Endocannabinoid Biosynthesis Proceeding through Glycerophospho-N-acyl Ethanolamine and a Role for α/β-Hydrolase 4 in This Pathway

Role and regulation of acylethanolamides in energy balance: focus on adipocytes and beta-cells

The endocannabinoid, arachidonoylethanolamide (AEA), and the peroxisome proliferator-activated receptor (PPAR)-alpha ligand, oleylethanolamide (OEA) produce opposite effects on lipogenesis. The regulation of OEA and its anti-inflammatory congener, palmitoylethanolamide (PEA), in adipocytes and pancreatic beta-cells has not been investigated. We report here the results of studies on acylethanolamide regulation in these cells during obesity and hyperglycaemia, and provide an overview of acylethanolamide role in metabolic control. We analysed by liquid chromatography-mass spectrometry OEA and PEA levels in: 1) mouse 3T3F442A adipocytes during insulin-induced differentiation, 2) rat insulinoma RIN m5F beta-cells kept in 'low' or 'high' glucose, 3) adipose tissue and pancreas of mice with high fat diet-induced obesity (DIO), and 4) in visceral fat or blood of obese or type 2 diabetes (T2D) patients. In adipocytes, OEA levels remain unchanged during differentiation, whereas those of PEA decrease significantly, and are under the negative control of both leptin and PPAR-gamma. PEA is significantly downregulated in subcutaneous adipose tissue of DIO mice. In RIN m5F insulinoma beta-cells, OEA and PEA levels are inhibited by 'very high' glucose, this effect being enhanced by insulin, whereas in cells kept for 24 h in 'high' glucose, they are stimulated by both glucose and insulin. Elevated OEA and PEA levels are found in the blood of T2D patients. Reduced PEA levels in hypertrophic adipocytes might play a role in obesity-related pro-inflammatory states. In beta-cells and human blood, OEA and PEA are down- or up-regulated under conditions of transient or chronic hyperglycaemia, respectively.

Tempo and mode in the endocannaboinoid system

The best-known endocannabinoid ligands, anandamide and 2-AG, signal at least seven receptors and involve ten metabolic enzymes. Genes for the receptors and enzymes were examined for heterogeneities in tempo (relative rate of evolution, RRE) and mode (selection pressure, Ka/Ks) in six organisms with sequenced genomes. BLAST identified orthologs as reciprocal best hits, and nucleotide alignments were performed with ClustalX and MacClade. Two bioinformatics platforms, LiKaKs (a distance-based LWL85 model) and SNAP (a parsimony-based NG86 model) made pairwise comparisons of orthologs in murids (rat and mouse) and primates (human and macaque). Mean RRE of the 18 endocannabinoid genes was significantly greater in murids than primates, whereas mean Ka/Ks did not differ significantly. Next we used FUGE (tree-based maximum-likelihood model) to compute human lineage-specific Ka/Ks calculations for 18 genes, which ranged from 1.11 to 0.00, in rank order from highest to lowest: PTPN22, NAAA, TRPV1, TRPA1, NAPE-PLD, MAGL, PPARgamma, FAAH1, COX2, FAAH2, ABDH4, CB2, GPR55, DAGLbeta, PPARalpha, TRPV4, CB1, DAGLalpha; differences were significant (p < 0.0001). Rat and mouse presented different rank orders (e.g., GPR55 generated the greatest Ka/Ks ratio). The 18 genes were then tested for recent positive selection (within 10,000 yr) using an extended haplotype homozygosity analysis of SNP data from the HapMap database. Significant evidence (p < 0.05) of a positive "selective sweep" was exhibited by PTPN22, TRPV1, NAPE-PLD, and DAGLalpha. In conclusion, the endocannabinoid system is collectively under strong purifying selection, although some genes show evidence of adaptive evolution.

TheN-Acylethanolamine-Mediated Regulatory Pathway in Plants

While cannabinoids are secondary metabolites synthesized by just a few plant species, N-acylethanolamines (NAEs) are distributed widely in the plant kingdom, and are recovered in measurable, bioactive quantities in many plant-derived products. NAEs in higher plants are ethanolamides of fatty acids with acyl-chain lenghts of C12-C(18) and zero to three C=C bonds. Generally, the most-abundant NAEs found in plants and vertebrates are similar, including NAE 16 : 0, 18 : 1, 18 : 2, and 18 : 3. Like in animal systems, NAEs are formed in plants from N-acylphosphatidylethanolamines (NAPEs), and they are hydrolyzed by an amidase to yield ethanolamine and free fatty acids (FFA). Recently, a homologue of the mammalian fatty acid amide hydrolase (FAAH-1) was identified in Arabidopsis thaliana and several other plant species. Overexpression of Arabidopsis FAAH (AtFAAH) resulted in plants that grew faster, but were more sensitive to biotic and abiotic insults, suggesting that the metabolism of NAEs in plants resides at the balance between growth and responses to environmental stresses. Similar to animal systems, exogenously applied NAEs have potent and varied effects on plant cells. Recent pharmacological approaches combined with molecular-genetic experiments revealed that NAEs may act in certain plant tissues via specific membrane-associated proteins or by interacting with phospholipase D-alpha, although other, direct targets for NAE action in plants are likely to be discovered. Polyunsaturated NAEs can be oxidized via the lipoxygenase pathway in plants, producing an array of oxylipin products that have received little attention so far. Overall, the conservation of NAE occurrence and metabolic machinery in plants, coupled with the profound physiological effects of elevating NAE content or perturbing endogenous NAE metabolism, suggest that an NAE-mediated regulatory pathway, sharing similarities with the mammalian endocannabinoid pathway, indeed exists.

N-Acylethanolamine metabolism interacts with abscisic acid signaling in Arabidopsis thaliana seedlings

N-Acylethanolamines (NAEs) are bioactive acylamides that are present in a wide range of organisms. In plants, NAEs are generally elevated in desiccated seeds, suggesting that they may play a role in seed physiology. NAE and abscisic acid (ABA) levels were depleted during seed germination, and both metabolites inhibited the growth of Arabidopsis thaliana seedlings within a similar developmental window. Combined application of low levels of ABA and NAE produced a more dramatic reduction in germination and growth than either compound alone. Transcript profiling and gene expression studies in NAE-treated seedlings revealed elevated transcripts for a number of ABA-responsive genes and genes typically enriched in desiccated seeds. The levels of ABI3 transcripts were inversely associated with NAE-modulated growth. Overexpression of the Arabidopsis NAE degrading enzyme fatty acid amide hydrolase resulted in seedlings that were hypersensitive to ABA, whereas the ABA-insensitive mutants, abi1-1, abi2-1, and abi3-1, exhibited reduced sensitivity to NAE. Collectively, our data indicate that an intact ABA signaling pathway is required for NAE action and that NAE may intersect the ABA pathway downstream from ABA. We propose that NAE metabolism interacts with ABA in the negative regulation of seedling development and that normal seedling establishment depends on the reduction of the endogenous levels of both metabolites.

Coevolution between cannabinoid receptors and endocannabinoid ligands

Genes for receptors and ligands must coevolve to maintain coordinated gene expression and binding affinities. Researchers have debated whether anandamide or 2-arachidonyl glycerol (2-AG) is a more "intrinsic" ligand of cannabinoid receptors. We addressed this debate with a coevolutionary analysis, by examining genes for CB1, CB2, and ten genes that encode ligand metabolic enzymes: abhydrolase domain containing 4 protein, cyclooxygenase 2, diacylglycerol lipase paralogs (DAGLalpha, DAGLbeta), fatty acid amide hydrolase paralogs (FAAH1, FAAH2), monoglyceride lipase, N-acylethanolamine acid amidase, NAPE-selective phospholipase D, and protein tyrosine phosphatase non-receptor type 22. Gene trees (cladograms) of CB1, CB2, and ligand enzymes were obtained by searching for orthologs (tBLASTn) in the genomes of nine phylogenetically diverse species, aligning ortholog sequences with ClustalX, and applying Bayesian analysis (MrBayes). Mirrored cladograms provided evidence of coevolution (i.e., parallel cladogenesis). Next we constructed phylograms of CB1, CB2, and the ten enzymes. Phylogram branch lengths were proportional to three sets of maximum likelihood metrics: all-nucleotide-substitutions and NS/SS ratios (using PAUP()), and Ka/Ks ratios (using FUGE). Spurious correlations in all-nucleotide-substitutions trees (due to phylogenetic bias) and in Ka/Ks ratio trees (due to simplistic modeling) were parsed. Branch lengths from equivalent branches in paired trees were correlated by linear regression. Regression analyses, mirrored cladograms, and phylogenetic profiles produced the same results: close associations between cannabinoid receptors and DAGL enzymes. Therefore we propose that cannabinoid receptors initially coevolved with a fatty acid ester ligand (akin to 2-AG) in ancestral metazoans, and affinity for fatty acid ethanolamide ligands (e.g., AEA) evolved thereafter.

The pharmacology of the cannabinoid system--a question of efficacy and selectivity

Our knowledge of the function of the cannabinoid system in the body has been aided by the availability of pharmacological agents that affect its function. This has been achieved by the design of agents that either directly interact with the receptor (agonists and antagonist/inverse agonists) and agents that indirectly modulate the receptor output by changing the levels of the endogenous cannabinoids (endocannabinoids). In this review, examples of the most commonly used receptor agonists, antagonists/inverse agonists, and indirectly acting agents (anandamide uptake inhibitors, fatty acid amide hydrolase inhibitors, monoacylglycerol lipase inhibitors) are given, with particular focus upon their selectivity and, in the case of the directly acting compounds, efficacy. Finally, the links between the endocannabinoid and cyclooxygenase pathways are explored, in particular, with respect to agents whose primary function is to inhibit cyclooxygenase activity, but which also interact with the endocannabinoid system.

The role of the endocannabinoid system in gametogenesis, implantation and early pregnancy

Maternal use of marijuana, in which the exocannabinoid Delta(9)-tetrahydrocannabinol is the most active psychoactive ingredient, is known to have adverse effects on various aspects of reproduction including ovulation, spermatogenesis, implantation and pregnancy duration. Endogenous cannabinoids of which Anandamide is the prototype are widely distributed in the body especially in the reproductive tract and pregnancy tissues and act through the same receptors as the receptor as Delta(9)-tetrahydrocannabinol. Anandamide, has been reported to have pleiotropic effects on human reproduction and in experimental animal models. It appears to be the important neuro-cytokine mediator synchronizing the embryo-endometrial development for timed implantation, the development of the embryo into the blastocyst and transport of the embryo across the fallopian tubes. The mechanisms by which it exerts these effects are unclear but could be via direct actions on the various sites within the reproductive system or its differential actions on vascular tone dependent. In this review article we bring together the current knowledge on the role of endoccanabinoids in reproduction and postulate on the potential mechanisms on how these affect reproduction. In addition, we examine its role on the endothelium and vascular smooth muscle as a potential mechanism for adverse pregnancy outcome.

Multiple pathways involved in the biosynthesis of anandamide

Endocannabinoids, including anandamide (arachidonoyl ethanolamide) have been implicated in the regulation of a growing number of physiological and pathological processes. Anandamide can be generated from its membrane phospholipid precursor N-arachidonoyl phosphatidylethanolamine (NAPE) through hydrolysis by a phospholipase D (NAPE-PLD). Recent evidence indicates, however, the existence of two additional, parallel pathways. One involves the sequential deacylation of NAPE by alpha,beta-hydrolase 4 (Abhd4) and the subsequent cleavage of glycerophosphate to yield anandamide, and the other one proceeds through phospholipase C-mediated hydrolysis of NAPE to yield phosphoanandamide, which is then dephosphorylated by phosphatases, including the tyrosine phosphatase PTPN22 and the inositol 5' phosphatase SHIP1. Conversion of synthetic NAPE to AEA by brain homogenates from wild-type and NAPE-PLD(-/-) mice can proceed through both the PLC/phosphatase and Abdh4 pathways, with the former being dominant at shorter (<10 min) and the latter at longer (60 min) incubations. In macrophages, the endotoxin-induced synthesis of anandamide proceeds uniquely through the phospholipase C/phosphatase pathway.

Microglia produce and hydrolyze palmitoylethanolamide

Microglial cell activation and migration play an important role in neuroinflammation propagation. While it is known that the lipid transmitter palmitoylethanolamide (PEA) regulates microglial migration by interacting with a cannabinoid-like receptor, the production and inactivation of this lipid by microglia has never been addressed directly. Here we show that the mouse microglial cell line BV-2 produces and hydrolyzes PEA. The carbamate compound URB602 inhibits PEA hydrolysis in BV-2 cell homogenates and increases PEA levels in intact cells, whereas the FAAH inhibitor URB597 and serine-hydrolase inhibitor MAFP do not affect PEA levels in intact cells. This unique pharmacological profile of inhibitors on PEA hydrolysis suggests the involvement of a previously undescribed enzyme that degrades PEA. This enzyme expressed by microglia constitutes a promising target for controlling the propagation of neuroinflammation.

Identification of a novel endocannabinoid-hydrolyzing enzyme expressed by microglial cells

The endocannabinoids (eCBs) anandamide and 2-arachidonoyl glycerol (2-AG) are inactivated by a two-step mechanism. First, they are carried into cells, and then anandamide is hydrolyzed by fatty acid amide hydrolase (FAAH) and 2-AG by monoacylglycerol lipase (MGL). Here we provide evidence for a previously undescribed MGL activity expressed by microglial cells. We found that the mouse microglial cell line BV-2 does not express MGL mRNA and yet efficiently hydrolyzes 2-AG. URB597 (3'-carbamoyl-biphenyl-3-yl-cyclohexylcarbamate) reduces this hydrolysis by 50%, suggesting the involvement of FAAH. The remaining activity is blocked by classic MGL inhibitors [[1,1-biphenyl]-3-yl-carbamic acid, cyclohexyl ester (URB602) and MAFP (methylarachidonyl fluorophosphate)] and is unaffected by inhibitors of COXs (cyclooxygenases), LOXs (lipooxygenases), and DGLs (diacylglycerol lipases), indicating the involvement of a novel MGL activity. Accordingly, URB602 leads to selective accumulation of 2-AG in intact BV-2 cells. Although MGL expressed in neurons is equally distributed between the cytosolic, mitochondrial, and nuclear fractions, the novel MGL activity expressed by BV-2 cells is enriched in mitochondrial and nuclear fractions. A screen for novel inhibitors of eCB hydrolysis identified several compounds that differentially block MGL, FAAH, and the novel MGL activity. Finally, we provide evidence for expression of the novel MGL by mouse primary microglia in culture. Our results suggest the presence of a novel, pharmacologically distinct, MGL activity that controls 2-AG levels in microglia.

Endocannabinoids and the regulation of their levels in health and disease

PURPOSE OF REVIEW

Endocannabinoids are defined as endogenous agonists of cannabinoid receptors, that is, of the two G-protein-coupled receptors for the Cannabis psychoactive principle Delta-tetra-hydrocannabinol. Two such endogenous mediators have been most thoroughly studied so far: anandamide and 2-arachidonoylglycerol. Here we review the mechanisms for the regulation of their levels under physiological and pathological conditions, and recent findings on their role in disease.

RECENT FINDINGS

It is becoming increasingly clear that, although both anandamide and 2-arachidonoyl-glycerol are produced and degraded 'on demand', the levels of these two compounds appear to be regulated in different, and sometimes even opposing, ways, often using redundant molecular mechanisms. Alterations of endocannabinoid levels have been found in both animal models of pain, neurological and neurodegenerative states, gastrointestinal disorders and inflammatory conditions, and in blood, cerebrospinal fluid and bioptic samples from patients with various diseases.

SUMMARY

Endocannabinoid levels appear to be transiently elevated as an adaptive reaction to re-establish normal homeostasis when this is acutely and pathologically perturbed. In some chronic conditions, however, this system also contributes to the progress or symptoms of the disorder. As a consequence, new therapeutic drugs are being designed from both stimulants and blockers of endocannabinoid action.

Predominant expression of lysosomal N-acylethanolamine-hydrolyzing acid amidase in macrophages revealed by immunochemical studies

Bioactive N-acylethanolamines, including anandamide (an endocannabinoid), N-palmitoylethanolamine (an anti-inflammatory substance), and N-oleoylethanolamine (an anorexic substance) are enzymatically hydrolyzed to fatty acids and ethanolamine. Fatty acid amide hydrolase plays a major role in this reaction. In addition, we cloned cDNA of an isozyme termed "N-acylethanolamine-hydrolyzing acid amidase (NAAA)" [K. Tsuboi, Y.-X. Sun, Y. Okamoto, N. Araki, T. Tonai, N. Ueda, Molecular characterization of N-acylethanolamine-hydrolyzing acid amidase, a novel member of the choloylglycine hydrolase family with structural and functional similarity to acid ceramidase, J. Biol. Chem. 280 (2005) 11082-11092]. Previous biochemical analyses suggested the expression of NAAA in macrophage cells and various rat tissues including lung and brain. To clarify the physiological significance of NAAA, here we immunochemically studied NAAA for the first time. We developed an antibody specific for rat NAAA, and by Western blotting revealed that NAAA is glycosylated and subjected to specific proteolysis. In alveolar macrophages isolated from rat lung, NAAA was immunocytochemically localized in lysosomes. In the whole lung tissue, only alveolar macrophages were immunostained for NAAA. Conformably, the mRNA and protein levels and activity of NAAA in alveolar macrophages were much higher than those in the whole lung tissue. In brain, intraventricular macrophages were positively stained with anti-NAAA antibody, while microglia appeared to be negative. These results strongly suggested the importance of macrophages as an expression site of NAAA in rat tissues.

Biochemistry and pharmacology of endovanilloids

Endovanilloids are defined as endogenous ligands and activators of transient receptor potential (TRP) vanilloid type 1 (TRPV1) channels. The first endovanilloid to be identified was anandamide (AEA), previously discovered as an endogenous agonist of cannabinoid receptors. In fact, there are several similarities, in terms of opposing actions on the same intracellular signals, role in the same pathological conditions, and shared ligands and tissue distribution, between TRPV1 and cannabinoid CB(1) receptors. After AEA and some of its congeners (the unsaturated long chain N-acylethanolamines), at least 2 other families of endogenous lipids have been suggested to act as endovanilloids: (i) unsaturated long chain N-acyldopamines and (ii) some lipoxygenase (LOX) metabolites of arachidonic acid (AA). Here we discuss the mechanisms for the regulation of the levels of the proposed endovanilloids, as well as their TRPV1-mediated pharmacological actions in vitro and in vivo. Furthermore, we outline the possible pathological conditions in which endovanilloids, acting at sometimes aberrantly expressed TRPV1 receptors, might play a role.

A shifted repertoire of endocannabinoid genes in the zebrafish (Danio rerio)

The zebrafish has served as a model organism for developmental biology. Sequencing its genome has expanded zebrafish research into physiology and drug-development testing. Several cannabinoid pharmaceuticals are in development, but expression of endocannabinoid receptors and enzymes remains unknown in this species. We conducted a bioinformatics analysis of the zebrafish genome using 17 human endocannabinoid genes as a reference set. Putative zebrafish orthologs were identified in filtered BLAST searches as reciprocal best hits. Orthology was confirmed by three in silico methods: phylogenetic testing, synteny analysis, and functional mapping. Zebrafish expressed orthologs of cannabinoid receptor 1, transient receptor potential channel vanilloid receptor 4, GPR55 receptor, fatty acid amide hydrolase 1, monoacylglycerol lipase, NAPE-selective phospholipase D, abhydrolase domain-containing protein 4, and diacylglycerol lipase alpha and beta; and paired paralogs of cannabinoid receptor 2, fatty acid amide hydrolase 2, peroxisome proliferator-activated receptor alpha, prostaglandin-endoperoxide synthase 2, and transient receptor potential cation channel subtype A1. Functional mapping suggested the orthologs of transient receptor potential vanilloid receptor 1 and peroxisome proliferator-activated receptor gamma lack specific amino acids critical for cannabinoid ligand binding. No orthologs of N-acylethanolamine acid amidase or protein tyrosine phosphatase, non-receptor type 22 were identified. In conclusion, the zebrafish genome expresses a shifted repertoire of endocannabinoid genes. In vitro analyses are warranted before using zebrafish for cannabinoid development testing.

The emerging functions of endocannabinoid signaling during CNS development

In the postnatal brain, endocannabinoids acting as retrograde messengers regulate the function of many synapses. By contrast, the understanding of endocannabinoid functions that regulate fundamental developmental processes such as cell proliferation, migration, differentiation and survival during patterning of the CNS is just beginning to unfold. Increasing the knowledge of basic developmental and signaling principles that are controlled by endocannabinoids will provide important insights into the molecular mechanisms that establish functional neuronal circuits in the brain. Moreover, determining the molecular basis of permanent modifications to cellular structure and intercellular communication imposed by cannabis smoking during pregnancy will provide novel therapeutic targets for alleviating pathogenic changes in affected offspring. Here, we summarize recent findings regarding the ontogeny of the endocannabinoid system in neurons that sculpt the temporal and spatial diversity of cellular functions during CNS development.

Endocannabinoids and the control of energy balance

Two receptors have been cloned to date for the psychotropic compound Delta(9)-tetrahydrocannabinol, and termed cannabinoid CB(1) and CB(2) receptors. Their endogenous ligands, the endocannabinoids, have also been identified. CB(1) receptors and endocannabinoids are present in brain structures controlling energy intake and in peripheral cells (hepatocytes, adipocytes, pancreatic islet cells) regulating energy homeostasis. CB(2) receptors are more abundant in lymphocytes and macrophages, and participate in immune and inflammatory reactions. Metabolic hormones and peptides regulate the levels of the endocannabinoids and, hence, the activity of cannabinoid receptors in several tissues in a seemingly coordinated way. The endocannabinoids, particularly after stress and brief food deprivation, act in turn as local modulators of the expression and action of neurotransmitters, hormones and adipokines involved in metabolic control. Endocannabinoid overactivity seems to accompany metabolic and eating disorders and to contribute to the development of abdominal obesity, dyslipidemia and hyperglycemia. Accordingly, clinical trials have shown that CB(1) receptor antagonists are efficacious at reducing not only food intake, but also abdominal adiposity and its metabolic sequelae.