Molecular mechanisms and cell signaling of 20-hydroxyeicosatetraenoic acid in vascular pathophysiology

Abstract

Cytochrome P450s enzymes catalyze the metabolism of arachidonic acid to epoxyeicosatrienoic acids (EETs), dihydroxyeicosatetraenoic acid and hydroxyeicosatetraeonic acid (HETEs). 20-HETE is a vasoconstrictor that depolarizes vascular smooth muscle cells by blocking K+ channels. EETs serve as endothelial derived hyperpolarizing factors. Inhibition of the formation of 20-HETE impairs the myogenic response and autoregulation of renal and cerebral blood flow. Changes in the formation of EETs and 20-HETE have been reported in hypertension and drugs that target these pathways alter blood pressure in animal models. Sequence variants in CYP4A11 and CYP4F2 that produce 20-HETE, UDP-glucuronosyl transferase involved in the biotransformation of 20-HETE and soluble epoxide hydrolase that inactivates EETs are associated with hypertension in human studies. 20-HETE contributes to the regulation of vascular hypertrophy, restenosis, angiogenesis and inflammation. It also promotes endothelial dysfunction and contributes to cerebral vasospasm and ischemia-reperfusion injury in the brain, kidney and heart. This review will focus on the role of 20-HETE in vascular dysfunction, inflammation, ischemic and hemorrhagic stroke and cardiac and renal ischemia reperfusion injury.

Figure 1. Pathways for the metabolism of arachidonic acid

Arachidonic acid is metabolized by enzymes of the cytochrome P450 (CYP) monooxygenase, cyclooxygenase and lipoxygenase pathways to epoxyeicosatrienoic acid (EETs), 20-hydroxytetraenoic acid (20-HETE), prostacyclin (PGI₂), thromboxane A2 (TXA₂), Prostaglandins (PGE₂ and PGF_2a), 5-, 12-, and 15-hydroxyeicosatetraenoic acids (HETEs) and leukotrienes. EETs are metabolized by soluble epoxide hydrolase (_SEH) to the corresponding dihydroxyeicosatrienoic acids (DHETEs). 20-HETE is metabolized by cyclooxygenase 2 (COX₂) to 20 hydroxy-prostaglandins (20-OH-PGE); conjugated by uridine 5′-diphosphoglucuronosyltransferase (UGT) in the liver to form a glucuronide and metabolized by alcohol dehydrogenase (ADH) to 20-carboxy-hydroxyeicosatetraenoic acid (20-COOH-HETE that is converted to shorter chain dicarboxylic acids by β-oxidation. Leukotrienes are metabolized by CYP4F2 enzyme to a less active 20-hydroxy metabolite (20-OH-LTB₄).

Figure 2. Diverse physiological and pathological effects of 20-HETE

20-HETE is a potent vasoconstrictor in the renal and cerebral circulation that potentiates the vasoconstrictor response to Ang II, endothelin and other Gq dependent vasoactive agents. It promotes vascular hypertrophy, endothelial dysfunction, vascular restenosis, angiogenesis and vascular inflammation. In blood, 20-HETE inhibits platelet aggregation. In the lung, 20-HETE is an endothelial dependent dilator that reduces pulmonary vascular resistance and lowers airway resistance. 20-HETE plays an important role in the myogenic and tubuloglomerular feedback responses of the renal afferent arteriole. It also inhibits sodium transport in the proximal tubule and thick ascending loop of Henle. Deficiencies in the renal formation of 20-HETE are associated with the development of salt-sensitive forms of hypertension and renal ischemia reperfusion injury. Overproduction of 20-HETE is linked to the development of polycystic kidney disease. 20-HETE plays an important role in the regulation of cerebral vascular tone and elevations in the production of 20-HETE is associated with cerebral vasospasm following subarachnoid hemorrhage and infarct size following ischemic injury. In the heart, elevations in 20-HETE promote the development of cardiac hypertrophy and infarct size following cardiac ischemia.

Figure 3. Mechanisms contributing to the vasoconstrictor action of 20-HETE

Increases in vascular stretch or administration of Gq coupled vasoconstrictor agonists activate phospholipase A (PLA) secondary to inositol triphosphate (IP3) mediated release of intracellular Ca²⁺. PLA₂ catalyzes the releases of arachidonic acid from membrane phospholipids and stimulates the formation of 20-hydroxyeicosatetraenoic acid (20-HETE). 20-HETE activates phosphokinase C to inhibit activity of the large conductance, calcium activated potassium channel (K_Ca) to depolarize the cell and increases the activities of the L-type calcium and TRPC6 channels to promote Ca²⁺ entry.

Figure 4. 20-HETE and endothelial dysfunction

Upregulation of the formation of 20-HETE in endothelial cells increases the formation of superoxide (O₂⁻) by stimulating NADPH oxidase activity. It also uncouples endothelial nitric oxide synthase (eNOS) to inhibit the formation of nitric oxide (NO) and increase the formation of superoxide. The rise in superoxide levels reacts with NO to form peroxynitrite and lower the bioavailability of NO and that promotes endothelial dysfunction.

Figure 5. Angiogenic effects of 20-HETE. Panel A and B

Implantation of a pellet containing angiogenic substances in the rabbit cornea induces growth of blood vessels. Panel C and D indicates that implantation of a 20-HETE mimetic is angiogenic and increases the growth of blood vessels in the cornea of rabbits. Panels E and F indicate that implantation of VEGF, FGF and other growth factors induce angiogenesis; administration of a 20-HETE synthesis inhibitor or a 20-HETE antagonist completely blocks the angiogenic effects of VEGF and other growth factors. Figure redrawn from data presented in Chen et al. (180) with permission.

Figure 6. Role of 20-HETE in hemorrhagic and ischemic stroke

Subarachnoid hemorrhage (SAH) leads to the release of free hemoglobin (Hb) and serotonin (5-HT) from clotting blood. Serotonin activates phospholipase A₂ in the cerebral vasculature to increase the release of arachidonic acid and the formation of 20-hydroxyeicosatetraenoic acid (20-HETE). Free hemoglobin scavenges NO and the fall in NO levels increase the activity of CYP4A enzymes and the formation of 20-HETE. The rise in 20-HETE levels contributes to acute and delayed vasospasm in SAH. 20-HETE levels are also elevated in cerebral tissue, CSF and plasma following ischemic stroke. The increase in 20-HETE levels was initially thought to promote infarct size by reducing blood flow in the penumbral regions, but subsequent work indicates that 20-HETE increases oxidative stress in ischemic neurons and promotes apoptosis and cell death.