ALOX15

ALOX15 (also termed arachidonate 15-lipoxygenase, 15-lipoxygenase-1, 15-LO-1, 15-LOX-1) is a polyunsaturated fatty acid-metabolizing enzyme that in humans is encoded by the ALOX15 gene located on chromosome 17p13.3. The 11 kilobase pair gene consists of 14 exons and 13 introns coding for a 75 kiloDalton protein composed of 662 amino acids. 15-LO is distinguished from another human 15-lipoxygenase enzyme, ALOX15B (15-lipoxygenase-2). Orthologs of ALOX15, termed Alox15, are widely distributed in animal and plant species.

Nomenclature

Human ALOX15 was initially named arachidonate 15-lipoxygenase or 15-lipoxygenase but subsequent studies uncovered a second human enzyme with 15-lipoxygenase activity as well as various non-human mammalian that were closely related to and therefore orthologs of human ALOX15 but nonetheless possess 12-lipoxygenase rather than 15-lipoxygenase activity. Consequently, human ALOX15 is now referred to as arachidonate-15-lipoxygenase-1, 15-lipoxygenase-1, 15-LOX-1, 15-LO-1, human 12/15-lipoxygenase, leukocyte-type arachidonate 12-lipoxygenase, or arachidonate omega-6 lipoxygenase; the second discovered human 15-lipoxygenase, a product of the ALOX15B gene, is termed ALOX15B, arachidonate 15-lipoxygenase 2, 15-lipoxygenase-2, 15-LOX-2, 15-LO-2, arachidonate 15-lipoxygenase type II, arachidonate 15-lipoxygenase, second type, and arachidonate 15-lipoxygenase; and mouse, rat, and rabbit rodent orthologs of human ALOX15, which share 74-81% amino acid identity with the human enzyme, are commonly termed Alox15, 12/15-lipoxygenase (i.e. 12/15-LOX, and 12/15-LO).

Human 15-LOX-1 (ALOX15) and 15-LOX-2 ALOX15B genes are located on chromosome 17; their product proteins have an amino acid sequence identity of only ~38%, differ in the polyunsaturated fatty acids that they prefer as substrates, and exhibit different product profiles when acting on the same substrates.

Tissue distribution

Human ALOX15 protein is highly expressed in circulating blood eosinophils, bronchial airway epithelial cells, mammary epithelial cells, the Reed-Sternberg cells of Hodgkin's lymphoma, cornea epithelial cells, reticulocytes, and dendritic cells; it is less strongly expressed in alveolar macrophages, tissue mast cells, tissue fibroblasts, circulating blood neutrophils, vascular endothelial cells, joint Synovial membrane cells, seminal fluid, prostate epithelium cells, and mammary ductal epithelial cells.

The distribution of Alox15 in sub-human primates and, in particular, rodents differs significantly from that of human ALOX15; this, along with there different principal product formation (e.g. 12-HETE rather than 15-HETE) has made the findings of Alox15 functions in rat, mouse, or rabbit models difficult to extrapolate to the function of ALOX15 in humans.

Enzyme activities

Lipoxygenase acitivity

ALOX15 and Alox15 enzymes are non-heme iron-containing dioxygenases. They catalyze the dioxygenation (i.e. attachment of molecular oxygen, O
₂, to polyunsaturated fatty acids (PUFA) that contain two carbon-carbon double bonds that are located between carbons 10 and 9 and 7 and 6 (as numbered counting backward from the last or omega [i.e. ω] carbon at the methyl end of the PUFA; these carbons are therefore also termed ω-10 and ω-9 and ω-7 and ω-6). In PUFAs that do not have a third carbon-carbon double bound between their ω-13 and ω-12 carbons, human ALOX15 forms ω-6 peroxy intermediates; in PUFAs that do have this third double bound, human ALOX15 makes the ω-6 peroxy intermediate but also small amounts of the ω-9 peroxy intermediate. Rodent Alox15 enzymes, in contrast, produce almost exclusively ω-9 peroxy intermediates. Concurrently, ALOX15 and Alox15 enzymes rearrange the carbon-carbon double bounds to bring them into a 1-2 and 3-4 diene configuration system and then reduce (see Redox sections on reduction) the peroxy residues to their hydroperoxy counterparts. The enzymes act with a high degree of Stereospecificity to form products of their naturally occurring substrates that have hydroperoxy residues in the S stereoisomer configuration.

Lipohydroperoxidase activity

Human ALOX15 can also convert the peroxy PUFA intermediate to a cyclic ether with a three-atom ring, i.e. an epoxide intermediate that is attacked by a water molecule to form epoxy-hydrpoxy PUFA products.

Leukotriene synthase activity

The PUFA epoxide intermediate of arachidonic acid may also be conjugated with glutathione to form Hepoxilins or analogs of hepoxilins.

Substrates, substrate metabolites, and metabolite activities

Among their physiological substrates, human and rodent AlOX15 enzymes act on linoleic acid, alpha-linolenic acid, gamma-linolenic acid, arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid when presented not only as free acids but also when incorporated as esters in phospholipids, glycerides, or Cholesteryl esters. The human enzyme is particularly active on linoleic acid, preferring it over arachidonic acid but is less active on PUFA that are esters within the cited lipids.

Human ALOX15 metabolizes arachidonic acid, which has 4 double bonds all of which are in the cis (see Cis–trans isomerism or Z as opposed to the trans or E configuration between carbons 5-6, 8-9, 11-12, and 14-15. The reaction forms 15(S)-hydroperoxy-5Z, 8Z, 11Z, 13E-eicosatetraenoic acid (15(S)-HpETE) and 12(S)-hydroperoxy-5Z, 8Z, 10E, 15S-eicosatetraenoic acid (12(S)-HpETE) with in a 15(S)-HpETE to 12(S)-HpETE product ratio of ~4-9 to 1. Both products may be rapidly reduced by ubiquitous cellular enzymes with peroxide-reducing activity to form their corresponding hydroxy analogs, 15(S)-HETE (see 15-hydroxyicosatetraenoic acid) and 12(S)-HETE (see 12-Hydroxyeicosatetraenoic acid). 15(S)-HpETE and 15(S)-HETE bind to and activate the Leukotriene B4 receptor 2, activate the Peroxisome proliferator-activated receptor gamma, and at high concentrations cause cells to generate toxic reactive oxygen species; one or more of these effects may be at least in part responsible for their ability to alter the growth of various times of human cancer cell lines, contract various types of blood vessels, and stimulate pathological fibrosis in pulmonary arteries and liver (see 15-Hydroxyicosatetraenoic acid##Activities of parent metabolites and their metabolites###15(S)-HpETE and 15(S)-HETE). 15(S)-HpETE and 15(S)-HETE esterified into the membrane phospholipids of the mitochondria in red blood cell precursors may signal for the intracellular degradation of these mitochondria and thereby the maturation of these precursors to red blood cells in mice. However, this pathway operates along with two other mitochondria-removing pathways and therefore does not appear essential for mouse red blood cell maturation.

15-(S)-HpETE and/or 15(S)-HETE are further metabolized by 15-LOX-1 or other lipoxygenases to various dihydroxy, epoxy-hydroxy, and trihydroxy compounds such as 8(S), 15(S)-dihydroxy-eicosatetraenoic acid (8(S); 15(S)-diHETE); 8(R),15(S)-diHETE; 5(S),15(S)-diHETE; 14(R), 15(S)-diHETE; 5-oxo-15(S)-hydroxy-ETE; certain hepoxilin isomers (e.g. 11S-hydroxy-14S, 15S-epoxy-5Z, 8Z, 12E-eicosatrienoic acid [14,15-HXA3] and 13R-hydroxy-14S, 15S-epoxy-5Z, 8Z, 11Z-eicosatrienoic acid [14,15-HXB3]); various eoxins (e.g. eoxin C4, 14,15-eoxin D4, and eoxin E4); and lipoxins such as LXA4 and LXB4. 5-oxo-15(S)-hydroxy-ETE is a potent stimulator of human eosinophil and neutrophil chemotaxis and is thereby suggested to contribute to human allergic and non-allergic inflammation responses (see 5-Hydroxyicosatetraenoic acid##Clinical significance###Inflammation and 5-Hydroxyicosatetraenoic acid##Clinical significance###Allergy). The hepoxilin isomers, similar to other hepoxilins viz., HXA3 and HXB3, may also contribute to the regulation of inflammation responses and to stimulating the secretion of insulin (see hepoxilins. The eoxins have been suggested to have pro-inflammatory actions that contribute to in severe asthma, aspirin-induced asthma attacks, and other allergy reactions and to be involved in the lymphoma of Hodgkins disease (see (Eoxins).

15(S)-HETE is oxidized by NAD+-dependent 15-hydroxyprostaglandin dehydrogenase (NAD+) to its keto analog, 15-oxo-ETE, which inhibits the growth of cultured Human umbilical vein endothelial cells and various human cancer cell lines; it is also has activities on THP1 cell line cells suggesting that it might act as an inhibitor of inflammatory and oxidative stress reactions (see15-Hydroxyicosatetraenoic acid##Activities of parent metabolites and their metabolites###15-oxo-ETE)

Human 15-LOX-1 prefers linoleic acid over arachidonic acid as its primary ω-6 polyunsaturated fatty acid (see Omega-6 fatty acid) substrate, oxygenating it at carbon 13 to form 13(S)-hydroperoxy-9Z, 11E-octadecaenoic acid (13-HpODE or 13(S)-HpODE) which may then be reduce to the corresponding hydroxy derivative, 13(S)-HODE or 13-HODE (see 13-Hydroxyoctadecadienoic acid). Non-human 15-LOX1 orthologs such as mouse 12/15-LOX and soybean 15-LOX form, in addition to 13(S)-HODE, metabolize linoleic acid to 9-hydroperoxy-10E, 12Z-octadecaenoic acid (9-HpODE or 9(S)-HpODE), which in mice is rapidly converted to 9(S)-HODE (9-HODE) (see 9-Hydroxyoctadecadienoic acid. The sited arachidonic and linoleic acid metabolites have significant bioactivities and potential but unproven clinical relevancies. 13(S)-HODE acts through Peroxisome proliferator-activated receptors and the TRPV1 and human GPR132 receptors to stimulate a variety of responses related to monocyte maturation, lipid metabolism, and neuron activation (see 13-Hydroxyoctadecadienoic acid##Activities of 13-HODEs; 9(S)-HODE is a marker for diseases involving oxidative stress and may contribute to this disease as well as to pain perception and atherosclerosis (see 9-Hydroxyoctadecadienoic acid##Biological and clinical relevancy of 9-HODEs).

Human 15-LOX-1 also acts on ω-3 polyunsaturated fatty acids (see Omega-3 fatty acid): it metabolizes α-linolenic acid to 13(S)-hydroperoxy-9Z, 11E, 15Z)-octadecatrienoic acid; eicosapentaenoic acid to 15(S)-hydroperoxy-5Z, 8Z, 11Z, 13E, 17Z-eicosatetraenoic acid (15(S)-HpEPA); and docosahexaenoic acid to 17(S)-hydroperoxy-4Z, 7Z, 10Z, 13Z, 15E, 19Z-docosahexaenoic acid (17-HpDHA) and to neuroprotectin D1 (i.e. 10(R), 17(S)-dihydroxy-4Z, 7Z, 11E, 13E, 15Z, 19Z-docosahexaenoic acid, also designated protectin D1). 15(S)-HpEPA and 17(S)-HpDHA are reduced to 15(S)-hydroxy-5Z, 8Z, 11Z, 13E, 17Z-eicosatetraenoic acid (15-HEPA or 15(S)-HEPA) and 17(S)-hydroxy-4Z, 7Z, 10Z, 13Z, 15E, 19Z-docosahexaenoic acid (17-HDHA or 17(S)-HDHA), respectively; all of these metabolites possess various and often overlapping bioactivities. The 15-(S)-hydroxyl metabolites of α-linolenic acid, eicosapentaenoic acid, docosahexaenoic acid, and protectin D1 have potent antinflammatory activities. Protectin D1 also has activities in the retina, in Alzheimer's disease, and in the protection of certain viral diseases (see Protectin D1##Functions of PD1. 17(S)-HDHA and its 17(S)-hydoperoxy percrusor are also potent inhibitors of the growth of cultured human prostate cancer cells.

15-LOX-1 also works in conjunction with other oxygenating enzymes to form a wide range of products that inhibit, limit, and resolve inflammatory reactions. 15-LOX-1 acts in series with: a) 5-LOX to metabolize arachidonic acid to lipoxins; b) with 5-LOX to metabolize docosahexaenoic acid to resolvins D1, D2, D3, D4, D5, and D6; and c) aspirin-treated cyclooxygenase-2 or cytochrome P450 enzymes to metabolize eicosapentaenoic acid to resolvin E3.

Clinical significance

Inflammatory diseases

À huge and growing number of studies in animal models suggest that 15-LOX-1 and its lipoxin, resolvin, and protectin metabolites act to inhibit, limit, and resolve diverse inflammatory diseases including periodontitis, peritonitis, sepsis, and other pathogen-induced inflammatory responses; in eczema, arthritis, asthma, cystic fibrosis, atherosclerosis, and adipose tissue inflammation; in the insulin resistance that occurs in obesity that is associated with diabetes and the metabolic syndrome; and in Alzheimer's disease. While these studies have not yet been shown to translate to human diseases, first and second generation synthetic resolvins and lipoxins, which unlike their natural analogs, are relatively resistant to metabolic inactivation, have been made and tested as inflammation inhibitors in animal models. These synthetic analogs may prove to be clinically useful for treating the cited human inflammatory diseases.

By metabolizing the ω-3 polyunsaturated fatty acids, eicosapentaenoic acid and docosahexaenoic acid, into 17-HpDHA, 17-HDHA, and the resolvins and protectins, 15-LOX-1's metabolic action is thought to be one mechanism by which dietary ω-3 polyunsaturated fatty acids, particularly fish oil, act to ameliorate inflammation, inflammation-related diseases, and certain cancers.

Asthma

15-LOX-1 and its 5-oxo-15-hydroxy-ETE and eoxin metabolites have been suggested as potential contributors to, and therefore targets for the future study and treatment of, human allergen-induced asthma, aspirin-induced asthma, and perhaps other allergic diseases.

Cancer

In colorectal, breast, and kidney cancers, 15-LOX-1 levels are low or absent compared to the cancers' normal tissue counterparts and/or these levels sharply decline as the cancers progress. These results, as well as a 15-LOX-1 transgene study on colon cancer in mice suggests but do not prove that 15-LOX-1 is a tumor suppressor.

By metabolizing ω-3 polyunsaturated fatty acids, eicosapentaenoic acid and docosahexaenoic acid, into lipoxins and resolvins, 15-LOX-1 is thought to be one mechanism by which dietary ω-3 polyunsaturated fatty acids, particularly fish oil, may act to reduce the incidence and/or progression of certain cancers. Indeed, the ability of docosahexaenoic acid to inhibit the growth of cultured human prostate cancer cells is totally dependent upon the expression of 15-LOX-1 by these cells and appears due to this enzyme's production of docosahexaenoic acid metabolites such as 17(S)-HpETE, 17(S)-HETE, and/or and, possibly, an isomer of protectin DX (10S, 17S-dihydroxy-4Z, 7Z, 11E, 13Z, 15E, 19Z-docosahexaenoic acid)

See also

• Arachidonate 15-lipoxygenase
• 15-Hydroxyicosatetraenoic acid
• 12-Hydroxyeicosatetraenoic acid