Vascular pharmacology of epoxyeicosatrienoic acids

Abstract

Epoxyeicosatrienoic acids (EETs) are cytochrome P450 metabolites of arachidonic acid that are produced by the vascular endothelium in responses to various stimuli such as the agonists acetylcholine (ACH) or bradykinin or by shear stress which activates phospholipase A(2) to release arachidonic acid. EETs are important regulators of vascular tone and homeostasis. In the modulation of vascular tone, EETs function as endothelium-derived hyperpolarizing factors (EDHFs). In models of vascular inflammation, EETs attenuate inflammatory signaling pathways in both the endothelium and vascular smooth muscle. Likewise, EETs regulate blood vessel formation or angiogenesis by mechanisms that are still not completely understood. Soluble epoxide hydrolase (sEH) converts EETs to dihydroxyeicosatrienoic acids (DHETs) and this metabolism limits many of the biological actions of EETs. The recent development of inhibitors of sEH provides an emerging target for pharmacological manipulation of EETs. Additionally, EETs may initiate their biological effects by interacting with a cell surface protein that is a G protein-coupled receptor (GPCR). Since GPCRs represent a common target of most drugs, further characterization of the EET receptor and synthesis of specific EET agonists and antagonist can be used to exploit many of the beneficial effects of EETs in vascular diseases, such as hypertension and atherosclerosis. This review will focus on the current understanding of the contribution of EETs to the regulation of vascular tone, inflammation, and angiogenesis. Furthermore, the therapeutic potential of targeting the EET pathway in vascular disease will be highlighted.

Figure 1

Overview of EET Biosynthesis and Metabolism. Arachidonic acid is metabolized by cytochrome P450 (CYP) epoxygenases to produce epoxyeicosatrienoic acids (EETs). There are four EET regioisomers which vary with the placement of the epoxide group. They include 5,6-, 8,9-, 11,12- and 14,15-EET. EETs are esterified to cellular membrane phospholipids following chain elongation, metabolized by β–oxidation to shorter carbon chain molecules or metabolized by soluble epoxide hydrolase (sEH) to their corresponding vicinal diols, the dihydroxyeicosatrienoic acids (DHETs). DH-EET = dihomo-EET; EHDD = epoxyheptadecadienoic acid

Figure 2

Chemical structures of epoxyeicosatrienoic acid (EET) agonists, an EET agonist/sEH inhibitor and EET antagonists. mSI = methylsulfonamide; EE8ZE = epoxyeicosa-8Z-enoic acid; EE5ZE = epoxyeicosa-5Z-enoic acid; E8ZE = eicosa-8Z-enoic acid; eEH = soluble epoxide hydrolase

Figure 3

Proposed mechanisms of epoxyeicosatrienoic acid (EET)-mediated hyperpolarization and relaxation. Stimulation by bradykinin, acetylcholine, or shear activates phospholipase in endothelial cell membranes to elicit the release of arachidonic acid. Arachidonic acid is metabolized by cytochrome P450 (CYP) epoxygenases of the 2C and 2J families to EETs. EETs diffuse to the smooth muscle cell to induce membrane hyperpolarization via activation of large conductance, calcium-sensitive potassium (BK_Ca) channels. This causes K efflux, an increase in membrane potential (E_m) or hyperpolarization and relaxation. EETs also act intracellularly in endothelial cells to promote Ca efflux through transient receptor potential (TRP) channels. Calcium activates small conductance (SK) and intermediate conductance (IK) K_Ca to cause hyperpolarization. Endothelial hyperpolarization spreads to the smooth muscle cell via gap junctions. R = putative EET receptor; Gs = stimulatory guanine nucleotide-binding protein.

Figure 4

A simplifed schematic of vascular effects of epoxyeicosatrienoic acids (EETs). Arrows denote stimulation, straight line without arrow denotes inhibition. Agonists or shear stress activate phospholipase (PL) in endothelial cell membranes. This releases arachidonic acid (AA) which is metabolized by cytochrome P450 (CYP) to EETs. EETs can diffuse to the vascular smooth muscle cell to cause relaxation and inhibit cell migration. EETs inhibit adhesion molecule expression on endothelial cells which decreases leukocyte adherence. EETs prevent platelet aggregation and adhesion. EETs promote angiogenesis by stimulating endothelial cell proliferation.

Figure 5

Vascular effects of exogenous EETs. The threshold concentration of EETs that produce the vascular effects are indicated on a concentration line. BK_Ca, large conductance, calcium-activated potassium; VCAM-1, vascular cell adhesion molecule 1; tPA, tissue plasminogen activator; ENaC, epithelial sodium channel; TRP, transient receptor potential; PMN, polymorphonuclear leukocyte