ATP allosterically activates the human 5-lipoxygenase molecular mechanism of arachidonic acid and 5(S)-hydroperoxy-6(E),8(Z),11(Z),14(Z)-eicosatetraenoic acid

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.

Partial Inhibition of Complex I Restores Mitochondrial Morphology and Mitochondria-ER Communication in Hippocampus of APP/PS1 Mice

Alzheimer's disease (AD) has no cure. Earlier, we showed that partial inhibition of mitochondrial complex I (MCI) with the small molecule CP2 induces an adaptive stress response, activating multiple neuroprotective mechanisms. Chronic treatment reduced inflammation, Aβ and pTau accumulation, improved synaptic and mitochondrial functions, and blocked neurodegeneration in symptomatic APP/PS1 mice, a translational model of AD. Here, using serial block-face scanning electron microscopy (SBFSEM) and three-dimensional (3D) EM reconstructions combined with Western blot analysis and next-generation RNA sequencing, we demonstrate that CP2 treatment also restores mitochondrial morphology and mitochondria-endoplasmic reticulum (ER) communication, reducing ER and unfolded protein response (UPR) stress in the APP/PS1 mouse brain. Using 3D EM volume reconstructions, we show that in the hippocampus of APP/PS1 mice, dendritic mitochondria primarily exist as mitochondria-on-a-string (MOAS). Compared to other morphological phenotypes, MOAS have extensive interaction with the ER membranes, forming multiple mitochondria-ER contact sites (MERCS) known to facilitate abnormal lipid and calcium homeostasis, accumulation of Aβ and pTau, abnormal mitochondrial dynamics, and apoptosis. CP2 treatment reduced MOAS formation, consistent with improved energy homeostasis in the brain, with concomitant reductions in MERCS, ER/UPR stress, and improved lipid homeostasis. These data provide novel information on the MOAS-ER interaction in AD and additional support for the further development of partial MCI inhibitors as a disease-modifying strategy for AD.

Fatty acids negatively regulate platelet function through formation of noncanonical 15-lipoxygenase-derived eicosanoids

The antiplatelet effect of polyunsaturated fatty acids is primarily attributed to its metabolism to bioactive metabolites by oxygenases, such as lipoxygenases (LOX). Platelets have demonstrated the ability to generate 15-LOX-derived metabolites (15-oxylipins); however, whether 15-LOX is in the platelet or is required for the formation of 15-oxylipins remains unclear. This study seeks to elucidate whether 15-LOX is required for the formation of 15-oxylipins in the platelet and determine their mechanistic effects on platelet reactivity. In this study, 15-HETrE, 15-HETE, and 15-HEPE attenuated collagen-induced platelet aggregation, and 15-HETrE inhibited platelet aggregation induced by different agonists. The observed anti-aggregatory effect was due to the inhibition of intracellular signaling including αIIbβ3 and protein kinase C activities, calcium mobilization, and granule secretion. While 15-HETrE inhibited platelets partially through activation of peroxisome proliferator-activated receptor β (PPARβ), 15-HETE also inhibited platelets partially through activation of PPARα. 15-HETrE, 15-HETE, or 15-HEPE inhibited 12-LOX in vitro, with arachidonic acid as the substrate. Additionally, a 15-oxylipin-dependent attenuation of 12-HETE level was observed in platelets following ex vivo treatment with 15-HETrE, 15-HETE, or 15-HEPE. Platelets treated with DGLA formed 15-HETrE and collagen-induced platelet aggregation was attenuated only in the presence of ML355 or aspirin, but not in the presence of 15-LOX-1 or 15-LOX-2 inhibitors. Expression of 15-LOX-1, but not 15-LOX-2, was decreased in leukocyte-depleted platelets compared to non-depleted platelets. Taken together, these findings suggest that 15-oxylipins regulate platelet reactivity; however, platelet expression of 15-LOX-1 is low, suggesting that 15-oxylipins may be formed in the platelet through a 15-LOX-independent pathway.

Characterization of Arachidonate 5S-Lipoxygenase from Danio rerio with High Activity for the Production of 5S- and 7S-Hydroxy Polyunsaturated Fatty Acids

A recombinant putative lipoxygenase (LOX) from Danio rerio (zebrafish), ALOX3c protein with 6-histidine tag, was purified using affinity chromatography, with a specific activity of 17.2 U mg^-1 for arachidonic acid (AA). The molecular mass of the native ALOX3c was 156 kDa composed of a 78-kDa dimer by gel-filtration chromatography. The product obtained from the conversion of AA was identified as 5S-hydroxyeicosatetraenoic acid (5S-HETE) by HPLC and LC-MS/MS analyses. The specific activity and catalytic efficiency of the LOX from D. rerio for polyunsaturated fatty acids (PUFAs) followed the order AA (17.2 U mg^-1, 1.96 s^-1 μM^-1) > docosahexaenoic acid (DHA, 13.6 U mg^-1, 0.91 s^-1 μM^-1) > eicosapentaenoic acid (EPA, 10.5 U mg^-1, 0.65 s^-1 μM^-1) and these values for AA were the highest among the 5S-LOXs reported to date. Based on identified products and substrate specificity, the enzyme is an AA 5S-LOX. The enzyme exhibited the maximal activity at pH 8.0 and 20 °C with 0.1 mM Zn²⁺ in the presence of 10 mM cysteine. Under these reaction conditions, 6.88 U mL^-1 D. rerio 5S-LOX converted 1.0 mM of AA, EPA, and DHA to 0.91 mM 5S-HETE, 0.72 mM 5S-hydroxyeicosapentaenoic acid (5S-HEPE), and 0.68 mM 7S-hydroxydocosahexaenoic acid (7S-HDHA) in 25, 30, and 25 min, corresponding to molar conversion rates of 91, 72, and 68% and productivities of 2.18, 1.44, and 1.63 mM h^-1, respectively. To the best of our knowledge, this study is the first to describe the bioconversion into 5S-HETE, 5S-HEPE, and 7S-HDHA for the application of biotechnological production.

Structures of Leukotriene Biosynthetic Enzymes and Development of New Therapeutics

Leukotrienes are potent immune-regulating lipid mediators with patho-genic roles in inflammatory and allergic diseases, particularly asthma. These autacoids also contribute to low-grade inflammation, a hallmark of cardiovascular, neurodegenerative, metabolic, and tumor diseases. Biosynthesis of leukotrienes involves release and oxidative metabolism of arachidonic acid and proceeds via a set of cytosolic and integral membrane enzymes that are typically expressed by cells of the innate immune system. In activated cells, these enzymes traffic and assemble at the endoplasmic and perinuclear membrane, together comprising a biosynthetic complex. Here we describe recent advances in our molecular understanding of the protein components of the leukotriene-synthesizing enzyme machinery and also briefly touch upon the leukotriene receptors. Moreover, we discuss emerging opportunities for pharmacological intervention and development of new therapeutics.

Reevaluation of a Bicyclic Pyrazoline as a Selective 15-Lipoxygenase V-Type Activator Possessing Fatty Acid Specificity

Regulation of lipoxygenase (LOX) activity is of great interest due to the involvement of the various LOX isoforms in the inflammatory process and hence many diseases. The bulk of investigations have centered around the discovery and design of inhibitors. However, the emerging understanding of the role of h15-LOX-1 in the resolution of inflammation provides a rationale for the development of activators as well. Bicyclic pyrazolines are known bioactive molecules that have been shown to display antibiotic and anti-inflammatory activities. In the current work, we reevaluated a previously discovered bicyclic pyrazoline h15-LOX-1 activator, PKUMDL_MH_1001 (written as 1 for this publication), and determined that it is inactive against other human LOX isozymes, h5-LOX, h12-LOX, and h15-LOX-2. Analytical characterization of 1 obtained in the final synthesis step identified it as a mixture of cis- and trans-diastereomers: cis-1 (12%) and trans-1 (88%); and kinetic analysis indicated similar potency between the two. Using compound 1 as the cis-trans mixture, h15-LOX-1 catalysis with arachidonic acid (AA) (AC50 = 7.8 +/- 1 μM, A max = 240%) and linoleic acid (AC50 = 5.3 +/- 0.7 μM, A max = 98%) was activated, but not with docosahexaenoic acid (DHA) or mono-oxylipins. Steady-state kinetics demonstrate V-type activation for 1, with a β value of 2.2 +/- 0.4 and an K x of 16 +/- 1 μM. Finally, it is demonstrated that the mechanism of activation for 1 is likely not due to decreasing substrate inhibition, as was postulated previously. 1 also did not affect the activity of the h15-LOX-1 selective inhibitor, ML351, nor did 1 affect the activity of allosteric effectors, such as 12S-hydroxy-5Z,8Z,10E,14Z-eicosatetraenoic acid (12S-HETE) and 14S-hydroperoxy-4Z,7Z,10Z,12E,16Z,19Z-docosahexaenoic acid (14S-HpDHA). These data confirm that 1 binds to a distinct activation binding site, as previously postulated. Future work should be aimed at the development of selective activators that are capable of activating h15-LOX-1 catalysis with DHA, thus enhancing the production of DHA-derived pro-resolution biomolecules.

Tissue Specific Dual RNA-Seq Defines Host-Parasite Interplay in Murine Visceral Leishmaniasis Caused by Leishmania donovani and Leishmania infantum

Visceral leishmaniasis is associated with hepato-splenomegaly and altered immune and hematological parameters in both preclinical animal models and humans. We studied mouse experimental visceral leishmaniasis caused by Leishmania infantum and Leishmania donovani in BALB/c mice using dual RNA-seq to investigate the transcriptional response of host and parasite in liver and spleen. We identified only 4 species-specific parasite expressed genes (SSPEGs; log2FC >1, FDR <0.05) in the infected spleen, and none in the infected liver. For the host transcriptome, we found 789 differentially expressed genes (DEGs; log2FC >1, FDR <0.05) in the spleen that were common to both infections, with IFNγ signaling and complement and coagulation cascade pathways highly enriched, and an additional 286 and 186 DEGs that were selective to L. donovani and L. infantum infection, respectively. Among those, there were network interactions between genes of amino acid metabolism and PPAR signaling in L. donovani infection and increased IL1β and positive regulation of fatty acid transport in L. infantum infection, although no pathway enrichment was observed. In the liver, there were 1,939 DEGs in mice infected with either L. infantum or L. donovani in comparison to uninfected mice, and the most enriched pathways were IFNγ signaling, neutrophil mediated immunity, complement and coagulation, cytokine-chemokine responses, and hemostasis. Additionally, 221 DEGs were selective in L. donovani and 429 DEGs in L. infantum infections. These data show that the host response for these two visceral leishmaniasis infection models is broadly similar, and ∼10% of host DEGs vary in infections with either parasite species. IMPORTANCE Visceral leishmaniasis (VL) is caused by two species of Leishmania parasites, L. donovani in the Old World and L. infantum in the New World and countries bordering the Mediterranean. Although cardinal features such as hepato-splenomegaly and alterations in blood and immune function are evident, clinical presentation may vary by geography, with for example severe bleeding often associated with VL in Brazil. Although animal models of both L. donovani and L. infantum have been widely used to study disease pathogenesis, a direct side-by-side comparison of how these parasites species impact the infected host and/or how they might respond to the stresses of mammalian infection has not been previously reported. Identifying common and distinct pathways to pathogenesis will be important to ensure that new therapeutic or prophylactic approaches will be applicable across all forms of VL.

Combined Machine Learning and GRID-Independent Molecular Descriptor (GRIND) Models to Probe the Activity Profiles of 5-Lipoxygenase Activating Protein Inhibitors

Leukotrienes (LTs) are pro-inflammatory lipid mediators derived from arachidonic acid (AA), and their high production has been reported in multiple allergic, autoimmune, and cardiovascular disorders. The biological synthesis of leukotrienes is instigated by transfer of AA to 5-lipoxygenase (5-LO) via the 5-lipoxygenase-activating protein (FLAP). Suppression of FLAP can inhibit LT production at the earliest level, providing relief to patients requiring anti-leukotriene therapy. Over the last 3 decades, several FLAP modulators have been synthesized and pharmacologically tested, but none of them could be able to reach the market. Therefore, it is highly desirable to unveil the structural requirement of FLAP modulators. Here, in this study, supervised machine learning techniques and molecular modeling strategies are adapted to vaticinate the important 2D and 3D anti-inflammatory properties of structurally diverse FLAP inhibitors, respectively. For this purpose, multiple machine learning classification models have been developed to reveal the most relevant 2D features. Furthermore, to probe the 3D molecular basis of interaction of diverse anti-inflammatory compounds with FLAP, molecular docking studies were executed. By using the most probable binding poses from docking studies, the GRIND model was developed, which indicated the positive contribution of four hydrophobic, two hydrogen bond acceptor, and two shape-based features at certain distances from each other towards the inhibitory potency of FLAP modulators. Collectively, this study sheds light on important two-dimensional and three-dimensional structural requirements of FLAP modulators that can potentially guide the development of more potent chemotypes for the treatment of inflammatory disorders.

Analysis of differentially expressed genes and signaling pathways involved in atherosclerosis and chronic obstructive pulmonary disease

Atherosclerosis is an important medical and social problem, and the keys to solving this problem are still largely unknown. A common situation in real clinical practice is the comorbid course of atherosclerosis with chronic obstructive pulmonary disease (COPD). Diseases share some common risk factors and may be closely linked pathogenetically.

METHODS

Bioinformatics analysis of datasets from Gene Expression Omnibus (GEO) was performed to examine the gene ontology (GO) of common differentially expressed genes (DEGs) in COPD and peripheral arterial atherosclerosis. DEGs were identified using the limma R package with the settings p < 0.05, corrected using the Benjamini & Hochberg algorithm and ǀlog 2FCǀ > 1.0. The GO, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment, and the protein-protein interaction (PPI) network analysis were performed with the detected DEGs.

RESULTS

The biological processes and signaling pathways involving common DEGs from airway epithelial datasets in COPD and tissue in peripheral atherosclerosis were identified. A total of 15 DEGs were identified, comprising 12 upregulated and 3 downregulated DEGs. The GO enrichment analysis demonstrated that the upregulated hub genes were mainly involved in the inflammatory response, reactive oxygen species metabolic process, cell adhesion, lipid metabolic process, regulation of angiogenesis, icosanoid biosynthetic process, and cellular response to a chemical stimulus. The KEGG pathway enrichment analysis demonstrated that the common pathways were Toll-like receptor signaling pathway, NF-kappa B signaling pathway, lipid and atherosclerosis, and cytokine-cytokine receptor interaction.

CONCLUSIONS

Biological processes and signaling pathways associated with the immune response may link the development and progression of COPD and atherosclerosis.

On the biosynthesis of specialized pro-resolving mediators in human neutrophils and the influence of cell integrity

Neutrophils are key players in inflammation initiation and resolution. Little attention has been paid to the detailed biosynthesis of specialized pro-resolving mediators (SPM) in these cells. We investigated SPM formation in human polymorphonuclear leukocytes (PMNL), in broken PMNL preparations and recombinant human 5-lipoxygenase (5-LO) supplemented with the SPM precursor lipids 15-Hydroxyeicosatetraenoic acid (15-HETE), 18-Hydroxyeicosapentaenoic acid (18-HEPE) or 17-Hydroxydocosahexaenoic acid (17-HDHA). In addition, the influence of 5-LO activating protein (FLAP) inhibition on SPM formation in PMNL was assessed. Intact human PMNL preferred ARA over DHA for lipid mediator formation. In contrast, in incubations supplemented with the SPM precursor lipids DHA-derived 17-HDHA was preferred over 15-HETE and 18-HEPE. SPM formation in the cells was dominated by 5(S),15(S)-diHETE (800 pmol/20 mio cells) and Resolvin D5 (2300 pmol/20 mio cells). Formation of lipoxins (<10 pmol/20 mio cells), E-series (<70 pmol/20 mio cells) and other D-series resolvins (<20 pmol/20 mio cells) was low and only detected after addition of the precursor lipids. Upon destruction of cell integrity, formation of lipoxins and 5(S),15(S)-diHETE increased while formation of 17-HDHA- and 18-HEPE-derived SPMs was attenuated. Recombinant 5-LO did not accept the precursors for SPM formation and FLAP inhibition prevented the formation of the 5-LO-dependent SPMs. Together with the data on FLAP inhibition our results point to unknown factors that control SPM formation in human neutrophils and also render lipoxin and 5(S),15(S)-diHETE formation independent of membrane association and FLAP when cellular integrity is destroyed.

Different forms of vitamin E and metabolite 13'-carboxychromanols inhibit cyclooxygenase-1 and its catalyzed thromboxane in platelets, and tocotrienols and 13'-carboxychromanols are competitive inhibitors of 5-lipoxygenase

Cyclooxygenase (COX-1 and COX-2)- and 5-lipoxygenase (5-LOX)-catalyzed biosynthesis of eicosanoids play important roles in inflammation and chronic diseases. The vitamin E family has four tocopherols and tocotrienols. We have shown that the metabolites of δ-tocopherol (δT) and δ-tocotrienol (δTE), i.e., δT-13'-carboxychromanol (COOH) and δTE-13'-COOH, respectively, inhibit COX-1/-2 and 5-LOX activity, but the nature of how they inhibit 5-LOX is not clear. Further, the impact of tocopherols and tocotrienols on COX-1/-2 or 5-LOX activity has not been fully delineated. In this study, we found that tocopherols and tocotrienols inhibited human recombinant COX-1 with IC50s of 1-12 µM, and suppressed COX-1-mediated formation of thromboxane in collagen-stimulated rat's platelets with IC50s of 8-50 µM. None of the vitamin E forms directly inhibited COX-2 activity. 13'-COOHs inhibited COX-1 and COX-2 enzyme activity with IC50s of 3-4 and 4-10 µM, respectively, blocked thromboxane formation in collagen- and ionophore-stimulated rats' platelets with IC50s of 1.5-2.5 µM, and also inhibited COX-2-mediated prostaglandins in stimulated cells. Using enzyme kinetics, we observed that δT-13'-COOH, δTE-13'-COOH and δTE competitively inhibited 5-LOX activity with Ki of 1.6, 0.8 and 2.2 µM, respectively. These compounds decreased leukotriene B₄ from stimulated neutrophil-like cells without affecting translocation of 5-LOX from cytosol to the nucleus. Our study reveals inhibitory effects of vitamin E forms and 13'-COOHs on COX-1 activity and thromboxane formation in platelets, and elucidates mechanisms underlying their inhibition of 5-LOX. These observations are useful for understanding the role of these compounds in disease prevention and therapy.

Docking and mutagenesis studies lead to improved inhibitor development of ML355 for human platelet 12-lipoxygenase

Human platelet 12-(S)-Lipoxygenase (12-LOX) is a fatty acid metabolizing oxygenase that plays an important role in platelet activation and cardiometabolic disease. ML355 is a specific 12-LOX inhibitor that has been shown to decrease thrombosis without prolonging hemostasis and protect human pancreatic islets from inflammatory injury. It has an amenable drug-like scaffold with nM potency and encouraging ADME and PK profiles, but its binding mode to the active site of 12-LOX remains unclear. In the current work, we combined computational modeling and experimental mutagenesis to propose a model in which ML355 conforms to the "U" shape of the 12-LOX active site, with the phenyl linker region wrapping around L407. The benzothiazole of ML355 extends into the bottom of the active site cavity, pointing towards residues A417 and V418. However, reducing the active site depth alone did not affect ML355 potency. In order to lower the potency of ML355, the cavity needed to be reduced in both length and width. In addition, H596 appears to position ML355 in the active site through an interaction with the 2-methoxy phenol moiety of ML355. Combined, this binding model suggested that the benzothiazole of ML355 could be enlarged. Therefore, a naphthyl-benzothiazole derivative of ML355, Lox12Slug001, was synthesized and shown to have 7.2-fold greater potency than ML355. This greater potency is proposed to be due to additional van der Waals interactions and pi-pi stacking with F414 and F352. Lox12Slug001 was also shown to be highly selective against 12-LOX relative to the other LOX isozymes and more importantly, it showed activity in rescuing human islets exposed to inflammatory cytokines with comparable potency to ML355. Further studies are currently being pursued to derivatize ML355 in order to optimize the additional space in the active site, while maintaining acceptable drug-like properties.

Molecular insights into lipoxygenases for biocatalytic synthesis of diverse lipid mediators

Oxylipins derived mainly from C20- and C22-polyunsaturated fatty acids (PUFAs), termed lipid mediators (LMs), are essential signalling messengers involved in human physiological responses associated with homeostasis and healing process for infection and inflammation. Some LMs involved in the resolution of inflammation and infection are termed specialized pro-resolving mediators (SPMs), which are generated by human M2 macrophages or polymorphonuclear leukocytes and have the potential to protect and treat hosts from bacterial and viral infections by phagocytosis activation. Lipoxygenases (LOXs) biosynthesize regio- and stereoselective LMs. Thus, understanding the regio- and stereoselectivities of LOXs for PUFAs at a molecular level is important for the biocatalytic synthesis of diverse LMs. Here, we elucidate the catalytic mechanisms and discuss regio- and stereoselectivities and their changes of LOXs determined by insertion direction and position of the substrate and oxygen at a molecular level for the biosynthesis of diverse human LMs. Recently, the biocatalytic synthesis of PUFAs to human LMs or analogues has been conducted using microbial LOXs. Such microbial LOXs involved in the biosynthesis of LMs are expected to exert significantly higher activity and stability than human LOXs. Diverse regio- and stereoselective LOXs can be obtained from microorganisms, which represent a wealth of genomic sources. We reconstruct the biosynthetic pathways of LOX-catalyzed LMs in humans and other organisms. Furthermore, we suggest the effective methods of biocatalytic synthesis of diverse human LMs from PUFAs or glucose by using microbial LOXs, increasing the stability and activity of LOXs, combining the reactions of LOXs, and constructing metabolic pathways.

Deciphering the Molecular Details of the Lipoxin Formation Mechanism in the 5(S),15(S)-DiHpETE Biosynthetic Pathway Catalyzed by Reticulocyte 15-Lipoxygenase-1

Chronic inflammation is now widely recognized to play important roles in many commonly occurring diseases, including COVID-19. The resolution response to this chronic inflammation is an active process governed by specialized pro-resolving mediators (SPMs) like the lipid mediators known as lipoxins. The biosynthesis of lipoxins is catalyzed by several lipoxygenases (LOXs) from arachidonic acid. However, the molecular details of the mechanisms involved are not well known yet. In this paper, we have combined molecular dynamics (MD) simulations and quantum mechanics/molecular mechanics (QM/MM) calculations to analyze how reticulocyte 15-LOX-1 catalyzes the production of lipoxins from 5(S),15(S)-diHpETE. Our results indicate that the dehydration mechanism from 5(S),15(S)-diHpETE, via the formation of an epoxide, presents huge energy barriers even though it was one of the two a priori synthetic proposals. This result is compatible with the fact that no epoxide has been directly detected as an intermediate in the catalytic formation of lipoxins from 5(S),15(S)-diHpETE. Conversely, the oxygenation of 5(S),15(S)-diHpETE at C₁₄ is feasible because there is an open channel connecting the protein surface with this carbon atom, and the energy barrier for oxygen addition through this channel is small. The analysis of the following steps of this mechanism, leading to the corresponding hydroperoxide at the 15-LOX-1 active site, indicates that the oxygenation mechanism will lead to the formation of lipoxinB₄ after the final action of a reductase. In contrast, our calculations are in agreement with experiments that lipoxinA₄ cannot derive from 5(S),15(S)-diHpETE by either of the two proposed mechanisms and that 5(S),15(S)-diHETE is not an intermediate of lipoxin biosynthesis catalyzed by 15-LOX-1.

Role of Human 15-Lipoxygenase-2 in the Biosynthesis of the Lipoxin Intermediate, 5S,15S-diHpETE, Implicated with the Altered Positional Specificity of Human 15-Lipoxygenase-1

The oxylipins, 5S,12S-dihydroxy-6E,8Z,10E,14Z-eicosatetraenoic acid (5S,12S-diHETE) and 5S,15S-dihydroxy-6E,8Z,11Z,13E-eicosatetraenoic acid (5S,15S-diHETE), have been identified in cell exudates and have chemotactic activity toward eosinophils and neutrophils. Their biosynthesis has been proposed to occur by sequential oxidations of arachidonic acid (AA) by lipoxygenase enzymes, specifically through oxidation of AA by h5-LOX followed by h12-LOX, h15-LOX-1, or h15-LOX-2. In this work, h15-LOX-1 demonstrates altered positional specificity when reacting with 5S-HETE, producing 90% 5S,12S-diHETE, instead of 5S,15S-diHETE, with kinetics 5-fold greater than that of h12-LOX. This is consistent with previous work in which h15-LOX-1 reacts with 7S-HDHA, producing the noncanonical, DHA-derived, specialized pro-resolving mediator, 7S,14S-diHDHA. It is also determined that oxygenation of 5S-HETE by h15-LOX-2 produces 5S,15S-diHETE and its biosynthetic kcat/KM flux is 2-fold greater than that of h15-LOX-1, suggesting that h15-LOX-2 may have a greater role in lipoxin biosynthesis than previously thought. In addition, it is shown that oxygenation of 12S-HETE and 15S-HETE by h5-LOX is kinetically slow, suggesting that the first step in the in vitro biosynthesis of both 5S,12S-diHETE and 5S,15S-diHETE is the production of 5S-HETE.

15-Lipoxygenase-1 biosynthesis of 7S,14S-diHDHA implicates 15-lipoxygenase-2 in biosynthesis of resolvin D5

The two oxylipins 7S,14S-dihydroxydocosahexaenoic acid (diHDHA) and 7S,17S-diHDHA [resolvin D5 (RvD5)] have been found in macrophages and infectious inflammatory exudates and are believed to function as specialized pro-resolving mediators (SPMs). Their biosynthesis is thought to proceed through sequential oxidations of DHA by lipoxygenase (LOX) enzymes, specifically, by human 5-LOX (h5-LOX) first to 7(S)-hydroxy-4Z,8E,10Z,13Z,16Z,19Z-DHA (7S-HDHA), followed by human platelet 12-LOX (h12-LOX) to form 7(S),14(S)-dihydroxy-4Z,8E,10Z,12E,16Z,19Z-DHA (7S,14S-diHDHA) or human reticulocyte 15-LOX-1 (h15-LOX-1) to form RvD5. In this work, we determined that oxidation of 7(S)-hydroperoxy-4Z,8E,10Z,13Z,16Z,19Z-DHA to 7S,14S-diHDHA is performed with similar kinetics by either h12-LOX or h15-LOX-1. The oxidation at C14 of DHA by h12-LOX was expected, but the noncanonical reaction of h15-LOX-1 to make over 80% 7S,14S-diHDHA was larger than expected. Results of computer modeling suggested that the alcohol on C7 of 7S-HDHA hydrogen bonds with the backbone carbonyl of Ile399, forcing the hydrogen abstraction from C12 to oxygenate on C14 but not C17. This result raised questions regarding the synthesis of RvD5. Strikingly, we found that h15-LOX-2 oxygenates 7S-HDHA almost exclusively at C17, forming RvD5 with faster kinetics than does h15-LOX-1. The presence of h15-LOX-2 in neutrophils and macrophages suggests that it may have a greater role in biosynthesizing SPMs than previously thought. We also determined that the reactions of h5-LOX with 14(S)-hydroperoxy-4Z,7Z,10Z,12E,16Z,19Z-DHA and 17(S)-hydroperoxy-4Z,7Z,10Z,13Z,15E,19Z-DHA are kinetically slow compared with DHA, suggesting that these reactions may be minor biosynthetic routes in vivo. Additionally, we show that 7S,14S-diHDHA and RvD5 have anti-aggregation properties with platelets at low micromolar potencies, which could directly regulate clot resolution.

Biosynthesis of the Maresin Intermediate, 13S,14S-Epoxy-DHA, by Human 15-Lipoxygenase and 12-Lipoxygenase and Its Regulation through Negative Allosteric Modulators

Human reticulocyte 15-lipoxygenase-1 (h15-LOX-1 or ALOX15) and platelet 12-lipoxygenase (h12-LOX or ALOX12) catalysis of docosahexaenoic acid (DHA) and the maresin precursor, 14S-hydroperoxy-4Z,7Z,10Z,12E,16Z,19Z-docosahexaenoic acid (14S-HpDHA), were investigated to determine their product profiles and relative rates in the biosynthesis of the key maresin intermediate, 13S,14S-epoxy-4Z,7Z,9E,11E,16Z,19Z-docosahexaenoic acid (13S,14S-epoxy-DHA). Both enzymes converted DHA to 14S-HpDHA, with h12-LOX having a 39-fold greater kcat/KM value (14.0 ± 0.8 s-1 μM-1) than that of h15-LOX-1 (0.36 ± 0.08 s-1 μM-1) and a 1.8-fold greater 14S-HpDHA product selectivity, 81 and 46%, respectively. However, h12-LOX was markedly less effective at producing 13S,14S-epoxy-DHA from 14S-HpDHA than h15-LOX-1, with a 4.6-fold smaller kcat/KM value, 0.0024 ± 0.0002 and 0.11 ± 0.006 s-1 μM-1, respectively. This is the first evidence of h15-LOX-1 to catalyze this reaction and reveals a novel in vitro pathway for maresin biosynthesis. In addition, epoxidation of 14S-HpDHA is negatively regulated through allosteric oxylipin binding to h15-LOX-1 and h12-LOX. For h15-LOX-1, 14S-HpDHA (Kd = 6.0 μM), 12S-hydroxy-5Z,8Z,10E,14Z-eicosatetraenoic acid (12S-HETE) (Kd = 3.5 μM), and 14S-hydroxy-7Z,10Z,12E,16Z,19Z-docosapentaenoic acid (14S-HDPAω-3) (Kd = 4.0 μM) were shown to decrease 13S,14S-epoxy-DHA production. h12-LOX was also shown to be allosterically regulated by 14S-HpDHA (Kd = 3.5 μM) and 14S-HDPAω-3 (Kd = 4.0 μM); however, 12S-HETE showed no effect, indicating for the first time an allosteric response by h12-LOX. Finally, 14S-HpDHA inhibited platelet aggregation at a submicrololar concentration, which may have implications in the benefits of diets rich in DHA. These in vitro biosynthetic pathways may help guide in vivo maresin biosynthetic investigations and possibly direct therapeutic interventions.

The Biosynthesis of Enzymatically Oxidized Lipids

Enzymatically oxidized lipids are a specific group of biomolecules that function as key signaling mediators and hormones, regulating various cellular and physiological processes from metabolism and cell death to inflammation and the immune response. They are broadly categorized as either polyunsaturated fatty acid (PUFA) containing (free acid oxygenated PUFA "oxylipins", endocannabinoids, oxidized phospholipids) or cholesterol derivatives (oxysterols, steroid hormones, and bile acids). Their biosynthesis is accomplished by families of enzymes that include lipoxygenases (LOX), cyclooxygenases (COX), cytochrome P450s (CYP), and aldo-keto reductases (AKR). In contrast, non-enzymatically oxidized lipids are produced by uncontrolled oxidation and are broadly considered to be harmful. Here, we provide an overview of the biochemistry and enzymology of LOXs, COXs, CYPs, and AKRs in humans. Next, we present biosynthetic pathways for oxylipins, oxidized phospholipids, oxysterols, bile acids and steroid hormones. Last, we address gaps in knowledge and suggest directions for future work.

Emerging Roles of 5-Lipoxygenase Phosphorylation in Inflammation and Cell Death

5-Lipoxygenase (ALOX5) is an iron-containing and nonheme dioxygenase that catalyzes the peroxidation of polyunsaturated fatty acids such as arachidonic acid. ALOX5 is the rate-limiting enzyme for the biosynthesis of leukotrienes, a family of proinflammatory lipid mediators derived from arachidonic acid. ALOX5 also make great contributions to mediating lipid peroxidation. In recent years, it has been discovered that ALOX5 plays a central role in cell death including apoptosis, pyroptosis, and ferroptosis, a newly discovered type of cell death. According to the previous studies, ALOX5 can regulate cell death in two ways: one is inflammation and the other is lipid peroxidation. Meanwhile, it has been shown that ALOX5 activity is regulated by several factors including protein phosphorylation, ALOX5-interactng protein, redox state, and metal ions such as iron and calcium. In this review, we aim to summarize the knowledge on the emerging roles of ALOX5 protein phosphorylation in the regulation of cell death and inflammation in order to explore a potential target for human diseases.

Development of multitarget agents possessing soluble epoxide hydrolase inhibitory activity

Over the last two decades polypharmacology has emerged as a new paradigm in drug discovery, even though developing drugs with high potency and selectivity toward a single biological target is still a major strategy. Often, targeting only a single enzyme or receptor shows lack of efficacy. High levels of inhibitor of a single target also can lead to adverse side effects. A second target may offer additive or synergistic effects to affecting the first target thereby reducing on- and off-target side effects. Therefore, drugs that inhibit multiple targets may offer a great potential for increased efficacy and reduced the adverse effects. In this review we summarize recent findings of rationally designed multitarget compounds that are aimed to improve efficacy and safety profiles compared to those that target a single enzyme or receptor. We focus on dual inhibitors/modulators that target the soluble epoxide hydrolase (sEH) as a common part of their design to take advantage of the beneficial effects of sEH inhibition.

5 S,15 S-Dihydroperoxyeicosatetraenoic Acid (5,15-diHpETE) as a Lipoxin Intermediate: Reactivity and Kinetics with Human Leukocyte 5-Lipoxygenase, Platelet 12-Lipoxygenase, and Reticulocyte 15-Lipoxygenase-1

The reaction of 5 S,15 S-dihydroperoxyeicosatetraenoic acid (5,15-diHpETE) with human 5-lipoxygenase (LOX), human platelet 12-LOX, and human reticulocyte 15-LOX-1 was investigated to determine the reactivity and relative rates of producing lipoxins (LXs). 5-LOX does not react with 5,15-diHpETE, although it can produce LXA₄ when 15-HpETE is the substrate. In contrast, both 12-LOX and 15-LOX-1 react with 5,15-diHpETE, forming specifically LXB₄. For 12-LOX and 5,15-diHpETE, the kinetic parameters are k_cat = 0.17 s^-1 and k_cat/ K_M = 0.011 μM^-1 s^-1 [106- and 1600-fold lower than those for 12-LOX oxygenation of arachidonic acid (AA), respectively]. On the other hand, for 15-LOX-1 the equivalent parameters are k_cat = 4.6 s^-1 and k_cat/ K_M = 0.21 μM^-1 s^-1 (3-fold higher and similar to those for 12-HpETE formation by 15-LOX-1 from AA, respectively). This contrasts with the complete lack of reaction of 15-LOX-2 with 5,15-diHpETE [Green, A. R., et al. (2016) Biochemistry 55, 2832-2840]. Our data indicate that 12-LOX is markedly inferior to 15-LOX-1 in catalyzing the production of LXB₄ from 5,15-diHpETE. Platelet aggregation was inhibited by the addition of 5,15-diHpETE, with an IC₅₀ of 1.3 μM; however, LXB₄ did not significantly inhibit collagen-mediated platelet activation up to 10 μM. In summary, LXB₄ is the primary product of 12-LOX and 15-LOX-1 catalysis, if 5,15-diHpETE is the substrate, with 15-LOX-1 being 20-fold more efficient than 12-LOX. LXA₄ is the primary product with 5-LOX but only if 15-HpETE is the substrate. Approximately equal proportions of LXA₄ and LXB₄ are produced by 12-LOX but only if LTA₄ is the substrate, as described previously [Sheppard, K. A., et al. (1992) Biochim. Biophys. Acta 1133, 223-234].

Leukotriene biosynthetic enzymes as therapeutic targets

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

Strict Regiospecificity of Human Epithelial 15-Lipoxygenase-2 Delineates Its Transcellular Synthesis Potential

Lipoxins are an important class of lipid mediators that induce the resolution of inflammation and arise from transcellular exchange of arachidonic acid (AA)-derived lipoxygenase products. Human epithelial 15-lipoxygenase-2 (h15-LOX-2), the major lipoxygenase in macrophages, has exhibited strict regiospecificity, catalyzing only the hydroperoxidation of carbon 15 of AA. To determine the catalytic potential of h15-LOX-2 in transcellular synthesis events, we reacted it with the three lipoxygenase-derived monohydroperoxy-eicosatetraenoic acids (HPETE) in humans: 5-HPETE, 12-HPETE, and 15-HPETE. Only 5-HPETE was a substrate for h15-LOX-2, and the steady-state catalytic efficiency (kcat/Km) of this reaction was 31% of the kcat/Km of AA. The only major product of h15-LOX-2's reaction with 5-HPETE was the proposed lipoxin intermediate, 5,15-dihydroperoxy-eicosatetraenoic acid (5,15-diHPETE). However, h15-LOX-2 did not react further with 5,15-diHPETE to produce lipoxins. This result is consistent with the specificity of h15-LOX-2 despite the increased reactivity of 5,15-diHPETE. Density functional theory calculations determined that the radical, after abstracting the C10 hydrogen atom from 5,15-diHPETE, had an energy 5.4 kJ/mol lower than that of the same radical generated from AA, demonstrating the facility of 5,15-diHPETE to form lipoxins. Interestingly, h15-LOX-2 does react with 5S,6R-diHETE, forming LipoxinA4, indicating the gemdiol does not prohibit h15-LOX-2 reactivity. Taken together, these results demonstrate the strict regiospecificity of h15-LOX-2 that circumscribes its role in transcellular synthesis.

Impact of myeloperoxidase-derived oxidants on the product profile of human 5-lipoxygenase

Human 5-lipoxygenase (5-LOX) oxidizes arachidonic acid to 5S-hydroperoxy-6 E,8 Z,11 Z,14 Z-eicosatetraenoic acid (5-HpETE) and leukotriene (LT) A4. In neutrophils, LTA4 is further converted to the potent chemoattractant LTB4. These cells also contain the heme enzyme myeloperoxidase (MPO), which produces several potent oxidants such as hypochlorous acid (HOCl), which are involved in pathogen defense and immune regulation. Here, we addressed the question whether MPO-derived oxidants are able to affect the activity of 5-LOX and the product profile of this enzyme. Human 5-LOX was incubated with increasing amounts of HOCl or HOBr. Afterward, arachidonic acid metabolites of 5-LOX were analyzed by reverse-phase high-performance liquid chromatography as well as by liquid chromatography-electrospray ionization-tandem mass spectrometry. The incubation of 5-LOX with the MPO-derived oxidants significantly changed the product profile of 5-LOX. Thereby, HOCl and HOBr increased the ratio of 5-H(p)ETE to 6-trans-LTB4 in a concentration-dependent manner. At low oxidant concentrations, there was a strong decrease in the yield of 6-trans-LTB4, whereas 5-HpETE did not change or increased. Additionally, the formation of 8-HpETE and 12-HpETE by 5-LOX rose slightly with increasing HOCl and HOBr. Comparable results were obtained with the MPO-H2O2-Cl(-) system when glucose oxidase and glucose were applied as a source of H2O2. This was necessary because of a strong impairment of 5-LOX activity by H2O2. In summary, MPO-derived oxidants showed a considerable impact on 5-LOX, impairing the epoxidation of 5-HpETE, whereas the hydroperoxidation of arachidonic acid was unaffected. Apparently, this was caused by an oxidative modification of critical amino acid residues of 5-LOX. Further work is necessary to assess the specific type and position of oxidation in the substrate-binding cavity of 5-LOX and to specify whether this interaction between 5-LOX and MPO-derived oxidants also takes place in stimulated neutrophils.