Biosynthesis and inactivation of the endocannabinoid 2-arachidonoylglycerol in circulating and tumoral macrophages

Leptin-regulated endocannabinoids are involved in maintaining food intake

Leptin is the primary signal through which the hypothalamus senses nutritional state and modulates food intake and energy balance. Leptin reduces food intake by upregulating anorexigenic (appetite-reducing) neuropeptides, such as alpha-melanocyte-stimulating hormone, and downregulating orexigenic (appetite-stimulating) factors, primarily neuropeptide Y. Genetic defects in anorexigenic signalling, such as mutations in the melanocortin-4 (ref. 5) or leptin receptors, cause obesity. However, alternative orexigenic pathways maintain food intake in mice deficient in neuropeptide Y. CB1 cannabinoid receptors and the endocannabinoids anandamide and 2-arachidonoyl glycerol are present in the hypothalamus, and marijuana and anandamide stimulate food intake. Here we show that following temporary food restriction, CB1 receptor knockout mice eat less than their wild-type littermates, and the CB1 antagonist SR141716A reduces food intake in wild-type but not knockout mice. Furthermore, defective leptin signalling is associated with elevated hypothalamic, but not cerebellar, levels of endocannabinoids in obese db/db and ob/ob mice and Zucker rats. Acute leptin treatment of normal rats and ob/ob mice reduces anandamide and 2-arachidonoyl glycerol in the hypothalamus. These findings indicate that endocannabinoids in the hypothalamus may tonically activate CB1 receptors to maintain food intake and form part of the neural circuitry regulated by leptin.

The uptake by cells of 2-arachidonoylglycerol, an endogenous agonist of cannabinoid receptors

It is not yet clear if the endocannabinoid 2-arachidonoylglycerol (2-AG) is transported into cells through the same membrane transporter mediating the uptake of the other endogenous cannabinoid, anandamide (N-arachidonoylethanolamine, AEA), and whether this process (a) is regulated by cells and (b) limits 2-AG pharmacological actions. We have studied simultaneously the facilitated transport of [14C]AEA and [3H]2-AG into rat C6 glioma cells and found uptake mechanisms with different efficacies but similar affinities for the two compounds (Km 11.0 +/- 2.0 and 15.3 +/- 3.1 microM, Bmax 1.70 +/- 0.30 and 0.24 +/- 0.04 nmol.min-1.mg protein-1, respectively). Despite these similar Km values, 2-AG inhibits [14C]AEA uptake by cells at concentrations (Ki = 30.1 +/- 3.9 microM) significantly higher than those required to either 2-AG or AEA to inhibit [3H]2-AG uptake (Ki = 18.9 +/- 1.8 and 20.5 +/- 3.2 microM, respectively). Furthermore: (a) if C6 cells are incubated simultaneously with identical concentrations of [14C]AEA and [3H]2-AG, only the uptake of the latter compound is significantly decreased as compared to that observed with [3H]2-AG alone; (b) the uptake of [14C]AEA and [3H]2-AG by cells is inhibited with the same potency by AM404 (Ki = 7.5 +/- 0.7 and 10.2 +/- 1.7 microM, respectively) and linvanil (Ki = 9.5 +/- 0.7 and 6.4 +/- 1.2 microM, respectively), two inhibitors of the AEA membrane transporter; (c) nitric oxide (NO) donors enhance the uptake of both [14C]AEA and [3H]2-AG, thus suggesting that 2-AG action can be regulated through NO release; (d) AEA and 2-AG induce a weak release of NO that can be blocked by a CB1 cannabinoid receptor antagonist, and significantly enhanced in the presence of AM404 and linvanil, thus suggesting that transport into C6 cells limits the action of both endocannabinoids.

Evidence for an endocannabinoid system in the central nervous system of the leech Hirudo medicinalis

In invertebrates, like Hydra and sea urchins, evidence for a functional cannabinoid system was described. The partial characterization of a putative CB1 cannabinoid receptor in the leech Hirudo medicinalis led us to investigate the presence of a complete endogenous cannabinoid system in this organism. By using gas chromatography-mass spectrometry, we demonstrate the presence of the endocannabinoids anandamide (N-arachidonoylethanolamine, 21.5+/-0.7 pmol/g) and 2-arachidonoyl-glycerol (147.4+/-42.7 pmol/g), and of the biosynthetic precursor of anandamide, N-arachidonylphosphatidyl-ethanolamine (16.5+/-3.3 pmol/g), in the leech central nervous system (CNS). Anandamide-related molecules such as N-palmitoylethanolamine (32.4+/-1.6 pmol/g) and N-linolenoylethanolamine (5.8 pmol/g) were also detected. We also found an anandamide amidase activity in the leech CNS cytosolic fraction with a maximal activity at pH 7 and little sensitivity to typical fatty acid amide hydrolase (FAAH) inhibitors. Using an antiserum directed against the amidase signature sequence, we focused on the identification and the localization of the leech amidase. Firstly, leech nervous system protein extract was subjected to Western blot analysis, which showed three immunoreactive bands at ca. approximately 42, approximately 46 and approximately 66 kDa. The former and latter bands were very faint and were also detected in whole homogenates from the coelenterate Hydra vulgaris, where the presence of CB1-like receptors, endocannabinoids and a FAAH-like activity was reported previously. Secondly, amidase immunocytochemical detection revealed numerous immunoreactive neurons in the CNS of three species of leeches. In addition, we observed that leech amidase-like immunoreactivity matches to a certain extent with CB1-like immunoreactivity. Finally, we also found that stimulation by anandamide of this receptor leads, as in mammals, to inhibition of cAMP formation, although this effect appeared to be occurring through the previously described anandamide-induced and CB1-mediated activation of nitric oxide release. Taken together, these results suggest the existence of a complete and functional cannabinoid system in leeches.

Human platelets bind and degrade 2-arachidonoylglycerol, which activates these cells through a cannabinoid receptor

The endocannabinoid 2-arachidonoylglycerol (2-Delta(4)Ach-Gro) activates human platelets in platelet-rich plasma at physiological concentrations. The activation was inhibited by selective antagonists of CB(1) and CB(2) cannabinoid receptors, but not by acetylsalicylic acid. Human platelets can metabolize 2-Delta(4)Ach-Gro by internalization through a high affinity transporter (K(m) = 300 +/- 30 nM, V(max) = 10 +/- 1 pmol.min(-1).mg protein(-1)), followed by hydrolysis by a fatty acid amide hydrolase (K(m) = 8 +/- 1 microM, V(max) = 400 +/- 50 pmol.min(-1).mg protein(-1)). The anandamide transport inhibitor AM404, and anandamide itself, were ineffective on 2-Delta(4)Ach-Gro uptake by platelets, whereas anandamide competitively inhibited 2-Delta(4)Ach-Gro hydrolysis (inhibition constant = 10 +/- 1 microM). Platelet activation by 2-Delta(4)Ach-Gro was paralleled by an increase of intracellular calcium and inositol-1,4,5-trisphosphate, and by a decrease of cyclic AMP. Moreover, treatment of preloaded platelet-rich plasma with 2-Delta(4)Ach-Gro induced an approximately threefold increase in [(3)H]2-Delta(4)Ach-Gro release, according to a CB receptor-dependent mechanism. On the other hand, ADP and collagen counteracted the activation of platelets by 2-Delta(4)Ach-Gro, whereas 5-hydroxytryptamine (serotonin) enhanced and extended its effects. Remarkably, ADP and collagen also reduced [(3)H]2-Delta(4)Ach-Gro release from 2-Delta(4)Ach-Gro-activated platelets, whereas 5-hydroxytryptamine further increased it. These findings suggest a so far unnoticed interplay between the peripheral endocannabinoid system and physiological platelet agonists.

Gas chromatography-mass spectrometry analysis of endogenous cannabinoids in healthy and tumoral human brain and human cells in culture

Endocannabinoids are lipid mediators thought to modulate central and peripheral neural functions. We report here gas chromatography-electron impact mass spectrometry analysis of human brain, showing that lipid extracts contain anandamide and 2-arachidonoylglycerol (2-AG), the most active endocannabinoids known to date. Human brain also contained the endocannabinoid-like compounds N-oleoylethanolamine, N-palmitoylethanolamine and N-stearoylethanolamine. Anandamide and 2-AG (0.16 +/- 0.05 and 0.10 +/- 0.05 nmol/mg protein, respectively) represented 7.7% and 4.8% of total endocannabinoid-like compounds, respectively. N-Palmitoyethanolamine was the most abundant (50%), followed by N-oleoyl (23.6%) and N-stearoyl (13.9%) ethanolamines. A similar composition in endocannabinoid-like compounds was found in human neuroblastoma CHP100 and lymphoma U937 cells, and also in rat brain. Remarkably, human meningioma specimens showed an approximately six-fold smaller content of all N-acylethanolamines, but not of 2-AG, and a similar decrease was observed in a human glioblastoma. These ex vivo results fully support the purported roles of endocannabinoids in the nervous system.

Anandamide and 2-arachidonoylglycerol inhibit fatty acid amide hydrolase by activating the lipoxygenase pathway of the arachidonate cascade

Treatment of intact human neuroblastoma CHP100 cells with anandamide (arachidonoylethanolamide, AEA) or 2-arachidonoylglycerol (2-AG) inhibits intracellular fatty acid amide hydrolase (FAAH). This effect was not associated with covalent modifications of FAAH, since specific inhibitors of farnesyltransferase, kinases, phosphatases, glycosyltransferase or nitric oxide synthase were ineffective. Electrophoretic analysis of (33)P-labelled proteins, Western blot with anti-phosphotyrosine antibodies, and glycan analysis of cellular proteins confirmed the absence of covalent modifications of FAAH. The inhibition by AEA was paralleled by an increased arachidonate release, which was not observed upon treatment of cells with linoleoylethanolamide, palmitoylethanolamide, or oleoylethanolamide. Moreover, cell treatment with AEA or 2-AG increased the activity of cyclooxygenase and 5-lipoxygenase, and the hydro(pero)xides generated from arachidonate by lipoxygenase were shown to inhibit FAAH, with inhibition constants in the low micromolar range. Consistently, inhibitors of 5-lipoxygenase, but not those of cyclooxygenase, significantly counteracted the inhibition of FAAH by AEA or 2-AG.

Endocannabinoids and fatty acid amides in cancer, inflammation and related disorders

The long history of the medicinal use of Cannabis sativa and, more recently, of its chemical constituents, the cannabinoids, suggests that also the endogenous ligands of cannabinoid receptors, the endocannabinoids, and, particularly, their derivatives may be used as therapeutic agents. Studies aimed at correlating the tissue and body fluid levels of endogenous cannabinoid-like molecules with pathological conditions have been started and may lead to identify those diseases that can be alleviated by drugs that either mimic or antagonize the action of these substances, or modulate their biosynthesis and degradation. Hints for the therapeutic applications of endocannabinoids, however, can be obtained also from our previous knowledge of marijuana medicinal properties. In this article, we discuss the anti-tumor and anti-inflammatory activity of: (1) the endocannabinoids anandamide (arachidonoylethanolamide) and 2-arachidonoyl glycerol; (2) the bioactive fatty acid amides palmitoylethanolamide and oleamide; and (3) some synthetic derivatives of these compounds, such as the N-acyl-vanillyl-amines. Furthermore, the possible role of cannabimimetic fatty acid derivatives in the pathological consequences of cancer and inflammation, such as cachexia, wasting syndrome, chronic pain and local vasodilation, will be examined.

The movement of N-arachidonoylethanolamine (anandamide) across cellular membranes

This review presents and explores the hypothesis that N-arachidonoylethanolamine (AEA, also called anandamide) is transported across cellular membranes by a process that is protein-mediated. Support for this hypothesis comes from experiments demonstrating that cellular accumulation of extracellularly applied AEA is saturable, time and temperature dependent and exhibits selective inhibition by various structural analogs of AEA. The accumulation of AEA is cell specific; data is presented demonstrating that several cell types, including the bovine adrenal zona glomerulosa cell, exhibit very high capacity for AEA accumulation while others, such as the HeLa cell, have a very low capacity. The transport process has the characteristics of facilitated diffusion; it is bi-directional, not dependent on either ATP or extracellular sodium and exhibits the trans effect of flux coupling. Several important questions remain to be answered regarding the carrier, including its molecular structure and its role in the release and inactivation of endogenously produced AEA.

The fatty acid amide hydrolase (FAAH)

The topic of this review is fatty acid amide hydrolase (FAAH), one of the best-characterized enzymes involved in the hydrolysis of bioactive lipids such as anandamide, 2-arachidonoylglycerol (2-AG), and oleamide. Herein, we discuss the nomenclature, the various assays that have been developed, the relative activity of the various substrates and the reversibility of the enzyme reactions catalyzed by FAAH. We also describe the cloning of the enzyme from rat and subsequent cDNA isolation from mouse, human, and pig. The proteins and the mRNAs from different species are compared. Cloning the enzyme permitted the purification and characterization of recombinant FAAH. The conserved regions of FAAH are described in terms of sequence and function, including the amidase domain which contains the serine catalytic nucleophile, the hydrophobic domain important for self association, and the proline rich domain region, which may be important for subcellular localization. The distribution of FAAH in the major organs of the body is described as well as regional distribution in the brain and its correlation with cannabinoid receptors. Since FAAH is recognized as a drug target, a large number of inhibitors have been synthesized and tested since 1994 and these are reviewed in terms of reversibility, potency, and specificity for FAAH and cannabinoid receptors.

2-Arachidonoylglycerol and the cannabinoid receptors

2-Arachidonoylglycerol (2-AG) is a unique molecular species of monoacylglycerol isolated from rat brain and canine gut as an endogenous cannabinoid receptor ligand (Sugiura, T., Kondo, S., Sukagawa, A., Nakane, S., Shinoda, A., Itoh, K., Yamashita, A., Waku, K., 1995. 2-Arachidonoylglycerol: a possible endogenous cannabinoid receptor ligand in brain. Biochem. Biophys. Res. Commun. 215, 89-97; Mechoulam, R., Ben-Shabat, S., Hanus, L., Ligumsky, M., Kaminski, N. E., Schatz, A.R., Gopher, A., Almog, S., Martin, B.R., Compton, D.R., Pertwee, R.G., Giffin, G., Bayewitch, M., Brag, J., Vogel, Z., 1995. Identification of an endogenous 2-monoglyceride, present in canine gut, that binds to cannabinoid receptors. Biochem. Pharmacol. 50, 83-90). 2-AG binds to the cannabinoid receptors (CB1 and CB2) and exhibits a variety of cannabimimetic activities in vitro and in vivo. Recently, we found that 2-AG induces Ca(2+) transients in NG108-15 cells, which express the CB1 receptor, and in HL-60 cells, which express the CB2 receptor, through a cannabinoid receptor- and Gi/Go-dependent mechanism. Based on the results of structure-activity relationship experiments, we concluded that 2-AG but not anandamide is the natural ligand for both the CB1 and the CB2 receptors and both receptors are primarily 2-AG receptors. Evidences are gradually accumulating that 2-AG is a physiologically essential molecule, although further detailed studies appear to be necessary to determine relative importance of 2-AG and anandamide in various animal tissues. In this review, we described mainly our previous and current experimental results, as well as those of others, concerning the tissue levels, bioactions and metabolism of 2-AG.

Cannabinoid receptors and the regulation of immune response

Cannabinoid research underwent a tremendous increase during the last 10 years. This progress was made possible by the discovery of cannabinoid receptors and the endogenous ligands for these receptors. Cannabinoid research is developing in two major directions: neurobehavioral properties of cannabinoids and the impact of cannabinoids on the immune system. Recent studies characterized the cannabinoid-induced response as a very complex process because of the involvement of multiple signalling pathways linked to cannabinoid receptors or effects elicited by cannabinoids without receptor participation. The objective of this review is to present this complexity as it applies to immune response. The functional properties of cannabinoid receptors, signalling pathways linked to cannabinoid receptors and the modulation of immune response by cannabinoid receptor ligands are discussed. Special attention is given to 'endocannabinoids' as immunomodulatory molecules.

Endocannabinoids as cardiovascular modulators

Cannabinoids, the bioactive constituents of the marijuana plant and their synthetic and endogenous analogs cause not only neurobehavioral, but also cardiovascular effects. The most important component of these effects is a profound decrease in blood pressure and heart rate. Although multiple lines of evidence indicate that the hypotensive and bradycardic effects of anandamide and other cannabinoids are mediated by peripherally located CB1 cannabinoid receptors, anandamide can also elicit vasodilation in certain vascular beds, which is independent of CB1 or CB2 receptors. Possible cellular mechanisms underlying these effects and the cellular sources of vasoactive anandamide are discussed.

Oxygenation of the endocannabinoid, 2-arachidonylglycerol, to glyceryl prostaglandins by cyclooxygenase-2

Cyclooxygenases (COX) play an important role in lipid signaling by oxygenating arachidonic acid to endoperoxide precursors of prostaglandins and thromboxane. Two cyclooxygenases exist which differ in tissue distribution and regulation but otherwise carry out identical chemical functions. The neutral arachidonate derivative, 2-arachidonylglycerol (2-AG), is one of two described endocannabinoids and appears to be a ligand for both the central (CB1) and peripheral (CB2) cannabinoid receptors. Here we report that 2-AG is a substrate for COX-2 and that it is metabolized as effectively as arachidonic acid. COX-2-mediated 2-AG oxygenation provides the novel lipid, prostaglandin H(2) glycerol ester (PGH(2)-G), in vitro and in cultured macrophages. PGH(2)-G produced by macrophages is a substrate for cellular PGD synthase, affording PGD(2)-G. Pharmacological studies reveal that macrophage production of PGD(2)-G from endogenous sources of 2-AG is calcium-dependent and mediated by diacylglycerol lipase and COX-2. These results identify a distinct function for COX-2 in endocannabinoid metabolism and in the generation of a new family of prostaglandins derived from diacylglycerol and 2-AG.

Anandamide induces apoptosis in human cells via vanilloid receptors. Evidence for a protective role of cannabinoid receptors

The endocannabinoid anandamide (AEA) is shown to induce apoptotic bodies formation and DNA fragmentation, hallmarks of programmed cell death, in human neuroblastoma CHP100 and lymphoma U937 cells. RNA and protein synthesis inhibitors like actinomycin D and cycloheximide reduced to one-fifth the number of apoptotic bodies induced by AEA, whereas the AEA transporter inhibitor AM404 or the AEA hydrolase inhibitor ATFMK significantly increased the number of dying cells. Furthermore, specific antagonists of cannabinoid or vanilloid receptors potentiated or inhibited cell death induced by AEA, respectively. Other endocannabinoids such as 2-arachidonoylglycerol, linoleoylethanolamide, oleoylethanolamide, and palmitoylethanolamide did not promote cell death under the same experimental conditions. The formation of apoptotic bodies induced by AEA was paralleled by increases in intracellular calcium (3-fold over the controls), mitochondrial uncoupling (6-fold), and cytochrome c release (3-fold). The intracellular calcium chelator EGTA-AM reduced the number of apoptotic bodies to 40% of the controls, and electrotransferred anti-cytochrome c monoclonal antibodies fully prevented apoptosis induced by AEA. Moreover, 5-lipoxygenase inhibitors 5,8,11,14-eicosatetraynoic acid and MK886, cyclooxygenase inhibitor indomethacin, caspase-3 and caspase-9 inhibitors Z-DEVD-FMK and Z-LEHD-FMK, but not nitric oxide synthase inhibitor Nomega-nitro-l-arginine methyl ester, significantly reduced the cell death-inducing effect of AEA. The data presented indicate a protective role of cannabinoid receptors against apoptosis induced by AEA via vanilloid receptors.

Comparative biology of the endocannabinoid system possible role in the immune response

In this review we discuss data showing that the endogenous cannabinoid system, represented by cannabinoid receptors, endogenous cannabinoid receptor ligands and enzymes for the biosynthesis and degradation of these ligands, is conserved throughout evolution from coelenterates to man. This signaling system has been suggested to play several roles in animals, including the regulation of cell development and growth, nervous functions, reproduction and feeding behavior. In this article, however, we shall describe with more detail the possible function of the endogenous cannabinoid system in the modulation of immune response in organisms from the lower to the higher levels of animal evolution.

Structure-activity relationships among N-arachidonylethanolamine (Anandamide) head group analogues for the anandamide transporter

Two putative endocannabinoids, N-arachidonylethanolamine (AEA) and 2-arachidonylglycerol, are inactivated by removal from the extracellular environment by a process that has the features of protein-mediated facilitated diffusion. We have synthesized and studied 22 N-linked analogues of arachidonylamide for the purpose of increasing our understanding of the structural requirements for the binding of ligands to the AEA transporter. We have also determined the affinities of these analogues for both the CB(1) cannabinoid receptor and fatty acid amide hydrolase (FAAH). We have identified several structural features that enhance binding to the AEA transporter in cerebellar granule cells. We have confirmed the findings of others that replacing the ethanolamine head group with 4-hydroxybenzyl results in a high-affinity ligand for the transporter. However, we find that the same molecule is also a competitive inhibitor of FAAH. Similarly, replacement of the ethanolamine of AEA with 3-pyridinyl also results in a high-affinity inhibitor of both the transporter and FAAH. We conclude that the structural requirements for ligand binding to the CB(1) receptor and binding to the transporter are very different; however, the transporter and FAAH share most, but not all, structural requirements.

Anandamide uptake by human endothelial cells and its regulation by nitric oxide

Anandamide (AEA) has vasodilator activity, which can be terminated by cellular re-uptake and degradation. Here we investigated the presence and regulation of the AEA transporter in human umbelical vein endothelial cells (HUVECs). HUVECs take up AEA by facilitated transport (apparent K(m) = 190 +/- 10 nm and V(max) = 45 +/- 3 pmol. min(-1).mg(-1) protein), which is inhibited by alpha-linolenoyl-vanillyl-amide and N-(4-hydroxyphenyl)-arachidonoylamide, and stimulated up to 2.2-fold by nitric oxide (NO) donors. The NO scavenger hydroxocobalamin abolishes the latter effect, which is instead enhanced by superoxide anions but inhibited by superoxide dismutase and N-acetylcysteine, a precursor of glutathione synthesis. Peroxynitrite (ONOO(-)) causes a 4-fold activation of AEA transport into cells. The HUVEC AEA transporter contributes to the termination of a typical type 1 cannabinoid receptor (CB(1)) -mediated action of AEA, i.e. the inhibition of forskolin-stimulated adenylyl cyclase, because NO/ONOO(-) donors and alpha-linolenoyl-vanillyl-amide/N-(4-hydroxyphenyl)-arachidonoylamide were found to attenuate and enhance, respectively, this effect of AEA. Consistently, activation of CB(1) cannabinoid receptors by either AEA or the cannabinoid HU-210 caused a stimulation of HUVEC inducible NO synthase activity and expression up to 2.9- and 2. 6-fold, respectively. Also these effects are regulated by the AEA transporter. HU-210 enhanced AEA uptake by HUVECs in a fashion sensitive to the NO synthase inhibitor Nomega-nitro-l-arginine methyl ester. These findings suggest a NO-mediated regulatory loop between CB(1) cannabinoid receptors and AEA transporter.

Down-regulation of anandamide hydrolase in mouse uterus by sex hormones

Endocannabinoids are an emerging class of lipid mediators, which mimic several effects of cannabinoids. Anandamide (arachidonoylethanolamide) is a major endocannabinoid, which has been shown to impair pregnancy and embryo development. The activity of anandamide is controlled by cellular uptake through a specific transporter and intracellular degradation by the enzyme anandamide hydrolase (fatty acid amide hydrolase, FAAH). We characterized FAAH in mouse uterus by radiochromatographic and immunochemical techniques, showing that the enzyme is confined to the epithelium and its activity decreases appreciably during pregnancy or pseudopregnancy because of lower gene expression at the translational level. Ovariectomy prevented the decrease in FAAH, and both progesterone and estrogen further reduced its basal levels, suggesting hormonal control of the enzyme. Anandamide was shown to induce programmed cell death in mouse blastocysts, through a pathway independent of type-1 cannabinoid receptor. Blastocysts, however, have a specific anandamide transporter and FAAH, which scavenge this lipid. Taken together, these results provide evidence of an interplay between endocannabinoids and sex hormones in pregnancy. These findings may also be relevant for human fertility, as epithelial cells from healthy human uterus showed FAAH activity and expression, which in adenocarcinoma cells was increased fivefold.

Anandamide induces apoptosis of PC-12 cells: involvement of superoxide and caspase-3

Anandamide (arachidonoylethanolamide), an endogenous cannabinoid receptor ligand has been suggested to have physiological role in mammalian nervous system. However, little is known about the role of anandamide on neuronal cells. Here, we demonstrate that anandamide causes death of PC-12 cells, showing marked DNA condensation and fragmentation, appearance of cells at sub-G(0)/G(1) and redistribution of phosphatidyl serine, the hallmark features of apoptosis. Anandamide raised intracellular superoxide level and CPP32-like protease activity in PC-12 cells markedly. Furthermore, antioxidant N-acetyl cysteine prevented anandamide-induced superoxide anion formation and cell death, implying that intracellular superoxide is a novel mediator of anandamide-induced apoptosis of PC-12 cells.

Cardiovascular effects of endocannabinoids--the plot thickens

Cannabinoids, the bioactive ingredients of the marijuana plant, are best known for their psychoactive properties, but they also influence other physiological processes, such as cardiovascular variables. Endocannabinoids are recently identified lipid mediators that act as natural ligands at cannabinoid receptors and mimic most of the biological effects, including the cardiovascular actions, of plant-derived cannabinoids. In experimental animals, the most prominent component of the cardiovascular effects of cannabinoids is prolonged hypotension and bradycardia. This review focuses on the possible mechanisms underlying these effects. The emerging evidence suggesting that endocannabinoids may be involved in the peripheral regulation of vascular tone under certain conditions is also discussed.

Biochemistry and pharmacology of the endocannabinoids arachidonylethanolamide and 2-arachidonylglycerol

The purpose of this review is to discuss the cellular synthesis and inactivation of two putative endogenous ligands of the cannabinoid receptor, N-arachidonylethanolamine (AEA) and 2-arachidonylglycerol (2-AG). Both ligands are synthesized by neurons and brain tissue in response to increased intracellular calcium concentrations. Both ligands are substrates for fatty acid amide hydrolase (FAAH). Both AEA and 2-AG bind to the neuronal form of the cannabinoid receptor (CB1). AEA binds the receptor with moderate affinity and has the characteristics of a partial agonist, whereas, 2-AG binds with low affinity but exhibits full efficacy. Two possible physiological roles of the endocannabinoids and the CB1 receptor are discussed: the regulation of gestation and the regulation of gastrointestinal motility.

Anandamide amidohydrolase (fatty acid amide hydrolase)

Anandamide (N-arachidonoylethanolamine) loses its cannabimimetic activity when it is hydrolyzed to arachidonic acid and ethanolamine by the catalysis of an enzyme referred to as anandamide amidohydrolase or fatty acid amide hydrolase. Cravatt's group and our group cloned cDNA of the enzyme from rat, human, mouse and pig, and the primary structures revealed that the enzymes belong to an amidase family characterized by the amidase signature sequence. The recombinant enzyme acted not only as an amidase for anandamide and oleamide, but also as an esterase for 2-arachidonoylglycerol. The reversibility of the enzymatic anandamide hydrolysis and synthesis was also confirmed with a purified recombinant enzyme. Several fatty acid derivatives like methyl arachidonyl fluorophosphonate potently inhibited the enzyme. The enzyme was distributed widely in mammalian organs such as liver, small intestine and brain. However, the anandamide hydrolyzing enzyme found in human megakaryoblastic cells was catalytically distinct from the previously known enzyme.

Cannabimimetic fatty acid derivatives in cancer and inflammation

Evidence for the role of the cannabimimetic fatty acid derivatives (CFADs), i.e. anandamide (arachidonoylethanolamide, AEA), 2-arachidonoylglycerol (2-AG) and palmitoylethanolamide (PEA), in the control of inflammation and of the proliferation of tumor cells is reviewed here. The biosynthesis of AEA, PEA, or 2-AG can be induced by stimulation with either Ca(2+) ionophores, lipopolysaccharide, or platelet activating factor in macrophages, and by ionomycin or antigen challenge in rat basophilic leukemia (RBL-2H3) cells (a widely used model for mast cells). These cells also inactivate CFADs through re-uptake and/or hydrolysis and/or esterification processes. AEA and PEA modulate cytokine and/or arachidonate release from macrophages in vitro, regulate serotonin secretion from RBL-2H3 cells, and are analgesic in some animal models of inflammatory pain. However, the involvement of endogenous CFADs and cannabinoid CB(1) and CB(2) receptors in these effects is still controversial. In human breast and prostate cancer cells, AEA and 2-AG, but not PEA, potently inhibit prolactin and/or nerve growth factor (NGF)-induced cell proliferation. Vanillyl-derivatives of anandamide, such as olvanil and arvanil, exhibit even higher anti-proliferative activity. These effects are due to suppression of the levels of the 100 kDa prolactin receptor or of the high affinity NGF receptors (trk), are mediated by CB(1)-like cannabinoid receptors, and are enhanced by other CFADs. Inhibition of adenylyl cyclase and activation of mitogen-activated protein kinase underlie the anti-mitogenic actions of AEA. The possibility that CFADs act as local inhibitors of the proliferation of human breast cancer is discussed here.

Human mast cells take up and hydrolyze anandamide under the control of 5-lipoxygenase and do not express cannabinoid receptors

Human mast cells (HMC-1) take up anandamide (arachidonoyl-ethanolamide, AEA) with a saturable process (K(m)=200+/-20 nM, V(max)=25+/-3 pmol min(-1) mg protein(-1)), enhanced two-fold over control by nitric oxide-donors. Internalized AEA was hydrolyzed by a fatty acid amide hydrolase (FAAH), whose activity became measurable only in the presence of 5-lipoxygenase, but not cyclooxygenase, inhibitors. FAAH (K(m)=5.0+/-0.5 microM, V(max)=160+/-15 pmol min(-1) mg protein(-1)) was competitively inhibited by palmitoylethanolamide. HMC-1 cells did not display a functional cannabinoid receptor on their surface and neither AEA nor palmitoylethanolamide affected tryptase release from these cells.

Cardiovascular effects of 2-arachidonoyl glycerol in anesthetized mice

Cannabinoids, including the endogenous ligand anandamide, elicit pronounced hypotension and bradycardia through the activation of CB1 cannabinoid receptors. A second endogenous cannabinoid, 2-arachidonoyl glycerol (2-AG), has been proposed to be the natural ligand of CB1 receptors. In the present study, we examined the effects of 2-AG on mean arterial pressure and heart rate in anesthetized mice and assessed the role of CB1 receptors through the use of selective cannabinoid receptor antagonists and CB1 receptor knockout (CB1(-/-)) mice. In control ICR mice, intravenous injections of 2-AG or its isomer 1-AG elicit dose-dependent hypotension and moderate tachycardia that are unaffected by the CB1 receptor antagonist SR141716A. The same dose of SR141716A (6 nmol/g IV) completely blocks the hypotensive effect and attenuates the bradycardic effect of anandamide. 2-AG elicits a similar hypotensive effect, resistant to blockade by either SR141716A or the CB2 antagonist SR144528, in both CB1(-/-) mice and their homozygous (CB1(+/+)) control littermates. In ICR mice, arachidonic acid (AA, 15 nmol/g IV) elicits hypotension and tachycardia, and indomethacin (14 nmol/g IV) inhibits the hypotensive effect of both AA and 2-AG. Synthetic 2-AG incubated with mouse blood is rapidly (<2 minutes) and completely degraded with the parallel appearance of AA, whereas anandamide is stable under the same conditions. A metabolically stable ether analogue of 2-AG causes prolonged hypotension and bradycardia in ICR mice, and both effects are completely blocked by SR141716A, whereas the same dose of 2-AG-ether does not influence blood pressure and heart rate in CB1(-/-) mice. These findings are interpreted to indicate that exogenous 2-AG is rapidly degraded in mouse blood, probably by a lipase, which masks its ability to interact with CB1 receptors. Although the observed cardiovascular effects of 2-AG probably are produced by an arachidonate metabolite through a noncannabinoid mechanism, the CB1 receptor-mediated cardiovascular effects of a stable analogue of 2-AG leaves open the possibility that endogenous 2-AG may elicit cardiovascular effects through CB1 receptors.

Evidence that 2-arachidonoylglycerol but not N-palmitoylethanolamine or anandamide is the physiological ligand for the cannabinoid CB2 receptor. Comparison of the agonistic activities of various cannabinoid receptor ligands in HL-60 cells

We examined the effect of 2-arachidonoylglycerol, an endogenous cannabinoid receptor ligand, on the intracellular free Ca(2+) concentrations in HL-60 cells that express the cannabinoid CB2 receptor. We found that 2-arachidonoylglycerol induces a rapid transient increase in intracellular free Ca(2+) concentrations in HL-60 cells. The response was affected by neither cyclooxygenase inhibitors nor lipoxygenase inhibitors, suggesting that arachidonic acid metabolites are not involved. Consistent with this notion, free arachidonic acid was devoid of any agonistic activity. Importantly, the Ca(2+) transient induced by 2-arachidonoylglycerol was blocked by pretreatment of the cells with SR144528, a CB2 receptor-specific antagonist, but not with SR141716A, a CB1 receptor-specific antagonist, indicating the involvement of the CB2 receptor but not the CB1 receptor in this cellular response. G(i) or G(o) is also assumed to be involved, because pertussis toxin treatment of the cells abolished the response. We further examined the structure-activity relationship. We found that 2-arachidonoylglycerol is the most potent compound among a number of naturally occurring cannabimimetic molecules. Interestingly, anandamide and N-palmitoylethanolamine, other putative endogenous ligands, were found to be a weak partial agonist and an inactive ligand, respectively. These results strongly suggest that the CB2 receptor is originally a 2-arachidonoylglycerol receptor, and 2-arachidonoylglycerol is the intrinsic natural ligand for the CB2 receptor that is abundant in the immune system.

Suppression of nerve growth factor Trk receptors and prolactin receptors by endocannabinoids leads to inhibition of human breast and prostate cancer cell proliferation

Anandamide and 2-arachidonoylglycerol (2-AG), two endogenous ligands of the CB1 and CB2 cannabinoid receptor subtypes, inhibit the proliferation of PRL-responsive human breast cancer cells (HBCCs) through down-regulation of the long form of the PRL receptor (PRLr). Here we report that 1) anandamide and 2-AG inhibit the nerve growth factor (NGF)-induced proliferation of HBCCs through suppression of the levels of NGF Trk receptors; 2) inhibition of PRLr levels results in inhibition of the proliferation of other PRL-responsive cells, the prostate cancer DU-145 cell line; and 3) CB1-like cannabinoid receptors are expressed in HBCCs and DU-145 cells and mediate the inhibition of cell proliferation and Trk/PRLr expression. Beta-NGF-induced HBCC proliferation was potently inhibited (IC50 = 50-600 nM) by the synthetic cannabinoid HU-210, 2-AG, anandamide, and its metabolically stable analogs, but not by the anandamide congener, palmitoylethanolamide, or the selective agonist of CB2 cannabinoid receptors, BML-190. The effect of anandamide was blocked by the CB1 receptor antagonist, SR141716A, but not by the CB2 receptor antagonist, SR144528. Anandamide and HU-210 exerted a strong inhibition of the levels of NGF Trk receptors as detected by Western immunoblotting; this effect was reversed by SR141716A. When induced by exogenous PRL, the proliferation of prostate DU-145 cells was potently inhibited (IC50 = 100-300 nM) by anandamide, 2-AG, and HU-210. Anandamide also down-regulated the levels of PRLr in DU-145 cells. SR141716A attenuated these two effects of anandamide. HBCCs and DU-145 cells were shown to contain 1) transcripts for CB1 and, to a lesser extent, CB2 cannabinoid receptors, 2) specific binding sites for [3H]SR141716A that could be displaced by anandamide, and 3) a CB1 receptor-immunoreactive protein. These findings suggest that endogenous cannabinoids and CB1 receptor agonists are potential negative effectors of PRL- and NGF-induced biological responses, at least in some cancer cells.