15-S-hydroxyeicosatetraenoic acid (15-(S)-HETE), a product of 15-LOX-2 that binds to PPARgamma. We identified an active steroid nuclear receptor half-site present in the 15-LOX-2 promoter fragment F-5 (-618/+177) that can interact with PPARgamma. After forced expression of wild-type PPARgamma, 15-(S)-HETE (1 microM) decreased F-5 reporter activity in PrECs whereas forced expression of 15-LOX-2 resulted in 15-(S)-HETE production which enhanced F-5 activity in PC-3. In contrast, the expression of dominant-negative PPARgamma reversed the transcriptional activation of F-5 by enhancing it 202-fold in PrEC or suppressing it in PC-3; the effect in PC-3 was positively increased 150-fold in the presence of 15-(S)-HETE (1 microM). Peroxisome proliferator-activated receptor gamma interacted with 15-LOX-2 promoter sequences in pulldown experiments using biotinylated 15-LOX-2 (-560/-596 bp) oligonucleotides. In gelshift analyses PPARgamma and orphan receptor RORalpha were shown to interact with the F-5 fragment in PC-3 cells. These data suggest that crosstalk mechanisms exist between the 15-LOX-2 gene and PPARgamma to counterbalance expression and help explain the inverse relationship of these genes in normal versus cancer cells.

15(S)-Lipoxygenase-2 Mediates Arachidonic Acid-stimulated Adhesion of Human Breast Carcinoma Cells through the Activation of TAK1, MKK6, and p38 MAPK

The dietary cis-polyunsaturated fatty acid, arachidonic acid, stimulates adhesion of metastatic human breast carcinoma cells (MDA-MB-435) to the extracellular matrix, but the molecular mechanisms by which fatty acids modify the behavior of these cells are unclear. Exposure to arachidonic acid activates multiple signaling pathways. Activation of p38 mitogen-activated protein kinase (p38 MAPK) is required for increased cell adhesion to type IV collagen, and this activation is sensitive to inhibitors of lipoxygenases, suggesting a requirement for arachidonic acid metabolism. The goals of the current study were to identify the one or more key metabolites of arachidonic acid that are responsible for activation of p38 MAPK and to elucidate the upstream kinases that lead to p38 MAPK activation. High performance liquid chromatographic analysis revealed that MDA-MB-435 cells metabolize exogenous arachidonic acid predominantly to 15(S)-hydroxyeicosatetraenoic acid (15(S)-HETE). Immunoblot analysis with antibodies specific to 15(S)-lipoxygenase-1 (LOX-1) and 15(S)-lipoxygenase-2 (LOX-2) demonstrated the expression of 15-LOX-2, but not 15-LOX-1, in these tumor cells. A LOX inhibitor, nordihydroguaiaretic acid, attenuated production of 15(S)-HETE and inhibited the phosphorylation of p38 MAPK following exposure to arachidonic acid. In contrast, overexpression of LOX-2 sensitized the cells to the addition of arachidonic acid, leading to increased activation of p38 MAPK. Addition of exogenous 15(S)-HETE to MDA-MB-435 cells stimulated cell adhesion to type IV collagen and activated the p38 MAPK pathway, including the upstream kinases transforming growth factor-beta1-activated protein kinase-1 (TAK1) and MAPK kinase 6. Transfection of these cells with a dominant negative form of TAK1 blocked arachidonic acid-stimulated p38 MAPK phosphorylation. These data demonstrate that 15(S)-LOX-2 generation of 15(S)-HETE activates specific growth factor receptor-related signaling pathways, thereby initiating signal transduction events leading to increased cell adhesion to the extracellular matrix.

Cell-autonomous induction of functional tumor suppressor 15-lipoxygenase 2 (15-LOX2) contributes to replicative senescence of human prostate progenitor cells

Normal human prostatic (NHP) epithelial cells undergo senescence in vitro and in vivo, but little is known about the tissue-specific molecular mechanisms. Here we first characterize young primary NHP cells as CK5(+)/CK18(+) intermediate basal cells that also express several other putative stem/progenitor cell markers including p63, CD44, alpha2beta1, and hTERT. When cultured in serum- and androgen-free medium, NHP cells gradually lose the expression of these markers, slow down in proliferation, and enter senescence. Several pieces of evidence implicate 15-lipoxygenase 2 (15-LOX2), a molecule with a restricted tissue expression and most abundantly expressed in adult human prostate, in the replicative senescence of NHP cells. First, the 15-LOX2 promoter activity and the mRNA and protein levels of 15-LOX2 and its multiple splice variants are upregulated in serially passaged NHP cells, which precede replicative senescence and occur in a cell-autonomous manner. Second, all immortalized prostate epithelial cells and prostate cancer cells do not express 15-LOX2. Third, PCa cells stably transfected with 15-LOX2 or 15-LOX2sv-b, a splice variant that does not possess arachidonate-metabolizing activity, show a passage-related senescence-like phenotype. Fourth, infection of early-passage NHP cells with retroviral vectors encoding 15-LOX2 or 15-LOX2sv-b induces partial cell-cycle arrest and big and flat senescence-like phenotype. Finally, 15-LOX2 protein expression in human prostate correlates with age. Together, these data suggest that 15-LOX2 may represent an endogenous prostate senescence gene and its tumor-suppressing functions might be associated with its ability to induce cell senescence.

Hypoxia Increases LDL Oxidation and Expression of 15-Lipoxygenase-2 in Human Macrophages

Objective— Macrophage-mediated oxidation of low-density lipoprotein (LDL) by enzymes, such as the lipoxygenases, is considered of major importance for the formation of oxidized LDL during atherogenesis. Macrophages have been identified in hypoxic areas in atherosclerotic plaques.

Methods and Results— To investigate the role of hypoxia in macrophage-mediated LDL oxidation, we incubated human monocyte-derived macrophages with LDL under normoxic (21% O 2 ) or hypoxic (0% O 2 ) conditions. The results showed that hypoxic macrophages oxidized LDL to a significantly higher extent than normoxic cells. Interestingly, the mRNA and protein expression of 15-lipoxygenase-2 (15-LOX-2) as well as the activity of this enzyme are elevated in macrophages incubated at hypoxia. Both the unspliced 15-LOX-2 and the spliced variant 15-LOX-2sv-a are found in macrophages. In addition, 15-LOX-2 was identified in carotid plaques in some macrophage-rich areas but was only expressed at low levels in nondiseased arteries.

Conclusions— In summary, these observations show for the first time that 15-LOX-2 is expressed in hypoxic macrophages and in atherosclerotic plaques and suggest that 15-LOX-2 may be one of the factors involved in macrophage-mediated LDL oxidation at hypoxia.

Evidence that Sp1 positively and Sp3 negatively regulate and androgen does not directly regulate functional tumor suppressor 15-lipoxygenase 2 (15-LOX2) gene expression in normal human prostate epithelial cells

In this project, we studied the gene regulation of 15-lipoxygenase 2 (15-LOX2), the most abundant arachidonate-metabolizing LOX in adult human prostate and a negative cell-cycle regulator in normal human prostate (NHP) epithelial cells. Through detailed in silico promoter examination and promoter deletion and activity analysis, we found that several Sp1 sites (i.e., three GC boxes and one CACCC box) in the proximal promoter region play a critical role in regulating 15-LOX2 expression in NHP cells. Several pieces of evidence further suggest that the Sp1 and Sp3 proteins play a physiologically important role in positively and negatively regulating the 15-LOX2 gene expression, respectively. First, mutations in the GC boxes affected the 15-LOX2 promoter activity. Second, both Sp1 and Sp3 proteins were detected in the protein complexes that bound the GC boxes revealed by electrophoretic mobility shift assay. Third, importantly, inhibition of Sp1 activity or overexpression of Sp3 both inhibited the endogenous 15-LOX2 mRNA expression. Since 15-LOX2 is normally expressed in the prostate luminal epithelial cells, we subsequently explored whether androgen/androgen receptor may directly regulate its gene expression. The results indicate that androgen does not directly regulate 15-LOX2 gene expression. Together, these observations provide insight on how 15-LOX2 gene expression may be regulated in NHP cells.

15-hydroxy-eicosatetraenoic acid arrests growth of colorectal cancer cells via a peroxisome proliferator-activated receptor gamma-dependent pathway

Peroxisome proliferator-activated receptor gamma (PPARgamma) inhibits cell growth via promoting apoptosis. Human colorectal cancer tissues had abundant PPARgamma but the incidence of apoptosis was very low, suggesting a defect in the PPARgamma pathway. Here, we found that 15-hydroxy-eicosatetraenoic acid (15S-HETE), an endogenous ligand for PPARgamma, was significantly decreased in the serum of patients with colorectal cancer. Treatment of colon cancer cells with 15S-HETE inhibited cell proliferation and induced apoptosis, which was preceded by an increase in TGF-beta-inducible early gene (TIEG) and a decrease in Bcl-2. The action of 15S-HETE could be blocked when PPARgamma was suppressed. Overexpression of Bcl-2 prevented the apoptosis. The levels of TIEG and 15-lipoxygenase (15-LOX), the enzyme responsible for 15S-HETE production, was decreased in colorectal cancer. Therefore, colorectal cancer is associated with decreased 15S-HETE. Treatment of colon cancer cells with 15S-HETE inhibits cell proliferation and induces apoptosis in a PPARgamma-dependent pathway involving augmentation of TIEG and reduction of Bcl-2 expression.

Subcellular localization and tumor-suppressive functions of 15-lipoxygenase 2 (15-LOX2) and its splice variants

15-Lipoxygenase 2 (15-LOX2), the most abundant arachidonate (AA)-metabolizing enzyme expressed in adult human prostate, is a negative cell-cycle regulator in normal human prostate epithelial cells. Here we study the subcellular distribution of 15-LOX2 and report its tumor-suppressive functions. Immunocytochemistry and biochemical fractionation reveal that 15-LOX2 is expressed at multiple subcellular locations, including cytoplasm, cytoskeleton, cell-cell border, and nucleus. Surprisingly, the three splice variants of 15-LOX2 we previously cloned, i.e. 15-LOX2sv-a/b/c, are mostly excluded from the nucleus. A potential bi-partite nuclear localization signal (NLS),203RKGLWRSLNEMKRIFNFRR221, is identified in the N terminus of 15-LOX2, which is retained in all splice variants. Site-directed mutagenesis reveals that this putative NLS is only partially involved in the nuclear import of 15-LOX2. To elucidate the relationship between nuclear localization, enzymatic activity, and tumor suppressive functions, we established PCa cell clones stably expressing 15-LOX2 or 15-LOX2sv-b. The 15-LOX2 clones express 15-LOX2 in the nuclei and possess robust enzymatic activity, whereas 15-LOX2sv-b clones show neither nuclear protein localization nor AA-metabolizing activity. To our surprise, both 15-LOX2- and 15-LOX2sv-b-stable clones proliferate much slower in vitro when compared with control clones. More importantly, when orthotopically implanted in nude mouse prostate, both 15-LOX2 and 15-LOX2sv-b suppress PC3 tumor growth in vivo. Together, these results suggest that both 15-LOX2 and 15-LOX2sv-b suppress prostate tumor development, and the tumor-suppressive functions apparently do not necessarily depend on AA-metabolizing activity and nuclear localization.

Reduced 15S-lipoxygenase-2 expression in esophageal cancer specimens and cells and upregulation in vitro by the cyclooxygenase-2 inhibitor, NS398

Alterations in arachidonic acid metabolism are involved in human carcinogenesis. Cyclooxygenase (COX) and lipoxygenase (LOX) are key enzymes in this metabolism. We analyzed the expression of 15S-lipoxygenase-2 (15-LOX-2) mRNA and protein in surgical specimens from normal (N=37) and malignant (63) esophageal tissues using in situ hybridization and immunohistochemistry (IHC), and in normal (1), premalignant (1), and malignant (5) esophageal cell lines using Northern and Western blotting. 15-LOX-2 was expressed in normal esophageal epithelial cells (EECs) at the highest levels, whereas an SV40-immortalized HET-1A line and three of five esophageal cancer cell lines failed to express it at detectable levels. 15-LOX-2 was detected in 76% (28/37) of the normal esophageal mucosae, but only in 46% (29/63) of the cancer specimens using IHC (P<.01). Transient transfection of 15-LOX-2 expression vectors into esophageal cancer cells significantly inhibited the proliferation of 15-LOX-2-negative cancer cells. The COX-2 inhibitor, NS398, induced 15-LOX-2 expression in esophageal cancer cells, which is associated with reduced cell viability. This study demonstrated that 15-LOX-2 expression is lost in esophageal cancers and that the induction of 15-LOX-2 can inhibit cancer cell proliferation. Further investigation of the effects of nonsteroidal anti-inflammatory drugs on 15-LOX-2 expression and apoptosis in esophageal cancer cells may be warranted.

Gene expression and immunolocalization of 15-lipoxygenase isozymes in the airway mucosa of smokers with chronic bronchitis

15-lipoxygenase (15-LO) has been implicated in the inflammation of chronic bronchitis (CB), but it is unclear which of its isoforms, 15-LOa or 15-LOb, is primarily involved. To detect 15-LO gene (mRNA) and protein expression, we have applied in situ hybridization (ISH) and immunohistochemistry (IHC), respectively, to bronchial biopsies obtained from 7 healthy nonsmokers (HNS), 5 healthy smokers (HS), and 8 smokers with CB, and additionally include the airways of lungs resected from 11 asymptomatic smokers (AS) and 11 smokers with CB. Compared with HNS, biopsies in CB demonstrated increased numbers of 15-LOa mRNA+ cells (median: HNS = 31.3/mm(2) versus CB = 84.9/mm(2), P < 0.01) and protein+ cells (HNS = 2.9/mm(2) versus CB = 32.1/mm(2), P < 0.01). The HS group also showed a significant increase in protein+ cells (HNS = 2.9/mm(2) versus HS = 14/mm(2), P < 0.05). In the resected airways, 15-LOa protein+ cells in the submucosal glands of the CB group were more numerous than in the AS group (AS = 33/mm(2) versus CB = 208/mm(2); P < 0.001). 15-LOa mRNA+ and protein+ cells consistently outnumbered 15-LOb by approximately 7- and 5-fold, respectively (P < 0.01). Quantitative reverse transcriptase polymerase chain reaction of complementary biopsies confirmed the increased levels of 15-LOa in CB compared with that in either HNS or HS (P < 0.05). There was no difference between the subject groups with respect to 15-LOb expression. The numbers of cells expressing mRNA for 15-LOa in CB showed a positive association with those expressing interleukin (IL)-4 mRNA (r = 0.80; P < 0.01). We conclude that the upregulation of 15-LO activity in the airways of HS and of smokers with CB primarily involves the 15-LOa isoform: the functional consequences of its association the upregulation of IL-4 in chronic bronchitis requires further study.

Mammalian arachidonate 15-lipoxygenases structure, function, and biological implications

Lipoxygenases (LOXs) constitute a heterogeneous family of lipid peroxidizing enzymes capable of oxygenating polyunsaturated fatty acids to their corresponding hydroperoxy derivatives. In mammals, LOXs are classified with respect to their positional specificity of arachidonic acid oxygenation into 5-, 8-, 12-, and 15-LOXs. Arachidonate 15-LOXs may be sub-classified into a reticulocyte-type (type-1) and an epidermis-type (type-2) enzyme. Since the leukocyte-type 12-LOXs are very similar to the reticulocyte-type 15-LOXs, these enzymes are designated 12/15-LOXs. Several LOX isoforms, in particular the reticulocyte-type 15-LOX and the human 5-LOX, are well characterized with respect to their structural and functional properties On the other hand, the biological role of most LOX-isozymes including the reticulocyte-type 15-LOC is far from clear. This review is intended to summarize the recent developments in 15-LOX research with particular emphasis to molecular enzymology and regulation of gene expression. In addition, the major hypotheses on the physiological and patho-physiological roles of 15-LOXs will be discussed briefly.

Evidence that arachidonate 15-lipoxygenase 2 is a negative cell cycle regulator in normal prostate epithelial cells

15-Lipoxygenase 2 (15-LOX2) is a recently cloned human lipoxygenase that shows tissue-restricted expression in prostate, lung, skin, and cornea. The protein level and enzymatic activity of 15-LOX2 have been shown to be down-regulated in prostate cancers compared with normal and benign prostate tissues. The biological function of 15-LOX2 and the role of loss of 15-LOX2 expression in prostate tumorigenesis, however, remain unknown. We report the cloning and functional characterization of 15-LOX2 and its three splice variants (termed 15-LOX2sv-a, 15-LOX2sv-b, and 15-LOX2sv-c) from primary prostate epithelial cells. Western blotting with multiple primary prostate cell strains and prostate cancer cell lines reveals that the expression of 15-LOX2 is lost in all prostate cancer cell lines, accompanied by decreased enzymatic activity revealed by liquid chromatography/tandem mass spectrometry analyses. Further experiments show that the loss of 15-LOX2 expression results from transcriptional repression caused by mechanism(s) other than promoter hypermethylation or histone deacetylation. Subsequent functional studies indicate the following: 1) the 15-LOX2 product, 15(S)-hydroxyeicosatetraenoic acid, inhibits prostate cancer cell cycle progression; 2) 15-LOX2 expression in primary prostate epithelial cells is inversely correlated with cell cycle; and 3) restoration of 15-LOX2 expression in prostate cancer cells partially inhibits cell cycle progression. Taken together, these results suggest that 15-LOX2 could be a suppressor of prostate cancer development, which functions by restricting cell cycle progression.

15-Lipoxygenase-2 expression in benign and neoplastic sebaceous glands and other cutaneous adnexa

15-Lipoxygenase-2 has a limited tissue distribution in epithelial tissues, with mRNA detected in skin, cornea, lung, and prostate. It was originally cloned from human hair rootlets. In this study the distribution of 15-lipoxygenase-2 was characterized in human skin using immunohistochemistry and in situ hybridization. Strong uniform 15-lipoxygenase-2 in situ hybridization (n = 6) and immunostaining (n = 16) were observed in benign cutaneous sebaceous glands, with expression in differentiated secretory cells. Strong 15-lipoxygenase-2 immunostaining was also observed in secretory cells of apocrine and eccrine glands. Variable reduced immunostaining was observed in skin-derived sebaceous neoplasms (n = 8). In the eyelid, Meibomian glands were uniformly negative for 15-lipoxygenase-2 in all cases examined (n = 9), and sebaceous carcinomas apparently derived from Meibomian glands were also negative (n = 12). The mechanisms responsible for differential expression in cutaneous sebaceous vs eyelid Meibomian glands remain to be established. In epidermis, positive immunostaining was observed in the basal cell layer in normal skin, whereas five examined basal cell carcinomas were negative. Thus, the strongest 15-lipoxygenase-2 expression is in the androgen regulated secretory cells of sebaceous, apocrine, and eccrine glands. This compares with the prostate, in which 15-lipoxygenase-2 is expressed in differentiated prostate secretory cells (and reduced in the majority of prostate adenocarcinomas). The product of 15-lipoxygenase-2, 15-hydroxyeicosatetraenoic acid, may be a ligand for the nuclear receptor peroxisome proliferator activated receptor-gamma, which is expressed in sebocytes, and contribute to secretory differentiation in androgen regulated tissues such as prostate and sebaceous glands.

Tryptic digestion of soybean lipoxygenase-1 generates a 60 kDa fragment with improved activity and membrane binding ability

Lipoxygenases are key enzymes in the metabolism of unsaturated fatty acids. Soybean lipoxygenase-1 (LOX-1), a paradigm for lipoxygenases isolated from different sources, is composed of two domains: a approximately 30 kDa N-terminal domain and a approximately 60 kDa C-terminal domain. We used limited proteolysis and gel-filtration chromatography to generate and isolate a approximately 60 kDa fragment of LOX-1 ("mini-LOX"), produced by trypsin cleavage between lysine 277 and serine 278. Mini-LOX was subjected to N-terminal sequencing and to electrophoretic, chromatographic, and spectroscopic analysis. Mini-LOX was found to be more acidic and more hydrophobic than LOX-1, and with a higher content of alpha-helix. Kinetic analysis showed that mini-LOX dioxygenates linoleic acid with a catalytic efficiency approximately 3-fold higher than that of LOX-1 (33.3 x 10(6) and 10.9 x 10(6) M(-1) x s(-1), respectively), the activation energy of the reaction being 4.5 +/- 0.5 and 8.3 +/- 0.9 kJ x mol(-1) for mini-LOX and LOX-1, respectively. Substrate preference, tested with linoleic, alpha-linolenic, and arachidonic acids, and with linoleate methyl ester, was the same for LOX-1 and mini-LOX, and also identical was the regio- and stereospecificity of the products generated thereof, analyzed by reversed-phase and chiral high-performance liquid chromatography, and by gas chromatography/mass spectrometry. Mini-LOX was able to bind artificial vesicles with higher affinity than LOX-1, but the binding was less affected by calcium ions than was that of LOX-1. Taken together, these results suggest that the N-terminal domain of soybean lipoxygenase-1 might be a built-in inhibitor of catalytic activity and membrane binding ability of the enzyme, with a possible role in physio(patho)logical conditions.

Qualitative and quantitative analysis of lipoxygenase products in bovine corneal epithelium by liquid chromatography-mass spectrometry with an ion trap

Electrospray ionization ion trap mass spectra of 5-, 12-, and 15-hydroperoxyeicosatetraenoic (HPETE), hydroxyeicosatetraenoic (HETE), and ketoeicosatetraenoic (KETE) acids were recorded. The HPETE were partly dehydrated to the corresponding KETE in the heated capillary of the mass spectrometer. 12-HPETE and 15-HPETE were also converted to KETE by collision-induced dissociation (CID) in the ion trap, whereas CID of 5-HPETE yielded little formation of 5-KETE. Subcellular fractions of bovine corneal epithelium were incubated with arachidonic acid (AA) and the metabolites were analyzed. 15-HETE and 12-HETE were consistently formed, whereas significant accumulation of HPETE and KETE was not detected. Biosynthesis of 12- and 15-HETE was quantified with octadeuterated 12-HETE and 15-HETE as internal standards. The average biosynthesis of 15-HETE and 12-HETE from 30 microM AA by the cytosol was 38 +/- 8 and below 3 ng/mg protein/30 min, respectively, which increased to 78 +/- 21 and 10 +/- 4 ng/mg protein/30 min in the presence of 1 mM free Ca2+. The microsomal biosynthesis was unaffected by Ca2+. The microsomes metabolized AA to 15-HETE as the main metabolite at a low protein concentration (0.3 mg/mL), whereas 12-HETE and 15-HETE were formed in a 2:1 ratio at a combined rate of 0.7 +/- 0.2 microg/mg protein/30 min at a high protein concentration (1.8 mg/mL). The level of 12-HETE in corneal epithelial cells was 50 +/- 13 pg/mg tissue, whereas the endogenous amount of 15-HETE was low or undetectable (<3 pg/mg tissue). Incubation of corneas for 20 min at 37 degrees C before processing selectively increased the amounts of 12-HETE in the epithelium fourfold to approximately 0.2 ng/mg tissue. We conclude that 12-HETE is the main endogenously formed lipoxygenase product of bovine corneal epithelium.