Epoxyeicosatrienoic acids (FA0308)

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The NADPH-dependent epoxidation of arachidonic acid by cytochrome P450 (CYP450) leads to the formation of epoxyeicosatrienoic acids (EETs) (Fig.19).

Figure 19: Metabolism of arachidonic acid to epoxyeicosatrienoic acid and to hydroxyeicosatetraenoic acid (Spiecker et al. 2005)
Figure 19: Metabolism of arachidonic acid to epoxyeicosatrienoic acid and to hydroxyeicosatetraenoic acid (Spiecker et al. 2005)

Four regioisomeric cis-epoxyeicosatienoic acids have been reported: 5,6-, 8,9-, 11,12-, and 14,15-EET (Spiecker et al. 2005). All forms are present at similar concentration in heart, endothelium and human plasma and they act anti-inflammatory, antioxidative, antimigratory and profibrinolytic. Soluble epoxide hydrolase (sEH) converts EETs into less biologically active and more stable metabolites, dihydroxyeicosatrienoic acids (DHETs).

Several mammalian CYP isoforms that generate EETs (CYP1A, CYP2B, CYP2C, CYP2G, and CYP2J, CYP2N, CYP4A) differ in their selectivity for regioisomers, catalytic efficiency and tissue distribution. The most predominant epoxygenase isoforms participating in EET production belong to the CYP2 gene family and whereas CYP2C isoforms regulate EETs biosynthesis in human liver and kidney, CYP2J isoforms have been reported to be involved in epoxidation of endogenous arachidonic acid in human and rat heart (Wu et al. 1996,1997).

EETs have numerous functions in the endothelial cell, including protection from hypoxia-reoxygenation injury, inhibition of cytokine-induced cellular adhesion molecule expression, and activation of tissue plasminogen activator (tPA) expression. In the vascular smooth muscle cell, 20-hydroxyeicosatetraenoic acid (HETE) is the major product of cytochrome P450-(CYP4A and CYP4F) catalyzed arachidonic acid metabolism.

Membrane stretch and vasoactive agents activate phospholipase C (PLC) leading to release of IP3 and diacylglycerol (DAG). Increased intracellular Ca²+, resulting from inositol 1,4,5-triphosphate (IP3) release, signals the activation of Ca²+-sensitive PLA2 and DAG lipase to stimulate the formation of 20-HETE. Inhibition of CYP4A enzymes and therefore 20-HETE formation by nitric oxide (NO) is an important mechanism for maintaining basal tone in the vascular smooth muscle cell. The pathway by which 20-HETE inhibits large-conductance Ca²+-activated K+-channels (BKCa) involves protein kinase C (PKC) and Raf/mitogen-activated protein kinase (MAPK). Inhibition of BKCa elevates the membrane potential, which enhances Ca²+-entry via L-type Ca²+-channels, and vasoconstrictions. Endothelial-derived EET can activate BKCa leading to membrane hyperpolarization and vasodilation. The multiple and often opposing effects of EETs and 20-HETE in vascular endothelial and smooth muscle cells illustrate the intricate regulation of these complex biological pathways by cytochrome P450-derived eicosanoids.

EETs may represent the putative endothelial-derived hyperpolarizing factor (EDHF) that relaxes VSMC by opening large-conductance, Ca²+-activated K+ channels (BKCa) in the coronary vessels (Pratt 2001, Fisslthaler 1999) a process that envolves Gα-dependent activation of BKCa channel alpha subunit and requires ADP-ribosylation of Gαs (Li 1999, Fukao 2001).

EETs possess potent inflammatory effects by inhibiting cytokine-induced endothelial cell adesion molecule expression and preventing leukocyte adhesion to the vascular wall (Node 1999,Campbell 2000, Zeldin 2000). The mechanism involves NFκB and IκB-kinase. EETs also increase tPA expression and enhance fibrinolityic activity via a mechanism that involves activation of Gαs, increases intracellular cAMP and cAMP response element (CRE)-mediated tPA promoter activation (Node 2001). EETs were shown to protect EC against hypoxic injury (Yang 2001) and to effect L-type and P/Q type Ca²+ channels (Qu 2001,Chen 1999) and to suppress urinary Na+-excretion (Brand-Schieber 2000). 14,15-EET inhibits apoptosis induced by serum withdrawal, H2O2, etoposide and excess AA in renal proximal tubular epithelial cells (Chen 2001). The mechanism involves PI-3 kinase and PKB/AKT but not MAPK and mice overexpressing 14,15 epoxygenase (CYP102F87V mutant) are protected against induced apoptosis. In renal epithelial cells 14,15-EET is a potent mitogen that requires Src kinase, MAPK, ERK1/2 and PI-3 kinase (Chen 1998). 14,15-EET increases Src kinase activity and overexpression of C-terminal Src-kinase, which inhibits a family of kinases, and blocked EET-induced Tyr-phosphorylation and mitogenesis (Chen 2000).

EETs may act via cell surface receptors (K1=226 nM) that are attenuated by cAMP and PKA activation (Wong 2000). Once formed, EETs can be further metabolized along a number of pathways (Zeldin 2001, Fang 2000, Fang 2001, Widstrom 2001) including hydratation by soluble epoxide hydrolase (SEH), conjugation by glutathione S-transferase, oxidation by cytochrome P450s and cyclooxygenases, esterification to glycerophospholipids, conversion to chain shortened epoxy-fatty acids via peroxisomal β-oxydation, affinities of heart-, liver- and intestinal FABPs were 20-fold greater for EETs that for DHET (dihydroxyeicosatrienoic acid).

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