Leukotriene signaling in the extinct human subspecies Homo denisovan and Homo neanderthalensis. Structural and functional comparison with Homo sapiens

p53 Activates the Lipoxygenase Activity of ALOX15B via Inhibiting SLC7A11 to Induce Ferroptosis in Bladder Cancer Cells

Bladder cancer is a malignant tumor of the urinary system and is one of the most common cancers worldwide. Lipoxygenases are closely related to the development of various cancers. However, the relationship between lipoxygenases and p53/SLC7A11-dependent ferroptosis in bladder cancer has not been reported. Here, we aimed to investigate the roles and internal mechanisms of lipid peroxidation and p53/SLC7A11-dependent ferroptosis in the development and progression of bladder cancer. First, ultraperformance liquid chromatography-tandem mass spectrometry was performed to measure the metabolite production of lipid oxidation in patients' plasma. The metabolic changes in patients with bladder cancer were discovered, revealing that stevenin, melanin, and octyl butyrate were upregulated. Then, the expressions of lipoxygenase family members were measured to screen out candidates with significant changes in bladder cancer tissues. Among various lipoxygenases, ALOX15B was significantly downregulated in bladder cancer tissues. Moreover, p53 and 4-hydroxynonenal (4-HNE) levels were decreased in bladder cancer tissues. Next, sh-ALOX15B, oe-ALOX15B, or oe-SLC7A11 plasmids were constructed and transfected into bladder cancer cells. Then, the p53 agonist Nutlin-3a, tert-butyl hydroperoxide, iron chelator deferoxamine, and the selective ferroptosis inhibitor ferr1 were added. The effects of ALOX15B and p53/SLC7A11 on bladder cancer cells were evaluated by in vitro and in vivo experiments. We revealed that knockdown of ALOX15B promoted bladder cancer cell growth, which was also found to protect bladder cancer cells from p53-induced ferroptosis. Furthermore, p53 activated ALOX15B lipoxygenase activity by suppressing SLC7A11. Taken together, p53 activated the lipoxygenase activity of ALOX15B via inhibiting SLC7A11 to induce ferroptosis in bladder cancer cells, which provided insight into the molecular mechanism of the occurrence and development of bladder cancer.

The Reaction Specificity of Mammalian ALOX15 Orthologs is Changed During Late Primate Evolution and These Alterations Might Offer Evolutionary Advantages for Hominidae

Arachidonic acid lipoxygenases (ALOXs) have been implicated in the immune response of mammals. The reaction specificity of these enzymes is decisive for their biological functions and ALOX classification is based on this enzyme property. Comparing the amino acid sequences and the functional properties of selected mammalian ALOX15 orthologs we previously hypothesized that the reaction specificity of these enzymes can be predicted based on their amino acid sequences (Triad Concept) and that mammals, which are ranked in evolution below gibbons, express arachidonic acid 12-lipoxygenating ALOX15 orthologs. In contrast, Hominidae involving the great apes and humans possess 15-lipoxygenating enzymes (Evolutionary Hypothesis). These two hypotheses were based on sequence data of some 60 mammalian ALOX15 orthologs and about half of them were functionally characterized. Here, we compared the ALOX15 sequences of 152 mammals representing all major mammalian subclades expressed 44 novel ALOX15 orthologs and performed extensive mutagenesis studies of their triad determinants. We found that ALOX15 genes are absent in extant Prototheria but that corresponding enzymes frequently occur in Metatheria and Eutheria. More than 90% of them catalyze arachidonic acid 12-lipoxygenation and the Triad Concept is applicable to all of them. Mammals ranked in evolution above gibbons express arachidonic acid 15-lipoxygenating ALOX15 orthologs but enzymes with similar specificity are only present in less than 5% of mammals ranked below gibbons. This data suggests that ALOX15 orthologs have been introduced during Prototheria-Metatheria transition and put the Triad Concept and the Evolutionary Hypothesis on a much broader and more reliable experimental basis.

A role of Gln596 in fine-tuning mammalian ALOX15 specificity, protein stability and allosteric properties

His596 of human ALOX12 has been suggested to interact with the COO^--group of arachidonic acid during ALOX catalysis. In mammalian ALOX15 orthologs Gln596 occupies this position and this amino acid exchange might contribute to the functional differences between the two ALOX-isoforms. To explore the role of Gln596 for ALOX15 functionality we mutated this amino acid to different residues in rabbit and human ALOX15 and investigated the impact of these mutations on structural, catalytic and allosteric enzyme properties. To shed light on the molecular basis of the observed functional alterations we performed in silico substrate docking studies and molecular dynamics simulations and also explored the impact of Gln596 exchange on the protein structure. The combined theoretical and experimental data suggest that Gln596 may not directly interact with the COO^--group of arachidonic acid. In contrast, mutations at Gln596 destabilize the secondary and tertiary structure of ALOX15 orthologs, which may be related to a disturbance of the electrostatic interaction network with other amino acids in the immediate surrounding. Moreover, our MD-simulations suggest that the geometry of the dimer interface depends on the structure of substrate bound inside the substrate-binding pocket and that Gln596Ala exchange impairs the allosteric properties of the enzyme. Taken together, these data indicate the structural and functional importance of Gln596 for ALOX15 catalysis.

Functional Characterization of Knock-In Mice Expressing a 12/15-Lipoxygenating Alox5 Mutant Instead of the 5-Lipoxygenating Wild-Type Enzyme

Aims: Most mammalian genomes involve several genes encoding for functionally distinct arachidonate lipoxygenase (ALOX isoforms). Proinflammatory leukotrienes are formed via the ALOX5 pathway, but 12/15-lipoxygenating ALOX isoforms have been implicated in the biosynthesis of pro-resolving mediators. In vitro mutagenesis of the triad determinants abolished the leukotriene synthesizing activity of ALOX5, but the biological consequences of these alterations have not been studied. To fill this gap, we created Alox5 knock-in mice, which express the 12/15-lipoxygenating Phe359Trp + Ala424Ile + Asn425Met Alox5 triple mutant and characterized its phenotypic alterations. Results: The mouse Alox5 triple mutant functions as arachidonic acid 15-lipoxygenating enzyme, which also forms 12S-hydroxy and 8S-hydroxy arachidonic acid. In contrast to the wild-type enzyme, the triple mutant effectively oxygenates linoleic acid to 13S-hydroxy linoleic acid (13S-HODE), which functions as activating ligand of the type-2 nuclear receptor peroxisome proliferator-activated receptor gamma (PPARγ). Knock-in mice expressing the mutant enzyme are viable, fertile, and develop normally. The mice cannot synthesize proinflammatory leukotrienes but show significantly attenuated plasma levels of lipolytic endocannabinoids. When aging, the animals gained significantly more body weight, which may be related to the fivefold higher levels of 13-HODE in the adipose tissue. Innovation: These data indicate for the first time that in vivo mutagenesis of the triad determinants of mouse Alox5 abolished the biosynthetic capacity of the enzyme for proinflammatory leukotrienes and altered the catalytic properties of the protein favoring the formation of 13-HODE. Conclusion:In vivo triple mutation of the mouse Alox5 gene impacts the body weight homeostasis of aging mice via augmented formation of the activating PPARγ ligand 13-HODE.

The lipoxygenase pathway of Tupaia belangeri representing Scandentia. Genomic multiplicity and functional characterization of the ALOX15 orthologs in the tree shrew

The tree shrew (Tupaia belangeri) is a rat-sized mammal, which is more closely related to humans than mice and rats. However, the use of tree shrew to explore the patho-mechanisms of human inflammatory disorders has been limited since nothing is known about eicosanoid metabolism in this mammalian species. Eicosanoids are important lipid mediators exhibiting pro- and anti-inflammatory activities, which are biosynthesized via lipoxygenase and cyclooxygenase pathways. When we searched the tree shrew genome for the presence of cyclooxygenase and lipoxygenase isoforms we found copies of functional COX1, COX2 and LOX genes. Interestingly, we identified four copies of ALOX15 genes, which encode for four structurally distinct ALOX15 orthologs (tupALOX15a-d). To explore the catalytic properties of these enzymes we expressed tupALOX15a and tupALOX15c as catalytically active proteins and characterized their enzymatic properties. As predicted by the Evolutionary Hypothesis of ALOX15 specificity we found that the two enzymes converted arachidonic acid predominantly to 12S-HETE and they also exhibited membrane oxygenase activities. However, their reaction kinetic properties (K_M for arachidonic acid and oxygen, T- and pH-dependence) and their substrate specificities were remarkably different. In contrast to mice and humans, tree shrew ALOX15 isoforms are highly expressed in the brain suggesting a role of these enzymes in cerebral function. The genomic multiplicity and the tissue expression patterns of tree shrew ALOX15 isoforms need to be considered when the results of in vivo inflammation studies obtained in this animal are translated into the human situation.

Evolutionary alteration of ALOX15 specificity optimizes the biosynthesis of antiinflammatory and proresolving lipoxins

ALOX15 (12/15-lipoxygenase) orthologs have been implicated in maturational degradation of intracellular organelles and in the biosynthesis of antiinflammatory and proresolving eicosanoids. Here we hypothesized that lower mammals (mice, rats, pigs) express 12-lipoxygenating ALOX15 orthologs. In contrast, 15-lipoxygenating isoforms are found in higher primates (orangutans, men), and these results suggest an evolution of ALOX15 specificity. To test this hypothesis we first cloned and characterized ALOX15 orthologs of selected Catarrhini representing different stages of late primate evolution and found that higher primates (men, chimpanzees) express 15-lipoxygenating orthologs. In contrast, lower primates (baboons, rhesus monkeys) express 12-lipoxygenating enzymes. Gibbons, which are flanked in evolution by rhesus monkeys (12-lipoxygenating ALOX15) and orangutans (15-lipoxygenating ALOX15), express an ALOX15 ortholog with pronounced dual specificity. To explore the driving force for this evolutionary alterations, we quantified the lipoxin synthase activity of 12-lipoxygenating (rhesus monkey, mouse, rat, pig, humIle418Ala) and 15-lipoxygenating (man, chimpanzee, orangutan, rabbit, ratLeu353Phe) ALOX15 variants and found that, when normalized to their arachidonic acid oxygenase activities, the lipoxin synthase activities of 15-lipoxygenating ALOX15 variants were more than fivefold higher (P < 0.01) [corrected]. Comparative molecular dynamics simulations and quantum mechanics/molecular mechanics calculations indicated that, for the 15-lipoxygenating rabbit ALOX15, the energy barrier for C13-hydrogen abstraction (15-lipoxygenation) was 17 kJ/mol lower than for arachidonic acid 12-lipoxygenation. In contrast, for the 12-lipoxygenating Ile418Ala mutant, the energy barrier for 15-lipoxygenation was 10 kJ/mol higher than for 12-lipoxygenation. Taken together, our data suggest an evolution of ALOX15 specificity, which is aimed at optimizing the biosynthetic capacity for antiinflammatory and proresolving lipoxins.

Down-regulation of a signaling mediator in association with lowered plasma arachidonic acid levels in individuals with autism spectrum disorders

Previous studies have indicated that the altered composition of polyunsaturated fatty acids (PUFAs) might contribute to the pathophysiology of autism spectrum disorder (ASD). We examined the relationship between the plasma fatty acid levels, expressed as μg/ml, and the plasma levels of biomarkers of AA-related signaling mediators, such as ceruloplasmin, transferrin and superoxide dismutase, and assessed the behavioral symptoms of 30 individuals with ASD (mean age, 13.6 ± 4.3 years old) compared with 20 age- and gender-matched normal controls (mean age, 13.2 ± 5.4 years old) using Aberrant Behavior Checklists (ABC). The plasma levels of EPA and the plasma ratios of EPA/AA were significantly higher, while the plasma levels of AA and metabolites, such as 5,8,11,14-eicosatetraenoic acid, adrenic acid, and ceruloplasmin (Cp), were significantly lower in the 30 individuals with ASD compared with the 20 normal controls. The ABC scores were significantly increased in the ASD group compared with those of the control group. Thus, the results of the present study revealed that reduced plasma levels of AA and metabolites in association with high plasma EPA/AA ratios might down-regulate AA-related signaling mediators, such as Cp. Subsequently, reduced plasma Cp levels might reduce the protective capacity for brain damage, resulting in the pathophysiology underlying the behavioral symptoms in individuals with ASD. These findings suggest that reduced plasma AA levels may downregulate Cp.

Structural and functional biology of arachidonic acid 15-lipoxygenase-1 (ALOX15)

Lipoxygenases (LOX) form a family of lipid peroxidizing enzymes, which have been implicated in a number of physiological processes and in the pathogenesis of inflammatory, hyperproliferative and neurodegenerative diseases. They occur in two of the three domains of terrestrial life (bacteria, eucarya) and the human genome involves six functional LOX genes, which encode for six different LOX isoforms. One of these isoforms is ALOX15, which has first been described in rabbits in 1974 as enzyme capable of oxidizing membrane phospholipids during the maturational breakdown of mitochondria in immature red blood cells. During the following decades ALOX15 has extensively been characterized and its biological functions have been studied in a number of cellular in vitro systems as well as in various whole animal disease models. This review is aimed at summarizing the current knowledge on the protein-chemical, molecular biological and enzymatic properties of ALOX15 in various species (human, mouse, rabbit, rat) as well as its implication in cellular physiology and in the pathogenesis of various diseases.

Evolutionary aspects of lipoxygenases and genetic diversity of human leukotriene signaling

Leukotrienes are pro-inflammatory lipid mediators, which are biosynthesized via the lipoxygenase pathway of the arachidonic acid cascade. Lipoxygenases form a family of lipid peroxidizing enzymes and human lipoxygenase isoforms have been implicated in the pathogenesis of inflammatory, hyperproliferative (cancer) and neurodegenerative diseases. Lipoxygenases are not restricted to humans but also occur in a large number of pro- and eucaryotic organisms. Lipoxygenase-like sequences have been identified in the three domains of life (bacteria, archaea, eucarya) but because of lacking functional data the occurrence of catalytically active lipoxygenases in archaea still remains an open question. Although the physiological and/or pathophysiological functions of various lipoxygenase isoforms have been studied throughout the last three decades there is no unifying concept for the biological importance of these enzymes. In this review we are summarizing the current knowledge on the distribution of lipoxygenases in living single and multicellular organisms with particular emphasis to higher vertebrates and will also focus on the genetic diversity of enzymes and receptors involved in human leukotriene signaling.