13-Hydroxyoctadecadienoic acid (13-HODE) is the commonly used term for 13(S)-hydroxy-9Z,11E-octadecadienoic acid (13(S)-HODE). The production of 13(S)-HODE is often accompanied by the production of its stereoisomer, 13(R)-hydroxy-9Z,11E-octadecadienoic acid (13(R)-HODE). The adjacent figure gives the structure for the (S) stereoisomer of 13-HODE. Two other naturally occurring 13-HODEs that may accompany the production of 13(S)-HODE are its cis-trans (i.e., 9E,11E) isomers viz., 13(S)-hydroxy-9E,11E-octadecadienoic acid (13(S)-EE-HODE) and 13(R)-hydroxy-9E,11E-octadecadienoic acid (13(R)-EE-HODE). Studies credit 13(S)-HODE with a range of clinically relevant bioactivities; recent studies have assigned activities to 13(R)-HODE that differ from those of 13(S)-HODE; and other studies have proposed that one or more of these HODEs mediate physiological and pathological responses, are markers of various human diseases, and/or contribute to the progression of certain diseases in humans. Since, however, many studies on the identification, quantification, and actions of 13(S)-HODE in cells and tissues have employed methods that did not distinguish between these isomers, 13-HODE is used here when the actual isomer studied is unclear.
A similar set of 9-Hydroxyoctadecadienoic acid (9-HODE) metabolites (i.e., 9(S)-HODE), 9(R)-HODE, 9(S)-EE-HODE), and 9(R)-EE-HODE) occurs naturally and particularly under conditions of oxidative stress forms concurrently with the 13-HODEs; the 9-HODEs have overlapping and complementary but not identical activities with the 13-HODEs. Some recent studies measuring HODE levels in tissue have lumped the four 9-HODEs with the four 13-HODEs to report only on total HODEs (tHODEs). tHODEs have been proposed to be markers for certain human diseases. Other studies have lumped together the 9-(S), 9(R), 13 (S)-, and 13(R)-HODEs along with the two ketone metabolites of these HODEs, 13-oxoODE (13-oxo-9Z,12E-octadecadienoic acid) and 9-oxoODE, reporting only on total OXLAMs (oxidized linoleic acid metabolites); the OXLAMs have been implicated in working together to signal for pain perception.
Pathways making 13-HODEs
15-Lipoxygenase 1 15-lipoxygenase 1 (ALOX15), while best known for converting the 20 carbon polyunsaturated fatty acid, arachidonic acid, into a series of 15-hydroxylated arachidonic acid metabolites (see 15-hydroxyicosatetraenoic acid), actually prefers as its substrate the 18 carbon polyunsaturated fatty acid, linoleic acid, over arachidonic acid, converting it to 13-hydroperoxy-9Z,11E-octadecadienoic acid (13-HpODE). The enzyme acts in a highly stereospecific manner, forming 13(S)-hydroperoxy-9Z,11E-octadecadienoic acid (13(S)-HpODE) but comparatively little or no 13(R)-hydroperoxy-9Z,11E-octadecadienoic acid (13(R)-HpODE) -. In cells, 13(S)-HpODE is rapidly reduced by peroxidases to 13(S)-HODE. ALOX15 is fully capable of metabolizing the linoleic acid that is bound to phospholipid or cholesterol to form 13(S)-HpODE-bound phospholipids and cholesterol that are rapidly converted to their corresponding 13(S)-HODE-bound products.
15-lipoxygenase 2 15-lipoxygenase type 2 (ALOX15B) strongly prefers arachidonic acid over linoleic acid and in consequence is relatively poor in metabolizing linoleic acid to 13(S)-HpODE (which is then converted to 13(S)-HODE) compared to 15-lipoxygenase 1; nonetheless, it can contribute to the production of these metabolites.
Cyclooxygenases 1 and 2 Cyclooxygenase 1 (COX-1) and cyclooxygenase 2 (COX-2) metabolize linoleic acid to 13(S)-HODE with COX-2 exhibiting a higher preference for linoleic acid and therefore producing far more of this product than its COX-1 counterpart; consequently, COX-2 appears to be the principle COX making 13(S)-HODE in cells expressing both enzymes. Concurrently with their production of 13(S)-HODE, these enzymes also produce smaller amounts of 9(R)-HODE.
Cytochrome P450 Cytochrome P450 microsomal enzymes metabolize linoleic acid to a mixture of 13-HODEs and 9-HODEs; these reactions produce racemic mixtures in which the R stereoisomer predominates, for instance by a R/S ratio of 80%/20% for both 13-HODE and 9-HODE in human liver microsomes.
Free radical and singlet oxygen oxidations Oxidative stress in cells and tissues produces Free radical and singlet oxygen oxidations of linoleic acid to generate 13-HpODEs, 9-HpODEs, 13-HODEs, and 9-HODEs; these non-enzymatic reactions produce or are suspected but not proven to produce approximately equal amounts of their S and R stereoisomers. Free radical oxidations of linoleic acid also produce 13-EE-HODE, 9-hydroxy-10E,12-E-octadecadienoic acid, 9-hydroxy-10E,12-Z-octadecadienoic acid, and 11-hydroxy-9Z,12Z-octadecadienoic acid while singlet oxygen attacks on linoleic acid produce (presumably) racemic mixtures of 9-hydroxy-10E,12-Z-octadecadienoic acid, 10-hydroxy-8E,12Z-octadecadienoic acid, and 12-hydroxy-9Z-13-E-octadecadienoic acid. 4-Hydroxynonenal (i.e. 4-hydroxy-2E-nonenal or HNE) is also a peroxidation product of 13-HpODE. Since oxidative stress commonly produces both free radicals and singlet oxygen, most or all of these products may form together in tissues undergoing oxidative stress. Free radical and singlet oxygen oxidations of linoleic acid produce a similar set of 13-HODE metabolites (see 9-Hydroxyoctadecadienoic acid). Studies attribute these oxidations to be major contributors to 13-HODE production in tissues undergoing oxidative stress including in humans sites of inflammation, steatohepatitis, cardiovascular disease-related atheroma plaques, neurodegenerative disease, etc. (see oxidative stress).
Metabolism of 13(S)-HODE
Like most polyunsaturated fatty acids and mono-hydroxyl polyunsaturated fatty acids, 13(S)-HODE is rapidly and quantitatively incorporated into phospholipids; the levels of 13(S)-HODE esterified to the sn-2 position of phosphatidylcholine, phosphatidylinositol, and phosphatidylethanolamine in human psoriasis lesions are significantly lower than those in normal skin; this chain shortening pathway may be responsible for inactivating 13(S)-HODE. 13(S)-HODE is also metabolized by peroxisome-dependent β-oxidations to chain-shortened 16-carbon, 14-carbon, and 12-carbon products which are released from the cell; this chain-shortening pathway may serve to inactive and dispose of 13(S)-HODE.
13(S)-HODE is oxidized to 13-oxo-9Z,11E-octadecadienoic acid (13-oxo-HODE or 13-oxoODE) by a NAD+-dependent 13-HODE dehydrogenase, the protein for which has been partially purified from rat colon. The formation of 13-oxo-ODE may represent the first step in 13(S)-HODEs peroxisome-dependent chain shortening but 13-oxo-ODE has its own areas of biological importance: it accumulates in tissues, is bioactive, and may have clinically relevance as a marker for and potential contributor to human disease. 13-Oxo-ODE itself may react with glutathione in a non-enzymatic Michael reaction or a glutathione transferase-dependent reaction to form 13-oxo-ODE products containing an 11 trans double bound and glutathione attached to carbon 9 in a mixture of S and R diastereomers; these two diastereomers are major metabolites of 13(S)-HODE in cultured HT-29 human colon cancer cells. Colonic mucosal explants from Sprague-Dawley rats and human colon cancer HT29 cells add glutathione to 13-oxo-ODE in a Michael reaction to form 13-oxo-9-glutatione-11(E)-octadecenoic acid; this conjugation reaction appears to be enzymatic and mediated by a glutathione transferase. Since this conjugate may be rapidly exported from the cell and has not yet been characterized for biological activity, it is not clear if this transferase reaction serves any function beyond removing 13-oxo-ODE from the cell to limit its activity.
Activities
Stimulation of peroxisome proliferator-activated receptors 13-HODE, 13-oxoODE, and 13-EE-HODE (along with their 9-HODE counterparts) directly activate peroxisome proliferator-activated receptor gamma (PPARγ). This activation appears responsible for the ability of 13-HODE (and 9-HODE) to induce the transcription of PPARγ-inducible genes in human monocytes as well as to stimulate the maturation of these cells to macrophages. 13(S)-HODE (and 9(S)-HODE) also stimulate the activation of peroxisome proliferator-activated receptor beta (PPARβ) in a model cell system; 13-HODE (and 9-HODE) are also proposed to contribute to the ability of oxidized low-density lipoprotein (LDL) to activate PPARβl: LDL containing phospholipid-bound 13-HODE (and 9-HODE) is taken up by the cell and then acted on by phospholipases to release the HODEs which in turn directly activate PPARβl.
Stimulation of TRPV1 receptor 13(S)-HODE, 13(R)-HODE and 13-oxoODE, along with their 9-HODE counterparts, also act on cells through TRPV1. TRPV1 is the transient receptor potential cation channel subfamily V member 1 receptor (also termed capsaicin receptor or vanilloid receptor 1). These 6 HODEs, dubbed, oxidized linoleic acid metabolites (OXLAMs), individually but also and possibly to a greater extent when acting together, stimulate TRPV1-dependent responses in rodent neurons, rodent and human bronchial epithelial cells, and in model cells made to express rodent or human TRPV1. This stimulation appears due to a direct interaction of these agents on TRPV1 although reports disagree on the potencies of the (OXLAMs) with, for example, the most potent OXLAM, 9(S)-HODE, requiring at least 10 micromoles/liter or a more physiological concentration of 10 nanomoles/liter to activate TRPV1 in rodent neurons. The OXLAM-TRPV1 interaction is credited with mediating pain sensation in rodents (see below).
Stimulation of GPR132 receptor 13(S)-HpODE, and 13(S)-HODE directly activate human (but not mouse) GPR132 (G protein coupled receptor 132, also termed G2A) in Chinese hamster ovary cells made to express these receptors; they are, however, far weaker GPR132 activators than 9(S)-HpODE or 9(S)-HODE. GPR132 was initially described as a pH sensing receptor; the role(s) of 13(S)-HpODE and 13(S)-HODE as well as 9(S)-HpODE, 9(S)HODE, and a series or GPR132-activating arachidonic acid hydroxy metabolites (i.e. HETEs) in activating G2A under the physiological and pathological conditions in which G2A is implicated (see GPR132 for a lists of these conditions) have not yet been determined. This determination, as it might apply to humans, is made difficult by the inability of these HODEs to activate rodent GPR132 and therefore to be analyzed in rodent models.
Involvement in mitochondria degradation In the maturation of the red blood cell lineage (see erythropoiesis) from mitochondria-bearing reticulocytes to mature mitochondria-free erythrocytes in rabbits, the mitochondria accumulate phospholipid-bound 13(S)-HODE in their membranes due to the action of a lipoxygenase which (in rabbits, mice, and other sub-primate vertebrates) directly metabolizes linoleic acid-bound phospholipid to 13(S)-HpODE-bound phospholipid which is rapidly reduced to 13(S)-HODE-bound phospholipid. It is suggested that the accumulation of phospholipid-bound 13(S)-HpODE and/or 13(S)-HODE is a critical step in rendering mitochondria more permeable thereby triggering their degradation and thence maturation to erythrocytes. However, functional inactivation of the phospholipid-attacking lipoxygenase gene in mice does not cause major defects in erythropoiesis. It is suggested that mitochondrial degradation proceeds through at least two redundant pathways besides that triggered by lipoxygenase-dependent formation of 13(S)-HpODE- and 13(S)-HODE-bound phospholipids viz., mitochondrial digestion by autophagy and mitochondrial exocytosis. In all events, formation of 13(S)-HODE bound to phospholipid in mitochondrial membranes is one pathway by which they become more permeable and thereby subject to degradation and, as consequence of their release of deleterious elements, to cause cell injury.
Stimulation of blood leukocytes