Pathways of epoxyeicosatrienoic acid metabolism in endothelial cells. Implications for the vascular effects of soluble epoxide hydrolase inhibition

Epoxyeicosatrienoic acids and/or their metabolites promote hypoxic response of cells

Epoxyeicosatrienoic acids (EETs), including 5,6-EET, 8,9-EET, 11,12-EET, and 14,15-EET, are produced by cytochrome P450 (P450) such as CYP2C8 and 2C9; and they are hydrolyzed to dihydroxyeicosatrienoic acids (DHETs) by epoxide hydrolase. Particular interest in the epoxygenase reaction has developed because of the potent biological activities (modulation of vascular tone and anti-inflammatory activity, etc.) attributed to EETs. We focused on a new biological function of EETs and DHETs, which induce vascular endothelial growth factor (VEGF) and erythropoietin (EPO) under hypoxia. Human hepatoma cells, Hep3B, and human umbilical artery endothelial cells (HUAEC) were used in this study. An inhibitor of phospholipase A(2), methyl arachidonyl fluorophosphonate (MAFP), and inhibitors of P450s inhibited the VEGF and EPO induction of HUAEC and Hep3B, respectively, under hypoxia. Overexpression of CYP2C8 in Hep3B induced EPO and VEGF under hypoxia. Sulfaphenazole, an inhibitor of CYP2C8/2C9 suppressed luciferase promoter activity with the hypoxia response element (HRE) of VEGF in HUAEC. Exogenous 11,12-EET and 14,15-DHET induced reporter activity in HUAEC and Hep3B cells concomitant with increased levels of hypoxia-inducible factor-1alpha (HIF-1alpha), which is a key factor in the hypoxia response, but 11,12-DHET and 14,15-EET did not. These results suggested that EETs and DHETs play an important role in the hypoxia response of cells.

Role of CYP2C9 and CYP2C19 polymorphisms in patients with atherosclerosis

The arachidonic acid metabolizing CYP enzymes with prominent roles in vascular regulation are epoxygenases of the two gene family which generate epoxyeicosatrienoic acids. Carriers of CYP2C9 mutant alleles exhibit a diminished CYP2C9 metabolic capacity leading to decreased endothelium-derived hyperpolarizing factors (EDHF) synthesis and an increased risk for atherosclerosis. We investigated whether the polymorphisms of CYP2C9/19 are related with atherosclerosis. We examined 108 patients having angioraphically > or =70 coronary artery narrowing and 90 healthy controls. CYPC2C9/19*2 and CYP2C9/19*3 alleles were investigated in both patients and controls by a real time PCR instrument. There was no significant difference in the distribution of the CYP2C9*2/*3 alleles between cases and the controls. We found that smoker patients having CYP2C9*2 heterozygote genotype have 3.7-fold risk of developing atherosclerosis. CYP2C19*3 heterozygote alleles are more frequent in patients than in controls (10.2%, 5.6% respectively) and it is related with a three-fold risk of atherosclerosis (odds ratio (OR) = 3.75, confidence interval (CI) = 0.75-18.65). It becomes clear that cigarette smoking can cause almost all major diseases prevalent today, such as cancer or heart disease. This inter-subject variability in cigarette-induced pathologies is partly mediated by genetic variants of genes that may participate in detoxification processes, e.g., cytochrome P450 (CYP), cellular susceptibility to toxins, such as p53, or disease development such as atherosclerosis.

Structural characterization of monohydroxyeicosatetraenoic acids and dihydroxy- and trihydroxyeicosatrienoic acids by ESI-FTICR

The fragmentation characteristics of monohydroxyeicosatetraenoic acids and dihydroxy- and trihydroxyeicosatrienoic acids were investigated by electrospray ionization Fourier transform ion cyclotron resonance (FTICR) mass spectrometry using sustained off-resonance irradiation collision-induced dissociation (SORI-CID) and infrared multiphoton dissociation (IRMPD). The fragmentation patterns of these compounds were associated with the number and positions of the hydroxyl substituents. The fragmentation is more complicated with increasing number of the hydroxyl groups of the compounds. In general, the major carbon-carbon cleavage of [M - H](-) ions occurred at the alpha-position to the hydroxyl group, and the carbon-carbon cleavage occurred when there was a double-bond at the beta-position to the hydroxyl group. SORI-CID and IRMPD produced some common fragmentation patterns; however, each technique provided some unique patterns that are useful for structural identification of these compounds. This study demonstrated the application of FTICR via the identification of regioisomers of trihydroxyeicosatrienoic acids in rabbit aorta samples.

Roles of epoxyeicosatrienoic acids in vascular regulation and cardiac preconditioning

Continuing investigations of the roles of cytochrome P450 (CYP) arachidonic acid epoxygenase metabolites in the regulation of cardiovascular physiology and pathophysiology have revealed their complex and diverse biological effects. Often these metabolites demonstrate protective properties that are revealed during cardiovascular disease. In this regard, the epoxyeicosatrienoic acids (EETs) are an emerging target for pharmacological manipulation aimed at enhancing their cardiac and vascular protective mechanisms. This review will focus on the role of EETs in the regulation of vascular tone, with emphasis on the coronary circulation, their role in limiting platelet aggregation, vascular inflammation and EET contribution to preconditioning of the ischemic myocardium. Production and metabolism of EETs as well as their specific cellular signaling mechanisms are discussed.

Altered Soluble Epoxide Hydrolase Gene Expression and Function and Vascular Disease Risk in the Stroke-Prone Spontaneously Hypertensive Rat

Soluble epoxide hydrolase (sEH) metabolizes epoxyeicosatrienoic acids and represents a novel therapeutic target in cardiovascular disease treatment. We investigated the relationship among sequence variation in the sEH gene (Ephx2), sEH function, and risk of end-organ injury in strains of spontaneously hypertensive rat (SHRs) differing in their susceptibility to develop brain vascular disease. Brain Ephx2 expression was significantly lower in stroke-prone (SHR/A3) than in stroke-resistant (SHR/N) SHRs (5-fold; P <0.0001). Resequencing of the Ephx2 promoter in the 2 strains identified 3 polymorphisms that significantly influenced promoter transcriptional activity in vitro. Measurements of brain sEH enzyme activity and plasma levels of arachidonate and linoleate metabolites of sEH further suggested significant differences between the 2 strains. Ratios of epoxyoctadecenoic acids to dihydroxyoctadecenoic acids were significantly higher, indicating a lower sEH activity in SHR/A3 than in SHR/N ( P <0.0001). Plasma dihydroxyeicosatrienoic acid levels were lower in SHR/A3 than in SHR/N ( P <0.0001), but plasma epoxyeicosatrienoic acids levels were similar in the 2 strains. Association analysis of Ephx2 polymorphism in the F2 progeny of an SHR/A3×SHR/N cross showed that animals carrying the SHR/A3 allele of Ephx2 had a greater risk of stroke and associated urinary proteinuria than animals that do not. Investigation of patterns of allelic similarities and differences among multiple stroke-prone and stroke-resistant SHR substrains showed that Ephx2 belongs to a haplotype block shared among all of the stroke-prone but no stroke-resistant substrains. These data support a role for Ephx2 polymorphism on sEH gene expression and function and risk of end-organ injury in the stroke-prone SHR.

EET displays anti-inflammatory effects in TNF-alpha stimulated human bronchi: putative role of CPI-17

The aim of the present study was to investigate the anti-inflammatory effects of 14,15-epoxyeicosatrienoic acid (EET) on reactivity and Ca(2+) sensitivity in TNF-alpha-stimulated human bronchi. Tension measurements performed on either control, TNF-alpha-, or TNF-alpha + EET-pretreated bronchi revealed that 100 nM 14,15-EET pretreatments significantly reduced the reactivity of TNF-alpha-pretreated tissues to contractile agonists. EET also normalized the relaxing response to isoproterenol in TNF-alpha-treated bronchi. Pretreatment with 100 nM 14,15-EET prevented TNF-alpha-induced IkappaBalpha degradation, as demonstrated by an increase in IkappaBalpha protein levels on Western blot analysis. The anti-inflammatory properties of EET were mediated by the inhibition of IkappaBalpha degradation, suggesting a lower activation of NF-kappaB. The Ca(2+) sensitivity of TNF-alpha-stimulated bronchi was also evaluated on beta-escin-permeabilized preparations. Observed mean responses demonstrated that EET pretreatments abolished Ca(2+) hypersensitivity developed by TNF-alpha-stimulated bronchial explants. Moreover, 14,15-EET significantly reduced PDBu-induced Ca(2+) sensitivity in TNF-alpha-stimulated bronchi. Western blot and RT-PCR analyses revealed that CPI-17 protein and transcript levels were increased in TNF-alpha-treated bronchi, as opposed to being decreased in the presence of 14,15-EET. This eicosanoid also reduced U-46619-induced Ca(2+) sensitivity, which is related to the activation of Rho-kinase pathway. These results were also correlated with an increase in protein staining and transcription level of p116(Rip), a RhoA inhibitory-binding protein. Altogether, these data demonstrate that 14,15-EET is a potent modulator of the hyperreactivity triggered by TNF-alpha in human airway smooth muscle cells.

Origins of blood volume change due to glutamatergic synaptic activity at astrocytes abutting on arteriolar smooth muscle cells

The cellular mechanisms that couple activity of glutamatergic synapses with changes in blood flow, measured by a variety of techniques including the BOLD signal, have not previously been modelled. Here we provide such a model, that successfully accounts for the main observed changes in blood flow in both visual cortex and somatosensory cortex following their stimulation by high-contrast drifting grating or by single whisker stimulation, respectively. Coupling from glutamatergic synapses to smooth muscle cells of arterioles is effected by astrocytes releasing epoxyeicosatrienoic acids (EETs) onto them, following glutamate stimulation of the astrocyte. Coupling of EETs to the smooth muscle of arterioles is by means of potassium channels in their membranes, leading to hyperpolarization, relaxation and hence an increase in blood flow. This model predicts a linear increase in blood flow with increasing numbers of activated astrocytes, but a non-linear increase with increasing glutamate release.

TRPV4 initiates the acute calcium-dependent permeability increase during ventilator-induced lung injury in isolated mouse lungs

We have previously implicated calcium entry through stretch-activated cation channels in initiating the acute pulmonary vascular permeability increase in response to high peak inflation pressure (PIP) ventilation. However, the molecular identity of the channel is not known. We hypothesized that the transient receptor potential vanilloid-4 (TRPV4) channel may initiate this acute permeability increase because endothelial calcium entry through TRPV4 channels occurs in response to hypotonic mechanical stress, heat, and P-450 epoxygenase metabolites of arachidonic acid. Therefore, permeability was assessed by measuring the filtration coefficient (K(f)) in isolated perfused lungs of C57BL/6 mice after 30-min ventilation periods of 9, 25, and 35 cmH(2)O PIP at both 35 degrees C and 40 degrees C. Ventilation with 35 cmH(2)O PIP increased K(f) by 2.2-fold at 35 degrees C and 3.3-fold at 40 degrees C compared with baseline, but K(f) increased significantly with time at 40 degrees C with 9 cmH(2)O PIP. Pretreatment with inhibitors of TRPV4 (ruthenium red), arachidonic acid production (methanandamide), or P-450 epoxygenases (miconazole) prevented the increases in K(f). In TRPV4(-/-) knockout mice, the high PIP ventilation protocol did not increase K(f) at either temperature. We have also found that lung distention caused Ca(2+) entry in isolated mouse lungs, as measured by ratiometric fluorescence microscopy, which was absent in TRPV4(-/-) and ruthenium red-treated lungs. Alveolar and perivascular edema was significantly reduced in TRPV4(-/-) lungs. We conclude that rapid calcium entry through TRPV4 channels is a major determinant of the acute vascular permeability increase in lungs following high PIP ventilation.

Cytochrome P450 eicosanoids are activators of peroxisome proliferator-activated receptor alpha

Cytochrome P450 (P450) eicosanoids regulate vascular tone, renal tubular transport, cellular proliferation, and inflammation. Both the CYP4A omega-hydroxylases, which catalyze 20-hydroxyeicosatetraenoic acid (20-HETE) formation, and soluble epoxide hydrolase (sEH), which catalyzes epoxyeicosatrienoic acid (EET) degradation to the dihydroxyeicosatrienoic acids (DHETs), are induced upon activation of peroxisome proliferator-activated receptor alpha (PPARalpha) by fatty acids and fibrates. In contrast, the CYP2C epoxygenases, which are responsible for EET formation, are repressed after fibrate treatment. We show here that P450 eicosanoids can bind to and activate PPARalpha and result in the modulation of PPARalpha target gene expression. In transactivation assays, 14,15-DHET, 11,2-EET, and 20-HETE were potent activators of PPARalpha. Gel shift assays showed that EETs, DHETs, and 20-HETE induced PPARalpha-specific binding to its cognate response element. Expression of apolipoprotein A-I was decreased 70% by 20-HETE, whereas apolipoprotein A-II expression was increased up to 3-fold by 11,12-EET, 14,15-DHET, and 20-HETE. In addition, P450 eicosanoids induced CYP4A1, sEH, and CYP2C11 expression, suggesting that they can regulate their own levels. Given that P450 eicosanoids have multiple cardiovascular effects, pharmacological modulation of their formation and/or degradation may yield therapeutic benefits.

Cardiovascular therapeutic aspects of soluble epoxide hydrolase inhibitors

Soluble epoxide hydrolase (sEH) is an enzyme responsible for the conversion of lipid epoxides to diols by the addition of water. Biological actions on the cardiovascular system that are attributed to epoxides include vasodilation, antiinflammatory actions and vascular smooth muscle cell antimigratory actions. Conversion of arachidonic acid epoxides to diols by sEH diminishes the beneficial cardiovascular properties of these epoxyeicosano-ids. Cardiovascular diseases in animal models and humans have been associated with decreased epoxygenase activity or increased sEH activity and these changes are responsible for the progression of the disease state. More recently, sEH gene polymorphisms in the human population have been associated with increased risk for cardiovascular diseases. Thus the biological actions of epoxyeicosanoids and the sEH enzyme are ideal therapeutic targets for cardiovascular diseases. The rapid development of 1,3-disubstituted urea based sEH inhibitors over the past five years has resulted in a number of studies demonstrating cardiovascular protection. sEH inhibitors have antihypertensive and antiinflammatory actions and have been demonstrated to decrease cerebral ischemic and renal injury in rat models of hypertension. These findings of beneficial actions in animal models of disease position the sEH enzyme as a promising therapeutic target for cardiovascular diseases.

Arachidonic acid metabolites as endothelium-derived hyperpolarizing factors

The endothelium regulates vascular tone through the release of a number of soluble mediators, including NO, prostaglandin I2, and endothelium-derived hyperpolarizing factor. Epoxyeicosatrienoic acids are cytochrome P450 epoxygenase metabolites of arachidonic acid. They are synthesized by the vascular endothelium and open calcium-activated potassium channels, hyperpolarize the membrane, and relax vascular smooth muscle. Endothelium-dependent relaxations to acetylcholine, bradykinin, and shear stress that are not inhibited by cyclooxygenase and NO synthase inhibitors are mediated by the endothelium-derived hyperpolarizing factor. In arteries from experimental animals and humans, the non-NO, non-prostaglandin-mediated relaxations and endothelium-dependent hyperpolarizations are blocked by cytochrome P450 inhibitors, calcium-activated potassium channel blockers, and epoxyeicosatrienoic acid antagonists. Acetylcholine and bradykinin stimulate epoxyeicosatrienoic acid release from endothelial cells and arteries. These findings indicate that epoxyeicosatrienoic acids act as endothelium-derived hyperpolarizing factors and regulate arterial tone.

20-Carboxy-arachidonic acid is a dual activator of peroxisome proliferator-activated receptors α and γ

20-carboxy-arachidonic acid (20-COOH-AA) is a metabolite of 20-hydroxyeicosatetraenoic acid (20-HETE), an eicosanoid produced from arachidonic acid by cytochrome P450 (CYP) omega-oxidases. Alcohol dehydrogenases convert 20-HETE to 20-COOH-AA, and we now find that a microsomal preparation containing recombinant human CYP4F3B converts arachidonic acid to 20-HETE and 20-COOH-AA. Studies with transfected COS-7 cell expression systems indicate that 20-COOH-AA activates peroxisome proliferators-activated receptor (PPAR) alpha and PPARgamma. 20-COOH-AA was twice as potent as either 20-HETE or ciglitazone in stimulating PPARgamma-mediated luciferase expression. While 20-COOH-AA also was more potent than 20-HETE in increasing PPARalpha-mediated luciferase expression, the increase was only half as much as that produced by Wy-14643. 20-COOH-AA did not increase PPARalpha or PPARgamma expression in the transfected cells. Radiolabeled 20-COOH-AA was detected intracellularly when the COS-7 cells were incubated with either [3H]20-COOH-AA or [3H]20-HETE, and binding studies indicated that [3H]20-COOH-AA bound to the isolated ligand binding domains of PPARalpha (Kd=0.87+/-0.12 microM) and PPARgamma (Kd=1.7+/-0.5 microM). These findings suggest that 20-COOH-AA, a relatively stable metabolite of 20-HETE, might function as an endogenous dual activator of PPARalpha and PPARgamma.

Action of epoxyeicosatrienoic acids on cellular function

Epoxyeicosatrienoic acids (EETs), which function primarily as autocrine and paracrine mediators in the cardiovascular and renal systems, are synthesized from arachidonic acid by cytochrome P-450 epoxygenases. They activate smooth muscle large-conductance Ca(2+)-activated K(+) channels, producing hyperpolarization and vasorelaxation. EETs also have anti-inflammatory effects in the vasculature and kidney, stimulate angiogenesis, and have mitogenic effects in the kidney. Many of the functional effects of EETs occur through activation of signal transduction pathways and modulation of gene expression, events probably initiated by binding to a putative cell surface EET receptor. However, EETs are rapidly taken up by cells and are incorporated into and released from phospholipids, suggesting that some functional effects may occur through a direct interaction between the EET and an intracellular effector system. In this regard, EETs and several of their metabolites activate peroxisome proliferator-activated receptor alpha (PPARalpha) and PPARgamma, suggesting that some functional effects may result from PPAR activation. EETs are metabolized primarily by conversion to dihydroxyeicosatrienoic acids (DHETs), a reaction catalyzed by soluble epoxide hydrolase (sEH). Many potentially beneficial actions of EETs are attenuated upon conversion to DHETs, which do not appear to be essential under routine conditions. Therefore, sEH is considered a potential therapeutic target for enhancing the beneficial functions of EETs.

Oxygenation of omega-3 fatty acids by human cytochrome P450 4F3B: effect on 20-hydroxyeicosatetraenoic acid production

Cytochrome P450 (CYP) omega-oxidases convert arachidonic acid (AA) to 20-hydroxyeicosatetraenoic acid (20-HETE), a lipid mediator that modulates vascular tone. We observed that a microsomal preparation containing recombinant human CYP4F3B, which converts AA to 20-HETE, converted eicosapentaenoic acid (EPA) to 20-OH-EPA. Likewise, docosahexaenoic acid (DHA) was converted to 22-OH-DHA, indicating that human CYP4F3B also can oxidize 22-carbon omega-3 fatty acids. Consistent with these findings, addition of 0.5-5 microM EPA, DHA or omega-3 docosapentaenoic acid (DPA) to incubations containing 0.5 microM [3H]AA inhibited [3H]20-HETE production by 15-65%. [3H]20-OH-EPA was rapidly taken up by COS-7 cells, and almost all of the incorporated radioactivity remained as unmodified 20-OH-EPA. The 20-OH-EPA stimulated luciferase activity in COS-7 cells that express peroxisome proliferator-activated receptor alpha, indicating that this EPA metabolite may function as a lipid mediator. These findings suggest that some functional effects of omega-3 fatty acid supplementation may be due to inhibition of 20-HETE formation or the conversion of EPA to the corresponding omega-oxidized product.

Cytochrome P450 4F subfamily: at the crossroads of eicosanoid and drug metabolism

The cytochrome P450 4F (CYP4F) subfamily has over the last few years come to be recognized for its dual role in modulating the concentrations of eicosanoids during inflammation as well as in the metabolism of clinically significant drugs. The first CYP4F was identified because it catalyzed the hydroxylation of leukotriene B(4) (LTB(4)) and since then many additional members of this subfamily have been documented for their distinct catalytic roles and functional significance. Recent evidence emerging in relation to the temporal change of CYP4F expression in response to injury and infection supports an important function for these isozymes in curtailing inflammation. Their tissue-dependent expression, isoform-based catalytic competence and unique response to the external stimuli imply a critical role for them to regulate organ-specific functions. From this standpoint variations in relative CYP4F levels in humans may have direct influence on the metabolic outcome through their ability to generate and/or degrade bioactive eicosanoids or therapeutic agents. This review covers the enzymatic characteristics and regulatory properties of human and rodent CYP4F isoforms and their physiological relevance to major pathways in eicosanoid and drug metabolism.

Role of soluble epoxide hydrolase in postischemic recovery of heart contractile function

Cytochrome P450 epoxygenases metabolize arachidonic acid to epoxyeicosatrienoic acids (EETs) which are converted to dihydroxyeicosatrienoic acids (DHETs) by soluble epoxide hydrolase (Ephx2, sEH). To examine the functional role of sEH in the heart, mice with targeted disruption of the Ephx2 gene were studied. Hearts from sEH null mice have undetectable levels of sEH mRNA and protein and cannot convert EETs to DHETs. sEH null mice have normal heart anatomy and basal contractile function, but have higher fatty acid epoxide:diol ratios in plasma and cardiomyocyte cell culture media compared with wild type (WT). sEH null hearts have improved recovery of left ventricular developed pressure (LVDP) and less infarction compared with WT hearts after 20 minutes ischemia. Perfusion with the putative EET receptor antagonist 14,15-epoxyeicosa-5(Z)-enoic acid (10 to 100 nmol/L) before ischemia abolishes this cardioprotective phenotype. Inhibitor studies demonstrate that perfusion with phosphatidylinositol-3 kinase (PI3K) inhibitors wortmannin (200 nmol/L) or LY294002 (5 micromol/L), the ATP-sensitive K+ channel (K(ATP)) inhibitor glibenclamide (1 micromol/L), the mitochondrial K(ATP) (mitoK(ATP)) inhibitor 5-hydroxydecanoate (100 to 200 micromol/L), or the Ca2+-sensitive K+ channel (K(Ca)) inhibitor paxilline (10 micromol/L) abolishes the cardioprotection in sEH null hearts. Consistent with increased activation of the PI3K cascade, sEH null mice exhibit increased cardiac expression of glycogen synthase kinase-3beta (GSK-3beta) phospho-protein after ischemia. Together, these data suggest that targeted disruption of sEH increases the availability of cardioprotective EETs that work by activating PI3K signaling pathways and K+ channels.

Role of epoxyeicosatrienoic acids in protecting the myocardium following ischemia/reperfusion injury

Cardiomyocyte injury following ischemia-reperfusion can lead to cell death and result in cardiac dysfunction. A wide range of cardioprotective factors have been studied to date, but only recently has the cardioprotective role of fatty acids, specifically arachidonic acid (AA), been investigated. This fatty acid can be found in the membranes of cells in an inactive state and can be released by phospholipases in response to several stimuli, such as ischemia. The metabolism of AA involves the cycloxygenase (COX) and lipoxygenase (LOX) pathways, as well as the less well characterized cytochrome P450 (CYP) monooxygenase pathway. Current research suggests important differences with respect to the cardiovascular actions of specific CYP mediated arachidonic acid metabolites. For example, CYP mediated hydroxylation of AA produces 20-hydroxyeicosatetraenoic acid (20-HETE) which has detrimental effects in the heart during ischemia, pro-inflammatory effects during reperfusion and potent vasoconstrictor effects in the coronary circulation. Conversely, epoxidation of AA by CYP enzymes generates 5,6-, 8,9-, 11,12- and 14,15-epoxyeicosatrienoic acids (EETs) that have been shown to reduce ischemia-reperfusion injury, have potent anti-inflammatory effects within the vasculature, and are potent vasodilators in the coronary circulation. This review aims to provide an overview of current data on the role of these CYP pathways in the heart with an emphasis on their involvement as mediators of ischemia-reperfusion injury. A better understanding of these relationships will facilitate identification of novel targets for the prevention and/or treatment of ischemic heart disease, a major worldwide public health problem.

Evolutionary genomics of the HAD superfamily: understanding the structural adaptations and catalytic diversity in a superfamily of phosphoesterases and allied enzymes

The HAD (haloacid dehalogenase) superfamily includes phosphoesterases, ATPases, phosphonatases, dehalogenases, and sugar phosphomutases acting on a remarkably diverse set of substrates. The availability of numerous crystal structures of representatives belonging to diverse branches of the HAD superfamily provides us with a unique opportunity to reconstruct their evolutionary history and uncover the principal determinants that led to their diversification of structure and function. To this end we present a comprehensive analysis of the HAD superfamily that identifies their unique structural features and provides a detailed classification of the entire superfamily. We show that at the highest level the HAD superfamily is unified with several other superfamilies, namely the DHH, receiver (CheY-like), von Willebrand A, TOPRIM, classical histone deacetylases and PIN/FLAP nuclease domains, all of which contain a specific form of the Rossmannoid fold. These Rossmannoid folds are distinguished from others by the presence of equivalently placed acidic catalytic residues, including one at the end of the first core beta-strand of the central sheet. The HAD domain is distinguished from these related Rossmannoid folds by two key structural signatures, a "squiggle" (a single helical turn) and a "flap" (a beta hairpin motif) located immediately downstream of the first beta-strand of their core Rossmanoid fold. The squiggle and the flap motifs are predicted to provide the necessary mobility to these enzymes for them to alternate between the "open" and "closed" conformations. In addition, most members of the HAD superfamily contains inserts, termed caps, occurring at either of two positions in the core Rossmannoid fold. We show that the cap modules have been independently inserted into these two stereotypic positions on multiple occasions in evolution and display extensive evolutionary diversification independent of the core catalytic domain. The first group of caps, the C1 caps, is directly inserted into the flap motif and regulates access of reactants to the active site. The second group, the C2 caps, forms a roof over the active site, and access to their internal cavities might be in part regulated by the movement of the flap. The diversification of the cap module was a major factor in the exploration of a vast substrate space in the course of the evolution of this superfamily. We show that the HAD superfamily contains 33 major families distributed across the three superkingdoms of life. Analysis of the phyletic patterns suggests that at least five distinct HAD proteins are traceable to the last universal common ancestor (LUCA) of all extant organisms. While these prototypes diverged prior to the emergence of the LUCA, the major diversification in terms of both substrate specificity and reaction types occurred after the radiation of the three superkingdoms of life, primarily in bacteria. Most major diversification events appear to correlate with the acquisition of new metabolic capabilities, especially related to the elaboration of carbohydrate metabolism in the bacteria. The newly identified relationships and functional predictions provided here are likely to aid the future exploration of the numerous poorly understood members of this large superfamily of enzymes.

20-Hydroxyeicosatetraenoic acid is a potent dilator of mouse basilar artery: role of cyclooxygenase

20-Hydroxyeicosatetraenoic acid (20-HETE), an arachidonic acid (AA) metabolite synthesized by cytochrome P-450 omega-oxidases, is reported to produce vasoconstriction in the cerebral circulation. However, we find that like 14,15-epoxyeicosatrienoic acid (14,15-EET), 20-HETE produces dilation of mouse basilar artery preconstricted with U-46619 in vitro. Indomethacin inhibited the vasodilation produced by 20-HETE but not by 14,15-EET, suggesting a cyclooxygenase (COX)-dependent mechanism. Metabolic studies indicated several mechanisms that may play a role in this process. Mouse brain endothelial cells (MBEC) converted 20-HETE to 20-OH-PGE(2), which was as potent as PGE(2) in dilating the basilar artery. 20-HETE also stimulated AA release and PGE(2) and 6-keto-PGF(1alpha) production in MBEC. Furthermore, the basilar artery converted 20-HETE to 20-COOH-AA, which also produced COX-dependent dilation of the basilar artery. 20-COOH-AA increased AA release and PGE(2) and 6-keto-PGF(1alpha) production by the MBEC, but to a lesser extent than 20-HETE. Whereas the conversion of 20-HETE to 20-OH-PGE(2) and production of endogenous prostaglandins probably are primarily responsible for vasodilation, the production of 20-COOH-AA also may contribute to this process.

Emerging role of epoxyeicosatrienoic acids in coronary vascular function

The importance of endothelium-derived nitric oxide in coronary vascular regulation is well-established and the loss of this vasodilator compound is associated with endothelial dysfunction, tissue hypoperfusion and atherosclerosis. Numerous studies indicate that the endothelium produces another class of compounds, the epoxyeicosatrienoic acids (EETs), which may partially compensate for the loss of nitric oxide in cardiovascular disease. The EETs are endogenous lipids which are derived through the metabolism of arachidonic acid by cytochrome P450 epoxygenase enzymes. Also, EETs hyperpolarize vascular smooth muscle and induce dilation of coronary arteries and arterioles, and therefore may be endogenous mediators of coronary vasomotor tone and myocardial perfusion. In addition, EETs have been shown to inhibit vascular smooth muscle migration, decrease inflammation, inhibit platelet aggregation and decrease adhesion molecule expression, therefore representing an endogenous protective mechanism against atherosclerosis. Endogenous EETs are degraded to less active dihydroxyeicosatrienoic acids by soluble epoxide hydrolase. Pharmacological inhibition of soluble epoxide hydrolase has received considerable attention as a potential approach to enhance EET-mediated vascular protection, and several compounds have appeared promising in recent animal studies. The present review discusses the emerging role of EETs in coronary vascular function, as well as recent advancements in the development of pharmacological agents to enhance EET bioavailability.

The cloning and characterization of a soluble epoxide hydrolase in chicken

The mammalian soluble epoxide hydrolase (sEH) plays a role in the regulation of blood pressure and vascular homeostasis through its hydrolysis of the endothelial-derived messenger molecules, the epoxyeicosatrienoic acids. This study reports the cloning and expression of a sEH homolog from chicken liver. The resulting 63-kDa protein has an isoelectric point of 6.1. The recombinant enzyme displayed epoxide hydrolase activity when assayed with [3H]-trans-1,3-diphenylpropene oxide (t-DPPO), as well as trans-9,10-epoxystearate and the cis-8,9-, 11,12-, and 14,15- epoxyeicosatrienoic acids. The chicken enzyme displayed a lower kcat:Km for t-DPPO than the mammalian enzymes. The enzyme was sensitive to urea-based inhibitors developed for mammalian sEH. Such compounds could be used to study the role of chicken sEH in conditions in which endothelial-derived vasodilation is believed to be impaired, such as pulmonary hypertension syndrome.

Human soluble epoxide hydrolase: structural basis of inhibition by 4-(3-cyclohexylureido)-carboxylic acids

X-ray crystal structures of human soluble epoxide hydrolase (sEH) complexed with four different dialkylurea inhibitors bearing pendant carboxylate "tails" of varying length have been determined at 2.3-3.0 A resolution. Similarities among inhibitor binding modes reinforce the proposed roles of Y381 and/or Y465 as general acids that protonate the epoxide ring of the substrate in concert with nucleophilic attack of D333 at the electrophilic epoxide carbon. Additionally, the binding of these inhibitors allows us to model the binding mode of the endogenous substrate 14,15-epoxyeicosatrienoic acid. Contrasts among inhibitor binding modes include opposite orientations of inhibitor binding in the active-site hydrophobic tunnel. Alternative binding orientations observed for this series of inhibitors to human sEH, as well as the binding of certain dialkylurea inhibitors to human sEH and murine sEH, complicate the structure-based design of human sEH inhibitors with potential pharmaceutical applications in the treatment of hypertension. Thus, with regard to the optimization of inhibitor designs targeting human sEH, it is critical that human sEH and not murine sEH be utilized for inhibitor screening, and it is critical that structures of human sEH-inhibitor complexes be determined to verify inhibitor binding orientations that correlate with measured affinities.

An Epoxide Hydrolase Inhibitor, 12-(3-Adamantan-1-yl-ureido)dodecanoic Acid (AUDA), Reduces Ischemic Cerebral Infarct Size in Stroke-Prone Spontaneously Hypertensive Rats

Soluble epoxide hydrolase (sEH) inhibitors have been demonstrated to have cardiovascular protective actions. This hydrolase enzyme converts fatty acid epoxides to their corresponding diols, and this conversion can alter the biologic activity of these metabolites. We hypothesized that 12-(3-adamantan-1-yl-ureido)dodecanoic acid (AUDA), a sEH inhibitor, would protect stroke-prone spontaneously hypertensive rats from cerebral ischemia. AUDA was administered to 6-week-old male rats for 6 weeks, during which blood pressure was measured by telemetry. Cerebral ischemia was induced by middle cerebral artery occlusion, the size of the cerebral infarct was assessed after 6 hours of ischemia, and the results were expressed as a percentage of the hemisphere infarcted (%HI). Vascular structure and function were assessed using a pressurized arteriograph. Plasma levels of AUDA at the end of the treatment period averaged 5.0 +/- 0.4 ng/mL, and the urinary excretion rate was 99 +/- 21 ng/d. AUDA-treated rats had significantly smaller cerebral infarcts than control rats (36 +/- 4% vs 53 +/- 4% HI, treated versus control, P < 0.05, n = 6). This difference occurred independently of changes in blood pressure. AUDA treatment increased the passive compliance of the cerebral vessels but had no effect on vascular structure. The results of this study provide novel evidence suggesting that the sEH inhibitor AUDA is a possible therapeutic agent for ischemic stroke.

The antiinflammatory effect of laminar flow: the role of PPARgamma, epoxyeicosatrienoic acids, and soluble epoxide hydrolase

We previously reported that laminar flow activates peroxisome proliferator-activated receptor gamma (PPARgamma) in vascular endothelial cells in a ligand-dependent manner that involves phospholipase A2 and cytochrome P450 epoxygenases. In this study, we investigated whether epoxyeicosatrienoic acids (EETs), the catalytic products of cytochrome P450 epoxygenases, are PPARgamma ligands. Competition and direct binding assays revealed that EETs bind to the ligand-binding domain of PPARgamma with K(d) in the microM range. In the presence of adamantyl-ureido-dodecanoic acid (AUDA), a soluble epoxide hydrolase (sEH)-specific inhibitor, EETs increased PPARgamma transcription activity in endothelial cells and 3T3-L1 preadipocytes. Inclusion of AUDA in the perfusing media enhanced, but overexpression of sEH reduced, the laminar flow-induced PPARgamma activity. Furthermore, laminar flow augmented cellular levels of EETs but decreased sEH at the levels of mRNA, protein, and activity. Blocking PPARgamma by GW9662 abolished the EET/AUDA-mediated antiinflammatory effect, which indicates that PPARgamma is an effector of EETs.

Attenuation of Vascular Smooth Muscle Cell Proliferation by 1-Cyclohexyl-3-dodecyl Urea Is Independent of Soluble Epoxide Hydrolase Inhibition

Epoxyeicosatrienoic acid(s) (EET) have variable hemodynamic, anti-inflammatory, and growth regulatory effects, and inhibitors of their regulatory enzyme, soluble epoxide hydrolase (sEH), can mimic these effects. For this reason, sEH inhibitors are being studied as potential pharmaceuticals for the treatment of hypertension, atherosclerosis, and inflammatory diseases. We now show that a highly selective urea-based sEH inhibitor 1-cyclohexyl-3-dodecyl urea (CDU) attenuates human aortic vascular smooth muscle (HVSM) cell proliferation independently of any effect on sEH. CDU also inhibits endothelial cells when stimulated with basic fibroblast growth factor or serum. In addition, we demonstrate that EET, as well as several newer generation sEH inhibitors and a urea-based weak sEH inhibitor, do not affect proliferation in HVSM cells. Structure-activity relationships demonstrate that the addition of an acid group to the dodecyl carbon chain, changing the cyclohexyl group to an adamantyl group, and shortening the carbon chain to two carbons all abolish the antiproliferative effect. Our finding that a highly selective urea-based inhibitor of sEH can alter biology independently of its putative target enzyme suggests that there may be other useful properties of this class of compounds unrelated to their influence on epoxyeicosanoids. In addition, our results show that caution should be used when attempting to infer conclusions of EET biology based solely on the effects these inhibitors in tissue culture models, especially when used at micromolar concentrations.

14,15-Dihydroxyeicosatrienoic acid activates peroxisome proliferator-activated receptor-alpha

Epoxyeicosatrienoic acids (EETs), lipid mediators synthesized from arachidonic acid by cytochrome P-450 epoxygenases, are converted by soluble epoxide hydrolase (SEH) to the corresponding dihydroxyeicosatrienoic acids (DHETs). Originally considered as inactive degradation products of EETs, DHETs have biological activity in some systems. Here we examined the capacity of EETs and DHETs to activate peroxisome proliferator-activated receptor-alpha (PPARalpha). We find that among the EET and DHET regioisomers, 14,15-DHET is the most potent PPARalpha activator in a COS-7 cell expression system. Incubation with 10 microM 14,15-DHET produced a 12-fold increase in PPARalpha-mediated luciferase activity, an increase similar to that produced by the PPARalpha agonist Wy-14643 (20 microM). Although 10 microM 14,15-EET produced a threefold increase in luciferase activity, this was abrogated by the SEH inhibitor dicyclohexylurea. 14-Hexyloxytetradec-5(Z)-enoic acid, a 14,15-EET analog that cannot be converted to a DHET, did not activate PPARalpha. However, PPARalpha was activated by 2-(14,15-epoxyeicosatrienoyl)glycerol, which was hydrolyzed and the released 14,15-EET converted to 14,15-DHET. COS-7 cells incorporated 14,15-[3H]DHET from the medium, and the cells also retained a small amount of the DHET formed during incubation with 14,15-[3H]EET. Binding studies indicated that 14,15-[3H]DHET binds to the ligand binding domain of PPARalpha with a Kd of 1.4 microM. Furthermore, 14,15-DHET increased the expression of carnitine palmitoyltransferase 1A, a PPARalpha-responsive gene, in transfected HepG2 cells. These findings suggest that 14,15-DHET, produced from 14,15-EET by the action of SEH, may function as an endogenous activator of PPARalpha.

Induction of cyclooxygenase-2 by anandamide in cerebral microvascular endothelium

Anandamide (AEA), an endogenous cannabinoid receptor agonist, is a potent vasodilator in the cerebral microcirculation. AEA is converted to arachidonic acid (AA) by fatty acid amidohydrolase (FAAH), and the conversion of AA to prostaglandins has been proposed as a potential mechanism for the vasodilation. Although AEA stimulated prostaglandin production by mouse cerebral microvascular endothelial cells, no [(3)H]prostaglandins were produced when these cells were incubated with [3H]AEA. Incubation with R(+)-methanandamide (MAEA), a stable analogue of AEA that is not a substrate for FAAH, produced a similar increase in PGE2 production as AEA. The PGE2 production induced by either AEA or MAEA was completely inhibited by NS-398, a selective cyclooxygenase (COX)-2 inhibitor, suggesting that COX-2 was induced. AEA and MAEA increased the expression of COX-2 protein in a time-dependent manner. This increase occurred as early as 1 h and reached maximum at 2 h. Induction of COX-2 protein by AEA was partially inhibited by AM-251, a selective cannabinoid receptor-1 antagonist. Furthermore, AEA increased COX-2 promoter activity approximately twofold above baseline in a fragment ranging from -1432 to +59, the full-length of the COX-2 promoter, and the increase in COX-2 promoter activity produced by AEA was partially inhibited by AM-251. These results indicate that AEA increased COX-2 expression at the transcriptional level through, at least in part, a cannabinoid receptor-1-mediated mechanism in cerebral microvascular endothelium.

Epoxide hydrolases: mechanisms, inhibitor designs, and biological roles

Organisms are exposed to epoxide-containing compounds from both exogenous and endogenous sources. In mammals, the hydration of these compounds by various epoxide hydrolases (EHs) can not only regulate their genotoxicity but also, for lipid-derived epoxides, their endogenous roles as chemical mediators. Recent findings suggest that the EHs as a family represent novel drug discovery targets for regulation of blood pressure, inflammation, cancer progression, and the onset of several other diseases. Knowledge of the EH mechanism provides a solid foundation for the rational design of inhibitors, and this review summarizes the current understanding of the catalytic mechanism of the EHs. Although the overall EH mechanism is now known, the molecular basis of substrate selectivity, possible allosteric regulation, and many fine details of the catalytic mechanism remain to be solved. Finally, recent development in the design of EH inhibitors and the EH biological role are discussed.

Activation of peroxisome proliferator-activated receptor alpha by substituted urea-derived soluble epoxide hydrolase inhibitors

Soluble epoxide hydrolase (sEH) plays a major role in regulating vascular epoxyeicosatrienoic acid metabolism and function, and substituted urea derivatives that inhibit sEH activity reduce blood pressure in hypertensive rats. We found that substituted urea derivatives containing a dodecanoic acid group, besides effectively inhibiting sEH, increased peroxisome proliferator-activated receptor (PPAR) alpha activity. In PPARalpha transfected COS-7 cells, treatment with 10 microM N-cyclohexyl-N'-dodecanoic acid urea (CUDA) or N-adamantanyl-N'-dodecanoic acid urea (AUDA) produced 6- and 3-fold increases, respectively, in PPARalpha activation. Neither CUDA nor AUDA activated PPARdelta or PPARgamma directly, indicating selectivity for PPARalpha. CUDA did not alter PPARalpha protein expression, and it competitively inhibited the binding of Wy-14643 (pirinixic acid) to the ligand binding domain of PPARalpha, suggesting that it functions as a PPARalpha ligand. CUDA and AUDA were metabolized to chain-shortened beta-oxidation products, a process that reduced their potency as sEH inhibitors and their ability to bind and activate PPARalpha. N,N'-Dicylclohexylurea and N-cyclohexyl-N'-dodecylurea, sEH inhibitors that do not contain a carboxylic acid group, did not activate PPARalpha. In HepG2 cells, CUDA increased the expression of the PPARalpha-responsive gene carnitine palmitoyltransferase 1A. We conclude that CUDA and AUDA, by virtue of their carboxylic acid substitution, activate PPARalpha in addition to potently inhibiting sEH. Further development of these compounds could lead to a class of agents with hypotensive and lipid-lowering properties that may be valuable for the prevention and treatment of cardiovascular disease.

Metabolism of anandamide in cerebral microvascular endothelial cells

Anandamide (N-arachidonoylethanolamine, AEA), an endogenous cannabinoid receptor agonist, causes potent vasodilation in the cerebral circulation through an endothelial-dependent or -independent mechanism. We have investigated the processing of [3H]AEA in cultured mouse cerebral microvascular endothelial cells (MEC) in order to better understand its mechanism of action in the cerebral vasculature. These cells took up anandamide very quickly, reaching a maximum value in 5 min and remaining at that level for at least 8 h. Analysis of the cell lipids demonstrated that, in addition to free anandamide, radioactivity was incorporated into phosphatidylcholine (PC), phosphatidylinositol (PI), and phosphatidylethanolamine (PE) in a time-dependent manner. Analysis of the hydrolyzed cell lipids indicated that anandamide was converted to arachidonic acid, a process that was inhibited by the selective fatty acid amide hydrolase inhibitor oleyl trifluoromethyl ketone (OTMK). Phospholipase A2 (PLA2) hydrolysis of the PC, PI, and PE fractions indicated that the arachidonic acid formed from anandamide was esterified predominately into sn-2 position of the endothelial phospholipids. Furthermore, anandamide and arachidonic acid were released when the cells were incubated with A23187. These results suggest that the biological activity of anandamide might be regulated by its rapid uptake and calcium-dependent release in endothelial cells, and conversion of anandamide to arachidonic acid might serve as an inactivation process in the cerebral microcirculation.

Attenuation of tobacco smoke-induced lung inflammation by treatment with a soluble epoxide hydrolase inhibitor

Changes in the lungs due to smoking include inflammation, epithelial damage, and remodeling of the airways. Airway inflammation is likely to play a critical role in the genesis and progression of tobacco smoke-induced airway disease. Soluble epoxide hydrolase (sEH) is involved in the metabolism of endogenous chemical mediators that play an important role in inflammation. Epoxyeicosatrienoic acids (EETs) have demonstrated antiinflammatory properties, and hydrolysis of these epoxides by sEH is known to diminish this activity. To examine whether acute tobacco smoke-induced inflammation could be reduced by a sEH inhibitor, 12-(3-adamantane-1-yl-ureido)-dodecanoic acid n-butyl ester was given by daily s.c. injection to spontaneously hypertensive rats exposed to filtered air or tobacco smoke for a period of 3 days (6 h/day). Acute exposure to tobacco smoke significantly increased by 3.2-fold (P <0.05) the number of cells recovered by bronchoalveolar lavage. The sEH inhibitor significantly decreased total bronchoalveolar lavage cell number by 37% in tobacco smoke-exposed rats with significant reductions noted in neutrophils, alveolar macrophages, and lymphocytes. A combination of sEH inhibitor and EETs was more significant in its ability to further reduce tobacco smoke-induced inflammation compared with the sEH inhibitor alone. The sEH inhibitor led to a shift in some plasma epoxides and diols that are consistent with the hypothetical action of these compounds. We conclude that an sEH inhibitor, in the presence or absence of EETs, can attenuate, in part, inflammation associated with acute exposure to tobacco smoke.

Soluble epoxide hydrolase is a main effector of angiotensin II-induced hypertension

The soluble epoxide hydrolase (sEH) metabolizes vasodilatory epoxyeicosatrienoic acids (EETs) to their di-hydroxy derivatives. We hypothesized that the metabolism of EETs by the sEH contributes to angiotensin II-induced hypertension and tested the effects of a water-soluble sEH inhibitor, 12-(3-adamantan-1-yl-ureido) dodecanoic acid (AUDA) on blood pressure. AUDA (130 microg/mL in drinking water) did not affect blood pressure in normotensive animals but markedly lowered it in mice with angiotensin II-induced hypertension (1 mg/kg per day). The effect of AUDA was accompanied by an increase in urinary salt and water excretion. Intravenous application of AUDA (8 mg/kg) acutely lowered blood pressure and heart rate in animals with angiotensin II-induced hypertension but failed to affect blood pressure in animals with phenylephrine-induced hypertension (29 mg/kg per day). AUDA (0.1 micromol/L) selectively lowered vascular resistance in an isolated perfused kidney preparation from angiotensin II-pretreated mice but not from control mice. In the perfused hind limb and in isolated carotid arteries from angiotensin II-treated mice, AUDA was without effect. The omega-hydroxylase inhibitor N-methylsulfonyl-12,12-dibromododec-11-enamide, which attenuates formation of the potent vasoconstrictor 20-hydroxyeicosatetraenoic acid, decreased tone in carotid arteries from angiotensin II-treated but not from control mice. These data demonstrate that the decrease in blood pressure observed after sEH inhibition in angiotensin II-induced hypertension can be attributed to an initial reduction in heart rate followed by pressure diuresis resulting from increased perfusion of the kidney. Direct vasodilatation of resistance arteries in skeletal muscles does not appear to contribute to the antihypertensive effects of sEH inhibition in mice.

Effect of soluble epoxide hydrolase inhibition on epoxyeicosatrienoic acid metabolism in human blood vessels

We investigated the effects of soluble epoxide hydrolase (sEH) inhibition on epoxyeicosatrienoic acid (EET) metabolism in intact human blood vessels, including the human saphenous vein (HSV), coronary artery (HCA), and aorta (HA). When HSV segments were perfused with 2 micromol/l 14,15-[3H]EET for 4 h, >60% of radioactivity in the perfusion medium was converted to 14,15-dihydroxyeicosatrienoic acid (DHET). Similar results were obtained with endothelium-denuded vessels. 14,15-DHET was released from both the luminal and adventitial surfaces of the HSV. When HSVs were incubated with 14,15-[3H]EET under static (no flow) conditions, formation of 14,15-DHET was detected within 15 min and was inhibited by the selective sEH inhibitors N,N'-dicyclohexyl urea and N-cyclohexyl-N'-dodecanoic acid urea (CUDA). Similarly, CUDA inhibited the conversion of 11,12-[3H]EET to 11,12-DHET by the HSV. sEH inhibition enhanced the uptake of 14,15-[3H]EET and facilitated the formation of 10,11-epoxy-16:2, a beta-oxidation product. The HCA and HA converted 14,15-[3H]EET to DHET, and this also was inhibited by CUDA. These findings in intact human blood vessels indicate that conversion to DHET is the predominant pathway for 11,12- and 14,15-EET metabolism and that sEH inhibition can modulate EET metabolism in vascular tissue.

11,12-epoxyeicosatrienoic acid (11,12-EET): structural determinants for inhibition of TNF-alpha-induced VCAM-1 expression

A series of 11,12-EET analogues were synthesized and compared using a human endothelial cell based TNF-alpha-induced VCAM-1 expression assay. The resulting data were used to map a putative recognition/binding domain for 11,12-EET.

Human CYP4F3s are the main catalysts in the oxidation of fatty acid epoxides

CYP4F isoforms are involved in the oxidation of important cellular mediators such as leukotriene B4 (LTB4) and prostaglandins. The proinflammatory agent LTB4 and cytotoxic leukotoxins have been associated with several inflammatory diseases. We present evidence that the hydroxylation of Z 9(10)-epoxyoctadecanoic, Z 9(10)-epoxyoctadec-Z 12-enoic, and Z 12(13)-epoxyoctadec-Z 9-enoic acids and that of monoepoxides from arachidonic acid [epoxyeicosatrienoic acid (EET)] is important in the regulation of leukotoxin and EET activity. These three epoxidized derivatives from the C18 family (C18-epoxides) were converted to 18-hydroxy-C18-epoxides by human hepatic microsomes with apparent Km values of between 27.6 and 175 microM. Among recombinant P450 enzymes, CYP4F2 and CYP4F3B catalyzed mainly the omega-hydroxylation of C18-epoxides with an apparent Vmax of between 0.84 and 15.0 min(-1), whereas the apparent Vmax displayed by CYP4F3A, the isoform found in leukocytes, ranged from 3.0 to 21.2 min(-1). The rate of omega-hydroxylation by CYP4A11 was experimentally found to be between 0.3 and 2.7 min(-1). CYP4F2 and CYP4F3 exhibited preferences for omega-hydroxylation of Z 8(9)-EET, whereas human liver microsomes preferred Z 11(12)-EET and, to a lesser extent, Z 8(9)-EET. Moreover, vicinal diol from both C18-epoxides and EETs were omega-hydroxylated by liver microsomes and by CYP4F2 and CYP4F3. These data support the hypothesis that the human CYP4F subfamily is involved in the omega-hydroxylation of fatty acid epoxides. These findings demonstrate that another pathway besides conversion to vicinal diol or chain shortening by beta-oxidation exists for fatty acid epoxide inactivation.

Role of EETs in regulation of endothelial permeability in rat lung

This study tested the hypothesis that epoxyeicosatrienoic acids (EETs) derived from arachidonic acid via P-450 epoxygenases are soluble factors linking depletion of endoplasmic reticulum Ca2+ stores and store-dependent regulation of endothelial cell (EC) permeability in rat lung. EC permeability was measured via the capillary filtration coefficient ( Kf,c) in isolated, perfused rat lungs. 14,15-EET and 5,6-EET increased EC permeability, a response that was significantly different from that of 8,9-EET, 11,12-EET, and vehicle control. The permeability response to 14,15-EET was not significantly attenuated by the nonspecific Ca2+ channel blocker Gd3+ ( P = 0.068). In lungs perfused with low [Ca2+], 14,15-EET tended to increase EC permeability, although a significant increase in Kf,c was observed only following Ca2+ add-back. As positive control, we showed that the 3.7-fold increase in Kf,c evoked by thapsigargin (TG), a known activator of store depletion-induced Ca2+ entry, was blocked by both Gd3+ and low [Ca2+] buffer. Nonetheless, the permeability response to TG could not be blocked by the phospholipase A2 inhibitors mepacrine or methyl arachidonyl fluorophosphonate or the P-450 epoxygenase inhibitors 17-octadecynoic acid or propargyloxyphenyl hexanoic acid. Similarly, combined pretreatment with ibuprofen and dicyclohexylurea to block EET metabolism had no effect on the permeability response to TG. We conclude that EETs have a heterogeneous impact on EC permeability. Despite a requirement for Ca2+ entry with both TG and 14,15-EET, our data suggest that distinct signaling pathways or heterogeneity in EC responsiveness is responsible for the observed EC injury evoked by EETs and store depletion in the isolated rat lung.

Polymorphism of the Soluble Epoxide Hydrolase Is Associated With Coronary Artery Calcification in African-American Subjects

Background— Modulation of endogenous epoxide levels by soluble epoxide hydrolase (sEH) in the endothelium represents an important mechanism in the regulation of cardiovascular function. We examined the relationship between a common, functional polymorphism of the human sEH gene and coronary artery calcification (CAC) in young, largely asymptomatic African-American and non-Hispanic white subjects.

Methods and Results— Multiple logistic regression and Tobit regression models were used to assess the relationship between the sEH Arg287Gln polymorphism and presence and quantity of CAC. Models adjusting for race (except in race-specific analyses), age, sex, smoking, body mass index, systolic blood pressure, LDL cholesterol, and HDL cholesterol were estimated. Allele and genotype frequency distributions were not significantly different between the 2 ethnic groups ( P =0.22; P =0.17, respectively). The Arg287Gln polymorphism of the sEH gene was a significant predictor of CAC status in African-American participants, either alone or after adjusting for other risk factors. African-American subjects with at least 1 copy of the Gln287 allele had a 2-fold greater risk of having CAC compared with those not carrying this allele (95% CI, 1.1 to 2.9; P =0.02). There was no relationship between Arg287Gln polymorphism and the probability of having CAC in white participants (OR, 0.8; 95% CI, 0.5 to 1.3; P =0.49). Inferences from multivariable Tobit regression were similar to those obtained in the logistic regression models, indicating that the Arg287Gln polymorphism was a significant independent predictor of both presence and quantity of CAC in African-American but not white subjects.

Conclusions— These data suggest an intriguing and possibly novel role for sEH in the pathogenesis of atherosclerosis, which deserves additional investigation.

20-Hydroxyeicosatetraenoic Acid (20-HETE) Metabolism in Coronary Endothelial Cells

We have investigated the role of endothelial cells in the metabolism of 20-hydroxyeicosatetraenoic acid (20-HETE), a vasoactive mediator synthesized from arachidonic acid by cytochrome P450 omega-oxidases. Porcine coronary artery endothelial cells (PCEC) incorporated 20-[(3)H]HETE primarily into the sn-2 position of phospholipids through a coenzyme A-dependent process. The incorporation was reduced by equimolar amounts of arachidonic, eicosapentaenoic or 8,9-epoxyeicosatrienoic acids, but some uptake persisted even when a 10-fold excess of arachidonic acid was available. The retention of 20-[(3)H]HETE increased substantially when methyl arachidonoyl fluorophosphonate, but not bromoenol lactone, was added, suggesting that a Ca(2+)-dependent cytosolic phospholipase A(2) released the 20-HETE contained in PCEC phospholipids. Addition of calcium ionophore A23187 produced a rapid release of 20-[(3)H]HETE from the PCEC, a finding that also is consistent with a Ca(2+)-dependent mobilization process. PCEC also converted 20-[(3)H]HETE to 20-carboxy-arachidonic acid (20-COOH-AA) and 18-, 16-, and 14-carbon beta-oxidation products. 20-COOH-AA produced vasodilation in porcine coronary arterioles, but 20-HETE was inactive. These results suggest that the incorporation of 20-HETE and its subsequent conversion to 20-COOH-AA in the endothelium may be important in modulating coronary vascular function.

Vascular localization of soluble epoxide hydrolase in the human kidney

Epoxyeicosatrienoic acids are cytochrome P-450 metabolites of arachidonic acid with multiple biological functions, including the regulation of vascular tone, renal tubular transport, cellular proliferation, and inflammation. Epoxyeicosatrienoic acids are converted by soluble epoxide hydrolase into the corresponding dihydroxyeicosatrienoic acids, and epoxyeicosatrienoic acid hydration is regarded as one mechanism whereby their biological effects are eliminated. Previous animal studies indicate that soluble epoxide hydrolase plays an important role in the regulation of renal eicosanoid levels and systemic blood pressure. To begin to elucidate the mechanism of these effects, we determined the cellular localization of soluble epoxide hydrolase in human kidney by examining biopsies taken from patients with a variety of non-end-stage renal diseases, as well as those without known renal disease. Immunohistochemical staining of acetone-fixed kidney biopsy samples revealed that soluble epoxide hydrolase was preferentially expressed in the renal vasculature with relatively low levels in the surrounding tubules. Expression of soluble epoxide hydrolase was evident in renal arteries of varying diameter and was localized mostly in the smooth muscle layers of the arterial wall. Western blot analysis and functional assays confirmed the expression of soluble epoxide hydrolase in the human kidney. There were no obvious differences in soluble epoxide hydrolase expression between normal and diseased human kidney tissue in the samples examined. Our results indicate that soluble epoxide hydrolase is present in the human kidney, being preferentially expressed in the renal vasculature, and support an essential role for this enzyme in renal hemodynamic regulation and its potential utility as a target for therapeutic intervention.

CYP2U1, a novel human thymus- and brain-specific cytochrome P450, catalyzes omega- and (omega-1)-hydroxylation of fatty acids

Long chain fatty acids have recently emerged as critical signaling molecules in neuronal, cardiovascular, and renal processes, yet little is presently known about the precise mechanisms controlling their tissue distribution and bioactivation. We have identified a novel cytochrome P450, CYP2U1, which may play an important role in modulating the arachidonic acid signaling pathway. Northern blot and real-time PCR analysis demonstrated that CYP2U1 transcripts were most abundant in the thymus and the brain (cerebellum), indicating a specific physiological role for CYP2U1 in these tissues. Recombinant human CYP2U1 protein, expressed in baculovirus-infected Sf9 insect cells, was found to metabolize arachidonic acid exclusively to two region-specific products as determined by liquid chromatography-mass spectrometry. These metabolites were identified as 19- and 20-hydroxy-modified arachidonic acids by liquid chromatography-tandem mass spectrometry analysis. In addition to omega/omega-1 hydroxylation of arachidonic acid, CYP2U1 protein also catalyzed the hydroxylation of structurally related long chain fatty acid (docosahexaenoic acid) but not fatty acids such as lauric acid or linoleic acid. This is the first report of the cloning and functional expression of a new human member of P450 family 2, CYP2U1, which metabolizes long chain fatty acids. Based on the ability of CYP2U1 to generate bioactive eicosanoid derivatives, we postulate that CYP2U1 plays an important physiological role in fatty acid signaling processes in both cerebellum and thymus.

Up-regulation of endothelial nitric-oxide synthase by endothelium-derived hyperpolarizing factor involves mitogen-activated protein kinase and protein kinase C signaling pathways

Cytochrome P450 (P450)-dependent metabolites of arachidonic acid, the epoxyeicosatrienoic acids (EETs), are proposed to be endothelium-derived hyperpolarizing factors (EDHF) that affect vascular tone; however, the effects of EDHF on endothelial-derived nitric oxide biosynthesis remain unknown. We examined the regulation of endothelial nitric-oxide synthase (eNOS) by EDHF and investigated the relevant signaling pathways involved. The P450 epoxygenases CYP102 F87V mutant, CYP2C11-CYPOR, and CYP2J2 were transfected into cultured bovine aortic endothelial cells, and the effects of endogenously formed or exogenously applied EETs on eNOS expression and activity were assessed. Transfection with the P450 epoxygenases led to increased eNOS protein expression, an effect that was attenuated by cotreatment with the P450 inhibitor 17-ODYA. Northern analysis demonstrated that P450 transfection led to increased eNOS mRNA levels consistent with an effect at the pretranslational level. P450 epoxygenase transfection resulted in increased eNOS activity as measured by the conversion of L-arginine to L-citrulline. Addition of synthetic EETs (50-200 nM) to the culture media also increased eNOS expression and activity. Treatment with mitogen-activated protein kinase (MAPK), MAPK kinase, and protein kinase C inhibitors apigenin, 2'-amino-3'-methoxyflavone (PD98059), and 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), respectively, significantly inhibited the effects of P450 transfection on eNOS expression. Overexpression of P450 epoxygenases or addition of synthetic EETs increased Thr495 phosphorylation of eNOS, an effect that was inhibited by both apigenin and PD98059. Overexpression of P450 epoxygenases in rats resulted in increased aortic eNOS expression, providing direct evidence that EDHF can influence vascular eNOS levels in vivo. Based on this data, we conclude that EDHF up-regulates eNOS via activation of MAPK and protein kinase C signaling pathways.

The Telltale Structures of Epoxide Hydrolases

Traditionally, epoxide hydrolases (EH) have been regarded as xenobiotic-metabolizing enzymes implicated in the detoxification of foreign compounds. They are known to play a key role in the control of potentially genotoxic epoxides that arise during metabolism of many lipophilic compounds. Although this is apparently the main function for the mammalian microsomal epoxide hydrolase (mEH), evidence is now accumulating that the mammalian soluble epoxide hydrolase (sEH), despite its proven role in xenobiotic metabolism, also has a central role in the formation and breakdown of physiological signaling molecules. In addition, a certain class of microbial epoxide hydrolases has recently been identified that is an integral part of a catabolic pathway, allowing the use of specific terpens as sole carbon sources. The recently available x-ray structures of a number of EHs mirror their respective functions: the microbial terpen EH differs in its fold from the canonical alpha/beta hydrolase fold of the xenobiotic-metabolizing mammalian EHs. It appears that the latter fold is the perfect solution for the efficient detoxification of a large variety of structurally different epoxides by a single enzyme, whereas the smaller microbial EH, which has a particularly high turnover number with its prefered substrate, seems to be the better solution for the hydrolysis of one specific substrate. The structure of the sEH also includes an additional catalytic domain that has recently been shown to possess phosphatase activity. Although the physiological substrate for this second active site has not been identified so far, the majority of known phosphatases are involved in signaling processes, suggesting that the sEH phosphatase domain also has a role in the regulation of physiological functions.

In vitro metabolism of the mammalian soluble epoxide hydrolase inhibitor, 1-cyclohexyl-3-dodecyl-urea

The metabolism of the soluble epoxide hydrolase (sEH) inhibitor, 1-cyclohexyl-3-dodecyl-urea (CDU), was studied in rat and human hepatic microsomes. The microsomal metabolism of CDU enhanced sEH inhibition potency of the reaction mixture and resulted in the formation of several metabolites. During the course of this study, a sensitive and specific high-performance liquid chromatography with tandem mass spectrometry analytical method was developed to investigate simultaneously the production of these metabolites. In both rat and human hepatic microsomes, CDU was ultimately transformed into the corresponding omega-carboxylate; however, the rodent tissue appeared to perform this transformation more rapidly. After a 60-min incubation in rat hepatic microsomes, the percentage of residual CDU, the omega-carboxylate, and the intermediary omega-hydroxyl were about 20%, 20%, and 50%, respectively. Carbon monoxide inhibited the metabolism of CDU by rat hepatic microsomes, suggesting that the initial step is catalyzed by cytochrome P450. Further metabolism was enhanced by the addition of NAD, suggesting that dehydrogenases are associated with intermediate metabolic steps. Regardless, the ultimate product of microsomal metabolism, 12-(3-cyclohexyl-ureido)-dodecanoic acid, is also an excellent sEH inhibitor with several hundred-fold higher solubility, supporting the hypothesis that CDU has prodrug characteristics. These findings will facilitate the rational design and optimization of sEH inhibitors with better physical properties and improved metabolic stability.

Differences in positional esterification of 14,15-epoxyeicosatrienoic acid in phosphatidylcholine of porcine coronary artery endothelial and smooth muscle cells

Epoxyeicosatrienoic acids (EETs) are readily incorporated into phospholipids of smooth muscle cells (SMC) and endothelial cells (EC). Incorporation of EETs into intact porcine coronary arteries potentiates EC-dependent relaxation, but not vasorelaxation induced by agents that act solely on SMC. To explore the potential mechanisms responsible for this difference, porcine coronary artery SMC and EC preloaded with [3H]14,15-EET were treated with calcium ionophore A23187. Although the amount of EET incorporated into EC and SMC was similar, A23187 stimulated a five-fold increase in release of radioactivity from EC, but only a 21% increase in release from SMC. Thin layer chromatography (TLC) examination of cell lipids demonstrated that > 70% of the incorporated radioactivity was present in phosphatidylcholine (PC) in both SMC and BC. After treatment of EC PC with PLA2, TLC analysis indicated that approximately equal to 75% of radioactivity was present as free EET, and 25% of radioactivity was present as lyso-PC. Therefore, most of the 14,15-EET was esterified into the sn-2 position of PC in EC. However, in SMC, approximately equal to 70% of radioactivity was present as lyso-PC after PLA2 treatment, indicating that the EET was predominately esterified into the sn-1 position. In contrast, all of the 14,15-EET was esterified into the sn-2 position of PI in both EC and SMC. These results suggest that the preferential incorporation of 14,15-EET into the sn-1 position of PC in SMC may help to explain the greater retention of the compound in SMC, while incorporation into the sn-2 position of PC in EC may facilitate agonist-induced 14,15-EET release and potentiation of EC-dependent porcine coronary artery relaxation.

The N-terminal domain of mammalian soluble epoxide hydrolase is a phosphatase

The mammalian soluble epoxide hydrolase (sEH) is an enzyme with multiple functions, being implicated in detoxification of xenobiotic epoxides as well as in regulation of physiological processes such as blood pressure. The enzyme is a homodimer, in which each subunit is composed of two domains. The 35-kDa C-terminal domain has an alpha/beta hydrolase fold and harbors the catalytic center for the EH activity. The 25-kDa N-terminal domain has a different alpha/beta fold and belongs to the haloacid dehalogenase superfamily of enzymes. The catalytic properties of the enzyme reported so far can all be explained by the action of the C-terminal domain alone. The function of the N-terminal domain, other than in structural stabilization of the dimer, has therefore remained unclear. By structural comparison of this domain to other haloacid dehalogenase family members, we identified a putative active site containing all necessary components for phosphatase activity. Subsequently, we found rat sEH hydrolyzed 4-nitrophenyl phosphate with a rate constant of 0.8 s(-1) and a K(m) of 0.24 mM. Recombinant human sEH lacking the C-terminal domain also displayed phosphatase activity. Presence of a phosphatase substrate did not affect epoxide turnover nor did epoxides affect dephosphorylation by the intact enzyme, indicating both catalytic sites act independently. The enzyme was unable to hydrolyze 4-nitrophenyl sulfate, suggesting its role in xenobiotic metabolism does not extend beyond phosphates. Thus, we propose this domain participates instead in the regulation of the physiological functions associated with sEH.

Human coronary endothelial cells convert 14,15-EET to a biologically active chain-shortened epoxide

Cytochrome P-450 epoxygenase-derived epoxyeicosatrienoic acids (EETs) play an important role in the regulation of vascular reactivity and function. Conversion to the corresponding dihydroxyeicosatrienoic acids (DHETs) by soluble epoxide hydrolases is thought to be the major pathway of EET metabolism in mammalian vascular cells. However, when human coronary artery endothelial cells (HCEC) were incubated with (3)H-labeled 14,15-EET, chain-shortened epoxy fatty acids, rather than DHET, were the most abundant metabolites. After 4 h of incubation, 23% of the total radioactivity remaining in the medium was converted to 10,11-epoxy-hexadecadienoic acid (16:2), a product formed from 14,15-EET by two cycles of beta-oxidation, whereas only 15% was present as 14,15-DHET. Although abundantly present in the medium, 10,11-epoxy-16:2 was not detected in the cell lipids. Exogenously applied (3)H-labeled 10,11-epoxy-16:2 was neither metabolized nor retained in the cells, suggesting that 10,11-epoxy-16:2 is a major product of 14,15-EET metabolism in HCEC. 10,11-Epoxy-16:2 produced potent dilation in coronary microvessels. 10,11-Epoxy-16:2 also potently inhibited tumor necrosis factor-alpha-induced production of IL-8, a proinflammatory cytokine, by HCEC. These findings implicate beta-oxidation as a major pathway of 14,15-EET metabolism in HCEC and provide the first evidence that EET-derived chain-shortened epoxy fatty acids are biologically active.

Stereospecific activation of cardiac ATP-sensitive K(+) channels by epoxyeicosatrienoic acids: a structural determinant study

The heart is richly endowed with K(ATP) channels, which function as biological sensors, regulating membrane potentials and electrical excitability in response to metabolic alterations. We recently reported that the cytochrome P450 metabolites of arachidonic acid, epoxyeicosatrienoic acids (EETs), potently activate cardiac K(ATP) channels by reducing channel sensitivity to ATP. In the present study, we further demonstrate that 11(S),12(R)-EET activated the cardiac K(ATP) channels with an EC(50) of 39.5 nM, whereas 11(R),12(S)-EET was totally inactive. In addition, 11(S),12(R)-EET but not 11(R),12(S)-EET hyperpolarized the resting membrane potentials and shortened the duration of cardiomyocyte action potentials. By studying homologs and analogs of 11,12-EET, we also found that all four EET regioisomers are equipotent activators of the K(ATP) channels, reducing the ATP sensitivity by more than 10-fold; however, neither altered chain length, double bond number, epoxide position, nor methylation of the carboxyl group affected channel inhibitions by ATP. All the fatty epoxides studied are potent K(ATP) channel activators, but the omega-3 homolog was particularly potent, reducing ATP sensitivity 27-fold. Together, the results indicate that the presence of an epoxide group in a particular three-dimensional configuration is a critical determinant for K(ATP) channel activation, and its effect is augmented by a double bond at omega-3 position. The results also suggest that fatty epoxides are important modulators of cardiac electrical excitability.

The CYP4A isoforms hydroxylate epoxyeicosatrienoic acids to form high affinity peroxisome proliferator-activated receptor ligands

Cytochromes P450 of the CYP2C and CYP4A gene subfamilies metabolize arachidonic acid to 5,6-, 8,9-, 11,12-, and 14,15-epoxyeicosatrienoic acids (EETs) and to 19- and 20-hydroxyeicosatetraenoic acids (HETEs), respectively. Abundant functional studies indicate that EETs and HETEs display powerful and often opposing biological activities as mediators of ion channel activity and regulators of vascular tone and systemic blood pressures. Incubation of 8,9-, 11,12-, and 14,15-EETs with microsomal and purified forms of rat CYP4A isoforms led to rapid NADPH-dependent metabolism to the corresponding 19- and 20-hydroxylated EETs. Comparisons of reaction rates and catalytic efficiency with those of arachidonic and lauric acids showed that EETs are one of the best endogenous substrates so far described for rat CYP4A isoforms. CYP4A1 exhibited a preference for 8,9-EET, whereas CYP4A2, CYP4A3, and CYP4A8 preferred 11,12-EET. In general, the closer the oxido ring is to the carboxylic acid functionality, the higher the rate of EET metabolism and the lower the regiospecificity for the EET omega-carbon. Analysis of cis-parinaric acid displacement from the ligand-binding domain of the human peroxisome proliferator-activated receptor-alpha showed that omega-hydroxylated 14,15-EET bound to this receptor with high affinity (K(i) = 3 +/- 1 nm). Moreover, at 1 microm, the omega-alcohol of 14,15-EET or a 1:4 mixture of the omega-alcohols of 8,9- and 11,12-EETs activated human and mouse peroxisome proliferator-activated receptor-alpha in transient transfection assays, suggesting a role for them as endogenous ligands for these orphan nuclear receptors.

Difference of gene expression profiles in spontaneous hypertensive rats and Wistar-Kyoto rats from two sources

Spontaneously hypertensive rats (SHR) are a well-known animal model for hypertension. We have previously identified eleven differentially expressed genes in the kidneys between SHR/Hos and Wistar-Kyoto rats (WKY/Hos) using an oligonucleotide microarray and analyzed the correlation between these genes and hypertension. In the present study, we analyzed the differentially expressed genes in the kidneys between SHR/NCrj and WKY/NCrj obtained from an other source to clarify the common and/or specific gene expression between the different sources. Furthermore, expression changes in the representative genes were characterized by Northern blot analysis using samples prepared from a third source, the Izm strain. The comparison revealed quite different changes in the differentially expressed genes among them. Sequence analysis of one of the differentially expressed genes, cytosolic epoxide hydrolase, revealed that two haplotypes could in part explain the expression level. Our study showed the complex nature of the genetic heterogeneity between SHR and WKY from different sources.