Oxidation of the endogenous cannabinoid arachidonoyl ethanolamide by the cytochrome P450 monooxygenases: physiological and pharmacological implications

Abstract

Arachidonoyl ethanolamide (anandamide) is an endogenous amide of arachidonic acid and an important signaling mediator of the endocannabinoid system. Given its numerous roles in maintaining normal physiological function and modulating pathophysiological responses throughout the body, the endocannabinoid system is an important pharmacological target amenable to manipulation directly by cannabinoid receptor ligands or indirectly by drugs that alter endocannabinoid synthesis and inactivation. The latter approach has the possible advantage of more selectivity, thus there is the potential for fewer untoward effects like those that are traditionally associated with cannabinoid receptor ligands. In that regard, inhibitors of the principal inactivating enzyme for anandamide, fatty acid amide hydrolase (FAAH), are currently in development for the treatment of pain and inflammation. However, several pathways involved in anandamide synthesis, metabolism, and inactivation all need to be taken into account when evaluating the effects of FAAH inhibitors and similar agents in preclinical models and assessing their clinical potential. Anandamide undergoes oxidation by several human cytochrome P450 (P450) enzymes, including CYP3A4, CYP4F2, CYP4X1, and the highly polymorphic CYP2D6, forming numerous structurally diverse lipids, which are likely to have important physiological roles, as evidenced by the demonstration that a P450-derived epoxide of anandamide is a potent agonist for the cannabinoid receptor 2. The focus of this review is to emphasize the need for a better understanding of the P450-mediated pathways of the metabolism of anandamide, because these are likely to be important in mediating endocannabinoid signaling as well as the pharmacological responses to endocannabinoid-targeting drugs.

Fig. 1.

Biosynthesis and degradation of anandamide. The formation of the anandamide precursor C20:4-NAPE is catalyzed by NAT, which transfers an arachidonoyl group from the sn-1 position of a phospholipid, such as phosphatidylcholine (PC), to the amino group of phosphatidylethanolamine (PE). Several pathways act upon C20:4-NAPE to produce anandamide, including 1) direct conversion by NAPE-PLD; 2) PLC-catalyzed formation of a phospho-NAE species, which is subsequently converted to anandamide via the action of phosphatases, including PTPN22 and SH2-containing inositol-5-phosphatase (SHIP1); and 3) sPLA2 or ABHD4-catalyzed formation of lyso-NAPE, followed by ABHD4-catalyzed formation of a GP-NAE species that is subsequently converted to anandamide by the phosphodiesterase GDE1. The enzymatic inactivation of anandamide is carried out by membrane-bound FAAH, which forms arachidonic acid (AA) and EA.

Fig. 2.

Oxidative pathways for the metabolism of anandamide catalyzed by cytochrome P450s and epoxide hydrolases. Anandamide undergoes epoxidation by CYP3A4, CYP2D6, and CYP4X1 to form four EET-EAs. The hydroxylation of anandamide is carried out by CYP4F2 or CYP2D6 to form 20-HETE-EA. The EET-EAs can be further metabolized by microsomal and soluble epoxide hydrolase (EH) to form the corresponding DHET-EAs or by CYP2D6 to form the corresponding HEET-EAs.

Fig. 3.

Potential protective actions of 5,6-EET-EA in pathological conditions of the liver and CNS. Anandamide (AEA) is metabolized by FAAH to arachidonic acid (AA) and by cytochrome P450 (CYP) to 5,6-EET-EA, which is in turn metabolized by epoxide hydrolase (EH) to 5,6-DHET-EA. Administration of FAAH or EH inhibitors could lead to an increase in the formation of endogenously produced 5,6-EET-EA by affecting its synthesis or its degradation. CB2, which is primarily expressed on immune cells, is activated by 5,6-EET-EA, and this activation may lead to important anti-inflammatory events that could alter the pathological outcome in both acute and chronic conditions affecting the liver and the CNS.