Cytochrome P450 epoxygenase pathway of polyunsaturated fatty acid metabolism

Abstract

Polyunsaturated fatty acids (PUFA) are oxidized by cytochrome P450 epoxygenases to PUFA epoxides which function as potent lipid mediators. The major metabolic pathways of PUFA epoxides are incorporation into phospholipids and hydrolysis to the corresponding PUFA diols by soluble epoxide hydrolase. Inhibitors of soluble epoxide hydrolase stabilize PUFA epoxides and potentiate their functional effects. The epoxyeicosatrienoic acids (EETs) synthesized from arachidonic acid produce vasodilation, stimulate angiogenesis, have anti-inflammatory actions, and protect the heart against ischemia-reperfusion injury. EETs produce these functional effects by activating receptor-mediated signaling pathways and ion channels. The epoxyeicosatetraenoic acids synthesized from eicosapentaenoic acid and epoxydocosapentaenoic acids synthesized from docosahexaenoic acid are potent inhibitors of cardiac arrhythmias. Epoxydocosapentaenoic acids also inhibit angiogenesis, decrease inflammatory and neuropathic pain, and reduce tumor metastasis. These findings indicate that a number of the beneficial functions of PUFA may be due to their conversion to PUFA epoxides. This article is part of a Special Issue entitled "Oxygenated metabolism of PUFA: analysis and biological relevance".

Keywords: Arachidonic acid (AA); Docosahexaenoic acid (DHA); Eicosapentaenoic acid (EPA); Epoxydocosapentaenoic acid (EpDPE); Epoxyeicosatetraenoic acid (EpETE); Epoxyeicosatrienoic acid (EET).

Fig. 1

EET synthesis in endothelial cells. In response to bradykinin or other vasodilators, AA contained in endothelial phospholipids is released by a Ca²⁺-activated phospholipase A₂ (PLA₂). CYP epoxygenases oxidize the AA to EETs in a reaction that utilizes NADPH and O₂, and the EETs are released from the cell. Although CYP epoxygenases can synthesize all four EET regioisomers as illustrated in the figure, 11,12- and 14,15-EET are the most abundant EETs produced by the endothelium and many other tissues. DHA or EPA epoxygenation by CYP epoxygenases has not been investigated in intact cells, so it is uncertain whether this receptor-mediated production mechanism also applies to ω-3 PUFA epoxides.

Fig. 2

Epoxides derived from LA and ω-3 PUFAs. The most prominent regioisomers include 9,10-epoxyoctadecaenoic acid (9,10-EpOME) from LA,17,18-epoxyeicosatetraenoic acid (17,18-EpETE) from EPA and 19,20-epoxydocosapentaenoic acid (19,20-EpDPE) from DHA. Although minor, 13,14-EpDPE is a potent activator of coronary BK_Ca channels and reduces inflammatory and neuropathic pain.

Fig. 3

Pathways of cellular EET metabolism. The illustration is for 14,15-EET, but the other EET regioisomers also are metabolized by the major pathways indicated by thick arrows with the exception of 5,6-EET which is a poor substrate for sEH. Cytosolic FABP binds to EETs, which may enhance EET uptake, although requirement for a transporter has not been demonstrated. Following uptake, the EET is either incorporated into phospholipids or hydrolyzed by soluble epoxide hydrolase (sEH) to the corresponding dihydroxyeicosatrienoic acids (DHET) which is excreted to the extracellular matrix. The mechanism of DHET exit from the cell has not been determined. Incorporation of the EET into phospholipids is mediated by a membrane-bound lysophospholipid acyltransferase (LPAT) that requires ATP and CoA. The membrane-incorporated EET is released by a phospholipases (PLA₂) and can be either hydrolyzed by sEH or converted to minor metabolites. The minor metabolites include the partial β-oxidation product 10,11-Ep-16:2, chain elongation product 16,17-Ep-22:3 and ω-oxidation product 14,15-EET(20-OH). Appreciable amounts of these metabolites are produced only if sEH is deficient or inhibited, and some of the minor metabolites have been observed only in one cell type.

Fig. 4

Mechanisms of action of EETs. 11,12-EET is used in this illustration. Many EET functions occur through a membrane receptor-dependent mechanism which activates signal transduction pathways that modulate ion channels and transcription factors in the target cell. This mechanism is indicated by the solid arrows. Some EET responses may occur through intracellular effects or direct interactions with ion channels. Because the physiological relevance of these direct acting mechanisms is less substantial, they are illustrated by the dashed arrows. Listed are the signal transduction pathways, ion channels and transcription factors that are targeted by EETs in various cells, and the functional responses that occur in the cardiovascular, renal, and nervous system.