Endocannabinoid oxygenation by cyclooxygenases, lipoxygenases, and cytochromes P450: cross-talk between the eicosanoid and endocannabinoid signaling pathways.

The discovery of the cannabinoid receptors CB1 and CB2 in 19881,2 and 1990,(3) respectively, and of the endogenous cannabinoid ligands (endocannabinoids) arachidonoylethanolamide (AEA) and 2-arachidonoylglycerol (2-AG) in 1992(4) and 1995,(5) respectively, represented major strides in the understanding of cannabinoid physiology and pharmacology. The realization that both endocannabinoids are derivatives of arachidonic acid (AA) also revealed a potential interrelationship between the endocannabinoid and eicosanoid signaling systems that is just beginning to be unraveled. In this review, we explore what is known about the interplay between the two lipid signaling networks and discuss the challenges and opportunities offered by this new field of inquiry. 1.1. Eicosanoid Biosynthetic Pathways AA is an ω-6 tetraunsaturated fatty acid that is a component of mammalian cell membrane phospholipids, where it is predominantly esterified at the sn-2 position. AA’s role in eicosanoid signaling was first discovered in 1964, when Van Dorp et al. and Bergstrom et al. showed that incubation of the radiolabeled fatty acid with bull seminal vesicles led to the formation of prostaglandin E2 (PGE2).6,7 Since that time, we have come to appreciate that a wide range of stimuli (depending on cell type, tissue context, and physiologic state) can trigger the activation of cytosolic phospholipase A2 (cPLA2) and/or other phospholipases, leading to the release of free AA from phospholipid pools. The free fatty acid is then subject to oxidative metabolism by cyclooxygenase 1 and/or 2 (COX-1 and/or COX-2), leading to the formation of the endoperoxide PGH2. Tissue-specific metabolism of PGH2 by a group of PG synthases yields the biologically active PGs (PGE2, PGD2, PGF2α), prostacyclin (PGI2), and thromboxane A2 (TxA2) (Figure ​(Figure11a).8,9 Alternatively, free AA may be metabolized by one of a variety of lipoxygenases (LOXs) that catalyze regio- and stereospecific oxygenation, yielding hydro-peroxyeicosatetraenoic acids (HPETEs). These compounds are enzymatically or chemically reduced to the corresponding hydroxy-eicosatetraenoic acids (HETEs) or undergo further metabolism. Multiple lipoxygenations produce the lipoxins,(10) or in the case of 5-HPETE, epoxidation followed by hydrolysis or glutathione adduction yields the leukotrienes (LTs)9,11 (Figure ​(Figure1b).1b). Finally, free AA may be oxidized at each of its double bonds or at the ω-terminus by cytochromes P450, leading to the epoxyeicosatrienoic acids (EETs) or HETEs(12) (Figure ​(Figure1c).1c). Members of each of these classes of compounds possess a unique range of biological activities. Open in a separate window Figure 1 (a) Cyclooxygenase pathway of AA metabolism. AA is converted to PGG2 at the cyclooxygenase active site of COX-1 or COX-2 and is then reduced to PGH2 at the peroxidase active site. PGH2 spontaneously decomposes to yield PGE2 or PGD2, but these compounds, as well as PGF2α, PGI2, and TxA2, are also produced enzymatically by specific synthases. PGI2 and TXA2 are unstable and spontaneously decompose to yield 6-keto-PGF1α and TXB2, respectively. The ethanolamide and glyceryl ester of PGH2 produced from the metabolism of AEA and 2-AG, respectively, are converted to the same range of eicosanoid products as PGH2 with the exception of the TXA2 analogue. (b) Lipoxygenase pathway of AA metabolism. AA is converted by LOX enzymes to position-specific HPETEs, which are then reduced to the corresponding HETEs. 5-LOX also converts 5-HPETE to LTA4, which may then be metabolized to LTB4 or LTC4. The glutathionyl moiety of LTC4 is subject to enzymatic hydrolysis, yielding LTD4 and LTE4. The actions of multiple LOX enzymes lead to the formation of lipoxins. An example is provided for the synthesis of lipoxin A4 through multiple steps catalyzed by 5-LOX and either 15-LOX or 12-LOX. (c) Cytochromes P450 catalyze the epoxygenation of AA at each of the double bonds, producing the range of products shown.