Keizo Waku

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 Ca2+ concentrations in HL-60 cells that express the cannabinoid CB2 receptor. We found that 2-arachidonoylglycerol induces a rapid transient increase in intracellular free Ca2+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 Ca2+ 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. Gi or Go 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.

Evidence That the Cannabinoid CB1 Receptor Is a 2-Arachidonoylglycerol Receptor STRUCTURE-ACTIVITY RELATIONSHIP OF 2-ARACHIDONOYLGLYCEROL, ETHER-LINKED ANALOGUES, AND RELATED COMPOUNDS

An endogenous cannabimimetic molecule, 2-arachidonoylglycerol, induces a rapid, transient increase in intracellular free Ca2+ concentrations in NG108–15 cells through a cannabinoid CB1 receptor-dependent mechanism. We examined the activities of 24 relevant compounds (2-arachidonoylglycerol, its structural analogues, and several synthetic cannabinoids). We found that 2-arachidonoylglycerol is the most potent compound examined so far: its activity was detectable from as low as 0.3 nm, and the maximal response induced by 2-arachidonoylglycerol exceeded the responses induced by others. Activities of HU-210 and CP55940, potent cannabinoid receptor agonists, were also detectable from as low as 0.3 nm, whereas the maximal responses induced by these compounds were low compared with 2-arachidonoylglycerol. Anandamide was also found to act as a partial agonist in this assay system. We confirmed that free arachidonic acid failed to elicit a response. Furthermore, we found that a metabolically stable ether-linked analogue of 2-arachidonoylglycerol possesses appreciable agonistic activity, although its activity was apparently lower than that of 2-arachidonoylglycerol. We also confirmed that pretreating cells with various cannabinoid receptor agonists nullified the response induced by 2-arachidonoylglycerol, whereas pretreating cells with other neurotransmitters or neuromodulators did not affect the response. These results strongly suggested that the cannabinoid CB1 receptor is originally a 2-arachidonoylglycerol receptor, and 2-arachidonoylglycerol is the intrinsic physiological ligand for the cannabinoid CB1 receptor.

Synaptically Driven Endocannabinoid Release Requires Ca2+-Assisted Metabotropic Glutamate Receptor Subtype 1 to Phospholipase C β4 Signaling Cascade in the Cerebellum

Endocannabinoids mediate retrograde signaling and modulate synaptic transmission in various regions of the CNS. Depolarization-induced elevation of intracellular Ca2+ concentration causes endocannabinoid-mediated suppression of excitatory/inhibitory synaptic transmission. Activation of Gq/11-coupled receptors including group I metabotropic glutamate receptors (mGluRs) also causes endocannabinoid-mediated suppression of synaptic transmission. However, precise mechanisms of endocannabinoid production initiated by physiologically relevant synaptic activity remain to be determined. To address this problem, we made whole-cell recordings from Purkinje cells (PCs) in mouse cerebellar slices and examined their excitatory synapses arising from climbing fibers (CFs) and parallel fibers (PFs). We first characterized three distinct modes to induce endocannabinoid release by analyzing CF to PC synapses. The first mode is strong activation of mGluR subtype 1 (mGluR1)-phospholipase C (PLC) β4 cascade without detectable Ca2+ elevation. The second mode is Ca2+ elevation to a micromolar range without activation of the mGluR1-PLCβ4 cascade. The third mode is the Ca2+-assisted mGluR1-PLCβ4 cascade that requires weak mGluR1 activation and Ca2+ elevation to a submicromolar range. By analyzing PF to PC synapses, we show that the third mode is essential for effective endocannabinoid release from PCs by excitatory synaptic activity. Furthermore, our biochemical analysis demonstrates that combined weak mGluR1 activation and mild depolarization in PCs effectively produces 2-arachidonoylglycerol (2-AG), a candidate of endocannabinoid, whereas either stimulus alone did not produce detectable 2-AG. Our results strongly suggest that under physiological conditions, excitatory synaptic inputs to PCs activate the Ca2+-assisted mGluR1-PLCβ4 cascade, and thereby produce 2-AG, which retrogradely modulates synaptic transmission to PCs.

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.

2‐Arachidonoylglycerol, an endogenous cannabinoid receptor agonist: identification as one of the major species of monoacylglycerols in various rat tissues, and evidence for its generation through Ca2+‐dependent and ‐independent mechanisms

The molecular species compositions of monoacylglycerols obtained from various rat tissues were examined by reverse‐phase high‐performance liquid chromatography (HPLC) and gas chromatography‐mass spectrometry (GC‐MS) analyses. We confirmed that 2‐arachidonoylglycerol, an endogenous cannabinoid receptor agonist, is one of the most abundant molecular species of monoacylglycerols in the brain. Substantial amounts of 2‐arachidonoylglycerol were also found in the liver, spleen, lung and kidney, but the levels were considerably lower than that in the brain. We found that a small amount of 2‐arachidonoylglycerol was generated in a brain homogenate during incubation in the absence of Ca2+. Importantly, the generation of 2‐arachidonoylglycerol was markedly augmented in the presence of Ca2+, suggesting that Ca2+ plays a key role in regulation of the generation of 2‐arachidonoylglycerol in this tissue.

Different fatty chain compositions of alkenylacyl, alkylacyl and diacyl phospholipids in rabbit alveolar macrophages: High amounts of arachidonic acid in ether phospholipids

High levels of ether phospholipids were found in rabbit alveolar macrophages. Choline phosphoglycerides (CPG) contained a significant amount of alkylacyl compound (32.5%). On the other hand, ethanolamine phosphoglyceride (EPG) included a very large amount of alkenylacyl compounds (61.2%). Small amounts of alkenylacyl CPG and alkylacyl EPG were also observed. The occurrence of a high amount of alkylacyl CPG may be related to the synthesis or release of platelet-activating factor (PAF) from macrophages. Fatty chains at the 1- and 2-positions in each lipid class of CPG or CPG or alkenylacyl EPG were several other. Particularly, the levels of 20∶4 (arachidonic acid) in alkylacyl CPG or alkenylacyl EPG were several times higher than those in corresponding diacyl phospholipids. Large portions of 20∶4-containing species have alkenyl or alkyl ether moieties at their 1-position in both CPG (73.6%) and EPG (85.9%). These results suggest the importance of ether-containing phospholipids in rabbit alveolar macrophages.

Occurrence of lysophosphatidic acid and its alkyl ether-linked analog in rat brain and comparison of their biological activities toward cultured neural cells.

Rat brain was found to contain substantial amounts of potent bioactive lipids lysophosphatidic acid (acyl LPA) (3.73 nmol/g tissue) and lysoplasmanic acid (alkyl LPA) (0.44 nmol/g tissue). The presence of alkyl LPA was confirmed by mild alkaline hydrolysis analysis and by gas chromatography/mass spectrometry analysis of the trimethylsilyl derivative. This is the first clear evidence of the occurrence of an alkyl LPA in nature. The predominant molecular species of acyl LPA are 18:1-, 18:0- and 16:0-containing species (46. 9, 22.5 and 18.8%, respectively). A significant amount of a 20:4-containing species (7.2%) was also detected in the acyl LPA fraction. We also confirmed that rat brain alkyl LPA consists of 16:0-, 18:0- and 18:1-containing species. Noticeably, either acyl or alkyl LPA is capable of stimulating neuroblastomaxglioma hybrid NG108-15 cells to elicit a Ca(2+) transient, the potencies being almost the same. Both acyl and alkyl LPAs also induce cell rounding upon addition to the cells. These results suggest that acyl and alkyl LPAs play important physiological roles as intercellular signaling molecules as well as the roles as metabolic intermediates in the nervous system.

2‐Arachidonoylglycerol, an endogenous cannabinoid receptor ligand, induces the migration of EoL‐1 human eosinophilic leukemia cells and human peripheral blood eosinophils

2‐Arachidonoylglycerol (2‐AG) is an endogenous cannabinoid receptor ligand. To date, two types of cannabinoid receptors have been identified: the CB1 receptor, abundantly expressed in the brain, and the CB2 receptor, expressed in various lymphoid tissues such as the spleen. The CB1 receptor has been assumed to play an important role in the regulation of synaptic transmission, whereas the physiological roles of the CB2 receptor remain obscure. In this study, we examined whether the CB2 receptor is present in human eosinophils and found that the CB2 receptor is expressed in human peripheral blood eosinophils. In contrast, human neutrophils do not contain a significant amount of the CB2 receptor. We then examined the effect of 2‐AG on the motility of eosinophils. We found that 2‐AG induces the migration of human eosinophilic leukemia EoL‐1 cells. The migration evoked by 2‐AG was abolished in the presence of SR144528, a CB2 receptor antagonist, or by pretreatment of the cells with pertussis toxin, suggesting that the CB2 receptor and Gi/o are involved in the 2‐AG‐induced migration. The migration of EoL‐1 cells induced by 2‐AG was suggested to be a result of chemotaxis. In contrast to 2‐AG, neither anandamide nor free arachidonic acid elicited the migration. Finally, we examined the effect of 2‐AG on human peripheral blood eosinophils and neutrophils and found that 2‐AG induces migration of eosinophils but not neutrophils. These results suggest that the CB2 receptor and its endogenous ligand 2‐AG may be closely involved in allergic inflammation accompanied by the infiltration of eosinophils.

COA-independent transfer of arachidonic acid from 1,2-diacyl-sn-glycero-3-phosphocholine to 1-0-alkyl-sn-glycero-3-phosphocholine (lyso plateletactivating factor) by macrophage microsomes

Macrophage microsomes catalyzed the transfer of arachidonic acid (20:4) from 1,2-diacyl-glycerophosphocholine (GPC) to 1-alkyl-GPC (lyso platelet-activating factor). This enzyme reaction did not require the presence of cofactors such as Co A. Free arachidonic acid or linoleic acid-labeled phospholipids failed to act as the acyl donor. These results suggest that the reaction is a CoA-independent direct transfer of arachidonic acid. This arachidonoyl transacylation system may play a very important role in the metabolism of lyso platelet-activating factor and also in the elimination or release of arachidonic acid from diacyl-GPC.

Acyltransferases and transacylases that determine the fatty acid composition of glycerolipids and the metabolism of bioactive lipid mediators in mammalian cells and model organisms

Over one hundred different phospholipid molecular species are known to be present in mammalian cells and tissues. Fatty acid remodeling systems for phospholipids including acyl-CoA:lysophospholipid acyltransferases, CoA-dependent and CoA-independent transacylation systems, are involved in the biosynthesis of these molecular species. Acyl-CoA:lysophospholipid acyltransferase system is involved in the synthesis of phospholipid molecular species containing sn-1 saturated and sn-2 unsaturated fatty acids. The CoA-dependent transacylation system catalyzes the transfer of fatty acids esterified in phospholipids to lysophospholipids in the presence of CoA without the generation of free fatty acids. The CoA-dependent transacylation reaction in the rat liver exhibits strict fatty acid specificity, i.e., three types of fatty acids (20:4, 18:2 and 18:0) are transferred. On the other hand, CoA-independent transacylase catalyzes the transfer of C20 and C22 polyunsaturated fatty acids from diacyl phospholipids to various lysophospholipids, especially ether-containing lysophospholipids, in the absence of any cofactors. CoA-independent transacylase is assumed to be involved in the accumulation of PUFA in ether-containing phospholipids. These enzymes are involved in not only the remodeling of fatty acids, but also the synthesis and degradation of some bioactive lipids and their precursors. In this review, recent progresses in acyltransferase research including the identification of the enzymes genes are described.