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Arachidonic acid cytochrome P450 epoxygenase pathway

2008; Elsevier BV; Volume: 50; Linguagem: Inglês

10.1194/jlr.r800038-jlr200

ISSN

1539-7262

Autores

Arthur A. Spector,

Tópico(s)

Hormonal Regulation and Hypertension

Resumo

Cytochrome P450 (CYP) epoxygenases convert arachidonic acid to four epoxyeicosatrienoic acid (EET) regioisomers, 5,6-, 8,9-, 11,12-, and 14,15-EET, that function as autacrine and paracrine mediators. EETs produce vascular relaxation by activating smooth muscle large-conductance Ca2+-activated K+ channels (BKCa). In addition, they have anti-inflammatory effects on blood vessels and in the kidney, promote angiogenesis, and protect ischemic myocardium and brain. CYP epoxygenases also convert eicosapentaenoic acid to vasoactive epoxy-derivatives, and endocannabinoids containing 11,12- and 14,15-EET are formed. Many EET actions appear to be initiated by EET binding to a membrane receptor that activates ion channels and intracellular signal transduction pathways. However, EETs also are taken up by cells, are incorporated into phospholipids, and bind to cytosolic proteins and nuclear receptors, suggesting that some functions may occur through direct interaction of the EET with intracellular effector systems. Soluble epoxide hydrolase (sEH) converts EETs to dihydroxyeicosatrienoic acids (DHETs). Because this attenuates many of the functional effects of EETs, sEH inhibition is being evaluated as a mechanism for increasing and prolonging the beneficial actions of EETs. Cytochrome P450 (CYP) epoxygenases convert arachidonic acid to four epoxyeicosatrienoic acid (EET) regioisomers, 5,6-, 8,9-, 11,12-, and 14,15-EET, that function as autacrine and paracrine mediators. EETs produce vascular relaxation by activating smooth muscle large-conductance Ca2+-activated K+ channels (BKCa). In addition, they have anti-inflammatory effects on blood vessels and in the kidney, promote angiogenesis, and protect ischemic myocardium and brain. CYP epoxygenases also convert eicosapentaenoic acid to vasoactive epoxy-derivatives, and endocannabinoids containing 11,12- and 14,15-EET are formed. Many EET actions appear to be initiated by EET binding to a membrane receptor that activates ion channels and intracellular signal transduction pathways. However, EETs also are taken up by cells, are incorporated into phospholipids, and bind to cytosolic proteins and nuclear receptors, suggesting that some functions may occur through direct interaction of the EET with intracellular effector systems. Soluble epoxide hydrolase (sEH) converts EETs to dihydroxyeicosatrienoic acids (DHETs). Because this attenuates many of the functional effects of EETs, sEH inhibition is being evaluated as a mechanism for increasing and prolonging the beneficial actions of EETs. Epoxyeicosatrienoic acids (EET) are epoxide derivatives of arachidonic acid. They are formed by cytochrome P450 (CYP) epoxygenases and function as lipid mediators. Epoxidation can occur at any of the four double bonds of arachidonic acid, giving rise to four regioisomers, 5,6-, 8,9-, 11,12-, and 14,15-EET. EETs are synthesized in the endothelium and activate large-conductance Ca2+-activated K+ channels (BKCa), causing hyperpolarization of the vascular smooth muscle and vasorelaxation. Thus, EETs function as an endothelium-derived hyperpolarizing factor (EDHF) in a number of vascular beds, including the coronary and renal circulations, producing a decrease in blood pressure. Soluble epoxide hydrolase (sEH), which converts EETs to dihydroxyeicosatrienoic acids (DHETs), attenuates many of the functional effects of EETs. These seminal findings have been described in a number of detailed reviews (1Campbell W.B. Harder D.R. Endothelium derived hyperpolarizing factors and vascular cytochrome P450 metabolites of arachidonic acid in the regulation of tone.Circ. Res. 1999; 84: 484-488Crossref PubMed Scopus (189) Google Scholar, 2Capdevila J.H. Falck J.R. Harris R.C. Cytochrome P450 and arachidonic acid bioactivation: molecular and functional properties of arachidonic acid monooxygenases.J. Lipid Res. 2000; 41: 163-181Abstract Full Text Full Text PDF PubMed Google Scholar, 3Roman R. P-450 metabolites of arachidonic acid in the control of cardiovascular function.Physiol. Rev. 2002; 82: 131-185Crossref PubMed Scopus (1156) Google Scholar, 4Spector A.A. Norris A.W. Action of epoxyeicosatrienoic acids on cellular function.Am. J. Physiol. Cell Physiol. 2007; 292: C996-C1012Crossref PubMed Scopus (376) Google Scholar). Recent results with cultured cells and animal models indicate that EETs have additional potentially beneficial effects on the vascular system, heart, kidneys, and nervous system, and many current studies are directed at these actions (5Campbell W.B. Falck J.R. Arachidonic acid metabolites as endothelium-derived hyperpolarizing factors.Hypertension. 2007; 49: 590-596Crossref PubMed Scopus (175) Google Scholar, 6Michaelis U.R. Fleming I. From endothelium-derived hyperpolarizing factor (EDHF) to angiogenesis: epoxyeicosatrienoic acids (EETs) and cell signaling.Pharmacol. Ther. 2006; 111: 584-595Crossref PubMed Scopus (101) Google Scholar, 7Larsen B.T. Campbell W.B. Gutterman D.D. Beyond vasodilation: non-vascular roles for epoxyeicosatrienoic acids in the cardiovascular system.Trends Pharmacol. Sci. 2007; 28: 32-38Abstract Full Text Full Text PDF PubMed Scopus (68) Google Scholar, 8Seubert J.M. Zeldin D.C. Nithipatikom K. Gross G.J. Role of epoxyeicosatrienoic acids in protecting myocardium following ischemic/reperfusion injury.Prostagl. other Lipid Mediat. 2007; 82: 50-59Crossref PubMed Scopus (121) Google Scholar, 9Fleming I. Vascular cytochrome p450 enzymes: physiology and pathophysiology.Trends Cardiovasc. Med. 2008; 18: 20-25Crossref PubMed Scopus (90) Google Scholar). The other current emphasis is on sEH inhibition as a therapeutic strategy for increasing the beneficial effects of EETs (10Imig J.D. Epoxide hydrolase and epoxygenase metabolites as therapeutic targets for renal diseases.Am. J. Physiol. Renal Physiol. 2005; 289: F496-F503Crossref PubMed Scopus (190) Google Scholar, 11Chiamvimonvat N. Ho C.M. Tsai H.J. Hammock B.D. The soluble epoxide hydrolase as a pharmaceutical target for hypertension.J. Cardiovasc. Pharmacol. 2007; 50: 225-237Crossref PubMed Scopus (141) Google Scholar).EET SYNTHESIS, METABOLISM, AND FUNCTIONEETs are synthesized by cells that express CYP epoxygenase activity. As illustrated in Fig. 1, these enzymes act on arachidonic acid released from phospholipids when cytosolic phospholipase A2 (cPLA2) is activated (12Spector A.A. Fang X. Snyder G.D. Weintraub N.L. Epoxyeicosatrienoic acids (EETs): metabolism and biochemical function.Prog. Lipid Res. 2004; 43: 55-90Crossref PubMed Scopus (481) Google Scholar). The epoxygenase inserts an oxygen atom on a carbon attached to one of the double bonds of arachidonic acid, and the double bond is reduced as the epoxide forms. Each CYP epoxygenase produces several regioisomers, with one form usually predominating. Thus, epoxygenases that mainly produce 14,15-EET also produce a moderate amount of 11,12-EET and a small amount of 8,9-EET. Each regioisomer contains two R/S enantiomeric forms in different proportions (2Capdevila J.H. Falck J.R. Harris R.C. Cytochrome P450 and arachidonic acid bioactivation: molecular and functional properties of arachidonic acid monooxygenases.J. Lipid Res. 2000; 41: 163-181Abstract Full Text Full Text PDF PubMed Google Scholar, 4Spector A.A. Norris A.W. Action of epoxyeicosatrienoic acids on cellular function.Am. J. Physiol. Cell Physiol. 2007; 292: C996-C1012Crossref PubMed Scopus (376) Google Scholar). Because the regioisomers have a number of similar metabolic and functional properties, EETs are generally considered as a single class of compounds. This is an oversimplification, and there are quantitative and even qualitative differences in the actions of the various regioisomers (4Spector A.A. Norris A.W. Action of epoxyeicosatrienoic acids on cellular function.Am. J. Physiol. Cell Physiol. 2007; 292: C996-C1012Crossref PubMed Scopus (376) Google Scholar). For example, 14,15-EET is the best substrate for sEH (4Spector A.A. Norris A.W. Action of epoxyeicosatrienoic acids on cellular function.Am. J. Physiol. Cell Physiol. 2007; 292: C996-C1012Crossref PubMed Scopus (376) Google Scholar), and 11,12-EET is the only regioisomer that inhibits basolateral K+ channels in the renal cortical collecting duct (13Wang Z. Wei Y. Falck J.R. Atcha K.R. Wang W.H. Arachidonic acid inhibits K channels in the cortical collecting duct via cytochrome P-450 epoxygenase-dependent metabolic pathways.Am. J. Physiol. Renal Physiol. 2008; 294: F1441-F1447Crossref PubMed Scopus (12) Google Scholar).EETs are taken up by many different kinds of cells (4Spector A.A. Norris A.W. Action of epoxyeicosatrienoic acids on cellular function.Am. J. Physiol. Cell Physiol. 2007; 292: C996-C1012Crossref PubMed Scopus (376) Google Scholar, 12Spector A.A. Fang X. Snyder G.D. Weintraub N.L. Epoxyeicosatrienoic acids (EETs): metabolism and biochemical function.Prog. Lipid Res. 2004; 43: 55-90Crossref PubMed Scopus (481) Google Scholar), and purified heart and liver cytoplasmic fatty acid binding proteins (FABP) bind EETs (14Widstrom R.L. Norris A.W. Veer J.Van Der Spector A.A. Fatty acid binding proteins inhibit hydration of epoxyeicosatrienoic acids by soluble epoxide hydrolase.Biochemistry. 2003; 42: 11762-11767Crossref PubMed Scopus (34) Google Scholar). This suggests that FABP may increase EET desorption from the cell membrane and thereby facilitate its uptake into the cell, as well as modulate EET incorporation into phospholipids and its availability to sEH for DHET formation (4Spector A.A. Norris A.W. Action of epoxyeicosatrienoic acids on cellular function.Am. J. Physiol. Cell Physiol. 2007; 292: C996-C1012Crossref PubMed Scopus (376) Google Scholar, 12Spector A.A. Fang X. Snyder G.D. Weintraub N.L. Epoxyeicosatrienoic acids (EETs): metabolism and biochemical function.Prog. Lipid Res. 2004; 43: 55-90Crossref PubMed Scopus (481) Google Scholar). Because conversion to DHET attenuates many of the physiological actions of EETs, binding to FABP may increase the intracellular retention of EETs and thereby prolong their functional effectiveness. Although not shown in Fig. 1, the cells that synthesize EETs also have the capacity to incorporate EETs into phospholipids and convert them to DHETs.EETs undergo β-oxidation, forming 16-carbon epoxy-derivatives that accumulate in the extracellular fluid, and they can be chain-elongated to form 22-carbon derivatives that are incorporated into phospholipids (15Fang X. Weintraub N.L. Oltman C.L. Stoll L.L. Kaduce T.L. Harmon S. Dellsperger K.C. Morisseau C. Hammock B.D. Spector A.A. Human coronary endothelial cells convert 14,5-EET to a biologically active chain-shortened epoxide.Am. J. Physiol. Heart Circ. Physiol. 2002; 283: H2306-H2314Crossref PubMed Scopus (44) Google Scholar). As illustrated in Fig. 2, the availability of EETs to these metabolic pathways increases when sEH is inhibited (16Fang X. Kaduce T.L. Weintraub N.L. Harmon S. Teesch L.M. Dellsperger K.C. Morisseau C. Thompson D.A. Hammock B.D. Spector A.A. Pathways of epoxyeicosatrienoic acid metabolism in endothelial cells. Implications for the vascular effects of soluble epoxide hydrolase inhibition.J. Biol. Chem. 2001; 276: 14867-14874Abstract Full Text Full Text PDF PubMed Scopus (175) Google Scholar). The 22-carbon elongation product of 14,15-EET, which is an epoxide derivative of adrenic acid, relaxes bovine coronary artery preparations by hyperpolarizing smooth muscle in a manner similar to EETs (17Yi X.Y. Gauthier K.M. Cui L. Nithipatikom K. Falck J.R. Campbell W.B. Metabolism of adrenic acid to vasodilatory 1α,1β-dihomo-epoxyeicosatrienoic acids by bovine coronary arteries.Am. J. Physiol. Heart Circ. Physiol. 2007; 292: H2265-H2274Crossref PubMed Scopus (23) Google Scholar).Fig. 2sEH inhibition enhances EET function. Conversion of EET to DHET by sEH is the main pathway of EET metabolism. This attenuates most functional effects of EETs, making sEH a logical target for increasing and prolonging the actions of EETs. sEH inhibition decreases DHET formation and leads to intracellular EET accumulation. This results in more EET incorporation into phospholipids and utilization by other metabolic pathways, including β-oxidation and chain-elongation. Functional responses are increased because of the larger amounts of intracellular unesterified EET and EET-containing phospholipids. Furthermore, more EET is released when intracellular phospholipids are hydrolyzed, maintaining the increased intracellular concentration of unesterified EET.View Large Image Figure ViewerDownload Hi-res image Download (PPT)EETs produce autacrine and paracrine effects, but these actions often are overlooked in experimental studies because CYP epoxygenases are labile in cultured cells and are inhibited under conditions where H2O2 is formed (18Larsen B.T. Gutterman D.D. Sato A. Toyama A. Campbell W.B. Zeldin D.C. Manthati V.L. Falck J.R. Miura H. Hydrogen peroxide inhibits cytochrome p450 epoxygenases: interaction between two endothelium-derived hyperpolarizing factors.Circ. Res. 2008; 102: 59-67Crossref PubMed Scopus (84) Google Scholar). The paracrine effects are produced by the EET released into the extracellular fluid (Fig. 1). While the autocrine effects probably also are produced by the EET initially released into the extracellular fluid, the possibility that they result from intracellular actions of EET that is retained in the cell following synthesis cannot be excluded.CELLULAR MECHANISM OF ACTIONThree mechanisms have been proposed to explain the cellular actions of EETs (4Spector A.A. Norris A.W. Action of epoxyeicosatrienoic acids on cellular function.Am. J. Physiol. Cell Physiol. 2007; 292: C996-C1012Crossref PubMed Scopus (376) Google Scholar). Two involve EET binding to cell-surface receptors, and the other is an intracellular mechanism. A substantial amount of chemical and functional data supports the likelihood that EETs bind to a selective EET receptor that is coupled by a G-protein to intracellular signal transduction pathways (4Spector A.A. Norris A.W. Action of epoxyeicosatrienoic acids on cellular function.Am. J. Physiol. Cell Physiol. 2007; 292: C996-C1012Crossref PubMed Scopus (376) Google Scholar, 19Yang W. Tuniki V.R. Anjaiah S. Falck J.R. Hillard C.J. Campbell W.B. Characterization of epoxyeicosatrienoic acid binding site in U937 membranes using a novel radiolabeled agonist, 20-125I-14,15-epoxyeicosa-8(Z)-enoic acid.J. Pharmacol. Exp. Ther. 2008; 324: 1019-1027Crossref PubMed Scopus (72) Google Scholar). However, this possibility remains open to question because the putative EET receptor has not yet been identified or cloned. A second mechanism involves EET binding to receptors for other lipid-soluble agonists that also function by coupling to intracellular signaling pathways. The third possibility, an intracellular mechanism, is based on the fact that EETs have many characteristics of long-chain fatty acids. This mechanism involves uptake of the EET by the cell, with the cell-associated EET directly interacting with ion channels, signaling proteins, or transcription factors. Support for an intracellular mechanism of action stems from the fact that EETs are incorporated into cell phospholipids and bind to cytoplasmic FABPs and peroxisome proliferator-activated receptor (PPAR)γ (4Spector A.A. Norris A.W. Action of epoxyeicosatrienoic acids on cellular function.Am. J. Physiol. Cell Physiol. 2007; 292: C996-C1012Crossref PubMed Scopus (376) Google Scholar, 12Spector A.A. Fang X. Snyder G.D. Weintraub N.L. Epoxyeicosatrienoic acids (EETs): metabolism and biochemical function.Prog. Lipid Res. 2004; 43: 55-90Crossref PubMed Scopus (481) Google Scholar, 14Widstrom R.L. Norris A.W. Veer J.Van Der Spector A.A. Fatty acid binding proteins inhibit hydration of epoxyeicosatrienoic acids by soluble epoxide hydrolase.Biochemistry. 2003; 42: 11762-11767Crossref PubMed Scopus (34) Google Scholar, 20Liu Y. Zhang Y. Schmelzer K. Lee T.S. Fang X. Zhu Y. Spector A.A. Gill S. Morisseau C. Hammock B.D. al et The anti-inflammatory effect of laminar flow: the role of PPARγ, epoxyeicosatrienoic acids, and soluble epoxide hydrolase.Proc. Natl. Acad. Sci USA. 2005; 102: 16747-16752Crossref PubMed Scopus (240) Google Scholar).INCORPORATION INTO PHOSPHOLIPIDSThe functional significance of EET incorporation into cell phospholipids is uncertain. EETs are incorporated primarily into the sn-2 position of the phospholipids, and cell culture studies indicate that a substantial amount of the incorporation is subsequently released even when no stimulus is applied to the cultures (4Spector A.A. Norris A.W. Action of epoxyeicosatrienoic acids on cellular function.Am. J. Physiol. Cell Physiol. 2007; 292: C996-C1012Crossref PubMed Scopus (376) Google Scholar, 12Spector A.A. Fang X. Snyder G.D. Weintraub N.L. Epoxyeicosatrienoic acids (EETs): metabolism and biochemical function.Prog. Lipid Res. 2004; 43: 55-90Crossref PubMed Scopus (481) Google Scholar). This suggests that EET incorporation may temporarily alter the properties of membrane microdomains and thereby transiently affect the function of proteins located in these domains. Alternatively, the incorporation of EETs may modulate phospholipid-dependent signal transduction systems. EETs also are hydrolyzed rapidly from the phospholipids by a Ca2+-stimulated mechanism (12Spector A.A. Fang X. Snyder G.D. Weintraub N.L. Epoxyeicosatrienoic acids (EETs): metabolism and biochemical function.Prog. Lipid Res. 2004; 43: 55-90Crossref PubMed Scopus (481) Google Scholar, 16Fang X. Kaduce T.L. Weintraub N.L. Harmon S. Teesch L.M. Dellsperger K.C. Morisseau C. Thompson D.A. Hammock B.D. Spector A.A. Pathways of epoxyeicosatrienoic acid metabolism in endothelial cells. Implications for the vascular effects of soluble epoxide hydrolase inhibition.J. Biol. Chem. 2001; 276: 14867-14874Abstract Full Text Full Text PDF PubMed Scopus (175) Google Scholar). Therefore, EETs contained in phospholipids may constitute an intracellular storage pool that is available for immediate release when the cell is activated (12Spector A.A. Fang X. Snyder G.D. Weintraub N.L. Epoxyeicosatrienoic acids (EETs): metabolism and biochemical function.Prog. Lipid Res. 2004; 43: 55-90Crossref PubMed Scopus (481) Google Scholar).Another possibility is that phospholipids containing EET are substrates for the production of other lipid mediators. Kidney and spleen produce 2-epoxyeicosatrienoylglycerols that contain 11,12-EET or 14,15-EET (21Chen J.K. Chen J. Imig J.D. Wei S. Hackey D.L. Guthi J.S. Falck J.R. Capdevila J.H. Harris R.C. Identification of novel endogenous cytochrome p450 arachidonate metabolites with high affinity for cannabinoid receptors.J. Biol. Chem. 2008; 283: 24511-24524Abstract Full Text Full Text PDF Scopus (60) Google Scholar). 2-Epoxyeicosatrienoylglycerol are endocannabinoids that activate CB1 and CB2 receptors, and 2-(14,15)-EG produces proliferation of renal proximal tubule cells by causing the release of ligands that activate the epidermal growth factor receptor (22Chen J. Chen J.K. Falck J.R. Guthi J.S. Anjaiah S. Capdevila J.H. Harris R.C. Mitogenic activity and signaling mechanism of 2-(14,15-epoxyeicosatrienoyl)glycerol, a novel cytochrome p450 arachidonate metabolite.Mol. Cell. Biol. 2007; 27: 3023-3034Crossref PubMed Scopus (27) Google Scholar). In addition, phospholipids containing EET are the likely substrates for the EET-ethanolamide synthesized in the liver and kidney (23Snider N.T. Kornilov A.M. Kent U.M. Hollenberg P.F. Anandamide metabolism by human liver and kidney microsomal cytochrome p450 enzymes to form hydroxyeicosatetraenoic and epoxyeicosatrienoic acid ethanolamides.J. Pharmacol. Exp. Ther. 2007; 321: 590-597Crossref PubMed Scopus (82) Google Scholar).Finally, EET incorporation into phospholipids may serve to lower the intracellular unesterified EET concentration and thereby be part of the mechanism that terminates its functional effects. The subsequent gradual hydrolysis of the EET from the phospholipids might then make the EET available so it can be efficiently inactivated by sEH or β-oxidation.EET-ACTIVATED SIGNALING PATHWAYSThe functional effects of EETs have been observed to occur through a number of different signal transduction pathways (4Spector A.A. Norris A.W. Action of epoxyeicosatrienoic acids on cellular function.Am. J. Physiol. Cell Physiol. 2007; 292: C996-C1012Crossref PubMed Scopus (376) Google Scholar). The most effective regioisomer that produces the EDHF effect in the coronary circulation, 11,12-EET, functions through a cAMP-dependent process that activates vascular smooth muscle BKCa channels. This paracrine mechanism involves the Gαs protein, adenylyl cyclase activation, and an increase in cAMP (5Campbell W.B. Falck J.R. Arachidonic acid metabolites as endothelium-derived hyperpolarizing factors.Hypertension. 2007; 49: 590-596Crossref PubMed Scopus (175) Google Scholar). A similar pathway involving ADP-ribosylation of Gαs, an increase in cAMP, and protein kinase A activation produces EET-stimulated vasodilation of preglomerular renal microvessels (24Carroll M.A. Doumod A.B. Li J. Cheng M.K. Falck J.R. McGiff J.C. Adenosine 2A receptor vasodilation of rat preglomerular microvessels is mediated by EETs that activate the cAMP/PKA pathway.Am. J. Physiol. Renal Physiol. 2006; 291: F155-F161Crossref PubMed Scopus (61) Google Scholar). Likewise, a cAMP-protein kinase A mechanism mediates the EET-stimulated increase in StAR protein and steroid hormone production (25Wang X. Shen C.L. Dyson M.T. Yin X. Schiffer A.B. Grammas P. Stocco J.M. The involvement of epoxygenase metabolites of arachidonic acid in cAMP-stimulated steroidogenesis and steroidogenic acute regulatory protein gene expression.J. Endocrinol. 2006; 190: 871-878Crossref PubMed Scopus (31) Google Scholar). While the EET-stimulated relaxation of renal afferent arterioles also is mediated by cAMP-dependent activation of BKCa channels, the response in this preparation involves an increase in phosphoprotein phosphatase 2A (26Imig J.D. Dimitropoulou C. Reddy D.S. White R.E. Falck J.R. Afferent arteriole dilation to 11,12-EET analogs involves PP2A activity and Ca2+-activated K+ channels.Microcirculation. 2008; 15: 137-150Crossref PubMed Scopus (58) Google Scholar).EET activation of endothelial Trp channels is an alternative mechanism proposed for the coronary EDHF response (27Fleming I. Reuben A. Poop R. Fisslthaler B. Schrodt S. Sander A. Haendeler J. Falck J.R. Morisseau C. Hammock B.D. al et Epoxyeicosatrienoic acids regulate Trp channel dependent Ca2+ signaling and hyperpolarization in endothelial cells.Arterioscler. Thromb. Vasc. Biol. 2007; 27: 2612-2618Crossref PubMed Scopus (147) Google Scholar). Trp channel activation produces Ca2+ influx and endothelial K+ channel activation, and the hyperpolarized endothelium triggers relaxation of the vascular smooth muscle.EETs have anti-inflammatory and antiapoptotic actions in the endothelium (7Larsen B.T. Campbell W.B. Gutterman D.D. Beyond vasodilation: non-vascular roles for epoxyeicosatrienoic acids in the cardiovascular system.Trends Pharmacol. Sci. 2007; 28: 32-38Abstract Full Text Full Text PDF PubMed Scopus (68) Google Scholar). The anti-inflammatory effect produced by 11,12-EET occurs through a signaling pathway that inhibits NF-κB activation (28Spiecker M. Liao J.K. Vascular protective effects of cytochrome P450 epoxygenase-derived eicosanoids.Arch. Biochem. Biophys. 2005; 433: 413-420Crossref PubMed Scopus (151) Google Scholar). 8,9-EET, 11,12-EET, and 14,15-EET inhibit endothelial apoptosis, but this occurs through activation of a PI3K/Akt pathway that inhibits Erk1/2 dephosphorylation (29Yang S. Lin L. Chen J.X. Lee C.R. Seubert J.M. Wang Y. Wang H. Chao Z.R. Tao D.D. Gong J.P. al et Cytochrome P450 epoxygenases protect endothelial cells from apoptosis induced by tumor necrosis factor alpha via MAPK and PI3K/Akt signaling pathway.Am. J. Physiol. Heart Circ. Physiol. 2007; 293: H142-H151Crossref PubMed Scopus (107) Google Scholar). Anti-inflammatory effects of EETs also have been observed in the kidney (10Imig J.D. Epoxide hydrolase and epoxygenase metabolites as therapeutic targets for renal diseases.Am. J. Physiol. Renal Physiol. 2005; 289: F496-F503Crossref PubMed Scopus (190) Google Scholar).Several different signaling pathways have been implicated in the angiogenic effect of EETs (4Spector A.A. Norris A.W. Action of epoxyeicosatrienoic acids on cellular function.Am. J. Physiol. Cell Physiol. 2007; 292: C996-C1012Crossref PubMed Scopus (376) Google Scholar, 6Michaelis U.R. Fleming I. From endothelium-derived hyperpolarizing factor (EDHF) to angiogenesis: epoxyeicosatrienoic acids (EETs) and cell signaling.Pharmacol. Ther. 2006; 111: 584-595Crossref PubMed Scopus (101) Google Scholar). 11,12-EET has been observed to stimulate angiogenesis by activating an EphB4-coupled PI3K/Akt pathway (30Webber A.C. Popp R. Korff U.Thomas Michaelis R. Urbich C. Busse R. Fleming I. Cytochrome P450 2C9-induced angiogenesis is dependent on EphB4.Arterioscler. Thronb. Vasc. Biol. 2008; 28: 1123-1129Crossref PubMed Scopus (30) Google Scholar), whereas others find that it functions by activating sphingosine kinase-1 (31Yan G. Chen S. You B. Sun J. Activation of sphingosine kinase-1 mediates induction of endothelial cell proliferation and angiogenesis by epoxyeicosatrienoic acids.Cardiovasc. Res. 2008; 78: 308-314Crossref PubMed Scopus (57) Google Scholar). Likewise, different pathways are reported to produce 14,15-EET-mediated angiogenesis. A Src activated PI3K/Akt pathway coupled to FGF-2 expression and mTOR-S6K1 activation has been observed in one study (32Zhang B. Cao H. Rao G.N. Fibroblast growth factor-2 is a downstream mediator of phosphatidylinositol 3-kinase-Akt signaling in 14,15-epoxyeicosatrienoic acid-induced angiogenesis.J. Biol. Chem. 2006; 281: 905-914Abstract Full Text Full Text PDF PubMed Scopus (68) Google Scholar), whereas Src-stimulated tyrosine phosphorylation of STAT-3, which binds to the VEGF promoter, has been observed in another study with 14,15-EET (33Cheranov S.Y. Karpurapu M. Wang D. Zhang B. Venema R.C. Rao G.N. An essential role for SRC-activated STAT-3 in 14,15-EET-induced VGEF expression and angiogenesis.Blood. 2008; 111: 5581-5591Crossref PubMed Scopus (99) Google Scholar).11,12- and 14,15-EET have cardioprotective effects during reoxygenation of ischemic myocardium, and they decrease infarct size (8Seubert J.M. Zeldin D.C. Nithipatikom K. Gross G.J. Role of epoxyeicosatrienoic acids in protecting myocardium following ischemic/reperfusion injury.Prostagl. other Lipid Mediat. 2007; 82: 50-59Crossref PubMed Scopus (121) Google Scholar, 34Gross G.J. Gauthier K.M. Moore J. Falck J.R. Hammock B.D. Campbell W.B. Nithipatikom K. Effect of the selective EET antagonist, 14,15-EEZE, on cardioprotection produced by exogenous or endogenous EETs in canine hearts.Am. J. Physiol. Heart Circ. Physiol. 2008; 294: H2838-H2844Crossref PubMed Scopus (79) Google Scholar). Several mechanisms have been reported to mediate the cardioprotective effect, including activation of myocardial KATP channels by decreasing their sensitivity to ATP (35Wang X.L. Lu T. Cao S. Shah V.H. Lee H.C. Inhibition of ATP binding to the carboxyl terminus of Kir 6.2 by epoxyeicosatrienoic acids.Biochim. Biophys. Acta. 2006; 1761: 1041-1049Crossref PubMed Scopus (7) Google Scholar), activation of KATP channels by triggering a burst of reactive oxygen species (36Gross G.J. Hsu A. Falck J.R. Nithipatikom K. Mechanism by which epoxyeicosatrienoic acids (EETs) elicit cardioprotection in rat hearts.J. Mol. Cell. Cardiol. 2007; 42: 687-691Abstract Full Text Full Text PDF PubMed Scopus (87) Google Scholar), and activation of a PI3K/Akt pro-survival pathway (37Dhanasekaran A. Gruenloh S.K. Buonaccorsi J.N. Zhang R. Gross G.R. Falck J.R. Patel P.K. Jacobs E.R. Medhora M. Multiple antiapoptotic targets of PI3K/Akt survival pathway are activated by epoxyeicosatrienoic acids to protect cardiomyocytes from hypoxia/anoxia.Am. J. Physiol. Heart Circ. Physiol. 2008; 294: H724-H735Crossref PubMed Scopus (147) Google Scholar).EETs are synthesized in the brain by astrocytes through a mechanism linked to mGluR and adenosine A(2B) receptors and are involved in neurovascular coupling (38Shi Y. Liu X. Gebremedhin D. Falck J.R. Harder D.R. Koehler R.C. Interaction of mechanisms involving epoxyeicosatrienoic acids, adenosine receptors, and metabotropic glutamate receptors in neurovascular coupling in rat whisker barrel cortex.J. Cereb. Blood Flow Metab. 2008; 28: 111-125Crossref PubMed Scopus (65) Google Scholar). They also produce antinociception by activating β-endorphin and met-enkephalin that interact with μ- and δ-opioid receptors (39Terashvili M. Tseng L.F. Wu H.E. Narayanan J. Hart L.M. Falck J.R. Pratt P.F. Harder D.R. Antinociception produced by 14,15-epoxyeicosatrienoic acid is mediated by the activation of beta-endorphin and met-enkephalin in the rat ventrolateral periaqueductal gray matter.J. Pharmacol. Exp. Ther. 2008; 326: 614-622Crossref PubMed Scopus (73) Google Scholar). In addition, EETs reduce brain ischemia and infarct size in stroke (40Zhang W. Koerner I.P. Noppens R. Grafe M. Tsai H.J. Morisseau C. Luria A. Hammock B.D. Falck J.R. Alkayed N.J. Soluble epoxide hydrolase: a novel therapeutic target in stroke.J. Cereb. 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