Epoxyeicosatrienoic acid

The epoxyeicosatrienoic acids or EETs are signaling molecules formed within various types of cells by the metabolism of arachidonic acid by a specific subset of Cytochrome P450 enzymes termed cytochrome P450 epoxygenases. These nonclassic eicosanoids are generally short-lived, being rapidly converted from epoxides to less active or inactive dihydroxy-eicosatrienoic acids (diHETrEs) by a widely distributed cellular enzyme, Soluble epoxide hydrolase (sEH), also termed Epoxide hydrolase 2. The EETs consequently function as transiently acting, short-range hormones; that is, they work locally to regulate the function of the cells that produce them (i.e. they are autocrine agents) or of nearby cells (i.e. they are paracrine agents). The EETs have been most studied in animal models where they show the ability to lower blood pressure possibly by a) stimulating arterial vasorelaxation and b) inhibiting the kidney's retention of salts and water to decrease intravascular blood volume. In these models, EETs prevent arterial occlusive diseases such as heart attacks and brain strokes not only by their anti-hypertension action but possibly also by their anti-inflammatory effects on blood vessels, their inhibition of platelet activation and thereby blood clotting, and/or their promotion of pro-fibrinolytic removal of blood clots. With respect to their effects on the heart, the EETs are often termed cardio-protective. Beyond these cardiovascular actions that may prevent various cardiovascular diseases, studies have implicated the EETs in the pathological growth of certain types of cancer and in the physiological and possibly pathological perception of neuropathic pain. While studies to date imply that the EETs, EET-forming epoxygenases, and EET-inactivating sEH can be manipulated to control a wide range of human diseases, clinical studies have yet to prove this. Determination of the role of the EETS in human diseases is made particularly difficult because of the large number of EET-forming epoxygenases, large number of epoxygenase substrates other than arachidonic acid, and the large number of activities, some of which may be pathological or injurious, that the EETs possess. The epoxyeicosatrienoic acids or EETs are signaling molecules formed within various types of cells by the metabolism of arachidonic acid by a specific subset of Cytochrome P450 enzymes termed cytochrome P450 epoxygenases. These nonclassic eicosanoids are generally short-lived, being rapidly converted from epoxides to less active or inactive dihydroxy-eicosatrienoic acids (diHETrEs) by a widely distributed cellular enzyme, Soluble epoxide hydrolase (sEH), also termed Epoxide hydrolase 2. The EETs consequently function as transiently acting, short-range hormones; that is, they work locally to regulate the function of the cells that produce them (i.e. they are autocrine agents) or of nearby cells (i.e. they are paracrine agents). The EETs have been most studied in animal models where they show the ability to lower blood pressure possibly by a) stimulating arterial vasorelaxation and b) inhibiting the kidney's retention of salts and water to decrease intravascular blood volume. In these models, EETs prevent arterial occlusive diseases such as heart attacks and brain strokes not only by their anti-hypertension action but possibly also by their anti-inflammatory effects on blood vessels, their inhibition of platelet activation and thereby blood clotting, and/or their promotion of pro-fibrinolytic removal of blood clots. With respect to their effects on the heart, the EETs are often termed cardio-protective. Beyond these cardiovascular actions that may prevent various cardiovascular diseases, studies have implicated the EETs in the pathological growth of certain types of cancer and in the physiological and possibly pathological perception of neuropathic pain. While studies to date imply that the EETs, EET-forming epoxygenases, and EET-inactivating sEH can be manipulated to control a wide range of human diseases, clinical studies have yet to prove this. Determination of the role of the EETS in human diseases is made particularly difficult because of the large number of EET-forming epoxygenases, large number of epoxygenase substrates other than arachidonic acid, and the large number of activities, some of which may be pathological or injurious, that the EETs possess. EETS are epoxide eicosatrienoic acid metabolites of arachidonic acid (a straight chain Eicosatetraenoic acid, omega-6 fatty acid). Arachidonic acid has 4 cis double bonds (see Cis–trans isomerism which are abbreviated with the notation Z in the IUPAC Chemical nomenclature used here. These double bonds are located between carbons 5-6, 8-9, 11-12, and 14-15; arachidonic acid is therefore 5Z,8Z,11Z,14Z-eicosatetraenoic acid. Cytochrome P450 epoxygenases attack these double bonds to form their respective eicosatrienoic acid epoxide regioisomers (see Structural isomer, section on position isomerism (regioisomerism)) viz., 5,6-EET (i.e. 5,6-epoxy-8Z,11Z,14Z-eicosatrienoic acid), 8,9-EET (i.e. 8,9-epoxy-5Z,11Z,14Z-eicosatrienoic acid), 11,12-EET (i.e. 11,12-epoxy-5Z,8Z,14Z-eicosatrienoic acid), or, as drawn in the attached figure, 14,15-EET (i.e. 14,15-epoxy-5Z,8Z,11Z-eicosatrienoic acid). The enzymes generally form both R/S enantiomers at each former double bond position; for example, cytochrome P450 epoxidases metabolize arachidonic acid to a mixture of 14R,15S-EET and 14S,15R-EET. The cytochrome P450 (CYP) superfamily of enzymes is distributed broadly throughout bacteria, archaea, fungi, plants, animals, and even viruses (see Cytochrome P450). The superfamily comprises more than 11,000 genes categorized into 1,000 families. Humans have 57 putatively active CYP genes and 58 CYP pseudogenes; only a relatively few of the active CYP genes code for EET-forming epoxygenases, i.e. protein enzymes with the capacity to attach atomic oxygen (see Allotropes of oxygen#Atomic oxygen) to the carbon-carbon double bonds of unsaturated long chain fatty acids such as arachidonic acid. The CYP epoxygenases fall into several subfamilies including CYP1A, CYP2B, CYP2C, CYP2E, CYP2J, and within the CYP3A sub family, CYP3A4; in humans, CYP2C8, CYP2C9, CYP2C19, CYP2J2, and possibly CYP2S1 isoforms are the main producers of EETs although CYP2C9, CYP2C18, CYP3A4, CYP4A11, CYP4F8, and CYP4F12 are capable of producing the EETs and may do so in certain tissues. The CYP epoxygenases can epoxidize any of the double bounds in arachidonic acid but most of them are relatively selective in that they make appreciable amounts of only one or two EETs with 11,12-EET and 14,15-EET accounting for 67%-80% of the product made by the cited CYP epoxidases as well as the main EETs made by mammalian tissues. CYP2C9, CYP2J9, and possibly the more recently characterized CYP2S1 appear to be the main produces of the EETs in humans with CYP2C9 being the main EET producer in vascular endothelial cells and CYP2J9 being highly expressed (although less catalytically active than CYP2C) in heart muscle, kidneys, pancreas, lung, and brain. CYP2S1 is expressed in macrophages, liver, lung, intestine, and spleen and is abundant in human and mouse atherosclerosis (i.e. Atheroma) plaques as well as inflamed tonsils. ETEs are commonly produced by the stimulation of specific cell types. The stimulation causes arachidonic acid to be released from the sn-2 position of cellular phospholipids through the action of Phospholipase A2-type enzymes and subsequent attack of the released arachidonic acid by a CYP epoxidase. In a typical example of this mechanism, bradykinin or acetylcholine acting through their respective Bradykinin receptor B2 and muscarinic acetylcholine receptor M1 or muscarinic acetylcholine receptor M3 stimulate vascular endothelial cells to make and release EETs. The CYP epoxygenases, similar to essentially all CYP450 enzymes, are involved in the metabolism of diverse xenobiotics and natural compounds. Since many of these same compounds also induce increases in the levels of the epoxygenases, CYP oxygenase levels and consequently EET levels in humans vary widely and are highly dependent on their recent consumption history. In cells, the EETs are rapidly metabolized by a cytosolic soluble epoxide hydrolase (sEH) which adds water (H2O) across the epoxide to form their corresponding Vicinal (chemistry) diol dihydroxyeicosatrienoic acids (diHETrEs or DHETs), i.e. sEH converts 14,15-ETE to 14,15-dihydroxy-eicosatrienoic acid (14,15-diHETrE), 11,12-ETE to 11,12-diHETrE, 8,9-ETE to 8,9-diHETrE, and 5,6-ETE to 5,6-diHETrE. The product diHETrEs, like their epoxy precursors, are enantiomer mixtures; for instance, sEH converts 14,15-ETE to a mixture of 14(S),15(R)-diHETrE and 14(R),15(S)-diHETrE. However, 5,6-EET is a relatively poor substrate for sEH and in cells is more rapidly metabolized by cyclooxygenase-2 to form 5,6-epoxy-prostaglandin F1α. Since the diHETrE products are as a rule generally far less active than their epoxide precursors, the sEH pathway of EET metabolism is regarded as a critical EET-inactivating pathway. In some instances, however, the diHETrEs have been found to possess appreciable activity as indicated in the Biological activities section below. Membrane-bound Microsomal epoxide hydrolase (mEH or Epoxide hydrolase 1 ) can metabolize EETs to their dihydroxy products but is regarded as not contributing significantly to EET inactivation in vivo except perhaps in brain tissue where mEH activity levels far outstrip those of sEH. Furthermore, two other human sEH, epoxide hydrolases 3 and 4 (see epoxide hydrolase), have been defined but their role in attacking EETs (and other epoxides) in vivo has not yet been determined. Besides these four epoxide hydrolase pathways, EETs may be acylated into phospholipids in an Acylation-like reaction. This pathway may serve to limit the action of EETs or store them for future release. EETs are also inactivated by being further metabolized though three other pathways: Beta oxidation, Omega oxidation, and elongation by enzymes involved in Fatty acid synthesis. These alternate to sEH pathways of EET metabolism ensure that blockade of sEH with drugs can increase EET levels only moderately in vivo.