Glycerylprostaglandin synthesis by resident peritoneal macrophages in response to a zymosan stimulus

Temporal dissociation of COX-2-dependent arachidonic acid and 2-arachidonoylglycerol metabolism in RAW264.7 macrophages

Cyclooxygenase-2 converts arachidonic acid to prostaglandins (PGs) and the endocannabinoid, 2-arachidonoylglycerol (2-AG), to PG glyceryl esters (PG-Gs). The physiological function of PG biosynthesis has been extensively studied, but the importance of the more recently discovered PG-G synthetic pathway remains incompletely defined. This disparity is due in part to a lack of knowledge of the physiological conditions under which PG-G biosynthesis occurs. We have discovered that RAW264.7 macrophages stimulated with Kdo2-lipid A (KLA) produce primarily PGs within the first 12 h followed by robust PG-G synthesis between 12 h and 24 h. We suggest that the amount of PG-Gs quantified is less than actually synthesized, because PG-Gs are subject to a significant level of hydrolysis during the time course of synthesis. Inhibition of cytosolic phospholipase A2 by giripladib does not accelerate PG-G synthesis, suggesting the differential time course of PG and PG-G synthesis is not due to the competition between arachidonic acid and 2-AG. The late-phase PG-G formation is accompanied by an increase in the level of 2-AG and a concomitant decrease in 18:0-20:4 diacylglycerol (DAG). Inhibition of DAG lipases by KT-172 decreases the levels of 2-AG and PG-Gs, indicating that the DAG-lipase pathway is involved in delayed 2-AG metabolism/PG-G synthesis. These results demonstrate that physiologically significant levels of PG-Gs are produced by activated RAW264.7 macrophages well after the production of PGs plateaus.

Zymosan-Induced Murine Peritonitis Is Associated with an Increased Sphingolipid Synthesis without Changing the Long to Very Long Chain Ceramide Ratio

Sphingolipids are key molecules in inflammation and defense against pathogens. Their role in dectin-1/TLR2-mediated responses is, however, poorly understood. This study investigated the sphingolipidome in the peritoneal fluid, peritoneal cells, plasma, and spleens of mice after intraperitoneal injection of 0.1 mg zymosan/mouse or PBS as a control. Samples were collected at 2, 4, 8, and 16 h post-injection, using a total of 36 mice. Flow cytometry analysis of peritoneal cells and measurement of IL-6, IL-1β, and TNF-α levels in the peritoneal lavages confirmed zymosan-induced peritonitis. The concentrations of sphingoid bases, dihydroceramides, ceramides, dihydrosphingomyelins, sphingomyelins, monohexosylceramides, and lactosylceramides were increased after zymosan administration, and the effects varied with the time and the matrix measured. The greatest changes occurred in peritoneal cells, followed by peritoneal fluid, at 8 h and 4 h post-injection, respectively. Analysis of the sphingolipidome suggests that zymosan increased the de novo synthesis of sphingolipids without change in the C14-C18:C20-C26 ceramide ratio. At 16 h post-injection, glycosylceramides remained higher in treated than in control mice. A minor effect of zymosan was observed in plasma, whereas sphinganine, dihydrosphingomyelins, and monohexosylceramides were significantly increased in the spleen 16 h post-injection. The consequences of the observed changes in the sphingolipidome remain to be established.

The Immune Endocannabinoid System of the Tumor Microenvironment

Leukocytes are part of the tumor microenvironment (TME) and are critical determinants of tumor progression. Because of the immunoregulatory properties of cannabinoids, the endocannabinoid system (ECS) may have an important role in shaping the TME. Members of the ECS, an entity that consists of cannabinoid receptors, endocannabinoids and their synthesizing/degrading enzymes, have been associated with both tumor growth and rejection. Immune cells express cannabinoid receptors and produce endocannabinoids, thereby forming an “immune endocannabinoid system”. Although in vitro effects of exogenous cannabinoids on immune cells are well described, the role of the ECS in the TME, and hence in tumor development and immunotherapy, is still elusive. This review/opinion discusses the possibility that the “immune endocannabinoid system” can fundamentally influence tumor progression. The widespread influence of cannabinoids on immune cell functions makes the members of the ECS an interesting target that could support immunotherapy.

Aspects of Prostaglandin Glycerol Ester Biology

The Cyclooxygenase enzymes (COX-1 and COX-2) incorporate 2 molecules of O₂ into arachidonic acid (AA), resulting in an array of bioactive prostaglandins. However, much work has been done showing that COX-2 will perform this reaction on several different AA-containing molecules, most importantly, the endocannabinoid 2-arachidonoylglycerol (2-AG). The products of 2-AG oxygenation, prostaglandin glycerol esters (PG-Gs), are analogous to canonical prostaglandins. This chapter reviews the literature detailing the production, metabolism, and bioactivity of these compounds, as well as their detection in intact animals.

Signal integration and information transfer in an allosterically regulated network

A biological reaction network may serve multiple purposes, processing more than one input and impacting downstream processes via more than one output. These networks operate in a dynamic cellular environment in which the levels of network components may change within cells and across cells. Recent evidence suggests that protein concentration variability could explain cell fate decisions. However, systems with multiple inputs, multiple outputs, and changing input concentrations have not been studied in detail due to their complexity. Here, we take a systems biochemistry approach, combining physiochemical modeling and information theory, to investigate how cyclooxygenase-2 (COX-2) processes simultaneous input signals within a complex interaction network. We find that changes in input levels affect the amount of information transmitted by the network, as does the correlation between those inputs. This, and the allosteric regulation of COX-2 by its substrates, allows it to act as a signal integrator that is most sensitive to changes in relative input levels.

Detection of Cyclooxygenase-2-Derived Oxygenation Products of the Endogenous Cannabinoid 2-Arachidonoylglycerol in Mouse Brain

Cyclooxygenase-2 (COX-2) catalyzes the formation of prostaglandins, which are involved in immune regulation, vascular function, and synaptic signaling. COX-2 also inactivates the endogenous cannabinoid (eCB) 2-arachidonoylglycerol (2-AG) via oxygenation of its arachidonic acid backbone to form a variety of prostaglandin glyceryl esters (PG-Gs). Although this oxygenation reaction is readily observed in vitro and in intact cells, detection of COX-2-derived 2-AG oxygenation products has not been previously reported in neuronal tissue. Here we show that 2-AG is metabolized in the brain of transgenic COX-2-overexpressing mice and mice treated with lipopolysaccharide to form multiple species of PG-Gs that are detectable only when monoacylglycerol lipase is concomitantly blocked. Formation of these PG-Gs is prevented by acute pharmacological inhibition of COX-2. These data provide evidence that neuronal COX-2 is capable of oxygenating 2-AG to form a variety PG-Gs in vivo and support further investigation of the physiological functions of PG-Gs.

Prostaglandin E2 glyceryl ester is an endogenous agonist of the nucleotide receptor P2Y6

Cyclooxygenase-2 catalyses the biosynthesis of prostaglandins from arachidonic acid but also the biosynthesis of prostaglandin glycerol esters (PG-Gs) from 2-arachidonoylglycerol. Previous studies identified PG-Gs as signalling molecules involved in inflammation. Thus, the glyceryl ester of prostaglandin E2, PGE2-G, mobilizes Ca2+ and activates protein kinase C and ERK, suggesting the involvement of a G protein-coupled receptor (GPCR). To identify the endogenous receptor for PGE2-G, we performed a subtractive screening approach where mRNA from PGE2-G response-positive and -negative cell lines was subjected to transcriptome-wide RNA sequencing analysis. We found several GPCRs that are only expressed in the PGE2-G responder cell lines. Using a set of functional readouts in heterologous and endogenous expression systems, we identified the UDP receptor P2Y6 as the specific target of PGE2-G. We show that PGE2-G and UDP are both agonists at P2Y6, but they activate the receptor with extremely different EC50 values of ~1 pM and ~50 nM, respectively. The identification of the PGE2-G/P2Y6 pair uncovers the signalling mode of PG-Gs as previously under-appreciated products of cyclooxygenase-2.

Assay of Endocannabinoid Oxidation by Cyclooxygenase-2

The endocannabinoids, 2-arachidonoylglycerol (2-AG) and arachidonylethanolamide (AEA), are endogenous ligands for the cannabinoid receptors (CB1 and CB2) and are implicated in a wide array of physiological processes. These neutral arachidonic acid (AA) derivatives have been identified as efficient substrates for the second isoform of the cyclooxygenase enzyme (COX-2). A diverse family of prostaglandin glycerol esters (PG-Gs) and prostaglandin ethanolamides (PG-EAs) is generated by the action of COX-2 (and downstream prostaglandin synthases) on 2-AG and AEA. As the biological importance of the endocannabinoid system becomes more apparent, there is a tremendous need for robust, sensitive, and efficient analytical methodology for the endocannabinoids and their metabolites. In this chapter, we describe methodology suitable for carrying out oxygenation of endocannabinoids by COX-2, and analysis of products of endocannabinoid oxygenation by COX-2 and of endocannabinoids themselves from in vitro and cell assays.

Competition and allostery govern substrate selectivity of cyclooxygenase-2

Cyclooxygenase-2 (COX-2) oxygenates arachidonic acid (AA) and its ester analog, 2-arachidonoylglycerol (2-AG), to prostaglandins (PGs) and prostaglandin glyceryl esters (PG-Gs), respectively. Although the efficiency of oxygenation of these substrates by COX-2 in vitro is similar, cellular biosynthesis of PGs far exceeds that of PG-Gs. Evidence that the COX enzymes are functional heterodimers suggests that competitive interaction of AA and 2-AG at the allosteric site of COX-2 might result in differential regulation of the oxygenation of the two substrates when both are present. Modulation of AA levels in RAW264.7 macrophages uncovered an inverse correlation between cellular AA levels and PG-G biosynthesis. In vitro kinetic analysis using purified protein demonstrated that the inhibition of 2-AG oxygenation by high concentrations of AA far exceeded the inhibition of AA oxygenation by high concentrations of 2-AG. An unbiased systems-based mechanistic model of the kinetic data revealed that binding of AA or 2-AG at the allosteric site of COX-2 results in a decreased catalytic efficiency of the enzyme toward 2-AG, whereas 2-AG binding at the allosteric site increases COX-2's efficiency toward AA. The results suggest that substrates interact with COX-2 via multiple potential complexes involving binding to both the catalytic and allosteric sites. Competition between AA and 2-AG for these sites, combined with differential allosteric modulation, gives rise to a complex interplay between the substrates, leading to preferential oxygenation of AA.

Regulation of inflammation by cannabinoids, the endocannabinoids 2-arachidonoyl-glycerol and arachidonoyl-ethanolamide, and their metabolites

2-Arachidonoyl-glycerol (2-AG) and arachidonyl-ethanolamide (AEA) are endocannabinoids that have been implicated in many physiologic disorders, including obesity, metabolic syndromes, hepatic diseases, pain, neurologic disorders, and inflammation. Their immunomodulatory effects are numerous and are not always mediated by cannabinoid receptors, reflecting the presence of an arachidonic acid (AA) molecule in their structure, the latter being the precursor of numerous bioactive lipids that are pro- or anti-inflammatory. 2-AG and AEA can thus serve as a source of AA but can also be metabolized by most eicosanoid biosynthetic enzymes, yielding additional lipids. In this regard, enhancing endocannabinoid levels by using endocannabinoid hydrolysis inhibitors is likely to augment the levels of these lipids that could regulate inflammatory cell functions. This review summarizes the metabolic pathways involved in the biosynthesis and metabolism of AEA and 2-AG, as well as the biologic effects of the 2-AG and AEA lipidomes in the regulation of inflammation.

Substrate-selective COX-2 inhibition as a novel strategy for therapeutic endocannabinoid augmentation

Pharmacologic augmentation of endogenous cannabinoid (eCB) signaling is an emerging therapeutic approach for the treatment of a broad range of pathophysiological conditions. Thus far, pharmacological approaches have focused on inhibition of the canonical eCB inactivation pathways - fatty acid amide hydrolase (FAAH) for anandamide and monoacylglycerol lipase (MAGL) for 2-arachidonoylglycerol. We review here the experimental evidence that cyclooxygenase-2 (COX-2)-mediated eCB oxygenation represents a third mechanism for terminating eCB action at cannabinoid receptors. We describe the development, molecular mechanisms, and in vivo validation of 'substrate-selective' COX-2 inhibitors (SSCIs) that prevent eCB inactivation by COX-2 without affecting prostaglandin (PG) generation from arachidonic acid (AA). Lastly, we review recent data on the potential therapeutic applications of SSCIs with a focus on neuropsychiatric disorders.

COX-2-derived endocannabinoid metabolites as novel inflammatory mediators

Cyclooxygenase-2 (COX-2) is an enzyme that plays a key role in inflammatory processes. Classically, this enzyme is upregulated in inflammatory situations and is responsible for the generation of prostaglandins (PGs) from arachidonic acid (AA). One lesser-known property of COX-2 is its ability to metabolize the endocannabinoids, N-arachidonoylethanolamine (AEA) and 2-arachidonoylglycerol (2-AG). Endocannabinoid metabolism by COX-2 is not merely a means to terminate their actions. On the contrary, it generates PG analogs, namely PG-glycerol esters (PG-G) for 2-AG and PG-ethanolamides (PG-EA or prostamides) for AEA. Although the formation of these COX-2-derived metabolites of the endocannabinoids has been known for a while, their biological effects remain to be fully elucidated. Recently, several studies have focused on the role of these PG-G or PG-EA in vivo. In this review we take a closer look at the literature concerning these novel bioactive lipids and their role in inflammation.

Cyclooxygenase metabolism mediates vasorelaxation to 2-arachidonoylglycerol (2-AG) in human mesenteric arteries

Objective

The vasorelaxant effect of 2-arachidonoylglycerol (2-AG) has been well characterised in animals. 2-AG is present in human vascular cells and is up-regulated in cardiovascular pathophysiology. However, the acute vascular actions of 2-AG have not been explored in humans.

Approach

Mesenteric arteries were obtained from patients receiving colorectal surgery and mounted on a myograph. Arteries were contracted and 2-AG concentration–response curves were carried out. Mechanisms of action were characterised pharmacologically. Post hoc analysis was carried out to assess the effects of cardiovascular disease/risk factors on 2-AG responses.

Results

2-AG caused vasorelaxation of human mesenteric arteries, independent of cannabinoid receptor or transient receptor potential vanilloid-1 activation, the endothelium, nitric oxide or metabolism via monoacyglycerol lipase or fatty acid amide hydrolase. 2-AG-induced vasorelaxation was reduced in the presence of indomethacin and flurbiprofen, suggesting a role for cyclooxygenase metabolism 2-AG. Responses to 2-AG were also reduced in the presence of Cay10441, L-161982 and potentiated in the presence of AH6809, suggesting that metabolism of 2-AG produces both vasorelaxant and vasoconstrictor prostanoids. Finally, 2-AG-induced vasorelaxation was dependent on potassium efflux and the presence of extracellular calcium.

Conclusions

We have shown for the first time that 2-AG causes vasorelaxation of human mesenteric arteries. Vasorelaxation is dependent on COX metabolism, activation of prostanoid receptors (EP₄ & IP) and ion channel modulation. 2-AG responses are blunted in patients with cardiovascular risk factors.

Identification of palmitoyl protein thioesterase 1 in human THP1 monocytes and macrophages and characterization of unique biochemical activities for this enzyme

The profiles of serine hydrolases in human and mouse macrophages are similar yet different. For instance, human macrophages express high levels of carboxylesterase 1 (CES1), whereas mouse macrophages have minimal amounts of the orthologous murine CES1. On the other hand, macrophages from both species exhibit limited expression of the canonical 2-arachidonoylglycerol (2-AG) hydrolytic enzyme, MAGL. Our previous study showed CES1 was partly responsible for the hydrolysis of 2-AG (50%) and prostaglandin glyceryl esters (PG-Gs) (80-95%) in human THP1 monocytes and macrophages. However, MAGL and other endocannabinoid hydrolases, FAAH, ABHD6, and ABHD12, did not have a role because of limited expression or no expression. Thus, another enzyme was hypothesized to be responsible for the remaining 2-AG hydrolysis activity following chemical inhibition and immunodepletion of CES1 (previous study) or CES1 gene knockdown (this study). Here we identified two candidate serine hydrolases in THP1 cell lysates by activity-based protein profiling (ABPP)-MUDPIT and Western blotting: cathepsin G and palmitoyl protein thioesterase 1 (PPT1). Both proteins exhibited electrophoretic properties similar to those of a serine hydrolase in THP1 cells detected by gel-based ABPP at 31-32 kDa; however, only PPT1 exhibited lipolytic activity and hydrolyzed 2-AG in vitro. Interestingly, PPT1 was strongly expressed in THP1 cells but was significantly less reactive than cathepsin G toward the activity-based probe, fluorophosphonate-biotin. KIAA1363, another serine hydrolase, was also identified in THP1 cells but did not have significant lipolytic activity. On the basis of chemoproteomic profiling, immunodepletion studies, and chemical inhibitor profiles, we estimated that PPT1 contributed 32-40% of 2-AG hydrolysis activity in the THP1 cell line. In addition, pure recombinant PPT1 catalyzed the hydrolysis of 2-AG, PGE2-G, and PGF2α-G, although the catalytic efficiency of hydrolysis of 2-AG by PPT1 was ~10-fold lower than that of CES1. PPT1 was also insensitive to several chemical inhibitors that potently inhibit CES1, such as organophosphate poisons and JZL184. This is the first report to document the expression of PPT1 in a human monocyte and macrophage cell line and to show PPT1 can hydrolyze the natural substrates 2-AG and PG-Gs. These findings suggest that PPT1 may participate in endocannabinoid metabolism within specific cellular contexts and highlights the functional redundancy often exhibited by enzymes involved in lipid metabolism.

Oxidized PLs and vascular inflammation

Oxidized PLs (OxPLs) generated in health and disease are now recognized as important mediators of cellular signalling. There is an increasing body of evidence showing that PL peroxidation is not only increased in vascular disorders, but is also a physiological event of relevance to coagulation, innate immunity, and self-tolerance. Nonenzymatically formed OxPLs generated during chronic inflammation is an uncontrolled event, generating hundreds of diverse structures, and prone to more deleterious bioactivities. In contrast, enzymatic formation of OxPLs is tightly regulated, involving receptors and intracellular signaling, acting as part of the normal physiological response to injury in order to restore homeostasis. In the present review, the major nonenzymatic OxPLs structures found during vascular inflammation are summarized, along with a brief description of their known biological activities. Also, we review what is currently known about enzymatic formation of OxPLs by acutely activated immune cells and their signaling actions under homeostatic and pathological conditions.

2-Arachidonoyl-glycerol- and arachidonic acid-stimulated neutrophils release antimicrobial effectors against E. coli, S. aureus, HSV-1, and RSV

The endocannabinoid 2-AG is highly susceptible to its hydrolysis into AA, which activates neutrophils through de novo LTB(4) biosynthesis, independently of CB activation. In this study, we show that 2-AG and AA stimulate neutrophils to release antimicrobial effectors. Supernatants of neutrophils activated with nanomolar concentrations of 2-AG and AA indeed inhibited the infectivity of HSV-1 and RSV. Additionally, the supernatants of 2-AG- and AA-stimulated neutrophils strongly impaired the growth of Escherichia coli and Staphylococcus aureus. This correlated with the release of a large amount (micrograms) of α-defensins, as well as a limited amount (nanograms) of LL-37. All the effects of AA and 2-AG mentioned above were prevented by inhibiting LTB(4) biosynthesis or by blocking BLT(1). Importantly, neither CB(2) receptor agonists nor antagonists could mimic nor prevent the effects of 2-AG, respectively. In fact, qPCR data show that contaminating eosinophils express ∼100-fold more CB(2) receptor mRNA than purified neutrophils, suggesting that CB(2) receptor expression by human neutrophils is limited and that contaminating eosinophils are likely responsible for the previously documented CB(2) expression by freshly isolated human neutrophils. The rapid conversion of 2-AG to AA and their subsequent metabolism into LTB(4) promote 2-AG and AA as multifunctional activators of neutrophils, mainly exerting their effects by activating the BLT(1). Considering that nanomolar concentrations of AA or 2-AG were sufficient to impair viral infectivity, this suggests potential physiological roles for 2-AG and AA as regulators of host defense in vivo.

New families of bioactive oxidized phospholipids generated by immune cells: identification and signaling actions

Phospholipids are of critical importance in mammalian cell biology, both through providing a permeability barrier and acting as substrates for synthesis of lipid mediators. Recently, several new families of bioactive lipids were identified that form through the enzymatic oxidation of membrane phospholipids in circulating innate immune cells and platelets. These comprise eicosanoids attached to phosphatidylethanolamine and phosphatidylcholine and form within 2-5 minutes of cell activation by pathophysiologic agonists, via the coordinated action of receptors and enzymes. In this review, we summarize what is currently known regarding their structures, mechanisms of formation, cell biology, and signaling actions. We show that phospholipid oxidation by acutely activated immune cells is a controlled event, and we propose a central role in regulating membrane biology and innate immune function during health and disease. We also review the mass spectrometry methods used for identification of the lipids and describe how these approaches can be used for discovery of new lipid mediators in complex biologic samples.

Inactivation of lipid glyceryl ester metabolism in human THP1 monocytes/macrophages by activated organophosphorus insecticides: role of carboxylesterases 1 and 2

Carboxylesterases (CES) have important roles in pesticide and drug metabolism and contribute to the clearance of ester-containing xenobiotics in mammals. Tissues with the highest levels of CES expression are the liver and small intestine. In addition to xenobiotics, CES also harness their broad substrate specificity to hydrolyze endobiotics, such as cholesteryl esters and triacylglycerols. Here, we determined if two human CES isoforms, CES1 and CES2, hydrolyze the endocannabinoids 2-arachidonoylglycerol (2AG) and anandamide (AEA), and two prostaglandin glyceryl esters (PG-Gs), which are formed by COX-mediated oxygenation of 2AG. We show that recombinant CES1 and CES2 efficiently hydrolyze 2AG to arachidonic acid (AA) but not amide-containing AEA. Steady-state kinetic parameters for CES1- and CES2-mediated 2AG hydrolysis were, respectively, kcat, 59 and 43 min(-1); Km, 49 and 46 μM; and kcat/Km, 1.2 and 0.93 μM(-1) min(-1). kcat/Km values are comparable to published values for rat monoacylglycerol lipase (MAGL)-catalyzed 2AG hydrolysis. Furthermore, we show that CES1 and CES2 also efficiently hydrolyze PGE2-G and PGF2α-G. In addition, when cultured human THP1 macrophages were treated with exogenous 2AG or PG-G (10 μM, 1 h), significant quantities of AA or PGs were detected in the culture medium; however, the ability of macrophages to metabolize these compounds was inhibited (60-80%) following treatment with paraoxon, the toxic metabolite of the insecticide parathion. Incubation of THP1 cell lysates with small-molecule inhibitors targeting CES1 (thieno[3,2-e][1]benzothiophene-4,5-dione or JZL184) significantly reduced lipid glyceryl ester hydrolase activities (40-50% for 2AG and 80-95% for PG-Gs). Immunodepletion of CES1 also markedly reduced 2AG and PG-G hydrolase activities. These results suggested that CES1 is in part responsible for the hydrolysis of 2AG and PG-Gs in THP1 cells, although it did not rule out a role for other hydrolases, especially with regard to 2AG metabolism since a substantial portion of its hydrolysis was not inactivated by the inhibitors. An enzyme (Mr 31-32 kDa) of unknown function was detected by serine hydrolase activity profiling of THP1 cells and may be a candidate. Finally, the amounts of in situ generated 2AG and PG-Gs in macrophages were enhanced by treating the cells with bioactive metabolites of OP insecticides. Collectively, the results suggest that in addition to MAGL and fatty-acid amide hydrolase (FAAH), which have both been documented to terminate endocannabinoid signaling, CES may also have a role. Furthermore, since PG-Gs have been shown to possess biological activities in their own right, CES may represent an important enzyme class that regulates their in vivo levels.

(R)-Profens are substrate-selective inhibitors of endocannabinoid oxygenation by COX-2

Cyclooxygenase-2 (COX-2) catalyzes the oxygenation of arachidonic acid and the endocannabinoids 2-arachidonoylglycerol and arachidonoylethanolamide. Evaluation of a series of COX-2 inhibitors revealed that many weak competitive inhibitors of arachidonic acid oxygenation are potent inhibitors of endocannabinoid oxygenation. (R) enantiomers of ibuprofen, naproxen and flurbiprofen, which are considered to be inactive as COX-2 inhibitors, are potent 'substrate-selective inhibitors' of endocannabinoid oxygenation. Crystal structures of the COX-2–(R)-naproxen and COX-2–(R)-flurbiprofen complexes verified this unexpected binding and defined the orientation of the (R) enantiomers relative to (S) enantiomers. (R)-Profens selectively inhibited endocannabinoid oxygenation by lipopolysaccharide-stimulated dorsal root ganglion (DRG) cells. Substrate-selective inhibition provides new tools for investigating the role of COX-2 in endocannabinoid oxygenation and a possible explanation for the ability of (R)-profens to maintain endocannabinoid tone in models of neuropathic pain.

Structural determinants for calcium mobilization by prostaglandin E2 and prostaglandin F2alpha glyceryl esters in RAW 264.7 cells and H1819 cells

2-Arachidonoylglycerol is oxygenated by cyclooxygenase-2 to form prostaglandin glyceryl esters. Previous work in this laboratory has suggested that PGE(2)-G activates a novel G protein-coupled receptor in a murine macrophage-like cell line, RAW 264.7. To probe the structural determinants for the putative receptor in RAW 264.7 cells, a panel of 10 analogs was tested for their ability to increase intracellular calcium. These analogs included PGE(2)- and PGF(2alpha)-ethanolamide, 4 PGE(2) glyceryl ester analogs, and 4 PGF(2alpha) glyceryl ester analogs. The glyceryl ester analogs differed in the positioning of the hydroxyl groups in the glycerol moiety and the type of linker (ester, amide, or thioester) of the prostaglandin to the glycerol moiety. Compounds were also evaluated in a human non-small cell lung cancer cell line (H1819). The glycerol moiety was required for the calcium response. All glyceryl ester analogs but not ethanolamides caused a concentration-dependent increase in calcium levels in both RAW 264.7 and H1819 cells. An amide or ester linkage was preferable to a thioester linkage. The EC(50) values did not significantly change when the positioning of the hydroxyls was varied. This calcium response induced by the glyceryl ester analogs appears to be independent of the putative hydrolysis products, PGE(2) and PGF(2alpha), and appears to represent a novel signaling pathway.

Identification of atypical ether-linked glycerophospholipid species in macrophages by mass spectrometry

A large scale profiling and analysis of glycerophospholipid species in macrophages has facilitated the identification of several rare and atypical glycerophospholipid species. By using liquid chromatography tandem mass spectrometry and comparison of the elution and fragmentation properties of the rare lipids to synthetic standards, we were able to identify an array of ether-linked phosphatidylinositols (PIs), phosphatidic acids, phosphatidylserines (PSs), very long chain phosphatidylethanolamines (PEs), and phosphatidylcholines (PCs) as well as phosphatidylthreonines (PTs) and a wide collection of odd carbon fatty acid-containing phospholipids in macrophages. A comprehensive qualitative analysis of glycerophospholipids from different macrophage cells was conducted. During the phospholipid profiling of the macrophage-like RAW 264.7 cells, we identified dozens of rare or previously uncharacterized phospholipids, including ether-linked PIs, PSs, and glycerophosphatidic acids, PTs, and PCs and PTs containing very long polyunsaturated fatty acids. Additionally, large numbers of phospholipids containing at least one odd carbon fatty acid were identified. Using the same methodology, we also identified many of the same species of glycerophospholipids in resident peritoneal macrophages, foam cells, and murine bone marrow derived macrophages.

Glucocorticoid-regulated crosstalk between arachidonic acid and endocannabinoid biochemical pathways coordinates cognitive-, neuroimmune-, and energy homeostasis-related adaptations to stress

Arachidonic acid and its derivatives constitute the major group of signaling molecules involved in the innate immune response and its communication with all cellular and systemic aspects involved on homeostasis maintenance. Glucocorticoids spread throughout the organism their influences over key enzymatic steps of the arachidonic acid biochemical pathways, leading, in the central nervous system, to a shift favoring the synthesis of anti-inflammatory endocannabinoids over proinflammatory metabolites, such as prostaglandins. This shift modifies local immune-inflammatory response and neuronal activity to ultimately coordinate cognitive, behavioral, neuroendocrine, neuroimmune, physiological, and metabolic adjustments to basal and stress conditions. In the hypothalamus, a reciprocal feedback between glucocorticoids and arachidonate-containing molecules provides a mechanism for homeostatic control. This neurochemical switch is susceptible to fine-tuning by neuropeptides, cytokines, and hormones, such as leptin and interleukin-1beta, assuring functional integration between energy homeostasis control and the immune/stress response.

The pharmacology and therapeutic relevance of endocannabinoid derived cyclo-oxygenase (COX)-2 products

The discovery of anandamide and 2-arachidonyl glycerol (2-AG) as naturally occurring mammalian endocannabinoids has had important and wide-reaching therapeutic implications. This, to a large extent, ensues from the complexity of endocannabinoid biology. One facet of endocannabinoid biology now receiving increased attention is the cyclo-oxygenase-2 (COX-2) derived oxidation products. Anandamide and 2-AG are oxidized to a range of PG-ethanolamides and PG-glyceryl esters that closely approaches that of the prostaglandins (PGs) formed from arachidonic acid. The pharmacology of these electrochemically neutral PG-ethanolamides (prostamides) and PG-glyceryl esters appears to be unique. No meaningful interaction with natural or recombinant prostanoid receptors is apparent. Nevertheless, in certain cells and tissues, prostamides and PG-glyceryl esters exert potent effects. The recent discovery of selective antagonists for the putative prostamide receptor has been a major advance in further establishing prostamide pharmacology as an entity distinct from prostanoid receptors. Since discovery of the prototype prostamide antagonist (AGN 204396), rapid progress has been made. The latest prostamide antagonists (AGN 211334-6) are 100 times more potent than the prototype and are, therefore, sufficiently active to be used in living animal studies. These compounds will allow a full evaluation of the role of prostamides in health and disease. To date, the only therapeutic application for prostamides is in glaucoma. The prostamide analog, bimatoprost, being the most effective ocular hypotensive drug currently available. Interestingly, PGE(2)-glyceryl ester and its chemically stable analog PGE(2)-serinolamide also lower intraocular pressure in dogs. Nevertheless, the therapeutic future of PGE(2)-glyceryl ester is more likely to reside in inflammation.

Prostaglandin I2 plays a key role in zymosan-induced mouse pleurisy

Zymosan, the cell wall of Saccharomyces cerevisiae, induces innate immune responses involving prostanoid production and complement activation. However, the roles of prostanoids in zymosan-induced inflammation and their interaction with the complement system remain to be determined. To clarify these issues, we examined zymosan-induced pleurisy in mice lacking receptors for prostaglandin (PG) E(2) (EP(-/-) mice) or PGI(2) (IP(-/-) mice). Zymosan-induced exudate formation was significantly reduced in IP(-/-) mice compared with wild-type (WT) mice, whereas none of the EP(-/-) mice (EP(1)(-/-), EP(2)(-/-), EP(3)(-/-), and EP(1)(-/-)(4) mice) showed any significant difference from WT mice. Furthermore, indomethacin, an inhibitor of prostanoid biosynthesis, suppressed exudate formation in WT mice to almost the same level as that of IP(-/-) mice. Accordingly, significant production of PGI(2) in the pleural cavity, suggested to be cyclooxygenase-2-dependent, was observed after zymosan injection. Complement activation in the pleural cavity after zymosan injection was confirmed, and preinjection of cobra venom factor (CVF), to deplete blood complement C3, was significantly suppressed after zymosan-induced exudate formation in WT mice. Simultaneous treatment with indomethacin and CVF further suppressed exudate formation in WT mice compared with each treatment alone. Because, some degree of exudate formation was still observed, other factor(s) seem to be involved. However, platelet-activating factor, a promising candidate as one such factor, was not involved in zymosan-induced exudate formation. These results clearly indicate that the PGI(2)-IP system together with the complement system plays a key role in exudate formation in zymosan-induced pleurisy.

Non-redundant functions of cyclooxygenases: oxygenation of endocannabinoids

The two cyclooxygenase (COX) enzymes catalyze the oxygenation of arachidonic acid to prostaglandin endoperoxides, which are the common intermediates in the biosynthesis of the bioactive lipids prostaglandins and thromboxane. COX-1 and COX-2 are approximately 60% identical in amino acid sequence, exhibit highly homologous three-dimensional structures, and appear functionally similar at the biochemical level. Recent work has uncovered a subtle functional difference between the two enzymes, namely the ability of COX-2 to efficiently utilize neutral derivatives (esters and amides) of arachidonic acid as substrates. Foremost among these neutral substrates are the endocannabinoids 2-arachidonoylglycerol and arachidonoylethanolamide. This raises the possibility that COX-2 oxygenation plays a role in a novel signaling pathway dependent on agonist-induced release of endocannabinoids and their selective oxygenation by COX-2. Among the products of COX-2 oxygenation of endocannabinoids are glyceryl prostaglandins, some of which (e.g. glyceryl prostaglandin E(2) and glyceryl prostaglandin I(2)) exhibit interesting biological activities in inflammatory, neurological, and vascular systems. These compounds are produced in intact cells stimulated with physiological agonists and have been isolated from in vivo sources. Important concepts relevant to the hypothesis of a COX-2-selective signaling pathway are presented.

Prostaglandin E2 glycerol ester, an endogenous COX-2 metabolite of 2-arachidonoylglycerol, induces hyperalgesia and modulates NFkappaB activity

BACKGROUND AND PURPOSE

Recombinant cyclooxygenase-2 (COX-2) oxygenates 2-arachidonoylglycerol (2-AG) in vitro. We examined whether prostaglandin E2 glycerol ester (PGE2-G), a COX-2 metabolite of 2-AG, occurs endogenously and affects nociception and immune responses.

EXPERIMENTAL APPROACH

Using mass spectrometric techniques, we examined whether PGE2-G occurs in vivo and if its levels are altered by inhibition of COX-2, monoacylglycerol (MAG) lipase or inflammation induced by carrageenan. We also examined the effects of PGE2-G on nociception in rats and NFkappaB activity in RAW264.7 cells.

KEY RESULTS

PGE2-G occurs endogenously in rat. Its levels were decreased by inhibition of COX-2 and MAG lipase but were unaffected by carrageenan. Intraplantar administration of PGE2-G induced mechanical allodynia and thermal hyperalgesia. In RAW264.7 cells, PGE2-G and PGE2 produced similar, dose-related changes in NFkappaB activity. PGE2-G was quickly metabolized into PGE2. While the effects of PGE2 on thermal hyperalgesia and NFkappaB activity were completely blocked by a cocktail of antagonists for prostanoid receptors, the same cocktail of antagonists only partially antagonized the actions of PGE2-G.

CONCLUSIONS AND IMPLICATIONS

Thermal hyperalgesia and immunomodulation induced by PGE2-G were only partially mediated by PGE2, which is formed by metabolism of PGE2-G. PGE2-G may function through a unique receptor previously postulated to mediate its effects. Taken together, these findings demonstrate that 2-AG is oxygenated in vivo by COX-2 producing PGE2-G, which plays a role in pain and immunomodulation. COX-2 could act as an enzymatic switch by converting 2-AG from an antinociceptive mediator to a pro-nociceptive prostanoid.

Glucocorticoids shift arachidonic acid metabolism toward endocannabinoid synthesis: a non-genomic anti-inflammatory switch

Glucocorticoids are capable of exerting both genomic and non-genomic actions in target cells of multiple tissues, including the brain, which trigger an array of electrophysiological, metabolic, secretory and inflammatory regulatory responses. Here, we have attempted to show how glucocorticoids may generate a rapid anti-inflammatory response by promoting arachidonic acid-containing endocannabinoids biosynthesis. According to our hypothesized model, non-genomic action of glucocorticoids results in the global shift of membrane lipid metabolism, subverting metabolic pathways toward the synthesis of the anti-inflammatory endocannabinoids, anandamide (AEA) and 2-arachidonoyl-glycerol (2-AG), and away from arachidonic acid production. Post-transcriptional inhibition of cyclooxygenase-2 (COX(2)) synthesis by glucocorticoids assists this mechanism by suppressing the synthesis of pro-inflammatory prostaglandins as well as endocannabinoid-derived prostanoids. In the central nervous system (CNS) this may represent a major neuroprotective system, which may cross-talk with leptin signaling in the hypothalamus allowing for the coordination between energy homeostasis and the inflammatory response.

The contribution of cyclooxygenase-2 to endocannabinoid metabolism and action

The development of sensitive analytical methods for measurement of endocannabinoids, their metabolites, and related lipids, has underlined the complexity of the endocannabinoid system. A case can be made for an 'endocannabinoid soup' (akin to the inflammatory soup) whereby the net effect of a pathological state and/or a pharmacological intervention on this system is the result not only of changes in endocannabinoid levels but also of their metabolites and related compounds that affect their function. With respect to the metabolism of anandamide and 2-arachidonoylglycerol, the main hydrolytic enzymes involved are fatty acid amide hydrolase and monoacylglycerol lipase. However, other pathways can come into play when these are blocked. Cyclooxygenase-2 derived metabolites of anandamide and 2-arachidonoylglycerol have a number of properties, including effects upon cell viability, contraction of the cat iris sphincter (an effect mediated by a novel receptor), mobilization of calcium and modulation of synaptic transmission. Nonsteroidal anti-inflammatory agents, whose primary mode of action is the inhibition of cyclooxygenase, can also interact with the endocannabinoid system both in vitro and in vivo. Other enzymes, such as the lipoxygenase and cytochrome P450 oxidative enzymes, can also metabolize endocannabinoids and produce biologically active compounds. It is concluded that sensitive analytical methods, which allow for measurement of endocannabinoids and related lipids, should provide vital information as to the importance of these alternative metabolic pathways when the primary hydrolytic endocannabinoid metabolizing enzymes are inhibited.

COX-2 suppresses tissue factor expression via endocannabinoid-directed PPARdelta activation

Although cyclooxygenase (COX)-2 inhibitors (coxibs) are effective in controlling inflammation, pain, and tumorigenesis, their use is limited by the recent revelation of increased adverse cardiovascular events. The mechanistic basis of this side effect is not well understood. We show that the metabolism of endocannabinoids by the endothelial cell COX-2 coupled to the prostacyclin (PGI₂) synthase (PGIS) activates the nuclear receptor peroxisomal proliferator–activated receptor (PPAR) δ, which negatively regulates the expression of tissue factor (TF), the primary initiator of blood coagulation. Coxibs suppress PPARδ activity and induce TF expression in vascular endothelium and elevate circulating TF activity in vivo. Importantly, PPARδ agonists suppress coxib-induced TF expression and decrease circulating TF activity. We provide evidence that COX-2–dependent attenuation of TF expression is abrogated by coxibs, which may explain the prothrombotic side-effects for this class of drugs. Furthermore, PPARδ agonists may be used therapeutically to suppress coxib-induced cardiovascular side effects.

Hydrolysis of Prostaglandin Glycerol Esters by the Endocannabinoid-Hydrolyzing Enzymes, Monoacylglycerol Lipase and Fatty Acid Amide Hydrolase

Cyclooxygenase-2 (COX-2) can oxygenate the endocannabinoids, arachidonyl ethanolamide (AEA) and 2-arachidonylglycerol (2-AG), to prostaglandin-H2-ethanolamide (PGH2-EA) and -glycerol ester (PGH2-G), respectively. Further metabolism of PGH2-EA and PGH2-G by prostaglandin synthases produces a variety of prostaglandin-EA's and prostaglandin-G's nearly as diverse as those derived from arachidonic acid. Thus, COX-2 may regulate endocannabinoid levels in neurons during retrograde signaling or produce novel endocannabinoid metabolites for receptor activation. Endocannabinoid-metabolizing enzymes are important regulators of their action, so we tested whether PG-G levels may be regulated by monoacylglycerol lipase (MGL) and fatty acid amide hydrolase (FAAH). We found that PG-Gs are poor substrates for purified MGL and FAAH compared to 2-AG and/or AEA. Determination of substrate specificity demonstrates a 30-100- and 150-200-fold preference of MGL and FAAH for 2-AG over PG-Gs, respectively. The substrate specificity of AEA compared to those of PG-Gs was approximately 200-300 fold higher for FAAH. Thus, PG-Gs are poor substrates for the major endocannabinoid-degrading enzymes, MGL and FAAH.

Lipid profiling reveals glycerophospholipid remodeling in zymosan-stimulated macrophages

Comprehensive lipid profiling by mass spectrometry provides comparative data on the relative distribution of individual glycerophospholipids within each of the major classes. Application of this method to the analysis of glycerophospholipid remodeling in murine primary resident peritoneal macrophages (RPMs) during zymosan phagocytosis reveals significant decreases in the levels of every major arachidonic acid (20:4)-containing species of phosphatidylcholine (GPCho) and in selected 20:4-containing phosphatidylinositol (GPIns) and phosphatidylglycerol (GPGro) species. No net changes in 20:4-containing phosphatidylethanolamine (GPEtn) species were detected. Pretreatment of RPMs with LPS resulted in subtle changes in the magnitude and kinetics of the response but had no effect on the overall pattern of zymosan-induced glycerophospholipid remodeling. Inhibition of prostaglandin (PG) synthesis with indomethacin reduced the magnitude of the changes in 20:4-containing diacyl but not alkyl acyl species. Blockade of 20:4 reacylation with thimerosal had no effect on the magnitude of the zymosan-induced changes in GPCho, GPIns, or GPGro species but revealed decreases in the level of alkyl acyl GEtn species. RAW264.7 cells contain much lower levels of phospholipid 20:4 than do RPMs and synthesize PGs poorly in response to zymosan. Pretreatment with granulocyte-macrophage colony stimulating factor, lipopolysaccharide, and interferon-gamma substantially increased the extent of 20:4 mobilization and PG synthesis in these cells. However, under conditions of maximal zymosan-dependent PG synthesis, the only glycerophospholipid that exhibited a significant change was a 20:4-containing plasmenyl GPEtn. These results suggest that GPCho is the major ultimate source of 20:4 that is mobilized in zymosan-stimulated RPMs but that 20:4 mobilization may involve the intermediate turnover of alkyl acyl GPEtn species.

Lipid profiling reveals arachidonate deficiency in RAW264.7 cells: Structural and functional implications

Glycerophospholipids containing arachidonic acid (20:4) serve as the precursors for an array of biologically active lipid mediators, most of which are produced by macrophages. We have applied mass spectrometry-based lipid profiling technology to evaluate the glycerophospholipid structure and composition of two macrophage populations, resident peritoneal macrophages and RAW264.7 cells, with regard to their potential for 20:4-based lipid mediator biosynthesis. Fatty acid analysis indicated that RAW264.7 cells were deficient in 20:4 (10 +/- 1 mol %) compared to peritoneal macrophages (26 +/- 1 mol %). Mass spectrometry of total glycerophospholipids demonstrated a marked difference in the distribution of lipid species, including reduced levels of 20:4-containing lipids, in RAW264.7 cells compared to peritoneal macrophages. Enrichment of RAW264.7 cells with 20:4 increased the fatty acid to 20 +/- 1 mol %. However, the distribution of the incorporated 20:4 remained different from that of peritoneal macrophages. RAW264.7 cells pretreated with granulocyte-macrophage colony stimulating factor followed by lipopolysaccharide and interferon-gamma mobilized similar quantities of 20:4 and produced similar amounts of prostaglandins as peritoneal macrophages treated with LPS alone. LPS treatment resulted in detectable changes in specific 20:4-containing glycerophospholipids in peritoneal cells, but not in RAW264.7 cells. 20:4-enriched RAW264.7 cells lost 88% of the incorporated fatty acid during the LPS incubation without additional prostaglandin synthesis. These results illustrate that large differences in glycerophospholipid composition may exist, even in closely related cell populations, and demonstrate the importance of interpreting the potential for lipid-mediator biosynthesis in the context of overall glycerophospholipid composition.

Targeted cyclooxygenase gene (ptgs) exchange reveals discriminant isoform functionality

The prostaglandin G/H synthase enzymes, commonly termed COX-1 and COX-2, differ markedly in their responses to regulatory stimuli and their tissue expression patterns. COX-1 is the dominant source of "housekeeping" prostaglandins, whereas COX-2 synthesizes prostaglandins of relevance to pain, inflammation, and mitogenesis. Despite these distinctions, the two enzymes are remarkably conserved, and their subcellular distributions overlap considerably. To address the functional interchangeability of the two isozymes, mice in which COX-1 is expressed under COX-2 regulatory elements were created by a gene targeting "knock-in" strategy. In macrophages from these mice, COX-1 was shown to be lipopolysaccharide-inducible in a manner analogous to COX-2 in wild-type macrophages. However, COX-1 failed to substitute effectively for COX-2 in lipopolysaccharide-induced prostaglandin E2 synthesis at low concentrations of substrate and in the metabolism of the endocannabinoid 2-arachidonylglycerol. The marked depression of the major urinary metabolite of prostacyclin in COX-2 null mice was only partially rescued by COX-1 knock-in, whereas the main urinary metabolite of prostaglandin E2 was rescued totally. Replacement with COX-1 partially rescued the impact of COX-2 deletion on reproductive function. The renal pathology consequent to COX-2 deletion was delayed but not prevented, whereas the corresponding peritonitis was unaltered. Insertion of COX-1 under the regulatory sequences that drive COX-2 expression indicated that COX-1 can substitute for some COX-2 actions and rescue only some of the consequences of gene disruption. Manipulation of COX-2 also revealed a preference for coupling with distinct downstream prostaglandin synthases in vivo. These mice will provide a valuable reagent with which to elucidate the distinct roles of the COX enzymes in mammalian biology.

Zymosan-induced glycerylprostaglandin and prostaglandin synthesis in resident peritoneal macrophages: roles of cyclo-oxygenase-1 and -2

COX [cyclo-oxygenase; PG (prostaglandin) G/H synthase] oxygenates AA (arachidonic acid) and 2-AG (2-arachidonylglycerol) to endoperoxides that are converted into PGs and PG-Gs (glycerylprostaglandins) respectively. In vitro, 2-AG is a selective substrate for COX-2, but in zymosan-stimulated peritoneal macrophages, PG-G synthesis is not sensitive to selective COX-2 inhibition. This suggests that COX-1 oxygenates 2-AG, so studies were carried out to identify enzymes involved in zymosan-dependent PG-G and PG synthesis. When macrophages from COX-1-/- or COX-2-/- mice were treated with zymosan, 20-25% and 10-15% of the PG and PG-G synthesis observed in wild-type cells respectively was COX-2 dependent. When exogenous AA and 2-AG were supplied to COX-2-/- macrophages, PG and PG-G synthesis was reduced as compared with wild-type cells. In contrast, when exogenous substrates were provided to COX-1-/- macrophages, PG-G but not PG synthesis was reduced. Product synthesis also was evaluated in macrophages from cPLA(2alpha) (cytosolic phospholipase A2alpha)-/- mice, in which zymosan-induced PG synthesis was markedly reduced, and PG-G synthesis was increased approx. 2-fold. These studies confirm that peritoneal macrophages synthesize PG-Gs in response to zymosan, but that this process is primarily COX-1-dependent, as is the synthesis of PGs. They also indicate that the 2-AG and AA used for PG-G and PG synthesis respectively are derived from independent pathways.

The endocannabinoid system: current pharmacological research and therapeutic possibilities

In the relatively short period of time since the discovery of cannabinoid receptors and their endogenous ligands, the endocannabinoids, an intensive research effort has resulted in the identification of agents that affect all aspects of the endocannabinoid system. The cannabinoid(1) receptor antagonist rimonabant is in phase III clinical trials for the treatment of obesity and as an aid to smoking cessation, and cannabinoid(2) receptor agonists are promising in animal models of inflammatory and neuropathic pain. In the present MiniReview, the endocannabinoid system is described from a pharmacological perspective. The main topics covered are: the mechanism of action of cannabinoid(2) receptor agonists; identification of the endocannabinoid(s) involved in retrograde signalling; the elusive mechanism(s) of endocannabinoid uptake; therapeutic possibilities for fatty acid amide hydrolase inhibitors; and the cyclooxygenase-2 and lipoxygenase-derived biologically active metabolites of the endocannabinoids.

Structural and functional differences between cyclooxygenases: Fatty acid oxygenases with a critical role in cell signaling

Cyclooxygenase (COX) catalyzes the first two steps in the conversion of arachidonic acid (AA) to prostaglandins (PGs). The reaction mechanism is well-defined and supported by extensive structural data. There are two isoforms of COX, which are nearly indistinguishable in structure and mechanism, however, COX-2 oxygenates neutral derivatives of AA that are poor substrates for COX-1. The best neutral substrate is 2-arachidonylglycerol, oxygenation of which produces an array of prostaglandin glyceryl esters (PG-Gs) that is nearly as diverse as the PGs. The mobilization of Ca2+ by subnanomolar concentrations of PGE2-G in RAW264.7 cells suggests the existence of a distinct receptor, and the formation of PG-Gs by zymosan-stimulated macrophages indicates that these species may be formed in vivo. These findings suggest that PG-Gs comprise a new class of lipid mediators, and that oxygenation of neutral derivatives of AA is a distinct function for the COX-2 isoform.