5-Hydroxyeicosatetraenoic acid

5(S)-HETE family members share a common receptor target for stimulating cells that differs from the receptors targeted by the other major products of ALOX5, i.e., leukotriene B4, leukotriene C4, leukotriene D4, leukotriene E4, lipoxin A4, and lipoxin B4. It and other members of the 5(S)-HETE family stimulate cells primarily by binding and thereby activating a dedicated G protein-coupled receptor, the oxoeicosanoid receptor 1 (i.e. OXER1, also termed the OXE, OXE-R, hGPCR48, HGPCR48, or R527 receptor).[12][16] OXER1 couples to the G protein complex composed of the Gi alpha subunit (Gαi) and G beta-gamma complex (Gβγ); when bound to a 5-(S)-HETE family member, OXER1 triggers this G protein complex to dissociate into its Gαi and Gβγ components with Gβγ appearing to be the component responsible for activating the signal pathways which lead to cellular functional responses.[12] The cell-activation pathways stimulated by OXER1 include those mobilizing calcium ions and activating MAPK/ERK, p38 mitogen-activated protein kinases, cytosolic phospholipase A2, PI3K/Akt, and protein kinase C beta and epsilon.[12][17] The relative potencies of 5-oxo-ETE, 5-oxo-15(S)-HETE, 5(S)-HETE, 5(S),15(S)-diHETE, 5-oxo-20-hydroxy-ETE, 5(S),20-diHETE, and 5,15-dioxo-ETE in binding to, activating, and thereby stimulating cell responses through the OXER1 receptor are ~100, 30, 5-10, 1-3, 1-3, 1, and <1, respectively.[3][17][18]

Progress in proving the role of the 5-HETE family of agonists and their OXER1 receptor in human physiology and disease has been made difficult because mice, rats, and the other rodents so far tested lack OXER1. Rodents are the most common in vivo models for investigating these issues. OXER1 is expressed in non-human primates, a wide range of other mammals, and various fish species and a model of allergic airways disease in cats, which express OXER1 and make 5-oxo-ETE, has recently been developed for such studies.[17][19] In any event, cultured mouse MA-10 Leydig cells, while responding to 5-oxo-ETE, lack OXER1. It is suggested that this cell's, as well as mouse and other rodent, responses to 5-oxo-ETE are mediated by a receptor closely related to OXER11 viz., the mouse niacin receptor 1, Niacr1. Niacr1, an ortholog of OXER1, is a G protein-coupled receptor for niacin, and responds to 5-oxo-ETE.[20] It has also been suggested that one or more of the mouse hydroxycarboxylic acid (HCA) family of the G protein-coupled receptors, HCA1 (GPR81), HCA2 (GPR109A), and HCA3 (GPR109B), which are G protein-coupled receptors for fatty acids may be responsible for rodent responses to 5-oxo-ETE.[20] It is possible that human cellular responses to 5-oxo-ETE and perhaps its analogs may involve, at least in isolated instances, one or more of these receptors.

5-Oxo-15(S)-hydroxy-ETE and to a lesser extent 5-oxo-ETE but not 5(S)-HETE also bind to and activate peroxisome proliferator-activated receptor gamma (PPARγ). Activation of OXER1 receptor and PPARγ by the oxo analogs can have opposing effects on cells. For example, 5-oxo-ETE-bound OXER1 stimulates while 5-oxo-ETE-bound PPARγ inhibits the proliferation of various types of human cancer cell lines.[21]

5(S)-HETE acylated into the phosphatidylethanolamines fraction of human neutrophil membranes is associated with the inhibition of these cells from forming neutrophil extracellular traps, i.e. extracellular DNA scaffolds which contain neutrophil-derived antimicrobial proteins that circulate in blood and have the ability to trap bacteria. It seems unlikely that this inhibition reflects involvement of OXER1.[22] 5-Oxo-ETE relaxes pre-contracted human bronchi by a mechanism that does not appear to involve OXER1 but is otherwise undefined.[17][23]

5(S)-HETE and other family members were first detected as products of arachidonic acid made by stimulated human polymorphonuclear neutrophils (PMN), a leukocyte blood cell type involved in host immune defense against infection but also implicated in aberrant pro-inflammatory immune responses such as arthritis; soon thereafter they found to be active also in stimulating these cells to migrate (i.e. chemotaxis), degranulate (i.e. release the anti-bacterial and tissue-injuring contents of their granules), produce bacteriocidal and tissue-injuring reactive oxygen species, and mount other pro-defensive as well as pro-inflammatory responses of the innate immune system. For example, the gram-negative bacterium, Salmonella tryphimurium, and the outer surface of gram-negative bacteria lipopolysaccharide, promote the production of 5(S)-HETE and 5-oxo-ETE by human neutrophils. The family members stimulate another blood cell of the innate immunity system, the human monocyte, acting synergistically with the pro-inflammatory CC chemokines, monocyte chemotactic protein-1 and monocyte chemotactic protein-3, to stimulate monocyte function. 5-Oxo-ETE also stimulates two other cell types that share responsibility with the PMN for regulating inflammation, the human lymphocyte and dendritic cell. And, in vivo studies, the injection of 5-oxo-ETE into the skin of human volunteers causes the local accumulation of PMN and monocyte-derived macrophages.[17] Furthermore, the production of one or more 5(S)-HETE family members as well as the expression of orthologs of the human OXER1 receptor occur in various mammalian species including dogs, cats, cows, sheep, elephants, pandas, opossums, and ferrets and in several species of fish; for example, cats undergoing experimentally induced asthma accumulate 5-oxo-ETE in their lung lavage fluid, feline leucocytes make as well as respond to 5-oxo-ETE by an oxer1-dependent mechanism; and an OXER1 ortholog and, apparently, 5-oxo-ETE are necessary for the inflammatory response to tissue damage caused by osmolarity insult in zebrafish.[12][24][19]

These results given above suggest that members of the 5-oxo-ETE family and the OXER1 receptor or its orthologs may contribute to protection against microbes, the repair of damaged tissues, and pathological inflammatory responses in humans and other animal species.[12] However, an OXER1 ortholog is absent in mice and other rodents; while rodent tissues do exhibit responsiveness to 5-oxo-ETE, the lack of an oxer1 or other clear 5-oxoETE receptor in such valued animal models of diseases as rodents has impeded progress in our understanding of the physiological and pathological roles of 5-oxo-ETE.[19]

The following human cell types or tissues that are implicated in allergic reactivity produce 5-HETE (stereoisomer typically not defined): alveolar macrophages isolated from asthmatic and non-asthmatic patients, basophils isolated from blood and challenged with anti-IgE antibody, mast cells isolated from lung, cultured pulmonary artery endothelial cells, isolated human pulmonary vasculature, and allergen-sensitized human lung specimens challenged with specific allergen.[17][25] Additionally, cultured human airway epithelial cell lines, normal bronchial epithelium, and bronchial smooth muscle cells convert 5(S)-HETE to 5-oxo-ETE in a reaction that is greatly increase by oxidative stress, which is a common component in allergic inflammatory reactions.[17] Finally, 5-HETE is found in the bronchoalveolar lavage fluid of asthmatic humans and 5-oxo-ETE is found in the bronchoalveolar lavage fluid of cats undergoing allergen-induced bronchospasm.[17][19][26]

Among the 5-HETE family of metabolites, 5-oxo-ETE is implicated as the most likely member to contribute to allergic reactions. It has exceptionally high potency in stimulating the chemotaxis, release of granule-bound tissue-injuring enzymes, and production of tissue-injuring reactive oxygen species of a cell type involved in allergic reactions, the human eosinophil granulocyte.[17] It is also exceptionally potent in stimulating eosinophils to activate cytosolic phospholipase A2 (PLA2G4A) and possibly thereby to form platelet-activating factor (PAF) as well as metabolites of the 5-HETE family.[17][27] PAF is itself a proposed mediator of human allergic reactions which commonly forms concurrently with 5-HETE family metabolites in human leukocytes and acts synergistically with these metabolites, particularly 5-oxo-ETE, to stimulate eosinophils.[17][28][29][30] 5-Oxo-ETE also cooperates positively with at least four other potential contributors to allergic reactions, RANTES, eotaxin, granulocyte macrophage colony-stimulating factor, and granulocyte colony-stimulating factor in stimulating human eosinophils and is a powerful stimulator of chemotaxis in another cell type contributing to allergic reactions, the human basophil granulocyte.[17] Finally, 5-oxo-ETE stimulates the infiltration of eosinophils into the skin of humans following its intradermal injection (its actions are more pronounced in asthmatic compared to healthy subjects) and when instilled into the trachea of Brown Norway rats causes eosinophils to infiltrate lung.[17] These results suggest that the 5-oxo-ETE made at the initial tissue site of allergen insult acting through the OXER1 on target cells attracts circulating eosinophils and basophils to lung, nasal passages, skin, and possibly other sites of allergen deposition to contribute to asthma, rhinitis, and dermatitis, and other sites of allergic reactivity.[17][31]

The role of 5-HETE family agonists in the bronchoconstriction of airways (a hallmark of allergen-induced asthma) in humans is currently unclear. 5-HETE stimulates the contraction of isolated human bronchial muscle, enhances the ability of histamine to contract this muscle, and contracts guinea pig lung strips.[32] 5-Oxo-ETE also stimulates contractile responses in fresh bronchi, cultured bronchi, and cultured lung smooth muscle taken from guinea pigs but in direct contrast to these studies is reported to relax bronchi isolated from humans.[23][33][34] The latter bronchi contractile responses were blocked by cyclooxygenase-2 inhibition or a thromboxane A2 receptor antagonist and therefore appear mediated by 5-oxo-ETE-induced production of this thromboxane. In all events, the relaxing action of 5-oxo-ETE on human bronchi does not appear to involve OXER1.[17]

The 5-oxo-ETE family of agonists have also been proposed to contribute to the growth of several types of human cancers. This is based on their ability to stimulate certain cultured human cancer cell lines to proliferate, the presence of OXER1 mRNA and/or protein in these cell lines, the production of 5-oxo-ETE family members by these cell lines, the induction of cell death (i.e. apoptosis) by inhibiting 5-lipoxygenase in these cells, and/or the overexpression of 5-lipoxygenase in tissue taken from the human tumors. Human cancers whose growth has been implicated by these studies as being mediated at least in part by a member(s) of the 5-oxo-ETE family include those of the prostate, breast, lung, ovary, and pancreas.[17][21][35][36]

5(S)-HETE and 5(S)-HpETE stimulate the production of progesterone by cultured rat ovarian glomerulosa cells[37] and enhance the secretion of progesterone and testosterone by cultured rat testicular Leydig cells.[38] Both metabolites are made by cyclic adenosine monophosphate-stimulated MA-10 mouse Leydig cells; stimulate these cells to transcribe steroidogenic acute regulatory protein, and in consequence produce the steroids.[39][40] The results suggest that trophic hormones (e.g., leutenizing hormone, adrenocorticotropic hormone) stimulate these steroid producing cells to make 5(S)-HETE and 5(S)-HpEPE which in turn increase the synthesis of steroidogenic acute regulatory protein; the latter protein promotes the rate-limiting step in steroidogenesis, transfer of cholesterol from the outer to the inner membrane of mitochondria and thereby acts in conjunction with trophic hormone-induce activation of protein kinase A to make progesterone and testosterone.[41] This pathway may also operate in humans: Human H295R adrenocortical cells do express OXER1 and respond to 5-oxo-ETE by an increasing the transcription of steroidogenic acute regulatory protein messenger RNA as well as the production of aldosterone and progesterone by an apparent OXER1-dependent pathway.[20]

Rat and mouse cells lack OXER1. It has been suggested that the cited mouse MA-10 cell responses to 5-oxo-ETE are mediated by an ortholog to OXER1, mouse niacin receptor 1, Niacr1, which is a G protein-coupled receptor mediating the activity of niacin, or by one or more of the mouse hydroxycarboxylic acid (HCA) family of the G protein-coupled receptors, HCA1 (GPR81), HCA2 (GPR109A), and HCA3 (GPR109B), which are G protein-coupled receptors for fatty acids.[20] In any event, Human H295R adrenocortical cells do express OXER1 and respond to 5-oxo-ETE by an increasing the transcription of steroidogenic acute regulatory protein messenger RNA as well as the production of aldosterone and progesterone by an apparent OXER1-dependent pathway.[20]

In an in vitro mixed culture system, 5(S)-HETE is released by monocytes to stimulate, at sub-nanomolar concentrations, osteoclast-dependent bone reabsorption.[42] It also inhibits morphogenetic protein-2 (BMP-2)-induced bone-like nodule formation in mouse calvarial organ cultures.[43] These results allow that 5(S)-HETE and perhaps more potently, 5-oxo-ETE contribute to the regulation of bone remodeling.

5(S)-HETE is: elevated in the human uterus during labor;[44] at 3–150 nM, increases both the rates of spontaneous contractions and overall contractility of myometrial strips obtained at term but prior to labor from human lower uterine segments;[45] and in an in vitro system crosses either amnion or intact amnion-chorion-decidua and thereby may along with prostaglandin E2 move from the amnion to uterus during labor in humans.[46] These studies allow that 5(S)-HETE, perhaps in cooperation with established role of prostaglandin E2, may play a role in the onset of human labor.

5(S)-HETE is reported to modulate tubuloglomerular feedback.[47] 5(S)-HpETE is also reported to inhibit the Na⁺/K⁺-ATPase activity of synaptosome membrane preparations prepared from rat cerebral cortex and may thereby inhibit synapse-dependent communications between neurons.[48]

5(S)-HETE acylated into phosphatidylethanolamine is reported to increase the stimulated production of superoxide anion and interleukin-8 release by isolated human neutrophils and to inhibit the formation of neutrophil extracellular traps (i.e. NETS); NETS trap blood-circulating bacteria to assist in their neutralization.[22] 5(S)-HETE esterified to phosphatidylcholine and glycerol esters by human endothelial cells is reported to be associated with the inhibition of prostaglandin production.[49]