No abstract

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 Ca2+-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 α (PPARα) and PPARγ, 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.

Cytochrome P450 (CYP) epoxygenases convert arachidonic acid to four epoxyeicosatrienoic acid (EET) regioisomers, 5,6-, 8,9-, 11,12-, and 14,15-EET, that function as autacrine and paracrine mediators. EETs produce vascular relaxation by activating smooth muscle large-conductance Ca 21 -activated K 1 channels (BK Ca ). In addition, they have anti-inflammatory effects on blood vessels and in the kidney, promote angiogenesis, and protect ischemic myocardium and brain. CYP epoxygenases also convert eicosapentaenoic acid to vasoactive epoxy-derivatives, and endocannabinoids containing 11,12-and 14,15-EET are formed. Many EET actions appear to be initiated by EET binding to a membrane receptor that activates ion channels and intracellular signal transduction pathways. However, EETs also are taken up by cells, are incorporated into phospholipids, and bind to cytosolic proteins and nuclear receptors, suggesting that some functions may occur through direct interaction of the EET with intracellular effector systems. Soluble epoxide hydrolase (sEH) converts EETs to dihydroxyeicosatrienoic acids (DHETs). Because this attenuates many of the functional effects of EETs, sEH inhibition is being evaluated as a mechanism for increasing and prolonging the beneficial actions of EETs. Epoxyeicosatrienoic acids (EET) are epoxide derivatives of arachidonic acid. They are formed by cytochrome P450 (CYP) epoxygenases and function as lipid mediators. Epoxidation can occur at any of the four double bonds of arachidonic acid, giving rise to four regioisomers, 5,6-, 8,9-, 11,12-, and 14,15-EET. EETs are synthesized in the endothelium and activate large-conductance Ca 21 -activated K 1 channels (BK Ca ), causing hyperpolarization of the vascular smooth muscle and vasorelaxation. Thus, EETs function as an endothelium-derived hyperpolarizing factor (EDHF) in a number of vascular beds, including the coronary and renal circulations, producing a decrease in blood pressure. Soluble epoxide hydrolase (sEH), which converts EETs to dihydroxyeicosatrienoic acids (DHETs), attenuates many of the functional effects of EETs. These seminal findings have been described in a number of detailed reviews (1-4). Recent results with cultured cells and animal models indicate that EETs have additional potentially beneficial effects on the vascular system, heart, kidneys, and nervous system, and many current studies are directed at these actions (5-9). The other current emphasis is on sEH inhibition as a therapeutic strategy for increasing the beneficial effects of EETs (10, 11). EET SYNTHESIS, METABOLISM, AND FUNCTIONEETs are synthesized by cells that express CYP epoxygenase activity. As illustrated in Fig. 1, these enzymes act on arachidonic acid released from phospholipids when cytosolic phospholipase A 2 (cPLA 2 ) is activated (12). The epoxygenase inserts an oxygen atom on a carbon attached to one of the double bonds of arachidonic acid, and the double bond is reduced as the epoxide forms. Each CYP epoxygenase produces several regioisomers, with one for...

We previously reported that laminar flow activates peroxisome proliferator-activated receptor ␥ (PPAR␥) 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 PPAR␥ ligands. Competition and direct binding assays revealed that EETs bind to the ligandbinding domain of PPAR␥ with Kd in the M range. In the presence of adamantyl-ureido-dodecanoic acid (AUDA), a soluble epoxide hydrolase (sEH)-specific inhibitor, EETs increased PPAR␥ 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 PPAR␥ activity. Furthermore, laminar flow augmented cellular levels of EETs but decreased sEH at the levels of mRNA, protein, and activity. Blocking PPAR␥ by GW9662 abolished the EET͞AUDA-mediated antiinflammatory effect, which indicates that PPAR␥ is an effector of EETs. endothelial cells ͉ shear stressA therosclerosis preferentially localizes in branches and curved regions of the arterial tree, where the blood flow is disturbed. In contrast, the straight parts of vessels exposed to nondisturbed laminar flow have few lesions (1). The focal distribution of atherosclerotic lesions has been proposed to be related to the proinflammatory effect of disturbed flow imposed on the endothelium vs. the antiinflammatory effect of laminar flow. In vitro studies using flow channels with cultured endothelial cells (ECs) revealed that disturbed flow induces a number of molecules involved in inflammation, including chemoattractants, adhesion molecules, and cytokines (2, 3). However, prolonged exposure to laminar flow suppresses the cytokine-stimulated or oxidized low-density lipoprotein (LDL)-stimulated inflammatory response in ECs (4).Recent studies showed that the nuclear receptor peroxisome proliferator-activated receptor ␥ (PPAR␥) is involved in antiinflammatory effects in the artery wall (5, 6). The activation of PPAR␥ in cultured ECs suppresses the NF-B-mediated expression of molecules such as vascular cell adhesion molecule 1, intercellular adhesion molecule 1, and endothelin 1 that are involved in the inflammatory response (7,8). Troglitazone, a synthetic PPAR␥ ligand, attenuates the formation of lesions in both apolipoprotein E-and low-density lipoprotein receptordeficient mice (7, 9), due in part to the reduction of monocytes͞ macrophages homing to the plaques.We previously demonstrated that laminar flow activates PPAR␥ in a ligand-dependent manner, which exerts an antiinflammatory effect in ECs. Furthermore, we showed that such induction of PPAR␥ ligands involves phospholipase A2 and cytochrome P450 epoxygenases (CYPs) (10). Epoxyeicosatrienoic acids (EETs), the main products of arachidonic acid catalyzed by CYPs, have been reported to dilate coronary arteries by hyperpolarizing vascular smooth muscles (11) and to exe...