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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.

Epoxyeicosatrienoic acids (EETs) are considered to be endothelium-derived hyperpolarizing factors, and are potent activators of the large-conductance, Ca(2+)-activated K(+) (BK(Ca)) channel in vascular smooth muscle. Here, we investigate the signal transduction pathway involved in the activation of BK(Ca) channels by 11,12-EET and 11,12-EET stable analogs in rat mesenteric vascular smooth muscle cells. 11,12-EET and the 11,12-EET analogs, 11-nonyloxy-undec-8(Z)-enoic acid (11,12-ether-EET-8-ZE), 11-(9-hydroxy-nonyloxy)-undec-8(Z)-enoic acid (11,12-ether-EET-8-ZE-OH) and 11,12-trans-oxidoeicosa-8(Z)-enoic acid (11,12-tetra-EET-8-ZE), caused vasorelaxation of mesenteric resistance arteries. Mesenteric myocyte whole-cell (perforated-patch) currents were substantially (approximately 150%) increased by 11,12-EET and 11,12-EET analogs. Single-channel recordings were conducted to identify the target for 11,12-EET. 11,12-EET and 11,12-EET analogs also increased mesenteric myocyte BK(Ca) channel activity in cell-attached patches. Similar results were obtained in cell-free patches. Baseline mesenteric myocyte BK(Ca) channel activity (NPo) in cell-free patches averaged less than 0.001 at +50 mV and 11,12-EET (1 micromol/L) increased NPo to 0.03+/-0.02 and 11,12-EET analogs (1 micromol/L) increased NPo to 0.09+/-0.006. Inhibition of protein phosphatase 2A (PP2A) activity with okadaic acid (10 nmol/L) completely reversed 11,12-EET stimulated BK(Ca) channel activity and greatly attenuated 11,12-ether-EET-8-ZE mesenteric resistance artery vasorelaxation. 11,12-EET and 11,12-EET analogs increased mesenteric myocyte PP2A activity by 3.5-fold. Okadaic acid and the EET inhibitor, 14,15-epoxyeicosa-5(Z)-enoic acid (14,15-EEZE) inhibited the 11,12-EET mediated increase in PP2A activity. These findings provide initial evidence that PP2A activity contributes to 11,12-EET and 11,12-EET analog activation of mesenteric resistant artery BK(Ca) channels and vasorelaxation.

5,6-Epoxyeicosatrienoic acid (5,6-EET) is a cytochrome P450 epoxygenase metabolite of arachidonic acid that causes vasorelaxation. However, investigations of its role in biological systems have been limited by its chemical instability. We developed a stable agonist of 5,6-EET, 5-(pentadeca-3(Z),6(Z),9(Z)-trienyloxy)pentanoic acid (PTPA), in which the 5,6-epoxide was replaced with a 5-ether. PTPA obviates chemical and enzymatic hydrolysis. In bovine coronary artery rings precontracted with U46619, PTPA (1 nmol/L to 10 μmol/L) induced concentration-dependent relaxations, with maximal relaxation of 86±5% and EC 50 of 1 μmol/L. The relaxations were inhibited by the cyclooxygenase inhibitor indomethacin (10 μmol/L; max relaxation 43±9%); the ATP-sensitive K + channel inhibitor glybenclamide (10 μmol/L; max relaxation 49±6%); and the large conductance calcium-activated K + channel inhibitor iberiotoxin (100 nmol/L; max relaxation 38±6%) and abolished by the combination of iberiotoxin with indomethacin or glybenclamide or increasing extracellular K + to 20 mmol/L. Whole-cell outward K + current was increased nearly 6-fold by PTPA (10 μmol/L), which was also blocked by iberiotoxin. Additionally, we synthesized 5-(pentadeca-6(Z),9(Z)-dienyloxy)pentanoic acid and 5-(pentadeca-3(Z),9(Z)-dienyloxy)pentanoic acid (PDPA), PTPA analogs that lack the 8,9 or 11,12 double bonds of arachidonic acid and therefore are not substrates for cyclooxygenase. The PDPAs caused concentration-dependent relaxations (max relaxations 46±13% and 52±7%, respectively; EC 50 1μmol/L), which were not altered by glybenclamide but blocked by iberiotoxin. These studies suggested that PTPA induces relaxation through 2 mechanisms: (1) cyclooxygenase-dependent metabolism to 5-ether–containing prostaglandins that activate ATP-sensitive K + channels and (2) activation of smooth muscle large conductance calcium-activated K + channels. PDPAs only activate large conductance calcium-activated K + channels.

Epoxyeicosatrienoic acids (EETs) are candidate endothelium-derived hyperpolarizing factors that demonstrate a wide range of biological effects. The presence of both cis - and trans -EETs in rat plasma was identified with HPLC-electrospray ionization tandem mass spectrometry in this study. The total EETs in plasma are 38.2 ng/ml with cis -EETs representing 21.4 ± 0.4 ng/ml and trans -EETs 16.8 ± 0.4 ng/ml. EETs in RBCs were estimated to be 20.2 ng/10 9 RBCs, which corresponds to 200 ng in RBCs contained in 1 ml blood. RBC incubation with 10 mM tert -butyl hydroperoxide resulted in 4.4-fold increase of total cis -EETs (from 9.2 to 40.2 ng/10 9 RBCs) and 5.5-fold increase of total trans -EETs (from 11.0 to 60.8 ng/10 9 RBCs). EETs were released (2 ng/ml) from RBCs after incubation at 37 °C for 10 min even after being washed 3 times, indicating that RBCs are reservoirs of plasma EETs. The identification of cis - and trans -EETs in RBCs and in plasma as well as their release from RBCs suggest a vasoregulatory role of RBCs in view of their potent vasoactivity.

Arachidonic acid metabolites contribute to the regulation of vascular tone and therefore tissue blood flow. The vascular endothelium metabolizes arachidonic acid by cytochrome P450 epoxygenases to epoxyeicosatrienoic acids or EETs. The placement of the epoxide group can occur on any of the double bonds of arachidonic acid resulting in four EET regioisomers; 5,6-, 8,9-, 11,12- and 14,15-EET. In the vasculature, EETs are key components of cellular signaling cascades that culminate in the activation of smooth muscle potassium channels to induce membrane hyperpolarization and vascular relaxation. In some vasculatures such as bovine coronary arteries, EET regioisomers are equipotent in inducing relaxations, while in other arteries, a specific EET regioisomer induces relaxation while others do not. Therefore, the position of the double bonds and/or the epoxide group may alter vascular agonist activity. This observation suggests that small alterations in the chemical structure of EETs can significantly impact vascular activity. To explore this hypothesis, we synthesized a series of EET analogs and characterized their vasodilator agonist and antagonist activity in bovine coronary arteries. In this chapter, we first review the mechanisms of EET-dependent relaxations in bovine coronary arteries to familiarize the reader with the role of EETs in these arteries. The second component is a synopsis of the functional characterization of the 14,15-EET analogs and the resulting description of structural components required for vascular dilator activity. Lastly, we discussed the characterization of three 14,15-EET analogs with specific EET-antagonist activity and compared this to the activity of similar 11,12-EET analogs. These studies have revealed that specific structural components of the 14,15-EET molecule are critical for dilator activity and that alteration of these components influences agonist activity and may confer antagonist properties.

A novel eicosanoid, 5,6-trans-epoxy-8Z,11Z,14Z-eicosatrienoic acid (5,6-trans-EET), was identified in rat red blood cells. Characterization of 5,6-trans-EET in the sn-2 position of the phospholipids was accomplished by hydrolysis with phospholipase A(2) followed by gas chromatography/mass spectrometry as well as electrospray ionization-tandem mass spectrometry analyses. The electron ionization spectrum of 5,6-erythro-dihydroxyeicosatrienoic acid (5,6-erythro-DHET), converted from 5,6-trans-EET in the samples, matches that of the authentic standard. Hydrogenation of the extracted 5,6-erythro-DHET with platinum(IV) oxide/hydrogen resulted in an increase of the molecular mass by 6 daltons and the same retention time shift as an authentic standard in gas chromatography, suggesting the existence of three olefins as well as the 5,6-erythro-dihydroxyl structure in the metabolite. Match of retention times by chromatography indicated identity of the stereochemistry of the red blood cell 5,6-erythro-DHET vis à vis the synthetic standard. High pressure liquid chromatography-electrospray ionization-tandem mass spectrometry analysis of the phospholipase A(2)-hydrolyzed lipid extracts from red blood cells revealed match of the mass spectrum and retention time of the compound with the authentic 5,6-trans-EET standard, providing direct evidence of the existence of 5,6-trans-EET in red blood cells. The presence of other trans-EETs was also demonstrated. The ability of both 5,6-trans-EET and its product 5,6-erythro-DHET to relax preconstricted renal interlobar arteries was significantly greater than that of 5,6-cis-EET. In contrast, 5,6-cis-EET and 5,6-trans-EET were equipotent in their capacity to inhibit collagen-induced rat platelet aggregation, whereas 5,6-erythro-DHET was without effect. We propose that the red blood cells serve as a reservoir for epoxides which on release may act in a vasoregulatory capacity.

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.